glucocorticoids ( gcs ) are among the most effective drugs used in the treatment of hematopoietic malignancies of the lymphoid lineage in virtue of their ability to induce apoptosis of these cancerous cells [ 13 ] .
the main hematopoietic cancer types that respond well to gc therapy include t acute lymphoblastic leukemia ( t - all ) , chronic b lymphocytic leukemia ( cll ) , multiple myeloma ( mm ) , hodgkin 's lymphoma ( hl ) , and non - hodgkin 's lymphoma ( nhl ) .
gcs appear , however , to have little value in the treatment of acute or chronic myeloid leukemia ( aml / cml ) .
a major drawback of gc therapy is the gradual development of resistance to gc during treatment that limits the clinical utility of this drug .
poor response to a 7-day monotherapy with the gc prednisone is one of the strongest predictors of adverse outcomes in the treatment of pediatric all [ 2 , 4 ] .
a great challenge today is to develop strategies that can overcome the drug resistant phenotype . for this purpose
it is important to understand the underlying mechanisms of gc resistance and the signaling pathways regulating apoptosis induced by gcs . besides inducing apoptosis of lymphoid cells , gcs
gc treatment produces rapid symptomatic improvements , including relief of fever , sweats , lethargy , weakness , and other nonspecific effects of cancer.gcs decrease the severity of chemotherapy - induced emesis .
gcs are also used in the clinics for other medical conditions such as autoimmune diseases , asthma , ulcerative colitis , chronic obstructive pulmonary disease , kidney diseases , and rheumatologic disorders due to their strong anti - inflammatory and immunosuppressive properties .
gc therapy is hampered by a variety of metabolic and medical complications , including insulin resistance , diabetes , hypertension , glaucoma , osteoporosis , and osteonecrosis with increased risk of bone fractures [ 510 ] .
diabetes may develop by direct gc - mediated induction of apoptosis in insulin - producing beta cells of the langerhans islets [ 1113 ] , and osteoporosis may develop due to apoptosis of osteoblasts [ 1416 ] .
gcs also suppress cell growth and proliferation processes in the brain [ 17 , 18 ] . besides being used as monotherapy at high dosages , gcs
are frequently combined with other chemotherapeutic drugs to achieve rapid and more efficient therapeutic effects . for the treatment of t - all ,
gcs such as prednisone , methylprednisolone , and dexamethasone are usually used in combination with other chemotherapeutic drugs such as vincristine , daunorubicine , l - asparaginase , cytosine arabinoside , doxorubicin , and cyclophosphamide .
this multidrug regimen prolongs remission , minimizes the long - term use of prednisone , and thus reduces the steroid - mediated adverse effects .  
typical b - cell chronic lymphocytic leukemia ( cll ) in the early stage of progression responds well to combination chemotherapy including an alkylating agent ( such as chlorambucil ) plus or minus prednisolone.advanced stages of the disease often require the addition of an anthracycline and a vinca alkaloid for successful therapy .
one commonly used combination is cyclophosphamide , doxorubicin , vincristine , and prednisolone , a drug combination termed chop .
rituximab , a chimeric monoclonal antibody directed against the b - cell specific antigen cd20 , is often added to the therapy , which is here termed r - chop .
rituximab is also combined with fludarabine and cyclophosphamide in the treatment of cll [ 19 , 20 ] .
another antibody proved to be efficient against cll in combination with methylprednisolone is alemtuzumab , which targets cd52 .
the immunomodulatory drug lenalidomide shows also good activity in relapse / refractory or treatment - nave cll [ 23 , 24 ] .
chop is also used for non - hodgkin 's lymphomas and anaplastic large cell lymphoma ( alcl ) .
, prednisone has been used in combination with carmustine , vincristine ( oncovin ) , procarbazine ( mopp ) , and rituximab .
recently , brentuximab vedotin ( adcetris ) , an antibody directed towards cd30 conjugated with the anti - tubulin chemotherapeutic agent monomethyl auristatin e , has been approved for the treatment of hodgkin 's lymphoma and systemic anaplastic large cell lymphoma .
cd30 expression is restricted to only a relative small population of activated t and b cells , and therefore this treatment is expected to be more selective for cd30-positive tumor cells .
another monoclonal antibody entered the clinics is epratuzumab , which targets cd22 and is proved to be efficient in the treatment of adult non - hodgkin 's lymphoma as a single agent or in combination with chemotherapy . a phase ii clinical trial showed that combining epratuzumab with rituximab and chop ( er - chop )
may have a favorable response on diffusing large b - cell non - hodgkin lymphoma ( dlbcl ) .  
multiple myeloma ( mm ) has frequently been treated with vincristine , doxorubicine ( adriamycin ) , and dexamethasone ( vad ) or prednisone / melphalan .
bortezomib ( velcade ) , lenalidomide , and to a lesser extend thalidomide have proven efficient in the treatment of mm in combination with dexamethasone .
lenalidomide is a 4-amino - glutamyl analogue of thalidomide that lacks the neurological side effects of thalidomide and has emerged as a drug with activity against various hematological malignancies [ 27 , 28 ] .
bortezomib is a selective inhibitor of the 26s proteasome that stabilizes many cell cycle - regulatory proteins .
the antitumor effects of bortezomib in lymphoid tumors have been attributed to nfb inhibition through stabilization of its inhibitor ib .
other tumors that have been treated with combination chemotherapy involving a gc include medulloblastoma , primitive neuroectodermal tumors , and ependymomas . one major obstacle in the therapy of lymphoid malignancies is the appearance of gc resistant cells .
drug resistance may occur at the level of the glucocorticoid receptor ( gr ) or through alterations in downstream regulatory pathways . in
most gc - resistant all primary biopsy specimens , gr was found to be functional , suggesting that pharmacological intervention may restore drug sensitivity .
several strategies have been developed that aim to overcome drug resistance through specifically targeting anti - apoptotic pathways .
gc resistance may occur due to overexpression of anti - apoptotic proteins of the bcl-2 superfamily [ 30 , 31 ] . among these ,
small molecules that target the anti - apoptotic proteins of the bcl-2 family are attractive drugs that should be able to overcome gc resistance .
one example is abt-737 , a bh3 mimetic that inhibits the pro - survival function of bcl-2 , bcl - xl , and bcl - w and induces apoptosis in a variety of cancer cell types including leukemias [ 3335 ] .
treatment of the lymphoma - prone e-myc transgenic mice with abt-737 prevented the development of myc - driven lymphomagenesis , understating the need for these anti - apoptotic proteins .
combined use of abt-737 and the dual specificity pi3k / mtor inhibitor pi-103 led to loss of c - myc expression and apoptosis of burkitt 's lymphoma cells , whose tumorigenicity is driven by overexpression of the c - myc gene .  
the pro - apoptotic effect of abt-737 in cll depends on sufficient amount of bcl-2 that tonically sequesters the pro - apoptotic bim protein .
also , the sensitivity of lymphoma cell lines to bcl-2 antagonism is directly related to the amount of bcl-2 primed with bim .
the sequestration of bim may explain the marked chemosensitivity of cll and follicular lymphoma ( fl ) that express abundant bcl-2
abt-737 potentiated the effect of vincristine , dexamethasone , and l - asparaginase ( vxl ) treatment on all cells   and could potentiate the effect of the vxl combination in chemoresistant human primary all xenografts .
this study also shows a synergistic effect between the three components of the vxl regimen .
an additive effect was observed in primary mm cells when abt-737 was combined with dexamethasone [ 41 , 42 ] .  
abt-263 ( navitoclax ) is a second generation , orally bioavailable small molecule bcl-2 family protein inhibitor that has entered clinical trials with promising efficacy on cll [ 4346 ] .
abt-263 has been shown to have synergistic effects with r - chop treatment on mantle cell lymphoma .
the pro - apoptotic bim that is displaced from bcl-2 by abt-737 , becomes captured by either bfl-1 or mcl-1
. the resistance could be overcome by decreasing the mcl-1 level with the cyclin - dependent kinase ( cdk ) inhibitors flavopiridol and pha767491 , or by inhibiting mtor complex 1 ( mtorc1 )   or glycolysis [ 49 , 50 ] .   another approach to overcome mcl-1-dependent resistance is to use the small molecule obatoclax ( gx15 - 070 ) that has entered clinical trials in the combined treatment of various hematopoietic neoplasms [ 5153 ] .
obatoclax disrupts the interaction between mcl-1 and its pro - apoptotic counterparts including bak , bax , and noxa [ 54 , 55 ] .
obatoclax and flavopiridol synergized in overcoming drug resistance in human myeloma cells through a mechanism involving bim and noxa .
the multikinase inhibitor sorafenib could synergize with obatoclax in inducing apoptosis in acute myeloid leukemia ( aml ) through downregulating mcl-1 .
obatoclax could overcome gc resistance in all through induction of apoptosis and autophagy , an effect that depends on the pro - apoptotic bak and to a certain extent also on beclin-1 [ 58 , 59 ] , a mammalian orthologue of yeast atg6 that plays a central role in autophagy .
under certain conditions , cell death induced by obatoclax and gc may be executed in the absence of both bax and bak . under these conditions ,
necroptosis ensues necroptosis ensues , a process mediated by rip-1 ( receptor - interacting protein-1 ) kinase and the cylindromatosis deubiquitinase cyld .
it may promote either cell death or cell survival dependent on its ubiquitinated state , which is regulated by cyld and a20 , two nfb target genes .
altogether , there is a general consensus that obatoclax might be a favorable drug that ought to be combined with dexamethasone / prednisone and/or rapamycin to overcome gc resistance in all cells and other hematological lymphoid malignancies .  

1.2.1.3 . overcoming bcl-2-mediated resistance with small molecular inhibitors of xiap ( x - linked inhibitor of apoptosis ) .
bcl-2-mediated resistance in cll may also be overcome by small molecular inhibitors of the anti - apoptotic xiap ( x - linked inhibitor of apoptosis ) when exposed to trail [ 62 , 63 ] .
xiap and the cellular ciaps 1 and 2 are expressed at high levels in cll cells [ 62 , 63 ] .
xiap inhibitors enhanced bcl-2 cleavage and induced a conformational change in bax . similarly , xiap inhibitors sensitized all for cd95-induced apoptosis . in patients with t - all ,
xiap inhibition using the low - molecular - weight smac mimetic lbw242 resulted in increased prednisone - induced apoptosis in vitro .  
another anti - apoptotic protein that negatively regulates gc - induced apoptosis is notch1 [ 6668 ] .
notch1 is indispensable for normal t - cell development [ 6971 ] and is an attractive target in the treatment of hematopoietic malignancies of the t lineage .
mice transplanted with bone marrow cells transduced with a constitutively active form of notch1 develop t - cell neoplasms , while mice transgenic for constitutively active form of notch3 develop thymic lymphomas .
acute lymphoblastic t - cell leukemia is frequently associated with increased notch signaling [ 7579 ] , which may be caused by the chromosomal translocation t(7 ; 9)(q34 ; q34.3 ) , gain - of - function mutations of notch1 , and/or mutations in fbw7 ( f - box and wd repeat domain - containing 7 ) , a negative regulator of notch1 .  
one approach to avoid notch activation is to prevent its cleavage by the -secretase complex using -secretase inhibitors ( gsi ) .
rather , gsi can enhance the pro - apoptotic effect of gcs and other chemotherapeutic agents including the mtor inhibitor rapamycin [ 84 , 88 ] .
gsi does not overcome gc resistance in t - all deficient for pten [ 89 , 90 ] , supposedly due to elevated akt activity .
the constitutive akt activation in the absence of pten leads to increased glucose metabolism and bypasses the requirement of notch signaling to sustain cell growth . in this context
it should be noted that notch1 by itself may upregulate the p13k / akt pathway via its target gene hes1 .
as pten is a target of several micrornas that are often expressed abnormally in cancer ( see section  2.4.2.3 ) , resistance to gsi may be far more prevalent .
nevertheless , gsi compounds , such as pf-03084014 , have entered clinical trials for refractory t - all .
preclinical data do show a synergistic effect between gsi inhibition and gc in reducing xenografted t - all tumor burden .
another concern associated with the clinical use of gsis is severe toxicity to various organs at therapeutic doses , which may be explained by the broad action of notch1 as well as -secretase on various biological systems .
the simultaneous use of gcs may prevent the gsi - induced gastrointestinal toxicity via inhibition of goblet cell metaplasia .
a more specific inhibition of notch1 can be achieved by the sahm1 peptide that prevents notch - mediated transcription by interfering with the mastermind - notch interaction essential for notch - mediated transcription of target genes .
the effect of this peptide on gc sensitivity awaits examination as well as its toxicity . since notch signaling
is intertwined with the pi3k / akt / mtor signaling axis [ 9496 ] , the inhibition of the latter has proven to be more efficient in overcoming gc resistance ( see section 1.2.3 ) and would be a better therapeutic choice . accumulating data show that gc therapy can affect the activity of several protein kinases , and , vice versa , many protein kinases can affect gc - induced apoptosis [ 30 , 31 , 9799 ] .
the mtor signaling pathway is frequently activated and found to be essential for cell growth and survival in lymphoid malignancies [ 100106 ] .
gc resistance frequently appears in malignant cells due to aberrant activation of various protein kinases that exert anti - apoptotic effects [ 30 , 31 , 67 , 97 , 107109 ] .
one strategy to overcome gc resistance would be to prevent the activities of the pi3k / akt / mtor , mek1/erk1/2 , and other activated protein kinase pathways .
the mtor inhibitor rapamycin especially has proven efficient in sensitizing human gc - resistant t - all , b - all , mm , and npm - alk ( nucleophosmin - anaplastic lymphoma kinase)-dlbcl to gc - induced apoptosis [ 110117 ] .
the combinatory therapy of rapamycin with dexamethasone was proven to be effective also in pten - negative cells . a lower dose of dexamethasone was sufficient for reducing t - all burden in a xenograft model when used together with rapamycin . one major drawback with rapamycin therapy
is its immunosuppressive function , which adds to the immunosuppressive function of gcs . the dual pi3k / mtor inhibitor nvp - bez235 synergistically enhanced cytotoxicity of dexamethasone , doxorubicine , and cytosine arabinoside ( arac ) , even in gc - resistant all cells .
the broad - acting protein kinase staurosporine was especially effective in overcoming gc resistance in mouse lymphomas that overexpressed notch-1 , bcl-2 , and/or bcl - xl .
this sensitization was achieved through prevention of akt - mediated inhibition of gsk3   and induction of the pro - apoptotic nur77 . however , staurosporine was less effective on human t - all cell lines ( unpublished data ) , which could rather be sensitized to gc by rapamycin . in order to choose the right kinase inhibitor for combinatory therapy ,
the cyclin - dependent kinase ( cdk ) inhibitors flavopiridol ( alvocidib ) , bms-387032 ( sns-032 ) , sunitinib , and sorafenib are currently under clinical trials for relapsed / refractory cll .
these include vandetanib ( zd6474 ) , bosutinib ( ski-606 ) , tki258 ( chir-258 ) , pazopanib ( gw786034 ) , and axitinib ( ag013736 ) .
chir-258 , a potent inhibitor of flt3 ( fms - like tyrosine kinase receptor-3 ) , c - kit tyrosine kinase , and fibroblast growth factor receptor 3 ( fgfr3 ) , prevented cell growth of fgfr3-positive human multiple myeloma cell lines and augmented their sensitivity to gc - induced apoptosis .
importantly , neither interleukin-6 ( il-6 ) nor stromal cells conferred resistance to chir-258 .   other protein kinase inhibitors with more cell - type specific effects have been developed , which are expected to have less adverse effects . the classical example for efficient use of a specific protein kinase inhibitor in the clinics is the bcr - abl kinase inhibitor sti-572 ( imatinib ) used for the treatment of chronic myelogenic leukemia ( cml ) . a similar strong response of a single agent
was observed in alk - anaplastic large cell lymphoma ( alcl ) patients treated with crizotinib , an inhibitor of the alk tyrosine kinase .
another promising target is the b - cell receptor ( bcr ) signaling , which is important during b - cell oncogenesis and is a key to the survival of malignant b cells , including cll and dlbcl [ 125 , 126 ] .
the survival of dlbcl may depend on the nonligand - dependent ( tonic ) signals from the bcr .
the bcr signaling can be targeted with small molecular inhibitors directed against bruton 's tyrosine kinase ( btk ) , spleen tyrosine kinase ( syk ) , or phosphoinositide 3-kinase ( pi3k ) isoform p110 ( pi3k ) , all being efficient in the treatment of cll . targeting btk with the inhibitor pci-32765 leads to disruption of bcr signaling and was effective in a preclinical model of b cell non - hodgin 's lymphoma [ 127 , 128 ] .
pci-32765 seems also to be promising for the treatment of cll [ 128131 ] and mm .
importantly , pci-32765 induced apoptosis in cll cells even in the presence of various exogenous stimuli , including cd40l , baff , il-6 , and il-4 and when cultivated together with stromal cells .
two other btk inhibitors , ibrutinib and avl-263 , are also under investigation for cll .
the syk ( spleen tyrosine kinase ) inhibitor fostamatinib had clinical activity in non - hodgkin lymphoma and cll .
syk is a cytoplasmic tyrosine kinase that is important for immunoreceptor signaling in b cells .
syk has also been shown to be critical for the survival and maintenance of mature normal and malignant b cells [ 125 , 134 ] and is frequently expressed at high levels in follicular lymphoma .
the pi3k inhibitor gs-1101 ( cal-101 ) had preclinical and clinical activity against cll , mantle cell lymphoma , and mm [ 121 , 129 , 136138 ] . while the pi3k and  isoforms are ubiquitously expressed , pi3k expression is largely restricted to hematopoietic cells , where it plays a role in b - cell homeostasis and function .
the effect of the btk , syk , and pi3k kinase inhibitors on the sensitivity to gcs warrants investigations .  
accordi et al .   found aberrant activation of protein kinases in poor prognosis pediatric b - cell precursor - all patients .
the p56 ( lymphocyte cell - specific tyrosine kinase ) activity was enhanced in patients with poor clinical response to prednisone with respect to those with good response .
p56 is a nonreceptor tyrosine kinase of the src oncogene family mostly expressed in t cells where it plays an essential role in activation and development , and in some b cells .
its activity is negatively regulated by the membrane - bound tyrosine kinase csk ( c - src tyrosine kinase ) .
the p56 inhibitor dasatinib ( bms-354825 ) was shown to enhance apoptosis induction by dexamethasone in otherwise gc - resistant cll cells .
this finding concurs with the observation by sade et al .   showing that notch - mediated resistance of a mouse lymphoma cell line
a synergistic effect was observed between the aurora a kinase inhibitor mnl8237 ( alisertib ) and dexamethasone .  
ampk ( amp activated protein kinase ) activation has a dual effect on cell death and survival , which contextually depends on signaling alterations with related oncogenic pathways .
however , in all and cll , activation of ampk by aicar ( 5-aminoimidazole-4-carboxamide riboside or acadesine ) , a cell - permeable nucleotide , induces growth inhibition and apoptosis [ 146148 ] .
however , aicar prevented glucocorticoid - induced apoptosis   and thus can not be combined with steroids in the treatment of lymphoid malignancies .   of note , inhibition of either bcl-2 family members , notch1 , or
the akt / mtor survival pathways was independently sufficient for sensitizing resistant cells to gc , suggesting a tight crosstalk between these pathways , interruption of one of them being sufficient for abrogating the resistant phenotype .
however , it is likely that using a combination of these three strategies together with gc should lead to a more efficient therapy , which may require lower dosages with reduced adverse effects .
in order to develop strategies to overcome gc resistance , it is essential to understand the signaling network regulating gc - induced apoptosis .
main factors affecting the response to gc include the basal and inducible gr expression levels , the induction of and basal expression of genes involved in the intrinsic apoptotic pathway , the ability of gr to translocate to the mitochondria , the activity of gsk3 ( glycogen synthase kinase 3 ) , the general protein kinase activation profile of the cell prior to and following gc therapy , the expression profile of anti - apoptotic proteins , and the activities of pro - survival signaling pathways .
the main traits will only be briefly described here as these have been extensively reviewed elsewhere [ 30 , 31 , 99 , 150153 ] , and the scope of this paper is to provide updated data with a specific focus on the microrna world that has emerged to comprise important regulators of most biological processes .
numerous factors have been shown to affect gc responsiveness by regulating glucocorticoid receptor ( gr ) activity and expression level .
these include gr co - activators and corepressors [ 154 , 155 ] , gr splice variants [ 156159 ] , gr isoforms [ 160 , 161 ] , and regulators of gc nucleocytoplasmic shuttle [ 162164 ] .  
the transcription of human gr is regulated by at least 11 different promoters ( 1a1 , 1a2 , 1a3 , 1b , 1c , 1d , 1e , 1f , 1h , 1i , and 1j ) [ 155 , 165 ] , seven of them being embedded in a highly enriched cpg island region subjected to methylation and harbor single nucleotide polymorphisms ( snps ) that affect their activity .
promoter 1a is involved in the upregulation of gr by gc in some kinds of t cells , while downregulated in other cell types [ 167169 ] .
gc resistance in primary pediatric t- and b - all could not be correlated with either basal or stimulated expression of the 1a- , 1b , or 1c transcripts .  
the gr expression level prior and following gc therapy affects drug responsiveness . the cellular response
to gcs depends on sufficient gr expression [ 30 , 171179 ] , and resistance to gc therapy has been associated with downregulation and loss of gr expression in malignant plasma cells [ 180 , 181 ] .
however , most primary all cells showed upregulation of gr expression upon prednisolone treatment regardless of their phenotype or sensitivity to gc - induced apoptosis , suggesting that other factors are more dominant for conferring a gc - resistant phenotype in these cells [ 29 , 170 , 182184 ] .
many glucocorticoid - regulated genes ( e.g. , fkbp5 and socs1 ) were upregulated by dexamethasone in all primary all xenografts tested , suggesting for a functional gr in these leukemic cells .
also , beesley et al .   observed that receptor mutation is not a common mechanism of gc resistant in primary all .
however , the minor c allele of rs10482605 ( 1c ) has been associated with a higher complication rate in childhood all .
a bcli polymorphism in the nr3c1 gene was associated with increased lymphocyte response to methylprednisolone .
also , initial good responder cells may develop resistance upon repeated gc dosages , a phenomenon that sometimes occurs due to downregulation of gr [ 156 , 179 , 188 , 189 ] .
posttranslational modifications of gr are another way of regulating its target gene specificity and involve several cell - signaling cascades [ 30 , 190 , 191 ] .
gr can be phosphorylated at ser211 by cdks and p38 map kinase , and at ser226 by jnk .
phosphorylation of gr modulates its transcriptional activity , alters its protein stability and subcellular location [ 192195 ] .
gr phosphorylation appears to be cell - cycle dependent [ 196 , 197 ] and may affect gc - sensitivity of t - all cells [ 98 , 195 ] .
, gr is mostly located to the cytosol sequestered to heat - shock protein complexes [ 30 , 162 ] .
following gc binding to gr , the receptor undergoes phosphorylation , dissociates from the heat - shock complexes , dimerizes , and translocates to the nucleus where it either promotes or represses a whole series of genes .
transcriptional activation is either directly mediated by binding of gr to glucocorticoid response elements ( gres ) , or through interaction with other transcription factors such as forkhead transcription factors , thereby increasing their transcriptional activity on target genes .
gr may repress gene expression either through binding to negative gres ( ngres ) or through interaction with and inhibition of the transcription factors activating protein-1 ( ap-1 ) and nfb .
the o - glcnac transferase ( ogt ) was found to be involved in gc - mediated transrepression .
hundreds of genes are regulated by gcs [ 199203 ] , and some genes are differentially regulated in gc - sensitive versus gc - resistant cells [ 29 , 199 , 204 ] .  
of special importance is the induction of the pro - apoptotic bim ( bh3-only b - cell lymphoma 2 ( bcl-2 ) interacting mediator of cell death ; or bcl2l11bcl-2-like apoptosis initiator-11 ) for achieving the propensity to undergo apoptosis in response to gc [ 29 , 30 , 67 , 205208 ] .
the central role of bim in gc - induced apoptosis is understated by the partial gc response of bim thymocytes , and gc resistance of lymphoma cells after knocking down bim [ 67 , 207 ] .
bim is often expressed at high basal levels in lymphoid cells [ 30 , 120 , 209 , 210 ] , and in these cells there is no further need for upregulating bim in order to achieve an apoptotic response to gcs [ 30 , 59 ] . however , in several t - all and b - all cells , an upregulation of bim in response to gcs is an absolute must , especially when the basal level is low .
bim was shown to be upregulated in gc - sensitive primary t - all samples , but not in resistant ones [ 29 , 182 ] . also , a comparison of established t - all cell lines , bim was upregulated in the sensitive ones only . when sufficient bim expression can not be achieved , gc resistance pursued .
a significantly lower bim expression was detected in high risk childhood all patients who exhibited slow early response to a standard 4-drug induction regimen compared with patients who responded rapidly .
homozygous deletion of bim has been seen in many mantle cell lymphomas   and silencing of bim by promoter methylation and mutation is common in b - cell lymphomas . however , in pediatric all , no correlation between bim cpg methylation and gc resistance was found . rather , gc resistance in primary pediatric all samples correlated with decreased histone h3 acetylation .
the histone deacetylase inhibitor vorinostat relieved bim repression and exerted synergistic antileukemic efficacy with dexamethasone both in vitro and in vivo using a xenograft model .
bim has been shown to be a prognostic biomarker for early prednisolone response in pediatric all .
bim is a potent pro - apoptotic protein belonging to the bcl-2 protein family [ 215 , 216 ] .
bim binds to the pro - survival proteins bcl-2 , bcl - xl , and mcl-1 , thereby allowing bax and bak to promote apoptosis .
bim may also directly bind to bax and bak , triggering a conformational change required for their subsequent oligomerization on the mitochondrial outer membrane .
bim appears in various alternative splice variants , which exhibit different intrinsic toxicities and modes of regulation . in gc - resistant primary cll
, bim was upregulated by dexamethasone , but failed to activate bax and bak due to exclusive sequestration to bcl-2 .
bim may cooperate with the pro - apoptotic puma ( p53 upregulated modulator of apoptosis ) in mediating apoptosis induced by dexamethasone . in b - lymphoid cells ,
other pro - apoptotic members of the bcl-2 family that is not directly upregulated by gcs , but may contribute to the cell death response , include bid , bad , and noxa .
also the thioredoxin - interacting protein txnip ( vdup1/tbp-2 ) has been shown to be upregulated by gc and could contribute to gc - induced apoptosis in one mouse lymphoma cell line . during gc monotherapy of childhood all , gc was found to repress the expression of the pro - apoptotic pmaip / noxa , which could be one mechanism leading to impaired gc sensitivity .
bam ,  is also induced by gcs in all cells , but its importance in gc - induced apoptosis is still not defined .
bim expression is tightly regulated both at the transcription and posttranscriptional levels [ 215 , 218 ] ( figure 1 ) .
rather , gc - induced bim expression in lymphoid cells requires p38 activation and is mediated by the forkhead transcription factor foxo3a / fkhr - l1 .
foxo3a has also been shown to promote bim transcription in various other cellular systems [ 227229 ] and may cooperate with runx1 ( runt - related transcription factor 1 ) .
differential recruitment of foxo3a to the bim promoter was observed after dexamethasone treatment of gc - sensitive versus gc - resistant childhood all xenografts .
foxo3 was found to be an immediate early gr target , whose transcription is further enhanced by stimuli that activate the amp - activated protein kinase ampk .
the activity of foxo transcription factors is tightly regulated , inhibited by akt and erk signaling , while promoted by p38 signaling [ 232236 ] .
the ribosomal protein s6 kinase ( rsk ) activated downstream of erk1/2 , phosphorylates bimel , providing a binding site for the f - box proteins beta - transducin repeat containing protein ( trcp)1 and trcp2 , which promote the polyubiquitination of bimel .
the erk1/2-mediated phosphorylation of bimel at ser69 facilitates optimal phosphorylation by rsk at ser93 , ser94 , and ser98 and this motif serves as the binding sites for trcp1/2 . while erk1/2 lowers the affinity of bim for mcl-1 and bcl - xl and targets bim for degradation , phosphorylation of bim by jnk increases the pro - apoptotic activity of bim [ 240 , 241 ] .
gcs may repress erk1/2 activity through upregulation of mitogen - activated protein kinase phosphatase 1 ( mkp-1 ) .
several drugs that inhibit the erk1/2 and pkb / akt pathways may facilitate upregulation of bim expression .
additional signals are required , such as simultaneous inhibition of the pkb / akt pathway or the downstream mammalian target of rapamycin ( mtor ) kinase .
apoptosis may be induced in a variety of all cells when cotreated with dexamethasone and a mek / erk inhibitor or an akt inhibitor [ 67 , 108 , 243 ] .
early studies by the thompson research group noticed that c - jun played a role in gc - induced apoptosis .
an increase in c - jun was observed in gc - sensitive , but not gc - resistant t - all cell lines , while c - fos and jund were unaffected by the steroid .
chen et al .   reconfirmed that c - jun was upregulated by gcs in gc - sensitive , but not gc - resistant all cells .
they further showed that c - jun is recruited to the ap-1 site of the bim promoter upon gc treatment .
another study showed that dexamethasone - induced bim expression was decreased in cells harboring a dominant - negative c - jun , suggesting a role for c - jun in the upregulation of bim .
a p38 inhibitor prevented dexamethasone - induced expression of runx2 , c - jun , and bim , suggesting that p38-mapk activation acts upstream to the induction of these three molecules . another level of bim regulation is through micrornas .
bim transcription is repressed by the mir-17~92 microrna cluster , which , in turn , is repressed by gcs .
thus , one mechanism by which gcs upregulate bim is through repression of mir-17~92 . of note , the mir-17~92 cluster is often overexpressed or amplified in human cancers [ 247252 ] , thereby preventing the upregulation of bim required for an apoptotic response .
another microrna that suppresses bim expression is mir-26a , which is frequently upregulated in t - all patients . in gastric cancer , mir-106a~363 targets bim .
the mir-106a~363 cluster located at chromosome xq26.2 is the paralogue of mir-17~92 and encodes for mir-363 , mir-106a , and mir-20b . in hepatocellular carcinoma ,
mir-25 of the mir-106b~25 cluster targets bim . also , the mir-106b~25 cluster , which includes mir-106b , mir-93 and mir-25 , is a paralogue of the mir-17~92 cluster and located on chromosome 7 within the thirteenth intron of the protein - coding gene mcm7 . also , the foxo transcription factors , important for bim upregulation , are regulated by micrornas   ( figure 2 ) .
foxo1 and foxo3 transcripts might be targeted by mir-182 [ 258261 ] , mir-1 , mir-27a , mir-96 , and mir-155 [ 263 , 264 ] .
mir-155 plays a role in the activation and function of b and t lymphocytes [ 265 , 266 ] ( see section 3.1.6 ) .
mir-182 expression was higher in gc - resistant cells in comparison to gc sensitive ones .
increased expression of mir-182 reduced total foxo3a expression in t - all cells with consequent lower bim expression . foxo3a and bim increased upon downregulation of mir-182 , suggesting that mir-182 is involved in conferring gc resistance .
the expression of the mir-182~96~183 cluster was induced in splenocytes from mouse with experimental systemic lupus erythematosus ( sle ) , suggesting a role of these micrornas in the breakdown of immunological tolerance and the manifestation of chronic autoimmune inflammation .
this microrna cluster was also upregulated upon t - cell activation by an il-2-dependent manner .
prevention of the expression of the mir-182~96~183 cluster led to increased foxo1 expression and limited population expansion of activated t - helper cells , due to increased cell death .
vice versa , foxo3a was found to negatively regulate the oncomir mir-21 , which may be one mechanism by which foxo3a regulates apoptosis .
as mir-21 targets pten [ 269 , 270 ] , activation of foxo3 by gcs   may be one mechanism responsible for the gc - induced reduction in akt activity . besides function as a transcription factor in the nucleus , gr was found to translocate to the mitochondria in gc - sensitive , but not gc - resistant , lymphoma cell lines .
gr was also found to translocate to the mitochondria in gc - sensitive thymocytes [ 272 , 273 ] .
although there is one paper describing an interaction between gr and bcl-2 in the mitochondria , gc - induced mitochondrial gr translocation in gc - sensitive thymocytes and lymphoma cells proceeded in the absence of bcl-2 .
exclusive overexpression of gr in the mitochondria was sufficient for inducing apoptosis , suggesting that mitochondrial gr may contribute to gc - induced apoptosis .
glucocorticoid treatment inhibited complex i and complex iii of the electron transport chain , and the mitochondria was found to be the primary source of h2o2 production required for gc - induced apoptosis of lymphoma cells [ 275 , 276 ] .
gcs may interact with the mitochondrial thioredoxin trx2 , a redox regulator , and directly modulate mitochondrial gene transcription .
several mitochondrial metabolite and protein transporters and two subunits of the atp synthase were downregulated in t - all and precursor b - all cells at the gene expression level by dexamethasone .
these changes were observed in gc - sensitive , but not gc - resistant , cells .
corticosterone and other steroids were found to directly act on mitochondria to inhibit mitochondrial atp production by suppressing electron transfer from nadh to the electron transfer chain through complex i .
the cellular protein kinase network ( kinome ) has critical influence on the gc sensitivity of lymphoid cells [ 30 , 31 , 97 , 281 ] .
below , i will provide data supporting an involvement of gsk3 ( glycogen synthase kinase 3 ) in gc - induced apoptosis , and the antagonism of its activity by protein kinases such as akt and mtor , which leads to gc resistance .  
the activity of gsk3 was found to be essential for gc - induced apoptosis [ 67 , 282 ] .
gsk3 inhibitors prevented gc - induced apoptosis , and gc resistance frequently occurs through inhibition of gsk activity . reactivating gsk3 by using inhibitors of the pi3k - akt or mtor pathways sensitized gc - resistant cells to gc - induced apoptosis [ 67 , 108 , 115 , 116 , 243 , 283 ] .
gsk3 was found to interact with gr in the absence of ligand and released from gr following exposure to gc .
gsk3 also regulates gr transcriptional activity of bim , iap1 ( inhibitor of apoptosis 1 ) , and gilz ( glucocorticoid - induced leucine zipper ) [ 282 , 284 ] .
the pi3k / akt and mtor signaling pathways are frequently hyperactivated in gc - resistant t - all [ 104 , 286 , 287 ] and is associated with poor prognosis and chemotherapeutic resistance in pediatric b - precursor all .
mtor is a crucial regulator of cell metabolism , growth , and proliferation and mtor is positively regulated by pi3k / akt and notch1 [ 96 , 289 ] , while negatively regulated by the tuberous sclerosis tumor suppressor complex ( tsc1/tsc2 ) .
activation of mtor contributes to tumor cell survival in alk ( anaplastic lymphoma kinase)-positive alcl ( anaplastic large cell lymphoma ) , mantle cell lymphoma , childhood b - precursor all , t - all , and aml .
akt and mtor confer drug resistance by phosphorylating a series of targets [ 292 , 293 ] .
phosphorylation and inactivation of gsk3 is a major cause for gc resistance   that can be overcome by reactivating gsk3 , for example , by akt inhibitors or mtor inhibitors .
as mentioned in section 1.2.3 , the mtor inhibitor rapamycin is efficient in overcoming gc resistance in various lymphoid malignancies .
gc resistance can also be overcome in akt - active lymphoma cells by inhibiting src members ( e.g. , by pp1 ) , pi3k ( e.g. , wortmannin ) , or an akt inhibitor [ 67 , 68 ] .
combination of gc with rapamycin or gc with obatoclax led to reduced akt phosphorylation at ser473 , suggesting that mtor may also act upstream to akt .
gcs could also independent of other cytotoxic agents reduce mtor activity in lymphoid cells . low - dose arsenic trioxide could sensitize gc - resistant all to dex through an akt - dependent pathway .
inhibition of mtor with rapamycin , which binds to fkbp12 , leads to increased bim expression and overcomes ras - dependent survival signals .
synergy between mtor inhibitors ( e.g. , rapamycin ( sirolimus ) and cci-779 ( temsirolimus ) ) and other chemotherapeutic agents has been observed in b- and t - lineage all cell lines and preclinical models [ 96 , 298 ] .  

the akt activity is negatively regulated by pten ( phosphatase and tensin homolog deleted on chromosome 10 ) , a tumor suppressor gene that is suppressed , mutated , or deleted at high frequency in a large number of cancers .
pten mutations or deletions are frequent in t - all and pten deletions are associated with less favorable outcome in t - all [ 104 , 300 ] .
for instance , treatment of t - all with gamma secretase inhibitor ( gsi ) was only efficient if the cells expressed functional pten .
one mechanism by which notch confers gc resistance is through pten inhibition leading to akt activation .
pten specifically catalyzes the dephosphorylation of 3-phosphate of the inositol ring in phosphatidylinositol ( 3,4,5)-triphosphate ( pip3 ) resulting in the biphosphate product phosphatidyl ( 4,5)-biphosphate ( pip2 ) .
pip3 is a second messenger generated by pi3k that binds to the pleckstrin homology ( ph ) domain of akt , which allows its phosphorylation and activation by the 3-phosphoinositide - dependent protein kinase 1 ( pdk1 ) .  

regulation of pten stability by phosphorylation and ubiquitination . taken into account the important role of pten in determining drug sensitivity ,
pten expression has been attributed to epigenetic events such as promoter methylation [ 302 , 303 ] . at the posttranslational level , phosphorylation and ubiquitination decrease pten protein levels , while oxidation and acetylation reduce pten activity .
rak phosphorylation of pten at tyr336 stabilizes the pten protein , while phosphorylation at thr366 , ser370 , ser380 , thr382 , and ser385 by casein kinase 2 ( ck2 ) and gsk3 reduces its stability [ 305 , 306 ] .
pten is regulated by the protooncogene ubiquitin ligase nedd4 - 1 ( neural precursor cell expressed , developmentally downregulated 4 ) that promotes pten for proteasomal degradation .
in multiple human cancer samples where the genetic background of pten was normal , but its protein level was low , nedd4 - 1 was highly expressed . upon tcr / cd28 stimulation of t cells , pten undergoes inactivation by nedd4 - 1 .
the association between pten and nedd4 could be impeded by the e3 ubiquitin ligase cbl - b ( casitas - b - lineage lymphoma protein - b ) .
cbl - b t cells show elevated akt activity , which was abrogated by simultaneous deficiency in nedd4 .
pten is also negatively regulated by the anti - apoptotic xiap ( x - linked inhibitor of apoptosis ) that promotes pten for polyubiquitination and proteosomal degradation .
induction of apoptosis in b - cll by arsenic trioxide was shown to lead to activation of c - jun - nh2 terminal kinase ( jnk ) , inactivation of akt and nfb , xiap downregulation , and pten upregulation .
two other e3 ligases downregulating pten include wwp2 ( ww - domain containing protein-2 or aip-2 , atrophin-1-interacting protein 2 ) , and chip ( chaperone - associated e3 ligase c terminus of hsc70-interacting protein ) .
pten expression can also be repressed by a range of micrornas including the mir-17~92 cluster [ 247 , 248 ] , mir-106b~25 , mir-21 , mir-26a [ 253 , 315 ] , mir-29b , mir-214 [ 317 , 318 ] , mir-216a and mir-217 , mir-212 , mir-221 , and mir-222   ( figure 3 ) .
bcl-2 and bcl - xl are anti - apoptotic proteins residing in the mitochondrial outer membrane and in the endoplasmic reticulum .
they prevent apoptosis of various chemotherapeutic drugs including gcs by capturing pro - apoptotic members of the bcl-2 superfamily , including bim , bax , and bak [ 215 , 322 , 323 ] .
bcl-2 may also regulate gene expression [ 324 , 325 ] , cell cycle [ 326328 ] , activate erk1/2 [ 324 , 329 ] , and modulate the activities of transcription factors such as p53 , e2f , nfb , and notch [ 332 , 333 ] .
this may be explained by the ability of bcl-2 to modulate p27 expression and promote g0 arrest [ 325 , 327 , 331 , 335 , 336 ] .
long - term exposure to gcs could overcome resistance caused by either bcl-2 or bcl - xl [ 30 , 120 , 337 ] .
overexpression of bcl-2 is common in leukemias and lymphomas [ 338341 ] . in follicular lymphoma ( fl ) and
diffuse large b - cell lymphoma ( dlbcl ) , bcl-2 upregulation is commonly due to the t(14,18)(q32 ; q21 ) translocation , which places the bcl-2 gene under the control of ig heavy chain enhancers [ 342344 ] .  

overexpression of bcl-2 is common in cll due to the loss or downregulation of the human chromosome 13q14 locus , which harbors the mir-15a and mir-16 - 1 cluster .
overexpression of mir-15a and mir-16 - 1 in cll cells led to cleavage of procaspase-9 and parp ( poly - adp - ribose polymerase ) and activation of the intrinsic apoptosis pathway .
these two micrornas could serve as natural antisense bcl-2 actors that have potential use in the therapy of bcl-2 overexpressing tumors .
the tumor - suppressor mir-34a , a pivotal member of the p53 network , also downregulates bcl-2 [ 347 , 348 ] , which may be one mechanism by which p53 activation leads to downregulation of bcl-2 .
it also targets mcl-1 and bcl - w , and indirectly bcl - xl by attenuating il-6/stat-3 ( signal transducer and activator of transcription 3 ) signaling pathway [ 350 , 352 ] .
mir-125b may function both as tumor suppressor and as an oncogene   and has been widely considered as conferring drug resistance , among others by downregulating bak1 ( bcl-2 antagonist killer 1 ) [ 353355 ] and bmf . over - expression of mir-125b could induce leukemia in a mouse model .
mir-181a / b that shows altered expression in cll could also target bcl-2 , besides acting on mcl-1 and xiap [ 358360 ] .
bcl - xl can be targeted by the tumor suppressor microrna let-7   and mir-491 .
a predominant feature of the gene expression signature leading to gc resistance in all was found to be elevated expression of the anti - apoptotic mcl-1 ( myeloid cell leukemia sequence 1 ) [ 364 , 365 ] .
mcl-1 expression is especially high in mll - rearranged all , which represents an unfavorable type of leukemia that is often highly resistant to gcs .
mcl-1 is also frequently overexpressed in b - cell and mantle - cell lymphomas , cml , cll , and mm .
mcl-1 is an anti - apoptotic protein that sequesters the pro - apoptotic proteins tbid , bim , puma , noxa , and bak . besides preventing gc - induced apoptosis
, mcl-1 confers resistance to trail ( tumor necrosis factor - related apoptosis inducing ligand)-induced cell death .  

mcl-1 differs from bcl-2 and bcl - xl in having a short protein turnover regulated by the 26s proteasome and its expression is tightly regulated . unlike bcl-2 ,
rapamycin , a mtor inhibitor that sensitizes resistant all cells to gc , reduces the expression level of mcl-1 [ 113 , 287 ] .
the degradation of mcl-1 depends on gsk3-mediated phosphorylation of mcl-1 at ser159 [ 369 , 370 ] .
e3 ubiquitin ligases implicated in the regulation of mcl-1 include mule ( mcl-1-ubiquitinase ligase e3 ) , scf ( skp1/cul1/f - box protein -transducin repeat - containing protein ) , and fbw7 ( f - box and wd repeat domain - containing 7 ) which is part of the skp1-cullin1-f - box ( scf ) e3 ligase complex . the deubiquitinase usp9x ( ubiquitin specific peptidase 9 x - linked ) is an important regulator of mcl-1 stability . silencing of usp9x resulted in loss of mcl-1 .
high levels of mcl-1 correlated with elevated usp9x expression in follicular lymphoma , diffuse large b - cell lymphoma , and some other cancer samples . increased expression of usp9x mrna was associated with poor prognosis of multiple myeloma .
overexpression of noxa triggered an increase in the mule / mcl-1 interaction in parallel to a decrease in mule / usp9x complex formation . in an
akt - driven , e-myc lymphoma mouse model , translational regulation of mcl-1 by mtor has been implicated in promoting lymphomagenesis .
as gc may activate gsk3   and gsk3 inhibits mtor through phosphorylation of tsc2   and promotes mcl-1 degradation [ 369 , 370 ] , mcl-1 expressing lymphoid cells may ultimately undergo apoptosis if the exposure time to gc is sufficiently long .
this may explain why many mcl-1-positive all cells exhibit delayed response to gcs , and not complete resistance [ 67 , 108 ] .
also , the anti - apoptotic function of mcl-1 appears to require simultaneous expression of other anti - apoptotic bcl-2 family members .
similarly , overexpression of mcl-1 in bcl-2- and bcl - xl - negative mouse double positive thymic lymphoma cells did not confer gc resistance upon these cells .
usually , mcl-1 is expressed together with other anti - apoptotic proteins in gc - resistant lymphoid malignancies . 

mcl-1 is also regulated by micrornas ( figure 2 ) , including mir-29a , mir-29b [ 381383 ] , mir-101 , mir-125b , mir-181a / b [ 358 , 385 ] , mir-133b , mir-193b , and mir-512 .
alk - positive anaplastic large cell lymphomas ( alcl ) express low levels of mir-29a , whose downregulation requires an active npm - alk kinase , and may probably also be due to methylation repression . enforced mir-29a expression reduced mcl-1 expression in alcl cells and reduced tumor growth in a xenografted model .
mir-29b is downregulated in primary mm and aml samples and forced overexpression of mir-29b - induced apoptosis in mm and aml cells [ 381 , 383 ] .
mir-29b overexpression also downregulated the expression of the dna methyltransferase isoforms dnmt1 , dnmt3a , and 3b .
the global dna hypomethylation induced by mir-29b led to reexpression of tumor suppressor genes such as the cdk inhibitor p15 .
altogether , these data propose that targeting mcl-1 with micrornas such as mir-29 represents a potential tool to constrict tumor growth of mcl-1 positive lymphomas .
gcs release ca from the endoplasmic reticulum into the cytosol , which in turn increases the amount of mitochondrial ca .
elevated expression of calcium - binding proteins s100a8 and s100a9 and of the anti - apoptotic mcl-1 ( myeloid cell leukemia-1 ) inhibits the free cytosolic ca and mitochondrial ca signals , respectively , thereby imposing gc resistance [ 287 , 365 , 389 , 390 ] .
downregulation of s100a8 and s100a9 by the src kinase inhibitor pp2 sensitized mll - arranged all cells otherwise resistant to prednisolone - induced cell death .
bcl-2 inhibits apoptosis in part by decreasing the size of ca stores in the endoplasmic reticulum resulting in reduced ca transfer to the mitochondria [ 391393 ] .
one mechanism is through interaction of bcl-2 with ip3r ( inositol 1,4,5-triphosphate ( insp3 ) receptor ) , which is the principle er ca release channel in most cell types .
also , bcl - xl and mcl-1 act in part by inhibiting ip3r [ 393 , 395 , 396 ] .
an increase in hydrogen peroxide ( h2o2 ) is a necessary signal for gc - induced apoptosis .
the mitochondria is the source of this signal , gcs inhibit complex i and complex iii of the electron transport chain .
expression of anti - oxidant defense proteins such as manganese superoxide dismutase , thioredoxin , and catalase prevents gc - induced apoptosis [ 276 , 398400 ] .
the anti - apoptotic bcl-2 may regulate the mitochondrial redox state in cancer cells [ 323 , 401 ] .
notch is frequently activated in t - all cells , which may be due to mutations in notch1 ( gain - of - function ) and/or in the e3 ligase fbw7 that targets notch1 for degradation [ 7678 , 80 , 81 , 402405 ] .
for example lnx1 ( ligand of numb - protein x1 ) is a positive regulator of notch signaling through degradation of numb , a membrane - associated protein that inhibits the function of the notch receptor .
neuralized ( neur ) and mind bomb ( mib ) promote the monoubiquitination and endocytosis of delta [ 409 , 410 ] .
itch binds to the n - terminal portion of the notch intracellular domain via its ww domains and promotes ubiquitination of icn - notch1 through its hect ubiquitin ligase domain .
recent studies showed that notch1 can be activated in leukemic cells through interaction with bone marrow stromal cells that express notch receptors and ligands [ 412 , 413 ] .
interaction with bone marrow stroma is also a mechanism for notch activation in multiple myeloma .
cyclin e , which is targeted for degradation by fbw7 [ 415 , 416 ] , is expressed at higher levels in early relapsed pediatric b - cell precursor all patients , who usually show an unfavorable prognosis .
notch1 prevents gc - induced apoptosis , among others , through activation of p56 , which activates the pi3k - akt axis , and through the transactivation of its target genes deltex and hes1 .
deltex is a ring - domain ubiquitin ligase that may affect notch activity , and its overexpression prevents gc - induced apoptosis .
activation of the pro - survival pi3k / akt / mtor pathway by notch has also been observed in other studies [ 95 , 106 , 419 , 420 ] and may be responsible for notch - mediated inhibition of the p53 tumor suppressor gene .
another mechanism by which notch1 protects t - all cells from gc - induced apoptosis , is through the anti - apoptotic gimap5/ian5 ( gtpase of the immunity - associated protein / immune - associated nucleotide - binding protein 5 ) [ 421 , 422 ] . giamp5/ian5
interacts with bcl-2 and bcl - xl and inhibits apoptosis during t - cell development   and is highly expressed in human b - cell lymphoid malignancies .
it is localized within the mitochondria and endoplasmic reticulum ( er ) and regulates mitochondrial integrity .
gimap has been linked to immunological diseases such as t - cell lymphopenia and autoimmune diseases .
notch also activates nfb signaling [ 74 , 427 ] and induces c - myc expression [ 428430 ] , both contributing to apoptotic resistance .
this resistance can be overcome by the simultaneous exposure of the cells to src inhibitors , pi3k / akt inhibitors , or mtor inhibitors [ 67 , 68 ] , understating the importance of the protein kinase network in regulating the effects of notch1 on gc - induced apoptosis .  
a recent report showed that gc sensitivity of t - all is associated with gr - mediated inhibition of notch1 expression .
the serum- and glucocorticoid - inducible kinase 1 ( sgk1 ) was also shown to control notch1 signaling by downregulating its protein stability through fbw7 ubiquitin ligase .
sgk1 phosphorylates fbw7 at ser227 , an effect inducing icn - notch1 ubiquitination and degradation . despite gc resistance induced by notch , notch- and fbw7-mutated
t - all shows in general a favorable response to gc therapy and in some studies , but not all , also exhibits a better prognosis [ 405 , 433436 ] .
this may be related to the fact that gcs may overcome notch - dependent drug resistance , and in these t - all cases the cell survival depends on notch signaling .
various micrornas negatively regulate fbw7 expression including mir-27a , mir-182 , mir-363~92 , and mir-223 [ 253 , 438 , 439 ] and may increase the expression of fbw7-regulated target genes including notch1 , mcl-1 , c - jun , c - myc , and cyclin e . mir-451 and mir-709 suppressed oncogenesis in notch1-induced mouse t - all .
mir-150 , which is upregulated upon thymocyte maturation , targets notch3 and thus regulates t - cell proliferation and survival .
the p53-induced mir-34a also targets the notch1 receptor as well as its ligand dll1 ( delta like-1 ) [ 443 , 444 ] .
prevention of notch activation in cutaneous t - cell lymphoma ( ctlc ) by gsi ( -secretase inhibitor ) treatment led to alterations in the microrna profile of the cell . among others ,
mir-27a , mir92b , mir-181a , mir-18a , mir-19b , mir-222 , and mir-221 were downregulated , while mir-122 and mir-214 upregulated .
mir-27a targets fbw7/hcdc4 [ 253 , 438 , 439 ] , the substrate recognition component of the scf ( skp1-cullin - f - box ) ubiquitin ligase complex that targets notch1 for degradation . the repressive effect of mir-27a on fbw7 mrna is especially pronounced at the g2/m and early g1 phases .
other targets of mir-27a includes zbtb10 ( zinc finger and btb domain containing 10 ) , which acts as a repressor of sp ( specificity proteins ) transcription factors and induces g1 arrest , and the myt-1 kinase , which inhibits the transition through g2-m by enhanced phosphorylation and inactivation of cdc2 ( cdk1 , cyclin - dependent kinase 1 ) .
upregulation of mir-122 by gsi seems to be mediated by p53 and has an antagonistic effect on apoptosis through activation of akt .
c - myc is , among others , a target of notch [ 428430 ] and has broad effects on tumorigenesis   and modulates gc - induced apoptosis [ 99 , 448 ] .
conditional overexpression of c - myc in hematopoietic cells in mice culminated in the formation of malignant t - cell lymphomas and acute myeloid leukemias .
these authors demonstrated a role for the pi3k / akt axis in c - myc activation .
dysregulation of the c - myc gene is a common trait of burkitt 's lymphoma due to chromosomal translocations , the most frequent one being t(8 ; 14)(q24:q32 ) involving c - myc and igh ( immunoglobulin heavy locus ) [ 451453 ] .
other hematopoietic malignancies characterized with c - myc overexpression include diffuse large b - cell lymphoma ( dlbcl ) , follicular lymphoma , cll , b - cell lymphoma , and aml [ 454459 ] .
earlier studies have shown that dexamethasone - induced apoptosis of a t - all cell line was associated with c - myc suppression [ 460 , 461 ] .
the gc - mediated down - regulation of c - myc expression was initially thought to be one mechanism that contributes to apoptosis .
not all studies have confirmed this finding , which may be explained by the many signaling pathways induced by gcs .
c - myc uses distinct mechanisms for activating and repressing gene expression . for transcriptional activation , c - myc dimerizes with max and
transcriptional repression is achieved through protein - protein interactions , where it antagonizes the activity of positive regulators of transcriptions .
the c - myc - mediated upregulation of mir-17 and mir-20a ( belonging to the mir-17~92 cluster ) negatively regulates e2f1 translation by targeting the 3-utr of e2f1 mrna and may therefore fine tune the direct myc - mediated transcriptional activation of e2f1 , allowing a tightly regulated proliferative signal   ( figure 4 ) .
e2f1 - 3 also binds to the promoter of the mir-17~92 cluster and activates its transcription , thus generating an autoregulatory feedback loop .
another target of the mir-17~92 cluster is cyclin d1 , which also induces the expression of mir-17 and mir-20a by binding to the promoter regulatory region of the mir-17~92 cluster .
the gc - induced down - regulation of mir-17~92   should actually stimulate e2f1 expression , which under certain circumstances may exert pro - apoptotic effects .
e2f1 may promote apoptosis through transcriptional activation of the pro - apoptotic mir-15a~16 cluster   and by activating jnk . in a b - cell lymphoma model , c - myc down - regulated a series of micrornas , an action that may contribute to tumorigenesis .
the c - myc mediated repression of the mir-30 cluster   may affect autophagy , as beclin-1 expression is regulated by mir-30a .
some of the pro - autophagy activity of cancer therapy is mediated through down - regulation of mir-30a .
also the down - regulation of mir-15a and mir-16 by c - myc   is of interest as these micrornas are deleted or downregulated in over two - thirds of individuals with cll , and they target the anti - apoptotic bcl-2 gene [ 345 , 346 ] .
a third mirna downregulated by c - myc is the tumor suppressor let-7 mirna cluster , which targets , among others , the ras oncogene , hmga2 ( high mobility group a2 ) [ 477 , 478 ] , bcl - xl , cdc25a , cdk6 ( cyclin - dependent kinase 6 ) , and cyclin d2 .
other mirnas repressed by myc include mir-22 , mir-23a / b , mir-26a / b , mir-29a / b / c , mir-34a , mir-146a , mir-150 , and mir-195 [ 465 , 473 , 480 ] .
mir-26a levels were found to be reduced in various b - cell lymphomas , especially burkitt lymphoma   as well as various solid tumors [ 481 , 482 ] .
b - cll , which does not have a prominent pathological role of c - myc , showed higher expression of mir-26a than myc - dependent burkitt lymphoma .
mir-26 restoration in burkitt lymphoma or nasopharyngeal carcinomas reduced proliferation and colony formation through g1 arrest and repression of the histone - lysine n - methyltransferase ezh2 , a global regulator of gene expression [ 465 , 481 , 483 ] . the tumor - suppression function was only seen in myc - transformed cells , but not in v - abl transformed cells [ 465 , 483 ] . however , in t - all , mir-26a was one of five micrornas that independently promoted tumorigenesis through inhibition of pten .
in the background of activating mutations in notch1 , mir-26a overexpression decreased the latency of t - all .
forced overexpression of mir-34a , mir-150 , and mir-15a/16 - 1 attenuated in vivo tumor growth of myc - induced b - cell lymphoma .
mir-34a is a crucial component of the p53 tumor suppressor network with potential anti - proliferative and pro - apoptotic activity [ 484486 ] .
c - myc transcriptionally induces lin28b , which is an rna - binding protein that suppresses the maturation of let-7 family microrna precursors [ 487 , 488 ] .
lin28 is involved in stem cell maintenance [ 489491 ] and is a marker of cancer stem cells .
the effect of autophagy on the cellular response to chemotherapy is dual . under certain conditions ,
autophagy acts as a pro - survival mechanism to protect cancer cells from chemotherapy , whereas under other circumstances , autophagy mediates the therapeutic effects of the anticancer agents .
another important regulator of autophagy is the activity of mtor ( mammalian target of rapamycin ) , which is a central element signaling cell growth and enhancing protein translation . when this kinase is inhibited , autophagy is promoted .
it should be noted that beclin-1 may play a dual role in both regulating autophagy and apoptosis , thus being at the cross - road between these two physiological processes .
beclin-1 has recently been recognized as a bh3-only protein interacting with bcl-2 , bcl - xl and mcl-1 [ 59 , 60 , 494496 ] .
one report provides evidence that after initiating apoptosis , beclin-1 is cleaved by caspases and the n - terminal fragment of beclin can inhibit autophagy , while the c - terminal fragment can amplify mitochondrial - mediated apoptosis .
perturbation of beclin-1 cleavage by knockin mutation phenocopied the autophagy induction observed in apoptosis - defective cancer cells and rendered chemotherapy resistance both in vitro and in vivo . a role for beclin in regulating tumorigenesis has been demonstrated in mice with heterozygous disruption of beclin-1 .
dlbcl expressing high beclin-1 levels had a favorable clinical outcome with r - chop treatment than those with low beclin-1 expression .
gcs have been shown to promote autophagy in lymphocyte cell lines and primary t - all cells [ 501 , 502 ] .
one mechanism for induction of autophagy is through upregulation of the mtor - inhibitory stress protein dig2 ( dexamethasone - induced gene 2 ) , also known as rtp801 and redd1 ( regulated in development and dna damage responses 1 ) .
mtor inhibition by dexamethasone was demonstrated by reduced phosphorylation of s6k ( 70kd ribosomal protein s6 kinase 1 ) , a member of the rsk family of serine / threonine kinases .
dig2 releases tsc2 from 14 - 3 - 3 , thereby promoting the assembly of the tsc1/tsc2 complex , which inhibits mtor .
dig2 knockout thymocytes underwent more extensive dexamethasone - induced cell death , suggesting that autophagy promotes cell survival .
however , rapamycin , an inhibitor of mtor and inducer of autophagy , strongly sensitizes resistant mm and t - all cells to gc - induced apoptosis [ 59 , 111 , 116 , 117 ] , suggesting that induction of autophagy does not always combat apoptosis .
it could be that the higher degree of autophagy induced by rapamycin itself may be pro - apoptotic .
bonapace et al .   showed that rapamycin induces an autophagy - dependent necroptosis , which is required for childhood t - all to overcome gc resistance .
necroptosis is a form of programmed necrosis that occurs when apoptosis is abortive due to caspase inhibition .
the gc - mediated necroptosis was mediated by rip-1 ( receptor - interacting protein-1 ) and cyld ( cylindromatosis ) .
mir-19 , which is frequently overexpressed in t - all patients and cell lines , represses cyld expression .
obatoclax , a putative antagonist of bcl-2 family members , could also sensitize t - all cells to gc - induced apoptosis through induction of autophagy .
this effect was associated with dissociation of the autophagy inducer beclin-1 from mcl-1 and decreased mtor activity .
the apoptosis induced by gc in combination with obatoclax or rapamycin could be prevented by the autophagy inhibitors 3-methyladenine and bafilomycin .
cdkn2/p16 , which acts as a g0/g1 cycle inhibitor , is frequently lost in t - all [ 507 , 508 ] and predicts relapse in children with all [ 508510 ] .
p16 sensitizes t - all cell lines to gc - induced apoptosis through induction of bbc3/puma and repression of mcl-1 and bcl-2 .
noxa was repressed in p16 transgenic cells , which could be a result of the simultaneous repression of e2f1 due to retinoblastoma protein and p130 activation .
diffuse large b - cell lymphoma with cdkn2a deletion had a poor prognosis under r - chop treatment .
also , myc gene arrangement in diffuse large b - cell lymphoma patients had a poor prognosis with r - chop chemotherapy .
during the last decade , micrornas have become the focus of having a central role in the pathogenesis of cancer including lymphoid malignancies , besides their role in regulating gene expression during cell division , development , and differentiation [ 514523 ] .
micrornas are short noncoding rnas that induce posttranscriptional gene silencing through base pairing with the 3 untranslated region ( utr ) of their target mrnas , thereby inhibiting their translation , with subsequent reduced protein levels [ 524 , 525 ]
. bases 27 or 28 of the microrna are primary contributors to target specificity and are referred to as the microrna seed region .
the micrornas are usually transcribed by rna polymerase ii , and sometimes by rna polymerase iii , into long primary precursor transcripts referred to as pri - mirnas .
mirna are encoded by one arm of a stem loop structure embedded in introns or , less frequently , exons of protein - coding or noncoding transcripts . in the nucleus
, the pri - mirnas stem loop is cleaved by the nuclear rnase iii enzyme drosha together with its cofactor dgcr8 ( digeorge syndrome critical region 8)/pasha ( the microprocessor complex ) to generate ~70 nucleotides long precursors called pre - mirnas . in some cases , an entire intron consists of such a stem loop structure , which is released by the splicing machinery in a drosha - independent manner .
pre - mirnas are exported by rangtp / exportin-5 to the cytoplasm , where they are further processed by dicer , another rnase iii enzyme , to generate ~22 base pair microrna duplexes that enter effector complexes called mirisc ( mirna - containing rna - induced silencing complex ) . here , they are converted into single - stranded mature mirnas that target mrnas and thereby affect their translation and stability [ 516 , 528 , 529 ] .
cancer cells frequently display reduced levels of micrornas that act as tumor suppressors , while expressing elevated levels of oncogenic micrornas , called  oncomirs  that promote tumor development by negatively regulating tumor suppressor genes and/or genes that control cell differentiation and apoptosis .
there are variations in the microrna expression pattern described between the various scientific reports , which can be explained by the use of different internal standards , different controls for comparison , and the use of sample materials of malignant cells at different developmental stage and at different ontogeny tumor grade .
virtually every step in hematopoiesis seems to be finely tuned by specific micrornas [ 514 , 530533 ] .
conditional deletion of dicer expression in the t - cell compartment resulted in impaired t - cell development and diminished regulatory t - cell function [ 534536 ] , and ablation of dicer in the b - cell compartment attenuates b - cell development and alters the antibody repertoire .
it should be noted that there exists an alternative microrna processing pathway that is independent of dicer , but dependent on argonaute-2 .
microrna expression is dynamically regulated during thymocyte development , with different enriched micrornas expressed at each developmental stage   ( table 1 ) .
it should be emphasized that the cd4cd8 ( double positive , dp ) thymocytes are the most gc - sensitive thymocyte population [ 540542 ] .
dicer - deficient dp thymocytes expressed higher levels of cd69 and tcr ( t - cell receptor ) , but lower levels of bcl-2 .
the dicer - deficient thymocytes were more prone to apoptosis than control cells [ 539 , 543 ] , understating the role of micrornas in regulating cell survival .
some micrornas , such as mir-146a and mir-182 , play a dominant role in the regulation of the innate and adaptive immune responses , respectively [ 544 , 545 ] . according to neilson et al .
, the pro - apoptotic mir-15b is almost not expressed at the immature dn1 ( double negative 1 ) thymocyte stage but becomes gradually upregulated in dn3 and dn4 , and further in dp cells .
the pro - apoptotic mir-16 is also low in dp1 and reaches a maximum in dn4 cells , with a reduction upon transition to dp cells .
the oncogenic mir-21 is expressed at the highest level in dn1 and becomes reduced upon transition to dn3 and is almost not expressed in dp cells .
mir-181a / b is expressed at the highest level in dp thymocytes , together with mir-92 and mir-350 .
it should be noted that in this study the expression of each microrna was determined relative to the general microrna pool of each subpopulation . since the amount of total microrna becomes strongly reduced upon transition from dn4 to dp ( a drop from 32000 to 5200 copies / cell ) ,
the absolute microrna number in each cell population differs , which can be demonstrated by the mir-181a transcript .
while mir-181a presents 15.6% of the microrna in dp cells and 6.7% and 5% in dn3 and dn4 , respectively , the numbers of mir-181a copies in these three populations were estimated to be 810 in dp , 1400 in dn3 , and 1600 in dn4 .
.   showed that mir-181a is expressed at dn1 and becomes upregulated during dn2 and dn3 and then downregulated at dn4 .
mir-181 is still significantly expressed in dp cells , albeit at a slight lesser extent than in dn4 and becomes downregulated upon differentiation to the sp ( single positive ) stage .
performed an extensive microrna profiling to identify micrornas specifically expressed in b - cell subsets during peripheral b - cell differentiation .
notably , mir-18a , mir-28 , mir-15a~16 - 1 , and mir-181 are expressed at higher levels in centroblasts ( germinal center b lymphocytes ) compared with memory b cells , whereas mir-101c , mir-150 , mir-29a , b , c , and mir-23a~24 are enriched in memory b cells .
mir-17~92 , mir-363~106a , and mir25~106b are highly expressed in all b - cell subtypes .
the high level of mir-15a~16 - 1 in germinal center b - cells corresponds with low bcl-2 expression in these cells .
mir-223 is highly expressed in nave and memory b cells , but not in centroblasts .
mir-181a represses the expression of bcl-2 , cd69 , and the t - cell receptor ( tcr ) -chain .
mir-181a augments the strength of tcr signaling and down - regulates several phosphatases including dusp5 , dusp6 , shp-2 , and ptpn22 that regulate the sensitivity of t cells to antigens .
the down - regulation of ptpn22 by mir-181a led to elevated phosphorylation of p56 at y394 and the down - regulation of dusp5/6 to increased erk activation .
the normally high levels of mir-181a maintain t - cell tolerance to self - peptide / mhc molecules , with a reduction in this microrna increasing the number of self - reactive t cells . also , dampening mir-181a expression using antagomir-181a impaired positive selection with about a 70% reduction of mature cd4 sp thymocytes .
thus , mir-181a plays a role in regulating tcr response during t - cell development .
recently , mir-181a-1/b-1 , but notmir-181a-2b-2andmir-181-c / d , was found to control the development of normal thymic t cells and leukemia cells .
conditional deletion of mir-181ab1 allele resulted in 50%75% decrease in cellularity in the thymus and a significant reduction in the percentage of dp cells .
mir-181a expression decreased during the dn3a to dn3b transition during -selection , and loss ofmir-181ab1resulted in a reduction in the percentage of dn3 and dn4 cells that expressed intracellular tcr- , while pret expression in dn3 thymocytes was normal .
other alterations occurring upon tcr / cd28 co - stimulation includes the upregulation of the mir-466 family , mir-574 , mir-346 , mir-214 , mir-155 , and mir-709 , and the down - regulation of the mir-29 family , mir-15a , mir-15b , mir-16 , mir-146b , mir-142 , mir-27a , mir-150 , and let-7 family . chen et al
.   showed that mir-181 is expressed in the b - lymphoid cells of the mouse bone marrow , and its ectopic overexpression in hematopoietic stem / progenitor cells significantly increased b - cell production .
mir-181 also affects the development of nk cells through targeting the nemo - like kinase ( nlk ) , an inhibitor of notch signaling .
mir-181 targets the rna - binding protein lin28 , thereby disrupting the lin28-let-7 reciprocal regulatory loop , with concomitant upregulation of let-7 and differentiation of megakaryocytes .
mir-150 is highly expressed in mature and resting lymphocytes , but not in their progenitors [ 547 , 553 ] .
overexpression of mir-150 led to a block in b - cell formation at the pro - b to pre - b - cell transition by downregulating c - myb , among other targets , suggesting for a role for this microrna in b - cell differentiation . within the lymphoid lineage the choice between t and b cells
the t - cell population level was unaffected by overexpression of mir-150 in hematopoietic progenitor cells , while the mature b - cell levels were strongly reduced .
mir-150 drives megakaryocyte - erythrocyte progenitor ( mep ) cells towards megakaryocytes at the expense of erythroid cells .
mir-150 also regulates the development of nk ( natural killer ) and inkt ( invariant nk ) cells .
mice with target deletion in mir-150 had a defect in their ability to generate mature nk cells , while overexpression of mir-150 resulted in a substantial reduction in inkt in the thymus and in the peripheral lymphoid organs , supposedly through targeting of c - myb by mir-150 .
mir-125b affects t - cell differentiation through regulation of ifn ( interferon  ) , il-2r , il-10r , and prdm1/blimp1 ( b lymphocyte - induced maturation protein-1 ) .
ectopic expression of mir-125b in nave lymphocytes inhibited differentiation to effector cells . during normal b - cell development , mir-125b is enriched in germinal center b cells and keeps the transcription factor irf4 and prdm1/blimp1 down , while mir-223 is enriched in memory b cells , where it targets the transcription factor lmo2 , which is specifically expressed in germinal center b cells .
irf4 and prdm1/blimp1 expression are repressed in centroblasts , but is necessary for differentiation into memory and plasma cells [ 593 , 594 ] .
overexpression of mir-125b alone in mice causes an aggressive , transplantable myeloid leukemia . before leukemia , these mice did not display elevation of white blood cells in the spleen or bone marrow , rather the hematopoietic compartment showed lineage - skewing , with myeloid cell numbers dramatically increase and b - cell numbers severely diminished .
mir-125b targets lin28a , an induced pluripotent stem cell gene . knockdown of lin28a led to hematopoietic lineage - skewing similar to ectopic mir-125b overexpression , with increased myeloid and decreased b - cell number .
mir-155 is also important for lymphopoiesis and for preserving normal immune system responses [ 266 , 597599 ] .
mir-155 is processed within the second exon of the nonprotein - encoding gene bic ( b - cell integration cluster ) .
mir-155 is upregulated upon tcr / cd28 costimulation in mouse t cells , and in macrophages by several tlr ( toll - like receptor ) pathways .
b cells require mir-155 for normal production of isotype - switched , high - affinity antibodies and for a memory response .
the transcription factor pu.1 , which down - regulates igg1 levels , is a target gene of mir-155 in b cells .
this may explain the reduced amount of circulating igg1 in mir-155 knockout mice . as with b cells
, it seems that mir-155 is involved in t - cell differentiation [ 266 , 597 ] .
nave t cells derived from mir-155 knockout mice showed increased propensity to differentiate into th2 rather than th1 cells , with the concomitant production of th2 cytokines such as il-4 , il-5 , and il-10 [ 266 , 597 ] .
one explanation for this biased development of th2 cells might be the mir-155 mediated targeting of c - maf ( musculoaponeurotic fibrosarcoma ) , a transcription factor that transactivates the il-4 gene . with regard to the acute immune response , the t cells had an impaired response and showed attenuated il-2 and ifn release in response to antigens [ 266 , 597 ] .
mir-155 is upregulated by the transcription factor foxp3 and critical for t regulatory cell function .
mice overexpressing mir-155 in the b - cell lineage results in preleukemic pre - b - cell proliferation in the spleen and bone marrow , followed later in life by b - cell malignancy .
the mir-17~92 cluster located on chromosome 13 at locus q31.3 is essential for b - cell development .
the expression of mir-17~92 peaked in pre - b cells , where it inhibited cell death .
it is expressed at higher levels in normal germinal center b cells compared to nave and memory b cells .
knockout of mir-17~92 leads to increased bim expression and inhibits b - cell development at the pro - b to pre - b transition , a step also blocked by mir-150 .
mice overexpressing the mir-17~92 cluster in lymphocytes developed lymphoproliferative disease and autoimmunity and they died prematurely .
these animals were found to have increased numbers of activated b cells , and a higher ratio of activated cd4 t cells versus cd8 t cells .
the enhanced proliferation and survival of b and t cells may result from the down - regulation of bim and pten .
mir-17~92 expression is strongly induced after activation of cd8 t cells , which is critical for the rapid clonal expansion of these cells . however , following the clonal expansion , mir-17~92 is downregulated and further silenced during memory development .
malignant lymphomas arise from normal b- or t - cell counterparts at different ontogeny stages and commonly continue to express gene signatures inherited from their nontransformed cellular progenitors .
extensive mirna profiling studies have been performed on various lymphoid malignancies , including t - all , cutaneous t - cell lymphoma , cll [ 557 , 563 ] , pre - b - all [ 557 , 605 , 607 ] , diffuse large b - cell lymphoma ( dlbcl ) [ 564 , 580582 , 608610 ] , anaplastic large cell lymphoma ( alcl ) , multiple myeloma ( mm ) [ 574 , 611 , 612 ] , mantle cell lymphoma ( mcl ) [ 583 , 591 , 613 ] , burkitt lymphoma ( bl ) [ 564 , 614 ] , and follicular lymphoma ( fl ) [ 135 , 582 , 615 ] .
a comprehensive study aimed to integrate the many mirnas upregulated in t - all into a microrna - transcription factor coregulatory network was performed by ye et al . .
a short description of some important micrornas in malignant lymphoid diseases is described below and summarized in tables 2 and 3 . in general ,
t - all is characterized by upregulation of the mir-17~92 cluster , mir-26a , mir-128a / b , mir-146a , mir-181a / b , mir-150 , and mir-155 , while let-7b and mir-223 are downregulated [ 253 , 557561 ] . 
 3.2.1.1 .
the mir-19 , mir-20a , mir-92a , and mir-17 especially of the mir-17~92 cluster are upregulated in t - all .
all six mirnas mir-17 , mir-18a , mir-19a , mir-20a , mir-19b , and mir-92a , of the mir-17~92 cluster promoted leukemogenesis in notch1-induced t - all in vivo [ 253 , 616 ] . among them
, the mir-19 family has been considered the key oncogenic component [ 248 , 506 , 617 ] .
the mir-17~92 cluster is located within a fragile site that is frequently amplified in a range of hematopoietic malignancies .
mir-19 represses notch1 , pten , hoxa9 , cyld , runx1 , e2f1 , and bcl2l11 ( bim ) [ 506 , 616 , 620 ] .
posttranslational inactivation of pten by mir-19 promotes activation of the pi3k / akt pathway , and incontrollable proliferation of t cells [ 104 , 105 ] . increased akt signaling antagonizes gc - induced apoptosis by several mechanisms , including phosphorylation of foxo3a , thus preventing its nuclear translocation and transcriptional activation of bim , and through inactivation of gsk3 , which is essential for gc - induced apoptosis [ 30 , 67 , 97 ] .
hoxa9 is a leukemogenic homeoprotein in t - all , and a target gene of the oncogenic mll - af4 fusion protein .
high expression of mir-196b was found in pediatric all with aberrant activation of hoxa genes .
notch1 plays a vital role in t - cell development and transformation , and about 50% of primary t - all samples show abnormal notch1 expression .
downstream transcriptional targets of notch1 include hes1 and c - myc , the former affecting the pi3k / akt and nfb signaling pathways [ 624 , 625 ] .
c - myc is a potent and direct transcriptional activator of mir-17~92 , leading to modulation of e2f1 expression .
gcs repress the expression of mir-17~92 , which may be one means to overcome the tumorigenicity of t - all cells and to elevate bim expression .
in contrast to mir-19 , mir-451 , and mir-709 are potent suppressors of oncogenesis in notch1-induced mouse t - all .
mir-451 represses c - myc , while mir-709 represses ras - grf-1 that acts upstream to ras and prevents akt activation .
both mir-451 and mir-709 are transcriptional targets of the bhlh e2a tumor suppressor , which is degraded upon notch1 induction in mouse t - all cells [ 626 , 627 ] .
repression of tumor suppressor mir-451 is essential for notch1-induced oncogenesis in a murine model of t - all .
human t - alls with activating notch1 mutations have decreased mir-451 and increased c - myc levels compared with t - alls with wild - type notch1 .
one mechanism of the tumor suppressive action of mir-451 could be through down - regulation of the pi3k / akt survival pathway .  

primary t - all cells also express elevated levels of mir-26a that suppresses pten and bim . mir-26a
mir-26a was found to be repressed by c - myc in a mouse lymphoma model , leading to enhanced expression of the ezh2 oncogene , a component of the polycomb repressive complex 2 .
mir-146a , mir-181a / c , and mir-221 were associated with overall survival in all patients .
however , overexpression studies of mir-146a in transplanted bone marrow cells suggest a tumor - suppressive role for this microrna .
mir-146a knockout mice developed massive myeloproliferation followed by hematopoietic tumors , including myeloid sarcomas and lymphomas [ 635 , 636 ] . the myeloproliferative phenotype correlated with enhanced nfb signaling .
mir-146a suppresses the nfb activators irak1 ( interleukin 1 receptor - associated kinase 1 ) and traf6 ( tnf receptor - associated factor 6 ) [ 635 , 637 , 638 ] .
a negative feedback loop exists between nfb and mir-146 . whereas mir-146 represses nfb signaling , nfb signaling upregulates mir-146 .  

mir-181a family members are highly expressed in t - all leukemia cells and downregulated during remission .
deletion ofmir-181a-1/b-1expression inhibits the development ofnotch1 oncogene - induced t - all in a mouse model .
mir-181a / bcontrols the strength and threshold of notch activity in tumorigenesis in part by dampening multiple negative feedback regulators downstream of notch and pre - t - cell receptor ( tcr ) signaling pathways .  

mir-124a has been shown to be downregulated in more than 50% of all cases and associated with higher relapse rate and mortality rate .
a comparison study of primary cll samples with normal unstimulated or cpg - stimulated b cells showed high similarities between cll and activated b cells , including upregulation of mir-34a , mir-155 , and mir-342 and down - regulation of mir-103 and mir-181a / b .
activation of normal b cells led to reduced mir-23a , mir-23b , mir-24 , mir-27b , mir-181a / b , and mir-223 and increased mir-155 with all activation agents used .
mir-150 was reduced during b - cell activation , whereas it was upregulated in most cll cases .
the latter confirms the study of fulci et al . , but is opposed to wang et al
ectopic mir-150 expression increased cell death in pro - b cells , while mir-150 deficiency led to b - cell expansion and an enhanced humoral immune response .
some differences in mir-150 are observed between the mutated versus unmutated igvh ( immunoglobulin heavy chain variable - region genes ) subgroups , where expression is higher at the average in the mutated igvh subgroup .
cll cases with unmutated igvh or with high expression levels of zap-70 ( 70kd zeta - associated protein ) show an unfavorable course with rapid progression in comparison to patients with a mutated igvh .
two research groups [ 565 , 566 ] observed decreased levels of mir-29c and mir-223 in cll with zap70 and igvh unmutated status .
calin et al .   observed that the unmutated igvh cll subgroup exhibited high levels of tcl-1 due to low expression of mir-29 and mir-181 that negatively regulate this oncogene . mir-181 and mir-29 might therefore be considered to have tumor - suppressor functions .
tcl-1 functions as a coactivator of akt , and b - cell forced expression of tcl-1 in transgenic mice resulted in tumors that resembled cll .
cll with unmutated igvh and high expression of zap-70 showed also relative high expression of mir-15a , mir-16 - 1 , mir-16 - 2 , mir-195 , mir-23b , mir-155 , mir24 - 1 , and mir-146 , while low expression of mir-223 , mir-29a-2 , mir-29b-2 , and mir-29c . in an aggressive subtype of cll with abnormalities in the tp53 gene ,
cll cases with good prognostic features are typically characterized by down - regulation of mir-15a and mir-16 - 1 [ 643 , 708 ] , located at the 13q14.3 locus .
these mirnas map to a region between exon 2 and 5 of the leu2 gene .
deletion of 13q14.3 ( del(13q ) ) is the most common cytogenetic abnormality in cll occurring in more than 50% of the cases and implies for a favorable prognosis .
this deletion occurs also frequently in mm patients .deletion in mice of the 13q14-minimal deleted region , which encompasses the mir-15a~16 cluster , caused the development of indolent b - cell - autonomous , clonal lymphoproliferative disorders , recapitulating the spectrum of cll - associated phenotypes observed in humans .
repression of mir-15a and mir-16 - 1 , as well as mir-29b , in cll may also be mediated by histone deacetylases ( hdacs ) .
hdac inhibition triggered the accumulation of the transcriptionally activating chromatin modification h3k4me2 and restored the expression of mir-15a , mir-16 - 1 , and mir-29b .
both mir-15a and mir-16 - 1 negatively regulate bcl-2 , and mir-29 targets mcl-1 [ 381 , 382 ] .
the expression of bcl-2 in cll cases is inversely correlated with the expression of mir-15a and mir-16 - 1 [ 563 , 711 ] .
other targets of mir-15/16 include chek1 , cyclind1 , cyclind2 , and cdc25a [ 525 , 645 ] .
overexpression of mir-15a and mir-16 - 1 induced cell cycle arrest at g1/g0 in an rb - dependent manner . a germ - line mutation in the primary precursor of mir-15a/16 - 1 that impairs their processing was observed in familial cll patients .
targeting deletion of mir-15a~16 in mice led to the development of a spectrum of diseases resembling cll - associated lymphoproliferation in humans , including cll , cd5 monoclonal b - cell lymphocytosis , and cd5 non - hodgkin 's lymphomas .
the new zealand black ( nzb ) mouse that harbor a point mutation in the 3-flanking region of mir-16 that leads to reduced mir-16 expression and develops symptoms similar to b - cll in humans , further confirming the tumor suppressor function of this locus .  

underexpression of mir-181a / b was associated with shorter overall survival in cll , while higher levels of mir-181a were associated with a shorter time from diagnosis to initial therapy . during the course of cll progression ,
the mir-181a / b levels were decreased , which inversely correlated with increased levels of its target genes mcl-1 and bcl-2 .
the study of marton et al .   showed consistent underexpression of mir-181a , as well as let-7a and mir-30d in all cll cases studied .
however , increased expression of mir-181a / b was associated with favorable outcome in patients with cytogenetically normal acute myeloid leukemia ( aml ) .
ectopic overexpression of mir-181a / b into primary cll increased fludarabine - sensitivity in p53 wild - type cells , but not in cll with attenuated p53 response .
the importance of the mir-181 target mcl-1 in cll survival was demonstrated by rapid apoptosis of cll cells following sirna - mediated down - regulation of mcl-1 , and by the mcl-1 transgenic mice , which developed b - cell lymphoma .
thus , low mir-181 and mir-29 expression in cll could confer drug resistance through upregulation of mcl-1 expression .  

the mir-29 family consisting of mir-29a and mir-29b seems to play a dual role in tumorigenesis . on the one hand , mir-29a and mir-29b
are downregulated in mantle cell lymphoma , aggressive cll samples ( high zap-70 with unmutated igvh ) [ 659 , 710 , 716 ] , alk - positive anaplastic large cell lymphomas ( alcl ) , mm , and aml . on the other hand , mir-29a and b
are expressed at higher degree in indolent cll ( low zap-70/mutated igvh ) than in normal cd19 cells [ 563 , 569 , 716 ] . mir-29c
together with mir-223 down - regulation is associated with higher tumor burden , disease aggressiveness , and poor prognosis in cll . forced overexpression of mir-29b
the tumor suppressor activity of mir-29 may be achieved through targeting cell cycle regulators and oncogenes such as cdk6 , dna methyltransferase 3a ( dnm3a ) and 3b ( dnmt3b ) , mcl-1 , and tcl1a [ 382 , 569 , 583 , 717 ] .
another tumor suppressor function of mir-29 is mediated through activation of p53 , which is achieved by targeting p85 ( the regulatory subunit of pi3k kinase ) and cdc42 ( a rho family gtpase ) . however , in another setting mir-29 acts as an oncogene .
mir29a overexpression in immature and mature b cells promoted cll development , and transplantation of mir-29-transduced hematopoietic stem and progenitor cells into irradiated mice resulted in myeloproliferative disease and aml .
one mechanism for the oncogenic feature of mir-29 could be through repression of the tumor suppressor cell - adhesion molecule peroxidasin homologue ( pxdn ) .
thus , depending on the cellular contexts , mir-29 can function as an oncogene or a tumor suppressor .  

mir-221 and mir-222 are expressed at higher levels in cll with unmutated igvh and high expression of zap-70 , the most aggressive cll subtype with poor prognosis .
these micrornas may contribute to oncogenesis by targeting the cdk inhibitor p27 [ 695 , 696 , 718 , 719 ] , foxo3a [ 720 , 721 ] , apaf-1 [ 721 , 722 ] , p57 , bmf , pten , and timp3 ( tissue inhibitor of metalloproteinase 3 ) .
in other cll cases , the mir-222 was found to be lower than that of normal cd19 cells .
the p53 target mir-34a is decreased in cll patients with 11q deletions , leading to increased zap-70 expression .
mir-34a also targets bcl-2 [ 348 , 484 ] , and the e2f1 and b - myb oncogenes in cll .
members of the mir-17~92 polycistron are upregulated in b - cell lymphoma , as well as mir-155 [ 469 , 568 , 685 ] .
adoptive transfer of hematopoietic stem cells bearing a truncated portion of the mir-17~92 polycistron in c - myc transgenic mice resulted in a more rapid onset of malignant b - cell lymphomas .
transgenic overexpression of the entire mir-17~92 in the murine hematopoietic compartment led to the development of lymphoproliferative disease and increased lethality .
the negative regulation of bim by the mir-17~92 cluster seems to be a major mechanism by which b - cell lymphomas evade apoptosis .
silencing of mir-17 and mir-20a in mantle cell lymphoma led to upregulation of the cyclin - dependent kinase ( cdk ) inhibitor p21 , suggesting that p21 is an essential target of the mir-17~92 cluster during b - cell lymphomagenesis .
overexpression of c - myc mrna together with mir-17 - 5p / mir-20a was associated with a more aggressive behavior in mantle cell lymphoma .
mir-17~92 confers chemoresistance in mantle cell lymphoma through activation of the pi3k / akt pathway .
mir-21 is commonly upregulated in cll   as well as cml   and many other cancer cell types .
forced overexpression of mir-21 under the control of the nestin promoter resulted in severe pre - b - cell lymphoma .
mir-21 overexpression potentiated lung tumorigenesis of a constitutively activated k - ras proto - oncogene .
mir-21 is an oncomir that promotes tumorigenesis by targeting a range of genes involved in regulating cell proliferation and/or survival , including pten , sprouty ( spry2 ) , pdcd4 ( programmed cell death 4 ) , tpm1 ( tropomyosin 1 ) , and human dna muts homolog 2 ( hmsh2 ) . in glioblastoma cells , mir-21 also targets a network of p53 pathways , tgf , and mitochondrial tumor suppressor genes .
pdcd4 inhibits ap-1-mediated transactivation   and negatively regulates the pro - survival ral guanine - nucleotide dissociation stimulator ( ralgds ) signaling pathways [ 517 , 729 ] .
pdcd4 is a tumor suppressor that is upregulated during apoptosis   and downregulated in several cancer forms [ 737739 ] .
spouty , which is downregulated by mir-21 , negatively regulates the c - raf pro - survival signaling pathway .  

overexpression of mir-125b in cll - derived cell lines resulted in the repression of many transcripts encoding enzymes implicated in cell metabolism .
these authors proposed that mir-125b acts as a regulator for the adaptation of cell metabolism to a transformed state .  

one microrna consistently downregulated in most b - lymphomas is mir-150 , which is proposed to act as a tumor suppressor [ 523 , 547 , 589 ] .
mice lacking mir-150 have increased expression of its target transcription factor c - myb , which plays an important role in lymphocyte development and maturation .
premature expression of mir-150 blocked the transition from pro - b to the pre - b stage .
overexpression of mir-150 in nk / t lymphomas increased apoptosis and reduced cell proliferation , with concomitant reduction in dkc1 ( dyskeratosis congenita 1 ) and akt2 , reduced akt phosphorylation , and elevated levels of bim and p53 .  

mir-155 is overexpressed in many b - cell lymphomas including cll , primary mediastinal b - cell lymphoma ( pmbl ) , aggressive activated b - cell like ( abc ) subtype of dlbcl , hodgkin 's lymphoma , and pediatric burkitt 's lymphoma , but is almost absent in adult burkitt 's lymphoma [ 515 , 566 , 568 , 576 , 583 , 584 , 587 , 602 , 685 , 686 ] .
c - myb ( v - myb myeloblastosis viral oncogene homolog ) , which is overexpressed in a subset of cll patients , associates with the promoter of mir-155 host genes ( mir155hg , also known as bic , b - cell integration cluster ) and stimulates its transcription .
forced overexpression of mir-155 in b cells ( e-mir-155 transgenic mice ) led to initial preleukemic pre - b - cell proliferation followed by frank b - cell malignancy .
the mir-155 orthologue mir - k12 - 11 in kaposi sarcoma - associated herpes virus ( kshv ) has been associated with b - cell tumors .
mir-155 is essential for immune function and is strongly induced in activated t and b cells .
mir-155 represses sh2-domain containing inositol-5-phosphatase-1 ( ship-1 ) , which is a critical phosphatase that negatively downmodulates akt pathway and is involved in normal b cell development . thus , sustained overexpression of mir-155 in b cells unblocks akt activity , inducing b - cell development .
c - maf in lymphocytes , and hgal and smad5 in diffuse large b - cell lymphoma ( dlbcl ) [ 741 , 742 ] .
hgal , a germinal center ( gc)-specific gene , inhibits lymphocyte and lymphoma cell motility by activating rhoa signaling cascade   and by interacting with actin and myosin proteins .
smad5 is a bone morphogenetic protein ( bmp)-responsive transcription factor and is activated by various cytokines .
dlbcl expressing high levels of mir-155 concomitant with low hgal expression showed high aggressiveness and cell dissemination .
thus , down - regulation of smad5 in diffuse large b - cell lymphoma defines a unique mechanism used by the cancerous cells to escape tgf growth inhibitory effects . in breast cancer ,
as foxo3a is a positive regulator of the pro - apoptotic bim essential for gc - induced apoptosis [ 227229 ] , mir-155 overexpression may prevent bim upregulation . in one study , mir-93 , mir-25 , mir-92 ,
mir-19a / b , mir-181a / b , and mir-32 were shown to be significantly overexpressed , while let7-b , let7-i , let7-c , mir-29a , and -29b significantly downregulated in mm .
.   found decreased expression of mir-15a~16 and increased expression of mir-222 , mir-221 , mir-382 , and mir-181a / b in their mm samples .
also , the 13q14.3 locus containing the mir-15a and mir-16 - 1 is sometimes deleted in mm [ 345 , 746748 ] .
the absence of mir-15a expression and overexpression of mir-181a / b correlated with worse prognosis of mm .
antagonists especially to mir-19a / b and mir-181a / b ( antagomirs ) suppressed tumor growth of human myeloma cells implanted into nude mice .
some differential mirna expression was observed between malignant mm and mgus ( monoclonal gammopathy of undetermined significance ) , which is the precancerous state preceding mm .
mgus show already upregulation of mir-21 , mir-106~25 , mir-181a / b , mir-1 , and mir-133a , while during the progression to malignant multiple myeloma mir-17~92 , mir-32 , mir-193b~365 are upregulated and mir-192~194~215 and mir-15a~16 are downregulated [ 574 , 576 , 577 ] .
the upregulation of mir-17~92 could be related to the upregulation of c - myc observed during mm progression [ 750 , 751 ] .
upregulation of mir-1 and mir-133a correlated with t(14 ; 16 ) translocation in mm cases , suggesting that deregulation of microrna expression could be associated with chromosomal abberations .
higher expression of dicer was associated with improved progression - free survival in symptomatic mm cases . the global increase in microrna expression in high - risk mm patients with poor prognosis
was associated with increased expression of argonaute ( ago2/elf2c2 ) , a master regulator of mirna maturation and function [ 753 , 754 ] .
adhesion of multiple myeloma to bone marrow stroma triggers cytokine production and enhances cell proliferation and resistance to chemotherapy through il-6-induced activation of nfb , pi3k / akt , and stat3 pathways .
it should be noted that these pro - survival pathways antagonize gc - induced apoptosis in mm [ 756760 ] .
mir-19a and mir-19b that are part of the mir-17~92 cluster downregulate socs-1 ( suppressor of cytokine signaling-1 ) , a gene frequently silenced in mm that plays a critical role as inhibitor of il-6 growth signaling , thus enforcing the il-6-induced survival signals .  

the oncogenic mir-21 is upregulated in mm patient samples and cell lines [ 574 , 579 , 652 ] . in il-6-dependent
ectopic mir-21 expression was sufficient to sustain growth of il-6-dependent cell lines in the absence of il-6 .
mir-21 is upregulated in a nfb - dependent manner in mm cells upon cell adhesion to bone marrow stromal cells . combining mir-21 inhibition with dexamethasone
the p300-cbp - associated factor ( pcaf ) was found to be a target of the combined action of the mir106b~25 cluster and mir-32 .
mir106b~25 , mir-17 , and mir-20a target the cdkn1a1/p21 cell cycle regulator , which prevents cell cycle progression in general and prevents the growth of mm cells [ 764 , 765 ] .  

mir-15a~16 is a pro - apoptotic microrna that targets bcl-2 , cyclin d1 , cyclin d2 , and cdc25a [ 346 , 748 , 766768 ] .
overexpression of mir-15a~16 in mm led to inhibition of akt3 , ribosomal protein s6 , map kinases , and the nfb - activator map3kip3 , ultimately resulting in an antiproliferative effect and apoptosis .
the anti - mm effect of mir-15a~16 was observed even in the context of the bone marrow microenvironment . mir-15a~16
reduced vegf secretion from mm cells , thereby reducing mm cell - induced pro - angiogenic activity on endothelial cells .
vegf represents one of the major pro - angiogenic cytokines responsible for the induction of neoangiogenesis in mm patients [ 769 , 770 ] .
a distinct microrna profile could distinguish between alk and alk subtypes of alcl , an aggressive form of non - hodgkin 's lymphoma ( nhl ) belonging to the t - cell lineage .
more than 80% of alk alcl harbor the t(2 ; 5)(p23 ; q35 ) translocation , resulting in the expression of the chimeric nucleophosmin ( npm)-alk .
the constitutive alk activity leads to the activation of many different growth - promoting and anti - apoptotic pathways including pi3k / akt / mtor , jak / stat , c - jun , junb , and c - myc . the prognosis of alk alcl is worse [ 772 , 773 ] .
alk alcl has a high cure rate with chop treatment , in contrast to alk cells that are relative resistant .
five members of the mir-17~92 cluster were expressed higher in alk alcl , whereas mir-155 was expressed more than 10-fold higher in alk alcl .
the upregulation of mir-17~92 cluster in alk alcl cells is in agreement with the observation that c - myc is expressed in alk alcl and absent from alk samples .
mir-101 targets mtor , mcl-1 , and the histone methyltransferase ezh2 [ 778 , 779 ] . inhibition of mtor , which is targeted by mir-101 , led to reduced tumor growth in engrafted alcl mouse models .
mir-29a and mir29b down - regulation in alk alcl confer apoptotic resistance due to mcl-1 upregulation [ 380 , 587 ] .
another microrna that has been implicated in npm - alk - driven oncogenicity is mir-135b .
mir-135b inhibition reduced tumor angiogenesis and growth in vivo , suggesting that targeting this microrna has therapeutic potential .
a 9-mirna signature ( mir-146b-5p , mir-146a , mir-21 , mir-155 , mir-500 , mir-222 , mir-363 , mir-574 - 3p , and mir-574 - 5p ) could differentiate the diffuse large b - cell lymphoma ( dlbcl ) , the most common subtype of non - hodgkin 's lymphoma , into abc ( activated b - cell ) or gcb ( germinal center b - cell ) subtypes , with a general higher expression in the abc subtype .
another study   found that mir-331 , mir-151 , mir-28 , and mir-454 were upregulated in the gcb type , whereas mir-222 , mir-144 , mir-451 , and mir-221 upregulated in the abc type .
the microrna expression of both gcb - like and abc - like cells was more similar to germinal center lymphocytes than memory b - cells .
the region encoding the mir-17~92 cluster was more commonly amplified in gcb - like than abc - like dlbcl .
lawrie et al .   identified 3 mirnas , mir-155 , mir-21 , and mir-221 , more highly expressed in abc type than gcb type cells .
expression of mir-155 and mir-21 was also higher in nonmalignant abc than in gcb cells .
patients with gcb dlbcl have longer overall survival and event - free survival compared with patients with an abc phenotype when treated with r - chop [ 782 , 783 ] . increased expression of mir-18a in dlbcl was associated with a shorter os ( overall survival ) of patients receiving r - chop regimen .
increased expression of mir-181a was associated with longer pfs ( progression - free survival ) , while increased expression of mir-222 was associated with shorter pfs . in dlbcl , mir-181a regulates foxp1 ( forkhead box protein p1 ) and mgmt ( o - methylguanine - dna methyltransferase ) expression in dlbcl cells .
foxp1 is expressed in normal activated b cells , mantle zone b cells , and some germinal center b cells [ 784 , 785 ] .
foxp1 is recurrently targeted by chromosomal translocations involving the immunoglobulin heavy chain locus in marginal zone lymphomas and dlbcl , suggesting a potential role for foxp1 in lymphomagenesis [ 786 , 787 ] .
foxp1 has in some studies been shown to be associated with poor prognosis and survival [ 788 , 789 ] .
the ability of mir-181a to reduce mgmt protein expression may contribute to better cyclophosphamide chemosensitivity .
mir-222 is part of the mir-221/mir-222 cluster , which is highly expressed in abc - like dlbcl cell lines   and abc - like dlbcl tumors .
mir-222 regulates the expression of the stem cell factor c - kit , and the cyclin - dependent kinase inhibitors p27 and p57 [ 695 , 698 ] .
high expression of mir-222 was associated with inferior overall survival and progression - free survival .
fl is characterized by high mir-9 , mir-138 , mir-20a / b , and mir-155 expression [ 135 , 564 , 582 ] .  

mir-9 targets also the transcription factor prdm1/blimp1 in lymphoma and may contribute to the phenotype maintenance and pathogenesis of lymphoma cells by interfering with normal b - cell terminal differentiation [ 582 , 608 ] .
brdm1/blimp1 and bcl6 are critical regulators of germinal center b - cell differentiation [ 594 , 792 , 793 ] .
brdm1/blimp1 and bcl6 are expressed in a mutual exclusive pattern and evidence suggests that they repress each other in germinal center b cells [ 792 , 794 ] .
a marked decrease of brdm1/blimp1 and an increase of bcl6 were observed in follicular lymphoma cells , which might be related to the increased mir-9 levels in these cells .
mutations in brdm1/blimp1 are frequently found in activated b cell ( abc)-like dlbcl [ 790 , 795 ] .
the malignant hodgkin 's lymphoma cells are usually derived from b cells , but have lost the expression of typical b - cell genes .
multiple signaling pathways are deregulated , including nfb , jak ( janus kinase)/stat ( signal transducer and activator of transcription ) , pi3k / akt , erk , notch1 , and receptor tyrosine kinases .
patients with low mir-135a expression had a higher probability of relapse and a shorter disease - free survival .
mir-135a targets jak2 , a cytoplasmic tyrosine kinase involved in a subset of cytokine receptor signaling pathways
. transfection of pre - mir-135a into classical hl ( chl ) caused apoptosis and decreased cell growth .
the mir-135a - mediated jak2 down - regulation led to decreased bcl - xl expression , a downstream effector of jak2 .
about 40%60% of hodgkin 's lymphomas have ebv ( epstein - barr virus ) associated with the malignant cells .
since mir-155 is overexpressed in hodgkin 's lymphoma   and promotes b - cell lymphoma formation [ 602 , 798 , 799 ] , ebv may be important in the pathogenesis of chl .
micrornas have been shown to modulate gr expression in neuronal tissue [ 649 , 800 , 801 ] .
mir-18 and mir-124a especially reduced gr - mediated events in addition to decreasing gr protein levels .
upregulation of the mir-17~92 has causally been related to small cell lung cancer [ 802 , 803 ] , where reduced gr levels have been associated with gc resistance .
activation of the gr - responsive glucocorticoid - induced leucine zipper ( gilz ) was impaired by mir-124a and -18 overexpression , while mirs-22 , -328 , and -524 did not have any effect .
of note , mir-124 regulates hes1 expression in p19 teratocarcinoma cells , a transcription factor that negatively regulate gr expression .
while mir-130b , -181a , and -636 have putative complimentary binding sites in the 3-utr of gr , only mir-130b was found to down - regulate endogeneous gr protein expression in the multiple myeloma cell line mm.1 .
the mir-130b , -181a , and -636 were differentially expressed between gc - sensitive and gc - resistant mm.1 cell lines .
overexpression of mir-130b in mm.1s cells resulted in decreased expression of endogeneous gr , decreased induction of the gr - target gene gilz , and induction of gc resistance .
expression of mir-130b was therefore suggested to be a potential biomarker for patients who could be refractory to gc therapy . in gastric cancers , mir-130b regulated the tumor suppressor gene runx3 .
mir-130b may also down - regulate p21 , resulting in inhibition of cellular senescence [ 676 , 806 ] .
another study   showed that elevated mir-142 expression in human t - all cells confers gc resistance by reducing the gr expression level .
other mechanism for the oncogenic role of mir-142 might be explained by its targeting of adenylyl cyclase 9 mrna   leading to reduced production of cyclic adenosine monophosphate ( camp ) production with concomitant inhibition of the protein kinase a ( pka ) signaling pathway .
the reduction in camp levels and reduced pka activity caused by mir-142 relieve the inhibitory effect of pka on t - leukemic cell proliferation .
t - all with poor prognosis expressed higher levels of mir-142 than those with good prognosis .
also , mir-142 was expressed at higher levels in relapsed t - all than newly diagnosed samples .
transfection of mir-142 inhibitor increased gr expression levels and sensitized t - all cells to gc - induced apoptosis .
these findings are in accord with previous findings showing a synergistic effect of camp mimetics on gc - induced apoptosis [ 99 , 460 , 807 ] .
camp signaling can also be negatively regulated by phosphodiesterase 4b ( pde4b ) that is frequently overexpressed in diffuse large b - cell lymphoma ( dlbcl ) .
pharmacological inhibition of pde4 in a xenograft model of human lymphoma unleashed camp effects , inhibited akt , and restored gc sensitivity .
pde4 inhibitors may thus improve the clinical outcome of patients with b - cell malignancies .
triptolide , a drug that overcomes dexamethasone - resistance in human multiple myeloma cells , was found to regulate gr expression in the mm1.s cell line by downregulating the expression of mir-142 and mir-181a . mir-142 and mir-181a mimetics slightly attenuated , whereas mir-142 and mir-181a inhibitors enforced gc - induced apoptosis of mm1.s cells .
mir-181a / b can also increase gc - induced apoptosis in virtue of their ability to repress the expression of the anti - apoptotic bcl-2 , mcl-1 , and xiap proteins [ 385 , 539 , 810 ] .  
.   showed that broad microrna repression occurs during gc - induced apoptosis of rat thymocytes .
this repression was associated with reduced expression of both nuclear ( drosha and dgcr8/pasha ) and cytoplasmic ( dicer ) microrna processing enzymes .
silencing of dicer in two human leukemic cell lines ( cem - c7 and ectopic gr-overexpressed jurkat cells ) led to enhanced sensitivity to gc - induced apoptosis .
global downregulation of microrna levels , especially the mir-17 family , by gcs was also observed in gc - sensitive all cell lines , with concomitant upregulation of bim .
later studies showed that gcs selectively upregulate and downmodulate specific mirnas   that can not be explained by altered dicer expression .
one polycistron cluster repressed by gcs is mir-17~92 [ 648 , 657 ] , which regulates bim expression [ 246 , 247 ] .
this microrna cluster also represses pten , a negative regulator of the pi3k / akt signaling pathway .
the gc - mediated downregulation of mir-17~92 might be one mechanism responsible for the gc - induced dephosphorylation of akt .
primary thymocytes derived from mice transgenic for the mir-17~92 polycistron members in the lymphocyte compartment exhibited diminished sensitivity to gc - induced apoptosis in lymphocytes , further supporting a role for gc - induced repression of mir-17~92 in promoting apoptosis .
observed that gcs reduced mir-17 family expression in 50% of primary gc - sensitive all , but not in any of the gc - resistant ones .
overexpression of mir-17~92 attenuated gc - induced cell death , while inhibition of mir-17~92 increased the sensitivity to gc .
they also reported that in a pre - b all cell line , a 10-hour dexamethasone treatment led to a reduction in mir-142 and mir-27a , while mir-9 was induced .
there is also some evidence that gcs can reduce mir-27a expression in mouse muscle cells .
rainer et al .   reported an induction of the myeloid - specific mir-223 and the apoptosis and cell - cycle arrest inducing mir-15~16 clusters by gc in a subset of b- and t - all cells , together with downregulation of the mir-17~92 complex .
the mirnas of the mir-15~16 family are encoded in two clusters ( 15a~16 - 1 and 15b~16 - 2 ) embedded in the dleu2 ( deleted in leukemia 2 ) and smc4 loci , respectively [ 766 , 812 ] .
they have been implicated in cell - cycle arrest   and in cell death / survival decisions , the latter supposedly by targeting bcl-2 .
other micrornas affected by gcs in pediatric all include upregulation of mir-548d-1 and repression of mir-128b along with mir-106b~25~93 , the paralogue of mir-17~92 .
it is still not known whether the gc - induced upregulation of mir-223 affects gc - induced apoptosis .
increased expression of mir-223 is involved in the differentiation of myeloid precursors into granulocytes such as neutrophils [ 701 , 814 ] . during granulopoiesis ,
mir-223 targets e2f1 , which in turn represses mir-223 expression , creating an autoregulatory negative feedback loop .
a negative feedback loop also exists between mir-223 and the transcription factor nfi - a .
mir-223 is positively regulated by c / ebp during differentiation to granulocytes   and negatively regulated by aml1/eto in leukemia cells .
moreover , mir-223 targets the myeloid elf-1-like factor ( mef)-2c and igfr ( insulin - like growth factor receptor ) , which may account for some of its negative regulation of granulocyte proliferation . through suppression of igf-1r , the downstream pi3k / akt / mtor / p70s6k pathway is suppressed , with consequent inhibition of cell proliferation . mir-223 attenuates hematopoietic cell proliferation and positively regulates mir-142 through lmo2 isoforms and c / ebp .
mir-223 knockout mice showed increased numbers of granulocyte progenitors in the bone marrow and hypermature neutrophils in the circulation , suggesting that mir-223 is involved in the negative regulation of maturation rather than differentiation of granulocytes . mir-223
may also target fbw7 [ 705 , 816 ] , a negative regulator of the anti - apoptotic mcl-1 .
dexamethasone treatment of thymocytes led to upregulation of mir-150 and mir-342 , while mir-181a and mir-181d were downregulated .
lif is a member of the il-6 cytokine family expressed in thymic epithelial cells and t lymphocytes , which elevates gc levels following lps exposure and is responsible for thymic atropy induced by stress [ 817819 ] .
a recent report showed that gcs could prevent lipopolysaccharide ( lps)-mediated inflammatory responses in macrophages by downregulating mir-155 .
overexpression of mir-155 reversed the suppressive action of gcs , while inhibition of mir-155 exhibited an effect similar to that of gcs on lps - treated macrophages , suggesting that gc - induced repression of mir-155 is one mechanism for the immunosuppressive function of gc .
mir-155 is transcribed from b - cell integration cluster ( bic ) [ 584 , 691 ] and targets among others socs1 ( suppressor of cytokine signaling 1 ) , which negatively regulates jak / stat signaling .
gcs also prevented the lps - mediated upregulation of mir-146 , mir-147 , mir-148 , mir-32b , and mir-301 in macrophages . in the brain
, gcs prevents bdnf ( brain - derived neurotrophic factor)-regulated synaptic function through suppression of mir-132 expression .
mir-132 is increased by bdnf and is involved in promotion of neuronal outgrowth . in some carcinoma cell lines , dexamethasone was shown to down - regulate mir-27b , mir-148a , and mir-451 . from all we have learned above , any microrna that modulates
any of the many factors regulating gc - induced apoptosis may affect the apoptotic response to gcs ( figures 16 ) .
these include micrornas that affect gr expression ( e.g. , mir-18 , mir-124a , mir-130b , mir-142 , and mir-181a ) , those affecting bim expression ( mir-26a , mir-93 , mir-17~92 , mir-106a~363 , and mir-106b~25 ) or its transcription factor foxo3 ( e.g. , mir-1 , mir-21 , mir-27a , mir-96 , mir-135b , mir-155 , and mir-182 ) , those affecting pten expression ( mir-17~92 , mir-106b~25 , mir-21 , mir-26a , mir-29b , mir-214 , mir-216a , mir-217 , mir-221 , and mir-222 ) or mtor ( e.g. , mir-101 ) , and those downregulating directly or indirectly the anti - apoptotic proteins bcl-2 , bcl - xl , mcl-1 , xiap , and cyld ( e.g. , mir-15a~16 , mir-181a / b , mir-34a , mir-125b , mir-29a / b / c , mir-101 , mir-133b , mir-193b , mir-512 , let-7 , and mir-491 ) . the effect of some of these micrornas on gc - sensitivity has already been described above and will not be repeated here .
rather , i will present here some data from primary samples showing the influence of micrornas on clinical outcome .
a study searching for differential mirnas expression in all relapse cells versus childhood all with complete remission showed significant associations for mir-708 , mir-223 , and mir-27a with individual relapse - free survival .
for samples at relapse versus diagnosis , the most differentially expressed micrornas included mir-223 , mir-23a , let-7 g , mir-181 , mir-708 , and mir-130b , while comparison of complete response with diagnostic samples showed differential expression pattern of mir-27a , mir-223 , mir-23a , mir-181 , and mir-128b . among these micrornas , mir-223 , mir-128b , mir-23a , and let-7 g
were downregulated in the relapse samples compared with complete response samples , while mir-181 family members , mir-708 , and mir-130b were upregulated in the relapse samples .
it should be remained here that mir-130b targets gr , runx3 , and p21 , and mir-223 is upregulated by gcs   and targets igfr   and e2f1 .
e2f1 has a dual role in cell - cycle control , as it affects several cell processes .
it can either act as a tumor - suppressor or oncogene depending on the cellular context .
thus , the upregulation of mir-130b together with downregulation of mir-223 may contributes to gc resistance .
mir-708 was the most upregulated microrna in the relapse samples , whereas mir-223 was significantly downregulated , suggesting that these two micrornas may have important role in pediatric all relapse .
moreover , upregulation of mir-708 was found to be associated with the in vivo gc therapy response and with disease risk stratification in childhood all .
standard and middle risk stratification groups had a higher mir-708 expression at diagnosis than the high risk group .
interestingly , mir-708 was low in high relapse patients at diagnosis , while specimens of relapsed samples showed abundance of mir-708 , suggesting for an upregulation of mir-708 during disease progression .
foxo3 , that is critical for hematopoietic stem cell self - renewal and mediates the initial apoptotic response [ 825827 ] , contains a conserved mir-708 response element in its 3-utr .
foxo3 can act as either an oncogene or a tumor suppressor in leukemia [ 828 , 829 ] .
foxo3 transcriptional activity was found to prevent b - cll and cml proliferation [ 825 , 828 ] .
foxo3a is also targeted by other micrornas , including mir-27a ( see section 2.2.6 ) .
moreover , mir-27a directly regulates the drug - resistant factor p - glycoprotein , and overexpression of mir-27a increased sensitivity of leukemia cells to doxorubicin .
mir-27a is relevant to treatment outcome in vivo and may be involved in relapse of both lymphocytic leukemia and myeloid leukemia .
low expression of mir-27a might promote all relapse [ 655 , 658 ] . on the contrary ,
mir-27a exerts oncogenic effects by regulating zbtb10 [ 446 , 830 ] and fbw7 [ 253 , 438 ] .
mir-128b , which was higher in relapse all and at diagnosis compared to complete response , has been reported to confer drug resistance in many cancers including all [ 672 , 673 ] .
both mir-27a and mir-128b might target bmi1 , a transcription factor of the polycomb - group gene necessary for hematopoietic stem cell ( hsc ) and leukemia stem - cell self - renewal [ 831 , 832 ] .
deletion of bmi1 inhibits self - renewal of tumor stem cells and prevents leukemia recurrence .
a role for mir-128 and mir-221 in regulating gc sensitivity in cells from mll - af4 all patients has been proposed . mir-128b and mir-221 are downregulated in mll - arranged all relative to other types of all . the mll gene is located at 11q23 , a site frequently involved in chromosomal translocations in aggressive human lymphoid and myeloid leukemias . as a result of chromosomal translocations ,
a portion of mll becomes fused to one among more than 40 different partner proteins .
mll - af4 all , which results from the translocation between mll and af4 , is associated with gc resistance and has a poor prognosis [ 834 , 835 ] .
re - expression of mr-128 and mir-221 in cultured mll - af4 all cells sensitized them to gcs .
mir-128 targets mll , af4 , and the mll - af4 pusion protein resulting in lower expression of hoxa9 , whereas mir-221 downregulates cdkn1b ( cyclin - dependent kinase inhibitor 1b , p27 ) , another gene transcriptionally activated by mll - af4 as well as the wild - type mll protein .
the targeting of different proteins may explain the cooperative effect of mir-128b and mir-221 on gc sensitization .
it should be noted that mir-221 in other settings , for example , cll , has anti - apoptotic effects and functions as an oncogene . in light of the multiple effects of various micrornas on cell survival and apoptosis
, modulating microrna expression in tumor cells is an attractive approach for sensitizing the tumor cells to chemotherapeutic drugs .
inhibition of specific micrornas is performed by using antisense sequences ( termed antagomirs ) targeting the microrna guide stand that blocks the interaction with the microrna recognition elements within the 3-utr of the target mrna genes . to increase their binding affinity and stability in biological fluids , the antagomirs
are often modified with 2-o - methyl- , phosphorothioate , or locked nucleic acid substitutions . to overexpress micrornas ,
one potential use of micrornas is to repress the expression of mll - af4 fusion protein in all that is responsible for gc resistance .
this fusion protein can be repressed through overexpression of mir-143 , or mir-128 together with mir-221 .
the latter combination was shown to sensitize the mll - af4-carrying all cells to gcs .  
another promising approach is to target mir-155 , an oncogenic microrna often correlated with poor prognosis .
they showed that overexpression of mir-155 in lymphoid tissues resulted in disseminatedlymphomacharacterized by a clonal , transplantable pre - b - cell population of neoplastic lymphocytes .
systemic delivery of antisense peptide nucleic acids encapsulated in unique polymer nanoparticles inhibited mir-155 and slowed the growth of these pre - b - cell tumors in vivo .
glucocorticoid - induced apoptosis appears to be a complex process involving several signaling pathways ( figure 6 ) .
these include ( 1 ) transactivation of pro - apoptotic genes ( importantly bim ) ; ( 2 ) alterations in microrna expression ( upregulation of mir-15~16 that targets the pro - apoptotic bcl-2 ; mir-223 that targets igfr ; mir-150 that targets akt and notch , while suppressing mir-17~92 that prevents bim and pten translation ) ; ( 3 ) direct action of gr on the mitochondria ( including mitochondrial gr translocation and production of reactive oxygen species within the mitochondria ) ; ( 4 ) activation of the protein kinases gsk3 and p38 ; ( 5 ) activation of the foxo3a transcription factor that upregulates bim ; ( 6 ) inhibition of the notch1 , pi3/akt / mtor , and erk1/2 survival pathways .
altered microrna expression in malignant cells may modulate many of these processes thereby imposing apoptotic resistance ( figures 16 ) .
gc - resistant lymphoid cells might be divided into two major subgroups according to the underlying mechanism of resistance .
the first group consists of cancer cells whose drug resistance can be overcome by exposing the cells to gcs in combination with drugs that target protein kinases such as akt , mtor , src , alk , and/or bcr , or drugs antagonizing bcl-2 , bcl - xl , mcl-1 , c - myc , or notch .
these lymphoid malignancies show in general a more favorable response to combined gc therapy and in many cases may be explained by their growth dependency on these signaling molecules .
the second group of gc - resistant cells exhibits an intrinsic defect in the gc - mediated apoptotic process and can thus not be turned sensitive to this drug .
it is important to distinguish between these two subgroups prior to therapy initiation in order to choose the right drug combination .
a diagnostic test needs to be developed that can distinguish between the different resistance backgrounds .
recently , burnsides et al .   have developed an ex vivo stimulation assay that determines the ability of leukocytes to upregulate anti - inflammatory genes such as gilz and fkbp51 following exposure to dexamethasone .
it is reasonable that a similar test may be developed to gene profiling lymphoid malignancies prior to and following gc treatment , where upregulation of the pro - apoptotic bim gene would be a favorable predictor .
also , bim induction may be measured after combining gc with a protein kinase inhibitor .
simultaneous expression profiling of micrornas , notch1 , and bcl-2 family proteins together with the activated protein kinase status in the malignant cell would provide valuable information for choosing the proper drug combination . a predictor for a good gc response would be to determine the ability of gcs to downregulate mir-17~92 and upregulate mir-15~16 , mir-150 , and mir-223 .
a tentative therapeutic approach would be to modulate the microrna status of the cell using microrna mimics or antagomirs as described in section 4.4 .
what we have learned from the studies described in this paper is that it seems that in general it would be favorable to augment the expression of mir-29 , mir-27 , mir-15a~16 , mir-34a , mir-150 , and let-7 , while suppressing mir-155 , mir-181 , mir-182 , mir-21 , and mir-221/222 as well as mir-17~92 .
obviously , an initial microrna profiling should be performed , and the cancer - type classification should be considered .
while down - regulation of mir-181 may suppress the growth of t - all and mm , augmented mir-181 expression prevents the growth of unmutated igvh cll cases .
also , mir-26a has a dual effect . its overexpression prevents growth of c - myc - positive burkitt lymphoma , while it must be downregulated in notch - positive t - all to achieve growth inhibition . mir-451 and mir-709 could prevent growth of notch - positive t - all . a reduction in mir-142 , and maybe also of mir-708 , which is highly expressed in relapsed childhood t - all , is anticipated to improve t - all therapy . for classical hl , mir-135a may cause apoptosis . in conclusion , in certain types of lymphoid malignancies
, gc resistance may be overcome by relieving the inhibitory effects of protein kinases and bcl-2 family members .
both the activity of protein kinases and the expression of bcl-2 members are affected by the microrna network .
modulation of microrna expression might increase gc drug responsiveness and thus improve the therapy of lymphoid malignancies .
 the initial response of lymphoid malignancies to glucocorticoids ( gcs ) is a critical parameter predicting successful treatment . 
 although being known as a strong inducer of apoptosis in lymphoid cells for almost a century , the signaling pathways regulating the susceptibility of the cells to gcs are only partly revealed . 
 there is still a need to develop clinical tests that can predict the outcome of gc therapy . in this paper , 
 i discuss important parameters modulating the pro - apoptotic effects of gcs , with a specific emphasis on the microrna world comprised of small players with big impacts . 
 the journey through the multifaceted complexity of gc - induced apoptosis brings forth explanations for the differential treatment response and raises potential strategies for overcoming drug resistance . 