bluuebunny/crossref_metadata_2025 huggingface dataset:
Phosphatidylinositol 4-kinase, but not phosphatidylinositol 3-kinase, is present in GLUT4-containing vesicles isolated from rat skeletal muscle
Authors: KRISTIANSEN S⊘ren , RAMLAL Toolsie , KLIP Amira
Published: August 2015

Insulin stimulates the rate of glucose transport into muscle and adipose cells by translocation of glucose transporter (GLUT4)-containing vesicles from an intracellular storage pool to the surface membrane. This event is mediated through the insulin receptor substrates (IRSs), which in turn activate phosphatidylinositol (PI) 3-kinase isoforms. It has been suggested that insulin causes attachment of PI 3-kinases to the intracellular GLUT4-containing vesicles in rat adipose cells. Furthermore, it has also been shown that GLUT4-containing vesicles in adipose cells contain a PI 4-kinase. In the present study we investigate whether GLUT4-containing vesicles isolated from rat skeletal muscle display PI 3-kinase and/or PI 4-kinase activities. Insulin stimulation caused a rapid increase (5–15-fold increase compared with control) in the intracellular cytosolic IRS-1-associated PI-3 kinase activity. This PI 3-kinase activity was also present in a membrane preparation containing the insulin-regulatable pool of GLUT4 transporters. However, when GLUT4-containing vesicles were isolated by immunoprecipitation from basal and insulin-stimulated (3 min) skeletal muscle, the vesicles displayed PI 4-kinase, but not PI 3-kinase, activity. Insulin did not regulate the PI 4-kinase activity in the GLUT4-containing vesicles. In conclusion, GLUT4-containing vesicles from rat skeletal muscle contain a PI 4-kinase, but not a PI 3-kinase. It is suggested that, in skeletal muscle, insulin causes activation of the IRS/PI 3-kinase complex in an intracellular membrane compartment associated closely with the GLUT4-containing vesicles, but not in the GLUT4-containing vesicles themselves.
AMP-activated protein kinase is activated by low glucose in cell lines derived from pancreatic β cells, and may regulate insulin release
Authors: SALT Ian P. , JOHNSON Gabriele , ASHCROFT Stephen J. H. , HARDIE D. Grahame
Published: August 2015

The role of the AMP-activated protein kinase (AMPK) cascade in the glucose-sensitive pancreatic β cell lines HIT-T15 and INS-1 was addressed. In both cell types, removal of glucose leads to a > 5-fold activation of AMPK activity. Activation of AMPK was due to phosphorylation, since the effect was reversed by protein phosphatase treatment of the extracts, and was restored by re-addition of MgATP and the purified upstream kinase. When the effects of different concentrations of medium glucose were examined, insulin secretion and AMPK activity were inversely related, and varied over the same concentration range. The activation in response to glucose removal appeared to be due to changes in the concentration of the known regulators of the cascade, i.e. AMP and ATP, since AMPK activation was associated with a large increase in the cellular AMP/ATP ratio, and the two parameters varied over the same range of glucose concentrations. In late-passage HIT-T15 cells that had lost the glucose-dependent insulin secretion response, both AMPK activity and the AMP/ATP ratio also became insensitive to the extracellular glucose concentration. Treatment of INS-1 cells, but not HIT-T15 cells, with AICA riboside (5-aminoimidazole-4-carboxamide riboside) results in accumulation of the ribotide, ZMP (AICA riboside monophosphate), and activation of AMPK. AICA riboside treatment of INS-1 cells, and of isolated rat islets, had both inhibitory and stimulatory effects on insulin secretion. These results show that in β cell lines the AMP-activated protein kinase, like its yeast homologue the SNF1 complex, can respond to the level of glucose in the medium, and may be involved in regulating insulin release.
Activation of the protein tyrosine phosphatase SHP2 via the interleukin-6 signal transducing receptor protein gp130 requires tyrosine kinase Jak1 and limits acute-phase protein expression
Authors: SCHAPER Fred , GENDO Cornelia , ECK Monika , SCHMITZ Jochen , GRIMM Carsten , ANHUF Dirk , KERR Ian M. , HEINRICH Peter C.
Published: August 2015

Stimulation of the interleukin-6 (IL-6) signalling pathway occurs via the IL-6 receptor–glycoprotein 130 (IL-6R–gp130) receptor complex and results in the regulation of acute-phase protein genes in liver cells. Ligand binding to the receptor complex leads to tyrosine phosphorylation and activation of Janus kinases (Jak), phosphorylation of the signal transducing subunit gp130, followed by recruitment and phosphorylation of the signal transducer and activator of transcription factors STAT3 and STAT1 and the src homology domain (SH2)-containing protein tyrosine phosphatase (SHP2). The tyrosine phosphorylated STAT factors dissociate from the receptor, dimerize and translocate to the nucleus where they bind to enhancer sequences of IL-6 target genes. Phosphorylated SHP2 is able to bind growth factor receptor bound protein (grb2) and thus might link the Jak/STAT pathway to the ras/raf/mitogen-activated protein kinase pathway. Here we present data on the dose-dependence, kinetics and kinase requirements for SHP2 phosphorylation after the activation of the signal transducer, gp130, of the IL-6-type family receptor complex. When human fibrosarcoma cell lines deficient in Jak1, Jak2 or tyrosine kinase 2 (Tyk2) were stimulated with IL-6–soluble IL-6R complexes it was found that only in Jak1-, but not in Jak 2- or Tyk2-deficient cells, SHP2 activation was greatly impaired. It is concluded that Jak1 is required for the tyrosine phosphorylation of SHP2. This phosphorylation depends on Tyr-759 in the cytoplasmatic domain of gp130, since a Tyr-759 → Phe exchange abrogates SHP2 activation and in turn leads to elevated and prolonged STAT3 and STAT1 activation as well as enhanced acute-phase protein gene induction. Therefore, SHP2 plays an important role in acute-phase gene regulation.
Kinetic isotope effects of peptidylglycine α-hydroxylating mono-oxygenase reaction
Authors: TAKAHASHI Kenichi , ONAMI Tetsuo , NOGUCHI Masato
Published: August 2015

Many bioactive polypeptides or neuropeptides possess a C-terminal α-amide group as a critical determinant for their optimal bioactivities. The amide functions are introduced by the sequential actions of peptidylglycine α-hydroxylating mono-oxygenase (PHM; EC 1.14.17.3) and peptidylamidoglycollate lyase (PAL; EC 4.3.2.5) from their glycine-extended precursors. In the present study we examined the kinetic isotope effects of the frog PHM reaction by competitive and non-competitive approaches. In the competitive approach we employed the double-label tracer method with d-Tyr-[U-14C]Val-Gly, d-Tyr-[3,4-3H]Val-[2,2-2H2]-Gly, and d-Tyr-Val-(R,S)[2-3H]Gly as substrates, and we determined the deuterium and tritium effects on Vmax/Km as 1.625±0.041 (mean±S.D.) and 2.71±0.16 (mean±S.D.), respectively. The intrinsic deuterium isotope effect (Dk) on the glycine hydroxylation reaction was estimated to be 6.5–10.0 (mean 8.1) by the method of Northrop [Northrop (1975) Biochemistry 14, 2644–2651]. In the non-competitive approach with N,N-dimethyl-1,4-phenylenediamine as a reductant, however, the deuterium effect on Vmax (DV) was approximately unity, although the deuterium effect on Vmax/Km (DV/K) was comparable to that obtained by the competitive approach. These results indicated that DV was completely masked by the presence of one or more steps much slower than the glycine hydroxylation step and that DV/K was diminished from Dk by a large forward commitment to catalysis. The addition of PAL, however, increased the apparent DV from 1.0 to 1.2, implying that the product release step was greatly accelerated by PAL. These results suggest that the product release is rate-limiting in the overall PHM reaction. The large Dk indicated that the glycine hydroxylation catalysed by PHM might proceed in a stepwise mechanism similar to that proposed for the dopamine β-hydroxylase reaction [Miller and Klinman (1983) Biochemistry 22, 3091–3096].
Fates of endocytosed somatostatin sst2 receptors and associated agonists
Authors: KOENIG Jennifer A. , KAUR Rejbinder , DODGEON Iain , EDWARDSON J. Michael , HUMPHREY Patrick P. A.
Published: August 2015

Somatostatin agonists are rapidly and efficiently internalized with the somatostatin sst2 receptor. The fate of internalized agonists and receptors is of critical importance because the rate of ligand recycling back to the cell surface can limit the amount of radioligand accumulated inside the cells, whereas receptor recycling might be of vital importance in providing the cell surface with dephosphorylated, resensitized receptors. Furthermore the accumulation of radioisotope-conjugated somatostatin agonists inside cancer cells resulting from receptor-mediated internalization has been used as a treatment for cancers that overexpress somatostatin receptors. In the present study, radio-iodinated agonists at the sst2 somatostatin receptor were employed to allow quantitative analysis of the fate of endocytosed agonist. After endocytosis, recycling back to the cell surface was the main pathway for both 125I-labelled somatostatin-14 (SRIF-14) and the more stable agonist 125I-labelled cyclo(N-Me-Ala-Tyr-d-Trp-Lys-Abu-Phe) (BIM-23027; Abu stands for aminobutyric acid), accounting for 75–85% of internalized ligand when re-endocytosis of radioligand was prevented. We have shown that there is a dynamic cycling of both somatostatin agonist ligands and receptors between the cell surface and internal compartments both during agonist treatment and after surface-bound agonist has been removed, unless steps are taken to prevent the re-activation of receptors by recycled agonist. Internalization leads to increased degradation of 125I-labelled SRIF-14 but not 125I-labelled BIM-23027. The concentration of recycled agonist accumulating in the extracellular medium was sufficient to re-activate the receptor, as measured both by the inhibition of forskolin-stimulated adenylate cyclase and the recovery of surface receptor number after internalization.
Regulation of protein-synthesis elongation-factor-2 kinase by cAMP in adipocytes
Authors: DIGGLE Tricia A. , REDPATH Nicholas T. , HEESOM Kate J. , DENTON Richard M.
Published: August 2015

Treatment of primary rat epididymal adipocytes or 3T3-L1 adipocytes with various agents which increase cAMP led to the phosphorylation of eukaryotic translation elongation factor-2 (eEF-2). The increase in eEF-2 phosphorylation was a consequence of the activation of eEF-2 kinase (eEF-2K), which is a Ca2+/calmodulin-dependent kinase. eEF-2K was shown to be essentially inactive at less than 0.1 µM free Ca2+ when measured in cell-free extracts. Treatment of adipocytes with isoproterenol induced Ca2+-independent eEF-2K activity, and an 8–10-fold activation of eEF-2K was observed at Ca2+ concentrations of less than 0.1 µM. Increased cAMP in 3T3-L1 adipocytes led to the inhibition of total protein synthesis and decreased the rate of polypeptide-chain elongation. We also show that the phosphorylation of eEF-2 and the activity of eEF-2K are insulin-regulated in adipocytes. These results demonstrate a novel mechanism for the control of protein synthesis by hormones which act by increasing cytoplasmic cAMP.
Prolonged activation of the mitogen-activated protein kinase pathway promotes DNA synthesis in primary hepatocytes from p21Cip-1/WAF1-null mice, but not in hepatocytes from p16INK4a-null mice
Authors: AUER Kelly L. , PARK Jong-Sung , SETH Prem , COFFEY Robert J. , DARLINGTON Gretchen , ABO Arie , McMAHON Martin , DePINHO Ronald A. , FISHER Paul B. , DENT Paul
Published: August 2015

In primary rat hepatocytes, prolonged activation of the p42/44 mitogen-activated protein kinase (MAPK) pathway is associated with a decrease in DNA synthesis and increased expression of the cyclin-dependent kinase inhibitor (CKI) proteins p21Cip-1/WAF1 and p16INK4a. To evaluate the relative importance of these CKIs in mediating this response, we determined the impact of prolonged MAPK activation on DNA synthesis in primary cultures of hepatocytes derived from mice embryonically deleted (null) for either p21Cip-1/WAF1 or p16INK4a. When MAPK was activated in wild-type mouse hepatocytes for 24 h, via infection with a construct to express an inducible oestrogen receptor–Raf-1 fusion protein (ΔRaf:ER), the expression of p21Cip-1/WAF1 and p16INK4a CKI proteins increased, cyclin-dependent kinase 2 (cdk2) and cdk4 activities decreased, and DNA synthesis decreased. Inhibition of RhoA GTPase function increased the basal expression of p21Cip-1/WAF1 and p27Kip-1 but not p16INK4a, and enhanced the ability of MAPK signalling to decrease DNA synthesis. Ablation of the expression of CCAATT enhancer-binding protein α (C/EBPα), but not of the expression of C/EBPβ, decreased the ability of MAPK signalling to induce p21Cip-1/WAF1. When MAPK was activated in p16INK4a-null hepatocytes for 24 h, the expression of p21Cip-1/WAF1 increased, cdk2 and cdk4 activities decreased and DNA synthesis decreased. In contrast with these findings, prolonged activation of the MAPK pathway in hepatocytes from p21Cip-1/WAF1-null mice enhanced cdk2 and cdk4 activities and caused a large increase in DNA synthesis, despite elevated expression of p16INK4a. Inhibition of RhoA GTPase activity in p21Cip-1/WAF1-null cells partly blunted both the basal levels of DNA synthesis and the ability of prolonged MAPK signalling to increase DNA synthesis. Expression of anti-sense p21Cip-1/WAF1 in either wild-type or p16INK4a-null hepatocytes decreased the ability of prolonged MAPK signalling to increase the expression of p21Cip-1/WAF1, and permitted MAPK signalling to increase both cdk2 and cdk4 activities and DNA synthesis. These results argue that the ability of prolonged MAPK signalling to inhibit DNA synthesis in hepatocytes requires the expression of p21Cip-1/WAF1, and that the increased expression of p16INK4a has a smaller role in the ability of this stimulus to mediate growth arrest. Our results also suggest that RhoA function can modulate DNA synthesis in primary hepatocytes via the expression of p21Cip-1/WAF1 and p27Kip-1.
Differential modulation of transcriptional activity of oestrogen receptors by direct protein–protein interactions with retinoid receptors
Authors: SONG Mi-Ryoung , LEE Soo-Kyung , SEO Young-Woo , CHOI Hueng-Sik , LEE Jae Woon , LEE Mi-Ock
Published: August 2015

Control of oestradiol-responsive gene regulation by oestrogen receptors (ERs) may involve complex cross-talk with retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Recently, we have shown that ERα directly interacts with RARα and RXRα through their ligand binding domains (LBDs). In the present work, we extend these results by showing that ERβ binds similarly to RARα and RXRα but not to the glucocorticoid receptor, as demonstrated by the yeast two-hybrid tests and glutathione S-transferase pull-down assays. These direct interactions were also demonstrated in gel-shift assays, in which the oestrogen response element (ERE) binding by ERα was enhanced by the RXRα LBD but was abolished by the RARα LBD. In addition, we showed that RARα and RXRα bound the ERE as efficiently as ERα, suggesting that competition for DNA binding may affect the transactivation function of the ER. In transient transfection experiments, co-expression of RARα or RXRα, along with ERα or ERβ, revealed differential modulation of the ERE-dependent transactivation, which was distinct from the results when each receptor alone was co-transfected. Importantly, when the LBD of RARα was co-expressed with ERα, transactivation of ERα on the ERE was repressed as efficiently as when wild-type RARα was co-expressed. Furthermore, liganded RARα or unliganded RXRα enhanced the ERα transactivation, suggesting the formation of transcriptionally active heterodimer complexes between the ER and retinoid receptors. Taken together, these results suggest that direct protein–protein interactions may play major roles in the determination of the biological consequences of cross-talk between ERs and RARα or RXRα.
Interaction of the type I methyltransferase M.EcoR124I with modified DNA substrates: sequence discrimination and base flipping
Authors: MERNAGH Darren R. , TAYLOR Ian A. , KNEALE G. Geoff
Published: August 2015

We have analysed the DNA–protein contacts made between the type I DNA methyltransferase M.EcoR124I and its recognition sequence. The effects of base modifications have been probed by measuring the affinity of M.EcoR124I for the modified sequences relative to that for the wild-type sequence by using gel-retardation competition assays. These results, along with those from methylation interference footprinting and photo-affinity cross-linking have identified the location of potential DNA contacts within the DNA recognition site. Substitution of 6-thioguanosine for each of the three specific guanines in the recognition sequence leads to a large (10–20-fold) decrease in the strength of DNA binding, indicating the importance of hydrogen-bonding interactions in the major groove of DNA. In contrast, replacement of either (or both) of the adenines at the target site for methylation by the enzyme, to produce either a base pair mismatch or loss of the base, leads to a marked increase in DNA-binding affinity. The results strongly support the proposal that type I methyltransferases employ a base-flipping mechanism to methylate their target base.
A conserved motif in the yeast nucleolar protein Nop2p contains an essential cysteine residue
Authors: KING Michelle , TON Duy , REDMAN Kent L.
Published: August 2015

Nop2p is an essential nucleolar protein in Saccharomyces cerevisiae that is involved in large ribosomal subunit assembly. It has substantial homology with human p120, the proliferation-associated nucleolar antigen that is overexpressed in many human cancers. A motif containing an invariant Pro–Cys dipeptide is found in Nop2p, p120 and the bacterial Fmu proteins. A total of nine conserved residues, including Pro423 and Cys424, were individually altered in Nop2p by site-directed mutagenesis. Nop2p function was abolished by conversion of Cys424 into either alanine or serine. All of the other Nop2p mutations tested sustained yeast viability, including glycine replacement of Pro423 and the conversion of a second conserved cysteine into alanine. The crucial role of Cys424 in Nop2p is intriguing, due to the critical roles that cysteine residues adjacent to a proline have in a number of nucleotide-modifying enzymes.
Age-related changes in the synthesis of link protein and aggrecan in human articular cartilage: implications for aggregate stability
Authors: BOLTON Mark C. , DUDHIA Jayesh , BAYLISS Michael T.
Published: August 2015

The rates of incorporation of radiolabelled leucine into aggrecan and link protein have been measured in human articular cartilage of different ages. Aggrecan and link protein were purified in the A1 fraction of CsCl gradients as a result of their ability to form high-buoyant-density proteoglycan aggregates with hyaluronic acid. Separation of the aggrecan from the link protein was achieved by Mono Q anion-exchange chromatography. The rates of synthesis of both aggrecan and link protein decreased with age. The age-related decrease in synthesis of aggrecan was paralleled by a decrease in the rate of sulphate incorporation into glycosaminoglycan chains. The synthesis of link protein decreased with age to a greater extent than that of aggrecan such that the ratio of the rates of link protein to aggrecan synthesis decreased from 1 in immature cartilage to 0.2 in mature cartilage. The age-related decrease in link protein synthesis is controlled at least in part by transcriptional or post-trancriptional mechanisms, as shown by the accompanying age-related decrease in link-protein mRNA. The absence of any age-related decrease in aggrecan mRNA suggests that the decrease in synthesis of aggrecan core protein is controlled by a translational mechanism. Measurement of the total tissue content of aggrecan and link protein by radioimmunoassay revealed an age-related increase in the accumulation of these matrix proteins, even though their de novo synthesis was decreasing. This illustrates the importance that the regulation of extracellular post-translational modification also has in controlling the overall turnover of the cartilage matrix.
Platelet-derived-growth-factor stimulation of the p42/p44 mitogen-activated protein kinase pathway in airway smooth muscle: role of pertussis-toxin-sensitive G-proteins, c-Src tyrosine kinases and phosphoinositide 3-kinase
Authors: CONWAY Ann-Marie , RAKHIT Soma , PYNE Susan , PYNE Nigel J.
Published: August 2015

The mechanism used by the platelet-derived growth factor receptor (PDGFR) to activate the mitogen-activated- protein-kinase (p42/p44 MAPK) pathway was investigated in cultured airway smooth muscle (ASM) cells. We have found that pertussis toxin (PTX, which was used to inactivate the heterotrimeric G-protein Gi) induced an approx. 40–50% decrease in the activation of c-Src and p42/p44 MAPK by PDGF. An essential role for c-Src was confirmed using the c-Src inhibitor, PP1, which abolished p42/p44 MAPK activation (PP1 and PTX were without effect on PDGFR tyrosine phosphorylation). Furthermore, the PTX-dependent decrease in c-Src and p42/p44 MAPK activation appeared correlated. These findings suggest that the PDGFR can utilize the PTX-sensitive G-protein, Gi, to regulate c-Src and subsequent p42/p44 MAPK activation. Phosphoinositide 3-kinase (PI3K) has been shown by others to be involved in p42/p44 MAPK activation. This is confirmed here by experiments which showed that PI3K inhibitors (wortmannin and LY294002) reduced the activation of p42/p44 MAPK by PDGF. PI3K activity was increased in Grb-2 immunoprecipitates from PDGF-stimulated cells and was decreased by pretreating these cells with PTX. These findings show that Gi might also promote Grb-2–PI3K complex formation and that Grb-2 may be a site at which PI3K is integrated into the p42/p44 MAPK cascade. In conclusion, our results demonstrate that Gi enables the PDGFR to signal more efficiently to p42/p44 MAPK, and this appears to be achieved through the regulation of c-Src and Grb-2/PI3K, which are intermediates in the p42/p44 MAPK cascade.
Enhancement of chemotactic peptide-induced activation of phosphoinositide 3-kinase by granulocyte–macrophage colony-stimulating factor and its relation to the cytokine-mediated priming of neutrophil superoxide-anion production
Authors: KODAMA Takeshi , HAZEKI Kaoru , HAZEKI Osamu , OKADA Taro , UI Michio
Published: August 2015

Incubation of human neutrophils with a chemotactic peptide [N-formylmethionyl-leucylphenylalanine (fMLP)] gave rise to an increase in the phosphoinositide 3-kinase (PI3K) activity, phosphorylation of p47phoxand superoxide-anion (O2-) generation in the same fMLP-concentration-dependent manner. These responses to fMLP were markedly enhanced when the cells had been incubated for 10 min before the addition of fMLP with increasing concentrations of granulocyte–macrophage colony-stimulating factor (GM-CSF) that were only slightly effective themselves. Wortmannin, an inhibitor of PI3K, suppressed all of these fMLP actions in the same concentration-dependent manner in either GM-CSF-primed or non-primed cells. Sustained activation of protein kinase C by the addition of PMA caused marked phosphorylation of p47phoxand respiratory burst itself without activation of PI3K. This strong action of PMA was not primed by GM-CSF. The chemotactic peptide was without effect in pertussis-toxin-treated cells, indicating that its actions are mediated by βγ-subunits liberated from toxin-susceptible heterotrimeric Gi proteins (Gβγ). Thus one of the mechanisms of GM-CSF-mediated priming of fMLP-induced respiratory burst is synergistic activation of wortmannin-sensitive PI3K by Gβγ in the presence of tyrosine-phosphorylated proteins in GM-CSF-treated cells, as recently indicated in a cell-free system [Kurosu, Maehama, Okada, Yamamoto, Hoshino, Fukui, Ui, Hazeki and Katada (1997) J. Biol. Chem. 272, 24252–24256]. GM-CSF primed fMLP-induced MAP (mitogen-activated protein) kinase activation enormously as well. The MAP kinase activation was primed even in the presence of wortmannin, indicating that PI3K was not the sole site where tyrosine kinase-related and Gβγ-mediated intracellular signals converge to elicit the priming. The GM-CSF priming of fMLP-induced PI3K activation and O2- generation was much smaller in magnitude in neutrophils in which cAMP accumulated upon incubation with prostaglandin E1 than in the cells without the nucleotide accumulation. Thus the GM-CSF priming site, in addition to PI3K, might be just the target of cAMP-dependent protein kinase A in fMLP-initiated signalling cascades or could be localized immediately downstream thereof.
[Difluro(phosphono)methyl]phenylalanine-containing peptide inhibitors of protein tyrosine phosphatases
Authors: DESMARAIS Sylvie , FRIESEN Richard W. , ZAMBONI Robert , RAMACHANDRAN Chidambaram
Published: August 2015

Peptides containing the non-hydrolysable phosphotyrosine analogue 4-[difluro(phosphono)methyl]phenylalanine [Phe(CF2P)] were synthesized and tested as inhibitors of the protein tyrosine phosphatases (PTPs) PTP1B, CD45, PTPβ, LAR and SHP-1. We have identified peptides containing two adjacent Phe(CF2P) residues as potent inhibitors of PTPs. The tripeptide having the sequence Glu-Phe(CF2P)-Phe(CF2P) is a potent and selective inhibitor of PTP1B. This peptide inhibits PTP1B with an IC50 of 40 nM, which is at least 100-fold lower than with other PTPs. A second tripeptide, Pro-Phe(CF2P)-Phe(CF2P), is most potent against PTPβ, with an IC50 of 200 nM, and inhibits PTP1B with an IC50 of 300 nM. These data suggest that it is possible to develop selective, active-site-directed, reversible, potent inhibitors of PTPs.
Mildly oxidized low-density lipoproteins decrease early production of interleukin 2 and nuclear factor κB binding to DNA in activated T-lymphocytes
Authors: CASPAR-BAUGUIL Sylvie , TKACZUK Jean , HAURE Marie-José , DURAND Martine , ALCOUFFE Julie , THOMSEN Mogens , SALVAYRE Robert , BENOIST Hervé
Published: August 2015

Activated T-lymphocytes are found early in atherosclerosis lesions, but little is known about their role. Oxidized low-density lipoproteins (oxLDLs) are considered to be involved in the pathogenesis of the lesions, and we have previously demonstrated that oxLDLs inhibit not only interleukin (IL)-2-receptor expression on the surface of in vitro-activated T-lymphocytes but also their proliferation. We have now investigated the effect of oxLDLs on blast differentiation, on IL-2 synthesis and on the activation of the nuclear factor κB (NF-κB) system in activated lymphocytes. Mildly oxLDLs (50 and 100 µg/ml) decreased the number of lymphoblasts and the level of IL-2 concentration in the culture supernatants after activation of lymphocytes by phytohaemagglutinin and PMA+ionomycin. The inhibition of IL-2 production was observed in the CD3+ T-lymphocyte cytoplasm as early as 4 h after activation by PMA+ionomycin. The study of NF-κB showed that oxLDLs led to a decrease of activation-induced p65/p50 NF-κB heterodimer binding to DNA, whereas the presence of the constitutive nuclear form of p50 dimer was unchanged. This was correlated with an unchanged level of the active form of the cytosolic inhibitor protein IκB-α. Taken together, these observations suggest that the immunosuppressive effect of oxLDLs might operate via a dysregulation of the T-lymphocyte activation mechanisms.
Interaction sites of the C-terminal region of the cGMP phosphodiesterase inhibitory subunit with the GDP-bound transducin α-subunit
Authors: LIU Yu , ARSHAVSKY Vadim Y. , RUOHO Arnold E.
Published: August 2015

In the present report, the region of interaction between the GDP-bound α-subunit of transducin (αt.GTP) and the cGMP phosphodiesterase inhibitory γ-subunit (Pγ) has been studied. It is widely accepted that the αt.GTP is the active form of transducin and that the GDP-bound transducin α-subunit (αt.GDP) is the inactive form. We have reported previously that the binding region of the C-terminal of Pγ on αt.GTP is in a region between the exposed face of the α3 and α4 helices of αt.GTP [Liu, Arshavsky and Ruoho (1996) J. Biol. Chem. 271, 26900–26907]. We now report that N-[(3-[125I]iodo-4-azidophenylpropionamido-S-(2-thiopyridyl)]cysteine ([125I]ACTP)-derivatized Pγ (at Cys-68) reversibly undergoes a unique disulphide exchange of the radioiodinated moiety N-(3-[125I]iodo-4-azidophenylpropionamido)cysteine ([125I]APC) from Cys-68 of Pγ to αt.GDP but not to the guanosine 5´-(γ-thio)-triphosphate (GTP[S])-bound transducin α-subunit (αt-GTP[S]). The specificity of the interaction was demonstrated by the fact that exchange was protected by the functionally active Cys-68 → Ala Pγ mutant, and by pretreatment of the αt.GDP with the βγ-subunit of transducin. Chemical cleavage and amino acid sequencing demonstrated that the [125I]ACTP-derived Pγ specifically transferred the [125I]APC group to Cys-250 and Cys-210 of αt.GDP. These data indicate that the C-terminal region (especially Cys-68–Trp-70) of Pγ interacts with αt.GDP on the exposed interface between α2/β4 and α3/β5 of the α-subunit of transducin. Disulphide exchange was also observed with the α-subunit of holotransducin but this was only approx. 60% of that of pure αt.GDP. The variation in the binding pattern between αt.GDP and αt.GTP with the C-terminal region of Pγ may contribute to the functional difference between the GDP- and GTP-bound states.
In vitro analysis of the zinc-finger motif in human replication protein A
Authors: DONG Jiaowang , PARK Jang-Su , LEE Suk-Hee
Published: August 2015

Human replication protein A (RPA) is composed of 70, 34 and 11 kDa subunits (p70, p34 and p11 respectively) and functions in all three major DNA metabolic processes: replication, repair and recombination. Recent deletion analysis demonstrated that the large subunit of RPA, p70, has multiple functional domains, including a DNA polymerase α-stimulation domain and a single-stranded DNA-binding domain. It also contains a putative metal-binding domain of the 4-cysteine type (Cys-Xaa4-Cys-Xaa13-Cys-Xaa2-Cys) that is highly conserved among eukaryotes. To study the role of this domain in DNA metabolism, we created various p70 mutants that lack the zinc-finger motif (by Cys → Ala substitutions). Mutation at the zinc-finger domain (ZFM) abolished RPA's function in nucleotide excision repair (NER), but had very little impact on DNA replication. The failure of zinc-finger mutant RPA in NER may be explained by the observation that wild-type RPA significantly stimulated DNA polymerase δ activity, whereas only marginal stimulation was observed with zinc-finger mutant RPA. We also observed that ZFM reduced RPA's single-stranded DNA-binding activity by 2–3-fold in the presence of low amounts of RPA. Interestingly, the ZFM abolished phosphorylation of the p34 subunit by DNA-dependent protein kinase, but not that by cyclin-dependent kinase. Taker together, our results strongly suggest a positive role for RPA's zinc finger domain in its function.
Changes in the protein pattern of H1 histones associated with apoptotic DNA fragmentation
Authors: KRATZMEIER Martin , ALBIG Werner , MEERGANS Thomas , DOENECKE Detlef
Published: August 2015

Characteristic steps in the course of cellular apoptosis are the induction of chromatin condensation and cleavage of the DNA, leading to the formation of oligomers of nucleosomes. Since the H1 histones represent functional elements that are essential for the generation of highly condensed chromatin structures, we analysed the total cellular H1 histones of five leukaemic and three solid human tumour cell lines, comparing the H1 pattern of exponentially growing cells with that of apoptotic cells. For the induction of apoptosis, cell lines were treated with the water-soluble camptothecin derivative, topotecan (a topoisomerase I inhibitor), or with an apoptosis-inducing monoclonal anti-CD95 (Fas/APO-1) antibody. Total histone H1 proteins were isolated by extraction with 5% perchloric acid and were analysed by means of capillary zone electrophoresis (CZE) separation. The identities of the peaks representing different histone H1 subtypes on CZE electropherograms were confirmed by analysis of preparations (recombinant proteins purified from transformed yeast used as internal standards) mixed with each of the subtypes respectively. The progress of topotecan- or anti-CD95-induced cell death was monitored by flow cytometry analysis and also by agarose electrophoresis of fragmented DNA. During early apoptosis of three of these cell lines, we observed the induction of internucleosomal DNA cleavage and, simultaneously, a typical change in the histone H1 protein pattern, leading to an increase in the relative amounts of histone subtypes H1.4 and H1.5. Upon apoptosis induction, these changes were only observed in correlation with the occurrence of DNA fragmentation, thus possibly reflecting a prerequisite for DNA accessibility and/or endonuclease activity.
A novel 17β-hydroxysteroid dehydrogenase in the fungus Cochliobolus lunatus: new insights into the evolution of steroid-hormone signalling
Authors: LANIŠNIK RIŽNER Tea , MOELLER Gabriele , THOLE Hubert H. , ŽAKELJ-MAVRIČ Marija , ADAMSKI Jerzy
Published: August 2015

17β-Hydroxysteroid dehydrogenase (17β-HSD) from the filamentous fungus Cochliobolus lunatus (17β-HSDcl) catalyses the reduction of steroids and of several o- and p-quinones. After purification of the enzyme, its partial amino acid sequence was determined. A PCR fragment amplified with primers derived from peptide sequences was generated for screening the Coch. lunatus cDNA library. Three independent full-length cDNA clones were isolated and sequenced, revealing an 810-bp open reading frame encoding a 270-amino-acid protein. After expression in Escherichia coli and purification to homogeneity, the enzyme was found to be active towards androstenedione and menadione, and was able to form dimers of Mr 60000. The amino acid sequence of the novel 17β-HSD demonstrated high homology with fungal carbonyl reductases, such as versicolorin reductase from Emericella nidulans (Aspergillus nidulans; VerA) and Asp. parasiticus (Ver1), polyhydroxynaphthalene reductase from Magnaporthe grisea, the product of the Brn1 gene from Coch. heterostrophus and a reductase from Colletotrichum lagenarium, which are all members of the short-chain dehydrogenase/reductase superfamily. 17β-HSDcl is the first discovered fungal 17β-hydroxysteroid dehydrogenase belonging to this family. The primary structure of this enzyme may therefore help to elucidate the evolutionary history of steroid dehydrogenases.
Importance of splicing for prosaposin sorting
Authors: MADAR-SHAPIRO Liora , PASMANIK-CHOR Metsada , VACCARO Anna Maria , DINUR Tama , DAGAN Arie , GATT Shimon , HOROWITZ Mia
Published: August 2015

The prosaposin gene encodes a 70 kDa protein. This protein might either reach the lysosomes and get processed there to four peptides, which are activators of known lysosomal enzymes, or be secreted by cells as a 70 kDa protein, recently anticipated to have several biological activities. The human prosaposin gene has a 9 bp exon (exon 8) that is alternatively spliced, thus encoding three prosaposin forms: one with an extra three amino acid residues, one with an extra two residues and a third form with no extra residues. With the aim of testing whether there is an association between the alternative splicing and the differential sorting of prosaposins, we cloned two human prosaposin cDNA forms in a T7/EMC/vaccinia virus-derived vector and expressed them in human cells. The results indicated that the prosaposin containing the three extra residues accumulated faster and in greater amounts in the medium, whereas the prosaposin with no extra residues was mainly destined for lysosomes. Point mutations created by mutagenesis in vitro in the 9 bp stretch had a diverse effect on prosaposin secretion. When supplied to cells in the medium, both prosaposins were endocytosed and reached the lysosomes, where they were processed to active saposin B and saposin C. The activities of the saposins were monitored qualitatively and quantitatively. Quantitatively, lipids were extracted from the cells, separated on TLC and measured fluorimetrically. Qualitatively, cells were detected by fluorescence microscopy.
Formation of apolipoprotein-specific high-density lipoprotein particles from lipid-free apolipoproteins A-I and A-II
Authors: CLAY Moira A. , CEHIC Daniel A. , PYLE Diana H. , RYE Kerry-Anne , BARTER Philip J.
Published: August 2015

We have shown previously that apolipoprotein A (apoA)-I-containing high-density lipoprotein (HDL) particles are formed by the conjugation of lipid-free apoA-I with lipids derived from other lipoprotein fractions in a process dependent on non-esterified fatty acids, generated by the lipolysis of very-low-density lipoprotein (VLDL) or provided exogenously. In the present study, we show that this process is also able to generate HDL particles containing apoA-II (A-II HDL) and both apoA-I and apoA-II (A-I/A-II HDL). When lipid-free apoA-II was incubated with either VLDLs and lipoprotein lipase or LDLs and sodium oleate, a significant proportion of the apoA-II was recovered in the HDL density fraction. This was associated with the formation of several populations of HDL-sized particles with pre-β2 electrophoretic mobility, which contained phospholipids and unesterified cholesterol as their main lipid constituents. When both lipid-free apoA-I and lipid-free apoA-II were incubated with LDL and sodium oleate, both apolipoproteins were recovered in HDLs that contained phospholipids and unesterified cholesterol as their main lipids. Two populations of particles had diameters of 7.4 and 10.8 nm and pre-β2-migration; there was also a population of pre-β1-migrating particles of diameter 4.7 nm. ApoA-I and apoA-II were both present in the larger HDLs, whereas only apoA-I was present in the smaller particles. Immunoaffinity chromatography on an anti-(apoA-I)–Sepharose column revealed that 10–20% of the apoA-II resided in particles that also contained apoA-I. The majority of the A-I/A-II HDL were present in a population of pre-β2 particles of 10.8 nm diameter. These results in vitro illustrate a potential mechanism for the formation of HDLs containing both apoA-I and apoA-II.
Occurrence and expression of members of the ferritin gene family in cowpeas
Authors: WARDROP Alison J. , WICKS Ronald E. , ENTSCH Barrie
Published: August 2015

Ferritin gene expression has been demonstrated in a variety of plants including maize, Arabidopsis, cowpeas, soybeans, beans and peas. Most available evidence shows that the mature protein is located in plastids and its production is under gene transcriptional control. In maize, two different ferritin genes have been identified; they were found to express protein under different physiological conditions. Only single gene products have been found until now in the other plants, with the exception of cowpeas (Vigna unguiculata). Our previous work with cowpeas [Wicks and Entsch (1993) Biochem. Biophys. Res. Commun. 192, 813–819] showed the existence of a family of at least three ferritin genes, each coding for a protein subunit with a unique amino acid sequence. Here we report the discovery of a fourth active gene in cowpeas and present the full cDNA sequences for two of the four known members of the cowpea gene family. We also provide preliminary evidence for a family of ferritin genes in soybeans (Glycine max) related to that in cowpeas. We conclude that a family of genes is probably present in all higher plants. We have used quantitative reverse transcriptase-mediated PCR to show that each of the four members of the cowpea ferritin gene family expresses mRNA in leaves and roots under normal growth with a complete nutrient supply. The results clearly show a marked differential pattern of mRNA levels formed during development from the four genes. We conclude that the composition of plant ferritin molecules from plant leaf extracts is probably a complex mixture of subunits, which might be different in roots and in leaves.
L-ATP is recognized by some cellular and viral enzymes: does chance drive enzymic enantioselectivity?
Authors: VERRI Annalisa , MONTECUCCO Alessandra , GOSSELIN Gilles , BOUDOU Valérie , IMBACH Jean-Louis , SPADARI Silvio , FOCHER Federico
Published: August 2015

We demonstrate that l-ATP is recognized by some enzymes that are involved in the synthesis of nucleotides and nucleic acids. l-ATP, as well as its natural d-enantiomer, acts as a phosphate donor in the reaction catalysed by human deoxycytidine kinase, whereas it is not recognized by either enantioselective human thymidine kinase or non-enantioselective herpes virus thymidine kinase. l-ATP strongly inhibits (Ki 80 µM) the synthesis of RNA primers catalysed by DNA primase associated with human DNA polymerase α, whereas RNA synthesis catalysed by Escherichia coli RNA polymerase is completely unaffected. Moreover, l-ATP competitively inhibits ATP-dependent T4 DNA ligase (Ki 25 µM), suggesting that it interacts with the ATP-binding site of the enzyme. Kinetic studies demonstrated that l-ATP cannot be used as a cofactor in the ligase-catalysed joining reaction. On the other hand, l-AMP is used by T4 DNA ligase to catalyse the reverse reaction, even though a high level of intermediate circular nicked DNA molecules accumulates. Our results suggest that a lack of enantioselectivity of enzymes is more common than was believed a few years ago, and, given the absence of selective constraints against l-nucleosides in Nature, this may depend on chance more than on evolutionary strategy.
Incorporation of iron by the unusual dodecameric ferritin from Listeria innocua
Authors: STEFANINI Simonetta , CAVALLO Stefano , MONTAGNINI Benedetta , CHIANCONE Emilia
Published: August 2015

The polypeptide chain that assembles into the unusual dodecameric shell of Listeria innocua apoferritin lacks the ferroxidase centre characteristic of H-type mammalian chains, but is able to catalyse both Fe(II) oxidation and nucleation of the iron core. A cluster of five carboxylate residues, which correspond in part to the site of iron core nucleation typical of L-type mammalian ferritins, has been proposed to be involved in both functions. The features of the iron uptake kinetics and of Fe(II) autoxidation in the presence of citrate followed spectrophotometrically confirm this assignment. In Listeria the kinetics of iron uptake is hyperbolic at low Fe(II)-to-dodecamer ratios and becomes sigmoidal when iron exceeds 150 Fe(II) atoms per dodecamer, namely when a fast crystal growth phase follows a slow initial nucleation step. Iron autoxidation in the presence of citrate displays a similar behaviour. Thus the time course is sigmoidal at low citrate-to-Fe ratios at which Fe(III) polymerization is predominant, but is hyperbolic at ligand concentrations high enough to prevent polymerization. The marked inhibitory effect of Tb(III) on the kinetics of iron incorporation confirms that carboxylates provide the iron ligands in L. innocua apoferritin. Iron uptake followed in steady-state fluorescence experiments allows one to distinguish Fe(II) binding and oxidation from the subsequent movement of Fe(III) into the apoferritin cavity as in mammalian ferritins despite the different localization of the tryptophan residues.
Presence of enolase in the M-band of skeletal muscle and possible indirect interaction with the cytosolic muscle isoform of creatine kinase
Authors: FOUCAULT Georges , VACHER Monique , MERKULOVA Tatyana , KELLER Angelica , ARRIO-DUPONT Martine
Published: August 2015

Glycerol-skinned skeletal muscle fibres retain the defined sarcomeric structure of the myofibrils. We show here that a small fraction of two enzymes important for energy metabolism, the cytosolic muscle isoform of creatine kinase (EC 2.7.3.2), MM-creatine kinase (MM-CK), and enolase (EC 4.2.1.11), remains bound to skinned fibres. CK is slowly exchangeable, whereas enolase is firmly bound. Two-dimensional gel electrophoresis followed by Western blot analyses demonstrates that both α (ubiquitous) and β (muscle-specific) subunits of enolase are present in these preparations. Enolase and CK were co-localized at the M-band of the sarcomeres, as observed by indirect immunofluorescence and confocal microscopy. Cross-linking experiments were performed on skinned fibres with three bifunctional succinimidyl esters of different lengths and yielded a protein complex of 150 kDa that reacted with antibodies directed against either M-CK or β-enolase. The cross-linking efficiency was greatest for the longest reagent and zero for the shortest one. The length of the cross-linker giving a covalent complex between the two enzymes does not support the notion of a direct interaction between M-CK and enolase. This is the first demonstration of the presence of an enzyme of energy metabolism other than CK at the M-band of myofibres.
Predominance of the acylation route in the metabolic processing of exogenous sphingosine in neural and extraneural cells in culture
Authors: RIBONI Laura , BASSI Rosaria , PRINETTI Alessandro , VIANI Paola , TETTAMANTI Guido
Published: August 2015

The metabolic fate of exogenous [3H]sphingosine was investigated in five types of cultured cells: primary cultures of neurons and astrocytes, murine and human neuroblastoma cells and human skin fibroblasts. After administration of 40 nM [3-3H]sphingosine into a cell-conditioned medium containing fetal calf serum, all cell types rapidly and efficiently incorporated the long-chain base in a time-dependent fashion. In all cases, after a 120 min pulse, the amount of radioactivity taken up was in the range of the endogenous sphingosine content. However, unchanged [3H]sphingosine represented only a very minor portion of the label incorporated into cells throughout the pulse period (10–120 min), indicating rapid and efficient sphingosine metabolism in these cells. Most of the [3H]sphingosine taken up was metabolically processed, either by degradation (assessed as 3H2O release into the culture medium) or by N-acylation (mainly to radioactive ceramide, sphingomyelin, neutral glycolipids and gangliosides). [3H]Sphingosine 1-phosphate accounted for less than 2% of the total radioactivity incorporated in all cases. Throughout the pulse period and in all cell types, 3H-labelled organic metabolites largely prevailed over 3H2O, indicating that N-acylation is the major metabolic fate of sphingosine in these cells under apparently physiological conditions. These results are consistent with the notion that sphingosine has a rapid turnover in the cells studied, and indicate that regulation of the basal level of this bioactive molecule occurs mainly through N-acylation.