# A Role for PPARγ in the Regulation of Cytokines in  Immune Cells and Cancer

Authors: Xiao Yi Yang; Li Hua Wang; William L. Farrar
Journal: PPAR Research
 (2008)

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## Abstract

Peroxisome proliferator-activated receptorγ (PPARγ) is a ligand-activated transcription factor and a member of the nuclear receptor superfamily. PPARγ and its ligands appear to serve diverse biological functions. In addition to the well-studied effects of PPARγ on metabolism and cellular differentiation, abundant evidence suggests that PPARγ is an important regulator of the immune system and cancers. Since cytokines are not only key modulators of inflammation with pro- and anti-inflammatory functions but they also can either stimulate or inhibit tumor growth and progression, this review summarizes the role for PPARγ in the regulation of cytokine production and cytokine-mediated signal transduction pathways in immune cells and cancer.


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## Body

## 1. Introduction

Peroxisome proliferator-activated receptors (PPARs) are
members of the nuclear receptor superfamily [1–6]. PPARs exist in three
isoforms, PPARα, PPARβ/δ, and PPARγ, which are encoded by different genes
and harbour isotype-specific expression patterns and functions. PPARs were
initially identified as mediators of peroxisome proliferation in rodent liver,
where PPARα plays the major role. However, none of
the PPARs could be attributed to peroxisome proliferation in humans [7–10].
Among the various subtypes of PPARs, PPARγ is the best characterized receptor in
humans. There are at least two PPARγ
isoforms derived from the alternative promoters, PPARγ1 and PPARγ2. PPARγ2
isoform is longer than PPARγ1 by additional 30 N-terminal amino acids [11, 12].
Synthetic ligands including the thiazolidinedione (TZD) class of drugs,
L-tyrosine-based compounds, and diindolymethanes as well as natural ligands
including a broad range of polyunsaturated fatty acids 9- and
13-hydroxyoctadecadienoic acid (9- and 13-HODE) and the eicosanoids 15-deoxy-Δ12,14-prostaglandin J2
(15-d-PGJ2) function as efficacious PPARγ activators [13–15].PPARγ is expressed at high levels in adipose
tissue and is an important regulator of adipocyte differentiation, which
functions as a ligand-dependent, sequence-specific activator of transcription.
Expression of PPARγ in immune system was initially
documented in 1994. Kliewer et al. reported that PPARγ is expressed at high levels in mouse
spleen [8].  Greene et al. detected the
expression of PPARγ2 in normal neutrophils and peripheral
blood lymphocytes by Northern blot analysis in 1995 [9]. Monocytes and macrophages
were the first cells of the immune system in which the physical
presence and anti-inflammatory properties of PPARs were first
described [16, 17]. Subsequently, PPARγ has been reported to exist in other immune cell types of hematopoietic origin, including T lymphocytes [18–22], B lymphocytes [23], NK cells [24], dendritic cells [25–28], eosinophils [29], and mast cells [30–32].Multiple
lines of evidence suggest that PPARs, especially PPARγ, are known to be expressed or overexpressed in
several cancers such as epithelial tumor cells, renal cell carcinoma cells,
myeloid and lymphoid malignancies, and multiple myeloma cells [33–37]. Ligands
of PPARγ have been shown to promote
differentiation and to inhibit cell growth and induce apoptosis in several
types of human cancer, including colon cancer [38–40], breast cancer [41, 42],
lung cancer [43], prostate cancer [44, 45], gastric cancer [46], liposarcoma
[47, 48], and leukaemia [49], supporting a role for PPARγ ligands as potential tumor suppressors
in PPARγ-dependent or -independent manner
[50, 51], although several murine models suggest that, under certain
circumstances, PPARγ ligands may stimulate cancer formation
[36].The cytokines are a large family of
secreted molecules consisting of more than 100 peptides or glycoproteins.  Each cytokine is secreted by particular cell
types in response to a variety of stimuli and produces a characteristic
constellation of effects on the growth, motility, differentiation, or function
of its target cells. Cytokines can act in an autocrine manner to affect the
behavior of the cell that releases the cytokine and/or in a paracrine manner to
affect the behavior of adjacent cells. Moreover, some cytokines are stable
enough to act in an endocrine manner to affect the behavior of distant cells,
although this depends on their abilities to enter the circulation and their
half-life in the blood. Cytokines are especially important for regulating
immune and inflammatory responses with pro- and anti-inflammatory functions,
and have crucial functions in controlling both the innate and adaptive arms of
the immune response. Not only do cytokines govern the development and
homeostasis of lymphocytes, but they also direct the differentiation of helper
T cells and promote the generation of memory cells [52]. During formation and
development of tumor, the mixture of cytokines that is produced in the tumor
microenvironment has an important role in cancer pathogenesis. Cytokines can
either stimulate or inhibit tumor growth and progression [53–57]. Specific
polymorphisms in cytokine genes are associated with an increased risk of cancer
[58]. Cytokines are produced by immune cells as a host response to cellular
stress caused by either exogenous or endogenous agents to control and minimize
cellular damage. However, an uncontrolled and sustained generation of cytokines
can lead to altered cell growth, differentiation, and apoptosis. Therefore, cytokines are a linker among immunity,
inflammation, and cancer [59].In addition to their antiproliferative and
proapoptotic activities on immune cells and cancer cells, effects of PPARs and their ligands in immune
system and cancer cells may be mediated through influencing cytokine
production or cytokine-mediated signal transduction pathways. Conversely, the
expression of PPARs is also modulated by cytokines. In this review, we recapitulate molecular mechanisms on PPARs
regulating cytokine production or cytokine-mediated signal transduction and
cell responses, and enumerate their physiological and pathological consequences
in immune responses, inflammation, and cytokine-responsive tumors.


## 2. Mechanism(s) of Cytokine Gene Regulation by PPARγ

Like other
nuclear receptors, the structure of PPARs is comprised of: an amino-terminal
activation function, AF-1 (A/B domain), which can activate transcription in a
ligand-independent fashion, the DNA-binding domain (DBD), a hinge region, and a
carboxy-terminal ligand-binding domain (LBD) [1–3, 60, 61]. The DBD allows them
to bind to and activate target genes, thus defining them as transcription
factors. The LBD also contains a second activation function (AF-2) that maps to
a surface-exposed hydrophobic pocket, proving a docking site for coregulatory
proteins, and modulates their activities, making them hormone-dependent
transcription factors. Upon ligand binding, PPARs heterodimerize with retinoid
X receptors (RXRs) and form a complex that translocates to the nucleus and
regulates gene expression. This heterodimeric complex binds to peroxisome
proliferator response elements (PPREs) located within the promoter regions of
target genes that consist of a direct repetition of the consensus AGGTCA
half-site spaced by one or two nucleotides (DR1 or DR2). In addition to the
heterodimer complex, it has been reported that a host of accessory proteins,
named “coactivators” or “corepressors,” bind to the nuclear receptors PPAR/RXR
in a ligand-dependent manner and impact the transcriptional process by either
remodeling chromatin structure and/or acting as adapter molecules that link the
nuclear receptor complex to key transcriptional machinery. Ligand binding to
PPARs appears to trigger conformational changes that permit their dissociation
from corepressors and favor their association with coactivators. The
coactivators possess or recruit histone acetyltransferase activity to the
transcription site. Subsequently, acetylation of histone proteins alters
chromatin structure, thereby facilitating the binding of RNA polymerase and the
initiation of transcription. In the absence of ligand, PPARγ has the potential to silence genes to
which it is bound by recruiting transcriptional corepressor complexes and
repress gene expression [1–5, 62, 63, 64].Surprisingly, most of the effects of PPARs
on cytokine expression result from crosstalk with other transcriptional factors
through nongenomic transrepressive mechanisms. It is well known that some key
transcriptional factors such as nuclear factor of activated T cells (NFAT), nuclear
factor-kappa B (NF-κB), GATA-3, T-bet, AP-1, or signal
transducers and activators of transcription (STAT) regulate the expression of
cytokine genes. Transrepression by PPARs can occur either by inhibiting the
binding of transcriptional factors to DNA through direct protein to protein
interactions or by sequestrating cofactors necessary to their activity.  A protein-to-protein interaction between PPARs
and other transcription factors completely prevents these transcription factors
from binding to their own response elements and therefore blocks their
transcriptional activation of cytokine genes [63, 64]. Activation of PPARγ negatively influences the production of inflammatory cytokines such as tumor necrosis factor-alpha (TNFα), Interleukin (IL)-6, and IL-1β by macrophages. A well-established
example is PPARγ coassociation with NFAT, a T-cell
specific transcription factor, in regulation of IL-2 gene expression [18]. The
transcription factor NFAT plays an essential role in gene expression of IL-2 by
T lymphocytes and is also involved in the proliferation of peripheral T
lymphocytes. Therefore, we evaluated transcriptional activity and DNA binding
of NFAT to determine whether NFAT might be a target for negative regulation of
T-cell activation by PPARγ ligands. Utilizing the gel-shift experiment,
we found that PPARγ ligands significantly inhibited the
specific binding of NFAT probe corresponding to the human IL-2 promoter. The
transcriptional activation of the reporter construct directed by the NFAT
distal site of the IL-2 promoter was abrogated by 15-d-PGJ2 or ciglitazone
in the presence of PPARγ over expression. We further tested for
complex formation between PPARγ and NFAT in a coimmunoprecipitation
experiment. The NFAT can be coprecipitated with PPARγ in T cells induced by PMA/PHA and 15-d-PGJ2 or ciglitazone.  Furthermore, the
addition of anti-PPARγ antibody induced high-affinity binding of
extracts to the NFAT probes as determined by using an electronic mobility
shift assay, demonstrating that removal of PPARγ with this antiserum increases the target
specificity of NFAT. This data indicated that a direct physical protein-protein
interaction occurs between nuclear receptor PPARγ and transcription factors NFAT, in turn
inhibiting transcription of IL-2 in T lymphocytes.


## 3. Crosstalk of PPARγ with Cytokine-Mediated Signal Transduction Pathways

Cytokines induce a variety of biological responses by binding
to specific cell surface receptors and activating cytoplasmic signal
transduction pathways, such as the Janus kinase-signal transducer and activator
of transcription (JAK-STAT) pathway, which transmits information received from
extracellular polypeptide signals, through transmembrane receptors, directly to
target gene promoters in the nucleus, providing a mechanism for transcriptional
regulation without second messengers [65–74]. JAKs bind specifically to
intracellular domains of cytokine receptor signaling chains and catalyze
ligand-induced phosphorylation of themselves and of intracellular tyrosine
residues on the receptor, creating STAT docking sites. Phosphorylation of STATs
on activating tyrosine residues leads to STAT homo- and heterodimerization.
STAT dimers are rapidly transported from the cytoplasm to the nucleus and are
competent for DNA binding.  Binding of
the activated STAT dimer to a target promoter initials formation of a primary
transcription complex and dramatically increases the transcription rate from
this promoter of target gene. Transcription of target genes induced by the STAT
dimers reflects an intrinsic ability of STAT transcriptional activation domains
to recruit nuclear coactivators that mediate chromatin modifications and
communication with the core promoters [73].Several lines of evidence indicated that
activated PPARγ crosstalks with
cytokine-mediated signal transduction pathways in modulation of immune
responses and tumor cell growth and apoptosis [75–82]. Interestingly, in the
case of interactions between PPARγ and STAT3
[83–87], two structurally distinct PPARγ ligands
suppress IL-6 activated-STAT3 through the divergent types of crosstalk
including direct or a corepressor SMRT-mediated association (see Figure 1). The
15-d-PGJ2 is a naturally occurring ligand with low affinity of PPARγ, whereas a
class of antidiabetic drugs known as thiazolidinediones is a type of high-affinity
synthetic ligands of PPARγ. Because the
ligand-binding pocket is not static, each PPARγ ligand has the
potential to induce a different conformation of the receptor. Additionally, a non-PPARγ-dependent mechanism
may be involved in the difference between the effects of 15-d-PGJ2 and the
thiazolidinediones on STAT3.  Therefore,
it is reasonable that these two structurally distinct PPARγ agonists
suppress IL-6 activated STAT3 through diverse molecular mechanisms. The multiplicity of crosstalk between nuclear receptors and other
transcriptional factors is an important factor that contributes to both signal
diversification and specification.Figure 1
PPARγ crosstalk with IL-6-activated STAT3 signaling
pathway. Upon IL-6 binding, the IL-6R/gp130 dimer induces phosphorylation
of JAK1,3, which in turn phosphorylates STAT3. The phosphorylated STAT3
dimerizes and translocates to the nucleues, where they bind to the STAT3
binding element (SBE) in the responsive gene to initiate transcription. Two
structurally distinct PPARγ agonists suppress
IL-6-activated STAT3 through diverse molecular mechanisms. 15-d-PGJ2 enhances direct
physical protein-protein interaction between PPARγ and phosphorylated STAT3 and represses IL-6 signaling by
inhibiting the binding of STAT3 to target promoters; Troglitazone inhibits the
interaction between PPARγ and the corepressor SMRT, thereby inducing the
redistribution of SMRT from PPARγ to activated STAT3, in turn transcriptionally inactivating
STAT3 signaling.Direct protein-protein interaction between
transcription factors and ligand-activated nuclear receptors has been shown
involved in the regulation of some transcription factors. In multiple myeloma
cells, we demonstrated that upon 15-d-PGJ2 binding, PPARγ indeed interacted with phosphorylated
STAT3 and represses IL-6 signaling by inhibiting the binding of STAT3 to target
genes [84]. Ligand-induced activation
of PPARγ induces growth arrest by antagonizing the prosurvival signaling
cascade induced by IL-6. PPARγ impedes IL-6 signaling by inhibiting the
transcription of a number of STAT3-regulated genes such as mcl-1 and
c-myc that are important in
cell growth and survival. The exact mechanism through which PPARγ represses
STAT3 has not been fully elucidated. PPARγ has been shown to physically
associate with STAT3, which may inhibit STAT3 from binding DNA or possibly
facilitate the export of STAT3 out of the nucleus. However, certain agonists
that induced growth arrests of these cells did not induce SMRT to dissociate
from PPARγ, suggesting that this nuclear hormone receptor may use numerous
mechanisms to inhibit multiple myeloma cell growth.An alternative mechanism for PPARγ-mediated STAT3 repression has also been
suggested, in which PPARγ agonist treatment of multiple myeloma cells induces
the corepressor protein SMRT to dissociate from PPARγ; SMRT could then complex
with and inhibit the transcriptional activities of STAT3. The
corepressor SMRT has also to be demonstrated to mediate PPARγ downregulation of STAT3 in multiple
myeloma cells. PPARγ can form weak interactions with the
corepressor NCoR/SMRT complex. PPARγ cannot bind to DNA while it is
associated with the corepressor complex. After ligand binding, PPARγ disassociates from the corepressor
complex, and then binds to DNA through a peroxisome proliferator response
element. We first clarified that treatment of MM cells with troglitazone
decreased association of SMRT with PPARγ, which results in redistribution of
corepressor SMRT from PPARγ to activated STAT3. Furthermore, this
interaction between SMRT and IL-6-activated STAT3 can be attenuated by a PPARγ antagonist GW9662, confirming the
specificity of the exchange of corepressor SMRT induced by the liganded PPARγ. Recruitment of SMRT, which is
associated with histone deacetylase, by STAT3 leads to transcriptionally
inactivating STAT3 and consequently downregulating IL-6 mediated MM cell growth
and gene expression. These observations support that coactivators or
corepressors function is not only for regulation of the ligand-dependent DNA
binding and transcriptional activities of nuclear receptors themselves but also
acts as a bridge protein to modulate nuclear receptors crosstalk with other
transcription factors.


## 4. PPAR Regulation of Cytokine in Immune Cells

The immune response can be classified into two fundamental
types: innate and adaptive immunity. The innate immune response functions as the first line of defense against
infection. It consists of soluble factors, such as complement proteins, and
diverse cellular components including granulocytes (basophils, eosinophils, and
neutrophils), mast cells, macrophages, dendritic cells, and natural killer
cells. The adaptive immune response is slower to develop but manifests as
increased antigenic specificity and memory. It consists of antibodies, B cells,
and CD4+ and CD8+ T lymphocytes. Natural killer T cells
and γδT cells are cytotoxic lymphocytes that straddle the interface of innate and
adaptive immunity [57]. In immune responses innate and adaptive immunity
are interlocked and complement each other.Signaling in the immune system can be
either a direct interaction of cells or be mediated by cytokines and antibodies
that are carrying signals to all cells with the appropriate receptors. Although
PPARγ involvement in the regulation of innate
immunere sponses has been studied since the late 1990s [16, 17],
only recently it has the role of PPARγ in adaptive immunity been
investigated [18–32]. Here, we focus on PPARγ regulation of cytokine-mediated immune
responses in immune cells.


### 4.1. PPAR and IL-2

IL-2 is an
autocrine and paracrine growth factor that is secreted by activated T
lymphocytes and is essential for clonal T cell proliferation. Although
originally described as a potent T cell growth factor in vitro, the main
nonredundant role of IL-2 in vivo
is now known to be the maintenance of peripheral T cell tolerance. As well as
promoting the proliferation and survival of recently activated effector T
cells, IL-2 also plays a critical role in regulatory T cell (Treg) homeostasis
and has been variously described as promoting the thymic development,
peripheral homeostasis and suppressive function of Tregs. These observations,
stemming largely from studies on various murine models of IL-2 and IL-2
receptor deficiency, have prompted a greater understanding of the
protolerogenic nature of IL-2 dependent signaling.Greene et al. detected the expression of
PPARγ2 in normal neutrophils and peripheral
blood lymphocytes in 1995
[9]. In
human peripheral blood T cells, we detected inhibition of PHA-induced
proliferation and IL-2 production by 15-d-PGJ2 and TZD troglitazone
in a dose-dependent manner [18]. When PPARγ2 wild type expression vector was
transfected into Jurkat cells, we found that troglitazone and 15-d-PGJ2 inhibited transcription and production of IL-2 in Jurkat cells in a
PPARγ-dependent manner. Cotransfection assays with PPARγ and PPRE-driven/IL-2
promoter luciferase reporter constructs revealed that the inhibitory effects of
troglitazone and 15-d-PGJ2 on IL-2 promoter activity are dependent
on the expression and activation of PPARγ. Finally, we demonstrated that
activated PPARγ inhibited the DNA-binding and activity of transcription factor
NFAT regulating the IL-2 promoter in T cells.Clark et al. described the expression and
function of PPARγ in mouse T-lymphocytes [20]. They
demonstrated that murine SJL-derived Th1 clones and freshly isolated T cell-enriched
splenocytes from SJL mice express PPARγ1 mRNA but not PPARγ2. To test its functional significance,
they used two PPARγ ligands, 15-d-PGJ2 and a
TZD, ciglitazone. Both ligands could inhibit antigen-induced and anti-CD3
antibody-induced T cell proliferative responses of T cell clones, and the
freshly isolated T cell enriched splenocytes. In these studies, it was also
demonstrated that the two PPARγ ligands mediated inhibition of IL-2
secretion by the T cell clones, whereas inhibition of IL-2 induced proliferation
was not detected.


### 4.2. PPAR and IL-4

IL-4 is a pleiotropic and multifunctional cytokine produced
by activated T cells, mast cells, and basophils [88]. IL-4 plays a
critical role in regulating the outcome of an immunere sponse by
facilitating the differentiation of CD4+ T cells into
IL-4-producing T helper (Th) type 2 cells and suppressing the
differentiation of interferon-γ producing Th1 cells, thereby
favoring humoral immune responses [89]. Regulation of IL-4 gene
expression, therefore, is critically important for the
differentiation of Th2 cells and Th2-dependent immune responses
[90]. Dysregulated expression of IL-4-producing cells has been
linked with autoimmune and allergic diseases [91].In T cells, IL-4 gene expression is
regulated at the transcriptional level by both ubiquitous and cell
type-restricted factors, including NF-AT, c-Maf, GATA-3, STAT6,
JunB, and other transcription factors [90]. These factors interact with a
proximal promoter region composed of multiple regulatory elements
that can both positively and negatively affect transcriptional
activation. IL-4 gene transcription is mediated by subset-specific
transcription factors such as GATA-3 and c-Maf during the
differentiation of naive T cells into Th2 cells. A phase of
short-term gene transcription, elicited by the interaction of
differentiated T cells with antigen, requires the antigen-induced transcription
factor NFAT. Treatment of CD4+ T cells with ciglitazone
or 15-d-PGJ2 triggered the physical association between PPARγ and NFATc1, resulting in
IL-4 promoter inhibition and decreased IL-4 production [92].Huang et al. [93]
reported that IL-4 induces expression of PPARγ and 12/15-lipoxygenase in
macrophages, suggesting the potential of coordinated induction of both receptor
and activating ligands. Therefore, it appears likely that PPARγ is a key factor
in regulating at least some aspects of macrophage lipid metabolism and
primarily as a repressor of inflammatory responses. The ways how these two processes
are connected, and the contribution of macrophage specific PPARγ-induced gene
expression and transrepression to inflammatory responses in vivo remains to be explored.We reported an
interesting PPARγ ligand-mediated immunoregulatory
circuit between monocyte/macrophages and T cells [19]. Traditionally, T helper
cells can be divided into two functional subsets consisting of Th1 and Th2
cells on the basis of the immunoregulatory cytokines that these T
cells produce. Some of these immunoregulatory cytokines possess
cross-regulatory properties and can enhance or suppress cytokine
production by Th1 or Th2 subset. Thp cells are the pluripotent
precursors of Th1 and Th2 cells. Moreover, the development of either
Th1 or Th2 helper cells is believed to be determined by the effects
of cytokines directly on helper Thp cells. IL-4 is principally
produced by helper T cells of theTh2 phenotype. IL-4 has been shown
to induce 12/15 lipoxygenase in monocytes/macrophages, which converts
arachidonic acid into several metabolic products, including the potential PPARγ ligand 13-HODE [93].
Based on this finding, we tested the relevance of the regulation of soluble
mediators (PPARγ ligands) released by IL-4 treated
monocytes/macrophages on T cell activation. The medium of macrophages cultured
with or without IL-4 was added to T cells stimulated with anti-CD3 or PHA/PMA.
We found that T cells with the conditioned medium from IL-4-treated macrophages
produced significantly less IL-2. The medium of IL-4-treated macrophages
contained a sufficient amount of 13-HODE and anti-13-HODE antibody could
neutralize the inhibitory effects of the IL-4-conditional medium on T cell IL-2
production. Using human T lymphocytes and the PPARγ-transfected Jurkat T cells, we
demonstrated the specific inhibition by 13-HODE of the transcription factors
NFAT and NF-κB, the IL-2 promoter reporter, and IL-2
production. These observations led us to hypothesize that IL-4, produced by Th2
cells, may indirectly affect the production of IL-2 by Thp or Th1
helper cells by inducing the production of these potential PPARγ ligands by macrophage 12/15-lipoxygenase,
which in turn interferes with the subsequent development of T helper cells (see Figure 2) [19].Figure 2
PPARγ regulation of cytokine-mediated immunoregulatory
circuit between monocytes/macrophages and T lymphocytes. T helper (Th)
lymphocytes can be traditionally divided into two functional subsets consisting
of Th1 and Th2 cells on the basis of the immunoregulatory cytokines
that these T cells produce. Thp cells are the pluripotent precursors
of Th1 and Th2 cells. IL-4 is principally produced by helper T cells of theTh2 phenotype. IL-4
can induce the upregulation of expression of the enzyme 12/15 lipoxygenase in
monocytes/macrophages, providing a potential PPARγ-specific ligands 13-HODE. The mediator
secreted by monocytes can be taken up by neighboring Thp or Th1 cells and
activate PPARγ in these cells. Since NFAT and NF-κB
bind to the promoter region of the IL-2 gene and are needed to activate IL-2
transcription in T cells, the ligand-dependent binding of PPARγ to NFAT and NF-κB correlates the dissociation of NFAT
and NF-κB from IL-2 promoter, thus inhibiting gene expression of IL-2 in Thp
or Th1 cells.Since many complicated pathological
situations cannot be simply explained by the Th1 cell and Th2 cell paradigm,
efforts to resolve these issues in recent years have resulted in the discovery
of many new T helper cell subsets such as Treg cell and Th17 cell subsets.
Therefore, it is interesting to explore further how PPARγ regulates these new Th subsets (see Section 5).


### 4.3. PPAR and IFNγ

IFNγ plays a central role in inflammatory reactions
and is predominately produced by CD4, CD8, and NK cells. IFNγ drives inflammatory reactions by
stimulating the release of NO, TNF-α, and IL-1β bymonocytes/macrophages. IFNγ is also a major effector cytokine, responsible
for driving cell-mediated immunity and mediating organ-specific
autoimmunity. Recent studies have shown that PPARγ ligands inhibit IFNγ production by T lymphocytes; however,
the mechanism underlying this observation has not been clarified
[94]. Based on previous studies, PPARγ ligands could indirectly decrease IFNγ by inhibiting activation of T
cells, production of IL-2, or induction of apoptosis, or inhibiting IL-12 production
by antigen-presenting cells [95–98].Cunard et al. demonstrated that PPARγ is expressed in both murine CD4 and CD8 cells
and that PPARγ ligands directly decrease IFNγ expression by murine and transformed
T cell lines. In contrast, GW9662, a PPARγ antagonist, increases IFNγ expression. Transient transfection studies
reveal that PPARγ ligands, in a PPARγ-dependent manner, potently repress an
IFNγ promoter construct. Repression localizes to
the distal conserved sequence of the minimal IFNγ promoter. They also demonstrate
that PPARγ acts on the minimal IFNγ promoter by interfering with c-Jun
activation. These studies suggest that many of the observed
anti-inflammatory effects of PPARγ ligands may be related to direct inhibition of
IFNγ by PPARγ [94].


## 4.1. PPAR and IL-2

IL-2 is an
autocrine and paracrine growth factor that is secreted by activated T
lymphocytes and is essential for clonal T cell proliferation. Although
originally described as a potent T cell growth factor in vitro, the main
nonredundant role of IL-2 in vivo
is now known to be the maintenance of peripheral T cell tolerance. As well as
promoting the proliferation and survival of recently activated effector T
cells, IL-2 also plays a critical role in regulatory T cell (Treg) homeostasis
and has been variously described as promoting the thymic development,
peripheral homeostasis and suppressive function of Tregs. These observations,
stemming largely from studies on various murine models of IL-2 and IL-2
receptor deficiency, have prompted a greater understanding of the
protolerogenic nature of IL-2 dependent signaling.Greene et al. detected the expression of
PPARγ2 in normal neutrophils and peripheral
blood lymphocytes in 1995
[9]. In
human peripheral blood T cells, we detected inhibition of PHA-induced
proliferation and IL-2 production by 15-d-PGJ2 and TZD troglitazone
in a dose-dependent manner [18]. When PPARγ2 wild type expression vector was
transfected into Jurkat cells, we found that troglitazone and 15-d-PGJ2 inhibited transcription and production of IL-2 in Jurkat cells in a
PPARγ-dependent manner. Cotransfection assays with PPARγ and PPRE-driven/IL-2
promoter luciferase reporter constructs revealed that the inhibitory effects of
troglitazone and 15-d-PGJ2 on IL-2 promoter activity are dependent
on the expression and activation of PPARγ. Finally, we demonstrated that
activated PPARγ inhibited the DNA-binding and activity of transcription factor
NFAT regulating the IL-2 promoter in T cells.Clark et al. described the expression and
function of PPARγ in mouse T-lymphocytes [20]. They
demonstrated that murine SJL-derived Th1 clones and freshly isolated T cell-enriched
splenocytes from SJL mice express PPARγ1 mRNA but not PPARγ2. To test its functional significance,
they used two PPARγ ligands, 15-d-PGJ2 and a
TZD, ciglitazone. Both ligands could inhibit antigen-induced and anti-CD3
antibody-induced T cell proliferative responses of T cell clones, and the
freshly isolated T cell enriched splenocytes. In these studies, it was also
demonstrated that the two PPARγ ligands mediated inhibition of IL-2
secretion by the T cell clones, whereas inhibition of IL-2 induced proliferation
was not detected.


## 4.2. PPAR and IL-4

IL-4 is a pleiotropic and multifunctional cytokine produced
by activated T cells, mast cells, and basophils [88]. IL-4 plays a
critical role in regulating the outcome of an immunere sponse by
facilitating the differentiation of CD4+ T cells into
IL-4-producing T helper (Th) type 2 cells and suppressing the
differentiation of interferon-γ producing Th1 cells, thereby
favoring humoral immune responses [89]. Regulation of IL-4 gene
expression, therefore, is critically important for the
differentiation of Th2 cells and Th2-dependent immune responses
[90]. Dysregulated expression of IL-4-producing cells has been
linked with autoimmune and allergic diseases [91].In T cells, IL-4 gene expression is
regulated at the transcriptional level by both ubiquitous and cell
type-restricted factors, including NF-AT, c-Maf, GATA-3, STAT6,
JunB, and other transcription factors [90]. These factors interact with a
proximal promoter region composed of multiple regulatory elements
that can both positively and negatively affect transcriptional
activation. IL-4 gene transcription is mediated by subset-specific
transcription factors such as GATA-3 and c-Maf during the
differentiation of naive T cells into Th2 cells. A phase of
short-term gene transcription, elicited by the interaction of
differentiated T cells with antigen, requires the antigen-induced transcription
factor NFAT. Treatment of CD4+ T cells with ciglitazone
or 15-d-PGJ2 triggered the physical association between PPARγ and NFATc1, resulting in
IL-4 promoter inhibition and decreased IL-4 production [92].Huang et al. [93]
reported that IL-4 induces expression of PPARγ and 12/15-lipoxygenase in
macrophages, suggesting the potential of coordinated induction of both receptor
and activating ligands. Therefore, it appears likely that PPARγ is a key factor
in regulating at least some aspects of macrophage lipid metabolism and
primarily as a repressor of inflammatory responses. The ways how these two processes
are connected, and the contribution of macrophage specific PPARγ-induced gene
expression and transrepression to inflammatory responses in vivo remains to be explored.We reported an
interesting PPARγ ligand-mediated immunoregulatory
circuit between monocyte/macrophages and T cells [19]. Traditionally, T helper
cells can be divided into two functional subsets consisting of Th1 and Th2
cells on the basis of the immunoregulatory cytokines that these T
cells produce. Some of these immunoregulatory cytokines possess
cross-regulatory properties and can enhance or suppress cytokine
production by Th1 or Th2 subset. Thp cells are the pluripotent
precursors of Th1 and Th2 cells. Moreover, the development of either
Th1 or Th2 helper cells is believed to be determined by the effects
of cytokines directly on helper Thp cells. IL-4 is principally
produced by helper T cells of theTh2 phenotype. IL-4 has been shown
to induce 12/15 lipoxygenase in monocytes/macrophages, which converts
arachidonic acid into several metabolic products, including the potential PPARγ ligand 13-HODE [93].
Based on this finding, we tested the relevance of the regulation of soluble
mediators (PPARγ ligands) released by IL-4 treated
monocytes/macrophages on T cell activation. The medium of macrophages cultured
with or without IL-4 was added to T cells stimulated with anti-CD3 or PHA/PMA.
We found that T cells with the conditioned medium from IL-4-treated macrophages
produced significantly less IL-2. The medium of IL-4-treated macrophages
contained a sufficient amount of 13-HODE and anti-13-HODE antibody could
neutralize the inhibitory effects of the IL-4-conditional medium on T cell IL-2
production. Using human T lymphocytes and the PPARγ-transfected Jurkat T cells, we
demonstrated the specific inhibition by 13-HODE of the transcription factors
NFAT and NF-κB, the IL-2 promoter reporter, and IL-2
production. These observations led us to hypothesize that IL-4, produced by Th2
cells, may indirectly affect the production of IL-2 by Thp or Th1
helper cells by inducing the production of these potential PPARγ ligands by macrophage 12/15-lipoxygenase,
which in turn interferes with the subsequent development of T helper cells (see Figure 2) [19].Figure 2
PPARγ regulation of cytokine-mediated immunoregulatory
circuit between monocytes/macrophages and T lymphocytes. T helper (Th)
lymphocytes can be traditionally divided into two functional subsets consisting
of Th1 and Th2 cells on the basis of the immunoregulatory cytokines
that these T cells produce. Thp cells are the pluripotent precursors
of Th1 and Th2 cells. IL-4 is principally produced by helper T cells of theTh2 phenotype. IL-4
can induce the upregulation of expression of the enzyme 12/15 lipoxygenase in
monocytes/macrophages, providing a potential PPARγ-specific ligands 13-HODE. The mediator
secreted by monocytes can be taken up by neighboring Thp or Th1 cells and
activate PPARγ in these cells. Since NFAT and NF-κB
bind to the promoter region of the IL-2 gene and are needed to activate IL-2
transcription in T cells, the ligand-dependent binding of PPARγ to NFAT and NF-κB correlates the dissociation of NFAT
and NF-κB from IL-2 promoter, thus inhibiting gene expression of IL-2 in Thp
or Th1 cells.Since many complicated pathological
situations cannot be simply explained by the Th1 cell and Th2 cell paradigm,
efforts to resolve these issues in recent years have resulted in the discovery
of many new T helper cell subsets such as Treg cell and Th17 cell subsets.
Therefore, it is interesting to explore further how PPARγ regulates these new Th subsets (see Section 5).


## 4.3. PPAR and IFNγ

IFNγ plays a central role in inflammatory reactions
and is predominately produced by CD4, CD8, and NK cells. IFNγ drives inflammatory reactions by
stimulating the release of NO, TNF-α, and IL-1β bymonocytes/macrophages. IFNγ is also a major effector cytokine, responsible
for driving cell-mediated immunity and mediating organ-specific
autoimmunity. Recent studies have shown that PPARγ ligands inhibit IFNγ production by T lymphocytes; however,
the mechanism underlying this observation has not been clarified
[94]. Based on previous studies, PPARγ ligands could indirectly decrease IFNγ by inhibiting activation of T
cells, production of IL-2, or induction of apoptosis, or inhibiting IL-12 production
by antigen-presenting cells [95–98].Cunard et al. demonstrated that PPARγ is expressed in both murine CD4 and CD8 cells
and that PPARγ ligands directly decrease IFNγ expression by murine and transformed
T cell lines. In contrast, GW9662, a PPARγ antagonist, increases IFNγ expression. Transient transfection studies
reveal that PPARγ ligands, in a PPARγ-dependent manner, potently repress an
IFNγ promoter construct. Repression localizes to
the distal conserved sequence of the minimal IFNγ promoter. They also demonstrate
that PPARγ acts on the minimal IFNγ promoter by interfering with c-Jun
activation. These studies suggest that many of the observed
anti-inflammatory effects of PPARγ ligands may be related to direct inhibition of
IFNγ by PPARγ [94].


## 5. PPAR Regulation of Cytokines in Th17 and Regulatory T Cells

Recently, Th17 cells and CD4+ CD25+ regulatory T (Treg) cells have been described as two distinct T helper cell subsets
from Th1 and Th2 cells. Th17 cells play critical roles in the development of
autoimmunity and allergic reactions by producing IL-17 and, to a lesser extent,
TNF-α and IL-6 [99, 100], while Treg cells expressing the forkhead/winged helix
transcription factor (Foxp3) have an anti-inflammatory role and maintain
tolerance to self components by contact-dependent suppression or releasing
anti-inflammatory cytokines [transforming growth factor (TGF)-β1 and IL-10],
therefore, the balance between Th17 and Treg may be important in the
development/prevention of inflammatory and autoimmune diseases [101, 102].


### 5.1. PPAR, IL-17 and Th17 Cells

Production of IL-17 is a defining feature of a recently
identified class of effector T cells termed Th17 cells [99, 100]. Th17 cells
act as a distinct effector subset and secrete the signature cytokine
IL-17, a proinflammatory cytokine that recruits and activates
neutrophils, enhances T cell priming, and promotes the release of
inflammatory mediators. Th17 cells provide defense against
extracellular bacteria, mediate inflammation, and are critical for many
types of autoinflammatory disorders (i.e., experimental autoimmune
encephalomyelitis, type II collagen-induced arthritis, inflammatory
bowel disease, and psoriasis). The discovery and initial characterization of
these Th17 cells have provided a potential explanation for various
chronic disease pathologies that were unclear with an understanding
of only the Th1 and Th2 cell subsets.IL-10-deficient (IL-10-/-) mice
spontaneously develop inflammatory bowel disease with a Th1-polarized cytokine
pattern. In addition to showing high colonic expression of the Th1-derived
cytokine IFNγ, IL-10-/- mice also show
high expression of IL-17. Lytle et al. observed that rosiglitazone, a
high-affinity ligand for PPARγ, had its greatest effect in suppressing
IL-17 production in IL-10 knockout mice [103].
Interestingly, the PPARα ligand fenofibrate has been shown to
repress IL-17 expression in cultured splenocytes activated by PMA plus
ionomycin and by Th17 cells in a pathogenic CD4+ T cell line
cultured from C3H Bir mice treated with cecal bacterial antigens [104].


### 5.2. PPAR, TGFβ and Treg Cells

At least two subtypes of CD4+ CD25+ regulatory T cells (Tregs) have been described: thymically derived natural Tregs (nTregs) and inducible Tregs (iTregs) generated peripherally from
CD4+ CD25− T effector
cells (Teff) [100, 101]. Induced Treg are more functionally and phenotypically heterogeneous in comparison to natural Treg and can be subdivided into:
induced Foxp3+ Tregs, Th3, and Tr1. Which signals drive Treg cell
proliferation in the tumor setting? TGFβ is the cytokine that is thought to
foster Treg-cell amplification [101]. Both tumor cells directly or “tumor
educated” immune cells can locally produce large amounts of TGFβ [102]. Some mouse and rat tumors
actively induce myeloid immature dentritic cells to secrete TGFβ and this promotes Treg cell
proliferation. There is also substantial evidence that indicates the
involvement of TGFβ in Treg cell conversion. Wohlfert et
al. have used ciglitazone, a synthetic PPARγ ligands, to characterize the
relationship between PPARγ ligands and both iTregs and
nTregs. They reported that ciglitazone-activated PPARγ enhances the TGFβ-dependent conversion of naive T cells
into Foxp3+-induced Tregs in vitro,
although the mechanism by which PPARγ enhances Treg activity remains unknown
[105]. Hontecillas and Bassaganya-Riera have used PPARγ deficient CD4+ cells
obtained from tissue-specific PPARγ null mice to investigate the role of
endogenous PPARγ on Treg and effector CD4+ T
cell function. They demonstrated that only PPARγ-expressing Treg was able to completely prevent inflammation
induced by effector cells of either genotype, suggesting that PPARγ expression and/or activation by
endogenous agonists is required for optimal Treg function [106].


## 5.1. PPAR, IL-17 and Th17 Cells

Production of IL-17 is a defining feature of a recently
identified class of effector T cells termed Th17 cells [99, 100]. Th17 cells
act as a distinct effector subset and secrete the signature cytokine
IL-17, a proinflammatory cytokine that recruits and activates
neutrophils, enhances T cell priming, and promotes the release of
inflammatory mediators. Th17 cells provide defense against
extracellular bacteria, mediate inflammation, and are critical for many
types of autoinflammatory disorders (i.e., experimental autoimmune
encephalomyelitis, type II collagen-induced arthritis, inflammatory
bowel disease, and psoriasis). The discovery and initial characterization of
these Th17 cells have provided a potential explanation for various
chronic disease pathologies that were unclear with an understanding
of only the Th1 and Th2 cell subsets.IL-10-deficient (IL-10-/-) mice
spontaneously develop inflammatory bowel disease with a Th1-polarized cytokine
pattern. In addition to showing high colonic expression of the Th1-derived
cytokine IFNγ, IL-10-/- mice also show
high expression of IL-17. Lytle et al. observed that rosiglitazone, a
high-affinity ligand for PPARγ, had its greatest effect in suppressing
IL-17 production in IL-10 knockout mice [103].
Interestingly, the PPARα ligand fenofibrate has been shown to
repress IL-17 expression in cultured splenocytes activated by PMA plus
ionomycin and by Th17 cells in a pathogenic CD4+ T cell line
cultured from C3H Bir mice treated with cecal bacterial antigens [104].


## 5.2. PPAR, TGFβ and Treg Cells

At least two subtypes of CD4+ CD25+ regulatory T cells (Tregs) have been described: thymically derived natural Tregs (nTregs) and inducible Tregs (iTregs) generated peripherally from
CD4+ CD25− T effector
cells (Teff) [100, 101]. Induced Treg are more functionally and phenotypically heterogeneous in comparison to natural Treg and can be subdivided into:
induced Foxp3+ Tregs, Th3, and Tr1. Which signals drive Treg cell
proliferation in the tumor setting? TGFβ is the cytokine that is thought to
foster Treg-cell amplification [101]. Both tumor cells directly or “tumor
educated” immune cells can locally produce large amounts of TGFβ [102]. Some mouse and rat tumors
actively induce myeloid immature dentritic cells to secrete TGFβ and this promotes Treg cell
proliferation. There is also substantial evidence that indicates the
involvement of TGFβ in Treg cell conversion. Wohlfert et
al. have used ciglitazone, a synthetic PPARγ ligands, to characterize the
relationship between PPARγ ligands and both iTregs and
nTregs. They reported that ciglitazone-activated PPARγ enhances the TGFβ-dependent conversion of naive T cells
into Foxp3+-induced Tregs in vitro,
although the mechanism by which PPARγ enhances Treg activity remains unknown
[105]. Hontecillas and Bassaganya-Riera have used PPARγ deficient CD4+ cells
obtained from tissue-specific PPARγ null mice to investigate the role of
endogenous PPARγ on Treg and effector CD4+ T
cell function. They demonstrated that only PPARγ-expressing Treg was able to completely prevent inflammation
induced by effector cells of either genotype, suggesting that PPARγ expression and/or activation by
endogenous agonists is required for optimal Treg function [106].


## 6. PPAR Regulation of Cytokines in Cancer Cells

Cytokines that are released in response to infection, inflammation, and
immunity can function to inhibit tumor development and progression.
Alternatively, cancer cells can respond to host-derived cytokines that promote
growth, attenuate apotosis, and facilitate invasion and metastasis. Proinflammatory
cytokines implicated in carcinogenesis include IL-1, IL-6, IL-15, colony
stimulating factors, TNF-α, and the macrophage migration
inhibitory factor. A unique immune response signature, consisting predominantly
of humoral cytokines, promotes metastasis in hepatocellular carcinoma.
Likewise, a signature consisting of 11 cytokine genes in the lung environment
predicted lymph node metastasis and prognosis of lung adenocarcinoma with IL-8 and TNF-α as the top 2 genes for predicting
prognosis. IL-8 was originally described as a monocyte-derived
neutrophil chemotactic factor that specifically attracted neutrophils and was
renamed due to its multiple function. IL-8 can have angiogenic
activities in several cancers including nonsmall cell lung cancer and can
function as a positive autocrine growth factor.
Both TNF-α
and IL-6 contributed to the
chemically induced skin tumors and lymphomas in mice. Collectively, cytokines
are considered as a linker between inflammation and cancer [55–57].A considerable amount of research has shown that PPARγ ligands suppress the
proliferation rates of many types of cancer cells, particularly those derived
from liposarcoma, colon cancer, breast cancer, prostate cancer, myeloid
leukemia, glioblastoma, and many others. Various in vitro studies have shown
that treatment of many types of cancer cells with TZD resulted in the induction
of cell differentiation or apoptosis as well as improvement in levels of
various markers for invasion and metastasis. Furthermore, activation of PPARγ by glitazones inhibits angiogenesis and neovascularization both in vitro
and in vivo and blocks the release of vascular endothelial growth factor from
smooth muscle cells [107, 108]. In addition to the above direct
antiproliferative and proapoptotic activities on cancer cells, effects of PPARs and their ligands in cancer
cells may function through influencing cytokine production or
cytokine-mediated signal transduction pathways. The mechanisms are probably linked to: (1) PPAR ligands may
sensitize cancer cells to the antitumor effects of cytokines such as TNFα, (2) PPAR ligands may suppress
production of cytokines for tumor cell growth, and (3) PPAR ligands may affect
tumor microenvironment by regulation of Treg through influencing associated
cytokines. A good example is that PPARγ ligands suppress multiple myeloma
through inhibiting IL-6 and IL-6 activated signal pathway in both PPARγ-dependent and -independent manner.


### 6.1. PPAR and IL-6

Interleukin-6 (IL-6) is a cytokine with
multiple biologic activities on a variety of cells. IL-6 plays a major role in
the response to injury or infection and is involved in the immune response,
inflammation, and hematopoiesis. Its deregulation impacts numerous disease
states, including many types of cancer. Consequently, modulating IL-6 may be an
innovative therapeutic strategy in several diseases. IL-6 is a pleiotropic
cytokine that is involved in the physiology of virtually every organ system. Aberrant
expression of this cytokine has been implicated in diverse human illnesses,
most notably inflammatory and autoimmune disorders, coronary artery and
neurologic disease, gestational problems, and neoplasms. In cancer, high levels
of circulating IL-6 are observed in almost every type of tumor studied and
predict a poor outcome. Furthermore, elevated IL-6 levels are associated
strongly with several of the striking phenotypic features of cancer. Several
molecules have been developed recently that target the biologic function of
IL-6. Early results in the clinic suggest that this strategy may have a
significant salutary impact on diverse tumors. The field of cytokine research
has yielded a deep understanding of the fundamental role of IL-6 and its
receptor in health and disease. Therapeutic targeting of IL-6 and its receptor
in cancer has strong biologic rationale, and there is preliminary evidence
suggesting that targeting of the IL-6 system may be beneficial in the treatment
of cancer [109].One of the most studied tumor types in
relation to IL-6 is multiple myeloma, a malignancy of differentiated
B-lymphocytes. Multiple myeloma is characterized by accumulation of clonal
plasma cells in the bone marrow, accounts for 10% of all hematologic cancers,
and remains an incurable hematological malignancy [110–112]. Recently, we investigated how PPARγ
ligands suppress IL-6 gene expression through crosstalk between PPARγ
and NF-κB
or between PPARγ
and C/EBPβ [86]. C/EBPβ and NF-κB bind to the promoter region of the IL-6
gene, and their cooperation is needed to activate IL-6 transcription. The
nuclear receptor PPARγ can be activated by troglitazone.
Predominately, the complex between C/EBPβ and troglitazone-bound PPARγ leads to decreased DNA binding and
transactivation of C/EBPβ, inhibiting gene expression of IL-6. In addition,
PGC-1, a coactivator, is shared by both PPARγ and NF-κB. After activation by ligands,
ligand-bound PPARγ competes for the limited amounts of
PGC-1. Therefore, NF-κB dissociates with PGC-1 and decreases
NF-κB DNA-binding and transactivation,
leading to blocked IL-6 transcription. In the case of 15-d-PGJ2 inhibition of
IL-6 transcription, although 15-d-PGJ2 also shares the above ligand-bound PPARγ downregulation mechanisms on C/EBPβ and
NF-κB, 15-d-PGJ2, compared with
troglitazone, prefers to use PGC-1 as a bridging protein to associate with NF-κB. In addition, 15-d-PGJ2 inactivates
NF-κB through decreasing phosphorylation of
IKK and IκB in PPARγ-independent manner. The molecular
mechanisms of PPARγ ligands on the regulation of multiple
transcription factors have proven, not surprisingly, complex. Given that IL-6
is the key growth and survival factor of multiple myeloma cells, and is
particularly involved in the origin of all benign and malignant plasma cell
expansions as well as MM cell resistance, the effects and targets of the PPARγ ligands on aspects of multiple myeloma
biology and bone marrow stromal cells may be clinically relevant.


## 6.1. PPAR and IL-6

Interleukin-6 (IL-6) is a cytokine with
multiple biologic activities on a variety of cells. IL-6 plays a major role in
the response to injury or infection and is involved in the immune response,
inflammation, and hematopoiesis. Its deregulation impacts numerous disease
states, including many types of cancer. Consequently, modulating IL-6 may be an
innovative therapeutic strategy in several diseases. IL-6 is a pleiotropic
cytokine that is involved in the physiology of virtually every organ system. Aberrant
expression of this cytokine has been implicated in diverse human illnesses,
most notably inflammatory and autoimmune disorders, coronary artery and
neurologic disease, gestational problems, and neoplasms. In cancer, high levels
of circulating IL-6 are observed in almost every type of tumor studied and
predict a poor outcome. Furthermore, elevated IL-6 levels are associated
strongly with several of the striking phenotypic features of cancer. Several
molecules have been developed recently that target the biologic function of
IL-6. Early results in the clinic suggest that this strategy may have a
significant salutary impact on diverse tumors. The field of cytokine research
has yielded a deep understanding of the fundamental role of IL-6 and its
receptor in health and disease. Therapeutic targeting of IL-6 and its receptor
in cancer has strong biologic rationale, and there is preliminary evidence
suggesting that targeting of the IL-6 system may be beneficial in the treatment
of cancer [109].One of the most studied tumor types in
relation to IL-6 is multiple myeloma, a malignancy of differentiated
B-lymphocytes. Multiple myeloma is characterized by accumulation of clonal
plasma cells in the bone marrow, accounts for 10% of all hematologic cancers,
and remains an incurable hematological malignancy [110–112]. Recently, we investigated how PPARγ
ligands suppress IL-6 gene expression through crosstalk between PPARγ
and NF-κB
or between PPARγ
and C/EBPβ [86]. C/EBPβ and NF-κB bind to the promoter region of the IL-6
gene, and their cooperation is needed to activate IL-6 transcription. The
nuclear receptor PPARγ can be activated by troglitazone.
Predominately, the complex between C/EBPβ and troglitazone-bound PPARγ leads to decreased DNA binding and
transactivation of C/EBPβ, inhibiting gene expression of IL-6. In addition,
PGC-1, a coactivator, is shared by both PPARγ and NF-κB. After activation by ligands,
ligand-bound PPARγ competes for the limited amounts of
PGC-1. Therefore, NF-κB dissociates with PGC-1 and decreases
NF-κB DNA-binding and transactivation,
leading to blocked IL-6 transcription. In the case of 15-d-PGJ2 inhibition of
IL-6 transcription, although 15-d-PGJ2 also shares the above ligand-bound PPARγ downregulation mechanisms on C/EBPβ and
NF-κB, 15-d-PGJ2, compared with
troglitazone, prefers to use PGC-1 as a bridging protein to associate with NF-κB. In addition, 15-d-PGJ2 inactivates
NF-κB through decreasing phosphorylation of
IKK and IκB in PPARγ-independent manner. The molecular
mechanisms of PPARγ ligands on the regulation of multiple
transcription factors have proven, not surprisingly, complex. Given that IL-6
is the key growth and survival factor of multiple myeloma cells, and is
particularly involved in the origin of all benign and malignant plasma cell
expansions as well as MM cell resistance, the effects and targets of the PPARγ ligands on aspects of multiple myeloma
biology and bone marrow stromal cells may be clinically relevant.


## 7. Conclusions

Most proinflammatory
cytokines produced by either host immune cells or tumor cells themselves
promote tumor development. By contrast, proapoptotic and anti-inflammatory
cytokines usually interfere with tumor development [55].  There is emerging evidence that the nuclear
receptor PPARγ interacts with transcriptional factors
to modulate cytokine production and action in immunity, inflammation,
autoimmune diseases, and tumors. PPARγ regulation may occur at the levels of
gene expression of cytokines themselves and their receptors or
cytokine-mediated signaling transduction pathways in immune cells and cancer.
The crosstalk between PPARs and cytokine signaling pathways mediating
inflammatory effects at the cellular level is also effective to induce the
expression of PPAR genes.  The molecular
basis of this interaction has remained elusive, despite the proposal of several
distinct mechanisms. One of the most important mechanistic aspects is
protein-protein interaction through a direct or cofactor-mediated indirect
manner. On the basis of insights into the mechanisms on interaction between these two distinct
families of transcriptional factors activated by different signaling pathways, new
targeting drug design and/or therapeutic strategies will be discovered and
developed for treatment of cytokine-related diseases ranging from inflammation
to cancer.



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*Source: 961753-2008-06-12.xml*