# PPAR Alpha Regulation of the Immune Response and Autoimmune Encephalomyelitis

Authors: Yuhong Yang; Anne R. Gocke; Amy Lovett-Racke; Paul D. Drew; Michael K. Racke
Journal: PPAR Research
 (2008)

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## Abstract

PPARs are members of the steroid hormone nuclear receptor superfamily and play an important role in the regulation of lipid metabolism, energy balance, artherosclerosis and glucose control. Recent studies suggest that they play an important role in regulating inflammation. This review will focus on PPAR-α regulation of the immune response. We describe how PPAR-α regulates differentiation of T cells by transactivation and/or interaction with other transcription factors. Moreover, PPAR-α agonists have been shown to ameliorate experimental autoimmune encephalomyelitis (EAE) in mice, suggesting that they could provide a therapy for human autoimmune diseases such as multiple sclerosis.


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## Body

## 1. Peroxisome Proliferator-Activated Receptors (PPARs)

Peroxisome  proliferator-activated
receptors  (PPARs) are members of the steroid hormone nuclear receptor
superfamily. So far, there are three isoforms that have been identified and
cloned, including PPAR-α, PPAR-β/δ, and PPAR-γ, and they exhibit
different tissue distribution as well as different ligand
specificities. PPAR-α is mainly expressed in hepatocytes, cardiac
myocytes, and proximal tubular epithelial cells of the kidney. PPAR-γ expression occurs in adipose tissues and
colonic mucosal epithelium. PPAR-δ is highly expressed in the
placenta and large intestine. They can be activated by polyunsaturated fatty
acids [1–4].PPARs are ligand-activated
nuclear receptors and they have been extensively studied in
the regulation ofgenes involved in glucose and lipid metabolism. They have been thought to play an
important role in the regulation of lipid metabolism, energy  balance,
inflammation, artherosclerosis and glucose control. Like other nuclear receptor
family members, all three members of the PPAR family have distinct
functional domains responsible for ligand binding, DNA binding, and
coactivator/corepressor binding. They bind to direct repeat 1 (DR1) elements or
peroxisome proliferators response elements (PPREs) in the promoter region of target genes and
drive the transcription of these target genes [1].Recent studies have shown that
PPARs, including PPAR-α and γ, play a very important role in the regulation of inflammatory responses,
through mechanisms involving transactivation or transrepression of gene
expression through activation of transcription factors, including NF-κB, AP1, and NFAT. In this review, we will focus on the regulation of PPAR-α on immune responses and their agonists as a potential treatment for
autoimmune demyelinating diseases such as multiple sclerosis.Approximately 350 000 people in the
United States
have physician-diagnosed multiple sclerosis (MS) [5]. It is the leading cause
of neurologic  disability in the United
States
in young adults after trauma, thus
most patients suffer from the effects of MS for most of their adult life. The cause of MS remains unknown. An autoimmune process for MS is hypothesized because it shares characteristics of inflammation and demyelination with its animal model, EAE. Epidemiologic studies and studies examining the disease in
identical twins also suggest that both environment and genetics influence
expression of the disease and play a role in disease pathogenesis [6]. There
are now six drugs approved for use in the treatment of MS by the FDA, however
none of these agents are a cure for the disease, so the need for better
treatment strategies for MS remains [7–10]. In addition, the unfortunate
expression of progressive multifocal
leukoencephalopathy (PML) in MS patients treated with nataluzimab
highlights the need for medications with a proven safety record [11–13].Several animal models have been used to study MS. In some of these models,
disease is induced by viruses, such as Theiler’s virus or Borna disease virus
[14]. Of the EAE models, the most commonly studied are those established in the
Lewis rat and in several susceptible mouse strains. Murine EAE results in a relapsing-remitting
disease, similar to the early phase of disease for most MS patients, whereas
EAE in the Lewis rat is a monophasic illness. In chronic murine EAE, the
pathology observed in the white matter shows much more demyelination than the
Lewis rat model, again being more reminiscent of the pathology seen in the CNS
of patients with MS. With the advent of transgenic and homologous recombination
technology, it is increasingly clear that many powerful molecular tools are
becoming available to study the immune response in pathologic processes such as
EAE.


## 2. Regulation of Immune Responses by PPAR-α

### 2.1. PPAR-α Expression in Immune Cells

PPAR-α is predominantly expressed in
tissues exhibiting high catabolic rates of fatty acids (liver, heart,
kidney, and muscle). However, recent studies have shown that it is also
expressed in immune cells.Monocytes and Macrophages
Chinetti et al. [15] showed that
PPAR-α is expressed in undifferentiated monocytes
and in differentiated human monocyte-derived macrophages. PPAR-α is constitutively expressed in the cytoplasm,
whereas PPAR-γ is predominantly localized in the nucleus.
They both were shown to be transcriptionally active after ligand binding to
their receptors. Both PPAR-α and PPAR-γ ligands induce apoptosis of
macrophages following activation with tumor necrosis factor-α/interferon-γ.T and B Lymphocytes
PPAR-α has been reported to be expressed in T
and B lymphocytes [16, 17]. Jones et al. [16] demonstrated that T and B
lymphocytes constitutively express PPAR-α and PPAR-γ. PPAR-α is the predominant isoform expressed in
lymphocytes, whereas PPAR-γ dominates in all cell types of the
myeloid lineage. However, PPAR-α and PPAR-γ are differentially expressed following
lymphocyte activation. PPAR-α expression was downregulated following
T-cell activation while PPAR-γ expression increased under the same
activating conditions. Exposure to specific ligand determined that PPAR-α in lymphocytes effectively
transactivates a peroxisome proliferator response element (PPRE) reporter
construct. Ligand activation of lymphocyte PPAR-α antagonized NF-κB. These observations suggested that a
functional PPAR-α exists within T cells and B
lymphocytes.Langerhans Cells
Epidermal
Langerhans cells (LCs)
play a pivotal role in initiating and maintaining primary immune responses in
the skin. Dubrac
et al. [18] showed that PPAR-α is expressed in immature LC and
downregulated in mature LC. Pharmacologic PPAR-α activation inhibits LC maturation,
migratory capacity, cytokine expression, and the ability to drive T-cell proliferation.
Moreover, PPAR-α activation inhibits NF-κB but not stress-activated protein
kinase/JNK, p38MAPK, and ERK1/2. This study suggested that PPAR-α activation by endogenous ligands may
provide a molecular signal that allows LC to remain in an immature state.


### 2.2. PPAR-α Regulation of Inflammation and Cytokine Production

The study of PPAR-α deficient mice revealed the
relationship between PPAR-α and inflammation. Devchand et al. [19]
demonstrated that lack of PPAR-α activity increases inflammatory
responses. They showed that inflammation due to inflammatory agents, including
arachidonic acid and LTB4, is prolonged in PPAR-α deficient mice as compared to wild-type
mice. The β and γ PPAR subtypes did not compensate for a lack of
PPAR-α in an LTB4-mediated inflammatory
response.Delerive et al. [20] showed another
possible mechanism of PPAR-α regulating inflammation. They
demonstrated that PPAR-α negatively regulates the vascular
inflammatory gene response by negative cross-talk with the transcription
factors NF-κB and AP-1. They showed that aortic explants
isolated from PPAR-α-null mice display an exacerbated
response to inflammatory stimuli, such as lipopolysaccharide (LPS), as
demonstrated by increased IL-6 secretion.Cytokines are one of the major
factors directing T-cell differentiation and play an important role in the
pathogenesis of autoimmune diseases. Recent studies have shown that PPAR-α regulates the expression of cytokines
which are critical in autoimmune disease (see below). Splenocytes harvested
from PPAR-α agonist, WY14,643, fed and pMOG(35–55)
immunized mice showed impaired production of IFN-γ, IL-6, and TNF-α despite similar proliferative responses,
following in vitro restimulation with pMOG(35–55). It was also observed that
IL-4 expression in cultures of mitogen-activated splenocytes was increased [21].Lee et al. [22] reported that the PPAR-α agonist, Fenofibrate, repressed IL-17
and interferon-gamma expression and improved colitis in IL-10-deficient mice.
PPAR-α was found to be expressed in
lymphocytes, macrophages, and crypt and surface epithelial cells of the colon.
Colonic expression of interferon-gamma and IL-17 genes was decreased in IL-10
deficient mice, when the mice were treated with fenofibrate. Fenofibrate also
repressed interferon-gamma and IL-17 expression in isolated T cells, the
expression of the genes encoding the chemokines, CXCL10, CCL2, and CCL20, and
repressed CXCL10 gene promoter activity in tumor necrosis factor-alpha-treated
HT-29 cells.Jones et al. [23] reported that unliganded PPAR-α suppressed T-bet expression and decreased IFN-γ production in T cells. They
demonstrated that activated CD4(+) T cells lacking PPAR-α produce increased levels of IFN-γ, but significantly lower levels of IL-2
when compared with activated wild-type CD4(+) T cells.Another study by Dasgupta et al. [24] suggested that PPAR-α increased the activity of GATA-3 and
inhibited expression of T-bet, which would be in agreement with prior studies
which showed that PPAR-α agonists increased IL-4 production by T
cells. Interestingly, this study also suggested that the PPAR-α agonist gemfibrozil could inhibit
clinical signs of EAE in mice deficient in PPAR-α, with concomitant upregulation of IL-4
and inhibition of IFN-γ [24]. This study did not indicate
whether the same changes in T-bet and GATA-3 expression also occurred in PPAR-α deficient mice.Delerive et al. [20] showed fibrate treatment represses IL-6 mRNA levels in
LPS-stimulated aortas of PPAR-α wild-type, but not of PPAR-α-null mice, demonstrating a role for PPAR-α in this fibrate action. In human aortic
smooth muscle cells, fibrates inhibit IL-1-induced IL-6 gene expression.


### 2.3. Possible Mechanisms

Like other transcription factors, PPARs are able to positively regulate gene
expression by binding to PPRE as a heterodimer with the retinoic
acid X receptor (RXR). In the unliganded state, PPARs are associated
with a nuclearreceptor corepressor. In addition, heat shock
protein-90 and the hepatitis virus B X-associated protein 2 have been shown to
associate with PPAR-α and negatively regulate subsequent gene
activation [25, 26]. Upon activation, the PPARs undergo a
conformational change that results in the dissociation from the
corepressor, enabling the PPAR to bind nuclear receptor coactivators. These
coactivators
then act to reorganize the chromatin templates allowing the basal
transcription machinery to gain access to the promoter regions
driving transcription of target genes.In our lab, we have investigated the
mechanism by which the PPAR-α agonist gemfibrozil induces immune
deviation and protects mice from EAE. Similar to the studies by Dasgupta [24],
we demonstrated that treatment with gemfibrozil increases GATA-3 and decreases
T-bet expression in vitro and directly ex-vivo. These changes correlated
with an increase in nuclear PPAR-α expression.
Moreover, the protective effects of gemfibrozil in EAE were shown to be
partially dependent on IL-4 and to occur in a receptor-dependent manner. PPAR-α was shown to
regulate the IL-4 and IL-5 genes and bound the IL-4 promoter in the presence of
steroid receptor coactivator-1 (SRC-1), suggesting transactivation of the IL-4
gene (Figure 1) [27].Figure 1
A model for PPAR-α-mediated protection in EAE.
In the presence of PPAR-α agonists, PPAR-α heterodimerizes with RXR, dissociates from its nuclear corepressor complex, associates with a coactivator complex, and binds to PPREs in the promoter region of IL-4 and/or IL-5. The transactivation of IL-4/IL-5 leads to increased expression of GATA-3 which in turn results in decreased T-bet expression and downregulation of the Th1/Th17 inflammatory response. This shift in the immune response to a Th2-like phenotype results in amelioration of EAE.PPARs cannot only induce but also
repress gene transcription. One recent study showed a sumoylation-dependent
pathway mediating transrepression of inflammatory response genes by PPAR-γ in macrophages [28]. The initial step
of this pathway involves ligand-dependent sumoylation of the PPAR-γ ligand-binding domain, which targets
PPAR-γ to nuclear receptor corepressor
(NCoR)-histone deacetylase-3 (HDAC3) complexes on inflammatory gene promoters.
This in turn prevents recruitment of the ubiquitylation/19S proteasome machinery that normally mediates the
signal-dependent removal of corepressor complexes required for gene activation.
As a result, NCoR complexes are not cleared from the promoter and target genes
are maintained in a repressed state. This mechanism provides an explanation for
how an agonist-bound nuclear receptor can be converted from an activator of
transcription to a promoter-specific repressor of NF-κB target genes. However, so far there is
no evidence showing that PPAR-α is able to repress target genes by this
sumoylation-dependent pathway.Activated PPAR-α has been demonstrated to exert
anti-inflammatory activities through its ability to antagonize other signaling pathways, in part through the interaction with other
transcription factors, including NF-κB, AP-1, and STATs
(see below).Spencer et al. [29] have demonstrated that therapeutic treatment of aged
mice with PPAR-α activating agents corrected abnormal
nuclear NF-κB activity, redu ced
lipid peroxide levels, and eliminated the dysregulated expression of cytokines
and other genes under NF-κB control.Delerive et al. showed activation of PPAR-α represses both c-Jun- and p65-induced
transcription of the human IL-6 promoter. Glutathione S-transferase (GST) pull-down
experiments demonstrated that PPAR-α physically interacts with c-Jun, p65,
and CBP [20]. They further showed that fibrates, synthetic PPAR-α activators, induced the expression of
the inhibitory protein IκBα in human aortic smooth muscle cells as
well as in primary human hepatocytes. They demonstrated that fibrates induced IκBα in liver in vivo and that this action
required PPAR-α. Furthermore, fibrate treatment induced
IκBα protein expression in the cytoplasm and
also enhanced IL-1β-induced accumulation of IκBα protein in the nucleus [30]. These
actions of fibrates on IκBα expression were accompanied by a
decrease in NF-κB DNA binding activity. They further
demonstrated that induction of IκBα gene transcription by PPAR-α is DNA binding-independent. They
demonstrated that PPAR-α potentiates p65-stimulated IκBα transcription in a ligand-dependent
manner. PPAR-α activation of IκBα transcription requires the NF-κB and Sp1 sites within the IκBα promoter. PPAR-α activation enhances the occupancy of
the NF-κB response element in IκBα promoter in vivo. VDR-interacting
protein 205 (DRIP205) is required to regulate IκBα promoter activity [31].PPAR-α was also found to negatively regulate
the transcription of T-bet. T-bet is a key regulator of the IFNγ gene in
Th1 cells. The induction of T-bet expression in CD4(+) T cells was determined
to be positively influenced by p38 mitogen-activated protein (MAP) kinase
activation, and the presence of unliganded PPAR-α effectively suppressed the
phosphorylation of p38 MAP kinase. The activation of PPAR-α with highly specific ligands relaxed
its capacity to suppress p38 MAP kinase phosphorylation and promoted T-bet
expression [23]. This observation conflicts with the observation of Dasgupta
[24] and our own work.Lee et al. found that four PPAR-α activators suppressed
lipopolysaccharide-stimulated STAT1 phosphorylation and nuclear factor binding
to γ-interferon-activated
sequence/interferon-α-stimulated response element sites known
to contain STAT binding sites. PPAR-α activators also suppressed
lipopolysaccharide-stimulated tumor necrosis factor-α and monocyte chemoattractant protein-1
transcription and release [32].In addition to PPAR-α dependent transcriptional regulation,
Selim et al. [33] showed that fibrates upregulate TRB3 in lymphocytes
independent of PPAR-α by augmenting CCAAT/enhancer-binding
protein β (C/EBP-β) expression. They demonstrated that
fibrates upregulate TRB3 expression (a protein that interferes with
insulin-induced activation of AKT), in mitogen-activated lymphocytes of both
wild type and knockout mice, suggesting that upregulation of this protein
occurs in a PPAR-α-independent manner.Dasgupta et al. [24] showed gemfibrozil
inhibited the encephalitogenicity of MBP-primed T cells and switched the immune
response from a Th1 to a Th2 profile independent of PPAR-α. Gemfibrozil consistently inhibited the
expression and DNA-binding activity of T-bet, and stimulated the expression and
DNA-binding activity of GATA-3, a key regulator of IL-4. Gemfibrozil treatment
decreased the number of T-bet-positive T cells and increased the number of GATA-3-positive
T cells in the spleens of donor mice. Gemfibrozil was shown to have an
inhibitory effect on the invasion of T-bet-positive T cells into the spinal
cord of EAE mice. Furthermore, they demonstrate that the differential effect of
gemfibrozil on the expression of T-bet and GATA-3 was due to its inhibitory
effect on NO production.


## 2.1. PPAR-α Expression in Immune Cells

PPAR-α is predominantly expressed in
tissues exhibiting high catabolic rates of fatty acids (liver, heart,
kidney, and muscle). However, recent studies have shown that it is also
expressed in immune cells.Monocytes and Macrophages
Chinetti et al. [15] showed that
PPAR-α is expressed in undifferentiated monocytes
and in differentiated human monocyte-derived macrophages. PPAR-α is constitutively expressed in the cytoplasm,
whereas PPAR-γ is predominantly localized in the nucleus.
They both were shown to be transcriptionally active after ligand binding to
their receptors. Both PPAR-α and PPAR-γ ligands induce apoptosis of
macrophages following activation with tumor necrosis factor-α/interferon-γ.T and B Lymphocytes
PPAR-α has been reported to be expressed in T
and B lymphocytes [16, 17]. Jones et al. [16] demonstrated that T and B
lymphocytes constitutively express PPAR-α and PPAR-γ. PPAR-α is the predominant isoform expressed in
lymphocytes, whereas PPAR-γ dominates in all cell types of the
myeloid lineage. However, PPAR-α and PPAR-γ are differentially expressed following
lymphocyte activation. PPAR-α expression was downregulated following
T-cell activation while PPAR-γ expression increased under the same
activating conditions. Exposure to specific ligand determined that PPAR-α in lymphocytes effectively
transactivates a peroxisome proliferator response element (PPRE) reporter
construct. Ligand activation of lymphocyte PPAR-α antagonized NF-κB. These observations suggested that a
functional PPAR-α exists within T cells and B
lymphocytes.Langerhans Cells
Epidermal
Langerhans cells (LCs)
play a pivotal role in initiating and maintaining primary immune responses in
the skin. Dubrac
et al. [18] showed that PPAR-α is expressed in immature LC and
downregulated in mature LC. Pharmacologic PPAR-α activation inhibits LC maturation,
migratory capacity, cytokine expression, and the ability to drive T-cell proliferation.
Moreover, PPAR-α activation inhibits NF-κB but not stress-activated protein
kinase/JNK, p38MAPK, and ERK1/2. This study suggested that PPAR-α activation by endogenous ligands may
provide a molecular signal that allows LC to remain in an immature state.


## 2.2. PPAR-α Regulation of Inflammation and Cytokine Production

The study of PPAR-α deficient mice revealed the
relationship between PPAR-α and inflammation. Devchand et al. [19]
demonstrated that lack of PPAR-α activity increases inflammatory
responses. They showed that inflammation due to inflammatory agents, including
arachidonic acid and LTB4, is prolonged in PPAR-α deficient mice as compared to wild-type
mice. The β and γ PPAR subtypes did not compensate for a lack of
PPAR-α in an LTB4-mediated inflammatory
response.Delerive et al. [20] showed another
possible mechanism of PPAR-α regulating inflammation. They
demonstrated that PPAR-α negatively regulates the vascular
inflammatory gene response by negative cross-talk with the transcription
factors NF-κB and AP-1. They showed that aortic explants
isolated from PPAR-α-null mice display an exacerbated
response to inflammatory stimuli, such as lipopolysaccharide (LPS), as
demonstrated by increased IL-6 secretion.Cytokines are one of the major
factors directing T-cell differentiation and play an important role in the
pathogenesis of autoimmune diseases. Recent studies have shown that PPAR-α regulates the expression of cytokines
which are critical in autoimmune disease (see below). Splenocytes harvested
from PPAR-α agonist, WY14,643, fed and pMOG(35–55)
immunized mice showed impaired production of IFN-γ, IL-6, and TNF-α despite similar proliferative responses,
following in vitro restimulation with pMOG(35–55). It was also observed that
IL-4 expression in cultures of mitogen-activated splenocytes was increased [21].Lee et al. [22] reported that the PPAR-α agonist, Fenofibrate, repressed IL-17
and interferon-gamma expression and improved colitis in IL-10-deficient mice.
PPAR-α was found to be expressed in
lymphocytes, macrophages, and crypt and surface epithelial cells of the colon.
Colonic expression of interferon-gamma and IL-17 genes was decreased in IL-10
deficient mice, when the mice were treated with fenofibrate. Fenofibrate also
repressed interferon-gamma and IL-17 expression in isolated T cells, the
expression of the genes encoding the chemokines, CXCL10, CCL2, and CCL20, and
repressed CXCL10 gene promoter activity in tumor necrosis factor-alpha-treated
HT-29 cells.Jones et al. [23] reported that unliganded PPAR-α suppressed T-bet expression and decreased IFN-γ production in T cells. They
demonstrated that activated CD4(+) T cells lacking PPAR-α produce increased levels of IFN-γ, but significantly lower levels of IL-2
when compared with activated wild-type CD4(+) T cells.Another study by Dasgupta et al. [24] suggested that PPAR-α increased the activity of GATA-3 and
inhibited expression of T-bet, which would be in agreement with prior studies
which showed that PPAR-α agonists increased IL-4 production by T
cells. Interestingly, this study also suggested that the PPAR-α agonist gemfibrozil could inhibit
clinical signs of EAE in mice deficient in PPAR-α, with concomitant upregulation of IL-4
and inhibition of IFN-γ [24]. This study did not indicate
whether the same changes in T-bet and GATA-3 expression also occurred in PPAR-α deficient mice.Delerive et al. [20] showed fibrate treatment represses IL-6 mRNA levels in
LPS-stimulated aortas of PPAR-α wild-type, but not of PPAR-α-null mice, demonstrating a role for PPAR-α in this fibrate action. In human aortic
smooth muscle cells, fibrates inhibit IL-1-induced IL-6 gene expression.


## 2.3. Possible Mechanisms

Like other transcription factors, PPARs are able to positively regulate gene
expression by binding to PPRE as a heterodimer with the retinoic
acid X receptor (RXR). In the unliganded state, PPARs are associated
with a nuclearreceptor corepressor. In addition, heat shock
protein-90 and the hepatitis virus B X-associated protein 2 have been shown to
associate with PPAR-α and negatively regulate subsequent gene
activation [25, 26]. Upon activation, the PPARs undergo a
conformational change that results in the dissociation from the
corepressor, enabling the PPAR to bind nuclear receptor coactivators. These
coactivators
then act to reorganize the chromatin templates allowing the basal
transcription machinery to gain access to the promoter regions
driving transcription of target genes.In our lab, we have investigated the
mechanism by which the PPAR-α agonist gemfibrozil induces immune
deviation and protects mice from EAE. Similar to the studies by Dasgupta [24],
we demonstrated that treatment with gemfibrozil increases GATA-3 and decreases
T-bet expression in vitro and directly ex-vivo. These changes correlated
with an increase in nuclear PPAR-α expression.
Moreover, the protective effects of gemfibrozil in EAE were shown to be
partially dependent on IL-4 and to occur in a receptor-dependent manner. PPAR-α was shown to
regulate the IL-4 and IL-5 genes and bound the IL-4 promoter in the presence of
steroid receptor coactivator-1 (SRC-1), suggesting transactivation of the IL-4
gene (Figure 1) [27].Figure 1
A model for PPAR-α-mediated protection in EAE.
In the presence of PPAR-α agonists, PPAR-α heterodimerizes with RXR, dissociates from its nuclear corepressor complex, associates with a coactivator complex, and binds to PPREs in the promoter region of IL-4 and/or IL-5. The transactivation of IL-4/IL-5 leads to increased expression of GATA-3 which in turn results in decreased T-bet expression and downregulation of the Th1/Th17 inflammatory response. This shift in the immune response to a Th2-like phenotype results in amelioration of EAE.PPARs cannot only induce but also
repress gene transcription. One recent study showed a sumoylation-dependent
pathway mediating transrepression of inflammatory response genes by PPAR-γ in macrophages [28]. The initial step
of this pathway involves ligand-dependent sumoylation of the PPAR-γ ligand-binding domain, which targets
PPAR-γ to nuclear receptor corepressor
(NCoR)-histone deacetylase-3 (HDAC3) complexes on inflammatory gene promoters.
This in turn prevents recruitment of the ubiquitylation/19S proteasome machinery that normally mediates the
signal-dependent removal of corepressor complexes required for gene activation.
As a result, NCoR complexes are not cleared from the promoter and target genes
are maintained in a repressed state. This mechanism provides an explanation for
how an agonist-bound nuclear receptor can be converted from an activator of
transcription to a promoter-specific repressor of NF-κB target genes. However, so far there is
no evidence showing that PPAR-α is able to repress target genes by this
sumoylation-dependent pathway.Activated PPAR-α has been demonstrated to exert
anti-inflammatory activities through its ability to antagonize other signaling pathways, in part through the interaction with other
transcription factors, including NF-κB, AP-1, and STATs
(see below).Spencer et al. [29] have demonstrated that therapeutic treatment of aged
mice with PPAR-α activating agents corrected abnormal
nuclear NF-κB activity, redu ced
lipid peroxide levels, and eliminated the dysregulated expression of cytokines
and other genes under NF-κB control.Delerive et al. showed activation of PPAR-α represses both c-Jun- and p65-induced
transcription of the human IL-6 promoter. Glutathione S-transferase (GST) pull-down
experiments demonstrated that PPAR-α physically interacts with c-Jun, p65,
and CBP [20]. They further showed that fibrates, synthetic PPAR-α activators, induced the expression of
the inhibitory protein IκBα in human aortic smooth muscle cells as
well as in primary human hepatocytes. They demonstrated that fibrates induced IκBα in liver in vivo and that this action
required PPAR-α. Furthermore, fibrate treatment induced
IκBα protein expression in the cytoplasm and
also enhanced IL-1β-induced accumulation of IκBα protein in the nucleus [30]. These
actions of fibrates on IκBα expression were accompanied by a
decrease in NF-κB DNA binding activity. They further
demonstrated that induction of IκBα gene transcription by PPAR-α is DNA binding-independent. They
demonstrated that PPAR-α potentiates p65-stimulated IκBα transcription in a ligand-dependent
manner. PPAR-α activation of IκBα transcription requires the NF-κB and Sp1 sites within the IκBα promoter. PPAR-α activation enhances the occupancy of
the NF-κB response element in IκBα promoter in vivo. VDR-interacting
protein 205 (DRIP205) is required to regulate IκBα promoter activity [31].PPAR-α was also found to negatively regulate
the transcription of T-bet. T-bet is a key regulator of the IFNγ gene in
Th1 cells. The induction of T-bet expression in CD4(+) T cells was determined
to be positively influenced by p38 mitogen-activated protein (MAP) kinase
activation, and the presence of unliganded PPAR-α effectively suppressed the
phosphorylation of p38 MAP kinase. The activation of PPAR-α with highly specific ligands relaxed
its capacity to suppress p38 MAP kinase phosphorylation and promoted T-bet
expression [23]. This observation conflicts with the observation of Dasgupta
[24] and our own work.Lee et al. found that four PPAR-α activators suppressed
lipopolysaccharide-stimulated STAT1 phosphorylation and nuclear factor binding
to γ-interferon-activated
sequence/interferon-α-stimulated response element sites known
to contain STAT binding sites. PPAR-α activators also suppressed
lipopolysaccharide-stimulated tumor necrosis factor-α and monocyte chemoattractant protein-1
transcription and release [32].In addition to PPAR-α dependent transcriptional regulation,
Selim et al. [33] showed that fibrates upregulate TRB3 in lymphocytes
independent of PPAR-α by augmenting CCAAT/enhancer-binding
protein β (C/EBP-β) expression. They demonstrated that
fibrates upregulate TRB3 expression (a protein that interferes with
insulin-induced activation of AKT), in mitogen-activated lymphocytes of both
wild type and knockout mice, suggesting that upregulation of this protein
occurs in a PPAR-α-independent manner.Dasgupta et al. [24] showed gemfibrozil
inhibited the encephalitogenicity of MBP-primed T cells and switched the immune
response from a Th1 to a Th2 profile independent of PPAR-α. Gemfibrozil consistently inhibited the
expression and DNA-binding activity of T-bet, and stimulated the expression and
DNA-binding activity of GATA-3, a key regulator of IL-4. Gemfibrozil treatment
decreased the number of T-bet-positive T cells and increased the number of GATA-3-positive
T cells in the spleens of donor mice. Gemfibrozil was shown to have an
inhibitory effect on the invasion of T-bet-positive T cells into the spinal
cord of EAE mice. Furthermore, they demonstrate that the differential effect of
gemfibrozil on the expression of T-bet and GATA-3 was due to its inhibitory
effect on NO production.


## 3. PPAR Alpha and Autoimmune Encephalomyelitis

Organ-specific autoimmune diseases, such as multiple sclerosis (MS) and its
animal model, are mediated by IFN-γ and/or IL-17 producing CD4 T helper cells.
Since PPAR-α regulates inflammation and cytokine
production, PPAR-α agonists have been tested as a
potential treatment for autoimmune diseases.Lovett-Racke et al. [34] demonstrated that PPAR-α agonists can be used as a therapy for
autoimmune disease. They demonstrated that PPAR-α agonists can increase the production of
the Th2 cytokine, IL-4, and suppress proliferation by TCR transgenic T cells
specific for the myelin basic protein Ac1-11, as well as reduce NO production by microglia. Oral
administration of gemfibrozil and fenofibrate inhibited clinical signs of
experimental autoimmune encephalomyelitis. More importantly, gemfibrozil was
shown to shift the cytokine secretion of human T-cell lines by inhibiting IFN-γ and promoting IL-4 secretion. These
results suggest that PPAR-α agonists, such as gemfibrozil and
fenofibrate, may be attractive candidates for use in human inflammatory
conditions such as multiple sclerosis.In another study, Dasgupta et al. [24] demonstrated that gemfibrozil
ameliorates relapsing-remitting EAE independent of PPAR-α. They showed that clinical signs of
EAE, infiltration of mononuclear cells, and demyelination were significantly
lower in mice receiving gemfibrozil, suggesting gemfibrozil may find
therapeutic use in multiple sclerosis.Interestingly, PPAR-α expression in T cells was suggested to
mediate gender differences in development of T-cell-mediated autoimmunity. Dunn
et al. [35] showed that PPAR-α is more abundant in male as compared
with female CD4(+) cells and that its expression is sensitive to androgen
levels. Upon induction of EAE, male PPAR-α (−/−) mice developed more severe
clinical signs than females. These results suggest that males are less prone to
develop Th1-mediated autoimmunity because they have higher T-cell expression of
PPAR-α.Xu et al. [36] investigated the effects of PPAR-α agonists on primary mouse microglia, a
cell type implicated in the pathology of MS and EAE. They demonstrated that
PPAR-α agonists inhibited the secretion of
IL-1β, TNF-α, IL-6, and IL-12 p40 and the chemokine
MCP-1 by LPS-stimulated microglia. Retinoid X receptors (RXRs) physically
interact with PPAR-α receptors, and the resulting
heterodimers regulate the expression of PPAR-responsive genes. They
demonstrated that the PPAR-α agonists ciprofibrate, fenofibrate,
gemfibrozil, and WY14,643 inhibited NO production by stimulated microglia in a
dose-dependent manner. Furthermore, a combination of 9-cis RA and the PPAR-α agonist fenofibrate cooperatively
inhibited NO production by these cells. This study suggested that PPAR-α and RXR agonists might have benefit as
a therapy in MS, where activated microglia are believed to contribute to
disease pathology.Other than microglia cells, they also investigated the effects of PPAR-α agonists on primary mouse astrocytes [37].
They observed similar inhibition on cytokine production by PPAR-α agonists. PPAR-α agonists inhibited the secretion of
TNF-α, IL-1β, and IL-6 by LPS-stimulated astrocytes.
Additionally, fenofibrate inhibited NF-κB DNA binding activity, suggesting a
mechanism by which PPAR-α agonists may regulate the expression of
genes encoding these proinflammatory molecules. Retinoid X receptors (RXRs)
physically interact with PPAR-α receptors, and the resulting
heterodimers regulate the expression of PPAR-responsive genes.They further demonstrated that fenofibrate suppression of EAE was associated
with decreased expression of IL-12 family cytokine mRNAs as well as mRNAs
encoding TLR4, CD14, and MyD88. They showed that the PPAR-α agonist fenofibrate inhibited the
secretion of IL-12p40, IL-12p70 (p35/p40), IL-23 (p19/p40), and IL-27p28 by
lipopolysaccharide-stimulated microglia. Furthermore, fenofibrate inhibited
microglial expression of CD14 which plays a critical role in TLR signaling [38].


## 4. Conclusion

The functional expression of PPAR-α by several immune cell types suggests
that this receptor may play a very important role in regulation of immune
responses. Recent studies demonstrate that PPAR-α regulates different aspects of immune
responses, including inflammation and cytokine production. Moreover, several
studies showed evidence that PPAR-α agonists have potent effects in
regulating immune responses and ameliorating EAE. However, the detailed
mechanisms have not been completely delineated. Better understanding of the
molecular mechanism by which PPAR-α regulates cytokine pathways in immune
cells will be very helpful for further development of PPAR-α agonists as a therapy for autoimmune
diseases.



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*Source: 546753-2008-07-14.xml*