# PPAR Regulation of Inflammatory Signaling in CNS Diseases

Authors: John J. Bright; Saravanan Kanakasabai; Wanida Chearwae; Sharmistha Chakraborty
Journal: PPAR Research
 (2008)

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## Abstract

Central nervous system (CNS) is an immune privileged site, nevertheless inflammation associates with many CNS diseases. Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear hormone receptors that regulate immune and inflammatory responses. Specific ligands for PPARα, γ, and δ isoforms have proven effective in the animal models of multiple sclerosis (MS), Alzheimer's disease, Parkinson's disease, and trauma/stroke, suggesting their use in the treatment of neuroinflammatory diseases. The activation of NF-κB and Jak-Stat signaling pathways and secretion of inflammatory cytokines are critical in the pathogenesis of CNS diseases. Interestingly, PPAR agonists mitigate CNS disease by modulating inflammatory signaling network in immune cells. In this manuscript, we review the current knowledge on how PPARs regulate neuroinflammatory signaling networks in CNS diseases.


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## Body

## 1. Introduction

The
central nervous  system  (CNS) was thought to be an immune privileged site due to
the ability of blood-brain-barrier (BBB) to shield immune cell entry and protect
from the constantly changing circulatory milieu. Nevertheless, activated immune
cells readily traverse the BBB, secrete inflammatory cytokines, and mediate many
CNS diseases. Neuroinflammatory diseases present major challenges to the health
care system and impose substantial economic costs around the world. Current
treatments targeting clinical symptoms of CNS diseases have modest therapeutic
values in patients. Significant progress has been made in recent years in developing
therapeutic strategies for the treatment of neuroinflammatory diseases.


## 2. Neuroinflammatory Diseases

The innate
and adaptive immunity evoked during infection in the CNS often leads to the
development of neuroinflammatory diseases [1–3]. The mounting evidence suggests
that neuroinflammatory diseases such as multiple sclerosis (MS), Alzheimer’s
disease (AD), trauma, and ischemia/stroke can occur in the absence of infection.
MS is an inflammatory demyelinating disease of the CNS with clinical symptoms ranging
from pain to paralysis and the patients becoming wheel-chair bound for rest of
their lives [4]. Although the etiology of MS is not known, it is generally
viewed as a neural antigen-specific T cell-mediated autoimmune disease [4–6].
Experimental allergic encephalomyelitis (EAE) is an autoimmune disease model of
MS, commonly used to study the mechanism of disease pathogenesis and to test
the efficacy of potential therapeutic agents for the treatment of MS. In AD,
the deposition of beta-amyloid (Aβ) and plaque formation in the CNS associate
with inflammation resulting in neuronal death, progressive deterioration of
cognitive functions, and memory loss [7, 8]. Traumatic brain injury (TBI), spinal
cord injury, and ischemic stroke also display neuroinflammation associated
secondary tissue damage in the CNS [9, 10]. The pathogenesis of neuroinflammatory
diseases involves the orchestrated interaction of immune cells resulting in tissue
injury to the CNS [6, 11]. Although the exact mechanisms are not known, recent evidence
suggests the use of peroxisome proliferator-activated receptor (PPAR) agonists
in the treatment of neuroinflammatory diseases.


## 3. PPAR Isoforms and Their Ligands

PPAR is
a family of ligand-dependent nuclear hormone receptor transcription factors
that play key roles in the regulation of immune and inflammatory responses [12].
Structure-function analyses revealed that PPARs are composed of a DNA-binding
domain (DBD) linked to the C-terminal ligand-binding domain (LBD) by a hinge
region (Figure 1) [13, 14]. PPARs stimulate gene expression
through binding to peroxisome-proliferator response elements (PPREs), present
in the promoter regions of the target genes. In the absence of ligands, the heterodimers
physically associate with corepressors and suppress gene transcription [14, 15].
Upon ligand binding, the coactivators replace corepressors and activate gene
expression [16, 17]. PPARα, PPARβ/δ, and PPARγ are three structurally
homologous isotypes found in various species which display distinct
physiological and pharmacological functions [18]. The PPARα is
expressed in liver, kidney, intestine, heart, skeletal muscle, adrenal gland, pancreas,
and
brain. PPARα is involved in acetylcholine metabolism, excitatory
neurotransmission, and oxidative stress defense [19]. PPARα also regulates
lipid metabolism and energy homeostasis through its ability to stimulate the
breakdown of fatty acids and cholesterol, driving gluconeogenesis and reduction
in serum triglyceride levels
[19]. While polyunsaturated
fatty acids activate all three isoforms of PPARs with different affinities, each isotype has its own ligand binding property [20].
Fibrates, WY14643, and GW7647 are PPARα agonists commonly used for the treatment
of hypertriglyceridemia [19].(a) Functional domains of PPAR isoforms. N, N-terminus; DBD: DNA-binding domain; LBD: ligand-binding domain. The numbers represent percentage identity to human PPARα. (b) PPAR/RXR binds to PPREDR-1 promoter regions. Binding of agonists leads to heterodimerization, recruitment of coactivator and transcriptional activation of target genes.
(a)(b)PPARβ/δ is ubiquitously expressed in
all cell types including immature oligodendrocytes and promotes differentiation
and myelination in the CNS
[21–23]. PPARβ/δ null mice show
an altered myelination of corpus  callosum,
suggesting its
role in brain function [24]. PPARβ/δ regulates transcriptional activation of
Acyl-CoA synthetase 2, a key enzyme in fatty acid utilization, suggesting its
role in lipid metabolism in the brain. Prostagladin I2, GW0742,
GW501516, and GW7842 are PPARβ/δ agonists which induce fatty acid oxidation in
muscle [25]. PPARγ expression is  detected in adipose tissue intestinal mucosa,
retina, skeletal muscle, heart, liver, and lymphoid organs [26]. PPARγ is
expressed in microglia and astrocytes and regulates inflammation in the CNS [27, 28]. Eicosanoids and
prostaglandin J2 (15d-PGJ2) are the naturally occurring PPARγ ligands, and
thiazolidinedones (TZDs) including pioglitazone (Actos) and rosiglitazone
(Avandia) are Food and Drug Administration (FDA) approved synthetic drugs for
the treatment of type II diabetes [29]. Recent
studies have shown the use of PPAR agonists in the treatment of many neuroinflammatory
diseases.


## 4. Therapeutic Effects of PPAR Agonists in CNS Diseases

The
therapeutic effects of PPAR agonists have been tested in many different
neuroinflammatory diseases (Table1). The use of PPARγ agonists in the treatment of MS
has been tested in EAE model by different groups [30–33]. In vivo treatment
with synthetic PPARγ ligand, troglitazone, ameliorates EAE by reducing the infiltration
of leukocytes in the CNS [34]. Two other studies also showed that in vivo
treatment with PPARγ ligands, 15d-PGJ2 and ciglitazone, ameliorates
EAE [30, 31]. Oral treatment with pioglitazone inhibits chronic progressive and
relapsing forms of EAE even when administered at the peak of disease [35, 36],
suggesting their use of PPARγ agonists in the treatment of
MS. PPARγ-deficient heterozygous mice develop an exacerbated EAE with increased CNS inflammation and demyelination [37]. A recent report also
showed that PPARγ antagonists, bisphenol A diglycidyl ether (BADGE),
and 2-chloro-5 nitro-N-(4 pyridyl) benzamide (T007) reversed the inhibition of EAE
by PPARγ agonists, further suggesting
the physiological role of PPARγ in the pathogenesis of EAE [38].Table 1
Role of PPARs in the regulation of neuroinflammatory diseases.
CNS diseaseInflammatory responseEffect of PPAR agonistsMultiple sclerosisActivation of macrophage, microglia and
dendritic cells; infiltration of Th1/Th17 cells in the CNS; induction of NF-κB and Jak-Stat
pathway and release of IL-12, IFNγ, IL-17 and
other cytokines in the CNSPPARα, δ and γ agonists ameliorate  EAE by inhibiting inflammationAlzheimer’s diseaseBeta-amyloid (Aβ) accumulation leads to CNS
inflammation via TNFα and NF-κB pathway and secretion of inflammatory
cytokinesPPARγ ligands reduce neuronal loss in animal
models of ADInfectionDuring bacterial, viral, fungal and parasitic infection, activated
APC and T cells release TNFα, IFNγ, iNOS, IL-2, IL-6 and induce inflammation
via NF-κB, Stat and AP-1 signaling pathwaysPPAR agonists regulate infection associated inflammationTraumaCNS injury results in the activation of resident microglia and
astrocytes resulting inflammation through secretion of TNFα, prostaglandin
and COX-2 and mediate inflammation via NF-κB, Stat1 and AP-1 pathwaysPPARα, δ and γ ligands regulate inflammatory response in traumaIschemia/strokeIschemic stroke associates with recruitment and activation of
macrophages and neutrophils via increased expression of VCAM-1, ICAM-1, IL-6,
IL-8 and COX-2 through Stat-1PPARγ ligands reduce the infarct size in animal modelsEpidemiological
studies suggest a reduced risk of AD among the users of nonsteroidal anti-inflammatory
drugs (NSAID) [39, 40].
Treatment with pioglitazone and
rosiglitazone significantly reduced the lesion size, motor neuron
loss, myelin loss, astrogliosis, microglial activation, and chronic thermal
hyperalgesia in spinal cord injury [41]. In a rat model of AD induced by
cortical Aβ injection, ciglitazone and pioglitazone suppressed the clinical
symptoms significantly. In the amyloid precursor protein (APP) transgenic model
of AD, treatment with pioglitazone reduced the plaque burden by affecting the
production, clearance, and homeostasis of Aβ in the CNS [42].
A clinical trial involving 500
AD patients showed significant
improvement in cognition following treatment with rosiglitazone for 6 months,
suggesting its use in the treatment of AD [43]. Recent evidence also suggests
that NSAIDs such as ibuprofen may delay or prevent the development of Parkinson’s
disease (PD) [44, 45]. Moreover, PPARγ is expressed in the CNS of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine
(MPTP)-induced model of PD [24] and treatment with pioglitazone protected the
animals from neuronal cell death [46]. Similar results were also generated
using lipopolysaccharide (LPS)-induced inflammation model of dopaminergic
neurodegeneration in rat, where pioglitazone treatment effectively reduced
inflammation, oxidative stress, and restored mitochondrial function [47]. Treatment with
pioglitazone also extends the survival of superoxide
dismutase-1 (SOD1-G93A) transgenic animal model of amylotrophic lateral
sclerosis (ALS) [36, 48–51].The
effects of PPAR agonists in reducing deleterious inflammatory responses suggest
their use in the treatment of trauma, spinal cord injury, and stroke. Experimental
evidence suggests that the Pro12Ala polymorphism of PPARγ2 is associated with a
reduced risk for ischemic stroke
[52] and treatment with TZDs
and 15d-PGJ2 cause neuroprotection in animal models of stroke. Treatment with PPARγ
agonists also reduce the infarct volumes and improve sensorimotor function in a
rodent model of middle cerebral artery occlusion (MCAO)
[53, 54]. Similar effects were observed following oral or intracerebrovascular
administration of
PPARγ agonists [55, 56].
TZD-unrelated PPARγ agonist L-796,449 decreases
infarct size and improves neurological scores after MCAO in the rat brain [57].
Treatment with PPARγ antagonist T0070907 increased
the infarct size and reversed rosiglitazone-induced protection after stroke. A
small clinical trial has revealed improved functional recovery after stroke in
diabetic patients receiving pioglitazone or rosiglitazone compared to patients
not receiving TZD therapy
[58]. A recent clinical trial demonstrated
that pioglitazone significantly reduced the combined risk of myocardial
infarction and stroke-associated death in high-risk patients with type 2
diabetes
[59].Recent
studies have demonstrated the beneficial effects of PPARα agonists in the treatment of neuroinflammatory
diseases. Oral treatment with gemfibrozil protects mice from EAE [60]. The
tyrosine hydroxylase (TH)-positive SNpc cells express PPARα and in vivo
treatment with PPARα agonist, fenofibrate, protects mice from MPTP-induced
inflammation and neuronal loss. In vivo treatment with PPARα agonist,
fenofibrate and WY-14643, reduced the infarct size
in mouse
models of stroke [61, 62]. This effect was absent in PPARα deficient mice,
reinforcing receptor dependency of the observed effects
. Treatment
with PPARα agonist, fenofibrate,
decreases the neurological deficit induced by traumatic brain injury (TBI)
caused by lateral fluid percussion of brain in rats [63]. Fenofibrate also
reduces brain edema and ICAM-1 expression and induces neurological recovery
associated with a reduction of the brain lesion. Anti-inflammatory therapies
showed neuroprotective effects after spinal cord injury in rodents [64, 65]. Moreover,
oral treatment with selective PPARβ/δ agonist GW0742 exerted beneficial effects
in the MOGp35-55-induced EAE model [66]. GW0742 reduced the severity of EAE
even when administered at the peak of clinical disease [66]. PPAR β/δ null mice
exhibit significantly greater infarct sizes than wild type animals suggesting its role in stroke [67].


## 5. PPAR Agonists Regulate Inflammatory Cytokines in CNS Diseases

The
anti-inflammatory effects of PPARγ agonists have been extensively
studied in CNS diseases (Table 2). While the inflammatory cytokines, IL-1β, IL-6, TNFα, IL-12, IL-23, IL-27, IFNγ, and IL-17, mediate the
pathogenesis of CNS diseases, anti-inflammatory cytokines, IL-4, IFNβ, TGFβ, and IL-10, confer recovery in
MS and its animal model, EAE [68–70]. In EAE model of MS, PPARγ agonists decrease the TNFα
mRNA expression in antigen-specific T cell in vitro [71]. Other studies have shown that 15d-PGJ2 inhibits
EAE in association with inhibition of T-cell proliferation and secretion of
inflammatory cytokines including IFNγ, IL-10, and IL-4 in culture [31–34]. PPARγ agonists, 15d-PGJ2 and ciglitazone,
block IL-12 signaling through Jak-Stat pathway leading to Th1 differentiation
in T cells. Pioglitazone also suppresses IFNγ secretion in spleen T cells following
stimulation with MOGp35-55 in vitro
[30]. The activation of resident glial cells and infiltration of leukocytes
contribute to demyelination in EAE and MS. The chemokines and chemokine
receptors promote the trafficking and entry of immune cells across blood-brain
barrier into the CNS in EAE and MS [71–74]. Whereas, PPARγ agonists,
troglitazone and pioglitazone, reduce the expression of MCP1 [33], IP-10
(CXCL3), MIG, I-TAC, MIP1α, and RANTES [27] that contribute to reduced
infiltration of immune cells in the MOG-induced EAE [73, 74]. Moreover, the
surface molecules such as MHC class II, CD40, CD28, and ICAM enhance the
disease pathogenesis and CTLA4 inhibits EAE/MS [75]. Negative regulation of
adhesion molecules may also account for reduced brain infiltration observed in
PPARγ agonists treated EAE mice.Table 2
Role of PPARs in the regulation of inflammatory signaling pathways in CNS
diseases.
Tissue
DistributionPPAR AgonistsEffect and Mode
of Action in CNS diseasesPPARα  Expressed in liver, heart, kidney, large
intestine, skeletal muscle and astrocytes. PPARα knockout  mice develop severe LPS-induced
inflammationPalmitic
acid, linoleic acid, stearic acid, palmitoleic acid, oleic acid, 8-HETE,
Wy-14643, clofibrate, nafenopin, bezafibrate, fenofibratePPARα agonists
inhibit Aβ induced expression
of TNFα, IL-6, IL-4
and infiltration of CD4+ T cells in the CNS of AD; reduce ICAM-1
expression and oxidative damage in stroke; protect MPTP-induced loss of
neurons in PD; protect mice from EAE by inhibiting IFNγ, TNFα and IL-6
production in stroke, cerebral ischemia and MS modelsPPARβ/δ
Expressed ubiquitously in brain, adipose
tissue and skin. PPARβ/δ knockout mice show reduced fat depositionProstacyclin,
PGI2, GW0742, GW501516, GW7842, L165041PPARβ/δ agonists reduce the severity of EAE and stroke by inhibiting NF-κB and Jak-Stat
signaling pathways in immune cells from MS and stroke modelsPPARγ  Expressed in heart, muscle, colon, kidney, pancreas, spleen, macrophage,
intestine, adipose tissue  and liver.
PPARγ knockouts are
embryonically lethalProstaglandin
J2, thiazolidinediones,  pioglitazone,
rosiglitazone, GW78456, WY14,643, GW7647PPARγ agonists inhibit
T-cell proliferation, IFNγ, IL-10 and IL-4 production through blocking NF-κB,
AP-1 and Jak-Stat pathways in CNS diseases models of AD, PD, Trauma, MS, ALS,
stroke and ischemiaThe
immunomodulatory effects of PPARγ agonists were tested in human peripheral blood mononuclear cells (PBMCs). In vitro treatment of
cells with pioglitazone, ciglitazone, and GW347845 abolished proliferation and
cytokine secretion accompanied by DNA condensation and down-regulation of
bcl-2, suggesting the induction of apoptosis in activated T lymphocytes. MS
patients showed a decrease in the expression of PPARγ in immune cells and a
reduction in the anti-inflammatory effects of pioglitazone when compared to
healthy controls [35, 36, 76]. However, treatment of immune cells derived from
diabetic patients with pioglitazone in
vitro or by oral treatment in vivo increased the expression and DNA-binding
activity of PPARγ [77]. In MS patients, pioglitazone increased the DNA binding
activity of PPARγ and decreased the NF-κB activity by increasing IκBα. Activated
microglial cells were significantly reduced at sites of neurodegeneration in
pioglitazone-treated SOD1-G93A mice, as were the protein levels of COX-2 and
iNOS. The mRNA levels of the suppressor of cytokine signaling 1 and 3 genes were
also increased by pioglitazone [48], but their functional significance is not
well known.In vivo
treatment with PPARγ agonists suppresses Aβ-evoked
microglial activation and inflammatory cytokine expression, iNOS expression and
NO production, and inhibition of COX-2 in Aβ-evoked animal models of AD and
APP [42]. PPARγ agonists also suppress Aβ-mediated activation of microglia in vitro [40–43]. The expression of
iNOS in neurons resulted in neuronal cell death which was prevented by
activation of PPARγ in vitro
and in vivo [42, 43].
Neuroinflammatory changes accompanied by activation of microglia and astrocytes
and expression of TNFα, IL
-1β, and iNOS play a pivotal role in PD [46].
An increase in infiltrating CD8+ T lymphocytes and IFNγ+ cells were also
reported in the CNS with PD. Pioglitazone decreased microglia and astrocyte
activation and reduced the number of iNOS-positive cells in the CNS [47]. In
trauma, macrophages, and neutrophils are involved in the early stages of
inflammation followed by leukocyte recruitment via VCAM-1, ICAM-1, IL-6, IL-8,
and COX-2 [75]. The leukocytes and microglia mount inflammatory responses with
elevated expression of cytokines, chemokines, adhesion molecules, iNOS, COX-2,
and other inflammatory mediators that exacerbate the tissue damage [61].
Treatment with pioglitazone significantly reduced the induction of
inflammatory genes, IL-1β, IL-6, monocyte chemoatractant
protein-1, and intracellular adhesionmolecule-1. The PPARγ antagonist, 2-chloro-5-nitro-N-phenyl-benzamide
(GW9662), prevented the neuroprotective effect of pioglitazone [48],
suggesting the involvement of PPARγ-dependent mechanisms in the
regulation of inflammation and new therapeutic avenue for the treatment of MS.The
expression and activation of PPARα in T lymphocytes decreases IL-2 production
and proliferation. PPARα-null mice show an augmented LPS-induced inflammatory
response and oral treatment with gemfibrozil reduced CD4+ lymphocyte and
macrophage infiltration into the CNS of mice with EAE. Several agonists of
PPARα, including gemfibrozil and ciprofibrate, decreased murine lymphocyte
proliferation in a concentration-dependent manner, in vitro [60]. The gemfibrozil and ciprofibrate-induced IL-4
production in murine and human lymphocytes, whereas IFNγ production was
decreased. WY14,643, a synthetic PPARα agonist, reduced the IgG response in
mice with EAE and impaired generation of IFNγ, TNFα, and IL-6 in response to
MOG peptide in vitro. PPARα and
PPARβ/δ are expressed in astrocytes, while the latter are present more in
oligodendrocytes, thus playing a role in the process of remyelination [66]. In
AD, the neuroinflammatory components include resident microglia,  astrocyte, the
complement system, cytokines, and chemokines. Microglia and astrocytes generate
beta-amyloid protein that stimulate proinflammatory cytokines in AD brain. PPARα agonists inhibitAβ-stimulated expression of TNFα and IL-6 in a dose dependent
manner [40, 42]. In trauma and spinal cord injury-induced edema, neutrophil
infiltration and immunoreactivity to TNFα were augmented with a worsened
recovery of limb function in PPARα knockout than wild type mice.
CNS injury leads to rapid recruitment of microglia, macrophage, and astrocytes
that secrete IL-1, TNFα, iNOS, PGs, and COX-2 [63]. Fenofibrate promotes
neurological recovery by decreasing iNOS, COX2, MMP9 expression, and
antioxidant effect in TBI. Although PPAR agonists
inhibit neuroinflammation in many CNS diseases, their modes of action are not
well characterized.


## 6. PPAR Agonists Regulate IL-12 Family Cytokines in CNS Diseases

IL-12,
IL-23, and IL-27 are three IL-12 family cytokines produced by macrophage,
microglia, and dendritic cells in the CNS. IL-12 is a 70 kD heterodimeric
cytokine composed of p40 and p35 subunits encoded by two different genes that
play a critical role in the differentiation of neural antigen-specific Th1
cells in EAE [78, 79]. We and others have shown earlier that in vivo treatment
with neutralizing anti-IL-12p40 antibody prevents EAE [79]. Furthermore,
therapeutic intervention of IL-12-signaling was effective in preventing EAE. We
have shown that PPARγ agonists inhibit IL-12 production,
IL-12 signaling, and differentiation of Th1 cells in EAE [30]. We have also
shown that PPARγ-deficient heterozygous mice
develop an exacerbated EAE in association with an augmented Th1 response [37],
suggesting a physiological role for PPARγ in the regulation of IL-12/IFNγ axis in CNS demyelination. IL-23
is a heterodimeric cytokine composed of a common IL-12p40 subunit and an
IL-23p19 subunit specific to IL-23 encoded by two different genes [70]. Signaling
through its receptor, composed of IL-12Rβ1 and IL-23R, IL-23 induces the
activation of Jak-Stat pathways and differentiation of IL-17 producing (Th17)
cells from memory Th1 cells, leading to the pathogenesis of EAE [80]. Targeted
disruption of IL-23p19 in mice was effective in preventing the pathogenesis of
EAE [70, 81] and suggested that the IL-23/IL-17 axis plays a critical role in
the pathogenesis of CNS inflammation and demyelination. Although IL-6 and TGFβ [82] are important mediators
of Th17 differentiation in culture, their physiological role in activating Th17
cells in CNS disease is not known (Figure 2).Figure 2
Regulation of neuroinflammation by PPAR agonists in CNS diseases. CD40/TLR induce the activation of NF-κB pathway leading to expression of IL-12 family cytokines from APCs which in turn signal through Jak-Stat pathway in T cells leading to Th1/Th17 differentiation and development of CNS diseases. PPAR agonists modulate signaling and transcription in APC and T cells thereby preventing CNS diseases.IL-27
is another heterodimeric cytokine consisting of EBI3 and p28 encoded by two
different genes. IL-27 receptor is composed of WSX-1 and gp130 molecules that
mediate IL-27-induced activation of the Jak-Stat pathway in naive CD4+ T cells
[83]. In vivo treatment with anti-IL-27 antibody ameliorates EAE, suggesting
its role in the pathogenesis of Th1 cell-mediated autoimmune diseases. Recent
studies have also shown that IL-27 and IFNγ are potent inducers of T-bet,
a T-box protein transcription factor, in T cells. Targeted disruption of T-bet
or siRNA inhibition of T-bet was sufficient to prevent the pathogenesis of EAE,
suggesting the critical role of IL-27/IFNγ/T-bet axis in the pathogenesis
of demyelination [84]. PPARγ agonists regulate IL-27/IFNγ/T-bet axis in EAE. Interestingly,
recent studies have shown that EBI3 can also heterodimerize with IL-12p35 to
form IL-35 in CD4+-Foxp3+ regulatory T cells and functions as a potent anti-inflammatory
cytokine [85]. Although PPARγ has been shown to upregulate
Treg cells in vitro [86], the role of PPAR in the development of Treg cells or
production of IL-35 in EAE/MS or other CNS diseases is not known.


## 7. PPAR Agonists Regulate NF-κB Signaling Pathways in CNS Diseases

The IL-12 family
cytokines are produced by macrophage, microglia, and dendritic cells in
response to autoantigens, TLR ligands, and CD40 ligands [87]. In earlier studies, we and others have shown
that autoimmune cells secrete IL-12 in response to antigens and that this
response was inhibited by treatment with PPARγ agonists [30]. PPARγ agonists also inhibit
LPS and CD40L-induced secretion of IL-12 from macrophage, microglia, and
dendritic cells. The induction of IL-12/IL-23 gene expression involves activation
of the NF-κB signaling pathway in antigen-presenting cells [88]. NF-κB is a heterodimeric transcription factor
composed of p50 and p65 subunits from the Rel family of proteins. It is
sequestered in the cytoplasm as an inactive complex when associated with its
inhibitor, IκB.Upon stimulation with specific
inducers, IκB is phosphorylated and degraded through
proteosome-mediated pathways. The activated NF-κB then translocates into the nucleus and binds
to specific 10 bp response elements of the IL-12, IL-23, and IL-27 genes [88, 89] Activation of NF-κB is a complex process involving the successive action of
proximal NF-κB-inducing kinase (NIK) and the IκB kinases,
IKKα, IKKβ, and IKKγ [90]. The expression of the IL-12 p40 subunit is controlled
by proximal cis-acting elements (NF-κB half site) interacting with NF-κB family
members [91]. Inhibitors of IL-12 gene expression, including retinoids, acetyl
salicylic acid, and 1,25 dihydroxyvitamin D3, block NF-κB activation and bind within
the IL-12p40 promoter [92, 93]. The inhibition of NF-κB pathway leading to the
expression of IL-12 family cytokines by PPAR agonists suggests this be a
mechanism by which PPAR agonists regulate CNS diseases (Figure 3).Figure 3
Regulation of NF-κB pathway by PPAR agonists in CNS diseases. The activation of microglia, macrophage and dendritic cells through toll-like receptor, CD40 or cytokine associated NF-κB pathway leads to secretion of inflammatory cytokines leading to pathogenesis of CNS diseases. PPAR agonists inhibit NF-κB pathway resulting in inhibition of CNS diseases.The NF-κB family of proteins (RelA/p65,
RelB, c-Rel, p50, p52) are widely expressed in the CNS [94] and activated in a
number of CNS inflammatory diseases. Microglia plays a pivotal role in immune
surveillance and host defense against infectious agents in the CNS. NF-κB, JNK, and p38 pathways are
responsible for F-actin architecture during microglial activation. In AD, NF-κB activation is increased when
compared to control brain. The brain samples from PD patients showed an
increased nuclear p65 (RelA) in dopaminergic neurons when compared to age
matched controls [95]. The spinal cord samples from  ALS patients with
degenerating motor neurons showed increased NF-κB activation in astrocytes that
are controlled by c-jun, and JNK/SAPK kinases [96]. In MS patients, NF-κB and c-jun activities are
increased in chronic lesions. PPARs are expressed in microglial cells and PPARγ agonists act as negative
regulators for elements that contain Stat binding sites. While the inflammatory
cascade is mediated via both NF-κB and JNK pathways, PPARγ agonists increase
the levels of IκB-α and IκB-β and reduce the nuclear translocation of NF-κB [97].
While the induction of NF-κB promotes postischemic
inflammation, PPAR agonists prevent postischemic inflammation and neuronal
damage by inhibiting NF-κB pathway. Further analyses indicate
that L-796,449 inhibits NF-κB signaling through both PPARγ-dependent and independent
pathways. In addition, spinal cord injury (SCI) associated neuronal damage was
less severe in NF-κB knockout mice. PPARγ induces
transrepression of NF-κB-induced inflammatory genes through their association
with corepressor complexes [96].


## 8. PPAR Agonists Regulate Jak-Stat Signaling Pathway in CNS Diseases

The
orchestrated interaction of APCs and T cells in the CNS leads to activation of Jak-Stat
signaling pathway, secretion of inflammatory cytokines, and pathogenesis of
neuroinflammatory diseases. The antigen-induced proliferation of T cells is a
two-step process in which signaling through T cell receptor (signal 1) drives T
cells from resting G0 to activated G1 phase of the cell cycle, whereas
signaling through IL-2 or IL-12 receptor (second signal) is required for T
cells to transit from G1 to S/G2/M phase of the cell cycle (proliferation).
IL-12 is a potent inducer of G1 to S/G2/M phase transition and differentiation
of Th1 cells that are critical in the pathogenesis of EAE and other CNS
diseases. IL-12 signals through IL-12 receptorβ1 and β2, members of the gp130 cytokine receptor
super-family, expressed primarily on activated NK cells and T cells. Coexpression
of IL-12Rβ1 and β2 leads to the formation of high affinity IL-12 receptors [87].
Signaling through its receptor, IL-12 induces tyrosine phosphorylation and
activation of Jak2, Tyk2, Stat3, and Stat4 in T and NK cells [98, 99]. Activation
of the Jak-Stat pathway leads to transcription of IL-12 response genes
associated with proliferation, Th1 differentiation, and IFNγ production. IL-23
receptor is composed of common IL-12Rβ1 and a specific IL-23 receptor subunit
[100]. Signaling through its receptor, IL-23 induces the activation of Jak2,
Tyk2, Stat1, Stat3, Stat4, and Stat5 in T cells [98]. Activation of the
Jak-Stat pathway leads to transcription of IL-23 response genes, including
IL-17, which are associated with proliferation of memory T cells [101], whereas
IL-27 and IFNγ activate a specific Jak-Stat pathway in T
cells, resulting in the induction of T-bet in naive T cells [102]. Modulation
of cytokine signaling by targeting protein tyrosine kinases or transcription
factors has been considered a novel strategy for the treatment of autoimmune
diseases [103, 104]. We have shown
earlier that the blockade of IL-12 signaling through Jak-Stat pathway by
treatment with a Jak-2 inhibitor, tyrphostin AG490, quercetin, vitamin D, and
curcumin inhibits Th1 differentiation and pathogenesis of EAE [105–108]. We
have also shown recently that PPARγ agonists inhibit
IL-12-induced tyrosine phosphorylation of Jak2, Tyk2, Stat3, and Stat4 in T
cells, differentiation of Th1 cells and pathogenesis of EAE [30]. These findings suggest that IL-12
signaling through the Jak-Stat pathway is a molecular target in the regulation
of autoimmune diseases. Recent studies have shown that the transcription
factors such as Stat4 and T-bet are involved in the pathogenesis of EAE/MS, whereas
Stat6 mediates recovery. While the induction
of Stat1 and Stat3 promotes postischemic inflammation, and Stat-1 knockout mice
develop less severe stroke lesions in the CNS [32], activation of PPARs prevents
postischemic inflammation and neuronal damage (Figure 4).Figure 4
IL-12, IL-23 and IL-27 are heterodimeric cytokines signals through Jak-Stat pathway and induce Th1/Th17 differentiation and T-bet expression in T cells. Treatment with PPAR agonists regulate these responses in T cells resulting in inhibition of CNS inflammation.The
exact mechanism by which PPAR agonists negatively regulate neuroinflammation,
and in particular, the Jak-Stat signaling pathway is not known. Suppressor of
cytokine signaling (SOCS) proteins are negative regulators of Jak-Stat pathway.
While PPARγ agonists inhibit Jak-Stat
pathway in astrocytes and microglial cells, they rapidly induce the expression of
SOCS 1 and 3, which in turn inhibit Jak activity in glial cells [109]. In
addition, PPAR agonist can modulate Jak-Stat pathway through activation of Src
homology 2 domain-containing protein phosphatase 2 (SHP2) in immune cells, thereby
inhibiting neuroinflammatory diseases.


## 9. Conclusion

The neuroinflammatory diseases such as multiple sclerosis, Alzhimer’s disease, stroke, and
trauma are common health problems
affecting more than five percent of the population worldwide. While the exact
mechanisms are not known, the immune cell activation and secretion of
inflammatory cytokines, involving NF-κB and Jak-Stat signaling pathways, play critical roles in the
pathogenesis of many CNS diseases. Thus,
interfering with the signaling network could be an effective approach in the
treatment of MS and other neuroinflammatory diseases. PPAR
is a family of nuclear receptor transcription factors that regulate CNS
diseases by modulating neuroinflammatory signaling network. Since PPAR agonists
are already in human use, they are likely to prove useful in the treatment of MS
and other neuroinflammatory diseases in the near future.



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*Source: 658520-2008-07-28.xml*