# PPAR-δ in Vascular Pathophysiology

Authors: Nanping Wang
Journal: PPAR Research
 (2008)

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## Abstract

Peroxisome proliferator-activated receptors belong to the superfamily of ligand-dependent nuclear receptor transcription factors, which include three subtypes: PPAR-α, β/δ, and γ. PPAR-δ, play important roles in the regulation of cell growth and differentiation as well as tissue wound and repair. Emerging evidence has also demonstrated that PPAR-δ is implicated in lipids and glucose metabolism. Most recently, the direct effects of PPAR-δ on cardiovascular processes such as endothelial function and angiogenesis have also been investigated. Therefore, it is suggested that PPAR-δ may have critical roles in cardiovascular pathophysiology and is a potential target for therapeutic intervention of cardiovascular disorders such as atherosclerosis.


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## Body

## 1. Introduction

Peroxisome
proliferator-activated receptors (PPARs) are members of nuclear
receptor/ligand-activated transcription factors superfamily. PPAR
subfamily consists of 3 subtypes: PPAR-α, -β/δ, and -γ.
PPARs form heterodimers with a retinoid X receptor (RXR) and bind to
specific PPAR-responsive elements (PPREs) to regulate target gene expression.
In the absence of specific ligands, the PPAR-RXR heterodimer forms repressive
complex with corepressors and histone deacetylases. Upon ligand binding, the
receptor undergoes conformational changes that cause the dissociation of
repressors, the recruitment of coactivators, and the activation of gene
transcription [1]. Recently, extensive studies
have been performed to characterize the biological and pathophysiological roles
of PPAR-α
and -γ, which are
pharmacological targets of the clinical interventions for dyslipidemia and type
2 diabetes, respectively. Fibrates class of lipid-lowering drugs such as
fenofibrate and gemfibrozil are agonists for PPAR-α. Thiazolidinedione class of
insulin sensitizers including troglitazone (Rezulin), rosiglitazone (Avandia),
and pioglitazone (Actos) are specific ligands for PPAR-γ [2].
PPAR-α, primarily
expressed in liver, muscles, heart, and kidney,
plays a key role in fatty acid catabolism such
as β-oxidation.
PPAR-γ is highly
expressed in fat, controls adipogenesis, and regulates insulin action. However,
relatively little has been known with regards to the function of PPAR-δ, the only
subtype of PPARs that is not a target of current drug. Newly developed
synthetic ligands and genetically modified mouse models for PPAR-δ have rapidly
advanced our understanding of the important roles of PPAR-δ in tissue
development, repair, inflammation, and metabolism [3–7]. Most recently, the direct
effects of PPAR-δ
on cardiovascular processes such as endothelial function and angiogenesis have
also been investigated. In this review, we will focus on the recent
advancements regarding the roles of PPAR-δ in vascular pathophysiological
processes.


## 2. Materials and Methods

### 2.1. Gene and Protein

PPAR-δ, also known as nuclear
hormone receptor 1 (NUC1), PPAR-β, or NR1C2, was first cloned in 1992
[8, 9]. The PPAR-δ gene is
mapped to human chromosome 6p21.2-p21.1 and has 11 exons, spanning 35
kilobase-pair [10]. Like other PPARs, PPAR-δ protein has a
modular structure consisted of 5 regions: an N-terminal region (A/B), a
DNA-binding domain (C), a flexible hinge region (D), ligand-binding domain (E),
and a C-terminal region (F). X-ray crystallographic study revealed that PPAR-δ has an exceptionally
large ligand binding pocket, which maybe related to the promiscuous
accommodation of a large range of mostly amphipathic ligands [11, 12].


### 2.2. Endogenous and Synthetic Ligands

Several 14- to 18-carbon saturated fatty acids and 16- to
20-carbon polyunsaturated fatty acids can bind PPAR-δ [13–15].
Naturally occurring or synthetic eicosanoids such as prostaglandin A1
and carbaprostacyclin have been shown to bind and activate PPAR-δ [16]. Very low-density lipoprotein (VLDL) derived
has also been demonstrated to activate the PPAR-δ target genes in a receptor-dependent
manner [17]. Since these agonists
activate PPAR-δ
all with affinities at molar range, it raises a question as to whether these
are bona fide physiological ligands for PPAR-δ. However, many of the above-mentioned
agonists under the physiological or pathological conditions are either released
by the vessels, such as PGI2, or being exposed to vascular endothelium, such as
VLDL. It would be intriguing to examine whether PPAR-δ in the vessel wall is activated in
vivo. In addition, it was recently shown that retinoic acid, a ligand for
retinoic acid receptor, also can activate the PPAR-δ with nanomolar affinity without
affecting the other two subtypes of PPARs [18]. This finding expanded our
understanding of the mechanisms of PPAR-δ activation.Recently, several synthetic ligands
have been reported to selectively activate PPAR-δ. The PPAR-δ agonists reported
to date were discovered using several strategies: GW501516 and GW-0742
(GlaxoSmithKline) were optimized from a library of hydrophobic carboxylates [19];
L165461 (Merck) was derived from an in silico approach [20]. These derivatives of
phenoxyacetic acid are the highly selective PPAR-δ ligands with a nanomolar
affinity and 1000-fold selectivity over PPAR-α and -γ. Other PPAR-δ agonists including KD3010 (Kalypsys)
and MBX-8025 (Metabolex) are currently in clinical development. On the other
hand, the development of these specific agonists has greatly aided the
investigation in the biological functions of PPAR-δ. A selective antagonist for PPAR-δ, GSK0660, has
also been recently demonstrated as it by itself exhibits inverse agonist activity and  competes
with agonist in a cellular context [21] (Table 1).Table 1
Ligands for PPAR-δ.
LigandsNatureAffinityClinical statusNatural agonistsLinoleic acidDietary fatty acidμMOleic acidDietary fatty acidμMArachidonic acidDietary fatty acidμMEicosapentaenoic acidDietary fatty acidμMDocosahexaenoic acidDietary fatty acidμMProstaglandin A1Endogenous prostaglandinμMSynthetic agonistsCarbaprostacyclinSynthetic stable PGI2 analogueμMIliprostProstacyclin analogueμMCompound FPhenoxyacetic acid derivativenML165,041Phenoxyacetic acid derivativenMGW501516Phenoxyacetic acid derivativenMGW0742Phenoxyacetic acid derivativenMKD2010not disclosednMPhase IMBX-8025not disclosednMPhase IISynthetic dual agonistsCompound 23Dual agonist forγ and δnMSynthetic antagonistGSK0660Antagonist forδnM


### 2.3. Effects on the Vessel Wall

Although nearly ubiquitously
expressed with highest levels in placenta, skeletal muscles, and adipose
tissue, PPARδ is also expressed in the vascular cells including endothelial
cells [22], smooth muscle cells, and
macrophages. Particularly, a number of studies during the past 2 years have
demonstrated that PPAR-δ plays direct roles in various basic
vascular processes such as apoptosis, survival, angiogenesis, and inflammation.


### 2.4. Endothelial Apoptosis

Vascular endothelium, when unperturbed, is
considered to provide a relatively nonadhesive and nonthrombotic interface.
This characteristic is likely essential to physiological homeostasis. However,
endothelial cells (ECs) can undergo apoptosis in vitro in response to a variety
of pathophysiological conditions including hypoxia, proinflammatory cytokines,
bacterial endotoxins, and atherogenic risk factors such as homocysteine and
lipoproteins [23, 24]. EC apoptosis has been
implicated in numerous pathophysiological processes, such as angiogenesis,
thrombosis, and atherosclerosis. On the other hand, ECs produce a plethora of
bioactive molecules to maintain vascular homeostasis. Among those, prostacyclin
(PGI2) protects ECs from apoptosis. Although PPAR-δ has been previously documented to
protect against the hypertonicity-induced apoptosis in renal cells [25] and the growth factor
deprivation- or anoikis-induced apoptosis in keratinocytes [6], its role in vascular cells
has been recently demonstrated. Liou et al. showed that PGI2 protects ECs from H2O2-induced
apoptosis via the action of PPAR-δ. By inducing the expression of its
target gene 14-3-3alpha, PPAR-δ prevents Bad-triggered apoptosis [26]. Treatment with L165041 or
overexpression of PPAR-δ also has a similar effect. In addition,
small interfering RNA-mediated knockdown of PPAR-δ abrogated the antiapoptotic effect,
suggesting that the
antiapoptotic role of PPAR-δ appeared to be specific for and
dependent on the endogenous PPAR-δ receptor. Interestingly, gene
expression of 14-3-3epsilon was also induced by PPAR-δ through a PPRE-independent mechanism
and an interaction between PPAR-δ and CCAAT/enhancer binding protein
(C/EBP) [27].


### 2.5. Endothelial Activation

When exposed to
proinflammatory stimuli such as tumor necrosis factor (TNF) or
lipopolysaccharide (LPS), normally quiescent endothelium undergoes a phenotypic
change, which is characterized by induction of proinflammatory and procoagulant
factors such as adhesion molecules and tissue factor. Such a phenotypic
conversion, referred to as EC activation, is implicated in a number of
proinflammatory diseases including atherosclerosis and thrombosis [28]. PPAR-α and -γ have been previously shown to suppress
EC expression of proinflammatory adhesion molecules such as intercellular
adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1),
E-selectin, monocyte chemoattactant protein-1 (MCP-1), and ensuing recruitment
of leukocytes [22, 29–32]. However, there has also been evidence
that suggests a proinflammatory role of PPAR-α or -γ [33, 34].
Recent studies suggested that PPAR-δ also plays a role in inflammatory
processes and atherosclerosis. In macrophages, the PPAR-δ agonist GW0742 inhibited lipopolysaccharide (LPS)-induced expression
of proinflammatory genes, such as cyclooxygenase (COX)-2 and inducible nitric
oxide synthase (iNOS) [35]. GW0742 reduced atherosclerotic lesions and decreased
the expression of MCP-1 and ICAM-1 in the aorta of LDLR−/− mice [36, 37]. Given the beneficial effects
of PPAR-δ
agonists on lipid profiles, it is likely that PPAR-δ agonists may inhibit endothelial
activation by improving dyslipidemia. A direct anti-inflammatory effect was
also demonstrated. In EAhy926 cells, Rival et al. found that L-165041, at high
a concentration up to 100 μM, inhibited TNF-α-induced VCAM-1 and MCP-1 expressions [38]. In primary culture of human
umbilical vein ECs (HUVECs), specific agonists GW0742 and GW501516 inhibited
the TNF-α-
or interleukin-1β-induced expression of adhesion molecules and the monocyte adhesion to ECs.
PPAR-δ agonist
induced the gene expression of antioxidative enzymes, such as superoxide
dismutase-1, catalase, and thioredoxin, and it decreased reactive oxygen species
production in ECs. Unexpectedly, the anti-inflammatory effect not only
persisted but it also was further enhanced
after the decrease of PPAR-δ expression by siRNA knockdown [39]. Given the evidence that the
ligand binding caused the dissociation of the transcription repressor BCL-6
from PPAR-δ
and the subsequent association of BCL-6 with the VCAM-1 promoter region, this
seemingly paradoxical result could be plausibly interpreted with the previously
proposed PPAR-δ/Bcl-6
interaction action mode: the synthetic ligand binds to PPAR-δ and recruits
the coactivators to replace the corepressors such as Bcl-6. The released
corepressors relocate to repress the transcription of proinflammatory genes
such as VCAM-1 and E-selectin and thus contribute to the vascular protection.Ghosh et al. recently showed that the metabolism of
endocannabinoids by the endothelial COX-2 coupled to the prostacyclin synthase
activates PPAR-δ,
which negatively regulates the expression of tissue factor (TF), the primary
initiator of blood coagulation [40]. As COX-2 inhibitors
suppressed PPAR-δ
activity and induced TF expression, these results may help explaining the
prothrombotic adverse effects of the cox-2 inhibitors rofecoxib and valdecoxib [41].


### 2.6. Angiogenesis

Angiogenesis is referred to
the formation of new capillaries from the existing blood vessels. Physiological
angiogenesis is involved in wound healing and aerobic exercise, whereas
pathological or therapeutic angiogenesis is implicated in cardiovascular
diseases, diabetic complications, inflammatory diseases, and cancers [42]. Recent studies
have also linked
metabolic homeostasis to angiogenesis and further interrogate the potential
effects of PPARs on the angiogenic process
[43, 44]. An earlier study
demonstrated that the PPAR-δ agonist GW501516 dose-dependently
stimulates HUVEC proliferation with increased mRNA expression of vascular
endothelial growth factor αand its receptor flt-1 [45]. Later, GW501516 was shown to
promote endothelial tube formation on an extracellular matrigel, EC outgrowth
in a murine aortic ring model, and increased angiogenesis in the implanted matrigel
plug assay in vivo through a PPAR-δ- and VEGF-dependent manner [22]. Most recently, Gaudel
et al. found that
treatment with GW0742 or muscle-specific overexpression of PPAR-δ promoted
angiogenesis in mouse skeletal muscle [46]. Besides, arising from
sprouts on existing vessels, vessels also arise from endothelial
progenitorcells (EPCs), a process referred to as vasculogenesis [47]. Culminating evidence further
suggests that
circulating EPCs is
capable of stimulating angiogenesis [48]. A recent study showed that
the proangiogenic effects of human EPCs are in part dependent on the
biosynthesis and release of PGI2, and subsequent
activation of PPAR-δ
[48, 49]. Furthermore,
functional genomic approach provided evidence that silencing of PPAR-δ in
the tumor microenvironment impairs angiogenesis and tumor growth, identifying
PPAR-δ as one of a
few hub nodes in the angiogenic network
[49, 50]. Up-to-date results have been largely consistent and pointed
toward a proangiogenic activity of PPAR-δ. In corroboration with this,
Müller-Brüsselbach et al. found that the growth of syngeneic PPAR-δ wild-type tumors was
impaired in PPAR-δ−/−  mice, concomitant with a
reduced blood flow and hyperplastic vascular structures, suggesting that PPAR-δ maybe required in tumor ECs for
the formation of functionally mature vessels [51]. Nevertheless, a full understanding of the specific
roles of PPAR-δ
in specific scenarios of angiogenesis will be imperative for a safe and
rational therapeutic strategy.


### 2.7. Smooth Muscle Cells

PPAR-δ
is expressed in SMCs and is induced in response to platelet-derived growth
factor (PDGF) in SMCs, which involved the phosphatidylinositol 3-kinase/Akt signaling pathway.
Initial study using overexpression showed that PPAR-δ increased SMC proliferation, indicating
a proliferation-promoting effect in SMCs [52]. However, L-165041, a selective PPAR-δ agonist, inhibited SMC proliferation and migration
via inhibition of the PDGF-induced expression of cyclin D1 and cyclin-dependent
kinase (CDK) 4 and cell cycle progression  [53]. In SMCs, GW501516 increased
the expression of transforming growth factor-β1 (TGF-β1) and the effect seemed to depend on
endogenous PPAR-δ. Subsequently, TGF-β1 was likely
responsible for suppression of the IL1β-induced expression of MCP-1 and
proliferation of SMCs [54]. In rats, administration of
L-165041 decreased neointima formation in balloon-injured carotid arteries [53]. Thus, synthetic PPAR-δ agonists appear to have antiproliferative and
anti-inflammatory properties in SMCs. This is consistent with previous
reports that adenovirus-mediated gene transfer of prostacyclin synthase, which
produces the endogenous PPAR-δ ligand PGI2, inhibited SMCs
proliferation and intimal hyperplasia [55–57].


### 2.8. Macrophages

Macrophage
infiltration in vessel wall is known to play important role in
atherogenesis. PPAR-δ is expressed in macrophages. During past years, the role of
PPAR-δ in macrophage biology has been extensively studied. However,
existing results still remain controversial. Oliver Jr. et al. showed
that GW501516 in a human monocytic cell line increased the expression of ATP-binding
cassette A1 (ABCA1) and Apo AI-mediated cholesterol efflux [58]. However, Vosper et
al. found that a different PPAR-δ agonist, compound F,
increased
lipid
accumulation in both human primary macrophages and THP-1 cells. Compound F
induced the expression of genes involved in lipid uptake and storage such as class
A and B scavenger receptors (SRA and CD36) but repressed key genes involved in
lipid metabolism and efflux such as Apo E and cholesterol 27-hydroxylase [59]. In mouse macrophages, neither
genetic loss of PPAR-δ nor treatment with the PPAR-δ agonists GW501516 or
GW0742 significantly influenced cholesterol efflux or accumulation
[36, 60]. Beside the effects on lipid trafficking, PPARδ agonists
have a potent anti-inflammatory effect in macrophages. Welch et al. first
demonstrated that, in mouse peritoneal macrophages, PPAR-δ agonist
GW0742 inhibited LPS-induced expression of COX-2 and iNOS
[32, 35]. Recently, Barish et al. found that GW501516 in mouse
macrophages suppressed the gene induction of MCP-1, -3, -5 by IL-1, interferon-γ (IFN-γ), and phorbol
ester. The agonist treatment also inhibited transendothelial migration of THP-1
cells [61]. The anti-inflammatory
effects of the agonist was lost in the receptor-deficient macrophages [60]. However, in other cell types
such as epithelial cells, eosinophils, neutrophils, and lymphocytes, the PPAR-δ agonist was
ineffective in inhibiting inflammatory processes, indicating that the effect is
cell-type-specific [62].


### 2.9. Atherosclerosis

To date, several studies have been reported regarding the
roles of PPAR-δ in atherosclerosis in different mouse models with different
approaches. Lee et al. transplanted PPAR-δ-null bone marrow progenitor
cells into LDL receptor-null (LDLR−/−) mice. Unexpectedly, the
adoptive transfer of PPAR-δ-null macrophages led to a
less severe atherosclerosis, suggesting that endogenous PPAR-δ maybe proatherogenic. Although overexpression or
deletion of PPAR-δ
in macrophages suggested that PPAR-δ is proinflammatory, the agonist
GW501516 decreased MCP-1, seemingly having an opposite effect [60]. To reconcile this contradiction, they postulated an
unconventional ligand-dependent transcriptional mechanism, which switches PPAR-δ between a “proinflammatory” and “anti-inflammatory”: in the
absence of ligand, PPAR-δ sequesters
a transcriptional repressor of inflammatory responses such as Bcl-6, permitting
induction of proinflammatory genes; in the presence of ligand, PPAR-δ releases the repressor, which is then free to exert its
anti-inflammatory effects. Following this loss-of-function approach, two
independent studies examined the effect of the PPAR-δ agonist GW0742 on atherogenesis in high fat- and
cholesterol-fed LDLR−/− mice and yielded divergent results. In the
first study, Li et al. found that GW7842 decreased gene expression of
proinflammatory cytokines and adhesion molecules within atherosclerotic lesions
but failed to alter the progression of atherosclerosis after 14 weeks of
treatment (5 mg−1 kg−1 day−1). In another one, Graham et al. used female
LDLR−/− mice fed with a diet that
induced moderate levels of hypercholesterolemia and observed that GW0742 reduced the lesion
size at a higher dose (60 mg−1 kg−1 day−1)
after 10 weeks of
treatment  [37]. Discrepancy between
these two studies may be caused by differences in the levels of
hypercholesterolemia and different drug doses used. However, the
anti-inflammatory effect was generally consistent in both studies, regardless
the different effects on the lesion sizes. It is likely that the
anti-inflammatory properties of the PPAR-δ agonists on the vessel wall
per se are not
sufficient to attenuate the progression of atherosclerotic lesions if it is not
corroborated by an efficient improvement of metabolic abnormalities. This
notion is supported by the data from recently published results. Most recently,
GW501516, which has a potent lipid-modifying capacity, has also been
demonstrated to have a clear antiatherosclerotic property in apoE−/− mice. Barish et al. showed that GW501516 significantly reduced
atherosclerotic lesions with an increase in HDL level and a reduced expression
of chemokines in the aorta and in macrophages [61]. Furthermore, in a model of angiotensin II-accelerated atherosclerosis (LDLR−/− mice), Takata et al. confirmed the atheroprotective effect of GW0742
[63]. After 4 weeks of treatment, GW0742 at both doses (1 and 10 mg−1 kg−1 day−1) significantly inhibited the
Ang II induction of atherosclerosis without altering blood pressure. This
beneficial effect was likely mediated via the potent anti-inflammatory property
since GW0742 increased vascular expression of Bcl-6, the regulators of G protein-coupled
signaling (RGS4 and 5) in the artery and suppressed Ang II-induced activation
of p38 and ERK in macrophages. However, the metabolic effect of GW0742 may also
have contributed to the atheroprotective outcome because GW0742 significantly
reduced plasma levels of insulin, glucose, leptin, and decreased triglycerides [63]. Overall, studies in mouse models suggest that PPAR-δ may have an attractive therapeutic target for the treatment
of atherosclerosis.


### 2.10. Cardiovascular Risk Factors

In addition to the direct effects on the vessel wall, PPAR-δ also has profound effects on various
metabolic parameters associated with cardiovascular diseases such as obesity,
dyslipidemia, and insulin resistance.


### 2.11. Obesity

PPAR-δ deficiency causes embryonic lethality due to
a placental defect.
Some surviving PPAR-δ null mice had reduced fat mass [3], indicating a role of PPAR-δ in adipogenesis. Transgenic
mice specifically expressing VP16-PPAR-δ, a constitutively active
receptor, in adipose tissue had a reduced body weight, fat mass, and lower
levels of circulating free fatty acids and triglycerides [64]. These animals were less susceptible to high-fat diet-induced
obesity. In contrast, PPAR-δ null mice were more prone to weight gain on a high-fat diet.
Similarly, GW501516
ameliorated diet-induced obesity [65]. It has been known that PPAR-δ activates genes
involved in
fatty acid
oxidation and energy dissipation, such as carnitine
palmitoyltransferase 1 (CPT1), acyl-CoA oxidase (AOX), and long chain acyl-CoA
dehydrogenase (LCAD), uncoupling proteins. In addition to the direct effects on
obesity, PPAR-δ
agonists may also have regulatory effects on adipokine profile. For example,
administration of L-165041 in rats increased the expression of visfatin and
adiponectin, which are known to improve insulin sensitivity and are
vasoprotective, but decreased the production of resistin in visceral adipose
tissue [66].


### 2.12. Dyslipidemia

Increased levels of
LDL and triglycerides and decreased HDL in plasma are independent risk factors
for atherosclerosis and associated with metabolic syndrome as well. Recent
studies have demonstrated that activation of PPAR-δ may modify lipid profile in animal
models as well as in human. Oliver Jr. et al. first reported that GW501516
significantly improved dyslipidemia in obese primates with an increase in HDL
and a decrease in LDL cholesterol and triglycerides [58]. The beneficial effect of
GW0742 and L-165041
on HDL level was also observed in obese and nonobese mice
[67, 68]. In addition to enhancing
fatty acid oxidation in muscles, PPAR-δ agonists upregulated expression of
ABCA1 in several types of cells, which may lead to an increase in HDL and
cholesterol reverse transport. In intestinal cells, it also inhibited gene
expression of Niemann-Pick
C1-like 1, the key molecule for cholesterol absorption [67]. However, the precise mechanisms underlying these
lipid-modifying effects still remain to be elucidated. In a small number of
healthy human volunteers, GW501516 has been reported to increase HDL
cholesterol level and improved the triglycerides clearance [69].


### 2.13. Insulin Resistance and Glucose Homeostasis

It has been previously
known that, in obese primates, GW501516 declined fasting insulin level [58]. GW501516 treated ob/ob mice also
showed a significantly improved glucose tolerance and with a lower postprandial
levels of plasma glucose and insulin [65]. In cultured myotubes, PPAR-δ agonists were found to directly stimulate
glucose uptake independent of insulin action. The agonist-stimulated glucose
uptake in myotubes appeared to require AMP-activated protein kinase (AMPK) but
not PPAR-δ
[70, 71]. However, GW501516 had no acute effect on
glucose transport in rat skeletal muscles [72]. Recently, Lee et al. showed that PPAR-δ−/− mice were glucose intolerant. Euglycemic
hyperinsulinemic clamp experiments showed that GW501516 improved insulin
sensitivity in multiple tissues including hepatic and peripheral tissues. The agonist suppresses hepatic
glucose output and increases glucose disposal [73]. Gene array analysis suggested that PPAR-δ might ameliorate hyperglycemia by increasing glucose flux
through the pentose phosphate pathway, which is known to enhance de novo fatty
acid synthesis. Thus, it could be a
concern whether PPAR-δ improves hyperglycemia at
the cost of exacerbation of hepatosteatosis, a problem commonly associated with
metabolic syndrome and diabetes. However, recent studies demonstrated that
PPAR-δ also had a beneficial effect on hepatosteatosis. In a
diet-induced mouse model of nonalcoholic steaohepatitis, GW501516 reduced
triglycerides accumulation in the livers. In a most recent study, it was
demonstrated that PPAR-δ suppressed hepatic
lipogenesis via the induction of insulin-induced gene-1 (insig-1) and the
inhibition of lipogenic sterol-regulatory element binding prortein-1 (SREBP-1)
activation. In obese diabetic mice, hepatic overexpression of PPAR-δ ameliorated hepatosteatosis [74]. Since Insig-1 is a critical regulator of lipid homeostasis,
identification of the insig-1 as a target gene of PPAR-δ may also facilitate our understanding of the profound
effects of PPAR-δ activation on adipogenesis
and lipid metabolism
[75, 76]. Recently, in a double-blind and randomized study, PPAR-δ agonist (10 mg o.d. GW501516) was given to a small number of
healthy overweight subjects. The results showed that treatment with GW501516
for 2 weeks significantly reduced liver fat content by 20% without increasing
oxidative stress [77].


### 2.14. Gene Polymorphisms

Skogsberg et al.
initially described 4 polymorphisms:−409C/T (in the promoter region), +73C/T
(exon 1), +255A/G (exon 3), and +294T/C (exon 4). The +294T/C polymorphism
showed a significant association with a metabolic trait. The homozygotes for
the C allele had a higher plasma LDL and a tendency toward higher risk of CHD compared with
homozygous carriers of the T-allele
[78, 79]. In addition, there is a highly
significant association between the rare C allele and lower plasma HDL
concentrations in the female patients with mixed hyperlipidemia. Associations were also found for the
C-allele with coronary heart disease and body mass
index (BMI)
[80, 81]. Chen et al.
also demonstrated that, in “lipoprotein and coronary atherosclerosis study”
(LCAS) subjects, the PPAR-δ SNPs were strongly associated with the dyslipidemia, the responses
to statin, and the atherosclerotic lesions [82]. Vänttinen et
al. investigated the effects of the PPAR-δ gene SNPs on tissue
glucose uptake and suggested that the SNPs regulate insulin sensitivity
primarily in skeletal muscles [83]. The PPAR-δ SNPs were genotyped in
type II diabetes subjects and normal control. Although no significant
association was detected with the risk of type II diabetes, several SNPs were
associated with fasting plasma glucose and BMI [84]. In addition,
the association has been found between the PPAR-δ SNPs and metabolic
syndrome, and the association was influenced by dietary fat intake [85].


## 2.1. Gene and Protein

PPAR-δ, also known as nuclear
hormone receptor 1 (NUC1), PPAR-β, or NR1C2, was first cloned in 1992
[8, 9]. The PPAR-δ gene is
mapped to human chromosome 6p21.2-p21.1 and has 11 exons, spanning 35
kilobase-pair [10]. Like other PPARs, PPAR-δ protein has a
modular structure consisted of 5 regions: an N-terminal region (A/B), a
DNA-binding domain (C), a flexible hinge region (D), ligand-binding domain (E),
and a C-terminal region (F). X-ray crystallographic study revealed that PPAR-δ has an exceptionally
large ligand binding pocket, which maybe related to the promiscuous
accommodation of a large range of mostly amphipathic ligands [11, 12].


## 2.2. Endogenous and Synthetic Ligands

Several 14- to 18-carbon saturated fatty acids and 16- to
20-carbon polyunsaturated fatty acids can bind PPAR-δ [13–15].
Naturally occurring or synthetic eicosanoids such as prostaglandin A1
and carbaprostacyclin have been shown to bind and activate PPAR-δ [16]. Very low-density lipoprotein (VLDL) derived
has also been demonstrated to activate the PPAR-δ target genes in a receptor-dependent
manner [17]. Since these agonists
activate PPAR-δ
all with affinities at molar range, it raises a question as to whether these
are bona fide physiological ligands for PPAR-δ. However, many of the above-mentioned
agonists under the physiological or pathological conditions are either released
by the vessels, such as PGI2, or being exposed to vascular endothelium, such as
VLDL. It would be intriguing to examine whether PPAR-δ in the vessel wall is activated in
vivo. In addition, it was recently shown that retinoic acid, a ligand for
retinoic acid receptor, also can activate the PPAR-δ with nanomolar affinity without
affecting the other two subtypes of PPARs [18]. This finding expanded our
understanding of the mechanisms of PPAR-δ activation.Recently, several synthetic ligands
have been reported to selectively activate PPAR-δ. The PPAR-δ agonists reported
to date were discovered using several strategies: GW501516 and GW-0742
(GlaxoSmithKline) were optimized from a library of hydrophobic carboxylates [19];
L165461 (Merck) was derived from an in silico approach [20]. These derivatives of
phenoxyacetic acid are the highly selective PPAR-δ ligands with a nanomolar
affinity and 1000-fold selectivity over PPAR-α and -γ. Other PPAR-δ agonists including KD3010 (Kalypsys)
and MBX-8025 (Metabolex) are currently in clinical development. On the other
hand, the development of these specific agonists has greatly aided the
investigation in the biological functions of PPAR-δ. A selective antagonist for PPAR-δ, GSK0660, has
also been recently demonstrated as it by itself exhibits inverse agonist activity and  competes
with agonist in a cellular context [21] (Table 1).Table 1
Ligands for PPAR-δ.
LigandsNatureAffinityClinical statusNatural agonistsLinoleic acidDietary fatty acidμMOleic acidDietary fatty acidμMArachidonic acidDietary fatty acidμMEicosapentaenoic acidDietary fatty acidμMDocosahexaenoic acidDietary fatty acidμMProstaglandin A1Endogenous prostaglandinμMSynthetic agonistsCarbaprostacyclinSynthetic stable PGI2 analogueμMIliprostProstacyclin analogueμMCompound FPhenoxyacetic acid derivativenML165,041Phenoxyacetic acid derivativenMGW501516Phenoxyacetic acid derivativenMGW0742Phenoxyacetic acid derivativenMKD2010not disclosednMPhase IMBX-8025not disclosednMPhase IISynthetic dual agonistsCompound 23Dual agonist forγ and δnMSynthetic antagonistGSK0660Antagonist forδnM


## 2.3. Effects on the Vessel Wall

Although nearly ubiquitously
expressed with highest levels in placenta, skeletal muscles, and adipose
tissue, PPARδ is also expressed in the vascular cells including endothelial
cells [22], smooth muscle cells, and
macrophages. Particularly, a number of studies during the past 2 years have
demonstrated that PPAR-δ plays direct roles in various basic
vascular processes such as apoptosis, survival, angiogenesis, and inflammation.


## 2.4. Endothelial Apoptosis

Vascular endothelium, when unperturbed, is
considered to provide a relatively nonadhesive and nonthrombotic interface.
This characteristic is likely essential to physiological homeostasis. However,
endothelial cells (ECs) can undergo apoptosis in vitro in response to a variety
of pathophysiological conditions including hypoxia, proinflammatory cytokines,
bacterial endotoxins, and atherogenic risk factors such as homocysteine and
lipoproteins [23, 24]. EC apoptosis has been
implicated in numerous pathophysiological processes, such as angiogenesis,
thrombosis, and atherosclerosis. On the other hand, ECs produce a plethora of
bioactive molecules to maintain vascular homeostasis. Among those, prostacyclin
(PGI2) protects ECs from apoptosis. Although PPAR-δ has been previously documented to
protect against the hypertonicity-induced apoptosis in renal cells [25] and the growth factor
deprivation- or anoikis-induced apoptosis in keratinocytes [6], its role in vascular cells
has been recently demonstrated. Liou et al. showed that PGI2 protects ECs from H2O2-induced
apoptosis via the action of PPAR-δ. By inducing the expression of its
target gene 14-3-3alpha, PPAR-δ prevents Bad-triggered apoptosis [26]. Treatment with L165041 or
overexpression of PPAR-δ also has a similar effect. In addition,
small interfering RNA-mediated knockdown of PPAR-δ abrogated the antiapoptotic effect,
suggesting that the
antiapoptotic role of PPAR-δ appeared to be specific for and
dependent on the endogenous PPAR-δ receptor. Interestingly, gene
expression of 14-3-3epsilon was also induced by PPAR-δ through a PPRE-independent mechanism
and an interaction between PPAR-δ and CCAAT/enhancer binding protein
(C/EBP) [27].


## 2.5. Endothelial Activation

When exposed to
proinflammatory stimuli such as tumor necrosis factor (TNF) or
lipopolysaccharide (LPS), normally quiescent endothelium undergoes a phenotypic
change, which is characterized by induction of proinflammatory and procoagulant
factors such as adhesion molecules and tissue factor. Such a phenotypic
conversion, referred to as EC activation, is implicated in a number of
proinflammatory diseases including atherosclerosis and thrombosis [28]. PPAR-α and -γ have been previously shown to suppress
EC expression of proinflammatory adhesion molecules such as intercellular
adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1),
E-selectin, monocyte chemoattactant protein-1 (MCP-1), and ensuing recruitment
of leukocytes [22, 29–32]. However, there has also been evidence
that suggests a proinflammatory role of PPAR-α or -γ [33, 34].
Recent studies suggested that PPAR-δ also plays a role in inflammatory
processes and atherosclerosis. In macrophages, the PPAR-δ agonist GW0742 inhibited lipopolysaccharide (LPS)-induced expression
of proinflammatory genes, such as cyclooxygenase (COX)-2 and inducible nitric
oxide synthase (iNOS) [35]. GW0742 reduced atherosclerotic lesions and decreased
the expression of MCP-1 and ICAM-1 in the aorta of LDLR−/− mice [36, 37]. Given the beneficial effects
of PPAR-δ
agonists on lipid profiles, it is likely that PPAR-δ agonists may inhibit endothelial
activation by improving dyslipidemia. A direct anti-inflammatory effect was
also demonstrated. In EAhy926 cells, Rival et al. found that L-165041, at high
a concentration up to 100 μM, inhibited TNF-α-induced VCAM-1 and MCP-1 expressions [38]. In primary culture of human
umbilical vein ECs (HUVECs), specific agonists GW0742 and GW501516 inhibited
the TNF-α-
or interleukin-1β-induced expression of adhesion molecules and the monocyte adhesion to ECs.
PPAR-δ agonist
induced the gene expression of antioxidative enzymes, such as superoxide
dismutase-1, catalase, and thioredoxin, and it decreased reactive oxygen species
production in ECs. Unexpectedly, the anti-inflammatory effect not only
persisted but it also was further enhanced
after the decrease of PPAR-δ expression by siRNA knockdown [39]. Given the evidence that the
ligand binding caused the dissociation of the transcription repressor BCL-6
from PPAR-δ
and the subsequent association of BCL-6 with the VCAM-1 promoter region, this
seemingly paradoxical result could be plausibly interpreted with the previously
proposed PPAR-δ/Bcl-6
interaction action mode: the synthetic ligand binds to PPAR-δ and recruits
the coactivators to replace the corepressors such as Bcl-6. The released
corepressors relocate to repress the transcription of proinflammatory genes
such as VCAM-1 and E-selectin and thus contribute to the vascular protection.Ghosh et al. recently showed that the metabolism of
endocannabinoids by the endothelial COX-2 coupled to the prostacyclin synthase
activates PPAR-δ,
which negatively regulates the expression of tissue factor (TF), the primary
initiator of blood coagulation [40]. As COX-2 inhibitors
suppressed PPAR-δ
activity and induced TF expression, these results may help explaining the
prothrombotic adverse effects of the cox-2 inhibitors rofecoxib and valdecoxib [41].


## 2.6. Angiogenesis

Angiogenesis is referred to
the formation of new capillaries from the existing blood vessels. Physiological
angiogenesis is involved in wound healing and aerobic exercise, whereas
pathological or therapeutic angiogenesis is implicated in cardiovascular
diseases, diabetic complications, inflammatory diseases, and cancers [42]. Recent studies
have also linked
metabolic homeostasis to angiogenesis and further interrogate the potential
effects of PPARs on the angiogenic process
[43, 44]. An earlier study
demonstrated that the PPAR-δ agonist GW501516 dose-dependently
stimulates HUVEC proliferation with increased mRNA expression of vascular
endothelial growth factor αand its receptor flt-1 [45]. Later, GW501516 was shown to
promote endothelial tube formation on an extracellular matrigel, EC outgrowth
in a murine aortic ring model, and increased angiogenesis in the implanted matrigel
plug assay in vivo through a PPAR-δ- and VEGF-dependent manner [22]. Most recently, Gaudel
et al. found that
treatment with GW0742 or muscle-specific overexpression of PPAR-δ promoted
angiogenesis in mouse skeletal muscle [46]. Besides, arising from
sprouts on existing vessels, vessels also arise from endothelial
progenitorcells (EPCs), a process referred to as vasculogenesis [47]. Culminating evidence further
suggests that
circulating EPCs is
capable of stimulating angiogenesis [48]. A recent study showed that
the proangiogenic effects of human EPCs are in part dependent on the
biosynthesis and release of PGI2, and subsequent
activation of PPAR-δ
[48, 49]. Furthermore,
functional genomic approach provided evidence that silencing of PPAR-δ in
the tumor microenvironment impairs angiogenesis and tumor growth, identifying
PPAR-δ as one of a
few hub nodes in the angiogenic network
[49, 50]. Up-to-date results have been largely consistent and pointed
toward a proangiogenic activity of PPAR-δ. In corroboration with this,
Müller-Brüsselbach et al. found that the growth of syngeneic PPAR-δ wild-type tumors was
impaired in PPAR-δ−/−  mice, concomitant with a
reduced blood flow and hyperplastic vascular structures, suggesting that PPAR-δ maybe required in tumor ECs for
the formation of functionally mature vessels [51]. Nevertheless, a full understanding of the specific
roles of PPAR-δ
in specific scenarios of angiogenesis will be imperative for a safe and
rational therapeutic strategy.


## 2.7. Smooth Muscle Cells

PPAR-δ
is expressed in SMCs and is induced in response to platelet-derived growth
factor (PDGF) in SMCs, which involved the phosphatidylinositol 3-kinase/Akt signaling pathway.
Initial study using overexpression showed that PPAR-δ increased SMC proliferation, indicating
a proliferation-promoting effect in SMCs [52]. However, L-165041, a selective PPAR-δ agonist, inhibited SMC proliferation and migration
via inhibition of the PDGF-induced expression of cyclin D1 and cyclin-dependent
kinase (CDK) 4 and cell cycle progression  [53]. In SMCs, GW501516 increased
the expression of transforming growth factor-β1 (TGF-β1) and the effect seemed to depend on
endogenous PPAR-δ. Subsequently, TGF-β1 was likely
responsible for suppression of the IL1β-induced expression of MCP-1 and
proliferation of SMCs [54]. In rats, administration of
L-165041 decreased neointima formation in balloon-injured carotid arteries [53]. Thus, synthetic PPAR-δ agonists appear to have antiproliferative and
anti-inflammatory properties in SMCs. This is consistent with previous
reports that adenovirus-mediated gene transfer of prostacyclin synthase, which
produces the endogenous PPAR-δ ligand PGI2, inhibited SMCs
proliferation and intimal hyperplasia [55–57].


## 2.8. Macrophages

Macrophage
infiltration in vessel wall is known to play important role in
atherogenesis. PPAR-δ is expressed in macrophages. During past years, the role of
PPAR-δ in macrophage biology has been extensively studied. However,
existing results still remain controversial. Oliver Jr. et al. showed
that GW501516 in a human monocytic cell line increased the expression of ATP-binding
cassette A1 (ABCA1) and Apo AI-mediated cholesterol efflux [58]. However, Vosper et
al. found that a different PPAR-δ agonist, compound F,
increased
lipid
accumulation in both human primary macrophages and THP-1 cells. Compound F
induced the expression of genes involved in lipid uptake and storage such as class
A and B scavenger receptors (SRA and CD36) but repressed key genes involved in
lipid metabolism and efflux such as Apo E and cholesterol 27-hydroxylase [59]. In mouse macrophages, neither
genetic loss of PPAR-δ nor treatment with the PPAR-δ agonists GW501516 or
GW0742 significantly influenced cholesterol efflux or accumulation
[36, 60]. Beside the effects on lipid trafficking, PPARδ agonists
have a potent anti-inflammatory effect in macrophages. Welch et al. first
demonstrated that, in mouse peritoneal macrophages, PPAR-δ agonist
GW0742 inhibited LPS-induced expression of COX-2 and iNOS
[32, 35]. Recently, Barish et al. found that GW501516 in mouse
macrophages suppressed the gene induction of MCP-1, -3, -5 by IL-1, interferon-γ (IFN-γ), and phorbol
ester. The agonist treatment also inhibited transendothelial migration of THP-1
cells [61]. The anti-inflammatory
effects of the agonist was lost in the receptor-deficient macrophages [60]. However, in other cell types
such as epithelial cells, eosinophils, neutrophils, and lymphocytes, the PPAR-δ agonist was
ineffective in inhibiting inflammatory processes, indicating that the effect is
cell-type-specific [62].


## 2.9. Atherosclerosis

To date, several studies have been reported regarding the
roles of PPAR-δ in atherosclerosis in different mouse models with different
approaches. Lee et al. transplanted PPAR-δ-null bone marrow progenitor
cells into LDL receptor-null (LDLR−/−) mice. Unexpectedly, the
adoptive transfer of PPAR-δ-null macrophages led to a
less severe atherosclerosis, suggesting that endogenous PPAR-δ maybe proatherogenic. Although overexpression or
deletion of PPAR-δ
in macrophages suggested that PPAR-δ is proinflammatory, the agonist
GW501516 decreased MCP-1, seemingly having an opposite effect [60]. To reconcile this contradiction, they postulated an
unconventional ligand-dependent transcriptional mechanism, which switches PPAR-δ between a “proinflammatory” and “anti-inflammatory”: in the
absence of ligand, PPAR-δ sequesters
a transcriptional repressor of inflammatory responses such as Bcl-6, permitting
induction of proinflammatory genes; in the presence of ligand, PPAR-δ releases the repressor, which is then free to exert its
anti-inflammatory effects. Following this loss-of-function approach, two
independent studies examined the effect of the PPAR-δ agonist GW0742 on atherogenesis in high fat- and
cholesterol-fed LDLR−/− mice and yielded divergent results. In the
first study, Li et al. found that GW7842 decreased gene expression of
proinflammatory cytokines and adhesion molecules within atherosclerotic lesions
but failed to alter the progression of atherosclerosis after 14 weeks of
treatment (5 mg−1 kg−1 day−1). In another one, Graham et al. used female
LDLR−/− mice fed with a diet that
induced moderate levels of hypercholesterolemia and observed that GW0742 reduced the lesion
size at a higher dose (60 mg−1 kg−1 day−1)
after 10 weeks of
treatment  [37]. Discrepancy between
these two studies may be caused by differences in the levels of
hypercholesterolemia and different drug doses used. However, the
anti-inflammatory effect was generally consistent in both studies, regardless
the different effects on the lesion sizes. It is likely that the
anti-inflammatory properties of the PPAR-δ agonists on the vessel wall
per se are not
sufficient to attenuate the progression of atherosclerotic lesions if it is not
corroborated by an efficient improvement of metabolic abnormalities. This
notion is supported by the data from recently published results. Most recently,
GW501516, which has a potent lipid-modifying capacity, has also been
demonstrated to have a clear antiatherosclerotic property in apoE−/− mice. Barish et al. showed that GW501516 significantly reduced
atherosclerotic lesions with an increase in HDL level and a reduced expression
of chemokines in the aorta and in macrophages [61]. Furthermore, in a model of angiotensin II-accelerated atherosclerosis (LDLR−/− mice), Takata et al. confirmed the atheroprotective effect of GW0742
[63]. After 4 weeks of treatment, GW0742 at both doses (1 and 10 mg−1 kg−1 day−1) significantly inhibited the
Ang II induction of atherosclerosis without altering blood pressure. This
beneficial effect was likely mediated via the potent anti-inflammatory property
since GW0742 increased vascular expression of Bcl-6, the regulators of G protein-coupled
signaling (RGS4 and 5) in the artery and suppressed Ang II-induced activation
of p38 and ERK in macrophages. However, the metabolic effect of GW0742 may also
have contributed to the atheroprotective outcome because GW0742 significantly
reduced plasma levels of insulin, glucose, leptin, and decreased triglycerides [63]. Overall, studies in mouse models suggest that PPAR-δ may have an attractive therapeutic target for the treatment
of atherosclerosis.


## 2.10. Cardiovascular Risk Factors

In addition to the direct effects on the vessel wall, PPAR-δ also has profound effects on various
metabolic parameters associated with cardiovascular diseases such as obesity,
dyslipidemia, and insulin resistance.


## 2.11. Obesity

PPAR-δ deficiency causes embryonic lethality due to
a placental defect.
Some surviving PPAR-δ null mice had reduced fat mass [3], indicating a role of PPAR-δ in adipogenesis. Transgenic
mice specifically expressing VP16-PPAR-δ, a constitutively active
receptor, in adipose tissue had a reduced body weight, fat mass, and lower
levels of circulating free fatty acids and triglycerides [64]. These animals were less susceptible to high-fat diet-induced
obesity. In contrast, PPAR-δ null mice were more prone to weight gain on a high-fat diet.
Similarly, GW501516
ameliorated diet-induced obesity [65]. It has been known that PPAR-δ activates genes
involved in
fatty acid
oxidation and energy dissipation, such as carnitine
palmitoyltransferase 1 (CPT1), acyl-CoA oxidase (AOX), and long chain acyl-CoA
dehydrogenase (LCAD), uncoupling proteins. In addition to the direct effects on
obesity, PPAR-δ
agonists may also have regulatory effects on adipokine profile. For example,
administration of L-165041 in rats increased the expression of visfatin and
adiponectin, which are known to improve insulin sensitivity and are
vasoprotective, but decreased the production of resistin in visceral adipose
tissue [66].


## 2.12. Dyslipidemia

Increased levels of
LDL and triglycerides and decreased HDL in plasma are independent risk factors
for atherosclerosis and associated with metabolic syndrome as well. Recent
studies have demonstrated that activation of PPAR-δ may modify lipid profile in animal
models as well as in human. Oliver Jr. et al. first reported that GW501516
significantly improved dyslipidemia in obese primates with an increase in HDL
and a decrease in LDL cholesterol and triglycerides [58]. The beneficial effect of
GW0742 and L-165041
on HDL level was also observed in obese and nonobese mice
[67, 68]. In addition to enhancing
fatty acid oxidation in muscles, PPAR-δ agonists upregulated expression of
ABCA1 in several types of cells, which may lead to an increase in HDL and
cholesterol reverse transport. In intestinal cells, it also inhibited gene
expression of Niemann-Pick
C1-like 1, the key molecule for cholesterol absorption [67]. However, the precise mechanisms underlying these
lipid-modifying effects still remain to be elucidated. In a small number of
healthy human volunteers, GW501516 has been reported to increase HDL
cholesterol level and improved the triglycerides clearance [69].


## 2.13. Insulin Resistance and Glucose Homeostasis

It has been previously
known that, in obese primates, GW501516 declined fasting insulin level [58]. GW501516 treated ob/ob mice also
showed a significantly improved glucose tolerance and with a lower postprandial
levels of plasma glucose and insulin [65]. In cultured myotubes, PPAR-δ agonists were found to directly stimulate
glucose uptake independent of insulin action. The agonist-stimulated glucose
uptake in myotubes appeared to require AMP-activated protein kinase (AMPK) but
not PPAR-δ
[70, 71]. However, GW501516 had no acute effect on
glucose transport in rat skeletal muscles [72]. Recently, Lee et al. showed that PPAR-δ−/− mice were glucose intolerant. Euglycemic
hyperinsulinemic clamp experiments showed that GW501516 improved insulin
sensitivity in multiple tissues including hepatic and peripheral tissues. The agonist suppresses hepatic
glucose output and increases glucose disposal [73]. Gene array analysis suggested that PPAR-δ might ameliorate hyperglycemia by increasing glucose flux
through the pentose phosphate pathway, which is known to enhance de novo fatty
acid synthesis. Thus, it could be a
concern whether PPAR-δ improves hyperglycemia at
the cost of exacerbation of hepatosteatosis, a problem commonly associated with
metabolic syndrome and diabetes. However, recent studies demonstrated that
PPAR-δ also had a beneficial effect on hepatosteatosis. In a
diet-induced mouse model of nonalcoholic steaohepatitis, GW501516 reduced
triglycerides accumulation in the livers. In a most recent study, it was
demonstrated that PPAR-δ suppressed hepatic
lipogenesis via the induction of insulin-induced gene-1 (insig-1) and the
inhibition of lipogenic sterol-regulatory element binding prortein-1 (SREBP-1)
activation. In obese diabetic mice, hepatic overexpression of PPAR-δ ameliorated hepatosteatosis [74]. Since Insig-1 is a critical regulator of lipid homeostasis,
identification of the insig-1 as a target gene of PPAR-δ may also facilitate our understanding of the profound
effects of PPAR-δ activation on adipogenesis
and lipid metabolism
[75, 76]. Recently, in a double-blind and randomized study, PPAR-δ agonist (10 mg o.d. GW501516) was given to a small number of
healthy overweight subjects. The results showed that treatment with GW501516
for 2 weeks significantly reduced liver fat content by 20% without increasing
oxidative stress [77].


## 2.14. Gene Polymorphisms

Skogsberg et al.
initially described 4 polymorphisms:−409C/T (in the promoter region), +73C/T
(exon 1), +255A/G (exon 3), and +294T/C (exon 4). The +294T/C polymorphism
showed a significant association with a metabolic trait. The homozygotes for
the C allele had a higher plasma LDL and a tendency toward higher risk of CHD compared with
homozygous carriers of the T-allele
[78, 79]. In addition, there is a highly
significant association between the rare C allele and lower plasma HDL
concentrations in the female patients with mixed hyperlipidemia. Associations were also found for the
C-allele with coronary heart disease and body mass
index (BMI)
[80, 81]. Chen et al.
also demonstrated that, in “lipoprotein and coronary atherosclerosis study”
(LCAS) subjects, the PPAR-δ SNPs were strongly associated with the dyslipidemia, the responses
to statin, and the atherosclerotic lesions [82]. Vänttinen et
al. investigated the effects of the PPAR-δ gene SNPs on tissue
glucose uptake and suggested that the SNPs regulate insulin sensitivity
primarily in skeletal muscles [83]. The PPAR-δ SNPs were genotyped in
type II diabetes subjects and normal control. Although no significant
association was detected with the risk of type II diabetes, several SNPs were
associated with fasting plasma glucose and BMI [84]. In addition,
the association has been found between the PPAR-δ SNPs and metabolic
syndrome, and the association was influenced by dietary fat intake [85].


## 3. Conclusions

During the last few years, rapid progress has
been made with regards to the roles of PPAR-δ in vascular biology.
Emerging evidence supports the notion that activation of PPAR-δ may have profound effects on vascular homeostasis and
coronary artery diseases. These include both the direct actions in the vessel
wall and the beneficial effects on central metabolic pathways (Figure 1).
Importantly, PPAR-δ agonists have an
unprecedented role in raising HDL level in animals. In addition to a previously
reported function of PPAR-δ in increasing oxidative
muscle fibers and running endurance, a most recent study revealed that AMPK and
PPAR-δ pathways have synergistic effects in terms of
exercise-enhancing capacity [86]. The outcome of currently ongoing clinical trial is awaited
to prove its clinical efficacy in the treatment of dyslipidemia. Despite the
studies in rodent models point to a vascular-protective effect for PPAR-δ agonists, their efficacies in human coronary artery diseases remain to be
clarified. With regards to the effects of PPAR-δ on tumor angiogenesis and
the unsettled role in carcinogenesis, safety issues also call for attention
[87, 88]. Clearly, further studies are warranted to explore the roles
of PPAR-δ in cardiovascular
pathophysiology and to exploit this lipid-sensing receptor as a therapeutic
target for metabolic syndrome and its cardiovascular
complications.Figure 1
Activation of PPAR-δ may have profound effects on vascular homeostasis and
coronary artery diseases. These include both the direct actions in the vessel
wall and the indirect effects on multiple cardiovascular risk factors. PPAR-δ gene polymorphisms are also linked to cardiovascular
diseases.



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*Source: 164163-2009-01-06.xml*