# PPAR-α Ligands as Potential Therapeutic Agents for  Wet Age-Related Macular Degeneration

Authors: Marisol del V Cano; Peter L. Gehlbach
Journal: PPAR Research
 (2008)

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## Abstract

The peroxisome proliferator-activated receptors (PPAR's) are members of the steroid/thyroid nuclear receptor, superfamily of transcription factors. There are currently three known PPAR subtypes,α, β, and γ. The PPARs are now recognized participants in a number
of biological pathways some of which are implicated in the
pathogenesis of age-related macular degeneration (AMD). These
include immune modulation, lipid regulation, and
oxidant/antioxidant pathways important to the onset and
progression of “dry” AMD, and vascular endothelial
growth factor (VEGF) mediated pathways that stimulate choroidal
neovascularization (CNV), characteristic of “wet” AMD.
PPAR-α is found in retina and also on vascular cells
important to formation of CNV.  At this time, however, relatively
little is known about potential contributions of PPAR-α to the pathogenesis of dry and wet AMD. This review examines current literature for potential roles of PPAR-α in the pathogenesis and potential treatment of AMD with emphasis on prevention and treatment of wet AMD.


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## Body

## 1. Introduction

Age-related macular degeneration (AMD) is the leading cause of new blindness in the Western World and is
currently responsible for more than half of all legal blindness in the United States. There are
approximately 8 million people in the U.S. with early or intermediate stage AMD. Approximately one million of
these people will develop advanced disease within the next five
years [1–5]. Currently AMD
is estimated to affect about 50 million people worldwide. With aging of the
population this number is expected to double by the year 2020. Strategic approaches to management of AMD
include delaying onset and progression of nonneovascular “(dry)” disease; preventing
conversion from dry to wet disease and treatment of wet disease.While specific antioxidant vitamin
formulations are now known to delay progression of intermediate disease,
current treatment of AMD focuses largely on providing therapeutic intervention
following the progression of intermediate “(dry)” disease to late stage “(wet)”
disease. The neovascular (“wet” or
“exudative”) form of AMD can lead to rapid visual decline and accounts for
nearly 90% of vision lost. It is characterized by development of
pathologic choroidal neovascularization (CNV). Early strategies to ablate CNV
used thermal laser or photodynamic therapy. These are now less frequently used
as treatments that antagonize the effects of vascular endothelial growth factor
(VEGF), continue to enhance efficacy, and improve outcomes. Currently pegaptanib,
ranibizumab, and bevacizumab are
considered relatively safe and achieve therapeutic effects that may include inhibition/regression
of CNV, decreased vascular leakage, absorption of subretinal fluid, and
improved vision [6–10].The peroxisome proliferator-activated receptors (PPAR's)
are not, at the present time, known as direct treatment targets in the
management of AMD. Each represents a separate nuclear receptor of the steroid super-family
of ligand activated transcription factors that induce steroid hormones, thyroid
hormones, vitamin D, and retinoid acid receptor [11].  PPAR's comprise a family of three
ligand-activated transcription factors (α, β, and γ) that
are characterized by distinct function, ligand specificity, and tissue
distribution. The PPAR transcription factors regulate transcription of many
genes involved in differentiation, proliferation, and apoptosis, in a variety
of cell types.  During gene expression the
PPAR forms a heterodimer receptor complex with the 9-cis-retinoic acid receptor
(RXR). The PPAR/RXR heterodimer is associated with a multiprotein corepressor.  When a ligand or agonist binds to the
receptor, the corepressor complex dissociates. The heterodimer receptor then
binds with peroxisome proliferator response elements on the promoter domain of
target genes to stimulate transcription [12].Three distinct PPAR's had been identified in mammals,
PPAR-α, PPAR-γ, and PPAR-δ (also referred to as PPAR β). The
first PPAR entity identified was PPAR-α agonist, which has multiple functions that result in
an improved lipid profile, increasing high density lipoprotein cholesterol
(HDL-C), decreasing triglycerides and free fatty acids, and shifting low
density lipoprotein cholesterol (LDL-C) to
larger less atherogenic particles.  Each
of these improvements in the lipid profile is potentially beneficial and may in
theory lead to delay in AMD onset and progression thereby avoiding late stage
or “wet” disease.PPAR-α is transcribed from chromosome 22q12-13.1, is primarily
expressed in tissues with elevated mitochondrial and peroxisomal fatty acids β-oxidation
rates, such as liver, heart muscle, kidney, skeletal muscle, retina, and brown
fat [13–15] and may have a potential role in oxidant/antioxidant pathways now
strongly implicated in the pathogenesis of dry AMD.  PPAR-α is also present in cells of the arterial wall
associated with smooth muscle cells [16] and endothelial cells [17] and is
found in monocytes and macrophages [18] that participate in CNV formation,
characteristic of wet AMD [19]. The PPAR's are activated by a number of ligands
including eicosanoids and fatty acids. In addition, synthetic antidiabetic and lipid lowering fibrates have
been shown to activate PPAR-γ and PPAR-α, respectively. PPAR-α is the main target of fibrate drugs, a class of
amphipathic carboxylic acids (gemfibrozil, fenofibrate, clofibrate) used in
managing elevated triglycerides and cholesterol. PPAR-γ is highly expressed in adipose tissues and is
a key mediator of adipogenesis [20, 21] and glucose homeostasis [22]. Little is
known about the PPAR-δ which is expressed ubiquitously and has now been linked
to obesity.


## 2. PPARs in the Vasculature

In addition to well established roles for the PPAR's
in metabolic pathways, recent work suggests that the PPAR's may be involved in
vascular regulation.  Several groups have identified PPAR-γ and PPAR-α expression in monocytes/macrophages, vascular smooth
muscle cells, and endothelial cells [16–18]. In the endothelium, PPAR-γ has
been identified by PCR reaction [23], western blot and immunoprecipitation.
PPAR-α has been demonstrated in the vascular endothelium by
immunohistochemical technique [24]. While PPAR-γ has been widely studied for its antiangiogenic properties
[25], recent studies now indicate that PPAR-α may have antiangiogenic properties as well [26, 27],
a finding with potential therapeutic implications for wet AMD. PPAR-α agonists
have recently been shown to inhibit expression of VEGF receptor 2 (VEGFR2)
upregulation in neovascularization [26]. Varet et al. have demonstrated that
fenofibrate, a PPAR-α ligand, inhibits angiogenesis in vitro and in vivo.  They have also shown that fenofibrate reduces
endothelial cell growth rate, endothelial cell mediated wound repair, and capillary tube formation.  Interestingly
fenofibrate has been shown to inhibit bFGF-induced angiogenesis in vivo [27]. Simultaneous inhibition
of VEGFR2, bFGF, and VEGF would in theory have a profound effect on pathological angiogenesis in the eye.PPAR-α and PPAR-γ are associated with anti-inflammatory and
antioxidant activity [28–30] and have antiatherogenic effects [31]. Each
of these pathways is considered important to the onset and progression of early
AMD and to development of late choroidal neovascularization. PPAR-α
activators inhibit expression of vascular cell adhesion molecules on the
endothelium that are important for the development of new blood vessels and for
atherogenesis [32]. Experimental evidence suggests that the PPAR activators prevent
in vitro vascular muscle cell
growth [33], limit inflammatory responses [16], and are proapoptotic indicating
a potential role in vascular remodeling [34]. Such activity could theoretically
inhibit the transition from dry to wet AMD. PPAR-α agonists also inhibit interleukin-1-induced
production of interleukin-6 and prostaglandins [16]. Moreover, Delerive et al.
have demonstrated prolonged inflammatory responses and increased interleukin-6
production in aortic explants of PPAR-α deficient mice [35] underscoring the
anti-inflammatory potential of PPAR-α.


## 3. PPAR-α in Angiogenesis

Pathological angiogenesis leading to choroidal
neovascularization is pathognomonic of “wet” AMD. Angiogenesis is the formation
of new blood vessels from preexisting vessels and involves endothelial cell
proliferation, migration, and organization into new capillary tubes.
Pathological angiogenesis is integral to a number of prevalent ocular diseases
characterized by the development of ocular neovascularization including but not
limited to wet AMD, diabetic retinopathy, corneal neovascularization, the
occlusive retinal vasculopathies, and retinopathy of prematurity. Inhibitors of
ocular angiogenesis therefore have broad therapeutic implications for patients
with these diseases.Varet et al. demonstrated inhibition of angiogenesis
by the PPAR-α ligand fenofibrate [27]. The antiangiogenic
properties exhibited were characterized by a dose-dependent decrease in
endothelial cell proliferation and apoptosis.
Fenofibrates also reduced endothelial cell migration in vitro and capillary tube formation
in a matrigel assay. Meissner et al. have also reported a reduction in
endothelial cell proliferation, migration, and tube formation following
treatment with fenofibrates and also with the PPAR-α agonist
Wy14643 [26]. In further support of the evident antiangiogenic effect is the
observation that several PPAR-α agonists decrease expression of VEGF receptor 2
(VEGFR2) in human umbilical endothelial cells (HUVECs) [26].VEGFR2 is the most potent of the VEGF receptors. When
activated VEGFR2 initiates signaling that leads to endothelial cell
proliferation and also to expression of cytoprotective antiapoptotic molecules [36].
VEGFR2 is detectable only at relatively low levels in the adult vasculature; it
is markedly up regulated by blood vessels during chronic inflammation, hypoxia,
tumor growth, and wound repair. VEGFR2 and VEGF expression both increase as
part of the angiogenic response and this coordinate response is observed in wet
AMD as well as other ocular diseases characterized by pathological
neovascularization [37, 38]. VEGF has been identified in fibroblastic cells and
transdifferentiated RPE cells in surgically excised choroidal neovascular
membranes (CNV) [39, 40]. VEGF expression is also increased in macular RPE
cells in patients with AMD [41].
Vitreous VEGF levels are significantly higher in AMD patients with CNV
as compared to healthy controls [42]. VEGF production is also increased in RPE
cells, retinal vascular endothelial cells, retinal pericytes [43–45], and Muller cells [46]. The
endothelial cells of the retinal vasculature possess numerous high-affinity VEGF receptors.PPAR-α agonists have been associated with a reduction in
VEGF levels in OVCAR-3 tumor as well as in DISS-derived ascites [47]. They also
reduce microvessel density in these tumors. Other studies have similarly
demonstrated that a reduction in PPAR-α message and activity is associated with hypoxia [48].
Hypoxia-induced VEGF expression contributes to choroidal and retinal
neovascularization. The relative significance of the effect of PPAR-α on
VEGFR2 and VEGF expression in the setting of AMD is not yet known.


## 4. PPAR-α and Wet AMD

Fenofibrates and other PPAR-α
agonists are reported to decrease expression of VEGF and VEGFR2 that are
central to the VEGF/VEGFR signaling cascade and important to the development of
pathological CNV in AMD. Growth of experimentally induced CNV, via laser
rupture of Bruch's membrane in a rat model, is inhibited by intravitreous
treatment with a PPAR-γ agonist [49]. At the time of this writing, similar
data has not been reported for PPAR-α. Evaluation of this question is however supported by
evidence of PPAR-α reduction of VEGFR2 expression in endothelial cells
[26] and reported decreases in tissue VEGF levels [47]. PPAR-α
activators have also been shown to limit the expression of vascular cell
adhesion molecules in the endothelium, an early step in atherogenesis and an
important step in the development of new blood vessels [32]. Inhibition of CNV initiation
and early progression of CNV are therefore theoretical benefits of PPAR-α agonist
treatment. Described proapoptotic effects also suggest therapeutic roles in
early CNV development or late regression of CNV [34].With reports that the PPAR's limit inflammatory as
well as oxidative responses and improve lipid profiles [16, 28, 29, 35, 37, 50], it is
tempting to speculate on a potential role in delaying onset and progression of nonneovascular “(dry)” disease,
thereby potentially preventing latter “wet” stages of disease. There is a
substantial literature linking oxidative damage to dry AMD pathogenesis [51].
PPAR-α could theoretically inhibit AMD progression via effects on oxidative pathways. It has been
previously reported that PPAR-α activation induces the expression and activation of
antioxidant enzymes, such as super oxide dismutase and glutathione peroxidase
[29]. It has also been reported that PPAR-α agonists are neuroprotective in the CNS, and that
this neuroprotection has been associated with a decrease in cerebral oxidative
stress. Consumption of direct acting antioxidants to provide protection to the
retina and the RPE is supported by the AREDS clinical trial that has added
antioxidant formulation to the routine care of dry AMD. Whether the antioxidant
effects of PPAR-α activation are comparable to those of AREDS formulation is not known.Because fenofibrates are orally administered and have
an established safety profile in the treatment of atherosclerosis,
investigations pertaining to the impact of oral therapy on oxidative stress,
VEFGR2, VEGF, and CNV growth are important. It is also important to consider
examining for potential beneficial effects on onset and progression of nonneovascular “(dry)” disease
and conversion from dry to wet disease. These and other factors support a hypothesis
that asks whether PPAR-α may play a therapeutic role in either prevention or
treatment of wet AMD.


## 5. Summary

AMD remains the leading cause of new blindness
in people over 65 years of age and is the leading cause of new blindness in the
Western World. The conversion of dry AMD to wet AMD is associated with most of
the attendant visual decline. Currently a variety of antiangiogenic treatments
directed at halting CNV growth and leakage are the mainstay of therapy. The
most frequently injected agent ranibizumab (Lucentis) results in stabilization of
visual acuity at the pretreatment level for a majority of patients and results
in improvement of visual acuity by 3 or more lines in about 1/3 of those
treated. The therapy does not however restore visual acuity to normal levels in
the majority of those treated. Moreover, therapy with ranibizumab and other
currently available VEGF antagonists requires frequent intravitreous injections
and is associated with significant expense, some risk, and for most, incomplete
recovery of vision.An oral therapy with an established safety
profile that favorably modified VEGF/VEGFR signaling and increased the
antioxidant capacity could significantly impact the therapy of wet AMD. Taken
collectively, the PPAR's demonstrate favorable biological activity in
pathophysiological pathways relevant to the onset and progression of nonneovascular and neovascular age-related
macular degeneration. The relative importance of the PPAR-α pathway in AMD is
not yet known. There is, however, sufficient preliminary evidence to support
further study of a potential role for PPAR-α pathway modulation as an adjuvant
or primary treatment in AMD.



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