# PPAR-γ, Microglial Cells, and Ocular Inflammation:  New Venues for Potential Therapeutic Approaches

Authors: Fiorella Malchiodi-Albedi; Andrea Matteucci; Antonietta Bernardo; Luisa Minghetti
Journal: PPAR Research
 (2008-02-20)

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## Abstract

The last decade has witnessed an increasing interest for the role played by the peroxisome proliferator-activated receptor-γ (PPAR-γ) in controlling inflammation in peripheral organs as well as in the brain. Activation of PPAR-γ has been shown to control the response of microglial cells, the main macrophage population found in brain parenchyma, and limit the inflammation. The anti-inflammatory capacity of PPAR-γ agonists  has led to the hypothesis that PPAR-γ might be targeted to modulate  degenerative brain diseases in which inflammation has been increasingly recognized as a significant component. Recent experimental evidence suggests  that PPAR-γ agonists could be exploited to treat ocular diseases such as diabetic retinopathy, age-related macular degeneration, autoimmune uveitis, and optic neuritis where inflammation  has relevant role. Additional  PPAR-γ agonist beneficial effects could involve amelioration of retinal microcirculation and inhibition of neovascularization. However,  PPAR-γ activation could, in some instances,  aggravate the ocular pathology, for example, by increasing the synthesis of vascular endothelial growth factor, a proangiogenic factor that could trigger a vicious circle and further deteriorate retinal perfusion. The development of new in vivo and in vitro models to study ocular inflammation and how to modulate for the eye benefit will be instrumental for the search of  effective therapies.


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## Body

## 1. Introduction

The peroxisome proliferator-activated receptor-γ
(PPAR-γ) is a ligand-inducible transcription factor that
belongs to a large superfamily comprising the nuclear receptors for steroids,
thyroid hormones, and retinoids. The PPAR-γ and the two closely related PPAR-α and PPAR-δ
(also known as β,  NUC-1, or FAAR) are activated by naturally occurring fatty acids and act as sensors
that regulate whole body metabolism in response to the dietary intake by
controlling lipid and carbohydrate metabolism and lipid storage [1]. All three
PPARs,  once agonist-activated, form heterodimers with retinoic X receptors and
regulate specific target gene transcription by binding to specific DNA regions
(peroxisome proliferator response elements,  PPREs) or by a mechanism
independent of PPRE binding,  termed transrepression,  which begins to be
unravelled [2].Because of
their role in the regulation of genes involved in lipid and carbohydrate
metabolism,  PPARs deeply affect lipid homeostasis and insulin sensitivity [3, 4]. The serum glucose lowering activity
of PPAR-γ has lead to the development of
specific PPAR-γ agonists  for the treatment of type-2 diabetes and the
metabolic syndrome [5]. PPAR-γ agonists such as thiazolidinediones (TZD),
including pioglitazone (Actos) and rosiglitazone (Avandia),
increase insulin sensitivity thereby improving glycaemic control,  but also
modify lipidemic profile and decrease blood pressure [6–9]. On the other hand,  fibrates,  which are PPAR-α
agonists,  are prevalently antilipidemic drugs, and therapeutic benefits of PPAR-α and PPAR-γ activations,
which only are minimally overlapping, have
generated interest in dual agonists that target both receptors,  thus offering
improved insulin sensitivity and lipidemic control in the same molecule [10, 11]. This would provide a therapeutic
tool against diabetes and the metabolic syndrome.The three PPARs share a high homology,  but differ for
tissue distribution and ligand specificity. PPAR-α is mainly expressed in
tissues with high catabolic rates of fatty acids,  such as the liver,  muscle, and heart,  whereas PPAR-δ shows a much wider distribution. PPAR-γ is highly expressed in adipose tissue,  where
it plays a central role in the regulation of adipocyte differentiation [12],  and
in cells of the immune system,  including lymphocytes and macrophages. In
peripheral monocytes,  PPAR-γ expression is
induced during the process of extravasation from blood vessels into the
tissues,  and in the course of activation by pro-inflammatory
stimuli,  suggesting that PPAR-γ is important
for promoting monocyte-macrophage differentiation and activation and,  thus,
controlling inflammation [13–16]. As for macrophages of peripheral tissues,
PPAR-γ regulates the activation of microglial cells,
the main macrophage population found in brain parenchyma,  and increasing evidence indicates that PPAR-γ might
modulate brain inflammation and neurodegeneration [17]  and be exploited as
valuable therapeutic target in neurological diseases [18]. Indeed,  brain
inflammation is increasingly viewed as a target for treating neurological
diseases,  not only in classical infectious and immune-mediate disorders such as
meningitis or multiple sclerosis,  but also in stroke,  trauma, and
neurodegenerative diseases that were not originally considered to be
inflammatory [19, 20].In a
similar way,  inflammation could represent an important target to treat ocular
diseases. In the study of ophthalmology,  the classical subdivision of pathology
textbooks in metabolic,  inflammatory,  hemodynamic, and degenerative
disorders appears artificial and does not reflect the complexity of conditions,
where inflammation,  dysmetabolic and hemodynamic disorders, and
neurodegeneration often conspire to the development of diseases. Paradigmatic
example is diabetic retinopathy (DR),  where a metabolic derangement
(hyperglycemia) triggers a pathologic pathway,  characterized initially by
inflammation (leukostasis,  enhanced retinal vascular permeability,  Muller cell, and microglial  activation),  followed by microvasculature alterations and
ischemia (proliferative DR),  eventually leading to degeneration of neural
retina and visual loss. To this complexity,  a simplicity in the natural history
may correspond and the course of different retinal diseases may at a certain
stage converge toward a similar evolution. For example,  pathologic
neovascularization may be the same and ominous outcome of DR,  age-related
macular degeneration (AMD), and autoimmune uveitis,  conditions that are very far
from each other from the point of view of etiology.In the present article,  we will
first briefly review the immune cells that participate to the ocular
inflammation,  mainly microglia,  and the role of PPAR-γ in controlling their functions. In a second
part,  we will consider three conditions,  where inflammation has a relevant function,
microglia is involved, and the role of PPARs has been taken
into consideration: DR,  AMD, and optic neuritis (ON).


## 2. Microglial Cells and Other Cell Populations of the Immune Respose in the Eye.

Glial cells are the primary participants in the
formation of scars in response to retinal or ocular injury and diseases. In
addition,  under normal conditions,  they carry out a variety of supportive
functions for the neurons with which they are closely related. Glial cells
include astrocytes,  oligodendrocytes,  the retina-specific Muller-glial cells, and microglia,  which are considered the main immune resident cells.Retinal
microglia,  like their counterpart in the brain,  belong to the myeloid lineage
and their myeloid progenitors enter the nervous system primarily during
embryonic and fetal periods of development. During embryogenesis,  microglial
precursors migrate to the retina before retinal vascularization and differentiate
into ramified,  quiescent microglia typical of adult healthy retina. A second
population of phagocytes,  which express macrophage markers,  invades the retinal later through the developing vasculature and remains associated with the blood vessels (see below). In the adult retina,  microglia
are distributed through most of the retinal layers,  including outer plexiform
layer,  outer nuclear layer,  inner plexiform layer,  ganglion cell layer,  and
nerve fiber layer. Engraftment experiments have
shown that they display some proliferative capacity and have a slow turnover in
respect of other macrophage populations [21]. Disturbances in the number or
distribution of these cells disrupt the normal development of the eye and its
related structures. Ritter and collaborators [22] have recently reported that
myeloid progenitors migrate to vascular regions of the retina where they
differentiate into microglia and facilitate the normalization of the
vasculature,  thus underlining a main role of microglial cells in promoting and
maintaining retinal vasculature during development.Microglia show particular capacity of interaction with
retinal cells,  supervising the immune environment (see [23] and references therein). As for microglia in the brain
parenchyma,  retinal microglial cells are immunocompetent cells,  able to remove
the debris created during normal eye development or degenerative conditions by
phagocytosis and to mount an inflammatory and immune response against ocular
injury,  infection, and disease.Under normal conditions,  microglia are characterized by a
downregulated phenotype when compared to
other macrophage populations of peripheral tissues. The maintenance of
microglia in this “inhibited” state is crucial for the regulation of the immune
state of the retina,  which has to maintain tissue homeostasis while preventing
the destructive potential of inflammatory and immune response. The complexity
of the several intraocular structures on which the correct vision is dependent
renders the eye particularly vulnerable to the reactions of the immune system
against invading pathogens or ocular injury. To prevent that a defensive
reaction can transform into a threat to vision in itself,  the eye is equipped
with several regulatory mechanisms,  which contribute to make the eye an
“immune-privileged” site [24]. As
recently described for the brain parenchyma [25],  the immune privilege is not an absolute or an immutable
state,  but rather it is the result of the active interplay among specialized
cellular elements and specific microenvironment characteristics,  and it can be
overcome in several instances. Among the main features that account for the ocular immune privilegeare is the presence of
blood-ocular barriers (the blood-aqueous barrier and the blood-retinal
barrier),  which are physical barriers between
the local blood vessels and most parts of the eye itself,  and the
peculiar characteristics of the resident immune cells,  namely, microglia, which are largely dependent on the presence of immunomodulatory factors in the
aqueous humor and on the cross-talk between microglia and retinal cells.
Several “ligand-receptor-” type interactions between retinal
cells and microglia contribute to maintaining microglia in a nonactivated
state. Among these,  the glycoprotein CD200,  which in the retina is extensively
expressed in neurons and endothelial cells,  and the cognate ligand CD200L on
microglia [39],  and the
neuronal chemokine fractalkine (or CX3CL1) and its microglial receptor CX3CR1 [40].In spite of their apparent “dormant” state,  resting
microglia actively monitor the surrounding microenvironment with extremely
motile processes and protrusions,  entering in contact with other cellular
elements and sensing alterations in the nearby environment,  to which they rapidly react. Microglial  activation
comprises morphological changes,  such as cellular hypertrophy,  retraction of
processes, and expression of surface markers,  as well as functional changes, including
proliferation,  migration,  phagocytosis, and production of bioactive
molecules. Activated microglia have been described in several forms of retinal
injury or disease (see Table1),  in which they are
believed to play major roles,  either protective or detrimental. Indeed,
activated microglia can,  on one side,  remove the degenerating neurons and
contribute to re-establish tissue integrity; on the
other side, they can secrete proinflammatory cytokines such interleukin (IL)-1β,  IL-3,
IL-6,  tumour necrosis factor (TNF)-α, and interferon (IFN)-γ,  which can be toxic to neurons and
photoreceptors [41, 42] or to other cellular targets such as oligodendrocytes [43, 44].
In addition,  several of these microglial products can up-regulate
the expression of vascular cell adhesion molecules and chemokines [45–47],  thus
promoting the recruitment of lymphocytes and macrophages,  and enhancing the
immune-mediated tissue damage [23, 48]. In this context,  molecules that can
enable the control of microglial activation represent valuable tools to
counteract the detrimental effects of inflammation and immune response while
fostering those necessary for healing.Table 1
Retinal pathologies characterized by microglial activation.
PathologyReferencesDiabetic retinopathy[26–29]Glaucomatous optic nerve degeneration[30–33]Human retinitis pigmentosa[34]Age-related macular degeneration[34, 35]Retinal ischemia and reperfusion injury[36, 37]Retinal degeneration[14, 20, 38]In addition to microglia,  other cell types contribute
to the immune response in the eye. The perivascular macrophages reside outside
the blood-ocular barrier,  in the space that separates the endothelium of the
retinal capillaries and retinal pigment epithelium (RPE). Because of their anatomical
location,  they escape the tight control to which retinal microglia are
subjected and their morphology and immunophenotype are very similar to those of
macrophages of peripheral tissues. In close proximity,  but separate from
perivascular macrophages are the pericytes,  which are believed to be essential
as structural support in microcirculation. In addition,  together with
astrocytes and Muller glia,  they are considered to play a major role in
maintaining the inner blood-retinal barrier [49]. These cells,
of mesodermal origin,  are enclosed within the basal lamina on the abluminal
surface of endothelial cells and contain contractile proteins. Pericytes have
been shown to control vessel constriction and retinal blood flow [50],  and are
involved in several pathological conditions,  including hypoxia,  hypertension, and DR. Their activation,  since the very early phases of disease,  is thought
contribute to the disruption of the blood-retinal barrier [51]. Finally,  the RPE cells are important in ocular immune response and in
maintaining the eye immune privilege. These cells form a monolayer between the
neuroretina and the choroids and are the essential component of the outer
blood-retinal barrier. One of the main characteristics of RPE cells is the
presence of tight junctions at the apical side of their lateral membrane,  which
render the monolayer impermeable for macromolecules and limit access of blood
components to the retina. In addition to several important supportive
functions,  including regulation of transport of nutrients to the photoreceptors,
phagocytosis of damaged or old rod outer segments, and production of
growth factors,  RPE cells contribute to the immune and inflammatory response of
the retina by expressing major histocompatibility complex (MHC) antigens,
adhesion molecules, and a variety of cytokines,  which
may either promote or enhance immune responses or down-regulate
them [52].In addition to the cell types so far described,  a
novel population of dendritic cells has been recently reported in normal mouse
retina,  distinguishable by the cell types by the extent of specific surface
antigens and anatomical tissue location [53].


## 3. Diabetic Retinopathy

Diabetic
retinopathy (DR) is one of the most serious complications of diabetes and the
leading cause of blindness among working-age adults. DR symptoms are mostly
due to the vascular alterations that affect the retina. The early events are increased
blood flow and abnormal vessel permeability,  due to the impairment of blood-retinal
barrier. They are caused by hyperglycemia and the other metabolic consequences
of excess glucose disposal. As the disease progresses,  retinal vasculopathy
develops,  showing loss of pericytes,  smooth muscle and endothelial cell death, and microaneurysm formation,  resulting in areas of ischemia in the retina. At
this stage,  up-regulation of proangiogenic
factors in ischemic retina,  such as vascular endothelial growth factor (VEGF),
initiates a vicious circle of neovascularization (proliferative DR),  characterized
by enhanced vascular leakage and formation of new,  weak,  and prone-to-break
blood vessels,  which further deteriorates retinal perfusion,  worsens ischemia
and eventually leads to visual loss.Although
the pathogenetic cascade connecting these events is still unclear,  evidence
suggesting a role for inflammation in DR is accumulating,  supporting the
involvement of both chemical mediators and inflammatory cells in the
pathogenesis of the disease [54]. Elevated
levels of proinflammatory cytokines,  such as IL-1β,  IL-6 and IL-8, and TNF-α and
vascular cell adhesion molecule-1,  have been found in the vitreous of patients
with proliferative DR [55–57]. Increased
VEGF and IL-6 levels were detected in the aqueous humor of diabetic patients
with macular edema [58]. TNF-α was
found in epiretinal membranes of proliferative DR [59]. Data from experimental models are in line with these
observations. In streptozotocin (STZ)-induced diabetic rats,  changes in retinal
blood vessel permeability,  which characterizes the early phases of DR, are
paralleled by increase in the level of the intercellular cell adhesion
molecule-1 (ICAM-1),  which facilitates the trafficking of leukocytes [60],  and pro-inflammatory mediators,  such as TNF-α and
cyclo-oxygenase-2 (COX-2) [61, 62]. In the same
animal model,  an increased level of IL-1β has been observed and put in relation to upregulated inducible nitric oxide
synthase (iNOS) [63]. Mice
deficient in the leukocyte adhesion molecules CD18 and ICAM-1 demonstrate
significantly fewer adherent leukocytes in the retinal vasculature after
induction of diabetes with STZ [54]. According
to some authors,  VEGF could be responsible for the initiation of the
inflammatory cascade,  as its administration in vivo was found
to induce retinal ICAM-1 and endothelial NOS (eNOS) expression [64, 65]. As far as inflammatory cells are
concerned,  microglia seem to be mostly involved. Microglial  activation appears
early in the course of DR,  before the onset of overt neuronal cell death [62]. In STZ-induced diabetic rats,  hypertrophic microglia
were observed one month after the onset of diabetes [66],  with significant increase also in cell number [67]. In mice with alloxan-induced diabetes,  changes in
microglial cell morphology were the first detectable cellular modifications,
apparently preceding ganglion cell apoptosis and increase in blood barrier
permeability [68]. Treatment
of STZ-induced diabetic rats with minocycline,  a semisynthetic tetracycline
that counteracts microglial  activation,  besides decreasing the expression of proinflammatory cytokines,  decreased
caspase-3 levels [62],  suggesting
a potential neuroprotective antiapoptotic effect of inhibition of microglial  activation.Considering
the role of inflammation in the pathogenesis of DR,  it has been suggested that
PPAR-γ ligands exert therapeutic effects also as
modulators of inflammation,  besides providing glycemic control [69]. In diabetic patients,  PPAR-γ agonists reduce several markers of
inflammation,  such as serum levels of C-reactive protein,  IL-6,  monocyte
chemoattractant protein-1 (MCP-1),
plasminogen activator inhibitor-1,  soluble CD40
ligand,  and matrix metalloproteinase-9 [70–75]. In addition,  they have been shown
to induce the suppression of activated NFκB and decrease ROS generation in blood
mononuclear cells [70, 73].Modulation
of the inflammatory process has also been studied in DR in in vivo models.
In streptozotocin-induced DR,  rosiglitazone was shown to inhibit both retinal
leukostasis and retinal leakage [76]. The effect
was not accompanied by downregulation
of proinflammatory cytokines,  such as
TNF-α,  although the adhesion molecule ICAM was found reduced. Nitric Oxide (NO)
of endothelial origin regulates ocular blood flow. In the endothelial dysfunction,
which characterizes the early stages of DR,  a reduction in the bioavailability
of NO may contribute to impairment of ocular hemodynamics [77]. In bovine aortic endothelial cells,  troglitazone
increased NO production in a dose- and time-dependent manner with no
modifications in eNOS expression [78]. A study
focused on NO production in pericytes showed that PPAR-γ is constitutively expressed in retinal
pericytes and that troglitazone increases NO production and iNOS expression in
a PPAR-γ-dependent manner,
an effect which is opposite to what observed in cultured microglia [79, 80]. This study suggests that PPAR-γ agonists,  in addition to improving insulin
sensitivity,  might also improve retinal microcirculation in early DR [81]. However,  NO is a double-edged sword. Overproduction of NO by neuronal NOS is
supposed to contribute to retinal injury in ischemia [82, 83]. Thus,  although in DR early phase an increase in NO may
contribute to the improvement of retinal microcirculation,  in proliferative DR
a beneficial effect is doubtful. A further reason of concern is represented by
TZD effects on VEGF. Several in vivo and in vitro
studies have reported increased expression of VEGF in response to PPAR-γ ligands. TZDs have been found to upregulate VEGF in human vascular
muscle cells [84],  in 3T3-L1
adipocytes [85],  in cultured
cardiac myofibroblasts [86]. In bovine
aortic endothelial cells treated with troglitazone,  NO increase was accompanied
by upregulation of VEGF and
its receptor,  KDR/Flk-1 [78]. Administration
of pioglitazone [87] and
troglitazone [85] also
significantly increased plasma VEGF levels in diabetic patients. Considering
the role played by VEGF in the development and progression of DR,  caution has
been suggested in the use of PPAR-γ ligands in
patients with advanced disease [85, 87]. However,  in
partial disagreement with the results above reported,  antiangiogenic properties
of PPAR agonists have been shown both in in vitro and
in vivo
models [35, 88–90]. In neonatal mice,  where ischemia was used as a model of retinal
neovascularization,  intravitreous injection of rosiglitazone or troglitazone
inhibited development of new retinal vessels [91]. In the same study,  TZDs have been found to inhibit retinal
endothelial cell proliferation,  migration, and tube formation
in response to VEGF treatment [91]. Further
studies are therefore required to clarify the issue.


## 4. Age-Related Macular Degeneration

Age-related
macular degeneration (AMD) is the leading cause of vision loss in the elderly
in the western world. It is characterized by degeneration of the macula,  the
central area of the retina with the highest concentration of cone
photoreceptors,  responsible for visual acuity and color vision.
Histopathologically,  the early phase of AMD is characterized by formation of
drusen,  deposits of lipid and cellular debris that are found between the RPE
cells and Bruch’s membrane,  possibly as a result of RPE degeneration or,  as
recently proposed [92],
microglial infiltration and transformation in foam cells. As the disease
proceeds,  photoreceptor degeneration and,  in the most aggressive cases,
choroidal neovascularization (CNV) intervene, with growth of new blood vessels from the choroids into the
subretinal space. Two major clinical phenotypes of AMD are recognized: nonexudative
(dry type),  and exudative (wet type). The latter more frequently develops into
CNV.AMD is a complex,
multifactorial disease and both genetic
and environmental factors may contribute at some level. In the pathogenesis of
the disease,  both altered angiogenesis and inflammation play a role. The study
of pathologic angiogenesis in the retina has focused on two main factors: the
angiogenic VEGF [93, 94] and the antiangiogenic PEDF [94–96],  although a number of other
factors are implicated(for a
review,  see [97]).
It is widely agreed that in CNV an
imbalance between angiogenic and anti-angiogenic factors takes place,  but what
disrupts this delicate equilibrium is still unclear. Several
lines of evidence point to inflammation as a pathogenetic mechanism. Many risk
factors for AMD are related to inflammation, including
environmental factors,  such as smoking and low intake of omega-3 fatty acid [98, 99],  and genetic factors,  such as
polymorphisms of complement factor H [100–102]
and the chemokine receptor CX3CR,  which is expressed by microglia and
mediates migration and adhesion in response to its ligand fractalkine or CX3CL1 [103]. Increased serum levels of IL-6 and
C-reactive protein have been found to be related with progression of AMD [104]. More recently,  IL-6 receptor neutralization has shown
to lead to decrease in the expression of inflammatory mediators,  such as the
chemokine MCP-1,  the adhesion molecule ICAM-1,  and VEGF,  and to reduce
macrophage infiltration into CNV in in vivo model of
the disease [105]. Inflammatory
mediators,  such as macrophage chemoattractants and activated complement
components,  especially C3a and C5a,  are also found in drusen samples from AMD
patients [106–108]. A role
for complement in the development of the disease has been suggested [34]. In line with this hypothesis,  it has been observed
that genetic ablation of receptors for C3a or C5a reduced VEGF expression,
leukocyte recruitment,  and CNV [109].Activation of microglia and
infiltration of macrophages have been reported in
the human AMD as well as in experimental CNV [110–112]. In transgenic mice lacking CX3CR1,  microglia migrate
defectively and accumulate in the subretinal space,  evoking morphological and
pathological features similar to those observed in human AMD. In addition,
laser-induced CNV was exacerbated in these mice [92]. A controversy exists regarding the origin of activated
retinal mononuclear phagocytes, that is, whether they are
resident microglia [113, 114] or
blood-derived bone marrow macrophages [46, 115]. In support
of the latter hypothesis, it should be noted that systemic
depletion of macrophages using clodronate-filled liposomes blocked neovascularization
[116, 117]. However,
the role of macrophages is still debated,  since some studies suggest an antiangiogenic role for macrophages. For
example,  mice lacking CC chemokine ligand 2 (CCL2) or its receptor,  both
involved in chemoattraction of macrophages and/or microglia,  show drusen-like
deposits and CNV,  suggesting that macrophage recruitment may protect against
AMD [118]. In addition,  mice lacking
IL-10,  an anti-inflammatory cytokine known to control
macrophage/microglia functions,  had significantly reduced neovascularization
and increased macrophage infiltrates compared to wild type,  in a laser-induced
model of CNV. In these experiments,  prevention of macrophage entry into the eye
promoted neovascularization while direct injection of macrophages significantly
inhibited CNV.As mentioned earlier,  beside
mononuclear phagocytic cells,  RPE cells have also a role in the inflammatory
and angiogenetic process,  as a major source of VEGF and PEDF. In addition,
there is a cross-talk between RPE and macrophages. It has been shown that
macrophages in CNV are immunopositive for VEGF,  TNF-α, and IL-1β [119]. The latter factors can induce the secretion  of IL-8 and MCP-1 in RPE cells  in vitro
[120, 121]. MCP-1 is,
in turn,  involved in the recruitment of macrophages [122],  thus closing the circle. Indeed,  in surgically excised CNV
specimens,  RPE was found
to express VEGF and MCP-1 and macrophages were immunolabeled for VEGF [123].The interest in the role of
PPARs in AMD has been mainly focused on their activities as modulators of
angiogenesis. PPAR agonists have shown antiangiogenic properties both in in vitro and
in vivo
models [35, 88, 89]. It has been
shown that choroidal ECs and RPE cells express PPAR-γ and that PPAR-γ ligands inhibit their response to VEGF,
without apparent toxicity to the adjacent retina,  in a laser-induced model of
CNV [90]. Decrease in angiogenesis
apparently takes place by inhibition of VEGF,  since PPAR-α agonists are found
to inhibit endothelial VEGFR2 expression [124]. An opposite role has been recently described for PPARδ,  which induced endothelial proliferation and
angiogenesis in vitro,
through a VEGF-dependent mechanism [125]. The
natural ligand 15-deoxy-Δ12, 14-prostaglandin
J2 (15d-PGJ2) was found to protect a human RPE cell line
from oxidative stress by elevating GSH and enhancing MAPK activation,  but such
activity was independent of its PPAR-γ binding
activity [126]. The roles of
infiltrating macrophages and/or resident microglia in the pathogenesis of AMD
open the possibility that PPAR-γ agonists may
ameliorate the course of the disease also through the down-regulation
of several proinflammatory functions of these cells
[8] and reference therein,  including TNF-α and iNOS,  and
MHC-II expression.However,  possible beneficial
effects of PPAR-γ agonists in
the treatment of ocular inflammation and,  particularly,  of AMD need to be further verified. It is important to keep in mind that PPAR-γ is involved in the differentiation of
macrophages to foaming cells and PPAR-γ ligands can
induce expression of adipocyte lipid binding protein (ALBP/aP2),  a gene that is
highly expressed in vivo in macrophage/foam cells of human atherosclerotic
plaques [127]. Moreover,
activation of PPAR-γ has been
shown to reduce CCR2 expression in monocytes and their chemotaxis in response
to MCP-1 [128]. These
PPAR-γ mediated activities are of particular interest
in the view of the recent finding by Combadière et al. [92],  suggesting that subretinal microglial foam cells might
be the origin of drusen-like deposits and that accumulation of microglia in the
subretinal space may be a driving force in the pathogenesis of AMD.


## 5. Optic Neuritis and Related Disorders

Optic neuritis (ON),  an inflammatory,  demyelinating disease
of the optic nerve,  may be the initial symptom of multiple sclerosis (MS) or
appear in the course of the disease. In any event,  nearly half of MS patients
develop ON during the course of the disease. An idiopathic demyelinating
disorder of the optic nerve also occurs as NeuroMyelitis Optica (NMO) or Devic’s disease,  which is characterized by the coexistence of usually
bilateral and severe optic neuritis with spinal cord involvement and the presence
of a highly specific serum autoantibody marker (NMO-IgG),  recognizing the
transmembrane channel Aquaporin 4 [138, 139]. The
boundaries between NMO and MS are,  however,  rather imprecise,  from both the
clinical and pathologic points of view and it is still a matter of controversy
whether NMO should be considered a variant of MS or a separate entity [139, 140].Considering
their role in inflammation,  the possible therapeutic efficacy of PPAR-γ agonists has been studied in experimental
autoimmune encephalomyelitis (EAE),  an animal model of the disease where the autoimmune
reaction against myelin is induced in animals by active sensitization with
myelin components. Although several criticisms have been moved towards this
model,  EAE still provides a valuable tool for improving our understanding on
the pathogenesis and treatment of MS. EAE is also considered a model relevant
to the study of demyelinated diseases of the optic nerve [141, 142]. An additional animal model is
represented by T cell receptor transgenic mice specific for myelin
oligodendrocyte glycoprotein (MOG). These mice develop isolated optic neuritis
either spontaneously or after sensitization with suboptimal doses of MOG [143]. Therapeutic efficacy of PPAR-γ ligands has been demonstrated in terms of
suppression or amelioration of clinical symptoms and decrease of inflammatory
signs (see Table 2). Although the anti-inflammatory
activities of PPAR-γ agonists are
complex and multifaceted,  evidence has been provided suggesting a direct action
of PPAR-γ agonists on
microglia/mononuclear phagocytic cells. Indeed,  taking part in both innate and
adaptive immune responses,  microglia and mononuclear
phagocytes are deeply implicated in the complex inflammatory cascade associated
with MS. Their role has been recently and extensively reviewed [144, 145]. The PPAR-γ natural agonist 15d-PGJ2 [146] and the PPAR-α agonist gemfibrozil [133] were found to significantly reduce macrophage
infiltration in the lesions. A decreased number of IL-1β-positive cells were found in EAE brain of mice
treated with GW0742 and a
PPAR-δ agonist and this observation was considered
indicative of a reduction of glial activation [134]. PPAR-γ inhibition of
microglial cell activation is also supported by in vitro experiments
[8, 79, 80, 147–152].Table 2
PPAR agonists and EAE.
AgonistsBiological activityReceptorReferencesTroglitazoneAmelioration
of clinical symptoms. Reduced expression of proinflammatory
cytokines,  IL1β
and TNF-αPPAR-γ[129]Ciglitazone, 15d-PGJ2Decrease
of severity and duration  of clinical
paralysis. Decrease of CNS inflammation and
demyelination. Decrease of IL-12 productionPPAR-γ[130]15d-PGJ2Delay
in the onset and decrease in the severity of disease.
Reduction of Con A- and MBP Ac1–11-reactive,
IFN-α- and IL-4-secreting cellsPPAR-γ[131]PioglitazoneDecreased
mRNA levels of iNOS and the chemokines MIP1 and RANTES in the central nervous
systemPPAR-γ[132]Gemfibrozil and fenofibrateDose-dependent
suppression of lymphocyte proliferation. Promotion of  IL-4 production and inhibition of IFN-γ productionPPAR-α[133]GW0742Improvement
of clinical recovery. Reduction of glial activationPPAR-δ[134]Ciglitazone, 15d-PGJ2Amelioration
of clinical and pathological symptoms. Inhibition of neural antigen-specific T cell proliferationPPAR-γ[135]GemfibroilReduction
of incidence and clinical signs. Inhibition of the infiltration of
inflammatory cells into the CNS. Reduced expression of proinflammatory molecules such as iNOS,  IL-1,  IL-6, and TNF-αno PPAR-γ[136]PioglitazonePrevention
of relapse episodes and reduction of mean clinical scores during the
treatment period. Decrease of IFN-γ levelsPPAR-γ[137]Notwithstanding
the amount of data regarding a therapeutic activity of PPAR agonists in EAE,
clinical studies are lacking and report on their clinical use in MS or ON is
still anecdotical [153]. Clinical
trials are however in course with pioglitazone and rosiglitazone.


## 6. Conclusions

The promising results obtained in
experimental models of ocular diseases and the recent advancements in the
knowledge of the pathogenic mechanisms driving ocular damage and vision loss
strongly point to PPAR-γ as a valuable
target to control inflammation and treat invalidating diseases such as DR,  AMD, and ON. Given the complexity of the phenomena that can be influenced by PPAR-γ activation,  involving not only inflammation
but also retinal microcirculation,
neovascularization, and transformation of activated microglia in foam cells  contributing to drusen-like deposits,  further
studies are mandatory for a correct evaluation of pro and cons of using PPAR-γ agonists in ocular disease treatment. The
PPAR-γ agonists could also find other
important applications in controlling the adverse effects of inflammation that
can put at risk the eye integrity and the correct vision. As an example,   some of the adverse reactions described after
liquid artificial vitreous replacement use in vitreoretinal surgery are a
consequence of inflammatory reaction and activation of  mononuclear phagocytic cells [154],  suggesting that the use of PPAR-γ agonists could be very advantageous in controlling
the inflammatory response to biomaterials.



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*Source: 295784-2008-03-11.xml*