# PPARγ in Kidney Physiology and Pathophysiology

Authors: Éva Kiss-Tóth; Tamás Rőszer
Journal: PPAR Research
 (2008)

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## Abstract

Involvement of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ) in kidney physiology has been explored recently. Synthetic PPARγ ligands can ameliorate the diabetic kidney disease through different mechanisms, involving inhibition of mesangial cell growth, reduction of mesangial matrix, and cytokine production of glomerular cells as well as promoting endothelial cell survival within the kidney glomeruli. Activation of PPARγ has additional profibrotic consequences, which can contribute to wound healing in diabetic glomerulonephritis. Beside many beneficial effects, PPARγ activation, however, can lead to severe water retention, a common side effect of thiazolidinedione therapy. This unwanted effect is due to the activation of PPARγ in the mesonephric distal collecting system, where PPARγ positively regulates sodium and water resorbtion leading to the expansion of interstitial fluid volume. Recent studies indicate that PPARγ is also involved in the normal kidney development, renal lipid metabolism, and activation of the renin-angiotensin system. In this paper, we give a synopsis of the current knowledge on PPARγ functions in kidney phyisology and pathophysiology.


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## Body

## 1. Introduction

The nuclear receptor peroxisome
proliferator-activated receptor gamma (PPARγ) regulates transcription of various genes involved in
lipid uptake, fatty acid metabolism, and glucose homeostasis [1], therefore, the
modulation of PPARγ action is of intense
interests in the medication of insulin resistance and related metabolic disorders
[2–7]. Pharmacological
activation of PPARγ facilitates the glucose
and free fatty acid flux from striated muscle fibers to adipocytes and reduces
liver gluconeogenesis by which PPARγ exerts antidiabetic
benefits [1, 5, 6]. PPARγ signaling can also
influence the expression of insulin-dependent glucose transport (GLUT) proteins
[8], and can induce the production of hormone-like substances in adipose cells
(e.g., resistin and adipokines) promoting insulin responsiveness [1]. Recent
studies indicate that impaired insulin sensitivity of skeletal muscle and white
adipose tissue can be a consequence of a chronic subclinical inflammation [2–6]. Activation of
PPARγ in macrophages has anti-inflammatory
effects, by which PPARγ ligands can reduce the
local low-grade inflammation and consequent insulin resistance of muscle and
adipose tissues [5, 6]. Thiazolidinediones (TZDs), synthetic ligands of PPARγ are clinically proven
insulin sensitizers with antiinflammatory benefits. Nowadays, TZD therapy is a
widely used medication strategy of type 2 diabetes and related diseases [5, 6].Beside beneficial
effects of TZD therapy in insulin resistance, edema and water retention also
frequently occurs as secondary effects of PPARγ activation [9, 10]. The understanding of TZD side effects highly
facilitated the basic research on PPARγ and kidney physiology. As a result, several fundamental findings on the
involvement of PPARγ in fluid homeostasis have been explored in the recent years [1, 10–18]. These
findings indicate that PPARγ is involved in the regulation of sodium and water resorbtion of the distal
collecting ducts of the kidney which explains the unwanted TZD effects on
interstitial fluid volume regulation [9, 17, 18]. Due to the anti-inflammatory
roles of PPARγ activation, the receptor is involved in the attenuation of
glomerulonephritis, which is also a potent therapeutic value of TZDs [10–16].Many other roles
are also attributed to PPARγ in normal kidney development, lipid metabolism, and endocrine functions [19].
In this paper, we
give a synopsis of PPARγ actions as well as the PPARγ-independent effects of synthetic PPARγ ligands in kidney phyisology and pathophysiology.


## 2. PPARγ in the Filtration Units of the Kidney

### 2.1. Diabetic Kidney Disease Is Coupled to Impaired Mesangial Cell Functions

In the latest years, several articles explored the
conneciton of PPARγ and the impaired function of the kidney filtration units in
diabetic kidney disease [10, 12–15]. More than
30% of patients with juvenile or maturity onset diabetes mellitus develop
clinically evident diabetic glomerulopathy within 10–20 years of the
diabetes onset [16, 20]. After years of poor glycemic control, the structure of
the glomerular walls get scarred and permeability changes can develop which are
core features of the diabetic glomerulosclerosis or glomerulonephritis [20]. The disease is characterized
by the strong accumulation of extracellular matrix proteins (Figure 1) and
deposition of type IV collagen in the glomerular mesangium leading to the
expansion of mesangial matrix and glomerular size [10, 12–16, 21, 22].
Elevated glomerular size can manifest in kidney hypertrophy [20]. Alterations
of the glomerular morphology lead to fluid filtration deficits, albuminuria,
glucosuria, and finally reduction of glomerular filtration [21–30].Figure 1
Roles of PPARγ in the filtration units of the kidney. The kidney
capsules (a) contain the glomerular capillaries covered with podocytes (pc). In the
wall of the afferent arterioles, modified smooth muscle cells form the
juxtaglomerular system (jg). The filtrated urine is guided to the proximal
tubules (pt). The distal tubules (dt) can return to the cortical kidney capsules
and their epithelial layers
serve as a chemosensory region, the macula densa (labeled with red). (b) PPARγ activation affects either
podocyte (pc), mesangial cell (ms), or endothel cell (en) functions. (c) Periodic
acid-Schiff (PAS) stained sections of a normal kidney capsule in mouse. (d) Glomerulonephritis
in high-fat diet fed mouse and (e) type
2 diabetic (db/db) mouse, showing intensive PAS staining of the expanded
mesangial matrix, thickening of glomerular walls, and enlargement of kidney
capsules. (f) Summary of PPARγ-mediated cellular events
in mesangial cells, podocytes, kidney macrophages, and glomerular endothel
cells.Glomerular mesangial cells have
a central role in the development of diabetic glomerulonephritis (Figure1(b)),
since these cells can overproduce the extracellular matrix proteins of the
glomerular mesangium in response to chronic hyperglycemia [11–13, 21].


### 2.2. Effects of PPARγ Activation in Mesangial Cells

Activation of PPARγ as well as PPARα in mesangial cells can attenuate the overproduction of the mesangial
matrix (Figure 1(f)), as it has been described in animal models of diabetic
nephropathy [21, 22]. Diabetes in apolipoprotein-E (ApoE)-deficient mice is
associated with a significant accumulation of extracellular matrix proteins and
increased immunostaining for collagen IV in the glomerular compartments
(Figures 1(d), 1(e)). Treatment with rosiglitazone results in a significant
reduction in collagen IV deposition [21]. In Otsuka Long-Evans Tokushima Fatty (OLETF),
type 2 diabetic rats glomerular hypertrophy correlates well with the expression
of large quantities of the Bcl-2 protein, an apoptosis-suppressing molecule in
the mesangial cells [22]. This finding suggests that persistent proliferation
and prolonged survival of the mesangial cells can also contribute to the
supernormal matrix secretion in glomerulopathy. The gene
encoding Bcl-2 has a PPAR response element by which PPARγ can increase Bcl-2 mRNA
transcription. However, some reports have indicated that TZD treatment can
decrease the level of Bcl-2 and induce apoptosis independently of PPARγ[22].TZDs cannot only reduce
glomerular cross-sectional area and the mesangial matrix size as well as collagen IV synthesis but also enhance the tumor growth factor beta-1 (TGF-β1)
positive staining areas in the kidney of OLETF rats [22]. TGF-β seems to be a central molecule in the PPAR agonist
action [10, 12, 22]. This growth factor activates several intracellular signal
transduction systems involved in the regulation of the extracellular matrix
biosynthesis (Figure 1(f)), including mitogen-activated protein kinases (MAPKs),
the extracellular signal-regulated kinases (ERKs), the c-jun NH2-terminal
kinases, diacylglycerol/protein kinase C extracellular signal-regulated kinase
pathway, and the p38 MAPK [23–30]. PPARγ agonists besides their
anti-inflammatory effect can inhibit TGF-β expression leading to a repression
in glomerular proliferation [16, 22, 30]. PPARγ also has a direct effect
on key extracellular matrix regulators as plasminogen activator inhibitor-1
(PAI-1). PAI-1 is a member of the serine protease inhibitor superfamily and it
can inhibit proteolysis of the extracellular matrix, leading to matrix
accumulation and sclerosis. PPARγ agonists may inhibit
PAI-1 transcription by antagonizing the activities of activator protein-1
(AP-1) and nuclear factor κB [23–31].The presence of TGF-β1 in the
mesangial cells refers to a mechanism by which high-glucose milieu induces
inflammatory and profibrotic cytokine production in glomerular cells (Figure 1(f)).
In diabetic nephropathy, mesangial cells as well as podocytes and interstitial
cells can secrete monocyte chemoattractant protein-1 (MCP-1) and TGF-β1 which
may initiate macrophage infiltration into the kidney [10, 12–15]. The number
of infiltrated machrophages is being increased both in the glomeruli and the
renal interstitium with the development of diabetic kidney disease in OLETF
rats. TZDs have an anti-inflammatory effect in the peripheral tissues,
therefore treatment with pioglitazone or rosiglitazone decreases macrophage
infiltration of the kidney [19, 22, 30].
MCP-1 can also influence the alternative macrophage activation.
Alternatively activated macrophage release factors such as IL-1ra/IL-1F3,
IL-10, and TGF-β [31–33]. TGF-β
functions indirectly to promote extracellular matrix building by inducing
nearby kidney fibroblasts to produce matrix components [34]. The alternatively
activated macrophages themselves
produce extracellular matrix components, as fibronectin and a cross-linking
enzyme transglutaminase (Figure 1(f)), as well as osteopontin, which is
involved in cell adhesion to the matrix [32, 35]. The molecules secreted by the
alternatively activated macrophages can promote wound repair due to their
anti-inflammatory, fibrotic, proliferative, and angiogenic activities [32–35].


### 2.3. Role of PPARγ in Podocytes and Capillaries  in Glomerulonephritis

Podocyte injury is also among the primary events
in early development of the glomerulosclerosis [33, 36]. A decrease in podocyte
number in type 2 diabetic Pima Indians correlates closely with those patients
who have microalbuminuria, the earliest manifestation of diabetic nephropathy [37].
High-glucose treatment or the epithelial cell toxin puromycin aminonucleosid
(PAN) supplementation induces podocyte injury and PPARγ upregulation in podocyte culture [37]. This increase of PPARγ is counterregulatory
and might promote podocyte healing and repair. Pioglitazone treatment of
podocytes can inhibit expression or phosphorylation of cell proliferation and antiapoptotic proteins
(e.g., p27Kip1, p42 MAPK, Bcl-2) which can be one major molecular
mechanism behind the therapeutic potential of TZDs on high glucose-induced
hypertrophy of podocytes [22, 23].Microangiopathy of glomerular capillaries is also a hallmark
of the diabetic nephropathy [10, 12–15, 33]. Endothelial
growth and survival are regulated by two factors, vascular endothelial growth factor (VEGF) and
angioprotein which are also expressed by podocytes (Figure 1(f)). PPARγ agonists can protect
glomerular capillaries against
injury both by increasing podocyte VEGF expression and by decreasing Aglp4 [38].
TZDs, therefore, can prevent angiopathy of the capillaries in the glomeruli,
one causing event of progressive kidney disease.


## 2.1. Diabetic Kidney Disease Is Coupled to Impaired Mesangial Cell Functions

In the latest years, several articles explored the
conneciton of PPARγ and the impaired function of the kidney filtration units in
diabetic kidney disease [10, 12–15]. More than
30% of patients with juvenile or maturity onset diabetes mellitus develop
clinically evident diabetic glomerulopathy within 10–20 years of the
diabetes onset [16, 20]. After years of poor glycemic control, the structure of
the glomerular walls get scarred and permeability changes can develop which are
core features of the diabetic glomerulosclerosis or glomerulonephritis [20]. The disease is characterized
by the strong accumulation of extracellular matrix proteins (Figure 1) and
deposition of type IV collagen in the glomerular mesangium leading to the
expansion of mesangial matrix and glomerular size [10, 12–16, 21, 22].
Elevated glomerular size can manifest in kidney hypertrophy [20]. Alterations
of the glomerular morphology lead to fluid filtration deficits, albuminuria,
glucosuria, and finally reduction of glomerular filtration [21–30].Figure 1
Roles of PPARγ in the filtration units of the kidney. The kidney
capsules (a) contain the glomerular capillaries covered with podocytes (pc). In the
wall of the afferent arterioles, modified smooth muscle cells form the
juxtaglomerular system (jg). The filtrated urine is guided to the proximal
tubules (pt). The distal tubules (dt) can return to the cortical kidney capsules
and their epithelial layers
serve as a chemosensory region, the macula densa (labeled with red). (b) PPARγ activation affects either
podocyte (pc), mesangial cell (ms), or endothel cell (en) functions. (c) Periodic
acid-Schiff (PAS) stained sections of a normal kidney capsule in mouse. (d) Glomerulonephritis
in high-fat diet fed mouse and (e) type
2 diabetic (db/db) mouse, showing intensive PAS staining of the expanded
mesangial matrix, thickening of glomerular walls, and enlargement of kidney
capsules. (f) Summary of PPARγ-mediated cellular events
in mesangial cells, podocytes, kidney macrophages, and glomerular endothel
cells.Glomerular mesangial cells have
a central role in the development of diabetic glomerulonephritis (Figure1(b)),
since these cells can overproduce the extracellular matrix proteins of the
glomerular mesangium in response to chronic hyperglycemia [11–13, 21].


## 2.2. Effects of PPARγ Activation in Mesangial Cells

Activation of PPARγ as well as PPARα in mesangial cells can attenuate the overproduction of the mesangial
matrix (Figure 1(f)), as it has been described in animal models of diabetic
nephropathy [21, 22]. Diabetes in apolipoprotein-E (ApoE)-deficient mice is
associated with a significant accumulation of extracellular matrix proteins and
increased immunostaining for collagen IV in the glomerular compartments
(Figures 1(d), 1(e)). Treatment with rosiglitazone results in a significant
reduction in collagen IV deposition [21]. In Otsuka Long-Evans Tokushima Fatty (OLETF),
type 2 diabetic rats glomerular hypertrophy correlates well with the expression
of large quantities of the Bcl-2 protein, an apoptosis-suppressing molecule in
the mesangial cells [22]. This finding suggests that persistent proliferation
and prolonged survival of the mesangial cells can also contribute to the
supernormal matrix secretion in glomerulopathy. The gene
encoding Bcl-2 has a PPAR response element by which PPARγ can increase Bcl-2 mRNA
transcription. However, some reports have indicated that TZD treatment can
decrease the level of Bcl-2 and induce apoptosis independently of PPARγ[22].TZDs cannot only reduce
glomerular cross-sectional area and the mesangial matrix size as well as collagen IV synthesis but also enhance the tumor growth factor beta-1 (TGF-β1)
positive staining areas in the kidney of OLETF rats [22]. TGF-β seems to be a central molecule in the PPAR agonist
action [10, 12, 22]. This growth factor activates several intracellular signal
transduction systems involved in the regulation of the extracellular matrix
biosynthesis (Figure 1(f)), including mitogen-activated protein kinases (MAPKs),
the extracellular signal-regulated kinases (ERKs), the c-jun NH2-terminal
kinases, diacylglycerol/protein kinase C extracellular signal-regulated kinase
pathway, and the p38 MAPK [23–30]. PPARγ agonists besides their
anti-inflammatory effect can inhibit TGF-β expression leading to a repression
in glomerular proliferation [16, 22, 30]. PPARγ also has a direct effect
on key extracellular matrix regulators as plasminogen activator inhibitor-1
(PAI-1). PAI-1 is a member of the serine protease inhibitor superfamily and it
can inhibit proteolysis of the extracellular matrix, leading to matrix
accumulation and sclerosis. PPARγ agonists may inhibit
PAI-1 transcription by antagonizing the activities of activator protein-1
(AP-1) and nuclear factor κB [23–31].The presence of TGF-β1 in the
mesangial cells refers to a mechanism by which high-glucose milieu induces
inflammatory and profibrotic cytokine production in glomerular cells (Figure 1(f)).
In diabetic nephropathy, mesangial cells as well as podocytes and interstitial
cells can secrete monocyte chemoattractant protein-1 (MCP-1) and TGF-β1 which
may initiate macrophage infiltration into the kidney [10, 12–15]. The number
of infiltrated machrophages is being increased both in the glomeruli and the
renal interstitium with the development of diabetic kidney disease in OLETF
rats. TZDs have an anti-inflammatory effect in the peripheral tissues,
therefore treatment with pioglitazone or rosiglitazone decreases macrophage
infiltration of the kidney [19, 22, 30].
MCP-1 can also influence the alternative macrophage activation.
Alternatively activated macrophage release factors such as IL-1ra/IL-1F3,
IL-10, and TGF-β [31–33]. TGF-β
functions indirectly to promote extracellular matrix building by inducing
nearby kidney fibroblasts to produce matrix components [34]. The alternatively
activated macrophages themselves
produce extracellular matrix components, as fibronectin and a cross-linking
enzyme transglutaminase (Figure 1(f)), as well as osteopontin, which is
involved in cell adhesion to the matrix [32, 35]. The molecules secreted by the
alternatively activated macrophages can promote wound repair due to their
anti-inflammatory, fibrotic, proliferative, and angiogenic activities [32–35].


## 2.3. Role of PPARγ in Podocytes and Capillaries  in Glomerulonephritis

Podocyte injury is also among the primary events
in early development of the glomerulosclerosis [33, 36]. A decrease in podocyte
number in type 2 diabetic Pima Indians correlates closely with those patients
who have microalbuminuria, the earliest manifestation of diabetic nephropathy [37].
High-glucose treatment or the epithelial cell toxin puromycin aminonucleosid
(PAN) supplementation induces podocyte injury and PPARγ upregulation in podocyte culture [37]. This increase of PPARγ is counterregulatory
and might promote podocyte healing and repair. Pioglitazone treatment of
podocytes can inhibit expression or phosphorylation of cell proliferation and antiapoptotic proteins
(e.g., p27Kip1, p42 MAPK, Bcl-2) which can be one major molecular
mechanism behind the therapeutic potential of TZDs on high glucose-induced
hypertrophy of podocytes [22, 23].Microangiopathy of glomerular capillaries is also a hallmark
of the diabetic nephropathy [10, 12–15, 33]. Endothelial
growth and survival are regulated by two factors, vascular endothelial growth factor (VEGF) and
angioprotein which are also expressed by podocytes (Figure 1(f)). PPARγ agonists can protect
glomerular capillaries against
injury both by increasing podocyte VEGF expression and by decreasing Aglp4 [38].
TZDs, therefore, can prevent angiopathy of the capillaries in the glomeruli,
one causing event of progressive kidney disease.


## 3. PPARγ in the Distal Collecting System

### 3.1. Expression of PPARγ in the Nephron Ducts

Under physiological
conditions, PPARγ is dominantly expressed
in the collecting system of the mammalian urinary tract, including connective
renal tubules and collecting ducts (Figure 2(a)). PPARγ is abundant in the inner renal medulla (Figures 2(b), 2(c))
and localized to the epithelial layer starting from medullary collecting ducts to
the urothelium of the ureter and the bladder [39–41]. PPARγ also occurs in renal medullary
interstitial cells [39]. The PPARγ partner RXRα has a complimentary
distribution in the collecting ducts [42]. The connective tubules and
collective ducts are parts of the distal collecting system, where hormone-regulated
ion exchange and water resorbtion takes place and provides the balance of interstitial fluid volume
(Figure 2(e)). If aldosterone is present, sodium is resorbed and potassium is
secreted. Sodium transport is followed by passive water resorbtion, therefore,
this mechanism regulates the
total electrolite and water volume in the body [43].
The epithelium of the collecting ducts is responsive to antidiuretic hormone.
If the hormone is present, the epithelia becomes permeable to water. The distal
collecting system is, therefore, a major site of fluid volume regulation.Figure 2
Roles of PPARγ in the collecting system of the kidney. (a) Expression
of PPARγ is confined to the distal
collecting system (labeled with green) including connective tubules (cn) and
collective ducts (ct). (b) Hematoxylin and esoin stained cross-sections of the
kidney medulla showing numerous collective ducts (ct). (c) Fluorescent PPARγ immunostaining in the
same region of the kidney. (d) Oil red-O stained sections of the distal tubules
(dt) showing severe lipid accumulation in type 2 diabetic (db/db) mice. (e) Summary
of PPARγ functions in the
collective system.


### 3.2. Embryology and Phylogenetic Homologies of PPARγ Expressing Collecting Ducts

In mammals, the
development of the kidney collecting system differs from the other excretory
parts of the kidney [44]. Collecting ducts and tubules are formed by the ureteric bud, which is an
outgrowth of the dorsomedial wall of the mesonephric duct. The proliferating
mesonephric bud penetrates the developing metanephric tissues and dilates
forming the primitive renal pelvis and calyces. The further subdivisions of the
calyces form the presumptive collecting ducts [44]. According to the recent
literature, PPARγ expression is mainly
confined to the collecting system of the kidney [39, 41, 45–55], which has a
mesonephric origin (Figure 2(a)). A lower expression of PPARγ1 in the proximal tubules,
which are derived from the metanephric tissue, has been indicated in the rat
kidney [56] while in mesangial cells and podocytes of the kidney capsules PPARγ is upregulated only under
pathological conditions as chronic hyperglycemia or glomerulonephritis [10, 12–15]. The
distribution pattern of PPARγ suggests that PPARγ may have been coupled to
the mesonpehros in the vertebrate phylogeny. Supporting this possibility, the
kidney of teleost fishes, which is a functioning mesonephros and a phylogenic
homolog of the mammalian collecting system, contains all of the three PPAR
isoforms [57–59]. Like their
mammalian homologs, fish PPARs bind to a variety of natural PPAR response
elements (PPREs) present in the promoters of mammalian or piscine genes.


### 3.3. Role of PPARγ in the Balance of Fluid Homeostasis

As its distribution
pattern suggests, the clinically most relevant function of PPARγ is the modulation of
electrolyte and water resorbtion [17, 18, 41, 60].  Edema and fluid retention are common and
serious side effects of TZD therapy, which are due to supernormal sodium
resorbtion and consequent interstitial fluid volume expansion [9, 32]. Since
PPARγ is a significant target for TZDs and it is predominantly expressed in the
collecting ducts, critical sites for the control of fluid metabolism, its
possible involvement in fluid metabolism has been recently elucidated. PPARγ activation can modulate
sodium resorbtion through the stimulation of epthelial sodium channels and the
Na+/K+-ATPase system [41, 60]. Additionally, TZDs can
ditsurb the renin-angiotensin-aldosterone system also (Figure 2(e)). In human collecting duct cell culture PPARγ activation enhances the
expression of cell surface epithelial sodium channels which can facilitate the
sodium resorbtion ability of the tubular cells [41]. The role of PPARγ in the regulation of
sodium resorbtion has been also confirmed by studies carried out on mice with
collecting duct-specific ablation of PPARγ [17, 18]. These studies show a
critical role for PPARγ in systemic fluid retention through the regulation of
renal sodium transport, and that the adverse effects of TZD in fluid metabolism
are indeed PPARγ-dependent. A gene encoding for the gamma subunit of the
epithelial sodium channel has been identified as a critical PPARγ target gene
in the control of electrolyte and water resorbtion of the collecting ducts
(Figure 2(e)).


### 3.4. Proliferation and Metabolism of Kidney Epithelia and Effects of PPARγ

PPARγ has some additional
functions in the collecting system of the kideny. During embryogenesis, the
expression of PPARγ in urothelium [41, 46, 55]
suggests its possible involvement in the urothelial proliferation and
differentiation. In cultured rat kidney epithelial cells, both troglitazone and
15d-PGJ2 significantly inhibit cell proliferation and
dramatically alter cell shape by induction of cell process formation
[19, 41]. TZDs or PPARγ overexpression induces
the Klotho gene expression in mouse kidneys and renal epithelial cell culture
promoting insulin sensitivity and reducing cellular aging [46].The PPARγ ligand TZDs alter not
only cellular growth and survival but also metabolic processes of the kidney
collecting duct epithelia including carbohydrate, lipid metabolism, and
albumine transport [19, 47]. TZDs can activate PPARγ-regulated genes as well
as P-ERK and AMP-activated protein kinase pathways which modulate
gluconeogenesis, cellular acidosis, glutamine metabolism, and ammoniagenesis of
porcine tubular cells [19]. It is possible that modulation of kidney
carbohydrate metabolism by TZDs has a beneficial role in the glycemic control [19].
Interestingly some in vitro
studies with kidney epithelial cells of the opossum have revealed that TZD
affects protein handling of tubular epithelia also [47]. Rosiglitazone,
ciglitazone, and troglitazone can inhibit the uptake of FITC-labeled albumin by
tubular epithelial cells in a dose-dependent manner without any cytotoxic
effect. Unexpectedly, in tubular cells overexpressing PPARγ or in cells treated with
the PPARγ antagonist GW9662, albumin
handling cannot be affected. Similarly, the PPARγ ligand 15d-PGJ2, which is
structurally unrelated to TZDs, has no effect on albumin uptake [47]. Albumin
handling of tubular cells can be, therefore, affected by TZDs independently
from PPARγ. Effects of TZDs on
tubular protein uptake, however, can be physiologically less relevant than the
benefits of TZD administration on glomerular functions which conseqeuntly
reduce albuminuria.PPARγ is also involved in the
renal lipid metabolism (Figures 2(d), 2(e)). Abnormal renal lipid synthesis
plays a role in the pathogenesis of diabetic nepropathy [48]. Renal lipid
deposits in glomerulosclerosis have been mentioned even in the first
description of the diabetic kidney alterations by Kimmelstiel and Wilson in 1936
[49]. Lipid deposits are present in the kidney of diabetic humans as well as of
diabetes model rodents [48–55, 61, 62]. In diabetic
animals upregulation of kidney SREBP-1, the key enzyme of fatty acid synthesis
can lead to the renal accumulation of lipids as well as mesangial matrix
expansion and kidney hypertrophy [51, 52]. Elevated levels of plasma lipids
also can contribute to renal fat deposition and facilitate the development of
glomerulosclerosis [53]. High glucose concentration can also increase SREBP-1 expression
in cultured rat mesangial cells, suggesting that impaired glycemic control can
disturb renal lipid metabolism through altered SREBP-1 gene expression, which
is regulated by PPARγ [51]. It is possible that
the transcriptional activity of PPARγ in the duct cells is
upregulated by insulin and C-protein, a protein fragment of proinsulin [54].
Both insulin and C-peptide can induce a concentration-dependent activation of
PPARγ and both agents can
augment the TZD-stimulated PPARγ activity giving the possibility
that hyperinsulinemia in type 2 diabetes can augment PPARγ as well as PPARγ-regulated SREBP-1 gene
functions.Renal
lipid accumulation, however, not only  is a consequence of the hyperglycemia or
dyslipidemia but also can predispose or provoke glomerulonephritis. Recent in vitro studies suggest that low-density
lipoproteins and very low-density lipoproteins induce upregulation of growth
factors, TGF-β, and matrix proteins in
cultured renal mesangial and tubular cells [55, 61]. This direct effect of lipids
on gene expression of kidney cells can initiate the development of mesangial
matrix expansion which is a hallmark of glomerulonephritic syndrome. In mice
with upregulated SREBP-1 expression, the signs of glomerulonephritis as
albuminuria, renal cholesterol, and triglyceride deposits occur without changes
in glucose homeostasis or serum lipid levels [51]. In these SREBP-1 transgenic
mice, the elevated renal lipid content is coupled with increased TGF-β and vascular endothelial
growth factor (VEGF) expression [51]. VEGF plays a pivotal role in the
pathogenesis of glomerulosclerosis [63]. PPARγ haploinsufficiency as
well as Pro12Ala (P12A) allele polymorphism of PPARγ has a protective role in the development of
diabetic nephropathy [64]. In mice with heterozygous PPARγ mutation, high-fat diet
results in a less severe nephropathy and lipid depositions than in wild type
animals [45].


## 3.1. Expression of PPARγ in the Nephron Ducts

Under physiological
conditions, PPARγ is dominantly expressed
in the collecting system of the mammalian urinary tract, including connective
renal tubules and collecting ducts (Figure 2(a)). PPARγ is abundant in the inner renal medulla (Figures 2(b), 2(c))
and localized to the epithelial layer starting from medullary collecting ducts to
the urothelium of the ureter and the bladder [39–41]. PPARγ also occurs in renal medullary
interstitial cells [39]. The PPARγ partner RXRα has a complimentary
distribution in the collecting ducts [42]. The connective tubules and
collective ducts are parts of the distal collecting system, where hormone-regulated
ion exchange and water resorbtion takes place and provides the balance of interstitial fluid volume
(Figure 2(e)). If aldosterone is present, sodium is resorbed and potassium is
secreted. Sodium transport is followed by passive water resorbtion, therefore,
this mechanism regulates the
total electrolite and water volume in the body [43].
The epithelium of the collecting ducts is responsive to antidiuretic hormone.
If the hormone is present, the epithelia becomes permeable to water. The distal
collecting system is, therefore, a major site of fluid volume regulation.Figure 2
Roles of PPARγ in the collecting system of the kidney. (a) Expression
of PPARγ is confined to the distal
collecting system (labeled with green) including connective tubules (cn) and
collective ducts (ct). (b) Hematoxylin and esoin stained cross-sections of the
kidney medulla showing numerous collective ducts (ct). (c) Fluorescent PPARγ immunostaining in the
same region of the kidney. (d) Oil red-O stained sections of the distal tubules
(dt) showing severe lipid accumulation in type 2 diabetic (db/db) mice. (e) Summary
of PPARγ functions in the
collective system.


## 3.2. Embryology and Phylogenetic Homologies of PPARγ Expressing Collecting Ducts

In mammals, the
development of the kidney collecting system differs from the other excretory
parts of the kidney [44]. Collecting ducts and tubules are formed by the ureteric bud, which is an
outgrowth of the dorsomedial wall of the mesonephric duct. The proliferating
mesonephric bud penetrates the developing metanephric tissues and dilates
forming the primitive renal pelvis and calyces. The further subdivisions of the
calyces form the presumptive collecting ducts [44]. According to the recent
literature, PPARγ expression is mainly
confined to the collecting system of the kidney [39, 41, 45–55], which has a
mesonephric origin (Figure 2(a)). A lower expression of PPARγ1 in the proximal tubules,
which are derived from the metanephric tissue, has been indicated in the rat
kidney [56] while in mesangial cells and podocytes of the kidney capsules PPARγ is upregulated only under
pathological conditions as chronic hyperglycemia or glomerulonephritis [10, 12–15]. The
distribution pattern of PPARγ suggests that PPARγ may have been coupled to
the mesonpehros in the vertebrate phylogeny. Supporting this possibility, the
kidney of teleost fishes, which is a functioning mesonephros and a phylogenic
homolog of the mammalian collecting system, contains all of the three PPAR
isoforms [57–59]. Like their
mammalian homologs, fish PPARs bind to a variety of natural PPAR response
elements (PPREs) present in the promoters of mammalian or piscine genes.


## 3.3. Role of PPARγ in the Balance of Fluid Homeostasis

As its distribution
pattern suggests, the clinically most relevant function of PPARγ is the modulation of
electrolyte and water resorbtion [17, 18, 41, 60].  Edema and fluid retention are common and
serious side effects of TZD therapy, which are due to supernormal sodium
resorbtion and consequent interstitial fluid volume expansion [9, 32]. Since
PPARγ is a significant target for TZDs and it is predominantly expressed in the
collecting ducts, critical sites for the control of fluid metabolism, its
possible involvement in fluid metabolism has been recently elucidated. PPARγ activation can modulate
sodium resorbtion through the stimulation of epthelial sodium channels and the
Na+/K+-ATPase system [41, 60]. Additionally, TZDs can
ditsurb the renin-angiotensin-aldosterone system also (Figure 2(e)). In human collecting duct cell culture PPARγ activation enhances the
expression of cell surface epithelial sodium channels which can facilitate the
sodium resorbtion ability of the tubular cells [41]. The role of PPARγ in the regulation of
sodium resorbtion has been also confirmed by studies carried out on mice with
collecting duct-specific ablation of PPARγ [17, 18]. These studies show a
critical role for PPARγ in systemic fluid retention through the regulation of
renal sodium transport, and that the adverse effects of TZD in fluid metabolism
are indeed PPARγ-dependent. A gene encoding for the gamma subunit of the
epithelial sodium channel has been identified as a critical PPARγ target gene
in the control of electrolyte and water resorbtion of the collecting ducts
(Figure 2(e)).


## 3.4. Proliferation and Metabolism of Kidney Epithelia and Effects of PPARγ

PPARγ has some additional
functions in the collecting system of the kideny. During embryogenesis, the
expression of PPARγ in urothelium [41, 46, 55]
suggests its possible involvement in the urothelial proliferation and
differentiation. In cultured rat kidney epithelial cells, both troglitazone and
15d-PGJ2 significantly inhibit cell proliferation and
dramatically alter cell shape by induction of cell process formation
[19, 41]. TZDs or PPARγ overexpression induces
the Klotho gene expression in mouse kidneys and renal epithelial cell culture
promoting insulin sensitivity and reducing cellular aging [46].The PPARγ ligand TZDs alter not
only cellular growth and survival but also metabolic processes of the kidney
collecting duct epithelia including carbohydrate, lipid metabolism, and
albumine transport [19, 47]. TZDs can activate PPARγ-regulated genes as well
as P-ERK and AMP-activated protein kinase pathways which modulate
gluconeogenesis, cellular acidosis, glutamine metabolism, and ammoniagenesis of
porcine tubular cells [19]. It is possible that modulation of kidney
carbohydrate metabolism by TZDs has a beneficial role in the glycemic control [19].
Interestingly some in vitro
studies with kidney epithelial cells of the opossum have revealed that TZD
affects protein handling of tubular epithelia also [47]. Rosiglitazone,
ciglitazone, and troglitazone can inhibit the uptake of FITC-labeled albumin by
tubular epithelial cells in a dose-dependent manner without any cytotoxic
effect. Unexpectedly, in tubular cells overexpressing PPARγ or in cells treated with
the PPARγ antagonist GW9662, albumin
handling cannot be affected. Similarly, the PPARγ ligand 15d-PGJ2, which is
structurally unrelated to TZDs, has no effect on albumin uptake [47]. Albumin
handling of tubular cells can be, therefore, affected by TZDs independently
from PPARγ. Effects of TZDs on
tubular protein uptake, however, can be physiologically less relevant than the
benefits of TZD administration on glomerular functions which conseqeuntly
reduce albuminuria.PPARγ is also involved in the
renal lipid metabolism (Figures 2(d), 2(e)). Abnormal renal lipid synthesis
plays a role in the pathogenesis of diabetic nepropathy [48]. Renal lipid
deposits in glomerulosclerosis have been mentioned even in the first
description of the diabetic kidney alterations by Kimmelstiel and Wilson in 1936
[49]. Lipid deposits are present in the kidney of diabetic humans as well as of
diabetes model rodents [48–55, 61, 62]. In diabetic
animals upregulation of kidney SREBP-1, the key enzyme of fatty acid synthesis
can lead to the renal accumulation of lipids as well as mesangial matrix
expansion and kidney hypertrophy [51, 52]. Elevated levels of plasma lipids
also can contribute to renal fat deposition and facilitate the development of
glomerulosclerosis [53]. High glucose concentration can also increase SREBP-1 expression
in cultured rat mesangial cells, suggesting that impaired glycemic control can
disturb renal lipid metabolism through altered SREBP-1 gene expression, which
is regulated by PPARγ [51]. It is possible that
the transcriptional activity of PPARγ in the duct cells is
upregulated by insulin and C-protein, a protein fragment of proinsulin [54].
Both insulin and C-peptide can induce a concentration-dependent activation of
PPARγ and both agents can
augment the TZD-stimulated PPARγ activity giving the possibility
that hyperinsulinemia in type 2 diabetes can augment PPARγ as well as PPARγ-regulated SREBP-1 gene
functions.Renal
lipid accumulation, however, not only  is a consequence of the hyperglycemia or
dyslipidemia but also can predispose or provoke glomerulonephritis. Recent in vitro studies suggest that low-density
lipoproteins and very low-density lipoproteins induce upregulation of growth
factors, TGF-β, and matrix proteins in
cultured renal mesangial and tubular cells [55, 61]. This direct effect of lipids
on gene expression of kidney cells can initiate the development of mesangial
matrix expansion which is a hallmark of glomerulonephritic syndrome. In mice
with upregulated SREBP-1 expression, the signs of glomerulonephritis as
albuminuria, renal cholesterol, and triglyceride deposits occur without changes
in glucose homeostasis or serum lipid levels [51]. In these SREBP-1 transgenic
mice, the elevated renal lipid content is coupled with increased TGF-β and vascular endothelial
growth factor (VEGF) expression [51]. VEGF plays a pivotal role in the
pathogenesis of glomerulosclerosis [63]. PPARγ haploinsufficiency as
well as Pro12Ala (P12A) allele polymorphism of PPARγ has a protective role in the development of
diabetic nephropathy [64]. In mice with heterozygous PPARγ mutation, high-fat diet
results in a less severe nephropathy and lipid depositions than in wild type
animals [45].


## 4. PPARγ Function in the Juxtaglomerular Apparatus

Kidney is not only an excretory organ but also serves endocrine
functions by the secretion of renin, a 37 kDa protein hormone produced by the
juxtaglomerular cells. Juxtaglomerular cells are modified smooth muscle cells
in the media of the afferent arteriole adjacent to the renal capsule (Figure1(a)).
Renin acts on a plasma protein called angiotensinogen, producing an inactive
decapeptide, the angiotensin I. This substance as a result of the action of a
converting enzyme present in high concentration in lung endothelial cells,
becoming an octapeptide called angiotensin II. Angiotensin II enhances the
secretion of aldosterone in the adrenal gland [65, 66]. The main targets of
aldosterone are the distal tubules, where it can regulate sodium reabsorption
(Figure 2(e)).Human renin gene enhancer is modulated by PPARγ activation [67, 68]. In
human renin-producing cell line CaLu-6, endogenous or pharmacological PPARγ agonists (unsaturated
fatty acids and TZDs) can stimulate renin mRNA transcription [67, 68].Although
renin production is facilitated by PPARγ activation, the
hypertensive effects of angiotensin II can be attenuated by TZDs [69–71]. In addition
to its role in controlling water and salt homeostasis, the inhibition of the
renin-angiotensin system reduces the incidence of type 2 diabetes in patients
with hypertension or congestive heart failure and also reduces the risk of
nephropathy in diabetic patients [71]. The mechanisms underlying these
protective effects appear to be complex and may involve an improvement of both
insulin sensitivity and insulin secretion. Recent works suggest that
aldosterone and mineralocorticoid receptors regulate PPARγ expression [72, 73].
Aldosterone as well as angiotensin receptor blockers appear to induce PPARγ activity in the adipose
tissue, which could explain the protective effect of the renin-angiotensin
system inhibition against the development of type 2 diabetes [71]. It is
unlikely, however, that the favorable effects of TZDs on diabetic nephropathy
would be related to a dierct effect on the renin-angiotensin system [74].


## 5. Cytotoxic Effects of PPARγ Ligands on Tubular Epithelial Cells

Synthetic PPARγ ligands are widely used
drugs for the treatment of insulin resistance. There is an evidence that these
drugs have beneficial effects on the improvement of metabolic parameters as proteinuria
in type 2 diabetes, however, some severe metabolic secondary effects have been
recognized [75–77].Increasing
number of synthetic PPARγ ligands is commercially available
today (e.g., troglitazone, rosiglitazone, pioglitazone, ciglitazone,
muraglitazar) for treatment of type 2 diabetes complications. Many reports have
described the side effects of them including antiproliferative and apoptotic
actions in cultures of renal proximal tubular cells [78], mesangial cells [79],
and interstitial fibroblasts [80]. Ciglitazone has a direct necrotic effect on
renal proximal tubular cells at a concentration range similar to its
therapeutical plasma levels. Interestingly, these cytotoxic effects are not
universal for all PPARγ agonists because
pioglitazone is not cytotoxic in the same cell lines [81]. Although
renoprotective effects
of dual PPARα and PPARγ activation have been reported in
type 2 diabetic animals [82], muraglitazar (a PPARα/γ dual agonist) can induce
multifocal urothelial necrosis and proliferation in young male rats which is
thought to be provoked by muraglitazar-associated changes in urine composition [83] .


## 6. Summary

PPARγ agonists have many beneficial
effects combined with their independent antiatherosclerotic actions and their
important effects on dyslipidemia and insulin resistance in the medication of
kidney disease coupled to diabetes [10, 12–15, 21, 82].
Activation of PPARγ attenuates diabetic
glomerulonephritis due to its anti-inflammatory and profibrotic effects [32–35]. PPARγ and PPARα have similar antidiabetic and renoprotective
effects, therefore administration of PPARα or PPARα/γ dual agonists may be also
useful for the prevention of kidney complications of type 1 as well as type 2
diabetes mellitus [13, 21, 82]. On the other hand, PPARγ signaling can facilitate
lipid accumulation or induce a direct necrotic cell death of tubular epithelial
cells, therefore synthetic PPARγ ligands, especially TZDs should
be used with a great foresight in the medication of insulin-resistant diabetes
mellitus [1, 48–54].The most recently discovered role of PPARγ in the positive regulation
of salt and water resorbtion have elucidated the pathomechanism of water
retention and edema in patients treated with TZDs, the widely used PPARγ agonists [17, 18]. Edema
and fluid retention can be fatal side effects of TZDs, which can be attenuated
by the combination of TZD therapy with diuretics [9]. The selective PPAR
modulator (SPPARM) approach has also been proposed as a method to avoid
unwanted complications of PPARγ ligands [9].Some comparative data suggest that PPARγ is coupled to the
mesonephric parts of the vertebrate kidney, therefore the involvement of PPARγ in the intesrtitial fluid
volume regulation can be an ancient and evolutionarily conserved role [56–58]. Some other
components of renal PPARγ activation, including the
function of PPARγ in the moduation of renal
endocrine functions, are still undefined [67, 74] indicating the timeliness of
future research in the field of PPARγ and kidney physiology.



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*Source: 183108-2009-03-10.xml*