# Peroxisome Proliferator-Activated Receptors in HCV-Related Infection

Authors: Sébastien Dharancy; Maud Lemoine; Philippe Mathurin; Lawrence Serfaty; Laurent Dubuquoy
Journal: PPAR Research
 (2009)

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## Abstract

The topic of peroxisome proliferator-activated receptors has been developed in the field of hepatology allowing envisaging therapeutic strategies for the most frequent chronic liver diseases such as chronic infection with hepatitis C virus (HCV). PPARs contribute to wide physiological processes within the liver such as lipid/glucid metabolisms, inflammatory response, cell differentiation, and cell cycle. In vitro experiments and animal studies showed that PPARα discloses anti-inflammatory property, and PPARγ discloses anti-inflammatory, antifibrogenic, and antiproliferative properties in the liver. Experimental and human studies showed impaired PPARs expression and function during HCV infection. The available nonhepatotoxic agonists of PPARs may constitute a progress in the therapeutic management of patients chronically infected with HCV.


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## Body

## 1. Introduction

Chronic infection  with the
hepatitis C virus (HCV) is a major cause of chronic liver disease worldwide,
affecting about 3% of the population
[1].
The natural history of HCV infection is characterized by a high rate of
progression to chronic hepatitis leading, in at least 20% of cases, to
cirrhosis and ultimately to hepatocellular carcinoma [2, 3]. The precise mechanisms underlying HCV-related
liver injury are not well understood but involve a cell-mediated immune
response with lymphocytic infiltration leading to a chronic inflammatory
response and progressive scale fibrosis which are dependent on proinflammatory
and fibrogenic mediators. HCV infection is also characterized by disruption of lipid and glucid metabolisms leading to hepatocyte fat
accumulation (also called hepatic steatosis) and an increase risk of diabetes [4–7]. These latter are under dependence, at least in part, of peroxisome
proliferator-activated receptors functions.Peroxisome
proliferator-activated receptors (PPARs) belong to the nuclear receptor superfamily and require heterodimerization
with receptor X for retinoids (RXR) in order to function [8]. The first PPAR identified was murine PPARα [9]. Since its
description in 1990, a family of homologous receptors has been identified in various species. The PPAR
family includes three subtypes: PPARα (NR1C1), PPARβ (PPARδ or NR1C2), and PPARγ  (NR1C3) [10]. PPARα/γ, together with their obligate partner RXR, are the
three main nuclear receptors expressed in the liver. These receptors
contribute to the great diversity of physiological processes in the liver, such
as control of lipid and glucid metabolisms, inflammatory responses, and cellular differentiation
and proliferation. PPAR activation has been associated with anti-inflammatory
and antifibrotic functions in the liver. In light of their multiple activities,
PPARs quickly became considered as therapeutic targets of the most widespread
human metabolic diseases (obesity, diabetes, and atherosclerosis). The
development of new nonhepatotoxic ligands made
it possible to use PPARs as new therapeutic targets in hepatology. For these
reasons, recent discoveries in the field of PPARs are of intense interest not
only to fundamental researchers but to clinicians as well. This review
will first describe the main functions of PPARs within the liver. Then the
interactions between PPARs and HCV will be developed to provide a precise
picture of the potential role of PPARs in HCV pathophysiology and therapy.


## 2. Control of Lipid and Lipoprotein Metabolism

PPARα is strongly involved in the control of lipid
and lipoprotein metabolisms.
PPARα−/− mice display minimal accumulation of
triglycerides in the liver under fed conditions, but manifest an exaggerated
steatotic response to fasting [11]. Moreover, PPARα−/− mice fed a high-fat diet showed massive
accumulation of lipids in the liver, highlighting its crucial role in lipid
metabolism [12]. Intracellular fatty
acid (FA) concentrations are controlled, in part, by regulation of the FA import
and export system. PPARs control the uptake of FA in liver; for example, PPAR
agonists control expression of the FA transport protein (FATP-1) responsible
for FA transport across the cell membrane, FA translocase (FAT/CD36), and the
hepatic cytosolic FA binding protein (L-FABP) involved in FA trafficking [13]. The absence of
induction of FATP-1 and FAT/CD36 mRNA in liver by PPARα activators has been well demonstrated in PPARα−/− mice. PPARα and -γ also prevent the efflux of FA by promoting
their activation into fatty acyl CoA thioesters by the acyl-CoA synthetase ACS [14, 15]. In addition, PPARα stimulates activated FA catabolism by the
peroxisomal, microsomal, and mitochondrial β-oxidation systems. It regulates the expression
of genes coding for enzymes such as acyl-CoA oxidase (AOX), the first and main
enzyme in the classic peroxisomal β-oxidation system. It also activates the transcription
of genes encoding for classical microsomal β-oxidation system cytochrome P450 CYP4A (A1 and
A3) isoforms, and stimulates activated FA catabolism by mitochondrial β-oxidation. It is generally accepted that oxidation
of long-chain FA is regulated at the level of the liver carnitine
palmitoyl-transferase I (LCPT1), whose product catalyzes the initial step in
long-chain FA import into mitochondria. PPARα has been demonstrated to affect FA import into
mitochondria by upregulating expression of the LCPT1 gene [14]. Concerning PPARγ, treatment of HepG2 with troglitazone, a
synthetic agonist, specifically increased recycling by peroxisomal β-oxidation of C18 to C16 FAs, and the
interconversion of long-chain FAs was associated with reduced de novo lipogenesis. It is
conceivable that these effects of troglitazone on oxidation, interconversion,
and synthesis-saturated FA play a major role in cellular energy metabolism,
membrane lipid composition and turnover [16].It
has been convincingly established that PPARα activation increases plasma HDL cholesterol and
enhances reverse cholesterol transport via the induction of hepatic
apolipoprotein A-I and apolipoprotein A-II expression in human liver.
Nevertheless, PPARα activators affect HDL metabolism in an opposite
manner in rodents and humans. While fibrate treatment of rats lowers plasma
HDL, an increase is generally observed in humans; such an increase in HDL
plasma levels is related, at least in part, to changes in apoAI and apoA-II
gene expression in liver. ApoA-I and apoA-II gene transcription is upregulated
in humans and repressed in rodents by PPARα activators. Treatment of human primary hepatocytes with fibrates
increases mRNA levels and secretion of apoA-I. Moreover, in patients with familial combined hyperlipoproteinemia, ciprofibrate
administration enhances production of apoA-I [17].


## 3. Anti-Inflammatory and Immunomodulatory Properties of PPAR in the Liver

PPARα exert catabolic
function through peroxisomal, microsomal, and mitochondrial β-oxydation pathways,
and therefore allows the use of FA by hepatocytes as energetic substrate. Thus,
it degrades several lipid inflammatory mediators (prostaglandins, leukotriens)
using these metabolic pathways [18, 19]. There are numerous experimental
and clinical evidences in favor of anti-inflammatory activity of PPAR in the
liver. PPARα and PPARγ were shown to
negatively interfere
with the nuclear factor (NF)-κB, signal transducers
and activators of transcription (STAT), and activating protein 1 (AP-1)
signalling pathways in primary hepatocyte cultures and monocytes/macrophages [20, 21]. Delerive et al. showed
that fibrates, which are the main synthetic agonists of PPARα induced the expression
of IκBα protein in primary
human hepatocytes, whereas neither IκB kinase activity nor
the degradation rate of IκBα was affected. The
consequence is that the downstream inflammatory responses genes such as IL-2,
IL-6, IL-8, TNFα, and metalloproteases
are inhibited [20]. PPARα can also control
hepatic inflammation by regulating hemostatic factors and acute-phase response
proteins in hepatocytes [22].T-lymphocytes, natural killer cells, and
monocytes/macrophages express significant amount of PPARα and γwhen activated, and they inhibit the production of
inflammatory and immunomodulatory activity cytokines [23–28]. In lymphocytes, immunomodulatory activity is supported by a
decrease in nuclear factor of activated T-cells (NFAT) activity which regulate IL-2
promoter [23, 29]. PPARγ inhibit the proliferation
of T-lymphocytes and the production of IFNγ, TNFα, and IL-2 [30].Animal studies confirmed the role of PPARα and PPARγ in inflammation control
showing that PPARα−/− mice
display a prolonged response to inflammation induced by arachidonic acid or zymosan
[31–33]. Interestingly, PPARα−/− and PPARγ+/− mice displayed an exacerbated sensibility to hepatitis in different
models of liver injury [34, 35].


## 4. Antifibrogenic Activities of PPAR in the Liver

Hepatic
stellate cells   (HSCs) are the main cells
responsible for liver modulation, by producing the protein of the extracellular
matrix [36, 37]. In response to liver
injury, HSC changes from a quiescent to an activated phenotype. This activation
process includes a phenotypic change to a myofibroblast-like cell, an increased
proliferation rate, loss of retinoid stores, and increased production of
extracellular matrix proteins, chemokines, cytokines, and contractility.
Interestingly, HSC activation is associated with impaired expression of PPARγ and with a decrease in binding to the peroxisome
proliferator response element (PPRE) in vivo, whereas NFκB and AP-1 binding is increased. HSC activation
is reversed by treatment with PPARγ agonists in vitro, suggesting antifibrogenic activity
[38, 39]. PPARγ activation dose-dependently inhibited HSC
proliferation and chemotaxis induced by platelet-derived growth factor. This
effect was independent of changes in postreceptor signalling or expression of
c-fos and c-myc, and was associated with inhibition of cell progression beyond
the G1 phase. PPARγ activation also resulted in complete inhibition
of the expression of monocyte chemotactic protein (MCP1), a potent
chemoattractant for monocytes and T-lymphocytes [40]. All things
considered, PPARγ activation in vitro resulted in decreases in
HSC proliferation, migration, and chemokine expression, three pivotal actions
relevant to the process of liver wound healing and fibrogenesis. A study performed
in a model of liver fibrosis induced by dimethylnitrosamine or carbon
tetrachloride showed that oral administration of pioglitazone or rosiglitazone
reduces hepatic extracellular matrix deposition and HSC activation [41]. PPARα ligands may
also have rescue effects on hepatic fibrosis. Using thioacetamide models of
liver cirrhosis, animals treated with a diet containing one of the two PPARα
ligands, Wy-14 643
(WY) or fenofibrate, displayed reduced hepatic fibrosis. PPARα ligands
had an antifibrotic action in the thioacetamine model of liver cirrhosis,
probably due to an antioxidant effect of enhanced catalase expression and
activity in the liver [42].


## 5. Interaction between HCV and PPARs

Liver
inflammation, hepatocyte fat accumulation, and diabetes are three pivotal
hallmarks in the natural history of chronic HCV infection which are at
least in part controlled by PPARs. Thus, it was tempting for several teams to
investigate whether PPARα/γ may be impaired and/or nonfunctional, and might
therefore play a role in the physiopathology of HCV infection.Numerous studies have demonstrated that PPARα/γ were physiologically highly expressed by parenchymal
hepatic cells, where they play a pivotal anti-inflammatory and metabolic role.
Several teams have demonstrated, using in
vitroand ex vivo approaches, a decrease
expression and functional activity of PPARα in hepatocytes during HCV infection, which may
contribute to the pathogenesis of the disease. (1) During chronic HCV infection in humans [43, 44], (2) in an experimental model of transgenic
mouse [4], and (3) in transfected
hepatocytes expressing the capsid protein of HCV [4, 43, 44]. These abnormalities may be
the consequence of viral capsid inhibiting transcriptional activity of genes
involved in lipid metabolism of infected host cell [4, 43, 44].In
the same line, De Gottardi et al. showed that (1) PPARγ expression was
significantly lower in genotype 3 compared with genotype 1 HCV infection and (2) steatosis was
associated to decreased levels of PPARγ in genotype 1 HCV infection.
In this study, there was no significant relationship between PPARs mRNA levels
and liver activity or fibrosis. The presences of steatosis and hepatitis C
virus genotype 3 were both associated with a significant downregulation of
PPARs [44].The
ways in which impaired expression of PPARα/γ contribute to the pathogenesis of HCV hepatitis
are numerous. As previously described, PPARα activators play a regulatory role in the
inflammatory response at the hepatic level by inhibiting cytokines and
acute-phase inflammatory protein production by negatively interfering with NF-κB and AP-1 pathways. Clinical evidence for the
anti-inflammatory role of PPARα in the liver was provided by the
characterization of PPARα−/− mice which were more susceptible to various hepatotoxic compounds
and had delayed liver regeneration compared to their wild-type littermates. A
recent study also demonstrated that activation of PPARα limits the expression of the TH1 cytokine
profile and TNFα production by human CD4-positive T-cells, a
cytokine pattern known to substantially contribute to HCV pathogenesis.Preliminary
human data reinforce interest in this topic. Two pilot studies suggested
therapeutic effects of bezafibrate, an agonist of PPARα/β/γ, during chronic HCV infection. In the first open
trial, 30 patients unresponsive to interferon monotherapy were treated for 6
months with 400 mg/day [45]. At endpoint, viral
load and liver enzymes were significantly decreased, suggesting
anti-inflammatory and antiviral activities of bezafibrate. More recently, 7
patients received 400 mg/day of bezafibrate for 8 weeks. Once again, ALT, AST,
and viral load were lowered compared to baseline [46]. Taken together, these
results may argue in favor of restoration of transcriptional activity of the
hepatoprotective PPARα and/or PPARγ receptors during chronic HCV infection.
Pioglitazone, an agonist of PPARγ, is currently evaluated in patient with chronic
HCV infection and insulin resistance (http://clinicaltrials.gov/show/NCT00189163).


## 6. Conclusion

Chronic infection with HCV is a major cause of chronic
liver disease worldwide and is associated with chronic liver inflammation,
hepatocyte fat accumulation, and diabetes. Nuclear hormone receptors PPARα and γ are
involved in the transduction of metabolic and nutritional signals into
transcriptional responses in order to maintain liver homeostasis. Evidence exists
in favor of a
decreased expression and functional activity of PPARα/γ in hepatocytes during HCV infection related to HCV
core protein, suggesting that HCV may have evolved strategies to prevent
inducing, or to overcome, an efficient response by the host. This decreased
expression of PPARα/γ may be involved in the pathogenesis of HCV infection
through an alteration of the protective effects of these nuclear receptors against
hepatic inflammation and fibrosis. Taken together, all these considerations suggest
that PPARα/γ may represent new potential therapeutic targets in
HCV infection.



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