# PPARs Mediate Lipid Signaling in Inflammation and Cancer

Authors: Liliane Michalik; Walter Wahli
Journal: PPAR Research
 (2008)

---

## Abstract

Lipid mediators can trigger physiological responses by activating nuclear hormone receptors, such as the peroxisome proliferator-activated receptors (PPARs). PPARs, in turn, control the expression of networks of genes encoding proteins involved in all aspects of lipid metabolism. In addition, PPARs are tumor growth modifiers, via the regulation of cancer cell apoptosis, proliferation, and differentiation, and through their action on the tumor cell environment, namely, angiogenesis, inflammation, and immune cell functions. Epidemiological studies have established that tumor progression may be exacerbated by chronic inflammation. Here, we describe the production of the lipids that act as activators of PPARs, and we review the roles of these receptors in inflammation and cancer. Finally, we consider emerging strategies for therapeutic intervention.


---

## Body

## 1. Introduction

Signal lipids are known  to trigger systemic physiological responses, to control
inflammatory reactions, and to regulate key cellular processes, such as cellular
energy metabolism, cell survival, proliferation, migration, and differentiation
[1]. Among these lipids, fatty acids,
diverse fatty  acid derivatives, some eicosanoids, and sterol derivatives are
modulators of gene expression via binding and activation of the
nuclear hormone receptors (NHRs) peroxisome proliferator-activated receptors (PPARs), liver X
receptors (LXRs), and farnesoid X receptor (FXR) [2].
These transcription factors control genes that regulate lipid
homeostasis [2] and, for PPARs in particular,
inflammatory responses [3]. Disturbance of lipid signaling
and/or NHR pathways promotes the progression of a long list of imbalances and
diseases, such as obesity, type 2 diabetes, chronic inflammation,
cardiovascular diseases, cancer, hypertension, degenerative diseases,
autoimmune diseases, and a few others [1, 2]. Important
cross-regulation exists between lipid signaling and NHR pathways, which
generates a variety of responses dependent on signaling networks that are often
tissue-specific [1].In
this paper, we propose an integrated view of the production of the lipids that
activate PPARs, and of the functions of these receptors in inflammation and
cancer. We conclude with comments on therapeutic opportunities.The
three PPAR isotypes (PPARα
or NR1C1, PPARβ/δ
or NR1C2, and PPARγ
or NR1C3) share a high degree of structural similarity with all members of the
nuclear hormone receptor superfamily [4–6]. The cellular
and systemic roles that have been attributed to PPARs extend far beyond the
control of hepatic peroxisome proliferation in the rodents after which they
were initially named [2, 3, 7]. PPARs exhibit
isotype-specific tissue expression patterns, with PPARα expressed at high levels in organs with a
significant catabolism of fatty acids, PPARβ/δ
in all cell types analyzed so far with levels depending on the extent of cell
proliferation and differentiation, and with PPARγ found at high levels in the adipose tissues
and lower levels in colon, immune cells, and other tissues [8]. Transcriptional regulation by
PPARs requires heterodimerization with the retinoid X receptor (RXR), and
interactions with coregulator complexes [9–11]. When activated
by a ligand, the PPAR:RXR dimer controls transcription via binding to the
peroxisome proliferator response element (PPRE) in the regulatory region of
target genes [9]. The selective action of PPARs in
different tissues results from the combination, at a given time point, between
expression levels of each of the three PPAR and RXR isotypes, affinity for a specific
regulatory PPRE, ligand production by lipid-modifying enzymes, and cofactor
availabilities [12].


## 2. Production of Endogenous PPAR Ligands

The
prevalent point of view today is that the three PPAR isotypes function, in a
broad sense, as lipid sensors that translate lipid signals from different
origins into responses whose aim is to maintain energy homeostasis, in response
to the different physiologic challenges to which the body is exposed. However,
the connection between lipid metabolism pathways and PPAR responses was only
recently unveiled. The production and nature of the endogenous ligands or
mediators of PPAR activation have not been well characterized although it is
known that many lipid-modifying enzymes are involved. The pathways that
generate these lipid signals from fatty acids, which also serve as PPAR
ligands, are
recapitulated in Figure1.Figure 1
PPARs
are mediators  of lipid signaling in inflammation and cancer. Lipid
mediators originate from and
participate in the control of physiological and pathophysiological situations.
Many lipid-modifying enzymes are involved in the production of PPAR ligands. The cyclooxygenases (COX), lipoxygenases
(LO), epoxygenases/cytochrome (CYP)/P450s enzymes, and the lipases use either
fatty acids, triglycerides, or phospholipids as substrates to generate PPAR
ligands, which are guided to their receptors by the cytoplasmic fatty acid
binding proteins (FABPs). PPARs translate these lipid
signals into responses, which maintain energy
homeostasis, regulate inflammation and modify tumor growth. Among the
pathways involved in inflammation and cancer, PPARs interact with COX2, NF-κB,
MAPKs, and PTEN.  PPARα
and γinhibit COX2 expression, thereby reducing the
production of their own ligands. Conversely, PPARβ/δ
is thought to activate COX2 expression, generating a positive feedback loop by
increasing the production of PPAR ligands. PPARs reduce inflammation by
inhibiting NF-κB,
a major pathway that links chronic inflammation to cancer promotion. Several modes of interactions between PPARs and MAPKs have been reported, but
the relevance and consequences of such crosstalks are unclear. Finally, PPARβ/δ
and γdecrease and increase the expression of the
tumor suppressor PTEN (phosphatase and tensin homologue deleted from chromosome
10), respectively.  PPARγ activation of PTEN is thought to potentiate
its tumor suppressor function, whereas PPARβ/δ
would have the opposite effect.ω-3
and ω-6
polyunsaturated fatty acids are stored in membrane phospholipids and lipid
bodies, and are released by cytosolic phospholipase A2 (cPLA2) [13]. ω-6 fatty acids, predominantly arachidonic acids,
are abundant in the western diet and they are often converted to leukotrienes,
prostaglandins, and other cyclooxygenase or lipoxygenase products [13].  They regulate cellular functions
with inflammatory, atherogenic, and prothrombotic effects [13]. The ω-3
fatty acids, such as docosahexaenoic acid and eicosapentaenoic acid, are also
substrates for cyclooxygenases and lipoxygenases. Interestingly, ω-3 fatty acid-derived eicosanoids antagonize
the proinflammatory effects of ω-6
fatty acids by downregulating inflammatory and lipid synthesis genes, and by
stimulating fatty acid degradation [13]. Many eicosanoids bind to PPARs
and control tissue homeostasis and inflammation [3, 14].The
epoxygenases are a group of microsomal cytochrome P450s (CYP) enzymes that
convert arachidonic acid to epoxyeicosatrienoic acids (EETs), which function
primarily as autocrine and paracrine mediators in the cardiovascular and renal
systems [15]. These mediators, which are
unstable and are rapidly metabolized in most tissues, have important roles in
cellular migration and proliferation, and in inflammation. Although their
mechanism(s) of action is not fully understood, the epoxygenase pathway can
generate potent ligands for the PPARs, which participate in antiatherogenic,
antithrombotic, and cardioprotective processes that may be targeted by new
therapeutic developments in vascular and inflammatory disorders [16].The
various lipases have unique pattern of expression, distinct biological actions,
and preferred substrate from which they release diverse products [17]. They preferentially hydrolyze
triglycerides versus phospholipids, and use lipoproteins, such as very
low-density lipoproteins (VLDLs), low-density lipoproteins (LDLs), and high-density
lipoproteins (HDLs), as substrates [17]. Hydrolysis of triglycerides
within triglyceride-rich lipoproteins by the lipoprotein lipase (LPL) results
in the  transfer of lipids and apolipoproteins to HDLs. In turn, hepatic lipase (HL) hydrolyzes HDL
triglyceride and phospholipids, generating smaller lipid-depleted HDL
particles. Finally, endothelial lipase (EL) might hydrolyze HDL phospholipids,
thus promoting HDL catabolism [17]. Lipases generate various
lipolytic products such as fatty acids with different chain lengths and degrees
of saturation, as well as other molecules such as monoacylglycerol. While fatty
acids can be oxidized in order to gain energy, or alternatively stored in fat,
they can also direct transcriptional responses. PPAR activation, as a
consequence of lipolysis, underscores a key role of the functional interplay
between lipases and lipoproteins. It was reported that LPL acts on circulating
lipoproteins to generate PPARα ligands
that induce endothelial vascular cell adhesion molecule 1 (VCAM1) [18]. LPL can release
HODEs, which are known as PPARα
agonists, from electronegative LDL, thereby reversing the proinflammatory
responses of this lipoprotein. Similarly, HDL hydrolysis, and to a lesser extent
hydrolysis of LDL and VLDL, by EL can also activate PPARα [19, 20]. In macrophages,
VLDL regulates gene expression through activation of PPARβ/δ,
an activation that depends on the release of the VLDL triglycerides by LPL [21]. An additional
lipase, named as adipose triglyceride lipase, desnutrin, iPLA2ζ, or transport secretion protein 2, was
identified more recently. It increases the availability of fatty acids from
VLDL, resulting in increased PPARβ/δ
activity [22–24]. Obviously, the
combination of a variety of lipases and lipoproteins and the resulting
distribution in the organism of fatty acids and their often short-lived
derivatives did not enable a precise characterization of their impact on PPAR
functions as a whole. Furthermore, activation of PPARs by ligands produced by
the different lipid signaling enzymes can lead to a feedback stimulation or
inhibition of the expression of these enzymes (see Section 3).


## 3. Guiding Ligands to PPARs: Roles of FABPs

Both
fatty acid binding proteins (FABPs) and retinoic acid binding proteins (CRABPs)
belong to an evolutionarily conserved family of intracellular proteins [25]. Various functions have been
attributed to these proteins, including cellular uptake and transport of fatty
acids, the targeting of fatty acids to specific metabolic pathways, and the
regulation of gene expression and cell growth [26]. Interestingly, FABPs are thought
to deliver ligands to the PPARs. For
instance, specific interactions with fatty acid-loaded adipocyte FABP (FABP4)
and keratinocyte FABP (FABP5) selectively enhance the activity of PPARγ and
PPARβ/δ, respectively [27]. In this
function, FABPs relocate to the nucleus when bound to ligands that are
selective for the PPAR isotype they activate, and thus FABPs mediate the transcriptional activities of their
own ligands. Retinoic acid receptors
(RARs) belong to the same type-2 class of receptors as PPARs in the nuclear
receptor superfamily [12]. A coevolution between the fatty
acid and retinoid-binding protein families and the RAR and PPAR families can be
postulated, which has promoted the emergence of a mechanism for directing a
ligand to the appropriate receptor. The two associated systems, FABPs-PPARs and
CRABPs-RAR, show some promiscuity at the expense of specificity, but in favor
of an increased diversity in transcriptional responses. Depending on the ratio
of FABP5 to CRABP-II, RA activates RAR or PPARβ/δ. Surprisingly, when the FABP5-to-CRABP-II
ratio is high, RA serves as a physiological ligand for PPARβ/δ, which broadens
the spectrum of physiological regulation due to the activity of this receptor
in an unexpected way [28, 29]. The key issue
raised by these studies concerns the importance of the role of directed ligand
transport in nuclear receptor activation, and ligand-dependent crosstalk
between different receptor types [29]. Overruling
ligand selectivity between receptor categories by this mechanism might promote
a promiscuity that may contribute significantly to the pleiotropic effects of
key members of the nuclear receptor superfamily [28, 29].Similarly to the genes encoding lipid-signaling
enzymes, the expression of FABPs is controlled by PPARs in specific situations.
L-FABP is highly expressed in the liver and small intestine, where it
plays an essential role in controlling cellular fatty acid flux. Its expression
is increased by both the fibrate hypolipidemic drugs and LCFAs. The different
PPAR isotypes (α,
β/δ,
and γ)
promote the upregulation by FAs of the gene encoding L-FABP in vitro, while PPARα is an important regulator of L-FABP in the
liver, but not in the intestine [30, 31]. In contrast,
only PPARβ/δ
is able to upregulate the gene encoding L-FABP in the intestine of PPARα-null
mice. Thus, PPARβ/δ
contributes to metabolic adaptation of the small intestine to changes in the lipid
content of the diet [30, 31]. In summary,
FABPs bind PPAR ligands within the cytoplasm, channel this cargo to the
respective nuclear receptors, and by so doing influence their activation, which
sometimes regulates their own expression [32].


## 4. PPARs in Inflammation and Cancer

Although acute inflammation is a
necessary process aimed at protecting the organism after an injury or an
infection, unresolved chronic inflammation may promote cancer formation by
providing an appropriate environment for tumor growth [33, 34]. Mechanisms that
link inflammation and cancer have only recently been studied, but
epidemiological studies show a convincing association between them (see [33–36] and references therein).
For example, hepatitis is often followed by the development of hepatocarcinoma,
ulcerative colitis is a risk factor for colon cancer, and inflammation due to
infection by Helicobacter pylori precedes the majority of gastric cancers [34]. In the lungs also, the risk of
developing lung cancer is higher in patients suffering from asthma or from
chronic bronchitis [37, 38].The role of immune cells
in tumor development is not yet fully understood. Although inflammatory
mediators may promote cancer development, immune cells can also secrete
cytokines that can limit tumor progression [33–35]. Data collected
from mouse models suggest that the role of the immune system in cancer is
likely to depend on the profile of cytokines secreted by the immune cells.
Modifying this profile may contribute to the development of new treatments [33]. Based on present knowledge, the
NF-κB
and COX2 pathways have emerged as important links between inflammation and
cancer (reviewed in [36, 39–42]). Consistent
with inflammation and COX2 favoring the development of tumors, long-term use of
NSAIDs, albeit at relatively high doses, prevents colorectal tumor development [43].The roles of PPARs in tumor
development are still unclear and their pro- or anticarcinogenic effects remain
open to discussion (reviewed in [7, 44]). PPAR activity
has been associated with numerous cancer types in organs such as the liver,
colon, skin, prostate, breast, and lung (reviewed in [7, 45]). The mechanisms
reported so far suggest that the anticarcinogenic activity of PPARs is due to
direct effects in the cancer cells themselves, such as inhibition of the cell
cycle, activation of cell differentiation, or cell death (reviewed in [7, 45]). But in
addition to these functions, one can speculate that PPARs may have non-cell
autonomous effects by acting on the tumor environment. In fact, PPARs regulate
inflammatory processes [3, 46, 47], and they
fulfill vital regulatory functions in cells that are important components of
the tumor stroma, such as immune or endothelial cells [35, 48–51]. In line with
the link between inflammation and cancer promotion, we provide below an
overview of PPARs’ involvement in organs in which inflammatory pathways and
cancer development are known to have been connected, namely, the skin and the
digestive tract.


### 4.1. Skin, Inflammation, and Cancer

An analysis
of various models of PPAR activation or invalidation shows that PPARs are not
absolutely indispensable for normal epidermal maturation and renewal, but that
they accelerate mouse and human keratinocyte differentiation, as well as mouse
epidermal barrier recovery after disruption (reviewed in [52, 53]). In addition, PPARα
and PPARβ/δ activation regulates human hair follicle
survival and mouse hair follicle growth, respectively, whereas the roles of
PPARs in the sebaceous glands remain unclear (reviewed in [52]).After an
injury, skin repair involves the recruitment of inflammatory cells, the
migration and proliferation of keratinocytes, activation of dermal fibroblasts,
and angiogenesis [54]. Though undetectable in the
interfollicular epidermis of healthy rodent skin, the expression of PPARα
and PPARβ/δ
is reactivated in the epidermis at the edges of skin wounds [55]. The expression
of  PPARα
is upregulated early after the injury, but the signal involved is
unknown. The study of genetically modified mice showed that no, or low, PPARα
activity results in impaired inflammatory reaction, which causes a transient
delay in healing [55, 56]. The
upregulation of PPARβ/δ
expression, as well as the production of an unknown endogenous agonist, is
triggered by proinflammatory cytokines, such as TNF-α [57], whereas TGFβ-1
signaling is responsible for the repression of inflammatory-induced PPARβ/δ
expression at the end of the healing process [58]. The completion
of skin healing in the PPARβ/δ-null
animals is delayed, mostly because of impaired epithelialization due to
apoptosis and defects in keratinocyte adhesion and migration [55, 59, 60]. Consistent with
decreased healing efficiency in its absence, prolonged expression of PPARβ/δ
accelerated wound closure [61, 62], whereas
premature downregulation of PPARβ/δ
expression temporarily delayed wound closure [62]. In summary, PPARα and PPARβ/δ
both promote the healing of skin wounds. PPARα prevents exacerbated early inflammation, while
PPARβ/δ,
whose expression and activity are increased by inflammatory cytokines, enhances
keratinocyte survival and migration.Inflammatory
skin disorders are usually characterized by keratinocyte hyperproliferation and
aberrant differentiation, as observed in psoriasis [63, 64]. Moreover,
numerous lipid molecules, which are potent activators of PPARs, are produced in
the psoriatic lesions where they accumulate  [65]. Consistent with
stimulated expression by inflammatory cytokines after skin injury in the mouse  [57], the PPARβ/δ levels are
particularly high in the hyperproliferative lesional skin of psoriatic patients [66], while those of
PPARα
and PPARγ
remain unchanged, or even decrease  [65, 66]. Overall, PPAR
activation reduces inflammation in skin disorders  [53]. It is well
documented that PPARα
activation is beneficial in mouse models of hyperproliferative epidermis [67], in models of
irritant and allergic dermatitis [68], and in a model
of atopic dermatitis [69]. Interestingly, PPARα
may be the molecular target of the antiallergic and anti-inflammatory effects
of palmitoylethanolamide, a natural fatty acid derivative present in murine
skin [70]. PPARγ
activation also has beneficial consequences in various models of psoriatic
skin, such as in organ cultures, in a model of
human psoriatic skin transplant, and in murine models of keratinocyte
hyperproliferation  [71, 72]. Despite these
promising studies in models of psoriatic skin, PPARα, PPARβ/δ, or PPARγ
activation did not improve skin homeostasis when locally applied on psoriatic
plaques [73, 74]. However, PPARγ
agonists thiazolidinediones efficiently normalized skin homeostasis when
orally administrated to patients suffering from psoriasis (reviewed in [75, 76]), suggesting that
their beneficial effects are most likely due to systemic anti-inflammatory
functions of PPARγ.The skin is constantly exposed to many types of aggression, including carcinogens such as xenobiotics or UV. Much remains to be explored regarding PPAR functions in skin cancers, either squamous or basal cell carcinomas (tumors of keratinocyte origin) or melanomas (tumors of melanocyte origin) (reviewed in [52]). Activation of PPARα and PPARγ reduces proliferation and stimulates differentiation of cultured melanocytes [77, 78]. Several PPARγ agonists inhibit the proliferation of human malignant melanomas [79], and the PPARα agonist fenofibrate has antimetastatic effects on melanoma tumors in vivo in a hamster model [80]. Interestingly, combined treatment with the PPARγ agonist pioglitazone, the COX2 inhibitor rofecoxib, and angiostatic chemotherapy stabilized or even reversed chemorefractory melanoma progression, though in only 11% of the treated patients [81]. In a search for genetic factors that may increase melanoma risk, correlation between PPARγ variants and melanoma development in a Caucasian population indicated that PPARγ polymorphisms are an unlikely risk factor for melanoma development in this population [82]. In tumors of keratinocyte origin, increased expression of PPARβ/δ was reported in head and neck squamous carcinoma [83]. In a mouse model of DMBA/TPA-induced skin tumors, PPARβ/δ-null animals showed enhanced tumor formation, suggesting that PPARβ/δ attenuates tumor development. A possible mechanism of this effect is that, by activating the expression of ubiquitin C, PPARβ/δ activates the ubiquitin degradation pathway that is critical for the breakdown of many proteins involved in cell cycle progression [84]. Another proposed mechanism is the downregulation by PPARβ/δ of protein kinase Cα (PKCα) activity, thereby also inhibiting keratinocyte proliferation [85]. However, the selective ablation of PPARβ/δ in keratinocytes did not have any incidence on the development of DMBA/TPA-induced skin tumors, suggesting that PPARβ/δ may exert its tumor modifier activity by acting on the tumor environment [49, 86]. It is worth noting that PPARα activators prevented DMBA/TPA-induced skin tumors when locally applied to mouse skin [87], and reduced UV-induced inflammation in human skin, which is a risk factor for further development of UV-induced skin cancers [88]. On the contrary, the activation of PPARγ did not prevent the development of UV- or DMBA/TPA-induced skin tumors [89], despite increased susceptibility of PPARγ+/− and keratinocyte-selective PPARγ-null mice to DMBA-mediated carcinogenesis [86, 90]. Finally, UV treatment of a human keratinocyte cell line induced the production of an unknown PPARγ activator [91], but the relevance of this observation remains unclear.Taken together, these many observations underscore the implications of PPARs in inflammatory skin disorders, UV-induced inflammation, and tumor development. So far, PPARγ activation in patients has proven efficient to treat psoriasis, but other therapeutical applications remain to be explored and defined, particularly in the field of carcinogenesis.


### 4.2. Digestive Tract Inflammation

Inflammatory bowel diseases (IBDs) are inflammatory diseases affecting the small or the large intestine [92]. Crohn's disease and ulcerative colitis are the best known forms of IBDs although their causes remain unclear. In their acute phase, IBDs are characterized by acute inflammation, involving the recruitment of immune cells and an elevated production of cytokines. Under chronic conditions, abnormal intestinal epithelium morphology and scarring develop. In various animal models of IBD, the activation of PPARα or PPARγ has anti-inflammatory effects in the intestine, resulting in decreased production of inflammatory markers and slower progression of colitis [93–96]. In these models, PPARγ is the best studied isotype. With the exception of one contradictory study showing that long-term pretreatment with a PPARγ agonist aggravated colitis [97], the preventive activation of PPARγ was efficient, whereas the efficacy of ligand administration after the onset of the disease was dependent on the levels of PPARγ [95, 98–100]. PPARγ activation also prevented colon damage caused by immobilization-induced stress [101]. Conversely, enhanced susceptibility to colitis was observed in mice with reduced PPARγ levels or activity [95, 102–105]. The bases of the protective action of PPARγ in colitis are reduced proinflammatory cytokine production, attenuated expression of ICAM-1 and COX-2, inhibition of NF-κB and JNK/p38 MAPK, and modification of immune cell activity
[44, 95, 98, 99, 102, 105–107]. In patients suffering from active ulcerative colitis, a twelve-week treatment with the PPARγ agonist rosiglitazone efficiently cured four out of fifteen patients [108]. Furthermore PPARγ is thought to be one of the molecular targets underlying the beneficial anti-inflammatory effect of 5-aminosalicylic acid, a drug widely used to treat inflammatory bowel diseases (IBDs) [109]. Together, these treatments confirm PPARγ as a potential target in IBDs. The beneficial role of PPARγ activation in inflammatory diseases of the digestive tract may not be limited to the intestine, but seems to extend to gastritis and pancreatitis, an inflammation of the gastric mucosa and pancreas, respectively. In several models of gastritis or gastric ulcers, activation of PPARγ attenuates mucosa damage and accelerates healing, via reduction of inflammation, apoptosis, and lipid peroxidation [110–115]. As in the stomach, PPARγ activity is beneficial in various animal models of pancreatitis, reducing inflammation, restoring exocrine pancreas functions, and limiting chronic pancreatitis development [116–121].In addition  to its already
mentioned anti-inflammatory effects, PPARα protects the intestine from colitis-induced
permeability [122]. So far, the benefits of PPARβ/δ
activation in colitis are poorly documented [44].
One report suggested that PPARβ/δ-null mice exhibit more severe damage in a model of
DSS-induced colitis, whereas a PPARβ/δ agonist had no protective or deleterious effect
when administrated to PPARβ/δ-wt or -null animals [123]. This observation suggests not
only  that PPARβ/δ
protects wt animals against DSS-colitis, but also that this protective effect may be ligand-independent or triggered by a so far nonidentified ligand.The liver is an additional target organ of PPARs
for the control of inflammation. Prolonged
liver inflammation, which is deleterious, usually activates hepatic stellate
cells (HSCs), also known as Ito cells or lipocytes, which proliferate,
transdifferentiate into myofibroblasts, and produce excess extracellular
matrix, finally leading to severe fibrosis and end-stage cirrhosis [124]. Animal models suggest that
limiting, or even reversing, fibrosis may be possible by reducing inflammation,
enhancing HSC apoptosis, blocking HSC transdifferentiation, or stimulating ECM
degradation [124]. Although PPARβ/δ
activation seems to enhance fibrosis via activation of HSC [125], increasing PPARα
or PPARγ
activity appears to have antifibrotic effects.
PPARα
reduces inflammation and oxidative stress [126, 127], and PPARγ
decreases  HSC proliferation, reverses
their profibrotic activity, and counteracts the TGFβ1-induced
production of collagen [128–136]. Recently, PPARγ
activity in human hepatic stellate cells has been shown to be inhibited by
acetaldehyde, the major product of ethanol oxidation and one of the main
mediators of alcohol-induced liver fibrosis [137].In conclusion, manipulating the balance of PPAR isotype activities is an interesting therapeutic concept when used to control inflammation of the digestive tract and associated glands.


### 4.3. Digestive Tract and Cancer

As the literature
includes extensive recent reviews on the interaction between PPARs and Wnt/Apc,
known to play a major role in colorectal cancer progression [7, 138], this paragraph
will focus on data dealing with chronic inflammation as a risk factor for colon
carcinogenesis. Inflammatory bowel diseases, particularly ulcerative colitis,
increase the risk of colorectal cancer in patients [139]. As discussed above, PPARγ
activation has protective effects in animal models of ulcerative colitis
(reviewed in [140]). Moreover, activation of PPARα
and PPARγ
in rodents reduced the formation of aberrant crypt foci, a risk factor for
colon cancer [94]. However, the PPARγ
agonists pioglitazone and rosiglitazone had no effect on the development of tumors
in a mouse model of azoxymethane/dextran sodium sulfate-induced colon cancer,
whereas in the same study the anti-inflammatory 5-ASA reduced the number and
the size of the tumors [141], showing that
PPARγ
is certainly not the only target of 5-ASA. However, in a different study, COX2
inhibitors, the PPARγ
agonist troglitazone and, to a lesser extent, the PPARα agonist bezafibrate, reduced the development
of adenocarcinoma in a mouse model of azoxymethane/dextran sodium
sulfate-induced colon cancer [142, 143].Chronic inflammation finally leading
to cancer may also arise from infections, as in the stomach where infection byHelicobacter pylori is a common risk
factor for gastric cancer [144]. PPARγ expression is increased in gastric epithelia
infected by Helicobacter pylori. The
consequences of upregulated PPARγ
expression are unknown, but it may contribute to reducing inflammation [145]. The treatment of gastric cancer
patients with the COX2 inhibitor rofecoxib correlated with increased levels of
PPARγ
in the tumor [146]. An
epidemiological study performed in a restricted region of Japan suggested that
the Pro12Ala variant of PPARγ,
which is less active than the wt protein, might be associated with increased
risk of gastric cancer [147].Pancreatic cancer is still lethal in
most cases, due to the lack of early markers and specific symptoms and because
of aggressive tumor growth and resistance to treatments [148]. While PPARγ activation shows beneficial anti-inflammatory
effects in the pancreas, the consequences of such activation in patients with
pancreatic cancer are unknown. In
vitro data show that PPARγ
inhibits pancreatic cell proliferation, which would be beneficial, but also
suggest that PPARγ
may activate angiogenesis through induced VEGF expression, which would be
detrimental (reviewed in [148]). In one in vivo study, however, the PPARγ agonist pioglitazone prevented cancer in a
hamster model [149]. In human
patients, a high level of PPARγ
expression correlated with high-grade pancreatic carcinoma [150]. The mechanism
responsible for this effect remains unknown.


### 4.4. Age-Related Diseases

Oxidative stress and
inflammation increase with age, and further enhancement by environmental
factors is thought to favor the development of age-related diseases and
cancers. Although this is not fully clear in human, slight caloric restriction
diet may retard these processes. The roles of PPARs in age-related inflammation
and associated diseases have been reviewed recently in [151–153]. In short, PPARs
are thought to be involved in age-related inflammation, caloric restriction
physiology, and longevity. Increased inflammation levels during aging are
correlated to decreased PPAR activity. Conversely, administration of the PPARα
activator Wy14,643 improved the redox balance and reduced inflammation in aged
mice [154, 155]. A similar
inhibition of age-related inflammation was observed in rat kidney after feeding
with a PPARγ
agonist [156]. Interestingly,
among flavonoids found in fruits and vegetables, which have been associated
with decreased risk of inflammation-mediated diseases, some are PPARγ
agonists that are known to decrease proinflammatory mediator production. For
instance, curcumin, a naturally occurring compound in turmeric, has been used
in India for centuries as an anti-inflammatory agent. It is thought to be a
PPARγ
activator and was suggested to have beneficial effect on colorectal cancer when
taken on a daily basis [152, 157].


## 4.1. Skin, Inflammation, and Cancer

An analysis
of various models of PPAR activation or invalidation shows that PPARs are not
absolutely indispensable for normal epidermal maturation and renewal, but that
they accelerate mouse and human keratinocyte differentiation, as well as mouse
epidermal barrier recovery after disruption (reviewed in [52, 53]). In addition, PPARα
and PPARβ/δ activation regulates human hair follicle
survival and mouse hair follicle growth, respectively, whereas the roles of
PPARs in the sebaceous glands remain unclear (reviewed in [52]).After an
injury, skin repair involves the recruitment of inflammatory cells, the
migration and proliferation of keratinocytes, activation of dermal fibroblasts,
and angiogenesis [54]. Though undetectable in the
interfollicular epidermis of healthy rodent skin, the expression of PPARα
and PPARβ/δ
is reactivated in the epidermis at the edges of skin wounds [55]. The expression
of  PPARα
is upregulated early after the injury, but the signal involved is
unknown. The study of genetically modified mice showed that no, or low, PPARα
activity results in impaired inflammatory reaction, which causes a transient
delay in healing [55, 56]. The
upregulation of PPARβ/δ
expression, as well as the production of an unknown endogenous agonist, is
triggered by proinflammatory cytokines, such as TNF-α [57], whereas TGFβ-1
signaling is responsible for the repression of inflammatory-induced PPARβ/δ
expression at the end of the healing process [58]. The completion
of skin healing in the PPARβ/δ-null
animals is delayed, mostly because of impaired epithelialization due to
apoptosis and defects in keratinocyte adhesion and migration [55, 59, 60]. Consistent with
decreased healing efficiency in its absence, prolonged expression of PPARβ/δ
accelerated wound closure [61, 62], whereas
premature downregulation of PPARβ/δ
expression temporarily delayed wound closure [62]. In summary, PPARα and PPARβ/δ
both promote the healing of skin wounds. PPARα prevents exacerbated early inflammation, while
PPARβ/δ,
whose expression and activity are increased by inflammatory cytokines, enhances
keratinocyte survival and migration.Inflammatory
skin disorders are usually characterized by keratinocyte hyperproliferation and
aberrant differentiation, as observed in psoriasis [63, 64]. Moreover,
numerous lipid molecules, which are potent activators of PPARs, are produced in
the psoriatic lesions where they accumulate  [65]. Consistent with
stimulated expression by inflammatory cytokines after skin injury in the mouse  [57], the PPARβ/δ levels are
particularly high in the hyperproliferative lesional skin of psoriatic patients [66], while those of
PPARα
and PPARγ
remain unchanged, or even decrease  [65, 66]. Overall, PPAR
activation reduces inflammation in skin disorders  [53]. It is well
documented that PPARα
activation is beneficial in mouse models of hyperproliferative epidermis [67], in models of
irritant and allergic dermatitis [68], and in a model
of atopic dermatitis [69]. Interestingly, PPARα
may be the molecular target of the antiallergic and anti-inflammatory effects
of palmitoylethanolamide, a natural fatty acid derivative present in murine
skin [70]. PPARγ
activation also has beneficial consequences in various models of psoriatic
skin, such as in organ cultures, in a model of
human psoriatic skin transplant, and in murine models of keratinocyte
hyperproliferation  [71, 72]. Despite these
promising studies in models of psoriatic skin, PPARα, PPARβ/δ, or PPARγ
activation did not improve skin homeostasis when locally applied on psoriatic
plaques [73, 74]. However, PPARγ
agonists thiazolidinediones efficiently normalized skin homeostasis when
orally administrated to patients suffering from psoriasis (reviewed in [75, 76]), suggesting that
their beneficial effects are most likely due to systemic anti-inflammatory
functions of PPARγ.The skin is constantly exposed to many types of aggression, including carcinogens such as xenobiotics or UV. Much remains to be explored regarding PPAR functions in skin cancers, either squamous or basal cell carcinomas (tumors of keratinocyte origin) or melanomas (tumors of melanocyte origin) (reviewed in [52]). Activation of PPARα and PPARγ reduces proliferation and stimulates differentiation of cultured melanocytes [77, 78]. Several PPARγ agonists inhibit the proliferation of human malignant melanomas [79], and the PPARα agonist fenofibrate has antimetastatic effects on melanoma tumors in vivo in a hamster model [80]. Interestingly, combined treatment with the PPARγ agonist pioglitazone, the COX2 inhibitor rofecoxib, and angiostatic chemotherapy stabilized or even reversed chemorefractory melanoma progression, though in only 11% of the treated patients [81]. In a search for genetic factors that may increase melanoma risk, correlation between PPARγ variants and melanoma development in a Caucasian population indicated that PPARγ polymorphisms are an unlikely risk factor for melanoma development in this population [82]. In tumors of keratinocyte origin, increased expression of PPARβ/δ was reported in head and neck squamous carcinoma [83]. In a mouse model of DMBA/TPA-induced skin tumors, PPARβ/δ-null animals showed enhanced tumor formation, suggesting that PPARβ/δ attenuates tumor development. A possible mechanism of this effect is that, by activating the expression of ubiquitin C, PPARβ/δ activates the ubiquitin degradation pathway that is critical for the breakdown of many proteins involved in cell cycle progression [84]. Another proposed mechanism is the downregulation by PPARβ/δ of protein kinase Cα (PKCα) activity, thereby also inhibiting keratinocyte proliferation [85]. However, the selective ablation of PPARβ/δ in keratinocytes did not have any incidence on the development of DMBA/TPA-induced skin tumors, suggesting that PPARβ/δ may exert its tumor modifier activity by acting on the tumor environment [49, 86]. It is worth noting that PPARα activators prevented DMBA/TPA-induced skin tumors when locally applied to mouse skin [87], and reduced UV-induced inflammation in human skin, which is a risk factor for further development of UV-induced skin cancers [88]. On the contrary, the activation of PPARγ did not prevent the development of UV- or DMBA/TPA-induced skin tumors [89], despite increased susceptibility of PPARγ+/− and keratinocyte-selective PPARγ-null mice to DMBA-mediated carcinogenesis [86, 90]. Finally, UV treatment of a human keratinocyte cell line induced the production of an unknown PPARγ activator [91], but the relevance of this observation remains unclear.Taken together, these many observations underscore the implications of PPARs in inflammatory skin disorders, UV-induced inflammation, and tumor development. So far, PPARγ activation in patients has proven efficient to treat psoriasis, but other therapeutical applications remain to be explored and defined, particularly in the field of carcinogenesis.


## 4.2. Digestive Tract Inflammation

Inflammatory bowel diseases (IBDs) are inflammatory diseases affecting the small or the large intestine [92]. Crohn's disease and ulcerative colitis are the best known forms of IBDs although their causes remain unclear. In their acute phase, IBDs are characterized by acute inflammation, involving the recruitment of immune cells and an elevated production of cytokines. Under chronic conditions, abnormal intestinal epithelium morphology and scarring develop. In various animal models of IBD, the activation of PPARα or PPARγ has anti-inflammatory effects in the intestine, resulting in decreased production of inflammatory markers and slower progression of colitis [93–96]. In these models, PPARγ is the best studied isotype. With the exception of one contradictory study showing that long-term pretreatment with a PPARγ agonist aggravated colitis [97], the preventive activation of PPARγ was efficient, whereas the efficacy of ligand administration after the onset of the disease was dependent on the levels of PPARγ [95, 98–100]. PPARγ activation also prevented colon damage caused by immobilization-induced stress [101]. Conversely, enhanced susceptibility to colitis was observed in mice with reduced PPARγ levels or activity [95, 102–105]. The bases of the protective action of PPARγ in colitis are reduced proinflammatory cytokine production, attenuated expression of ICAM-1 and COX-2, inhibition of NF-κB and JNK/p38 MAPK, and modification of immune cell activity
[44, 95, 98, 99, 102, 105–107]. In patients suffering from active ulcerative colitis, a twelve-week treatment with the PPARγ agonist rosiglitazone efficiently cured four out of fifteen patients [108]. Furthermore PPARγ is thought to be one of the molecular targets underlying the beneficial anti-inflammatory effect of 5-aminosalicylic acid, a drug widely used to treat inflammatory bowel diseases (IBDs) [109]. Together, these treatments confirm PPARγ as a potential target in IBDs. The beneficial role of PPARγ activation in inflammatory diseases of the digestive tract may not be limited to the intestine, but seems to extend to gastritis and pancreatitis, an inflammation of the gastric mucosa and pancreas, respectively. In several models of gastritis or gastric ulcers, activation of PPARγ attenuates mucosa damage and accelerates healing, via reduction of inflammation, apoptosis, and lipid peroxidation [110–115]. As in the stomach, PPARγ activity is beneficial in various animal models of pancreatitis, reducing inflammation, restoring exocrine pancreas functions, and limiting chronic pancreatitis development [116–121].In addition  to its already
mentioned anti-inflammatory effects, PPARα protects the intestine from colitis-induced
permeability [122]. So far, the benefits of PPARβ/δ
activation in colitis are poorly documented [44].
One report suggested that PPARβ/δ-null mice exhibit more severe damage in a model of
DSS-induced colitis, whereas a PPARβ/δ agonist had no protective or deleterious effect
when administrated to PPARβ/δ-wt or -null animals [123]. This observation suggests not
only  that PPARβ/δ
protects wt animals against DSS-colitis, but also that this protective effect may be ligand-independent or triggered by a so far nonidentified ligand.The liver is an additional target organ of PPARs
for the control of inflammation. Prolonged
liver inflammation, which is deleterious, usually activates hepatic stellate
cells (HSCs), also known as Ito cells or lipocytes, which proliferate,
transdifferentiate into myofibroblasts, and produce excess extracellular
matrix, finally leading to severe fibrosis and end-stage cirrhosis [124]. Animal models suggest that
limiting, or even reversing, fibrosis may be possible by reducing inflammation,
enhancing HSC apoptosis, blocking HSC transdifferentiation, or stimulating ECM
degradation [124]. Although PPARβ/δ
activation seems to enhance fibrosis via activation of HSC [125], increasing PPARα
or PPARγ
activity appears to have antifibrotic effects.
PPARα
reduces inflammation and oxidative stress [126, 127], and PPARγ
decreases  HSC proliferation, reverses
their profibrotic activity, and counteracts the TGFβ1-induced
production of collagen [128–136]. Recently, PPARγ
activity in human hepatic stellate cells has been shown to be inhibited by
acetaldehyde, the major product of ethanol oxidation and one of the main
mediators of alcohol-induced liver fibrosis [137].In conclusion, manipulating the balance of PPAR isotype activities is an interesting therapeutic concept when used to control inflammation of the digestive tract and associated glands.


## 4.3. Digestive Tract and Cancer

As the literature
includes extensive recent reviews on the interaction between PPARs and Wnt/Apc,
known to play a major role in colorectal cancer progression [7, 138], this paragraph
will focus on data dealing with chronic inflammation as a risk factor for colon
carcinogenesis. Inflammatory bowel diseases, particularly ulcerative colitis,
increase the risk of colorectal cancer in patients [139]. As discussed above, PPARγ
activation has protective effects in animal models of ulcerative colitis
(reviewed in [140]). Moreover, activation of PPARα
and PPARγ
in rodents reduced the formation of aberrant crypt foci, a risk factor for
colon cancer [94]. However, the PPARγ
agonists pioglitazone and rosiglitazone had no effect on the development of tumors
in a mouse model of azoxymethane/dextran sodium sulfate-induced colon cancer,
whereas in the same study the anti-inflammatory 5-ASA reduced the number and
the size of the tumors [141], showing that
PPARγ
is certainly not the only target of 5-ASA. However, in a different study, COX2
inhibitors, the PPARγ
agonist troglitazone and, to a lesser extent, the PPARα agonist bezafibrate, reduced the development
of adenocarcinoma in a mouse model of azoxymethane/dextran sodium
sulfate-induced colon cancer [142, 143].Chronic inflammation finally leading
to cancer may also arise from infections, as in the stomach where infection byHelicobacter pylori is a common risk
factor for gastric cancer [144]. PPARγ expression is increased in gastric epithelia
infected by Helicobacter pylori. The
consequences of upregulated PPARγ
expression are unknown, but it may contribute to reducing inflammation [145]. The treatment of gastric cancer
patients with the COX2 inhibitor rofecoxib correlated with increased levels of
PPARγ
in the tumor [146]. An
epidemiological study performed in a restricted region of Japan suggested that
the Pro12Ala variant of PPARγ,
which is less active than the wt protein, might be associated with increased
risk of gastric cancer [147].Pancreatic cancer is still lethal in
most cases, due to the lack of early markers and specific symptoms and because
of aggressive tumor growth and resistance to treatments [148]. While PPARγ activation shows beneficial anti-inflammatory
effects in the pancreas, the consequences of such activation in patients with
pancreatic cancer are unknown. In
vitro data show that PPARγ
inhibits pancreatic cell proliferation, which would be beneficial, but also
suggest that PPARγ
may activate angiogenesis through induced VEGF expression, which would be
detrimental (reviewed in [148]). In one in vivo study, however, the PPARγ agonist pioglitazone prevented cancer in a
hamster model [149]. In human
patients, a high level of PPARγ
expression correlated with high-grade pancreatic carcinoma [150]. The mechanism
responsible for this effect remains unknown.


## 4.4. Age-Related Diseases

Oxidative stress and
inflammation increase with age, and further enhancement by environmental
factors is thought to favor the development of age-related diseases and
cancers. Although this is not fully clear in human, slight caloric restriction
diet may retard these processes. The roles of PPARs in age-related inflammation
and associated diseases have been reviewed recently in [151–153]. In short, PPARs
are thought to be involved in age-related inflammation, caloric restriction
physiology, and longevity. Increased inflammation levels during aging are
correlated to decreased PPAR activity. Conversely, administration of the PPARα
activator Wy14,643 improved the redox balance and reduced inflammation in aged
mice [154, 155]. A similar
inhibition of age-related inflammation was observed in rat kidney after feeding
with a PPARγ
agonist [156]. Interestingly,
among flavonoids found in fruits and vegetables, which have been associated
with decreased risk of inflammation-mediated diseases, some are PPARγ
agonists that are known to decrease proinflammatory mediator production. For
instance, curcumin, a naturally occurring compound in turmeric, has been used
in India for centuries as an anti-inflammatory agent. It is thought to be a
PPARγ
activator and was suggested to have beneficial effect on colorectal cancer when
taken on a daily basis [152, 157].


## 5. Crosstalk between PPARs and Pathways Relevant to Cancer and Inflammation

It is obvious from the above that PPARs interact with numerous pathways involved in cancer development (reviewed in [7, 45, 158]). For instance, PPARα regulates the expression of miRNA let-7C in hepatocytes, a tumor suppressor gene that regulates cancer cell proliferation. PPARβ/δ is a downstream target of two pathways often involved in colon cancer development, namely, the Ras and the APC-β-catenin pathways. PPARβ/δ also controls the PTEN/Pi3K/Akt pathway,
whose  actors are  often associated with cancer, and promotes cell migration via activation of the Rho-GTPases [60]. Finally, PPARγ activation can induce growth arrest, differentiation, or apoptosis in many cancer cells [7].In the next sections, we summarize the interaction of PPARs with the main pathways involved in the control of inflammatory responses and cancer development  [3, 46].


### 5.1. COX2 as a Link to Lipid Mediators

Cyclooxygenases (COX) are the enzymes that catalyze the first steps
of the production of prostaglandins from arachidonic acid. The COX1 isoform is constitutively expressed in most tissues, whereas the expression of COX2 is induced in inflamed tissues and in tumors. Genetic, epidemiological, and pharmacological evidence supports the hypothesis that elevated COX2 activity is involved in tumor progression (reviewed in [159–161]). Laboratory experiments as well as clinical studies have shown that COX2 inhibitors are promising antitumoral compounds to combine with other anticancer treatments. However, there is a need to develop new compounds with reduced risk of cardiovascular side effects (reviewed in [40, 159, 161, 162]). Antitumoral activity of COX2 inhibitors most probably results from a combination of effects on angiogenesis, apoptosis, tumor cell invasiveness, and inflammation. Interestingly, PPARα and γactivation may help in inhibiting the activity of COX2 by reducing its expression. PPARα agonists prevented PMA-induced expression of COX2 and VEGF [163], and the PPARγ agonist ciglitazone decreased the expression of COX2 and cJun in a colorectal cancer cell line [164]. COX2 can also modify PPAR activity since some of the COX-2-produced fatty acid derivatives are PPAR activators. COX2 has been proposed to modify the activity of PPARβ/δ in colorectal cancer by producing activators such as PGI2 [165–167] or PGE2, which indirectly increase PPARβ/δ activity [168]. In human cholangiocarcinoma cell lines, activation of PPARβ/δ was shown to increase cell proliferation by increasing the expression of COX2 and thus the production of PGE2 [169]. In this model, PGE2 is meant to subsequently activate PPARβ/δ indirectly via cPLA2α, thereby triggering a positive feedback loop controlling cholangiocarcinoma cell proliferation. Inhibiting COX2 is likely to result in decreased PPAR activity. This was in fact demonstrated in hair follicle growth of murine skin, during which inhibition of COX2 replicates the phenotype of PPARβ/δ-null animals [170]. However, increased PPARγ activity by COX2 inhibitors was also reported, although the mechanism remains unknown (reviewed in [148]). The COX2 and PPAR pathways are certainly interconnected, but to what extent the PPAR activity contributes to the COX2 cancer promotion function is unclear. However, drug-combined modification of PPAR activity in inflammation and cancer is an interesting therapeutic prospect.


### 5.2. NF-κB Links Inflammation to Cancer

The NF-κB pathway is an important link between
inflammation and cancer (see [41]; reviewed in [36, 42]). The three
PPARs are able to antagonize this pathway, via their transactivation or
transrepression activities, thereby leading to the repression of several genes
involved in inflammation [3, 44, 47]. In colon cancer
cell lines, the PPARγ
agonist 15d-PGJ2 attenuated the production of IL-1β-induced IL-8 and MCP-1 by inhibition of NF-κB
activity [96], and induced
apoptosis via NF-κB
and Bcl-2 [171]. In the liver,
the disruption of NF-κB
signaling resulted in the suppression of PPARα-increased expression during a high-fat diet,
whereas, in parallel, an increase in PPARγ expression was observed. In these mice, liver
steatosis (a consequence of decreased FA oxidation and increased expression of
genes involved in lipogenesis), inflammation, and development of liver cancer were
aggravated [172]. Animal  and
preclinical studies showed that an ω-3 fatty acid supplement to the diet should
provide a useful complement to cancer therapy, slowing down progression of
various tumors and improving patients’ quality of life [173]. Among the mechanisms proposed
for these beneficial effects, ω-3 fatty acids repress the NF-κB function and Bcl-2 expression, which in turn
leads to decreased COX2 expression and restoration of functional apoptosis [173]. In addition to PPARs regulating
the activity of NF-κB,
the p65 subunit of the latter was shown to inhibit the transcriptional activity
of PPARγ
on adipocyte gene expression [174] and of the three
PPARs in transfected keratinocytes [65], suggesting that a reciprocal
regulation between the two pathways exists.


### 5.3. MAPK Pathway as a Major Player in Carcinogenesis

The MAPK pathway is activated by
cytokines, and its overactivation is found in the vast majority of cancer cells
and tumors (reviewed in [175]). Phosphorylation of PPARα
and PPARγ
by this pathway increases or decreases their transcriptional activity,
respectively (reviewed in [9, 176]). The
physiological impact of the regulation of PPAR activity through phosphorylation
has mostly been addressed for PPARα
and γregarding insulin signaling and fatty acid
metabolism, but the impact of this modification on inflammation or cancer is
currently not documented [9, 176]. Nevertheless,
PPAR and MAPK crosstalk has been described in immune or cancer cells. In its
unliganded form, PPARα
suppressed p38 MAPK phosphorylation in CD4(+) T cells. Ligand activation
reversed this inhibition, resulting in the expression of the transcription
factor of T cells (T-bet), a marker of Th1 inflammatory responses [177]. The PPARγ
agonist rosiglitazone attenuated TNBS-induced colitis via inhibition of the
activity of the MAPKs p38 and the c-Jun N-terminal kinase (JNK), and of NF-κB,
thereby limiting the expression of proinflammatory genes [95]. In a human
colon cancer cell line, PPARγ
activation was reported to increase the expression of caveolin1, a protein that
is linked to cancer development [178]. This induction seemed to result
from an activation of the MAPK pathway by PPARγ. In another study, the activation of PPARγ
in turn activated the Rho-GTPase/MEK1/ERK1/2 cascade, resulting in
morphological changes and increased motility in rat intestinal epithelial cells
[179]. In lung cancer
cell lines, the PPARγ
agonist troglitazone induced cell differentiation, probably via activation of
Erk1/2 [180, 181]. In addition,
the Erk5-dependent activation of PPARγ
seemed to be responsible for the antitumorigenic effect of the Wnt signaling
pathway [182]. PPARβ/δ
also interacts with the MAPK pathway. When activated by TNFα, the MAPK pathway induced the expression of
the PPARβ/δ gene in inflamed keratinocytes [57]. Once activated
by a ligand produced in parallel, PPARβ/δ
facilitates keratinocyte survival and migration. Interestingly, both the
expression of PPARβ/δ
and the activity of the MAPK pathway are elevated in many tumors [7, 175]. Whether the
expression of PPARβ/δ
is stimulated by this pathway in cancers remains to be investigated. Finally,
anti-inflammatory effects of the MEK5/Erk5 pathway in a muscle cell line are
due to inhibition of NF-κB
and are thought to involve PPARβ/δ
activation [183].Crosstalk between PPARs and MEKs,
the upstream regulators of the MAPK, has also been described [184]. It has been suggested that MEK1
interacts with PPARγ,
thereby causing PPARγ
delocalization from the nucleus to the cytoplasm [185]. Interestingly,
PPARγ
was described as mainly cytoplasmic in human biopsies of salivary duct
carcinoma and breast cancer [186, 187]. Although the
significance of this shuttling is unclear, it should decrease PPARγ
transactivation functions.


### 5.4. PTEN/Pi3K Pathway and its Target mTOR

The phosphatase and tensin homologue deleted from chromosome
10 (PTEN) is a tumor suppressor whose activity is lost in many human
cancers. PTEN is a lipid and protein phosphatase whose main substrate
is the PIP3 produced by the Pi3K. Through its phosphatase activity,
PTEN antagonizes PiK3 activity and inhibits the Pi3K/Akt pathway
involved in the regulation of apoptosis, cell proliferation and growth, and metabolism [188, 189]. The mammalian target of rapamycin (mTOR), one of the targets of the PTEN/Pi3K pathway, is a conserved kinase that regulates central cellular functions in response to environmental signals, such as transcription and translation, mRNA and protein turnover, or autophagy (reviewed in [190, 191]). Impaired mTOR pathway is often associated with tumorigenesis [1]. PPARβ/δ was shown to indirectly inhibit the expression of PTEN in keratinocytes, thereby activating the Pi3K/Akt pathway, which enabled keratinocyte survival [59]. In lung carcinoma cells, the activation of PPARβ/δ stimulated cell proliferation, via decreased expression of PTEN and activation of NF-κB and Pi3K/Akt [192, 193]. While PPARβ/δ decreases PTEN expression, PPARγ activation has the opposite effect. In a model of allergic inflammation in mouse lung, PPARγ agonists decreased inflammation, most probably via increased PTEN expression, and reduced PiP3 levels as well as Akt and NFκB activities
[194]. Treatment of lung carcinoma cell lines with rosiglitazone decreased proliferation via PPARγ-dependent upregulation of PTEN and inhibition of Akt activity, and also via PPARγ-independent inhibition of the mTOR pathway [195, 196]. PPARγ-independent inhibition of mTOR by TZD was also reported in keratinocytes [197]. In this model, TZD inhibited the mitogenic effect of IGF via indirect inhibition of mTOR, a mechanism which may be involved in TZD-mediated inhibition of skin tumor development in transgenic mice overexpressing IGF.In a hepatocarcinoma cell line, PPARγ activation by rosiglitazone inhibited cell migration through increased expression of PTEN [198]. Rosiglitazone also had important anticarcinogenic effects in some highly aggressive anaplastic thyroid cancer cell lines. In these cells, rosiglitazone induced apoptosis, cell cycle inhibition, differentiation, and decreased anchorage-independent growth and migration. This was at least partially due to upregulation of PTEN and inhibition of Akt activity, which antagonized IGF-1 effects necessary for the progression of thyroid cancers [199].In summary, PPARβ/δ and γare both regulators of the expression of PTEN, and interact with the mTOR pathway. PPARβ/δ decreases PTEN expression, whereas PPARγ activates this tumor suppressor gene.


## 5.1. COX2 as a Link to Lipid Mediators

Cyclooxygenases (COX) are the enzymes that catalyze the first steps
of the production of prostaglandins from arachidonic acid. The COX1 isoform is constitutively expressed in most tissues, whereas the expression of COX2 is induced in inflamed tissues and in tumors. Genetic, epidemiological, and pharmacological evidence supports the hypothesis that elevated COX2 activity is involved in tumor progression (reviewed in [159–161]). Laboratory experiments as well as clinical studies have shown that COX2 inhibitors are promising antitumoral compounds to combine with other anticancer treatments. However, there is a need to develop new compounds with reduced risk of cardiovascular side effects (reviewed in [40, 159, 161, 162]). Antitumoral activity of COX2 inhibitors most probably results from a combination of effects on angiogenesis, apoptosis, tumor cell invasiveness, and inflammation. Interestingly, PPARα and γactivation may help in inhibiting the activity of COX2 by reducing its expression. PPARα agonists prevented PMA-induced expression of COX2 and VEGF [163], and the PPARγ agonist ciglitazone decreased the expression of COX2 and cJun in a colorectal cancer cell line [164]. COX2 can also modify PPAR activity since some of the COX-2-produced fatty acid derivatives are PPAR activators. COX2 has been proposed to modify the activity of PPARβ/δ in colorectal cancer by producing activators such as PGI2 [165–167] or PGE2, which indirectly increase PPARβ/δ activity [168]. In human cholangiocarcinoma cell lines, activation of PPARβ/δ was shown to increase cell proliferation by increasing the expression of COX2 and thus the production of PGE2 [169]. In this model, PGE2 is meant to subsequently activate PPARβ/δ indirectly via cPLA2α, thereby triggering a positive feedback loop controlling cholangiocarcinoma cell proliferation. Inhibiting COX2 is likely to result in decreased PPAR activity. This was in fact demonstrated in hair follicle growth of murine skin, during which inhibition of COX2 replicates the phenotype of PPARβ/δ-null animals [170]. However, increased PPARγ activity by COX2 inhibitors was also reported, although the mechanism remains unknown (reviewed in [148]). The COX2 and PPAR pathways are certainly interconnected, but to what extent the PPAR activity contributes to the COX2 cancer promotion function is unclear. However, drug-combined modification of PPAR activity in inflammation and cancer is an interesting therapeutic prospect.


## 5.2. NF-κB Links Inflammation to Cancer

The NF-κB pathway is an important link between
inflammation and cancer (see [41]; reviewed in [36, 42]). The three
PPARs are able to antagonize this pathway, via their transactivation or
transrepression activities, thereby leading to the repression of several genes
involved in inflammation [3, 44, 47]. In colon cancer
cell lines, the PPARγ
agonist 15d-PGJ2 attenuated the production of IL-1β-induced IL-8 and MCP-1 by inhibition of NF-κB
activity [96], and induced
apoptosis via NF-κB
and Bcl-2 [171]. In the liver,
the disruption of NF-κB
signaling resulted in the suppression of PPARα-increased expression during a high-fat diet,
whereas, in parallel, an increase in PPARγ expression was observed. In these mice, liver
steatosis (a consequence of decreased FA oxidation and increased expression of
genes involved in lipogenesis), inflammation, and development of liver cancer were
aggravated [172]. Animal  and
preclinical studies showed that an ω-3 fatty acid supplement to the diet should
provide a useful complement to cancer therapy, slowing down progression of
various tumors and improving patients’ quality of life [173]. Among the mechanisms proposed
for these beneficial effects, ω-3 fatty acids repress the NF-κB function and Bcl-2 expression, which in turn
leads to decreased COX2 expression and restoration of functional apoptosis [173]. In addition to PPARs regulating
the activity of NF-κB,
the p65 subunit of the latter was shown to inhibit the transcriptional activity
of PPARγ
on adipocyte gene expression [174] and of the three
PPARs in transfected keratinocytes [65], suggesting that a reciprocal
regulation between the two pathways exists.


## 5.3. MAPK Pathway as a Major Player in Carcinogenesis

The MAPK pathway is activated by
cytokines, and its overactivation is found in the vast majority of cancer cells
and tumors (reviewed in [175]). Phosphorylation of PPARα
and PPARγ
by this pathway increases or decreases their transcriptional activity,
respectively (reviewed in [9, 176]). The
physiological impact of the regulation of PPAR activity through phosphorylation
has mostly been addressed for PPARα
and γregarding insulin signaling and fatty acid
metabolism, but the impact of this modification on inflammation or cancer is
currently not documented [9, 176]. Nevertheless,
PPAR and MAPK crosstalk has been described in immune or cancer cells. In its
unliganded form, PPARα
suppressed p38 MAPK phosphorylation in CD4(+) T cells. Ligand activation
reversed this inhibition, resulting in the expression of the transcription
factor of T cells (T-bet), a marker of Th1 inflammatory responses [177]. The PPARγ
agonist rosiglitazone attenuated TNBS-induced colitis via inhibition of the
activity of the MAPKs p38 and the c-Jun N-terminal kinase (JNK), and of NF-κB,
thereby limiting the expression of proinflammatory genes [95]. In a human
colon cancer cell line, PPARγ
activation was reported to increase the expression of caveolin1, a protein that
is linked to cancer development [178]. This induction seemed to result
from an activation of the MAPK pathway by PPARγ. In another study, the activation of PPARγ
in turn activated the Rho-GTPase/MEK1/ERK1/2 cascade, resulting in
morphological changes and increased motility in rat intestinal epithelial cells
[179]. In lung cancer
cell lines, the PPARγ
agonist troglitazone induced cell differentiation, probably via activation of
Erk1/2 [180, 181]. In addition,
the Erk5-dependent activation of PPARγ
seemed to be responsible for the antitumorigenic effect of the Wnt signaling
pathway [182]. PPARβ/δ
also interacts with the MAPK pathway. When activated by TNFα, the MAPK pathway induced the expression of
the PPARβ/δ gene in inflamed keratinocytes [57]. Once activated
by a ligand produced in parallel, PPARβ/δ
facilitates keratinocyte survival and migration. Interestingly, both the
expression of PPARβ/δ
and the activity of the MAPK pathway are elevated in many tumors [7, 175]. Whether the
expression of PPARβ/δ
is stimulated by this pathway in cancers remains to be investigated. Finally,
anti-inflammatory effects of the MEK5/Erk5 pathway in a muscle cell line are
due to inhibition of NF-κB
and are thought to involve PPARβ/δ
activation [183].Crosstalk between PPARs and MEKs,
the upstream regulators of the MAPK, has also been described [184]. It has been suggested that MEK1
interacts with PPARγ,
thereby causing PPARγ
delocalization from the nucleus to the cytoplasm [185]. Interestingly,
PPARγ
was described as mainly cytoplasmic in human biopsies of salivary duct
carcinoma and breast cancer [186, 187]. Although the
significance of this shuttling is unclear, it should decrease PPARγ
transactivation functions.


## 5.4. PTEN/Pi3K Pathway and its Target mTOR

The phosphatase and tensin homologue deleted from chromosome
10 (PTEN) is a tumor suppressor whose activity is lost in many human
cancers. PTEN is a lipid and protein phosphatase whose main substrate
is the PIP3 produced by the Pi3K. Through its phosphatase activity,
PTEN antagonizes PiK3 activity and inhibits the Pi3K/Akt pathway
involved in the regulation of apoptosis, cell proliferation and growth, and metabolism [188, 189]. The mammalian target of rapamycin (mTOR), one of the targets of the PTEN/Pi3K pathway, is a conserved kinase that regulates central cellular functions in response to environmental signals, such as transcription and translation, mRNA and protein turnover, or autophagy (reviewed in [190, 191]). Impaired mTOR pathway is often associated with tumorigenesis [1]. PPARβ/δ was shown to indirectly inhibit the expression of PTEN in keratinocytes, thereby activating the Pi3K/Akt pathway, which enabled keratinocyte survival [59]. In lung carcinoma cells, the activation of PPARβ/δ stimulated cell proliferation, via decreased expression of PTEN and activation of NF-κB and Pi3K/Akt [192, 193]. While PPARβ/δ decreases PTEN expression, PPARγ activation has the opposite effect. In a model of allergic inflammation in mouse lung, PPARγ agonists decreased inflammation, most probably via increased PTEN expression, and reduced PiP3 levels as well as Akt and NFκB activities
[194]. Treatment of lung carcinoma cell lines with rosiglitazone decreased proliferation via PPARγ-dependent upregulation of PTEN and inhibition of Akt activity, and also via PPARγ-independent inhibition of the mTOR pathway [195, 196]. PPARγ-independent inhibition of mTOR by TZD was also reported in keratinocytes [197]. In this model, TZD inhibited the mitogenic effect of IGF via indirect inhibition of mTOR, a mechanism which may be involved in TZD-mediated inhibition of skin tumor development in transgenic mice overexpressing IGF.In a hepatocarcinoma cell line, PPARγ activation by rosiglitazone inhibited cell migration through increased expression of PTEN [198]. Rosiglitazone also had important anticarcinogenic effects in some highly aggressive anaplastic thyroid cancer cell lines. In these cells, rosiglitazone induced apoptosis, cell cycle inhibition, differentiation, and decreased anchorage-independent growth and migration. This was at least partially due to upregulation of PTEN and inhibition of Akt activity, which antagonized IGF-1 effects necessary for the progression of thyroid cancers [199].In summary, PPARβ/δ and γare both regulators of the expression of PTEN, and interact with the mTOR pathway. PPARβ/δ decreases PTEN expression, whereas PPARγ activates this tumor suppressor gene.


## 6. Conclusions

In numerous cancer types, PPARs
regulate autonomous processes in tumor cells, such as apoptosis, proliferation,
and differentiation, by interacting with major pathways involved in
carcinogenesis. They also act on the tumor cell environment, modifying
angiogenesis, inflammation, and immune cell functions (reviewed in
[3, 7, 45, 48–51]). Not
surprisingly, their activation has complex consequences, in which the
contribution of tumor cell-autonomous versus nonautonomous mechanisms remains
to be evaluated. Whether PPARs are pro- or anticarcinogenic actors is still open
to discussion, and may depend not only on the origin and genetics of the tumor
cell, but also on the nature of the host tissue and inflammation levels.
Although the possible carcinogenic or toxic effects of PPAR activation remain
an unresolved issue, PPARs nevertheless constitute valuable therapeutic targets
(reviewed in [7, 200]). The use of
PPARα
and PPARγ
agonists is increasing in the treatment of a constantly expanding number of
diseases related to the metabolic syndrome. In this context, although their
supposedly carcinogenic or toxic effects have to be carefully monitored, PPARs
are important therapeutic targets. Many valuable approaches are now under
investigation in order to better understand the mechanisms of adverse effects,
and to develop better compounds. In
vivo models, such as tissue or cell-type selective PPAR knock-out mice,
as well as humanized animals carrying the human PPAR genes, will certainly help
in sorting out the various actions of PPARs in inflammation and cancer. In
addition, the development of selective PPAR modulators (SPPARMs), rather than
PPAR full agonists, which would retain most of the benefits while reducing the
adverse effects of PPAR activation, is a promising approach. For all these
reasons, PPARs are certainly useful pharmaceutical targets to be explored
further in the context of inflammation and/or cancer therapy.



---

*Source: 134059-2008-12-21.xml*