The Role of Peroxisome Proliferator-Activated Receptors in Colorectal Cancer

Abstract

Colorectal cancer is one of the most common cancers in the world. Dietary fat intake is a major risk factor for colorectal cancer. Some nuclear hormone receptors play an important role in regulating nutrient metabolism and energy homeostasis. Among these receptors, special attention has been focused on the role of peroxisome proliferator-activated receptors (PPARs) in colorectal cancer, because PPARs are involved in regulation of lipid and carbohydrate metabolism. PPARs are ligand-activated intracellular transcription factors. The PPAR subfamily consists of three subtypes encoded by distinct genes named PPARα, PPARβ/δ, and PPARγ. PPARγ is the most extensively studied subtype of PPARs. Even though many investigators have studied the expression and clinical implications of PPARs in colorectal cancer, there are still many controversies about the role of PPARs in colorectal cancer. In this paper, the recent progresses in understanding the role of PPARs in colorectal cancer are summarized.

1. Introduction

Colorectal cancer is one of the most common cancers in the world. Its incidence appears to be increasing, particularly in developed countries [1–3]. Colorectal carcinogenesis results from the loss of the normal regulatory pathways involved in cell proliferation and cell death. Especially, molecular alterations of multiple pathways including Wnt (Wingless type)/adenomatous polyposis coli (APC), cyclooxygenase-2 (COX-2), and Ras are known to play important roles in progression of colorectal cancer. Recent progresses in the development of new chemotherapeutic agents have improved the prognosis of colorectal cancer patients [4]. However, for most patients with advanced colorectal cancer, it is still difficult to achieve a complete remission, especially with surgery or chemotherapy. Therefore, significant effort has been exerted to identify novel drug targets for both the prevention and treatment of colorectal cancer.

The peroxisome proliferator-activated receptors (PPARs) belong to members of the nuclear hormone receptor superfamily including receptors for steroid, retinoid, vitamin D, and thyroid hormones [5]. PPARs have received the attention of investigators interested in studying about the intracellular pathways that control signal transduction and gene transcription since their discovery in 1990. The name of PPARs was derived from its property to proliferate peroxisomes in rodent liver, where PPARα plays the major role. However, none of the PPARs could be contributed to peroxisome proliferation in humans [6]. PPARs are metabolic regulators involved in the regulation of glucose and lipid homeostasis. Ligand-activated PPAR forms heterodimer with the retinoid X receptor (RXR) and binds to a PPAR response element (PPRE) to regulate the transcription of numerous target genes [7, 8]. The target genes are involved in cell differentiation, proliferation, immune/inflammation response, and lipid metabolism. PPAR subfamily consists of three members such as PPARα, PPARβ/δ, and PPARγ. PPAR isoforms consist of activation domain (A/B), DNA-binding domain (C), hinge region (D), and ligand-binding domain (E). Each subtype has different characteristics as summarized in Figure 1. PPARα is expressed in brown adipose tissue, liver, kidney, heart, skeletal muscle, and enterocyte. Ligands for PPARα are fibrates, leukotriene B4, and so on. PPARα is involved in peroxisome proliferation, lipid catabolism, lipid-lowering effect, anti-inflammation, keratinocyte differentiation and proliferation, and skin wound healing. PPARβ/δ is ubiquitously expressed and is involved in reverse cholesterol transport, cell proliferation, apoptosis, and so on. PPARγ is expressed in adipose tissue, colon, immune system, hematopoietic cells, and retina. PPARγ is involved in lipid anabolism, adipocyte differentiation, control of inflammation, macrophage maturation, embryo implantation, and molecular targets of antidiabetic thiazolidinediones (Reviewed in [9]). Of the three PPARs identified to now, PPARγ represents the most promising target in view of the many reports implicating this molecule in cancer cell growth.

Figure 1.

Summary of ligands and functions of each PPAR.

2. The Role of PPARα in Colorectal Cancer

Although the procarcinogenic effects of PPARα in rodent hepatocarcinoma are evident, less is known about the role of PPARα in human colorectal cancer. Previous studies showed that activation of PPARα by exogenous agonists causes inhibition of tumor cell growth in cell lines derived from colorectal cancer [10]. However, there is no evidence showing that PPARα expression is elevated in human cancers. Recent studies have shown that aspirin and other nonsteroidal anti-inflammatory drugs reduce the relative risk of developing colorectal cancers [11, 12]. The products of COX activity are known to be involved in carcinogenesis [13–15]. COX-2 is not expressed in most normal tissues but is induced upon stimulation by inflammatory agents, and also by oncogenes, growth factors, carcinogens, and tumor promoters [16–21]. Overexpression of COX-2 contributes to colorectal carcinogenesis by promoting the invasiveness of malignant cells, inhibiting apoptosis, and supporting angiogenesis [22–24]. Furthermore, human colorectal carcinoma patients with COX-2 positive tumors show a significantly poorer prognosis than those with tumors negative for COX-2 [25]. It was recently demonstrated that bile acids, particularly secondary bile acids such as lithocholic acid and chenodeoxycholic acid, can stimulate cell proliferation [26] and act as tumor promoters in colon carcinogenesis [27, 28]. Previous reports have suggested that endogenous bile acids are ligands for nuclear receptors such as farnesoid X receptor (FXR), pregnane X receptor (PXR), and vitamin D receptor (VDR) [29–32]. A recent study reported that bile acids also induce the expression of the PPARα gene via activation of FXR and leads to expression of COX-2 contributing to colorectal carcinogenesis [33]. These data suggest that PPARα has the tumor-promoting activity.

There is a growing importance of chemotherapy for malignant colon cancers. However, resistance to anticancer drugs is still a major obstacle in the failure of chemotherapy in colorectal cancer patients. Tong et al. demonstrated that decreased expression of PPARα confers resistance to hydroxycamptothecin, an inhibitor of topoisomerase I [34]. Thus, they suggest that increased expression of PPARα is necessary to overcome hydroxycamptothecin resistance even though its reason is not clarified.

3. The Role of PPARγ in Colorectal Cancer

The PPARγ is a ligand-activated transcription factor of the nuclear receptor superfamily [35, 36] and is expressed in a variety of malignant tissues including prostate, breast, and colon [37–41]. Upon activation, PPARγ forms heterodimer with RXR and mediate transcriptional activation by binding to the PPRE [7, 8]. In the inactive state, association of various corepressor molecules with PPARγ (e.g., nuclear receptor corepressor or silencing mediator for retinoid receptor and thyroid hormone receptors) prevents this complex from binding to DNA. For transcriptional transactivation of PPARγ, recruitment of coactivators (e.g., CCAAT/enhancer-binding protein, cyclic adenosine monophosphate response-element-binding protein, steroid receptor coactivator-1, receptor-interacting protein 140, PPARγ coactivator-1, and PPARγ binding protein) is required which replace co-repressors from the heterodimer complex. Transcriptional transrepression occurs through a genome independent mechanism and is mediated via physical association of the heterodimer with other activated transcription factors (STAT, NF-κB, and AP-1) thereby blocking their functions (reviewed in [42]). PPARγ has been known to be related to inflammation, immune response, and pathogenesis of some disorders including obesity, atherosclerosis, cancer, and so on [43]. There are natural ligands for PPARγ, including long-chain polyunsaturated fatty acids, eicosanoids, components of oxidized low density lipoproteins (oxLDL) [44], and oxidized alkyl phospholipids. The prostaglandin J₂ derivative, 15d-PGJ₂ is the most potent endogenous ligand for the PPARγ receptor. The antidiabetic thiazolidinedione (TZD) class of drugs including troglitazone, rosiglitazone, pioglitazone and ciglitazone are synthetic ligands for PPARγ [44]. Recent studies have focused on the effect of PPARγ ligands as anticancer agents. However, there are still controversies about the antitumor activity of PPARγ agonists. Thus, this paper describes the role of PPARγ in colorectal cancer and its detailed mechanisms clarified until now.

3.1. The Role of PPARγ as a Tumor Suppressor in Colorectal Cancer

Several studies have focused on the putative association between the various polymorphisms and mutations of the PPARγ gene and the occurrence of cancer. It was described that 4 somatic PPARγ gene mutations resulting in reducing its function occurred in 55 sporadic colon cancers [45]. However, Ikezoe et al. [46] analyzed 397 clinical samples and cell lines including colon, breast, and lung cancers for mutations of PPARγ gene and showed the absence of PPARγ gene mutations. These data suggest that PPARγ mutations may occur in cancers but they are rare.

There has been substantial accumulation of experimental data supporting that synthetic PPARγ ligands as well as 15d-PGJ₂ induce apoptosis in several types of cancer cells [41, 43]. Although increasing evidence has established that PPARγ agonist induces growth arrest in cancer cells, the molecular mechanism of the growth inhibition by PPARγ agonist is not well understood and complicated. This paper describes some of the molecular mechanisms for anticancer activity of PPARγ (Table 1 and Figure 2) as follows.

Table 1.

Potential molecular mechanisms for anticancer activity of PPARγ.

Actions and molecular mechanisms	References
(1) Inhibition of cell proliferation and induction of apoptosis
(1) Upregulation of PTEN	[47, 48]
(2) Downregulation of survivin	[49]
(3) Downregulation of XIAP	[50, 51]
(4) Suppression of NF-κB	[52]
(5) Up-regulation of cyclin-dependent kinase  (CDK) inhibitors, down-regulation of CDK, and  down-regulation of cyclin D1	[51, 53–57]
(6) Down-regulation of COX-2	[51]
(7) Up-regulation of Krüppel-like Factor4 (KLF4)	[58, 59]
(8) Up-regulation of Bax and down-regulation of Bcl-2	[51, 60, 61]
(9) Inhibition of telomerase activity and hTERT expression	[62]
(2) Induction of cellular differentiation
Up-regulation of E-cadherin and Drg-1 gene expression	[63]
(3) Inhibition of angiogenesis
(1) Down-regulation of vascular endothelial growth factor (VEGF)	[64]
(2) Down-regulation of matrix metalloproteinases (MMP)	[65]
(3) Down-regulation of iNOS and COX-2	[66–72]
(4) Down-regulation of proinflammatory mediators	[73]

Figure 2.

Potential molecular mechanisms for PPARγ as tumor suppressor in colorectal cancer.

3.1.1. Inhibition of Cell Proliferation and Induction of Apoptosis

(1) Upregulation of PTEN —

The Phosphatase and Tensin Homolog (PTEN) tumor suppressor gene modulates several cellular functions, including cell migration, survival, and proliferation by inhibiting phosphatidylinositol 3-kinase (PI-3K)-mediated signaling cascades [74]. Previous studies have demonstrated that rosiglitazone, a synthetic ligand for PPARγ, upregulates PTEN expression in Caco2 colorectal cancer cells [47]. Dai et al. also show that treatment of colon cancer cells with rosiglitazone stimulates expression of tumor suppressor gene PTEN. This effect is probably mediated through the binding of PPARγ on PPRE in the promoter of PTEN [48]. Inhibition of the PI-3K/Akt pathway by increased PTEN expression is believed to underlie this effect of the PPARγ ligand.

(2) Downregulation of Survivin —

Survivin is one of the inhibitors of apoptosis protein (IAP) family since it is overexpressed in almost every human tumor that has been studied, but is barely detectable in most normal adult tissues [75]. Overexpression of survivin is associated with poor clinical outcome with reduced tumor cell apoptosis in patients with colorectal cancer [76, 77]. PPARγ agonist GW7845 induced cell death through downregulation of survivin in colorectal cancer cells [49].

(3) Downregulation of X-Linked Inhibitor of Apoptosis (XIAP) —

XIAP can inhibit apoptosis by binding and thereby inactivating caspases including caspase-9 and the effector caspases (-3 and -7) [78]. Qiao et al. showed that 15d-PGJ₂ and troglitazone mediate XIAP downregulation in colon cancer cells by facilitating ubiquitination and proteasomal degradation [50]. In addition, Lee et al. demonstrated that pioglitazone induces apoptosis through downregulation of XIAP via unknown mechanism in colorectal cancer cell lines [51].

(4) Suppression of NF-κB and GSK-3β —

The transcription factor NF-κB is involved in the regulation of various genes, including metalloproteinases (MMPs), inflammatory response genes, and a number of antiapoptotic genes including cIAP1, cIAP2, and glycogen synthase kinase-3 (GSK-3) [79]. Its activation is also associated with cell proliferation, cell cycle progression, promotion of tumor growth, angiogenesis, and metastasis through the expression of genes participating in malignant conversion and tumor promotion [80–82]. Ban et al. showed that PPARγ agonist, troglitazone inhibits colon cancer cell growth via inactivation of NF-κB by suppressing GSK-3β activity [52].

(5) Upregulation of Cyclin-Dependent Kinase (CDK) Inhibitors, Downregulation of CDK and Downregulation of Cyclin D1 —

Interestingly, CDK5 protein expression and kinase activity were significantly inhibited by ciglitazone, which was associated with ciglitazone-induced antiproliferation in colon cancer HT-29 cells [53]. Cyclin D1 is involved in G1/S progression and increased proliferation. PPARγ activation in intestinal epithelial cells results in the inhibition of cell cycle and S-phase entry though a decrease in cyclin D1 expression [54, 55]. PPARγ ligand treatment not only decreases the protein level of cyclin D1, but also increases the CDK inhibitors p21^CIP and p27^KIP1 through both increased transcriptional activity and inhibition of proteasome degradation in colorectal cancer cells [56, 57]. Ciglitazone also inhibited G1/S cell cycle progression through upregulation of p27 and inhibition of Cdk2 activity in HT-29 cells [56]. Fajas et al. [83] suggested that PPARγ activation in the presence of RB results in G1 arrest, whereas in the absence of RB, cells accumulate in G2/M, endoduplicate, and undergo apoptosis. Lee et al. [51] also showed that pioglitazone treatment leads to G2/M block through downregulation of cyclin B1 and cdc2 and upregulation of p21 in RB-deficient human colorectal cancer SNU-C4 and SNU-C2A cells. Thus, these studies suggest that the antiproliferative or proapoptotic effects of PPARγ agonist are associated with its ability to regulate the expression of various genes which are involved in controlling the cell cycle and cell survival/death.

(6) Downregulation of COX-2 —

Most of the current studies showed that COX-2 contributes to tumorigenesis through various mechanisms and overexpression of COX-2 can stimulate tumor growth, invasion, and metastasis [84, 85]. A previous study showed that pioglitazone induces apoptosis through the downregulation of COX-2, activation of caspase-3, downregulation of Bcl-2 and upregulation of Bax in RB-deficient human colorectal cancer cells [51].

(7) Upregulation of Krüppel-Like Factor 4 (KLF4) —

KLF4 is a member of the Krüppel-like zinc finger transcription factor family. It is extensively expressed in the epithelial cells of the gastrointestinal tract [86–88]. Over-expression of KLF4 in colon cancer cells caused inhibition of DNA synthesis and cell growth [89, 90]. Zhi and Tseng demonstrated that 15d-PGJ₂ inhibits proliferation of HT-29 human colon cancer cells and induces upregulation of KLF4 mRNA and protein through the activation of MEK/ERK and STAT-dependent pathway [58]. They provided a novel mechanism for the antitumorigenic actions of 15d-PGJ₂. In addition, rosiglitazone treatment of colorectal cancer cells caused to G1 arrest because increased expression of KLF4 by rosiglitazone leads to increased expression of p21 and decreased expression of cyclin D1 [59]. These data suggest that KLF4 is a nodal player in a network of PPARγ-regulated genes.

(8) Upregulation of Bax and Downregulation of Bcl-2 —

In colon cancer cells, treatment of the PPARγ ligands (pioglitazone, troglitazone) induces apoptosis through upregulation of the proapoptotic protein Bax and downregulation of the antiapoptpotic protein Bcl-2 [51, 60, 61]. Alternative expression of Bax and Bcl-2 causes apoptosis by the release of cytochrome c and subsequent activation of several effector caspases.

(9) Inhibition of Telomerase Activity and hTERT Expression through Modulation of the Myc/Mad/Max Network —

The telomerase stabilizes telomere length by adding TTAGGG repeats to telomeres [91, 92]. Telomerase activity has been detected in almost all human tumors [93, 94] but not in adjacent normal cells [95, 96]. Human telomerase is composed of human telomerase RNA, telomerase-associated protein 1 and human telomerase reverse transcriptase (hTERT) [91, 97]. The forced expression of hTERT in normal human cells has been reported to increase their lifespan [98], while the expression of dominant-negative hTERT in human cancer cells has been known to inhibit telomerase and cause telomere shortening [99, 100]. A recent study shows that 15d-PGJ₂ and rosiglitazone inhibit Caco-2 colon cancer cell proliferation through the inhibition of telomerase activity and hTERT expression. In addition, it was demonstrated that the inhibition of hTERT expression in Caco-2 cells depends on the downregulation of c-Myc and the upregulation of Mad 1 by PPARγ ligands [62].

3.1.2. Induction of Cellular Differentiation

PPARγ has been demonstrated to induce differentiation in solid tumors both in vitro and in vivo [101]. In colon cancer cells, activation of PPARγ by troglitazone treatment inhibits growth and metastasis through differentiation-promoting effects, such as the marked increase in p21 Waf-1, developmentally regulated GTP-binding protein 1 (DRG-1), and E-cadherin in human colon cancer cells [63]. These effects involve modulation of the E-cadherin/β-catenin system and upregulation of Drg-1 gene expression.

3.1.3. Inhibition of Angiogenesis

Angiogenesis, a formation of new capillaries from the preexisting vessels, is a complex process involved in the degradation of the basement membrane by cellular proteases, the penetration and migration of endothelial cells into the extracellular matrix, endothelial cell proliferation, tube formation, and vessel stabilization [102]. Inhibition of angiogenesis may contribute to the mechanism by which PPARγ agonists halt the cancer process. Several studies demonstrated that PPARγ agonist inhibits angiogenesis through the following mechanisms.

(1) Downregulation of Vascular Endothelial Growth Factor (VEGF) —

VEGF is involved in angiogenesis [103, 104]. VEGF expression is increased in several cancers including colorectal and other tumors [105, 106]. It was shown that rosiglitazone inhibited angiogenesis via the downregulation of VEGF and VEGF mRNA in pancreatic cancer xenografts [64].

(2) Downregulation of Matrix Metalloproteinases (MMPs) —

The process of cancer cell invasion is dependent on the degradation of the extracellular matrix (ECM) by MMPs. MMPs are a family of proteases cleaving several macromolecules of the ECM [107]. 15d-PGJ₂ has been reported to have inhibitory effects on the proliferation and invasiveness of colon cancer cell lines which are associated with G1 cell cycle arrest and downregulation of MMP-7 synthesis [65].

(3) Downregulation of iNOS and COX-2 —

It has been shown that both COX-2 and inducible nitric oxide synthase (iNOS) are overexpressed in various human cancers [108]. It was reported that iNOS is associated with altered expression of important modulators of angiogenesis [108]. 15d-PGJ₂ downregulates iNOS [66–68] and COX-2 [69–71]. The expression of COX-2 and iNOS is regulated by NF-κB. The recent several studies have demonstrated that 15d-PGJ₂ can act as a negative regulator of proinflammatory signaling through blocking the NF-κB activation pathway at multiple levels via covalent modification of NF-κB or its regulators [72]. Thus, antiangiogenic effects of 15d-PGJ₂ might be associated with disruption of NF-κB and subsequent blockade of iNOS and COX-2 expression.

(4) Downregulation of Proinflammatory Mediators —

The potential mechanism of angiogenesis inhibition by 15d-PGJ₂ may involve downregulation of pro-inflammatory mediators. Both physiological and pathological angiogenesis can be stimulated by pro-inflammatory cytokines, such as IL-1 and TNF-α. Certain cytokines (e.g., IL-6 and CSF-1) can influence the phenotype and the function of tumor-associated macrophages and indirectly stimulate tumor invasiveness and angiogenesis [109]. Tumor-associated macrophages play an important role in tumor progression due to production of several angiogenic factors, such as VEGF, IL-8, inflammatory cytokines (IL-1 and IL-10) and proteases (MMP-2 and MMP-9) [109]. Thus, 15d-PGJ₂ inhibits angiogenesis through suppression of such pro-inflammatory cytokines [73]. Induction of several pro-inflammatory cytokines, such as TNF-α, IL-1, and IL-8, is regulated at the transcription level by NF-κB. It is still unclear whether 15d-PGJ₂ exerts an anti-angiogenic effect through inhibition of NF-κB-dependent induction of pro-inflammatory mediators or through downregulation of cancer cell-derived pro-inflammatory cytokine release which is NF-κB-independent. Hence, further investigations are necessary to clarify the signaling pathways that delineate the anti-angiogenic effects of 15d-PGJ₂.

3.2. The Role of PPARγ as a Tumor Promoter in Colorectal Cancer

In contrast to above described, PPARγ has been known to have procarcinogenic activity such as stimulation of tumor cell growth and induction of angiogenesis. This review describes some mechanisms for it as summarized in Table 2 and Figure 3.

Table 2.

Potential molecular mechanisms for procarcinogenic activity of PPARγ.

Actions and molecular mechanisms	References
(1) Stimulation of tumor cell growth
(1) Up-regulation of β-catenin and c-Myc expression	[110]
(2) Up-regulation of COX-2	[111]
(2) Induction of angiogenesis
(1) Up-regulation of VEGF and VEGF receptor	[112–114]
(2) Up-regulation of MMP-1	[115]

Figure 3.

Potential molecular mechanisms for PPARγ as tumor promoter in colorectal cancer.

3.2.1. Stimulation of Tumor Cell Growth

Although the majority of publications indicate that PPARγ agonists have potent antiproliferative properties in several types of cancer cells, there are some reports demonstrating the cell growth promoting effects of 15d-PGJ₂ and other PPARγ ligands. It was shown that activation of PPARγ by troglitazone increased the frequency and the size of colon tumors in C57BL/6J-APC^Min/+ mice [116, 117]. In addition, a recent study shows that low concentration of 15d-PGJ₂ and pioglitazone can promote the growth of APC-mutated HT-29 colon cancer cells in vitro and in vivo [110].

(1) Upregulation of β-Catenin and c-Myc Expression —

The Wnt/β-catenin pathway plays a critical role in the development of colon cancer [118]. Choi et al. showed that low concentrations of 15d-PGJ₂ and pioglitazone promote the HT-29 colon cancer cells in vitro and in vivo through increase in β-catenin and c-Myc expression [110].

(2) Upregulation of COX-2 —

As previously mentioned, 15d-PGJ₂ is one of the major final products of COX-2. Since abnormal overexpression of COX-2 was observed in several cancer cells, COX-2 has been shown to contribute to carcinogenesis by promoting cell proliferation and angiogenesis as well as by protecting cells from apoptosis [119]. The regulation of COX-2 synthesis occurs mainly at the transcriptional level, although mRNA stabilization is also involved. A recent study has shown that 15d-PGJ₂ enhances COX-2 expression through ROS-Akt-driven AP-1 activation in human breast cancer cells [111].

3.2.2. Induction of Angiogenesis

It has been reported that PPARγ agonist can induce angiogenesis in various cell lines. Several studies provided some molecular mechanisms for induction of angiogenesis by PPARγ agonist. Here, this paper summarizes the potential molecular mechanisms for enhanced metastasis and invasion by PPARγ agonist clarified until now.

(1) Upregulation of Expression of VEGF and VEGF Receptors —

It was shown that the mRNA expression of VEGF was augmented by 15d-PGJ₂ and troglitazone in vascular smooth muscle cell, human monocytes/macrophages, human colorectal cancer cells and human coronary artery endothelial cells [112, 113]. More recently, 15d-PGJ₂ and troglitazone have been reported to increase the expression of VEGF and its receptors (Flt-1 and KDR) in myofibroblasts [114].

(2) Upregulation of MMP-1 —

Kim et al. reported that 15d-PGJ₂ enhances the angiogenesis by upregulation of MMP-1 [115]. MMP-1 is a major proteinase degrading native fibrillar collagens. MMP-1 is produced by a variety of cell types, including endothelium. It is implicated in several pathological processes such as tumor invasion and restenosis [120]. In addition, Kim et al. suggested that iron may contribute to increased metastasis and invasiveness by 15d-PGJ₂ in human breast cancer cells [115]. Thus, these studies suggest the regulation of MMP-1 expression by 15d-PGJ₂ may be more complex than expected.

4. The Role of PPARβ/δ in Colorectal Cancer

PPARβ/δ is also expressed in the colon and can be activated by fatty acids. In recent studies, it was shown that PPARβ/δ plays a central role in the differentiation of Paneth cells and innate immunity [121]. The role of PPARβ/δ in colorectal cancer is more controversial than that of PPARγ. Recent studies have shown that PPARβ/δ is involved in the pathogenesis of colorectal cancer [122]. Inactivation of APC upregulates PPARβ/δ expression in colorectal cancer cells [122]. It has also been reported that PPARβ/δ levels increase in colorectal tumor after treatment with the potent carcinogen azoxymethane (AOM) [123]. The increased expression of PPARβ/δ could potentially be activated by endogenous ligands such as COX-derived prostacyclin [123]. It was proposed that PPARβ/δ activation would initiate the expression of target genes, which still remain to be identified, and enhance cell growth. In support of this model, PPARβ/δ-null HCT116 cells have reduced tumorigenecity in a xenograft model [124]. PPARβ/δ expression levels in colorectal cancers are higher than in normal mucosa, supporting the hypothesis that APC suppresses activity of β-catenin/Tcf-4 transcription of target genes, including PPARβ/δ, c-myc, and cyclin D1 [122, 123]. PPARβ/δ expression and activity are also induced by oncogenic K-ras in rat intestinal epithelial cells [125]. These studies support a procarcinogenic role of PPARβ/δ in colorectal cancer.

A few mechanisms have been proposed to explain the procarcinogenic effect of PPARβ/δ. Di-Poï et al. suggested that PPARβ/δ activation increases the expression of 3-phosphoinositide-dependent-protein kinae 1 (PDPK1) and integrin-linked kinase (ILK), and decreases the expression of PTEN, causing increased phosphorylation of AKT, leading to antiapoptotic signaling and enhanced cell survival [126]. Another related mechanism is derived from the observation that ligand activation of PPARβ/δ increases the expression of VEGF through a PPARβ/δ-dependent mechanism, causing increased phosphorylation of AKT, which promotes cell survival by blocking apoptosis [127]. In addition, Kwak et al. [128] demonstrated that PPARβ/δ-binding aptamers suppressed transcription from natural promoters of VEGF-A and COX-2 and inhibited tumorigenic potential of colon-cancer cells. These data suggest that PPARβ/δ play an important role in transcription of tumor-promoting genes such as VEGF-A and COX-2.

However, other studies conflict with those reports. Targeted deletion of APC alleles reduces PPARβ/δ expression in mouse intestine [129]. PPARβ/δ expression in human colorectal cancers or intestinal polyps of APC^Min/+ mice are either unchanged or downregulated as compared with normal controls (reviewed in [130, 131]).

The conflicting results about the effect of PPARβ/δ on intestinal tumorigenesis in APC^Min/+- and AOM-treated mice may be related to differences in the specific targeting strategy employed to delete PPARβ/δ [127]. Deletion of PPARβ/δ exon 4 and/or 5, which encodes an essential portion of the DNA-binding domain, is thought to disrupt PPARβ/δ function as a nuclear transcriptional factor and to inhibit colonic carcinogenesis [127, 132]. Increased expression of VEGF in colon tumors was suppressed by loss of PPARβ/δ expression [133]. These findings indicate that PPARβ/δ has an important role in promoting colonic tumorigenesis. The deletion of exon 8 [134, 135], the last PPARβ/δ exon, is postulated to generate a hypomorphic PPARβ/δ protein that remains at least partly functional.

In a mouse mammary tumor model, treatment with the PPARβ/δ agonist GW501516 accelerated tumor formation, while a PPARγ agonist GW7845 delayed tumor growth [136]. This observation suggests that there are distinct mechanistic differences between PPARγ and PPARβ/δ in regulating tumor progression. A recent study showed that PPARβ/δ confers resistance to PPARγ-induced apoptosis by increasing the expression of survivin [49].

Recently, Yang et al. [137] showed that the specific knockdown of PPARβ/δ in colon-cancer cell lines results in more malignant morphologies, larger colonies and less CEA production, and enhances cell-fibronectin adhesion, without effects on cell invasion and migration. These findings indicate that PPARβ/δ may facilitate differentiation and inhibit the cell-fibronectin adhesion of colon cancer, having a protective role in the carcinogenesis and progression of colon cancer. Further immunohistochemistry data reveal that the expression of PPARβ/δ is closely associated with the differentiation and tumor-node-metastasis stage of rectal cancer. It was also shown that PGI₂ and L-165041, a synthetic PPARδ ligand, activate PPARδ and upregulate PPARδ-mediated 14-3-3ε expression. 14-3-3ε binds and sequesters Bad in cytosol. PGI₂-induced 14-3-3ε upregulation is accompanied by augmented Bad sequestration and protects HT-29 cells from Bad-triggered mitochondrial leakage of proapoptotic factors and the consequent apoptosis [138].

5. Conclusion and Future Directions

Even though the extensive studies to clarify the role of PPARs in colorectal cancer using several PPAR agonists and gene knockout experiments were performed, there are still many controversies about them. PPAR ligands induce many physiological changes, including increased oxidation of fatty acids, which contributes to decreasing serum lipids and reducing body weight; and inhibition of inflammatory signaling. There are good reasons to suggest that PPAR agonists should be potential candidates for treating and preventing colorectal cancer, because obesity and chronic inflammation are major risk factors for colorectal cancer. It is interesting to note that there is an overlap in target genes regulated by each PPAR, but the physiological effects induced by selective PPAR agonists are unique owing to the complexity of the PPAR-dependent and the PPAR-independent effects that each agonist induces. To completely understand the role of PPARs in colorectal cancer, it is necessary to dissect the complex regulation of PPAR expression and to examine interactions of each PPAR with other nuclear receptors and signalling molecules involved in cell proliferation and cell death in the near future.