Activators of the nuclear receptor PPARgamma enhance colon polyp formation

A novel peroxisome proliferator-activated receptor gamma isoform with dominant negative activity generated by alternative splicing

We examined the peroxisome proliferator-activated receptor gamma (PPARG) locus in an attempt to identify expressed sequence tags and/or conserved non-coding sequences in the intron sequences containing open reading frames and potentially able to encode new proteins. We identified a new PPARG transcript, defined gammaORF4, which harbors a readthrough in intron 4. The expected translated protein lacks the ligand-binding domain encoded by exons 5 and 6. We identified the transcript in human tumor cell lines and tissues, synthesized the cDNA, and cloned it in expression vectors. Using transient transfections, we found that gammaORF4 cDNA is translated into a predominantly nuclear protein that does not transactivate a reporter gene. Moreover, the isoform is dominant negative versus PPARgamma. Interestingly, gammaORF4 was expressed in vivo in a series of sporadic colorectal cancers. In some cases, it was expressed, albeit at lower levels, also in the mucosa adjacent to the tumors, suggesting that it may be related to tumorigenesis. A tumorigenic effect of gammaORF4 is in line with our finding that gammaORF4 has not only lost the capacity to restrain cell growth but has acquired the potential to stimulate it. In conclusion, this study demonstrates that gammaORF4 is expressed in vivo, that it has lost some PPARgamma properties, and that it affects PPARgamma functioning. The ability to counteract PPARgamma suggests that gammaORF4 plays a role in the pathogenesis of colorectal cancers.

Dietary (n-3) polyunsaturated fatty acids inhibit HER-2/neu-induced breast cancer in mice independently of the PPARgamma ligand rosiglitazone

Overexpression of human epidermal growth factor receptor 2 (HER-2/neu) characterizes a molecular subtype of breast cancer associated with poor clinical outcome. Preventive strategies for HER-2/neu-positive breast cancer, which is often estrogen and progesterone receptor negative, remain undefined. Activators of peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear hormone receptor also expressed in breast cancer, hold potential as cancer prevention agents. PPARgamma ligands include specific fatty acids and synthetic compounds, such as the thiazolidinediones, which appear to inhibit cell proliferation and tumorigenesis. We hypothesized that a thiazolidinedione, rosiglitazone, may serve as a chemopreventive agent for HER-2/neu-associated mammary carcinogenesis, but that efficacy may be influenced by dietary fat content. We studied the effects of diets enriched with corn or fish oil (25% of energy) with and without rosiglitazone (12 g/kg) in a 2 x 2 factorial design on mammary tumorigenesis in murine mammary tumor virus (MMTV)-HER-2/neu transgenic mice. Despite in vitro evidence of antiproliferative effects in an MMTV-HER-2/neu tumor cell line, rosiglitazone did not affect mammary carcinogenesis in vivo. Interestingly, fish oil-based diets markedly suppressed breast tumor incidence (57% of mice vs. 87% of corn oil-fed mice, P = 0.0001) as well as tumor multiplicity (P = 0.001) and mammary gland dysplasia (P = 0.001). These findings demonstrate a potent preventive effect of (n-3) PUFA on HER-2/neu-mediated mammary carcinogenesis, without interaction with a synthetic PPARgamma activator. Further studies focusing on the mechanisms by which (n-3) fatty acids suppress HER-2/neu signaling pathways involved in the pathogenesis of breast cancer are warranted.

Peroxisome proliferator-activated receptor delta and gamma agonists differentially alter tumor differentiation and progression during mammary carcinogenesis

Peroxisome proliferator-activated receptor (PPAR) represents a ligand-dependent nuclear receptor family that regulates multiple metabolic processes associated with fatty acid beta-oxidation, glucose utilization, and cholesterol transport. These and other receptor-mediated actions pertain to their role in hypolipidemic and antidiabetic therapies and as potential targets for cancer chemopreventive agents. The present study evaluated the chemopreventive activity of two highly potent and selective PPARgamma and PPARdelta agonists in a progestin- and carcinogen-induced mouse mammary tumorigenesis model. Animals treated with the PPARgamma agonist GW7845 exhibited a moderate delay in tumor formation. In contrast, animals treated with the PPARdelta agonist GW501516 showed accelerated tumor formation. Significantly, tumors from GW7845-treated mice were predominantly ductal adenocarcinomas, whereas tumors from GW501516-treated animals were adenosquamous and squamous cell carcinomas. Gene expression analysis of tumors arising from GW7845- and GW501516-treated mice identified expression profiles that were distinct from each other and from untreated control tumors of the same histopathology. Only tumors from mice treated with the PPARgamma agonist expressed estrogen receptor-alpha in luminal transit cells, suggesting increased ductal progenitor cell expansion. Tumors from mice treated with the PPARdelta agonist exhibited increased PPARdelta levels and activated 3-phosphoinositide-dependent protein kinase-1 (PDK1), which co-associated, suggesting a link between the known oncogenic activity of PDK1 in mammary epithelium and PPARdelta activation. These results indicate that PPARdelta and PPARgamma agonists produce diverse, yet profound effects on mammary tumorigenesis that give rise to distinctive histopathologic patterns of tumor differentiation and tumor development.

Molecular and cellular effects of cis-9, trans-11-conjugated linoleic acid in enterocytes: Effects on proliferation, differentiation, and gene expression

It has been hypothesized that dietary conjugated linoleic acids (CLA) may inhibit colon tumorigenesis. The aim of our study was to investigate the cellular and molecular effects of cis-9 (9Z), trans-11 (11E)-CLA on the proliferation, differentiation, interaction with peroxisome proliferator-activated receptors (PPARs), and expression of genes relevant in the APC-beta-catenin-TCF4 signalling pathway in human HT-29 and Caco-2 colon cells. We found that 9Z,11E-CLA inhibited the proliferation of HT-29 and Caco-2 cells. Trans-vaccenic acid (VA) showed no antiproliferative effects at all. We determined that 9Z,11E-CLA induced cell differentiation as measured by intestinal alkaline phosphatase (IAP) enzyme activity in Caco-2 cells, mRNA expression of IAP, and activation of a 5' flanking region of IAP. The 9Z,11E-CLA activated human PPARdelta as measured in a reporter gene assay. Treatment of HT29 cells in the poliferation phase with 9Z,11E-CLA repressed mRNA-expression of proliferation genes such as c-myc, cyclin D1 and c-jun in a concentration dependent manner. The promoter activities of c-myc and AP1 were also inhibited after incubation with 9Z,11E-CLA. beta-Catenin mRNA and protein expression was also repressed by the treatment with 9Z,11E-CLA. In addition, the mRNA expression of PPARdelta was repressed by treatment of the HT-29 cells with 9Z,11E-CLA. We conclude that 9Z,11E-CLA has an antiproliferative effect at the cellular and molecular levels in human colon cells. The results indicate that the preventive effects of CLA in the development of colon cancer may be due to their downregulation of some target genes of the APC-beta-catenin-TCF-4- and PPARdelta signalling pathway.

Chemopreventive Effect of Peroxisome Proliferator–Activated Receptor γ on Gastric Carcinogenesis in Mice

Peroxisome proliferator-activated receptor gamma (PPARgamma) is known to be expressed in several cancers, and the treatment of these cancer cells with PPARgamma ligands often induces cell differentiation and apoptosis. Recently, the chemopreventive potential of PPARgamma ligands on colon carcinogenesis was reported, although the effect of PPARgamma on colon carcinogenesis and the mechanism of the effect remain controversial. In this study, we attempted to elucidate the role of PPARgamma in gastric carcinogenesis and explored the possible use of PPARgamma ligand as a chemopreventive agent for gastric cancer. N-methyl-N-nitrosourea (MNU, 240 ppm) was given in drinking water for 10 weeks to induce gastric cancer in PPARgamma wild-type (+/+) and heterozygous-deficient (+/-) mice, followed by treatment with PPARgamma ligand [troglitazone, 0.15% (w/w) in powder food] or the vehicle alone for 42 weeks. At the end of the experiment, PPARgamma (+/-) mice were more susceptible to MNU-induced gastric cancer than wild-type (+/+) mice (89.5%/55.5%), and troglitazone significantly reduced the incidence of gastric cancer in PPARgamma (+/+) mice (treatment 55.5%/vehicle 9%) but not in PPARgamma (+/-) mice. The present study showed that (a) PPARgamma suppresses gastric carcinogenesis, (b) the PPARgamma ligand troglitazone is a potential chemopreventive agent for gastric carcinogenesis, and (c) troglitazone's chemopreventive effect is dependent on PPARgamma.

Critical Role of Peroxisome Proliferator-Activated Receptor γ on Anoikis and Invasion of Squamous Cell Carcinoma

PURPOSE

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays a important role in various physiological functions. We examined whether PPARgamma is expressed in primary squamous cell carcinoma and lymph node metastasis and whether PPARgamma is a potential target for tumor therapy.

EXPERIMENTAL DESIGN AND RESULTS

A high-level expression of PPARgamma was observed in tumor cells of human primary squamous cell carcinoma, lymph node metastasis, and squamous cell carcinoma cell lines. Treatment with PPARgamma-specific antagonists, but not agonists, caused apoptotic cell death on squamous cell carcinoma cell lines in a concentration-dependent manner. Small interfering RNA for PPARgamma also inhibited cell adhesion and growth of squamous cell carcinomas. The phosphorylation of focal adhesion kinase (FAK) was decreased by treatment with PPARgamma antagonists, and resulted in decreases in phosphorylation of Erk and mitogen-activated protein kinase. Furthermore, PPARgamma antagonists decreased the adhesion of squamous cell carcinomas into fibronectin-coated plates, indicating the inhibition of interaction between squamous cell carcinomas and fibronectin. Expression of integrin alpha5, a counter adhesion molecule for fibronectin, was inhibited by the treatment with PPARgamma antagonists. These results indicate that the decrease in integrin alpha5 and following inhibition of cell adhesion may cause the inhibition of FAK signaling pathways. PPARgamma antagonists also strongly inhibited invasion of squamous cell carcinoma via down-regulation of CD151 expression.

CONCLUSIONS

The cell death caused by the PPARgamma antagonists was a result of direct interference with cell adhesion "anoikis" involving intracellular FAK signaling pathways. These results imply a potentially important and novel role for the inhibition of PPARgamma function via the use of specific antagonists in the treatment of squamous cell carcinoma and the prevention of tumor invasion and metastasis.

Ciglitazone induces early cellular proliferation and NF-kappaB transcriptional activity in colon cancer cells through p65 phosphorylation

While it is well established that peroxisome proliferator activated receptor gamma (PPARgamma) ligands inhibit cell growth and induce apoptosis of colon cancer cells with a prolonged treatment (3-5 days), the early effects of PPARgamma exposure are less clear. In this report, we demonstrate that the PPARgamma ligand, ciglitazone, induces proliferation of HT-29 and Caco-2 colon cancer cells transiently (<48 h) prior to a decrease in cell proliferation (>72 h). Associated with this cellular proliferation phase, we observed an increase in NF-kappaB transcriptional activity. Ciglitazone exposure did not affect NF-kappaB DNA binding but rather, increased phosphorylation of p65 as well as the recruitment of the co-activator CBP. Pre-treatment of HT-29 cells with wortmannin, a phosphatidylinositol 3-kinase (PI3K) kinase inhibitor, inhibited ciglitazone-induced p65 phosphorylation, NF-kappaB transcriptional activity and cell proliferation. Interestingly, ciglitazone inhibited PPAR transcriptional activity, suggesting this early proliferative effect is PPRE independent. These data suggest that the early proliferative phase of PPARgamma ligand exposure is associated with activation of NF-kappaB by p65 phosphorylation and cofactor recruitment and not through increased DNA binding.

Suppression of colitis-related mouse colon carcinogenesis by a COX-2 inhibitor and PPAR ligands

Background

It is generally assumed that inflammatory bowel disease (IBD)-related carcinogenesis occurs as a result of chronic inflammation. We previously developed a novel colitis-related mouse colon carcinogenesis model initiated with azoxymethane (AOM) and followed by dextran sodium sulfate (DSS). In the present study we investigated whether a cyclooxygenase (COX)-2 inhibitor nimesulide and ligands for peroxisome proliferator-activated receptors (PPARs), troglitazone (a PPARγ ligand) and bezafibrate (a PPARα ligand) inhibit colitis-related colon carcinogenesis using our model to evaluate the efficacy of these drugs in prevention of IBD-related colon carcinogenesis.

Methods

Female CD-1 (ICR) mice were given a single intraperitoneal administration of AOM (10 mg/kg body weight) and followed by one-week oral exposure of 2% (w/v) DSS in drinking water, and then maintained on the basal diets mixed with or without nimesulide (0.04%, w/w), troglitazone (0.05%, w/w), and bezafibrate (0.05%, w/w) for 14 weeks. The inhibitory effects of dietary administration of these compounds were determined by histopathological and immunohistochemical analyses.

Results

Feeding with nimesulide and troglitazone significantly inhibited both the incidence and multiplicity of colonic adenocarcinoma induced by AOM/DSS in mice. Bezafibrate feeding significantly reduced the incidence of colonic adenocarcinoma, but did not significantly lower the multiplicity. Feeding with nimesulide and troglitazone decreased the proliferating cell nuclear antigen (PCNA)-labeling index and expression of β-catenin, COX-2, inducible nitric oxide synthase (iNOS) and nitrotyrosine. The treatments increased the apoptosis index in the colonic adenocarcinoma. Feeding with bezafibrate also affected these parameters except for β-catenin expression in the colonic malignancy.

Conclusion

Dietary administration of nimesulide, troglitazone and bezafibrate effectively suppressed the development of colonic epithelial malignancy induced by AOM/DSS in female ICR mice. The results suggest that COX-2 inhibitor and PPAR ligands could serve as an effective agent against colitis-related colon cancer development.

Selective activation of PPARgamma in breast, colon, and lung cancer cell lines

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays a critical albeit poorly defined role in the development and progression of several cancer types including those of the breast, colon, and lung. A PPAR response element (PPRE) reporter assay was utilized to evaluate the selective transactivation of PPARgamma in 10 different cell lines including normal mammary epithelial, breast, lung, and colon cancer cells. Cells were treated with one of four compounds including rosglitizone (Ros), ciglitizone (Cig), 15-deoxy-Delta(12,14)-prostaglandin J2 (PGJ2), or GW 9662 (GW). We observed differences in transactivation between cell lines from different tissue origin, across cell lines from a single tissue type, and selective modulation of PPARgamma within a single cell line by different ligands. Interestingly, GW, a PPARgamma antagonist in adipocytes, enhanced PPRE reporter activation in normal mammary epithelial cells while it had virtually no effect in any of the cancer cell lines tested. Within each cancer type, individual cell lines were found to respond differently to distinct PPARgamma ligands. For instance, Ros, Cig, and PGJ2 were all potent agonist of PPARgamma transactivation in lung adenocarcinoma cell lines while these same ligands had no effect in squamous cell or large cell carcinomas of the lung. Message levels of PPARgamma and retinoid X receptor alpha (RXRalpha) in the individual cell lines were quantitated by real time-polymerase chain reaction (RT-PCR). The ratio of PPARgamma to RXRalpha was predictive of how cells responded to co-treatment of Ros and 9-cis-retinoic acid, an RXRalpha agonist, in two out of three cell lines tested. These data indicate that PPARgamma can be selectively modulated and suggests that it may be used as a therapeutic target for individual tumors.

Inhibition of beta-oxidative respiration is a therapeutic window associated with the cancer chemo-preventive activity of PPARgamma agonists

We demonstrate expression and coordinate induction of PPARgamma and lipogenic enzymes (HMG-CoA synthase, HMG-CoA reductase and fatty acid synthase) in a murine lung alveolar carcinoma cell line (Line 1) treated with the PPARgamma agonist troglitazone (TRO) [0-100 microM]. We postulate that TRO induces a shift in cellular energy metabolism towards fatty acid oxidation (beta-oxidative respiration). Accordingly, co-treatment with TRO [30 microM] and increasing concentrations of trimetazidine (TMZ) [0.1-3 mM], an inhibitor of beta-oxidation, results in a dose dependent decrease cellular ATP levels and a dose dependent induction of apoptosis. These findings, suggest that inhibition of beta-oxidative respiration is a therapeutic window associated with the cancer chemo-preventive activity of PPARgamma agonists.

Thiazolidenediones mediate apoptosis in prostate cancer cells in part through inhibition of Bcl-xL/Bcl-2 functions independently of PPARgamma

Certain members of the thiazolidenedione family of the peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, such as troglitazone and ciglitazone, exhibit antitumor effects; however, the underlying mechanism remains inconclusive. This study shows that the effect of these thiazolidenedione members on apoptosis in prostate cancer cells is independent of PPARgamma activation. First, close structural analogues of thiazolidenediones, whereas devoid of PPARgamma activity, retain the ability to induce apoptosis with equal potency. Second, both PC-3 (PPARgamma-expressing) and LNCaP (PPARgamma-deficient) cells are sensitive to apoptosis induction by troglitazone and its PPARgamma-inactive analogue irrespective of their PPARgamma expression status. Third, rosiglitazone and pioglitazone, potent PPARgamma agonists, show marginal effects on apoptosis even at high concentrations. Evidence indicates that the apoptotic effect of troglitazone, ciglitazone, and their PPARgamma-inactive analogues 5-[4-(6-hydroxy-2,5,7,8-tetramethyl-chroman-2-ylmethoxy)-benzylidene]-2,4-thiazolidine-dione (Delta2-TG) and 5-[4-(1-methyl-cyclohexylmethoxy)-benzylidene]-thiazolidine-2,4-dione, respectively, is in part attributable to their ability to inhibit the anti-apoptotic functions of Bcl-xL and Bcl-2. Treatment of PC-3 cells with troglitazone or Delta2-TG led to reduced association of Bcl-2 and Bcl-xL with Bak, leading to caspase-dependent apoptosis. Bcl-xL overexpression protects LNCaP cells from apoptosis induction by troglitazone and Delta2-TG in an expression level-dependent manner. Considering the pivotal role of Bcl-xL/Bcl-2 in regulating mitochondrial integrity, this new mode of mechanism provides a framework to account for the PPARgamma-independent action of thiazolidenediones in inducing apoptosis in cancer cells. Moreover, dissociation of these two pharmacologic activities provides a molecular basis to develop novel Bcl-xL/Bcl-2 inhibitors, of which the proof of principle is illustrated by a Delta2-TG analogue with potent in vivo antitumor activities.

Hic-5 regulates an epithelial program mediated by PPARgamma

PPARgamma is a dominant regulator of fat cell differentiation. However, this nuclear receptor also plays an important role in the differentiation of intestinal and other epithelial cell types. The mechanism by which PPARgamma can influence the differentiation of such diverse cell lineages is unknown. We show here that PPARgamma interacts with Hic-5, a coactivator protein expressed in gut epithelial cells. Hic-5 and PPARgamma colocalize to the villus epithelium of the small intestine, and their expression during embryonic gut development correlates with the transition from endoderm to a specialized epithelium; expression of both these factors is reduced in tumors. Forced expression of Hic-5 in colon cancer cells enhances the PPARgamma-mediated induction of several gut epithelial differentiation/maturation markers such as L-FABP, kruppel-like factor 4 (KLF4), and keratin 20. siRNA directed against Hic-5 specifically reduces PPARgamma-mediated induction of gut epithelial genes in colon cells and in an ex vivo model of embryonic gut differentiation. Finally, forced expression of Hic-5 during 3T3-L1 preadipocyte differentiation inhibits adipogenesis while inducing inappropriate expression of several mRNAs characteristic of gut epithelium in these mesenchymal cells. These results indicate that Hic5 is an important component in determining an epithelial differentiation program induced by PPARgamma.

15d-PGJ2: the anti-inflammatory prostaglandin?

15-Deoxy-Delta-12,14-prostaglandin J2 (15d-PGJ2) is the most recently discovered prostaglandin. This cyclopentanone, the dehydration end product of PGD2, differs from other prostaglandins in several respects. There is no specific prostaglandin synthase (PGS) leading to 15d-PGJ2 production and no specific 15d-PGJ2 receptor has been identified to date. Instead, 15d-PGJ2 has been shown to act via PGD2 receptors (DP1 and DP2) and through interaction with intracellular targets. In particular, 15d-PGJ2 is recognized as the endogenous ligand for the intranuclear receptor PPARgamma. This property is responsible for many of the 15d-PGJ2 anti-inflammatory functions. In this review, we summarize the current understanding of 15d-PGJ2 synthesis, biology and main effects both in molecular physiology and pathological states.

Modified fatty acids and their possible therapeutic targets in malignant diseases

Fatty acids and other lipids have multiple roles in the cell, functioning as structural components, participating in intracellular signalling and serving as metabolic fuel. Various compounds that influence cellular lipid metabolism can reduce the growth of malignant cells, and dietary as well as pharmacological strategies for modulating lipid metabolism have therefore been suggested as possible approaches for cancer prevention and treatment. By chemically modifying fatty acids (e.g., butyrates, retinoids), new potential anticancer agents have been produced that possess increased metabolic stability and more specific and potent biological activity compared to the natural fatty acids. Possible therapeutic targets for such modified fatty acids include: i) Histone deacetylase; ii) nuclear hormone receptors (retinoid receptors), peroxisome proliferator-activated receptors; iii) cyclooxygenase-2; iv) intracellular signalling involving protein farnesylation and Ras activation; and v) various mitochondrial functions. Although several fatty acid derivatives have been thoroughly investigated in experimental models, clinical data on toxicity and pharmacological interactions are not available for the majority of these agents. However, several promising novel compounds are now being evaluated in preclinical and early clinical studies, and future research will hopefully reveal new formulations and therapy schedules that will improve the outcome of patients with malignant disorders.

Activation of PPAR{gamma} by curcumin inhibits Moser cell growth and mediates suppression of gene expression of cyclin D1 and EGFR

Colorectal cancer is a leading cause of cancer-related morbidity and mortality in the United States. Curcumin, the yellow pigment in turmeric, possesses inhibitory effects on growth of a variety of tumor cells by reducing cell proliferation and inducing apoptosis. Effects of the peroxisome proliferator-activated receptor-gamma (PPARgamma) on stimulating cell differentiation and on inducing cell cycle arrest have attracted attention from the perspective of treatment and prevention of cancer. The aim of this study was to elucidate the mechanisms by which curcumin inhibits colon cancer cell growth. In the present report, we observed that curcumin, in a dose-dependent manner, inhibited the growth of Moser cells, a human colon cancer-derived cell line, and stimulated the trans-activating activity of PPARgamma. Further studies demonstrated that activation of PPARgamma was required for curcumin to inhibit Moser cell growth. Activation of PPARgamma mediated curcumin suppression of the expression of cyclin D1, a critical protein in the cell cycle, in Moser cells. In addition, curcumin blocked EGF signaling by inhibiting EGF receptor (EGFR) tyrosine phosphorylation and suppressing the gene expression of EGFR mediated by activation of PPARgamma. In addition to curcumin reduction of the level of phosphorylated PPARgamma, inhibition of cyclin D1 expression played a major and significant role in curcumin stimulation of PPARgamma activity in Moser cells. Taken together, our results demonstrated for the first time that curcumin activation of PPARgamma inhibited Moser cell growth and mediated the suppression of the gene expression of cyclin D1 and EGFR. These results provided a novel insight into the roles and mechanisms of curcumin in inhibition of colon cancer cell growth and potential therapeutic strategies for treatment of colon cancer.

The peroxisome proliferator-activated receptor gamma ligand rosiglitazone delays the onset of inflammatory bowel disease in mice with interleukin 10 deficiency

AIMS

To test whether the peroxisome proliferator-activated receptor gamma (PPARgamma) ligand rosiglitazone (Ro) has therapeutic activity in the IL-10(-/-) mouse model of inflammatory bowel disease (IBD), and to identify the cellular targets and molecular mechanisms of Ro action.

METHODS

The progression of spontaneous chronic colitis in IL-10(-/-) mice was compared in 5-week-old mice fed a standard diet with or without Ro for 12 weeks. The possible therapeutic effect of Ro was also tested over a 6-week interval in older IL-10(-/-) mice with established IBD.

RESULTS

Treatment with Ro slowed the onset of spontaneous IBD in IL-10(-/-) mice. Crypt hyperplasia, caused by increased mitotic activity of crypt epithelial cells, was also delayed by Ro. Treatment with Ro significantly decreased expression of interferon gamma (IFNgamma), interleukin 17 (IL-17), tumor necrosis factor alpha, and the inducible nitric oxide synthase mRNA in the colon, whereas expression of IL-12p40 was unchanged. PPARgamma was detected in epithelial cells throughout the crypts and surface. Ro increased expression of PPARgamma protein in these cells, suggesting the existence of a positive feedback loop that would potentiate its action in these cells. Ro also specifically increased expression of a novel PPAR target, aquaporin-8 (AQP8), in differentiated colonic epithelial surface cells, demonstrating that PPARgamma is not only present but also regulates gene expression in these cells in vivo. Finally, Ro was ineffective in improving disease activity in older IL-10(-/-) mice with established IBD.

CONCLUSIONS

PPARgamma is expressed, and the PPARgamma ligand Ro regulates gene expression in colonic epithelial cells. As a single agent, Ro works best for disease prevention in the IL-10(-/-) mouse model for IBD.

Review article: the potential of combinational regimen with non-steroidal anti-inflammatory drugs in the chemoprevention of colorectal cancer

Non-steroidal anti-inflammatory drugs are chemopreventive agents in colorectal cancer. Non-steroidal anti-inflammatory drugs do not, however, offer complete protection against adenoma and carcinoma development. There is increasing interest in combining non-steroidal anti-inflammatory drugs with agents that target specific cell signalling pathways in malignant and premalignant cells. This review aims to describe the current knowledge regarding the efficacy of peroxisome proliferator-activated receptor-gamma ligands, cholesterol synthesis inhibitors (statins), epidermal growth factor signalling inhibitors and tumour necrosis factor-related apoptosis-inducing ligand against colorectal neoplasms and the rationale for combining these drugs with non-steroidal anti-inflammatory drugs to improve efficacy in the chemoprevention of colorectal cancer, a PUBMED computer search of the English language literature was conducted to identify relevant papers published before July 2004. Peroxisome proliferator-activated receptor-gamma ligands and statins, both in clinical use, reduce the growth rate of human colon cancer cells in vitro and in rodents models. In vitro, preclinical in vivo and clinical studies have shown efficacy of epidermal growth factor signalling inhibition in colorectal cancer. In vitro, tumour necrosis factor-related apoptosis-inducing ligand induces apoptosis in human colon cancer cells, but not in normal cells. These drugs have all been shown to interact with non-steroidal anti-inflammatory drugs in colorectal cancer cells and/or in rodent models. Combinational regimen are a promising strategy for the chemoprevention of colorectal cancer and should be further explored.

Peroxisome proliferator-activated receptor expression and activation in normal human colonic epithelial cells and tubular adenomas

OBJECTIVE

Peroxisome proliferator-activated receptor (PPAR) ligands, widely used in type 2 diabetes treatment, have variably been shown to promote or prevent colon tumor formation in animal models and cell lines, but their role in normal human colon is unknown. The aim of this study was to determine PPAR expression and function in normal human colonic epithelial cells and tubular adenomas.

MATERIAL AND METHODS

Short-term cultures of normal human colonic epithelial cells were established from biopsies obtained in 42 patients with normal colonoscopy. PPAR and adipophilin mRNA expression was assessed by real-time RT-PCR. PPARs were activated by ligands for PPAR alpha (Wy-14643), PPAR delta (GW-501516) and PPAR gamma (rosiglitazone or troglitazone). Cell viability was measured using the methyltetrazoleum assay, proliferation by thymidine incorporation, and DNA profiles by flow cytometry. PPAR mRNA levels in tubular adenomas or metaplastic polyps (n=12) were compared with those in controls.

RESULTS

PPAR alpha and gamma were consistently expressed in normal colonocytes while no PPAR delta expression could be detected. PPAR gamma activation induced a 7.5-fold increase in adipophilin expression (a PPAR-activated gene). PPAR gamma activation had no effect on viability or DNA profiles, but led to a 25% significant decrease in cell proliferation. Finally, a selective and significant 2.5-fold decrease in PPAR alpha expression was observed in tubular adenomas, but not in metaplastic polyps, compared to controls.

CONCLUSIONS

Our findings support the view that PPAR gamma ligands act as anti-proliferative agents rather than as promoters of tumorigenesis in normal human colon. Moreover, they raise interest in investigation of PPAR alpha as a therapeutic target to prevent adenoma formation.

Growth Stimulation of COX-2–Negative Pancreatic Cancer by a Selective COX-2 Inhibitor

Cyclooxygenase 2 (COX-2) inhibitors are promising antiangiogenic agents in several preclinical models. The aim of the present study was to evaluate the effect of selective COX-2 inhibitors on vascular endothelial growth factor (VEGF) production in vitro and angiogenesis and growth of pancreatic cancer in vivo, focusing on putative differences between COX-2–negative and COX-2–positive tumors. VEGF production and angiogenesis in vitro were determined by ELISA and endothelial cell migration assay. To determine whether the effect of COX-2 inhibitors was mediated by peroxisome proliferator–activated receptor γ (PPAR-γ), we used a dominant-negative PPAR-γ and a pharmacologic inhibitor. In vitro findings were validated in a pancreatic cancer animal model. Microvessel density was assessed by CD31 immunostaining. Intratumoral prostaglandin and VEGF levels were measured by mass spectroscopy and ELISA. Selective COX-2 inhibitors had a concentration-dependent effect on VEGF production in vitro. Higher concentrations increased VEGF levels and stimulated angiogenesis by activating PPAR-γ. In vivo, nimesulide increased VEGF production by cancer cells in COX-2–positive and COX-2–negative pancreatic tumors. In COX-2–negative pancreatic cancer, this effect was associated with an increase in angiogenesis and growth. In COX-2–positive pancreatic cancer, the nimesulide-induced increase of VEGF production by the cancer cells was offset by a decrease in VEGF production by the nonmalignant cell types leading to reduced tumor angiogenesis and growth. Selective COX-2 inhibitors had opposite effects on growth and angiogenesis in pancreatic cancer depending on COX-2 expression. These findings imply that assessing the COX-2 profile of the pancreatic tumor is mandatory before initiating therapy with a selective COX-2 inhibitor.

The Wnt/beta-catenin signaling pathway targets PPARgamma activity in colon cancer cells

Control of colon cell fate in adenocarcinomas is disrupted, in part, due to aberrant Wnt/beta-catenin signaling. The nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma) has been implicated in the development of colon cancers. In the adenomatous polyposis coli multiple intestinal neoplasia (APCMin) mouse cancer model, PPARgamma expression in the colonic mucosa is markedly altered. In addition, PPARgamma protein levels are elevated, possibly through sequestration by activated beta-catenin in colon cancer cell lines. Induction of the Wnt/beta-catenin pathway by LiCl also elevated PPARgamma levels and induced PPARgamma-dependent reporter and endogenous target genes. Mechanistically, PPARgamma, through interactions with beta-catenin and T cell transcription factor (Tcf)-4, may be a determinant of cell fate and is likely a target of the Wnt pathway in cancer cells.

PPARalpha agonists stimulate progastrin production in human colorectal carcinoma cells

The three subtypes of peroxisome proliferator activated-receptors (PPARalpha, delta and gamma) control the storage and metabolism of fatty acids. Treatment of rats with the PPARalpha ligand ciprofibrate increases serum gastrin concentrations, and several lines of evidence suggest that non-amidated gastrins act as growth factors for the colonic mucosa. The aim of the present study was to investigate the expression of PPARs and the effect of PPAR ligands on gastrin production and cell proliferation in human colorectal carcinoma (CRC) cell lines. mRNAs for all three PPAR subtypes were detected by PCR in all CRC cell lines tested. The concentrations of progastrin, but not of glycine-extended or amidated gastrin, measured by radioimmunoassay in LIM 1899 conditioned media and cell extracts were significantly increased by treatment with the PPARalpha ligand clofibrate. Similar increases in progastrin were seen following treatment with the PPARalpha ligands ciprofibrate and fenofibrate, but not with bezafibrate, gemfibrozil or Wy 14643. The PPARgamma agonist rosiglitazone had no significant effect on progastrin production. The PPARalpha ligand clofibrate also stimulated proliferation of the LIM 1899 cell line. We conclude that some PPARalpha ligands increase progastrin production by the human CRC cell line LIM 1899, and that clofibrate increases proliferation of LIM 1899 cells. These studies have revealed a relationship between PPARs and gastrin, two regulatory molecules implicated in the pathogenesis of CRC.

Peroxisome proliferator-activated receptor-γ (PPARγ) inhibits tumorigenesis by reversing the undifferentiated phenotype of metastatic non-small-cell lung cancer cells (NSCLC)

Pharmacological activators of peroxisome proliferator-activated receptor-gamma (PPAR(gamma)) have been shown to inhibit growth of lung tumors largely through growth inhibition and induction of apopotosis. However, since many of these agents engage other effectors, the role of (PPAR(gamma) in lung tumorigenesis remains poorly defined. To specifically examine PPAR(gamma)-mediated events, non-small-cell lung cancer (NSCLC) cells overexpressing PPAR(gamma) were established. Overexpression of PPAR(gamma) in H2122 adenocarcinoma cells (H2122-PPAR(gamma)) blocked anchorage-independent growth compared to cells transfected with empty vector (H2122-LNCX), but had no significant effect on cell proliferation or apoptosis under standard tissue culture conditions. Orthotopic implantation of H2122-PPAR(gamma) cells into the lungs of nude rats inhibited tumor growth and metastasis in vivo and prolonged survival compared to implantation of H2122-LNCX cells. Consistent with these findings, H2122-PPAR(gamma) cells had an impaired invasiveness as assessed in Transwell assays. In a three-dimensional culture system, H2122-PPAR(gamma) cells formed polarized spheroid structures similar to those observed with normal lung epithelial cells. H2122-LNCX cells formed nonpolarized aggregate structures and did not show any of these epithelial properties. These data indicate that inhibitory effects of PPAR(gamma) on lung tumorigenesis involve selective inhibition of invasive metastasis, and activation of pathways that promote a more differentiated epithelial phenotype.

Peroxisome proliferator-activated receptor-gamma calls for activation in moderation: lessons from genetics and pharmacology

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a prototypical member of the nuclear receptor superfamily and integrates the control of energy, lipid, and glucose homeostasis. PPARgamma can bind a variety of small lipophilic compounds derived from metabolism and nutrition. These ligands, in turn, determine cofactor recruitment to PPARgamma, regulating the transcription of genes in a variety of metabolic pathways. PPARgamma is the main target of the thiazolidinedione class of insulin-sensitizing drugs, which are currently a mainstay of therapy for type 2 diabetes. However, this therapy has a number of side effects. Here, we review the clinical consequences of PPARgamma polymorphisms in humans, as well as several studies in mice using general or tissue-specific knockout techniques. We also discuss the recent pharmacological literature describing a variety of new PPARgamma partial agonists and antagonists, as well as pan-PPAR agonists. The results of these studies have added to the understanding of PPARgamma function, allowing us to hypothesize a general mechanism of PPARgamma action and speculate on future trends in the use of PPARgamma as a target in the treatment of type II diabetes.

Peroxisome proliferator-activated receptor gamma agonists promote TRAIL-induced apoptosis by reducing survivin levels via cyclin D3 repression and cell cycle arrest

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising cancer therapy that preferentially induces apoptosis in cancer cells. However, many neoplasms are resistant to TRAIL by mechanisms that are poorly understood. Here we demonstrate that human breast cancer cells, but not normal mammary epithelial cells, are dramatically sensitized to TRAIL-induced apoptosis and caspase activation by peroxisome proliferator-activated receptor gamma (PPARgamma) agonists of the thiazolidinedione (TZD) class. Although TZDs do not significantly alter the expression of components of the TRAIL signaling pathway, they profoundly reduce protein levels of cyclin D3, but not other D-type cyclins, by decreasing cyclin D3 mRNA levels and by inducing its proteasomal degradation. Importantly, both TRAIL sensitization and reduction in cyclin D3 protein levels induced by TZDs are likely PPARgamma-independent because a dominant negative mutant of PPARgamma did not antagonize these effects of TZDs, nor were they affected by the expression levels of PPARgamma. TZDs also inhibit G(1) to S cell cycle progression. Furthermore, silencing cyclin D3 by RNA interference inhibits S phase entry and sensitizes breast cancer cells to TRAIL, indicating a key role for cyclin D3 repression in these events. G(1) cell cycle arrest sensitizes breast cancer cells to TRAIL at least in part by reducing levels of the anti-apoptotic protein survivin: ectopic expression of survivin partially suppresses apoptosis induced by TRAIL and TZDs. We also demonstrate for the first time that TZDs promote TRAIL-induced apoptosis of breast cancer in vivo, suggesting that this combination may be an effective therapy for cancer.

Peroxisome proliferator-activated receptor gamma-dependent and -independent growth inhibition of gastrointestinal tumour cells

Peroxisome proliferator-activated receptor gamma (PPARgamma) acts as a ligand-activated transcription factor. Although ligand-induced cellular differentiation and growth inhibition have been mostly studied on human cancers expressing PPARgamma, it is unclear if the transcriptional activation of PPARgamma is the main mechanism of growth inhibition. In this study, we investigated whether there is a link between growth inhibitory effect and transcriptional activation of PPARgamma in several gastrointestinal tumour cell lines. The transcriptional activation potential of PPARgamma was assessed by reporter gene assay employing a PPRE-luciferase vector, and growth inhibitory effect of PPARgamma was investigated by (3)H-thymidine incorporation assay, in the presence or absence of thiazolidinedione ligands, rosiglitazone and troglitazone. As expected, in the case of cell lines positive for the transcriptional activation potential of PPARgamma (T.Tn, MKN-45 and LoVo), both the ligands induced growth inhibition. However, in case of some other cell lines negative for the transcriptional activation potential of PPARgamma (TT, AGS and HCT-15), troglitazone still showed a growth inhibitory effect. Administration of the PPARgamma antagonist GW9662 did not reverse this growth inhibitory activity of troglitazone. The introduction of dominant negative mutants of PPARgamma did not suppress the activity either. These observations suggest that while rosiglitazone inhibits cellular growth predominantly through transcriptional activation of PPARgamma, troglitazone can induce it both in PPARgamma-dependent and -independent pathways.

Pleiotropic effects of thiazolidinediones: taking a look beyond antidiabetic activity

Thiazolidinediones (TZD) [Troglitazone (TRO), Pioglitazone (PGZ), Rosiglitazone, (RGZ)] are a novel class of antidiabetic drugs for patients with Type-2 diabetes mellitus (T2DM) able to decrease blood glucose, working through a reduction of insulin resistance. The family of TZD exerts its effect specifically bound to peroxisome proliferator-activated receptor y (PPARy). This is a member of the nuclear hormone receptor superfamily of ligand-dependent transcription factors, together with PPARalpha and deltabeta. Although PPARgamma is essentially expressed in adipose tissue, it has also been found in endothelial cells, macrophages, vascular smooth muscle cells, glomerular mesangial cells, hepatic stellate cells and in several cancer cell lines. In these cells, the PPARgamma activation by TZD determines modulatory effects on growth factor release, production of cytokine, cell proliferation and migration, extracellular matrix remodeling and control on cell cycle progression and differentiation. In addition, TZD have been shown to have a potent antioxidant effect. This review, taking a quick look beyond the antidiabetic activity of PPARgamma, shows the dramatic ranging of medical implications that the use of TZD could have modulating the PPARgamma activity in several diseases with a strong social impact, such as insulin resistance syndrome, chronic inflammation, atherosclerosis and cancer.

Transforming growth factor-beta (TGF-beta) activates cytosolic phospholipase A2alpha (cPLA2alpha)-mediated prostaglandin E2 (PGE)2/EP1 and peroxisome proliferator-activated receptor-gamma (PPAR-gamma)/Smad signaling pathways in human liver cancer cells. A novel mechanism for subversion of TGF-beta-induced mitoinhibition

Transforming growth factor-beta (TGF-beta) potently inhibits the growth of human epithelial cells. However, neoplastic epithelial cells become resistant to TGF-beta-mediated mitoinhibition, and the mechanisms for this alteration during tumorigenesis are not fully understood. This study was designed to determine whether there is an association between the cytosolic phospholipase A2alpha (cPLA2alpha)-controlled eicosanoid metabolism and the growth response to TGF-beta in human liver cancer cells. TGF-beta treatment induced simultaneous Smad-mediated gene transcription and phosphorylation of cPLA2alpha. Whereas Smad activation inhibited tumor cell growth, phosphorylation of cPLA2 alpha promoted growth and counteracted Smad-mediated mitoinhibition. TGF-beta1 failed to prevent the growth of cells with high basal expression of cPLA2alpha, but inhibition of cPLA2 alpha, cyclooxygenase-2 (COX-2), or EP1 receptor restored mitoinhibition by TGF-beta1 in these cells. These results suggest that resistance of tumor cells to TGF-beta-mediated mitoinhibition involves activation of cPLA2alpha/COX-2/EP1 signaling. Furthermore, the TGF-beta1-induced Smad transcriptional activity and mitoinhibition were blocked by overexpression of cPLA2alpha or peroxisome proliferator-activated receptor-gamma (PPAR-gamma) but enhanced by depletion of cPLA2alpha or PPAR-gamma. These findings, along with the observations that cPLA2alpha activates PPAR-gamma and that PPAR-gamma binds Smad3, illustrate novel cPLA2alpha/COX-2/EP1 and cPLA2alpha/PPAR-gamma/Smad signaling pathways that counteract the mitoinhibition by TGF-beta in human cancer cells.

Cyclin D1 genetic heterozygosity regulates colonic epithelial cell differentiation and tumor number in ApcMin mice

Constitutive beta-catenin/Tcf activity, the primary transforming events in colorectal carcinoma, occurs through induction of the Wnt pathway or APC gene mutations that cause familial adenomatous polyposis. Mice carrying Apc mutations in their germ line (ApcMin) develop intestinal adenomas. Here, the crossing of ApcMin with cyclin D1-/- mice reduced the intestinal tumor number in animals genetically heterozygous or nullizygous for cyclin D1. Decreased tumor number in the duodenum, intestines, and colons of ApcMin/cyclin D1+/- mice correlated with reduced cellular proliferation and increased differentiation. Cyclin D1 deficiency reduced DNA synthesis and induced differentiation of colonic epithelial cells harboring mutant APC but not wild-type APC cells in vivo. In previous studies, the complete loss of cyclin D1 through homozygous genetic deletion conveyed breast tumor resistance. The protection of mice, genetically predisposed to intestinal tumorigenesis, through cyclin D1 heterozygosity suggests that modalities that reduce cyclin D1 abundance could provide chemoprotection.

Promotion of colon tumors in C57BL/6J-APC(min)/+ mice by thiazolidinedione PPARgamma agonists and a structurally unrelated PPARgamma agonist

Thiazolidinedione PPARgamma agonists (troglitazone and rosiglitazone) were previously shown to promote colon tumor formation in C57BL/6J-APC(min)/+ mice, a model for human familial adenomatous polyposis. This study was conducted to determine if another thiazolidinedione PPARgamma agonist, pioglitazone, and a PPARgamma agonist structurally unrelated to the thiazolidinedione family, NID525, (a tetrazole-substituted phenoxymethylquinolone), would also promote colon tumors in this mouse model. Mice were treated in-feed with the thiazolidinediones troglitazone (150 mg/kg/day), rosiglitazone (20 mg/kg/day), or pioglitazone (150 mg/kg/day), or with NID525 (150 mg/kg/day) for 8 weeks. An increased incidence in colon tumors compared to controls was observed for all of the thiazolidinedione-treated groups as well as the NID525-treated group. These results indicate that the tumor-promoting effect of PPARgamma agonists in the colon of C57BL/6J-APC(min)/+ mice is likely related to the pharmacological activity of this group of drugs and not the thiazolidinedione structure.

1,1-Bis(3'-indolyl)-1-(p-substitutedphenyl)methanes induce peroxisome proliferator-activated receptor gamma-mediated growth inhibition, transactivation, and differentiation markers in colon cancer cells

1,1-Bis(3'indolyl)-1-(p-substitutedphenyl)methanes containing p-trifluoromethyl (DIM-C-pPhCF)), p-t-butyl (DIM-C-pPhtBu), and p-phenyl (DIM-C-pPhC6H5) groups induce peroxisome proliferator-activated receptor gamma (PPARgamma)-mediated transactivation in HT-29, HCT-15, RKO, and SW480 colon cancer cell lines. Rosiglitazone also induces transactivation in these cell lines and inhibited growth of HT-29 cells, which express wild-type PPARgamma but not HCT-15 cells, which express mutant (K422Q) PPARgamma. In contrast, DIM-C-pPhCF3, DIM-C-pPhtBu, and DIM-C-pPhC6H5 inhibited growth of both HT-29 and HCT-15 cells with IC50 values ranging from 1 to 10 micromol/L. Rosiglitazone and diindolylmethane (DIM) analogues did not affect expression of cyclin D1, p21, or p27 protein levels or apoptosis in HCT-15 or HT-29 cells but induced keratin 18 in both cell lines. However, rosiglitazone induced caveolins 1 and 2 in HT-29 but not HCT-15 cells, whereas these differentiation markers were induced by DIM-C-pPhCF3 and DIM-C-pPhC6H5 in both cell lines. Because overexpression of caveolin 1 is known to suppress colon cancer cell and tumor growth, the growth inhibitory effects of rosiglitazone and the DIM compounds are associated with PPARgamma-dependent induction of caveolins.

Antineoplastic effects of peroxisome proliferator-activated receptor gamma agonists

Peroxisome proliferator-activated receptors (PPAR) are members of a superfamily of nuclear hormone receptors. Activation of PPAR isoforms elicits both antineoplastic and anti-inflammatory effects in several types of mammalian cells. PPARs are ligand-activated transcription factors and have a subfamily of three different isoforms: PPAR alpha, PPAR gamma, and PPAR beta/delta. All isoforms heterodimerise with the 9-cis-retinoic acid receptor RXR, and play an important part in the regulation of several metabolic pathways, including lipid biosynthesis and glucose metabolism. Endogenous ligands of PPAR gamma include long-chain polyunsaturated fatty acids, eicosanoid derivates, and oxidised lipids. Newly developed synthetic ligands include thiazolidinediones-a group of potent PPAR gamma agonists and antidiabetic agents. Here, we review PPAR gamma-induced antineoplastic signalling pathways, and summarise the antineoplastic effects of PPAR gamma agonists in different cancer cell lines, animal models, and clinical trials.

Mitochondrial respiratory chain dysfunction, a non-receptor-mediated effect of synthetic PPAR-ligands: biochemical and pharmacological implications

Peroxisome proliferator activated receptors (PPARs) are a class of nuclear receptors involved in lipid and glucidic metabolism, immune regulation, and cell differentiation. Many of their biological activities have been studied by using selective synthetic activators (mainly fibrates and thiazolidinediones) which have been already employed in therapeutic protocols. Both kinds of drugs, however, showed pharmacotoxicological profiles, which cannot be ascribed by any means to receptor activation. To better understand these non-receptorial or extrareceptorial aspects, the effect of different PPAR-ligands on the metabolic status of human HL-60 cell line has been investigated. At this regard, NMR analysis of cell culture supernatants was accomplished in order to monitor modifications at the level of cell metabolism. Cell growth and chemiluminescence assays were employed to verify cell differentiation. Results showed that all the considered PPAR-ligands, although with different potencies and independently from their PPAR binding specificity, induced a significant derangement of the mitochondrial respiratory chain consisting in a strong inhibition of NADH-cytochrome c reductase activity. This derangement has been shown to be strictly correlated to the adaptive metabolic modifications, as evidenced by the increased formation of lactate and acetate, due to the stimulation of anaerobic glycolysis and fatty acid beta-oxidation. It is worthy noting that the mitochondrial dysfunction appeared also linked to the capacity of any given PPAR-ligand to induce cell differentiation. These data could afford an explanation of biochemical and toxicological aspects related to the therapeutic use of synthetic PPAR-ligands and suggest a revision of PPAR pathophysiologic mechanisms.

Hepatocyte-specific Pten deficiency results in steatohepatitis and hepatocellular carcinomas

PTEN is a tumor suppressor gene mutated in many human cancers, and its expression is reduced or absent in almost half of hepatoma patients. We used the Cre-loxP system to generate a hepatocyte-specific null mutation of Pten in mice (AlbCrePten(flox/flox) mice). AlbCrePten(flox/flox) mice showed massive hepatomegaly and steatohepatitis with triglyceride accumulation, a phenotype similar to human nonalcoholic steatohepatitis. Adipocyte-specific genes were induced in mutant hepatocytes, implying adipogenic-like transformation of these cells. Genes involved in lipogenesis and beta-oxidation were also induced, possibly as a result of elevated levels of the transactivating factors PPARgamma and SREBP1c. Importantly, the loss of Pten function in the liver led to tumorigenesis, with 47% of AlbCrePten(flox/flox) livers developing liver cell adenomas by 44 weeks of age. By 74-78 weeks of age, 100% of AlbCrePten(flox/flox) livers showed adenomas and 66% had hepatocellular carcinomas. AlbCrePten(flox/flox) mice also showed insulin hypersensitivity. In vitro, AlbCrePten(flox/flox) hepatocytes were hyperproliferative and showed increased hyperoxidation with abnormal activation of protein kinase B and MAPK. Pten is thus an important regulator of lipogenesis, glucose metabolism, hepatocyte homeostasis, and tumorigenesis in the liver.

PPAR trilogy from metabolism to cancer

PURPOSE OF REVIEW

This review highlights recent advances related to malignancies in the field of peroxisome proliferator-activated receptors (PPARs). It also discusses the implications of cancer research and therapy.

RECENT FINDINGS

In the last few years, genetic evidence has implicated the PPARs, specifically PPARgamma and PPARbeta/delta, in tumorigenesis. Also, new insights into the regulation of the nuclear hormone receptors have emerged.

SUMMARY

Exciting research in PPAR biology has established these nuclear factors as key regulators of metabolism and energy homeostasis. Evidence indicates that PPARs can also affect the pathogenesis and development of tumors. However, the type of effects observed thus far appears to depend on the experimental context. As a result, the findings are generating much debate, as PPAR agonists are widespread targets in the treatment of metabolic disorders such as diabetes and dyslipidemia. Here, we summarize the most recent advances in this field, outline the conflicting reports and discuss their overall implications in cancer research.

Dissociation of oxidant production by peroxisome proliferator-activated receptor ligands from cell death in human cell lines

Ligands of peroxisome proliferator-activated receptors (PPARs) come from a diverse group of chemicals that include pharmaceutical drugs, phthalate plasticizers, steroids, and pesticides. PPAR ligands exhibit a number of effects, including an ability to induce apoptosis in some systems. The mechanism(s) underlying the induction of apoptosis is not known. The current study examined the ability of Wy14643, a fibrate and PPARalpha agonist, and ciglitazone, a thiazolidinedione and PPARgamma agonist, to induce apoptosis as well as the production of oxidants in human Jurkat T cells that express all PPAR isoforms. Treatment with increasing doses of Wy14643 caused a substantial time-dependent increase in the overall oxidant status (as reflected by increased dichlorofluorescein fluorescence) of Jurkat cells without any change in viability except at the highest dose and longest time. Ciglitazone also caused a dose- and time-dependent increase in oxidant production. However, although the extent of this production was less than that seen with Wy14643, ciglitazone caused a dose- and time-dependent increase in apoptosis that could not be inhibited by antioxidants. Confocal micrographs of Jurkat cells loaded with dichlorofluorescein diacetate or dihydrorhodamine 123 and treated with Wy14643 or ciglitazone revealed a punctate pattern of fluorescence at early time points suggestive of a mitochondrial origin for these oxidants. Rotenone and antimycin A prevented Wy14643- but not ciglitazone-induced oxidant production. Other relatively specific PPARgamma agonists (15delta-PGJ2, and troglitazone), but not nonspecific agonists (bezafibrate and conjugated linoleic acid), were also able to induce oxidant production in Jurkat cells. These data, as well as the findings that oxidant production could be induced by Wy14643 in A549 cells that lack PPARalpha, and could not be blocked in Jurkat cells by the PPARalpha inhibitor MK886, indicate oxidant formation is unrelated to PPARalpha. These data also suggest that oxidant production induced by PPARalpha ligands originates in the mitochondria.

PPARs and the complex journey to obesity

Obesity and the related disorders of dyslipidemia and diabetes (components of syndrome X) have become global health epidemics. Over the past decade, the elucidation of key regulators of energy balance and insulin signaling have revolutionized our understanding of fat and sugar metabolism and their intimate link. The three 'lipid-sensing' peroxisome proliferator-activated receptors (PPAR-alpha, PPAR-gamma and PPAR-delta) exemplify this connection, regulating diverse aspects of lipid and glucose homeostasis, and serving as bona fide therapeutic targets. With molecular underpinnings now in place, new pharmacologic approaches to metabolic disease and new questions are emerging.

Inhibition of HER-kinase activation prevents ERK-mediated degradation of PPARgamma

R-etodolac, a nonsteroidal anti-inflammatory drug, inhibits the progression of CWRSA6 androgen-independent and LuCaP-35 androgen-dependent prostate cancer xenograft growth through downregulation of cyclin D1 expression via the PPARgamma pathway. PPARgamma protein degradation, observed post-R-etodolac treatment, resulted from phospho-MAP kinase (p44/42) induction by R-etodolac negatively regulating PPARgamma function. Negative regulation of PPARgamma was overcome by a combination regimen of R-etodolac with the HER-kinase axis inhibitor, rhuMab 2C4, which demonstrated an additive antitumor effect. We further show that the inhibition of HER-kinase activity by rhuMab 2C4 is sufficient to inhibit PPARgamma protein degradation. This study introduces a novel concept of an in vivo crosstalk between the HER-kinase axis and PPARgamma pathways, ultimately leading to negative regulation of PPARgamma activity and tumor growth inhibition.

Hepatocyte-specific Pten deficiency results in steatohepatitis and hepatocellular carcinomas

PTEN is a tumor suppressor gene mutated in many human cancers, and its expression is reduced or absent in almost half of hepatoma patients. We used the Cre-loxP system to generate a hepatocyte-specific null mutation of Pten in mice (AlbCrePten(flox/flox) mice). AlbCrePten(flox/flox) mice showed massive hepatomegaly and steatohepatitis with triglyceride accumulation, a phenotype similar to human nonalcoholic steatohepatitis. Adipocyte-specific genes were induced in mutant hepatocytes, implying adipogenic-like transformation of these cells. Genes involved in lipogenesis and beta-oxidation were also induced, possibly as a result of elevated levels of the transactivating factors PPARgamma and SREBP1c. Importantly, the loss of Pten function in the liver led to tumorigenesis, with 47% of AlbCrePten(flox/flox) livers developing liver cell adenomas by 44 weeks of age. By 74-78 weeks of age, 100% of AlbCrePten(flox/flox) livers showed adenomas and 66% had hepatocellular carcinomas. AlbCrePten(flox/flox) mice also showed insulin hypersensitivity. In vitro, AlbCrePten(flox/flox) hepatocytes were hyperproliferative and showed increased hyperoxidation with abnormal activation of protein kinase B and MAPK. Pten is thus an important regulator of lipogenesis, glucose metabolism, hepatocyte homeostasis, and tumorigenesis in the liver.

Approaches to understanding the importance and clinical implications of peroxisome proliferator-activated receptor gamma (PPARgamma) signaling in prostate cancer

The development and maintenance of the prostate are dependent upon a complex series of interactions occurring between the epithelial and stromal tissues (Hayward and Cunha [2000]: Radiol. Clin. N. Am. 38:1-14). During the process of prostatic carcinogenesis, there are progressive changes in the interactions of the nascent tumor with its surrounding stroma and extracellular matrix. These include the development of a reactive stromal phenotype and the possible promotion, by stromal cells, of epithelial proliferation and loss of differentiated function (Hayward et al. [1996]: Ann. N. Y. Acad. Sci. 784:50-62; Grossfeld et al. [1998]: Endocr. Related Cancer 5:253-270; Rowley [1998]: Cancer Metastasis Rev. 17:411-419; Tuxhorn et al. [2002]: Clin. Cancer Res. 8:2912-2923). Many molecules play an as yet poorly defined role in establishing and maintaining a growth quiescent glandular structure in the adult. Peroxisome proliferator-activated receptor gamma (PPARgamma) is a candidate regulator of prostatic epithelial differentiation and may play a role in restricting epithelial proliferation. PPARgamma agonists are relatively non-toxic and have been used with limited success to treat some prostate cancer patients. We would propose that a more complete understanding of PPARgamma biology, particularly in the context of appropriate stromal-epithelial and host-tumor interactions would allow for the selection of patients most likely to benefit from this line of therapy. In particular, it seems reasonable to suggest that the patients most likely to benefit may be those with relatively indolent low stage disease for whom this line of therapy could be a useful additive to watchful waiting.

PPAR gamma signaling exacerbates mammary gland tumor development

Breast cancer cell lines that express the nuclear peroxisome proliferator-activated receptor gamma (PPAR gamma) can be prompted to undergo growth arrest and differentiation when treated with synthetic PPAR gamma ligands. To evaluate the therapeutic potential of increased PPAR gamma signaling in vivo, we generated transgenic mice that express a constitutively active form of PPAR gamma in mammary gland. These mice are indistinguishable from their wild-type littermates. However, when bred to a transgenic strain prone to mammary gland cancer, bigenic animals develop tumors with greatly accelerated kinetics. Surprisingly, in spite of their more malignant nature, bigenic tumors are more secretory and differentiated. The molecular basis of this tumor-promoting effect may be an increase in Wnt signaling, as ligand activation of PPAR gamma potentiates Wnt function in an in vivo model of this pathway. These results suggest that once an initiating event has taken place, increased PPAR gamma signaling serves as a tumor promoter in the mammary gland.

Thiazolidinediones increase arachidonic acid release and subsequent prostanoid production in a peroxisome proliferator-activated receptor γ-independent manner

Thiazolidinedione, peroxisome proliferator-activated receptor gamma (PPARgamma) agonist, has been used as an anti-diabetic drug and as an useful tool to elucidate multiple PPARgamma functions by in vitro and in vivo studies. We investigated the effects of thiazolidinediones on prostanoid production in lipopolysaccharide-stimulated cells. The high concentrations (>10 microM) of rosiglitazone and pioglitazone significantly increased lipopolysaccharide-stimulated prostanoid production such as thromboxane A2 and prostaglandin E2. However, PPARgamma antagonist could not inhibit them. In PPARgamma-deficient cells, thiazolidinediones increased prostaglandin E2 production. Thiazolidinediones increased arachidonic acid (AA) release from the cell membrane by not stimulating AA releasing process involving several phospholipase A2s but inhibiting AA reuptaking process. The expression of cyclooxygenase-1 and cyclooxygenase-2 were not affected by thiazolidinediones. In this study, we demonstrated that high concentrations of TZDs increased AA release by the inhibition of AA reuptaking process, leading to subsequent increase in the prostanoid production in a PPARgamma-independent manner. This mechanism provides useful information for the elucidation of multiple PPARgamma functions and diabetic drug therapy.

Peroxisome proliferator-activated receptor-delta attenuates colon carcinogenesis

Peroxisome proliferator-activated receptor-delta (PPAR-delta; also known as PPAR-beta) is expressed at high levels in colon tumors, but its contribution to colon cancer is unclear. We examined the role of PPAR-delta in colon carcinogenesis using PPAR-delta-deficient (Ppard(-/-)) mice. In both the Min mutant and chemically induced mouse models, colon polyp formation was significantly greater in mice nullizygous for PPAR-delta. In contrast to previous reports suggesting that activation of PPAR-delta potentiates colon polyp formation, here we show that PPAR-delta attenuates colon carcinogenesis.

Therapeutic potential of thiazolidinediones as anticancer agents

Thiazolidinediones (TZDs) are synthetic ligands that activate the nuclear receptor peroxisome proliferator-activated receptor-gamma (PPAR-gamma). These compounds are widely used in the treatment of Type 2 diabetes. TZDs have antitumour activity in a wide variety of experimental cancer models, in vitro and in vivo, by affecting the cell cycle, induction of cell differentiation and apoptosis as well as by inhibiting tumour angiogenesis. These effects are mediated through both PPAR-gamma-dependent and -independent pathways depending on concentration and tumour cell type. Angiogenesis inhibition mechanisms of TZDs include directly inhibiting endothelial cell proliferation and migration as well as decreasing tumour cell vascular endothelial growth factor production. Further studies suggest that TZDs may be effective in prevention of certain cancers and in the treatment of cancer as adjuvant therapy.

Activation of nuclear hormone receptor peroxisome proliferator-activated receptor-delta accelerates intestinal adenoma growth

We treated Apc(min) mice, which are predisposed to intestinal polyposis, with a selective synthetic agonist of peroxisome proliferator-activated receptor-delta (PPAR-delta). Exposure of Apc(min) mice to the PPAR-delta ligand GW501516 resulted in a significant increase in the number and size of intestinal polyps. The most prominent effect was on polyp size; mice treated with the PPAR-delta activator had a fivefold increase in the number of polyps larger than 2 mm. Our results implicate PPAR-delta in the regulation of intestinal adenoma growth.