Peroxisome-proliferator-activated receptors and cancers: complex stories

Effects of Thiazolidinediones on Differentiation, Proliferation, and Apoptosis

Thiazolidinediones induce adipocyte differentiation and thereby limit proliferative potential; hence, early investigations focused on their ability to modulate cellular proliferation and apoptosis. Several lines of evidence indicate significant thiazolidinedione-mediated antitumor activity. An emerging view is that some antitumor effects are totally or partially peroxisome proliferator-activated receptor-γ (PPARγ) dependent, whereas others are PPARγ independent. The aim of this review is to examine the current evidence about the molecular mechanisms by which thiazolidinediones augment cellular differentiation, inhibit cellular proliferation, and induce apoptosis. We first address the role of thiazolidinediones and/or PPARγ on Wnt/β-catenin signaling pathway as it affects cellular differentiation and then discuss other pathways that are also involved in differentiation as well as proliferation and apoptosis. (Mol Cancer Res 2007;5(6):523–30)

Clofibrate-induced changes in the liver, heart, brain and white adipose lipid metabolome of Swiss-Webster mice

Peroxisome proliferator activated receptor alpha (PPARα) agonists are anti-hyperlipidemic drugs that influence fatty acid combustion, phospholipid biosynthesis and lipoprotein metabolism. To evaluate impacts on other aspects of lipid metabolism, we applied targeted metabolomics to liver, heart, brain and white adipose tissue samples from male Swiss-Webster mice exposed to a 5 day, 500 mg/kg/day regimen of i.p. clofibrate. Tissue concentrations of free fatty acids and the fatty acid content of sphingomyelin, cardiolipin, cholesterol esters, triglycerides and phospholipids were quantified. Responses were tissue-specific, with changes observed in the liver > heart ≫ brain > adipose. These results indicate that liver saturated fatty acid-rich triglycerides feeds clofibrate-induced monounsaturated fatty acid (MUFA) synthesis, which were incorporated into hepatic phospholipids and sphingomyelin. In addition, selective enrichment of docosahexeneoic acid in the phosphatidylserine of liver (1.7-fold), heart (1.6-fold) and brain (1.5-fold) suggests a clofibrate-dependent systemic activation of phosphatidylserine synthetase 2. Furthermore, the observed ~20% decline in cardiac sphingomyelin is consistent with activation of a sphingomeylinase with a substrate preference for polyunsaturate-containing sphingomyelin. Finally, perturbations in the liver, brain, and adipose cholesterol esters were observed, with clofibrate exposure elevating brain cholesterol arachidonyl-esters ~20-fold. Thus, while supporting previous findings, this study has identified novel impacts of PPARα agonist exposure on lipid metabolism that should be further explored.

Thiazolidinediones and the risk of lung, prostate, and colon cancer in patients with diabetes

PURPOSE

Peroxisome proliferator-activated receptor gamma (PPARgamma) mediates cell cycle arrest and adipocyte differentiation; has tumor suppressor activity in liposarcoma, lung, and prostate cancers; and suppresses colonic polyp formation in adenomatous polyposis coli (APC)min/+ mice. To assess the influence of thiazolidinediones (TZDs), which are PPAR ligands used to treat diabetes mellitus, a retrospective analysis of a database from 10 Veteran Affairs medical centers was conducted.

PATIENTS AND METHODS

Data on male patients 40 years and older diagnosed to have diabetes mellitus between 1997 and 2003 were obtained from the Veterans Integrated Services Network 16 (VISN 16) data warehouse. Subsequent diagnoses of colorectal, lung, and prostate cancer and use of TZD, other antidiabetic agents, and insulin were identified. Cox regression with time-dependent covariates was used to estimate the association between TZD use and cancer risk. Relative risks were adjusted for confounders (age, race/ethnicity, body mass index, use of insulin, and other oral antidiabetic agents).

RESULTS

Of 87,678 individuals, 1,137 had colorectal cancer, 3,246 had prostate cancer, and 1,371 had lung cancer. We observed a 33% reduction in lung cancer risk among TZD users compared with nonusers after adjusting for confounder interactions (relative risk, 0.67; 95% CI, 0.51 to 0.87). The risk reduction for colorectal and prostate cancers did not reach statistical significance.

CONCLUSION

TZD use was associated with reduced risk of lung cancer. Further studies are warranted to confirm our findings.

Synergy between PPARgamma ligands and platinum-based drugs in cancer

PPARgamma is a member of the nuclear receptor family for which agonist ligands have antigrowth effects. However, clinical studies using PPARgamma ligands as a monotherapy failed to show a beneficial effect. Here we have studied the effects of PPARgamma activation with chemotherapeutic agents in current use for specific cancers. We observed a striking synergy between rosiglitazone and platinum-based drugs in several different cancers both in vitro and using transplantable and chemically induced "spontaneous" tumor models. The effect appears to be due in part to PPARgamma-mediated downregulation of metallothioneins, proteins that have been shown to be involved in resistance to platinum-based therapy. These data strongly suggest combining PPARgamma agonists and platinum-based drugs for the treatment of certain human cancers.

Proteomic profile of mouse fibroblasts with a targeted disruption of the peroxisome proliferator activated receptor-β/δ gene

The peroxisome proliferator activated receptor-beta (PPARbeta) plays an essential role in lipid metabolism, immune modulation, differentiation and cell proliferation. There is also strong evidence for a function in oncogenesis and tumor vascularization, but the underlying molecular mechanisms remain elusive. In the present study, we have used fibroblasts derived from Pparb wild-type and null mice to determine by 2-DE and PMF analysis the contribution of PPARbeta to the protein profile of fibroblasts. Thirty-one differentially expressed proteins of different functional categories were identified. For at least two proteins a role in tumorigenesis and/or tumor vascularization has previously been reported: while the calcium intracellular channel-4 (CLIC4) was expressed at lower levels in Pparb null cells, expression of the cellular retinol binding protein 1 (CRBP1) was enhanced. Clic4 and Crbp1 gene expression patterns observed in different experimental settings in vitro and in vivo confirmed the proteomics data. Our findings indicate that the expression of a defined set of proteins is altered in fibroblasts and endothelial cells from Pparb null mice, that this is due to aberrant gene regulation, and that the altered expression of these proteins is consistent with the tumor vascularization phenotype of Pparb null mice.

Activation of peroxisome proliferator-activated receptor gamma contributes to the survival of T lymphoma cells by affecting cellular metabolism

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a metabolic regulator involved in maintaining glucose and fatty acid homeostasis. Besides its metabolic functions, the receptor has also been implicated in tumorigenesis. Ligands of PPARgamma induce apoptosis in several types of tumor cells, leading to the proposal that these ligands may be used as antineoplastic agents. However, apoptosis induction requires high doses of ligands, suggesting the effect may not be receptor-dependent. In this report, we show that PPARgamma is expressed in human primary T-cell lymphoma tissues and activation of PPARgamma with low doses of ligands protects lymphoma cells from serum starvation-induced apoptosis. The prosurvival effect of PPARgamma was linked to its actions on cellular metabolic activities. In serum-deprived cells, PPARgamma attenuated the decline in ATP, reduced mitochondrial hyperpolarization, and limited the amount of reactive oxygen species (ROS) in favor of cell survival. Moreover, PPARgamma regulated ROS through coordinated transcriptional control of a set of proteins and enzymes involved in ROS metabolism. Our study identified cell survival promotion as a novel activity of PPARgamma. These findings highlight the need for further investigation into the role of PPARgamma in cancer before widespread use of its agonists as anticancer therapeutics.

Control of peroxisome proliferator-activated receptor fate by the ubiquitinproteasome system

Peroxisome proliferator-activated receptor (PPAR) alpha, gamma, and delta belong to the nuclear hormone receptor superfamily of ligand-activated transcription factors. PPARs regulate metabolic, developmental, and differentiation pathways and play important roles in human diseases, such as diabetes, atherosclerosis, cancer, and chronic inflammation. PPARs are the targets of drugs of widespread clinical use and represent promising targets for discovery of new therapeutics. The interaction of PPARs with the ubiquitin-proteasome system (UPS) has been the subject of limited investigation. The UPS plays an important role in regulating the levels and modulating ligand-dependent and-independent activity of nuclear receptors. This review highlights the current knowledge regarding the interactions of the UPS with PPARs and focuses on the differential regulation of the level and activity of the PPAR isotypes by the UPS in response to selective ligands. Understanding the connections between the UPS and PPARs can provide insights in the actions of existing drugs and raise the possibilities for development of more effective PPAR-based therapeutics.

International Union of Pharmacology. LXI. Peroxisome proliferator-activated receptors

The three peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors of the nuclear hormone receptor superfamily. They share a high degree of structural homology with all members of the superfamily, particularly in the DNA-binding domain and ligand- and cofactor-binding domain. Many cellular and systemic roles have been attributed to these receptors, reaching far beyond the stimulation of peroxisome proliferation in rodents after which they were initially named. PPARs exhibit broad, isotype-specific tissue expression patterns. PPARalpha is expressed at high levels in organs with significant catabolism of fatty acids. PPARbeta/delta has the broadest expression pattern, and the levels of expression in certain tissues depend on the extent of cell proliferation and differentiation. PPARgamma is expressed as two isoforms, of which PPARgamma2 is found at high levels in the adipose tissues, whereas PPARgamma1 has a broader expression pattern. Transcriptional regulation by PPARs requires heterodimerization with the retinoid X receptor (RXR). When activated by a ligand, the dimer modulates transcription via binding to a specific DNA sequence element called a peroxisome proliferator response element (PPRE) in the promoter region of target genes. A wide variety of natural or synthetic compounds was identified as PPAR ligands. Among the synthetic ligands, the lipid-lowering drugs, fibrates, and the insulin sensitizers, thiazolidinediones, are PPARalpha and PPARgamma agonists, respectively, which underscores the important role of PPARs as therapeutic targets. Transcriptional control by PPAR/RXR heterodimers also requires interaction with coregulator complexes. Thus, selective action of PPARs in vivo results from the interplay at a given time point between expression levels of each of the three PPAR and RXR isotypes, affinity for a specific promoter PPRE, and ligand and cofactor availabilities.

Block of nuclear receptor ubiquitination. A mechanism of ligand-dependent control of peroxisome proliferator-activated receptor delta activity

Peroxisome proliferator-activated receptor delta (PPARdelta) is a ligand-activated transcription factor involved in many physiological and pathological processes. PPARdelta is a promising therapeutic target for metabolic, chronic inflammatory, and neurodegenerative disorders. However, limited information is available about the mechanisms that control the activity of this nuclear receptor. Here, we examined the role of the ubiquitinproteasome system in PPARdelta turnover. The receptor was ubiquitinated and subject to rapid degradation by the 26 S proteasome. Unlike most nuclear receptors that are degraded upon ligand binding, PPARdelta ligands inhibited the ubiquitination of the receptor, thereby preventing its degradation. Ligand binding was required for inhibition of the ubiquitination since disruption of the ligand binding domain abolished the effect. Site-directed mutagenesis showed that the DNA binding domain was also required, indicating that ligands preferentially stabilized the DNA-bound receptor. In contrast, the activation function-2 domain and co-repressor binding site were not involved in ligand-induced stabilization. Block of ubiquitination by ligands may be an essential step to avoid rapid degradation of a receptor, like PPARdelta, with a very short half-life and sustain its transcriptional activity once it is engaged in transcriptional activation complexes.

Peroxisome proliferator-activated receptor beta/delta as a therapeutic target for metabolic diseases

The peroxisome proliferator-activated receptor (PPAR) family comprises three distinct isotypes: PPARalpha, PPARbeta/delta and PPARgamma. PPARs are nuclear hormone receptors that mediate the effects of fatty acids and their derivatives at the transcriptional level. Until recently, the characterisation of the important role of PPARalpha in fatty acid oxidation and of PPARgamma in lipid storage contrasted with the sparse information concerning PPARbeta/delta. However, evidence is now emerging for a role of PPARbeta/delta in tissue repair and energy homeostasis. Experiments with tissue-specific overexpression of PPARbeta/delta or treatment of mice with selective PPARbeta/delta agonists demonstrated that activation of PPARbeta/delta in vivo increases lipid catabolism in skeletal muscle, heart and adipose tissue and improves the serum lipid profile and insulin sensitivity in several animal models. PPARbeta/delta activation also prevents the development of obesity and improves cholesterol homeostasis in obesity-prone mouse models. These new insights into PPARbeta/delta functions suggest that targeting PPARbeta/delta may be helpful for treating disorders associated with the metabolic syndrome. Although these perspectives are promising, several independent and contradictory reports raise concerns about the safety of PPARbeta/delta ligands with respect to tumourigenic activity in the gut. Thus, it appears that further exploration of PPARbeta/delta functions is necessary to better define its potential as a therapeutic target.

Arachidonic and docosahexaenoic acids reduce the growth of A549 human lung-tumor cells increasing lipid peroxidation and PPARs

Polyunsaturated fatty acids (PUFAs) play an important role in both induction and prevention of carcinogenic process. It is well known that several types of neoplastic cells show decreased total PUFA content, contributing to their resistance to chemotherapy and lipid peroxidation. In the light of this, human lung cancer A549 cells, with low PUFA content, were exposed to arachidonic or docosahexaenoic acid to investigate the effect of n-6 and n-3 PUFAs on growth and elucidate underlying mechanisms. The bulk of the results showed that both n-6 PUFAs and n-3 PUFAs decrease human lung-tumor cell growth in a concentration-dependent manner, inducing cell death mainly evident at 100microM concentration. The mechanism underlying the antiproliferative effect of n-6 and n-3 PUFAs appeared to be the same, involving changes in fatty acid composition of biomembranes, production of cytostatic aldehydes derived from lipid peroxidation and able to affect DNA-binding activity of AP-1, and induction of PPAR. From these results it may be hypothesized that n-6 PUFAs, like n-3 PUFAs, are able to inhibit tumor growth.

Ligand activation of peroxisome proliferator-activated receptor-beta/delta(PPARbeta/delta) inhibits cell growth of human N/TERT-1 keratinocytes

The functional role of peroxisome proliferator-activated receptor-beta(PPARbeta; also referred to as PPARdelta) in epidermal cell growth remains controversial. Recent evidence suggests that ligand activation of PPARbeta/delta increases cell growth and inhibits apoptosis in epidermal cells. In contrast, other reports suggest that ligand activation of PPARbeta/delta leads to the induction of terminal differentiation and inhibition of cell growth. In the present study, the effect of the highly specific PPARbeta/delta ligand GW0742 on cell growth was examined using a human keratinocyte cell line (N/TERT-1) and mouse primary keratinocytes. Ligand activation of PPARbeta/delta with GW0742 prevented cell cycle progression from G1 to S phase and attenuated cell proliferation in N/TERT-1 cells. Despite specifically activating PPARbeta/delta as revealed by target gene induction, no changes in PTEN, PDK and ILK expression or downstream phosphorylation of Akt were found in either N/TERT-1 cells or primary keratinocytes. Further, altered cell growth resulting from serum withdrawal and the induction of caspase-3 activity by ultraviolet radiation were unchanged in the absence of PPARbeta/delta expression and/or the presence of GW0742. While no changes in the expression of mRNAs encoding cell cycle control proteins were found in response to GW0742, a significant decrease in the level of ERK phosphorylation was observed. Results from these studies demonstrate that ligand activation of PPARbeta/delta does not lead to an anti-apoptotic effect in either human or mouse keratinocytes, but rather, leads to inhibition of cell growth likely through the induction of terminal differentiation.

Combined simulation and mutagenesis analyses reveal the involvement of key residues for peroxisome proliferator-activated receptor alpha helix 12 dynamic behavior

The dynamic properties of helix 12 in the ligand binding domain of nuclear receptors are a major determinant of AF-2 domain activity. We investigated the molecular and structural basis of helix 12 mobility, as well as the involvement of individual residues with regard to peroxisome proliferator-activated receptor alpha (PPARalpha) constitutive and ligand-dependent transcriptional activity. Functional assays of the activity of PPARalpha helix 12 mutants were combined with free energy molecular dynamics simulations. The agreement between the results from these approaches allows us to make robust claims concerning the mechanisms that govern helix 12 functions. Our data support a model in which PPARalpha helix 12 transiently adopts a relatively stable active conformation even in the absence of a ligand. This conformation provides the interface for the recruitment of a coactivator and results in constitutive activity. The receptor agonists stabilize this conformation and increase PPARalpha transcription activation potential. Finally, we disclose important functions of residues in PPARalpha AF-2, which determine the positioning of helix 12 in the active conformation in the absence of a ligand. Substitution of these residues suppresses PPARalpha constitutive activity, without changing PPARalpha ligand-dependent activation potential.

RXR: From Partnership to Leadership in Metabolic Regulations

Vitamin A signaling occurs through nuclear receptors recognizing diverse forms of retinoic acid (RA). The retinoic acid receptors (RARs) bind all-trans RA and its 9-cis isomer (9-cis RA). They convey most of the activity of RA, particularly during embryogenesis. The second subset of receptors, the rexinoid receptors (RXRs), binds 9-cis RA only. However, RXRs are obligatory DNA-binding partners for a number of nuclear receptors, broadening the spectrum of their biological activity to the corresponding nuclear receptor-signaling pathways. The present chapter more particularly focuses on RXR-containing transcriptional complexes for which RXR is not only a structural component necessary for DNA binding but also acts as a ligand-activated partner. After positioning RXR among the nuclear receptor superfamily in the first part, we will give an overview of three major signaling pathways involved in metabolism, which are sensitive to RXR activation: LXR:RXR, FXR:RXR, and PPAR:RXR. The third and last part is focused on RXR signaling and its potential role in metabolic regulation. Indeed, while the nature of the endogenous ligand for RXR is still in question, as we will discuss herein, a better understanding of RXR activities is necessary to envisage the potential therapeutic applications of synthetic RXR ligands.

Effect of ligand troglitazone on peroxisome proliferator-activated receptor γ expression and cellular growth in human colon cancer cells

AIM: To investigate the effect of troglitazone on pe-roxisome proliferator-activated receptor γ (PPARγ) expression and cellular growth in human colon cancer HCT-116 and HCT-15 cells and to explore the related molecular mechanism.

METHODS: Human colon cancer HCT-116 and HCT-15 cells cultured in vitro were treated with troglitazone. Reverse transcription-polymerase chain reaction (RT-PCR) and Western blot were employed to detect the effect of troglitazone on PPARγ expression. The proliferative activity was determined by MTT assay, cell cycle and apoptosis were detected by flow cytometry. Apoptosis-related genes, cell cycle regulatory genes and p53 were examined by RT-PCR and Western blot respectively.

RESULTS: The expression of PPARγ in colon cancer HCT-116 and HCT-15 cells was up-regulated by troglitazone. Troglitazone inhibited proliferation, induced apoptosis and cell cycle G1 arrest in colon cancer cells. Troglitazone induced p53 expression in HCT-116 cells, but not in HCT-15 cells. The down-regulation of survivin and bcl-2 was found in both cell lines and up-regulation of bax was found only in HCT-116 cells, being consistent with growth inhibition in HCT-116 cells but not in HCT-15 cells. Troglitazone increased expression of p21^WAF1/CIP1 (p21), p27^KIP1 (p27) and reduced cyclin D1 in HCT-116 cells while only a minor decrease of cyclin D1 was found in HCT-15 cells.

CONCLUSION: Troglitazone is an inductor of PPARγ in colon cancer cells and inhibits PPARγ-dependently proliferation, which may attribute to cell cycle G1 arrest and apoptosis in colon cancer cells. Troglitazone may induce p53-independent apoptosis and p53-dependent expression of p21 and p27. Depending on cell background, different activation pathways may exist in colon cancer cells.

Cellular and molecular consequences of peroxisome proliferator-activated receptor-gamma activation in ovarian cancer cells

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a ligand-activated transcription factor. In addition to its canonical role in lipid and glucose metabolism, PPAR-gamma controls cell proliferation, death, and differentiation in several tissues. Here we have examined the expression of PPAR-gamma in ovarian tumors and the cellular and molecular consequences of its activation in ovarian cancer cells. PPAR-gamma was expressed in a large number of epithelial ovarian tumors and cell lines. The PPAR-gamma ligand ciglitazone inhibited the growth and clonogenic survival of ovarian cancer cells, inducing cell cycle arrest and cell death. Growth inhibition by ciglitazone was reversed by the PPAR-gamma antagonist GW9662, indicating the involvement of PPAR-gamma-dependent mechanisms. Microarray-based gene profiling revealed complex changes in the transcriptional program of ovarian cancer cells on treatment with ciglitazone and identified multiple pathways that may contribute to PPAR-gamma ligands' antitumor activity. Genes upregulated by ciglitazone were predominantly associated with metabolic, differentiation, and tumor-suppressor pathways, whereas downregulated genes were involved in cell proliferation, cell cycle, cell organization, and steroid biosynthesis. Collectively, our data indicate that PPAR-gamma activation by selective agonists is a valid strategy for ovarian cancer therapy and prevention, and should be tested alone and in combination with other anticancer drugs.

The Insulin-like Growth Factor-binding Protein 1 Gene Is a Primary Target of Peroxisome Proliferator-activated Receptors

Insulin-like growth factor-binding protein 1 (IGFBP-1) is a biomarker for metabolic and hyperproliferative diseases. At the same time, the nuclear receptors peroxisome proliferator-activated receptors (PPARs) are known for their critical role in the development of both the metabolic syndrome and various cancers. Here we demonstrate, in human hepatocellular carcinoma cells and in normal mouse liver, that IGFBP-1 mRNA expression is under the primary control of PPAR ligands. We applied an improved in silico screening approach for PPAR response elements (PPREs) and identified five candidate PPREs located within 10 kb of the transcription start site (TSS) of the IGFBP-1 gene. Chromatin immunoprecipitation assays showed that, in living cells, the genomic region containing the most proximal PPRE, at position -1200 (relative to the TSS), preferentially associates with multiple PPAR subtypes and various other components of the transcriptional apparatus, which include their heterodimerizing partner, retinoid X receptor, as well as phosphorylated RNA polymerase II, co-repressor, co-activator, and mediator proteins. Moreover, further chromatin immunoprecipitation assays demonstrated that the TSS regions of the IGFBP-1 gene and those of the related IGFBP-2, -5, and -6, but not of IGFBP-3 and -4 genes, bind PPARs as well. We also show that these additional PPAR binding genes contain a number of candidate PPREs and that their mRNA levels respond quickly to the presence of PPAR ligands, indicating that they are also primary PPAR target genes.

Pioglitazone inhibits androgen production in NCI-H295R cells by regulating gene expression of CYP17 and HSD3B2

Thiazolidinediones (TZDs) such as pioglitazone and rosiglitazone are widely used as insulin sensitizers in the treatment of type 2 diabetes. In diabetic women with polycystic ovary syndrome, treatment with pioglitazone or rosiglitazone improves insulin resistance and hyperandrogenism, but the mechanism by which TZDs down-regulate androgen production is unknown. Androgens are synthesized in the human gonads as well as the adrenals. We studied the regulation of androgen production by analyzing the effect of pioglitazone and rosiglitazone on steroidogenesis in human adrenal NCI-H295R cells, an established in vitro model of steroidogenesis of the human adrenal cortex. Both TZDs changed the steroid profile of the NCI-H295R cells and inhibited the activities of P450c17 and 3betaHSDII, key enzymes of androgen biosynthesis. Pioglitazone but not rosiglitazone inhibited the expression of the CYP17 and HSD3B2 genes. Likewise, pioglitazone repressed basal and 8-bromo-cAMP-stimulated activities of CYP17 and HSD3B2 promoter reporters in NCI-H295R cells. However, pioglitazone did not change the activity of a cAMP-responsive luciferase reporter, indicating that it does not influence cAMP/protein kinase A/cAMP response element-binding protein pathway signaling. Although peroxisome proliferator-activated receptor gamma (PPARgamma) is the nuclear receptor for TZDs, suppression of PPARgamma by small interfering RNA technique did not alter the inhibitory effect of pioglitazone on CYP17 and HSD3B2 expression, suggesting that the action of pioglitazone is independent of PPARgamma. On the other hand, treatment of NCI-H295R cells with mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) inhibitor 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD98059) enhanced promoter activity and expression of CYP17. This effect was reversed by pioglitazone treatment, indicating that the MEK/ERK signaling pathway plays a role in regulating androgen biosynthesis by pioglitazone.

Intestinal-specific PPARgamma deficiency enhances tumorigenesis in ApcMin/+ mice

Multiple investigations of the effects of peroxisome proliferator-activated receptor gamma (PPARgamma) ligands on colon cancer have produced contradictory results. While some studies demonstrated increased numbers of colonic polyps in Apc(Min/+) mice treated with various thiazolidinedione (TZD) PPARgamma ligands, others reported amelioration of tumor multiplicity and progression in both Apc(Min/+) mice and in mice with chemically-induced colon cancer. Here, we addressed the role of PPARgamma in murine intestinal tumorigenesis using gene knockout methodology. We found that either heterozygous or homozygous intestinal-specific PPARgamma deficiency enhanced the number of Apc(Min/+) tumors in both the small intestine and colon, especially in the colon, where PPARgamma deficiency also modulated tumor incidence. Gender significantly affected tumor multiplicity independent of PPARgamma genotype. Female Apc(Min/+) mice developed more tumors in the small intestine and more tumors overall, whereas male Apc(Min/+) mice developed more tumors in the colon. Nevertheless, intestinal PPARgamma deficiency enhanced tumorigenesis irrespective of gender. Our results suggest that PPARgamma functions as a tumor resistance factor in the mouse intestine and warrant further investigation of the PPARgamma-dependent and independent actions of TZDs in cancer.

Manganese treatment modulates the expression of peroxisome proliferator-activated receptors in astrocytoma and neuroblastoma cells

Peroxisome proliferator-activated receptors (PPARs) play roles in neural cells by regulating energy balance, cell proliferation and anti-oxidant responses although the molecular mechanisms underlying such roles are unclear. Chronic exposure to excess manganese (Mn) leads to neurotoxicity, although Mn-induced neurotoxic mechanisms have not been fully elucidated. We hypothesized Mn neurotoxicity differentially alters the expression of PPARs. We investigated the effects of manganese chloride treatment (0.01-4 mM) on protein expression of PPAR isoforms (alpha, beta, and gamma) in human astrocytoma (U87) and neuroblastoma (SK-N-SH) cells. The two cell types expressed the 3 PPAR isoforms differentially: their expression of the PPARs was altered by Mn-treatment. Furthermore, nuclear and cytosolic fractions derived from the 2 cell types, with and without Mn-treatment, exhibited marked differences in the protein content of PPARs. Our results constitute the first demonstration that the PPAR signaling pathway may assume pathophysiological importance in Mn neurotoxicity.

What are cyclooxygenases and lipoxygenases doing in the driver's seat of carcinogenesis?

Substantial evidence supports a functional role for cyclooxygenase- and lipoxygenase-catalyzed arachidonic and linoleic acid metabolism in cancer development. Genetic intervention studies firmly established cause-effect relations for cyclooxygenase-2, but cyclooxygenase-1 may also be involved. In addition, pharmacologic cyclooxygenase inhibition was found to suppress carcinogenesis in both experimental mouse models and several cancers in humans. Arachidonic acid-derived eicosanoid or linoleic acid-derived hydro[peroxy]fatty acid signaling are likely to be involved impacting fundamental biologic phenomena as diverse as cell growth, cell survival, angiogenesis, cell invasion, metastatic potential and immunomodulation. However, long chain unsaturated fatty acid oxidation reactions indicate antipodal functions of distinct lipoxygenase isoforms in carcinogenesis, i.e., the 5- and platelet-type 12-lipoxygenase exhibit procarcinogenic activities, while 15-lipoxygenase-1 and 15-lipoxygenase-2 may suppress carcinogenesis.

PPARbeta/delta regulates paneth cell differentiation via controlling the hedgehog signaling pathway

BACKGROUND & AIMS

All 4 differentiated epithelial cell types found in the intestinal epithelium derive from the intestinal epithelial stem cells present in the crypt unit, in a process whose molecular clues are intensely scrutinized. Peroxisome proliferator-activated receptor beta (PPARbeta) is a nuclear hormone receptor activated by fatty acids and is highly expressed in the digestive tract. However, its function in intestinal epithelium homeostasis is understood poorly.

METHODS

To assess the role of PPARbeta in the small intestinal epithelium, we combined various cellular and molecular approaches in wild-type and PPARbeta-mutant mice.

RESULTS

We show that the expression of PPARbeta is particularly remarkable at the bottom of the crypt of the small intestine where Paneth cells reside. These cells, which have an important role in the innate immunity, are strikingly affected in PPARbeta-null mice. We then show that Indian hedgehog (Ihh) is a signal sent by mature Paneth cells to their precursors, negatively regulating their differentiation. Importantly, PPARbeta acts on Paneth cell homeostasis by down-regulating the expression of Ihh, an effect that can be mimicked by cyclopamine, a known inhibitor of the hedgehog signaling pathway.

CONCLUSIONS

We unraveled the Ihh-dependent regulatory loop that controls mature Paneth cell homeostasis and its modulation by PPARbeta. PPARbeta currently is being assessed as a drug target for metabolic diseases; these results reveal some important clues with respect to the signals controlling epithelial cell fate in the small intestine.

Chemoprevention of gastric cancer

Gastric cancer is the second commonest cause of cancer-associated death in the world. The high mortality is largely attributed to the huge number of at-risk individuals as well as the delay in presentation. Hence, chemoprevention of gastric cancer appears to be the most promising approach in reducing the incidence and mortality related to this cancer. Among various chemoprevention strategies, Helicobacter pylori eradication is the one being most extensively examined. Results from several large-scale prospective randomized studies, however, showed marginal benefits of H. pylori eradication on regression of premalignant gastric lesions. Moreover, there is no significant reduction in gastric cancer incidence. Similarly, ascorbic acid and/or beta-carotene supplementation have borderline effects on premalignant gastric lesions. Based on epidemiological data, the use of non-steroidal anti-inflammatory drugs is associated with a reduced risk of stomach cancer. Future studies should evaluate the role of other chemopreventive agents, particularly specific cyclooxygenase-2 inhibitors, in reducing the risk of gastric cancer.

Polymorphisms in the genes involved in the arachidonic acid-pathway, fish consumption and the risk of colorectal cancer

The objective of this study on colorectal cancer was to investigate the associations between SNPs in the genes involved in the arachidonic acid (AA)-pathway, their haplotypes and colorectal cancer. Moreover, interactions between SNPs and fish consumption were considered. In this study, a total of 508 cases and 772 controls were included, originating from 2 prospective cohorts, the Monitoring Project on Cardiovascular Disease Risk Factors (PPHV) and Diagnostisch Onderzoek Mammacarcinoom (DOM). Genotypes of 23 SNPs in 7 candidate genes were determined and the modifying effect of fish consumption was considered. A protective effect of the minor allele of SNP V102V in PTGS2 was observed (odds ratio (OR), 0.37; 95% confidence intervals (CI), 0.16-0.87). The haplotype representing this allele showed a weaker inverse association, indicating that 2 alleles are necessary to obtain this protective effect. Fish consumption data was available for 209 cases and 418 controls. Increased fish consumption was inversely associated with cancer, although not significant (OR, 0.83; 95% CI, 0.57-1.20). Despite the substantial reductions in cancer risk for some genotypes in combination with high fish intake, no significant interactions between any SNP studied and fish consumption were observed. We have previously described an association between colorectal adenomas and SNP V102V in PTGS2 and have now confirmed this association for colorectal adenocarcinomas. Fish consumption of once a week or more might protect against colorectal cancer, but no significant interactions with SNPs in the genes involved in the AA-pathway could be detected within the study.

Antitumorigenic effect of Wnt 7a and Fzd 9 in non-small cell lung cancer cells is mediated through ERK-5-dependent activation of peroxisome proliferator-activated receptor gamma

The Wnt pathway is critical for normal development, and mutation of specific components is seen in carcinomas of diverse origins. The role of this pathway in lung tumorigenesis has not been clearly established. Recent studies from our laboratory indicate that combined expression of the combination of Wnt 7a and Frizzled 9 (Fzd 9) in Non-small Cell Lung Cancer (NSCLC) cell lines inhibits transformed growth. We have also shown that increased expression of peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits transformed growth of NSCLC and promotes epithelial differentiation of these cells. The goal of this study was to determine whether the effects of Wnt 7a/Fzd 9 were mediated through PPARgamma. We found that Wnt 7a and Fzd 9 expression led to increased PPARgamma activity. This effect was not mediated by altered expression of the protein. Wnt 7a and Fzd 9 expression resulted in activation of ERK5, which was required for PPARgamma activation in NSCLC. SR 202, a known PPARgamma inhibitor, blocked the increase in PPARgamma activity and restored anchorage-independent growth in NSCLC expressing Wnt 7a and Fzd 9. SR 202 also reversed the increase in E-cadherin expression mediated by Wnt 7a and Fzd 9. These data suggest that ERK5-dependent activation of PPARgamma represents a major effector pathway mediating the anti-tumorigenic effects of Wnt 7a and Fzd 9 in NSCLC.

PPARgamma insufficiency promotes follicular thyroid carcinogenesis via activation of the nuclear factor-kappaB signaling pathway

The molecular genetic events underlying thyroid carcinogenesis are poorly understood. Mice harboring a knock-in dominantly negative mutant thyroid hormone receptor beta (TRbetaPV/PV mouse) spontaneously develop follicular thyroid carcinoma similar to human thyroid cancer. Using this mutant mouse, we tested the hypothesis that the peroxisome proliferator-activated receptor gamma (PPARgamma) could function as a tumor suppressor in thyroid cancer in vivo. Using the offspring from the cross of TRbetaPV/+ and PPARgamma+/- mice, we found that thyroid carcinogenesis progressed significantly faster in TRbetaPV/PV mice with PPARgamma insufficiency from increased cell proliferation and reduced apoptosis. Reduced PPARgamma protein abundance led to the activation of the nuclear factor-kappaB signaling pathway, resulting in the activation of cyclin D1 and repression of critical genes involved in apoptosis. Treatment of TRbetaPV/PV mice with a PPARgamma agonist, rosiglitazone, delayed the progression of thyroid carcinogenesis by decreasing cell proliferation and activation of apoptosis. These results suggest that PPARgamma is a critical modifier in thyroid carcinogenesis and could be tested as a therapeutic target in thyroid follicular carcinoma.

A new melanoma antigen fatty acid-binding protein 7, involved in proliferation and invasion, is a potential target for immunotherapy and molecular target therapy

The identification of molecules that are preferentially expressed in melanoma cells and involved in their malignant phenotypes is important for understanding melanoma biology and the development of new diagnostic and therapeutic methods. By comparing the expression profile of a melanoma cell line with those of various normal tissues using GeneChip and by confirming the actual expression of the selected genes by reverse transcription-PCR and Northern and Western blot analyses, fatty acid-binding protein 7 (FABP7), which is frequently expressed in melanomas, was identified. Immunohistochemical examination revealed that FABP7 was expressed in 11 of 15 melanoma tissues. By down-regulating the FABP7 expression with FABP7-specific small interfering RNAs, in vitro cell proliferation and Matrigel invasion were suppressed in two of six melanoma cell lines. Overexpression of FABP7 in a FABP7-negative embryonic kidney cell line 293T by transfecting with the FABP7 cDNA resulted in enhanced cell proliferation and Matrigel invasion, indicating that FABP7 plays a role in the malignant phenotype of some melanoma cell lines. IgG antibodies specific for the phage or bacterial recombinant FABP7 protein were detected in 14 of 25 (56%) or in 8 of 31 (26%) sera from melanoma patients, respectively, but not in sera from healthy individuals, indicating that FABP7 is an immunogenic antigen in melanoma patients. These results showed that FABP7 is frequently expressed in melanoma, may be involved in cell proliferation and invasion, and may be a potential target for development of diagnostic and therapeutic methods.

Genetic regulation of unsaturated fatty acid composition in C. elegans

Delta-9 desaturases, also known as stearoyl-CoA desaturases, are lipogenic enzymes responsible for the generation of vital components of membranes and energy storage molecules. We have identified a novel nuclear hormone receptor, NHR-80, that regulates delta-9 desaturase gene expression in Caenorhabditis elegans. Here we describe fatty acid compositions, lifespans, and gene expression studies of strains carrying mutations in nhr-80 and in the three genes encoding delta-9 desaturases, fat-5, fat-6, and fat-7. The delta-9 desaturase single mutants display only subtle changes in fatty acid composition and no other visible phenotypes, yet the fat-5;fat-6;fat-7 triple mutant is lethal, revealing that endogenous production of monounsaturated fatty acids is essential for survival. In the absence of FAT-6 or FAT-7, the expression of the remaining desaturases increases, and this ability to compensate depends on NHR-80. We conclude that, like mammals, C. elegans requires adequate synthesis of unsaturated fatty acids and maintains complex regulation of the delta-9 desaturases to achieve optimal fatty acid composition.

The ratio of saturated to unsaturated fatty acids has a profound affect on the fluidity and function of cellular membranes. Animals, plants, and microorganisms regulate the synthesis of unsaturated fatty acids during changing environmental conditions, as well as in response to dietary nutrients. In this paper the authors use a combination of genetic and biochemical approaches to address the regulation of unsaturated fatty acid synthesis in the roundworm Caenorhabditis elegans. They identify a new transcription factor, NHR-80, that activates the expression of genes encoding delta-9 fatty acid desaturases, the enzymes responsible for catalyzing the insertion of double bonds into saturated fatty acid chains. These unsaturated fatty acids are critical components of membranes, as well as fat storage molecules. Experiments presented here demonstrate that the worms require adequate synthesis of unsaturated fatty acids for survival and that they maintain intricate regulation of the three delta-9 desaturase genes in response to different nutrients. Abnormalities in lipid metabolism lead to obesity and diabetes in humans; this study contributes to our understanding of the regulation of this metabolic pathway.

Variations in the peroxisome proliferator-activated receptor-gamma gene and melanoma risk

There is strong evidence to suggest that the peroxisome proliferator-activated receptor (PPAR)-gamma, a member of the nuclear receptor family of transcriptional regulators, mediates tumor suppressive activities in a variety of human cancers. Recently, PPARgamma agonists were found to inhibit growth of melanoma cell lines. Here, we tested the possibility that variations in the gene encoding PPARgamma (PPARG) influence melanoma risk. Two variations of PPARG (P12A[rs1801282] and C161T [rs3856806]) were investigated in two independent case-control studies with a total of 832 melanoma patients and 790 control individuals. In the first study, homozygous carriers of the rare *T allele of the C161T polymorphism in exon 6 of PPARG were significantly more common among patients with melanoma than among healthy subjects (6.0 vs. 2.0%; P=0.0096) and this association was independent of clinical risk factors such as skin type and nevus count (odds ratio 5.18; 95% confidence interval 1.68-15.96; P=0.0041). This finding, however, could not be replicated in the second case-control study. We therefore conclude that the investigated PPARG polymorphisms are not likely to constitute a significant risk factor for the development of melanoma among German Caucasians.

Peroxisome proliferator-activated receptor-gamma ligands for the treatment of breast cancer

Pioglitazone and rosiglitazone are thiazolidinediones used for the treatment of Type 2 diabetes mellitus. They modulate glucose and fat metabolism, mainly by binding to the nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR)-gamma. PPAR-gamma signalling is involved in a number of other disease conditions including cancer. In breast cancer cells, PPAR-gamma ligands inhibit proliferation and induce apoptosis both in vitro and in vivo. PPAR-gamma ligands also inhibit tumour angiogenesis and invasion. The only published clinical trial using a PPAR-gamma ligand in patients with metastatic breast cancer failed to show any clinical benefits. The mechanism of action of the thiazolidinediones in breast cancer cells is not fully understood but involves interactions with other nuclear hormone receptors, transcriptional co-activators and repressors as well as PPAR-gamma-independent effects. A better understanding of these mechanisms will be needed before PPAR-gamma ligands may be useful in the treatment of breast cancer patients.

Peroxisome-proliferator-activated receptor alpha agonists inhibit cyclo-oxygenase 2 and vascular endothelial growth factor transcriptional activation in human colorectal carcinoma cells via inhibition of activator protein-1

Recent evidence indicates that PPAR (peroxisome-proliferator-activated receptor) alpha ligands possess anti-inflammatory and antitumoural properties owing to their inhibitory effects on the expression of genes that are involved in the inflammatory response. However, the precise molecular mechanisms underlying these effects are poorly understood. In the present study, we show that tumour promoter PMA-mediated induction of genes that are significantly associated with inflammation, tumour growth and metastasis, such as COX-2 (cyclo-oxygenase 2) and VEGF (vascular endothelial growth factor), is inhibited by PPARalpha ligands in the human colorectal carcinoma cell line SW620. PPARalpha activators LY-171883 and WY-14,643 were able to diminish transcriptional induction of COX-2 and VEGF by inhibiting AP-1 (activator protein-1)-mediated transcriptional activation induced by PMA or by c-Jun overexpression. The actions of these ligands on AP-1 activation and COX-2 and VEGF transcriptional induction were found to be dependent on PPARalpha expression. Our studies demonstrate the existence of a negative cross-talk between the PPARalpha- and AP-1-dependent signalling pathways in these cells. PPARalpha interfered with at least two steps within the pathway leading to AP-1 activation. First, PPARalpha activation impaired AP-1 binding to a consensus DNA sequence. Secondly, PPARalpha ligands inhibited c-Jun transactivating activity. Taken together, these findings provide new insight into the anti-inflammatory and anti-tumoural properties of PPARalpha activation, through the inhibition of the induction of AP-1-dependent genes that are involved in inflammation and tumour progression.

Involvement of PPAR nuclear receptors in tissue injury and wound repair

Tissue damage resulting from chemical, mechanical, and biological injury, or from interrupted blood flow and reperfusion, is often life threatening. The subsequent tissue response involves an intricate series of events including inflammation, oxidative stress, immune cell recruitment, and cell survival, proliferation, migration, and differentiation. In addition, fibrotic repair characterized by myofibroblast transdifferentiation and the deposition of ECM proteins is activated. Failure to initiate, maintain, or stop this repair program has dramatic consequences, such as cell death and associated tissue necrosis or carcinogenesis. In this sense, inflammation and oxidative stress, which are beneficial defense processes, can become harmful if they do not resolve in time. This repair program is largely based on rapid and specific changes in gene expression controlled by transcription factors that sense injury. PPARs are such factors and are activated by lipid mediators produced after wounding. Here we highlight advances in our understanding of PPAR action during tissue repair and discuss the potential for these nuclear receptors as therapeutic targets for tissue injury.

Bioassay for the identification of natural product-based activators of peroxisome proliferator-activated receptor-gamma (PPARgamma): the marine sponge metabolite psammaplin A activates PPARgamma and induces apoptosis in human breast tumor cells

Peroxisome proliferator-activated receptors (PPARs), members of the nuclear hormone receptor (NHR) family, are ligand-activated transcription factors. Ligands (agonists) of PPARgamma have been shown to inhibit growth, promote terminal differentiation, and induce apoptosis in human breast tumor cells. A cell-based reporter assay was developed to examine extracts of terrestrial and marine organisms for the ability to activate PPARgamma. Bioassay-guided fractionation and isolation of an active extract from Pseudoceratina rhax yielded the known histone deacetylase (HDAC) inhibitor psammaplin A (1). Compound 1 activates PPARgamma in a MCF-7 cell-based reporter assay and induces apoptosis in human breast tumor cells in vitro. Molecular modeling studies suggest that 1 may interact with binding sites within the PPARgamma ligand-binding pocket. Therefore, in addition to its known effects on HDAC-mediated processes, activation of PPARgamma-regulated gene expression may play a role in the ability of 1 to induce apoptosis.

Oxidative metabolism of linoleic acid modulates PPAR-beta/delta suppression of PPAR-gamma activity

Peroxisome proliferator-activated receptors (PPARs) are transcription factors that strongly influence molecular events in normal and cancer cells. PPAR-beta/delta (PPAR-b/d) overexpression suppresses the activity of PPAR-gamma (PPAR-g) and PPAR-alpha. This interaction has been questioned, however, by studies with synthetic ligands of PPARs in PPAR-b/d-null cells, and it is not known whether an interaction between PPAR-b/d and PPAR-g exists, especially in relation to the signaling by natural PPAR ligands. Oxidative metabolites of linoleic and arachidonic acids are natural ligands of PPARs. 13-S-hydroxyoctadecadienoic acid (13-S-HODE), the main product of 15-lipoxygenase-1 (15-LOX-1) metabolism of linoleic acid, downregulates PPAR-b/d. We tested (a) whether PPAR-b/d expression modulates PPAR-g activity in experimental models of the loss and gain of PPAR-b/d function in colon cancer cells and (b) whether 15-LOX-1 formation of 13-S-HODE influences the interaction between PPAR-b/d and PPAR-g. We found that (a) 15-LOX-1 formation of 13-S-HODE promoted PPAR-g activity, (b) PPAR-b/d expression suppressed PPAR-g activity in models of both loss and gain of PPAR-b/d function, (c) 15-LOX-1 activated PPAR-g by downregulating PPAR-b/d, and (d) 15-LOX-1 expression induced apoptosis in colon cancer cells via modulating PPAR-b/d suppression of PPAR-g. These findings elucidate a novel mechanism of the signaling by natural ligands of PPARs, which involves modulating the interaction between PPAR-b/d and PPAR-g.

In vitro and in vivo effects of the PPAR-alpha agonists fenofibrate and retinoic acid in endometrial cancer

Fenofibrate, an agonist of PPAR-alpha, in doses above 25 μM, inhibits proliferation and induces apoptosis in Ishikawa endometrial cancer cells. We show that these effects are potentiated by retinoic acid, an agonist of the retinoid-X-receptor. DNA content analysis shows that G₁/S phase progression through the cell cycle is inhibited. Independent Component Analysis of gene microarray experiments demonstrated downregulation of Cyclin D1 (CCND1) and associated changes in cell cycle gene expression. Expression of PPAR-alpha mRNA was reduced by >75% using RNA-interference but this resulted in only minor changes in biological effects. A nude mouse model of endometrial carcinoma was used to investigate the effect of fenofibrate in vivo but failed to show consistent inhibition of tumour growth.

The combination of fenofibrate and retinoic acid is a potent inhibitor of Ishikawa endometrial cancer cell growth in vitro.

PPARbeta/delta selectively induces differentiation and inhibits cell proliferation

Peroxisome proliferator-activated receptor (PPAR) beta-null mice exhibit exacerbated epithelial cell proliferation and enhanced sensitivity to skin carcinogenesis, suggesting that ligand activation of PPARbeta will inhibit keratinocyte proliferation. By using of a highly specific ligand (GW0742) and the PPARbeta-null mouse model, activation of PPARbeta was found to selectively induce keratinocyte terminal differentiation and inhibit keratinocyte proliferation. Additionally, GW0742 was found to be anti-inflammatory due to inhibition of myeloperoxidase activity, independent of PPARbeta. These data suggest that ligand activation of PPARbeta could be a novel approach to selectively induce differentiation and inhibit cell proliferation, thus representing a new molecular target for the treatment of skin disorders resulting from altered cell proliferation such as psoriasis and cancer.

Protective effect of nonsteroidal anti-inflammatory drugs on colorectal adenomas is modified by a polymorphism in peroxisome proliferator-activated receptor delta

OBJECTIVE

Nonsteroidal anti-inflammatory drugs (NSAIDs) are associated with a decreased risk of colorectal tumors. Single nucleotide polymorphisms (SNPs) in target genes of NSAID action, and their haplotypes, might modulate this protective effect.

METHODS

A case-control study including 724 cases and 682 controls was used to evaluate the effect of NSAIDs on colorectal adenoma risk in The Netherlands, a country in which NSAID use is relatively low. Cases and controls were classified according to presence or absence of endoscopy-proven, pathology-confirmed colorectal adenomas, ever in their lives. Thirteen SNPs in four genes (PPARdelta, PPARgamma, PTGS1 and PTGS2) were genotyped in 787 subjects (384 cases and 403 controls).

RESULTS

Compared to non-regular users (< 12 times/year), regular users of NSAIDs (> or = 12 times/year) had a lower risk of colorectal adenomas (odds ratio (OR): 0.75, 95% confidence interval (CI): 0.56-0.99). The results were similar for aspirin only. We found an interaction between SNP c.-789C>T in PPARdelta and NSAID use (P=0.03). The protective effect of NSAIDs was strengthened for regular users with the PPARdelta CT or TT genotypes (OR: 0.35, 95%CI: 0.11-1.13), whereas a positive association was observed for non-regular users with these genotypes (OR: 2.24, 95%CI: 1.06-4.73) as compared to non-regular users with the CC genotype. Also, a statistically significant interaction between a major haplotype containing the minor allele of this SNP and NSAID use was observed.

CONCLUSIONS

This study confirms the protective effect of NSAIDs and suggests a modulating effect of a SNP in the promoter of PPARdelta.

Induction of PPARbeta and prostacyclin (PGI2) synthesis by Raf signaling: failure of PGI2 to activate PPARbeta

A role for the nuclear receptor peroxisome proliferator-activated receptor-beta (PPARbeta) in oncogenesis has been suggested by a number of observations but its precise role remains elusive. Prostaglandin I2 (PGI2, prostacyclin), a major arachidonic acid (AA) derived cyclooxygenase (Cox) product, has been proposed as a PPARbeta agonist. Here, we show that the 4-hydroxytamoxifen (4-OHT) mediated activation of a C-Raf-estrogen receptor fusion protein leads to the induction of both the PPARbeta and Cox-2 genes, concomitant with a dramatic increase in PGI2 synthesis. Surprisingly, however, 4-OHT failed to activate PPARbeta transcriptional activity, indicating that PGI2 is insufficient for PPARbeta activation. In agreement with this conclusion, the overexpression of ectopic Cox-2 and PGI2 synthase (PGIS) resulted in massive PGI2 synthesis but did not activate the transcriptional activity of PPARbeta. Conversely, inhibition of PGIS blocked PGI2 synthesis but did not affect the AA mediated activation of PPARbeta. Our data obtained with four different cell types and different experimental strategies do not support the prevailing opinion that PGI2 plays a significant role in the regulation of PPARbeta.

PPAR alpha inhibits vascular smooth muscle cell proliferation underlying intimal hyperplasia by inducing the tumor suppressor p16INK4a

Vascular SMC proliferation is a crucial event in occlusive cardiovascular diseases. PPARalpha is a nuclear receptor controlling lipid metabolism and inflammation, but its role in the regulation of SMC growth remains to be established. Here, we show that PPARalpha controls SMC cell-cycle progression at the G1/S transition by targeting the cyclin-dependent kinase inhibitor and tumor suppressor p16(INK4a) (p16), resulting in an inhibition of retinoblastoma protein phosphorylation. PPARalpha activates p16 gene transcription by both binding to a canonical PPAR-response element and interacting with the transcription factor Sp1 at specific proximal Sp1-binding sites of the p16 promoter. In a carotid arterial-injury mouse model, p16 deficiency results in an enhanced SMC proliferation underlying intimal hyperplasia. Moreover, PPARalpha activation inhibits SMC growth in vivo, and this effect requires p16 expression. These results identify an unexpected role for p16 in SMC cell-cycle control and demonstrate that PPARalpha inhibits SMC proliferation through p16. Thus, the PPARalpha/p16 pathway may be a potential pharmacological target for the prevention of cardiovascular occlusive complications of atherosclerosis.

Prostate cancer cell type-specific involvement of the VDR and RXR in regulation of the human PTHrP gene via a negative VDRE

Parathyroid hormone-related protein (PTHrP) increases the growth and osteolytic potential of prostate cancer cells, making it important to control PTHrP expression in these cells. We show that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and its non-hypercalcemic analog, EB1089, decrease PTHrP mRNA and cellular protein levels in the androgen-dependent human prostate cancer cell line LNCaP and its androgen-independent derivative, the C4-2 cell line. This effect is mediated via a negative Vitamin D response element (nVDREhPTHrP) within the human PTHrP gene and involves an interaction between nVDREhPTHrP and the Vitamin D receptor (VDR). The retinoid X receptor (RXR) is a frequent heterodimeric partner of the VDR. We show that RXRalpha forms part of the nuclear protein complex that interacts with nVDREhPTHrP along with the VDR in LNCaP and C4-2 cells. We also show that the RXR ligand, 9-cis-retinoic acid, downregulates PTHrP mRNA levels; this decrease is more pronounced in LNCaP than in C4-2 cells. In addition, 9-cis-retinoic acid enhances the 1,25(OH)2D3-mediated downregulation of PTHrP expression in both cell lines; this effect also is more pronounced in LNCaP cells. Proliferation of LNCaP, but not C4-2, cells is decreased by 9-cis-retinoic acid. Promoter activity driven by nVDREhPTHrP cloned upstream of the SV40 promoter and transiently transfected into LNCaP and C4-2 cells is downregulated in response to 1,25(OH)2D3 and EB1089 in both cell lines. Co-treatment with these compounds and 9-cis-retinoic acid further decreases CAT activity in LNCaP, but not C4-2, cells. These results indicate that PTHrP gene expression is regulated by 1,25(OH)2D3 in a cell type-specific manner in prostate cancer cells.

Antineoplastic effects of the DNA methylation inhibitor hydralazine and the histone deacetylase inhibitor valproic acid in cancer cell lines

Background

Among the epigenetic alterations occurring in cancer, DNA hypermethylation and histone hypoacetylation are the focus of intense research because their pharmacological inhibition has shown to produce antineoplastic activity in a variety of experimental models. The objective of this study was to evaluate the combined antineoplastic effect of the DNA methylation inhibitor hydralazine and the histone deacetylase inhibitor valproic acid in a panel of cancer cell lines.

Results

Hydralazine showed no growth inhibitory effect on cervical, colon, breast, sarcoma, glioma, and head & neck cancer cell lines when used alone. On the contrary, valproic acid showed a strong growth inhibitory effect that is potentiated by hydralazine in some cell lines. Individually, hydralazine and valproic acid displayed distinctive effects upon global gene over-expression but the number of genes over-expressed increased when cells were treated with the combination. Treatment of HeLa cells with hydralazine and valproic acid lead to an increase in the cytotoxicity of gemcitabine, cisplatin and adriamycin. A higher antitumor effect of adriamycin was observed in mice xenografted with human fibrosarcoma cells when the animals were co-treated with hydralazine and valproic acid.

Conclusion

Hydralazine and valproic acid, two widely used drugs for cardiovascular and neurological conditions respectively have promising antineoplastic effects when used concurrently and may increase the antitumor efficacy of current cytotoxic agents.

From molecular action to physiological outputs: peroxisome proliferator-activated receptors are nuclear receptors at the crossroads of key cellular functions

Peroxisome proliferator-activated receptors (PPARs) compose a family of three nuclear receptors which act as lipid sensors to modulate gene expression. As such, PPARs are implicated in major metabolic and inflammatory regulations with far-reaching medical consequences, as well as in important processes controlling cellular fate. Throughout this review, we focus on the cellular functions of these receptors. The molecular mechanisms through which PPARs regulate transcription are thoroughly addressed with particular emphasis on the latest results on corepressor and coactivator action. Their implication in cellular metabolism and in the control of the balance between cell proliferation, differentiation and survival is then reviewed. Finally, we discuss how the integration of various intra-cellular signaling pathways allows PPARs to participate to whole-body homeostasis by mediating regulatory crosstalks between organs.

Inhibition of chemically induced skin carcinogenesis by sulindac is independent of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta)

Inhibition of cyclooxygenase-2 (COX2) by non-steroidal anti-inflammatory drugs (NSAID) is known to suppress skin carcinogenesis. It was further suggested that inhibition of COX2-derived prostaglandins by NSAIDs could reduce levels of putative endogenous ligands of peroxisome proliferator-activated receptor-beta (PPARbeta), and these ligands could potentiate tumorigenesis. However, it is currently unclear whether ligand activation of PPARbeta either inhibits or potentiates carcinogenesis. The present studies were designed to examine the mechanism of NSAID-mediated chemoprevention in skin, and, in particular, to determine the role of PPARbeta in this process. A two-stage skin carcinogenicity bioassay was performed using wild-type and PPARbeta-null mice that were fed either a control diet or one containing 0.32 g sulindac/kg diet. Significant inhibition of chemically induced skin carcinogenesis was observed in both wild-type and PPARbeta-null mice, and this was associated with a marked decrease in the concentration of skin prostaglandins including PGE(2) and PGI(2). Results from these studies demonstrate that inhibition of COX2 by dietary sulindac in mouse skin can effectively inhibit chemically induced skin carcinogenesis, and suggest that the mechanism underlying this chemopreventive effect is independent of PPARbeta. Additionally, results from these studies do not support the hypothesis that ligand activation of PPARbeta by COX-derived metabolites potentiates chemically induced skin carcinogenesis.

Peroxisome proliferator-activated receptor-gamma Pro12Ala polymorphism, Helicobacter pylori infection and non-cardia gastric carcinoma in Chinese

BACKGROUND

Peroxisome proliferator-activated receptor gamma inhibits the growth and induces apoptosis of gastric cancer cells. A common polymorphism at codon 12 of this gene (Pro12Ala) has been shown to confer protection against diabetes and colorectal cancer.

AIM

To study the association between peroxisome proliferator-activated receptor gamma gene polymorphism, Helicobacter pylori infection and gastric cancer in Chinese.

METHODS

One hundred and four consecutive patients with non-cardia gastric adenocarcinoma and 104 matched controls were examined. Peroxisome proliferator-activated receptor gamma Pro12Ala polymorphism was analysed by polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

The frequency of peroxisome proliferator-activated receptor gamma G (Ala12) allele was significantly higher among cancer patients (19.2%) than in control (8.7%; OR 2.5, 95% CI 1.1-5.8). While H. pylori infection was more prevalent in gastric cancer patients (OR 3.0; 95% CI 1.6-5.7), the combination of peroxisome proliferator-activated receptor gamma G allele and H. pylori infection further increased the risk of gastric cancer (OR 12.8, 95% CI 3.2-50.5). The presence of the Ala12 did not increase the risk of gastric cancer in H. pylori-negative subjects.

CONCLUSION

Our study suggests the potential association between peroxisome proliferator-activated receptor gamma polymorphism and H. pylori infection in the development of non-cardia gastric cancer.

Growth inhibition and apoptosis in human Philadelphia chromosome-positive lymphoblastic leukemia cell lines by treatment with the dual PPARalpha/gamma ligand TZD18

Treatment of adult Philadelphia chromosome-positive lymphocytic leukemia is rarely successful. We report here the effects of TZD18, a novel dual ligand specific for peroxisome proliferator-activated receptor alpha and gamma (PPARalpha/gamma) on Ph(+) lymphocytic leukemia cell lines BV173, SD1, and SupB-15. Exposure of these cells to TZD18 resulted in growth inhibition in a dose- and time-dependent manner that was associated with G(1) cell cycle arrest. This effect was much stronger than that mediated by the PPARgamma ligand pioglitazone (PGZ), which also belongs to the thiazolidinediones (TZD) class of ligands. However, it may not be mediated through PPARgamma or PPARalpha activation because antagonists of PPARgamma and PPARalpha cannot reverse it. Study of the key regulators of cell cycle progression by Western blot analysis showed that the expression of the cyclin-dependent kinase inhibitor (CDKI) p27(kip1), but not that of p21(cip1), was enhanced, whereas that of c-Myc, cyclin E, cyclin D2, and cyclin-dependent kinases 2 and 4 (CDK-2 and CDK-4) was decreased when these cells were treated with TZD18 (10 or 20 microM). Therefore, the up-regulation of p27(kip1) and the down-regulation of CDK-2 and CDK-4 may, at least in part, account for the G(1) cell cycle arrest. Furthermore, a remarkable induction of apoptosis was observed in the cells treated with this dual ligand. No obvious alteration of bcl-2 protein level occurred, but bax was up-regulated in these TZD18-treated cells. Activation of caspase 8 and caspase 9 by TZD18 was also observed. Importantly, NF-kappaB DNA-binding activity was markedly decreased by the TZD18 treatment. In addition, TZD18 enhanced the growth inhibitory effect of imatinib, a specific tyrosine kinase inhibitor therapeutically used in the treatment of Ph(+) leukemia. Overall, our findings strongly suggest that TZD18 may offer a new therapeutic approach to aid in the treatment of Ph(+) lymphocytic leukemia.

A novel partial agonist of peroxisome proliferator-activated receptor-gamma (PPARgamma) recruits PPARgamma-coactivator-1alpha, prevents triglyceride accumulation, and potentiates insulin signaling in vitro

Partial agonists of peroxisome proliferator-activated receptor-gamma (PPARgamma), also termed selective PPARgamma modulators, are expected to uncouple insulin sensitization from triglyceride (TG) storage in patients with type 2 diabetes mellitus. These agents shall thus avoid adverse effects, such as body weight gain, exerted by full agonists such as thiazolidinediones. In this context, we describe the identification and characterization of the isoquinoline derivative PA-082, a prototype of a novel class of non-thiazolidinedione partial PPARgamma ligands. In a cocrystal with PPARgamma it was bound within the ligand-binding pocket without direct contact to helix 12. The compound displayed partial agonism in biochemical and cell-based transactivation assays and caused preferential recruitment of PPARgamma-coactivator-1alpha (PGC1alpha) to the receptor, a feature shared with other selective PPARgamma modulators. It antagonized rosiglitazone-driven transactivation and TG accumulation during de novo adipogenic differentiation of murine C3H10T1/2 mesenchymal stem cells. The latter effect was mimicked by overexpression of wild-type PGC1alpha but not its LXXLL-deficient mutant. Despite failing to promote TG loading, PA-082 induced mRNAs of genes encoding components of insulin signaling and adipogenic differentiation pathways. It potentiated glucose uptake and inhibited the negative cross-talk of TNFalpha on protein kinase B (AKT) phosphorylation in mature adipocytes and HepG2 human hepatoma cells. PGC1alpha is a key regulator of energy expenditure and down-regulated in diabetics. We thus propose that selective recruitment of PGC1alpha to favorable PPARgamma-target genes provides a possible molecular mechanism whereby partial PPARgamma agonists dissociate TG accumulation from insulin signaling.