Peroxisome proliferator-activated receptor gamma is induced during differentiation of colon epithelium cells

Peroxisome proliferator-activated receptors (PPARs) and associated transcription factors in colon cancer: reduced expression of PPARγ-coactivator 1 (PGC-1)

Peroxisome proliferator-activated receptors (PPARs) alpha,beta/delta and gamma are fatty acid sensitive transcription factors that have been implicated in colorectal cancer. To better understand their role, we studied the expression levels of all PPAR-isoforms and transcriptional partners such as the retinoid X receptor alpha (RXRalpha) and PPARgamma-coactivator-1 (PGC-1) by means of real-time PCR in 17 patients with colon cancer. While a heterogeneous pattern was observed for the expression level of the PPAR-isoforms alpha,beta/delta and gamma, the coactivator PGC-1 was significantly decreased in 15 of 17 tumors. Taken together our data suggest that the transcriptional activity of PPARgamma may not only be decreased by mutation but also by downregulation of the coactivator PGC-1 of PPARgamma.

Induction of differentiation and peroxisome proliferator-activated receptor gamma expression in colon cancer cell lines by troglitazone

PURPOSE

We investigated the relationship between the effects of troglitazone (TGZ) on cellular growth, differentiation and apoptosis induction, and the induction of peroxisome proliferator-activated receptor (PPAR) gamma in three human colon cancer cell lines, HCT-15, DLD-1and LoVo.

METHODS

Viable cell number was evaluated by the Alamar blue assay and apoptotic cell death by TUNEL methods. Expression of PPARgamma mRNA and protein was examined by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, respectively. The differentiation markers of colonic mucosa, villin and MUC2 mRNAs, were analyzed by real-time RT-PCR.

RESULTS

HCT-15 and DLD-1 cells proliferated rapidly while LoVo cells grew slowly. TGZ dose-dependently inhibited the proliferation of all the cell lines, and also induced apoptotic cell death. High expression of PPARgamma mRNA and protein was demonstrated in DLD-1 and LoVo cells before TGZ treatment. After the treatment, PPARgamma mRNA and protein levels were increased in HCT-15 and LoVo cells. Villin and MUC2 mRNAs were increased by TGZ treatment in HCT-15 cells while villin mRNA was repressed in LoVo cells. Changes in expression of PPARgamma, villin or MUC2 mRNAs were not observed in DLD-1 cells.

CONCLUSIONS

These results suggest that PPARgamma levels are not correlated with the rates of cell proliferation. Differentiation induction by TGZ was only observed in the cell lines with enhanced PPARgamma expression.

The role of peroxisome proliferator-activated receptor gamma in colon cancer and inflammatory bowel disease

OBJECTIVE

Review the role and therapeutic potential of peroxisome proliferator-activated receptor (PPAR) gamma in colonic disorders.

DATA SOURCES

Recent peer-reviewed scientific literature focusing on PPAR gamma in the colon.

STUDY SELECTION

Research reports using animal models, cultured cell lines, and clinical material were examined for content related to the role of PPAR gamma in normal colon cell function, colon cancer, and inflammatory bowel disease. Issues concerned with potential therapeutic use were also considered.

DATA SYNTHESIS

Key points pertaining to PPAR function and involvement in colon pathology were extracted and noted. Potential compromises to therapeutic utility are identified.

CONCLUSIONS

The emerging important role of PPAR gamma in normal tissue homeostasis and pathologic outcomes suggests this receptor is a good candidate as a drug target. Several potential problems with this approach will require further investigation prior to widespread recommendations for modulation of PPAR as an efficacious therapy for cancer, chemoprevention of colon cancer, or treatment of inflammatory bowel disease. The widespread use of PPAR gamma ligands for management of type 2 diabetes (such as the glitazone class of drugs including rosiglitazone and pioglitazone) may provide a fortuitous assessment of the efficacy of long-term PPAR modulation.

Proteome analysis reveals novel proteins associated with proliferation and differentiation of the colorectal cancer cell line Caco-2

Here, we describe a proteomics approach to study protein expression changes in differentiating Caco-2 cells. Caco-2 is a colorectal carcinoma cell line, which upon differentiation loses its tumorigenic phenotype and displays characteristics of mature enterocytes, including brush borders with microvilli. Cells were grown in culture flasks and harvested at different stages of differentiation (days post-confluence: -3, 0, 3, 7, 10, 14, and 18). Two-dimensional gel electrophoresis was used to analyse proteome changes. Approximately 1400 protein spots were detected within the Caco-2 proteome, within the pH 4-7 range. Two-dimensional gel electrophoresis allowed for the detection of 18 proteins from which the levels of expression were found to be associated with differentiation. Of these proteins, 11 were identified by means of MALDI-TOF or NANO-ESI-MS/MS mass spectrometry and include liver fatty acid binding protein (FABL), three forms of alpha-enolase (ENOA), nucleoside diphosphate kinase A (NDKA), cofilin-1 (COF1), translationally controlled tumour protein (TCTP), mitochondrial 60-kDa heat shock protein (CH60), probable protein disulfide isomerase (ER60), creatine kinase B (KCRB), and glutathione S-transferase alpha (GTA1). Thus, proteomics revealed that the differentiation-related change in phenotype of Caco-2 involves changes in a variety of distinct biochemical pathways. Some of these proteins have not been shown before to be associated with Caco-2 differentiation (ER60; COF1; CH60; NDKA; TCTP and ENOA). Therefore, processes related to protein folding and disulfide bridge formation, cytoskeleton formation and maintenance, nucleotide metabolism, glycolysis as well as tumorigenesis-associated proteins may be involved in Caco-2 differentiation. Changes in the expression of CH60, TCTP, GTA1, NDKA, and FABL have also been reported to be associated with in vivo colon carcinogenesis. These findings illustrate that a combination of proteomics and cell culture is a useful approach to find markers for Caco-2 differentiation, which could contribute to the comprehension of the process of colon carcinogenesis.

PPARgamma controls cell proliferation and apoptosis in an RB-dependent manner

The nuclear receptor PPARgamma is implicated in the control of cell proliferation and apoptosis. However, the molecular mechanisms by which it controls these processes remain largely elusive. We show here that PPARgamma activation in the presence of the retinoblastoma protein (RB) results in the arrest of cells at the G1 phase of the cell cycle, whereas in the absence of RB, cells accumulate in G2/M, endoreduplicate, and undergo apoptosis. Through the use of HDAC inhibitors and coimmunoprecipitations, we furthermore demonstrate that the effects of RB on PPARgamma-mediated control of the cell cycle and apoptosis depend on the recruitment of histone deacetylase 3 (HDAC3) to PPARgamma. In combination, these data hence demonstrate that the effects of PPARgamma on cell proliferation and apoptosis are dependent on the presence of an RB-HDAC3 complex.

Peroxisome proliferator-activated receptor gamma-mediated differentiation: a mutation in colon cancer cells reveals divergent and cell type-specific mechanisms

Activation of the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits cell growth and induces differentiation in both adipocyte and epithelial cell lineages, although it is unclear whether this occurs through common or cell-type specific mechanisms. We have identified four human colon cancer cell lines that do no undergo growth inhibition or induce markers of differentiation after exposure to PPARgamma agonists. Sequence analysis of the PPARgamma gene revealed that all four cell lines contain a previously unidentified point mutation in the ninth alpha-helix of the ligand binding domain at codon 422 (K422Q). The mutant receptor did not exhibit any defects in DNA binding or retinoid X receptor heterodimerization and was transcriptionally active in an artificial reporter assay. However, only retroviral transduction of the wild-type (WT), but not mutant, receptor could restore PPARgamma ligand-induced growth inhibition and differentiation in resistant colon cancer cell lines. In contrast, there was no difference in the ability of fibroblast cells expressing WT or K422Q mutant receptor to undergo growth inhibition, express adipocyte differentiation markers, or uptake lipid after treatment with a PPARgamma agonist. Finally, analysis of direct PPARgamma target genes in colon cancer cells expressing the WT or K422Q mutant allele suggests that the mutation may disrupt the ability of PPARgamma to repress the basal expression of a subset of genes in the absence of exogenous ligand. Collectively, these data argue that codon 422 may be a part of a co-factor(s) interaction domain necessary for PPARgamma to induce terminal differentiation in epithelial, but not adipocyte, cell lineages and argues that the receptor induces growth inhibition and differentiation via cell lineage-specific mechanisms.

Peroxisome proliferator-activated receptor-gamma upregulates caveolin-1 and caveolin-2 expression in human carcinoma cells

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear receptor for eicosanoids that promotes differentiation of human epithelial and mesenchymal cells in vitro and in vivo. PPARgamma was proposed as a target for drug-induced differentiation therapy of cancer. Caveolin-1 is a constituent of plasma membrane caveolae in epithelial cells that is often downregulated upon oncogenic transformation. Caveolin-1 has growth-inhibitory activities and its disruption is sufficient to induce transformation in fibroblasts. Herein we have tested the hypothesis that caveolins are transcriptional target genes for PPARgamma. In human HT-29 colon carcinoma cells, thiazolidinedione PPARgamma ligands increased the levels of caveolin-1 and caveolin-2 proteins two to fivefold in a concentration-dependent manner within 24 h. In human MCF-7 breast adenocarcinoma cells, nonthiazolidinedione PPARgamma ligands elevated caveolin-2 protein three to fourfold, while the thiazoli-dinediones were less effective. Caveolin-1 mRNA levels were found to be upregulated by PPARgamma ligands already after 3 h in both the cell lines. Ectopic expression of a dominant-negative PPARgamma construct attenuated ligand-induced upregulation of caveolins in both HT-29 and HEK-293T cells, indicating that ligand action is mediated by PPARgamma. Ligand-treated MCF-7 cells exhibited a differentiated phenotype, as evinced by analysis of cell-specific differentiation markers: protein levels of maspin were elevated and perinuclear lipid droplets accumulated. In contrast, in HT-29 cells, caveolin expression was not correlated with differentiation. Interestingly, PPARgamma partially cofractionated in lipid rafts and could be coimmunoprecipitated from cell lysates with caveolin-1, indicating that PPARgamma and caveolin-1 may coexist in a complex. Our data indicate that PPARgamma participates in the regulation of caveolin gene expression in human carcinoma cells and suggest that caveolin-1 may mediate some of the phenotypic changes induced by this nuclear receptor in cancer cells. These findings may have potentially important functional implications in the context of cancer differentiation therapy and multidrug resistance.

Impaired expression of peroxisome proliferator-activated receptor gamma in ulcerative colitis

BACKGROUND & AIMS

The peroxisome proliferator-activated receptor gamma (PPAR gamma) has been proposed as a key inhibitor of colitis through attenuation of nuclear factor kappa B (NF-kappa B) activity. In inflammatory bowel disease, activators of NF-kappa B, including the bacterial receptor toll-like receptor (TLR)4, are elevated. We aimed to determine the role of bacteria and their signaling effects on PPAR gamma regulation during inflammatory bowel disease (IBD).

METHODS

TLR4-transfected Caco-2 cells, germ-free mice, and mice devoid of functional TLR4 (Lps(d)/Lps(d) mice) were assessed for their expression of PPAR gamma in colonic tissues in the presence or absence of bacteria. This nuclear receptor expression and the polymorphisms of gene also were assessed in patients with Crohn's disease (CD) and ulcerative colitis (UC), 2 inflammatory bowel diseases resulting from an abnormal immune response to bacterial antigens.

RESULTS

TLR4-transfected Caco-2 cells showed that the TLR4 signaling pathway elevated PPAR gamma expression and a PPAR gamma-dependent reporter in an I kappa kappa beta dependent fashion. Murine and human intestinal flora induced PPAR gamma expression in colonic epithelial cells of control mice. PPAR gamma expression was significantly higher in the colon of control compared with Lps(d)/Lps(d) mice. Although PPAR gamma levels appeared normal in patients with CD and controls, UC patients displayed a reduced expression of PPAR gamma confined to colonic epithelial cells, without any mutation in the PPAR gamma gene.

CONCLUSIONS

These data showed that the commensal intestinal flora affects the expression of PPAR gamma and that PPAR gamma expression is considerably impaired in patients with UC.

Role of peroxisome proliferator-activated receptor gamma and retinoid X receptor heterodimer in hepatogastroenterological diseases

The peroxisome proliferator-activated receptor gamma (PPARgamma) and its partner the retinoid X receptor (RXR) are two nuclear receptors that are expressed mainly in adipose tissue and which have a role in lipid metabolism and insulin sensitisation. New sites of PPARgamma/RXR expression have been described, especially in the intestinal tract, pancreas, and liver. Concomitantly, new functions have been attributed to this heterodimer in regulation of inflammation, by its inhibition of nuclear factor (NF)-kappaB and via stress-kinase pathways. These new sites and functions of PPARgamma/RXR have led to novel ideas about pathophysiology of different inflammatory digestive diseases and to development of innovative treatment strategies with PPARgamma activators.

Peroxisome proliferator-activated receptor-gamma is a target of nonsteroidal anti-inflammatory drugs mediating cyclooxygenase-independent inhibition of lung cancer cell growth

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit the growth of different cancer cell types, suggesting a broad role for their cyclooxygenase (COX) targets and eicosanoid products in tumor cell growth. Sulindac sulfide, a COX inhibitor, inhibited the growth of non-small-cell lung cancers (NSCLC) both in soft agar and as xenografts in nude mice. Importantly, the concentration of sulindac sulfide required to inhibit NSCLC cell growth greatly exceeded the concentration required to inhibit prostaglandin (PG) E(2) synthesis in NSCLC cells, suggesting that NSAID inhibition of cell growth is mediated by additional targets distinct from COX. Both sulindac sulfide and ciglitazone, a defined peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist, stimulated a promoter construct containing a PPAR response element linked to luciferase and potently inhibited NSCLC cell growth at similar concentrations, indicating a role for PPARgamma as a target of NSAID action in these cells. Overexpression of PPARgamma in NSCLC cells strongly inhibited the transformed growth properties of the cells, providing a molecular confirmation of the results obtained with the PPARgamma agonists. Increased expression of PPARgamma, as well as ciglitazone and sulindac sulfide induced expression of E-cadherin, which has been linked to increased differentiation of NSCLC. Despite the fact that SCLC cell lines expressed little or no cytosolic phospholipase A(2), COX-1, or COX-2, sulindac sulfide and PPARgamma agonists also inhibited the transformed growth of these lung cancer cells. We propose that PPARgamma serves as a target for NSAIDs that accounts for COX-independent inhibition of lung cancer cell growth.

Anti-tumor mechanisms of valproate: a novel role for an old drug

Valproic acid (VPA, 2-propylpentanoic acid) is an established drug in the long-term therapy of epilepsy. During the past years, it has become evident that VPA is also associated with anti-cancer activity. VPA not only suppresses tumor growth and metastasis, but also induces tumor differentiation in vitro and in vivo. Several modes of action might be relevant for the biological activity of VPA: (1) VPA increases the DNA binding of activating protein-1 (AP-1) transcription factor, and the expression of genes regulated by the extracellular-regulated kinase (ERK)-AP-1 pathway; (2) VPA downregulates protein kinase C (PKC) activity; (3) VPA inhibits glycogen synthase kinase-3beta (GSK-3beta), a negative regulator of the Wnt signaling pathway; (4) VPA activates the peroxisome proliferator-activated receptors PPARgamma and delta; (5) VPA blocks HDAC (histone deacetylase), causing hyperacetylation. The findings elucidate an important role of VPA for cancer therapy. VPA might also be useful as low toxicity agent given over long time periods for chemoprevention and/or for control of residual minimal disease.

Peroxisome proliferator-activated receptor gamma agonist ligands stimulate a Th2 cytokine response and prevent acute colitis

Peroxisome proliferator-activated receptor gamma (PPARgamma), a member of a nuclear transcription factor family, has been previously demonstrated to have antiinflammatory activity. The effects of PPARgamma activation in the development of an immune response are less well characterized. Through evaluation of PPARgamma heterozygote mice (PPARgamma(+/-) and specific PPARgamma agonist ligand binding, we evaluated the immunologic effects of PPARgamma activation in a well-described model of colitis. Increased susceptibility to dextran sodium sulfate (DSS)-induced colitis as defined by body weights, histologic injury, and survival was observed in the PPARgamma(+/-) mice in comparison to wild-type mice. Three different PPARgamma ligands (troglitazone, pioglitazone, and rosiglitazone) demonstrated beneficial dose-related treatment effects when administered prior to the onset of colitis. However, no protection was observed when PPARgamma ligand activation occurred after the onset of colitis. The reduction in DSS-induced inflammation noted with PPARgamma ligand treatment was associated with decreased interferon-gamma and tumor necrosis factor-alpha and increased interleukin (IL)-4 and IL- 10 levels as assessed by quantitative reverse transcriptase-polymerase chain reaction. Consistent with this shift towards a T helper (Th2) cytokine dominance, PPARgamma ligand treatment stimulated increased GATA-3 expression. These results indicate that the protective effects exhibited by PPARgamma ligands in intestinal inflammation may be due to immune deviation away from Th1 and towards Th2 cytokine production.

The small heterodimer partner interacts with the liver X receptor alpha and represses its transcriptional activity

The small heterodimer partner SHP (NR0B2) is an unusual nuclear receptor that lacks the typical DNA binding domain common to most nuclear receptors. SHP has been reported to act as a corepressor for several nuclear receptors, but its exact mechanism of action is still elusive. Here we show that SHP can interact with the liver X receptors LXRalpha (NR1H3) and LXRbeta (NR1H2), as demonstrated by glutathione-S-transferase pull-down assays, mammalian two-hybrid, and coimmunoprecipitation experiments. In transfection assays, SHP inhibits the expression of an artificial reporter driven by an LXR-response element and represses the transcriptional activation by LXR of the human ATP-binding cassette transporter 1 (ABCA1) promoter. Treatment of Caco-2 cells with bile acids, which activate farnesoid X receptor and subsequently induce SHP, leads to the repression of the human ABCG1 gene, an established LXR target gene. These results demonstrate that SHP is able to interact with LXR and to modulate its transcriptional activity.

Controversy: PPARgamma as a target for treatment of colorectal cancer

Colorectal cancer (CRC) represents a significant cause of morbidity and mortality worldwide. Recently, ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma) have exhibited promise in the treatment of CRC. For example, activation of PPARgamma reduces the proliferation of cultured CRC cells grown in vitro or in vivo using the nude mouse xenograft model of tumor growth. Furthermore, agonists of the receptor also reduce the development of preneoplastic lesions in a model of carcinogen-induced CRC in rats. However, ligands for the receptor paradoxically enhance intestinal adenoma formation in another murine model of intestinal polyposis, the APC(Min) mice. These disparate results may be due to the inherent limitations of the APC(Min) mouse as a model for humans with CRC. Finally, genetic studies identifying loss of function mutations of PPARgamma in human CRC specimens strongly suggest a tumor suppressive role for the receptor during the development of CRC.

Eicosapentaenoic acid induces mRNA expression of peroxisome proliferator-activated receptor gamma

OBJECTIVE

To verify whether polyunsaturated fatty acids (PUFAs) can regulate the expression of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) in human adipose tissue.

RESEARCH METHODS AND PROCEDURES

The effect of various PUFAS on PPARgamma1 and -gamma2 mRNA expression was investigated in freshly isolated adipocytes prepared from fat samples obtained during surgery. PPARgamma mRNA levels were also determined in subcutaneous adipose tissue biopsies of 11 obese women, in the fasting state, to search for in vivo associations between PPARgamma expression and plasma PUFA concentrations. PPARgamma mRNA levels were determined by reverse-transcription competitive polymerase chain reaction.

RESULTS

Eicosapentaenoic acid (EPA) significantly increased PPARgamma1 mRNA levels in isolated adipocytes, without affecting the expression of PPARgamma2. The other tested fatty acids (linolenic acid, docosahexaenoic acid and omega-6 PUFAs) had no effect. The effect of EPA was dependent on the concentration (maximal effect after 6 hours with 50 microM) and was not reproduced by activators of the different members of the PPAR family. In addition, a strong positive correlation was found between plasma EPA concentrations and PPARgamma mRNA levels in adipose tissue of obese subjects.

DISCUSSION

Our results demonstrate that adipose tissue PPARgamma1 mRNA concentration is positively regulated by EPA, suggesting that the composition of dietary lipids may affect PPARgamma gene expression in vivo in humans. These data also suggest that an induction of the expression of this nuclear receptor isoform might be involved in the mechanism of action of EPA and in some of its beneficial effects.

PPARgamma agonists inhibit cell growth and suppress the expression of cyclin D1 and EGF-like growth factors in ras-transformed rat intestinal epithelial cells

Peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits the growth of several types of cancer cells. However, the mechanisms by which this occurs are poorly understood. The goal of the present study was to investigate the effects of PPARgamma on mutated ras-induced cell growth, activation of transcription factors and expression of genes associated with cellular transformation in rat intestinal epithelial cells. A human PPARgamma cDNA was introduced to the activated H-ras-transfected IEC-6 cells (IECras) and 1 clone (IECrasPR82) that stably expresses both activated ras and PPARgamma was obtained. Thiazolidinedione derivatives such as troglitazone and rosiglitazone, selective ligands for PPARgamma, inhibited the cellular growth of IECrasPR82 cells in a time-dependent manner and induced G1 cell cycle arrest. Treatment with troglitazone (20 microM) decreased the expression of cyclin D1, heparin-binding epidermal growth factor-like growth factor (HB-EGF) and amphiregulin and suppressed the promoter activities of cyclin D1 and HB-EGF. Furthermore, a luciferase assay and an electrophoretic mobility shift assay showed that thiazolidinedione derivatives suppressed the transcriptional activities of AP-1 and Ets, both of which play crucial roles in the expression of cyclin D1 and HB-EGF. These findings suggest that reduction of EGF-like growth factors and cyclin D1 through the suppression of AP-1 and Ets may be 1 mechanism whereby PPARgamma inhibits their growth.

PPAR gamma/RXR alpha heterodimers are involved in human CG beta synthesis and human trophoblast differentiation

Recent studies performed with null mice suggested a role of either RXR alpha or PPAR gamma in murine placental development. We report here that both PPAR gamma and RXR alpha are strongly expressed in human villous cytotrophoblasts and syncytiotrophoblasts. Moreover, specific ligands for RXRs or PPAR gamma (but not for PPAR alpha or PPAR delta) increase both human CG beta transcript levels and the secretion of human CG and its free beta-subunit. When combined, these ligands have an additive effect on human CG secretion. Pan-RXR and PPAR gamma ligands also have an additive effect on the synthesis of other syncytiotrophoblast hormones such as human placental lactogen, human placental GH, and leptin. Therefore, in human placenta, PPAR gamma/RXR alpha heterodimers are functional units during cytotrophoblast differentiation into the syncytiotrophoblast in vitro. Elements located in the regulatory region of the human CG beta gene (beta 5) were found to bind RXR alpha and PPAR gamma from human cytotrophoblast nuclear extracts, suggesting that PPAR gamma/RXR alpha heterodimers directly regulate human CG beta transcription. Altogether, these data show that PPAR gamma/RXR alpha heterodimers play an important role in human placental development.

Target genes of peroxisome proliferator-activated receptor gamma in colorectal cancer cells

Activation of the nuclear hormone peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits cell growth and promotes differentiation in a broad spectrum of epithelial derived tumor cell lines. Here we utilized microarray technology to identify PPARgamma gene targets in intestinal epithelial cells. For each gene, the induction or repression was seen with two structurally distinct PPARgamma agonists, and the change in expression could be blocked by co-treatment with a specific PPARgamma antagonist. A majority of the genes could be regulated independently by a retinoid X receptor specific agonist. Genes implicated in lipid transport or storage (adipophilin and liver fatty acid-binding protein) were also activated by agonists of PPAR subtypes alpha and/or delta. In contrast, PPARgamma-selective targets included genes linked to growth regulatory pathways (regenerating gene IA), colon epithelial cell maturation (GOB-4 and keratin 20), and immune modulation (neutrophil-gelatinase-associated lipocalin). Additionally, three different genes of the carcinoembryonic antigen family were induced by PPARgamma. Cultured cells treated with PPARgamma ligands demonstrated an increase in Ca(2+)-independent, carcinoembryonic antigen-dependent homotypic aggregation, suggesting a potential role for PPARgamma in regulating intercellular adhesion. Collectively, these results will help define the mechanisms by which PPARgamma regulates intestinal epithelial cell biology.

Regulation of p85alpha phosphatidylinositol-3-kinase expression by peroxisome proliferator-activated receptors (PPARs) in human muscle cells

Regulation of p85a phosphatidylinositol-3-kinase (p85alphaPI-3K) expression by peroxisome proliferator-activated receptor (PPAR) activators was studied in human skeletal muscle cells. Activation of PPARgamma or PPARbeta did not modify the expression of p85alphaPI-3K. In contrast, activation of PPARalpha increased p85alphaPI-3K mRNA. This effect was potentiated by 9-cis-retinoic acid, an activator of RXR. Up-regulation of p85alphaPI-3K gene expression resulted in a rise in p85alphaPI-3K protein level and in an increase in insulin-induced PI3-kinase activity. According to the role of p85alphaPI-3K in insulin action, these results suggest that drugs with dual action on both PPARgamma and PPARalpha can be of interest for the treatment of insulin resistance.

Attenuation of colon inflammation through activators of the retinoid X receptor (RXR)/peroxisome proliferator-activated receptor gamma (PPARgamma) heterodimer. A basis for new therapeutic strategies

The peroxisome proliferator-activated receptor gamma (PPARgamma) is highly expressed in the colon mucosa and its activation has been reported to protect against colitis. We studied the involvement of PPARgamma and its heterodimeric partner, the retinoid X receptor (RXR) in intestinal inflammatory responses. PPARgamma(1/)- and RXRalpha(1/)- mice both displayed a significantly enhanced susceptibility to 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis compared with their wild-type littermates. A role for the RXR/PPARgamma heterodimer in the protection against colon inflammation was explored by the use of selective RXR and PPARgamma agonists. TNBS-induced colitis was significantly reduced by the administration of both PPARgamma and RXR agonists. This beneficial effect was reflected by increased survival rates, an improvement of macroscopic and histologic scores, a decrease in tumor necrosis factor alpha and interleukin 1beta mRNA levels, a diminished myeloperoxidase concentration, and reduction of nuclear factor kappaB DNA binding activity, c-Jun NH(2)-terminal kinase, and p38 activities in the colon. When coadministered, a significant synergistic effect of PPARgamma and RXR ligands was observed. In combination, these data demonstrate that activation of the RXR/PPARgamma heterodimer protects against colon inflammation and suggest that combination therapy with both RXR and PPARgamma ligands might hold promise in the clinic due to their synergistic effects.

Peroxisome proliferator-activated receptor- gamma expression in human malignant and normal brain, breast and prostate-derived cells

The constitutive and gamma -linolenic acid (GLA)-induced expression of peroxisome proliferator-activated receptor gamma (PPAR gamma) immunoreactive protein in a panel of human malignant brain (U87MG, T98G); breast (MCF-7, MB MDA-231, MB MDA 435) and prostate (ALVA, DU-145, LNCaP, PC3) cell lines have been compared with those for their normal cell counterparts, the human normal astrocyte (NHA), mammary epithelial (HMEC) and prostate epithelial (PrEC) cells, respectively. Constitutive levels of expression for PPAR gamma protein were significantly higher in the malignant cell lines relative to their normal cells. GLA supplementation did not affect the protein expression in malignant cells but caused 6- and 3-fold increases in normal breast and prostate cells, respectively. Since activation of PPAR gamma protein in some human malignant cell lines has been demonstrated to induce tumour cell death, these findings signal the need to exploit the significantly elevated expression of this protein in the therapy of human cancer.

Hormonal signaling and transcriptional control of adipocyte differentiation

Recent advances regarding the biology of adipose tissue have identified the adipocyte as an important mediator in many physiologic and pathologic processes regarding energy metabolism. Consideration for a central role of adipose tissue in the development of obesity, cardiovascular disease and noninsulin-dependent diabetes mellitus has resulted in new incentives toward understanding the complexities of adipocyte differentiation. Current knowledge of this process includes a cascade of transcriptional events that culminate in the expression of peroxisome proliferator-activated receptor-gamma (PPARgamma) and CCAAT/enhancer binding protein-alpha (C/EBPalpha). These prominent adipogenic transcription factors have been shown to regulate, directly or indirectly, the gene expression necessary for the development of the mature adipocyte. Hormonal and nutritional signaling that impinges on these trans-acting factors provides a molecular link between lipids and lipid-related compounds and the gene expression important for glucose and lipid homeostasis. Knowledge concerning the transcriptional events mediating adipocyte differentiation provides a basis for understanding the physiologic processes associated with adipose tissue as well as for the development of therapeutic interventions in obesity and its related disorders.

PPAR-gamma is selectively upregulated in Caco-2 cells by butyrate

The peroxisome proliferator-activated receptor (PPAR)-gamma is a nuclear lipid-activable receptor controlling the expression of genes involved in lipid metabolism and adipocyte differentiation. In order to investigate the possible role of PPAR-gamma in the differentiation of intestinal epithelial cells, we examined its expression in the human colon carcinoma cell line Caco-2, which undergoes rapid cell differentiation in the presence of butyrate. PPARs were quantified on mRNA level by RT competitive multiplex PCR, the corresponding proteins were determined by Western blot. In contrast to PPAR-alpha and PPAR-beta, PPAR-gamma mRNA and protein increased significantly in butyrate-treated Caco-2 cells in a dose- and time-dependent manner. This effect was butyrate-specific, since no change in PPAR-gamma expression could be observed after incubation with propionate or valerate. Activation of PPAR-gamma by ciglitazone further increased butyrate-induced cell differentiation dose-dependently. These data demonstrate a role for PPAR-gamma in the regulation of cell differentiation in Caco-2 cells.