Activators of the nuclear receptor PPARgamma enhance colon polyp formation

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.

Suppression of polyamine catabolism by activated Ki-ras in human colon cancer cells

An activated Ki-ras was expressed in the human colon adenocarcinoma cell line Caco-2 to study the effects of Ki-ras oncogene on polyamine metabolism during gastrointestinal tumorigenesis. Multiple clones selected for expression of the mutant Ki-ras transgene displayed a suppression of transcription of a key catabolic enzyme in polyamine catabolism spermidine/spermine N1-acetyltransferase (SSAT). Gene expression analysis, with cDNA microarrays, showed that Ki-ras transfected clones had decreased levels of expression, compared to mock transfected cells, of peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the nuclear hormone receptor family and an important regulator of cell proliferation and differentiation. The activated Ki-ras suppressed SSAT expression by a mechanism involving the PPARgamma response element (PPRE) located at +48 bp relative to the transcription start site of the SSAT gene. Transient expression of the PPARgamma protein in Ki-ras expressing Caco-2 clones, or treatment with the PPARgamma ligand ciglitazone, led to an increase in the SSAT promoter activity. A MEK1/2 inhibitor PD98059 induced transcription of both PPARgamma and SSAT genes in the activated Ki-ras clones, suggesting that the mitogen-activated protein kinases (MAPKs) were involved in the regulation of SSAT expression by PPARgamma. We concluded that mutated Ki-ras suppressed SSAT via a transcriptional mechanism involving the PPARgamma signaling pathway.

Role of Peroxisome Proliferator-Activated Receptors in Inflammation Control

Peroxisome proliferator-activated receptors (PPARs) were discovered over a decade ago, and were classified as orphan members of the nuclear receptor superfamily. To date, three PPAR subtypes have been discovered and characterized (PPAR $\alpha$, $\beta/\delta$, $\gamma$ ). Different PPAR subtypes have been shown to play crucial roles in important diseases and conditions such as obesity, diabetes, atherosclerosis, cancer, and fertility. Among the most studied roles of PPARs is their involvement in inflammatory processes. Numerous studies have revealed that agonists of PPAR $\alpha$ and PPAR $\gamma$ exert anti-inflammatory effects both in vitro and in vivo. Using the carrageenan-induced paw edema model of inflammation, a recent study in our laboratories showed that these agonists hinder the initiation phase, but not the late phase of the inflammatory process. Furthermore, in the same experimental model, we recently also observed that activation of PPAR $\delta$ exerted an anti-inflammatory effect. Despite the fact that exclusive dependence of these effects on PPARs has been questioned, the bulk of evidence suggests that all three PPAR subtypes, PPAR $\alpha, \delta, \gamma$, play a significant role in controlling inflammatory responses. Whether these subtypes act via a common mechanism or are independent of each other remains to be elucidated. However, due to the intensity of research efforts in this area, it is anticipated that these efforts will result in the development of PPAR ligands as therapeutic agents for the treatment of inflammatory diseases.

Peroxisome-proliferator-activated receptor gamma suppresses Wnt/beta-catenin signalling during adipogenesis

The Wnt/beta-catenin signalling pathway appears to operate to maintain the undifferentiated state of preadipocytes by inhibiting adipogenic gene expression. To define the mechanisms regulating suppression of Wnt/beta-catenin signalling, we analysed the beta-catenin expression in response to activation of transcription factors that regulate adipogenesis. The results show an extensive down-regulation of nuclear beta-catenin that occurs during the first few days of differentiation of 3T3-L1 preadipocytes and coincides with the induction of the adipogenic transcription factors, C/EBPbeta (CCAAT-enhancer-binding protein) and PPARgamma (peroxisome-proliferator-activated receptor). To assess the role of each of these factors in this process, we conditionally overexpressed C/EBPbeta in Swiss mouse fibroblasts using the TET-off system. Abundant expression of C/EBPbeta alone had minimal effect on beta-catenin expression, whereas expression of C/EBPbeta, in the presence of dexamethasone, induced PPARgamma expression and caused a measurable decrease in beta-catenin. In addition, exposure of cells expressing both C/EBPbeta and PPARgamma to a potent PPARgamma ligand resulted in an even greater decrease in beta-catenin by mechanisms that involve the proteasome. Our studies also suggest a reciprocal relationship between PPARgamma activity and beta-catenin expression, since ectopic production of Wnt-1 in preadipocytes blocked the induction of PPARgamma gene expression. Moreover, by suppressing beta-catenin expression, ectopic expression of PPARgamma in Wnt-1-expressing preadipocytes rescued the block in adipogenesis after their exposure to the PPARgamma ligand, troglitazone.

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.

Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands as bifunctional regulators of cell proliferation

Peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the ligand-activated nuclear receptor superfamily, plays a key role in mediating differentiation of adipocytes and regulating fat metabolism. PPARgamma has been implicated in the pathophysiology of atherosclerosis, inflammation, obesity, diabetes, immune response, and ageing. Recently, it has been shown that activation of PPARgamma by J(2) series cyclopentenone prostaglandins (cyPGs), especially 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) or synthetic agents, such as antidiabetic thiazolidinediones, causes anti-proliferation, apoptosis, differentiation, and anti-inflammation of certain types of cancer cells. The anti-proliferative effects of PPARgamma activators are associated with de novo synthesis of proteins involved in regulating the cell cycle and cell survival/death. Anti-inflammatory effects of 15d-PGJ(2) are associated with interruption of nuclear factor-kappaB and subsequent blockade of inflammatory gene expression. Furthermore, 15d-PGJ(2) at nontoxic doses induce expression of phase II detoxification or stress-responding enzymes, which may confer cellular resistance or adaptation to oxidative stress. The presence of a reactive alpha,beta-unsaturated carbonyl moiety in the cyclopentenone ring of 15d-PGJ(2) is important for part of biological functions this cyPG has. Recently, attention has been focused on the anti-proliferative activity of nonsteroidal anti-inflammatory drugs (NSAIDs) in cancerous or transformed cells, which is mediated through interaction with PPARgamma irrespective of their ability to inhibit COX-2. Despite the fact that abnormally elevated COX-2 is associated with resistance to cell death, induction of apoptosis by certain NSAIDs is accompanied by up-regulation of COX-2 expression. This commentary focuses on dual effects of the typical PPARgamma agonist 15d-PGJ(2) on cell proliferation and growth, and its possible involvement in the NSAID-induced COX-2 expression and apoptosis.

Dose-dependent suppression of hyperlipidemia and intestinal polyp formation in Min mice by pioglitazone, a PPAR gamma ligand

In our previous study, a peroxisome proliferator-activated receptor gamma (PPAR gamma) agonist, pioglitazone, suppressed both hyperlipidemia and intestinal polyp formation in Apc(1309) mice at doses of 100 and 200 ppm in the diet. In contrast, it has been reported that doses of 1500 or 2000 ppm of another PPAR gamma agonist, troglitazone, enhanced colon polyp development in Min mice. In the present study, we therefore investigated the effects of a wide range of pioglitazone doses on both hyperlipidemia and intestinal polyp formation in Min mice. Serum triglycerides and very low density lipoprotein (VLDL) cholesterol in the basal diet group were elevated to levels 13-15 times higher than those in the wild-type counterparts at 20 weeks of age. They were reduced dose-dependently by treatment with 100, 200, 400 and 1600 ppm pioglitazone from 6-20 weeks of age with suppression to almost the wild-type level at the highest dose. Moreover, up-regulation of the liver mRNA levels for lipoprotein lipase (LPL) was evident in the pioglitazone-treated animals. Dose-dependent reduction of intestinal polyps was observed in Min mice given 100-1600 ppm for 14 weeks, total numbers being decreased to 63-9% of the control value. A suppressive effect of pioglitazone on colon polyp formation was also found. The PPAR gamma agonist, pioglitazone, may thus be a promising candidate chemopreventive agent for colon cancer.

15d-PGJ2 inhibits cell growth and induces apoptosis of MCG-803 human gastric cancer cell line

AIM: To investigate the influence of peroxisome proliferator-activated receptor γ (PPARγ) ligand, 15-deoxy-△12, 14-prostaglandin J₂ (15dPGJ₂) on the proliferation and apoptosis of MCG-803 human gastric cancer cell lines.

METHODS: Cell proliferation was measured by ³H-TdR assay. Apoptosis was determined by ELISA and TUNEL staining. Protein and mRNA level of bcl-2 family and COXs were measured by Western blotting and Northern blotting respectively. PGE₂ production was examined by RIA.

RESULTS: 15dPGJ₂ inhibited cell growth and induced apoptosis of MCG-803 cells. The COX-2 and bcl-2/bax ratios were decreased following 15dPGJ₂ treatment. The PGE₂ production in supernatants was also decreased. These changes were in a dose-dependent manner.

CONCLUSION: 15dPGJ₂ may be a useful therapeutic agent for the treatment of gastric cancer.

Thiazolidinediones in type 2 diabetes mellitus: current clinical evidence

Type 2 diabetes mellitus is characterised by insulin resistance as well as progressive pancreatic beta cell dysfunction. The cornerstone of current oral blood-glucose lowering therapy consists of metformin, which primarily lowers hepatic glucose production, and the sulphonylureas that act by stimulating pancreatic beta-cells to secrete insulin. Recently, a novel class of agents, the thiazolidinediones, has been introduced that favourably influence insulin sensitivity and possibly also pancreatic beta-cell function. The thiazolidinediones are synthetic ligands that bind to the nuclear peroxisome proliferator-activated receptor-gamma and exert their action by activating transcription of genes that, among others, regulate adipocyte differentiation and adipogenesis as well as glucose and lipid metabolism. To date, the precise mechanisms underlying the actions of thiazolidinediones are largely unknown. When given as monotherapy or in combination with sulphonylureas, metformin or insulin in patients with type 2 diabetes, the currently available thiazolidinediones (rosiglitazone and pioglitazone) ameliorate glycaemic control, by lowering fasting and postprandial blood glucose levels, and improve insulin sensitivity in placebo-controlled trials. They seem to have differential effects on dyslipidaemia in patients with type 2 diabetes; rosiglitazone increases total cholesterol as well as high-density lipoprotein (HDL) and low-density lipoprotein cholesterol levels and affects plasma triglyceride levels depending on the baseline values, whereas pioglitazone lowers triglycerides and increases HDL cholesterol levels. The adverse events of both agents that occur with greater frequency than in patients treated with placebo are fluid retention and oedema. As demonstrated, mainly in preclinical studies to date, rosiglitazone and pioglitazone possess beneficial effects on other cardiovascular risk factors associated with the insulin resistance syndrome. Thus, these agents were shown to decrease blood pressure, enhance myocardial function and fibrinolysis, as well as possess anti-inflammatory and other beneficial vascular effects. Long-term efficacy and surveillance of this promising class of drugs in patients, however, still need to be demonstrated in outcome trials.

Genetic deficiency in Pparg does not alter development of experimental prostate cancer

The role of the nuclear peroxisome proliferator-activated receptor (PPAR)-gamma in cancer has been a subject of debate. The identification of loss-of-function mutations in PPARG in colon and prostate tumors has led to the idea that this gene may function as a tumor suppressor. We have directly tested this notion using a mouse model of prostate cancer. Neither hemizygous deletion of Pparg nor complete ablation of Ppara influenced the development of prostate cancer in our experimental context.

[PPARgamma-gene therapy using an adenovirus vector for inflammatory bowel disease]

Peroxisome proliferator-activated receptor gamma (PPARgamma) is one of the nuclear receptors that plays a central role in adipocyte differentiation and insulin sensitivity. Recently, PPARgamma has also been recognized as a suppressive regulator of inflammation in the gastrointestinal tract. We summarize here the therapeutic benefits of PPARgamma-gene therapy using a replication-deficient adenovirus vector expressing PPARgamma (AdRGD-PPARgamma). We demonstrate that PPARgamma- protein levels are decreased in dextran sodium sulfate-induced colitis and restored in this model by intraperitoneal administration of the AdRGD-PPARgamma. Treatment with AdRGD-PPARgamma and PPARgamma-specific ligands resulted in a marked amelioration of tissue inflammation associated with the colitis, including reduction in intercellular adhesion molecule-1, cyclooxygenase-2, and tumor necrosis factor-alpha expression. Our results suggest that gene delivery of PPARgamma may open up a novel therapeutic approach for inflammatory bowel diseases such as Crohn's disease and ulcerative colitis.

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.

Cyclin D1 repression of peroxisome proliferator-activated receptor gamma expression and transactivation

The cyclin D1 gene is overexpressed in human breast cancers and is required for oncogene-induced tumorigenesis. Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor selectively activated by ligands of the thiazolidinedione class. PPAR gamma induces hepatic steatosis, and liganded PPAR gamma promotes adipocyte differentiation. Herein, cyclin D1 inhibited ligand-induced PPAR gamma function, transactivation, expression, and promoter activity. PPAR gamma transactivation induced by the ligand BRL49653 was inhibited by cyclin D1 through a pRB- and cdk-independent mechanism, requiring a region predicted to form an helix-loop-helix (HLH) structure. The cyclin D1 HLH region was also required for repression of the PPAR gamma ligand-binding domain linked to a heterologous DNA binding domain. Adipocyte differentiation by PPAR gamma-specific ligands (BRL49653, troglitazone) was enhanced in cyclin D1(-/-) fibroblasts and reversed by retroviral expression of cyclin D1. Homozygous deletion of the cyclin D1 gene, enhanced expression by PPAR gamma ligands of PPAR gamma and PPAR gamma-responsive genes, and cyclin D1(-/-) mice exhibit hepatic steatosis. Finally, reduction of cyclin D1 abundance in vivo using ponasterone-inducible cyclin D1 antisense transgenic mice, increased expression of PPAR gamma in vivo. The inhibition of PPAR gamma function by cyclin D1 is a new mechanism of signal transduction cross talk between PPAR gamma ligands and mitogenic signals that induce cyclin D1.

Aminosalicylates and colorectal cancer in IBD: a not-so bitter pill to swallow

Inflammatory bowel disease (IBD) is associated with an increased risk of developing intestinal cancer at sites of chronic inflammation. Aminosalicylates, including both sulfasalazine and mesalamine, are the most commonly prescribed anti-inflammatory agents prescribed in IBD. On balance, the body of literature to date suggests that aminosalicylates confer some protection against the development of colonic neoplasia in patients with IBD and in a variety of models, including in the noninflamed gut. This latter observation implies that aminosalicylates may be of chemopreventive value in normal as well as IBD individuals. The current review examines and gives an overview of the evidence from a variety of sources, including epidemiological, in vivo and in vitro studies that have investigated the potential anticancer effects of aminosalicylates.

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.

Induction and repression of peroxisome proliferator-activated receptor alpha transcription by coregulator ARA70

In an effort to understand transcriptional regulation by the peroxisome proliferator-activated receptor alpha (PPARalpha), we investigated the ability of a number of transcriptional coactivators to enhance PPARalpha:retinoic acid receptor (RXR) mediated transcription. We identified ARA70, a coactivator of the androgen receptor and PPARgamma, as a ligand-enhanced coactivator of PPARalpha in the prostate cancer cell line DU145. In prostate cancer cells, ARA70 demonstrated the strongest enhancement of PPARalpha transcription among the coactivators examined. Mutation of the N-terminal of the PPARalpha ligandbinding domain dramatically reduced the ability of ARA70 to enhance PPARalpha:RXR transcription. ARA70 was able to physically interact with both the wild-type and mutant PPARalpha as determined by coimmunoprecipitation. However, in the adrenal cell line Y1, ARA70 behaved as a repressor of PPARalpha while still able to coactivate PPARgamma.

The Potential Contributions of Chronic Inflammation to Lung Carcinogenesis

A number of lines of evidence suggests that chronic inflammation contributes to the process of carcinogenesis. In this article, this theme is explored with particular emphasis on the involvement of inflammation in the development of lung cancer. A number of molecular pathways activated in chronic inflammation may contribute to lung carcinogenesis. The challenge is to conceptualize a cohesive picture of this complex biology that allows for effective pharmaceutical intervention. Initial therapeutic efforts involve strategies to block single pathways, such as with cyclooxygenase (COX) activity. However, the more that is learned about the consequences of COX activity, the more evident are the relationships of this enzyme to other classes of regulatory molecules such as the potent nuclear factor-kB. In light of this emerging picture, more global intervention strategies, such as with drug combinations, may be essential for success. Further basic study is essential to sort out possible molecular relationships and to permit elucidation of the most critical regulatory circuits. Given the complexity of these molecular interactions, well-designed clinical trials that specifically evaluate the precise effects of particular antiinflammatory drugs on lung carcinogenesis will also be critical to sort out the complexity and to validate successful approaches to arresting lung carcinogenesis.

Thiazolidinediones -- some recent developments

The role of thiazolidinediones (currently rosiglitazone and pioglitazone) in the treatment of Type 2 diabetes is firmly established. The mechanism of action involves binding to the peroxisome proliferator-activated receptor-gamma, a transcription factor that regulates the expression of specific genes especially in fat cells but also other cell types such as endothelial cells, macrophages and monocytes, vascular smooth muscle cells and colonic epithelium. Thiazolidinediones have been shown to interfere with expression and release of mediators of insulin resistance originating in adipose tissue (e.g., increased free fatty acids, decreased adiponectin) in a way that results in net improvement of insulin sensitivity (i.e., in muscle and liver). A direct or indirect effect on AMP-dependent protein kinase may also be involved. Prevention of lipid accumulation in tissues critical to glycaemia such as visceral adipocytes, liver, muscle and beta-cells at the expense of lipids accumulating at the less harmful subcutaneous site may be central to their net metabolic effect. The sustained beneficial effect of troglitazone on beta-cell function in women with previous gestational diabetes in addition to the insulin-sensitising properties point to an important role of this class of drugs in the prevention of Type 2 diabetes. Original safety concerns based on animal and in vitro studies (e.g., fatty bone marrow transformation, colonic cancer, adipogenic transdifferentiation of blood cells) remain theoretical issues but become less pressing practically with prolonged uneventful clinical use. Hepatotoxicity for troglitazone and fluid retention, which can aggravate pre-existing heart failure, are the most important side effects. In summary, with the thiazolidinediones, a novel concept for the treatment of insulin resistance and possibly preservation of beta-cell function is available that could become effective in the prevention of Type 2 diabetes. Moreover, their anti-inflammatory properties also make them interesting in the prevention and treatment of atherosclerosis and possibly other inflammatory conditions (e.g., inflammatory bowel disease). Long-term data will be necessary for a final risk-benefit assessment of these substances.

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.

Effects of peroxisome proliferator-activated receptor gamma ligands ciglitazone and 15-deoxy-delta 12,14-prostaglandin J2 on rat cultured cerebellar granule neuronal viability

Peroxisome proliferator-activated receptor gamma (PPARgamma) has been the focus of studies assessing its potential neuroprotective role. These studies have shown either neuroprotection or neurotoxicity by PPARgamma ligands. Comparison of these studies is complicated by the use of different PPARgamma ligands, mechanisms of neurotoxicity induction, and neuronal cell type. In this study, we compared the effects of the synthetic PPARgamma ligand ciglitazone with an endogenous PPARgamma ligand, 15-deoxy-delta(12,14)-prostaglandin J(2) (15-deoxy PGJ(2)), on inherent neurotoxicity and neuroprotection using a reduction in extracellular KCl in rat cultured cerebellar granule neurons (CGN). We also assessed the effects of these ligands on c-Jun protein expression, which is up-regulated on induction of low-KCl-mediated neuronal apoptosis as well as being associated with PPAR in other cell types. We showed that PPARgamma mRNA is expressed in CGN cultures and observed ciglitazone- and 15-deoxy PGJ(2)-mediated inherent neurotoxicity that was concentration and time dependent. c-Jun was only modestly increased in the presence of ciglitazone but was markedly up-regulated by 15-deoxy PGJ(2) after 12 hr. Treatment of CGN cultures with ciglitazone simultaneous with KCl withdrawal resulted in a modest, time-dependent neuroprotection. Such neuroprotection after KCl withdrawal was not observed with 15-deoxy PGJ(2). Despite the absence of neuroprotection, 15-deoxy PGJ(2) markedly inhibited the early up-regulation of c-Jun during KCl withdrawal. These studies suggest that ciglitazone and 15-deoxy PGJ(2) have markedly different effects on inherent and low-KCl-induced toxicity and c-Jun expression in CGN, indicating potential non-PPARgamma mechanisms.

15-deoxy-delta12,14 prostaglandin J2 synergizes with phorbol ester to induce proliferation in Swiss 3T3 cells independently of peroxisome proliferator-activated receptor gamma and PGD2 receptors

15-deoxy-Delta(12,14) prostaglandin J(2) (15dPGJ(2)), a peroxisome proliferator-activated receptor gamma (PPARgamma) ligand, induced synergistic stimulation of DNA synthesis in the presence of phorbol dibutyrate (PDB) in Swiss 3T3 cells. This effect was dose-dependent and the maximum response was obtained at 2 microM 15dPGJ(2), although higher concentrations of 15dPGJ(2) were cytotoxic. Furthermore, 15dPGJ(2) synergizes with PDB to induce cell-cycle progression and cyclin D(1) expression. Rosiglitazone and ciglitazone, two other agonists of PPARgamma, did not synergize with PDB to induce DNA synthesis, suggesting that activation of PPARgamma is not involved in 15dPGJ(2)-induced DNA synthesis. 15dPGJ(2) neither increased the levels of cAMP, nor changed the phosphorylation state of CREB, nor induced calcium mobilization, indicating that 15dPGJ(2) effects are independent of prostaglandin D(2) receptor (DP1 and DP2). Moreover, 15dPGJ(2) did not induce activation of PKB/AKT or activation of extracellular signal-regulated kinase (ERK). These results establish a proliferative role for 15dPGJ(2) in Swiss 3T3 cells independent of the activation of PPARgamma or the PGD(2) receptors.

High-level expression of cutaneous fatty acid-binding protein in prostatic carcinomas and its effect on tumorigenicity

The expression of cutaneous fatty acid-binding protein (C-FABP) in prostate tissues was examined by immunohistochemistry. Among the 76 cases, all seven (100%) normal tissues were unstained. Of the 35 benign prostatic hyperplasia (BPH), 25 (71.4%) specimens were unstained and 10 (28.6%) were stained positively. For the 34 prostatic carcinomas, the C-FABP expression was remarkably increased: 25 (73.5%) samples stained positively, and only nine (26.5%) were unstained. Transfection of a vector expressing an antisense C-FABP transcript into the PC-3M prostatic cancer cells yielded two transfectant lines: PC-3M-CFABP-1 and PC-3M-CFABP-3, producing, respectively, a 3.8- and a 6.9-fold reduction in C-FABP levels. Comparing with the control transfectants, the in vitro invasiveness of both PC-3M-CFABP-1 and PC-3M-CFABP-3 was significantly reduced. When tested in nude mouse, the average size of tumours produced by PC-3M-CFABP-1 and by PC-3M-CFABP-3 was reduced by 2.9- and 4.2-fold respectively, in comparison with that of tumours produced by the control transfectants. Analysis showed that the decreased vascular endothelial growth factor (VEGF) and microvessel densities in the tumours were associated with the reduced C-FABP. These data show that C-FABP is increased in prostatic carcinoma cells and suppression of its expression can significantly inhibit the tumorigenicity, probably by reducing the expression of VEGF.

A novel PPAR gamma gene therapy to control inflammation associated with inflammatory bowel disease in a murine model

BACKGROUND & AIMS

Peroxisome proliferator-activated receptor gamma (PPAR gamma) is one of the nuclear receptors that plays a central role in adipocyte differentiation and insulin sensitivity. PPAR gamma has also recently been recognized as an endogenous regulator of intestinal inflammation. However, its levels are decreased during chronic inflammation in human and mice, thus limiting PPAR gamma ligand therapy during established disease. We sought to determine whether this decrease in PPAR gamma could be counteracted by a gene therapy approach.

METHODS

We characterized PPAR gamma levels in experimental colitis associated with dextran sodium sulfate administration to mice. In this model, the therapeutic benefits of PPAR gamma gene therapy using a replication-deficient adenovirus vector expressing PPAR gamma (Ad-PPAR gamma) was assessed.

RESULTS

PPAR gamma protein levels were decreased in whole colonic tissue, lamina propria lymphocytes, and peritoneal exudate cells during the course of colitis. PPAR gamma gene delivery using Ad-PPAR gamma restored responsiveness to a PPAR gamma ligand, resulting in marked amelioration of tissue inflammation associated with the colitis, which included attenuation of intercellular adhesion molecule-1, cyclooxygenase-2 and tumor necrosis factor-alpha expression.

CONCLUSIONS

Our results suggest that gene delivery of PPAR gamma can be used to restore and/or enhance endogenous anti-inflammatory processes that are normally operative in mammalian tissues such as in the colon.

Liver-specific disruption of PPARgamma in leptin-deficient mice improves fatty liver but aggravates diabetic phenotypes

To elucidate the function of PPARgamma in leptin-deficient mouse (ob/ob) liver, a PPARgamma liver-null mouse on an ob/ob background, ob/ob-PPARgamma(fl/fl)AlbCre(+), was produced using a floxed PPARgamma allele, PPARgamma(fl/fl), and Cre recombinase under control of the albumin promoter (AlbCre). The liver of ob/ob-PPARgamma(fl/fl)AlbCre(+) mice had a deletion of exon 2 and a corresponding loss of full-length PPARgamma mRNA and protein. The PPARgamma-deficient liver in ob/ob mice was smaller and had a dramatically decreased triglyceride (TG) content compared with equivalent mice lacking the AlbCre transgene (ob/ob-PPARgamma(fl/fl)AlbCre(-)). Messenger RNA levels of the hepatic lipogenic genes, fatty acid synthase, acetyl-CoA carboxylase, and stearoyl-CoA desaturase-1, were reduced in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice, and the levels of serum TG and FFA in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice were significantly higher than in the control ob/ob-PPARgamma(fl/fl)AlbCre(-) mice. Rosiglitazone treatment exacerbated the fatty liver in ob/ob-PPARgamma(fl/fl)AlbCre(-) mice compared with livers from nonobese Cre(-) mice; there was no effect of rosiglitazone in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice. The deficiency of hepatic PPARgamma further aggravated the severity of diabetes in ob/ob mice due to decreased insulin sensitivity in muscle and fat. These data indicate that hepatic PPARgamma plays a critical role in the regulation of TG content and in the homeostasis of blood glucose and insulin resistance in steatotic diabetic mice.

Liver-specific disruption of PPARgamma in leptin-deficient mice improves fatty liver but aggravates diabetic phenotypes

To elucidate the function of PPARgamma in leptin-deficient mouse (ob/ob) liver, a PPARgamma liver-null mouse on an ob/ob background, ob/ob-PPARgamma(fl/fl)AlbCre(+), was produced using a floxed PPARgamma allele, PPARgamma(fl/fl), and Cre recombinase under control of the albumin promoter (AlbCre). The liver of ob/ob-PPARgamma(fl/fl)AlbCre(+) mice had a deletion of exon 2 and a corresponding loss of full-length PPARgamma mRNA and protein. The PPARgamma-deficient liver in ob/ob mice was smaller and had a dramatically decreased triglyceride (TG) content compared with equivalent mice lacking the AlbCre transgene (ob/ob-PPARgamma(fl/fl)AlbCre(-)). Messenger RNA levels of the hepatic lipogenic genes, fatty acid synthase, acetyl-CoA carboxylase, and stearoyl-CoA desaturase-1, were reduced in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice, and the levels of serum TG and FFA in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice were significantly higher than in the control ob/ob-PPARgamma(fl/fl)AlbCre(-) mice. Rosiglitazone treatment exacerbated the fatty liver in ob/ob-PPARgamma(fl/fl)AlbCre(-) mice compared with livers from nonobese Cre(-) mice; there was no effect of rosiglitazone in ob/ob-PPARgamma(fl/fl)AlbCre(+) mice. The deficiency of hepatic PPARgamma further aggravated the severity of diabetes in ob/ob mice due to decreased insulin sensitivity in muscle and fat. These data indicate that hepatic PPARgamma plays a critical role in the regulation of TG content and in the homeostasis of blood glucose and insulin resistance in steatotic diabetic mice.

Peroxisome proliferator-activated receptor gamma and transforming growth factor-beta pathways inhibit intestinal epithelial cell growth by regulating levels of TSC-22

Peroxisome proliferator-activated receptor gamma (PPARgamma) and transforming growth factor-beta (TGF-beta) are key regulators of epithelial cell biology. However, the molecular mechanisms by which either pathway induces growth inhibition and differentiation are incompletely understood. We have identified transforming growth factor-simulated clone-22 (TSC-22) as a target gene of both pathways in intestinal epithelial cells. TSC-22 is member of a family of leucine zipper containing transcription factors with repressor activity. Although little is known regarding its function in mammals, the Drosophila homolog of TSC-22, bunched, plays an essential role in fly development. The ability of PPARgamma to induce TSC-22 was not dependent on an intact TGF-beta1 signaling pathway and was specific for the gamma isoform. Localization studies revealed that TSC-22 mRNA is enriched in the postmitotic epithelial compartment of the normal human colon. Cells transfected with wild-type TSC-22 exhibited reduced growth rates and increased levels of p21 compared with vector-transfected cells. Furthermore, transfection with a dominant negative TSC-22 in which both repressor domains were deleted was able to reverse the p21 induction and growth inhibition caused by activation of either the PPARgamma or TGF-beta pathways. These results place TSC-22 as an important downstream component of PPARgamma and TGF-beta signaling during intestinal epithelial cell differentiation.

Troglitazone, a peroxisome proliferator-activated receptor gamma (PPAR gamma ) ligand, selectively induces the early growth response-1 gene independently of PPAR gamma. A novel mechanism for its anti-tumorigenic activity

Troglitazone (TGZ) is a peroxisome proliferator-activated receptor gamma (PPAR gamma) ligand that has pro-apoptotic activity in human colon cancer. Although TGZ binds to PPAR gamma transcription factors as an agonist, emerging evidence suggests that TGZ acts independently of PPAR gamma in many functions, including apoptosis. Early growth response-1 (Egr-1) transcription factor has been linked to apoptosis and shown to be activated by extracellular signal-regulated kinase (ERK). We investigated whether TGZ-induced apoptosis may be related to Egr-1 induction, because TGZ has been known to induce ERK activity. Our results show that Egr-1 is induced dramatically by TGZ but not by other PPAR gamma ligands. TGZ affects Egr-1 induction at least by two mechanisms; TGZ increases Egr-1 promoter activity by 2-fold and prolongs Egr-1 mRNA stability by 3-fold. Inhibition of ERK phosphorylation in HCT-116 cells abolishes the Egr-1 induction by TGZ, suggesting its ERK-dependent manner. Further, the TGZ-induced Egr-1 expression results in increased promoter activity using a reporter system containing four copies of Egr-1 binding sites, and TGZ induces Egr-1 binding activity to Egr-1 consensus sites as assessed by gel shift assay. In addition, TGZ induces ERK-dependent phosphorylation of PPAR gamma, resulting in the down-regulation of PPAR gamma activity. The fact that TGZ-induced apoptosis is accompanied by the biosynthesis of Egr-1 suggests that Egr-1 plays a pivotal role in TGZ-induced apoptosis in HCT-116 cells. Our results suggest that Egr-1 induction is a unique property of TGZ compared with other PPAR gamma ligands and is independent of PPAR gamma activation. Thus, the up-regulation of Egr-1 may provide an explanation for the anti-tumorigenic properties of TGZ.

Advances in understanding the regulation of apoptosis and mitosis by peroxisome-proliferator activated receptors in pre-clinical models: relevance for human health and disease

Peroxisome proliferator activated receptors (PPARs) are a family of related receptors implicated in a diverse array of biological processes. There are 3 main isotypes of PPARs known as PPARalpha, PPARbeta and PPARgamma and each is organized into domains associated with a function such as ligand binding, activation and DNA binding. PPARs are activated by ligands, which can be both endogenous such as fatty acids or their derivatives, or synthetic, such as peroxisome proliferators, hypolipidaemic drugs, anti-inflammatory or insulin-sensitizing drugs. Once activated, PPARs bind to DNA and regulate gene transcription. The different isotypes differ in their expression patterns, lending clues on their function. PPARalpha is expressed mainly in liver whereas PPARgamma is expressed in fat and in some macrophages. Activation of PPARalpha in rodent liver is associated with peroxisome proliferation and with suppression of apoptosis and induction of cell proliferation. The mechanism by which activation of PPARalpha regulates apoptosis and proliferation is unclear but is likely to involve target gene transcription. Similarly, PPARgamma is involved in the induction of cell growth arrest occurring during the differentiation process of fibroblasts to adipocytes. However, it has been implicated in the regulation of cell cycle and cell proliferation in colon cancer models. Less in known concerning PPARbeta but it was identified as a downstream target gene for APC/beta-catenin/T cell factor-4 tumor suppressor pathway, which is involved in the regulation of growth promoting genes such as c-myc and cyclin D1. Marked species and tissue differences in the expression of PPARs complicate the extrapolation of pre-clinical data to humans. For example, PPARalpha ligands such as the hypolipidaemic fibrates have been used extensively in the clinic over the past 20 years to treat cardiovascular disease and side effects of clinical fibrate use are rare, despite the observation that these compounds are rodent carcinogens. Similarly, adverse clinical responses have been seen with PPARgamma ligands that were not predicted by pre-clinical models. Here, we consider the response to PPAR ligands seen in pre-clinical models of efficacy and safety in the context of human health and disease.

Novel agents for the prevention of breast cancer: targeting transcription factors and signal transduction pathways

Transformation of breast cells occurs through loss or mutation of tumor suppressor genes, or activation or amplification of oncogenes, leading to deregulation of signal transduction pathways, abnormal amplification of growth signals, and aberrant expression of genes that ultimately transform the cells into invasive cancer. The goal of cancer preventive therapy, or "chemoprevention," is to eliminate premalignant cells or to block the progression of normal cells into cancer. Multiple alterations in signal pathways and transcription factors are observed in mammary gland tumorigenesis. In particular, estrogen receptor (ER) deregulation plays a critical role in breast cancer development and progress, and targeting ER with selective ER modulators (SERMs) has achieved significant reduction of breast cancer incidence in women at high risk for breast cancer. However, not all breast cancer is prevented by SERMs, because 30-40% of the tumors are ER-negative. Other receptors for retinoids, vitamin D analogs and peroxisome proliferator-activiator, along with transcription factors such as AP-1, NF-kappaB, and STATs (signal transducers and activators of transcription) affect breast tumorigenesis. This is also true for the signal transduction pathways, for example cyclooxygenase 2 (Cox-2), HER2/neu, mitogen-activated protein kinase (MAPK), and PI3K/Akt. Therefore, proteins in pathways that are altered during the process of mammary tumorigenesis may be promising targets of future chemopreventive drugs. Many newly-developed synthetic or natural compounds/agents are now under testing in preclinical studies and clinical trials. Receptor selective retinoids, receptor tyrosine kinase inhibitors (TKIs), SERMs, Cox-2 inhibitors, and others are some of the promising novel agents for the prevention of breast cancer. The chemopreventive activity of these agents and other novel signal transduction inhibitors are discussed in this chapter.

Role of PPAR.GAMMA. in the development of the central nervous system

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor that plays a central role in adipocyte differentiation and insulin sensitivity. Recently, a diversity of the action of PPARgamma on many other cell types or organs is indicated. We summarize here the possible role of PPARgamma in the development of the murine central nervous system. Expressions of PPARgamma in newborn or adult mouse brain are extremely low, but high in embryo or fetal mouse brain. Furthermore, we investigated the role of PPARgamma in proliferation or differentiation of neural stem cells (NSCs) isolated from murine embryonic brains, because NSCs are considered to be a major source of neurons in developmental brains. Administrations of PPARgamma-specific ligands on the NSCs from wild-type mice resulted in the stimulation of cell growth. On the other hand, administration of PPARgamma-antagonist showed the cell death and apoptosis of NSCs. These results may indicate that PPARgamma plays an important role during the early stage of the development of the central nervous system.

Thrombomodulin expression by THP-1 but not by vascular endothelial cells is upregulated by pioglitazone

Thrombomodulin-protein C pathway is a major anti-thrombotic mechanism present in endothelial cells (EC), and an important modulator of inflammation. Peroxisomal proliferator activated receptor-gamma (PPARgamma) expressed in monocytes/macrophages may have a role in cell differentiation. Since the expression of thrombomodulin (TM) by monocytes is upregulated during differentiation into macrophages, we investigated the effect of pioglitazone, a thiazolidinedione (TZD) that is a synthetic ligand of PPARgamma, on the expression of TM by a human monocyte/macrophage cell line; human acute monocytic leukemia (THP-1) cells. Pioglitazone dose-dependently upregulated TM antigen expression by THP-1 cells accompanied by an upregulation of TM cofactor activity for thrombin-dependent protein C activation. Thrombomodulin mRNA expression in THP-1 cells was also upregulated by pioglitazone, whereas tissue factor (TF) mRNA expression was not induced at all. Treatment cells with a natural PPARgamma ligand, 15-deoxy-delta12,14-prostaglandin J(2) (PGJ2), also enhanced TM protein expression. PGF(2alpha) an agent known to inactivate PPARgamma, diminished the stimulatory effect of pioglitazone and PGJ2 on TM protein expression. In contrast, pioglitazone had no effect on TM antigen expression by human umbilical vein ECs. These results suggest that PPARgamma activation in macrophages may counteract potentially prothrombotic and putative inflammatory properties in activated macrophages.

Docosahexaenoic acid suppresses the activity of peroxisome proliferator-activated receptors in a colon tumor cell line

Fatty acids are generally considered as agonists for peroxisome proliferator-activated receptors (PPARs). Fatty acids have been shown to bind to and transactivate PPARs; it is not known whether fatty acids act as generalized agonists for PPARs in different cell types, and thus, stimulate the expression of PPAR-regulated target genes. Here, we investigated the potency of unsaturated fatty acids on transactivation of PPRE, DNA-binding activity of PPARs, and the expression of a PPAR-regulated gene product, CD36. Docosahexaenoic acid (DHA) suppressed the basal and PPAR agonist-induced transactivation of PPRE, and DNA binding of PPARs in colon tumor cells (HCT116). The suppression of PPAR transactivation by DHA leads to reduced expression of CD36 in HCT116 cells and human monocytic cells (THP-1) as determined by promoter reporter gene assay and flow cytometric analysis. Our results demonstrate that DHA and other unsaturated fatty acids act as antagonists instead of agonists for transactivation of PPRE and PPAR-regulated gene expression in the cell lines tested. These results suggest that PPAR-mediated gene expression and cellular responses can be dynamically modulated by different types of dietary fatty acids consumed.

Peroxisome proliferator-activated receptor gamma ligand troglitazone induces cell cycle arrest and apoptosis of hepatocellular carcinoma cell lines

BACKGROUND

Ligand activation of peroxisome proliferator-activated receptor gamma (PPARgamma) results in the inhibition of proliferation of various cancer cells. The aim of this study is to investigate the mechanisms of cell growth inhibition of hepatocellular carcinoma (HCC) cell lines by the PPARgamma ligand, troglitazone.

METHODS

Six HCC cell lines were used to study the effects of troglitazone on cell growth by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay, on cell cycle by flow cytometry, and on the cell cycle-regulating factors of late G1 phase by Western blotting. Apoptosis assays were performed by flow cytometry using membrane, nuclear, cytoplasmic, and mitochondrial markers. Caspase inhibitors were used to analyze the mechanisms of apoptosis induced by troglitazone.

RESULTS

Troglitazone showed a potent dose-dependent effect on the growth inhibition of all six HCC cell lines, which were suppressed to under 50% of control at the concentration of 10 micromol/L. The growth inhibition was linked to the G1 phase cell cycle arrest through the up-expression of the cyclin-dependent kinase inhibitors, p21 and p27 proteins, and the hypophosphorylation of retinoblastoma protein. Troglitazone induced apoptosis by caspase-dependent (mitchondrial transmembrane potential decrease, cleavage of poly [adenosine diphosphate ribose] polymerase, 7A6 antigen exposure, Bcl-2 decrease, and activation of caspase 3) and caspase-independent (phosphatidylserine externalization) mechanisms.

CONCLUSIONS

Our data suggest that ligand activation of PPARgamma by troglitazone or modified analogs of the thiazolidinedione class of drugs is a novel target for effective therapy against HCC, because of the significant antiproliferative and programmed cell death induction capabilities demonstrated by troglitazone.

Lack of association of the codon 12 polymorphism of the peroxisome proliferator-activated receptor gamma gene with breast cancer and body mass

A principal hypothesized mechanism underlying breast carcinogenesis involves oestrogen-induced cell proliferation. In addition to its well-established role in the transcriptional regulation of genes required for adipocyte differentiation, the peroxisome proliferator-activated receptor gamma (PPARgamma) may be involved in transcriptional down-regulation of aromatase, a key enzyme in oestrogen biosynthesis. Furthermore, specific agonists for PPARgamma induce differentiation and suppress markers of malignancy in breast cancer cells in vitro. We investigated the association of the Pro12Ala PPARgamma polymorphism with breast cancer in a case-control study nested within the prospective Nurses' Health Study. Included were 725 incident cases of breast cancer diagnosed after blood collection through 1996 and 953 matched controls. In addition to breast cancer, the association of the PPARgamma Pro12Ala polymorphism with breast cancer risk factors, body mass index (BMI), weight gain since age 18 years, plasma hormones [oestrone sulphate, oestrone, oestradiol, androstenedione, testosterone, dehydroepiandrosterone (DHEA) and DHEA sulphate] and plasma lipids (total cholesterol and high density lipoprotein) was analysed. No significant association was observed between PPARgamma Pro12Ala polymorphism and either incident breast cancer (odds ratio = 1.08, 95% confidence interval = 0.85-1.38 for Ala allele carriers compared to non-carriers), plasma hormones, plasma cholesterol, BMI, weight gain since age 18 years or waist-to-hip ratio. To our knowledge, this is the first study to investigate the role of the Pro12Ala PPARgamma polymorphism in cancer. We did not find evidence to support a role for this polymorphism in breast cancer susceptibility. Furthermore, similar to others, we did not find evidence to suggest that Pro12Ala PPARgamma polymorphism is directly associated with body mass or weight gain.

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.

15-Deoxy-delta12,14-prostaglandin J2-induced apoptosis does not require PPARgamma in breast cancer cells

Naturally occurring derivatives of arachidonic acid are potent agonists for the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma) and block cancer cell proliferation through the induction of apoptosis. We have previously reported that induction of apoptosis using cyclopentenone prostaglandins of the J series, including 15deoxydelta(12,14)PGJ(2) (15dPGJ(2)), is associated with a high degree of PPAR-response element (PPRE) activity and requires early de novo gene expression in breast cancer cells. In the current study, we used pharmacologic and genetic approaches to test the hypothesis that PPARgamma is required for 15dPGJ(2)-induced apoptosis. The PPARgamma agonists 15dPGJ(2), trogliltazone (TGZ), and GW7845, a synthetic and highly selective tyrosine-based PPARgamma agonist, all increased transcriptional activity of PPARgamma, and expression of CD36, a PPARgamma-dependent gene. Transcriptional activity and CD36 expression was reduced by GW9662, a selective and irreversible PPARgamma antagonist, but GW9662 did not block apoptosis induced by 15dPGJ(2). Moreover, dominant negative expression of PPARgamma blocked PPRE transcriptional activity, but did not block 15dPGJ(2)-induced apoptosis. These studies show that while 15dPGJ(2) activates PPRE-mediated transcription, PPARgamma is not required for 15dPGJ(2)-induced apoptosis in breast cancer cells. Other likely mechanisms through which cyclopentenone prostaglandins induce apoptosis of cancer cells are discussed.

APC-dependent suppression of colon carcinogenesis by PPARgamma

Activation of PPARgamma by synthetic ligands, such as thiazolidinediones, stimulates adipogenesis and improves insulin sensitivity. Although thiazolidinediones represent a major therapy for type 2 diabetes, conflicting studies showing that these agents can increase or decrease colonic tumors in mice have raised concerns about the role of PPARgamma in colon cancer. To analyze critically the role of this receptor, we have used mice heterozygous for Ppargamma with both chemical and genetic models of this malignancy. Heterozygous loss of PPARgamma causes an increase in beta-catenin levels and a greater incidence of colon cancer when animals are treated with azoxymethane. However, mice with preexisting damage to Apc, a regulator of beta-catenin, develop tumors in a manner insensitive to the status of PPARgamma. These data show that PPARgamma can suppress beta-catenin levels and colon carcinogenesis but only before damage to the APC/beta-catenin pathway. This finding suggests a potentially important use for PPARgamma ligands as chemopreventative agents in colon cancer.

Apoptosis induced by activation of peroxisome-proliferator activated receptor-gamma is associated with Bcl-2 and NF-kappaB in human colon cancer

Peroxisome-proliferator activated receptor-gamma (PPARgamma) has been demonstrated to exert an inhibitory effect on cell growth in most cell types studied, but its role in colon cancer is still uncertain. The molecular mechanism between the activation of PPARgamma and its consequence is unknown. In the present report, we show that the expression of PPARgamma was significantly increased in tumor tissues from human colon cancer compared with non-tumor tissues and that PPARgamma ligands, 15-Deoxy-delta(12,14)prostaglandin J2 or ciglitizone, induced apoptosis in HT-29 cells, a human colon cancer cell line. The occurrence of apoptosis induced by PPARgamma ligands was sequentially accompanied by reduced levels of NF-kappaB and Bcl-2. Over-expression of Bcl-2 significantly protected the cells from apoptosis. This study suggested that a PPARgamma-Bcl-2 feedback loop may function to control the life-death continuum in colonic cells and that a deficiency in generation of PPARgamma ligands may precede the development of human colon cancer.

Dietary polyunsaturated fatty acids and colorectal neoplasia

Epidemiology has implicated dietary fat in mortality associated with some of the most common forms of cancer, including those affecting the intestinal tract, breast and prostate. Polyunsaturated fatty acids, and arachidonate in particular, have been unequivocally linked to experimental colorectal carcinogenesis. Dietary, pharmacologic and genetic manipulation of tissue arachidonic acid and its conversion to bioactive lipids has provided insights into pathogenic mechanisms as well as compelling evidence to support rational preventative and therapeutic methods of disease intervention. While it is clear that conversion of arachidonate to prostaglandins and other bioactive lipids contributes significantly to tumorigenesis in the intestinal tract and other organs, it is also clear that no single metabolic pathway or lipid in this complex biochemical network is solely responsible for dietary or pharmacologic benefits evident in epidemiologic studies. We will review some of these data and provide a summary of our own work showing that conversion of arachidonate to prostaglandin E2 contributes significantly to tumor growth through the modulation of apoptosis and cellular proliferation.

Enhanced colon carcinogenesis induced by azoxymethane in min mice occurs via a mechanism independent of beta-catenin mutation

The multiple intestinal neoplasia (min) mouse is a well-established cancer model in which loss of a single copy of the APC protein predisposes mice to the development of numerous tumors in the intestine. We have developed a novel variation of the min mouse model by using azoxymethane (AOM) to cause an increase in tumor incidence, number and size. Thus, treatment of min mice with AOM resulted in 2.6-, 6.3- and 5.9-fold increases in overall tumor incidence, multiplicity and size, respectively, when compared to wild type C57BL/6J mice treated with AOM. Furthermore, adenocarcinomas of the colon, which are otherwise relatively rare in min mice, increased in incidence (P<0.004), multiplicity (P<0.005), and size (P<0.02) in the AOM-treated min mice when compared to control untreated min mice. Of these adenocarcinomas, the number of poorly plus moderately differentiated adenocarcinomas was also significantly higher in the AOM-treated min mice (P<0.008). Thirty-seven histopathologically verified colon tumors (eight adenomas, five carcinoma in situ and 24 adenocarcinomas) induced in min mice and in C57BL/6J mice after treatment with or without AOM were analyzed for mutations in the beta-catenin gene or de novo mutations in the Apc gene. No mutations in the beta-catenin gene were found in any of colon tumors in min mice with or without treatment with AOM. However, mutations in either the beta-catenin gene or the Apc gene were found in tumors induced in C57BL/6J mice by AOM. These results suggest that mutations in the beta-catenin gene are less contributory to tumor development in min mice, as is the case in familial adenomatous polyposis (FAP) in humans. However, de novo mutations in either the Apc or beta-catenin gene can play a role in tumor development in C57BL/6J mice treated with AOM. The differences in mutation status between min and C57BL/6J mice may indicate different genetic pathways for developing colon tumors. These two experimental systems may, therefore, be useful animal models of human colon carcinomas in patients with FAP and in patients with sporadic colon carcinomas.

A phase II study of troglitazone, an activator of the PPARgamma receptor, in patients with chemotherapy-resistant metastatic colorectal cancer

PURPOSE

Troglitazone, a potent activator of the peroxisome proliferator-activated receptor-gamma, induces tumor differentiation in human liposarcomas and causes regression of tumors that are derived from human colon cancer cells in nude mice. We therefore assessed the efficacy of troglitazone in the treatment of metastatic colon cancer in humans.

METHODS

Twenty-five patients with metastatic colorectal cancer were treated with oral troglitazone. Patients were followed up for evidence of toxicity, tumor response, and survival.

RESULTS

The treatment was well tolerated: no grade 3/4 treatment-related toxicities were observed. However, no objective tumor responses were noted, and all 25 patients had progressive disease as their best response to therapy. The median progression-free survival time was only 1.6 months, and the median survival time was 3.9 months.

DISCUSSION

Troglitazone is not an active agent for the treatment of metastatic colorectal cancer.

Optimizing the diabetic formulary: beyond aspirin and insulin

Type 2 diabetes mellitus (DM) afflicts nearly 16 million persons in the U.S. Forty million people have impaired glucose tolerance and thus have a 10% annualized risk of developing type 2 DM. These prevalence rates are estimated to double within the next two decades. At-risk groups appear to be the elderly and minorities including African, Hispanic and Native Americans. While the epidemiologic underpinnings for this increase are not fully realized, there has been a parallel increase in societal obesity, sedentary lifestyle and a marked increase in type 2 DM among children. Although there have recently been noteworthy advances in the field of cardiovascular medicine, cardiovascular case fatalities remain the leading cause of death among diabetic patients. According to national health statistics, there continues to be a downward trend in cardiovascular mortality and morbidity. This observation has not been consistently noted among patients with DM and has led many to re-evaluate current treatment goals and pharmacologic regimens for the at-risk patients with type 2 DM. This shifting treatment paradigm for diabetic patients has led to a ratcheting down of targeted risk factor goals including low-density lipoprotein cholesterol, systolic and diastolic blood pressure, and serum glucose levels with a requisite increase in the number of pharmacologic agents being administered. This review focuses on the current adjunctive pharmacologic treatment regimen that is well suited for patients with type 2 DM.

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.

Natural ligands of PPARgamma: are prostaglandin J(2) derivatives really playing the part?

The peroxisome proliferator-activated receptor (PPAR) family was discovered from an orphan nuclear receptor approach, and thereafter, three subtypes were identified, namely PPARalpha, PPARbeta or PPARgamma and PPARgamma. The two former seem to regulate lipid homeostasis, whereas the latter is involved, among others, in glucose homeostasis and adipocyte differentiation. PPARs were pharmacologically characterised first using peroxisome proliferators such as clofibrates, which demonstrate moderate affinity (efficiency at micromolar concentrations) and low PPARalpha/delta versus PPARgamma specificity. Hence, several laboratories have started the search for potent and subtype-specific natural PPAR activators. In this respect, prostaglandin (PG)-related compounds were identified as good PPARgamma agonists with varying specificity, the most notable PPAR ligand being 15-deoxy-Delta12-14-PGJ2 (15d-PGJ2). Recently, an oxidized phosphatidylcholine was identified as a potent alternative (patho)physiological natural ligand of PPARgamma. In the present review, we discuss the different PPARgamma-dependent and -independent biological effects of the PG PPARgamma ligands and the concern about their low potency in molecular models as compared with thiazolidinediones (TZDs), a family of potent (nanomolar) synthetic PPARgamma ligands. Finally, the oxidized lipids are presented as a novel and interesting alternative for discovering potent PPARgamma activators in order to understand more in details the implications of PPARgamma in various pathophysiological conditions.

Alpha-Methylacyl-CoA racemase: a novel tumor marker over-expressed in several human cancers and their precursor lesions

alpha-Methylacyl-CoA racemase (AMACR) is a mitochondrial and peroxisomal enzyme involved in the metabolism of branched-chain fatty acid and bile acid intermediates. Recently, AMACR has been demonstrated to be over-expressed in localized and metastatic prostate cancer, suggesting that it may be an important tumor marker. This study examines AMACR expression in a variety of human cancers and their precursor lesions. A survey of online Expressed Sequence Tags (ESTs) and Serial Analysis of Gene Expression (SAGE) databases revealed that AMACR was over-expressed in multiple cancers. The findings were confirmed by AMACR immunohistochemistry performed on several tissue microarrays containing common human tumors, including prostate, colon, and breast. Based on prior work, AMACR protein expression was divided into two categories: negative (negative to weak staining intensity) and positive (moderate to strong staining intensity). AMACR protein over-expression was found in a number of cancers, including colorectal, prostate, ovarian, breast, bladder, lung, and renal cell carcinomas, lymphoma, and melanoma. Greatest over-expression was seen in colorectal and prostate cancer with positive staining in 92% and 83% cases, respectively. AMACR over-expression was present in 44% of breast cancer cases. AMACR was also over-expressed in precursor lesions. Sixty-four percent of high-grade prostatic intraepithelial neoplasia and 75% colonic adenomas demonstrated positive AMACR protein expression. Reverse transcriptase-polymerase chain reaction for AMACR using laser capture microdissected prostate tissue confirmed gene over-expression at the mRNA level. In conclusion, our study suggests that AMACR is potentially an important tumor marker for several cancers and their precursor lesions, especially those linked to high-fat diets.

An inducible pathway for degradation of FLIP protein sensitizes tumor cells to TRAIL-induced apoptosis

TRAIL (Apo2 ligand) is a member of the tumor necrosis factor (TNF) family of cytokines that induces apoptosis. Because TRAIL preferentially kills tumor cells, sparing normal tissues, interest has emerged in applying this biological factor for cancer therapy in humans. However, not all tumors respond to TRAIL, raising questions about resistance mechanisms. We demonstrate here that a variety of natural and synthetic ligands of peroxisome proliferator-activated receptor-gamma (PPAR gamma) sensitize tumor but not normal cells to apoptosis induction by TRAIL. PPAR gamma ligands selectively reduce levels of FLIP, an apoptosis-suppressing protein that blocks early events in TRAIL/TNF family death receptor signaling. Both PPAR gamma agonists and antagonists displayed these effects, regardless of the levels of PPAR gamma expression and even in the presence of a PPAR gamma dominant-negative mutant, indicating a PPAR gamma-independent mechanism. Reductions in FLIP and sensitization to TRAIL-induced apoptosis were also not correlated with NF-kappa B, further suggesting a novel mechanism. PPAR gamma modulators induced ubiquitination and proteasome-dependent degradation of FLIP, without concomitant reductions in FLIP mRNA. The findings suggest the existence of a pharmacologically regulated novel target of this class of drugs that controls FLIP protein turnover, and raise the possibility of combining PPAR gamma modulators with TRAIL for more efficacious elimination of tumor cells through apoptosis.