Use of the peroxisome proliferator-activated receptor (PPAR) gamma ligand troglitazone as treatment for refractory breast cancer: a phase II study

The insulin receptor: a new target for cancer therapy

A large body of evidences have shown that both the IGF-I receptor (IGF-IR) and the insulin receptor (IR) play a role in cancer development and progression. In particular, IR overactivation by IGF-II is common in cancer cells, especially in dedifferentiated/stem-like cells. In spite of these findings, until very recently, only IGF-IR but not IR has been considered a target in cancer therapy. Although several preclinical studies have showed a good anti-cancer activity of selective anti-IGF-IR drugs, the results of the clinical first trials have been disappointing. In fact, only a small subset of malignant tumors has shown an objective response to these therapies. Development of resistance to anti-IGF-IR drugs may include upregulation of IR isoform A (IR-A) in cancer cells and its overactivation by increased secretion of autocrine IGF-II. These findings have led to the concept that co-targeting IR together with IGF-IR may increase therapy efficacy and prevent adaptive resistance to selective anti-IGF-IR drugs. IR blockade should be especially considered in tumors with high IR-A:IGF-IR ratio and high levels of autocrine IGF-II. Conversely, insulin sensitizers, which ameliorate insulin resistance associated with metabolic disorders and cancer treatments, may have important implications for cancer prevention and management. Only few drugs co-targeting the IR and IGF-IR are currently available. Ideally, future IR targeting strategies should be able to selectively inhibit the tumor promoting effects of IR without impairing its metabolic effects.

Peroxisome proliferator-activated receptor-gamma activation inhibits tumor metastasis by antagonizing Smad3-mediated epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) was shown to confer tumor cells with abilities essential for metastasis, including migratory phenotype, invasiveness, resistance to apoptosis, evading immune surveillance, and tumor stem cell traits. Therefore, inhibition of EMT can be an important therapeutic strategy to inhibit tumor metastasis. Here, we show that activation of peroxisome proliferator-activated receptor γ (PPAR-γ) inhibits transforming growth factor β (TGF-β)-induced EMT in lung cancer cells and prevents metastasis by antagonizing Smad3 function. Activation of PPAR-γ by synthetic ligands (troglitazone and rosiglitazone) or by a constitutively active form of PPAR-γ prevents TGF-β-induced loss of E-cadherin expression and inhibits the induction of mesenchymal markers (vimentin, N-cadherin, fibronectin) and matrix metalloproteases. Consistently, activation of PPAR-γ also inhibited EMT-induced migration and invasion of lung cancer cells. Furthermore, effects of PPAR-γ ligands were attenuated by siRNA-mediated knockdown of PPAR-γ, indicating that the ligand-induced responses are PPAR-γ dependent. Selective knockdown of Smad2 and Smad3 by siRNA showed that TGF-β-induced EMT is Smad3 dependent in lung cancer cells. Activation of PPAR-γ inhibits TGF-β-induced Smad transcriptional activity but had no effect on the phosphorylation or nuclear translocation of Smads. Consistently, PPAR-γ activation prevented TGF-β-induced transcriptional repression of E-cadherin promoter and inhibited transcriptional activation of N-cadherin promoter. Finally, treatment of mice with troglitazone or knockdown of Smad3 in tumor cells significantly inhibited TGF-β-induced experimental metastasis in SCID-Beige mice. Together, with the low toxicity profile of PPAR-γ ligands, our data show that these ligands may serve as potential therapeutic agents to inhibit metastasis.

Steroid receptor coactivator 1 deficiency increases MMTV-neu mediated tumor latency and differentiation specific gene expression, decreases metastasis, and inhibits response to PPAR ligands

Background

The peroxisome proliferator activated receptor (PPAR) subgroup of the nuclear hormone receptor superfamily is activated by a variety of natural and synthetic ligands. PPARs can heterodimerize with retinoid X receptors, which have homology to other members of the nuclear receptor superfamily. Ligand binding to PPAR/RXRs results in recruitment of transcriptional coactivator proteins such as steroid receptor coactivator 1 (SRC-1) and CREB binding protein (CBP). Both SRC-1 and CBP are histone acetyltransferases, which by modifying nucleosomal histones, produce more open chromatin structure and increase transcriptional activity. Nuclear hormone receptors can recruit limiting amounts of coactivators from other transcription factor binding sites such as AP-1, thereby inhibiting the activity of AP-1 target genes. PPAR and RXR ligands have been used in experimental breast cancer therapy. The role of coactivator expression in mammary tumorigenesis and response to drug therapy has been the subject of recent studies.

Methods

We examined the effects of loss of SRC-1 on MMTV-neu mediated mammary tumorigenesis.

Results

SRC-1 null mutation in mammary tumor prone mice increased the tumor latency period, reduced tumor proliferation index and metastasis, inhibited response to PPAR and RXR ligands, and induced genes involved in mammary gland differentiation. We also examined human breast cancer cell lines overexpressing SRC-1 or CBP. Coactivator overexpression increased cellular proliferation with resistance to PPAR and RXR ligands and remodeled chromatin of the proximal epidermal growth factor receptor promoter.

Conclusions

These results indicate that histone acetyltransferases play key roles in mammary tumorigenesis and response to anti-proliferative therapies.

In vitro and in vivo therapeutic efficacy of the PPAR-γ agonist troglitazone in combination with cisplatin against malignant pleural mesothelioma cell growth

Malignant pleural mesothelioma (MPM), an aggressive and refractory tumor type, is increasing in frequency throughout the world. Peroxisome proliferator activated receptor-γ (PPAR-γ) agonists have anticancer activity against several cancer cell lines in vitro and in vivo. However, there have been no reports that PPAR-γ agonists induce growth inhibition of MPM cell lines. In this study, we investigated the inhibitory effect of a PPAR-γ agonist in combination with an anticancer agent on MPM cell growth in vitro and in vivo. We examined the therapeutic efficacy of the PPAR-γ agonist troglitazone (TGZ) in combination with cisplatin against a human MPM cell line, both in vitro and orthotopically inoculated into severe combined immunodeficient (SCID) mice. Troglitazone (TGZ) alone inhibited MPM cell growth in vitro in a dose-dependent manner via induction of G1 cell cycle arrest and apoptosis. The combination of TGZ and cisplatin showed an additive inhibitory effect on MPM cell growth compared to treatment with either individual drug. Treatment with 500 mg/kg or 1000 mg/kg TGZ effectively inhibited the production of thoracic tumors and pleural effusion in EHMES-10 cell-bearing SCID mice. Moreover, treatment with 500 mg/kg TGZ in combination with 3 mg/kg cisplatin more effectively prolonged survival compared to treatment with either individual drug. These results suggest that TGZ in combination with cisplatin may become a novel therapy for MPM.

Troglitazone suppresses telomerase activity independently of PPARgamma in estrogen-receptor negative breast cancer cells

Background

Breast cancer is one the highest causes of female cancer death worldwide. Many standard chemotherapeutic agents currently used to treat breast cancer are relatively non-specific and act on all rapidly dividing cells. In recent years, more specific targeted therapies have been introduced. It is known that telomerase is active in over 90% of breast cancer tumors but inactive in adjacent normal tissues. The prevalence of active telomerase in breast cancer patients makes telomerase an attractive therapeutic target. Recent evidence suggests that telomerase activity can be suppressed by peroxisome proliferator activated receptor gamma (PPARγ). However, its effect on telomerase regulation in breast cancer has not been investigated.

Methods

In this study, we investigated the effect of the PPARγ ligand, troglitazone, on telomerase activity in the MDA-MB-231 breast cancer cell line. Real time RT-PCR and telomerase activity assays were used to evaluate the effect of troglitazone. MDA-MB-231 cells had PPARγ expression silenced using shRNA interference.

Results

We demonstrated that troglitazone reduced the mRNA expression of hTERT and telomerase activity in the MDA-MB-231 breast cancer cell line. Troglitazone reduced telomerase activity even in the absence of PPARγ. In agreement with this result, we found no correlation between PPARγ and hTERT mRNA transcript levels in breast cancer patients. Statistical significance was determined using Pearson correlation and the paired Student's t test.

Conclusions

To our knowledge, this is the first time that the effect of troglitazone on telomerase activity in breast cancer cells has been investigated. Our data suggest that troglitazone may be used as an anti-telomerase agent; however, the mechanism underlying this inhibitory effect remains to be determined.

Diabetes and cancer: a consensus report

Epidemiologic evidence suggests that cancer incidence is associated with diabetes as well as certain diabetes risk factors and diabetes treatments. This consensus statement of experts assembled jointly by the American Diabetes Association and the American Cancer Society reviews the state of science concerning 1) the association between diabetes and cancer incidence or prognosis, 2) risk factors common to both diabetes and cancer, 3) possible biologic links between diabetes and cancer risk, and 4) whether diabetes treatments influence risk of cancer or cancer prognosis. In addition, key unanswered questions for future research are posed.

Advances in combination of antiangiogenic agents targeting VEGF-binding and conventional chemotherapy and radiation for cancer treatment

Despite great efforts and resources being devoted to treatment, the incidence and mortality of numerous cancers have not decreased in recent decades. This is a result of the resistance of cancer cells to chemotherapeutic agents and radio-therapy. The development of antiangiogenic agents that target vascular endothelial growth factor (VEGF) provides a new option for treatment of cancer. Major advances have been achieved with cancer therapy based on antiangiogenic VEGF-targeted agents in the past few years, and some of the recently approved therapies are now being used in daily clinical practice. A further challenge is finding a more efficacious combination of antiangiogenic VEGF-targeted therapies and conventional radio- and chemotherapies. This review outlines the current preclinical and clinical cancer treatments using optimized combinations of antiangiogenic VEGF-targeted agents and conventional radiochemotherapy and highlights that better scheduling for the combination of radiochemotherapy and antiangiogenic VEGF-targeted agents should be developed to achieve better treatment outcomes.

Anticancer Role of PPARgamma Agonists in Hematological Malignancies Found in the Vasculature, Marrow, and Eyes

The use of targeted cancer therapies in combination with conventional chemotherapeutic agents and/or radiation treatment has increased overall survival of cancer patients. However, longer survival is accompanied by increased incidence of comorbidities due, in part, to drug side effects and toxicities. It is well accepted that inflammation and tumorigenesis are linked. Because peroxisome proliferator-activated receptor (PPAR)-gamma agonists are potent mediators of anti-inflammatory responses, it was a logical extension to examine the role of PPARgamma agonists in the treatment and prevention of cancer. This paper has two objectives: first to highlight the potential uses for PPARgamma agonists in anticancer therapy with special emphasis on their role when used as adjuvant or combined therapy in the treatment of hematological malignancies found in the vasculature, marrow, and eyes, and second, to review the potential role PPARgamma and/or its ligands may have in modulating cancer-associated angiogenesis and tumor-stromal microenvironment crosstalk in bone marrow.

Differential effects of PPARgamma activation by the oral antidiabetic agent pioglitazone in Barrett's carcinoma in vitro and in vivo

BACKGROUND AND PURPOSE

The nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a key transcription factor regulating genes involved in adipogenesis, glucose homeostasis and cell differentiation. Moreover, PPARgamma has been demonstrated to control proliferation and apoptosis in various cancer cells. We investigated the biological effects of PPARgamma activation by the oral antidiabetic agent pioglitazone in Barrett's adenocarcinoma cells in vitro and in vivo.

RESULTS

PPARgamma mRNA and protein were overexpressed in endoscopic biopsies of Barrett's epithelium and the human Barrett's adenocarcinoma cancer cell line OE33 as compared to normal esophagus and stomach and the esophageal squamous epithelium cancer cell line Kyse-180. PPARgamma activation by pioglitazone in OE33 cells in vitro led to reduced cell growth by induction of apoptosis. Effects of systemic PPARgamma activation by the thiazolidinedione pioglitazone on tumor cell proliferation and apoptosis were then assessed in vivo in nude mice bearing transplantable Barrett's adenocarcinomas derived from OE33 cells. Unexpectedly, enhanced growth of OE33 derived transplantable adenocarcinomas was observed in Balb/c nu/nu mice upon systemic pioglitazone treatment due to increased cell proliferation.

CONCLUSION

These results indicate that PPARgamma is involved in the molecular pathogenesis of Barrett's adenocarcinoma formation and growth. However, activation of PPARgamma exerts differential effects on growth of Barrett's adenocarcinoma cells in vitro and in vivo emphasizing the importance of additional cell context specific factors and systemic metabolic status for the modulation of PPARgamma action in vivo.

In vivo effects of rosiglitazone in a human neuroblastoma xenograft

BACKGROUND

Neuroblastoma (NB) is the most common extra-cranial solid tumour in infants. Unfortunately, most children present with advanced disease and have a poor prognosis. There is in vitro evidence that the peroxisome proliferator-activated receptor gamma (PPARgamma) might be a target for pharmacological intervention in NB. We have previously demonstrated that the PPARgamma agonist rosiglitazone (RGZ) exerts strong anti-tumoural effects in the human NB cell line, SK-N-AS. The aim of this study was to evaluate whether RGZ maintains its anti-tumoural effects against SK-N-AS NB cells in vivo.

METHODS AND RESULTS

For this purpose, tumour cells were subcutaneously implanted in nude mice, and RGZ (150 mg kg(-1)) was administered by gavage daily for 4 weeks. At the end of treatment, a significant tumour weight inhibition (70%) was observed in RGZ-treated mice compared with control mice. The inhibition of tumour growth was supported by a strong anti-angiogenic activity, as assessed by CD-31 immunostaining in tumour samples. The number of apoptotic cells, as determined by cleaved caspase-3 immunostaining, seemed lower in RGZ-treated animals at the end of the treatment period than in control mice, likely because of the large tumour size observed in the latter group.

CONCLUSIONS

To our knowledge, this is the first demonstration that RGZ effectively inhibits tumour growth in a human NB xenograft and our results suggest that PPARgamma agonists may have a role in anti-tumoural strategies against NB.

New troglitazone derivatives devoid of PPARγ agonist activity display an increased antiproliferative effect in both hormone-dependent and hormone-independent breast cancer cell lines

Numerous recent studies indicate that most anticancer effects of PPARγ agonists like thiazolidinediones are the result of PPARγ-independent pathways. These conclusions were obtained by several approaches including the use of thiazolidinedione derivatives like Δ2-Troglitazone (Δ2-TGZ) that does not activate PPARγ. Since biotinylation has been proposed as a mechanism able to increase the specificity of drug delivery to cancer cells which could express a high level of vitamin receptor, a biotinylated derivative of Δ2-TGZ (bΔ2-TGZ) has been synthetized. In the present article, we have studied the in vitro effects of this molecule on both hormone-dependent (MCF-7) and hormone-independent (MDA-MB-231) breast cancer cells. In both cell lines, bΔ2-TGZ was more efficient than Δ2-TGZ to decrease cell viability. bΔ2-TGZ was also more potent than Δ2-TGZ to induce the proteasomal degradation of cyclin D1 in both cell lines and those of ERα in MCF-7 cells. However, in competition experiments, the presence of free biotin in the culture medium did not decrease the antiproliferative action of bΔ2-TGZ. Besides, other compounds that had no biotin but that were substituted at the same position of the phenolic group of the chromane moiety of Δ2-TGZ decreased cell viability similarly to bΔ2-TGZ. Hence, we concluded that the increased antiproliferative action of bΔ2-TGZ was not due to biotin itself but to the functionalization of the terminal hydroxyl group. This should be taken into account for the design of new thiazolidinedione derivatives able to affect not only hormone-dependent but also hormone-independent breast cancer cells in a PPARγ-independent pathway.

Activating peroxisome proliferator-activated receptor gamma mutant promotes tumor growth in vivo by enhancing angiogenesis

Peroxisome proliferator-activated receptor gamma (PPARgamma) is expressed in a variety of cancer cells. The addition of ligand activates the receptor by inducing a conformational change in the receptor, which can be recapitulated by mutation. To investigate the role of activated PPARgamma signaling in breast cancer, we compared the function of a constitutively active PPARgamma (PgammaCA) mutant with the wild-type PPARgamma in ErbB2-induced mammary tumorigenesis in vivo. Tumor cells transduced with either PPARgamma or PgammaCA were implanted into immunocompetent FVB mice. Enhanced tumor growth was observed in PgammaCA-transduced cells, which was associated with increased angiogenesis and endothelial stem cells as evidenced by increased number of cells stained with von Willebrand factor, c-Kit, CD133, and CD31. Genome-wide expression profiling identified a group of genes within the angiogenesis pathway, including Angptl4, as targets of activated PPARgamma; PgammaCA also induced Angptl4 protein secretion in ErbB2-transformed mammary epithelial cells. Angptl4 promoted vascular endothelial cell migration; conversely, immunodepletion of Angptl4 reduced PgammaCA-mediated cellular migration. Collectively, these studies suggest that activated PPARgamma induces Angptl4 to promote tumor growth through enhanced angiogenesis in vivo.

Troglitazone reverses the multiple drug resistance phenotype in cancer cells

A major problem in treating cancer is the development of drug resistance. We previously demonstrated doxorubicin (DOX) resistance in K562 human leukemia cells that was associated with upregulation of glyoxalase 1 (GLO-1) and histone H3 expression. The thiazolidinedione troglitazone (TRG) downregulated GLO-1 expression and further upregulated histone H3 expression and post-translational modifications in these cells, leading to a regained sensitivity to DOX. Given the pleiotropic effects of epigenetic changes in cancer development, we hypothesized that TRG may downregulate the multiple drug resistance (MDR) phenotype in a variety of cancer cells. To test this, MCF7 human breast cancer cells and K562 cells were cultured in the presence of low-dose DOX to establish DOX-resistant cell lines (K562/DOX and MCF7/DOX). The MDR phenotype was confirmed by Western blot analysis of the 170 kDa P-glycoprotein (Pgp) drug efflux pump multiple drug resistance protein 1 (MDR-1), and the breast cancer resistance protein (BCRP). TRG markedly decreased expression of both MDR-1 and BCRP in these cells, resulting in sensitivity to DOX. Silencing of MDR-1 expression also sensitized MCF7/DOX cells to DOX. Use of the specific and irreversible peroxisome proliferator-activated receptor gamma (PPARγ) inhibitor GW9662 in the nanomolar range not only demonstrated that the action of TRG on MCF/DOX was PPARγ-independent, but indicated that PPARγ may play a role in the MDR phenotype, which is antagonized by TRG. We conclude that TRG is potentially a useful adjunct therapy in chemoresistant cancers.

Obesity and breast cancer: the roles of peroxisome proliferator-activated receptor-γ and plasminogen activator inhibitor-1

Breast cancer is the most prominent cancer among females in the United States. There are a number of risk factors associated with development of breast cancer, including consumption of a high-fat diet and obesity. Plasminogen activator inhibitor-1 (PAI-1) is a cytokine upregulated in obesity whose expression is correlated with a poor prognosis in breast cancer. As a key mediator of adipogenesis and regulator of adipokine production, peroxisome proliferator-activated receptor-γ (PPAR-γ) is involved in PAI-1 expression from adipose tissue. We summarize the current knowledge linking PPAR-γ and PAI-1 expression to high-fat diet and obesity in the risk of breast cancer.

Structurally diverse peroxisome proliferator-activated receptor agonists induce apoptosis in human uro-epithelial cells by a receptor-independent mechanism involving store-operated calcium channels

OBJECTIVES

Peroxisome proliferator-activated receptors (PPARs) are implicated in epithelial cell proliferation and differentiation, but investigation has been confounded by potential off-target effects of some synthetic PPAR ligands. Our aim was to determine mechanisms underlying the pro-apoptotic effect of synthetic PPAR agonists in normal human bladder uro-epithelial (urothelial) cells and to reconcile this with the role of PPARs in urothelial cytodifferentiation.

MATERIALS AND METHODS

Normal human urothelial (NHU) cells were grown as non-immortal lines in vitro and exposed to structurally diverse agonists ciglitazone, troglitazone, rosiglitazone (PPARgamma), ragaglitazar (PPARalpha/gamma), fenofibrate (PPARalpha) and L165041 (PPARbeta/delta).

RESULTS

NHU cells underwent apoptosis following acute exposure to ciglitazone, troglitazone or ragaglitazar, but not fenofibrate, L165041 or rosiglitazone, and this was independent of ERK or p38 MAP-kinase activation. Pro-apoptotic agonists induced sustained increases in intracellular calcium, whereas removal of extracellular calcium altered the kinetics of ciglitazone-mediated calcium release from sustained to transient. Cell death was accompanied by plasma-membrane disruption, loss of mitochondrial membrane-potential and caspase-9/caspase-3 activation. PPARgamma-mediated apoptosis was unaffected following pre-treatment with PPARgamma antagonist T0070907 and was strongly attenuated by store-operated calcium channel (SOC) inhibitors 2-APB and SKF-96365.

CONCLUSIONS

Our results provide a mechanistic basis for the ability of some PPAR agonists to induce death in NHU cells and demonstrate that apoptosis is mediated via PPAR-independent mechanisms, involving intracellular calcium changes, activation of SOCs and induction of the mitochondrial apoptotic pathway.

PPAR-γ agonists and their effects on IGF-I receptor signaling: Implications for cancer

It is now well established that the development and progression of a variety of human malignancies are associated with dysregulated activity of the insulin-like growth factor (IGF) system. In this regard, promising drugs have been developed to target the IGF-I receptor or its ligands. These therapies are limited by the development of insulin resistance and compensatory hyperinsulinemia, which in turn, may stimulate cancer growth. Novel therapeutic approaches are, therefore, required. Synthetic PPAR-γ agonists, such as thiazolidinediones (TZDs), are drugs universally used as antidiabetic agents in patients with type 2 diabetes. In addition of acting as insulin sensitizers, PPAR-γ agonists mediate in vitro and in vivo pleiotropic anticancer effects. At least some of these effects appear to be linked with the downregulation of the IGF system, which is induced by the cross-talk of PPAR-γ agonists with multiple components of the IGF system signaling. As hyperinsulinemia is an emerging cancer risk factor, the insulin lowering action of PPAR-γ agonists may be expected to be also beneficial to reduce cancer development and/or progression. In light of these evidences, TZDs or other PPAR-γ agonists may be exploited in those tumors "addicted" to the IGF signaling and/or in tumors occurring in hyperinsulinemic patients.

Peroxisome proliferator-activated receptor gamma pathway targeting in carcinogenesis: implications for chemoprevention

The peroxisome proliferator-activated receptor (PPAR) gamma is one member of the nuclear receptor superfamily that contains in excess of 80 described receptors. PPARgamma activators are a diverse group of agents that range from endogenous fatty acids or derivatives (linolenic, linoleic, and 15-deoxy-Delta(12,14)-prostaglandin J(2)) to Food and Drug Administration-approved thiazolidinedione drugs [pioglitazone (Actos) and rosiglitazone (Avandia)] for the treatment of diabetes. Once activated, PPARgamma will preferentially bind with retinoid X receptor alpha and signal antiproliferative, antiangiogenic, and prodifferentiation pathways in several tissue types, thus making it a highly useful target for down-regulation of carcinogenesis. Although PPAR-gamma activators show many anticancer effects on cell lines, their advancement into human advanced cancer clinical trials has met with limited success. This article will review translational findings in PPARgamma activation and targeting in carcinogenesis prevention as they relate to the potential use of PPARgamma activators clinically as cancer chemoprevention strategies.

Differentiation therapy of leukemia: 3 decades of development

A characteristic feature of leukemia cells is a blockade of differentiation at a distinct stage in cellular maturation. In the 1970s and 1980s, studies demonstrating the capabilities of certain chemicals to induce differentiation of hematopoietic cell lines fostered the concept of treating leukemia by forcing malignant cells to undergo terminal differentiation instead of killing them through cytotoxicity. The first promising reports on this notion prompted a review article on this subject by us 25 years ago. In this review, we revisit this interesting field of study and report the progress achieved in the course of nearly 3 decades. The best proof of principle for differentiation therapy has been the treatment of acute promyelocytic leukemia with all-trans retinoic acid. Attempts to emulate this success with other nuclear hormone ligands such as vitamin D compounds and PPARgamma agonists or different classes of substances such as hematopoietic cytokines or compounds affecting the epigenetic landscape have not been successful on a broad scale. However, a multitude of studies demonstrating partial progress and improvements and, finally, the new powerful possibilities of forward and reverse engineering of differentiation pathways by manipulation of transcription factors support the continued enthusiasm for differentiation therapy of leukemia in the future.

Fluorine-18 labeling and biodistribution studies on peroxisome proliferator-activated receptor-gamma ligands: potential positron emission tomography imaging agents

INTRODUCTION

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is an important regulator of lipid metabolism; it controls the differentiation of preadipocytes and is also found at high levels in small metastatic tumors. In this report, we describe the radiochemical synthesis and evaluation of two (18)F-labeled analogs of the potent and selective PPARgamma agonist farglitazar.

MATERIALS AND METHODS

The isomeric aromatic fluorine-substituted target compounds [(2S)-(2-benzoylphenylamino)-3-(4-(2-[2-(4-[(18)F]fluorophenyl)-5-methyloxazol-4-yl]ethoxy)-phenyl)propionic acid ([(18)F]-1) and (2S)-[2-(4-fluorobenzoyl)phenylamino]-3-(4-[2-(5-methyl-2-phenyloxazol-4-yl)ethoxy]-phenyl)propionic acid ([(18)F]-2)] were prepared in fluorine-18-labeled form, respectively, by radiofluorination of an iodonium salt precursor or by Ullmann-type condensation with 2-iodo-4'-[(18)F]fluorobenzophenone after nucleophilic aromatic substitution with [(18)F]fluoride ion. Each compound was obtained in high specific activity and good radiochemical yield.

RESULTS AND DISCUSSION

(18)F-1 and (18)F-2 have high and selective PPARgamma binding affinities comparable to that of the parent molecule farglitazar, and they were found to have good metabolic stability. Tissue biodistribution studies of (18)F-1 and (18)F-2 were conducted, but PPARgamma-mediated uptake of both agents was minimal.

CONCLUSION

This study completes our first look at an important class of PPARgamma ligands as potential positron emission tomography (PET) imaging agents for breast cancer and vascular disease. Although (18)F-1 and (18)F-2 have high affinities for PPARgamma and good metabolic stability, their poor target-tissue distribution properties, which likely reflect their high lipophilicity combined with the low titer of PPARgamma in target tissues, indicate that they have limited potential as PPARgamma PET imaging agents.

In vitro and in vivo anti-melanoma effects of ciglitazone

Activation of PPARgamma by synthetic ligands, thiazolidinediones, inhibits the proliferation of cancer cells. In this report, focusing our attention on ciglitazone, we show that ciglitazone inhibits melanoma growth by inducing apoptosis and cell-cycle arrest, whereas normal melanocytes are resistant to ciglitazone. In melanoma cells, ciglitazone-induced apoptosis is associated with caspase activations and a loss of mitochondrial membrane potential. Induction of cell-cycle arrest by ciglitazone is associated with changes in expression of key cell-cycle regulators such as p21, cyclin D1, and pRB hypophosphorylation. Cell-cycle arrest occurs at low ciglitazone concentrations and through a PPARgamma-dependent pathway, whereas the induction of apoptosis is caused by higher ciglitazone concentrations and independently of PPARgamma. These results allow an effective molecular dissociation between proapoptotic effects and growth inhibition evoked by ciglitazone in melanoma cells. Finally, we show that in vivo treatment of nude mice by ciglitazone dramatically inhibits human melanoma xenograft development. The data presented suggest that ciglitazone might be a better candidate for clinical trials in melanoma treatment than the thiazolidinediones currently used in the treatment of type 2 diabetes, such as rosiglitazone, which is devoid of a proapoptotic PPARgamma-independent function.

Peroxisome proliferator-activated receptor gamma agonist pioglitazone prevents the hyperglycemia caused by phosphatidylinositol 3-kinase pathway inhibition by PX-866 without affecting antitumor activity

The phosphatidylinositol 3-kinase (PI3K)/Akt signaling cascade is an important component of the insulin signaling in normal tissues leading to glucose uptake and homeostasis and for cell survival signaling in cancer cells. Hyperglycemia is an on-target side effect of many inhibitors of PI3K/Akt signaling including the specific PI3K inhibitor PX-866. The peroxisome proliferator-activated receptor gamma agonist pioglitazone, used to treat type 2 diabetes, prevents a decrease in glucose tolerance caused by acute administration of PX-866. Our studies have shown that pioglitazone does not inhibit the antitumor activity of PX-866 in A-549 non-small cell lung cancer and HT-29 colon cancer xenografts. In vitro studies also showed that pioglitazone increases 2-[1-(14)C]deoxy-D-glucose uptake in L-6 muscle cells and prevents inhibition of 2-deoxyglucose uptake by PX-866. Neither pioglitazone nor PX-866 had an effect on 2-deoxyglucose uptake in A-549 lung cancer cells. In vivo imaging studies using [18F]2-deoxyglucose (FDG) positron emission tomography showed that pioglitazone increases FDG accumulation by normal tissue but does not significantly alter FDG uptake by A-549 xenografts. Thus, peroxisome proliferator-activated receptor gamma agonists may be useful in overcoming the increase in blood glucose caused by inhibitors of PI3K signaling by preventing the inhibition of normal tissue insulin-mediated glucose uptake without affecting antitumor activity.

Clinical Use of PPARgamma Ligands in Cancer

The role of PPARγ in adipocyte differentiation has fueled intense interest in the function of this steroid nuclear receptor for regulation of malignant cell growth and differentiation. Given the antiproliferative and differentiating effects of PPARγ ligands on liposarcoma cells, investigation of PPARγ expression and ligand activation in other solid tumors such as breast, colon, and prostate cancers ensued. The anticancer effects of PPARγ ligands in cell culture and rodent models of a multitude of tumor types suggest broad applicability of these agents to cancer therapy. This review focuses on the clinical use of PPARγ ligands, specifically the thiazolidinediones, for the treatment and prevention of cancer.

The prince and the pauper. A tale of anticancer targeted agents

Cancer rates are set to increase at an alarming rate, from 10 million new cases globally in 2000 to 15 million in 2020. Regarding the pharmacological treatment of cancer, we currently are in the interphase of two treatment eras. The so-called pregenomic therapy which names the traditional cancer drugs, mainly cytotoxic drug types, and post-genomic era-type drugs referring to rationally-based designed. Although there are successful examples of this newer drug discovery approach, most target-specific agents only provide small gains in symptom control and/or survival, whereas others have consistently failed in the clinical testing. There is however, a characteristic shared by these agents: -their high cost-. This is expected as drug discovery and development is generally carried out within the commercial rather than the academic realm. Given the extraordinarily high therapeutic drug discovery-associated costs and risks, it is highly unlikely that any single public-sector research group will see a novel chemical "probe" become a "drug". An alternative drug development strategy is the exploitation of established drugs that have already been approved for treatment of non-cancerous diseases and whose cancer target has already been discovered. This strategy is also denominated drug repositioning, drug repurposing, or indication switch. Although traditionally development of these drugs was unlikely to be pursued by Big Pharma due to their limited commercial value, biopharmaceutical companies attempting to increase productivity at present are pursuing drug repositioning. More and more companies are scanning the existing pharmacopoeia for repositioning candidates, and the number of repositioning success stories is increasing. Here we provide noteworthy examples of known drugs whose potential anticancer activities have been highlighted, to encourage further research on these known drugs as a means to foster their translation into clinical trials utilizing the more limited public-sector resources. If these drug types eventually result in being effective, it follows that they could be much more affordable for patients with cancer; therefore, their contribution in terms of reducing cancer mortality at the global level would be greater.

Regression of drug-resistant lung cancer by the combination of rosiglitazone and carboplatin

PURPOSE

Current therapy for lung cancer involves multimodality therapies. However, many patients are either refractory to therapy or develop drug resistance. KRAS and epidermal growth factor receptor (EGFR) mutations represent some of the most common mutations in lung cancer, and many studies have shown the importance of these mutations in both carcinogenesis and chemoresistance. Genetically engineered murine models of mutant EGFR and KRAS have been developed that more accurately recapitulate human lung cancer. Recently, using cell-based experiments, we showed that platinum-based drugs and the antidiabetic drug rosiglitazone (PPARgamma ligand) interact synergistically to reduce cancer cell and tumor growth. Here, we directly determined the efficacy of the PPARgamma/carboplatin combination in these more relevant models of drug resistant non-small cell lung cancer.

EXPERIMENTAL DESIGN

Tumorigenesis was induced by activation of either mutant KRAS or EGFR. Mice then received either rosiglitazone or carboplatin monotherapy, or a combination of both drugs. Change in tumor burden, pathology, and evidence of apoptosis and cell growth were assessed.

RESULTS

Tumor burden remained unchanged or increased in the mice after monotherapy with either rosiglitazone or carboplatin. In striking contrast, we observed significant tumor shrinkage in mice treated with these drugs in combination. Immunohistochemical analyses showed that this synergy was mediated via both increased apoptosis and decreased proliferation. Importantly, this synergy between carboplatin and rosiglitazone did not increase systemic toxicity.

CONCLUSIONS

These data show that the PPARgamma ligand/carboplatin combination is a new therapy worthy of clinical investigation in lung cancers, including those cancers that show primary resistance to platinum therapy or acquired resistance to targeted therapy.

Biomarkers for early effects of carcinogenic dual-acting PPAR agonists in rat urinary bladder urotheliumin vivo

Small-molecule agonists of the peroxisome proliferator-activated receptor (PPAR) alpha and gamma isoforms (dual-acting PPAR agonists) can cause urothelial cancers in rodents. Rats were dosed orally for 16 days with bladder carcinogenic (ragaglitazar) as well as non-bladder carcinogenic (fenofibrate and rosiglitazone) PPAR agonists and protein changes were assayed in the urinary bladder urothelium by Western blotting. Dose levels reflected 10-20 x human exposure, and the ragaglitazar dose was in the carcinogenic range. Ragaglitazar induced expression of the transcription factor Egr-1, phosphorylation of the c-Jun transcription factor and phosphorylation of the ribosomal S6 protein were observed. These changes were also observed in rats dosed with either rosiglitazone or fenofibrate. However, the protein changes were stronger (Egr-1 induction) or of a longer duration (S6 phosphorylation) in ragaglitazar-treated animals. Animals co-administered fenofibrate (a specific PPARalpha agonist) and rosiglitazone (a specific PPARgamma agonist) exhibited Egr-1 and S6 protein changes more similar to those induced by ragaglitazar (a dual-acting PPARalpha/gamma agonist) than either fenofibrate or rosiglitazone alone. The findings suggest that ragaglitazar causes Egr-1, c-Jun and S6 protein changes in the urothelium by a mechanism involving PPARalpha as well as PPARgamma, and that the Egr-1, c-Jun and S6 protein changes might have potential biomarker value.

Troglitazone enhances tamoxifen-induced growth inhibitory activity of MCF-7 cells

Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands have been identified as a potential source of therapy for human cancers. However, PPARgamma ligands have a limitation for breast cancer therapy, since estrogen receptor alpha (ER(alpha)) negatively interferes with PPARgamma signaling in breast cancer cells. Here we show that ER(alpha) inhihits PPARgamma transactivity and ER(alpha)-mediated inhibition of PPARgamma transactivity is blocked by tamoxifen, an estrogen receptor blocker. The activation of ER(alpha) with 17-beta-estradiol blocked PPRE transactivity induced by troglitazone, a PPARgamma ligand, indicating the resistance of ER(alpha)-positive breast cancer cells to troglitazone. Indeed, troglitazone inhibited the growth of ER(alpha)-negative MDA-MB-231 cells more than that of ER(alpha)-positive MCF-7 cells. Combination of troglitazone with tamoxifen led to a marked increase in growth inhibition of ER(alpha)-positive MCF-7 cells compared to either agent alone. Our data indicates that troglitazone enhances the growth inhibitory activity of tamoxifen in ER(alpha)-positive MCF-7 cells.

Involvement of PPARα in the growth inhibitory effect of arachidonic acid on breast cancer cells

Epidemiological studies suggest that dietary PUFA may influence breast cancer progression.n-3 PUFA are generally known to exert antitumour effects, whereas reports relative ton-6 PUFA anti-carcinogen effects are controversial. Arachidonic acid (AA; 20 : 4n − 6) and its metabolites have been shown to inhibit the growth of human breast cancer cell lines, even if the downstream mechanisms by which AA may influence carcinogenesis remain unresolved. We explored the molecular basis for AA influence on proliferation, signal transduction and apoptosis in two human breast cancer cell lines, MCF-7 and MDA-MB-231. In both cell lines AA inhibited cell growth in a dose-dependent manner, even if MDA-MB-231 was somewhat more growth-inhibited than MCF-7. AA decreased extracellular signal-regulated protein kinase 1/2 phosphorylation level, and positively modulated PPARγ and PPARα expression, with only a slight effect against PPARβ/δ. In addition, AA increased Bak (an apoptosis-regulating protein) expression and reduced procaspase-3 and -9 levels only in MDA-MB-231 cells, thus indicating that the growth inhibitory effect can be correlated with apoptosis induction. In both cell lines the use of a specific antagonist made it possible to establish a relationship between AA growth inhibitory effect and PPARα involvement. AA decreases cell proliferation most likely by inducing apoptosis in MDA-MB-231 cells, while in the MCF-7 cell line the growth inhibitory activity can be attributed to the inhibition of the signal transduction pathway involved in cell proliferation. In both cases, the results here presented suggest PPARα as a possible contributor to the growth inhibitory effect of AA.

Rosiglitazone attenuates insulin-like growth factor 1 receptor survival signaling in PC-3 cells

PPARgamma, a member of the peroxisome proliferator-activated receptor family, is overexpressed in prostate cancer. Natural and synthetic ligands of PPARgamma via genomic and nongenomic actions promote cell cycle arrest and apoptosis of several prostate cancer cells, in vitro. Insulin-like growth factor 1 (IGF-1) inhibits the adriamycin-induced apoptosis of PC-3 human prostate cancer cells. Therefore, we have analyzed the ability of two PPARgamma ligands,15dPGJ2 and rosiglitazone, a natural and a synthetic PPARgamma ligand, respectively, to increase the adriamycin-induced cytotoxicity of PC-3 cells and to suppress the IGF-1 survival effect on adriamycin-induced apoptosis of PC-3 cells. Our data revealed that both the PPARgamma ligands increased the adriamycin-induced cytostasis of PC-3 cells, however, only rosiglitazone added to the adriamycin-induced apoptosis of PC-3 cells. In addition, rosiglitazone attenuated the type I IGF receptor (IGF-1R) survival signaling on adriamycin-induced apoptosis of PC-3 cells via its nongenomic action on ERK1/2 and AKT phosphorylation. Because the IGF-1R signaling is probably the most important host tissue (bone) metastasis microenvironment-related survival signaling for prostate cancer cells, we conclude that rosiglitazone effects on IGF-1R-mediated activation of ERK1/2 and AKT could have clinical implications for the management of androgen ablation-refractory and chemotherapy-resistant advanced prostate cancer with bone metastasis.

Fat and beyond: the diverse biology of PPARgamma

The nuclear receptor PPARgamma is a ligand-activated transcription factor that plays an important role in the control of gene expression linked to a variety of physiological processes. PPARgamma was initially characterized as the master regulator for the development of adipose cells. Ligands for PPARgamma include naturally occurring fatty acids and the thiazolidinedione (TZD) class of antidiabetic drugs. Activation of PPARgamma improves insulin sensitivity in rodents and humans through a combination of metabolic actions, including partitioning of lipid stores and the regulation of metabolic and inflammatory mediators termed adipokines. PPARgamma signaling has also been implicated in the control of cell proliferation, atherosclerosis, macrophage function, and immunity. Here, we review recent advances in our understanding of the diverse biological actions of PPARgamma with an eye toward the expanding therapeutic potential of PPARgamma agonist drugs.

Immunostaining for peroxisome proliferator gamma distinguishes dedifferentiated liposarcoma from other retroperitoneal sarcomas

Dedifferentiated liposarcoma can be readily diagnosed by the juxtaposition of a well-differentiated liposarcoma to a nonlipogenic sarcoma. However, if the lipogenic component is not abundant due to surgical sampling or small biopsy, dedifferentiated liposarcoma can be difficult to distinguish from other poorly different sarcomas. Peroxisome proliferator-activated receptor gamma (PPAR-gamma) is a nuclear hormone receptor that plays a critical role in adipocyte differentiation. Prior studies have not only demonstrated PPAR-gamma mRNA in various subtypes of liposarcoma but have also shown that adipocyte differentiation can be induced in some liposarcomas by a PPAR-gamma agonist. In the present study, we investigated whether immunostaining for PPAR-gamma can be used to distinguish dedifferentiated liposarcoma from other retroperitoneal sarcomas. We examined a series of 40 dedifferentiated liposarcoma and compared the staining for PPAR-gamma to a series of 24 retroperitoneal sarcomas that lacked lipogenic differentiation. A monoclonal antibody against PPAR-gamma was used to stain formalin-fixed paraffin-embedded tissue. Specific nuclear immunostaining was present in 37/40 (93%) of the dedifferentiated liposarcoma and 6/24 (25%) of the other sarcomas (two leiomyosarcomas and four undifferentiated sarcomas). Interestingly, immunostaining for CDK4 and/or MDM2 was identified in three of the four PPAR-gamma-positive undifferentiated sarcomas, raising the possibility that these may represent dedifferentiated liposarcoma. This is the first study demonstrating the utility of PPAR-gamma immunohistochemistry in the diagnosis of dedifferentiated liposarcoma in tissue sections. Although not completely specific, the presence of PPAR-gamma staining, in combination with histologic findings and other markers, can aid in the diagnosis of dedifferentiated liposarcoma, particularly on small biopsies that may not sample the well-differentiated component.

Minireview: nuclear receptors and breast cancer

Until recently, the study of nuclear receptor (NR) function in breast cancer biology has been largely limited to estrogen and progesterone receptors. The development of reliable gene expression arrays, real-time quantitative RT-PCR, and immunohistochemical techniques for studying NR superfamily members in primary human breast cancers has now revealed the presence and potential importance of several additional NRs in the biology of breast cancer. These include receptors for steroid hormones (including androgens and corticosteroids), fat-soluble vitamins A and D, fatty acids, and xenobiotic lipids derived from diet. It is now clear that after NR activation, both genomic and nongenomic NR pathways can coordinately activate growth factor signaling pathways. Advances in our understanding of both NR functional networks and epithelial cell growth factor signaling pathways have revealed a frequent interplay between NR and epithelial cell growth factor family signaling that is clinically relevant to breast cancer. Understanding how growth factor receptors and their downstream kinases are activated by NRs (and vice-versa) is a central goal for maximizing treatment opportunities in breast cancer. In addition to the estrogen receptor, it is predicted that modulating the activity of other NRs will soon provide novel prevention and treatment approaches for breast cancer patients.

COX-2 and PPARgamma expression are potential markers of recurrence risk in mammary duct carcinoma in-situ

BACKGROUND

In women with duct carcinoma in-situ (DCIS) receiving breast conservation therapy (BCT), in-breast recurrences are seen in approximately 10%, but cannot be accurately predicted using clinical and histological criteria. We performed a case-control study to identify protein markers of local recurrence risk in DCIS.

METHODS

Women treated for DCIS with BCT, who later developed in-breast recurrence (cases) were matched by age and year of treatment to women who remained free of recurrence (controls).

RESULTS

A total of 69 women were included in the study, 31 cases and 38 controls. Immunohistochemical evaluation of DCIS tissue arrays was performed for estrogen receptor, progesterone receptor, HER-2/neu, cyclin D1, p53, p21, cycloxygenase-2 (COX-2) and peroxisome proliferator activated receptor gamma (PPARgamma). Two markers were significantly different between cases and controls on univariate analysis: strong COX-2 expression was associated with increased risk of recurrence, with 67% vs. 24% positivity in cases and controls p = 0.006; and nuclear expression of PPARgamma was associated with protection from recurrence with 4% vs. 27% positivity in cases and controls, p = 0.024. In a multivariate model which included size, grade, COX-2 and PPARgamma positivity, we found COX-2 positivity to be a strong independent risk factor for recurrence (OR 7.90, 95% CI 1.72-36.23)., whereas size and grade were of borderline significance. PPARgamma expression continued to demonstrate a protective trend, (OR 0.14, 95% CI 0.06-1.84).

CONCLUSION

Our findings suggest that COX-2 and PPARgamma should be investigated further as biologic markers to predict DCIS recurrence, particularly since they are also potential therapeutic targets.

Disruption of ERalpha signalling pathway by PPARgamma agonists: evidences of PPARgamma-independent events in two hormone-dependent breast cancer cell lines

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor that can be activated by natural ligands such as 15-deoxy-delta(12,14)-prostaglandin J2 (15d-PGJ(2)) as well as synthetic drugs such as thiazolidinediones. The treatment of human breast cancer cell lines with PPARgamma agonists is known to have antiproliferative effects but the role of PPARgamma activation in the process remains unclear. In the present study, we investigated the effects of four PPARgamma agonists, Rosiglitazone (RGZ), Ciglitazone (CGZ), Troglitazone (TGZ) and the natural agonist 15d-PGJ(2), on estrogen receptor alpha (ERalpha) signalling pathway in two hormone-dependent breast cancer cell lines, MCF-7 and ZR-75-1. In both of them, TGZ, CGZ and 15d-PGJ(2) induced an inhibition of ERalpha signalling associated with the proteasomal degradation of ERalpha. ZR-75-1 cells were more sensitive than MCF-7 cells to these compounds. Treatments that induced ERalpha degradation inhibited cell proliferation after 24 h. In contrast, 24 h exposure to RGZ, the most potent activator of PPARgamma disrupted neither ERalpha signalling nor cell proliferation. 9-cis retinoic acid never potentiated the proteasomal degradation of ERalpha. PPARgamma antagonists (T0070907, BADGE and GW 9662) did not block the proteolysis of ERalpha in MCF-7 and ZR-75-1 cells treated with TGZ. ERalpha proteolysis still occurred in case of PPARgamma silencing as well as in case of treatment with the PPARgamma-inactive compound Delta2-TGZ, demonstrating a PPARgamma-independent mechanism. The use of thiazolidinedione derivatives able to trigger ERalpha degradation by a PPARgamma-independent pathway could be an interesting tool for breast cancer therapy.

A Role for the PPARgamma in Cancer Therapy

In 1997, the first published reports highlighted PPARgamma as a novel cancer therapeutic target regulating differentiation of cancer cells. A subsequent flurry of papers described these activities more widely and fuelled further enthusiasm for differentiation therapy, as the ligands for the PPARgamma were seen as well tolerated and in several cases well-established in other therapeutic contexts. This initial enthusiasm and promise was somewhat tempered by contradictory findings in several murine cancer models and equivocal trial findings. As more understanding has emerged in recent years, a renaissance has occurred in targeting PPARgamma within the context of either chemoprevention or chemotherapy. This clarity has arisen in part through a clearer understanding of PPARgamma biology, how the receptor interacts with other proteins and signaling events, and the mechanisms that modulate its transcriptional actions. Equally greater translational understanding of this target has arisen from a clearer understanding of in vivo murine cancer models. Clinical exploitation will most likely require precise and quantifiable description of PPARgamma actions, and resolution of which targets are the most beneficial to target combined with an understanding of the mechanisms that limits its anticancer effectiveness.

Synergistic Effects of PPARgamma Ligands and Retinoids in Cancer Treatment

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily. The activation of PPARs by their specific ligands is regarded as one of the promising strategies to inhibit cancer cell growth. However, recent clinical trials targeting several common cancers showed no beneficial effect when PPAR ligands are used as a monotherapy. Retinoid X receptors (RXRs), which play a critical role in normal cell proliferation as a master regulator for nuclear receptors, preferentially form heterodimers with PPARs. A malfunction of RXRalpha due to phosphorylation by the Ras/MAPK signaling pathway is associated with the development of certain types of human malignancies. The activation of PPARgamma/RXR heterodimer by their respective ligands synergistically inhibits cell growth, while inducing apoptosis in human colon cancer cells when the phosphorylation of RXRalpha was inhibited. We herein review the synergistic antitumor effects produced by the combination of the PPAR, especially PPARgamma, ligands plus other agents, especially retinoids, in a variety of human cancers. We also focus on the phosphorylation of RXRalpha because the inhibition of RXRalpha phosphorylation and the restoration of its physiological function may activate PPAR/RXR heterodimer and, therefore, be a potentially effective and critical strategy for the inhibition of cancer cell growth.

Peroxisome proliferator-activated receptors: new players in the field of reproduction

Peroxisome proliferator-activated receptors (PPAR) are members of the nuclear hormone receptor superfamily. Synthetic ligands to one family member, PPARgamma, are currently widely used as treatment for chronic diseases such as diabetes type II and other insulin resistances, e.g. as seen in polycystic ovary syndrome (PCOS). Moreover, novel approaches employing knock-out mice demonstrated that PPARgamma seems to play a key role in placental and fetal development. This review describes recent insights into the role of PPARs in human reproduction with specific reference to infertility, placental maturation and fetal development as well as disturbed pregnancy. Further, we highlight the current knowledge on synthetic ligands to PPARgamma used as a treatment in women with PCOS.

Involvement of PPAR gamma and E-cadherin/beta-catenin pathway in the antiproliferative effect of conjugated linoleic acid in MCF-7 cells

Conjugated linoleic acid (CLA) is a naturally occurring fatty acid, which has been shown to exert beneficial effects against breast carcinogenesis. It has been reported that CLA could modulate cellular proliferation and differentiation through the activation of peroxisome proliferator-activated receptors (PPARs). Among different PPAR isotypes, PPAR gamma is involved in growth inhibition of transformed cells. Ligands of PPAR gamma are considered as potential anticancer drugs, so CLA was tested for its ability to induce PPAR gamma expression in MCF-7 breast cancer cells. The effects of CLA and of a specific synthetic PPAR gamma antagonist were evaluated on cell growth as well as on parameters responsible for cell growth regulation. We demonstrated here that CLA stimulated the expression of PPAR gamma to levels up to control and caused PPAR gamma translocation into the nucleus. Furthermore, the overexpression of PPAR gamma positively correlates with the inhibition of cell proliferation and with the modulation of ERK signaling induced by CLA; in all cases the administration of the antagonist reverted CLA effects. The PPAR-signaling pathway is connected with the beta-catenin/E-cadherin pathway, thus we evaluated CLA effects on the expression and cellular distribution of these proteins, which are involved in cell adhesion and responsible for invasive behavior. The treatment with CLA determined the up-regulation and the redistribution of beta-catenin and E-cadherin and the antagonist reverted only the effect on beta-catenin. These studies indicate that CLA regulates PPAR gamma expression by selectively acting as an agonist and may influence cell-cell adhesion and invasiveness of MCF-7 cells.

PPARgamma activation by thiazolidinediones (TZDs) may modulate breast carcinoma outcome: the importance of interplay with TGFbeta signalling

The thiazolidinediones (TZDs) are a class of synthetic antidiabetic drugs exerting its action primarily upon acti-vation of the peroxisome proliferator-activated receptor-γ (PPARγ). Given the widespread incidence of diabetes type II and lifelong exposure of these patients to TZDs, there is a possibility that chronic treatment with TZD modifies clinical phenotypes of other common human diseases, for example breast carcinoma. There is evidence that TZDs act as breast carcinoma suppression agents, at least in the in vitro and animal models. Stimulation of the PPARγ by TZDs interferes with oestrogen receptor signalling, STAT5B and NF-κB signalling cascades. On the other hand, TZDs repress TGFβ signalling, a well-known suppressor of the initial stages of breast carcinoma development. Another layer of complexity arises at the later stages of tumour development, when TGFβ acts as a tumour promoter: its overexpression is associated with poor prognosis, higher degree of tumour vascularization and metastasis. Longitudinal studies of breast carcinoma development in chronic TZD users are needed. In this review, we dissect possible interplays between chronic exposure of breast tis-sue to TZDs and TGFβ signalling and predict influence of TZD exposure on cancer-related clinical outcome.

Synergy between PPARgamma ligands and platinum-based drugs in cancer

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

Transactivation of ERalpha by Rosiglitazone induces proliferation in breast cancer cells

In the present study, we demonstrate that Rosiglitazone (Rosi), a thiazolidinedione and PPARgamma agonist, induces ERE (Estrogen Receptor Response Element) reporter activity, pS2 (an endogenous ER gene target) expression, and proliferation of ER positive breast cancer (MCF-7) cells. By performing a dose-response assay, we determined that high concentrations of Rosi inhibit proliferation, while low concentrations of Rosi induce proliferation. Using the anti-estrogen ICI, ER negative breast cancer (MDA-MB-231) cells, and a prostate cancer cell line (22Rv1) deficient in both ERalpha and PPARgamma, we determined that Rosiglitazone-induced ERE reporter activation and proliferation is through an ERalpha dependent mechanism. Rosiglitazone-induced ERE activation is also dependent on activation of the Extracellular Signal-Regulated Kinase-Mitogen Activated Protein Kinase (ERK-MAPK) pathway, since it is inhibited by co-treatment with U0126, a specific inhibitor of this pathway. We also demonstrate that when ERalpha and PPARgamma are both present, they compete for Rosi, inhibiting each others transactivation. To begin to unravel the pharmacological mechanism of Rosi-induced ER activation, sub-maximally effective concentrations of E(2) were used in combination with increasing concentrations of Rosi in luciferase reporter assays. From these assays it appears that E(2) and Rosi both activate ERalpha via similar pharmacological mechanisms. Furthermore sub-maximally effective concentrations of E(2) and Rosi additively increase both ERE reporter activity and MCF-7 cell proliferation. The results of this study may have clinical relevancy for Rosi's use both as an anti-diabetic in post-menopausal women and as an anti-cancer drug in women with ER positive breast cancer.

Peroxisome proliferator-activated receptor gamma: a novel target for cancer therapeutics?

Peroxisome proliferator-activated receptors are ligand-activated intracellular transcription factors that have been implicated in important biological processes such as inflammation, tissue remodeling and atherosclerosis. Emerging information also implicates peroxisome proliferator-activated receptors in oncogenesis. Peroxisome proliferator-activated receptor gamma, the best studied of the peroxisome proliferator-activated receptors, modulates the proliferation and apoptosis of many cancer cell types, and it is expressed in many human tumors including lung, breast, colon, prostate and bladder. Natural and synthetic agents capable of activating peroxisome proliferator-activated receptor gamma have been found to inhibit cancer cell growth in vitro and in animal models. These agents, however, are not specific and both peroxisome proliferator-activated receptor gamma-dependent and peroxisome proliferator-activated receptor gamma-independent pathways involved in their effects have been identified. Together, these studies, coupled with a few clinical trials, suggest that peroxisome proliferator-activated receptor gamma is a novel target for the development of new and effective anticancer therapies.

Pilot study of rosiglitazone therapy in women with breast cancer: effects of short-term therapy on tumor tissue and serum markers

PURPOSE

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a steroid nuclear receptor that is activated by natural compounds such as specific fatty acids and synthetic drugs such as thiazolidinedione antidiabetic agents. Expressed in normal and malignant mammary epithelial cells, activation of PPARgamma is associated with antiproliferative effects on human breast cancer cells in preclinical studies. The purpose of this study was to test the hypothesis that PPARgamma ligand therapy might inhibit tumor growth and progression in human breast cancer.

EXPERIMENTAL DESIGN

We conducted a pilot trial of short-term (2-6 weeks) treatment with the thiazolidinedione rosiglitazone in 38 women with early-stage (T(is)-T(2), N(0-1), M(0)) breast cancer, administered between the time of diagnostic biopsy and definitive surgery.

RESULTS

Short-term treatment with rosiglitazone (8 mg/d) did not elicit significant effects on breast tumor cell proliferation using Ki67 expression as a measure of cell proliferation and surrogate marker of tumor growth and progression. In pretreatment tumors notable for nuclear expression of PPARgamma by immunohistochemistry, down-regulation of nuclear PPARgamma expression occurred following rosiglitazone administration (P = 0.005). No PPARG mutations were identified, and the incidence of P12A and H446H polymorphisms did not differ relative to U.S. controls (P = 0.5). Treatment with rosiglitazone resulted in increased serum adiponectin (P < 0.001), decreased insulin levels (P = 0.005), and increased insulin sensitivity (P = 0.004). Rosiglitazone was well tolerated without serious adverse events.

CONCLUSION

Our data indicate that short-term rosiglitazone therapy in early-stage breast cancer patients leads to local and systemic effects on PPARgamma signaling that may be relevant to breast cancer.

Mechanisms of Disease: adipokines and breast cancer—endocrine and paracrine mechanisms that connect adiposity and breast cancer

A vast number of epidemiological studies suggest an important, but still controversial, role for obesity and adipose tissue mass in breast cancer risk and an association with tumor phenotype. The main conclusions from these studies raise the possibility that the adipose tissue can act as an effector organ that influences both cancer risk and tumor behavior. Here we also review heterotypic mechanisms in breast-cancer tumorigenesis; these mechanisms involve soluble secreted factors from peritumoral cells, extracellular-matrix components and interactions between stromal cells and tumor cells that create a specific and local peritumoral microenvironment. As a special focus, we discuss the increasing evidence for a role of peritumoral adipose tissue and secreted adipokines (such as adiponectin and leptin) in breast cancer; furthermore, the cellular and molecular basis of the peritumoral 'desmoplastic' tissue reaction observed in breast cancer is reviewed in detail.

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

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

Synthesis and Binding Affinity of a Fluorine-Substituted Peroxisome Proliferator-Activated Gamma (PPARγ) Ligand as a Potential Positron Emission Tomography (PET) Imaging Agent

The peroxisome proliferator-activated receptor gamma (PPARgamma) is an important regulator of lipid metabolism and the differentiation of pre-adipocytes. Thus, imaging PPARgamma in vivo using positron-emission tomography (PET) might be useful in assessing lipid metabolism disorders and identifying tumor cell differentiation. A fluorine-substituted PPARgamma ligand from tyrosine-benzophenone class, compound 1, has a very high affinity for PPARgamma receptor (Ki = 0.14 nM). To develop this compound as a PPARgamma PET imaging agent, we investigated synthetic routes suitable for its labeling with the short-lived PET radionuclide fluorine-18 (t1/2 = 110 min). To obtain the high specific activity material needed for receptor imaging with this isotope, reactions need to proceed efficiently, within a short time, starting from fluoride ion at the tracer level. The most promising approach involves introduction of fluorine into a suitable benzophenone precursor, followed by efficient coupling of this intermediate with the heterocyclic tyrosine component using a copper-catalyzed Ullmann-type condensation.