Magnitude of peroxisome proliferator-activated receptor-gamma activation is associated with important and seemingly opposite biological responses in breast cancer cells

Diabetes and Cancer: The Epidemiological and Metabolic Associations

Diabetes mellitus, commonly known as diabetes, and cancer are two of the most common diseases plaguing the world today. According to the Centers for Disease Control and Prevention (CDC), there are currently more than 20 million people with diabetes in the United States [1]. According to the International Agency for Research on Cancer (IARC), there were around 18 million people diagnosed with cancer, with approximately ten million deaths globally in 2018 [2]. Given the prevalence and deadliness of diabetes and cancer, these two diseases have long been the focus of many researchers with the goal of improving treatment outcomes. While diabetes and cancer may seem to be two very different diseases at first glance, they share several similarities, especially regarding their metabolic characteristics. This chapter discusses the similarities and relationships between the metabolism of diabetes, especially type 2 diabetes (T2D), and cancer, including their abnormal glucose and amino acid metabolism, the contribution of hyperglycemia to oncogenic mutation, and the contribution of hyperinsulinemia to cancer progression. Investigating the metabolic interplay between diabetes and cancer in an effort to exploit this connection for cancer treatment has the potential to significantly improve clinical efficacy.

Melatonin suppresses milk fat synthesis by inhibiting the mTOR signaling pathway via the MT1 receptor in bovine mammary epithelial cells

Milk fat content is an important criterion for assessing milk quality and is one of the main target traits of dairy cattle breeding. Recent studies have shown the importance of melatonin in regulating lipid metabolism, but the potential effects of melatonin on milk fat synthesis in bovine mammary epithelial cells (BMECs) remain unclear. Here, we showed that melatonin supplementation at 10 μmol/L significantly downregulated the mRNA expression of lipid metabolism-related genes and resulted in lower lipid droplet formation and triglyceride accumulation. Moreover, melatonin significantly upregulated melatonin receptor subtype melatonin receptor 1a (MT1) gene expression, and the negative effects of melatonin on milk fat synthesis were reversed by treatment with the nonselective MT1/melatonin receptor subtype melatonin receptor 1b (MT2) antagonist. However, a selective MT2 antagonist did not modify the negative effects of melatonin on milk fat synthesis. In addition, KEGG analysis revealed that melatonin inhibition of milk fat synthesis may occur via the mTOR signaling pathway. Further analysis revealed that melatonin significantly suppressed the activation of the mTOR pathway by restricting the phosphorylation of mTOR, 4E-BP1, and p70S6K, and the inhibition of melatonin on milk fat synthesis was reversed by mTOR activator MHY1485 in BMECs. Furthermore, in vivo experiments in Holstein dairy cows showed that exogenous melatonin significantly decreased milk fat concentration. Our data from in vitro and in vivo studies revealed that melatonin suppresses milk fat synthesis by inhibiting the mTOR signaling pathway via the MT1 receptor in BMECs. These findings lay a foundation to identify a new potential means for melatonin to modulate the fat content of raw milk in Holstein dairy cows.

Diabetes Mellitus and Risk of Hepatocellular Carcinoma

The occurrence of hepatocellular carcinoma (HCC) is two to three times higher in patients with diabetes mellitus (DM), the prevalence of which is increasing sharply worldwide. The purpose of this review was to describe clinical links between DM and HCC and potential biological mechanisms that may account for this association. We evaluated the role of potential pathways that could account for the development of HCC with different etiologies in the presence of DM. In addition, we also briefly discuss the potential effect of other factors such as type and dosage of antidiabetic medicines and duration of DM on HCC risk.

The Role of PPARγ in the Transcriptional Control by Agonists and Antagonists

In recent years, peroxisome proliferator-activated receptor gamma (PPARγ) has been reported to be a target for the treatment of type II diabetes. Furthermore, it has received attention for its therapeutic potential in many other human diseases, including atherosclerosis, obesity, and cancers. Recent studies have provided evidence that the endogenously produced PPARγ antagonist, 2,3-cyclic phosphatidic acid (cPA), which is similar in structure to lysophosphatidic acid (LPA), inhibits cancer cell invasion and metastasis in vitro and in vivo. We recently observed that cPA negatively regulates PPARγ function by stabilizing the binding of the corepressor protein, silencing mediator of retinoic acid and thyroid hormone receptor. We also showed that cPA prevents neointima formation, adipocyte differentiation, lipid accumulation, and upregulation of PPARγ target gene transcription. We then analyzed the molecular mechanism of cPA's action on PPARγ. In this paper, we summarize the current knowledge on the mechanism of PPARγ-mediated transcriptional activity and transcriptional repression in response to novel lipid-derived ligands, such as cPA.

Troglitazone induces extracellular matrix and cytoskeleton remodeling in mouse collecting duct cells

Peroxisome proliferator-activated receptor (PPARγ) has been shown to have a protective role in the nephron through its ability to inhibit a transforming growth factor- (TGF-β) mediated fibrotic response. In contrast, PPARγ was also shown to induce a mesenchymal transformation in epithelial intestinal cells. A fibrotic response in the collecting duct has only recently been established; however, the entire collecting duct has not been fully examined. Inner medullary collecting duct cells (IMCD-K2) and mouse cortical collecting duct cells (M1), representing the cortical and medullary collecting duct, were exposed to 5–10 μM troglitazone for 24 hours. Troglitazone resulted in an elongated morphology, 60% decreases in E-cadherin and β-catenin, a 35% decrease in α-catenin, and a 1.5-fold increase in fibronectin. These effects were not reversed with PPARγ antagonists or affected with PPARγ overexpression. Our results indicate that troglitazone induced a mesenchymal-like transformation in M1 and IMCD-K2 epithelial cells independently of PPARγ.

Peroxisome Proliferator-Activated Receptor-gamma Ligands: Potential Pharmacological Agents for Targeting the Angiogenesis Signaling Cascade in Cancer

Peroxisome proliferator-activated receptor-γ (PPAR-γ) has currently been considered as molecular target for the treatment of human metabolic disorders. Experimental data from in vitro cultures, animal models, and clinical trials have shown that PPAR-γ ligand activation regulates differentiation and induces cell growth arrest and apoptosis in a variety of cancer types. Tumor angiogenesis constitutes a multifaceted process implicated in complex downstream signaling pathways that triggers tumor growth, invasion, and metastasis. In this aspect, accumulating in vitro and in vivo studies have provided extensive evidence that PPAR-γ ligands can function as modulators of the angiogenic signaling cascade. In the current review, the crucial role of PPAR-γ ligands and the underlying mechanisms participating in tumor angiogenesis are summarized. Targeting PPAR-γ may prove to be a potential therapeutic strategy in combined treatments with conventional chemotherapy; however, special attention should be taken as there is also substantial evidence to support that PPAR-γ ligands can enhance angiogenic phenotype in tumoral cells.

The Critical Role of PPARgamma in Human Malignant Melanoma

The past 30 years have only seen slight improvement in melanoma therapy. Despite a wide variety of therapeutic options, current survival for patients with metastatic disease is only 6-8 months. Part of the reason for this treatment failure is the broad chemoresistance of melanoma, which is due to an altered survival capacity and an inactivation of apoptotic pathways. Several targetable pathways, responsible for this survival/apoptosis resistance in melanoma, have been described and current research has focused on mechanism inactivating these pathways. As PPARgamma was shown to be constitutively active in several tumour entities and PPARgamma agonists extent strong anticancer effects, the role of PPARgamma as a possible target for specific anticancer strategy was investigated in numerous studies. However, only a few studies have focused on the effects of PPARgamma agonists in melanoma, showing conflicting results. The use of PPARgamma agonists in melanoma therapy has to be carefully weighted against considerable, undesirable side effects, as their mode of action is not fully understood and even pro-proliferative effects have been described. In the current review, we discuss the role of PPARs, in particular PPARgamma in melanoma and their potential role as a molecular target for melanoma therapy.

Omega-3 Fatty Acids and PPARgamma in Cancer

Omega-3 (or n-3) polyunsaturated fatty acids (PUFAs) and their metabolites are natural ligands for peroxisome proliferator receptor activator (PPAR)gamma and, due to the effects of PPARgamma on cell proliferation, survival, and differentiation, are potential anticancer agents. Dietary intake of omega-3 PUFAs has been associated with a reduced risk of certain cancers in human populations and in animal models. In vitro studies have shown that omega-3 PUFAs inhibit cell proliferation and induce apoptosis in cancer cells through various pathways but one of which involves PPARgamma activation. The differential activation of PPARgamma and PPARgamma-regulated genes by specific dietary fatty acids may be central to their distinct roles in cancer. This review summarizes studies relating PUFAs to PPARgamma and cancer and offers a new paradigm relating an n-3 PUFA through PPARgamma to the expression of the cell surface proteoglycan, syndecan-1, and to the death of cancer cells.

Specific thiazolidinediones inhibit ovarian cancer cell line proliferation and cause cell cycle arrest in a PPARγ independent manner

BACKGROUND

Peroxisome Proliferator Activated Receptor gamma (PPARγ) agonists, such as the thiazolinediones (TZDs), have been studied for their potential use as cancer therapeutic agents. We investigated the effect of four TZDs--Rosiglitazone (Rosi), Ciglitazone (CGZ), Troglitazone (TGZ), and Pioglitazone (Pio)--on ovarian cancer cell proliferation, PPARγ expression and PPAR luciferase reporter activity. We explored whether TZDs act in a PPARγ dependent or independent manner by utilizing molecular approaches to inhibit or overexpress PPARγ activity.

PRINCIPAL FINDINGS

Treatment with CGZ or TGZ for 24 hours decreased proliferation in three ovarian cancer cell lines, Ovcar3, CaOv3, and Skov3, whereas Rosi and Pio had no effect. This decrease in Ovcar3 cell proliferation was due to a higher fraction of cells in the G(0)/G(1) stage of the cell cycle. CGZ and TGZ treatment increased apoptosis after 4 hours of treatment but not after 8 or 12 hours. Treatment with TGZ or CGZ increased PPARγ mRNA expression in Ovcar3 cells; however, protein levels were unchanged. Surprisingly, luciferase promoter assays revealed that none of the TZDs increased PPARγ activity. Overexpression of wild type PPARγ increased reporter activity. This was further augmented by TGZ, Rosi, and Pio indicating that these cells have the endogenous capacity to mediate PPARγ transactivation. To determine whether PPARγ mediates the TZD-induced decrease in proliferation, cells were treated with CGZ or TGZ in the absence or presence of a dominant negative (DN) or wild type overexpression PPARγ construct. Neither vector changed the TZD-mediated cell proliferation suggesting this effect of TZDs on ovarian cancer cells may be PPARγ independent.

CONCLUSIONS

CGZ and TGZ cause a decrease in ovarian cancer cell proliferation that is PPARγ independent. This concept is supported by the finding that a DN or overexpression of the wild type PPARγ did not affect the changes in cell proliferation and cell cycle.

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.

Thiazolidinediones alter growth and epithelial cell integrity, independent of PPAR-γ and MAPK activation, in mouse M1 cortical collecting duct cells

Peroxisome proliferator-activated receptor (PPAR)-γ is highly expressed in the collecting duct (CD), yet little is known about the effects of PPAR-γ ligands, thiazolidinediones (TZDs), on CD cell structure and function. M1 mouse cortical CD cells were treated with 5 μM troglitazone (TRO) and rosiglitazone (ROSI). First, growth was measured by [(3)H]thymidine and [(3)H]leucine incorporation, as well as analysis of cyclin D1 and the CDK inhibitor p27 by Western blot. [(3)H]thymidine incorporation was reduced by 56 and 24% by TRO and ROSI at 6 h, and [(3)H]leucine by 21 and 10%. A similar growth inhibition was also observed after 24 h for thymidine, but leucine was reduced by 48 and 24%, respectively. Likewise, cyclin D1 was diminished 60% by TRO, and p27 was elevated 1.6- and 1.7-fold in response to TRO and ROSI. Next, epithelial cell integrity was assessed by measuring different markers by Western blot analysis. While fibronectin and α-smooth muscle actin levels were unchanged, by 24 h E-cadherin was decreased by 50%, and β-catenin levels were reduced 2- and 1.5-fold in response to TRO and ROSI, respectively. GW9662, a PPAR-γ antagonist, did not reverse any of the TZD responses in M1 cells. Of interest, phosho-p38 levels were also elevated 2-fold in response to TRO and 2.3-fold to ROSI, but MAPK inhibition by PD98059 or SB203580 caused an additive inhibition of cell growth and did not alter E-cadherin or β-catenin in response to TZDs. Finally, apoptotic death was assessed by Western blot, but cleaved caspase-3 levels were unchanged from 15 min to 24 h in response to TZDs, and TRO did not affect cell viability or reactive oxygen species generation. Our data suggest that TZDs cause a disruption of M1 cell integrity that is preceded by an inhibition of cell growth. This response is independent of p38 or PPAR-γ activation.

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.

PPARs in Irradiation-Induced Gastrointestinal Toxicity

The use of radiation therapy to treat cancer inevitably involves exposure of normal tissues. Although the benefits of this treatment are well established, many patients experience distressing complications due to injury to normal tissue. These side effects are related to inflammatory processes, and they decrease therapeutic benefit by increasing the overall treatment time. Emerging evidence indicates that PPARs and their ligands are important in the modulation of immune and inflammatory reactions. This paper discusses the effects of abdominal irradiation on PPARs, their role and functions in irradiation toxicity, and the possibility of using their ligands for radioprotection.

Prognostic and therapeutic potential of nuclear receptors in head and neck squamous cell carcinomas

Head and neck squamous cell carcinomas are among the most common neoplasms worldwide and characterized by local tumor aggressiveness, high rate of early recurrences, development of metastasis, and second primary cancers. Despite modern therapeutic strategies and sophisticated surgical management, overall survival-rates remained largely unchanged over the last decades. Thus, the need for novel treatment options for this tumor entity is undeniable. A key event in carcinogenesis is the uncontrolled modulation of genetic programs. Nuclear receptors belong to a large superfamily of transcription factors implicated in a broad spectrum of physiological and pathophysiological processes, including cancer. Several nuclear receptors have also been associated with head and neck cancer. This review will summarize their mode of action, prognostic/therapeutic relevance, as well as preclinical and clinical studies currently targeting nuclear receptors in this tumor entity.

Peroxisome proliferator-activated receptor-γ is essential in the pathogenesis of gastric carcinoma

AIM: To investigate whether peroxisome proliferator-activated receptor γ (PPAR-γ) is expressed in human gastric carcinoma and whether PPAR-γ is a potential target for gastric carcinoma therapy.

METHODS: PPAR-γ protein in gastric carcinoma was examined by immunohistochemistry. In the gastric carcinoma cell line MGC803, PPAR-γ, survivin, Skp2 and p27 protein and mRNA were examined by Western blotting and real-time reverse transcription-polymerase chain reaction, respectively; proliferation was examined by MTT; apoptosis was examined by chromatin staining with Hoechst 33342 and fluorescence activated cell sorting (FACS). and cell cycle was examined by FACS; the knockdown of PPAR-γ was done by RNA interference.

RESULTS: A high level of expression of PPAR-γ was observed in human gastric carcinoma and in a human gastric carcinoma cell line MGC803. The PPAR-γ agonist 15-deoxy-Δ12,14-prostaglandin J₂ (15d-PGJ₂) inhibited growth, and induced apoptosis and G₁/G₀ cell cycle arrest in MGC803 cells in a concentration-dependent and time-dependent manner. The effect of 15d-PGJ₂ on MGC803 cells was not reversed by the selective and irreversible antagonist GW9662 for PPAR-γ. Furthermore, survivin and Skp2 expression were decreased, whereas p27 expression was enhanced following 15d-PGJ₂ treatment in a dose-dependent manner in MGC803 cells. Interestingly, we also found that small interfering RNA for PPAR-γ inhibited growth and induced apoptosis in MGC803 cells. The inhibition of PPAR-γ function may be a potentially important and novel modality for treatment and prevention of gastric carcinoma.

CONCLUSION: A PPAR-γ agonist inhibited growth of human gastric carcinoma MGC803 cells by inducing apoptosis and G₁/G₀ cell cycle arrest with the involvement of survivin, Skp2 and p27 and not via PPAR-γ.

Rat Urinary Bladder Carcinogenesis by Dual-Acting PPARalpha + gamma Agonists

Despite clinical promise, dual-acting activators of PPARalpha and gamma (here termed PPARalpha+gamma agonists) have experienced high attrition rates in preclinical and early clinical development, due to toxicity. In some cases, discontinuation was due to carcinogenic effect in the rat urothelium, the epithelial layer lining the urinary bladder, ureters, and kidney pelvis. Chronic pharmacological activation of PPARalpha is invariably associated with cancer in rats and mice. Chronic pharmacological activation of PPARgamma can in some cases also cause cancer in rats and mice. Urothelial cells coexpress PPARalpha as well as PPARgamma, making it plausible that the urothelial carcinogenicity of PPARalpha+gamma agonists may be caused by receptor-mediated effects (exaggerated pharmacology). Based on previously published mode of action data for the PPARalpha+gamma agonist ragaglitazar, and the available literature about the role of PPARalpha and gamma in rodent carcinogenesis, we propose a mode of action hypothesis for the carcinogenic effect of PPARalpha+gamma agonists in the rat urothelium, which combines receptor-mediated and off-target cytotoxic effects. The proposed mode of action hypothesis is being explored in our laboratories, towards understanding the human relevance of the rat cancer findings, and developing rapid in vitro or short-term in vivo screening approaches to faciliate development of new dual-acting PPAR agonist compounds.

Down-regulation of PPARgamma1 suppresses cell growth and induces apoptosis in MCF-7 breast cancer cells

BACKGROUND

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear hormone receptor superfamily and is highly expressed in many human tumors including breast cancer. PPARgamma has been identified as a potential target for breast cancer therapy based on the fact that its activation by synthetic ligands affects the differentiation, proliferation, and apoptosis of cancer cells. However, the controversial nature of current studies and disappointing results from clinical trials raise questions about the contribution of PPARgamma signaling in breast cancer development in the absence of stimulation by exogenous ligands. Recent reports from both in vitro and in vivo studies are inconsistent and suggest that endogenous activation of PPARgamma plays a much more complex role in initiation and progression of cancer than previously thought.

RESULTS

We have previously demonstrated that an increase in expression of PPARgamma1 in MCF-7 breast cancer cells is driven by a tumor-specific promoter. Myc-associated zinc finger protein (MAZ) was identified as a transcriptional mediator of PPARgamma1 expression in these cells. In this study, using RNA interference (RNAi) to inhibit PPARgamma1 expression directly or via down-regulation of MAZ, we report for the first time that a decrease in PPARgamma1 expression results in reduced cellular proliferation in MCF-7 breast cancer cells. Furthermore, we demonstrate that these changes in proliferation are associated with a significant decrease in cell transition from G1 to the S phase. Using a dominant-negative mutant of PPARgamma1, Delta462, we confirmed that PPARgamma1 acts as a pro-survival factor and showed that this phenomenon is not limited to MCF-7 cells. Finally, we demonstrate that down-regulation of PPARgamma1 expression leads to an induction of apoptosis in MCF-7 cells, confirmed by analyzing Bcl-2 expression and PARP-1 cleavage.

CONCLUSION

Thus, these findings suggest that an increase in PPARgamma1 signaling observed in breast cancer contributes to an imbalance between proliferation and apoptosis, and may be an important hallmark of breast tumorigenesis. The results presented here also warrant further investigation regarding the use of PPARgamma ligands in patients who are predisposed or already diagnosed with breast cancer.

Peroxisome proliferator-activated receptor ligand MCC-555 suppresses intestinal polyps in ApcMin/+ mice via extracellular signal-regulated kinase and peroxisome proliferator-activated receptor-dependent pathways

A large body of studies has suggested that peroxisome proliferator-activated receptor gamma (PPARgamma) ligands, such as thiazolidinedione, are potent candidates for chemopreventive agents. MCC-555 is a PPARgamma/alpha dual agonist and has been shown previously to induce apoptosis in vitro; however, the molecular mechanisms by which MCC-555 affects antitumorigenesis in vivo are poorly understood. In this study, we explored the antitumorigenic effects of MCC-555 both in cell culture and in Apc-deficient mice, an animal model for human familial adenomatous polyposis. MCC-555 increased MUC2 expression in colorectal and lung cancer cells, and treatment with the PPARgamma antagonist GW9662 revealed that MUC2 induction by MCC-555 was mediated in a PPARgamma-dependent manner. Moreover, MCC-555 increased transcriptional activity of human and mouse MUC2 promoters. Subsequently, treatment with MCC-555 (30 mg/kg/d) for 4 weeks reduced the number of small intestinal polyps to 54.8% of that in control mice. In agreement with in vitro studies, enhanced Muc2 expression was observed in the small intestinal tumors of Min mice treated with MCC-555, suggesting that MUC2 expression may be associated at least in part with the antitumorigenic action of MCC-555. In addition, highly phosphorylated extracellular signal-regulated kinase (ERK) was found in the intestinal tumors of MCC-555-treated Min mice, and inhibition of the ERK pathway by a specific inhibitor markedly suppressed MCC-555-induced Muc2 expression in vitro. Overall, these results indicate that MCC-555 has a potent tumor suppressor activity in intestinal tumorigenesis, likely involving MUC2 up-regulation by ERK and PPARgamma pathways.

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.

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.

The non-genomic crosstalk between PPAR-gamma ligands and ERK1/2 in cancer cell lines

Peroxisome proliferator activated receptors (PPARs) are members of the nuclear receptor superfamily acting as transcription factors. PPAR-gamma, one of the three PPAR subtypes, is expressed in many malignant and non-malignant cells and tissues. PPAR-gamma ligands influence cancer biology via both genomic as well as non-genomic events. The non-genomic action of PPAR-gamma ligands, including the activation of MAPK signaling pathways, is under intense investigation. In the presence of PPAR-gamma ligands, a rapid phosphorylation of ERK1/2 is observed in many cancer cell lines. Activated ERK1/2 elicits rapid, non-genomic cellular effects and can directly repress PPAR-gamma transcriptional activity by phosphorylation. This paper reviews the interrelation of PPAR-gamma ligands and activated ERK1/2, in relation to their antineoplastic actions in cancer cell lines, which may offer the potential for improved anticancer therapies.

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.

Troglitazone and pioglitazone interactions via PPAR-γ-independent and -dependent pathways in regulating physiological responses in renal tubule-derived cell lines

Troglitazone (Tro) and pioglitazone (Pio) activation of peroxisome proliferator-activated receptor (PPAR)-γ and PPAR-γ-independent pathways was studied in cell lines derived from porcine renal tubules. PPAR-γ-dependent activation of PPAR response element-driven luciferase gene expression was observed with Pio at 1 μM but not Tro at 1 μM. On the other hand, PPAR-γ-independent P-ERK activation was observed with 5 μM Tro but not with Pio (5–20 μM). In addition, Pio (1–10 μM) increased metabolic acid production and activated AMP-activated protein kinase (AMPK) associated with decreased mitochondrial membrane potential, whereas Tro (1–20 μM) did not. These results are consistent with three pathways through which glitazones may act in effecting metabolic processes (ammoniagenesis and gluconeogenesis) as well as cellular growth: 1) PPAR-γ-dependent and PPAR-γ-independent pathways, 2) P-ERK activation, and 3) mitochondrial AMPK activation. The pathways influence cellular acidosis and glucose and glutamine metabolism in a manner favoring reduced plasma glucose in vivo. In addition, significant interactions can be demonstrated that enhance some physiological processes (ammoniagenesis) and suppress others (ligand-mediated PPAR-γ gene expression). Our findings provide a model both for understanding seemingly opposite biological effects and for enhancing therapeutic potency of these agents.

Cyclooxygenase-2 in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer and the third leading cause of cancer related mortality worldwide. The incidence of HCC is rising worldwide, especially in the United States. The overall survival of patients with HCC is grim and currently no efficient secondary prevention or systemic treatments are available. Recent evidence suggests that COX-2 signaling is implicated in hepatocarcinogenesis and COX-2 inhibitors prevent HCC cell growth in vitro and in animal models. However, given the recently reported side effect associated with some of the COX-2 inhibitors, it is imperative to develop chemotherapeutic strategy that simultaneously targets COX-2 and other related key molecules in hepatocarcinogenesis or to utilize agents inhibiting COX-2 signaling in conjunction with other standard chemotherapy or radiation therapy. Such combinational therapeutic approaches are expected to provide synergistic anti-tumor effect with lesser side effect. In this regard, the recently delineated interplay between COX-2-derived PG signaling and other growth-regulatory pathways such as EGFR, Met, iNOS, VEGF and n-3 polyunsaturated fatty acids is expected to provide important therapeutic implications. This review summarizes the recent advances in understanding the mechanisms for COX-2-derived PG signaling in hepatocarcinogenesis and focuses on the newly unveiled interactions between PG cascade and other key signaling pathways that coordinately regulate HCC growth. Understanding these mechanisms and interplays will facilitate the development of more effective chemopreventive and therapeutic strategies.

Inhibitors of arachidonic acid metabolism act synergistically to signal apoptosis in neoplastic cells

Arachidonic acid (AA) and its metabolites are intimately linked to carcinogenesis. Inhibitors of AA metabolic enzymes have demonstrated anti-carcinogenic effects in vivo and induce apoptosis of many cancer cell lines in vitro. The mechanism by which AA influences carcinogenesis, however, remains unresolved. The current study explores the growth inhibitory potential of Triacsin C, PLT-98625, and NS-398 which inhibit three distinct metabolic enzymes that control intracellular AA levels: fatty acid coenzyme-A ligase 4 (FACL-4), coenzyme-A independent transacylase (CoA-IT), and cyclooxygenase (COX), respectively. Results reveal the anti-proliferative effects of these inhibitors in a number of human cancer cell lines. Further studies in the SK-MES-1 cell line demonstrate that all three inhibitors induce accumulation of unesterified AA which correlates with induction of apoptosis. Addition of exogenous AA also induces apoptosis. Furthermore, in combination, these inhibitors act cooperatively to induce AA accumulation which correlates to a synergistic reduction in cell viability. Taken together, these results suggest that accumulation of unesterified AA is a common mechanism in the induction of cancer cell apoptosis by various inhibitors of AA metabolism, confirm that previously described AA remodeling pathways are valid in cancer cells, and indicate that combination treatment strategies utilizing these inhibitors may represent a novel approach to blocking cancer cell growth. Further study is required to determine the downstream pathway(s) whereby high cellular burdens of unesterified AA promote apoptosis.

Cyclooxygenase-2 and prostaglandin signaling in cholangiocarcinoma

Cholangiocarcinoma is a highly malignant epithelial neoplasm arising within the biliary tract and its incidence and mortality is rising. Early diagnosis is difficult and there is presently no effective treatment. Significant progress has been made over the past several years in defining the link between COX-2 and cholangiocarcinogenesis. Selective COX-2 inhibitors have been shown to inhibit cholangiocarcinoma cell growth in vitro and in animal models. However, recently, concerns have been raised about the cardiovascular side effect associated with some COX-2 inhibitors utilized at relatively high dose for antitumor chemoprevention, despite that these inhibitors have a proven safety profile when given as monotherapy to arthritis patients. Therefore, there is an urgent and practical need to develop novel chemopreventive strategy that simultaneously targets COX-2 signaling and other related key molecules in cholangiocarcinogenesis, such as EGFR or utilization of agents inhibiting COX-2 signaling in conjunction with other standard chemotherapy or radiation therapy; these approaches are expected to provide synergistic anti-tumor effect with lesser side effect. In this context, the recently delineated interplay between COX-2-derived PG signaling and other growth-regulatory pathways, such as EGFR, ErbB2, IL-6/GP130, HGF/Met, TGF-beta/Smad, and iNOS is expected to provide important therapeutic implications. This review will summarize the recent advances in understanding the mechanisms for COX-2-derived PG signaling in cholangiocarcinogenesis and focus on the newly unveiled interactions between PG cascade and other key signaling pathways that coordinately regulate cholangiocarcinoma growth. Knowledge on these aspects will help develop more effective therapeutic strategy targeting COX-2 and related key signaling molecules.

Cyclooxygenase-2 and prostaglandin signaling in cholangiocarcinoma

Cholangiocarcinoma is a highly malignant epithelial neoplasm arising within the biliary tract and its incidence and mortality is rising. Early diagnosis is difficult and there is presently no effective treatment. Significant progress has been made over the past several years in defining the link between COX-2 and cholangiocarcinogenesis. Selective COX-2 inhibitors have been shown to inhibit cholangiocarcinoma cell growth in vitro and in animal models. However, recently, concerns have been raised about the cardiovascular side effect associated with some COX-2 inhibitors utilized at relatively high dose for antitumor chemoprevention, despite that these inhibitors have a proven safety profile when given as monotherapy to arthritis patients. Therefore, there is an urgent and practical need to develop novel chemopreventive strategy that simultaneously targets COX-2 signaling and other related key molecules in cholangiocarcinogenesis, such as EGFR or utilization of agents inhibiting COX-2 signaling in conjunction with other standard chemotherapy or radiation therapy; these approaches are expected to provide synergistic anti-tumor effect with lesser side effect. In this context, the recently delineated interplay between COX-2-derived PG signaling and other growth-regulatory pathways, such as EGFR, ErbB2, IL-6/GP130, HGF/Met, TGF-beta/Smad, and iNOS is expected to provide important therapeutic implications. This review will summarize the recent advances in understanding the mechanisms for COX-2-derived PG signaling in cholangiocarcinogenesis and focus on the newly unveiled interactions between PG cascade and other key signaling pathways that coordinately regulate cholangiocarcinoma growth. Knowledge on these aspects will help develop more effective therapeutic strategy targeting COX-2 and related key signaling molecules.

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

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

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

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

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

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

PPARgamma expression in breast cancer: clinical value and correlation with ERbeta

AIMS

To examine the expression of peroxisome proliferator-activated receptor gamma (PPARgamma) in invasive breast carcinoma in relation to known clinicopathological features, ERbeta, and relapse-free and overall patient survival. PPARgamma is a ligand-activated transcriptional factor that regulates the transcription of various target genes and has been implicated in human breast cancer.

METHODS AND RESULTS

We performed immunohistochemistry to detect PPARgamma, ERalpha, PR and ERbeta in 170 infiltrative breast carcinomas. The results were subjected to statistical analysis. PPARgamma was detected in the cytoplasm of 58% of breast carcinoma samples. PPARgamma did not differ with regard to any of the clinicopathological parameters except for histological grade, to which it was found to be inversely correlated (P = 0.019), and ERbeta, to which it was positively related (P = 0.016). As regards relapse-free survival, in univariate statistical analysis PPARgamma was found to exert a marginally favourable impact on all the patients (P = 0.076), but a strong one on patients with ductal carcinoma (P = 0.027), whereas Cox's regression analysis depicted PPARgamma to be an independent prognosticator for patients with ductal carcinoma (P = 0.039). No association was found between PPARgamma expression and overall survival.

CONCLUSION

These results indicate the favourable impact of PPARgamma expression on disease-free survival of patients with ductal breast carcinoma and its possible cooperation with ERbeta in exerting that favourable effect.

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

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

GW9662, a potent antagonist of PPARgamma, inhibits growth of breast tumour cells and promotes the anticancer effects of the PPARgamma agonist rosiglitazone, independently of PPARgamma activation

Peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the nuclear receptor superfamily, is activated by several compounds, including the thiazolidinediones. In addition to being a therapeutic target for obesity, hypolipidaemia and diabetes, perturbation of PPARgamma signalling is now believed to be a strategy for treatment of several cancers, including breast. Although differential expression of PPARgamma is observed in tumours compared to normal tissues and PPARgamma agonists have been shown to inhibit tumour cell growth and survival, the interdependence of these observations is unclear. This study demonstrated that the potent, irreversible and selective PPARgamma antagonist GW9662 prevented activation of PPARgamma and inhibited growth of human mammary tumour cell lines. Controversially, GW9662 prevented rosiglitazone-mediated PPARgamma activation, but enhanced rather than reversed rosiglitazone-induced growth inhibition. As such, these data support the existence of PPARgamma-independent pathways and question the central belief that PPARgamma ligands mediate their anticancer effects via activation of PPARgamma.

Troglitazone Acts on Cellular pH and DNA Synthesis through a Peroxisome Proliferator-Activated Receptor γ-Independent Mechanism in Breast Cancer-Derived Cell Lines

PURPOSE

The purpose of this study was to assess whether troglitazone (TRO) would induce cellular acidosis by inhibiting Na(+)/H(+) exchanger (NHE) 1 in breast carcinoma-derived cell lines and, if so, whether cellular acidosis would be associated with a reduction in proliferation.

EXPERIMENTAL DESIGN

Intracellular pH (pH(i)) and acid extrusion capacity after an exogenous acid load were assayed using (2, 7)-biscarboxyethyl-5(6)-carboxyfluorescein in MCF-7 and MDA-MB-231 cells treated with TRO. Radiolabeled thymidine incorporation was used to assess DNA synthesis. Peroxisome proliferator-activated receptor (PPAR) gamma involvement was assessed using an antagonist and PPARgamma(-/-) NIH3T3 cells.

RESULTS

TRO induced a prompt (<4 minute) and severe cellular acidosis in both MCF-7 (7.54 +/- 0.23 to 6.77 +/- 0.06; P < 0.001) and MDA-MB-231 cells (7.38 +/- 0.18 to 6.89 +/- 0.25; P < 0.05) after 12 minutes, without increasing acid production. Acid extrusion as assessed by the response to an exogenous acid load (NH(4)Cl pulse) was markedly blunted (MDA-MB-231, P < 0.01) or eliminated (MCF-7, P < 0.001). Chronic exposure to TRO resulted in NHE1 activity reduction (P < 0.05) and a dose-dependent decrease in DNA synthesis (<75% inhibition at 100 micromol/L; P < 0.001 and P < 0.01 for MCF-7 and MDA-MB-231, respectively) associated with a decreased number of viable cells. TRO-mediated inhibition of proliferation was not reversed by the presence of the PPARgamma inhibitor GW9662 and was demonstrable in PPARgamma(-/-) NIH3T3 cells, consistent with a PPARgamma-independent mechanism.

CONCLUSIONS

TRO induces marked cellular acidosis in MCF-7 and MDA-MD-231 cells. Sustained acidosis is consonant with decreased proliferation and growth that is not reversed by a PPARgamma antagonist. Our results support a NHE-mediated action of TRO that exerts its effect independent of PPARgamma.

Antineoplastic effects of peroxisome proliferator-activated receptor gamma agonists

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

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

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

PPAR trilogy from metabolism to cancer

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

PPAR gamma signaling exacerbates mammary gland tumor development

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

Therapeutic potential of thiazolidinediones as anticancer agents

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

15-Deoxy-delta12,14-PGJ2: endogenous PPARgamma ligand or minor eicosanoid degradation product?

15-Deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) affects gene transcription by activating PPARgamma and by covalent addition to transcription factors and signaling molecules, However, it is not known whether the high concentrations of 15d-PGJ2 required for these responses are consistent with physiological levels. A new study suggests that in vivo 15d-PGJ2 levels are actually several orders of magnitude below the levels required to induce many of the biological effects attributed to this molecule.

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.

PPARgamma ligands inhibit primary tumor growth and metastasis by inhibiting angiogenesis

Several drugs approved for a variety of indications have been shown to exhibit antiangiogenic effects. Our study focuses on the PPARgamma ligand rosiglitazone, a compound widely used in the treatment of type 2 diabetes. We demonstrate, for the first time to our knowledge, that PPARgamma is highly expressed in tumor endothelium and is activated by rosiglitazone in cultured endothelial cells. Furthermore, we show that rosiglitazone suppresses primary tumor growth and metastasis by both direct and indirect antiangiogenic effects. Rosiglitazone inhibits bovine capillary endothelial cell but not tumor cell proliferation at low doses in vitro and decreases VEGF production by tumor cells. In our in vivo studies, rosiglitazone suppresses angiogenesis in the chick chorioallantoic membrane, in the avascular cornea, and in a variety of primary tumors. These results suggest that PPARgamma ligands may be useful in treating angiogenic diseases such as cancer by inhibiting angiogenesis.

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.

PPARgamma ligands inhibit primary tumor growth and metastasis by inhibiting angiogenesis

Several drugs approved for a variety of indications have been shown to exhibit antiangiogenic effects. Our study focuses on the PPARgamma ligand rosiglitazone, a compound widely used in the treatment of type 2 diabetes. We demonstrate, for the first time to our knowledge, that PPARgamma is highly expressed in tumor endothelium and is activated by rosiglitazone in cultured endothelial cells. Furthermore, we show that rosiglitazone suppresses primary tumor growth and metastasis by both direct and indirect antiangiogenic effects. Rosiglitazone inhibits bovine capillary endothelial cell but not tumor cell proliferation at low doses in vitro and decreases VEGF production by tumor cells. In our in vivo studies, rosiglitazone suppresses angiogenesis in the chick chorioallantoic membrane, in the avascular cornea, and in a variety of primary tumors. These results suggest that PPARgamma ligands may be useful in treating angiogenic diseases such as cancer by inhibiting angiogenesis.