Peroxisome proliferator-activated receptor delta as a molecular target to regulate lung cancer cell growth

The role of peroxisome proliferator-activated receptors in the tumor microenvironment, tumor cell metabolism, and anticancer therapy

Peroxisome proliferator-activated receptors (PPARs) have been extensively studied for over 3 decades and consist of three isotypes, including PPARα, γ, and β/δ, that were originally considered key metabolic regulators controlling energy homeostasis in the body. Cancer has become a leading cause of human mortality worldwide, and the role of peroxisome proliferator-activated receptors in cancer is increasingly being investigated, especially the deep molecular mechanisms and effective cancer therapies. Peroxisome proliferator-activated receptors are an important class of lipid sensors and are involved in the regulation of multiple metabolic pathways and cell fate. They can regulate cancer progression in different tissues by activating endogenous or synthetic compounds. This review emphasizes the significance and knowledge of peroxisome proliferator-activated receptors in the tumor microenvironment, tumor cell metabolism, and anti-cancer treatment by summarizing recent research on peroxisome proliferator-activated receptors. In general, peroxisome proliferator-activated receptors either promote or suppress cancer in different types of tumor microenvironments. The emergence of this difference depends on various factors, including peroxisome proliferator-activated receptor type, cancer type, and tumor stage. Simultaneously, the effect of anti-cancer therapy based on drug-targeted PPARs differs or even opposes among the three peroxisome proliferator-activated receptor homotypes and different cancer types. Therefore, the current status and challenges of the use of peroxisome proliferator-activated receptors agonists and antagonists in cancer treatment are further explored in this review.

The Potential Role of PPARs in the Fetal Origins of Adult Disease

The fetal origins of adult disease (FOAD) hypothesis holds that events during early development have a profound impact on one’s risk for the development of future adult disease. Studies from humans and animals have demonstrated that many diseases can begin in childhood and are caused by a variety of early life traumas, including maternal malnutrition, maternal disease conditions, lifestyle changes, exposure to toxins/chemicals, improper medication during pregnancy, and so on. Recently, the roles of Peroxisome proliferator-activated receptors (PPARs) in FOAD have been increasingly appreciated due to their wide variety of biological actions. PPARs are members of the nuclear hormone receptor subfamily, consisting of three distinct subtypes: PPARα, β/δ, and γ, highly expressed in the reproductive tissues. By controlling the maturation of the oocyte, ovulation, implantation of the embryo, development of the placenta, and male fertility, the PPARs play a crucial role in the transition from embryo to fetus in developing mammals. Exposure to adverse events in early life exerts a profound influence on the methylation pattern of PPARs in offspring organs, which can affect development and health throughout the life course, and even across generations. In this review, we summarize the latest research on PPARs in the area of FOAD, highlight the important role of PPARs in FOAD, and provide a potential strategy for early prevention of FOAD.

Targeting Nuclear Receptors in Lung Cancer—Novel Therapeutic Prospects

Lung cancer, the second most commonly diagnosed cancer, is the major cause of fatalities worldwide for both men and women, with an estimated 2.2 million new incidences and 1.8 million deaths, according to GLOBOCAN 2020. Although various risk factors for lung cancer pathogenesis have been reported, controlling smoking alone has a significant value as a preventive measure. In spite of decades of extensive research, mechanistic cues and targets need to be profoundly explored to develop potential diagnostics, treatments, and reliable therapies for this disease. Nuclear receptors (NRs) function as transcription factors that control diverse biological processes such as cell growth, differentiation, development, and metabolism. The aberrant expression of NRs has been involved in a variety of disorders, including cancer. Deregulation of distinct NRs in lung cancer has been associated with numerous events, including mutations, epigenetic modifications, and different signaling cascades. Substantial efforts have been made to develop several small molecules as agonists or antagonists directed to target specific NRs for inhibiting tumor cell growth, migration, and invasion and inducing apoptosis in lung cancer, which makes NRs promising candidates for reliable lung cancer therapeutics. The current work focuses on the importance of various NRs in the development and progression of lung cancer and highlights the different small molecules (e.g., agonist or antagonist) that influence NR expression, with the goal of establishing them as viable therapeutics to combat lung cancer.

Diagnostic value of PPARδ and miRNA-17 expression levels in patients with non-small cell lung cancer

The PPARδ gene codes protein that belongs to the peroxisome proliferator-activated receptor (PPAR) family engaged in a variety of biological processes, including carcinogenesis. Specific biological and clinical roles of PPARδ in non-small cell lung cancer (NSCLC) is not fully explained. The association of PPARα with miRNA regulators (e.g. miRNA-17) has been documented, suggesting the existence of a functional relationship of all PPARs with epigenetic regulation. The aim of the study was to determine the PPARδ and miR-17 expression profiles in NSCLC and to assess their diagnostic value in lung carcinogenesis. PPARδ and miR-17 expressions was assessed by qPCR in NSCLC tissue samples (n = 26) and corresponding macroscopically unchanged lung tissue samples adjacent to the primary lesions served as control (n = 26). PPARδ and miR-17 expression were significantly lower in NSCLC than in the control (p = 0.0001 and p = 0.0178; respectively). A receiver operating characteristic (ROC) curve analysis demonstrated the diagnostic potential in discriminating NSCLC from the control with an area under the curve (AUC) of 0.914 for PPARδ and 0.692 for miR-17. Significant increase in PPARδ expression in the control for current smokers vs. former smokers (p = 0.0200) and increase in miR-17 expression in control tissue adjacent to adenocarcinoma subtype (p = 0.0422) were observed. Overexpression of miR-17 was observed at an early stage of lung carcinogenesis, which may suggest that it acts as a putative oncomiR. PPARδ and miR-17 may be markers differentiating tumour tissue from surgical margin and miR-17 may have diagnostic role in NSCLC histotypes differentiation.

The pleiotropic peroxisome proliferator activated receptors: Regulation and therapeutics

The Peroxisome proliferator-activated receptors (PPARs) are key regulators of metabolic events in our body. Owing to their implication in maintenance of homeostasis, both PPAR agonists and antagonists assume therapeutic significance. Understanding the molecular mechanisms of each of the PPAR isotypes in the healthy body and during disease is crucial to exploiting their full therapeutic potential. This article is an attempt to present a rational analysis of the multifaceted therapeutic effects and underlying mechanisms of isotype-specific PPAR agonists, dual PPAR agonists, pan PPAR agonists as well as PPAR antagonists. A holistic understanding of the mechanistic dimensions of these key metabolic regulators will guide future efforts to identify novel molecules in the realm of metabolic, inflammatory and immunotherapeutic diseases.

PPAR Beta/Delta and the Hallmarks of Cancer

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor family. Three different isoforms, PPAR alpha, PPAR beta/delta and PPAR gamma have been identified. They all form heterodimers with retinoic X receptors to activate or repress downstream target genes dependent on the presence/absence of ligands and coactivators or corepressors. PPARs differ in their tissue expression profile, ligands and specific agonists and antagonists. PPARs attract attention as potential therapeutic targets for a variety of diseases. PPAR alpha and gamma agonists are in clinical use for the treatment of dyslipidemias and diabetes. For both receptors, several clinical trials as potential therapeutic targets for cancer are ongoing. In contrast, PPAR beta/delta has been suggested as a therapeutic target for metabolic syndrome. However, potential risks in the settings of cancer are less clear. A variety of studies have investigated PPAR beta/delta expression or activation/inhibition in different cancer cell models in vitro, but the relevance for cancer growth in vivo is less well documented and controversial. In this review, we summarize critically the knowledge of PPAR beta/delta functions for the different hallmarks of cancer biological capabilities, which interplay to determine cancer growth.

Vascular PPARβ/δ Promotes Tumor Angiogenesis and Progression

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, which function as transcription factors. Among them, PPARβ/δ is highly expressed in endothelial cells. Pharmacological activation with PPARβ/δ agonists had been shown to increase their angiogenic properties. PPARβ/δ has been suggested to be involved in the regulation of the angiogenic switch in tumor progression. However, until now, it is not clear to what extent the expression of PPARβ/δ in tumor endothelium influences tumor progression and metastasis formation. We addressed this question using transgenic mice with an inducible conditional vascular-specific overexpression of PPARβ/δ. Following specific over-expression of PPARβ/δ in endothelial cells, we induced syngenic tumors. We observed an enhanced tumor growth, a higher vessel density, and enhanced metastasis formation in the tumors of animals with vessel-specific overexpression of PPARβ/δ. In order to identify molecular downstream targets of PPARβ/δ in the tumor endothelium, we sorted endothelial cells from the tumors and performed RNA sequencing. We identified platelet-derived growth factor receptor beta (Pdgfrb), platelet-derived growth factor subunit B (Pdgfb), and the tyrosinkinase KIT (c-Kit) as new PPARβ/δ -dependent molecules. We show here that PPARβ/δ activation, regardless of its action on different cancer cell types, leads to a higher tumor vascularization which favors tumor growth and metastasis formation.

Role of FABP7 in tumor cell signaling

Lipids are major molecules for the function of organisms and are involved in the pathophysiology of various diseases. Fatty acids (FAs) signaling and their metabolism are some of the most important pathways in tumor development, as lipids serve as energetic sources during carcinogenesis. Fatty acid binding proteins (FABPs) facilitate FAs transport to different cell organelles, modulating their metabolism along with mediating other physiological activities. FABP7, brain-typed FABP, is thought to be an important molecule for cell proliferation in healthy as well as diseased organisms. Several studies on human tumors and tumor-derived cell lines put FABP7 in the center of tumorigenesis, and its high expression level has been reported to correlate with poor prognosis in different tumor types. Several types of FABP7-expressing tumors have shown an up-regulation of cell signaling activity, but molecular mechanisms of FABP7 involvement in tumorigenesis still remain elusive. In this review, we focus on the expression and function of FABP7 in different tumors, and possible mechanisms of FABP7 in tumor proliferation and migration.

Induction of apoptosis in prostate cancer by ginsenoside Rh2

The therapeutic action of ginsenoside Rh2 on several cancer models has been reported. This study aimed to evaluate its apoptotic effect on prostate cancer and the underlying mechanism. Cultured DU145 cells were treated with Rh2 (5 × 10^-5 to 1 × 10^-4 M), peroxisome proliferator-activated receptor-delta (PPAR-delta) antagonist GSK0660 (1 × 10^-6 to 5 × 10^-6 M); or small interfering RNA (siRNA) of PPAR-delta. The treatment effects were evaluated with cell viability assay, life/death staining and flow cytometry for apoptosis. Immunostaining was used for reactive oxygen species (ROS) and superoxide detection. Western blot analysis for PPAR-delta and signal transducer and activator of transcription 3 (STAT3) protein expression were performed. The results showed that Rh2 significantly decreased DU145 cell survival and increased cell apoptosis. ROS and superoxide induction, PPAR-delta up-regulation and phosphorylated STAT3 (p-STAT3) down-regulation by Rh2 were demonstrated. GSK0660 partially but significantly inhibited the Rh2-induced apoptosis and restored cell viability. Treatment with siRNA reversed the Rh2-induced apoptosis as well as changes in PPAR-delta and p-STAT3 expression. In conclusion, our findings have demonstrated that ginsenoside Rh2 induces prostate cancer DU145 cells apoptosis through up-regulation of PPAR-delta expression which is associated with p-STAT3 up-regulation and ROS/superoxide induction. Rh2 may be potentially useful in the treatment of prostate cancer.

ADAMTS1-mediated targeting of TSP-1 by PPARδ suppresses migration and invasion of breast cancer cells

Migration and invasion of cancer cells into surrounding tissue is a key stage of cancer metastasis. Here, we show that peroxisome proliferator-activated receptor (PPAR) δ regulates migration and invasion of human breast cancer cells via thrombospondin-1 (TSP-1) and its degrading protease, a disintegrin and metalloprotease domains with thrombospondin motifs 1 (ADAMTS1). Activation of PPARδ by GW501516, a specific ligand for PPARδ, led to marked inhibition in the cell migration and TSP-1 expression of breast cancer. These effects were suppressed by small interfering RNA-mediated knock-down of ADAMTS1, indicating that ADAMTS1 is involved in PPARδ-mediated inhibition of migration and TSP-1 expression in breast cancer cells. In addition, ligand-activated PPARδ upregulated expression of ADAMTS1 at the transcriptional level via binding of PPARδ to a direct repeat-1 site within the ADAMTS1 gene promoter. Furthermore, ligand-activated PPARδ suppressed invasion of breast cancer cells in an ADAMTS1-dependent manner. Taken together, these results demonstrate that PPARδ suppresses migration and invasion of breast cancer cells by downregulating TSP-1 in a process mediated by upregulation of ADAMTS1.

PPAR Agonists for the Prevention and Treatment of Lung Cancer

Lung cancer is the most common and most fatal of all malignancies worldwide. Furthermore, with more than half of all lung cancer patients presenting with distant metastases at the time of initial diagnosis, the overall prognosis for the disease is poor. There is thus a desperate need for new prevention and treatment strategies. Recently, a family of nuclear hormone receptors, the peroxisome proliferator-activated receptors (PPARs), has attracted significant attention for its role in various malignancies including lung cancer. Three PPARs, PPARα, PPARβ/δ, and PPARγ, display distinct biological activities and varied influences on lung cancer biology. PPARα activation generally inhibits tumorigenesis through its antiangiogenic and anti-inflammatory effects. Activated PPARγ is also antitumorigenic and antimetastatic, regulating several functions of cancer cells and controlling the tumor microenvironment. Unlike PPARα and PPARγ, whether PPARβ/δ activation is anti- or protumorigenic or even inconsequential currently remains an open question that requires additional investigation. This review of current literature emphasizes the multifaceted effects of PPAR agonists in lung cancer and discusses how they may be applied as novel therapeutic strategies for the disease.

Prostaglandin terminal synthases as novel therapeutic targets

Non-steroidal anti-inflammatory drugs (NSAIDs) exert their anti-inflammatory and anti-tumor effects by reducing prostaglandin (PG) production via the inhibition of cyclooxygenase (COX). However, the gastrointestinal, renal and cardiovascular side effects associated with the pharmacological inhibition of the COX enzymes have focused renewed attention onto other potential targets for NSAIDs. PGH₂, a COX metabolite, is converted to each PG species by species-specific PG terminal synthases. Because of their potential for more selective modulation of PG production, PG terminal synthases are now being investigated as a novel target for NSAIDs. In this review, I summarize the current understanding of PG terminal synthases, with a focus on microsomal PGE synthase-1 (mPGES-1) and PGI synthase (PGIS). mPGES-1 and PGIS cooperatively exacerbate inflammatory reactions but have opposing effects on carcinogenesis. mPGES-1 and PGIS are expected to be attractive alternatives to COX as therapeutic targets for several diseases, including inflammatory diseases and cancer.

Expression Profiling Identifies Bezafibrate as Potential Therapeutic Drug for Lung Adenocarcinoma

Drug-induced gene expression patterns that invert disease profiles have recently been illustrated to be a new strategy for drug-repositioning. In the present study, we validated this approach and focused on prediction of novel drugs for lung adenocarcinoma (AC), for which there is a pressing need to find novel therapeutic compounds. Firstly, connectivity map (CMap) analysis computationally predicted bezafibrate as a putative compound against lung AC. Then this hypothesis was verified by in vitro assays of anti-proliferation and cell cycle arrest. In silico docking evidence indicated that bezafibrate could target cyclin dependent kinase 2(CDK2), which regulates progression through the cell cycle. Furthermore, we found that bezafibrate can significantly down-regulate the expression of CDK2 mRNA and p-CDK2. Using a nude mice xenograft model, we also found that bezafibrate could inhibit tumor growth of lung AC in vivo. In conclusion, this study proposed bezafibrate as a potential therapeutic option for lung AC patients, illustrating the potential of in silico drug screening.

[Screening and identification of novel drug-resistant genes in CD133+ and CD133- lung adenosarcoma cells using cDNA microarray]

背景与目的

肿瘤干细胞可能是肿瘤多药耐药的主要原因，CD133是目前较为公认的肿瘤干细胞标记物。本研究旨在应用功能分类基因芯片筛选CD133⁺和CD133^-肺腺癌细胞中差异表达的肿瘤耐药基因，寻求新的肺癌耐药相关基因。

方法

免疫磁珠分选法分选A549细胞，采用功能分类基因芯片筛选CD133⁺和CD133^-肺腺癌细胞中差异表达的肿瘤耐药基因，并使用RT-qPCR验证。顺铂半数有效抑制浓度（half inhibiting concentration, IC₅₀）、阿霉素IC₅₀作用A549细胞48 h后，RT-qPCR检测肿瘤耐药基因CYP2C19、CYP2D6、CYP2E1、GSK3α、PPARα和PPARβ/δ的表达变化。

结果

共筛查出31个差异表达的肿瘤耐药基因，与CD133^-细胞相比，CD133⁺细胞有30个基因表达上调，1个基因表达下调。RT-qPCR结果与芯片一致。A549细胞经1.97 μg/mL顺铂或0.61 μg/mL阿霉素作用48 h后，CYP2C19、CYP2D6、CYP2E1、GSK3α、PPARα和PPARβ/δ等肿瘤耐药基因表达上调。

结论

利用功能分类基因芯片筛选出31个可能与CD133⁺肺腺癌细胞耐药相关的基因，其中CYP2C19、CYP2D6、CYP2E1、GSK3α、PPARα和PPARβ/δ为新发现的肺癌耐药相关基因。

PPARα activation can help prevent and treat non-small cell lung cancer

Non-small cell lung cancer (NSCLC) not amenable to surgical resection has a high mortality rate, due to the ineffectiveness and toxicity of chemotherapy. Thus, there remains an urgent need of efficacious drugs that can combat this disease. In this study, we show that targeting the formation of proangiogenic epoxyeicosatrienoic acids (EET) by the cytochrome P450 arachidonic acid epoxygenases (Cyp2c) represents a new and safe mechanism to treat NSCLC growth and progression. In the transgenic murine K-Ras model and human orthotopic models of NSCLC, we found that Cyp2c44 could be downregulated by activating the transcription factor PPARα with the ligands bezafibrate and Wyeth-14,643. Notably, both treatments reduced primary and metastatic NSCLC growth, tumor angiogenesis, endothelial Cyp2c44 expression, and circulating EET levels. These beneficial effects were independent of the time of administration, whether before or after the onset of primary NSCLC, and they persisted after drug withdrawal, suggesting the benefits were durable. Our findings suggest that strategies to downregulate Cyp2c expression and/or its enzymatic activity may provide a safer and effective strategy to treat NSCLC. Moreover, as bezafibrate is a well-tolerated clinically approved drug used for managing lipidemia, our findings provide an immediate cue for clinical studies to evaluate the utility of PPARα ligands as safe agents for the treatment of lung cancer in humans.

Ligand-activated peroxisome proliferator-activated receptor β/δ modulates human endometrial cancer cell survival

Endometrial cancer is the fourth most common malignancy among women and is a major cause of morbidity contributing to approximately 8,200 annual deaths in the USA. Despite advances to the understanding of endometrial cancer, novel interventions for the disease are necessary given that many tumors become refractory to therapy. As a strategy to identify novel therapies for endometrial carcinoma, in this study, we examined the contribution of the peroxisome proliferator-activated receptor β/δ (PPARβ/δ) to endometrial cancer cell proliferation and apoptosis. We found that when activated with the highly selective PPARβ/δ agonists, GW0742 and GW501516, PPARβ/δ inhibited the proliferation and markedly induced the apoptosis of three endometrial cancer cell lines. The specificity of the PPARβ/δ-induced effects on cell proliferation and apoptosis was demonstrated using PPARβ/δ-selective antagonists and PPARβ/δ small interfering RNA in combination with PPARβ/δ-selective agonists. Furthermore, we showed that PPARβ/δ activation increased phosphatase and tensin homolog expression, which led to protein kinase B (AKT) and glycogen synthase kinase-3β (GSK3β) dephosphorylation, and increased β-catenin phosphorylation associated with its degradation. Overall, our data suggest that the antitumorigenic effect of PPARβ/δ activation in endometrial cancer is mediated through the negative regulation of the AKT/GSK3β/β-catenin pathway. These findings warrant further investigation of PPARβ/δ as a therapeutic target in endometrial cancer.

Dissecting the role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in colon, breast, and lung carcinogenesis

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) is a promising drug target since its agonists increase serum high-density lipoprotein; decrease low-density lipoprotein, triglycerides, and insulin associated with metabolic syndrome; improve insulin sensitivity; and decrease high fat diet-induced obesity. PPARβ/δ agonists also promote terminal differentiation and elicit anti-inflammatory activities in many cell types. However, it remains to be determined whether PPARβ/δ agonists can be developed as therapeutics because there are reports showing either pro- or anti-carcinogenic effects of PPARβ/δ in cancer models. This review examines studies reporting the role of PPARβ/δ in colon, breast, and lung cancers. The prevailing evidence would suggest that targeting PPARβ/δ is not only safe but could have anti-carcinogenic protective effects.

Peroxisome proliferator-activated receptors and cancer: challenges and opportunities

Peroxisome proliferator-activated receptors (PPARs), members of the nuclear hormone receptor superfamily, function as transcription factors and modulators of gene expression. These actions allow PPARs to regulate a variety of biological processes and to play a significant role in several diseases and conditions. The current literature describes frequently opposing and paradoxical roles for the three PPAR isotypes, PPARα, PPARβ/δ and PPARγ, in cancer. While some studies have implicated PPARs in the promotion and development of cancer, others, in contrast, have presented evidence for a protective role for these receptors against cancer. In some tissues, the expression level of these receptors and/or their activation correlates with a positive outcome against cancer, while, in other tissue types, their expression and activation have the opposite effect. These disparate findings raise the possibility of (i) PPAR receptor-independent effects, including effects on receptors other than PPARs by the utilized ligands; (ii) cancer stage-specific effect; and/or (iii) differences in essential ligand-related pharmacokinetic considerations. In this review, we highlight the latest available studies on the role of the various PPAR isotypes in cancer in several major organs and present challenges as well as promising opportunities in the field.

The Role of PPAR Ligands in Controlling Growth-Related Gene Expression and their Interaction with Lipoperoxidation Products

Peroxisome proliferators-activated receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. The three PPAR isoforms (α, γ and β/δ) have been found to play a pleiotropic role in cell fat metabolism. Furthermore, in recent years, evidence has been found regarding the antiproliferative, proapoptotic, and differentiation-promoting activities displayed by PPAR ligands, particularly by PPARγ ligands. PPAR ligands affect the expression of different growth-related genes through both PPAR-dependent and PPAR-independent mechanisms. Moreover, an interaction between PPAR ligands and other molecules which strengthen the effects of PPAR ligands has been described. Here we review the action of PPAR on the control of gene expression with particular regard to the effect of PPAR ligands on the expression of genes involved in the regulation of cell-cycle, differentiation, and apoptosis. Moreover, the interaction between PPAR ligands and 4-hydroxynonenal (HNE), the major product of the lipid peroxidation, has been reviewed.

Xenobiotic metabolism, disposition, and regulation by receptors: from biochemical phenomenon to predictors of major toxicities

To commemorate the 50th anniversary of the Society of Toxicology, this special edition article reviews the history and current scope of xenobiotic metabolism and transport, with special emphasis on the discoveries and impact of selected "xenobiotic receptors." This overall research realm has witnessed dynamic development in the past 50 years, and several of the key milestone events that mark the impressive progress in these areas of toxicological sciences are highlighted. From the initial observations regarding aspects of drug metabolism dating from the mid- to late 1800's, the area of biotransformation research witnessed seminal discoveries in the mid-1900's and onward that are remarkable in retrospect, including the discovery and characterization of the phase I monooxygenases, the cytochrome P450s. Further research uncovered many aspects of the biochemistry of xenobiotic metabolism, expanding to phase II conjugation and phase III xenobiotic transport. This led to hallmark developments involving integration of genomic technologies to elucidate the basis for interindividual differences in response to xenobiotic exposures and discovery of nuclear and soluble receptor families that selectively "sense" the chemical milieu of the mammalian cell and orchestrate compensatory changes in gene expression programming to accommodate complex xenobiotic exposures. This review will briefly summarize these developments and investigate the expanding roles of xenobiotic receptor biology in the underlying basis of toxicological response to chemical agents.

Peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) acts as regulator of metabolism linked to multiple cellular functions

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors. They function as ligand activated transcription factors. They exist in three isoforms, PPARalpha, PPARbeta (formerly PPARdelta), and PPARgamma. For all PPARs lipids are endogenous ligands, linking them directly to metabolism. PPARs form heterodimers with retinoic X receptors, and, upon ligand binding, modulate gene expression of downstream target genes dependent on the presence of co-repressors or co-activators. This results in cell-type specific complex regulations of proliferation, differentiation and cell survival. Specific synthetic agonists for all PPARs are available. PPARalpha and PPARgamma agonists are already in clinical use for the treatment of hyperlipidemia and type 2 diabetes, respectively. More recently, PPARbeta activation came into focus as an interesting novel approach for the treatment of metabolic syndrome and associated cardiovascular diseases. Although the initial notion was that PPARbeta is expressed ubiquitously, more recently extensive investigations have been performed demonstrating high PPARbeta expression in a variety of tissues, e.g. skin, skeletal muscle, adipose tissue, inflammatory cells, heart, and various types of cancer. In addition, in vitro and in vivo studies using specific PPARbeta agonists, tissue-specific over-expression or knockout mouse models have demonstrated a variety of functions of PPARbeta in adipose tissue, muscle, skin, inflammation, and cancer. We will focus here on functions of PPARbeta in adipose tissue, skeletal muscle, heart, angiogenesis and cancer related to modifications in metabolism and the identified underlying molecular mechanisms.

A prostacyclin agonist with thromboxane inhibitory activity for airway allergic inflammation in mice

BACKGROUND

ONO-1301 is a novel drug that acts as a prostacyclin agonist with thromboxane A(2) (TxA(2)) synthase inhibitory activity. We investigated the effect of ONO-1301 on development of airway allergic inflammation.

METHODS

Mice sensitized and challenged to ovalbumin (OVA) received ONO-1301, OKY-046 (TxA(2) synthase inhibitor), beraprost, a prostacyclin receptor (IP) agonist, ONO-1301 plus CAY10449 (selective IP antagonist) or vehicle during the challenge period. Twenty-four hours after the OVA challenge, airway hyperresponsiveness (AHR) to methacholine was assessed and bronchoalveolar lavage was performed. Lung specimens were excised for goblet cell staining and analysis of lung dendritic cells (DCs). Bone marrow-derived dendritic cells (BMDCs) were generated, in the presence or absence of drugs, for analysis of DC function.

RESULTS

Mice that received ONO-1301 showed significantly lower AHR, airway eosinophilia, T-helper type 2 cytokine levels, mucus production and lung DCs numbers than vehicle-treated mice. These effects of ONO-1301 were mostly reversed by CAY10449. BMDCs treated with ONO-1301 alone showed lower DC functions, such as expression of costimulatory factors or stimulation to spleen T cells.

CONCLUSIONS

These data suggest that ONO-1301 may suppress AHR and airway allergic inflammation through modulation of DCs, mainly mediated through the IP receptor. This agent may be effective as an anti-inflammatory drug in the treatment of asthma.

Nicotine stimulates PPARbeta/delta expression in human lung carcinoma cells through activation of PI3K/mTOR and suppression of AP-2alpha

We previously showed that nicotine stimulates non-small cell lung carcinoma (NSCLC) cell proliferation through nicotinic acetylcholine receptor (nAChR)-mediated signals. Activation of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) has also been shown to induce NSCLC cell growth. Here, we explore the potential link between nicotine and PPARbeta/delta and report that nicotine increases the expression of PPARbeta/delta protein; this effect was blocked by an alpha7 nAChR antagonist (alpha-bungarotoxin), by alpha7 nAChR short interfering RNA, and by inhibitors of phosphatidylinositol 3-kinase (PI3K; wortmannin and LY294002) and mammalian target of rapamycin (mTOR; rapamycin). In contrast, this effect was enhanced by PUN282987, an alpha7 nAChR agonist. Silencing of PPARbeta/delta attenuated the stimulatory effect of nicotine on cell growth, which was overcome by transfection of an exogenous PPARbeta/delta expression vector. Of note, nicotine induced complex formation between alpha7 nAChR and PPARbeta/delta protein and increased PPARbeta/delta gene promoter activity through inhibition of AP-2alpha as shown by reduced AP-2alpha binding using electrophoretic gel mobility shift and chromatin immunoprecipitation assays. In addition, silencing of Sp1 attenuated the effect of nicotine on PPARbeta/delta. Collectively, our results show that nicotine increases PPARbeta/delta gene expression through alpha7 nAChR-mediated activation of PI3K/mTOR signals that inhibit AP-2alpha protein expression and DNA binding activity to the PPARbeta/delta gene promoter. Sp1 seems to modulate this process. This study unveils a novel mechanism by which nicotine promotes human lung carcinoma cell growth.

Sorting out the functional role(s) of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) in cell proliferation and cancer

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) has many beneficial physiological functions ranging from enhancing fatty acid catabolism, improving insulin sensitivity, inhibiting inflammation and increasing oxidative myofibers allowing for improved athletic performance. Thus, given the potential for targeting PPARbeta/delta for the prevention and/or treatment of diseases including diabetes, dyslipidemias, metabolic syndrome and cancer, it is critical to clarify the functional role of PPARbeta/delta in cell proliferation and associated disorders such as cancer. However, there is considerable controversy whether PPARbeta/delta stimulates or inhibits cell proliferation. This review summarizes the literature describing the influence of PPARbeta/delta on cell proliferation, with an emphasis toward dissecting the data that give rise to opposing hypotheses. Suggestions are offered to standardize measurements associated with these studies so that interlaboratory comparisons can be accurately assessed.

The beneficial role of vitamin D and its analogs in cancer treatment and prevention

BACKGROUND

Cancer is the leading cause of death in the United States, and the probability of developing cancer increases dramatically with age. Interestingly, vitamin D deficiency is also recognized more often in people of advanced ages. A potential relationship between vitamin D deficiency and cancer has been reported in the literature.

METHOD

Review Medline database literature and discuss the relationship between vitamin D status and cancer.

RESULTS

Environmental (including seasonal and geographic) and genetic factors contribute to the development of both vitamin D deficiency and cancer. The vitamin D receptor is present in many tissues, especially in malignant cells, and may contribute to the successful use of vitamin D and its analogs in the treatment of some cancer patients.

CONCLUSION

Further investigation of the role of vitamin D in the treatment of cancer is warranted.

PPARδ activity in cardiovascular diseases: A potential pharmacological target

Activation of peroxisome proliferator-activated receptors (PPARs), and particularly of PPARα and PPARγ, using selective agonists, is currently used in the treatment of metabolic diseases such as hypertriglyceridemia and type 2 diabetes mellitus. PPARα and PPARγ anti-inflammatory, antiproliferative and antiangiogenic properties in cardiovascular cells were extensively clarified in a variety of in vitro and in vivo models. In contrast, the role of PPARδ in cardiovascular system is poorly understood. Prostacyclin, the predominant prostanoid released by vascular cells, is a putative endogenous agonist for PPARδ, but only recently PPARδ selective synthetic agonists were found, improving studies about the physiological and pathophysiological roles of PPARδ activation. Recent reports suggest that the PPARδ activation may play a pivotal role to regulate inflammation, apoptosis, and cell proliferation, suggesting that this transcriptional factor could become an interesting pharmacological target to regulate cardiovascular cell apoptosis, proliferation, inflammation, and metabolism.

Effect of ligand activation of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) in human lung cancer cell lines

There is compelling evidence that peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) mediates terminal differentiation and is associated with inhibition of cell growth. However, it was recently suggested that growth of two human lung cancer cell lines is enhanced by PPARbeta/delta. The goal of the present study was to provide insight in resolving this controversy. Therefore, the effect of ligand activation of PPARbeta/delta in A549 and H1838 human lung cancer cell lines was examined using two high affinity PPARbeta/delta ligands. Ligand activation of PPARbeta/delta caused up-regulation of a known PPARbeta/delta target gene, angiopoietin-like 4 (Angptl4) but did not influence expression of phosphatase and tensin homolog (PTEN) or phosphorylation of protein kinase B (Akt), and did not affect cell growth. Results from this study demonstrate that two human lung cancer cell lines respond to ligand activation of PPARbeta/delta by modulation of target gene expression (Angptl4), but fail to exhibit significant modulation of cell proliferation.

Activation of peroxisome proliferator-activated receptor beta/delta induces lung cancer growth via peroxisome proliferator-activated receptor coactivator gamma-1alpha

We previously demonstrated that a selective agonist of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta), GW501516, stimulated human non-small cell lung carcinoma (NSCLC) growth, partly through inhibition of phosphatase and tensin homolog deleted on chromosome 10 expression. Here, we show that GW501516 also decreases the phosphorylation of AMP-activated protein kinase alpha (AMPKalpha), a major regulator of energy metabolism. This was mediated through specific activation of PPARbeta/delta, as a PPARbeta/delta small interfering RNA inhibited the effect. However, AMPKalpha did not mediate the growth-promoting effects of GW501516, as silencing of AMPKalpha did not inhibit GW501516-induced cell proliferation. Instead, we found that GW501516 stimulated peroxisome proliferator-activated receptor coactivator gamma (PGC)-1alpha, which activated the phosphatidylinositol 3 kinase (PI3-K)/Akt mitogenic pathway. An inhibitor of PI3-K, LY294002, had no effect on PGC-1alpha, consistent with PGC-1alpha being upstream of PI3-K/Akt. Of note, an activator of AMPKalpha, 5-amino-4-imidazole carboxamide riboside, inhibited the growth-promoting effects of GW501516, suggesting that although AMPKalpha is not responsible for the mitogenic effects of GW501516, its activation can oppose these events. This study unveils a novel mechanism by which GW501516 and activation of PPARbeta/delta stimulate human lung carcinoma cell proliferation, and suggests that activation of AMPKalpha may oppose this effect.

Role of peroxisome-proliferator-activated receptor beta/delta (PPARbeta/delta) in gastrointestinal tract function and disease

PPARbeta/delta (peroxisome-proliferator-activated receptor beta/delta) is one of three PPARs in the nuclear hormone receptor superfamily that are collectively involved in the control of lipid homoeostasis among other functions. PPARbeta/delta not only acts as a ligand-activated transcription factor, but also affects signal transduction by interacting with other transcription factors such as NF-kappaB (nuclear factor kappaB). Constitutive expression of PPARbeta/delta in the gastrointestinal tract is very high compared with other tissues and its potential physiological roles in this tissue include homoeostatic regulation of intestinal cell proliferation/differentiation and modulation of inflammation associated with inflammatory bowel disease and colon cancer. Analysis of mouse epithelial cells in the intestine and colon has clearly demonstrated that ligand activation of PPARbeta/delta induces terminal differentiation. The PPARbeta/delta target genes mediating this effect are currently unknown. Emerging evidence suggests that PPARbeta/delta can suppress inflammatory bowel disease through PPARbeta/delta-dependent and ligand-independent down-regulation of inflammatory signalling. However, the role of PPARbeta/delta in colon carcinogenesis remains controversial, as conflicting evidence suggests that ligand activation of PPARbeta/delta can either potentiate or attenuate this disease. In the present review, we summarize the role of PPARbeta/delta in gastrointestinal physiology and disease with an emphasis on findings in experimental models using both high-affinity ligands and null-mouse models.

Ligand activation of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) attenuates carbon tetrachloride hepatotoxicity by downregulating proinflammatory gene expression

Peroxisome proliferator-activated receptor (PPAR) beta/delta-null mice exhibit exacerbated hepatotoxicity in response to administration of carbon tetrachloride (CCl(4)). To determine whether ligand activation of the receptor protects against chemical toxicity in the liver, wild-type and PPARbeta/delta-null mice were administered CCl(4) with or without coadministration of the highly specific PPARbeta/delta ligand GW0742. Biomarkers of liver toxicity, including serum alanine aminotransferase (ALT) and hepatic tumor necrosis factor (TNF) alpha mRNA, were significantly higher in CCl(4)-treated PPARbeta/delta-null mice compared to wild-type mice. Hepatic expression of TNF-like weak inducer of apoptosis receptor (TWEAKr) and S100 calcium-binding protein A6 (S100A6/calcyclin), genes involved in nuclear factor kappa B signaling, was higher in the CCl(4)-treated PPARbeta/delta-null mice compared to wild-type mice. GW0742 treatment resulted in reduced serum ALT concentration and lower expression of CCl(4)-induced TNF-alpha, S100A6, monocyte chemoattractant protein-1 (MCP1), and TWEAKr in wild-type mice, and these effects were not observed in PPARbeta/delta-null mice. Expression of TNF-alpha was higher in PPARbeta/delta-null primary hepatocytes in response to interleukin-1beta treatment compared to wild-type hepatocytes, but GW0742 did not significantly modulate TNF-alpha expression in hepatocytes from either genotype. While PPARbeta/delta-null hepatic stellate exhibited higher rates of proliferation compared to wild-type cells, GW0742 did not affect alpha-smooth muscle actin expression in these cells. Combined, these findings demonstrate that ligand activation of PPARbeta/delta protects against chemically induced hepatotoxicity by downregulating expression of proinflammatory genes. Hepatocytes and hepatic stellate cells do not appear to directly mediate the inhibitory effects of ligand activation of PPARbeta/delta in liver, suggesting the involvement of paracrine and autocrine events mediated by hepatic cells.

Peroxisome proliferator-activated receptor beta/delta expression and activation in lung cancer

Peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) is a ligand-binding inducible transcriptional factor linked to carcinogenesis. Important functions of PPARbeta/delta were demonstrated in series of human epithelial cancers; however, its role in lung cancer remains controversial. We investigated the differential expression level and localization of PPARbeta/delta in tumors and adjacent normal lung tissue, and the effect of PPARbeta/delta activation on lung cancer cell proliferation and apoptosis. PPARbeta/delta was expressed in all studied human non-small cell lung cancers, and strong PPARbeta/delta immunoreactivity was observed in epithelial cells of more than 75% of studied lung tumors. PPARbeta/delta expression was consistently limited to the cancer cells in tumor tissue, while in adjacent normal lung tissue it was limited predominantly to the mononuclear cells. We found that ligand-binding activation of PPARbeta/delta stimulates cell proliferation (an effect that was blocked by a dominant-negative construct of PPARbeta/delta), stimulates anchorage-independent cell growth, and inhibits apoptosis in lung cancer cell lines. Importantly, the activation of PPARbeta/delta induces Akt phosphorylation correlated with up-regulation of PDK1, down-regulation of PTEN, and increased expression of Bcl-xL and COX-2. These findings indicate that PPARbeta/delta exerts proliferative and anti-apoptotic effects via PI3K/Akt1 and COX-2 pathways. In conclusion, PPARbeta/delta is strongly expressed in the majority of lung cancers, and its activation induces proliferative and survival response in non-small cell lung cancer.

PPARbeta/delta agonist stimulates human lung carcinoma cell growth through inhibition of PTEN expression: the involvement of PI3K and NF-kappaB signals

Recent studies suggest that activation of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) promotes cancer cell survival. We previously demonstrated that a selective PPARbeta/delta agonist, GW501516, stimulated human non-small cell lung carcinoma (NSCLC) cell growth. Here, we explore the mechanisms responsible for this effect. We show that GW501516 decreased phosphate and tensin homolog deleted on chromosome 10 (PTEN), a tumor suppressor known to decrease cell growth and induce apoptosis. Activation of PPARbeta/delta and phosphatidylinositol 3-kinase (PI3K)/Akt signaling was associated with inhibition of PTEN. GW501516 increased NF-kappaB DNA binding activity and p65 protein expression through activation of PPARbeta/delta and PI3K/Akt signals and enhanced the physical interactions between PPARbeta/delta and p65 protein. Conversely, inhibition of PI3K and silencing of p65 by small RNA interference (siRNA) blocked the effect of GW501516 on PTEN expression and on NSCLC cell proliferation. GW501516 also inhibited IKBalpha protein expression. Silencing of IKBalpha enhanced the effect of GW501516 on PTEN protein expression and on cell proliferation. It also augmented the GW501516-induced complex formation of PPARbeta/delta and p65 proteins. Overexpression of PTEN suppressed NSCLC cell growth and eliminated the effect of GW501516 on phosphorylation of Akt. Together, our observations suggest that GW501516 induces the proliferation of NSCLC cells by inhibiting the expression of PTEN through activation of PPARbeta/delta, which stimulates PI3K/Akt and NF-kappaB signaling. Overexpression of PTEN overcomes this effect and unveils PPARbeta/delta and PTEN as potential therapeutic targets in NSCLC.

Targeted disruption of the galectin-3 gene results in decreased susceptibility to NNK-induced lung tumorigenesis: an oligonucleotide microarray study

PURPOSE

Galectin-3, a beta-galactoside-binding animal lectin is a multifunctional protein, which regulates cell growth, cell adhesion, cell proliferation, angiogenesis, and apoptosis, and in turn contributes to tumorigenesis and metastasis. The aim of this study was to clarify the role or related mechanisms of galectin-3 in lung carcinogenesis.

METHODS

We administrated 4-(methylnitrosamino)-1-(3-pyridyle)-1-butanone (NNK), a powerful chemical carcinogen into galectin-3 wild-type (gal3+/+) and galectin-3 knock-out (gal3-/-) CD1 mice by intraperitoneal injection, examined the expression status of 22,690 mouse genes of the NNK-induced tumors using Affymetrix GeneChip mouse expression 430 A arrays, and then analyzed functional network and gene ontology by Ingenuity Pathway Analysis. Real-time PCR was also employed to partially confirm the genechip data.

RESULTS

Compared with the gal3+/+ mice, the incidence of lung tumors was significantly low in gal3-/- mice after 32 weeks (28.6 vs 52.1%, P < 0.05). Pathway analysis indicated that galectin-3 up-regulated carcinogenesis-related genes (e.g. B-cell receptor, ERK/MAPK, and PPAR signalings) in normal condition, and lung cancer and NNK-induced gene expression associated with cellular growth (e.g. Wnt/beta-catenin signaling) or immunological disease (e.g. EGF and PDGF signalings) in lung carcinogenesis with or without the galectin-3 control, respectively.

CONCLUSION

Disrupted galectin-3 may attenuate the lung carcinogenesis due to its regulatory role in the B-cell receptor, ERK/MAPK, and PPAR signal pathways.

Regulation of Cell Proliferation and Differentiation by PPARbeta/delta

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) is a ligand-activated transcription factor with essential functions in the regulation of lipid catabolism, glucose homeostasis, and inflammation, which makes it a potentially relevant drug target for the treatment of major human diseases. In addition, there is strong evidence that PPARbeta/delta modulates oncogenic signaling pathways and tumor growth. Consistent with these observations, numerous reports have clearly documented a role for PPARbeta/delta in cell cycle control, differentiation, and apoptosis. However, the precise role of PPARbeta/delta in tumorigenesis and cell proliferation remains controversial. This review summarizes our current knowledge and proposes a model corroborating the discrepant data in this area of research.

Peroxisome proliferator-activated receptor-beta/delta protects against chemically induced liver toxicity in mice

Potential functional roles for the peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) in skeletal muscle fatty acid catabolism and epithelial carcinogenesis have recently been described. Whereas PPARbeta/delta is expressed in liver, its function in this tissue is less clear. To determine the role of PPARbeta/delta in chemically induced liver toxicity, wild-type and PPARbeta/delta-null mice were treated with azoxymethane (AOM) and markers of liver toxicity examined. Bile duct hyperplasia, regenerative hyperplasia, and increased serum alanine aminotransferase (ALT) were found in AOM-treated PPARbeta/delta-null mice, and these effects were not observed in similarly treated wild-type mice. Exacerbated carbon tetrachloride (CCl(4)) hepatoxicity was also observed in PPARbeta/delta-null as compared with wild-type mice. No differences in messenger RNAs (mRNAs) encoding cytochrome2E1 required for the metabolic activation of AOM and CCl(4) were observed between wild-type or PPARbeta/delta-null mice in response to CCl(4). Significant differences in the expression of genes reflecting enhanced nuclear factor kappa B (NF-kappaB) activity were noted in PPARbeta/delta-null mice.

CONCLUSION

Results from these studies show that PPARbeta/delta is protective against liver toxicity induced by AOM and CCl(4), suggesting that this receptor is hepatoprotective against environmental chemicals that are metabolized in this tissue.

Inflammation and lung carcinogenesis: applying findings in prevention and treatment

Lung carcinogenesis is a complex process requiring the acquisition of genetic mutations that confer the malignant phenotype as well as epigenetic alterations that may be manipulated in the course of therapy. Inflammatory signals in the lung cancer microenvironment can promote apoptosis resistance, proliferation, invasion, metastasis, and secretion of proangiogenic and immunosuppressive factors. Here, we discuss several prototypical inflammatory mediators controlling the malignant phenotype in lung cancer. Investigation into the detailed molecular mechanisms underlying the tumor-promoting effects of inflammation in lung cancer has revealed novel potential drug targets. Cytokines, growth factors and small-molecule inflammatory mediators released in the developing tumor microenvironment pave the way for epithelial-mesenchymal transition, the shift from a polarized, epithelial phenotype to a highly motile mesenchymal phenotype that becomes dysregulated during tumor invasion. Inflammatory mediators within the tumor microenvironment are derived from neoplastic cells as well as stromal and inflammatory cells; thus, lung cancer develops in a host environment in which the deregulated inflammatory response promotes tumor progression. Inflammation-related metabolic and catabolic enzymes (prostaglandin E(2) synthase, prostaglandin I(2) synthase and 15-hydroxyprostaglandin dehydrogenase), cell-surface receptors (E-type prostaglandin receptors) and transcription factors (ZEB1, SNAIL, PPARs, STATs and NF-kappaB) are differentially expressed in lung cancer cells compared with normal lung epithelial cells and, thus, may contribute to tumor initiation and progression. These newly discovered molecular mechanisms in the pathogenesis of lung cancer provide novel opportunities for targeted therapy and prevention in lung cancer.

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands inhibit growth of UACC903 and MCF7 human cancer cell lines

The development of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands for the treatment of diseases including metabolic syndrome, diabetes and obesity has been hampered due to contradictory findings on their potential safety. For example, while some reports show that ligand activation of PPARbeta/delta promotes the induction of terminal differentiation and inhibition of cell growth, other reports suggest that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation. Some of the contradictory findings could be due in part to differences in the ligand examined, the presence or absence of serum in cell cultures, differences in cell lines or differences in the method used to quantify cell growth. For these reasons, this study examined the effect of ligand activation of PPARbeta/delta on cell growth of two human cancer cell lines, MCF7 (breast cancer) and UACC903 (melanoma) in the presence or absence of serum using two highly specific PPARbeta/delta ligands, GW0742 or GW501516. Culturing cells in the presence of either GW0742 or GW501516 caused upregulation of the known PPARbeta/delta target gene angiopoietin-like protein 4 (ANGPTL4). Inhibition of cell growth was observed in both cell lines cultured in the presence of either GW0742 or GW501516, and the presence or absence of serum had little influence on this inhibition. Results from the present studies demonstrate that ligand activation of PPARbeta/delta inhibits the growth of both MCF7 and UACC903 cell lines and provide further evidence that PPARbeta/delta ligands are not mitogenic in human cancer cell lines.

Ligand activation of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) and inhibition of cyclooxygenase 2 (COX2) attenuate colon carcinogenesis through independent signaling mechanisms

Cyclooxygenase (COX) 2-derived prostaglandin E(2) (PGE(2)) promotes colorectal carcinoma growth and invasion, and inhibition of COX2 by non-steroidal anti-inflammatory drugs is known to inhibit these processes. There is controversy regarding the effect of ligand activation of peroxisome proliferator-activated receptor (PPAR)-beta/delta on colon carcinogenesis, although collective evidence from independent laboratories suggest that ligand activation of PPARbeta/delta leads to the induction of terminal differentiation coupled with inhibition of cell growth in a variety of models. The present study examined the hypothesis that ligand activation of PPARbeta/delta and inhibition of COX2 attenuate colon cancer through independent mechanisms and that combining these two mechanisms will enhance this inhibition. Colon cancer was induced by administering azoxymethane to wild-type and PPARbeta/delta-null mice. Cohorts of mice were treated with GW0742 (a PPARbeta/delta ligand), nimesulide (a COX2 inhibitor) or a combination of GW0742 and nimesulide. Inhibition of COX2 by nimesulide attenuated colon cancer and ligand activation of PPARbeta/delta by GW0742 had inhibitory effects. However, the combined treatment of GW0742 and nimesulide did not cause an enhancement in the attenuation of colon cancer. Mechanistically, the effects of these compounds occurred through independent mechanisms as increased levels of differentiation markers as a result of ligand activation of PPARbeta/delta were not found with COX2 inhibition, and a reduction in PGE(2) levels resulting from COX2 inhibition was not observed in response to ligand activation of PPARbeta/delta. Results from these studies effectively dissociate COX2 inhibition and PPARbeta/delta activity during colon carcinogenesis.

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands do not potentiate growth of human cancer cell lines

Ligands for peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) increase skeletal muscle fatty acid catabolism, improve insulin sensitivity, increase serum high-density lipoprotein cholesterol, elicit anti-inflammatory activity and induce terminal differentiation. Contradictory findings are also reported suggesting that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation, by inhibiting apoptosis through phosphorylation of Akt and by increasing cyclooxygenase-2 (COX2) and vascular endothelial growth factor (VEGF) expression. The contradictory findings could be due to differences in the model system (cancer cell line versus in vivo), differences in cell culture conditions (with and without serum) or differences in ligands. The present study examined the effect of two different PPARbeta/delta ligands (GW0742 and GW501516) in human cancer cell lines (HT29, HCT116, LS-174T, HepG2 and HuH7) cultured in the presence or absence of serum and compared in vitro analysis with in vivo analysis. Neither PPARbeta/delta ligand increased cell growth or phosphorylation of Akt and no increase in the expression of VEGF or COX2 were detected in any cancer cell line in the presence or absence of serum. Similarly, liver, colon and colon polyps from mice administered these PPARbeta/delta ligands in vivo did not exhibit changes in these markers. Results from these studies demonstrate that serum withdrawal and/or differences in ligands do not underlie the disparity in responses reported in the literature. The quantitative nature of the present findings are inconsistent with the hypothesis that cancer cell lines respond differentially as compared with normal cells, and provide further evidence that PPARbeta/delta ligands do not potentiate tumorigenesis.

Extracellular signal-regulated kinase is a target of cyclooxygenase-1-peroxisome proliferator-activated receptor-delta signaling in epithelial ovarian cancer

The underlying causes of epithelial ovarian cancer (EOC) are unclear, and treatment options for patients with advanced disease are limited. There is evidence that the use of nonsteroidal anti-inflammatory drugs is associated with decreased risk of developing EOC. Nonsteroidal anti-inflammatory drugs inhibit cyclooxygenase (COX)-1 and COX-2, which catalyze prostaglandin biosynthesis. We previously showed that mouse and human EOCs have increased levels of COX-1, but not COX-2, and a COX-1-selective inhibitor, SC-560, attenuates prostaglandin production and tumor growth. However, the downstream targets of COX-1 signaling in EOC are not yet known. To address this question, we evaluated peroxisome proliferator-activated receptor delta (PPARdelta) expression and function in EOC. We found that EOC cells express high levels of PPARdelta, and neutralizing PPARdelta function reduces tumor growth in vivo. More interestingly, aspirin, a nonsteroidal anti-inflammatory drug that preferentially inhibits COX-1, compromises PPARdelta function and cell growth by inhibiting extracellular signal-regulated kinases 1/2, members of the mitogen-activated protein kinase family. Our study, for the first time, shows that whereas PPARdelta can be a target of COX-1, extracellular signal-regulated kinase is a potential target of PPARdelta. The ability of aspirin to inhibit EOC growth in vivo is an exciting finding because of its low cost, lack of cardiovascular side effects, and availability.

Carbacyclin induces carnitine palmitoyltransferase-1 in cardiomyocytes via peroxisome proliferator-activated receptor (PPAR) δ independent of the IP receptor signaling pathway

Prostacyclin (PGI2) and its analogues exert cardioprotective effects via the rhodopsin type membrane PGI2 receptor, IP. Peroxisome proliferator-activated receptor (PPAR) delta is a nuclear receptor abundantly expressed in cardiomyocytes and plays a pivotal role in maintaining constitutive mitochondrial fatty acid beta-oxidation (FAO). Recently, a novel signaling pathway of PGI2 via PPARdelta has been demonstrated in non-cardiac tissues. We therefore examined whether carbacyclin (cPGI2), a PGI2 analogue, up-regulates transcriptional expression of carnitine palmitoyltransferase-1 (CPT-1), the rate-limiting enzyme in mitochondrial FAO, via PPARdelta in cardiomyocytes. Intraperitoneal injection of cPGI2 increased CPT-1 mRNA expression in murine hearts. Transcriptional activity was evaluated by PPAR responsive element (PPRE)-luciferase reporter gene assay in cultured neonatal rat cardiomyocytes. CPT-1 mRNA expression and PPRE promoter activity were significantly increased by cPGI2 in a concentration-dependent manner, where PPRE has been mapped to the promoter region of the CPT-1 gene. Moreover, the elevation of CPT-1 mRNA expression and PPRE promoter activity by cPGI2 was not abolished by H-89, a potent protein kinase A inhibitor, but was significantly inhibited in cardiomyocytes over-expressing a dominant-negative type of PPARdelta. Furthermore, electrophoretic mobility shift assays demonstrated that binding of PPARdelta to PPRE in the CPT-1 gene promoter is enhanced in response to cPGI2 stimulation. In addition, down-regulation of CPT-1 mRNA expression in cardiomyocytes subjected to hypoxia was attenuated by cPGI2. These results indicate that cPGI2 induces CPT-1 mRNA expression through PPARdelta, independent of the IP receptor signaling pathway, suggesting a possibility that cPGI2 modulates cardiac energy metabolism by activating FAO via PPARdelta.

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

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

PPARdelta status and mismatch repair mediated neoplasia in the mouse intestine

Background

Therapeutic regulation of PPARδ activity using selective agonists has been proposed for various disorders. However, the consequences of altered peroxisome proliferator-activated receptor delta (PPARδ) activity in the context of intestinal tumourigenesis remain somewhat unclear. Contradictory evidence suggesting PPARδ either attenuates or potentiates intestinal neoplasia. To further investigate the PPARδ dependency of intestinal tumourigenesis, we have analysed the consequences of PPARδ deficiency upon intestinal neoplasia occurring in mice with impaired mismatch DNA repair.

Methods

Mice deficient for both PPARδ and the mismatch repair gene Mlh1 were produced and the incidence and severity of intestinal neoplasia recorded.

Results

No significant differences between the control genotypes and the double mutant genotypes were recorded indicating that deficiency of PPARδ does not modify impaired mismatch repair induced neoplasia.

Conclusion

In contrast with the previously observed acceleration of intestinal neoplasia in the context of the Apc^Min/+ mouse, PPARδ deficiency does not alter the phenotype of mismatch repair deficiency. This data supports the notion that PPARδ is not required for adenoma formation and indicate that any pro-tumourigenic effect of PPARδ inactivation may be highly context dependent.