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

Peroxisome Proliferator-Activated Receptors as Molecular Links between Caloric Restriction and Circadian Rhythm

The circadian rhythm plays a chief role in the adaptation of all bodily processes to internal and environmental changes on the daily basis. Next to light/dark phases, feeding patterns constitute the most essential element entraining daily oscillations, and therefore, timely and appropriate restrictive diets have a great capacity to restore the circadian rhythm. One of the restrictive nutritional approaches, caloric restriction (CR) achieves stunning results in extending health span and life span via coordinated changes in multiple biological functions from the molecular, cellular, to the whole-body levels. The main molecular pathways affected by CR include mTOR, insulin signaling, AMPK, and sirtuins. Members of the family of nuclear receptors, the three peroxisome proliferator-activated receptors (PPARs), PPARα, PPARβ/δ, and PPARγ take part in the modulation of these pathways. In this non-systematic review, we describe the molecular interconnection between circadian rhythm, CR-associated pathways, and PPARs. Further, we identify a link between circadian rhythm and the outcomes of CR on the whole-body level including oxidative stress, inflammation, and aging. Since PPARs contribute to many changes triggered by CR, we discuss the potential involvement of PPARs in bridging CR and circadian rhythm.

Peroxisome Proliferator-Activated Receptors and Caloric Restriction-Common Pathways Affecting Metabolism, Health, and Longevity

Caloric restriction (CR) is a traditional but scientifically verified approach to promoting health and increasing lifespan. CR exerts its effects through multiple molecular pathways that trigger major metabolic adaptations. It influences key nutrient and energy-sensing pathways including mammalian target of rapamycin, Sirtuin 1, AMP-activated protein kinase, and insulin signaling, ultimately resulting in reductions in basic metabolic rate, inflammation, and oxidative stress, as well as increased autophagy and mitochondrial efficiency. CR shares multiple overlapping pathways with peroxisome proliferator-activated receptors (PPARs), particularly in energy metabolism and inflammation. Consequently, several lines of evidence suggest that PPARs might be indispensable for beneficial outcomes related to CR. In this review, we present the available evidence for the interconnection between CR and PPARs, highlighting their shared pathways and analyzing their interaction. We also discuss the possible contributions of PPARs to the effects of CR on whole organism outcomes.

PPARβ/δ Agonist GW501516 Inhibits Tumorigenicity of Undifferentiated Nasopharyngeal Carcinoma in C666-1 Cells by Promoting Apoptosis

Activation of peroxisome proliferator-activated receptor β/δ (PPARβ/δ) had been linked to inhibition on the proliferation and apoptosis in a few cancer cell lines. However, limited data exists regarding the role of PPARβ/δ in nasopharyngeal carcinoma (NPC). This study was undertaken to determine the effect of PPARβ/δ on cell proliferation, anchorage-dependent clonogenicity, and ectopic xenografts in the human NPC cell lines. Gene and protein expression of PPARβ/δ were reduced specifically in the poor- and un-differentiated NPC cell lines as compared with the control NP-69 cells. Ligand activation of PPARβ/δ by GW501516, a specific PPARβ/δ selective agonist, inhibited cell proliferation and colony formation strikingly, and induced a G2/M phase arrest in the EBV positive undifferentiated NPC C666-1 cells relative to the control cells. Moreover, GW501516 induced C666-1 cell apoptosis in a caspase and BAX dependent manner. In accordance with the in vitro result, GW501516 significantly suppressed the ectopic NPC xenograft tumorigenicity that derived from the C666-1 NPC cells in BALB/c nu/nu mice. This effect is greatly associated with its inhibition on the gene and protein expression of integrin-linked kinase (ILK) through activation of the AMPKα-dependent signaling pathways. Collectively, we showed that PPARβ/δ expression is in reverse correlation with the degree of differentiation in the NPC cell lines, and revealed the anti-tumorigenic effects of GW501516 in NPC cells by activation of AMPKα. This study suggested that PPARβ/δ targeting molecules may be useful for the poor-, and particularly un-differentiated NPC chemoprevention.

Targeting protein kinase-b3 (akt3) signaling in melanoma

INTRODUCTION

Deregulated Akt activity leading to apoptosis inhibition, enhanced proliferation and drug resistance has been shown to be responsible for 35-70% of advanced metastatic melanomas. Of the three isoforms, the majority of melanomas have elevated Akt3 expression and activity. Hence, potent inhibitors targeting Akt are urgently required, which is possible only if (a) the factors responsible for the failure of Akt inhibitors in clinical trials is known; and (b) the information pertaining to synergistically acting targeted therapeutics is available. Areas covered: This review provides a brief introduction of the PI3K-Akt signaling pathway and its role in melanoma development. In addition, the functional role of key Akt pathway members such as PRAS40, GSK3 kinases, WEE1 kinase in melanoma development are discussed together with strategies to modulate these targets. Efficacy and safety of Akt inhibitors is also discussed. Finally, the mechanism(s) through which Akt leads to drug resistance is discussed in this expert opinion review. Expert opinion: Even though Akt play key roles in melanoma tumor progression, cell survival and drug resistance, many gaps still exist that require further understanding of Akt functions, especially in the (a) metastatic spread; (b) circulating melanoma cells survival; and

Activation of Peroxisome Proliferator–Activated Receptor-δ as Novel Therapeutic Strategy to Prevent In-Stent Restenosis and Stent Thrombosis

Objective—

Drug-eluting coronary stents reduce restenosis rate and late lumen loss compared with bare-metal stents; however, drug-eluting coronary stents may delay vascular healing and increase late stent thrombosis. The peroxisome proliferator–activated receptor-delta (PPARδ) exhibits actions that could favorably influence outcomes after drug-eluting coronary stents placement.

Approach and Results—

Here, we report that PPARδ ligand–coated stents strongly reduce the development of neointima and luminal narrowing in a rabbit model of experimental atherosclerosis. Inhibition of inflammatory gene expression and vascular smooth muscle cell (VSMC) proliferation and migration, prevention of thrombocyte activation and aggregation, and proproliferative effects on endothelial cells were identified as key mechanisms for the prevention of restenosis. Using normal and PPARδ-depleted VSMCs, we show that the observed effects of PPARδ ligand GW0742 on VSMCs and thrombocytes are PPARδ receptor dependent. PPARδ ligand treatment induces expression of pyruvate dehydrogenase kinase isozyme 4 and downregulates the glucose transporter 1 in VSMCs, thus impairing the ability of VSMCs to provide the increased energy demands required for growth factor–stimulated proliferation and migration.

Conclusions—

In contrast to commonly used drugs for stent coating, PPARδ ligands not only inhibit inflammatory response and proliferation of VSMCs but also prevent thrombocyte activation and support vessel re-endothelialization. Thus, pharmacological PPARδ activation could be a promising novel strategy to improve drug-eluting coronary stents outcomes.

PXR Mediated Protection against Liver Inflammation by Ginkgolide A in Tetrachloromethane Treated Mice

The pregnane X receptor (PXR), a liver and intestine specific receptor,, has been reported to be related with the repression of inflammation as well as activation of cytochromosome P450 3A (CYP3A) expression. We examined the effect of PXR on tetrachloromethane (CCl4)-induced mouse liver inflammation in this work. Ginkgolide A, one main component of Ginkgo biloba extracts (GBE), activated PXR and enhanced PXR expression level, displayed both significant therapeutic effect and preventive effect against CCl4-induced mouse hepatitis. siRNA-mediated decrease of PXR expression significantly reduced the efficacy of Ginkgolide A in treating CCl4-induced inflammation in mice. Flavonoids, another important components of GBE, were shown anti-inflammatory effect in a different way from Ginkgolide A which might be independent on PXR because flavonoids significantly inhibited CYP3A11 activities in mice. The results indicated that anti-inflammatory effect of PXR might be mediated by enhancing transcription level of IκBα through binding of IκBα. Inhibition of NF-κB activity by NF-κB-specific suppressor IκBα is one of the potential mechanisms of Ginkgolide A against CCl4-induced liver inflammation.

Establishing the Role of PPARβ/δ in Carcinogenesis

The role of the nuclear hormone receptor peroxisome proliferator-activated receptor β/δ (PPARβ/δ) in carcinogenesis is controversial because conflicting studies indicate that it both inhibits and promotes tumorigenesis. In this review, we focus on recent studies on PPARβ/δ including the significance of increased or decreased PPARβ/δ expression in cancers; a range of opposing mechanisms describing how PPARβ/δ agonists, antagonists, and inverse agonists regulate tumorigenesis and/or whether there may be cell context-specific mechanisms; and whether activating or inhibiting PPARβ/δ is feasible for cancer chemoprevention and/or therapy. Research questions that need to be addressed are highlighted to establish whether PPARβ/δ can be effectively targeted for cancer chemoprevention.

Antitumor action of the peroxisome proliferator-activated receptor-γ agonist rosiglitazone in hepatocellular carcinoma

The inhibition of apoptosis in cancer cells is the major pathological feature of hepatic carcinoma. Rosiglitazone (RGZ), a ligand for peroxisome proliferator-activated receptor γ (PPAR-γ), has been shown to induce apoptosis in hepatic carcinoma cells. However, the mechanism underlying this effect remains to be elucidated. The present study aimed to investigate the effect of RGZ on cell viability and apoptosis, and its mechanisms in cultured HepG2 cells using MTT assay, flow cytometry and western blotting. The results revealed that treatment with RGZ may attenuate HepG2 cell viability and induce the apoptosis of the cells. The mechanism of RGZ-induced apoptosis involves an increase in the level of activated PPAR-γ (p-PPAR-γ) and a decrease in p85 and Akt expression. In addition, the PPAR-γ antagonist GW9662 suppressed the effect of RGZ in the HepG2 cells. Taken together, the results suggest that RGZ induces the apoptosis of HepG2 cells through the activation of PPAR-γ, suppressing the activation of the PI3K/Akt signaling pathway. Such mechanisms may contribute to the favorable effects of treatment using RGZ in HepG2 cells.

Targeting Peroxisome Proliferator-Activated Receptor-β/δ (PPARβ/δ) for Cancer Chemoprevention

The role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in cancer remains contentious due in large part to divergent publications indicating opposing effects in different rodent and human cell culture models. During the past 10 years, some facts regarding PPARβ/δ in cancer have become clearer, while others remain uncertain. For example, it is now well accepted that (1) expression of PPARβ/δ is relatively lower in most human tumors as compared to the corresponding non-transformed tissue, (2) PPARβ/δ promotes terminal differentiation, and (3) PPARβ/δ inhibits pro-inflammatory signaling in multiple in vivo models. However, whether PPARβ/δ is suitable to target with natural and/or synthetic agonists or antagonists for cancer chemoprevention is hindered because of the uncertainty in the mechanism of action and role in carcinogenesis. Recent findings that shed new insight into the possibility of targeting this nuclear receptor to improve human health will be discussed.

Opposite cross-talk by oleate and palmitate on insulin signaling in hepatocytes through macrophage activation

Chronic low grade inflammation in adipose tissue during obesity is associated with an impairment of the insulin signaling cascade. In this study, we have evaluated the impact of palmitate or oleate overload of macrophage/Kupffer cells in triggering stress-mediated signaling pathways, in lipoapoptosis, and in the cross-talk with insulin signaling in hepatocytes. RAW 264.7 macrophages or Kupffer cells were stimulated with oleate or palmitate, and levels of M1/M2 polarization markers and the lipidomic profile of eicosanoids were analyzed. Whereas proinflammatory cytokines and total eicosanoids were elevated in macrophages/Kupffer cells stimulated with palmitate, enhanced arginase 1 and lower leukotriene B4 (LTB4) levels were detected in macrophages stimulated with oleate. When hepatocytes were pretreated with conditioned medium (CM) from RAW 264.7 or Kupffer cells loaded with palmitate (CM-P), phosphorylation of stress kinases and endoplasmic reticulum stress signaling was increased, insulin signaling was impaired, and lipoapoptosis was detected. Conversely, enhanced insulin receptor-mediated signaling and reduced levels of the phosphatases protein tyrosine phosphatase 1B (PTP1B) and phosphatase and tensin homolog (PTEN) were found in hepatocytes treated with CM from macrophages stimulated with oleate (CM-O). Supplementation of CM-O with LTB4 suppressed insulin sensitization and increased PTP1B and PTEN. Furthermore, LTB4 decreased insulin receptor tyrosine phosphorylation in hepatocytes, activated the NFκB pathway, and up-regulated PTP1B and PTEN, these effects being mediated by LTB4 receptor BTL1. In conclusion, oleate and palmitate elicit an opposite cross-talk between macrophages/Kupffer cells and hepatocytes. Whereas CM-P interferes at the early steps of insulin signaling, CM-O increases insulin sensitization, possibly by reducing LTB4.

Polymorphisms of peroxisome proliferator-activated receptors and survival of lung cancer and upper aero-digestive tract cancers

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) are transcriptional factors involved in several biological processes such as inflammation, cancer growth, progression and apoptosis that are important in lung and upper aero-digestive tract (UADT) cancer outcomes. Nonetheless, there are no published studies of the relationship between PPARs gene polymorphisms and survival of patients with lung cancer or UADT cancers.

METHODS

1212 cancer patients (611 lung, 303 oral, 100 pharyngeal, 90 laryngeal, and 108 esophageal) were followed for a median duration of 11 years. We genotyped three potentially functional single nucleotide polymorphisms (SNPs) using Taqman - rs3734254 of the gene PPARD and rs10865710 and rs1801282 of the gene PPARG - and investigated their associations with lung and UADT cancer survival using Cox regression. A semi-Bayesian shrinkage approach was used to reduce the potential for false positive findings when examining multiple associations.

RESULTS

The variant homozygote CC (vs. TT) of PPARD rs3734254 was inversely associated with mortality of both lung cancer (adjusted hazard ratio [aHR]=0.63, 95% confidence interval [CI]=0.42, 0.96) and UADT cancers (aHR=0.51, 95% CI=0.27, 0.99). Use of the semi-Bayesian shrinkage approach yielded a posterior aHR for lung cancer of 0.66 (95% posterior limits=0.44, 0.98) and a posterior aHR for UADT cancers of 0.58 (95% posterior limits=0.33, 1.03).

CONCLUSION

Our findings suggest that lung-cancer patients with the CC variant of PPARD rs3734254 may have a survival advantage over lung-cancer patients with other gene variants.

PPARδ induces estrogen receptor-positive mammary neoplasia through an inflammatory and metabolic phenotype linked to mTOR activation

The peroxisome proliferator-activated receptor-δ (PPARδ) regulates a multitude of physiological processes associated with glucose and lipid metabolism, inflammation, and proliferation. One or more of these processes are potential risk factors for the ability of PPARδ agonists to promote tumorigenesis in the mammary gland. In this study, we describe a new transgenic mouse model in which activation of PPARδ in the mammary epithelium by endogenous or synthetic ligands resulted in progressive histopathologic changes that culminated in the appearance of estrogen receptor- and progesterone receptor-positive and ErbB2-negative infiltrating ductal carcinomas. Multiparous mice presented with mammary carcinomas after a latency of 12 months, and administration of the PPARδ ligand GW501516 reduced tumor latency to 5 months. Histopathologic changes occurred concurrently with an increase in an inflammatory, invasive, metabolic, and proliferative gene signature, including expression of the trophoblast gene, Plac1, beginning 1 week after GW501516 treatment, and remained elevated throughout tumorigenesis. The appearance of malignant changes correlated with a pronounced increase in phosphatidylcholine and lysophosphatidic acid metabolites, which coincided with activation of Akt and mTOR signaling that were attenuated by treatment with the mTOR inhibitor everolimus. Our findings are the first to show a direct role of PPARδ in the pathogenesis of mammary tumorigenesis, and suggest a rationale for therapeutic approaches to prevent and treat this disease.

Transcriptional and Non-Transcriptional Functions of PPARβ/δ in Non-Small Cell Lung Cancer

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is a nuclear receptor involved in regulation of lipid and glucose metabolism, wound healing and inflammation. PPARβ/δ has been associated also with cancer. Here we investigated the expression of PPARβ/δ and components of the prostaglandin biosynthetic pathway in non-small cell lung cancer (NSCLC). We found increased expression of PPARβ/δ, Cox-2, cPLA₂, PGES and VEGF in human NSCLC compared to normal lung. In NSCLC cell lines PPARβ/δ activation increased proliferation and survival, while PPARβ/δ knock-down reduced viability and increased apoptosis. PPARβ/δ agonists induced Cox-2 and VEGF transcription, suggesting the existence of feed-forward loops promoting cell survival, inflammation and angiogenesis. These effects were seen only in high PPARβ/δ expressing cells, while low expressing cells were less or not affected. The effects were also abolished by PPARβ/δ knock-down or incubation with a PPARβ/δ antagonist. Induction of VEGF was due to both binding of PPARβ/δ to the VEGF promoter and PI3K activation through a non-genomic mechanism. We found that PPARβ/δ interacted with the PI3K regulatory subunit p85α leading to PI3K activation and Akt phosphorylation. Collectively, these data indicate that PPARβ/δ might be a central element in lung carcinogenesis controlling multiple pathways and representing a potential target for NSCLC treatment.

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.

Prognostic value of nuclear factor κ B expression in patients with advanced cervical cancer undergoing radiation therapy followed by hysterectomy

Aims

The nuclear factor κ B (NF-κB) family comprises transcription factors that promote the development and progression of cancer. The NF-κB pathway is induced by radiation therapy and may be related to tumour radioresistance. The aim of this study was to evaluate the expression of NF-κB as a predictor of the response to radiotherapy and its value as a prognostic marker.

Methods

A retrospective analysis was performed in a series of 32 individuals with stage IB2 and IIB cervical cancer who underwent radiotherapy, followed by radical hysterectomy, from January 1992 to June 2001. NF-κB-p65 and NF-κB-p50 expression was examined by immunohistochemistry in biopsies from all patients before radiotherapy and in 12 patients with residual tumours after radiotherapy.

Results

16 (50%) patients had residual disease after radical hysterectomy. The median follow-up time was 73.5 months, and the 5-year overall survival was 66.5%. Before radiotherapy, cytoplasmic expression of NF-κB-p65 and NF-κB-p50 was noted in 91% and 97% of cases, respectively, versus 59% of cases with nuclear expression of these subunits. Cytoplasmic expression of NF-κB-p65 and NF-κB-p50 in the residual tumours after radiotherapy was observed in 50% of cases; 75% of cases with residual tumours had nuclear expression of NF-κB-p50 versus none with NF-κB-p65. NF-κB-p65 and NF-κB-p50 did not correlate with the risk of residual tumours after radiotherapy or recurrence or death.

Conclusions

These data suggest that NF-κB does not predict the response to radiotherapy and does not correlate with poor outcomes in advanced cervical cancer.

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.

Nuclear factor-κB: a key regulator in health and disease of lungs

Rel/NF-κB transcription factors play a key role in modulating the response of immunoregulatory genes including cytokines and chemokines, cell adhesion molecules, acute phase proteins, and anti-microbial peptides. Furthermore, an array of genes important for angiogenesis, tumor invasion and metastasis is also regulated by nuclear factor-κB (NF-κB). Close association of NF-κB with inflammation and tumorigenesis makes it an attractive target for basic research as well as for pharmaceutical industries. Studies involving various animal and cellular models have revealed the importance of NF-κB in pathobiology of lung diseases. This review (a) describes structures, activities, and regulation of NF-κB family members; (b) provides information which implicates NF-κB in pathogenesis of pulmonary inflammation and cancer; and (c) discusses information about available synthetic and natural compounds which target NF-κB or specific components of NF-κB signal transduction pathway and which may provide the foundation for development of effective therapy for lung inflammation and bronchogenic carcinomas.

Activation of peroxisome proliferator-activated receptor-{delta} enhances regenerative capacity of human endothelial progenitor cells by stimulating biosynthesis of tetrahydrobiopterin

The mechanisms underlying the regenerative capacity of endothelial progenitor cells (EPCs) are not fully understood. We hypothesized that biosynthesis of tetrahydrobiopterin is an important mechanism responsible for the stimulatory effects of peroxisome proliferator-activated receptor-δ (PPARδ) activation on regenerative function of human EPCs. Treatment of human EPCs with a selective PPARδ agonist GW501516 for 24 hours increased the levels of mRNA, protein, and enzymatic activity of GTP cyclohydrolase I (GTPCH I), as well as the production of tetrahydrobiopterin. The effects of GW501516 were mediated by suppression of PTEN expression, thereby increasing phosphorylation of AKT. The AKT signaling also mediated GW501516-induced phosphorylation of endothelial NO synthase. In addition, activation of PPARδ significantly enhanced proliferation of EPCs. This effect was abolished by the GTPCH I inhibitor, 2,4-diamino-6-hydroxypyrimidine, or genetic inactivation of GTPCH I with small interfering RNA but not by inhibition of endothelial NO synthase with N(G)-nitro-l-arginine methyl ester. Supplementation of NO did not reverse 2,4-diamino-6-hydroxypyrimidine-inhibited 5-bromodeoxyuridine incorporation. Furthermore, transplantation of human EPCs stimulated re-endothelialization in a mouse model of carotid artery injury. Pretreatment of EPCs with GW501516 significantly enhanced the ability of transplanted EPCs to repair denuded endothelium. GTPCH I-small interfering RNA transfection significantly inhibited in vivo regenerative capacity of EPCs stimulated with GW501516. Thus, in human EPCs, activation of PPARδ stimulates expression and activity of GTPCH I and biosynthesis of tetrahydrobiopterin via PTEN-AKT signaling pathway. This effect enhances the regenerative function of EPCs.

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.

Non-genomic loss of PTEN function in cancer: not in my genes

Loss of function of the phosphatase and tensin homolog (PTEN) tumour suppressor contributes to the development of many cancers. However, in contrast to classical models of tumour suppression, partial loss of PTEN function appears to be frequently observed in the clinic. In addition, studies of both humans and mice with reductions in PTEN gene dosage indicate that even partial loss of PTEN function is sufficient to promote some cancer types, particularly in the breast. PTEN expression appears to be tightly controlled both transcriptionally and post-transcriptionally, with several recent studies implicating oncogenic microRNAs in PTEN suppression. The lipid phosphatase activity of PTEN can also be regulated post-translationally via inhibitory phosphorylation, ubiquitination or oxidation. Here we discuss these multiple mechanisms of PTEN regulation. We also put into context recent proposals that changes in this regulation can drive tumour development and address the accompanying evidence for their clinical significance.

Induction of metastatic gastric cancer by peroxisome proliferator-activated receptorδ activation

Peroxisome proliferator-activated receptorδ (PPARδ) regulates a multiplicity of physiological processes associated with glucose and lipid metabolism, inflammation, and proliferation. One or more of these processes likely create risk factors associated with the ability of PPARδ agonists to promote tumorigenesis in some organs. In the present study, we describe a new gastric tumor mouse model that is dependent on the potent and highly selective PPARδ agonist GW501516 following carcinogen administration. The progression of gastric tumorigenesis was rapid as determined by magnetic resonance imaging and resulted in highly metastatic squamous cell carcinomas of the forestomach within two months. Tumorigenesis was associated with gene expression signatures indicative of cell adhesion, invasion, inflammation, and metabolism. Increased PPARδ expression in tumors correlated with increased PDK1, Akt, β-catenin, and S100A9 expression. The rapid development of metastatic gastric tumors in this model will be useful for evaluating preventive and therapeutic interventions in this disease.

PPARalpha is essential for microparticle-induced differentiation of mouse bone marrow-derived endothelial progenitor cells and angiogenesis

Background

Bone marrow-derived endothelial progenitor cells (EPCs) are critical for neovascularization. We hypothesized that microparticles (MPs), small fragments generated from the plasma membrane, can activate angiogenic programming of EPCs.

Methodology/Principal Findings

We studied the effects of MPs obtained from wild type (MPs^PPARα+/+) and knock-out (MPs^{PPARα−/−}) mice on EPC differentiation and angiogenesis. Bone marrow-derived cells were isolated from WT or KO mice and were cultured in the presence of MPs^PPARα+/+ or MPs^{PPARα−/−} obtained from blood of mice. Only MPs^PPARα+/+ harboring PPAR^α significantly increased EPC, but not monocytic, differentiation. Bone marrow-derived cells treated with MPs^PPARα+/+ displayed increased expression of pro-angiogenic genes and increased in vivo angiogenesis. MPs^PPARα+/+ increased capillary-like tube formation of endothelial cells that was associated with enhanced expressions of endothelial cell-specific markers. Finally, the effects of MPs^PPARα+/+ were mediated by NF-κB-dependent mechanisms.

Conclusions/Significance

Our results underscore the obligatory role of PPARα carried by MPs for EPC differentiation and angiogenesis. PPARα-NF-κB-Akt pathways may play a pivotal stimulatory role for neovascularization, which may, at least in part, be mediated by bone marrow-derived EPCs. Improvement of EPC differentiation may represent a useful strategy during reparative neovascularization.

Chronic expression of PPAR-delta by oligodendrocyte lineage cells in the injured rat spinal cord

The transcription factor peroxisome proliferator-activated receptor (PPAR)-delta promotes oligodendrocyte differentiation and myelin formation in vitro and is prevalent throughout the brain and spinal cord. Its expression after injury, however, has not been examined. Thus, we used a spinal contusion model to examine the spatiotemporal expression of PPAR-delta in naïve and injured spinal cords from adult rats. As previously reported, PPAR-delta was expressed by neurons and oligodendrocytes in uninjured spinal cords; PPAR-delta was also detected in NG2 cells (potential oligodendrocyte progenitors) within the white matter and gray matter. After spinal cord injury (SCI), PPAR-delta mRNA and protein were present early and increased over time. Overall PPAR-delta+ cell numbers declined at 1 day post injury (dpi), likely reflecting neuron loss, and then rose through 14 dpi. A large proportion of NG2 cells expressed PPAR-delta after SCI, especially along lesion borders. PPAR-delta+ NG2 cell numbers were significantly higher than naive by 7 dpi and remained elevated through at least 28 dpi. PPAR-delta+ oligodendrocyte numbers declined at 1 dpi and then increased over time such that >20% of oligodendrocytes expressed PPAR-delta after SCI compared with approximately 10% in uninjured tissue. The most prominent increase in PPAR-delta+ oligodendrocytes was along lesion borders where at least a portion of newly generated oligodendrocytes (bromodeoxyuridine+) were PPAR-delta+. Consistent with its role in cellular differentiation, the early rise in PPAR-delta+ NG2 cells followed by an increase in new PPAR-delta+ oligodendrocytes suggests that this transcription factor may be involved in the robust oligodendrogenesis detected previously along SCI lesion borders.

Anticancer actions of PPARγ ligands: Current state and future perspectives in human lung cancer

Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent nuclear transcription factors and members of the nuclear receptor superfamily. Of the three PPARs identified to date (PPARγ, PPARβ/δ, and PPARα), PPARγ has been studied the most, in part because of the availability of PPARγ agonists (also known as PPARγ ligands) and its significant effects on the management of several human diseases including type 2 diabetes, metabolic syndrome, cardiovascular disease and cancers. PPARγ is expressed in many tumors including lung cancer, and its function has been linked to the process of lung cancer development, progression and metastasis. Studies performed in gynogenic and xenograft models of lung cancer showed decreased tumor growth and metastasis in animals treated with PPARγ ligands. Furthermore, data are emerging from retrospective clinical studies that suggest a protective role for PPARγ ligands on the incidence of lung cancer. This review summarizes the research being conducted in this area and focuses on the mechanisms and potential therapeutic effects of PPARγ ligands as a novel anti-lung cancer treatment strategy.

Faithfull modeling of PTEN loss driven diseases in the mouse

A decade of work has indisputably defined PTEN as a pivotal player in human health and disease. Above all, PTEN has been identified as one of the most commonly lost or mutated tumor suppressor genes in human cancers. For this reason, the generation of a multitude of mouse models has been an invaluable strategy to dissect the function and consequences-of-loss of this essential, evolutionary conserved lipid phosphatase in tumor initiation and progression.In this chapter, we will summarize the mouse models that have allowed us to faithfully recapitulate features of human cancers and to highlight the network of connections between the PTEN signaling cascade and other oncogenic or tumor suppressive pathways.Notably, PTEN represents one of the most extensively modeled genes involved in human cancer and exemplifies the strength of genetic mouse modeling as an approach to gain information aimed to improve our understanding of and ability to alleviate human disease.

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.

Paired box gene 8-peroxisome proliferator-activated receptor-gamma fusion protein and loss of phosphatase and tensin homolog synergistically cause thyroid hyperplasia in transgenic mice

Approximately 35% of follicular thyroid carcinomas and a small fraction of follicular adenomas are associated with a t(2;3)(q13;p25) chromosomal translocation that fuses paired box gene 8 (PAX8) with the peroxisome proliferator-activated receptor-gamma gene (PPARG), resulting in expression of a PAX8-PPARgamma fusion protein, PPFP. The mechanism by which PPFP contributes to follicular thyroid neoplasia is poorly understood. Therefore, we have created mice with thyroid-specific expression of PPFP. At 1 yr of age, 25% of PPFP mice demonstrate mild thyroid hyperplasia. We bred these mice to mice with thyroid-specific single-allele deletion of the tumor suppressor Pten, denoted ThyPten(+/-). In humans, PTEN deletion is associated with follicular adenomas and carcinomas, and in mice, deletion of one Pten allele causes mild thyroid hyperplasia. We found that PPFP synergizes with ThyPten(+/-) to cause marked thyroid hyperplasia, but carcinomas were not observed. AKT phosphorylation was increased as expected in the ThyPten(+/-) thyroids, and also was increased in the PPFP thyroids and in human PPFP follicular cancers. Staining for the cell cycle marker Ki-67 was increased in the PPFP, ThyPten(+/-), and PPFP;ThyPten(+/-) thyroids compared with wild-type thyroids. Several genes with increased expression in PPFP cancers also were found to be increased in the thyroids of PPFP mice. This transgenic mouse model of thyroidal PPFP expression exhibits properties similar to those of PPFP thyroid cancers. However, the mice develop thyroid hyperplasia, not carcinoma, suggesting that additional events are required to cause follicular thyroid cancer.

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.

PPARdelta agonist attenuates alcohol-induced hepatic insulin resistance and improves liver injury and repair

BACKGROUND/AIMS

Chronic ethanol exposure impairs liver regeneration due to inhibition of insulin signaling and oxidative injury. PPAR agonists function as insulin sensitizers and anti-inflammatory agents. We investigated whether treatment with a PPARdelta agonist could restore hepatic insulin sensitivity, survival signaling, and regenerative responses vis-a-vis chronic ethanol feeding.

METHODS

Adult rats were fed isocaloric liquid diets containing 0% or 37% ethanol, and administered a PPARdelta agonist by i.p. injection. We used liver tissue to examine histopathology, gene expression, oxidative stress, insulin signaling, and regenerative responses to 2/3 hepatectomy.

RESULTS

Chronic ethanol feeding caused insulin resistance, increased oxidative stress, lipid peroxidation, DNA damage, and hepatocellular injury in liver. These effects were associated with reduced insulin receptor binding and affinity, impaired survival signaling through PI3K/Akt/GSK3beta, and reduced expression of insulin responsive genes mediating energy metabolism and tissue remodeling. PPARdelta agonist treatment reduced ethanol-mediated hepatic injury, oxidative stress, lipid peroxidation, and insulin resistance, increased signaling through PI3K/Akt/GSK3beta, and enhanced the regenerative response to partial hepatectomy.

CONCLUSIONS

PPARdelta agonist administration may attenuate the severity of chronic ethanol-induced liver injury and ethanol's adverse effects on the hepatic repair by restoring insulin responsiveness, even in the context of continued high-level ethanol consumption.

Targeted genetic disruption of peroxisome proliferator-activated receptor-delta and colonic tumorigenesis

Peroxisome proliferator-activated receptor-delta (PPAR-delta) is overexpressed in human colon cancer, but its contribution to colonic tumorigenesis is controversial. We generated a mouse model in which PPAR-delta was genetically disrupted in colonic epithelial cells by targeted deletion of exon 4. Elimination of colon-specific PPAR-delta expression was confirmed by real-time reverse transcription-polymerase chain reaction (real-time RT-PCR), immunoblotting, and activity assays. Mice with and without targeted PPAR-delta genetic disruption (10-11 mice per group) were tested for incidence of azoxymethane-induced colon tumors. The effects of targeted PPAR-delta deletion on vascular endothelial growth factor expression were determined by real-time RT-PCR. Targeted PPAR-delta genetic disruption inhibited colonic carcinogenesis: Mice with PPAR-delta((-/-)) colons developed 98.5% fewer tumors than wild-type mice (PPAR-delta((-/-)) vs wild-type, mean = 0.1 tumors per mouse vs 6.6 tumors per mouse, difference = 6.5 tumors per mouse, 95% confidence interval = 4.9 to 8.0 tumors per mouse, P < .001, two-sided test). Increased expression of vascular endothelial growth factor in colon tumors vs normal colon was suppressed by loss of PPAR-delta expression. These findings indicate that PPAR-delta has a crucial role in promoting colonic tumorigenesis.

PPARs Mediate Lipid Signaling in Inflammation and Cancer

Lipid mediators can trigger physiological responses by activating nuclear hormone receptors, such as the peroxisome proliferator-activated receptors (PPARs). PPARs, in turn, control the expression of networks of genes encoding proteins involved in all aspects of lipid metabolism. In addition, PPARs are tumor growth modifiers, via the regulation of cancer cell apoptosis, proliferation, and differentiation, and through their action on the tumor cell environment, namely, angiogenesis, inflammation, and immune cell functions. Epidemiological studies have established that tumor progression may be exacerbated by chronic inflammation. Here, we describe the production of the lipids that act as activators of PPARs, and we review the roles of these receptors in inflammation and cancer. Finally, we consider emerging strategies for therapeutic intervention.

The PTEN-PI3K pathway: of feedbacks and cross-talks

The tumor suppressor PTEN was originally identified as a negative regulator of the phosphoinositide 3-kinase (PI3K) signaling, a main regulator of cell growth, metabolism and survival. Yet this function of PTEN is extremely relevant for its tumor-suppressive ability, albeit the recent characterization of many PI3K-independent tumor-suppressive activities. PI3K-mediated PIP(3) production leads to the activation of the canonical AKT-mTORC1 pathway. The implications of this signaling cascade in health and disease have been underscored by the high number of regulators within the pathway whose alterations give rise to different malignancies, including familiar syndromes, metabolic dysfunctions and cancer. Moreover, PI3K is tightly buffered at multiple levels by downstream components, which have turned this signaling pathway literally upside down. PI3K and its downstream components in turn cross-talk with a number of other pathways, thereby leading to a complex network of signals that may have dramatic consequences when perturbed. Here, we review the current status of the PTEN-PI3K signaling pathway with special emphasis on the most recent data on targets and regulation of the PTEN-PI3K axis. This provides novel provocative therapeutic implications based on the targeted modulation of PI3K-cross-talking signals.

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.

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.