Cannabidiolic acid activates the expression of the PPARß/δ target genes in MDA-MB-231 cells.

Cannabidiolic acid (CBDA) can activate peroxisome proliferator-activated receptor-α (PPARα) and PPARγ. Whether CBDA can activate PPARß/δ has not been examined sufficiently to date. Since previous studies showed that triple-negative breast cancer cells respond to activation of PPARß/δ, the present study examined the effect of CBDA in MDA-MB-231 cells and compared the activities of CBDA with known PPARß/δ agonists/antagonists. Expression of the PPARß/δ target genes angiopoietin-like 4 (ANGPTL4) and adipocyte differentiation-related protein (ADRP) was increased by CBDA. Interestingly, ligand activation of PPARß/δ with GW501516 caused an increase in expression of both ANGPTL4 and ADRP, but the magnitude of this effect was markedly increased when co-treated with CBDA. Specificity of these effects were confirmed by showing that CBDA-induced expression of ANGPTL4 and ADRP is mitigated in the presence of either a PPARß/δ antagonist or an inverse agonist. Results from these studies suggest that CBDA can synergize with PPARß/δ and might interact with endogenous agonists that modulate PPARß/δ function.

Targeting Peroxisome Proliferator-Activated Receptor-ß/δ (PPARß/δ) for the Treatment or Prevention of Alcoholic Liver Disease.

Excessive, chronic alcohol consumption can lead to alcoholic liver disease. The etiology of alcoholic liver disease is multifactorial and is influenced by alterations in gene expression and changes in fatty acid metabolism, oxidative stress, and insulin resistance. These events can lead to steatosis, fibrosis, and eventually to cirrhosis and liver cancer. Many of these functions are regulated by peroxisome proliferator-activated receptors (PPARs). Thus, it is not surprising that PPARs can modulate the mechanisms that cause alcoholic liver disease. While the roles of PPARα and PPARγ are clearer, the role of PPARß/δ in alcoholic liver disease requires further clarification. This review summarizes the current understanding based on recent studies that indicate that PPARß/δ can likely be targeted for the treatment and/or the prevention of alcoholic liver disease.

Interplay Between the Host, the Human Microbiome, and Drug Metabolism.

The human microbiome is composed of four major areas including intestinal, skin, vaginal, and oral microbiomes, with each area containing unique species and unique functionalities. The human microbiome may be modulated with prebiotics, probiotics, and postbiotics to potentially aid in the treatment of diseases like irritable bowel syndrome, bacterial vaginosis, atopic dermatitis, gingivitis, obesity, or cancer. There is also potential for many of the inhabitants of the human microbiome to directly modulate host gene expression and modulate host detoxifying enzyme activity like cytochrome P450s (CYPs), dehydrogenases, and carboxylesterases. Therefore, the microbiome may be important to consider during drug discovery, risk assessment, and dosing regimens for various diseases given that the human microbiome has been shown to impact host detoxification processes.

Regulatory mechanisms mediated by peroxisome proliferator-activated receptor-ß/δ in skin cancer.

Considerable progress has been made during the past 20 years towards elucidating the role of peroxisome proliferator-activated receptor-ß/δ (PPARß/δ) in skin cancer. In 1999, the original notion that PPARß/δ was involved with epithelial cell function was postulated based on a correlation between PPARß/δ expression and the induction of messenger RNAs encoding proteins that mediate terminal differentiation in keratinocytes. Subsequent studies definitively revealed that PPARß/δ could induce terminal differentiation and inhibit proliferation of keratinocytes. Molecular mechanisms have since been discovered to explain how this nuclear receptor can be targeted for preventing and treating skin cancer. This includes the regulation of terminal differentiation, mitotic signaling, endoplasmic reticulum stress, and cellular senescence. Interestingly, the effects of activating PPARß/δ can preferentially target keratinocytes with genetic mutations associated with skin cancer. This review provides the history and current understanding of how PPARß/δ can be targeted for both nonmelanoma skin cancer and melanoma and postulates how future approaches that modulate PPARß/δ signaling may be developed for the prevention and treatment of these diseases.

Flipping a citrate switch on liver cancer cells.

Energy homeostasis and oncogenic signaling are critical determinants of the growth of human liver cancer cells, providing a strong rationale to elucidate the regulatory mechanisms for these systems. A new study reports that loss of solute carrier family 13 member 5, which transports citrate across cell membranes, halts liver cancer cell growth by altering both energy production and mammalian target of rapamycin signaling in human liver cancer cell lines and in both an in vitro and in vivo model of liver tumors, suggesting a new target for liver cancer chemoprevention and/or chemotherapy.

Lipid metabolism and lipophagy in cancer.

The tumor microenvironment can be hypoxic, acidic, and deficient in nutrients, thus causing the metabolism of tumor cells as well as the neighboring stromal cells to be remodelled to facilitate tumor survival, proliferation, and metastasis. Abnormal tumor lipid metabolism is a fairly new field, which has received attention in the past few years. Cross-talk between tumor cells and tumor-associated stromal cells modulates the high metabolic needs of the tumor. Fatty acid turnover is high in tumor cells to meet the energy as well as synthetic requirements of the growing tumor. Lipolysis of lipids stored in lipid droplets was earlier considered to be solely carried out by cytosolic lipases. However recent studies demonstrate that lipophagy (autophagic degradation of lipids by acidic lipases) serves as an alternate pathway for the degradation of lipid droplets. Involvement of lipophagy in lipid turnover makes it a crucial player in tumorigenesis and metastasis. In this review we discuss the metabolic reprogramming of tumor cells with special focus on lipid metabolism. We also address the lipid turnover machinery in the tumor cell, especially the lipophagic pathway. Finally, we integrate the current understanding of lipophagy with tumor lipid metabolism.

The Ron Receptor Tyrosine Kinase Regulates Macrophage Heterogeneity and Plays a Protective Role in Diet-Induced Obesity, Atherosclerosis, and Hepatosteatosis.

Obesity is a chronic inflammatory disease mediated in large part by the activation of inflammatory macrophages. This chronic inflammation underlies a whole host of diseases including atherosclerosis, hepatic steatosis, insulin resistance, type 2 diabetes, and cancer, among others. Macrophages are generally classified as either inflammatory or alternatively activated. Some tissue-resident macrophages are derived from yolk sac erythromyeloid progenitors and fetal liver progenitors that seed tissues during embryogenesis and have the ability to repopulate through local proliferation. These macrophages tend to be anti-inflammatory in nature and are generally involved in tissue remodeling, repair, and homeostasis. Alternatively, during chronic inflammation induced by obesity, bone marrow monocyte-derived macrophages are recruited to inflamed tissues, where they produce proinflammatory cytokines and exacerbate inflammation. The extent to which these two populations of macrophages are plastic in their phenotype remains controversial. We have demonstrated previously that the Ron receptor tyrosine kinase is expressed on tissue-resident macrophages, where it limits inflammatory macrophage activation and promotes a repair phenotype. In this study, we demonstrate that Ron is expressed in a subpopulation of macrophages during chronic inflammation induced by obesity that exhibit a repair phenotype as determined by the expression of arginase 1. In addition, we demonstrate that the Ron receptor plays a protective role in the progression of diet-induced obesity, hepatosteatosis, and atherosclerosis. These results suggest that altering macrophage heterogeneity in vivo could have the potential to alleviate obesity-associated diseases.

The PPARα-dependent rodent liver tumor response is not relevant to humans: addressing misconceptions.

A number of industrial chemicals and therapeutic agents cause liver tumors in rats and mice by activating the nuclear receptor peroxisome proliferator-activated receptor α (PPARα). The molecular and cellular events by which PPARα activators induce rodent hepatocarcinogenesis have been extensively studied elucidating a number of consistent mechanistic changes linked to the increased incidence of liver neoplasms. The weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis is summarized here. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators. The key events (KE) identified in the MOA are PPARα activation (KE1), alteration in cell growth pathways (KE2), perturbation of hepatocyte growth and survival (KE3), and selective clonal expansion of preneoplastic foci cells (KE4), which leads to the apical event-increases in hepatocellular adenomas and carcinomas (KE5). In addition, a number of concurrent molecular and cellular events have been classified as modulating factors, because they potentially alter the ability of PPARα activators to increase rodent liver cancer while not being key events themselves. These modulating factors include increases in oxidative stress and activation of NF-kB. PPARα activators are unlikely to induce liver tumors in humans due to biological differences in the response of KEs downstream of PPARα activation. This conclusion is based on minimal or no effects observed on cell growth pathways and hepatocellular proliferation in human primary hepatocytes and absence of alteration in growth pathways, hepatocyte proliferation, and tumors in the livers of species (hamsters, guinea pigs and cynomolgus monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Despite this overwhelming body of evidence and almost universal acceptance of the PPARα MOA and lack of human relevance, several reviews have selectively focused on specific studies that, as discussed, contradict the consensus opinion and suggest uncertainty. In the present review, we systematically address these most germane suggested weaknesses of the PPARα MOA.

Regulation of Cytochrome P450 2B10 (CYP2B10) Expression in Liver by Peroxisome Proliferator-activated Receptor-ß/δ Modulation of SP1 Promoter Occupancy.

Alcoholic liver disease is a pathological condition caused by overconsumption of alcohol. Because of the high morbidity and mortality associated with this disease, there remains a need to elucidate the molecular mechanisms underlying its etiology and to develop new treatments. Because peroxisome proliferator-activated receptor-ß/δ (PPARß/δ) modulates ethanol-induced hepatic effects, the present study examined alterations in gene expression that may contribute to this disease. Chronic ethanol treatment causes increased hepatic CYP2B10 expression inPparß/δ+/+ mice but not in Pparß/δ-/- mice. Nuclear and cytosolic localization of the constitutive androstane receptor (CAR), a transcription factor known to regulate Cyp2b10 expression, was not different between genotypes. PPARγ co-activator 1α, a co-activator of both CAR and PPARß/δ, was up-regulated in Pparß/δ+/+ liver following ethanol exposure, but not in Pparß/δ-/- liver. Functional mapping of the Cyp2b10 promoter and ChIP assays revealed that PPARß/δ-dependent modulation of SP1 promoter occupancy up-regulated Cyp2b10 expression in response to ethanol. These results suggest that PPARß/δ regulates Cyp2b10 expression indirectly by modulating SP1 and PPARγ co-activator 1α expression and/or activity independent of CAR activity. Ligand activation of PPARß/δ attenuates ethanol-induced Cyp2b10 expression in Pparß/δ+/+ liver but not in Pparß/δ-/- liver. Strikingly, Cyp2b10 suppression by ligand activation of PPARß/δ following ethanol treatment occurred in hepatocytes and was mediated by paracrine signaling from Kupffer cells. Combined, results from the present study demonstrate a novel regulatory role of PPARß/δ in modulating CYP2B10 that may contribute to the etiology of alcoholic liver disease.

Peroxisome proliferator-activated receptor-ß/δ modulates mast cell phenotype.

The peroxisome proliferator-activated receptor-ß/δ (PPARß/δ) is known to have multiple anti-inflammatory effects, typically observed in endothelial cells, macrophages, T cells and B cells. Despite the fact that mast cells are important mediators of inflammation, to date, the role of PPARß/δ in mast cells has not been examined. Hence, the present study examined the hypothesis that PPARß/δ modulates mast cell phenotype. Bone-marrow-derived mast cells (BMMCs) and peritoneal mast cells from Pparß/δ+/+ mice expressed higher levels of high-affinity IgE receptor (FcεRI) compared with Pparß/δ-/- mice. BMMCs from Pparß/δ+/+ mice also exhibited dense granules, associated with higher expression of enzymes and proteases compared with Pparß/δ-/- mice. Resting BMMCs from Pparß/δ+/+ mice secreted lower levels of inflammatory cytokines, associated with the altered activation of phospholipase Cγ1 and extracellular signal-regulated kinases compared with Pparß/δ-/- mice. Moreover, the production of cytokines by mast cells induced by various stimuli was highly dependent on PPARß/δ expression. This study demonstrates that PPARß/δ is an important regulator of mast cell phenotype.

Peroxisome proliferator-activated receptor-ß/δ inhibits human neuroblastoma cell tumorigenesis by inducing p53- and SOX2-mediated cell differentiation.

Neuroblastoma is a common childhood cancer typically treated by inducing differentiation with retinoic acid (RA). Peroxisome proliferator-activated receptor-ß/δ, (PPARß/δ) is known to promote terminal differentiation of many cell types. In the present study, PPARß/δ was over-expressed in three human neuroblastoma cell lines, NGP, SK-N-BE(2), and IMR-32, that exhibit high, medium, and low sensitivity, respectively, to retinoic acid-induced differentiation to determine if PPARß/δ and retinoic acid receptors (RARs) could be jointly targeted to increase the efficacy of treatment. All-trans-RA (atRA) decreased expression of SRY (sex determining region Y)-box 2 (SOX2), a stem cell regulator and marker of de-differentiation, in NGP and SK-N-BE(2) cells with inactive or mutant tumor suppressor p53, respectively. However, atRA did not suppress SOX2 expression in IMR-32 cells carrying wild-type p53. Over-expression and/or ligand activation of PPARß/δ reduced the average volume and weight of ectopic tumor xenografts from NGP, SK-N-BE(2), or IMR-32 cells compared to controls. Compared with that found with atRA, PPARß/δ suppressed SOX2 expression in NGP and SK-N-BE(2) cells and ectopic xenografts, and was also effective in suppressing SOX2 expression in IMR-32 cells that exhibit higher p53 expression compared to the former cell lines. Combined, these observations demonstrate that activating or over-expressing PPARß/δ induces cell differentiation through p53- and SOX2-dependent signaling pathways in neuroblastoma cells and tumors. This suggests that combinatorial activation of both RARα and PPARß/δ may be suitable as an alternative therapeutic approach for RA-resistant neuroblastoma patients.

Four-week dietary supplementation with 10- and/or 15-fold basal choline caused decreased body weight in Sprague Dawley rats.

Choline is an essential nutrient utilized for phosphatidylcholine biosynthesis and lipoprotein packaging and secretion. Recently, choline supplementation has been used by athletes and the public for weight loss. However, the potential toxicological impact of choline dietary supplementation requires further investigation. This study examined the effects of choline dietary supplementation in Sprague Dawley rats for 4 weeks. Rats were fed diets containing basal choline levels (control) or 5-, 10-, or 15-fold (5×, 10×, or 15×) basal diet concentration. In groups fed choline-supplemented diets, there were no toxicologically relevant findings in clinical observations, food intake, clinical chemistry, liver weights, or liver histopathology. However, decreased mean body weights (8.5-10.2%) and body weight gains (24-31%) were noted for the 10× choline-supplemented (females only) and 15× choline-supplemented (both sexes) groups relative to the control groups from day 3 onward. These body weight effects were not related to a persistent reduction in average food intake. Serum cholesterol was increased in the 15× choline-supplemented male rats relative to the controls, an expected effect of choline supplementation; however, there were no changes in the serum cholesterol of female rats. Serum choline concentrations were increased in female rats relative to the male rats across all treatment groups. The maximum tolerated dose for male and female rats were the 15× and 10× choline supplements, respectively, based on decreased mean body weight and body weight gains. This study supported the conclusions of a clinical trial that showed a high choline diet can decrease body weight in humans.

Peroxisome Proliferator-activated Receptor-D (PPARD) Coordinates Mouse Spermatogenesis by Modulating Extracellular Signal-regulated Kinase (ERK)-dependent Signaling.

Ppard(-/-) mice exhibit smaller litter size compared with Ppard(+/+) mice. To determine whether peroxisome proliferator-activated receptor-D (PPARD) could possibly influence this phenotype, the role of PPARD in testicular biology was examined. Atrophic testes and testicular degeneration were observed in Ppard(-/-) mice compared with Ppard(+/+) mice, indicating that PPARD modulates spermatogenesis. Higher expression of p27 and decreased expression of proliferating cellular nuclear antigen in Sertoli cells were observed in Ppard(+/+) mice as compared with Ppard(-/-) mice, and these were associated with decreased Sertoli cell number in Ppard(+/+) mice. Cyclin D1 and cyclin D2 expression was lower in Ppard(+/+) as compared with Ppard(-/-) mice. Ligand activation of PPARD inhibited proliferation of a mouse Sertoli cell line, TM4, and an inverse agonist of PPARD (DG172) rescued this effect. Temporal inhibition of extracellular signal-regulated kinase (ERK) activation by PPARD in the testis was observed in Ppard(+/+) mice and was associated with decreased serum follicle-stimulating hormone and higher claudin-11 expression along the blood-testis barrier. PPARD-dependent ERK activation also altered expression of claudin-11, p27, cyclin D1, and cyclin D2 in TM4 cells, causing inhibition of cell proliferation, maturation, and formation of tight junctions in Sertoli cells, thus confirming a requirement for PPARD in accurate Sertoli cell function. Combined, these results reveal for the first time that PPARD regulates spermatogenesis by modulating the function of Sertoli cells during early testis development.

Omics Approaches To Probe Microbiota and Drug Metabolism Interactions.

The drug metabolism field has long recognized the beneficial and sometimes deleterious influence of microbiota in the absorption, distribution, metabolism, and excretion of drugs. Early pioneering work with the sulfanilamide precursor prontosil pointed toward the necessity not only to better understand the metabolic capabilities of the microbiota but also, importantly, to identify the specific microbiota involved in the generation and metabolism of drugs. However, technological limitations important for cataloging the microbiota community as well as for understanding and/or predicting their metabolic capabilities hindered progress. Current advances including mass spectrometry-based metabolite profiling as well as culture-independent sequence-based identification and functional analysis of microbiota have begun to shed light on microbial metabolism. In this review, case studies will be presented to highlight key aspects (e.g., microbiota identification, metabolic function and prediction, metabolite identification, and profiling) that have helped to clarify how the microbiota might impact or be impacted by drug metabolism. Lastly, a perspective of the future of this field is presented that takes into account what important knowledge is lacking and how to tackle these problems.

The nuclear receptor peroxisome proliferator-activated receptor-ß/δ (PPARß/δ) promotes oncogene-induced cellular senescence through repression of endoplasmic reticulum stress.

Endoplasmic reticulum (ER) stress and ER stress-associated unfolded protein response (UPR) can promote cancer cell survival, but it remains unclear whether they can influence oncogene-induced senescence. The present study examined the role of ER stress in senescence using oncogene-dependent models. Increased ER stress attenuated senescence in part by up-regulating phosphorylated protein kinase B (p-AKT) and decreasing phosphorylated extracellular signal-regulated kinase (p-ERK). A positive feed forward loop between p-AKT, ER stress, and UPR was discovered whereby a transient increase of ER stress caused reduced senescence and promotion of tumorigenesis. Decreased ER stress was further correlated with increased senescence in both mouse and human tumors. Interestingly, H-RAS-expressing Pparß/δ null cells and tumors having increased cell proliferation exhibited enhanced ER stress, decreased cellular senescence, and/or enhanced tumorigenicity. Collectively, these results demonstrate a new role for ER stress and UPR that attenuates H-RAS-induced senescence and suggest that PPARß/δ can repress this oncogene-induced ER stress to promote senescence in accordance with its role as a tumor modifier that suppresses carcinogenesis.

Modulation of aryl hydrocarbon receptor (AHR)-dependent signaling by peroxisome proliferator-activated receptor ß/δ (PPARß/δ) in keratinocytes.

Whether peroxisome proliferator-activated receptor ß/δ (PPARß/δ) reduces skin tumorigenesis by altering aryl hydrocarbon receptor (AHR)-dependent activities was examined. Polycyclic aromatic hydrocarbons (PAH) increased expression of cytochrome P4501A1 (CYP1A1), CYP1B1 and phase II xenobiotic metabolizing enzymes in wild-type skin and keratinocytes. Surprisingly, this effect was not found in Pparß/δ-null skin and keratinocytes. Pparß/δ-null keratinocytes exhibited decreased AHR occupancy and histone acetylation on the Cyp1a1 promoter in response to a PAH compared with wild-type keratinocytes. Bisulfite sequencing of the Cyp1a1 promoter and studies using a DNA methylation inhibitor suggest that PPARß/δ promotes demethylation of the Cyp1a1 promoter. Experiments with human HaCaT keratinocytes stably expressing shRNA against PPARß/δ also support this conclusion. Consistent with the lower AHR-dependent activities in Pparß/δ-null mice compared with wild-type mice, 7,12-dimethylbenz[a]anthracene (DMBA)-induced skin tumorigenesis was inhibited in Pparß/δ-null mice compared with wild-type. Results from these studies demonstrate that PPARß/δ is required to mediate complete carcinogenesis by DMBA. The mechanisms underlying this PPARß/δ-dependent reduction of AHR signaling by PAH are not due to alterations in the expression of AHR auxiliary proteins, ligand binding or AHR nuclear translocation between genotypes, but are likely influenced by PPARß/δ-dependent demethylation of AHR target gene promoters including Cyp1a1 that reduces AHR accessibility as shown by reduced promoter occupancy. This PPARß/δ/AHR crosstalk is unique to keratinocytes and conserved between mice and humans.

Aryl hydrocarbon receptor antagonism attenuates growth factor expression, proliferation, and migration in fibroblast-like synoviocytes from patients with rheumatoid arthritis.

Rheumatoid arthritis (RA) is a chronic autoimmune disease with high morbidity and mortality. Within the inflammatory milieu, resident fibroblast-like synoviocytes (FLS) in the synovial tissue undergo hyperplasia, which leads to joint destruction. Epidemiologic studies and our previous research suggest that activation of the aryl hydrocarbon receptor (AHR) pathway plays an instrumental role in the inflammatory and destructive RA phenotype. In addition, our recent studies implicate the AHR in the regulation of the expression of several growth factors in established tumor cell lines. Thus, under inflammatory conditions, we hypothesized that the AHR is involved in the constitutive and inducible expression of several growth factors, FLS proliferation and migration, along with protease-dependent invasion in FLS from patients with RA (RA-FLS). Treatment with the AHR antagonist GNF351 inhibits cytokine-induced expression of vascular endothelial growth factor-A (VEGF-A), epiregulin, amphiregulin, and basic fibroblast growth factor mRNA through an AHR-dependent mechanism in both RA-FLS and FLS. Secretion of VEGF-A and epiregulin from RA-FLS was also inhibited upon GNF351 treatment. RA-FLS cell migration, along with cytokine-induced RA-FLS cell proliferation, was significantly attenuated by GNF351 exposure. Treatment of RA-FLS with GNF351 mitigated cytokine-mediated expression of matrix metalloproteinase-2 and -9 mRNA and diminished the RA-FLS invasive phenotype. These findings indicate that inhibition of AHR activity may be a viable therapeutic target in amelioration of disease progression in RA by attenuating growth factor release; FLS proliferation, migration, and invasion; and inflammatory activity.

Mode of action framework analysis for receptor-mediated toxicity: The peroxisome proliferator-activated receptor alpha (PPARα) as a case study.

Several therapeutic agents and industrial chemicals induce liver tumors in rodents through the activation of the peroxisome proliferator-activated receptor alpha (PPARα). The cellular and molecular events by which PPARα activators induce rodent hepatocarcinogenesis has been extensively studied and elucidated. This review summarizes the weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis and identifies gaps in our knowledge of this MOA. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators including a phthalate ester plasticizer di(2-ethylhexyl) phthalate (DEHP) and the drug gemfibrozil. While biologically plausible in humans, the hypothesized key events in the rodent MOA, for PPARα activators, are unlikely to induce liver tumors in humans because of toxicodynamic and biological differences in responses. This conclusion is based on minimal or no effects observed on growth pathways, hepatocellular proliferation and liver tumors in humans and/or species (including hamsters, guinea pigs and cynomolgous monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Overall, the panel concluded that significant quantitative differences in PPARα activator-induced effects related to liver cancer formation exist between rodents and humans. On the basis of these quantitative differences, most of the workgroup felt that the rodent MOA is "not relevant to humans" with the remaining members concluding that the MOA is "unlikely to be relevant to humans". The two groups differed in their level of confidence based on perceived limitations of the quantitative and mechanistic knowledge of the species differences, which for some panel members strongly supports but cannot preclude the absence of effects under unlikely exposure scenarios.

Activation of Peroxisome Proliferator-Activated Receptor-ß/δ (PPARß/δ) in Keratinocytes by Endogenous Fatty Acids.

Nuclear hormone receptors exist in dynamic equilibrium between transcriptionally active and inactive complexes dependent on interactions with ligands, proteins, and chromatin. The present studies examined the hypothesis that endogenous ligands activate peroxisome proliferator-activated receptor-ß/δ (PPARß/δ) in keratinocytes. The phorbol ester treatment or HRAS infection of primary keratinocytes increased fatty acids that were associated with enhanced PPARß/δ activity. Fatty acids caused PPARß/δ-dependent increases in chromatin occupancy and the expression of angiopoietin-like protein 4 (Angptl4) mRNA. Analyses demonstrated that stearoyl Co-A desaturase 1 (Scd1) mediates an increase in intracellular monounsaturated fatty acids in keratinocytes that act as PPARß/δ ligands. The activation of PPARß/δ with palmitoleic or oleic acid causes arrest at the G2/M phase of the cell cycle of HRAS-expressing keratinocytes that is not found in similarly treated HRAS-expressing Pparb/d-null keratinocytes. HRAS-expressing Scd1-null mouse keratinocytes exhibit enhanced cell proliferation, an effect that is mitigated by treatment with palmitoleic or oleic acid. Consistent with these findings, the ligand activation of PPARß/δ with GW0742 or oleic acid prevented UVB-induced non-melanoma skin carcinogenesis, an effect that required PPARß/δ. The results from these studies demonstrate that PPARß/δ has endogenous roles in keratinocytes and can be activated by lipids found in diet and cellular components.

COX-2 suppresses tissue factor expression via endocannabinoid-directed PPARdelta activation.

Although cyclooxygenase (COX)-2 inhibitors (coxibs) are effective in controlling inflammation, pain, and tumorigenesis, their use is limited by the recent revelation of increased adverse cardiovascular events. The mechanistic basis of this side effect is not well understood. We show that the metabolism of endocannabinoids by the endothelial cell COX-2 coupled to the prostacyclin (PGI(2)) synthase (PGIS) activates the nuclear receptor peroxisomal proliferator-activated receptor (PPAR) delta, which negatively regulates the expression of tissue factor (TF), the primary initiator of blood coagulation. Coxibs suppress PPARdelta activity and induce TF expression in vascular endothelium and elevate circulating TF activity in vivo. Importantly, PPARdelta agonists suppress coxib-induced TF expression and decrease circulating TF activity. We provide evidence that COX-2-dependent attenuation of TF expression is abrogated by coxibs, which may explain the prothrombotic side-effects for this class of drugs. Furthermore, PPARdelta agonists may be used therapeutically to suppress coxib-induced cardiovascular side effects.