High-affinity peroxisome proliferator-activated receptor β/δ-specific ligands with pure antagonistic or inverse agonistic properties

Roles of PPAR activation in cancer therapeutic resistance: Implications for combination therapy and drug development

Therapeutic resistance is a major obstacle to successful treatment or effective containment of cancer. Peroxisome proliferator-activated receptors (PPARs) play an essential role in regulating energy homeostasis and determining cell fate. Despite of the pleiotropic roles of PPARs in cancer, numerous studies have suggested their intricate relationship with therapeutic resistance in cancer. In this review, we provided an overview of the roles of excessively activated PPARs in promoting resistance to modern anti-cancer treatments, including chemotherapy, radiotherapy, targeted therapy, and immunotherapy. The mechanisms through which activated PPARs contribute to therapeutic resistance in most cases include metabolic reprogramming, anti-oxidant defense, anti-apoptosis signaling, proliferation-promoting pathways, and induction of an immunosuppressive tumor microenvironment. In addition, we discussed the mechanisms through which activated PPARs lead to multidrug resistance in cancer, including drug efflux, epithelial-to-mesenchymal transition, and acquisition and maintenance of the cancer stem cell phenotype. Preliminary studies investigating the effect of combination therapies with PPAR antagonists have suggested the potential of these antagonists in reversing resistance and facilitating sustained cancer management. These findings will provide a valuable reference for further research on and clinical translation of PPAR-targeting treatment strategies.

Fatty acid desaturation by stearoyl-CoA desaturase-1 controls regulatory T cell differentiation and autoimmunity

The imbalance between pathogenic and protective T cell subsets is a cardinal feature of autoimmune disorders such as multiple sclerosis (MS). Emerging evidence indicates that endogenous and dietary-induced changes in fatty acid metabolism have a major impact on both T cell fate and autoimmunity. To date, however, the molecular mechanisms that underlie the impact of fatty acid metabolism on T cell physiology and autoimmunity remain poorly understood. Here, we report that stearoyl-CoA desaturase-1 (SCD1), an enzyme essential for the desaturation of fatty acids and highly regulated by dietary factors, acts as an endogenous brake on regulatory T-cell (Treg) differentiation and augments autoimmunity in an animal model of MS in a T cell-dependent manner. Guided by RNA sequencing and lipidomics analysis, we found that the absence of Scd1 in T cells promotes the hydrolysis of triglycerides and phosphatidylcholine through adipose triglyceride lipase (ATGL). ATGL-dependent release of docosahexaenoic acid enhanced Treg differentiation by activating the nuclear receptor peroxisome proliferator-activated receptor gamma. Our findings identify fatty acid desaturation by SCD1 as an essential determinant of Treg differentiation and autoimmunity, with potentially broad implications for the development of novel therapeutic strategies and dietary interventions for autoimmune disorders such as MS.

The Role of PPARs in Breast Cancer

Breast cancer is a malignant tumor with high morbidity and lethality. Its pathogenesis is related to the abnormal expression of many genes. The peroxisome proliferator-activated receptors (PPARs) are a class of ligand-dependent transcription factors in the nuclear receptor superfamily. They can regulate the transcription of a large number of target genes, which are involved in life activities such as cell proliferation, differentiation, metabolism, and apoptosis, and regulate physiological processes such as glucose metabolism, lipid metabolism, inflammation, and wound healing. Further, the changes in its expression are associated with various diseases, including breast cancer. The experimental reports related to "PPAR" and "breast cancer" were retrieved from PubMed since the discovery of PPARs and summarized in this paper. This review (1) analyzed the roles and potential molecular mechanisms of non-coordinated and ligand-activated subtypes of PPARs in breast cancer progression; (2) discussed the correlations between PPARs and estrogen receptors (ERs) as the nuclear receptor superfamily; and (3) investigated the interaction between PPARs and key regulators in several signaling pathways. As a result, this paper identifies PPARs as targets for breast cancer prevention and treatment in order to provide more evidence for the synthesis of new drugs targeting PPARs or the search for new drug combination treatments.

Thiophene-Based Compounds with Potential Anti-Inflammatory Activity

Rheumatoid arthritis, arthrosis and gout, among other chronic inflammatory diseases are public health problems and represent major therapeutic challenges. Non-steroidal anti-inflammatory drugs (NSAIDs) are the most prescribed clinical treatments, despite their severe side effects and their exclusive action in improving symptoms, without effectively promoting the cure. However, recent advances in the fields of pharmacology, medicinal chemistry, and chemoinformatics have provided valuable information and opportunities for development of new anti-inflammatory drug candidates. For drug design and discovery, thiophene derivatives are privileged structures. Thiophene-based compounds, like the commercial drugs Tinoridine and Tiaprofenic acid, are known for their anti-inflammatory properties. The present review provides an update on the role of thiophene-based derivatives in inflammation. Studies on mechanisms of action, interactions with receptors (especially against cyclooxygenase (COX) and lipoxygenase (LOX)), and structure-activity relationships are also presented and discussed. The results demonstrate the importance of thiophene-based compounds as privileged structures for the design and discovery of novel anti-inflammatory agents. The studies reveal important structural characteristics. The presence of carboxylic acids, esters, amines, and amides, as well as methyl and methoxy groups, has been frequently described, and highlights the importance of these groups for anti-inflammatory activity and biological target recognition, especially for inhibition of COX and LOX enzymes.

Cannabidiolic acid dampens the expression of cyclooxygenase-2 in MDA-MB-231 breast cancer cells: Possible implication of the peroxisome proliferator-activated receptor β/δ abrogation

A growing body of experimental evidence strongly suggests that cannabidiolic acid (CBDA), a major component of the fiber-type cannabis plant, exerts a variety of biological activities. We have reported that CBDA can abrogate cyclooxygenase-2 (COX-2) expression and its enzymatic activity. It is established that aberrant expression of COX-2 correlates with the degree of malignancy in breast cancer. Although the reduction of COX-2 expression by CBDA offers an attractive medicinal application, the molecular mechanisms underlying these effects have not fully been established. It has been reported that COX-2 expression is positively controlled by peroxisome proliferator-activated receptor β/δ (PPARβ/δ) in some cancerous cells, although there is "no" modulatory element for PPARβ/δ on the COX-2 promoter. No previous studies have examined whether an interaction between PPARβ/δ-mediated signaling and COX-2 expression exists in MDA-MB-231 cells. We confirmed, for the first time, that COX-2 expression is positively modulated by PPARβ/δ-mediated signaling in MDA-MB-231 cells. CBDA inhibits PPARβ/δ-mediated transcriptional activation stimulated by the PPARβ/δ-specific agonist, GW501516. Furthermore, the disappearance of cellular actin stress fibers, a hallmark of PPARβ/δ and COX-2 pathway activation, as evoked by the GW501516, was effectively reversed by CBDA. Activator protein-1 (AP-1)-driven transcriptional activity directly involved in the regulation of COX-2 was abrogated by the PPARβ/δ-specific inverse agonists (GSK0660/ST-247). Thus, it is implicated that there is positive interaction between PPARβ/δ and AP-1 in regulation of COX-2. These data support the concept that CBDA is a functional down-regulator of COX-2 through the abrogation of PPARβ/δ-related signaling, at least in part, in MDA-MB-231 cells.

Human Endometrial Stromal Cell Differentiation is Stimulated by PPARβ/δ Activation: New Targets for Infertility?

CONTEXT

Implantation is a reproductive bottleneck in women, regulated by fluctuations in ovarian steroid hormone concentrations. However, other nuclear receptor ligands are modifiers of endometrial differentiation leading to successful pregnancy. In the present study we analyzed the effects of peroxisome-proliferator-activated receptor β/δ (PPARβ/δ) activation on established cellular biomarkers of human endometrial differentiation (decidualization).

OBJECTIVE

The objective of this work is to test the effects of PPARβ/δ ligation on human endometrial cell differentiation.

DESIGN

Isolated primary human endometrial stromal cells (ESCs) were treated with synthetic (GW0742) or natural (all trans-retinoic acid, RA) ligands of PPARβ/δ, and also with receptor antagonists (GSK0660, PT-S58, and ST247) in the absence or presence of decidualizing hormones (10 nM estradiol, 100 nM progesterone, and 0.5 mM dibutyryl cAMP [3',5'-cyclic adenosine 5'-monophosphate]). In some cases interleukin (IL)-1β was used as an inflammatory stimulus. Time course and dose-response relationships were evaluated to determine effects on panels of well characterized in vitro biomarkers of decidualization.

RESULTS

PPARβ/δ, along with estrogen receptor α (ERα) and PR-A and PR-B, were expressed in human endometrial tissue and isolated ESCs. GW0742 treatment enhanced hormone-mediated ESC decidualization in vitro as manifested by upregulation of prolactin, insulin-like growth factor-binding protein 1, IL-11, and vascular endothelial growth factor (VEGF) secretion and also increased expression of ERα, PR-A and PR-B, and connexin 43 (Cx43). RA treatment also increased VEGF, ERα, PR-A, and PR-B and an active, nonphosphorylated isoform of Cx43. IL-1β and PPARβ/δ antagonists inhibited biomarkers of endometrial differentiation.

CONCLUSION

Ligands that activate PPARβ/δ augment the in vitro expression of biomarkers of ESC decidualization. By contrast, PPARβ/δ antagonists impaired decidualization markers. Drugs activating these receptors may have therapeutic benefits for embryonic implantation.

The PPAR Ω Pocket: Renewed Opportunities for Drug Development

The past decade of PPARγ research has dramatically improved our understanding of the structural and mechanistic bases for the diverging physiological effects of different classes of PPARγ ligands. The discoveries that lie at the heart of these developments have enabled the design of a new class of PPARγ ligands, capable of isolating central therapeutic effects of PPARγ modulation, while displaying markedly lower toxicities than previous generations of PPARγ ligands. This review examines the emerging framework around the design of these ligands and seeks to unite its principles with the development of new classes of ligands for PPARα and PPARβ/δ. The focus is on the relationships between the binding modes of ligands, their influence on PPAR posttranslational modifications, and gene expression patterns. Specifically, we encourage the design and study of ligands that primarily bind to the Ω pockets of PPARα and PPARβ/δ. In support of this development, we highlight already reported ligands that if studied in the context of this new framework may further our understanding of the gene programs regulated by PPARα and PPARβ/δ. Moreover, recently developed pharmacological tools that can be utilized in the search for ligands with new binding modes are also presented.

MNK1 signaling induces an ANGPTL4-mediated gene signature to drive melanoma progression

The BRAFV600E mutation occurs in more than 50% of cutaneous melanomas, and results in the constitutive activation of the mitogen-activated protein kinases (MAPK) pathway. MAP kinase-interacting serine/threonine-protein kinase 1 and 2 (MNK1/2) are downstream effectors of the activated MAPK pathway, and important molecular targets in invasive and metastatic cancer. Despite the well-known role of MNK1 in regulating mRNA translation, little is known concerning the impact of its aberrant activation on gene transcription. Here, we show that changes in the activity, or abundance, of MNK1 result in changes in the expression of pro-oncogenic and pro-invasive genes. Among the MNK1-upregulated genes, we identify Angiopoietin-like 4 (ANGPTL4), which in turn promotes an invasive phenotype via its ability to induce the expression of matrix metalloproteinases (MMPs). Using a pharmacologic inhibitor of MNK1/2, SEL201, we demonstrate that BRAFV600E-mutated cutaneous melanoma cells are reliant on MNK1/2 for invasion and lung metastasis.

PPARβ/δ recruits NCOR and regulates transcription reinitiation of ANGPTL4

In the absence of ligands, the nuclear receptor PPARβ/δ recruits the NCOR and SMRT corepressors, which form complexes with HDAC3, to canonical target genes. Agonistic ligands cause dissociation of corepressors and enable enhanced transcription. Vice versa, synthetic inverse agonists augment corepressor recruitment and repression. Both basal repression of the target gene ANGPTL4 and reinforced repression elicited by inverse agonists are partially insensitive to HDAC inhibition. This raises the question how PPARβ/δ represses transcription mechanistically. We show that the PPARβ/δ inverse agonist PT-S264 impairs transcription initiation by decreasing recruitment of activating Mediator subunits, RNA polymerase II, and TFIIB, but not of TFIIA, to the ANGPTL4 promoter. Mass spectrometry identifies NCOR as the main PT-S264-dependent interactor of PPARβ/δ. Reconstitution of knockout cells with PPARβ/δ mutants deficient in basal repression results in diminished recruitment of NCOR, SMRT, and HDAC3 to PPAR target genes, while occupancy by RNA polymerase II is increased. PT-S264 restores binding of NCOR, SMRT, and HDAC3 to the mutants, resulting in reduced polymerase II occupancy. Our findings corroborate deacetylase-dependent and -independent repressive functions of HDAC3-containing complexes, which act in parallel to downregulate transcription.

Peroxisome proliferator-activated receptor β/δ does not regulate glucose uptake and lactose synthesis in bovine mammary epithelial cells cultivated in vitro

The hypothesis of the study was that inhibition of PPARβ/δ increases glucose uptake and lactose synthesis in bovine mammary epithelial cells by reducing the expression of the glucose transporter mRNA destabiliser calreticulin. Three experiments were conducted to test the hypothesis using immortalised bovine mammary alveolar (MACT) and primary bovine mammary (PBMC) cells. In Experiment 1, the most effective dose to inhibit PPARβ/δ activity among two synthetic antagonists (GSK-3787 and PT-s58) was assessed using a gene reporter assay. In Experiment 2, the effect on glucose uptake and lactose synthesis was evaluated by measuring glucose and lactose in the media and expression of related key genes upon modulation of PPARβ/δ using GSK-3787, the synthetic PPARβ/δ agonist GW-501516, or a combination of the two in cells cultivated in plastic. In Experiment 3, the same treatments were applied to cells cultivated in Matrigel and glucose and lactose in media were measured. In Experiment 1 it was determined that a significant inhibition of PPARβ/δ in the presence or absence of fetal bovine serum was achieved with ≥ 1000 nm GSK-3787 but no significant inhibition was observed with PT-s58. In Experiment 2, inhibition of PPARβ/δ had no effect on glucose uptake and lactose synthesis but they were both increased by GW-501516 in PBMC. The mRNA abundance of PPARβ/δ target gene pyruvate dehydrogenase kinase 4 was increased but transcription of calreticulin was decreased (only in MACT cells) by GW-501516. Treatment with GSK-3787 did not affect the transcription of measured genes. No effects on glucose uptake or lactose synthesis were detected by modulation of PPARβ/δ activity on cells cultivated in Matrigel. The above data do not provide support for the original hypothesis and suggest that PPARβ/δ does not play a major role in glucose uptake and lactose synthesis in bovine mammary epithelial cells.

The Anticancer Potential of Peroxisome Proliferator-Activated Receptor Antagonists

The effects on cancer-cell proliferation and differentiation mediated by peroxisome proliferator-activated receptors (PPARs) have been widely studied, and pleiotropic outcomes in different cancer models and under different experimental conditions have been obtained. Interestingly, few studies report and little preclinical evidence supports the potential antitumor activity of PPAR antagonists. This review focuses on recent findings on the antitumor in vitro and in vivo effects observed for compounds able to inhibit the three PPAR subtypes in different tumor models, providing a rationale for the use of PPAR antagonists in the treatment of tumors expressing the corresponding receptors.

Bladder-cancer-associated mutations in RXRA activate peroxisome proliferator-activated receptors to drive urothelial proliferation

RXRA regulates transcription as part of a heterodimer with 14 other nuclear receptors, including the peroxisome proliferator-activated receptors (PPARs). Analysis from TCGA raised the possibility that hyperactive PPAR signaling, either due to PPAR gamma gene amplification or RXRA hot-spot mutation (S427F/Y) drives 20-25% of human bladder cancers. Here, we characterize mutant RXRA, demonstrating it induces enhancer/promoter activity in the context of RXRA/PPAR heterodimers in human bladder cancer cells. Structure-function studies indicate that the RXRA substitution allosterically regulates the PPAR AF2 domain via an aromatic interaction with the terminal tyrosine found in PPARs. In mouse urothelial organoids, PPAR agonism is sufficient to drive growth-factor-independent growth in the context of concurrent tumor suppressor loss. Similarly, mutant RXRA stimulates growth-factor-independent growth of Trp53/Kdm6a null bladder organoids. Mutant RXRA-driven growth of urothelium is reversible by PPAR inhibition, supporting PPARs as targetable drivers of bladder cancer.

Low-density lipoprotein docosahexaenoic acid nanoparticles induce ferroptotic cell death in hepatocellular carcinoma

Low-density lipoprotein nanoparticles reconstituted with the natural omega-3 fatty acid, docosahexaenoic acid (LDL-DHA), have been reported to selectively kill hepatoma cells and reduce the growth of orthotopic liver tumors in the rat. To date, little is known about the cell death pathways by which LDL-DHA nanoparticles kill tumor cells. Here we show that the LDL-DHA nanoparticles are cytotoxic to both rat hepatoma and human hepatocellular carcinoma (HCC) cell lines. Following LDL-DHA treatment both rat and human HCC cells experience pronounced lipid peroxidation, depletion of glutathione and inactivation of the lipid antioxidant glutathione peroxidase-4 (GPX4) prior to cell death. Inhibitor studies revealed that the treated HCC cells die independent of apoptotic, necroptotic or autophagic pathways, but require the presence of cellular iron. These hallmark features are consistent and were later confirmed to reflect ferroptosis, a novel form of nonapoptotic iron-dependent cell death. In keeping with the mechanisms of ferroptosis cell death, GPX4 was also found to be a central regulator of LDL-DHA induced tumor cell killing. We also investigated the effects of LDL-DHA treatments in mice bearing human HCC tumor xenografts. Intratumoral injections of LDL-DHA severely inhibited the growth of HCC xenografts long term. Consistent with our in vitro findings, the LDL-DHA treated HCC tumors experienced ferroptotic cell death characterized by increased levels of tissue lipid hydroperoxides and suppression of GPX4 expression.

CONCLUSION

LDL-DHA induces cell death in HCC cells through the ferroptosis pathway, this represents a novel molecular mechanism of anticancer activity for LDL-DHA nanoparticles.

PPAR-delta promotes survival of chronic lymphocytic leukemia cells in energetically unfavorable conditions

Targeting the mechanisms that allow chronic lymphocytic leukemia (CLL) cells to survive in harsh cancer microenvironments should improve patient outcomes. The nuclear receptor peroxisome proliferator activated receptor delta (PPARδ) sustains other cancers, and in silico analysis showed higher PPARD expression in CLL cells than normal lymphocytes and other hematologic cancers. A direct association was found between PPARδ protein levels in CLL cells and clinical score. Transgenic expression of PPARδ increased the growth and survival of CD5⁺ Daudi cells and primary CLL cells in stressful conditions including exhausted tissue culture media, low extracellular glucose, hypoxia and exposure to cytotoxic drugs. Glucocorticoids and synthetic PPARδ agonists up-regulated PPARD expression and also protected Daudi and primary CLL cells from metabolic stressors. Survival in low glucose was related to increased antioxidant expression, substrate utilization and mitochondrial performance, and was reversed by genetic deletion and synthetic PPARδ antagonists. These findings suggest PPARδ conditions CLL cells to survive in harsh microenvironmental conditions by reducing oxidative stress and increasing metabolic efficiency. Targeting PPARδ may be beneficial in the treatment of CLL.

PPARβ/δ in human cancer

The nuclear receptor factor peroxisome proliferator-activated receptor (PPARβ/δ) can regulate its target genes by transcriptional activation or repression through both ligand-dependent and independent mechanism as well as by interactions with other transcription factors. PPARβ/δ exerts essential regulatory functions in intermediary metabolism that have been elucidated in detail, but clearly also plays a role in inflammation, differentiation, apoptosis and other cancer-associated processes, which is, however, mechanistically only partly understood. Consistent with these functions clinical associations link the expression of PPARβ/δ and its target genes to an unfavorable outcome of several human cancers. However, the available data do not yield a clear picture of PPARβ/δ's role in cancer-associated processes and are in fact partly controversial. This article provides an overview of this research area and discusses the role of PPARβ/δ in cancer in light of the complex mechanisms of its transcriptional regulation and its potential as a druggable anti-cancer target.

A transcriptome-based global map of signaling pathways in the ovarian cancer microenvironment associated with clinical outcome

BACKGROUND

Soluble protein and lipid mediators play essential roles in the tumor environment, but their cellular origins, targets, and clinical relevance are only partially known. We have addressed this question for the most abundant cell types in human ovarian carcinoma ascites, namely tumor cells and tumor-associated macrophages.

RESULTS

Transcriptome-derived datasets were adjusted for errors caused by contaminating cell types by an algorithm using expression data derived from pure cell types as references. These data were utilized to construct a network of autocrine and paracrine signaling pathways comprising 358 common and 58 patient-specific signaling mediators and their receptors. RNA sequencing based predictions were confirmed for several proteins and lipid mediators. Published expression microarray results for 1018 patients were used to establish clinical correlations for a number of components with distinct cellular origins and target cells. Clear associations with early relapse were found for STAT3-inducing cytokines, specific components of WNT and fibroblast growth factor signaling, ephrin and semaphorin axon guidance molecules, and TGFβ/BMP-triggered pathways. An association with early relapse was also observed for secretory macrophage-derived phospholipase PLA2G7, its product arachidonic acid (AA) and signaling pathways controlled by the AA metabolites PGE2, PGI2, and LTB4. By contrast, the genes encoding norrin and its receptor frizzled 4, both selectively expressed by cancer cells and previously not linked to tumor suppression, show a striking association with a favorable clinical course.

CONCLUSIONS

We have established a signaling network operating in the ovarian cancer microenvironment with previously unidentified pathways and have defined clinically relevant components within this network.

Deregulation of PPARβ/δ target genes in tumor-associated macrophages by fatty acid ligands in the ovarian cancer microenvironment

The nuclear receptor peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is a lipid ligand-inducible transcription factor associated with macrophage polarization. However, its function in tumor-associated macrophages (TAMs) has not been investigated to date. Here, we report the PPARβ/δ-regulated transcriptome and cistrome for TAMs from ovarian carcinoma patients. Comparison with monocyte-derived macrophages shows that the vast majority of direct PPARβ/δ target genes are upregulated in TAMs and largely refractory to synthetic agonists, but repressible by inverse agonists. Besides genes with metabolic functions, these include cell type-selective genes associated with immune regulation and tumor progression, e.g., LRP5, CD300A, MAP3K8 and ANGPTL4. This deregulation is not due to increased expression of PPARβ/δ or its enhanced recruitment to target genes. Instead, lipidomic analysis of malignancy-associated ascites revealed high concentrations of polyunsaturated fatty acids, in particular linoleic acid, acting as potent PPARβ/δ agonists in macrophages. These fatty acid ligands accumulate in lipid droplets in TAMs, thereby providing a reservoir of PPARβ/δ ligands. These observations suggest that the deregulation of PPARβ/δ target genes by ligands of the tumor microenvironment contributes to the pro-tumorigenic polarization of ovarian carcinoma TAMs. This conclusion is supported by the association of high ANGPTL4 expression with a shorter relapse-free survival in serous ovarian carcinoma.

Synthesis, in vitro evaluation, and molecular modeling investigation of benzenesulfonimide peroxisome proliferator-activated receptors α antagonists

Recent evidences suggest a moderate activation of Peroxisome Proliferator-Activated Receptors (PPARs) could be favorable in metabolic diseases, reducing side effects given from full agonists. PPAR partial agonists and antagonists represent, to date, interesting tools to better elucidate biological processes modulated by these receptors. In this work are reported new benzenesulfonimide compounds able to block PPARα, synthesized and tested by transactivation assays and gene expression analysis. Some of these compounds showed a dose-dependent antagonistic behavior on PPARα, submicromolar potency, different profiles of selectivity versus PPARγ, and a repressive effect on CPT1A expression. Dockings and molecular dynamics on properly selected benzenesulfonimide derivatives furnished fresh insights into the molecular determinant most likely responsible for PPARα antagonism.

Synthesis of 5-trifluoromethyl-2-sulfonylpyridine PPARβ/δ antagonists: Effects on the affinity and selectivity towards PPARβ/δ

The covalent modification of peroxisome-proliferator activated receptor β/δ (PPARβ/δ) is part of the mode of action of 5-trifluoromethyl-2-sulfonylpyridine PPARβ/δ antagonists such as GSK3787 and CC618. Herein, the synthesis and in vitro biological evaluation of a range of structural analogues of the two antagonists are reported. The new ligands demonstrate that an improvement in the selectivity of 5-trifluoromethyl-2-sulfonylpyridine antagonists towards PPARβ/δ is achievable at the expense of their immediate affinity for PPARβ/δ. However, their putatively covalent and irreversible mode of action may ensure their efficacy over time, as observed in time-resolved fluorescence resonance energy transfer (TR-FRET)-based ligand displacement assays.

Saturated fatty acids regulate retinoic acid signalling and suppress tumorigenesis by targeting fatty acid-binding protein 5

Long chain fatty acids (LCFA) serve as energy sources, components of cell membranes, and precursors for signalling molecules. Here we show that these biological compounds also regulate gene expression and that they do so by controlling the transcriptional activities of the retinoic acid (RA)-activated nuclear receptors RAR and PPARβ/δ. The data indicate that these activities of LCFA are mediated by FABP5 which delivers ligands from the cytosol to nuclear PPARβ/δ. Both saturated and unsaturated LCFA (SLCFA, ULCFA) bind to FABP5, thereby displacing RA and diverting it to RAR. However, while SLCFA inhibit, ULCFA activate the FABP5/PPARβ/δ pathway. We show further that, by concomitantly promoting activation of RAR and inhibiting the activation of PPARβ/δ, SLCFA suppress the oncogenic properties of FABP5-expressing carcinoma cells in cultured cells and in vivo. The observations suggest that compounds that inhibit FABP5 may constitute a new class of drugs for therapy of certain types of cancer.

α1-Antitrypsin Combines with Plasma Fatty Acids and Induces Angiopoietin-like Protein 4 Expression

α1-Antitrypsin (A1AT) purified from human plasma upregulates expression and release of angiopoietin-like protein 4 (Angptl4) in adherent human blood monocytes and in human lung microvascular endothelial cells, providing a mechanism for the broad immune-regulatory properties of A1AT independent of its antiprotease activity. In this study, we demonstrate that A1AT (Prolastin), a potent inducer of Angptl4, contains significant quantities of the fatty acids (FA) linoleic acid (C18:2) and oleic acid (C18:1). However, only trace amounts of FAs were present in preparations that failed to increase Angplt4 expression, for example, A1AT (Zemaira) or M-type A1AT purified by affinity chromatography. FA pull-down assays with Western blot analysis revealed a FA-binding ability of A1AT. In human blood-adherent monocytes, A1AT-FA conjugates upregulated expression of Angptl4 (54.9-fold, p < 0.001), FA-binding protein 4 (FABP4) (11.4-fold, p < 0.001), and, to a lesser degree, FA translocase (CD36) (3.1-fold, p < 0.001) relative to A1AT devoid of FA (A1AT-0). These latter effects of A1AT-FA were blocked by inhibitors of peroxisome proliferator-activated receptor (PPAR) β/δ (ST247) and PPARγ (GW9662). When compared with controls, cell pretreatment with ST247 diminished the effect of A1AT-LA on Angptl4 mRNA (11.6- versus 4.1-fold, p < 0.001) and FABP4 mRNA (5.4- versus 2.8-fold, p < 0.001). Similarly, preincubation of cells with GW9662 inhibited inducing effect of A1AT-LA on Angptl4 mRNA (by 2-fold, p < 0.001) and FABP4 mRNA (by 3-fold, p < 0.001). Thus, A1AT binds to FA, and it is this form of A1AT that induces Angptl4 and FABP4 expression via a PPAR-dependent pathway. These findings provide a mechanism for the unexplored area of A1AT biology independent of its antiprotease properties.

The transcriptional PPARβ/δ network in human macrophages defines a unique agonist-induced activation state

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is a lipid ligand-inducible transcription factor with established metabolic functions, whereas its anti-inflammatory function is poorly understood. To address this issue, we determined the global PPARβ/δ-regulated signaling network in human monocyte-derived macrophages. Besides cell type-independent, canonical target genes with metabolic and immune regulatory functions we identified a large number of inflammation-associated NFκB and STAT1 target genes that are repressed by agonists. Accordingly, PPARβ/δ agonists inhibited the expression of multiple pro-inflammatory mediators and induced an anti-inflammatory, IL-4-like morphological phenotype. Surprisingly, bioinformatic analyses also identified immune stimulatory effects. Consistent with this prediction, PPARβ/δ agonists enhanced macrophage survival under hypoxic stress and stimulated CD8⁺ T cell activation, concomitantly with the repression of immune suppressive target genes and their encoded products CD274 (PD-1 ligand), CD32B (inhibitory Fcγ receptor IIB) and indoleamine 2,3-dioxygenase 1 (IDO-1), as well as a diminished release of the immune suppressive IDO-1 metabolite kynurenine. Comparison with published data revealed a significant overlap of the PPARβ/δ transcriptome with coexpression modules characteristic of both anti-inflammatory and pro-inflammatory cytokines. Our findings indicate that PPARβ/δ agonists induce a unique macrophage activation state with strong anti-inflammatory but also specific immune stimulatory components, pointing to a context-dependent function of PPARβ/δ in immune regulation.

Integrative and systemic approaches for evaluating PPARβ/δ (PPARD) function

The peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptors that function as transcription factors regulating the expression of genes involved in cellular differentiation, development, metabolism and also tumorigenesis. Three PPAR isotypes (α, β/δ and γ) have been identified, among which PPARβ/δ is the most difficult to functionally examine due to its tissue-specific diversity in cell fate determination, energy metabolism and housekeeping activities. PPARβ/δ acts both in a ligand-dependent and -independent manner. The specific type of regulation, activation or repression, is determined by many factors, among which the type of ligand, the presence/absence of PPARβ/δ-interacting corepressor or coactivator complexes and PPARβ/δ protein post-translational modifications play major roles. Recently, new global approaches to the study of nuclear receptors have made it possible to evaluate their molecular activity in a more systemic fashion, rather than deeply digging into a single pathway/function. This systemic approach is ideally suited for studying PPARβ/δ, due to its ubiquitous expression in various organs and its overlapping and tissue-specific transcriptomic signatures. The aim of the present review is to present in detail the diversity of PPARβ/δ function, focusing on the different information gained at the systemic level, and describing the global and unbiased approaches that combine a systems view with molecular understanding.

Synthesis, biological evaluation and molecular modeling studies of the PPARβ/δ antagonist CC618

Herein, we describe the synthesis, biological evaluation and molecular docking of the selective PPARβ/δ antagonist (4-methyl-2-(4-(trifluoromethyl)phenyl)-N-(2-(5-(trifluoromethyl)-pyridin-2-ylsulfonyl)ethyl)thiazole-5-carboxamide)), CC618. Results from in vitro luciferase reporter gene assays against the three known human PPAR subtypes revealed that CC618 selectively antagonizes agonist-induced PPARβ/δ activity with an IC50 = 10.0 μM. As observed by LC-MS/MS analysis of tryptic digests, the treatment of PPARβ/δ with CC618 leads to a covalent modification of Cys249, located centrally in the PPARβ/δ ligand binding pocket, corresponding to the conversion of its thiol moiety to a 5-trifluoromethyl-2-pyridylthioether. Finally, molecular docking is employed to shed light on the mode of action of the antagonist and its structural consequences for the PPARβ/δ ligand binding pocket.

Involvement of covalent interactions in the mode of action of PPARβ/δ antagonists

Investigations on the mode of action of several different chemical modulators of the peroxisome proliferator-activated receptor β/δ (PPARβ/δ) have been reported using MS and NMR experiments.

The inverse agonist DG172 triggers a PPARβ/δ-independent myeloid lineage shift and promotes GM-CSF/IL-4-induced dendritic cell differentiation

The stilbene derivative (Z)-2-(2-bromophenyl)-3-{[4-(1-methylpiperazine)amino]phenyl}acrylonitrile (DG172) was developed as a highly selective inhibitory peroxisome proliferator-activated receptor (PPAR)β/δ ligand. Here, we describe a novel PPARβ/δ-independent, yet highly specific, effect of DG172 on the differentiation of bone marrow cells (BMCs). DG172 strongly augmented granulocyte-macrophage-colony-stimulating factor (GM-CSF)-induced differentiation of primary BMCs from Ppard null mice into two specific populations, characterized as mature (CD11c(hi)MHCII(hi)) and immature (CD11c(hi)MHCII(lo)) dendritic cells (DCs). IL-4 synergized with DG172 to shift the differentiation from MHCII(lo) cells to mature DCs in vitro. The promotion of DC differentiation occurred at the expense of differentiation to granulocytic Gr1(+)Ly6B(+) cells. In agreement with these findings, transcriptome analyses showed a strong DG172-mediated repression of genes encoding neutrophilic markers in both differentiating wild-type and Ppard null cells, while macrophage/DC marker genes were up-regulated. DG172 also inhibited the expression of transcription factors driving granulocytic differentiation (Cebpe, Gfi1, and Klf5), and increased the levels of transcription factors promoting macrophage/DC differentiation (Irf4, Irf8, Spib, and Spic). DG172 exerted these effects only at an early stage of BMC differentiation induced by GM-CSF, did not affect macrophage-colony-stimulating factor-triggered differentiation to macrophages and had no detectable PPARβ/δ-independent effect on other cell types tested. Structure-function analyses demonstrated that the 4-methylpiperazine moiety in DG172 is required for its effect on DC differentiation, but is dispensable for PPARβ/δ binding. Based on these data we developed a new compound, (Z)-2-(4-chlorophenyl)-3-[4-(4-methylpiperazine-1-yl)phenyl]acrylonitrile (DG228), which enhances DC differentiation in the absence of significant PPARβ/δ binding.

Genetic ablation of the fatty acid-binding protein FABP5 suppresses HER2-induced mammary tumorigenesis

The fatty acid-binding protein FABP5 shuttles ligands from the cytosol to the nuclear receptor PPARβ/δ (encoded for by Pparδ), thereby enhancing the transcriptional activity of the receptor. This FABP5/PPARδ pathway is critical for induction of proliferation of breast carcinoma cells by activated epidermal growth factor receptor (EGFR). In this study, we show that FABP5 is highly upregulated in human breast cancers and we provide genetic evidence of the pathophysiologic significance of FABP5 in mammary tumorigenesis. Ectopic expression of FABP5 was found to be oncogenic in 3T3 fibroblasts where it augmented the ability of PPARδ to enhance cell proliferation, migration, and invasion. To determine whether FABP5 is essential for EGFR-induced mammary tumor growth, we interbred FABP5-null mice with MMTV-ErbB2/HER2 oncomice, which spontaneously develop mammary tumors. FABP5 ablation relieved activation of EGFR downstream effector signals, decreased expression of PPARδ target genes that drive cell proliferation, and suppressed mammary tumor development. Our findings establish that FABP5 is critical for mammary tumor development, rationalizing the development of FABP5 inhibitors as novel anticarcinogenic drugs.

Regulation of TAK1/TAB1-mediated IL-1β signaling by cytoplasmic PPARβ/δ

The peroxisome proliferator-activated receptor subtypes PPARα, PPARβ/δ, PPARγ are members of the steroid hormone receptor superfamily with well-established functions in transcriptional regulation. Here, we describe an unexpected cytoplasmic function of PPARβ/δ. Silencing of PPARβ/δ expression interferes with the expression of a large subset of interleukin-1β (IL-1β)-induced target genes in HeLa cells, which is preceded by an inhibition of the IL-1β-induced phosphorylation of TAK1 and its downstream effectors, including the NFκBα inhibitor IκBα (NFKBIA) and the NFκBα subunit p65 (RELA). PPARβ/δ enhances the interaction between TAK1 and the small heat-shock protein HSP27, a known positive modulator of TAK1-mediated IL-1β signaling. Consistent with these findings, PPARβ/δ physically interacts with both the endogenous cytoplasmic TAK1/TAB1 complex and HSP27, and PPARβ/δ overexpression increases the TAK1-induced transcriptional activity of NFκB. These observations suggest that PPARβ/δ plays a role in the assembly of a cytoplasmic multi-protein complex containing TAK1, TAB1, HSP27 and PPARβ/δ, and thereby participates in the NFκB response to IL-1β.

PPARβ/δ regulates glucocorticoid- and sepsis-induced FOXO1 activation and muscle wasting

FOXO1 is involved in glucocorticoid- and sepsis-induced muscle wasting, in part reflecting regulation of atrogin-1 and MuRF1. Mechanisms influencing FOXO1 expression in muscle wasting are poorly understood. We hypothesized that the transcription factor peroxisome proliferator-activated receptor β/δ (PPARβ/δ) upregulates muscle FOXO1 expression and activity with a downstream upregulation of atrogin-1 and MuRF1 expression during sepsis and glucocorticoid treatment and that inhibition of PPARβ/δ activity can prevent muscle wasting. We found that activation of PPARβ/δ in cultured myotubes increased FOXO1 activity, atrogin-1 and MuRF1 expression, protein degradation and myotube atrophy. Treatment of myotubes with dexamethasone increased PPARβ/δ expression and activity. Dexamethasone-induced FOXO1 activation and atrogin-1 and MuRF1 expression, protein degradation, and myotube atrophy were inhibited by PPARβ/δ blocker or siRNA. Importantly, muscle wasting induced in rats by dexamethasone or sepsis was prevented by treatment with a PPARβ/δ inhibitor. The present results suggest that PPARβ/δ regulates FOXO1 activation in glucocorticoid- and sepsis-induced muscle wasting and that treatment with a PPARβ/δ inhibitor may ameliorate loss of muscle mass in these conditions.

Inverse PPARβ/δ agonists suppress oncogenic signaling to the ANGPTL4 gene and inhibit cancer cell invasion

Besides its established functions in intermediary metabolism and developmental processes, the nuclear receptor peroxisome proliferator-activated receptor β/δ (PPARβ/δ) has a less defined role in tumorigenesis. In the present study, we have identified a function for PPARβ/δ in cancer cell invasion. We show that two structurally divergent inhibitory ligands for PPARβ/δ, the inverse agonists ST247 and DG172, strongly inhibit the serum- and transforming growth factor β (TGFβ)-induced invasion of MDA-MB-231 human breast cancer cells into a three-dimensional matrigel matrix. To elucidate the molecular basis of this finding, we performed chromatin immunoprecipitation sequencing (ChIP-Seq) and microarray analyses, which identified the gene encoding angiopoietin-like 4 (ANGPTL4) as the major transcriptional PPARβ/δ target in MDA-MB-231 cells, previously implicated in TGFβ-mediated tumor progression and metastatic dissemination. We show that the induction of ANGPTL4 by TGFβ and other oncogenic signals is strongly repressed by ST247 and DG172 in a PPARβ/δ-dependent fashion, resulting in the inhibition of ANGPTL4 secretion. This effect is attributable to these ligands' ability to induce a dominant transcriptional repressor complex at the site of transcription initiation that blocks preinitiation complex formation through an histone deacetylase-independent, non-canonical mechanism. Repression of ANGPTL4 transcription by inverse PPARβ/δ agonists is functionally linked to the inhibition of cancer cell invasion into a three-dimensional matrix, as (i) invasion of MDA-MB-231 cells is critically dependent on ANGPTL4 expression, (ii) recombinant ANGPTL4 stimulates invasion, and (iii) reverses the inhibitory effect of ST247 and DG172. These findings indicate that a PPARβ/δ-ANGPTL4 pathway is involved in the regulation of tumor cell invasion and that its pharmacological manipulation by inverse PPARβ/δ agonists is feasible.

The role of peroxisome proliferator-activated receptors in carcinogenesis and chemoprevention

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that are involved in regulating glucose and lipid homeostasis, inflammation, proliferation and differentiation. Although all of these functions might contribute to the influence of PPARs in carcinogenesis, there is a distinct need for a review of the literature and additional experimentation to determine the potential for targeting PPARs for cancer therapy and cancer chemoprevention. As PPAR agonists include drugs that are used for the treatment of metabolic diseases, a more complete understanding of the roles of PPARs in cancer will aid in determining any increased cancer risk for patients undergoing therapy with PPAR agonists.