Integrating nuclear receptor mobility in models of gene regulation

The mode of action of nuclear receptors in living cells is an actively investigated field but much remains hypothetical due to the lack, until recently, of methods allowing the assessment of molecular mechanisms in vivo. However, these last years, the development of fluorescence microscopy methods has allowed initiating the dissection of the molecular mechanisms underlying gene regulation by nuclear receptors directly in living cells or organisms. Following our analyses on peroxisome proliferator activated receptors (PPARs) in living cells, we discuss here the different models arising from the use of these tools, that attempt to link mobility, DNA binding or chromatin interaction, and transcriptional activity

Attenuation of Colon Inflammation through Activators of the Retinoid X Receptor (Rxr)/Peroxisome Proliferator–Activated Receptor γ (Pparγ) Heterodimer: A Basis for New Therapeutic Strategies

The peroxisome proliferator–activated receptor γ (PPARγ) is highly expressed in the colon mucosa and its activation has been reported to protect against colitis. We studied the involvement of PPARγ and its heterodimeric partner, the retinoid X receptor (RXR) in intestinal inflammatory responses. PPARγ1/− and RXRα1/− mice both displayed a significantly enhanced susceptibility to 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis compared with their wild-type littermates. A role for the RXR/PPARγ heterodimer in the protection against colon inflammation was explored by the use of selective RXR and PPARγ agonists. TNBS-induced colitis was significantly reduced by the administration of both PPARγ and RXR agonists. This beneficial effect was reflected by increased survival rates, an improvement of macroscopic and histologic scores, a decrease in tumor necrosis factor α and interleukin 1β mRNA levels, a diminished myeloperoxidase concentration, and reduction of nuclear factor κB DNA binding activity, c-Jun NH2-terminal kinase, and p38 activities in the colon. When coadministered, a significant synergistic effect of PPARγ and RXR ligands was observed. In combination, these data demonstrate that activation of the RXR/PPARγ heterodimer protects against colon inflammation and suggest that combination therapy with both RXR and PPARγ ligands might hold promise in the clinic due to their synergistic effects

Transcriptional profiling reveals divergent roles of PPARalpha and PPARbeta/delta in regulation of gene expression in mouse liver.

Little is known about the role of the transcription factor peroxisome proliferator-activated receptor (PPAR) beta/delta in liver. Here we set out to better elucidate the function of PPARbeta/delta in liver by comparing the effect of PPARalpha and PPARbeta/delta deletion using whole genome transcriptional profiling and analysis of plasma and liver metabolites. In fed state, the number of genes altered by PPARalpha and PPARbeta/delta deletion was similar, whereas in fasted state the effect of PPARalpha deletion was much more pronounced, consistent with the pattern of gene expression of PPARalpha and PPARbeta/delta. Minor overlap was found between PPARalpha- and PPARbeta/delta-dependent gene regulation in liver. Pathways upregulated by PPARbeta/delta deletion were connected to innate immunity and inflammation. Pathways downregulated by PPARbeta/delta deletion included lipoprotein metabolism and various pathways related to glucose utilization, which correlated with elevated plasma glucose and triglycerides and reduced plasma cholesterol in PPARbeta/delta-/- mice. Downregulated genes that may underlie these metabolic alterations included Pklr, Fbp1, Apoa4, Vldlr, Lipg, and Pcsk9, which may represent novel PPARbeta/delta target genes. In contrast to PPARalpha-/- mice, no changes in plasma free fatty acid, plasma beta-hydroxybutyrate, liver triglycerides, and liver glycogen were observed in PPARbeta/delta-/- mice. Our data indicate that PPARbeta/delta governs glucose utilization and lipoprotein metabolism and has an important anti-inflammatory role in liver. Overall, our analysis reveals divergent roles of PPARalpha and PPARbeta/delta in regulation of gene expression in mouse liver

Contribution à la morphologie comparée du cloaque et des glandes épidermoïdes de la région cloacale chez les Lépidosauriens.

Little is known about the role of the transcription factor PPARß/d in liver. Here we set out to better elucidate the function of PPARß/d in liver by comparing the effect of PPARa and PPARß/d deletion using whole genome transcriptional profiling and analysis of plasma and liver metabolites. In fed state, the number of genes altered by PPARa and PPARß/d deletion was similar, whereas in fasted state the effect of PPARa deletion was much more pronounced, consistent with the pattern of gene expression of PPARa and PPARß/d. Minor overlap was found between PPARa- and PPARß/d-dependent gene regulation in liver. Pathways upregulated by PPARß/d deletion were connected to innate immunity. Pathways downregulated by PPARß/d deletion included lipoprotein metabolism and various pathways related to glucose utilization, which correlated with elevated plasma glucose and triglycerides and reduced plasma cholesterol in PPARß/d-/- mice. Downregulated genes that may underlie these metabolic alterations included Pklr, Fbp1, Apoa4, Vldlr, Lipg, and Pcsk9, which may represent novel PPARß/d target genes. In contrast to PPARa-/- mice, no changes in plasma FFA, plasma ß-hydroxybutyrate, liver triglycerides and liver glycogen were observed in PPARß/d-/- mice. Our data indicate a role for PPARß/d in hepatic glucose utilization and lipoprotein metabolism but not in the adaptive response to fasting

Effect PPARb/d knockout in white adipose tissue

Analysis of white adipose tissue of PPARb/d knockout mice. Data may point towards putative target genes of PPARb/d and thus the function of PPARb/d in white adipose tissue. Datasets were used to identify glycogen synthase 2 as novel PPAR target

Critical roles of PPARβ/δ in keratinocyte response to inflammation

The immediate response to skin injury is the release of inflammatory signals. It is shown here, by use of cultures of primary keratinocytes from wild-type and PPARβ/δ(−/−) mice, that such signals including TNF-α and IFN-γ, induce keratinocyte differentiation. This cytokine-dependent cell differentiation pathway requires up-regulation of the PPARβ/δ gene via the stress-associated kinase cascade, which targets an AP-1 site in the PPARβ/δ promoter. In addition, the pro-inflammatory cytokines also initiate the production of endogenous PPARβ/δ ligands, which are essential for PPARβ/δ activation and action. Activated PPARβ/δ regulates the expression of genes associated with apoptosis resulting in an increased resistance of cultured keratinocytes to cell death. This effect is also observed in vivo during wound healing after an injury, as shown in dorsal skin of PPARβ/δ(+/+) and PPARβ/δ(+/−) mice

Attenuation of colon inflammation through activators of the retinoid X receptor (RXR)/peroxisome proliferator-activated receptor gamma (PPARgamma) heterodimer. A basis for new therapeutic strategies.

The peroxisome proliferator-activated receptor gamma (PPARgamma) is highly expressed in the colon mucosa and its activation has been reported to protect against colitis. We studied the involvement of PPARgamma and its heterodimeric partner, the retinoid X receptor (RXR) in intestinal inflammatory responses. PPARgamma(1/)- and RXRalpha(1/)- mice both displayed a significantly enhanced susceptibility to 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis compared with their wild-type littermates. A role for the RXR/PPARgamma heterodimer in the protection against colon inflammation was explored by the use of selective RXR and PPARgamma agonists. TNBS-induced colitis was significantly reduced by the administration of both PPARgamma and RXR agonists. This beneficial effect was reflected by increased survival rates, an improvement of macroscopic and histologic scores, a decrease in tumor necrosis factor alpha and interleukin 1beta mRNA levels, a diminished myeloperoxidase concentration, and reduction of nuclear factor kappaB DNA binding activity, c-Jun NH(2)-terminal kinase, and p38 activities in the colon. When coadministered, a significant synergistic effect of PPARgamma and RXR ligands was observed. In combination, these data demonstrate that activation of the RXR/PPARgamma heterodimer protects against colon inflammation and suggest that combination therapy with both RXR and PPARgamma ligands might hold promise in the clinic due to their synergistic effects

Prostaglandin E(2) promotes colorectal adenoma growth via transactivation of the nuclear peroxisome proliferator-activated receptor delta.

Cyclooxygenase-derived prostaglandin E(2) (PGE(2)) is the predominant prostanoid found in most colorectal cancers (CRC) and is known to promote colon carcinoma growth and invasion. However, the key downstream signaling pathways necessary for PGE(2)-induced intestinal carcinogenesis are unclear. Here we report that PGE(2) indirectly transactivates PPARdelta through PI3K/Akt signaling, which promotes cell survival and intestinal adenoma formation. We also found that PGE(2) treatment of Apc(min) mice dramatically increased intestinal adenoma burden, which was negated in Apc(min) mice lacking PPARdelta. We demonstrate that PPARdelta is a focal point of crosstalk between the prostaglandin and Wnt signaling pathways which results in a shift from cell death to cell survival, leading to increased tumor growth