No abstract

The metabolism of vitamin A and the diverse effects of its metabolites are tightly controlled by distinct retinoid-generating enzymes, retinoid-binding proteins and retinoid-activated nuclear receptors. Retinoic acid regulates differentiation and metabolism by activating the retinoic acid receptor and retinoid X receptor (RXR), indirectly influencing RXR heterodimeric partners. Retinoic acid is formed solely from retinaldehyde (Rald), which in turn is derived from vitamin A. Rald currently has no defined biologic role outside the eye. Here we show that Rald is present in rodent fat, binds retinol-binding proteins (CRBP1, RBP4), inhibits adipogenesis and suppresses peroxisome proliferator-activated receptor-c and RXR responses. In vivo, mice lacking the Rald-catabolizing enzyme retinaldehyde dehydrogenase 1 (Raldh1) resisted diet-induced obesity and insulin resistance and showed increased energy dissipation. In ob/ob mice, administrating Rald or a Raldh inhibitor reduced fat and increased insulin sensitivity. These results identify Rald as a distinct transcriptional regulator of the metabolic responses to a high-fat diet.Although vitamin A and its metabolite retinoic acid have therapeutic applications, frequent side effects limit their use 1-3 . In clinical trials involving β-carotene supplementation, worrisome increases in cardiovascular events and mortality have been noted, despite evidence suggesting possible beneficial vascular effects of this treatment 3 . These variable responses to retinoids probably derive from the fact that β-carotene and vitamin A (retinol) and their major metabolites-retinaldehyde (Rald) and retinoic acid-regulate diverse cellular responses, including development, immune function and vision 4,5 . The tight control of retinoid biology is evident in the elaborate system that governs the absorption, formation, transportation and action of these structurally and functionally distinct retinoid metabolites. Despite this, retinoids

Natural and synthetic agonists of the peroxisome proliferatoractivated receptor ␥ (PPAR␥) regulate adipocyte differentiation, glucose homeostasis, and inflammatory responses. Although effects on adipogenesis and glucose metabolism are genetically linked to PPAR␥, the PPAR␥ dependence of antiinflammatory responses of these substances is less clear. Here, we have used a combination of mRNA expression profiling and conditional disruption of the PPAR␥ gene in mice to characterize programs of transcriptional activation and repression by PPAR␥ agonists in elicited peritoneal macrophages. Natural and synthetic PPAR␥ agonists, including the thiazolidinedione rosiglitazone (Ro), modestly induced the expression of a surprisingly small number of genes, several of which were also induced by a specific PPAR␦ agonist. The majority of these genes encode proteins involved in lipid homeostasis. In contrast, Ro inhibited induction of broad subsets of lipopolysaccharide and IFN-␥ target genes in a genespecific and PPAR␥-dependent manner. At high concentrations, Ro inhibited induction of lipopolysaccharide target genes in PPAR␥-deficient macrophages, at least in part by activating PPAR␦. These studies establish overlapping transactivation and transrepression functions of PPAR␥ and PPAR␦ in macrophages and suggest that a major transcriptional role of PPAR␥ is negative regulation of specific subsets of genes that are activated by T helper 1 cytokines and pathogenic molecules that signal through pattern recognition receptors. These findings support a physiological role of PPAR␥ in regulating both native and acquired immune responses. P PAR␥ is a member of the nuclear receptor superfamily of ligand-dependent transcription factors that regulates adipocyte differentiation and glucose homeostasis (1-3). Although the endogenous ligands that regulate peroxisome proliferatoractivated receptor (PPAR) ␥ activity in vivo remain poorly characterized, several naturally occurring polyunsaturated fatty acids and their metabolites have been identified that activate PPAR␥, including products that are generated through the actions of specific lipoxygenases (e.g., 13-hydroxyoctadecadienoic acid and 15-hydroxyeicosatetraenoic acid) and prostaglandin synthases (e.g., 15 deoxy-⌬ 12,14 prostaglandin J 2 ) (4-7). In addition, numerous synthetic PPAR␥ agonists have been identified, including the thiazolidinedione class of drugs used clinically in the treatment of type 2 diabetes mellitus (1, 3).Natural and synthetic PPAR␥ ligands have been also been shown to exert antiinflammatory effects in models of atherosclerosis (8-10), inflammatory bowel disease (11, 12), and allergic encephalomyelitis (13-15). The investigation of potential antiinflammatory effects of PPAR␥ agonists in these settings was initially based on studies demonstrating that they could inhibit transcriptional activation of inflammatory response genes by activators such as lipopolysaccharide (LPS), IL-1␤, and IFN-␥ in macrophages and other cell types (16-18). PPAR␥ expression is dramatically up...

The molecular mechanisms regulating secretion of the orexigenic-glucoregulatory hormone ghrelin remain unclear. Based on qPCR analysis of FACS-purified gastric ghrelin cells, highly expressed and enriched 7TM receptors were comprehensively identified and functionally characterized using in vitro, ex vivo and in vivo methods. Five Gαs-coupled receptors efficiently stimulated ghrelin secretion: as expected the β1-adrenergic, the GIP and the secretin receptors but surprisingly also the composite receptor for the sensory neuropeptide CGRP and the melanocortin 4 receptor. A number of Gαi/o-coupled receptors inhibited ghrelin secretion including somatostatin receptors SSTR1, SSTR2 and SSTR3 and unexpectedly the highly enriched lactate receptor, GPR81. Three other metabolite receptors known to be both Gαi/o- and Gαq/11-coupled all inhibited ghrelin secretion through a pertussis toxin-sensitive Gαi/o pathway: FFAR2 (short chain fatty acid receptor; GPR43), FFAR4 (long chain fatty acid receptor; GPR120) and CasR (calcium sensing receptor). In addition to the common Gα subunits three non-common Gαi/o subunits were highly enriched in ghrelin cells: GαoA, GαoB and Gαz. Inhibition of Gαi/o signaling via ghrelin cell-selective pertussis toxin expression markedly enhanced circulating ghrelin. These 7TM receptors and associated Gα subunits constitute a major part of the molecular machinery directly mediating neuronal and endocrine stimulation versus metabolite and somatostatin inhibition of ghrelin secretion including a series of novel receptor targets not previously identified on the ghrelin cell.