No abstract

A novel nonthiazolidinedione dual peroxisome proliferator-activated receptor (PPAR)-␣/␥ agonist, LY465608, was designed to address the major metabolic disturbances of type 2 diabetes. LY465608 altered PPAR-responsive genes in liver and fat of db/db mice and dose-dependently lowered plasma glucose in hyperglycemic male Zucker diabetic fatty (ZDF) rats, with an ED 50 for glucose normalization of 3.8 mg ⅐ kg -1 ⅐ day -1 . Metabolic improvements were associated with enhanced insulin sensitivity, as demonstrated in female obese Zucker (fa/fa) rats using both oral glucose tolerance tests and hyperinsulinemic-euglycemic clamps. Further characterization of LY465608 revealed metabolic changes distinct from a selective PPAR-␥ agonist, which were presumably due to the concomitant PPAR-␣ agonism, lower respiratory quotient, and less fat accumulation, despite a similar impact on glycemia in male ZDF rats. In addition to these alterations in diabetic and insulin-resistant animals, LY465608 dose-dependently elevated HDL cholesterol and lowered plasma triglycerides in human apolipoprotein A-I transgenic mice, demonstrating that this compound significantly improves primary cardiovascular risk factors. Overall, these studies demonstrate that LY465608 beneficially impacts multiple facets of type 2 diabetes and associated cardiovacular risk, including those facets involved in the development of micro-and macrovascular complications, which are the major sources for morbidity and mortality in these patients.

Fenofibrate is clinically successful in treating hypertriglyceridemia and mixed hyperlipidemia presumably through peroxisome proliferator-activated receptor ␣ (PPAR␣)-dependent induction of genes that control fatty acid ␤-oxidation. Lipid homeostasis and cholesterol metabolism also are regulated by the nuclear oxysterol receptors, liver X receptors ␣ and ␤ (LXR␣ and LXR␤). Here we show that fenofibrate ester, but not fenofibric acid, functions as an LXR antagonist by directly binding to LXRs. Likewise, ester forms, but not carboxylic acid forms, of other members of the fibrate class of molecules antagonize the LXRs. The fibrate esters display greater affinity for LXRs than the corresponding fibric acids have for PPAR␣. Thus, these two nuclear receptors display a degree of conservation in their recognition of ligands; yet, the acid/ester moiety acts as a chemical switch that determines PPAR␣ versus LXR specificity. Consistent with its LXR antagonistic activity, fenofibrate potently represses LXR agonist-induced transcription of hepatic lipogenic genes. Surprisingly, fenofibrate does not repress LXR-induced transcription of various ATP-binding cassette transporters either in liver or in macrophages, suggesting that fenofibrate manifests variable biocharacter in the context of differing gene promoters. These findings provide not only an unexpected mechanism by which fenofibrate inhibits lipogenesis but also the basis for examination of the pharmacology of an LXR ligand in humans.Originally developed as hypolipidemic therapeutic agents, the fibrate class of molecules has been extensively characterized as ligands for the nuclear receptor, peroxisome proliferator-activated receptor ␣ (PPAR␣) 1 (1). In humans, the major pharmacological effects of PPAR␣ activation by fibrates are the reduction of plasma cholesterol and triglyceride levels. Mechanistically, decreases in triglyceride-rich plasma lipoprotein levels could occur via accelerated catabolism, decreased synthesis, or both. Indeed, activation of PPAR␣ by fibrates promotes ␤-oxidation of fatty acids in the peroxisome as well as in the mitochondria, thus reducing the fatty acid available to the liver for triglyceride synthesis (2).Conversely, under appropriate metabolic states, such as those when starved animals are fed a high carbohydrate diet, increased lipogenesis in liver occurs in part by means of transcriptional activation of genes associated with de novo fatty acid biosynthesis, such as fatty acid synthase (FAS), via the transcription factor, sterol regulatory element-binding protein-1 (SREBP1) (3-5). Coincidentally, recent studies revealed that both oxysterols and synthetic agonists for the nuclear receptor liver X receptor (LXR) indirectly activate the lipogenic program by LXR-mediated induction of SREBP1, thereby leading to the coordinate expression of major lipogenic enzymes and profound elevation of triglyceride levels in liver (6 -8). LXR responsiveness of the FAS promoter also is attributed to a conserved LXR/RXR binding site within the 5Ј-flanking re...

Natural products have been identified as ligands for a number of members of the nuclear hormone receptor (NHR) superfamily. Often these natural products are used as dietary supplements to treat myriad ailments ranging from perimenopausal hot flashes to hypercholesterolemia and reduced cognitive function. Examples of some natural product ligands for NHRs include genestein (estrogen receptors NR3A1 and NR3A2), guggulsterone (farnesoid X receptor NR1H4), and St. John's wort (pregnane X receptor, NR1I2). In this study, we identified the first nonoxysterol natural product that functions as a ligand for the liver X receptor (LXR␣ and LXR␤; NR1H3, NR1H2), a NHR that acts as the receptor for oxysterols and plays a key role in regulation of cholesterol metabolism and transport as well as glucose metabolism. We show that paxilline, a fungal metabolite, is an efficacious agonist of both LXR␣ and LXR␤ in biochemical and in vitro cell-based assays. Paxilline binds directly to both receptors and is an activator of LXR-dependent transcription in cell-based reporter assays. We also demonstrate that paxilline binding to the receptors results in efficient activation of transcription of two physiological LXR target genes, ABCA1 and SREBP. The discovery of paxilline, the first reported nonoxysterol natural product ligand of the LXRs, may provide insight into the mechanism of ligand recognition by these receptors and reaffirms the utility of examining natural product libraries for identifying novel NHR ligands.

The structural basis of the pharmacology enabling the use of glucocorticoids as reliable treatments for inflammation and autoimmune diseases has been augmented with a new group of glucocorticoid receptor (GR) ligands. Compound 10, the archetype of a new family of dibenzoxepane and dibenzosuberane sulfonamides, is a potent anti-inflammatory agent with selectivity for the GR versus other steroid receptors and a differentiated gene expression profile versus clinical glucocorticoids (lower GR transactivation with comparable transrepression). A stereospecific synthesis of this chiral molecule provides the unique topology needed for biological activity and structural biology. In vivo activity of 10 in acute and chronic models of inflammation is equivalent to prednisolone. The crystal structure of compound 10 within the GR ligand binding domain (LBD) unveils a novel binding conformation distinct from the classic model adopted by cognate ligands. The overall conformation of the GR LBD/10 complex provides a new basis for binding, selectivity, and anti-inflammatory activity and a path for further insights into structure-based ligand design.