The nuclear receptors liver X receptor α (LXRα) (NR1H3) and LXRβ (NR1H2) are important regulators of genes involved in lipid metabolism, including ABCA1,ABCG1, and sterol regulatory element-binding ...protein-1c (SREBP-1c). Although it has been demonstrated that oxysterols are LXR ligands, little is known about the identity of the physiological activators of these receptors. Here we confirm earlier studies demonstrating a dose-dependent induction of ABCA1 and ABCG1 in human monocyte-derived macrophages by cholesterol loading. In addition, we show that formation of 27-hydroxycholesterol and cholestenoic acid, products of CYP27 action on cholesterol, is dependent on the dose of cholesterol used to load the cells. Other proposed LXR ligands, including 20(S)-hydroxycholesterol, 22(R)-hydroxycholesterol, and 24(S),25-epoxycholesterol, could not be detected under these conditions. A role for CYP27 in regulation of cholesterol-induced genes was demonstrated by the following findings. 1) Introduction of CYP27 into HEK-293 cells conferred an induction of ABCG1 and SREBP-1c; 2) upon cholesterol loading, CYP27-expressing cells induce these genes to a greater extent than in control cells; 3) in CYP27-deficient human skin fibroblasts, the induction of ABCA1 in response to cholesterol loading was ablated; and 4) in a coactivator association assay, 27-hydroxycholesterol functionally activated LXR. We conclude that 27-hydroxylation of cholesterol is an important pathway for LXR activation in response to cholesterol overload.
Increased levels of triglyceride-rich lipoproteins provoke lipid accumulation in the artery wall, triggering early inflammatory responses central to atherosclerosis like endothelial adhesion molecule ...expression. The endogenous mechanisms limiting such reactions remain poorly defined. Lipoprotein lipase (LPL) plays a central role in lipid metabolism by hydrolyzing triglyceride rich lipoproteins and releasing fatty acids. We found that LPL treatment reversed tumor necrosis factor α and very low-density lipoprotein (VLDL)-stimulated endothelial vascular cell adhesion molecule 1 (VCAM1) induction and VCAM1 promoter responses, thus recapitulating effects reported with synthetic peroxisome proliferator-activated receptor (PPAR) agonists. In fact, these LPL effects on VCAM1 were absent in endothelial cells isolated from PPARα-deficient mice. This finding suggests a novel antiinflammatory role for LPL. Further studies reveal specificity for PPAR activation through lipolysis in regards to lipoprotein substrate (VLDL >> LDL > HDL), PPAR isoform (PPARα >> PPARδ > PPARγ), and among fatty acid-releasing lipases. These PPAR responses required intact LPL catalytic activity. In vivo, transgenic mice overexpressing LPL had increased peroxisome proliferation, but not in the genetic absence of PPARα. Although human plasma possesses minimal PPARα activation despite containing abundant free fatty acids, marked PPARα activation is seen with human plasma after LPL is added in vitro or systemically released in vivo. These data suggest a previously uncharacterized pathway in which the key lipolytic enzyme LPL can act on circulating lipoproteins to generate PPARα ligands, providing a potentially important link between lipoprotein metabolism and distal PPARα transcriptional effects.
Antidiabetic thiazolidinediones (TZDs) and non-TZD compounds have been shown to serve as agonists of the peroxisome proliferator-activated receptor γ (PPARγ). Here, we report the identification and ...characterization of a novel non-TZD selective PPARγ modulator (nTZDpa). nTZDpa bound potently to PPARγ with high selectivity vs. PPARα or PPARδ. In cell-based assays for transcriptional activation, nTZDpa served as a selective, potent PPARγ partial agonist and was able to antagonize the activity of PPARγ full agonists. nTZDpa also displayed partial agonist effects when its ability to promote adipogenesis in 3T3-L1 cells was evaluated. Assessment of protein conformation using protease protection or solution nuclear magnetic resonance spectroscopy methods showed that nTZDpa produced altered PPARγ conformational stability vs. full agonists, thereby establishing a physical basis for its observed partial agonism. DNA microarray analysis of RNA from 3T3-L1 adipocytes treated with nTZDpa or several structurally diverse PPARγ full agonists demonstrated qualitative differences in the affected gene expression profile for nTZDpa. Chronic treatment of fat-fed, C57BL/6J mice with nTZDpa or a TZD full agonist ameliorated hyperglycemia and hyperinsulinemia. However, unlike the TZD, nTZDpa caused reductions in weight gain and adipose depot size. Feed efficiency was also substantially diminished. Unlike TZDs, nTZDpa did not cause cardiac hypertrophy in mice. When a panel of PPARγ target genes was examined in white adipose tissue, nTZDpa produced a different in vivo expression pattern vs. the full agonist. These findings establish that novel selective PPARγ modulators can produce altered receptor conformational stability leading to distinctive gene expression profiles, reduced adipogenic cellular effects, and potentially improved in vivo biological responses. Such compounds may lead to preferred therapies for diabetes, obesity, or metabolic syndrome.
This study sought to determine whether the adipose depot-specific (subcutaneous SF vs. visceral VF) action of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists on fat deposition ...extends to the expression of lipoprotein lipase (LPL) and other key adipose lipid metabolism genes, and whether changes in LPL impact triglyceridemia. Rats were fed a standard diet or an obesity-promoting diet for 3 weeks, with or without treatment with COOH, a nonthiazolidinedione PPAR-gamma agonist. Treatment effects were essentially similar in both dietary cohorts. COOH did not affect weight gain, but increased SF (inguinal) fat mass twofold and reduced VF (retroperitoneal) accretion by half. Corresponding depot-specific alterations were observed in mRNA levels of the glucocorticoid-activating enzyme 11beta-hydroxysteroid dehydrogenase 1 (11beta-HSD-1) and the thermogenic modulator uncoupling protein 1 (UCP-1). COOH increased brown adipose tissue (BAT) weight and LPL availability by five- to eightfold. In rats refed standard diet after a 24-h fast, COOH reduced the insulin excursion by half. The agonist increased SF LPL activity and mRNA levels, but had no effect on VF LPL. The two- to threefold postprandial increase in plasma triglycerides (TGs) was abrogated in COOH-treated rats, likely in part because of increased LPL in SF and BAT. Thus PPAR-gamma agonist treatment had a powerful, site-specific effect on adipose metabolism and lipid deposition, and greatly impacted the postprandial handling of TG-rich lipoproteins. These depot-specific effects may be mediated by differential regulation of key metabolic genes, including LPL, 11beta-HSD-1, and UCP-1.
The design and synthesis of a novel class of 2,3-dihydrobenzofuran-2-carboxylic acids as highly potent and subtype-selective PPARα agonists are reported. Systematic study of structure−activity ...relationships has identified several key structural elements within this class for maintaining the potency and subtype selectivity. Select compounds were evaluated in animal models of dyslipidemia using Syrian hamsters and male Beagle dogs, and all these compounds displayed excellent cholesterol- and triglyceride-lowering activity at dose levels that were much lower than the marketed weak PPARα agonist fenofibrate.
The LXR nuclear receptors are intracellular sensors of cholesterol excess and are activated by various oxysterols. LXRs have been shown to regulate multiple genes of lipid metabolism, including ABCA1 ...(formerly known asABC1). ABCA1 is a lipid pump that effluxes cholesterol and phospholipid out of cells. ABCA1 deficiency causes extremely low high density lipoprotein (HDL) levels, demonstrating the importance of ABCA1 in the formation of HDL. The present work shows that the acetyl-podocarpic dimer (APD) is a potent, selective agonist for both LXRα (NR1H3) and LXRβ (NR1H2). In transient transactivation assays, APD was ∼1000-fold more potent, and yielded ∼6-fold greater maximal stimulation, than the widely used LXR agonist 22-(R)-hydroxycholesterol. APD induced ABCA1mRNA levels, and increased efflux of both cholesterol and phospholipid, from multiple cell types. Gas chromatography-mass spectrometry measurements demonstrated that APD stimulated efflux of endogenous cholesterol, eliminating any possible artifacts of cholesterol labeling. For both mRNA induction and stimulation of cholesterol efflux, APD was found to be more effective than was cholesterol loading. Taken together, these data show that APD is a more effective LXR agonist than endogenous oxysterols. LXR agonists may therefore be useful for the prevention and treatment of atherosclerosis, especially in the context of low HDL levels.
A series of selective PPARγ modulators (SPPARγMs) and their development from hPPARγ active screening leads
1 and
2 is described. SPPARγM
24 displayed robust anti-diabetic activity with an improved ...therapeutic window in comparison to a PPARγ full agonist in a rodent efficacy model.
Routine screening for human PPAR ligands yielded compounds
1 and
2, both of which were sub-micromolar hPPARγ agonists. Synthetic modifications of these leads led to a series of potent substituted 3-benzyl-2-methyl indoles, a subset of which were noted to be selective PPARγ modulators (SPPARγMs). SPPARγM
24 displayed robust anti-diabetic activity with an improved therapeutic window in comparison to a PPARγ full agonist in a rodent efficacy model.
The synthesis and structure−activity relationships of novel series of α-aryloxyphenylacetic acids as PPARα/γ dual agonists are reported. The initial search for surrogates of the ester group in the ...screen lead led first to the optimization of a subseries with a ketone moiety. Further efforts to modify the ketone subseries led to the design and synthesis of two new subseries containing fused heterocyclic ring systems. All these analogues were characterized by their “super” PPARα agonist activity and weak or partial agonist activity on PPARγ in PPAR-GAL4 transactivation assays despite their similar binding affinities for both receptors. The cocrystal structures of compounds 7 and rosiglitazone with PPARγ-LBD were compared, and significant differences were found in their interactions with the receptor. Select analogues in each subseries were further evaluated for in vivo efficacy. They all showed excellent anti-hyperglycemic efficacy in a d b/d b mouse model and hypolipidemic activity in hamster and dog models without provoking the typical PPARγ-associated side effects in the rat tolerability assay.
A series of 2-aryloxy-2-methyl-propionic acid compounds and related analogues were designed, synthesized, and evaluated for their PPAR agonist activities. ...2-(5,7-Dipropyl-3-trifluoromethyl)-benzisoxazol-6-yloxy-2-methylpropionic acid (4) was identified as a PPARα/γ dual agonist with relative PPARα selectivity and demonstrated potent efficacy in lowering both glucose and lipids in animal models without causing body weight gain. The PPARα activity of 4 appeared to have played a significant role in lowering glucose levels in db/db mice.
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A series of metabolically robust N-benzyl-indole selective PPARγ modulators with either a 3-benzoyl or 3-benzisoxazoyl moiety have been identified. In vitro, these compounds are ...partial agonists and exhibit reduced adipogenesis in human adipocytes. In vivo, these SPPARγMs result in potent glucose lowering in db/db mice and attenuate increases in heart weight and brown adipose tissue that is typically observed in rats upon treatment with PPARγ full agonists.
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