Peroxisome proliferator-activated receptor delta promotes very low-density lipoprotein-derived fatty acid catabolism in the macrophage.

Significant attention has focused on the role of low-density lipoprotein (LDL) in the pathogenesis of atherosclerosis. However, recent advances have identified triglyceride-rich lipoproteins [e.g., very LDL (VLDL)] as independent risk predictors for this disease. We have previously demonstrated peroxisome proliferator-activated receptor (PPAR)delta, but not PPARgamma, is the major nuclear VLDL sensor in the macrophage, which is a crucial component of the atherosclerotic lesion. Here, we show that, in addition to beta-oxidation and energy dissipation, activation of PPARdelta by VLDL particles induces key genes involved in carnitine biosynthesis and lipid mobilization mediated by a recently identified TG lipase, transport secretion protein 2 (also named desnutrin, iPLA2zeta, and adipose triglyceride lipase), resulting in increased fatty acid catabolism. Unexpectedly, deletion of PPARdelta results in derepression of target gene expression, a phenotype similar to that of ligand activation, suggesting that unliganded PPARdelta suppresses fatty acid utilization through active repression, which is reversed upon ligand binding. This unique transcriptional mechanism assures a tight control of the homeostasis of VLDL-derived fatty acid and provides a therapeutic target for other lipid-related disorders, including dyslipidemia and diabetes, in addition to coronary artery disease.

A chemical, genetic, and structural analysis of the nuclear bile acid receptor FXR.

The farnesoid X receptor (FXR) functions as a bile acid (BA) sensor coordinating cholesterol metabolism, lipid homeostasis, and absorption of dietary fats and vitamins. However, BAs are poor reagents for characterizing FXR functions due to multiple receptor independent properties. Accordingly, using combinatorial chemistry we evolved a small molecule agonist termed fexaramine with 100-fold increased affinity relative to natural compounds. Gene-profiling experiments conducted in hepatocytes with FXR-specific fexaramine versus the primary BA chenodeoxycholic acid (CDCA) produced remarkably distinct genomic targets. Highly diffracting cocrystals (1.78 A) of fexaramine bound to the ligand binding domain of FXR revealed the agonist sequestered in a 726 A(3) hydrophobic cavity and suggest a mechanistic basis for the initial step in the BA signaling pathway. The discovery of fexaramine will allow us to unravel the FXR genetic network from the BA network and selectively manipulate components of the cholesterol pathway that may be useful in treating cholesterol-related human diseases.

Genetic profiling defines the xenobiotic gene network controlled by the nuclear receptor pregnane X receptor.

The orphan nuclear receptor pregnane X receptor (PXR) is essential for the transcriptional regulation of hepatic xenobiotic enzymes including the cytochrome 3A isoenzymes. These enzymes are central to the catabolism and clearance of most endogenous sterol metabolites (endobiotics) and a vast diversity of foreign compounds (xenobiotics) including pharmaceuticals, pesticides, and toxins encountered through diet and environmental exposure. To explore a broader role of PXR in the mammalian xenobiotic response, we have conducted a unique microarray gene profiling analysis on liver samples derived from PXR knockout mice and mice expressing a constitutively active variant, VP-hPXR. This genetically guided expression analysis enables targeting and restriction of the PXR response to liver, and is devoid of side effects resulting from drugs and their metabolites. As with pharmacological studies, receptor-dependent genes include both phase I and phase II metabolic enzymes, as well as certain drug and anion transporters as principal PXR targets. Moreover, comparative analysis of data from both genetic and pharmacological arrays reveals a core network that represents a genetic description of the xenobiotic response.

Regulation of a xenobiotic sulfonation cascade by nuclear pregnane X receptor (PXR).

The nuclear receptor PXR (pregnane X receptor) protects the body from hepatotoxicity of secondary bile acids such as lithocholic acid (LCA) by inducing expression of the hydroxylating cytochrome P450 enzyme CYP3A and promoting detoxification. We found that activation of PXR also increases the activity and gene expression of the phase II conjugating enzyme dehydroepiandrosterone sulfotransferase (STD) known to sulfate LCA to facilitate its elimination. This activation is direct and appears to extend to other xenobiotic sulfotransferases as well as to 3'-phosphoadenosine 5'-phosphosulfate synthetase 2 (PAPSS2), an enzyme that generates the donor cofactor for the reaction. Because sulfation plays an important role in the metabolism of many xenobiotics, prescription drugs, and toxins, we propose that PXR serves as a master regulator of the phase I and II responses to facilitate rapid and efficient detoxification and elimination of foreign chemicals.