Hepatic peroxisomal fatty acid beta-oxidation is regulated by liver X receptor alpha.

Peroxisomes are the exclusive site for the beta-oxidation of very-long-chain fatty acids of more than 20 carbons in length (VLCFAs). Although the bulk of dietary long-chain fatty acids are oxidized in the mitochondria, VLCFAs cannot be catabolized in mitochondria and must be shortened first by peroxisomal beta-oxidation. The regulation of peroxisomal, mitochondrial, and microsomal fatty acid oxidation systems in liver is mediated principally by peroxisome proliferator-activated receptor alpha (PPARalpha). In this study we provide evidence that the liver X receptor (LXR) regulates the expression of the genetic program for peroxisomal beta-oxidation in liver. The genes encoding the three enzymes of the classic peroxisomal beta-oxidation cycle, acyl-coenzyme A (acyl-CoA) oxidase, enoyl-CoA hydratase/L-3-hydroxyacyl-CoA dehydrogenase, and 3-ketoacyl-CoA thiolase, are activated by the LXR ligand, T0901317. Accordingly, administration of T0901317 in mice promoted a dose-dependent and greater than 2-fold increase in the rate of peroxisomal beta-oxidation in the liver. The LXR effect is independent of PPARalpha, because T0901317-induced peroxisomal beta-oxidation in the liver of PPARalpha-null mice. Interestingly, T0901317-induced peroxisomal beta-oxidation is dependent on the LXRalpha isoform, but not the LXRbeta isoform. We propose that induction of peroxisomal beta-oxidation by LXR agonists may serve as a counterregulatory mechanism for responding to the hypertriglyceridemia and liver steatosis that is promoted by potent LXR agonists in vivo; however, additional studies are warranted.