Hepatic fatty acid transporter Cd36 is a common target of LXR, PXR, and PPARγ in promoting steatosis

Background & Aims: Liver X receptor (LXR) is known to promote hepatic lipogenesis by activating the lipogenic transcriptional factor sterol regulatory element-binding protein (Srebp). Pregnane X receptor (PXR), a previously known "xenobiotic receptor," could mediate a Srebp-independent lipogenic pathway by activating the free fatty acid uptake transporter Cd36. The goal of this study is to investigate further the role of Cd36 in hepatic steatosis. Methods: Wildtype, LXR transgenic, PXR transgenic, and Cd36 null mice were used to study the regulation of Cd36 and other hepatic lipogenic genes and the implication of this regulation in hepatic steatosis. Promoter sequences of Cd36 and peroxisome proliferator-activated receptor (PPAR) gamma were cloned, and their respective regulation by LXR and PXR was investigated by combinations of receptor-DNA binding and reporter gene assays. Results: We showed that genetic (transgene) or pharmacologic (ligands) activation of LXR induced Cd36. Promoter analysis established Cd36 as a novel transcription target of LXR alpha. Moreover, the hepatic steatosis induced by LXR agonists was largely abolished in U36 null mice. We also showed that PPAR gamma, a positive regulator of Cd36, is a transcriptional target of PXR, suggesting that PXR can regulate Cd36 directly or through its activation of PPAR gamma. Interestingly, both LXR-mediated Cd36 regulation and PXR-mediated PPAR gamma regulation are liver specific. Conclusions: We conclude that U36 is a shared target of LXR, PXR, and PPAR gamma. The network of CD36 regulation by LXF, PXR, and PPAR gamma establishes this free fatty acid transporter as a common target of orphan nuclear receptors in their mediation of lipid homeostasis.