Direct and indirect mechanisms for regulation of fatty acid synthase gene expression by liver X receptors

The nuclear receptors LXRalpha and LXRbeta have been implicated in the control of lipogenesis and cholesterol homeostasis. Ligand activation of these receptors in vivo induces expression of the LXR target gene SREBP-1c and increases plasma triglyceride levels. Expression of fatty acid synthase (FAS), a central enzyme in de novo lipogenesis and an established target of the SREBP-1 pathway, is also induced by LXR ligands. The effects of LXR ligands on FAS expression have been proposed to be entirely secondary to the induction of SREBP-1c. We demonstrate here that LXRs regulate FAS expression through direct interaction with the FAS promoter as well as through activation of SREBP-1c expression. Induction of FAS expression in HepG2 cells by LXR ligands is reduced, but not abolished, under conditions where SREBP processing is suppressed. Moreover, LXR ligands induce FAS expression in CHO-7 cells without altering expression of SREBP-1. We demonstrate that in addition to tandem SREBP sites, the FAS promoter contains a high affinity binding site for the LXR/RXR heterodimer that is conserved in diverse animal species including birds, rodents, and humans. The LXR and SREBP binding sites independently confer LXR responsiveness on the FAS promoter, and maximal induction requires both transcription factors. Transient elevation of plasma triglyceride levels in mice treated with a synthetic LXR agonist correlates with transient induction of hepatic FAS expression. These results indicate that the LXR signaling pathway modulates FAS expression through distinct but complementary mechanisms and suggest that the FAS gene may be a critical target in the control of lipogenesis by LXRs.