Abstract 636: The Nuclear Receptor FXR Uncouples the Actions of Srebp2 and MiR-33

Abstract

The role of microRNAs in fine-tuning gene expression has now been well established, but the mechanisms for microRNA regulation at the transcriptional level remain understudied. The physiologic context in which microRNAs are acting is likely linked to the function of these same microRNAs and how they are regulated. Here we identify miR-33 as a direct target gene of the bile acid nuclear receptor FXR. We show that pharmacologic FXR activation induces miR-33 in wild-type, but not FXR knockout mice. MiR-33 is located in the Srebp2 locus, and we show that FXR activation also induces the expression of Srebp2 mRNA. Using ChIP-Seq, we identify at least one functional FXRE in an intronic region of Srebp2, demonstrating that miR-33 and Srebp2 are both direct FXR targets. Although FXR activation increases Srebp2 mRNA levels, we also demonstrate that FXR in fact decreases Srebp2 proteolytic processing, which determines its transcriptional activity. This decreased activity is a result of the FXR dependent induction of the integral ER membrane protein Insig2, which is itself also a direct FXR target. Together these results identify a molecular mechanism for the transcriptional regulation of Srebp2 and miR-33, as well as the mechanism for the decreased activity of Srebp2 processing in the ER by FXR. The best characterized target of miR-33 is ABCA1. We have previously demonstrated that FXR activation decreases ABCA1 protein levels via the induction of miR-144, a microRNA that also targets ABCA1. Our new findings suggest that FXR induces two independent microRNAs that function together to repress ABCA1 and reduce HDL levels.