Abstract 76: MicroRNA-144 Regulates Hepatic ABCA1 and Plasma HDL Following Activation of the Nuclear Receptor FXR

Abstract

Rationale The bile acid receptor Farnesoid-X-Receptor (FXR) regulates many aspects of lipid metabolism by various complex and not fully understood molecular mechanisms. We set out to investigate the molecular mechanisms for FXR-dependent regulation of lipid and lipoprotein metabolism. Objective To identify FXR-regulated microRNAs that were subsequently involved in regulating lipid metabolism. Methods and Results ATP binding cassette transporter A1 (ABCA1) is a major determinant of plasma High Density Lipoprotein (HDL)-cholesterol levels. Here we show that activation of the nuclear receptor FXR in vivo increases hepatic levels of miR-144, which in turn lower hepatic ABCA1 and plasma HDL levels. We identified two complementary sequences to miR-144 in the 3’ untranslated region (UTR) of ABCA1 mRNA that are necessary for miR-144-dependent regulation. Overexpression of miR-144 in vitro decreased both cellular ABCA1 protein and cholesterol efflux to lipid-poor apolipoprotein A-I (ApoA-I) protein, whilst overexpression in vivo reduced hepatic ABCA1 protein and plasma HDL- cholesterol. Conversely, silencing miR-144 in mice increased hepatic ABCA1 protein and HDL- cholesterol. In addition, we utilized tissue-specific FXR deficient mice to show that induction of miR-144 and FXR-dependent hypolipidemia requires hepatic, but not intestinal FXR. Finally, we identified functional FXR response elements (FXREs) upstream of the miR-144 locus, consistent with direct FXR regulation. Conclusion In conclusion, we have identified a pathway involving FXR, miR-144 and ABCA1 that together regulate plasma HDL cholesterol. This pathway may be therapeutically targeted in the future in order to increase HDL levels.