Abstract 22: OSBPL6 is a Novel miR-33 Target Gene Involved in Cellular Cholesterol Efflux

Cellular lipid homeostasis is fundamental to human health. Recent studies indicate that microRNAs (miRNAs) play important roles in the post-transcriptional regulation of lipid metabolism genes. This is illustrated by the discovery, by our group and others, of miR-33 as a central regulator of multiple aspects of lipid metabolism, including cholesterol efflux, fatty acid oxidation, and plasma levels of HDL cholesterol and VLDL triglycerides. In a study of miR-33 antagonism in non-human primates, we identified the most highly de-repressed miR-33 target gene as OSBPL6 (oxysterol binding protein like 6). A member of the OSBP family of cytosolic proteins that bind cholesterol/oxysterols, OSBPL6 contains an ER-targeting FFAT motif as well as a predicted plasma membrane-targeting pleckstrin homology domain, suggesting a putative role for OSBPL6 in vesicular traffic between these compartments. Notably, the OSBPL6 gene resides in a chromosome 2 locus associated with a predisposition to CAD in two major human heart disease studies (Family Heart Study, Framingham Heart Study). We therefore sought to further understand the regulation and function of OSBPL6 in cellular lipid homeostasis. Results We show that OSBPL6 is induced in macrophages and hepatocytes in response to cholesterol loading in vitro, as well as in the livers of LDLR-/- mice and non-human primates fed a high fat diet. Overexpression of miR-33 reduces OSBPL6-3’UTR activity by 40%, and this repression is relieved by mutation of the putative miR-33 binding site in the 3’UTR, confirming that OSBPL6 is a direct target of miR-33. Accordingly, transfection of HEPG2 cells with miR-33 mimic decreases mRNA expression of Osbpl6, but not the related family member Osbpl1. Notably, siRNA-mediated knockdown of OSBPL6 in cholesterol-loaded HEPG2 cells or THP-1 macrophages impairs cholesterol efflux to both apoA-I and HDL, and conversely, OSBPL6 overexpression enhances cholesterol efflux to these acceptors. Conclusion These data identify OSBPL6 as a novel miR-33 target gene in mice and humans that regulates cellular cholesterol trafficking and efflux via the ABC transporters, further highlighting the coordinated regulation by miR-33 of pathways that promote cellular cholesterol clearance.