The adaptor protein GIPC1 stabilizes the scavenger receptor SR-B1 and increases its cholesterol uptake

The scavenger receptor class B type 1 (SR-B1), a high-density lipoprotein (HDL) receptor, is a membrane glycoprotein that mediates selective uptake of HDL-cholesterol and cholesterol ester (CE) into cells. SR-B1 is subject to posttranslational regulation; however, the underlying mechanisms still remain obscure. Here, we identified a novel SR-B1-interacting protein, GIPC1 (GAIP-interacting protein, C terminus 1) that interacts with SR-B1 and stabilizes SR-B1 by negative regulation of its proteasomal and lysosomal degradation pathways. The physiological interaction between SR-B1 and GIPC1 was supported by co-immunoprecipitation of wild-type and mutant GIPC1 constructs in SR-B1 +/- GIPC1 overexpressing cells, in native liver cells, and in mouse liver tissues. Overexpression of GIPC1 increased endogenous SR-B1 protein levels, subsequently increasing selective HDL-cholesterol/CE uptake and cellular triglyceride (TG) and total cholesterol (TC) levels, whereas silencing of GIPC1 in the mouse liver was associated with blunted hepatic SR-B1 levels, elevated plasma TG and TC, and attenuated hepatic TG and TC content. A positive correlation was identified between GIPC1 and SR-B1 expression, and both expressions of GIPC1 and SR-B1 from human liver samples were inversely correlated with body mass index (BMI) from human subjects. We therefore conclude that GIPC1 plays a key role in the stability and function of SR-B1 and can also effectively regulate hepatic lipid and cholesterol metabolism. These findings expand our knowledge of the regulatory roles of GIPC1 and suggest that GIPC1 exerts a major effect on cell surface receptors such as SR-B1 and its associated hepatic lipid and cholesterol metabolic processes.