Discovery of a novel regulatory pathway in cholesterol metabolism

Abstract

We recently identified and synthesized a novel sulfated oxysterol, 5-cholesten-3β, 25-diol 3-sulphate (25HC3S), whose concentration increased dramatically in the mitochondria and the nuclei of primary rat hepatocytes in response to over-expression of cholesterol mitochondria delivery protein, StarD1. This oxysterol was also found in human liver nuclei. The addition of synthesized 25HC3S to HepG2 cell culture increased cholesterol uptake and inhibited expression of HMG-CoA reductase, suggesting this nuclear oxysterol is a regulatory molecule. The present study demonstrates a pathway for biosynthesis of 25HC3S in hepatocytes. Assays using mitochondria isolated from rat liver and CYP27A1− knock out mouse liver indicated that 25-hydroxycholesterol is synthesized by CYP27A1 in mitochondria. Incubation of mitochondrial and cytosol fractions resulted in synthesis of the sulfated oxysterol. RT-PCR analysis showed the presence of the highly regulated hydroxycholesterol sulfotransferase 2B1b (SULT2B1b) in hepatocytes. Its expression could be down-regulated by addition of 25HC3S but not by 25-hydroxycholesterol. The results indicate that the mitochondria synthesize 25-OH-cholesterol which is subsequently 3β-sulfonated to form 25HC3S. The current findings imply the presence of a regulatory pathway in hepatocytes which play an important role in cholesterol homeostasis.