Overexpression of CYP27 in hepatic and extrahepatic cells: role in the regulation of cholesterol homeostasis

In the liver, sterol 27-hydroxylase (CYP27) participates in the classic and alternative pathways of bile acid biosynthesis from cholesterol (Chol). In extrahepatic tissues, CYP27 converts intracellular Chol to 27-hydroxycholesterol (27OH-Chol), which may regulate the activity of 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA-R). This study attempts to better define the role of CYP27 in the maintenance of Chol homeostasis in hepatic and extrahepatic cells by overexpressing CYP27 in Hep G2 cells and Chinese hamster ovary (CHO) cells through infection with a replication-defective recombinant adenovirus encoding for CMV-CYP27. After infection, CYP27 mRNA and protein levels increased dramatically. CYP27 specific activity also increased two- to fourfold in infected cells (P less than or equal to 0.02), with a marked increase in conversion of [C-14]Chol to [C-14]27OH-Chol (similar to 150%; P less than or equal to 0.01). Accumulation of 27OH-Chol in CHO cells was associated with a 50% decrease in HMG-CoA-R specific activity (P less than or equal to 0.02). In infected Hep G2 cells, the significant increase in bile acid synthesis (46%; P less than or equal to 0.006), which prevented the accumulation of intracellular 27OH-Chol, resulted in increased HMG-CoA-R activity (183%; P less than or equal to 0.02). Overexpression of CYP27 in Hep G2 cells also increased acyl CoA-cholesterol acyltransferase (71%, P less than or equal to 0.02) and decreased cholesteryl ester hydrolase (55%, P less than or equal to 0.02). In conclusion, CYP27 generates different physiological responses depending on cell type and presence or absence of bile acid biosynthetic pathways.