Effects of CYP7A1 overexpression on cholesterol and bile acid homeostasis

The initial and rate-limiting step in the classic pathway of bile acid bios ynthesis is 7 alpha -hydroxylation of cholesterol, a reaction catalyzed by cholesterol 7 alpha -hydroxylase (CYP7A1). The effect of CYP7A1 overexpress ion on cholesterol homeostasis in human liver cells has not been examined. The specific aim of this study was to determine the effects of overexpressi on of CYP7A1 on key regulatory steps involved in hepatocellular cholesterol homeostasis, using primary human hepatocytes (PHH) and HepG2 cells. Overex pression of CYP7A1 in HepG2 cells and PHH was accomplished by using a recom binant adenovirus encoding a CYP7A1 cDNA (AdCMV-CYP7A1). CYP7A1 overexpress ion resulted in a marked activation of the classic pathway of bile acid bio synthesis in both PHH and HepG2 cells. In response, there was decreased HMG -CoA-reductase (HMGR) activity, decreased acyl CoA:cholesterol acyltransfer ase (ACAT) activity, increased cholesteryl ester hydrolase (CEH) activity, and increased low-density lipoprotein receptor (LDLR) mRNA expression. Chan ges observed in HMGR, ACAT, and CEH mRNA levels paralleled changes in enzym e specific activities. More specifically, LDLR expression, ACAT activity, a nd CEH activity appeared responsive to an increase in cholesterol degradati on after increased CYP7A1 expression. Conversely, accumulation of the oxyst erol 7 alpha -hydroxycholesterol in the microsomes after CYP7A1 overexpress ion was correlated with a decrease in HMGR activity.