Studies on the regulatory roles of cholesterol and bile acids

Sammanfattning: Cholesterol is essential for normal growth and development in mammals. However, excess cholesterol can be harmful leading to diseases such as atherosclerosis and gallstones. Once formed the ring structure of cholesterol cannot be catabolised by the body and must therefore be converted to other compounds for excretion. In most vertebrates the faecal route of cholesterol excretion is through the formation of bile acids (BA) and the solubilisation of free cholesterol in bile. Cholesterol is converted into the primary bile acids cholic acid (CA) and chenodeoxycholic acid, the ratio of which is determined by the enzyme sterol 12 a -hydroxylase (CYP8B1). Both cholesterol and BAs regulate their own synthesis, and other biological processes, through the actions of transcription factors such as the sterol regulatory element binding proteins (SREBP), the liver X receptor and the farnesoid X receptor. A Cyp8b1 knockout mouse model has been created, which lacks the ability to synthesize CA. Using this model we investigated the regulatory effects of CA and cholesterol on metabolism and found that a reduction in CA resulted in decreased cholesterol absorption, increased cholesterol synthesis and was protective against large increases in hepatic cholesterol levels. Our results suggest an inhibitor of CYP8B1 may reduce cholesterol levels and thereby reduce the development of cardiovascular disease. The gene involved in cholesterol synthesis most responsive to regulation by CA and cholesterol treatment was squalene epoxidase (Sqle). Therefore the promoter of this gene was investigated further. In analogy to the human SQLE promoter the murine promoter was activated by SREBPs. A 205bp region of the promoter, containing three novel sterol regulatory elements, was found to be responsible for SREBP-2 regulation. While investigating the promoters of other genes involved in cholesterol synthesis mevalonate kinase was found to share a short common promoter with the cob(I)alamin adenotransferase gene (MMAB) catalysing the synthesis of AdoCbl, a cofactor for methylmalonyl CoA mutase. In mice the expression of Mmab was increased by statin treatment. Therefore, statins may represent a novel treatment for patients with methylmalonic aciduria type B, resulting from mutations in this gene.