Studies on the regulatory role of cholic acid in cholesterol metabolism

Sammanfattning: Cholesterol is essential for normal growth and development in both man and mouse. However, excess cholesterol can be harmful leading to diseases such as atherosclerosis and gallstones. Once formed cholesterol cannot be catabolised by the body and must therefore be converted to other compounds for excretion. In both man and mouse the faecal route of cholesterol excretion, through bile acid synthesis and solubilisation of free cholesterol in bile is the major pathway for the removal of excess cholesterol. Cholesterol is converted into the two primary bile acids, cholic acid (CA) and chenodeoxycholic acid (CDCA). The ratio of CA to CDCA is determined by the enzyme sterol 12alpha-hydroxylase (CYP 8B 1). A CYP8B1 knockout mouse model was created', which completely lacks the ability to synthesise CA. These, CYP8B1 knockout mice appear phenotypically normal although they have an increased bile acid pool size comprising of only CDCA and its secondary bile acids. We investigated the effect that a loss of CA has on cholesterol absorption, hepatic de novo cholesterol synthesis, accumulation of free and esterifled cholesterol in the liver and plasma cholesterol concentration. One result from this study was that squalene epoxidase (SE) was one of the most highly regulated genes tested at the mRNA level. Therefore the regulation of the murine SE promoter was investigated. In analogy to the human SE promoter the murine promoter was activated by SREBPs with SREBP-2 being responsible for the greatest activation. However, while the SRE element reported to be responsible for this regulation by SREBP2 in the human SE promoter is conserved in the mouse it is not responsive to SREBP-2 in this species. Through deletions and mutagenesis of the murine SE promoter a region of 205bp was found to be responsive to SREBP-2 regulation. Two binding sites for NF-Y, a know co-factor for SREBP2, were also conserved in the mouse, rat and human SE promoters. In the mouse promoter both NFY sites bound to NF-Y and were required for SREBP-2 activation.