Studies on the molecular regulation of bile acids and lipoproteins in human liver

Sammanfattning: Elevated lipid levels in the bloodstream, in particular high levels of LDL cholesterol, are a major risk factor for the development of atherosclerosis. The liver has a central role in cholesterol metabolism and is the only organ capable of a quantitative elimination of cholesterol from the body. Cholesterol can be eliminated both through excretion of free cholesterol in bile and through conversion into bile acids. The aim of this thesis was to improve our understanding of how the metabolism of bile acids and lipoproteins are regulated in human liver both in vivo and in vitro. In the first part of the studies, the effect of stimulation or inhibition of bile acid synthesis on the metabolism of bile acids and lipoproteins were investigated in human liver. Stimulation and inhibition of bile acid synthesis were achieved by treating patients with cholestyramine or chenodeoxycholic acid, respectively. Following stimulation of bile acid synthesis, the mRNA levels for HNF4á were up-regulated and positively correlated to the expression levels of CYP7A1, the rate limiting enzyme in bile acid synthesis. From the study we can conclude that HNF4á appears to be a key factor in the regulation of bile acid formation in humans. When bile acid synthesis is altered, the metabolism of triglycerides, cholesterol and lipoproteins is also affected. Following cholestyramine treatment the mRNA expression of LDLR, HMGCoAR and PCSK9 increased and the expression of these genes was positively correlated to the expression of SREBP2. The positive correlation between the expression of PCSK9 and SREBP2 indirectly suggests that SREBP2 regulates PCSK9 transcription in humans, in vivo. In the last part of the studies, we used primary human hepatocytes to investigate the effect of ethanol on the synthesis of bile acids. Following addition of ethanol to the cells, the formation of bile acids, especially cholic acid, was stimulated. The increased bile acid synthesis can provide one explanation for the observed reduction of cardiovascular- and gallstone disease following ethanol intake.