Studies on cholesterol homeostasis by modulation of lipoproteins and bile acids

Excess plasma cholesterol, in particular high levels of LDL cholesterol, is a major risk factor for the development of atherosclerosis. The liver has a central role in the regulation of cholesterol metabolism: i) a major part of the cholesterol present in the body is synthesized in the liver; ii) the level of plasma lipoproteins is regulated by liver uptake and secretion of these particles and iii) the liver is essential for elimination of cholesterol from the body as such or by its conversion into bile acids.

In this thesis we have investigated how different modifications of lipoproteins and bile acids affect cholesterol metabolism both in vivo and in vitro.

Stimulation and inhibition of bile acid synthesis was studied in patients treated with cholestyramine or chenodeoxycholic acid (CDCA). Following stimulation of bile acid synthesis with cholestyramine the mRNA levels for HNF4alpha were upregulated and positively correlated to the expression levels of CYP7A1, the rate limiting enzyme in bile acid synthesis. The results were further supported by the observation that CDCA treatment of primary human hepatocytes reduces the expression of HNF4alpha. From this study we can conclude that HNF4alpha appears to be a key factor in the regulation of bile acid formation in humans.

When bile acid synthesis is altered, the metabolism of cholesterol and lipoproteins is also affected. Following cholestyramine treatment the mRNA expression of LDLR and PCSK9 increased and in the opposite situation when patients were treated with CDCA, a reduction of LDLR and HMGCoAR expression was observed. The expression of these three genes was positively correlated to the expression of SREBP2 which indirectly suggests that SREBP2 regulates PCSK9 transcription in humans, in vivo.

It was previously reported that alcohol intake can reduce the risk for development of cardiovascular and gallstone disease, one possible explanation for the reduction could be an increased conversion of cholesterol into bile acids. In the third paper, primary human hepatocytes were used to investigate the effect of ethanol on the synthesis of bile acids. Following addition of ethanol to the cells the formation of bile acids was stimulated. In particular the synthesis of cholic acid increased, which suggests a stimulation of the neutral pathway for bile acid synthesis. The molecular mechanisms behind the effect remain to be elucidated.

Thyroid hormones (TH) regulate cholesterol metabolism but their use as lipid-lowering drugs are restricted because of negative cardiac effects. TH mimetic compounds modulating TH receptor beta have been designed as potential drugs since they reduce serum cholesterol while avoiding apparent deleterious effects. In Paper IV the TH receptor modulator KB3495 was investigated for its potential to lower serum cholesterol and treat atherosclerosis. In ApoE deficient mice KB3495, alone or in combination with atorvastatin, reduced atherosclerosis and lowered the cholesterol content in aorta, liver and skin. A reduced cholesterol synthesis, increased bile acid formation and induced fecal excretion of bile acids and neutral sterols was observed. Thus, the combined treatment of KB3495 and statins could provide a possible strategy to treat atherosclerosis.

List of scientific papers

I. Abrahamsson A, Gustafsson U, Ellis E, Nilsson LM, Sahlin S, Björkhem I, Einarsson C (2005). "Feedback regulation of bile acid synthesis in human liver: importance of HNF-4alpha for regulation of CYP7A1." Biochem Biophys Res Commun 330(2): 395-9
https://pubmed.ncbi.nlm.nih.gov/15796896

II. Nilsson LM, Abrahamsson A, Sahlin S, Gustafsson U, Angelin B, Parini P, Einarsson C (2007). "Bile acids and lipoprotein metabolism: effects of cholestyramine and chenodeoxycholic acid on human hepatic mRNA expression." Biochem Biophys Res Commun 357(3): 707-11. Epub 2007 Apr 9
https://pubmed.ncbi.nlm.nih.gov/17448444

III. Nilsson LM, Sjövall J, Strom S, Bodin K, Nowak G, Einarsson C, Ellis E (2007). "Ethanol stimulates bile acid formation in primary human hepatocytes." Biochem Biophys Res Commun 364(4): 743-7. Epub 2007 Oct 15
https://pubmed.ncbi.nlm.nih.gov/17976534

IV. Nilsson LM, Rehnmark S, Davoodpour P, Larsson L, Malm J, Parini P (2010). "The Thyroid Receptor agonist beta modulator KB3495 reduces atherosclerosis independently of ApoB-cholesterol in ApoE depleted mice." (Manuscript)