Studies on cholesterol and bile acid metabolism in relation to plasma lipoproteins

Abstract

The metabolism of cholesterol and bile acids is tightly controlled but only partially characterized. The liver is responsible for most of the clearance and catabolism of plasma cholesterol, and the hepatocyte expression of LDL receptors is central in this process. The major pathways for net excretion of cholesterol from the body are through biliary excretion as free cholesterol or after conversion to bile acids. Through activation of the nuclear receptor FXR and the G protein-coupled receptor TGR5, bile acids regulate not only their own homeostasis but also lipid, glucose and energy metabolism. The overall aim of this thesis was to further characterize the regulation of cholesterol and bile acid metabolism in relation to plasma lipoproteins.

Bile acid synthesis is regulated by negative feedback mechanisms. Bile acids mediate inhibition of the rate limiting enzyme in bile acid synthesis, CYP7A1, through activation of FXR in the liver and in the intestine via the FXR-SHP pathway and the FXR-FGF19-FGFR pathway, respectively. In paper I, we demonstrate that circulating FGF19 is markedly influenced by transintestinal flux of bile acids whereas its proposed role in the suppression of bile acid synthesis and triglyceride levels may not always apply.

PCSK9 regulates the number of LDL receptors by targeting the LDL receptors for degradation. In paper II, we show that circulating PCSK9 has a diurnal rhythm synchronous with cholesterol synthesis. We also describe how fasting strongly reduces circulating PCSK9 and reduces cholesterol synthesis, whereas plasma LDL-cholesterol is unchanged in healthy subjects. Furthermore we show that a ketogenic diet induces cholesterol synthesis and increase plasma cholesterol whereas circulating PCSK9 is unaltered.

Diet composition and eating patterns influence cholesterol metabolism. Knowledge of underlying mechanisms provides the ability to modulate regulation and homeostasis in a desired direction. In paper III, we show that a vegan diet reduces serum total cholesterol and LDL-cholesterol. In paper IV, we further explore possible mechanism(s) implicated in this dietary effect, but cannot find evidence for any involvement of PCSK9 or altered bile acid or cholesterol synthesis. In summary, the work presented in this thesis contributes to the understanding of lipid metabolism, and may in a longer term assist in the development of better treatment for lipid disorders.