Studies on the regulation of hepatic and intestinal lipid metabolism

Abstract: Disturbances in the lipid homeostasis can resu lt in a wide variety of different diseases and symptoms. This thesis is focused on the regulation of chol esterol metabolism and its relation to atherosclerosis development, and the effect of mitochondrial (mt)DNA depletion on hepatic ? -oxidation of fatty acids. ApoE deficient mice spontaneously develop atherosclerosis due to accumulation of chylomicron remnants in th e circulation, a process highly dependent on intestinal cholesterol absorption. The lack of functional ApoE prevents receptor mediated uptake of lipoproteins, leading to ve ry high levels of circulating cholesterol. We used two approaches to test the in fluence of plasma cholesterol levels on atherosclerosis development in ApoE defi cient mice. In paper I we disrupted the Cyp8b1 gene to disable the endogenous synthesis of cholic acid and thereby reduce the intestinal absorption of cholesterol. C yp8b1/ApoE double knoc kout mice displayed reduced atherosclerotic lesions in aorta, reduced cholesterol levels in plasma and liver and reduced intestinal cholesterol absorp tion compared to ApoE knockout mice. In paper II we treated ApoE deficient mice with the thyroid receptor ? modulator GC-1. Supplementation with thyroid hormone re duces plasma cholesterol in humans and rodents; this effect is mediated by the thyroid receptor isoform ? (TR ? ). We found that oral administration of GC-1 reduced atherosc lerosis after 20 weeks of treatment. This reduction was accompanied by in creased hepatic LDL-receptor expression, bile acid synthesis and fecal bile acid excretion, but serum cholesterol leve ls were not reduced. The onset and development of atherosclerosis was likely delayed by short-term (one to ten weeks) treatment effects, such as incr eased fecal excretion of neutral sterols, reduced intestinal cholesterol absorption and reduced plasma cholesterol levels, which were not significant after long-term (20 weeks) treatment. Increased HDL-cholesterol and reduced intestinal cholesterol absorption may lead to improved anti-atherosclerotic effects on cholesterol metabolism, but the LXR-induced increase in HDL-cholesterol may depend on the intestinal cholesterol absorption. In paper III we investigated this by oral administration of the LXR agonist GW3965 and the cholesterol absorption inhi bitor ezetimibe. The combined treatment effectively reduced the intestinal choles terol absorption and increased serum HDL- cholesterol and ApoAI. Neither intestin al nor hepatic ABCA1 protein expression increased, but hepatic ApoaI expression was induced. Th e LXR-driven increase in HDL-cholesterol and ApoAI was thus indepe ndent of both intestinal cholesterol absorption and increased intestinal or hepa tic ABCA1 protein. The combined treatment effectively increased fecal excretion of cholesterol. Thymidine kinase (TK) 2 deficien cy leads to mtDNA depletion. TK2 deficient mice have been developed and disp lay symptoms of defect fat metabolism. In paper IV we studied the lipid phenotype of TK2 deficient mice in search for treatment options. TK2 deficient mice displaye d elevated plasma levels of cholesterol, free fatty acids and long-chain acylcarnitine s. In mice with hepatic mtDNA levels below 20%, the blood sugar and the ketone levels dropped. These mice also exhibited reduced hepatic mitochondrial ? -oxidation and CPT activity, indicating reduced capacity to metabolize long-chain acylcarnit ines. Hepatic ATP synthesis rates were unaltered in 14 days old TK2 defici ent mice compared to controls.