Quantifying anomalous intestinal sterol uptake, lymphatic transport, and biliary secretion in Abcg8−/− mice

✍ Scribed by Helen H. Wang; Shailendra B. Patel; Martin C. Carey; David Q.-H. Wang

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 481 KB
• Volume: 45
• Category: Article
• ISSN: 0270-9139
• DOI: 10.1002/hep.21579

✦ Synopsis

Sitosterolemia is caused by mutations in either ABCG5 or ABCG8, but simultaneous mutations of these genes have never been observed. To explore whether ABCG8, the sterol efflux (hemi-)transporter, plays a major role in determining intestinal absorption efficiency and hepatic secretion rates of cholesterol and sitostanol, we performed direct measurements of the absorption and lymphatic transport of these sterols in mice with chronic biliary and lymphatic fistulae, as well as the transport rates of radiolabeled cholesterol and sitostanol from plasma high-density lipoprotein (HDL) into bile in male Abcg8 ؊/؊ and wild-type mice. We observed that the absorption and lymphatic transport rates of radiolabeled cholesterol and sitostanol were increased by Ϸ40% and Ϸ500%, respectively, in Abcg8 ؊/؊ mice in the setting of constant intraduodenal infusion of micellar taurocholate and lecithin. Both strains displayed identical intestinal Npc1l1 expression levels and small intestinal transit rates. After 45 minutes of intraduodenal infusion, acute intestinal uptake rates of trace [ 14 C]cholesterol and [ 3 H]sitostanol were essentially similar in both groups of mice with intact biliary secretion. Furthermore, in wild-type mice, mass transport rate of [ 3 H]sitostanol from plasma HDL into bile was significantly faster than that of [ 14 C]cholesterol; however, no [ 3 H]sitostanol and only traces of [ 14 C]cholesterol were detected in bile of Abcg8 ؊/؊ mice. Conclusion: Deletion of the Abcg8 gene alone significantly increases the mass of intestinal cholesterol and sitostanol absorption and reduces but does not eliminate hepatic secretion of cholesterol. Moreover, the mutation has no influence on acute uptake of cholesterol and sitostanol by the enterocyte nor small intestinal transit time. (HEPATOLOGY 2007;45:998-1006.)

S

itosterolemia is a rare autosomal, recessively inherited disorder that is characterized mainly by elevated plasma levels of plant sterols (phytosterols) and with normal or only moderately increased cholesterol levels. 1 Sitosterolemic patients display hyperabsorption of cholesterol and phytosterols and reduced secretion of these sterols into bile. 2,3 Patel et al. 4 were the first to map the sitosterolemia locus, STSL, to human chromosome 2p21, between D2S2294 and D2S2298. Using a positional cloning approach as well as microarray analysis of murine complementary DNAs from intestines and livers of mice treated with a liver X receptor (LXR) agonist, 2 groups 5,6 independently identified the adjacent genes, ABCG5 and ABCG8, encoding adenosine triphosphatebinding cassette (ABC) hemi-transporters that are mutated in sitosterolemia. Unlike other ABC transporters that encode proteins with 12-transmembrane domains, these 2 proteins function as a unique heterodimer forming a 12-transmembrane protein complex essential for sterol transport activity. 7 Heterodimerization of ABCG5 and ABCG8 is also necessary for their translocation from endoplasmic reticulum to the plasma membrane. 8 Based on these observations, as well as the similarity of the clinical and biochemical phenotypes, it has been proposed that both ABCG5 and ABCG8 function as obligate heterodimers in the transmembrane "flipping" of sterols. 9 A mouse model with disruption of both Abcg5 and Abcg8 results in a phenotype similar to that of human disease. 10 Abbreviations: ABC, adenosine triphosphate-binding cassette (transporter); HDL, high-density lipoprotein; LXR, liver X receptor.