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

Abstract

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 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. 10Abbreviations: ABC, adenosine triphosphate-binding cassette (transporter); HDL, high-density lipoprotein; LXR, liver X receptor. From the

Evaluation of the Contribution of the ATP Binding Cassette Transporter, P-glycoprotein, to in Vivo Cholesterol Homeostasis

“…The dimer formed by the half transporters ABCG5 and ABCG8 is the primary efflux mechanism for unmodified cholesterol from cells of the small intestine and the liver. Deletion of ABCG5 and ABCG8 does not completely ablate cholesterol efflux suggesting that a secondary mechanism for the removal of cholesterol may exist in mice 3,4 , with confirmatory findings in humans 5 .…”

Biliary lipids and cholesterol gallstone disease

“…However, ABCG5/ABCG8 requires a functioning ABCB4, indicating the necessity for a hydrophobic "sink" for solubilizing the secreted cholesterol molecules. Gallstone formation is observed in ABCG5 or ABC/ G8 knockout mice challenged with a lithogenic diet (20), suggesting that an ABCG5/ABCG8-independent pathway for biliary cholesterol secretion exists.…”

Plant sterols and stanols: Their role in health and disease

“…Absence of ABCG5, ABCG8 or both results in an elevation of plant sterols in the tissues and blood and a failure to excrete sterols into bile [24][25][26][27][28][29][30][31] . Over expression of ABCG5 and ABCG8 in mice transgenic for the human genes led to a super saturation of cholesterol in bile and protected animals against atherosclerosis.…”