Genetic inactivation of NPC1L1 protects against sitosterolemia in mice lacking ABCG5/ABCG8

Abstract

Mice lacking Niemann-Pick C1-Like 1 (NPC1L1) (NPC1L1 2/2 mice) exhibit a defect in intestinal absorption of cholesterol and phytosterols. However, wild-type (WT) mice do not efficiently absorb and accumulate phytosterols either. Cell-based studies show that NPC1L1 is a much weaker transporter for phytosterols than cholesterol. In this study, we examined the role of NPC1L1 in phytosterol and cholesterol trafficking in mice lacking ATP-binding cassette (ABC) transporters G5 and G8 (G5/G8 2/2 mice). G5/G82/2 mice develop sitosterolemia, a genetic disorder characterized by the accumulation of phytosterols in blood and tissues. We found that mice lacking ABCG5/G8 and NPC1L1 [triple knockout (TKO) mice] did not accumulate phytosterols in plasma and the liver. TKO mice, like G5/G8 2/2 mice, still had a defect in hepatobiliary cholesterol secretion, which was consistent with TKO versus NPC1L1 2/2 mice exhibiting a 52% reduction in fecal cholesterol excretion. Because fractional cholesterol absorption was reduced similarly in NPC1L1 2/2 and TKO mice, by subtracting fecal cholesterol excretion in TKO mice from NPC1L1 2/2 mice, we estimated that a 25g NPC1L1 2/2 mouse may secrete about 4 mmol of cholesterol daily via the G5/G8 pathway. In conclusion, NPC1L1 is essential for phytosterols to enter the body in mice. The two ATP-binding cassette (ABC) half-transporters, G5 and G8, reside at the canalicular membrane of hepatocytes and the apical membrane of absorptive enterocytes where they function as heterodimers to transport cholesterol and noncholesterol sterols into the bile canaliculus and the gut lumen for fecal excretion (1-4). Mutations in either ABCG5 (G5) or ABCG8 (G8) cause sitosterolemia, a rare autosomal recessive disorder (5, 6). The hallmark of sitosterolemia is the accumulation of sitosterol and campesterol, two major plant-derived sterols, in the blood and tissues (7-10). In sitosterolemic patients and mice, the plasma cholesterol concentrations follow fluctuations in dietary cholesterol intake (2,11,12). Many sitosterolemic individuals develop xanthomas and premature coronary heart disease as a result of sterol deposition in the skin, tendons, and coronary arteries (13-15).Ezetimibe, a cholesterol absorption inhibitor (16), has been recently shown to effectively reduce the plasma phytosterol levels in sitosterolemic patients and mice (17,18). It can also completely reverse xanthomatosis when used in combination with low-dose cholestyramine therapy in a sitosterolemic patient (19). These findings suggest two possibilities: 1) there is a common pathway for intestinal absorption of cholesterol and phytosterols; or 2) ezetimibe simultaneously targets two independent transporters: one for cholesterol and the other for phytosterols.In search of the molecular target of ezetimibe, Altmann et al. (20) identified a polytopic transmembrane protein named Niemann-Pick C1-Like 1 (NPC1L1), which localizes at the brush border membrane of the small intestine and mediates intestinal absorption of cholesterol. Disrupti...

ACAT2 and ABCG5/G8 are both required for efficient cholesterol absorption in mice: evidence from thoracic lymph duct cannulation

“…Another protein that is expressed exclusively in the same two cell types is acyl CoA:cholesterol acyltransferase type 2 (ACAT2), a cholesterol-esterifying enzyme residing in the endoplasmic reticulum (ER) membrane ( 12 ). Mice lacking G5G8 have phenotypes resembling patients with sitosterolemia, including increased fractional absorption of noncholesterol sterols, marked accumulation of sitosterol and campesterol in the blood and liver, reduced levels of biliary cholesterol, and various hemolytic disorders ( 7,13 ). These fi ndings suggested that the physiological role of G5G8 is to limit phytosterol accumulation in the body by limiting its absorption in the intestine and by promoting its secretion from the liver.…”

“…These fi ndings suggested that the physiological role of G5G8 is to limit phytosterol accumulation in the body by limiting its absorption in the intestine and by promoting its secretion from the liver. Because sitosterolemic patients and G5G8 knockout (G5G8 Ϫ / Ϫ ) mice can still maintain the same rank order of absorption effi ciency (cholesterol > campesterol > sitosterol), it has been suggested that other proteins independent of G5G8 are responsible for the selectivity of cholesterol over phytosterol absorption ( 13,14 ).…”

Niemann-Pick C1-Like 1 deletion in mice prevents high-fat diet-induced fatty liver by reducing lipogenesis

“…After 18 weeks on an HFD, mice were used for the determination of fecal neutral sterol excretion and intestinal cholesterol absorption as we have described previously ( 28 ).…”

Potential Effects of NPC1L1 Polymorphisms in Protecting against Clinical Disease in a Chinese Family with Sitosterolaemia

“…For example, the dietary sterol intake may influence the plasma sterol levels, although the two patients lived in the same household and reported consuming a similar diet. Animal studies shown that the genetic inactivation of NPC1L1 protects against sitosterolaemia in mice lacking Abcg5/Abcg8 15) . In humans, rare variants in NPC1L1 have been reported to be associated with reduced sterol absorption and plasma LDL-C levels 16) .…”