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

Abstract

This article is available online at http://www.jlr.org Beginning in 1974, the identifi cation of sitosterolemia ( 1 ), a rare recessive disorder characterized by elevated plasma and tissue concentrations of phytosterols, has focused attention on the basic molecular processes that govern how the body normally absorbs animal-derived dietary cholesterol while excluding all other similarly structured plantderived sterols, generally called phytosterols. Phytosterols, including campesterol, stigmasterol, and sitosterol, differ from cholesterol mainly in possessing one or two additional carbons in side chains at C24. The average North American diet contains about equal amounts of cholesterol and phytosterols ( ‫ف‬ 150-400 mg/day) ( 2-5 ), yet <5% of phytosterols are absorbed compared with ‫ف‬ 50% of cholesterol ( 4, 6 ). Thus in healthy individuals, sensitive mechanisms exist that allow the body to distinguish among modestly different sterol structures.In general, different species of sterols have different absorption effi ciencies; the closer the structural similarity to the cholesterol molecule, the higher the percentage absorption ( 4, 7 ). Even mildly hypercholesterolemic patients have serum concentrations of phytosterols that are 500 (campesterol) to 20,000 (sitosterol) times lower than that of cholesterol. Patients with sitosterolemia have increased fractional sterol absorption rates and impaired biliary secretion of neutral sterols, resulting in accumulations of these sterols in the blood and tissues ( 1, 8 ), premature atherosclerosis, and tendon/skin xanthomatosis ( 8, 9 ). Sitosterolemia has been linked to mutations in either adenosine triphosphate-binding cassette transporter G5 or G8 Abstract The metabolic fate of newly absorbed cholesterol and phytosterol is orchestrated through adenosine triphosphate-binding cassette transporter G5 and G8 heterodimer (G5G8), and acyl CoA:cholesterol acyltransferase 2 (ACAT2). We hypothesized that intestinal G5G8 limits sterol absorption by reducing substrate availability for ACAT2 esterifi cation and have attempted to defi ne the roles of these two factors using gene deletion studies in mice. Male ACAT2 ؊ / ؊ mice, whereas it was up to 6.8% in G5G8؊ / ؊ and DKO mice. G5G8 ؊ / ؊ mice also produced chylomicrons with ‫ف‬ 70% less cholesterol ester mass than WT mice. In contrast to expectations, the data demonstrated that the absence of G5G8 led to decreased intestinal cholesterol esterifi cation and reduced cholesterol transport efficiency. Intestinal G5G8 appeared to limit the absorption of phytosterols; ACAT2 more effi ciently esterifi ed cholesterol than phytosterols. The data indicate that handling of sterols by the intestine involves both G5G8 and ACAT2 but that an additional factor (possibly Niemann-Pick C1-like 1) may be key in determining absorption effi ciency. Abbreviations: ATF-6, activating transcription factor 6; CE, cholesterol ester; DDIT-3, DNA damage-inducible transcript 3; ER, endoplasmic reticulum; FC, free cholesterol; FP, free phytosterol; G5G8, adenosin...

ABCG5/G8 Deficiency in Mice Reduces Dietary Triacylglycerol and Cholesterol Transport into the Lymph

“…Because G5/G8 is presumed to efflux sterols into the intestinal lumen, its deficiency would expectedly accumulate cholesterol in the mucosa. Also, since G5/G8 is expressed differentially along the length of the intestine [42], which is thought to contribute to differential degrees of absorption [20,21,43], we speculated that G5/G8 deficiency would accumulate lipids differently in the various intestinal segments. However, we found that G5/G8 deficiency did not affect the mucosal accumulation of cholesterol, except slightly in the duodenum (Fig.…”

“…In regards to sterol absorption, G5/G8 is well-established to limit the absorption of plant sterols [6,11,12,[19][20][21]. However, the extent to which G5/G8 limits cholesterol absorption is unclear.…”

Differing rates of cholesterol absorption among inbred mouse strains yield differing levels of HDL-cholesterol

“…Inside the enterocytes, the cholesterol is reesterifi ed by ACAT2, packaged with triglyceride and apoB-48 into chylomicrons by microsomal triglyceride transfer protein (MTTP), and then secreted into the circulation ( 22 ). Additionally, the ATP binding cassette transporter proteins ABCG5 and ABCG8 have been shown to be involved in resecretion of the enterocyte cholesterol back into the lumen ( 23,24 ). Other factors, including the rate of gastric emptying, have been shown to alter the rate of cholesterol absorption between inbred mouse strains ( 25 ).…”

Intestinal nuclear receptors in HDL cholesterol metabolism

“…Along the apical membrane of enterocytes, the heterodimer ABCG5/G8 counteracts the NPC1L1-mediated sterol uptake (29)(30)(31)(32) by promoting FC effl ux back into the intestinal lumen, with the net result of cholesterol absorption inhibition. Thoracic lymph duct cannulation studies have provided clear evidence that effi cient cholesterol absorption requires both ACAT2 and ABCG5/G8 heterodimer, with the latter being responsible for the apical effl ux of newly absorbed FC into the gut lumen, thus limiting the substrate availability for the esterification reaction ( 27 ).…”

“…FC is soluble in membrane phospholipids and represents an important structural component of cellular membranes; by contrast, CE is mostly insoluble in membranes and must be packaged into lipid droplets or incorporated into the core of lipoprotein particles for export out of the cell. Studies in ACAT2 knockout mice have reported that the lack of CE formation results in reduced cholesterol absorption ( 24,25 ), and that CE formation by ACAT2 selectively utilizes chylomicron particles for quantitative transport of newly absorbed cholesterol out of the enterocyte into the lymphatic system, and subsequently into the body ( 26,27 ). Along the basolateral membrane of the enterocyte, CE gets to be assembled into the core of chylomicron particles and be secreted into the lymph in a microsomal transfer protein (MTP)-dependent manner.…”