Evidence for multiple complementary pathways for efficient cholesterol absorption in mice

Abstract

Apolipoprotein B (apoB)-dependent and apoBindependent pathways for cholesterol transport have been described in cultured cells. Here, we show that the apoBindependent pathway involves apoA-I-containing high density lipoproteins (HDLs). Cholesterol secretion by the HDLs, but not by the apoB pathway, was significantly reduced in primary enterocytes isolated from chow-and cholesterolfed apoA-I ؊ / ؊ mice. These enterocytes were capable of cholesterol efflux when apoA-I was provided extracellularly. In apoA-I ؊ / ؊ mice, the absorption of a bolus of cholesterol was similar in control and apoA-I ؊ / ؊ mice fed chow or highcholesterol diet. However, short-term studies revealed that cholesterol absorption was occurring over longer lengths of the intestine, and cholesterol but not triglyceride transport to the plasma and liver in chow-and cholesterol-fed apoA-I ؊ / ؊ mice was significantly reduced. These studies indicate that in apoA-I deficiency, there is a delay in cholesterol absorption, but cholesterol is eventually absorbed because of the compensatory apoB pathway. Nonetheless, long-term studies involving multiple feedings showed significant reduction in cholesterol absorption after 4 days. We propose that multiple compensatory mechanisms ensure efficient cholesterol absorption in mice. Cholesterol absorption is defined as the transport of cholesterol from intestinal lumen to plasma (1, 2). There are three major steps in this process. The first step occurs in the lumen of the intestine and involves emulsification and hydrolysis of dietary fat and cholesteryl esters by bile acids and pancreatic enzymes. Genetic ablation of cholesteryl ester hydrolase (3) and pancreatic triglyceride lipase (4) has been shown to decrease cholesterol absorption, emphasizing their role in the hydrolysis of dietary fat in the intestinal lumen. The second step is the uptake of free cholesterol by enterocytes. Several candidate proteins that can facilitate cholesterol uptake by enterocytes have been identified. Current emerging evidence suggests that Niemann-Pick C1-like 1 (NPC1L1) might play a key role in this process. NPC1L1 gene deletion reduces cholesterol absorption in mice (5, 6). Furthermore, ezetimibe, a cholesterol absorption inhibitor, was shown to be ineffective in the absence of NPC1L1. The third step is the assembly of cholesterol and cholesteryl esters with lipoproteins in the enterocytes. The importance of apolipoprotein B (apoB) lipoproteins, mainly chylomicrons, in this process has been appreciated for a long time (7). Recently, we have provided evidence for multiple, independently regulated pathways for cholesterol transport across the intestinal epithelial cells (8). These pathways were broadly classified into apoB-dependent and apoB-independent pathways. However, it is not known whether such mechanisms play a role in cholesterol absorption in vivo. To test the hypothesis that multiple pathways contribute to efficient cholesterol absorption in vivo, we used apoA-I knockout mice as a model system.ApoA-I is the ma...

RETRACTED: Crystal structure of human apolipoprotein A-I: Insights into its protective effect against cardiovascular diseases

“…ApoA-I interacts with ABCA1 and accepts FC in the first step, acts as a cofactor for LCAT in the second, and mediates interaction of HDL with SR-B1 in the third (6)(7)(8). Additionally, HDL and apoA-I play a significant role in cholesterol absorption (9), modulate endothelial function, inhibit platelet-activating factor synthesis, stimulate prostacyclin synthesis, bind to bacterial lipopolysaccharide, and inhibit low-density lipoproteininduced monocyte chemotaxis (10).…”

Small Intestine but Not Liver Lysophosphatidylcholine Acyltransferase 3 (Lpcat3) Deficiency Has a Dominant Effect on Plasma Lipid Metabolism

“…Based on studies of the small intestine in neonatal mice, two groups of researchers proposed models for the assembly of Lpcat3-related apoB-containing lipoproteins (9,10). However, cholesterol secretion in cultured primary enterocytes is mediated by both apoB-dependent and -independent pathways (14). In this study, we found that Lpcat3 deficiency in primary enterocytes reduced not only cholesterol uptake (Fig.…”

“…Primary Enterocyte Preparation and Plasma Membrane Isolation-Primary enterocytes were isolated according to two previous reports (14,16). The plasma membrane was isolated from primary enterocytes according to our protocol (17).…”

“…Cholesterol absorption studies were performed with male mice using the conventional fecal dual-isotope ratio method (14). Absorption of triglycerides and phospholipids was measured with male mice using our reported method (15).…”

“…Enterocytes secrete cholesterol as a part of apoB lipoproteins into lymph and as a part of HDL into the circulation (14,19). The latter is influenced by the ABCA1 level (19).…”