NPC1L1 and cholesterol transport

Abstract

The polytopic transmembrane protein, Niemann-Pick C1-Like 1 (NPC1L1), is enriched in the apical membrane of small intestine absorptive enterocytes where it mediates extracellular sterol transport across the brush border membrane. It is essential for intestinal sterol absorption and is the molecular target of ezetimibe, a potent cholesterol absorption inhibitor that lowers blood cholesterol in humans. NPC1L1 is also highly expressed in human liver. The hepatic function of NPC1L1 may be to limit excessive biliary cholesterol loss. NPC1L1-dependent sterol uptake seems to be a clathrin-mediated endocytic process and is regulated by cellular cholesterol content. Recently, NPC1L1 inhibition has been shown to have beneficial effects on components of the metabolic syndrome, such as obesity, insulin resistance, and fatty liver, in addition to atherosclerosis.

Figure 1. Amino acid sequence and predicted topological structure of human Niemann–Pick C1-like 1 (NPC1L1)

The conserved N-terminal “NPC1” domain is shown with red residues. Two potential YXXØ endocytic motifs are outlined in blue squares. Residues in dark circles denote the sterol-sensing domain. The luminal portion of NPC1L1 also has extensive N-linked glycosylation sites which are highlighted in green. The N-terminal 21 amino acids are assumed to be the signal peptide and are not shown in this figure.

Figure 2. Proposed model of NPC1L1-dependent cholesterol uptake

NPC1L1 is enriched at the plasma membrane when cellular cholesterol levels are low. Extracellular cholesterol is recruited to the NPC1L1-containing plasma membrane microdomain by binding to NPC1L1 or other mechanisms. When cholesterol content increases to a threshold in the microdomain, it is sensed by NPC1L1's SSD, which then triggers NPC1L1 protein conformational changes and subsequent internalization of the NPC1L1/cholesterol-containing membrane microdomain via clathrin-mediated endocytosis. Two potential YXXØ motifs known to facilitate the clathrin-mediated endocytosis of many plasma membrane proteins via interaction with the μ subunit of AP2 are highlighted in blue squares. NPC1L1 and its sterol cargo are dissociated in the sorting endosome and/or the endocytic recycling compartment, freeing NPC1L1 to be recycled back to the plasma membrane to take up additional cholesterol, particularly during cholesterol deprivation. Ezetimibe interacts with the second extracellular loop of NPC1L1, causing conformational changes of NPC1L1 protein, thereby inhibiting cholesterol uptake.

Figure 3. Role of NPC1L1 and ABCG5/G8 in enterohepatic cholesterol recirculation

Biliary and dietary sterols are mixed in the gut lumen and solubulized by bile acids (BA) and phospholipids (PL) to form mixed micelles. NPC1L1 absorbs sterols, including free cholesterol (FC), from these mixed micelles at the apical surface of the enterocyte. Sterols in enterocytes can be pumped out to the gut lumen by the action of the heterodimeric ATP binding cassette transporters G5 and G8 (ABCG5/G8), or can be esterified by acyl-coenzyme A: cholesterol acyltransferase 2 (ACAT2) to yield cholesteryl ester (CE) for assembly into chylomicrons for secretion into lymph. In the hepatocyte, ABCG5/G8 at the canalicular membrane pumps free cholesterol into the bile canaliculus. This action is opposed by NPC1L1, which reabsorbs the FC back into the hepatocyte. Thus, in both the enterocyte and hepatocyte, sterol efflux by ABCG5/G8 is opposed by NPC1L1-dependent sterol influx.