Niemann-pick C1-like 1 (NPC1L1) protein in intestinal and hepatic cholesterol transport

Abstract

Increased blood cholesterol is an independent risk factor for atherosclerotic cardiovascular disease. Cholesterol homeostasis in the body is controlled mainly by endogenous synthesis, intestinal absorption, and hepatic excretion. Niemann-Pick C1-Like 1 (NPC1L1) is a polytopic transmembrane protein localized at the apical membrane of enterocytes and the canalicular membrane of hepatocytes. It functions as a sterol transporter to mediate intestinal cholesterol absorption and counter-balances hepatobiliary cholesterol excretion. NPC1L1 is the molecular target of ezetimibe, a potent cholesterol absorption inhibitor that is widely used in treating hypercholesterolemia. Recent findings suggest that NPC1L1 deficiency or ezetimibe treatment also prevents diet-induced hepatic steatosis and obesity in addition to reducing blood cholesterol. Future studies should focus on molecular mechanisms underlying NPC1L1-dependent cholesterol transport and elucidation of how a cholesterol transporter modulates the pathogenesis of metabolic diseases.

Figure 1

The role of Niemann-Pick C1-Like 1 (NPC1L1) in cholesterol transport in the small intestine and liver. In the lumen of the small intestine, unesterified free cholesterol (FC) from dietary intake and biliary secretion is solubilized in mixed micelles containing bile acids (BA) and phospholipids (PL). This solubilization is critical for the diffusion of FC across the unstirred water layer to reach intestinal brush border membranes, where FC is taken up into enterocytes by the apically localized NPC1L1 protein. Ezetimibe (Zetia) can inhibit this NPC1L1-dependent cholesterol uptake. The majority of absorbed and endogenously synthesized cholesterol is transported to the endoplasmic reticulum, where it is converted to cholesterol ester (CE) by acyl-CoA:cholesterol acyltransferase 2 (ACAT2) and is then assembled into chylomicrons in a microsomal triglyceride transfer protein (MTP)-dependent manner for secretion into the circulation via the lymphatic system. Unesterified FC may be transported back to the intestinal lumen by the apically localized heterodimeric sterol transporter ABCG5/ABCG8 [ATP-binding cassette (ABC) transporters G5 and G8]. FC may also be transported into the circulation as a constituent of high-density lipoprotein (HDL) via ABCA1 located at the basolateral membrane of enterocytes. In the liver, cholesterol can be synthesized locally or taken up by hepatocytes from circulating lipoproteins such as low-density lipoprotein (LDL), HDL, and chylomicron remnants via LDL receptor (LDLR), HDL receptor scavenger receptor class B type I (SR-BI), and LDLR-related protein (LRP) localized at the basolateral membrane of hepatocytes. A large amount of FC is converted to bile acids for hepatobiliary secretion. A proportion of FC is esterified by ACAT2 and then packaged into nascent very low density lipoprotein (VLDL) particles in an MTP-dependent manner for secretion into the circulation as a constituent of VLDL. Unmetabolized FC can be transported to ABCA1 localized at the sinusoidal membrane of hepatocytes for the biogenesis of HDL or to ABCG5/ABCG8 localized at the canalicular membrane of hepatocytes for direct secretion into bile. In humans and nonhuman primates, NPC1L1 is also localized at the apical membrane of hepatocytes, where it may counterbalance the function of ABCG5/ABCG8 by transporting newly secreted biliary cholesterol back into hepatocytes, thereby preventing excessive loss of endogenous cholesterol.

Figure 2

The proposed cellular basis for Niemann-Pick C1-Like 1 (NPC1L1)-dependent cholesterol uptake. The polytopic transmembrane NPC1L1 protein is synthesized in the rough endoplasmic reticulum (ER) and then traffics to the plasma membrane via the Golgi apparatus. When cellular cholesterol is low, the protein preferentially localizes at the apical membrane of enterocytes and the canalicular membrane of hepatocytes, the two membrane domains that are exposed to high concentrations of unesterified free cholesterol (FC) in mixed micelles. On the basis of current knowledge about NPC1L1, we propose the following mechanism for NPC1L1-dependent cholesterol uptake. Through direct binding of FC to the extracellular N-terminal domain of NPC1L1, NPC1L1 may recruit FC to the membrane microdomain, where the sterol-sensing domain (SSD) of NPC1L1 is localized. When FC is enriched at this microdomain to a certain extent, it can be sensed by NPC1L1’s SSD, resulting in conformational changes of the protein, which exposes NPC1L1’s YXXØ motif and causes the direct binding of this motif to the μ2 subunit of adaptor protein complex 2 in the clathrin-mediated endocytic pathway and subsequent internalization of NPC1L1-containing and FC-rich membrane microdomains. The intracellular sorting of NPC1L1 in these internalized membrane microdomains may follow typical vesicular trafficking itineraries, moving from sorting endosome (SE) to late endosome (LE) to lysosome for degradation. A large proportion of NPC1L1 may be trafficked to the endocytic recycling compartment (ERC) and then recycled back to the cell surface for another round of sterol-dependent endocytosis. During this intracellular sorting, FC may be dissociated from SE or ERC and may be carried via vesicles or proteins to the ER for esterification and assembly into chylomicrons in the intestine and very low density lipoprotein (VLDL) particles in the liver.

Figure 3

Proposed mechanisms for Niemann-Pick C1-Like 1 (NPC1L1) deficiency or ezetimibe (Zetia) treatment to prevent nonalcoholic fatty liver disease (NAFLD). (a) NPC1L1 inhibition blocks intestinal absorption and promotes biliary excretion of free cholesterol (FC). Therefore, NPC1L1 may prevent hepatic steatosis by reducing hepatic cholesterol content and cholesterol-dependent activation of liver X receptor (LXR), a nuclear receptor that promotes hepatic lipogenesis. (b) Reduced lipid accumulation in the liver as a result of NPC1L1 inhibition may improve liver insulin signaling by reducing hepatic content of lipotoxic lipids, endoplasmic reticulum (ER) stress, and/or the production of proinflammatory cytokines and reactive oxygen species (ROS). Such events thereby inhibit simple steatosis progression to nonalcoholic steatohepatitis (NASH) and blunt diet-induced insulin resistance and the development of hyperinsulinemia, a condition that promotes hepatic lipogenesis via a mechanism dependent on a membrane-bound transcription factor, sterol regulatory element–binding protein (SREBP)-1c. (c) For unknown reasons, NPC1L1 inhibition prevents diet-induced obesity, which may further protect the liver from steatosis by improving whole-body insulin sensitivity. (d) NPC1L1 inhibition dramatically reduces blood insulin concentrations. It is unclear if this reduction is attributable solely to improved insulin sensitivity. NPC1L1 inhibition may somehow directly impair insulin secretion by pancreatic β cells, thereby preventing the liver from undergoing insulin-driven lipogenesis.