Novel function of Niemann-Pick C1-like 1 as a negative regulator of Niemann-Pick C2 protein

Abstract

The hepatic expression of Niemann-Pick C1-like 1 (NPC1L1), which is a key molecule in intestinal cholesterol absorption, is high in humans. In addition to NPC1L1, NiemannPick C2 (NPC2), a secretory cholesterol-binding protein involved in intracellular cholesterol trafficking and the stimulation of biliary cholesterol secretion, is also expressed in the liver. In this study, we examined the molecular interaction and functional association between NPC1L1 and NPC2. In vitro studies with adenovirus-based or plasmid-mediated gene transfer systems revealed that NPC1L1 negatively regulated the protein expression and secretion of NPC2 without affecting the level of NPC2 messenger RNA. Experiments with small interfering RNA against NPC1L1 confirmed the endogenous association of these proteins. In addition, endocytosed NPC2 could compensate for the reduction of NPC2 in NPC1L1-overexpressing cells, and this demonstrated that the posttranscriptional regulation of NPC2 was dependent on a novel ability of NPC1L1 to inhibit the maturation of NPC2 and accelerate the degradation of NPC2 during its maturation. Furthermore, to confirm the physiological relevance of NPC1L1-mediated regulation, we analyzed human liver specimens and found a negative correlation between the protein levels of hepatic NPC1L1 and hepatic NPC2. Conclusion: NPC1L1 down-regulates the expression and secretion of NPC2 by inhibiting its maturation and accelerating its degradation. NPC2 functions as a regulator of intracellular cholesterol trafficking and biliary cholesterol secretion; therefore, in addition to its role in cholesterol re-uptake from the bile by hepatocytes, hepatic NPC1L1 may control cholesterol homeostasis via the downregulation of NPC2. (HEPATOLOGY 2012;55:953-964)

Lecithin:Cholesterol Acyltransferase Deficiency Protects against Cholesterol-induced Hepatic Endoplasmic Reticulum Stress in Mice

“…In the chow-fed mice, the major changes are in the normalization of expressions of various lipid transporters, including intestinal NPC1L1, hepatic NPC1L1 protein and Npc1l1 mRNA, and Abcg8 mRNA, and partial resolution of Abcg5 mRNA. In addition, we also observed a significant increase in NPC2 protein that may be a result of a reduction in NPC1L1 protein (21). In turn, this increase may further augment the ABCG5/G8-mediated cholesterol efflux.…”

“…8, e and f). To further address the significance of the changes in hepatic NPC1L1 expressions, we measured the hepatic Niemann-Pick C2 protein levels as recent reports have suggested that the soluble NPC2 protein also participates in stimulating biliary cholesterol efflux in an ABCG5/G8-dependent manner and that its hepatic and biliary levels are negatively modulated by hepatic NPC1L1 (20,21). We observed a significant 70% reduction in the hepatic NPC2 level in the LdlrϪ/ϪxLcatϩ/ϩ mice, and this was also significantly reversed by 2.2-fold in the LdlrϪ/ϪxLcatϪ/Ϫ mice (Fig.…”

Identification of hepatic NPC1L1 as an NAFLD risk factor evidenced by ezetimibe‐mediated steatosis prevention and recovery

“…Immunoblot analyses were performed as described in our previous report with minor modifications. Briefly, liver lysate samples were separated by SDS‐PAGE and transferred to an Immobilon‐P PVDF membrane (Millipore Corp., Bedford, MA) by electroblotting at 15 V for 51 minutes.…”

“…Importantly, the human liver can secrete cholesterol into bile, but can also re‐uptake the biliary‐secreted cholesterol. The latter process is mediated by the Niemann‐Pick C1‐Like 1 (NPC1L1) protein, a cholesterol transporter that is expressed on the bile canalicular membrane in humans . Thus, we hypothesized that hepatic NPC1L1 could exacerbate NAFLD, including steatosis.…”

Bile Formation and Secretion

“…Cholesterol excretion in bile is counteracted by Nieman-Pick C1-like 1 (NPC1L1) protein on the canalicular membrane while NPC2, a cholesterol-binding protein secreted by the biliary system, is a positive regulator of biliary cholesterol secretion by stimulating ABCG5/G8- mediated cholesterol transport (594). Hepatic NPC1L1 may control cholesterol homeostasis via the downregulation of NPC2 (592). …”