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, Niemann-Pick 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 down-regulation of NPC2.
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Article: Bile formation and secretion.[Show abstract] [Hide abstract]
ABSTRACT: Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions. © 2013 American Physiological Society. Compr Physiol 3:1035-1078, 2013.Comprehensive Physiology. 07/2013; 3(3):1035-78.
- GEN. 01/2013; 67(1):49-57.
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ABSTRACT: The formation of cholesterol gallstones is a very complex and polygenic disorder that involves an alteration of the secretion of bile lipids, cholesterol crystallization, important immunological reactions in the gallbladder tissue, formation of biliary sludge composed of mucin, and inadequate gallbladder motility. The search for a therapeutic target is oriented towards decreasing bile secretion and intestinal absorption of cholesterol, in which Niemann-Pick C1L1 (NPC1L1) proteins play an important role. In basic and clinical studies, regulating the expression of these proteins can reduce intestinal, liver, plasma and bile cholesterol levels, a therapeutic effect that would be useful not only for treating the disease, but to prevent it, given the large quantity of risk factors. We discuss these effects in this review and propose NPC1L1 proteins as future therapeutic targets of cholesterol gallstones disease.European journal of pharmacology 01/2014; · 2.59 Impact Factor