NPC L expression is both tissue and species
NPC1L1 expression is both tissue- and species-specific. Consistent with its function, NPC1L1 is mainly localized to the brush border membrane of enterocytes and the canalicular membrane of hepatocytes. NPC1L1, which is expressed in the liver of humans and rats but not mice, mediates cholesterol NIBR189 through clathrin/adaptin 2 (AP2)-dependent vesicular endocytosis (Fig. 2) . NPC1L1 contains a YVNXXF (X represents any amino acid) internalization motif located in the C-terminal cytoplasmic tail . As a specific recognition site for the clathrin adaptor Numb, the YVNXXF sequence is highly conserved among mammalian NPC1L1 proteins but absent in its homolog NPC1. Binding of free cholesterol (FC) to the sterol binding pocket causes dissociation of the NPC1L1 C-terminal cytoplasmic tail from the plasma membrane. The YVNXXF internalization motif is exposed and then recognized by Numb. Subsequently, the clathrin/AP2 complex is recruited to Numb, thereby triggering vesicular endocytosis . The endocytic vesicles are transported to the ERC where FC is released and NPC1L1 recycles to the plasma membrane. Myosin Vb is an important motor protein that is essential for vesicular trafficking in various mammalian tissues . Rab11a and Rab11 family-interacting protein 2 (Rab11-FIP2) promote the binding of myosin Vb to the cargo proteins of endocytic recycling vesicles . The myosin Vb/Rab11a/Rab11-FIP2 triple complex was shown to mediate NPC1L1 transport from the ERC to plasma membrane . LIM domain and actin binding 1 (LIMA1) is also known as sterol regulatory element-binding protein (SREBP)-3 but shows little sequence similarity to SREBP-1 or -2. A recent study revealed that LIMA1 serves as a scaffold protein linking myosin Vb to NPC1L1 and thereby promotes NPC1L1 translocation to the cell surface . A rare frameshift variant (K306fs) in the LIMA1 gene that was identified in a Chinese family of Kazakh ethnicity leads to a significant decrease in intestinal cholesterol absorption and circulating LDL cholesterol (LDL-C) levels . Similarly, mice with intestine-specific LIMA1 deficiency display lower cholesterol absorption and are resistant to diet-induced hypercholesterolemia . After entering into enterocytes, the vast majority of FC is converted to cholesterol esters (CEs) by acyl CoA:cholesterol acyltransferase (ACAT)-2 in the endoplasmic reticulum (ER). CEs are then incorporated into chylomicrons together with triglyceride (TG), FC and apoB48, which is then secreted into the lymphatic system for distribution throughout the body. Accumulating data show that NPC1L1 exerts a proatherogenic effect. For example, in low-density lipoprotein (LDL) receptor (Ldlr) mice with gastrointestinal tract-specific overexpression of NPC1L1, cholesterol feeding markedly promotes intestinal cholesterol absorption and causes hypercholesterolemia, an independent risk factor for CVD . Moreover, elevated plasma cholesterol is restricted to VLDL and LDL fractions, which are related to increased secretion of apoB100 and apoB48 . These findings suggest that NPC1L1-mediated intestinal cholesterol absorption may function as a primary determinant of apoB-containing atherogenic lipoproteins. Conversely, mice lacking NPC1L1 exhibit a 70% reduction in cholesterol absorption and are resistant to hypercholesterolemia and atherosclerosis [29,30]. Genetic variations have been frequently reported in the NPC1L1 gene [, , ]. A recent study by Muendlein et al. revealed that genetic variants of NPC1L1, especially rs55837134, showed a predictive impact on cardiovascular events in a cohort of 984 Caucasian patients with coronary artery disease (CAD) . In addition, administration of Lactobacillus acidophilus ATCC 4356 reduces plasma VLDL and LDL levels and protects against atherosclerosis in apoE–/– mice by inhibiting NPC1L1-mediated absorption of intestinal cholesterol . NPC1L1 expression is tightly regulated by several transcriptional factors. SREBP-2 is a membrane-bound transcriptional factor that is one of the master regulators of cholesterol homeostasis . SREBP-2 stimulates NPC1L1 transcription by binding to two sterol regulatory elements in its promoter region in human intestinal Caco-2 cells and HuH7 hepatoma cells [37,38]. Bisphenol A, an environmental chemical that humans are commonly exposed to, markedly increases NPC1L1 expression in Caco-2 cells through, at least in part, the SREBP-2 pathway . Interestingly, overexpression of SREBP-2 and hepatocyte nuclear factor (HNF) 4α synergistically induces NPC1L1 transcription in HepG2 cells and Caco-2 cells, although HNF4α alone has no this effect . However, HNF1α, a downstream effector of HNF4α, was found to directly transactivate NPC1L1 gene in HuH7 cells . The nuclear receptor liver receptor homolog-1 (LRH-1) has a binding site in the NPC1L1 promoter and can lead to synergistic transcriptional activation of NPC1L1 when coexpressed with SREBP2 in HepG2 cells . On the other hand, cyclic AMP-responsive element-binding protein H (CREBH) that is a basic leucine zipper domain transcriptional factor and highly expressed in the hepatic and intestinal tissues  may act as a transcriptional repressor of NPC1L1 .