HNF1α and SREBP2 are important regulators of NPC1L1 in human liver

Abstract

This article is available online at http://www.jlr.org target of the cholesterol-lowering drug ezetimibe ( 1, 2 ). Mice defi cient in NPC1L1 have ‫ف‬ 70% reduction in cholesterol absorption ( 1 ) and resistance to diet-induced hypercholesterolemia ( 3 ). NPC1L1 is widely expressed in many human tissues, with the highest expression in small intestine and in the liver ( 3, 4 ). In mice and rats, npc1l1 is predominantly expressed in the small intestine, whereas all others tissues showed expression levels <10% of the intestinal expression ( 1, 5 ). The exact function of NPC1L1 in the human liver is currently unknown. It was recently reported that NPC1L1 facilitates the uptake of free cholesterol from the culture medium in human ( 6 ) and rat ( 7 ) hepatoma cells. Previous reports also showed that NPC1L1 localizes to the canalicular membrane in hepatocytes ( 6,8 ). Transgenic mice overexpressing human NPC1L1 in the liver had dramatically decreased biliary cholesterol concentration, which was returned to normal with ezetimibe treatment ( 8 ). This suggests that hepatic NPC1L1 could be another target of ezetimibe in humans.Several genes involved in cholesterol synthesis and uptake are regulated by sterol regulatory element binding protein 2 (SREBP2). Activation of SREBP2 is dependent on the cholesterol status of the cell ( 9 ). When cellular cholesterol levels are low, SREBP2 is proteolytically cleaved to release the N-terminal portion to generate the mature form that can enter the nucleus and bind to sterol regulatory elements (SREs) or E-boxes in the promoter of various genes and affect gene expression ( 9, 10 ).Hepatic nuclear factors (HNFs) 1 and 4 are expressed in various organs, including the liver, intestine, and panAbstract Niemann-Pick C1-like 1 (NPC1L1), a key regulator of intestinal cholesterol absorption, is highly expressed in human liver. Here, we aimed to gain more insight into mechanisms participating in its hepatic regulation in humans. Correlation analysis in livers from Chinese patients with and without gallstone disease revealed strong positive correlations between NPC1L1 and sterol regulatory element binding protein 2 (SREBP2) ( r = 0.74, P < 0.05) and between NPC1L1 and hepatic nuclear factor ␣ (HNF4 ␣ ) ( r = 0.53, P < 0.05) mRNA expression. HNF4 ␣ is an upstream regulator of HNF1 ␣ ; thus, we also tested whether HNF1 ␣ participates in the regulation of NPC1L1. We showed a dose-dependent regulation by SREBP2 on the NPC1L1 promoter activity and mRNA expression in HuH7 cells. Chromatin immunoprecipitation assay confi rmed the binding of SREBP2 to the promoter in vivo. Surprisingly, HNF4 ␣ slightly decreased the NPC1L1 promoter activity but had no effect on its gene expression. By contrast, HNF1 ␣ increased the promoter activity and the gene expression, and an important HNF1 binding site was identifi ed within the human NPC1L1 promoter. ChIP assays confi rmed that HNF1 ␣ can bind to the NPC1L1 promoter in vivo. Niemann-Pick C1-like 1 (NPC1L1) is a key regulator of intestinal cholesterol absorption a...

Mechanisms and genetic determinants regulating sterol absorption, circulating LDL levels, and sterol elimination: implications for classification and disease risk

“…Moreover, they accumulated less cholesterol in both their bile (7.36 ± 0.58 mol/ml versus 11.1 ± 1.01 mol/ml, P < 0.05) and liver (79% lower, P < 0.01) than the control mice receiving the same cholesterol-enriched diet. Although these results support the premise that human NPC1L1 variants could modulate LDL-C levels by directly affecting NPC1L1 transcript numbers, their translatability, or both, other data reveal a disconnect between determinants of NPC1L1 gene expression ( 55,56 ) and NPC1L1 protein levels ( 57 ). Thus, in a crossover study involving 22 men with modestly raised LDL-C (i.e., >50 th percentile value for their age (38.1 ± 9.8 years), it was found that atorvastatin (12 weeks, 40 mg/day or placebo) increased intestinal NPC1L1 expression by 18.7% ( P = 0.03), as judged by RNA concentrations in duodenum biopsy samples.…”

Up-Regulation Mttp and Apob Gene Expression in Rat Liver is Related to Post-Lipectomy Hypertriglyceridemia

“…This suggests that the up-regulation of proteins involved in the assembly and secretion of TAG-rich lipoproteins may contribute to an increase in the serum concentration of TAGs after lipectomy. Taking into account the results of previous studies in which HNF1α and HNF4α were shown to be transcriptional activators of Mttp and Apob genes [38,39], and our hereby presented findings, we postulate that these hepatocyte nuclear factors (HNFs) might contribute to the hypertriglyceridemia observed in lipectomized rats. This is consistent with the results of a recent study in which lipectomized rats presented with higher concentrations of circulating TAGs and higher liver levels of HNF4α mRNA than the sham-operated controls [13].…”

“…Based on the data reported previously, indicating that Mttp , ApoB , Fasn and Srebf-1 are target genes for HNFs [38,39,48] and the results presented in this paper, we propose a mechanistic scheme showing how HNFs may affect circulating TAG concentrations in lipectomy rats (Fig. 6).…”

Steatohepatitis in laboratory opossums exhibiting a high lipemic response to dietary cholesterol and fat

“…Sterol-responsive element-binding protein-2 (SREBP2) activates the transcription of the LDLR gene and all of the genes that encode cholesterol synthesis enzymes (13). In addition, the NPC1L1 gene has been shown to be activated by SREBP2 (26). Higher levels of SREBP2 mRNA (2.8-fold) in low responders were associated with higher levels of LDLR and NPC1L1 mRNAs, except HMGCR mRNA, in low responders.…”