Pathophysiological importance of bile cholesterol reabsorption: hepatic NPC1L1-exacerbated steatosis and decreasing VLDL-TG secretion in mice fed a high-fat diet.

BACKGROUND: Non-alcoholic fatty liver disease (NAFLD) is one of the most common liver diseases worldwide, although its pathogenesis remains to be elucidated. A recent study revealed that hepatic Niemann-Pick C1-Like 1 (NPC1L1), a cholesterol re-absorber from bile to the liver expressed on the bile canalicular membrane, is an exacerbation factor of NAFLD. Indeed, transgenic mice with hepatic expression of human NPC1L1 under a liver-specific promoter (L1-Tg mice) developed steatosis with a high-fat diet (HFD) containing cholesterol within a few weeks. However, the mechanism underlying diet-induced hepatic NPC1L1-mediated lipid accumulation is poorly defined. METHODS: To achieve a deeper understanding of steatosis development in L1-Tg mice, the biochemical features of hepatic NPC1L1-mediated steatosis were investigated. Hemizygous L1-Tg mice and wild-type littermate controls fed a HFD or control-fat diet were used. At the indicated time points, the livers were evaluated for cholesterol and triglyceride (TG) contents as well as mRNA levels of hepatic genes involved in the maintenance of lipid homeostasis. The hepatic ability to secrete very low-density lipoprotein (VLDL)-TG was also investigated. RESULTS: Unlike the livers of wild-type mice that have little expression of hepatic Npc1l1, the livers of L1-Tg mice displayed time-dependent changes that indicated steatosis formation. In steatosis, there were three different stages of development: mild accumulation of hepatic cholesterol and TG (early stage), acceleration of hepatic TG accumulation (middle stage), and further accumulation of hepatic cholesterol (late stage). In the early stage, between WT and L1-Tg mice fed a HFD for 2 weeks, there were no significant differences in the hepatic expression of Pparα, Acox1, Fat/Cd36, Srebf1, and Srebf2; however, the hepatic ability to secrete VLDL-TG decreased in L1-Tg mice (P < 0.05). Furthermore, this decrease was completely prevented by administration of ezetimibe, an NPC1L1-selective inhibitor. CONCLUSION: Hepatic NPC1L1 exacerbates diet-induced steatosis, which was accompanied by decreased hepatic ability of VLDL-TG secretion. The obtained results provide a deeper understanding of L1-Tg mice as a promising NAFLD animal model that is able to re-absorb biliary-secreted cholesterol similar to humans. Furthermore, this work supports further studies of the pathophysiological impact of re-absorbed biliary cholesterol on the regulation of hepatic lipid homeostasis.

Identification and characterization of NPC1L1 variants in Uygur and Kazakh with extreme low-density lipoprotein cholesterol.

BACKGROUND: Plasma levels of low-density lipoprotein cholesterol (LDL-C) are a major risk factor for cardiovascular disease and are influenced by both heredity and dietary habits. The Niemann-Pick C1 like 1 (NPC1L1) protein mediates efficient dietary cholesterol absorption and contributes to variations in human LDL-C levels. METHODS: In the present study, using high throughput sequencing we identified three non-synonymous (NS) variations and 64 synonymous variations in the NPC1L1 gene from subsets of Chinese Han, Uygur and Kazakh populations with high or low LDL-C. Subsequently, three NS variations encoding R174H, V177I and V1284L substitutions were observed only in Uygur and Kazakh individuals with limited maximal plasma LDL-C levels. RESULTS: In further experiments, we investigated cholesterol-regulated recycling and glycosylation and stability of these NS NPC1L1 variants. However, no significant differences between WT and variant NPC1L1 proteins were observed using in vivo assays in mouse livers with adenovirus-mediated expression, demonstrating that none of the three NPC1L1 NS variants caused decreased uptake of biliary cholesterol. CONCLUSIONS: Simultaneously, these data indicate that R174H, V177I and V1284L NPC1L1 variations in high or low LDL-C individuals may not directly influence cholesterol absorption by NPC1L1.