Cholesterol, Its Metabolites, and Nonalcoholic Steatohepatitis

非酒精性脂肪性肝炎（NASH）中导致脂肪的代谢途径

The metabolic pathways in nonalcoholic steatohepatitis (NASH) that contribute to fatty liver’s progression to inflammation are being intensely studied. One pathway which has been overlooked is the CYP27A1 initiated ‘alternative pathway.’ This pathway is responsible for the metabolism of cholesterol to intracellular regulatory oxysterols. The pathway’s subsequent 7α- hydroxylation of these oxysterols and their lipotoxic metabolites by CYP7B1 is what is believed to eliminate their regulatory effects. However, what controls cell levels of these oxysterols and their known metabolites remains incompletely defined. Furthermore, the potential for these alternative pathway cholesterol metabolites in eliciting cytotoxicity and inflammation in fatty liver has not been studied. A preliminary study in a ‘fatty liver’ mouse model showed that overexpression of the mitochondrial cholesterol delivery protein, StARD1, increased mitochondrial CYP27A1 cholesterol metabolism, and, led to a dramatic reduction in hepatic cholesterol, triglycerides, and free fatty acids levels. Unanticipated, we found significant down regulation of hepatic CYP7B1 expression coupled with high levels of the regulatory oxysterols, 25- hydroxycholesterol(25HC), 27-hydroxycholesterol(27HC), and 24(S)-hydroxycholesterol(24HC) and a marked increase in LFTs. Based upon these observations, we postulated that low CYP7B1 and increased oxysterol levels may represent a pathway to inflammation in fatty liver. More specifically, chronic down-regulation of CYP7B1 leading to chronically increased oxysterol levels may play a role in the transition from fatty liver to inflammation as seen in NASH. Supportive of this hypothesis we found significant suppression of CYP7B1 expression correlated to an increase in 27HC levels in human steatotic livers obtained from the NIH liver tissue cell distribution system. Furthermore, in Western diet fed mice, we have now shown that a low CYP7B1 is associated with an increase in 27HC levels and subsequent inflammasome activation as determined by increased IL-1B levels; outlining a pathway from fatty liver to inflammation as occurs in NASH. These observations also provided evidence for CYP7B1 being a key regulator of the levels and ratio of the three CYP27A1 generated oxysterols (24HC/25HC/27HC), and their hydroxylated metabolites In conclusion: we hypothesize that in nonalcoholic fatty liver (NAFL) there is chronic suppression of CYP7B1. The chronic effects of increased levels of cholesterol metabolites as controlled by down-regulated CYP7B1 have injurious effects, and represent the major driving force for transition from NAFL to steatohepatitis via inflammasome activation. The means by which CYP7B1 is suppressed in NAFL is unknown. We propose three specific aims to define the pathway: Aim1: To define the CYP27A1 initiated ‘alternative pathway’ of oxysterol/BAS. More specifically, more clearly define the pathway metabolites and quantitate their levels/ratios under different metabolic conditions to elucidate their role in mediating liver inflammation. Aim2: To investigate the regulation of CYP7B1 in NAFL, and its control over intracellular levels of regulatory and potentially toxic cholesterol metabolites Aim3: To demonstrate that chronically increased hepatic levels of oxysterols and their metabolites that occur in fatty liver represent a driving force to hepatic inflammation.