Cholesterol, Its Metabolites, and Nonalcoholic Steatohepatitis

非酒精性脂肪性肝炎（NASH）中导致脂肪肝的代谢途径 肝脏炎症的进展正在被深入研究。一种途径， 被忽视的是CYP 27 A1启动的“替代途径”。这条通路负责 胆固醇代谢为细胞内调节性氧化固醇。该途径的后续7α- 据信这些氧固醇及其脂毒性代谢物被CYP 7 B1羟基化 以消除其监管影响。然而，是什么控制了这些氧化固醇的细胞水平， 其已知的代谢物仍不完全确定。此外，这些潜力 脂肪肝中引起细胞毒性和炎症的旁路途径胆固醇代谢物 尚未被研究。 在“脂肪肝”小鼠模型中的初步研究表明， 线粒体胆固醇转运蛋白StARD 1增加线粒体CYP 27 A1 胆固醇代谢，并导致肝脏胆固醇，甘油三酯， 和游离脂肪酸水平。出乎意料的是，我们发现肝细胞凋亡的显著下调， CYP 7 B1表达与高水平的调节性氧化固醇，25- 羟基胆固醇（25 HC）、27-羟基胆固醇（27 HC）和24（S）-羟基胆固醇（24 HC） LFT也明显增加根据这些观察，我们假设， CYP 7 B1和氧化固醇水平升高可能代表脂肪肝炎症的途径。 更具体地说，CYP 7 B1的慢性下调导致氧化固醇的慢性增加 水平可能在从脂肪肝到炎症的转变中起作用，如NASH中所见。 支持这一假设，我们发现CYP 7 B1表达的显着抑制 与从NIH肝脏获得的人脂肪变性肝脏中27 HC水平的增加相关 组织细胞分布系统此外，在西方饮食喂养的小鼠中，我们现在已经表明， 低CYP 7 B1与27 HC水平升高和随后的炎性小体相关 通过增加IL-1B水平确定的活化;概述了从脂肪肝到 如NASH中发生的炎症。这些观察结果也为CYP 7 B1是 CYP 27 A1产生的三种氧化固醇的水平和比例的关键调节剂 (24HC/25 HC/27 HC）及其羟基化代谢产物 结论：我们假设在非酒精性脂肪肝（NAFL）中， 抑制CYP 7 B1。胆固醇代谢物水平升高的慢性效应， 由下调的CYP 7 B1控制，具有损伤作用，并代表了主要的驱动因素。 通过炎性小体激活从NAFL转变为脂肪性肝炎的力量。的手段 CYP 7 B1在NAFL中被抑制的原因尚不清楚。我们提出三个具体目标， 途径： 目的1：明确CYP 27 A1启动的氧化甾醇/BAS的“旁路途径”。更 具体而言，更清楚地定义途径代谢物，并在 不同的代谢条件，以阐明它们在介导肝脏炎症中的作用。 目的2：探讨CYP 7 B1在非酒精性脂肪肝中的调节作用， 调节性和潜在毒性胆固醇代谢物的细胞内水平 目的3：证明氧化固醇及其代谢产物的肝脏水平长期升高 在脂肪肝中出现的代谢物代表了肝脏炎症的驱动力。

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.