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 目前正在对肝脏的炎症进展进行深入研究。其中一条途径是 被忽视的是由CYP27A1启动的“替代路径”。这条路径负责 胆固醇代谢为细胞内调节性氧化甾醇。这条通路随后的7个α- 这些氧化甾醇及其脂毒代谢物被认为是被CYP7B1羟基化的 以消除他们的监管影响。然而，是什么控制了这些氧固醇和细胞的水平 它们的已知代谢物仍未完全定义。此外，这些技术的潜力 替代途径胆固醇代谢产物在诱导脂肪肝细胞毒性和炎症中的作用 还没有被研究过。 在“脂肪肝”小鼠模型中的初步研究表明，过度表达 线粒体胆固醇传递蛋白，StARD1，增加线粒体CYP27A1 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，我们发现肝脏显著下调 CYP7B1的表达伴随着高水平的调节氧固醇，25- 羟基胆固醇(25HC)、27-羟基胆固醇(27HC)和24(S)-羟基胆固醇(24HC) 以及LFTs的显著增加。基于这些观察结果，我们推测Low 细胞色素P7B1和氧固醇水平升高可能是脂肪肝炎症的一种途径。 更具体地说，细胞色素P7B1的慢性下调会导致氧固醇值的慢性升高 水平可能在脂肪肝向炎症的转变中发挥作用，就像在NASH中看到的那样。 支持这一假设，我们发现显著抑制了细胞色素P7B1的表达 与从NIH肝脏获得的人类脂肪变性肝脏中27HC水平的增加有关 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CYP7B1与27HC水平升高和随后的炎症体相关 由IL-1B水平升高确定的激活；勾勒出从脂肪肝到 炎症如发生在NASH中。这些观察也为细胞色素P7B1是 三种细胞色素P27A1生成的氧化甾醇的水平和比例的关键调节因素 (24HC/25HC/27HC)及其羟化代谢物 结论：我们假设在非酒精性脂肪肝(NAFL)中存在慢性 抑制细胞色素P7B1的表达。胆固醇代谢产物水平升高的慢性影响 由下调的CYP7B1控制的具有破坏性的影响，并代表主要驱动 通过炎性小体激活从NAFL转变为脂肪性肝炎的力量。手段是通过 目前尚不清楚哪种细胞色素P7B1在NAFL中被抑制。我们提出了三个具体目标来定义 途径： 目的：明确细胞色素P27A1启动的氧固醇/BAS的“替代途径”。更多 具体地说，更清楚地定义途径代谢物，并量化其水平/比率 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：探讨细胞色素P7B1在非酒精性脂肪肝中的调控作用。 细胞内调节性和潜在毒性胆固醇代谢物的水平 目的：证明慢性升高的肝脏氧固醇水平及其影响因素。 脂肪肝中的代谢物是肝脏炎症的驱动力。

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.