目的:肝脏胆固醇积累和自噬缺陷导致脂肪肝患者肝细胞损伤。胆汁酸合成是肝脏胆固醇分解代谢的主要途径。本研究旨在了解胆固醇和胆汁酸代谢与肝脏自噬活性之间的分子联系。
方法:在细胞模型中研究胆固醇和胆固醇7α-羟化酶(CYP7A1)表达对自噬和溶酶体功能的影响。在小鼠中研究了破坏肝肠胆汁酸循环对肝自噬的影响及其机制。
结果:结果首先显示了游离胆固醇和胆固醇酯对肝自噬的差异调节,细胞游离胆固醇的适度增加,但不是胆固醇酯,溶酶体功能受损,并导致明显的自溶酶体积累。我们发现CYP7A1诱导,通过在小鼠中喂食胆甾胺或在肝细胞中腺病毒介导的CYP7A1表达,引起强烈的自噬诱导。机械上,我们发现CYP7A1的表达显著减弱了雷帕霉素(mTOR)的生长因子/AKT信号通路激活,而不是在体外和体内对mTOR的氨基酸信号传导。代谢组学分析进一步发现,CYP7A1诱导不仅降低了肝胆固醇,而且改变了磷脂和鞘脂的组成。总的来说,这些结果表明CYP7A1诱导可能通过改变膜脂质组成来干扰AKT/mTOR信号传导的生长因子激活。最后,我们表明,在西方饮食喂养小鼠中,消胆胺可恢复受损的肝脏自噬并改善代谢稳态.
结论:本研究确定了一种选择性诱导肝自噬的新型CYP7A1-AKT-mTOR信号轴,这有助于改善肝细胞完整性和代谢稳态。
OBJECTIVE: Hepatic cholesterol accumulation and autophagy defects contribute to hepatocyte injury in fatty liver disease. Bile acid synthesis is a major pathway for cholesterol catabolism in the liver. This study aims to understand the molecular link between cholesterol and bile acid metabolism and hepatic autophagy activity.
METHODS: The effects of cholesterol and cholesterol 7α-hydroxylase (CYP7A1) expression on autophagy and lysosome function were studied in cell models. The effects and mechanism of disrupting enterohepatic bile acid circulation on hepatic autophagy were studied in mice.
RESULTS: The results first showed differential regulation of hepatic autophagy by free cholesterol and cholesterol ester, whereby a modest increase of cellular free cholesterol, but not cholesterol ester, impaired lysosome function and caused marked autolysosome accumulation. We found that CYP7A1 induction, either by cholestyramine feeding in mice or adenovirus-mediated CYP7A1 expression in hepatocytes, caused strong autophagy induction. Mechanistically, we showed that CYP7A1 expression markedly attenuated growth factor/AKT signaling activation of mechanistic target of rapamycin (mTOR), but not amino acid signaling to mTOR in vitro and in vivo. Metabolomics analysis further found that CYP7A1 induction not only decreased hepatic cholesterol but also altered phospholipid and sphingolipid compositions. Collectively, these results suggest that CYP7A1 induction interferes with growth factor activation of AKT/mTOR signaling possibly by altering membrane lipid composition. Finally, we showed that cholestyramine feeding restored impaired hepatic autophagy and improved metabolic homeostasis in Western diet-fed mice.
CONCLUSIONS: This study identified a novel CYP7A1-AKT-mTOR signaling axis that selectively induces hepatic autophagy, which helps improve hepatocellular integrity and metabolic homeostasis.