Division of Gastroenterology and Hepatology, Department of Medicine, Indiana University School of Medicine, Indianapolis, IN, USA.
J Investig Med. 2013 Feb;61(2):270-7. doi: 10.2310/JIM.0b013e31827c2795.
Carbohydrate response element-binding protein (ChREBP) is a transcription factor involved in hepatic lipogenesis. Its function is in part under the control of AMP-activated protein kinase (AMPK) and protein phosphatase 2A (PP2A). Given known effects of ethanol on AMPK and PP2A, it is plausible that ethanol might enhance fatty acid synthesis by increasing the activity of ChREBP. We hypothesized that another potential pathway of ethanol-induced hepatic steatosis is mediated by activation of ChREBP.
The effects of ethanol on ChREBP were assessed in hepatoma cells and in C57BL/6J mice fed with the Lieber-DeCarli diet.
When the cells were exposed to ethanol (50 mM) for 24 hours, the activity of a liver pyruvate kinase (LPK) promoter-luciferase reporter was increased by ∼4-fold. Ethanol feeding of mice resulted in the translocation of ChREBP from cytosol to the nucleus. Protein phosphatase 2A activity was increased in the liver of ethanol-fed mice by 22%. We found no difference in the levels of hepatic Xu-5-P between ethanol-fed mice and controls. Transfection of a constitutively active AMPK expression plasmid suppressed the basal activity of the LPK luciferase reporter and abolished the effect of ethanol on the reporter activity. However, transfection of rat hepatoma cells with a dominant-negative AMPK expression plasmid induced basal LPK luciferase activity by only ∼20%. The effect of ethanol on ChREBP was attenuated in the presence of okadaic acid, an inhibitor of PP2A.
The effects of ethanol on AMPK and PP2A may result in activation of ChREBP, providing another potential mechanism for ethanol-induced hepatic steatosis. However, additional okadaic acid-insensitive effects appear to be important as well.
碳水化合物反应元件结合蛋白（ChREBP）是一种参与肝脂肪生成的转录因子。其功能部分受 AMP 激活的蛋白激酶（AMPK）和蛋白磷酸酶 2A（PP2A）的控制。鉴于乙醇对 AMPK 和 PP2A 的已知影响，乙醇可能通过增加 ChREBP 的活性来增强脂肪酸合成。我们假设乙醇诱导肝脂肪变性的另一个潜在途径是通过激活 ChREBP 介导的。
在肝癌细胞和用 Lieber-DeCarli 饮食喂养的 C57BL/6J 小鼠中评估乙醇对 ChREBP 的影响。
当细胞暴露于乙醇（50mM）24 小时时，肝丙酮酸激酶（LPK）启动子-荧光素酶报告基因的活性增加了约 4 倍。乙醇喂养小鼠导致 ChREBP 从细胞质易位到细胞核。乙醇喂养小鼠的肝蛋白磷酸酶 2A 活性增加了 22%。我们发现乙醇喂养小鼠和对照组之间肝 Xu-5-P 的水平没有差异。转染组成型活性 AMPK 表达质粒可抑制 LPK 荧光素酶报告基因的基础活性，并消除乙醇对报告基因活性的影响。然而，转染大鼠肝癌细胞的显性失活 AMPK 表达质粒仅诱导基础 LPK 荧光素酶活性增加约 20%。在存在 okadaic 酸（PP2A 的抑制剂）的情况下，乙醇对 ChREBP 的作用减弱。
乙醇对 AMPK 和 PP2A 的影响可能导致 ChREBP 激活，为乙醇诱导的肝脂肪变性提供了另一个潜在机制。然而，其他 okadaic 酸不敏感的作用似乎也很重要。
