Department of Surgery, Stanford University School of Medicine, Stanford, California 94305-5148, USA.
J Biol Chem. 2013 Jun 14;288(24):17214-24. doi: 10.1074/jbc.M112.445965. Epub 2013 Apr 25.
Numerous liver diseases are associated with extensive oxidative tissue damage. It is well established that Wnt/β-catenin signaling directs multiple hepatocellular processes, including development, proliferation, regeneration, nutrient homeostasis, and carcinogenesis. It remains unexplored whether Wnt/β-catenin signaling provides hepatocyte protection against hepatotoxin-induced apoptosis. Conditional, liver-specific β-catenin knockdown (KD) mice and their wild-type littermates were challenged by feeding with a hepatotoxin 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) diet to induce chronic oxidative liver injury. Following the DDC diet, mice with β-catenin-deficient hepatocytes demonstrate increased liver injury, indicating an important role of β-catenin signaling for liver protection against oxidative stress. This finding was further confirmed in AML12 hepatocytes with β-catenin signaling manipulation in vitro using paraquat, a known oxidative stress inducer. Immunofluorescence staining revealed an intense nuclear FoxO3 staining in β-catenin-deficient livers, suggesting active FoxO3 signaling in response to DDC-induced liver injury when compared with wild-type controls. Consistently, FoxO3 target genes p27 and Bim were significantly induced in β-catenin KD livers. Conversely, SGK1, a β-catenin target gene, was significantly impaired in β-catenin KD hepatocytes that failed to inactivate FoxO3. Furthermore, shRNA-mediated deletion of FoxO3 increased hepatocyte resistance to oxidative stress-induced apoptosis, confirming a proapoptotic role of FoxO3 in the stressed liver. Our findings suggest that Wnt/β-catenin signaling is required for hepatocyte protection against oxidative stress-induced apoptosis. The inhibition of FoxO through its phosphorylation by β-catenin-induced SGK1 expression reduces the apoptotic function of FoxO3, resulting in increased hepatocyte survival. These findings have relevance for future therapies directed at hepatocyte protection, regeneration, and anti-cancer treatment.
许多肝脏疾病都与广泛的氧化组织损伤有关。众所周知，Wnt/β-catenin 信号通路指导多种肝细胞过程，包括发育、增殖、再生、营养稳态和癌变。目前尚不清楚 Wnt/β-catenin 信号通路是否为肝细胞提供了对肝毒素诱导的细胞凋亡的保护。条件性肝特异性β-catenin 敲低（KD）小鼠及其野生型同窝仔鼠用肝毒素 3,5-二乙氧羰基-1,4-二氢吡啶（DDC）饮食喂养，以诱导慢性氧化肝损伤。在 DDC 饮食后，β-catenin 缺陷型肝细胞的小鼠显示出肝损伤增加，表明β-catenin 信号通路在肝脏对氧化应激的保护中起着重要作用。这一发现通过体外使用百草枯（一种已知的氧化应激诱导剂）对 AML12 肝细胞进行β-catenin 信号转导操纵得到了进一步证实。免疫荧光染色显示，β-catenin 缺陷型肝脏中 FoxO3 的核染色强烈，表明与野生型对照组相比，DDC 诱导的肝损伤时 FoxO3 信号通路处于活跃状态。一致地，β-catenin KD 肝脏中 FoxO3 的靶基因 p27 和 Bim 明显诱导。相反，β-catenin KD 肝细胞中的 SGK1（β-catenin 的靶基因）明显受损，无法使 FoxO3 失活。此外，FoxO3 的 shRNA 介导缺失增加了肝细胞对氧化应激诱导的细胞凋亡的抵抗，证实了 FoxO3 在应激肝脏中的促凋亡作用。我们的研究结果表明，Wnt/β-catenin 信号通路是肝细胞对氧化应激诱导的细胞凋亡的保护所必需的。FoxO3 的磷酸化通过β-catenin 诱导的 SGK1 表达抑制 FoxO3 的凋亡功能，从而增加肝细胞的存活。这些发现与未来针对肝细胞保护、再生和抗癌治疗的治疗策略有关。
