Autophagy inhibits oxidative stress and tumor suppressors to exert its dual effect on hepatocarcinogenesis

Abstract

The role of autophagy in carcinogenesis is controversial and apparently complex. By using mice with hepatocyte-specific knockout of Atg5, a gene essential for autophagy, we longitudinally studied the role of autophagy in hepatocarcinogenesis. We found that impairing autophagy in hepatocytes would induce oxidative stress and DNA damage, followed by the initiation of hepatocarcinogenesis, which could be suppressed by the antioxidant N-acetylcysteine. Interestingly, these mice developed only benign tumors with no hepatocellular carcinoma (HCC), even after the treatment with diethylnitrosamine, which induced HCC in wild-type mice. The inability of mice to develop HCC when autophagy was impaired was associated with the induction of multiple tumor suppressors including p53. Further analysis indicated that the induction of p53 was associated with the DNA-damage response. Tumorigenesis studies using an established liver tumor cell line confirmed a positive role of autophagy in tumorigenesis and a negative role of p53 in this process when autophagy was impaired. Our studies thus demonstrate that autophagy is required to maintain healthy mitochondria and to reduce oxidative stress and DNA damage to prevent the initiation of hepatocarcinogenesis. However, once hepatocarcinogenesis has been initiated, its presence is also required to suppress the expression of tumor suppressors to promote the development of HCC. Autophagy (i.e., macroautophagy) is important for cells to remove protein aggregates and damaged organelles. Its dysfunction can cause a variety of diseases including cancers. 1,2 However, its role in carcinogenesis is apparently complex, as it has been shown in different reports to positively or negatively regulate carcinogenesis. 3,4 Autophagy apparently can function as a tumor suppressor, as the gene encoding Beclin-1, a component of the phosphatidylinositol-3-kinase class III (PI3KC3) complex that is essential for the initiation of autophagy, is often monoallelically deleted or mutated in breast, ovarian and prostate cancers. 5 Frameshift mutations in Atg2B, Atg5, Atg9B and Atg12 autophagy genes are also often found in gastric and colorectal cancers with microsatellite instability. 6 The tumor suppressor role of autophagy is further supported by the studies using mouse models. It has been shown that the monoallelic deletion of the Beclin-1 gene in mice induced tumor lesions in various tissues, 7 Atg4C-knockout (KO) mice had increased susceptibility to carcinogens for the development of fibrosarcomas 8 and the systemic mosaic KO of Atg5 and the liver-specific KO of Atg7 in mice led to the development of benign liver adenomas. 9,10 Autophagy has also been shown to promote tumor growth. It has been shown that autophagy can enhance the survival of tumor cells in the hypoxic regions of solid tumors. 11 It has also been shown that in cells expressing oncogenic Ras, autophagy is required to promote tumorigenesis by maintaining oxidative metabolism or facilitating glycolysis. 12,13 Moreover, it has also been demonstrate...

Mitophagy Controls the Activities of Tumor Suppressor p53 to Regulate Hepatic Cancer Stem Cells

“…We had previously produced mice with hepatocyte-specific knockout of the ATG5 gene. These mice developed benign liver tumors with a high frequency with no detectable HCC, not even with the treatment of the carcinogen diethylnitrosamine (DEN), which induced HCC with a high frequency in wild-type mice (Tian et al, 2015). To ensure that the effect of autophagy on hepatic CSCs was not specific to HepG2 cell, we also analyzed the liver tumors isolated from ATG5 -knockout mice and control mice that had been treated with DEN.…”

“…Previous studies indicated that autophagy was essential for benign hepatic tumors to progress into malignant HCC (Takamura et al, 2011; Tian et al, 2015). However, the mechanism for this requirement was unclear.…”

“…It may function as a tumor suppressor by preventing the accumulation of dysfunctional mitochondria, which can lead to increased oxidative stress and DNA damage (Tian et al, 2015), and the accumulation of the p62 sequestosome protein, which also promotes oxidative stress and tumor growth (Mathew et al, 2009). Autophagy may also function as a tumor promoter to alleviate metabolic stress during tumorigenesis and suppress the expression of tumor suppressors (Guo et al, 2013; Rosenfeldt et al, 2013; Tian et al, 2015), and impairing autophagy can impede hepatocarcinogenesis and prevent benign hepatic tumors from becoming malignant hepatocellular carcinoma (HCC) (Takamura et al, 2011; Tian et al, 2015), prevent low-grade pre-malignant pancreatic neoplastic lesions from progressing into high-grade pancreatic intraepithelial neoplasia and the pancreatic ductal adenocarcinoma (Rosenfeldt et al, 2013), and alter the fate of pulmonary tumors from adenomas and carcinomas to benign oncocytomas (Guo et al, 2013). Interestingly, in the above studies of hepatic, pancreatic and pulmonary tumors with impaired autophagy, tumor progression was restored or partially restored if the expression of the tumor suppressor p53 was suppressed, suggesting that autophagy may promote tumorigenesis via the control of p53 activities.…”

Control of YAP/TAZ Activity by Metabolic and Nutrient-Sensing Pathways

“…For example, the recent discovery that MST1/2 (Hippo kinases) sustain autophagosome formation by phosphorylating LC3 [64] might suggest that there could be intimate crosstalk between Hippo signaling and autophagy in regulating YAP/ TAZ. Interestingly, blockade of autophagy by liver-specific knockout of Atg5 or Atg7 induces potent liver overgrowth [65,66], which is also observed upon inactivation of several Hippo pathway components (MST1/2, NF2, and SAV/WW45) or upon YAP activation [67][68][69].…”

“…For example, the recent discovery that MST1/2 (Hippo kinases) sustain autophagosome formation by phosphorylating LC3 [64] might suggest that there could be intimate crosstalk between Hippo signaling and autophagy in regulating YAP/ TAZ. Interestingly, blockade of autophagy by liver-specific knockout of Atg5 or Atg7 induces potent liver overgrowth [65,66], which is also observed upon inactivation of several Hippo pathway components (MST1/2, NF2, and SAV/WW45) or upon YAP activation [67][68][69].The YAP/TAZ inhibitory effects of autophagy indicate a tumor-suppressive function of autophagy; however, in many tumors autophagy favors cancer cell survival under stress, and can thus have protumorigenic functions [70,71] that are not easily compatible with a universal YAP/TAZ Trends in Cell Biology, Month Year, Vol. xx, No.…”