Department of Pathology, Johns Hopkins University School of Medicine, 1550 East Orleans Street, Baltimore, MD 21231, United States.
Department of Environmental Health Sciences, Johns Hopkins Bloomberg School of Public Health, 615 N. Wolfe Street, Baltimore, MD 21205, United States.
Free Radic Biol Med. 2018 May 20;120:407-413. doi: 10.1016/j.freeradbiomed.2018.04.009. Epub 2018 Apr 9.
Autophagy can serve as a mechanism for survival of cells during nutrient deprivation by recycling cellular macromolecules and organelles transiently to provide essential metabolic substrates. However, autophagy itself causes metabolic stress to cells, and other cellular protective mechanisms likely cooperate with autophagy to promote cell survival during nutrient deprivation. In this study, we explored protective mechanisms in breast cancer cells in the setting of glucose deprivation. While breast cancer cells (MCF7 and T47D) survive in glucose-free medium for three days or more, autophagy is induced in this setting. Blocking autophagy pharmacologically with chloroquine or by knock-out of an essential autophagy gene, such as Beclin 1 or ATG7, markedly reduces the ability of cells to survive during glucose deprivation. Autophagy previously was shown to degrade p62, a protein that sequesters KEAP1, and KEAP1 in turn sequesters Nrf2, a master regulator of the antioxidant response. Hence, we investigated how the Nrf2 signaling pathway might be affected by glucose deprivation and autophagy. We found that while glucose deprivation does cause decreased cellular levels of p62, Nrf2 protein levels and activity unexpectedly increase in this setting. Moreover, this increase in Nrf2 activity provides important protection to breast cancer cells during glucose deprivation, since siRNA knockdown of Nrf2 markedly impairs survival during glucose deprivation. Antioxidants, N-acetyl cysteine and glutathione also protect these cells during glucose deprivation, leading us to conclude that Nrf2 signaling via its antioxidant activity has a critical and previously undescribed role of protecting cells during glucose deprivation-induced autophagy.
自噬可以作为细胞在营养缺乏时生存的一种机制，通过回收细胞大分子和细胞器来短暂提供必要的代谢底物。然而，自噬本身会给细胞带来代谢压力，其他细胞保护机制可能与自噬合作，促进营养缺乏时细胞的存活。在这项研究中，我们探讨了葡萄糖剥夺环境下乳腺癌细胞的保护机制。虽然乳腺癌细胞（MCF7 和 T47D）在无葡萄糖培养基中可以存活三天或更长时间，但在这种环境下会诱导自噬。用氯喹或通过敲除必需的自噬基因（如 Beclin 1 或 ATG7）来抑制自噬，会显著降低细胞在葡萄糖剥夺期间的存活能力。先前的研究表明，自噬会降解 p62，p62 是一种能将 KEAP1 隔离的蛋白，而 KEAP1 反过来又将 Nrf2 隔离，Nrf2 是抗氧化反应的主要调节因子。因此，我们研究了 Nrf2 信号通路在葡萄糖剥夺和自噬中的可能影响。我们发现，虽然葡萄糖剥夺确实会导致细胞内 p62 水平降低，但 Nrf2 蛋白水平和活性在这种情况下出乎意料地增加。此外，Nrf2 活性的增加为乳腺癌细胞在葡萄糖剥夺期间提供了重要的保护，因为 Nrf2 的 siRNA 敲低显著损害了葡萄糖剥夺期间的存活。抗氧化剂 N-乙酰半胱氨酸和谷胱甘肽也在葡萄糖剥夺期间保护这些细胞，这使我们得出结论，通过其抗氧化活性的 Nrf2 信号在葡萄糖剥夺诱导的自噬中对保护细胞具有关键且以前未描述的作用。
