Impaired autophagy due to constitutive mTOR activation sensitizes TSC2-null cells to cell death under stress

It has been well documented that cells deficient in either TSC1 or TSC2 are highly sensitive to various cell death stimuli. In this study, we utilized the TSC2(-/-) mouse embryonic fibroblasts (MEFs) to study the involvement of autophagy in the enhanced susceptibility of TSC2-null cells to cell death. We first confirmed that both TSC1-null and TSC2-null MEFs are more sensitive to apoptosis in response to amino acid starvation (EBSS) and hypoxia. Second, we found that both the basal and inducible autophagy in TSC2(-/-) MEFs is impaired, mainly due to constitutive activation of mTORC1. Third, suppression of autophagy by chloroquine and Atg7 knockdown sensitizes TSC2(+/+) cells, but not TSC2(-/-) cells, to EBSS induced cell death. Conversely, the inhibition of mTORC1 by raptor knockdown and rapamycin activates autophagy and subsequently rescues TSC2(-/-) cells. Finally, in starved cells, nutrient supplementations (insulin-like growth factor-1 (IGF-1) and leucine) enhanced cell death in TSC2(-/-) cells, but reduced cell death in TSC2(+/+) cells. Taken together, these data indicate that constitutive activation of mTORC1 in TSC2(-/-) cells leads to suppression of autophagy and enhanced susceptibility to stress-mediated cell death. Our findings thus provide new insights into the complex relationships among mTOR, autophagy and cell death, and support the possible autophagy-targeted intervention strategies for the treatment of TSC-related pathologies.