The Role Of TSC1and TSC2Tumor Suppressors In ER Stress Response

TSC1and TSC2tumor suppressors inhibit cell growth. Loss-of-function mutations of either TSC1or TSC2gene result in growth of benign tumors in multiple tissues. The TSC1and TSC2gene products interact with each other to form a functional TSC1-TSC2complex. TSC1-TSC2complex activates the GTPase activity of Rheb, leading to the decrease of GTP-bound Rheb, therefore inhibits mTORC1. In TSC1or TSC2mutant tumors, Rheb and mTORC1is constitutively active.We found that TSC1or TSC2null cells are hypersensitive to ER stress and undergo apoptosis. TSC1or TSC2null cells show elevated eIF2a phosphorylation, an early ER stress response marker, at both basal and induced conditions (thapsigargin, tunicamycin, or MG132treatment). However, induction of other ER stress response markers, such as ATF4and CHOP, is severely compromised. Thapsigargin or tunicamycin treatment activate ATF6in normal cells. In TSC1or TSC2null cells, the activation of ATF6is dramatically decreased. The XBP1level induced by MG132is lower in TSC1-/-cells. The mRNA level of ATF4and CHOP are lower in TSC1-/-cells.Rapamycin blocks glucose starvation induced apoptosis, but not ER stress induced apoptosis. The sensitivity to ER stress induced apoptosis is blocked by raptor knockdown, and the defects in ER stress response are restored as well. Overexpression of Rheb sensitizes cells to ER stress induced apoptosis as TSC1or TSC2mutations. Both overexpression and knockdown of Rheb result in sensitivity to MG132induced apoptosis.Caspase-12, caspase-9, and caspase-3are activated during ER stress induced apoptosis. Activation of caspase-12is more dramatically induced by thapsigargin, compared to that induced by tunicamycin. In TSC1-/-MEF, ER stress still increase autophagy.The role of TSC1and TSC2in ER stress response may contribute to the benign nature of TSC1or TSC2mutated tumors (tuberous sclerosis complex). TSC1and TSC2are important for cells to trigger a full spectrum of ER stress response. Activation of Rheb and mTORC1lead to a truncated ER stress response and sensitize cells to ER stress induced apoptosis. This study indicates a potential therapeutic application of use ER stress agents, such as Velcade, for treatment of TSC and tumors with low TSC1/2activity.