Cancer Cell Survival Following DNA Damage-mediated Premature Senescence Is Regulated by Mammalian Target of Rapamycin (mTOR)-dependent Inhibition of Sirtuin 1

Jung Ho Back,Hamid Reza Rezvani,Yucui Zhu,Véronique Guyonnet-Duperat,Mohammad Athar,Desiree Ratner,Arianna L Kim

doi:10.1074/jbc.m111.240598

Abstract

DNA-damaging agents can induce premature senescence in cancer cells, which contributes to the static effects of cancer. However, senescent cancer cells may re-enter the cell cycle and lead to tumor relapse. Understanding the mechanisms that control the viability of senescent cells may be helpful in eliminating these cells before they can regrow. Treating human squamous cell carcinoma (SCC) cells with the anti-cancer compounds, resveratrol and doxorubicin, triggered p53-independent premature senescence by invoking oxidative stress-mediated DNA damage. This process involved the mTOR-dependent phosphorylation of SIRT1 at serine 47, resulting in the inhibition of the deacetylase activity of SIRT1. SIRT1 phosphorylation caused concomitant increases in p65/RelA NF-κB acetylation and the expression of an anti-apoptotic Bfl-1/A1. SIRT1 physically interacts with the mTOR-Raptor complex, and a single amino acid substitution in the TOS (TOR signaling) motif in the SIRT1 prevented Ser-47 phosphorylation and Bfl-1/A1 induction. The pharmacologic and genetic inhibition of mTOR, unphosphorylatable S47A, or F474A TOS mutants restored SIRT1 deacetylase activity, blocked Bfl-1/A1 induction, and sensitized prematurely senescent SCC cells for apoptosis. We further show that the treatment of UVB-induced SCCs with doxorubicin transiently stabilized tumor growth but was followed by tumor regrowth upon drug removal in p53(+/-)/SKH-1 mice. The subsequent treatment of stabilized SCCs with rapamycin decreased tumor size and induced caspase-3 activation. These results demonstrate that the inhibition of SIRT1 by mTOR fosters survival of DNA damage-induced prematurely senescent SCC cells via Bfl-1/A1 in the absence of functional p53.

Highlights

Tions, chemotherapeutic drugs, and oxidative stress can cause an acutely inducible, telomere-independent, stress-responsive form of cellular senescence, termed premature senescence (PS)2 [3, 4]
RES Induces PS in p53-deficient Human squamous cell carcinoma (SCC) Cells—We have shown previously that A431 cells treated with RES accumulate in the G1 phase of the cell cycle [19]
RES for the indicated time periods, determined by Western blotting. 50 ␮g of total protein was used per lane. ␤-Actin was used as an internal loading control

Summary

Introduction

Tions, chemotherapeutic drugs, and oxidative stress can cause an acutely inducible, telomere-independent, stress-responsive form of cellular senescence, termed premature senescence (PS)2 [3, 4]. C, increases in p-mTOR(S2448) and p-SIRT1(S47) during PS induction in A431 cells treated with 30 ␮M RES for the indicated time periods, assessed by Western blotting. Anti-apoptotic Bfl-1/A1 Is Important for the Survival of PIS Cells, and Its Induction Is Mediated by p65/RelA and p52 NF-␬B—To gain insight into potential target genes regulated in RES-induced prematurely senescent A431 cells, a cDNA microarray was performed on A431 cells treated with 100 ␮M RES for 48 or 72 h.

Results

Conclusion