Eukaryotic initiation factor 2?--a downstream effector of mammalian target of rapamycin--modulates DNA repair and cancer response to treatment.

ABSTRACT: In an effort to circumvent resistance to rapamycin--an mTOR inhibitor--we searched for novel rapamycin-downstream-targets that may be key players in the response of cancer cells to therapy. We found that rapamycin, at nM concentrations, increased phosphorylation of eukaryotic initiation factor (eIF) 2? in rapamycin-sensitive and estrogen-dependent MCF-7 cells, but had only a minimal effect on eIF2? phosphorylation in the rapamycin-insensitive triple-negative MDA-MB-231 cells. Addition of salubrinal--an inhibitor of eIF2? dephosphorylation--decreased expression of a surface marker associated with capacity for self renewal, increased senescence and induced clonogenic cell death, suggesting that excessive phosphorylation of eIF2? is detrimental to the cells' survival. Treating cells with salubrinal enhanced radiation-induced increase in eIF2? phosphorylation and clonogenic death and showed that irradiated cells are more sensitive to increased eIF2? phosphorylation than non-irradiated ones. Similar to salubrinal--the phosphomimetic eIF2? variant--S51D--increased sensitivity to radiation, and both abrogated radiation-induced increase in breast cancer type 1 susceptibility gene, thus implicating enhanced phosphorylation of eIF2? in modulation of DNA repair. Indeed, salubrinal inhibited non-homologous end joining as well as homologous recombination repair of double strand breaks that were induced by I-SceI in green fluorescent protein reporter plasmids. In addition to its effect on radiation, salubrinal enhanced eIF2? phosphorylation and clonogenic death in response to the histone deacetylase inhibitor--vorinostat. Finally, the catalytic competitive inhibitor of mTOR--Ku-0063794--increased phosphorylation of eIF2? demonstrating further the involvement of mTOR activity in modulating eIF2? phosphorylation. These experiments suggest that excessive phosphorylation of eIF2? decreases survival of cancer cells; making eIF2? a worthy target for drug development, with the potential to enhance the cytotoxic effects of established anti-neoplastic therapies and circumvent resistance to rapalogues and possibly to other drugs that inhibit upstream components of the mTOR pathway.

SUBMITTER: Tuval-Kochen L 

PROVIDER: S-EPMC3808413 | biostudies-literature | 2013

REPOSITORIES: biostudies-literature