title: Eukaryotic initiation factor 2α--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) 2α in rapamycin-sensitive and estrogen-dependent MCF-7 cells, but had only a minimal effect on eIF2α phosphorylation in the rapamycin-insensitive triple-negative MDA-MB-231 cells. Addition of salubrinal--an inhibitor of eIF2α 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 eIF2α is detrimental to the cells' survival. Treating cells with salubrinal enhanced radiation-induced increase in eIF2α phosphorylation and clonogenic death and showed that irradiated cells are more sensitive to increased eIF2α phosphorylation than non-irradiated ones. Similar to salubrinal--the phosphomimetic eIF2α variant--S51D--increased sensitivity to radiation, and both abrogated radiation-induced increase in breast cancer type 1 susceptibility gene, thus implicating enhanced phosphorylation of eIF2α 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 eIF2α phosphorylation and clonogenic death in response to the histone deacetylase inhibitor--vorinostat. Finally, the catalytic competitive inhibitor of mTOR--Ku-0063794--increased phosphorylation of eIF2α demonstrating further the involvement of mTOR activity in modulating eIF2α phosphorylation. These experiments suggest that excessive phosphorylation of eIF2α decreases survival of cancer cells; making eIF2α 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.
authors.affiliation: Cancer Research Center, Chaim Sheba Medical Center, Tel-Hashomer, Ramat-Gan, Israel , ; Sackler School of Medicine, Tel-Aviv University, Tel-Aviv, Israel .
authors.forename: Liron
authors.initials: L
authors.lastname: Tuval-Kochen
authors.forename: Shoshana
authors.initials: S
authors.lastname: Paglin
authors.forename: Gilmor
authors.initials: G
authors.lastname: Keshet
authors.forename: Yaniv
authors.initials: Y
authors.lastname: Lerenthal
authors.forename: Charles
authors.initials: C
authors.lastname: Nakar
authors.forename: Tamar
authors.initials: T
authors.lastname: Golani
authors.forename: Amos
authors.initials: A
authors.lastname: Toren
authors.forename: Joachim
authors.initials: J
authors.lastname: Yahalom
authors.forename: Raphael
authors.initials: R
authors.lastname: Pfeffer
authors.forename: Yaacov
authors.initials: Y
authors.lastname: Lawrence
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D045744:Cell Line, Tumor; D016922:Cellular Senescence / Q000187:drug effects; D002934:Cinnamates / Q000494:pharmacology; D004260:DNA Repair / Q000187:drug effects*; D004273:DNA, Neoplasm / Q000235:genetics* / Q000378:metabolism; D015252:Deoxyribonucleases, Type II Site-Specific / Q000235:genetics / Q000378:metabolism; D019008:Drug Resistance, Neoplasm / Q000187:drug effects / Q000235:genetics* / Q000528:radiation effects; D015852:Eukaryotic Initiation Factor-2 / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D005260:Female; D005720:Gamma Rays; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects* / Q000528:radiation effects; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D006877:Hydroxamic Acids / Q000494:pharmacology; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D009025:Morpholines / Q000494:pharmacology; D057786:Peptidomimetics / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects / Q000528:radiation effects; D011743:Pyrimidines / Q000494:pharmacology; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D013890:Thiourea / Q000031:analogs & derivatives / Q000494:pharmacology; D019076:Transgenes; D000077337:Vorinostat
chemical_list: D000970:Antineoplastic Agents; D002934:Cinnamates; D004273:DNA, Neoplasm; D015852:Eukaryotic Initiation Factor-2; D006877:Hydroxamic Acids; D008565:Membrane Proteins; D009025:Morpholines; D057786:Peptidomimetics; D011743:Pyrimidines; C488377:SFXN4 protein, human; D029701:Saccharomyces cerevisiae Proteins; C496827:salubrinal; D049452:Green Fluorescent Proteins; D000077337:Vorinostat; C541932:Ku 0063794; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; C037725:SCEI protein, S cerevisiae; D015252:Deoxyribonucleases, Type II Site-Specific; D013890:Thiourea; D020123:Sirolimus