title: c-Myc augments gamma irradiation-induced apoptosis by suppressing Bcl-XL.
abstract: Alterations in MYC and p53 are hallmarks of cancer. p53 coordinates the response to gamma irradiation (gamma-IR) by either triggering apoptosis or cell cycle arrest. c-Myc activates the p53 apoptotic checkpoint, and thus tumors overexpressing MYC often harbor p53 mutations. Nonetheless, many of these cancers are responsive to therapy, suggesting that Myc may sensitize cells to gamma-IR independent of p53. In mouse embryo fibroblasts (MEFs) and in E micro -myc transgenic B cells in vivo, c-Myc acts in synergy with gamma-IR to trigger apoptosis, but alone, when cultured in growth medium, it does not induce a DNA damage response. Surprisingly, c-Myc also sensitizes p53-deficient MEFs to gamma-IR-induced apoptosis. In normal cells, and in precancerous B cells of E micro -myc transgenic mice, this apoptotic response is associated with the suppression of the antiapoptotic regulators Bcl-2 and Bcl-X(L) and with the concomitant induction of Puma, a proapoptotic BH3-only protein. However, in p53-null MEFs only Bcl-X(L) expression was suppressed, suggesting levels of Bcl-X(L) regulate the response to gamma-IR. Indeed, Bcl-X(L) overexpression blocked this apoptotic response, whereas bcl-X-deficient MEFs were inherently and selectively sensitive to gamma-IR-induced apoptosis. Therefore, MYC may sensitize tumor cells to DNA damage by suppressing Bcl-X.
mesh_terms: D000818:Animals; D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins; D015152:Blotting, Northern; D002470:Cell Survival; D002478:Cells, Cultured; D020552:Comet Assay; D004249:DNA Damage; D018076:DNA, Complementary / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D005720:Gamma Rays; D049452:Green Fluorescent Proteins; D015151:Immunoblotting; D008164:Luminescent Proteins / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D018613:Microscopy, Confocal; D009154:Mutation; D009369:Neoplasms / Q000378:metabolism; D011506:Proteins / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D051020:bcl-X Protein
chemical_list: D051017:Apoptosis Regulatory Proteins; C438378:BBC3 protein, human; C494812:Bcl2l1 protein, mouse; D018076:DNA, Complementary; D008164:Luminescent Proteins; D011506:Proteins; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D016271:Proto-Oncogene Proteins c-myc; D016159:Tumor Suppressor Protein p53; D051020:bcl-X Protein; D049452:Green Fluorescent Proteins