RESUMEN
The c-Myc oncoprotein is a transcription factor involved in cellular transformation as well as apoptotic cell death. We show here that over-expression of c-Myc delivered by an adenovirus vector up-regulates endogenous proapoptotic bax mRNA and protein expression in human cells. In contrast, the cytotoxic tumor necrosis factor-related apoptosis-inducing ligand induces cell death without up-regulating bax expression. c-Myc/Max heterodimers bind to canonical E-box elements located in the bax promoter region as demonstrated by electrophoretic mobility shift analysis and DNaseI foot-printing assays. Analysis of bax regulatory region mutants suggests a model involving myc-dependent activation as well as relief of repression through distinct E-box elements. c-Myc-null cells are deficient in bax-promoter activation as compared with wild-type c-Myc-expressing cells. Overexpression of c-Myc in serum-starved human or mouse embryonic cells leads to apoptosis which is significantly reduced in the presence of growth factor-containing serum. c-Myc-induced apoptosis appears to be deficient in bax-null as compared with bax-wild-type mouse embryonic fibroblasts. The results suggest that the cell death-promoting gene bax is directly downstream of c-Myc in a pathway leading to apoptosis.
Asunto(s)
Apoptosis/fisiología , Proteínas Proto-Oncogénicas c-bcl-2 , Proteínas Proto-Oncogénicas c-myc/fisiología , Proteínas Proto-Oncogénicas/fisiología , Adenoviridae/genética , Animales , Apoptosis/genética , Regulación de la Expresión Génica , Genes myc , Vectores Genéticos/genética , Humanos , Ratones , Proteínas Proto-Oncogénicas/biosíntesis , Proteínas Proto-Oncogénicas/genética , Proteínas Proto-Oncogénicas c-myc/biosíntesis , Proteínas Proto-Oncogénicas c-myc/genética , ARN Mensajero/biosíntesis , ARN Mensajero/genética , Ratas , Transducción de Señal/fisiología , Factores de Transcripción/biosíntesis , Factores de Transcripción/genética , Factores de Transcripción/fisiología , Transcripción Genética , Transfección , Células Tumorales Cultivadas , Regulación hacia Arriba , Proteína X Asociada a bcl-2RESUMEN
The c-Myc oncoprotein is a transcription factor involved in cellular transformation. We previously found (M. V. Blagosklonny, et al., Cancer Res., 57: 320-325, 1997) that exposure of human SkBr3 breast cancer and LNCaP prostate cancer cells to 12-O-tetradecanoylphorbol-13-acetate (TPA) led to a growth arrest associated with the up-regulation of the cyclin-dependent kinase inhibitor p21(WAF1/cIP1) and the inhibition of c-Myc expression. We show here that exogenous c-Myc inhibits p21 expression in SkBr3 and LNCaP cells induced to enter into S-phase. p27 expression was not increased from basal levels in TPA-treated growth-arrested cells. A time course after infection of TPA-arrested cells using a c-Myc-expressing adenovirus revealed that the inhibition of p21 expression preceded entry into S-phase. In contrast, after infection by E2F-1-expressing adenovirus, p21 expression was reduced after the cells entered S-phase. Overexpression of c-Myc reduced the levels of endogenous p21 mRNA, and transfection of c-Myc repressed p21-promoter luciferase-reporter gene expression. The results suggest that the down-regulation of p21 expression may contribute to c-Myc-dependent entry into S-phase, possibly in situations in which growth arrest is associated with increased p21 expression.