Adenovirus-mediated E2F-1 gene transfer inhibits MDM2 expression and efficiently induces apoptosis in MDM2-overexpressing tumor cells

The oncoprotein MDM2 binds and inactivates p53. MDM2 also binds to the tumo r suppressor pRB, as well as E2F-1, E2F-1 is a transcription factor that re gulates S phase entry and has been shown to cause apoptosis in some cell ty pes when overexpressed, To investigate the effect of adenovirus-mediated E2 F-1 overexpression, MDM2-overexpressing tumor cell lines were treated by mo ck infection, infection with an adenoviral vector expressing beta galactosi dase, or E2F-1 (Ad5CMV-E2F-1), Western blot analysis confirmed significant overexpression of E2F-1 in Ad5CMV-E2F-1-infected cells. E2F-1 overexpressio n resulted in marked growth inhibition and rapid loss of cell viability. Ad 5CMV-E2F-1 infection resulted in early S phase entry, followed by apoptotic cell death. E2F-1 overexpression was associated with a marked decrease in MDM2 levels and no evidence of increased Bar levels, whereas p53 and Bcl-2 levels remained undetectable. Cleavage of poly-ADP-ribose polymerase and ca spase 3/CPP32 implicated activation of the caspase cascade in E2F-1-mediate d apoptosis, These results indicate that adenovirus-mediated E2F-1 overexpr ession in MDM2-overexpressing tumor cells results in decreased MDM2 express ion and widespread apoptosis, Because MDM2-overexpressing tumors are often resistant to p53 gene therapy, adenovirus-mediated E2F-1 gene therapy may b e a promising alternative strategy.