E1A inhibition of radiation-induced NF-kappa B activity through suppression of IKK activity and I kappa B degradation, independent of Akt activation

Activation of the transcription factor nuclear factor kappaB (NF-kappaB) ha s been implicated in the protection of cells from apoptosis. We have shown previously that the adenovirus type 5 E1A sensitizes cells to radiation-ind uced apoptosis by inhibiting NF-kappaB activity. However, the exact mechani sm of inhibition is not known. In this study, we compared the activity of i nhibitor of nuclear factor-kappaB (I kappaB) kinase (IKK) and the degradati on of I kappaB alpha in E1A transfectants and parental human cancer cells a fter ionizing radiation treatment. We found that radiation-induced IKK acti vity and I kappaB alpha degradation were inhibited in the E1A transfectants . Recently, Akt has been implicated in NF-kappaB activation. To test whethe r Akt is regulated by E1A and is involved in radiation- induced NF-kappaB a ctivity, we examined the phosphorylation status of Akt in the E1A transfect ants and parental cells and in irradiated cells. The results indicated that radiation induced Akt phosphorylation and that E1A inhibited basal but not radiation-induced Akt phosphorylation. We additionally examined radiation- induced NF-kappaB activity in cells stably transfected with a dominant-nega tive, inactive Akt and in parental cancer cells treated with a phosphatidyl inositol 3-kinase inhibitor, wortmannin. We found that dominant-negative Ak t and wortmannin did not block radiation-induced NF-kappaB activity. Thus, our results suggest that inhibition of IKK activity and I kappaB degradatio n is the predominant mechanism for E1A-mediated inhibition of radiation-ind uced NF-kappaB activity and that radiation-induced Akt activation cannot be inhibited by E1A and is likely independent of radiation-induced NF-kappaB activity.