Stimulation of adenovirus early gene expression by phorbol ester: its possible mechanism

Treatment of Hela cells infected with adenovirus 5 wild type (Ad5WT) with the tumor-promoting phorbol ester TPA (12-O-tetradecanoyl phorbol-13-acetate), accelerated as well as stimulated expression of viral early genes EII and EIII but not that of EIA. TPA treatment of HeLa cells infected with dl312, an Ad5 EIA deletion mutant, activated expression of EIII but not EII. Stimulation of EII and EIII expression was blocked by H7 (1-5-isoquinolinyl sulfonyl-2-methyl piperazine), a specific inhibitor of protein kinase c (PKc). Nuclear run off assays demonstrated that TPA exerted a stimulatory effect at the level of transcription. PKc inhibitor alone reduced transcription of early genes in the absence of TPA activation. Phosphorylation of EIA 35 kDa but not 40- to 45-kDa proteins was dramatically increased by TPA. Three cellular proteins of 200, 24, and 20 kDa which coprecipitated with EIA proteins underwent enhanced and preferential phosphorylation by activated PKc. Inhibitor of PKc blocked phosphorylation of cellular proteins and reduced phosphorylation of EIA 35 kDa but not EIA 40- to 45-kDa proteins. These results tend to indicate that TPA stimulates adenovirus early gene expression through activation of protein kinase c and further suggest but do not prove that this may be due to specific phosphorylation of EIA 35 kDa and cellular proteins of 200, 24, and 20 kDa.