Transforming potential of the adenovirus type 5 E4orf3 protein

Previous observations that the adenovirus type 5 (Ad5) E4orf6 and E4orf3 ge ne products have redundant effects in viral lyric infection together with t he recent findings that E4orf6 possesses transforming potential prompted us to investigate the effect of E4orf3 expression on the transformation of pr imary rat cells in combination with adenovirus E1 oncogene products. Our re sults demonstrate for the first time that E4orf3 can cooperate with adenovi rus E1A and E1A plus E1B proteins to transform primary baby rat kidney cell s, acting synergistically with E4orf6 in the presence of E1B gene products. Transformed rat cells expressing E4orf3 exhibit morphological alterations, higher growth rates and saturation densities, and increased tumorigenicity compared with transformants expressing E1 proteins only. Consistent with p revious results for adenovirus-infected cells, the E4orf3 protein is immuno logically restricted to discrete nuclear structures known as PML oncogenic domains (PODs) in transformed rat cells. As opposed to E4orf6, the ability of E4orf3 to promote oncogenic cell growth is probably not linked to a modu lation of p53 functions and stability. Instead, our results indicate that t he transforming activities of E4orf3 are due to combinatorial effects that involve the binding to the adenovirus 55-kDa E1B protein and the colocaliza tion with PODs independent from interactions with the PML gene product. The se data fit well with a model in which the reorganization of PODs may trigg er a cascade of processes that cause uncontrolled cell proliferation and ne oplastic growth. In sum, our results provide strong evidence for the idea t hat interactions with PODs by viral proteins are linked to oncogenic transf ormation.