The adenovirus E4orf6 protein contributes to malignant transformation by antagonizing E1A-induced accumulation of the tumor suppressor protein p53

The adenovirus type 5 (Ad5) E4orf6 protein promotes focus formation of prim ary baby rat kidney (BRK) cells in cooperation with Ad5 E1 proteins. This a ctivity is most likely related to the ability of the E4orf6 protein to bind to p53 and modulate its tumor suppressor functions. In this study we repor t that transformed BRK cells that stably express E40rf6 in addition to E1A and E1B (ABS cells) displayed multiple additional properties commonly assoc iated with a high grade of oncogenic transformation compared to cells expre ssing only E1A and E1B (AB cells). These properties included morphological alterations, markedly enhanced growth rates and growth to much higher satur ation densities. Following injection into nude mice ABS-derived tumors exhi bited accelerated growth and, based on histopathological criteria, proofed to be much more malignant compared to tumors generated by AB cells. Interes tingly, these highly transformed properties of ABS cells correlated with a dramatic reduction of p53 steady-state levels which inversely correlated wi th E40rf6 expression. From these results we conclude that expression of the Ad5 E40rf6 protein (i) confers additional transformed in vitro properties to primary rat cells expressing the Ad5 Fl proteins, and (ii) increases the tumorigenic and malignant potential of these cells in vivo. Our data sugge st that the Ad5 E40rf6 protein enhances the intrinsic ability of E1-transfo rmed rat cells to grow in a neoplastic state by completely inactivating p53 tumor suppressor function in combination with the E1A and E1B proteins.