Several E4 region functions influence mammary tumorigenesis by human adenovirus type 9

Among oncogenic adenoviruses, human adenovirus type 9 (Ad9) is unique in el iciting exclusively estrogen-dependent mammary tumors in rats and in not re quiring viral E1 region transforming genes for tumorigenicity. Instead, stu dies with hybrid viruses generated between Ad9 and the closely related nont umorigenic virus Ad26 have roughly localized an Ad9 oncogenic determinant(s ) to a segment of the viral E4 region containing open reading frame 1 (E4-O RF1), E4-ORF2, and part of E4-ORF3. Although subsequent findings have shown that E4-ORF1 codes for an oncoprotein essential for tumorigenesis by Ad9, it is not known,whether other E4 region functions may similarly play a role in this process. We report here that new results with Ad9/Ad26 hybrid viru ses demonstrated that the minimal essential Ad9 E4-region DNA sequences inc lude portions of both E4-ORF1 and E4-ORF2. Investigations with Ad9 mutant v iruses additionally shelved that the E4-ORF1 protein and certain E4-ORF2 DN A sequences are necessary for Ad9-induced tumorigenesis, whereas the E4-ORF 2 and E4-ORF3 proteins are not. In fact, the E4-ORF3 protein was found to a ntagonize this process. Also pertinent was that certain crucial nucleotide differences between Ad9 and Ad26 within E4-ORF1 and E4-ORF2 were found to b e silent with respect to the amino acid sequences of the corresponding prot eins. Furthermore, supporting a prominent role for the E4-ORF1 oncoprotein in Ad9-induced tumorigenesis, an E1 region-deficient Ad5 vector that expres ses the Ad9 but not the Ad26 E4-ORF1 protein was tumorigenic in rats and, l ike Ad9, promoted solely mammary tumors. These findings argue that the E4-O RF1 oncoprotein is the major oncogenic determinant of Ad9 and that an undef ined regulatory element(s) within the E4 region represents a previously uni dentified second function likewise necessary for tumorigenesis by this viru s.