Wild-type adenovirus type 5 transforming genes function as transdominant suppressors of oncogenesis in mutant adenovirus type 5 transformed rat embryo fibroblast cells

Abstract

Transformation of cloned rat embryo fibroblast (CREF) cells with the host-range adenovirus type 5 (Ad5) mutant, H5hr1, results in transformants with a fibroblastic morphology which displays a cold-sensitive transformation phenotype and oncogenic potential in both nude mice and syngeneic rats. In contrast, wild-type (wt) Ad5 transformed CREF cells are epithelioid in morphology, temperature independent for transformation, and nontumorigenic. The present studies were conducted to analyze the contribution of the mutated E1A and E1B regions of H5hr1 in regulating the biological properties of H5hr1-transformed CREF cells. CREF cells were constructed which contain the mutated E1A and E1B transforming regions of H5hr1 and either a wt Ad5 E1A gene, a wt Ad5 E1B gene, or both a wt Ad5 E1A and a wt E1B gene. A wt Ad5 E1A gene was sufficient in reversing the cold-sensitive transformation phenotype. By using a wt Ad5 E1A gene under the transcriptional control of a dexamethasone-inducible mouse mammary tumor virus promoter, a direct suppressive effect of wt Ad5 E1A on colony formation in monolayer culture and agar growth of H5hr1-transformed cells was demonstrated. Expression of a wt Ad5 E1A, a wt Ad5 E1B, or both wt transforming genes in H5hr1-transformed CREF cells also suppressed oncogenicity. The ability or inability to form tumors in animals was found not to correlate with sensitivity to natural killer cell-mediated lysis. These results indicate that both the wt Ad5 E1A and wt Ad5 E1B genes can function as dominant suppressors of the oncogenic process when coexpressed in H5hr1-transformed CREF cells. This effect does not require large quantities of wt Ad5 E1A or E1B transforming proteins, nor is it directly related to the acquisition of a natural killer cell cytolysis-susceptible phenotype.