Suppression of the progression phenotype in somatic cell hybrids occurs in the absence of altered adenovirus type 5 gene expression

In the present study we have analyzed the genetic regulation of increased expression of transformation-associated traits, a process termed progression, in adenovirus type 5 (Ad5)-transformed secondary rat embryo cells. Somatic cell hybrids were formed between a highly progressed neomycin-resistant Ad5-transformed cloned cell line (E11-NMT^neo) and an untransformed chloramphenicol-resistant rat embryo fibroblast cell line (CREF^cap). Parental E11-NMT^neo cells grew with high efficiency in agar, exhibited reduced ¹²⁵I-epidermal growth factor (EGF) binding, and were tumorigenic in nude mice. Parental CREF^cap cells exhibited phenotypes opposite to those of E11-NMT^neo cells. A high proportion (84%) of the presumptive hybrid cell types obtained after fusion and genetic selection (G418 and chloramphenicol) displayed a flat morphological phenotype intermediate between CREF^cap and E11-NMT^neo cells, suggesting that a trans-dominant extinction phenomenon had occurred. Two hybrids with a round morphology (R), which still exhibited the progressed phenotype, and two hybrids with a flat morphology (F), which had lost the progressed phenotype, were chosen for detailed analysis. Both R hybrids grew efficiently in agar, exhibited low ¹²⁵I-EGF binding, and were tumorigenic in nude mice, whereas both F hybrids grew poorly in agar, displayed increased ¹²⁵I-EGF binding in comparison with E11-NMT^neo and R hybrids, and were nontumorigenic in nude mice. An analysis of the viral DNA integration patterns and the rates of transcription, steady-state mRNA accumulation, and relative levels of the Ad5 E1A and E1B gene products revealed no differences among the parental and hybrid cells. These studies indicate that normal CREF cells may contain a suppressor gene(s) which can inhibit the expression of specific traits of the progression phenotype in Ad5-transformed cells and that this suppression is not associated with changes in the expression of Ad5 transforming genes.