A common cellular pathway for v‐mos and v‐Ki‐ ras is not required for v‐Ki‐ ras ‐induced tumorigenicity in a nonmalignant, v‐ mos ‐expressing revertant cell

A revertant cell line was selected from Moloney sarcoma virus‐transformed BALB/c cells after long‐term treatment with type I interferon. Despite an actively transcribed and transferable v‐ mos gene, these revertant cells were nontumorigenic in nude mice. The functionality of the mos protein was investigated, focusing on the α2(1) collagen promoter regulation, which is known to be affected by mos ‐induced trans ‐acting factors. Both in transient expression assays and after stable integration into the cellular genome, the transfected α2(1) collagen promoter fused to the cat reporter gene was activated in the revertant while being downregulated in the original transformed cells. In retransformation assays of the revertant by Moloney sarcoma virus strains homologous to the original transforming virus, no detectable change was noted in the in vitro phenotype or in tumorigenicity. These results reveal that the mos ‐directed factors were no longer effective on their specific targets. Thus, the R.MSVIF cell could be either an oncoprotein ‐deficient or a target‐related revertant. Attempts at retransformation with unrelated sarcoma viruses bearing v‐ sis , v‐ fms , or v‐ fos oncogenes were also negative. In contrast, tumorigenicity was obtained with the unrelated Kirsten sarcoma virus without any change in the revertant morphology or collagen expression. These findings showed that the common pathway blocked by the reversion and shared by v‐ mos and v‐ ras was not required for ras ‐induced tumorigenicity.