Activating Ras mutations fail to ensure efficient replication of adenovirus mutants lacking VA-RNA

Adenoviruses lacking their PKR-antagonizing VA RNAs replicate poorly in primary cells. It has been suggested that these virus recombinants still replicate efficiently in tumor cells with Ras mutations and might therefore be useful in tumor therapy. The ability of interferon-sensitive viruses to grow in Ras-mutant tumor cells is generally ascribed to a postulated inhibitory effect of mutant Ras on PKR. We have constructed a set of isogenic adenoviruses that lack either or both VA RNA species, and tested virus replication in a variety of cell species with different Ras status. In tendency, VA-less viruses replicated with higher efficiency in Ras-mutant cells, as compared to cell lines without Ras mutation. However, several exceptions to this rule were observed, arguing against a direct inhibition of PKR by mutant Ras. Phosphorylation of the PKR-substrate eIF2alpha was observed regardless of the Ras mutational status, upon infection with VA-less adenoviruses in the presence of interferon, but also upon addition of the PKR activator polyIC to cells. When comparing two isogenic cell lines that differ solely with regard to the presence or absence of mutant Ras, no difference was observed concerning the replication of VA-less adenoviruses or the phosphorylation of eIF2alpha. We conclude that mutant Ras does not directly affect eIF2alpha phosphorylation or the replication of interferon-sensitive adenoviruses. These results strongly suggest that the Ras mutational status is insufficient to predict the oncolytic effect of interferon-sensitive viruses. We propose that Ras mutations predispose tumor cells to undergo secondary changes that sometimes enable the replication of interferon-sensitive viruses.