Elevated expression of the junB proto-oncogene is essential for v-fos induced transformation of Rat-1 cells

We previously described the isolation of non-tumorigenic revertants from mutagenized populations of v-fos-transformed Rat-1 cells (Zarbl et al., 1987). In the present study we examined the possibility that the revertant phenotype resulted from mutations that altered the expression or activities of the c-jun or junB proto-oncogenes. The results demonstrated that levels of the c-jun mRNA and protein were unchanged in the revertants when compared to the transformed parental cells, and ectopic overexpression of c-jun failed to retransform the revertants. Although one mutant allele was detected in revertant EMS-1-19, overexpression of this mutant allele failed to inhibit v-fos induced cell transformation. Together these results indicated that the revertant phenotype did not result from altered expression or mutations in the c-jun gene. In contrast to the results obtained with c-jun, the levels of junB mRNA and protein were found to be reduced two- or threefold in revertant EMS-1-19. Ectopic overexpression of junB induced transformation of revertant EMS-1-19, but failed to transform Rat-1 cells. Moreover, about 10% of v-fos transformed cells transfected with vectors that express antisense junB mRNA acquired a non-transformed phenotype. Together these results indicate that expression of junB above a threshold level is essential for v-fos-induced transformation of Rat-1 fibroblasts.