Positive autoregulation of ras genes expression in fibroblasts

Abstract

We have studied the effect of ectopic overexpression of a ras gene on the expression of the other two members of the ras gene family. We obtained NIH3T3 cell lines stably transfected with inducible H- ras and N- ras oncogenes. The expression of these genes is driven by a glucocorticoid-responsive promoter and the addition of dexamethasone resulted in a dramatic induction (10–20-fold) of H- or N- ras mRNA, peaking 4 h after hormone addition. The induction of the expression of ras oncogenes resulted in a transformed phenotype. In quiescent NIH3T3 cells transfected with inducible H- ras oncogenes, the induction of H-Ras was followed 12 h later by a 3-fold increase in the mRNA expression of endogenous K- ras and N- ras. Similarly, in NIH3T3 transfected with inducible N- ras oncogene, the induction of N- ras was followed by an increase in the expression of endogenous K- and H- ras genes. Interestingly, the effect was not limited to the mutated N- ras, as a similar result was obtained in cells transfected with N- ras proto-oncogene. The induction of ras genes expression was not linked to cell cycle progression as it was reproduced in cells arrested in S-phase by pretreatment with hydroxyurea. These results suggest the presence of a positive cross-regulation in the expression among the members of the Ras family. This effect could play a role in Ras-mediated carcinogenesis.