THE RAS-RELATED SMALL GTP-BINDING PROTEIN RHOB IS IMMEDIATE-EARLY INDUCIBLE BY DNA-DAMAGING TREATMENTS

The low molecular weight GTP-binding proteins RhoA, RhoB, and RhoC are characterized as specific substrates for the ADP-ribosyltransferase C3 from Clostridium botulinum and are supposed to be involved in the organization of the microfilamental network and transformation. rhoB is known to be immediate-early inducible by growth factors and protein-tyrosine kinases. Since increasing evidence indicates overlapping of growth factor- and UV-induced signal pathways, we studied the effect of UV light and other genotoxic agents on early rhoB transcription. Within 30 min after UV irradiation of NIH3T3 cells, the amount of rhoB mRNA increased 3-4-fold. Elevated rhoB mRNA was accompanied by an increase in RhoB protein, as detected by C3-mediated [P-32]ADP-ribosylation. The transcription inhibitor actinomycin D prevented the UV-induced increase in rhoB mRNA and proved rhoB mRNA to be unstable with a half life of similar to 20 min. Transcriptional activation of rhoB by UV light was confirmed by run-on analysis. The increase in rhoB mRNA after UV irradiation was prevented by inhibitors of protein kinase A (H9) and C (H7, Go18). The tyrosine kinase inhibitor genistein did not affect UV induction of rhoB. In addition to UV, N-methyl-N-nitrosourea and the cytostatic drug cisplatin evoked rhoB response. Cycloheximide was likewise effective in increasing the amount of rhoB mRNA, whereas Bt(2)cAMP, 12-O-tetradecanoylphorbol-13-acetate, and retinoic acid were without effect. Prior down-regulation of signaling by la 12-O-tetradecanoylphorbol-13-acetate and serum pretreatment reduced UV-stimulated rhoB expression. The data indicate that rhoB represents a novel DNA damage-inducible function involved in early steps of signal transduction upon genotoxic stress.