TRANSFORMATION BY RAS MODIFIES AP1 COMPOSITION AND ACTIVITY

The Ras proteins play a central role in regulating cell growth and the ir mutation can lead to abnormal proliferation. To analyse the potenti al link betwen AP1 activity, encoded by members of the jun and fos gen e families, and Ras-mediated cellular transformation, we have studied several NIH3T3 clones which overexpress the Ha-Ras or Ki-Ras oncogenes . These transformed fibroblasts accumulated higher levels of cJun, Jun B, Fra1 and Fra2 proteins relative to their normal counterparts. They also displayed increased AP1 DNA binding activity which was predominan tly composed of cJun and Fra1 containing diners. Following serum stimu lation of Ras clones, the elevated levels of cJun and Fra1 remained st eady,,while the induction of JunB and Fra2 was partially attenuated. M oreover, deregulated Ras signaling resulted in a complete loss of the serum inducibility of cFos and FosB. Ectopic co-expression of cJun and Fra1 in NIH3T3 fibroblasts led to a transformed phenotype, attenuatio n of cFos serum inducibility, increased AP1 activity and Cyclin D1 acc umulation, all characteristics of oncogenic Ras expressing cells. Thes e results demonstrate that cJun and Fra1 are crucial mediators of the Ras-transformation process.