PLATELET-DERIVED GROWTH-FACTOR INCREASES THE TURNOVER OF GTP GDP ON RAS IN PERMEABILIZED FIBROBLASTS/

The potent mitogen platelet-derived growth factor (PDGF) induced a rap id increase in Ras.GTP in permeabilized human and murine fibroblasts. The effect was initiated by both PDGF-AA acting exclusively through PD GF alpha-receptors, and by PDGF-BB interacting with both alpha- and be ta-type receptors. The dose-response curves suggest that both receptor types mediate the response. PDGF-dependent Ras activation, measured a s increased formation of Ras.GTP, was rapid and reversible. At 37-degr ees-C the effect had a duration of around 10 min. The PDGF-dependent i ncrease in Ras.GTP was followed by a simultaneous increase in Ras.GDP. Under no experimental condition could a relative increase in Ras.GTP be detected. 0.5 muM GDP and 0.5 muM GTP were equally potent competing for the formation of Ras.[alpha-P-32]GTP upon PDGF stimulation. Furth ermore, when the basal nucleotide exchange rate on Ras was elevated by omission of Mg2+ from the medium, PDGF had no further effect on the f ormation of Ras.GTP. We therefore conclude that PDGF activates Ras thr ough a mechanism leading to an increased nucleotide exchange on Ras.