Induction of membrane ruffling by growth factors in morphologically TPA-resistant Balb/c3T3 TR4 cells

To investigate the biological characteristics of a Balb/c3T3 variant TR4 clone which is morphologically resistant to TPA and hypersensitive to v-src induced metastasis, we compared the responsiveness of the variant and its parent cells to growth factor-induced membrane ruffling. When the confluent cells were stimulated with PDGF, membrane ruffling was rapidly induced in TR4 but not in the parent cell cultures. In TR4 cells, membrane ruffling was observed under a phase-contrast microscope within 2 min after the addition of PDGF, reaching the maximum 5 min later and thereafter decreased gradually to the control level. There were no apparent differences in 125I-PDGF binding kinetics between TR4 and parent cells. Similar membrane ruffling was induced by other growth factors such as insulin, IGF-I, acidic or basic FGF but not by EGF or alpha- and beta-TGF, only in TR4 cells. When TR4 cells were incubated with TPA just before stimulation with these growth factors, growth factor-induced membrane ruffling was completely inhibited. Also, 5 out of 6 clones of stable fusion cells between TR4 and parent cells showed the parental type of responses to TPA and growth factors, indicating that the TR4 phenotype is recessive. These results suggest that the variant TR4 cells may acquire the genetic and recessive alteration of a cellular factor which is responsible for the regulation of growth factor-mediated membrane ruffling and that this genetic alteration occurs at a common step downstream of growth factor-mediated cascades, rather than at their receptor level.