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PDGF-independent activation of PDGF-beta receptors in NIH-3T3 cells transformed by c-met protooncogene.

We have previously reported that c-met protooncogene, a member of a new class of receptor tyrosine-kinase gene family, is transforming when overexpressed in NIH-3T3 cells. In this paper, we report that the c-met protooncogene-transformed cells proliferate in a serum- and growth factor-free medium and exhibit constitutive tyrosine phosphorylation of several cellular proteins including the met protooncogene-encoded p145 and p185. Further investigations revealed platelet-derived growth factor (PDGF)-independent phosphorylation of PDGF-beta receptors in the transformed cells. Phosphoamino acid analysis revealed phosphorylation of PDGF receptors at tyrosine and serine residues. The PDGF receptor phosphorylation is unlikely to occur via autocrine production of PDGF since we could not detect PDGF activity both at the RNA level and at a functional protein level. Additionally, phospholipase C-gamma (PLC-gamma) a substrate of activated PDGF receptors, was found to be physically associated with PDGF receptors in the absence of PDGF stimulation in transformed cells. Furthermore, PDGF receptors coimmunoprecipitated along with PLC-gamma. Taken together, our results demonstrate a PDGF-independent phosphorylation and activation of PDGF-beta receptor in NIH-3T3 cells transformed by c-met protooncogene.