No requirement for Src family kinases for PDGF signaling in fibroblasts expressing SV40 large T antigen

A growing body of literature suggests that the ubiquitously expressed Src f amily kinases (Src, Fyn and Yes) are required for agents such as platelet-d erived growth factor (PDGF) to stimulate DNA synthesis. Yet Klinghoffer and colleagues recently presented evidence that fibroblasts derived from mice null for Src, Fyn and Yes responded normally to PDGF (Klinghoffer et al,, 1 999, EMBO J,, 18: 2459-2471), What is the reason for this discrepancy? We n oted that Klinghoffer ef al, (1999) used SV40 large T antigen (largeT) to f acilitate derivation of cell lines from the embryos. We therefore tested th e effect of largeT on PDGF receptor signaling, We found that expression of largeT overcame the inhibitory effects of interfering forms of both Ras (N1 7Ras) and Src (SrcK-). Furthermore, injection of SrcK- or the cst.1 antibod y (which inhibits Src, Fyn and Yes) failed to inhibit PDGF-stimulated DNA s ynthesis in NIH3T3 cells expressing dominant negative p53, and fibroblasts derived from p53 null embryos. These data suggest firstly that caution shou ld be used in interpretation of experiments conducted in cell lines express ing largeT, and secondly that the role of Src family kinases in growth fact or signaling may be to oppose the effects of negative growth regulators suc h as p53.