SRC family protein tyrosine kinases and the cell cycle

We are interested in the roles that Src family protein tyrosine kinases play during the cell cycle. Previous work has shown that Src family kinases are activated as cells exit quiescence in response to some growth factors. Using microinjection to introduce catalytically inactive, dominant negative, forms of Src and Fyn, as well as an antibody that neutralises Src, Fyn and Yes, we have shown that Src family kinases are required for DNA synthesis in response to several growth factors. In general, those growth factors that we have examined (including PDGF, CSF-1 and EGF) whose receptors are tyrosine kinases act through Src family kinases. However, growth factors whose receptors are coupled to G proteins (LPA and bombesin) do not have an absolute requirement for Src family kinases. We have investigated the role of Src family kinases in the response to PDGF, CSF-1 and EGF in more detail. We find that the block to DNA synthesis induced by dominant negative Src can be rescued by heterologous expression of Myc, but not Fos and Jun. In contrast, the inhibition caused by dominant negative Ras can be overcome by Fos and Jun, but not by Myc. These findings suggest that Src and Ras are involved in separate PDGF-initiated signal transduction pathways, both of which are required for DNA synthesis. We are currently elaborating how the activation of Src family kinases results in the transcriptional activation of Myc. Src is transiently activated as fibroblasts enter mitosis. We find that also Fyn and Yes, two other members of Src family present in fibroblasts, are also activated during mitosis. We used the microinjection technique to show that a signal transduction cascade, involving Src family kinases, controls entry into mitosis. For this, the SH2 domain of Src is required, and we are currently investigating this pathway in more detail by identifying G2 specific, SH2 domain binding proteins.