Functional dissection of transformation by c-Src and v-Src

The c-src proto-oncogene product, c-Src, is frequently over-expressed and activated in various human malignant cancers, implicating a role for c-Src in cancer progression. To verify the role of c-Src, we analyzed the transforming ability of c-Src in mouse embryonic fibroblasts that lack Csk, a negative regulator of Src family kinases. Although Csk deficiency is not sufficient for cell transformation, c-Src over-expression induced characteristic transformed phenotypes including anchorage-independent growth and tumorigenecity. These phenotypes were dose-dependently inhibited by the re-expression of Csk, indicating that there is a certain threshold for c-Src transformation, which is determined by the c-Src : Csk ratio. In contrast to v-Src, c-Src induced the phosphorylation of a limited number of cellular proteins and elicited a restricted change in gene expression profiles. The activation of some critical targets for v-Src transformation, such as STAT3, was not significantly induced by c-Src transformation. Several genes that are involved in cancer progression, that is, cyclin D1 and HIF-1 alpha, were induced by v-Src, but not by c-Src. Furthermore, v-Src tumors exhibited aggressive growth and extensive angiogenesis, while c-Src tumors grew more slowly accompanied by the induction of hematomas. These findings demonstrate that c-Src has the potential to induce cell transformation, but it requires coordination with an additional pathway(s) to promote tumor progression in vivo.