V-SRC ACTIVATES MITOGEN-RESPONSIVE TRANSCRIPTION FACTOR EGR-1 VIA SERUM RESPONSE ELEMENTS

Activating the protein-tyrosine kinase activity of v-Src in murine fibroblasts leads to increased expression of Egr-1, a mitogen-responsive transcription factor. v-Src-induced expression of Egr-1 is independent of protein synthesis and is controlled at the level of transcription. Target sequences responsive to v-Src-induced signals were investigated using deletion mutant analysis of the Egr-1 promoter. Upstream Egr-1 promoter sequences linked to a reporter gene were cotransfected with a v-Src expression vector into NIH 3T3 cells. v-Src-enhanced gene expression from the Egr-1 promoter was dependent upon the presence of CC(A/T)6GG elements. The CC(A/T)6GG motif forms the core element of serum response elements (SREs) and is the binding site for serum response factor. The Egr-1 promoter sequences responsive to v-Src contained four SREs. Sequential deletion of these SREs reduced v-Src responsiveness to basal transcription levels. A single SRE from this region was able to confer v-Src responsiveness to a heterologous promoter, and a mutation to the CC(A/T)6GG box of this SRE abolished v-Src-enhanced gene expression. Thus, an early response of v-Src-induced intracellular signaling is the transcriptional activation of a growth factor-responsive transcription factor via an SRE.