Co-operative effect of c-Src tyrosine kinase and Stat3 in activation of hepatocyte growth factor expression in mammary carcinoma cells

We have previously shown coexpression of hepatocyte growth factor (HGF) and its receptor Met in the invasive tumor front of human breast carcinomas. W e have also demonstrated secretion of HGF, constitutive activation of Met, and increased invasion in a murine breast carcinoma cell line, SP1. These o bservations suggest the presence of an HGF autocrine loop in some breast ca rcinoma cells, which confers increased survival, growth, and invasiveness d uring tumor progression and metastasis. c-Src tyrosine kinase, which is cri tical in regulating the expression of many genes, is activated in SP1 carci noma cells, as well as in most human breast cancers. We therefore examined the role of c-Src kinase in HGF expression in breast carcinoma cells. Expre ssion of activated c-Src in SP1 cells increased transcription from the HGF promoter and expression of HGF mRNA and protein, while dominant negative c- Src had the opposite effect. Using deletion analysis, we showed that the re gion between -254 and -70 base pairs was required for c-Src responsiveness of the HGF promoter. This region contains two putative consensus sequences (at -110 and -149 base pairs) for the Stat3 transcription factor, which bin d protein complexes containing Stat3 (but not Stat1, -5A, or -5B), Coexpres sion of activated c-Src and Stat3 synergistically induced strong HGF promot er activity in SP1 cells, as well as in a nonmalignant epithelial cell line , HC11 (HGF negative). c-Src kinase activity correspondingly increased the tyrosine 705 phosphorylation and DNA binding affinity of Stat3 (but not Sta t1, -5A, or -5B). Collectively, our data indicate a cooperative effect of c -Src kinase and Stat3 in the activation of HGF transcription and protein ex pression in breast carcinoma cells, This process may be important in overri ding the strong repression of HGF expression in nonmalignant epithelium, an d thereby promote tumorigenesis.