We have previously shown coexpression of hepatocyte growth factor (HGF) and its receptor Met in the invasive tumor front of human breast carcinomas. We have also demonstrated secretion of HGF, constitutive activation of Met, and increased invasion in a murine breast carcinoma cell line, SP1. These observations suggest the presence of an HGF autocrine loop in some breast carcinoma cells, which confers increased survival, growth, and invasiveness during tumor progression and metastasis. c-Src tyrosine kinase, which is critical in regulating the expression of many genes, is activated in SP1 carcinoma 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. Expression of activated c-Src in SP1 cells increased transcription from theHGFpromoter and expression of HGF mRNA and protein, while dominant negative c-Src had the opposite effect. Using deletion analysis, we showed that the region between −254 and −70 base pairs was required for c-Src responsiveness of theHGFpromoter. This region contains two putative consensus sequences (at −110 and −149 base pairs) for the Stat3 transcription factor, which bind protein complexes containing Stat3 (but not Stat1, -5A, or -5B). Coexpression of activated c-Src and Stat3 synergistically induced strongHGFpromoter 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 Stat1, -5A, or -5B). Collectively, our data indicate a cooperative effect of c-Src kinase and Stat3 in the activation ofHGFtranscription and protein expression in breast carcinoma cells. This process may be important in overriding the strong repression ofHGFexpression in nonmalignant epithelium, and thereby promote tumorigenesis.
Partial purification and characterization of epidermal growth factor in human breast milk.
