Identification of a hepatocyte growth factor autocrine loop in a murine mammary carcinoma

Constitutive activation of growth factor receptors through autocrine/paracrine mechanisms occurs frequently in human cancers and is thought to play an important role in carcinogenesis. We have demonstrated previously that hepatocyte growth factor (HGF) is a potent mitogenic factor for murine mammary carcinoma (SP1) cells in vitro. We report here an autocrine HGF loop in SP1 cells. HGF receptor/Met is expressed in SP1 cells and is constitutively tyrosine phosphorylated. The phosphorylation of HGF receptor/Met is inhibited when cells are exposed to suramin or anti-HGF IgG. This finding suggests that constitutive tyrosine phosphorylation of HGF receptor/Met is sustained by an extracellular factor, most likely HGF. Using Northern blot and Western blot analysis, we detected expression of a 6-kb HGF mRNA in SP1 cells and a M(r) 85,000 HGF protein in SP1-conditioned medium, respectively. In vitro translation of mRNA from SP1 cells and metabolic labeling confirmed expression and synthesis of HGF by SP1 cells. SP1 cells also invade through Matrigel-coated transwell membranes in an in vitro invasion assay, and invasion of these cells was inhibited by neutralizing anti-HGF IgG. In addition, SP1-conditioned medium induced scatter activity of Madin-Darby canine kidney epithelial cells, and this activity was inhibited by neutralizing anti-HGF IgG. We have also shown that several signaling molecules including phosphatidylinositol 3-kinase, Src, focal adhesion kinase, and phospholipase C-gamma in SP1 cells are constitutively tyrosine phosphorylated, suggesting that coexpression of HGF and HGF receptor/Met may in part contribute to sustained tyrosine phosphorylation of these cytoplasmic proteins in SP1 cells. Our observations in the SP1 model suggest that HGF contributes to growth and invasive phenotypes of mammary carcinomas via both paracrine and autocrine mechanisms.