Kit, a tyrosine kinase growth factor receptor, and its ligand, stem cell factor (SCF), are commonly coexpressed in breast cancer. We have previously shown that MCF7 cells (that naturally express SCF) transfected with a c-kit expression vector exhibit enhanced growth in serum-free medium supplemented with IGF-1. Consequently, we wished to examine the interaction of Kit/SCF with additional growth factors important in the biology of breast cancer. MCF7 transfectants expressing Kit, cultured in serum-free medium supplemented with EGF, displayed more than twice the growth of controls at identical EGF concentrations. Similar responses were seen in the presence of heregulin alpha. The specificity of the Kit-mediated response was illustrated by a reduction in heregulin-stimulated growth in the presence of a monoclonal antibody directed against the Kit receptor. In addition, EGF- and heregulin-stimulated growth of the ZR75-1 cell line that naturally coexpresses Kit and SCF was also inhibited by the Kit blocking antibody. Preliminary investigations into the signal transduction pathways activated by these growth factors revealed that SCF activated both the Ras-MAP kinase and phosphatidyl-inositol-3-kinase (PI3 kinase) pathway. Both EGF and heregulin activated MAPK but to a lesser degree than SCF, and combination of SCF with these growth factors resulted in enhanced MAPK activation. Assessment of PI3K pathway activation using antiphospho-Akt antibodies revealed that EGF was a poor activator of Akt; activation of this pathway was markedly enhanced by the addition of SCF. Heregulin activated Akt and addition of SCF provided no further activation. Taken together these results suggest that coexpression of SCF and Kit may enhance responsiveness to erbB ligands by enhancing activation of the MAPK and PI3K pathways.