Coexpression of c-kit and stem cell factor in breast cancer results in enhanced sensitivity to members of the EGF family of growth factors

Kit, a tyrosine kinase growth factor receptor, and its ligand, stem cell fa ctor (SCF), are commonly coexpressed in breast cancer. We have previously s hown 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. M CF7 transfectants expressing Kit, cultured in serum-free medium supplemente d with EGF, displayed more than twice the growth of controls at identical E GF concentrations. Similar responses were seen in the presence of heregulin alpha. The specificity of the Kit-mediated response was illustrated by a r eduction in heregulin-stimulated growth in the presence of a monoclonal ant ibody directed against the Kit receptor. In addition, EGF- and heregulin-st imulated growth of the ZR75-1 cell line that naturally coexpresses Kit and SCF was also inhibited by the Kit blocking antibody. Preliminary investigat ions into the signal transduction pathways activated by these growth factor s revealed that SCF activated both the Ras-MAP kinase and phosphatidyl-inos itol-3-kinase (PI3 kinase) pathway. Both EGF and heregulin activated MAPK b ut to a lesser degree than SCF, and combination of SCF with these growth fa ctors resulted in enhanced MAPK activation. Assessment of PI3K pathway acti vation using anti-phospho-Akt antibodies revealed that EGF was a poor activ ator of Akt; activation of this pathway was markedly enhanced by the additi on of SCF. Heregulin activated Akt and addition of SCF provided no further activation. Taken together these results suggest that coexpression of SCF a nd Kit may enhance responsiveness to erbB ligands by enhancing activation o f the MAPK and PI3K pathways.