EPIDERMAL-GROWTH-FACTOR-DEPENDENT ACTIVATION OF THE SRC-FAMILY KINASES

The precise role of src-type kinases as signal transducers has been un der intensive investigation but only in a few instances has their role been revealed in any detail. Thus, src, fyn and yes are activated upo n stimulation by platelet-derived growth factor or colony-stimulating factor in cells expressing high levels of these receptors. Activation of src-family kinases by other receptor tyrosine kinases such as the e pidermal-growth-factor (EGF) receptor has not been directly demonstrat ed. In this report, we demonstrate EGF-dependent activation of src-fam ily tyrosine kinases in NIH3T3 cells overexpresssing the human EGF rec eptor. Activation is rapid (<1 min) and persistent (up to 16 h). Furth ermore, we show a correlation between the level of EGF receptor expres sed and the degree of src-family kinase activation. We show that src-f amily kinase activity is also activated by addition of ECF to PC12 cel ls, which endogenously express relatively high levels of EGF receptor. Most strikingly, we show that A431 cells, which endogenously express very high levels of EGF receptor, show 10-fold elevated src-family kin ase activity as compared to DHER14 cells, and that this activity is co nstitutive. This activity is completely blocked by AG1478, a specific inhibitor of the EGF-receptor tyrosine kinase activity, pointing to a direct link between overexpression of the EGF receptor and enhanced sr c-family kinase activity. Our findings suggest that EGF-dependent src- family kinase activity is detectable only when the levels of EGF recep tor reach a specific level. Additionally, high levels of EGF receptor, as in A431 cells, may contribute to the elevated activation of src-fa mily kinases. Sustained src-family kinase activation, similar to that seen in v-src-transformed cells, may play a role in tumorogenesis and tumor maintenance.