Increasing evidence suggests that the epidermal growth factor receptor (EGFR) and c-Src tyrosine kinase families have critical roles in the development of breast neoplasias. Both families are overexpressed or activated in many breast tumor samples and cell lines. Heterocomplexes have been detected between EGFR (HER-1) and c-Src and between p185neu (HER-2) and c-Src in human breast tumor cell lines and tumor samples. In addition, HER-1 is oncogenic in EGF-stimulated fibroblasts and Src family members are required for EGF-induced mitogenesis. These findings suggest that members of these two families may cooperate during tumor development. The hypothesis was confirmed when overexpression of HER-1 and c-Src in 10T1/2 fibroblasts resulted in a synergistic increase in tumor formation in nude mice. Enhanced tumor formation correlated with heterocomplexes between c-Src and HER-1, novel c-Src-dependent phosphorylations on HER-1 and elevated in vivo tyrosyl phosphorylation of receptor substrates. To study the mechanism of this synergy and how this interaction may be involved in mammary malignancies, it is proposed to examine human breast tumor samples and cell lines for heterocomplex formation between c-Src and HER-1, the presence of novel phosphorylations on the receptor, and enhanced kinase activity. A structure/function analysis of the domains of c-Src and HER-1 that are required for the synergism will be carried out in 10T1/2 cells. Similar studies will be performed with HER-2 and c-Src. Finally, c-Src from a number of tumor samples and cell lines will be examined to determine if established or novel mechanisms of activation are occurring, with the goal of determining whether diagnostic reagents would recognize activated vs. inactivated enzyme.
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