The class of mammalian biologically active polypeptides called growth factors influence the proliferation, differentiation, motility, maintenance and apoptosis of target cells. On-going studies in my laboratory are directed towards understanding the mechanism of action and biological role of growth factors which signal through the ErbB family of receptors . To address these issues, we utilize techniques common to protein biochemistry, cell biology and molecular biology. The ErbB family of receptors which include the epidermal growth factor receptor (EGFR), ErbB2, ErbB3 and ErbB4 mediate the biological actions of a family of growth factors which are structurally related to EGF. Members of this family of mitogens such as EGF and amphiregulin require the presence of the EGFR on cells for signaling, whereas signal transduction by the heregulins (HRGs; neu differentiation factor, neuregulin, acetylcholine receptor-inducing activity, glial growth factor) is initiated via an interaction with either ErbB3 or ErbB4. Recently, we have studied AR and EGF mitogenic signaling in EGFR-devoid NR6 fibroblasts which ectopically express either wild type EGFR (WT) or a truncated EGFR which lacks the three major sites of autophosphorylation (c'1000). COOH-terminal truncation of the EGFR significantly impairs the ability of AR to (i) stimulate DNA synthesis, (ii) elicit Elk-1 transactivation, and (iii) generate sustained enzymatic activation of mitogen-activated protein kinase. EGFR truncation had no significant effect on AR binding to receptor, but did result in defective GRB2 adaptor function. In contrast, EGFR truncation did not impair EGF mitogenic signaling and in c'1000 cells EGF was able to stimulate the association of ErbB2 with GRB2 and SHC. Elk-1 transactivation was monitored when either ErbB2 or a truncated dominant-negative ErbB2 mutant (ErbB2 1-813) was overexpressed in cells. Overexpression of full length ErbB2 resulted in strong constitutive transactivation of Elk-1 in c'1000, but only slightly stimulated Elk-1 in WT or parental NR6 cells. Conversely, overexpression of ErbB2 1-813 inhibited EGF-stimulated Elk-1 transactivation in c'1000, but not in WT cells. Thus, the cytoplasmic tail of the EGFR plays a critical role in AR mitogenic signaling, but is dispensible for EGF, since EGF-activated truncated EGFRs can signal through ErbB2. The laboratory is currently focusing on two important projects. The first project is attempting to understand the molecular basis for EGF-induced mitogenic signaling by a kinase-inactive EGFR. The second project is directed towards understanding how the protein-tyrosine phosphatase SHP-2 functions as a positive mediator of ligand-stimulated activation of mitogen-activated protein kinase (Map Kinase) by the ErbB receptors.
