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 epidermal growth factor receptor (EGFR). To address these issues, we utilize techniques common to protein biochemistry, cell biology and molecular biology. Our laboratory has a particular interest in one of the growth factors which signals via the EGFR, called amphiregulin. However, research on amphiregulin has been hindered due to the fact that a biologically active recombinant form of amphiregulin had not been successfully generated. To this end a series of recombinant amphiregulins were produced representing various forms which would arise from differential processing of the amphiregulin precursor. Biochemical and biological analyses of the different amphiregulins revealed a single 87 amino acid residue form with bioactivity which was very comparable to naturally derived growth factor. These results also demonstrated that the original structure of amphiregulin which was published in the journal Science was incorrect. We have also studied the ability of EGF and amphiregulin to activate signal transducers and activators of transcription (STAT) proteins which play a critical role in EGFR-mediated expression of early response genes. We provided the first evidence that (i) EGFR-mediated STAT activation does not require the JAK1 tyrosine kinase; (ii) the intrinsic tyrosine kinase activity of the EGFR is essential for STAT activation, and (iii) tyrosine phosphorylation sites within the EGFR are not required for STAT activation. A recent focus of the laboratory involves the role that the phosphatase PTP1D (SHP-2) plays in EGF-induced mitogenesis. It is known that PTP1D is essential to EGF stimulated DNA synthesis in mammalian cells and data derived from Drosophila implies that membrane targeting of the phosphatase is critical to this function. We have proven that EGF induces coupling of PTP1D to the adaptor protein GRB2 via the COOH-terminal Src homology 3 (SH3) domain of GRB2. This is the first demonstration of a growth factor- or cytokine-induced coupling of a protein through an SH3 domain and suggests that GRB2 functions to target PTP1D, in addition to the nucleotide exchange factor son-of-sevenless (Sos), to the plasma membrane in response to EGF.
