Many important biological processes are regulated by tyrosyl phosphorylation, which is controlled by the opposing actions of protein-- tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs). Abnormal regulation of these pathways can lead to diseases such as cancer. A complete understanding of cellular and whole organism regulation by tyrosyl phosphorylation requires defining how specific PTKs and PTPs interact. Such understanding may lead to the development of new, more specific therapeutic reagents for treating human disease. The long range goal of this research program is to understand the biological functions and mechanism of action of the SH2 domain-containing tyrosine phosphatase SHP2 in hematopoietic cells. Previous studies by several laboratories, including our own, established SHP2 as a vital component of signaling pathways downstream of cytokine receptors, hematopoietic growth factor receptors and multi- chain immune recognition receptors. These studies suggested that a 97kD phosphotyrosyl protein was a major target/regulator of SHP2 action, but the identity of this protein and its functions had remained unclear. During the initial funding period, we identified, purified and cloned p97. p97 is a relative of Gab1 and Drosophila Dos, a regulator of the fly homolog of SHP-2; thus, we renamed p97 as Gab2. Analyses of the effects of wild type and mutant versions of Gab2 suggest that it is a key regulator of cytokine signal transduction and also participates in signaling downstream of other types of hematopoietic cell receptors. Our studies establish that SHP2 acts at more than one step in cytokine receptor signal transduction: one requires Gab2 binding and the other is Gab-2 independent. The Ciab2-independent pathway is required for MAP kinase activation, whereas Gab2, acting via SHP2, regulates a novel signaling pathway downstream of/or parallel to MAP kinase activation but upstream of cytokine-induced gene expression. Our recent studies show that Gab2 is recruited to cytokine receptors via a novel complex involving Shc, and then provides the major route to PI-3 kinase activation for those receptors that do not bind PI3K directly. We will elucidate the details of the Shc/Gab2 pathway in cytokine signaling. and determine what other pathways are regulated by Gab2/PI3K complexes. Next, we will define how SHP2 regulates MAP kinase activation in a Gab-2- independent manner and how the Gab2/SHP2 complex regulates immediate early gene transcription. We will characterize the hematopoietic effects of """"""""true-null"""""""" SHP2 knockout mice and Gab2-/- mice to determine the physiological function of these molecules in vivo. These studies should yield new insights into how tyrosyl phosphorylation is controlled. clarify how specific PTPs contribute to regulation, and identify a new signaling pathway regulating cytokine-induced gene expression. Since abnormal regulation of cytokine signaling is implicated in leukemogenesis and some carcinomas, our studies may also suggest new targets for therapeutic intervention in human neoplastic disease.
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