Identification of JAK family tyrosine kinases as receptor-associated signaling molecules for growth hormone (GH) receptors (GHR) and the many other members of the cytokine receptor superfamily was a major step forward in our understanding of the cellular basis for GH actions. Upon GH binding, GHR-associated JAK2 is activated and phosphorylates tyrosines within JAK2 itself and GHR. These phosphotyrosines form binding sites for a variety of signaling molecules whose recruitment to and/or activation by JAK2-GHR complexes initiates the signaling pathways that lead to the diverse physiological responses to GH. While some signaling molecules bind to phosphotyrosines within GHR, little is known about the signaling molecules that bind to phosphotyrosines within JAK2, despite the presence of 49 tyrosines within JAK2. SH2-B is one of the few known JAK2 binding proteins. In response to GH, SH2-B binds to JAK2, is phosphorylated by JAK2 and enhances GH-induced activation of JAK2. Independent of its ability to activate JAK2, SH2-B enhances GH regulation of the actin cytoskeleton and cell motility. Finally, SH2-B shuttles between the cytoplasm and the nucleus, and enhances the nuclear export of forkhead transcription factors.
The aim of this proposal is to test the hypothesis that SH2-B is a multi-functional adapter/scaffolding protein that enhances GH activation of JAK2, enhances GH regulation of the actin cytoskeleton and regulates GH responses in the nucleus.
Aim 1 lexamines the mechanism by which SH2-B activates JAK2.
Aim 2 will further define the role of SH2-B in GH-mediated changes in the actin cytoskeleton. Multiple proteins implicated in the regulation of the actin cytoskeleton have been tentatively identified as SH2-Bbeta binding proteins. Their binding to SH2-B will be confirmed and the effect of SH2-B on their function and subcellular location determined.
Aim 3 will examine whether SH2-B regulates transcription by shuttling proteins into or out of the nucleus. Finally, Aim 4 will use a variety of protein depletion techniques to identify the physiological role for SH2-B in GH action, including siRNA, SH2-B"""""""" mice, and murine embryo fibroblasts and tissues from SH2-B -/- mice. These studies will provide insight into the role of SH2-B in the function of GH and other cytokines, hormones and growth factors (e.g. interferon-gamma, insulin, nerve growth factor) that also utilize SH2-B as a signaling protein. Such insight is relevant to understanding the mechanism(s) by which GH regulates body growth and metabolism, and other SH2-B activating ligands contribute to, prevent and/or alleviate symptoms of a variety of diseases including various cancers, diabetes, multiple sclerosis, and diseases of the immune system.
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