One of the most exciting recent advances in understanding signal transduction by membrane receptors is the identification of the JAK family of tyrosine kinases as receptor-associated signaling molecules for growth hormone (GH) receptors and the many other members of the cytokine/hematopoietin receptor superfamily. Upon GH binding, GH-receptor-associated JAK2 is activated and phosphorylates tyrosines within JAK2 itself and GH receptor. These phosphotyrosines form binding sites for a variety of signaling molecules whose recruitment to and/or activation by JAK2-GH receptor complexes initiates the signaling pathways that ultimately lead to the diverse physiological response to GH. Some signaling molecules bind to phosphotyrosines within GH receptor. However, little is known about the signaling molecules that bind to phosphotyrosines within JAK2, despite the presence of multiple (49) tyrosines within JAK2. The yeast 2-hybrid system was used to identify a new splicing variant of SH2-B (SH2- Bbeta) as a JAK2 binding protein. In response to GH, SH2-Bbeta binds to JAK2 and is tyrosyl phosphorylated. Prelim studies also suggest that SH2-Bbeta stimulates JAK2 autophosphorylation and mediates GH induced changes in actin polymerization.
The aim of this proposal is to test the hypothesis that SH2-Bbeta functions both as an activator of JAK2 and as an adapter molecule that recruits to activated JAK2-GH receptor complexes signaling molecules that regulate the cytoskeleton. Specifically, the mechanism by which JAK2 binds to and tyrosyl phosphorylates SH2- Bbeta will be elucidated. Regions of JAK2 and SH2-Bbeta required for JAK2-SH2-Bbeta complex formation and tyrosines within SH2- Bbeta that are phosphorylated by JAK2 in response to GH will be determined. The ability of SH2-Bbeta to enhance GH-stimulated JAK2 activity will be examined. Signaling molecules that bind to both the proline-rich, N-terminal half of SH2-Bbeta and to the phosphorylated tyrosines in SH2-Bbeta will be identified. Finally, cellular responses that lie downstream of SH2-Bbeta will be identified, with an initial emphasis on identifying pathways that may mediate GH- induced changes in the cytoskeleton. These studies will provide insight into the role of SH2-Bbeta in the function of GH and other cytokines, hormones and growth factors (e.g. interferon-gamma, insulin, platelet-derived growth factor) that also utilize SH2-Bbeta as a signaling molecule. Such insight is relevant to understanding the mechanisms by which GH regulates body growth and metabolism, and other SH2-Bbeta activating ligands contribute to, prevent and/or alleviate symptoms of a variety of diseases, including various cancers, diabetes, multiple sclerosis, various developmental abnormalities, and diseases of the immune and hematopoietic systems.
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