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Growth hormone-induced tyrosine phosphorylation of EGF receptor as an essential element leading to MAP kinase
1Third Department of Internal Medicine, Faculty of Medicine, University of Tokyo, Japan.
Abstract:
GH binding to its receptor, which belongs to the cytokine receptor superfamily, activates Janus kinase (JAK) 2 tyrosine kinase, thereby activating a number of intracellular key proteins such as STAT (signal transducers and activators of transcription) proteins and mitogen-activated protein (MAP) kinases, which finally lead to GH's biological actions including gene expression. In contrast to receptor tyrosine kinases, the signalling pathways leading to MAP kinase activation by GH are poorly understood but appear to involve Grb2 and Shc. We now show that GH stimulated tyrosine phosphorylation of epidermal growth factor receptor (EGFR) and its association with Grb2, and concomitantly stimulated MAP kinase activity in liver, a major target tissue. Expression of EGFR and its mutants into CHO-GH receptor (GHR) cells revealed that GH-induced full activation of MAP kinase and c-fos expression required tyrosine phosphorylation sites of EGFR but not its intrinsic tyrosine kinase activity. Moreover, by also using dominant negative JAK2 and in vitro kinase assay, we demonstrated that tyrosine 1068 of EGFR was evidently one of the major phosphorylation and Grb2 binding sites stimulated by GH via JAK2. These data suggest that the role of EGFR in GH signalling is to be phosphorylated by JAK2, thereby providing docking sites for Grb2 and activating MAP kinases and gene expression. This novel cross talk pathway may provide the first example of the hormone and cytokine receptor superfamily transducing signals via associated nonreceptor tyrosine kinase by phosphorylating growth factor receptor and utilizing it as a docking protein independent of its receptor tyrosine kinase activity.
Insights
Growth hormone (GH) signaling activates MAP kinase through epidermal growth factor receptor (EGFR) phosphorylation by JAK2. This novel pathway reveals EGFR acts as a docking protein, independent of its kinase activity, for GH-induced gene expression.
Area of Science:
- Endocrinology and Signal Transduction
- Molecular Cell Biology
Background:
- Growth hormone (GH) receptor activation initiates intracellular signaling cascades, including JAK2-STAT and MAP kinase pathways, mediating GH's biological effects.
- The precise mechanisms by which GH activates MAP kinases, particularly involving Grb2 and Shc, remain incompletely understood, contrasting with receptor tyrosine kinase pathways.
Purpose of the Study:
- To elucidate the role of epidermal growth factor receptor (EGFR) in GH-induced mitogen-activated protein (MAP) kinase activation and gene expression.
- To investigate the involvement of Janus kinase 2 (JAK2) in the phosphorylation and activation of EGFR within the GH signaling pathway.
Main Methods:
- Stimulation of GH receptor signaling in liver tissue and cell lines (CHO-GHR cells) to assess EGFR tyrosine phosphorylation and Grb2 association.
- Expression of wild-type and mutant EGFR in CHO-GHR cells to determine the necessity of EGFR tyrosine phosphorylation sites and kinase activity for MAP kinase activation.
- Utilized dominant-negative JAK2 and in vitro kinase assays to identify JAK2 as the kinase responsible for EGFR phosphorylation at specific sites.
Main Results:
- GH stimulation induced tyrosine phosphorylation of EGFR and its association with Grb2, leading to increased MAP kinase activity in liver.
- GH-induced MAP kinase activation and c-fos expression were dependent on EGFR tyrosine phosphorylation sites but not its intrinsic kinase activity.
- JAK2 was identified as the key kinase phosphorylating EGFR at tyrosine 1068, facilitating Grb2 binding and subsequent MAP kinase activation.
Conclusions:
- EGFR functions as a docking protein in GH signaling, becoming phosphorylated by JAK2 to recruit Grb2 and activate MAP kinases, independent of EGFR's own kinase function.
- This study reveals a novel cross-talk mechanism where a cytokine receptor superfamily member (GH receptor) signals through an associated non-receptor tyrosine kinase (JAK2) to phosphorylate and utilize a growth factor receptor (EGFR) for signal transduction.