Growth hormone-induced tyrosine phosphorylation of EGF receptor as an essential element leading to MAP kinase activation and 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.