Growth hormone-induced alteration in ErbB-2 phosphorylation status in 3T3-F442A fibroblasts

The growth hormone receptor (GHR), a cytokine receptor superfamily member, requires the JAK2 tyrosine kinase for signaling. We now examine functional interactions between growth hormone (GH) and epidermal growth factor (EGF) in 3T3-F442A fibroblasts. Although EGF enhanced ErbB-2 tyrosine phosphoryla tion, GH, while causing retardation of its migration on SDS-polyacrylamide gel electrophoresis, decreased ErbE-2's tyrosine phosphorylation, GH-induce d retardation was reversed by treatment of anti-ErbB-2 precipitates with bo th alkaline phosphatase and protein phosphatase 2A suggesting that GH induc ed serine/threonine phosphorylation of ErbB-2. Both GH-induced shift in Erb B-2 migration and GH-induced MAP kinase activation were unaffected by a pro tein kinase C inhibitor but were blocked by the mitogen-activated protein k inase/extracellular signal-regulated kinase kinase 1 (MEK1) inhibitor, PD98 059. Notably, leukemia inhibitory factor, but not interferon-gamma, also pr omoted ErbB-2 shift and mitogen-activated protein kinase activation. Cotrea tment with EGF and GH versus EGF alone resulted in a 35% decline in acute E rbB-2 tyrosine 1248 autophosphorylation, a marked decline (approximately 50 %) in DNA synthesis, and substantially decreased cyclin D1 expression. We c onclude that in 3T3-F442A cells, 1) the GH-induced decrease in ErbB-2 tyros ine phosphorylation correlates with MEK1/mitogen-activated protein kinase a ctivity and 2) GH antagonizes EGF induced DNA synthesis and cyclin D1 expre ssion in a pattern consistent with its alteration in ErbB-2 phosphorylation status.