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Extracellular Signal-Regulated Kinase 1/2 Activation by Myometrial Oxytocin Receptor Involves G_qGß and Epidermal Growth Factor Receptor Tyrosine Kinase Activation

Department of Biochemistry and Molecular Biology, University of Texas Medical School Houston, Houston, Texas 77030

Address all correspondence and requests for reprints to: Barbara M. Sanborn, Ph.D., Department of Biomedical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523. E-mail: Barbara.Sanborn{at}colostate.edu.

The mechanisms by which oxytocin (OT) stimulates extracellular signal-regulated kinase 1/2 (ERK1/2) are only partially understood. OT receptor (OTR) signals predominantly through G_q, but ERK1/2 phosphorylation (ERK1/2-P) in PHM1 myometrial cells was not eliminated by inhibition of downstream effectors such as phospholipase C or protein kinase C. Inconsistent with a G_i-coupled response, pertussis toxin inhibition of OT-induced ERK1/2-P was reversed by the protein kinase A inhibitors Rp-cAMPS and KT5720. Consistent with an inhibitory role for protein kinase A, pertussis toxin pretreatment raised cellular cAMP and 8-(4-chlorophenylthio)-cAMP inhibited OT-induced ERK1/2-P. Attenuation of the OT response by the Gß scavenger carboxyl terminus of the ß-adrenergic receptor kinase implicated a Gß-mediated pathway. In both COSM6 cells overexpressing OTR (OTR-COSM6) and in PHM1 cells, the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor AG1478 markedly reduced OT-induced ERK1/2-P, whereas the platelet-derived growth factor receptor tyrosine kinase inhibitor AG1296 had no effect. Furthermore, OT increased EGFR tyrosine phosphorylation in OTR-COSM6 cells, which was inhibited by AG1478 or EGTA plus thapsigargin pretreatment. AG1478 did not affect inositol 1,4,5-triphosphate production by OT or protein kinase C-stimulated ERK1/2-P but completely blocked ionomycin-induced ERK1/2-P and EGFR tyrosine phosphorylation. In both OTR-COSM6 and PHM1 cells, EGTA reduced OT-stimulated ERK1/2-P; no ERK1/2-P was observed when intracellular calcium increases were blocked by pretreatment with thapsigargin plus EGTA. These data are consistent with activation of a Gß-mediated pathway as a consequence of G_q activation in myometrium and OTR-COSM6 cells that results in increased ERK1/2-P. This pathway involves both EGFR activation and an influence of calcium.

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