Gonadotropin-releasing hormone (GnRH) is secreted from hypothalamic GnRH neurons and stimulates a GnRH receptor in gonadotroph cells and GnRH neurons.

The receptor for gonadotropin-releasing hormone (GnRH) is highly expressed in hypothalamic GnRH neurons, as well as in anterior pituitary gonadotrophs. In our previous study, we found that stimulation of the GnRH receptor activated protein kinase D1 (PKD1), and PKD1 was involved in the Fyn-mediated activation of proline-rich tyrosine kinase 2 (Pyk2) in cultured GnRH neurons (GT1-7 cells). In the present study, we examined the molecular mechanisms of Pyk2 activation and the interaction of Pyk2 and Fyn in GT1-7 cells. Experiments with site-directed mutants of Pyk2 indicated that tyrosine 402 (Tyr402) was phosphorylated both by autophosphorylation and by Fyn, whereas Tyr579 was phosphorylated mainly by Fyn. We found that dasatinib, a Src family inhibitor, enhanced the interaction of Pyk2 and Fyn. Experiments with site-directed mutants of Pyk2 and Fyn indicated that dasatinib enhanced the binding of Pyk2 autophosphorylated at Tyr402 and the Src homology 2 domain in Fyn. Our present data may suggest that fully activated Pyk2 dissociates from Fyn after Fyn-mediated phosphorylation of Pyk2 at sites other than Tyr402 and Tyr579.

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The receptor for gonadotropin‐releasing hormone (GnRH) is highly expressed in hypothalamic neurons. It has been reported that GnRH treatment of cultured GnRH neurons (GT1‐7 cells) activated proline‐rich tyrosine kinase 2 (Pyk2), and Pyk2 was involved in the activation of extracellular signal‐regulated protein kinase 1 (ERK1) and ERK2 (ERK1/2). In the present study, we first examined the possibility that GnRH treatment might activate epidermal growth factor receptor (EGFR). We found that activation of EGFR after GnRH treatment for 5 min was much less than after EGF or heparin‐binding EGF treatment. Next, we examined whether or not Pyk2 bound to growth factor receptor‐binding protein 2 (Grb2). We overexpressed FLAG‐fused Pyk2 in GT1‐7 cells, and immunoprecipitated Pyk2 using an anti‐FLAG antibody. The binding of Pyk2 to Grb2 was detected only after GnRH treatment. In contrast, a site‐directed mutant of Pyk2 wherein tyrosine 881 was mutated to phenylalanine did not bind to Grb2. Studies with small interfering RNA and inhibitors indicated that the activation of Grb2/Ras/Raf/MEK was a major pathway to ERK1/2 activation after the short‐term treatment of GT1‐7 cells with GnRH.