Abstract
Gonadotropin-releasing hormone (GnRH) is secreted from hypothalamic neurons (GnRH neurons). GnRH neurons have a GnRH receptor belonging to the G-protein-coupled receptors. The stimulation of this receptor activates extracellular signal-regulated kinase (ERK). In the present study, we found that epidermal growth factor receptor (EGFR) and ErbB4 were expressed in immortalized GnRH neurons (GT1-7 cells). AG1478, a relatively specific inhibitor of the ErbB family, and small interfering RNA (siRNA) for ErbB4 inhibited the GnRH-induced activation of ERK in GT1-7 cells, suggesting that EGFR and ErbB4 were necessary for the activation. In addition, GnRH induced the cleavage of ErbB4 and accumulation of an 80-kDa fragment. After treatment of the cells with 50 nM GnRH for 5 min, about 80% of ErbB4 was cleaved. Biotinylation of cell surface proteins revealed that more than 70% of the cell surface ErbB4 was cleaved by GnRH treatment. A higher concentration and longer treatment were necessary for GnRH to induce ErbB4 cleavage than ERK activation. TAPI-2, an inhibitor of tumor necrosis factor-α-converting enzyme (TACE), and siRNA for TACE inhibited the cleavage of ErbB4, suggesting that TACE was involved. After ErbB4 cleavage, the activation of ERK by neuregulin 1 was almost completely inhibited. These results suggest that the down-regulation of ErbB4 expression is induced by G-protein-coupled receptor stimulation.