Stimulation of mitogen-activated protein kinase pathway in rat somatotrophs by growth hormone-releasing hormone

Growth hormone-releasing hormone (GHRH) is an important regulator of somatotroph development and function. However, GHRH signaling is still not completely understood. Signaling through the mitogen-activated protein kinase (MAPK) pathway has been observed in a wide variety of cell types but has not been explored as a mediator of GHRH action. In this study, we examined the phosphorylation of MAPK pathway intermediates in response to GHRH. After treatment of the GH4 rat somatotroph cell line with rGHRH (10⁷ M) for 2.5 min, there was robust phosphorylation of MAPK not seen in vehicle-treated cells. Treatment of HeLa cells with GHRH resulted in no activation of MAPK, but activation was conferred by transfection with the GHRH receptor cDNA. MAPK activation by GHRH was dose dependent from 1 to 100 nM, was evident at 2.5 min, peaked at 5 min, and returned to baseline by 20 min. Pretreatment of GH4 cells with somatostatin analog BIM23014 or the MEK1 inhibitor PD98095 prevented the activation of MAPK. Finally, treatment with GHRH increased GH4 proliferation in culture, and this response was prevented by pretreatment with BIM 23014 and PD98095. These results indicate that GHRH activates the MAPK pathway. Furthermore, activation of MAPK may mediate, at least in part, the effects of GHRH on somatotroph cell line proliferation. The findings support the concept that multiple pathways mediate the effects of GHRH.