Activation of the luteinizing hormone β promoter by gonadotropin-releasing hormone requires c-Jun NH2-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by gonadotropin-releasing hormone (GnRH) in the gonadotrope cell line L beta T2 was investigated. Treatment with gonadotropin-releasing hormone agonist (GnRHa) activates extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK), Activation of ERK by GnRHa occurred within 5 min, and declined thereafter, whereas activation of JNK by GnRHa occurred with a different time frame, i.e. it was detectable at 5 min, reached a plateau at 30 min, and declined thereafter. GnRHa-induced ERK activation was dependent on protein kinase C or extracellular and intracellular Ca2+, whereas GnRHa-induced JNK activation was not dependent on protein kinase C or on extracellular or intracellular Ca2+. To determine whether a mitogen-activated protein kinase family cascade regulates rat luteinizing hormone beta (LH beta) promoter activity, we transfected the rat LH beta (-156 to +7)luciferase construct into L beta T2 cells. GnRH activated the rat LH beta promoter activity in a time-dependent manner. Neither treatment with a mitogen-activated protein kinase/ERK kinase (MEK) inhibitor, PD98059, nor cotransfection with a catalytically inactive form of a mitogen-activated protein kinase construct inhibited the induction of the rat LH beta promoter by GnRH, Furthermore, cotransfection with a dominant negative Ets had no effect on the response of the rat LH beta promoter to GnRH. On the other hand, cotransfection with either dominant negative JNK or dominant negative c-Jun significantly inhibited the induction of the rat LH beta promoter by GnRH. In addition, GnRH did not induce either the rat LH beta promoter activity in L beta T2 cells transfected stably with dominant negative c-Jun, These results suggest that GnRHa differentially activates ERK and JNK, and a JNK cascade is necessary to elicit the rat LH beta promoter activity in a c-Jun-dependent mechanism in L beta T2 cells.