Salmon gonadotropin IIβ subunit promoter contains multiple DNA elements responsible for stimulation by gonadotropin-releasing hormone through protein kinase C-dependent and -independent pathways

Abstract

Gonadotropin-releasing hormone (GnRH) stimulates gonadotropin (GTH) production by activating GTH subunit gene transcription. In salmonid fish, the expression of the β subunit gene of GTH II (sGTH IIβ) is stimulated by GnRH at the final stages of reproduction. DNA elements required for the GnRH stimulation were examined by analyzing sGTH IIβ promoter activity by transfection studies in a gonadotrope-derived cell line, αT3-1. A GnRH analog (GnRHa) specifically stimulated the sGTH IIβ promoter (3358 bp) expression 3.6-fold, while phorbol myristate acid (PMA) stimulated it 6.2–9-fold. Analysis of a series of 5′-deletion mutants has revealed that a proximal region (−258 to −199) was important in GnRHa stimulation through protein kinase C (PKC)-independent signal transduction pathways, because an internal deletion mutant (Δ(246−217)/3358) showed a significant decrease in the level of GnRHa stimulation, but showed no change in stimulation by PMA. A large upstream region (−3358 to −1260) showed an enhancing activity of the GnRHa stimulation, and a far upstream 530 bp segment in this region (−3358 to −2829) may be responsible for this activity. The present results suggest that sGTH IIβ gene may be controlled by GnRH through multiple DNA elements including those responsive to PKC-dependent and -independent signal transduction pathways.