Activation of protein kinase C β gene expression by gonadotropin-releasing hormone in αT3-1 cell line: Role of Ca²⁺ and autoregulation by protein kinase C

The gonadotroph-derived αT3-1 cell line was used to investigate the effect of gonadotropin-releasing hormone (GnRH) upon conventional protein kinase C subtypes (cPKCs) gene expression. Addition of the stable analog [D-Trp⁶]GnRH (GnRH-A, 0.1 nw) resulted in a rapid increase (30 min) of the steady state levels of PKCβ, but not PKCα, mRNA levels, while PKCγ is not expressed in the cells. The rapid stimulatory effect of GnRH-A was blocked by pretreatment with actinomycin D or with the GnRH antagonist (D-pGlu¹, pClPhe²,D-Trp^3,6)GnRH and was not mimicked by thyrotropin-releasing hormone. Addition of the PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted also in a rapid (30 min) and selective increase in PKCβ, but not PKCα, mRNA levels. In contrast, the calcium ionophore, ionomycin, increased rapidly (30 min) both PKCα and PKCβ mRNA levels, and its stimulatory effect on PKCβ was not additive with that of TPA. The rapid stimulatory effect of GnRH-A was blocked by the PKC inhibitor bisindolylmaleimide (GF 109203X) or by down-regulation of endogenous PKC. Similarly, the rapid effect of GnRH-A was abolished by the intracellular Ca²⁺ chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) or by removal of extracellular Ca²⁺. Stimulation of PKCβ mRNA levels by ionomycin was only reduced by GF 109203X and was not affected by down-regulation of PKC. In contrast the effect of TPA on PKCβ mRNA levels was reduced by BAPTA and abolished by removal of Ca²⁺. We conclude that Ca²⁺ and PKC act sequentially during GnRH-A-induced PKCβ gene expression and that PKCβ gene expression induced by GnRH-A is autoregulated by PKC.