Protein Kinase Cδ as Gonadotropin-Releasing Hormone Target Isoenzyme in the αT3-1 Gonadotrope Cell Line

Abstract

We investigated the kinetics of gonadotropin-releasing hormone (GnRH)-induced activation of the protein kinase C (PKC) δ isoform in αT3-1 gonadotrope cells. Results were evaluated in subcellular fractions and whole-cell lysates using specific antibodies recognizing either non- or ( trans- and auto-)phosphorylated forms of the kinase at Thr505 and Ser643 residues modulating stability and/or activation of the enzyme. Under basal conditions, and in contrast to PKC Ε, PKC δ was mainly associated with the membrane compartment. GnRH (10^–7 M) elicited further and rapid membrane translocation and time-dependent phosphorylation at both sites of PKC δ. The neuropeptide’s effects did not show a refractory period after short but successive GnRH stimulation and were abolished by the GnRH antagonist, antide. Sustained GnRH stimulation (2–6 h) provoked rapid down-regulation of PKC δ. Antide, by inhibiting the initial processes (translocation, phosphorylation), counteracted the degradation of the enzyme. Proteolytic processing of PKC δ was shown to mainly involve proteasome activity. Indeed, specific proteasome inhibitors prevented GnRH-elicited kinase depletion and induced membrane accumulation of the enzyme in a phosphorylated (Thr505, Ser643) form. Thus, GnRH may regulate time-dependent cell responses by modulating the phosphorylation/activation state of its signal transduction effector proteins, and by maintaining their appropriate expression balance via proteolytic processes involving the proteasome system.