THE ROLE OF THE CALPAIN-CALPASTATIN SYSTEM IN THYROTROPIN-RELEASING HORMONE-INDUCED SELECTIVE DOWN-REGULATION OF A PROTEIN-KINASE-C ISOZYME, NPKC-EPSILON, IN RAT PITUITARY GH(4)C(1) CELLS

We have examined the mechanism for the selective down-regulation of protein kinase C epsilon (nPKC epsilon) in rat pituitary GH(4)C(1) cells responding to thyrotropin-releasing hormone (TRH) stimulation. Among various low molecular weight protease inhibitors examined, only a cysteine protease inhibitor (calpain inhibitor I, N-acetyl-Leu-Leu-norleucinal) blocked the down-regulation of nPKC epsilon. Furthermore, the introduction of a synthetic calpastatin peptide, an exclusively specific inhibitor of calpain, into the cells also reduced the down-regulation, suggesting the involvement of calpain among all the intracellular cysteine proteases in this process. In accordance, we observed TRH-induced translocation of m-calpain from the cytosol to the membrane and the concomitant up regulation of calpastatin isoforms; presumably, the former represents activation of the protease initiating the kinase degradation, while the latter constitutes a negative feedback system protecting the cells from activated calpain. These results suggest that in GH(4)C(1) cells, TRH mobilizes both protease (m-calpain) and inhibitor (calpastatin) as a strictly regulating system for the nPKC epsilon pathway mediating TRH signals.