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
A. Eto et al., 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, The Journal of biological chemistry, 270(42), 1995, pp. 25115-25120
We have examined the mechanism for the selective down-regulation of pr
otein 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 cyst
eine protease inhibitor (calpain inhibitor I, N-acetyl-Leu-Leu-norleuc
inal) blocked the down-regulation of nPKC epsilon. Furthermore, the in
troduction 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 cy
steine proteases in this process. In accordance, we observed TRH-induc
ed translocation of m-calpain from the cytosol to the membrane and the
concomitant up regulation of calpastatin isoforms; presumably, the fo
rmer represents activation of the protease initiating the kinase degra
dation, while the latter constitutes a negative feedback system protec
ting the cells from activated calpain. These results suggest that in G
H(4)C(1) cells, TRH mobilizes both protease (m-calpain) and inhibitor
(calpastatin) as a strictly regulating system for the nPKC epsilon pat
hway mediating TRH signals.