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 GH4C1 cells.
We have examined the mechanism for the selective down-regulation of protein kinase C epsilon (nPKC epsilon) in rat pituitary GH4C1 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 GH4C1 cells, TRH mobilizes both protease (m-calpain) and inhibitor (calpastatin) as a strictly regulating system for the nPKC epsilon pathway mediating TRH signals.
Eto
A
A
Department of Molecular Biology, Tokyo Metropolitan Institute of Medical Science, Japan.
Akita
Y
Y
Saido
T C
TC
Suzuki
K
K
Kawashima
S
S
D000595:Amino Acid Sequence; D000818:Animals; D002135:Calcium-Binding Proteins / Q000502:physiology*; D002154:Calpain / Q000502:physiology*; D002460:Cell Line; D015536:Down-Regulation; D007527:Isoenzymes / Q000032:analysis*; D008969:Molecular Sequence Data; D010902:Pituitary Gland / Q000201:enzymology*; D011480:Protease Inhibitors / Q000494:pharmacology; D011493:Protein Kinase C / Q000032:analysis*; D051381:Rats; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D002135:Calcium-Binding Proteins; D007527:Isoenzymes; D011480:Protease Inhibitors; D013973:Thyrotropin-Releasing Hormone; C033668:calpastatin; D011493:Protein Kinase C; D002154:Calpain