Differential expression of protein kinase C isoforms in glial and neuronal cells. Translocation and down-regulation of PKC isoforms in C6 glioma and NG 108-15 hybrid cells: effects of extracellular Ca(2+)-depletion.

Protein kinase C (PKC), the major receptor for tumor-promoting phorbol esters, consists of a family of at least 12 distinct lipid-regulated enzymes. We examined the expression and regulation of PKC isoforms in C6-glioma and NG 108-15 hybrid cells. Western blot analysis indicated that both cell lines express four PKC isoforms, PKC alpha, PKC delta, PKC epsilon and PKC zeta. The expression of PKC alpha and PKC delta in C6-glioma cells was more abundant than NG 108-15 cells, however, PKC epsilon in NG 108-15 was more abundant than C6-glioma cells in which PKC epsilon was almost undetectable. Treatment of both cells with TPA for 10 min resulted in the translocation of PKC alpha, PKC delta and PKC epsilon to the membrane fraction. When the intact cells were treated with Ca(2+)-free, EGTA containing physiological saline solution, the membrane bound conventional PKC alpha (cPKC alpha) was greatly reduced and cytosolic cPKC alpha was only slightly increased. However, neither membrane bound nor cytosolic new PKC delta (nPKC delta), nPKC epsilon and atypical PKC zeta (aPKC zeta) was affected by extracellular Ca2+ depletion. In this condition, the translocation of cPKC alpha, nPKC delta and nPKC epsilon induced by TPA still occurred, however, that of cPKC alpha was reduced more than in the normal condition. After long-term treatment (17 h) with TPA, cPKC alpha, nPKC delta and nPKC epsilon were down-regulated both in the cytosol and membrane. The phenomena of cPKC alpha were confirmed by measuring the PKC activity with histone as the substrate. From in vitro endogenous phosphorylation studies, a 31 kDa substrate protein phosphorylation in C6 glioma cell membrane and 31 and 26 kDa proteins in NG 108-15 cell membrane were increased in the translocation but disappeared in the down-regulation of PKC.