POSSIBLE ROLE OF CA-2+-INDEPENDENT PROTEIN-KINASE-C ISOZYME, NPKC-EPSILON, IN THYROTROPIN-RELEASING HORMONE-STIMULATED SIGNAL TRANSDUCTION - DIFFERENTIAL DOWN-REGULATION OF NPKC-EPSILON IN GH4C1 CELLS

Protein kinase C (PKC) molecular species of GH4C1 cells were analyzed after separation by hydroxyapatite column chromatography. A novel Ca2+-independent PKC, nPKC e{open}, was identified together with two conventional Ca2+-dependent PKCs, PKCα and βII by analysis of kinase and phorbol ester-binding activities, immunoblotting using isozyme-specific antibodies, and Northern blotting. These PKCs are down-regulated differently when cells are stimulated by outer stimuli; phorbol esters deplete PKCβII and nPKC e{open} from the cells more rapidly than PKC α, whereas thyrotropin-releasing hormone (TRH) at 200 nM depletes nPKCe{open} exclusively with a time course similar to that induced by phorbol esters. However, translocation of PKCα and βII to the membranes is elicited by both TRH and phorbol esters. These results suggest that TRH and phorbol ester activate PKC α and βII differently but that nPKC e{open} is stimulated similarly by both stimuli. Thus, in GH4C1 cells, Ca2+-independent nPKC e{open} may play a crucial role distinct from that mediated by Ca2+-dependent PKC α and βII in a cellular response elicited by both TRH and phorbol esters. © 1990 Academic Press, Inc.