Overproduction of a Ca(2+)-independent protein kinase C isozyme, nPKC epsilon, increases the secretion of prolactin from thyrotropin-releasing hormone-stimulated rat pituitary GH4C1 cells

Abstract

Rat pituitary GH4C1 cells express protein kinase C (PKC) transcripts for cPKC alpha, cPKC beta II, nPKC delta, nPKC epsilon, nPKC eta, and aPKC zeta, but not for cPKC gamma or nPKC theta. Of the transcripts produced, the nPKC epsilon isoform is the most abundant. Transfection of GH4C1 cells with an expression plasmid containing nPKC epsilon cDNA leads to the transient overexpression of cellular nPKC epsilon and confers enhanced phorbol ester binding activity. Transient expression of an inactive point mutant (nPKC epsilon K-->R) of nPKC epsilon, where Lys436 at the putative ATP-binding site is replaced with Arg, also confers elevated binding activity. However, only overproduction of the wild type in transfected cells increases the basal levels and stimulates the secretion of prolactin (PRL) by 12-O-tetradecanoylphorbol-13-acetate or thyrotropin-releasing hormone (TRH). In stable clones overexpressing nPKC epsilon, immunocytofluorescence and immunoblot experiments indicated that TRH causes the rapid translocation and down-regulation of an appreciable fraction of nPKC epsilon. Both the basal and TRH-stimulated levels of PRL secretion are clearly correlated with the expression level of nPKC epsilon but not with the TRH receptor densities in these clones. The dose dependence of TRH-stimulated secretion were similar in all cells overexpressing cPKC alpha, cPKC beta II, nPKC epsilon, and nPKC delta, but the enhancement of PRL secretion was specific for the overproduction of nPKC epsilon, no effect was found when other isozymes were overproduced. These findings clearly demonstrate that the expression level of nPKC epsilon in GH4C1 cells is rate-limiting for basal and TRH-stimulated PRL secretion, and they provide the first direct evidence that nPKC epsilon plays a key role in hormonal secretory processes.