Expression of four protein kinase C isoforms in rat fibroblasts. Differential alterations in ras-, src-, and fos-transformed cells.

In the accompanying study (Borner, C.B., Guadagno, S. N., and Weinstein, I. B. (1992) J. Biol. Chem. 267, 12892-12899) we found that R6 embryo fibroblasts express four isoforms of PKC, cPKC alpha, nPKC epsilon, nPKC delta, and nPKC zeta whose subcellular distribution, activation, and down-regulation are differentially regulated. Furthermore, we demonstrated that overproduction of an exogenous cPKC beta I isoform in these cells (R6-PKC3) altered the TPA-induced down-regulation of nPKC delta and nPKC epsilon. In this paper we show that transformation of R6 or R6-PKC3 cells with a variety of different oncogenes results in differential alterations in expression of individual PKC isoforms. R6 or R6-PKC3 cells transformed by an activated c-H-ras oncogene displayed a marked increase in the expression of both cPKC alpha and nPKC delta, decreased expression of nPKC epsilon, and no change in the expression of nPKC zeta. These alterations occurred at both the mRNA and protein levels but did not significantly affect the subcellular distribution of any of the four isoforms. Studies using actinomycin D and nuclear run-off assays indicated that the increased expression of cPKC alpha in ras-transformed cells was due to increased de novo transcription rather than increased mRNA stability. Qualitatively similar, but less extensive changes in the expression of the four PKC isoforms were seen in v-fos-transformed R6-PKC3 cells. Decreased expression of nPKC epsilon was also seen in the v-src-transformed R6- and R6-PKC3 lines; however, the cellular level of cPKC beta I appeared to be a limiting factor in mediating the effects of v-src on the increased expression of cPKC alpha and nPKC delta. Interestingly, no major changes in the levels of expression of any of the four PKC isoforms were found when R6 cells were transformed by myc, neu/erb-B2, or mos oncogenes. These results demonstrate that transformation of R6 cells by the oncogenes ras, src, and fos differentially alter the expression of three isoforms of PKC in the same host cell, and they suggest that individual isoforms may play distinct roles in mediating cellular transformation by specific oncogenes.