ras downregulation of protein kinase C mRNA in C3H 10T1/2 fibroblasts.

C3H 10T1/2 fibroblasts transformed by oncogenic ras have lower levels of protein kinase C (PKC) activity and protein. It was previously suggested that elevated levels of diacylglycerol in ras-transformed fibroblasts lead to activation-induced proteolysis of cellular PKC. We found that stable expression of T24ras in C3H 10T1/2 fibroblasts resulted in a significant decrease in levels of PKC alpha and PKC epsilon mRNA. Using C3H 10T1/2 cell lines in which the levels of activated ras can be exogenously regulated (by addition of zinc to induce the expression of a metallothionein-promoted human Ha-ras oncogene), we examined the temporal dependence of oncogenic ras expression on PKC downregulation. In these cells, downregulation of PKC protein and activity was induced but was not preceded by activation of PKC. The downregulation of PKC levels correlated with the appearance of a highly transformed morphology and was seen only at high levels of ras expression. In the inducible cells, the decrease in levels of PKC alpha mRNA had the same dependence on the levels of ras expression as did protein downregulation. These experiments provide evidence that downregulation of PKC protein levels by expression of oncogenic Ha-ras in C3H 10T1/2 fibroblasts is primarily due to altered transcriptional regulation. Because the downregulation of PKC was coupled with the onset of morphological transformation, the data suggest that this downregulation is involved in or facilitates the maintenance of a ras-transformed phenotype in C3H 10T1/2 fibroblasts.