The presence in a mouse T cell line of a 97-kDa protein kinase C (PKC) with characteristics similar to known members of the novel PKC subgroup and its possible role in lymphocyte gene expression

The receptor for tumor-promoting phorbol esters has been shown to be the Ca⁺²/phospholipid dependent enzyme protein kinase C (PKC). There are two major groups of PKC, the conventional PKC isotypes (α, βI, βII, γ) and the novel Ca⁺²-independent PKC (δ, ε, ζ, η). Phorbol esters previously have been demonstrated to increase human IFN-γ gene expression after treatment of a murine T cell line (Cl 9) that has been transfected with human IFN-γ genomic DNA. In contrast, treatment with Ca⁺² ionophore alone or in combination with phorbol ester did not enhance IFN-γ production in a synergistic manner above the level obtained with phorbol ester treatment alone. To determine whether the lack of effect of Ca⁺² ionophore is due to a defect in PKC, we compared the level of PKC autophosphorylation in the mouse T cell line (Cl 9), a mouse epidermal cell line (JB6), and purified rat brain PKC by in vitro kinase assays. The results demonstrate that instead of the expected 80-kDa autophosphorylated PKC band seen in purified rat brain PKC or mouse JB6 cell lysates, only a novel 97-kDa Ca⁺²-independent phosphoprotein was observed in Cl 9 cells. To ascertain if there was any nucleic acid sequence similarity to PKCε, we hybridized Cl 9 poly(A⁺) RNA with a cloned fragment of the PKCε gene and observed two hybridizing RNA bands (4.4 and 4.0 kb). Our results suggest that the 97-kDa phosphoprotein is similar to, but not identical with, PKCε and is the major PKC expressed in the Cl 9 murine T cell line. These data suggested that the 97-kDa PKC may be responsible for the induction of both the transfected human IFN-γ gene and the endogenous murine IL-2R α-chain.