Selective post-transcriptional down-regulation of protein kinase C isoenzymes in leukemic T cells chronically treated with phorbol ester

Binding to, and activation of, protein kinase C (PKC) by phorbol ester (PE) tumor promoters may underlie their tumor-promoting activity. To study the effects of long-term PE treatment on regulation of cellular PKC, we adapted the human leukemic T cell line, Jurkat (JK), to continuous growth in the presence of PE. Such cells (JK^PE) displayed loss of CD2 and CD3 cell surface molecules, known to play an important role in agonist-stimulated T cell activation, unresponsiveness to stimuli that induce interleukin 2 (IL2) receptor expression or IL2 production, change in the expression of several cell cycle-regulated genes, and a 6-fold reduction in cellular PKC enzymatic activity. This reduction was accompanied by the disappearance of a major ≈82-kDa immunoreactive protein in JK^PE cytosol when cell extracts were imniunoblotted with a polyclonal anti-PKC peptide antibody crossreactive with the PKC isoenzymes, α, β, and γ. Analysis with additional anti-peptide antibodies specific for α, β, or γ PKC indicated that all three types of PKC are expressed in JK cells; however, JK^PE cells lost a major ≈82 kDa immunoreactive cytosolic protein detectable with anti-PKCa antibody. In contrast, levels of expression and subcellular distribution of immunoreactive PKC/8 and PKC7, as well as levels of mRNA specific for the three PKC isoenzymes, were not significantly affected by chronic PE treatment. These results indicate that PE-mediated reduction of PKC in JK^PE cells is selective and occurs at the protein, not mRNA, level, and support the notion that distinct isoenzymes encoded by the PKC multigene family may be independently regulated. Moreover, the correlation between phenotypic and functional changes on one hand, and the selective reduction of PKCα on the other, raises the possibility that expression of CD2 and/or CD3 and functional activation in JKPE cells are preferentially regulated by PKCα.