Phorbol-ester-induced stable changes in the regulation of DNA synthesis and intracellular pH are accompanied by altered expression of protein kinase C in the monoblastoid cell line U-937.

Abstract

12-O-tetradecanoylphorbol-13-acetate (TPA)-induced changes in cytoplasmic pH, cytoplasmic Ca2+-concentration, rate of DNA synthesis, and concentration and activity of protein kinase C (PKC) were studied in human monoblastoid cell lines. The cell line U-937 GTB was compared to the subline U-937 RES (adapted to growth in the presence of 10(-9) M TPA) and another subline U-937 RESREV (U-937 RES grown in TPA-free medium) established in order to analyze the stability of the TPA-induced differences. TPA induced half maximal inhibition of DNA synthesis in the wild-type U-937 GTB cell line at 10(-9) M, whereas 10 times higher concentrations of phorbol ester were needed for a corresponding inhibition of the U-937 RES and U-937 RESREV lines. Furthermore, the U-937 RES cells exhibited a decreased sensitivity to TPA, and the U-937 RESREV cells did not respond at all to this agent with regard to cytoplasmic alkalinization by an intracellular mechanism independent of Na+/H+ exchange. A Na+-dependent system for extrusion of protons, which was activated by the Ca2+ ionophore ionomycin, was also severely depressed as a result of TPA-adaptation. The concentration of PKC, measured by immunoblotting, was reduced by 34 and 24% in U-937 RES and U-937 RESREV cells, respectively, as compared to the wild-type U-937 GTB line. The corresponding reductions in PKC activity were 32 and 54% when histone III-S was used as substrate. The data suggest that adaptation to growth in the presence of TPA results in stable modifications of several parameters, which are assumed to be involved in the regulation of proliferation and differentiation. Furthermore, the data from the U-937 RESREV cells question a causal relationship between cytoplasmic alkalinization and control of proliferation.