Dissimilar Effects of Phorbol Ester and Diacylglycerol Derivative on Protein Kinase Activity in the Monoblastoid U937 Cell

Mechanism in addition to protein kinase C activation may mediate 12- O -tetradecanoylphorbol-13-acetate (TPA) stimulated differentiation of leukemic cells. We compared the effect of pretreating intact monoblastoid U937 cells with TPA or the diacylglycerol derivative, 1-oleoyl-2-acetylglycerol (OAG), by studying the protein kinase C dependent and independent histone phosphotransferase activity, the phosphorylation of endogenous substrates, and the ability to stimulate differentiation. In cellular fractions derived from cells treated with TPA or OAG, cytosolic protein kinase C activity decreased. In the detergent extracted particulate fraction, TPA produced a time and dose dependent decrease in protein kinase C activity. In contrast, OAG increased particulate protein kinase C activity. In addition, the particulate fraction derived from cells treated with TPA exhibited increased phosphatidyl serine and diolein independent histone phosphotransferase activity as well as an increase in the phosphorylation of two endogenous substrates with molecular weights of 120,000 and 80,000. OAG did not mimic these effects. When exposed to 32P-labeled intact cells, OAG and TPA stimulated phosphorylation of three substrates. Thus, the inability of OAG to mimic the effects of TPA was not due to lack of protein kinase C activation. TPA, but not OAG, stimulated differentiation of the U937 cell to a monocyte-like cell. These data demonstrate that TPA and OAG have dissimilar effects on protein kinase activity and differentiation in the U937 monoblastoid cell.