ag-213 and Leukemia--Promyelocytic--Acute

An eosinophilic substrain of HL-60 cells (HL-60#7) predominantly synthesized cysteinyl leukotrienes after stimulation with the calcium ionophore A23187. Activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) specifically attenuated cysteinyl leukotriene production without affecting the biosynthesis of non-cysteinyl leukotrienes. The inhibition of cysteinyl leukotriene biosynthesis was prevented only by specific PKC inhibitors (staurosporine and bisindolylmaleimide) but not by inhibitors of tyrosine kinases (genistein, tyrphostin 47, and herbimycin A), protein kinase A (KT5720), or the oxidative burst (apocynin). Similar results were obtained when LTC4 synthase enzymatic activity was measured directly in the presence of saturating concentrations of exogenously added substrates. Therefore, the inhibitory effects of PKC activation on cysteinyl leukotriene formation in intact cells was attributable to effects on the LTC4 synthase enzyme. The mechanism of inhibition of LTC4 synthase by PKC activation was determined by kinetic analysis to be noncompetitive in both eosinophil-like HL-60#7 cells and monocytic THP-1 cells. Contrary to the effect of PKC activation on cysteinyl leukotriene biosynthesis, the formation of prostaglandin E2 and thromboxane B2 was elevated twofold to threefold after PMA treatment, which was prevented by the PKC inhibitor, staurosporine. We propose a regulatory model in which PKC activation shifts the profile of eicosanoid mediators produced by eosinophils from cysteinyl leukotrienes to prostanoids.

Topics: Alkaloids; Calcimycin; Cysteine; Down-Regulation; Eosinophils; Glutathione Transferase; Humans; Leukemia, Promyelocytic, Acute; Leukotrienes; Nitriles; Phenols; Prostaglandins; Protein Kinase C; Staurosporine; Tetradecanoylphorbol Acetate; Tumor Cells, Cultured; Tyrphostins

Experimental evidence suggests that hematopoietic growth factors promote cell survival by suppressing apoptosis or programmed cell death. Since interleukin 3 (IL-3) and granulocyte-macrophage colony stimulating factor (GM-CSF) induce tyrosine phosphorylation of a common set of proteins in the factor-dependent cell line M07e, we have investigated whether growth-factor-induced tyrosine phosphorylation is involved in the promotion of cell survival and suppression of apoptosis. Experiments were carried out with the leukemic cell lines HL-60 and M07e and the tyrosine kinase inhibitors genistein and tyrphostin AG82. Both the tyrosine kinase inhibitors induced apoptosis of HL-60 and M07e cells. This was indicated by the appearance of DNA degradation and morphologic evidence of nuclear condensation and fragmentation. It was also confirmed by flow cytometry of DNA, which showed apoptotic cells as a fraction of cells characterized by a diminished DNA stainability, represented on the DNA frequency histograms as a distinct peak below the G0/G1 population. Kinase inhibitors also reduced the fraction of cells in the S phase of the cell cycle. That tyrphostin specifically inhibited tyrosine kinases was further suggested by the prevention of its effects by the tyrosine phosphatase inhibitor sodium orthovanadate (vanadate), at least during the first 18-24 h of treatment. The incomplete prevention of genistein effects by vanadate suggests that genistein is a less specific inhibitor of tyrosine kinases than tyrphostin, and may also act as an inhibitor of topoisomerase II. Vanadate also prevented apoptosis and reduction of the S phase in M07e cells cultured for 24 h in the absence of growth factors. These results suggest that tyrosine phosphorylation is an essential step in IL-3 and GM-CSF signal transduction. Since in our experimental model the effects of tyrosine kinase inhibition and growth factor deprivation could be reversed by concomitant inhibition of tyrosine phosphatases, it is suggested that a balance between tyrosine kinases and tyrosine phosphatases establishes whether a cell will survive or undergo apoptosis.

Topics: Apoptosis; Catechols; Cell Cycle; Cell Division; Flow Cytometry; Genistein; Granulocyte-Macrophage Colony-Stimulating Factor; Humans; Interleukin-3; Isoflavones; Leukemia, Megakaryoblastic, Acute; Leukemia, Promyelocytic, Acute; Nitriles; Phosphorylation; Protein Tyrosine Phosphatases; Protein-Tyrosine Kinases; Tumor Cells, Cultured; Tyrosine; Tyrphostins; Vanadates