A 5-LIPOXYGENASE INHIBITOR AT MICROMOLAR CONCENTRATION RAISES INTRACELLULAR CALCIUM IN U937 CELLS PRIOR TO THEIR PHYSIOLOGICAL CELL-DEATH

Micromolar MK886, a selective inhibitor of 5-lipoxygenase at nanomolar concentration, induces physiologic cell death in U937 and chronic mye logenous leukemia blast cells. When U937 cells were challenged with 10 mu M MK886, an acute, biphasic and sustained rise in intracellular Ca 2+ occurred, as determined with Fura-2. The initial increase was follo wed by a transient decline and a larger rise due to an influx of exter nal Ca2+. The first increase and part of the subsequent rise also deve loped in Ca2+-free medium, identifying their origin from intracellular stores. The intracellular Ca2+ concentration of U937 cells that remai ned after culture for 24 or 48 h with 5 or 10 mu M MK886 was not relia bly altered from the control values of 130 +/- 8.3 nM. Under similar c onditions MK886 did not increase cytosol Ca2+ of a human prostate (PC3 ) cell line examined in suspension. The increase in intracellular Ca2 in response to MK886 in calcium-containing medium was confirmed with an ACAS laser spectrometer. U937 cytosol pH was measured with the fluo rescent probe BCECF, but no persistent acute or chronic change was ind uced by MK886. The rapid and sustained rise in Ca2+ induced by MK886 i s an early event in U937 cells which subsequently undergo physiologic cell death characterized in many by apoptosis.