A 5-lipoxygenase inhibitor at micromolar concentration raises intracellular calcium in U937 cells prior to their physiologic cell death

Micromolar MK886, a selective inhibitor of 5-lipoxygenase at nanomolar concentration, induces physiologic cell death in U937 and chronic myelogenous leukemia blast cells. When U937 cells were challenged with 10 mu M MK886, an acute, biphasic and sustained rise in intracellular Ca2+ occurred, as determined with Fura-2. The initial increase was followed by a transient decline and a larger rise due to an influx of external Ca2+. The first increase and part of the subsequent rise also developed in Ca2+-free medium, identifying their origin from intracellular stores. The intracellular Ca2+ concentration of U937 cells that remained after culture for 24 or 48 h with 5 or 10 mu M MK886 was not reliably altered from the control values of 130 +/- 8.3 nM. Under similar conditions 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 fluorescent probe BCECF, but no persistent acute or chronic change was induced by MK886. The rapid and sustained rise in Ca2+ induced by MK886 is an early event in U937 cells which subsequently undergo physiologic cell death characterized in many by apoptosis.