Hydroperoxide-induced Increases in Intracellular Calcium Due to Annexin VI Translocation and Inactivation of Plasma Membrane Ca2+-ATPase

Oxidative stress can cause changes in intracellular free calcium concentration ([Ca2+]i) that resemble those occurring under normal cell signaling. In the alveolar macrophage, hydroperoxide-induced elevation of [Ca2+]i modulates the respiratory burst and other important physiologic functions. The source of Ca2+ released by hydroperoxide is intracellular but separate from the endoplasmic reticulum pool released by receptor-mediated stimuli (Hoyal, C. R., Gozal, E., Zhou, H., Foldenauer, K., and Forman, H. J. (1996) Arch. Biochem. Biophys. 326, 166-171). Previous studies in other cells have suggested that mitochondria are a potential source of oxidant-induced [Ca2+]i elevation. In this study we have identified another potential source of hydroperoxide-releasable intracellular calcium, that bound to annexin VI on the inner surface of the plasma membrane. Translocation of annexin VI from the membrane during exposure to t-butyl hydroperoxide matched elevation of [Ca2+]i as a function of time and t-butyl hydroperoxide concentration. The translocation was possibly due to a combination of ATP depletion and oxidative modification of membrane lipids and proteins. A sustained increase in [Ca2+]i occurring > 50 pmol/10(6) cells (50 microM under these conditions) appeared to be a consequence of membrane Ca2+-ATPase dysfunction. These results suggest that exposure to oxidative stress results in early alterations to the plasma membrane and concomitant release of Ca2+ into the cytosol. In addition it suggests a mechanism for participation of annexin VI translocation that may underlie the alterations in macrophage function by oxidative stress.