1,25-Dihydroxyvitamin D 3 increases the toxicity of hydrogen peroxide: the role of calcium and heat shock

Abstract

1,25-Dihydroxyvitamin D 3 [1,25-(OH) 2D 3] increases synthesis of heat shock proteins in monocytes and U937 cells and protects these cells from thermal injury. We therefore examined whether 1,25-(OH) 2D 3 would also modulate the susceptibility to H 2O 2-induced oxidative stress. Prior incubation for 24 h with 1,25-(OH) 2D 3 (25 pM or higher) produced unexpected increased H 2O 2 toxicity. Since cellular Ca 2+ may be a mediator of cell injury, we investigated the effects of altering extracellular Ca 2+ ([Ca 2+ e) on 1,25-(OH) 2D 3-enhanced H 2O 2 toxicity, as well as the effects of 1,25-(OH) 2D 3 and H 2O 2 on cytosolic-free Ca 2+ concentration ([Ca 2+] f). Basal [Ca 2+] f in medium containing 1.5 mM Ca 2+ as determined by fura-2 fluorescence was higher in 1,25-(OH) 2D 3-pretreated cells than control cells (137 versus 112 nM, p < 0.005). H 2O 2 induced a rapid increase in [Ca 2+] f (to > 300 nM) in both 1,25-(OH) 2D 3-treated and control cells, which was prevented by a reduction in [Ca 2+] e to less than basal [Ca 2+] f. The 1,25-(OH) 2D 3-induced increase in H 2O 2 toxicity was also prevented by preincubation with 1,25-(OH) 2D 3 in Ca 2+-free medium or by exposing the cells to H 2O 2 in the presence of EGTA. Preexposure of cells to 45°C for 20 min, 4 h earlier, partially prevented the toxic effects of H 2O 2 particularly in 1,25-(OH) 2D 3-treated cells, even in the presence of physiological levels of [Ca 2+] e. Thus, 1,25-(OH) 2D 3 potentiates H 2O 2-induced injury probably by increasing cellular Ca 2+ stores. The protective effects of heat shock are probably exerted at a site distal to the toxic effects of Ca 2+. The 1,25-(OH) 2D 3-induced amplification of the heat shock response likely represents a mechanism for counteracting the Ca 2+-associated enhanced susceptibility of oxidative injury due to 1,25-(OH) 2D 3.