Cross talk between increased intracellular zinc (Zn2+) and accumulation of reactive oxygen species in chemical ischemia

Abstract

Both zinc (Zn2+) and reactive oxygen species (ROS) have been shown to accumulate during hypoxic-ischemic stress and play important roles in pathological processes. To understand the cross talk between the two of them, here we studied Zn2+ and ROS accumulation by employing fluorescent probes in HeLa cells to further the understanding of the cause and effect relationship of these two important cellular signaling systems during chemical-ischemia, stimulated by oxygen and glucose deprivation (OGD). We observed two Zn2+ rises that were divided into four phases in the course of 30 min of OGD. The first Zn2+ rise was a transient, which was followed by a latent phase during which Zn2+ levels recovered; however, levels remained above a basal level in most cells. The final phase was the second Zn2+ rise, which reached a sustained plateau called Zn2+ overload. Zn2+ rises were not observed when Zn2+ was removed by TPEN (a Zn2+ chelator) or thapsigargin (depleting Zn2+ from intracellular stores) treatment, indicating that Zn2+ was from intracellular storage. Damaging mitochondria with FCCP significantly reduced the second Zn2+ rise, indicating that the mitochondrial Zn2+ accumulation contributes to Zn2+ overload. We also detected two OGD-induced ROS rises. Two Zn2+ rises preceded two ROS rises. Removal of Zn2+ reduced or delayed OGD- and FCCP-induced ROS generation, indicating that Zn2+ contributes to mitochondrial ROS generation. There was a Zn2+-induced increase in the functional component of NADPH oxidase, p47phox, thus suggesting that NADPH oxidase may mediate Zn2+-induced ROS accumulation. We suggest a new mechanism of cross talk between Zn2+ and mitochondrial ROS through positive feedback processes that eventually causes excessive free Zn2+ and ROS accumulations during the course of ischemic stress.