Activation of the Ste20-like Oxidant Stress Response Kinase-1 during the Initial Stages of Chemical Anoxia-induced Necrotic Cell Death

Signal transduction mechanisms activated during the early stages of necrotic cell death are poorly characterized. We have recently identified the Sterile 20 (Ste20)-like oxidant stress response kinase-1, SOK-1, which is a member of the Ste20 kinase family. We report that SOK-1 is markedly activated as early as 20 min after chemical anoxia induced by exposure of Madin-Darby canine kidney or LLC-PK1 renal tubular epithelial cells to 2-deoxyglucose (2-DG) and any one of three inhibitors of the electron transport chain, cyanide (CN), rotenone, or antimycin A. Since oxidant stress activates SOK-1, we postulated that reactive oxygen species (ROS), which are produced by the electron transport chain during chemical anoxia, might be responsible for SOK-1 activation. The time course of CN/2-DG-induced SOK-1 activation and of production of ROS, measured in cells loaded with dichlorofluorescein, were compatible with a role for ROS in SOK-1 activation. Furthermore, preincubation of LLC-PK1 cells with three unrelated scavengers of ROS, pyrrolidine dithiocarbamate, pyruvate, or nordihydroguaiaretic acid, reduced both cellular oxidant stress and activation of SOK-1 by CN/2-DG. An increase in cytosolic free [Ca2+] ([Ca2+]i) was necessary but not sufficient for CN/2-DG-induced activation of SOK-1. Preincubation of cells with BAPTA-AM prevented activation of SOK-1. Incubation of cells with thapsigargin or the calcium ionophore, A23187, had no effect on SOK-1 activity, but preincubation of cells with either of these agents markedly enhanced CN/2-DG-induced activation of SOK-1 (20-fold versus 7-fold). In summary, chemical anoxia activates SOK-1 via an oxidant stress-dependent mechanism that is both critically dependent upon and markedly amplified by an increase in [Ca2+]i. This requirement for dual inputs of oxidant stress and an increase in [Ca2+]i may prevent inappropriate activation of the kinase by milder degrees of oxidant stress, which are insufficient to generate an increase in [Ca2+]i. The activation of SOK-1 may be one of the cell's earliest responses to inducers of necrotic cell death.