Initiation of caspase‐independent death in mouse mesangial cells by Cd2+: Involvement of p38 kinase and CaMK‐II

Abstract Cadmium (Cd) is a toxic metal with multiple effects on cell signaling and cell death. We studied the effects of Cd 2+ on quiescent mouse mesangial cells in serum‐free conditions. Cadmium induces cell death over 6 h through annexin V+ states without or with causing uptake of propidium iodide, termed apoptotic and apoptosis‐like death, respectively. Little or no necrosis is observed, and cell death is caspase‐independent and associated with nuclear translocation of the apoptosis‐inducing factor, AIF. We previously showed that Cd 2+ increased phosphorylation of Erk and CaMK‐II, and CaMK‐II activation increased cell death in an Erk‐independent manner. Here we demonstrate that Cd 2+ increases Jnk and p38 kinase phosphorylation, and inhibition of p38—but not of Jnk—increases cell viability by suppressing apoptosis in preference to apoptosis‐like death. Neither p38 kinase nor CaMK‐II inhibition protects against a decrease in mitochondrial membrane potential, ψ, indicating that kinase‐mediated death is either independent of, or involves events downstream of a mitochondrial pathway. However, both the antioxidant N ‐acetyl cysteine (NAC) and the mitochondrial membrane‐stabilizing agent cyclosporine A (CsA) partially preserve ψ, suppress activation of p38 kinase, and partially protect the cells from Cd 2+ ‐induced death. Whereas the effect of CsA is on apoptosis, NAC acts on apoptosis‐like death. Inhibition of glutathione synthesis exacerbates a Cd 2+ ‐dependent increase in cellular peroxides and favors apoptosis‐like death over apoptosis. The caspase‐independence of these modes of cell death is not due to an absence of this machinery in the mesangial cells: when they are exposed to Cd 2+ for longer periods in the presence of serum, procaspase‐3 and PARP are cleaved and caspase inhibition is protective. We conclude that Cd 2+ can kill mesangial cells by multiple pathways, including caspase‐dependent and ‐independent apoptotic and apoptosis‐like death. Necrosis is not prominent. Activation of p38 kinase and of CaMK‐II by Cd 2+ are associated with caspase‐independent apoptosis that is not dependent on mitochondrial destabilization. J. Cell. Physiol. 217: 307–318, 2008. © 2008 Wiley‐Liss, Inc.