Role and regulation of activation of caspases in cisplatin-induced injury to renal tubular epithelial cells

Background. Cellular and molecular mechanisms responsible for cisplatin-induced nephrotoxicity to renal tubular epithelial cells are not well understood. Although caspases play a critical role in the execution of the cell death pathway, their specific role in toxic injury to renal tubular epithelial cells has not been elucidated previously. Methods. The role of caspases in cisplatin-induced injury was determined using caspase inhibitors and p35 transfected LLC-PK1 cells. The Akt/PKB phosphorylation pathway was studied for the regulation of caspase activation in these cells. Results. The activation of initiator caspases-8, -9 and -2, and executioner caspase-3 began after eight hours of cisplatin treatment, thereafter markedly increased in a time (8 to 24 hours) and dose-dependent manner (0 to 200 mu mol/L). Proinflammatory caspase-1 did not show cisplatin-induced activation. Inhibition of caspase-3 by over expressing cowpox virus p35 protein or alternatively by the peptide inhibitor DEVD-CHO provided marked protection against cell death and partial protection against DNA damage. We then examined the role of the Akt/PKB phosphorylation pathway in regulation of cisplatin-induced caspase activation. There was a marked induction of Akt/PKB phosphorylation in a time (0 to 8 hours) and dose-dependent (0 to 200 mu mol/L) manner during the course of cisplatin injury. Cisplatin-induced Akt/PKB activation was associated with Bad phosphorylation, suggesting induction of a cell survival signal mediated by the Bcl-2 family member, Bad. Wortmannin or LY294002, two structurally dissimilar inhibitors of phosphatidylinositol 3'-kinase (PI-3 kinase), abolished both cisplatin-induced Akt phosphorylation and Bad phosphorylation, and promoted cisplatin-induced early and accelerated activation of caspase-3 and caspase-9, but not of caspase-8 and caspase-1, indicating that inhibition of the Akt/PKB phosphorylation pathway enhances the mitochondrial-dependent activation of caspases. The impact of enhanced activation of caspases by wortmannin or LY294002 was reflected on accelerated cisplatin-induced cell death. Conclusions. These studies demonstrate differential activation and role of caspases in cisplatin injury, and provide the first evidence of cisplatin-induced induction of the Akt/PKB phosphorylation pathway, inhibition of which enhances activation of caspase-3 and caspase-9.