Regulation of Caspase-3 and -9 Activation in Oxidant Stress to Renal Tubular Epithelial Cells by Forkhead Transcription Factors, Bcl-2 Proteins and Mitogen- Activated Protein Kinases

Cytotoxicity to renal tubular epithelial cells (RTE) is dependent on the relative response of cell survival and cell death signals triggered by the injury. Forkhead transcription factors, Bcl-2 family member Bad, and mitogen-activated protein kinases are regulated by phosphorylation that plays crucial roles in determining cell fate. We examined the role of phosphorylation of these proteins in regulation of H 2 O 2-induced caspase activation in RTE. The phosphorylation of FKHR, FKHRL, and Bcl-2 family member Bad were markedly increased in response to oxidant injury, and this increase was associated with elevated levels of basal phosphorylation of Akt/protein kinase B. PI-3 kinase inhibitors abolished this phosphorylation and also decreased expression of antiapoptotic proteins Bcl-2 and BclxL. Inhibition of phosphorylation of forkhead proteins resulted in a marked increase in proapoptotic protein Bim. These downstream effects of PI-3 kinase inhibition promoted the oxidant-induced activation of caspase-3 and-9, but not caspase-8 and-1. The impact of enhanced activation of caspases by PI-3 kinase inhibition was reflected on accelerated oxidant-induced cell death. Oxidant stress also induced marked phosphorylation of ERK ½, P38, and JNK kinases. Inhibition of ERK ½ phosphorylation but not P38 and JNK kinase increased caspase-3 and-9 activation; however, this activation was far less than induced by inhibition of Akt phosphorylation. Thus, Akt-mediated phosphorylation pathway, ERK signaling and the antiapoptotic Bcl-2 proteins distinctly regulate caspase activation during oxidant injury to RTE. These studies suggest that enhancing renal-specific survival signals may lead to preservation of renal function during oxidant injury.