Role of PI3-kinase–dependent Bad phosphorylation and altered transcription in cytokine-mediated neutrophil survival

Abstract

Phosphoinositide 3-kinase (PI3-kinase)–dependent phosphorylation of the proapoptotic Bcl-2 family member Bad has been proposed as an important regulator of apoptotic cell death. To understand the importance of this pathway in nontransformed hematopoietic cells, we have examined the effect of survival cytokines on PI3-kinase activity and Bad expression and phosphorylation status in human neutrophils. Granulocyte macrophage–colony-stimulating factor (GM-CSF) and tumor necrosis factor-α (TNF-α) both reduced the rate of apoptosis in neutrophils cultured in vitro for 20 hours. Coincubation with the PI3-kinase inhibitor LY294002, which in parallel experiments abolished GM-CSF–primed, fMLP-stimulated superoxide anion production and GM-CSF–stimulated PtdIns(3,4,5)P3accumulation, inhibited the GM-CSF and TNF-α survival effect. In contrast, the MAP kinase kinase (MEK1/2) inhibitor PD98059 and the protein kinase A inhibitor H-89 had only a marginal effect on GM-CSF–mediated neutrophil survival. GM-CSF substantially increased Bad phosphorylation at Ser112 and Ser136 and increased the cytosolic accumulation of Bad. GM-CSF also regulated Bad at a transcription level with a marked decrease in mRNA levels at 4 hours. TNF-α caused a biphasic effect on the rate of morphologic apoptosis, which corresponded to an early increase, and a late inhibition, of Bad mRNA levels. LY294002 inhibited GM-CSF– and TNF-α–mediated changes in Bad phosphorylation and mRNA levels. These data suggest that the survival effect of GM-CSF and TNF-α in neutrophils is caused by a PI3-kinase–dependent phosphorylation and cytosolic translocation of Bad, together with an inhibition of Bad mRNA levels. This has important implications for the regulation of neutrophil apoptosis in vivo.