NF-κB activation is a critical regulator of human granulocyte apoptosis in vitro

Abstract

During beneficial inflammation, potentially tissue-damaging granulocytes undergo apoptosis before being cleared by phagocytes in a non-phlogistic manner. Here we show that the rate of constitutive apoptosis in human neutrophils and eosinophils is greatly accelerated in both a rapid and concentration-dependent manner by the fungal metabolite gliotoxin, but not by its inactive analog methylthiogliotoxin. This induction of apoptosis was abolished by the caspase inhibitor zVAD-fmk, correlated with the inhibition of nuclear factor-kappa B (NF-κB), and was mimicked by a cell permeable inhibitory peptide of NF-κB, SN-50; other NF-κB inhibitors, curcumin and pyrrolidine dithiocarbamate; and the proteasome inhibitor, MG-132. Gliotoxin also augmented dramatically the early (2-6 h) pro-apoptotic effects of tumor necrosis factor-α (TNF-α) in neutrophils and unmasked the ability of TNF- α to induce eosinophil apoptosis. In neutrophils, TNF-α caused a gliotoxin- inhibitable activation of an inducible form of NF-κB, a response that may underlie the ability of TNF-α to delay apoptosis at later times (12-24 h) and limit its early killing effect. Furthermore, cycloheximide displayed a similar capacity to enhance TNF-α induced neutrophil apoptosis even at time points when cycloheximide alone had no proapoptotic effect, suggesting that NF-κB may regulate the production of protein(s) which protect neutrophils from the cytotoxic effects of TNF-α. These data shed light on the biochemical and molecular mechanisms regulating human granulocyte apoptosis and, in particular, indicate that the transcription factor NF-κB plays a crucial role in regulating the physiological cell death pathway in granulocytes.