NF-κB Regulation in Human Neutrophils by Nuclear IκBα: Correlation to Apoptosis

Neutrophils are among the first circulating leukocytes involved in acute inflammatory processes. Transcription factor NF-kappaB plays a key role in the inflammatory response, regulating the expression of proinflammatory and anti-apoptotic genes. Recently we have shown that human neutrophils contain a significant amount of NF-kappaB inhibitor, IkappaBalpha, in the nucleus of unstimulated cells. The present objective was to examine the mechanisms controlling the nuclear content of IkappaBalpha in human neutrophils and to determine whether increased accumulation of IkappaBalpha in the nucleus is associated with increased neutrophil apoptosis. We show for the first time that neutrophil stimulation with pro-inflammatory signals results in degradation of IkappaBalpha that occurs in both cytoplasm and nucleus. Prolonged (2-h) stimulation with TNF and LPS induces resynthesis of IkappaBalpha that is again translocated to the nucleus in human neutrophils, but not in monocytic cells. Leptomycin B, a specific inhibitor of nuclear export, increases nuclear accumulation of IkappaBalpha in stimulated neutrophils by blocking the IkappaBalpha nuclear export, and this is associated with inhibition of NF-kappaB activity, induction of caspase-3 activation, and apoptosis. Based on our data we present a new model of NF-kappaB regulation in human neutrophils by nuclear IkappaBalpha. Our results demonstrate that the NF-kappaB activity in human neutrophils is regulated by mechanisms clearly different from those in monocytes and other human cells and suggest that the increased nuclear content of IkappaBalpha in human neutrophils might represent one of the underlying mechanisms for the increased apoptosis in these cells.