For whom the bell tolls: An in-depth analysis of the mechanism of granulocyte apoptosis.

摘要

Neutrophil apoptosis is controlled by a complex balance of pro- and anti-apoptotic signals and it appears that deregulation of these signals can result in tissue damage and disease. Through our experiments we found that engagement of the β2 integrin, an essential component for normal neutrophil function, either in a clustered or activated state is a potent inhibitor of spontaneous apoptosis. β2-integrin engagement inhibits apoptosis via the activation of the anti-apoptotic Akt and Erk pathways, although this occurred differentially as clustering of the β2-integrin resulted in the activation of both Akt and Erk while activation of the integrin only resulted in Akt activation. Four other important conclusions came about through these initial studies on the role of the β2-integrin on neutrophil apoptosis. Primarily we determined that the combination of anti- and pro-apoptotic stimuli results in a significant increase in intracellular oxidants which inactivate Akt in a SHIP (SH2 containing inositol-5 phosphatase) dependent manner. Secondly we realized that the inactivation of Akt left the cells susceptible to enhanced apoptotic processes in part due to loss of Bax serine phosphorylation. Serine phosphorylation of Bax promotes its heterodimerization to anti-apoptotic Bcl-2 family members, thus keeping it in an inactive state in the cytoplasm. Thirdly we established that the difference in Erk signaling between integrin clustering and clustering with subsequent activation was due to oxidants resultant upon β2-integrin is activated. One major difference between neutrophils and eosinophils is their response to glucocorticoids. While eosinophils are highly susceptible to glucocorticoid-induced apoptosis, the neutrophil life span is actually enhanced. We attributed the enhancement of eosinophil apoptosis to the production of oxidants with subsequent JNK activation. We also found that GM-CSF combated these effects via the synthesis of XIAP.; In all we have elucidated a number of mechanisms for the regulation of neutrophil apoptosis in both normal and inflamed environments and have provided insight into the potential regulation of the rapid spontaneous apoptotic process these cells undergo. Lastly we have begun to understand some of the apparent differences existing between neutrophil and eosinophil apoptosis.