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Analyses of neutrophil death mechanisms have revealed many similarities with other cell types; however, a few important molecular features make these cells unique executors of cell death mechanisms. For instance, in order to fight invading pathogens, neutrophils possess a potent machinery to produce reactive oxygen species (ROS), the phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Evidence is emerging that these ROS are crucial in the execution of most neutrophil cell death mechanisms. Likewise, neutrophils exhibit many diverse granules that are packed with cytotoxic mediators. Of those, cathepsins were recently shown to activate pro-apoptotic B-cell lymphoma-2 (Bcl-2) family members and caspases, thus acting on apoptosis regulators. Moreover, neutrophils have few mitochondria, which hardly participate in ATP synthesis, as neutrophils gain energy from glycolysis. In spite of relatively low levels of cytochrome c in these cells, the mitochondrial death pathway is functional. In addition to these pecularities defining neutrophil death pathways, neutrophils are terminally differentiated cells, hence they do not divide but undergo apoptosis shortly after maturation. The initial trigger of this spontaneous apoptosis remains to be determined, but may result from low transcription and translation activities in mature neutrophils. Due to the unique biological characteristics of neutrophils, pharmacological intervention of inflammation has revealed unexpected and sometimes disappointing results when neutrophils were among the prime target cells during therapy. In this study, we review the current and emerging models of neutrophil cell death mechanisms with a focus on neutrophil peculiarities.

Class IA phosphoinositide 3-kinases (PI3Ks) signal downstream of tyrosine kinases and Ras and control a wide variety of biological responses. In mammals, these heterodimeric PI3Ks consist of a p110 catalytic subunit (p110␣, p110␤, or p110␦) bound to any of five distinct regulatory subunits (p85␣, p85␤, p55␥, p55␣, and p50␣, collectively referred to as ''p85s''). The relative expression levels of p85 and p110 have been invoked to explain key features of PI3K signaling. For example, free (i.e., non-p110-bound) p85␣ has been proposed to negatively regulate PI3K signaling by competition with p85/p110 for recruitment to phosphotyrosine docking sites. Using affinity and ion exchange chromatography and quantitative mass spectrometry, we demonstrate that the p85 and p110 subunits are present in equimolar amounts in mammalian cell lines and tissues. No evidence for free p85 or p110 subunits could be obtained. Cell lines contain 10,000 -15,000 p85/p110 complexes per cell, with p110␤ and p110␦ being the most prevalent catalytic subunits in nonleukocytes and leukocytes, respectively. These results argue against a role of free p85 in PI3K signaling and provide insights into the nonredundant functions of the different class IA PI3K isoforms.quantitative mass spectrometry ͉ signaling ͉ protein stability ͉ gene knockout ͉ lipid kinase P hosphoinositide 3-kinases (PI3Ks) generate lipid second messengers that serve as membrane docking sites for a variety of downstream effector proteins such as protein kinases, regulators of small GTPases, and scaffolding proteins (1, 2). The class IA PI3Ks are heterodimers consisting of a p110 catalytic subunit and a smaller regulatory subunit with Src-homology 2 (SH2) domains. Mammals have three catalytic subunits (p110␣, p110␤, p110␦) and five regulatory subunits (p85␣, p85␤, p55␥, p55␣, p50␣) (1, 2). Under experimental conditions, each p110 isoform can bind any p85 isoform with no apparent preference (see among others, refs. 3 and 4). p85s have a dual effect on the p110 subunits because they stabilize the thermally labile p110s but also conformationally inhibit their catalytic activity (5). Upon cellular stimulation, SH2 domain-mediated recruitment of p85/p110 complexes to Tyr phosphorylated (pY) membraneproximal proteins serves dual functions: It positions p110 in proximity with its substrates, and the engagement of the p85 SH2 domains relieves p85-mediated inhibition of p110, thus increasing enzymatic activity of p110 (6, 7).Experiments using gene-targeted mice and p110 isoformselective inhibitors have uncovered nonredundant physiological functions of the p110 isoforms. These functions include insulin signaling in metabolic tissues (p110␣; refs. 8 and 9), integrin signaling in platelets (p110␤; ref. 10) and signaling through a variety of receptors in leukocytes (p110␦; refs. 11-15). The mechanisms of this nonredundant signaling are not fully understood. Indeed, p110 isoforms have high homology in their primary sequence, interact nonselectively with the different p85s, and have the same lipid...

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