Jean Parodo

Pre–B cell colony–enhancing factor inhibits neutrophil apoptosis in experimental inflammation and clinical sepsis

Pre-B cell colony-enhancing factor (PBEF) is a highly conserved 52-kDa protein, originally identified as a growth factor for early stage B cells. We show here that PBEF is also upregulated in neutrophils by IL-1beta and functions as a novel inhibitor of apoptosis in response to a variety of inflammatory stimuli. Induction of PBEF occurs 5-10 hours after LPS exposure. Prevention of PBEF translation with an antisense oligonucleotide completely abrogates the inhibitory effects of LPS, IL-1, GM-CSF, IL-8, and TNF-alpha on neutrophil apoptosis. Immunoreactive PBEF is detectable in culture supernatants from LPS-stimulated neutrophils, and a recombinant PBEF fusion protein inhibits neutrophil apoptosis. PBEF is also expressed in neutrophils from critically ill patients with sepsis in whom rates of apoptosis are profoundly delayed. Expression occurs at higher levels than those seen in experimental inflammation, and a PBEF antisense oligonucleotide significantly restores the normal kinetics of apoptosis in septic polymorphonuclear neutrophils. Inhibition of apoptosis by PBEF is associated with reduced activity of caspases-8 and -3, but not caspase-9. These data identify PBEF as a novel inflammatory cytokine that plays a requisite role in the delayed neutrophil apoptosis of clinical and experimental sepsis.

Hypertonicity prevents lipopolysaccharide-stimulated CD11b/CD18 expression in human neutrophils in vitro: role for p38 inhibition.

BACKGROUND Neutrophil sequestration in the lungs plays an important role in the development of acute respiratory distress syndrome. We previously reported that hypertonic saline resuscitation attenuated lung injury after hemorrhagic shock and lipopolysaccharide (LPS) by abolishing neutrophil CD11b up-regulation. We investigated the mechanism underlying this effect. METHODS Human neutrophils were exposed to LPS in the presence or absence of hypertonicity or SB203580 (p38 inhibitor). CD11b and CD14 were studied by immunofluorescence and p38 phosphorylation by immunoblotting. RESULTS Hypertonicity had no effect on CD11b or CD14, caused a weak p38 phosphorylation, and completely prevented the LPS-induced p38 phosphorylation and CD11b up-regulation. p38 inhibition also abrogated CD11b up-regulation by LPS. CONCLUSION MAPKp38 is important in CD11b regulation by LPS. The inhibitory effect of hypertonicity on the LPS-mediated effect may contribute to its protective anti-inflammatory effect observed in vivo. Transient hypertonicity might minimize organ injury in diseases characterized by neutrophil-mediated damage such as ARDS.

Granulocytic differentiation of HL-60 cells results in spontaneous apoptosis mediated by increased caspase expression

HL‐60 cells differentiating into neutrophil‐like cells die an apoptotic death in vitro. Susceptibility to apoptosis is associated with decreased Bcl‐2 protein and mRNA expression; however, the effect of differentiation on the expression of pro‐apoptotic caspases is unknown. Spontaneous apoptosis occurred 6 days after retinoic acid treatment. Western blotting showed loss of Bcl‐2 by day 7, and new expression of ICE (caspase 1) and CPP32 (caspase 3) protein by day 2. Northern analysis demonstrated loss of Bcl‐2 mRNA and increases in ICE mRNA by day 2; CPP32 mRNA was unchanged. Differential Bcl‐2 and ICE mRNA expression was also found when granulocytic differentiation was stimulated by DMSO. Differentiated HL‐60 cell lysates exhibited functional ICE proteolytic activity. De novo caspase expression was responsible for the development of spontaneous apoptosis, since specific inhibitors of ICE (YVAD‐CMK) and CPP32 (DEVD‐CHO), inhibited retinoic acid induced spontaneous apoptosis. Functional maturation and susceptibility to apoptosis are both inducible and linked in this granulocyte precursor cell line.

Phagocytosis of Candida albicans induces apoptosis of human neutrophils.

Neutrophil-mediated inflammation is terminated through the programmed cell death or apoptosis of the neutrophil, a process that can be inhibited by soluble mediators released during an inflammatory response. It has been reported, however, that the phagocytosis of intact bacteria can accelerate apoptosis. We evaluated the effects of the phagocytosis of a common nosocomial pathogen, Candida albicans, on the expression of apoptosis. Phagocytosis of killed Candida induced a dose-dependent increase in the apoptosis of normal neutrophils after 18 h of in vitro culture, from 40.7+/-9.1% to 81.7+/-4.5%, while supernatants from neutrophil:Candida co-cultures actually inhibited apoptosis. Induction of apoptosis was not dependent on phagocytosis, since opsonization of yeast with serum failed to increase apoptosis, while inhibition of phagocytosis with latrunculin B resulted in a slightly increased apoptotic rate. Increased apoptosis induced by Candida was associated with increased activity of the membrane-associated apoptotic enzyme, caspase 8, and with increased expression of the active form of the key executioner caspase, caspase 3. Increased apoptosis was associated with depletion of intracellular glutathione (GSH), and could be inhibited by the addition of exogenous GSH. These data demonstrate an important physiologic role for host-pathogen interactions in the resolution of inflammation and suggest that the response to an invading pathogen is an important stimulus to the restoration of normal immunologic homeostasis.

Dynamic Regulation of Neutrophil Survival through Tyrosine Phosphorylation or Dephosphorylation of Caspase-8

Efficient expression of innate immunity is critically dependent upon the capacity of the neutrophil to be activated rapidly in the face of an acute threat and to involute once that threat has been eliminated. Here we report a novel mechanism regulating neutrophil survival dynamically through the tyrosine phosphorylation or dephosphorylation of caspase-8. Caspase-8 is tyrosine-phosphorylated in freshly isolated neutrophils but spontaneously dephosphorylates in culture, in association with the progression of constitutive apoptosis. Phosphorylation of caspase-8 on Tyr-310 facilitates its interaction with the Src-homology domain 2 containing tyrosine phosphatase-1 (SHP-1) and enables SHP-1 to dephosphorylate caspase-8, permitting apoptosis to proceed. The non-receptor tyrosine kinase, Lyn, can phosphorylate caspase-8 on Tyr-397 and Tyr-465, rendering it resistant to activational cleavage and inhibiting apoptosis. Exposure to lipopolysaccharide reduces SHP-1 activity and binding to caspase-8, caspase-8 activity, and rates of spontaneous apoptosis. SHP-1 activity is reduced and Lyn increased in neutrophils from patients with sepsis, in association with profoundly delayed apoptosis; inhibition of Lyn can partially reverse this delay. Thus the phosphorylation and dephosphorylation of caspase-8, mediated by Lyn and SHP-1, respectively, represents a novel, dynamic post-translational mechanism for the regulation of neutrophil apoptosis whose dysregulation contributes to persistent neutrophil survival in sepsis.

Impaired apoptotic death signaling in inflammatory lung neutrophils is associated with decreased expression of interleukin-1beta converting enzyme family proteases (caspases)

BACKGROUND Fas and tumor necrosis factor receptor 1 (TNFR1) are membrane proteins that signal for apoptotic cell death by downstream activation of proteins of the interleukin-1 beta converting enzyme (ICE) family. Spontaneous apoptosis is delayed in neutrophils activated by transmigration into an inflammatory focus. In this study we evaluated the effects of transmigration on Fas and TNFR1-induced apoptosis and apoptotic gene expression. METHODS Sprague-Dawley rats were killed 4 hours after intratracheal challenge with 500 micrograms lipopolysaccharide (LPS). Neutrophils isolated from the systemic circulation (circulation) or bronchoalveolar lavage fluid (lung) were incubated with or without an agonistic antibody to Fas (clone CH-11, 100 ng/ml) or TNF (10 ng/ml) for 24 hours. Apoptosis and Fas expression were assessed by flow cytometry. Expression of the antiapoptotic protein Bcl-2 and proapoptotic proteins ICE and CPP32 were measured by Western blots. RESULTS Neutrophils transmigrating into the lung in response to LPS showed delayed apoptosis compared with circulating neutrophils and failed to undergo apoptosis in response to anti-Fas antibody or TNF-alpha. Fas expression was unaltered; however, TNFR1 expression was reduced. Bcl-2 was not detected in either group; both the pro- and active forms of ICE and active CPP32 were significantly decreased in lung neutrophils. The specific ICE inhibitor, YVAD-CMK, partially blocked the increased rates of apoptosis resulting from engagement of Fas or TNFR1. CONCLUSIONS Neutrophil transmigration retards apoptosis through engagement of the death receptors Fas and TNFR1. This refractory state is associated with reduced levels of proapoptotic proteins. Blunted responsiveness to physiologic apoptotic stimuli prolongs neutrophil functional survival during acute inflammation and may contribute to the tissue injury associated with acute respiratory distress syndrome.

Tyrosine Phosphorylation of Caspase-8 Abrogates Its Apoptotic Activity and Promotes Activation of c-Src

Src family tyrosine kinases (SFKs) phosphorylate caspase-8A at tyrosine (Y) 397 resulting in suppression of apoptosis. In addition, the phosphorylation of caspase-8A at other sites including Y465 has been implicated in the regulation of caspase-8 activity. However, the functional consequences of these modifications on caspase-8 processing/activity have not been elucidated. Moreover, various Src substrates are known to act as potent Src regulators, but no such role has been explored for caspase-8. We asked whether the newly identified caspase-8 phosphorylation sites might regulate caspase-8 activation and conversely, whether caspase-8 phosphorylation might affect Src activity. Here we show that Src phosphorylates caspase-8A at multiple tyrosine sites; of these, we have focused on Y397 within the linker region and Y465 within the p12 subunit of caspase-8A. We show that phosphomimetic mutation of caspase-8A at Y465 prevents its cleavage and the subsequent activation of caspase-3 and suppresses apoptosis. Furthermore, simultaneous phosphomimetic mutation of caspase-8A at Y397 and Y465 promotes the phosphorylation of c-Src at Y416 and increases c-Src activity. Finally, we demonstrate that caspase-8 activity prevents its own tyrosine phosphorylation by Src. Together these data reveal that dual phosphorylation converts caspase-8 from a pro-apoptotic to a pro-survival mediator. Specifically, tyrosine phosphorylation by Src renders caspase-8 uncleavable and thereby inactive, and at the same time converts it to a Src activator. This novel dynamic interplay between Src and caspase-8 likely acts as a potent signal-integrating switch directing the cell towards apoptosis or survival.

Activated neutrophils induce epithelial cell apoptosis through oxidant-dependent tyrosine dephosphorylation of caspase-8.

Activated neutrophils can injure host cells through direct effects of oxidants on membrane phospholipids, but an ability to induce apoptotic cell death has not previously been reported. We show that neutrophils activated in vivo in patients who have sustained multiple trauma or in vitro by exposure to bacterial lipopolysaccharide promote epithelial cell apoptosis through SHP-1-mediated dephosphorylation of epithelial cell caspase-8. Epithelial cell apoptosis induced by circulating neutrophils from patients who had sustained serious injury depended on the generation of neutrophil-derived reactive oxygen intermediates and was blocked by inhibition of NADPH oxidase or restoration of intracellular glutathione. Caspase-8 was constitutively tyrosine phosphorylated in a panel of resting epithelial cells, but underwent SHP-1-dependent dephosphorylation in response to hydrogen peroxide, activated neutrophils, or inhibition of Src kinases. Cells transfected with a mutant caspase-8 in which tyrosine residues at Tyr397 or Tyr465 are replaced by nonphosphorylatable phenylalanine underwent accelerated apoptosis, whereas either mutation of these residues to phosphomimetic glutamic acid or transfection with the Src kinases Lyn or c-Src inhibited hydrogen peroxide-induced apoptosis. Exposure to either hydrogen peroxide or lipopolysaccharide-stimulated neutrophils increased phosphorylation and activity of the phosphatase SHP-1, increased activity of caspases 8 and 3, and accelerated epithelial cell apoptosis. These observations reveal a novel mechanism for neutrophil-mediated tissue injury through oxidant-dependent, SHP-1-mediated dephosphorylation of caspase-8 resulting in enhanced epithelial cell apoptosis.