Lipid Rafts Regulate Lipopolysaccharide-induced Activation of Cdc42 and Inflammatory Functions of the Human Neutrophil

New players in TLR-mediated innate immunity: PI3K and small Rho GTPases

Toll-like receptors (TLRs) play a crucial role in the innate immune system as a first line of defense against pathogens. TLR activation in phagocytes produces pro-inflammatory cytokines and chemokines that contribute directly to elimination of infectious agents and activation of adaptive immune responses. However, a sustained inflammatory response can result in tissue damage and generalized sepsis. This review summarizes the complex and sometimes conflicting links of TLR signaling with two important regulators of immune cells functions: phosphoinositide 3-kinases (PI3Ks) and small GTPases of the Rho family. A unified model of hierarchical organization of these signaling participants is still premature, given that the tools for delineating how control of TLRmediated pathways is achieved are just emerging. Critical progress in our understanding of spatial-temporal propagation of TLR signaling will certainly be provided in the near future by pharmacological targeting of PI3Ks using recently characterized, second-generation PI3K inhibitors in combination with gene-targeting strategies for PI3K subunits and Rho GTPases targeted to the murine myeloid compartment.

RhoA and p38 MAPK mediate apoptosis induced by cellular cholesterol depletion

Cholesterol is essential for cell viability, and homeostasis of cellular cholesterol is crucial to various cell functions. Here we examined the effect of cholesterol depletion on apoptosis and the mechanisms underlying this effect in NIH3T3 cells. We show that chronic cholesterol depletion achieved with lipoprotein-deficient serum (LPDS) and 25-hydroxycholesterol (25-HC) treatment resulted in a significant increase in cellular apoptosis and caspase-3 activation. This effect is not due to a deficiency of nonsterol isoprenoids, intermediate metabolites of the cholesterol biosynthetic pathway, but rather to low cholesterol levels, since addition of cholesterol together with LPDS and 25-HC nearly abolished apoptosis, whereas addition of farnesyl pyrophosphate or geranylgeranyl-pyrophosphate did not reverse the cell viability loss induced by LPDS plus 25-HC treatment. These effects were accompanied by an increase in ERK, JNK and p38 MAPK activity. However, only the inhibition of p38 MAPK with the specific inhibitor SB203580 or the overexpression of a kinase defective MKK6 resulted in a significant decrease in apoptosis and caspase-3 cleavage induced by cholesterol depletion. Furthermore, LPDS plus 25-HC increased RhoA activity, and this effect was reversed by addition of exogenous cholesterol. Finally, overexpression of the dominant negative N19RhoA inhibited p38 MAPK phosphorylation and apoptosis induced by low cholesterol levels. Together, our results demonstrate that cholesterol depletion induces apoptosis through a RhoA- and p38 MAPK-dependent mechanism.

Impairment of SHP-1 down-regulation in the lipid rafts of human neutrophils under GM-CSF stimulation contributes to their age-related, altered functions

It has been shown that the functions and the rescue from apoptosis by proinflammatory mediators of polymorphonuclear leukocytes (PMN) tend to diminish with aging. Here, we investigated the role of protein tyrosine phosphatases (PTP), especially Src homology domain-containing protein tyrosine phosphatase-1 (SHP-1), in the age-related, altered PMN functions under granulocyte macrophage-colony stimulating factor (GM-CSF) stimulation. The inhibition of PTP suggested a differential effect of GM-CSF on phosphatase activity in modulating PMN functions with aging. The down-regulation of phosphatase activity of immunopurified SHP-1 from lipid rafts of PMN of young donors was found significantly altered at 1 min of stimulation with aging. In young donors, SHP-1 is displaced from lipid rafts at 1 min of stimulation, whereas in the elderly, SHP-1 is constantly present. We assessed in PMN lipid rafts the phosphorylation of tyrosine and serine residues of SHP-1, which regulates its activity. We observed an alteration in the phosphorylation of tyrosine and serine residues of SHP-1 in PMN of elderly subjects, suggesting that GM-CSF was unable to inhibit SHP-1 activity by serine phosphorylation. GM-CSF activates Lyn rapidly, and we found alterations in its activation and translocation to the lipid rafts with aging. We also demonstrate that SHP-1 in the PMN of elderly is constantly recruited to Lyn, which cannot be relieved by GM-CSF. In contrast, in the young, the resting recruitment could be relieved by GM-CSF. Our results suggest an alteration of the SHP-1 modulation by GM-CSF in lipid rafts of PMN with aging. These alterations could contribute to the decreased GM-CSF effects on PMN.

Plasma membrane organization is essential for balancing competing pseudopod- and uropod-promoting signals during neutrophil polarization and migration

Exposure of neutrophils to chemoattractant induces cell polarization and migration. These behaviors require the asymmetric activation of distinct signaling pathways and cytoskeletal elements in the protruding pseudopod at the front of cells and the retracting uropod at the rear. An important outstanding question is, how does the organization of the plasma membrane participate in establishing asymmetry during polarization and migration? To answer this question, we investigated the function of cholesterol, a lipid known to influence membrane organization. Using controlled cholesterol depletion, we found that a cholesterol-dependent membrane organization enabled cell polarization and migration by promoting uropod function and suppressing ectopic pseudopod formation. At a mechanistic level, we showed that cholesterol was directly required for suppressing inappropriate activation of the pseudopod-promoting Gi/PI3-kinase signaling pathway. Furthermore, cholesterol was required for dampening Gi-dependent negative feedback on the RhoA signaling pathway, thus enabling RhoA activation and uropod function. Our findings suggest a model in which a cholesterol-dependent membrane organization plays an essential role in the establishment of cellular asymmetry by balancing the activation and segregating the localization of competing pseudopod- and uropod-inducing signaling pathways during neutrophil polarization and migration.

Structural organization of the neutrophil NADPH oxidase: phosphorylation and translocation during priming and activation

The reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is part of the microbicidal arsenal used by human polymorphonuclear neutrophils (PMNs) to eradicate invading pathogens. The production of a superoxide anion (O2-) into the phagolysosome is the precursor for the generation of more potent products, such as hydrogen peroxide and hypochlorite. However, this production of O2- is dependent on translocation of the oxidase subunits, including gp91phox, p22phox, p47phox, p67phox, p40phox, and Rac2 from the cytosol or specific granules to the plasma membrane. In response to an external stimuli, PMNs change from a resting, nonadhesive state to a primed, adherent phenotype, which allows for margination from the vasculature into the tissue and chemotaxis to the site of infection upon activation. Depending on the stimuli, primed PMNs display altered structural organization of the NADPH oxidase, in that there is phosphorylation of the oxidase subunits and/or translocation from the cytosol to the plasma or granular membrane, but there is not the complete assembly required for O2- generation. Activation of PMNs is the complete assembly of the membrane-linked and cytosolic NADPH oxidase components on a PMN membrane, the plasma or granular membrane. This review will discuss the individual components associated with the NADPH oxidase complex and the function of each of these units in each physiologic stage of the PMN: rested, primed, and activated.

Binding of Amyloidogenic Transthyretin to the Plasma Membrane Alters Membrane Fluidity and Induces Neurotoxicity

Transthyretin (TTR) can deposit as amyloid in the peripheral nervous system and induce a peripheral neuropathy. We examined the mechanism of TTR amyloid neurotoxicity on SH-SY5Y neuroblastoma cells. Wild-type (WT) TTR and two amyloidogenic mutants (V30M and L55P) were expressed in Escherichia coli. Incubation (aging) of WT TTR at 37 degrees C for 1 week caused no significant aggregation. However, there was a significant increase in the extent of amyloid fibril formation after the amyloidogenic mutants had been aged. L55P TTR aggregated more readily than V30M TTR. Both amyloidogenic mutants were neurotoxic after aging. The order of neurotoxicity was as follows: L55P > V30M > WT. As binding of amyloid proteins to the plasma membrane may cause cytotoxicity, we studied the binding of TTR to a plasma membrane-enriched preparation from SH-SY5Y cells by surface plasmon resonance. All three forms bound to the plasma membrane through electrostatic interactions. The binding of the amyloidogenic mutants was increased by aging. The amount of binding correlated closely with the amount of aggregation and with the cytotoxicity of each form. As membrane fluidity can influence cell viability, we also examined the effect of TTR on membrane fluidity using a fluorescence anisotropy method. Binding of the amyloidogenic TTR mutants increased membrane fluidity, and once again, the order of potency was as follows: L55P > V30M > WT. These results demonstrate that TTR can bind to the plasma membrane and cause a change in membrane fluidity. Altered membrane fluidity may be the cause of the neurotoxicity.

Regulation of innate immunity by Rho GTPases

Leukocytes are key cellular components of innate immunity. These phagocytic cells respond to bacteria at sites of infection through chemotactic sensing and directed motility regulated by Rho GTPases. The development of sensitive probes of Rho GTPase dynamics has provided insights into the temporal and spatial aspects of GTPase regulation during chemotaxis and subsequent microbial phagocytosis. The resulting destruction of ingested bacteria by means of reactive oxygen species (ROS) depends on a Rac-regulated "molecular switch" that is modulated by antagonistic crosstalk involving Cdc42. Recent studies of leukocytes derived from Rac1- and Rac2-knockout mice have shown that these highly homologous GTPases have unique biological roles. An understanding of the biochemical basis for such distinct activities should provide novel insights into the molecular details of Rho GTPase function and regulation in innate immunity.

The expression and roles of Toll-like receptors in the biology of the human neutrophil

Neutrophils are amongst the first immune cells to arrive at sites of infection, where they initiate antimicrobial and proinflammatory functions, which serve to contain infection. Sensing and defeating microbial infections are daunting tasks as a result of their molecular heterogeneity; however, Toll-like receptors (TLRs) have emerged as key components of the innate-immune system, activating multiple steps in the inflammatory reaction, eliminating invading pathogens, and coordinating systemic defenses. Activated neutrophils limit infection via the phagocytosis of pathogens and by releasing antimicrobial peptides and proinflammatory cytokines and generating reactive oxygen intermediates. Through the production of chemokines, they additionally recruit and activate other immune cells to aid the clearance of the microbes and infected cells and ultimately, mount an adaptive immune response. In acute inflammation, influx of neutrophils from the circulation leads to extremely high cell numbers within tissues, which is exacerbated by their delayed, constitutive apoptosis caused by local inflammatory mediators, potentially including TLR agonists. Neutrophil apoptosis and safe removal by phagocytic cells limit tissue damage caused by release of neutrophil cytotoxic granule contents. This review addresses what is currently known about the function of TLRs in the biology of the human neutrophil, including the regulation of TLR expression, their roles in cellular recruitment and activation, and their ability to delay apoptotic cell death.

Fc gammaRIIIB stimulation promotes beta1 integrin activation in human neutrophils

The molecular stimuli involved in receptor-induced integrin activation are still poorly defined. We have investigated the role of receptors for the Fc portion of immunoglobulin G molecules (Fc gammaR) on activation of integrins in human neutrophils. Cross-linking of Fc gammaRIIA induced an increase in surface expression of beta2 integrins but had no effect on beta1 integrins. In contrast, cross-linking of Fc gammaRIIIB not only increased beta2 integrins on the cell surface but also induced beta1 integrin activation, as indicated by an increase in binding to fibronectin and the appearance of an activation epitope detected by the monoclonal antibody 15/7. The Fc gammaRIIIB-induced increase of beta2 integrins required Src-family tyrosine kinases, Syk kinase, and phosphatidylinositol-3 kinase (PI-3K), as the corresponding, specific inhibitors, PP2, Piceatannol, and LY294002, completely blocked it. Contrary to this, Fc gammaRIIIB-induced beta1 integrin activation was not blocked by PP2 or LY294002. It was, however, enhanced by Piceatannol. After Fc gammaRIIIB cross-linking, colocalization of Fc gammaRIIIB and active beta1 integrins was detected on the neutrophil membrane. These data show, for the first time, that cross-linking of Fc gammaRIIIB induces an inside-out signaling pathway that leads to beta1 integrin activation. This activation is independent of Src-family kinases, and PI-3K and may be induced in part by the interaction of Fc gammaRIIIB with beta1 integrins.

A role for hydroxy-methylglutaryl coenzyme a reductase in pulmonary inflammation and host defense

RATIONALE

A growing literature indicates that hydroxy-methylglutaryl coenzyme A reductase inhibitors (statins) modulate proinflammatory cellular signaling and functions. No studies to date, however, have addressed whether statins modulate pulmonary inflammation triggered by aerogenic stimuli or whether they affect host defense.

OBJECTIVES

To test whether lovastatin modulates LPS-induced pulmonary inflammation and antibacterial host defense.

METHODS

To address these questions, and to confirm any effect of statins as dependent on inhibition of hydroxy-methylglutaryl coenzyme A reductase, we treated C57Bl/6 mice with three oral doses of 10 mg/kg lovastatin (or vehicle) and three intraperitoneal doses of 10 mg/kg mevalonic acid (or saline), and then exposed them to the following: (1) aerosolized LPS, (2) intratracheal keratinocyte-derived chemokine (KC), or (3) intratracheal Klebsiella pneumoniae.

MEASUREMENTS AND MAIN RESULTS

LPS- and KC-induced airspace neutrophils were reduced by lovastatin, an effect that was blocked by mevalonic acid cotreatment. Lovastatin was also associated with reduced parenchymal myeloperoxidase and microvascular permeability, and altered airspace and serum cytokines after LPS. Native pulmonary clearance of K. pneumoniae was inhibited by lovastatin and extrapulmonary dissemination was enhanced, both reversibly with mevalonic acid. Ex vivo studies of neutrophils isolated from lovastatin-treated mice confirmed inhibitory effects on Rac activation, actin polymerization, chemotaxis, and bacterial killing.

CONCLUSION

Lovastatin attenuates pulmonary inflammation induced by aerosolized LPS and impairs host defense.