Inhibitory action of Lipoxins on neutrophil migration

Inhibitory action of Lipoxins on neutrophil migration

Deregulated neutrophilic inflammation and chronic infection lead to progressive destruction of the airways in cystic fibrosis (CF). In normal tissues, lipoxins are endogenous anti-inflammatory lipid mediators in regulation of neutrophilic inflammation [1]. In CF, production of lipoxins is impaired [2], [3].

One striking feature of CF airways is the progressive accumulation of neutrophils. This "acute inflammation" never converts to a more "chronic" pattern. There is certainly an excess of chemoattractants such as Interleukin-8 (IL-8) and Leukotriene B4 recovered in bronchoalveolar lavage fluid. When present in excess, neutrophils and their products actually impair the host's ability to clear bacterial infection [4].

Colonized by bacteria, the CF lung contains a range of potent neutrophil chemoattractants, including the host-derived inflammatory mediators IL-8 and Leukotriene B4. N-formyl-Met-Leu-Phe peptide (fMLP), produced by bacteria, usually stimulates neutrophils to migrate by a mechanism that is mediated by alpha-M/beta-2 integrin (MAC-1), whereas IL-8 and Leukotriene B4 stimulate neutrophils to migrate using an alternative, MAC-1-independent pathway, that is mediated by alpha-L/beta-2 integrin (LFA-1) [5], [6], [7], [8], [9], [10]. Over 70% of migrating neutrophils from CF patients appeared to favor this, LFA-1-dependent, migratory route [11], [12].

The circulating neutrophils from normal tissues express two receptors for IL-8, Interleukin 8 receptor alpha (IL8RA) and Interleukin 8 receptor beta (IL8RB). In contrast, neutrophils from patients with acute and chronic pulmonary inflammation have decreased expression of IL8RB, and only IL8RA is the functionally dominant receptor on these neutrophils [7], [13].

In response to infection or tissue injury, arachidonic acid produces proinflammatory Leukotriene B4 that also induces neutrophil recruitment and acute inflammation [4], [14], [15].

In normal airways, arachidonic acid also produces antiinflammatory lipoxins. Lipoxins mediate switch to chronic inflammation and promote resolution [15], [16], [17]. In CF the inflammatory response remains persistently neutrophilic that leads to tissue injury and further infection. This may be attributed to a documented defect in the generation of lipoxins [1], [2], [3].

Lipoxins are bioactive eicosanoids derived from arachidonic acid. In contrast to proinflammatory leukotrienes and prostaglandins, lipoxins (Lipoxin A4 and 15-epi-LXA4) display potent antiinflammatory actions, including attenuation of neutrophil adhesion to endothelial cells [1], [18].

IL-8, Leukotriene B4 and Lipoxins (Lipoxin A4 and 15-epi-LXA4) interact with highly specific and distinct G protein-coupled membrane receptors [19], [20], to evoke opposing leukocyte responses, including Lipoxin-induced inhibition of chemoattractant-initiated migration of neutrophils [21], [22], [23].

Leukotriene B4 binds to the Leukotriene B4 receptor (LTBR1) that via G-protein alpha-i family and G-protein beta/gamma subunits activates Phosphatidylinositol 3-kinase (PI3K reg class IB (p101) and PI3K cat class IB (p110-gamma)) signaling [24], [25], [26], [27], [28], [29], [30].

IL-8 binding to IL8RA also stimulates PI3K cat class IB (p110-gamma) that phosphorylates the membrane lipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) to phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) [31].

PtdIns(3,4,5)P3 recruits and activates diverse cytosolic effectors, including Phospholipase D1 (PLD1) [32], 3-phosphoinositide dependent protein kinase-1 (PDPK1) [33], v-Akt murine thymoma viral oncogene homologs (AKT) [34], [35], Protein kinase C zeta (PKC-zeta) [36] and Phosphatidylinositol 3,4,5-trisphosphate-dependent RAC exchanger 1 (PREX1) [37], [38]. PREX1 is the main guanine nucleotide exchange factors for the Ras-related C3 botulinum toxin substrates 1 and 2 (Rac1 and Rac2) in neutrophils [31], [37], [39], [40]. Rac1 and Rac2 stimulate the kinase activity of p21-activated kinase 1 (PAK1) that is important for regulating neutrophil chemotactic responsiveness [41], [42].

PDPK1, in turn, phosphorylates and activates AKT, PKC-zeta and PAK1 [43], [44].

Lipoxin A4 and 15-epi-LXA4 interact with the Formyl peptide receptor-like 1 (FPRL1) [1], [16], [17] that transduces counter-regulatory signals in part via intracellular polyisoprenyl phosphate remodeling. Presqualene diphosphate is a polyisoprenyl phosphate in human neutrophils that is rapidly converted to Presqualene monophosphate upon cell activation. Phosphatidic acid phosphatase type 2 domain containing 2 (PPAPDC2) is presqualene diphosphate phosphatase that converts Presqualene diphosphate to Presqualene monophosphate [45]. In human neutrophils, Leukotriene-induced LTBR1 signaling initiates a rapid decrease in Presqualene diphosphate levels, probably through PPADC2 activation, to promote proinflammatory cell response, whereas Lipoxin-induced FPRL1 signaling dramatically blocks Presqualene diphosphate turnover to Presqualene monophosphate, probably through PPADC2 inhibition, to prevent neutrophil activation [15], [46].

Presqualene diphosphate, but not Presqualene monophosphate, directly inhibits PLD1 and PI3K cat class IB (p110-gamma) [15], [27], [46], [47], [48], [49].

PLD1 hydrolyzes membrane Phosphatidylcholines to generate Phosphatidic acid that is a powerful activator of PKC-zeta [50], [51], [52].

PKC-zeta has been shown to control lymphocyte alpha-L/beta-2 integrin rapid lateral mobility induced by chemokines [53], [54].

Phosphatidic acid also activates Type I phosphatidylinositol-4-phosphate 5-kinases (PIP5KI) that catalyze the synthesis of PtdIns(4,5)P2 [55], [56], which, in turn, mediates Talin activation of alpha-L/beta-2 integrin required for neutrophil transendothelial migration [34], [57], [58], [59]. This migration is mediated via binding of neutrophil alpha-L/beta-2 integrin to endothelial Ligand intercellular adhesion molecule-1 (ICAM-1) [10], [54], [60], [61].

Cell motility also requires polarized rearrangements of the actin/myosin cytoskeleton. PAK1 regulates directional cell motility through its effects on regulatory light chains of Myosin II (MRLC) [62].

Downstream of Rac1, PAK1 activates LIMK1, which, in turn, regulates the actin cytoskeletal reorganization through the phosphorylation and inactivation of the actin-depolymerizing factor Cofilin [63], [64]. Actin-organizing complex (Arp2/3) nucleates new Actin filaments from the sides of preexisting filaments. This interaction requires phosphorylation of Arp2/3 complex by PAK1, which promotes Actin polymerization [65].

PDPK1 and AKT also phosphorylate PAK1 that can regulate cell migration [66], [67], [68].

Leukotriene B4 can also induce neutrophil migration by Reactive oxygen species (ROS)/ Extracellular signal-regulated kinases 1 and 2 (ERK1/2)-linked cascade [69]. Leukotriene B4 signaling activates the NADPH oxidase that catalyzes the production of Superoxide anion (O(2)(-)), from which other ROS, including Hydrogen peroxide, are derived [70], [71], [72]. ERK1/2 activated by Hydrogen peroxide [69], [73] can modulate actin/myosin cytoskeleton remodeling via regulation of Myosin II phosphorylation. ERK1/2 can phosphorylate and inactivate the Myosin light chain phosphatase (MLCP) [74], which attenuates Myosin light chains (MELC) and Myosin regulatory light chains (MRLC) phosphorylation [75]. In addition, ERK1/2 can phosphorylate and activate Myosin light chain kinase (MYLK1) [76]. Myosin II function is regulated by phosphorylation of the MRLC by Myosin light chain kinases (MLCK) that promotes myosin ATPase activity and polymerization of actin cables. This results in generating contractile force necessary for cell motility [77].

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