Focus on carbon monoxide: a modulator of neutrophil oxidants and elastase spatial localization?

GASEOUS SIGNAL MEDIATORS include nitric oxide (NO), hydrogen sulfide, and carbon monoxide (CO) (3, 20, 35). Both NO and hydrogen sulfide play important roles in inflammation, with the existing evidence supporting them as components of the normal biochemical milieu that stabilize the vasculature against leukocyte adhesion, extravasation, and vascular permeability (25, 35). Unlike G protein-coupled receptors, gasotransmitters are membrane-permeant gases that modulate several forms of cell signaling by chemically binding numerous intracellular targets (25, 26, 35, 36). CO is formed in several tissues through the action of two types of hemoxygenases (HOs): an inducible form HO-1 and constituitive forms HO-2 and -3 (HO-3 is involved in O2 sensing) (19). HO-1 is the 32-kDa form of HO, synthesized in response to heat stress (hence, heat shock protein32), oxidants, cytokines, and environmental stress (e.g., metals). HOs catabolize heme to bilivervidin (rapidly converted to bilirubin by biliverdin reductase), Fe , and CO. While biliverdin and bilirubin formed by HO-1 and -2 are major antioxidants, with potency comparable with glutathione, many studies suggest that the induction of HO-1 expression in inflammation is adaptive through CO-mediated effects. CO is a potent physiological regulator of vascular tone, like NO, and exhibits some similar properties. For example, CO and NO both bind Hb, but NO binds Hb 1,500 more avidly than CO. NO also enhances the binding of CO to heme (the rate of carboxy-Hb formation is 10–15 greater at equivalent levels of CO and NO). NO also induces 30 more cGMP than CO and sensitizes guanylate cyclase to CO stimulation (3, 29). Whereas NO is a highly reactive free radical that forms NO-metal complexes, CO is a stable, water soluble gas that resists oxidation and reduction reactions. Thus CO and NO interact in toxicity and cell signaling, and the concept of CO-NO synergy is gaining acceptance as an HO-1-NO-CO axis (3, 17). CO activates several important signaling systems besides guanylate cyclase (22), including p38 MAPK, phosphatidylinositol 3-kinase/Akt, STATs, hypoxia-inducible factor-1a, Ca -activated K channels (40), and Toll-like receptor-2, -5, and -9 (24). CO binds cytochromes in mitochondrial complex IV (a3), reducing electron (e ) transfer to produce O2 and other oxidants (45). CO also indirectly modulates peroxisome proliferator-activated receptoractivity (2). CO exerts diverse and cell-specific effects on several physiological processes and pathological phenomena including circadian rhythms (via neuronal pas domain protein 2/brain and muscle ARNT-like protein 1) (13), inhibition of PDGF-induced smooth muscle proliferation (via reduced ERK1/2 and cyclin-D activity) (32), neurotransmission/long-term potentiation (by activating cGMPdependent kinases in locus coeruleus neurons) (18, 28, 34), platelet inhibition (5), apoptosis (by inhibiting caspase-3 and -7 and increased STAT-3) (42, 43), and inflammation. CO protects the vasculature against many forms of inflammatory injury including vasoconstriction and atherosclerosis (19, 23), cardiac and graft rejection (30, 31), arteriosclerosis (27), and LPS-induced cytokine expression (1, 16, 41). Paradoxically, CO may activate anti-inflammatory/preconditioning pathways by oxidant-dependent signals (1, 41). The current study by Mizuguchi et al. (22a) shows that septic lung injury (cecal ligation/puncture) leads to pulmonary entrapment and infiltration of neutrophils [polymorphonuclear leukocyte (PMN)]. This injury was blocked by the ruthenium-based CO-releasing molecule (CORM-3). The effect of CORM-3 was CO and cell specific since only PMN infiltration was blocked; these effects were not observed with inactive CORM-3, and the macrophage density in lungs was unaltered. CO can activate guanylate cyclase and cGMP/PKG signaling and may regulate several PMN behaviors including migration, apoptosis, and myeloperoxidase release (6, 33, 38, 39). A recent study by Ciuman et al. (11) shows that activated PMNs lose soluble guanylate cyclase and PKG, presumably decreasing CO-induced cGMP signaling (11). Moreover, PKG can phosphorylate and inhibit soluble guanylate cyclase activity (44), suggesting that other signaling pathways may be involved in the observed effects of CO. Whereas CO at high concentrations is toxic, lower levels of CO only slightly reduce mitochondrial respiration, yielding nontoxic amounts of “signal” oxidants that may trigger adaptation and preconditioning. PMNs are thought of as having few mitochondria, deriving most of their energy from glycolysis, allowing them to function in low ambient O2 and conserve O2 for respiratory burst (14). Studies by Fossati et al. (14) suggest that PMNs actually possess an intricate mitochondrial network and that mitochondrial membrane potential is an essential regulator of PMN motility, cytoskeleton, and apoptosis. CO-induced intracellular reactive oxygen species (ROS) might reflect CO-mediated uncoupling of mitochondrial e flow (12), leading to the production of signal oxidants (3); at least some of these signal pathways may be anti-inflammatory (1, 4). When PMN e flow is disrupted (e.g., by an uncoupler like FCCP) or by CO, PMNs also undergo morphologic/cytoskeletal reorganization, decrease chemotaxis (formyl-methionyl-leucylphenylalanine and LPS), and initiate apoptosis. The protective effects of CO produced by CORM-3 (reduced lung injury, solute permeability, PMN binding, and infiltration) are associated with increased intracellular and extracellular oxidants measured using two different reporters: L-012 (extracellular O2 ) and dihydrorhodamine (cytosolic OH /ONOO ). Using L-012, Mizuguchi et al. (22a) show that SOD-inhibitable O2 production by PMN is increased by CO; previous studies show that neither the PMN respiratory burst nor phagocytic capacity is blocked by mitochondrial inhibition (e.g., by CO), more consistent with Address for reprint requests and other correspondence: J. Alexander, LSU Health Sciences Ctr., 1501 Kings Hwy., Shreveport, LA 71130-3932 (e-mail: [email protected]). Am J Physiol Heart Circ Physiol 297: H902–H904, 2009; doi:10.1152/ajpheart.00587.2009.