Hydrogen sulfide (H ₂ S) is increasingly recognized as an important signaling molecule in the cardiovascular and nervous systems. Recently, H ₂ S donors were reported to induce neutrophil apoptosis and to suppress expression of some leukocyte and endothelial adhesion molecules. Using rats, we examined the possibility that H ₂ S is an endogenous regulator of key inflammatory events at the leukocyte‐endothelial interface. Via intravital microscopy, we observed that H ₂ S donors (NaHS and Na ₂ S) inhibited aspirin‐induced leukocyte adherence in mesenteric venules (ED50 of 5.0 µmol/kg for Na2S), likely via activation of ATP‐sensitive K ⁺ (KATP) channels. Inhibition of endogenous H ₂ S synthesis elicited leukocyte adherence. Leukocyte infiltration in an air pouch model was also suppressed by H ₂ S donors (NaHS, Lawesson's reagent, and N‐acetylcysteine; ED50 of 42.7, 1.3, and 29.9 µmol/kg, respectively) and exacerbated by inhibition of endogenous H ₂ S synthesis. Carrageenan‐induced paw edema was suppressed by H ₂ S donors (NaHS and Na ₂ S; ED50s of 35 and 28 µmol/kg, respectively) to the same extent as by diclofenac and enhanced by an inhibitor of H ₂ S synthesis. Suppression of edema formation by H ₂ S donors was mimicked by a KATP channel agonist and reversed by an antagonist of this channel. These results suggest that endogenous H ₂ S is an important mediator of acute inflammation, acting at the leukocyte‐endothelium interface. These findings have important implications for anti‐inflammatory drug development.—Zanardo, R. C. O., Brancaleone, V., Distrutti, E., Fiorucci, S., Cirino, G., Wallace, J. L. Hydrogen sulfide is an endogenous modulator of leukocyte‐mediated inflammation. FASEB J. 20, E1411–E1418 (2006)

Objective—Recent studies have demonstrated that hydrogen sulfide (H₂S) is produced within the vessel wall froml-cysteine regulating several aspects of vascular homeostasis. H₂S generated from cystathione γ-lyase (CSE) contributes to vascular tone; however, the molecular mechanisms underlying the vasorelaxing effects of H₂S are still under investigation.Methods and Results—Using isolated aortic rings, we observed that addition ofl-cysteine led to a concentration-dependent relaxation that was prevented by the CSE inhibitorsdl-propargylglyicine (PAG) and β-cyano-l-alanine (BCA). Moreover, incubation with PAG or BCA resulted in a rightward shift in sodium nitroprusside-and isoproterenol-induced relaxation. Aortic tissues exposed to PAG or BCA contained lower levels of cGMP, exposure of cells to exogenous H₂S or overexpression of CSE raised cGMP concentration. RNA silencing of CSE expression reduced intracellular cGMP levels confirming a positive role for endogenous H₂S on cGMP accumulation. The ability of H₂S to enhance cGMP levels was greatly reduced by the nonselective phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. Finally, addition of H₂S to a cell-free system inhibited both cGMP and cAMP breakdown.Conclusion—These findings provide direct evidence that H₂S acts as an endogenous inhibitor of phosphodiesterase activity and reinforce the notion that this gasotransmitter could be therapeutically exploited.

The human formyl-peptide receptor (FPR)-2 is a G protein-coupled receptor that transduces signals from lipoxin A₄, annexin A1, and serum amyloid A (SAA) to regulate inflammation. In this study, we report the creation of a novel mouse colony in which the murine FprL1 FPR2 homologue, Fpr2, has been deleted and describe its use to explore the biology of this receptor. Deletion of murine fpr2 was verified by Southern blot analysis and PCR, and the functional absence of the G protein-coupled receptor was confirmed by radioligand binding assays. In vitro, Fpr2^−/− macrophages had a diminished response to formyl-Met-Leu-Phe itself and did not respond to SAA-induced chemotaxis. ERK phosphorylation triggered by SAA was unchanged, but that induced by the annexin A1-derived peptide Ac2–26 or other Fpr2 ligands, such as W-peptide and compound 43, was attenuated markedly. In vivo, the antimigratory properties of compound 43, lipoxin A₄, annexin A1, and dexamethasone were reduced notably in Fpr2^−/− mice compared with those in wild-type littermates. In contrast, SAA stimulated neutrophil recruitment, but the promigratory effect was lost following Fpr2 deletion. Inflammation was more marked in Fpr2^−/− mice, with a pronounced increase in cell adherence and emigration in the mesenteric microcirculation after an ischemia–reperfusion insult and an augmented acute response to carrageenan-induced paw edema, compared with that in wild-type controls. Finally, Fpr2^−/− mice exhibited higher sensitivity to arthrogenic serum and were completely unable to resolve this chronic pathology. We conclude that Fpr2 is an anti-inflammatory receptor that serves varied regulatory functions during the host defense response. These data support the development of Fpr2 agonists as novel anti-inflammatory therapeutics.

Angiopoietins (Angs) are endothelium-selective ligands that exert most of their actions through the Tie-2 receptor. It is widely accepted that Ang-1 promotes the structural integrity of blood vessels and exhibits anti-inflammatory properties. In contrast, the role of Ang-2 remains less clear because it has been shown to behave as a Tie-2 agonist or antagonist under different experimental conditions. To define the role of Ang-2 in acute inflammation, we studied the effects of recombinant Ang-2 administration in vivo. We show herein that Ang-2, but not Ang-1, induces edema formation in the mouse paw in a dose-dependent manner; the edema seems to be fast-peaking (maximum at 30 min) and resolves within 4 h. The effect of Ang-2 is blocked by the coadministration with a soluble form of the Tie-2 receptor or Ang-1. NO and prostaglandin E₂ levels in mouse paw following the injection of Ang-2 remained unaltered, suggesting that the action of Ang-2 does not involve these mediators. In addition, Ang-2 exerted a weak stimulatory effect on leukocyte migration in the mouse paw. Similarly, Ang-2 injected into the mouse air pouch produced only a modest effect on cell extravasation that peaked at 30 min. However, when cell migration was elicited using zymosan, Ang-2 significantly inhibited leukocyte migration. We conclude that Ang-2 by itself stimulates the extravasation of cell-poor fluid, but in the presence of ongoing inflammation it reduces cellular infiltration in tissues.