Endogenous adenosine formation can regulate human neutrophil function

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

Effects of adenosine on bacterial lipopolysaccharide- and interleukin 1-induced nitric oxide release from equine articular chondrocytes

OBJECTIVE

To determine whether adenosine influences the in vitro release of nitric oxide (NO) from differentiated primary equine articular chondrocytes.

SAMPLE POPULATION

Articular cartilage harvested from the metacarpophalangeal and metatarsophalangeal joints of 11 horses (3 to 11 years old) without history or clinical signs of joint disease.

PROCEDURE

Chondrocytes were isolated, plated at a high density (10(5) cells/well), and treated with adenosine, the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA), bradykinin, or other agents that modify secondary messenger pathways alone or in combination with bacterial lipopolysaccharide (LPS) or recombinant human interleukin-1alpha (rhIL-1alpha). Nitric oxide release was measured indirectly by use of the Griess reaction and was expressed as micromol of nitrite in the supernatant/microg of protein in the cell layer. Inducible nitric oxide synthase (iNOS) activity was determined by measuring the conversion of radiolabeled arginine to radiolabeled citrulline.

RESULTS

Treatment of chondrocytes with adenosine alone had no significant effect on NO release. However, adenosine and NECA inhibited LPS- and rhIL-1alpha-induced NO release. This response was mimicked by forskolin, which acts to increase adenylate cyclase activity, but not by the calcium ionophore A23187 Treatment of chondrocytes with phorbol myristate acetate, which acts to increase protein kinase C activity, potentiated LPS-induced NO release. Adenosine treatment also significantly inhibited the LPS-induced increase in iNOS activity.

CONCLUSIONS AND CLINICAL RELEVANCE

Adenosine and the nonspecific adenosine receptor agonist NECA inhibited inflammatory mediator-induced release of NO from equine articular chondrocytes. Modulation of adenosine receptor-mediated pathways may offer novel methods for treatment of inflammation in horses with joint disease.

Modulation of the chemotactic peptide- and immunoglobulin G-triggered respiratory burst in human neutrophils by exogenous and endogenous adenosine

The effects of exogenous and endogenous adenosine on the production of oxygen metabolites in neutrophils triggered by the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) or immunoglobulin G (IgG)-opsonized yeast particles, were investigated. By using luminol-enhanced chemiluminescence, we found that adenosine A1 receptor activation did not affect, whereas adenosine A receptor activation, through a mechanism involving the cyclic AMP (cAMP)-protein kinase A signalling pathway, both inhibited the fMLP- and IgG-triggered respiratory burst. The adenosine-induced inhibition was however more pronounced after exposure to fMLP than to IgG-yeast. Stimulation with fMLP caused an extracellular accumulation of endogenous adenosine, which indicates that this event is a negative-feedback mechanism preventing an uncontrolled activation of chemoattractant-stimulated neutrophils. On the contrary, exposure of neutrophils to IgG-yeast did not appear to accumulate extracellular adenosine, probably due to increased adenosine deaminase activity during phagocytosis. In conclusion, this work accentuates the importance of adenosine, both exogenously applied and endogenously formed, as an inflammatory agent modulating the respiratory burst during the different phases in neutrophil activation.