ADENOSINE INHIBITS ACTIN DYNAMICS IN HUMAN NEUTROPHILS - EVIDENCE FORTHE INVOLVEMENT OF CAMP

The mechanisms by which adenosine regulates the inflammatory reaction are poorly characterized. In this study we investigated the effects of adenosine on neutrophil actin polymerization elicited by the chemotac tic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) or IgG opso nized yeast particles. We used bodipy-phallacidin staining in combinat ion with flow cytometry and found that adenosine markedly reduced acti n polymerization triggered by IgG-yeast, whereas the effect on the fML P-response was less pronounced. Similar or even more pronounced effect s were obtained with the adenosine A(2) receptor agonist 5'-N-ethylcar boxamidoadenosine (NECA), suggesting an A(2) receptor-mediated mechani sm. The following observations indicate that the A(2) receptor-induced effects involve the cAMP-protein kinase A (PKA) signaling pathway: (1 ) a combination of NECA and the cAMP-specific phosphodiesterase (PDE) inhibitor Ro 20-1724 raised the cAMP content in both unstimulated and stimulated neutrophils and also further inhibited the actin dynamics; (2) the PKA inhibitor H89 reversed the inhibitory effects of NECA on t he actin dynamics; (3) Ro 20-1724, isoproterenol and dibutyryl cAMP (D BcAMP) reduced actin polymerization in almost the same way as NECA did . NECA together with Ro 20-1724 impaired the fMLP-induced shape change s and cortical accumulation of actin filaments. In contrast, HS9 poten tiated the fMLP-induced formation of a submembranous ring of actin fil aments. Neutrophils phagocytosing yeast particles in the presence of N ECA and Ro 20-1724 were predominantly round in shape, and their abilit y to extend actin-rich pseudopods around the prey was reduced. These e ffects were partly antagonized by H89. In correlation with the effects on actin polymerization, NECA more effectively diminished IgG-induced upregulation of the beta 2 integrin CD11b/CD18 than such upregulation induced by fMLP. The inhibitory effects of A(2)-receptor activation o n actin dynamics and beta 2 integrin expression in neutrophils exposed to IgG-yeast were also associated with a cAMP-dependent reduction of the phagocytic capacity. In conclusion, we show that adenosine inhibit s actin dynamics and shape changes in neutrophils via a cAMP-dependent pathway. This finding further characterizes the mechanisms by which a denosine functions as an important modulator of the inflammatory respo nse.