Cross-talk between cAMP and formylmet-leu-phe in human neutrophils: phosphorylation of a 52,000 molecular weight protein

Evidence for ligand-specific conformations of the histamine H(2)-receptor in human eosinophils and neutrophils

The histamine H(2)-receptor (H(2)R) couples to G(S)-proteins and induces adenylyl cyclase-mediated cAMP accumulation. In human neutrophils and eosinophils, the H(2)R reduces chemotactic peptide-stimulated superoxide anion (O(2)(-)) formation. However, pharmacological characterization of the H(2)R in these cells is far from being complete. The aim of this study was to provide a comprehensive profiling of the H(2)R in neutrophils and eosinophils. Histamine inhibited O(2)(-) formation in human neutrophils more effectively than in eosinophils. H(2)R agonists mimicked the effects of histamine and H(2)R antagonists blocked the effects of histamine. We noticed multiple discrepancies in the potencies and efficacies of H(2)R agonists with respect to cAMP accumulation and inhibition of O(2)(-) formation in both cell types. There were also differences in the antagonist profiles between cAMP accumulation and inhibition of O(2)(-) formation in neutrophils. Moreover, the pharmacological profile of the recombinant H(2)R did not match the H(2)R profile in native cells. The H(2)R sequence identified in human neutrophils corresponds to the published H(2)R sequence, excluding the exclusive expression of a new H(2)R isoform as explanation for the differences. Very likely, the differences between ligands are explained by the existence of ligand-specific receptor conformations with unique affinities, potencies and efficacies. Thus, our data provide evidence for the notion that the concept of ligand-specific receptor conformations can be extended from recombinant systems to native cells.

Involvement of protein kinase C and of protein phosphatases 1 and/or 2A in p47 phox phosphorylation in formylmet-Leu-Phe stimulated neutrophils: studies with selective inhibitors RO 31-8220 and calyculin A

Previously employed non-selective protein kinase inhibitors yielded inconclusive results regarding involvement of protein kinase C (PKC) in phosphorylation of 47 kDa protein (p47 phox) in intact neutrophils stimulated with physiologic agonists of superoxide generation. In the present study, phosphorylation of p47 phox in formylMet-Leu-Phe (fMLP) stimulated neutrophils was potently inhibited in the presence of 0.3 microM RO 31-8220, a selective inhibitor of PKC. These results provide experimental evidence in support of the currently considered essential involvement of PKC in p47 phox phosphorylation in response to physiologic stimulation of neutrophil surface receptors. The fMLP-induced phosphorylation of p47 phox was enhanced and prolonged by calyculin A, a specific inhibitor of protein phosphatases of types 1 and 2A, and such enhanced phosphorylation was also effectively inhibited by RO 31-8220. Our results suggest that the extent and duration of p47 phox phosphorylation in intact fMLP-stimulated neutrophils is probably controlled by a balance between the activities of PKC, on the one hand, and of protein phosphatase(s) of type(s) 1 and/or 2A, on the other. Effects of RO 31-8220 and of calyculin A on the fMLP-induced p47 phox phosphorylation were paralleled by similar effects on superoxide release. Calyculin A and RO 31-8220 were also used to study signal transduction by a post-receptor agonist of superoxide generation, a calcium ionophore A23187. The results of the latter study indicated that PKC was activated in A23187-stimulated neutrophils and was essentially involved in superoxide generation and p47 phox phosphorylation. Further, these results suggested that protein phosphatase(s) of type(s) 1 and/or 2A were also activated in A23187-signalling pathway, and limited the extent of superoxide release and p47 phox phosphorylation.