Zusammenfassung
The histamine H4-receptor (H4R) is a classic pertussis toxin-sensitive Gi-protein-coupled receptor that mediates increases in intracellular calcium concentration (Ca2+i). The presence of the H4R in human eosinophils has been rigorously documented by several independent groups. It has also been suggested that the H4R is expressed in human monocytes, but this suggestion hinges, in part, on H4R ...
Zusammenfassung
The histamine H4-receptor (H4R) is a classic pertussis toxin-sensitive Gi-protein-coupled receptor that mediates increases in intracellular calcium concentration (Ca2+i). The presence of the H4R in human eosinophils has been rigorously documented by several independent groups. It has also been suggested that the H4R is expressed in human monocytes, but this suggestion hinges, in part, on H4R antibodies with questionable specificity. This situation prompted us to reinvestigate H4R expression in human monocytes. As positive control, we studied HEK293T cells stably expressing the human H4R (hH4R). In these cells, HA and the H4R agonist UR-PI376 (2-cyano-1-4-(1H-imidazol-4-yl)butyl-3-(2-phenylthio)ethylguanidine) induced pertussis toxin-sensitive Ca2+i increases. However, in quantitative real-time PCR studies we failed to detect hH4R mRNA in human monocytes and U937 promonocytes. In human monocytes, ATP and N-formyl-L-methionyl-L-leucyl-L-phenylalanine increased Ca2+i, but HA, UR-PI376 and 5-methylhistamine (a dual H4R/H2R agonist) did not. In U937 promonocytes and differentiated U937 cells, HA increased Ca2+i, but this increase was mediated via the H1R. In conclusion, there is no evidence for the presence of H4R in human monocytes.
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