Abstract

The G_i/o-coupled histamine H₄ receptor is highly expressed in hemopoietic cells and is a promising new target for the treatment of chronic inflammatory diseases. 1-[(5-Chloro-1H-indol-2-yl)carbonyl]-4-methyl-piperazine (JNJ7777120) has been described as a selective antagonist at the H₄ receptor and is widely used to characterize the physiological role of the H₄ receptor. We have investigated the pharmacological properties of JNJ7777120 using two distinct downstream signaling measurements, G protein activation and β-arrestin recruitment. The H₄ receptor agonists histamine and clobenpropit, but not JNJ7777120, were able to induce [³⁵S]GTPγS binding in membranes prepared from U2OS-H₄ cells. Thioperamide, a dual H₃/H₄ receptor antagonist, and JNJ7777120 were both able to inhibit the [³⁵S]GTPγS binding induced by clobenpropit. Agonists and antagonists specific for other members of the histamine receptor family had no effect in this assay format. Histamine and clobenpropit increased β-arrestin recruitment to the H₄ receptor in a concentration-dependent manner. This β-arrestin recruitment could be inhibited by preincubation with thioperamide. We were surprised to find that preincubation with the H₄-selective antagonist JNJ7777120 potentiated rather than antagonized the response to a submaximal concentration of clobenpropit. JNJ7777120 treatment alone resulted in an increase in β-arrestin recruitment, which again could be inhibited by preincubation with thioperamide. Schild analysis demonstrated competitive antagonism between thioperamide and both clobenpropit and JNJ7777120. Histamine and clobenpropit had comparable potencies for both [³⁵S]GTPγS binding and β-arrestin recruitment, suggesting little difference in the levels of receptor reserve between the two assays. In conclusion, we have demonstrated that JNJ7777120 recruits β-arrestin to the H₄ receptor, independent of G protein activation.