THE H(1) RECEPTOR AGONIST 2-(3-CHLOROPHENYL)HISTAMINE ACTIVATES G(I) PROTEINS IN HL-60 CELLS THROUGH A MECHANISM THAT IS INDEPENDENT OF KNOWN HISTAMINE-RECEPTOR SUBTYPES

In dibutyryl-cAMP-differentiated HL-60 cells, histamine H-1 and formyl peptide receptors mediate increases in the cytosolic Ca2+ concentrati on ([Ca2+](i)) via pertussis toxin-sensitive G proteins of the G(i) fa mily. We compared the effects of 2-(3-chlorophenyl)-histamine (CPH) [2 -[2-(3-chlorophenyl)-1H-imidazol-4-yl] ethanamine], one of the most po tent and selective H-1 receptor agonists presently available, with tho se of histamine and N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fML P) in these cells. CPH increased [Ca2+](i) through Ca2+ mobilization a nd Ca2+ influx. Unlike histamine-induced rises in [Ca2+](i), those ind uced by CPH were not desensitized in a homologous manner, and there wa s no cross-desensitization between CPH and histamine. Like fMLP, CPH a ctivated phospholipases C and D, tyrosine phosphorylation, superoxide anion formation, and azurophilic granule release. The effects of CPH o n [Ca2+](i), phospholipase D, and superoxide anion formation were inhi bited by pertussis toxin. CPH and fMLP stimulated high affinity GTP hy drolysis by G(i) proteins in HL-60 membranes. They also enhanced bindi ng of guanosine-5'-O-(3-thio)triphosphate and GTP azidoanilide to, and cholera toxin-catalyzed ADP-ribosylation of, G(i) protein alpha subun its. Histamine receptor antagonists did not inhibit the stimulatory ef fects of CPH, and CPH did not reduce fMLP binding in HL-60 membranes. Our data suggest that CPH activates G(i) proteins in HL-60 cells throu gh a receptor agonist-like mechanism that is, however, independent of known histamine receptor subtypes and formyl peptide receptors. CPH ma y be an agonist at an as yet unknown histamine receptor subtype or, by analogy with other cationic-amphiphilic substances, may activate G pr oteins directly. Future studies will have to take into consideration t he fact that CPH, in addition to activating H-1 receptors, may show ot her, most unexpected, stimulatory effects on G protein-mediated signal transduction processes.