Over-expression of the Regulator of the Signaling of G Protein-Coupled Receptors RGS9-2 reduces the signaling elicited by the human histamine H3 receptor in HEK-293T cells

In HEK-293T cells transiently transfected with the human histamine H3 receptor (hH3R), we studied the effect of over-expressing the human RGS9-2 protein on H3R-mediated stimulation of [35S]-GTP{gamma}S binding and inhibition of forskolin-induced cAMP formation. Maximal specific binding (Bmax) of [3H]-N-methyl-histamine to cell membranes was 468 {+/-} 12 and 442 {+/-} 38 fmol/mg protein for HEK-293T-hH3R and HEK-293T-hH3R/hRGS9-2 cells, respectively, with dissociation constants (Kd) 2.57 nM and 3.38 nM. The H3R agonist immepip stimulated [35S]-GTP{gamma}S binding with similar potency and efficacy (Emax 146.3 {+/-} 4.4 % and 150.0 {+/-} 5.3 % of basal, pEC50 8.57 {+/-} 0.26 and 9.00 {+/-} 0.33, respectively), but was significantly less efficacious to inhibit forskolin-induced cAMP accumulation in HEK-293T-hH3R/hRGS9 cells (-19.2 {+/-} 5.3 versus -37.7 {+/-} 5.1 % in HEK-293T-hH3R cells) with no significant difference in potency (pEC50 9.60 {+/-} 0.14 and 9.07 {+/-} 0.29, respectively). These results indicate that in HEK-293T cells hRGS9-2 regulates hH3R445 signaling downstream G protein activation.