Coupling of Serotonin 5-HT1B Receptors to Activation of Mitogen-Activated Protein Kinase (ERK-2) and p70 S6 Kinase Signaling Systems

Abstract: Little is known about the coupling of serotonin 5-HT_1B receptors to cellular signals other than cyclic AMP. In the present studies, the activation by 5-HT_1B receptors of p70 S6 kinase and the mitogen-activated protein kinase (MAP kinase) ERK-2 was investigated. Studies were performed by using both nontransfected Chinese hamster ovary (CHO) cells, which express endogenous receptors at a very low density, and a stable transfected CHO cell line expressing 5-HT_1B receptors at 230 fmol/mg of membrane protein, a density similar to that expressed in cortex. In nontransfected cells, 5-HT was found to stimulate a greater than twofold increase in MAP kinase activity with an EC₅₀ of 20 n M . Reflecting increased density of receptors, 5-HT caused a greater than eightfold activation of ERK-2 in transfected cells with an EC₅₀ of 2 n M . 5-HT was found to also stimulate p70 S6 kinase in both nontransfected and transfected cells. The stimulation was sixfold in both types of cells, but the EC₅₀ for 5-HT was fourfold lower in transfected cells. The coupling of 5-HT_1B receptors to ERK-2 and to p70 S6 kinase was inhibited by pertussis toxin, inhibitors of phosphatidylinositol 3-kinase, and by the inhibitor of MAP kinase kinase PD098059. Activation of p70 S6 kinase, but not ERK-2, was also inhibited by rapamycin. These findings demonstrate that 5-HT_1B receptors couple to ERK-2 and p70 S6 kinase through overlapping, but nonidentical, pathways.