Calmodulin-dependent regulation of the catalytic function of the human serotonin transporter in placental choriocarcinoma cells.

We investigated the involvement of calmodulin-dependent cellular processes in the regulation of the human serotonin transporter in placental choriocarcinoma cells. Treatment of JAR and BeWo cells with the selective calmodulin antagonist 1,3-dihydro-1-[1-((4-methyl-4H, 6H-pyrrolo[1,2-a][4,1]-benzoxapin-4-yl)-methyl)-4-piperindinyl+ ++]- 2H-benzimidazol-2-one (CGS93-43B (CGS)) for 1 h decreased the imipramine-sensitive serotonin transport activity markedly. The inhibitory effect was specific and was reproducible with other calmodulin antagonists. The basal serotonin transport activity as well as the activity that was stimulated by cholera toxin were inhibited to a similar extent. The CGS-induced inhibition was accompanied by a decrease in the maximal velocity and in the affinities of the transporter for Na+ and Cl- and an increase in the affinity for serotonin. There was, however, no change in the Na+/Cl-/serotonin stoichiometry. The inhibition of the transport activity induced by the treatment of intact cells with CGS was observable in plasma membrane vesicles isolated from these cells. Treatment with CGS had no effect on steady state levels of the serotonin transporter mRNAs nor on the transporter density in the plasma membrane. We conclude that the serotonin transporter is regulated by calmodulin-dependent processes in human placental choriocarcinoma cells involving posttranslational modification, most likely phosphorylation/dephosphorylation, of the transporter protein.