CYCLIC AMP-INDEPENDENT UP-REGULATION OF THE HUMAN SEROTONIN TRANSPORTER BY STAUROSPORINE IN CHORIOCARCINOMA CELLS

Treatment of confluent cultures of JAR human placental choriocarcinoma cells with staurosporine caused a marked stimulation of serotonin tra nsport activity in these cells, The stimulatory effect was noticeable at nanomolar concentrations of staurosporine, and a treatment time of >4 h was required for staurosporine to elicit the effect, At 40 nM and with a treatment time of 16 h, the stimulation of the transport activ ity was 3.5-6.0-fold, None of the several other protein kinase inhibit ors tested had similar effect except KT 5720, a protein kinase A inhib itor, which showed a small but significant (similar to 1.4-fold) stimu latory effect at a concentration of 5 mu M. Blockade of RNA synthesis and protein synthesis in the cells prevented completely the stimulatio n of the trans port activity induced by staurosporine. The stimulation was observed not only in intact cells but also in plasma membrane ves icles prepared from staurosporine-treated cells, The stimulation was a ccompanied by a 5-7-fold increase in the steady state levels of the tr ans porter specific mRNAs, by a 7-fold increase in the maximal velocit y of the transport process, and by a 6-fold increase in the transporte r density in the plasma membrane, Even though both staurosporine and c holera toxin had similar effects on the serotonin transport activity i n these cells, the effect was not additive when the cells were treated with both reagents together. While treatment of the cells with choler a toxin markedly elevated intracellular levels of cAMP, staurosporine did not have any effect on the cellular levels of this cyclic nucleoti de. It is concluded that staurosporine up-regulates the serotonin tran sport activity in JAR cells by increasing the steady state levels of t he serotonin transporter mRNA and by the consequent increase in the tr ansporter density in the plasma membrane and that the process involves a cAMP-independent signaling pathway.