Activation of 5-HT_1A receptors expressed in NIH-3T3 cells induces focus formation and potentiates EGF effect on DNA synthesis

NIH-3T3 fibroblasts have been transfected with human serotonin 5-HT_1A receptors. Clonal cell lines expressed between 40 and 500 fmol receptor/mg. 5-HT_1A agonists strongly inhibited nonstimulated- as well as forskolin- or isoproterenol-stimulated adenylyl cyclase. The effects of 5-HT_1A receptor activation on cell growth were investigated. 5-HT_1A agonists accelerated cell division, generated foci, and increased DNA synthesis. The stimulation of [³H]thymidine incorporation was much stronger when tyrosine kinase receptors were activated concomitantly. Cyclic AMP (cAMP) elevating agents inhibited DNA synthesis induced by all mitogens tested. The mitogenic activity of 5-HT_1A agonists did not seem to be linked to adenylyl cyclase inhibition because 1) we were not able to measure any decrease in intracellular cAMP levels under the conditions of DNA synthesis assay and 2) 2′,5′-dideoxyadenosine, which strongly inhibited adenylyl cyclase, was not mitogenic and did not modify the mitogenic effects of 5-HT_1A agonists. Pertussis toxin completely blocked potentiation of epidermal growth factor effect induced by 8-hydroxy-di-(n-propyl)aminotetralin, a 5-HT_1A agonist, but only partially blocked the one induced by insulin. In conclusion, in transfected NIH-3T3 cells, transforming and mitogenic effects of 5-HT_1A agonists involve a pertussis toxin-sensitive G protein but do not seem to be linked to adenylyl cyclase inhibition.