Evidence for receptor subtype cross-talk in the mitogenic action of serotonin on human small-cell lung carcinoma cells

We previously reported a significant mitogenic effect of serotonin (5-hydroxytryptamine, 5-HT) on human small-cell lung carcinoma cells (SCLC, GLC-8), mediated by both 5-HT1D and 5-HT1A receptors. Here we investigate possible interactions between the two receptor subtypes. Dose-effect curves obtained by simultaneously applying equipotent concentrations of the selective 5-HT1A agonist 8-OH-DPAT and the selective 5-HT1D receptor agonist sumatriptan are shifted to the right, although maximal effects are additive. The nonselective 5-HT antagonist metergoline displays higher potency when both receptor subtypes are activated. The 5-HT1D receptor antagonist GR127935 is markedly more potent against sumatriptan than against the sensitive portion of 5-HT effect. Indeed, both GR127935 and the 5-HT1A antagonist spiperone shift the EC50 for the residual effect of 5-HT from approximately 300 to 120-150 nM, suggesting that blocking one receptor subtype may facilitate activation of the other. Preincubation with either 8-OH-DPAT or sumatriptan suppresses the mitogenic response to the other specific receptor agonist; suppression is complete within 10 min at 37 degrees C, and is not observed when the preincubation is done at 4 degrees C. Measurements of adenylate cyclase activity do not help in interpreting the results. Conversely, measurements of MAP kinase activity reveals biphasic activation with a delayed activation at 1 h, and reproduce the suppression of the effect of the second drug by 15 min preincubation. These findings constitute the first evidence of a reciprocal negative interference between human 5-HT1A and 5-HT1D receptors, and indicate that SCLC GLC-8 cells simultaneously express both receptor subtypes. Mere reciprocal antagonism of the drugs employed cannot account for these data. We suggest that in this cell system cross-talk occurs in the transduction pathways of the two receptor subtypes.