Ras involvement in signal transduction by the serotonin 5-HT2B receptor

The family of serotonin 5-HT2 receptors stimulates the phospholipase C second messenger pathway via the α subunit of the G(q) GTP-binding protein. Here, we show that agonist stimulation of the 5-HT2B receptor subtype stably expressed in the mouse fibroblast LMTK^- cell line causes a rapid and transient activation of the protooncogene product p21(ras) as measured by an increase in GTP-bound Ras in response to serotonin. Furthermore, 5-HT2B receptor stimulation activates p42(mapk)/p44(mapk) (ERK2/ERK1) mitogen- activated protein kinases as assayed by phosphorylation of myelin basic protein. Antibodies against p21(ras), Gα(q), -β, or -γ₂ subunits of the GTP-binding protein inhibit MAP kinase-dependent phosphorylation. The MAP kinase activation is correlated with a stimulation of cell division by serotonin. In addition to this mitogenic action, transforming activity of serotonin is mediated by the 5-HT2B receptor since its expression in LMTK^- cells is absolutely required for foci formation and for these foci to form tumors in nude mice. Finally, we detected expression of the 5-HT2B receptor in spontaneous human and Mastomys natalensis carcinoid tumors and, similar to the 5-HT2B receptor transfected cells, the Mastomys tumor cells are also responsive to serotonin with similar coupling to p21(ras) activation.