The natural mutation encoding a C terminus-truncated 5-hydroxytryptamine2B receptor is a gain of proliferative functions

Although potentially implicated in several physiological functions, few functional mutations have been identified in the human 5-hydroxytryptamine (HT)(2B) receptor gene. A heterozygous mutation R393X in the 5-HT2B receptor was recently identified in a patient diagnosed with pulmonary hypertension after intake of the anorexigenic dexfenfluramine. Although reported to generate a lack of function, this C terminus-truncated 5-HT2B receptor should somehow affect transduction pathways relevant to pulmonary hypertension. In our study, we investigated putative modifications in transduction of the R393X- mutated 5-HT2B receptor. In stably transfected cells, we confirmed the loss of inositol 1,4,5-trisphosphate stimulation caused by the G(alpha q) uncoupling, despite conserved ligand affinity between the normal and mutated receptors. We also observed a partial loss of nitric-oxide synthase stimulation. However, the truncated R393X receptor presented 1) a strong gain of efficacy in cell proliferation as assessed by mitogen-activated protein kinase activity and thymidine incorporation, 2) a preferential coupling to G(alpha 13) as shown by blocking antiserum, and 3) an apparent lack of internalization upon agonist stimulation as observed by confocal microscopy. This work demonstrates that, in the 5-HT2B receptor, the C terminus, including the palmitoylation and phosphorylation sites, is absolutely required for proper transduction and internalization. For the first time, we show that the lack of C terminus can generate a switch of coupling to G alpha(13), a reduced NO synthase activation, and an increase in cell proliferation. All these modifications are relevant in pathophysiological vasoconstriction.