title: Signal transduction mechanism of biased ligands at histamine H2 receptors.
abstract: 7TMRs (seven-transmembrane receptors) exist as conformational collections in which different conformations would lead to differential downstream behaviours such as receptor phosphorylation, G-protein activation and receptor internalization. In this context, a ligand may cause differential activation of some, but not all, of the signalling events, which are associated to a particular receptor, and it would lead to biased agonism. The aim of the present study was to investigate whether H2R (histamine H2 receptor) ligands, described as inverse agonists because of their negative efficacy at modulating adenylate cyclase, could display some positive efficacy concerning receptor desensitization, internalization or even signalling through an adenylate-cyclase-independent pathway. Our present findings indicate that treatment with H2R inverse agonists leads to receptor internalization in HEK (human embryonic kidney)-293T transfected cells, by a mechanism mediated by arrestin and dynamin, but independent of GRK2 (G-protein-coupled receptor kinase 2)-mediated phosphorylation. On the other hand, we prove that two of the H2R inverse agonists tested, ranitidine and tiotidine, also induce receptor desensitization. Finally, we show that these ligands are able to display positive efficacy towards the ERK1/2 (extracellular-signal-regulated kinase 1/2) pathway by a mechanism that involves Gβγ and PI3K (phosphoinositide 3-kinase)-mediated signalling in both transfected HEK-293T cells and human gastric adenocarcinoma cells. These results point to the aspect of pluridimensional efficacy at H2R as a phenomenon that could be extended to naïve cells, and challenge previous classification of pharmacologically relevant histaminergic ligands.
authors.forename: Natalia
authors.initials: N
authors.lastname: Alonso
authors.forename: Federico
authors.initials: F
authors.lastname: Monczor
authors.affiliation: †Laboratorio de Farmacología de Receptores, Cátedra de Química Medicinal, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Junin 956 PP (1113), Buenos Aires, Argentina.
authors.forename: Emiliana
authors.initials: E
authors.lastname: Echeverría
authors.forename: Carlos
authors.initials: C
authors.lastname: Davio
authors.forename: Carina
authors.initials: C
authors.lastname: Shayo
authors.forename: Natalia
authors.initials: N
authors.lastname: Fernández
mesh_terms: D045744:Cell Line, Tumor; D054314:Drug Inverse Agonism; D057809:HEK293 Cells; D006635:Histamine H2 Antagonists / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D008024:Ligands; D011899:Ranitidine / Q000378:metabolism / Q000494:pharmacology; D011970:Receptors, Histamine H2 / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*
chemical_list: D006635:Histamine H2 Antagonists; D008024:Ligands; D011970:Receptors, Histamine H2; D011899:Ranitidine