Dual modulation of glucocorticoid receptor activity by histamine H2 receptor signaling. Involvement of Rap, ERK and cAMP

Abstract

There are reports describing the interaction between membrane G-protein coupled receptor signaling and glucocorticoid receptor (GR) transcriptional activity. We have already reported that the signaling of the Gαs-coupled histamine H2 receptor (H2r) increased GR transcriptional activity. The aim of the present work was to study the molecular mechanisms of this effect. HEK293 cells were transfected with plasmids coding to H2r, GR and a GR-driven reporter gene TAT3-Luc. While pretreatment with 10 μM amthamine, an H2r agonist which augments cAMP levels, increased dexamethasone (dex)-induced GR activity in a 50% (p<0.05), raising cAMP levels with 25 μM forskolin reduced dex-induced GR activity in a 30% (p<0.05). This discrepancy indicates that H2r regulation of GR activity is not strictly mediated by cAMP pathway, suggesting the involvement of other signaling partners. It has been described that H2r activation with amthamine also triggers ERK1/2 phosphorylation. In fact, treatment with the MEK inhibitor UO126 prevented amthamine potentiation of GR activity, pointing to ERK as a relevant player in the potentiation effect. Moreover, pretreatment with 10 μM of the H2r inverse agonists, famotidine and ranitidine, both of which decrease cAMP levels and increase ERK phosphorylation, boosted dex-induced GR activity to almost the quadruple. Trying to elucidate the role of other signaling proteins, cells were transfected with Rap-GAP, an inactivator of the small G-protein Rap. In this system, amthamine also lost its potentiating effect. The whole of our results points to a dual parallel regulation of the GR transcriptional activity: an inhibitory effect mediated by cAMP and an enhancing effect mediated by Rap and ERK proteins. Considering the co-expression of H2r and GR in several physiological systems and the widespread use of their ligands, the interaction described herein could have an impact on glucocorticoid based therapy and grants further research.