Effects of Histamine H1 receptor signaling on glucocorticoid system. Role of canonical and non-canonical pathways

ZAPPIA, CARLOS DANIEL; GRANJA GALEANO, GINA; FERNANDEZ NATALIA; FITZSIMONS CARLOS; MONCZOR FEDERICO

Bariloche, Rio Negro

Simposio; SISTAM - The Second South American Spring Symposium in Signal Transduction and Molecular Medicine; 2012

Previous studies have indicated a possible interaction between G-protein coupled receptors (GPCRs) signaling and glucocorticoid receptor (GR) activation. The histamine H1 receptor (H1R, a GPCR coupled to Galphaq/11) and the GR are within the most common pharmacological targets. Furthermore, under many circumstances, ligands of these two receptors are coadministered as antiallergic in current human prescriptions. In this context, the present work aims to study the possible modulation of GR activity by ligands of the histamine H1 receptor. We cotransfected HEK293 cells with plasmids expressing the H1R, the GR, and an artificial glucocorticoid reporter gene containing 3 repetitions of the GRE present in the TAT promoter (TAT3-Luc). In this cell system, histamine significantly increased dexamethasone-induced transcriptional response in a dose-dependent manner by a 300% without affecting dexamethasone pEC50 (11.6±0.2). To investigate whether Gbeta/gamma subunits could be involved in the effects induced by H1R activation, we overexpressed Galpha tranducin as a Gbeta/gamma scavenger. Intriguingly, under this conditions histamine potentiation of dexamethasone response was first abolished and then switched to inhibition. This result suggests that histamine exerts a dual effect on GR activity, a potentiating effect dependent on Gbeta/gamma, and an inhibitory effect that becomes apparent when Gbeta/gamma subunits are sequestered. Cotransfection of cells with several combinations of Gbeta and Ggamma subunits resulted in the identification of Gbeta2gamma5 and Gbeta2gamma11 as responsible for the boosting effect of histamine on GR-dependent transcriptional activity. On the other hand, using specific pharmacological blockers and a dominant negative mutant of the small G protein Rac (RacN17), we show that the inhibitory effect induced by histamine is mediated by PLC but not PKC, and Rac. In order to evaluate this phenomenon in a naïve cell system, we evaluated Dex induced ThBD expression in A549 pulmonary cells. Both histamine and an H1 specific agonist incremented the relative expression of the gene mediated by Dex (70%), and that increase was higher (150%) in beta2/gamma5 presence. We identified a novel molecular mechanism of interaction between the H1R and the GR, which ligands are among the most widely prescribed and over the counter-sold drugs in the world. Because of their widespread use to treat non-life-threatening conditions, there is a trend to use them as long-term therapy for atopic diseases, allergic asthma, and allergic rhinitis. Considering that these ligands are coadministered and sometimes coexist in pharmaceutical formulations (e.g. dexamethasone+loratadine), the assessment of potential interactions is of vital importance to prevent serious side effects.