Activated protein phosphatase PP2A by formoterol enhances nuclear translocation of glucocorticoid receptor induced by budesonide

Introduction: We have reported that formoterol (FM), a long-acting β2- adrenoceptor agonist, restores corticosteroid (CS) sensitivity by activation of a serine/threonine protein phosphatase PP2A (ERJ 2009;34:583s). However, the molecular mechanisms how FM activates PP2A and restores CS sensitivity have not been elucidated. Aims: To investigate the mechanism of PP2A activation by FM and the involvement of PP2A in glucocorticoid receptor (GR) nuclear translocation induced by CS budesonide (BUD). Methods: Phosphatase activity of immunopurified PP2A from U937 monocytic cells was measured by fluorescence-based assay. A549 lung epithelial cells, without functional β2-adrenoceptor, were used as control cells. Direct effect of FM was evaluated using PP2A immunopurified from cell membrane and recombinant PP2A. Nuclear/cytoplasmic GR ratios under PP2A inhibition by okadaic acid (OA) or overexpression were determined by western-blot. Results: FM enhanced PP2A activity in both U937 and A549 cells and the effects were not blocked by a β2-adrenoceptor inhibitor (ICI-118551). FM directly activated PP2A-immunoprecipitates in the membrane and recombinant PP2A. PP2A was detected in GR-immunoprecipitates. PP2A inactivation by OA reduced GR nuclear translocation by BUD and abrogated FM-mediated increase of GR translocation while PP2A overexpression enhanced BUD-induced GR translocation and further increased enhancement of GR translocation by FM. Conclusions: FM directly activates PP2A in a β2-adrenoceptor-independent manner. PP2A associated with GR enhances GR nuclear translocation by BUD. This mechanism may contribute to the clinical benefits of BUD+FM combination therapy.