Ligand-independent activation of the glucocorticoid receptor by β2-adrenergic receptor agonists in primary human lung fibroblasts and vascular smooth muscle cells

The glucocorticoid receptor (GR) is a ubiquitously expressed transcription factor present in most cell types. Upon ligand binding, the GR is activated and translocates into the nucleus where it transmits the anti-inflammatory actions of glucocorticoids. Here, we describe the ligand-independent activation of GR by the beta(2)-adrenergic receptor (beta(2)-AR) agonists, salbutamol and salmeterol, in primary human lung fibroblasts and vascular smooth muscle cells. Immunohistochemistry demonstrated expression of GR and the beta(2)-AR by fibroblasts and vascular smooth muscle cells. Treatment of the cells with the beta(2)-AR agonists, salbutamol or salmeterol, resulted in translocation of GR into the nucleus beginning at 30 min, as shown by immunohistochemistry and Western blotting of cytosolic and nuclear cell extracts. In comparison, activation of GR induced by the corticosteroids dexamethasone and fluticasone occurred at the same time after treatment (30 min) but resulted in a more complete depletion of GR from the cytosolic compartment. Electrophoretic mobility shift assays confirmed that nuclear GR, activated by both beta(2)-AR agonists and glucocorticoids, actively bound to the GR consensus sequence (GR element). Functional activation of the GR was confirmed by a Luciferase reporter gene assay, using a GR driven promoter fragment from the p21((WAF1/CIP1)) gene. The effects of the beta(2)-AR agonists, salbutamol and salmeterol, were dependent upon binding to the beta(2)-AR, because blocking of beta(2)-AR with propranolol abrogated GR activation. GR activation appeared to involve cAMP, In summary, these data show that beta(2)-AR agonists are potent activators of GR, Ligand-independent activation of GR by beta(2)-AR agonists may substantially mediate the anti-inflammatory actions of these drugs observed in vitro and in vivo.