Pharmacological characterization of glucocorticoid receptors in primary human bronchial epithelial cells

Bronchial epithelial cells play an important role in amplifying and perpetuating airway inflammation and may be a target for inhaled steroids. We have characterized glucocorticoid receptors in primary human bronchial epithelial cells. Northern and western blot analyses demonstrated the expression of glucocorticoid receptor mRNA and protein, respectively, in primary bronchial epithelial cells. The activity of these receptors was shown using a radioligand binding assay. High-affinity binding with pharmacological specificity was demonstrated for [3H]dexamethasone. The equilibrium dissociation constant (Kd) and density of binding sites (Bmax) for [3H]dexamethasone determined from saturation isotherms were 4.4 nM x/divided by 0.95 (SEM) and 30.1 fmol/mg protein +/-6.4 (SEM). Glucocorticoid receptors were activated by dexamethasone as assessed using a glucocorticoid-responsive reporter plasmid, pTAT3-CAT. Transfection of primary human bronchial epithelial cells with this reporter plasmid resulted in 35-fold activation of transcription following dexamethasone stimulation (10(-6) M). The glucocorticoid receptor antagonist RU-486 (mifepristone) significantly counteracted the effect of dexamethasone on glucocorticoid receptor activation, indicating that the dexamethasone effect is specific and is mediated through the glucocorticoid receptor. In summary, our study demonstrated that primary cultures of human bronchial epithelial cells possess glucocorticoid receptors that function as a ligand-activated transcriptional regulator. The presence of glucocorticoid receptors confers their responsiveness to glucocorticoids and indicates that the airway epithelium may be a target for the anti-inflammatory effects of inhaled steroids.