GLUCOCORTICOID RECEPTOR SIGNALING IN A BRONCHIAL EPITHELIAL-CELL LINE

Glucocorticoids are an effective anti-inflammatory therapy for the tre atment of asthma. The anti-inflammatory effects of glucocorticoids may be due to the inhibition of transcription factors that regulate cytok ine synthesis. Because of the potential role of the bronchial epitheli um in asthmatic inflammation and the possibility that this cell may be the main target of inhaled glucocorticoids, we have characterized glu cocorticoid receptors (GR) and GR signaling in the human bronchial epi thelial cell line BEAS-2B. Western blot analysis and radioligand bindi ng studies demonstrated that BEAS-2B cells have functional GR that bin d to dexamethasone (Dex) (dissociation constant = 5.6 nM and maximal d ensity of binding sites = 228 +/- 3.3 fmol/mg protein). GR were activa ted by Dex as assessed using a glucocorticoid-responsive reporter plas mid. Transfection of BEAS-2B cells with an activator protein-1 (AP-1) reporter construct followed by 12-O-tetradecanoylphorbol-13-acetate (T PA) treatment resulted in a fivefold induction of reporter gene activi ty. Transfection with a nuclear factor (NF)kappa B reporter construct followed by tumor necrosis factor-alpha (TNF-alpha) treatment resulted in a 10-fold induction of reporter gene activity. Dex (10(-7) M) mark edly repressed both the induced AP-1 and NF-kappa B activity. The GR a ntagonist RU-486 inhibited the repressive effect of Dex on TNF-alpha-i nduced NF-KB activity by 81% but only counteracted the repressive effe ct of Dex on TPA-induced AP-1 activity by 43%. These studies demonstra te that cross-signaling between AP-1 and NF-kappa B with GR may explai n the anti-inflammatory properties of glucocorticoids in airway epithe lial cells.