Interaction between glucocorticoids and β2 agonists on bronchial airway smooth muscle cells through synchronised cellular signalling

Background Increased airway smooth muscle bulk is a pathological feature of asthma. Asthma is well controlled by the combined inhalation of glucocorticoids and beta(2)-adrenoceptor agonists. The basic molecular mechanism of the interaction of the two drugs on proliferation of airway smooth muscle cells is yet to be identified. Our aim was to elucidate how glucocorticoids and beta(2) agonists affect the growth of human bronchial airway smooth muscle cells. Methods We assessed the effect of formoterol and budesonide on the activation and function of transcription factors by immunohistochemistry, western blotting, DNA mobility shift assay, and a luciferase reporter gene assay. The effect of the drugs and the involvement of specific transcription factors on cell proliferation was ascertained by direct cell count and confirmed by thymidine incorporation. Findings Both classes of drugs (10(-8) mol/L) activated C/EBP-alpha and the glucocorticoid receptor with different kinetic profiles, and inhibited proliferation. The combination of lower doses of drugs (10(-12) to 10(-9) mol/L) resulted in a synchronised activation of the transcription factors and an enhanced antiproliferative effect. The action of the drugs alone or in combination on transcription-factor activity and proliferation was suppressed by either depletion of C/EBP-alpha or in the presence of a glucocorticoid-receptor blocker. Interpretation Our findings could provide one explanation for the interaction of beta(2)-agonists and glucocorticoids at a molecular level, and indicate that the concentration of inhaled glucocorticoids can be reduced when combined with beta(2) agonists, minimising the side-effects of the drugs.