Effects of dexamethasone on cytokine and phorbol ester stimulated c-Fos and c-Jun DNA binding and gene expression in human lung

Abstract

Glucocorticosteroids have a wide variety of effects which result in the long-term dampening of inflammatory responses. An important site of steroid action may be on the control of the activator protein-1 (AP-1) binding to deoxyribonucleic acid (DNA). AP-1 is a proinflammatory transcription factor composed of a heterodimer of Fos and Jun proto-oncogenes, which can be induced by phorbol esters and various cytokines. We have examined the hypothesis that dexamethasone may inhibit inflammation via an effect on AP-1 activation in human lung tissue. The effect of dexamethasone on the phorbol eater and cytokine activation of AP1 and its monomers was examined in human lung tissue obtained from transplantation donors. AP-1 activation was measured by its ability to bind DNA, its localization in the nucleus by Western blotting, and the levels of fos and jun messenger ribonucleic acids (mRNAs) using Northern blotting. The phorbol ester, phorbol myristate acetate (PMA), caused a significant 2-3 fold increase in AP-1 DNA binding, which was sustained for 24 h and completely attenuated by co-incubation with dexamethasone. Dexamethasone alone caused a 40% decrease in AP-1 DNA binding. Dexamethasone modulated the expression of both c-jun and c-fos mRNA and produced long-term (24 h) 40% reduction in both mRNAs when compared to control tissues. PMA induced a rapid and prolonged increase in c-Fos and c-Jun nuclear localization, which was not attenuated by co-incubation with dexamethasone. Levels of c-Fos and c-Jun protein, within the nucleus and cytoplasm were unchanged for up to 3 h following dexamethasone treatment alone, but were greatly reduced at 24 h. This suggests that in human lung, the short-term effect of glucocorticoids is to prevent AP-1 DNA binding within the nucleus via protein-protein interactions. In the long-term, glucocorticoids reduce the level of Fos and Jun available for AP-1 formation. This may be an important molecular mechanism of steroid action in asthma and other chronic inflammatory lung diseases.