Glucocorticoids suppress hypoxia‐induced COX‐2 and hypoxia inducible factor‐1α expression through the induction of glucocorticoid‐induced leucine zipper

Background and Purpose The COX‐2/PGE2 pathway in hypoxic cancer cells has important implications for stimulation of inflammation and tumourigenesis. However, the mechanism by which glucocorticoid receptors (GRs) inhibit COX‐2 during hypoxia has not been elucidated. Hence, we explored the mechanisms underlying glucocorticoid‐mediated inhibition of hypoxia‐induced COX‐2 in human distal lung epithelial A549 cells. Experimental Approach The expressions of COX‐2 and glucocorticoid‐induced leucine zipper (GILZ) in A549 cells were determined by Western blot and/or quantitative real time‐PCR respectively. The anti‐invasive effect of GILZ on A549 cells was evaluated using the matrigel invasion assay. Key Results The hypoxia‐induced increase in COX‐2 protein and mRNA levels and promoter activity were suppressed by dexamethasone, and this effect of dexamethasone was antagonized by the GR antagonist RU486. Overexpression of GILZ in A549 cells also inhibited hypoxia‐induced COX‐2 expression levels and knockdown of GILZ reduced the glucocorticoid‐mediated inhibition of hypoxia‐induced COX‐2 expression, indicating that the inhibitory effects of dexamethasone on hypoxia‐induced COX‐2 are mediated by GILZ. GILZ suppressed the expression of hypoxia inducible factor (HIF)‐1α at the protein level and affected its signalling pathway. Hypoxia‐induced cell invasion was also dramatically reduced by GILZ expression. Conclusion and Implications Dexamethasone‐induced upregulation of GILZ not only inhibits the hypoxic‐evoked induction of COX‐2 expression and cell invasion but further blocks the HIF‐1 pathway by destabilizing HIF‐1α expression. Taken together, these findings suggest that the suppression of hypoxia‐induced COX‐2 by glucocorticoids is mediated by GILZ. Hence, GILZ is a potential key therapeutic target for suppression of inflammation under hypoxia.