Human lung adenocarcinoma cells, A549, possess the capacity of expressing both cyclooxygenase-2 (COX-2) and NAD+-linked 15-hydroxyprostaglandin dehydrogenase (15-PGDH). Resting cells express little COX-2 but significant levels of 15-PGDH. Interleukin (IL) 1beta, tumor necrosis factor-alpha (TNF-alpha) or phorbol ester [phorbol 12-myristate 13-acetate (PMA)] induced the expression of COX-2, as revealed by western blot analysis. Combination of PMA and IL-1beta or TNF-alpha induced synergistically the expression of COX-2. Interestingly, cytokines and cytokine plus PMA-induced expression of COX-2 were accompanied by a downregulation of 15-PGDH. This was evident from both the western blot analysis and activity assay of 15-PGDH. It appears that the higher the expression of COX-2 was induced, the lower the expression of 15-PGDH was found. This was further supported by the observation that overexpression of COX-2 but not COX-1 by adenovirus-mediated approach led to a decrease in 15-PGDH expression, indicating the specificity of COX-2. Furthermore, downregulation of the IL-1beta-induced expression of COX-2 by silencing RNA (siRNA) approach resulted in an increase in the expression of 15-PGDH by COX-2-siRNA but not by COX-1-siRNA, indicating that it was indeed the expression of COX-2 attenuating the expression of 15-PGDH. The IL-1beta-induced reduction of the expression of 15-PGDH was shown not to be mediated by COX-2-derived products since the presence of COX-2 inhibitors did not block the attenuation of the expression of 15-PGDH. Exogenous PGE2 also did not induce the reduction of the expression of 15-PGDH. However, overexpression of 15-PGDH by transfection with recombinant plasmid encoding 15-PGDH or adenovirus-mediated approach attenuated IL-1beta-induced expression of COX-2. On the contrary, downregulation of 15-PGDH expression by 15-PGDH-siRNA or 15-PGDH-antisense approach resulted in an increase in IL-1beta-induced expression of COX-2 but not that of COX-1. In fact, it was further observed that A549 clones expressing different degrees of 15-PGDH showed also different levels of COX-2 expression after IL-1beta induction. The levels of IL-1beta-induced COX-2 expression appeared to correlate inversely with those of 15-PGDH expression in the cells. These results support the contention that COX-2 and 15-PGDH are regulated reciprocally in A549 cells.