The recent finding of a link between cyclooxygenase-2 (COX-2) and p-glycoprotein expression suggests that COX-2 is involved in the development of the multidrug resistance (MDR) phenotype. MDR-associated protein 1 (MRP1) is another major MDR-related protein that is frequently overexpressed in cancer patients, including those with lung cancer. Based on our observation that among four human epithelial lung cell lines both MRP1 and COX-2 protein were highly expressed only in A549 cells, we have investigated whether COX-2 regulates the expression of MRP1. The COX-2 inhibitor celecoxib down-regulated the expression of MRP1 protein in A549 cells, which was accompanied by increased accumulation and enhanced cytotoxicity of doxorubicin, an MRP1 substrate. However, enforced expression of COX-2 in human H460 lung carcinoma cell lines, which express minimal level of COX-2, did not cause enhancement in MRP1 expression. Celecoxib down-regulation of MRP1 was observed independent of COX-2 expression. Moreover, in COX-2-overexpressing cell lines, celecoxib down-regulation of MRP1 was observed only at a concentration far exceeding that required for inhibiting COX activity, and exogenous addition of prostaglandin E(2) did not restore MRP1 expression. These results suggest that celecoxib down-regulates MRP1 expression in human lung cancer cells in a COX-independent manner. The use of celecoxib for adjuvant therapy in lung cancer patients may contribute to their decreased resistance to chemotherapeutic drugs transported by MRP1.