Cyclooxygenase-2 (COX-2) Inhibitors Reduce Immune Tolerance through Indoleamine 2,3-dioxygenase (IDO)

ABSTRACT

PURPOSE : Cyclooxygenase-2 (COX-2) and its metabolite, PGE2 affect multiple tumorigenesis, including angiogenesis, invasion, and tumor-induced immune suppression. Their overexpression is association with impaired immune cell function in many tumors. Indoleamine 2,3-dioxygenase (IDO) is an emerging immuno-regulatory enzyme that can catalyze the initial rate-limiting step in tryptophan catabolism, by causing tryptophan depletion can block T lymphocyte activation, and thus, enable tumor cells to escape from immune system. Although the potential of immunosuppression associated with tumorproduced COX-2 has been suggested, the mechanism of immunosuppression in tumor immunology is not yet well defined. Thus, we hypothesized that the tumor immunity of COX-2 could be partly due to IDO-dependent immune tolerance. To test this hypothesis, we evaluated IDO expression in cancer cells treated with selective COX-2 inhibitor. MATERIALS AND METHODS : The A549 human adenocarcinoma cell line, murine Lewis lung carcinoma (LLC) cell line and C57Bl/6 mice were used for in vitro and in vivo studies. In vitro studies, A549 cells were treated with various concentrations of COX-2 inhibitor (PTPBS) or PGE2. IDO enzyme activity and protein expression were checked by IDO enzyme activity assay and Western blotting. In vivo study, the 20 mice were randomized into normal control, LLC inoculated control, and low and high selective COX-2 inhibitor (celecoxib 25 or 250 mg/kg/day) treated LLL inoculated mice groups (n=5 per group). At one month, mice were sacrificed and tumor mass was isolated for quantification of IDO expression by immunohistochemical stain and western blotting. RESULTS : In vitro studies, PTPBS treated A549 cells showed a significant decreased in IDO enzyme activity and expression but PGE2 treated A549 cells showed increased in IDO expression. In vivo studies, the tumor mass and lung metastasis were attenuated by celecoxib (respectively, p<0.05, p<0.01). Compared with the LLC inoculated control group, mice treated with celecoxib had significant reductions in IDO expression of tumor mass (IDO immunohistochemical stain and western blotting ). CONCLUSION : The present study reveals that COX-2 inhibitor serves to restore the tumor-induced IDO expression and promotes antitumor reactivity in an immunocompetent murine lung cancer model. These findings further support the suggestion that COX-2 inhibitor is a potential pharmacological immunotherapy in cancer