Modulation of doxorubicin induced MDR1 overexpression in human myeloid AML blasts by cyclooxygenase-inhibitors

Multidrug Resistance (MDR) is known as a serious problem in the treatment of acute myeloid leukaemias (AML). MDR is frequently associated with decreased drug accumulation in cancer cells caused by the ATP-dependent multidrug transporter MDR1. Unfortunately the anthracycline Doxorubicin (Dox), an important drug hi the therapy of pediatric AML, is a known substrate and strong inducer of mdrl-type P-glycoprotein (P-gp) expression. Since previous studies with rat heapto-cytes indicated that cyclooxygenase-2 (COX-2), a key enzyme in arachidonic acid metabolism, is involved in mdrl gene up-regulation, we were interested in whether Dox-induced MDR1 overexpression in human leukaemia cells is also dependent on COX-2-activity and thus might be overcome with selective COX-2-inhibitors. In the present study, the human AML cell line HL-60 exhibited basal MDR1 and COX-2 mRNA and functional P-gp expression. As expected, P-gp expression and MDR1-dependent transport activity were markedly increased (~15-fold) in the presence of 0.15µg/ml Dox. Interestingly, Dox-mediated up-regulation of functional P-gp expression was almost reduced to basal levels by pre-incubation with the COX-2-preferential inhibitor meloxicam (10, 100 and 500 nM), indicating that Dox-induced MDR1 up-regulation might be COX-2-dependent. Subsequent PGE2-liberation studies indeed demonstrated significant induction of PGE2-liberation by Dox (from undetectable PGE2-levels to -800 pg/ml after 3d of incubation). In addition, MDR1 mRNA expression in HL-60 cells was induced in the presence of PGEa (3 µg/ml). Furthermore, MTT-assays demonstrated dose-dependent enhancement of the cyto-static effect of Dox by pre-incubation with meloxicam in HL-60 cells (maximum 76% in the presence of l µM meloxicam at day 3) and in primary blasts of a child with AML. Meloxicam alone had no effect on cell viability. In contrast, in mono-nuclear cells from a healthy donor and in isolated CD34+ stem cells no modulating effect of meloxicam on cytostatic potential of Dox was evident. In conclusion, these data suggest that COX-dependent liberation of PGE2 is involved in Dox-mediated MDR1 up-regulation. COX-inhibitors thus could help to overcome MDR in tumor cells, specifically in the therapy of pediatric AML. Additional experiments with AML-blasts are currently underway to verify the results in primary tumor cells