Down-regulation of catalase gene expression in the doxorubicin-resistant AML subline AML-2/DX100

A major obstacle to successful cancer chemotherapy is the development of mu ltidrug resistance (MDR). The previous study revealed that a doxorubicin-re sistant AML subline (AML-2/DX100) overexpressed an MDR-associated protein ( MRP) but not P-glycoprotein. The AML-2/DX100 also showed various levels of resistance to daunorubicin and vincristine but was paradoxically sensitive to hydrogen peroxide (5-fold), t-butyl hydroperoxide (3-fold), and paraquat (2-fold) when compared to the drug-sensitive parental AML-S cells (AML-2/W T). We compared the activities of antioxidant enzymes to detoxify reactive oxygen species (ROS), including superoxide dismutases, glutathione S-transf erase, catalase, glutathione reductase, glutathione peroxidase, and glucose -6-phosphate dehydrogenase in both AML-2/WT and AML-2/DX100. Interestingly, of these antioxidant enzymes, catalase activity of AML-2/DX100 decreased s ignificantly to about one-third that of AML-2/WT (P < 0.000005). The decrea sed activity of catalase was due to reduced expression of the catalase gene ; confirmed by Western blot and reverse transcription-polymerase chain reac tion (RT-PCR) analyses. The decreased activity of catalase was maintained e ven in the absence of doxorubicin for 3 months as well as by the treatment of probenecid, an MRP inhibitor. In addition, there was no difference in ca talase activity between HL-60 and another MRP-overexpressing subline HL-60/ Adr. Taken together, the paradoxical increase in the sensitivity of an MRP- overexpressing AML-2/DX100 in response to peroxides and paraquat is due to the down-regulation of catalase gene expression, which totally independent of overexpression of MRP. It is therefore possible that decreased catalase activity could be exploited as an Achilles' heel in resistant cells such as this. (C) 2001 Academic Press.