INDUCTION OF DT-DIAPHORASE IN CANCER CHEMOPREVENTION AND CHEMOTHERAPY

DT-diaphorase (EC 1.6.99.2) is a flavoprotein that catalyses two-elect ron reduction of quinones, quinone imines, and nitrogen oxides. It is a Phase II detoxifying enzyme that can detoxify chemically reactive me tabolites, and may be important in an early cellular defense against t umorigenesis. DT-diaphorase is also an activating enzyme for bioreduct ive antitumor agents like mitomycin C (MMC) and EO9. DT-diaphorase is induced in many tissues by a wide variety of compounds including dithi olethiones and isothiocyanates. Dithiolethiones are chemoprotective ag ents against a variety of chemical carcinogens in animal models, and t he dithiolethione analogue, oltipraz, is currently in Phase I and Phas e II clinical chemoprevention trials. Similarly, the isothiocyanate de rivative, sulforaphane, blocks the formation of carcinogen-induced mam mary tumors in rats. The low toxicity of these inducers of DT-diaphora se makes them suitable for use as chemopreventive agents in high-risk individuals. Cells with elevated DT-diaphorase levels are generally mo re sensitive to bioreductive antitumor agents. Thus, we suggested that the antitumor efficacy of bioreductive agents can be enhanced by sele ctive induction of DT-diaphorase in tumor cells compared with normal c ells. We showed that 1,2-dithiole-3-thione (D3T) can increase the leve l of DT-diaphorase activity and the cytotoxic activity of bioreductive agents in mouse lymphoma cells without increasing these activities in normal mouse marrow cells. D3T also increased DT-diaphorase activity in 24 of 33 human tumor cell lines representing nine tissue types with no obvious relationships between the tumor type, or the base level of DT-diaphorase activity, and the ability to increase enzyme activity. A series of dithiolethione analogues and dietary components were also shown to be good inducers of DT-diaphorase in human tumor cells. D3T i ncreased DT diaphorase activity in normal human bone marrow and kidney cells but the increases were small in these cells. Combination treatm ent with D3T and EO9 increased cell kill in HL-60 human leukemia cells compared with EO9 alone, but had no effect on EO9 toxicity in normal human kidney cells. Similarly, D3T increased tumor cell kill by EO9 in H661 human lung cancer cells and by MMC in T47D human breast cancer c ells. Thus, inducers of DT-diaphorase may play an important role in ca ncer chemoprevention programs and may also be useful in enhancing the antitumor efficacy of bioreductive agents.