DT-diaphorase (EC 1.6.99.2) is a
flavoprotein that catalyses two-electron reduction of
quinones,
quinone imines, and
nitrogen oxides. It is a Phase II detoxifying
enzyme that can detoxify chemically reactive metabolites, and may be important in an early cellular defense against
tumorigenesis.
DT-diaphorase is also an activating
enzyme for bioreductive
antitumor agents like
mitomycin C (MMC) and
EO9.
DT-diaphorase is induced in many tissues by a wide variety of compounds including dithiolethiones and
isothiocyanates. Dithiolethiones are chemoprotective agents against a variety of chemical
carcinogens in animal models, and the dithiolethione analogue,
oltipraz, is currently in Phase I and Phase II clinical
chemoprevention trials. Similarly, the
isothiocyanate derivative,
sulforaphane, blocks the formation of
carcinogen-induced mammary
tumors in rats. The low toxicity of these inducers of
DT-diaphorase makes them suitable for use as chemopreventive agents in high-risk individuals. Cells with elevated
DT-diaphorase levels are generally more sensitive to bioreductive
antitumor agents. Thus, we suggested that the antitumor efficacy of bioreductive agents can be enhanced by selective induction of
DT-diaphorase in
tumor cells compared with normal cells. We showed that
1,2-dithiole-3-thione (D3T) can increase the level 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 increased
DT-diaphorase activity in normal human bone marrow and kidney cells but the increases were small in these cells. Combination treatment 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 cells. Thus, inducers of
DT-diaphorase may play an important role in
cancer chemoprevention programs and may also be useful in enhancing the antitumor efficacy of bioreductive agents.