NAD(P)H : quinone oxidoreductase activity is the principal determinant of beta-lapachone cytotoxicity

beta-Lapachone activates a novel apoptotic response in a number of cell lin es. We demonstrate that the enzyme NAD(P)H:quinone oxidoreductase (NQO1) su bstantially enhances the toxicity of beta-lapachone. NQO1 expression direct ly correlated with sensitivity to a 4-h pulse of beta-lapachone in a panel of breast cancer cell lines, and the NQO1 inhibitor, dicoumarol, significan tly protected NQO1-expressing cells from all aspects of beta-lapachone toxi city. Stable transfection of the NQO1-deficient cell line, MDA-MB-468, with an NQO1 expression plasmid increased apoptotic responses and lethality aft er beta-lapachone exposure. Dicoumarol blocked both the apoptotic responses and lethality. Biochemical studies suggest that reduction of beta-lapachon e by NQO1 leads to a futile cycling between the quinone and hydroquinone fo rms, with a concomitant loss of reduced NAD(P)H. In addition, the activatio n of a cysteine protease, which has characteristics consistent with the neu tral calcium-dependent protease, calpain, is observed after beta-lapachone treatment. This is the first definitive elucidation of an intracellular tar get for beta-lapachone in tumor cells. NQO1 could be exploited for gene the rapy, radiotherapy, and/or chemopreventive interventions, since the enzyme is elevated in a number of tumor types (i.e. breast and lung) and during ne oplastic transformation.