␤-Lapachone activates a novel apoptotic response in a number of cell lines. We demonstrate that the enzyme NAD(P)H:quinone oxidoreductase (NQO1) substantially enhances the toxicity of ␤-lapachone. NQO1 expression directly correlated with sensitivity to a 4-h pulse of ␤-lapachone in a panel of breast cancer cell lines, and the NQO1 inhibitor, dicoumarol, significantly protected NQO1-expressing cells from all aspects of ␤-lapachone toxicity. Stable transfection of the NQO1-deficient cell line, MDA-MB-468, with an NQO1 expression plasmid increased apoptotic responses and lethality after ␤-lapachone exposure. Dicoumarol blocked both the apoptotic responses and lethality. Biochemical studies suggest that reduction of ␤-lapachone by NQO1 leads to a futile cycling between the quinone and hydroquinone forms, with a concomitant loss of reduced NAD(P)H. In addition, the activation of a cysteine protease, which has characteristics consistent with the neutral calciumdependent protease, calpain, is observed after ␤-lapachone treatment. This is the first definitive elucidation of an intracellular target for ␤-lapachone in tumor cells. NQO1 could be exploited for gene therapy, radiotherapy, and/or chemopreventive interventions, since the enzyme is elevated in a number of tumor types (i.e. breast and lung) and during neoplastic transformation.

␤-Lapachone (␤-Lap) triggers apoptosis in a number of human breast and prostate cancer cell lines through a unique apoptotic pathway that is dependent upon NQO1, a two-electron reductase. Downstream signaling pathway(s) that initiate apoptosis following treatment with ␤-Lap have not been elucidated. Since calpain activation was suspected in ␤-Lap-mediated apoptosis, we examined alterations in Ca 2؉ homeostasis using NQO1-expressing MCF-7 cells. ␤-Lap-exposed MCF-7 cells exhibited an early increase in intracellular cytosolic Ca 2؉, from endoplasmic reticulum Ca 2؉ stores, comparable to thapsigargin exposures. 1,2-Bis-(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid-acetoxymethyl ester, an intracellular Ca 2؉ chelator, blocked early increases in Ca 2؉ levels and inhibited ␤-Lap-mediated mitochondrial membrane depolarization, intracellular ATP depletion, specific and unique substrate proteolysis, and apoptosis. The extracellular Ca 2؉ chelator, EGTA, inhibited later apoptotic end points (observed >8 h, e.g. substrate proteolysis and DNA fragmentation), suggesting that later execution events were triggered by Ca 2؉ influxes from the extracellular milieu. Collectively, these data suggest a critical, but not sole, role for Ca 2؉ in the NQO1-dependent cell death pathway initiated by ␤-Lap. Use of ␤-Lap to trigger an apparently novel, calpain-like-mediated apoptotic cell death could be useful for breast and prostate cancer therapy. ␤-Lap1 is a naturally occurring compound present in the bark of the South American Lapacho tree. It has antitumor activity against a variety of human cancers, including colon, prostrate, promyelocytic leukemia, and breast (1-3). ␤-Lap was an effective agent (alone and in combination with taxol) against human ovarian and prostate xenografts in mice, with little host toxicity (4). We recently demonstrated that ␤-Lap kills human breast and prostate cancer cells by apoptosis, a cytotoxic response significantly enhanced by NAD(P)H:quinone oxidoreductase (NQO1, E.C. 1.6.99.2) enzymatic activity (5).2 ␤-Lap cytotoxicity was prevented by co-treatment with dicumarol (an NQO1 inhibitor) in NQO1-expressing breast and prostate cancer cells (5).2 NQO1 is a cytosolic enzyme elevated in breast cancers (6) that catalyzes a two-electron reduction of quinones (e.g. ␤-Lap, menadione), utilizing either NADH or NADPH as electron donors. Reduction of ␤-Lap by NQO1 presumably leads to a futile cycling of the compound, wherein the quinone and hydroquinone form a redox cycle with a net concomitant loss of reduced NAD(P)H (5).Apoptosis is an evolutionarily conserved pathway of biochemical and molecular events that underlie cell death processes involving the stimulation of intracellular zymogens. The process is a genetically programmed form of cell death involved in development, normal turnover of cells, and in cytotoxic responses to cellular insults. Once apoptosis is initiated, biochemical and morphological changes occur in the cell. These changes include: DNA fragmentation, chromatin condensation, cytoplasmic membran...

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Beta-lapachone (beta-Lap) triggers apoptosis in a number of human breast and prostate cancer cell lines through a unique apoptotic pathway that is dependent upon NQO1, a two-electron reductase. Recently, our laboratory showed that beta-lap-exposed MCF-7 cells exhibited an early increase in intracellular cytosolic Ca(2+) from endoplasmic reticulum stores, and that BAPTA-AM (an intracellular Ca(2+) chelator) blocked these early increases and partially inhibited all aspects of beta-lap-induced apoptosis. We now show that exposure of NQO1-expressing breast cancer cells to beta-lap stimulates a unique proteolytic apoptotic pathway involving mu-calpain activation. No apparent activation of m-calpain was noted. Upon activation, mu-calpain translocated to the nucleus concomitant with specific nuclear proteolytic events. Apoptotic responses in beta-lap-exposed NQO1-expressing cells were significantly delayed and survival enhanced by exogenous over-expression of calpastatin, a natural inhibitor of mu- and m-calpains. Furthermore, purified mu-calpain cleaved PARP to a unique fragment (approximately 60 kDa), not previously reported for calpains. We provide evidence that beta-lap-induced, mu-calpain-stimulated apoptosis does not involve any known apoptotic caspases; the activated fragments of caspases were not observed after beta-lap exposures, nor were there any changes in the pro-enzyme forms as measured by Western blot analyses. The ability of beta-lap to trigger an apparently novel, p53-independent, calpain-mediated apoptotic cell death further support the development of this drug for improved breast cancer therapy.