Several papers have reported that low level of genistein (<8 μM), the major bioactive component of isoflavones, stimulates the growth of MCF-7 cells. In the present study, we found that genistein-induced growth stimulation of MCF-7 cells is inhibited in the presence of Cu2+ (5 μM). Genistein induces the release of nitric oxide in MCF-7 cells in a concentration-dependent manner. The release of nitric oxide was inhibited by N-G-nitro-L-arginine methyl ester, suggesting the possibility of the activation of nitric oxide synthase. The growth of MCF-7 cells also increases in the presence of low levels of sodium nitriprusside (<10 muM), a nitric oxide donor compound, while high levels (>25 muM) are toxic. The sodium nitroprusside-induced growth of MCF-7 cells is drastically suppressed in the presence of Cu2+ (5 muM). This parallel behavior between Cu2+-genistein and Cu2+-sodium nitroprusside mixtures suggests that Cu2+ and/or copper-protein complexes, that may be formed in the media, may be reacting with nitric oxide or nitric oxide-derived reactive species. The products of these reactions may be responsible for the toxic effects of these mixtures. In contrast, the effect of curcumin that inhibits the growth of both estrogen receptor-positive and -negative breast tumor cells appreciably decreased in the presence of Cu2+. Since copper is known to overwhelmingly bind with proteins, present data suggest that an increase in copper-protein moieties or complexes formed in the serum containing media and their reactions with nitric oxide may be responsible for their toxic effects. Further studies are needed to characterize these reactions. (C) 2003 Elsevier Inc. All rights reserved.