Isothiocyanates (ITCs) are important anticarcinogenic phytochemicals abundant in cruciferous vegetables, especially broccoli sprouts. Recent studies have revealed that ITCs exert cytotoxic effects on various cancer cell lines through the inhibition of cell growth, induction of cell cycle arrest and activation of apoptosis, and that some ITCs might be selective exclusively towards cancer cells. In this study we compared the effects of major ITCs, sulforaphane (SFN) and its sulfide analog erucin (ERN) , on breast cancer (MCF7) and normal human mammary epithelial cells (HMEpC). We determined that SFN and ERN inhibited cell proliferation ~2-fold more potently in MCF7 than in HMEpC cells (SRB assay, 72h, IC50 SFN: 11 µ M and 19 µ M; ERN: 28 µ M and 46 µ M , respectively). We also found that ITCs blocked MCF7 cells in prometaphase stage of mitosis by disrupting spindle formation and preventing proper chromosome segregation through the suppression of microtubule dynamics (15 µM, 20 h incubation, immunofluorescence microscopy). Interestingly, SFN and ERN did not induce mitotic arrest in HMEpC cells at these concentrations; however, some aberrant mitotic spindles with abnormal chromosome segregation were observed. Cell cycle analysis by flow cytometry confirmed that SFN and ERN induced G2/M phase delay in MCF7 cells ( 15 µ M SFN, 24 h: 46 ± 1 % in G2/M vs. 29 ± 1 % control ), while the cell cycle of HMEpC cells was not affected. Furthermore, both SFN and ERN induced microtubule depolymerization in interphase MCF7 and HMEpC cells in a concentration dependent manner. MCF7 cell growth inhibition was accompanied by a significant time- and dose-dependent induction of apoptosis by both ITCs. Surprisingly, high concentrations of SFN and ERN (>15 µM, 24h) induced significant killing of HMEpC cells. Our results indicate that low concentrations of SFN and ERN may trigger cell cycle arrest in mitosis by perturbing normal microtubule polymerization through suppressing of microtubule dynamic instability, and leading to apoptosis in MCF7 cells. However, high concentrations of ITCs also inhibited proliferation of HMEpC cells and disrupted cytoplasmic microtubule polymerization without apparent blocking of cells in mitosis at 24h. These results indicate that ITCs are active in both normal and cancerous cells. Further experiments are under way to elucidate the mechanisms of action of ITCs on human breast cancer vs. normal human mammary epithelial cells. Our data lead to the important questions regarding proper implementation and suitability of isothiocyanates in cancer chemoprevention. Supported by California Breast Cancer Research Program dissertation award # 12GB-0137.
[Fifth AACR International Conference on Frontiers in Cancer Prevention Research, Nov 12-15, 2006]