Carcinogenesis Advance Access originally published online on February 28, 2007
Carcinogenesis 2007 28(7):1471-1477; doi:10.1093/carcin/bgm044
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Quantitative combination effects between sulforaphane and 3,3'-diindolylmethane on proliferation of human colon cancer cells in vitro
Division of Toxicology and Cancer Risk Factors, German Cancer Research Center (DKFZ), C010-2 Chemoprevention, Im Neuenheimer Feld 280, 69120 Heidelberg, Germany
* To whom correspondence should be addressed. Tel: +49 6221 42 33 06; Fax: +49 6221 42 33 59; Email: c.gerhauser{at}dkfz.de
Isothiocyanates (ITCs) and indoles derived from cruciferous vegetables possess growth-inhibiting and apoptosis-inducing activities in cancer cell lines in vitro. ITCs like sulforaphane (SFN) are cytotoxic, whereas indoles including indole-3-carbinol or its condensation product 3,3'-diindolylmethane (DIM) are acting by cytostatic mechanisms in human colon cancer cell lines. In the present study, we have investigated the impact of defined combinations of SFN and DIM (ratio 1:4, 1:2, 1:1, 2:1 and 4:1) on cell proliferation, cell-cycle progression and apoptosis induction in cultured 40-16 colon carcinoma cells. Calculations of combination effects were based on the method of Chou et al. (1984) Adv. Enzyme Regul., 22, 27–55, and were expressed as a combination index (CI) with CI < 1, CI = 1 or CI > 1 representing synergism, additivity or antagonism, respectively. Interestingly, at a total drug concentration of 2.5 µM, all combinations of SFN and DIM were antagonistic. With increasing concentrations, the antagonistic effect gradually turned into a synergistic interaction at the highest combined cytotoxic concentration of 40 µM. Cell-cycle analyses with SFN:DIM ratios of 1:1, 1:2 and 1:4 and total concentrations between 10 and 25 µM confirmed antagonism at low and additive effects at higher doses. SFN (10 µM) in combination with DIM (10 µM) resulted in strong G2/M cell-cycle arrest, which was not observed with either compound alone. Our results indicate that cytotoxic concentrations of SFN:DIM combinations affect cell proliferation synergistically. At low total concentrations (below 20 µM), which are physiologically more relevant, the combined broccoli compounds showed antagonistic interactions in terms of cell growth inhibition. These data stress the need for elucidating mechanistic interactions for better predicting beneficial health effects of bioactive food components.
Abbreviations: CI, combination index; DIM, 3,3'-diindolylmethane; IC50, half-maximal inhibitory concentration; ITC, isothiocyanate; PARP, poly (ADP-ribose) polymerase; PEITC, phenethyl isothiocyanate; SFN, sulforaphane
Received October 20, 2006; revised February 19, 2007; accepted February 22, 2007.