Mechanisms of action of the chemopreventive agent indole-3-carbinol in breast cell lines

The chemopreventive agent indole-3-carbinol (I3C) is found in cruciferous vegetables such as broccoli, cabbage and Brussels sprouts. Clinical trials have shown I3C to favourably alter urinary 2-OH:16a estrone ratios, which may prove beneficial for decreased risk of ER+ve breast cancer. However, molecular mechanisms for potential chemopreventive effects in ER-ve breast cancer have yet to be elucidated.;I3C inhibited proliferation of the immortalised HBL 100, and tumour-derived MDA MB468, T47D and MCF7 breast cell lines. Approximate IC50s were 122mNm 33mM, 73mM and 67mM respectively, with MDA MB468 cells exhibiting four-fold greater inhibition than HBL 100 cells.;I3C caused significant induction of apoptosis in the MDA MB468 cells only, possibly explaining this difference in sensitivity. This was not due to immediate effects upon members of the Bcl-2 family, although downregulation of the anti-apoptotic Bcl-2 protein may have contributed to apoptosis at later time points.;I3C caused a dose-dependent decrease in Akt phosphorylation and activity in the MDA MB468 cells, which possessed constitutive Akt activity caused by loss of PTEN expression. I3C was shown to similarly inhibit Akt phosphorylation cell line, but not in the high PTEN/low phospho-Akt expressing Du145 prostate cells or the HBL 100 line.;I3C inhibited PI3K activity in the MDA MB468 cells (50mM) and HBL 100 cells (500mM) when added directly into an in vitro kinase assay. Induction of apoptosis in the MDA MB468 and LNCaP cells by the PI3K inhibitor LY294002 supported the hypothesis that inhibition of PI3K signalling may contribute to apoptosis. However, induction of apoptosis by I3C also occurred via an Akt-independent pathway that may involve NF-kB, as I3C, but not LY294002, decreased NF-kB DNA-binding (as shown by EMSA) in the MDA MB468 cells.