Therapeutic intervention of experimental breast cancer bone metastasis by indole-3-carbinol in SCID-human mouse model

Several lines of experimental evidence have suggested that chemokine receptor CXCR4, a metastasis-promoting molecule, may play important roles in breast cancer bone metastasis. There is emerging evidence linking CXCR4 to matrix metalloproteinases (MMP) as well as their regulator nuclear factor-kappaB (NF-kappaB), a key transcription factor, which is known to activate metastasis-promoting molecules for many types of malignancies, including breast cancer. A recent study also showed that promoter region of CXCR4 has several NF-kappaB-binding sites, suggesting that there may be a cross-talk between CXCR4 and NF-kappaB. We have shown previously that indole-3-carbinol (I3C), a natural compound present in vegetables of the genus Brassica, can inhibit NF-kappaB in breast cancer cells. However, there are no reports in the literature showing any effect of I3C on CXCR4 expression in vitro and in vivo. We therefore examined whether I3C could inhibit bone metastasis of breast cancer by inhibiting CXCR4 and MMP-9 expression mediated via the inhibition of the NF-kappaB signaling pathway. Here, we have modified the severe combined immunodeficient (SCID)-human mouse model of experimental bone metastasis for use with the MDA-MB-231 breast cancer cell line. In this animal model, we found that I3C significantly inhibited MDA-MB-231 bone tumor growth, and our results were correlated with the down-regulation of NF-kappaB. Moreover, we found that I3C significantly inhibited the expression of multiple genes involved in the control of metastasis and invasion in vitro and in vivo, especially the expression of CXCR4 and MMP-9 along with pro-MMP-9, with concomitant decrease in Bcl-2 and increase in the proapoptotic protein Bax. From these results, we conclude that the CXCR4/NF-kappaB pathway is critical during I3C-induced inhibition of experimental breast cancer bone metastasis. These results also suggest that I3C could be a promising agent for the prevention and/or treatment of breast cancer bone metastasis in the future.