Indole-3-carbinol and tamoxifen cooperate to arrest the cell cycle of MCF-7 human breast cancer cells

The current options for treating breast cancer are limited to excision surg ery, general chemotherapy, radiation therapy, and, in a minority of breast cancers that rely on estrogen far their growth, antiestrogen therapy. The n aturally occurring chemical indole-3-carbinol (I3C), found in vegetables of the Brassica genus, is a promising anticancer agent that we have shown pre viously to induce a G(1) cell cycle arrest of human breast cancer cell line s, independent of estrogen receptor signaling. Combinations of I3C and the antiestrogen tamoxifen cooperate to inhibit the growth of the estrogen-depe ndent human MCF-7 breast cancer cell line more effectively than either agen t alone. This more stringent growth arrest was demonstrated by a decrease I n adherent and anchorage-independent growth, reduced DNA synthesis, and a s hift into the G(1) phase of the cell cycle. A combination of I3C and tamoxi fen also caused a more pronounced decrease in cyclin-dependent kinase (CDK) 2-specific enzymatic activity than either compound alone but had no effect on CDK2 protein expression. Importantly, treatment with I3C and tamoxifen ablated expression of the phosphorylated retinoblastoma protein (Rb), an en dogenous substrate far the G(1) CDKs, whereas either agent alone only parti ally inhibited endogenous Rb phosphorylation, Several lines of evidence sug gest that I3C works through a mechanism distinct from tamoxifen, I3C failed to compete with estrogen for estrogen receptor binding, and it specificall y down-regulated the expression of CDK6. These results demonstrate that I3C and tamoxifen work through different signal transduction pathways to suppr ess the growth of human breast cancer cells and may, therefore, represent a potential combinatorial therapy for estrogen-responsive breast cancer.