Combined Luteolin and Indole-3-Carbinol Synergistically Constrains ERα-Positive Breast Cancer by Dual Inhibiting Estrogen Receptor Alpha and Cyclin-Dependent Kinase 4/6 Pathway in Cultured Cells and Xenograft Mice

SIMPLE SUMMARY: Anti-cancer effects of bioactive compounds have been extensively investigated; however, the effective dosages of the bioactive compounds are too high to be obtained by oral intake. Our study aimed to assess if combined two bioactive compounds, luteolin (LUT) and indole-3-carbinol (I3C), at low dosages that LUT or I3C along has no significant effect, synergistically exerts anti-breast cancer. We confirmed that combined LUT and I3C synergistically suppressed estrogen receptor-alpha positive breast cancer in cultured cells and cells-derived xenograft mice. Our results also indicated two possible molecular pathways involving the synergistic effects of the combination of LUT and I3C. Our findings provide a practical approach to treat or prevent breast cancer by combining two bioactive compounds. ABSTRACT: The high concentrations of individual phytochemicals in vitro studies cannot be physiologically achieved in humans. Our solution for this concentration gap between in vitro and human studies is to combine two or more phytochemicals. We screened 12 phytochemicals by pairwise combining two compounds at a low level to select combinations exerting the synergistic inhibitory effect of breast cancer cell proliferation. A novel combination of luteolin at 30 μM (LUT30) and indole-3-carbinol 40 μM (I3C40) identified that this combination (L30I40) synergistically constrains ERα(+) breast cancer cell (MCF7 and T47D) proliferation only, but not triple-negative breast cancer cells. At the same time, the individual LUT30 and I3C40 do not have this anti-proliferative effect in ERα(+) breast cancer cells. Moreover, this combination L30I40 does not have toxicity on endothelial cells compared to the current commercial drugs. Similarly, the combination of LUT and I3C (LUT10 mg + I3C10 mg/kg/day) (IP injection) synergistically suppresses tumor growth in MCF7 cells-derived xenograft mice, but the individual LUT (10 mg/kg/day) and I3C (20 mg/kg/day) do not show an inhibitory effect. This combination synergistically downregulates two major therapeutic targets ERα and cyclin dependent kinase (CDK) 4/6/retinoblastoma (Rb) pathway, both in cultured cells and xenograft tumors. These results provide a solid foundation that a combination of LUT and I3C may be a practical approach to treat ERα(+) breast cancer cells after clinical trials.