02.04 Comparison of Baicalein with Metformin on mTOR Pathway Inhibition in Triple-Negative Breast Cancer

A. H. Choi1, Q. Xing1, J. Yan1, W. Wen1, E. S. Han2, J. H. Yim1 1City Of Hope National Medical Center,Division Of Surgical Oncology,Duarte, CA, USA 2City Of Hope National Medical Center,Division Of Gynecologic Oncology,Duarte, CA, USA

Introduction: The mammalian target of rapamycin (mTOR) signaling pathway controls fundamental cellular processes, including cell proliferation and metabolism. mTOR phosphorylates ribosomal protein S6 kinase 1 (S6K1), activating ribosomal protein S6 (S6), which is involved in cellular proliferation and glucose metabolism. Metformin has been shown to disrupt the mTOR pathway and is currently the subject of an ongoing phase III randomized trial as adjuvant treatment in breast cancer (NCT01101438). The flavone baicalein, which is present in the herb thyme and in asian herbal remedies, appears to be non-toxic in animals and humans. We have previously demonstrated that baicalein effectively inhibits cancer cell proliferation by inhibiting mTOR signaling. The objective of this study was to compare the effect of baicalein with metformin in triple-negative breast cancer.

Methods: MDA-MB-468, a triple-negative breast cancer cell line, was treated with baicalein (0-80 uM) and metformin (0-10,000 uM) for 24-72 hours. Total protein lysates were obtained for immunoblot analysis of the mTOR pathway mediators S6K1 and S6, as well as the mTOR inhibitor DNA Damage-Inducible Transcript 4 (DDIT4). Cytotoxicity after 72 hours of treatment was evaluated by MTT assay.

Results: Baicalein demonstrated dose-dependent growth inhibition of MDA-MB-468 in micromolar concentrations. By comparison, metformin required over a 60-fold higher concentration to achieve similar cytotoxicity (Figure 1A). Dose-dependent inhibition of phospho-S6K1 (pS6K1) and phospho-S6 (pS6) were observed following treatment with both baicalein and metformin. However, baicalein-induced phosphorylation blockade was more effective at a far lower concentration. The activity of DDIT4, an mTOR inhibitor, increased with higher concentrations of baicalein (Figure 1B).

Conclusion: Baicalein effectively inhibits growth of triple-negative breast cancer in vitro. Dose-dependent pS6K1 and S6 inhibition, as well as increased DDIT4 levels, were observed at micromolar concentrations. Baicalein, a flavone that appears to be non-toxic in humans, may represent a novel treatment for triple-negative breast cancer.