Introduction:

The thyroid hormone (TH) pathway influences cell growth and it may be a novel target for breast cancer (BC) therapy. However, various TH receptor (THR) isoforms exist and some have opposing functions thus complicating the role of THs in BC. Our previous work suggests that THR alpha1 (THRα1) promotes TH-mediated BC proliferation. Here we offer a mechanism that explains these findings. We first evaluated the role of THs on BC cell line proliferation using escalating doses of tri-iodothyronine (T3) and thyroxine (T4). Next, propylthiouracil (PTU) was used to block T4 conversion to T3 demonstrating that observed proliferation was TH dependent. Finally, the anti-proliferative efficacy of a THRα1 inhibitor, dronedarone, was used to demonstrate the necessity of THRα1 downstream of TH signalling. Dronedarone was selected because it is an FDA approved anti-arrhythmic drug, which may also serve as a novel anti-cancer agent.

Methods:

The effect of increasing concentrations of T3 and T4 on the proliferation of 3 BC cell lines (MCF 7, MD-MB-231, BT-474) in 10% charcoal-stripped phenol-free serum were evaluated at 24 and 48 hours following standard MTT-assay protocols. Increasing doses of PTU and dronedarone were added to cells with 200uM T3 or T4 to measure proliferation of MCF-7 and MD-MB-231 cells using MTT assays.

Results:

There was a statistically significant increase in the proliferation of MCF7, MD-MB-231 and BT-474 cells with the addition of T3 and T4 at 24 and 48 hours in a dose dependent manner. Of note, BT-474 cells had a significantly slower proliferation rate and required 9 days of incubation prior to detection of proliferation.

The addition of PTU reduced proliferation by 50% in MCF7 and MD-MB-231 cells in the presence of T4 at roughly 5 mM compared to the presence of T3 which had an IC50 of 12mM. This shows that TH signalling is mediated by T4 conversion to T3 in these cell lines. PTU alone reduced cell proliferation by 50% at concentrations above 12 mM, suggesting that PTU may also act through other mechanisms.

Cell proliferation was not affected by the presence of dronedarone alone, but the combination of 10μM dronedarone and T3 or T4 significantly reduced proliferation of both MCF7 and MD-MB-231 cells. These results suggest that THRα1 is required for TH mediated cell proliferation.

A library of compounds similar in structure to dronedarone is being designed to offer additional candidates for more potent THRα1 inhibition. The properties and structures of these novel compounds will be presented.

Conclusion:

The proliferation of several BC cell lines increases in response to exogenous T3 and T4 when grown in hormone depleted growth media. These results demonstrate the role of specific events contributing to TH-mediated cell proliferation, including T4 conversion to T3 and the necessity of downstream THRα1. Ultimately, in conjunction with our previous work, we propose that THRα1 inhibition is a novel target for cancer therapy.

Citation Format: Katarzyna Joanna Jerzak, Jessica G. Cockburn, John Hassel, Kathleen I. Pritchard, Sukhbinder K. Dhesy-Thind, Bane Anita. Thyroid hormone induced proliferation of breast cancer cell lines: A novel approach to hormone therapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3787.