Thyroid Hormones Enhance the Growth of Estrogen Receptor-Positive Breast Cancers

Thyroid disorders have been associated with breast cancer. In fact, breast cancer is the most common secondary malignancy in female patients with thyroid cancer (1). Moreover, hyperthyroidism is associated with an 11% increased risk of breast cancer in women (2). Importantly, up to 30% of patients with breast cancer are treated with thyroid hormone replacement therapy (THRT) for overt or subclinical hypothyroidism (3). These observations, coupled with the preclinical data showing growth stimulatory effects of thyroid hormones (THs) in various cancer models (4), formed the rationale for the study by Wahdan-Alaswad et al. aimed at investigating the role of THRT on the outcome of patients with nonmetastatic breast cancer (3).

The authors conducted an observational study analyzing the association between THRT, disease-free survival (DFS), and disease-specific survival (DSS) in two cohorts of patients with nonmetastatic breast cancer. The first cohort consistent of 820 patients followed for a median of 10 years and treated for breast cancer between 1962 and1993, with THRT implemented in 69 patients. The second cohort included 160 patients treated more recently (between 2006 and 2009) and followed for a median of 8.8 years, with 50 patients exposed to THRT. The data on the age, tumor size, presence or absence of steroid (estrogen and/or progesterone) receptors (SR+/SR-), and treatment regimen were incorporated in the multivariate model analyzing the association between DFS/DSS and THRT at 5 and 10 years. To better understand the results of the observational cohort study, the authors performed functional in vitro and in vivo experiments to investigate the molecular mechanisms underlying TH effects on breast cancer cells and to test the interactions between estrogen receptors (ERs) and TH receptors (THRs).

In patients with SR+ breast cancer, THRT was associated with a significantly increased risk of recurrence (DFS RR, 2.9; P<0.001) and death (DSS RR, 3.4; P<0.001), independent of age, tumor size and grade, while THRT in patients with SR- breast cancer was not associated with worse outcomes. Moreover, patients with SR+ breast cancer undergoing therapy with aromatase inhibitor combined with THRT were characterized by a shorter DFS (P<0.042) and a higher 10-year recurrence rate of 14%, as compared with 2% in patient treated with the aromatase inhibitor alone.The functional in vitro and in vivo studies revealed growth stimulatory effects of monotherapy with TH or estrogens that were further potentiated with combination therapy in ER+ breast cancer cell lines and mice xenografts. The RNA-Seq analysis revealed that combination therapy was associated with a significant activation of the cell cycle, mismatch repair, homologous recombination, and DNA replication signaling, as well as induced thyroid-specific genes and estrogen-mediated signatures. These effects were abrogated by the knockdown or inhibition of ER and/or THRa, suggesting that cross-talk and nuclear colocalization of ERs and THRs are major drivers of pro-oncogenic signaling in the ER+ breast cancer model.

The study reveals clinically significant associations between THRT and worse outcomes in patients with nonmetastatic SR+ breast cancer that are likely driven by interactions between the nuclear ERs and THRs, leading to upregulation of pro-oncogenic signaling. These results suggest that overuse of THRT in patients with hypothyroidism and concurrent breast cancer should be avoided.