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Background: Breast cancer (BC) and thyroid disease are well-recognized comorbidities. Hyperthyroidism and supraphysiologic thyroid hormone (TH) have been shown to promote BC incidence. We recently reported that thyroid hormone replacement therapy (THRT) was significantly and independently associated with shortened disease-free and overall survival, as well as endocrine resistance only in patients with steroid receptor-positive (SR+) BC (1). TH markedly upregulated estrogen and cell cycle signaling in vivo and in vitro and promoted dedifferentiation to basaloid and pre-stem phenotypes. Metformin (Met) attenuated this shift. Mechanisms of TH-mediated endocrine therapy resistance in ER+ BC are the focus of this report. Design: Two clinical cohorts of early-stage lymph node-negative (LN-) SR+ BC patients (n=820 and n=160) were used to determine the effect of THRT on overall survival using Kaplan-Meier methods. Bi-directional cross-talk between TH and E2 was tested using different BC cell lines, ER+ PDX in vivo models, in vitro methods, and publically available in silico data for modeling. Results: Our results show that E2+TH increases cell proliferation, enhances cell cycle, and hormone-associated oncogenic signaling in SR+/ER+ BC. Given that high expression of THRA is associated with poor prognosis in SR+ BC, knockdown of THRA and ESR1 reduced cell proliferation in ER+ BC cells. ER+ PDX tumors were implanted into NSG mice containing E2 pellet and subsequently treated with TH, Tamoxifen (Tam), Fulvestrant (ICI) or Met. Our data show that TH-mediated endocrine resistance only in the E2+TH+Tam treated tumors (P&amp;lt;0.0001 vs E2+Tam alone). Both ICI and Met provided significant attenuation of tumor growth in vivo. RNAseq analysis of E2+TH+Tam tumors show an increase in pro-oncogenic signaling (Wnt/Fizzled, MMPs, and TCL/LEFT). Our data suggest that the use of Tam did not dampen tumor growth whereas a full ER-antagonist (ICI) or Met attenuated E2-TH mediated cross-talk and tumor growth. Conclusions: These findings suggest that TH+Tam may enhance oncogenic signaling and is associated with a significantly increase in mortality risk in ER+/SR+ BC tumors. Exogenous TH adversely affects SR+ BC and not SR- BC. Understanding the mechanism of cross-talk between TH and E2 allows us to define novel therapeutic strategies that will facilitate rapid clinical application for ER+ BC patients currently taking THRT and anti-estrogen treatments. Reference: (1) Wahdan-Alaswad et. al. Clin Cancer Res October 23 2020 DOI: 10.1158/1078-0432.CCR-20-264.

journal of the endocrine society

Thyroid Hormone and Estrogen Promote Endocrine Resistance, Proliferation, Dedifferentiation, and Cancer Stem Cells in Steroid Receptor-Positive Breast Cancers