Rapid Induction of the Unfolded Protein Response and Apoptosis by Estrogen Mimic TTC-352 for the Treatment of Endocrine-Resistant Breast Cancer

Authors

Balkees Abderrahman
Philipp Y Maximov
Ramona F Curpan
Sean W Fanning
Jay S Hanspal
Ping Fan
Charles E Foulds
Yue Chen
Anna Malovannaya
Antrix Jain
Rui Xiong
Geoffrey L Greene
Debra A Tonetti
Gregory R J Thatcher
V Craig Jordan

Publication Date

1-1-2021

Journal

Molecular Cancer Therapeutics

DOI

10.1158/1535-7163.MCT-20-0563

PMID

33177154

PMCID

PMC7790886

PubMedCentral® Posted Date

7-1-2021

PubMedCentral® Full Text Version

Author MSS

Published Open-Access

yes

Keywords

Apoptosis, Benzothiazoles, Biomarkers, Tumor, Breast Neoplasms, Cell Survival, DNA, Neoplasm, Drug Resistance, Neoplasm, Endoribonucleases, Estrogens, Female, Fluorescence, Gene Expression Regulation, Neoplastic, Hormones, Humans, Ligands, MCF-7 Cells, Models, Biological, Molecular Dynamics Simulation, Protein Binding, Protein Domains, Protein Serine-Threonine Kinases, Receptors, Estrogen, Thermodynamics, Transcription, Genetic, Unfolded Protein Response, X-Box Binding Protein 1, Selective Human Estrogen Receptor Partial Agonists (ShERPAs), estetrol (E4), estrogen receptor (ER), coactivators, unfolded protein response, apoptosis

Abstract

Patients with long-term estrogen-deprived breast cancer (BC), after resistance to tamoxifen or aromatase inhibitors develops, can experience tumor regression when treated with estrogens. Estrogen’s anti-tumor effect is attributed to apoptosis via the estrogen receptor (ER). Estrogen treatment can have unpleasant gynecological and non-gynecological adverse events thus the development of safer estrogenic agents remains a clinical priority. Here, we study synthetic selective estrogen mimics (SEMs) BMI-135 and TTC-352, and the naturally-occurring estrogen estetrol (E4), which are proposed as safer estrogenic agents compared to 17β-estradiol (E2), for the treatment of endocrine-resistant BC. TTC-352 and E4 are being evaluated in BC clinical trials. Cell viability assays, real-time polymerase chain reaction, immunoblotting, ERE DNA pull downs, Mass spectrometry, X-ray crystallography, docking and molecular dynamic simulations, live cell imaging, and annexin V staining were conducted in 11 biologically-different BC models. Results were compared with the potent full agonist E2, less potent full agonist E4, the benchmark partial agonist triphenylethylene bisphenol (BPTPE), and antagonists 4-hydroxytamoxifen and endoxifen. We report ERα’s regulation and coregulators’ binding profiles with SEMs and E4. We describe TTC-352’s pharmacology as a weak full agonist and anti-tumor molecular mechanisms. This study highlights TTC-352’s benzothiophene scaffold that yields an H-bond with Glu353, which allows Asp351-to-helix 12 (H12) interaction; sealing ERα’s ligand binding domain, recruiting E2-enriched coactivators, and triggering rapid ERα-induced unfolded protein response (UPR) and apoptosis, as the basis of its anti-cancer properties. BPTPE’s phenolic OH yields an H-Bond with Thr347, which disrupts Asp351-to-H12 interaction; delaying UPR and apoptosis, and increasing clonal evolution risk.

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