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

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. Estrogen9s 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 ERα9s regulation and coregulators9 binding profiles with SEMs and E4. We describe TTC-3529s pharmacology as a weak full agonist and anti-tumor molecular mechanisms. This study highlights TTC-3529s benzothiophene scaffold that yields an H-bond with Glu353, which allows Asp351-to-helix 12 (H12) interaction; sealing ERα9s 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. BPTPE9s phenolic OH yields an H-Bond with Thr347, which disrupts Asp351-to-H12 interaction; delaying UPR and apoptosis, and increasing clonal evolution risk.