Abstract P139: An estrogen receptor beta agonist liquiritigenin potentiates inhibition of hormone-dependent breast-cancer growth by cholesterol biosynthesis inhibitor RO 48-8071

Abstract

Abstract Hormone-dependent breast cancer represents approximately 70% of all diagnosed breast cancers. Estrogens and progestins stimulate breast cancer cell proliferation, angiogenesis and metastasis. Estrogen receptor (ER)-alpha positive breast cancers are treated with anti-estrogens and aromatase inhibitors, but resistance to these agents invariably develops during the course of therapy. Our goal was to identify novel treatment strategies to suppress breast cancer growth and avoid drug resistance. Cholesterol biosynthesis plays a vital role in tumor cell growth. Cholesterol is an integral part of membrane structure and is also a precursor of steroid hormones that are synthesized by tumor cells and cause cellular proliferation by interacting with specific receptors. Cancer cells have elevated levels of cholesterol, which could be responsible for anti-hormone resistance. During studies to determine the effects of the cholesterol biosynthesis inhibitor RO 48-8071 ([4’-[6-(Allylmethylamino)hexyloxy]-4-bromo-2’-fluorobenzophenone fumarate] (RO), which inhibits oxidosqualene cyclase (OSC), an enzyme that acts downstream of HMG-CoA reductase to convert 2, 3-monoepoxysqualene to lanosterol (a key step in the biosynthesis of cholesterol), it was serendipitously discovered that RO degraded ER-alpha (ERa), and elevated levels of ER-beta (ERb), a known tumor suppressor. We tested RO alone and in combination with liquiritigenin (LQ), a selective agonist for ERb, to ascertain their inhibitory effects against tumor cell progression. RO or LQ alone reduced significantly the in vitro viability of MCF-7 and BT-474 breast-cancer cells. BT-474 cells were more sensitive than MCF-7 cells to LQ (IC50 0.15 uM vs 0.25 mM, respectively). RO (5-10 uM) + LQ (0.1-0.3 mM) treatment reduced cell viability in an additive manner in MCF-7 cells and in a synergistic manner in BT-474 cells. Combination treatment was significantly more effective than treatment with a single agent. Administration of RO (10 mg/kg/iv) or LQ (20 mg/kg/ip) significantly inhibited growth of tamoxifen-resistant BT-474 tumor xenografts in vivo; combination treatment was even more effective. Both agents appeared to be non-toxic in vivo. Mechanistically, RO, LQ, or RO + LQ reduced ERa but induced ERb expression in tumor xenografts. Both compounds significantly reduced expression of markers of angiogenesis and increased apoptosis in tumor xenografts; use of RO + LQ significantly enhanced the effects observed with a single agent. The anti-tumor properties of RO may in part be due to an off-target effect that reduces ERa and increases ERb, the latter then interacting with LQ to more effectively promote its anti-proliferative effects. The potent anti-tumor properties of RO and LQ, particularly when used together, suggest that this combination has substantial potential as a means by which to clinically manage breast-cancer growth, including in disease types that are tamoxifen resistant. Citation Format: Yayun Liang, Salman Hyder. An estrogen receptor beta agonist liquiritigenin potentiates inhibition of hormone-dependent breast-cancer growth by cholesterol biosynthesis inhibitor RO 48-8071 [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P139.