The Estrogen Receptor Beta Agonist Liquiritigenin Enhances Growth Inhibition of TNBC by the Cholesterol Biosynthesis Inhibitor RO 48-8071

Author(s): Yayun Liang, Benford Mafuvadze, Salman M. Hyder

Purpose: Since many human TNBC develop resistance to aggressive forms of chemotherapy, our goal was to identify novel, less toxic treatment strategies that prevent drug resistance in TNBC. Previously, we established that RO 48-8071 ([4’-[6-(Allylmethylamino)hexyloxy]-4- bromo-2’-fluorobenzophenone fumarate] [RO], a small-molecule inhibitor of oxidosqualene cyclase (OSC), a key enzyme in the biosynthesis of cholesterol, inhibited TNBC growth of TNBC and induced tumor suppressor ERβ in TNBC cells. Consequently, we studied the effects of RO, together with liquiritigenin (LQ), a naturally occurring plant product that is an ERβ agonist, on TNBC progression.

Methods: Sulforhodamine B assays were used to measure viability of cultured MDA-MB-231 and BT-20 human TNBC cells in the presence of either RO and LQ alone or in combination. Tumor xenografts in nude mice permitted the evaluation of tumor growth following treatment with RO, LQ or a combination of RO and LQ. Estrogen receptor expression, apoptosis and levels of angiogenesis markers in tumor-xenograft tissues were assessed immunohistochemically.

Results: Both RO and LQ significantly reduced the viability of MDAMB- 231 and BT-20 TNBC cells in vitro. RO + LQ treatment reduced cell viability to a greater extent than treatment with either individual agent. Administration of RO, LQ, and a combination of RO and LQ significantly inhibited the growth of MDA-MB-231 tumor xenografts in vivo. RO, LQ, and RO + LQ increased ERβ expression in vivo and significantly reduced the expression of angiogenesis markers. Both RO and LQ increased apoptosis in tumor xenografts; a combination of RO + LQ significantly enhanced apoptosis compared with levels observed in response to a single agent.

Conclusion: The ERβ ligand LQ significantly enhanced RO-mediated inhibition of TNBC cell viability in vitro, while also significantly increasing the inhibitory effects of RO on the growth of tumor xenografts in vivo. The anti-tumor properties of RO may in part be due to an offtarget effect that increases ERβ which may then interact with LQ to promote anti-proliferative effects. Anti-tumor effects included inhibition of angiogenesis and induction of apoptosis. A combination of RO and LQ may have potential as a novel treatment strategy against TNBC.