SAT-299 Growth Suppression of Triple-negative Breast Cancer Cells By the Cholesterol Biosynthesis Inhibitor RO 48-8071 is Associated With Induction of Estrogen Receptor-Beta

Abstract Disclosure: Y. Liang: None. S.M. Hyder: None. Approximately 15-20% of human breast cancers are classified as triple-negative (TNBC), because they are devoid of estrogen and progesterone receptors (ER, PR), as well as Her-2-neu (epidermal growth factor receptors), the three identifiable molecular targets for chemotherapy. Patients diagnosed with TNBCs are treated with conventional toxic chemotherapy regimens, which are non-specific and almost always lead to resistance. These patients have poor prognosis due to the emergence of drug-resistant cells that lead to tumor metastasis. Estrogens (E) are essential steroid hormones that regulate several physiological functions in humans. The diverse biological effects of E are mediated by the specific ER, ERα and ERβ. While TNBC are devoid of ERα, the anti-proliferative ERβ protein has recently been shown to be present in TNBC cells. It is thus possible that by modulating its activity in TNBC cells, ERβ might be exploited to trigger an anti-proliferative response. Previously we found that inhibition of oxidosqualene cyclase, an enzyme involved in cholesterol biosynthesis, by RO 48-8071 ([4’-[6-(Allylmethylamino)hexyloxy]-4-bromo-2’-fluorobenzophenone fumarate]; RO), a small-molecule inhibitor used to block cholesterol synthesis, induced ERβ protein in hormone-dependent breast cancer cells and reduced their proliferation. Further studies showed that RO (1-50 μM) also inhibited proliferation of TNBC cells and reduced cell viability. With this in mind, we conducted studies to ascertain whether RO might also elevate ERβ in TNBC. Treatment of MDA-MB-231 and BT-20 cells with RO (25 μM) for 6 h resulted in up-regulation of ERβ protein, but not ERα protein. Treatment of TNBC cells with RO (10 or 25 μM for 6 to 24 h) also increased ERβ mRNA levels. ERβ specific ligands were used to determine whether modulation of ERβ activity in TNBC cells controls cell viability, as determined by SRB assay. An ERβ antagonist (PHTPP) prevented RO-dependent loss of cell viability, while DPN, an ERβ agonist, increased the antiproliferative effects of RO. RO also reduced growth of TNBC BT20 tumor xenografts in nude mice. IHC of RO-treated tumor sections from BT-20 xenografts showed increased levels of ERβ but not ERα protein. In conclusion, RO is a potent inhibitor of TNBC whose action appears to be in part due to an off-target effect that increases the level of anti-proliferative ERβ in TNBC cells. Combinations of RO and ERβ agonists should be considered for TNBC therapy. Presentation: Saturday, July 12, 2025