Oxidosqualene Cyclase Inhibitor RO 48-8071 Increases Functional ERβ Levels in Triple- Negative Breast Cancer Cells

Author(s): Yayun Liang and Salman M. Hyder

Purpose: In most human breast cancers, tumor cell proliferation can be suppressed with anti-hormones or drugs targeting Her-2-neu. TNBC lack ER, PR and Her-2-neu and thus are not susceptible to traditional targeted therapies. TNBC are therefore treated with toxic chemotherapies and eventually develop resistance that leads to metastasis. Our goal was to identify alternative targets in TNBC that might be regulated to suppress progression of the disease. Previously we found that inhibition of oxidosqualene cyclase, an enzyme involved in cholesterol biosynthesis, by RO 48-8071 reduced proliferation of hormone-dependent breast cancer cells. RO 48-8071 induced anti-proliferative ERβ protein in these cells. We recently reported that RO 48-8071 also blocked progression of TNBC disease. With this in mind, we conducted studies to ascertain whether RO 48-8071 might also elevate ERβ protein in TNBC.

Methods: 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 cholesterol biosynthesis), was used for this study. ERβ and ERα protein expression in whole-cell extracts from TNBC cells treated with RO was determined by Western blotting. ERβ specific ligands were used to determine whether modulation of ERβ activity can control cell viability, as determined by SRB assay. RO treated TNBC BT20 tumor xenografts in nude mice were studied to determine whether levels of ERβ increased in vivo concomitant with reduced tumor growth.

Results: In vitro or in vivo exposure of TNBC to RO induced the antiproliferative protein ERβ, but not ERα, in TNBC cells. An ERβ antagonist prevented RO-dependent loss of cell viability, while agonists of ERβ increased the antiproliferative effects of RO.

Conclusions: RO is a potent inhibitor of TNBC and the anti-tumor properties of RO appear to be in part due to an off-target effect that increases the level of anti-proliferative protein ERβ in TNBC cells.