5,6-Epoxy-cholesterols contribute to the anticancer pharmacology of Tamoxifen in breast cancer cells

Tamoxifen (Tam) is a selective estrogen receptor modulator (SERM) that remains one of the major drugs used in the hormonotherapy of breast cancer (BC). In addition to its SERM activity, we recently showed that the oxidative metabolism of cholesterol plays a role in its anticancer pharmacology. We established that these effects were not regulated by the ER but by the microsomal antiestrogen binding site/ cholesterol-5,6-epoxide hydrolase complex (AEBS/ChEH). The present study aimed to identify the oxysterols that are produced under Tam treatment and to define their mechanisms of action. Tam and PBPE (a selective AEBS/ChEH ligand) stimulated the production and the accumulation of 5,6a-epoxycholesterol (5,6a-EC), 5,6a-epoxy-cholesterol-3b- sulfate (5,6-ECS), 5,6b-epoxy-cholesterol (5,6b-EC) in MCF-7 cells through a ROS-dependent mechanism, by inhibiting ChEH and inducing sulfation of 5,6a-EC by SULT2B1b. We showed that only 5,6a-EC was responsible for the induction of triacylglycerol (TAG) biosynthesis by Tam and PBPE, through the modulation of the oxysterol receptor LXRb. The cytotoxicity mediated by Tam and PBPE was triggered by 5,6b-EC through an LXRb-independent route and by 5,6-ECS through an LXRb-dependent mechanism. The importance of SULT2B1b was confirmed by its ectopic expression in the SULT2B1b(-) MDA-MB-231 cells, which became sensitive to 5,6a-EC, Tam or PBPE at a comparable level to MCF-7 cells. This study established that 5,6-EC metabolites contribute to the anticancer pharmacology of Tam and highlights a novel signaling pathway that points to a rationale for re-sensitizing BC cells to Tam and AEBS/ChEH ligands.