Interrogating Estrogen Signaling Pathways in Human ER-Positive Breast Cancer Cells Forming Bone Metastases in Mice

Breast cancer bone metastases (BMET) are incurable, primarily osteolytic, and occur most commonly in estrogen receptor-α positive (ER+) breast cancer. ER+ human breast cancer BMET modeling in mice has demonstrated an estrogen (E₂)-dependent increase in tumor-associated osteolysis and bone-resorbing osteoclasts, independent of estrogenic effects on tumor proliferation or bone turnover, suggesting a possible mechanistic link between tumoral ERα-driven osteolysis and ER+ bone progression. To explore this question, inducible secretion of the osteolytic factor, parathyroid hormone–related protein (PTHrP), was utilized as an in vitro screening bioassay to query the osteolytic potential of estrogen receptor- and signaling pathway–specific ligands in BMET-forming ER+ human breast cancer cells expressing ERα, ERß, and G protein–coupled ER. After identifying genomic ERα signaling, also responsibility for estrogen’s proliferative effects, as necessary and sufficient for osteolytic PTHrP secretion, in vivo effects of a genomic-only ER agonist, estetrol (E₄), on osteolytic ER+ BMET progression were examined. Surprisingly, while pharmacologic effects of E₄ on estrogen-dependent tissues, including bone, were evident, E₄ did not support osteolytic BMET progression (vs robust E₂ effects), suggesting an important role for nongenomic ER signaling in ER+ metastatic progression at this site. Because bone effects of E₄ did not completely recapitulate those of E₂, the relative importance of nongenomic ER signaling in tumor vs bone cannot be ascertained here. Nonetheless, these intriguing findings suggest that targeted manipulation of estrogen signaling to mitigate ER+ metastatic progression in bone may require a nuanced approach, considering genomic and nongenomic effects of ER signaling on both sides of the tumor/bone interface.