Effect of 17β-estradiol on estrogen receptor negative breast cancer cells in an osteolytic mouse model

Abstract

&NA; 17&bgr;‐Estradiol (E2) promotes metastasis of triple negative breast cancer cells to bone. Recent studies show many triple negative breast cancer cell lines lacking the 66 kDa estrogen receptor (ER) alpha (ER&agr;66) or its splice variant ER&agr;46, express another splice variant, ER&agr;36 associated with membrane‐mediated rapid actions of the hormone. qPCR and western blot confirmed that MCF7 cells possessed ER&agr; splice variants ER&agr;66, ER&agr;46 and ER&agr;36, while ER‐negative breast cancer cells MDA‐MB‐231 possessed only ER&agr;36. MDA‐MB‐231 breast cancer cells were implanted into medullary canals of ovariectomized female athymic nude mice femurs (N = 8 mice/treatment). To examine the effect of E2 on osteolysis, mice were treated with 0.72 mg E2 or placebo via 60 day release osmotic pumps implanted subcutaneously. Legs were examined by Faxitron through the course of the study, and by microCT and histology after 8 weeks. Greater occurrence of osteolysis and pathologic fracture was observed in E2‐treated animals compared to placebo, and microCT demonstrated less bone volume remaining in MDA‐MB‐231 treated legs compared to contralateral control legs, as well as E2‐treated animals compared to placebo. E2‐treated animals had significantly greater tumor volume compared to placebo. Large nests of anaplastic tumor cells with eroded cortical margin were observed in E2‐treated animals compared to placebo. MDA MB 231 breast cancer cells positive for ER&agr;36 but negative for ER&agr;46/66 had enhanced osteolysis, pathologic fractures, and tumor volume in an in vivo osteolytic mouse model when treated with 17&bgr;‐estradiol compared to placebo, demonstrating a role for ER&agr;36 in bone tumor progression.