Antiestrogen Resistant Cell Lines Expressing Estrogen Receptor α Mutations Upregulate the Unfolded Protein Response and are Killed by BHPI

Outgrowth of metastases expressing ER alpha mutations Y537S and D538G is common after endocrine therapy for estrogen receptor alpha (ER alpha) positive breast cancer. The effect of replacing wild type ER alpha in breast cancer cells with these mutations was unclear. We used the CRISPR-Cas9 genome editing system and homology directed repair to isolate and characterize 14 T47D cell lines in which ER alpha Y537S or ER alpha D538G replace one or both wild-type ER alpha genes. In 2-dimensional, and in quantitative anchorage-independent 3-dimensional cell culture, ER alpha Y537S and ER alpha D538G cells exhibited estrogen-independent growth. A progestin further increased their already substantial proliferation in micromolar 4-hydroxytamoxifen and fulvestrant/ICI 182,780 (ICI). Our recently described ER alpha biomodulator, BHPI, which hyperactivates the unfolded protein response (UPR), completely blocked proliferation. In ER alpha Y537S and ER alpha D538G cells, estrogen-ER alpha target genes were constitutively active and partially antiestrogen resistant. The UPR marker sp-XBP1 was constitutively activated in ER alpha Y537S cells and further induced by progesterone in both cell lines. UPR-regulated genes associated with tamoxifen resistance, including the oncogenic chaperone BiP/GRP78, were upregulated. ICI displayed a greater than 2 fold reduction in its ability to induce ER alpha Y537S and ER alpha D538G degradation. Progestins, UPR activation and perhaps reduced ICI-stimulated ER alpha degradation likely contribute to antiestrogen resistance seen in ER alpha Y537S and ER alpha D538G cells.