ER reactivation following long-term estrogen deprivation drives cell death mediated by an unfolded protein response
Session type: Proffered paper
Theme: Diagnosis and therapy
Suppression of estrogen receptor (ER) signaling with use of adjuvant endocrine therapies has significantly improved outcomes for ER+ breast cancer (BC) patients. However, prior to the introduction of the anti-estrogen tamoxifen, high-dose estrogen was used to treat advanced/metastatic ER+ BC with similar efficacy. Prior studies have shown that long-term estrogen deprivation sensitizes cells to estrogen therapy. Thus, high-dose estrogen may be used to treat anti-estrogen resistant tumors.
To model response to estrogen therapy in anti-estrogen-resistant ER+ BC, fulvestrant was withdrawn from MCF-7 cells with acquired resistance to fulvestrant (fulv;MCF-7/FR). Alternatively, long-term estrogen deprived MCF-7 cells (MCF-7/LTED) were treated with 1nM 17b-estradiol. To examine ER reactivation in vivo, mice bearing C7-2-HI or C4-HI murine ER+ mammary tumors or WHIM16 patient-derived xenografts were treated with 17b-estradiol. Growth, apoptosis, and phenotypic alterations in response to ER reactivation were assessed.
During adaptation to ER inhibition, FR and LTED cells upregulate ER expression compared to parental controls, resulting in hypersensitivity to 17b-estradiol. ER transcriptional activity resumes following withdrawal of fulv from FR cells, or 17b-estradiol treatment of LTED cells. While low doses (1pM) of 17b-estradiol stimulate cell proliferation, treatment with 1nM 17b-estradiol, which promotes growth in parental cells, induces apoptosis in LTED cells. In vivo, 17b-estradiol inhibits growth of C7-2-HI and C4-HI murine tumors, and WHIM16 patient-derived xenografts, concomitant with resumed ER activity.
ER re-engagement results in activation of the unfolded protein response (UPR) and p53. Knockdown of p53 rescues from both fulv-withdrawal- and 17b-estradiol-induced apoptosis. Similarly, knockdown of IRE1, a UPR mediator shown to initiate p53-driven apoptosis, protects against 17b-estradiol.
Long-term adaptation to the absence of ER signaling results in alteration of sensitivity to resumed ER function, driving UPR activation and leading to subsequent growth inhibition and apoptosis. Therefore, estrogen therapy should be considered as an alternative treatment for patients with anti-estrogen resistant disease.