Abstract P3-04-06: ER reactivation rapidly elicits cell death effects in anti-estrogen-resistant breast cancer
Authors: Hosford SR , Kettenbach AN , Varn FS , Cheng C , Miller TW 
Published: March 2017

Antagonism of estrogen receptor (ER) transcriptional activity using adjuvant anti-estrogen therapies has improved disease outcomes in many patients with ER+ breast cancer. However, cancer recurs in 1/3 of patients within 15 years of follow-up. While anti-estrogens can slow the progression of metastatic disease, this disease is almost uniformly fatal. Prior to the development of tamoxifen, high-dose estrogens were used to treat late stage breast cancer with response rates similar to those achieved with tamoxifen. Increased efficacy of estrogen therapy was observed in women who were farther past menopause, suggesting that tumor adaptation to low-estrogen conditions is associated with response to estrogen therapy. Similarly, withdrawal of anti-estrogen therapy in patients with anti-estrogen-resistant disease has shown clinical benefit.

MCF-7 cells with acquired resistance to fulvestrant (fulv; FR), and long-term estrogen-deprived (LTED) MCF-7 and HCC-1428 cells overexpress ER compared to parental controls. Upon withdrawal of fulv in FR cells or treatment with 17b-estradiol in LTED cells, ER transcriptional activity is re-engaged at higher levels than in parental cells, concomitant with drastically decreased cell proliferation and increased apoptosis in endocrine-resistant lines. ER reactivation coincides with an unfolded protein response (UPR) following fulv withdrawal (FR) or E2 treatment (LTED). However, treatment of LTED cells with a proteasome inhibitor protects against apoptosis induced by E2 treatment. Prior studies in other cancer subtypes have shown that proteasome inhibitor treatment can prevent expression of pro-apoptotic FasL, which is upregulated following ER reactivation in FR and LTED cells. Alternatively, inhibition of the proteasome may prevent degradation of anti-apoptotic Bcl-2 family proteins including Mcl-1, which is downregulated following FW in FR cells.

The WHIM16 PDX model was derived from a post-menopausal patient with anti-estrogen-resistant ER+/PR+ breast cancer that responded to 17b-estradiol therapy. WHIM16 PDX tumors grown in ovariectomized mice rapidly, completely regress upon 17b-estradiol treatment. Tumor regression is paralleled by increased Src activation, which is associated with ER turnover and has been implicated in 17b-estradiol-induced apoptosis. Src activation is also observed in FR cells following fulv withdrawal, and in LTED cells treated with E2. Treatment of LTED cells with the Src inhibitor dasatinib protects against E2-induced apoptosis, indicating Src activity may be required for the anti-cancer effects of 17b-estradiol.

Upon withdrawal of 17b-estradiol, clinically silent (non-palpable) WHIM16 tumors resume growth; however, tumors remain sensitive to repeat administration of 17b-estradiol. Long-term fulv-withdrawn FR cells show restored sensitivity to fulv, indicating that cycling of estrogen and anti-estrogen therapies may be an effective treatment strategy.

Citation Format: Hosford SR, Kettenbach AN, Varn FS, Cheng C, Miller TW. ER reactivation rapidly elicits cell death effects in anti-estrogen-resistant breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-04-06.