Abstract BS3-2: The unfolded protein response: A protective pathway and therapeutic target in breast cancer
Authors: Shapiro DJ 
Published: March 2017

Abstract
We identified BHPI, a potent first-in-class non-competitive estrogen receptor α (ERα) biomodulator that kills therapy-resistant ER positive cancer cells. In mouse xenografts, BHPI induced rapid and dramatic regression of breast cancers. We also demonstrated BHPI's effectiveness in cancer cells containing ER mutations Y537S and D538G that are common in metastatic breast cancer. We used the CRISPR-Cas9 genome editing system to isolate T47D breast cancer cell lines in which both copies of wild type ER are replaced with ERY537S or ERD538G. In anchorage independent 3-dimensional culture, the ERY537S and ERD538G cells exhibit robust proliferation in micromolar 4-hydroxytamoxifen and fulvestrant/Faslodex/ICI 182,780, but are killed by nanomolar BHPI. BHPI is so effective because it works by distorting a newly identified normal pathway of estrogen-ER action. BHPI hyperactivates the endoplasmic reticulum stress sensor, the unfolded protein response (UPR). Estrogen-ER rapidly elicits moderate and transient anticipatory activation of the UPR. Activation of this conserved pathway is critical for estrogen and epidermal growth factor (EGF) induced gene expression and cell proliferation and for VEGF regulated angiogenesis and cell survival. Moreover, in analysis of data from approximately 1,000 ER positive breast cancer patients, activation of a UPR gene signature at diagnosis was tightly correlated with subsequent tumor recurrence, tamoxifen resistance and a poor prognosis.
A major mechanism by which tumors develop resistance to therapy is overexpression of pumps, such as multidrug resistance protein 1 (MDR1)/p-glycoprotein, that carry out ATP-dependent pumping of anticancer drugs out of tumor cells. Nontoxic small molecule inhibitors targeting MDR1 remain elusive. Instead, we used BHPI's novel action in cancer cells to target MDR1's substrate, ATP. Acting via ER, BHPI hyperactivates the anticipatory UPR pathway leading to an ATP-depleting futile cycle in which there is ATP-dependent pumping of calcium into the endoplasmic reticulum (EnR), but the EnR calcium channels opened by BHPI-ER action allow the calcium to leak back out. This futile cycle rapidly depletes intracellular ATP, inactivating MDR1 in multidrug resistant breast and ovarian cancer cells. BHPI restored sensitivity of multidrug resistant breast and ovarian cancer cells to killing by therapeutically relevant concentrations of several anticancer drugs. In an orthotopic ovarian cancer xenograft model using OVCAR-3 cells, that are highly resistant to diverse anticancer drugs, the taxane paclitaxel was ineffective and the paclitaxel-treated tumors were uniquely prone to metastasize to adjacent tissue. BHPI alone strongly reduced tumor growth. Notably, tumors were undetectable in mice treated with BHPI plus paclitaxel. Compared to control ovarian tumors, after the combination therapy, plasma levels of the cancer biomarker, CA125, were at least several hundred fold lower; moreover, CA125 levels progressively declined to undetectable.
Targeting drug resistance in ER positive tumors through BHPI-mediated, UPR-dependent, ATP depletion represents a promising new therapeutic strategy. (Support: NIH RO1 DK 071909, DOD BCRP W81XWH-13)
Citation Format: Shapiro DJ. The unfolded protein response: A protective pathway and therapeutic target in 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 BS3-2.