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Title:Identifying new actions of estrogen receptor targetable with small molecules
Author(s):Andruska, Neal
Director of Research:Shapiro, David J.
Doctoral Committee Chair(s):Shapiro, David J.
Doctoral Committee Member(s):Hergenrother, Paul J.; Bolton, Eric C.; Chen, Lin-Feng
Department / Program:Biochemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):ovarian cancer
drug development
breast cancer
Estrogen Receptor
Abstract:Estrogens, acting via estrogen receptor α (ERα), stimulate the proliferation and metastatic potential of breast cancers, and likely some ovarian, endometrial, and cervical cancers. Endocrine therapy targeting these cancers often leads to development of resistance. Ovarian cancers do not respond to endocrine therapy. The presence of ERα in many resistant tumors suggested the existence of additional modes of ERα action that could be targeted with small molecule biomodulators. To identify new small molecule inhibitors that target the estrogen-ERα axis in cancer cells resistant to current therapies, we developed and implemented an unbiased pathway-directed screen of ~150,000 small molecules. Using screening and functional validation, I identified BHPI, a potent non-competitive small molecule ERα inhibitor. At 100 nM, BHPI completely inhibited estrogen-induced proliferation in ERα containing breast, ovarian and endometrial cancer cells with no effect at 10,000 nM in counterpart ERα negative cells. BHPI effectively targets cancer cells that no longer depend on estrogens for growth, kills drug-resistant breast and ovarian cancer cells, and restores paclitaxel sensitivity to multi-drug resistant cancer cells. In a mouse xenograft, BHPI at 15 mg/kg daily for 10 days induced rapid and substantial regression of 48/52 large tumors and was not toxic. BHPI’s effectiveness in a broad range of ERα-containing breast, ovarian and endometrial cancer cells is due to its ability to target the endoplasmic reticulum stress sensor pathway, the unfolded protein response (UPR). In ERα positive cancer cells, BHPI opens the endoplasmic reticulum IP3R calcium channel, triggering efflux of calcium into the cytosol, strongly activating all three arms of the UPR. BHPI activates the PERK arm of the UPR, which leads to phosphorylation of eukaryotic initiation factor 2α (eIF2α), and potent inhibition of protein synthesis. To restore endoplasmic reticulum calcium, calcium-ATPase pumps are activated, but the calcium rapidly leaks back out through the open IP3R channel, creating an ATP-depleting futile cycle. ATP depletion activates the energy sensor, AMPK. The influx of calcium into the cytosol leads to phosphorylation and inactivation of eukaryotic elongation factor 2 (eEF2), inhibiting the elongation step of protein synthesis. Inhibiting protein synthesis at a second site prevents synthesis of chaperones and other proteins that normally resolve UPR stress. BHPI kills ERα positive cancer cells by triggering a lethal cascade encompassing sustained activation of the UPR, persistent inhibition of protein synthesis, and depletion of ATP stores. BHPI selectively targets the UPR in ERα positive cancer cells by distorting a previously unknown ability of estrogen-ERα to stimulate a weak and transient UPR activation that protects cancer cells from subsequent stress. This anticipatory activation of the UPR by estrogen-ERα is essential for estrogen-ERα-induced gene expression and cell proliferation, and is a new paradigm by which estrogens promote therapy-resistance and tumor progression. Bioinformatic analysis of data from ≈1,000 ERα positive breast tumors shows that estrogen-ERα and UPR activity become elevated during tumor development and that a UPR gene signature is a powerful new prognostic biomarker predictive of resistance to tamoxifen therapy, time to relapse, and overall survival. We employed a novel approach to identifying new pathways and cancer biomarkers in which unbiased high throughput screening is used to “interrogate the cell”. For a well-studied protein, such as ERα, it was unclear whether this approach could identify new pathways and biomarkers and promising new drug candidates. These studies demonstrate the potential of small molecules identified through targeted cell-based screening to reveal and validate new pathways of action, therapeutic drug targets, and small molecule therapeutic candidates; even in a system as intensively studied as ERα positive breast cancer.
Issue Date:2015-01-08
Rights Information:Copyright 2015 Neal Andruska
Date Available in IDEALS:2015-07-22
Date Deposited:May 2015

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