GREB1

Abstract: The function of estrogen in breast cancer proliferation and progression is likely to be due to the expression of a repertoire of genes regulated by estrogen receptor (ER). Using suppression subtractive hybridization, we have isolated a set of 14 estrogen-responsive genes that was differentially expressed in MCF7 cells stimulated by beta-estradiol as compared with unstimulated cells. Tamoxifen repressed the expression of all 14 estrogen-responsive genes. Thirteen of the genes were induced within 6 h of estrogen treatment, indicating that these were early response genes in the ER-regulated pathway. PDZK1 and a new gene, GREB1, demonstrated a significant correlation with ER phenotype in a panel of breast cancer cell lines. Treatment with cycloheximide indicated that ER directly controls GREB1 expression. Three cDNAs (GREB1a, GREB1b, and GREB1c) were isolated by screening a MCF7 cDNA library. These three cDNAs of GREB1 shared extensive sequences through the open reading frame but had divergent 5' untranslated regions, indicating the possibility of multiple promoters regulated by beta-estradiol. Studies in primary breast cancers showed that PDZK1 and GREB1 were overexpressed in ER-positive breast cancers as compared with ER-negative breast cancers by 19-fold and 3.5-fold, respectively. GREB1 was also induced by beta-estradiol in the ER-positive endometrial cell line ECC-1. The pattern of expression suggests a critical role for these two genes in the response of tissues and tumors to beta-estradiol.

Abstract: Therapies targeting estrogenic stimulation in estrogen receptor-positive (ER+) breast cancer (BC) reduce mortality, but resistance remains a major clinical problem. Molecular studies have shown few high-frequency mutations to be associated with endocrine resistance. In contrast, expression profiling of primary ER+ BC samples has identified several promising signatures/networks for targeting. To identify common adaptive mechanisms associated with resistance to aromatase inhibitors (AIs), we assessed changes in global gene expression during adaptation to long-term estrogen deprivation (LTED) in a panel of ER+ BC cell lines cultured in 2D on plastic (MCF7, T47D, HCC1428, SUM44 and ZR75.1) or in 3D on collagen (MCF7) to model the stromal compartment. Furthermore, dimethyl labelling followed by LC-MS/MS was used to assess global changes in protein abundance. The role of target genes/proteins on proliferation, ER-mediated transcription and recruitment of ER to target gene promoters was analysed. The cholesterol biosynthesis pathway was the common upregulated pathway in the ER+ LTED but not the ER– LTED cell lines, suggesting a potential mechanism dependent on continued ER expression. Targeting the individual genes of the cholesterol biosynthesis pathway with siRNAs caused a 30–50 % drop in proliferation. Further analysis showed increased expression of 25-hydroxycholesterol (HC) in the MCF7 LTED cells. Exogenous 25-HC or 27-HC increased ER-mediated transcription and expression of the endogenous estrogen-regulated gene TFF1 in ER+ LTED cells but not in the ER– LTED cells. Additionally, recruitment of the ER and CREB-binding protein (CBP) to the TFF1 and GREB1 promoters was increased upon treatment with 25-HC and 27-HC. In-silico analysis of two independent studies of primary ER+ BC patients treated with neoadjuvant AIs showed that increased expression of MSMO1, EBP, LBR and SQLE enzymes, required for cholesterol synthesis and increased in our in-vitro models, was significantly associated with poor response to endocrine therapy. Taken together, these data provide support for the role of cholesterol biosynthesis enzymes and the cholesterol metabolites, 25-HC and 27-HC, in a novel mechanism of resistance to endocrine therapy in ER+ BC that has potential as a therapeutic target.

Abstract: // Martina S.J. McDermott 1 , Alexander A. Chumanevich 1 , Chang-uk Lim 1 , Jiaxin Liang 1 , Mengqian Chen 1 , Serena Altilia 1 , David Oliver 1 , James M. Rae 2 , Michael Shtutman 1 , Hippokratis Kiaris 1 , Balazs Győrffy 3 , Igor B. Roninson 1 and Eugenia V. Broude 1 1 Department of Drug Discovery and Biomedical Sciences, South Carolina College of Pharmacy, University of South Carolina, Columbia, SC, USA 2 University of Michigan Medical School, Ann Arbor, MI, USA 3 MTA TTK Lendulet Cancer Biomarker Research Group, Semmelweis University 2nd Department of Pediatrics, Budapest, Hungary Correspondence to: Eugenia V. Broude, email: // Keywords : CDK8, estrogen receptor, breast cancer, transcription, estrogen independence Received : January 04, 2017 Accepted : January 17, 2017 Published : January 29, 2017 Abstract Hormone therapy targeting estrogen receptor (ER) is the principal treatment for ER-positive breast cancers. However, many cancers develop resistance to hormone therapy while retaining ER expression. Identifying new druggable mediators of ER function can help to increase the efficacy of ER-targeting drugs. Cyclin-dependent kinase 8 (CDK8) is a Mediator complex-associated transcriptional regulator with oncogenic activities. Expression of CDK8, its paralog CDK19 and their binding partner Cyclin C are negative prognostic markers in breast cancer. Meta-analysis of transcriptome databases revealed an inverse correlation between CDK8 and ERα expression, suggesting that CDK8 could be functionally associated with ER. We have found that CDK8 inhibition by CDK8/19-selective small-molecule kinase inhibitors, by shRNA knockdown or by CRISPR/CAS9 knockout suppresses estrogen-induced transcription in ER-positive breast cancer cells; this effect was exerted downstream of ER. Estrogen addition stimulated the binding of CDK8 to the ER-responsive GREB1 gene promoter and CDK8/19 inhibition reduced estrogen-stimulated association of an elongation-competent phosphorylated form of RNA Polymerase II with GREB1. CDK8/19 inhibitors abrogated the mitogenic effect of estrogen on ER-positive cells and potentiated the growth-inhibitory effects of ER antagonist fulvestrant. Treatment of estrogen-deprived ER-positive breast cancer cells with CDK8/19 inhibitors strongly impeded the development of estrogen independence. In vivo treatment with a CDK8/19 inhibitor Senexin B suppressed tumor growth and augmented the effects of fulvestrant in ER-positive breast cancer xenografts. These results identify CDK8 as a novel downstream mediator of ER and suggest the utility of CDK8 inhibitors for ER-positive breast cancer therapy.

Abstract: ARV-471, an estrogen receptor (ER) alpha PROTAC, is a hetero-bifunctional molecule that facilitates the interactions between estrogen receptor alpha and an intracellular E3 ligase complex, leading to the ubiquitylation and subsequent degradation of estrogen receptors via the proteasome. ARV-471 robustly degrades ER in ER-positive breast cancer cell lines with a half-maximal degradation concentration (DC50) of ˜ 2 nM. PROTAC-mediated ER degradation decreases the expression of classically-regulated ER-target genes (PR, GREB1, TFF) and inhibits cell proliferation of ER-dependent cell lines (MCF7, T47D). Additionally, ARV-471 degrades clinically-relevant ESR1 variants (Y537S and D538G) and inhibits growth of cell lines expressing those variants. In an immature rat uterotrophic model, ARV-471 degrades rat uterine ER and demonstrates no agonist activity. Daily, oral-administration of single agent ARV-471 (3, 10, and 30 mpk) leads to significant tumor volume regressions of estradiol-dependent MCF7 xenografts and concomitant tumor ER protein reductions of >90% at study termination. Moreover, when a CDK4/6 inhibitor is combined with ARV-471 in the MCF7 model, even more pronounced tumor growth inhibition is observed (˜130% TGI), accompanied by significant reductions in ER protein levels. In an ESR1 Y537S, hormone-independent patient-derived xenograft model, ARV-471 at 10 mpk completely inhibited growth and also reduced mutant ER protein levels. Taken together, the preclinical data of ARV-471 supports its continued development as a best-in-class oral ER PROTAC-degrader. Citation Format: Flanagan JJ, Qian Y, Gough SM, Andreoli M, Bookbinder M, Cadelina G, Bradley J, Rousseau E, Willard R, Pizzano J, Crews CM, Crew AP, Taylor I, Houston J. ARV-471, an oral estrogen receptor PROTAC degrader for breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-04-18.