Abstract P2-16-05: Targeting EZH2 to overcome chemoresistance in Triple Negative Breast Cancers Employing Combinatorial Approach
Authors: Shankar Eswar , SNC Sridhar , Rima Xilal Y. , Kushwaha Prem , Verma Shiv , Sarathy Gautam K. , kulkarni Anagh , Allimuthu Dharmaraja , Reátegui Eduardo , Gupta Sanjay , Ramaswamy Bhuvaneswari 
Published: March 2023

Objective: Triple Negative Breast Cancers (TNBC) are highly invasive and 46% of patients develop distant metastases, leading to higher mortality. Chemotherapy remains the most efficacious option, however, there is still a largely unmet need to identify novel therapeutic targets in TNBC to increase treatment options and improve patient outcomes, survival. Enhancer of zeste homologue 2 (EZH2), a member of the catalytic subunit of the polycomb repressive complex 2, is a histone methyltransferase and its canonical function is to methylate lysine 27 of histone 3. EZH2 is a potential driver of TNBC metastasis, and its high expression strongly associates with the TNBC phenotype as compared with other molecular subtypes of breast cancer. Despite the advancement in the discovery of inhibitors for EZH2 that attenuate its catalytic activity. Currently, several EZH2 inhibitors are under development and undergoing clinical trials and these compounds have been proved to be effective in the treatment of hematological malignancies, sarcomas and malignant rhabdoid tumors. However these inhibitors do not affect the intrinsic protein stability of EZH2, but typically competes with the cofactor S-adenosylmethionine (SAM) and binds to the SET domain of EZH2. Hence, the EZH2 inhibitors are only effective for some malignant blood tumors, and have poor efficacy for solid tumors, such as TNBC. Several studies have shown the involvement of neurotransmitter dopamine in proliferation, apoptosis, tumor angiogenesis, and drug resistance among different cancers, including the breast1. Dopamine D1 receptor activation in TNBC cell line induces apoptosis, autophagy and phosphorylation of eukaryotic translation initiation factor 2-alpha (eIF2a)2. Also, D1R agonists inhibits the invasion of breast cancer cell lines MDA-MB-231 and BT-20 and regress mammary tumors3. The oncogenic nature of EZH2 in driving aggressiveness in TNBC cells led us to simultaneously target dopamine D1 receptor and EZH2 to completely ablate tumor growth and metastasis. Methods: Schrodinger protein modeling software was employed for docking studies. TNBC cells MDA-MB-231 cells were treated with the EZH2 inhibitor GSK126 and/or D1R agonists A77636 and SKF38393 for assessing cell viability, migration. Immuno precipitation to investigate if the combination could disrupt the PRC2 complex in TNBC cells. To model tumor burden and the effects of combination therapy on tumor growth in vitro, we tested the combinatory effect of GSK126 and D1R agonists in a 3D culture system of MDA-MB-231 cells encapsulated in calcium-alginate microgels seeded from a microfluidic droplet generator. Cell death was determined by Yo-pro-Propidium Iodide staining. Result: In silico analysis confirmed that D1R agonists exhibited strong stabilization of protein structures upon binding to the EZH2 catalytic active site, albeit with slightly weaker affinity than GSK126. Combination treatment with GSK126 and dopamine agonists A77636 or SKF 38393 led to significant and synergistic inhibition of cell viability (p&lt; 0.01), migration, invasion, and EZH2 activity (p&lt; 0.01) in TNBC cells. In addition, MDA-MB-231 cells formed tumor spheroids that were subjected to the EZH2 inhibitor alongside dopamine agonists, indicating tumor reduction relative to single agent treatment in 3D culture (p &lt; 0.0001). The combination also led to increased necrotic cell death (p &lt; 0.05). Immunoprecipitation of EZH2 in the presence of GSK126 and SKF38393 and A77636 dissociated the physical interaction between EZH2, EED, and SUZ12 in MDA-MB-231 cells. Conclusion: Our data suggest that the combination of GSK126 and Dopamine D1 agonists synergistically inhibits TNBC proliferation by disrupting EZH2 functions leading to necrotic cell death. (This work is supported by DOD: W81XWH2010065, for Eswar Shankar). 1) PMID: 21531818 2) PMID: 29773888 3) PMID: 26477316

Citation Format: Eswar Shankar, Sridhar SNC, Xilal Y. Rima, Prem Kushwaha, Shiv Verma, Gautam K. Sarathy, Anagh kulkarni, Dharmaraja Allimuthu, Eduardo Reátegui, Sanjay Gupta, Bhuvaneswari Ramaswamy. Targeting EZH2 to overcome chemoresistance in Triple Negative Breast Cancers Employing Combinatorial Approach. [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-16-05.