Abstract 3883: Overcoming tamoxifen resistance by re-activating cAMP-Ca2+−ROS-ferroptosis axis in ER+ breast cancer
Authors: Alam Rashedul , Saatci Ozge , Uner Meral Bugadyci , Huynh-Dam Kim-Tuyen , Cetin Metin , Gedik Mustafa Emre , Belder Nevin , Bal Hilal , Tokat Unal Metin , Marangoni Elisabetta , Aksoy Sercan , Uner Aysegul , Akyol Aytekin , Sahin Ozgur 
Published: April 2023

Abstract
Estrogen receptor (ER) positive breast cancer accounts for about 75% of all breast cancer cases. Tamoxifen has been the mainstay therapy for over 40 years that is used to treat early, locally advanced, and metastatic ER-positive breast cancers. Despite the initial clinical success, approximately 20-30% of high-risk, advanced ER-positive breast cancer patients are resistant to tamoxifen or develop resistance over time which significantly decreases survival. Long-noncoding RNAs (lncRNAs) are a class of non-coding RNAs that are more than 200 nucleotides in length and are involved in tumorigenesis via carrying out diverse functions by interacting with DNA, RNA or proteins. Here, we identified a lncRNA (LINC00152) that is upregulated in tamoxifen resistant (TamR) cell lines by RNA sequencing, and we validated its upregulation in endocrine resistant, ER-positive patient-derived xenografts (PDXs) as well as in tamoxifen-treated ER-positive breast cancer patient tumors. We performed in-situ hybridization of the LINC00152 in our own cohort of tamoxifen-treated ER+ breast cancer patients and demonstrated a significant association of high LINC00152 expression with worse disease-free survival and treatment response. Inhibiting LINC00152 using either siRNAs or anti-sense oligonucleotides or shRNAs restored tamoxifen sensitivity in TamR cells. Mechanistically, we demonstrated that LINC00152 knockdown in combination with tamoxifen increases the expression of TRPC1, a calcium channel located on the plasma membrane, and thereby causes the cytoplasmic accumulation of excessive amounts of Ca2+, which ultimately leads to the generation of reactive oxygen species (ROS), lipid peroxidation and cell death by ferroptosis. Notably, we identified a cyclic AMP (cAMP)-specific phosphodiesterase, PDE4D as a novel interactor of LINC00152. We showed that PDE4D mRNA is stabilized by LINC00152 and reduces cAMP levels which then decreases the intracellular Ca2+ and blocks ROS generation, lipid peroxidation, and ferroptosis, ultimately leading to tamoxifen resistance. Therefore, targeting the LINC00152-PDE4D-cAMP-Ca2+ axis may represent a promising therapeutic approach to tackle tamoxifen resistance and improve clinical outcome in refractory ER-positive breast cancer.
Citation Format: Rashedul Alam, Ozge Saatci, Meral Bugadyci Uner, Kim-Tuyen Huynh-Dam, Metin Cetin, Mustafa Emre Gedik, Nevin Belder, Hilal Bal, Unal Metin Tokat, Elisabetta Marangoni, Sercan Aksoy, Aysegul Uner, Aytekin Akyol, Ozgur Sahin. Overcoming tamoxifen resistance by re-activating cAMP-Ca2+−ROS-ferroptosis axis in ER+ breast cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3883.