Abstract 3646: New functional role of tamoxifen in breast cancer immunomodulation: role of RACK7 in activation of type I interferon signaling and CEACAM1/TIM-3-dependent immunosuppression
Authors: Aberin Marvin A. , Lee Yu-Ling (Pony) , Hsu Xin-Guo , Shen Chen-Yang , Chang Yao-Ming , Lin Kun-Yuan , Guha Chandan , Wang Shu-Ping 
Published: June 2023

Abstract
Despite advancements of immunotherapy against various cancers, breast cancer still retains poor response to immune checkpoint blockade (ICB) therapy. Therefore, the identification of promising target or new strategy to enhance ICB therapy in breast cancer is crucial. Here, we uncover that the DNA-damaging potential of tamoxifen (TAM) can shape the unfavorable but ready-to-fire tumor immune microenvironment in breast cancer. We discover that a long-term TAM administration unexpectedly induces JAK/STAT signaling and Type I interferon (IFN)-stimulated gene expression to promote T-cell infiltration. Furthermore, TAM also induces the expression of CEACAM1, which acts as an alternative T-cell inhibitory ligand via binding to TIM-3, a “checkpoint” receptor expressed on CD4+ and CD8+ T cells. The chromatin ‘‘reader’’ RACK7/ZMYND8, which functions as a transcriptional repressor of IFN-stimulated gene (ISG) and a critical factor in DNA repair, is found to be downregulated upon tamoxifen exposure and involved in the tamoxifen-mediated cellular modulation. We demonstrate that TAM in conjunction with RACK7-knockdown (KD) triggers robust upregulation of ISGs and CEACAM1 in both estrogen receptor-positive (ER+) and triple negative breast cancer (TNBC) cells. This immunomodulatory effect lead by loss of RACK7 is specific to TAM treatment, and is not observed when combined with other endocrine therapeutics. TAM combined with RACK7-KD promotes mitochondrial DNA damage, which leads to accumulation of cytosolic DNA and subsequent activation of the cGAS/STING pathway. The murine breast orthotopic models with TS/A, EO771 and 4T1 cells further demonstrate that TAM-mediated immunomodulatory in conjunction with RACK7-KD evokes cytokine/chemokine secretion and further induces T-cell infiltration into tumor microenvironment. However, the tumor killing effect is limited due to promotion of T-cell exhaustion from CEACAM1-TIM-3 interaction between tumor and T cells. Thus, our study indicates that targeting CEACAM1-TIM-3 interaction is crucial for TAM-mediated tumor immune response. This brings promising therapeutic approach with TAM by combining RACK7-KD with blockage to CEACAM1-TIM-3 interaction in breast cancer. The resistance of tamoxifen treatment may be overcome, and RACK7 may serve as both a therapeutic target and a biomarker to enable ICB therapy.
Citation Format: Marvin A. Aberin, Yu-Ling (Pony) Lee, Xin-Guo Hsu, Chen-Yang Shen, Yao-Ming Chang, Kun-Yuan Lin, Chandan Guha, Shu-Ping Wang. New functional role of tamoxifen in breast cancer immunomodulation: role of RACK7 in activation of type I interferon signaling and CEACAM1/TIM-3-dependent immunosuppression. [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 3646.