Abstract 1334: MerTK tumorigenesis and immune evasion mechanisms in triple negative breast cancer
Authors: Iida Mari , Kostecki Kourtney , Kranjac Carlene A. , Glitchev Christine , Yang David T. , Wheeler Deric L. 
Published: June 2022

Abstract
Triple negative breast cancer (TNBC) cells lack estrogen, progesterone, and HER2 receptors. TNBC tends to be more aggressive than most types of breast cancer and is more likely to recur even after surgery and treatment. Recently, a few targeted therapies were approved for TNBC, but additional molecular targeting agents are still needed. Previous studies have found that activated MerTK is associated with many types of human cancers, including breast cancer. In this study, we focused on MerTK expression and its tumorigenesis and immunoregulatory functions in TNBC. We first examined MerTK expression levels in human TNBC tissue samples by analyzing tissue microarrays (TMA) using immunohistochemistry (IHC). IHC revealed that ~40% of TNBC samples expressed high levels of MerTK. We further examined MerTK expression levels in ten TNBC cell lines by immunoblot analysis. Three cell lines (BT549, MDAMB231 and MDAMB436) showed strong MerTK expression levels. Next, to evaluate MerTK’s effect on cell proliferation, we treated MDAMB231 cells with MerTK siRNA, which inhibited cell proliferation by 20% compared to siNon-target (siNT). These data indicate that TNBC cells remain dependent on MerTK-activated signaling pathways for proliferation and survival. To further investigate the impact of MerTK expression in TNBC cells, we stably overexpressed MerTK in the TNBC cell line SUM102, which naturally has very low MerTK levels. The results indicated that overexpression of MerTK in SUM102 cells led to: 1) increased proliferation in vitro 2) robust tumor growth, 3) marked migration potential and 4) increased metastasis in mice. Collectively, these results demonstrate that activation of a MerTK-driven pathway could be involved in tumorigenesis in TNBC. To understand the role of MerTK in cell signaling and pro-tumor immunity, we utilized a cytokine array to examine which molecules were impacted in MerTK-overexpressing SUM102 cells. The results showed that production of CXCL4, CD74, and IL-1α was increased, and production of CXCL10 was decreased in MerTK-overexpressing SUM102 cells as compared to control cells. Interestingly, PD-L1 was upregulated and MHC class I was downregulated in MerTK-overexpressing SUM102 cells as compared to control cells. These preliminary findings suggested that MerTK-overexpressing TNBC could be creating a favorable immune microenvironment for proliferation of tumor cells. To further explore the cytokine, PD-L1, and MHC class I results, we overexpressed MerTK in 4T1 murine TNBC-like cells. Immunoblot analysis showed that PD-L1 was upregulated in MerTK-overexpressing 4T1 cells. We will utilize this cell line to evaluate tumor growth and immunosuppression in the tumor microenvironment in BALB/C mice.
Citation Format: Mari Iida, Kourtney Kostecki, Carlene A. Kranjac, Christine Glitchev, David T. Yang, Deric L. Wheeler. MerTK tumorigenesis and immune evasion mechanisms in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1334.