Abstract P2-04-06: Targeting of the unfolded protein response signaling arms differentially regulates macrophage proliferation, plasticity, and breast cancer cell clearance

The unfolded protein response (UPR) is an endoplasmic reticulum stress pathway controlled by the protein chaperone, glucose-regulated protein 78 (GRP78), to mediate inositol-requiring enzyme 1 (IRE1), PKR-like endoplasmic reticulum kinase (PERK), and activating transcription factor-6 (ATF6) signaling. UPR signaling has been shown to be upregulated in many different types of cancers, including breast cancer and melanoma, and is associated with the development of therapeutic resistance. These data suggest the importance of targeting UPR signaling as a possible cancer therapy. We have previously shown GRP78 to be upregulated in human breast tumor samples and leads to endocrine targeted therapy resistance. We recently showed inhibiting GRP78 in human orthotopic xenografts potentiates tamoxifen therapy effectiveness in sensitive tumors and restores endocrine therapy responsiveness in resistant tumors. In these GRP78-inhibited tumors there was a significant increase of CD68 positive macrophage population, suggesting that targeting UPR signaling has critical effects on the tumor microenvironment. Therefore, consideration of each UPR signaling component and how it effects the different cellular compartments of the tumor microenvironment need to be investigated to optimally induce both an antitumor immune effect and inhibit tumor epithelial cell growth. We now show deletion of GRP78, IRE1, and PERK through RNAi differentially regulates macrophage polarization. Specifically, PERK inhibition enhances macrophage proliferation and macrophage-mediated phagocytosis of 4T1 breast cancer cells, but not GRP78 or IRE1 inhibition. Targeting UPR signaling in the breast cancer cells also differentially affected macrophage cytolytic capacity; Specific breast cancer cell inhibition of IRE1 or GRP78 enhanced macrophage-mediated phagocytosis. Conditioned media from control or GRP78 silenced ZR-75-1 breast cancer cells indicated reciprocal regulation of CD206 and CD80; suggesting regulation of macrophage plasticity by GRP78-controlled secreted factors. GRP78 targeting in mice resulting in a cytokine shift and increased tumoral CD80+/CD68+ cells, suggesting that GRP78 inhibition favors a M1-like macrophage profile. Inhibition of UPR components in both macrophage and breast cancer cells, similar to what would be observed in systemic cancer therapies, indicated that either PERK or GRP78 inhibition enhances macrophage cytolytic clearance of breast cancer cells. Taken together, these data suggest that targeting GRP78 or PERK promotes an anti-tumor immune response by either directly promoting macrophage cytolytic activity (PERK targeting) or indirectly by shifting tumoral cytokine secretion (GRP78 targeting). Citation Format: Cook KL, Wilson A, Westwood B, Soto-Pantoja DR. Targeting of the unfolded protein response signaling arms differentially regulates macrophage proliferation, plasticity, and breast cancer cell clearance [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-04-06.