SUSD2 promotes tumor-associated macrophage recruitment by increasing levels of MCP-1 in breast cancer

Tumor-associated macrophages (TAMs) play a role in tumor angiogenesis and are recruited into the tumor microenvironment (TME) by secreted chemokines, including Monocyte Chemoattractant Protein-1 (MCP-1/CCL2). Angiogenesis is required to sustain proliferation and enable metastasis of breast cancer (BCa) cells. Understanding the underlying mechanisms of TAM recruitment would allow for the identification of desperately needed novel drug targets. Sushi Domain Containing 2 (SUSD2), a transmembrane protein on BCa cells, was previously shown to promote tumor angiogenesis in a murine model. To identify the role of SUSD2 in angiogenesis, 175 human breast tumors were surveyed by immunohistochemical analysis for the presence of SUSD2 and macrophages. Tumors with high levels of SUSD2 staining contained 2-fold more TAMs, mainly of the M2 pro-angiogenic phenotype. An in vitro co-culture model system was developed by differentiating SC monocytes into SC M0 macrophages. A 2-fold increase in polarized M2 macrophages was observed when M0 macrophages were incubated with SUSD2-expressing BCa cells compared to cancer cells that do not contain SUSD2. Since MCP-1 is known to recruit macrophages, levels of MCP-1 were compared between SUSD2-expressing MDA-MB-231 and MBA-MB-231-vector control cell lines. MCP-1 RNA, intracellular protein and secreted MCP-1 were all significantly increased compared to the vector control. Knockdown of SUSD2 in SKBR3 resulted in significantly decreased levels of secreted MCP-1. Consistently, increased levels of MCP-1 were observed in Susd2-expressing tumors generated from an in vivo isogeneic mouse model compared to the vector control tumors. Because SUSD2 recruits macrophages into the TME and promotes M2 polarization, inhibiting the function of SUSD2 may be an effective therapy for breast cancer patients.