Myeloid-derived PROS1 inhibits tumor metastasis by curbing inflammation

Introduction: The proto-oncogenic receptor tyrosine kinases Tyro3, Axl, and MerTK (TAM), are hyper-activated in various cancers, driving tumor cell proliferation, invasion, survival and drug-resistance. TAMs are also expressed by many immune cells, including macrophages, dendritic cells, natural killer (NK) cells and T cells, where they are important negative regulators of inflammation, suggesting their activation within the tumor microenvironment (TME) may affect tumorigenesis. Host macrophages express PROS1 and are a dominant compartment of the TME supporting tumor growth and metastasis.

Material and Methods: Here we assess whether PROS1 in myeloid cells plays a role in tumor growth and metastasis. For this, we generated Pros1-cKO mice genetically deleting PROS1 expression in myeloid cells by crossing Pros1^fl/fl mice to LysM-Cre⁺ mice (Pros1-cKO). Syngeneic Lewis Lung Carcinoma (LLC) or mammary breast cancer AT3 cells were injected into Pros1-cKO and littermate control mice. 21 days later, mice were sacrificed and assayed for primary tumor growth and metastasis and for inflammatory signaling. Tissues were analysed histologically and molecularly for gene expression by RT-qPCR. To specifically evaluate the role of PROS1 in bone marrow-derived macrophages (BMDMs) and their impact on tumorigenesis, cancer cells were educated with conditioned medium (C.M) derived either from Pros1-cKO BMDMs or controls (Pros1^fl/fl) and were analyzed molecularly for the activation state of key oncogenic pathways.

Results and Discussion: Upon tumor challenge, primary tumor growth wasn’t significantly affected. In contrast, the metastatic burden in lungs of Pros1-cKO mice was significantly enhanced compared to controls in both lung and breast cancer models. Lack of PROS1 in the myeloid compartment alone was sufficient to induce a local inflammation in the lungs of Pros1-cKO mice with increased levels of pro-inflammatory cytokines, rendering the lungs permissive for tumor cell colonization. Remarkably, replacing myeloid cells by PROS1-expressing bone marrow reversed the inflammatory status in the lungs. We find that PROS1-deficient BMDMs increased cancer cell aggressiveness through secreted factors, which enhanced the activation of the AKT and ERK oncogenic pathways within LLC and AT3 cancer cells. Finally, addition of purified PROS1 to Pros1-deficient BMDMs rescued their pro inflammatory profile and suppressed their pro-metastatic properties both in-vitro and in-vivo.

Conclusion: Altogether, our findings reveal PROS1 as a key regulator of inflammation in macrophages. We show that PROS1 acts as a molecular switch altering the macrophage effect on metastasis through inflammation. We suggest PROS1 may serve as a novel anti-cancer therapeutic strategy in certain cancer settings.