An Anti-Metastatic Role for Macrophage-Derived PROS1

Introduction

Host macrophages were shown to support primary tumor growth and contribute to increased metastasis through various secreted molecules, which modulate inflammation. Protein S (PROS1) expressed by immune cells, activates the TAM family of receptor tyrosine kinases, comprising of TYRO3, AXL and MERTK, to suppress inflammation in immune cells. Therefore, their inactivation is thought to mount a significant anti-tumor immune response, and subsequent inhibition of tumor growth. Here, we investigate the role of the TAM agonist PROS1 in macrophages as a mediator of inflammation, and its impact on tumor growth and metastasis.

Materials and Methods

We genetically deleted PROS1 expression in myeloid cells (Pros1-cKO) by crossing Pros1fl/fl mice to LysM-Cre+ mice. Pros1-cKO mice and littermate controls were challenged with syngeneic Lewis Lung Carcinoma (LLC) or orthotopic mammary breast AT3 tumor cells. Three weeks later, we assessed primary tumor growth and lung metastasis, and performed histological and molecular analysis for gene expression by RT-qPCR. The impact of PROS1 on inflammatory gene expression by bone marrow-derived macrophages (BMDMs) was evaluated by RT-qPCR and ELISA. The effect of BMDM-derived PROS1 on BMDM-secreted factors and on tumor cells was assessed both in-vitro and in-vivo following incubation of conditioned medium with cancer cells.

Results and Discussion

Primary tumor size in control and in Pros1-cKO mice was similar, By contrast, lung metastasis was significantly enhanced in Pros1-cKO mice using both lung and breast cancer models, pointing to an anti-metastatic effect of PROS1. Deletion of PROS1 in the myeloid compartment alone was sufficient to induce inflammation in the lungs of Pros1-cKO mice, with increased levels of pro-inflammatory cytokines. Moreover, PROS1-deficient macrophages increase tumor cell aggressiveness and lung colonization through secreted factors, and induce oncogenic pathways in both lung and mammary tumor cells. Finally, supplementation of exogenous PROS1 rescues this phenotype both in-vitro and in-vivo.

Conclusion

PROS1 in the host macrophages attenuates inflammation and is a potent anti-metastatic factor, implying its possible implementation in inhibiting metastasis.