Macrophage signaling cascade determines breast cancer distal metastasis.

Macrophages are abundantly found in the tumor microenvironment and enhance malignancy. At distal metastatic sites, our previous studies identified a distinct population of metastasis-associated macrophages (MAMs) that promote tumor cell extravasation, seeding and persistent growth. These macrophages were derived from inflammatory monocytes recruited by CCL2/CCR2 chemokine signaling and directly promote tumour cell extravasation through VEGF production. Our recent studies identified two subsequent signaling pathways that mediate the retention and function of MAMs after recruitment that are critical for tumour cell survival and persistent growth. Specifically, activation of CCR2 signaling turns on CCL3 expression in MAMs. Genetic deletion of CCL3 or its receptor CCR1 in macrophages reduces lung metastasis, and MAM accumulation in tumor-challenged lung. Adoptive transfer of wild type monocytes restores lung metastasis in Ccl3 deficient mice. Mechanistically, Ccr1 deficiency prevents MAM retention in the lung by reducing direct MAM–tumour cell interactions. On the other hand, MAMs turn on cell surface expression of FMS-like tyrosine kinase 1 (FLT1) upon differentiation from monocytes. Using several genetic models of Flt1 deficiency, we show that macrophage specific FLT1 signaling is critical for breast tumor distal metastatic potential. FLT1 specific inhibitory antibody significantly inhibits the metastatic seeding and persistent growth but does not affect the recruitment or retention of MAMs. Furthermore, we identified that FLT1 signaling regulates a set of downstream inflammatory response genes including Colony Stimulating Factor 1 (CSF1) a central regulator of macrophage biology. Using a genetic gain-of-function approach we show that CSF1 mediated autocrine signaling in MAMs is downstream of FLT1 and can restore the tumor-promoting activity in MAMs even when FLT1 is inhibited. Together, our data indicates CCL3/CCR1 and FLT1 signalling pathways are critical in regulating MAMs and promoting breast cancer distal metastasis, and suggests the therapeutic potential of targeting these pathways in treating metastatic disease.