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 promotes tumor cell extravasation, seeding and persistent growth. These macrophages were derived from circulating inflammatory monocytes recruited by CCL2/CCR2 chemokine signaling and directly promote tumor cell extravasation and metastatic seeding in vivo through VEGF production. Our recent studies identified that MAMs express high levels of cell surface FMS-like tyrosine kinase 1 (FLT1, also known as VEGFR1) after their recruitment. Blockade of FLT1 signaling using specific inhibitory antibodies significantly inhibited the metastatic seeding and persistent growth. Using several genetic models of Flt1 deficiency, we show that macrophage specific FLT1 signaling is critical for breast tumor distal metastatic potential. FLT1 is not expressed by other hematopoietic cells and its inhibition did not affect the recruitment of MAMs, which indicated that specific FLT1 signaling in MAMs are important for their metastasis promoting functions. Indeed, we identified that FLT1 regulates a set of 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 has been inhibited. Together, our data established a link between inflammation and cancer metastasis and suggested the therapeutic potential of targeting these pathways in treating metastatic disease.

Citation Format: Bin-Zhi Qian, Hui Zhang, Jiufeng Li, Eun-Jin Yeo, Neil O. Carragher, Anne R. Bresnick, Richard A. Lang, Jeffrey W. Pollard. Macrophage FLT1 mediated inflammatory response determines breast cancer distal metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr A13.