Macrophages are abundantly found in the tumor microenvironment enhance malignancy. At distal metastatic sites a distinct population of metastasis associated macrophages (MAMs) promotes tumor cell extravasation, seeding and persistent growth. Compared with lung resident macrophages, MAMs associated with breast cancer lung metastasis in pre-clinical models express high levels of cell surface FMS-like tyrosine kinase 1 (Flt1). Blockade of FLT1 signaling using specific inhibitory antibodies significantly inhibited the metastatic seeding and persistent growth of both human and murine breast tumor cell. This was confirmed using a genetic model of Flt1 tyrosine kinase domain (TK) ablation. Using bone marrow transplantation techniques, we further showed that this metastasis inhibitory effect was due to the lack of FLT1 signaling in hematopoietic cells. Furthermore, macrophage specific ablation of the tyrosine kinase domain of Flt1 by crossing Flt1 flox/flox mice with macrophage restricted Cre mice (Csf1r-Cre) also significantly inhibited tumor cell metastatic potential. Since Flt1 is not expressed by other hematopoietic cells and FLT1 inhibition did not affect the recruitment of MAMs, it indicated that specific FLT1 signaling in MAMs are important for their metastasis promoting functions. FLT1 regulated genes were identified by comparing gene expression profiles using microarray techniques of FACS sorted MAMs from mice treated with control and anti-FLT1 inhibitory antibody. Bioinformatics analysis showed that inflammatory response genes were highly enriched in these genes. Together, our studies indicated that FLT1 signaling is required for the activation of metastasis associated macrophages and regulates a set of inflammatory genes that promote breast tumor distal metastasis.

Citation Format: Bin-Zhi Qian, Richard A. Lang, Jeffrey W. Pollard. Macrophage FLT1 signaling activates an inflammatory response that promotes breast cancer distal metastasis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1531. doi:10.1158/1538-7445.AM2013-1531