Generation of myeloid-derived lymphatic endothelial cell progenitors (M-LECPs) by TLR4-mediated inflammation and de novo VEGFR-3 signaling in breast cancer

Breast cancer is the second leading cause of cancer-related death in women in the United States. Complications that lead to mortality of cancer patients are associated with tumor metastasis. Specifically, lymphatic metastasis in breast cancer patients strongly correlates with poor patient survival and this process is facilitated by the formation of new tumor lymphatic vessels termed lymphangiogenesis. Previously, our lab reported that lymphangiogenesis was promoted by a distinct subset of bone marrow (BM)-derived myeloid cells that co-express lymphatic-specific markers designated as myeloid-derived endothelial cell progenitors (M-LECPs). Furthermore, our lab has generated M-LECP in vitro from a mouse macrophage cell line (RAW264.7) by LPS stimulation. Taken together, these data suggest that chronically inflamed sites drive M-LECP differentiation and that these cells can contribute to the formation of new lymphatic vessels and promote lymph node metastasis. Evidence supporting this hypothesis was indicated by high levels of circulating M-LECP in peripheral blood of breast cancer patients but undetectable levels in healthy donors, cancer-free donors. Additionally, the generation of M-LECP was prompted through TLR4-signaling pathway, and de novo expression of VEGFR-3 and VEGF-C. This co-expression produces an autocrine loop essential for pro-lymphatic reprogramming in both primary human monocytes and the immature monocytic cell line, THP-1. Taken together, these data indicate the major regulatory role of TLR4 in inflammation-driven lymphangiogenesis involves the recruitment and differentiation of M-LECP, a process that may promote lymphatic metastasis.