Live imaging of breast tumors shows macrophage-dependent induction and TMEM-mediated enrichment of cancer stem cells during metastatic dissemination

Abstract

Cancer stem cells (CSCs) play an important role during metastasis, but the dynamic behavior and induction mechanisms of CSCs are not well understood. We employed high-resolution intravital microscopy using a CSC biosensor to directly observe CSCs in live mice with mammary tumors. CSCs display the slow-migratory, invadopod-rich phenotype that is the hallmark of disseminating tumor cells. CSCs are enriched near macrophages, particularly near macrophage-containing intravasation sites called Tumor Microenvironment of Metastasis (TMEM) doorways. A dramatic enrichment of CSCs occurs on association with TMEM doorways, contributing to the finding that CSCs represent ∼>60% of circulating tumor cells. Mechanistically, stemness is induced in non-stem cancer cells upon their direct contact with macrophages via Notch signaling. In breast cancers from patients, the density of TMEM doorways correlates strongly with the proportion of cancer cells expressing stem cell markers, indicating that in human breast cancer TMEM doorways are not only cancer cell intravasation portals but also CSC programming sites.One Sentence SummaryIntravital imaging reveals macrophage-mediated induction of cancer stem cells in vivo and their dramatic enrichment on dissemination through TMEM doorways.

Macrophages Promote Tumor Cell Extravasation across an Endothelial Barrier through Thin Membranous Connections

“…The importance of macrophages in promoting metastatic dissemination and growth in primary as well as secondary breast cancers is well-known (3,5,6). In this work, we focused our attention on the role of macrophages in tumor cell extravasation and, in particular on the physical interaction between macrophages and tumor cells that reside on opposing sides of the lung vasculature.…”

“…Metastasis results from a sequence of events that starts in the primary tumor and allows tumor cells to intravasate into the blood stream and use the circulatory system to reach distant organs (2)(3)(4). Viable circulating tumor cells (CTCs) that extravasate into the parenchyma of distant organs can then either colonize the organ (leading to overt metastases) or survive in a dormant state while still retaining the ability to produce metastases at a later time, depending on the microenvironment (5,6).…”

“…These macrophages mediate intravasation by creating a local transient opening in the blood vessel (12). Perivascular macrophages are also responsible for inducing tumor stemness and dormancy that prepares the tumor cells to colonize and survive at distant organs (13) (5). Other data indicating the importance of macrophages in metastases is the requirement of colonystimulating factor 1 (CSF-1) for metastatic dissemination, an essential factor for macrophage survival and proliferation (14,15).…”