Abstract
Cancer stem cells (CSCs) play an important role during metastasis, but the dynamic behavior and induction mechanisms of CSCs are not well understood. Here, we employ 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. Substantial 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-Jagged signaling. In breast cancers from patients, the density of TMEM doorways correlates 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.
Highlights
Cancer stem cells (CSCs) play an important role during metastasis, but the dynamic behavior and induction mechanisms of CSCs are not well understood
In vivo time-lapse imaging revealed that these CSCs demonstrate a slow-migratory, invasive invadopodium-rich phenotype, which is the hallmark of disseminating tumor cells[36]
Previous work has shown that the slow-migratory invadopodium-rich, invasive phenotype is present in the transendothelial migration competent cancer cells expressing high levels of MenaINV18,19,36
Summary
Cancer stem cells (CSCs) play an important role during metastasis, but the dynamic behavior and induction mechanisms of CSCs are not well understood. Stemness is induced in non-stem cancer cells upon their direct contact with macrophages via Notch-Jagged signaling. CSCs have an enhanced ability to self-renew, which makes them uniquely capable of initiating and sustaining primary and metastatic tumor growth[3] Consistent with this concept, a high proportion of CSCs in primary tumors is associated with poor prognosis and increased metastatic relapse[4]. Cell differentiation trajectories are largely unidirectional in normal development, it is clear that cellular hierarchies are much more plastic in the context of tissue injury and cancer, and that some microenvironmental signals can induce non-stem cancer cells to acquire a stem-like phenotype[10,11,12,13]. The complex interplay of so many potential contributors to the CSC niche makes in vivo observation important as a starting point for understanding and mechanistically dissecting
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