Real-Time Imaging Reveals Local, Transient Vascular Permeability, and Tumor Cell Intravasation Stimulated by TIE2hi Macrophage–Derived VEGFA

Abstract

Dissemination of tumor cells is an essential step in metastasis. Direct contact between a macrophage, Mena over-expressing tumor cell and endothelial cell [Tumor MicroEnvironment of Metastasis (TMEM)], correlates with metastasis in breast cancer patients. Here we show, using intravital high-resolution two-photon microscopy, that transient vascular permeability and tumor cell intravasation occur simultaneously and exclusively at TMEM. The hyperpermeable nature of tumor vasculature is described as spatially and temporally heterogeneous. Using real-time imaging we observed that vascular permeability is transient, restricted to TMEM, and required for tumor cell dissemination. VEGFA signaling from Tie2Hi TMEM macrophages causes local loss of vascular junctions, transient vascular permeability and tumor cell intravasation, demonstrating a role for TMEM within the primary mammary tumor. These data provide insight into the mechanism of tumor cell intravasation and vascular permeability in breast cancer, explaining the value of TMEM density as a predictor of distant metastatic recurrence in patients.

Cancer Metastasis: Perivascular Macrophages Under Watch

“…Similar to macrophage-specifi c Vegfa inactivation, the physical elimination of the tumor macrophages decreased the number of circulating cancer cells in the mice ( 5 ). This fi nding is in line with previous observations and supports the notion that macrophages facilitate the initial steps of tumor metastasis, at least in mouse models of breast cancer ( 1 ( 1 ).…”

“…For example, perivascular macrophages that express the angiopoietin receptor TIE2 are proangiogenic ( 2,3 ) and molecularly distinct from other tumor macrophage subpopulations ( 4 ). In this issue of Cancer Discovery , Harney and colleagues ( 5 ) use intravital microscopy to analyze vascular permeability at sites where TIE2-expressing macrophages (TEM) make contacts with the endothelial cells (EC) that line the tumor blood vessels. They found higher vascular permeability at these sites, compared with vessel segments lacking macrophages.…”

“…An important result of the study ( 5 ) was that the peak of vascular permeability at the TMEM site was both spatially and temporally associated with the attraction of neighboring cancer cells, some of which could enter the bloodstream and leave the tumor. Diffusion of plasma proteins from leaky vessels is known to attract cancer cells and enhance their motility ( 1 ).…”

“…Diffusion of plasma proteins from leaky vessels is known to attract cancer cells and enhance their motility ( 1 ). Furthermore, the authors noted that the molecular junctions that normally seal adjacent ECs to limit vascular leakiness were more frequently disrupted at the hyperpermeable TMEM sites ( 5 ). Therefore, the cancer cells could more easily fi nd their way to the systemic circulation through these temporarily opened gates.…”

The Role of EphB4 and Ephrin-B2 Interactions in Prostate Cancer Models of Intravasation and Extravasation

“…Using this method, Harney et al (2015) showed that intravasation can occur simultaneously when a macrophage, tumour cell and an endothelial cell are in direct contact, because the VEGFA signalling from macrophages induces local, transient vascular permeability and subsequent tumour cell intravasation [63].…”