To migrate into the lungs, cancer cells in the bloodstream must cross the lung's endothelial-cell barrier. A study shows that cancer cells can achieve this feat by signalling to induce endothelial-cell death. See Letter p.215
Cancer cells often migrate from where the cancer initially formed, to colonize other parts of the body in a process called metastasis, which is associated with poor clinical prognosis. On page 215, Strilic et al.1 uncover a surprising mechanism that migrating cancer cells in the bloodstream use to cross the lung's barrier of endothelial cells. The authors show that cancer cells send a signal that makes endothelial cells undergo a type of cell-death program called necroptosis (also known as programmed necrosis). Once in the lung, the cancer cells form lethal metastatic colonies.
The past three decades have provided increasing evidence that supports the key roles of endothelial cells in the formation and progression of tumours to a malignant state that has a poor prognosis for the patient2,3,4. Tumour cells rely heavily on the endothelial cells of blood vessels to enable continued tumour growth, because tumours need blood vessels to obtain oxygen and nutrients and expel metabolic waste.
Tumour cells exploit and manipulate endothelial cells by using intricate signalling mechanisms, such as those involving protein factors, secreted by tumours, that attract and remodel endothelial cells. Remodelling of blood vessels by tumour-derived proteins can enable cancer cells that reside in the primary site of tumour growth to enter the blood circulation, providing an escape route for the cells to reach distant organs5,6. After entering the bloodstream, cancer cells must cross the endothelial barriers that prevent them from entering other organs (Fig. 1a). In certain tissues, such as the lung or brain, the interface between the tissue and the bloodstream is relatively impenetrable to tumour cells6.
Strilic and colleagues' work began with an observation made when tumour cells and endothelial cells were cultured together in vitro. The researchers noted that such co-culture leads to an increase in endothelial-cell death. However, rather than exhibiting the typical cell-shape changes and molecular features of apoptosis, the most common form of programmed cell death, the dying endothelial cells exhibited features associated with another cell-death program called necroptosis. For example, the dying cells exhibited compromised cell-membrane integrity, as monitored by dye uptake.
To confirm the observed cell-death mechanism, the authors inhibited proteins that mediate necroptosis7, and found that this inhibited tumour-induced endothelial-cell death, whereas perturbing apoptotic signalling did not. They found that this necroptotic cell-death program was activated in both human and mouse endothelial cells exposed to a wide variety of cancer cell lines. Moreover, intravenous injection of mouse melanoma skin-cancer or lung-cancer cells into mice caused lung endothelial cells to undergo necroptotic death.
What advantage does killing endothelial cells afford tumour cells? Strilic and colleagues carried out in vitro experiments in which they inhibited necroptosis and observed reduced tumour-cell migration across an endothelial-cell monolayer, leading the authors to propose that tumour-induced necroptosis enhanced tumour-cell migration across the endothelial barrier. The authors made similar findings in in vivo experiments using genetic inactivation of the RIPK3 kinase enzyme, a necroptosis regulator, in endothelial cells. Inactivation of RIPK3 prevented endothelial-cell death, and reduced the ability of cancer cells to cross the endothelial barrier and enter the lung. Metastastic tumour-colony formation was reduced upon genetic or pharmacological inhibition of endothelial-cell necroptosis, indicating that tumour-induced endothelial-cell killing exerted control over metastasis.
How does endothelial-cell death enhance tumour-cell migration across the endothelial barrier? Strilic and colleagues propose various mechanisms. Tumour cells could migrate through gaps left in the endothelial barrier by dead endothelial cells. Another possibility is that damage-associated molecular pattern molecules (DAMPs), such as ATP released from necroptotic endothelial cells, could act on neighbouring endothelial cells to open the endothelial barrier by enabling tumour-cell migration between neighbouring endothelial cells that are usually bound together to form an impermeable barrier, and/or these signals could act directly on tumour cells to enhance their migration across the barrier8.
How do tumour cells induce endothelial necroptosis? The authors used a combination of molecular, pharmacological and genetic approaches to show that amyloid precursor protein (APP) on the surface of tumour cells induces necroptotic cell death by interacting with death receptor 6 (DR6) on endothelial cells (Fig. 1b). Consistent with this, pharmacological inhibition of DR6 signalling — achieved by injecting mice with a 'decoy' version of the DR6 receptor — inhibited metastasis.
Strilic et al. provide compelling evidence to support the existence of intricate signalling interactions between migrating tumour cells in the bloodstream and the blood-vessel endothelium that promote tumour-cell metastatic migration into tissue and subsequent tissue colonization. These findings raise a series of intriguing issues. Only a small fraction of endothelial cells cultured in vitro with tumour cells are induced to undergo necroptotic death. Discovering the molecular determinant that governs which endothelial cells die is a key challenge. The authors reveal that only approximately 10% of endothelial cells express DR6 and are thus susceptible to APP-mediated cell death.
It will be important to understand the mechanisms that regulate which fraction of endothelial cells express DR6, and whether cancer cells can regulate the susceptibility of endothelial cells to necroptosis by modulating DR6 expression on the cells. Microscopy analysis of human tumours could be used to reveal whether an increased fraction of DR6-expressing endothelial cells is associated with the propensity for lung metastatic progression. Perhaps molecular signals from the endothelium to tumour cells regulate expression or cleavage of APP on tumour cells — thus having an effect on endothelial-cell necroptosis. Such endothelial-cell-derived signals have roles in epithelial-cell fate and function9.
In addition to the mechanisms proposed by the authors, another mechanism by which endothelial-cell necroptosis might enhance tumour migration into tissue could be mediated by ATP. Release of ATP from dying endothelial cells might promote the survival of tumour cells during their migration through the endothelial barrier into the tissue10 — a process that can cause traumatic tumour-cell deformation and death. Live-cell microscopy imaging of tumour- and endothelial-cell dynamics during this interaction11 may be an ideal means of determining which of the intriguing potential cellular mechanisms proposed by the authors might underlie tumour-cell migration across the endothelial barrier. Footnote 1
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Alarcón, C., Tavazoie, S. Endothelial-cell killing promotes metastasis. Nature 536, 154–155 (2016). https://doi.org/10.1038/nature19465
Nature Communications (2019)