The evidence for and against different modes of tumour cell extravasation in the lung: diapedesis, capillary destruction, necroptosis, and endothelialization

Current biological implications and clinical relevance of metastatic circulating tumor cells

Metastatic disease and cancer recurrence are the primary causes of cancer-related deaths. Circulating tumor cells (CTCs) and disseminated tumor cells (DTCs) are the driving forces behind the spread of cancer cells. The emergence and development of liquid biopsy using rare CTCs as a minimally invasive strategy for early-stage tumor detection and improved tumor management is a promising advancement in recent years. However, before blood sample analysis and clinical translation, precise isolation of CTCs from patients' blood based on their biophysical properties, followed by molecular identification of CTCs using single-cell multi-omics technologies is necessary to understand tumor heterogeneity and provide effective diagnosis and monitoring of cancer progression. Additionally, understanding the origin, morphological variation, and interaction between CTCs and the primary and metastatic tumor niche, as well as and regulatory immune cells, will offer new insights into the development of CTC-based advanced tumor targeting in the future clinical trials.

Extravasation of immune and tumor cells from an endothelial perspective

Crossing the vascular endothelium is a necessary stage for circulating cells aiming to reach distant organs. Leukocyte passage through the endothelium, known as transmigration, is a multistep process during which immune cells adhere to the vascular wall, migrate and crawl along the endothelium until they reach their exit site. Similarly, circulating tumor cells (CTCs), which originate from the primary tumor or reseed from early metastatic sites, disseminate using the blood circulation and also must cross the endothelial barrier to set new colonies in distant organs. CTCs are thought to mimic arrest and extravasation utilized by leukocytes; however, their extravasation also requires processes that, from an endothelial perspective, are specific to cancer cells. Although leukocyte extravasation relies on maintaining endothelial impermeability, it appears that cancer cells can indoctrinate endothelial cells into promoting their extravasation independently of their normal functions. In this Review, we summarize the common and divergent mechanisms of endothelial responses during extravasation of leukocytes (in inflammation) and CTCs (in metastasis), and highlight how these might be leveraged in the development of anti-metastatic treatments.

New Dawn for Atherosclerosis: Vascular Endothelial Cell Senescence and Death

Endothelial cells (ECs) form the inner linings of blood vessels, and are directly exposed to endogenous hazard signals and metabolites in the circulatory system. The senescence and death of ECs are not only adverse outcomes, but also causal contributors to endothelial dysfunction, an early risk marker of atherosclerosis. The pathophysiological process of EC senescence involves both structural and functional changes and has been linked to various factors, including oxidative stress, dysregulated cell cycle, hyperuricemia, vascular inflammation, and aberrant metabolite sensing and signaling. Multiple forms of EC death have been documented in atherosclerosis, including autophagic cell death, apoptosis, pyroptosis, NETosis, necroptosis, and ferroptosis. Despite this, the molecular mechanisms underlying EC senescence or death in atherogenesis are not fully understood. To provide a comprehensive update on the subject, this review examines the historic and latest findings on the molecular mechanisms and functional alterations associated with EC senescence and death in different stages of atherosclerosis.

Visualizing cancer extravasation: from mechanistic studies to drug development

Metastasis is a multistep process that accounts for the majority of cancer-related death. By the end of metastasize dissemination, circulating tumor cells (CTC) need to extravasate the blood vessels at metastatic sites to form new colonization. Although cancer cell extravasation is a crucial step in cancer metastasis, it has not been successfully targeted by current anti-metastasis strategies due to the lack of a thorough understanding of the molecular mechanisms that regulate this process. This review focuses on recent progress in cancer extravasation visualization techniques, including the development of both in vitro and in vivo cancer extravasation models, that shed light on the underlying mechanisms. Specifically, multiple cancer extravasation stages, such as the adhesion to the endothelium and transendothelial migration, are successfully probed using these technologies. Moreover, the roles of different cell adhesive molecules, chemokines, and growth factors, as well as the mechanical factors in these stages are well illustrated. Deeper understandings of cancer extravasation mechanisms offer us new opportunities to escalate the discovery of anti-extravasation drugs and therapies and improve the prognosis of cancer patients.

Response of the neurovascular unit to brain metastatic breast cancer cells

Therapeutic resistance of cerebral secondary tumours largely depends on unique aspects linked to the neurovascular unit, especially cerebral endothelial cells and astrocytes. By using advanced microscopy techniques, here we explored novel mechanisms related to the neurovascular unit during extravasation and proliferation of triple negative breast cancer cells in the brain. Metastatic mammary carcinoma cells arrested and elongated within one hour in cerebral microvessels, but their number decreased by almost 80% in the first two days. Interestingly, malignant cells induced vasoconstriction and development of intraluminal endothelial plugs, which isolated invading cells from the circulation. During diapedesis - which usually took place on day four and five after inoculation of the tumour cells - continuity of cerebral endothelial tight junctions remained intact, indicating migration of cancer cells through the transcellular pathway. In addition, metastatic cells induced formation of multiluminal vessels and claudin-5-positive endothelial blebs. However, even severe endothelial blebbing could be reversed and the vessel morphology was restored shortly after the tumour cells completed transendothelial migration. Similar to neuro-inflammatory leukocytes, tumour cells migrated not only through the endothelial layer, but through the glia limitans perivascularis as well. Nevertheless, along with the growth of metastatic lesions by co-option of pre-existing capillaries, astrocytes and astrocyte end-feet were gradually expelled from the vessels to the border of the tumour. Taken together, we identified previously unknown mechanisms involved in the reaction of brain resident cells to invading breast cancer cells. Our results contribute to a better understanding of the complex cross-talk between tumour cells and host cells in the brain, which is essential for the identification of new therapeutic targets in this devastating disease.

Necroptotic cell death in anti-cancer therapy

Necroptosis is one the best-characterized forms of regulated necrosis. Necroptosis is mediated by the kinase activities of receptor interacting protein kinase-1 and receptor interacting protein kinase-3, which eventually lead to the activation of mixed lineage kinase domain-like. Necroptosis is characterized by rapid permeabilization of the plasma membrane, which is associated with the release of the cell content and subsequent exposure of damage-associated molecular patterns (DAMPs) and cytokines/chemokines. This release underlies the immunogenic nature of necroptotic cancer cells and their ability to induce efficient anti-tumor immunity. Triggering necroptosis has become especially important in experimental cancer treatments as an alternative to triggering apoptosis because one of the hallmarks of cancer is the blockade or evasion of apoptosis. In this review, we discuss recent advances in necroptosis research and the functional consequences of necroptotic cancer cell death, with focus on its immunogenicity and its role in the activation of anti-tumor immunity. Next, we discuss the molecular mechanisms of phosphatidylserine exposure during necroptosis and its role in the recognition of necroptotic cells. We also highlight the complex role of the necroptotic pathway in tumor promotion and suppression and in metastasis. Future studies will show whether necroptosis is truly a better strategy to overcome apoptosis resistance and provide the insights needed for development of novel treatment strategies for cancer.

Hostile takeover: how tumours hijack pre-existing vascular environments to thrive

An increasing body of evidence suggests that solid tumours do not require the generation of new blood vessels, i.e. angiogenesis, to successfully grow, and to colonize normal tissue. Instead, many tumour cells make the best use of what they find: pre-existing blood vessels of the host. In these cases, the host vasculature is incorporated by the growing tumour, resulting in a new organ consisting of malignant and non-malignant cell types. In consequence, pre-existing vessels are exploited by the tumour for optimal access to oxygen and nutrients. In this perspective article, the argument is made that tumour cells might gain even more: that is, access to the very special microenvironment of the perivascular niche. Here, specific cues for invasion, metastasis, survival, stem-like features, dormancy and, potentially, also immune escape exist - for non-malignant and malignant cells alike. The consequence of the hijacking of normal blood vessels and their perivascular niches by tumours is that antiangiogenic agents have little chance to work, and that tumour cells are better protected from the adverse effects of cytotoxic and targeted therapies. Thus, disturbing vascular hijacking could make tumours less resistant to established therapies. Concepts of how to do this are just starting to be explored. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.