Neutrophil extracellular trap (NET) formation, first described in 2004 as a previously unknown strategy of neutrophils to fight microbes, has attracted an increasing interest in the research community. NETs are formed when neutrophils externalize their decondensed chromatin together with content from their azurophilic granules. In addition to their role in defense against microbes, NETs have been implicated as mediators of pathology in sterile inflammation, such as cancer and autoimmunity, and their potential as therapeutic targets is actively explored. However, targeting of NETs is challenging since the beneficial effects of their removal need to be balanced against the potential harmful loss of their function in microbial defense. Moreover, depending on the stimuli or species, NETs can be formed via distinct mechanisms and are not always made up of the same components, making direct comparisons between various studies challenging. This review focuses on the role of NETs in cancer-associated pathology, such as thrombosis, organ dysfunction, and metastasis. Different strategies to target NETs, by either preventing their formation or degrading existing ones, are also discussed.

Cancer produces a variety of collateral effects in patients beyond the malignancy itself, including threats to distal organ functions. However, the basis for such effects, associated with either primary or metastatic tumors, are generally poorly understood. In this study, we show how heart and kidney vascular function is impaired by neutrophils that accumulate in those tissues as a result of tumor formation in two different transgenic mouse models of cancer (RIP1-Tag2 model of insulinoma and MMTV-PyMT model of breast cancer). Neutrophil depletion by systemic administration of an anti-Gr1 antibody improved vascular perfusion and prevented vascular leakage in kidney vessels. We also observed the accumulation of platelet–neutrophil complexes, a signature of neutrophil extracellular traps (NET), in the kidneys of tumor-bearing mice that were completely absent from healthy nontumor-bearing littermates. NET accumulation in the vasculature was associated with upregulation of the proinflammatory adhesion molecules ICAM-1, VCAM-1, and E-selectin, as well as the proinflammatory cytokines IL1β, IL6, and the chemokine CXCL1. Administering DNase I to dissolve NETs, which have a high DNA content, restored perfusion in the kidney and heart to levels seen in nontumor-bearing mice, and also prevented vessel leakage in the blood vasculature of these organs. Taken together, our findings strongly suggest that NETs mediate the negative collateral effects of tumors on distal organs, acting to impair vascular function, and to heighten inflammation at these sites. Cancer Res; 75(13); 2653–62. ©2015 AACR.

A large proportion of cancer-related deaths are caused by thrombosis and general organ failure. One example is acute renal failure, a major cause of morbidity and mortality in cancer patients. Surprisingly, however, little is known about the situation in organs that are not targets for metastasis or affected by the primary tumor. Recently, neutrophil extracellular traps (NET) were implicated in tumor-induced effects on distant organs unaffected by the actual tumor cells. Formation of NETs (NETosis) was identified a decade ago as a mechanism by which the innate immune system protects us from infections, especially in situations with sepsis. NETs are formed when neutrophils externalize their nuclear DNA together with antimicrobial granule proteins and form a web-like structure that can trap and kill microbes. It is now becoming increasingly clear that NETs also form under noninfectious inflammatory conditions like cancer, thrombosis, autoimmunity, and diabetes and significantly contribute to disease development. The existence of NET-dissolving drugs like heparin and DNase I, already in clinical use, and recent development of specific inhibitors of protein-arginine deiminase 4 (PAD4), an enzyme required for NET formation, should enable clinical targeting of NETosis. Preventing NETosis in cancer could provide a strategy to counteract tumor-induced thrombosis and organ failure as well as to suppress metastasis. Cancer Res; 1–5. ©2016 AACR.

Platelet-derived growth factor B (PDGFB) plays a crucial role in recruitment of PDGF receptor β–positive pericytes to blood vessels. The endothelium is an essential source of PDGFB in this process. Platelets constitute a major reservoir of PDGFB and are continuously activated in the tumor microenvironment, exposing tumors to the plethora of growth factors contained in platelet granules. Here, we show that tumor vascular function, as well as pericyte coverage is significantly impaired in mice with conditional knockout of PDGFB in platelets. A lack of PDGFB in platelets led to enhanced hypoxia and epithelial-to-mesenchymal transition in the primary tumors, elevated levels of circulating tumor cells, and increased spontaneous metastasis to the liver or lungs in two mouse models. These findings establish a previously unknown role for platelet-derived PDGFB, whereby it promotes and maintains vascular integrity in the tumor microenvironment by contributing to the recruitment of pericytes. Significance: Conditional knockout of PDGFB in platelets demonstrates its previously unknown role in the maintenance of tumor vascular integrity and host protection against metastasis.

Renal insufficiency is a frequent cancer-associated problem affecting more than half of all cancer patients at the time of diagnosis. To minimize nephrotoxic effects the dosage of anticancer drugs are reduced in these patients, leading to sub-optimal treatment efficacy. Despite the severity of this cancer-associated pathology, the molecular mechanisms, as well as therapeutic options, are still largely lacking. We here show that formation of intravascular tumor-induced neutrophil extracellular traps (NETs) is a cause of kidney injury in tumor-bearing mice. Analysis of clinical biomarkers for kidney function revealed impaired creatinine clearance and elevated total protein levels in urine from tumor-bearing mice. Electron microscopy analysis of the kidneys from mice with cancer showed reversible pathological signs such as mesangial hypercellularity, while permanent damage such as fibrosis or necrosis was not observed. Removal of NETs by treatment with DNase I, or pharmacological inhibition of the enzyme peptidylarginine deiminase 4 (PAD4), was sufficient to restore renal function in mice with cancer. Tumor-induced systemic inflammation and impaired perfusion of peripheral vessels could be reverted by the PAD4 inhibitor. In conclusion, the current study identifies NETosis as a previously unknown cause of cancer-associated renal dysfunction and describes a novel promising approach to prevent renal failure in individuals with cancer.