Neutrophil Extracellular Traps: when the immune system promotes metastatic dissemination of cancer cells.

During infections, neutrophils (a white blood cell type) release in the extracellular space the so-called neutrophil extracellular traps (NETs) to trap the microorganisms. NETs are made of DNA and proteins.

In cancer, NETs are modestly present in primary tumors, while are abundant in metastatic lesions, especially in liver –but also in lung, bone and brain– as well as in the blood of patients with liver metastases.

Main finding. NETs play a significant role in liver metastasis formation, as chemoattractant of metastatic cells through the CCDC25 protein expressed on the membrane of cancer cells; CCDC25 thus mediates NETs-induced promotion of cancer cell migration and metastatic dissemination.

Experimental details. In patients with breast and colon cancer, high levels of NETs deriving from neutrophils infiltrating the liver were detected in the liver before the appearance of liver metastases –likely facilitating liver metastasis development itself– and before the increase of NETs levels in the blood. In mouse models of breast cancer, the absence of the enzyme required for NET formation reduced liver metastatic lesions.

In vitro experiments showed that NETs promote breast cancer cell migration and adhesion through the interaction of NET DNA with CCDC25 protein (coil-coil domain containing protein 25), a protein expressed on the plasma membrane of cancer cells. So basically, CCDC25 works as a “sensor” used by cancer cells to “sense” NET presence. The role of CCDC25 in this “sensing” mechanism is crucial, as it affects NETs-induced cancer cell adhesion and migration: knock out of CCDC25 in breast cancer cells, indeed, abolishes cells’ adhesion, NETs-induced cytoskeleton remodelling of cancer cells, cancer cell movement (chemotaxis) towards NETs and, moderately, cancer cell proliferation.

Interestingly, CCDC25 was also involved in the enhanced migration and adhesion of cancer cells induced when cells are in contact with extracellular DNA (for instance DNA from apoptotic cells), especially when the extracellular DNA carried a specific modification, the guanosine oxidation, which is a typical hallmark of NET DNA.

In vivo experiments with mouse models demonstrated the role of CCDC25 in promoting NETs-mediated metastases: mouse transplantation of breast and colon cancer cells knocked out for CCDC25 resulted in reduced NET-induced lung and liver metastases. Similarly, antibodies specifically targeting CCDC25 inhibited NETs-elicited cancer cell migration, adhesion and cytoskeleton remodelling (in vitro) and reduced liver metastases (in vivo). On the other hand, enforcing CCDC25 expression promoted metastasis formation.

In patients, CCDC25 is expressed in several cancer types, highly expressed in breast and colon cancer, especially at the invasive front of tumors, and closely interacting with NETs in liver metastases of breast cancer patients. Notably, high levels of CCDC25 are associated with reduced patient long-term survival.

What is the signalling elicited by NET stimulation downstream of CCDC25? Upon NET stimulation, CCDC25 interacts with the ILK protein (integrin-linked kinase) which, in turn, recruits beta-Parvin protein. Beta-Parvin then activates RAC1 and CDC42, resulting into cytoskeleton remodelling and chemotaxis of cancer cells, ultimately promoting metastases. 

Conclusions. On one side, the authors showed that CCDC25 enable cancer cells to sense the extracellular DNA of the NETs. When cancer cells sense the NETs, metastasis formation is promoted, suggesting that CCDC25 may be a therapeutic target to prevent/halt metastatic dissemination. On the other side, the observed increased levels of NETs in blood may represent a biomarker to predict the risk of liver metastases in patients with early stage breast cancer.