Most cancer patients die from cancer that recurs after spreading to a different tissue, rather than from their original tumor. After successful treatment of the original tumor, cancer cells that have disseminated to other sites can undergo dormancy, remaining viable but not proliferating. In breast, prostate, and other cancers, cancer cells can remain dormant and clinically undetectable for years and even decades before recurring, or awakening, as metastatic cancer. Little is known about what might initiate cancer awakening, and this in turn reduces our opportunities to prevent metastasis.

Epidemiological studies have suggested that inflammation is linked to a higher risk of breast cancer recurrence after a period of clinical dormancy. Smoking, which causes chronic lung inflammation, is also associated with a higher risk of recurrence. However, whether inflammation can cause awakening is not clear. Inflammatory cells, such as neutrophils, can provide many different signals that promote cancer progression. Neutrophils can kill harmful microorganisms by the release of neutrophil extracellular traps (NETs) into the extracellular space. NETs are scaffolds of DNA with associated cytotoxic proteins and proteases [e.g., neutrophil elastase (NE) and matrix metalloproteinase 9 (MMP9)]. NETs induced by bacteria or by cancer cells can promote metastasis, but the mechanism by which this occurs is not known. In this study, we tested whether NETs formed during lung inflammation could induce awakening.

We found that sustained experimental lung inflammation—induced by either tobacco smoke exposure or nasal instillation of lipopolysaccharide (LPS)—converted dormant cancer cells to aggressive lung metastases in mice. Both types of sustained inflammation also caused the formation of NETs. Inhibiting NET formation or digesting the NETs’ DNA scaffold prevented conversion of single disseminated cancer cells to growing metastases in mouse models of breast and prostate cancer. The NET DNA bound to the extracellular matrix (ECM) protein laminin, thus bringing two NET-associated proteases, NE and MMP9, to their substrate. This in turn facilitated a sequential cleavage of laminin, first by NE and then by MMP9. The NET-mediated proteolytic remodeling of laminin revealed an epitope that triggered proliferation of dormant cancer cells through integrin activation and FAK/ERK/MLCK/YAP signaling. We generated a blocking antibody against NET-remodeled laminin, and this antibody prevented or reduced tobacco smoke exposure– or LPS-induced inflammation from awakening dormant cancer cells in mice.

Our data implicate NETs and NET-mediated ECM remodeling as critical mediators of inflammation-induced awakening in mouse models of dormancy. We propose that NETs awaken cancer by concentrating neutrophil proteases at the ECM protein laminin, allowing for sequential proteolytic remodeling of laminin and leading to integrin-mediated signaling in the cancer cells. Our findings set the stage for epidemiological studies to test possible links among inflammation or smoking, NETs, and recurrence after dormancy in human patients. If such links can be established, we envision that approaches similar to the ones used in mouse models in our study could be used to target NETs and their downstream effectors to reduce the risk of cancer recurrence in human patients.

Lung inflammation—caused, for instance, by tobacco smoke exposure—leads to NET formation. Two proteases on the NETs sequentially cleave the extracellular …