Neutrophils Pave the Way for Chronic Stress-Induced Cancer Metastasis

Chronic stress has been linked to metastasis across different types of cancers, yet mechanistic details have been lacking to explain this link (1). A recent study published in Cancer Cell provides molecular insights on how chronic stress promotes metastasis (2). The study found that stress hormones such as glucocorticoids (GCs) triggered neutrophil extracellular trap (NET) formation, structures composed of extracellular DNA commonly used to trap invading microorganisms. Importantly, NETs are also known to have pro-metastatic roles (3), and the work published in Cancer Cell demonstrates that targeting NETs in chronically stressed mice reduced breast cancer metastases.

The study was led by Xue-Yan He, PhD, then a postdoctoral fellow in the laboratory of Mikala Egeblad, PhD, at Cold Spring Harbor Laboratory. Dr. He was awarded the 2021 AACR-AstraZeneca Breast Cancer Research Fellowship for her project, “Targeting stress-induced neutrophil extracellular traps (NETs) prevents metastasis.” While both chronic stress and NETs had separately been linked to metastasis, it remained a mystery whether the two worked in conjunction at the molecular level to promote metastasis. Dr. He’s publication is the first demonstration of stress-induced NET formation and reveals a pathway by which chronic stress mediates metastasis. Moreover, while stress management has many benefits for healthy individuals, these findings provide evidence that stress reduction may improve cancer treatment outcomes, such as by reducing the risk of metastasis.

For their research, Dr. He and colleagues utilized mice that developed primary mammary tumors. After primary tumor resection, mice were exposed to chronic restraint stress to model an environment of predictable, constant stress. This experimental setup allowed the research team to study how stress affected the outgrowth of disseminated cancer cells (DCCs) originating from the primary tumor. The researchers found that chronic stress exposure elevated plasma GC levels and was accompanied by increased metastases to lung tissue. Knockout of the glucocorticoid receptor (GR) specifically in cancer cells did not reduce metastases to the lung, signifying that stress did not increase the cancer cells’ metastatic abilities.

Instead, multiple lines of evidence showed that chronic stress exposure induced a pro-metastatic microenvironment in the lungs. Bulk RNA sequencing (RNA-seq) of lung tissue from stressed mice and subsequent gene ontology analysis indicated that stress upregulated extracellular matrix (ECM) organization and downregulated the adaptive immune response, among other affected pathways. Subsequently, the researchers found that stressed mice had increased deposition of fibronectin in the lungs, an ECM protein known to promote metastasis (3). Furthermore, the lungs of stressed mice also had increased neutrophil infiltration, a feature of pro-metastatic niches (4). Experiments with stress exposed, neutrophil-depleted mice confirmed the critical role of neutrophils in mediating stress-induced metastasis.

After uncovering the central role of neutrophils in stress-induced metastasis, the researchers aimed to identify the biological changes in neutrophils driving this process. Gene expression analysis identified an upregulation of oxidative genes, leading the researchers to investigate NET levels as oxidative genes are known to regulate NET formation. Ex vivo and in vivo studies showed that GCs released during stress, and consequent GR activation in neutrophils, induced NET formation. Further studies revealed that targeting NETs directly with DNase I, a NET-digesting enzyme, strikingly prevented many of the stress-related effects in vivo: fibronectin deposition, cell proliferation, and lung metastases all decreased in stressed mice treated with DNase I.

To test whether these findings could be linked to real-world patient data, the researchers utilized their RNA-seq data to create a chronic stress exposure gene signature and compared it to a database of tumor gene expression from breast cancer patients. It was found that patient tumors with this gene expression signature had reduced overall survival, especially for estrogen and progesterone positive breast cancers.

Thinking about her results on a larger scale, Dr. He shared that “our findings suggest that stress-induced changes in the host, such as immune dysregulation, may contribute to metastasis promotion. Understanding the impact of stress on cancer progression is crucial for identifying patients at risk of recurrence and developing strategies to prevent metastasis. By reducing stress for cancer patients, we may potentially improve cancer treatment outcomes.”

Dr. He acknowledged the support the AACR fellowship provided in propelling her career, noting that “as an international postdoctoral fellow, I was ineligible to apply for most government-funded grants. However, the AACR grant provided an opportunity for international postdoctoral fellows like me. During the final funded year, I actively sought an independent investigator position. This AACR fellowship played a critical role in establishing my recognition as an early-stage scientist and has been instrumental in building my career.” After completing her postdoctoral fellowship, Dr. He accepted an Assistant Professor position at Washington University School of Medicine in St. Louis, where she runs an independent research group focused on understanding how chronic stress impacts tumor progression.

References

1. Dai S, Mo Y, Wang Y, Xiang B, Liao, Q, Zhou M, et al. Chronic stress promotes cancer development. Front. Oncol. 2020;10:1492. doi: 10.3389/fonc.2020.01492.
2. He X-Y, Gao Y, Ng D, Michalopoulou E, George S, Adrover JM, et al. Chronic stress increases metastasis via neutrophil-mediated changes to the microenvironment. Cancer Cell. 2024;42:474-486. doi: 10.1016/j.ccell.2024.01.013.
3. Chen Y, Hu H, Tan S, Dong Q, Fan X, Wang Y, et al. The role of neutrophil extracellular traps in cancer progression, metastasis and therapy. 2022;11:99. Exp. Hematol. Oncol. doi: 10.1186/s40164-022-00345-3.
4. Erler JT, Bennewith KL, Cox TR, Lang G, Bird D, Koong A, et al. Hypoxia-induced lysyl oxidase is a critical mediator of bone marrow cell recruitment to form the premetastatic niche. Cancer Cell. 2009;15:35-44. doi: 10.1016/j.ccr.2008.11.012.
5. Wculek SK, Malanchi I. Neutrophils support lung colonization of metastasis-initiating breast cancer cells. Nature. 2015;528:413-417. doi: 10.1038/nature16140.