Study: Metastasis is affected by wound healing and inflammation in study on mice

IN-DEPTH REVIEW OF RESEARCH

Study background:

Among breast cancer patients, an elevated risk of metastasis occurs 6-12 months after surgery to remove primary tumors. Cancerous cells that grow into a metastasized tumor could have:

1. broken off from the primary tumor and migrated at the time of surgery.
2. migrated prior to surgery and were quiescent (inactive) until triggered later to grow.

Some studies estimate that up to one-third of patients with localized breast cancer may have potential cancer cells that exist in other sites without growing but have the capacity to metastasize later. Metastatic cells may be dormant because of physiological cues that limit cell replication or limit blood supply. A current hypothesis—an idea examined by the authors of this study—is that the immune system may restrict these cells. Their experimental design involved leaving tumors in place and testing the impact of wound healing at other sites. Because these experiments would be unethical in humans, they used a mouse model system for their experiments.

Researchers of this study wanted to know:

Do wound-healing responses associated with surgery contribute to cancer metastasis?

Study findings:  

This study had two major findings about the spread of cancer cells in mice:

1. Wounding without tumor removal was sufficient to allow growth of cancer cells at distant sites.
2. Treatment with the anti-inflammatory medication reduced growth of distant cancer cells.

Study design

Surgery and metastasis

Jordan Krall and colleagues from the Whitehead Institute and other Boston institutions used a strain of mice that were treated with mouse mammary cancer cells to look at the frequency and growth of cells when the rodents were wounded. The cancer cells were not surgically removed but left in place to see how frequently they grew into larger tumors. Tumors at distant sites developed more frequently in mice that were wounded than in those that were unwounded.

Researchers first created a model system of metastasis in mice. After injecting mouse mammary cancer cells into mice, they observed that the cancer cells first grew into small tumors, but over time these cells were rejected and fewer tumors were seen.  This mimics dormancy of cancer cells that has been hypothesized in humans. The growth and rejection of tumor cells did not occur in mice that had faulty immune systems; instead these immunocompromised mice had more frequent tumors. Researchers hypothesized that the immune systems of the mice responded to the cancer cells and made them dormant.

Researchers then tested if surgical wounding could alter dormancy of cancer cells in these mice. They surgically wounded mice, and one week later, as before, they injected the same mice with mammary cancer cells. They then observed that tumors grew 6 times more often in the wounded mice than in the unwounded mice.

To confirm these results, the researchers used several standard wounding protocols and a second type of cancer cells—melanoma cells—and saw similar results. They also reversed the order of the experiments, injecting mice with cancer cells before wounding them (or not) 1 week later. In these experiments, they observed tumors more frequently in the wounded mice than in the unwounded mice.

Immune system involvement

The researchers then tried to identify which aspect of the immune response correlated with initial dormancy of cancer cells and which affected the release from dormancy during wound healing. In this mouse model system, tumor growth was correlated with fewer T-cells and an increase in circulating leukocytes and inflammatory monocytes (types of white blood cells). These changes in the immune system suggested to researchers that a systemic inflammatory response might be responsible for the increased growth of cancer cells. A similar sort of immune response has been observed in human breast cancer patients after surgery.

Affect of anti-inflammatory drug

The researchers then tested whether standard treatment to reduce systemic inflammatory responses reduced tumor growth. They treated some mice with meloxicam, a non-steroidal anti-inflammatory drug, before and/or 3 days after surgical wounding. One week later, researchers then injected these mice with mammary cancer cells and evaluated tumor growth. Mice that were wounded and treated with the NSAID had smaller tumors than mice that were wounded but were not given the NSAID. Unwounded mice treated with the NSAID had similar tumor growth as mice that did not get the NSAID, indicating that the NSAID itself doesn't reduce tumor growth. Overall, researchers concluded that reducing inflammation also reduces tumor burden in this mouse model system. They suggest that this data supports the idea that inflammation triggered by surgery may be responsible for triggering the outgrowth of distant cancerous cells, and that reducing inflammation responses may reduce the potential for outgrowth into metastases.

Limitations:

Although the experiments in this study were well-controlled and thorough, they were done in mice, which may respond differently than humans. The researchers noted a limitation of their mouse model system: the tumors were not a true metastasis from a primary tumor in the animal; injected mammary cancer cells were used as a proxy for true metastases.

Researchers also acknowledged that they were unable to evaluate cancer metastases at some sites in the mice, namely bone marrow, lung or brain, due to technical limitations. Metastases originating in these locations may respond differently. Additionally, they were unable to distinguish the immune effects on the cancer cells themselves, versus impact on angiogenesis (development of new blood supply) or microenvironment at distant sites.

Further studies in human patients are needed to determine if these results apply to people. Several clinical trials are now looking at the impact of NSAIDs on breast tumor metastasis in humans. The REACT trial in Europe, which is examining the impact of the NSAID Celecoxib on tumor growth and survival, has completed enrollment and expects outcomes to be reported in 2022. Another clinical trial in Korea is examining the impact of ketorolac and ibuprofen on metastasis, compared to no treatment or acetominophen.

Conclusions:

This research suggests that postoperative wound healing in mice may alter the immune system's inflammatory response. This may tip the balance between immune system components that may have previously kept cancer cells dormant and those of the inflammatory response that may trigger metastasis growth. By experimenting with mice, researchers could tease apart impacts of surgery by not removing tumors and separately causing a wound, a set of experiments that are ethically impossible in human patients. The observation that a NSAID given to mice may reduce the size or frequency of tumor outgrowth is interesting, but whether or not wound healing affects metastases similarly in humans remains undetermined. NSAIDs are widely used in the clinical setting; however, while this study’s findings are promising, until clinical trials in humans are completed and evaluated, it is unclear if this will be a useful approach.

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Posted 5/17/18