Chemo helps breast cancer cells get their ‘foot in the door’ to the lungs
Study in mice finds drug makes blood vessel walls leaky and sticky
Editor's note: This story was amended on Nov. 16. See below for a message to patients from The Ohio State University Comprehensive Cancer Center.
A new study using mouse models adds to the evidence that chemotherapy can enhance cancer’s spread beyond the primary tumor, showing how one chemo drug allows breast cancer cells to squeeze through and attach to blood vessel linings in the lungs.
The method used in experiments with mice was specifically designed to trace a series of steps in a chemo drug’s changes to non-cancer cells. Scientists pre-treated healthy mice with the chemotherapy agent and gave them intravenous injections of breast cancer cells four days later.
Within three hours of injection, the cancer cells were penetrating weakened junctions between blood vessel cells in the lungs and binding to those vessels’ underlining structure – avoiding being washed away by blood flow.
“This is the key step giving cancer cells a foot in the door at a secondary site,” said Tsonwin Hai, professor of biological chemistry and pharmacology at The Ohio State University and senior author of the study. “The whole point of this pre-treatment model is to ask the question: Does chemotherapy affect normal cells in such a way that they may impact the behavior of cancer cells? The answer is yes.”
The study was published online recently in the International Journal of Molecular Sciences.
Hai has studied the underpinnings of cancer metastasis for years, previously finding that activation of ATF3, a stress gene, in immune cells is a crucial link between stress and cancer’s spread and that the chemo drug paclitaxel sets off molecular changes in immune cells that allow breast cancer cells to escape from a tumor
Hai, who holds a PhD in biochemistry from the Massachusetts Institute of Technology and completed postdoctoral research at Harvard University before joining Ohio State’s faculty in 1990, was invited to give a TEDxOhioStateUniversity talk in 2020. She noted in her presentation that three decades of ATF3 research in her lab suggested this stress-related gene may be an important target to suppress as a way to inhibit cancer’s spread. She also described the chemo paradox shown in the literature by various pre-clinical mouse models: that chemotherapy is effective at killing cancer cells, but also has effects on non-cancer cells that may be exploited by cancer cells to help them survive.
This new study zeroed in on the molecular effects of the chemo drug cyclophosphamide on non-cancer cells before there is any cancer present, focusing on the lungs as the site of metastasis.
“The reason for focusing on the non-cancer cells in the lung – the second site in our model – rather than that at the primary tumors is because cancer cells’ escape from a primary tumor is not a late event – it can actually happen very early on,” Hai said.
Researchers injected one dose of the chemo into mice and waited four days for the animals to metabolize and excrete the drug. They then gave the mice intravenous injections of breast cancer cells, allowing them to travel to the lungs.
Once in the lungs, the cancer cells were more likely to latch onto blood vessel walls if the animals were pre-treated with chemo. Researchers identified two reasons: First, spaces had opened up between cells in the vessel lining. Beyond that, a second material under those cells, called the basement membrane, had changed properties in a way that let cancer cells latch on so they wouldn’t be whisked away by blood flow.
“The endothelial cells lining the inner side of the blood vessel are like a brick wall, and each brick is tightly adhered to the next one,” said Hai. “What we found when we treated mice with chemotherapy is that it makes the vessel leaky, so the tight junction is not as tight anymore and the cancer cells can squeeze themselves through the brick layer.”
“We also found that chemotherapy modified the underlying basement membrane so once the cancer cells squeeze through, they find a place to grab onto.”
In control mice that did not receive chemotherapy, fewer cancer cells adhered to the blood vessel walls, Hai said.
The research team determined that cyclophosphamide’s presence led to an increase in levels of an enzyme in the blood called MMP-2, and that increase induced changes to the basement membrane that allowed cancer cells to attach to the blood vessel lining (see figure).
For decades, scientists focused on the effects of chemo on the intrinsic properties of cancer cells. Only in the last 10 years or so have researchers uncovered the effects of chemotherapy on non-cancer cells and their contribution to cancer metastasis in animal models.
“It’s a cautionary note for the use of chemotherapy, and may partly explain why chemotherapy is often most effective for subsets of patients or when used in conjunction with other treatment modalities.” However, Hai emphasized that much more work is required before extrapolating the findings in mice to humans.
Hai noted that this emerging area of pre-clinical research using animal models by her lab and others is intended to reveal the molecular mechanisms behind the positive and negative effects of chemotherapy, with the hope to improve cancer treatments in the future for people facing this difficult life challenge.
This work was supported in part by the U.S. Department of Defense. Co-authors are Justin Middleton and Subhakeertana Sivakumar, both of Ohio State.
Hai elaborated that chemotherapy is an important treatment modality and is potentially curative for many cancer patients. Her note of caution reflects her opinion and is not intended as medical advice. Her opinion is informed by the accumulating experimental data on chemo’s paradoxical effect of enhancing cancer progression in animal models. Although this literature is primarily from mouse models, there are correlative data at the molecular and cellular levels derived from a limited number of cancer patient samples. As emphasized in the main text, much more work is required before extrapolating the findings in mice to humans. Patients who are concerned by this report should consult with their health care providers to make informed decisions.
Message to patients from The Ohio State University Comprehensive Cancer Center
On behalf of breast medical oncologists and the breast cancer translational research group, the Comprehensive Cancer Center would like to provide some context and perspective regarding chemotherapy in breast cancer. As oncologists treating breast cancer patients we do our best to support informed decision making, which includes advising them of the latest research. When faced with decisions about accepting life-saving chemotherapy, we understand the patient’s fear of side-effects are weighed against the benefits. In weighing the decision, the over-riding considerations we advise is what we have learned from more than 40 years of human studies and clinical research. While chemotherapy is not perfect, what we learn in the laboratory to make treatments better should not be a consideration when deciding to be treated with chemotherapy. Experimental mouse studies are important to advance our understanding of how cancer progresses or returns, but mice are not people and too often laboratory research does not apply to people. This particular preclinical model was studied by Dr. Hai using mice that do not replicate real world cancer metastasis and does not test real world treatment for women with breast cancer. We do not believe that this research should be a consideration against the use of chemotherapy by women and their physicians who are discussing breast cancer treatment options.