A new study adds evidence that chemotherapy increases the spread of cancer beyond the primary tumor, and shows how a chemotherapy drug allows breast cancer cells to push through and adhere to blood vessel deposits in the lungs.
The research in mice leaves no doubt that the chemo-agent caused changes in non-cancer cells that enable this process. Researchers pretreated healthy mice with the chemotherapy drug and gave them intravenous injections of breast cancer cells four days later.
Within three hours after the injection, the cancer cells penetrated into weakened connections between blood vessel cells in the lungs and bound to the underlying structure of these vessels – and avoided being washed away by the blood flow.
“This is the key step that gives cancer cells a foot inside the door in a secondary location,” said Tsonwin Hai, a professor of biological chemistry and pharmacology at Ohio State University and senior author of the study. “The whole point of our pretreatment model is to ask the question: Does chemotherapy affect normal cells in such a way that they will turn around and help cancer cells? The answer is yes.
“It’s a warning about the use of chemotherapy.”
The study was recently published online in the International Journal of Molecular Sciences.
Hai has been researching the basics of cancer metastases for years and has previously found that activation of a specific gene in immune cells is a crucial link between stress and cancer spread, and that the chemo-drug paclitaxel triggers molecular changes in immune cells that allow breast cancer cells to escape from a tumor.
This new study focused on the effects of the chemo-drug cyclophosphamide on non-cancer cells before any cancer is present, focusing on the lungs as the metastasis site.
Researchers injected a dose of the chemo into mice and waited four days for the animals to metabolize and excrete the substance. They then gave the mice intravenous injections of breast cancer cells so that they could travel to the lungs.
Once in the lungs, the cancer cells were more likely to attach to the blood vessel walls if the animals were pre-treated with chemo. Researchers identified two causes: First, spaces had opened up between cells in the vascular lining. In addition, another material beneath these cells, called the basement membrane, had altered properties in a way that allowed the cancer cells to lock in place so that they would not be carried away by the blood flow.
“The endothelial cells that line the inside of the blood vessel are like a brick wall, and each brick is tightly glued to the next,” said Hai, also an investigator at Ohio State’s Comprehensive Cancer Center. “What we found out when we treated mice with chemotherapy is that it leaks the tub so that the tight connection is no longer so tight and the cancer cells can push themselves through the brick layer.
“We also found that chemotherapy modified the underlying basement membrane, so once the cancer cells penetrate, they find a place to seize.”
In control mice that did not receive chemotherapy, the adhesion of the cancer cells to the blood vessel walls was relatively minimal, Hai said.
The research team determined that the presence of cyclophosphamide led to an increase in the level of an enzyme in the blood called MMP-2, and that the increase induced changes in the basement membrane that allowed cancer cells to bind to the lining of the blood vessel (see figure above).
For decades, researchers have focused on the effects of chemo on the inherent properties of cancer cells, enabling cells to survive, resist chemotherapy, and spread. Only in the last 10 years or so have researchers discovered the effects of chemotherapy on non-cancer cells and their contribution to metastases.
“We focused here on how chemo affects the non-cancer cells in the lung – the second place in our model – rather than on the primary tumors, because the escape of cancer cells from a primary tumor is not a late event – it can actually happen very early,” said Hai. “Our data revealed that chemo acts on non-cancer cells and initiates changes in the lungs so that they can already adhere very well within three hours of the arrival of the cancer cells.
“The effect of chemotherapy on non-cancer cells actually changes these cells, and these changes help cancer cells develop.”
This work was partially supported by the US Department of Defense. Co-authors are Justin Middleton and Subhakeertana Sivakumar, both from Ohio State.