Cancer cells work together to source nutrients from their environment -- a cooperative process that was previously overlooked by scientists but may be a promising target for treating cancer.
"We identified cooperative interactions among cancer cells that allow them to proliferate," said Carlos Carmona-Fontaine, an associate professor of biology at New York University and the senior author of the study published in Nature. "Thinking about the mechanisms that tumor cells exploit can inform future therapies."
Scientists have long known that cancer cells compete with one another for nutrients and other resources. Over time, a tumor becomes more and more aggressive as it becomes dominated by the strongest cancer cells.
However, ecologists also know that living organisms cooperate, particularly under harsh conditions. For instance, penguins form tight huddles to conserve heat during the extreme cold of winter, with their huddles growing in size as temperatures drop -- a behavior not observed during warmer months. Similarly, microorganisms such as yeast work together to find nutrients, but only when facing starvation.
Cancer cells also need nutrients to thrive and replicate into life-threatening tumors, but they live in environments where nutrients are scarce. Is it possible that cancer cells work together to scavenge for resources?
To determine whether cancer cells cooperate, the researchers tracked the growth of cells from different types of tumors. Using a robotic microscope and image analysis software they developed, they quickly counted millions of cells under hundreds of conditions over time. This approach allowed them to examine tumor cultures at a range of densities -- from sparsely populated dishes to those crowded with cancer cells -- and with different levels of nutrients in the environment.
It is well-known that cells uptake amino acids in a competitive manner. But when the cancer cells studied were starved of amino acids such as glutamine, all of the cell types tested showed a strong need to work together to acquire the available nutrients.
"Surprisingly, we observed that limiting amino acids benefited larger cell populations, but not sparse ones, suggesting that this is a cooperative process that depends on population density," said Carmona-Fontaine. "It became really clear that there was true cooperation among tumor cells."
Through additional experiments with skin, breast, and lung cancer cells, the researchers determined that a key source of nutrients for cancer cells comes from oligopeptides -- pieces of proteins made up of small chains of amino acids -- found outside the cell.
"Where this process becomes cooperative is that instead of grabbing these peptides and ingesting them internally, we found that tumor cells secrete a specialized enzyme that digests these peptides into free amino acids," explained Carmona-Fontaine. "Because this process happens outside the cells, the result is a shared pool of amino acids that becomes a common good."
Determining the enzyme secreted by cancer cells, called CNDP2, was an important discovery. The researchers identified the enzyme by testing different drugs to see if they kept tumor cells from digesting oligopeptides into free amino acids. When the drug bestatin was applied to cancer cells, inhibiting the function of CNDP2, cells were unable to feed on oligopeptides and were driven to extinction.
Now that the researchers knew that CNDP2 was a factor behind the cooperative process feeding cancer cells, they could test what happens when the enzyme is missing.
The researchers used the genetic editing technology CRISPR to delete -- or "knock out" -- the Cndp2 gene (which produces the CNDP2 enzyme) in tumor cells. Then, they looked at how these cells formed tumors in mice and compared their growth to tumors formed by identical cells still carrying the Cndp2 gene. The growth of the knockout tumors was significantly reduced, a difference that was even more pronounced when the deletion of Cndp2 was combined with restricting the tumor's access to amino acids using diets low in these nutrients. They were also able to reduce the growth of tumors with normal CNDP2 by combining these diets with bestatin, a combination that could help in the clinic.
"Because we've removed their ability to secrete the enzyme and to use the oligopeptides in their environment, cells without CNDP2 can no longer cooperate, which prevents tumor growth," said Carmona-Fontaine. "Competition is still critical for tumor evolution and cancer progression, but our study suggests that cooperative interactions within tumors are also important."
The researchers hope that their findings will help inform cancer treatments that target cooperation among cancer cells -- "a conceptual contribution that will have an impact in the clinic," said Carmona-Fontaine. For instance, the drug bestatin has been safely used in humans for decades as an add-on to chemotherapy, particularly in Japan, but has limited effectiveness on its own.
"We hope that a clearer understanding of this mechanism can help us make drugs more targeted and more effective," said Carmona-Fontaine.
Additional study authors include Gizem Guzelsoy, Setiembre Elorza, Manon Ros, Logan Schachtner, Spencer Hobson-Gutierrez, and Parker Rundstrom of NYU's Center for Genomics and Systems Biology; Makiko Hayashi, Ray Pillai, and Thales Papagiannakopoulos of the Perlmutter Cancer Center at NYU Langone Health; Julia Brunner and Lydia Finley of Memorial Sloan Kettering Cancer Center; William Walkowicz of Chemitope Glycopeptide; and Maxime Deforet of Sorbonne Université.
The research was supported by the National Cancer Institute (DP2 CA250005), American Cancer Society (RSG-21-179-01-TBE), Pew Charitable Trusts (00034121), an NIH Developmental Genetics Training Grant (5T32HD007520-20), Memorial Sloan Kettering's Marie-Josée Kravis Women in Science Endeavor, and the New York Stem Cell Foundation, as well as NIH support to NYU Langone's Proteomics Laboratory (P30CA016087) and Memorial Sloan Kettering (CA008748).
