Abstract
A recent study indicates that the protein Eya3 influences the adaptive immune response to promote tumor growth, not only decreasing the number of tumor-infiltrating CD8+ T cells, but also driving their exhaustion through PD-L1 upregulation. These findings could spur new strategies to treat triple-negative breast cancer, which currently lacks targeted therapies.
Researchers at the University of Colorado in Denver have discovered that the protein Eya3 promotes tumor growth by influencing the adaptive immune response. Their findings may lead to new therapeutic targets for triple-negative breast cancer (TNBC).
Previously, Eya proteins had been implicated in breast tumor progression and metastasis largely through their intrinsic tyrosine phosphatase activity, as well as through their interactions with Six1. Little was known, however, about the separate threonine phosphatase domain on each Eya, other than its function being linked with innate immunity.
“The general thinking about Eyas and cancer has been in terms of proliferation, migration, and other cell-autonomous phenotypes,” says senior author Heide Ford, PhD. “Experiments to explore their activity in tumors have also been in immunocompromised animals. We reasoned that if these proteins can mediate innate immunity, what if they play a key role in regulating antitumor immune responses?”
To test this hypothesis, Ford's team knocked down Eya3 in two murine TNBC cell lines, then injected these cells into syngeneic, immunocompetent mice. Compared with control cells in which Eya3 remained intact, this knockdown significantly delayed tumor progression; however, cell proliferation rates didn't change. “What did change,” Ford notes, “were the numbers of tumor-infiltrating CD8+ T cells”—in both models, tumors formed from cells lacking Eya3 had considerably more of this T-cell subset than control tumors.
The researchers reported that intact Eya3 spurred tumor growth by suppressing CD8+ T cells, which could otherwise attack and eradicate tumor cells. Not only were cytotoxic T cells decreased in number, but Eya3 also affected their function: A higher percentage expressed various exhaustion markers, including Tim3, compared with the T cells in mice without Eya3.
This observation prompted Ford and her group to examine PD-L1 levels, which were increased on the surface of Eya3-expressing tumor cells. They then showed that, via its threonine phosphatase domain, Eya3 dephosphorylates a specific residue, T58, on c-Myc. This stabilizes the latter and ultimately upregulates PD-L1, allowing tumor cells to evade surveillance by the immune system's adaptive arm.
“I think this is the first demonstration of Eya proteins regulating adaptive immunity in any context,” Ford says. “We didn't expect this series of results, but the data pretty much hit us on the head with its clarity.”
The team extrapolated their findings from mice to humans by examining data from The Cancer Genome Atlas. They found a strong correlation between Eya3 and PD-L1 expression not only in TNBC, but in all other breast cancer subtypes. High Eya3 levels also significantly correlated with lower estimates of CD8+ T cells in these patients' tumors.
So far, therapeutic targeting of Eyas has mostly centered on disrupting their tyrosine phosphatase activity, Ford says. Her work offers a different angle—going after the threonine phosphatase domain instead. This will likely be challenging, she acknowledges, having recently reported that Eya3's threonine phosphatase activity is not intrinsic; rather, it requires partnering with another protein, PP2A.
“We can develop small-molecule inhibitors to directly hit Eya3's tyrosine phosphatase domain, but any approach targeting its other end will be much less straightforward,” Ford explains. With colleague Rui Zhao, PhD, “we're now trying to solve the structure of Eya3 and PP2A bound together, to see what the interface looks like.”
To Angela DeMichele, MD, of the University of Pennsylvania in Philadelphia, this study not only sheds light on how TNBC evades immune recognition, but “is meaningful to our thinking about how we might use immunotherapy, which has been disappointing in breast cancer, more effectively.”
“Basic research to uncover immunologic interactions within all breast cancer subtypes is sorely needed, particularly for triple-negative disease, given the lack of targeted therapies,” she adds. “We're still several steps away from being able to implement [Ford's findings] therapeutically, but this foundation is essential to making progress in the field.” –Alissa Poh
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