The secret signals tumor cell clusters use to drive metastasis

It’s not the cancer in your breast that kills you, it’s the breast cancer that sets up shop in other organs like your liver or lungs or brain and starts growing uncontrollably.

Metastatic tumors, also called advanced or secondary cancer, are potent killers. In the U.S. alone, nearly 43,000 people die each year from breast cancer that’s metastasized. Metastatic prostate cancer kills around 33,000 a year and metastatic lung cancer claims a whopping 135,720.

And while science hasn’t yet figured out how to stop metastasis, a team of researchers at Fred Hutchinson Cancer Research Center have figured out that tumor cell clusters — clumps of anywhere from three to dozens of cells that break off from a primary tumor and travel through the lymph or bloodstream — are aggressive drivers of metastatic, or stage 4, disease.

Today, that team has a new paper out in the journal Cell outlining their discovery of a secret space between the cells of each microscopic cluster where the signal to grow is co-opted and miscommunicated to other cells. They’re now working on new ways to block that signal — call it growth factor “fake news” — in order to stop metastasis in its tracks.

“Metastasis forms in many ways,” said Dr. Kevin Cheung, a Hutch translational researcher who also treats breast cancer patients at Seattle Cancer Care Alliance. “In this paper we show that inducing cancer cells to become clusters gives them a 500-fold increase in the cells’ ability to form metastasis. We sought to understand why.”

A ‘nanoscale microenvironment’

For the past several years, the Cheung Lab, including the study’s first author and Fred Hutch / University of Washington molecular biology grad student Emma Wrenn, have been plumbing the biology of tumor cell clusters, trying to get to the heart of the mechanism that drives the formation of metastatic tumors, or “mets.”

They study the deadly process using mouse and patient-derived organoid models of breast cancer (in lab dishes), confirming their findings with high-resolution electron microscopy and other imaging techniques.

In this new paper, Cheung and team shared the first-ever views of a secret signaling chamber they call “nanolumina,” (lumina are “inside spaces”) where tumor cells communicate with each other as a group, a process called collective signaling.

“It’s sort of a nanoscale microenvironment, a localized signaling environment between cells,” Cheung said. “And it turns out cancer cells are communicating with each other via a signaling molecule called epigen that promotes the outgrowth of those cells in contact with it.”

The Cheung Lab found that when clustered, tumor cells are somehow able to co-opt epigen signaling, using the molecule to broadcast bad information to its neighboring cells, much like an untrustworthy media outlet might transmit misinterpreted facts and science.

In both instances, it causes big problems.

“It acts like a switch between migration and proliferation,” Cheung said. “When epigen is present, they stay put. They start to divide rapidly and grow. We see this in [tissue] cultures; in human tumors; in in vivo and in metastatic mouse models. If you get rid of it — via epigen knockdown or suppression — the cells migrate more efficiently.”

Without epigen, “they still cluster — but instead of growing, those clusters migrate,” Wrenn said. “This suggests epigen helps cells decide whether to grow or move.”

Cheung said their findings definitely point to epigen being a molecule of interest.

“It suggests we could target collective signaling in patients,” he said. “It’s very early days but we’re excited to think this could be a new means of treating breast cancer. That’s the dream.”

‘Profound’ reduction in metastatic growth

For decades, scientists believed metastasis started with a single cancer cell that “crawled away and set up shop,” Wrenn said.

But Cheung Lab research shows tumor cell clusters are a prime metastatic culprit.

“Not only does cancer metastasize as clusters, clusters are more powerful than single cancer cells,” she said. “They travel through the body in a clump and resist hostile signals from the immune systems or the organs where they’re setting up. Within the nanolumina, though, they can share pro-growth signals with each other that have the potential to supersede the other hostile signals.”

To demonstrate how clusters flex their collective power, the team ran a number of experiments.

Using a mouse model of an aggressive breast cancer, they tracked and compared the metastatic growth of single cells versus tumor cell clusters. They divvied cancer cells into groups, letting each group grow for zero hours, six hours, 24 hours and, finally, 24 hours followed by separation back into single cells.

They then injected cells from the separate samples into mice.

Tumor cell clusters that grew for six hours formed 140-fold more lung mets than an equal number of single cells. And clusters that grew for 24 hours formed 536-fold more lung mets than an equal number of single cells. Tumor cell clusters also showed much higher capacity for survival than single cancer cells.

Using high-powered microscopes, they looked at the miniscule clusters and discovered tiny spaces between the cells. Follow-up experiments identified epigen as a key signaling molecule in these tiny spaces and found that suppressing it “profoundly” reduced primary tumor and metastatic outgrowth.

“Whether it’s the primary tumor or crawling through the body or within a metastatic tumor, they have a secret signal between themselves that says, ‘Keep growing,’” Wrenn said. “Obviously, we don’t want that.”