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Science Spotlight

A key to breast cancer metastasis lurks in the tiny cavities between cells

From the Cheung Lab, Breast and Ovary Cancers Program, Cancer Consortium

“How does 1 + 1 become more than 2, or more like 500?” asks Dr. Kevin Cheung, a physician-scientist and Associate Professor at the Fred Hutchinson Cancer Research Center, and member of the Cancer Consortium’s Breast & Ovary Cancer Program. “When cancer cells are traveling individually they have dismal ability to form metastases. Yet when the same numbers of cancer cells are clustered together, they acquire many-fold, up to 500-fold in [our] work, greater ability to form distant metastases.” This observation motivated Dr. Emma Wrenn, a former graduate student in the Cheung Lab, to take a closer look at the mechanisms behind metastasis. “The vast majority of cancer patients die from metastatic cancer that has spread from the original tumor to other organs. Our goal is to understand this process better so we can develop therapies that specifically target metastasis to help these patients,” she explains. Dr. Wrenn led the Cheung Lab in a study published in Cell that found a critical modulator of metastasis may lie in the nooks and crannies between cells.

In this study, the Cheung Lab explored what causes clusters of tumor cells to be so successful in seeding metastases. First, they broke apart tumor cells and let them re-aggregate over time, measuring what genes were upregulated at different stages of aggregation by using RNA sequencing. As they aggregated, the tumor cells turned on sequential waves of genes with unique functions, ranging from cell migration and wound healing in the early waves to DNA replication and metabolism at later stages. Genes that were necessary for cluster-based metastasis were identified by knocking down individual genes upregulated during the early wave of re-aggregation and injecting those tumor cell clusters into mice to look for metastasis. Using this method, the authors zeroed in on one specific gene: Knockdown of the growth factor epigen almost completely abolished their metastatic potential.

Clusters of cells that dislodge from breast cancer tumors circulate through the body, seeding metastatic outgrowths in other organs. Tiny junctions between these cells, dubbed “nanolumen” by the Cheung Lab, contain growth factors such as “Epigen” that are responsible for cell-cell signaling. Wrenn et al found that clusters with high concentrations of Epigen had higher outgrowth, whereas Epigen-low clusters were more migratory but less proliferative. The Cheung Lab also profiled multiple triple negative breast cancer subtypes and found they had different concentrations of Epigen, which might explain differences in disease progression. Image provided by Dr. Emma Wrenn.
Clusters of cells that dislodge from breast cancer tumors circulate through the body, seeding metastatic outgrowths in other organs. Tiny junctions between these cells, dubbed “nanolumen” by the Cheung Lab, contain growth factors such as “epigen” that are responsible for cell-cell signaling. Wrenn et al. found that clusters with high concentrations of epigen had higher outgrowth, whereas epigen-low clusters were more migratory but less proliferative. The Cheung Lab also profiled multiple triple negative breast cancer subtypes and found they had different concentrations of epigen, which might explain differences in disease progression. Image provided by Dr. Emma Wrenn.

“We next looked at these clusters more closely with the advanced electron microscopy and super-resolution microscopy available at the [Fred] Hutch and what we found was really surprising,” said Dr. Cheung. “There were these intercellular interdigitating pockets or intercellular spaces between tumor cells which we describe as nanolumina”. According to Dr. Wrenn, “these nanolumina act as shared reservoirs for the growth factor epigen. Blocking tumor cell clusters from expressing and sharing epigen greatly reduced their growth in vitro and in vivo, suggesting that nanolumina and the cell-cell signaling they generate play a key role during metastatic outgrowth.” The Cheung Lab found that metastasizing tumor cells are constantly monitoring their surroundings and communicating with each other via signaling factors such as epigen, passing notes back and forth in these tiny crevices between cells. Knocking down epigen in these nanolumina, like snatching the note paper out of the cells’ “hands”, destroyed their ability to talk and listen to each other and thus their ability to cooperatively metastasize to new tissues.

The discovery of nanolumina in breast cancer tumor cell clusters is a major step for both cancer biology and basic sciences. “Tumor cells utilize collective intercellular signaling to promote metastasis - most certainly at the outgrowth phase but also potentially at earlier steps along the process,” said Dr. Cheung. “Whether nano lumens and collective signaling are dynamically regulated and by what mechanisms are questions that will need new molecular tools, technologies, and reporter systems.” “We also don't know if these are cancer-specific structures,” chimed in Dr. Wrenn. “It may be that normal, developing breast tissue also uses nanolumina to regulate growth but that these are co-opted or dysregulated in cancer to generate uncontrolled proliferation.” Now that they’ve discovered and briefly characterized these nanolumina, the Cheung Lab is eager to continue their spelunking and connect their results to potential cancer therapeutics. “Nanolumina are likely to contain a number of factors that perhaps like epigen participate in collective signaling,” said Dr. Cheung. “We are now doing the hard work of defining nano lumens as a target for cancer therapy and possibly an instigator of therapy resistance.”

The Cheung Lab is grateful to be a part of the Cancer Consortium, where they can combine their laboratory expertise and experiments with samples generously donated by cancer patients. According to Dr. Wrenn, “We can make a lot of progress using laboratory models like cell lines and genetically engineered mice, but ultimately we need to know if what we are studying is relevant to actual human patients. Through collaborations with other members of the Cancer Consortium at UW and the SCCA we were able to generate a pipeline to get fresh biopsied breast cancer samples from patients that we could immediately test these hypotheses on. This collaborative effort, and the generosity of the patients who donated their tissues, greatly strengthened the scientific foundation of our work.” Dr. Cheung agrees that the Cancer Consortium has contributed to multiple long-term projects within the lab, adding that “we are united by the common goal of getting to ‘cancer zero’…[and] the Cancer Consortium has helped foster dialogue and fresh ideas about how to combat metastatic breast cancer.”

This study was supported by the Department of Defense Breast Cancer Research Program, the Burroughs Wellcome Fund Career Award for Medial Scientists, the Breast Cancer Research Foundation, the V Foundation, the Phi Beta Psi Sorority, Seattle Translational Tumor Research, and the Shared Resources of the FHCRC/UW Cancer Consortium.

Cancer Consortium members Dr. Kevin Cheung, Dr. Habib Rahbar, and Dr. Savannah Partridge contributed to this work.

ED Wrenn, A Yamamoto, BM Moore, Y Huang, M McBirney, AJ Thomas, E Greenwood, YF Rabena, H Rahbar, SC Partridge, and KJ Cheung. 2020. Regulation of collective metastasis by nanolumenal signaling. Cell. 183: 395-410.