Fighting cancer at its tissue of origin is difficult enough, but when tumor cells disseminate or travel to other places in the body and establish residency, this challenge becomes even greater. This is especially true for brain metastases originating from breast cancer, which occur in up to 30% of women with metastatic triple negative breast cancer. Due to a number of obstacles, there has been little progress made in treating already established breast cancer brain metastases. Researchers in the lab of Dr. Cyrus Ghajar, in the Public Health Sciences and Human Biology Divisions, decided to take a different approach. In their recent study published in Nature Cancer, the Ghajar group sought to understand what makes a cell proliferate and metastasize once in the brain, instead of staying dormant. This study, led by postdoctoral researcher Dr. Jinxiang (David) Dai, used live cell imaging in the brain and found that escape from dormancy was the rate-limiting step towards brain metastasis. Dai et al. uncovered that specific interactions between tumor cells and astrocytes dictate whether or not a breast tumor cell proliferates and metastasizes in the brain.
Taking an unbiased approach to understand why a disseminated breast tumor cell would grow or not, the Ghajar group used intravital imaging with two photon microscopy to visualize first, the behavior and fate of tumor cells in the brain overtime, and second, how they interact with the microenvironment. To understand how cells that remain dormant behave differently from those that form metastases, the researchers utilized two breast cancer cell lines that travel to the brain after intracardiac delivery but remain dormant. They contrasted these cells’ behavior with derivatives of these cell lines that readily travel to and proliferate in the brain. The authors found that similar numbers of both breast cancer cell types were able to travel to the brain and survive. Interestingly, once in the brain, all cells occupied vascular niches, but only some cells began to proliferate while others remained dormant. This finding uncoupled dissemination from metastasis, revealing that dormancy was the rate-limiting step in brain metastasis. Careful imaging of the same cells every few days for up to two months uncovered that those cells that remained dormant maintained physical interactions with the endfeet of astrocytes- specialized star-shaped cells critical for the central nervous system. Tumor cells where the astrocyte endfeet had been stripped away became proliferative and formed metastases. These critical findings suggested a previously unidentified suppressive role that astrocytes play in preventing brain tumor metastasis. But what drives this suppression?
To determine how astrocytes could suppress metastasis, the Ghajar team used a culture-based system that mimics the brain’s perivascular niche. Using this system, they identified 24 extracellular matrix (ECM) proteins that were brain-specific and became enriched only when astrocytes were present in the culture. The authors then performed a gain-of-function screen and asked which of these factors were capable of suppressing breast tumor cell growth. Laminin-211, a protein predominantly expressed in astrocytes in the mouse brain, was the only factor able to suppress breast tumor cell outgrowth substantially and significantly. Consistent with this finding, an astrocyte-specific knockout of laminin-211 resulted in brain metastases in mouse models. The researchers reasoned that laminin-211 must be signaling through a receptor specifically on the breast tumor cell to suppress its growth. They knocked down three potential laminin-211 receptors and found that similar to laminin-211 mutant mice, knockdown of the dystroglycan receptor also promoted brain metastases. Finally, the authors asked how this astrocyte-specific signaling through laminin-211 and dystroglycan was able to suppress proliferation in breast tumor cells in the brain. They found that membrane bound dystroglycan interacts with the transcription factor YAP, which promotes cell growth. However, dystroglycan sequesters YAP from the nucleus and prevents it from activating proliferation, whereas constitutively active, nuclear YAP promotes brain metastases. Collectively, this work identifies a mechanism by which astrocyte-deposited laminin-211 drives disseminated tumor cell dormancy by inducing the dystroglycan receptor, present on tumor cells, to sequester YAP and prevent pro-metastatic functions. Future work in the Ghajar lab focuses on dystroglycan, a protein decorated by sugars which are needed for astrocytes to contact and suppress the tumor cell. Specifically, they are asking if and how these sugars might get lost over time, resulting in loss of tumor suppressive functions.
Dr. Ghajar highlighted that identifying that this tumor suppressive function of astrocytes is also likely true in humans was only made possible due to the selflessness of a metastatic breast cancer patient. This person “was adamant that their body be donated to research so that no one else had to suffer” from this disease, enabling this key discovery. Dr. Ghajar also mentions the importance of “having the flexibility of startup funds from Fred Hutch” that allowed them to ask high risk questions and “allowed [them] to fail,” until they eventually got some things right and uncovered a unique mechanism for how disseminated breast tumor cells remain dormant in the brain.
This work was supported by start-up funds provided by the FHCRC, National Institutes of Health, National Institute of Cancer, Department of Defense (DoD), Breast Cancer Research Program (BCRP), the Comparative Medicine, Experimental Histopathology and Genomics Shared Resources of the Fred Hutch/University of Washington Cancer Consortium, the Huntsman Cancer Foundation, Halt Cancer at X, the Susan G. Komen Foundation, the Swiss National Science and Deutsche Krebshilfe (German Cancer Aid).
UW/Fred Hutch Cancer Consortium members Cyrus Ghajar, Jason Bielas and Eric Holland contributed to this research.
Dai J, Cimino PJ, Gouin KH 3rd, Grzelak CA, Barrett A, Lim AR, Long A, Weaver S, Saldin LT, Uzamere A, Schulte V, Clegg N, Pisarsky L, Lyden D, Bissell MJ, Knott S, Welm AL, Bielas JH, Hansen KC, Winkler F, Holland EC, Ghajar CM. Astrocytic laminin-211 drives disseminated breast tumor cell dormancy in brain. Nat Cancer. 2022 Jan;3(1):25-42. doi: 10.1038/s43018-021-00297-3. Epub 2021 Dec 24. PMID: 35121993.