Active Remodeling of Capillary Endothelium via Cancer Cell-Derived MMP9 Promotes Metastatic Brain Colonization.

Crossing the blood-brain barrier is a crucial, rate-limiting step of brain metastasis. Understanding of the mechanisms of cancer cell extravasation from brain microcapillaries is limited as the underlying cellular and molecular processes cannot be adequately investigated using in vitro models and endpoint in vivo experiments. Using ultrastructural and functional imaging, we demonstrate that dynamic changes of activated brain microcapillaries promote the mandatory first steps of brain colonization. Successful extravasation of arrested cancer cells occurred when adjacent capillary endothelial cells (EC) entered into a distinct remodeling process. After extravasation, capillary loops were formed, which was characteristic of aggressive metastatic growth. Upon cancer cell arrest in brain microcapillaries, matrix-metalloprotease 9 (MMP9) was expressed. Inhibition of MMP2/9 and genetic perturbation of MMP9 in cancer cells, but not the host, reduced EC projections, extravasation, and brain metastasis outgrowth. These findings establish an active role of ECs in the process of cancer cell extravasation, facilitated by cross-talk between the two cell types. This extends our understanding of how host cells can contribute to brain metastasis formation and how to prevent it. SIGNIFICANCE: Tracking single extravasating cancer cells using multimodal correlative microscopy uncovers a brain seeding mechanism involving endothelial remodeling driven by cancer cell-derived MMP9, which might enable the development of approaches to prevent brain metastasis. See related commentary by McCarty, p. 1167.

Identification and Characterization of Cancer Cells That Initiate Metastases to the Brain and Other Organs.

Specific biological properties of those circulating cancer cells that are the origin of brain metastases (BM) are not well understood. Here, single circulating breast cancer cells were fate-tracked during all steps of the brain metastatic cascade in mice after intracardial injection over weeks. A novel in vivo two-photon microscopy methodology was developed that allowed to determine the specific cellular and molecular features of breast cancer cells that homed in the brain, extravasated, and successfully established a brain macrometastasis. Those BM-initiating breast cancer cells (BMIC) were mainly originating from a slow-cycling subpopulation that included only 16% to 20% of all circulating cancer cells. BMICs showed enrichment of various markers of cellular stemness. As a proof of principle for the principal usefulness of this approach, expression profiling of BMICs versus non-BMICs was performed, which revealed upregulation of NDRG1 in the slow-cycling BMIC subpopulation in one BM model. Here, BM development was completely suppressed when NDRG1 expression was downregulated. In accordance, in primary human breast cancer, NDRG1 expression was heterogeneous, and high NDRG1 expression was associated with shorter metastasis-free survival. In conclusion, our data identify temporary slow-cycling breast cancer cells as the dominant source of brain and other metastases and demonstrates that this can lead to better understanding of BMIC-relevant pathways, including potential new approaches to prevent BM in patients. IMPLICATIONS: Cancer cells responsible for successful brain metastasis outgrowth are slow cycling and harbor stemness features. The molecular characteristics of these metastasis-initiating cells can be studied using intravital microscopy technology.

Involvement of platelet-derived VWF in metastatic growth of melanoma in the brain.

BACKGROUND: The prognosis of patients with brain metastases (BM) is poor despite advances in our understanding of the underlying pathophysiology. The high incidence of thrombotic complications defines tumor progression and the high mortality rate. We, therefore, postulated that von Willebrand factor (VWF) promotes BM via its ability to induce platelet aggregation and thrombosis. METHODS: We measured the abundance of VWF in the blood and intravascular platelet aggregates of patients with BM, and determined the specific contribution of endothelial and platelet-derived VWF using in vitro models and microfluidics. The relevance for the brain metastatic cascade in vivo was demonstrated in ret transgenic mice, which spontaneously develop BM, and by the intracardiac injection of melanoma cells. RESULTS: Higher levels of plasma VWF in patients with BM were associated with enhanced intraluminal VWF fiber formation and platelet aggregation in the metastatic tissue and peritumoral regions. Platelet activation triggered the formation of VWF multimers, promoting platelet aggregation and activation, in turn enhancing tumor invasiveness. The absence of VWF in platelets, or the blocking of platelet activation, abolished platelet aggregation, and reduced tumor cell transmigration. Anticoagulation and platelet inhibition consistently reduced the number of BM in preclinical animal models. CONCLUSIONS: Our data indicate that platelet-derived VWF is involved in cerebral clot formation and in metastatic growth of melanoma in the brain. Targeting platelet activation with low-molecular-weight heparins represents a promising therapeutic approach to prevent melanoma BM.

Local blood coagulation drives cancer cell arrest and brain metastasis in a mouse model.

Clinically relevant brain metastases (BMs) frequently form in cancer patients, with limited options for effective treatment. Circulating cancer cells must first permanently arrest in brain microvessels to colonize the brain, but the critical factors in this process are not well understood. Here, in vivo multiphoton laser-scanning microscopy of the entire brain metastatic cascade allowed unprecedented insights into how blood clot formation and von Willebrand factor (VWF) deposition determine the arrest of circulating cancer cells and subsequent brain colonization in mice. Clot formation in brain microvessels occurred frequently (>95%) and specifically at intravascularly arrested cancer cells, allowing their long-term arrest. An extensive clot embedded ∼20% of brain-arrested cancer cells, and those were more likely to successfully extravasate and form a macrometastasis. Mechanistically, the generation of tissue factor-mediated thrombin by cancer cells accounted for local activation of plasmatic coagulation in the brain. Thrombin inhibition by treatment with low molecular weight heparin or dabigatran and an anti-VWF antibody prevented clot formation, cancer cell arrest, extravasation, and the formation of brain macrometastases. In contrast, tumor cells were not able to directly activate platelets, and antiplatelet treatments did reduce platelet dispositions at intravascular cancer cells but did not reduce overall formation of BMs. In conclusion, our data show that plasmatic coagulation is activated early by intravascular tumor cells in the brain with subsequent clot formation, which led us to discover a novel and specific mechanism that is crucial for brain colonization. Direct or indirect thrombin and VWF inhibitors emerge as promising drug candidates for trials on prevention of BMs.