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.

The PI3K/Akt/mTOR pathway as a preventive target in melanoma brain metastasis

BACKGROUND

Brain metastases (BM) are a frequent complication of malignant melanoma (MM), with limited treatment options and poor survival. Prevention of BM could be more effective and better tolerated than treating established BM in various conditions.

METHODS

To investigate the temporospatial dynamics of PI3K/Akt/mTOR (PAM) pathway activation during BM formation and the preventive potential of its inhibition, in vivo molecular imaging with an Akt biosensor was performed, and long-term intravital multiphoton microscopy through a chronic cranial window in mice.

RESULTS

In vivo molecular imaging revealed invariable PAM pathway activation during the earliest steps of brain colonization. In order to perform a long-term intravascular arrest and to extravasate, circulating MM cells needed to activate their PAM pathway during this process. However, the PAM pathway was quite heterogeneously activated in established human brain metastases, and its inhibition with the brain-penetrant PAM inhibitor GNE-317 resulted in only modest therapeutic effects in mice. In contrast, giving GNE-317 in preventive schedules that included very low doses effectively reduced the growth rate and number of BM in two MM mouse models over time, and led to an overall survival benefit. Longitudinal intravital multiphoton microscopy found that the first, rate-limiting steps of BM formation-permanent intravascular arrest, extravasation, and initial perivascular growth-are most vulnerable to dual PI3K/mTOR inhibition.

CONCLUSION

These findings establish a key role of PAM pathway activation for critical steps of early metastatic brain colonization and reveal its pharmacological inhibition as a potent avenue to prevent the formation of clinically relevant BM.

BSCI-11. Targeting PI3K/Akt/mTOR pathway to prevent melanoma brain metastasis

Background

Patients developing brain metastasis (BM) still face a poor survival due to limited treatment options. BM prevention using low dose drug schedules could be a more potent strategy with less side effects than treating established BM. This could add a real benefit to the ongoing challenge of facing the frequent BM formation in high-risk malignant melanoma (MM) patients.

Methods

Aiming to study the dynamics of PI3K/Akt/mTOR (PAM) pathway activation during the brain metastatic cascade, in vivo molecular imaging with an Akt biosensor was performed. Long-term intravital multiphoton microscopy through a chronic cranial window in mice was employed to investigate timing and effectiveness of PAM pathway inhibition for BM prevention.

Results

In vivo molecular imaging revealed the activation of PAM pathway as a prerequisite for extravasation of circulating MM cells in the brain. However, established human BM present with heterogeneous activation of the PAM pathway. Moreover, in two MM mouse models, PAM pathway inhibition with the brain-penetrant dual PI3K/mTOR inhibitor GNE-317 resulted in only moderate effects on established BM. In contrast, giving low dose GNE-317 in a preventive schedule successfully reduced growth rate and number of BM in both mouse models. Longitudinal intravital multiphoton microscopy suggests that the first, rate-limiting, steps of BM formation can be effectively targeted by dual PI3K/mTOR inhibition.

Conclusion

PAM pathway activation is key for the critical early steps of MM metastatic brain colonization. These findings reveal that early PAM pathway inhibition is a promising strategy to prevent the formation of clinically relevant BM.