Astrocyte involvement in brain metastasis: from biological mechanisms to therapeutic strategies

Brain metastases (BrMs) remain a major clinical challenge with poor prognosis. While the brain was once viewed as a passive site for tumor colonization, it is now clear that the brain microenvironment—specifically the role of astrocytes—is a primary driver of metastatic success. Astrocytes, the most abundant glial cells in the central nervous system, act as dynamic regulators that can either suppress or facilitate the growth of metastatic tumor cells depending on the stage of disease and the signals they receive.

Astrocytes contribute to the metastatic cascade through several key mechanisms. Initially, they may act as a defense mechanism, using plasminogen activators to induce apoptosis in invading tumor cells. However, metastatic cells often reprogram these astrocytes into a reactive state. Once activated, these reactive astrocytes (RAs) facilitate tumor progression by:

• Modulating the Blood-Brain Barrier (BBB): Through the secretion of factors like CCL2 and TNF-α, RAs can increase BBB permeability, allowing circulating tumor cells to extravasate into the brain parenchyma.
• Establishing a Pro-metastatic Niche: RAs secrete various cytokines and growth factors (e.g., CHI3L1, HGF, and JAG1) that promote tumor cell survival, proliferation, and stem-like properties.
• Forming Gap Junctions: Tumor cells exploit gap junctions (composed of connexins like Cx43) to establish direct communication with astrocytes. This allows for the exchange of molecules like cGAMP, which activates pro-survival signaling pathways in both the tumor and the astrocyte, ultimately leading to chemoresistance.

Given their central role in supporting metastases, astrocytes have emerged as a promising therapeutic target. Current research is focused on disrupting the communication between tumor cells and astrocytes. Strategies include using gap junction inhibitors (e.g., meclofenamate), STAT3 inhibitors to block pro-metastatic signaling, and S1P3 receptor antagonists to stabilize the BBB. Additionally, advanced drug delivery systems, such as microenvironment-customized nanoparticles, are being developed to target these interactions while minimizing systemic toxicity. Future clinical success will likely depend on identifying specific pro-metastatic astrocyte subpopulations and combining these targeted therapies with standard treatments like immune checkpoint blockers.