The Effect of Inflammatory Cytokines on the Blood-Brain Barrier and Breast Cancer Brain Metastasis Formation

Brain metastasis (BM) is a common complication in patients with breast cancer. The blood-brain barrier (BBB) plays a crucial role in maintaining central nervous system (CNS) homeostasis, but this protective structure becomes compromised due to inflammation caused by metastatic breast tumors. Inflammatory cytokines, secreted by various cells within the tumor microenvironment, can impair BBB function. The BBB consists of several cell types, primarily endothelial cells (ECs) and astrocytes, whose functions can be disrupted by inflammatory signals. In this study, we investigated the impact of three inflammatory proteins—Chitinase-3-like 1 (CHI3L1), Lipocalin-2 (LCN-2), and Matrix Metalloproteinase-9 (MMP-9)—secreted by cells within the tumor microenvironment, including cancer cells, neutrophils, and macrophages. Our results demonstrated that these cytokines did not increase the migration velocity of ECs. Additionally, we examined their effect on angiogenesis, observing that the combination of CHI3L1, LCN-2, and MMP-9 increased the number of branches compared to individual cytokines while in other aspects did not change significantly. To further understand their impact on BBB integrity, we used a tranwell migration assay to mimic the BBB structure, and CAL51 cancer cells passage through the membrane evaluated. The 100 ng/ml MMP-9 increased BBB permeability in comparison with 80 ng/ml CHI3L1. Meanwhile, astrocyte viability remained unchanged under various concentrations of LCN-2 and MMP-9, while with CHI3L1 it changed significantly. Immunohistochemistry analysis of brain tissues from highly invasive mammary tumors revealed no significant differences in vessel coverage. However, further investigations are necessary to assess the impact of these inflammatory cytokines on claudin-5 expression, a critical tight junction (TJ) protein. Reduced claudin-5 expression may impair the paracellular pathway, facilitating cancer cell extravasation across the BBB. This study highlights the role of inflammatory cytokines in promoting EC migration, angiogenesis, and BBB permeability, contributing to breast cancer brain metastasis. Future research focusing on tight junction integrity may provide new insights into preventing metastatic invasion through the BBB.