PhD student Hong Kong Polytechnic University, United States
Introduction:: Breast cancer preferentially metastasizes to specific organs, including brain tissue, which considerably deteriorates patient survival. Previous studies indicate the pre-existence of metastatic cells within the primary tumor and the significance of the mechanical microenvironment in tumor cell functions. However, the influence of local niche mechanics within the primary tumor on breast cancer brain metastasis remains poorly understood.
Materials and Methods:: In this study, ultrasound elastography was adopted to measure the distribution of local niche stiffness within primary breast tumors. Hydrogels with different stiffnesses were prepared to mimic these soft and stiff local niches. When breast cancer cells with no metastatic preference were cultured on these substrates for different durations, various in vitro assays were utilized to analyze the brain metastatic potential, including gene expression, cerebral endothelium adhesion, blood-brain barrier (BBB) transmigration, and the survival and proliferation in brain tissue. Further, the brain metastatic ability of these mechanically conditioned cells was examined by in vivo intracardiac and intracranial injection.
Results, Conclusions, and Discussions:: The primary breast tumor exhibits significant mechanical heterogeneity and contains local soft and stiff niches. Persistent culture of breast cancer cells with no metastatic preference on the soft substrates that mimic local soft niches promotes the expressions of brain metastasis genes and the molecular signature of neuron. In vitro analysis shows that soft niche-primed cells exhibit enhanced ability in cerebral endothelium adhesion, BBB transmigration, and survival and outgrowth in the extracted mouse brain tissues. Importantly, soft niche-primed cells preferentially colonize brain tissue and generate the largest brain metastases after intracardiac injection into nude mice compared to control cells and the cells cultured on stiff substrates. The elevated brain metastasis may be mediated by the enhanced ability of these cells to extravasate out of the BBB in brain tissue and outgrow in the brain microenvironment in the mouse model. Mechanistically, the RNA-seq analysis shows the up-regulation of histone deacetylation and enrichment of histone deacetylases (HDACs) in soft niche-primed cells. In particular, soft niches promote HDAC3 activity, inhibition of which abolishes the soft niche-induced acquisition of brain metastasis ability. Pharmacologic inhibition of HDAC3 effectively antagonizes brain metastasis formation of soft niche-primed cells in vivo. Further, soft niches enhance histone deacetylation through the suppression of actomyosin-mediated nuclear mechanotransduction. Extended disruption of actin cytoskeleton promotes HDAC3 activity and the acquisition of brain metastasis ability, while the activation of actomyosin on soft substrates inhibits HDAC3 activity and up-regulation of brain metastasis ability. In summary, our study demonstrates that local soft niches within the primary tumor promote breast cancer brain metastasis through mechanotransduction-mediated HDAC3 activation, unveiling a regulatory role of primary tumor mechanics in metastatic preference.
Acknowledgements (Optional): : We acknowledge the support from National Natural Science Foundation of China (Project no. 11972316), Shenzhen Science and Technology Innovation Commission (Project no. JCYJ20200109142001798, SGDX2020110309520303, and JCYJ20220531091002006), General Research Fund of Hong Kong Research Grant Council (PolyU 15214320), Health and Medical Research Fund (18191421), and the internal grant from the Hong Kong Polytechnic University (1-ZE2M, 1-CD75, and 1-ZVY1).
