The Role of Biomechanical Cues in Mechanotransduction and Breast Cancer Metastasis

Breast cancer metastasis to the brain is one of the most lethal forms of metastases. Metastasis is regarded as a non-random process governed by several biomechanical factors including tissue stiffness. As brain tissue is ultrasoft and extremely heterogeneous compared to breast cancer primary sites; how are breast cancer cells able to colonize the vastly different microenvironment of the brain? As a key protein of the Hippo pathway, YAP is regarded as a mechanotransducer that is sensitive to changes in substrate stiffness. Its biochemical activity is intertwined with Piezo1, a mechanosensitive ion channel activated through plasma membrane deformation. To impact cellular function, YAP enters the nucleus and binds to the TEAD transcription domain triggering downstream expression of proteins involved in cell motility, wound healing, and metastasis. In this work, triple- negative breast cancers (TNBC) were shown to experience greater migration rates on stiff surfaces compared to soft PDMS substrates. Concurrently, cells showed YAP nuclear localization in a stiffness dependent manner. Then, mechanical characterization of human brain tissue was performed to characterize the stiffness heterogeneity in the brain associated with region specific metastasis. Five to six regions of the brain from two different patients showed similar patterns of stiffness heterogeneity with the anterior regions being generally stiffer than posterior regions. As Piezo1 is directly linked with detecting changes in biomechanical stimuli, it was used as a readout of surface stiffness to examine if cells in the brain could detect different regional stiffnesses. Comparisons of grey and white matter showed no significant difference in Piezo1 expression. As a drug screening framework, molecular dynamic simulations were performed to evaluate drug efficacy on well- characterized inflammatory mediators that are implicated in metastasis. These findings contribute to understanding the gap in knowledge surrounding the interplay between tissue stiffness and YAP mechanotransduction in the context of breast-to-brain metastasis.