Shear Stress-Mediated Tumor-Endothelial Cross Talk Regulates the Angiogenic Potential of Breast Tumors In Vitro

The structural and functional abnormalities of the tumor vasculature generate regions of elevated interstitial fluid pressure and aberrant flow shear stress within the tumor microenvironment. While research has shown that the hydrodynamics of the tumor vasculature reduce transport and uptake of therapeutic agents, the underlying mechanisms by which fluid forces regulate vascular organization are not well known. Understanding the reciprocal interaction between tumor and endothelial cells to mediate angiogenesis, and the role of flow shear stress on this process, may offer insight into the design of improved therapeutic strategies to control vascularized tumors. Instrumental to this is the development of physiologically relevant models that enable tumor-endothelial co-culture under dynamic conditions. By integrating tissue-engineering strategies with cancer biology, micro-scale fluid mechanics, and optical flow diagnostics, the goal of this research was to develop a 3D in vitro microfluidic culture model to investigate tumor-endothelial cross talk under physiologically relevant flow shear stress. This objective was motivated by early findings demonstrating a contact-independent, paracrine-mediated mechanism by which endothelial cells enhance tumor-expressed angiogenic factors during 2D, static co-culture. The 3D tumor vascular model consists of a central microchannel embedded within a type I collagen hydrogel, through which a range of normal (4 dyn/cm^2), low (1 dyn/cm^2) and high (10 dyn/cm^2) microvascular wall shear stresses (WSS) were introduced. Endothelial cells lining the microchannel lumen form a confluent endothelium across which soluble growth factors are exchanged with tumor cells in the gel. Microscopic particle image velocimetry ("-PIV) was integrated within the model to enable noninvasive optical measurement of velocity profiles and quantification of WSS, which were then correlated with angiogenic potential. Results demonstrate that endothelial permeability decreases as a function of increasing WSS, while co-culture with tumor cells increases permeability. This response is likely due to shear stress-mediated endothelial cell alignment and tumor-VEGF-induced permeability. In addition, high WSS (10 dyn/cm^2) significantly down-regulates tumor-expressed angiogenic factors, suggesting flow shear stress-mediates endothelial cross talk with surrounding tumor cells. Collectively, this research demonstrates the utility of the 3D in vitro microfluidic culture model as a versatile platform for elucidating the role of tumor-relevant hydrodynamic stress on cellular response.