Distinct kinetic and mechanical properties govern mucin 16- and podocalyxin-mediated tumor cell adhesion to E- and L-selectin in shear flow.

Selectin-mediated tumor cell tethering to host cells, such as vascular endothelial cells, is a critical step in the process of cancer metastasis. We recently identified sialofucosylated mucin16 (MUC16) and podocalyxin (PODXL) as the major functional E- and L-selectin ligands expressed on the surface of metastatic pancreatic cancer cells. While the biophysics of leukocyte binding to selectins has been well studied, little is known about the mechanics of selectin-mediated adhesion pertinent to cancer metastasis. We thus sought to evaluate the critical parameters of selectin-mediated pancreatic tumor cell tethering and rolling. Using force spectroscopy, we characterized the binding interactions of MUC16 and PODXL to E- and L-selectin at the single-molecule level. To further analyze the response of these molecular interactions under physiologically relevant regimes, we used a microfluidic assay in conjunction with a mathematical model to study the biophysics of selectin-ligand binding as a function of fluid shear stress. We demonstrate that both MUC16 and PODXL-E-selectin-mediated interactions are mechanically stronger than like L-selectin interactions at the single-molecule level, and display a higher binding frequency at all contact times. The single-molecule kinetic and micromechanical properties of selectin-ligand bonds, along with the number of receptor-ligand bonds needed to initiate tethering, regulate the average velocity of ligand-coated microspheres rolling on selectin-coated surfaces in shear flow. Understanding the biophysics of selectin-ligand bonds and their responses to physiologically relevant shear stresses is vital for developing diagnostic assays and/or preventing the metastatic spread of tumor cells by interfering with selectin-mediated adhesion.