Current evidence indicates that tumor cell adhesion to the microvasculature in host organs during formation of distant metastases is a complex process involving various types of cell adhesion molecules. Recent results have shown that stabilization of tumor cell adhesion to the microvascular vessel wall is a very important step for successful tumor cell migration and colonization of host organs. We are beginning to understand the influences of fluid flow and local shear forces on these adhesive interactions and cellular responses within the circulation. Mechanosensory molecules or molecular complexes can transform shear forces acting on circulating tumor cells into intracellular signals and modulate cell signaling pathways, gene expression and other cellular functions. Flowing tumor cells can interact with microvascular endothelial cells mediated mainly by selectins, but the strength of these bonds is relatively low and not sufficient for stable cell adhesions. Integrin-mediated tumor cell adhesion and changes in the binding affinity of these adhesion molecules appear to be required for stabilized tumor cell adhesion and subsequent cell migration into the host organ. Failure of the conformational affinity switch in integrins results in breaking of these bonds and recirculation or mechanical damage of the tumor cells. Various cell signaling molecules, such as focal adhesion kinase, pp60src or paxillin, and cytoskeletal components, such as actin or microtubules, appear to be required for tumor cell adhesion and its stabilization under hydrodynamic conditions of fluid flow.
