Tyrosine phosphorylation of 40 kDa proteins in osteoblastic cells after mechanical stimulation of beta1-integrins

Using a method for the mechanical stimulation of cells which was adapted from one developed by Wang and Ingber employing magnetic microbeads [Wang, N. D., D. E. Ingber: Control of cytoskeletal mechanics by extracellular matrix, cell shape, and mechanical tension. Biophys. J. 66, 2181-2189 (1994)], mechanical stress could be applied to specific receptors on the cell surface. To achieve this, ferromagnetic microbeads coated with different ligands were magnetized after adhesion to the cells. The beads were then 'twisted' using a second magnetic field oriented perpendicular to the magnetizing one. Contrary to most current methods, it was possible to confer the strain without deforming the cell as a whole, thus being able to observe the individual reactions of transmembrane receptors to mechanical stress. An increase in tyrosine phosphorylation of proteins migrating at approximately 40 kDa could be observed as a reaction to stress on the beta1-subunits of the integrin family, while stress to other transmembrane molecules like the transferrin or low density lipoprotein receptors with no connection to the cytoskeleton did not give this reaction. Fibroblastic cells showed, contrary to osteoblastic cells, no reaction to stress applied on transmembrane proteins.