HUMAN OSTEOCLAST-LIKE CELLS SELECTIVELY RECOGNIZE LAMININ ISOFORMS, AN EVENT THAT INDUCES MIGRATION AND ACTIVATES CA2+ MEDIATED SIGNALS

Osteoclast precursors are chemotactically attracted to sites of bone r esorption via migration pathways that include transendothelial crossin g in blood capillaries. Transendothelial migration involves poorly und erstood interactions with basal lamina molecules, including laminins. To investigate osteoclast-laminin interactions, we used human osteocla st-like cell lines obtained from giant cell tumors of bone (GCT 23 and GCT 24). These cell lines are a well-characterized model for osteocla st functions, such as bone resorption and the behaviour of osteoclast precursors. Both GCT cell lines adhered to laminin-2 (merosin) coated wells in standard adhesion assays, but failed to adhere to laminin-1 ( EHS-laminin). By light microscopy, GCT cells on laminin-2 were partial ly spread, with a motile morphology. None of the anti-integrin antibod ies tested inhibited GCT cells adhesion to laminin-2. Peptides contain ing the integrin adhesion site RGD or the laminin adhesion sequence IK VAV did not inhibit GCT cell adhesion to laminin-2. By immunofluoresce nce, beta(1) integrins were organized in focal adhesions. However, in the presence of monensin this reorganization of beta(1) integrins was abolished, indicating that it was probably due to secretion of fibrone ctin by GCT cells subsequent to adhesion to laminin-2. GCT cells trans migrated through membranes coated with laminin-2, much more efficientl y than through membranes coated with collagen. Migration was induced b y osteocalcin, as a chemoattractant, in a dose-dependent manner. At lo w osteocalcin concentrations, transmigration was detectable on laminin -2 but not collagen. In cells loaded with fura-2, a sharp increase in intracellular Ca2+ was detected upon addition of soluble laminin-2, bu t not laminin-1, due to release from thapsigargin-dependent intracellu lar stores. In summary, osteoclasts may recognize laminin isoforms dif ferentially. Initial adhesion to laminin-2 appears to be due to integr in-independent mechanisms. Such adhesion, though, may trigger secretio n of fibronectin that could then support spreading and efficient chemo tactic migration. These mechanisms may play an important role in facil itating chemotactic migration of osteoclast precursors toward the bone surface.