Caspase-dependent cleavage of cadherins and catenins during osteoblast apoptosis

As transmembrane, Ca²⁺-dependent cell-cell adhesion molecules, cadherins play a central role in tissue morphogenesis and homeostasis. Stable adhesion is dependent on interactions of the cytoplasmic domain of the cadherins with a group of intracellular proteins, the catenins. In the present study, we have detected the expression of α-,β-, and γ-catenins in human osteoblasts, which assemble with cadherins to form two distinct complexes containing cadherin and α-catenin, with either β- or γ-catenin. In osteoblasts undergoing apoptosis, proteolytic cleavage of N-cadherin and β- and γ- catenins but not α-catenin was associated with the activation of caspase-3 and prevented by the caspase inhibitor Z-VAD-fmk. The pattern of cadherin/catenin cleavage detected in apoptotic osteoblasts was reproduced in vitro by recombinant caspase-3. The presence of a 90-kDa extracellular domain fragment of N-cadherin in conditioned medium from apoptotic cells indicates that additional extracellular or membrane-associated proteases also are activated. Disruption of N-cadherin-mediated cell-cell adhesion with function-blocking antibodies induced osteoblast apoptosis, activation of caspases, and cleavage of β-catenin. These findings provide compelling evidence that N-cadherin-mediated cell-cell adhesion promotes osteoblast survival and suggest that the underlying mechanism may involve activation of β-catenin signaling.