Unoccupied αvβ3 integrin regulates osteoclast apoptosis by transmitting a positive death signal

Cell/matrix detachment is a general inducer of programmed cell death, an event mediated by loss of integrin/ligand association. Because alpha(v)beta(3) is the major integrin expressed by the osteoclast, we asked whether its occupancy promotes survival of the resorptive cell. Thus, we generated wild-type preosteoclasts and placed them on selective matrix proteins. Consistent with the posture that alpha(v)beta(3) occupancy promotes survival, preosteoclasts plated on native collagen, a matrix not recognized by the integrin, undergo apoptosis 4-fold faster than those on the alpha(v)beta(3) ligand, vitronectin. To further explore the role of alpha(v)beta(3) in osteoclast apoptosis, wild-type and beta(3)(-/-) preosteoclasts were suspended and apoptosis determined, with time. beta(3)(-/-) preosteoclasts, in suspension, undergo a rate of apoptosis only 40 - 60% of that of their wild-type counterparts, indicating that unoccupied alpha(v)beta(3) transmits a positive death signal that we find regulated by caspase-8. Attesting to specificity of the unoccupied integrin-transmitted death signal, apoptosis in the absence of alpha(v)beta(3) is mediated by capsase-9. We have shown that the resorptive defect of beta(3)(-/-) osteoclasts is rescued by wild-type beta(3) cDNA but not by one bearing a (SP)-P-752 mutation. To determine whether the same holds true regarding osteoclast apoptosis, we constructed lentivirus vectors encoding green fluorescent protein, wild- type beta(3), or beta(3)(S752P). Once again, native beta(3)(-/-) preosteoclasts were protected against apoptosis. Similar to its effect on bone resorption, transduced wild- type beta(3) normalizes the apoptotic rate of beta(3)(-/-) preosteoclasts. Unexpectedly, however, beta(3)(S752P) transductants also die at a rate indistinguishable from wild type. Thus, unoccupied alpha(v)beta(3) integrin regulates osteoclast apoptosis via a component of the integrin that is different than that regulating resorption.