INHIBITION OF AVIAN OSTEOCLAST BONE-RESORPTION BY MONOCLONAL-ANTIBODY121F - A MECHANISM INVOLVING THE OSTEOCLAST FREE-RADICAL SYSTEM

Osteoclasts generate high levels of superoxide anions during bone reso rption that contribute to the degradative process, although excessive levels of this free radical may be damaging. One mechanism for their r emoval is via superoxide dismutase (SOD), a protective superoxide scav enging enzyme. We have previously described a novel developmentally re gulated 150 kDa plasma membrane glycoprotein of avian osteoclasts whic h is reactive with the osteoclast-specific monoclonal antibody (Mab) 1 21F and is related immunologically, biochemically, and in protein sequ ence to mitochondrial Mn2+/Fe2+ SOD. We hypothesized that this unusual osteoclast surface component may be involved in protection against su peroxides generated during active bone resorption. Increasing concentr ations of monovalent Fab fragments prepared hom Mab 121F, but not thos e from another antiosteoclast Mab designated 29C, markedly inhibited b oth bone particle and bone pit resorption by avian osteoclasts, while reducing tartrate-resistant acid phosphatase activity and causing the morphological contraction of osteoclasts on bone. Thus, the SOD-relate d membrane antigen may be essential for osteoclast bone resorption. Os teoclast superoxide production, monitored kinetically by cytochrome c reduction and histochemically by nitroblue tetrazolium reduction stain ing, was significantly greater in the presence of 121F, but not 29C, F ab treatment. Furthermore, the release of another free radical known a s nitric oxide, which is produced by osteoclasts, can scavenge superox ides, and acts to potently inhibit osteoclast bone resorption, was dos e-dependently increased by 121F Fab in resorbing osteoclast cultures. Therefore, Mab 121F binding may block the potential protective functio n of the osteoclast plasma membrane SOD-related glycoprotein, leading to a rapid elevation of superoxide levels and a subsequent rise in ost eoclast nitric oxide release, feedback messages which may be sensed by the osteoclast as signals to cease active bone resorption.