Protein geranylgeranylation is required for osteoclast formation, function, and survival: Inhibition by bisphosphonates and GGTI-298

Bisphosphonates are the important class of antiresorptive drugs used in the treatment of metabolic bone diseases. Although their molecular mechanism o f action has not been fully elucidated, recent studies have shown that the nitrogen-containing bisphosphonates can inhibit protein prenylation in macr ophages in vitro. In this study, we show that the nitrogen-containing bisph osphonates risedronate, zoledronate, ibandronate, alendronate, and pamidron ate (but not the neon nitrogen-containing bisphosphonates clodronate, etidr onate, and tiludronate) prevent the incorporation of [C-14]mevalonate into prenylated (farnesylated and geranylgeranylated) proteins in purified rabbi t osteoclasts, The inhibitory effect of nitrogen-containing bisphosphonates on bone resorption is likely to result largely fi om the loss of geranylge ranylated proteins rather than loss of farnesylated proteins in osteoclasts , because concentrations of GGTI-298 (a specific inhibitor of geranylgerany l transferase I) that inhibited protein geranylgeranylation in purified rab bit osteoclasts prevented osteoclast formation in murine bone marrow cultur es, disrupted the osteoclast cytoskeleton, inhibited bone resorption, and i nduced apoptosis in isolated chick and rabbit osteoclasts in vitro. By cont rast, concentrations of FTI-277 (a specific inhibitor of farnesyl transfera se) that prevented protein farnesylation in purified rabbit osteoclasts had little effect on osteoclast morphology or apoptosis and did Mot inhibit bo ne resorption, These results therefore show the molecular mechanism of acti on of nitrogen-containing bisphosphonate drugs in osteoclasts and highlight the fundamental importance of geranylgeranylated proteins in osteoclast fo rmation and function.