PROTEIN-SYNTHESIS IS REQUIRED FOR CASPASE ACTIVATION AND INDUCTION OFAPOPTOSIS BY BISPHOSPHONATE DRUGS

The exact mechanisms of action of antiresorptive bisphosphonate drugs remain unclear, although they may inhibit bone resorption by mechanism s that can lead to osteoclast apoptosis. These drugs also cause apopto sis in J774 macrophages, probably as a consequence of inhibition of pr otein prenylation. However, the molecular pathways that lead to apopto sis are not known. In some cells, apoptosis induced by statins (other inhibitors of protein prenylation) is dependent on protein synthesis. The aim of this study was to further characterize the kinetics and bio chemical features of bisphosphonate-induced apoptosis, including the d ependence on protein synthesis. Alendronate-induced apoptosis in J774 cells occurred after similar to 16 hr of treatment, although shorter e xposures to the drug followed by incubation in bisphosphonate-free med ium also committed cells to apoptosis. The appearance of apoptotic cel ls was associated with the appearance of caspase-3-like activity. Apop tosis induced by bisphosphonate or mevastatin was found to be dependen t on protein synthesis because cycloheximide inhibited chromatin conde nsation, DNA fragmentation and activation of caspase-3-like protease o r proteases. Protein synthesis was required for events that lead to co mmitment to apoptosis but not for the execution phase because cyclohex imide did not prevent apoptosis when added greater than or equal to 15 hr after the start of alendronate treatment. Furthermore, staurospori ne-induced caspase-3-like activity and apoptosis in J774 cells could n ot be prevented by cycloheximide. These observations demonstrate that activation of caspase-3-like proteases and inhibition of commitment to apoptosis by cycloheximide are common features of apoptotic cell deat h induced by inhibitors of protein prenylation such as bisphosphonates .