Bisphosphonates are currently the most important class of
antiresorptive drugs used for the treatment of
metabolic bone diseases. Although the molecular targets of
bisphosphonates have not been identified, these compounds inhibit
bone resorption by mechanisms that can lead to osteoclast apoptosis.
Bisphosphonates also induce apoptosis in mouse J774 macrophages in vitro, probably by the same mechanisms that lead to osteoclast apoptosis. We have found that, in J774 macrophages,
nitrogen-containing
bisphosphonates (such as
alendronate,
ibandronate, and
risedronate) inhibit post-translational modification (prenylation) of
proteins, including the
GTP-binding protein Ras, with farnesyl or geranylgeranyl
isoprenoid groups.
Clodronate did not inhibit protein prenylation.
Mevastatin, an inhibitor of 3-hydroxy-3-methylglutatyl (
HMG)-CoA reductase and hence the biosynthetic pathway required for the production of
farnesyl pyrophosphate and
geranylgeranyl pyrophosphate, also caused apoptosis in J774 macrophages and murine osteoclasts in vitro. Furthermore,
alendronate-induced apoptosis, like
mevastatin-induced apoptosis, could be suppressed in J774 cells by the addition of
farnesyl pyrophosphate or
geranylgeranyl pyrophosphate, while the effect of
alendronate on osteoclast number and
bone resorption in murine calvariae in vitro could be overcome by the addition of
mevalonic acid. These observations suggest that
nitrogen-containing
bisphosphonate drugs cause apoptosis following inhibition of post-translational prenylation of
proteins such as Ras. It is likely that these potent antiresorptive
bisphosphonates also inhibit
bone resorption by preventing protein prenylation in osteoclasts and that
enzymes of the
mevalonate pathway or
prenyl protein transferases are the molecular targets of the
nitrogen-containing
bisphosphonates. Furthermore, the data support the view that
clodronate acts by a different mechanism.