Nitrogen-containing bisphosphonates were shown to cause macrophage apoptosis by inhibiting enzymes in the biosynthetic pathway leading from mevalonate to cholesterol. This study suggests that, in osteoclasts, geranylgeranyl diphosphate, the substrate for prenylation of most GTP binding proteins, is likely to be the crucial intermediate affected by these bisphosphonates. We report that murine osteoclast formation in culture is inhibited by both lovastatin, an inhibitor of hydroxymethylglutaryl CoA reductase, and alendronate. Lovastatin effects are blocked fully by mevalonate and less effectively by geranylgeraniol whereas alendronate effects are blocked partially by mevalonate and more effectively by geranylgeraniol. Alendronate inhibition of bone resorption in mouse calvaria also is blocked by mevalonate whereas clodronate inhibition is not. Furthermore, rabbit osteoclast formation and activity also are inhibited by lovastatin and alendronate. The lovastatin effects are prevented by mevalonate or geranylgeraniol, and alendronate effects are prevented by geranylgeraniol. Farnesol and squalene are without effect. Signaling studies show that lovastatin and alendronate activate in purified osteoclasts a 34-kDa kinase. Lovastatin-mediated activation is blocked by mevalonate and geranylgeraniol whereas alendronate activation is blocked by geranylgeraniol. Together, these findings support the hypothesis that alendronate, acting directly on osteoclasts, inhibits a rate-limiting step in the cholesterol biosynthesis pathway, essential for osteoclast function. This inhibition is prevented by exogenous geranylgeraniol, probably required for prenylation of GTP binding proteins that control cytoskeletal reorganization, vesicular fusion, and apoptosis, processes involved in osteoclast activation and survival.

There is a close interaction between the processes involved in osteogenesis and hemopoiesis. In developing bone, the osteoclasts, cells of hemopoietic origin, resorb and invade the calcified cartilage rudiment. As a result, the primitive marrow cavity is formed and hemopoiesis initiates. Osteogenic cells—osteoblasts and osteocytes—control the development and activity of the osteoclasts through the local release of factors. One factor responsible for this osteoblast‐osteoclast interaction is colony‐stimulating factor‐1 (CSF‐1). Studies performed on the osteopetrotic op/op mouse mutant have established that this factor is essential for proliferation and differentiation of the osteoclasts. Expression of CSF‐1 receptors by mature osteoclasts and osteoclast precursors strongly suggests that CSF‐1 action is exerted directly on cells of this lineage. In vivo, CSF‐1 synthesis by osteoblasts is temporally and spatially related to sites of osteoclast development. Thus CSF‐1 may represent one of the factors responsible for coupling hemopoiesis to osteogenesis. In vitro, osteoblasts express at least 4 transcripts encoding either a secreted or a membrane‐bound form of CSF‐1. At the protein level, osteoblasts in vitro synthesize the membrane‐bound form and secrete the majority of CSF‐1 as a proteoglycan, a small fraction of which is integrated into the matrix. These different molecular forms may locally restrict the biological action of this cytokine. Indeed, injection of recombinant human CSF‐1 in op/op mutants does not correct the osteoclast deficiency in the metaphyseal spongiosa of long bones, and sclerosis persists at this site. Similarly, the deficiency of some tissue macrophage populations in op/op mice is only partially or not at all corrected by injection of CSF‐1. The expression of CSF‐1 receptors by mature osteoclasts may imply that CSF‐1 also influences their bone resorbing activity. Indeed, CSF‐1 has been shown to induce osteoclast fusion, spreading, and survival. These findings suggest that CSF‐1 is essential for the proliferation, differentiation, activity, and survival of tissue macrophages and osteoclasts, cells involved in tissue turnover. Furthermore, they corroborate the view that both osteoclasts and tissue macrophages stem from a CSF‐1‐dependent common precursor along the macrophage lineage. Mol Reprod Dev 46:75–84, 1997. © 1997 Wiley‐Liss, Inc.