Bisphosphonates are the most widely used class of drug for inhibiting osteoclast-mediated bone loss, but their effectiveness at preventing joint destruction in rheumatoid arthritis has generally been disappointing. We examined whether the ability of bisphosphonates to induce osteoclast apoptosis and inhibit bone resorption in vitro is influenced by the cytokine receptor activator of nuclear factor-kappa B ligand (RANKL), an important mediator of inflammation-induced bone loss.
Rabbit osteoclasts were treated with the bisphosphonates clodronate or alendronate for up to 48 hours in the absence or presence of RANKL. Changes in cell morphology and induction of apoptosis were examined by scanning electron microscopy, whilst resorptive activity was determined by measuring the area of resorption cavities. Changes in the level of anti-apoptotic proteins, including Mcl-1, Bcl-2, and Bcl-x>L, were determined in rabbit osteoclasts and in cytokine-starved mouse osteoclasts by Western blotting.
RANKL significantly attenuated the ability of both clodronate and alendronate to induce osteoclast apoptosis and inhibit bone resorption. Treatment of rabbit osteoclasts with RANKL was associated with an increase in the anti-apoptotic protein Mcl-1 but not Bcl-2. A role for Mcl-1 in osteoclast survival was suggested using osteoclasts generated from mouse bone marrow macrophages in the presence of RANKL + macrophage colony-stimulating factor (M-CSF) since cytokine deprivation of mouse osteoclasts caused a rapid loss of Mcl-1 (but not Bcl-2 or Bcl-xL), which preceded the biochemical and morphological changes associated with apoptosis. Loss of Mcl-1 from mouse osteoclasts could be prevented by factors known to promote osteoclast survival (RANKL, M-CSF, tumour necrosis factor-alpha [TNF-alpha], or lipopolysaccharide [LPS]).
RANKL protects osteoclasts from the apoptosis-inducing and anti-resorptive effects of bisphosphonates in vitro. The ability of RANKL (and other pro-inflammatory factors such as TNF-alpha and LPS) to increase the level of Mcl-1 in osteoclasts may explain the lack of effectiveness of some bisphosphonates in preventing inflammation-induced bone loss.