Tumor necrosis factor prevents alendronate‐induced osteoclast apoptosis in vivo by stimulating Bcl‐xL expression through Ets‐2

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Abstract
Objective To investigate why bisphosphonates are less effective at preventing focal bone loss in rheumatoid arthritis (RA) patients than in those with generalized osteoporosis, and the mechanisms involved. Methods The response of osteoclasts to alendronate (ALN) in tumor necrosis factor–transgenic (TNF‐Tg) mice that develop erosive arthritis and in wild‐type littermates was studied. TNF‐Tg and wild‐type mice were given ALN, and the osteoclast numbers in the inflamed joints and in the long bones were compared. The expression levels of Bcl‐xL in the osteoclasts of TNF‐Tg and wild‐type mice were examined by immunostaining. The effect of overexpression of Bcl‐xL and Ets‐2 proteins on ALN‐induced osteoclast apoptosis was determined using an in vitro osteoclast survival assay and retrovirus transfer approach. Results ALN reduced osteoclast numbers in the metaphyses by 97%, but by only 46% in the adjacent inflamed joints. Bcl‐xL expression was markedly higher in osteoclasts in the joints than in those in the metaphyses of TNF‐Tg mice. Bcl‐xL or Ets‐2 overexpression protected osteoclasts from ALN‐induced apoptosis, and TNF stimulated Bcl‐xL and Ets‐2 expression in osteoclasts. Overexpression of Ets‐2 increased Bcl‐xL messenger RNA in osteoclasts, while a dominant‐negative form of the Ets‐2 blocked the protective effect of Bcl‐xL or TNF on ALN‐induced apoptosis. Conclusion The reduced efficacy of bisphosphonates to stop bone erosion in the inflamed joints of RA patients may result from local high levels of TNF up‐regulating Ets‐2 expression in osteoclasts, which in turn stimulates Bcl‐xL expression in them and reduces their susceptibility to bisphosphonate‐induced apoptosis.