Credit: Images courtesy of G. R. Heiland

In healthy joints, the balance between bone resorption (mediated by osteoclasts) and bone formation (mediated by osteoblasts) is maintained; however, during inflammation, such as occurs in chronic arthritis, disturbance of this homeostasis tips the balance towards increased bone resorption. “Systemic bone loss in patients with rheumatoid arthritis causes fractures and high morbidity,” explains Jochen Zwerina, from the University of Erlangen-Nuremberg, Germany, “but the reasons for bone loss during chronic arthritis are not well understood.” A paper from Zwerina's group, published in the Annals of the Rheumatic Diseases, sheds some light on the pathophysiology of bone loss in this disease, and shows that the Wnt inhibitor Dickkopf-1 (Dkk-1) might be a therapeutic target for mitigating systemic bone loss in inflammatory arthritis.

Proinflammatory cytokines, including tumor necrosis factor (TNF) and interleukin (IL)-1, have been shown to directly activate osteoclasts and also to block bone formation. As Zwerina says, “Prior to this study, we showed that TNF and IL-1 are potent mediators of systemic bone loss in animal models of arthritis, and that the Wnt signalling pathway might be important for TNF-mediated bone loss.”

Here, the authors investigated the effects of blocking the function of Dkk-1 in TNF transgenic (hTNFtg) mice. These mice, which overexpress TNF, develop generalized osteopenia and inflammatory arthritis. Heterozygous hTNFtg mice and their wild-type littermates were treated for 4 weeks with vehicle alone (placebo), anti-TNF antibodies, anti-Dkk-1 antibodies, or anti-TNF and anti-Dkk-1 (aDkk-1) antibodies, and then histomorphometric and histological analyses were performed. As expected, trabecular bone mass was lower in the transgenic mice than the wild-type mice. Treatment of the hTNFtg mice with aDkk-1 antibodies led to normalization of bone mass, resulting from increases in the number and thickness of bony trabeculae as well as normalization of mineral apposition rate.

In this setting, therefore, Dkk-1 blockade seems to stop TNF-induced bone loss, but does this result from enhanced bone formation or interference with bone resorption? Additional analyses showed that the areas of bone covered with osteoblasts were decreased and the areas of bone covered with osteoclasts were increased in hTNFtg mice in comparison with wild-type controls. Treatment with aDkk-1 antibodies resulted in normalization of the numbers of these cells per bone surface area in the transgenic mice. In addition, blockade of Dkk-1 led to increased levels of β-catenin protein and osteoprotegerin (an inhibitor of osteoclast formation) and osteocalcin (a bone-building molecule) messenger RNA, suggesting that this treatment leads to increased bone formation and to reduced bone resorption.

Next, the researchers performed primary osteoblast culture experiments, using osteoblasts isolated from wild-type mice, to further dissect the signaling pathways downstream of Dkk-1. TNF stimulation of osteoblasts resulted in increased levels of Dkk-1, which were followed by increased levels of sclerostin (SOST), another Wnt-antagonist. Blockade of Dkk-1 in these cultures interfered with TNF-induced upregulation of SOST, and also appeared to enhance osteoblastogenesis by blocking TNF-induced downregulation of alkaline phosphatase activity. Addition of recombinant Dkk-1 protein resulted in increased levels of SOST. On the basis of these experiments, the authors concluded that Dkk-1 might have a role in osteoblast differentiation by directly controlling the expression of SOST. This idea was also supported by the results of in vivo experiments: osteocyte SOST mRNA and protein levels, and levels of osteocyte cell death, were higher in hTNFtg mice than in littermate controls; blockade of Dkk-1 led to fewer osteocytes dying and to reduced levels of SOST in these cells. Zwerina explains, “We found that TNF potently downregulates bone formation and upregulates expression of the Wnt antagonists Dkk-1 and SOST. Dkk-1 blockade protects against bone loss, despite ongoing inflammation, and so we conclude that Dkk-1 blockade may be considered as a treatment approach for inflammatory bone loss.” He adds that, “Our future work will focus on the mechanisms of impaired bone formation during arthritis.”