These findings indicate that low bone formation and expression of Dkk-1 trigger inflammatory bone loss. Dkk-1 blocks osteoblast differentiation, induces sclerostin expression and leads to osteocyte death. Inhibition of Dkk-1 may thus be considered as a potent strategy to protect bone from inflammatory damage.

Mice deficient in epidermal growth factor receptor (Egfr−/− mice) are growth retarded and exhibit severe bone defects that are poorly understood. Here we show that EGFR-deficient mice are osteopenic and display impaired endochondral and intramembranous ossification resulting in irregular mineralization of their bones. This phenotype is recapitulated in mice lacking EGFR exclusively in osteoblasts, but not in mice lacking EGFR in osteoclasts indicating that osteoblasts are responsible for the bone phenotype. Experiments are presented demonstrating that signaling via EGFR stimulates osteoblast proliferation and inhibits their differentiation by suppression of the IGF-1R/mTOR-pathway via ERK1/2-dependent up-regulation of IGFBP-3. Osteoblasts from Egfr−/− mice show increased levels of IGF-1R and hyperactivation of mTOR-pathway proteins, including enhanced phosphorylation of 4E-BP1 and S6. The same changes are also seen in Egfr−/− bones. Importantly, pharmacological inhibition of mTOR with rapamycin decreases osteoblasts differentiation as well as rescues the low bone mass phenotype of Egfr−/− fetuses. Our results demonstrate that suppression of the IGF-1R/mTOR-pathway by EGFR/ERK/IGFBP-3 signaling is necessary for balanced osteoblast maturation providing a mechanism for the skeletal phenotype observed in EGFR-deficient mice.

Objective Reduced vitamin D intake has been linked to increased susceptibility to develop rheumatoid arthritis (RA) and vitamin D defi ciency is associated with increased disease activity in RA patients. The pathophysiological role of vitamin D in joint infl ammation is, however, unclear. Methods To determine the infl uence of absent vitamin D signalling in chronic arthritis, vitamin D receptor (VDR)-defi cient mice were crossed with human tumour necrosis factor (TNF) transgenic mice (hTNFtg), which spontaneously develop chronic arthritis.

Objective Tumor necrosis factor (TNF) drives bone destruction, but it also inhibits new bone formation by inducing Dkk‐1, an inhibitor of the Wnt pathway. Accordingly, blocking of Dkk‐1 reverses the phenotype in experimental arthritis from a pattern of bone destruction to a pattern of bone formation. To delineate the potential role of Dkk‐1 in the structural phenotype of human arthritis, we analyzed the expression of Dkk‐1 and its regulation by proinflammatory cytokines in the inflamed peripheral joints of patients with spondyloarthritis (SpA) and rheumatoid arthritis (RA). Methods Expression of Dkk‐1 and proinflammatory cytokines was determined by enzyme‐linked immunosorbent assay and microarray analysis in synovial fluid (SF) and synovial tissue, respectively. Regulation of Dkk‐1 production by proinflammatory cytokines was assessed in fibroblast‐like synoviocyte (FLS) cultures. Results TNF and interleukin‐1β (IL‐1β) levels, were higher in RA SF than in SpA SF (P < 0.001 for both), whereas levels of IL‐6 were not. Levels of Dkk‐1 were similar in SpA SF and RA SF and were not correlated with TNF and IL‐1β levels. However, Dkk‐1 levels showed an inverse correlation with IL‐6 levels in both SpA SF (r = −0.31, P = 0.04) and RA SF (r = −0.39, P = 0.01); this result was reproduced at the messenger RNA level in synovial tissue. In vitro experiments with FLS confirmed that Dkk‐1 production was strongly induced by TNF but clearly suppressed by IL‐6. Moreover, IL‐6 was able to suppress the TNF‐induced up‐regulation of Dkk‐1 production by FLS. Conclusion The inverse correlation of Dkk‐1 levels with IL‐6 levels observed in vivo in the inflamed joints was mirrored by the differential regulation of Dkk‐1 production by TNF and IL‐6 in vitro. The relative balance between these and other factors in the arthritic joints may determine functional Wnt signaling and tissue remodeling.