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a b s t r a c tAlthough anti-tumor necrosis factor (TNF)-a treatments efficiently block inflammation in ankylosing spondylitis (AS), they are inefficient to prevent excessive bone formation. In AS, ossification seems more prone to develop in sites where inflammation has resolved following anti-TNF therapy, suggesting that TNF-a indirectly stimulates ossification. In this context, our objectives were to determine and compare the involvement of Wnt proteins, which are potent growth factors of bone formation, in the effects of TNF-a on osteoblast function. In human mesenchymal stem cells (MSCs), TNF-a significantly increased the levels of Wnt10b and Wnt5a. Associated with this effect, TNF-a stimulated tissue-non specific alkaline phosphatase (TNAP) and mineralization. This effect was mimicked by activation of the canonical bcatenin pathway with either anti-Dkk1 antibodies, lithium chloride (LiCl) or SB216763. TNF-a reduced, and activation of b-catenin had little effect on expression of osteocalcin, a late marker of osteoblast differentiation. Surprisingly, TNF-a failed to stabilize b-catenin and Dkk1 did not inhibit TNF-a effects. In fact, Dkk1 expression was also enhanced in response to TNF-a, perhaps explaining why canonical signaling by Wnt10b was not activated by TNF-a. However, we found that Wnt5a also stimulated TNAP in MSCs cultured in osteogenic conditions, and increased the levels of inflammatory markers such as COX-2. Interestingly, treatment with anti-Wnt5a antibodies reduced endogenous TNAP expression and activity. Collectively, these data suggest that increased levels of Dkk1 may blunt the autocrine effects of Wnt10b, but not that of Wnt5a, acting through non-canonical signaling. Thus, Wnt5a may be potentially involved in the effects of inflammation on bone formation.

Tumor necrosis factor (TNF)-a and interleukin (IL)-1b stimulate tissue non-specific alkaline phosphatase (TNAP) activity and mineralization in cultures of vascular smooth muscle cells (VSMCs). They are, therefore, considered as stimulators of vascular calcification in the context of atherosclerosis and diabetes type 2. In contrast, although ankylosing spondylitis (AS) leads to the formation of syndesmophytes, which are ectopic ossifications from entheses (where ligaments, tendons and capsules are attached to bone), anti-TNF-a therapies fail to block bone formation in this disease. In this context, our aims were to compare the effects of TNF-a and IL-1b on TNAP activity and mineralization in entheseal cells and VSMCs. Organotypic cultures of mouse ankle entheses were treated or not with TNF-a and IL-1b for 5 days. Micro-computed tomography was performed to determine trabecular bone parameters, and histology to assess TNAP activity and mineralization. Human mesenchymal stem cells cultured in pellets in chondrogenic conditions and human VSMCs were also used to determine the effects of cytokines on TNAP activity and expression, measured by quantitative PCR. In organotypic cultures, TNF-a and IL-1b significantly reduced the tibia BV/TV ratio. They also inhibited TNAP activity in entheseal chondrocytes in situ, and in mouse and human chondrocytes in vitro. In contrast, TNF-a stimulated TNAP expression and activity in human VSMCs. These differences were likely due to cell-specific effects of peroxisome proliferator-activated receptor g (PPARg), which is inhibited by TNF-a. Indeed, in human chondrocytes and VSMCs, the PPARg inhibitor GW-9662 displayed the same opposite effects as TNF-a on TNAP expression. In conclusion, whereas TNF-a and IL-1b stimulate TNAP activity in VSMCs, they inhibit it in entheseal cells in situ and on chondrocytes in vitro. The identification of PPARg as a likely mediator of cytokine effects deserves consideration for future research on the mechanisms of ectopic ossification.

1 IL-33 is expressed in human osteoblasts, but has no direct effect on bone remodelingsuggested that expression of IL-33, in contrast to that of IL-1β, is not repressed by PPARγ, likely explaining why IL-33, but not IL-1β, is expressed in adipocytes. The IL-33 receptor ST2L is not constitutively expressed in human bone marrow stromal cells, osteoblasts or CD14-positive monocytes, and IL-33 has no effect on these cells. In addition, although ST2L mRNA is induced by TNF-α and IL-1β in bone marrow stromal cells, IL-33 has the same effects as TNF-α and IL-1β, and, therefore, the biological activity of IL-33 may be redundant in this system. In agreement with this hypothesis, MC3T3-E1 osteoblast-like cells constitutively express ST2L mRNA, and in these cells IL-33 and TNF-α/IL-1β similarly decrease osteocalcin RNA levels in these cells. In conclusion, our results suggest that IL-33 has no direct effects on normal bone remodeling.

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