IL-6 induces chondrocyte catabolism mainly via Stat3 signalling, a pathway activated in cartilage from joint subjected to DMM. Systemic blockade of IL-6 or STAT-3 can alleviate DMM-induced OA in mice.

Low oxygen tension (hypoxia) regulates chondrocyte differentiation and metabolism. Hypoxia-inducible factor 1α (HIF1α) is a crucial hypoxic factor for chondrocyte growth and survival during development. The major metalloproteinase matrix metalloproteinase 13 (MMP13) is also associated with chondrocyte hypertrophy in adult articular cartilage, the lack of which protects from cartilage degradation and osteoarthritis (OA) in mice. MMP13 is up-regulated by the Wnt/β-catenin signaling, a pathway involved in chondrocyte catabolism and OA. We studied the role of HIF1α in regulating Wnt signaling in cartilage and OA. We used mice with conditional knockout of Hif1α (ΔHif1α chon ) with joint instability. Specific loss of HIF1α exacerbated MMP13 expression and cartilage destruction. Analysis of Wnt signaling in hypoxic chondrocytes showed that HIF1α lowered transcription factor 4 (TCF4)-β-catenin transcriptional activity and inhibited MMP13 expression. Indeed, HIF1α interacting with β-catenin displaced TCF4 from MMP13 regulatory sequences. Finally, ΔHif1α chon mice with OA that were injected intraarticularly with PKF118-310, an inhibitor of TCF4-β-catenin interaction, showed less cartilage degradation and reduced MMP13 expression in cartilage. Therefore, HIF1α-β-catenin interaction is a negative regulator of Wnt signaling and MMP13 transcription, thus reducing catabolism in OA. Our study contributes to the understanding of the role of HIF1α in OA and highlights the HIF1α-β-catenin interaction, thus providing new insights into the impact of hypoxia in articular cartilage.hypoxia-inducible factor 1α | chondrocyte | osteoarthritis | Wnt signaling | matrix metalloprotease 13

Objective. Wnt signaling is a master regulator of joint homeostasis, but its role in osteoarthritis (OA) remains unclear. This study was undertaken to characterize the activation of Wnt/␤-catenin in knee joints of mice with OA and to assess how inhibiting this pathway in bone could affect cartilage.Methods. OA was induced by partial meniscectomy in Topgal mice and in transgenic mice overexpressing Dkk-1 under the control of the 2.3-kb Col1a1 promoter (Col1a1-Dkk-1-Tg mice). Wnt/␤-catenin activation was assessed by X-Gal staining at baseline and at weeks 4, 6, and 9. Cartilage and bone damage was analyzed in Col1a1-Dkk-1-Tg mice with OA at week 6. Primary chondrocytes and cartilage explants were used to assess the effect of Dkk-1 on cartilage catabolism.Results. In meniscectomized Topgal mice, Wnt was mainly activated in osteocytes from the subchondral bone at week 6 after OA induction, as well as in osteophytes and synovium at week 4. Chondrocytes from damaged zones expressed X-Gal from week 4. Dkk-1 expression was high in chondrocytes in control mouse knees (mean ؎ SEM 84.2 ؎ 3.1%) but decreased greatly in knees of meniscectomized mice from week 4 (mean ؎ SEM 14.4 ؎ 3.8%). The OA score was lower in meniscectomized Col1a1-Dkk-1-Tg mice at week 6 compared with wild-type mice (5.1 ؎ 0.6 versus 8.4 ؎ 0.6; P ‫؍‬ 0.002). Subchondral bone fraction and osteophyte volume were decreased. However, cartilage explants from Col1a1-Dkk-1-Tg mice showed proteoglycan loss and increased NITEGE expression. Expression of vascular endothelial growth factor (VEGF) was reduced in osteoblasts from Col1a1-Dkk-1-Tg mice, thereby decreasing expression of messenger RNA for matrix metalloproteinases in chondrocytes.Conclusion. Wnt activation in OA affects the whole joint, particularly bone. Selective inhibition of this pathway in bone by Dkk-1 decreased OA severity through VEGF inhibition.

IntroductionSclerostin is a Wnt inhibitor produced by osteocytes that regulates bone formation. Because bone tissue contributes to the development of osteoarthritis (OA), we investigated the role of sclerostin in bone and cartilage in a joint instability model in mice.MethodsTen-week-old SOST-knockout (SOST-KO) and wild-type (WT) mice underwent destabilization of the medial meniscus (DMM). We measured bone volume at the medial femoral condyle and osteophyte volume and determined the OA score and expression of matrix proteins. Primary murine chondrocytes were cultured with Wnt3a and sclerostin to assess the expression of matrix proteins, proteoglycan release and glycosaminoglycan accumulation.ResultsSclerostin was expressed in calcified cartilage of WT mice with OA. In SOST-KO mice, cartilage was preserved despite high bone volume. However, SOST-KO mice with DMM had a high OA score, with increased expression of aggrecanases and type X collagen. Moreover, SOST-KO mice with OA showed disrupted anabolic–catabolic balance and cartilage damage. In primary chondrocytes, sclerostin addition abolished Wnt3a-increased expression of a disintegrin and metalloproteinase with thrombospondin motifs, matrix metalloproteinases and type X collagen by inhibiting the canonical Wnt pathway. Moreover, sclerostin inhibited Wnt-phosphorylated c-Jun N-terminal kinase (JNK) and rescued the expression of anabolic genes. Furthermore, sclerostin treatment inhibited both Wnt canonical and non-canonical JNK pathways in chondrocytes, thus preserving metabolism.ConclusionSclerostin may play an important role in maintaining cartilage integrity in OA.Electronic supplementary materialThe online version of this article (doi:10.1186/s13075-015-0540-6) contains supplementary material, which is available to authorized users.

Sclerostin, mainly produced by osteocytes, is now considered a major regulator of bone formation. Identified from patients with a low bone mass, sclerostin inhibits the Wnt pathway by binding to LRP5/6 and subsequently increases bone formation. Sclerostin may also play a role in the mediation of systemic and local factors such as calcitriol, PTH, glucocorticoids and tumor necrosis factor-alpha. Circulating sclerostin levels increase with age and with the decline of kidney function. However, they are surprisingly higher in patients with a high bone mineral density, suggesting that sclerostin may be a relevant marker of the pool of mature osteocytes. The anti-anabolic properties lead to the development of anti-sclerostin biotherapies that are under current evaluation. The results of these clinical trials will open new promising opportunities for the treatment of osteoporosis and bone fragility fractures.

Cartilage loss in osteoarthritis (OA) results from altered local production of growth factors and metalloproteases (MMPs). Furin, an enzyme involved in the protein maturation of MMPs, might regulate chondrocyte function. Here, we tested the effect of furin on chondrocyte catabolism and the development of OA. In primary chondrocytes, furin reduced the expression of MMP-13, which was reversed by treatment with the furin inhibitor α1-PDX. Furin also promoted the activation of Smad3 signaling, whereas activin receptor-like kinase 5 (ALK5) knockdown mitigated the effects of furin on MMP-13 expression. Mice underwent destabilization of the medial meniscus (DMM) to induce OA, then received furin (1 U/mice), α1-PDX (14 µg/mice) or vehicle. In mice with DMM, the OA score was lower with furin than vehicle treatment (6.42 ± 0.75 vs 9.16 ± 0.6, p < 0.01), and the number of MMP-13(+) chondrocytes was lower (4.96 ± 0.60% vs 20.96 ± 8.49%, p < 0.05). Moreover, furin prevented the increase in ALK1/ALK5 ratio in cartilage induced by OA. Conversely, α1-PDX had no effect on OA cartilage structure. These results support a protective role for furin in OA by maintaining ALK5 receptor levels and reducing MMP-13 expression. Therefore, furin might be a potential target mediating the development of OA.

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