Osteoarthritis
Figure 6 Inhibition of MKK7–JNK signalling delays chondrocyte senescence and OA development. (A) Representative images and quantification of SA- β Gal staining and immunofluorescence staining of p-JNK in primary chondrocytes from Fbxw7 KO mice and controls after elongation strain loading for 24 hours with or without DTP3 treatment. n=5 per group. Scale bar: 50 µm. (B) Representative images and quantification of safranin O/fast green staining and immunofluorescence staining of p-JNK, p16 INK4a , p21 and TUNEL in chondrocytes of DTP3-treated mice (intraperitoneal injection, 10 mg/ kg, every other day) at 4 weeks post-DMM surgery. n=10 per group. Scale bars: 100 µm (first row) and 50 µm (the rest of the rows). (C) Safranin O/ fast green staining and immunofluorescence staining of p-JNK of joints from DTP3-treated Fbxw7 KO mice at 4 weeks post-DMM surgery. Quantitative analysis of the OARSI score and p-JNK-positive chondrocytes are shown on the right. n=5 per group. Scale bars: 100 µm (first row) and 50 µm (second row). (D) Schematic diagram representing molecular pathways in which excessive mechanical loading induces OA development through FBXW7. Data are shown as mean±SD. Statistical analyses were conducted using two-way analysis of variance followed by Sidak’s multiple comparison test (A), Student’s t-test (B and C), or non-parametric Mann-Whitney U tests (OARSI score) (B,C). Boxed area is enlarged in the bottom right corner. *P<0.05, **P<0.01. DMM, destabilisation of the medial meniscus; FBXW7, F-box and WD repeat domain containing 7; OA, osteoarthritis; OARSI, Osteoarthritis Research Society International; TUNEL, terminal deoxynucleotidyl transferase dUTP nick end labelling.
cartilage chondrocytes, which represents a novel mechanism for chondrocyte senescence during OA. To identify FBXW7 downstream signalling during chondro- cyte senescence and OA development, we screened out MKK7, and one MAPK kinase was shown to activate JNK. 43 MAPKs, including the JNK and p38MAPK signalling pathways, have been suggested to be extensively involved in OA. 44 JNK signalling can be phosphorylated by activating MKK4 or MKK7, while MKK4 can activate p38MAPK and JNK, whereas MKK7 is specifically involved in only JNK activation. 45–47 We found that MKK7, but not MKK4, participated in mechanical overloading-induced JNK activation. Although JNK signalling plays a key role in cell proliferation, differentiation and apoptosis in response to stress,
the present study was that FBXW7 was markedly downregu- lated in chondrocytes by excessive mechanical loading both in vitro and in vivo. FBXW7 deletion in chondrocytes resulted in the senescence and degeneration of articular cartilage and exac- erbation of OA. In addition, we detected senescent chondro- cytes in articular cartilage of patients with OA and DMM OA mice, which were enhanced by FBXW7 deletion and reversed by FBXW7 overexpression. However, no significant changes in chondrocyte senescence were observed between Fbxw7 KO mice and controls at a young age, indicating that FBXW7 defi- ciency alone was not sufficient to induce cell senescence, unless under the context of mechanical loading or old age. Therefore, we propose that mechanical overloading reduces FBXW7 in
Zhang H, et al . Ann Rheum Dis 2022; 81 :676–686. doi:10.1136/annrheumdis-2021-221513
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