Abstract
IntroductionEvents normally taking place in the terminal chondrocyte differentiation in the growth plate are also observed during osteoarthritis (OA) development, suggesting that molecules, such as Wnts and bone morphogenetic proteins (BMPs) regulating chondrocyte activity in the growth plate, may play a key role in osteoarthritis pathogenesis. The aim of the study was to investigate the possible cross-talk between BMP-2 and Wnt/β-catenin pathways in OA progression.MethodsLow-density-lipoprotein receptor-related protein 5 (LRP-5) and 6, BMP-2, -4, and -7, bone morphogenetic protein receptor-IA and IB (BMPR-IA and BMPR-IA), lymphoid enhancer factor-1 (LEF-1), and transcription factor 4 (TCF-4) expression levels were investigated in normal and osteoarthritic chondrocytes. LRP-5, β-catenin (phospho and active form), matrix metalloproteinases (MMPs) 7, 9, 13, 14, ADAMTS-4, 5, as well as collagen X (COL10A1) expression levels were evaluated after LRP-5 silencing in BMP-2-treated chondrocytes. The investigation of Smad1/5/8 binding to LRP-5 promoter was assessed with chromatin immunoprecipitation (ChIP). Furthermore, we evaluated the effect of experimental activation of the Wnt/β-catenin pathway with LiCl and LEF-1 silencing, in LiCl-treated chondrocytes, on matrix metalloproteinases (MMPs) 7, 9, 13, 14, ADAMTS-4, 5, and collagen X (COL10A1) expression, as well as possible interactions between LEF-1 and MMPs and COL10A1 promoters by using a ChIP assay.ResultsLRP-5, BMP-2, BMP-4, BMPR-IA, and LEF-1 mRNA and protein expression levels were found to be significantly upregulated in osteoarthritic chondrocytes compared with normal. We showed that treatment of cultured chondrocytes with BMP-2 resulted in increased β-catenin nuclear translocation and LRP-5 expression and that the BMP-2-induced LRP-5 upregulation is mediated through Smad1/5/8 binding on LRP-5 promoter. LRP-5 silencing reduced nuclear β-catenin protein levels, MMPs and collagen X expression, whereas increased phospho-β-catenin protein levels in BMP-2-treated chondrocyte. Furthermore, we demonstrated that activation of the Wnt/β-catenin signaling pathway by LiCl and LEF-1 downregulation by using siRNA regulates MMP-9, 13, 14, ADAMTS-5, and COL10A1 expression, evidenced by the observed strong binding of LEF-1 to MMP-9, 13, 14, ADAMTS-5 and COL10A promoters.ConclusionsOur findings suggest, for the first time to our knowledge, that BMP-2-induced Wnt/β-catenin signaling activation through LRP-5 may contribute to chondrocyte hypertrophy and cartilage degradation in osteoarthritis.
Highlights
Events normally taking place in the terminal chondrocyte differentiation in the growth plate are observed during osteoarthritis (OA) development, suggesting that molecules, such as Wnts and bone morphogenetic proteins (BMPs) regulating chondrocyte activity in the growth plate, may play a key role in osteoarthritis pathogenesis
We found that osteoarthritic chondrocytes had significantly higher lipoprotein receptor-related protein 5 (LRP-5), BMP-2, BMP-4, bone morphogenetic protein receptor-IA (BMPR-IA), lymphoid enhancer factor-1 (LEF-1) mRNA, and protein expression compared with normal (P < 0.05; Figure 2a, b, c)
We found that COL2A1 and aggrecan were significantly upregulated in normal the effect of BMP-2 on Wnt/b-catenin signaling-pathway activation after treatment with BMP-2 in normal and osteoarthritic chondrocytes
Summary
Events normally taking place in the terminal chondrocyte differentiation in the growth plate are observed during osteoarthritis (OA) development, suggesting that molecules, such as Wnts and bone morphogenetic proteins (BMPs) regulating chondrocyte activity in the growth plate, may play a key role in osteoarthritis pathogenesis. Osteoarthritis (OA) is a progressively degenerative joint disorder characterized by extracellular matrix degradation, articular cartilage loss, and osteophyte formation [1]. It is considered a major health problem worldwide, causing chronic disability in elderly people [2]. In growth-plate chondrogenesis, chondrocytes become hypertrophic, expressing collagen X, remove the collagen matrix through the production of MMP-13, and die by apoptosis and are replaced by bone [5,6]. Chondrocytes of permanent cartilage reside at the ends of the long bones and do not mature into the hypertrophic state, preventing terminal differentiation by an unknown mechanism [7]. During osteoarthritis, chondrocytes lose the stable phenotype and undergo changes that occur in terminal differentiated growth-plate chondrocytes, such as high expression of MMP-13 and collagen X [8,9,10]
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