Bone morphogenetic protein-2-induced Wnt/b- catenin signaling pathway activation through enhanced low-density-lipoprotein receptor- related protein 5 catabolic activity contributes to hypertrophy in osteoarthritic chondrocytes

Introduction: Events 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/b-catenin pathways in OA progression. Methods: Low-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, b-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/b-catenin pathway with LiCl and LEF-1 silencing, in LiCltreated 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. Results: LRP-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 b-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 b-catenin protein levels, MMPs and collagen X expression, whereas increased phospho-bcatenin protein levels in BMP-2-treated chondrocyte. Furthermore, we demonstrated that activation of the Wnt/bcatenin 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. Conclusions: Our findings suggest, for the first time to our knowledge, that BMP-2-induced Wnt/b-catenin signaling activation through LRP-5 may contribute to chondrocyte hypertrophy and cartilage degradation in osteoarthritis. * Correspondence: [email protected] Laboratory of Cytogenetics and Molecular Genetics, University of Thessaly, School of Medicine, Mezourlo, Larissa, 41100, Greece Full list of author information is available at the end of the article Papathanasiou et al. Arthritis Research & Therapy 2012, 14:R82 http://arthritis-research.com/content/14/2/R82 © 2012 Papathanasiou et al.; licensee BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.