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Arthritis Rheum. 2011 Dec;63(12):3865-75. doi: 10.1002/art.30625.

R-spondins are newly recognized players in osteoarthritis that regulate Wnt signaling in osteoblasts.

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1
Unité de Recherche en Arthrose, Centre de Recherche du Centre Hospitalier de l'Université de Montréal, Hôpital Notre-Dame, Montréal, Québec, Canada.

Abstract

OBJECTIVE:

Clinical and in vitro studies suggest that altered osteogenesis or bone remodeling is involved in the progression and/or onset of osteoarthritis (OA). Wnt signaling plays a key role in osteogenesis via the canonical Wnt/β-catenin signaling pathway. Two of the R-spondins, Rspo-1 and Rspo-2, a family of 4 proteins unrelated to other Wnt ligands that act as Wnt agonists, are present in bone tissues. The purpose of this study was to investigate the potential role of Rspo-1 and Rspo-2 in OA osteoblasts.

METHODS:

Primary human normal and OA osteoblasts were prepared from tibial plateaus. The expression of Rspo-1 and Rspo-2 was evaluated by quantitative reverse transcription-polymerase chain reaction analysis. Western blot analysis was used to determine Rspo-2, β-catenin, and phospho-β-catenin levels. Wnt/β-catenin signaling was evaluated using the TOPflash T cell factor (TCF)/lymphoid enhancer factor (LEF) luciferase reporter assay. Mineralization was evaluated by alizarin red staining.

RESULTS:

The expression of Rspo-1 was similar in normal and OA osteoblasts, whereas the expression and production of Rspo-2 were reduced in OA osteoblasts due to elevated levels of transforming growth factor β1 in these cells. The reduced Wnt-3a-dependent TOPflash TCF/LEF luciferase reporter activity in OA osteoblasts as compared to normal osteoblasts was corrected by the addition of recombinant human Rspo-2. Wnt-3a-dependent β-catenin levels were also corrected in OA osteoblasts by Rspo-2 addition. Wnt-3a alone increased the mineralization of OA osteoblasts, which was further increased by Rspo-2.

CONCLUSION:

Reduced Rspo-2 levels in OA osteoblasts are responsible, at least in part, for their reduced Wnt/β-catenin signaling and abnormal mineralization. As Rspo-2 is a secreted soluble protein, this could lead to potential new avenues of treatment of OA.

PMID:
22127703
DOI:
10.1002/art.30625
[Indexed for MEDLINE]
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