Abstract
Cyclooxygenase-2 (COX-2) is known to modulate bone metabolism, including bone formation and resorption. Because cartilage serves as a template for endochondral bone formation and because cartilage development is initiated by the differentiation of mesenchymal cells into chondrocytes (Ahrens et al., 1977; Sandell and Adler, 1999; Solursh, 1989), it is of interest to know whether COX-2 expression affect chondrocyte differentiation. Therefore, we investigated the effects of COX-2 protein on differentiation in rabbit articular chondrocyte and chick limb bud mesenchymal cells. Overexpression of COX-2 protein was induced by the COX-2 cDNA transfection. Ectopic expression of COX-2 was sufficient to causes dedifferentiation in articular chondrocytes as determined by the expression of type II collagen via Alcian blue staining and Western blot. Also, COX-2 overexpression caused suppression of SOX-9 expression, a major transcription factor that regulates type II collagen expression, as indicated by the Western blot and RT-PCR. We further examined ectopic expression of COX-2 in chondrifying mesenchymal cells. As expected, COX-2 cDNA transfection blocked cartilage nodule formation as determined by Alcian blue staining. Our results collectively suggest that COX-2 overexpression causes dedifferentiation in articular chondrocytes and inhibits chondrogenic differentiation of mesenchymal cells.
Highlights
Chondrocytes in cartilage are differentiated from mesenchymal cells during embryonic development
Ectopic expression of COX-2 or IL-1β caused a dramatic loss of sulfated proteoglycan and type II collagen as determined Alcian blue staining and immunohistochemical staining, respectively (Figure 2)
We previously demonstrated that IL-1β induced COX-2 expression and dedifferentiation in primary culture chondrocytes (Kim, 2003)
Summary
Chondrocytes in cartilage are differentiated from mesenchymal cells during embryonic development. Differentiated chondrocytes, which are the only cell type found in normal mature cartilage, synthesize sufficient amounts of cartilage-specific extracellular matrix (ECM) to maintain matrix integrity. This homeostasis is destroyed in degenerative diseases, such as osteoarthritis and rheumatoid arthritis (Sandell and Aigner, 2001). The differentiated phenotype is unstable both in vivo and in vitro and lost by a process designated “dedifferentiation” upon exposure of cells to IL-1β (Goldring et al, 1994; DemoorFossard et al, 1998), nitric oxide (Amin and Abramson, 1998), or retinoic acid (Cash et al, 1997; Weston et al, 2000) and during serial monolayer culture (Lefebvre et al, 1990; Yoon et al, 2002).
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