During osteoarthritis (OA) chondrocytes show deviant behavior resembling terminal differentiation of growth-plate chondrocytes, characterized by elevated MMP-13 expression. The latter is also a hallmark for OA. TGF-beta is generally thought to be a protective factor for cartilage, but it has also displayed deleterious effects in some studies. Recently, it was shown that besides signaling via the ALK5 (activin-like kinase 5) receptor, TGF-beta can also signal via ALK1, thereby activating Smad1/5/8 instead of Smad2/3. The Smad1/5/8 route can induce chondrocyte terminal differentiation. Murine chondrocytes stimulated with TGF-beta activated the ALK5 receptor/Smad2/3 route as well as the ALK1/Smad1/5/8 route. In cartilage of mouse models for aging and OA, ALK5 expression decreased much more than ALK1. Thus, the ALK1/ALK5 ratio increased, which was associated with changes in the respective downstream markers: an increased Id-1 (inhibitor of DNA binding-1)/PAI-1 (plasminogen activator inhibitor-1) ratio. Transfection of chondrocytes with adenovirus overexpressing constitutive active ALK1 increased MMP-13 expression, while small interfering RNA against ALK1 decreased MMP-13 expression to nondetectable levels. Adenovirus overexpressing constitutive active ALK5 transfection increased aggrecan expression, whereas small interfering RNA against ALK5 resulted in increased MMP-13 expression. Moreover, in human OA cartilage ALK1 was highly correlated with MMP-13 expression, whereas ALK5 correlated with aggrecan and collagen type II expression, important for healthy cartilage. Collectively, we show an age-related shift in ALK1/ALK5 ratio in murine cartilage and a strong correlation between ALK1 and MMP-13 expression in human cartilage. A change in balance between ALK5 and ALK1 receptors in chondrocytes caused changes in MMP-13 expression, thereby causing an OA-like phenotype. Our data suggest that dominant ALK1 signaling results in deviant chondrocyte behavior, thereby contributing to age-related cartilage destruction and OA.

Objective. Wnt signaling pathway proteins are involved in embryonic development of cartilage and bone, and, interestingly, developmental processes appear to be recapitulated in osteoarthritic (OA) cartilage. The present study was undertaken to characterize the expression pattern of Wnt and Fz genes during experimental OA and to determine the function of selected genes in experimental and human OA.Methods. Longitudinal expression analysis was performed in 2 models of OA. Levels of messenger RNA for genes from the Wnt/␤-catenin pathway were determined in synovium and cartilage, and the results were validated using immunohistochemistry. Effects of selected genes were assessed in vitro using recombinant protein, and in vivo by adenoviral overexpression.Results. Wnt-induced signaling protein 1 (WISP-1) expression was strongly increased in the synovium and cartilage of mice with experimental OA. Osteoarthritis (OA) results in the destruction of cartilage and bone, ultimately leading to loss of joint function. The cause of the disease is largely unknown, although obesity, genetic factors, and injury have all been associated with increased risk of OA (1,2). Although it is likely that in most cases the initial events leading to OA occur within the cartilage or subchondral bone (3,4), the synovial tissue of many OA patients shows a changed morphology, with a marked inflammatory phenotype (5,6). Wnt-16 andLoss of articular cartilage extracellular matrix is thought to be mediated by matrix metalloproteinases (MMPs) and aggrecanases (ADAMTS-4 and ADAMTS-5), which are likely the most important groups of enzymes in the breakdown of extracellular

BMP-2 and TGF-beta stimulate PG synthesis and PG content with different kinetics, and these factors have different chondro-inductive properties.

This study shows that overexpression of active TGF-beta1 in the knee joint results in OA-like changes and suggests the synovial lining cells contribute to the chondro-osteophyte formation.

Multiple intra-articular injections of TGF-beta induce changes in articular cartilage and surrounding tissues that have strong resemblance to features of experimental and spontaneous osteoarthritis in mice, suggesting a role for TGF-beta in the OA process.

Osteoarthritis has as main characteristics the degradation of articular cartilage and the formation of new bone at the joint edges, so-called osteophytes. In this study enhanced expression of TGF-β1 and -β3 was detected in developing osteophytes and articular cartilage during murine experimental osteoarthritis. To determine the role of endogenous TGF-β on osteophyte formation and articular cartilage, TGF-β activity was blocked via a scavenging soluble TGF-β-RII. Our results clearly show that inhibition of endogenous TGF-β nearly completely prevented osteophyte formation. In contrast, treatment with recombinant soluble TGF-β-RII markedly enhanced articular cartilage proteoglycan loss and reduced the thickness of articular cartilage. In conclusion, we show for the first time that endogenous TGF-β is a crucial factor in the process of osteophyte formation and has an important function in protection against cartilage loss.

The modulation of interleukin 1 (IL-1) effects on proteoglycan metabolism in intact murine patellar cartilage by transforming growth factor P (TGF-I) was investigated in vitro and in vivo. In vitro TGF-,1 (400 pmol/) had no effect on basal proteoglycan degradation. Proteoglycan degradation induced by IL-1, however, was suppressed by TGF-j in serum free medium alone and in medium supplemented with 0.5 ,ug/mi insulin-like growth factor 1. This suggests a specific regulatory role for TGF-I under pathological conditions. In contrast with the suppression of breakdown, synthesis of proteoglycans was stimulated by TGF-iS for both basal and IL-1 suppressed proteoglycan synthesis in cultures without insulin-like growth factor. In the presence of insulin-like growth factor no extra effect of TGF-(8 on proteoglycan synthesis was observed. With insulin-like growth factor, however, TGF-P potentiated the ex vivo recovery of IL-1 induced suppression of proteoglycan synthesis. Analogous to the in vitro effects, TGF-I8 injected intraarticularly suppressed IL-1 induced proteoglycan degradation. Furthermore, TGF-, injected into the joint counteracted IL-1 induced suppression of proteoglycan synthesis. This indicates that in vivo also TGF-, can ameliorate the deleterious effects of IL-1 on the cartilage matrix.

Objectives-Transforming growth factor-,l (TGF-4) has been shown to antagonise interleukin-1 (IL-1) effects in different systems. Investigations were carried out to study whether TGF-,B1 modulates IL-1 induced inflammation and IL-1 effects on articular cartilage in the murine knee joint.Methods-IL-1, TGF-01 or both factors together were injected into the knee joint. Inflammation was studied in whole knee histological sections. Patellar cartilage proteoglycan synthesis was measured using 35S-sulphate incorporation while patellar cartilage glycosaminoglycan content was determined with automated image analysis on joint sections. Results-Co-injection of TGF-I1 and IL-1 resulted in synergistic attraction of inflammatory celis. In contrast, TGF-(l1 counteracted IL-1 induced suppression of articular cartilage proteoglycan synthesis. Proteoglycan depletion was similar shortly after the last injection of IL-1 or IL-i/TGF-pll, but accelerated recovery was found with the combination at later days. This protective effect of TGF-,1 could not be demonstrated in older mice.Conclusions-TGF-41 aggravates IL-1 induced knee joint inflammation, but counteracts the deleterious effects of IL-1 on articular cartilage proteoglycan synthesis and content. The data indicate that TGF-, I could play an important part in articular cartilage restoration after IL-I induced proteoglycan depletion.