Increased IL-6 receptor expression and signaling in ageing cartilage can be explained by loss of TGF-ß-mediated IL-6 receptor suppression.

OBJECTIVE: Osteoarthritis (OA) development is strongly associated with ageing, possibly due to age-related changes in transforming growth factor-ß (TGF-ß) signaling in cartilage. Recently, we showed that TGF-ß suppresses interleukin (IL)-6 receptor (IL-6R) expression in chondrocytes. As IL-6 is involved in cartilage degeneration, we hypothesized that age-related loss of TGF-ß signaling results in increased IL-6R expression and signaling in ageing cartilage. DESIGN: Bovine articular cartilage was collected and immediately processed to study age-related changes in IL-6R expression using qPCR and IHC (age-range: 0.5-14 years). Moreover, cartilage from young and aged cows was stimulated with rhIL-6 and/or rhTGF-ß1 to measure IL-6-induced p-STAT3 using Western blot. Expression of STAT3-responsive genes was analyzed using qPCR. RESULTS: Expression of IL-6 receptor (bIL-6R) significantly increased in cartilage upon ageing (slope: 0.32, 95%CI: 0.20-0.45), while expression of glycoprotein 130 (bGP130) was unaffected. Cartilage stimulation with IL-6 showed increased induction of p-STAT3 upon ageing (slope: 0.14, 95%CI: 0.08-0.20). Furthermore, IL-6-mediated induction of STAT3-responsive genes like bSOCS3 and bMMP3 was increased in aged compared to young cartilage. Interestingly, the ability of TGF-ß to suppress bIL6R expression in young cartilage was lost upon ageing (slope: 0.21, 95%CI: 0.13-0.30). Concurrently, an age-related loss in TGF-ß-mediated suppression of IL-6-induced p-STAT3 and bSOCS3 expression was observed. CONCLUSIONS: Ageing results in enhanced IL-6R expression and subsequent IL-6-induced p-STAT3 signaling in articular cartilage. This is likely caused by age-related loss of protective TGF-ß signaling, resulting in loss of TGF-ß-mediated IL-6R suppression. Because of the detrimental role of IL-6 in cartilage, this mechanism may be involved in age-related OA development.

IL-37 diminishes proteoglycan loss in human OA cartilage: donor-specific link between IL-37 and MMP-3.

OBJECTIVE: A hallmark of osteoarthritis (OA) is degradation of articular cartilage proteoglycans. In isolated human OA chondrocytes, the anti-inflammatory cytokine Interleukin-37 (IL-37) lowers the expression of the proteolytic MMP and ADAMTS enzymes, which mediate this degradation. Therefore, we investigated if IL-37 protects against proteoglycan loss in freshly obtained human OA explants. MATERIAL AND METHODS: Human OA cartilage explants were incubated with IL-37. Release of sulphated proteoglycans (sGAGs) was measured with the dimethylmethylene-blue assay. Production and degradation of newly synthesized proteoglycans was measured using 35S-sulphate. Proteoglycan and proteolytic enzyme expression were analyzed by qPCR and Western Blot. Proteolytic activity was determined by measuring MMP- and ADAMTS-generated aggrecan neo-epitopes with ELISA and by using MMP-3-, MMP-13- or ADAMTS-5-inhibitors. RESULTS: Over time, a linear release of sGAGs from OA cartilage was measured. IL-37 reduced this release by 87 µg/ml (24%) 95%CI [21.04-141.4]. IL-37 did not affect 35S-sulphate incorporation or proteoglycan gene expression. In contrast, IL-37 reduced loss of 35S-sulphate labeled GAGs and reduced MMP-3 protein expression, indicating that IL-37 inhibits proteoglycan degradation. Remarkably, we observed two groups of patients; one group in which MMP-3-inhibition lowered sGAG release, and one group in which ADAMTS5-inhibition had this effect. Remarkably, IL-37 was only functional in the group of patients that responded to MMP-3-inhibition. CONCLUSION: We identified a relationship between IL-37 and reduced sGAG loss in OA cartilage. Most likely, this effect is mediated by inhibition of MMP-3 expression. These results suggest that IL-37 could be applied as therapy in a subgroup of OA patients, in which cartilage degradation is mediated by MMP-3.

Osteoarthritis year in review 2016: biology.

This review highlights a selection of literature in the area of osteoarthritis biology published between the 2015 and 2016 Osteoarthritis Research Society International (OARSI) World Congress. Highlights were selected from a pubmed search covering cartilage, bone, inflammation and pain. A personal selection was made based, amongst other things, on topics presented during the 2015 conference. This covers circadian rhythm, TGF-ß signaling, autophagy, SIRT6, exercise, lubricin, TLR's, pain and NGF. Furthermore, in this review we have made an effort to connect these seemingly distant topics into one scheme of connections between them, revealing a theoretical big picture underneath.

TGF-ß is a potent inducer of Nerve Growth Factor in articular cartilage via the ALK5-Smad2/3 pathway. Potential role in OA related pain?

OBJECTIVE: Pain is the main problem for patients with osteoarthritis (OA). Pain is linked to inflammation, but in OA a subset of patients suffers from pain without inflammation, indicating an alternative source of pain. Nerve Growth Factor (NGF) inhibition is very efficient in blocking pain during OA, but the source of NGF is unclear. We hypothesize that damaged cartilage in OA releases Transforming Growth Factor-ß (TGF-ß), which in turn stimulates chondrocytes to produce NGF. DESIGN: Murine and human chondrocyte cell lines, primary bovine and human chondrocytes, and cartilage explants from bovine metacarpal joints and human OA joints were stimulated with TGF-ß1 and/or Interleukin-1 (IL-1)ß. We analyzed NGF expression on mRNA level with QPCR and stained human OA cartilage for NGF immunohistochemically. Cultures were additionally pre-incubated with inhibitors for TAK1, Smad2/3 or Smad1/5/8 signaling to identify the TGF-ß pathway inducing NGF. RESULTS: NGF expression was consistently induced in higher levels by TGF-ß than IL-1 in all of our experiments: murine, bovine and human origin, in cell lines, primary chondrocytes and explants cultures. TAK1 inhibition consistently reduced TGF-ß-induced NGF whereas it fully blocked IL-1ß-induced NGF expression. In contrast, ALK5-Smad2/3 inhibition fully blocked TGF-ß-induced NGF expression. Despite the large variation in basal NGF in human OA samples (mRNA and histology), TGF-ß exposure led to a consistent high level of NGF induction. CONCLUSION: We show for the first time that TGF-ß induces NGF expression in chondrocytes, in a ALK5-Smad2/3 dependent manner. This reveals a potential alternative non-inflammatory source of pain in OA.

Inducible chondrocyte-specific overexpression of BMP2 in young mice results in severe aggravation of osteophyte formation in experimental OA without altering cartilage damage.

OBJECTIVES: In osteoarthritis (OA) chondrocytes surrounding lesions express elevated bone morphogenetic protein 2 (BMP2) levels. To investigate the functional consequence of chondrocyte-specific BMP2 expression, we made a collagen type II dependent, doxycycline (dox)-inducible BMP2 transgenic mouse and studied the effect of elevated BMP2 expression on healthy joints and joints with experimental OA. METHODS: We cloned a lentivirus with BMP2 controlled by a tet-responsive element and transfected embryos of mice containing a collagen type II driven cre-recombinase and floxed rtTA to gain a mouse expressing BMP2 solely in chondrocytes and only upon dox exposure (Col2-rtTA-TRE-BMP2). Mice were treated with dox to induce elevated BMP2 expression. In addition, experimental OA was induced (destabilisation of the medial meniscus model) with or without dox supplementation and knee joints were isolated for histology. RESULTS: Dox treatment resulted in chondrocyte-specific upregulation of BMP2 and severely aggravated formation of osteophytes in experimental OA but not in control mice. Moreover, elevated BMP2 levels did not result in alterations in articular cartilage of young healthy mice, although BMP2-exposure did increase VDIPEN expression in the articular cartilage. Strikingly, despite apparent changes in knee joint morphology due to formation of large osteophytes there were no detectible differences in articular cartilage: none with regard to structural damage nor in Safranin O staining intensity when comparing destabilisation of the medial meniscus with or without dox exposure. CONCLUSIONS: Our data show that chondrocyte-specific elevation of BMP2 levels does not alter the course of cartilage damage in an OA model in young mice but results in severe aggravation of osteophyte formation.

Physiological and excessive mechanical compression of articular cartilage activates Smad2/3P signaling.

OBJECTIVE: Transforming growth factor beta (TGF-ß) in articular cartilage can signal via two routes, the ALK5/Smad2/3P and the ALK1/Smad1/5/8P route, the first being protective and the latter favoring chondrocyte terminal differentiation. Since biomechanical factors are known to play an essential role in osteoarthritis (OA) initiation and progression, we investigated if excessive mechanical compression can alter TGF-ß signaling in cartilage shifting it from ALK5/Smad2/3P to ALK1/Smad1/5/8P pathway, favoring terminal differentiation of chondrocytes. DESIGN: Articular cartilage explants were harvested from bovine metacarpophalangeal joints. After equilibration, explants were subjected to unconfined dynamic mechanical compression (1 Hz) with 3 MPa (physiological) or 12 MPa (excessive) stress. After different time intervals samples were frozen and mRNA levels of selected genes were examined using real-time polymerase chain reaction. RESULTS: In articular cartilage compressed with 3 MPa and also 12 MPa stress the expression of Smad2/3P responsive genes bSerpine1, bSmad7 and bAlk5 was up-regulated, whereas the expression of Smad1/5/8P responsive gene bId1 was down-regulated. Furthermore, the expression of bTgfb1 was significantly up-regulated in both compression groups. When ALK5/Smad2/3P pathway was blocked with a selective ALK4/5/7 inhibitor, the effect of excessive mechanical compression on bSmad7 and bAlk5 expression was prevented. CONCLUSIONS: Here we show that excessive mechanical compression alone is not able to shift TGF-ß signaling toward the ALK1/Smad1/5/8P pathway. In contrast, we show that mechanical compression not only with physiological but also with excessive stress can activate Smad2/3P signaling, which is known to be protective for articular cartilage and to block chondrocyte terminal differentiation.

Gene expression analysis of murine and human osteoarthritis synovium reveals elevation of transforming growth factor ß-responsive genes in osteoarthritis-related fibrosis.

OBJECTIVE: Synovial fibrosis is a major contributor to joint stiffness in osteoarthritis (OA). Transforming growth factor ß (TGFß), which is elevated in OA, plays a key role in the onset and persistence of synovial fibrosis. However, blocking of TGFß in OA as a therapeutic intervention for fibrosis is not an option since TGFß is crucial for cartilage maintenance and repair. Therefore, we undertook the present study to seek targets downstream of TGFß for preventing OA-related fibrosis without interfering with joint homeostasis. METHODS: Experiments were performed to determine whether genes involved in extracellular matrix turnover were responsive to TGFß and were elevated in OA-related fibrosis. We analyzed gene expression in TGFß-stimulated human OA synovial fibroblasts and in the synovium of mice with TGFß-induced fibrosis, mice with experimental OA, and humans with end-stage OA. Gene expression was determined by microarray, low-density array, or quantitative polymerase chain reaction analysis. RESULTS: We observed an increase in expression of procollagen genes and genes encoding collagen crosslinking enzymes under all of the OA-related fibrotic conditions investigated. Comparison of gene expression in TGFß-stimulated human OA synovial fibroblasts, synovium from mice with experimental OA, and synovium from humans with end-stage OA revealed that the genes PLOD2, LOX, COL1A1, COL5A1, and TIMP1 were up-regulated in all of these conditions. Additionally, we confirmed that these genes were up-regulated by TGFß in vivo in mice with TGFß-induced synovial fibrosis. CONCLUSION: Most of the up-regulated genes identified in this study would be poor targets for therapy development, due to their crucial functions in the joint. However, the highly up-regulated gene PLOD2, responsible for the formation of collagen crosslinks that make collagen less susceptible to enzymatic degradation, is an attractive and promising target for interference in OA-related synovial fibrosis.

Osteoarthritis-related fibrosis is associated with both elevated pyridinoline cross-link formation and lysyl hydroxylase 2b expression.

OBJECTIVE: Fibrosis is a major contributor to joint stiffness in osteoarthritis (OA). We investigated several factors associated with the persistence of transforming growth factor beta (TGF-ß)-induced fibrosis and whether these factors also play a role in OA-related fibrosis. DESIGN: Mice were injected intra-articularly (i.a.) with an adenovirus encoding either TGF-ß or connective tissue growth factor (CTGF). In addition, we induced OA by i.a. injection of bacterial collagenase into the right knee joint of C57BL/6 mice. mRNA was isolated from the synovium for Q-PCR analysis of the gene expression of various extracellular matrix (ECM) components, ECM degraders, growth factors and collagen cross-linking-related enzymes. Sections of murine knee joints injected with Ad-TGF-ß or Ad-CTGF or from experimental OA were stained for lysyl hydroxylase 2 (LH2). The number of pyridinoline cross-links per triple helix collagen in synovium biopsies was determined with high-performance liquid chromatography (HPLC). RESULTS: Expression of collagen alpha-1(I) chain precursor (Col1a1), tissue inhibitor of metalloproteinases 1 (TIMP1) and especially procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2b (Plod2b) were highly upregulated by TGF-ß but not by CTGF. Elevated expression of Plod2b mRNA was associated with high lysyl hydroxylase 2 (LH2) protein staining after TGF-ß overexpression and in experimental OA. Furthermore, in experimental OA the number of hydroxypyridinoline cross-links was significant increased compared to control knee joints. CONCLUSIONS: Our data show that elevated LH2b expression is associated with the persistent nature of TGF-ß-induced fibrosis. Also in experimental OA, LH2b expression as well as the number of hydroxypyridinoline cross-link were significantly upregulated. We propose that LH2b, and the subsequent increase in pyridinoline cross-links, is responsible for the persistent fibrosis in experimental OA.

Bone morphogenetic proteins and articular cartilage: To serve and protect or a wolf in sheep clothing's?

OBJECTIVE: Alterations in chondrocyte differentiation and matrix remodeling play a central role in osteoarthritis (OA). Chondrocyte differentiation and remodeling are amongst others regulated by the so-called Bone Morphogenetic Proteins (BMPs). Although BMPs are considered protective for articular cartilage these factors can also be involved in chondrocyte hypertrophy and matrix degradation. This review is focused on these opposed roles of BMPs in OA development and progression. METHODS: Peer reviewed publications published prior to August 2009 were searched in the Pubmed database. Articles that were relevant for the role of endogenous BMPs in OA were selected. Since good quality reviews on the application of BMP supplementation in cartilage tissue engineering have been described this subject has not been covered in this review. RESULTS: BMPs can stimulate both chondrocyte matrix synthesis and chondrocyte terminal differentiation. The latter results in elevated matrix metalloproteinase-13 (MMP-13) production. Stimulation of matrix synthesis will be protective for cartilage while elevated MMP-13 activity will drive matrix degradation. What action of BMPs is dominant in OA is not yet elucidated and their role might be different in patient subgroups. CONCLUSION: BMPs can be protective for articular cartilage but can, due to their effect on chondrocyte differentiation, have harmful effects on articular cartilage and contribute to OA progression.

TGF-beta signaling in chondrocyte terminal differentiation and osteoarthritis: modulation and integration of signaling pathways through receptor-Smads.

OBJECTIVE: Chondrocytes and alteration in chondrocyte differentiation play a central role in osteoarthritis. Chondrocyte differentiation is amongst others regulated by members of the transforming growth factor-beta (TGF-beta) superfamily. The major intracellular signaling routes of this family are via the receptor-Smads. This review is focused on the modulation of receptor-Smad signaling and how this modulation can affect chondrocyte differentiation and potentially osteoarthritis development. METHODS: Peer reviewed publications published prior to April 2009 were searched in the Pubmed database. Articles that were relevant for the role of TGF-beta superfamily/Smad signaling in chondrocyte differentiation and for differential modulation of receptor-Smads were selected. RESULTS: Chondrocyte terminal differentiation is stimulated by Smad1/5/8 activation and inhibited the by Smad2/3 pathway, most likely by modulation of Runx2 function. Several proteins and signaling pathways differentially affect Smad1/5/8 and Smad2/3 signaling. This will result in an altered Smad1/5/8 and Smad2/3 balance and subsequently have an effect on chondrocyte differentiation and osteoarthritis development. CONCLUSION: Modulation of receptor-Smads signaling can be expect to play an essential role in both the regulation of chondrocyte differentiation and osteoarthritis development and progression.

Resemblance of osteophytes in experimental osteoarthritis to transforming growth factor beta-induced osteophytes: limited role of bone morphogenetic protein in early osteoarthritic osteophyte formation.

OBJECTIVE: Osteoarthritis (OA) is characterized by cartilage damage, synovial fibrosis, and osteophyte formation. Both transforming growth factor beta (TGFbeta) and bone morphogenetic protein 2 (BMP-2) can induce the formation of osteophytes during OA, but their specific role in this process is unclear. The purpose of this study was to investigate the respective contributions of TGFbeta and BMP-2 to OA. METHODS: Mouse knee joints injected with adenovirus (Ad-TGFbeta or Ad-BMP-2) were compared histologically with knee joints from murine models of OA (joints injected with collagenase and joints from STR/Ort mice with spontaneous OA). To further investigate the role of BMP during osteophyte formation, adenovirus Ad-Gremlin was injected into knee joints that had previously been injected with Ad-TGFbeta or collagenase. RESULTS: BMP-2 induced early osteophytes, which bulged from the growth plates on the femur and grew on top of the patella, whereas TGFbeta induced early osteophyte formation on the bone shaft beneath the collateral ligament on the femur as well as on top of the patella. The pattern of osteophyte formation during experimental OA closely resembled that of TGFbeta-induced osteophyte formation, but differed from the pattern induced by BMP-2. Ad-Gremlin proved to be able to totally block BMP-2-induced osteophyte formation. However, blocking BMP activity inhibited neither TGFbeta-induced nor experimental OA-associated osteophyte formation. CONCLUSION: Our findings demonstrate that the role of BMP during the onset of TGFbeta-induced and experimental OA-induced osteophyte formation is limited. The latter finding does not rule out a role of BMP during osteophyte maturation.

TGF-beta and osteoarthritis.

OBJECTIVE: Cartilage damage is a major problem in osteoarthritis (OA). Growth factors like transforming growth factor-beta (TGF-beta) have great potential in cartilage repair. In this review, we will focus on the potential therapeutic intervention in OA with TGF-beta, application of the growth factor TGF-beta in cartilage repair and on the side effects of TGF-beta treatment that could occur. METHODS: This review summarizes peer-reviewed articles published in the PubMed database before November 2006. In addition, this review is supplemented with recent data of our own group on the use of TGF-beta as a cartilage reparative factor in OA. RESULTS: TGF-beta is crucial for cartilage maintenance and lack there of results in OA-like changes. Moreover, TGF-beta supplementation can enhance cartilage repair and is therefore a potential therapeutic tool. However, application of TGF-beta supplementation provides problems in other tissues of the joint and results in fibrosis and osteophyte formation. This can potentially be overcome by local inhibition of TGF-beta at sites of unwanted side-effects or by blocking downstream mediators of TGF-beta that are important for the induction of fibrosis or osteophyte formation. CONCLUSION: Current understanding of TGF-beta suggests that it essential for cartilage integrity and that it is a powerful tool to prevent or repair cartilage damage. The side-effects that occur with TGF-beta supplementation can be overcome by local inhibition of TGF-beta itself or downstream mediators.

Connective tissue growth factor/CCN2 overexpression in mouse synovial lining results in transient fibrosis and cartilage damage.

OBJECTIVE: Characteristics of osteoarthritis (OA) include cartilage damage, fibrosis, and osteophyte formation. Connective tissue growth factor (CTGF; also known as CCN2), is found in high levels in OA chondrocytes and is frequently involved in fibrosis, bone formation, and cartilage repair. The present study was therefore undertaken to investigate the potential role of CTGF in OA pathophysiology. METHODS: We transfected the synovial lining of mouse knee joints with a recombinant adenovirus expressing human CTGF and measured synovial fibrosis and proteoglycan content in cartilage on days 1, 3, 7, 14, and 28. Messenger RNA (mRNA) expression in synovium and cartilage was measured on days 3, 7, and 21. RESULTS: CTGF induced synovial fibrosis, as indicated by accumulation of extracellular matrix and an increase in procollagen type I-positive cells. The fibrosis reached a maximum on day 7 and had reversed by day 28. Levels of mRNA for matrix metalloproteinase 3 (MMP-3), MMP-13, ADAMTS-4, ADAMTS-5, tissue inhibitor of metalloproteinases 1 (TIMP-1), and transforming growth factor beta were elevated in the fibrotic tissue. TIMP-1 expression was elevated on day 3, while expression of other genes did not increase until day 7 or later. CTGF induced proteoglycan depletion in cartilage as early as day 1. Maximal depletion was observed on days 3-7. Cartilage damage was reduced by day 28. A high level of MMP-3 mRNA expression was found in cartilage. CTGF overexpression did not induce osteophyte formation. CONCLUSION: CTGF induces transient fibrosis that is reversible within 28 days. Overexpression of CTGF in knee joints results in reversible cartilage damage, induced either by the high CTGF levels or via factors produced by the CTGF-induced fibrotic tissue.

Expression of transforming growth factor-beta (TGFbeta) and the TGFbeta signalling molecule SMAD-2P in spontaneous and instability-induced osteoarthritis: role in cartilage degradation, chondrogenesis and osteophyte formation.

BACKGROUND: The primary feature of osteoarthritis is cartilage loss. In addition, osteophytes can frequently be observed. Transforming growth factor-beta (TGFbeta) has been suggested to be associated with protection against cartilage damage and new cartilage formation as seen in osteophytes. OBJECTIVE: To study TGFbeta and TGFbeta signalling in experimental osteoarthritis to gain insight into the role of TGFbeta in cartilage degradation and osteophyte formation during osteoarthritis progression. METHODS: Histological sections of murine knee joints were stained immunohistochemically for TGFbeta3 and phosphorylated SMAD-2 (SMAD-2P). Expression patterns were studied in two murine osteoarthritis models, representing spontaneous (STR/ort model) and instability-associated osteoarthritis (collagenase-induced instability model). RESULTS: TGFbeta3 and SMAD-2P staining was increasingly reduced in cartilage during osteoarthritis progression in both models. Severely damaged cartilage was negative for TGFbeta3. In contrast, bone morphogenetic protein-2 (BMP-2) expression was increased. In chondrocyte clusters, preceding osteophyte formation, TGFbeta3 and SMAD-2P were strongly expressed. In early osteophytes, TGFbeta3 was found in the outer fibrous layer, in the peripheral chondroblasts and in the core. Late osteophytes expressed TGFbeta3 only in the fibrous layer. SMAD-2P was found throughout the osteophyte at all stages. In the late-stage osteophytes, BMP-2 was strongly expressed. CONCLUSION: Data show that lack of TGFbeta3 is associated with cartilage damage, suggesting loss of the protective effect of TGFbeta3 during osteoarthritis progression. Additionally, our results indicate that TGFbeta3 is involved in early osteophyte development, whereas BMP might be involved in late osteophyte development.

Reduced transforming growth factor-beta signaling in cartilage of old mice: role in impaired repair capacity.

Osteoarthritis (OA) is a common joint disease, mainly effecting the elderly population. The cause of OA seems to be an imbalance in catabolic and anabolic factors that develops with age. IL-1 is a catabolic factor known to induce cartilage damage, and transforming growth factor (TGF)-beta is an anabolic factor that can counteract many IL-1-induced effects. In old mice, we observed reduced responsiveness to TGF-beta-induced IL-1 counteraction. We investigated whether expression of TGF-beta and its signaling molecules altered with age. To mimic the TGF-beta deprived conditions in aged mice, we assessed the functional consequence of TGF-beta blocking. We isolated knee joints of mice aged 5 months or 2 years, half of which were exposed to IL-1 by intra-articular injection 24 h prior to knee joint isolation. Immunohistochemistry was performed, staining for TGF-beta1, -2 or -3, TGF-betaRI or -RII, Smad2, -3, -4, -6 and -7 and Smad-2P. The percentage of cells staining positive was determined in tibial cartilage. To mimic the lack of TGF-beta signaling in old mice, young mice were injected with IL-1 and after 2 days Ad-LAP (TGF-beta inhibitor) or a control virus were injected. Proteoglycan (PG) synthesis (35S-sulfate incorporation) and PG content of the cartilage were determined. Our experiments revealed that TGF-beta2 and -3 expression decreased with age, as did the TGF-beta receptors. Although the number of cells positive for the Smad proteins was not altered, the number of cells expressing Smad2P strongly dropped in old mice. IL-1 did not alter the expression patterns. We mimicked the lack of TGF-beta signaling in old mice by TGF-beta inhibition with LAP. This resulted in a reduced level of PG synthesis and aggravation of PG depletion. The limited response of old mice to TGF-beta induced-IL-1 counteraction is not due to a diminished level of intracellular signaling molecules or an upregulation of intracellular inhibitors, but is likely due to an intrinsic absence of sufficient TGF-beta receptor expression. Blocking TGF-beta distorted the natural repair response after IL-1 injection. In conclusion, TGF-beta appears to play an important role in repair of cartilage and a lack of TGF-beta responsiveness in old mice might be at the root of OA development.