Induction of interleukin-1 in articular cartilage by explantation and cutting

Articular chondrocyte culturing for cell-based cartilage repair: needs and perspectives

Articular cartilage displays a limited capacity of self-regeneration after injury. Thus, the biology of this tissue and its cellular components - the chondrocytes - has become the focus of several investigations, driven by tissue engineering and the basic and clinical research fields, aiming to ameliorate the present clinical approaches to cartilage repair. In this work, we present a brief recapitulation of the events that lead to cartilage development during the skeletal embryonal growth. The intrinsic phenotypic plasticity of the mesenchymal precursors and the adult chondrocytes is evaluated, dependent on the cell source, its physiopathological state, and as a function of the donor's age. The phenotypic changes induced by the basic culturing techniques are also taken into account, thus highlighting the phenotypic plasticity of the chondrocyte as the main property which could couple the differentiation process to the repair process. Chondrocyte proliferation and the contemporary maintenance of the chondrogenic differentiation potential are regarded as the two primary goals to be achieved in order to fulfill the quantitative needs of the clinical applications and the qualitative requirements of a properly repaired tissue. In this light, the effects of several growth factors and medium supplements are investigated. Finally, the latest improvements in culturing conditions and their possible clinical applications are presented as well.

Prostaglandin E2 synthesis in cartilage explants under compression: mPGES-1 is a mechanosensitive gene

Knee osteoarthritis (OA) results, at least in part, from overloading and inflammation leading to cartilage degradation. Prostaglandin E2 (PGE₂) is one of the main catabolic factors involved in OA. Its synthesis is the result of cyclooxygenase (COX) and prostaglandin E synthase (PGES) activities whereas NAD+-dependent 15 hydroxy prostaglandin dehydrogenase (15-PGDH) is the key enzyme implicated in the catabolism of PGE₂. For both COX and PGES, three isoforms have been described: in cartilage, COX-1 and cytosolic PGES are constitutively expressed whereas COX-2 and microsomal PGES type 1 (mPGES-1) are inducible in an inflammatory context. COX-3 (a variant of COX-1) and mPGES-2 have been recently cloned but little is known about their expression and regulation in cartilage, as is also the case for 15-PGDH. We investigated the regulation of the genes encoding COX and PGES isoforms during mechanical stress applied to cartilage explants. Mouse cartilage explants were subjected to compression (0.5 Hz, 1 MPa) for 2 to 24 hours. After determination of the amount of PGE₂ released in the media (enzyme immunoassay), mRNA and proteins were extracted directly from the cartilage explants and analyzed by real-time RT-PCR and western blotting respectively. Mechanical compression of cartilage explants significantly increased PGE₂ production in a time-dependent manner. This was not due to the synthesis of IL-1, since pretreatment with interleukin 1 receptor antagonist (IL1-Ra) did not alter the PGE₂ synthesis. Interestingly, COX-2 and mPGES-1 mRNA expression significantly increased after 2 hours, in parallel with protein expression, whereas COX-3 and mPGES-2 mRNA expression was not modified. Moreover, we observed a delayed overexpression of 15-PGDH just before the decline of PGE₂ synthesis after 18 hours, suggesting that PGE₂ synthesis could be altered by the induction of 15-PGDH expression. We conclude that, along with COX-2, dynamic compression induces mPGES-1 mRNA and protein expression in cartilage explants. Thus, the mechanosensitive mPGES-1 enzyme represents a potential therapeutic target in osteoarthritis.

Toll-like receptors and chondrocytes: The lipopolysaccharide-induced decrease in cartilage matrix synthesis is dependent on the presence of toll-like receptor 4 and antagonized by bone morphogenetic protein 7

OBJECTIVE

To assess the presence of Toll-like receptors (TLRs) 1-9 in human articular cartilage, and to investigate the effects of lipopolysaccharide (LPS)-induced activation of TLR-4 on biosynthetic activity and matrix production by human articular chondrocytes.

METHODS

TLRs 1-9 were assessed in human articular cartilage by reverse transcription-polymerase chain reaction (RT-PCR); TLR-4 was also analyzed by Western blotting and immunohistochemistry. Articular chondrocytes were isolated from human donors and from wild-type or TLR-4(-/-) mice. Chondrocyte monolayer cultures were incubated with interleukin-1beta (IL-1beta) and LPS in the absence or presence of bone morphogenetic protein 7 (BMP-7) and IL-1 receptor antagonist (IL-1Ra). Neosynthesis of sulfated glycosaminoglycans (sGAG) was measured by (35)S-sulfate incorporation. Endogenous gene expression of cartilage markers as well as IL-1beta was examined using RT-PCR. The involvement of p38 kinase or p44/42 kinase (ERK-1/2) in LPS-mediated TLR-4 signaling was investigated by immunoblotting, RT-PCR, and sGAG synthesis.

RESULTS

TLRs 1-9 were found on the messenger RNA (mRNA) level in human articular chondrocytes. The presence of TLR-4 was also observed on the protein level. In murine and human articular chondrocytes, but not in chondrocytes derived from TLR-4(-/-) mice, stimulation with LPS resulted in a decrease in total proteoglycan synthesis. IL-1beta mRNA expression was increased by TLR-4 activation, whereas expression of aggrecan and type II collagen was significantly decreased. The presence of BMP-7 and IL-1Ra antagonized the anti-anabolic effects of LPS. Blocking of p38, but not ERK-1/2, resulted in inhibition of both LPS-mediated IL-1beta gene expression and the negative effects of LPS on matrix biosynthesis.

CONCLUSION

These data demonstrate the presence of TLRs in human articular cartilage. The suppressive effects of LPS on cartilage biosynthetic activity are dependent on the presence of TLR-4, are governed, at least in part, by an up-regulation of IL-1beta, and are mediated by p38 kinase. These in vitro data indicate an anti-anabolic effect of TLR-4 in articular chondrocytes that may hamper cartilage repair in various joint diseases.

The mammalian chitinase-like lectin, YKL-40, binds specifically to type I collagen and modulates the rate of type I collagen fibril formation

YKL-40 is expressed in arthritic cartilage and produced in large amounts by cultured chondrocytes, but its exact role is unclear, and the identities of its physiological ligands remain unknown. Purification of YKL-40 from resorbing bovine nasal cartilage and chondrocyte monolayers demonstrated the existence of three isoforms, a major and minor form from resorbing cartilage and a third species from chondrocytes. Affinity chromatography experiments with purified YKL-40 demonstrated specific binding of all three forms to collagen types I, II, and III, thus identifying collagens as potential YKL-40 ligands. Binding to immobilized type I collagen was inhibited by soluble native ligand, but not heat-denatured ligand, confirming a specific interaction. Binding of the chondrocyte-derived species to type I collagen was also demonstrated by surface plasmon resonance analysis, and the dissociation rate constant was calculated (3.42 x 10(-3) to 4.50 x 10(-3) s(-1)). The chondrocyte-derived species was found to prevent collagenolytic cleavage of type I collagen and to stimulate the rate of type I collagen fibril formation in a concentration-dependent manner. By contrast, the cartilage major form had an inhibitory effect on type I collagen fibrillogenesis. Digestion with N-glycosidase F, endoglycosidase H and lectin blotting did not reveal any difference in the carbohydrate component of these two YKL-40 species, indicating that this does not account for the opposing effects on fibril formation rate.

Inflammatory Cytokines Induce Production of CHI3L1 by Articular Chondrocytes

Elevated levels of CHI3L1 (chitinase-3-like protein 1) are associated with disorders exhibiting increased connective tissue turnover, such as rheumatoid arthritis, osteoarthritis, scleroderma, and cirrhosis of the liver. This secreted protein is not synthesized in young healthy cartilage, but is produced in cartilage from old donors or patients with osteoarthritis. The molecular processes governing the induction of CHI3L1 are currently unknown. To elucidate the molecular events involved in CHI3L1 synthesis, we investigated two models of articular chondrocytes: neonatal rat chondrocytes, which do not express CHI3L1, and human chondrocytes, which express CHI3L1 constitutively. In neonatal rat chondrocytes, the inflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 potently induced steady-state levels of CHI3L1 mRNA and protein secretion. Treatment of chondrocytes with TNF-alpha for as little as 1 h was sufficient for sustained induction up to 72 h afterward. Using inhibitors selective for the major signaling pathways implicated in mediating the effects of TNF-alpha and interleukin-1, only inhibition of NF-kappaB activation was effective in curtailing cytokine-induced expression, including after removal of the cytokine, indicating that induction and continued production of CHI3L1 are controlled mainly by this transcription factor. Inhibition of NF-kappaB signaling also abolished constitutive expression by human chondrocytes. Thus, induction and continued secretion of CHI3L1 in chondrocytes require sustained activation of NF-kappaB. Selective induction of CHI3L1 by cytokines acting through NF-kappaB coupled with the known restriction of the catabolic responses by CHI3L1 in response to these inflammatory cytokines represents a key regulatory feedback process in controlling connective tissue turnover.

Proteomic Analysis of Articular Cartilage Shows Increased Type II Collagen Synthesis in Osteoarthritis and Expression of Inhibin βA (Activin A), a Regulatory Molecule for Chondrocytes

We show that proteomic analysis can be applied to study cartilage pathophysiology. Proteins secreted by articular cartilage were analyzed by two-dimensional SDS-PAGE and mass spectrometry. Cartilage explants were cultured in medium containing [35S]methionine/cysteine to radiolabel newly synthesized proteins. To resolve the cartilage proteins by two-dimensional electrophoresis, it was necessary to remove the proteoglycan aggrecan by precipitation with cetylpyridinium chloride. 50-100 radiolabeled protein spots were detected on two-dimensional gels of human cartilage cultures. Of 170 silver-stained proteins identified, 19 were radiolabeled, representing newly synthesized gene products. Most of these were known cartilage constituents. Several nonradiolabeled cartilage proteins were also detected. The secreted protein pattern of explants from 12 osteoarthritic joints (knee, hip, and shoulder) and 14 nonosteoarthritic adult joints were compared. The synthesis of type II collagen was strongly up-regulated in osteoarthritic cartilage. Normal adult cartilage synthesized little or no type II collagen in contrast to infant and juvenile cartilage. Potential regulatory molecules novel to cartilage were identified; pro-inhibin betaA and processed inhibin betaA (which dimerizes to activin A) were produced by all the osteoarthritic samples and half of the normals. Connective tissue growth factor and cytokine-like protein C17 (previously only identified as an mRNA) were also found. Activin induced the tissue inhibitor for metalloproteinases-1 in human chondrocytes. Its expression was induced in isolated chondrocytes by growth factors or interleukin-1. We conclude that type II collagen synthesis in articular cartilage is down-regulated at skeletal maturity and reactivated in osteoarthritis in attempted repair and that activin A may be an anabolic factor in cartilage.