Enhanced breakdown of bovine articular cartilage proteoglycans by conditioned synovial medium in vitro: a possible role for prostaglandins

Do arachidonic acid and its metabolites, secreted by rheumatoid and osteoarthritic synovial tissue, account for the strong inhibition of DNA synthesis in cultured human articular chondrocytes? A novel approach to the mechanism of tissue damage

AIMS

To further characterize factors secreted in vitro by osteoarthritic and rheumatoid arthritis synovial membranes that inhibit DNA synthesis by cultured human articular chondrocytes, and extend these findings to synovial fluid.

MATERIAL AND METHODS

Synovial tissue, synovial fluid and articular cartilage were obtained at surgery from two patients suffering rheumatoid arthritis and two other patients suffering from osteoarthritis. Synovial tissue was incubated in DMEM, then condition media and synovial fluids were extracted with methanol. Methanol extracts and extracted residues (hyaluronic acid, proteins) were assayed for their capacity to inhibit DNA synthesis in articular chondrocytes. Methanol extracts were also fractionated by thin layer chromatography on silica-coated plates and recovered fractions similarly tested.

RESULTS

All extracts exhibited strong and concentration-dependent inhibition of [3H]-thymidine incorporation. The most potent inhibition was obtained with the extracts from rheumatoid joints and the least potent inhibition was with synovial fluids. The removal of active substances with methanol leaves an inactive residue. Methanol extraction does not alter the mitogenic activity of five exogenous growth factors and two cytokines, thus suggesting that such activity is entirely due to lipids. The bulk of anti-mitotic factors extracted by methanol co-migrate when fractionated by thin layer chromatography on silica-coated plates with arachidonic acid and its lipo-oxygenase metabolites.

IN CONCLUSION

Inflamed synovium produces and releases lipids, most probably arachidonic acid metabolites that inhibit cell proliferation thus limiting inflammation and pannus formation in arthritis.

NSAIDs and cartilage metabolism

Data from experimental studies on the influence of non-steroidal anti-inflammatory drugs (NSAIDs) on cartilage is reviewed. There is evidence from this data that some NSAIDs exert a harmful effect, but also that other NSAIDs do not influence cartilage metabolism or could even have a moderate beneficial effect. So far the experimental data must be interpreted with great caution to the clinical situation and no firm conclusions could be drawn from the few clinical reports available. However, the high frequency of other side effects of NSAIDs in the elderly calls a restrictive prescription of these drugs to osteoarthritic patients.

What's happened to catabolin?

Unlike other areas of research into arthritis, it is pleasing to record independent work leading to convergence of ideas instead of divergence and increasing complexity. Catabolin is now identified with interleukin-1, although there is case for the retention of the original name to describe its chondrocyte-stimulating function with subsequent matrix degradation. In 10 years, catabolin/interleukin-1 research has yielded much information about cellular interactions within the arthritic joint and has provided a probe with which the intricacies of matrix metabolism are being unravelled. In future, there are prospects of characterizing the naturally occurring inhibitors and developing new compounds which influence the actions of catabolin/IL-1 to the benefit of the patient with arthritis.

Bradykinin, a new potential mediator of inflammation-induced bone resorption. Studies of the effects on mouse calvarial bones and articular cartilage in vitro

The effect of bradykinin and desArg9-bradykinin on bone was studied in cultures of calvarial bones taken from 6-7-day-old mice. Bradykinin, at and above a 3-nM concentration, caused a dose-dependent stimulation of bone mineral mobilization and matrix degradation. Bradykinin-stimulated resorption was inhibited by calcitonin, an increased concentration of phosphate in the culture medium, hydrocortisone, dexamethasone, indomethacin, meclofenamic acid, naproxen, and 5, 8, 11, 14-eicosatetraenoic acid. The results suggest that bradykinin stimulates osteoclast-mediated bone resorption by a process that is dependent on endogenous prostaglandin production. The stimulatory effect of bradykinin, but not of parathyroid hormone and prostaglandin E2, was potentiated by the angiotensin-converting enzyme inhibitor, BPP5a. Treatment with carboxypeptidase B did not affect the capacity of the peptide to stimulate 45Ca release. DesArg9-bradykinin (1 mumole/liter) stimulated 45Ca release to the same degree as did bradykinin. Bradykinin (3 microM) did not affect the degradation of cartilage proteoglycans, as assessed by the release of 35S-sulfate from prelabeled calf articular cartilage in organ culture. These findings suggest that generation of bradykinin in inflammatory lesions of rheumatoid arthritis and periodontitis may contribute to the bone resorptive process seen in the joints and alveolar bone; however, bradykinin does not directly activate chondrocytes into a catabolic state.

Chondrocyte activation in response to factor(s) produced by a continuous line of lapine synovial fibroblasts

We have established a permanent line of lapine synovial fibroblasts called HIG-82. Upon appropriate stimulation, these cells mimicked primary cultures of lapine synovial cells in producing substances which activated primary cultures of lapine articular chondrocytes. Activated chondrocytes secreted prostaglandin E2 (PGE2) and latent neutral collagenase, gelatinase, and caseinase, but not acid hydrolases, into their culture media. PGE2 itself did not activate the chondrocytes. Heating the crude, synovial-conditioned media at 70 degrees C for 30 min reduced their activating activity by 49.3 +/- 20.5% (n = 7). Production of PGE2 by chondrocytes was maximal during the first day of exposure to synovial conditioned media, whereas the production of neutral proteinases peaked during the second day. All the chondrocyte-stimulating activity was present in a fraction of Mr 10,000-25,000. Unlike the crude conditioned medium, this partially-purified material retained full activity following heating to 70 degrees C for 30 min. These data indicate that synovial fibroblasts (type B synoviocytes) are a source of chondrocyte activator(s) and that neutral, but not acid, proteinases may be involved in extracellular proteolysis which leads to the resorption of the cartilaginous matrix seen in bioassays of catabolin.

Incorporation of [35S]sulphate and [3H]thymidine into isolated bovine articular chondrocytes in vitro. Effect of autogenous conditioned synovial medium

The present work is a part of a study series concerned with the metabolic events taking place in the synovial joint tissues. We have previously reported that synovial products not only enhance the degradation of articular cartilage proteoglycans but also inhibit the synthesis of articular cartilage proteoglycans, and that these cellular interactions may be partly modulated by arachidonic acid metabolites. In the experiments reported here we show that the addition of conditioned synovial medium to confluent monolayer cultures of articular chondrocytes not only inhibits the synthesis of proteoglycans, measured as the incorporation of radiosulphate into cartilage proteoglycans but also inhibits the synthesis of DNA, measured as the incorporation of radiothymidine into the chondrocytes, in a dose-dependent manner. In our culture system, calf knee-joint synovial tissue produces PGE, and it has been reported that previous exposure to PGE2 affects the ability of the cell to respond to that same hormone. In these experiments, previous exposure to conditioned synovial medium affected neither the inhibition of the incorporation of radiosulphate nor the inhibition of the incorporation of radiothymidine.

Effect of indomethacin and hydrocortisone upon joint tissue in vitro. Incorporation of [35S]sulphate into chondrocyte proteoglycans

It has previously been shown that products from the synovial tissue influence the behaviour of the chondrocyte in such a way that, not only is the degradation enhanced, but the synthesis of cartilage proteoglycans too is inhibited. In the case of cartilage degradation, these cellular interactions within the joint tissues have been shown to be partly modulated by arachidonic acid metabolites. The experiments reported here show that these cellular interactions cannot be completely ruled out in the case of cartilage synthesis either. Conditioned medium from synovial tissue (control SM) reduced the incorporation of [35S]sulphate into the monolayer cultures of chondrocytes. This reduction was not affected with the simultaneous addition of indomethacin (1.4 X 10(-6) mol/l) to the chondrocyte cultures. However, conditioned synovial medium from synovial tissue that had been cultured with indomethacin (1.4 X 10(-5) mol/l) enhanced radiosulphate incorporation, somewhat, as compared with control SM. Control SM together with hydrocortisone (2.2 X 10(-8) mol/l, 2.2 X 10(-7) mol/l, 2.2 X 10(-6) mol/l) reduced radiosulphate incorporation into the chondrocyte cultures in a dose-dependent manner. Conditioned synovial medium from synovial tissue that had been cultured with hydrocortisone (2.2 X 10(-8) mol/l, 2.2 X 10(-7) mol/l, 2.2 X 10(-6) mol/l) enhanced radiosulphate incorporation, as compared with control SM.

Continuous mechanical pressure and joint tissue. Effect of synovial membrane products and indomethacin in vitro

We have previously reported that a continuous mechanical pressure of approximately 30 kgfcm-2 greatly inhibited the degradation of articular proteoglycans in calves. In the present report it is shown that indomethacin (5 micrograms/ml) failed to inhibit cartilage degradation caused by mechanical pressure. It is therefore suggested that prostaglandins do not mediate the inhibition of cartilage degradation exerted by continuous mechanical pressure. We have also reported previously that bovine-conditioned synovial medium (SM) greatly enhances the degradation of calf articular cartilage proteoglycans. We now show that the catabolic activity of SM was greatly (but not entirely) inhibited by the application of a continuous mechanical pressure. Simultaneous addition of indomethacin (5 micrograms/ml) to the cartilage cultures did not affect this result. However, the addition of indo-SM (SM from synovial tissue cultured in the presence of 5 micrograms/ml indomethacin) to cartilage cultures subjected to continuous mechanical pressure did not increase cartilage degradation, compared with the addition of DMEM alone. A hypothesis is proposed that continuous mechanical pressure may cause a change in the state of synovial factor receptors on the membranes of chondrocytes.

Nonsteroidal anti-inflammatory drugs and modulation of cartilaginous changes in osteoarthritis and rheumatoid arthritis. Clinical implications

Nonsteroidal anti-inflammatory drugs have a potential for modifying the complex pathophysiologic events leading to cartilage destruction in various forms of arthritis. Following an evaluation of basic mechanisms in the pathogenesis of cartilaginous destructive lesions, the effects of nonsteroidal anti-inflammatory drugs on normal chondrocyte metabolism are discussed. Their capacity to modulate cartilage and bone lesions in experimental forms of arthritis is addressed, as is the manner in which they may modify the pathophysiology of cartilage destruction in human forms of arthritis. Different classes of nonsteroidal anti-inflammatory drugs produce different effects in certain in vivo or in vitro settings.

Synthesis of articular cartilage proteoglycans by isolated bovine chondrocytes. Effect of autogenous conditioned synovial medium in vitro

It was previously reported that bovine conditioned synovial medium (SM) has a catabolic activity on articular cartilage proteoglycans in an organ culture system. The question thus arose whether SM had an anabolic effect as well on articular cartilage proteoglycans. In this report it is shown that addition of autogenous SM to isolated bovine articular chondrocytes inhibited the synthesis of proteoglycans as measured by the incorporation of radiosulphate. Already after 4 hours an inhibition was seen in the pericellular (matrix) chondroitin sulphate (MA-CS) fraction, and in the culture medium chondroitin sulphate (CM-CS) fraction it was significant after 72 hours. When the synovial tissue was cultured with indomethacin (1.4 X 10(-5) mol/l) and this indo-SM was added to the chondrocyte cultures, the ability of the chondrocytes to incorporate [35S]sulphate was decreased further. It therefore seems evident that the products from the synovial tissue influences the behaviour of the chondrocyte in such a way that not only is the degradation enhanced but also the synthesis of cartilage proteoglycans is inhibited.