The effect of retinoic acid on proteoglycan biosynthesis in bovine articular cartilage cultures

Identification of RALDH2 as a visually regulated retinoic acid synthesizing enzyme in the chick choroid

PURPOSE

All-trans-retinoic acid (atRA) has been implicated in the local regulation of scleral proteoglycan synthesis in vivo. The purpose of the present study was to identify the enzymes involved in the synthesis of atRA during visually guided ocular growth, the cells involved in modulation of atRA biosynthesis in the choroid, and the effect of choroid-derived atRA on scleral proteoglycan synthesis.

METHODS

Myopia was induced in White leghorn chicks by form deprivation for 10 days, followed by up to 15 days of unrestricted vision (recovery). Expression of atRA synthesizing enzymes was evaluated by semiquantitative qRT-PCR, in situ hybridization, and immunohistochemistry. atRA synthesis was measured in organ cultures of isolated choroids using LC-tandem MS quantification. Scleral proteoglycan synthesis was measured in vitro by the incorporation of (35)SO(4) in CPC-precipitable glycosaminoglycans. RESULTS; RALDH2 was the predominant RALDH transcript in the choroid (> 100-fold that of RALDH3). RALDH2 mRNA was elevated after 12 and 24 hours of recovery (60% and 188%, respectively; P < 0.01). The atRA concentration was significantly higher in cultures of choroids from 24-hour to 15-day recovering eyes than in paired controls (-195%; P < 0.01). Choroid conditioned medium from recovering choroids inhibited proteoglycan synthesis to 43% of controls (P < 0.02, paired t-test; n = 16) and produced a relative inhibition corresponding to a RA concentration of 7.20 × 10(-8) M.

CONCLUSIONS

The results of this study suggest that RALDH2 is the major retinal dehydrogenase in the chick choroid and is responsible for increased atRA synthesis in response to myopic defocus.

Inhibitors of collagenase but not of gelatinase reduce cartilage explant proteoglycan breakdown despite only low levels of matrix metalloproteinase activity

Aims-To investigate the level of matrix metalloproteinase activity during the time-course of cartilage explant proteoglycan breakdown; to determine the effects of selective small-molecule inhibitors of matrix metalloproteinases on proteoglycan degradation.Methods-The levels of matrix metalloproteinase activity in cartilage explant cultures and conditioned media were monitored by use of a quenched fluorescent substrate. The constants for inhibition of certain matrix metalloproteinases by a series of synthetic inhibitors were determined. Bovine and human cartilage explant cultures were treated with interleukin-1, tumor necrosis factor or retinoic acid and the amount of proteoglycan released into the culture medium in the absence and presence of the inhibitors was quantified. Control experiments, examining the inhibition of other proteinases, and investigating possible toxic or non-specific effects of the inhibitors, were carried out.Results-The profile of inhibition of proteoglycan release suggested the involvement of interstitial collagenase-like, rather than gelatinase- or possibly stromelysin-like, proteinases. No evidence was found for toxic or non-specific mechanisms of inhibition. Very low levels of activity of the known matrix metalloproteinases were present during the time-course of aggrecan breakdown.Conclusions-A novel collagenase-like proteinase(s) may be involved in cartilage proteoglycan breakdown. Gelatinase-type matrix metalloproteinases do not seem to be involved in this process. Specific collagenase inhibitors may be therapeutically efficacious in the treatment of arthritis.

Keratan sulphate in the interglobular domain has a microstructure that is distinct from keratan sulphate elsewhere on pig aggrecan

The microstructure of keratan sulphate purified from the interglobular domain, the keratan sulphate-rich region and total aggrecan was compared using fluorophore-assisted-carbohydrate-electrophoresis. Keratan sulphate in the interglobular domain was substantially less sulphated than keratan sulphate elsewhere on aggrecan, based on the ratio of unsulphated: monosulphated disaccharides generated by endo-beta-galactosidase digestion, and the ratio of monosulphated: disulphated disaccharides generated by keratanase II digestion. The ratio of unsulphated: monosulphated: disulphated disaccharides was 1:4:5 for keratan sulphate from total aggrecan and the keratan sulphate-rich region, but only 1:0.9:0.8 for the interglobular domain. These results show that keratan sulphate in the interglobular domain of pig aggrecan has a microstructure that is distinct from keratan sulphate in the keratan sulphate-rich region.

Calcium pentosan polysulfate inhibits the catabolism of aggrecan in articular cartilage explant cultures

OBJECTIVE

The catabolism of aggrecan and loss of aggrecan fragments from articular cartilage is a key event in the pathogenesis of arthritic diseases such as osteoarthritis. The catabolism of aggrecan is mediated by the specific proteolytic activity termed aggrecanase. The aim of this study was to investigate the effect of the chondroprotective agent calcium pentosan polysulfate (CaPPS) on the aggrecanase-mediated catabolism of aggrecan.

METHODS

The catabolism of 35S-labeled aggrecan and loss of tissue glycosaminoglycans (GAGs) were investigated using bovine articular cartilage explant cultures maintained in medium containing varying concentrations of CaPPS (1-100 microg/ml) in the presence or absence of 10(-6)M retinoic acid or 7 ng/ml recombinant human interleukin-1alpha (rHuIL-1alpha). In addition, the effect of CaPPS on the degradation of aggrecan monomers by aggrecanase activity present in conditioned medium from joint capsule explant cultures was investigated.

RESULTS

CaPPS inhibited the catabolism of 35S-labeled aggrecan in a dose-dependent manner, particularly when retinoic acid or rHuIL-1alpha was used to stimulate aggrecan catabolism. These effects were reflected in the tissue levels of GAG remaining in these cultures at the end of the experiment. CaPPS inhibited the degradation of aggrecan monomers by soluble aggrecanase activity.

CONCLUSION

CaPPS inhibits the catabolism of aggrecan by articular cartilage in a dose-dependent manner, particularly when the processes responsible for aggrecan loss are stimulated. This effect occurs, at least in part, through direct inhibition of aggrecanase activity. CaPPS did not adversely affect overall chondrocyte metabolism, as shown by the incorporation of 35S-sulfate and 3H-leucine into macromolecules and by lactate production in cartilage explant cultures.

Bovine joint capsule and fibroblasts derived from joint capsule express aggrecanase activity

Bovine joint capsule was maintained in explant culture in the presence of bovine aggrecan monomer and it was shown that the aggrecan monomer was degraded. Amino-terminal sequence analysis of the resulting aggrecan core protein fragments revealed that the core protein was cleaved at five specific sites attributed to glutamyl endopeptidases referred to as aggrecanase activity. Fibroblast cultures were established from explant cultures of joint capsule and when these cells were exposed to aggrecan, cleavage of the core protein of aggrecan at the aggrecanase sites was observed. Inclusion of either retinoic acid or interleukin-1alpha in medium of either joint capsule explant cultures or fibroblast cultures did not increase the rate of cleavage of exogenous aggrecan present in the culture medium. When aggrecan monomer was incubated with conditioned medium from explant cultures of joint capsule maintained in medium, degradation could be detected after 10 min. After a 6-h incubation period the same fragments of aggrecan core protein were observed as those for tissue or cells incubated directly with aggrecan monomer. RT-PCR analysis of mRNA extracted from joint capsule fibroblasts showed that these cells express both aggrecanase-1 and -2 [ADAMTS-2 (Tang) and ADAMTS-5].

Physical and biological regulation of proteoglycan turnover around chondrocytes in cartilage explants. Implications for tissue degradation and repair

The development of clinical strategies for cartilage repair and inhibition of matrix degradation may be facilitated by a better understanding of (1) the chondrocyte phenotype in the context of a damaged extracellular matrix, and (2) the roles of biochemical and biomechanical pathways by which matrix metabolism is mediated. Using methods of quantitative autoradiography, we examined the cell-length scale patterns of proteoglycan deposition and turnover in the cell-associated matrices of chondrocytes in adult bovine and calf cartilage explants. Results highlight a rapid turnover in the pericellular matrix, which may indicate spatial organization of PG metabolic pools, and specific biomechanical roles for different matrix regions. Subsequent to injurious compression of calf explants, which resulted in grossly visible tissue cracks and caused a decrease in the number of viable chondrocytes within explants, cell-mediated matrix catabolic processes appeared to increase, resulting in apparently increased rates of proteoglycan turnover around active cells. Furthermore, the influences of cell-stimulatory factors such as IL-1 beta appeared to be delayed in their effects subsequent to injurious compression, suggesting interactions between biomechanical and biochemical pathways of PG degradation. These results may provide a useful reference point in the development of in vitro models for cartilage injury and disease, and hint at possible new approaches in the development of cartilage repair strategies.

A serine proteinase inactivator inhibits chondrocyte-mediated cartilage proteoglycan breakdown occurring in response to proinflammatory cytokines

The role played by serine proteinases with trypsin-like specificity in chondrocyte-mediated cartilage proteoglycan breakdown was investigated by use of a selective proteinase inactivator, 7-amino-4-chloro-3-(-3-isothiureidopropoxy)isocoumarin, in explant culture systems. This compound was a rapid inactivator of urokinase-type plasminogen activator. It potently inhibited interleukin 1- and tumor necrosis factor-stimulated proteoglycan release from both nasal and articular cartilage. Its less potent inhibition of basal and retinoic acid-stimulated release appeared to be due to cytotoxic effects. The functional half-life of the inactivator in culture medium was 95 min, and its concentration in cartilage was 2.5-fold higher than in the surrounding medium. Following spontaneous hydrolysis the breakdown products of the inactivator were unable to inhibit proteoglycan release. Trypsin-like activity was demonstrated by enzyme histochemistry to be chondrocyte-associated and inhibited by the serine proteinase inactivator. Cell-associated and secreted plasminogen activator activity was detected by zymography. These results suggest the involvement of a serine proteinase(s) with trypsin-like specificity, possibly urokinase-type plasminogen activator, in chondrocyte-mediated cartilage proteoglycan breakdown occurring as a result of stimulation with proinflammatory cytokines. Basal proteoglycan breakdown may occur via a different pathway. Our findings point to a pathological role for serine proteinase(s) in the development of cartilage diseases such as arthritis, possibly in a cascade which results in the activation of the enzyme(s) directly responsible for proteoglycan breakdown. It remains to be shown whether the target serine proteinase is urokinase-type plasminogen activator.

Proteoglycan breakdown from bovine nasal cartilage is increased, and from articular cartilage is decreased, by extracellular ATP

The addition of ATP, but not ADP or AMP, to the culture media of bovine nasal cartilage explants caused an acceleration in the rate of proteoglycan loss from the tissue. The ATP-stimulated loss of proteoglycan was not inhibited by the IL1-receptor antagonist protein, but was partially inhibited by the presence of ADP or AMP. The proteolytic events resulting from the presence of ATP were found to be similar to those following treatment with IL1, in that inhibitors of the cysteine-peptidase cathepsin B, serine-proteinases with trypsin-like specificity, and of some of the matrixins, could all prevent proteoglycan loss, which was mediated, at least in part, by the action of 'aggrecanase'. In contrast to its effects on nasal cartilage, ATP inhibited basal and stimulated proteoglycan release from articular cartilage. Both ADP and AMP had no effect on proteoglycan release in articular cartilage but enhanced the response to ATP when added concurrently. We conclude that extracellular ATP, probably acting via P2-purinoceptors, stimulates proteoglycan breakdown from bovine nasal cartilage and thus, may have a role in diseases which primarily involve destruction of non-articular cartilage. Extracellular ATP has, in contrast, a chondroprotective effect on bovine articular cartilage.

Inhibition of cartilage degradation and changes in physical properties induced by IL-1beta and retinoic acid using matrix metalloproteinase inhibitors

Bovine cartilage explants were treated with 100 ng/ml recombinant human interleukin-1beta (IL-1beta) or 1 microM all-trans retinoic acid (RA) and changes in biochemical, biomechanical, and physicochemical properties were assessed. Additionally, samples cultured with IL-1beta or RA were treated with 4 microM recombinant human tissue inhibitor of metalloproteinases-1 (TIMP-1) or a synthetic metalloproteinase inhibitor (L-758,354) to inhibit this degradation. Treatment with IL-1beta or RA each resulted in >90% GAG loss after 8 days in culture. Addition of TIMP or L-758,354 to the culture media inhibited IL-1beta-induced loss of tissue GAG by 40 and 65%, respectively, and inhibited RA-induced GAG loss by 35 and 65%, respectively. Analysis of degradation products in the culture media using a G1 antibody indicated that IL-1beta- and RA-treated plugs released 68-kDa fragments of aggrecan, corresponding to a segment of the aggrecan core protein from the G1 domain to the C-terminus NITEGE, consistent with "aggrecanase" activity. Release of the G1 fragment was inhibited by treatment with L-758,354. Both IL-1beta and RA induced significant loss of hyaluronan from cartilage explants after 8 days of exposure and HA loss was also inhibited by addition of L-756,354 to the culture media. IL-1beta, but not RA, induced a significant increase in swelling ratio (wet weight in 0.01 M NaCl normalized to wet weight in DMEM) after 8 days in culture, consistent with degradation of the collagen network, and the increase in tissue swelling was inhibited by treatment with TIMP-1 or L-758,354. Exposure to IL-1beta or RA resulted in significant changes in cartilage physical properties including streaming potential, equilibrium modulus, hydraulic permeability, and electrokinetic coupling coefficient after 8 days in culture, and these changes were inhibited by 40-90% by exposure to TIMP and by 50-90% by exposure to L-758,354. Measurement of dynamic streaming potential showed that changes due to treatment with IL-1beta alone were highly dependent in compression frequency, with dramatic changes seen at high frequency prior to changes in mechanical properties, and little initial change seen at low frequency. Streaming potential and equilibrium modulus of explants treated with RA decreased to 10% of their initial values after 8 days in culture, but decreased to only 40 and 90%, respectively, when treated with RA plus TIMP-1.

Effects of proteoglycan modification on mineral formation in a differentiating chick limb-bud mesenchymal cell culture system

In the presence of 4 mM inorganic phosphate, differentiating chick limb-bud mesenchymal cells plated in micromass cultures form a mineralized matrix resembling that of chick calcified cartilage. To test the hypothesis that cartilage proteoglycans are inhibitors of cell mediated mineralization, the synthesis, content, and turnover of proteoglycans were altered in this system, and the extent of mineralization and properties of the mineral crystals examined. In all cases where the proteoglycan synthesis or proteoglycans present were modified to provide fewer or smaller molecules, mineralization was enhanced. Specifically, when proteoglycan synthesis was blocked by treatment with 10(-10) M retinoic acid, extensive mineral deposition occurred on a matrix devoid of both proteoglycans and cartilage nodules. The crystals, which formed rapidly, were relatively large in size based on analysis by X-ray diffraction or FT-1R microspectroscopy, and were more abundant than in controls. When 2.5 or 5 mM xylosides were used to cause the synthesis of smaller proteoglycans, the extent of mineral accretion was also increased relative to controls; however, the matrix was less affected, and the extent of mineral deposition and the size of the crystals were not as markedly altered as in the case of retinoic acid. Modification of existing proteoglycans by either chondroinase ABC or hyaluronidase treatment similarly resulted in increased mineral accretion (based on 45Ca uptake or total Ca uptake) relative to cultures in which the proteoglycan content was not manipulated. Crystals were more abundant and larger than in control mineralizing cultures. In contrast, when proteoglycan degradation by metalloproteases was inhibited by metal chelation with o-phenanthroline, the Ca accretion at early time points was increased, but as mineralization progressed, Ca accumulation decreased. These data provide evidence that in this culture system, proteoglycans are inhibitors of mineralization.

Cyclic loading is harmful to articular cartilage from which proteoglycans have been partially depleted by retinoic acid

We studied whether cyclic loading is harmful to degraded cartilage. Sets of four cartilage-bearing sesamoid bones were dissected from 5-year old cows. One bone from each set was cultured for 17 h in control medium to serve as an ex vivo control. The three others were cultured for 1 week in control medium to which 0, 10 or 300 ng/mL retinoic acid (RAc), which depletes the cartilage matrix of proteoglycans, had been added. Two were then cultured for another week in control medium. During the last week, one of the two was subjected to a cyclic load (1 MPa, 0.2 Hz). Following treatment with RAc, glycosaminoglycan content and synthesis were significantly decreased, as confirmed by safranin O staining and autoradiography. They were further diminished by loading during the second week of culture. Increased amounts of 3-B-3(-)epitope were found in cartilage that had been treated with 300 ng/mL RAc and then loaded. While loading cartilage matrix that was only slightly degraded proved to be damaging, loading severely degraded cartilage matrix apparently induced osteoarthritic-like changes.

Sulfate incorporation into organic bone matrix of the tibiotarsus of broiler chicks is reduced by excess dietary methionine

Two experiments were conducted in broiler chicks to determine whether dietary imbalances of sulfur amino acids (SAA), vitamin A, or interactions between the two nutrients could influence organic bone matrix metabolism measured with L-[35S]-methionine. In the first experiment, in vivo incorporation of 35S into the tibiotarsal bone matrix of 2-wk-old birds was unaffected by vitamin A treatment of 10 and 100 times the requirement when compared with that of birds receiving recommended amounts of vitamin A. However, 35S incorporation was significantly reduced by increasing the SAA concentration of the diet to 1.5 times the requirement relative to lysine. In the second experiment, in vitro incorporation of 35S, derived from L-[35S]-methionine, into bone matrix was reduced in birds consuming a diet containing 1.5 times the methionine requirement relative to lysine (Diet HS) when compared with those receiving .75 (Diet LS), 1.0 (Diet NS), or 1.25 (Diet MS) times the requirement. Birds consuming Diet LS incorporated significantly more 35S into organic bone matrix than birds consuming the other three diets. Although the ratio of SAA to lysine was that recommended (.76:1), on a weight basis the concentration of SAA in diet NS was relatively high (11.48 g/kg diet) compared with the NRC (1984) recommendation of 9.3 g/kg diet. The results show that excess SAA can affect organic bone matrix metabolism and suggest that SAA may play a role in the etiology of tibial dyschondroplasia. They also indicate the importance of distinguishing between nutrient content of the diet expressed as a ratio and that expressed on a weight basis.

Cytokine modulation of plasminogen activator inhibitor-1 (PAI-1) production by human articular cartilage and chondrocytes. Down-regulation by tumor necrosis factor alpha and up-regulation by transforming growth factor-B basic fibroblast growth factor

Recombinant human cytokines were examined for their effects on plasminogen activator inhibitor-1 (PAI-1) production by human articular cartilage and chondrocyte monolayer cultures. Cartilage and chondrocytes were cultured with and without added cytokines and the conditioned media assayed for PAI-1 by a specific enzyme-linked immunosorbent assay, and mRNA levels determined by Northern blot analysis. Tumor necrosis factor alpha (TNF alpha) reduced, and transforming growth factor-beta (TGF-beta) and basic fibroblast growth factor (bFGF) increased, the levels of PAI-1 antigen and mRNA in the culture fluids and cell extracts, respectively. The effects of TNF alpha and TGF-beta on PAI-1 antigen levels were both time- and concentration-dependent; optimum doses being 10-100 pM TNF alpha and 0.4-0.8 nM TGF-beta, with each cytokine exerting its effect on PAI-1 antigen levels within 8 h of addition to culture. TNF alpha (and interleukin-1 alpha) also countered the effects of TGF-beta and bFGF. The anti-inflammatory drugs, indomethacin and dexamethasone, did not appear to modulate PAI-1 levels in cultures of cartilage tissue. The inhibition of PAI-1 levels by cytokines and reagents which stimulate cartilage resorption (i.e., TNF alpha, interleukin-1 alpha, retinoic acid) and enhancement by cytokines which counter it (i.e., TGF-beta, bFGF) further implicate plasminogen activator in the mechanism(s) of cartilage degradation in diseases such as arthritis.

The interaction between retinoic acid and the transforming growth factors-beta in calf articular cartilage organ cultures

In calf articular cartilage organ cultures, retinoic acid depressed proteoglycan anabolism to levels approximately 10% of control values and increased their catabolism approximately 14-fold at concentrations of 1 x 10(-8) and 1 x 10(-6) M, respectively, leading to a severe depletion of this component from the extracellular matrix (95% loss in 3 weeks). These effects were powerfully antagonized by maximal levels of transforming growth factors-beta (TGF-beta s) 1, 2, and 3, leading to preservation of matrix components. At a concentration of 1 x 10(-8) M retinoic acid, the TGF-beta s restored anabolism to control levels and lowered catabolic rates greater than 3-fold. While the TGF-beta s increased protein synthesis 2- to 3-fold over controls, retinoic acid alone did not change protein synthesis, as determined by incorporation of [3H]serine. Nevertheless, retinoic acid effectively antagonized the stimulation of protein synthesis by TGF-beta and restored control levels of synthesis at 1 x 10(-7) M. Analysis of proteins, labeled using [3H]serine and [35S]sulfate as precursors, by SDS-PAGE revealed that large molecular weight proteins (greater than 100 kDa) were not detectable in retinoic-acid-treated cultures, but treatment with the TGF-beta s restored these components in coincubation cultures, again supporting the antagonistic role of the polypeptide effectors on retinoid action. Treatment of the cultures with retinoic acid elevated levels of TGF-beta 2 synthesis, but not TGF-beta 1. While the role of the newly synthesized TGF-beta 2 in the set of events elicited by retinoic acid in articular cartilage is unclear, the results establish an intrinsic metabolic link between the isoprenoid and TGF-beta in articular cartilage. We propose that the retinoids and TGF-beta s are integral parts of a regulatory network that controls homeostasis, resorption, or growth, depending on their relative contributions.

The secretion of the tissue inhibitor of metalloproteinases (TIMP) by human synovial fibroblasts is modulated by all-trans-retinoic acid

The matrix metalloproteinases are a family of enzymes involved in the turnover of the connective tissues. The regulation of these enzymes is complex, involving the control of synthesis, the activation of proenzyme forms and the presence of specific inhibitors. Retinoids have been reported to inhibit the production of metalloproteinases by human and rabbit synovial fibroblasts and by human skin fibroblasts. The production of the highly specific tissue inhibitor of metalloproteinases (TIMP) by connective tissue cells may be crucial in the regulation of connective tissue breakdown and this present study was undertaken to determine if retinoic acid (RA) could modulate TIMP and collagenase production by synovial fibroblasts. The results show that RA at concentrations from 10(-7) to 10(-5) M significantly stimulated the secretion of TIMP by two of three human synovial cell lines. The effect of mononuclear cell factor (MCF) on TIMP and collagenase levels was also investigated. The apparent reduction of collagenase levels in the presence of RA, could result from a failure to accurately measure this enzyme in the presence of increased levels of TIMP.

Effects of isotretinoin (13-cis-retinoic acid) on the development of mouse limbs in vivo and in vitro

Isotretinoin (13-cis-RA) is known to be teratogenic in humans and laboratory animals. The relatively low potency of 13-cis-RA in NRMI mice in comparison to the all-trans isomer has been proposed to be due to minimal transfer across the placenta (Creech-Kraft et al., '87). To further delineate the teratogenic potential of 13-cis-RA, a dose-response, temporal study was conducted in vivo and in vitro using submerged limb culture and image analysis evaluation of development. Dose-dependent embryotoxicity was produced by treatment on GD 7, while later treatments produced inconsistent effects on resorption rate and fetal weight. Treatment on either GD 7 or GD 8 produced a number of malformations in dose-dependent manner. Most common were tail and cleft palate defects, which were produced by 13-cis-RA on each of the days tested (GD 7-GD 11), with peak malformations occurring on GD 9 and GD 10 for tail and cleft palate, respectively. Most limb defects were produced after GD 10 and GD 11 exposure. The observed frequency of defects confirmed that in ICR mice 13-cis-RA is about 10-fold less potent than all-trans-RA as a limb teratogen (Kwasigroch and Kochhar, '80; Kochhar and Penner, '87). Effects observed via image analysis following maintenance of limbs in serum-free culture medium were dose dependent. Low dose treatment produced occasional polydactyly. The intermediate dose caused somewhat variable region-dependent increases in cartilaginous bone anlagen area. The high dose of 13-cis-RA produced irregular limb outlines, a reduction in bone anlagen area, and an inhibition of alcian blue staining of cartilage without affecting morphogenesis of bone anlagen. These results confirm that, when the effects of the administered doses are evaluated, 13-cis-RA is a much less potent teratogen in comparison to the all-trans isomer. More importantly, the results show that retinoids can enhance (at low and intermediate doses), depress (at high doses), or eliminate (high dose) chondrogenenic expression during limb morphogenesis in vitro. This indicates that retinoids such as 13-cis-RA can manipulate events in development in a variety of ways (i.e., produce malformations, interfere with chondrogenic expression without affecting morphogenesis, and stimulate growth) in a dose- and time-dependent manner. Although the ability of RA to act as a true morphogen has recently been questioned (Wanek et al., '91; Noji et al., '91), the results presented here support the position that RA can modulate the development of the limb (and probably other organ systems) in several vertebrate species.

The small proteoglycans of cartilage matrix

The small proteoglycans (PGs) of cartilage matrix represent a small fraction of the total mass of PGs, but with a small size they can be present in equivalent moles to the large PGs. Three types of PGs with a wide skeletal and extraskeletal distribution, biglycan (PGI), decorin (PGII) and fibromodulin have distinct but homologous core proteins containing leucin-rich sequences. Carbohydrate substituants (one or two chondroitin sulfate/dermatan sulfate chains for decorin and biglycan respectively, chains of keratan sulfate for fibromodulin and oligosaccharides) present variations from tissue to tissue and with age and other factors. Decorin and fibromodulin appear to interact with collagen and to participate in the regulation of collagen matrices. In vitro experiments indicate a role for small PGs in adhesion, multiplication, differentiation, and migration of cells. Recent data on molecular biology of the small PGs contribute to a better understanding of their functions and make the evaluation of their role in hereditary diseases.

Turnover of proteoglycans in articular-cartilage cultures. Characterization of proteoglycans released into the medium

By using an e.l.i.s.a. method it was demonstrated that the majority of proteoglycans released into the medium of both control and retinoic acid-treated explant cultures of bovine articular cartilage did not contain a hyaluronate-binding region. This supports our previous findings [Campbell & Handley (1987) Arch. Biochem. Biophys. 258, 143-155] that proteoglycans released into the medium of both cultures were of smaller hydrodynamic size, more polydisperse and unable to form aggregates with hyaluronate. Analysis of 35S-labelled core proteins associated with proteoglycans released into the medium of both cultures by using SDS/polyacrylamide-gel electrophoresis and fluorography indicated the presence of a series of core-protein bands (Mr approx. 300,000, 230,000, 215,000, 200,000, 180,000, 140,000, 135,000, 105,000, 85,000 and 60,000) compared with three core proteins derived from the proteoglycans remaining in the matrix (Mr 300,000, 230,000 and 215,000). Further analysis of the core proteins released into the medium indicated that the larger core proteins associated with medium proteoglycans contain both chondroitin sulphate and keratan sulphate glycosaminoglycans whereas the smaller core proteins contain only chondroitin sulphate chains. These experiments provide definitive evidence that the loss of proteoglycans from the matrix involves proteolytic cleavage at various sites along the proteoglycan core protein.

Biochemical and morphological studies of steady state and lipopolysaccaride treated bovine articular cartilage explant cultures

Explants of bovine articular cartilage were cultured for up to 50 days in 20% fetal calf serum in the presence or absence of the endotoxin lipopolysaccharide (LPS); or in various protocols involving different treatment times with LPS followed by recovery times in the absence of LPS. Cultures were measured in terms of rates of proteoglycan synthesis (incorporation of [35S]sulfate), proteoglycan contents and collagen contents. Histological sections were prepared for both light and electron microscopy. In fetal calf serum, the rates of synthesis and contents of proteoglycans per collagen remained constant, while for LPS treated cultures both parameters decreased. For recovery groups, the rates of proteoglycan synthesis increased during the time of recovery if the LPS treatment times were relatively short (2 weeks or less) and if the tissue was obtained from younger animals; net increase in proteoglycan contents occurred infrequently if at all during recovery protocols. Histological examinations revealed that chondrocytes in cultures maintained in fetal calf serum appeared normal with large stores of glycogen. In LPS treated cultures, chondrocytes were depleted of glycogen stores and contained numerous lipid droplets. In recovery cultures, chondrocytes replenished their glycogen contents, but the lipid droplets remained. For both LPS treated and recovery groups the extracellular matrix was depleted of proteoglycans with time in culture. The results provide further evidence for the ability of this explant culture system to maintain steady state metabolic parameters for proteoglycan metabolism over long time periods and for its utility to study reagents which regulate or perturb these parameters.

The effect of retinoic acid on proteoglycan turnover in bovine articular cartilage cultures

This paper describes proteoglycan catabolism by adult bovine articular cartilage treated with retinoic acid as a means of stimulating the loss of this macromolecule from the extracellular matrix of cartilage. Addition of retinoic acid (10(-12)-10(-6) M) to adult bovine articular cartilage which had been labeled with [35S]sulfate for 6 h after 5 days in culture, resulted in a dose-dependent increase in the rate of loss of 35S-labeled proteoglycans from the matrix of the tissue. Concomitant with this loss was a decrease in the proteoglycan content of the tissue. Incubation of cultures treated with 1 microM retinoic acid, at 4 degrees C, or with 0.5 mM cycloheximide, resulted in a significant decrease in the rate of retinoic acid-induced loss of proteoglycans and demonstrated cellular involvement in this process. Analysis of the 35S-labeled proteoglycans remaining in the matrix showed that the percentage of radioactivity associated with the small proteoglycan species extracted from the matrix of articular cartilage explants labeled with [35S]sulfate after 5 days in culture was 15% and this increased to 22% in tissue maintained in medium alone. In tissue treated with 1 microM retinoic acid for 6 days, the percentage of radioactivity associated with the small proteoglycan was 58%. Approximately 93% of the 35S-labeled proteoglycans released into the medium of control and retinoic acid-treated cultures was recovered in high density fractions after CsCl gradient centrifugation and eluted on Sepharose CL-2B as a broad peak with a Kav of 0.30-0.37. Less than 17% of these proteoglycans was capable of aggregating with hyaluronate. These results indicate that in both control and retinoic acid-treated cultures the larger proteoglycan species is lost to the medium at a greater rate than the small proteoglycan species. The effect of retinoic acid on proteoglycan turnover was shown to be reversible. Cartilage cultures maintained with retinoic acid for 1 day then switched to medium with 20% (v/v) fetal calf serum for the remainder of the culture period exhibited decreased rates of loss of 35S-labeled proteoglycans from the matrix and increased tissue hexuronate contents to levels near those observed in tissue maintained in medium with 20% (v/v) fetal calf serum throughout. Furthermore, following switching to 20% (v/v) fetal calf serum, the relative proportions of the 35S-labeled proteoglycan species remaining in the matrix of these cultures were similar to those of control cultures.