Secretion of chondroitin SO4 by monolayer cultures of chick embryo chondrocytes

Effects of the mycotoxin nivalenol on bovine articular chondrocyte metabolism in vitro

Objective

Kashin-Beck Disease (KBD) is an endemic, age-related degenerative osteoarthropathy and its cause is hypothesised to involve Fusarium mycotoxins. This study investigated the Fusarium mycotoxin Nivalenol (NIV) on the metabolism of bovine articular chondrocytes in vitro.

Design

The effect 0.0–0.5 µg/ml NIV on transcript levels of types I and II collagen, aggrecan, matrix metalloproteinases (MMPs), a disintegrin and metalloproteinase with thrombospondin motif (ADAMTS) and the tissue inhibitors of MMPs (TIMPs) was investigated using quantitative PCR. Amounts of sulphated glycosaminoglycans, MMPs and TIMPs were assessed using the Dimethylmethylene Blue assay, gelatin zymography and reverse gelatin zymography respectively. Cytoskeletal organisation was analysed using confocal microscopy and cytoskeletal gene and protein levels were measured by quantitative PCR and Western blot analysis, respectively.

Results

NIV caused a dose-dependent increase in aggrecan transcription with a concomitant retention of sGAG in the cell lysate. Furthermore, NIV significantly increased MMPs-2, -3 & -9, ADAMTS-4 and -5, and TIMP-2 and -3 transcript levels but inhibited type I collagen, MMP 1 and TIMP 1 mRNA levels. NIV promoted extensive cytoskeletal network remodelling, particularly with vimentin where a dose-dependent peri-nuclear aggregation occurred.

Conclusion

NIV exposure to chondrocytes decreased matrix deposition, whilst enhancing selective catabolic enzyme production, suggesting its potential for induction of cellular catabolism. This NIV-induced extracellular matrix remodelling may be due to extensive remodelling/disassembly of the cytoskeletal elements. Collectively, these findings support the hypothesis that trichothecene mycotoxins, and in particular NIV, have the potential to induce matrix catabolism and propagate the pathogenesis of KBD.

Involvement of a cysteine protease in the secretion process of human xylosyltransferase I

Xylosylation of core proteins takes place in the Golgi-apparatus as the transfer of xylose from UDP-xylose to specific serine residues in proteoglycan core proteins. This initial and rate-limiting step in glycosaminoglycan biosynthesis is catalyzed by human xylosyltransferase I (XT-I). XT-I is proteolytically cleaved from the Golgi surface and shed in its active form into the extracellular space. The secreted, circulating glycosyltransferase represents a serum biomarker for various diseases with an altered proteoglycan metabolism, whereas a physiological function of secreted XT-I is still unknown. To shed light on the secretion process of XT-I and on its biological function, the cleavage site was examined and the group of proteases involved in the cleavage was identified in this study. The peptide mass fingerprint from partly purified secreted XT-I revealed the cleavage site to be localized in the aminoterminal 231 amino acids. The addition of a cysteine protease inhibitor cocktail to cells recombinantly expressing XT-I led to a concentration-dependent shift of enzyme activity towards the cell lysates attended by consistent total intracellular and extracellular XT-I activities. In conclusion, our findings provide a first insight into the XT-I secretion process regulated by a cysteine protease and may contribute to understanding the biological and pathological role of this process.

Glucosamine inhibits the synthesis of glycosaminoglycan chains on vascular smooth muscle cell proteoglycans by depletion of ATP

Glucosamine via GlcNAc is a precursor for the synthesis of glycosaminoglycan (GAG) chains on proteoglycans. We previously found that proteoglycans synthesized and secreted by vascular smooth muscle cells (VSMC) in the presence of supplementary glucosamine had GAG of decreased not increased size. We investigated the possibility that the inhibition of GAG chains synthesis on proteoglycans might be related to cellular ATP depletion. Confluent primate VSMCs were exposed to glucosamine, azide, or 2-deoxyglucose (2-DG). Each of these agents depleted cell ATP content by 25-30%. All agents decreased (35)S-SO(4) incorporation and reduced the size of the proteoglycans, decorin and biglycan as assessed by SDS-PAGE. On withdrawal of the glucosamine, azide or 2-DG ATP levels and proteoglycan synthesis returned towards baseline values. Glucosamine decreased glucose uptake and consumption suggesting that ATP depletion was due preferential phosphorylation of glucosamine over glucose. Thus, glucosamine inhibition of proteoglycan synthesis is due, at least in part, to depletion of cellular ATP content.

Serum xylosyltransferase: a new biochemical marker of the sclerotic process in systemic sclerosis

UDP-D-xylose:proteoglycan core protein beta-D-xylosyltransferase (EC2.4.2.26) is the initial enzyme in the biosynthesis of chondroitin sulfate and dermatan sulfate proteoglycans in fibroblasts and chondrocytes. Secretion of xylosyltransferase into the extracellular space was determined in cultured human dermal fibroblasts. A more than 6-fold accumulation of xylosyltransferase activity in cell culture supernatant was observed (day 1, 0.6 microU per 106 cells; day 9, 4.1 microU per 106 cells); however, intracellular xylosyltransferase activity remained at a constant level (0.4 microU per 106 cells). Exposure of human chondrocytes to colchicine led to a 3-fold decreased level of xylosyltransferase and chondroitin-6-sulfate concentration in cell culture. Specific xylosyltransferase activity and chondroitin-6-sulfate concentration decreased in a concentration-dependent manner and in parallel in culture medium and accumulated 5-fold in cell lysates indicating that xylosyltransferase is secreted simultaneously into the extracellular space with chondroitin sulfate proteoglycans. Xylosyltransferase activities were determined in serum samples of 30 patients with systemic sclerosis. Xylosyltransferase activities in female (mean value 1.28 mU per liter, 90% range 1.10-1.55 mU per liter) and male patients (mean 1.39 mU per liter, 90% range 1.16-1. 57 mU per liter) with systemic sclerosis were significantly increased in comparison with blood donors of a corresponding age. Furthermore, xylosyltransferase activity was correlated with the clinical classification of systemic sclerosis. Female patients with diffuse cutaneous systemic sclerosis showed higher serum xylosyltransferase activities than patients with limited systemic sclerosis. These results confirm that the increase of proteoglycan biosynthesis in sclerotic processes of scleroderma is closely related to an elevated xylosyltransferase activity in blood and demonstrate the validity of xylosyltransferase as an additional diagnostic marker for determination of sclerotic activity in systemic sclerosis.

Molecular cloning and expression of chick chondrocyte chondroitin 6-sulfotransferase

Chondroitin 6-sulfotransferase (C6ST) catalyzes the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate to position 6 of the N-acetylgalactosamine residue of chondroitin. The enzyme has been purified previously to apparent homogeneity from the serum-free culture medium of chick chondrocytes. The purified enzyme also catalyzed the sulfation of keratan sulfate. We have now cloned the cDNA of the enzyme. This cDNA contains a single open reading frame that predicts a protein composed of 458 amino acid residues. The protein predicts a Type II transmembrane topology similar to other glycosyltransferases and heparin/heparan sulfate N-sulfotransferase/N-deacetylases. Evidence that the predicted protein corresponds to the previously purified C6ST was the following: (a) the predicted sequence of the protein contains all of the known amino acid sequence, (b) when the cDNA was introduced in a eukaryotic expression vector and transfected in COS-7 cells, both the C6ST activity and the keratan sulfate sulfotransferase activity were overexpressed, (c) a polyclonal antibody raised against a fusion peptide, which was expressed from a cDNA containing the sequence coding for 150 amino acid residues of the predicted protein, cross-reacted to the purified C6ST, and (d) the predicted protein contained six potential sites for N-glycosylation, which corresponds to the observation that the purified C6ST is an N-linked glycoprotein. The amino-terminal amino acid sequence of the purified protein was found in the transmembrane domain, suggesting that the purified protein might be released from the chondrocytes after proteolytic cleavage in the transmembrane domain.

Coordinate inhibition of alkaline phosphatase and type X collagen syntheses by 1,25-dihydroxyvitamin D3 in primary cultured hypertrophic chondrocytes

The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (2.3 x 10(-12) - 1.4 x 10(-6) [M]) on alkaline phosphatase, collagen, and cell proliferation were examined in primary cultured hypertrophic chondrocytes prepared from the distal epiphyseal growth plate of the tibias of 12-day chick embryos. 1,25(OH)2D3 showed time- and dose-dependent inhibitory effects on the alkaline phosphatase and collagen levels. The inhibition of alkaline phosphatase activity became detectable at 2 x 10(-11) [M] and reached 10% of control at 10(-7) [M]. The concentration of 1,25(OH)2D3 giving a 50% inhibition of the enzyme level was approximately 3 x 10(-10) [M]. Of the two extracellular collagen pools, a cell-associated matrix pool showed a more dramatic decrease (to 10% of control) than a culture medium pool (to 50% of control) at increased 1,25(OH)2D3 concentrations. The degree of inhibition was different for each type of chondrocyte-specific collagen (types II, IX, X, and XI). Types II and IX were inhibited in a parallel manner to only 60-80% of control. On the other hand, types X and XI were more greatly reduced up to 10% of control, and their dose-dependent inhibitory curves were similar to that of alkaline phosphatase. On cell proliferation, 1,25(OH)2D3 had a biphasic effect: stimulation at 10(-10)-10(-8) [M] and inhibition at higher levels. The results revealed the significant involvement of 1,25(OH)2D3 in the metabolism of two probable calcification-related products, alkaline phosphatase and type X collagen.

The influence of bone and marrow on cartilage hypertrophy and degradation during 30-day serum-free culture of the embryonic chick tibia

In this study, an organ culture system is defined which demonstrates complete loss of cartilage matrix from embryonic chick tibiae. Efficient loss of the cartilage matrix occurs within 30 days of serum-free culture only when the intact tibiae containing bone, marrow, and cartilage tissue are cultured. During organ culture nonhypertrophic chondrocytes become hypertrophic and stain positively for type X collagen and alkaline phosphatase. The cartilage loses Safranin O staining, and finally all cartilage matrix disappears leaving the bony collar and marrow cells. If the tibial cartilage is separated from the bony collar and cultured alone in serum-free medium, the nonhypertrophic chondrocytes also hypertrophy; the matrix loses Safranin O staining; however, some components of the matrix including type X collagen still remain after 30 days. In the presence of serum, the chondrocytes will hypertrophy but cartilage degradation is not evident. The results of this study support the conclusions that 1) hypertrophy is inherently programmed in the chondrocyte and 2) while Safranin O staining of cartilage cultured alone is diminished in serum-free organ culture, the degradation of cartilage is complete only when bone and marrow are also present.

Secretion of chondroitin 6-sulfotransferase and chondroitin 4-sulfotransferase from cultured chick embryo chondrocytes

We found that chondroitin 6-sulfotransferase and chondroitin 4-sulfotransferase were released into the culture medium from the cultured chick embryo chondrocytes. Since the release of the sulfotransferases was observed not only in serum-supplemented medium but also in serum-free medium, the released sulfotransferases were unlikely to be derived from serum. Addition of ascorbate to the serum-free medium supported the continuous release of the sulfotransferases. Monensin, which is known to cause dilatation of the Golgi apparatus and to inhibit sulfation of proteoglycan, was found to affect the release of the sulfotransferases. In the presence of 10(-6) M monensin, chondroitin 6-sulfotransferase activity in the cell layer was decreased to less than one tenth of the control, and the rate of the release of the activity became much smaller than the control after the initial rapid release. The activity of chondroitin 4-sulfotransferase was also affected by monensin, but the reduction of the chondroitin 4-sulfotransferase activity in the cell layer was not so great as the reduction of chondroitin 6-sulfotransferase activity. Unlike to the microsomal sulfotransferases, both chondroitin 6-sulfotransferase and chondroitin 4-sulfotransferase released into the culture medium were retained in the soluble fraction after centrifugation at 100,000 x g for 60 min, and were not activated by detergent. pH optimum and requirements for sulfhydryl compounds of the released sulfotransferases were similar to those observed previously in the chondroitin sulfotransferases from chick embryo cartilage and from cultured chick embryo chondrocytes. These results suggest that chondroitin sulfotransferases, which are localized in the Golgi apparatus, may be secreted to the extracellular space in a soluble form under the culture conditions.

Effects of glutathione depletion on the synthesis of proteoglycan and collagen in cultured chondrocytes

We studied the effect of the depletion of glutathione on the synthesis of proteoglycan and collagen in cultured chick chondrocytes. When the cultured chondrocytes were incubated with 1 mM buthionine sulfoximine (BSO), a specific inhibitor of gamma-glutamyl-cysteine synthetase, the intracellular glutathione level markedly dropped within 12 h with no loss of cell viability. Incorporation of 35SO2-4 into proteoglycan was lowered in the presence of BSO. When the 35S-labeled proteoglycans were separated into two fractions by glycerol density gradient centrifugation, the inhibitory effect of BSO on the synthesis of proteoglycan was greater in the fast-sedimenting proteoglycan fraction, which consisted mainly of cartilage specific large proteoglycan (PG-H), than in the slowly sedimenting proteoglycan fraction. The inhibition by BSO of the synthesis of core protein-free glycosaminoglycan chains primed by p-nitrophenyl-beta-D-xyloside was smaller than the inhibition of the synthesis of proteoglycan. Analysis of glycosaminoglycans labeled with [3H]glucosamine indicated that the treatment of chondrocytes with BSO resulted in a small increase in the proportion of synthesis of hyaluronic acid to the synthesis of total glycosaminoglycan. The incorporation of [3H]proline into collagen was also inhibited by BSO. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the 3H-labeled collagen showed that, in the presence of BSO, processing of Type II collagen appeared to slow down and the proportion of Type X collagen synthesis was reduced.

Intrinsic and extrinsic controls of the hypertrophic program of chondrocytes in the avian columella

Immunohistochemical studies of the chick columella have shown that the extracellular matrix of this ossicular cartilage template is composed largely of type II collagen. As development proceeds, synthesis of type X collagen, a hypertrophic cartilage-specific molecule, is initiated by endochondral chondrocytes within the zone of cartilage cell hypertrophy. Subsequently, these cells and their surrounding extracellular matrix are removed, resulting in marrow cavity formation. We have examined which of these processes are programmed within the columella chondrocytes themselves, and which require involvement of exogenous factors. Prehypertrophic columella from 12-day chick embryos were grown either in organ culture on Nuclepore filters or as explants on the chorioallantoic membrane of host embryos. Chondrocytes from the same source were grown in monolayer cell cultures. In both organ culture and cell culture, chondrocytes developed to the stage at which some of them entered the hypertrophic program and initiated the production of type X collagen as determined by immunofluorescence histochemistry with a monoclonal antibody specific for that collagen type. The organ cultures, however, did not progress to the next stage, in which detectable removal of the type X collagen-containing matrix occurs. When identical columella were grown on the chorioallantoic membrane of host chicks, the type X collagen-containing matrix which formed was rapidly removed, resulting in the formation of a marrow cavity. Thus, progression of endochondral chondrocytes to the deposition of type X collagen-containing matrix seems to be programmed within the cells themselves. Subsequent removal of this matrix requires the involvement of exogenous factors.

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.

Preparation and high-performance liquid chromatography of 3'-phosphoadenosine-5'-phospho[35S]sulfate with a predetermined specific activity

3'-Phosphoadenosine-5'-phospho[35S]sulfate (PAP35S) was prepared by incubating ATP and carrier-free H2(35)SO4 with a 100,000g supernatant fraction prepared from chick embryo chondrocytes. The product was partially purified by paper electrophoresis and mixed with unlabeled PAPS to give a solution of PAP35S with a specific activity and a concentration approximating those required for the desired metabolic studies. The product was analyzed by high-performance liquid chromatography on an anion-exchange column to determine the proportion of the 35SO4 cpm and A260 material found in the PAPS and other contaminating nucleotides. The PAP35S was purified further by preparative high-performance liquid chromatography. The exact specific activity of the PAP35S was then determined by using this PAP35S preparation as the SO4 donor in a sulfotransferase reaction using a microsomal preparation from the chick embryo chondrocytes as the enzyme and an 3H-labeled oligosaccharide as the SO4 acceptor. The sulfated oligosaccharide was then isolated and the number of 3H and 35SO4 counts per minute in this product were used to calculate the specific activity of the donor. The features of this generally useful approach for preparing PAP35S of any desired specific activity and concentration are discussed.

Isolation and characterization of osteogenic cells derived from first bone of the embryonic tibia

Osteogenesis in the embryonic long bone rudiment occurs initially within an outer periosteal membrane and subsequently inside the cartilaginous core as a consequence of the endochondral ossification process. In order to investigate the development of these two different mechanisms of bone formation, embryonic chick tibial cell isolates were prepared from sites of first periosteal bone formation and from the immediately underlying hypertrophic cartilaginous core region. Mid-diaphyseal periosteal collars and the corresponding cartilage core were microdissected free from Hamburger-Hamilton stage 35 (Day 9) chick tibias and separately digested with a trypsin-collagenase enzyme mixture. The released cell populations were cultivated in vitro and characterized by morphological analysis, histochemical localization of alkaline phosphatase, alizarin red S staining for mineral deposition, growth rate [( 3H]thymidine uptake), and proteoglycan content. Results of these studies showed that periosteal collar cell cultures form nodule-like structures that stain positive with alkaline phosphatase and alizarin red S. Light and electron microscopic observation revealed cell and matrix morphologies similar to that of intact periosteum. The nodules were composed of plump cell types embedded within a mineralized matrix surrounded by a fibroblastic cell layer. Core cartilage cell cultures displayed typical characteristics of the hypertrophic state in their visual appearance and proteoglycan composition. The formation of osseous-like structures in periosteal collar cell cultures but not in core chondrocyte cell cultures demonstrates the relatively autonomous nature of intramembranous ossification while emphasizing the dependence of the endochondral ossification process upon an intact vascularized environment present in the developing tibia.

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.

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.