Sulfate incorporation by articular chondrocytes in monolayer culture

Some highlights in the emergence of modern concepts of osteoarthritis

The present-day concept that osteoarthritis may be amenable to biological modification rather than a hopeless expression of old age or injury has historical roots in the period of 1935 through the early 1970s. One root was the structural and chemical delineation of the connective tissues: discovery of the proteoglycans and multiple molecular species of collagen. Another was the recognition of the ability of mature articular chondrocytes to replicate themselves rather than being terminally differentiated. A third was the elucidation of the engineering physiology of the joint: the role of matrix hydrophilia to the material properties of articular cartilage and biolubrication. Each root has direct relevance to ongoing therapeutic approaches to degenerative joint disease. The early epidemiological studies of Kellgren and Lawrence evolved into new techniques for testing their validity in clinical practice. Along the way there was a rich 2-way interaction between scientists and clinicians in arriving at these ideas.

Stimulation of matrix formation in rabbit chondrocyte cultures by ascorbate. 1. Effect of ascorbate analogs and beta-aminopropionitrile

The most consistent effects of 0.2 mM L-ascorbate on monolayer cultures of rabbit articular chondrocytes were a diversion of incorporated radiosulfate into a pericellular matrix and enhancement of cell proliferation. Only with certain batches of fetal bovine serum (FBS) was there a cell-for-cell increase of proteoglycan synthesis. These actions increased as the cell inoculum rose from 0.5 to 2 x 10(5) cells/T25 flask. Maximal effects of ascorbate and D-isoascorbate were found over a range of 0.05-0.2 mM. L-Dehydroascorbic acid was less effective than either, and no stimulatory action was exerted by L-cysteine, glutathione, dithiothreitol, methylene blue, or phenazine methosulfate. Ascorbate increased the hypro:pro ratio of newly synthesized proteins. beta-Aminopropionitrile (1 mM) reduced the proportion of [3H]hydroxyproline and [35S]O4-proteoglycans in the ascorbate-supplemented matrix 31 and 7%, respectively. In corresponding electronmicrographs, the number of pericellular filaments was reduced. We conclude: (a) Ascorbate has a general anabolic effect on chondrocytes in culture and enhances matrix assembly through mechanisms other than its redox function; (b) deposition of proteoglycans in the matrix is not simply the result of mechanical entrapment by allysine- or hydroxyallysine-derived cross-linking of collagen; and (c) contradictory reports on the subject result from variations in the serum employed, inoculum density, and concentration of ascorbate.

Autocrine/paracrine mechanism of insulin-like growth factor-1 secretion, and the effect of insulin-like growth factor-1 on proteoglycan synthesis in bovine intervertebral discs

The present study was undertaken to investigate the effect of insulin-like growth factor-1 on proteoglycan synthesis and the autocrine/paracrine mechanism involving insulin-like growth factor-1 in the bovine coccygeal intervertebral disc. Insulin-like growth factor-1 stimulated proteoglycan synthesis in cultured cells of the nucleus pulposus of bovine intervertebral discs in a dose-dependent manner, and the effect was inhibited by an anti-insulin-like growth factor-1 monoclonal antibody. In situ hybridization histochemistry revealed the expression of insulin-like growth factor-1 mRNA in the cultured cells, and its production in these cells was demonstrated by radioimmunoassay. Insulin-like growth factor-1 receptor in the cultured cells was also demonstrated immunohistochemically. Scatchard analysis using an [125I]insulin-like growth factor-1 binding assay showed that the cells cultured in monolayer had a single type of insulin-like growth factor-1 receptor, whose affinity and number were estimated to be 7.38 x 10(8)/M and 9.27 x 10(4)/cell, respectively. These results suggest that insulin-like growth factor-1 stimulates proteoglycan synthesis in cells of the nucleus pulposus and that these cells in culture have an insulin-like growth factor-1 autocrine/paracrine mechanism. The expressions of insulin-like growth factor-1 mRNA and insulin-like growth factor-1 receptor in disc tissue were greater in cells of the nucleus pulposus of fetal bovine intervertebral discs than in those of the adult discs. These findings suggest that the action of autocrine/paracrine insulin-like growth factor-1 is more active in cells of the young nucleus pulposus than in cells of mature subjects.

Estrogen metabolism, not biosynthesis, in rabbit articular cartilage and isolated chondrocytes: a preliminary study

Because serum estrogen levels are associated with the presence of osteoarthritis, and cartilage tissue is known to contain estrogen receptors, it is of interest to determine the extent to which estrogen is biosynthesized and/or metabolized in cartilage tissue or isolated chondrocytes. In this preliminary study, using a sensitive assay method, estrogen synthetase (aromatase) was undetectable in articular cartilage or isolated chondrocytes in culture from immature female rabbits. However, estrogen metabolism, specifically estrogen 17 beta-hydroxysteroid dehydrogenase activity, was detected in homogenized cartilage tissue, and at substantially higher specific activities in freshly isolated chondrocytes. These fresh chondrocytes, assayed in culture without any exogenous cofactor, demonstrated a significantly higher activity for converting the weak estrogen, estrone, to the more potent estrogen, estradiol. Chondrocytes grown to confluence in culture had very low estrogen 17 beta-hydroxysteroid dehydrogenase specific activity. Homogenized cartilage tissue, tested only with added NADPH as cofactor, also showed a preference for estradiol as the principal product, but this may have been primarily due to the use of reduced cofactor. If subsequent experiments confirm the presence of estrogen 17 beta-hydroxysteroid dehydrogenase activity, and its preference for converting estrone into estradiol, in human cartilage tissue and chondrocytes, this could have substantial implications in the estrogen dependency of osteoarthritis.

Effect of hylan on cartilage and chondrocyte cultures

The protective role of hylan, a hyaluronan [hyaluronic acid (HA)] derivative, was studied in explanted bovine cartilage and isolated chondrocytes. Cartilage and chondrocytes were exposed to degradative enzymes (lysate from activated polymorphonuclear leukocytes), oxygen-derived free radicals (ODFR), conditioned media from mononuclear cells (MCCM), and interleukin-1 (IL-1), in the presence and absence of hylan. The effect of HA was also studied. In cartilage explants susceptibility to pertubation was evaluated in terms of 35S release and proteoglycan depletion and was compared to control cultures; high viscosity hylan was found to reduce 35S release in cartilage explants caused by degradative enzymes, ODFR, MCCM, and IL-1. The hylan effect was reversible and viscosity-dependent. In chondrocyte cultures, high viscosity hylan was effective in reducing cell injury caused by degradative enzymes and ODFR. The data suggest that the glycosaminoglycan hylan, as well as native HA, may mediate exposure to and/or response to stimuli associated with initiation of degenerative processes in cartilage tissues.

Enhanced sulfated-proteoglycan core protein synthesis by incubation of rabbit chondrocytes with recombinant transforming growth factor-beta 1

Rabbit articular chondrocytes were incubated with recombinant transforming-growth-factor-beta 1 (rhTGF-beta 1) and its effect on newly synthesized proteoglycan measured. rhTGF-beta 1 stimulated proteoglycan synthesis at a concentration as low as 5 ng/ml without further increases in radiosulfate incorporation up to 50 ng/ml. The quantitative increase in radiosulfate incorporation in rh-TGF-beta 1-treated chondrocytes was greater in the cell-associated culture compartment than in the medium compartment. rhTGF-beta 1 promoted an increased proteoglycan retention in the cell-associated compartment as evidenced by an increase in the t1/2 of retention from 8 h to 11 h. Specific enhanced synthesis of [35S]-methionine-labeled core proteins was seen in rh-TGF-beta 1-treated chondrocytes. rh-TGF-beta 1 increased the synthesis of the 2 core proteins derived from hydrodynamically large proteoglycans. They possessed apparent molecular weights of greater than 480 kD and 390 kD after 3-5% acrylamide gel electrophoresis. A compartmental analysis revealed that the cell-associated culture compartment contained only the larger of the 2 core proteins derived from large proteoglycans. Two other core proteins with apparent molecular weights 52 kD and 46 kD were also stimulated by rhTGF-beta 1. These results indicated that TGF-beta probably plays a significant role in stimulating proteoglycan core protein synthesis in articular chondrocytes and therefore may be an important growth factor in the restoration of cartilage extracellular matrix after injury.

Bacillus Calmette-Guérin (BCG) influences cell proliferation and glycosaminoglycans of chondrocyte cultures

There are only few reports on the correlation between bacterial products and the GAG pattern of cartilage. Mycobacteria bovis (BCG) were applied to chondrocyte monolayer cultures for one week. The following parameters did change: cell proliferation increased, glycosaminoglycan synthesis and secretion decreased, hyaluronic acid in secreted and cell-associated glycosaminoglycans increased, a correlation between the degree of these changes and the degree of cell differentiation seems to exist. The contact of bacteria like BCG to chondrocytes may change the cellular metabolism. On the tissue level this may injure articular cartilage and thus support the concept of predamaged cartilage that is readily susceptible to further degradation.

Chondrocytes may activate macrophages

It is demonstrated that conditioned media of articular chondrocytes cultured with Bacilli Calmette-Guérin are capable of activating macrophages. This activation reaches different levels and is expressed both by different cell survival and enhanced synthesis of glycosaminoglycans that remain cell-associated (exp. group III) or are secreted (exp. group II). The reason of these variances is unknown. - A protein factor is supposed to be present in variable concentration or activity. Interleukin-1 could not be correlated with the results.

Cryopreserved articular chondrocytes grow in culture, maintain cartilage phenotype, and synthesize matrix components

For osteochondral allograft transplantation to be successful, chondrocytes must survive preservation and retain their capacity to produce normal matrix components: proteoglycans and Type II collagen. Clinical success with osteochondral allograft transplantation has created an increased demand for supplies of suitable cartilage-bearing grafts. This demand has stimulated attempts to find successful methods for low temperature storage of cartilage for "banking" and heightened interest in cartilage cryobiology. In order to achieve the maximum viability of cryopreserved articular cartilage, previous comprehensive studies have focused on rates and temperatures of freezing, cryoprotective agents, and methods and influences of thawing. This study presents evidence that cryopreserved articular chondrocytes maintain their ability to grow in tissue culture following thawing and to produce normal matrix components. Chondrocytes isolated from Japanese white rabbits were divided into groups of fresh controls and experimental cryopreserved cells. Cells were incubated in dimethylsulfoxide, frozen at a rate of -1 degrees C/min, stored at -79 degrees C, rapidly thawed, and plated for culture. Growth rates were comparable in all groups. In all groups, typical chondroid characteristics were maintained throughout 14 days of culture. Typical cartilage phenotypic characteristics included maintenance of polygonal and rhomboidal cells, cell aggregation, proteoglycan production, and Type II collagen synthesis. This investigation strongly indicates that articular chondrocyte cryopreservation yields viable, functional cells and although these results cannot be directly extrapolated to intact adult articular cartilage, they do give further support for low temperature banking of cartilage-bearing allografts for transplantation.

Cell culture of rabbit meniscal fibrochondrocytes II. Sulfated proteoglycan synthesis

Rabbit meniscal fibrochondrocytes were grown in vitro under culture conditions previously shown to foster growth of this cell type. Regardless of the culture regimen employed, the cells synthesized sulfated proteoglycans which could be differentiated by their solubility when dialyzed against water. The water soluble proteoglycans (WSPG) were monomeric in nature and could be separated into sub-types based on their hydrodynamic size when analyzed by gel-filtration chromatography. The water insoluble proteoglycans (WIPG) appeared to represent hyaluronic acid-dependent aggregates of the larger of the two WSPG. The proteoglycans contained approximately 87% chondroitin sulfate and 5% dermatan sulfate. Keratan sulfate could not be detected. Addition of ascorbate to the culture medium did not change the amount or the hydrodynamic size of the proteoglycan aggregates but did alter the quantity of the larger WSPG monomer synthesized depending upon the culture regimen used. Thus, these cells are capable of expressing their differentiated phenotype in short-term monolayer cell culture.

DNA repair by articular chondrocytes. V. O6 methylguanine-acceptor protein activity in resting and cultured rabbit and human chondrocytes

The level of O6-methylguanine acceptor protein activity was examined in rabbit and human articular chondrocytes of different ages. The activity per microgram of DNA in rabbit chondrocytes was 5-fold lower than in humans. There was no age-dependent decrease in the activity of resting or cultured chondrocytes of either species. The values for resting cells were comparable to those of cultured cells. The lack of age-related differences in methyltransferase activity, in contrast to nucleotide excision repair [Mech. Ageing Dev. 32 (1985) 39-55], indicates that separate repair systems behave differently with respect to chronological aging. The methyl transferase activity may be more essential for survival of articular chondrocytes and therefore more highly conserved with age.

Rabbit chondrocytes maintained in serum-free medium. I. Synthesis and secretion of hydrodynamically-small proteoglycans

The biosynthesis of sulfated proteoglycan in vitro by rabbit articular chondrocytes in first passage monolayer culture maintained in fetal bovine serum (FBS) or in serum-free conditions was compared. Neosynthesized proteoglycan in the culture medium in the most dense fraction of an associative CsCl density gradient (fraction dAl) declined with increasing time under serum-free conditions, but not when cells were maintained in the presence of serum. After one day, the major peak of incorporated 35SO4 in medium fraction dAl eluted as a retarded peak (Kav 0.28) on Sepharose CL-2B, whether cells were maintained under serum-free or serum-containing conditions. The hydrodynamic size of proteoglycan monomer fraction dAlDl obtained after one day of exposure to serum-free culture media was smaller than dAlDl from serum-containing cultures. The hydrodynamic size of dAlDl obtained from serum-free culture media became even progressively smaller after 2 and 3 days' exposure to these conditions. Hydrodynamically small sulfated proteoglycans were identified in the cell-associated dAlDl fraction as early as one day after switching chondrocytes from serum-containing to serum-free medium. Culture medium fraction dAlDl from serum-free culture medium aggregated poorly when incubated with human hyaluronic acid (HA) in the presence of bovine link protein or when dialysed against bovine nasal cartilage proteoglycan aggregate. Proteoglycan monomer from serum-containing medium reaggregated more efficiently under both conditions. No change in the size of glycosaminoglycan chains was seen in the smaller proteoglycan subpopulations, nor was there any indication of marked changes in the glycosaminoglycan types.

Stimulation of sulfated-proteoglycan synthesis by forskolin in monolayer cultures of rabbit articular chondrocytes

Forskolin, a plant cardiotonic diterpene, stimulated proteoglycan biosynthesis by chondrocytes in monolayer culture. The quantitative increase in proteoglycans was dependent on the concentration of forskolin, but was relatively independent of the presence of serum. At forskolin concentrations that stimulated proteoglycan synthesis, a significant stimulation of adenylate cyclase and cAMP was also measured. The quantitative increase in proteoglycans was characterized, qualitatively, by an increased deposition of newly synthesized proteoglycan in the cell-associated fraction. An analysis of the most dense proteoglycans (fraction dA1) in the cell-associated fraction showed that more of the proteoglycans eluted in the void volume of a Sepharose CL-2B column, indicating that an increased amount of proteoglycan aggregate was synthesized in forskolin-treated cultures. The proteoglycan monomer dA1D1 secreted into the culture medium of forskolin-stimulated cultures overlapped in hydrodynamic size with that of control cultures, although cultures stimulated with forskolin and phosphodiesterase inhibitors produced even larger proteoglycans. The hydrodynamic size of 35SO4 and 3H-glucosamine-labelled glycosaminoglycans isolated from the dA1D1 fraction of the culture medium was greater in forskolin-treated chondrocytes, especially from those in which phosphodiesterase inhibitors had been added. These results indicated that forskolin, a direct activator of chondrocyte adenylate cyclase mimicked the effects of cAMP analogues on chondrocyte proteoglycan synthesis previously reported. These results implicate activation of adenylate cyclase as a regulatory event in the biosynthesis of cartilage proteoglycans, and more specifically in the production of hydrodynamically larger glycosaminoglycans.

The effect of sodium selenite on chondrocytes in monolayer culture

The effect of sodium selenite on DNA and sulfated proteoglycan synthesis by cultured rabbit articular and growth plate chondrocytes was studied as an in vitro model for Kashin-Beck disease. The selenium content of a defined medium (DMEM, fibroblast growth factor, insulin, and dexamethasone) was below the limit of detection by isotope dilution mass spectrometry. The chondrocytes were viable in the Se-free basal medium. Selenite over a range of 5 X 10(-9) M to 5 X 10(-7) M had no stimulatory effect on DNA or sulfated proteoglycan synthesis by either type of chondrocyte or skin fibroblasts. Proliferation of bovine endothelial cells was enhanced by 5 X 10(-7) M Se. At Se concentrations of greater than or equal to 10(-6) M, there was progressive inhibition of cell growth and radiosulfate incorporation of the connective tissue cells; bovine endothelial cells were more resistant. Twice equimolar concentrations of vitamins C and E exerted no protective effect against the cytotoxicity of higher concentrations of Se. Se supplementation also failed to stimulate growth of human infant chondrocytes. The model enabled simulation of conditions of hyposelenosis below those encountered in nature. The data provide no evidence that chondrocytes have idiosyncratic requirements for Se, and do not support the hypothesis that Se deficiency is a major etiologic factor in Kashin-Beck disease.

An assay for inorganic pyrophosphate in chondrocyte culture using anion-exchange high-performance liquid chromatography and radioactive orthophosphate labeling

A method is described for determination of inorganic pyrophosphate (PPi) in cell culture medium and in rabbit articular chondrocytes grown in the presence of radioactive orthophosphate (32Pi). Intra- and extracellular 32PPi formed was measured using high-performance liquid chromatographic (HPLC) separation of the PPi from orthophosphate (Pi) and other phosphate-containing compounds. The chromatographic separation on a weak anion-exchange column is based on the extent to which various phosphate compounds form complexes with Mg2+ at low pH and the rate at which such formation occurs. These complexes are eluted more readily than the uncomplexed compounds. Best results were obtained using a simultaneous gradient of Mg2+ ions and ionic strength. In this case separation of small amounts of PPi from a large excess of Pi was possible without prior removal of Pi or extraction of the PPi fraction. The assay is also useful for measurement of inorganic pyrophosphatase activity. The sensitivity of the assay depends on the specific activity of the added 32Pi and on the culture conditions, but is comparable with the most sensitive of the enzymatic assays. Sample preparation, particularly deproteinization, proved to be of importance. The losses of PPi which occur during procedures of this sort due to hydrolysis and coprecipitation were quantitated.

In-vitro culture of human chondrocytes from adult subjects

An agarose gel matrix was utilized to grow chondrocytes from human donors of various ages in cell culture. The chondrocytes produced the pericellular matrix characteristic for such cells and synthesized collagen type II as well as glyco-saminoglycans. The latter exhibit the typical distribution pattern of the respective articular cartilage matrix. The electron-microscopic appearance of the cultured chondrocytes closely resembles that of chondrocytes in sections of the original cartilage.

Fine epitope specifity of murine monoclonal antibodies reacting with rabbit cartilage proteoglycan

The fine epitope specificity of two murine monoclonal antibodies (HP 2G2 and HP 4D3)1 raised against rabbit xiphoid cartilage proteoglycan monomer (fraction AlDlDl) was determined by solid-phase radioimmune assay utilizing native and heat-denatured (50 degrees C, 30 min) AlDlDl as antigen. Competitive inhibition assays using trypsin-digested native AlDlDl fragments resolved by DEAE-cellulose chromatography and isopycnic CsCl density gradient ultracentrifugation showed that HP 2G2 reacted with a recurring epitope on the core protein, whereas HP 4D3 reacted with intact native and heat-denatured AlDlDl, but not tryptic peptides of AlDlDl. However, HP 4D3 was competitively inhibited from binding to native intact AlDlDl by the clostripain limit digestion products of AlDlDl partially purified by Sepharose CL-2B chromatography. HP 4D3 when added to rabbit but not human chondrocytes in culture affected the incorporation of 35SO4 into proteoglycans found in the most dense CsCl density gradient fraction (A1) under associative conditions.

Change in collagen synthesis of human chondrocyte culture. I. Development of a human model, demonstration of collagen type conversion by immunofluorescence

A research system constituted entirely of components of human origin was developed to study conversion of collagen synthesis by human chondrocytes. Type specificity of affinity chromatography-purified antibodies to human type II or type I collagen was proven by ELISA inhibition and immunofluorescence analysis. Human chondrocytes were isolated from articular cartilage and kept in monolayer cultures for eight subpassages. Conversion of type II to type I synthesis by chondrocytes was investigated by immunofluorescence. Staining with anti-type II collagen antibodies could be detected during primary cultures and in the first subpassage, whereas staining with anti-type I collagen antibodies occurred beginning from the end of primary cultures and was present up to the eighth subpassage. Results are compared to observations obtained in animal systems and their relevance to conditions in osteoarthritis is discussed.

DNA repair by articular chondrocytes. III. Unscheduled DNA synthesis following ultraviolet light irradiation of resting cartilage

Unscheduled DNA synthesis (UDS) was demonstrated both in fresh slices of rabbit articular cartilage and in resting chondrocytes dissociated from it. Uptake of [3H]thymidine by chondrocytes treated with 10 mM hydroxyurea and irradiated with 254 nm ultraviolet light (20 J/m2) was measured by autoradiography. The UDS in the resting chondrocytes was minuscule compared with that of chondrocytes growing in monolayer culture. It was greater in resting cells from 3- than in 18-month old rabbits but persisted in animals up to 52 months of age. In both resting and cultured chondrocytes, UDS was greatly inhibited by aphidicolin (5 micrograms/ml). The finding of age-related decrease in UDS of resting chondrocytes, contrasted with previously demonstrated lack of it in dividing ones, raises questions about in vitro artefacts in studies of cellular senescence.

Secretion of an articular cartilage proteoglycan-degrading enzyme activity by murine T lymphocytes in vitro

Destruction of articular cartilage is the hallmark of inflammatory arthritides. Enzymes elaborated by mononuclear cells infiltrating the synovium mediate, in part, the degradation of the cartilage extracellular matrix. Since mononuclear cells are the dominant cell type found in chronic inflammatory synovitis, we investigated whether interaction of immune mononuclear cells with antigen initiated the synthesis and secretion of a proteoglycan-degrading enzyme activity. Proteoglycan-degrading enzyme activity was monitored by the capacity of murine spleen cell conditioned medium to release [3H]serine/35SO4 incorporated into rabbit cartilage proteoglycan monomer fraction (A1D1), and by the relative change in specific viscosity of bovine nasal cartilage proteoglycan monomer. The results demonstrated that both virgin and immune mononuclear cells spontaneously generated proteoglycan-degrading enzyme activity and that cellular activation and proliferation induced by the antigen keyhole limpet hemocyanin or the mitogen phytohemagglutinin was not required. Kinetic studies demonstrated stable release of the enzyme activity over 72 h. Cell separation studies showed that T lymphocytes, a thymoma line, and macrophages separately produced proteoglycan-degrading enzyme activity. The enzyme activity has been partially characterized and appears to belong to a class of neutral pH metal-dependent proteinases. These observations, the first to demonstrate that T lymphocytes secrete an enzyme capable of degrading cartilage proteoglycan, raise the possibility that this enzyme activity contributes to cartilage extracellular matrix destruction in vivo. Moreover, these data support the conclusion that production of this enzyme by T lymphocytes is independent of an antigen-specific stimulus.

Effect of ascorbic acid on secreted proteoglycans from rabbit articular chondrocytes

Addition of ascorbic acid (25, 50 100 micrograms/ml) to cultures of rabbit articular chondrocytes did not change the total amount of proteoglycans produced. However, it induced an increased retention of these macromolecules in the pericellular fraction. The size of the proteoglycan subunits and the length of glycosaminoglycan chains, released in the medium, were not modified on exposure to ascorbic acid (25 micrograms/ml). On the other hand, the rate of non-sulfated chondroitin was increased 2.5-fold, whereas chondroitin-4-sulfate was depressed 1.5-fold.

Induction of bone xenografts of rabbit growth plate chondrocytes in the nude mouse

Subcutaneous transplantation of growth plate chondrocytes isolated enzymatically from the proximal tibia of 6-week-old rabbits into athymic (nu/nu) mice resulted in the formation of cartilaginous nodules. Calcification of the matrix was first seen after 48 hrs, and endochondral ossification at 12 days. The mineral first occurred about hypertrophic cells. Histochemical alkaline phosphatase activity was concentrated in pericellular collars at the same location. Immunofluorescence examination with rabbit anti-mouse lymphocyte serum disclosed that the bulk of the osteoblasts was derived from the mouse. A small quantity of mouse antigen was present in the cartilage matrix at its junction with bone. It presumably diffused into the cartilaginous interface from the host, but the possibility that some chondrocytes were of murine origin has not been excluded. Five of six grafts of cells grown to confluence in monolayer culture for 10 to 14 days became ossified. The ability to induce mineralization declined in subculture. Chondrocytes killed by heating to 56 degrees did not induce calcified cartilage or bone.

Effect of alkyltins on rabbit articular and growth-plate chondrocytes in monolayer culture

The effect of four different alkyltins (trimethyltin, triethyltin, dibutyltin, and dioctyltin) on the metabolism of rabbit articular and growth-plate chondrocytes was investigated using a monolayer cell-culture system. In most instances the compounds tested exhibited a general cytotoxic effect on these cells, inhibiting the synthesis of both DNA and sulfated proteoglycans. The effect of these compounds on proteoglycan synthesis was both quantitative and qualitative, as demonstrated by CsCl isopycnic density gradient centrifugation and gel exclusion chromatographic techniques. However, certain tin compounds tested, at specific concentrations, exerted a stimulatory effect on chondrocyte proliferation. Regarding DNA synthesis, growth-plate chondrocytes were more sensitive to the effect of the triethyltin, dibutyltin, and dioctyltin than were articular chondrocytes. The data are discussed in relation to the possible effects of the alkyltins on skeletal growth and development as well as the mechanism of action of the alkyltins at the molecular level.

The in vitro cell culture age and cell density of articular chondrocytes alter sulfated-proteoglycan biosynthesis

The effect of cell culture age and concomitant changes in cell density on the biosynthesis of sulfated-proteoglycan by rabbit articular chondrocytes in secondary monolayer culture was studied. Low density (LD, 2 d), middle density (MD, 5-7 d), and high density (HD, 12-15 d) cultures demonstrated changes in cellular morphology and rates of DNA synthesis. DNA synthesis was highest at LD to MD densities, but HD cultures continued to incorporate [3H]-thymidine. LD cultures incorporated 35SO4 into sulfated-proteoglycans at a higher rate than MD or LD cultures. The qualitative nature of the sulfated-proteoglycans synthesized at the different culture ages were analyzed by assessing the distribution of incorporated 35SO4 in associative and dissociative CsCl density gradients and by elution profiles on Sepharose CL-2B. Chondrocytes deposited into the extracellular matrix (cell-associated fraction) 35SO4-labeled proteoglycan aggregate. More aggregated proteoglycan was found in the MD and HD cultures than at LD. A 35SO4-labeled aggregated proteoglycan of smaller hydrodynamic size than that found in the cell-associated fraction was secreted into the culture medium at each culture age. The proteoglycan monomer (A1D1) of young and older cultures had similar hydrodynamic sizes at all cell culture ages and cell densities. The glycosaminoglycan chains of A1D1 were hydrodynamically larger in the younger LD cultures than in the older HD cultures and consisted of only chondroitin 6 and 4 sulfate chains. A small amount of chondroitin 4,6 sulfate was detected, but no keratan sulfate was measured. The A1D2 fractions of young LD cultures contained measurable amounts of dermatan sulfate; no dermatan sulfate was found in older MD or HD cultures. These studies indicated that chondrocytes at LD synthesized a proteoglycan monomer with many of the characteristics of young immature articular cartilage of rabbits. These results also indicated that rapidly dividing chondrocytes were capable of synthesizing proteoglycans which form aggregates with hyaluronic acid. Culture age and cell density appears primarily to modulate the synthesis of glycosaminoglycan types and chain length. Whether or not these glycosaminoglycans are found on the same or different core proteins remains to be determined.

Susceptibility of interstitial rabbit collagens to rabbit articular chondrocyte collagenase

Conditioned culture media from confluent rabbit articular chondrocytes maintained in serum-free monolayer culture contained metal-dependent neutral pH collagenolytic activity degrading Type I, II and III rabbit [125I]-labeled collagens. This collagenolytic activity degraded Type II collagen more slowly than Type I collagen and Type III collagen at 37 degrees C. By contrast, collagenolysis by chondrocyte cytosolic protein, lysosomal granule protein and residual lysosomal membrane protein was highly specific for Type II collagen. Although collagenolytic activity against all the collagen isotypes tested was predominantly in a latent form after 24 h of culture, increasing levels of constitutive collagenolytic activity was measured with increasing culture time. These results are consistent with a differential degradation of rabbit interstitial collagens by rabbit chondrocyte collagenase. The data suggest a cellular compartmentalization of collagenolytic activity with specificity toward Type II collagen.

Effect of piroxicam on structure and function of joint cartilage

In vitro experiments were performed on isolated articular chondrocytes under the influence of piroxicam. It was demonstrated that this non-steroidal anti-inflammatory agent affected neither cell proliferation nor the incorporation of 35SO4 into matrix macromolecules. Dogs were treated with piroxicam for 8 weeks. Morphological studies were performed on the tissues of the knee joints. Macroscopic and light microscopic investigations revealed no structural differences between the tissues (synovial membrane and articular cartilage) of control and treated dogs. Even at the ultrastructural level no alterations in the cartilage were observed. The capacity of chondrocytes to incorporate 35SO4 under in vitro conditions was identical in control and experimental animals. It is concluded that piroxicam has no adverse effect on chondrocytes under in vitro conditions or on articular cartilage structure in the in vivo model.

Stimulation of cyclic AMP in chondrocyte cultures: effects on sulfated-proteoglycan synthesis

The effects of N6,O2'-dibutyryladenosine 3':5'-cyclic monophosphate (DBcAMP), 8-bromoadenosine 3':5'-cyclic monophosphate (8Br-cAMP), 3':5'-cyclic monophosphate (cAMP), L-isoproterenol and L-epinephrine on sulfated-proteoglycan synthesis by rabbit articular chondrocytes were compared. DBcAMP and 8Br-cAMP in the presence or absence of 3-isobutyl-1-methylxanthine (IBMX) stimulated sulfated-proteoglycan biosynthesis after 20 h of incubation. cAMP had no significant effect. Both DBcAMP and 8Br-cAMP increased the hydrodynamic size of the newly synthesized proteoglycan monomer (A1D1) relative to control cultures. By contrast, although isoproterenol and epinephrine stimulated total cAMP synthesis, neither stimulated sulfated-proteoglycan synthesis. Whereas intracellular cAMP accumulated after incubation with DBcAMP and 8Br-cAMP, this was not the case with isoproterenol whether IBMX was present or not. Thus, stimulation of sulfated-proteoglycan synthesis by cAMP analogues in chondrocyte cultures appears to be dependent on increased intracellular cAMP accumulation rather than total cAMP biosynthesis.

Biosynthesis of sulfated proteoglycan in vitro by cells derived from human osteochondrophytic spurs of the femoral head

Cells derived from organ-explant culture of the cartilaginous component of osteochondrophytic spurs of human femoral heads were incubated with [35S]-sulfate in order to study sulfated proteoglycan biosynthesis in vitro. Secondary monolayer cultures incorporated [35S]-sulfate into macromolecules which were recovered in the bottom fraction (dA1) of a CsCl gradient after ultracentrifugation in associative buffer (0.5 M guanidine X HCl). The incorporated [35S]-sulfate in fraction dA1 from the culture medium eluted in two peaks with average partition coefficients (Kav) of 0.14 and 0.45 respectively, on Sepharose CL-2B eluted with dissociative buffer (4 M guanidine X HCl). A significant percentage of incorporated [35S]-sulfate was found in the medium dA4 fraction (44%). The Kav of this fraction on Sepharose CL-2B was 0.66 with a shoulder of incorporated [35S]-sulfate at Kav, 0.22. In contrast to the culture medium, cellular CsCl gradient fractions dA1-dA3 showed Kav's on Sepharose CL-2B ranging from 0.63-0.75. Cellular fraction dA4 was even more polydisperse. A dD1 fraction (proteoglycan monomer) prepared by CsCl ultracentrifugation in dissociative buffer of [35S]-sulfate labelled culture medium eluted with a Kav of 0.25 on Sepharose CL-2B identical to the Kav of bovine nasal cartilage A1D1 and human tissue osteophyte A1D1 chromatographed under identical conditions. Glycosaminoglycan analysis demonstrated significant amounts of chondroitin 6- and 4-sulfate in unfractionated culture medium and in those proteoglycan fractions generated from culture medium (dA1, dA2 and dD1). In contrast, cellular fractions dA1-dA3 and medium fraction dA4 were enriched in dermatan sulfate. The size of the [35S]-sulfated glycosaminoglycan chains analyzed by Sepharose CL-6B chromatography showed considerable polydispersity (Kav range, 0.29-0.52). The results of this study indicated that cells derived from the cartilaginous component of human osteophyte synthesized several distinct populations of sulfated proteoglycans. These results may reflect the heterogeneity of cells which grow out from osteophyte organ explants and become established in monolayer culture.

Proteoglycan aggregate formation by articular chondrocytes. Decrease in link-protein synthesis during culture

The synthesis of link-stabilized proteoglycan aggregates by rabbit articular chondrocytes was investigated by [35S]sulphate labelling of primary monolayer cultures maintained for up to 21 days. (1) At all culture times the cells secreted a high-molecular-weight cartilage-type proteoglycan monomer of which 75%-80% formed aggregates with hyaluronic acid. (2) At 2 days of culture all of the aggregates were in link-stabilized form, but by 21 days only 5% were link-stabilized, as shown by displacement of monomers from the aggregate by hyaluronic acid oligosaccharides. (3) The addition of purified link protein to 21-day culture medium increased the proportion of link-stable aggregate from 5% to 70%. (4) Analysis of [3H]serine-labelled proteoglycan aggregates in the medium showed a marked decrease with culture time in the ratio of 3H-labelled link protein to 3H-labelled core protein present. The results suggest that the secretion of proteoglycan monomers and link protein by articular chondrocytes changes independently during prolonged monolayer culture.

Xenografts of articular chondrocytes in the nude mouse

Subcutaneous transplantation of articular chondrocytes isolated enzymatically from immature rabbits and dogs into athymic (nu/nu) mice resulted in the formation of hyaline cartilaginous nodules. Graft rejection was seen when the cells were injected into heterozygous (nu/+) mice. Radiosulfate-labeled proteoglycan extracted from the xenografts had a high buoyant density and was digested by chondroitinase ABC. A monomeric preparation of proteoglycan (A1-D1) contained a small quantity of aggregate as assessed by gel chromatography and gel electrophoresis. Almost no incorporation of 3H-thymidine was found by autoradiography. The matrix did not become calcified over the course of 42 days. The nude mouse system lends itself to testing a variety of problems in the biology of cartilage. These include the redifferentiation of chondrocytes following dedifferentiation in vitro. Species differences were found in this regard. The nodules formed by rabbit articular chondrocytes, grown in monolayer culture for up to 14 days, had a hyaline chondroid character. Dog chondrocytes that had "dedifferentiated" during 14 days of culture prior to transplantation, formed a graft that had a sparse fibrous rather than hyaline matrix.

Estradiol and tamoxifen stimulation of lapine articular chondrocyte prostaglandin synthesis

The effect of estradiol and tamoxifen on prostaglandin (PG) synthesis by rabbit articular chondrocytes in secondary monolayer cultures was investigated. Radioimmunoassay for PGE2, PGF2 alpha, 6-oxo-PGF1 alpha and thromboxane B2 was performed on media from cultures containing estradiol and tamoxifen (10-12M-10-7M). Radiometric thin-layer chromatography was also carried out. The time course of estradiol/tamoxifen effect on chondrocyte PG synthesis was evaluated and its relationship to cell density in culture examined. Estradiol stimulated the synthesis of PGs by chondrocytes. Stimulation was noted at picomolar concentrations of estradiol without further stimulation at markedly higher concentrations. In time studies, after a lag, the effect of estradiol was present fully by 5 hrs, remained steady for 24 hrs and then declined by 48 hrs. Estradiol stimulation of PG synthesis was dependent upon chondrocyte culture plating density. Tamoxifen stimulated chondrocyte PG synthesis to relatively lower levels than estradiol. The characteristics of estradiol/tamoxifen stimulation of chondrocyte PG synthesis suggest a mechanism involving estradiol cytoplasmic receptors.

'Gelatinase-like' activity from articular chondrocytes in monolayer culture

In addition to releasing collagenase and proteoglycanase activity, rabbit articular chondrocytes in monolayer culture released into the culture medium, latent, neutral enzyme activity which when activated by p-aminophenylmercuric acetate degraded fluorescein-labeled polymeric rat tail tendon Type I collagen and the tropocollagen TCA and TCB fragments of human Type II collagen into smaller peptides at 37 degrees C. Enzyme activity was abolished if p-aminophenylmercuric acetate-activated culture medium was preincubated with 1.10-phenanthroline, a metal chelator. Thus, articular chondrocytes in monolayer culture are capable of producing neutral proteinases which acting together can result in complete degradation of tendon and cartilage collagen to small peptides.

DNA repair by articular chondrocytes. I. Unscheduled DNA synthesis following ultraviolet irradiation in monolayer culture

The hypothesis that aging of articular chondrocytes at a cellular level results from loss of DNA repair capability was studied by measuring unscheduled DNA synthesis (UDS). Cultured rabbit and human articular chondrocytes were irradiated with 254 nm ultraviolet light (20 J/m2) following treatment with 10 mM hydroxyurea. Neither the "in vitro senescence" nor spontaneous transformation that developed during serial passage of rabbit chondrocytes was accompanied by diminution of UDS. Synthesis of sulfated glycosaminoglycans declined more rapidly than the ability of the cells to divide. Levels of UDS by chondrocytes from old donors, rabbit or human, were comparable to those of younger individuals. UDS was greater in human than rabbit chondrocytes. Similar data have been reported previously for dermal fibroblasts but do not necessarily indicate that there is a direct or causative relationship between UDS capability and the longevity of mammalian species. X-Irradiation of rabbit chondrocytes or cartilage explants, in doses up to 40 000 rads, yielded no measurable UDS.

A comparison of the effects of two gold-containing therapeutic agents on articular chondrocyte growth in vitro

Chondrocyte structure and function under the influence of two gold-containing therapeutic agents, aurothioglucose and triethylphosphine gold, were studied in a monolayer culture system for cultivating lapine articular chondrocytes. The functional parameters investigated were chondrocyte proliferation and the incorporation of 35SO4 as indicator of glycosaminoglycan synthesis. Aurothioglucose (0.2, 1 and 10 micrograms/ml) failed to affect either parameter and caused no cytotoxic effect detectable with the light microscope. Triethylphosphine gold (1.5 and 3 micrograms/ml) prevented subcultured chondrocytes from forming monolayers and was cytotoxic to chondrocytes present in established monolayers, studied with the light and scanning electron microscope. A 0.3 microgram/ml concentration did not alter chondrocyte proliferation or sulphate incorporation and was not cytotoxic.

Matrix synthesis in high density cultures of bovine epiphyseal plate chondrocytes

Bovine epiphyseal plate chondrocytes were cultured by a method combining both suspension culture and high density monolayers. The matrix synthesized by the cultured cells was analyzed at fifteen days for glycosaminoglycan, proteoglycan, and collagen content. In the cell culture product glycosaminoglycan distribution was: 65% chondroitin-6-sulfate, 18% chondroitin-4-sulfate, 15% keratan sulfate, and less than 2% dermatan sulfate. Essentially all the radioactive sulfate in labelled specimens was present in high molecular weight aggregates. The collagen which was synthesized co-migrated with Type II collagen standard. Parallel analysis showed the matrix of cultured cells to be similar to that of intact epiphyseal plate tissue. This study demonstrates the ability to grow epiphyseal plate chondrocytes in a cell culture system which allows matrix synthesis similar to that seen in vivo.

Effect of purified growth factors on rabbit articular chondrocytes in monolayer culture. I. Dna synthesis

The ability of purified growth factors, insulin, ascorbate, and several other compounds to stimulate DNA synthesis by rabbit articular chondrocytes was studied in monolayer culture. Platelet-derived growth factor (1 U/ml), pituitary fibroblast growth factor (1-100 ng/ml), and epidermal growth factor (1-50 ng/ml) were stimulatory in a basal medium supplemented with 1% heat-inactivated fetal bovine serum. Insulin, 1-50 micrograms/ml, has small growth-promoting effects but acted synergistically with platelet-derived, pituitary fibroblast, and epidermal growth factors. Increasing concentrations of serum up to 10% enhanced the growth-promoting action of the purified factors, but not of insulin. There were indications of cooperation between insulin and bovine serum albumin and dexamethasone. Ascorbate (0.2 mM) reduced or had little growth-promoting action in the basal medium. At 5 and 10% serum concentrations, however, ascorbate promoted DNA synthesis as effectively as the purified growth factors. No significant stimulatory effect was shown by transferrin, thrombin, L-glutamine, putrescine, selenous acid, dexamethasone, 7S nerve growth factor, or multiplication-stimulating activity.

The effects of nickel ions on articular chondrocyte growth in monolayer culture

Monolayer cultures of lapine articular chondrocytes were used to study the effects of nickel ions (Ni++) on chondrocyte proliferative capacity, proteoglycan synthesis and cellular morphology. Nickel depressed chondrocyte proliferation at the highest concentration tested (100 mumol/l; P less than 0.01) and also exerted a dose-dependent inhibition of proteoglycan synthesis (50 and 100 mumol/l; P less than 0.01). Despite both these effects, no evidence of chondrocyte damage was detectable at the light-microscopical level. The possible significance of the nickel-induced reduction of articular chondrocyte proteoglycan synthesis for the functional integrity of the residual cartilage following hemiarthroplasty operations is discussed.

Response of articular chondrocytes to pituitary fibroblast growth factor (FGF)

Rabbit chondrocytes from pooled articular joints have been delineated by their time of attachment to culture flasks after initiation of primary monolayer culture, either attached (48-AT) or floating (48-F) after 48 hours. A general population of chondrocytes (attached after 72 hours, 72-AT) was also studied. The growth-promoting activity of pituitary fibroblast growth factor (FGF) and its effect on sulfated-proteoglycan synthesis was studied on each chondrocyte population in secondary monolayer culture. 3H-thymidine incorporation during a 1-hour pulse was stimulated by FGF (100 ng/ml) in each chondrocyte population. The response of AT-72 chondrocytes to FGF required an additional fetal bovine serum supplement, while 48-F cells responded independent of serum. The response of 48-AT chondrocytes to FGF (100 ng/ml) during a 1-hour pulse with 3H-thymidine was increased in low serum (0.5-2.0%) rather than when high serum (8-10%) was present in the culture medium. FGF reduced 35SO4 incorporation into sulfated-proteoglycans in the 48-AT and 48-F chondrocyte populations, but not in the 72-AT population. The reduction in 35SO4 incorporation in the 48-AT and 48-F chondrocytes was not characterized by alterations in the hydrodynamic size of the sulfated-proteoglycans as measured by Sepharose CL-2B chromatography nor by changes in the types of sulfated-glycosaminoglycans produced. These results indicated that FGF produced quantitative rather than qualitative alterations in chondrocyte sulfated-proteoglycan synthesis. The latter appears uncoupled from the growth-promoting activity of FGF on chondrocytes.

Synthesis of cartilage matrix by mammalian chondrocytes in vitro. I. Isolation, culture characteristics, and morphology

We describe the isolation and the ultrastructural characteristics of adult bovine articular chondrocytes in vitro. Slices of bovine articular cartilage undergo sequential digestions with pronase and collagenase in order to release cells. Chondrocytes are plated at high density (1 x 10(5) cells/cm2) in culture dishes or roller bottles with Ham's F-12 medium, supplemented with 10% fetal bovine serum. Before culture, chondrocytes are freed of surrounding territorial matrix. Within the first few days of culture they re-establish a territorial matrix. As time progresses, chondrocytes synthesize both territorial and extraterritorial matrices. The matrices are rich in collagen fibrils and ruthenium red-positive proteoglycans. These features are most apparent in mass roller cultures in which aggregates of cells and matrix appear as long streaks and nodules. This morphology reveals an organization of chondrocytes and their matrices that is similar to that of the parent articular cartilage in vivo.

Correlation of the biosynthesis of prostaglandin and cyclic AMP in monolayer cultures of rabbit articular chondrocytes

We have utilized ionophores to test whether stimulation of chondrocyte prostaglandin biosynthesis is accompanied by an increase in cyclic nucleotide levels in these cells. Radioimmunoassay of prostaglandin E2, 6-oxo-prostaglandin F1 alpha (the stable metabolite of prostaglandin I2) and prostaglandin F2 alpha showed that synthesis of each was stimulated by the divalent-cation ionophore, A23187 after short-term incubation (1-7 min) in serum-free medium. No stimulation of thromboxane B2 was detected. Two monovalent ionophores, lasalocid and monensin failed to stimulate prostaglandin biosynthesis after short-term incubation. Ionophore A23187-stimulated prostaglandin biosynthesis was variably and partially inhibited by sodium meclofenamate, indomethacin and aspirin, but not by sodium salicylate. Ionophore A23187-stimulated prostaglandin biosynthesis was accompanied by a 7.5-fold increase in cyclic AMP levels after 15 min. Sodium meclofenamate, indomethacin and aspirin which inhibited prostaglandin E2 biosynthesis also reduced cyclic AMP levels. Exogenous prostaglandin E2 (1 microgram/ml) stimulated cyclic AMP biosynthesis, which was not inhibited by aspirin. These results indicated that prostaglandins can be considered as one of the local effectors controlling cyclic AMP production in articular cartilage.

Stimulation of DNA synthesis by ascorbate in cultures of articular chondrocytes

The addition of 0.2 mM Na L-ascorbate increased the incorporation of 3H-thymidine by rabbit articular chondrocytes in cell and organ culture. The stimulatory response of explants to ascorbate was potentiated by pretreatment of the cartilage with 0.2% clostridial collagenase (type 1) or trypsin for 15-30 minutes. In explants there was a latent period of 3 to 4 days before increased labeling of the nuclei could be detected. The effect was transient and declined after 8 days of culture. It was more evident in organ cultures of immature (3-month-old) than 2- to 3-year-old rabbits. Age differences were not detected in cell cultures. Explants of adult human articular cartilage were stimulated by ascorbate when the medium was supplemented with 10% fresh human serum but not by fetal bovine serum. The findings indicated that synthesis of DNA by articular chondrocytes in situ is regulated by responsiveness of the cells proper to compounds such as vitamin C, by properties of the extracellular matrix, and by factors in the serum. Ascorbate was cytotoxic at concentrations greater than 0.2 mM in the presence of certain batches of serum.

Selective emergence of differentiated chondrocytes during serum-free culture of cells derived from fetal rat calvaria

Cells dispersed from the chondrocranial portions of fetal rat calvaria proliferated and performed specialized functions during primary culture in a chemically defined medium. Mature cultures were typified by multilayered clusters of redifferentiating cartilage cells. Flattened cells that lacked distinguishing features occupied areas between the clusters. Alkaline phosphate-enriched, ultrastructurally typical chondrocytes within the clusters were encased in a dense extracellular matrix that stained prominently for chondroitin sulfate proteoglycans. This matrix contained fibrils measuring 19 nm in diameter, which were associated with proteoglycan granules that preferentially bound ruthenium red. A progressive increase in the number of cells indicated the proliferation of certain elements in the primary culture. The cells in primary culture were biochemically as well as morphologically heterogeneous since they were found to synthesize type I and type II collagens. Homogeneous populations of redifferentiated chondrocytes were recovered as floating cells and were shown to express the chondrocyte phenotype in secondary culture. Subcultured cells synthesized type II collagen and its precursors almost exclusively and incorporated 35SO4 into proteoglycan monomer and aggregates to a greater degree than the cells in primary culture. The pattern of proteoglycan monomer and aggregate labeling resembled that of intact cartilage segments and bovine articular chondrocytes. Skin fibroblasts harvested from the same rat fetuses failed to proliferate when maintained under identical conditions. Hence, exogenous hormones, growth factors, and protein are not required for chondrocyte growth and maturation.