Effect of purified growth factors on rabbit articular chondrocytes in monolayer culture. II. Sulfated proteoglycan synthesis

Growth factor transgenes interactively regulate articular chondrocytes

Adult articular chondrocytes lack an effective repair response to correct damage from injury or osteoarthritis. Polypeptide growth factors that stimulate articular chondrocyte proliferation and cartilage matrix synthesis may augment this response. Gene transfer is a promising approach to delivering such factors. Multiple growth factor genes regulate these cell functions, but multiple growth factor gene transfer remains unexplored. We tested the hypothesis that multiple growth factor gene transfer selectively modulates articular chondrocyte proliferation and matrix synthesis. We tested the hypothesis by delivering combinations of the transgenes encoding insulin-like growth factor I (IGF-I), fibroblast growth factor-2 (FGF-2), transforming growth factor beta1 (TGF-β1), bone morphogenetic protein-2 (BMP-2), and bone morphogenetic protien-7 (BMP-7) to articular chondrocytes and measured changes in the production of DNA, glycosaminoglycan, and collagen. The transgenes differentially regulated all these chondrocyte activities. In concert, the transgenes interacted to generate widely divergent responses from the cells. These interactions ranged from inhibitory to synergistic. The transgene pair encoding IGF-I and FGF-2 maximized cell proliferation. The three-transgene group encoding IGF-I, BMP-2, and BMP-7 maximized matrix production and also optimized the balance between cell proliferation and matrix production. These data demonstrate an approach to articular chondrocyte regulation that may be tailored to stimulate specific cell functions, and suggest that certain growth factor gene combinations have potential value for cell-based articular cartilage repair.

Growth factor regulation of growth factors in articular chondrocytes

Several lines of evidence indicate that polypeptide growth factors are important in articular cartilage homeostasis and repair. It is not yet clear how these growth factors are regulated. We tested the hypothesis that the growth factors responsible for regulating cartilage are themselves regulated by growth factors. We delivered insulin-like growth factor I (IGF-I), fibroblast growth factor-2 (FGF-2), and/or transforming growth factor-beta1 (TGF-beta1) to adult bovine articular chondrocytes in primary culture and measured the resulting changes in IGF-I, FGF-2, and TGF-beta1 gene expression and protein production. These growth factors differentially regulated their own and each others gene expression and protein production. In concert, they regulated each other in an interactive fashion. Their interactions ranged from inhibitory to synergistic. The time course of the regulatory effects differed among the individual growth factors and combinations. Growth factor-induced changes in growth factor protein production by articular chondrocytes generally corresponded to the changes in gene expression patterns. These studies suggest that interactions among IGF-I, FGF-2, and TGF-beta1 substantially modulate their regulatory functions. The results may help guide the application of growth factors to articular cartilage repair.

Action of fibroblast growth factor-2 on the intervertebral disc

Introduction

Fibroblast growth factor 2 (FGF2) is a growth factor that is immediately released after cartilage injury and plays a pivotal role in cartilage homeostasis. In human adult articular cartilage, FGF2 mediates anti-anabolic and potentially catabolic effects via the suppression of proteoglycan (PG) production along with the upregulation of matrix-degrading enzyme activity. The aim of the present study was to determine the biological effects of FGF2 in spine disc cells and to elucidate the complex biochemical pathways utilized by FGF2 in bovine intervertebral disc (IVD) cells in an attempt to further understand the pathophysiologic processes involved in disc degeneration.

Methods

We studied the effect of FGF2 on IVD tissue homeostasis by assessing MMP-13 expression (potent matrix-degrading enzyme), PG accumulation, and PG synthesis in the bovine spine IVD, as well as evaluating whether FGF2 counteracts known anabolic factors such as BMP7. To understand the molecular mechanisms by which FGF2 antagonizes BMP7 activity, we also investigated the signaling pathways utilized by FGF2 in bovine disc tissue.

Results

The primary receptor expressed in bovine nucleus pulposus cartilage is FGFR1, and this receptor is upregulated in degenerative human IVD tissue compared with normal IVD tissue. Stimulation of bovine nucleus pulposus cells cultured in monolayer with FGF2 augmented the production of MMP-13 at the transcriptional and translational level in a dose-dependent manner. Stimulation of bovine nucleus pulposus cells cultured in alginate beads for 21 days with FGF2 resulted in a dose-dependent decrease in PG accumulation, due at least in part to the inhibition of PG synthesis. Further studies demonstrate that FGF2 (10 ng/ml) antagonizes BMP7-mediated acceleration of PG production in bovine nucleus pulposus cells via the upregulation of noggin, an inhibitor of the transforming growth factor beta/bone morphogenetic protein signaling pathway. Chemical inhibitor studies showed that FGF2 utilizes the mitogen-activated protein kinase and NF-κB pathways to upregulate noggin, serving as one potential mechanism for its anti-anabolic effects.

Conclusion

FGF2 is anti-anabolic in bovine spine disc cells, revealing the potential of FGF2 antagonists as unique biologic treatments for both prevention and reversal of IVD degeneration.

Basic fibroblast growth factor inhibits the anabolic activity of insulin-like growth factor 1 and osteogenic protein 1 in adult human articular chondrocytes

OBJECTIVE

To determine the effects of basic fibroblast growth factor (bFGF) on the chondrocyte anabolic activity promoted by insulin-like growth factor 1 (IGF-1) and osteogenic protein 1 (OP-1).

METHODS

Human articular chondrocytes were cultured in alginate beads or as cartilage explants in serum-free medium with or without IGF-1 (100 ng/ml), OP-1 (100 ng/ml), or bFGF (0-100 ng/ml). Cell survival, proliferation, proteoglycan synthesis, and total proteoglycan accumulation were measured after 21 days of culture in alginate beads, and proteoglycan synthesis was measured in explants.

RESULTS

Cell survival was not altered by bFGF at any dose, and chondrocyte proliferation was stimulated only at doses above 1 ng/ml. When combined with IGF-1, 1 ng/ml of bFGF stimulated proliferation to 170% of control, but when combined with IGF-1 and OP-1, proliferation increased to 373% of control. Doses of bFGF of 100 ng/ml decreased total proteoglycan levels accumulated per cell by 60% compared with control and also inhibited the ability of IGF-1 or OP-1 to increase proteoglycan production. Likewise, sulfate incorporation in response to IGF-1 and OP-1 alone or together was completely inhibited by 50 ng/ml bFGF in both alginate and explant cultures.

CONCLUSION

The anabolic activity of IGF-1 and OP-1, alone and in combination, is significantly inhibited by bFGF. The results suggest that excessive release of bFGF from the cartilage matrix during injury, with loading, or in arthritis could contribute to increased proliferation and reduced anabolic activity in articular cartilage.

Growth factors and treatment of intervertebral disc degeneration

STUDY DESIGN

Literature review.

OBJECTIVE

To review the most recent findings of the effects of growth factors on the intervertebral disc and, further, to discuss trends in the biologic repair of the degenerated intervertebral disc.

SUMMARY OF BACKGROUND DATA

Since early in 1990, advancements in molecular biology and cell culture technology have enabled researchers to accumulate knowledge about the in vitro actions of growth factors on intervertebral disc cells. More recently, the use of growth factors for the biologic regeneration of the intervertebral disc is of increasing interest to the orthopedic field, and indeed, some preliminary in vivo studies have proven their efficacy.

METHODS

Based on a literature search conducted using available databases, such as the National Library of Medicine, as well as data presented at scientific conferences held in the past 2 years, primarily in the United States, the current status of biologic therapy for disc degeneration using growth factors was summarized.

RESULTS

With increasing evidence to support the feasibility of biologically regenerating intervertebral disc tissues, the clinical application of growth factors has become more plausible. The effects of growth factors on the metabolism of intervertebral disc cells or tissues have been extensively studied using in vitro approaches. More recently, the efficacy of an injection of growth factor protein to reverse disc regeneration has been shown in vivo using a small animal disc degeneration model. The confirmation of those effects and a detailed dose-response study, as well as a long-term safety study, in a large animal model is highly anticipated. Hopefully, the expansion of the clinical use of improved imaging techniques for the early detection of disc degeneration and promising results about the effects of growth factors on intervertebral disc regeneration will benefit the human population in the near future.

CONCLUSIONS

The results from these in vitro and in vivo studies reviewed here clearly suggest the potential usefulness of growth factor injections as a new approach to restore intervertebral disc degeneration at an early stage.

Tumor necrosis factor alpha and epidermal growth factor act additively to inhibit matrix gene expression by chondrocyte

The failure of chondrocytes to replace the lost extracellular matrix contributes to the progression of degenerative disorders of cartilage. Inflammatory mediators present in the joint regulate the breakdown of the established matrix and the synthesis of new extracellular matrix molecules. In the present study, we investigated the effects of tumor necrosis factor alpha (TNF-alpha) and epidermal growth factor (EGF) on chondrocyte morphology and matrix gene expression. Chondrocytes were isolated from distal femoral condyles of neonatal rats. Cells in primary culture displayed a cobblestone appearance. EGF, but not TNF-alpha, increased the number of cells exhibiting an elongated morphology. TNF-alpha potentiated the effect of EGF on chondrocyte morphology. Individually, TNF-alpha and EGF diminished levels of aggrecan and type II collagen mRNA. In combination, the effects of TNF-alpha and EGF were additive, indicating the involvement of discrete signaling pathways. Cell viability was not compromised by TNF-alpha or by EGF, alone or in combination. EGF alone did not activate NF-kappaB or alter NF-kappaB activation by TNF-alpha. Pharmacologic studies indicated that the effects of TNF-alpha and EGF alone or in combination were independent of protein kinase C signaling, but were dependent on MEK1/2 activity. Finally, we analyzed the involvement of Sox-9 using a reporter construct of the 48 base pair minimal enhancer of type II collagen. TNF-alpha attenuated enhancer activity as expected; in contrast, EGF did not alter either the effect of TNF-alpha or basal activity. TNF-alpha and EGF, acting through distinct signaling pathways, thus have additive adverse effects on chondrocyte function. These findings provide critical insights into the control of chondrocytes through the integration of multiple extracellular signals.

Differentiation status-dependent regulation of cyclooxygenase-2 expression and prostaglandin E2 production by epidermal growth factor via mitogen-activated protein kinase in articular chondrocytes

Although large amounts of epidermal growth factor (EGF) are found in the synovial fluids of arthritic cartilage, the role of EGF in arthritis is not clearly understood. This study investigated the effect of EGF on differentiation and on inflammatory responses such as cyclooxygenase-2 (COX-2) expression and prostaglandin E(2) (PGE(2)) production in articular chondrocytes. EGF caused a loss of differentiated chondrocyte phenotype as demonstrated by inhibition of type II collagen expression and proteoglycan synthesis. EGF also induced COX-2 expression and PGE(2) production. EGF-induced dedifferentiation was caused by EGF receptor-mediated activation of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) but not p38 kinase, whereas the activation of both ERK1/2 and p38 kinase was necessary for COX-2 expression and PGE(2) production. Neither the inhibition of COX-2 expression and PGE(2) production nor the addition of exogenous PGE(2) affected EGF-induced dedifferentiation. However, COX-2 expression and PGE(2) production were significantly enhanced in chondrocytes that were dedifferentiated by serial subculture, and EGF also potentiated COX-2 expression and PGE(2) production, although these cells were less sensitive to EGF. Dedifferentiation-induced COX-2 expression and PGE(2) production were mediated by ERK1/2 and p38 kinase signaling. Our results indicate that EGF in articular chondrocytes stimulates COX-2 expression and PGE(2) production via ERK and p38 kinase signaling in association with differentiation status.

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.

The use of growth factors in cartilage repair

Articular cartilage, which enables smooth gliding of joints during skeletal motion, is vulnerable to injuries and degenerative diseases over time. Bone growth factors have a role in the preservation of the cartilage matrix. This article reviews the potential to treat cartilage damage for bone morphogenetic proteins, insulin-like growth factors, hepatocyte growth factor, basic fibroblast growth factor, and transforming growth factor beta.

Outside-in signaling in the chondrocyte. Nitric oxide disrupts fibronectin-induced assembly of a subplasmalemmal actin/rho A/focal adhesion kinase signaling complex

Elevated levels of fibronectin (Fn) in articular cartilage have been linked to the progression of both rheumatoid and osteoarthritis. In this study, we examined intracellular events which follow ligation of Fn to its receptor, the integrin alpha5beta1. In addition, we examined the regulatory influence of nitric oxide on these events, since this free radical has been implicated in cartilage degradation. Exposure of chondrocytes to Fn-coated beads resulted in the circumferential clustering of the alpha5beta1 integrin receptor, which was accompanied by the subplasmalemmal assembly of a focal activation complex comprised of F-actin, the tyrosine kinase, focal adhesion kinase (FAK), the ras related G protein rho A, as well as tyrosine-phosphorylated proteins. Treatment with exogenous nitric oxide (NO) or catabolic cytokines which induce nitric oxide synthase blocked the assembly of F-actin, FAK, rho A and tyrosine-phosphorylated proteins while not affecting the total number of beads bound per cell nor the clustering of alpha5beta1 integrin. Use of a cGMP antagonist (Rp-8-Br cGMPS) or cGMP agonist (Sp-cGMPS) either abolished or mimicked the NO effect, respectively. Adherence of chondrocytes to fibronectin enhanced proteoglycan synthesis by twofold (vs. albumin). In addition, basic fibroblast growth factor (FGF) and insulin growth factor (IGF-1) induced proteoglycan synthesis in chondrocytes adherent to Fn but not albumin suggesting a costimulatory signal transduced by alpha5betal and the FGF receptor. Both constitutive and FGF stimulated proteoglycan synthesis were completely inhibited by nitric oxide. These data indicate that the ligation of alpha5beta1 in the chondrocyte induced the intracellular assembly of an activation complex comprised of the cytoplasmic tail of alpha5beta1 integrin, actin, and the signaling molecules rho A and FAK. We show that NO inhibits the assembly of the intracellular activation complex and the synthesis of proteoglycans, but has no effect on the extracellular aggregation of alpha5beta1 integrin. These observations provide a basis by which nitric oxide can interfere with chondrocyte functions by affecting chondrocyte-matrix interactions.

Platelet-derived growth factor and vascular endothelial growth factor expression in disc herniation tissue: and immunohistochemical study

Angiogenesis is essential in tissue growth and regeneration. There are several factors that are able to stimulate vascular endothelial cell growth, including platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF). Disc herniation tissue (DHT) contains vascular ingrowth, which promotes granulation tissue formation. In this study we observed 50 disc herniations for PDGF and VEGF immunoreactivity. PDGF immunopositivity was detected in 38 samples (78%). In 28 samples (56%) there were PDGF immunopositive capillaries, PDGF immunopositive disc cells were detected in 19 samples (38%) and PDGF immunopositive fibroblasts in 6 DHT samples (12%). VEGF immunopositive capillaries were identified in 44 DHT samples (88%). For neither growth factor was immunopositivity dependent on preoperative radicular pain duration. In extrusions (n = 25) VEGF immunopositive capillaries were detected in 23 samples (92%) and PDGF immunopositivity in 21 samples (84%). PDGF immunopositivity was more commonly associated with capillaries than with nuclei of disc cells. In sequesters (n = 20) VEGF immunopositive capillaries were identified in all samples and PDGF immunopositivity in 16 (80%). As in extrusions, PDGF immunoreaction was more prevalent in capillaries than in disc cells. Patient age did not relate to VEGF expression. In all age groups it was higher than 80%. Thus capillaries in disc herniation tissue are evidently newly formed and our results demonstrate that PDGF and VEGF participate in the neovascularization process. The presence of PDGF in fibroblasts and in disc cells suggests that this growth factor regulates the function of these cells, possibly the proliferation of the cells and the production of extracellular matrix components.

Effects of basic fibroblast growth factor, transforming growth factor-beta 1, insulin-like growth factor-1, and insulin on human osteoarthritic articular cartilage explants

This study evaluated the effects of basic fibroblast growth factor, transforming growth factor-beta 1, insulin-like growth factor-1, and insulin on the incorporation of thymidine and sulfate in human osteoarthritic articular cartilage. Tissue explants were obtained from 11 patients undergoing total knee arthroplasty and were categorized as nonfibrillated or fibrillated cartilage. The explants were cultured for 22 days, with changes of medium and growth factor every 72 hours, and labeled with [3H]thymidine and [35S]sulfate. Growth factors were used in the following concentrations: basic fibroblast growth factor at 1, 10, and 100 ng/ml; transforming growth factor-beta 1 at 0.5, 5, and 50 ng/ml; insulin-like growth factor-1 at 0.15, 1.5, and 15 ng/ml; and insulin at 0.05, 0.5, and 5 micrograms/ml. Basic fibroblast growth factor decreased thymidine incorporation to 70% and sulfate incorporation to less than 20% that of the growth factor-free controls. Transforming growth factor-beta 1 had no significant effect on thymidine incorporation, whereas the concentrations studied inhibited sulfate incorporation to approximately 40% that of the controls. At the concentrations tested, insulin-like growth factor-1 had no significant effect on incorporation of either thymidine or sulfate. In contrast, insulin significantly stimulated the incorporation of both. Compared with growth factor-free controls, insulin maximally increased thymidine incorporation by a factor (+/- SEM) of 2.36 +/- 0.47 and 1.69 +/- 0.19 in nonfibrillated and fibrillated explants, respectively; sulfate incorporation was maximally increased 1.60 +/- 0.24 and 1.92 +/- 0.29-fold for nonfibrillated and fibrillated explants, respectively. Of the factors tested, insulin demonstrated the greatest promise for promoting a synthetic response that may contribute to the regeneration of osteoarthritic cartilage.

Calcium signaling in endothelin- and platelet-derived growth factor-stimulated chondrocytes

In bovine articular chondrocytes, endothelin (ET) and platelet-derived growth factor (PDGF) receptors mediate agonist-induced increases in inositol phosphate hydrolysis, cytoplasmic calcium concentration ([Ca2+]i), and mitogenesis. In most cells, ET stimulated nonoscillatory [Ca2+]i elevations with dose-dependent increases in both spike and plateau amplitudes. However, about 15% of cells showed oscillatory Ca2+ responses with a constant frequency and variable shape and duration of spiking. ET-1 and ET-2 were more potent than ET-3 in stimulating [Ca2+]i responses in inhibiting the specific binding of 125I-ET-1 and 125I-ET-3 and in promoting internalization of the receptor-ligand complex, consistent with actions through endothelin ETA receptors. Similar nonoscillatory and oscillatory patterns of Ca2+ responses were observed in PDGF-stimulated cells. In cells showing nonoscillatory Ca2+ responses to ET-1, subsequent stimulation with PDGF was frequently followed by the development of an oscillatory Ca2+ response. Nonoscillatory responses to both agonists were only slightly reduced in Ca(2+)-deficient medium, but the oscillatory responses were critically dependent on Ca2+ entry. Ca2+ spiking was not altered in the presence of the voltage-sensitive Ca2+ channel blocker, nifedipine; also, depolarization of chondrocytes by high K+ did not induce [Ca2+]i responses, confirming that voltage-sensitive calcium channels are not expressed in these cells. At high agonist concentrations, ET- but not PDGF-stimulated cells underwent rapid desensitization. Activation of ETA and PDGF receptors was associated with differential stimulation of thymidine incorporation; ET-1 induced a low-amplitude bell-shaped dose-response curve; PDGF induced a sustained sigmoidal and dose-dependent rise. These data indicate that two distinct types of Ca(2+)-mobilizing receptors initiate similar patterns of [Ca2+]i responses but have different capacities to maintain and reinitiate the Ca2+ signaling, as well as to promote mitogenesis.

Cytokines and proteoglycans

Cytokines play an important regulatory role in the metabolism of proteoglycans. Proteoglycans are found in plasma membranes, but predominantly in the extra-cellular matrix. In the latter they are quantitatively and qualitatively essential components. Especially in a tissue like cartilage without any blood vessels, the cells are dependent on cytokines for the communication among themselves in the extra-cellular matrix and also for communication with the 'outside world'. Various cytokines have been found to be able to penetrate the extra-cellular matrix and inhibit, respectively stimulate the proteoglycan synthesis. Also, the degradation of proteoglycans can be stimulated, respectively inhibited by several cytokines. In addition, some cytokines have been found which regulate the effects of the other cytokines. With respect to proteoglycan metabolism a complex cytokine network is emerging. Furthermore it is becoming increasingly clear that proteoglycans are connected to the cytokine network by their own bioactive functions. First, they possibly possess cytokine activities themselves. Second, they can function as receptors, protectors, inactivators and storage ligands for cytokines. So the proteoglycans are clearly involved in the feedback signalling from the extra-cellular matrix to the cells that are synthesizing this extra-cellular matrix. Together with agonistic or antagonistic cytokines they are involved in the regulation of proteoglycan turnover during balanced or unbalanced metabolism in normal, respectively pathological situations.

Platelet-derived growth factor and transforming growth factor-beta 1 differentially affect the synthesis of biglycan and decorin by monkey arterial smooth muscle cells

Platelet-derived growth factor (PDGF) and transforming growth factor-beta 1 (TGF-beta 1), two growth-regulatory peptides with opposite effects on arterial smooth muscle cell (ASMC) proliferation, were examined for their influence on the synthesis of two small chondroitin sulfate/dermatan sulfate proteoglycans (CS/DS PGs) called biglycan and decorin. Quiescent ASMCs treated with either PDGF or TGF-beta 1 for 24 hours increased [35S]sulfate incorporation into biglycan 3.3- and 2.9-fold, respectively, whereas the incorporation of [35S]sulfate into decorin was not significantly affected. Treatment with TGF-beta 1 but not PDGF more than doubled the steady-state level of messenger RNA (mRNA) transcripts hybridizing to a complementary DNA (cDNA) encoding biglycan. Both growth factors had little or no effect on steady-state levels of mRNA transcripts hybridizing to a decorin cDNA. Incorporation of [35S]sulfate into biglycan glycosaminoglycan (GAG) was maximal by 12 to 18 hours after either PDGF or TGF-beta 1 addition. Both PDGF and TGF-beta 1 increased the molecular sizes of biglycan and decorin. This increase was a result of the synthesis of longer GAG chains substituted on the core proteins of both PGs. PDGF but not TGF-beta 1 led to an increase of more than twofold in the ratio of 6'- to 4'-sulfated disaccharides in these newly synthesized GAG chains. These results indicate that PDGF and TGF-beta 1 have specific but different effects on the synthesis of small CS/DS PGs by monkey ASMCs in culture.

EGF abrogation-induced fusilli-form dysmorphogenesis of Meckel's cartilage during embryonic mouse mandibular morphogenesis in vitro

Mutations associated with genes of the EGF superfamily are implicated in facial malformations arising from abnormal development of the first branchial arch. EGF and EGF receptor (EGFr) transcripts are expressed in the mouse embryonic first branchial arch and derivatives from E9 through E15. EGF transcripts are localized to ectomesenchymal cells associated with precartilage, cartilage, bone and tooth-forming cells. EGF and EGFr proteins co-localize to the same cells suggesting an autocrine regulation. To test whether EGF effects the timing and positional information required for Meckel's cartilage (MC) and tooth development, we cultured E10 mandibular explants in serumless, chemically defined medium with either antisense or sense EGF oligodeoxynucleotides. Antisense inhibition of EGF expression produces bilaterally symmetrical Fusilli-form dysmorphogenesis of MC and decreases tooth bud size; these effects are reversed by the addition of exogenous EGF to the culture medium. Tyrphostin RG 50864, which inhibits EGF receptor kinase activity, inhibits EGF stimulation of tyrosine phosphorylation in a concentration-dependent manner and severely retards mandibular development yet increases tooth size. These findings support the hypothesis that endogenous EGF and EGF-like proteins provide signalling to regulate the size and shape both of cartilage and tooth formation during craniofacial morphogenesis.

Cytokines and proteoglycans

Cytokines play an important regulatory role in the metabolism of proteoglycans. Proteoglycans are found in plasma membranes, but predominantly in the extra-cellular matrix. In the latter they are quantitatively and qualitatively essential components. Especially in a tissue like cartilage without any blood vessels, the cells are dependent on cytokines for the communication among themselves in the extra-cellular matrix and also for communication with the 'outside world'. Various cytokines have been found to be able to penetrate the extra-cellular matrix and inhibit, respectively stimulate the proteoglycan synthesis. Also, the degradation of proteoglycans can be stimulated, respectively inhibited by several cytokines. In addition, some cytokines have been found which regulate the effects of the other cytokines. With respect to proteoglycan metabolism a complex cytokine network is emerging. Furthermore it is becoming increasingly clear that proteoglycans are connected to the cytokine network by their own bioactive functions. First, they possibly possess cytokine activities themselves. Second, they can function as receptors, protectors, inactivators and storage ligands for cytokines. So the proteoglycans are clearly involved in the feedback signalling from the extra-cellular matrix to the cells that are synthesizing this extra-cellular matrix. Together with agonistic or antagonistic cytokines they are involved in the regulation of proteoglycan turnover during balanced or unbalanced metabolism in normal, respectively pathological situations.

Altered proteoglycan synthesis via the false acceptor pathway can be dissociated from beta-D-xyloside inhibition of proliferation

beta-D-Xylosides have been used to perturb proteoglycan (PG) synthesis to elucidate the function of PGs in a number of cellular processes, including proliferation, migration, and differentiation. This study was designed to examine whether specific xylosides affect the proliferation of several different cell types and, if so, whether this effect is dependent on altered PG synthesis via the false acceptor pathway. Both methylumbelliferyl beta-D-xylopyranoside and p-nitrophenyl beta-D-xylopyranoside (PNP beta-xyloside) inhibit cell proliferation and modulate PG synthesis; however, the alpha form of PNP xyloside which does not perturb PG synthesis inhibits the proliferation of cultured cells on a molar basis equally as well as the beta form. Conversely, beta-methyl xylopyranoside stimulates the synthesis of free glycosaminoglycan chains equally as well as PNP beta-xyloside and yet has no measurable effect on cell proliferation at comparable doses, indicating that cells can grow normally while experiencing disruption of their proteoglycan metabolism. At doses ranging from 0.5 to 5 mM, PNP beta-xyloside arrests cells in the G1 phase of the cell cycle at the same time point as serum starvation. It also delays the exist of cycling cells from the S phase. This treatment is not cytotoxic and is rapidly reversed by the replacement of PNP beta-xyloside containing medium with control medium. Dimethyl sulfoxide, the most commonly used solvent for beta-xyloside in proteoglycan studies, potentiates the inhibitory effect of PNP beta-xyloside on cell proliferation. These results indicate that the perturbation of PG synthesis via the false acceptor pathway can be uncoupled from control of cell proliferation.

Low plasma levels of insulin-like growth factor I in Perthes' disease. A controlled study of 59 consecutive children

We studied Insulin-like Growth Factor I (IGF I) plasma levels, standing height, and weight in 59 consecutive children with Perthes' disease and 59 matched controls. The plasma-IGF I levels, measured by radioimmunoassay after acid ethanol extraction, were reduced in affected children during the first 2 years after the diagnosis of Perthes' disease. Partially paralleling the alterations in IGF I plasma levels, there was a tendency towards growth arrest and impaired weight-gain during early-stage disease, followed by catch-up growth and increased weight-gain. No relation was found between degree of femoral head involvement, according to Catterall (1971), and IGF I plasma-levels or body mass. Our data may reflect an impaired synthesis or release of IGF I relative to age in Perthes' disease, or changes in the affinity or metabolism of IGF binding proteins. The observed changes seem to be of a temporary nature.

Effect of insulin on the altered production of proteoglycans in rib cartilage of experimentally diabetic rats

Costal cartilage from experimentally diabetic rats, labeled in vivo or in vitro with [35S]sulfate, was shown to incorporate less label into proteoglycans than cartilage from nondiabetic rats. Analyses of guanidine HCl cartilage extracts by gel chromatography on Sepharose CL-2B showed two major peaks at Kav approximately 0.4 and 0.8 (peaks I and II, respectively). Cartilage extracts from the diabetic rats contained predominantly peak II proteoglycans, while 60 and 55%, respectively, of the total 35S-labeled proteoglycans extracted from control cartilage labeled in vivo and in vitro with [35S]sulfate were present in peak I. After insulin treatment of the diabetic rats, the relative amount of peak I 35S-labeled proteoglycans synthesized in vivo was increased to 70%. The overall in vivo incorporation of [35S]sulfate into proteoglycans was also stimulated in diabetic rats treated with insulin to levels above those found for control rats. Thus, diabetes-induced changes in the biosynthesis of rat costal cartilage proteoglycans may be alleviated by normalization of the diabetic state by insulin treatment. However, addition of insulin (10(-5)-10(-9) M) to the culture medium did not affect the amount of 35S-labeled proteoglycans synthesized in vitro or the relative amounts of peak I proteoglycans produced by control or diabetic cartilage, suggesting that insulin does not have a direct effect on proteoglycan production. Moreover, no decrease in the amount of 35S-labeled proteoglycans produced was found when glucose at high concentrations was present in the culture medium. However, the presence of rat serum resulted in an increase in the amount of 35S-labeled proteoglycans produced by both control and diabetic cartilage, demonstrating that the cartilage explants were metabolically responsive to stimulatory factors.

Differential modulation of growth and phenotypic expression of chondrocytes in sparse and confluent cultures by growth factors in cartilage

The growth-promoting actions of cartilage extracts (CE) on rabbit cultured chondrocytes were studied to assess the role of local acting growth factors in the generation and expansion of highly differentiated cells. In the present study, DNA synthesis and proteoglycan synthesis in the cultured chondrocytes were monitored by flow cytofluorometry and double-isotope autoradiography by using [3H]thymidine and [35S]sulfate. We report here that actions of the same set of growth factors extracted from cartilage evokes differential cellular responses depending upon cell density. Growth factors in the optimal dose of CE (2 micrograms/ml) or epidermal growth factor (EGF, 40 ng/ml) did not reveal such a cell density-dependent effect on cellular proliferation. However, growth factors in CE induced proteoglycan synthesis selectively in nonproliferating and expressing cells in confluent culture.

Ascorbic acid stimulates production of glycosaminoglycans in cultured fibroblasts

The effect of ascorbic acid on collagen synthesis is well characterized. Proteoglycans and their attached glycosaminoglycans are components of the extracellular matrix closely associated with collagen fibers. We examined whether ascorbic acid also plays a role in glycosaminoglycan production. Synthesis and deposition of glycosaminoglycans into the extracellular matrix and secretion into the media were followed in human skin fibroblasts cultured in the presence and absence of ascorbic acid. Specific glycosaminoglycans were identified and quantitated by differential enzyme digestion, ion-exchange column chromatography, and cellulose-acetate electrophoresis. No major qualitative changes in glycosaminoglycans were observed. However, quantitatively, synthesis of glycosaminoglycans increased 30 to 90%, and deposition into the extracellular matrix increased 80% in the presence of ascorbic acid. This effect was only in part secondary to decreased levels of collagen, and the diminished capacity of underhydroxylated collagen to bind proteoglycans. The effect of ascorbic acid on extracellular macromolecules is thus more pervasive than previously assumed.

The effect of growth factors and synovial fluid on chondrogenesis in perichondrium

Reconstruction of cartilage with perichondrium depends on the chondrogenic property of the perichondrial fibrocytes. The present investigation concerns the conditions for the differentiation of fibrocytes into chondrocytes both in vivo and in vitro. For the in vivo studies specimens of rib and auricular perichondrium from adult rabbits were wrapped round silicon rods which were enclosed in dialysis bags. One was placed in the suprapatellar pouch of the knee joint and one was placed intraperitoneally in each rabbit. After two months the bags were extracted, the perichondrium prepared for microscopic examination, and the chondrogenesis evaluated. In vitro the perichondrium was divided into small pieces and incubated with tissue culture medium. The medium was supplemented with fetal calf serum, together with epidermal growth factor, platelet derived growth factor, synovial fluid, or with human serum albumin (control group). After three weeks the explants were prepared for microscopy. Chondrogenesis was judged by the degree of cellular enlargement, capsule formation, deposition of matrix, and activation of the outer fibrocytic layer. In vivo, good cartilage development was found in all specimens placed in the knee joint but, in those placed intraperitoneally, little if any chondrogenesis was seen. In vitro profound differentiation occurred in all cultures supplemented with epidermal growth factor and platelet derived growth factor. An equivalent differentiation was found in perichondrium that had been incubated with synovial fluid. We conclude that the differentiation of perichondrial fibrocytes is initiated in vitro by growth factors. In addition, we have shown that synovial fluid contains factors that promote and enhance the development of cartilage from perichondrium.

Insulin-like growth factor stimulation of chondrocyte proteoglycan synthesis by human synovial fluid

We investigated the role of insulin-like growth factors (IGFs) as regulating factors of cartilage metabolism in human synovial fluid (SF), using a bovine explant culture system that was shown to respond to recombinant IGF-1 in vitro. SF from rheumatoid arthritis (RA) patients and from control patients was found to stimulate chondrocyte proteoglycan synthesis in bovine articular cartilage. A monoclonal antibody directed primarily against IGF-1 (and to some extent, IGF-2) partially blocked the stimulatory action of serum and totally blocked the stimulation by SF. These findings indicate that IGFs are major regulating factors of cartilage proteoglycan synthesis in human SF. In addition, we measured serum and SF levels of IGF-1 in RA patients and control patients, using a radioimmunoassay. No difference in immunoreactive serum IGF-1 was detected between patients and controls. The IGF-1 levels in SF were consistently lower than in serum, for both patient groups. No differences in IGF-1 concentration were found between RA and non-RA SF. The relevance of these data with respect to joint inflammation is discussed.

Growth factor stimulation of adult articular cartilage

We have examined the effect of peptide growth factors on DNA and proteoglycan synthesis by adult bovine articular cartilage in organ culture. The actions of somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I), insulin, epidermal growth factor (EGF), and fibroblast growth factor (FGF) from bovine pituitary were investigated individually and in combination. FGF stimulated a 10-fold increase in tritiated thymidine incorporation while other factors used individually did not influence mitotic activity. Used in concert, insulin with EGF and insulin with FGF acted synergistically in stimulating DNA synthesis 20-fold and 40-fold, respectively. All of these growth factors, acting individually, significantly enhanced radiosulfate incorporation. This stimulation was additive for Sm-C/IGF-I in combination with EGF or FGF, but not with insulin. These data indicate that adult bovine articular chondrocytes possess the capacity to augment both mitotic and differentiated cell functions in response to growth factors. The data further suggest that, with the exception of insulin and Sm-C/IGF-I, which appear to share a common mechanism of action, these factors produce their cellular effects via different receptor or postreceptor pathways.

Proteoglycan biosynthesis by rabbit articular chondrocytes treated with D-penicillamine

Rabbit articular chondrocytes in confluent monolayer cultures were treated with D-Penicillamine (D-Pen) during 3 or 5 days. The [35S]-sulfate incorporation in neosynthesized proteoglycans was not modified by D-Pen doses ranging from 50 to 800 micrograms/ml. After treatment during 5 days with D-Pen concentrations of 50 or 400 micrograms/ml, the chemical characteristics of proteoglycans from medium and cell-layer were determined. The aggregation capacity of proteoglycans from medium, the monomer molecular size, the glycosaminoglycan chain length and the relative rates of the different glycosaminoglycans (chondroitins, chondroitin 6-sulfate, chondroitin 4-sulfate, hyaluronic acid) remained unchanged. These results suggest that D-Pen does not alter some of the cartilage mechanical properties due to the presence of proteoglycans.

Effect of insulin on sulfated proteoglycan synthesis in cultured smooth muscle cells from pig aorta

The effect of insulin upon proteoglycan synthesis was studied in cultured smooth muscle cells from pig aorta blocked in the G0 phase by serum deprivation. Insulin enhanced [35S]sulfate incorporation into cell layer and medium-secreted proteoglycans. The increase in incorporation of the precursor was not due to a mitogenic response by smooth muscle cells to the hormone and the specific radioactivity of proteoglycans showed that the stimulation reflected a real increase in sulfated proteoglycan synthesis. Maximal stimulation was observed, for the cell layer as well as for the medium, 40 h after the addition of 1.7 x 10(-7) M insulin and reached respectively 65 and 53%. This stimulation was about 80 and 60% of the level achieved with 10% fetal calf serum for cell layer and medium-secreted proteoglycans, respectively. The half-maximal effect was attained, for both the cell layer and the medium, in the presence of 2.1 x 10(-9) M insulin. Proteoglycans secreted into the medium, in the presence of 1.7 x 10(-8) M insulin for 40 h, showed a higher proportion of complexes (24%) than those synthesized in control medium (11%) and at least 95% of the monomers from culture treated with insulin were characterized by a smaller hydrodynamic size than those synthesized by cells maintained in control medium. This decrease in the size of proteoglycans was partly due to a decrease in the size of their glycanic chains.

Human arthritic synovial fluid influences proteoglycan biosynthesis and degradation in organ culture of bovine nasal cartilage

The influence of synovial fluid and serum from patients with inflammatory joint disease on proteoglycan metabolism was studied in organ culture of bovine nasal cartilage. Proteoglycan biosynthesis, i.e. incorporation of [35S]-sulphate, was reduced after addition of synovial fluid from rheumatoid arthritis and reactive arthritis patients. Also some rheumatoid arthritis sera but no reactive arthritis serum reduced the biosynthesis compared to control sera. Proteoglycan degradation, i.e. release of proteoglycans prelabelled with [35S]-sulphate, as well as release of proteoglycans determined by chemical methods, was highest under the influence of rheumatoid arthritis synovial fluid. This effect appears to represent an activity truly stimulating degradation, since added control serum did not prevent the effect. The lowest proteoglycan degradation was observed in culture medium only. Addition of synovial fluid compared to addition of control serum did not increase proteoglycan degradation in freeze-killed cartilage indicating that the effect requires living cells. The findings are consistent with the presence in synovial fluid of mediators stimulating the chondrocytes both to activate proteoglycan degradation and to reduce proteoglycan biosynthesis.

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.

Stimulation of matrix formation in rabbit chondrocyte cultures by ascorbate. 2. Characterization of proteoglycans

The effect of ascorbate on the proteoglycans synthesized by rabbit articular chondrocytes was studied in first- and third-passage cultures for 12 and 26 days total duration, respectively. L-Ascorbate (0.2 mM) was added daily to half of the flasks after attachment of the cells. The cultures were labeled with Na2[35S]O4 or [14C]-glucosamine and [3H]-proline. Proteoglycans were isolated from the media and pericellular matrices by dissociative extraction and associative density gradient centrifugation. There was a large decline in the amount of proteoglycan synthesized between early and late cultures. Ascorbate increased the DNA content, amount of radiosulfate incorporated into glycosaminoglycans per microgram of DNA, and the proportion of labeled proteoglycan in the pericellular fraction of both short- and long-term cultures. The proteoglycans of the media and matrices of all cultures, with and without ascorbate, eluted as aggregates under associative column chromatographic conditions. The proteoglycans of 26-day cultures exhibited a higher degree of polydispersity in size than those of the short-term culture and contained small amounts of keratan (2-5%) and dermatan sulfate (4-8%) as assessed by keratanase and chondroitinase digestions, respectively. The effect of ascorbate, therefore, was to increase the amount of proteoglycan formed and to direct it into matrix deposition rather than to alter its quality.

Fibroblast growth factor stimulates colony formation of differentiated chondrocytes in soft agar

The effect of fibroblast growth factor (FGF) on the growth of chondrocytes in soft agar was examined. FGF induced colony formation by chick embryo and rabbit chondrocytes. The colony-forming efficiency of FGF-exposed chondrocytes was similar to that of Rous sarcoma virus-transformed chondrocytes (15-20%). Other mitogenic agents tested, such as epidermal growth factor, insulin, insulin-like growth factor-l, and platelet-derived growth factor, induced very low levels of colony formation. The induction of growth in soft agar of chondrocytes by FGF was not due to cells' phenotypic transformation, because chondrocytes grown in soft agar with FGF retained the ability to synthesize cartilage-characteristic proteoglycan. FGF did not induce growth in soft agar of chondrocytes whose phenotypic expression was suppressed by retinoic acid or 5-bromodeoxyuridine. In addition, FGF did not induce growth in soft agar of primary fibroblasts and normal rat kidney (NRK) cells. These results suggest that FGF selectively stimulates growth of differentiated chondrocytes in soft agar.

The growth of cartilage cells in vitro and the effect of intermittent compressive force. A histological evaluation

Chick epiphyseal chondrocytes are isolated from 15 day old embryos and subsequently grown as small high density cultures (aggregates consisting of 2 X 10(5) cells), which can be handled individually. Histological evaluation showed a considerable increase in the dimensions of the nucleus and the cells 2-6 hours after initiating the growth of the chondrocytes by adding 10% serum to the culture medium. After 24 hours of culture the proportion of the matrix in the aggregate was increased from 0% to about 45%. In a series of experiments these aggregates were exposed to intermittent compressive force (ICF) with a peak value of 130 mbar above ambient and a frequency of 0.3 Hz. The nuclear and cell-dimensions of ICF-exposed chondrocytes were significantly larger than under control conditions when measured after 24 and 48 hours. Concomitantly, chondrocytes in ICF-exposed aggregates synthesized and deposited significantly more matrix in the aggregates as compared to controls. This model can be used to further study the direct effect of mechanical force on the synthesis and deposition of matrix components by cartilage cells in vitro.

In vitro cell proliferation and proteoglycan synthesis of rabbit meniscal fibrochondrocytes as a function of age and sex

Meniscal fibrochondrocytes from male and female New Zealand white rabbits, ages 6, 12, and 24 months, were grown in primary and secondary monolayer cell culture. Neither age nor sex affected the majority of their cell culture characteristics. Cells from young males (6 months old) synthesized greater amounts of sulfated proteoglycans than did those from young females, but by 2 years of age, this result was reversed. All age groups synthesized 2 classes of proteoglycans, based on hydrodynamic size, but the ratio of the 2 classes changed as a function of age. Overall, the meniscal fibrochondrocytes from both skeletally immature and skeletally mature rabbits of both sexes were capable of proliferation and matrix synthesis in vitro.

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.

Articular cartilage cultured with catabolin (pig interleukin 1) synthesizes a decreased number of normal proteoglycan molecules

A homogeneous preparation of catabolin from pig leucocytes caused a reversible dose-dependent (0.01-1 nM) decrease in the synthesis of proteoglycan in slices of pig articular cartilage cultured in serum-free medium. The monomers that were synthesized and secreted in the presence of catabolin had the same average hydrodynamic size and ability to aggregate as the controls, and the core protein was substituted with the same number of glycosaminoglycan chains. The chains were the same average length and charge as normal and were sulphated to the same extent as the controls. Newly synthesized extracellular proteoglycan was not preferentially degraded. A 2-3-fold increase in glycosaminoglycan synthesis occurred in control and catabolin-treated cartilage in the presence of beta-D-xyloside (1 mM), more than 80% being secreted into the medium as free chains. Decreased incorporation of sulphate was not reversed in the presence of lysosomal-enzyme inhibitors, and there was no evidence in pulse-chase experiments of increased intracellular degradation of glycosaminoglycan chains before secretion. It is concluded that catabolin-treated cartilage synthesizes a smaller number of normal proteoglycan molecules.

Sulfated proteoglycan synthesis by confluent cultures of rabbit costal chondrocytes grown in the presence of fibroblast growth factor

We examined the effect of fibroblast growth factor (FGF) on proteoglycan synthesis by rabbit costal chondrocyte cultures maintained on plastic tissue culture dishes. Low density rabbit costal chondrocyte cultures grown in the absence of FGF gave rise at confluency to a heterogeneous cell population composed of fibroblastic cells and poorly differentiated chondrocytes. When similar cultures were grown in the presence of FGF, the confluent cultures organized into a homogenous cartilage-like tissue composed of rounded cells surrounded by a refractile matrix. The cell ultrastructure and that of the pericellular matrix were similar to those seen in vivo. The expression of the cartilage phenotype in confluent chondrocyte cultures grown from the sparse stage in the presence vs. absence of FGF was reflected by a fivefold increase in the rate of incorporation of [35S]sulfate into proteoglycans. These FGF effects were only observed when FGF was present during the cell logarithmic growth phase, but not when it was added after chondrocyte cultures became confluent. High molecular weight, chondroitin sulfate proteoglycans synthesized by confluent chondrocyte cultures grown in the presence of FGF were slightly larger in size than that produced by confluent cultures grown in the absence of FGF. The major sulfated glycosaminoglycans associated with low molecular weight proteoglycan in FGF-exposed cultures were chondroitin sulfate, while in cultures not exposed to FGF they were chondroitin sulfate and dermatan sulfate. Regardless of whether or not cells were grown in the presence or absence of FGF, the 6S/4S disaccharide ratio of chondroitin sulfate chains associated with high and low molecular weight proteoglycans synthesized by confluent cultures was the same. These results provide evidence that when low density chondrocyte cultures maintained on plastic tissue culture dishes are grown in the presence of FGF, it results in a stimulation of the expression and stabilization of the chondrocyte phenotype once cultures become confluent.

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.

Growth requirements of low-density rabbit costal chondrocyte cultures maintained in serum-free medium

The factors required for the active proliferation of low-density rabbit costal chondrocytes exposed to 9:1 (v/v) mixture of Dulbecco's modified Eagle's medium and Ham's F12 medium have been defined. Low-density primary cultures of rabbit costal chondrocytes proliferated actively when the medium was supplemented with high-density lipoprotein (300 micrograms/ml), transferrin (60 micrograms/ml), fibroblast growth factor (FGF) (1 ng/ml), hydrocortisone (10(-6) M), and epidermal growth factor (EGF) (30 ng/ml). Insulin, although it slightly decreased the final cell density, was required for reexpression of the cartilage phenotype at confluence. Optimal proliferation of low-density chondrocyte cultures was only observed when dishes were coated with an extracellular matrix (ECM) produced by cultured corneal endothelial cells, but not on plastic. Furthermore, serum-free chondrocyte cultures seeded at low density and maintained on ECM-coated dishes gave rise to a homogeneous cartilage-like tissue composed of spherical cells. These chondrocytes therefore seem to provide a good experimental system for analyzing factors involved in supporting proliferation of chondrocytes and their phenotypic expression.

The capacity of chondrocytes to respond to serum is enhanced by organ culture in the absence of serum, stimulated by serum, and modified by ascorbate

Cartilage slices maintained in organ culture have been shown to develop an enhanced capacity to respond to serum. The response was measured at the initiation of culture and after 3 and 7 days of culture in medium containing an inhibitor of DNA synthesis and 0, 1, or 16% serum. At these times, cartilage slices were washed to remove serum and inhibitor, and then exposed to various concentrations of serum for evaluation of DNA and proteoglycan synthesis. The range of the derived dose-response curves and the indicated sensitivity to low serum concentrations were the parameters used to evaluate the response capacity. Response capacity increased gradually, reaching a maximum after 8 days of culture. Considerable enhancement was obtained after maintenance in the absence of serum using both DNA and proteoglycan synthesis as markers. Additional, graded enhancement of response capacity was obtained when the cartilage slices were maintained in 1 or 16% serum. The effects of maintenance in serum were much greater when DNA synthesis rather than proteoglycan synthesis was used to measure the response. However, this serum-dependent enhancement was only prominent when ascorbate was present during the dose-response assay. Ascorbate caused a similar but less-marked increase in sensitivity to serum when proteoglycan synthesis was measured. The possibility that ascorbate may function as a cofactor during the progression phase of cell proliferation is discussed.

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.

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.