Age-related differences in the metabolism of proteoglycans in bovine articular cartilage explants maintained in the presence of insulin-like growth factor I

A novel in vitro bovine cartilage punch model for assessing the regeneration of focal cartilage defects with biocompatible bacterial nanocellulose

Introduction

Current therapies for articular cartilage defects fail to achieve qualitatively sufficient tissue regeneration, possibly because of a mismatch between the speed of cartilage rebuilding and the resorption of degradable implant polymers. The present study focused on the self-healing capacity of resident cartilage cells in conjunction with cell-free and biocompatible (but non-resorbable) bacterial nanocellulose (BNC). This was tested in a novel in vitro bovine cartilage punch model.

Methods

Standardized bovine cartilage discs with a central defect filled with BNC were cultured for up to eight weeks with/without stimulation with transforming growth factor-β1 (TGF-β1. Cartilage formation and integrity were analyzed by histology, immunohistochemistry and electron microscopy. Content, release and neosynthesis of the matrix molecules proteoglycan/aggrecan, collagen II and collagen I were also quantified. Finally, gene expression of these molecules was profiled in resident chondrocytes and chondrocytes migrated onto the cartilage surface or the implant material.

Results

Non-stimulated and especially TGF-β1-stimulated cartilage discs displayed a preserved structural and functional integrity of the chondrocytes and surrounding matrix, remained vital in long-term culture (eight weeks) without signs of degeneration and showed substantial synthesis of cartilage-specific molecules at the protein and mRNA level. Whereas mobilization of chondrocytes from the matrix onto the surface of cartilage and implant was pivotal for successful seeding of cell-free BNC, chondrocytes did not immigrate into the central BNC area, possibly due to the relatively small diameter of its pores (2 to 5 μm). Chondrocytes on the BNC surface showed signs of successful redifferentiation over time, including increase of aggrecan/collagen type II mRNA, decrease of collagen type I mRNA and initial deposition of proteoglycan and collagen type II in long-term high-density pellet cultures. Although TGF-β1 stimulation showed protective effects on matrix integrity, effects on other parameters were limited.

Conclusions

The present bovine cartilage punch model represents a robust, reproducible and highly suitable tool for the long-term culture of cartilage, maintaining matrix integrity and homoeostasis. As an alternative to animal studies, this model may closely reflect early stages of cartilage regeneration, allowing the evaluation of promising biomaterials with/without chondrogenic factors.

Aging and oxidative stress reduce the response of human articular chondrocytes to insulin-like growth factor 1 and osteogenic protein 1

OBJECTIVE

To determine the effects of aging and oxidative stress on the response of human articular chondrocytes to insulin-like growth factor 1 (IGF-1) and osteogenic protein 1 (OP-1).

METHODS

Chondrocytes isolated from normal articular cartilage obtained from tissue donors were cultured in alginate beads or monolayer. Cells were stimulated with 50-100 ng/ml of IGF-1, OP-1, or both. Oxidative stress was induced using tert-butyl hydroperoxide. Sulfate incorporation was used to measure proteoglycan synthesis, and immunoblotting of cell lysates was performed to analyze cell signaling. Confocal microscopy was performed to measure nuclear translocation of Smad4.

RESULTS

Chondrocytes isolated from the articular cartilage of tissue donors ranging in age from 24 years to 81 years demonstrated an age-related decline in proteoglycan synthesis stimulated by IGF-1 and IGF-1 plus OP-1. Induction of oxidative stress inhibited both IGF-1- and OP-1-stimulated proteoglycan synthesis. Signaling studies showed that oxidative stress inhibited IGF-1-stimulated Akt phosphorylation while increasing phosphorylation of ERK, and that these effects were greater in cells from older donors. Oxidative stress also increased p38 phosphorylation, which resulted in phosphorylation of Smad1 at the Ser(206) inhibitory site and reduced nuclear accumulation of Smad1. Oxidative stress also modestly reduced OP-1-stimulated nuclear translocation of Smad4.

CONCLUSION

These results demonstrate an age-related reduction in the response of human chondrocytes to IGF-1 and OP-1, which are 2 important anabolic factors in cartilage, and suggest that oxidative stress may be a contributing factor by altering IGF-1 and OP-1 signaling.

Insulin is essential for in vitro chondrogenesis of mesenchymal progenitor cells and influences chondrogenesis in a dose-dependent manner

PURPOSE

Insulin is a commonly used additive in chondrogenic media for differentiating mesenchymal stem cells (MSCs). The indispensability of other bioactive factors like TGF-β or dexamethasone in these medium formulations has been shown, but the role of insulin is unclear. The purpose of this study was to investigate whether insulin is essential for MSC chondrogenesis and if there is a dose-dependent effect of insulin on MSC chondrogenesis.

METHODS

We cultivated human MSCs in pellet culture in serum-free chondrogenic medium with insulin concentrations between 0 and 50 μg/ml and assessed the grade of chondrogenic differentiation by histological evaluation and determination of glycosaminoglycan (GAG), total collagen and DNA content. We further tested whether insulin can be delivered in an amount sufficient for MSC chondrogenesis via a drug delivery system in insulin-free medium.

RESULTS

Chondrogenesis was not induced by standard chondrogenic medium without insulin and the expression of cartilage differentiation markers was dose-dependent at insulin concentrations between 0 and 10 μg/ml. An insulin concentration of 50 μg/ml had no additional effect compared with 10 μg/ml. Insulin was delivered by a release system into the cell culture under insulin-free conditions in an amount sufficient to induce chondrogenesis.

CONCLUSIONS

Insulin is essential for MSC chondrogenesis in this system and chondrogenic differentiation is influenced by insulin in a dose-dependent manner. Insulin can be provided in a sufficient amount by a drug delivery system. Therefore, insulin is a suitable and inexpensive indicator substance for testing drug release systems in vitro.

Relationship between insulin-like growth factor-1 and radiographic disease in patients with primary osteoarthritis: a systematic review

OBJECTIVE

To evaluate the association between radiographic osteoarthritis (OA) and either serum insulin-like growth factor-1 (IGF-1) levels or IGF-1 gene polymorphisms in patients with primary OA.

METHODS

We conducted a systematic review of reported associations between circulating IGF-1 and/or IGF-1 gene polymorphisms and radiographic OA. Studies were eligible when: (1) investigating serum IGF-1 and/or IGF-1 gene polymorphisms in relation to prevalent or incident radiographic OA; (2) written in English; (3) full-text article or abstract; (4) patients had primary OA in knee, hip, hand or spine; (5) longitudinal, case-control or cross-sectional design. Quality assessment was done using a standardized criteria set. Best-evidence synthesis was performed based on guidelines on systematic review from the Cochrane Collaboration Back Review Group, using five evidence levels: strong, moderate, limited, conflicting and no evidence.

RESULTS

We included 11 studies with more than 3000 primary OA cases. Data on the relationship between serum IGF-1 and radiographic OA were inconsistent. Adjustment for body mass index (BMI) was often omitted. Of four high-quality studies, three studies reported no association, one study found significantly higher IGF-1 levels in OA patients compared to controls. Patients with IGF-1 gene promoter polymorphisms and a genetic variation at the IGF-1R locus had an increased OA prevalence compared to controls.

CONCLUSIONS

Observational data showed no association between serum IGF-1 and occurrence of radiographic OA (moderate level of evidence), and a positive relationship between IGF-1 gene polymorphisms and radiographic OA (moderate level of evidence); however the confounding effect of BMI was insufficiently addressed. Future well-designed prospective studies should further elaborate the role of the complex GH/IGF-1 system in primary OA.

Increased expression of the Akt/PKB inhibitor TRB3 in osteoarthritic chondrocytes inhibits insulin-like growth factor 1-mediated cell survival and proteoglycan synthesis

OBJECTIVE

The chondrocyte response to insulin-like growth factor 1 (IGF-1) is reduced with aging and in osteoarthritis (OA). IGF-1 signals through the phosphatidylinositol 3-kinase/Akt pathway. TRB3, a tribbles homolog, has been shown to inhibit IGF-1-mediated activation of Akt in HEK 293 cells. This study was undertaken to determine if TRB3 is expressed in chondrocytes, and whether the chondrocyte response to IGF-1 is reduced by TRB3.

METHODS

Human articular cartilage was obtained from normal tissue donors and from patients with OA at the time of knee replacement surgery. TRB3 was assessed in the tissue samples by reverse transcription-polymerase chain reaction, immunoblotting, and immunohistochemistry. Overexpression of TRB3 was induced by transient transfection to determine the effects of TRB3 on cell survival and proteoglycan synthesis.

RESULTS

TRB3 messenger RNA was detected in normal human chondrocytes. TRB3 protein levels were low in cells from normal cartilage but significantly increased in cells from OA cartilage. Incubation with 2 agents that induce endoplasmic reticulum stress, tunicamycin and thapsigargin, increased TRB3 levels in normal cells. Overexpression of TRB3 inhibited Akt phosphorylation and reduced chondrocyte survival and proteoglycan synthesis.

CONCLUSION

These results are the first to demonstrate that TRB3 is present in human chondrocytes, and that the level of TRB3 is increased in OA cartilage and in isolated OA chondrocytes. Because it is an inhibitor of Akt activation, elevated TRB3 production could play a role in the increased cell death and reduced response to IGF-1 observed in OA cartilage.

Regulation of immature cartilage growth by IGF-I, TGF-beta1, BMP-7, and PDGF-AB: role of metabolic balance between fixed charge and collagen network

Cartilage growth may involve alterations in the balance between the swelling tendency of proteoglycans and the restraining function of the collagen network. Growth factors, including IGF-I, TGF-beta1, BMP-7, and PDGF-AB, regulate chondrocyte metabolism and, consequently, may regulate cartilage growth. Immature bovine articular cartilage explants from the superficial and middle zones were incubated for 13 days in basal medium or medium supplemented with serum, IGF-I, TGF-beta1, BMP-7, or PDGF-AB. Variations in tissue size, accumulation of proteoglycan and collagen, and tensile properties were assessed. The inclusion of serum, IGF-I, or BMP-7 resulted in expansive tissue growth, stimulation of proteoglycan deposition but not of collagen, and a diminution of tensile integrity. The regulation of cartilage metabolism by TGF-beta1 resulted in tissue homeostasis, with maintenance of size, composition, and function. Incubation in basal medium or with PDGF-AB resulted in small volumetric and compositional changes, but a marked decrease in tensile integrity. These results demonstrate that the phenotype of cartilage growth, and the associated balance between proteoglycan content and integrity of the collagen network, is regulated differentially by certain growth factors.

In vitro model of full-thickness cartilage defect healing

Integration of the host-graft interface is implicated as one of the significant reasons for lack of complete healing in osteochondral grafting procedures for the treatment of cartilage lesions. We developed an in vitro model of cartilage healing in an osteochondral setting to study the effect of developmental age and collagenase treatment. Circular full-thickness vertical surgical incisions were made in the cartilaginous portion of cylindrical bovine osteochondral specimens. Two age groups were selected: Young (1-2 months old) and Older (6-8 months old). Cartilage integration across the surgical incisions was assessed by histologic analysis and by mechanical push-out testing at 2 and 4 weeks in culture. Histologic integration as well as peak push-out shear stress was significantly higher in older calf cartilage than in the young calf. Collagenase pretreatment in the older calf samples increased push-out strength at 4 weeks. Histologic integration correlated well with the mechanical push-out strength. Developmental age and time after injury affected the response to collagenase pretreatment. This osteochondral cartilage integration model can be useful to study factors that modulate healing of surgical replacement procedures in vitro, which may aid the development of newer approaches to promote the healing of cartilage defects.

Increased matrix metalloproteinase-13 production with aging by human articular chondrocytes in response to catabolic stimuli

Chondrocyte anabolic activity has been shown to decline with aging, but catabolic activity has received little attention. In this study, the effect of aging on the chondrocyte catabolic response was determined by stimulating isolated human chondrocytes with fibronectin fragments (FN-f) or interleukin-1beta and measuring matrix metalloproteinase-13 (MMP-13) production as a catabolic response. A significant age-related increase in chondrocyte MMP-13 production was noted. FN-f stimulation of MMP-13 expression was blocked using a nuclear factor kappa-B (NFkappaB) inhibitor suggesting a role for NFkappaB in this chondrocyte catabolic response. Chondrocyte production of the NFkappaB-regulated cytokine interleukin-1beta was also found to increase with donor age in unstimulated cells. These results demonstrate a significant age-related increase in chondrocyte catabolic responsiveness which could contribute to the development of osteoarthritis in older adults.

Aged bovine chondrocytes display a diminished capacity to produce a collagen-rich, mechanically functional cartilage extracellular matrix

Most fundamental studies in cartilage tissue engineering investigate the ability of chondrocytes from young animals to produce cartilaginous matrix under various conditions, while current clinical applications such as autologous chondrocyte implantation, use chondrocytes from donors that are decades past skeletal maturity. Previous investigations have suggested that several characteristics of primary chondrocytes are age-dependent but none have quantified cell proliferation, proteoglycan synthesis and accumulation, collagen synthesis and accumulation, compressive and tensile mechanical properties in order to examine the effects of donor age on all of these parameters. We enzymatically isolated primary bovine chondrocytes from fetal, young and aged animals and cultured these cells in agarose gels to assess the above-mentioned properties. We found that fetal and young (but still skeletally mature i.e. 18-month-old bovine) chondrocytes behaved similarly, while aged chondrocytes (5- to 7-year-old bovine) displayed diminished proliferation ( approximately 2x less), a slightly reduced proteoglycan accumulation per cell ( approximately 20%), and significantly less collagen accumulation per cell ( approximately 55%) compared to the younger cells. Histological observations and mechanical properties supported these findings, where a particularly significant reduction in tensile stiffness produced by aged chondrocytes compared to younger cells was observed. Our findings suggest that donor age is an important factor in determining the outcome and potential success when tissue-engineered cartilage is produced from articular chondrocytes. More specifically, primary chondrocytes from aged donors may not possess sufficient capacity to produce the extracellular matrix that is required for a mechanically resilient tissue.

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.

Combined effects of growth factors and static mechanical compression on meniscus explant biosynthesis

OBJECTIVE

To compare the actions of fibroblast growth factor-basic (bFGF), insulin-like growth factor-I (IGF-I), platelet derived growth factor-AB (PDGF-AB), and transforming growth factor-beta 1 (TGF-beta1) on bovine meniscus tissue explants with and without static mechanical compression.

DESIGN

Meniscus tissue explants were cultured in a serum-free environment supplemented with an individual growth factor (1) over a range of concentrations for 4 days, (2) at a single concentration for 2-14 days, and (3) at a single concentration for 4 days coupled with graded levels of static compression. Explants were analyzed for accumulation of newly synthesized proteoglycan and total protein as measured by 35S-sulfate and 3H-proline incorporation, respectively.

RESULTS

Over the range of chosen concentrations, TGF-beta1 was the most potent stimulator of both protein and proteoglycan production, whereas bFGF was the least effective stimulator. Over a 2-week period for all four growth factors, the stimulation of proteoglycan production was sustained while there was no stimulation of protein production during this period. The superposition of static mechanical compression inhibited matrix production in the presence of each anabolic factor, with comparable inhibition relative to uncompressed controls for all factors.

CONCLUSIONS

The growth factors chosen exhibited an anabolic effect on the meniscus tissue explants, encouraging matrix production and deposition. The addition of static mechanical compression produced comparable relative inhibition of matrix production for each growth factor, suggesting that static compression and growth factors may modulate meniscal fibrochondrocyte biosynthesis via distinct pathways.

Ultrastructural quantification of cell death after injurious compression of bovine calf articular cartilage

OBJECTIVE

It has been suggested that chondrocyte death by apoptosis may play a role in the pathogenesis of cartilage destruction in osteoarthritis, but the results of in-vivo and in-vitro investigations have been conflicting. To investigate further the cell death in our in-vitro model for traumatic joint injury, we performed a quantitative analysis by electron microscopy (EM) of cell morphology after injurious compression. For comparison, the TUNEL assay was also performed.

DESIGN

Articular cartilage explant disks were harvested from newborn calf femoropatellar groove. The disks were subjected to injurious compression (50% strain at a strain rate of 100%/s), incubated for 3 days, and then fixed for quantitative morphological analysis.

RESULTS

By TUNEL, the cell apoptosis rate increased from 7 +/- 2% in unloaded controls to 33 +/- 6% after injury (P=0.01; N=8 animals). By EM, the apoptosis rate increased from 5 +/- 1% in unloaded controls to 62 +/- 10% in injured cartilage (P=0.02, N=5 animals). Analysis by EM also identified that of the dead cells in injured disks, 97% were apoptotic by morphology.

CONCLUSIONS

These results confirm a significant increase in cell death after injurious compression and suggest that most cell death observed here was by an apoptotic process.

Mechanisms and kinetics of glycosaminoglycan release following in vitro cartilage injury

OBJECTIVE

Acute joint injury leads to increased risk for osteoarthritis (OA). Although the mechanisms underlying this progression are unclear, early structural, metabolic, and compositional indicators of OA have been reproduced using in vitro models of cartilage injury. This study was undertaken to determine whether glycosaminoglycan (GAG) loss following in vitro cartilage injury is mediated by cellular biosynthesis, activation of enzymatic activity, or mechanical disruption of the cartilage extracellular matrix.

METHODS

Immature bovine cartilage was cultured for up to 10 days. After 3 days, groups of samples were subjected to injurious mechanical compression (single uniaxial unconfined compression to 50% thickness, strain rate 100% per second). GAG release to the medium was measured, and levels were compared with those in location-matched, uninjured controls. The effects of medium supplementation with inhibitors of biosynthesis (cycloheximide), of matrix metalloproteinase (MMP) activity (CGS 27023A or GM 6001), and of aggrecanase activity (SB 703704) on GAG release after injury were assessed.

RESULTS

GAG release from injured cartilage was highest during the first 4 hours after injury, but remained higher than that in controls during the first 24 hours postinjury, and was not affected by inhibitors of biosynthesis or degradative enzymes. GAG release during the period 24-72 hours postinjury was similar to that in uninjured controls, but the MMP inhibitor CGS 27023A reduced cumulative GAG loss from injured samples between 1 day and 7 days postinjury. Other inhibitors of enzymatic degradation or biosynthesis had no significant effect on GAG release.

CONCLUSION

Injurious compression of articular cartilage induces an initially high rate of GAG release from the tissue, which could not be inhibited, consistent with mechanical damage. However, the finding that MMP inhibition reduced GAG loss in the days following injury suggests a potential therapeutic intervention.

Tensile mechanical properties of bovine articular cartilage: variations with growth and relationships to collagen network components

One approach to repairing articular defects is to regenerate cartilage by recapitulating the changes that occur during fetal and postnatal growth into adulthood, and to thereby restore functional biomechanical properties, especially those of the normally strong superficial region. The objectives of this study were (1) to characterize and compare tensile biomechanical properties of the superficial region of articular cartilage of the patellofemoral groove (PFG) and femoral condyle (FC) from bovine animals over a range of growth stages (third-trimester fetal, 1-3 week-old calf, and adult), and (2) to determine if these properties were correlated with collagen network components. With growth from the fetus to the adult, the equilibrium and dynamic tensile moduli and strength of cartilage samples increased by an average of 391-1060%, while the strain at the failure decreased by 43%. The collagen concentration (per wet weight) increased by 98%, and the pyridinoline cross-link concentration increased by 730%, while the glycosaminoglycan concentration remained unchanged or decreased slightly. Some growth-associated changes were location-specific, with tensile moduli and strength attaining higher values in the PFG than the FC. The growth-associated variation in tensile moduli and strength were associated strongly with variation in the contents of collagen and pyridinoline cross-link, but not sulfated glycosaminoglycan. The marked changes in the tensile properties and collagen network components of articular cartilage with growth suggest that such parameters may be used to evaluate the degrees to which regenerated cartilage recapitulates normal development and growth.

The effect of link peptide on proteoglycan synthesis in equine articular cartilage

The basal rate of in vitro proteoglycan (PG) synthesis in explants of equine articular cartilage was subject to considerable variation in animals of the same age but was greater in younger than older animals. Synthesis of PGs in explant cultures was stimulated by a synthetic link peptide, identical in sequence to the N-terminus of the link protein (LP) of PG aggregates, in a similar manner to that demonstrated previously for human articular cartilage [Biochem. Soc. Trans. 25 (1997) 427; Arthritis Rheum. 41 (1998) 157]. Stimulation occurred in tissue from animals ranging from 1 to 30 years old but older animals required higher concentrations of peptide to produce a measurable response. Synthesis of PGs increased in a concentration-dependent manner and was paralleled by increases in the ability of aggrecan monomers to form aggregates with hyaluronan (HA). In addition to its effect on synthesis of PGs, link peptide also increased synthesis of both aggrecan and LP mRNA. Cartilage explant and chondrocyte cultures secreted small amounts of biologically active interleukin 1 (IL 1) and secretion of this cytokine was reduced considerably by the addition of link peptide. Reduction in the activity of this catabolic cytokine coupled with the increased synthesis of mRNA for aggrecan and link peptide may be the mechanism by which link peptide exerts its positive effect on the rate of PG synthesis in articular cartilage.

The combination of insulin-like growth factor 1 and osteogenic protein 1 promotes increased survival of and matrix synthesis by normal and osteoarthritic human articular chondrocytes

OBJECTIVE

Although growth factor therapy could be an attractive method for stimulating the repair of damaged cartilage matrix, there is evidence that with aging and/or with the development of osteoarthritis (OA), articular chondrocytes may become unresponsive to growth factor stimulation. The aim of the current study was to compare the ability of insulin-like growth factor+(IGF-1) and osteogenic protein+(OP-1), alone and in combination, to stimulate human normal and OA chondrocytes in culture.

METHODS

Chondrocytes isolated by enzymatic digestion of cartilage obtained from subjects undergoing knee replacement for OA (n = 6) or from normal ankle joints of tissue donors (n = 7) were cultured in alginate beads in serum-free medium and treated for 21 days with 100 ng/ml IGF-1, 100 ng/ml OP-1, or both. Controls were treated with vehicle alone. The cultures were evaluated for cell survival, cell number by DNA analysis, matrix production by particle exclusion assay, and level of accumulated proteoglycan by dimethylmethylene blue assay.

RESULTS

After 21 days in serum-free alginate culture, survival of cells from OA cartilage was 65 +/- 2% (mean +/- SEM), while survival of cells from normal cartilage was significantly greater (82 +/- 3%). Treatment with either IGF-1 or OP-1 alone minimally improved survival, while the combination IGF +OP significantly improved survival, to 87 +/- 2% for OA cells and 95+/-1% for normal cells. Cell proliferation was noted only in the IGF+OP group; this was significant for both normal and OA cells ( approximately 2-fold increase in DNA levels). Matrix production, assessed by particle exclusion and by proteoglycan accumulation, was greatest in the cells treated with IGF + OP in both normal and OA cultures. When proteoglycan levels were corrected for cell numbers (mg proteoglycan/ng DNA), a significant increase over control was noted with OP-1 alone and IGF IGF-1 alone, in both normal and OA cultures, with the greatest levels in the combination group (3-fold increase over control).

CONCLUSION

OP-1 was more potent than IGF-1 in stimulating proteoglycan production in both normal and OA cells. However, the best results were obtained with the combination, suggesting that combined therapy with IGF-1 and OP-1 may be an effective strategy for treating OA cartilage damage.

High molecular weight hyaluronan promotes repair of IL-1 beta-damaged cartilage explants from both young and old bovines

OBJECTIVE

The addition of exogenous high molecular weight hyaluronic acid (HA) reverses cartilage damage caused by fibronectin fragments (Fn-fs) added to explant cultures of bovine and human cartilage and by Fn-fs in an experimental in vivo model of rabbit knee joint damage. Our objective was to test whether HA was also effective in an IL-1 damage model and whether this repair was stable and occurred in older bovine cartilage.

DESIGN

Bovine cartilage explants from 18-month-old or 6-year-old bovines in 10% serum/Dulbecco's modified Eagle's medium were exposed to Fn-f or to IL-1 and the ability of 1mg/ml HA of 800 kDa to block damage or promote restoration of proteoglycan (PG) after the damage was measured. The damage phase as well as the exposure to HA were varied.

RESULTS

Exposure of exogenous HA decreased Fn-f-mediated damage, but did not decrease IL-1 beta-induced cartilage damage. If explants from 18-month-old bovines were damaged by a 7-day exposure to Fn-f or IL-1 beta and then exposed for 7 days to HA, PG was restored. This reparative activity persisted up to 4 weeks after the removal of HA from the culture medium. The restoration of PG did not occur in 0.1% serum-free cultures, was less when the exposure to the Fn-f was doubled and failed when exposure to IL-1 beta was doubled. In explants from 6-year-old bovines damaged with IL-1 beta for 7 days, HA fully restored PG content to normal levels.

CONCLUSIONS

The reparative activities of HA occur not only in a Fn-f damage model, but also in an IL-1 damage model and occur with older bovine cartilage.

Development-associated differences in integrative cartilage repair: roles of biosynthesis and matrix

A recurring problem in tissue transplantation therapies for articular cartilage defects is the lack of integration between the implant and the host cartilage. Previous studies have shown that in vitro integration between explants of calf cartilage is markedly higher than that between fetal cartilage, despite similarly high levels of deposition of newly synthesized collagen. The aim of this study was to determine if cellular biosynthesis and extracellular matrix each contribute to these development-associated differences in integrative repair in vitro. The approach taken was to examine integration between specific combinations of cartilage explants that were apposed for two weeks. The cartilage matrix showed different propensities for repair, as integration of calf live cartilage to calf devitalized cartilage was greater than that of calf live cartilage to fetal devitalized cartilage. An inhibiting factor appeared to be present in fetal cartilage matrix since guanidine treatment of fetal devitalized cartilage was able to enhance its integration. The difference between integration to living cartilage and integration to devitalized cartilage, for calf and fetal tissue, indicated that the biosynthetic contribution to integration by calf cartilage was greater than the biosynthetic contribution by fetal cartilage. Thus, the increasing level of integration between fetal and fetal cartilage, fetal and calf cartilage, and calf and calf cartilage appeared to reflect both biosynthetic and matrix differences. Therapeutic strategies to enhance integration to cartilage may thus target both the extracellular components and the cellular biosynthetic activities of implants and host cartilage.

Integrative articular cartilage repair: dependence on developmental stage and collagen metabolism

OBJECTIVES

The objectives of this research were to determine whether the integrative repair of bovine cartilage explants was dependent on developmental stage, and whether observed differences in integration with developmental stage were related to deposition of newly synthesized collagen and lysyl oxidase-mediated collagen cross-linking.

METHODS

Pairs of fetal, newborn calf, and adult bovine cartilage blocks were cultured in partial apposition for 2 weeks in medium supplemented with serum, ascorbate, and [3H]proline. Following culture, mechanical integration between apposed cartilage blocks was assessed by measuring adhesive strength in a single-lap shear configuration. Formation and stabilization of newly synthesized protein and collagen was investigated by determination of [3H]proline and [3H]hydroxyproline in tissue digests and guanidine extracts.

RESULTS

Calf cartilage exhibited a relatively high integrative repair phenotype, achieving an adhesive strength that was three--four-fold that of adult or fetal specimens. The low and high integrative repair phenotypes appeared related in part to different levels of collagen biosynthesis, which was approximately four--five-fold higher in calf cartilage samples than in the adult. However, fetal cartilage also exhibited a high level of biosynthesis. The different integrative repair phenotypes were not associated with marked differences in the kinetics of chemical stabilization of newly synthesized collagen, as the proportion of incorporated [3H]proline and newly-formed [3H]hydroxyproline that was resistant to extraction by 4M guanidine-HCl following culture was similar for cartilage from all developmental stages. Integration of calf cartilage appeared to depend on lysyl oxidase-mediated collagen cross-link formation, since inclusion of beta-aminopropionitrile (BAPN) in the culture medium completely eliminated development of adhesive strength. BAPN treatment also increased the percentage of newly synthesized protein in the guanidine extracts from 10% to 36% of the total, and that of newly synthesized collagen from 2% to 20%, while having only slight inhibitory effects on overall protein and collagen biosynthesis.

CONCLUSION

The finding that cartilage exhibits enhanced integrative repair at a certain developmental stage suggests that it may ultimately be possible to enhance repair when needed in clinical situations.

Compressive properties and function-composition relationships of developing bovine articular cartilage

The composition of cartilage is known to change during fetal and postnatal development. The objectives of this study were to characterize the compressive biomechanical properties of the 1 mm thick articular layer of cartilage of the distal femur from third-trimester bovine fetuses, from 1 to 3 week old bovine calf and from young adult bovine knees, and to correlate these properties with tissue components. The confined compression modulus increased 180% from the fetus to the calf and adult. The hydraulic permeability at 45% offset compression (relative to the free-swelling thickness) decreased by 70% from fetus to adult. These development-associated changes in biomechanical properties were primarily associated with a marked (approximately 2-3-fold) increase during development in collagen content and no detectable change in glycosaminoglycan (GAG) content. A role for collagen in the compressive properties of cartilage and the gradual increase in collagen during development suggest that collagen metabolism is critical for cartilage tissue engineering and repair therapies.

Aging and the etiopathogenesis and treatment of osteoarthritis

Knee OA in the older adult is often the result of multiple factors, some of which are intrinsic to the joint tissues and related to aging and others, such as obesity, which are extrinsic and influence the rate of development and severity of the disease. Obesity and quadricep weakness are factors that may be modifiable. The focus in managing patients with knee OA should not only be centered on the improvement of pain but also improvement in function. A combination of simple analgesics and nonpharmacologic measures, including exercises to strengthen the knee extensors, should be employed. Patients also can benefit from aerobic exercises, such as a walking or swimming program and strategies for joint protection. When medications are needed, the least toxic drugs possible should be used given the increased susceptibility of older adults to unwanted side effects.

Articular chondrocytes from aging rats respond poorly to insulin-like growth factor-1: an altered signaling pathway

This study investigates the effect of insulin-like growth factor-1 (IGF-1) and phorbol 12-myrystate 13-acetate (PMA) on 3H-thymidine, 35SO(4) and 3H -glycine incorporations, adenosine 3':5'-cyclic monophosphate (cAMP) production and protein kinase C (PKC) activation in cultured rat articular chondrocyte monolayers (RACM) derived from animals of different ages. It was found that IGF-1 stimulates all these cellular functions in cultures derived from all age groups in a concentration dependent manner, although the cells from 14-month old animals responded poorly. IGF-1 also induces in cells from 1-month old rats an increase in the expression of mRNAs specific for aggrecan and type II collagen molecules as shown with RT-PCR. These effects are mediated via IGF-1 interaction with specific receptors because the monoclonal antibody against the receptor protein suppresses more than 60% of the ligand-induced DNA synthesis. PMA, a direct PKC activator, potentiated IGF-1-induced effects in all cells but much more strongly in cells from young than in cells from 14-month old animals. The age-related failure of RACM to respond adequately to IGF-1 was correlated with a decrease in IGF-1-induced cAMP production, and IGF-1-induced and PMA-induced PKC activations. These results show that IGF-1 regulates the synthesis of DNA, proteoglycans (PG) and collagen II at the level of transcription and suggest that the reduced response of cell monolayers derived from 14-month old rats to IGF-1 is probably due to a failure of old cells to adequately transduce IGF-1 receptor-generated downstream signaling.

The macromolecular characteristics of cartilage proteoglycans do not change when synthesis is up-regulated by link protein peptide

Previous studies have shown that a synthetic, unglycosylated analogue of the N-terminal peptide from link protein can function as a growth factor and up-regulate proteoglycan biosynthesis in explant cultures of normal human articular cartilage from a wide age range of subjects (McKenna et al., Arthritis Rheum. 41 (1998) 157-162). The present work further shows that link peptide increased proteoglycan synthesis by cartilage cultured in both the presence and absence of serum, suggesting that the mechanism of up-regulation may be different from that of insulin-like growth factors. The proteoglycans synthesised during stimulation with link peptide were of normal hydrodynamic size and the ratio of core protein to glycosaminoglycan side chains and the proportions of the large proteoglycan aggrecan to the small proteoglycans, decorin and biglycan, remained constant. Aggrecan molecules were equally capable of forming aggregates as those from control tissues and the relative proportions of decorin and biglycan were unchanged showing that both were co-ordinately up-regulated. These results confirmed that this novel peptide is a potent stimulator of proteoglycan synthesis by articular cartilage and showed that the newly synthesised proteoglycans were of normal composition.

Enhanced repair of extensive articular defects by insulin-like growth factor-I-laden fibrin composites

Stem cells indigenous to the cancellous spaces of the bone bed in an acute injury provide an important source of pluripotent cells for cartilage repair. Insulin-like growth factor-I facilitates chondrogenesis of bone marrow-derived stem cells in long-term culture and may enhance chondrogenesis in healing cartilage lesions in vivo. This study examined the impact of insulin-like growth factor-I, gradually released from fibrin clots polymerized in situ, on the recruitable stem-cell pool in a full-thickness critical cartilage defect model. Twelve full-thickness 15-mm cartilage lesions in the femoropatellar articulations of six young mature horses were repaired by an injection of autogenous fibrin containing 25 microg of human recombinant insulin-like growth factor-I or, in control joints, fibrin without the growth factor. All horses were killed at 6 months, and cartilage repair tissue and surrounding cartilage were assessed by histology, histochemistry, types I and II collagen immunohistochemistry, types I and II collagen in situ hybridization, and matrix biochemical determinations. White tissue filled grafted and control lesions, with the growth factor-treated defects being more completely filled and securely attached to the subchondral bone. A moderately improved chondrocyte population, more columnar cellular organization, and better attachment to the underlying bone were evident on histological evaluation of growth factor-treated defects. Type-II procollagen mRNA was abundantly present in the deeper half of the treated sections compared with moderate message expression in control tissues. Immunolocalization of type-II collagen showed a preponderance of the collagen in growth factor-treated lesions, confirming translation of type-II message to protein. Composite histologic healing scores for treated defects were significantly improved over those for control defects. DNA content in the cartilage defects was similar in treated and control joints. Matrix proteoglycan content was similar in treated and control defects and lower in the defects than in the intact surrounding and remote cartilage of the treated and control joints. The proportion of type-II collagen significantly increased in growth factor-treated tissues. Fibrin polymers laden with insulin-like growth factor-I improved the histologic appearance and the proportion of type-II collagen in healing, full-thickness cartilage lesions. However, none of the biochemical or morphologic features were consistent with those of normal articular cartilage.

Gas exchange is essential for bioreactor cultivation of tissue engineered cartilage

Tissue engineered cartilage can be grown in vitro if the necessary physical and biochemical factors are present in the tissue culture environment. Cell metabolism and tissue composition were studied for engineered cartilage cultured for 5 weeks using bovine articular chondrocytes, polymer scaffolds (5 mm diameter x 2 mm thick fibrous discs), and rotating bioreactors. Medium pH and concentrations of oxygen, carbon dioxide, glucose, lactate, ammonia, and glycosoaminoglycan (GAG) were varied by altering the exchange rates of gas and medium in the bioreactors. Cell-polymer constructs were assessed with respect to histomorphology, biochemical composition and metabolic activity. Low oxygen tension ( approximately 40 mmHg) and low pH ( approximately 6.7) were associated with anaerobic cell metabolism (yield of lactate on glucose, YL/G, of 2.2 mol/mol) while higher oxygen tension ( approximately 80 mmHg) and higher pH ( approximately 7.0) were associated with more aerobic cell metabolism (YL/G of 1.65-1.79 mol/mol). Under conditions of infrequent medium replacement (50% once per week), cells utilized more economical pathways such that glucose consumption and lactate production both decreased, cell metabolism remained relatively aerobic (YL/G of 1.67 mol/mol) and the resulting constructs were cartilaginous. More aerobic conditions generally resulted in larger constructs containing higher amounts of cartilaginous tissue components, while anaerobic conditions suppressed chondrogenesis in 3D tissue constructs.

Differentiated cellular function in fetal chondrocytes cultured with insulin-like growth factor-I and transforming growth factor-beta

This study examined fetal chondrocyte proliferation and function following exposure to transforming growth factor-beta and insulin-like growth factor-I. Fetal equine articular chondrocytes of the early third-trimester were isolated and cultured in monolayer conditions, then exposed to 0, 1, 5, or 10 ng/ml transforming growth factor-beta or 0, 10, 50, or 100 ng/ml insulin-like growth factor-I for 48 hours. Proliferative responses were assessed by cell counts and [3H]thymidine uptake into precipitable DNA. Differentiated chondrocyte metabolic activity was determined by sulfated glycosaminoglycan quantitation, 35[SO4] incorporation into precipitable glycosaminoglycan, and proteoglycan molecular sizing by CL-2B column chromatography. Morphological changes seen on phase-contrast microscopy included a larger proportion of rounded cells in monolayer cultures supplemented with insulin-like growth factor-I and cytotoxic changes in cells treated with transforming growth factor-beta. Both insulin-like growth factor-I and transforming growth factor-beta resulted in significant elevations of [3H]thymidine uptake; however, cell numbers did not rise sufficiently over the 48-hour culture period to reach significant levels. Maximum mitogenic responses were evident at 50 and 100 ng/ml insulin-like growth factor-I and 5 ng/ml transforming growth factor-beta. The production of proteoglycan was also enhanced (435%) by exposure to 50 ng/ml insulin-like growth factor-I, and an increased proportion of larger proteoglycan monomer species was evident in cultures treated with 50 and 100 ng/ml insulin-like growth factor-I. A similar dose-response was also evident in cultures treated with transforming growth factor-beta (maximal 164% increase with 5 ng/ml), although the presence of serum in the culture medium altered the pattern of enhanced proteoglycan synthesis to favor the lower concentration of 1 ng/ml (191%). Additionally, larger proteoglycan molecules were synthesized in response to high concentrations of transforming growth factor-beta in serum-free cultures. Significant biochemical changes resulted from the addition of transforming growth factor-beta to fetal chondrocyte cultures; however, monolayer cultures that were treated with transforming growth factor-beta and supplemented with serum began to develop cellular toxicity, including nuclear pyknosis and cytoplasmic fragmentation. Degenerative cellular changes were not evident in cultures treated with insulin-like growth factor-I, and significant differentiated metabolic activity resulted from the presence of insulin-like growth factor-I in the culture medium. These data suggest that the responses of fetal chondrocytes to insulin-like growth factor-I and transforming growth factor-beta were enhanced compared with the responses of chondrocytes derived from postnatal animals and that these metabolically active cells can be primed by endogenous or exogenous growth factors to provide enhanced articular function and repair.

Age-related changes in effects of insulin-like growth factor I on human osteoblast-like cells

The role of insulin-like growth factor I (IGF-I) in extracellular matrix metabolism was studied in both proliferating and confluent human osteoblast-like cultures derived from donors of different ages. In proliferating cultures, recombinant human (rh)IGF-I was found to increase the incorporation of [3H]thymidine in a dose- and age-dependent manner. To study cell proliferation dynamically, continuous growth curves with and without rhIGF-I were modelled by a modified logistic function. Increasing doses of rhIGF-I decreased the lag time and maximal growth rates, whereas plateau values decreased only at the highest dose (100 ng/ml). In post-proliferative cell strains, rhIGF-I (0.1-100 ng/ml) increased levels of type I collagen, biglycan and decorin, and to a smaller extent fibronectin and thrombospondin, whereas it decreased the levels of hyaluronan and a versican-like proteoglycan when protein and proteoglycan metabolism were followed by steady-state radiolabelling with [3H]proline, [3H]glucosamine or [35S]sulphate. These responses to rhIGF-I were found to be age-dependent, with osteoblast-like cells derived from younger patients being more responsive to rhIGF-I. When extracellular matrix turnover was analysed by pulse-chase experiments, rhIGF-I had no effect. The steady-state levels of collagen, decorin, hyaluronan and a versican-like proteoglycan for bone cells treated with rhIGF-I on day 7 in culture were equivalent to levels of these matrix components in untreated osteoblasts grown for 14 days. These results are consistent with rhIGF-I's altering cellular proliferative capacity and matrix synthesis, causing a change in the osteoblast differentiated state.

Articular cartilage destruction in experimental inflammatory arthritis: insulin-like growth factor-1 regulation of proteoglycan metabolism in chondrocytes

Rheumatoid arthritis, a disease of unknown aetiology, is characterized by joint inflammation and, in its later stages, cartilage destruction. Inflammatory mediators may exert not only suppression of matrix synthesis but also cartilage degradation, which eventually leads to severe cartilage depletion. Systemically and locally produced growth factors and hormones regulate cartilage metabolism. Alterations in levels of these factors or in their activity can influence the pathogenesis of articular cartilage destruction in arthritic joints. The main topic of the present review is the role of the anabolic factor insulin-like growth factor-1 in the regulation of chondrocyte metabolic functions in normal and in diseased cartilage. This is the most important growth factor that balances chondrocytes proteoglycan synthesis and catabolism to maintain a functional cartilage matrix. A brief overview of how chondrocytes keep the cartilage matrix intact, and how catabolic and anabolic factors are thought to be involved in pathological cartilage destruction precedes the review of the role of this growth factor in proteoglycan metabolism in cartilage.

Differential effects of serum, insulin-like growth factor-I, and fibroblast growth factor-2 on the maintenance of cartilage physical properties during long-term culture

The effects of fetal bovine serum, insulin-like growth factor-I, and fibroblast growth factor-2 on the regulation of the functional physical properties of adult bovine cartilage explants during an incubation period of 18-20 days was determined, and the relationship between the measured functional properties of the cartilage and the tissue composition was assessed. Cartilage disks were tested in the uniaxial radially confined configuration by the application of low amplitude oscillatory displacement and measurement of the resultant load and streaming potential. For the control cartilage terminated just after explant, the modulus was 0.39 +/- 0.28 MPa, the open circuit hydraulic permeability was 2.0 +/- 1.0 x 10(-15) m2/(Pa.sec), and the electrokinetic (streaming potential) coefficient was -2.3 +/- 0.6 mV/MPa. Incubation of cartilage in medium supplemented with serum or insulin-like growth factor-I resulted in maintenance of the modulus and electrokinetic coefficient, whereas incubation in basal medium or medium supplemented with fibroblast growth factor-2 led to a marked decrease from control values in the modulus and the amplitude of the electrokinetic coefficient. All of the culture conditions examined resulted in an increase in permeability that was not statistically significant. The variation in the electromechanical properties of all the cartilage samples tested was related to the density of tissue proteoglycan and collagen (hydroxyproline). The modulus was correlated with both the density of tissue proteoglycan (+0.014 MPa/[mg/ml]) and the density of tissue hydroxyproline (+0.008 MPa/[mg/ml]). The electrokinetic coefficient was also correlated with the density of proteoglycan (-0.080 [mV/MPa]/[mg/ml]) and the density of hydroxyproline (+0.064 [mV/MPa]/[mg/ml]). These data indicate that the regulation of chondrocyte matrix metabolism by growth factors can significantly affect the physical properties and function of cartilage.

Integrative repair of articular cartilage in vitro: adhesive strength of the interface region

The objective of this study was to quantify the strength of the repair tissue that forms at the interface between pairs of cartilage explants maintained in apposition in an in vitro culture system. Articular cartilage explants were harvested from calves and from adult bovine animals, dissected into uniform blocks, and incubated in pairs within a chamber that maintained a 4 x 5 mm area of tissue overlap. Following 1-3 weeks of incubation, integrative repair was assessed by testing samples in a tensile single-lap configuration to estimate adhesive strength. After incubation in medium containing 20% fetal bovine serum, the adhesive strength between pairs of calf cartilage blocks and pairs of adult bovine cartilage blocks increased at a rate of 7.0 and 10.5 kPa/week, respectively. This repair process appeared to be dependent on viable cells, since lyophilization of adult bovine cartilage before incubation completely inhibited the development of an interface with a measurable adhesive strength. The repair process was dependent on serum components in the medium. Incubation of sample pairs for 3 weeks in medium supplemented with 20% fetal bovine serum resulted in a relatively high proteoglycan content as well as a relatively high adhesive strength (34 kPa), whereas incubation in basal medium with or without 0.1% bovine serum albumin resulted in a 54-70% lower proteoglycan content and a 65-88% lower adhesive strength. Samples incubated for 3 weeks with serum also had a 20% higher DNA content than samples maintained in basal medium. Histological analysis indicated some cell division at the free surfaces of the explant and also occasional cells within the interface region between explants.

Proteoglycan metabolism of equine articular cartilage and its modulation by insulin-like growth factors

The effect of human recombinant insulin-like growth factor 1 (rhIGF-1) on proteoglycan (PG) metabolism of full thickness equine articular cartilage explants was investigated. PG synthesis was stimulated at all ages, but higher concentrations of rhIGF-1 were required for maximal stimulation of adult cartilage. There were no changes in the hydrodynamic size, electrophoretic heterogeneity or composition of proteoglycans isolated from rhIGF-1-stimulated cartilage. rhIGF-1 reduced the rate of turnover of both newly synthesized and endogenous proteoglycans in all ages of cartilage investigated. The structure of proteoglycan fragments retained within the matrix and those released into the culture medium was unaffected by IGF-1 stimulation, suggesting that this peptide is a key regulator of the proteoglycan composition of equine articular cartilage extracellular matrix.

Serum levels of insulin-like growth factor in subjects with osteoarthritis of the knee. Data from the Baltimore Longitudinal Study of Aging

OBJECTIVE

To examine the relationship between serum levels of insulin-like growth factor 1 (IGF-1) and osteoarthritis (OA) of the knee.

METHODS

Serum IGF-1 levels were compared in 162 male and 101 female subjects age > or = 20 stratified by presence of radiographic changes of OA of the knee.

RESULTS

Mean serum IGF-1 levels were significantly lower in subjects with knee OA; however, after adjustment for age-related changes in IGF-1 levels, these differences were no longer significant.

CONCLUSION

These data fail to support the hypothesis that serum IGF-1 levels are reduced in subjects with OA of the knee independent of the known age-related changes in these levels.

The recovery of articular cartilage in explant culture from interleukin-1 alpha: effects on proteoglycan synthesis and degradation

The cytokine interleukin-1 (IL-1) strongly inhibits proteoglycan biosynthesis and increases extracellular proteoglycan degradation in cartilage. In this study we investigated how quickly proteoglycan turnover recovered after IL-1 treatment. Porcine articular cartilage in explant culture was incubated with recombinant human (rh) IL-1 alpha, rh insulin-like growth factor (IGF)-1 or rh-transforming growth factor (TGF)-beta 1 for 3 days. RhIL-1 alpha strongly inhibited synthesis and promoted matrix degradation over 3 days, whereas rhIGF-1 stimulated proteoglycan synthesis, and rhTGF-beta 1 had no significant effect on synthesis. In the absence of serum, the recovery of proteoglycan synthesis after IL-1 treatment (10 ng/ml) for 3 days was extremely slow, and it remained 30% inhibited even after a further 8 days of recovery. Matrix degradation of proteoglycan assessed from the rate of release of glycosaminoglycan into the culture medium recovered more quickly and was greatly reduced within 3 days. Shorter exposure of explants to IL-1 alpha (8 h) resulted in less inhibition of synthesis, but it did not result in a more rapid rate of recovery following its removal. Treatment of cartilage with IGF-1 (20 ng/ml) or TGF-beta (10 ng/ml) during the recovery period increased the rate of recovery of both synthesis and degradation. Synthesis recovered to control rates within 6 days and degradation within 2 days. TGF-beta (10 ng/ml) was slightly more potent than IGF-1. Fetal calf serum (10% v/v) also promoted recovery in a similar way to the growth factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenotypic stability of bovine articular chondrocytes after long-term culture in alginate beads

Articular chondrocytes embedded in alginate gel produce de novo a matrix rich in collagens and proteoglycans. A major advantage of this culture system is that the cells can be recovered by chelating the calcium, which otherwise maintains the alginate in its gel state. Chondrocytes thus released are surrounded by tightly bound cell-associated matrix, which seems to correspond to the pericellular and territorial matrices identified in cartilage by electron microscopy. The cells and their associated matrix can be easily separated by mild centrifugation from more soluble matrix components derived principally from the ‘interterritorial’ matrix. This new cell culture system thus makes it possible to study the assembly and turnover of molecules present in two distinct matrix pools. Importantly, a significant proportion of the aggrecan molecules in each of these two pools can be extracted using a non-denaturing solvent, thereby making possible studies of the metabolism and turnover of native proteoglycan aggregates. We show in this report that chondrocytes isolated from the full depth of adult bovine articular cartilage and maintained for 8 months in alginate gel are still metabolically active and continue to synthesize cartilage-specific type II collagen and aggrecan. The cells did not synthesize large amounts of type I collagen or of the small nonaggregating proteoglycans as usually occurs when chondrocytes lose their phenotypic stability. After this extended period of time in culture, the cells were present as two populations exhibiting differences in size, shape and amount of extracellular matrix surrounding them. The first population was found only near the surface of the bead: these cells were flattened and surrounded by a matrix sparse in proteoglycans and collagen fibrils. The second population was found throughout the remaining depth of the bead: the cells were more round and almost always surrounded by a basket-like meshwork consisting of densely packed fibrils running tangential to the surface.

Differential rates of aggrecan synthesis and breakdown in different zones of the bovine growth plate

This study examines the basic metabolic events of aggrecan synthesis and breakdown in the growth plate at different depths and at different stages of development. Growth plate was harvested from the distal tibia of fetal and calf tissue and maintained as explants in serum-free-conditions. The tissue was sectioned into three equal depths (resting/proliferative zone, upper hypertrophic zone, and lower hypertrophic zone) and (a) cultured for three days with daily media change for studies of proteoglycan breakdown rates, or (b) incubated with [35S]-sulfate to determine relative rates of proteoglycan synthesis. Rates of both aggrecan synthesis and turnover were highest in the resting/proliferative zone compared to the upper or lower hypertrophic zones, and was greater in the calf compared to the fetal tissue. In situ hybridization studies showed that aggrecan gene expression in the cells of the resting/proliferative zone and the upper hypertrophic zones were similar, and was reduced in the deepest cells of the lower hypertrophic zone, adjacent to the zone of calcification. Proteoglycan structure was characterized by associative and dissociative Sepharose CL2B chromatography. These results showed that approximately 90% of the newly synthesized proteoglycan, and the total proteoglycan population, was able to aggregate and that the monomers were relatively large. The proteoglycan released into the media had a reduced ability to aggregate and the monomers were of a more variable size. These data support the hypothesis that the matrix proteoglycan content is controlled both by the rate of synthesis and breakdown, but in the lower regions the rate of synthesis may play a more dominant role. The higher metabolic activity of aggrecan in the calf than fetal growth plate may be a result of environmental stimuli (i.e., soluble mediators, loading) during different stages of development.

Maintenance of the synthesis of large proteoglycans in anatomically intact murine articular cartilage by steroids and insulin-like growth factor I

OBJECTIVES

The exact regulation of the synthesis of cartilage specific molecules, such as collagen type II and aggrecan, by articular chondrocytes is unknown, but growth factors and hormones probably play an important part. The effects of glucocorticosteroids (prednisolone and triamcinolone), in combination with insulin-like growth factor I (IGF-I), on the synthesis and hydrodynamic volume of proteoglycans from murine patellar cartilage were investigated.

METHODS

The in vitro effect of IGF-I and steroids on proteoglycan synthesis in murine patellar cartilage was evaluated by [35S]sulphate incorporation in combination with dissociative gel chromatography using a Sephacryl S-1000 column. The impact of in vivo prednisolone (0-5 mg/kg) on proteoglycan synthesis in murine patellar cartilage was analysed by [35S]sulphate incorporation immediately after dissection from the knee joint.

RESULTS

Prednisolone stimulated proteoglycan synthesis in murine patellar cartilage from normal knees and in cartilage from knees injected with papain in vitro in the absence and presence of IGF-I. Moreover, oral administration of prednisolone for seven days to C57Bl10 mice resulted in enhanced proteoglycan synthesis in patellar cartilage. The incubation of patellar cartilage for 48 hours without serum or growth factors led to the synthesis of proteoglycans with a smaller hydrodynamic volume than those synthesised immediately after dissection of the patellae. This could either be circumvented by the addition of IGF-I or by the addition of glucocorticosteroids (prednisolone or triamcinolone) to the culture medium.

CONCLUSIONS

These results show that in a dose range of 0.0003-0.3 mmol/l, glucocorticosteroids, like IGF-I, stimulate proteoglycan synthesis and maintain the synthesis of hydrodynamically large proteoglycans by chondrocytes from murine articular cartilage. This indicates that glucocorticosteroids might play a part in the preservation of matrix integrity in articular cartilage.