Correlation of the size of type II transforming growth factor beta (TGF-beta) receptor with TGF-beta responses of isolated bovine articular chondrocytes

TGFβ/BMP Signaling Pathway in Cartilage Homeostasis

Cartilage homeostasis is governed by articular chondrocytes via their ability to modulate extracellular matrix production and degradation. In turn, chondrocyte activity is regulated by growth factors such as those of the transforming growth factor β (TGFβ) family. Members of this family include the TGFβs, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs). Signaling by this protein family uniquely activates SMAD-dependent signaling and transcription but also activates SMAD-independent signaling via MAPKs such as ERK and TAK1. This review will address the pivotal role of the TGFβ family in cartilage biology by listing several TGFβ family members and describing their signaling and importance for cartilage maintenance. In addition, it is discussed how (pathological) processes such as aging, mechanical stress, and inflammation contribute to altered TGFβ family signaling, leading to disturbed cartilage metabolism and disease.

TGF-β1 upregulates the expression of hyaluronan synthase 2 and hyaluronan synthesis in culture models of equine articular chondrocytes

We investigated the effect of transforming growth factor beta 1 (TGF-β1) on equine hyaluronan synthase 2 (HAS2) gene expression and hyaluronan (HA) synthesis in culture models of articular chondrocytes. Equine chondrocytes were treated with TGF-β1 at different concentrations and times in monolayer cultures. In three-dimensional cultures, chondrocyte-seeded gelatin scaffolds were cultured in chondrogenic media containing 10 ng/mL of TGF-β1. The amounts of HA in conditioned media and in scaffolds were determined by enzyme-linked immunosorbent assays. HAS2 mRNA expression was analyzed by semi-quantitative reverse transcription polymerase chain reaction. The uronic acid content and DNA content of the scaffolds were measured by using colorimetric and Hoechst 33258 assays, respectively. Cell proliferation was evaluated by using the alamarBlue assay. Scanning electron microscopy (SEM), histology, and immunohistochemistry were used for microscopic analysis of the samples. The upregulation of HAS2 mRNA levels by TGF-β1 stimulation was dose and time dependent. TGF-β1 was shown to enhance HA and uronic acid content in the scaffolds. Cell proliferation and DNA content were significantly lower in TGF-β1 treatments. SEM and histological results revealed the formation of a cartilaginous-like extracellular matrix in the TGF-β1-treated scaffolds. Together, our results suggest that TGF-β1 has a stimulatory effect on equine chondrocytes, enhancing HA synthesis and promoting cartilage matrix generation.

Expression of TGF-β Signaling Regulator RBPMS (RNA-Binding Protein With Multiple Splicing) Is Regulated by IL-1β and TGF-β Superfamily Members, and Decreased in Aged and Osteoarthritic Cartilage

OBJECTIVE

RNA-binding protein with multiple splicing (RBPMS) has been shown to physically interact with Smads and enhance transforming growth factor-β (TGF-β)-mediated Smad2/3 transcriptional activity in mammalian cells. Objective of this study was to examine whether expression of RBPMS is regulated by interleukin-1β (IL)-1β and TGF-β superfamily growth factors and whether expression of RBPMS is altered during aging and experimental osteoarthritis.

METHODS

Expression of RBPMS protein was investigated in chondrocyte cell lines of murine (H4) and human (G6) origin using Western blot analysis. Regulation of RBPMS expression in H4 chondrocytes at mRNA level was done by reverse transcriptase-quantitative polymerase chain reaction. Furthermore, characterization of Smad signaling pathways regulating RBPMS expression was performed by blocking studies using small molecule inhibitors or by transfection studies with adenoviral vector constructs (constitutive-active ALK1 and constitutive-active ALK5). Expression of RBPMS in cartilage of different age groups of C57BL/6N mice (6 months and 20 months) and in a surgically induced osteoarthritis (OA) mouse model was analyzed using immunohistochemistry.

RESULTS

RBPMS was shown to be expressed in chondrocytes and cartilage of murine, human, and bovine origin. TGF-β inhibited RBPMS expression while BMP2 and IL-1β increased its expression. TGF-β-induced inhibition was blocked by ALK5 inhibitor. Overexpression of ca-ALK1 stimulated RBPMS expression. Moreover, RBPMS expression was found to be reduced with ageing and in OA pathogenesis.

CONCLUSIONS

Expression of RBPMS in chondrocytes is regulated by TGF-β superfamily members and IL-1β, indicating a counter-regulatory mechanism. Expression of RBPMS, in cartilage and its reduction during ageing and OA might suggest its potential role in the maintenance of normal articular cartilage.

Increase in ALK1/ALK5 ratio as a cause for elevated MMP-13 expression in osteoarthritis in humans and mice

During osteoarthritis (OA) chondrocytes show deviant behavior resembling terminal differentiation of growth-plate chondrocytes, characterized by elevated MMP-13 expression. The latter is also a hallmark for OA. TGF-beta is generally thought to be a protective factor for cartilage, but it has also displayed deleterious effects in some studies. Recently, it was shown that besides signaling via the ALK5 (activin-like kinase 5) receptor, TGF-beta can also signal via ALK1, thereby activating Smad1/5/8 instead of Smad2/3. The Smad1/5/8 route can induce chondrocyte terminal differentiation. Murine chondrocytes stimulated with TGF-beta activated the ALK5 receptor/Smad2/3 route as well as the ALK1/Smad1/5/8 route. In cartilage of mouse models for aging and OA, ALK5 expression decreased much more than ALK1. Thus, the ALK1/ALK5 ratio increased, which was associated with changes in the respective downstream markers: an increased Id-1 (inhibitor of DNA binding-1)/PAI-1 (plasminogen activator inhibitor-1) ratio. Transfection of chondrocytes with adenovirus overexpressing constitutive active ALK1 increased MMP-13 expression, while small interfering RNA against ALK1 decreased MMP-13 expression to nondetectable levels. Adenovirus overexpressing constitutive active ALK5 transfection increased aggrecan expression, whereas small interfering RNA against ALK5 resulted in increased MMP-13 expression. Moreover, in human OA cartilage ALK1 was highly correlated with MMP-13 expression, whereas ALK5 correlated with aggrecan and collagen type II expression, important for healthy cartilage. Collectively, we show an age-related shift in ALK1/ALK5 ratio in murine cartilage and a strong correlation between ALK1 and MMP-13 expression in human cartilage. A change in balance between ALK5 and ALK1 receptors in chondrocytes caused changes in MMP-13 expression, thereby causing an OA-like phenotype. Our data suggest that dominant ALK1 signaling results in deviant chondrocyte behavior, thereby contributing to age-related cartilage destruction and OA.

Creating a spectrum of fibrocartilages through different cell sources and biochemical stimuli

In this study a scaffoldless approach was employed with two different cell sources and biochemical stimuli to engineer a spectrum of fibrocartilages representative of the different regions of the knee meniscus. Constructs composed of 100% fibrochondrocytes (FC) or a 50:50 co-culture of fibrochondrocytes and chondrocytes (CC) were cultured in 10% fetal bovine serum medium or serum-free "chondrogenic" medium, each +/-10 ng/mL TGF-beta1 (+T). Constructs from these two cell groups and four culture conditions were cultured for 6 weeks. By varying the cell type and presence of the growth factor, GAG per dry weight of the constructs spanned that of native tissue, ranging 16-45% and 1-7% in the CC and FC groups, respectively. Collagen density was most dependent on cell type and was significantly lower than tissue values. The collagen I/II ratio could be manipulated by cell type and serum presence to span the native range, from 3.5 in the serum-free CC group to over 1,000 in the FC groups treated with serum-containing medium. Using the CC cell group in the presence of serum-free medium dramatically increased the compressive stiffness to 128 +/- 34 kPa, similar to native tissue. Similarly, serum-free medium or TGF-beta1 treatment enhanced the tensile modulus by an order of magnitude, up to 3,000 kPa. Using two cell sources and manipulating biochemical stimuli, a range of fibrocartilaginous neotissues was engineered. Fibrocartilages such as the knee meniscus are characterized by heterogeneity in matrix and functional properties, and this work demonstrates a strategy for recreating these heterogeneous tissues.

Influence of species and anatomical location on chondrocyte expansion

BACKGROUND

Bovine articular cartilage is often used to study chondrocytes in vitro. It is difficult to correlate in vitro studies using bovine chondrocytes with in vivo studies using other species such as rabbits and sheep. The aim of this investigation was to study the effect of species, anatomical location and exogenous growth factors on chondrocyte proliferation in vitro.

METHODS

Equine (EQ), bovine (BO) and ovine (OV) articular chondrocytes from metacarpophalangeal (fetlock (F)), shoulder (S) and knee (K) joints were cultured in tissue culture flasks. Growth factors (rh-FGFb: 10 ng/ml; rh-TGFbeta: 5 ng/ml) were added to the cultures at days 2 and 4. On day 6, cells were counted and flow cytometry analysis was performed to determine cell size and granularity. A three factor ANOVA with paired Tukey's correction was used for statistical analysis.

RESULTS

After 6 days in culture, cell numbers had increased in control groups of EQ-F, OV-S, OV-F and BO-F chondrocytes. The addition of rh-FGFb led to the highest increase in cell numbers in the BO-F, followed by EQ-F and OV-S chondrocytes. The addition of rh-TGFbeta increased cell numbers in EQ-S and EQ-F chondrocytes, but showed nearly no effect on EQ-K, OV-K, OV-S, OV-F and BO-F chondrocytes. There was an overall difference with the addition of growth factors between the different species and joints.

CONCLUSION

Different proliferation profiles of chondrocytes from the various joints were found. Therefore, we recommend performing in vitro studies using the species and site where subsequent in vivo studies are planned.

Isolation and multilineage differentiation of bovine bone marrow mesenchymal stem cells

The bone marrow harbors a population of mesenchymal stem cells (MSCs) that possess the potential to differentiate into bone, cartilage, and fat, and along other tissue pathways. To date, MSCs from various species have been studied. Despite the bovine experimental model being widely used in experiments in vivo and in vitro, only a limited amount of information regarding bovine MSCs is available. The aim of this study was to isolate and induce the multilineage mesenchymal differentiation of bovine MSCs, thereby initiating further research on these cells. Bovine MSCs were isolated from eight calves, and osteogenic, chondrogenic, and adipogenic differentiation was induced by using a combination of previously reported protocols for other species. The level of differentiation was evaluated by histological examination and by analyzing the expression of tissue-specific genes by a quantitative "real time" reverse transcription/polymerase chain reaction technique. Following osteoinduction, the isolated fibroblast-like cells transformed into cuboidal cells and formed alkaline-phosphatase-positive colonies; during differentiation, these colonies transformed into mineralized nodules. In addition, osteogenesis was followed by osteocalcin and collagen type I mRNA expression. Chondrogenesis was confirmed by the demonstration of collagen type II, aggrecan, and sox9 mRNA expression in the cells stimulated by transforming growth factor beta1 in monolayer culture. After being cultured in an adipogenesis-inducing medium, the MSCs responded by the accumulation of lipid vacuoles and the expression of adipocyte-specific genes. We have therefore demonstrated that cells harvested from bovine bone marrow are capable of in vitro extensive multiplication and multilineage differentiation, making them a relevant and invaluable model in the field of stem cell research.

Chondrogenic differentiation of bovine bone marrow mesenchymal stem cells in pellet cultural system

OBJECTIVE

Pluripotent mesenchymal stem cells (MSC) have been isolated and well characterized from several tissue sources, including bone marrow stroma. MSC from different animals showed slight differences in morphology and in the potential to differentiate. In the present study, we isolated MSC from bovine bone marrow and induced chondrogenesis in order to establish a new experimental model of stem cell research.

METHODS

Bone marrow was harvested from 8 calves. For inducing chondrogenesis, MSC were cultured in pellet culture system in a chemically defined medium supplemented with 0 and 10 ng/mL of transforming growth factor beta1 (TGF-beta1). Chondrogenic differentiation was evaluated by histological, immunohistochemical, and in situ hybridization techniques. The degrees of genes expression were measured by quantitative RT-PCR.

RESULTS

Metachromatic alcian blue staining and immunoreactivity for type II collagen were detected in both pellet groups (0 and 10 ng/mL TGF-beta1) after 7 days of culturing. In situ hybridization demonstrated strong expression of type II collagen and aggrecan mRNAs in the round cells located at the center region of pellets and at densely organized areas. On the other hand, type I collagen mRNA was strongly expressed in the superficial layer of the pellets. After 20 days of pellet culture, expression of type II collagen mRNA in the cells which were not treated by TGF-beta1 was 1.7-fold higher compared with that treated by TGF-beta1.

CONCLUSION

Independent, spontaneous chondrogenesis of bovine MSC in pellet culture occurred without addition of any external bioactive stimulators, namely factors from TGF-beta family, which were previously considered necessary.

Regulation of osteogenic proteins by chondrocytes

The purpose of this review is to summarize the current scientific knowledge of bone morphogenetic proteins (BMPs) in adult articular cartilage. We specifically focus on adult cartilage, since one of the major potential applications of the members of the BMP family may be a repair of adult tissue after trauma and/or disease. After reviewing cartilage physiology and BMPs, we analyze the data on the role of recombinant BMPs as anabolic agents in tissue formation and restoration in different in vitro and in vivo models following with the endogenous expression of BMPs and factors that regulate their expression. We also discuss recent transgenic modifications of BMP genes and subsequent effect on cartilage matrix synthesis. We found that the most studied BMPs in adult articular cartilage are BMP-7 and BMP-2 as well as transforming growth factor-beta (TGF-beta). There are a number of contradicting reports for some of these growth factors, since different models, animals, doses, time points, culture conditions and devices were used. However, regardless of the experimental conditions, only BMP-7 or osteogenic protein-1 (OP-1) exhibits the most convincing effects. It is the only BMP studied thus far in adult cartilage that demonstrates strong anabolic activity in vitro and in vivo with and without serum. OP-1 stimulates the synthesis of the majority of cartilage extracellular matrix proteins in adult articular chondrocytes derived from different species and of different age. OP-1 counteracts the degenerative effect of numerous catabolic mediators; it is also expressed in adult human, bovine, rabbit and goat articular cartilage. This review reveals the importance of the exploration of the BMPs in the cartilage field and highlights their significance for clinical applications in the treatment of cartilage-related diseases.

Marrow stimulating technique to augment meniscus repair

Several techniques exist to increase the rate of healing of meniscal tears after repair. We describe a simple arthroscopic technique of microfracture to the intercondylar notch. This technique can provide marrow elements to the site of meniscus repair to aid in meniscal healing at the time of repair.

Expression of bone morphogenetic protein 6 in healthy and osteoarthritic human articular chondrocytes and stimulation of matrix synthesis in vitro

OBJECTIVE

To elucidate the role of bone morphogenetic protein 6 (BMP-6) in human articular cartilage, we investigated whether BMP-6 is expressed in adult human articular chondrocytes and analyzed the potential stimulatory effects of BMP-6 on these cells. In addition, we investigated whether osteoarthritic (OA) and normal cartilage chondrocytes behave differently.

METHODS

Endogenous expression of the BMP-6 gene was examined by reverse transcription-polymerase chain reaction. BMP-6 protein was detected by Western immunoblotting. Chondrocytes were grown as monolayer cultures for 7 days in a chemically defined serum-free medium, in the absence or presence of recombinant BMP-6. Proteoglycan (PG) synthesis was measured by (35)S-sulfate incorporation into newly synthesized macromolecules. Cell proliferation was assessed by (3)H-thymidine incorporation.

RESULTS

BMP-6 was expressed in both healthy and OA chondrocytes at the messenger RNA and protein levels. Total PG synthesis was significantly increased after BMP-6 stimulation of healthy (mean +/- SEM 191 +/- 11%; P < 0.001) and OA (150 +/- 25%; P < 0.03) chondrocyte cultures. A direct comparison between healthy and OA samples revealed no significant difference. The proliferation rates of normal and OA chondrocytes were not affected by BMP-6 treatment.

CONCLUSION

BMP-6 is endogenously expressed in chondrocytes obtained from OA and normal adult human articular cartilage. Furthermore, BMP-6 has the potential to stimulate total PG synthesis in human articular chondrocytes derived from normal as well as OA joints. We conclude that the presence of BMP-6 in adult human articular cartilage indicates a functional role for this growth factor in the maintenance of joint integrity.

Growth factors and cartilage repair

Growth factors are obvious tools to enhance cartilage repair. Understanding of reactivities in normal and arthritic cartilage and potential side effects on other compartments in the joint will help to identify possibilities and limitations. Growth factor responses have been evaluated in normal and diseased murine knees. The main cartilage anabolic factor, insulinlike growth factor-1, shows great safety, but has little contribution in diseased cartilage because of insulinlike growth factor nonresponsiveness of arthritic chondrocytes. Transforming growth factor-beta can overrule interleukin-1 catabolic effects and can enhance cartilage repair in arthritic tissue, unlike bone morphogenetic protein-2 that only is capable of enhancing chondrocyte proteoglycan synthesis in the absence of interleukin-1. Transforming growth factor-beta and bone morphogenetic protein-2 induce chondrophyte formation at the margins of the joint. Studies with scavenging transforming growth factor beta soluble receptor identified endogenous transforming growth factor-beta involvement in spontaneous cartilage repair and chondrophyte and subsequent osteophyte formation in arthritic conditions. Osteophyte induction may hamper intraarticular transforming growth factor-beta application in the joint and warrants targeted growth factor application to cartilage lesion sites only.

Chondrocytic differentiation of mesenchymal stem cells sequentially exposed to transforming growth factor-beta1 in monolayer and insulin-like growth factor-I in a three-dimensional matrix

This study evaluated chondrogenesis of mesenchymal progenitor stem cells (MSCs) cultured initially under pre-confluent monolayer conditions exposed to transforming growth factor-beta1 (TGF-beta1), and subsequently in three-dimensional cultures containing insulin-like growth factor I (IGF-I). Bone marrow aspirates and chondrocytes were obtained from horses and cultured in monolayer with 0 or 5 ng of TGF-beta 1 per ml of medium for 6 days. TGF-beta 1 treated and untreated cultures were distributed to three-dimensional fibrin disks containing 0 or 100 ng of IGF-I per ml of medium to establish four treatment groups. After 13 days, cultures were assessed by toluidine blue staining, collagen types I and II in situ hybridization and immunohistochemistry, proteoglycan production by [35S]-sulfate incorporation, and disk DNA content by fluorometry. Mesenchymal cells in monolayer cultures treated with TGF-beta1 actively proliferated for the first 4 days, developed cellular rounding, and formed cell clusters. Treated MSC cultures had a two-fold increase in medium proteoglycan content. Pretreatment of MSCs with TGF-beta1 followed by exposure of cells to IGF-I in three-dimensional culture significantly increased the formation of markers of chondrocytic function including disk proteoglycan content and procollagen type II mRNA production. However, proteoglycan and procollagen type II production by MSC's remained lower than parallel chondrocyte cultures. MSC pretreatment with TGF-beta1 without sequential IGF-I was less effective in initiating expression of markers of chondrogenesis. This study indicates that although MSC differentiation was less than complete when compared to mature chondrocytes, chondrogenesis was observed in IGF-I supplemented cultures, particularly when used in concert with TGF-beta1 pretreatment.

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.

Recombinant human osteogenic protein 1 is a potent stimulator of the synthesis of cartilage proteoglycans and collagens by human articular chondrocytes

OBJECTIVE

To study the effects of recombinant human osteogenic protein-1 (rHuOP-1; bone morphogenetic protein-7) on proteoglycan and collagen synthesis by human articular chondrocytes.

METHODS

Articular chondrocytes from fetal, adolescent, and adult human donors were cultured in alginate beads for 4 days in a mixture of Ham's F-12, Dulbecco's modified Eagle's medium, 10% fetal bovine serum (FBS), then for an additional 3-10 days in the presence and absence of rHuOP-1, with and without FBS. Chondrocyte synthetic activity was measured as the amount of incorporation of 35S-sulfate into proteoglycans and 3H-proline into hydroxyproline. Sieve chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis were performed to identify specific proteoglycans and collagens.

RESULTS

Recombinant human OP-1 markedly stimulated the synthesis of proteoglycans (mostly aggrecan) and collagens (predominantly type II) by all chondrocyte preparations. This did not require the presence of FBS and was associated with continued expression of the chondrocyte phenotype.

CONCLUSION

Recombinant human OP-1 is a more potent stimulator of the synthesis of cartilage-specific molecules by human articular chondrocytes than are other factors tested for comparison, including TGF beta 1 and activin A.

Growth factor responsiveness of human articular chondrocytes in aging and development

OBJECTIVE

To examine growth factor responses of human articular chondrocytes in aging and development. We have previously established a growth factor response profile for adult human articular chondrocytes and shown that transforming growth factor beta (TGF beta) is the most potent mitogen among a variety of factors tested.

METHODS

Chondrocytes were isolated from human articular cartilage obtained from donors ages 11-83 years and tested in primary culture for proliferative responses to serum and recombinant preparations of the major chondrocyte growth factors. Proliferation was measured by 3H-thymidine incorporation and cell counting. Skeletal maturity of the young donors was determined by radiographic assessment of Risser's index.

RESULTS

Chondrocytes showed a continuous age-related decline in the proliferative response to serum. Analysis of recombinant growth factors showed that with increasing donor age, there was a decrease in the levels of DNA synthesis in response to all factors tested. In chondrocytes from adult donors, there was no change in the relative potencies of the different growth factors. The decrease in the levels of DNA synthesis as measured by 3H-thymidine incorporation corresponded to a reduced rate of in vitro cell replication with increasing donor age. In addition to the quantitative changes in the proliferative responses of chondrocytes with increasing age, there was a qualitative change in the pattern of growth factor responses during development. Cells from young donors (ages 10-20) responded better to platelet-derived growth factor, AA chain homodimer (PDGF-AA) than to TGF beta 1, while the inverse pattern was seen in cells from adult donors. This decrease in the response to PDGF-AA was significantly correlated with increasing skeletal maturity.

CONCLUSION

Chondrocyte growth factor responsiveness shows qualitative changes during development, and after skeletal maturity, there is a profound decline in the levels of DNA synthesis and cell replication in response to the known chondrocyte growth factors.

Cloning of an isoform of mouse TGF-beta type II receptor gene

A variant of transforming growth factor-beta type II receptor (TGF-beta RII) cDNA was isolated from a mouse brain cDNA library. The predicted receptor is identical to previously reported mouse TGF-beta RII except that the isoform has an insertion sequence of 25 amino acids in the predicted ligand-binding domain. By the use of reverse transcription-polymerase chain reaction (RT-PCR), transcripts for both isoforms were detected in all tissues and developing embryos examined. The isoform transiently expressed in COS cells showed a similar ligand-binding specificity to authentic TGF-beta RII. These results suggest that the mouse TGF-beta RII gene generates multiple isoforms, possibly by alternative splicing, as reported for activin type IIB receptor; and an isoform which has the extra sequence in the ligand-binding domain is also involved in the TGF-beta signal transduction.

Responsiveness of articular cartilage from normal and inflamed mouse knee joints to various growth factors

OBJECTIVE

Disturbed anabolic signalling might contribute to the decreased chondrocyte proteoglycan (PG) synthesis during joint inflammation. Articular cartilage obtained from mouse knee joints with experimentally-induced arthritis exhibits a state of nonresponsiveness towards stimulation of chondrocyte PG synthesis by insulin-like growth factor-1 (IGF-1). Investigations were carried out on the role of other growth factors apart from IGF-1 on regulation of chondrocyte PG synthesis under pathological conditions, that is, during repair after IL-1 exposure as well as during early and later arthritis.

METHODS

Mouse patellae were obtained from normal knee joints and joints injected with IL-1 or zymosan. The patellae were cultured with basic fibroblast growth factor [bFGF], platelet-derived growth factor [PDGF], epidermal growth factor [EGF] or transforming growth factor beta [TGF beta] for 24 hours in the presence or absence of IGF-1. Chondrocyte PG synthesis was measured by 35S-sulphate incorporation.

RESULTS

In normal cartilage none of the tested growth factors elicited stimulatory effects on the chondrocyte PG synthesis as caused by IGF-1. EGF and TGF beta even caused significant inhibition of chondrocyte PG synthesis. Combination of bFGF or PDGF with IGF-1 exerted significant additional stimulation of the 35S-sulphate incorporation. IL-1 exposed cartilage displayed reactivity to IGF-1 as well as to the other growth factors similar to control cartilage. Cartilage obtained from joints with experimentally-induced arthritis exhibited a state of nonresponsiveness towards all individually tested growth factors as well as growth factor combinations.

CONCLUSION

Arthritis causes nonresponsiveness to stimulation of chondrocyte PG synthesis by the tested growth factors, which might be caused by a general receptor function defect.