Bone morphogenetic protein-6 and parathyroid hormone-related protein coordinately regulate the hypertrophic conversion in mouse clonal chondrogenic EC cells, ATDC5

Signalling interaction between β-catenin and other signalling molecules during osteoarthritis development

Osteoarthritis (OA) is the most prevalent disorder of synovial joint affecting multiple joints. In the past decade, we have witnessed conceptual switch of OA pathogenesis from a 'wear and tear' disease to a disease affecting entire joint. Extensive studies have been conducted to understand the underlying mechanisms of OA using genetic mouse models and ex vivo joint tissues derived from individuals with OA. These studies revealed that multiple signalling pathways are involved in OA development, including the canonical Wnt/β-catenin signalling and its interaction with other signalling pathways, such as transforming growth factor β (TGF-β), bone morphogenic protein (BMP), Indian Hedgehog (Ihh), nuclear factor κB (NF-κB), fibroblast growth factor (FGF), and Notch. The identification of signalling interaction and underlying mechanisms are currently underway and the specific molecule(s) and key signalling pathway(s) playing a decisive role in OA development need to be evaluated. This review will focus on recent progresses in understanding of the critical role of Wnt/β-catenin signalling in OA pathogenesis and interaction of β-catenin with other pathways, such as TGF-β, BMP, Notch, Ihh, NF-κB, and FGF. Understanding of these novel insights into the interaction of β-catenin with other pathways and its integration into a complex gene regulatory network during OA development will help us identify the key signalling pathway of OA pathogenesis leading to the discovery of novel therapeutic strategies for OA intervention.

Strategies to Modulate the Redifferentiation of Chondrocytes

Because of the low self-healing capacity of articular cartilage, cartilage injuries and degenerations triggered by various diseases are almost irreversible. Previous studies have suggested that human chondrocytes cultured in vitro tend to dedifferentiate during the cell-amplification phase and lose the physiological properties and functions of the cartilage itself, which is currently a critical limitation in the cultivation of cartilage for tissue engineering. Recently, numerous studies have focused on the modulation of chondrocyte redifferentiation. Researchers discovered the effect of various conditions (extracellular environment, cell sources, growth factors and redifferentiation inducers, and gene silencing and overexpression) on the redifferentiation of chondrocytes during the in vitro expansion of cells, and obtained cartilage tissue cultured in vitro that exhibited physiological characteristics and functions that were similar to those of human cartilage tissue. Encouragingly, several studies reported positive results regarding the modulation of the redifferentiation of chondrocytes in specific conditions. Here, the various factors and conditions that modulate the redifferentiation of chondrocytes, as well as their limitations and potential applications and challenges are reviewed. We expect to inspire research in the field of cartilage repair toward the future treatment of arthropathy.

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.

Bone morphogenetic proteins for articular cartilage regeneration

Degeneration of articular cartilage (AC) tissue is the most common cause of osteoarthritis (OA) and rheumatoid arthritis. Bone morphogenetic proteins (BMPs) play important roles in bone and cartilage formation. This article reviews the experimental and clinical applications of BMPs in cartilage regeneration. Experimental evidence indicates that BMPs play an important role in protection against cartilage damage caused by inflammation or trauma, by binding to different receptor combinations and, consequently, activating different intracellular signaling pathways. Loss of function of BMP-related receptors contributes to the decreased intrinsic repair capacity of damaged cartilage and, thus, the multifunctional effects of BMPs make them attractive tools for the treatment of cartilage damage in patients with degenerative diseases. However, the development of BMP therapy as a treatment modality for cartilage regeneration has been hampered by certain factors, such as the eligibility of participants in clinical trials, financial support, drug delivery carrier safety, availabilities of effective scaffolds, appropriate selection of optimal dose and timing of administration, and side effects. Further research is needed to overcome these issues for future routine clinical applications. Research and development leading to the successful application of BMPs can initiate a new era in the treatment of cartilage degenerative diseases like OA.

Prostaglandin F2α receptor (FP) signaling regulates Bmp signaling and promotes chondrocyte differentiation

Prostaglandins are a group of lipid signaling molecules involved in various physiological processes. In addition, prostaglandins have been implicated in the development and progression of diseases including cancer, cardiovascular disease, and arthritis. Prostaglandins exert their effects through the activation of specific G protein-coupled receptors (GPCRs). In this report, we examined the role of prostaglandin F2α receptor (FP) signaling as a regulator of chondrocyte differentiation. We found that FP expression was dramatically induced during the differentiation of chondrocytes and was up-regulated in cartilages. Forced expression of FP in ATDC5 chondrogenic cell line resulted in the increased expression of differentiation-related genes and increased synthesis of the extracellular matrix (ECM) regardless of the presence of insulin. Similarly, PGF2α treatment induced the expression of chondrogenic marker genes. In contrast, knockdown of endogenous FP expression suppressed the expression of chondrocyte marker genes and ECM synthesis. Organ culture of cartilage rudiments revealed that PGF2α induces chondrocyte hypertrophy. Additionally, FP overexpression increased the levels of Bmp-6, phospho-Smad1/5, and Bmpr1a, while knockdown of FP reduced expression of those genes. These results demonstrate that up-regulation of FP expression plays an important role in chondrocyte differentiation and modulates Bmp signaling.

ATDC5: an excellent in vitro model cell line for skeletal development

The ATDC5 cell line is derived from mouse teratocarcinoma cells and characterized as a chondrogenic cell line which goes through a sequential process analogy to chondrocyte differentiation. Thus, it is regarded as a promising in vitro model to study the factors that influence cell behaviors during chondrogenesis. It also provides insights in exploring signaling pathways related to skeletal development as well as interactions with innovative materials. To date, over 200 studies have utilized ATDC5 to obtain lots of significant findings. In this review, we summarized the literature of ATDC5 related studies and emphasized the application of ATDC5 in chondrogenesis. In addition, the general introduction of ATDC5 including its derivation and characterization is covered in this article.

Comparative review of growth factors for induction of three-dimensional in vitro chondrogenesis in human mesenchymal stem cells isolated from bone marrow and adipose tissue

The ability of bone-marrow-derived mesenchymal stem cells (MSCs) and adipose-derived stem cells (ASCs) to undergo chondrogenic differentiation has been studied extensively, and it has been suggested that the chondrogenic potential of these stem cells differ from each other. Here, we provide a comprehensive review and analysis of the various growth factor induction agents for MSC and ASC three-dimensional in vitro chondrogenic differentiation. In general, the most common growth factors for chondrogenic induction come from the transforming growth factor beta (TGFbeta) superfamily. To date, the most promising growth factors for chondrogenesis appear to be TGFbeta-3 and bone morphogenetic protein (BMP)-6. A thorough review of the literature indicates that human MSCs (hMSCs) appear to exhibit the highest chondrogenic potential in three-dimensional culture in the medium containing both dexamethasone and TGFbeta-3. Some reports indicate that the addition of BMP-6 to TFGbeta-3 and dexamethasone further increases hMSC chondrogenesis, but these results are still not consistently supported. Induction of human ASC (hASC) chondrogenesis appears most successful when dexamethasone, TGFbeta-3, and BMP-6 are used in combination. However, to date, current formulations do not always result in stable differentiation to the chondrocytic lineage by hMSCs and hASCs. Continued research must be performed to examine the expression cascades of the TFGbeta superfamily to further determine the effects of each growth factor alone and in combination on these stem cell lines.

Stable subclones of the chondrogenic murine cell line MC615 mimic distinct stages of chondrocyte differentiation

Fourteen stable subclones derived from the murine chondrogenic cell line MC615 were established and characterised regarding their differentiation stages and responsivity to BMP2. Based on their gene expression profiles which revealed remarkable variances in Col2a1 and Col10a1 expression, subclones could be grouped into at least three distinct categories. Three representative subclones (4C3, 4C6 and 4H4) were further characterised with respect to gene expression pattern and differentiation capacity. These subclones resembled (i) weakly differentiated chondrogenic precursors, strongly responding to BMP2 stimulation (4C3), (ii) collagen II expressing chondrocytes which could be induced to undergo maturation (4C6) and (iii) mature chondrocytes expressing Col10a1 and other markers of hypertrophy (4H4). Interestingly, BMP2 administration caused Smad protein phosphorylation and stimulated Col10a1 expression in all clones, but induced Col2a1 expression only in precursor-like cells. Most remarkably, these clones maintained a stable gene expression profile at least until the 30th passage of subconfluent culture, but revealed reproducible changes in gene expression and differentiation pattern in long term high density cultures. Thus, the newly established MC615 subclones may serve as a potent new tool for investigations on the regulation of chondrocyte differentiation and function.

Conditional inactivation of Has2 reveals a crucial role for hyaluronan in skeletal growth, patterning, chondrocyte maturation and joint formation in the developing limb

The glycosaminoglycan hyaluronan (HA) is a structural component of extracellular matrices and also interacts with cell surface receptors to directly influence cell behavior. To explore functions of HA in limb skeletal development, we conditionally inactivated the gene for HA synthase 2, Has2, in limb bud mesoderm using mice that harbor a floxed allele of Has2 and mice carrying a limb mesoderm-specific Prx1-Cre transgene. The skeletal elements of Has2-deficient limbs are severely shortened, indicating that HA is essential for normal longitudinal growth of all limb skeletal elements. Proximal phalanges are duplicated in Has2 mutant limbs indicating an involvement of HA in patterning specific portions of the digits. The growth plates of Has2-deficient skeletal elements are severely abnormal and disorganized, with a decrease in the deposition of aggrecan in the matrix and a disruption in normal columnar cellular relationships. Furthermore, there is a striking reduction in the number of hypertrophic chondrocytes and in the expression domains of markers of hypertrophic differentiation in the mutant growth plates, indicating that HA is necessary for the normal progression of chondrocyte maturation. In addition, secondary ossification centers do not form in the central regions of Has2 mutant growth plates owing to a failure of hypertrophic differentiation. In addition to skeletal defects, the formation of synovial joint cavities is defective in Has2-deficient limbs. Taken together, our results demonstrate that HA has a crucial role in skeletal growth, patterning, chondrocyte maturation and synovial joint formation in the developing limb.

Novel late response genes of PTHrP in chondrocytes

To gain more insight into the downstream effectors of parathyroid hormone (PTH) related peptide (PTHrP) signaling in chondrocytes, we performed microarray analysis to identify late PTHrP response genes using the chondrogenic ATDC5 cell line and studied their response in the osteoblastic KS483 cell line and explanted metatarsals. At day 8 of micromass culture, ATDC5 cells have pre-hypertrophic-like characteristics and at this time point the cells were stimulated with PTHrP for 24 and 72 h and RNA was isolated. PTHrP treatment inhibited outgrowth of cartilage matrix and decreased the expression of Col10a1 mRNA, which is in line with the inhibitory effects of PTHrP on chondrocyte differentiation. Using cDNA microarray analysis, a list of 9 genes (p< 10(-3)) was generated, including 3 upregulated (IGFBP4, Csrp2, and Ecm1) and 6 downregulated (Col9a1, Col2a1, Agc, Hmgn2, Calm1, and Mxd4) response genes. Four out of 9 genes are novel PTHrP response genes and 2 out of 9 have not yet been identified in cartilage. Four out of 9 genes are components of the extra-cellular matrix and the remaining genes are involved in signal transduction and transcription regulation. The response to PTHrP was validated by quantitative PCR, using the same RNA samples as labeled in the microarray experiments and RNA samples isolated from a new experiment. In addition, we examined whether these genes also reacted to PTHrP in other PTHrP responsive models, like KS483 osteoblasts and explanted metatarsals. The expression of late PTHrP response genes varied between ATDC5 chondrocytes, KS483 osteoblasts and metatarsals, suggesting that the expression of late response genes is dependent on the cellular context of the PTHrP responsive cells.

Potent induction of chondrocytic differentiation of human adipose-derived adult stem cells by bone morphogenetic protein 6

OBJECTIVE

Recent studies have identified an abundant source of multipotent progenitor cells in subcutaneous human adipose tissue, termed human adipose-derived adult stem cells (ADAS cells). In response to specific media formulations, including transforming growth factor beta1 (TGFbeta1), these cells exhibit significant ability to differentiate into a chondrocyte-like phenotype, expressing cartilage-specific genes and proteins such as aggrecan and type II collagen. However, the influence of other growth factors on the chondrogenic differentiation of ADAS cells is not fully understood. This study was undertaken to investigate the effects of TGFbeta1, TGFbeta3, insulin-like growth factor 1, bone morphogenetic protein 6 (BMP-6), and dexamethasone, in various combinations, on the chondrogenic potential of ADAS cells in alginate beads.

METHODS

The chondrogenic response of alginate-encapsulated ADAS cells was measured by quantitative polymerase chain reaction, 3H-proline and 35S-sulfate incorporation, and immunolabeling for specific extracellular matrix components.

RESULTS

Significant differences in chondrogenesis were observed under the different culture conditions for all outcomes measured. Most notably, BMP-6 up-regulated AGC1 and COL2A1 expression by an average of 205-fold and 38-fold, respectively, over day-0 controls, while down-regulating COL10A1 expression by approximately 2-fold.

CONCLUSION

These findings suggest that BMP-6 is a potent inducer of chondrogenesis in ADAS cells, in contrast to mesenchymal stem cells, which exhibit increased expression of type X collagen and a hypertrophic phenotype in response to BMP-6. Combinations of growth factors containing BMP-6 may provide a novel means of regulating the differentiation of ADAS cells for applications in the tissue-engineered repair or regeneration of articular cartilage.

Novel early target genes of parathyroid hormone-related peptide in chondrocytes

We have performed microarray analysis to identify PTHrP target genes in chondrocytes. ATDC5 cells were cultured as micromasses to induce chondrocyte differentiation. On d 8 of culture, the cells had a prehypertrophic appearance. This time point was chosen for isolation of RNA at 0, 1, 2, and 4 h after a challenge with 10(-7) M PTHrP. Samples were subjected to a cDNA microarray using competition hybridization. A list of 12 genes (P < 10(-3)), the expression regulation of which by PTHrP was confirmed by quantitative PCR analysis, was generated. This included seven up-regulated and five down-regulated genes. Three genes were known to be involved in PTHrP regulation, and six were previously found in growth plate chondrocytes. Most of the genes (10 of 12) were implicated in signal transduction and regulation. PTHrP also induced expression of the up-regulated genes in KS483 osteoblasts, suggesting involvement in a more generalized response to PTHrP. The vast majority of the up-regulated genes (six of seven) contained cAMP response element-binding protein- and/or activating protein-1 transcription factor-binding sites in their promoter regions. Remarkably, a number of PTHrP-regulated genes contained signal transducer and activator of transcription factor (Stat)-binding sites in their promoters. In transient transfection assays, we show that PTHrP is able to positively regulate the activity of Stat3-specific and negatively regulate the activity of Stat5-specific promoter-reporter constructs in ATDC5 and UMR106 cells. In combination with the expression regulation of genes involved in Janus kinase/Stat signaling, this data suggest a previously unrecognized interaction between PTHrP and Janus kinase/Stat signaling.

Multiple functions of BMPs in chondrogenesis

The ability of bone morphogenetic proteins (BMPs) to promote chondrogenesis has been investigated extensively over the past two decades. Although BMPs promote almost every aspect of chondrogenesis, from commitment to terminal differentiation is well known, the mechanisms of BMP action in discrete aspects of endochondral bone formation have only recently begun to be investigated. In this review, we focus on in vivo studies that have identified interactions between BMP signaling pathways and key downstream targets of BMP action in chondrogenesis. We also discuss evidence regarding the potential roles of BMP receptors in mediating distinct aspects of chondrogenesis, and studies investigating the intersection of BMP pathways with other pathways known to coordinate the progression of chondrocytes through the growth plate. These studies indicate that both Smad-dependent and -independent BMP pathways are required for chondrogenesis, and that BMPs exert essential roles via regulation of the Indian hedgehog (IHH)/parathyroid hormone-related protein (PTHrP) and fibroblast growth factor (FGF) pathways in the growth plate.

Dlx5 is a positive regulator of chondrocyte differentiation during endochondral ossification

The process of endochondral ossification in which the bones of the limb are formed after generation of cartilage models is dependent on a precisely regulated program of chondrocyte maturation. Here, we show that the homeobox-containing gene Dlx5 is expressed at the onset of chondrocyte maturation during the conversion of immature proliferating chondrocytes into postmitotic hypertrophying chondrocytes, a critical step in the maturation process. Moreover, retroviral misexpression of Dlx5 during differentiation of the skeletal elements of the chick limb in vivo results in the formation of severely shortened skeletal elements that contain excessive numbers of hypertrophying chondrocytes which extend into ectopic regions, including sites normally occupied by immature chondrocytes. The expansion in the extent of hypertrophic maturation detectable histologically is accompanied by expanded and upregulated domains of expression of molecular markers of chondrocyte maturation, particularly type X collagen and osteopontin, and by expansion of mineralized cartilage matrix, which is characteristic of terminal hypertrophic differentiation. Furthermore, Dlx5 misexpression markedly reduces chondrocyte proliferation concomitant with promoting hypertrophic maturation. Taken together, these results indicate that Dlx5 is a positive regulator of chondrocyte maturation and suggest that it regulates the process at least in part by promoting conversion of immature proliferating chondrocytes into hypertrophying chondrocytes. Retroviral misexpression of Dlx5 also enhances formation of periosteal bone, which is derived from the Dlx5-expressing perichondrium that surrounds the diaphyses of the cartilage models. This suggests that Dlx5 may be involved in regulating osteoblast differentiation, as well as chondrocyte maturation, during endochondral ossification.

The zinc finger transcription factor Zfp60 is a negative regulator of cartilage differentiation

The differentiation of many mesenchyme-derived cells, including cells that form bone and cartilage, is regulated at the level of gene transcription, but many of the factors involved in this regulation remain to be identified. In this study, a modified RNA fingerprinting technique was used to identify the KRAB domain zinc finger transcription factor Zfp60 as a candidate regulator of cell differentiation in mouse calvaria primary cultures. The highest expression of Zfp60 mRNA in vivo was found between embryonic day 11 (E11) and E15 during mouse embryonic development, coinciding with stages of active organ formation. The expression of Zfp60 mRNA and protein was analyzed further in mouse embryos during skeletal development. The most prominent expression was found in prehypertrophic chondrocytes, where it coincides with the expression of key regulators of chondrocyte maturation, Indian hedgehog (Ihh), and the parathyroid hormone-related peptide (PTHrP) receptor. Zfp60 mRNA was also found transiently expressed during chondrogenesis of C1 cells in vitro, preceding collagen type X expression and cellular hypertrophy. Overexpression of Zfp60 inhibited cartilage differentiation in the chondrogenic ATDC5 cell line. These results suggest a role for Zfp60 as a negative regulator of gene transcription, specifically during the development and/or differentiation of chondrocytes.

Parathyroid hormone-related peptide and indian hedgehog expression patterns in naturally acquired equine osteochondrosis

Early changes in parathyroid hormone-related peptide (PTH-rP) and Indian hedgehog (Ihh) expression were examined in equine articular osteochondrosis (OC) as a model of a naturally acquired dyschondroplasia. Cartilage was harvested from OC-affected femoropatellar or scapulohumeral joints from immature horses and normal control horses of similar age. PTH-rP expression levels were assessed by semi-quantitative PCR, in situ hybridization, and immunohistochemistry. Ihh protein expression levels were assessed by immunohistochemistry. Elevated PTH-rP protein and mRNA expression were identified in the deeper layers of affected articular cartilage and the fibrous tissue of interposing clefts. These changes were confined to the chondrocytes in the OC-affected cartilage, which had significantly increased PTH-rP protein and mRNA expression when compared to control cartilages. Ihh protein expression showed similar distribution as PTH-rP in the deeper layers of articular cartilage; however, only a trend for increased Ihh immunostaining was evident in the OC cartilage when compared to the normal cartilage. Increased PTH-rP expression in prehypertrophic chondrocytes of diseased OC cartilage suggests a possible link between this peptide and the delayed ossification, which is a consistent histologic alteration in OC. More evidence is necessary to determine the role of Ihh in articular cartilage and if a similar feedback cycle exists as previously described for the growth plate.

FK506 induces chondrogenic differentiation of clonal mouse embryonic carcinoma cells, ATDC5

FK506 (Tacrolimus) and cyclosporin A exert their immunosuppressive effects via a common mechanism, calcineurin inhibition, after binding to intracellular proteins termed immunophilins: FK506-binding protein (FKBP) and cyclophilin. In this study, FK506 was found to induce chondrogenic differentiation of ATDC5 cells (clonal mouse embryonal carcinoma cells) in a concentration-dependent manner (0.1-1000 ng/ml). Immunohistochemical staining showed that ATDC5 cells induced to differentiate by FK506 produced proteoglycan and type II collagen, main components of the extracellular matrix of cartilage. Rapamycin, an immunosuppressant that binds to FKBP, antagonized the effect of FK506. Cyclosporin A did not induce chondrogenesis at concentrations up to 1000 ng/ml. Taken together, these results suggest that FK506 induces chondrogenic differentiation of ATDC5 cells via a calcineurin-independent mechanism, after binding to FKBP.

The release and activation of transforming growth factor beta2 associated with apoptosis of chick hypertrophic chondrocytes

The apoptosis of hypertrophic chondrocytes at the interface between growth cartilage and invading vessels is at the center of a series of critical events in endochondral formation. We have shown that the hypertrophy and apoptosis of chick chondrocytes in culture is associated with the release and activation of transforming growth factor beta2 (TGF-beta2). Supplementation of the culture medium with agents that influenced the maintenance of hypertrophic differentiation also influenced the release of TGF-beta2. A large proportion of the TGF-beta2 released from the cells was shown to be in an active form-particularly TGF-beta2 associated with the support matrix. Inhibition of apoptosis with a broad-spectrum caspase inhibitor inhibited activation of the matrix-associated TGF-beta2. However, inhibition of apoptosis did not diminish the release of TGF-beta2. Release of TGF-beta2 by chondrocytes at a late stage of their terminal differentiation and its activation in association with apoptosis may provide a mechanism controlling the processes of vascular invasion of growth cartilage and the deposition of bone matrix on nearby cartilage remnants.

A 30-base-pair element in the first intron of SOX9 acts as an enhancer in ATDC5

SOX9 is a transcription factor that is essential for chondrogenesis and testis differentiation, and haploinsufficiency of SOX9 causes campomelic dysplasia, severe skeletal malformation syndrome with variably penetrant XY sex reversal. Here we demonstrate that in several cell lines that express SOX9, 30-bp element in the first intron of human SOX9 gene act as a potential enhancer in the ATDC5 chondroprogenitor cell line, despite the apparent absence of cell-specific regulatory elements within a 5.5-kb promoter region. Deletion and site-specific mutational analyses reveal that the last 12 bp of the 30-bp element are critical for transcriptional activity, while 5'-half sequences are necessary for full transactivation. Gel retardation assays indicate the possible involvement of several binding factors along the length of this element. These results suggest that functionally interdependent elements in the 30-bp enhancer region of the first intron account for basal expression levels of Sox9 in ATDC5.

Molecular cloning and biological activity of a novel lysyl oxidase-related gene expressed in cartilage

We cloned a cDNA encoding a novel lysyl oxidase-related protein, named LOXC, by suppression subtractive hybridization between differentiated and calcified ATDC5 cells, a clonal mouse chondrogenic EC cell line. The deduced amino acid sequence of mouse LOXC consists of 757 amino acids and shows 50% identity with that of mouse lysyl oxidase. Northern blot analysis showed a distinct hybridization band of 5.4 kilobases, and Western blot analysis showed an immunoreactive band at 82 kilodaltons. Expression of LOXC mRNA was detected in osteoblastic MC3T3-E1 cells and embryonic fibroblast C3H10T1/2 cells, whereas none of NIH3T3 fibroblasts and myoblastic C2C12 cells expressed LOXC mRNA in vitro. Moreover, the LOXC mRNA and protein levels dramatically increased throughout a process of chondrogenic differentiation in ATDC5 cells. In vivo, LOXC gene expression was localized in hypertrophic and calcified chondrocytes of growth plates in adult mice. The conditioned media of COS-7 cells transfected with the full-length LOXC cDNA showed the lysyl oxidase activity in both type I and type II collagens derived from chick embryos, and these activities of LOXC were inhibited by beta-aminopropionitrile, a specific inhibitor of lysyl oxidase. Our data indicate that LOXC is expressed in cartilage in vivo and modulates the formation of a collagenous extracellular matrix.

BMP-6 enhances chondrogenesis in a subpopulation of human marrow stromal cells

Marrow stromal cells (MSCs) can differentiate into several mesenchymal lineages. MSCs were recently shown to form cartilage in micromass cultures with serum-free medium containing TGF-beta and dexamethasone. Here we found that addition of BMP-6 increased the weight of the pellets about 10-fold and they stained more extensively for proteoglycans. mRNAs for type II procollagen and type X collagen were detected at 1 week and the levels were increased at 3 weeks. We also compared two subpopulation of cultures of MSCs: Small and rapidly self-renewing cells (RS cells) and the large, more mature and slowly replicating cells (mMSCs). The cartilage pellets prepared from cultures enriched for RS cells were about 2.5-fold larger, stained more extensively for proteoglycans, and had levels of mRNA for type II procollagen that were 1.6-fold higher. Also, RS cells retained more of their chondrogenic potential as the cells were passaged.

Parathyroid hormone-related peptide inhibits the expression of bone morphogenetic protein-4 mRNA through a cyclic AMP/protein kinase A pathway in mouse clonal chondrogenic EC cells, ATDC5

The bone morphogenetic proteins (BMPs) play crucial roles in chondrogenic differentiation. Little is known, however, regarding the regulation of BMP gene expression. Here we examined the effect of parathyroid hormone-related peptide (PTHrP) (1-141), a full-length form of PTHrP molecules, on the expression of BMP-4 mRNA in clonal mouse chondrogenic EC cells, ATDC5. In differentiated ATDC5 cells, the expression of BMP-4 mRNA was inhibited by PTHrP (1-141), which stimulated cAMP accumulation and protein kinase A (PKA) activity in these cells. Dibutyryl cAMP, a permeable analog of cAMP, mimicked and H-89, a selective PKA inhibitor, blocked this effect of PTHrP (1-141). Moreover, actinomycin D attenuated the inhibition of BMP-4 mRNA expression by PTHrP (1-141). These results indicate that PTHrP (1-141) transcriptionally inhibits the expression of BMP-4 mRNA through a cAMP/PKA pathway in ATDC5 cells.

Parathyroid hormone-related peptide and bone: pathological and physiological aspects

Parathyroid hormone-related peptide (PTHrP) was initially discovered as a tumor-derived systemic factor which causes humoral hypercalcemia of malignancy. When overproduced and secreted by tumor cells, PTHrP acts on target organs such as bone and kidney to cause hypercalcemia through its 'PTH-like effects'. The hypercalcemic effects of PTHrP are attributed to its N-terminal portion (1-36) which shows a limited homology with PTH and is able to bind to the common PTH/PTHrP receptor. In contrast to such pathological effects as a humoral factor, PTHrP is now recognized as a locally active cytokine produced by a variety of tissues and cell types. Gene knockout experiments have revealed critical roles for PTHrP in a wide spectrum of physiological processes including chondrogenesis. It also significantly contributes to various pathological processes such as tumor metastasis to bone and bone destruction in arthropathies, acting as a bone-resorbing cytokine. Consistent with its divergent roles, regulation of PTHrP expression as well as its mode of action seems to be much more complex than its hormonal counterpart, PTH. In this article, we will briefly review the recent progress in our understanding of both physiological and pathological aspects of PTHrP biology, with a particular focus on its roles as a bone cytokine.

Requirement of autocrine signaling by bone morphogenetic protein-4 for chondrogenic differentiation of ATDC5 cells

Mouse EC cell line ATDC5 undergoes differentiation to form cartilage nodules via the cellular condensation stage in the presence of insulin. ATDC5 cells expressed transcripts for bone morphogenetic protein-4 (BMP-4), and type IA and type II BMP receptors. Moreover, cells retained responsiveness to BMP-4, which induced the formation of chondrocytes in the culture. When transfected with a kinase domain-truncated type IA BMP receptor construct, cells failed to undergo differentiation beyond the condensation stage even in the presence of insulin. The soluble form of type IA BMP receptor also blocked the formation of chondrocytes in a dose dependent manner. These lines of evidence suggested that autocrine BMP-4 signaling is required for the conversion of chondrogenic precursor cells into chondrocytes.