BMP-6 is an autocrine stimulator of chondrocyte differentiation

TGF-beta, BMPS, and their signal transducing mediators, Smads, in rat fracture healing

Smads are cytoplasmic signal transducers of transforming growth factor-beta (TGF-beta) and bone morphogenetic proteins (BMPs). Their relation to fracture healing is unknown. This study examined the temporal protein expression of Smads, together with TGF-beta and BMPs, using immunohistochemistry in a rodent fracture model. Over-expression of TGF-beta, BMPs-2, 4, and 7, common-mediator Smad (Smad4), and receptor-regulated Smads (Smads1, 2, 3, and 5) versus lower levels of inhibitory Smad (Smad6), were detected at day 3 in osteogenic cells in the thickened periosteum and bone marrow at the fracture sites. At day 10, Smad6 increased dramatically, Smad2, Smad3, and Smad4 remained elevated while Smad1 and Smad5 decreased in the fracture callus. Smad7 was expressed only in vascular endothelial cells. By day 28, when new bone had replaced the fracture callus, all the protein regulators decreased, approaching control levels. During fracture healing, the expression patterns of Smads1 and 5 were similar to that of BMPs-2 and 7 whereas the expression of Smads2 and 3 was parallel with that of TGF-beta. The Smad family, associated with BMPs and TGF-beta, may play an important role in the early stage of rat fracture healing.

Differential effects of osteogenic protein-1 (BMP-7) on gene expression of BMP and GDF family members during differentiation of the mouse MC615 chondrocyte cells

The mRNA expression patterns of several bone morphogenetic proteins (BMPs) and growth differentiation factors (GDFs) in long-term cultures of the clonal mouse chondrocyte cell line MC615 were examined. Distinct spatial and temporal patterns of expression of BMPs and GDFs were observed. The temporal orders of expression were correlated with those of several biochemical markers characteristic of chondrocytic cell differentiation. BMP-1, -2, -5, and -6 mRNA expression increased throughout the chondrogenic process and BMP-4 mRNA expression was not changed. GDF-1 and -3 mRNA expression increased throughout the chondrogenic process, and GDF-5, -6, -8, and -9 mRNA expressions were not changed. Effects of osteogenic protein-1 (OP-1, BMP-7) on the expression patterns of several other members of the BMP family and of the GDF family were also examined. OP-1 downregulated the BMP-1, -4, -5, and -6 mRNA expression by a maximal 3-, 5-, 2.5-, and 3-fold, respectively. The BMP-2 mRNA expression was not changed significantly by a low concentration of OP-1, but was increased at 200 ng/ml at day 7 of treatment. In contrast to the BMPs, OP-1 upregulated significantly the six GDF members examined (GDF-1, -3, -5, -6, -8, and -9) by three- to four-fold. Our findings demonstrate that OP-1 differentially regulates the mRNA expression of several related members of the BMP family and upregulates the mRNA expression of several members of the GDF family. The observations suggest that OP-1 action on cartilage differentiation involves a complex regulation of gene expression of several members of the BMP and the GDF family.

The role of the resting zone in growth plate chondrogenesis

In mammals, growth of long bones occurs at the growth plate, a cartilage structure that contains three principal layers: the resting, proliferative, and hypertrophic zones. The function of the resting zone is not well understood. We removed the proliferative and hypertrophic zones from the rabbit distal ulnar growth plate in vivo, leaving only the resting zone. Within 1 wk, a complete proliferative and hypertrophic zone often regenerated. Next, we manipulated growth plates in vivo to place resting zone cartilage ectopically alongside the proliferative columns. Ectopic resting zone cartilage induced a 90-degree shift in the orientation of nearby proliferative zone chondrocytes and seemed to inhibit their hypertrophic differentiation. Our findings suggest that resting zone cartilage makes important contributions to endochondral bone formation at the growth plate: 1) it contains stem-like cells that give rise to clones of proliferative chondrocytes; 2) it produces a growth plate-orienting factor, a morphogen, that directs the alignment of the proliferative clones into columns parallel to the long axis of the bone; and 3) it may also produce a morphogen that inhibits terminal differentiation of nearby proliferative zone chondrocytes and thus may be partially responsible for the organization of the growth plate into distinct zones of proliferation and hypertrophy.

Molecular analysis of expansion, differentiation, and growth factor treatment of human chondrocytes identifies differentiation markers and growth-related genes

This study is intended to optimise expansion and differentiation of cultured human chondrocytes by growth factor application and to identify molecular markers to monitor their differentiation state. We dissected the molecular consequences of matrix release, monolayer, and 3D-alginate culture, growth factor optimised expansion, and re-differentiation protocols by gene expression analysis. Among 19 common cartilage molecules assessed by cDNA array, six proved best to monitor differentiation. Instant down-regulation at release of cells from the matrix was strongest for COL 2A1, fibromodulin, and PRELP while LUM, CHI3L1, and CHI3L2 were expansion-related. Both gene sets reflected the physiologic effects of the most potent growth-inducing (PDGF-BB) and proteoglycan-inducing (BMP-4) factors. Only CRTAC1 expression correlated with 2D/3D switches while the molecular phenotype of native chondrocytes was not restored. The markers and optimised protocols we suggest can help to improve cell therapy of cartilage defects and chondrocyte differentiation from stem cell sources.

Differential temporal expression of members of the transforming growth factor beta superfamily during murine fracture healing

Fracture healing is a unique postnatal repair process in which the events of endochondral and intramembranous bone formation follow a definable temporal sequence. The temporal patterns of messenger RNA (mRNA) expression for members of the transforming growth factor beta (TGF-beta) superfamily were examined over a 28-day period of fracture healing in mouse tibias. Bone morphogenetic protein 2 (BMP-2) and growth and differentiation factor 8 (GDF8) showed maximal expression on day 1 after fracture, suggesting their roles as early response genes in the cascade of healing events. Restricted expression of GDF8 to day 1, in light of its known actions as a negative regulator of skeletal muscle growth, suggests that it may similarly regulate cell differentiation early in the fracture healing process. GDF5, TGF-beta2, and TGF-beta3 showed maximal expression on day 7, when type II collagen expression peaked during cartilage formation. In contrast, BMP-3, BMP-4, BMP-7, and BMP-8 showed a restricted period of expression from day 14 through day 21, when the resorption of calcified cartilage and osteoblastic recruitment were most active. TGF-beta1, BMP-5 and BMP-6, and GDF10 were constitutively expressed from day 3 to day 21. However, during the same time period, GDF3, GDF6, and GDF9 could not be detected, and GDF1 was expressed at extremely low levels. These findings suggest that several members of the TGF-beta superfamily are actively involved in fracture healing and although they are closely related both structurally and functionally, each has a distinct temporal expression pattern and potentially unique role in fracture healing.

Assessment of the gene expression profile of differentiated and dedifferentiated human fetal chondrocytes by microarray analysis

OBJECTIVE

To study the changes in patterns of gene expression exhibited by human chondrocytes as they dedifferentiate into fibroblastic cells in culture in order to better understand the mechanisms that control this process and its relationship to the phenotypic changes that occur in chondrocytes during the development of osteoarthritis (OA).

METHODS

Human fetal epiphyseal chondrocytes (HFCs) were cultured either on poly-(2-hydroxyethyl methacrylate)-coated plates (differentiated HFC cultures) or in plastic tissue culture flasks as monolayers (dedifferentiated HFC cultures). Following 11 days of culture under either condition, poly(A+) RNA was isolated from the two cell populations and subjected to a gene expression analysis using a microarray containing approximately 5,000 known human genes and approximately 3,000 expressed sequence tags (ESTs).

RESULTS

A > or =2-fold difference in the expression of 62 known genes and 6 ESTs was observed between the two cell types. The differences in expression of several of the genes detected by the microarray hybridization were confirmed by Northern analyses. Two transcription factor genes, TWIST and HIF-1alpha, and a cellular adhesion protein gene, cadherin 11, were markedly regulated in response to differentiation and dedifferentiation. Expression of these genes was also detected in adult normal and OA cartilage and chondrocytes. Analysis of the gene expression profile of HFCs revealed a complex pattern of gene expression, including many genes not yet reported to be expressed by chondrocytes.

CONCLUSION

Chondrocytes in monolayer become dedifferentiated, acquiring a fibroblast-like appearance and changing their pattern of gene expression from one of expression of chondrocyte-specific genes to one that resembles a fibroblastic or chondroprogenitor-like pattern. Changes in gene expression associated with the process of dedifferentiation of HFCs in vitro were observed in a wide variety of genes, including genes encoding extracellular matrix proteins, transcription factors, and growth factors. At least 3 of the genes that were regulated in response to dedifferentiation were also found to be expressed in adult normal and OA articular cartilage and chondrocytes.

A role for the BMP antagonist chordin in endochondral ossification

Bone morphogenetic proteins (BMPs) are ubiquitous regulators of cellular growth and differentiation. A variety of processes modulate BMP activity, including negative regulation by several distinct binding proteins. One such BMP antagonist chordin has a role in axis determination and neural induction in the early embryo. In this study, a role for chordin during endochondral ossification has been investigated. During limb development, Chordin expression was detected only at the distal ends of the skeletal elements. In cultured embryonic sternal chondrocytes, Chordin expression was related inversely to the stages of maturation. Further, treating cultured chondrocytes with chordin interfered with maturation induced by treatment with BMP-2. These results suggest that chordin may negatively regulate chondrocyte maturation and limb growth in vivo. To address this hypothesis, chordin protein was expressed ectopically in Hamburger-Hamilton (HH) stage 25-27 embryonic chick limbs. The phenotypic changes and alteration of gene expression in treated limbs revealed that overexpression of chordin protein delayed chondrocyte maturation in developing skeletal elements. In summary, these findings strongly support a role for chordin as a negative regulator of endochondral ossification.

The life cycle of chondrocytes in the developing skeleton

Cartilage serves multiple functions in the developing embryo and in postnatal life. Genetic mutations affecting cartilage development are relatively common and lead to skeletal malformations, dysfunction or increased susceptibility to disease or injury. Characterization of these mutations and investigation of the molecular pathways in which these genes function have contributed to an understanding of the mechanisms regulating skeletal patterning, chondrogenesis, endochondral ossification and joint formation. Extracellular growth and differentiation factors including bone morphogenetic proteins, fibroblast growth factors, parathyroid hormone-related peptide, extracellular matrix components, and members of the hedgehog and Wnt families provide important signals for the regulation of cell proliferation, differentiation and apoptosis. Transduction of these signals within the developing mesenchymal cells and chondrocytes results in changes in gene expression mediated by transcription factors including Smads, Msx2, Sox9, signal transducer and activator of transcription (STAT), and core-binding factor alpha 1. Further investigation of the interactions of these signaling pathways will contribute to an understanding of cartilage growth and development, and will allow for the development of strategies for the early detection, prevention and treatment of diseases and disorders affecting the skeleton.

Bone morphogenetic proteins, extracellular matrix, and mitogen-activated protein kinase signaling pathways are required for osteoblast-specific gene expression and differentiation in MC3T3-E1 cells

Osteoblasts secrete a complex extracellular matrix (ECM) containing collagenous and noncollagenous proteins, bone morphogenetic proteins (BMPs), and growth factors. Osteoblast-specific gene expression requires ascorbic acid (AA)-dependent assembly of a collagenous ECM. Matrix responsiveness requires an alpha2beta1 integrin-collagen interaction and mitogen-activated protein kinase (MAPK) activity, which phosphorylates and activates the osteoblast-specific transcription factor Cbfa1. This study examines interactions between this integrin/MAPK-mediated pathway and signals initiated by BMPs contained in the osteoblast matrix. MC3T3-E1 cells were shown to constitutively express BMP-2, BMP-4, and BMP-7. Noggin, a specific BMP inhibitor, reversibly blocked AA-induced gene expression, indicating that BMP production by MC3T3-E1 cells was necessary for differentiation. The ability of exogenously added BMP-2, BMP-4, or BMP-7 to stimulate osteocalcin (OCN) and bone sialoprotein (BSP) mRNAs or OCN promoter activity was synergistically increased in cells that were actively synthesizing an ECM (i.e., were grown in the presence of AA). A minimum of 4 days of ECM accumulation was required for this synergistic response to be observed. Neither BMP-7, AA, nor a combination of these two treatments had major effects on Cbfa1 messenger RNA (mRNA) or protein levels, as would be expected if regulation was mainly at the posttranscriptional level. U0126, a specific inhibitor of MAPK/extracellular signal-regulated kinase (MEK), blocked AA- or BMP-7/AA-dependent gene expression in a time- and dose-dependent manner that was closely correlated with inhibition of extracellular signal-regulated kinase (ERK) phosphorylation. This work establishes that autocrine BMP production as well as integrin-mediated cell-collagen interactions are both required for osteoblast differentiation, and both these pathways require MAP kinase activity.

PTHrP expression in chick sternal chondrocytes is regulated by TGF-beta through Smad-mediated signaling

PTHrP regulates the rate of chondrocyte differentiation during endochondral bone formation. The expression of PTHrP and its regulation by TGF-beta, BMP-2, and PTHrP was examined in upper sternal chondrocytes following 1, 3, and 5 days of continuous treatment. While TGF-beta stimulated the expression of PTHrP (5-fold), PTHrP caused a slight inhibition, and BMP-2 markedly inhibited PTHrP mRNA expression. The effect of these factors on PTHrP expression was not simply related to the maturational state of the cells, since BMP-2 increased, while both PTHrP and TGF-beta decreased the expression of type X collagen. TGF-beta isoforms 1, 2, and 3 all stimulated PTHrP expression. Signaling events involved in the induction of PTHrP by TGF-beta were further evaluated in a PTHrP-promoter CAT construct. The effect of TGF-beta, BMP-2, and PTHrP on the PTHrP-promoter paralleled their effects on mRNA expression, with TGF-beta significantly increasing CAT activity, BMP-2 decreasing CAT activity, and PTHrP having a minimal effect. Co-transfection of the TGF-beta signaling molecule, Smad 3, mimicked the effect of TGF-beta (induction of PTHrP promoter), while dominant negative Smad 3 inhibited the induction of the PTHrP promoter by TGF-beta. Furthermore, infection with a Smad 3-expressing retrovirus mimicked the effects of exogenously added TGF-beta, and induced PTHrP mRNA expression in the infected chondrocyte culture. In contrast, a dominant negative Smad 3 completely inhibited PTHrP promoter stimulation by TGF-beta, but only partially blocked the effect of TGF-beta on PTHrP mRNA synthesis. These findings demonstrate that PTHrP is expressed in chondrocytes undergoing endochondral ossification, and show regulation, at least in part, by TGF-beta through Smad mediated signaling events.

Regulation of chondrocyte terminal differentiation in the postembryonic growth plate: the role of the PTHrP-Indian hedgehog axis

Chondrocyte differentiation during embryonic bone growth is controlled by interactions between PTHrP and Indian hedgehog. We have now determined that the major components of this signaling pathway are present in the postembryonic growth plate. PTHrP was immunolocalized throughout the growth plate, and semiquantitative RT-PCR analysis of maturationally distinct chondrocyte fractions indicated that PTHrP, Indian hedgehog, and the PTH/PTHrP receptor were expressed at similar levels throughout the growth plate. However, patched, the hedgehog receptor, was more highly expressed in proliferating chondrocytes. Although all fractionated cells responded to PTHrP in culture by increasing thymidine incorporation and cAMP production and decreasing alkaline phosphatase activity, the magnitude of response was greatest in the proliferative chondrocytes. Bone morphogenetic proteins are considered likely intermediates in PTHrP signaling. Expression of bone morphogenetic protein-2 and 4--7 was detected within the growth plate, and PTHrP inhibited the expression of bone morphogenetic protein-4 and 6. Although organ culture studies indicated a possible paracrine role for epiphyseal chondrocyte-derived PTHrP in regulating growth plate chondrocyte differentiation, the presence within the postembryonic growth plate of functional components of the PTHrP-Indian hedgehog pathway suggests that local mechanisms intrinsic to the growth plate exist to control the rate of endochondral ossification.

Effect of recombinant human bone morphogenetic protein-2 on bone regeneration and osseointegration of dental implants

Recombinant human bone morphogenetic protein-2 (rhBMP-2) induced bone regeneration and osseointegration was evaluated in bony defects created within the hollow chamber of endosseous dental implants in 14 foxhound dogs. Bilateral extractions of mandibular premolars were performed and surgical implantation of 104 hollow cylinder implants followed after 8 weeks of healing. Experimental implants had their hollow chamber filled with 20 microg of rhBMP-2 delivered with a bovine collagen carrier, whereas the control implants had their apical chamber left empty. Dogs were followed for 2, 4, 8 and 12 weeks. Histomorphometric evaluation and immunohistochemical analysis were performed. Minimal bone was regenerated at 2 weeks for both groups. At 4 weeks, bone fill averaged 23.48% for the rhBMP-2 and 5.98% for the control group (P<0.05). At 8 weeks, mean bone fill was 20.94% and 7.75% for the rhBMP-2 and the controls, respectively (P<0.05). At 12 weeks, mean bone fill was 31.39% and 24.31% for the rhBMP-2 and control implants, respectively (P>0.05). Bone-implant contact (BIC) increased for both groups over time and at 8 weeks the rhBMP-2 BIC value was 18.65% and for the control 7.22% (P<0.05). At 12 weeks, the BIC was 43.78% and 21.05% for the rhBMP-2 and the control group, respectively (P<0.05). Immunohistochemical staining for type II collagen was positive only for parts of the collagen carrier and formation of cartilaginous intermediate was not observed in any of the specimens. The results suggest that, in confined defects adjacent to dental implants, rhBMP-2 can induce bone regeneration in close apposition to the implant surface.

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.

A murine osteosarcoma cell line with a potential to develop ossification upon transplantation

An osteosarcoma cell line has been established from a soft tissue tumor that occurred spontaneously in a BALB / c mouse. This cell line showed ossification when transplanted into syngeneic mice. To examine the mechanism of bone formation, the expression of mRNAs for osteoblastic and chondroblastic markers and factors associated with ossification has been investigated. In culture, the cells exhibited a spindle shape in the growth phase, but had a polygonal shape in the stationary phase. Reverse transcription-polymerase chain reaction analysis showed that the cells expressed mRNAs for pro-alpha1(I) chain of type I collagen, alkaline phosphatase, osteopontin, osteocalcin, and core binding factor alpha1, suggesting differentiation into the stage of osteoblasts during the stationary phase. After transplantation, histological examination revealed small foci of pale blue material and basophilic networks that were scattered in the tumor tissues at one week. The former stained positive with alcian blue, suggesting a chondroid matrix. Pro-alpha1(II) chain of type II collagen mRNA was expressed at one week. A large part of tumors at two and three weeks consisted of basophilic networks, which stained positive via von Kossa's method, indicating a calcified woven bone. In situ hybridization analysis showed strong expression of osteopontin and osteocalcin mRNAs in tumor cells surrounding the bone matrix. Bone morphogenetic protein-6 and -7 mRNAs were detected in transplanted tumors, but not in cultured cells. These results suggest that the cell line has the properties of an osteoblastic lineage when cultured in vitro and has an ossifying ability through endochondral bone formation processes when transplanted in vivo.

Reciprocal control of osteoblast/chondroblast and osteoblast/adipocyte differentiation of multipotential clonal human marrow stromal F/STRO-1(+) cells

The regulation of human bone marrow stromal precursor cell differentiation toward the chondrocyte, osteoblast or adipocyte lineages is not known. In this study, we assessed the lineage-specific differentiation and conversion of immortalized clonal F/STRO-1(+) A human fetal bone marrow stromal cells under the control of dexamethasone (Dex), indomethacin/insulin (Indo/Ins) and linoleic acid (LA). Under basal conditions, F/STRO-1(+) A cells expressed markers mRNAs or proteins of the osteoblast lineage [CBFA1, osteocalcin (OC), alkaline phosphatase (ALP), type 1 collagen], of the chondrocyte lineage (aggrecan, types 2, 9 and 10 collagen), and of the adipocyte lineage (PPARgamma2, C/EBPalpha, aP2, G3PDH, lipoprotein lipase, leptin). Treatment with Dex increased CBFA1, OC and ALP mRNA and protein levels. Exposure to LA enhanced expression of adipocytic genes and cytoplasmic triglycerides accumulation, and suppressed the Dex-induced stimulation of osteoblast marker genes. Indo/Ins stimulated the synthesis of aggrecan and type 2 collagen and increased types 9 and 10 collagen mRNA levels, and suppressed both basal and Dex-promoted expression of osteoblast markers. Conversely, stimulation of osteoblastogenesis by Dex suppressed both basal and Indo/Ins-stimulated chondrocyte genes. Thus, the clonal human fetal bone marrow stromal F/STRO-1(+) A cell line is a lineage-unrestricted common progenitor that expresses tripotential adipocyte, osteoblast or chondrocyte characteristics. Our data also show that differentiation towards one pathway in response to Dex, Indo/Ins and LA restricts expression of other lineage-specific genes, and provide evidence for a controlled reciprocal regulation of osteoblast/chondroblast and osteoblast/adipocyte differentiation of clonal F/STRO-1(+) human bone marrow stromal cells.

Direct inhibition of Indian hedgehog expression by parathyroid hormone (PTH)/PTH-related peptide and up-regulation by retinoic acid in growth plate chondrocyte cultures

Indian hedgehog (Ihh) is highly expressed in prehypertrophic chondrocytes in vivo and has been proposed to regulate the proliferation and maturation of chondrocytes and bone collar formation in the growth plate. In high-density cultures of rabbit growth-plate chondrocytes, Ihh mRNA was also expressed at the highest level in the prehypertrophic stage. To explore endogenous factors that regulate Ihh expression in chondrocytes, we examined the effects of various growth factors on Ihh mRNA expression in this system. Retinoic acid (RA) and bone morphogenetic protein-2 enhanced Ihh mRNA expression, whereas PTH/PTH-related peptide (PTHrP) markedly suppressed Ihh expression. RA at more than 10(-8) M induced the expression of Ihh and Patched 1 (Ptc1) within 3 h, before it increased the type X collagen mRNA level at 6-24 h. Cycloheximide blocked the up-regulation of Ihh by RA, indicating the requirement of de novo protein synthesis for this stimulation. These findings suggest that RA is involved in the up-regulation of Ihh during endochondral bone formation. In contrast to RA, PTH (1-84) at 10(-7) M abolished the mRNA expression of Ihh and Ptc1 within 2-4 h, before it suppressed the expression of type X collagen at 12-24 h. The inhibition of Ihh expression by PTH (1-84) did not require de novo protein synthesis. PTH (1-34), PTHrP (1-34), and (Bu)(2)cAMP also suppressed Ihh expression. On the other hand, Ihh has been reported to induce PTHrP synthesis in the perichondrium. Consequently, the direct inhibitory action of PTH/PTHrP on Ihh appears to be a negative feedback mechanism that prevents excess PTHrP accumulation in cartilage.

PTHrP modulates chondrocyte differentiation through AP-1 and CREB signaling

During the process of differentiation, chondrocytes integrate a complex array of signals from local or systemic factors like parathyroid hormone-related peptide (PTHrP), Indian hedgehog, bone morphogenetic proteins and transforming growth factor beta. While PTHrP is known to be a critical regulator of chondrocyte proliferation and differentiation, the signaling pathways through which this factor acts remain to be elucidated. Here we show that both cAMP response element-binding protein (CREB) and AP-1 activation are critical to PTHrP signaling in chondrocytes. PTHrP treatment leads to rapid CREB phosphorylation and activation, while CREB DNA binding activity is constitutive. In contrast, PTHrP induces AP-1 DNA binding activity through induction of c-Fos protein expression. PTHrP activates CRE and TRE reporter constructs primarily through PKA-mediated signaling events. Both signaling pathways were found to be important mediators of PTHrP effects on chondrocyte phenotype. Alone, PTHrP suppresses maturation and stimulates proliferation of the chondrocyte cultures. However, in the presence of dominant negative inhibitors of CREB and c-Fos, these PTHrP effects were suppressed, and chondrocyte maturation was accelerated. Moreover, in combination, the effects of dominant negative c-Fos and CREB are synergistic, suggesting interaction between these signaling pathways during chondrocyte differentiation.

Tel-2 is a novel transcriptional repressor related to the Ets factor Tel/ETV-6

We report here the isolation of Tel-2, a novel member of the Ets transcription factor family, with high homology to Tel/ETV-6. Tel-2 is the second mammalian member of the Tel Ets family subclass whose prototype Tel is involved in various chromosomal translocations in human cancers. Six differentially expressed alternative splice products of Tel-2 were characterized encoding different Tel-2 isoforms which either contain or lack the amino-terminal Pointed domain and also vary at the carboxyl terminus. In contrast to Tel, which is highly expressed in several different cell types and tissues, Tel-2 is only weakly expressed in a variety of tissues and cell types, including placenta, prostate, spleen, liver, and lung. Tel-2 binds to functionally relevant Ets-binding sites of several genes and only the Tel-2 isoform containing the Pointed domain and the DNA-binding domain acts as a strong repressor of transcription. The retinoic acid receptor alpha and bone morphogenetic protein-6B (BMP-6) genes are specifically repressed by Tel-2 indicating a function for Tel-2 as an inhibitor of differentiation. Due to the important involvement of Tel in human cancer and the location of Tel-2 within the MHC cluster region, Tel-2 might be involved in chromosomal translocations in human cancer as well.

Epigenetic regulation of human bone morphogenetic protein 6 gene expression in prostate cancer

Bone morphogenetic proteins (BMPs), belonging to the transforming growth factor-beta (TGF-beta) superfamily, are multifunctional molecules that regulate bone induction and organ development. Among BMPs, BMP-6 has been shown to be overexpressed in prostate cancer and is speculated to be associated with bone-forming skeletal metastasis. We investigated the regulatory mechanism of the BMP-6 gene expression in prostate cancer cell lines DU-145, LNCaP, PC-3, and PC-3M with regard to the methylation status of the CpG island in the 5' flanking region of the human BMP-6 gene. By sequence-specific analysis of methylated cytosines, we show here that the methylation status of the CpG loci around the Sp1 site of the BMP-6 promoter is related to its steady-state expression and an alternative splicing of messenger RNA (mRNA) in prostate cancer cell lines. Furthermore, a study of clinical cases of benign and malignant prostate lesion by in situ hybridization showed that BMP-6 expression was high at both primary and secondary sites in cases of advanced cancer with metastasis. Demethylation of the CpG loci around the Spl binding site was shown in cases with high BMP-6 expression by sequencing analysis of the methylated cytosine from paraffin-embedded materials. Our results suggested that during cancer progression, besides inactivation of tumor suppressor genes by hypermethylation, activation of certain genes like BMP-6 by selective demethylation was a common epigenetic event giving a variable character to the invading and metastasizing cancer cells.

Distribution of genes for bone morphogenetic protein-4, -6, growth differentiation factor-5, and bone morphogenetic protein receptors in the process of experimental spondylosis in mice

OBJECT

Because little is known about the molecular mechanisms underlying the process of spondylosis, the authors examined the extent of genetic localization of several members of bone morphogenetic protein (BMP) and BMP receptors in chondrogenesis during the process of inducing spondylosis in their previously established experimental mice model.

METHODS

Experimental spondylosis was induced in 5-week-old ICR mice. The cervical spine was harvested chronologically, and histological sections were prepared. Messenger RNA for BMP-4, growth and differentiation (GDF)-5, BMP-6, and BMP receptors (ALK-3, -6, and BMP-RII) was localized in the tissue sections by in situ hybridization. In the early stage, BMP-4-derived mRNA was localized mainly in cells in the anterior margin of the cervical discs, together with ALK-6 and BMP-RII mRNA. No GDF-5 and BMP-6 mRNA was detected at this stage. In the late stage, cells positive for BMP-4 decreased, whereas GDF-5 and BMP-6 mRNA were localized in cells undergoing chondrogenesis. The ALK-3 mRNA began to appear in this stage, as did ALK-6 and BMP-RII.

CONCLUSIONS

The localization of transcripts for BMP-4, -6, and GDF-5 as well as BMP receptors shown during the present experimental model indicate the possible involvement of molecular signaling by these BMPs in the chondrogenic progress in spondylosis.

BMP signaling stimulates chondrocyte maturation and the expression of Indian hedgehog

Mutant BMP receptors were transfected into cultured embryonic upper sternal chrondrocytes using retroviral vectors to determine if BMP signaling is required for chondrocyte maturation and the expression of a key regulatory molecule, Indian hedgehog (Ihh). Chondrocytes infected with replication competent avian retroviruses (RCAS) viruses carrying constitutive active (CA) BMPR-IA and BMPR-IB had enhanced expression of type X collagen and Ihh mRNA. Addition of PTHrP, a known inhibitor of chondrocyte maturation, abolished the expression of type X collagen, BMP-6, and Ihh mRNAs in control cells. In contrast, PTHrP treated cultures infected with of CA BMPR-IA or CA BMPR-IB had low levels of BMP-6 and type X collagen, but high levels of Ihh expression. Although dominant negative (DN) BMPR-IA had no effect, DN BMPR-IB inhibited the expression of type X collagen and BMP-6, and decreased alkaline phosphatase activity, even in the presence of exogenously added BMP-2 and BMP-6. DN BMPR-IB also completely blocked Ihh expression. Overall, the effect of DN BMPR-IB mimicked the effects of PTHrP. To determine if there is an autocrine role for the BMPs in chondrocyte maturation, the cultures were treated with noggin and follistatin, molecules that bind BMP-2/-4 and BMP-6/-7, respectively. While noggin and follistatin inhibited the effects of recombinant BMP-2 and BMP-6, respectively, they had only minimal effects on the spontaneous maturation of chondrocytes in culture, suggesting that more than one subgroup of BMPs regulates chondrocyte maturation. The results demonstrate that: (i) BMP signaling stimulates chondrocyte maturation; (ii) BMP signaling increases Ihh expression independent of maturational effects; and (iii) BMP signaling can partially overcome the inhibitory effects of PTHrP on maturation.

Uncoupling of chondrocyte death and vascular invasion in mouse galectin 3 null mutant bones

Galectin 3 is a beta-galactoside binding protein which localizes to the cytoplasm of proliferative, mature, and hypertrophic chondrocytes in the growth plate cartilage of developing long bones. To elucidate the function of galectin 3 during bone development, we examined the epiphyseal femurs and tibias of fetal mice carrying a null mutation for the galectin 3 gene. Detailed histological and ultrastructural studies identified abnormalities in the cells of the proliferative, mature, and hypertrophic zones and in the extracellular matrix of the hypertrophic zone, as well as a reduction in the total number of hypertrophic chondrocytes. The expression patterns of several chondrocyte and bone cell markers were analyzed and revealed a subtle modification of Ihh expression in the galectin 3 mutant growth plate. A striking difference was observed at the chondrovascular junction where many empty lacunae are present. In addition, large numbers of condensed chondrocytes exhibiting characteristic signs of cell death were found in the late hypertrophic zone, indicating that the rate of chondrocyte death is increased in the mutants. These results suggest a role for galectin 3 as a regulator of chondrocyte survival. In addition, this unique phenotype shows that the elimination of chondrocytes and vascular invasion can be uncoupled and indicates that galectin 3 may play a role in the coordination between chondrocyte death and metaphyseal vascularization.

Smad2 and 3 mediate transforming growth factor-beta1-induced inhibition of chondrocyte maturation

Transforming growth factor-beta (TGF-beta) is a multifunctional regulator of a variety of cellular functions, including proliferation, differentiation, matrix synthesis, and apoptosis. In growth plate chondrocytes, TGF-beta slows the rate of maturation. Because the current paradigm of TGF-beta signaling involves Smad proteins as downstream regulators of target genes, we have characterized their role as mediators of TGF-beta effects on chondrocyte maturation. Both Smad2 and 3 translocated to the nucleus upon TGF-beta1 signaling, but not upon BMP-2 signaling. Cotransfection experiments using the TGF-beta responsive and Smad3 sensitive p3TP-Lux luciferase reporter demonstrated that wild-type Smad3 potentiated, whereas dominant negative Smad3 inhibited TGF-beta1 induced luciferase activity. To confirm the role of Smad2 and 3 as essential mediators of TGF-beta1 effects on chondrocyte maturation, we overexpressed both wild-type and dominant negative Smad2 and 3 in virally infected chondrocyte cultures. Overexpression of both wild-type Smad2 and 3 potentiated the inhibitory effect of TGF-beta on chondrocyte maturation, as determined by colx and alkaline phosphatase activity, whereas dominant negative Smad2 and 3 blocked these effects. Wild-type and dominant negative forms of Smad3 had more pronounced effects than Smad2. Our results define Smad2 and 3 as key mediators of the inhibitory effect of TGF-beta1 signaling on chondrocyte maturation.

Sox-4 messenger RNA is expressed in the embryonic growth plate and regulated via the parathyroid hormone/parathyroid hormone-related protein receptor in osteoblast-like cells

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) exert potent and diverse effects in cells of the osteoblastic and chondrocytic lineages. However, downstream mediators of these effects are characterized inadequately. We identified a complementary DNA (cDNA) clone encoding the 5' end of the transcription factor Sox-4, using a subtracted cDNA library enriched in PTH-stimulated genes from the human osteoblast-like cell line OHS. The SOX-4 gene is a member of a gene family (SOX and SRY) comprising transcription factors that bind to DNA through their high mobility group (HMG)-type binding domain, and previous reports have implicated Sox proteins in various developmental processes. In situ hybridization of fetal and neonatal mouse hindlimbs showed that Sox-4 messenger RNA (mRNA) was expressed most intensely in the zone of mineralizing cartilage where chondrocytes undergo hypertrophy, and by embryonic day 17 (ED17), after the primary ossification center was formed, its expression was detected only in the region of hypertrophic chondrocytes. Sox-4 mRNA was detected in osteoblast-like cells of both human and rodent origin. In OHS cells, physiological concentrations (10(-10)-10(-9) M) of human PTH 1-84 [hPTH(1-84)] and hPT(1-34), but not hPTH(3-84), stimulated Sox-4 mRNA expression in a time-dependent manner, indicating involvement of the PTH/PTHrP receptor. Sox-4 transcripts also were detected in various nonosteoblastic human cell lines and tissues, in a pattern similar to that previously reported in mice. The presence of Sox-4 mRNA in hypertrophic chondrocytes within the mouse epiphyseal growth plate at sites that overlap or are adjacent to target cells for PTH and PTHrP, and its strong up-regulation via activated PTH/PTHrP receptors in OHS cells, makes it a promising candidate for mediating downstream effects of PTH and PTHrP in bone.

Bone morphogenetic protein (BMP) localization in developing human and rat growth plate, metaphysis, epiphysis, and articular cartilage

We assessed the distribution and relative staining intensity of bone morphogenetic protein (BMP)-1-7 by immunohistochemistry in tibial growth plates, epiphyses, metaphyses, and articular cartilage in one 21-week and one 22-week human fetus and in five 10-week-old Sprague-Dawley rats. In the rats, articular cartilage was also examined. BMP proteins were mostly cytoplasmic, with negligible matrix staining. Highest BMP levels were seen in (a) hypertrophic and calcifying zone chondrocytes of growth plate (BMP-1-7), (b) osteoblasts and/or osteoprogenitor fibroblasts and vascular cells of the metaphyseal cortex and medulla (BMP-1-6), (c) osteoclasts of the metaphysis and epiphysis (BMP-1,-4,-5, and -6), and (d) mid to deep zone articular chondrocytes of weanling rats (BMP-1-7). BMP staining in osteoclasts, an unexpected finding, was consistently strong with BMP-4, -5, and -6 but was variable and dependent on osteoclast location with BMP-2,-3, and -7. BMP-1-7 were moderately to intensely stained in vascular canals of human fetal epiphyseal cartilage by endothelial cells and pericytes. BMP-1,-3,-5,-6, and -7 were localized in hypertrophic chondrocytes adjacent to cartilage canals. We conclude that BMP expression is associated with maturing chondrocytes of growth plate and articular cartilage, and may play a role in chondrocyte differentiation and/or apoptosis. BMP appears to be expressed by osteoclasts and might be involved in the intercellular "cross-talk" between osteoclasts and neighboring osteoprogenitor cells at sites of bone remodeling.

Utilization of bone morphogenetic protein receptors during chondrocyte maturation

Cartilage from the upper, cephalic portion of embryonic chick sternums undergoes hypertrophy, while the lower, caudal portion of the sternum remains as cartilage. Bone morphogenetic proteins (BMPs) induce type X collagen (colX) in cultured upper but not lower sternal chondrocytes (LSCs). We have examined the utilization of BMP receptors (BMPRs) by upper sternal chondrocytes (USCs) and LSCs both by analyzing receptor expression and by overexpressing mutant BMPRs. Reverse-transcription polymerase chain reaction (RT-PCR) analyses indicate that both upper and lower chondrocytes produce messenger RNA (mRNA) for all three receptors: BMPR type IA (BMPR-IA), BMPR type IB (BMPR-IB), and BMPR type II (BMPR-II). Infection of USC with retroviral vectors expressing constitutively active (CA) BMPRs showed that CA-BMPR-IB, like exogenous BMP-4, induced both colX mRNA and elevated alkaline phosphatase (AP), while CA-BMPR-IA was markedly less potent. However, expression of activated receptors in LSC cultures resulted in only minimal induction of hypertrophic markers. Consistent with the results seen for CA receptors, dominant negative (DN) BMPR-IB blocked BMP-induced hypertrophy in USCs more effectively than DN-BMPR-IA. These results imply that the major BMPR required for BMP induction of chondrocyte hypertrophy is BMPR-IB, and that difference between permanent and prehypertrophic chondrocytes is not caused by absence of receptors required for BMP signaling.

Analysis of cartilage maturation using micromass cultures of primary chondrocytes

A micromass culture (MM-C) system of primary immature chondrocytes for functional analysis of soluble factors involved in the maturation step of cartilage was previously developed. Ectopically expressed BMP-2 was shown to induce the expression of the Ihh and Noggin genes. Here it is demonstrated that, upon longer culture, secreted bone morphogenetic protein-2 (BMP-2) further promotes the maturation step as judged by the induction of type X collagen and BMP-6 expression, which are known to be detectable in the later phase of cartilage maturation. Induction of all of these genes by secreted BMP-2 was not inhibited by ectopic expression of parathyroid hormone-related peptide (PTHrP) induced by retrovirus vector infection, although the same virus vector showed strong inhibitory effects on the expression of type X collagen gene or alkaline phosphatase activity in mature chondrocytes. These results suggest that the maturation-promoting activity exhibited by BMP-2 is dominant over the suppressive effect of PTHrP in immature chondrocytes. When the BMP-6 gene was introduced into the same virus vector as that used for BMP-2, it induced the same sets of genes (Ihh, Noggin, type X collagen and endogenous BMP-6) as BMP-2 did. These results also suggest that BMP-6 would autonomously maintain and/or promote a later stage of chondrocytic maturation.

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.

PTHrP expression in chondrocytes, regulation by TGF-beta, and interactions between epiphyseal and growth plate chondrocytes

Although PTHrP has been identified as a key regulator of chondrocyte differentiation in the growth plate, the factors directly regulating PTHrP expression have not been identified. Furthermore, while cells from the epiphysis are considered the physiologic source of PTHrP, the relative expression of PTHrP in epiphyseal and growth plate chondrocytes has not been defined. PTHrP expression was examined in chondrocytes isolated from 3- to 5-week-old chick long bones. The expression of PTHrP mRNA was 10-fold higher in epiphyseal chondrocytes compared to cells from the growth plate. Growth plate chondrocytes were isolated into populations with distinct maturational characteristics by countercurrent centrifugal elutriation and analyzed for PTHrP expression. The expression was highest in the least mature cells and progressively declined with the onset of maturation. The regulation of PTHrP expression was further examined in epiphyseal chondrocytes. Both TGF-beta1 and cis-retinoic acid stimulation markedly increased PTHrP mRNA levels, while BMP-2 and PTHrP stimulation decreased the expression of this transcript. The effects of TGF-beta1 (8.9-fold stimulation) and TGF-beta3 (9.2-fold) were slightly greater than the effects of TGF-beta2 (4.9-fold). The effect of TGF-beta was dose-dependent and increases could be detected after 68 h of treatment. To analyze the paracrine effect of epiphyseal and growth plate chondrocytes on each other, these cells were placed in coculture and the mRNA from each of the populations was harvested separately after 24 h. Following coculture the PTHrP mRNA levels increased in the epiphyseal cells while the expression of type X collagen and Indian hedgehog transcripts decreased in growth plate chondrocytes. The results demonstrate potentially important paracrine interactions between these cell populations, possibly mediated by TGF-beta and PTHrP.

Anabolic and destructive mediators in osteoarthritis

Osteoarthritis is a joint disease that is characterized by focal degradation of articular cartilage. In addition to the degeneration of articular cartilage, attempts at repair are found in the affected tissue. Cartilage cells (chondrocytes) play a key role, not only in the destructive process, but also in the repair response. It has become apparent that anabolic and catabolic mediators, released from chondrocytes themselves or from other joint cells, drive both destructive and repair activities in the osteoarthritic joint.

Bone morphogenetic protein-4 regulates its own expression in cultured osteoblasts

During development, bone morphogenetic proteins (BMPs) induce the differentiation of mesenchymal progenitor cells to enter into the osteoblastic lineage, and BMPs enhance osteoblastic function. BMPs and noggin, a specific binding protein that blocks BMP actions, are expressed by osteoblastic cells but there is limited information about regulation of BMP synthesis in skeletal cells. We tested for the expression and regulation of BMP-4 in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). BMP-4 caused a short-lived increase in BMP-4 mRNA followed by a marked inhibition of BMP-4 expression. The stimulatory effect was transcriptional, as determined by nuclear run-on assays, whereas the inhibitory effect was transcriptional and posttranscriptional, because longer BMP-4 exposure decreased its rate of transcription and shortened the half-life of BMP-4 mRNA in transcriptionally arrested Ob cells. BMP-2 and BMP-6 also inhibited BMP-4 mRNA levels. Transforming growth factor beta1 increased, whereas fibroblast growth factor-2, platelet-derived growth factor BB, and insulin-like growth factor I decreased BMP-4 mRNA in Ob cells. BMP-2 also was expressed by Ob cells and it was downregulated by BMP-2, BMP-4, and BMP-6. Noggin increased BMP-4 transcripts, suggesting autocrine control of BMP-4 expression. In conclusion, BMP-4 inhibits its own expression in Ob cells, a mechanism to limit BMP availability to osteoblasts.

Expression of the components of the insulin-like growth factor axis across the growth-plate

Linear bone growth occurs as the result of proliferation and differentiation of growth-plate chondrocytes. These two phases of chondrocyte growth are regulated separately, with insulin-like growth factor I (IGF-I) being the primary stimulator of proliferation. We studied the expression of the components of the growth hormone GH/IGF system to learn if this proliferative signal is altered as chondrocytes undergo differentiation. Growth-plate chondrocytes were isolated from fetal cows and fractionated on discontinuous Percoll gradients. Five populations were recovered, ranging from high density cells (proliferative chondrocytes) to low density cells (hypertrophic chondrocytes). Messenger RNAs (mRNAs) were analyzed by a reverse transcriptase/quantitative polymerase chain reaction (RT/qPCR) technique. Results showed that mRNA of IGF-I and IGF-II in proliferative chondrocytes was 32 and five fold more abundant, respectively, than in hypertrophic chondrocytes. Of the four major IGF-I mRNA transcripts, the class 1-Ea transcript was predominant. Messenger RNA levels for IGFBP-3, -4, and -5 were also reduced in hypertrophic chondrocytes. Levels of GH receptor, the type 1 IGF receptor, and IGF binding protein-2 (IGFBP-2) mRNAs were unchanged across the growth-plate. Since IGF-I and -II are potent stimulators of proliferation, the down-regulation of these genes may be necessary in order for hypertrophy to proceed.