Parathyroid hormone [PTH(1-34)] and parathyroid hormone-related protein [PTHrP(1-34)] promote reversion of hypertrophic chondrocytes to a prehypertrophic proliferating phenotype and prevent terminal differentiation of osteoblast-like cells

The chondrocyte

The chondrocyte is the resident cell of cartilage that is a prominent tissue in the embryo acting as a template for the development of skeletal elements. In the adult, the distribution of permanent cartilage is much more restricted and is necessary for mechanical support, growth and movement. The cell is isolated within a voluminous extracellular matrix (ECM) that is neither vascularised nor innervated. As a result, nutrient/waste exchange occurs through diffusion and, consequently, under normal and pathological conditions, the cell is unique in its ability to exist in a low oxygen tension environment. Partly as a result of these properties, the tissue has a low reparative potential that, in the case of articular cartilage, predisposes the tissue to degenerative conditions such as arthritis that is a significant clinical problem. Cellfacts. Cytoplasmically isolated. High matrix/cell volume ratio. Do not divide after skeletal maturity unless during pathology. Major contributor to growth of the body. Most energy requirements obtained through glycolysis.

PTHrP, PTHr, and FGFR3 are involved in the process of endochondral ossification in human osteophytes

To elucidate the process of endochondral ossification in human osteophytes we have studied the expression of parathyroid hormone-related protein (PTHrP), its receptor (PTHr), and fibroblast growth factor receptor 3 (FGFR3). Osteophytes from patients undergoing total knee replacement ( n=13), and fetal growth plate cartilages ( n=4) were processed for safranin O staining and immunohistochemistry. Chondrocytes and their matrix were preferentially stained for PTHrP in the middle and deep zones of the osteophytes examined. Ossified areas did not show a positive staining. In fetal joints the cartilaginous surface and the perichondrium as well as the osteoblasts in the trabecular bone were positive. PTHr was expressed at large in chondrocytes and osteoblasts of all osteophytes and fetal joints. Cells of the perichondrium were also positive. The FGFR3 antibody stained only single chondrocytes in some osteophytes, and groups of cells in others. In fetal samples, chondrocytes of the proliferating and the hypertrophic zone showed staining for FGFR3. This is the first report on the expression of PTHrP, PTHr, and FGFR3 in human osteophytes. As in fetal joints these mediators might regulate proliferation and differentiation of chondrocytes playing an important role in osteo(chondro)phyte growth.

A chondrogenesis-related lipocalin cluster includes a third new gene, CALgamma

We have previously reported the modulation, during chondrogenesis and/or inflammation, of two chicken genes laying in the same genomic locus and coding for two polypeptides of the lipocalin protein family, the extracellular fatty acid binding protein (ExFABP) and the chondrogenesis associated lipocalin beta (CALbeta). A third gene, located within the same cluster and coding for a new lipocalin, CALgamma, has been identified and is here characterized. Tissue distribution analyzed by real-time quantitative reverse transcriptase-polymerase chain reaction in chicken embryos shows a ubiquitous expression with predominant levels of mRNA transcripts in the liver and the brain. In the developing tibia, a high expression of CALgamma mRNA was evidenced by in situ hybridization within the pre-hypertrophic and the hypertrophic zones of the bone-forming cartilage. In agreement, dedifferentiated chondrocytes in vitro express the transcripts to the highest level when they re-differentiate reaching hypertrophy. Such peculiar developmental pattern of expression that is analogous to those already described for Ex-FABP and CALbeta suggests that all three proteins may act synergistically in the process of endochondral bone formation. Moreover, like Ex-FABP and CALbeta, CALgamma is also highly induced in dedifferentiated chondrocytes upon stimulation with lypopolysaccharides, indicating that the whole cluster quite possibly is transcriptionally activated not only in physiological morphogenic differentiation but also in pathological acute phase response.

Establishment of a novel chondrocytic cell line N1511 derived from p53-null mice

We established a clonal chondrocytic cell line N1511 derived from rib cartilage of a p53-null mouse. N1511 cells proliferated in polygonal shape and elicited differentiation at confluence when treated with combination of bone morphogenetic protein (BMP) 2 and insulin or parathyroid hormone (PTH) and dexamethasone. BMP-2/insulin-treated cells became refractile without forming cartilaginous nodules and reached terminal differentiation, became positive for alizarin red staining, and developed considerable ALP activity. In contrast, PTH/dexamethasone-treated cells formed Alcian blue-positive nodules but remained negative for alizarin red staining and ALP activity. Northern blot analysis revealed that BMP-2/insulin-treated cells sequentially expressed type II, IX, and X collagens, whereas PTH/dexamethasone-treated cells slowly expressed type II collagen and then type IX, and they did not exhibit type X collagen expression. These results show that BMP-2/insulin treatment induces full differentiation toward hypertrophy, whereas treatment with PTH/dexamethasone slows and limits differentiation. Recovery of p53 expression in N1511 cells by transient transfection inhibited cell proliferation, suggesting that cell proliferation could be regulated with p53 in this cell line. These results indicate that N1511 is the only cell line with known genetic mutation, which undergoes multiple steps of chondrocyte differentiation toward hypertrophy, and because proliferation could be regulated by expression of p53, N1511 could be an excellent model for studies of chondrogenesis, the function of p53, and genetic engineering of cartilage tissue.

Chondrocyte protein with a poly-proline region is a novel protein expressed by chondrocytes in vitro and in vivo

Chondrogenic differentiation is a multistep process entailing the sequential activation and inhibition of the expression of a number of genes. To identify genes preferentially expressed at the hypertrophic stage rather than early differentiation stages of chicken chondrocyte differentiation, a subtracted cDNA library was generated. Here we describe the characterization of a cDNA isolated from this library and that of the encoded protein referred to as Chondrocyte Protein with a Poly-proline Region (CHPPR). The cDNA coding for CHPPR hybridizes with a 3.0-kb mRNA expressed at extremely low levels in dedifferentiated chondrocytes, cultured in adherent conditions, at low levels in differentiating chondrocytes and at very high levels in hypertrophic chondrocytes in suspension culture. The Parathyroid Hormone peptide [PTH (1-34)] enhances accumulation of CHPPR mRNA in cultured chondrocytes. This 3.0-kb mRNA is also detectable in several chick embryo tissues but at a lower extent when compared to that present in cartilage and in hypertrophic chondrocytes. The CHPPR cDNA has a complete open reading frame coding for a polypeptide with a calculated mass of 35.6 kDa containing a proline-rich region with a PPLP motif (single-letter amino acid code). We demonstrate by Western blot analysis that two CHPPR isoforms are detected in the cell lysates from cultured chondrocytes when they are not in the culture medium; furthermore, we find that the CHPPR gene is expressed in vivo by chick embryo chondrocytes at higher levels in the prehypertrophic and hypertrophic zones.

Parathyroid hormone and parathyroid hormone-related protein exert both pro- and anti-apoptotic effects in mesenchymal cells

During bone formation, multipotential mesenchymal cells proliferate and differentiate into osteoblasts, and subsequently many die because of apoptosis. Evidence suggests that the receptor for parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP), the PTH-1 receptor (PTH-1R), plays an important role in this process. Multipotential mesenchymal cells (C3H10T1/2) transfected with normal or mutant PTH-1Rs and MC3T3-E1 osteoblastic cells were used to explore the roles of PTH, PTHrP, and the PTH-1R in cell viability relative to osteoblastic differentiation. Overexpression of wild-type PTH-1R increased cell numbers and promoted osteocalcin gene expression versus inactivated mutant receptors. Furthermore, the effects of PTH and PTHrP on apoptosis were dramatically dependent on cell status. In preconfluent C3H10T1/2 and MC3T3-E1 cells, PTH and PTHrP protected against dexamethasone-induced reduction in cell viability, which was dependent on cAMP activation. Conversely, PTH and PTHrP resulted in reduced cell viability in postconfluent cells, which was also dependent on cAMP activation. Further, the proapoptotic-like effects were associated with an inhibition of Akt phosphorylation. These data suggest that parathyroid hormones accelerate turnover of osteoblasts by promoting cell viability early and promoting cell departure from the differentiation program later in their developmental scheme. Both of these actions occur at least in part via the protein kinase A pathway.

CALbeta, a novel lipocalin associated with chondrogenesis and inflammation

We have previously demonstrated the association of the chicken lipocalin Ex-FABP with cartilage formation and inflammatory responses as a marker of these processes (Descalzi Cancedda et al., Biochim. Biophys. Acta 1482, 127-135, 2000). Here we report the isolation and characterisation of a new lipocalin gene laying upstream the Ex-FABP, thus representing the second member of a possible genomic cluster. This gene contains an open reading frame coding for a polypeptide of about 19 kDa. The amino-acid sequence revealed a conserved lipocalin secondary structure. Tissue distribution of the protein in developing embryos showed a preferential expression in the heart although mRNA transcripts could be detected also in muscle, lung and liver. The lowest expression was observed in the stomach, brain and skin. During endochondral formation of long bones, the protein is differentially distributed, as the transcripts, evidenced in the tibia by in situ hybridisation, are present in the hypertrophic cone of the cartilage and mostly absent in the area of the proliferating chondrocytes. Such developmental regulation was observed also in vitro in cultured chondrocytes where the transcripts were barely detectable in dedifferentiated cells but highly expressed in hypertrophic chondrocytes. The protein was also significantly induced by lipopolysaccharide stimulation of chondrocytes, indicating a possible involvement in acute phase response. Raising specific antibodies in a rabbit allowed validating, at the protein level, all the transcriptional data. Moreover, we gained evidence that the protein is actively secreted in the extracellular matrix surrounding the chondrocytes. Because of its peculiar expression in cartilage, this new protein was named chondrogenesis-associated lipocalin beta (thereafter referred to as CAL beta). The close similarity between Ex-FABP and CAL beta expression patterns supports the hypothesis of a genomic organisation in a cluster where both genes could be co-ordinately regulated.

Co-expression of parathyroid hormone-related protein (PTHrP) and PTH/PTHrP receptor in cartilaginous tumours: a marker for malignancy?

AIM

Parathyroid hormone-related protein (PTHrP) is one of the critical factors for the differentiation and growth of chondrocytes. We examined the correlation between the co-expression of PTHrP and PTH/PTHrP receptor and the grade of malignancy in cartilaginous tumours.

METHODS

We analysed PTHrP and PTH/PTHrP receptor expression in chondrosarcoma by immunohistochemistry and compared specific staining with the expression in benign cartilaginous tumours. There were 38 cartilaginous bone tumours consisting of 26 chondrosarcoma, six enchondroma and six osteochondroma. Chondrosarcoma were composed of 20 conventional chondrosarcoma (10 grade 1, seven grade 2, and three grade 3), two dedifferentiated chondrosarcoma, two clear cell chondrosarcoma, and two myxoid chondrosarcoma. We performed the standard peroxidase-labelled streptavidin-biotin detection method for immunohistochemistry using an antibody raised against PTHrP (1-14) and PTH/PTHrP receptor. The magnitude of receptor positivity of PTHrP and PTH/ PTHrP in each tumour was assessed as a percentage of PTHrP and PTH/PTHrP-positive cells per thousand tumour cells in the most histologically aggressive area of the tumour.

RESULTS

All chondrosarcoma, five of six enchondroma, and four of six osteochondroma showed PTHrP-positive cells, and all chondrosarcoma, five of six enchondroma and five of six osteochondroma showed PTHrP receptor-positive cells. The grade of malignancy correlated with the percentage of both PTHrP and PTH/PTHrP receptor-positive tumour cells (P < 0.0001, either). Each grade of chondrosarcoma showed statistically higher expression of both PTHrP and PTH/PTHrP receptor than benign cartilaginous tumour.

CONCLUSION

This is the first report of the co-expression of PTHrP and PTH/PTHrP receptor in chondrosarcoma. PTHrP and PTH/PTHrP receptor positivity may be valuable for differentiating between benign and malignant cartilaginous tumours.

Selective anabolic effects of muteins of mid-region PTH fragments on skeletal tissues of prepubertal rats

We have demonstrated the net anabolic potential of a mid-region fragment of human parathyroid hormone (hPTH), and a protease resistant mutein derived from it, to stimulate growth of skeletal-derived tissues. The fragment hPTH (28-48), lacking the N-terminal amino acids necessary for stimulation of adenylate cyclase, and therefore unable to stimulate bone resorption by osteoclasts, was compared with the protease-resistant double-mutein hPTH (28-48) F34M L37T, full-length hPTH (1-84), the protease resistant form hPTH (1-84) L37T, 17beta estradiol (E(2)), and the combination of mid-region fragments of PTH and E(2). The hormones, at concentrations spanning a 100-fold range, were given by 14 injections (6/week, excluding Saturday), to 17-day-old female Wistar-derived rats. At the low concentration of 200 ng/day of PTH (1-84), or the molar equivalent of the fragment, and 50 ng E(2), all the hormones increased significantly the specific activity of creatine kinase (CK; a marker of skeletal cell proliferation) in tibial diaphysis and epiphysis, the width of the cortical bone in the humeral diaphysis, and the number of cells in the proliferating zone of the humeral epiphyseal growth plate. At a 10-fold lower concentration of both PTH and E(2), CK specific activity was synergistically stimulated in both diaphyseal bone and epiphyseal cartilage. However, PTH mid-region fragments at a dose of 1 microg/day did not increase trabecular bone volume.

Selective differentiation of mammalian bone marrow stromal cells cultured on three-dimensional polymer foams

Bone marrow stromal cells (BMSC) are pluripotent progenitor cells that can regenerate different skeletal tissues in response to environmental signals. In this study, we used highly porous, structurally stable three-dimensional polymer foams in conjunction with specific regulatory molecules to selectively differentiate mammalian BMSC into either cartilaginous or bone-like tissues. Bovine BMSC were expanded in monolayers and cultured on 5-mm-diameter, 2-mm-thick foams made of poly(lactic-co-glycolic acid) and poly(ethylene glycol). Constructs maintained their original size and shape for up to 4 weeks of culture and supported BMSC growth and production of extracellular matrix (ECM). By proper use of chondrogenic (dexamethasone, insulin, transforming growth factor-beta1) or osteogenic (dexamethasone, beta-glycerophosphate) medium supplements, we could control whether the generated ECM was cartilaginous (containing collagen type II and sulfated glycosaminoglycans) or bone-like (containing osteocalcin, osteonectin, and mineralized foci). After 4 weeks of cultivation, cartilaginous and bone-like ECM were uniformly distributed throughout the construct volume and respectively represented 34.2 +/- 9.3% and 12.6 +/- 3.2% of the total available area. BMSC culture on poly(lactic-co-glycolic acid)/poly(ethylene glycol) foams provides a three-dimensional model system to study the development of mesenchymal tissues in vitro and has potential applications in engineering autologous grafts for skeletal tissue repair.

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.

Mitogen-activated protein kinase p38 mediates regulation of chondrocyte differentiation by parathyroid hormone

Parathyroid hormone (PTH) and its related peptide regulate endochondral ossification by inhibiting chondrocyte differentiation toward hypertrophy. However, the intracellular pathway for transducing PTH/PTH-related peptide signals in chondrocytes remains unclear. Here, we show that this pathway is mediated by mitogen-activated protein kinase (MAPK) p38. Incubation of hypertrophic chondrocytes with PTH (1-34) induces an inhibition of p38 kinase activity in a time- and dose-dependent manner. Inhibition of protein kinase C prevents PTH-induced p38 MAPK inhibition, whereas inhibition of protein kinase A has no effect. Thus, protein kinase C, but not protein kinase A, is required for the inhibition of p38 MAPK by PTH. Treatment of hypertrophic chondrocytes by PTH or by p38 MAPK inhibitor SB203580 up-regulates Bcl-2, suggesting that Bcl-2 lies downstream of p38 MAPK in the PTH signaling pathway. Inhibition of p38 MAPK in hypertrophic chondrocytes by either PTH, SB303580, or both together leads to a decrease of hypertrophic marker type X collagen mRNA and an increase of the expression of prehypertrophic marker cartilage matrix protein. Therefore, inhibition of p38 converts a hypertrophic cell phenotype to a prehypertrophic one, thereby preventing precocious chondrocyte hypertrophy. Taken together, these data suggest a major role for p38 MAPK in transmitting PTH signals to regulate chondrocyte differentiation.

The effect of chemotherapy on the growing skeleton

With the increasing use of high dose (poly)chemotherapy schedules in the treatment of childhood cancer it is particularly important to know the adverse effects of these treatments. Growth is a complex mechanism affected not only by chemotherapy but also by the malignancy itself as well as nutritional status, the use of corticosteroids and (cranial) radiation. In vitro and animal studies are often the most useful in determining the effect of a single chemotherapeutic agent on the growing skeleton. In vitro studies have shown doxorubicin, actinomycin D and cisplatin to have a direct effect on growth plate chondrocytes that in animals results in decreased growth and final height. Clinical studies with multiagent chemotherapy have demonstrated that antimetabolites decrease bone growth and final height. Childhood cancer survivors are at risk of a reduced bone mineral density, mainly due to methotrexate, ifosfamide and corticosteroids. This reduced bone mineral density persists into adult life and may increase bone fracture risk at an older age.