Demonstration of receptors for parathyroid hormone on cultured rabbit costal chondrocytes

Functional characterization of the parathyroid hormone 1 receptor in human periodontal ligament cells

OBJECTIVES

Intermittent parathyroid hormone (PTH) exerts anabolic effects on bone and has been approved for osteoporosis therapy. The dual actions of PTH are mediated primarily through the parathyroid hormone 1 receptor (PTH1R). Upon ligand binding, PTH1R activates diverse signaling pathways, including cAMP/protein kinase A (PKA)- and phospholipase C/protein kinase C (PLC/PKC)-dependent pathways. PTH1R has been abundantly studied in bone cells. Knowledge on PTH1R characteristics and physiology in periodontal ligament (PDL) cells is still in its infancy.

MATERIALS AND METHODS

We characterized PTH1R in PDL cells in terms of its cellular localization, binding affinity, and signal transduction and compared these characteristics to those of MG63 osteoblast-like cells.

RESULTS

PTH1R mRNA/protein was identified in PDL and MG63 cells. PTH1R was mainly localized on the plasma membrane, in vesicular structures inside the cell, and, to some extent, in the nucleus of both cell types. Binding characteristics of PTH1R were cell type specific, with PDL cells demonstrating a lower binding affinity. The response of cAMP and active PKC production in MG63 cells was dose dependent with increasing PTH(1-34) concentration, whereas in PDL cells, it was regulated biphasically. However, we observed a cross talk between the cAMP/PKA and PLC/PKC signaling pathways, which were regulated diametrically opposed at a given concentration of PTH(1-34).

CONCLUSION

These data indicate that, albeit the similarity in its subcellular distribution, PTH1R in PDL cells exhibits characteristics different from those in MG63 cells, pointing to the cell type specificity of this receptor.

CLINICAL RELEVANCE

The findings further elucidate the characteristics of PTH action in dental tissues and widen the theoretical basis for the development of anabolic treatment strategies.

Parathyroid hormone and transforming growth factor-beta1 coregulate chondrocyte differentiation in vitro

Parathyroid hormone (1-34) (PTH(1-34) and transforming growth factor-beta1 (TGF-beta1) regulate chondrocyte proliferation, differentiation, and matrix synthesis. Both proteins mediate their effects in a dose- and time-dependent manner, and the effects are cell maturation specific. Moreover, similar signaling pathways are used, suggesting that there may be cross talk leading to coregulated cell response. To test this hypothesis, confluent cultures of rat costochondral resting zone and growth zone chondrocytes were treated with 0.22, 0.44, or 0.88 ng/mL of rhTGF-beta1 for 24 h, followed by treatment with 10(-11) to 10(-8) M PTH(1-34) for 10 min or 24 h. [3H]-Thymidine incorporation, specific activity of alkaline phosphatase (AP), and [35S]-sulfate incorporation were measured. PTH(1-34) had no effect on [3H]-thymidine incorporation by growth zone cells pretreated with 0.22 or 0.44 ng/mL of TGF-beta1, but in cultures treated with 0.88 ng/mL, PTH(1-34) caused a dose-dependent decrease that was maximal at the lowest concentration tested. By contrast, PTH(1-34) stimulated [3H]-thymidine incorporation by resting zone cells, and this effect was additive with the stimulation caused by 0.22 ng/mL of TGF-beta1. PTH(1-34) caused a synergistic increase in AP in growth zone cells treated with 0.44 or 0.88 ng/mL of TGF-beta1, but not in cells treated with 0.22 ng/mL of TGF-beta1. It had no effect on AP in resting zone cells pretreated with any concentration of TGF-beta1. PTH(1-34) increased [35S]-sulfate incorporation in growth zone and resting zone cell cultures treated with 0.22 ng/mL of TGF-beta1 to levels seen in cultures treated with 0.88 ng/mL of TGF-beta1 alone. These results support the hypothesis that PTH(1-34) and TGF-beta1 coregulate growth plate chondrocytes and that the effects are cell maturation dependent.

Parathyroid hormone-related protein regulates extracellular matrix gene expression in cementoblasts and inhibits cementoblast-mediated mineralization in vitro

Parathyroid hormone-related protein (PTHrP) has been implicated in regulating tooth eruption and/or development. Formation of cementum, a mineralized tissue covering the tooth root surface, is a critical biological event for tooth root development. To test the hypothesis that PTHrP targets cementoblasts (CMs) and acts to regulate cementogenesis, CM cell lines were established and their responsiveness to PTHrP stimulation was determined, in vitro. First, subclones were derived from two immortalized murine cell populations that contained CMs; SV-CM/periodontal ligament (PDL) cells were obtained from the root surface of first mandibular molars of CD-1 mice and immortalized with SV40 T-antigen (TAg), and OC-CM cell population was established from OC-TAg transgenic mice in which their cells harbor an osteocalcin (OC and/or OCN) promoter-driving immortal gene SV40 TAg. Based on our previous in situ studies, CM subclones were identified as cells expressing bone sialoprotein (BSP) and OCN transcripts, while PDL cell lines were designated as cells lacking BSP and OCN messenger RNA (mRNA). CMs exhibited a cuboidal appearance and promoted biomineralization, both in vitro and in vivo. In contrast, PDL cells (PDL subclones) displayed a spindle-shaped morphology and lacked the ability to promote mineralized nodule formation, both in vitro and in vivo. Next, using these subclones, the effect of PTHrP on cementogenesis was studied. CMs, not PDL cells, expressed PTH/PTHrP receptor mRNA and exhibited PTHrP-mediated elevation in cyclic adenosine monophosphate (cAMP) levels and c-fos gene induction. PTHrP stimulation repressed mRNA expression of BSP and OCN in CMs and blocked CM-mediated mineralization, in vitro. Collectively, these data suggest that CMs possess PTH/PTHrP receptors and, thus, are direct targets for PTHrP action during cementogenesis and that PTHrP may serve as an important regulator of cementogenesis.

Parathyroid hormone-(1-34) enhances aggrecan synthesis via an insulin-like growth factor-I pathway

During endochondral bone formation, the growth plate chondrocytes proliferate, become hypertrophic, lose the cartilage phenotype, undergo mineralization, and provide a scaffold upon which subsequent longitudinal bone growth occurs. Parathyroid hormone (PTH), a calcium-regulating hormone, and parathyroid hormone-related peptide (PTHrP), which shares several properties with PTH, have profound effects on skeletal growth and new bone formation. In order to define further the mechanism by which PTH/PTHrP promotes the cartilage phenotype, chondrocytes isolated from the rib cages of developing rat embryos were evaluated for the biosynthesis of aggrecan. Cells treated with PTH-(1-34) for a 4-h period followed by a 20-h recovery period showed a significant increase in cartilage proteoglycan (aggrecan) synthesis in a dose-dependent manner. Only N-terminally intact PTH and PTHrP were effective in stimulating aggrecan synthesis. Addition of a neutralizing antibody to insulin-like growth factor-I (IGF-I) during PTH treatment resulted in the inhibition of PTH-stimulated aggrecan synthesis, whereas the addition of a neutralizing antibody to insulin-like growth factor-binding protein-2 (IGFBP-2) resulted in an increase in synthesis in both the control and PTH-treated cells. In addition, PTH treatment resulted in an increase in the mRNA for aggrecan, a reduction in IGFBP-3 mRNA, and no discernible changes in IGF-I mRNA levels, which was complemented by quantitative changes in IGFBP-3 and free IGF-I levels. The reciprocal relationship in the expression of aggrecan and IGFBP was further confirmed in chondrocytes from various gestational stages during normal development. Collectively, our results indicate that the effect of PTH may be mediated at least in part through the regulation of the IGF/IGFBP axis, by a decrease in the level of IGFBP-3, and an increase in free IGF-I levels. It is likely that the local increase in IGF-I may lead to an increase in cartilage type proteoglycan synthesis and maintenance of the cartilage phenotype. The consequence of the prolonged maintenance may be to halt mineralization while a new scaffolding is created.

Rapid and long-term effects of PTH(1-34) on growth plate chondrocytes are mediated through two different pathways in a cell-maturation-dependent manner

The aims of this study were to clarify the role of cell maturation stage on chondrocyte response to parathyroid hormone (PTH) by examining the effect of PTH(1-34) on alkaline-phosphatase-specific activity (ALPase) of chondrocyte cultures at two distinct stages of maturation, and to determine the signaling pathways used by the cells to mediate this effect. Confluent, fourth passage rat costochondral resting zone (RC) and growth zone (GC) chondrocytes were used. ALPase was measured in the cell layer, as well as in matrix vesicles (MV) and plasma membranes (PM), after the addition of 10(-7) 10(-11) mol/L bovine PTH(1-34), the active peptide, or bovine PTH(3-34), the inactive peptide, to the cultures. PTH(1-34) increased ALPase in the GC cultures at two separate times: between 5 and 180 min, with maximal stimulation at 10 min, and 36 to 48 h. In contrast, PTH(3-34) had no effect. At 10 min and 48 h, PTH(1-34) produced a dose-dependent increase in ALPase of both MV and PM isolated from GC cultures. Addition of forskolin and IBMX to increase cAMP increased ALPase in GC cultures to a level similar to that seen after addition of PTH(1-34). In contrast, the addition of PTH(1-34) to RC cells only increased ALPase between 5 and 60 min, with peak activity at 10 min. As with GC, PTH increased ALPase in both MV and PM. Moreover, the addition of PTH(3-34) or forskolin and IBMX had no effect on ALPase in RC. PTH(1-34) had no effect on GC protein kinase C (PKC) activity; however, the addition of PTH(1-34) to RC caused a dose-dependent increase in PKC activity. H8, an inhibitor of PKA, had no effect on PTH-stimulated ALPase in RC cells, but inhibited the PTH-dependent response in GC cells. In contrast, chelerythrine, an inhibitor of PKC activity, inhibited PTH-stimulated ALPase in RC cells, but had no effect on PTH-stimulated ALPase in GC cells. This study shows that the effect of PTH(1-34) on RC and GC cells is maturation dependent in terms of time course and mechanism. Whereas both cell types exhibit a rapid response to PTH, only GC cells show a long-term response. In GC, the effects of PTH are associated with changes in cAMP and may also involve at least one other pathway, whereas, in RC, the PTH effects appear to be associated with changes in PKC.

Structure and functional expression of a complementary DNA for porcine parathyroid hormone/parathyroid hormone-related peptide receptor

A complementary DNA (cDNA) encoding the receptor for porcine parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) was isolated from a porcine kidney cDNA library. The porcine PTH/PTHrP receptor is a 585 amino acid protein containing seven putative membrane-spanning domains. The porcine PTH/PTHrP receptor has amino acid identity of 95.6%, 80.4%, and 88.7% with human, opossum, and rat PTH/PTHrP receptors, respectively and 53.4% identity to the recently cloned human PTH2 receptor. The receptor cDNA was subsequently cloned into a mammalian cell expression vector (pRC/CMV) which contains a human cytomegalovirus promoter. A human kidney cell line (293), stably transfected with this vector, expressed the receptor at a high level and, when challenged with human PTH(1-34), increased cytoplasmic cAMP and inositol triphosphate production. Radioligand binding studies revealed that the receptor bound both human PTH(1-34), and PTHrP(1-36). Scatchard analyses of three clones showed that the cells harbor a single class of high affinity receptor (Kd = 1-4 nM for human PTH(1-34)) but had varying receptor numbers (10(5)-10(6) receptors/cell). In contrast to PTH(1-34), the [Arg2]PTH(1-34) analog bound to the porcine PTH/PTHrP receptor with low affinity and was a weak agonist for cAMP stimulation with the cloned receptor. These response characteristics differentiate the porcine receptor from the previously cloned rat and opossum PTH/PTHrP receptors.

Chondrogenic differentiation of clonal mouse embryonic cell line ATDC5 in vitro: differentiation-dependent gene expression of parathyroid hormone (PTH)/PTH-related peptide receptor

The regulatory role of parathyroid hormone (PTH)/PTH-related peptide (PTHrP) signaling has been implicated in embryonic skeletal development. Here, we studied chondrogenic differentiation of the mouse embryonal carcinoma-derived clonal cell line ATDC5 as a model of chondrogenesis in the early stages of endochondral bone development. ATDC5 cells retain the properties of chondroprogenitor cells, and rapidly proliferate in the presence of 5% FBS. Insulin (10 micrograms/ml) induced chondrogenic differentiation of the cells in a postconfluent phase through a cellular condensation process, resulting in the formation of cartilage nodules, as evidenced by expression of type II collagen and aggrecan genes. We found that differentiated cultures of ATDC5 cells abundantly expressed the high affinity receptor for PTH (Mr approximately 80 kD; Kd = 3.9 nM; 3.2 x 10(5) sites/cell). The receptors on differentiated cells were functionally active, as evidenced by a PTH-dependent activation of adenylate cyclase. Specific binding of PTH to cells markedly increased with the formation of cartilage nodules, while undifferentiated cells failed to show specific binding of PTH. Northern blot analysis indicated that expression of the PTH/PTHrP receptor gene became detectable at the early stage of chondrogenesis of ATDC5 cells, preceding induction of aggrecan gene expression. Expression of the PTH/PTHrP receptor gene was undetectable in undifferentiated cells. The level of PTH/PTHrP receptor mRNA was markedly elevated parallel to that of type II collagen mRNA. These lines of evidence suggest that the expression of functional PTH/PTHrP receptor is associated with the onset of chondrogenesis. In addition, activation of the receptor by exogenous PTH or PTHrP significantly interfered with cellular condensation and the subsequent formation of cartilage nodules, suggesting a novel site of PTHrP action.

Interaction of basic fibroblast growth factor with bovine growth plate chondrocytes

The basic fibroblast growth factor (bFGF) family of peptides influences a wide range of cellular actions. To better understand the possible role of bFGF in the growth plate, we have characterized the interaction of this growth factor with isolated bovine growth plate chondrocytes. Basic FGF interacts with two classes of binding sites on these cells. One is consistent with high-affinity bFGF receptors and the other with low-affinity heparin-like binding sites on the chondrocyte surface. Radiolabeled bFGF binding studies revealed approximately 4 x 10(6) binding sites per cell, with a Kd of approximately 42 nM. Graded concentrations of heparin or NaCl competed with [125I]-labeled bFGF in a dose-dependent fashion, reducing [125I]-labeled bFGF binding by 75 and 97%, respectively. The data suggest the presence of a high-capacity, low-affinity class of binding sites with the properties of a heparin-like moiety. Affinity cross-linking of [125I]-labeled bFGF to chondrocytes labeled two principal species with apparent molecular masses of 135 and 160 kDa. Labeled bFGF was specifically displaced from both species by subnanomolar concentrations of unlabeled bFGF. These high-affinity, low-capacity binding sites are characteristic of classical bFGF receptors. Binding of [125I]-labeled bFGF to these sites was also influenced by heparin, consistent with coregulation of binding to the two classes of binding sites. The data suggest that bFGF participates in the regulation of skeletal growth at the growth plate and that this regulation may involve bFGF interaction with at least two distinct classes of binding sites.

Structural changes in condylar cartilage following prolonged exposure to the human parathyroid hormone fragment (hPTH) 1-34 in vitro

This investigation presents the structural changes in condylar cartilage incubated in the presence of human parathyroid hormone (1-34) in an organ culture system for 6 to 12 days. Control cultures maintained their cartilaginous characteristics whereas human parathyroid hormone (1-34)-treated cultures revealed the following modifications: (1) The chondroprogenitor cell zone at the apical region of the explant underwent a substantial enlargement. The cells changed from a mesenchyme-like morphology into polygonal, glycogen-rich cells that were tightly attached to each other by a fibrillar intercellular matrix, but even by 12 days the apical region was comprised of healthy cells. (2) The mineralizing zone in the hypertrophic cartilage revealed a change in its cellular population. Hypertrophic chondrocytes were replaced by cells with amoeboid extensions and large numbers of secretory granules or vesicles. Based upon the above findings it appears that the chondroprogenitor cells that are initially stimulated to proliferate, are being suppressed from subsequent differentiation into chondroblasts; and that hypertrophic chondrocytes apparently undergo a dedifferentiation process followed by development into an as yet unknown cell population.

Bone morphogenetic proteins (BMP-2 and BMP-3) promote growth and expression of the differentiated phenotype of rabbit chondrocytes and osteoblastic MC3T3-E1 cells in vitro

We studied the effects of highly purified bone morphogenetic protein 2 and 3 (BMP-2 and -3) on growth plate chondrocytes and osteoblastic cells in vitro and compared to TGF-beta. A mixture of BMP-2 and 3 (BMPs) strongly stimulated DNA synthesis of chondrocytes in the presence of fibroblast growth factor (FGF). BMPs induced rapid maturation of chondrocytes at a growing stage: BMPs transformed the cells into rounded cells and induced marked accumulation of cartilage matrix; TGF-beta slightly reduced matrix accumulation and changed cell morphology into spindle-like in the presence of FGF. Moreover, exposure of chondrocytes to BMPs resulted in a dramatic increase of the putative approximately 80 kD PTH receptors expressed on the cell surface. In multilayered chondrocytes at the calcifying stage, BMPs stimulated alkaline phosphatase (ALPase) activity but TGF-beta inhibited it. In osteoblastic MC3T3-E1 cells, BMPs were found to be the most potent stimulator of ALPase activity thus far described: ALPase in the cells treated with approximately 100 ng/ml of BMPs reached 5- to 20-fold over the basal, whereas TGF-beta inhibited expression of ALPase activity in these cells. The stimulatory action of BMPs overrode the inhibition of ALPase activity by TGF-beta when the cells were incubated with TGF-beta and BMPs. BMPs also upregulated expression of the approximately 80 kD PTH receptor on the cells. These results suggest that BMPs have unique biologic activities in vitro that lead to growth and phenotypic expression of cells playing a critical role in endochondral bone formation.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)