Regulation of PTH/PTH-related protein receptor expression by endogenous PTH-related protein in the rat osteosarcoma cell line ROS 17/2.8

The N- and C-terminal domains of parathyroid hormone-related protein affect differently the osteogenic and adipogenic potential of human mesenchymal stem cells

Parathyroid hormone-related protein (PTHrP) is synthesized by diverse tissues, and its processing produces several fragments, each with apparently distinct autocrine and paracrine bioactivities. In bone, PTHrP appears to modulate bone formation in part through promoting osteoblast differentiation. The putative effect of PTH-like and PTH-unrelated fragments of PTHrP on human mesenchymal stem cell (MSCs) is not well known. Human MSCs were treated with PTHrP (1-36) or PTHrP (107-139) or both (each at 10 nM) in osteogenic or adipogenic medium, from the start or after 6 days of exposure to the corresponding medium, and the expression of several osteoblastogenic and adipogenic markers was analyzed. PTHrP (1-36) inhibited adipogenesis in MSCs and favoured the expression of osteogenic early markers. The opposite was observed with treatment of MSCs with PTHrP (107-139). Moreover, inhibition of the adipogenic differentiation by PTHrP (1-36) prevailed in the presence of PTHrP (107-139). The PTH/PTHrP type 1 receptor (PTH1R) gene expression was maximum in the earlier and later stages of osteogenesis and adipogenesis, respectively. While PTHrP (107-139) did not modify the PTH1R overexpression during adipogenesis, PTHrP (1-36) did inhibit it; an effect which was partially affected by PTHrP (7-34), a PTH1R antagonist, at 1 microM. These findings demonstrate that both PTHrP domains can exert varying effects on human MSCs differentiation. PTHrP (107-139) showed a tendency to favor adipogenesis, while PTHrP (1-36) induced a mild osteogenic effect in these cells, and inhibited their adipocytic commitment. This further supports the potential anabolic action of the latter peptide in humans.

Over-expression of parathyroid hormone Type 1 receptor confers an aggressive phenotype in osteosarcoma

Osteosarcoma is the most common primary bone malignancy in children and is associated with rapid bone growth. Parathyroid hormone-related peptide (PTHrP) signaling via parathyroid hormone Type 1 receptor (PTHR1) is important for skeletal development and is involved in bone metastases in other tumors. The aim of this study was to investigate the status of PTHrP/PTHR1 and its possible role in osteosarcoma. In a preliminary screening, a higher level of PTHR1 mRNA, but not PTHrP, was found in 4 osteosarcoma xenografts as compared with 4 standard cell lines, or 5 patient derived cell lines (p < 0.05) using quantitative RT-PCR. It was therefore extended to 55 patient specimens, in which a significantly higher level of PTHR1 mRNA was detected in metastatic or relapsed samples than those from primary sites (p < 0.01). Cell behavior caused by PTHR1 overexpression was further studied in vitro using PTHR1 transfected HOS cell line as a model. Over-expression of PHTR1 resulted in increased proliferation, motility and Matrigel invasion without addition of exogenous PTHrP suggesting an autocrine effect. Importantly, the aggressiveness in PTHR1-expressing cells was completely reversed by RNAi mediated gene knockdown. In addition, PTHR1 over-expression led to delayed osteoblastic differentiation and upregulation of genes involved in extracellular matrix production, such as TGF-beta1 and connective tissue growth factor. When cocultured with bone marrow derived monocytes, PTHR1 transfected HOS cells induced a greater number of osteoclasts. This study suggests that PTHR1 over-expression may promote osteosarcoma progression by conferring a more aggressive phenotype, and forming a more favorable microenvironment.

Osteoblast-derived PTHrP is a potent endogenous bone anabolic agent that modifies the therapeutic efficacy of administered PTH 1-34

Mice heterozygous for targeted disruption of Pthrp exhibit, by 3 months of age, diminished bone volume and skeletal microarchitectural changes indicative of advanced osteoporosis. Impaired bone formation arising from decreased BM precursor cell recruitment and increased apoptotic death of osteoblastic cells was identified as the underlying mechanism for low bone mass. The osteoporotic phenotype was recapitulated in mice with osteoblast-specific targeted disruption of Pthrp, generated using Cre-LoxP technology, and defective bone formation was reaffirmed as the underlying etiology. Daily administration of the 1-34 amino-terminal fragment of parathyroid hormone (PTH 1-34) to Pthrp+/- mice resulted in profound improvement in all parameters of skeletal microarchitecture, surpassing the improvement observed in treated WT littermates. These findings establish a pivotal role for osteoblast-derived PTH-related protein (PTHrP) as a potent endogenous bone anabolic factor that potentiates bone formation by altering osteoblast recruitment and survival and whose level of expression in the bone microenvironment influences the therapeutic efficacy of exogenous PTH 1-34.

Parathyroid hormone-related protein overexpression in the human colon cancer cell line HT-29 enhances adhesion of the cells to collagen type I

The present experiments examined the potential ability of parathyroid hormone-related protein (PTHrP) to influence growth of the human colon cancer cell HT-29 and the ability of the cell to adhere to several extracellular matrix (ECM) proteins found in normal tissues. Addition of PTHrP analogs, PTHrP (1-34), PTHrP (67-86), or PTHrP (107-139), to HT-29 cells in culture did not influence cell growth or the adhesion of the cells to wells coated with fibronectin, laminin, or collagen type I. Likewise, in HT-29 cells induced to overexpress PTHrP by stable transfection with PTHrP cDNA, compared to vector-transfected control HT-29 cells, no effect on cell growth occurred. However, in the transfected cells, the increased production of PTHrP significantly enhanced cell adhesion to type I collagen but not to fibronectin or laminin. The results raise the possibility that PTHrP might play a role in colon tumor invasion and metastasis by influencing cell adhesion to specific extracellular matrix proteins.

Parathyroid hormone potentiates nucleotide-induced [Ca2+]i release in rat osteoblasts independently of Gq activation or cyclic monophosphate accumulation. A mechanism for localizing systemic responses in bone

The regulation of tissue turnover requires the coordinated activity of both local and systemic factors. Nucleotides exist transiently in the extracellular environment, where they serve as ligands to P2 receptors. Here we report that the localized release of these nucleotides can sensitize osteoblasts to the activity of systemic factors. We have investigated the ability of parathyroid hormone (PTH), a principal regulator of bone resorption and formation, to potentiate signals arising from nucleotide stimulation of UMR-106 clonal rat osteoblasts. PTH receptor activation alone did not lead to [Ca(2+)](i) elevation in these cells, indicating no G(q) coupling, however, activation of G(q)-coupled P2Y(1) receptors resulted in characteristic [Ca(2+)](i) release. PTH potentiated this nucleotide-induced Ca(2+) release, independently of Ca(2+) influx. PTH-(1-31), which activates only G(s), mimicked the actions of PTH-(1-34), whereas PTH-(3-34), which only activates G(q), was unable to potentiate nucleotide-induced [Ca(2+)](i) release. Despite this coupling of the PTHR to G(s), cAMP accumulation or protein kinase A activation did not contribute to the potentiation. 3-Isobutyl-1-methylxanthine, but not forskolin effectively potentiated nucleotide-induced [Ca(2+)](i) release, however, further experiments proved that cyclic monophosphates were not involved in the potentiation mechanism. Costimulation of UMR-106 cells with P2Y(1) agonists and PTH led to increased levels of cAMP response element-binding protein phosphorylation and a synergistic effect was observed on endogenous c-fos gene expression following costimulation. In fact the calcium responsive Ca/cAMP response element of the c-fos promoter alone was effective at driving this synergistic gene expression. These findings demonstrate that nucleotides can provide a targeted response to systemic factors, such as PTH, and have important implications for PTH-induced signaling in bone.