Ablation of the PTHrP gene or the PTH/PTHrP receptor gene leads to distinct abnormalities in bone development

Activated parathyroid hormone/parathyroid hormone-related protein receptor in osteoblastic cells differentially affects cortical and trabecular bone

Parathyroid hormone (PTH), an important regulator of calcium homeostasis, targets most of its complex actions in bone to cells of the osteoblast lineage. Furthermore, PTH is known to stimulate osteoclastogenesis indirectly through activation of osteoblastic cells. To assess the role of the PTH/PTH-related protein receptor (PPR) in mediating the diverse actions of PTH on bone in vivo, we generated mice that express, in cells of the osteoblastic lineage, one of the constitutively active receptors described in Jansen's metaphyseal chondrodysplasia. In these transgenic mice, osteoblastic function was increased in the trabecular and endosteal compartments, whereas it was decreased in the periosteum. In trabecular bone of the transgenic mice, there was an increase in osteoblast precursors, as well as in mature osteoblasts. Osteoblastic expression of the constitutively active PPR induced a dramatic increase in osteoclast number in both trabecular and compact bone in transgenic animals. The net effect of these actions was a substantial increase in trabecular bone volume and a decrease in cortical bone thickness of the long bones. These findings, for the first time to our knowledge, identify the PPR as a crucial mediator of both bone-forming and bone-resorbing actions of PTH, and they underline the complexity and heterogeneity of the osteoblast population and/or their regulatory microenvironment.

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.

The PTH/PTHrP receptor in Jansen's metaphyseal chondrodysplasia

JMC is a rare autosomal dominant form of short limb dwarfism characterized by asymptomatic hypercalcemia and skeletal deformities, despite low PTH and PTHrP levels. This rare disorder is likely to be caused by activating mutations in the PTH/PTHrP receptor leading to ligand-independent cAMP accumulation. The analysis of genetically altered mice which lack either PTHrP or the PTH/PTHrP receptor, as well as of transgenic mice in which the mutant receptor is targeted to the growth plate, has provided a molecular explanation for the severe skeletal abnormalities seen in JMC. In addition, the study of this rare human disorder has further elucidated the fundamental role played by the PTH/PTHrP receptor in mediating both the paracrine/autocrine actions of PTHrP in growth plate development and bone elongation, as well as the endocrine actions of PTH. The insight gained from the study of this human disease model is likely to continue to provide an important tool to define the cellular and molecular mechanisms that mediate the biological roles of the PTH, PTHrP and their receptor.

Nuclear and nucleolar localization of parathyroid hormone-related protein

Parathyroid hormone-related protein (PTHrP) was first discovered as the factor causing hypercalcaemia produced by solid tumours frequently associated with the head and neck, breast, lung and kidney. The homology of its amino-terminus to parathyroid hormone (PTH; eight of the first 13 residues are identical), enables it to share the same receptor and perform similar biological functions to PTH. The sequences of PTHrP C-terminal to its PTH-like region confer functions such as transplacental calcium transport, renal bicarbonate excretion and in vitro osteoclast inhibition. Recent findings have shown that PTHrP is a nuclear/nucleolar protein in certain tissues and that this localization is cell cycle-regulated, mediated by the middle portion of the molecule, and involves the nuclear import receptor importin beta1. The present review discusses what is known about the pathway by which PTHrP localizes to the nucleus/nucleolus and the putative roles it may have there.

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.