Parathyroid hormone (PTH) down-regulates PTH/PTH-related protein receptor gene expression in UMR-106 osteoblast-like cells via a 3',5'-cyclic adenosine monophosphate-dependent, protein kinase A-independent pathway

Pleckstrin homology (PH) domain and Leucine Rich Repeat Phosphatase 1 (Phlpp1) Suppresses Parathyroid Hormone Receptor 1 (Pth1r) Expression and Signaling During Bone Growth

Endochondral ossification is tightly controlled by a coordinated network of signaling cascades including parathyroid hormone (PTH). Pleckstrin homology (PH) domain and leucine rich repeat phosphatase 1 (Phlpp1) affects endochondral ossification by suppressing chondrocyte proliferation in the growth plate, longitudinal bone growth, and bone mineralization. As such, Phlpp1-/- mice have shorter long bones, thicker growth plates, and proportionally larger growth plate proliferative zones. The goal of this study was to determine how Phlpp1 deficiency affects PTH signaling during bone growth. Transcriptomic analysis revealed greater PTH receptor 1 (Pth1r) expression and enrichment of histone 3 lysine 27 acetylation (H3K27ac) at the Pth1r promoter in Phlpp1-deficient chondrocytes. PTH (1-34) enhanced and PTH (7-34) attenuated cell proliferation, cAMP signaling, cAMP response element-binding protein (CREB) phosphorylation, and cell metabolic activity in Phlpp1-inhibited chondrocytes. To understand the role of Pth1r action in the endochondral phenotypes of Phlpp1-deficient mice, Phlpp1-/- mice were injected with Pth1r ligand PTH (7-34) daily for the first 4 weeks of life. PTH (7-34) reversed the abnormal growth plate and long-bone growth phenotypes of Phlpp1-/- mice but did not rescue deficits in bone mineral density or trabecular number. These results show that elevated Pth1r expression and signaling contributes to increased proliferation in Phlpp1-/- chondrocytes and shorter bones in Phlpp1-deficient mice. Our data reveal a novel molecular relationship between Phlpp1 and Pth1r in chondrocytes during growth plate development and longitudinal bone growth. © 2021 American Society for Bone and Mineral Research (ASBMR).

Sources of lumbar back pain during aging and potential therapeutic targets

Lumbar back pain during aging is a major clinical problem, the origins and underlying mechanisms of which are challenging to study. Degenerative changes occur in various parts of the functional spinal unit, such the vertebral endplate and intervertebral disc. The homeostasis of these structural components is regulated by signaling molecules, such as transforming growth factor-β and parathyroid hormone. Previous efforts to understand sources of lumbar back pain focused on sensory innervation in the degenerative intervertebral disc, but intervertebral disc degeneration is frequently asymptomatic. An in vivo mouse model of lumbar spine aging and degeneration, combined with genetic technology, has identified endplate innervation as a major source of lumbar back pain and a potential therapeutic target. In this review, we consider how each structural component of the functional spinal unit contributes to lumbar back pain, how the homeostasis of each component is regulated, and how these findings can be used to develop potential therapies.

Mangiferin suppresses human metastatic osteosarcoma cell growth by down-regulating the expression of metalloproteinases-1/2 and parathyroid hormone receptor 1

The study evaluates the protective effect of mangiferin on osteosarcoma cell proliferation and metastasis. Saos-2 and U2OS cells were treated with mangiferin (25, 50, 75 and 100 µM) for 72 h. Mangiferin reduced the cell viability, invasion, and cell adhesion and migration rate. Matrix metalloproteinases-2/9 (MMP-2/9) mRNA expression was reduced significantly, while the levels of tissue inhibitors of metalloproteinases-1/2 (TIMP-1/2) were elevated in Saos-2 and U2OS cells. Mangiferin treatment significantly reduced parathyroid hormone receptor 1 (PTHR1) mRNA and protein expression by more than 0.5-fold in both osteosarcoma cells. In addition, the immunofluorescent analysis also showed decreased PTHR1 expression following treatment with mangiferin. In summary, we have demonstrated that treatment with mangiferin reduces cell viability, proliferation, invasion, adhesion and migration, and induces apoptosis of osteosarcoma cells. Therefore, treatment with mangiferin can be effective agent in inhibiting growth and inducing apoptosis in osteosarcoma cells. Our experimental results provide evidence for the therapeutic effect of mangiferin in osteosarcoma cells.

Ciliary parathyroid hormone signaling activates transforming growth factor-β to maintain intervertebral disc homeostasis during aging

Degenerative disc disease (DDD) is associated with intervertebral disc degeneration of spinal instability. Here, we report that the cilia of nucleus pulposus (NP) cells mediate mechanotransduction to maintain anabolic activity in the discs. We found that mechanical stress promotes transport of parathyroid hormone 1 receptor (PTH1R) to the cilia and enhances parathyroid hormone (PTH) signaling in NP cells. PTH induces transcription of integrin α_vβ₆ to activate the transforming growth factor (TGF)-β-connective tissue growth factor (CCN2)-matrix proteins signaling cascade. Intermittent injection of PTH (iPTH) effectively attenuates disc degeneration of aged mice by direct signaling through NP cells, specifically improving intervertebral disc height and volume by increasing levels of TGF-β activity, CCN2, and aggrecan. PTH1R is expressed in both mouse and human NP cells. Importantly, knockout PTH1R or cilia in the NP cells results in significant disc degeneration and blunts the effect of PTH on attenuation of aged discs. Thus, mechanical stress-induced transport of PTH1R to the cilia enhances PTH signaling, which helps maintain intervertebral disc homeostasis, particularly during aging, indicating therapeutic potential of iPTH for DDD.

Sensory structures found in the jelly-like space between spinal discs release a hormone that helps preserve back health in aging mice. Xu Cao from Johns Hopkins University in Baltimore, Maryland, USA, and colleagues observed that levels of a critical growth factor declined in the space between adjacent vertebrae as mice aged, and that injecting a naturally occurring hormone that activates this growth factor could attenuate disc degeneration in older animals. The researchers showed, in response to mechanical stresses, receptor proteins that respond to this hormone relocate themselves to particular sensory organelles known as cilia that found within cells of the intervertebral core. That results in elevated hormone signaling—and drugs designed to have the same effect could help treat degenerative disc disease, one of the most common causes of chronic back pain.

Parathyroid hormone/parathyroid hormone-related peptide regulate osteosarcoma cell functions: Focus on the extracellular matrix (Review)

Osteosarcoma (OS) is a primary bone tumor of mesenchymal origin mostly affecting children and adolescents. The OS extracellular matrix (ECM) is extensively altered as compared to physiological bone tissue. Indeed, the main characteristic of the most common osteoblastic subtype of OS is non-mineralized osteoid production. Parathyroid hormone (PTH) is a polypeptide hormone secreted by the chief cells of the parathyroid glands. The PTH-related peptide (PTHrP) may be comprised of 139, 141 or 173 amino acids and exhibits considerate N-terminal amino acid sequence homology with PTH. The function of PTH/PTHrP is executed through the activation of the PTH receptor 1 (PTHR1) and respective downstream intracellular pathways which regulate skeletal development, bone turnover and mineral ion homeostasis. Both PTHR1 and its PTH/PTHrP ligands have been shown to be expressed in OS and to affect the functions of these tumor cells. This review aims to highlight the less well known aspects of PTH/PTHrP functions in the progression of OS by focusing on ECM-dependent signaling.

Intermittent parathyroid hormone (1-34) application regulates cAMP-response element binding protein activity to promote the proliferation and osteogenic differentiation of bone mesenchymal stromal cells, via the cAMP/PKA signaling pathway

The potential effects of intermittent parathyroid hormone (1-34) [PTH (1-34)] administration on bone formation have previously been investigated. A number of studies have suggested that the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway is associated with PTH-induced osteogenic differentiation. However, the precise signaling pathways and molecular mechanism by which PTH (1-34) induces the osteogenic differentiation of bone mesenchymal stromal cells (BMSCs) remain elusive. The purpose of the present study was to investigate the mechanism underlying the effect of intermittent PTH (1-34) application on the proliferation and osteogenic differentiation of BMSCs. BMSCs were randomly divided into four groups, as follows: Osteogenic medium (control group); osteogenic medium and intermittent PTH (1-34); osteogenic medium and intermittent PTH (1-34) plus the adenylyl cyclase activator forskolin; and osteogenic medium and intermittent PTH (1-34) plus the PKA inhibitor H-89. A cell proliferation assay revealed that PTH (1-34) stimulates BMSC proliferation via the cAMP/PKA pathway. Furthermore, reverse transcription-quantitative polymerase chain reaction, alkaline phosphatase activity testing and cell examination using Alizarin Red S staining demonstrated that PTH (1-34) administration promotes osteogenic differentiation and mineralization, mediated by the cAMP/PKA pathway. Crucially, the results of western blot analyses suggested that PTH (1-34) treatment and, to a greater degree, PTH (1-34) plus forskolin treatment caused an increase in phosphorylated cAMP response element binding protein (p-CREB) expression, but the effect of PTH on p-CREB expression was blocked by H-89. In conclusion, the current study demonstrated that intermittent PTH (1-34) administration regulates downstream proteins, particularly p-CREB, in the cAMP/PKA signaling pathway, to enhance the proliferation, osteogenic differentiation and mineralization of BMSCs.

Expression of osteoclastogenic factor transcripts in osteoblast-like UMR-106 cells after exposure to FGF-23 or FGF-23 combined with parathyroid hormone

As a bone-derived hormone, fibroblast growth factor-23 (FGF-23) negatively regulates phosphate and calcium metabolism, while retaining growth-promoting action for mesenchymal cell differentiation. Elevated FGF-23 levels, together with hyperparathyroidism, are often observed in chronic kidney disease, which is associated with impaired bone mineralization and enhanced bone resorption. Although overexpression of osteoblast-derived osteoclastogenic cytokines might contribute to this metabolic bone disease, whether FGF-23 alone and FGF-23 plus parathyroid hormone (PTH) directly modulated the expression of osteoblast-derived osteoclastogenic genes remained elusive. Herein, we demonstrated the direct effects of FGF-23 on proliferation and mRNA expression of osteoblast-specific differentiation and osteoclastogenic markers in rat osteoblast-like UMR-106 cells in the presence or absence of PTH. FGF-23 was found to suppress UMR-106 cell proliferation, while increasing FGF-23 expression, the latter of which suggested the presence of positive feedback regulation of FGF-23 expression in osteoblasts. FGF-23 also upregulated the mRNA expression of osteoblast differentiation markers (e.g., Runx2, osterix, AJ18, Dlx5, alkaline phosphatase, and osteopontin), osteoclastogenic factors (e.g., MCSF, MCP-1, IL-6, and TNF-α), and bone resorption regulators (RANKL and osteoprotegerin). However, combined PTH and FGF-23 exposure did not alter the levels of FGF-23-induced transcripts, suggesting that both hormones had no additive effect. In conclusion, FGF-23 directly suppressed osteoblast proliferation, while inducing osteoclastogenic gene expression in UMR-106 cells, and the FGF-23-induced transcripts were not altered by long-standing PTH exposure.

Zebrafish Bone and General Physiology Are Differently Affected by Hormones or Changes in Gravity

Teleost fish such as zebrafish (Danio rerio) are increasingly used for physiological, genetic and developmental studies. Our understanding of the physiological consequences of altered gravity in an entire organism is still incomplete. We used altered gravity and drug treatment experiments to evaluate their effects specifically on bone formation and more generally on whole genome gene expression. By combining morphometric tools with an objective scoring system for the state of development for each element in the head skeleton and specific gene expression analysis, we confirmed and characterized in detail the decrease or increase of bone formation caused by a 5 day treatment (from 5dpf to 10 dpf) of, respectively parathyroid hormone (PTH) or vitamin D3 (VitD3). Microarray transcriptome analysis after 24 hours treatment reveals a general effect on physiology upon VitD3 treatment, while PTH causes more specifically developmental effects. Hypergravity (3g from 5dpf to 9 dpf) exposure results in a significantly larger head and a significant increase in bone formation for a subset of the cranial bones. Gene expression analysis after 24 hrs at 3g revealed differential expression of genes involved in the development and function of the skeletal, muscular, nervous, endocrine and cardiovascular systems. Finally, we propose a novel type of experimental approach, the "Reduced Gravity Paradigm", by keeping the developing larvae at 3g hypergravity for the first 5 days before returning them to 1g for one additional day. 5 days exposure to 3g during these early stages also caused increased bone formation, while gene expression analysis revealed a central network of regulatory genes (hes5, sox10, lgals3bp, egr1, edn1, fos, fosb, klf2, gadd45ba and socs3a) whose expression was consistently affected by the transition from hyper- to normal gravity.

Ascorbic acid regulates osterix expression in osteoblasts by activation of prolyl hydroxylase and ubiquitination-mediated proteosomal degradation pathway

Mouse genetic studies reveal that ascorbic acid (AA) is essential for osteoblast (OB) differentiation and that osterix (Osx) was a key downstream target of AA action in OBs. To determine the molecular pathways for AA regulation of Osx expression, we evaluated if AA regulates Osx expression by regulating production and/or actions of local growth factors and extracellular matrix (ECM) proteins. Inhibition of actions of IGFs by inhibitory IGFBP-4, BMPs by noggin, and ECM-mediated integrin signaling by RGD did not block AA effects on Osx expression in OBs. Furthermore, blockade of components of MAPK signaling pathway had no effect on AA-induced Osx expression. Because AA is required for prolyl hydroxylase domain (PHD) activity and because PHD-induced prolyl-hydroxylation targets proteins to proteosomal degradation, we next tested if AA effect on Osx expression involves activation of PHD to hydroxylate and induce ubiquitin-proteosome-mediated degradation of transcriptional repressor(s) of Osx gene. Treatment of OBs with dimethyloxallyl glycine and ethyl 3, 4-dihydroxybenzoate, known inhibitors of PHD, completely blocked AA effect on Osx expression and OB differentiation. Knockdown of PHD2 expression by Lentivirus-mediated shRNA abolished AA-induced Osx induction and alkaline phosphatase activity. Furthermore, treatment of OBs with MG115, inhibitor of proteosomal degradation, completely blocked AA effects on Osx expression. Based on these data, we conclude that AA effect on Osx expression is mediated via a novel mechanism that involves PHD2 and proteosomal degradation of a yet to be identified transcriptional repressor that is independent of BMP, IGF-I, or integrin-mediated signaling in mouse OBs.

Decrease in the expression of the type 1 PTH/PTHrP receptor (PTH1R) on chondrocytes in animals with osteoarthritis

BACKGROUND

To evaluate the expression of the type 1 PTH/PTHrP receptor (PTH1R) on chondrocytes from hyaline cartilage over the course of osteoarthritis (OA).

METHODS

In 12 NZW rabbits, the anterior cruciate ligament (ACL) was resected to create anterior instability of the knee. In 12 control rabbits, only a sham operation, without resection of the ACL, was performed. Four animals from each group were killed at 3, 6, and 12 weeks. After opening the knee joint, OA was macroscopically graded and hyaline cartilage of the load-bearing area was evaluated histologically according to the Mankin scale and by immunostaining for PTH1R.

RESULTS

There was a positive linear correlation between the time after surgery and the macroscopic and histologic OA scores. The scores in the control group were constant over the time course. Immunostaining showed significantly less expression of PTH1R in the experimental compared to the control group after 6 (P < 0.05) and 12 weeks (P < 0.01). In the experimental group, a negative linear correlation between PTH1R expression and macroscopic and histologic grades was found.

CONCLUSIONS

The results show an in vivo decrease in the expression of PTH1R on chondrocytes over the time course of OA. Further studies are needed to evaluate whether new treatment approaches could evolve from this knowledge.

PTHrP Induces Notch Signaling in Periodontal Ligament Cells

Periodontal ligament (PDL) cells are known to play important roles in tooth eruption and alveolar bone metabolism. We previously reported that PTHrP increases RANKL expression in human PDL cells, suggesting that it promotes odontoclastic root resorption during tooth eruption. While it is known that Notch-related genes play a key role during bone development, the role of the Notch signaling pathway in PDL cells during tooth and bone resorption is less clear. We hypothesized that PTHrP induces a Notch ligand in PDL cells and thereby regulates osteo- and odontoclastogenesis. We found that PTHrP increased Notch1 ligand Jagged1 expression in human PDL cells in a dose- and time-dependent manner. PTHrP-induced Jagged1 up-regulation was mediated by PKA activation, but not by PKC. Jagged1 also promoted RANKL-induced osteoclastogenesis. These results demonstrate that PTHrP induces Jagged1 expression in PDL cells, leading to osteo- and odontoclastogenesis, and thus likely promoting tooth and alveolar bone resorption.

Parathyroid hormone synergizes with non-cyclic AMP pathways to activate the cyclic AMP response element

Parathyroid hormone (PTH) activates multiple signaling pathways following binding to the PTH1 receptor in osteoblasts. Previous work revealed a discrepancy between cAMP stimulation and CRE reporter activation of truncated PTH peptides, suggesting that additional signaling pathways contribute to activation of the CRE. Using a CRE-Luciferase reporter containing multiple copies of the CRE stably transfected into the osteoblastic cell line Saos-2, we tested the ability of modulators of alternative pathways to activate the CRE or block the PTH-induced activation of the CRE. Activators of non-cyclic AMP pathways, that is, EGF (Akt, MAPK, JAK/STAT pathways); thapsigargin (intracellular calcium pathway); phorbol myristate acetate (protein kinase C, PKC pathway) induced minor increases in CRE-luciferase activity alone but induced dramatic synergistic effects in combination with PTH. The protein kinase A (PKA) inhibitor H-89 (10 microM) almost completely blocked PTH-induced activation of the CRE-reporter. Adenylate cyclase inhibitors SQ 22536 and DDA had profound and time-dependent biphasic effects on the CRE response. The MAPK inhibitor PD 98059 partially inhibited basal and PTH-induced CRE activity to the same degree, while the PKC inhibitor bisindolylmaleimide (BIS) had variable effects. The calmodulin kinase II inhibitor KN-93 had no significant effect on the response to PTH. We conclude that non-cAMP pathways (EGF pathway, calcium pathway, PKC pathway) converge on, and have synergistic effects on, the response of a CRE reporter to PTH.

Simvastatin suppresses leptin expression in 3T3-L1 adipocytes via activation of the cyclic AMP-PKA pathway induced by inhibition of protein prenylation

Simvastatin inhibits 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, which catalyses conversion of HMG-CoA to mevalonate, a rate-limiting step in cholesterol synthesis. We demonstrated that simvastatin at 1 microM markedly inhibited adipocyte differentiation measured by Oil Red O staining in preadipocyte cells (3T3-L1), while expression of leptin, a marker of adipocyte differentiation, was suppressed by 1 muM simvastatin for up to 12 days of culture. Next, to elucidate mechanisms underlying the reduction of leptin expression induced by simvastatin, differentiated 3T3-L1 adipocytes were treated with various inhibitors with mevalonate or its metabolite in the presence or absence of simvastatin. Simvastatin time- and dose-dependently suppressed leptin mRNA expression. Heterogeneous nuclear RNA related to leptin mRNA was inhibited by 10 muM simvastatin, while stability of the mRNA was not changed by treatment with simvastatin in transcription-arrested 3T3-L1 cells. Simvastatin inhibition of leptin gene transcription was not abrogated by pre-treatment with cycloheximide, an inhibitor of protein synthesis. Addition of mevalonate or geranylgeranyl pyrophosphate (GGPP), a mevalonate metabolite, abolished simvastatin-induced inhibition of leptin expression in 3T3-L1 cells. Suppression of expression was observed upon addition of GGTI-298, a geranylgeranyl transferase I inhibitor, but not FTI-277, a farnesyl transferase inhibitor. Expression was suppressed by treatment with hydroxyfasudil, a protein prenylation inhibitor. Treatment with phosphatidylinositol 3-kinase (PI3K) inhibitors, LY294002 and wortmannin, reduced leptin expression in 3T3-L1 cells. Simvastatin dose-dependently increased intra-cellular cyclic AMP (cAMP) concentrations in 3T3-L1 cells, with maximal stimulation obtained at 10 muM. Addition of GGPP abolished simvastatin-induced stimulation of cAMP accumulation and protein kinase A (PKA) activity. H89, an inhibitor of PKA, completely abolished simvastatin-induced suppression of leptin expression. These results suggested that simvastatin reduced geranylgeranylprotein prenylation followed by deactivation of PI3K, leading to cAMP accumulation and subsequent activation of PKA in differentiated 3T3-L1 adipocytes. Finally, PKA inhibited leptin gene transcription without new protein synthesis.

Mechanisms underlying the long-term regulation of NHE3 by parathyroid hormone

The activity of the Na+/H+ exchanger NHE3 is regulated by a number of factors including parathyroid hormone (PTH). In the current study, we used a renal epithelial cell line, the opossum kidney (OKP) cell, to elucidate the mechanisms underlying the long-term effects of PTH on NHE3 transport activity and expression. We observed that NHE3 activity was reduced 6 h after addition of PTH, and this reduction persisted almost unaltered after 24 h. The decrease in activity was associated with diminished NHE3 cell surface expression at 6, 16, and 24 h after PTH addition, total cellular NHE3 protein at 16 and 24 h, and NHE3 mRNA abundance at 24 h. The lower levels of NHE3 mRNA were associated to a small, but significant, decrease in mRNA stability. Additionally, by analyzing the rat NHE3 gene promoter activity in OKP cells, we verified that the regulatory region spanning the segment −152 to +55 was mildly reduced under the influence of PTH. This effect was completely abolished by the presence of the PKA inhibitor KT 5720. In conclusion, long-term exposure to PTH results in reduction of NHE3 mRNA levels due to a PKA-dependent inhibitory effect on the NHE3 promoter and a small reduction of mRNA half-life, and decrease in the total amount of protein which is preceded by endocytosis of the apical surface NHE3. The decreased NHE3 expression is likely to be responsible for the reduction of sodium, bicarbonate, and fluid reabsorption in the proximal tubule consistently perceived in experimental models of PTH disorders.

Control of the SOST bone enhancer by PTH using MEF2 transcription factors

Expression of the osteocyte-derived bone formation inhibitor sclerostin in adult bone requires a distant enhancer. We show that MEF2 transcription factors control this enhancer and mediate inhibition of sclerostin expression by PTH.

INTRODUCTION

Sclerostin encoded by the SOST gene is a key regulator of bone formation. Lack of SOST expression is the cause for the progressive bone overgrowth disorders sclerosteosis and Van Buchem disease. We have previously identified a distant enhancer within the 52-kb Van Buchem disease deletion downstream of the SOST gene that is essential for its expression in adult bone. Furthermore, we and others have reported that SOST expression is suppressed by PTH. The aim of this study was to identify transcription factors involved in SOST bone enhancer activity and mediating PTH responsiveness.

MATERIALS AND METHODS

Regulation of the SOST enhancer and promoter was studied by luciferase reporter gene assays. Transcription factor binding sites were mapped by footprint analysis and functional mutation analyses using transient transfections of osteoblast-like UMR-106 cells that exhibit endogenous SOST expression. Specific transcription factor binding was predicted by sequence analysis and shown by gel retardation assays and antibody-induced supershifts. Expression of myocyte enhancer factors 2 (MEF2) was detected by in situ hybridization, quantitative RT-PCR (qPCR), and immunohistochemistry. The role of MEF2s in SOST expression was assessed by reporter gene assays and siRNA-mediated RNA knockdown.

RESULTS

PTH completely suppressed the transcriptional activity of the SOST bone enhancer but did not affect the SOST promoter. A MEF2 response element was identified in the bone enhancer. It was essential for transcriptional activation, bound MEF2 transcription factors, and mediated PTH responsiveness. Expression of MEF2s in bone was shown by qPCR, in situ hybridization, and immunohistochemistry. MEF2s and sclerostin co-localized in osteocytes. Enhancer activity was stimulated by MEF2C overexpression and inhibited by co-expression of a dominant negative MEF2C mutant. Finally, siRNA-mediated knockdown of MEF2A, C, and D suppressed endogenous SOST expression in UMR-106 osteoblast-like cells.

CONCLUSIONS

These data strongly suggest that SOST expression in osteocytes of adult bone and its inhibition by PTH is mediated by MEF2A, C, and D transcription factors controlling the SOST bone enhancer. Hence, MEF2s are implicated in the regulation of adult bone mass.

Upregulation of PTH receptor mRNA expression by dexamethasone in UMR-106 osteoblast-like cells

OBJECTIVES

Glucocorticoids influence receptor interactions of the parathyroid hormone (PTH) that are crucial for osteoblast function. As mechanisms linking receptor mRNA with glucocorticoids are incompletely understood, we investigated regulation of PTH receptor (PTH1R) mRNA expression in rat osteoblast-like UMR-106 cells by using dexamethasone (Dex), a synthetic glucocorticoid.

MATERIALS AND METHODS

UMR-106 cells were exposed to 10(-8) to 10(-5) M Dex, while some cells were also exposed to a transcriptional inhibitor (DRB) for 24 h with or without Dex. PTH-stimulated cyclicAMP activities were measured by an enzyme-linked immunosorbent assay. PTH1R mRNA was determined by Northern analysis. Transcriptional activities were measured as heretogeneous nuclear PTH1R RNA and also as luciferase activity in constructs, including the PTH1R gene promoter.

RESULTS

Dexamethasone dose-dependently increased PTH-stimulated adenylyl cyclase activity at 72 h. Dex markedly increased PTH1R mRNA accumulation, but did not change transcriptional activity. PTH1R mRNA stability was significantly increased by Dex in transcriptionally arrested cells.

CONCLUSION

In osteoblast-like cells, Dex induced upregulation of PTH1R mRNA followed by increased functional PTH receptor expression. This was caused by posttranscriptional mechanisms increasing mRNA stability.

Parathyroid-hormone-related protein induces expression of receptor activator of NF-{kappa}B ligand in human periodontal ligament cells via a cAMP/protein kinase A-independent pathway

Periodontal ligament (PDL) cells play important roles in root resorption of human deciduous teeth by odontoclasts (osteoclast-like cells). However, it is unclear how PDL cells regulate osteoclastogenesis. We examined the effects of PTHrP, TGF-beta, and EGF, which are all secreted by the tooth germ, on tartrate-resistant acid-phosphatase-positive (TRAP+) cell formation using co-cultures of human PDL cells and mouse spleen cells. Only PTHrP promoted TRAP+ cell formation in co-cultures. PTHrP induced receptor activator of NF-kappaB ligand (RANKL) mRNA expression and slightly reduced osteoprotegerin (OPG) expression in PDL cells. The cAMP/PKA inhibitors Rp-cAMP, H89, and PKI did not affect PTHrP-induced TRAP+ cell formation. The PKC inhibitor, Ro-32-0432, suppressed RANKL expression in PDL cells and PTHrP-induced TRAP+ cell formation. However, this inhibitor directly modulated the number of osteoclast precursors. Thus, PTHrP induces osteoclastogenesis by increasing the relative expression level of RANKL vs. OPG in PDL cells via a cAMP/PKA-independent pathway.

ABBREVIATIONS

PTHrP, parathyroid-hormone-related protein; TGF-beta, transforming growth factor-beta; EGF, epidermal growth factor; RANKL, receptor activator of NF-kappaB ligand; OPG, osteoprotegerin; PDL, periodontal ligament; TRAP, tartrate-resistant acid phosphatase; PKA, protein kinase A; PKC, protein kinase C; MAP, mitogen-activated protein; ERK, extracellular signal-regulated kinase; cAMP, cyclic Adenosine 3'5'-Monophosphate.

Identification of the promoter region of the parathyroid hormone receptor gene responsible for transcriptional suppression by insulin-like growth factor-I

We investigated parathyroid hormone (PTH)/PTH-related protein receptor (PTH1R) gene suppression induced by insulin-like growth factor (IGF)-I using a rat osteoblast-like cell line (UMR-106). Observations were made with PD98059, a specific ERK signaling pathway inhibitor, and UMR-106 cells transfected with dominant negative or constitutively active forms of MAP kinase kinase. IGF-I inhibited PTH1R gene expression via an ERK1/2 MAP kinase pathway. We cloned the 8-kb promoter region of the rat PTH1R gene and characterized the U3 promoter, a major IGF-I-responsive promoter among the two present in rat osteoblasts. The IGF-I-suppressive region was between +1 and +25, identical to the previously described PTH-suppressive region (PTHSR). Gel mobility-shift detected a specific DNA-protein complex decreased by IGF-I. Mutation involving a three base sequence (+1 to +3) among more than 3.5 kb constituting the PTH1R promoter region completely abolished IGF-I action. Thus, IGF-I signaling may act at the osteoblast exon U3 transcription initiation site to repress the transcriptional activity.

PTHrP signaling targets cyclin D1 and induces osteoblastic cell growth arrest

PTHrP control of the MC3T3-E1 cell cycle machinery showed that, during differentiation, PTHrP induced G1 growth arrest. Cyclin D1 was a critical mediator as a downstream effector of cAMP, PKC, and MAPK signaling, and the process was PKA-independent. The involvement of JunB has been found critical for PTHrP effects.

INTRODUCTION

PTH-related protein (PTHrP) has been implicated in the control of bone cell turnover, but the mechanisms underlying its effect on osteoblast proliferation and differentiation have not been clearly defined. The mechanisms by which PTHrP impacts cell cycle proteins and the role of signaling pathways in differentiated osteoblasts were studied.

MATERIALS AND METHODS

To elucidate the role of PTHrP, flow cytometric analyses were performed using MC3T3-E1 and primary mouse calvarial cells. Relative protein abundance (Western blot), physical association of partners (immunoprecipitation), and kinase activities (in vitro kinase assays using either GST-Rb or H1-histone as substrates) of cell cycle-associated proteins in vehicle and PTHrP-treated 7-day differentiated cells were determined. ELISA and/or Northern blot analyses were done to evaluate JunB and cyclin D1 expression. SiRNA-mediated gene silencing experiments were performed to silence JunB protein. Finally, inhibitors of cAMP, protein kinase A (PKA), protein kinase C (PKC), and mitogen-activated protein kinase (MAPK) were used to determine involvement of different signaling pathways.

RESULTS

PTHrP inhibited cyclin D1 protein expression 7-fold in a dose- and time-dependent manner and increased the level of p16 protein in differentiated osteoblasts. Additionally, PTHrP reduced cyclin D1-CDK4/CDK6 and CDK1 kinase activities. Forskolin, a cAMP agonist, mimicked PTHrP action, and the PKC inhibitor, GF109203X, slightly blocked downregulation of cyclin D1, implying involvement of both cAMP and PKC. U0126, a MAPK inhibitor, alone decreased cyclin D1 protein, suggesting that the basal cyclin D1 protein is MAPK dependent. H-89, a PKA inhibitor, did not alter the effect of PTHrP on cyclin D1, suggesting a PKA-independent mechanism. Finally, expression of JunB, an activating protein-1 transcription factor, was significantly upregulated, and silencing JunB (siRNA) partially reversed the cyclin D1 response, implying involvement of JunB in the PTHrP-mediated growth arrest of MC3T3-E1 cells.

CONCLUSION

PTHrP upregulates JunB and reduces cyclin D1 expression while inducing G1 cell cycle arrest in differentiated osteoblasts. Such regulation could be an important determinant of the life span and bone-forming activity of osteoblasts.