Increased bone formation by intermittent parathyroid hormone administration is due to the stimulation of proliferation and differentiation of osteoprogenitor cells in bone marrow

Towards an enhanced understanding of osteoanabolic effects of PTH-induced microRNAs on osteoblasts using a bioinformatic approach

In this study, we used a bioinformatic approach to construct a miRNA-target gene interaction network potentially involved in the anabolic effect of parathyroid hormone analogue teriparatide [PTH (1-34)] on osteoblasts. We extracted a dataset of 26 microRNAs (miRNAs) from previously published studies and predicted miRNA target interactions (MTIs) using four software tools: DIANA, miRWalk, miRDB, and TargetScan. By constructing an interactome of PTH-regulated miRNAs and their predicted target genes, we elucidated signaling pathways regulating pluripotency of stem cells, the Hippo signaling pathway, and the TGF-beta signaling pathway as the most significant pathways in the effects of PTH on osteoblasts. Furthermore, we constructed intersection of MTI networks for these three pathways and added validated interactions. There are 8 genes present in all three selected pathways and a set of 18 miRNAs are predicted to target these genes, according to literature data. The most important genes in all three pathways were BMPR1A, BMPR2 and SMAD2 having the most interactions with miRNAs. Among these miRNAs, only miR-146a-5p and miR-346 have validated interactions in these pathways and were shown to be important regulators of these pathways. In addition, we also propose miR-551b-5p and miR-338-5p for further experimental validation, as they have been predicted to target important genes in these pathways but none of their target interactions have yet been verified. Our wet-lab experiment on miRNAs differentially expressed between PTH (1-34) treated and untreated mesenchymal stem cells supports miR-186-5p from the literature obtained data as another prominent miRNA. The meticulous selection of miRNAs outlined will significantly support and guide future research aimed at discovering and understanding the crucial pathways of osteoanabolic PTH-epigenetic effects on osteoblasts. Additionally, they hold potential for the discovery of new PTH target genes, innovative biomarkers for the effectiveness and safety of osteoporosis-affected treatment, as well as novel therapeutic targets.

Tgif1-deficiency impairs cytoskeletal architecture in osteoblasts by activating PAK3 signaling

Osteoblast adherence to bone surfaces is important for remodeling bone tissue. This study demonstrates that deficiency of TG-interacting factor 1 (Tgif1) in osteoblasts results in altered cell morphology, reduced adherence to collagen type I-coated surfaces, and impaired migration capacity. Tgif1 is essential for osteoblasts to adapt a regular cell morphology and to efficiently adhere and migrate on collagen type I-rich matrices in vitro. Furthermore, Tgif1 acts as a transcriptional repressor of p21-activated kinase 3 (Pak3), an important regulator of focal adhesion formation and osteoblast spreading. Absence of Tgif1 leads to increased Pak3 expression, which impairs osteoblast spreading. Additionally, Tgif1 is implicated in osteoblast recruitment and activation of bone surfaces in the context of bone regeneration and in response to parathyroid hormone 1-34 (PTH 1-34) treatment in vivo in mice. These findings provide important novel insights in the regulation of the cytoskeletal architecture of osteoblasts.

PTH and the Regulation of Mesenchymal Cells within the Bone Marrow Niche

Parathyroid hormone (PTH) plays a pivotal role in maintaining calcium homeostasis, largely by modulating bone remodeling processes. Its effects on bone are notably dependent on the duration and frequency of exposure. Specifically, PTH can initiate both bone formation and resorption, with the outcome being influenced by the manner of PTH administration: continuous or intermittent. In continuous administration, PTH tends to promote bone resorption, possibly by regulating certain genes within bone cells. Conversely, intermittent exposure generally favors bone formation, possibly through transient gene activation. PTH's role extends to various aspects of bone cell activity. It directly influences skeletal stem cells, osteoblastic lineage cells, osteocytes, and T cells, playing a critical role in bone generation. Simultaneously, it indirectly affects osteoclast precursor cells and osteoclasts, and has a direct impact on T cells, contributing to its role in bone resorption. Despite these insights, the intricate mechanisms through which PTH acts within the bone marrow niche are not entirely understood. This article reviews the dual roles of PTH-catabolic and anabolic-on bone cells, highlighting the cellular and molecular pathways involved in these processes. The complex interplay of these factors in bone remodeling underscores the need for further investigation to fully comprehend PTH's multifaceted influence on bone health.

Comparative study in estrogen-depleted mice identifies skeletal and osteocyte transcriptomic responses to abaloparatide and teriparatide

Osteocytes express parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptors and respond to the PTHrP analog abaloparatide (ABL) and to the PTH 1-34 fragment teriparatide (TPTD), which are used to treat osteoporosis. Several studies indicate overlapping but distinct skeletal responses to ABL or TPTD, but their effects on cortical bone may differ. Little is known about their differential effects on osteocytes. We compared cortical osteocyte and skeletal responses to ABL and TPTD in sham-operated and ovariectomized mice. Administered 7 weeks after ovariectomy for 4 weeks at a dose of 40 μg/kg/d, TPTD and ABL had similar effects on trabecular bone, but ABL showed stronger effects in cortical bone. In cortical osteocytes, both treatments decreased lacunar area, reflecting altered peri-lacunar remodeling favoring matrix accumulation. Osteocyte RNA-Seq revealed that several genes and pathways were altered by ovariectomy and affected similarly by TPTD and ABL. Notwithstanding, several signaling pathways were uniquely regulated by ABL. Thus, in mice, TPTD and ABL induced a positive osteocyte peri-lacunar remodeling balance, but ABL induced stronger cortical responses and affected the osteocyte transcriptome differently. We concluded that ABL affected the cortical osteocyte transcriptome in a manner subtly different from TPTD, resulting in more beneficial remodeling/modeling changes and homeostasis of the cortex.

The effect of intermittent parathyroid hormone on bone lengthening: current evidence to inform future effective interventions

PURPOSE

Recent studies have demonstrated the positive effects of parathyroid hormone (PTH) on bone healing, and findings support the use of PTH to accelerate bone healing following distraction osteogenesis. The goal of this review was to compile and discuss the mechanisms potentially underlying the effects of PTH on newly formed bone following a bone-lengthening procedure incorporating all relevant evidence in both animal and clinical studies.

METHODS

This review summarized all evidence from in vivo to clinical studies regarding the effects of PTH administration on a bone-lengthening model. In addition, a comprehensive evaluation of what is currently known regarding the potential mechanisms underlying the potential benefits of PTH in bone lengthening was presented. Some controversial findings regarding the optimal dosage and timing of administration of PTH in this model were also discussed.

RESULTS

The findings demonstrated that the potential mechanisms associated with the action of PTH on the acceleration of bone regeneration after distraction osteogenesis are involvement in mesenchymal cell proliferation and differentiation, endochondral bone formation, membranous bone formation, and callus remodeling.

CONCLUSIONS

In the last 20 years, a number of animal and clinical studies have indicated that there is a prospective role for PTH treatment in human bone lengthening as an anabolic agent that accelerates the mineralization and strength of the regenerated bone. Therefore, PTH treatment can be viewed as a potential treatment to increase the amount of new calcified bone and the mechanical strength of the bone in order to shorten the consolidation stage after bone lengthening.

Pathophysiology of bone disease in chronic kidney disease: from basics to renal osteodystrophy and osteoporosis

Chronic kidney disease (CKD) is a highly prevalent disease that has become a public health problem. Progression of CKD is associated with serious complications, including the systemic CKD-mineral and bone disorder (CKD-MBD). Laboratory, bone and vascular abnormalities define this condition, and all have been independently related to cardiovascular disease and high mortality rates. The "old" cross-talk between kidney and bone (classically known as "renal osteodystrophies") has been recently expanded to the cardiovascular system, emphasizing the importance of the bone component of CKD-MBD. Moreover, a recently recognized higher susceptibility of patients with CKD to falls and bone fractures led to important paradigm changes in the new CKD-MBD guidelines. Evaluation of bone mineral density and the diagnosis of "osteoporosis" emerges in nephrology as a new possibility "if results will impact clinical decisions". Obviously, it is still reasonable to perform a bone biopsy if knowledge of the type of renal osteodystrophy will be clinically useful (low versus high turnover-bone disease). However, it is now considered that the inability to perform a bone biopsy may not justify withholding antiresorptive therapies to patients with high risk of fracture. This view adds to the effects of parathyroid hormone in CKD patients and the classical treatment of secondary hyperparathyroidism. The availability of new antiosteoporotic treatments bring the opportunity to come back to the basics, and the knowledge of new pathophysiological pathways [OPG/RANKL (LGR4); Wnt-ß-catenin pathway], also affected in CKD, offers great opportunities to further unravel the complex physiopathology of CKD-MBD and to improve outcomes.

Role of various pharmacologic agents in alveolar bone regeneration: A review

Alveolar bone and gingiva are components of the periodontium that house the tooth. It constantly adapts itself to the masticatory forces and position of the tooth. However, localized diseases like chronic periodontitis and certain systemic diseases destroy periodontal tissues, which include the alveolar bone. Various pharmacological agents are being explored for their pleiotropic properties to combat the destruction of alveolar bone. This review focuses on the role of pharmacological agents in alveolar bone regeneration.

The associations of gut microbiota, endocrine system and bone metabolism

Gut microbiota is of great importance in human health, and its roles in the maintenance of skeletal homeostasis have long been recognized as the "gut-bone axis." Recent evidence has indicated intercorrelations between gut microbiota, endocrine system and bone metabolism. This review article discussed the complex interactions between gut microbiota and bone metabolism-related hormones, including sex steroids, insulin-like growth factors, 5-hydroxytryptamine, parathyroid hormone, glucagon-like peptides, peptide YY, etc. Although the underlying mechanisms still need further investigation, the regulatory effect of gut microbiota on bone health via interplaying with endocrine system may provide a new paradigm for the better management of musculoskeletal disorders.

Continuous and intermittent parathyroid hormone administration promotes osteogenic differentiation and activity of programmable cells of monocytic origin

Bone healing deficiencies are challenging for orthopedic practice. The use of stem cells with scaffolds to treat bone tissue losses currently is popular for promoting regeneration of tissue. Programmable cells of monocytic origin (PCMO) may differentiate into three germ layers and may be a promising alternative treatment due to their stem cell-like properties. Parathyroid hormone (PTH) participates in bone metabolism. Intermittent administration of PTH promotes osteogenic activity of mesenchymal stem cdells (MSC). We investigated the osteogenic effects of continuous and intermittent administration of PTH on PCMO. Mononuclear cells were harvested from the peripheral blood of healthy donors. Isolated cells were cultured for six days in a de-differentiation medium. Indirect immunocytochemistry using anti-CD14, anti-CD45 and anti-CD90 primary antibodies, as well as electron microscopy were used to detect PCMO. PCMO then were cultured in an osteogenic differentiation medium supplemented with continuous or intermittent 50 ng/ml PTH. The PTH-free control group (CG), intermittent PTH treated group (IPG) and continuous PTH treated group (CPG) were cultured and assessed for their differentiation into osteogenic lineage cells by indirect immunocytochemistry using anti-collagen I, anti-osteonectin and anti-osteocalcin primary antibodies. Osteoblast-like cells obtained by continuous or intermittent PTH administration exhibited increased levels of collagen I, osteonectin and osteocalcin immunoreactivity. We found that continuous and intermittent PTH administration to PCMO enhanced their differentiation to osteogenic lineage cells and increased osteoblastic activity.

Procalcitonin is expressed in osteoblasts and limits bone resorption through inhibition of macrophage migration during intermittent PTH treatment

Intermittent injections of parathyroid hormone (iPTH) are applied clinically to stimulate bone formation by osteoblasts, although continuous elevation of parathyroid hormone (PTH) primarily results in increased bone resorption. Here, we identified Calca, encoding the sepsis biomarker procalcitonin (ProCT), as a novel target gene of PTH in murine osteoblasts that inhibits osteoclast formation. During iPTH treatment, mice lacking ProCT develop increased bone resorption with excessive osteoclast formation in both the long bones and axial skeleton. Mechanistically, ProCT inhibits the expression of key mediators involved in the recruitment of macrophages, representing osteoclast precursors. Accordingly, ProCT arrests macrophage migration and causes inhibition of early but not late osteoclastogenesis. In conclusion, our results reveal a potential role of osteoblast-derived ProCT in the bone microenvironment that is required to limit bone resorption during iPTH.

Parathyroid Hormone 1 Receptor Signaling in Dental Mesenchymal Stem Cells: Basic and Clinical Implications

Parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) are two peptides that regulate mineral ion homeostasis, skeletal development, and bone turnover by activating parathyroid hormone 1 receptor (PTH1R). PTH1R signaling is of profound clinical interest for its potential to stimulate bone formation and regeneration. Recent pre-clinical animal studies and clinical trials have investigated the effects of PTH and PTHrP analogs in the orofacial region. Dental mesenchymal stem cells (MSCs) are targets of PTH1R signaling and have long been known as major factors in tissue repair and regeneration. Previous studies have begun to reveal important roles for PTH1R signaling in modulating the proliferation and differentiation of MSCs in the orofacial region. A better understanding of the molecular networks and underlying mechanisms for modulating MSCs in dental diseases will pave the way for the therapeutic applications of PTH and PTHrP in the future. Here we review recent studies involving dental MSCs, focusing on relationships with PTH1R. We also summarize recent basic and clinical observations of PTH and PTHrP treatment to help understand their use in MSCs-based dental and bone regeneration.

The molecular etiology and treatment of glucocorticoid-induced osteoporosis

Glucocorticoid-induced osteoporosis (GIOP) is the most common form of secondary osteoporosis, accounting for 20% of osteoporosis diagnoses. Using glucocorticoids for >6 months leads to osteoporosis in 50% of patients, resulting in an increased risk of fracture and death. Osteoblasts, osteocytes, and osteoclasts work together to maintain bone homeostasis. When bone formation and resorption are out of balance, abnormalities in bone structure or function may occur. Excess glucocorticoids disrupt the bone homeostasis by promoting osteoclast formation and prolonging osteoclasts' lifespan, leading to an increase in bone resorption. On the other hand, glucocorticoids inhibit osteoblasts' formation and facilitate apoptosis of osteoblasts and osteocytes, resulting in a reduction of bone formation. Several signaling pathways, signaling modulators, endocrines, and cytokines are involved in the molecular etiology of GIOP. Clinically, adults ≥40 years of age using glucocorticoids chronically with a high fracture risk are considered to have medical intervention. In addition to vitamin D and calcium tablet supplementations, the major therapeutic options approved for GIOP treatment include antiresorption drug bisphosphonates, parathyroid hormone N-terminal fragment teriparatide, and the monoclonal antibody denosumab. The selective estrogen receptor modulator can only be used under specific condition for postmenopausal women who have GIOP but fail to the regular GIOP treatment or have specific therapeutic contraindications. In this review, we focus on the molecular etiology of GIOP and the molecular pharmacology of the therapeutic drugs used for GIOP treatment.

The diverse origin of bone-forming osteoblasts

Osteoblasts are the only cells that can give rise to bones in vertebrates. Thus, one of the most important functions of these metabolically active cells is mineralized matrix production. Because osteoblasts have a limited lifespan, they must be constantly replenished by preosteoblasts, their immediate precursors. Because disruption of the regulation of bone-forming osteoblasts results in a variety of bone diseases, a better understanding of the origin of these cells by defining the mechanisms of bone development, remodeling, and regeneration is central to the development of novel therapeutic approaches. In recent years, substantial new insights into the origin of osteoblasts-largely owing to rapid technological advances in murine lineage-tracing approaches and other single-cell technologies-have been obtained. Collectively, these findings indicate that osteoblasts involved in bone formation under various physiological, pathological, and therapeutic conditions can be obtained from numerous sources. The origins of osteoblasts include, but are not limited to, chondrocytes in the growth plate, stromal cells in the bone marrow, quiescent bone-lining cells on the bone surface, and specialized fibroblasts in the craniofacial structures, such as sutures and periodontal ligaments. Because osteoblasts can be generated from local cellular sources, bones can flexibly respond to regenerative and anabolic cues. However, whether osteoblasts derived from different cellular sources have distinct functions remains to be investigated. Currently, we are at the initial stage to aptly unravel the incredible diversity of the origins of bone-forming osteoblasts. © 2021 American Society for Bone and Mineral Research (ASBMR).

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

Emerging insights into the comparative effectiveness of anabolic therapies for osteoporosis

Over the past three decades, the mainstay of treatment for osteoporosis has been antiresorptive agents (such as bisphosphonates), which have been effective with continued administration in lowering fracture risk. However, the clinical landscape has changed as adherence to these medications has declined due to perceived adverse effects. As a result, decreases in hip fracture rates that followed the introduction of bisphosphonates have now levelled off, which is coincident with a decline in the use of the antiresorptive agents. In the past two decades, two types of anabolic agents (including three new drugs), which represent a novel approach to improving bone quality by increasing bone formation, have been approved. These therapies are expected to lead to a new clinical paradigm in which anabolic agents will be used either alone or in combination with antiresorptive agents to build new bone and reduce fracture risk. This Review examines the mechanisms of action for these anabolic agents by detailing their receptor-activating properties for key cell types in the bone and marrow niches. Using these advances in bone biology as context, the comparative effectiveness of these anabolic agents is discussed in relation to other therapeutic options for osteoporosis to better guide their clinical application in the future.

Kindlin-2 regulates skeletal homeostasis by modulating PTH1R in mice

In vertebrates, the type 1 parathyroid hormone receptor (PTH1R) is a critical regulator of skeletal development and homeostasis; however, how it is modulated is incompletely understood. Here we report that deleting Kindlin-2 in osteoblastic cells using the mouse 10-kb Dmp1-Cre largely neutralizes the intermittent PTH-stimulated increasing of bone volume fraction and bone mineral density by impairing both osteoblast and osteoclast formation in murine adult bone. Single-cell profiling reveals that Kindlin-2 loss increases the proportion of osteoblasts, but not mesenchymal stem cells, chondrocytes and fibroblasts, in non-hematopoietic bone marrow cells, with concomitant depletion of osteoblasts on the bone surfaces, especially those stimulated by PTH. Furthermore, haploinsufficiency of Kindlin-2 and Pth1r genes, but not that of either gene, in mice significantly decreases basal and, to a larger extent, PTH-stimulated bone mass, supporting the notion that both factors function in the same genetic pathway. Mechanistically, Kindlin-2 interacts with the C-terminal cytoplasmic domain of PTH1R via aa 474–475 and Gsα. Kindlin-2 loss suppresses PTH induction of cAMP production and CREB phosphorylation in cultured osteoblasts and in bone. Interestingly, PTH promotes Kindlin-2 expression in vitro and in vivo, thus creating a positive feedback regulatory loop. Finally, estrogen deficiency induced by ovariectomy drastically decreases expression of Kindlin-2 protein in osteocytes embedded in the bone matrix and Kindlin-2 loss essentially abolishes the PTH anabolic activity in bone in ovariectomized mice. Thus, we demonstrate that Kindlin-2 functions as an intrinsic component of the PTH1R signaling pathway in osteoblastic cells to regulate bone mass accrual and homeostasis.

Effects of intermittent treatment with parathyroid hormone (PTH) on osteoblastic differentiation and mineralization of mouse induced pluripotent stem cells in a 3D culture model

BACKGROUND/OBJECTIVES

PTH plays an important role in bone remodeling, and different actions have been reported depending on its administration method. iPSCs are promising as a cell source for regeneration of periodontal tissue due to their ability of proliferation and pluripotency. However, the effects of PTH on iPSCs remain mostly unknown. The purpose of this study was to investigate in vitro effects of parathyroid hormone (PTH) on osteoblastic differentiation of induced pluripotent stem cells (iPSCs) in a 3D culture model.

MATERIALS AND METHODS

Following embryoid body (EB) induction from mouse iPSCs (miPSCs), dissociated cells (miPS-EB-derived cells) were seeded onto atelocollagen sponge (ACS) in osteoblast differentiation medium (OBM). Cell-ACS constructs were divided into three groups: continuous treatment with human recombinant PTH (1-34) (PTH-C), intermittent PTH treatment (PTH-I) or OBM control. To confirm the expression of PTH receptor-1(PTH1R), the expression of Pth1r and cAMP production over time were assessed. Real-time PCR was used to assess the expression of genes encoding osterix (Sp7), runt-related transcription factor 2 (Runx2), collagen type 1 (Col1a1), and osteocalcin (Bglap) at different time points. Mineralization was assessed by von Kossa staining. Histochemical staining was used to analyze alkaline phosphatase (ALP) activity, and immunolocalization of SP7 and BGLAP was analyzed by confocal laser scanning microscopy (CLSM).

RESULTS

On days 7 and 14, expression of the Pth1r in miPS-EB-derived cells was increased in all groups. Production of cAMP, the second messenger of the PTH1R, tended to increase in the PTH-I group compared with PTH-C group on day 14. Expression of Col1a1 in the PTH-I group on day 14 was significantly higher than other groups. There was a time-dependent increase in the expression of Sp7 in all groups. On day 14, the expression level of Sp7 in the PTH-I group was significantly higher than other groups. In von Kossa staining, the PTH-I group showed higher level of staining compared with other groups on day 14, whereas the level was slightly attenuated in the PTH-C group. In histochemical staining, ALP-positive cells were significantly increased in the PTH-I group compared with other groups on day 14. In CLSM analysis, the numbers of SP7- and BGLAP-positive cells showed a gradual increase over time, and on day 14, a significantly greater SP7 expression was observed in the PTH-I group than other groups.

CONCLUSION

These results suggested that the intermittent PTH treatment promotes osteoblastic differentiation and mineralization of miPSCs in the ACS scaffold.

Enhancement of peri-implant bone formation via parathyroid hormone administration in a rat model at risk for medication-related osteonecrosis of the jaw

Purpose

Dental implant-associated medication-related osteonecrosis of the jaw has been frequently reported in patients administered bisphosphonates (BPs) to prevent osteoporosis. The aim of this study was to investigate the effect of intermittent administration of parathyroid hormone (PTH) on peri-implant bone in the maxillae of ovariectomized rats systemically administered BPs.

Methods

Thirty 8-week-old female Sprague-Dawley rats were randomly divided into 3 groups. The OVX-ZP group included ovariectomized rats administered 60 µg/kg of zoledronate once a week for 6 weeks and 30 µg/kg PTH after implant installation. The OVX-Z group included ovariectomized rats administered 60 µg/kg of zoledronate once a week for 6 weeks and saline after implant installation, and the control group included rats that underwent a sham operation and were then administered saline. Rats were sacrificed 4 weeks after implant placement for histomorphometric and micro-computed tomography (CT) analyses.

Results

The average bone area percentage was greater in the OVX-ZP group than in the OVX-Z group (53.4%±4.0% vs. 28.9%±9.5%, P=0.01). The bone-to-implant contact ratio was 50.8%±1.4% in the OVX-ZP group and 16.9%±2.4% in the OVX-Z group (P=0.012). The average bone volume ratio as shown on micro-CT was 31.3%±19.8% in the OVX-ZP group and 19.4%±9.3% in the OVX-Z group (P=0.045). The OVX-ZP and OVX-Z groups displayed similar trabecular thickness (0.06±0.004 mm vs. 0.06±0.002 mm) (P>0.05) and trabecular separation (0.21±0.02 mm vs. 0.29±0.13 mm) (P>0.05). However, the number of trabeculae in the OVX-ZP group was significantly higher than that in the OVX-Z group (4.3±1.33/mm³ vs. 2.2±0.19/mm³) (P=0.024).

Conclusions

The present findings indicate that intermittently-administered PTH can promote peri-implant bone formation and suggest that PTH administration may aid in effective treatment for medication-related osteonecrosis of the jaw after dental implantation.

Runx2 regulates cranial suture closure by inducing hedgehog, Fgf, Wnt and Pthlh signaling pathway gene expressions in suture mesenchymal cells

Cleidocranial dysplasia (CCD, #119600), which is characterized by hypoplastic clavicles, open fontanelles, supernumerary teeth and a short stature, is caused by heterozygous mutations in RUNX2. However, it currently remains unclear why suture closure is severely impaired in CCD patients. The closure of posterior frontal (PF) and sagittal (SAG) sutures was completely interrupted in Runx2+/- mice, and the proliferation of suture mesenchymal cells and their condensation were less than those in wild-type mice. To elucidate the underlying molecular mechanisms, differentially expressed genes between wild-type and Runx2+/- PF and SAG sutures were identified by microarray and real-time reverse transcription polymerase chain reaction analyses. The expression of hedgehog, Fgf, Wnt and Pthlh signaling pathway genes, including Gli1, Ptch1, Ihh, Fgfr2, Fgfr3, Tcf7, Wnt10b and Pth1r, which were directly regulated by Runx2, was reduced in the sutures, but not the calvarial bone tissues of Runx2+/- mice. Bone formation and suture closure were enhanced in an organ culture of Runx2+/- calvariae with ligands or agonists of hedgehog, Fgf, Wnt and Pthlh signaling, while they were suppressed and suture mesenchymal cell proliferation was decreased in an organ culture of wild-type calvariae with their antagonists. These results indicate that more than a half dosage of Runx2 is required for the proliferation of suture mesenchymal cells, their condensation and commitment to osteoblast-lineage cells, and the induction of hedgehog, Fgf, Wnt and Pthlh signaling pathway gene expressions in sutures, but not in calvarial bone tissues, and also that the activation of hedgehog, Fgf, Wnt and Pthlh signaling pathways is necessary for suture closure.

Parathyroid Hormone Shifts Cell Fate of a Leptin Receptor-Marked Stromal Population from Adipogenic to Osteoblastic Lineage

Intermittent parathyroid hormone (iPTH) treatment induces bone anabolic effects that result in the recovery of osteoporotic bone loss. Human PTH is usually given to osteoporotic patients because it induces osteoblastogenesis. However, the mechanism by which PTH stimulates the expansion of stromal cell populations and their maturation toward the osteoblastic cell lineage has not be elucidated. Mouse genetic lineage tracing revealed that iPTH treatment induced osteoblastic differentiation of bone marrow (BM) mesenchymal stem and progenitor cells (MSPCs), which carried the leptin receptor (LepR)-Cre. Although these findings suggested that part of the PTH-induced bone anabolic action is exerted because of osteoblastic commitment of MSPCs, little is known about the in vivo mechanistic details of these processes. Here, we showed that LepR⁺ MSPCs differentiated into type I collagen (Col1)⁺ mature osteoblasts in response to iPTH treatment. Along with osteoblastogenesis, the number of Col1⁺ mature osteoblasts increased around the bone surface, although most of them were characterized as quiescent cells. However, the number of LepR-Cre-marked lineage cells in a proliferative state also increased in the vicinity of bone tissue after iPTH treatment. The expression levels of SP7/osterix (Osx) and Col1, which are markers for osteoblasts, were also increased in the LepR⁺ MSPCs population in response to iPTH treatment. In contrast, the expression levels of Cebpb, Pparg, and Zfp467, which are adipocyte markers, decreased in this population. Consistent with these results, iPTH treatment inhibited 5-fluorouracil- or ovariectomy (OVX)-induced LepR⁺ MSPC-derived adipogenesis in BM and increased LepR⁺ MSPC-derived osteoblasts, even under the adipocyte-induced conditions. Treatment of OVX rats with iPTH significantly affected the osteoporotic bone tissue and expansion of the BM adipose tissue. These results indicated that iPTH treatment induced transient proliferation of the LepR⁺ MSPCs and skewed their lineage differentiation from adipocytes toward osteoblasts, resulting in an expanded, quiescent, and mature osteoblast population. © 2019 American Society for Bone and Mineral Research.

Stem cells in Osteoporosis: From Biology to New Therapeutic Approaches

Osteoporosis is a systemic disease that affects the skeleton, causing reduction of bone density and mass, resulting in destruction of bone microstructure and increased risk of bone fractures. Since osteoporosis is a disease affecting the elderly and the aging of the world's population is constantly increasing, it is expected that the incidence of osteoporosis and its financial burden on the insurance systems will increase continuously and there is a need for more understanding this condition in order to prevent and/or treat it. At present, available drug therapy for osteoporosis primarily targets the inhibition of bone resorption and agents that promote bone mineralization, designed to slow disease progression. Safe and predictable pharmaceutical means to increase bone formation have been elusive. Stem cell therapy of osteoporosis, as a therapeutic strategy, offers the promise of an increase in osteoblast differentiation and thus reversing the shift towards bone resorption in osteoporosis. This review is focused on the current views regarding the implication of the stem cells in the cellular and physiologic mechanisms of osteoporosis and discusses data obtained from stem cell-based therapies of osteoporosis in experimental animal models and the possibility of their future application in clinical trials.

Single injection of PTH improves osteoclastic parameters of remodeling at a stress fracture site in rats

Stress fractures (SFx) result from repetitive cyclical loading of bone. They are frequent athletic injuries and underlie atypical femoral fractures following long-term bisphosphonate (BP) therapy. We investigated the effect of a single PTH injection on the healing of SFx in the rat ulna. SFx was induced in 120 female Wistar rats (300 ± 15 g) during a single loading session. A single PTH (8 µg.100g^-1 ) or vehicle (VEH) saline injection was administered 24 h after loading. Rats were divided into four groups (n = 15) and ulnae were examined 1, 2, 6, or 10 weeks following SFx. Two Toluidine Blue and TRAP-stained sections of the SFx were examined for histomorphometric analysis using Osteomeasure™ software. An increase in osteoclast number (N.Oc) and perimeter (Oc.Pm) was observed two weeks following PTH treatment (p < 0.01). At 6 weeks, bone formation was the main activity in BMUs. At 10 weeks, the proportion of healing along the SFx line remained 50% greater in PTH groups (p = 0.839), leading to a 43% reduction in the porosity area of BMU (p = 0.703). The main effect of time was a significant variable along the entire SFx remodeling cycle, with significant interactions between time and treatment type affecting (N.Oc) (p = 0.047) and (Oc.Pm) (p = 0.002). We conclude that a single PTH injection increases osteoclastogenesis by the second week of the remodeling cycle in a SFx in vivo. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Short-term intermittent PTH 1-34 administration and bone marrow blood vessel ossification in Mature and Middle-Aged C57BL/6 mice

Intermittent parathyroid hormone (PTH) administration augments bone and progressive bone marrow blood vessel (BMBV) ossification occurs with advancing age. Since intermittent PTH administration augments bone, it may also serve to increase BMBV ossification. We assessed the influence of 5- and 10-days of intermittent PTH 1–34 administration on trabecular and cortical bone and BMBV ossification in mature (6–8 mon; n = 30) and middle-aged (10–12 mon; n = 30) male and female C57BL/6 mice. Mice were divided accordingly: control (CON) and 5-days (5dPTH) and 10-days (10dPTH) of PTH. Mice were given PBS (50 μl) or PTH 1–34 (43 μg/kg/d) for 5- and 10-consecutive days. Trabecular bone microarchitecture (i.e., BV/TV [%], Tb.Th [μm], Tb.N [/mm], and Tb.Sp [μm]) was assessed in the distal femoral metaphysis and cortical bone parameters (i.e., Ct.Th [μm] and CSMI [mm⁴]) at the femoral mid-shaft. BMBV ossification (i.e., ossified vessel volume [OsVV, %] and ossified vessel thickness [OsV.Th, μm]) was assessed in the medullary cavity of the femoral shaft. All parameters were determined by μCT. At this sample size, no gender-related differences were observed so female and male data were pooled. There were no main effects nor interactions for trabecular microarchitecture and Ct.Th. However, CSMI was larger (p < 0.05) in Middle-Age vs. Mature and larger (p < 0.05) in CON and 10dPTH vs. 5dPTH. OsVV tended (p = 0.057) to be higher (0.18 ± 0.04% vs. 0.09 ± 0.02%, respectively) and OsV.Th was higher (p < 0.05; 17.4 ± 1.6 μm vs. 12.1 ± 1.4 μm, respectively) in Middle-Aged vs. Mature mice. OsVV was not altered, but ossified vessels tended (p = 0.08) to be thicker in 10dPTH (17.6 ± 2.0 μm) vs. CON (12.5 ± 1.7 μm). No interactions were observed for OsVV and OsV.Th. In conclusion, this is the first report of ossified BMBV in C57BL/6 mice. The increased OsV.Th in Middle-Aged mice coincides with previous reports of increased OsVV in aged rats. The tendency of augmented OsV.Th in 10dPTH suggests that this treatment may ultimately impair the patency of bone marrow blood vessels.

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Bone marrow blood vessel (BMBV) ossification occurs in rats and humans.

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This is the first report of BMBV ossification in Mature and Middle-Aged mice.

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Intermittent PTH administration tended to thicken ossified BMBV.

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PTH treatment may ultimately impact the patency of bone marrow blood vessels.

Effects of Intermittent Parathyroid Hormone 1-34 Administration on Circulating Mesenchymal Stem Cells in Postmenopausal Osteoporotic Women

BACKGROUND Intermittent parathyroid hormone (PTH) 1-34 administration stimulates osteogenesis and increases bone marrow mesenchymal stem cell (MSC) density; however, its effect on the circulating MSCs is unknown. This study aimed to examine the effect of intermittent PTH 1-34 administration on circulating MSCs in the peripheral blood of postmenopausal osteoporotic women. MATERIAL AND METHODS Fifty-four postmenopausal osteoporotic women at high risk of fracture were enrolled and administered either teriparatide (PTH 1-34) or alendronate for 12 months. Whole blood samples were obtained at baseline, 1, 3, 6, and 12 months after initiation of treatment. Flow cytometry analyses were performed to identify circulating MSCs (CD73+, CD90+, CD105+, CD34-, and CD45-). Serum markers of bone formation, bone resorption, as well as bone mineral density (BMD) were serially measured. Circulating MSCs were isolated from peripheral blood of teriparatide treated women and cultured in osteogenic medium to examine their osteogenic differentiation potential. RESULTS Teriparatide treatment increased circulating MSCs to 141±96% (P<0.001) by month 1, persisting until month 12; this increase was positively associated with increases in bone formation and bone resorption biomarkers (at month 6) and spine BMD (at month 12). Furthermore, intermittent PTH 1-34 administration promoted in vitro osteogenic differentiation of circulating MSCs, evident from increased alkaline phosphatase (ALP) activity, ALP-expressing cell density, calcium deposition, and Runx-2, OSX, COL 1a1, and osteocalcin mRNA upregulation. CONCLUSIONS Intermittent PTH 1-34 administration increased circulating MSC density in women with postmenopausal osteoporosis and enhanced in vitro osteogenic differentiation potential of these cells.

Intermittent Parathyroid Hormone Injection Can Decrease Femoral Head Collapse in the Vascular Deprivation of Rat Femoral Head Model

BACKGROUNDS

Intermittent parathyroid hormone (intermittent PTH) injection has been shown to improve osteogenesis. We hypothesized that intermittent PTH injection could stimulate osteogenesis during the early phase of vascular deprivation-induced femoral neck osteonecrosis in a rat model.

MATERIALS AND METHODS

Eighteen Sprague-Dawley rats were divided into three groups (normal saline [CON], PTH 10 μg/kg [PTH-H], and PTH 1 μg/kg [PTH-L]) for 8 weeks by subcutaneous injection. All rats were sacrificed at postoperative 8 weeks, and all underwent a micro-computed tomography (μ-CT) examination for bone quality and quantity evaluation and histomorphometric analysis for microscopic histologic differences.

RESULTS

Under μ-CT examination, both the PTH-H and PTH-L groups revealed less bone resorption than the control group. The PTH-H group had a better bone protective effect than the PTH-L group. Bone mineral density was increased in the PTH-H and PTH-L groups compared to the control group. The uninjured left femoral head was enlarged in both PTH groups. The histologic examination showed that both PTH groups had new bone and cartilage formation. The control group had only dead bone without any osteogenesis.

CONCLUSION

Intermittent PTH injection could decrease bone resorption and improve bone density, compared to the control group, in vascular deprivation of the femoral head in a rat model. High-level intermittent PTH injection had a better effect than low-level intermittent PTH injection.

Parathyroid hormone for bone regeneration

Delayed healing and/or non-union occur in approximately 5-10% of the fractures that occur annually in the United States. Segmental bone loss increases the probability of non-union. Though grafting can be an effective treatment for segmental bone loss, autografting is limited for large defects since a limited amount of bone is available for harvest. Parathyroid hormone (PTH) is a key regulator of calcium homeostasis in the body and plays an important role in bone metabolism. Presently PTH is FDA approved for use as an anabolic treatment for osteoporosis. The anabolic effect PTH has on bone has led to research on its use for bone regeneration applications. Numerous studies in animal models have indicated enhanced fracture healing as a result of once daily injections of PTH. Similarly, in a human case study, non-union persisted despite treatment attempts with internal fixation, external fixation, and autograft in combination with BMP-7, until off label use of PTH1-84 was utilized. Use of a biomaterial scaffold to locally deliver PTH to a defect site has also been shown to improve bone formation and healing around dental implants in dogs and drill defects in sheep. Thus, PTH may be used to promote bone regeneration and provide an alternative to autograft and BMP for the treatment of large segmental defects and non-unions. This review briefly summarizes the unmet clinical need for improved bone regeneration techniques and how PTH may help fill that void by both systemically and locally delivered PTH for bone regeneration applications. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2586-2594, 2018.

Regulation of Bone Remodeling by Parathyroid Hormone

Parathyroid hormone (PTH) exerts profound effects on skeletal homeostasis through multiple cellular and molecular mechanisms. Continuous hyperparathyroidism causes net loss of bone mass, despite accelerating bone formation by osteoblasts. Intermittent treatment with PTH analogs represents the only Food and Drug Administration (FDA)-approved bone anabolic osteoporosis treatment strategy. Functional PTH receptors are present on cells of the osteoblast lineage, ranging from early skeletal stem cells to matrix-embedded osteocytes. In addition, bone remodeling by osteoclasts liberates latent growth factors present within bone matrix. Here, we will provide an overview of the multiple cellular and molecular mechanisms through which PTH influences bone homeostasis. Notably, net skeletal effects of continuous versus intermittent can differ significantly. Where possible, we will highlight mechanisms through which continuous hyperparathyroidism leads to bone loss, and through which intermittent hyperparathyroidism boosts bone mass. Given the therapeutic usage of intermittent PTH (iPTH) treatment for osteoporosis, particular attention will be paid toward mechanisms underlying the bone anabolic effects of once daily PTH administration.

Glucocorticoid-induced osteoporosis: an update

Glucocorticoid-induced osteoporosis is the most common secondary cause of osteoporosis and the resulting fractures cause significant morbidity. Following initiation of oral glucocorticoids, rapid bone loss occurs, and fracture risk increases within a few months in a dose-dependent manner. These adverse effects are due to inhibition of bone formation accompanied by an early but transient increase in bone resorption. Multiple mechanisms underlie these changes in bone remodeling; direct effects include upregulation of PPARγR2, increased expression of sclerostin and increased RANKL/OPG ratio, whilst hypogonadism, altered renal and intestinal calcium handling, and reduced production of insulin-like growth factor 1 also contribute. Fracture risk assessment should be performed as soon as possible after glucocorticoids are initiated and bone protective therapy started promptly in individuals at high-risk, with calcium and vitamin D supplements where appropriate. Oral bisphosphonates are currently regarded as first line options on the grounds of their low cost. However, teriparatide has been shown to be superior in its effects on BMD and vertebral fracture risk in glucocorticoid-treated individuals with osteoporosis and should be considered as an alternative first line option in high-risk patients.

Influence of human parathyroid hormone during orthodontic tooth movement and relapse in the osteoporotic rat model: A preliminary study

OBJECTIVE

To investigate the effects of parathyroid hormone (PTH) on tooth movement in ovariectomized (OVX) rats by comparing the tooth movement distance and relapse and by examining the alveolar bone microstructure.

MATERIALS AND METHODS

Thirty 8-week-old female rats were classified into 3 groups: sham-operated, OVX and ovariectomized rats injected with PTH (PTH). Eight weeks later, a closed-coil spring appliance was placed between the maxillary incisor and the first molar and then activated with 50 cN of force. During tooth movement, 30 μg/kg of PTH was administered 3 times per week in the PTH group. Tooth movement distances were measured weekly. Five rats in each group were killed after 3 weeks for microcomputerized tomographic analysis, and the remaining 5 rats in each group were killed at an additional 3 weeks after the removal of the appliance to measure relapsed distance.

RESULTS

The OVX group showed significantly greater tooth movement compared to those in the other 2 groups at 2 and 3 weeks (P < .05). The relapse distance and relapse percentage for the OVX group were higher; however, it did not differ significantly from the PTH group. On micro-CT analysis, bone volume/tissue volume ratio and bone mineral density in the PTH group were significantly greater than in the OVX group (P < .05).

CONCLUSIONS

Application of PTH did not promote tooth movement in OVX rat, however, did lead to decrease in relapse tendency. Therefore, the application of PTH during orthodontic treatment of patients with osteoporosis should be carefully considered.

Parathyroid Hormone Signaling in Osteocytes

Osteocytes are the most abundant cell type in bone and play a central role in orchestrating skeletal remodeling, in part by producing paracrine‐acting factors that in turn influence osteoblast and osteoclast activity. Recent evidence has indicated that osteocytes are crucial cellular targets of parathyroid hormone (PTH). Here, we will review the cellular and molecular mechanisms through which PTH influences osteocyte function. Two well‐studied PTH target genes in osteocytes are SOST and receptor activator of NF‐κB ligand (RANKL). The molecular mechanisms through which PTH regulates expression of these two crucial target genes will be discussed. Beyond SOST and RANKL, PTH/PTH‐related peptide (PTHrP) signaling in osteocytes may directly influence the way osteocytes remodel their perilacunar environment to influence bone homeostasis in a cell‐autonomous manner. Here, I will highlight novel, additional mechanisms used by PTH and PTHrP to modulate bone homeostasis through effects in osteocytes. © 2017 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Intermittent Administration of Parathyroid Hormone 1-34 Enhances Osteogenesis of Human Mesenchymal Stem Cells by Regulating Protein Kinase Cδ

Human mesenchymal stem cells (hMSCs) can differentiate into osteoblasts and are regulated by chemical cues. The recombinant N-terminal (1–34 amino acids) fragment of the parathyroid hormone (PTH (1–34)) is identified to promote osteogenesis. The osteoanabolic effects of intermittent PTH (1–34) treatment are linked to a complex consisting of signaling pathways; additionally, protein kinase C (PKC) act as mediators of multifunctional signaling transduction pathways, but the role of PKC δ (PKCδ), a downstream target in regulating osteoblast differentiation during intermittent administration of PTH (1–34) is less studied and still remains elusive. The purpose of this study is to examine the role of PKCδ during intermittent and continuous PTH (1–34) administration using osteoblast-lineage-committed hMSCs. Relative gene expression of osteoblast-specific genes demonstrated significant upregulation of RUNX2, type I Collagen, ALP, and Osterix and increased alkaline phosphatase activity in the presence of PTH (1–34). Intermittent PTH (1–34) administration increased PKC activity at day 7 of osteogenic differentiation, whereas inhibition of PKC activity attenuated these effects. In addition, the specific isoform PKCδ was activated upon treatment. These findings demonstrate that intermittent PTH (1–34) treatment enhances the osteogenesis of hMSCs by upregulating osteoblast-specific genes via PKCδ activation.

Parathyroid hormone regulates fates of murine osteoblast precursors in vivo

Teriparatide, a recombinant form of parathyroid hormone (PTH), is the only approved treatment for osteoporosis that increases the rate of bone formation. Teriparatide increases osteoblast numbers by suppressing osteoblast apoptosis and activating bone-lining cells. No direct evidence for teriparatide's actions on early cells of the osteoblast lineage has been demonstrated. Here, we have employed a lineage-tracing strategy that uses a tamoxifen-dependent, promoter-driven cre to mark early cells of the osteoblast lineage in adult mice. We show that teriparatide increases the numbers of osteoblast precursors and drives their differentiation into mature osteoblasts. Unexpectedly, following withdrawal of teriparatide therapy, bone marrow adipocytes increased dramatically in number. Some of these adipocytes derived from cells marked by Sox9-cre expression weeks earlier. Continued therapy with teriparatide prevented the appearance of adipocytes. Selective, inducible deletion of the PTH receptor in Sox9-cre cells demonstrated that PTH receptor expression is required for teriparatide-mediated increases in early osteoblast precursors. The increase in early precursors after teriparatide administration was associated with robust suppression of precursor apoptosis without affecting their rate of proliferation. Thus, teriparatide increases the numbers of early cells of the osteoblast lineage, hastens their differentiation into osteoblasts, and suppresses their differentiation into adipocytes in vivo.

Physiological Bone Remodeling: Systemic Regulation and Growth Factor Involvement

Bone remodeling is essential for adult bone homeostasis. It comprises two phases: bone formation and resorption. The balance between the two phases is crucial for sustaining bone mass and systemic mineral homeostasis. This review highlights recent work on physiological bone remodeling and discusses our knowledge of how systemic and growth factors regulate this process.

Mitogen-activated protein kinase phosphatase-1: function and regulation in bone and related tissues

In this review, we have highlighted work that has clearly demonstrated that mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1), a negative regulator of MAPKs, is an important signaling mediator in bone, muscle, and fat tissue homeostasis and differentiation. Further, we examined recent studies with particular focus on MKP-1 overexpression or deletion and its impact on tissues connected to bone. We also summarized regulation of MKP-1 by known skeletal regulators like parathyroid hormone (PTH)/PTH-related peptide (PTHrP) and bone morphogenic proteins. MKP-1's integration into the pathophysiological state of osteoporosis, osteoarthritis, rheumatoid arthritis, obesity, and muscular dystrophy are examined to emphasize possible involvement of MKP-1 both at the molecular level and in disease complications such as sarcopenia- or diabetes-related osteoporosis. We predict that understanding the mechanism of MKP-1-mediated signaling in bone-muscle-fat crosstalk will be a key in coordinating their activities and developing therapeutics to improve clinical outcomes for diseases associated with advanced age.

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.

Short-term intermittent administration of parathyroid hormone facilitates osteogenesis by different mechanisms in cancellous and cortical bone

Intermittent administration of human parathyroid hormone (1–34)[hPTH(1–34)] induces anabolic action on the bones. To understand the mechanism underlying the early phase of hPTH(1–34)-induced anabolic action, we investigated the expression profiles of osterix and sclerostin after short-term intermittent administration of hPTH(1–34) using immunohistochemistry in adult rats. In the cancellous bone, hPTH(1–34) administration greatly increased the number of osterix-positive cells in the bone marrow on day 1, but the cells gradually decreased on days 3 and 5. Injections of hPTH(1–34) induced no significant changes in the number of sclerostin-positive osteocytes in the cancellous bone. In the cortical bone, intermittent administration of hPTH(1–34) significantly reduced the number of sclerostin-positive osteocytes. The serum sclerostin level was downregulated and the osteocalcin level was upregulated on day 5 after intermittent administration of hPTH(1–34). Intermittent hPTH(1–34) injections increased osteoblast surface, osteoid thickness, and osteoid surface in cancellous bone, but not in cortical bone. This study suggested that the increase in osterix-positive osteoprogenitors in cancellous bone and the decrease in sclerostin-positive osteocytes in cortical bone play important roles in anabolic action on osteogenesis induced by short-term administration of hPTH(1–34).

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We analyzed the effects of hPTH(1–34) injection into rats at early phase.

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hPTH(1–34) injection increased the osterix-positive cells in bone marrow.

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hPTH(1–34) injection decreased sclerostin-positive cells in cortical bone.

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hPTH(1–34) exerts different effects in cancellous and cortical bone.

T cells, osteoblasts, and osteocytes: interacting lineages key for the bone anabolic and catabolic activities of parathyroid hormone

Osteoimmunology is a field of research dedicated to the study of the interactions between the immune system and bone. Among the cells of the immune system that regulate bone turnover and the responsiveness of bone cells to calciothropic hormones are bone marrow T lymphocytes. T cells secrete osteoclastogenic cytokines such as RANKL and TNF-α, as well as factors that stimulate bone formation, one of which is Wnt10b. In addition, T cells regulate the differentiation and life span of stromal cells (SCs) and their responsiveness to parathyroid hormone (PTH) via costimulatory molecules expressed on their surface. The conditioning effect of T cells on SCs is inherited by the osteoblastic and osteocytic progeny of SCs. As a result, osteoblastic cells of T cell-deficient mice have functional characteristics different from corresponding cells of T cell-replete mice. These differences include the ratio of RANKL/OPG produced in response to continuous PTH treatment, and the osteoblastogenic response to intermittent PTH treatment. This article reviews the evidence indicating that the effects of PTH are mediated not only by osteoblasts and osteocytes but also by T cells.

DMP-1-mediated Ghr gene recombination compromises skeletal development and impairs skeletal response to intermittent PTH

Bone minerals are acquired during growth and are key determinants of adult skeletal health. During puberty, the serum levels of growth hormone (GH) and its downstream effector IGF-1 increase and play critical roles in bone acquisition. The goal of the current study was to determine how bone cells integrate signals from the GH/IGF-1 to enhance skeletal mineralization and strength during pubertal growth. Osteocytes, the most abundant bone cells, were shown to orchestrate bone modeling during growth. We used dentin matrix protein (Dmp)-1-mediated Ghr knockout (DMP-GHRKO) mice to address the role of the GH/IGF axis in osteocytes. We found that DMP-GHRKO did not affect linear growth but compromised overall bone accrual. DMP-GHRKO mice exhibited reduced serum inorganic phosphate and parathyroid hormone (PTH) levels and decreased bone formation indices and were associated with an impaired response to intermittent PTH treatment. Using an osteocyte-like cell line along with in vivo studies, we found that PTH sensitized the response of bone to GH by increasing Janus kinase-2 and IGF-1R protein levels. We concluded that endogenously secreted PTH and GHR signaling in bone are necessary to establish radial bone growth and optimize mineral acquisition during growth.

Early Effects of Single and Low-Frequency Repeated Administration of Teriparatide, hPTH(1-34), on Bone Formation and Resorption in Ovariectomized Rats

Intermittent repeated administration of teriparatide (TPTD) has potent anabolic effects on bones in vivo. However, TPTD has both anabolic and catabolic effects on osteoblasts in vitro, and the mechanisms underlying its promotion of bone formation are unclear. This study aimed to elucidate the time-dependent changes in bone formation and resorption by examining changes in bone turnover markers and bone tissue over time after TPTD administration with low frequency in ovariectomized rats. The amount of serum osteocalcin, a bone formation marker, was transiently reduced after single TPTD administration, but increased thereafter, remaining increased for several days. In contrast, the amount of excreted urinary C-telopeptide, a bone resorption marker, increased transiently after single TPTD administration, and subsequently returned to control levels on the day after administration. Tissue histomorphometric analyses conducted 8 h after administration showed no changes in bone formation or bone resorption parameters. However, at 48 h, the bone formation parameters OS/BS and Ob.S/BS were increased, while the bone resorption parameter ES/BS was decreased. After repeated TPTD administration for 4 weeks, OS/BS, Ob.S/BS, and MS/BS increased, while Oc.S/BS decreased. Serum osteocalcin at 4 weeks after repeated administration was significantly correlated with OS/BS and Ob.S/BS. These present findings indicate that TPTD has dual, time-dependent effects on bone resorption and bone formation. Immediately after single administration, there was transient promotion of bone resorption and suppression of bone formation. However, sustained stimulation of bone formation occurred thereafter. Furthermore, these data suggest that this sustained bone formation led to anabolic effects with repeated TPTD administration.

Gender-Specific Differences in the Skeletal Response to Continuous PTH in Mice Lacking the IGF1 Receptor in Mature Osteoblasts

The primary goal of this study was to determine whether the IGF1R in mature osteoblasts and osteocytes was required for the catabolic actions of continuous parathyroid hormone (cPTH). Igf1r was deleted from male and female FVN/B mice by breeding with mice expressing cre recombinase under control of the osteocalcin promoter ((0CN) Igfr1(-/-) ). Littermates lacking the cre recombinase served as controls. PTH, 60 μg/kg/d, was administered continuously by Alzet minipumps for 4 weeks. Blood was obtained for indices of calcium metabolism. The femurs were examined by micro-computed tomography for structure, immunohistochemistry for IGF1R expression, histomorphometry for bone formation rates (BFR), mRNA levels by qPCR, and bone marrow stromal cell cultures (BMSC) for alkaline phosphatase activity (ALP(+) ), mineralization, and osteoblast-induced osteoclastogenesis. Whereas cPTH led to a reduction in trabecular bone volume/tissue volume (BV/TV) and cortical thickness in the control females, no change was found in the control males. Although trabecular BV/TV and cortical thickness were reduced in the (0CN) Igfr1(-/-) mice of both sexes, no further reduction after cPTH was found in the females, unlike the reduction in males. BFR was stimulated by cPTH in the controls but blocked by Igf1r deletion in the females. The (0CN) Igfr1(-/-) male mice showed a partial response. ALP(+) and mineralized colony formation were higher in BMSC from control males than from control females. These markers were increased by cPTH in both sexes, but BMSC from male (0CN) Igfr1(-/-) also were increased by cPTH, unlike those from female (0CN) Igfr1(-/-) . cPTH stimulated receptor activator of NF-κB ligand (RANKL) and decreased osteoprotegerin and alkaline phosphatase expression more in control female bone than in control male bone. Deletion of Igf1r blocked these effects of cPTH in the female but not in the male. However, PTH stimulation of osteoblast-driven osteoclastogenesis was blocked by deleting Igfr1 in both sexes. We conclude that cPTH is catabolic in female but not male mice. Moreover, IGF1 signaling plays a greater role in the skeletal actions of cPTH in the female mouse than in the male mouse, which may underlie the sex differences in the response to cPTH.

T cell-expressed CD40L potentiates the bone anabolic activity of intermittent PTH treatment

T cells are known to potentiate the bone anabolic activity of intermittent parathyroid hormone (iPTH) treatment. One of the involved mechanisms is increased T cell secretion of Wnt10b, a potent osteogenic Wnt ligand that activates Wnt signaling in stromal cells (SCs). However, additional mechanisms might play a role, including direct interactions between surface receptors expressed by T cells and SCs. Here we show that iPTH failed to promote SC proliferation and differentiation into osteoblasts (OBs) and activate Wnt signaling in SCs of mice with a global or T cell-specific deletion of the T cell costimulatory molecule CD40 ligand (CD40L). Attesting to the relevance of T cell-expressed CD40L, iPTH induced a blunted increase in bone formation and failed to increase trabecular bone volume in CD40L(-/-) mice and mice with a T cell-specific deletion of CD40L. CD40L null mice exhibited a blunted increase in T cell production of Wnt10b and abrogated CD40 signaling in SCs in response to iPTH treatment. Therefore, expression of the T cell surface receptor CD40L enables iPTH to exert its bone anabolic activity by activating CD40 signaling in SCs and maximally stimulating T cell production of Wnt10b.

Muscle-bone and fat-bone interactions in regulating bone mass: do PTH and PTHrP play any role?

Metabolic bone disease occurs when there is a net loss in bone density. Osteoporosis, the most common metabolic bone disease, is a devastating problem and an increasingly major public health issue. A substantial body of evidence in the elderly population indicates that a relationship exists between the components of body weight and various measures of bone/mass, density, and function. Both muscle and fat contribute to the body's total weight and the intimate associations of muscle, fat, and bone are known. But the close functional interactions between muscle and bone or fat and bone are largely unidentified and have drawn much attention in recent years. Each of these tissues not only responds to afferent signals from traditional hormone systems and the central nervous systems but also secretes factors with important endocrine functions. Studies suggest that during growth, development, and aging, the relationship of muscle and fat with the skeleton possibly governs bone homeostasis and turnover. A better understanding of the endocrine function and the cellular and molecular mechanisms and pathways linking muscle or adipose tissues with bone anabolism and catabolism is a new avenue for novel pathways for anabolic drug discovery. These in turn will likely lead to more rational therapy toward increasingly prevalent disorders like osteoporosis. In this review, some of the recent works on the interaction of bone with muscle and fat are highlighted, and in so doing the role of parathyroid hormone (PTH), and PTH-related peptide (PTHrP) is surveyed.

Roles of parathyroid hormone (PTH) receptor and reactive oxygen species in hyperlipidemia-induced PTH resistance in preosteoblasts

Bioactive lipids initiate inflammatory reactions leading to pathogenesis of atherosclerosis. Evidence shows that they also contribute to bone loss by inhibiting parathyroid hormone receptor (PTH1R) expression and differentiation of osteoblasts. We previously demonstrated that bone anabolic effects of PTH(1-34) are blunted in hyperlipidemic mice and that these PTH effects are restored by antioxidants. However, it is not clear which osteoblastic cell developmental stage is targeted by bioactive lipids. To investigate the effects of hyperlipidemia at the cellular level, hyperlipidemic Ldlr(-/-) mice were bred with Col3.6GFPtpz mice, in which preosteoblasts/osteoblasts carry a topaz fluorescent label, and with Col2.3GFPcyan mice, in which more mature osteoblasts/osteocytes carry a cyan fluorescent label. Histological analyses of trabecular bone surfaces in femoral as well as calvarial bones showed that intermittent PTH(1-34) increased fluorescence intensity in WT-Tpz mice, but not in Tpz-Ldlr(-/-) mice. In contrast, PTH(1-34) did not alter fluorescence intensity in femoral cortical envelopes of either WT-Cyan or Ldlr(-/-)-Cyan mice. To test the mechanism of PTH1R downregulation, preosteoblastic MC3T3-E1 cells were treated with bioactive lipids and the antioxidant Trolox. Results showed that inhibitory effects of PTH1R levels by bioactive lipids were rescued by pretreatment with Trolox. The inhibitory effects on expression of PTH1R as well as on PTH-induced osteoblastic genes were mimicked by xanthine/xanthine oxidase, a known generator of reactive oxygen species. These findings suggest an important role of the preosteoblastic development stage as the target and downregulation of PTH receptor expression mediated by intracellular oxidant stress as a mechanism in hyperlipidemia-induced PTH resistance.

Stem cell therapy for osteoporosis

Osteoporosis is a debilitating disease that affects millions of people worldwide. Current osteoporosis treatments are predominantly bone-resorbing drugs that are associated with several side effects. The use of stem cells for tissue regeneration has raised great hope in various fields of medicine, including musculoskeletal disorders. Stem cell therapy for osteoporosis could potentially reduce the susceptibility of fractures and augment lost mineral density by either increasing the numbers or restoring the function of resident stem cells that can proliferate and differentiate into bone-forming cells. Such osteoporosis therapies can be carried out by exogenous introduction of mesenchymal stem cells (MSCs), typically procured from bone marrow, adipose, and umbilical cord blood tissues or through treatments with drugs or small molecules that recruit endogenous stem cells to osteoporotic sites. The main hurdle with cell-based osteoporosis therapy is the uncertainty of stem cell fate and biodistribution following cell transplantation. Therefore, future advancements will focus on long-term engraftment and differentiation of stem cells at desired bone sites for tangible clinical outcome.

Profile of changes in bone turnover markers during once-weekly teriparatide administration for 24 weeks in postmenopausal women with osteoporosis

SUMMARY

Changes in bone turnover markers with weekly 56.5 μg teriparatide injections for 24 weeks were investigated in women with osteoporosis. Changes in bone turnover markers 24 h after each injection of teriparatide were constant. During the 24 week period, bone formation markers increased and baseline bone resorption marker levels were maintained.

INTRODUCTION

This study aimed to clarify the changes in bone turnover markers during 24 weeks of once-weekly teriparatide injections in postmenopausal women with osteoporosis.

METHODS

The 24 h changes in pharmacokinetics (PK), calcium metabolism, and bone turnover markers (serum osteocalcin, procollagen type I N-terminal propeptide (P1NP), urinary cross-linked N-telopeptide of type I collagen (NTX), deoxypiridinoline (DPD)) after each injection of 56.5 μg teriparatide at the data collection weeks (0, 4, 12, and 24 weeks) were investigated. The changes were evaluated by comparison with the data at 0 h in each data collection week.

RESULTS

Similar 24 h changes in each parameter after injection of teriparatide were observed in each data collection week. Serum calcium increased transiently, and intact PTH decreased 4-8 h after injection; serum calcium subsequently returned to baseline levels. Calcium and intact PTH levels decreased for 24 weeks. Although serum osteocalcin decreased at 24 h, it was significantly increased at 4 weeks. P1NP decreased transiently and then increased significantly at 24 h. P1NP was significantly increased at 4 weeks. Urinary NTX and DPD were significantly increased transiently and then decreased at 24 h. The urinary DPD level decreased significantly at 4 weeks.

CONCLUSIONS

Twenty-four hour changes in PK, calcium metabolism, and bone turnover markers showed the same direction and level after once-weekly teriparatide injections for 24 weeks, with no attenuation of the effect over time. After 24 weeks, the bone formation marker, serum osteocalcin, increased significantly, but the serum P1NP, did not. Bone resorption markers decreased or remained the same.

The sclerostin-independent bone anabolic activity of intermittent PTH treatment is mediated by T-cell-produced Wnt10b

Both blunted osteocytic production of the Wnt inhibitor sclerostin (Scl) and increased T-cell production of the Wnt ligand Wnt10b contribute to the bone anabolic activity of intermittent parathyroid hormone (iPTH) treatment. However, the relative contribution of these mechanisms is unknown. In this study, we modeled the repressive effects of iPTH on Scl production in mice by treatment with a neutralizing anti-Scl antibody (Scl-Ab) to determine the contribution of T-cell-produced Wnt10b to the Scl-independent modalities of action of iPTH. We report that combined treatment with Scl-Ab and iPTH was more potent than either iPTH or Scl-Ab alone in increasing stromal cell production of OPG, osteoblastogenesis, osteoblast life span, bone turnover, bone mineral density, and trabecular bone volume and structure in mice with T cells capable of producing Wnt10b. In T-cell-null mice and mice lacking T-cell production of Wnt10b, combined treatment increased bone turnover significantly more than iPTH or Scl-Ab alone. However, in these mice, combined treatment with Scl-Ab and iPTH was equally effective as Scl-Ab alone in increasing the osteoblastic pool, bone volume, density, and structure. These findings demonstrate that the Scl-independent activity of iPTH on osteoblasts and bone mass is mediated by T-cell-produced Wnt10b. The data provide a proof of concept of a more potent therapeutic effect of combined treatment with iPTH and Scl-Ab than either alone.

A novel flavonoid C-glucoside from Ulmus wallichiana preserves bone mineral density, microarchitecture and biomechanical properties in the presence of glucocorticoid by promoting osteoblast survival: a comparative study with human parathyroid hormone

PURPOSE

6-C-β-D-glucopyranosyl-(2S,3S)-(+)-5,7,3',4'-tetrahydroxydihydroflavonol (GTDF) is a novel compound isolated from Ulmus wallichiana, reported to have bone anabolic action in ovariectomized rats. Here, we studied the effect of GTDF in glucocorticoid (GC)-induced bone loss and its mode of action.

METHODS

Osteoblasts were cultured from rat calvaria or bone marrow to study apoptosis and differentiation by dexamethasone (Dex), methylprednisolone (MP), GTDF, quercetin and rutin. Female Sprague Dawley rats were treated with Dex or MP with or without GTDF or PTH. Efficacy was evaluated by bone microarchitecture using microcomputed tomography, determination of new bone formation by fluorescent labeling of bone and osteoblast apoptosis by co-labeling bone sections with Runx-2 and TUNEL. Serum osteocalcin was determined by ELISA.

RESULTS

GTDF preserved trabecular and cortical bones in the presence of Dex and MP and mitigated the MP-mediated suppression of serum osteocalcin. Co-administration of GTDF to MP rats increased mineral apposition, bone formation rates, bone biomechanical strength, reduced osteoblast apoptosis and increased osteogenic differentiation of bone marrow stromal cells compared to MP group, suggesting in vivo osteogenic effect of GTDF. These effects of GTDF were to a great extent comparable to PTH. GTDF prevented GC-induced osteoblast apoptosis by inhibiting p53 expression and acetylation, and activation of AKT but did not influence transactivation of GC receptor (GR).

CONCLUSIONS

GTDF protects against GC-induced bone loss by promoting osteoblast survival through p53 inhibition and activation of AKT pathways but not as a GR antagonist. GTDF has the potential in the management of GC-induced osteopenia.