Determination of dual effects of parathyroid hormone on skeletal gene expression in vivo by microarray and network analysis

The osteocytic actions of glucocorticoids on bone mass, mechanical properties, or perilacunar remodeling outcomes are not rescued by PTH(1-34)

Glucocorticoids (GC) and parathyroid hormone (PTH) are widely used therapeutic endocrine hormones where their effects on bone and joint arise from actions on multiple skeletal cell types. In osteocytes, GC and PTH exert opposing effects on perilacunar canalicular remodeling (PLR). Suppressed PLR can impair bone quality and joint homeostasis, including in GC-induced osteonecrosis. However, combined effects of GC and PTH on PLR are unknown. Given the untapped potential to target osteocytes to improve skeletal health, this study sought to test the feasibility of therapeutically mitigating PLR suppression. Focusing on subchondral bone and joint homeostasis, we hypothesize that PTH(1-34), a PLR agonist, could rescue GC-suppressed PLR. The skeletal effects of GC and PTH(1-34), alone or combined, were examined in male and female mice by micro-computed tomography, mechanical testing, histology, and gene expression analysis. For each outcome, females were more responsive to GC and PTH(1-34) than males. GC and PTH(1-34) exerted regional differences, with GC increasing trabecular bone volume but reducing cortical bone thickness, stiffness, and ultimate force. Despite PTH(1-34)'s anabolic effects on trabecular bone, it did not rescue GC's catabolic effects on cortical bone. Likewise, cartilage integrity and subchondral bone apoptosis, tartrate-resistant acid phosphatase (TRAP) activity, and osteocyte lacunocanalicular networks showed no evidence that PTH(1-34) could offset GC-dependent effects. Rather, GC and PTH(1-34) each increased cortical bone gene expression implicated in bone resorption by osteoclasts and osteocytes, including Acp5, Mmp13, Atp6v0d2, Ctsk, differences maintained when GC and PTH(1-34) were combined. Since PTH(1-34) is insufficient to rescue GC's effects on young female mouse bone, future studies are needed to determine if osteocyte PLR suppression, due to GC, aging, or other factors, can be offset by a PLR agonist.

The Molecular Mechanisms Underlying the Systemic Effects Mediated by Parathormone in the Context of Chronic Kidney Disease

Chronic kidney disease (CKD) stands as a prominent non-communicable ailment, significantly impacting life expectancy. Physiopathology stands mainly upon the triangle represented by parathormone-Vitamin D-Fibroblast Growth Factor-23. Parathormone (PTH), the key hormone in mineral homeostasis, is one of the less easily modifiable parameters in CKD; however, it stands as a significant marker for assessing the risk of complications. The updated "trade-off hypothesis" reveals that levels of PTH spike out of the normal range as early as stage G2 CKD, advancing it as a possible determinant of systemic damage. The present review aims to review the effects exhibited by PTH on several organs while linking the molecular mechanisms to the observed actions in the context of CKD. From a diagnostic perspective, PTH is the most reliable and accessible biochemical marker in CKD, but its trend bears a higher significance on a patient's prognosis rather than the absolute value. Classically, PTH acts in a dichotomous manner on bone tissue, maintaining a balance between formation and resorption. Under the uremic conditions of advanced CKD, the altered intestinal microbiota majorly tips the balance towards bone lysis. Probiotic treatment has proven reliable in animal models, but in humans, data are limited. Regarding bone status, persistently high levels of PTH determine a reduction in mineral density and a concurrent increase in fracture risk. Pharmacological manipulation of serum PTH requires appropriate patient selection and monitoring since dangerously low levels of PTH may completely inhibit bone turnover. Moreover, the altered mineral balance extends to the cardiovascular system, promoting vascular calcifications. Lastly, the involvement of PTH in the Renin-Angiotensin-Aldosterone axis highlights the importance of opting for the appropriate pharmacological agent should hypertension develop.

The effect of aquatic training and vitamin D3 supplementation on bone metabolism in postmenopausal obese women

Purpose

Despite prevalence of studies indicating the positive effect of land-based exercise on bone metabolism, there are limited findings regarding the effect of aquatic exercise. The present study aimed to evaluate the effects of aquatic training and vitamin D3 supplementation on femur bone mineral density (BMD), serum 25(OH)D, and parathyroid hormone (PTH) in postmenopausal obese women with vitamin D insufficiency.

Methods

40 postmenopausal obese women were randomly divided into four groups of aquatic training + vitamin D3 intake group; (ATD), aquatic training with placebo intake group (AT), vitamin D3 intake group (D), and control group with placebo intake (CON). AT groups performed aerobic aquatic exercises for 8 weeks. Vitamin D3 supplementation groups consumed oral dose of 4000 IU/d for 8 weeks.

Results

The femur BMD was significantly higher in the ATD than the AT and D and CON groups; in AT it was higher than the D and CON groups. Serum 25(OH)D level in the ATD was more than AT and CON, and in the D was more than the CON and AT. PTH in the ATD group was lower compared to AT, D, and CON groups. PTH was lower in the AT and D compared to the CON.

Conclusion

In postmenopausal obese women with vitamin D insufficiency or deficiency, combining vitamin D supplementation and aquatic training was the most effective method for improving bone metabolism; Vitamin D supplementation (alone) was not sufficient to affect some of bone metabolism indices; Aquatic training could not improve serum vitamin D. By priority, ATD, AT, and D indicated better bone related metabolism indices.

Early gene expression profiles of anabolic and catabolic molecules in murine bone after a single PTH injection

The current study examined the gene expression profiles of anabolic and catabolic molecules after a single parathyroid hormone (PTH) injection in mice. No significant changes were observed in alkaline phosphatase area/tissue volume, tartrate-resistant acid phosphatase-positive osteoclasts, or static bone histomorphometry parameters. However, a sudden and significant decrease in Runx2 expression occurred at 1.5 h post-injection followed by immediate elevation, while sclerostin level was initially downregulated but gradually recovered. Meanwhile, Rankl expression initially increased and then returned to baseline. The prolonged elevation of anabolic molecules and transient increase in catabolic molecules may contribute to the anabolic effect of PTH treatment.

Differential Effects of PTH (1-34), PTHrP (1-36), and Abaloparatide on the Murine Osteoblast Transcriptome

Teriparatide (PTH (1-34)), PTHrP (1-36), and abaloparatide (ABL) have been used for the treatment of osteoporosis, but their efficacy long term is significantly limited. The 3 peptides exert time- and dose-dependent differential responses in osteoblasts, leading us to hypothesize they may also differentially modulate the osteoblast transcriptome. Treatment of mouse calvarial osteoblasts with 1 nM of the peptides for 4 hours results in RNA sequencing data with PTH (1-34) regulating 367 genes, including 194 unique genes; PTHrP (1-36) regulating 117 genes, including 15 unique genes; and ABL regulating 179 genes, including 20 unique genes. There were 83 genes shared among all 3 peptides. Gene ontology analyses showed similarities in Wnt signaling, cAMP-mediated signaling, ossification, but differences in morphogenesis of a branching structure in biological processes; receptor ligand activity, transcription factor activity, and cytokine receptor/binding activity in molecular functions. The peptides increased Vdr, Cited1, and Pde10a messenger RNAs (mRNAs) in a pattern similar to Rankl, that is, PTH (1-34) greater than ABL greater than PTHrP (1-36). mRNA abundance of other genes, including Wnt4, Wnt7, Wnt11, Sfrp4, Dkk1, Kcnk10, Hdac4, Epn3, Tcf7, Crem, Fzd5, Ppp2r2a, and Dvl3, showed that some genes were regulated similarly by all 3 peptides; others were not. Finally, small interfering RNA knockdowns of SIK1/2/3 and CRTC1/2/3 in PTH (1-34)-treated cells revealed that Vdr and Wnt4 genes are regulated by salt-inducible kinases (SIKs) and CREB-regulated transcriptional coactivators (CRTCs), while others are not. Although many studies have examined PTH signaling in the osteoblast/osteocyte, ours is the first to compare the global effects of these peptides on the osteoblast transcriptome or to analyze the roles of the SIKs and CRTCs.

Sfrp4 is required to maintain Ctsk-lineage periosteal stem cell niche function

We have previously reported that the cortical bone thinning seen in mice lacking the Wnt signaling antagonist Sfrp4 is due in part to impaired periosteal apposition. The periosteum contains cells which function as a reservoir of stem cells and contribute to cortical bone expansion, homeostasis, and repair. However, the local or paracrine factors that govern stem cells within the periosteal niche remain elusive. Cathepsin K (Ctsk), together with additional stem cell surface markers, marks a subset of periosteal stem cells (PSCs) which possess self-renewal ability and inducible multipotency. Sfrp4 is expressed in periosteal Ctsk-lineage cells, and Sfrp4 global deletion decreases the pool of PSCs, impairs their clonal multipotency for differentiation into osteoblasts and chondrocytes and formation of bone organoids. Bulk RNA sequencing analysis of Ctsk-lineage PSCs demonstrated that Sfrp4 deletion down-regulates signaling pathways associated with skeletal development, positive regulation of bone mineralization, and wound healing. Supporting these findings, Sfrp4 deletion hampers the periosteal response to bone injury and impairs Ctsk-lineage periosteal cell recruitment. Ctsk-lineage PSCs express the PTH receptor and PTH treatment increases the % of PSCs, a response not seen in the absence of Sfrp4. Importantly, in the absence of Sfrp4, PTH-dependent increase in cortical thickness and periosteal bone formation is markedly impaired. Thus, this study provides insights into the regulation of a specific population of periosteal cells by a secreted local factor, and shows a central role for Sfrp4 in the regulation of Ctsk-lineage periosteal stem cell differentiation and function.

Differential effects of PTH (1-34), PTHrP (1-36) and abaloparatide on the murine osteoblast transcriptome

Teriparatide (PTH(1-34)) and its analogs, PTHrP(1-36) and abaloparatide (ABL) have been used for the treatment of osteoporosis, but their efficacy over long-term use is significantly limited. The 3 peptides exert time- and dose-dependent differential responses in osteoblasts, leading us to hypothesize that they may also differentially modulate the osteoblast transcriptome. We show that treatment of mouse calvarial osteoblasts with 1 nM of the 3 peptides for 4 h results in RNA-Seq data with PTH(1-34) regulating 367 genes, including 194 unique genes; PTHrP(1-36) regulating 117 genes, including 15 unique genes; and ABL regulating 179 genes, including 20 unique genes. There were 83 genes shared among all 3 peptides. Gene ontology analyses showed differences in Wnt signaling, cAMP-mediated signaling, bone mineralization, morphogenesis of a branching structure in biological processes; receptor ligand activity, transcription factor activity, cytokine receptor/binding activity and many other actions in molecular functions. The 3 peptides increased Vdr, Cited1 and Pde10a mRNAs in a pattern similar to Rankl , i.e., PTH(1-34) > ABL > PTHrP(1-36). mRNA abundance of other genes based on gene/pathway analyses, including Wnt4, Wnt7, Wnt11, Sfrp4, Dkk1, Kcnk10, Hdac4, Epha3, Tcf7, Crem, Fzd5, Pp2r2a , and Dvl3 showed that some genes were regulated similarly by all 3 peptides; others were not. Finally, siRNA knockdowns of SIK1/2/3 and CRTC1/2/3 in PTH(1-34)-treated cells revealed that Vdr and Wnt4 genes are regulated by SIKs and CRTCs, while others are not. Although many studies have examined PTH signaling in the osteoblast/osteocyte, ours is the first to examine the global effects of these peptides on the osteoblast transcriptome. Further delineation of which signaling events are attributable to PTH(1-34), PTHrP(1-36) or ABL exclusively and which are shared among all 3 will help improve our understanding of the effects these peptides have on the osteoblast and lead to the refinement of PTH-derived treatments for osteoporosis.

Mitochondrial β-oxidation of adipose-derived fatty acids by osteoblasts fuels parathyroid hormone-induced bone formation

The energetic costs of bone formation require osteoblasts to coordinate their activities with tissues, like adipose, that can supply energy-dense macronutrients. In the case of intermittent parathyroid hormone (PTH) treatment, a strategy used to reduce fracture risk, bone formation is preceded by a change in systemic lipid homeostasis. To investigate the requirement for fatty acid oxidation by osteoblasts during PTH-induced bone formation, we subjected mice with osteoblast-specific deficiency of mitochondrial long-chain β-oxidation as well as mice with adipocyte-specific deficiency for the PTH receptor or adipose triglyceride lipase to an anabolic treatment regimen. PTH increased the release of fatty acids from adipocytes and β-oxidation by osteoblasts, while the genetic mouse models were resistant to the hormone's anabolic effect. Collectively, these data suggest that PTH's anabolic actions require coordinated signaling between bone and adipose, wherein a lipolytic response liberates fatty acids that are oxidized by osteoblasts to fuel bone formation.

Pharmaceutical treatment of bone loss: From animal models and drug development to future treatment strategies

Animal models are fundamental to advance our knowledge of the underlying pathophysiology of bone loss and to study pharmaceutical countermeasures against it. The animal model of post-menopausal osteoporosis from ovariectomy is the most widely used preclinical approach to study skeletal deterioration. However, several other animal models exist, each with unique characteristics such as bone loss from disuse, lactation, glucocorticoid excess, or exposure to hypobaric hypoxia. The present review aimed to provide a comprehensive overview of these animal models to emphasize the importance and significance of investigating bone loss and pharmaceutical countermeasures from perspectives other than post-menopausal osteoporosis only. Hence, the pathophysiology and underlying cellular mechanisms involved in the various types of bone loss are different, and this might influence which prevention and treatment strategies are the most effective. In addition, the review sought to map the current landscape of pharmaceutical countermeasures against osteoporosis with an emphasis on how drug development has changed from being driven by clinical observations and enhancement or repurposing of existing drugs to today's use of targeted anti-bodies that are the result of advanced insights into the underlying molecular mechanisms of bone formation and resorption. Moreover, new treatment combinations or repurposing opportunities of already approved drugs with a focus on dabigatran, parathyroid hormone and abaloparatide, growth hormone, inhibitors of the activin signaling pathway, acetazolamide, zoledronate, and romosozumab are discussed. Despite the considerable progress in drug development, there is still a clear need to improve treatment strategies and develop new pharmaceuticals against various types of osteoporosis. The review also highlights that new treatment indications should be explored using multiple animal models of bone loss in order to ensure a broad representation of different types of skeletal deterioration instead of mainly focusing on primary osteoporosis from post-menopausal estrogen deficiency.

Attenuation of Alzheimer's brain pathology in 5XFAD mice by PTH1-34, a peptide of parathyroid hormone

BACKGROUND

Alzheimer's disease (AD) and osteoporosis are two distinct diseases but often occur in the same patient. Their relationship remains poorly understood. Studies using Tg2576 AD animal model demonstrate bone deficits, which precede the brain phenotypes by several months, arguing for the independence of bone deficits on brain degeneration and raising a question if the bone deficits contribute to the AD development. To address this question, we investigated the effects of PTH_1-34, a peptide of parathyroid hormone analog and a well-recognized effective anabolic therapy drug for patients with osteoporosis, on 5XFAD animal model.

METHODS

5XFAD mice, an early onset β-amyloid (Aβ)-based AD mouse model, were treated with PTH_1-34 intermittently [once daily injection of hPTH_1-34 (50 μg/Kg), 5 days/week, starting at 2-month old (MO) for 2-3 month]. Wild type mice (C57BL/6) were used as control. The bone phenotypes were examined by microCT and evaluated by measuring serum bone formation and resorption markers. The AD relevant brain pathology (e.g., Aβ and glial activation) and behaviors were assessed by a combination of immunohistochemical staining analysis, western blots, and behavior tests. Additionally, systemic and brain inflammation were evaluated by serum cytokine array, real-time PCR (qPCR), and RNAscope.

RESULTS

A reduced trabecular, but not cortical, bone mass, accompanied with a decrease in bone formation and an increase in bone resorption, was detected in 5XFAD mice at age of 5/6-month old (MO). Upon PTH_1-34 treatments, not only these bone deficits but also Aβ-associated brain pathologies, including Aβ and Aβ deposition levels, dystrophic neurites, glial cell activation, and brain inflammatory cytokines, were all diminished; and the cognitive function was improved. Further studies suggest that PTH_1-34 acts on not only osteoblasts in the bone but also astrocytes in the brain, suppressing astrocyte senescence and expression of inflammatory cytokines in 5XFAD mice.

CONCLUSIONS

These results suggest that PTH_1-34 may act as a senolytic-like drug, reducing systemic and brain inflammation and improving cognitive function, and implicate PTH_1-34's therapeutic potential for patients with not only osteoporosis but also AD.

The significance of forearm bone mineral density evaluation in determining surgical indications in primary hyperparathyroidism

PURPOSE

Forearm osteoporosis is a well-known complication of primary hyperparathyroidism (PHPT). However, measuring forearm bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA) at the distal radius is often neglected in clinical practice despite the fact that osteoporosis at any site is a criterion indicating surgery. We aimed to evaluate the importance and priority of forearm BMD and to determine its association with biochemical parameters.

MATERIAL AND METHODS

Three hundred fourteen patients (272 females, 42 males) with PHPT who had BMD measurements at 3 sites were recruited for this retrospective study. The effect on surgical indications of osteoporosis only in the forearm was evaluated. Group 1 (n=151) with forearm osteoporosis and group 2 (n=163) without were compared in terms of biochemical and clinical parameters.

RESULTS

In the overall study population, 165 of the 314 patients had osteoporosis in at least 1 site. Twenty seven percent (n=86/314) had osteoporosis only in the forearm, while the other 2 sites (lumbar spine and femoral neck) were normal or osteopenic. Surgery was indicated based on osteoporosis only in the forearm in 10% of patients (n=30/314). Corrected calcium and parathyroid hormone levels were significantly higher in group 1 than group 2 (p=0.001 and p<0.001, respectively) and were also negatively correlated with distal radius BMD, T-score and Z-score in the whole study group.

CONCLUSION

Including the distal radius in BMD measurement increased the number of patients diagnosed with osteoporosis and for whom surgery was indicated. Calcium and PTH were also more frequently elevated in patients with forearm osteoporosis. These results show that distal radius BMD is relevant to the management of PHPT.

Abaloparatide Has the Same Catabolic Effects on Bones of Mice When Infused as PTH (1-34)

Abaloparatide is a peptide analog of parathyroid hormone-related protein (PTHrP 1-34) and was approved in 2017 as the second osteoanabolic peptide for treating osteoporosis. We previously showed that intermittent abaloparatide is equally as effective as PTH (1-34). This study was designed to compare the catabolic effects of PTH (1-34) and abaloparatide on bone in young female wild-type mice. Two-month-old C57Bl/6J female mice were continuously infused with human PTH (1-34) or abaloparatide at 80 μg/kg BW/day or vehicle for 2 weeks. At euthanasia, DEXA-PIXImus was performed to assess bone mineral density (BMD) in the whole body, femurs, tibiae, and vertebrae. Bone turnover marker levels were measured in sera, femurs were harvested for micro-computer tomography (μCT) analyses and histomorphometry, and tibiae were separated into cortical and trabecular fractions for gene expression analyses. Our results demonstrated that the infusion of abaloparatide resulted in a similar decrease in BMD as infused PTH (1-34) at all sites. μCT and histomorphometry analyses showed similar decreases in cortical bone thickness and BMD associated with an increase in bone turnover from the increased bone formation rate found by in vivo double labeling and serum P1NP and increased bone resorption as shown by osteoclast numbers and serum cross-linked C-telopeptide. Trabecular bone did not show major changes with either treatment. Osteoblastic gene expression analyses of trabecular and cortical bone revealed that infusion of PTH (1-34) or abaloparatide led to similar and different actions in genes of osteoblast differentiation and activity. As with intermittent and in vitro treatment, both infused PTH (1-34) and abaloparatide similarly regulated downstream genes of the PTHR1/SIK/HDAC4 pathway such as Sost and Mmp13 but differed for those of the PTHR1/SIK/CRTC pathway. Taken together, at the same dose, infused abaloparatide causes the same high bone turnover as infused PTH (1-34) with a net resorption in female wild-type mice. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Mast Cell Chymase Has a Negative Impact on Human Osteoblasts

Mast cells have been linked to osteoporosis and bone fractures, and in a previous study we found that mice lacking a major mast cell protease, chymase, develop increased diaphyseal bone mass. These findings introduce the possibility that mast cell chymase can regulate bone formation, but the underlying mechanism(s) has not previously been investigated. Here we hypothesized that chymase might exert such effects through a direct negative impact on osteoblasts, i.e., the main bone-building cells. Indeed, we show that chymase has a distinct impact on human primary osteoblasts. Firstly, chymase was shown to have pronounced effects on the morphological features of osteoblasts, including extensive cell contraction and actin reorganization. Chymase also caused a profound reduction in the output of collagen from the osteoblasts, and was shown to degrade osteoblast-secreted fibronectin and to activate pro-matrix metallopeptidase-2 released by the osteoblasts. Further, chymase was shown to have a preferential impact on the gene expression, protein output and phosphorylation status of TGFβ-associated signaling molecules. A transcriptomic analysis was conducted and revealed a significant effect of chymase on several genes of importance for bone metabolism, including a reduction in the expression of osteoprotegerin, which was confirmed at the protein level. Finally, we show that chymase interacts with human osteoblasts and is taken up by the cells. Altogether, the present findings provide a functional link between mast cell chymase and osteoblast function, and can form the basis for a further evaluation of chymase as a potential target for intervention in metabolic bone diseases.

Monocyte Chemotactic Protein-1 (MCP1) Accumulation in Human Osteoclast Precursor Cultures

In vitro osteoclast methods require constant treatment with macrophage colony stimulating factor (M-CSF) to support precursor survival and addition of the differentiation agent receptor activator of NF-κB ligand (RANKL). Constant exposure to granulocyte macrophage colony stimulating factor (GM-CSF) suppresses human osteoclast formation in vitro. Addition of the chemokine monocyte chemotactic protein-1 (MCP1) to such cultures dramatically increases osteoclast formation and overcomes GM-CSF mediated suppression. We investigated the effect of M-CSF, GM-CSF and the combination of M-CSF and GM-CSF treatment on the expression of chemokines in human CD14+ cells in culture. Of assayed chemokines, MCP1 was the most abundant in terms of mRNA transcript and protein in M-CSF treated cultures and was suppressed by GM-CSF. MCP1 protein accumulated up to 50 ng/mL in culture medium, greatly exceeding other assayed chemokines. C-C chemokine receptor-2 (CCR2) is the receptor for MCP1: the formation of osteoclast-like cells was inhibited by constant exposure to the CCR2 antagonist RS102895, in part by decreasing expression of RANK, the receptor for RANKL.

Association between bone turnover markers, BMD and height loss of cemented vertebrae after percutaneous vertebroplasty in patients with osteoporotic vertebral compression fractures

Introduction

Percutaneous vertebroplasty (PVP) was recently performed for treating patients with osteoporotic vertebral compression fractures (OVCF). However, recompression of cemented vertebra with significant vertebral height loss occurred in the patients after PVP was observed during the follow-up period. The purpose is to explore the risk factors among several potential predictors for the height loss of treated vertebral bodies after PVP in patients with OVCF.

Methods

A study of 93 patients who had undergone PVP between May 1, 2016, and March 1, 2019, at the Spine Center of Huadong Hospital Affiliated to Fudan University was conducted. The fractured vertebral height loss ratio ≥ 15% at final follow-up were defined as cemented vertebra recompression. The following variables were measured and collected: age, gender, body mass index (BMI), bone mineral density (BMD), volume of bone cement injected, bone cement leakage, fractured vertebra segment, contact between bone cement and endplates, serum of calcium and phosphorus, and six kinds of bone turnover markers.

Results

Mann–Whitney U test and Univariate Logistic regression analysis showed that the cemented vertebra recompression was correlated with BMD, contact between bone cement and endplates, parathyroid hormone (PTH), and 25-hydroxy vitamin D3 (25-OH-D3). Following multivariate modeling, multiple factors logistic regression elucidated that high BMD (P < 0.001, OR = 0.089) and high level of serum 25-OH-D3 (P = 0.012, OR = 0.877) were negatively correlated with the cemented vertebra recompression after PVP.

Conclusion

Decreased BMD and lower level of serum 25-OH-D3 might be two critical and significant risk factors for the height loss of cemented vertebrae after PVP.

Sfrp4 and the Biology of Cortical Bone

PURPOSE OF REVIEW

Periosteal apposition and endosteal remodeling regulate cortical bone expansion and thickness, both critical determinants of bone strength. Yet, the cellular characteristics and local or paracrine factors that regulate the periosteum and endosteum remain largely elusive. Here we discuss novel insights in cortical bone growth, expansion, and homeostasis, provided by the study of Secreted Frizzled Receptor Protein 4 (Sfrp4), a decoy receptor for Wnt ligands.

RECENT FINDINGS

SFRP4 loss-of function mutations cause Pyle disease, a rare skeletal disorder characterized by cortical bone thinning and increased fragility fractures despite increased trabecular bone density. On the endosteal surface, Sfrp4-mediated repression of non-canonical Wnt signaling regulates endosteal resorption. On the periosteum, Sfrp4 identifies as a critical functional mediator of periosteal stem cell/progenitor expansion and differentiation. Analysis of signaling pathways regulating skeletal stem cells/progenitors provides an opportunity to advance our understanding of the mechanisms involved in cortical bone biology.

ROS-induced PADI2 downregulation accelerates cellular senescence via the stimulation of SASP production and NFκB activation

Cellular senescence is closely related to tissue aging including bone. Bone homeostasis is maintained by the tight balance between bone-forming osteoblasts and bone-resorbing osteoclasts, but it undergoes deregulation with age, causing age-associated osteoporosis, a main cause of which is osteoblast dysfunction. Oxidative stress caused by the accumulation of reactive oxygen species (ROS) in bone tissues with aging can accelerate osteoblast senescence and dysfunction. However, the regulatory mechanism that controls the ROS-induced senescence of osteoblasts is poorly understood. Here, we identified Peptidyl arginine deiminase 2 (PADI2), a post-translational modifying enzyme, as a regulator of ROS-accelerated senescence of osteoblasts via RNA-sequencing and further functional validations. PADI2 downregulation by treatment with H₂O₂ or its siRNA promoted cellular senescence and suppressed osteoblast differentiation. CCL2, 5, and 7 known as the elements of the senescence-associated secretory phenotype (SASP) which is a secretome including proinflammatory cytokines and chemokines emitted by senescent cells and a representative feature of senescence, were upregulated by H₂O₂ treatment or Padi2 knockdown. Furthermore, blocking these SASP factors with neutralizing antibodies or siRNAs alleviated the senescence and dysfunction of osteoblasts induced by H₂O₂ treatment or Padi2 knockdown. The elevated production of these SASP factors was mediated by the activation of NFκB signaling pathway. The inhibition of NFκB using the pharmacological inhibitor or siRNA effectively relieved H₂O₂ treatment- or Padi2 knockdown-induced senescence and osteoblast dysfunction. Together, our study for the first time uncover the role of PADI2 in ROS-accelerated cellular senescence of osteoblasts and provide new mechanistic and therapeutic insights into excessive ROS-promoted cellular senescence and aging-related bone diseases.

Bone Cells Differentiation: How CFTR Mutations May Rule the Game of Stem Cells Commitment?

Cystic fibrosis (CF)-related bone disease has emerged as a significant comorbidity of CF and is characterized by decreased bone formation and increased bone resorption. Both osteoblast and osteoclast differentiations are impacted by cystic fibrosis transmembrane conductance regulator (CFTR) mutations. The defect of CFTR chloride channel or the loss of CFTRs ability to interact with other proteins affect several signaling pathways involved in stem cell differentiation and the commitment of these cells toward bone lineages. Specifically, TGF-, nuclear factor-kappa B (NF-B), PI3K/AKT, and MAPK/ERK signaling are disturbed by CFTR mutations, thus perturbing stem cell differentiation. High inflammation in patients changes myeloid lineage secretion, affecting both myeloid and mesenchymal differentiation. In osteoblast, Wnt signaling is impacted, resulting in consequences for both bone formation and resorption. Finally, CFTR could also have a direct role in osteoclasts resorptive function. In this review, we summarize the existing literature on the role of CFTR mutations on the commitment of induced pluripotent stem cells to bone cells.

The molecular structure and role of CCL2 (MCP-1) and C-C chemokine receptor CCR2 in skeletal biology and diseases

Monocyte chemoattractant protein-1, also called chemokine (C-C motif) ligand 2 (CCL2) or small inducible cytokine A2, is an inflammatory mediator capable of recruiting monocytes, memory T cells, and dendritic cells. CCL2 is a member of the CC chemokine superfamily, which binds to its receptor, C-C motif chemokine receptor-2 (CCR2), for the induction of chemotactic activity and an increase of calcium influx. It exerts multiple effects on a variety of cells, including monocytes, macrophages, osteoclasts, basophils, and endothelial cells, and is involved in a diverse range of diseases. This review discusses the molecular structure and role of CCL2 and CCR2 in skeletal biology and disease. Molecular structure analyses reveal that CCL2 shares a conserved C-C motif; however, it has only limited sequence homology with other CCL family members. Likewise, CCR2, as a member of the G-protein-coupled seven-transmembrane receptor superfamily, shares conserved cysteine residues, but exhibits very limited sequence homology with other CCR family members. In the skeletal system, the expression of CCL2 is regulated by a variety of factors, such as parathyroid hormone/parathyroid hormone-related peptide, interleukin 1b, tumor necrosis factor-α and transforming growth factor-beta, RANKL, and mechanical forces. The interaction of CCL2 and CCR2 activates several signaling cascades, including PI3K/Akt/ERK/NF-κB, PI3K/MAPKs, and JAK/STAT-1/STAT-3. Understanding the role of CCL2 and CCR2 will facilitate the development of novel therapies for skeletal disorders, including rheumatoid arthritis, osteolysis and other inflammatory diseases related to abnormal chemotaxis.

Nanosurfacing Ti alloy by weak alkalinity-activated solid-state dewetting (AAD) and its biointerfacial enhancement effect

Nanoscale manipulation of material surfaces can create extraordinary properties, holding great potential for modulating the implant-bio interface for enhanced performance. In this study, a green, simple and biocompatible nanosurfacing approach based on weak alkalinity-activated solid-state dewetting (AAD) was for the first time developed to nano-manipulate the Ti6Al4V surface by atomic self-rearrangement. AAD treatment generated quasi-periodic titanium oxide nanopimples with high surface energy. The nanopimple-like nanostructures enhanced the osteogenic activity of osteoblasts, facilitated M2 polarization of macrophages, and modulated the cross-talk between osteoblasts and macrophages, which collectively led to significant strengthening of in vivo bone-implant interfacial bonding. In addition, the titanium oxide nanopimples strongly adhered to the Ti alloy, showing resistance to tribocorrosion damage. The results suggest strong nano-bio interfacial effects, which was not seen for the control Ti alloy processed through traditional thermal oxidation. Compared to other nanostructuring strategies, the AAD technique shows great potential to integrate high-performance, functionality, practicality and scalability for surface modification of medical implants.

Osteoblastic monocyte chemoattractant protein-1 (MCP-1) mediation of parathyroid hormone's anabolic actions in bone implicates TGF-β signaling

Parathyroid hormone (PTH) is necessary for the regulation of calcium homeostasis and PTH (1-34) was the first approved osteoanabolic therapy for osteoporosis. It is well established that intermittent PTH increases bone formation and that bone remodeling and several cytokines and chemokines play an essential role in this process. Earlier, we had established that the chemokine, monocyte chemoattractant protein-1 (MCP-1/CCL2), was the most highly stimulated gene in rat bone after intermittent PTH injections. Nevertheless, MCP-1 function in bone appears to be complicated. To identify the primary cells expressing MCP-1 in response to PTH, we performed in situ hybridization of rat bone sections after hPTH (1-34) injections and showed that bone-lining osteoblasts are the primary cells that express MCP-1 after PTH treatment. We previously demonstrated MCP-1's importance by showing that PTH's anabolic effects are abolished in MCP-1 null mice, further implicating a role for the chemokine in this process. To establish whether rhMCP-1 peptide treatment could rescue the anabolic effect of PTH in MCP-1 null mice, we treated 4-month-old wild-type (WT) mice with hPTH (1-34) and MCP-1^-/- mice with rhMCP-1 and/or hPTH (1-34) for 6 weeks. Micro-computed tomography (μCT) analysis of trabecular and cortical bone showed that MCP-1 injections for 6 weeks rescued the PTH anabolic effect in MCP-1^-/- mice. In fact, the combination of rhMCP-1 and hPTH (1-34) has a synergistic anabolic effect compared with monotherapies. Mechanistically, PTH-enhanced transforming growth factor-β (TGF-β) signaling is abolished in the absence of MCP-1, while MCP-1 peptide treatment restores TGF-β signaling in the bone marrow. Here, we have shown that PTH regulates the transcription of the chemokine MCP-1 in osteoblasts and determined how MCP-1 affects bone cell function in PTH's anabolic actions. Taken together, our current work indicates that intermittent PTH stimulates osteoblastic secretion of MCP-1, which leads to increased TGF-β signaling, implicating it in PTH's anabolic actions.

Activin type IIA decoy receptor and intermittent parathyroid hormone in combination overturns the bone loss in disuse-osteopenic mice

Damage of the lower motor neuron cell bodies or their axons results in reduced or abolished voluntary movement accompanied by a substantial loss of bone and muscle mass. Intermittent parathyroid hormone 1-34 (PTH) (teriparatide) is one of the most potent bone-anabolic treatment regimens. ActRIIA-mFc is an activin type IIA decoy receptor that increases bone mass mediated by inhibition of the activin receptor signaling pathway. We investigated whether PTH or ActRIIA-mFc alone or in combination could prevent loss of bone and muscle mass induced by injecting botulinum toxin A (BTX) into the right hind limb in mice. Seventy-two 16-week-old female C57BL/6 mice were allocated to the following groups: Baseline, Control, BTX, BTX + ActRIIA-mFc (10 mg/kg), BTX + PTH (100 μg/kg), and BTX + ActRIIA-mFc + PTH. The mice were sacrificed after three weeks of disuse and treatment. In contrast to monotherapy with PTH, ActRIIA-mFc alone or in combination with PTH was able partly or completely to prevent disuse-induced loss of whole femoral bone mass, trabecular thickness, and bone strength. Moreover, an additive effect of ActRIIA-mFc and PTH on areal bone mineral density and trabecular bone volume was found. In summary, ActRIIA-mFc and PTH in combination were more effective in preventing disuse-induced bone loss and deterioration of trabecular micro-architecture than either treatment alone.

Parathyroid Hormone and Bone Mineral Density: A Mendelian Randomization Study

PURPOSE

Accumulating evidence implicates parathyroid hormone (PTH) in the development of osteoporosis. However, the causal effect of PTH on bone mineral density (BMD) remains unclear. Thus, this study is aimed at exploring the association between the concentrations of serum PTH and BMD.

METHODS

The instrumental variables for PTH were selected from a large-scale genome-wide association study (GWAS; n = 29 155). Outcomes included BMD of the forearm (FA; n = 8143), femoral neck (FN; n = 33 297), lumbar spine (LS; n = 32 735), heel (HL; n = 394 929), and risk of fractures in these bones (n = 361 194). Furthermore, the BMD of 5 different age groups: 15 years or younger (n = 11 807), 15-30 (n = 4180), 30-45 (n = 10 062), 45-60 (n = 18 805), and 60 years or older (n = 22 504) were extracted from a GWAS meta-analysis study. The analyses were performed using the 2-sample Mendelian randomization method.

RESULTS

Mendelian randomization analysis revealed that the level of serum PTH was inversely associated with BMD of FA (95% CI: -0.763 to -0.016), FN (95% CI: -0.669 to -0.304), and LS (95% CI: -0.667 to -0.243). A causal relationship between serum PTH levels and BMD was observed in individuals aged 30-45 (95% CI: -0.888 to -0.166), 45-60 (95% CI: -0.758 to -0.232), and over 60 years (95% CI: -0.649 to -0.163).

MAIN CONCLUSIONS

This study demonstrated that the concentrations of serum PTH is inversely associated with BMD of several bones. Further analysis revealed site- and age-specific correlations between serum PTH levels and BMD, which implies that the levels of serum PTH contribute to the development of osteoporosis.

Intermittent Parathyroid Hormone Accelerates Stress Fracture Healing More Effectively Following Cessation of Bisphosphonate Treatment

Parathyroid hormone (PTH) and bisphosphonates (BPs), including alendronate (ALN), have opposing effects on bone dynamics. The extent to which PTH remains effective in the treatment of stress fracture (SFx) in the presence of an ongoing BP treatment has not been tested. SFx was induced in 150 female Wistar rats, divided into five equal groups (n = 30). All rats were pretreated with ALN (1 μg/kg^-1/day^-1) for 14 days prior to SFx induction, followed by ALN cessation or continuation for the duration of the experiment; this was combined with daily PTH (8 μg/100 g^-1/day^-1) on SFx induction for 14 days, followed by cessation or continuation of ALN after SFx induction or an equivalent vehicle as a control. Ulnas were examined 2 weeks or 6 weeks following SFx. Two toluidine blue- and two tartrate-resistant acid phosphatase-stained sections were examined for histomorphometric analysis using Osteomeasure software. There was a significant interaction between the effects of time and treatment type on the woven bone width and apposition rate, as well as an improvement in the woven bone architecture. However, woven bone variables remained unaffected by the cessation or continuation of ALN. Cessation of ALN increased osteoclast number when compared with the ALN-PTH continuation group (p = 0.006), and vehicle (p = 0.024) after 2 weeks. There was a significant interaction between the effects of time and treatment type on the number of osteoclasts per unit BMU area and length. The number of osteoclasts per unit BMU area and length was significantly greater in ALN cessation groups. It was concluded that intermittent short-duration iPTH treatment effectively increased remodeling of SFx with a concurrent BP treatment, provided that BP was ceased at the time of SFx. Our results could help develop shorter iPTH treatment protocols for the clinical management of SFxs and guide clinical decision-making to cease BP treatment in cases of SFx. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Parathyroid hormone

Parathyroid hormone is an essential regulator of extracellular calcium and phosphate. PTH enhances calcium reabsorption while inhibiting phosphate reabsorption in the kidneys, increases the synthesis of 1,25-dihydroxyvitamin D, which then increases gastrointestinal absorption of calcium, and increases bone resorption to increase calcium and phosphate. Parathyroid disease can be an isolated endocrine disorder or part of a complex syndrome. Genetic mutations can account for diseases of parathyroid gland formulation, dysregulation of parathyroid hormone synthesis or secretion, and destruction of the parathyroid glands. Over the years, a number of different options are available for the treatment of different types of parathyroid disease. Therapeutic options include surgical removal of hypersecreting parathyroid tissue, administration of parathyroid hormone, vitamin D, activated vitamin D, calcium, phosphate binders, calcium-sensing receptor, and vitamin D receptor activators to name a few. The accurate assessment of parathyroid hormone also provides essential biochemical information to properly diagnose parathyroid disease. Currently available immunoassays may overestimate or underestimate bioactive parathyroid hormone because of interferences from truncated parathyroid hormone fragments, phosphorylation of parathyroid hormone, and oxidation of amino acids of parathyroid hormone.

hPTH(3-34)(29-34) selectively activated PKC and mimicked osteoanabolic effects of hPTH(1-34)

Teriparatide (hPTH(1-34)) exhibits both osteoanabolic and osteocatabolic effects. We generated a novel PTH analog by duplicating the PTH(29-34) domain to hPTH(3-34) (named MY-1), which was identified to activate PKC but not PLC and cAMP/PKA signaling. It increased osteo-differentiation but did not affect osteoclastogenesis and RANKL expression in primary osteoblasts or bone marrow cells. MY-1 and hPTH(1-34) increased the synthesis and decreased the degradation οf β-catenin protein in osteoblasts, while PKC inhibitor blunted such effects. In vivo results indicated that intermittent MY-1 and hPTH(1-34) prevented bone loss in ovariectomized mice, and that MY-1 infusion increased bone volume in normal mice. Histological analysis observed more osteoclasts surrounding the cancellous bone surface in hPTH(1-34), but not MY-1 treated mice. We conclude that MY-1 mimicked the osteoanabolic but not the osteocatabolic effects of hPTH(1-34), which is related to PKC and β-catenin signaling. Such anabolic-only analog provides a new strategy to study PTH's versatile functions and design new medicines to treat osteoporosis and bone defects.

Abaloparatide at the Same Dose Has the Same Effects on Bone as PTH (1-34) in Mice

Abaloparatide, a novel analog of parathyroid hormone-related protein (PTHrP 1-34), became in 2017 the second osteoanabolic therapy for the treatment of osteoporosis. This study aims to compare the effects of PTH (1-34), PTHrP (1-36), and abaloparatide on bone remodeling in male mice. Intermittent daily subcutaneous injections of 80 μg/kg/d were administered to 4-month-old C57Bl/6J male mice for 6 weeks. During treatment, mice were followed by DXA-Piximus to assess changes in bone mineral density (BMD) in the whole body, femur, and tibia. At either 4 or 18 hours after the final injection, femurs were harvested for μCT analyses and histomorphometry, sera were assayed for bone turnover marker levels, and tibias were separated into cortical, trabecular, and bone marrow fractions for gene expression analyses. Our results showed that, compared with PTH (1-34), abaloparatide resulted in a similar increase in BMD at all sites, whereas no changes were found with PTHrP (1-36). With both PTH (1-34) and abaloparatide, μCT and histomorphometry analyses revealed similar increases in bone volume associated with an increased trabecular thickness, in bone formation rate as shown by P1NP serum level and in vivo double labeling, and in bone resorption as shown by CTX levels and osteoclast number. Gene expression analyses of trabecular and cortical bone showed that PTH (1-34) and abaloparatide led to different actions in osteoblast differentiation and activity, with increased Runx2, Col1A1, Alpl, Bsp, Ocn, Sost, Rankl/Opg, and c-fos at different time points. Abaloparatide seems to generate a faster response on osteoblastic gene expression than PTH (1-34). Taken together, abaloparatide at the same dose is as effective as PTH (1-34) as an osteoanabolic, with an increase in bone formation but also an increase in bone resorption in male mice. © 2019 American Society for Bone and Mineral Research.

Loss of p62 impairs bone turnover and inhibits PTH-induced osteogenesis

The p62 (also named sequestosome1/SQSTM1) is multidomain and multifunctional protein associated with several physiological and pathological conditions. A number of studies evidenced an involvement of p62 on the disruptive bone scenarios due to its participation in the inflammatory/osteoclastogenic pathways. However, so far, information regarding the function of p62 in the fine-tuned processes underpinning the bone physiology are not well-defined and are sometime discordant. We, previously, demonstrated that the intramuscular administration of a plasmid coding for p62 was able to contrast bone loss in a mouse model of osteopenia. Here, in vitro findings showed that the p62 overexpression in murine osteoblasts precursors enhanced their maturation while the p62 depletion by a specific siRNA, decreased osteoblasts differentiation. Consistently, the activity of osteoblasts from p62^-/- mice was reduced compared with wild-type. Also, morphometric analyses of bone from p62 knockout mice revealed a pathological phenotype characterized by a lower turnover that could be explained by the poor Runx2 protein synthesis in absence of p62. Furthermore, we demonstrated that the parathyroid hormone (PTH) regulates p62 expression and that the osteogenic effects of this hormone were totally abrogated in osteoblasts from p62-deficient mice. Therefore, these findings, for the first time, highlight the important role of p62 both for the basal and for PTH-stimulated bone remodeling.

Changes in gene expression in human skeletal stem cells transduced with constitutively active Gsα correlates with hallmark histopathological changes seen in fibrous dysplastic bone

Fibrous dysplasia (FD) of bone is a complex disease of the skeleton caused by dominant activating mutations of the GNAS locus encoding for the α subunit of the G protein-coupled receptor complex (Gsα). The mutation involves a substitution of arginine at position 201 by histidine or cysteine (Gsα^{R201H or R201C}), which leads to overproduction of cAMP. Several signaling pathways are implicated downstream of excess cAMP in the manifestation of disease. However, the pathogenesis of FD remains largely unknown. The overall FD phenotype can be attributed to alterations of skeletal stem/progenitor cells which normally develop into osteogenic or adipogenic cells (in cis), and are also known to provide support to angiogenesis, hematopoiesis, and osteoclastogenesis (in trans). In order to dissect the molecular pathways rooted in skeletal stem/progenitor cells by FD mutations, we engineered human skeletal stem/progenitor cells with the Gsα^R201C mutation and performed transcriptomic analysis. Our data suggest that this FD mutation profoundly alters the properties of skeletal stem/progenitor cells by pushing them towards formation of disorganized bone with a concomitant alteration of adipogenic differentiation. In addition, the mutation creates an altered in trans environment that induces neovascularization, cytokine/chemokine changes and osteoclastogenesis. In silico comparison of our data with the signature of FD craniofacial samples highlighted common traits, such as the upregulation of ADAM (A Disintegrin and Metalloprotease) proteins and other matrix-related factors, and of PDE7B (Phosphodiesterase 7B), which can be considered as a buffering process, activated to compensate for excess cAMP. We also observed high levels of CEBPs (CCAAT-Enhancer Binding Proteins) in both data sets, factors related to browning of white fat. This is the first analysis of the reaction of human skeletal stem/progenitor cells to the introduction of the FD mutation and we believe it provides a useful background for further studies on the molecular basis of the disease and for the identification of novel potential therapeutic targets.

Monocyte Chemoattractant Protein-1 (MCP-1/CCL2) Drives Activation of Bone Remodelling and Skeletal Metastasis

PURPOSE OF REVIEW

The purpose of this review is to explore the role of monocyte chemoattractant protein-1 (MCP-1 or CCL2) in the processes that underpin bone remodelling, particularly the action of osteoblasts and osteoclasts, and its role in the development and metastasis of cancers that target the bone.

RECENT FINDINGS

MCP-1 is a key mediator of osteoclastogenesis, being the highest induced gene during intermittent treatment with parathyroid hormone (iPTH), but also regulates catabolic effects of continuous PTH on bone including monocyte and macrophage recruitment, osteoclast formation and bone resorption. In concert with PTH-related protein (PTHrP), MCP-1 mediates the interaction between tumour-derived factors and host-derived chemokines to promote skeletal metastasis. In breast and prostate cancers, an osteolytic cascade is driven by tumour cell-derived PTHrP that upregulates MCP-1 in osteoblastic cells. This relationship between PTHrP and osteoblastic expression of MCP-1 may drive the colonisation of disseminated breast cancer cells in the bone. There is mounting evidence to suggest a pivotal role of MCP-1 in many diseases and an important role in the establishment of comorbidities. Coupled with its role in bone remodelling and the regulation of bone turnover, there is the potential for pathological relationships between bone disorders and bone-related cancers driven by MCP-1. MCP-1's role in bone remodelling and bone-related cancers highlights its potential as a novel anti-resorptive and anti-metastatic target.

Brief exposure to full length parathyroid hormone-related protein (PTHrP) causes persistent generation of cyclic AMP through an endocytosis-dependent mechanism

Parathyroid hormone (PTH)-related protein (PTHrP) (gene name Pthlh) was discovered as the factor responsible for the humoral hypercalcemia of malignancy. It shares such sequence similarity with PTH in the amino-terminal region that the two are equally able to act through a single G protein-coupled receptor, PTH1R. A number of biological activities are ascribed to domains of PTHrP beyond the amino-terminal domain. PTH functions as a circulating hormone, but PTHrP is generated locally in many tissues including bone, where it acts as a paracrine factor on osteoblasts and osteocytes. The present study compares how PTH and PTHrP influence cyclic AMP (cAMP) formation through adenylyl cyclase, the first event in cell activation through PTH1R. Brief exposure to full length PTHrP(1-141) in several osteoblastic cell culture systems was followed by sustained adenylyl cyclase activity for more than an hour after ligand washout. This effect was dose-dependent and was not found with shorter PTHrP or PTH peptides even though they were fully able to activate adenylyl cyclase with acute treatment. The persistent activation response to PTHrP(1-141) was seen also with later events in the cAMP/PKA pathway, including persistent activation of CRE-luciferase and sustained regulation of several CREB-responsive mRNAs, up to 24 h after the initial exposure. Pharmacologic blockade of endocytosis prevented the persistent activation of cAMP and gene responses. We conclude that full length PTHrP, the likely local physiological effector in bone, differs in intracellular action to PTH by undergoing endosomal translocation to induce a prolonged adenylyl cyclase activation in its target cells.

Computational model of the dual action of PTH - Application to a rat model of osteoporosis

This paper presents a pharmacokinetic/pharmacodynamic (PK/PD) model of the action of PTH(1-34) on bone modelling and remodelling, developed for quantitatively investigating the dose- and administration pattern-dependency of the bone tissue response to this drug. Firstly, a PK model of PTH(1-34) was developed, accounting for administration via subcutaneous injections. Subsequently, the PK model was coupled to a (mechanistic) bone cell population model of bone modelling and remodelling, taking into account the effects of PTH(1-34) on the differentiation of lining cells into active osteoblasts, on the apoptosis of active osteoblasts, and on proliferation of osteoblast precursors, as well as on the key regulatory pathways of bone cell activities. Numerical simulations show that the coupled PK/PD model is able to distinguish between continuous and intermittent administration patterns of PTH(1-34), in terms of yielding both catabolic bone responses (if drug administration is carried out continuously) and anabolic bone responses (if drug administration is carried out intermittently). The model also features a non-linear relation between bone gain and drug dose (as known from experiments); doubling the dose from 80 μg/kg/day to 160 μg/kg/day induced a 1.3-fold increase of the bone volume-to-total volume ratio. Furthermore, the model presented in this paper confirmed that bone modelling represents an essential mechanism of the anabolic response of bone to PTH(1-34) administration in rat models, and that the large amount of bone formation observed in such models cannot be explained via remodelling alone.

Intermittent parathyroid hormone promotes cementogenesis in a PKA- and ERK1/2-dependent manner

BACKGROUND

Intermittent parathyroid hormone (PTH) promotes cementogenesis and provides a promising biotherapeutic to rehabilitate resorbed roots. However, the underlying mechanisms remain inconclusive. Cyclic aenosine monophosphate (AMP)-dependent protein kinases A (PKA) and extracellular signal-regulated MAP kinases 1/2 (ERK1/2) are key regulators of bone remodeling. The present study aims to investigate whether PKA and ERK1/2 are involved in the process of intermittent PTH-promoted cementogenesis.

METHODS

Sprague-Dawley rats in experimental group (n = 30) received a daily subcutaneous injection of PTH and the control (n = 30) received placebo vehicle for 1, 2, 3, 4, and 5 weeks. Results were evaluated by hematoxylin and eosin and immunohistochemistry staining. In vitro, OCCM-30 cells were incubated with intermittent PTH. H89 and U0126 were used to determine the role of PKA and ERK1/2, respectively. The cementogenic results were analyzed by reverse transcription-polymerase chain reaction (RT-PCR), western blotting, alkaline phosphatase activity assay and Alizarin Red S staining. The interaction of PKA and p-ERK1/2 was determined by co-immunoprecipitation (Co-IP).

RESULTS

Intermittent PTH exerted anabolic effect on cellular cementum in developing teeth with elevated expression of osteocalcin, osteopontin, and PKA (catalytic subunit) in PTH injection group. The promoting effects of intermittent PTH on cementogenesis and osteogenic differentiation were abrogated by H89 and U0126 in vitro, respectively. Blocking of PKA pathway downregulated intermittent PTH-induced ERK1/2 phosphorylation.

CONCLUSIONS

Intermittent PTH promotes cementogenesis in a PKA- and ERK1/2-dependent manner. In this process, PKA and p-ERK1/2 interact with each other. These results support the future biotherapeutic applications of PTH in cementum resorption.

Physiology of Parathyroid Hormone

Parathyroid hormone (PTH) is the major secretory product of the parathyroid glands, and in hypocalcemic conditions, can enhance renal calcium reabsorption, increase active vitamin D production to increase intestinal calcium absorption, and mobilize calcium from bone by increasing turnover, mainly but not exclusively in cortical bone. PTH has therefore found clinical use as replacement therapy in hypoparathyroidism. PTH also may have a physiologic role in augmenting bone formation, particularly in trabecular and to some extent in cortical bone. This action has been applied to the clinic to provide anabolic therapy for osteoporosis.

Parathyroid hormone(1-34) and its analogs differentially modulate osteoblastic Rankl expression via PKA/SIK2/SIK3 and PP1/PP2A-CRTC3 signaling

Osteoporosis can result from the loss of sex hormones and/or aging. Abaloparatide (ABL), an analog of parathyroid hormone-related protein (PTHrP(1-36)), is the second osteoanabolic therapy approved by the United States Food and Drug Administration after teriparatide (PTH(1-34)). All three peptides bind PTH/PTHrP receptor type 1 (PTHR1), but the effects of PTHrP(1-36) or ABL in the osteoblast remain unclear. We show that, in primary calvarial osteoblasts, PTH(1-34) promotes a more robust cAMP response than PTHrP(1-36) and ABL and causes a greater activation of protein kinase A (PKA) and cAMP response element-binding protein (CREB). All three peptides similarly inhibited sclerostin (Sost). Interestingly, the three peptides differentially modulated two other PKA target genes, c-Fos and receptor activator of NF-κB ligand (Rankl), and the latter both in vitro and in vivo Knockdown of salt-inducible kinases (SIKs) 2 and 3 and CREB-regulated transcription coactivator 3 (CRTC3), indicated that all three are part of the pathway that regulates osteoblastic Rankl expression. We also show that the peptides differentially regulate the nuclear localization of CRTC2 and CRTC3, and that this correlates with PKA activation. Moreover, inhibition of protein phosphatases 1 and 2A (PP1/PP2A) activity revealed that they play a major role in both PTH-induced Rankl expression and the effects of PTH(1-34) on CRTC3 localization. In summary, in the osteoblast, the effects of PTH(1-34), PTHrP(1-36), and ABL on Rankl are mediated by differential stimulation of cAMP/PKA signaling and by their downstream effects on SIK2 and -3, PP1/PP2A, and CRTC3.

Regulation of Skeletal Homeostasis

Landmark advances in skeletal biology have arisen mainly from the identification of disease-causing mutations and the advent of rapid and selective gene-targeting technologies to phenocopy human disease in mice. Here, we discuss work on newly identified mechanisms controlling the remodeling of bone, communication of bone cells with cells of other lineages, and crosstalk between bone and vital organs as these relate to the therapeutic targeting of the skeleton.

FGF2 crosstalk with Wnt signaling in mediating the anabolic action of PTH on bone formation

The mechanisms of the anabolic effect of parathyroid hormone (PTH) in bone are not fully defined. The bone anabolic effects of PTH require fibroblast growth factor 2 (FGF2) as well as Wnt signaling and FGF2 modulates Wnt signaling in osteoblasts. In vivo PTH administration differentially modulated Wnt signaling in bones of wild type (WT) and in mice that Fgf2 was knocked out (Fgf2KO). PTH increased Wnt10b mRNA and protein in WT but not in KO mice. Wnt antagonist SOST mRNA and protein was significantly higher in KO group. However, PTH decreased Sost mRNA significantly in WT as well as in Fgf2KO mice, but to a lesser extent in Fgf2KO. Dickhopf 2 (DKK2) is critical for osteoblast mineralization. PTH increased Dkk2 mRNA in WT mice but the response was impaired in Fgf2KO mice. PTH significantly increased Lrp5 mRNA and phosphorylation of Lrp6 in WT but the increase was markedly attenuated in Fgf2KO mice. PTH increased β-catenin expression and Wnt/β-catenin transcriptional activity significantly in WT but not in Fgf2KO mice. These data suggest that the impaired bone anabolic response to PTH in Fgf2KO mice is partially mediated by attenuated Wnt signaling.

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In vivo PTH administration differentially modulated Wnt signaling in bones of WT and Fgf2KO mice.

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PTH treatment increased WNT10b and DKK2 expression in WT mice but the increase was blunted in Fgf2KO mice

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PTH increased Lrp5 mRNA and phosphorylation of Lrp6 in WT but the increase was markedly attenuated in Fgf2KO mice.

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PTH treatment increased β-catenin protein level and Wnt/β-catenin transcriptional activity in WT but not in Fgf2KO mice

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The impaired bone anabolic response to PTH in Fgf2KO mice is partially mediated by attenuated Wnt signaling.

Periostin: A Matricellular Protein With Multiple Functions in Cancer Development and Progression

Tumor microenvironment is considered nowadays as one of the main players in cancer development and progression. Tumor microenvironment is highly complex and consists of non-tumor cells (i.e., cancer-associated fibroblast, endothelial cells, or infiltrating leukocytes) and a large list of extracellular matrix proteins and soluble factors. The way that microenvironment components interact among them and with the tumor cells is very complex and only partially understood. However, it is now clear that these interactions govern and modulate many of the cancer hallmarks such as cell proliferation, the resistance to death, the differentiation state of tumor cells, their ability to migrate and metastasize, and the immune response against tumor cells. One of the microenvironment components that have emerged in the last years with strength is a heterogeneous group of multifaceted proteins grouped under the name of matricellular proteins. Matricellular proteins are a family of non-structural matrix proteins that regulate a variety of biological processes in normal and pathological situations. Many components of this family such as periostin (POSTN), osteopontin (SPP1), or the CNN family of proteins have been shown to regulate key aspect of tumor biology, including proliferation, invasion, matrix remodeling, and dissemination to pre-metastatic niches in distant organs. Matricellular proteins can be produced by tumor cells themselves or by tumor-associated cells, and their synthesis can be affected by intrinsic and/or extrinsic tumor cell factors. In this review, we will focus on the role of POSTN in the development and progression of cancer. We will describe their functions in normal tissues and the mechanisms involved in their regulation. We will analyze the tumors in which their expression is altered and their usefulness as a biomarker of tumor progression. Finally, we will speculate about future directions for research and therapeutic approaches targeting POSTN.

Lack of endogenous parathyroid hormone delays fracture healing by inhibiting vascular endothelial growth factor‑mediated angiogenesis

Intermittent low-dose injections of parathyroid hormone (PTH) have been reported to exert bone anabolic effects and to promote fracture healing. As an important proangiogenic cytokine, vascular endothelial growth factor (VEGF) is secreted by bone marrow mesenchymal stem cells (BMSCs) and osteoblasts, and serves a crucial regulatory role in the process of vascular development and regeneration. To investigate whether lack of endogenous PTH causes reduced angiogenic capacity and thereby delays the process of fracture healing by downregulating the VEGF signaling pathway, a PTH knockout (PTHKO) mouse fracture model was generated. Fracture healing was observed using X-ray and micro-computerized tomography. Bone anabolic and angiogenic markers were analyzed by immunohistochemistry and western blot analysis. The expression levels of VEGF and associated signaling pathways in murine BMSC-derived osteoblasts were measured by quantitative polymerase chain reaction and western blot analysis. The expression levels of protein kinase A (PKA), phosphorylated-serine/threonine protein kinase (pAKT), hypoxia-inducible factor-1α (HIF1α) and VEGF were significantly decreased in BMSC-derived osteoblasts from PTHKO mice. In addition, positive platelet endothelial cell adhesion molecule staining was reduced in PTHKO mice, as determined by immunohistochemistry. The expression levels of HIF1α, VEGF, runt-related transcription factor 2, osteocalcin and alkaline phosphatase were also decreased in PTHKO mice, and fracture healing was delayed. In conclusion, lack of endogenous PTH may reduce VEGF expression in BMSC-derived osteoblasts by downregulating the activity of the PKA/pAKT/HIF1α/VEGF pathway, thus affecting endochondral bone formation by causing a reduction in angiogenesis and osteogenesis, ultimately leading to delayed fracture healing.

RANKL/OPG system regulation by endogenous PTH and PTH1R/ATF4 axis in bone: Implications for bone accrual and strength in growing rats with mild uremia

Osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL), and parathyroid hormone (PTH) play a central role in the regulation of bone turnover in chronic kidney disease (CKD), but their influence on bone mineral density (BMD) and strength remains unclear, particularly in children. We studied the clinical significance of OPG and RANKL in relation to PTH, femur weight, BMD, and bone biomechanical properties in growing rats after one month (CKD-1) and three months (CKD-3) of surgically-induced mild CKD. Gene expression of parathyroid hormone 1 receptor (PTH1R) and activating transcription factor 4 (ATF4), major regulators of anabolic PTH response in bone, was also determined. Serum PTH and bone PTH1R/ATF4 expression was elevated in CKD-3 compared with other groups, and it positively correlated with femur weight, BMD, and the biomechanical properties of the femoral diaphysis reflecting cortical bone strength. In contrast, bone RANKL/OPG ratios were decreased in CKD-3 rats compared with other groups, and they were inversely correlated with PTH and the other abovementioned bone parameters. However, the PTH-PTH1R-ATF4 axis exerted an unfavorable effect on the biomechanical properties of the femoral neck. In conclusion, this study showed for the first time an inverse association between serum PTH and the bone RANKL/OPG system in growing rats with mild CKD. A decrease in the RANKL/OPG ratio, associated with PTH-dependent activation of the anabolic PTH1R/ATF4 pathway, seems to be responsible for the unexpected, beneficial effect of PTH on cortical bone accrual and strength. Simultaneously, impaired biomechanical properties of the femoral neck were observed, making this bone site more susceptible to fractures.

Elevated intracellular cAMP exacerbates vulnerability to oxidative stress in optic nerve head astrocytes

Glaucoma is characterized by a progressive loss of retinal ganglion cells and their axons, but the underlying biological basis for the accompanying neurodegeneration is not known. Accumulating evidence indicates that structural and functional abnormalities of astrocytes within the optic nerve head (ONH) have a role. However, whether the activation of cyclic adenosine 3',5'-monophosphate (cAMP) signaling pathway is associated with astrocyte dysfunction in the ONH remains unknown. We report here that the cAMP/protein kinase A (PKA) pathway is critical to ONH astrocyte dysfunction, leading to caspase-3 activation and cell death via the AKT/Bim/Bax signaling pathway. Furthermore, elevated intracellular cAMP exacerbates vulnerability to oxidative stress in ONH astrocytes, and this may contribute to axonal damage in glaucomatous neurodegeneration. Inhibition of intracellular cAMP/PKA signaling activation protects ONH astrocytes by increasing AKT phosphorylation against oxidative stress. These results strongly indicate that activation of cAMP/PKA pathway has an important role in astrocyte dysfunction, and suggest that modulating cAMP/PKA pathway has therapeutic potential for glaucomatous ONH degeneration.

Catabolic Effects of Human PTH (1-34) on Bone: Requirement of Monocyte Chemoattractant Protein-1 in Murine Model of Hyperparathyroidism

The bone catabolic actions of parathyroid hormone (PTH) are seen in patients with hyperparathyroidism, or with infusion of PTH in rodents. We have previously shown that the chemokine, monocyte chemoattractant protein-1 (MCP-1), is a mediator of PTH’s anabolic effects on bone. To determine its role in PTH’s catabolic effects, we continuously infused female wild-type (WT) and MCP-1^−/− mice with hPTH or vehicle. Microcomputed tomography (µCT) analysis of cortical bone showed that hPTH-infusion induced significant bone loss in WT mice. Further, μCT analysis of trabecular bone revealed that, compared with the vehicle-treated group, the PTH-treated WT mice had reduced trabecular thickness and trabecular number. Notably, MCP-1^−/− mice were protected against PTH-induced cortical and trabecular bone loss as well as from increases in serum CTX (C-terminal crosslinking telopeptide of type I collagen) and TRACP-5b (tartrate-resistant acid phosphatase 5b). In vitro, bone marrow macrophages (BMMs) from MCP-1^−/− and WT mice were cultured with M-CSF, RANKL and/or MCP-1. BMMs from MCP-1^−/− mice showed decreased multinucleated osteoclast formation compared with WT mice. Taken together, our work demonstrates that MCP-1 has a role in PTH’s catabolic effects on bone including monocyte and macrophage recruitment, osteoclast formation, bone resorption, and cortical and trabecular bone loss.

G protein-coupled receptors as anabolic drug targets in osteoporosis

Osteoporosis is a progressive bone disorder characterised by imbalance between bone building (anabolism) and resorption (catabolism). Most therapeutics target inhibition of osteoclast-mediated bone resorption, but more recent attention in early drug discovery has focussed on anabolic targets in osteoblasts or their precursors. Two marketed agents that display anabolic properties, strontium ranelate and teriparatide, mediate their actions via the G protein-coupled calcium-sensing and parathyroid hormone-1 receptors, respectively. This review explores their activity, the potential for improved therapeutics targeting these receptors and other putative anabolic GPCR targets, including Smoothened, Wnt/Frizzled, relaxin family peptide, adenosine, cannabinoid, prostaglandin and sphingosine-1-phosphate receptors.

Prevention of breast cancer skeletal metastases with parathyroid hormone

Advanced breast cancer is frequently associated with skeletal metastases and accelerated bone loss. Recombinant parathyroid hormone [teriparatide, PTH(1-34)] is the first anabolic agent approved in the US for treatment of osteoporosis. While signaling through the PTH receptor in the osteoblast lineage regulates bone marrow hematopoietic niches, the effects of anabolic PTH on the skeletal metastatic niche are unknown. Here, we demonstrate, using orthotopic and intratibial models of 4T1 murine and MDA-MB-231 human breast cancer tumors, that anabolic PTH decreases both tumor engraftment and the incidence of spontaneous skeletal metastasis in mice. Microcomputed tomography and histomorphometric analyses revealed that PTH increases bone volume and reduces tumor engraftment and volume. Transwell migration assays with murine and human breast cancer cells revealed that PTH alters the gene expression profile of the metastatic niche, in particular VCAM-1, to inhibit recruitment of cancer cells. While PTH did not affect growth or migration of the primary tumor, it elicited several changes in the tumor gene expression profile resulting in a less metastatic phenotype. In conclusion, PTH treatment in mice alters the bone microenvironment, resulting in decreased cancer cell engraftment, reduced incidence of metastases, preservation of bone microarchitecture and prolonged survival.

CCL2, CCL5, and IGF-1 participate in the immunomodulation of osteogenesis during M1/M2 transition in vitro

The modulation of macrophage phenotype from pro-inflammatory (M1) to tissue healing (M2) via exogenous addition of interleukin-4 (IL-4) facilitates osteogenesis; however, the molecular mediators underlying this phenomenon remain unknown. This study characterizes the IL-4-dependent paracrine crosstalk between macrophages and osteoprogenitors and its effect on osteogenesis in vitro. Primary murine M1 were co-cultured with MC3T3 cells (M1-MC3T3) in both transwell plates and direct co-cultures. To modulate M1 to M2, M1-MC3T3 were treated with IL-4 (20 ng/mL) at day 3 after seeding (M1 + IL-4-MC3T3). Selected molecular targets were assessed at days 3 and 6 after seeding at protein and mRNA levels. Mineralization was assessed at day 21. Transwell M1 + IL-4-MC3T3 significantly enhanced the secretion of CCL2/MCP-1, IGF-1 and to a lesser degree, CCL5/RANTES at day 6. At day 3, alkaline phosphatase (Alpl) was upregulated in direct M1-MC3T3. At day 6, Smurf2 and Insulin growth factor-1 (IGF-1) were downregulated and upregulated, respectively, in direct M1 + IL-4-MC3T3. Finally, M1 + IL-4-MC3T3 increased bone matrix mineralization compared with MC3T3 cells in transwell, but this was significantly less than M1-MC3T3. Taken together, macrophage subtypes enhanced the osteogenesis in transwell setting and the transition from M1 to M2 was associated with an increase in bone anabolic factors CCL2/MCP-1, CCL5/RANTES and IGF-1 in vitro. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 3069-3076, 2017.

Potential Role of Chemokines in Fracture Repair

Chemokines are a family of small cytokines that share a typical key structure that is stabilized by disulfide bonds between the cysteine residues at the NH₂-terminal of the protein, and they are secreted by a great variety of cells in several different conditions. Their function is directly dependent on their interactions with their receptors. Chemokines are involved in cell maturation and differentiation, infection, autoimmunity, cancer, and, in general, in any situation where immune components are involved. However, their role in postfracture inflammation and fracture healing is not yet well established. In this article, we will discuss the response of chemokines to bone fracture and their potential roles in postfracture inflammation and healing based on data from our studies and from other previously published studies.

Environmental Factors Impacting Bone-Relevant Chemokines

Chemokines play an important role in normal bone physiology and the pathophysiology of many bone diseases. The recent increased focus on the individual roles of this class of proteins in the context of bone has shown that members of the two major chemokine subfamilies-CC and CXC-support or promote the formation of new bone and the remodeling of existing bone in response to a myriad of stimuli. These chemotactic molecules are crucial in orchestrating appropriate cellular homing, osteoblastogenesis, and osteoclastogenesis during normal bone repair. Bone healing is a complex cascade of carefully regulated processes, including inflammation, progenitor cell recruitment, differentiation, and remodeling. The extensive role of chemokines in these processes and the known links between environmental contaminants and chemokine expression/activity leaves ample opportunity for disruption of bone healing by environmental factors. However, despite increased clinical awareness, the potential impact of many of these environmental factors on bone-related chemokines is still ill defined. A great deal of focus has been placed on environmental exposure to various endocrine disruptors (bisphenol A, phthalate esters, etc.), volatile organic compounds, dioxins, and heavy metals, though mainly in other tissues. Awareness of the impact of other less well-studied bone toxicants, such as fluoride, mold and fungal toxins, asbestos, and chlorine, is also reviewed. In many cases, the literature on these toxins in osteogenic models is lacking. However, research focused on their effects in other tissues and cell lines provides clues for where future resources could be best utilized. This review aims to serve as a current and exhaustive resource detailing the known links between several classes of high-interest environmental pollutants and their interaction with the chemokines relevant to bone healing.

CCL2/Monocyte Chemoattractant Protein 1 and Parathyroid Hormone Action on Bone

Chemokines are small molecules that play a crucial role as chemoattractants for several cell types, and their components are associated with host immune responses and repair mechanisms. Chemokines selectively recruit monocytes, neutrophils, and lymphocytes and induce chemotaxis through the activation of G protein-coupled receptors. Two well-described chemokine families (CXC and CC) are known to regulate the localization and trafficking of immune cells in cases of injury, infection, and tumors. Monocyte chemoattractant protein 1 (MCP-1/CCL2) is one of the important chemokines from the CC family that controls migration and infiltration of monocytes/macrophages during inflammation. CCL2 is profoundly expressed in osteoporotic bone and prostate cancer-induced bone resorption. CCL2 also regulates physiological bone remodeling in response to hormonal and mechanical stimuli. Parathyroid hormone (PTH) has multifaceted effects on bone, depending on the mode of administration. Intermittent PTH increases bone in vivo by increasing the number and activity of osteoblasts, whereas a continuous infusion of PTH decreases bone mass by stimulating a net increase in bone resorption. CCL2 is essential for both anabolic and catabolic effects of PTH. In this review, we will discuss the pharmacological role of PTH and involvement of CCL2 in the processes of PTH-mediated bone remodeling.

Fgf23 and parathyroid hormone signaling interact in kidney and bone

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone, suppressing renal phosphate reabsorption and vitamin D hormone synthesis in proximal tubules, and stimulating calcium reabsorption in distal tubules of the kidney. Here, we analyzed the long term sequelae of deficient Fgf23 signaling on bone and mineral metabolism in 9-month-old mice lacking both Fgf23 or Klotho and a functioning vitamin D receptor (VDR). To prevent hypocalcemia in VDR deficient mice, all mice were kept on a rescue diet enriched with calcium, phosphate, and lactose. VDR mutants were normocalcemic and normophosphatemic, and had normal tibial bone mineral density. Relative to VDR mutants, Fgf23/VDR and Klotho/VDR compound mutants were characterized by hypocalcemia, hyperphosphatemia, and very high serum parathyroid hormone (PTH). Despite ∼10-fold higher serum PTH levels in compound mutants, urinary excretion of phosphate and calcium as well as osteoclast numbers in bone remained unchanged relative to VDR mutants. The increase in plasma cAMP after hPTH(1-34) injection was similar in all genotypes. However, a 5-day infusion of hPTH(1-34) via osmotic minipumps resulted in reduced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) in bone and kidney of Fgf23/VDR and Klotho/VDR compound mutants, relative to VDR and WT controls. Similarly, the PTH-mediated ERK1/2 phosphorylation was reduced in primary osteoblasts isolated from Fgf23 and Klotho deficient mice, but was restored by concomitant treatment with recombinant FGF23. Collectively, our data indicate that the phosphaturic, calcium-conserving, and bone resorption-stimulating actions of PTH are blunted by Fgf23 or Klotho deficiency. Hence, FGF23 may be an important modulator of PTH signaling in bone and kidney.