Induction of osteoblast differentiation indexes by PTHrP in MG-63 cells involves multiple signaling pathways

To B (Bone Morphogenic Protein-2) or Not to B (Bone Morphogenic Protein-2): Mesenchymal Stem Cells May Explain the Protein's Role in Osteosarcomagenesis

Osteosarcoma (OS), a primary malignant bone tumor, stems from bone marrow-derived mesenchymal stem cells (BMSCs) and/or committed osteoblast precursors. Distant metastases, in particular pulmonary and skeletal metastases, are common in patients with OS. Moreover, extensive resection of the primary tumor and bone metastases usually leads to bone defects in these patients. Bone morphogenic protein-2 (BMP-2) has been widely applied in bone regeneration with the rationale that BMP-2 promotes osteoblastic differentiation of BMSCs. Thus, BMP-2 might be useful after OS resection to repair bone defects. However, the potential tumorigenicity of BMP-2 remains a concern that has impeded the administration of BMP-2 in patients with OS and in populations susceptible to OS with severe bone deficiency (e.g., in patients with genetic mutation diseases and aberrant activities of bone metabolism). In fact, some studies have drawn the opposite conclusion about the effect of BMP-2 on OS progression. Given the roles of BMSCs in the origination of OS and osteogenesis, we hypothesized that the responses of BMSCs to BMP-2 in the tumor milieu may be responsible for OS development. This review focuses on the relationship among BMSCs, BMP-2, and OS cells; a better understanding of this relationship may elucidate the accurate mechanisms of actions of BMP-2 in osteosarcomagenesis and thereby pave the way for clinically safer and broader administration of BMP-2 in the future. For example, a low dosage of and a slow-release delivery strategy for BMP-2 are potential topics for exploration to treat OS.

Polyethylenimine-Bisphosphonate-Cyclodextrin Ternary Conjugates: Supramolecular Systems for the Delivery of Antineoplastic Drugs

Bisphosphonates (BPs) are bone-binding molecules that provide targeting capabilities to bone cancer cells when conjugated with drug-carrying polymers. This work reports the design, synthesis, and biological evaluation of polyethyleneimine-BP-cyclodextrin (PEI-BP-CD) ternary conjugates with supramolecular capabilities for the loading of antineoplastic drugs. A straightforward, modular, and versatile strategy based on the click aza-Michael addition reaction of vinyl sulfones (VSs) allows the grafting of BPs targeting ligands and βCD carrier appendages to the PEI polymeric scaffold. The in vitro evaluation (cytotoxicity, cellular uptake, internalization routes, and subcellular distribution) for the ternary conjugates and their doxorubicin inclusion complexes in different bone-related cancer cell lines (MC3T3-E1 osteoblasts, MG-63 sarcoma cells, and MDA-MB-231 breast cancer cells) confirmed specificity, mitochondrial targeting, and overall capability to mediate a targeted drug transport to those cells. The in vivo evaluation using xenografts of MG-63 and MDA-MB-231 cells on mice also confirmed the targeting of the conjugates.

Cyanidin 3-O-Glucoside Induces the Apoptosis in the Osteosarcoma Cells through Upregulation of the PPARγ and P21: An <i>In Vitro</i> Study

BACKGROUND

Osteosarcoma (OS) is known as the malignant tumors in the bone. Cyanidin 3-OGlucoside (C3G) has a potential to induce the apoptotic cell death in different cancer cells; however, the mechanisms of action for C3G have not been clarified yet.

OBJECTIVE

In this study, the apoptotic effects of C3G on three different osteosarcoma cell lines including Saso-2, MG-63, and G-292 (clone A141B1) were investigated.

METHODOLOGY

The 24-hr IC50 of C3G for Saso-2, G-292, and MG-63 cells was evaluated by the MTT assay. Apoptosis induction in these cell lines after treatment with the C3G was approved by the Annexin V/PI flow cytometry. Changes at the mRNA expression level of PPARγ, P21, Bax, and Bcl-xl genes were investigated by real-time Polymerase Chain Reaction (PCR) technique, and P21 expression was further confirmed by the western blotting.

RESULTS

The MTT assay results demonstrated that the 24-hr IC50 of C3G was equal to 110μg/ml for Saso-2 and G-292 cells while it was about 140μg/ml for the MG-63 cells. The results of real-time PCR clearly showed that treatment of the cells with 24hrs IC50 of C3G caused the upregulation of PPARγ, P21, and Bax genes. Moreover, western blot analysis confirmed that P21 protein overexpressed endogenously after treatment of the cells with the C3G, and it was more upregulated in the MG-63 cells compared to the other cell lines.

CONCLUSION

According to the findings of the study, the C3G is a novel anti-osteosarcoma agent with the ability to induce the apoptosis in different osteosarcoma cells through upregulation of the PPARγ and P21 genes.

PLK2 modulation of enriched TAp73 affects osteogenic differentiation and prognosis in human osteosarcoma

There are three subtypes of undifferentiated human conventional osteosarcoma (HCOS): osteoblastic osteosarcoma (OOS), chondroblastic osteosarcoma (COS), and fibroblastic osteosarcoma (FOS). HCOS also exhibits heterogeneous pathological maldifferentiation in individual patients. Currently, the mechanism regulating HCOS differentiation remains unclear, and therapies are ineffective. Osteopontin (OPN) and osteocalcin (OCN) are markers of osteoblast maturation, and their expression is inhibited in HCOS. A previous study found that PLK2 inhibited TAp73 phosphorylation and consequent anti-OS function of TAp73 in OS cells with enriched TAp73. TAp73 was also reported to regulate bone cell calcification. Here, OOS was found to have higher TAp73 levels and PLK2 expression than those in COS, which is correlated with HCOS maldifferentiation according to Spearman analysis and affects patient prognosis according to Kaplan-Meier survival analysis. In the conventional OS cell-line Saos2 and in patient-derived xenograft OS (PDX-OS) cells, increased PLK2 expression owing to abundant TAp73 levels affected OPN and OCN content as measured by RT-PCR and Western blotting, and alizarin red staining showed that PLK2 affected calcium deposition in OS cells. In addition, PLK2 inhibition in PDX-OS cells prohibited clone formation, as indicated by a clonogenic assay, and sensitized OS cells to cisplatin (CDDP) (which consequently limited proliferation), as shown by the CCK-8 assay. In an established PDX animal model with abundant TAp73 levels, PLK2 inhibition or CDDP treatment prevented tumor growth and prolonged median survival. The combined therapeutic effect of PLK2 inhibition with CDDP treatment was better than that of either monotherapy. These results indicate that increased PLK2 levels due to enriched TAp73 affect osteogenic differentiation and maturation and OS prognosis. In conclusion, PLK2 is a potential target for differentiation therapy of OS with enriched TAp73.

Advances in differentiation therapy for osteosarcoma

Differentiation therapy involves the use of agents that can induce differentiation in cancer cells, with the irreversible loss of tumour phenotype. The application of differentiation therapy in osteosarcoma has made progress because of a better understanding of the potential links between differentiation defects and tumorigenesis. Here, we review recent studies on differentiation therapy for osteosarcoma, describing a variety of differentiation inducers. By highlighting these examples of drug-induced osteosarcoma cell differentiation, we can acquire unique insights into not only osteosarcoma treatment, but also novel approaches to transform differentiating drugs into more effective therapies for other solid tumours.

The potential of magnetic hyperthermia for triggering the differentiation of cancer cells

Magnetic hyperthermia is a potential technique for cancer therapy that exploits heat generated by magnetic nanoparticles to kill cancerous cells. Many studies have shown that magnetic hyperthermia is effective at killing cancer cells both in vitro and in vivo, however little attention has been paid to the cellular functioning of the surviving cells. We report here new evidence demonstrating the onset of thermally triggered differentiation in osteosarcoma cancer cells that survive magnetic hyperthermia treatment. This raises the possibility that in addition to causing cell death, magnetic hyperthermia could induce surviving cancer cells to form more mature cell types and thereby inhibit their capacity to self-renew. Such processes could prove to be as important as cell death when considering magnetic hyperthermia for treating cancer.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Odontogenic Potential of Parathyroid Hormone-related Protein (107-111) Alone or in Combination with Mineral Trioxide Aggregate in Human Dental Pulp Cells

INTRODUCTION

Parathyroid hormone-related protein plays an important role in bone remodeling. Its N-terminal domain parathyroid hormone-related protein (107-111) is called osteostatin (OST). OST has demonstrated osteogenic potential when combined with biomaterials such as hydroxyapatite or bioceramics. However, the odontogenic potential of OST has not yet been reported. Therefore, the aim of this study was to determine whether OST has an odontogenic effect or a synergistic effect with mineral trioxide aggregate (MTA) in human dental pulp cells (hDPCs) and to examine the underlying signaling mechanisms involved in OST-mediated odontogenic differentiation.

METHODS

Viability of hDPCs on stimulation with OST or MTA was measured. Real-time polymerase chain reaction and Western blot analyses were performed to evaluate the expression levels of odontogenic markers and the activation of extracellular signal-regulated kinase (ERK). To evaluate mineralized nodule formation, alkaline phosphatase (ALP) staining and alizarin red S staining were performed. Combined effects of OST and MTA were evaluated.

RESULTS

OST promoted odontogenic differentiation, as evidenced by the formation of mineralized nodules, induction of ALP activity, and upregulation of odontogenic markers (dentin sialophosphoprotein, dentin matrix protein-1, and ALP). Phosphorylation of ERK was increased by OST. However, ERK inhibitor (U0126) inhibited the increase in dentin sialophosphoprotein and dentin matrix protein-1 expression and mineralization induced by OST. A combination of MTA and OST upregulated odontogenic differentiation-associated gene expression and calcium nodule mineralization in hDPCs compared with MTA alone.

CONCLUSIONS

The present study revealed that OST can promote odontogenic differentiation and mineralization through activating the ERK signaling pathway. A combination of MTA and OST showed a synergistic effect compared with MTA alone in hDPCs.

RhoA inhibits the hypoxia-induced apoptosis and mitochondrial dysfunction in chondrocytes via positively regulating the CREB phosphorylation

Chondrocytes that are embedded within the growth plate or the intervertebral disc are sensitive to environmental stresses, such as inflammation and hypoxia. However, little is known about the molecular signalling pathways underlining the hypoxia-induced mitochondrial dysfunction and apoptosis in chondrocytes. In the present study, we firstly examined the hypoxia-induced apoptosis, mitochondrial dysfunction and the activation of cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB) signalling in human chondrocyte cell line, C28/I2 and then investigated the regulatory role of RhoA, a well-recognized apoptosis suppressor, in such process, with gain-of-function strategy. Our results indicated that hypoxia induced apoptosis and inhibited CREB phosphprylation in chondrocytes, meanwhile, the dysfunctional mitochondria with up-regulated mitochondrial superoxide and reactive oxygen species (ROS) levels, whereas with a reduced mitochondrial membrane potential (MMP) and Complex IV activity were observed in the hypoxia-treated C28/I2 cells. However, the overexpressed RhoA blocked the hypoxia-mediated reduction in CREB phosphprylation and inhibited the apoptosis induction, along with an ameliorated mitochondrial function in the hypoxia-treated C28/I2 cells. In conclusion, the present study confirmed the reduced CREB phosphorylation, along with the apoptosis induction and mitochondrial dysfunction in the hypoxia-treated chondrocyte cells. And the overexpression of RhoA ameliorated the hypoxia-induced mitochondrial dysfunction and apoptosis via blocking the hypoxia-mediated reduction in CREB phosphorylation.

Protective effect of α-lipoic acid against antimycin A cytotoxicity in MC3T3-E1 osteoblastic cells

Oxidative stress represents a major cause of cellular damage and death in the process of osteoporosis. Antimycin A (AMA) has been shown to stimulate mitochondrial superoxide anions and reactive oxygen species (ROS). α-Lipoic acid (α-LA) is a naturally occurring essential coenzyme in mitochondrial multienzyme complexes and acts as a key player in mitochondrial energy production. However, whether α-LA affects the cytotoxicity of AMA in osteoblastic cells is unknown. In this study, we investigated the protective effects of α-LA against AMA-induced cytotoxicity using the MC3T3-E1 osteoblast-like cell line. Our results indicated that α-LA treatment attenuated AMA-induced cytotoxicity and LDH release in a dose-dependent manner. Notably, a significant recovery effect of α-LA on mineralization inhibited by AMA was found. Our results also demonstrated that treatment with 50 μM AMA leads to a reduction of mitochondrial membrane potential (MMP) and the complex IV dysfunction, which was inhibited by pretreatment with α-LA in a dose-dependent manner. In addition, treatment with α-LA significantly reduced the generation of ROS and mitochondrial superoxide production induced by AMA. In addition, our result suggests that PI3K/Akt and CREB pathways are related to the protective effect of α-LA. Importantly, Hoechst 33258 staining results indicated that pretreatment with α-LA prevented AMA-induced apoptosis. Mechanistically, we found that α-LA prevents MC3T3-E1 cells from apoptosis through attenuating cytochrome C release and reducing the level of cleaved caspase-3.

Allicin inhibits oxidative stress-induced mitochondrial dysfunction and apoptosis by promoting PI3K/AKT and CREB/ERK signaling in osteoblast cells

Osteoporosis is a disease of the skeleton that is characterized by the loss of bone mass and degeneration of bone microstructure, resulting in an increased risk of fracture. Oxidative stress, which is known to promote oxidative damage to mitochondrial function and also cell apoptosis, has been recently indicated to be implicated in osteoporosis. However, there are few agents that counteract oxidative stress in osteoporosis. In the present study, the protective effects of allicin against the oxidative stress-induced mitochondrial dysfunction and apoptosis were investigated in murine osteoblast-like MC3T3-E1 cells. The results demonstrated that allicin counteracted the reduction of cell viability and induction of apoptosis caused by hydrogen peroxide (H₂O₂) exposure. The inhibition of apoptosis by allicin was confirmed by the inhibition of H₂O₂-induced cytochrome c release and caspase-3 activation. Moreover, the inhibition of apoptosis by allicin was identified to be associated with the counteraction of H₂O₂-induced mitochondrial dysfunction. In addition, allicin was demonstrated to be able to significantly ameliorate the repressed phosphoinositide 3-kinase (PI3K)/AKT and cyclic adenosine monophosphate response element-binding protein (CREB)/extracellular-signal-regulated kinase (ERK) signaling pathways by H₂O₂, which may also be associated with the anti-oxidative stress effects of allicin. In conclusion, allicin protects osteoblasts from H₂O₂-induced oxidative stress and apoptosis in MC3T3-E1 cells by improving mitochondrial function and the activation of PI3K/AKT and CREB/ERK signaling. The present study implies a promising role of allicin in oxidative stress-associated osteoporosis.

Controlling Arteriogenesis and Mast Cells Are Central to Bioengineering Solutions for Critical Bone Defect Repair Using Allografts

Although most fractures heal, critical defects in bone fail due to aberrant differentiation of mesenchymal stem cells towards fibrosis rather than osteogenesis. While conventional bioengineering solutions to this problem have focused on enhancing angiogenesis, which is required for bone formation, recent studies have shown that fibrotic non-unions are associated with arteriogenesis in the center of the defect and accumulation of mast cells around large blood vessels. Recently, recombinant parathyroid hormone (rPTH; teriparatide; Forteo) therapy have shown to have anti-fibrotic effects on non-unions and critical bone defects due to inhibition of arteriogenesis and mast cell numbers within the healing bone. As this new direction holds great promise towards a solution for significant clinical hurdles in craniofacial reconstruction and limb salvage procedures, this work reviews the current state of the field, and provides insights as to how teriparatide therapy could be used as an adjuvant for healing critical defects in bone. Finally, as teriparatide therapy is contraindicated in the setting of cancer, which constitutes a large subset of these patients, we describe early findings of adjuvant therapies that may present future promise by directly inhibiting arteriogenesis and mast cell accumulation at the defect site.

PTH Induces Systemically Administered Mesenchymal Stem Cells to Migrate to and Regenerate Spine Injuries

Osteoporosis affects more than 200 million people worldwide leading to more than 2 million fractures in the United States alone. Unfortunately, surgical treatment is limited in patients with low bone mass. Parathyroid hormone (PTH) was shown to induce fracture repair in animals by activating mesenchymal stem cells (MSCs). However, it would be less effective in patients with fewer and/or dysfunctional MSCs due to aging and comorbidities. To address this, we evaluated the efficacy of combination i.v. MSC and PTH therapy versus monotherapy and untreated controls, in a rat model of osteoporotic vertebral bone defects. The results demonstrated that combination therapy significantly increased new bone formation versus monotherapies and no treatment by 2 weeks (P < 0.05). Mechanistically, we found that PTH significantly enhanced MSC migration to the lumbar region, where the MSCs differentiated into bone-forming cells. Finally, we used allogeneic porcine MSCs and observed similar findings in a clinically relevant minipig model of vertebral defects. Collectively, these results demonstrate that in addition to its anabolic effects, PTH functions as an adjuvant to i.v. MSC therapy by enhancing migration to heal bone loss. This systemic approach could be attractive for various fragility fractures, especially using allogeneic cells that do not require invasive tissue harvest.

Talking among ourselves: paracrine control of bone formation within the osteoblast lineage

While much research focuses on the range of signals detected by the osteoblast lineage that originate from endocrine influences, or from other cells within the body, there are also multiple interactions that occur within this family of cells. Osteoblasts exist as teams and form extensive communication networks both on, and within, the bone matrix. We provide four snapshots of communication pathways that exist within the osteoblast lineage between different stages of their differentiation, as follows: (1) PTHrP, a factor produced by early osteoblasts that stimulates the activity of more mature bone-forming cells and the most mature osteoblast embedded within the bone matrix, the osteocyte; (2) sclerostin, a secreted factor, released by osteocytes into their extensive communication network to restrict the activity of younger osteoblasts on the bone surface; (3) oncostatin M, a member of the IL-6/gp130 family of cytokines, expressed throughout osteoblast differentiation and acting to stimulate osteoblast activity that works on a different receptor in the mature osteocyte compared to the preosteoblast; and (4) Eph/ephrins, cell-contact-dependent kinases, and the osteoblast-lineage-specific interaction of EphB4 and ephrinB2, which provides a checkpoint for entry to the late stages of osteoblast differentiation and restricts RANKL expression.

The dual face of parathyroid hormone and prostaglandins in the osteoimmune system

The microenvironment of bone marrow, an extraordinarily heterogeneous and dynamic system, is populated by bone and immune cells, and its functional dimension has been at the forefront of recent studies in the field of osteoimmunology. The interaction of both marrow niches supports self-renewal, differentiation, and homing of the hematopoietic stem cells and provides the essential regulatory molecules for osteoblast and osteoclast homeostasis. Impaired signaling within the niches results in a pathological tableau and enhances disease, including osteoporosis and arthritis, or the rejection of hematopoietic stem cell transplants. Discovering the anabolic players that control these mechanisms has become warranted. In this review, we focus on parathyroid hormone (PTH) and prostaglandins (PGs), potent molecular mediators, both of which carry out a multitude of functions, particularly in bone lining cells and T cells. These two regulators proved to be promising therapeutic agents when strictly clinical protocols on dose treatments were applied.

Functional characterization of the parathyroid hormone 1 receptor in human periodontal ligament cells

OBJECTIVES

Intermittent parathyroid hormone (PTH) exerts anabolic effects on bone and has been approved for osteoporosis therapy. The dual actions of PTH are mediated primarily through the parathyroid hormone 1 receptor (PTH1R). Upon ligand binding, PTH1R activates diverse signaling pathways, including cAMP/protein kinase A (PKA)- and phospholipase C/protein kinase C (PLC/PKC)-dependent pathways. PTH1R has been abundantly studied in bone cells. Knowledge on PTH1R characteristics and physiology in periodontal ligament (PDL) cells is still in its infancy.

MATERIALS AND METHODS

We characterized PTH1R in PDL cells in terms of its cellular localization, binding affinity, and signal transduction and compared these characteristics to those of MG63 osteoblast-like cells.

RESULTS

PTH1R mRNA/protein was identified in PDL and MG63 cells. PTH1R was mainly localized on the plasma membrane, in vesicular structures inside the cell, and, to some extent, in the nucleus of both cell types. Binding characteristics of PTH1R were cell type specific, with PDL cells demonstrating a lower binding affinity. The response of cAMP and active PKC production in MG63 cells was dose dependent with increasing PTH(1-34) concentration, whereas in PDL cells, it was regulated biphasically. However, we observed a cross talk between the cAMP/PKA and PLC/PKC signaling pathways, which were regulated diametrically opposed at a given concentration of PTH(1-34).

CONCLUSION

These data indicate that, albeit the similarity in its subcellular distribution, PTH1R in PDL cells exhibits characteristics different from those in MG63 cells, pointing to the cell type specificity of this receptor.

CLINICAL RELEVANCE

The findings further elucidate the characteristics of PTH action in dental tissues and widen the theoretical basis for the development of anabolic treatment strategies.

The Current and Future Therapies for Human Osteosarcoma

Osteosarcoma (OS) is the most common non-hematologic malignant tumor of bone in adults and children. As sarcomas are more common in adolescents and young adults than most other forms of cancer, there are a significant number of years of life lost secondary to these malignancies. OS is associated with a poor prognosis secondary to a high grade at presentation, resistance to chemotherapy and a propensity to metastasize to the lungs. Current OS management involves both chemotherapy and surgery. The incorporation of cytotoxic chemotherapy into therapeutic regimens escalated cure rates from <20% to current levels of 65-75%. Furthermore, limb-salvage surgery is now offered to the majority of OS patients. Despite advances in chemotherapy and surgical techniques over the past three decades, there has been stagnation in patient survival outcome improvement, especially in patients with metastatic OS. Thus, there is a critical need to identify novel and directed therapy for OS. Several Phase I trials for sarcoma therapies currently ongoing or recently completed have shown objective responses in OS. Novel drug delivery mechanisms are currently under phase II and III clinical trials. Furthermore, there is an abundance of preclinical research which holds great promise in the development of future OS-directed therapeutics. Our continuously improving knowledge of the molecular and cell-signaling pathways involved in OS will translate into more effective therapies for OS and ultimately improved patient survival. The present review will provide an overview of current therapies, ongoing clinical trials and therapeutic targets under investigation for OS.

Developmental cues for bone formation from parathyroid hormone and parathyroid hormone-related protein in an ex vivo organotypic culture system of embryonic chick femora

Enhancement and application of our understanding of skeletal developmental biology is critical to developing tissue engineering approaches to bone repair. We propose that use of the developing embryonic femur as a model to further understand skeletogenesis, and the effects of key differentiation agents, will aid our understanding of the developing bone niche and inform bone reparation. We have used a three-dimensional organotypic culture system of embryonic chick femora to investigate the effects of two key skeletal differentiation agents, parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP), on bone and cartilage development, using a combination of microcomputed tomography and histological analysis to assess tissue formation and structure, and cellular behavior. Stimulation of embryonic day 11 (E11) organotypic femur cultures with PTH and PTHrP initiated osteogenesis. Bone formation was enhanced, with increased collagen I and STRO-1 expression, and cartilage was reduced, with decreased chondrocyte proliferation, collagen II expression, and glycosaminoglycan levels. This study demonstrates the successful use of organotypic chick femur cultures as a model for bone development, evidenced by the ability of exogenous bioactive molecules to differentially modulate bone and cartilage formation. The organotypic model outlined provides a tool for analyzing key temporal stages of bone and cartilage development, providing a paradigm for translation of bone development to improve scaffolds and skeletal stem cell treatments for skeletal regenerative medicine.

The C-terminal fragment of parathyroid hormone-related peptide promotes bone formation in diabetic mice with low-turnover osteopaenia

BACKGROUND AND PURPOSE

Current data suggest that parathyroid hormone (PTH)-related peptide (PTHrP) domains other than the N-terminal PTH-like domain contribute to its role as an endogenous bone anabolic factor. PTHrP-107-139 inhibits bone resorption, a fact which has precluded an unequivocal demonstration of its possible anabolic action in vivo. We thus sought to characterize the osteogenic effects of this peptide using a mouse model of diabetic low-turnover osteopaenia.

EXPERIMENTAL APPROACH

PTHrP-107-139 was administered to streptozotocin-induced diabetic mice, with or without bone marrow ablation, for 13 days. Osteopaenia was confirmed by dual-energy X-ray absorptiometry and microcomputed tomography analysis. Histological analysis was performed on paraffin-embedded bone tissue sections by haematoxylin/eosin and Masson's staining, and tartrate-resistent acid phosphatase immunohistochemistry. Mouse bone marrow stromal cells and osteoblastic MC3T3-E1 cells were cultured in normal and/or high glucose (HG) medium. Osteogenic and adipogenic markers were assessed by real-time PCR, and PTHrP and the PTH(1) receptor protein expression by Western blot analysis.

KEY RESULTS

PTHrP-107-139 reversed the alterations in bone structure and osteoblast function, and also promoted bone healing after marrow ablation without affecting the number of osteoclast-like cells in diabetic mice. This peptide also reversed the high-glucose-induced changes in osteogenic differentiation in both bone marrow stromal cells and the more differentiated MC3T3-E1 cells.

CONCLUSIONS AND IMPLICATIONS

These findings demonstrate that PTHrP-107-139 promotes bone formation in diabetic mice. This mouse model and in vitro cell cultures allowed us to identify various anabolic effects of this peptide in this scenario.

FGF-23/Klotho signaling is not essential for the phosphaturic and anabolic functions of PTH

Parathyroid hormone (PTH) is widely recognized as a key regulator of mineral ion homeostasis. Daily intermittent administration of PTH is the only currently available anabolic therapy for bone disorders such as osteoporosis. Recent studies have shown that PTH increases transcription and secretion of fibroblast growth factor 23 (FGF-23), another important regulator of phosphate homeostasis and skeletal metabolism. However, the full relationship between PTH and FGF-23 is largely unknown. This study evaluated the effect of FGF-23/Klotho signaling on the phosphaturic and anabolic functions of PTH. Eight-day-old wild-type (WT) Fgf23(-/-) and Kl(-/-) mice were injected with 100 µg/kg PTH(1-34) or vehicle daily for a 2-week-period and then euthanized. Intermittent injection of PTH successfully reduced the serum phosphate levels and reversed the hyperphosphatemia of Fgf23(-/-) and Kl(-/-) mice. Bone changes were analyzed in the distal femur metaphysis by peripheral quantitative computed tomography (pQCT), micro-computed tomography (µCT), and histomorphometry. PTH treatment induced substantial increases in bone mineral density (BMD) and trabecular bone volume in each mouse genotype. Expression of osteoblastic marker genes, including Runx2, Col1, Alp, Ocn, and Sost, was similarly altered. In addition, primary osteoblasts were isolated and treated with 100 nM PTH in vitro. PTH treatment similarly induced cAMP accumulation and phosphorylation of ERK1/2 and CREB in the osteoblasts from each genotype. Taken together, our results demonstrate that FGF-23/Klotho signaling is not essential for the phosphaturic and anabolic functions of PTH, suggesting that PTH can function as a therapeutic agent to improve the skeletal quality of patients even in the presence of abnormal serum FGF-23 levels.

Multiple signaling pathways involved in stimulation of osteoblast differentiation by N-methyl-D-aspartate receptors activation in vitro

AIM

Glutamate receptors are expressed in osteoblastic cells. The present study was undertaken to investigate the mechanisms underlying the stimulation of osteoblast differentiation by N-methyl-D-aspartate (NMDA) receptor activation in vitro.

METHODS

Primary culture of osteoblasts was prepared from SD rats. Microarray was used to detect the changes of gene expression. The effect of NMDA receptor agonist or antagonist on individual gene was examined using RT-PCR. The activity of alkaloid phosphotase (ALP) was assessed using a commercial ALP staining kit.

RESULTS

Microarray analyses revealed that 10 genes were up-regulated by NMDA (0.5 mmol/L) and down-regulated by MK801 (100 μmol/L), while 13 genes down-regulated by NMDA (0.5 mmol/L) and up-regulated by MK801 (100 μmol/L). Pretreatment of osteoblasts with the specific PKC inhibitor Calphostin C (0.05 μmol/L), the PKA inhibitor H-89 (20 nmol/L), or the PI3K inhibitor wortmannin (100 nmol/L) blocked the ALP activity increase caused by NMDA (0.5 mmol/L). Furthermore, NMDA (0.5 mmol/L) rapidly increased PI3K phosphorylation, which could be blocked by pretreatment of wortmannin (100 nmol/L).

CONCLUSION

The results suggest that activation of NMDA receptors stimulates osteoblasts differentiation through PKA, PKC, and PI3K signaling pathways, which is a new role for glutamate in regulating bone remodeling.

Kaempferol protects MC3T3-E1 cells through antioxidant effect and regulation of mitochondrial function

Kaempferol, a natural flavonoid present in fruits, vegetables, and teas, provides beneficial effects for human health. In this study, we investigated the protective effects of kaempferol on antimycin A (AMA)-induced toxicity in osteoblast-like MC3T3-E1 cells. Exposure of MC3T3-E1 cells to AMA caused significant cell viability loss, as well as mitochondrial membrane potential dissipation, complex IV inactivation, intracellular calcium ([Ca²⁺](i)) elevation, and reactive oxygen species (ROS) production. Pretreatment with kaempferol prior to AMA exposure significantly reduced AMA-induced cell damage by preventing mitochondrial membrane potential dissipation, complex IV inactivation, [Ca²⁺](i) elevation, and ROS production. Kaempferol also induced the activation of PI3K (phosphoinositide 3-kinase), Akt (protein kinase B), and CREB (cAMP-response element-binding protein) inhibited by AMA, which result demonstrates that kaempferol utilizes the PI3K/Akt/CREB pathway to augment metabolic activity inhibited by AMA. All these data indicate that kaempferol may reduce or prevent osteoblasts degeneration in osteoporosis or other degenerative disorders.

Defective osteogenic differentiation in the development of osteosarcoma

Osteosarcoma (OS) is associated with poor prognosis due to its high incidence of metastasis and chemoresistance. It often arises in areas of rapid bone growth in long bones during the adolescent growth spurt. Although certain genetic conditions and alterations increase the risk of developing OS, the molecular pathogenesis is poorly understood. Recently, defects in differentiation have been linked to cancers, as they are associated with high cell proliferation. Treatments overcoming these defects enable terminal differentiation and subsequent tumor inhibition. OS development may be associated with defects in osteogenic differentiation. While early regulators of osteogenesis are unable to bypass these defects, late osteogenic regulators, including Runx2 and Osterix, are able to overcome some of the defects and inhibit tumor propagation through promoting osteogenic differentiation. Further understanding of the relationship between defects in osteogenic differentiation and tumor development holds tremendous potential in treating OS.

Timing, rather than the concentration of cyclic AMP, correlates to osteogenic differentiation of human mesenchymal stem cells

Previously, we demonstrated that protein kinase A (PKA) activation using dibutyryl-cAMP in human mesenchymal stem cells (hMSCs) induces in vitro osteogenesis and bone formation in vivo. To further investigate the physiological role of PKA in hMSC osteogenesis, we tested a selection of G-protein-coupled receptor ligands which signal via intracellular cAMP production and PKA activation. Treatment of hMSCs with parathyroid hormone, parathyroid hormone-related peptide, melatonin, epinephrine, calcitonin or calcitonin gene-related peptide did not result in accumulation of cAMP or induction of alkaline phosphatase (ALP) expression. The only ligand that did induce cAMP, prostaglandin E2, even inhibited ALP expression and mineralization, suggesting that physiological levels of cAMP may inhibit osteogenesis. Furthermore, intermittent exposure of hMSCs to dibutyryl-cAMP inhibited ALP expression, whereas we did not observe an inhibitive effect at low dibutyryl-cAMP concentrations. Taken together, our results demonstrate that cAMP can either stimulate or inhibit osteogenesis in hMSCs, depending on the duration, rather than the strength, of the signal provided.

The Role of RUNX2 in Osteosarcoma Oncogenesis

Osteosarcoma is an aggressive but ill-understood cancer of bone that predominantly affects adolescents. Its rarity and biological heterogeneity have limited studies of its molecular basis. In recent years, an important role has emerged for the RUNX2 "platform protein" in osteosarcoma oncogenesis. RUNX proteins are DNA-binding transcription factors that regulate the expression of multiple genes involved in cellular differentiation and cell-cycle progression. RUNX2 is genetically essential for developing bone and osteoblast maturation. Studies of osteosarcoma tumours have revealed that the RUNX2 DNA copy number together with RNA and protein levels are highly elevated in osteosarcoma tumors. The protein is also important for metastatic bone disease of prostate and breast cancers, while RUNX2 may have both tumor suppressive and oncogenic roles in bone morphogenesis. This paper provides a synopsis of the current understanding of the functions of RUNX2 and its potential role in osteosarcoma and suggests directions for future study.

Integration of phosphatidylinositol 3-kinase, Akt kinase, and Smad signaling pathway in BMP-2-induced osterix expression

Osterix (Osx), a BMP-2-regulated transcription factor, controls expression of genes essential for osteoblast differentiation. Using progressive deletion of the Osx promoter, we characterized a Smad binding element (SBE) between -552 and -839 bp from its transcription start site. Electrophoretic mobility shift assay and chromatin immunoprecipitation assay showed binding and in vivo recruitment of Smads 1 and 5 to the Osx SBE. Inactivation of PI 3-kinase by the pharmacologic inhibitor Ly294002 or by dominant negative (DN) enzyme significantly blocked BMP-2-induced Osx protein and mRNA expression and Osx transcription. Finally, both DN PI 3-kinase and DN Akt significantly attenuated Smad 5-dependent transcription of Osx, demonstrating the first evidence for a concerted action of PI 3-kinase/Akt signaling with BMP-specific Smads for expression of Osx.

Glimepiride induces proliferation and differentiation of rat osteoblasts via the PI3-kinase/Akt pathway

Glimepiride is a third-generation sulfonylurea agent and is widely used in the treatment of type 2 diabetes mellitus. In addition to the stimulatory effects on pancreatic insulin secretion, glimepiride has also been reported to have extrapancreatic functions including activation of PI3 kinase (PI3K) and Akt in rat adipocytes and skeletal muscle. PI3-kinase and Akt are important signaling molecules in the regulation of proliferation and differentiation in various cells. This study investigated the actions of glimepiride in rat osteoblasts and the role of PI3K/Akt pathway. Cell proliferation was determined by measuring absorbance at 550 nm. Supernatant assay was used for measuring alkaline phosphatase activity. Western blot analysis was used for determining collagen I, insulin receptor substrate-1/2, PI3K/Akt, and endothelial nitric oxide synthase expression. We found that glimepiride significantly enhanced proliferation and differentiation of osteoblasts and led to activation of several key signaling molecules including insulin receptor substrate-1/2, PI3K/Akt, and endothelial nitric oxide synthase. Furthermore, a specific inhibitor of PI3K abolished the stimulatory effects of glimepiride on proliferation and differentiation. Taken together, these observations provide concrete evidence that glimepiride activates the PI3K/Akt pathway; and this activation is likely required for glimepiride to stimulate proliferation and differentiation of rat osteoblasts.

Therapeutic Implications of PPARgamma in Human Osteosarcoma

Osteosarcoma (OS) is the most common nonhematologic malignancy of bone in children and adults. Although dysregulation of tumor suppressor genes and oncogenes, such as Rb, p53, and the genes critical to cell cycle control, genetic stability, and apoptosis have been identified in OS, consensus genetic changes that lead to OS development are poorly understood. Disruption of the osteogenic differentiation pathway may be at least in part responsible for OS tumorigenesis. Current OS management involves chemotherapy and surgery. Peroxisome proliferator-activated receptor (PPAR) agonists and/or retinoids can inhibit OS proliferation and induce apoptosis and may inhibit OS growth by promoting osteoblastic terminal differentiation. Thus, safe and effective PPAR agonists and/or retinoid derivatives can be then used as adjuvant therapeutic drugs for OS therapy. Furthermore, these agents have the potential to be used as chemopreventive agents for the OS patients who undergo the resection of the primary bone tumors in order to prevent local recurrence and/or distal pulmonary metastasis.

cAMP/PKA signaling inhibits osteogenic differentiation and bone formation in rodent models

We previously demonstrated that cAMP-mediated protein kinase A (PKA) activation induces in vitro osteogenesis and in vivo bone formation by human mesenchymal stem cells (hMSCs). To analyze the species-specific response of this phenomenon and to translate our findings into a clinical trial, suitable animal models and cell lines are desirable. In this report, we assessed whether PKA plays a similar proosteogenic role played by two commonly used PKA activators-N6,2'-O-dibutyryl-cAMP (db-cAMP) and 8-bromo cAMP (8b-cAMP)-in a number of model systems. To this end, we treated MC3T3-E1 cells, mouse calvarial osteoblasts, mouse MSCs, and rat MSCs with cAMP. We demonstrate that cAMP inhibits osteogenesis in rodent cell types, evidenced by inhibition of osteogenic markers such as alkaline phosphatase (ALP), osteocalcin (BGLAP), and collagen type 1 (COL1A1). In support of this, ex vivo-cultured mouse calvaria exposed to db-cAMP showed a reduction in bone volume. Interestingly, cAMP even stimulated adipogenic differentiation in rat MSCs. Taken together, our data demonstrate that cAMP inhibits osteogenesis in vitro and bone formation ex vivo in rodent models in contrast to our earlier findings in hMSCs. The species discrepancy in response to various osteogenic signals is a critical need to be tested in clinically relevant models to translate the fundamental findings in lower species level to clinical applications.

Critical role of activating transcription factor 4 in the anabolic actions of parathyroid hormone in bone

Parathyroid hormone (PTH) is a potent anabolic agent for the treatment of osteoporosis. However, its mechanism of action in osteoblast and bone is not well understood. In this study, we show that the anabolic actions of PTH in bone are severely impaired in both growing and adult ovariectomized mice lacking bone-related activating transcription factor 4 (ATF4). Our study demonstrates that ATF4 deficiency suppresses PTH-stimulated osteoblast proliferation and survival and abolishes PTH-induced osteoblast differentiation, which, together, compromise the anabolic response. We further demonstrate that the PTH-dependent increase in osteoblast differentiation is correlated with ATF4-dependent up-regulation of Osterix. This regulation involves interactions of ATF4 with a specific enhancer sequence in the Osterix promoter. Furthermore, actions of PTH on Osterix require this same element and are associated with increased binding of ATF4 to chromatin. Taken together these experiments establish a fundamental role for ATF4 in the anabolic actions of PTH on the skeleton.

PTHrP Induces Notch Signaling in Periodontal Ligament Cells

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

Role of parathyroid hormone-related protein in the decreased osteoblast function in diabetes-related osteopenia

A deficit in bone formation is a major factor in diabetes-related osteopenia. We examined here whether diabetes-associated changes in osteoblast phenotype might in part result from a decrease in PTH-related protein (PTHrP). We used a bone marrow ablation model in diabetic mice by multiple streptozotocin injections. PTHrP (1-36) (100 microg/kg, every other day) or vehicle was administered to mice for 13 d starting 1 wk before marrow ablation. Diabetic mice showed bone loss in both the intact femur and the regenerating tibia on d 6 after ablation; in the latter, this was related to decreased bone-forming cells, osteoid surface, and blood vessels, and increased marrow adiposity. Moreover, a decrease in matrix mineralization occurred in ex vivo bone marrow cultures from the unablated tibia from diabetic mice. These skeletal alterations were associated with decreased gene expression (by real-time PCR) of Runx2, osterix, osteocalcin, PTHrP, the PTH type 1 receptor, vascular endothelial growth factor and its receptors, and osteoprotegerin to receptor activator of nuclear factor-kappaB ligand mRNA ratio, and increased peroxisome proliferator-activated receptor-gamma2 mRNA levels. Similar changes were induced by hyperosmotic (high glucose or mannitol) medium in osteoblastic MC3T3-E1 cells, which were mimicked by adding a neutralizing anti-PTHrP antibody or PTH type 1 receptor antagonists to these cells in normal glucose medium. PTHrP (1-36) administration reversed these changes in both intact and regenerating bones from diabetic mice in vivo, and in MC3T3-E1 cells exposed to high glucose. These findings strongly suggest that PTHrP has an important role in the altered osteoblastic function related to diabetes.

Osteogenic BMPs promote tumor growth of human osteosarcomas that harbor differentiation defects

Osteosarcoma (OS) is the most common primary malignancy of bone. Here, we investigated a possible role of defective osteoblast differentiation in OS tumorigenesis. We found that basal levels of the early osteogenic marker alkaline phosphatase (ALP) activity were low in OS lines. Osteogenic regulators Runx2 and OSX, and the late marker osteopontin (OPN) expressed at low levels in most OS lines, indicating that most OS cells fail to undergo terminal differentiation. Furthermore, OS cells were refractory to osteogenic BMP-induced increases in ALP activity. Osteogenic BMPs were shown to upregulate early target genes, but not late osteogenic markers OPN and osteocalcin (OC). Furthermore, osteogenic BMPs failed to induce bone formation from human OS cells, rather effectively promoted OS tumor growth in an orthotopic OS model. Exogenous expression of early target genes enhanced BMP-stimulated OS tumor growth, whereas osteogenic BMP-promoted OS tumor growth was inhibited by exogenous Runx2 expression. These results suggest that alterations in osteoprogenitors may disrupt osteogenic differentiation pathway. Thus, identifying potential differentiation defects in OS tumors would allow us to reconstruct the tumorigenic events in osteoprogenitors and to develop rational differentiation therapies for clinical OS management.

Osteosarcoma development and stem cell differentiation

Osteosarcoma is the most common nonhematologic malignancy of bone in children and adults. The peak incidence occurs in the second decade of life, with a smaller peak after age 50. Osteosarcoma typically arises around the growth plate of long bones. Most osteosarcoma tumors are of high grade and tend to develop pulmonary metastases. Despite clinical improvements, patients with metastatic or recurrent diseases have a poor prognosis. Here, we reviewed the current understanding of human osteosarcoma, with an emphasis on potential links between defective osteogenic differentiation and bone tumorigenesis. Existing data indicate osteosarcoma tumors display a broad range of genetic and molecular alterations, including the gains, losses, or arrangements of chromosomal regions, inactivation of tumor suppressor genes, and the deregulation of major signaling pathways. However, except for p53 and/or RB mutations, most alterations are not constantly detected in the majority of osteosarcoma tumors. With a rapid expansion of our knowledge about stem cell biology, emerging evidence suggests osteosarcoma should be regarded as a differentiation disease caused by genetic and epigenetic changes that interrupt osteoblast differentiation from mesenchymal stem cells. Understanding the molecular pathogenesis of human osteosarcoma could ultimately lead to the development of diagnostic and prognostic markers, as well as targeted therapeutics for osteosarcoma patients.

Parathyroid hormone increases activating transcription factor 4 expression and activity in osteoblasts: requirement for osteocalcin gene expression

PTH is an important peptide hormone regulator of calcium homeostasis and osteoblast function. However, its mechanism of action in osteoblasts is poorly understood. Our previous study demonstrated that PTH activates mouse osteocalcin (Ocn) gene 2 promoter through the osteoblast-specific element 1 site, a recently identified activating transcription factor-4 (ATF4) -binding element. In the present study, we examined effects of PTH on ATF4 expression and activity as well as the requirement for ATF4 in the regulation of Ocn by PTH. Results show that PTH elevated levels of ATF4 mRNA and protein in a dose- and time-dependent manner. This PTH regulation requires transcriptional activity but not de novo protein synthesis. PTH also increased binding of nuclear extracts to osteoblast-specific element 1 DNA. PTH stimulated ATF4-dependent transcriptional activity mainly through protein kinase A with a lesser requirement for protein kinase C and MAPK/ERK pathways. Lastly, PTH stimulation of Ocn expression was lost by small interfering RNA down-regulation of ATF4 in MC-4 cells and Atf4(-/-) bone marrow stromal cells. Collectively, these studies for the first time demonstrate that PTH increases ATF4 expression and activity and that ATF4 is required for PTH induction of Ocn expression in osteoblasts.

Studies on the mechanisms of the skeletal anabolic action of endogenous and exogenous parathyroid hormone

Parathyroid hormone (PTH) has been viewed as catabolic for bone. Nevertheless, exogenous PTH is anabolic when administered intermittently, at a frequency that permits complete clearance between doses. In the fetus and neonate, endogenous PTH is required for normal trabecular bone formation. In older animals PTH produces net bone loss in fulfilling its calcium homeostatic role, whereas PTH-related peptide (PTHrP), acting in a paracrine/autocrine mode, is anabolic. The proliferative, differentiating, and anti-apoptotic effects of PTH on cells of the osteoblast lineage leading to anabolism can be direct, or indirect via release of local growth factors. The anabolic effect of PTH is also influenced by osteoclastic activity such that suppression of osteoclasts with anti-resorptive agents, concomitant to administering PTH, may enhance the anabolic effect by delaying a reactive osteoclastic response. In contrast, prolonged suppression of osteoclast activity prior to administering PTH appears to diminish molecular signals that increase the osteoblast pool and thereby reduces the anabolic efficacy of PTH. These observations may define the proper timing of the use of PTH as a therapeutic in diseases of bone loss. Finally, the capacity of exogenous PTH to modulate extra-osseous factors such as 1,25 dihydroxyvitamin D may also modulate its potency as an anabolic agent.

Extracellular calcium regulates parathyroid hormone-related peptide expression in osteoblasts and osteoblast progenitor cells

Parathyroid hormone-related peptide (PTHrP) has been shown to have anabolic effects on bone in women with postmenopausal osteoporosis. On the cellular level PTHrP promotes the recruitment of osteogenic cells and prevents apoptotic death of osteoblasts and osteocytes. The calcium concentration is considerably higher in the vicinity of resorbing osteoclasts than in the plasma. Therefore the osteoblasts are likely to be confronted by elevated extracellular calcium concentrations in the areas of resorptive activity. The present study was designed to assess the possibility that extracellular calcium could regulate PTHrP expression in osteoblastic cells. Adult human mesenchymal stem cells (hMSC) were cultured and differentiated by standard methods. The PTHrP release into the culture media was measured by an immunoradiometric assay and the expression of PTHrP, osteocalcin and Runx2 mRNA was assayed by real-time PCR. Increasing the extracellular calcium from 1 mM to 5 mM for 24 h resulted in a 4-6-fold increase in the PTHrP release. PTHrP mRNA was also increased by elevated calcium levels. The effect of calcium stimulation on PTHrP release could be seen within 60 min of treatment. The extracellular calcium sensing receptor (CaR) agonist neomycin mimicked the effects of calcium and the MEK/MAPK inhibitor PD98059 abolished the effect of calcium and neomycin. High extracellular calcium increased the mineralization of hMSC and the expression of osteocalcin, but this effect was not mimicked by neomycin. Our results show that in hMSC, elevated extracellular calcium levels increases both released PTHrP and PTHrP mRNA expression. The effect of calcium on PTHrP can be mimicked by activation of the CaR and can be diminished by inhibition of the MAPK signalling pathway.

Protein kinase C-zeta activation markedly enhances beta-cell proliferation: an essential role in growth factor mediated beta-cell mitogenesis

OBJECTIVE

Diabetes results from a deficiency of functional beta-cells. Previous studies have identified hepatocyte growth factor (HGF) and parathyroid hormone-related protein (PTHrP) as two potent beta-cell mitogens. The objective of this study is to determine 1) whether HGF and PTHrP have additive/synergistic effects on beta-cell growth and proliferation; 2) the signaling pathways through which these growth factors mediate beta-cell mitogenesis; and 3) whether activation of this/these signaling pathway(s) enhances human beta-cell replication.

RESEARCH DESIGN AND METHODS

We generated and phenotypically analyzed doubly transgenic mice overexpressing PTHrP and HGF in the beta-cell. INS-1 and primary mouse and human islet cells were used to identify mitogenic signaling pathways activated by HGF and/or PTHrP.

RESULTS

Combined overexpression of HGF and PTHrP in the beta-cell of doubly transgenic mice did not result in additive/synergistic effects on beta-cell growth and proliferation, suggesting potential cross-talk between signaling pathways activated by both growth factors. Examination of these signaling pathways in INS-1 cells revealed atypical protein kinase C (PKC) as a novel intracellular target activated by both HGF and PTHrP in beta-cells. Knockdown of PKC zeta, but not PKC iota/lambda, expression using specific small-interfering RNAs blocked growth factor-induced INS-1 cell proliferation. Furthermore, adenovirus-mediated delivery of kinase-dead PKC zeta completely inhibited beta-cell proliferation in primary islet cells overexpressing PTHrP and/or HGF. Finally, adenovirus-mediated delivery of constitutively active PKC zeta in mouse and human primary islet cells significantly enhanced beta-cell proliferation.

CONCLUSIONS

PKC zeta is essential for PTHrP- and HGF-induced beta-cell proliferation. PKC zeta activation could be useful in therapeutic strategies for expanding beta-cell mass in vitro and in vivo.

Effect of parathyroid-hormone-related protein on human platelet activation

Evidence suggests that PTHrP [PTH (parathyroid hormone)-related protein] can act as an inflammatory mediator in several pathological settings including cardiovascular disease. The aim of the present study was to determine whether PTHrP might be involved in human platelet activation. We used a turbidimetric method to determine platelet aggregation. The expression of PTH1R (PTH type 1 receptor) in human platelets was analysed by Western blot and flow cytometry analyses. PTHrP-(1–36) (10−7 mol/l) by itself failed to modify the activation of platelets. However, it significantly enhanced ADP-induced platelet activation, and also increased the ability of other agonists (thrombin, collagen and arachidonic acid) to induce platelet aggregation. H89 (10−6 mol/l) and 25×10−6 mol/l Rp-cAMPS (adenosine 3′,5′-cyclic monophosphorothioate Rp-isomer), two protein kinase A inhibitors, and 25×10−9 mol/l bisindolylmaleimide I, a protein kinase C inhibitor, partially decreased the enhancing effect of PTHrP-(1–36) on ADP-induced platelet activation. Meanwhile, 10−6 mol/l PTHrP-(7–34), a PTH1R antagonist, as well as 10−5 mol/l PD098059, a MAPK (mitogen-activated protein kinase) inhibitor, or a farnesyltransferase inhibitor abolished this effect of PTHrP-(1–36). Moreover, 10−7 mol/l PTHrP-(1–36) increased (2-fold over control) MAPK activation in human platelets. PTH1R was detected in platelets, and the number of platelets expressing it on their surface in patients during AMI (acute myocardial infarction) was not different from that in a group of patients with similar cardiovascular risk factors without AMI. Western blot analysis showed that total PTH1R protein levels were markedly higher in platelets from control than those from AMI patients. PTH1R was found in plasma, where its levels were increased in AMI patients compared with controls. In conclusion, human platelets express the PTH1R. PTHrP can interact with this receptor to enhance human platelet activation induced by several agonists through a MAPK-dependent mechanism.

Co-treatment of PTH with osteoprotegerin or alendronate increases its anabolic effect on the skeleton of oophorectomized mice

We examined the effects of 60 days of co-treatment of PTH with either OPG or alendronate in oophorectomized mice. Compared with PTH alone, co-treatment of PTH with either of these two mechanistically distinct anti-catabolics improved bone volume, mechanical strength, and appendicular and axial mineralization and prolonged the beneficial effect of PTH on BMD.

INTRODUCTION

Conflicting evidence exists as to whether the anabolic effect of PTH is inhibited by the action of anti-catabolics. To examine this issue, we assessed the effects of alendronate and osteoprotegerin (OPG), two anti-catabolics with different modes of action, on the anabolic activity of PTH(1-34) in the skeleton of 4-month-old oophorectomized mice.

MATERIALS AND METHODS

Mice treated with vehicle alone (PBS), alendronate alone (100 microg/kg/week), OPG alone (10 mg/kg twice a week), or PTH alone (80 microg/kg/day) were compared with each other and with animals administered PTH plus alendronate or PTH plus OPG. We assessed lumbar spine and femoral BMD at 0, 30, and 60 days. Contact radiography, histology, and histomorphometry, three-point bending assay of the femur, and serum osteocalcin and TRACP5b assays were performed at 2 months.

RESULTS

Although alendronate and OPG each suppressed bone turnover, at the doses used, this was more profound with OPG. Increases in lumbar spine and femoral BMD and in trabecular bone volume were at least as great with OPG as with alendronate, and mechanical indices of femoral bone strength improved only with OPG. Both produced a plateau in spine and femoral BMD increases by 30 days. Co-treatment of PTH with each anti-catabolic produced additive increases in BMD in the femur and supra-additive increases in the lumbar spine with no plateau effects. Neither anti-catabolic impeded the PTH-induced increase in bone volume or the increase in mechanical strength of the femur.

CONCLUSIONS

These studies show that the highly potent anti-catabolic OPG can produce dramatic increases in BMD and bone strength; that the temporal pattern of activity of bone formation and resorption modulators may have major influence on net skeletal accrual; and that, depending on timing, inhibition of osteoclastic activity may markedly augment the anabolic action of PTH.

Transient exposure to PTHrP (107-139) exerts anabolic effects through vascular endothelial growth factor receptor 2 in human osteoblastic cells in vitro

Intermittent administration of the N-terminal fragment of parathyroid hormone (PTH) and PTH-related protein (PTHrP) induces bone anabolic effects. However, the effects of the C-terminal domain of PTHrP on bone turnover remain controversial. We examined the putative mechanisms whereby this PTHrP domain can affect osteoblastic differentiation, using human osteosarcoma MG-63 cells and osteoblastic cells from human trabecular bone. Intermittent exposure to PTHrP (107-139), within 10-100 nM, for only <or=24 hours during cell growth stimulated alkaline phosphatase (ALP) and Runt homology domain protein (Runx2) activities as well as osteocalcin (OC) and osteoprotegerin (OPG) expression but inhibited receptor activator of nuclear factor kappaB (NF-kappaB) ligand. Continuous exposure to this PTHrP peptide reversed these effects. The stimulatory effects of transient treatment with PTHrP (107-139) on OC mRNA and/or OPG protein expression were unaffected by a neutralizing anti-insulin-like growth factor I antibody or [Asn(10), Leu(11), d-Trp(12)]PTHrP (7-34) in these cells. On the other hand, the former antibody and the latter PTHrP antagonist abrogated the PTHrP (1-36)-induced increase in these osteoblastic products. Transient exposure to PTHrP (107-139), in contrast to PTHrP (1-36), stimulated vascular endothelial growth factor receptor 2 (VEGFR2) mRNA levels in these cells. Moreover, induction of ALP activity as well as OC and OPG expression by PTHrP (107-139) was blunted by SU5614, a permeable tyrosine kinase inhibitor of VEGFR2. Protein kinase C (PKC) and extracellular signal-regulated kinase (ERK) inhibitors abolished the PTHrP (107-139)-stimulated VEGFR2 and OPG mRNA levels in these cells. These results indicate that intermittent exposure to PTHrP (107-139) exerts potential anabolic effects through the PKC/ERK pathway and, subsequently, VEGFR2 upregulation in vitro in human osteoblastic cells.

Osteoblastic Activation in the Hematopoietic Stem Cell Niche

Hematopoietic stem cells (HSC) are rare primitive cells capable of reconstituting all blood cell lineages throughout the life of an individual. The microenvironment in which stem cells reside is essential for their survival, self-renewal, and differentiation. This microenvironment, or HSC niche, has been difficult to define in bone and bone marrow, but recent studies from our laboratory and others have shown that osteoblasts, the bone-forming cells, are an essential regulatory component of this complex cellular network. We established that parathyroid hormone (PTH), through activation of the PTH/PTHrP receptor (PTH1R) in osteoblastic cells, could alter the HSC niche resulting in HSC expansion in vivo and in vitro and improving dramatically the survival of mice receiving bone marrow transplants. These findings are of great clinical appeal, because they suggest that a strategy aimed at modifying supportive cells in a stem cell niche can expand HSC. While a number of molecules have been found to be important for hematopoietic/osteoblastic interactions, we have focused on the Jagged1/Notch signaling pathway, which was necessary for the PTH-dependent HSC expansion. Since the Jagged1/Notch signaling pathway has been implicated in the microenvironmental control of stem cell self-renewal in several organ systems, definition of Jagged1 modulation, which is currently poorly understood, should provide additional molecular targets for stem cell regulation and advance the understanding of stem cell-microenvironmental interactions.

Multiple mechanisms are involved in inhibition of osteoblast differentiation by PTHrP and PTH in KS483 Cells

We examined the mechanism by which PTHrP and PTH inhibit KS483 osteoblastic differentiation. We show that PTHrP and PTH inhibit differentiation downstream of early BMP signaling and downregulated components of the hedgehog (Hh) signaling cascade. In addition, PTHrP and PTH repressed RunX2 and osx expression. Overexpression of either gene, however, could not relieve PTHrP and PTH's inhibitory actions. Our data suggest that multiple parallel mechanisms are involved in the inhibition of osteoblast differentiation and matrix mineralization by PTHrP and PTH.

INTRODUCTION

PTH-related peptide (PTHrP) and PTH are potent inhibitors of osteoblast differentiation in vitro by as yet unexplained mechanisms.

MATERIALS AND METHODS

We treated murine bone marrow stromal cells and the mesenchymal progenitor cell line KS483 with PTHrP and PTH in combination with either BMPs or hedgehog (Hh) and measured early and late markers of osteoblast differentiation and studied the expression of RunX2 and Osterix (osx). In addition, we examined the PTHrP and PTH response in stable KS483 cells overexpressing either RunX2 or osx.

RESULTS

PTHrP and PTH inhibited BMP- and Hh-induced osteogenesis downstream of early BMP signaling and by downregulation of components of the Hh signaling cascade. PTHrP and PTH prevented the upregulation of RunX2 expression associated with osteoblast differentiation in an indirect response. However, PTHrP and PTH could still inhibit differentiation, and particularly matrix mineralization, of cells expressing RunX2. In addition, PTHrP and PTH potently downregulated osx expression only in mature osteoblasts in an intermediate early response, but osx overexpression could not relieve the inhibitory effects of PTHrP and PTH on matrix mineralization.

CONCLUSIONS

Our data suggest that, besides transcriptional repression of RunX2 and osx, other mechanisms in parallel with or downstream of RunX2 and osx are involved in the inhibition of osteoblast differentiation and matrix mineralization by PTHrP and PTH in vitro.

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

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

ABBREVIATIONS

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