Parathyroid hormone and parathyroid hormone-related protein exert both pro- and anti-apoptotic effects in mesenchymal cells

The Development of Molecular Biology of Osteoporosis

Osteoporosis is one of the major bone disorders that affects both women and men, and causes bone deterioration and bone strength. Bone remodeling maintains bone mass and mineral homeostasis through the balanced action of osteoblasts and osteoclasts, which are responsible for bone formation and bone resorption, respectively. The imbalance in bone remodeling is known to be the main cause of osteoporosis. The imbalance can be the result of the action of various molecules produced by one bone cell that acts on other bone cells and influence cell activity. The understanding of the effect of these molecules on bone can help identify new targets and therapeutics to prevent and treat bone disorders. In this article, we have focused on molecules that are produced by osteoblasts, osteocytes, and osteoclasts and their mechanism of action on these cells. We have also summarized the different pharmacological osteoporosis treatments that target different molecular aspects of these bone cells to minimize osteoporosis.

Phenanthrenoid Coelogin Isolated from Coelogyne cristata Exerts Osteoprotective Effect Through MAPK-Mitogen-Activated Protein Kinase Signaling Pathway

Osteoporosis is a major health problem in postmenopausal women globally. This study determined the mechanism through which coelogin stimulates osteoblastogenesis and its osteoprotective and bone regenerating potential. Coelogin effect on primary calvarial osteoblast cells was determined by measuring alkaline phosphatase activity, mineralization, osteoblast survival, and apoptosis and protein expression studies. The osteoprotective effect of coelogin was also evaluated on osteopenic adult female Swiss mice. At autopsy, bones were collected for dynamic and histomorphometry studies. Serum samples were also collected for assessment of serum parameters. Coelogin treatment led to increased osteoblast proliferation, survival, differentiation, and mineralization in osteoblast cells. Coelogin supplementation to Ovx mice promoted new bone formation, prevented Ovx-induced deterioration of bone microarchitecture, and enhanced bone regeneration. In addition, signaling studies revealed that coelogin treatment activates the ER-Erk and Akt-dependent signaling pathways which stimulate the osteoblastogenesis in osteoblast cells.

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.

Handling Parathormone Receptor Type 1 in Skeletal Diseases: Realities and Expectations of Abaloparatide

Musculoskeletal disorders represent an elevated socioeconomic burden for developed aging societies. Osteoporosis (OP) has been treated with antiresorptive therapies or with teriparatide that was until recently the only anabolic therapy. However, approval of osteoporosis treatment in postmenopausal women with abaloparatide, which is an analog of parathyroid hormone-related peptide (PTHrP), has created a new alternative for OP management. The success of this new treatment is related to differential mechanisms of activation of PTH receptor type 1 (PTH1R) by abaloparatide and PTH. Here, we address the distinguishing mechanisms of PTH1R activation; the effects of PTH1R stimulation in osteoblast, osteocytes, and chondrocytes; the differences between PTH and abaloparatide actions on PTH1R; potential safety concerns; and future perspectives about abaloparatide use in other musculoskeletal disorders.

Proteolytic Regulation of Parathyroid Hormone-Related Protein: Functional Implications for Skeletal Malignancy

Parathyroid hormone-related protein (PTHrP), with isoforms ranging from 139 to 173 amino acids, has long been implicated in the development and regulation of multiple tissues, including that of the skeleton, via paracrine and autocrine signaling. PTHrP is also known as a potent mediator of cancer-induced bone disease, contributing to a vicious cycle between tumor cells and the bone microenvironment that drives the formation and progression of metastatic lesions. The abundance of roles ascribed to PTHrP have largely been attributed to the N-terminal 1-36 amino acid region, however, activities for mid-region and C-terminal products as well as additional shorter N-terminal species have also been described. Studies of the protein sequence have indicated that PTHrP is susceptible to post-translational proteolytic cleavage by multiple classes of proteases with emerging evidence pointing to novel functional roles for these PTHrP products in regulating cell behavior in homeostatic and pathological contexts. As a consequence, PTHrP products are also being explored as potential biomarkers of disease. Taken together, our enhanced understanding of the post-translational regulation of PTHrP bioactivity could assist in developing new therapeutic approaches that can effectively treat skeletal malignancies.

Metastatic Bone Disease in the Era of Bone-Targeted Therapy: Clinical Impact

Advances in the diagnosis and treatment of tumors by surgery, chemotherapy, biotherapy, radiotherapy and other modalities have increased the survival of cancer patients over the last 20 years. As a consequence, bone now represents the third most common site of metastatic involvement after the lung and liver. Approximately 20–25% of patients with neoplastic disease develop clinically evident bone metastases (BMs) during the natural course of their illness, with a further 50% of such lesions being identified during autopsy. BMs are the major cause of morbidity in cancer patients because of their epidemiological and clinical impact. Pain is the most frequent symptom in about 75% of patients but other serious complications can also occur, such as pathological fractures, spinal cord compression, hypercalcemia and bone marrow suppression. These complications worsen the patient's general condition and reduce patients’ mobility, facilitating the development of lung infections, skin ulcers, deep vein thrombosis, etc., and ultimately reducing prognosis and quality of life. The frequency of serious complications depends on the site and type of lesions and the treatment administered. Over the last 10 years, the introduction of bisphosphonates for the treatment of patients with BMs has led to a marked decrease in the frequency of complications, thus improving quality of life and clinical outcome. Furthermore, progress in understanding the pathophysiology of bone metastases has resulted in the development of new bone-targeted molecules such as denosumab. We therefore felt it would be useful to report on the epidemiological, clinical and economic impact of bone disease in a cancer setting.

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

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

Parathyroid Hormone (PTH) Induces Autophagy to Protect Osteocyte Cell Survival from Dexamethasone Damage

Background

Glucocorticoids (GC) have direct adverse effects on osteocytes, the most abundant bone cell type, and play an important role in osteonecrosis of the femoral head (ONFH). Teriparatide has been reported to be an effective treatment for ONFH. However, the underlying mechanism is unclear.

Material/Methods

An osteocyte cell line, MLO-Y4, was used under various doses of dexamethasone (Dex) with or without rhPTH (1–34). Cell viability, autophagy, and apoptosis markers and osteocyte characteristic mRNAs were investigated to better understand this phenomenon.

Results

Induction of apoptosis by Dex was increased in a time- and dose-dependent manner in MLO-Y4 cells. Autophagy markers (LC3-II and Beclin-1) were increased at the low dose of Dex (10⁻⁷ or 10⁻⁶ M) and decreased at the high dose (10⁻⁵ M). In MOL-Y4 cells, rhPTH (1–34) was shown to be protective against Dex-induced apoptosis. The upregulation of LC3-II and Beclin-1 and decreased level of Caspase-3 was observed in the rhPTH (1–34)-treated group compared with the Dex-only-treated group. Furthermore, the changes induced by Dex in osteocytes, such as increased SOST, RANKL, and DMP-1 mRNA level and decreased Destrin mRNA level, were reversed by rhPTH (1–34). A similar result was found in osteocyte-specific proteins sclerostin expression encoded by SOST mRNA, which acted as a bone formation inhibitor.

Conclusions

The self-activation of autophagy may be a protective mechanism against apoptosis induced by Dex. The protection effect of rhPTH (1–34) for GC-induced ONFH thus results, at least in part, from enhanced autophagy.

Macrophages and skeletal health

Bone is in a constant state of remodeling, a process which was once attributed solely to osteoblasts and osteoclasts. Decades of research has identified many other populations of cells in the bone that participate and mediate skeletal homeostasis. Recently, osteal macrophages emerged as vital participants in skeletal remodeling and osseous repair. The exact mechanistic roles of these tissue-resident macrophages are currently under investigation. Macrophages are highly plastic in response to their micro-environment and are typically classified as being pro- or anti-inflammatory (pro-resolving) in nature. Given that inflammatory states result in decreased bone mass, proinflammatory macrophages may be negative regulators of bone turnover. Pro-resolving macrophages have been shown to release anabolic factors and may present a target for therapeutic intervention in inflammation-induced bone loss and fracture healing. The process of apoptotic cell clearance, termed efferocytosis, is mediated by pro-resolving macrophages and may contribute to steady-state bone turnover as well as fracture healing and anabolic effects of osteoporosis therapies. Parathyroid hormone is an anabolic agent in bone that is more effective in the presence of mature phagocytic macrophages, further supporting the hypothesis that efferocytic macrophages are positive contributors to bone turnover. Therapies which alter macrophage plasticity in tissues other than bone should be explored for their potential to treat bone loss either alone or in conjunction with current bone therapeutics. A better understanding of the exact mechanisms by which macrophages mediate bone homeostasis will lead to an expansion of pharmacologic targets for the treatment of osteoporosis and inflammation-induced bone loss.

Parathyroid Hormone-Related Peptide: A Novel Endocrine Cardioprotective "Conditioning Mimetic"

An as-yet limited body of evidence suggests that calcium-regulating endocrine hormones-in particular, parathyroid hormone-related peptide (PTHrP)-may have unappreciated cardioprotective effects. The current review focuses on the concept that PTHrP may, via modulation of classic cardioprotective signaling pathways, provide a novel strategy to attenuate myocardial ischemia-reperfusion injury.

Lung carcinoma progression and survival versus amino- and carboxyl-parathyroid hormone-related protein expression

PURPOSE

Expression of the carboxyl PTHrP region of parathyroid hormone-related protein (PTHrP) is a positive prognostic indicator in women with lung cancer, but amino PTHrP is a negative indicator in other lung cancer patients. This project investigated whether PTHrP could be expressed as predominantly amino PTHrP or carboxyl PTHrP in individual lung carcinomas. It also assessed domain-specific effects on cancer progression and patient survival.

METHODS

PTHrP immunoreactivities were analyzed versus survival in a human lung cancer tissue microarray (TMA). Growth was compared in athymic mice for isogenic lung carcinoma xenografts differing in expression of amino and carboxyl PTHrP domains.

RESULTS

In the TMA, 33 of 99 patient tumors expressed only one PTHrP domain, while 54 expressed both. By Cox regression, the hazard ratio for cancer-specific mortality (95% confidence interval) was 2.6 (1.28-5.44) for amino PTHrP (P = 0.008) and 0.6 (0-2.58) for carboxyl PTHrP (P = 0.092). Xenografts of H358 lung adenocarcinoma cells that overexpressed amino PTHrP grew twice as fast as isogenic low PTHrP tumors in athymic mice, but growth of tumors expressing amino plus carboxyl PTHrP was not significantly different than growth of the control tumors. In summary, the presence of amino PTHrP signifies worse prognosis in lung cancer patients. In mouse xenografts, this effect was abrogated if carboxyl PTHrP was also present.

CONCLUSION

Amino PTHrP and carboxyl PTHrP can vary independently in different lung carcinomas. Carboxyl PTHrP may temper the stimulatory effect of amino PTHrP on cancer progression.

Preprogrammed Long-Term Systemic Pulsatile Delivery of Parathyroid Hormone to Strengthen Bone

Parathyroid hormone (PTH) is the only US Food and Drug Administration (FDA)-approved anabolic agent for the treatment of osteoporosis. The anabolic action of PTH depends on the mode of PTH administration. Pulsatile administration promotes bone formation, however continuous PTH exposure results in bone resorption. In addition, the therapeutic effect of PTH is optimal when the dose and duration fit the therapeutic window. Current PTH treatment requires daily injection, which is neither a convenient nor a favorable choice of patients. Here, an implantable and biodegradable device capable of long-term pulsatile delivery of PTH is developed as a patient-friendly alternative. The advanced materials and fabrication techniques developed in this work enable us to preprogram a pulsatile delivery device to systemically deliver 21 daily pulses of PTH that build bone in vivo. In addition, the device is biodegradable and absorbable in vivo so that no retraction procedure is needed. Therefore, this implantable and biodegradable pulsatile device holds promise to promote bone growth and treat various conditions of bone loss without the burden of daily injections or secondary surgeries.

Parathyroid hormone 1-34 and skeletal anabolic action: The use of parathyroid hormone in bone formation

Intermittently administered parathyroid hormone (PTH 1-34) has been shown to promote bone formation in both human and animal studies. The hormone and its analogues stimulate both bone formation and resorption, and as such at low doses are now in clinical use for the treatment of severe osteoporosis. By varying the duration of exposure, parathyroid hormone can modulate genes leading to increased bone formation within a so-called 'anabolic window'. The osteogenic mechanisms involved are multiple, affecting the stimulation of osteoprogenitor cells, osteoblasts, osteocytes and the stem cell niche, and ultimately leading to increased osteoblast activation, reduced osteoblast apoptosis, upregulation of Wnt/β-catenin signalling, increased stem cell mobilisation, and mediation of the RANKL/OPG pathway. Ongoing investigation into their effect on bone formation through 'coupled' and 'uncoupled' mechanisms further underlines the impact of intermittent PTH on both cortical and cancellous bone. Given the principally catabolic actions of continuous PTH, this article reviews the skeletal actions of intermittent PTH 1-34 and the mechanisms underlying its effect.

CITE THIS ARTICLE

L. Osagie-Clouard, A. Sanghani, M. Coathup, T. Briggs, M. Bostrom, G. Blunn. Parathyroid hormone 1-34 and skeletal anabolic action: The use of parathyroid hormone in bone formation. Bone Joint Res 2017;6:14-21. DOI: 10.1302/2046-3758.61.BJR-2016-0085.R1.

PTH(1-34) Affects Osteoprotegerin Production in Human PDL Cells in vitro

Since periodontal ligament (PDL) cells exhibit several osteoblastic traits, we hypothesized that human PDL cells will respond to hormonal stimulation in an osteoblast-like manner. Confluent and pre-confluent PDL cells from six patients were challenged with PTH(1-34). Cell number, ALP, osteocalcin, osteoprotegerin, and RANKL expression were determined. Intermittent PTH(1-34) treatment of confluent PDL cells caused a significant increase in proliferation, whereas differentiation and osteoprotegerin production decreased significantly. In pre-confluent PDL cells, this treatment regimen induced a biphasic decrease in proliferation, but a biphasic increase in differentiation and osteoprotegerin production. Continuous PTH(1-34) exposure enhanced proliferation but inhibited osteocalcin production in confluent cells and stimulated osteoprotegerin production in pre-confluent PDL cells. RANKL was hardly detectable and unaffected by PTH(1-34) treatment. These results indicate that human PDL cells respond to PTH(1-34) in an osteoblast-like manner, and that the PTH(1-34) effect depends on the maturation state of the cells and on the mode of administration.

Parathyroid Hormone-Related Protein Analogs as Osteoporosis Therapies

The only bone anabolic agent currently available for osteoporosis treatment is parathyroid hormone (PTH)-either its N-terminal 1-34 fragment or the whole molecule of 1-84 aminoacids-whose intermittent administration stimulates new bone formation by targeting osteoblastogenesis and osteoblast survival. PTH-related protein (PTHrP) is an abundant factor in bone which shows N-terminal homology with PTH and thus exhibits high affinity for the same PTH type 1 receptor in osteoblasts. Therefore, it is not surprising that intermittently administered N-terminal PTHrP peptides induce bone anabolism in animals and humans. Furthermore, the C-terminal region of PTHrP also elicits osteogenic features in vitro in osteoblastic cells and in various animal models of osteoporosis. In this review, we discuss the current concepts about the cellular and molecular mechanisms whereby PTHrP may induce anabolic actions in bone. Pre-clinical studies and clinical data using N-terminal PTHrP analogs are also summarized, pointing to PTHrP as a promising alternative to current bone anabolic therapies.

Calcium Sensing Receptor Function Supports Osteoblast Survival and Acts as a Co-Factor in PTH Anabolic Actions in Bone

Anabolic actions of PTH in bone involve increased deposition of mineralizing matrix. Regulatory feedback of the process may be important to maintain calcium homeostasis and, in turn, calcium may inform the process. This investigation clarified the role of calcium availability and the calcium sensing receptor (CaSR) in the anabolic actions of PTH. CaSR function promoted osteoblastic cell numbers, with lower cell numbers in post-confluent cultures of primary calvarial cells from Col1-CaSR knock-out (KO) mice, and for calvarial cells from wild-type (WT) mice treated with a calcilytic. Increased apoptosis of calvarial cells with calcilytic treatment suggested CaSR is critical for protection against stage-dependent cell death. Whole and cortical, but not trabecular, bone parameters were significantly lower in Col1-CaSR KO mice versus WT littermates. Intact Col1-CaSR KO mice had lower serum P1NP levels relative to WT. PTH treatment displayed anabolic actions in WT and, to a lesser degree, KO mice, and rescued the lower P1NP levels in KO mice. Furthermore, PTH effects on whole tibiae were inhibited by osteoblast-specific CaSR ablation. Vertebral body implants (vossicles) from untreated Col1-CaSR KO and WT mice had similar bone volumes after 4 weeks of implantation in athymic mice. These findings suggest that trabecular bone formation can occur independently of the CaSR, and that the CaSR plays a collaborative role in the PTH anabolic effects on bone. J. Cell. Biochem. 117: 1556-1567, 2016. © 2015 Wiley Periodicals, Inc.

Parathyroid hormone-related peptide protects cardiomyocytes from oxidative stress-induced cell death: First evidence of a novel endocrine-cardiovascular interaction

Although there is a growing interest in the molecular cross-talk between the endocrine and cardiovascular systems, the cardiac effects of calcium-regulating hormones (i.e., parathyroid hormone-related peptide (PTHrP)) have not been explored. In this study, we examined the effect of PTHrP on the viability of isolated adult mouse cardiomyocytes subjected to oxidative stress. Myocytes from 19 to 22 week old male 129J/C57BL6 mice were exposed to oxidative insult in the form of H2O2 which led to more than 70% loss of cell viability. Herein we demonstrate, for the first time, that pretreatment with 100 nM PTHrP prior to 100 μM H2O2 incubation prevents H2O2 -induced cell death by more than 50%. Immunoblot analysis revealed H2O2 induction of MKP-1 protein expression while PTHrP decreased MKP-1 expression. Moreover, myocytes derived from MKP1 KO mice were resistant to oxidative injury. No added benefit of PTHrP treatment was noted in MKP-1 null cardiomyocytes. Using specific pharmacological inhibitors we demonstrated that P-p38, P-ERK and P-AKT mediated PTHrP's cardioprotective action. These data provide novel evidence that: i) down-regulation of MKP1 affords profound protection against oxidative stress; and ii) PTHrP is cardioprotective, possibly via down-regulation of MKP-1 and activation of MAPK and PI3K/AKT signaling.

Role of the Parathyroid Hormone Type 1 Receptor (PTH1R) as a Mechanosensor in Osteocyte Survival

Osteocytes have a major role in the control of bone remodeling. Mechanical stimulation decreases osteocyte apoptosis and promotes bone accrual, whereas skeletal unloading is deleterious in both respects. PTH1R ablation or overexpression in osteocytes in mice produces trabecular bone loss or increases bone mass, respectively. The latter effect was related to a decreased osteocyte apoptosis. Here, the putative role of PTH1R activation in osteocyte protection conferred by mechanical stimulation was assessed. Osteocytic MLO-Y4 cells were subjected to mechanical stimuli represented by hypotonic shock (216 mOsm/kg) or pulsatile fluid flow (8 Hz, 10 dynes/cm(2)) for a short pulse (10 min), with or without PTH1R antagonists or after transfection with specific PTHrP or PTH1R siRNA. These mechanical stimuli prevented cell death induced within 6 hours by etoposide (50 μM), related to PTHrP overexpression; and this effect was abolished by the calcium antagonist verapamil (1 μM), a phospholipase C (PLC) inhibitor (U73122; 10 μM), and a PKA activation inhibitor, Rp-cAMPS (25 μM), in these cells. Each mechanical stimulus also rapidly induced β-catenin stabilization and nuclear ERK translocation, which were inhibited by the PTH1R antagonist PTHrP(7-34) (1 μM), or PTH1R siRNA, and mimicked by PTHrP(1-36) (100 nM). Mechanical stretching by hypotonic shock did not affect cAMP production but rapidly (<1 min) stimulated Ca(i)(2+) transients in PTH1R-overexpressing HEK-293 cells and in MLO-Y4 cells, in which calcium signaling was unaffected by the presence of a PTHrP antiserum or PTHrP siRNA but inhibited by knocking down PTH1R. These novel findings indicate that PTH1R is an important component of mechanical signal transduction in osteocytic MLO-Y4 cells, and that PTH1R activation by PTHrP-independent and dependent mechanisms has a relevant role in the prosurvival action of mechanical stimulus in these cells.

Molecular mechanisms associated with PTHrP-induced proliferation of colon cancer cells

Parathyroid Hormone-related Protein (PTHrP) is normally produced in many tissues and is recognized for its endocrine, paracrine, autocrine and intracrine modes of action. PTHrP is also implicated in different types of cancer and its expression correlates with the severity of colon carcinoma. Using the human colon cell line Caco-2 we recently obtained evidence that PTHrP, through a paracrine pathway, exerts a protective effect under apoptotic conditions. However, if exogenous PTHrP is able or not to induce the proliferation of these intestinal tumor cells is not known. We found that PTHrP treatment increases the number of live Caco-2 cells. The hormone induces the phosphorylation and nuclear translocation of ERK 1/2, α p38 MAPK, and Akt, without affecting JNK phosphorylation. In addition, PTHrP-dependent ERK phosphorylation is reverted when PI3K activity was inhibited. Following MAPKs nuclear translocation, the transcription factors ATF-1 and CREB were activated in a biphasic manner. In addition PTHrP induces the translocation into the nucleus of β-catenin, protein that plays key role in maintaining the growth and proliferation of colorectal cancer, and increases the amount of both positive cell cycle regulators c-Myc and Cyclin D. Studies with ERK1/2, α p38 MAPK, and PI3K specific inhibitors showed that PTHrP regulates Caco-2 cell proliferation via these signaling pathways. In conclusion, the results obtained in this work expand our knowledge on the role of exogenous PTHrP in intestinal tumor cells and identify the signaling pathways that are involved in the mitogenic effect of the hormone on Caco-2 cells.

Pleiotropy as the Mechanism for Evolving Novelty: Same Signal, Different Result

In contrast to the probabilistic way of thinking about pleiotropy as the random expression of a single gene that generates two or more distinct phenotypic traits, it is actually a deterministic consequence of the evolution of complex physiology from the unicellular state. Pleiotropic novelties emerge through recombinations and permutations of cell-cell signaling exercised during reproduction based on both past and present physical and physiologic conditions, in service to the future needs of the organism for its continued survival. Functional homologies ranging from the lung to the kidney, skin, brain, thyroid and pituitary exemplify the evolutionary mechanistic strategy of pleiotropy. The power of this perspective is exemplified by the resolution of evolutionary gradualism and punctuated equilibrium in much the same way that Niels Bohr resolved the paradoxical duality of light as Complementarity.

Parathyroid hormone: anabolic and catabolic actions on the skeleton

Parathyroid hormone (PTH) is essential for the maintenance of calcium homeostasis through, in part, its actions to regulate bone remodeling. While PTH stimulates both bone formation and bone resorption, the duration and periodicity of exposure to PTH governs the net effect on bone mass, that is whether it is catabolic or anabolic. PTH receptor signaling in osteoblasts and osteocytes can increase the RANKL/OPG ratio, increasing both osteoclast recruitment and osteoclast activity, and thereby stimulating bone resorption. In contrast, PTH-induced bone formation is explained, at least in part, by its ability to downregulate SOST/sclerostin expression in osteocytes, permitting the anabolic Wnt signaling pathway to proceed. The two modes of administration of PTH, that is, continuous vs. intermittent, can regulate, in bone cells, different sets of genes; alternatively, the same sets of genes exposed to PTH in sustained vs. transient way, will favor bone resorption or bone formation, respectively. This article reviews the effects of PTH on bone cells that lead to these dual catabolic and anabolic actions on the skeleton.

The lung alveolar lipofibroblast: an evolutionary strategy against neonatal hyperoxic lung injury

SIGNIFICANCE

Oxygen, the main mode of support for premature infants with immature lungs, can cause toxicity by producing reactive oxygen species (ROS) that disrupt homeostasis; yet, these same molecules were entrained to promote vertebrate lung phylogeny. By providing a deeper understanding of this paradox, we propose physiologically rational strategies to prevent chronic lung disease (CLD) of prematurity.

RECENT ADVANCES

To prevent neonatal hyperoxic lung damage biologically, we have exploited the alveolar defense mechanism(s) that evolutionarily evolved to combat increased atmospheric oxygen during the vertebrate water to land transition.

CRITICAL ISSUES

Over the course of vertebrate lung evolution, ROS promoted the formation of lipofibroblasts, specialized adepithelial cells, which protect the alveoli against oxidant injury; peroxisome proliferator-activated receptor gamma (PPARγ), the master switch for lipofibroblast differentiation, prevents such oxidant lung injury, both by directly promoting mesodermal differentiation and its antioxidant defenses, and indirectly by stimulating the developmental epithelial-mesenchymal paracrine interactions that have physiologically determined lung surfactant production in accord with the lung's phylogenetic adaptation to atmospheric oxygen, preventing Respiratory Distress Syndrome at birth.

FUTURE DIRECTIONS

The molecular strategy (PPARγ agonists) to prevent CLD of prematurity, proposed by us, although seems to be robust, effective, and safe under experimental conditions, it awaits detailed pharmacokinetic and pharmacodynamic studies for its safe and effective clinical translation to human infants. Antioxid. Redox Signal. 21, 1893-1904. "I have procured air [oxygen]…between five and six times as good as the best common air that I have ever met with." -Joseph Priestley, 1775.

Phosphate interacts with PTHrP to regulate endochondral bone formation

Phosphate and parathyroid hormone related peptide (PTHrP) are required for normal growth plate maturation. Hypophosphatemia impairs hypertrophic chondrocyte apoptosis leading to rachitic expansion of the growth plate; however, the effect of phosphate restriction on chondrocyte differentiation during endochondral bone formation has not been examined. Investigations were, therefore, undertaken to address whether phosphate restriction alters the maturation of embryonic d15.5 murine metatarsal elements. Metatarsals cultured in low phosphate media exhibited impaired chondrocyte differentiation, analogous to that seen with PTHrP-treatment of metatarsals cultured in control media. Because phosphate restriction acutely increases PTHrP expression in cultured metatarsals, studies were undertaken to determine if this increase in PTHrP plays a pathogenic role in the impaired chondrocyte differentiation observed under low phosphate conditions. In contrast to what was observed with wild-type metatarsal elements, phosphate restriction did not impair the differentiation of metatarsals isolated from PTHrP heterozygous or PTHrP knockout mice. In vivo studies in postnatal mice demonstrated that PTHrP haploinsufficiency also prevents the impaired hypertrophic chondrocyte apoptosis observed with phosphate restriction. To determine how signaling through the PTH/PTHrP receptor antagonizes the pro-apoptotic effects of phosphate, investigations were performed in primary murine hypertrophic chondrocytes. Receptor activation impaired phosphate-induced Erk1/2 phosphorylation specifically in the mitochondrial fraction and decreased levels of mitochondrial Bad, while increasing cytosolic phospho-Bad. Thus, these data demonstrate that phosphate restriction attenuates chondrocyte differentiation as well as impairing hypertrophic chondrocyte apoptosis and implicate a functional role for the PTH/PTHrP signaling pathway in the abnormalities in chondrocyte differentiation and hypertrophic chondrocyte apoptosis observed under phosphate restricted conditions.

Functional roles of the nuclear localization signal of parathyroid hormone-related protein (PTHrP) in osteoblastic cells

PTHrP is an important regulator of bone remodelling, apparently by acting through several sequence domains. We here aimed to further delineate the functional roles of the nuclear localization signal (NLS) comprising the 88-107 amino acid sequence of PTHrP in osteoblasts. PTHrP mutants from a human PTHrP (-36/+139) cDNA (wild type) cloned into pcDNA3.1 plasmid with deletion (Δ) of the signal peptide (SP), NLS, T(107), or T107A replacing T(107) by A(107) were generated and stably transfected into osteoblastic MC3T3-E1 cells. In these cells, intracellular trafficking, cell proliferation and viability, as well as cell differentiation were evaluated. In these transfected cells, PTHrP was detected in the cytoplasm and also in the nucleus, except in the NLS mutant. Meanwhile, the PTH type 1 receptor (PTH1R) accumulates in the cytoplasm except for the ΔSP mutant in which the receptor remains at the cell membrane. PTHrP-wild type cells showed enhanced growth and viability, as well as an increased matrix mineralization, alkaline phosphatase activity, and osteocalcin gene expression; and these features were inhibited or abolished in ΔNLS or ΔT(107) mutants. Of note, these effects of PTHrP overexpression on cell growth and function were similarly decreased in the ΔSP mutant after PTH1R small interfering RNA transfection or by a PTH1R antagonist. The present in vitro findings suggest a mixed model for PTHrP actions on osteoblastic growth and function whereby this protein needs to be secreted and internalized via the PTH1R (autocrine/paracrine pathway) before NLS-dependent shuttling to the nucleus (intracrine pathway).

Disruption of LRP6 in osteoblasts blunts the bone anabolic activity of PTH

Mutations in low-density lipoprotein receptor-related protein 6 (LRP6) are associated with human skeletal disorders. LRP6 is required for parathyroid hormone (PTH)-stimulated signaling pathways in osteoblasts. We investigated whether LRP6 in osteoblasts directly regulates bone remodeling and mediates the bone anabolic effects of PTH by specifically deleting LRP6 in mature osteoblasts in mice (LRP6 KO). Three-month-old LRP6 KO mice had a significant reduction in bone mass in the femora secondary spongiosa relative to their wild-type littermates, whereas marginal changes were found in femoral tissue of 1-month-old LRP6 KO mice. The remodeling area of the 3-month-old LRP6 KO mice showed a decreased bone formation rate as detected by Goldner's Trichrome staining and calcein double labeling. Bone histomorphometric and immumohistochemical analysis revealed a reduction in osteoblasts but little change in the numbers of osteoclasts and osteoprogenitors/osteoblast precursors in LRP6 KO mice compared with wild-type littermates. In addition, the percentage of the apoptotic osteoblasts on the bone surface was higher in LRP6 KO mice compared with wild-type littermates. Intermittent injection of PTH had no effect on bone mass or osteoblastic bone formation in either trabecular and cortical bone in LRP6 KO mice, whereas all were enhanced in wild-type littermates. Additionally, the anti-apoptotic effect of PTH on osteoblasts in LRP6 KO mice was less significant compared with wild-type mice. Therefore, our findings demonstrate that LRP6 in osteoblasts is essential for osteoblastic differentiation during bone remodeling and the anabolic effects of PTH.

Current perspectives on parathyroid hormone (PTH) and PTH-related protein (PTHrP) as bone anabolic therapies

Osteoporosis is characterized by low bone mineral density and/or poor bone microarchitecture leading to an increased risk of fractures. The skeletal alterations in osteoporosis are a consequence of a relative deficit of bone formation compared to bone resorption. Osteoporosis therapies have mostly relied on antiresorptive drugs. An alternative therapeutic approach for osteoporosis is currently available, based on the intermittent administration of parathyroid hormone (PTH). Bone anabolism caused by PTH therapy is mainly accounted for by the ability of PTH to increase osteoblastogenesis and osteoblast survival. PTH and PTH-related protein (PTHrP)-an abundant local factor in bone- interact with the common PTH type 1 receptor with similar affinities in osteoblasts. Studies mainly in osteoporosis rodent models and limited data in postmenopausal women suggest that N-terminal PTHrP peptides might be considered a promising bone anabolic therapy. In addition, putative osteogenic actions of PTHrP might be ascribed not only to its N-terminal domain but also to its PTH-unrelated C-terminal region. In this review, we discuss the underlying cellular and molecular mechanisms of the anabolic actions of PTH and the similar potential of PTH-related protein (PTHrP) to increase bone mass and improve bone regeneration.

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.

Transcriptomic and proteomic analyses in bone tumor cells: Deciphering parathyroid hormone-related protein regulation of the cell cycle and apoptosis

Giant cell tumor of bone (GCT) is an aggressive skeletal tumor characterized by local bone destruction, high recurrence rates, and metastatic potential. Previous works in our laboratory, including functional assays, have shown that neutralization of parathyroid hormone-related protein (PTHrP) in the cell environment inhibits cell proliferation and induces cell death in GCT stromal cells, indicating a role for PTHrP in cell propagation and survival. The objective of this study was to investigate the global gene and protein expression patterns of GCT cells in order to identify the underlying pathways and mechanisms of neoplastic proliferation provided by PTHrP in the bone microenvironment. Primary stromal cell cultures from 10 patients with GCT were used in this study. Cells were exposed to optimized concentrations of either PTHrP peptide or anti-PTHrP neutralizing antiserum and were analyzed with both cDNA microarray and proteomic microarray assays in triplicate. Hierarchical clustering and principal component analyses confirmed that counteraction of PTHrP in GCT stromal cells results in a clear-cut gene expression pattern distinct from all other treatment groups and the control cell line human fetal osteoblast (hFOB). Multiple bioinformatics tools were used to analyze changes in gene/protein expression and identify important gene ontologies and pathways common to this anti-PTHrP-induced regulatory gene network. PTHrP neutralization interferes with multiple cell survival and apoptosis signaling pathways by triggering both death receptors and cell cycle-mediated apoptosis, particularly via the caspase pathway, TRAIL pathway, JAK-STAT signaling pathway, and cyclin E/CDK2-associated G1/S cell cycle progression. These findings indicate that PTHrP neutralization exhibits anticancer potential by regulating cell-cycle progression and apoptosis in bone tumor cells, with the corollary being that PTHrP is a pro-neoplastic factor that can be targeted in the treatment of bone tumors.

PPARγ Signaling Mediates the Evolution, Development, Homeostasis, and Repair of the Lung

Epithelial-mesenchymal interactions mediated by soluble growth factors determine the evolution of vertebrate lung physiology, including development, homeostasis, and repair. The final common pathway for all of these positively adaptive properties of the lung is the expression of epithelial parathyroid-hormone-related protein, and its binding to its receptor on the mesenchyme, inducing PPARγ expression by lipofibroblasts. Lipofibroblasts then produce leptin, which binds to alveolar type II cells, stimulating their production of surfactant, which is necessary for both evolutionary and physiologic adaptation to atmospheric oxygen from fish to man. A wide variety of molecular insults disrupt such highly evolved physiologic cell-cell interactions, ranging from overdistention to oxidants, infection, and nicotine, all of which predictably cause loss of mesenchymal peroxisome-proliferator-activated receptor gamma (PPARγ) expression and the transdifferentiation of lipofibroblasts to myofibroblasts, the signature cell type for lung fibrosis. By exploiting such deep cell-molecular functional homologies as targets for leveraging lung homeostasis, we have discovered that we can effectively prevent and/or reverse the deleterious effects of these pathogenic agents, demonstrating the utility of evolutionary biology for the prevention and treatment of chronic lung disease. By understanding mechanisms of health and disease as an evolutionary continuum rather than as dissociated processes, we can evolve predictive medicine.

Twenty-five years of PTHrP progress: from cancer hormone to multifunctional cytokine

Twenty-five years ago a "new" protein was identified from cancers that caused hypercalcemia. It was credited for its ability to mimic parathyroid hormone (PTH), and hence was termed parathyroid hormone-related protein (PTHrP). Today it is recognized for its widespread distribution, its endocrine, paracrine, and intracrine modes of action driving numerous physiologic and pathologic conditions, and its central role in organogenesis. The multiple biological activities within a complex molecule with paracrine modulation of adjacent target cells present boundless possibilities. The protein structure of PTHrP has been traced, dissected, and deleted comprehensively and conditionally, yet numerous questions lurk in its past that will carry into the future. Issues of the variable segments of the protein, including the enigmatic nuclear localization sequence, are only recently being clarified. Aspects of PTHrP production and action in the menacing condition of cancer are emerging as dichotomies that may represent intended temporal actions of PTHrP. Relative to PTH, the hormone regulating calcium homeostasis, PTHrP "controls the show" locally at the PTH/PTHrP receptor throughout the body. Great strides have been made in our understanding of PTHrP actions, yet years of exciting investigation and discovery are imminent. © 2012 American Society for Bone and Mineral Research.

Cyclic AMP is both a pro-apoptotic and anti-apoptotic second messenger

The second messenger cyclic AMP (cAMP) can either stimulate or inhibit programmed cell death (apoptosis). Here, we review examples of cell types that show pro-apoptotic or anti-apoptotic responses to increases in cAMP. We also show that cells can have both such responses, although predominantly having one or the other. Protein kinase A (PKA)-promoted changes in phosphorylation and gene expression can mediate pro-apoptotic responses, such as in murine S49 lymphoma cells, based on evidence that mutants lacking PKA fail to undergo cAMP-promoted, mitochondria-dependent apoptosis. Mechanisms for the anti-apoptotic response to cAMP likely involve Epac (Exchange protein activated by cAMP), a cAMP-regulated effector that is a guanine nucleotide exchange factor (GEF) for the low molecular weight G-protein, Rap1. Therapeutic approaches that activate PKA-mediated pro-apoptosis or block Epac-mediated anti-apoptotisis may provide a means to enhance cell killing, such as in certain cancers. In contrast, efforts to block PKA or stimulate Epac have the potential to be useful in diseases settings (such as heart failure) associated with cAMP-promoted apoptosis.

The basic helix loop helix transcription factor Twist1 is a novel regulator of ATF4 in osteoblasts

Parathyroid hormone (PTH) is an essential regulator of endochondral bone formation and an important anabolic agent for the reversal of bone loss. PTH mediates its functions in part by regulating binding of the bone-related activating transcription factor 4 (ATF4) to the osteoblast-specific gene, osteocalcin. The basic helix-loop-helix (bHLH) factors Twist1 and Twist2 also regulate osteocalcin transcription in part through the interaction of the C-terminal "box" domain in these factors and Runx2. In this study, we discovered a novel function of PTH: its ability to dramatically decrease Twist1 transcription. Since ATF4 is a major regulator of the PTH response in osteoblasts, we assessed the mutual regulation between these factors and determined that Twist proteins and ATF4 physically interact in a manner that affects ATF4 DNA binding function. We mapped the interaction domain of Twist proteins to the C-terminal "box" domain and of ATF4, to the N-terminus. Furthermore, we demonstrate that Twist1 overexpression in osteoblasts attenuates ATF4 binding to the osteocalcin promoter in response to PTH. This study thus identifies Twist proteins as novel inhibitory binding partners of ATF4 and explores the functional significance of this interaction.

A naturally occurring rare analog of quercetin promotes peak bone mass achievement and exerts anabolic effect on osteoporotic bone

The effect of quercetin C-glucoside (QCG) on osteoblast function in vitro and bone formation in vivo was investigated. QCG supplementation promoted peak bone mass achievement in growing rats and new bone formation in osteopenic rats. QCG has substantial oral bioavailability. Findings suggest a significant bone anabolic effect of QCG.

INTRODUCTION

Recently, we showed that extracts of Ulmus wallichiana promoted peak bone mass achievement in growing rats and preserved trabecular bone mass and cortical bone strength in ovariectomized (OVx) rats. 3,3',4',5,7-Pentahydroxyflavone-6-C-β-D-glucopyranoside, a QCG, is the most abundant bioactive compound of U. wallichiana extract. We hypothesize that QCG exerts bone anabolic effects by stimulating osteoblast function.

METHODS

Osteoblast cultures were harvested from rat calvaria and bone marrow (BM) to study differentiation and mineralization. In vivo, growing female Sprague Dawley rats and OVx rats with osteopenia were administered QCG (5.0 or 10.0 mg kg(-1) day(-1)) orally for 12 weeks. Efficacy was evaluated by examining changes in bone microarchitecture using histomorphometric and microcomputed tomographic analyses and by determination of new bone formation by fluorescent labeling of bone. Plasma and BM levels of QCG were determined by high-performance liquid chromatography.

RESULTS

QCG was much more potent than quercetin (Q) in stimulating osteoblast differentiation, and the effect of QCG was not mediated by estrogen receptors. In growing rats, QCG increased BM osteoprogenitors, bone mineral density, bone formation rate, and cortical deposition. In osteopenic rats, QCG treatment increased bone formation rate and improved trabecular microarchitecture. Comparison with the sham group (ovary intact) revealed significant restoration of trabecular bone in osteopenic rats treated with QCG. QCG levels in the BM were ~50% of that of the plasma levels.

CONCLUSION

QCG stimulated modeling-directed bone accrual and exerted anabolic effects on osteopenic rats by direct stimulatory effect on osteoprogenitors likely due to substantial QCG delivery at tissue level following oral administration.

Catabolic and anabolic actions of parathyroid hormone on the skeleton

PTH, an 84-amino acid peptide hormone synthesized by the parathyroid glands, is essential for the maintenance of calcium homeostasis.While in its traditional metabolic role, PTH helps to maintain the serum calcium concentration within narrow, normal limits and participates as a determinant of bone remodeling, more specific actions, described as catabolic and anabolic are also well known. Clinically, the catabolic effect of PTH is best represented by primary hyperparathyroidism (PHPT), while the osteoanabolic effect of PTH is best seen when PTH or its biological amino-terminal fragment [PTH(1-34)] is used as a therapy for osteoporosis. These dual functions of PTH are unmasked under very specific pathological (PHPT) or therapeutic conditions. At the cellular level, PTH favors bone resorption, mostly by affecting the receptor activator of nuclear factor κ-B (RANK) ligand (RANKL)-osteoprotegerin- RANK system, leading to an increase in osteoclast formation and activity. Increased bone formation due to PTH therapy is explained best by its ability to enhance osteoblastogenesis and/or osteoblast survival. The PTH-induced bone formation is mediated, in part, by a decrease in SOST/sclerostin expression in osteocytes. This review focuses on the dual anabolic and catabolic actions of PTH on bone, situations where one is enhanced over the other, and the cellular and molecular mechanisms by which these actions are mediated.

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.

A novel quercetin analogue from a medicinal plant promotes peak bone mass achievement and bone healing after injury and exerts an anabolic effect on osteoporotic bone: the role of aryl hydrocarbon receptor as a mediator of osteogenic action

We recently reported that extracts made from the stem bark of Ulmus wallichiana promoted peak bone mass achievement in growing rats and preserved trabecular bone mass and cortical bone strength in ovariectomized (OVX) rats. Further, 6-C-β-D-glucopyranosyl-(2S,3S)-(+)-3',4',5,7-tetrahydroxyflavanol (GTDF), a novel flavonol-C-glucoside isolated from the extracts, had a nonestrogenic bone-sparing effect on OVX rats. Here we studied the effects of GTDF on osteoblast function and its mode of action and in vivo osteogenic effect. GTDF stimulated osteoblast proliferation, survival, and differentiation but had no effect on osteoclastic or adipocytic differentiation. In cultured osteoblasts, GTDF transactivated the aryl hydrocarbon receptor (AhR). Activation of AhR mediated the stimulatory effect of GTDF on osteoblast proliferation and differentiation. Furthermore, GTDF stimulated cAMP production, which mediated osteogenic gene expression. GTDF treatments given to 1- to 2-day-old rats or adult rats increased the mRNA levels of AhR target genes in calvaria or bone marrow stromal cells. In growing female rats, GTDF promoted parameters of peak bone accrual in the appendicular skeleton, including increased longitudinal growth, bone mineral density, bone-formation rate (BFR), cortical deposition, and bone strength. GTDF promoted the process of providing newly generated bone to fill drill holes in the femurs of both estrogen-sufficient and -deficient rats. In osteopenic OVX rats, GTDF increased BFR and significantly restored trabecular bone compared with the ovaries-intact group. Together our data suggest that GTDF stimulates osteoblast growth and differentiation via the AhR and promotes modeling-directed bone accrual, accelerates bone healing after injury, and exerts anabolic effects on osteopenic rats likely by a direct stimulatory effect on osteoprogenitors. Based on these preclinical data, clinical evaluation of GTDF as a potential bone anabolic agent is warranted.

Review: The diabetic bone: a cellular and molecular perspective

With the increasing worldwide prevalence of diabetes the resulting complications, their consequences and treatment will lead to a greater social and financial burden on society. One of the many organs to be affected is bone. Loss of bone is observed in type 1 diabetes, in extreme cases mirroring osteoporosis, thus a greater risk of fracture. In the case of type 2 diabetes, both a loss and an increase of bone has been observed, although in both cases the quality of the bone overall was poorer, again leading to a greater risk of fracture. Once a fracture has occurred, healing is delayed in diabetes, including nonunion. The reasons leading to such changes in the state of the bone and fracture healing in diabetes is under investigation, including at the cellular and the molecular levels. In comparison with our knowledge of events in normal bone homeostasis and fracture healing, that for diabetes is much more limited, particularly in patients. However, progress is being made, especially with the use of animal models for both diabetes types. Identifying the molecular and cellular changes in the bone in diabetes and understanding how they arise will allow for targeted intervention to improve diabetic bone, thus helping to counter conditions such as Charcot foot as well as preventing fracture and accelerating healing when a fracture does occur.

PTHrP induces autocrine/paracrine proliferation of bone tumor cells through inhibition of apoptosis

Giant Cell Tumor of Bone (GCT) is an aggressive skeletal tumor characterized by local bone destruction, high recurrence rates and metastatic potential. Previous work in our lab has shown that the neoplastic cell of GCT is a proliferating pre-osteoblastic stromal cell in which the transcription factor Runx2 plays a role in regulating protein expression. One of the proteins expressed by these cells is parathryroid hormone-related protein (PTHrP). The objectives of this study were to determine the role played by PTHrP in GCT of bone with a focus on cell proliferation and apoptosis. Primary stromal cell cultures from 5 patients with GCT of bone and one lung metastsis were used for cell-based experiments. Control cell lines included a renal cell carcinoma (RCC) cell line and a human fetal osteoblast cell line. Cells were exposed to optimized concentrations of a PTHrP neutralizing antibody and were analyzed with the use of cell proliferation and apoptosis assays including mitochondrial dehydrogenase assays, crystal violet assays, APO-1 ELISAs, caspase activity assays, flow cytometry and immunofluorescent immunohistochemistry. Neutralization of PTHrP in the cell environment inhibited cell proliferation in a consistent manner and induced apoptosis in the GCT stromal cells, with the exception of those obtained from a lung metastasis. Cell cycle progression was not significantly affected by PTHrP neutralization. These findings indicate that PTHrP plays an autocrine/paracrine neoplastic role in GCT by allowing the proliferating stromal cells to evade apoptosis, possibly through non-traditional caspase-independent pathways. Thus PTHrP neutralizing immunotherapy is an intriguing potential therapeutic strategy for this tumor.

Parathyroid Hormone-Related Protein (PTHrP): A Key Regulator of Life/Death Decisions by Tumor Cells with Potential Clinical Applications

Parathyroid hormone-related protein (PTHrP), classically regarded as the mediator of the humoral hypercalcemia of malignancy syndrome, is a polyhormone that undergoes proteolytic processing into smaller bioactive forms. These bioactive forms comprise an N-terminal- as well as midregion- and C-terminal peptides, which have been shown to regulate various biological events, such as survival, proliferation and differentiation, in diverse cell model systems, both normal and pathological. A number of experimental data have demonstrated that PTHrP is also able to modulate tumor-relevant phenotypic expressions, thereby playing a role in early and advanced tumorigenesis, and in the response to treatment. In particular, interest has mainly been focused on the effects of PTHrP on cell proliferation/apoptosis, migration and invasion, which are the main roles involved in cancer development in vivo. The objective of this review is to discuss collectively the literature data on the molecular and biochemical basis of the mechanisms underlying the different, and sometimes opposite, effects exerted by PTHrP on various neoplastic cytotypes, with some final comments on both present and potential utilization of PTHrP as a target for anti-cancer therapy.

Continuous elevation of PTH increases the number of osteoblasts via both osteoclast-dependent and -independent mechanisms

Sustained parathyroid hormone (PTH) elevation stimulates bone remodeling (ie, both resorption and formation). The former results from increased RANKL synthesis, but the cause of the latter has not been established. Current hypotheses include release of osteoblastogenic factors from osteoclasts or from the bone matrix during resorption, modulation of the production and activity of osteoblastogenic factors from cells of the osteoblast lineage, and increased angiogenesis. To dissect the contribution of these mechanisms, 6-month-old Swiss-Webster mice were infused for 5 days with 470 ng/h PTH(1-84) or 525 ng/h soluble RANKL (sRANKL). Both agents increased osteoclasts and osteoblasts in vertebral cancellous bone, but the ratio of osteoblasts to osteoclasts and the increase in bone formation was greater in PTH-treated mice. Cancellous bone mass was maintained in mice receiving PTH but lost in mice receiving sRANKL, indicating that maintenance of balanced remodeling requires osteoblastogenic effects beyond those mediated by osteoclasts. Consistent with this contention, PTH, but not sRANKL, decreased the level of the Wnt antagonist sclerostin and increased the expression of the Wnt target genes Nkd2, Wisp1, and Twist1. Furthermore, PTH, but not sRANKL, increased the number of blood vessels in the bone marrow. Weekly injections of the RANKL antagonist osteoprotegerin at 10 µg/g for 2 weeks prior to PTH infusion eliminated osteoclasts and osteoblasts and prevented the PTH-induced increase in osteoclasts, osteoblasts, and blood vessels. These results indicate that PTH stimulates osteoclast-dependent as well as osteoclast-independent (Wnt signaling) pro-osteoblastogenic pathways, both of which are required for balanced focal bone remodeling in cancellous bone.

Parathyroid hormone mediates hematopoietic cell expansion through interleukin-6

Parathyroid hormone (PTH) stimulates hematopoietic cells through mechanisms of action that remain elusive. Interleukin-6 (IL-6) is upregulated by PTH and stimulates hematopoiesis. The purpose of this investigation was to identify actions of PTH and IL-6 in hematopoietic cell expansion. Bone marrow cultures from C57B6 mice were treated with fms-like tyrosine kinase-3 ligand (Flt-3L), PTH, Flt-3L plus PTH, or vehicle control. Flt-3L alone increased adherent and non-adherent cells. PTH did not directly impact hematopoietic or osteoclastic cells but acted in concert with Flt-3L to further increase cell numbers. Flt-3L alone stimulated proliferation, while PTH combined with Flt-3L decreased apoptosis. Flt-3L increased blasts early in culture, and later increased CD45⁺ and CD11b⁺ cells. In parallel experiments, IL-6 acted additively with Flt-3L to increase cell numbers and IL-6-deficient bone marrow cultures (compared to wildtype controls) but failed to amplify in response to Flt-3L and PTH, suggesting that IL-6 mediated the PTH effect. In vivo, PTH increased Lin^- Sca-1⁺c-Kit⁺ (LSK) hematopoietic progenitor cells after PTH treatment in wildtype mice, but failed to increase LSKs in IL-6-deficient mice. In conclusion, PTH acts with Flt-3L to maintain hematopoietic cells by limiting apoptosis. IL-6 is a critical mediator of bone marrow cell expansion and is responsible for PTH actions in hematopoietic cell expansion.

PTH(1-34)-induced changes in RANKL and OPG expression by human PDL cells modify osteoclast biology in a co-culture model with RAW 264.7 cells

Parathyroid hormone (PTH) is widely accepted as an anabolic agent when administered intermittently. Here, we explored the influence of intermittent PTH(1-34) on the expression of local factors by human periodontal ligament (PDL) cells that modify osteoclast biology. This approach aimed at a further elucidation of the role of the hormone and of PDL cells in the regulation of periodontal tissue homeostasis and of repair processes. In a co-culture model of mature PDL cells and RAW 264.7 cells, intermittent PTH(1-34) induced an increased gene expression for tartrate-resistant acid phosphatase (+84%), cathepsin K (+56%), and vitronectin-receptor (+56%); and an enhanced resorptive activity of differentiated osteoclasts (+154%). These findings were correlated with a reduction of the osteoprotegerin (OPG)/receptor activator of nuclear factor kappaB ligand (RANKL) ratio in the presence of PTH(1-34; -44%). Similar results were obtained when RAW cells were cultured with the conditioned medium of PTH(1-34)-stimulated PDL cells. In contrast, when less mature PDL cells were co-cultured with RAW cells, PTH(1-34) induced an inhibition of osteoclastic differentiation (TRAP, -35%; cathepsin K, -28%; vitronectin-receptor, -35%), a reduction of the resorbed substrate area (-77%) and an increase of the OPG/RANKL ratio (+11%). The conditioned medium of PTH(1-34)-pretreated less mature PDL cells led to a down-regulation of the number and activity of multinucleated cells. These data indicate that intermittent PTH(1-34) modifies the expression of membrane-bound and secreted factors by PDL cells which then in turn alter osteoclast biology. The PDL cell response to PTH(1-34) is specific in terms of cell maturation and the mechanism involved.

Tumor-supportive and osteoclastogenic changes induced by breast cancer-derived factors are reversed by inhibition of {gamma}-secretase

During breast cancer metastasis to bone, tumor cells home to bone marrow, likely targeting the stem cell niche, and stimulate osteoclasts, which mediate osteolysis required for tumor expansion. Although osteoblasts contribute to the regulation of the hematopoietic stem cell niche and control osteoclastogenesis through production of proresorptive cytokine RANKL (receptor activator of NF-κB ligand), their role in cancer metastases to bone is not fully understood. C57BL/6J mouse bone marrow cells were treated for 3–12 days with ascorbic acid (50 μg/ml) in the presence or absence of 10% medium conditioned by breast carcinoma cells MDA-MB-231, 4T1, or MCF7. Treatment with cancer-derived factors resulted in a sustained 40–60% decrease in osteoblast differentiation markers, compared with treatment with ascorbic acid alone, and induced an osteoclastogenic change in the RANKL/osteoprotegerin ratio. Importantly, exposure of bone cells to breast cancer-derived factors stimulated the subsequent attachment of cancer cells to immature osteoblasts. Inhibition of γ-secretase using pharmacological inhibitors DAPT and Compound E completely reversed cancer-induced osteoclastogenesis as well as cancer-induced enhancement of cancer cell attachment, identifying γ-secretase activity as a key mediator of these effects. Thus, we have uncovered osteoblasts as critical intermediary of premetastatic signaling by breast cancer cells and pinpointed γ-secretase as a robust target for developing therapeutics potentially capable of reducing both homing and progression of cancer metastases to bone.

Foxo1, a novel regulator of osteoblast differentiation and skeletogenesis

Skeletogenesis depends on the activity of bone-forming cells derived from mesenchymal cells. The pathways that control mesenchymal cell differentiation are not well understood. We propose that Foxo1 is an early molecular regulator during mesenchymal cell differentiation into osteoblasts. In mouse embryos, Foxo1 expression is higher in skeletal tissues, while Foxo1 silencing has a drastic impact on skeletogenesis and craniofacial development, specially affecting pre-maxilla, nasal bone, mandible, tibia, and clavicle. Similarly, Foxo1 activity and expression increase in mouse mesenchymal cells under the influence of osteogenic stimulants. In addition, silencing Foxo1 blocks the expression of osteogenic markers such as Runx2, alkaline phosphatase, and osteocalcin and results in decreased culture calcification even in the presence of strong osteogenic stimulants. Conversely, the expression of these markers increases significantly in response to Foxo1 overexpression. One mechanism through which Foxo1 affects mesenchymal cell differentiation into osteoblasts is through regulation of a key osteogenic transcription factor, Runx2. Indeed, our results show that Foxo1 directly interacts with the promoter of Runx2 and regulates its expression. Using a tibia organ culture model, we confirmed that silencing Foxo1 decreases the expression of Runx2 and impairs bone formation. Furthermore, our data reveals that Runx2 and Foxo1 interact with each other and cooperate in the transcriptional regulation of osteoblast markers. In conclusion, our in vitro, ex vivo, and in vivo results strongly support the notion that Foxo1 is an early molecular regulator in the differentiation of mesenchymal cells into osteoblast.