Receptors for the carboxyl-terminal region of pth(1-84) are highly expressed in osteocytic cells

Quality of life in patients with hypoparathyroidism receiving standard treatment: an updated systematic review

PURPOSE

Hypoparathyroidism is defined by hypocalcemia with inappropriately normal or low parathyroid hormone levels. The current standard treatment consists of lifelong calcium and/ or vitamin D supplementation. Even while on stable treatment regimens, hypoparathyroid patients might still suffer from symptoms that can negatively impact their quality of life.

METHODS

A systematic literature review to identify the current knowledge regarding quality of life in patients with hypoparathyroidism receiving standard treatment was performed on November 1st, 2023. PubMed as well as Web of Science were searched. The systematic review is registered in PROSPERO (#CRD42023470924).

RESULTS

After removal of duplicates, 398 studies remained for title and abstract screening, after which 30 were included for full-text screening. After exclusion of seven studies with five studies lacking a control population, one using a non-validated questionnaire, and one being a subsample of the larger included study, 23 studies were included in this systematic review. The majority of the included studies used a guideline-conform definition of hypoparathyroidism, and the SF-36 was the most often applied tool. Almost all studies (87%) reported statistically significantly lower scores in at least one quality of life domain compared to a norm population or controls.

CONCLUSION

Patients with hypoparathyroidism receiving standard treatment report impairments in quality of life. The reasons for these impairments are probably multifaceted, making regular monitoring and the inclusion of various professionals necessary.

Impairments in quality of life and predictors of symptom burden in patients with hypoparathyroidism: results from a population-based survey

PURPOSE

To investigate the quality of life (QoL) in patients with hypoparathyroidism (hypoPT) compared to the general population and to identify sociodemographic and clinical factors that are associated with symptom burden.

METHODS

Patients with a diagnosis of hypoPT participated in an online survey. Information regarding the survey was distributed by treating physicians or a self-help organization. Quality of life was assessed using the EORTC QLQ-C30 and symptom burden using the Hypoparathyroid Patient Questionnaire (HPQ28). Multivariate linear regression analysis was used to compare QoL of hypoPT patients with the general population (adjusted for age, sex, education)and to identify factors associated with symptom burden.

RESULTS

Altogether, 264 hypoPT patients provided information on QoL and symptom burden. HypoPT was associated with worse cognitive (β = -15.9; p < 0.01) and emotional functioning (β = -12.3; p = 0.04) compared to the general population. The highest symptom burden in hypoPT patients was observed for the domains loss of vitality (mean: 61.4; SD: 21.9), pain and cramps (mean: 43.7; SD: 26.5), and numbness and tingling sensations (mean: 38.9; SD: 30.0). Female gender was associated with a higher symptom burden across all nine domains of the HPQ28, while longer disease duration was associated with a lower symptom burden in neurovegetative symptoms, loss of vitality, depression and anxiety, and depressive symptoms.

CONCLUSION

HypoPT patients have impaired QoL compared to the general population. Being female is strongly associated with high symptom burden.

Determination of parathyroid hormone: from radioimmunoassay to LCMS/MS

Parathyroid hormone (PTH) determination is of paramount importance for the exploration of diseases related with calcium metabolism and for the follow-up of patients suffering from bone and mineral disorders associated with chronic kidney diseases (CKD-MBD). Unfortunately, the biologically active form of PTH, i.e. 1-84 PTH, circulates in the blood stream with many fragments and post-translationally modified forms, which decreases the specificity of immunoassays. The assays used to measure PTH, either from 2nd or 3rd generation, are not standardised, which may lead to interpretation errors and clinical consequences. Reference ranges for PTH have neither been always correctly established and the stability of the peptide is also a matter of concern. Fortunately, these last years, newer techniques using mass spectrometry (either high resolution or triple quadripole) coupled with liquid chromatography have been developed, which will help to standardise the different assays. Indeed, PTH assays standardisation is one of the task of the IFCC Committee for Bone Metabolism. Such standardisation will allow a better consistency in the interpretation of the results and will promote studies aiming at the establishment of correct reference ranges.

The parathyroid glands and parathyroid hormone: Insights from PTH gene mutations

Nine mutations have been discovered in the parathyroid hormone (PTH) gene since it was initially sequenced in 1983. An autosomal dominant C18R mutation in the signal peptide was first reported in 1990, followed by an exon skipping mutation, leading to loss of exon 2 in 1992; the latter mutation prevents PTH biosynthesis, as exon 2 contains the initiation codon. The S23P and S23X mutations affecting the same residue were reported in 1999 and 2012, respectively, while in 2008, the somatic mutation, R83X, was detected in a parathyroid adenoma tissue sample from a patient with overt hyperparathyroidism. In 2013, the heterozygous p.Met1_Asp6del mutation was discovered incidentally in a case-control study, while another heterozygous mutation, M14K, was detected in the signal peptide 4 years later. In 2015, a homozygous R56C mutation was reported, and was the first hypoparathyroidism-causing mutation identified that affects the mature bioactive portion of PTH; this mutation has significantly contributed to the understanding of the molecular mechanisms involved in signal transduction through the PTH receptor. Recently, a novel homozygous S32P mutation was identified, which is also situated in the bioactive portion of PTH. The discovery of these nine mutations in the PTH gene and determination of the molecular mechanisms underlying their effects has provided deep insights into the synthesis, processing, and secretion of PTH. Future attempts to discover other such mutations will help elucidate as yet unknown functions of PTH, with potential clinical implications.

Quality of Life in Patients With Hypoparathyroidism After Treatment for Thyroid Cancer

PURPOSE

Surgical complications such as hypoparathyroidism (HPT) or vocal cord palsy are seldom assessed when the quality of life (QOL) in thyroid cancer patients is investigated. The aim of this study was to measure the QOL difference in thyroid cancer survivors with and without HPT.

METHODS

Participants for this analysis were enrolled in 13 countries from a study that pilot-tested a thyroid cancer-specific QOL instrument. They were included if they had been diagnosed with thyroid cancer at least 9 months previously. QOL was measured using the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire Core (EORTC QLQ-C30) and some items on HPT symptoms (eg, tingling in fingers or toes). HPT status and other clinical data were extracted from the patients' medical charts. Comparisons of QOL domains between patients with and without HPT were performed using Mann-Whitney U test. The occurrence of HPT-related symptoms was compared using chi-square tests. Multiple ordinal regression analysis was performed to evaluate factors that might affect QOL.

RESULTS

Eighty-nine patients participated in this study, 17 of whom were considered to have HPT. Patients in the HPT group reported significantly reduced QOL in 9 of the 15 scales of the EORTC QLQ-C30 compared to patients without HPT. Regression analysis showed that HPT was independently negatively associated with various scales of the QLQ-C30. Both groups showed a high prevalence of typical HPT symptoms.

CONCLUSION

Thyroid cancer patients with HPT report significantly impaired QOL compared to thyroid cancer survivors without HPT. The assessment of HPT should be considered when measuring QOL in thyroid cancer patients.

Quality of life of patients more than 1 year after surgery for thyroid cancer

PURPOSE

Patients with thyroid cancer are often assumed to have no quality of life (QOL) impairments after treatment because of thyroid cancer's good prognosis. However, the QOL implications of surgical complications and the necessity to take lifelong medication are seldom assessed.

METHODS

Patients who had surgery due to thyroid cancer at the University Medical Center Mainz between 2010 and 2015 and who had calcium or parathyroid hormone levels below the reference values immediately following surgery were eligible for this study. QOL was assessed using the EORTC QLQ-C30 and the thyroid cancer module EORTC QLQ-THY34. Multiple logistic regression was used to determine factors associated with a worse QOL compared with a general population.

RESULTS

A total of 75 (56%) of 134 eligible patients participated in the study. Patients with persistent/prolonged calcium or vitamin D intake reported worse QOL in the domains of global health, physical functioning, role functioning, emotional functioning, and insomnia than patients without current intake. Current calcium and vitamin D intake, higher education, living with a partner, and age had an effect on the odds of having worse QOL than the age- and sex-adjusted general population.

CONCLUSION

Prolonged calcium and/or vitamin D intake are negatively associated with certain domains of QOL in thyroid cancer patients who are at least 1 year post surgery. Assessment of calcium and vitamin D and diagnosis of hypoparathyroidism are therefore important for the follow-up of thyroid cancer survivors since it may affect their QOL.

PTH and PTHrP Actions on Bone

Parathyroid hormone (PTH), PTH-related peptide (PTHrP), PTHR, and their cognate G protein-coupled receptor play defining roles in the regulation of extracellular calcium and phosphate metabolism and in controlling skeletal growth and repair. Acting through complex signaling mechanisms that in many instances proceed in a tissue-specific manner, precise control of these processes is achieved. A variety of direct and indirect disease processes, along with genetic anomalies, can cause these schemes to become dysfunctional. Here, we review the basic components of this regulatory network and present both the well-established elements and emerging findings and concepts with the overall objective to provide a framework for understanding the elementary aspects of how PTH and PTHrP behave and as a call to encourage further investigation that will yield more comprehensive understanding of the physiological and pathological steps at play, with a goal toward novel therapeutic interventions.

[Anabolic action of teriparatide to osteoporotic patients]

Intermittent administration of human parathyroid hormone (PTH) [1-34], or teriparatide, has been used for osteoporotic treatment, which is available for daily and weekly administration for the osteoporotic patients in Japan, increasing bone mass and reduce bone fracture risk. In general, continuous PTH infusion shows catabolic effects in bone, while the intermittent administration of PTH results in anabolic action in osteoporotic patients. Intermittent PTH administration promotes preosteoblastic proliferation, as well as stimulates osteoblastic bone formation dependent on cell coupling with osteoclasts. Dosing frequency of PTH administration may affect resultant bone mass, and therefore, we have examined the anabolic effects of the high and low frequency of PTH administration using a mouse model. As a consequence, the high frequency of PTH administration accelerated the preosteoblastic proliferation with forming thick preosteoblastic network, osteoclastogenesis inside the preosteoblastic network, as well as osteoblastic bone formation. In contrast, the low frequency of PTH administration promoted osteoblastic bone formation, but, did not stimulate preosteoblastic proliferation and osteoclastogenesis. In addition, the high or low frequency of PTH administration demonstrated bone formation by manners of accelerated bone remodeling or bone remodeling/mini-modeling, respectively. Thus, the different dosing frequency of PTH administration may induce the different cellular mechanism of anabolic effects in bone.

Integrins in Osteocyte Biology and Mechanotransduction

PURPOSE OF REVIEW

Osteocytes are the main mechanosensitive cells in bone. Integrin-based adhesions have been shown to facilitate mechanotransduction, and therefore play an important role in load-induced bone formation. This review outlines the role of integrins in osteocyte function (cell adhesion, signalling, and mechanotransduction) and possible role in disease.

RECENT FINDINGS

Both β₁ and β₃ integrins subunits have been shown to be required for osteocyte mechanotransduction. Antagonism of these integrin subunits in osteocytes resulted in impaired responses to fluid shear stress. Various disease states (osteoporosis, osteoarthritis, bone metastases) have been shown to result in altered integrin expression and function. Osteocyte integrins are required for normal cell function, with dysregulation of integrins seen in disease. Understanding the mechanism of faulty integrins in disease may aid in the creation of novel therapeutic approaches.

Investigating Osteocytic Perilacunar/Canalicular Remodeling

PURPOSE OF REVIEW

In perilacunar/canalicular remodeling (PLR), osteocytes dynamically resorb, and then replace, the organic and mineral components of the pericellular extracellular matrix. Given the enormous surface area of the osteocyte lacuna-canalicular network (LCN), PLR is important for maintaining homeostasis of the skeleton. The goal of this review is to examine the motivations and critical considerations for the analysis of PLR, in both in vitro and in vivo systems.

RECENT FINDINGS

Morphological approaches alone are insufficient to elucidate the complex mechanisms regulating PLR in the healthy skeleton and in disease. Understanding the role and regulation of PLR will require the incorporation of standardized PLR outcomes as a routine part of skeletal phenotyping, as well as the development of improved molecular and cellular outcomes. Current PLR outcomes assess PLR enzyme expression, the LCN, and bone matrix composition and organization, among others. Here, we discuss current PLR outcomes and how they have been applied to study PLR induction and suppression in vitro and in vivo. Given the role of PLR in skeletal health and disease, integrated analysis of PLR has potential to elucidate new mechanisms by which osteocytes participate in skeletal health and disease.

Osteoporosis Changes Collagen/Apatite Orientation and Young's Modulus in Vertebral Cortical Bone of Rat

This study revealed the distinguished changes of preferential orientation of collagen and apatite and Young's modulus in two different types of osteoporotic bones compared with the normal bone. Little is known about the bone material properties of osteoporotic bones; therefore, we aimed to assess material properties in osteoporotic bones. 66 female Sprague-Dawley rats were used. We analyzed the volumetric bone mineral density, collagen/apatite orientation, and Young's modulus of fifth lumbar vertebral cortex for osteoporotic rats caused by ovariectomy (OVX), administration of low calcium and phosphate content (LCaP) diet, and their combination (OVX + LCaP), as well as sham-operated control. Osteocyte conditions were assessed by hematoxylin and eosin and immunohistochemical (matrix extracellular phosphoglycoprotein (MEPE) and dentin matrix protein 1 (DMP1)) staining. All osteoporotic animals showed bone loss compared with the sham-operated control. OVX improved craniocaudal Young's modulus by enhancing collagen/apatite orientation along the craniocaudal axis, likely in response to the elevated stress due to osteoporotic bone loss. Conversely, LCaP-fed animals showed either significant bone loss or degraded collagen/apatite orientation and Young's modulus. Osteocytes in LCaP and OVX + LCaP groups showed atypical appearance and MEPE- and DMP1-negative phenotype, whereas those in the OVX group showed similarity with osteocytes in the control group. This suggests that osteocytes are possibly involved in the osteoporotic changes in collagen/apatite orientation and Young's modulus. This study is the first to demonstrate that osteoporosis changes collagen/apatite orientation and Young's modulus in an opposite manner depending on the cause of osteoporosis in spite of common bone loss.

Quality of life in patients with hypoparathyroidism receiving standard treatment: a systematic review

PURPOSE

Hypoparathyroidism is a rare endocrine disorder for which replacement therapy of the missing parathyroid hormone is not the standard therapeutic option. Current standard treatment consists of calcium and vitamin D supplementation. The intake of calcium and vitamin D supplementation can lead to complications and therefore might negatively influence patients' quality of life.

METHODS

A systematic literature review was performed to assess the current knowledge on the influence of hypoparathyroidism on patients' quality of life. The literature search was conducted in PubMed and Web of Science; all relevant literature published by August 24, 2016, was included.

RESULTS

In total 372 records were found. After title and abstract screening, 14 studies remained for a full-text screening. The full-text screening resulted in five studies which were included into the systematic review. Comparing the results with a norm-based reference population, three studies reported lower SF-36 scores for hypoparathyroidism patients. Two studies showed a reduced quality of life in hypoparathyroidism patients when their results were compared to control populations.

CONCLUSION

Most hypoparathyroidism patients receiving standard treatment show stable calcium and vitamin D levels. However, hypoparathyroidism patients still report reduced quality of life and experience physical, mental, and emotional symptoms. Therefore, it is assumed that the lack of parathyroid hormone directly influences the patients' quality of life. This review indicates that patients with hypoparathyroidism have a reduced quality of life in comparison to norm-based populations or matched controls. Further studies are required to quantify the effect of hypoparathyroidism on patients' quality of life using disease-specific questionnaires and controlling for the co-morbidities and etiologies of the patients.

Osteocyte Mechanobiology

PURPOSE OF REVIEW

Over the past decades, osteocytes have emerged as mechano-sensors of bone and master regulators of bone homeostasis. This article summarizes latest research and progress made in understanding osteocyte mechanobiology and critically reviews tools currently available to study these cells.

RECENT FINDINGS

Whereas increased mechanical forces promote bone formation, decrease loading is always associated with bone loss and skeletal fragility. Recent studies identified cilia, integrins, calcium channels, and G-protein coupled receptors as important sensors of mechanical forces and Ca²⁺ and cAMP signaling as key effectors. Among transcripts regulated by mechanical forces, sclerostin and RANKL have emerged as potential therapeutic targets for disuse-induced bone loss. In this paper, we review the mechanisms by which osteocytes perceive and transduce mechanical cues and the models available to study mechano-transduction. Future directions of the field are also discussed.

Unique Regenerative Mechanism to Replace Bone Lost During Dietary Bone Depletion in Weanling Mice

The present study was undertaken to determine the mechanism whereby calcitropic hormones and mesenchymal stem cell progeny changes are involved in bone repletion, a regenerative bone process that restores the bone lost to calcium deficiency. To initiate depletion, weanling mice with a mixed C57BL/6 (75%) and CD1 (25%) genetic background were fed a calcium-deficient diet (0.01%) for 14 days. For repletion, the mice were fed a control diet containing 1.2% calcium for 14 days. Depletion decreased plasma calcium and increased plasma parathyroid hormone, 1,25(OH)2D (calcitriol), and C-terminal telopeptide of type I collagen. These plasma parameters quickly returned toward normal on repletion. The trabecular bone volume and connectivity decreased drastically during depletion but were completely restored by the end of repletion. This bone repletion process largely resulted from the development of new bone formation. When bromodeoxyuridine (BrdU) was administered in the middle of depletion for 3 days and examined by fluorescence-activated cell sorting at 7 days into repletion, substantial increases in BrdU incorporation were seen in several CD105 subsets of cells of osteoblastic lineage. When BrdU was administered on days 1 to 3 of repletion and examined 11 days later, no increases in BrdU were seen in these subsets. Additionally, osteocytes that stained positively for BrdU were increased during depletion. In conclusion, the results of the present study have established a unique regenerative mechanism to initiate bone repair during the bone insult. Calcium homeostatic mechanisms and the bone repletion mechanism are opposing functions but are simultaneously orchestrated such that both endpoints are optimized. These results have potential clinical relevance for disease entities such as type 2 osteoporosis.

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Evolution of parathyroid hormone receptor family and their ligands in vertebrate

The presence of the parathyroid hormones in vertebrates, including PTH, PTH-related peptide (PTHrP), and tuberoinfundibular peptide of 39 residues (TIP39), has been proposed to be the result of two rounds of whole genome duplication in the beginning of vertebrate diversification. Bioinformatics analyses, in particular chromosomal synteny study and the characterization of the PTH ligands and their receptors from various vertebrate species, provide evidence that strongly supports this hypothesis. In this mini-review, we summarize recent advances in studies regarding the molecular evolution and physiology of the PTH ligands and their receptors, with particular focus on non-mammalian vertebrates. In summary, the PTH family of peptides probably predates early vertebrate evolution, indicating a more ancient existence as well as a function of these peptides in invertebrates.

Effects of PTH(1-84) therapy on muscle function and quality of life in hypoparathyroidism: results from a randomized controlled trial

The effects of treatment with 100 μg parathyroid hormone (PTH) (1-84) or an identical placebo on muscle function and quality of life (QoL) was studied in hypoparathyroid patients. At baseline, we found reduced QoL but no myopathy in the patients. Six months of treatment did not improve QoL, and muscle strength decreased slightly.

INTRODUCTION

A reduced quality of life (QoL) and myopathy that may be due to the absence of PTH have been reported in patients with hypoparathyroidism (hypoPT).

METHODS

Sixty-two patients with chronic hypoPT were randomized to 6 months of treatment with either PTH(1-84) 100 μg/d s.c. or placebo, given as add-on therapy to conventional treatment. Muscle function and postural stability were investigated using a dynamometer chair, a stadiometer platform, the repeated chair stands test, the timed up and go test, and electromyography. QoL was assessed using the 36-item Short Form Health Survey and the WHO-5 Well-Being Index.

RESULTS

The mean age of the patients was 52 ± 11 years, and 85 % were females. At baseline, QoL was significantly reduced in comparison with norm-based scores. Compared with placebo, PTH did not improve QoL or muscle function. Rather, max force production decreased significantly by 30 % at elbow flexion in the PTH group compared with the placebo group. Moreover, there was a nonsignificant trend for muscle strength to decrease in the upper extremities and on knee extension in response to PTH. Treatment did not affect postural stability. Electromyography showed a slight decrease in the duration of motor unit potentials in the PTH group, indicating a tendency toward myopathy, which, however was not symptomatic.

CONCLUSIONS

Overall, our data do not support an immediate beneficial effect of PTH replacement therapy on muscle function or QoL. A high frequency of hypercalcemia among our patients may have compromised the potential beneficial effects of reversing the state of PTH insufficiency.

Fragmented Adipose Tissue Transplanted to Craniofacial Deformities Induces Bone Repair Associated with Immunoexpression of Adiponectin and Parathyroid Hormone 1-Receptor

Objective

This study analyzed the influence of autogenous white adipose tissue on bone matrix development in critical-size defects created in rabbit calvaria.

Materials and Methods

A 15-mm-diameter defect was created in the calvaria of 42 rabbits. Twenty-one rabbits were treated with 86 mm3 of immediate transplant of fragmented white subcutaneous adipose tissue (WSAT); the others constituted the control group (sham). The animals were euthanized at 7, 15, and 40 days postsurgery (n = 7), and the histological data were analyzed by histomorphometry and immunohistochemistry using the anti-adiponectin and parathyroid hormone 1-receptor (PTH1R) antibodies.

Results

The calvariae treated with fragmented WSAT demonstrated significant bone formation. These results coincided with the significant presence of immunopositivity to adiponectin and PTH1R in loci, which in turn coincided with the increase in bonelike matrix deposited both in fat tissue stroma and adipocytes' cytoplasm. In contrast, the control group revealed a small amount of bone-matrix deposition and presented scarce PTH1R expression and a lack of immunostain for adiponectin.

Conclusion

These results indicate that transplant of fragmented white subcutaneous adipose tissue may be an alternative to treatment of craniofacial bone deformities because adipose tissue suffers from osseous metaplasia and exhibits immunoexpression of the adiponectin and PTH1R, which are proteins associated with bone metabolism

Osteocyte biology and space flight

The last decade has seen an impressive expansion of our understanding of the role of osteocytes in skeletal homeostasis. These amazing cells, deeply embedded into the mineralized matrix, are the key regulators of bone homeostasis and skeletal mechano sensation and transduction. They are the cells that can sense the mechanical forces applied to the bone and then translate these forces into biological responses. They are also ideally positioned to detect and respond to hormonal stimuli and to coordinate the function of osteoblasts and osteoclasts through the production and secretion of molecules such as Sclerostin and RANKL. How osteocytes perceive mechanical forces and translate them into biological responses in still an open question. Novel "in vitro" models as well the opportunity to study these cells under microgravity condition, will allow a closer look at the molecular and cellular mechanisms of mechano transduction. This article highlights novel investigations on osteocytes and discusses their significance in our understanding of skeletal mechano transduction.

In vitro and in vivo approaches to study osteocyte biology

Osteocytes, the most abundant cell population of the bone lineage, have been a major focus in the bone research field in recent years. This population of cells that resides within mineralized matrix is now thought to be the mechanosensory cell in bone and plays major roles in the regulation of bone formation and resorption. Studies of osteocytes had been impaired by their location, resulting in numerous attempts to isolate primary osteocytes and to generate cell lines representative of the osteocytic phenotype. Progress has been achieved in recent years by utilizing in vivo genetic technology and generation of osteocyte directed transgenic and gene deficiency mouse models. We will provide an overview of the current in vitro and in vivo models utilized to study osteocyte biology. We discuss generation of osteocyte-like cell lines and isolation of primary osteocytes and summarize studies that have utilized these cellular models to understand the functional role of osteocytes. Approaches that attempt to selectively identify and isolate osteocytes using fluorescent protein reporters driven by regulatory elements of genes that are highly expressed in osteocytes will be discussed. In addition, recent in vivo studies utilizing overexpression or conditional deletion of various genes using dentin matrix protein (Dmp1) directed Cre recombinase are outlined. In conclusion, evaluation of the benefits and deficiencies of currently used cell lines/genetic models in understanding osteocyte biology underlines the current progress in this field. The future efforts will be directed towards developing novel in vitro and in vivo models that would additionally facilitate in understanding the multiple roles of osteocytes.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Parathyroid hormone (1-84) and teriparatide in the treatment of postmenopausal osteoporosis

Osteoporosis-related fractures are a major public health problem worldwide. Antiresorptive drugs, which work principally by suppressing bone resorption, are the established therapeutic approach for the prevention of fragility fractures in patients with osteoporosis. Parathyroid hormone and its analogs represent a new class of agents with anabolic effects on the skeleton. The results of double-blind, randomized, placebo-controlled trials have shown that both the full length, 84 amino acid parathyroid hormone and teriparatide, the parathyroid hormone fragment (1-34) increase bone mineral density and reduce the risk of fracture when administered intermittently to postmenopausal osteoporotic women. Therefore, these drugs should be considered an alternative therapy in postmenopausal osteoporosis.

Teriparatide - Indications beyond osteoporosis

Osteoporosis is a condition of impaired bone strength that results in an increased risk of fracture. The current and most popular pharmacological options for the treatment of osteoporosis include antiresorptive therapy, in particular, oral bisphosphonates (alendronate, risedronate, ibandronate). Anabolic agents like teriparatide have widened our therapeutic options. They act by directly stimulating bone formation and improving bone mass quantity and quality. Two forms of recombinant human parathyroid hormone (PTH) are available : full-length PTH (PTH 1-84; approved in the EU only) and the 1-34 N-terminal active fragment of PTH (teriparatide, US FDA approved). This review aims to discuss the benefits of teriparatide beyond the currently licensed indications like fracture healing, dental stability, osteonecrosis of jaw, hypoparathyroidism, and hypocalcemia.

PTH Assays: Understanding What We Have and Forecasting What We Will Have

Parathyroid hormone (PTH) assays have evolved continuously for the last 50 years. Since the first radioimmunoassay was described in 1963, several assays based on immunological identification have been published (first generation assays). The routine assays used nowadays are immunometric "sandwich-type". They are based on two different monoclonal antibodies, one amino-terminal and the other carboxyl terminal specific. These second generation assays are widely available and adapted to most of the automation platforms. The specificity of the amino terminal antibody defines if the immunometric assay measures only the bioactive PTH circulating form (including the first amino terminal amino acids) or the "intact" PTH, which includes, besides bioactive PTH, other "long" carboxyl-terminal forms, for example, 7-84-PTH. Assays for "intact" PTH are the most commonly available and the potential advantage of the bioactive PTH assays is still debatable. Next generation of assays will be based on different principles, mainly mass spectrometry in samples submitted to a prior purification and fragmentation steps. These assays will provide information about the whole spectra of PTH peptides in circulation, with a significant increase of the information regarding this biologically important peptide hormone.

Present at the beginning: a personal reminiscence on the history of teriparatide

The ability of parathyroid glandular extracts to stimulate bone acquisition in rodents was established in the 1920s, but interest in this action lay dormant for almost 50 years until application of contemporary laboratory methods permitted the large-scale production of an amino-terminal fragment of PTH, (1-34) hPTH (teriparatide), which was capable of carrying out all known actions of the full-length (1-84) PTH molecule. In the 1970s, largely stimulated by the efforts of a British pharmacologist, Dr. John Parsons, the scientific community began to revisit these anabolic actions and showed that single daily injections of teriparatide dramatically increased bone mass in several mammalian species and restored bone in oöphorectomized rats. Shortly thereafter, human studies confirmed a striking increase in trabecular bone mass and showed also that an important part of teriparatide's action is to increase cortical bone. Eli Lilly and Company conducted a formal registration trial in postmenopausal women with osteoporosis. The unexpected occurrence of osteosarcomas in Fisher 344 rats treated long-term with teriparatide provoked an abrupt cessation of that trial, but ambiguity concerning the relevance of this rat finding to human disease, combined with significant anti-fracture efficacy, led to FDA approval of teriparatide for men and postmenopausal women with osteoporosis "at high risk for fracture" in 2002. Subsequently, teriparatide has been approved also for treatment of patients with glucocorticoid-associated osteoporosis, and papers indicating utility of this agent for dental and orthopedic applications have begun to appear.

Targeted ablation of the PTH/PTHrP receptor in osteocytes impairs bone structure and homeostatic calcemic responses

Parathyroid hormone (PTH) is a major physiologic regulator of calcium, phosphorous, and skeletal homeostasis. Cells of the osteoblastic lineage are key targets of PTH action in bone, and recent evidence suggests that osteocytes might be important in the anabolic effects of PTH. To understand the role of PTH signaling through the PTH/PTHrP receptors (PPR) in osteocytes and to determine the role(s) of these cells in mediating the effects of the hormone, we have generated mice in which PPR expression is specifically ablated in osteocytes. Transgenic mice in which the 10 kb-Dmp1 promoter drives a tamoxifen-inducible Cre-recombinase were mated with animals in which exon 1 of PPR is flanked by lox-P sites. In these animals, osteocyte-selective PPR knockout (Ocy-PPR(cKO) mice) could be induced by administration of tamoxifen. Histological analysis revealed a reduction in trabecular bone and mild osteopenia in Ocy-PPR(cKO) mice. Reduction of trabeculae number and thickness was also detected by micro-computed tomography analysis whereas bone volume fraction (BV/TV%) was unchanged. These findings were associated with an increase in Sost and sclerostin expression. When Ocy-PPR(cKO) mice were subjected to a low-calcium diet to induce secondary hyperparathyroidism, their blood calcium levels were significantly lower than littermate controls. Moreover, PTH was unable to suppress Sost and sclerostin expression in the Ocy-PPR(cKO) animals, suggesting an important role of PTH signaling in osteocytes for proper bone remodeling and calcium homeostasis.

Teriparatide treatment complicated by malignant myeloma

Recombinant human parathyroid hormone (1-34) (rhPTH 1-34), teriparatide (Forsteo in Europe), is a new compound that has been introduced and shown to be successful in the treatment of osteoporosis. The mechanisms of action include a pulsative influence on the RANKL/OPG system resulting in osteoblast activation and increased bone formation by teriparatide. In malignant myeloma there is an imbalance between osteoclast and osteoblast activity with involvement of the RANKL/OPG system among others. We report a case with monoclonal gammopathy of uncertain significance (MGUS) who developed malignant myeloma after teriparatide treatment and we suggest that in addition to malignant myeloma and smouldering myeloma, MGUS should also be considered contraindicated for teriparatide treatment.

Heterogeneity of carboxyl-terminal parathyroid hormone circulating forms in patients with hyperparathyroidism due to end stage renal disease

OBJECTIVE

To study carboxyl-terminal (COOH) parathyroid hormone (PTH) circulating forms in patients with hyperparathyroidism due to end stage renal disease (ESRD).

METHODS

An immunometric assay that recognizes both intact and COOH PTH forms was developed. The assay, in conjunction with an intact assay, was used to measure PTH in serum samples obtained from 25 patients with hyperparathyroidism due to ESRD. Samples were also submitted to gel filtration chromatography in a Superdex((R)) 30 1.6 x 60 cm column, and the PTH content in the elution tubes, measured using both assays.

RESULTS

Values from 39.000 to 232.300 ng/mL (mean +/- sd = 101.680 +/- 45.330 ng/mL) were found using the COOH assay (PTH 39-84 was used as standard). Values obtained by the intact PTH assay ranged from 318 to 3.307 ng/mL (1.769 +/- 693 ng/mL) with a correlation between assays of 0.462 (p = 0.02). The elution profile obtained using the COOH assay showed a preponderance of forms with MW ranging from 8.500 to 4.500 daltons. The profiles obtained from the 25 patients were very similar.

CONCLUSIONS

In patients with hyperparathyroidism due to ESRD circulating PTH levels contain a broad range of molecular forms including COOH with MW ranging from 8.500 to 4.500 daltons. These forms are not recognized by the standard intact PTH assays. The correlation of these findings to the clinical aspects of bone disease in ESRD patients remains to be studied.

Parathyroid hormone, a uremic toxin

Despite the innovations in the treatment of secondary hyperparathyroidism, there are uremic patients with marked elevation in PTH levels. Uremic toxicity is in part attributable to the excess of circulating PTH. It has been known for many years that PTH may induce changes in cell calcium, a key intracellular signal required for normal cell function. The effect of PTH in dialysis patients is not limited to bone; the diversity of biologic effects of PTH is summarized in this review. In addition, the present review addresses other issues: (i) the presence of different circulating PTH fragments in uremic patients, (ii) the PTH assays currently utilized to measure circulating PTH, and (iii) the fact that some of the PTH effects seen in uremic patients may be due to the interaction of C-terminal PTH fragment with putative C-terminal PTH receptors.

Critical issues of PTH assays in CKD

Measurement of bioactive parathyroid hormone (PTH) is essential for the optimal management of secondary hyperparathyroidism and its associated bone disorders in chronic kidney disease (CKD) patients. For this purpose, three generations of increasingly specific PTH assays have been developed over the last 4 decades. To date, however, only second-generation PTH assays are most widely used, although these have been shown to cross-react with large PTH fragments having a partially preserved N-structure, mostly PTH(7-84). The newly developed third-generation PTH assays are believed to be the most specific means of measuring PTH(1-84), but their clinical utility remains debatable. More recently, these latter assays have also been shown to react with a new N-form of PTH, which has been identified in patients with severe hyperparathyroidism and parathyroid carcinoma. Progressive research in this area has advanced our understanding considerably regarding the circulating molecular forms of PTH and their pathophysiological roles in bone abnormalities associated with CKD. However, developing an ideal PTH assay continues to be difficult because of key issues such as the reliability of PTH as a surrogate marker for bone turnover, practicality of employing third-generation PTH assays, and unknown biological implications of N-PTH and other PTH fragments. Further research exploring these issues is mandatory to understand and optimally manage parathyroid disorders and bone abnormalities in CKD patients.

384-channel parallel microfluidic cytometer for rare-cell screening

We have constructed a 384-channel parallel microfluidic cytometer (PMC). The multichannel architecture allows 384 unique samples for a cell-based screen to be read out in approximately 6-10 min, about 30-times the speed of a conventional fluorescence-activated cytometer system (FACS). This architecture also allows the signal integration time to be varied over a larger range than is practical in single-channel FACS and is suitable for detection of rare-cells in a high background of negatives. The signal-to-noise advantages have been confirmed by using the system to count rare clonal osteocytes in the most difficult early stages of an expression-cloning screen for the carboxy-terminal parathyroid hormone receptor (CPTHR). This problem requires finding several dozen positive cells in a background of one million negatives. The system is automated around a scanning laser confocal detector and a 96-tip robotic pipettor and can maintain in vitro cultures on-system in 384-well plates. It is therefore directly practical for biology applications using existing high-throughput culture facilities. The PMC system lends itself to high-sample-number cytometry with an unusual capability for time synchronization and rare-cell sensitivity. A limited ability to handle large sample numbers has restricted applications of single-channel FACS in combinatorial cell assays; therefore the PMC could have a significant application in high-throughput screening.

Mammalian hibernation as a model of disuse osteoporosis: the effects of physical inactivity on bone metabolism, structure, and strength

Reduced skeletal loading typically leads to bone loss because bone formation and bone resorption become unbalanced. Hibernation is a natural model of musculoskeletal disuse because hibernating animals greatly reduce weight-bearing activity, and therefore, they would be expected to lose bone. Some evidence suggests that small mammals like ground squirrels, bats, and hamsters do lose bone during hibernation, but the mechanism of bone loss is unclear. In contrast, hibernating bears maintain balanced bone remodeling and preserve bone structure and strength. Differences in the skeletal responses of bears and smaller mammals to hibernation may be due to differences in their hibernation patterns; smaller mammals may excrete calcium liberated from bone during periodic arousals throughout hibernation, leading to progressive bone loss over time, whereas bears may have evolved more sophisticated physiological processes to recycle calcium, prevent hypercalcemia, and maintain bone integrity. Investigating the roles of neural and hormonal control of bear bone metabolism could give valuable insight into translating the mechanisms that prevent disuse-induced bone loss in bears into novel therapies for treating osteoporosis.

Hormonal, pH, and calcium regulation of connexin 43-mediated dye transfer in osteocytes in chick calvaria

Gap junctional intercellular communication among osteocytes in chick calvaria, their natural 3D environment, was examined using FRAP analysis. Cell-cell communication among osteocytes in chick calvaria was mediated by Cx43 and was regulated by extracellular pH, extracellular calcium ion concentration, and PTH.

INTRODUCTION

The intercellular network of communication among osteocytes is mediated by gap junctions. Gap junctional intercellular communication (GJIC) is thought to play an important role in integration and synchronization of bone remodeling. We hypothesized that extracellular pH (pH(o)) and extracellular calcium ion concentration ([Ca2+](e)), both of which are dynamically altered by osteoclasts during bone remodeling, affect GJIC among osteocytes. Using fluorescence replacement after photobleaching (FRAP) analysis, we examined the effect of changes in pH(o) and [Ca2+](e) and addition of PTH on GJIC in osteocytes in chick calvaria. Additionally, we examined the role of intracellular calcium on the regulation of GJIC among osteocytes.

MATERIALS AND METHODS

Anti-Connexin43 (Cx43) immunolabeling was used to localize gap junctions in chick calvaria. GJIC among osteocytes in chick calvariae was assessed using FRAP.

RESULTS

Cx43 immunoreactivity was detected in most of the osteocyte processes. FRAP analysis showed dye-coupling among osteocytes in chick calvariae. In untreated osteocytes, fluorescence intensity recovered 43.7 +/- 2.2% within 5 min after photobleaching. Pretreatment of osteocytes with 18 alpha-GA, a reversible inhibitor of GJIC, significantly decreased fluorescence recovery to 10.7 +/- 2.2%. When pH(o) was decreased from 7.4 to 6.9, fluorescence recovery significantly decreased from 43.3 +/- 2.9% to 19.7 +/- 2.3%. Conversely, when pH(o) was increased from 7.4 to 8.0, fluorescence recovery was significantly increased to 61.9 +/- 4.5%. When [Ca2+](e) was increased from 1 to 25 mM, fluorescence recovery was significantly decreased from 47.0 +/- 6.1% to 16.1 +/- 2.1%. In bone fragments exposed to 1.0-10 nM rPTH for 3 h, replacement of fluorescence was significantly increased to 60.7 +/- 7.2%. Chelating intracellular calcium ions affected GJIC regulation by [Ca2+](e) and PTH.

CONCLUSIONS

Our study of cell-cell communication between osteocytes in chick calvaria showed for the first time that GJIC among osteocytes is regulated by the extracellular environment and by hormonal stimulation during bone remodeling. This method may be more biologically relevant to living bone than current methods.

Full length parathyroid hormone (1-84) in the treatment of osteoporosis in postmenopausal women

Objective:

To review the pharmacological properties and the available clinical data of full length parathyroid hormone (PTH) in post-menopausal osteoporosis.

Sources:

A MEDLINE search was completed, together with a review of information obtained from the manufacturer and from the medicine regulatory agencies.

Study and data selection:

Studies were selected according to relevance and availability. Relevant information (design, objectives, patients’ characteristics, outcomes, adverse events, dosing, etc) was analyzed.

Results:

Different studies have shown that, when administered intermittently as a subcutaneous injection in the abdomen, PTH increases bone mineral density (BMD) and prevents vertebral fractures. On completion of PTH therapy (up to 24 months), there is evidence that sequential treatment with alendronate is associated with a therapeutic benefit in terms of increase in BMD. Further trials are necessary to determine long-term safety and the role of PTH in combination with other treatments for osteoporosis and the effect of repeated cycles of PTH followed by an anti-catabolic agent. There are currently no completed comparative trials with other osteoporosis treatments.

Conclusions:

Full length PTH, given intermittently as an abdominal subcutaneous injection, appears to be a safe and efficacious treatment option for high risk osteoporosis. More data are needed to determine its specific role in osteoporosis treatment.

Parathyroid hormone fragments inhibit active hormone and hypocalcemia-induced 1,25(OH)2D synthesis

Carboxyl (C)-terminal fragments of parathyroid hormone (PTH) oppose the calcemic, phosphaturic, and bone-resorbing effects of active hormone. To study the action of these fragments on 1,25(OH)(2)D (1,25-dihydroxyvitamin D) synthesis, we infused parathyroidectomized rats with human or rat active 1-34 or 1-84 PTH at doses selected to produce similar calcemic responses. Human active PTH influenced neither phosphate nor 1,25(OH)(2)D concentrations. However, active 1-34 rat PTH decreased phosphate to the same level as vehicle-treated rats and increased 1,25(OH)(2)D to very high levels, whereas active 1-84 PTH decreased phosphate but maintained 1,25(OH)(2)D. As the latter effect could have been due to C-terminal fragment generation during its metabolic breakdown, we infused a mixture of rat C-terminal fragments alone or with rat 1-34. The C-terminal fragments decreased 1,25(OH)(2)D and prevented hypocalcemic-induced 1,25(OH)(2)D synthesis. When infused with active rat 1-34, they lowered the 1,25(OH)(2)D level to that seen with intact rat 1-84. The C-terminal fragments did not influence either basal or rat 1-34- or 1-84-induced CYP27B1 mRNA levels, suggesting that their inhibitory effects on 1,25(OH)(2)D synthesis appears to be post-transcriptional.

Mitogen-activated protein kinase phosphatase 1/dual specificity phosphatase 1 mediates glucocorticoid inhibition of osteoblast proliferation

Steroid-induced osteoporosis is a common side effect of long-term treatment with glucocorticoid (GC) drugs. GCs have multiple systemic effects that may influence bone metabolism but also directly affect osteoblasts by decreasing proliferation. This may be beneficial at low concentrations, enhancing differentiation. However, high-dose treatment produces a severe deficit in the proliferative osteoblastic compartment. We provide causal evidence that this effect of GC is mediated by induction of the dual-specificity MAPK phosphatase, MKP-1/DUSP1. Excessive MKP-1 production is both necessary and sufficient to account for the impaired osteoblastic response to mitogens. Overexpression of MKP-1 after either GC treatment or transfection ablates the mitogenic response in osteoblasts. Knockdown of MKP-1 using either immunodepletion of MKP-1 before in vitro dephosphorylation assay or short interference RNA transfection prevents inactivation of ERK by GCs. Neither c-jun N-terminal kinase nor p38 MAPK is activated by the mitogenic cocktail in 20% fetal calf serum, but their activation by a DNA-damaging agent (UV irradiation) was inhibited by either GC treatment or overexpression of MKP-1, indicating regulation of all three MAPKs by MKP-1 in osteoblasts. However, an inhibitor of the MAPK/ERK kinase-ERK pathway inhibited osteoblast proliferation whereas inhibitors of c-jun N-terminal kinase or p38 MAPK had no effect, suggesting that ERK is the MAPK that controls osteoblast proliferation. Regulation of ERK by MKP-1 provides a novel mechanism for control of osteoblast proliferation by GCs.

Evolution of PTH assays

PTH metabolism is complex and the circulating forms include the intact 1-84 molecule as well as several carboxyl-terminal fragments. The first generation of PTH assays included several types of competitive assays, with specificities that spanned carboxyl, mid-region and amino-terminal portions of the molecule. The limitations of these assays and the methodological evolution led to the description of 2nd generation non-competitive immunometric assays for PTH in the late 80's, based on the recognition of the PTH molecule by two different antibodies, one directed against de amino-terminal and other against the carboxyl-terminal segments. The observation that in some circumstances "long" carboxyl-terminal segments were also measured by 2nd generation assays led to the development of 3rd generation assays based on amino-terminal specific antibodies that are specific for the first amino acids, measuring only the molecular forms that activate PTH1R. The practical and cost-benefit advantages of these assays are still debatable. The recent observation that carboxyl-terminal fragments of PTH have biological activity via a distinct receptor than PTH1R, points to the future need of more than one assay in order to evaluate parathyroid hormone function.

The biology of osteocytes

Osteocytes, the most abundant cell type in bone, remain the least characterized. Several theories have been proposed regarding their function, including osteolysis, sensing the strains produced in response to mechanical loading of bones, and producing signals that affect the function of osteoblasts and osteoclasts and hence, bone turnover. This review also discusses the role of osteocyte apoptosis in targeted bone remodeling and proposes that the occurrence of osteocyte apoptosis is consistent with the description of apoptosis as an essential homeostatic mechanism for the healthy maintenance of tissues.

Abnormal muscle and hematopoietic gene expression may be important for clinical morbidity in primary hyperparathyroidism

In primary hyperparathyroidism (PHPT), excess PTH secretion by adenomatous or hyperplastic parathyroid glands leads to elevated serum [Ca(2+)]. Patients present complex symptoms of muscular fatigue, various neuropsychiatric, neuromuscular, and cardiovascular manifestations, and, in advanced disease, kidney stones and metabolic bone disease. Our objective was to characterize changes in muscle and hematopoietic gene expression in patients with reversible mild PHPT after parathyroidectomy and possibly link molecular pathology to symptoms. Global mRNA profiling using Affymetrix gene chips was carried out in biopsies obtained before and 1 yr after parathyroidectomy in seven patients discovered by routine blood [Ca(2+)] screening. The tissue distribution of PTH receptor (PTHR1 and PTHR2) mRNAs were quantitated using real-time RT-PCR in unrelated persons to define PTH target tissues. Of about 10,000 expressed genes, 175 muscle, 169 hematological, and 99 bone-associated mRNAs were affected. Notably, the major part of muscle-related mRNAs was increased whereas hematological mRNAs were predominantly decreased during disease. Functional and molecular network analysis demonstrated major alterations of several tissue characteristic groups of mRNAs as well as those belonging to common cell signaling and major metabolic pathways. PTHR1 and PTHR2 mRNAs were more abundantly expressed in muscle and brain than in hematopoietic cells. We suggest that sustained stimulation of PTH receptors present in brain, muscle, and hematopoietic cells have to be considered as one independent, important cause of molecular disease in PHPT leading to profound alterations in gene expression that may help explain symptoms like muscle fatigue, cardiovascular pathology, and precipitation of psychiatric illness.

Effect of haemodialysis on markers of bone turnover in children

'Intact' parathyroid hormone (iPTH) assays are used to measure serum PTH levels in haemodialysis patients to diagnose and monitor secondary hyperparathyroidism and consequent renal osteodystrophy (ROD); these assays exhibit cross-reactivity with long carboxyl-terminal PTH fragments (C-PTH) that accumulate in end stage renal failure (ESRF) and antagonise the biological activity of the whole molecule, 1-84 PTH. The effects of haemodialysis on C-PTH are not known. We investigated how haemodialysis affects serum concentrations of calcium, iPTH, 1-84 PTH, C-PTH, and other markers of bone turnover; bone-specific alkaline phosphatase (BALP) and type 1 collagen cross-linked telopeptide (CTx). Fifteen patients, mean (range) age 13.9 (4.3-17.6) years, haemodialysed for a median of 16.3 (4-41) months, had pre- and post-dialysis serum samples collected for routine biochemistry, BALP, CTx, iPTH and 1-84 PTH assays. Changes to serum concentrations and relationships between these biochemical surrogate markers of ROD were investigated. Serum phosphate and PTH levels (measured by both assays) fell significantly during dialysis, whereas serum calcium, C-PTH, the 1-84 PTH: C-PTH ratio and BALP and CTx concentrations were not significantly changed. 1-84 PTH levels were related to pre but not post dialysis serum calcium levels and changes to 1-84 PTH levels during dialysis were related to changes in serum calcium levels. 1-84 PTH and iPTH were reduced by haemodialysis, whereas levels of BALP and CTx remained stable post-dialysis. The relationship between BALP and CTx and bone histology requires investigation to determine whether they are more useful markers of bone turnover in this patient group.

Treatment of skeletally mature ovariectomized rhesus monkeys with PTH(1-84) for 16 months increases bone formation and density and improves trabecular architecture and biomechanical properties at the lumbar spine

Histomorphometric studies of treatments for osteoporosis in humans are restricted to iliac crest biopsies. We studied the effects of PTH(1-84) treatment at the lumbar spine of skeletally mature ovariectomized rhesus monkeys. PTH increased bone turnover, rapidly normalized BMD, and increased vertebral compressive strength. PTH increased trabecular bone volume primarily by increasing trabecular number by markedly increasing intratrabecular tunneling.

INTRODUCTION

Histomorphometric studies of the anabolic properties of PTH(1-84) (PTH) and related peptides in human bone are restricted to iliac crest biopsies. The ovariectomized (OVX) monkey is an accepted model of human postmenopausal bone loss and was used to study the effects of PTH treatment at clinically relevant skeletal sites.

MATERIALS AND METHODS

Skeletally mature rhesus monkeys were OVX or sham-operated and, after a bone depletion period of 9 months, treated daily for 16 months with PTH (5, 10, or 25 microg/kg). Markers of bone formation (serum osteocalcin) and resorption (urine N-telopeptide [NTX]) and lumbar spine BMD were measured throughout the study. Trabecular architecture and vertebral biomechanical properties were quantified at 16 months.

RESULTS

PTH treatment induced dose-dependent increases in bone turnover but did not increase serum calcium. Osteocalcin was significantly increased above OVX controls by 1 month. NTX was significantly elevated at 1 month with the highest dose, but not until 12 months with the 5 and 10 microg/kg doses. Lumbar spine BMD was 5% lower in OVX than in sham animals when treatment was started. All PTH doses increased BMD rapidly, with sham levels restored by 3-7 months with 10 and 25 microg/kg and by 16 months with 5 microg/kg. PTH treatment increased trabecular bone volume (BV/TV), primarily by increasing trabecular number, and dose-dependently increased bone formation rate (BFR) solely by increasing mineralizing surface. The largest effects on BV/TV and yield load occurred with the 10 microg/kg dose. The highest dose reduced trabecular thickness by markedly increasing intratrabecular tunneling.

CONCLUSIONS

PTH treatment of OVX rhesus monkeys increased bone turnover and increased BV/TV, BMD, and strength at the lumbar spine. All PTH doses were safe, but the 10 microg/kg dose was generally optimal, possibly because the highest dose resulted in too marked a stimulation of bone remodeling.

Defining a noncarcinogenic dose of recombinant human parathyroid hormone 1-84 in a 2-year study in Fischer 344 rats

The carcinogenic potential of human parathyroid hormone 1-84 (PTH) was assessed by daily subcutaneous injection (0, 10, 50, 150 microg/kg/day) for 2 years in Fischer 344 rats. Histopathological analyses were conducted on the standard set of soft tissues, tissues with macroscopic abnormalities, selected bones, and bones with abnormalities identified radiographically. All PTH doses caused widespread osteosclerosis and significant, dose-dependent increases in femoral and vertebral bone mineral content and density. In the mid-and high-dose groups, proliferative changes in bone increased with dose. Osteosarcoma was the most common change, followed by focal osteoblast hyperplasia, osteoblastoma, osteoma and skeletal fibrosarcoma. The incidence of bone neoplasms was comparable in control and low-dose groups providing a noncarcinogenic dose for PTH of 10 microg/kg/day at a systemic exposure to PTH that is 4.6-fold higher than for a 100 microg dose in humans. The ability of PTH to interact with and balance the effects of both the PTH-1 receptor and the putative C-terminal PTH receptor, may lead to the lower carcinogenic potential observed with PTH than reported previously for teriparatide.

Parathyroid hormone assay: problems and opportunities

The assay of parathyroid hormone continues to remain problematic as a result of the presence in the circulation of a variety of parathyroid hormone (PTH) peptides derived from secretion and from peripheral metabolism. The detection of these PTH fragments to varying degrees leads to widely differing results in the various assays used, particularly in the setting of chronic kidney disease, where PTH fragments accumulate as glomerular filtration rate (GFR) falls. The differing results not only lead to problems in comparing values from various laboratories but also limit efforts to develop useful clinical practice guidelines. At the same time, research into the precise identification of the PTH fragments which contribute to the assay problems has uncovered a relatively new area of parathyroid research that has pointed to potential biologic activity of PTH peptides previously thought to be biologically inactive and which may act on a novel PTH receptor. These issues have brought new focus to the difficulties in standardization of PTH assays and have provoked efforts to provide standards to help in the characterization of PTH assays and to facilitate the development of clinical practice guidelines.