Parathyroid hormone secretion and action: evidence for discrete receptors for the carboxyl-terminal region and related biological actions of carboxyl- terminal ligands

The parathyroid hormone regulates skin tumour susceptibility in mice

Using a forward genetics approach to map loci in a mouse skin cancer model, we previously identified a genetic locus, Skin tumour modifier of MSM 1 (Stmm1) on chromosome 7, conferring strong tumour resistance. Sub-congenic mapping localized Parathyroid hormone (Pth) in Stmm1b. Here, we report that serum intact-PTH (iPTH) and a genetic polymorphism in Pth are important for skin tumour resistance. We identified higher iPTH levels in sera from cancer-resistant MSM/Ms mice compared with susceptible FVB/NJ mice. Therefore, we performed skin carcinogenesis experiments with MSM-BAC transgenic mice (Pth^MSM-Tg) and Pth knockout heterozygous mice (Pth^+/−). As a result, the higher amounts of iPTH in sera conferred stronger resistance to skin tumours. Furthermore, we found that the coding SNP (rs51104087, Val28Met) localizes in the mouse Pro-PTH encoding region, which is linked to processing efficacy and increased PTH secretion. Finally, we report that PTH increases intracellular calcium in keratinocytes and promotes their terminal differentiation. Taken together, our data suggest that Pth is one of the genes responsible for Stmm1, and serum iPTH could serve as a prevention marker of skin cancer and a target for new therapies.

Biochemical Testing Relevant to Bone

Laboratory biochemical testing is critical to the clinical understanding of bone disorders. Patients with skeletal diseases have underlying themes in their pathophysiology that would be impossible to detect without biochemical assessment of serum and urine minerals, vitamin D, parathyroid hormone, parathyroid hormone-related peptide, and bone turnover markers. Bone disorders are caused by abnormalities in signaling pathways that affect bone formation and resorption. Therapies for common bone diseases were developed in direct response to underlying biochemical abnormalities.

Bone mineral density in people living with HIV: a narrative review of the literature

Bone health status is largely absent in South Africa, the main reasons being the absence and cost-effectiveness of specific screening equipment for assessing bone mineral density (BMD). Various risk factors seem to play a role, some of which can be modified to change bone health status. Urbanisation is also a public health concern. Changing nutritional, as well as social behaviour, play integral roles in the prevalence and incidence of decreased BMD. Furthermore, human immunodeficiency virus (HIV) specifically, has a negative impact on BMD and although highly active antiretroviral therapy increases the prognosis for HIV-infected individuals, BMD still seem to decrease further. Dual energy X-ray absorptiometry is considered the gold standard for BMD assessment; however, recent developments have provided more cost-effective screening methods, among which heel quantitative ultrasound appears to be the most widely used in resource limited countries such as South Africa.

Positioning novel biologicals in CKD-mineral and bone disorders

Renal osteodystrophy (ROD), the histologic bone lesions of chronic kidney disease (CKD), is now included in a wider syndrome with laboratory abnormalities of mineral metabolism and extra-skeletal calcifications or CKD-mineral and bone disorders (CKD-MBD), to highlight the increased burden of mortality. Aging people, frequently identified as early CKD, could suffer from either the classical age-related osteoporosis (OP) or ROD. Distinguishing between these two bone diseases may not be easy without bone biopsy. In any case, besides classical therapies for ROD, nephrologists are now challenged by the possibility of using new drugs developed for OP. Importantly, while therapies for ROD mostly aim at controlling parathyroid secretion with bone effects regarded as indirect, new drugs for OP directly modulate bone cells activity. Thus, their action could be useful in specific types of ROD. Parathyroid hormone therapy, which is anabolic in OP, could be useful in renal patients with low turnover bone disease. Denosumab, the monoclonal antibody against receptor activator of NF-κB ligand (RANK-L) that inhibits osteoclast activity and proliferation, could be beneficial in cases with high turnover bone. Use of romosozumab, the monoclonal antibody against sclerostin, which both stimulates osteoblasts and inhibits osteoclasts, could allow both anabolic and anti-resorptive effects. However, we should not forget the systemic role now attributed to CKD-MBD. In fact, therapies targeting bone cells activity could also result in unpredicted extra-bone effects and affect cardiovascular outcomes. In conclusion, the new biologicals established for OP could be useful in renal patients with either OP or ROD. In addition, their potential non-bone effects warrant investigation.

Current Role and Application of Teriparatide in Fracture Healing of Osteoporotic Patients: A Systematic Review

Background

The use of osteoanabolic agents to facilitate fracture healing has been of heightened interest to the field of orthopaedic trauma. This study aimed to evaluate the evidence of teriparatide for fracture healing and functional recovery in osteoporotic patients.

Methods

We performed a literature search in PubMed, EMBASE, Web of Science, and the Cochrane Library using terms including “Fracture” [tiab] AND “Teriparatide [tiab] OR “PTH” [tiab].

Results

This systematic review included 6 randomized clinical trials, 4 well-controlled retrospective studies, and 1 retrospective post hoc subgroup analysis. Fracture location was 2 in pelvis, 3 in proximal femur, 1 in distal femur, 1 in shoulder, 2 in wrist and 2 in spine. The use of teriparatide yielded positive effects on radiographic bone healing in 6 studies, but was not associated with better radiographic outcome in 3. In terms of functional recovery, teriparatide injection was related with decrease in pain or shorter time to mobilization in 6 studies, but not related with pain numerical scale and mobility in 3.

Conclusions

Our findings suggest that teriparatide provide selective advantages to fracture healing or functional recovery in the management of osteoporotic fractures. A better understanding of the role of teriparatide on osteoporotic fractures requires greater evidences from large volume prospective trials.

Calcium and Bone Metabolism Indices

Calcium and inorganic phosphate are of critical importance for many body functions, thus the regulations of their plasma concentrations are tightly controlled by the concerted actions of reabsorption/excretion in the kidney, absorption in the intestines, and exchange from bone, the major reservoir for calcium and phosphate in the body. Parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D (1,25(OH)₂D) control calcium homeostasis, whereas PTH, 1,25(OH)₂D, and bone-derived fibroblast growth factor 23 (FGF 23) control phosphate homeostasis. Hypoparathyroidism can cause hypocalcemia and hyperphosphatemia, whereas deficient vitamin D actions can cause osteomalacia in adults and rickets in children. Hyperparathyroidism, alternatively, can cause hypercalcemia and hypophosphatemia. Laboratory tests of calcium, phosphate, PTH, and 25-hydroxyvitamin D are very useful in the diagnosis of abnormalities associated with calcium and/or phosphate metabolisms. Bone is constantly remodeled throughout life in response to mechanical stress and a need for calcium in extracellular fluids. Metabolic bone diseases such as osteoporosis, osteomalacia in adults or rickets in children, and renal osteodystrophy develop when bone resorption exceeds bone formation. Bone turnover markers (BTM) such as serum N-terminal propeptide of type I procollagen (P1NP) and C-terminal collagen cross-link (CTX) may be useful in predicting future fracture risk or monitoring the response to anti-resorptive therapy. There is a need to standardize sample collection protocols because certain BTMs exhibit large circadian variations and tend to be influenced by food intakes. In the United States, a project to standardize BTM sample collection protocols and to establish the reference intervals for serum P1NP and serum CTX is ongoing. We anticipate the outcome of this project to shine lights on the standardization of BTM assays, sample collection protocols, reference intervals in relation to age, sex, and ethnic origins, and clinical utilities of BTMs. This review will briefly discuss the regulations of calcium and phosphate homeostasis, laboratory's role in the diagnosis, and monitoring of bone and calcium metabolism, as well as the usefulness and controversies of the utilities of BTMs in the diagnosis and monitoring of metabolic bone diseases.

Parathyroid hormone metabolism and signaling in health and chronic kidney disease

Circulating parathyroid hormone (PTH) shows a complex relationship with hard outcomes in subjects with chronic kidney disease (CKD). Moreover, intervention studies directly targeting PTH failed to yield unequivocal results. Disturbed PTH metabolism, posttranslational modifications of PTH, and end-organ hyporesponsiveness to PTH may explain the poor performance of PTH as an outcome biomarker and precise target of therapy in the setting of CKD, at least in the gray middle target zone. PTH fragments accumulate in CKD patients and may exert effects that are distinct from, if not opposite to biointact (1-84)PTH. Posttranslational modification of PTH and especially oxidation may alter the interaction of PTH with its receptor. Its clinical relevance, however, remains a matter of ongoing debate. Less controversial is the issue of end-organ hyporesponsiveness to PTH. This phenomenon, formally referred to as PTH resistance, has long been recognized in CKD, but factors and mechanisms contributing to it remain poorly defined. Subsequent evidence identified downregulation of the PTH receptor and competing downstream signals as underlying pathophysiologic mechanisms. End-organ hyporesponsiveness to PTH in CKD, along with important analytical and biological variability, renders defining the PTH target range in CKD challenging. Although this may still be accomplished at the population level, it may prove to be very difficult at the individual level. This is a disillusioning thought in an era of personalized medicine. Parallel to the search of a functional and readily available assay quantifying PTH signaling tone or sensitivity, additional biomarkers (or a panel of biomarkers) should be formally evaluated.

Factors affecting parathyroid hormone levels in different types of primary aldosteronism

BACKGROUND

Recent studies have found that mild secondary hyperparathyroidism might be another clinical feature of patients with primary aldosteronims (PA), but whether serum parathyroid hormone level (PTH) is correlated with subtypes of PA and what contributes to the elevated PTH level remains unclear.

OBJECTIVE

To illustrate the changes of PTH in PA and to partly explain the mechanism of how the effects of aldosterone regulating the secretion of PTH in PA.

METHODS

We enrolled a total of 120 patients with primary hypertension (PH) and 242 patients with PA, which included 89 APAs (aldosterone-producing adenoma), 119 IHAs (idiopathic hyperaldosteronism) and 34 UAHs (unilateral adrenal hyperplasia). The plasma levels of aldosterone, renin activity, parathyroid hormone and markers associated with calcium metabolism were measured.

RESULTS

We found serum PTH level was significantly elevated in patients with PA compared with primary hypertension [9·0 (6·6, 11·7) vs 5·7 (4·4, 7·0)] pmol/l, P < 0·001]. However, no difference was found between the three PA subtypes (P > 0·05). Stepwise multiple regression analysis showed that in patients with PA, serum levels of K(+) and Ca(2+) were independently associated with serum PTH level. More importantly, elevated PTH level could be corrected either by unilateral adrenalectomy [9·9 (7·5, 12·8) vs 5·2 (4·4, 7·0) pmol/l, P < 0·001] or mineralocorticoid receptor (MR) antagonists treatment [11·7 (9·1, 13·4) vs 6·3 (5·1, 7·8) pmol/l, P < 0·001].

CONCLUSIONS

PTH level is elevated in PA patients and irrelevant with subtypes of PA. Serum K(+) and serum Ca(2+) level are main factors influence the plasma PTH level in PA patients. After medical or surgical treatment, PTH levels return to normal.

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.

Small Molecule Inhibited Parathyroid Hormone Mediated cAMP Response by N-Terminal Peptide Binding

Ligand binding to certain classes of G protein coupled receptors (GPCRs) stimulates the rapid synthesis of cAMP through G protein. Human parathyroid hormone (PTH), a member of class B GPCRs, binds to its receptor via its N–terminal domain, thereby activating the pathway to this secondary messenger inside cells. Presently, GPCRs are the target of many pharmaceuticals however, these drugs target only a small fraction of structurally known GPCRs (about 10%). Coordination complexes are gaining interest due to their wide applications in the medicinal field. In the present studies we explored the potential of a coordination complex of Zn(II) and anthracenyl–terpyridine as a modulator of the parathyroid hormone response. Preferential interactions at the N–terminal domain of the peptide hormone were manifested by suppressed cAMP generation inside the cells. These observations contribute a regulatory component to the current GPCR–cAMP paradigm, where not the receptor itself, but the activating hormone is a target. To our knowledge, this is the first report about a coordination complex modulating GPCR activity at the level of deactivating its agonist. Developing such molecules might help in the control of pathogenic PTH function such as hyperparathyroidism, where control of excess hormonal activity is essentially required.

Parathyroid hormone is associated with incident diabetes in white, but not black adults: The Atherosclerosis Risk in Communities (ARIC) Study

OBJECTIVE

Accumulating evidence has linked elevated parathyroid hormone (PTH) with insulin resistance, beta cell dysfunction and dysglycaemia, however, its role in the development of diabetes is largely unclear, particularly among non-whites. We sought to examine the association of PTH with the incidence of diabetes.

METHODS

We studied 8066 white and 2034 black adults aged 46-70 years at baseline (1990-92) from the ARIC Study with follow-up for incident diabetes ascertained during study visits conducted in 1993-95 and 1996-98. Hazard ratios (HR) and their 95% CIs for diabetes adjusted for demographics, lifestyle, and 25-hydroxyvitamin D were estimated according to PTH measured at baseline.

RESULTS

PTH was higher among blacks than whites (median [IQR], 43.8 [35.0-55.8] vs. 37.9 [30.4-47.3] pg/mL; P<0.001). During a median follow-up of 6 years, 498 white and 167 black participants developed diabetes. The association of PTH with diabetes varied significantly by race (P-interaction 0.02). PTH was not associated with risk for diabetes among black adults. Among whites, HRs according to quintiles of PTH were 1 (referent), 0.95 (0.71, 1.29), 0.95 (0.70, 1.28), 1.12 (0.84, 1.51), and 1.31 (0.98, 1.76) (P-trend 0.03). When a clinical cut-point for PTH was applied (≥65pg/mL; 5.7% of whites), the HR for diabetes among whites was 1.38 (1.01, 1.88). Results were similar when restricted to participants with normal baseline kidney function.

CONCLUSION

In this large, population-based study, elevated PTH was independently associated with risk for diabetes among white, but not black adults. Further studies are needed to elucidate the mechanisms that may underlie this differential association of PTH with diabetes across race groups.

Parathyroid hormone 1 receptor is essential to induce FGF23 production and maintain systemic mineral ion homeostasis

Parathyroid-hormone-type 1 receptor (PTH1R) is extensively expressed in key regulatory organs for systemic mineral ion homeostasis, including kidney and bone. We investigated the bone-specific functions of PTH1R in modulating mineral ion homeostasis by generating a novel mouse model in which PTH1R is ablated in the limb mesenchyme using Prx1Cre transgenic mice. Such ablation decreased FGF23 protein and serum levels by 50%, despite normal Fgf23 mRNA levels in long bones. Circulating calcium and PTH levels were unchanged, but inorganic phosphate and 1,25(OH)2D3 levels were significantly decreased and accompanied by elevated urinary calcium and phosphate wasting. Key renal genes for balancing mineral ion homeostasis, calbindinD28k, Klotho, and Napi2a were suppressed by 30-40%. Intermittent hPTH(1-34) injections increased Fgf23 mRNA (7.3-fold), Nurr1 mRNA (3.1-fold), and serum intact-FGF23 (1.6-fold) in controls, but failed to induce Fgf23, Nurr1 mRNA, or intact FGF23 production in mutants. Moreover, a significant elevation in serum C-terminal-FGF23 levels (4-fold) was detected in both genotypes. PTH markedly downregulated Galnt3 expression (2.7-fold) in controls but not in mutants. These results demonstrate the pivotal role of PTH1R in long bones to regulate systemic mineral ion homeostasis and the direct induction of FGF23 by PTH1R signaling.

Hypoparathyroidism: Replacement Therapy with Parathyroid Hormone

Hypoparathyroidism (HypoPT) is characterized by low serum calcium levels caused by an insufficient secretion of parathyroid hormone (PTH). Despite normalization of serum calcium levels by treatment with activated vitamin D analogues and calcium supplementation, patients are suffering from impaired quality of life (QoL) and are at increased risk of a number of comorbidities. Thus, despite normalization of calcium levels in response to conventional therapy, this should only be considered as an apparent normalization, as patients are suffering from a number of complications and calcium-phosphate homeostasis is not normalized in a physiological manner. In a number of recent studies, replacement therapy with recombinant human PTH (rhPTH(1-84)) as well as therapy with the N-terminal PTH fragment (rhPTH(1-34)) have been investigated. Both drugs have been shown to normalize serum calcium while reducing needs for activated vitamin D and calcium supplements. However, once a day injections cause large fluctuations in serum calcium. Twice a day injections diminish fluctuations, but don't restore the normal physiology of calcium homeostasis. Recent studies using pump-delivery have shown promising results on maintaining normocalcemia with minimal fluctuations in calcium levels. Further studies are needed to determine whether this may improve QoL and lower risk of complications. Such data are needed before replacement with the missing hormone can be recommended as standard therapy.

Quantitation of Parathyroid Hormone in Serum or Plasma by Liquid Chromatography-Tandem Mass Spectrometry

Parathyroid hormone (PTH), an 84 amino acid peptide hormone, is an important regulator of calcium homeostasis. Quantitation of PTH in serum is useful for the diagnosis of primary hyperparathyroidism, hypoparathyroidism, and for monitoring osteodystrophy in patients with renal failure. The biological activity of PTH arises from binding of PTH (N terminus) to its target receptor (D'Amour et al., Kidney Int 68: 998-1007, 2005). Several C-terminal and N-terminal fragments circulate in normal subjects. Recent studies have demonstrated that accurate quantitation of PTH fragments may be of clinical value. In this chapter a mass spectrometry based method for quantitation of PTH(1-84) is described. This method involves immunoaffinity capture of PTH followed by trypsinization and quantitation of PTH-specific tryptic peptides by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The N-terminal tryptic peptide, PTH(1-13) as surrogate of 1-84 PTH, is used for quantitation.

The dynamic skeleton

Adding to its well-known roles in locomotion and calcium balance, the skeleton has recently been appreciated as a true endocrine organ. Bone remodeling, a highly dynamic process, requires synchronized activities and crosstalk between bone cells. Discovery and characterization of the Wnt/β catenin pathway in bone formation, FGF23 regulation of phosphate homeostasis and osteocalcin in energy and glucose homeostasis have reframed our view of the skeleton from simply a target tissue of the endocrine system to an endocrine tissue itself. This comprehensive review provides an overview of these complex pathways, their application to human bone disorders and implications for developing diagnostic and therapeutic targets.

Parathyroid hormone, cognitive function and dementia: a systematic review

Background

Metabolic factors are increasingly recognized to play an important role in the pathogenesis of Alzheimer’s disease and dementia. Abnormal parathyroid hormone (PTH) levels play a role in neuronal calcium dysregulation, hypoperfusion and disrupted neuronal signaling. Some studies support a significant link between PTH levels and dementia whereas others do not.

Methods

We conducted a systematic review through January 2014 to evaluate the association between PTH and parathyroid conditions, cognitive function and dementia. Eleven electronic databases and citation indexes were searched including Medline, Embase and the Cochrane Library. Hand searches of selected journals, reference lists of primary studies and reviews were also conducted along with websites of key organizations. Two reviewers independently screened titles and abstracts of identified studies. Data extraction and study quality were performed by one and checked by a second reviewer using predefined criteria. A narrative synthesis was performed due to the heterogeneity of included studies.

Results

The twenty-seven studies identified were of low and moderate quality, and challenging to synthesize due to inadequate reporting. Findings from six observational studies were mixed but suggest a link between higher serum PTH levels and increased odds of poor cognition or dementia. Two case-control studies of hypoparathyroidism provide limited evidence for a link with poorer cognitive function. Thirteen pre-post surgery studies for primary hyperparathyroidism show mixed evidence for improvements in memory though limited agreement in other cognitive domains. There was some degree of cognitive impairment and improvement postoperatively in observational studies of secondary hyperparathyroidism but no evident pattern of associations with specific cognitive domains.

Conclusions

Mixed evidence offers weak support for a link between PTH, cognition and dementia due to the paucity of high quality research in this area.

Lipoprotein receptor-related protein 6 is required for parathyroid hormone-induced Sost suppression

Parathyroid hormone (PTH) suppresses the expression of the bone formation inhibitor sclerostin (Sost) in osteocytes by inducing nuclear accumulation of histone deacetylases (HDACs) to inhibit the myocyte enhancer factor 2 (MEF2)-dependent Sost bone enhancer. Previous studies revealed that lipoprotein receptor-related protein 6 (LRP6) mediates the intracellular signaling activation and the anabolic bone effect of PTH. Here, we investigated whether LRP6 mediates the inhibitory effect of PTH on Sost using an osteoblast-specific Lrp6-knockout (LRP6-KO) mouse model. An increased level of Sost mRNA expression was detected in femur tissue from LRP6-KO mice, compared to wild-type littermates. The number of osteocytes expressing sclerostin protein was also increased in bone tissue of LRP6-KO littermates, indicating a negative regulatory role of LRP6 on Sost/sclerostin. In wild-type littermates, intermittent PTH treatment significantly suppressed Sost mRNA expression in bone and the number of sclerostin(+) osteocytes, while the effect of PTH was much less significant in LRP6-KO mice. Additionally, PTH-induced downregulation of MEF2C and 2D, as well as HDAC changes in osteocytes, were abrogated in LRP6-KO mice. These data indicate that LRP6 is required for PTH suppression of Sost expression.

A potential kidney-bone axis involved in the rapid minute-to-minute regulation of plasma Ca2+

Background

Understanding the regulation of mineral homeostasis and function of the skeleton as buffer for Calcium and Phosphate has regained new interest with introduction of the syndrome “Chronic Kidney Disease-Mineral and Bone Disorder”(CKD-MBD). The very rapid minute-to-minute regulation of plasma-Ca²⁺ (p-Ca²⁺) takes place via an exchange mechanism of Ca²⁺ between plasma and bone. A labile Ca storage pool exists on bone surfaces storing excess or supplying Ca when blood Ca is lowered. Aim was to examine minute-to-minute regulation of p-Ca²⁺ in the very early phase of acute uremia, as induced by total bilateral nephrectomy and to study the effect of absence of kidneys on the rapid recovery of p-Ca²⁺ from a brief induction of acute hypocalcemia.

Methods

The rapid regulation of p-Ca²⁺ was examined in sham-operated rats, acute nephrectomized rats(NX), acute thyroparathyrectomized(TPTX) rats and NX-TPTX rats.

Results

The results clearly showed that p-Ca²⁺ falls rapidly and significantly very early after acute NX, from 1.23 ± 0.02 to 1.06 ± 0.04 mM (p < 0.001). Further hypocalcemia was induced by a 30 min iv infusion of EGTA. Control groups had saline. After discontinuing EGTA a rapid increase in p-Ca²⁺ took place, but with a lower level in NX rats (p < 0.05). NX-TPTX model excluded potential effect of accumulation of Calcitonin and C-terminal PTH, both having potential hypocalcemic actions. Acute TPTX resulted in hypercalcemia, 1.44 ± 0.02 mM and less in NX-TPTX rats,1.41 ± 0.02 mM (p < 0.05). Recovery of p-Ca²⁺ from hypocalcemia resulted in lower levels in NX-TPTX than in TPTX rats, 1.20 ± 0.02 vs.1.30 ± 0.02 (p < 0.05) demonstrating that absence of kidneys significantly affected the rapid regulation of p-Ca²⁺ independent of PTH, C-PTH and CT.

Conclusions

P-Ca²⁺ on a minute-to-minute basis is influenced by presence of kidneys. Hypocalcemia developed rapidly in acute uremia. Levels of p-Ca²⁺, obtained during recovery from hypocalcemia resulted in lower levels in acutely nephrectomized rats. This indicates that kidneys are of significant importance for the ‘set-point’ of p-Ca²⁺ on bone surface, independently of PTH and calcitonin. Our results point toward existence of an as yet unknown factor/mechanism, which mediates the axis between kidney and bone, and which is involved in the very rapid regulation of p-Ca²⁺.

Standardization of DiaSorin and Roche automated third generation PTH assays with an International Standard: impact on clinical populations

BACKGROUND

Standardization of parathyroid hormone (PTH) assays is a major issue, especially in hemodialyzed (HD) patients. Two automated third generation PTH assays (Roche Elecsys and DiaSorin Liaison) are now available. These assays are specific for the (1-84) PTH and do not cross-react with the (7-84) fragment, contrary to second generation (intact) assays. We aimed to calibrate the two methods against the WHO International PTH Standard (IS) 95/646 to see if the two assays could provide comparable results in a population of healthy subjects, HD patients and patients suffering from primary hyperparathyroidism (PHP).

METHODS

We selected 79 healthy subjects and two populations of patients presenting PTH disorders: 56 HD and 27 PHP patients. We reconstituted the IS in a pool of human serum containing undetectable levels of 1-84 PTH and prepared 13 serum standards ranging from 0 to 2000 pg/mL. The standards were run on the two instruments to calibrate the assays on the IS. The different populations were run before and after restandardization.

RESULTS

As these kits were differently calibrated, the results obtained after restandarization were significantly different. Restandardization process improved concordance between assays and, taking the analytical variability of the two kits into account, the results could be considered to be similar.

CONCLUSIONS

Restandardization of automated third generation PTH assays with the WHO 1-84 PTH Standard significantly reduces inter-method variability. Reference ranges and raw values are totally transposable from one method to the other in healthy subjects, but also in diseased patients, e.g., with HD or those suffering from PHP.

High-throughput RNA sequencing reveals the effects of 2,2',4,4' -tetrabromodiphenyl ether on retina and bone development of zebrafish larvae

Background

2,2′,4,4′-Tetrabromodiphenyl ether (BDE47) is a prevalent environmental pollutant and has been demonstrated to be a serious toxicant in both humans and animals, but little is known about the molecular mechanism underlying its toxic effect on the early development of vertebrates. BDE47-treated zebrafish larvae were found to present the light-related locomotion reduction in our previous study, therefore, we aimed to use high throughput sequencing to investigate the possible reasons from a transcriptomic perspective.

Results

By exposing zebrafish embryos/larvae to 5 μg/l and 500 μg/l BDE47, we measured the influence of BDE47 on the mRNA expression profiles of zebrafish larvae until 6 days post-fertilization, using Illumina HiSeq 2000 sequencing. Differential expression analysis and gene enrichment analysis respectively revealed that a great number of genes, and gene sets based on two popular terminologies, were affected by the treatment of 500 μg/l BDE47. Among them, BDE47 caused changes in the retinal metabolism and related biological processes involving eye morphogenesis and visual perception, as confirmed by disordered photoreceptor arrangement and thickened bipolar cell layer of larval retina from histological observations. Other altered genes such as pth1a and collaborative cathepsin family exhibited disrupted bone development, which was consistent with the body curvature phenotype. The transcriptome of larvae was not significantly affected by the treatment of 5 μg/l BDE47, as well as the treatment of DMSO vehicle.

Conclusions

Our results suggest that high BDE47 concentrations disrupt the eye and bone development of zebrafish larvae based on both transcriptomic and morphological evidences. The abnormal visual perception may result in the alteration of dark adaption, which was probably responsible for the abnormal larval locomotion. Body curvature arose from enhanced bone resorption because of the intensive up-regulation of related genes. We also proposed the larval retina as a novel potential target tissue for BDE47 exposure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-014-1194-5) contains supplementary material, which is available to authorized users.

Vitamin D and parathyroid hormone status in pregnancy: effect on insulin sensitivity, β-cell function, and gestational diabetes mellitus

CONTEXT

Previous studies have yielded conflicting findings on the relationship between vitamin D deficiency/insufficiency and gestational diabetes mellitus (GDM). We hypothesized that PTH may be an underlying factor relevant to this potential association.

OBJECTIVE

This study sought to evaluate the effect of vitamin D and PTH status on insulin sensitivity, β-cell function, and glycemia in pregnancy.

SETTING AND DESIGN

Five-hundred-twenty-four women underwent a glucose challenge test (GCT) and oral glucose tolerance test (OGTT) in late second/early third trimester. The GCT/OGTT identified 142 women with GDM, 94 with gestational impaired glucose tolerance, 163 with an abnormal GCT and normal OGTT, and 125 with normal GCT and OGTT.

MAIN OUTCOMES

Glycemia was assessed by glucose tolerance status and area under the glucose curve (AUC(gluc)) on the OGTT. Insulin sensitivity and β-cell function were assessed by Matsuda index and Insulin Secretion-Sensitivity Index-2 (ISSI-2), respectively.

RESULTS

There were 166 women (31.7%) with vitamin D deficiency (25-OH-D < 50 nmol/L), 178 (34%) with insufficiency (25-OH-D ≥ 50 nmol/L and < 75 nmol/L), and 180 (34.3%) with sufficiency (25-OH-D ≥ 75 nmol/L). Vitamin D status was not associated with Matsuda index, ISSI-2, AUC(gluc), or glucose tolerance status. In contrast, ISSI-2 decreased and AUC(gluc) increased across ascending tertiles of PTH (P = .06 and P = .002, respectively). Indeed, the prevalence of GDM progressively increased from the first (22.6%) to second (25.8%) to third (33.5%) tertile of PTH (P < .001). On logistic regression analyses, the third tertile of PTH was independently associated with GDM (adjusted OR = 1.82; 95% CI, 1.09-3.02; P = .022), whereas vitamin D deficiency and insufficiency were not significant predictors of GDM.

CONCLUSIONS

Increased PTH, rather than vitamin D deficiency/insufficiency, is independently associated with dysglycemia in pregnancy.

N-terminal phosphorylation of parathyroid hormone (PTH) abolishes its receptor activity

The parathyroid hormone (PTH) is an 84-residue peptide, which regulates the blood Ca(2+) level via GPCR binding and subsequent activation of intracellular signaling cascades. PTH is posttranslationally phosphorylated in the parathyroid glands; however, the functional significance of this processes is not well characterized. In the present study, mass spectrometric analysis revealed three sites of phosphorylation, and NMR spectroscopy assigned Ser1, Ser3, and Ser17 as modified sites. These sites are located at the N-terminus of the hormone, which is important for receptor recognition and activation. NMR shows further that the three phosphate groups remotely disturb the α-helical propensity up to Ala36. An intracellular cAMP accumulation assay elucidated the biological significance of this phosphorylation because it ablated the PTH-mediated signaling. Our studies thus shed light on functional implications of phosphorylation at native PTH as an additional level of regulation.

Intact parathyroid hormone and whole parathyroid hormone assay results disagree in hemodialysis patients under cinacalcet hydrochloride therapy

Background

The parathyroid gland secretes 1-84 and 7-84 parathyroid hormone (PTH) fragments, and its regulation is dependent on stimulation of the extracellular calcium-sensing receptor. While the intact PTH system detects both PTH fragments, the whole PTH system detects the 1-84PTH but not the 7-84PTH. Cinacalcet hydrochloride (CH) binds to calcium-sensing receptor as a calcimimetic. Here we investigated the role of CH treatment in the assessment of parathyroid gland function.

Methods

Stable adult dialysis patients for whom CH therapy was planned were included. Patients for whom CH therapy was not planned were simultaneously included as the control group.

Results

The CH group (n = 44) showed significantly higher circulating levels of Ca, intact PTH, and whole PTH, before the CH treatment than the control group (n = 112). The Ca, intact PTH, and whole PTH levels decreased along with the CH therapy, and the Ca levels became comparable in the 8th week of treatment and thereafter. The CH group in the 8th week and thereafter showed significantly lower whole/intact PTH ratios than the control group, while the whole/intact PTH ratio was not significantly different between before and during the CH therapy. A multiple regression analysis revealed that the whole/intact PTH ratio was almost constant, but both the serum Ca level and a CH therapy could potentially modify the fixed number. When the whole PTH levels were estimated by intact PTH levels using the relationship between them in the control group, the levels were clearly overestimated in the CH group.

Conclusions

Although the direct effect of CH on the whole/intact PTH ratio is masked by its hypocalcemic action, we could successfully demonstrate that the ratio in CH users is lower than that in the non-users with comparable levels of serum Ca. Evaluating parathyroid function with intact PTH according to the clinical practice guidelines in patients being treated with CH may lead to significant overestimation and subsequent overtreatment.

News on biomarkers in CKD-MBD

The increased awareness of the potential role played by mineral and bone disorder in the appearance of cardiovascular disease in renal patients has produced research efforts aimed at discovering possible pathogenic links. Accordingly, the diagnostic significance of the classic bone markers of mineral disorders and of the new markers in the setting of chronic kidney disease-mineral and bone disorders (CKD-MBD) needs to be re-evaluated along with increasing information. In this article we include classic markers of bone metabolism and some of the noncollagenous bone proteins that are gaining experimental and clinical significance in CKD-MBD. Among classic markers of secondary hyperparathyroidism and of renal osteodystrophy, we analyzed parathyroid hormone, alkaline phosphatase, tartrate-resistant acid phosphatase, and bone collagen-derived peptides. We underlined, for each, the relevance of parent proteins (peptides or isoforms) that affect assay methods and, eventually, the diagnostic or prognostic significance. Also, we considered their relationship with cardiovascular mortality. Among the numerous noncollagenous bone proteins, we examined matrix Gla protein (MGP), osteocalcin (OC), osteoprotegerin, and the small integrin-binding ligand N-linked glycoprotein family. For MGP and OC we report the relevant involvement with the process of calcification (MGP) and with glucose and energy metabolism (OC). Both of these proteins require vitamin K to become active and this is a specific problem in renal patients who frequently are deficient of this vitamin. Finally, recent acquisitions on the fascinating family of the small integrin-binding ligand N-linked glycoprotein proteins are recapitulated briefly to underline their potential clinical interest and their complex involvement with all aspects of CKD-MBD. Their diagnostic role in clinical practice awaits further studies.

Prognostic value of serum parathyroid hormone level in acute decompensated heart failure

BACKGROUND

Secondary hyperparathyroidism develops as a compensatory response to chronic heart failure (HF) and renal failure. The role of parathyroid hormone (PTH) level in acute decompensated HF remains unclear. The aim of this study was therefore to investigate the relationships among mortality, renal function, and serum PTH level in acute decompensated HF patients. METHODS AND RESULTS: A total of 266 consecutive patients admitted for acute decompensated HF without acute coronary syndrome (78±12 years; 48% male) were enrolled. Demographic, clinical, and laboratory characteristics were obtained on admission.During 1-year follow-up, 65 patients (24%) died. Serum PTH level on admission was within the normal range (10-65 pg/ml) in 108 patients (41%), of whom 39 (15%) had low-normal PTH (10-40 pg/ml). On Kaplan-Meier analysis all-cause mortality was significantly higher in patients with low-normal PTH than in those with high-normal (40-65 pg/ml) or high (>65 pg/ml) PTH (log-rank test). On univariate and multivariate Cox regression analysis, low-normal PTH was significantly associated with increased all-cause mortality (unadjusted HR, 2.88; 95% CI: 1.69-4.91; P<0.001; adjusted HR, 3.84; 95% CI: 1.54-9.57; P=0.004).

CONCLUSIONS

In patients with acute decompensated HF resulting in hospitalization, low-normal PTH on admission is associated with increased all-cause mortality, regardless of renal function.

Parathyroidectomy improves cardiovascular outcome in nondiabetic dialysis patients with secondary hyperparathyroidism

OBJECTIVE

Secondary hyperparathyroidism and its associated abnormalities in mineral metabolism and haemodynamic changes increase the cardiovascular risk in patients with end-stage renal disease (ESRD). Our objective was to determine the association of parathyroidectomy (PTX) with major cardiovascular events in nondiabetic dialysis patients with severe secondary hyperparathyroidism (SHPTH).

DESIGN AND PATIENTS

We performed a cohort study with fifty-three nondiabetic ESRD patients who were treated with maintenance haemodialysis and who had intact parathyroid hormone (PTH) levels > 800 pg/ml. Participants received either only medical therapy or medical therapy and total PTX with autotransplantation for SHPTH.

MEASUREMENTS

We evaluated the associations between PTX and major cardiovascular events including death, cerebrovascular accident and myocardial infarction. The biochemical and haemodynamic changes associated with PTX were measured.

RESULTS

During the mean follow-up of 72 months, twenty-three patients received only medical treatment (medical group) while thirty patients underwent PTX in addition to medical treatment (PTX group). The two groups were comparable in respect of baseline characteristics. PTX group was found to be associated with a reduced incidence of major cardiovascular events (P = 0·021). A multiple Cox regression analysis showed that the variable significantly associated with major cardiovascular events was treatment modality (medical therapy vs medical therapy and parathyroidectomy, hazard ratio = 26·12, 95% CI = 1·30-526·27, P = 0·033). Blood pressure, haemoglobin, alkaline phosphatase, calcium, phosphate and calcium × phosphate product significantly improved after PTX.

CONCLUSIONS

PTX was associated with better cardiovascular outcome in nondiabetic dialysis patients with severe SHPTH.

Impact of parathyroidectomy on cardiovascular outcomes and survival in chronic hemodialysis patients with secondary hyperparathyroidism. A retrospective study of 50 cases prior to the calcimimetics era

BACKGROUND

In chronic hemodialysis patients with secondary hyperparathyroidism, pathological modifications of bone and mineral metabolism increase the risk of cardiovascular morbidity and mortality. Parathyroidectomy, reducing the incidence of cardiovascular events, may improve outcomes; however, its effects on long-term survival are still subject of active research.

METHODS

From January 2004 to December 2006, 30 hemodialysis patients, affected by severe and unresponsive secondary hyperparathyroidism, underwent parathyroidectomy - 15 total parathyroidectomy and 15 total parathyroidectomy + subcutaneous autoimplantation. During a 5-year follow-up, patients did not receive a renal transplantation and were evaluated for biochemical modifications and major cardiovascular events - death, cardiovascular accidents, myocardial infarction and peripheral vascular disease. Results were compared with those obtained in a control group of 20 hemodialysis patients, affected by secondary hyperparathyroidism, and refusing surgical treatment, and following medical treatment only.

RESULTS

The groups were comparable in terms of age, gender, dialysis vintage, and comorbidities. Postoperative cardiovascular events were observed in 18/30 - 54% - surgical patients and in 4/20 - 20%- medical patients, with a mortality rate respectively of 23.3% in the surgical group vs. 15% in the control group. Parathyroidectomy was not associated with a reduced risk of cardiovascular morbidity and survival rate was unaffected by surgical treatment.

CONCLUSIONS

In secondary hyperparathyroidism hemodialysis patients affected by severe cardiovascular disease, surgery did not modify cardiovascular morbidity and mortality rates. Therefore, in secondary hyperparathyroidism hemodialysis patients, resistant to medical treatment, only an early indication to calcimimetics, or surgery, in the initial stage of chronic kidney disease - mineral bone disorders, may offer a higher long-term survival. Further studies will be useful to clarify the role of secondary hyperparathyroidism in determining unfavorable cardiovascular outcomes and mortality in hemodialysis population.

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.

Runx1 is critical for PTH-induced onset of mesenchymal progenitor cell chondrogenic differentiation

Parathyroid hormone (PTH) plays a critical role in the regulation of chondrogenesis. In this study, we have found for the first time that Runt-related transcription factor 1 (Runx1) contributes to PTH-induced chondrogenesis. Upon PTH treatment, limb bud mesenchymal progenitor cells in micromass culture showed an enhanced chondrogenesis, which was associated with a significant increase of chondrogenic marker gene expression, such as type II collagen and type X collagen. Runx1 was also exclusively expressed in cells treated with PTH at the onset stage of chondrogenesis. Knockdown of Runx1 completely blunted PTH-mediated chondrogenesis. Furthermore, PTH induced Runx1 expression and chondrogenesis were markedly reduced by inhibition of protein kinase A (PKA) signaling. Taken together, our present study indicates that chondrogenesis induced by PTH in mesenchymal progenitor cells is mediated by Runx1, which involves the activation of PKA. These data provide a novel insight into understanding the molecular mechanisms behind PTH-enhanced cartilage regeneration.

PTH replacement therapy of hypoparathyroidism

Hypoparathyroidism is characterized by hypocalcemia with inappropriately low parathyroid hormone (PTH) levels. Bone turnover is abnormally low and bone mineral density (BMD) is typically increased. Plasma calcium levels can be normalized by treatment with calcium supplements and vitamin D analogs, but bone turnover remains low and patients complain of a reduced quality of life (QoL). During recent years, a number of studies have shown that PTH replacement therapy (PTH-RT) may maintain calcium levels within the normal range, while the need for calcium and vitamin D supplements is reduced. In the initial response to subcutaneous PTH injections once or twice daily, bone turnover is overstimulated. BMD increases in cancellous bone, but decreases in cortical bone due to an increased porosity. Microcomputed tomography scans and histomorphometric studies on bone biopsies have shown changes similar to the well-known bone anabolic effects of PTH treatment in osteoporosis rather than a normalization of bone remodeling balancing the anabolic and catabolic effects of PTH. Most recently, continuous PTH delivery by pump was shown to increase the levels of bone markers into the normal range (without overstimulation of bone turnover) and with a normalization of renal calcium excretion. As PTH has a short plasma half-life, these findings indicate that exposure to PTH once or twice daily is not sufficient to reestablish a calcium homeostasis and bone metabolism that resembles normal physiology. Further studies should assess the effects of continuous PTH exposure by pump delivery (or multiple daily injections) on BMD and bone histology, as well as the effects of PTH-RT on indices of QoL.

From parathyroid hormone to cytosolic Ca2+ signals

PTHR1 (type 1 parathyroid hormone receptors) mediate the effects of PTH (parathyroid hormone) on bone remodelling and plasma Ca2+ homoeostasis. PTH, via PTHR1, can stimulate both AC (adenylate cyclase) and increases in [Ca2+]i (cytosolic free Ca2+ concentration), although the relationship between the two responses differs between cell types. In the present paper, we review briefly the mechanisms that influence coupling of PTHR1 to different intracellular signalling proteins, including the G-proteins that stimulate AC or PLC (phospholipase C). Stimulus intensity, the ability of different PTH analogues to stabilize different receptor conformations ('stimulus trafficking'), and association of PTHR1 with scaffold proteins, notably NHERF1 and NHERF2 (Na+/H+ exchanger regulatory factor 1 and 2), contribute to defining the interactions between signalling proteins and PTHR1. In addition, cAMP itself can, via Epac (exchange protein directly activated by cAMP), PKA (protein kinase A) or by binding directly to IP3Rs [Ins(1,4,5)P3 receptors] regulate [Ca2+]i. Epac leads to activation of PLCϵ, PKA can phosphorylate and thereby increase the sensitivity of IP3Rs and L-type Ca2+ channels, and cAMP delivered at high concentrations to IP3R2 from AC6 increases the sensitivity of IP3Rs to InsP3. The diversity of these links between PTH and [Ca2+]i highlights the versatility of PTHR1. This versatility allows PTHR1 to evoke different responses when stimulated by each of its physiological ligands, PTH and PTH-related peptide, and it provides scope for development of ligands that selectively harness the anabolic effects of PTH for more effective treatment of osteoporosis.

Acute and chronic regulation of circulating PTH: significance in health and in disease

Circulating human parathyroid hormone (PTH) is immunoheterogenous. It is composed of 80% carboxyl-terminal (C) fragments and of 20% PTH(1-84). This composition contrasts with the biological activity of the hormone, which is only related to PTH(1-84), creating a paradox between circulating PTH composition and PTH bioactivity. PTH molecular forms are either secreted by the parathyroid glands or generated by the peripheral metabolism of PTH(1-84) in the liver. The kidney has a major role in the disposal of C-PTH fragments. Secretion of PTH molecular forms by the parathyroid glands is highly regulated under a variety of clinical conditions, suggesting that C-PTH fragments could exert some biological effects of their own. Recent data suggest that C-PTH fragments can exert biological actions opposite to those of PTH(1-84) by acting on a C-PTH receptor not yet cloned. They can decrease calcium concentration, phosphate excretion, bone resorption and 1,25(OH)₂ synthesis. The clinical implications of this new concept are reviewed.

Prognostic role of serum parathyroid hormone levels in advanced prostate cancer patients undergoing zoledronic acid administration

BACKGROUND

Secondary hyperparathyroidism is frequent in prostate cancer patients with bone metastases, and this condition is worsened by the administration of potent bisphosphonates. Serum parathyroid hormone (PTH) elevation can impair the efficacy of these drugs in terms of survival.

METHODS

The prognostic role of elevated serum PTH levels at baseline and after 3 months of zoledronic acid administration was assessed prospectively in 643 bone metastatic prostate cancer patients enrolled in a prospective randomized, placebo-controlled study.

RESULTS

On multivariate analysis, after adjusting for major prognostic factors and bone turnover markers, elevated baseline serum PTH level was negatively associated with overall survival (hazard ratio [HR], 1.448; 95% confidence interval [CI], 1.045-2.006; p < .03) in zoledronic acid-treated patients but not in placebo-treated patients. In patients with normal baseline PTH levels, there was a trend but insignificant association between zoledronic acid administration and a better survival outcome than with placebo (HR, 0.81; 95% CI, 0.65-1.01; p = .065), whereas a trend in the opposite direction was observed in patients with elevated PTH levels (HR, 1.45; 95% CI, 0.87-2.39; p = .151); interaction test, p = .040. Elevated serum PTH level after 3 months of zoledronic acid treatment was not significantly associated with survival outcome.

CONCLUSIONS

Secondary hyperparathyroidism has a negative prognostic impact in metastatic prostate cancer patients undergoing zoledronic acid administration. Counteracting elevated PTH levels by adequate doses of vitamin D may improve the efficacy of this drug.

Aldosterone and parathyroid hormone: a precarious couple for cardiovascular disease

Animal and human studies support a clinically relevant interaction between aldosterone and parathyroid hormone (PTH) levels and suggest an impact of the interaction on cardiovascular (CV) health. This review focuses on mechanisms behind the bidirectional interactions between aldosterone and PTH and their potential impact on the CV system. There is evidence that PTH increases the secretion of aldosterone from the adrenals directly as well as indirectly by activating the renin-angiotensin system. Upregulation of aldosterone synthesis might contribute to the higher risk of arterial hypertension and of CV damage in patients with primary hyperparathyroidism. Furthermore, parathyroidectomy is followed by decreased blood pressure levels and reduced CV morbidity as well as lower renin and aldosterone levels. In chronic heart failure, the aldosterone activity is inappropriately elevated, causing salt retention; it has been argued that the resulting calcium wasting causes secondary hyperparathyroidism. The ensuing intracellular calcium overload and oxidative stress, caused by PTH and amplified by the relative aldosterone excess, may increase the risk of CV events. In the setting of primary aldosteronism, renal and faecal calcium loss triggers increased PTH secretion which in turn aggravates aldosterone secretion and CV damage. This sequence explains why adrenalectomy and blockade of the mineralocorticoid receptor tend to decrease PTH levels in patients with primary aldosteronism. In view of the reciprocal interaction between aldosterone and PTH and the potentially ensuing CV damage, studies are urgently needed to evaluate diagnostic and therapeutic strategies addressing the interaction between the two hormones.

Antagonists of LRP6 regulate PTH-induced cAMP generation

LRP6 is a common coreceoptor for different G protein-coupled seven-transmembrane receptors in production of cAMP. Extracelluar proteins sclerostin and DKK1, initially identified as antagonists for Wnt signaling by binding to LRP6, are negative regulators for bone formation. Here, we show that both sclerostin and DKK1 inhibit PTH-stimulated cAMP production. In addition, PTH suppresses expression of sclerostin in osteocytes in mice. We also found that sclerostin and DKK1 binds to LRP6 as antagonists to increase the availability of LRP6 to facilitate PTH signaling in a positive-feedback fashion. These studies reveal a previously unrecognized function of sclerostin and DKK1, which provides an alternative explanation for the application of sclerostin and DKK1 neutralization on enhancing bone formation as a potential therapy for skeletal diseases.

Parathyroid hormone measurement in chronic kidney disease--an evolving issue for the nephrologist and the clinical laboratorist: minireview

Parathyroid hormone (PTH) is the polypeptide hormone produced by the parathyroid glands, which plays a central role in calcium homeostasis. Circulating PTH must be measured regularly in patients with chronic kidney disease (CKD)--mineral and bone disorders (MBD) to monitor and to adapt treatment with the aim of maintaining PTH levels within a defined narrow range of optimal values for each stage of CKD. Often, for the nephrologists, it is not easy to determine what PTH levels are clinically appropriate. Moreover, the PTH determination also shows many criticisms from the laboratory point of view and there is a clear need to standardize PTH measurements in every phase of the process: pre-analytical, analytical and post-analytical. In this review, all these aspects are summarized with particular reference to the most recent opportunities to improve PTH assays quality on the whole. To this aim, a closer cooperation between nephrologists and clinical laboratories is undoubtedly necessary.

Peptide and protein drugs: the study of their metabolism and catabolism by mass spectrometry

Peptide and protein drugs have evolved in recent years into mainstream therapeutics, representing a significant portion of the pharmaceutical market. Peptides and proteins exhibit highly diverse structures, broad biological activities as hormones, neurotransmitters, structural proteins, metabolic modulators and therefore have a significant role as both therapeutics and biomarkers. Understanding the metabolism of synthetic or biotechnologically derived peptide and protein drugs is critical for pharmaceutical development as metabolism has a significant impact on drug efficacy and safety. Although the same principles of pharmacokinetics and metabolism of small molecule drugs apply to peptide and protein drugs, there are few notable differences. Moreover, the study of peptide and protein drug metabolism is a rather complicated process which requires sophisticated analytical techniques, and mass spectrometry based approaches have provided the capabilities for efficient and reliable quantification, characterization, and metabolite identification. This review article will focus on the current use of mass spectrometry for the study of the metabolism of peptide and protein drugs.

The role of the liver and kidneys in the pharmacokinetics of subcutaneously administered teriparatide acetate in rats

Teriparatide acetate, a synthetic polypeptide fragment consisting of human parathyroid hormone residues 1-34 [hPTH(1-34)], is a bone anabolic agent used to treat osteoporosis. The present study was conducted to characterise the pharmacokinetics of teriparatide acetate in rats after subcutaneous administration. Teriparatide was rapidly absorbed into the circulation and eliminated immediately. No intact teriparatide was detected in the urine. To elucidate the mechanism of teriparatide metabolism, we performed in vivo and in vitro studies using the radiolabelled bioactive analogue, [(125)I]-[Nle(8,18),Tyr(34)]-hPTH(1-34). After subcutaneous administration, the concentration of analogue metabolites increased in the plasma time-dependently. The concentration in the kidneys was more than 3-fold the concentration in the liver. In vitro analyses suggested that kidney radioactivity was associated with degraded bioactive analogue. In model rats, renal failure, but not hepatic failure, affected the pharmacokinetics of teriparatide acetate, which accounted for the decrease in the clearance of teriparatide. In conclusion, our results suggest that after subcutaneous administration of teriparatide acetate, teriparatide is rapidly absorbed and distributed to the liver or kidneys, where it is immediately degraded. The kidneys play a particularly important role in the distribution and metabolism of teriparatide, but not its excretion.

Primary hyperparathyroidism with low intact PTH levels in a 14-year-old girl

CONTEXT

Primary hyperparathyroidism (PHPT) is usually associated with hypercalcemia and inappropriately elevated serum PTH.

OBJECTIVE

Our objective was to identify the reason(s) for a low serum intact PTH in a child with PHPT.

SUBJECT AND METHODS

A 14-yr-old Caucasian girl presented with pancreatitis, nephrolithiasis, hypercalcemia ranging from 13.2 to 17.5 mg/dl, and a low serum intact PTH. She had an ultrasound and technetium-99m-sestamibi scintigraphy confirming the presence of a parathyroid adenoma.

RESULTS

The preoperative serum intact PTH assays performed at Quest Diagnostics, Nichols Institute, were low even after serial dilutions, whereas the intraoperative turbo PTH assay was elevated at 3618 pg/ml. C-terminal and cyclase-activating PTH assays for PTH were also elevated. PTH gene sequence analysis performed from DNA extracted both from the parathyroid adenoma and the patient's peripheral blood leukocytes was negative for a mutation in the PTH gene sequence.

CONCLUSIONS

The contrasting values on the intact PTH assay and the turbo PTH assay suggest that the adenoma was producing an aberrant PTH molecule that was not detected by the routine intact PTH assay. Because there was no change in PTH gene sequence, this could be indicative of a posttranslational change in the PTH molecule that would not be recognized solely by DNA sequencing. Therefore, a low or normal PTH measurement against the backdrop of clinical and biochemical hyperparathyroidism needs measurement with a variety of assays.

Parathyroid hormone analogues in the treatment of osteoporosis

Osteoporosis is characterized by the occurrence of fragility fractures. Over the past years, various treatment options have become available, mostly antiresorptive agents such as bisphosphonates. However, antiresorptive therapy cannot restore bone mass and structure that has been lost due to increased remodeling. In this case, recombinant human parathyroid hormone (PTH) analogues-the full-length PTH(1-84) or the shortened molecule PTH(1-34), which is also known as teriparatide-present the possibility of increasing the formation of new bone substance by virtue of their anabolic effects. The bone formation induced by PTH analogues not only increases BMD or bone mass but also improves the microarchitecture of the skeleton, thereby leading to improved strength of bone and increased mechanical resistance. Controlled trials have shown that both analogues significantly reduce the incidence of vertebral fractures, and PTH(1-34) also reduces the risk of nonvertebral fractures. The need for daily self-injection and the higher cost compared with other forms of treatment limit the widespread use of PTH analogues. Nevertheless, treatment with PTH analogues should be considered in postmenopausal women and men with severe osteoporosis, as well as in patients on established glucocorticoid treatment with a high fracture risk. Concurrent therapy with antiresorptive agents should be avoided, but sequential therapy with these agents might consolidate the beneficial effects on the skeleton.

Parathyroid hormone and the regulation of cell cycle in colon adenocarcinoma cells

Parathyroid hormone (PTH) functions as a major mediator of bone remodeling and as an essential regulator of calcium homeostasis. In this study, we investigated the role of PTH in the regulation of the cell cycle in human colon adenocarcinoma Caco-2 cells. Flow cytometry analysis revealed that PTH (10(-8)M, 12-24h) treatment increases the number of cells in the G0/G1 phase and diminishes the number in both phases S and G2/M. In addition, analysis by Western blot showed that the hormone increases the expression of the inhibitory protein p27Kip1 and diminishes the expression of cyclin D1, cyclin D3 and CDK6. However, the amounts of CDK4, p21Cip1, p15INK4B and p16INK4A were not different in the absence or presence of PTH. Inhibitors of PKC (Ro-318220, bisindolylmaleimide and chelerythine), but not JNK (SP600125) and PP2A (okadaic acid and calyculin A), reversed PTH response in Caco-2 cells. Taken together, our results suggest that PTH induces G0/G1 phase arrest of Caco-2 intestinal cells and changes the expression of proteins involved in cell cycle regulation via the PKC signaling pathway.

Calcimimetics inhibit renal pathology in rodent nephronophthisis

The development and progression of renal cysts appears to be driven by reduced cellular calcium and increased cyclic adenosine monophosphate (cAMP) from G-protein-coupled receptors. To test whether treatment with a calcimimetic that stimulates the G-protein-coupled calcium-sensing receptor might normalize cystic epithelial cell intracellular calcium and cAMP, thereby inhibiting cyst progression, we used pcy mice. These animals develop cysts principally in the collecting duct, as do humans with nephronophthisis (NPHP). We administered the calcimimetic R-568 mixed in their food at early or late stages in the pathogenesis of cyst formation. The treatment reduced cyst enlargement, and the early treatment inhibited development of renal fibrosis. Although the effect of later treatment was more modest, both stages of the disease responded positively to treatment. Additionally, R-568 decreased total kidney cAMP in the pcy mice and, in vitro, decreased cAMP levels and cell proliferation, while increasing intracellular calcium in immortalized human autosomal recessive polycystic kidney disease renal epithelial cells. The latter two effects were unique to R-568 and not replicated by raising extracellular calcium. Thus, treating pcy mice with R-568 was effective in reducing cyst progression in this rodent model of NPHP. Direct studies will be needed to determine whether these results can be applied to the human disease.