Human parathyroid hormone carboxyterminal peptide (53-84) stimulates alkaline phosphatase activity in dexamethasone-treated rat osteosarcoma cells in vitro

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.

Parathormone et maladie rénale chronique

The serum parathyroid hormone (PTH) rises in chronic kidney disease (CKD) and induces renal bone disease as well as other organ damage. The bone disease guidelines were released by the K-DOQI in 2003 in order to help physicians to improve bone management at all different CKD stages. However, many different PTH commercial assays are available today and some questions are raised concerning the interpretation, the validity and the practical choice of these different measurements. After reviewing PTH biosynthesis and metabolism, we will describe the regulation of different PTH fragments (particularly 1-84 and 7-84) and the various types of PTH assays. In compromised clinical situations, bone biopsy still remains the golden standard assessment of bone disease, and it will be helpful to clarify the interest of new 3rd generation PTH measurements. At present, we do not dispose of valid therapeutic recommendations using 3rd generation tests, as well as the relevance of the ratio PTH 1-84/7-84.

Identification of carboxyl-terminal peptide fragments of parathyroid hormone in human plasma at low-picomolar levels by mass spectrometry

For decades, researchers have tried to identify the primary structures of circulating carboxyl-terminal parathyroid hormone (C-PTH) peptide fragments that may be present at only picomolar levels in human plasma. Although immunoassays and radiosequencing techniques have provided valuable fragment characterizations, no analysis has successfully determined their exact primary structures. In this work, for the first time, four human C-PTH peptide fragments, hPTH(34-84), hPTH(37-84), hPTH(38-84), and hPTH(45-84), have been identified from human plasma using MS-based methods. C-PTH peptide fragments were isolated from plasma samples by immunoaffinity extraction. The eluate was analyzed by capillary LC fractionation followed by MALDI-TOF-MS or by on-line coupling of nano-LC with ESI-TOF-MS. Both the MALDI- and the ESI-based approaches were capable of detecting C-PTH peptide fragments in human plasma at <10 pmol/L. The MALDI-TOF approach was effective in preliminary searches for C-PTH peptide fragments, but the use of high laser power limited the resolution necessary for accurate C-PTH peptide identification. The high mass resolution (10,000) and accuracy (10 ppm) attained by the ESI-TOF approach enabled unambiguous identification of these peptides. The four C-PTH peptide fragments identified in plasma samples from patients with chronic renal insufficiency were also found in the plasma of healthy women receiving recombinant human PTH either by subcutaneous injection or by intravenous infusion. This newly developed analytical capability should greatly enhance the understanding of PTH metabolism and parathyroid gland function.

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.

The parathyroid polyhormone hypothesis revisited

The parathyroid polyhormone hypothesis holds that peptides derived from the metabolism of parathyroid hormone (PTH) (so-called C-terminal fragments) are themselves biologically active and that their effects are mediated by a novel 'C-terminal receptor.' The evidence supporting these assertions is extensive but remains inconclusive. This Commentary focuses on in vivo pharmacology studies that provide information relevant to understanding the physiological significance of C-terminal fragments. The more recent studies of this sort provide compelling evidence that the bioactivity of C-terminal fragments is likely to become physiologically relevant in settings of secondary hyperparathyroidism. In this condition, circulating levels of C-terminal fragments greatly exceed those of PTH. There is convincing evidence that the hypocalcemic effect of C-terminal fragments results from direct actions on the skeleton that inhibit bone resorption. On the other hand, there are few if any results of in vivo studies suggesting a role for C-terminal fragments in more physiological settings, at least when parameters associated with systemic calcium homeostasis are assessed.

The need for reliable serum parathyroid hormone measurements

Serum parathyroid hormone (PTH) is a recognized marker of bone remodeling in patients with renal osteodystrophy. However, identification of N-terminal truncated PTH fragments and a new form of PTH that interfere with second-generation PTH assays may be responsible for the great variability of PTH values and the difficulties of implementing the recommendations of the National Kidney Foundation/Kidney Disease Outcomes Quality Initiative.

Circulating PTH molecular forms: What we know and what we don't

Circulating parathyroid hormone (PTH) molecular forms have been identified by three generations of PTH assays after gel chromatography or high-performance liquid chromatography fractionation of serum. Carboxyl-terminal (C) fragments missing the amino-terminal (N) structure of PTH(1-84) were identified first. They represent 80% of circulating PTH in normal individuals and up to 95% in renal failure patients. They are regulated by calcium (Ca) slightly differently than PTH(1-84), occurring in a relatively smaller proportion relative to the latter in hypocalcemia but in a much larger proportion in hypercalcemia. Synthetic C-PTH fragments do not bind to the PTH/PTHrP type I receptor and are not implicated in the classical biological effect of PTH(1-84). They bind to a different C-PTH receptor and exert biological actions on bone that are opposite to those of PTH(1-84). The integrity of the distal C-structure appears to be important for these biological effects, and it is uncertain if all C-PTH fragments are intact up to position 84. A second category of C-PTH fragment has a partially preserved N-structure. They are called non-(1-84) PTH or N-truncated fragments. They react in Intact (I)-PTH assays but not in PTH assays with a 1-4 epitope. They are acutely regulated by Ca(2+) concentration. They also exert similar hypocalcemic and antiresorptive effects but have 10-fold greater affinity for the C-PTH receptor compared to other C-PTH fragments. Even if they represent only 10% of all C-PTH fragments, they could be as relevant biologically. An N form of PTH other than PTH(1-84) has been identified in the circulation. It reacts very well in PTH assays with a 1-4 epitope but poorly in I-PTH assay with a 12-18 epitope. It is oversecreted in severe primary and secondary hyperparathyroidism and in parathyroid cancers. Its biological activity is still unknown. Overall, these studies suggest that PTH(1-84) and C-PTH fragments are regulated differently to exert opposite biological effects on bone via two different receptors. This may serve to control bone turnover and Ca concentration more efficiently.

Difficulties in Achieving the K/DOQI Practice Guidelines for Bone and Mineral Metabolism

Hyperparathyroidism and alterations in bone and mineral metabolism are known to occur early in the course of chronic kidney disease (CKD). The etiology of these abnormalities is multifactorial. Phosphate retention and alterations in vitamin D metabolism play important roles. The recent practice guidelines from the National Kidney Foundation (NKF) have emphasized the need to approach these problems early in the course of CKD by measuring the levels of parathyroid hormone when glomerular filtration rates (GFRs) fall to less than 60 ml/min. If hyperparathyroidism is detected, then treatment must be undertaken to try to prevent its progression. Strict guidelines have been proposed for the management of these alterations in bone and mineral metabolism when kidney disease progresses to CKD stage V. Although the initial recommendations were often opinion based, evidence is accumulating to support these views. However, with our current therapeutic armamentarium, there is considerable difficulty in achieving these practice guidelines for calcium, phosphorus, calcium-phosphorus product, and parathyroid hormone on a sustained basis. New therapeutic agents are becoming available that will help with phosphorus control and control of hyperparathyroidism, and minimize the calcium load. Additional beneficial effects of vitamin D analogs are being uncovered and are being intensively investigated. It is hoped that with attention to these practice guidelines, control of the abnormalities of mineral metabolism will be improved and will lead to improved patient outcomes.

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

PTH is a major systemic regulator of the concentrations of calcium, phosphate, and active vitamin D metabolites in blood and of cellular activity in bone. Intermittently administered PTH and amino-terminal PTH peptide fragments or analogs also augment bone mass and currently are being introduced into clinical practice as therapies for osteoporosis. The amino-terminal region of PTH is known to be both necessary and sufficient for full activity at PTH/PTHrP receptors (PTH1Rs), which mediate the classical biological actions of the hormone. It is well known that multiple carboxyl-terminal fragments of PTH are present in blood, where they comprise the major form(s) of circulating hormone, but these fragments have long been regarded as inert by-products of PTH metabolism because they neither bind to nor activate PTH1Rs. New in vitro and in vivo evidence, together with older observations extending over the past 20 yr, now points strongly to the existence of novel large carboxyl-terminal PTH fragments in blood and to receptors for these fragments that appear to mediate unique biological actions in bone. This review traces the development of this field in the context of the evolution of our understanding of the "classical" receptor for amino-terminal PTH and the now convincing evidence for these receptors for carboxyl-terminal PTH. The review summarizes current knowledge of the structure, secretion, and metabolism of PTH and its circulating fragments, details available information concerning the pharmacology and actions of carboxyl-terminal PTH receptors, and frames their likely biological and clinical significance. It seems likely that physiological parathyroid regulation of calcium and bone metabolism may involve receptors for circulating carboxy-terminal PTH ligands as well as the action of amino-terminal determinants within the PTH molecule on the classical PTH1R.

PTH revisited11This paper is dedicated to Professor Thomas E. Andreoli. Pigmæi gigantum humeris impositi plusquam ipsi gigantes vident.22Original studies were supported by National Institutes of Health grant DK-54171

Recent investigations of parathyroid hormone (PTH) have advanced our understanding of its circulating forms as well as its action. It is now clear that first-generation immunoradiometric assays of so-called intact "PTH" not only measured full-length PTH(1-84) but also recognized large PTH fragments lacking the amino-terminus. New, second generation assays detect only full-length PTH. Under diverse pathological settings, second generation assays display lower levels of PTH (1-84). By measuring full-length PTH (bioactive PTH) and the combined full-length plus amino-terminal PTH fragments, the amount of non-PTH(1-84) in circulation can be estimated. The primary amino-terminal fragment is likely to be PTH(7-84). A considerable controversy surrounds the pathological significance of PTH(7-84) and its relation to adynamic bone disease. While these findings were emerging, other work uncovered the apparent basis by which PTH receptors signal through cAMP in some instances but through Ca/inositol phosphate in others. This signaling switch is dictated by the cytoplasmic adapter protein NHERF1 (EBP50), which is expressed in a cell-selective fashion. Other provocative findings may provide a means of unifying determinations of PTH(7-84) with the effects of NHERF1 on PTH receptor signaling. These latter studies reveal that in cells expressing NHERF1, PTH(7-84) has no effect on PTH receptor signaling or internalization. However, in cells lacking or expressing low levels of NHERF1, PTH(7-84) internalizes the PTH receptor without accompanying activation. Together, these findings suggest that the accumulation of PTH(7-84) in renal failure may lead to PTH resistance by internalizing and down-regulating PTH receptors.

Biologic Effects of Parathyroid Hormone Metabolites: Implications for Renal Bone Disease

Renal bone disease or renal osteodystrophy is the term used to describe the spectrum of histologic abnormalities encountered in patients with chronic kidney disease (CKD). The patterns of bone histology encountered in the setting of CKD range from states of high bone turnover, such as osteitis fibrosa, the result of hyperparathyroidism, to states of abnormally low bone turnover, such as adynamic bone.1 The major factors involved in the pathogenesis of secondary hyperparathyroidism include phosphate retention as glomerular filtration rate decreases and low levels of calcitriol as renal mass is reduced. Both of these factors may lower serum calcium and therefore stimulate parathyroid hormone (PTH) secretion. In addition to these indirect effects, low levels of calcitriol and high serum phosphorus have been shown to have direct effects on the parathyroid gland to increase PTH secretion. The pathogenetic factors involved in the development of adynamic bone disease are less well understood, but it appears that oversuppression of PTH and the use of vitamin D compounds may play a role. Thus, in the management of renal osteodystrophy, it is important to be able to monitor and treat hyperparathyroidism effectively while at the same time avoiding oversuppression of PTH. In this regard, accurate measurements of circulating PTH levels are an essential guide in the management of renal bone disease.

It is well accepted that PTH is present in the circulation in the form of both the intact 84-amino acid peptide and a variety of truncated fragments.2–4 Although the role of the intact PTH molecule as a major regulator of mineral ion homeostasis is well established, the actions of PTH fragments have remained poorly understood. In this review, we discuss the generation of PTH metabolites and the evidence supporting their biologic activity and their potential role in renal bone disease.

Trial to predict malignancy of affected parathyroid glands in primary hyperparathyroidism

Parathyroid cancer is rare but relatively frequent in Japan compared to Western countries. Surgical parathyroidectomy is the primary choice for radical treatment of primary hyperparathyroidism (pHPT), hence it is important to distinguish malignant from benign tumor in the determination of surgical indication as well as method of operation. However, it is not easy to diagnose parathyroid cancer prior to operation. In the present study, we analyzed the background data, biochemical data and bone mineral density (BMD) of 131 patients with pHPT (111 benign and 20 malignant). BMD of the lumbar spine and mid-radius was measured by dual-energy X-ray absorptiometry. Serum levels of calcium, alkaline phosphatase (ALP), and parathyroid hormone (PTH) were significantly higher in malignant group compared to benign one. The extent of elevation of mid PTH seemed to be higher than that of intact PTH in malignant group. Age-, gender-, and race-adjusted BMD of distal one-third of radius was significantly decreased in malignant group compared to benign one, although that of lumbar spine was not significantly different between the two groups, indicating that osteopenia was marked in the region which was rich in cortical bone in malignant group. On the other hand, serum levels of calcium, ALP, and mid PTH as well as age were selected as predictors of malignancy in univariate logistic regression analysis, while serum level of intact PTH was not selected. In conclusion, radial BMD was lower in malignant group compared to benign one in pHPT. Serum levels of calcium, ALP and mid PTH were useful to predict malignancy of affected parathyroid glands in pHPT patients.

Improved assessment of bone turnover by the PTH-(1-84)/large C-PTH fragments ratio in ESRD patients

BACKGROUND

The "intact" parathyroid hormone (PTH) assay recognizes PTH-(1-84) as well as amino terminally truncated PTH fragments, that is, large carboxyterminal PTH fragments (C-PTH fragments). The present study investigated whether the use of the plasma PTH-(1-84)/C-PTH fragment ratio enhances the noninvasive assessment of bone turnover in patients on dialysis.

METHODS

Bone biopsies and blood samples for determinations of routine indices of bone turnover and PTH peptides were obtained in 51 adult patients on dialysis not treated with drugs affecting bone such as vitamin D or corticosteroids. Blood levels of large C-PTH fragments were calculated by subtracting PTH-(1-84) from "intact" PTH. Patients were classified according to their levels of bone turnover based on histomorphometrically obtained results of activation frequency. Prediction of bone turnover by the various blood indices was done by using proper statistical methods. In addition, hypercalcemia was induced by calcium gluconate infusion in a subset of patients, and levels of PTH-(1-84), "intact" PTH, and PTH-(1-84)/C-PTH fragment ratio were determined.

RESULTS

The PTH-(1-84)/C-PTH fragment ratio was the best predictor of bone turnover. A ratio> 1 predicted high or normal bone turnover (sensitivity 100%), whereas a ratio <1 indicated a high probability (sensitivity 87.5%) of low bone turnover. Calcium infusion resulted in decrease in PTH-(1-84)/C-PTH fragment ratio.

CONCLUSIONS

The PTH-(1-84)/C-PTH fragment ratio predicts bone turnover with acceptable precision for biological measurements. Moreover, a change in serum calcium levels is one of the regulators of the relative amount of circulating PTH-(1-84) and its large C-PTH fragments.

The evolution of assays for parathyroid hormone

Recent progress in the assay of parathyroid hormone has revealed that commercially available assays for intact parathyroid hormone also measure additional parathyroid hormone peptides that appear to be truncated at the amino-terminal region and have the elution position on high-performance liquid chromatography of parathyroid hormone 7-84. Specific assays have been developed that measure only the true or 'whole', 84-amino-acid peptide. Such 'whole' parathyroid hormone assays have led to the discovery of new findings that suggest that parathyroid hormone fragments such as parathyroid hormone 7-84, which have hitherto been considered to be biologically inactive, may actually have biologic effects. These data, coupled with the emerging discovery of additional receptors for parathyroid hormone peptides, suggest that parathyroid hormone fragments might have potentially important actions, at least in the setting of renal failure.

Synthetic carboxyl-terminal fragments of parathyroid hormone (PTH) decrease ionized calcium concentration in rats by acting on a receptor different from the PTH/PTH-related peptide receptor

Even if the carboxyl-terminal (C-) fragments/intact (I-) PTH ratio is tightly regulated by the ionized calcium (Ca(2+)) concentration in humans and animals, in health and in disease, the physiological roles of C-PTH fragments and of the C-PTH receptor remain elusive. To explore these issues, we studied the influence of synthetic C-PTH peptides of various lengths on Ca(2+) concentration and on the calcemic response to human (h) PTH-(1-34) and hPTH-(1-84) in anesthetized thyroparathyroidectomized (TPTX) rats. We also looked at the capacity of these PTH preparations to react with the PTH/PTHrP receptor and with a receptor for the carboxyl (C)-terminal portion of PTH (C-PTH receptor) in rat osteosarcoma cells, ROS 17/2.8. The Ca(2+) concentration was reduced by 0.19 +/- 0.03 mmol/liter over 2 h in all TPTX groups. Infusion of solvent over 2 more h had no further effect on the Ca(2+) concentration (-0.01 +/- 0.01 mmol/liter), whereas infusion of hPTH-(7-84) or a fragment mixture [10% hPTH-(7-84) and 45% each of hPTH-(39-84) and hPTH-(53-84)] 10 nmol/h further decreased the Ca(2+) concentration by 0.18 +/- 0.02 (P<0.001) and 0.07+/-0.04 mmol/liter (P< 0.001), respectively. Infusion of hPTH-(1-84) or hPTH-(1-34) (1 nmol/h) increased the Ca(2+) concentration by 0.16 +/- 0.03 (P < 0.001) and 0.19 +/- 0.03 mmol/liter (P < 0.001), respectively. Adding hPTH-(7-84) (10 nmol/h) to these preparations prevented the calcemic response and maintained Ca(2+) concentrations equal to or below levels observed in TPTX animals infused with solvent alone. Adding the fragment mixture (10 nmol/h) to hPTH-(1-84) did not prevent a normal calcemic response, but partially blocked the response to hPTH-(1-34), and more than 3 nmol/h hPTH-(7-84) prevented it. Both hPTH-(1-84) and hPTH-(1-34) stimulated cAMP production in ROS 17/2.8 clonal cells, whereas hPTH-(7-84) was ineffective in this respect. Both hPTH-(1-84) and hPTH-(1-34) displaced (125)I-[Nle(8,18),Tyr(34)]hPTH-(1-34) amide from the PTH/PTHrP receptor, whereas hPTH-(7-84) had no such influence. Both hPTH-(1-84) and hPTH-(7-84) displaced (125)I-[Tyr(34)]hPTH-(19-84) from the C-PTH receptor, the former preparation being more potent on a molar basis, whereas hPTH-(1-34) had no effect. These results suggest that C-PTH fragments, particularly hPTH-(7-84), can influence the Ca(2+) concentration negatively in vivo and limit in such a way the calcemic responses to hPTH-(1-84) and hPTH-(1-34) by interacting with a receptor different from the PTH/PTHrP receptor, possibly a C-PTH receptor.

New members of the parathyroid hormone/parathyroid hormone receptor family: the parathyroid hormone 2 receptor and tuberoinfundibular peptide of 39 residues

The parathyroid hormone (PTH) family currently includes three peptides and three receptors. PTH regulates calcium homeostasis through bone and kidney PTH1 receptors. PTH-related peptide, probably also through PTH1 receptors, regulates skeletal, pancreatic, epidermal, and mammary gland differentiation and bladder and vascular smooth muscle relaxation and has a CNS role that is under investigation. Tuberoinfundibular peptide of 39 residues (TIP39) was recently purified from bovine hypothalamus based on selective PTH2 receptor activation. PTH2 receptor expression is greatest in the CNS, where it is concentrated in limbic, hypothalamic, and sensory areas, especially hypothalamic periventricular neurons, nerve terminals in the median eminence, superficial layers of the spinal cord dorsal horn, and the caudal part of the sensory trigeminal nucleus. It is also present in a number of endocrine cells. Thus TIP39 and PTH2 receptor-influenced functions may range from pituitary and pancreatic hormone release to pain perception. A third PTH-recognizing receptor has been found in zebrafish.

Parathyroid hormone-(1-34) enhances aggrecan synthesis via an insulin-like growth factor-I pathway

During endochondral bone formation, the growth plate chondrocytes proliferate, become hypertrophic, lose the cartilage phenotype, undergo mineralization, and provide a scaffold upon which subsequent longitudinal bone growth occurs. Parathyroid hormone (PTH), a calcium-regulating hormone, and parathyroid hormone-related peptide (PTHrP), which shares several properties with PTH, have profound effects on skeletal growth and new bone formation. In order to define further the mechanism by which PTH/PTHrP promotes the cartilage phenotype, chondrocytes isolated from the rib cages of developing rat embryos were evaluated for the biosynthesis of aggrecan. Cells treated with PTH-(1-34) for a 4-h period followed by a 20-h recovery period showed a significant increase in cartilage proteoglycan (aggrecan) synthesis in a dose-dependent manner. Only N-terminally intact PTH and PTHrP were effective in stimulating aggrecan synthesis. Addition of a neutralizing antibody to insulin-like growth factor-I (IGF-I) during PTH treatment resulted in the inhibition of PTH-stimulated aggrecan synthesis, whereas the addition of a neutralizing antibody to insulin-like growth factor-binding protein-2 (IGFBP-2) resulted in an increase in synthesis in both the control and PTH-treated cells. In addition, PTH treatment resulted in an increase in the mRNA for aggrecan, a reduction in IGFBP-3 mRNA, and no discernible changes in IGF-I mRNA levels, which was complemented by quantitative changes in IGFBP-3 and free IGF-I levels. The reciprocal relationship in the expression of aggrecan and IGFBP was further confirmed in chondrocytes from various gestational stages during normal development. Collectively, our results indicate that the effect of PTH may be mediated at least in part through the regulation of the IGF/IGFBP axis, by a decrease in the level of IGFBP-3, and an increase in free IGF-I levels. It is likely that the local increase in IGF-I may lead to an increase in cartilage type proteoglycan synthesis and maintenance of the cartilage phenotype. The consequence of the prolonged maintenance may be to halt mineralization while a new scaffolding is created.

Ablation of the PTHrP gene or the PTH/PTHrP receptor gene leads to distinct abnormalities in bone development

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) bind to and activate the same PTH/PTHrP receptor. Deletion of either the PTHrP gene or the PTH/PTHrP receptor gene leads to acceleration of differentiation of growth plate chondrocytes. To explore further the functional relationships of PTHrP and the PTH/PTHrP receptor, bones of knockout mice were analyzed early in development, and the phenotypes of double-knockout mice were characterized. One early phenotype is shared by both knockouts. Normally, the first chondrocytes to become hypertrophic are located in the centers of long bones; this polarity is greatly diminished in both these knockouts. The PTH/PTHrP receptor-deficient (PTH/PTHrP-R(-/-)) mice exhibited 2 unique phenotypes not shared by the PTHrP(-/-) mice. During intramembranous bone formation in the shafts of long bones, only the PTH/PTHrP-R(-/-) bones exhibit a striking increase in osteoblast number and matrix accumulation. Furthermore, the PTH/PTHrP-R(-/-) mice showed a dramatic decrease in trabecular bone formation in the primary spongiosa and a delay in vascular invasion of the early cartilage model. In the double-homozygous knockout mice, the delay in vascular invasion did not occur. Thus, PTHrP must slow vascular invasion by a mechanism independent of the PTH/PTHrP receptor.

3',5'-Cyclic adenosine monophosphate activation in osteoblastic cells: effects on parathyroid hormone-1 receptors and osteoblastic differentiation in vitro

PTH has anabolic and catabolic effects in bone through activation of the PTH-1 (PTH/PTHrP) receptor and the cAMP/protein kinase A pathway. The effects of agents that regulate cAMP in nontransformed osteoblasts in relation to cell differentiation have not been described. The purpose of this study was to determine the effects of PTH fragments with differing cAMP-stimulating activity, and nonPTH cAMP regulators on PTH-1 receptor expression and activity, and osteoblast differentiation in vitro using MC3T3-E1 and primary rat calvarial cells. PTH (1-34), but not PTH (53-84), (7-34), or PTHrP (107-139) treatment (24 h) resulted in down-regulation of steady-state messenger RNA for the PTH-1 receptor. Forskolin (a stimulator of cAMP accumulation) also down regulated the PTH-1 receptor, whereas 9-(tetrahydro-2-furyl) adenine (THFA) (an inhibitor of adenylyl cyclase) had no effect. Similarly, PTH (1-34) treatment for 48 h abolished PTHrP binding to cell surface receptors; however, neither the PTH analogs nor the cAMP regulating agents altered PTH binding or numbers of binding sites on osteoblastic cells. Basal levels of cAMP were reduced in cultured cells treated for 6 days with PTH (7-34) or THFA compared with controls. In contrast, PTH-stimulated cAMP levels were significantly increased in cultures treated with PTH (7-34) and THFA for 6 days during osteoblast differentiation and were decreased in cultures treated with PTH (1-34) and forskolin compared with controls. To evaluate effects of the cAMP pathway on osteoblast differentiation, cultures were treated continuously with PTH analogs and cAMP regulators during an 18-day differentiation regime, total RNA was isolated at multiple time points, and Northern blot analysis for osteocalcin (OCN) was performed. THFA and PTH (7-34)-treated cultures had increased OCN expression; whereas, PTH (1-34) and forskolin reduced OCN expression. Interestingly, PTH (7-34) and THFA-treated cultures had increased mineralized nodule formation, in contrast to PTH (1-34) and forskolin treatment, which reduced nodule formation. Similarly, calcium accumulation in cultures was significantly increased in the PTH (7-34) and THFA-treated cultures and reduced in the PTH (1-34) and forskolin-treated cultures. These data demonstrate that agents that increase cAMP down regulate PTH-1 receptor messenger RNA and inhibit osteoblast differentiation in vitro. Agents that reduce or block adenylyl cyclase or cAMP activity do not alter PTH-1 receptor expression or binding, but have striking effects on promoting osteoblast differentiation. We conclude that many effects of PTH on osteoblasts may be mimicked or antagonized by agents that alter cAMP activity and bypass the PTH-1 receptor.

Mapping of a carboxyl-terminal active site of parathyroid hormone by calcium-imaging

We recently showed that the C-terminal fragment PTH (52-84) effectively increases intracellular free calcium ([Ca2+]i) in a subset of growth plate chondrocytes not activated by the N-terminal PTH fragment (1-34). Here we characterize the active site on C-terminal PTH (52-84) with respect to calcium (Ca2+)-signaling and the mechanism involved by using synthetic PTH-subfragments in digital CCD ratio-imaging experiments. Our results show amino acids 73-76 to be the core region for increasing [Ca2+]i. Ryanodine (1 microM), caffeine (10 mM), lithium (2 mM), or cyclopiazonic acid (2-5 microM), agents that interfere with intracellular Ca2+ release, all failed to block PTH (52-84) induced [Ca2+]i increases. Depletion of extracellular calcium ([Ca2+]o) blocked PTH (52-84) induced [Ca2+]i increases, indicating a transmembrane Ca2+ influx. In contrast to voltage-gated and Ca2+ release activated Ca2+ influx, PTH (52-84) evoked Ca2+ influx was not blocked by nickel (1 mM). We conclude that PTH amino acids 73-76 are essential for activation of a nickel-insensitive Ca2+ influx pathway in growth plate chondrocytes that is likely to be of relevance for matrix calcification, a key step in endochondral bone formation.

Comparison of the effects of carboxyl-terminal parathyroid hormone peptide[53-84] and aminoterminal peptide[1-34] on mouse tooth germ in vitro

The biological activity of carboxyl-terminal parathyroid hormone peptide[53-84] (PTH[53-84]) and aminoterminal peptide [1-34] (PTH[1-34]) during cultivation of the lower molar tooth germ from mouse embryos was examined. PTH[1-34] increased alkaline phosphatase activity in the tooth germ at an early developmental stage (early to advanced bell stage), and decreased it at a late developmental stage (advanced to late bell stage). On the other hand, PTH[53-84] decreased alkaline phosphatase activity at the early stage and increased it at the late stage. Thus, both PTH fragments had distinct effects on tooth development in vitro, which were confirmed by histological observation of their marked effects on the formation of dentine and enamel. These results show that PTH[53-84] has biological activity in the murine tooth germ, opposite to the action of PTH[1-34].

Parathyroid hormone enhances early and suppresses late stages of osteogenic and chondrogenic development in a BMP-dependent mesenchymal differentiation system (C3H10T1/2)

The role of parathyroid hormone (PTH) upon osteo-/chondrogenic development was investigated in a bone morphogenetic protein (BMP)-dependent differentiation system involving the recombinant expression of BMPs in mesenchymal progenitor cells (C3H10T1/2). The constitutive expression of the PTH/PTH related protein receptor in this system led to a marked stimulation of chondrogenic and osteogenic development, while the permanent application of the ligand PTH(1-34) resulted in opposite responses by stimulating the early and suppressing the late stages of osteo-/chondrogenic development. These contrasting effects of PTH(1-34) on osteogenic and chondrocytic development seem, therefore, to depend on the cellular state of differentiation. The osteogenic and chondrocytic differentiation potential was substantiated histologically and by genetic analyses of marker genes like c-fos, alkaline phosphatase, osteocalcin, collagen alpha1(I), and collagen alpha1(II). The capacity to regulate osteogenic and chondrogenic development is located in the amino-terminal (1-34) region of the PTH molecule and seems to be mediated by the cyclic adenosine monophosphate signaling cascade. The application of other PTH domains like PTH(28-48) and PTH(53-84) did not exhibit significant responses. PTH acts as an essential factor in mesenchymal development controlling rates of differentiation into the osteogenic or chondrogenic lineage. The analysis of PTH effects in this system demonstrates the value of recombinant mesenchymal progenitor cells in the in vitro analysis of osteo-/chondrogenic development.

Parathyroid hormone increases circulating levels of fibronectin in vivo: modulating effect of ovariectomy

To explore the effect of PTH on circulating levels of fibronectin (FN), adult female Sprague-Dawley rats were implanted with Alzet minipumps prepared to deliver 7 pmol/h x kg BW of either human PTH (1-34) or human PTH (1-84). Both forms of the hormone led to significant and progressive increases in circulating levels of FN over the 72-h study period (P < 0.001). However, at every time point, circulating levels of FN with human PTH (hPTH) (1-84) infusion were significantly higher than with hPTH (1-34), such that at the end of the infusion, mean levels in the hPTH (1-34) group were 32.2 +/- 1.4 ng/ml, in the hPTH (1-84) group 93.8 +/- 5.4 ng/ml, and in the vehicle infused group 14.6 +/- 0.7 ng/ml. The greater agonist efficacy of hPTH (1-84) was not explained by differences in circulating levels of the hormones, and both forms of the hormone were equipotent at stimulating cAMP production by ROS 17/2.8 cells. However, hPTH (1-84) remained a more effective agonist than hPTH (1-34) at stimulating FN production in these cells (P < 0.001). Nephrectomy did not blunt the ability of PTH to increase circulating FN in vivo, indicating that the kidney was not the source of the FN produced in response to PTH. Pretreament with the potent bisphosphonate APD to block bone resorption also did not blunt the in vivo response to PTH. Parathyroidectomy did not blunt the response. Cultured fetal rat bones showed a significant 2.4-fold increase in FN production when treated with PTH. Consistent with our earlier in vitro studies (Endocrinology, 135: 1639-1644, 1994), estrogen deficiency, induced by ovariectomy, significantly diminished the ability of PTH to increase circulating FN levels in vivo (P < 0.001). We conclude that PTH increases circulating levels of FN in vivo and may be a physiologic regulator for the plasma form of this glycoprotein. The effects of PTH on circulating FN may reflect the anabolic properties of the hormone in bone and the blunted response following estrogen withdrawal could be a manifestation of the diminished bone formation vis-à-vis resorption seen in the estrogen deficient state.

C-terminal parathyroid hormone-related protein inhibits proliferation and differentiation of human osteoblast-like cells

Parathyroid hormone-related protein (PTHrP) is synthesized by osteoblasts, although its local role in bone is not completely understood. The C-terminal (107-111) region of PTHrP seems to be a potent inhibitor of osteoblastic bone resorption. We studied the effect of this PTHrP domain on the proliferation and synthesis of osteoblastic markers in osteoblast-like cells from adult human bone. We found that the human (h)PTHrP(107-139) fragment, between 10 fM and 10 nM, inhibited 3H-thymidine incorporation into these cells. The antiproliferative effect of the latter fragment, or that of hPTHrP(107-111), was similar to that induced by [Tyr34] hPTHrP(1-34) amide, bovine PTH(1-34), and hPTHrP(1-141), while hPTHrP(38-64) amide was ineffective. Human PTHrP(7-34) amide, at 10 nM, and 1 microM phorbol-12-myristate-13-acetate also significantly decreased DNA synthesis in human osteoblast-like cells. Neither hPTHrP(7-34) amide nor hPTHrP(107-139), at 10 nM, stimulated protein kinase A (PKA) activity in these cells. Moreover, 100 nM H-89, a PKA inhibitor, did not eliminate the inhibitory effect of hPTHrP(107-139) on these cells' growth. However 100 nM calphostin C, a PKC inhibitor, blunted this effect of PTHrP(107-139). In addition to their antimitogenic effect, hPTHrP(107-139) and hPTHrP(107-111) inhibited basal and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-stimulated alkaline phosphatase activity in these cells. Both fragments, like 1,25(OH)2D3, decreased C-terminal type I procollagen secretion into the cell-conditioned medium, but osteocalcin secretion by these cells was unaffected by the C-terminal PTHrP fragments. These findings suggest that PTHrP may act as a local regulator of bone formation.

Differential effects of parathyroid hormone fragments on collagen gene expression in chondrocytes

The effect of parathyroid hormone (PTH) in vivo after secretion by the parathyroid gland is mediated by bioactive fragments of the molecule. To elucidate their possible role in the regulation of cartilage matrix metabolism, the influence of the amino-terminal (NH2-terminal), the central, and the carboxyl-terminal (COOH-terminal) portion of the PTH on collagen gene expression was studied in a serum free cell culture system of fetal bovine and human chondrocytes. Expression of alpha1 (I), alpha1 (II), alpha1 (III), and alpha1 (X) mRNA was investigated by in situ hybridization and quantified by Northern blot analysis. NH2-terminal and mid-regional fragments containing a core sequence between amino acid residues 28-34 of PTH induced a significant rise in alpha1 (II) mRNA in proliferating chondrocytes. In addition, the COOH-terminal portion (aa 52-84) of the PTH molecule was shown to exert a stimulatory effect on alpha1 (II) and alpha1 (X) mRNA expression in chondrocytes from the hypertrophic zone of bovine epiphyseal cartilage. PTH peptides harboring either the functional domain in the central or COOH-terminal region of PTH can induce cAMP independent Ca2+ signaling in different subsets of chondrocytes as assessed by microfluorometry of Fura-2/AM loaded cells. These results support the hypothesis that different hormonal effects of PTH on cartilage matrix metabolism are exerted by distinct effector domains and depend on the differentiation stage of the target cell.

Domain-specific gene activation by parathyroid hormone in osteoblastic ROS17/2.8 cells

Parathyroid hormone (PTH)-mediated gene activation was assessed in the osteoblast-like rat cell line ROS17/2.8 with two PTH fragments harboring distinct activating domains: PTH-(1-34) and PTH-(28-48). The PTH response of genes expressed immediate early in the cell cycle or in the osteoblast developmental sequence was investigated. In addition, subtractive cloning was used to identify genes in ROS17/2.8 cells that are activated by the two PTH domains. PTH-(1-34) immediately increased the transcript levels of c-fos and c-jun at a considerably higher rate than PTH-(28-48). A significant immediate PTH effect on osteoblastic marker genes could not be detected, with the exception of elevated ornithine decarboxylase transcript levels. However, continuous application of PTH-(1-34) increased transcript levels of the osteoblast-specific osteocalcin gene and reduced those of other osteoblastic marker genes including alkaline phosphatase and the PTH/PTH-related peptide receptor. By subtractive cloning, nine cDNAs were isolated corresponding to mRNAs directly up-regulated by PTH-(1-34) or PTH-(28-48). Among these were a cyclic phosphodiesterase, a (cytosine 5)-methyltransferase, an 80-kDa protein kinase C substrate, junB, and a novel GC-binding protein. Three cDNAs are unknown at present. Interestingly, in all cases, the efficiency of gene activation by PTH-(28-48) was substantially lower in comparison with PTH-(1-34). PTH-mediated protein kinase C signaling in ROS17/2.8 cells may therefore constitute a minor pathway in comparison with the dominant cAMP/protein kinase A cascade.

Influence of Ca2+ concentration on the clearance and circulating levels of intact and carboxy-terminal iPTH in pentobarbital-anesthetized dogs

The role of hormone secretion and hormone clearance in the differential control of circulating levels of intact (I-) and carboxy-terminal (C-) immunoreactive parathyroid hormone (iPTH) was evaluated in 18 pentobarbital-anesthetized dogs. Catheters were installed in the aorta, left renal, and hepatic veins for sampling. Hepatic and renal blood flows were calculated from sulfobromophtalein (BSP) and p-aminohippuric acid (PAH) extraction and clearance. I- and C-iPTH were measured during a 1 h of infusion of CaCl2 or Na2EDTA. High-performance liquid chromatography (HPLC) profiles of I- and C-iPTH in and out of the liver and kidney were also obtained. Data on two dogs (one CaCl2 and one Na2EDTA infusion) were pooled for the analysis of one parathyroid function using a four-parameter mathematical model. Results obtained in the basal state and during analysis of the parathyroid function were also compared with those of 24 awakened dogs. Results are means +/- SD. Anesthetized dogs had lower levels of Ca2+ (1.29 +/- 0.03 vs. 1.34 +/- 0.04 mmol/l; p < 0.001) and higher levels of I- (11.5 +/- 5.7 vs. 3.0 +/- 1.9 pmol/l, p < 0.001) and C-iPTH (52 +/- 20.9 vs. 22.8 +/- 10.5 pmol/l; p < 0.001) than awakened dogs. Their stimulated (S) and nonsuppressible (NS) I-iPTH levels were increased 2- and 4-fold, respectively, while similar C-iPTH levels rose only 1.35- and 1.75-fold; this caused their S (4.4 +/- 0.7 vs. 6.8 +/- 1.9; p < 0.001) and NS (24.6 +/- 11.8 vs. 49.8 +/- 27.5; p < 0.05) C-iPTH/I-iPTH ratios to decrease. This was not explained by different renal clearance rates of I- and C-iPTH since both were similar at approximately 10 ml/kg/minute and unaffected by Ca2+ concentration. Clearance of all I- and C-iPTH HPLC molecular forms by the kidney appeared equal. A 50% decrease in the hepatic clearance of I-iPTH to approximately 12 ml/kg/minute in pentobarbital-anesthetized dogs, related to a lower hepatic blood flow, explained the higher levels of S and NS I-iPTH in these animals. I-iPTH hepatic clearance was unaffected by Ca2+ concentration. C-iPTH hepatic clearance was much lower at approximately 5 ml/kg/minute, abolished by hypercalcemia, and reduced by the influence of anesthesia on hepatic blood flow. This also explained the higher S C-iPTH levels in anesthetized animals. I-PTH(1-84) detected by the C-iPTH assay explained only 37.6% of the hepatic C-iPTH clearance in hypocalcemia and 73.3% in hypercalcemia. Overall, our results indicate that total C-iPTH clearance is about 40.2% that of I-iPTH in hypocalcemia and 41.3% in hypercalcemia. This would only explain a 2.4- to 2.5-fold difference in circulating levels of I- and C-iPTH if secretion rates were equal; the larger difference observed in S and NS C-iPTH/I-iPTH ratio values is thus mainly explained by different production rates.

Structurally diverse N-terminal peptides of parathyroid hormone (PTH) and PTH-related peptide (PTHRP) inhibit the Na+/H+ exchanger NHE3 isoform by binding to the PTH/PTHRP receptor type I and activating distinct signaling pathways

N-terminal peptides of parathyroid hormone (PTH) and PTH-related peptide (PTHRP) elicit a wide variety of biological responses in target cells, including the inhibition of Na+/H+ exchanger NHE3 activity in renal cells. This response is believed to be mediated by ligand binding to a common receptor (i.e. PTH/PTHRP receptor type I) and activation of cAMP-dependent and/or Ca2+/phospholipid-dependent protein kinases (PKA and PKC, respectively). However, the mechanism of action of these N-terminal peptides is now unclear because of recent data reporting the existence of additional receptor isoforms. Therefore, to directly examine the ligand binding and signaling characteristics of the PTH/PTHRP receptor type I and its ability to elicit a biological response, cDNAs encoding the rat type I receptor and the rat NHE3 isoform were transfected into Chinese hamster ovary (AP-1) cells that lack endogenous expression of these proteins. Competition binding assays using [125I-Tyr36]PTHRP-(1-36)-NH2 radioligand indicated that several biologically active human N-terminal PTH and PTHRP fragments (PTH-(1-34), PTH-(3-34), PTH-(28-42), PTH-(28-48), and PTHRP-(1-34)) were capable of binding to the type I receptor. Both PTH-(1-34) and PTHRP-(1-34) stimulated adenylate cyclase and PKC activities in these cells, whereas PTH-(3-34), PTH-(28-42), and PTH-(28-48) selectively enhanced only PKC activity. PTHRP-(1-16), a biologically inert fragment, was incapable of binding to this receptor and influencing either the PKA or PKC pathway. Furthermore, all the analogues with the exception of PTHRP-(1-16) inhibited NHE3 activity. Inhibition of PKC by the potent antagonist chelerythrine chloride abolished the depression of NHE3 activity by PTH-(3-34), PTH-(28-42), and PTH-(28-48) but did not alleviate the effects of PTH-(1-34). Likewise, antagonism of PKA by H-89 was unable to prevent the inhibition caused by PTH-(1-34). However, inhibition of both PKA and PKC by the nonselective protein kinase antagonist H-7 abolished the reduction of NHE3 activity by PTH-(1-34). These data indicate that discrete N-terminal analogues of PTH and PTHRP can interact with the classical PTH/PTHRP receptor type I and activate PKA and/or PKC. Activation of either signaling pathway independently leads to inhibition of NHE3.

Inositol 1-,4-,5-trisphosphate-dependent Ca2+ signaling by the recombinant human PTH/PTHrP receptor stably expressed in a human kidney cell line

We previously reported the preparation and partial characterization of a series of human embryonic kidney cell lines (HEK-293) stably expressing various numbers of the recombinant human (h) parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor (Rc). Using this expression system we examined ligand (PTH or PTHrP) binding characteristics and cyclic AMP responsiveness. We have now extended these studies to investigate the calcium signal transduction pathways activated by the hPTH/PTHrP Rc. In parental HEK-293 cells, which lack endogenous PTH/PTHrP Rc, incubation with hPTH(1-34) had no effect on cytosolic free Ca2+ concentration [Ca2+]i. In HEK-293 clone C-21, stably expressing approximately 400,000 Rc/cell, PTH stimulated an increase in [Ca2+]i by Ca2+ release from intracellular stores; PTH released Ca2+ exclusively from the IP3 sensitive Ca2+ pool. Unlike previous studies, the ability of PTH to elicit both cAMP responses and [Ca2+]i transients occurred over a wide range of Rc numbers (between 400,000 and 3000 Rc/cell); both responses were always observed at PTH concentrations in the same dose range although the magnitude of the responses decrease with Rc number. Pretreatment of C-21 cells with pertussis toxin for 24 h, which significantly enhanced PTH-stimulated cAMP accumulation, did not modulate PTH-stimulated [Ca2+]i transients. At each PTH concentration tested which resulted in increased cAMP levels, there was also an increase in [Ca2+]i transients. Treatment of C-21 cells with a battery of midregion and C-terminal PTH or PTHrP peptides showed no effect on either [Ca2+]i transients or cAMP accumulation, indicating a lack of functional interactions between these peptides and the form of the hPTH/PTHrP Rc stably expressed in these cells. Immunological analysis of G-protein expression demonstrated the presence of Gs, Gi, and Gq in all parental and transfected cells lines examined. Taken together, these data demonstrate that the hPTH/PTHrP Rc, stably expressed in HEK-293 cells, elicits responses in both the cAMP and IP3-dependent [Ca2+]i pathways and is responsive only to N-terminal PTH/PTHrP peptides.

Structure of recombinant human parathyroid hormone in solution using multidimensional NMR spectroscopy

The solution structure of human parathyroid hormone, in the form of recombinant prolyl-hPTH(1-84), has been investigated by multidimensional NMR spectroscopy under conditions (aqueous trifluoroethanol) which favour the structured-state of the protein. Spin systems were identified from 3D 1H DQF (double-quantum filtered)-COSY and TOCSY spectra and sequence-specific assignments were from 2D 1H phase-sensitive NOESY spectra. Signal overlap was resolved in a 3D-NOESY-TOCSY spectrum and assignments were confirmed with 2D NOESY-15N-HMQC (heteronuclear multiple-quantum coherence) spectra taken of a sample universally labeled with 15N. A satisfactory set of final structures was calculated from the quantitative NOE data using restrained molecular dynamics and energy minimization calculations. The N-terminus is dominated by three, well defined helices between Ser-3 to Asn-10, Ser-17 to Lys-27 and Asp-30 to Leu-37, while the most significant structural features in the C-terminus are a short, less-well defined helix between Asn-57 to Ser-62 and a series of loose turns. These two terminal units are joined by an unstructured mid-region. The molecule shows a tendency towards tertiary structure, defined by a number of long-range NOEs. A detailed RMS deviation analysis allowed the final refined structures to be classified into a limited ensemble of stable conformations that reflect the inherent flexibility of the hormone in solution.

Expression and characterization of a recombinant human parathyroid hormone partial agonist with antagonistic properties: Gly-hPTH(-1-->+84)

We have produced and characterized a hPTH analogue with an amino-terminal extension of glycine, Gly-hPTH(-1-->+84) (denoted Gly-hPTH). The hormone analogue was synthesized in E. coli strain BJ5183 transformed with the expression plasmid pKKPTH, extracted from the bacterial pellet and purified by reverse-phase high performance liquid chromatography. Its chemical nature, as determined by amino acid composition analysis, N-terminal amino acid analysis, and mass spectrometry, showed the 9480-Da Gly-hPTH as the predominant species. Because f-Met-Gly-hPTH was the expected form encoded by the plasmid construct, the results indicate that the f-Met residue was efficiently removed from the precurser form. The following functional characteristics of Gly-hPTH were demonstrated. 1) In cells transfected with the human PTH/PTHrP receptor, the receptor binding affinity was reduced threefold compared to the authentic hPTH(1-84) produced by Saccharomyces cerevisiae (apparent Kds: 8.4 and 2.7 nM, respectively). 2) Using the same cells, Gly-hPTH showed 27-fold reduced potency compared to hPTH(1-84) in stimulating intracellular cAMP production (EC50: 32 and 1.2 nM, respectively). 3) Gly-hPTH demonstrated antagonist activity by reducing hPTH-induced cAMP production by 33 +/- 5% (mean +/- SD) when tested at a 1:1 molar ratio. In these studies the recombinant authentic hPTH(1-84) was used as standard for comparisons, and it showed an equal receptor binding affinity and cAMP production as the chemically synthesized peptide [Nle8,18,Tyr34]bovinePTH(1-34)-NH2.

Studies on the effect of parathyroid hormone (1-84) on glucose output in the liver: comparison of effects in isolated hepatocytes and in perfused liver

This study was conducted to determine the action of parathyroid hormone (1-84) (PTH(1-84)) on glucose output both in perfused liver and in isolated hepatocytes. In isolated rat hepatocytes, PTH(1-84) stimulated glucose output in a concentration-dependent manner. The action was detected at 10(-11) M and, at 10(-9) M, PTH produced its maximal effect. The magnitude of the maximal effect of PTH(1-84) was about 65% of that of phenylephrine. In contrast, PTH(1-84) had no effect on glucose output in perfused rat liver. Concentration of PTH(1-84) in effluent of perfused liver was less than that in the inflow. However, when the effluent obtained from liver perfused with 10 nM PTH(1-84) was added to isolated hepatocytes, a considerable amount of glucose was released, which was reversed by PTH(7-34), a competitive inhibitor of PTH receptor. These results indicate that PTH(1-84) increases glucose output in isolated hepatocytes but not in intact liver. It is suggested that the action of PTH(1-84) is blocked in intact liver by a yet unknown mechanism.

Human parathyroid hormone: efficient synthesis in Escherichia coli using a synthetic gene, purification and characterization

Human parathyroid hormone is a peptide hormone consisting of 84 amino acid residues. Production of small proteins by direct expression in Escherichia coli is often unsuccessful owing to susceptibility of the mRNA and/or the product to endogenous enzymes. In this study, direct expression of the hormone at an excellent level (over 100 mg/L) has been achieved by using a suitably designed synthetic gene under the control of the T7 promoter. The protein produced in bacteria was extracted and easily purified in a good yield of 27 mg/L. The purified product was physico-chemically identified as intact human parathyroid hormone from the results of amino acid analysis, N-terminal sequencing, and peptide mapping using fast atom bombardment mass spectrometry. In biological assays the purified product stimulated adenylate cyclase in vitro, promoted bone growth and increased the serum osteocalcin in rats to the same extent as the authentic hormone.

Carboxyl-terminal parathyroid hormone peptide (53-84) elevates alkaline phosphatase and osteocalcin mRNA levels in SaOS-2 cells

Previous findings in our laboratory have shown that hPTH-(53-84) stimulates alkaline phosphatase activity in dexamethasone-treated ROS 17/2.8 cells. In the present study, we examined the effects of hPTH-(53-84) and hPTH-(1-34) on the expressions of alkaline phosphatase, osteocalcin, and collagen type I mRNA levels in the human osteosarcoma cell line SaOS-2. The effect of hPTH-(53-84) on alkaline phosphatase and osteocalcin message levels was dose dependent (ANOVA, p < 0.005 and p < 0.001, respectively), with significant stimulation observed at 10 nM. Treatment with 10 nM hPTH-(53-84) for 24 h resulted in significant 2- and 1.4-fold increases in mRNA levels for alkaline phosphatase and osteocalcin, respectively (p < 0.05), but had no effect on collagen type I expression. The increased alkaline phosphatase mRNA levels was associated with a 1.5-fold increase in enzyme activity (p < 0.05). In contrast, under similar incubation conditions, hPTH-(1-34) had no significant effects on alkaline phosphatase or osteocalcin mRNA levels. On the other hand, hPTH-(1-34) had dose-dependent stimulatory effects on collagen type I mRNA levels (ANOVA, p < 0.001), 10 nM hPTH-(1-34) stimulating collagen type I expression 1.6-fold (p < 0.05). The results indicate that carboxyl-terminal hPTH-(53-84) has direct and unique biologic effects in human osteoblast-like cells in culture.

The structure of human parathyroid hormone from a study of fragments in solution using 1H NMR spectroscopy and its biological implications

In order to gain insight into the structure of human parathyroid hormone (hPTH), four fragments [hPTH(1-34), hPTH(18-48), hPTH(28-48), and hPTH(53-84)], which cover all regions of the intact hormone, have been investigated by CD and NMR spectroscopy in combination with distance geometry, and restrained molecular dynamics and energy minimization calculations, under a variety of solution conditions. Significantly, all fragments showed little propensity to form stable structures in aqueous solution alone, and it was only on the addition of trifluoroethanol (TFE) that defined structural features were observed. In an extension of earlier work [Klaus et al. (1991) Biochemistry 30, 6936-6942], hPTH(1-34) in 70% trifluoroethanol (TFE) showed two helices that were longer than in 10% TFE, but essentially showed the same characteristics. Although overlap in the 1H NMR spectra prevented the determination of quantitative NOE data for residues 26-30, the combination of the alpha-proton chemical shift data and quantitative NOE data indicated the helices extend from residues 3 to 13 and 15 to 29. No evidence was found for interaction of the two helical regions. The nature and extent of this second helix in the intact hormone were better defined from the data for hPTH(18-48). Under limiting solution conditions, where the fragment assumed its maximum helical content, a well-defined helix was observed between residues 21 and 38 with a possible discontinuity between Leu-28 and Gln-29. There was little evidence of any form of secondary structure between Gly-38 and the terminus of this fragment, Ser-48. In keeping with this result, the shorter fragment, hPTH(28-48), showed little evidence of stable secondary structure on addition of TFE. From the alpha-proton chemical shifts residues 23-27 appeared to sustain helical structure more readily than the rest of molecule under all solution regimes in both hPTH(1-34) and hPTH(18-48). In contrast to the other two longer fragments hPTH(53-84) showed little propensity for helical secondary structure even at the highest TFE concentrations. However, there was evidence that the molecule did adopt a defined three-dimensional structure. Various long-range NOE's were observed in 10% TFE that allowed the calculation of an open tertiary structure consisting of an initial series of turns surrounded by a loop structure of several loose turns.(ABSTRACT TRUNCATED AT 400 WORDS)

Parathyroid hormone and glucagon compete for binding to low affinity sites on human skin fibroblasts

We have studied the binding sites for parathyroid hormone (PTH) present on normal human fibroblasts by studying these receptors with respect to (1) affinity and specificity towards various peptide hormones including human PTH(1-34) and porcine glucagon(1-29); (2) ability to mediate stimulation of cAMP production in response to these hormones; and (3) molecular size. Binding assays using 125I-labelled human PTH(1-34) and 125I-labelled porcine glucagon(1-29), and hormone stimulations of cAMP production were performed on confluent fibroblasts grown in 24-well dishes (passage 4-10). The molecular sizes of the binding sites for PTH and glucagon were assessed after cross-linking to the corresponding 125I-labelled ligand using the heterobifunctional reagent 1,4-difluoro-2,5-dinitrobenzene (DFDNB), by sodium dodecyl sulphate gel electrophoresis and autoradiography. The results demonstrate: (1) Biologically active PTH and glucagon, but not other peptide hormones tested, are equipotent competitors for binding on human fibroblasts to sites which have a relatively low affinity for these ligands (Kd approximately 0.8-2.4 x 10(-7) M); these sites have an apparent molecular weight of 95 kDa and are not linked to stimulation of cAMP production by PTH. (2) A distinct class of receptors for PTH with an apparent molecular weight of 60 kDa and which probably are linked to stimulation of cAMP production by PTH is also expressed by these cells; glucagon cannot compete with PTH for binding to these sites, and does not interfere with the stimulation of cAMP production by PTH. (3) Glucagon does not stimulate cAMP production in human fibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Dexamethasone-treated ROS 17/2.8 rat osteosarcoma cells are responsive to human carboxylterminal parathyroid hormone peptide hPTH (53-84): stimulation of alkaline phosphatase

We tested the effects of various parathyroid hormone (PTH) peptides on alkaline phosphatase (ALP) activity in the osteoblastic cell line ROS 17/2.8. In dexamethasone-treated ROS 17/2.8 cells there was a dose-related increase in ALP activity due to treatment with hPTH (53-84). ALP activity was stimulated by 10 nM hPTH (53-84) by a mean of 1.51 +/- 0.07-fold (P less than 0.001) in nine experiments, whereas the same dose of bPTH (1-34) and bPTH (1-84) inhibited enzyme activity to 0.36 +/- 0.02-fold (P less than 0.001) and 0.37 +/- 0.03-fold (P less than 0.001), respectively. Significant stimulation of ALP activity occurred with doses of hPTH (53-84) as low as 0.01 nM. There was no stimulation of enzyme activity by hPTH (53-84) in the absence of dexamethasone; the maximum ALP response to hPTH (53-84) occurred between 96 and 144 hours, and no significant effect was seen at time periods less than 96 hours. The optimum dose of dexamethasone required to enable the response to hPTH (53-84) was 10 nM. Carboxylterminal PTH fragments had a specific stimulatory effect on ALP activity in dexamethasone-treated ROS 17/2.8 cells, but the aminoterminal PTH effect appeared to be dominant, as the equimolar combination of bPTH (1-34) and hPTH (53-84) resulted in inhibition of ALP activity. Thus, in order for the effects of carboxylterminal fragments to be manifest, the cells would have to be stimulated under conditions in which the aminoterminal receptor is unoccupied; this could occur under some in vivo conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro response of neonatal condylar cartilage to simultaneous exposure to the parathyroid hormone fragments 1-34, 28-48, and 53-84 hPTH

Mandibular condylar explants of neonatal ICR mice were maintained as serum-free organ culture systems and were used to study the effects of three synthetic fragments of human parathyroid hormone (hPTH) on the morphology of the organ and its ability to incorporate [3H]thymidine. Forty-eight-hour incubation with hPTH (1-34), at a concentration of 0.5 microgram/ml caused an increase of 88% in DNA synthesis and a marked increase in the size of the chondroprogenitor zone. The mitogenic effect of hPTH (1-34) was decreased to 34% over control levels when the fragment hPTH (28-48) was added to the system. However, the addition of the latter fragment brought about a marked enhancement in the mineralization of the cartilaginous extracellular matrix along with the formation of an appreciable amount of new bone. The de novo osseous tissue was attached to the mineralized cartilage. When the carboxyl-terminal fragment hPTH (53-84) was added together with the other two fragments, the mitogenic effect of hPTH (1-34) was completely abolished and the respective cultures incorporated [3H]thymidine even less than untreated control cultures. Moreover, the addition of hPTH (53-84) to the culture system led to distinct structural features throughout the mineralized hypertrophic cartilage. The latter contained a mixture of cells within an unorganized extracellular matrix. Untreated control cultures lacked such structures, but contained the various cell zones as normally seen in neonatal condylar cartilage. Therefore, it seems reasonable to suggest that each of the three fragments tested induces a biological effect on neonatal cartilage and might be involved in the normal process of endochondral ossification.

Characterization of the interaction of parathyroid hormone with the mitochondrial ATPase

Parathyroid hormone (PTH) has been shown to bind specifically to the beta subunit of the mitochondrial ATPase on nitrocellulose blots. We have now examined this interaction further, using intact mitochondria, submitochondrial particles, and the purified F1 ATPase. With intact mitochondria, 1 microM concentrations of PTH and its biologically active 1-34 fragment activate the ATPase about 3-fold. This effect was reduced to a 1.4-fold activation with 3-34 and 7-34 fragments of the hormone, and oxidized PTH gave no detectable activity. Activation could only be observed below pH 7. PTH had no significant effect on the activity of the purified enzyme or on submitochondrial particles. However, specific binding of an iodinated PTH analog, [Nle 8,18-Tyr 34] bPTH (1-34) amide, was found with submitochondrial particles and the purified ATPase. Binding affinity with the purified enzyme was about 10(-3) that of the plasma membrane receptor, and the molar stoichiometry was close to 1:1 (PTH:intact enzyme). With submitochondrial particles the affinity was about 10-fold higher than with the purified enzyme. This binding was further examined with PTH derivatives and fragments, and compared to that seen in the plasma membrane receptor. Oxidation of methionine 18 in PTH reduced the affinity about 50%, oxidation of methionine 8 reduced the affinity 95%, and oxidation of both methionines further decreased affinity in both membranes and submitochondrial particles. However, when compared to the native hormone, the 3-34 and 7-34 PTH fragments had much higher affinity for the submitochondrial particles than for the plasma membranes. PTH also reduced chemical crosslinking of the ATP analog, p-fluorosulfonyl benzoyl 5'-adenosine, to the alpha subunit of this enzyme, but did not alter labeling of the enzyme with 3'-O-(4'-benzoyl) benzoyl ATP, suggesting that the hormone binds near a regulatory nucleotide binding site. Direct chemical crosslinking of PTH to the beta-subunit of the enzyme was attained with a cleavable, photoactivate crosslinker, sulfosuccinimidyl 2-(p-azidosalicylamido) ethyl-1,3-dithiopropionate. The crosslinked protein was cleaved with cyanogen bromide and the labeled fragments were sequenced. The labeled fragments were found to be segments of the protein which have previously been implicated as being close to the noncatalytic ATP binding sites.

iPTH values during hemodialysis: role of ionized Ca, dialysis membranes and iPTH assays

The evolution of serum iPTH concentration during hemodialysis was studied in eight patients who were dialyzed with cuprophane (Cu) and polyacrylonitrile membranes (PAN) during two four-hour sessions. Ca+(+) concentration in the dialysate was 1.37 mM/liter. iPTH was measured with an intact hormone immunoradiometric assay (I), with two late (L1, L2) and one mid (M) carboxylterminal immunoassays at the beginning and end of hemodialysis, from the arterial and venous sides of the extracorporeal unit. Results are means +/- SD. Serum Ca+(+) increased during dialysis with Cu (1.26 +/- 0.08 vs. 1.33 +/- 0.03 mmol/liter, P less than 0.05, without any change in the concentration of iPTH measured with L1, L2 or M, but with a 50% decrease in iPTH measured with I (21.8 +/- 19.2 vs. 10.3 +/- 9.0 pmol/liter, P less than 0.05). Serum Ca+(+) increased similarly with PAN (1.25 +/- 0.10 vs. 1.34 +/- 0.04 mmol/liter, P less than 0.01), but there was a greater than 50% decrease in iPTH concentration measurements for all four assays (I: 17.2 +/- 17 vs. 7.6 +/- 8.3 pmol/liter, P less than 0.05; L1: 92 +/- 75 vs. 36 +/- 32 pmol/liter, P less than 0.05; L2: 312 +/- 289 vs. 126 +/- 128 pmol/liter, P less than 0.01; M: 926 +/- 1074 vs. 373 +/- 422 pmol/liter, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)