Receptors for parathyroid hormone and parathyroid hormone-related peptide: exploration of their biological importance

Hypercalcemia during pregnancy: management and outcomes for mother and child

Diagnosing and treating hypercalcemia during pregnancy can be challenging due to both the physiological changes in calcium homeostasis and the underlying cause for the hypercalcemia. During pregnancy and lactation there is increased mobilization of calcium in the mother to meet the fetus' calcium requirements. Here we discuss the diagnostic challenges, management, and patient perspective of hypercalcemia during pregnancy in two particular cases and in other rare conditions causing hypercalcemia.

Analytical Differences in Intraoperative Parathyroid Hormone Assays

BACKGROUND

We compared the rates of intraoperative parathyroid hormone (PTH) decline using the Siemens Immulite^® Turbo PTH and Roche Elecsys^® short turnaround time PTH assays in 95 consecutive surgical patients to investigate analytical and turnaround time (TAT) differences between the tests performed in the operating room (OR) vs the central clinical chemistry laboratory (CCL).

METHODS

Serial blood samples from 95 patients undergoing parathyroidectomy were collected and measured using the 2 immunoassays. Specimens from the first 15 patients were measured simultaneously in the OR and CCL and used for the TAT study. In addition to 2 baseline samples, specimens were collected at 5, 10, and 15 min (for some patients, >15 min) after parathyroidectomy.

RESULTS

In the TAT study, a significant difference was observed (OR median 20 min vs CCL median 27 min; P < 0.05). Of the 95 patient series, slower rates of parathyroid hormone decrease were observed in approximately 20% of the patients when comparing the Roche with the Immulite immunoassay.

CONCLUSIONS

There was a slightly longer TAT in the CCL compared with running the assay directly within the OR (median difference of approximately 7 min). For a majority of the patients, both methods showed equivalent rates of PTH decline; however, for approximately 20% of the patients, there was a slower rate of PTH decline using the Roche assay.

Tolerance to sustained activation of the cAMP/Creb pathway activity in osteoblastic cells is enabled by loss of p53

The loss of p53 function is a central event in the genesis of osteosarcoma (OS). How mutation of p53 enables OS development from osteoblastic lineage cells is poorly understood. We and others have reported a key role for elevated and persistent activation of the cAMP/PKA/Creb1 pathway in maintenance of OS. In view of the osteoblast lineage being the cell of origin of OS, we sought to determine how these pathways interact within the context of the normal osteoblast. Normal osteoblasts (p53 WT) rapidly underwent apoptosis in response to acute elevation of cAMP levels or activity, whereas p53-deficient osteoblasts tolerated this aberrant cAMP/Creb level and activity. Using the p53 activating small-molecule Nutlin-3a and cAMP/Creb1 activator forskolin, we addressed the question of how p53 responds to the activation of cAMP. We observed that p53 acts dominantly to protect cells from excessive cAMP accumulation. We identify a Creb1-Cbp complex that functions together with and interacts with p53. Finally, translating these results we find that a selective small-molecule inhibitor of the Creb1-Cbp interaction demonstrates selective toxicity to OS cells where this pathway is constitutively active. This highlights the cAMP/Creb axis as a potentially actionable therapeutic vulnerability in p53-deficient tumors such as OS. These results define a mechanism through which p53 protects normal osteoblasts from excessive or abnormal cAMP accumulation, which becomes fundamentally compromised in OS.

Evolution of parathyroid hormone receptor family and their ligands in vertebrate

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

Functional characterization and evolution of PTH/PTHrP receptors: insights from the chicken

BACKGROUND

The parathyroid hormone (PTH)-family consists of a group of structurally related factors that regulate calcium and bone homeostasis and are also involved in development of organs such as the heart, mammary gland and immune system. They interact with specific members of family 2 B1 G-protein coupled receptors (GPCRs), which have been characterised in teleosts and mammals. Two PTH/PTHrP receptors, PTH1R and PTH2R exist in mammals and in teleost fish a further receptor PTH3R has also been identified. Recently in chicken, PTH-family members involved in calcium transport were characterized and specific PTHRs are suggested to exist although they have not yet been isolated or functionally characterized. The aim of this study is to further explore the evolution and function of the vertebrate PTH/PTHrP system through the isolation, phylogenetic analysis and functional characterization of the chicken receptors.

RESULTS

Two PTHRs were isolated in chicken and sequence comparison and phylogenetic analysis indicate that the chicken receptors correspond to PTH1R and PTH3R, which emerged prior to the teleost/tetrapod divergence since they are present in cartilaginous fish. The vertebrate PTH2R receptor and its ligand TIP39 have been lost from bird genomes. Chicken PTH1R and PTH3R have a divergent and widespread tissue expression and are also evident in very early embryonic stages of development. Receptor stimulation studies using HEK293 cells stably expressing the chicken PTH1R and PTH3R and monitoring cAMP production revealed they are activated by chicken 1-34 N-terminal PTH-family peptides in a dose dependent manner. PTH-L and PTHrP were the most effective peptides in activating PTH1R (EC(50) = 7.7 nM and EC(50) = 22.7 nM, respectively). In contrast, PTH-L (100 nM) produced a small cAMP accumulation on activation of PTH3R but PTHrP and PTH (EC(50) = 2.5 nM and EC(50) = 22.1 nM, respectively) readily activated the receptor. PTHrP also stimulated intracellular Ca(2+) accumulation on activation of PTH1R but not PTH3R.

CONCLUSION

Two PTHR homologues of the vertebrate PTH1R and PTH3R were isolated and functionally characterized in chicken. Their distinct pattern of expression during embryo development and in adult tissues, together with their ligand preference, suggests that they have acquired specific functions, which have contributed to their maintenance in the genome. PTH2R and its activating ligand, TIP39, are absent from bird genomes. Nonetheless identification of putative PTH2R and TIP39 in the genome of an ancient agnathan, lamprey, suggests the PTH/PTHrP ligand and receptor family was already present in an early basal paraphyletic group of vertebrates and during the vertebrate radiation diverged via gene/genome duplication and deletion events. Knowledge of the role PTH/PTHrP system in early vertebrates will help to establish evolution of function.

Parathyroid hormone-related peptide (PTHrP), parathyroid hormone/parathyroid hormone-related peptide receptor 1 (PTHR1), and MSX1 protein are expressed in central and peripheral giant cell granulomas of the jaws

OBJECTIVE

Parathyroid hormone-related peptide (PTHrP) binds to the parathyroid hormone receptor type 1 (PTHR1), which results in the activation of pathways in osteoblasts that promote osteoclastogenesis through the RANK/RANKL system. RANK/RANKL expression has been shown in central giant cell granuloma of the jaws but PTHrP/PTHR1 has not. MSX1 protein is a classical transcription regulator which promotes cell proliferation and inhibits cell differentiation by inhibiting master genes in tissues such as bone and muscle. It has been implicated in the pathogenesis of cherubism, and its expression has been reported in a single central giant cell granuloma (CGCG) case. We aimed, therefore, to study the expression of those proteins by the different cellular populations of central and peripheral giant cell granulomas (PGCGs) of the jaws.

STUDY DESIGN

Twenty cases of CGCG and 20 cases of PGCG of the jaws were retrospectively examined by immunohistochemistry for the percentage of positively staining cells to antibodies for PTHrP, PTHR1, and MSX1, using a semiquantitative method.

RESULTS

In both CGCG and PGCG of the jaws, PTHrP and PTHR1 were abundantly expressed by type I multinucleated giant cells (MGC) and mononucleated stromal cells (MSC) with vesicular nuclei, whereas type II MGC and MSC with pyknotic nuclei expressed those proteins to a lesser extent. In both CGCG and PGCG of the jaws, MSX1 was abundantly expressed by type I MGC and MSC but type II MGC did not express it. A statistically significant difference (P < .05) was observed between CGCG and PGCG in the expression of PTHrP in type II MGC and MSC with pyknotic nuclei and in the expression of PTHR1 in type II MGC.

CONCLUSIONS

We suggest that in CGCG and PGCG of the jaws, PTHrP-positive immature osteoblasts activate PTHR1-positive mature osteoblasts to produce RANKL which interacts with RANK on the PTHrP/PTHR1-positive osteoclast-precursor cells found in abundance in the stroma of giant cell lesions and induces osteoclastogenesis through the classic pathway. Cells of the jawbones, the periodontal ligament, or the dental follicle, originating from the neural crest, may be involved in the pathogenesis of giant cell lesions of the jaws. Further study is required for these suggestions to be proved.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

Plasma parathyroid hormone-related peptide and bone metabolism in periparturient dairy cows

The first weeks of lactation in dairy cows are characterised by elevated bone resorption. The connection between lactation and bone metabolism is still much discussed. In this work, changes in the concentration of plasma parathyroid hormone-related peptide (PTHrP) and markers of bone metabolism were studied in Holstein cows and heifers in the dry period and early lactation to determine the role of PTHrP in the relationship between the rate of bone remodelling and the onset of lactation in dairy cows. Blood samples were taken 14 days before calving ('D-14', n = 23) and then on day 10 ('D+10', n = 21) and day 30 after calving ('D+30', n = 23). Using enzyme immunoassay (EIA), the concentrations of PTHrP, parathyroid hormone (PTH), carboxyterminal cross-linked telopeptide of type I collagen (CTX) and oestradiol and the activity of bone specific alkaline phosphatase (BSALP) were determined. The results showed a statistically significant increase in plasma PTHrP (p < 0.005) and CTX (p < 0.0001) in cows on 'D+10' as compared to 'D-14' and CTX on 'D+30' as compared to 'D-14' (p < 0.0001). Significant negative correlations were found between the concentrations of PTHrP and oestradiol (r = -0.29, p < 0.05) and those of CTX and oestradiol (r = -0.54, p < 0.0001). In nonpregnant heifers (n = 6), the concentration of CTX and the activity of BSALP were significantly higher (p < 0.0001) than in dry cows. The observed increments of PTHrP and bone resorption after parturition reveal adaptations of bone metabolism to lactation in dairy cows.

Synthesis and characterization of novel biotinylated carboxyl-terminal parathyroid hormone peptides that specifically crosslink to the CPTH-receptor

Parathyroid hormone (PTH) regulates calcium, phosphorous and skeletal homeostasis via interaction with the G protein-coupled PTH/PTHrP receptor, which is fully activated by the amino-terminal 34 amino-acid portion of the hormone. Recent evidence points to the existence of another class of receptors for PTH that recognize the carboxyl (C)-terminal region of intact PTH (1-84) (CPTHRs) and are highly expressed by osteocytes. Here we report the synthesis and characterization of two novel bifunctional CPTH ligands that include benzoylphenylalanine (Bpa) substitutions near their amino-termini and carboxyl-terminal biotin moieties, as well as a tyrosine(34) substitution to enable radioiodination. These peptides are shown to bind to CPTHRs with affinity similar to that of PTH (1-84) and to be specifically and covalently crosslinked to CPTHRs upon exposure to ultraviolet light. Crosslinking to osteocytes or osteoblastic cells generates complexes of 80 and 220 kDa, of which the larger form represents an aggregate that can be resolved into the 80 kDa. The crosslinked products can be further purified using immunoaffinity and avidin-based affinity procedures. While the molecular structure of the CPTHR(s) remains undefined, these bifunctional ligands represent powerful new tools for use in isolating and characterizing CPTHR protein(s).

The parathyroid hormone family of peptides: structure, tissue distribution, regulation, and potential functional roles in calcium and phosphate balance in fish

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) are two factors that share amino acid sequence homology and act via a common receptor. In tetrapods, PTH is the main endocrine factor acting in bone and kidney to regulate calcium and phosphate. PTHrP is an essential paracrine developmental factor present in many tissues and is involved in the regulation of ossification, mammary gland development, muscle relaxation, and other functions. Fish apparently lack an equivalent of the parathyroid gland and were long thought to be devoid of PTH. Only in recent years has the existence of PTH-like peptides and their receptors in fish been firmly established. Two forms of PTH, two of PTHrP, and a protein with intermediate characteristics designated PTH-L are encoded by separate genes in teleost fish. Three receptors encoded by separate genes in fish mediate PTH/PTHrP actions, whereas only two receptors have so far been found in terrestrial vertebrates. PTHrP has been more intensively studied than PTH, from lampreys to advanced teleosts. It is expressed in many tissues and is present in high concentration in fish blood. Administration of this peptide alters calcium metabolism and has marked effects on associated gene expression and enzyme activity in vivo and in vitro. This review provides a comprehensive overview of the physiological roles, distribution, and molecular relationships of the piscine PTH-like peptides.

Identification of a PTH regulated gene selectively induced in vivo during PTH-mediated bone formation

The biological activities of parathyroid hormone (PTH) on bone are quite complex as demonstrated by its catabolic and anabolic activities on the skeleton. Although there have been many reports describing genes that are regulated by PTH in osteoblast cells, the goal of this study was to utilize a well-established in vivo PTH anabolic treatment regimen to identify genes that mediate bone anabolic effects of PTH. We identified a gene we named PTH anabolic induced gene in bone (PAIGB) that has been reported as brain and acute leukemia cytoplasmic (BAALC). Therefore, using the latter nomenclature, we have discovered that BAALC is a PTH-regulated gene whose mRNA expression was selectively induced in rat tibiae nearly 100-fold (maximal) by a PTH 1-34 anabolic treatment regimen in a time-dependent manner. Although BAALC is broadly expressed, PTH did not regulate BAALC expression in other PTH receptor expressing tissues and we find that the regulation of BAALC protein by PTH in vivo is confined to mature osteoblasts. Further in vitro studies using rat UMR-106 osteoblastic cells show that PTH 1-34 rapidly induces BAALC mRNA expression maximally by 4 h while the protein was induced by 8 h. In addition to being regulated by PTH 1-34, BAALC expression can also be induced by other bone forming factors including PGE(2) and 1,25 dihydroxy vitamin D(3). We determined that BAALC is regulated by PTH predominantly through the cAMP/PKA pathway. Finally, we demonstrate in MC3T3-E1 osteoblastic cells that BAALC overexpression regulates markers of osteoblast differentiation, including downregulating alkaline phosphatase and osteocalcin expression while inducing osteopontin expression. We also demonstrate that these transcriptional responses mediated by BAALC are similar to the responses elicited by PTH 1-34. These data, showing BAALC overexpression can mimic the effect of PTH on markers of osteoblast differentiation, along with the observations that BAALC is induced selectively with a bone anabolic treatment regimen of PTH (not a catabolic treatment regimen), suggest that BAALC may be an important mediator of the PTH anabolic action on bone cell function.

Parathyroid Hormone 7-84 Induces Hypocalcemia and Inhibits the Parathyroid Hormone 1-84 Secretory Response to Hypocalcemia in Rats with Intact Parathyroid Glands

Biologic effects of large C-terminal parathyroid hormone (PTH) fragments, opposite to those of N-terminal PTH, have been demonstrated. C-terminal PTH fragments are co-secreted with N-terminal PTH from the parathyroids. The aim of our study was to examine whether C-terminal PTH 7-84 regulates secretion of PTH 1-84 and affects the expression of genes of relevance for parathyroid function, PTH, calcium-sensing receptor (CaR), PTH type 1 receptor (PTHR1), and PTH-related peptide (PTHrP) genes in rat parathyroid glands. PTH 7-84 induced a significant decrease in plasma Ca2+ in rats with intact parathyroid glands. Despite the reduction of plasma Ca2+, no stimulation of PTH 1-84 secretion took place. Furthermore, the PTH 1-84 secretory response to EGTA-induced acute and severe hypocalcemia was significantly inhibited by PTH 7-84. During recovery from hypocalcemia, plasma Ca2+ levels were significantly lower in the PTH 7-84-treated group, as compared with the vehicle group, and at the same time plasma PTH 1-84 levels were significantly suppressed. The expression of PTH, CaR, PTHR1, and PTHrP genes in the rat parathyroid glands was not affected by PTH 7-84. The peripheral metabolism of PTH 1-84 was not affected by PTH 7-84. PTH 7-84 did not cross-react with the rat bioactive PTH 1-84 assay. In normal rats with intact parathyroid glands, PTH 7-84 inhibited the PTH 1-84 secretory response to hypocalcemia and induced a significant decrease in plasma Ca2+. These effects of PTH 7-84 on PTH 1-84 secretion and on plasma Ca2+ levels were not associated with significant changes in PTH, PTHR1, CaR, and PTHrP gene expressions in the rat parathyroid glands. It is hypothesized that PTH 7-84 regulates PTH secretion via an autocrine/paracrine regulatory mechanism.

Age-related changes in the response of intestinal cells to parathyroid hormone

The concept of the role(s) of parathyroid hormone (PTH), has expanded from that on acting on the classical target tissues, bone and kidney, to the intestine where its actions are of regulatory and developmental importance: regulation of intracellular calcium through modulation of second messengers and, activation of mitogenic cascades leading to cell proliferation. Several causes have been postulated to modify the hormone response in intestinal cells with ageing, among them, alterations of PTH receptor (PTHR1) binding sites, reduced expression of G proteins and hormone signal transduction changes. The current review summarizes the actual knowledge regarding the molecular and biochemical basis of age-impaired PTH receptor-mediated signaling in intestinal cells. A fundamental understanding of why PTH functions are impaired with age will enhance our understanding of its importance in intestinal cell physiology.

Topical PTH (1-34) is a novel, safe and effective treatment for psoriasis: a randomized self-controlled trial and an open trial

BACKGROUND

There continues to be a need to develop new pharmacological approaches for treating the common skin disease psoriasis. Human skin produces parathyroid hormone related peptide. This peptide is a potent inhibitor of epidermal cell growth.

OBJECTIVES

A programme was initiated to determine whether an agonist of this peptide's receptor, PTH (1-34), could be developed as a drug to treat psoriasis.

METHODS

PTH (1-34) was formulated in Novasome A cream. Fifteen adult patients with chronic plaque psoriasis who had failed to respond to at least one standard treatment were enrolled in a randomized double-blinded placebo self-controlled trial. The patients topically applied to a 25-cm2 psoriatic lesion 0.1 g of either Novasome A cream or Novasome A cream that contained 20 microg of PTH (1-34) twice a day for 2 months. At the end of the double-blind study, patients were enrolled in an open large area study. Ten patients applied PTH (1-34) (50 microg per 0.1 g) once daily to their psoriatic lesions. The patients were evaluated for their global improvement and calcium metabolism.

RESULTS

Novasome A cream enhanced the percutaneous absorption of PTH (1-34) in human skin in comparison with formulations in propylene glycol or normal saline. Psoriatic lesions treated with PTH (1-34) showed marked improvement in scaling, erythema and induration. There was a 67.3% improvement in the global severity score for the lesion treated with PTH (1-34) compared with the placebo-treated lesion, which only showed a 17.8% improvement. Ten patients topically applied PTH (1-34) on all of their lesions in a stepwise manner. A Psoriasis Area and Severity Index score analysis of all the patients revealed improvement of 42.6% (P < 0.02). None of the patients experienced hypercalcaemia or hypercalciuria or developed any side-effect to the medication.

CONCLUSIONS

Patients who were resistant to at least one standard therapy for psoriasis had a remarkable improvement in their psoriasis when they applied PTH (1-34) to their lesion(s). No untoward toxicity was observed in any of the subjects. This pilot study suggests that topical PTH (1-34) is a safe and effective novel therapy for psoriasis.

Determination of tissue and plasma concentrations of PTHrP in fish: development and validation of a radioimmunoassay using a teleost 1-34 N-terminal peptide

A specific and sensitive radioimmunoassay (RIA) for the N-terminus of sea bream (Sparus auratus) and flounder (Platichthys flesus) parathyroid hormone-related protein (PTHrP) was developed. A (1-34) amino-terminal sequence of flounder PTHrP was synthesized commercially and used as the antigen to generate specific antiserum. The same sequence with an added tyrosine (1-35(Tyr)) was used for iodination. Human (1-34) parathyroid hormone (PTH), human (1-34) PTHrP, and rat (1-34) PTHrP did not cross-react with the antiserum or displace the teleost peptide. Measurement of PTHrP in fish plasma was only possible after denaturing by heat treatment due to endogenous plasma binding activity. The minimum detectable concentration of (1-34) PTHrP in the assay was 2.5 pg/tube. The level of immunoreactive (1-34) PTHrP in plasma was 5.2+/-0.44 ng/ml (mean+/-SEM, n=20) for flounder and 2.5+/-0.29 ng/ml (n=64) for sea bream. Dilution curves of denatured fish plasma were parallel to the assay standard curve, indicating that the activity in the samples was indistinguishable immunologically from (1-34) PTHrP. Immunoreactivity was present, in order of abundance, in extracts of pituitary, oesophagus, kidney, head kidney, gills, intestine, skin, muscle, and liver. The pituitary gland and oesophagus contained the most abundant levels of PTHrP, 37.7+/-6.1 ng/g wet tissue and 2.3+/-0.7 ng/g wet tissue, respectively. The results suggest that in fish PTHrP may act in a paracrine and/or autocrine manner but may also be a classical hormone with the pituitary gland as a potential major source of the protein.

Expression of inducible cAMP early repressor is coupled to the cAMP-protein kinase A signaling pathway in osteoblasts

We previously showed that parathyroid hormone (PTH) induces inducible cAMP early repressor (ICER) in osteoblastic cells and mouse calvariae. PTH signaling in osteoblastic cells is transduced by PTH receptor 1, which is coupled to cAMP-protein kinase A (PKA), protein kinase C (PKC), and calcium signaling pathways. In the present study, we examined the role of these pathways in mediating PTH-induced ICER mRNA and protein expression in osteoblastic MC3T3-E1 cells. Using RT-PCR, we found that PTH(1-34), forskolin (FSK), and 8-bromo-cAMP (8Br-cAMP) induced ICER expression, while phorbol myristate acetate (PMA), ionomycin, and PTH(3-34) did not. Similar results were found for the induction of ICER protein. PKA inhibition by H89 markedly reduced PTH- and FSK-induced ICER expression, while PKC depletion by PMA had little effect. We also tested ICER induction by other osteotropic signaling agonists. Other cAMP-PKA pathway activators, such as PTH-related protein (PTHrP), induced ICER expression, while agents that signal through other pathways did not. PTHrP maximally induced ICER mRNA at 2-4 h, which then returned to baseline by 10 h. Finally, PTH, FSK, and PTHrP induced ICER in cultured mouse calvariae and osteoblastic ROS 17/2.8, UMR-106, and Pyla cells. We conclude that ICER expression in osteoblasts requires activation of the cAMP-PKA signaling pathway.

The investigation of hypocalcaemia and rickets

Our understanding of disorders that present with hypocalcaemia has advanced rapidly in the past decade. The molecular basis of many of these disorders and conditions associated with phosphate wasting has now been established. While many children will need specialist involvement, they often will present to general paediatricians, and appropriate investigations prior to intervention will enable early diagnosis. Not all children with hypocalcaemia and low or low normal parathyroid hormone levels have isolated hypoparathyroidism, and clinicians need to be aware of the potential for misdiagnosis. Outpatient departments and paediatric wards should have a readily accessible and comprehensive list of bloods that need to be taken when a child presents with hypocalcaemia or rickets.

Characterization of PTH/PTHrP receptor in rat duodenum: effects of ageing

In rat enterocytes, signaling through the parathyroid hormone (PTH)/PTH-related peptide receptor type 1(PTHR1) includes stimulation of adenylyl cyclase, increases of intracellular calcium, activation of phospholipase C, and the MAP kinase pathway, mechanisms that suffer alterations with ageing. The purpose of this study was to evaluate whether an alteration at the level of the PTH receptor (PTHR1) is the basis for impaired PTH signaling in aged rat enterocytes. Western Blot analysis with a specific monoclonal anti-PTHR1 antibody revealed that a 85 kDa PTH binding component, the size expected for the mature PTH/PTHrP receptor, localizes in the basolateral (BLM) and brush border (BBM) membranes of the enterocyte, being the protein expression about 7-fold higher in the BLM. Two other bands of 105 kDa (corresponding to highly glycosylated, incompletely processed receptor form) and 65 kDa (proteolytic fragment) were also seen. BLM PTHR1 protein expression significantly decreases with ageing, while no substantial decrease was observed in the BBM from old rats. PTHR1 immunoreactivity was also present in the nucleus where PTHR1 protein levels were similar in enterocytes from young and aged rats. Immunohistochemical analysis of rat duodenal sections showed localization of PTHR1 in epithelial cells all along the villus with intense staining of BBM, BLM, and cytoplasm. The nuclei of these cells were reactive to the PTHR1 antiserum, but not all cells showed the same nuclear staining. The receptor was also detected in the mucosae lamina propria cells, but was absent in globets cells from epithelia. In aged rats, PTHR1 immunoreactivity was diffused in both membranes and cytoplasm and again, PTH receptor expression was lower than in young animals, while the cell nuclei showed a similar staining pattern than in young rats. Ligand binding to PTHR1 was performed in purified BLM. rPTH(1-34) displaced [I(125)]PTH(1-34) binding to PTHR1 in a concentration-dependent fashion. In both, aged (24 months) and young (3 months) rats, binding of [I(125)]PTH was characterized by a single class of high-affinity binding sites. The affinity of the receptor for PTH was not affected by age. The maximum number of specific PTHR1 binding sites was decreased by 30% in old animals. The results of this study suggest that age-related declines in PTH regulation of signal transduction pathways in rat enterocytes may be due, in part, to the loss of hormone receptors.

Signal transduction in the parathyroid

PURPOSE OF REVIEW

To review recent developments regarding the mechanisms underlying the regulation of various aspects of parathyroid function.

RECENT FINDINGS

New studies published during the past year focused on calcium sensing via the calcium-sensing receptor, signal transduction within parathyroid cells, regulation of parathyroid hormone secretion, and the role of caveolae in calcium-sensing receptor-mediated signal transduction. In recent years, in-vitro and in-vivo studies have suggested a dominant role for the calcium-sensing receptor in the regulation of not only parathyroid hormone secretion but also parathyroid cellular proliferation by extracellular calcium. The development of a mouse model for primary hyperparathyroidism that over expresses cyclin D1 in the parathyroid gland provides an experimental system for studying the molecular basis for reduced calcium receptor expression and its role in the pathophysiology of primary hyperparathyroidism. There is also increasing evidence for the importance of vitamin D and the level of inorganic phosphate in regulating parathyroid function.

SUMMARY

Important advances are being made in understanding extracellular calcium- and calcium-sensing receptor-regulated signal transduction in the parathyroid but the subsequent steps coupling the calcium-sensing receptor to the control of parathyroid hormone secretion and parathyroid cellular proliferation remain to be fully elucidated.

Calcium-sensing receptors and parathyroid hormone-related protein in the caudal neurosecretory system of the flounder (Platichthys flesus)

The caudal neurosecretory system of the flounder (Platichthys flesus) has been examined by immunocytochemistry and in situ hybridization for the expression of parathyroid hormone-related protein (PTHrP) and calcium-sensing receptors (CaSR). The N-terminus nucleotide and deduced amino acid sequences of flounder PTHrP were determined and used to prepare oligonucleotide probes and homologous antiserum. The Dahlgren cells of the posterior spinal cord and their axons contained PTHrP protein which was also detected around the capillaries of the urophysis. PTHrP gene expression was abundant in the Dahlgren perikarya and axons in the spinal cord, but it was absent from nerve endings in the urophysis. Calcium-sensing receptor protein was present in the Dahlgren perikarya and axons, also with abundant gene expression, but there was neither protein nor mRNA in the urophysis. There were no apparent differences between freshwater- and seawater-adapted fish in either CaSR or PTHrP expression in the caudal neurosecretory system. These observations suggest that Dahlgren cells produce PTHrP which may be released from axons abutting capillaries in the urophysis. However, the sensing of ionic calcium appears to be confined to the perikarya of the Dahlgren cells in the spinal cord neuropil, suggesting that they are responsive to calcium in the central nervous system rather than the general circulation.

Parathyroid hormone-dependent signaling pathways regulating genes in bone cells

Parathyroid hormone (PTH) is an 84-amino-acid polypeptide hormone functioning as a major mediator of bone remodeling and as an essential regulator of calcium homeostasis. PTH and PTH-related protein (PTHrP) indirectly activate osteoclasts resulting in increased bone resorption. During this process, PTH changes the phenotype of the osteoblast from a cell involved in bone formation to one directing bone resorption. In addition to these catabolic effects, PTH has been demonstrated to be an anabolic factor in skeletal tissue and in vitro. As a result, PTH has potential medical application to the treatment of osteoporosis, since intermittent administration of PTH stimulates bone formation. Activation of osteoblasts by PTH results in expression of genes important for the degradation of the extracellular matrix, production of growth factors, and stimulation and recruitment of osteoclasts. The ability of PTH to drive changes in gene expression is dependent upon activation of transcription factors such as the activator protein-1 family, RUNX2, and cAMP response element binding protein (CREB). Much of the regulation of these processes by PTH is protein kinase A (PKA)-dependent. However, while PKA is linked to many of the changes in gene expression directed by PTH, PKA activation has been shown to inhibit mitogen-activated protein kinase (MAPK) and proliferation of osteoblasts. It is now known that stimulation of MAPK and proliferation by PTH at low concentrations is protein kinase C (PKC)-dependent in both osteoblastic and kidney cells. Furthermore, PTH has been demonstrated to regulate components of the cell cycle. However, whether this regulation requires PKC and/or extracellular signal-regulated kinases or whether PTH is able to stimulate other components of the cell cycle is unknown. It is possible that stimulation of this signaling pathway by PTH mediates a unique pattern of gene expression resulting in proliferation in osteoblastic and kidney cells; however, specific examples of this are still unknown. This review will focus on what is known about PTH-mediated cell signaling, and discuss the established or putative PTH-regulated pattern of gene expression in osteoblastic cells following treatment with catabolic (high) or anabolic (low) concentrations of the hormone.

Parathyroid hormone-related protein and its receptors: nuclear functions and roles in the renal and cardiovascular systems, the placental trophoblasts and the pancreatic islets

The cloning of the so-called 'parathyroid hormone-related protein' (PTHrP) in 1987 was the result of a long quest for the factor which, by mimicking the actions of PTH in bone and kidney, is responsible for the hypercalcemic paraneoplastic syndrome, humoral calcemia of malignancy. PTHrP is distinct from PTH in a number of ways. First, PTHrP is the product of a separate gene. Second, with the exception of a short N-terminal region, the structure of PTHrP is not closely related to that of PTH. Third, in contrast to PTH, PTHrP is a paracrine factor expressed throughout the body. Finally, most of the functions of PTHrP have nothing in common with those of PTH. PTHrP is a poly-hormone which comprises a family of distinct peptide hormones arising from post-translational endoproteolytic cleavage of the initial PTHrP translation products. Mature N-terminal, mid-region and C-terminal secretory forms of PTHrP are thus generated, each of them having their own physiologic functions and probably their own receptors. The type 1 PTHrP receptor, binding both PTH(1-34) and PTHrP(1-36), is the only cloned receptor so far. PTHrP is a PTH-like calciotropic hormone, a myorelaxant, a growth factor and a developmental regulatory molecule. The present review reports recent aspects of PTHrP pharmacology and physiology, including: (a) the identification of new peptides and receptors of the PTH/PTHrP system; (b) the recently discovered nuclear functions of PTHrP and the role of PTHrP as an intracrine regulator of cell growth and cell death; (c) the physiological and developmental actions of PTHrP in the cardiovascular and the renal glomerulo-vascular systems; (d) the role of PTHrP as a regulator of pancreatic beta cell growth and functions, and, (e) the interactions of PTHrP and calcium-sensing receptors for the control of the growth of placental trophoblasts. These new advances have contributed to a better understanding of the pathophysiological role of PTHrP, and will help to identify its therapeutic potential in a number of diseases.

Pathophysiological mechanisms of vascular calcification in end-stage renal disease

Vascular calcification has been clearly defined as a risk factor for cardiovascular mortality in the general population and is highly prevalent in end-stage renal disease (ESRD), where it is associated with a number of markers of increased mortality such as left ventricular hypertrophy. The pattern of calcification in ESRD is characterized by mineral deposition in the tunica media, in contrast to non-ESRD populations, where calcification of atheromatous plaque predominates. This difference may have important clinical implications. The pathophysiological mechanisms underlying both types of vascular calcification remain to be clarified; however, current evidence suggests that they are active processes rather than passive mineral precipitation, and the presence in the vasculature of cells expressing an osteoblastic phenotype may be of central importance. In ESRD, the presence of secondary and tertiary hyperparathyroidism, disordered calcium and phosphate homeostasis, and the use of vitamin D- and calcium-based treatments in its therapy may all contribute to vascular calcification. These issues and the impact on other current and future therapies have great importance for clinical nephrology, and a better understanding of vascular calcification through a focused research effort is essential.

Parathyroid hormone-related protein-(1-34) inhibits intrinsic pump activity of isolated murine lymph vessels

Parathyroid hormone-related protein (PTHrP) was originally found as a tumor-derived vasoactive factor and has also been known to produce significant relaxation of vascular smooth muscles. Thus effects of PTHrP-(1-34), a PTH receptor-binding domain, on spontaneous lymphatic pump activity was investigated in isolated pressurized lymph vessels of mice. Low concentrations (1 x 10(-10) and 3 x 10(-10) M) of PTHrP-(1-34) dilated lymph vessels and reduced the frequency of pump activity, whereas high concentrations (1 x 10(-9) to 1 x 10(-8) M) of PTHrP-(1-34) caused dilation with cessation of the lymphatic pump activity. N(omega)-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-5) M) but not indomethacin (1 x 10(-5) M) significantly reduced the PTHrP-(1-34)-induced inhibitory responses of the lymphatic pump activity. In the presence of L-NAME (3 x 10(-5) M) and L-arginine (1 x 10(-3) M), the L-NAME-induced inhibition in the PTHrP-(1-34)-mediated responses was significantly reduced. Glibenclamide (1 x 10(-6) M) significantly suppressed the inhibitory responses of the lymphatic pump activity induced by PTHrP-(1-34) and S-nitroso-N-acetyl-penicillamine. The PTHrP-(1-34)-mediated inhibitory responses were significantly reduced by treatment with PTHrP-(7-34) (1 x 10(-7) M). These results suggest that PTHrP-(1-34) inhibits spontaneous pump activity of the isolated lymph vessels via PTH receptors and that production and release of endogenous nitric oxide and activation of ATP-sensitive K(+) channels in the lymph vessels contribute to the PTHrP-(1-34)-mediated inhibitory responses of the lymphatic pump activity.

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

PTH is a potent systemic regulator of cellular differentiation and function in bone. It acts upon cells of the osteoblastic lineage via the G protein-coupled type-1 PTH/PTH-related peptide receptor (PTH1R). Carboxyl fragments of intact PTH(1-84) (C-PTH fragments) are cosecreted with it by the parathyroid glands in a calcium-dependent manner and also are generated via proteolysis of the hormone in peripheral tissues. Receptors that recognize C-PTH fragments (CPTHRs) have been described previously in osteoblastic and chondrocytic cells. To directly study CPTHRs in bone cells, we isolated clonal, conditionally transformed cell lines from fetal calvarial bone of mice that are homozygous for targeted ablation of the PTH1R gene and transgenically express a temperature-sensitive mutant SV40 T antigen. Cells with the highest specific binding of the CPTHR radioligand (125)I-[Tyr(34)]hPTH(19-84) exhibited a stellate, dendritic appearance suggestive of an osteocytic phenotype and expressed 6- to 10-fold more CPTHR sites/cell than did osteoblastic cells previously isolated from the same bones. In these osteocytic (OC) cells, expression of mRNAs for CD44, connexin 43, and osteocalcin was high, whereas that for alkaline phosphatase and cbfa-1/osf-2 was negligible. The CPTHR radioligand was displaced completely by hPTH(1-84), hPTH(19-84) and hPTH(24-84) (IC(50)s = 20-50 nM) and by hPTH(39-84) (IC(50) = 500 nM) but only minimally (24%) by 10,000 nM hPTH(1-34). CPTHR binding was down-regulated dose dependently by hPTH(1-84), an effect mimicked by ionomycin and active phorbol ester. Human PTH(1-84) and hPTH(39-84) altered connexin 43 expression and increased apoptosis in OC cells. Apoptosis induced by PTH(1-84) was blocked by the caspase inhibitor DEVD. We conclude that osteocytes, the most abundant cells in bone, may be principal target cells for unique actions of intact PTH(1-84) and circulating PTH C-fragments that are mediated by CPTHRs.

Aquaporin-5 expression, but not other peripheral lung marker genes, is reduced in PTH/PTHrP receptor null mutant fetal mice

Parathyroid hormone-related peptide (PTHrP) and the parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor are important developmental regulators of cell growth and differentiation in some organs. In lung, both the peptide and the receptor are expressed early in development and in alveolar cells in adults. In adult alveolar cells, PTHrP appears to promote the alveolar type II cell phenotype in vitro. Mice carrying null mutations in genes for either receptor or ligand die at birth of respiratory failure. To determine if absence of the PTH/PTHrP receptor alters morphogenesis or cellular differentiation of the distal lung, we analyzed the morphology and gene expression patterns in PTH/PTHrP receptor null mutant mice right before birth and compared them with wild-type and heterozygous null littermates. Using semiquantitative Northern blots, we observed that messenger RNA (mRNA) for aquaporin-5, the type I cell-specific water channel, was markedly decreased. The abundance of other marker mRNAs for type I and type II cell phenotypes, including T1alpha, surfactant proteins, and others, was unaltered. Gross morphology and lung pattern, assessed by in situ hybridization for surfactant protein C, were normal. We conclude therefore that, although signaling through this receptor may influence expression of specific lung genes, it does not play a major role in the general regulation of lung development and growth.