Nucleotide sequence of cloned cDNAs encoding chicken preproparathyroid hormone

Gene structure, transcripts and calciotropic effects of the PTH family of peptides in Xenopus and chicken

Background

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) belong to a family of endocrine factors that share a highly conserved N-terminal region (amino acids 1-34) and play key roles in calcium homeostasis, bone formation and skeletal development. Recently, PTH-like peptide (PTH-L) was identified in teleost fish raising questions about the evolution of these proteins. Although PTH and PTHrP have been intensively studied in mammals their function in other vertebrates is poorly documented. Amphibians and birds occupy unique phylogenetic positions, the former at the transition of aquatic to terrestrial life and the latter at the transition to homeothermy. Moreover, both organisms have characteristics indicative of a complex system in calcium regulation. This study investigated PTH family evolution in vertebrates with special emphasis on Xenopus and chicken.

Results

The PTH-L gene is present throughout the vertebrates with the exception of placental mammals. Gene structure of PTH and PTH-L seems to be conserved in vertebrates while PTHrP gene structure is divergent and has acquired new exons and alternative promoters. Splice variants of PTHrP and PTH-L are common in Xenopus and chicken and transcripts of the former have a widespread tissue distribution, although PTH-L is more restricted. PTH is widely expressed in fish tissue but from Xenopus to mammals becomes largely restricted to the parathyroid gland. The N-terminal (1-34) region of PTH, PTHrP and PTH-L in Xenopus and chicken share high sequence conservation and the capacity to modify calcium fluxes across epithelia suggesting a conserved role in calcium metabolism possibly via similar receptors.

Conclusions

The parathyroid hormone family contains 3 principal members, PTH, PTHrP and the recently identified PTH-L. In teleosts there are 5 genes which encode PTHrP (2), PTH (2) and PTH-L and in tetrapods there are 3 genes (PTHrP, PTH and PTH-L), the exception is placental mammals which have 2 genes and lack PTH-L. It is hypothesized that genes of the PTH family appeared at approximately the same time during the vertebrate radiation and evolved via gene duplication/deletion events. PTH-L was lost from the genome of eutherian mammals and PTH, which has a paracrine distribution in lower vertebrates, became the product of a specific endocrine tissue in Amphibia, the parathyroid gland. The PTHrP gene organisation diverged and became more complex in vertebrates and retained its widespread tissue distribution which is congruent with its paracrine nature.

Role of calcium channels in carboxyl-terminal parathyroid hormone receptor signaling

Parathyroid hormone (PTH), an 84-amino acid polypeptide, is a major systemic regulator of calcium homeostasis that activates PTH/PTHrP receptors (PTH1Rs) on target cells. Carboxyl fragments of PTH (CPTH), secreted by the parathyroids or generated by PTH proteolysis in the liver, circulate in blood at concentrations much higher than intact PTH-(1–84) but cannot activate PTH1Rs. Receptors specific for CPTH fragments (CPTHRs), distinct from PTH1Rs, are expressed by bone cells, especially osteocytes. Activation of CPTHRs was previously reported to modify intracellular calcium within chondrocytes. To further investigate the mechanism of action of CPTHRs in osteocytes, cytosolic free calcium concentration ([Ca2+]i) was measured in the PTH1R-null osteocytic cell line OC59, which expresses abundant CPTHRs but no PTH1Rs. [Ca2+]iwas assessed by single-cell ratiometric microfluorimetry in fura-2-loaded OC59 cells. A rapid and transient increase in [Ca2+]iwas observed in OC59 cells in response to the CPTH fragment hPTH-(53–84) (250 nM). No [Ca2+]isignal was observed in COS-7 cells, in which CPTHR binding also cannot be detected. Neither hPTH-(1–34) nor a mutant CPTH analog, [Ala55–57]hPTH-(53–84), that does not to bind to CPTHRs, increased [Ca2+]iin OC59 cells. The [Ca2+]iresponse to hPTH-(53–84) required the presence of extracellular calcium and was blocked by inhibitors of voltage-dependent calcium channels (VDCCs), including nifedipine (100 nM), ω-agatoxin IVA (10 nM), and ω-conotoxin GVIA (100 nM). We conclude that activation of CPTHRs in OC59 osteocytic cells leads to a rapid increase in influx of extracellular calcium, most likely through the opening of VDCCs.

Identification and characterization of cis-acting elements in the human and bovine PTH mRNA 3'-untranslated region

The human PTH mRNA 3'-UTR has a cis element homologous to the rat cis-acting instability element and a more proximal element identical to the single binding element identified in bovine PTH mRNA 3'-UTR. The function of the elements was shown in vitro.

INTRODUCTION

In the rat, Ca(2+) and phosphate regulate PTH mRNA stability by the interaction of trans-acting proteins with a defined cis-acting instability element in the distal region of the PTH mRNA 3'-untranslated region (UTR). This element has been characterized in the rat and is conserved in human, canine, feline, and murine 3'-UTRs but not in bovine and porcine 3'-UTRs.

MATERIALS AND METHODS

Parathyroid protein-binding assays to the PTH mRNA transcripts were performed. Functionality was studied in reporter genes that were transiently transfected into HEK293 cells.

RESULTS

Protein-RNA binding experiments identified an element in bovine PTH mRNA at the proximal end of the 3'-UTR that is different from the rat protein-binding element. The human 3'-UTR contains both elements, but only the distal element binds proteins. Functional studies with HEK293 cells transiently transfected with reporter genes containing the different elements and flanking nucleotides (nt) showed that the human distal element destabilized a reporter mRNA similar to the effect of this element in the rat. A reporter mRNA containing the single bovine PTH mRNA protein-binding element was also destabilized, and this was prevented by coexpression of AU-rich element binding factor 1 (AUF1).

CONCLUSION

Our results identify a new protein-binding element in the PTH mRNA 3'-UTR. In bovine PTH mRNA, it is the only element, and it is functional in destabilizing a reporter gene. It is also present in other species, including human PTH mRNA, where it is not functional, possibly because of differences in flanking sequences. The human PTH mRNA 3'-UTR distal element is highly homologous to the rat cis-acting instability element and destabilized a reporter gene, indicating its functionality. Therefore, different species have alternative cis-acting protein-binding elements that may determine the regulation of PTH mRNA stability in response to changes in serum calcium and phosphate.

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.

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.

Secretin, glucagon, gastric inhibitory polypeptide, parathyroid hormone, and related peptides in the regulation of the hypothalamus- pituitary-adrenal axis

Secretin, glucagon, gastric inhibitory polypeptide (GIP), and parathyroid hormone (PTH) belong, together with vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase (AC)-activating polypeptide, to a family of peptides (the VIP-secretin-glucagon family), which also includes growth hormone-releasing hormone and exendins. All the members of this peptide family possess a remarkable amino-acid sequence homology, and bind to G-protein-coupled receptors, whose signaling mechanism primarily involves AC/protein kinase A and phospholipase C/protein kinase C cascades. VIP and pituitary AC-activating polypeptide play a role in the regulation of the hypothalamus-pituitary-adrenal (HPA) axis, and in this review we survey findings that also other members of the VIP-secretin-glucagon family may have the same function. Secretin and secretin receptors are expressed in the hypothalamus and pituitary gland, and secretin inhibits adrenocorticotropic hormone (ACTH) release. No evidence is available for the presence of secretin receptors in adrenal glands, but secretin selectively depresses the glucocorticoid response to ACTH of dispersed zona fasciculata-reticularis (ZF/R) cells. Glucagon and glucagon-like peptide-1 are contained in the hypothalamus, and all the components of the HPA axis are provided with glucagon and glucagons-like-1 receptors. These peptides exert a short-term inhibitory effect on stress-induced pituitary ACTH release and depress the ZF/R cell response to ACTH by inhibiting the AC/protein kinase A cascade; they also stimulate hypothalamic arginine-vasopressin release. GIP receptors are present in the ZF/R of the normal adrenals, and are particularly abundant in some types of adrenocortical adenomas and hyperplasias. GIP, through the activation of the AC/protein kinase A cascade, evokes a sizeable glucocorticoid secretagogue effect, leading to the identification of a food/GIP-dependent Cushing's syndrome. PTH and PTH-related protein are expressed in the hypothalamus and pituitary gland, and PTH and PTH-related protein receptors in all the components of the HPA axis. Both peptides enhance ACTH and arginine-vasopressin release, as well as stimulate aldosterone and glucocorticoid secretion of dispersed zona glomerulosa and ZF/R cells, respectively. The involvement of growth hormone-releasing hormone and exendins in the functional regulation of the HPA axis has not yet been extensively investigated.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.

The (18-48)-fragment of human parathyroid hormone is a partial agonist for cAMP-dependent actions with retained mitogenic function

In the search of PTH agonists, PTH fragments were examined for both cAMP-dependent and -independent actions on bone-derived assay systems. In a periosteal cell culture system from the embryonic chick, hPTH (18-48) was capable of inducing cell proliferation in a similar manner to hPTH (1-34) and hPTH (28-48) at picomolar concentrations, whereas hPTH (53-84) was inactive. Near physiological concentrations were needed for the proliferative action of PTH which seems to argue against a role for cAMP as a main mediator for growth response. In the tibiae organ culture system of embryonic chick cAMP stimulation and calcium mobilization requires micromolar concentrations. hPTH (18-48) produced, at a concentration of 10(-6) mol/L, a 14-fold increase in cAMP synthesis and an 11-fold increase in calcium mobilization and was 100-fold less active than hPTH (1-34). hPTH (28-48) and hPTH (53-84) had no significant effect. These results demonstrate that hPTH (18-48) retains the ability to induce cell proliferation and exhibits partial agonist activity in the cAMP/PKA signal transduction pathway.

Chicken parathyroid hormone-related protein and its expression during embryologic development

A parathyroid hormone-related protein (PTHrP) is the probable cause of humoral hypercalcemia in malignancy, but its normal physiologic role remains unknown. Since current evidence suggests that PTHrP may have a role in embryonic development, we cloned a genomic fragment that encodes chicken PTHrP (cPTHrP) and studied the expression of PTHrP in developing chick embryos. Blot hybridization of chicken genomic DNA with a cPTHrP genomic DNA probe showed a single band, suggesting that a single-copy gene encodes cPTHrP. By screening a chicken genomic library with the human probe an open reading frame was identified that corresponds to the human PTHrP (hPTHrP) exon IV. Compared to the human sequence the 5' splice junction is highly conserved and the two predicted propeptide residues are identical. The sequence predicts a mature peptide of 139 amino acids; all of the first 21 and 94 of the first 112, but only 8 of the final 27 residues of cPTHrP are identical to the human sequence. The structural features required for PTH receptor binding and activation are highly conserved between chicken and hPTHrP. Poly(A)-enriched RNA from 3-15 day chicken embryos was surveyed by hybridization to the chicken probe. A hybridizing band of 1.45 kb was found in tissues derived from all three germ layers, including brain, heart, lung, liver, gizzard, intestine, chorioallantoic membrane, yolk sac, and skeletal muscle. An additional 1.2 kb hybridizing band was found in some tissues. The conservation of the PTHrP sequence between chicken and mammals supports the view that PTHrP has an important physiologic role.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic AMP-dependent inhibition of collagen synthesis in avian epiphyseal cartilage cells: effect of chicken and human parathyroid hormone and parathyroid hormone-related peptide

Avian cartilage cells derived from epiphyseal growth-plate and avian skin fibroblasts were cultured in vitro. Production of cAMP by cartilage cells was stimulated by the synthetic fragments (1-34) of chicken (cPTH), human (hPTH) parathyroid hormone and by parathyroid hormone-related peptide (PTHrP). The enhancement of cAMP production by any of the peptides could be blocked by the parathyroid hormone analogue (3-34)PTH, suggesting interaction with PTH specific receptors. When incubated with [3H]proline, both cell types released radiolabelled collagenase-digestible and non-digestible proteins into the medium. cPTH, hPTH, PTHrP, forskolin, prostaglandin E2 (PGE2) and 8-bromo cAMP inhibited collagen production in cartilage cells with only minor effects on non-collagenase digestible proteins. No effect of cAMP on collagen production by fibroblasts was observed. The present results provide additional evidence that avian growth-plate cartilage cells are targets for PTH, and are first to demonstrate the response of a non-mammalian system to mammalian PTHrP. The data suggest that collagen production by epiphyseal growth-plate cartilage cells is inhibited by PTH and that this inhibition is mediated by cAMP.