Structure and functional expression of a complementary DNA for porcine parathyroid hormone/parathyroid hormone-related peptide receptor

PTH and Vitamin D

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

Adrenal effects of teriparatide in the treatment of severe postmenopausal osteoporosis

The aim of our study was to investigate the effects of teriparatide on the hypophysis-adrenal axis in postmenopausal women. Treatment with teriparatide increased plasmatic and urinary levels of cortisol after 6 and 12 months. Our paper demonstrates a possible direct secretagogue effect of teriparatide on adrenals in osteoporotic postmenopausal women.

INTRODUCTION

Teriparatide, recombinant human parathyroid hormone (1-34) (rhPTH [1-34]), is approved for the treatment of osteoporosis in men and postmenopausal women at high risk for fracture. In literature, data regarding the secretagogue effect of PTH on adrenocortical cells are present on in vitro, but not on in vivo, studies. The aim of our study was to investigate the effects of teriparatide on the hypophysis-adrenal axis in postmenopausal women.

METHODS

Twenty postmenopausal women with severe osteoporosis were treated with teriparatide in a regimen of 20 μg daily, self-administered injections at bedtime for 12 months and a calcium and vitamin D supplementation. At the same time, 20 osteopenic women matched for age and body mass index with the patients were enrolled and treated only with calcium and vitamin D. In all subjects, calcium, adrenocorticotropic hormone (ACTH), and plasmatic and urinary cortisol were evaluated at baseline and after 6 and 12 months.

RESULTS

Treatment with teriparatide increased plasmatic and urinary levels of cortisol after 6 and 12 months, reaching statistical significance only after 1 year. Plasmatic levels of ACTH did not change significantly.

CONCLUSIONS

Our paper, for the first time, demonstrates a possible direct secretagogue effect of teriparatide on adrenals in osteoporotic postmenopausal women.

Direct action of parathyroid hormone-related peptide to enhance corticosterone production stimulated by adrenocorticotropic hormone in adrenocortical cells of hens

The presence of receptor for chicken parathyroid hormone-related peptide (cPTHrP) in the cortical cells of the adrenal gland of the hen was demonstrated by radioligand binding assays. When the cortical cells were incubated in vitro with chicken adrenocorticotropic hormone (cACTH) in the presence of cPTHrP, greater production of corticosterone was observed than when incubated with cACTH alone. The results suggest that PTHrP may act directly on the adrenocortical cells via its receptor binding and increase the response to ACTH for corticosterone secretion in the hen.

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.

Tuberoinfundibular peptide of 39 residues (TIP39): molecular structure and activity for parathyroid hormone 2 receptor

The neuropeptide TIP39 was recently purified from bovine hypothalamus based on the ability of the peptide to activate the parathyroid hormone 2 receptor (PTH2R) ( Nat. Neurosci. 2 (1999) 941). PTH2R is abundantly expressed in the nervous system, and its expression pattern suggests that it may play a role in modulation of pituitary function and in nociception. Towards understanding the physiological role of TIP39 and PTH2R, we cloned human, mouse and rat TIP39 gene. Our results revealed that: (1) the mature peptide is processed from a precursor; (2) TIP39 peptide is highly conserved among species; and (3) TIP39 from all species activates adenylyl cyclase and elevates intracellular calcium levels through PTH2R. We also defined and compared the structure-activity relationship of TIP39 on both activation of adenylyl cyclase and calcium mobilization pathways through PTH2R, finding common and differential determinants of TIP39 that are required for these pathways. Furthermore, we observed that TIP39 elevates intracellular calcium levels in primary dorsal root ganglion neurons whereas the peptide inactive on PTH2R do not, suggesting that TIP39 may activate these neurons important for nociception in vivo through PTH2R-dependent mechanisms.

Cell and molecular cell biology of melanin-concentrating hormone

Recent advances in the study of melanin-concentrating hormone (MCH) have depended largely on molecular biological techniques. In mammals, which have attracted the most attention, novel findings concern (i) the MCH gene, which can yield several peptides by either posttranslational cleavage or alternative splicing, as well as bidirectional transcription; (ii) the identification of two G protein-coupled MCH receptors in the brain and peripheral tissues; and (iii) the evidence for subpopulations of MCH neurons in the central nervous system, characterized by their chemical phenotypes, connections, and individual physiological responses to different physiological paradigms. The involvement of central MCH in various functions, including feeding, reproduction, stress, and behavior patterns, is reviewed. The stage during evolution at which MCH may have acquired hypophysiotrophic and hormonal functions in lower vertebrates is considered in light of morphological data. Evidence that MCH also has peripheral paracrine/autocrine effects in mammals is provided.

PTH and PTH-related peptide enhance steroid secretion from human adrenocortical cells

Parathyroid hormone (PTH) and PTH-related peptide (PTH-RP) are two hypercalcemic hormones that share a common receptor subtype, the PTH/PTH-RP receptor. PTH and PTH-RP concentration dependently enhanced basal aldosterone and cortisol secretion from dispersed human adrenocortical cells, with a maximal effective concentration (approximately 2-fold increase) of 10(-8) M. The secretagogue effect of 10(-8) M PTH or PTH-RP was abolished by the PTH/PTH-RP receptor antagonist [Leu11,D-Trp12]-PTH-RP-(7-34)-amide (10(-6) M). PTH and PTH-RP (10(-8) M) raised cAMP and inositol-triphosphate release by dispersed adrenocortical cells, and these effects were blocked by the adenylate cyclase inhibitor SQ-22536 (10(-4) M) and the phospholipase C (PLC) inhibitor U-73122 (10(-5) M), respectively. SQ-22536 (10(-4) M) and U-73122 (10(-5) M) partially inhibited aldosterone and cortisol response to 10(-8) M PTH and PTH-RP; when added together, they abolished it. Similar results were obtained by using the protein kinase (PK)A and PKC inhibitors H-89 and calphostin C (10(-5) M). It is concluded that PTH and PTH-RP exert a sizeable secretagogue action on the human adrenal cortex, probably acting through the PTH/PTH-RP receptor coupled with both adenylate cyclase/PKA- and PLC/PKC-dependent signaling cascades.

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.

Growth hormone (GH)-releasing factor differentially activates cyclic adenosine 3',5'-monophosphate- and inositol phosphate-dependent pathways to stimulate GH release in two porcine somatotrope subpopulations

Somatotropes comprise two morphologically and functionally distinct subpopulations of low (LD) and high (HD) density cells. We recently reported that GRF induces different patterns of increase in the cytosolic free Ca2+ concentration in single porcine LD and HD somatotropes, which for LD cells required not only Ca2+ influx but also intracellular Ca2+ mobilization. This suggested that GRF may activate multiple signaling pathways in pig LD and HD somatotropes to stimulate GH secretion. To address this question, we first assessed the direct GRF effect on second messenger activation in cultures of LD and HD cells by measuring cAMP levels and [3H]myo-inositol incorporation. Secondly, to determine the relative importance of cAMP- and inositol phosphate (IP)-dependent pathways, and of intra- and extracellular Ca2+, GRF-induced GH release from cultured LD and HD somatotropes was measured in the presence of specific blockers. GRF increased cAMP levels in both subpopulations, whereas it only augmented IP turnover in LD cells. Accordingly, adenylate cyclase inhibition by MDL-12,330A abolished GRF-stimulated GH release in both subpopulations, whereas phospholipase C inhibition by U-73122 only reduced this effect partially in LD cells. Likewise, blockade of Ca2+ influx with Cl2Co reduced GRF-stimulated GH secretion in both LD and HD somatotropes, whereas depletion of thapsigargin-sensitive intracellular Ca2+ stores only decreased the secretory response to GRF in LD cells. These results demonstrate that GRF specifically and differentially activates multiple signaling pathways in two somatotrope subpopulations to stimulate GH release. Thus, although the prevailing signaling cascade employed by GRF in both subpopulations is adenylate cyclase/cAMP/extracellular Ca2+, the peptide also requires activation of the phospholipase C/IP/intracellular Ca2+ pathway to exert its full effect in porcine LD somatotropes.

Comparison of recombinant human PTH(1-34) (LY333334) with a C-terminally substituted analog of human PTH-related protein(1-34) (RS-66271): In vitro activity and in vivo pharmacological effects in rats

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) are believed to exert their biological actions through binding and activation of a common cell surface receptor. Recently, an analog of PTHrP (RS-66271), was described that demonstrated reduced binding affinity for the PTH/PTHrP receptor compared with bovine PTH(1-34) but retained equal biological activity. The present study investigated the receptor binding affinities of synthetic RS-66271 and recombinant human PTH(1-34) (LY333334) and compared their in vitro and in vivo pharmacological effects. RS-66271 had one hundredth the activity of PTH(1-34) in competing for the binding of [125I] [Nle8,18, Tyr34]human PTH(1-34) to the human PTH/PTHrP receptor stably expressed in a human kidney cell line. Despite this reduced binding affinity, RS-66271 had equivalent activity in increasing both cAMP production in osteoblast-like cells and bone resorption in neonatal mouse calvariae. However, RS-66271 was 7. 6-fold less active in stimulating inositol phosphate production. For in vivo studies, young, male Fisher rats received a daily subcutaneous dose of either 10 or 40 microg/kg of peptide for 1, 2, or 4 weeks. Volumetric bone mineral density and total bone mineral content of the proximal tibia were determined by peripheral quantitative computerized tomography. Trabecular and cortical bone of the distal femur were analyzed for calcium and dry weight. Lumbar vertebrae (L4-L6) were analyzed by histomorphometry. Trabecular and cortical bone mass showed a dose- and time-dependent increase in the treated animals compared with the controls. These increases were evident as early as 1 week after initiation of dosing. There were no consistent significant differences in the comparative effects of PTH(1-34) and RS-66271 on the measured bone parameters. In conclusion, despite the reduced binding affinity of RS-66271 for the PTH/PTHrP receptor compared with human PTH(1-34), both peptides displayed similar in vitro and in vivo pharmacological effects.

Dual signaling and ligand selectivity of the human PTH/PTHrP receptor

Parathyroid hormone (PTH) activates PTH/PTH-related peptide-related receptors (PTHRs) to stimulate both adenylyl cyclase (AC) and phospholipase C (PLC). How these parallel signals mediate specific cellular and tissue responses to PTH, such as the complex anabolic versus catabolic actions of PTH on bone, remains unsettled. Previous studies of PTHR signaling and function employed mainly rodent or other cell lines that express endogenous PTHRs and, possibly, alternate species of PTH receptors. To preclude confounding effects of such receptors, we stably expressed recombinant human PTHRs (hPTHRs) at different levels of surface density in LLC-PK1 porcine renal epithelial cells that lack endogenous PTH responsiveness. hPTH(1-34) induced concentration-dependent activation of both AC and PLC via transfected hPTHRs. Maximal intensity of each signal increased with receptor density, but more hPTHRs were required for PLC than for AC activation. Coupling to AC was saturated at receptor densities too low to detect sustained PLC activation. hPTH(3-34), found by others to be a PLC/protein kinase C (PKC)-selective peptide in rat cells, did not activate PLC via human (or rat) PTHRs under conditions (1 microM peptide, 106 hPTHRs/cell) where hPTH(1-34) stimulated PLC severalfold. Other cellular responses that require PKC activation in these cells, such as sodium-dependent phosphate transport and cAMP-independent secretion of plasminogen activator, were induced by PTH(1-34) but not by hPTH(3-34) or hPTH(7-34). We conclude that amino-truncated PTH analogs reported to activate PKC cannot directly activate phosphatidylinositol-specific PLC via the human or rat PTHR and therefore that PTH receptors may access alternate, PLC-independent pathways of PKC activation in some target cells. The relative intensity of AC and PLC signaling via the hPTHR may be strongly regulated by changes in its surface expression.

Development of a scintillation proximity assay for high-throughput measurement of intact parathyroid hormone

A simple, high-throughput scintillation proximity assay (SPA) for parathyroid hormone (1-84) (PTH) has been developed. Fifteen commercially available N-terminal and six C-terminal anti-PTH antibodies were evaluated for detection of human PTH(1-84). Two C-terminal antibodies (CR1073M and 10-P55) gave the most consistent results. Using one of these antibodies (10-P55), an assay was developed with a sensitivity of 4 pg/ml for human and rat PTH(1-84). Porcine PTH(1-84) was not detectable. The intra-assay and inter-assay coefficients of variation for a 467 pg/ml sample were 6. 1 and 6.5%, respectively, and for a 21 pg/ml sample, 6.2 and 4.4%. Human PTH(1-34), while not detected in the assay, interfered with the detection of PTH(1-84). Smaller fragments [for example, human PTH(3-34)] and a C-terminal PTH fragment [PTH(53-84)] did not interfere in the assay. The procedure gave 106-110% recovery of human PTH(1-84) spiked into samples. Immunoreactive PTH concentrations in serum of rats administered EGTA were determined by SPA and by a commercially available PTH immunoassay. There was a good correlation between the two assays with significant increases in serum immunoreactive PTH concentrations at 15 and 30 min after EGTA injection and a rapid decrease to baseline values by 60 min. The SPA gives a high-throughput method for simply and accurately determining PTH(1-84) concentrations in serum.

Sequence and activity of parathyroid hormone/parathyroid hormone-related protein receptor promoter region in human osteoblast-like cells

The parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor gene has been characterized in various species. The structure of its promoter and the regulation of its expression in human tissues have, however, not been clearly established yet. We characterized the region upstream of the PTH/PTHrP receptor gene and investigated its promoter activity in the human osteoblast-like SaOS-2 cells. In this region, three untranslated exons were localized, U1 and U2 by using a kidney cDNA, and U3 by homology with the mouse gene. In human osteoblast-like cells, a distal promoter (P1) was found to be inactive, as evaluated by luciferase reporter gene assays, in contrast to the situation found in human and mouse kidney tissue. A second promoter (P2), previously described in mouse and human kidney, was shown to be active in human osteoblast-like SaOS-2 cells. We found a hitherto uncharacterized promoter (P3), closely upstream of the ATG start codon. The activities of P2 and P3 were not additive. These results provide important information on the structure of the 5' flanking region of the human PTH/PTHrP gene receptor.

Tissue-specific transcription start sites and alternative splicing of the parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor gene: a new PTH/PTHrP receptor splice variant that lacks the signal peptide

The PTH/PTHrP receptor gene is expressed in bone and kidney as well as in many other tissues. Using primer extension followed by rapid cloning of amplified complementary DNA ends, we have isolated new PTH/PTHrP receptor complementary DNAs with different splicing patterns and have characterized a new upstream transcription start site. Three 5' nontranslated exons, U3, U2 and U1, located 4.8, 2.5, and 1.2 kb upstream of the exon that encodes the putative signal peptide of the classical receptor (exon S), have been characterized. Four types of splicing patterns were recognized. Type I splicing pattern is transcribed from exon U1 and is spliced to exons S and E1; this pattern was found in most tissues tested. Types II, III, and IV splicing patterns are transcribed from exon U3 and have a restricted tissue distribution. Type II splice pattern, containing exons U3, U2, and S and type III splicing pattern, containing exon U3, U2, and E1 (skipping exon S), was found only in kidney. Type IV splice pattern, containing exon U3 and S was found both in kidney and ovary. Because the type III splice variant skips exon S, translation of this splice variant initiates at a different AUG codon. The type III splice variant was weakly expressed on the cell surface of COS-7 cells, as assessed by double antibody binding assay, and no detectable ligand binding was observed on intact cells. The type III splice variant, however, increased cAMP accumulation in COS-7 cells when challenged with PTH(1-34), PTH(1-84) and hPTHrP(1-36) with EC50s that are similar to those observed in COS-7 cells expressing the type I variant but with a maximum stimulation that was lower than that observed in COS-7 cells expressing the type I variant. These data indicate low levels of cell surface expression of the type III splice variant. Treatment of COS-7 cells with tunicamycin decreased the size of the type I splice variant from a broad band of 85 kDa to a compact band of about 60 kDa. The type III splice variant did not change in size in COS-7 cells treated with tunicamycin, indicating that the type III splice variant did not undergo any glycosylation step. In conclusion, the PTH/PTHrP receptor gene uses alternate promoters in a tissue-specific manner that results in several tissue-specific alternatively spliced transcripts. One of these transcripts, the type III splice variant, is expressed in kidney and lacks the signal peptide.