Developmental upregulation of human parathyroid hormone (PTH)/PTH-related peptide receptor gene expression from conserved and human-specific promoters

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.

Transcriptional regulation of a new variant of human platelet-derived growth factor receptor alpha transcript by E2F-1

Platelet-derived growth factors (PDGFs) and their receptors play an important role in cell proliferation, angiogenesis, and differentiation during normal development, and have also been implicated in tumorigenesis. In this study, we identified a novel variant of human PDGF receptor alpha mRNA (type II), which contains the same open reading frame as the known PDGF receptor alpha mRNA (type I) but a different 5'-untranslated region (5'-UTR). The 5'-UTR of the type II transcript was identified as a 363-bp exon located in intron 1 at position +1,210 to +1,572 relative to the transcriptional initiation site of the type I transcript. This type II transcript was expressed in a subset of human cell lines, such as MG-63 and MNNG/HOS cells. Moreover, transcription of the type II, but not the type I, was regulated by E2F-1 through an E2F-1-responsive site located at position +1,086/+1,093 downstream of the transcriptional initiation site of the type I transcript. Furthermore, epigenetic modulation might be involved in the expression of the type II transcript. Our findings provide new insights into the regulatory mechanism of PDGF receptor alpha transcription in normal and tumor cells.

A functional polymorphism in the PTHR1 promoter region is associated with adult height and BMD measured at the femoral neck in a large cohort of young caucasian women

The parathyroid hormone type 1 receptor (PTHR1) mediates the actions of parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHRP). Interacting with this receptor, PTHRP contributes to skeletal development through the regulation of chondrocyte proliferation and differentiation. Recently, a tetranucleotide repeat-(AAAG)( n )-in the P3 promoter of the PTHR1 gene has been shown to have functional activity in vitro, and homozygosity for (AAAG)(6), or the 6/6 genotype, has been associated with greater adult height compared to the 5/5 genotype. In this study, we evaluated the association of (AAAG)( n ) with height and bone mineral density (BMD) measured at lumbar spine (LS) and femoral neck (FN) in a cohort of 677 young caucasian women 18-35 years of age. Genomic DNA was amplified and genotyped by comparison with sequenced controls following electrophoretic separation through high-resolution polyacrylamide gels. Allele frequencies for (AAAG)( n ) were: 76.8% (n=5); 20.9% (n=6); 1.8% (n=7); 0.18% (n=8); 0.27% (n=9); 0.08% (n=2), and there was no evidence for Hardy-Weinberg disequilibrium. Analysis of variance showed that subjects bearing one or two (AAAG)(6) alleles (6/X & 6/6) were significantly taller (165.7+/-0.5 cm) than the others (X/X, 164.5+/-0.3 cm; P=0.034). This association was significant after adjusting for multiple covariates-current age, age at menarche, physical activity, smoking status, and intakes of caffeine and calcium. Comparison of genotype groups for BMD was not significant at LS, but BMD was significantly higher at FN in the group with at least one (AAAG)(6) allele (adjusted means: 1.021+/-0.008 vs. 0.999+/-0.006 g/cm(2), P=0.032). In conclusion, our data show that subjects bearing one or two (AAAG)(6) alleles are taller than subjects without, reinforcing the notion that in vivo variation in promoter activity of the PTHR1 gene may be a relevant genetic influence on final adult height and BMD.

Isogenic human cell lines for drug discovery: regulation of target gene expression by engineered zinc-finger protein transcription factors

Isogenic cell lines differing only in the expression of the protein of interest provide the ideal platform for cell-based screening. However, related natural lines differentially expressing the therapeutic target of choice are rare. Here the authors report a strategy for drug screening employing isogenic human cell lines in which the expression of the target protein is regulated by a gene-specific engineered zinc-finger protein (ZFP) transcription factor (TF). To demonstrate this approach, a ZFP TF activator of the human parathyroid hormone receptor 1 (PTHR1) gene was identified and introduced into HEK293 cells (negative for PTHR1). Following induction of ZFP TF expression, this cell line produced functional PTHR1 protein, resulting in a robust and ligand-specific cyclic adenosine monophosphate (cAMP) response. Reciprocally, the natural expression of PTHR1 observed in SAOS2 cells was dramatically reduced by the introduction of the appropriate PTHR1-specific ZFP TF repressor. Moreover, this ZFP-driven PTHR1 repression selectively eliminated the functional cAMP response invoked by known ligands of PTHR1. These data establish ZFP TF-generated isogenic lines as a general approach for the identification of therapeutic agents specific for the target gene of interest.

The (GT)n polymorphism and haplotype of the COL1A2 gene, but not the (AAAG)n polymorphism of the PTHR1 gene, are associated with bone mineral density in Chinese

Collagen type I alpha2 (COL1A2) and parathyroid hormone (PTH)/PTH-related peptide receptor (PTHR1) are two prominent candidate genes for bone mineral density (BMD). To test their importance for BMD variation in Chinese, we recruited 388 nuclear families composed of both parents and at least one healthy daughter with a total of 1,220 individuals, and simultaneously analyzed population stratification, total-family association, and within-family association between BMD at the spine and hip and the (GT)n marker in the intron 1 of the COL1A2 gene and the (AAAG)n marker in the P3 promoter of PTHR1 gene. We also performed these association analyses with haplotypes of the MspI and (GT)n polymorphisms in the COL1A2 gene. Significant within-family association was found between the M(GT)12 haplotype and trochanter BMD (P<0.001). Individuals with this haplotype have, on average, 9.53% lower trochanter BMD than the non-carriers. Suggestive evidence of the within-family association was detected between the (GT)17 allele and BMD at the spine (P=0.012), hip (P=0.011), femoral neck (P=0.032), trochanter (P=0.023), and intertrochanter (P=0.034). The association was confirmed by subsequent permutation tests. For the association, the proportion of phenotypic variance explained by the detected markers ranged from 1.2 to 3.9%, with the highest 3.9% at the trochanter for the M(GT)12 haplotype. This association indicates that there is strong linkage disequilibrium between the polymorphisms (MspI and GT repeat polymorphism) in the COL1A2 gene and a nearby quantitative trait locus (QTL) underlying BMD variation in Chinese, or the markers themselves may have an important effect on the variation of BMD. On the other hand, no significant within-family association, population stratification and total-family association between the PTHR1 polymorphism and BMD were found in our Chinese population.

Association between AAAG repeat polymorphism in the P3 promoter of the human parathyroid hormone (PTH)/PTH-related peptide receptor gene and adult height, urinary pyridinoline excretion, and promoter activity

The PTH/PTHrP receptor (PTHR1) plays an essential role in skeletal development and mediates many other functions of PTH and PTHrP. Human PTHR1 gene transcription is controlled by three promoters, P1-P3. The most proximal promoter, P3, is active in bone and osteoblast-like cell lines and accounts for the majority of renal transcripts in adults. We have identified a tetranucleotide repeat (AAAG)n polymorphism in the P3 promoter. In 214 unrelated Japanese, the repeat number (n) ranged from 3-8, with the AAAG5 allele being the most frequent (59%). In 55 unrelated Caucasians, n ranged from 5-7, and the frequency of the AAAG5 allele was 78%. The most frequent genotypes in a cohort of 85 young (18-20 yr) female Japanese were 5/5, 5/6, and 6/6. The 6/6 genotype was associated with greater height (5/5 vs. 6/6; P < 0.02) and lower urinary deoxypyridinoline and pyridinoline (P < 0.02), which are markers of bone resorption. The height of an additional 71 healthy female Japanese subjects, aged 14-17 yr, having genotype 5/5, 5/6, or 6/6 was also in the order of genotype 5/5 < 5/6 < 6/6 (5/5 vs. 6/6, P < 0.05). There was no significant difference in lumbar and femoral bone mineral density between genotypes. Likewise, there was no difference in circulating intact PTH levels between groups. The activity of P3 promoter-luciferase reporter constructs in transcription assays in 2 human osteoblast-like cell-lines varied according to repeat number, with AAAG6 being the least active. In conclusion, the P3 promoter (AAAG)n polymorphism is frequent in both Japanese and Caucasians and has potential as a linkage marker for the PTHR1 locus. In addition, it may influence the expression of the receptor in target tissues and have functional consequences on the developing skeleton.

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.

Parathyroid hormone (PTH) suppresses rat PTH/PTH-related protein receptor gene promoter

Parathyroid hormone (PTH) regulates osteoblasts via a G protein-linked PTH/PTH-related protein (PTHrP) receptor. PTH effects on PTH/PTHrP receptor gene expression were studied in UMR 106 osteoblast-like cells. In heterogeneous nuclear RNA and Northern analysis, PTH suppressed PTH/PTHrP receptor transcription. We cloned the 7-kb promoter region of the rat PTH/PTHrP receptor gene and transiently transfected chimeric deletion constructs containing the 5'-flanking region and the luciferase gene into UMR 106 cells. In transfected cells the minimal region for basal promoter activity was between positions -128 and +103. The 5'-flanking region of exon U1 contained several putative-binding sites for Sp1 and the myc-associated zinc finger protein (MAZ). The minimal PTH-suppressive region (PTHSR) was between +1 and +25 in exon U1, but the 5'-flanking region or Sp1 and MAZ-binding sites also were required for PTH-mediated repression. By gel mobility shift assay PTH markedly decreased binding of PTHSR-protein complex in UMR 106 cells. The mutation experiments showed that the most critical sequence for the repression of PTH was 5'-GGGGGAGGGGAG-3' (+1 to +12) of PTHSR. This represents the first characterization of a PTH-suppressive region of the PTH/PTHrP receptor gene in rat.

Tissue-specific and ubiquitous promoters direct the expression of alternatively spliced transcripts from the calcitonin receptor gene

The gene encoding the murine calcitonin receptor (mCTR) was isolated, and the exon/intron structure was determined. Analysis of transcripts revealed novel cDNA sequences, new alternative exon splicing in the 5'-untranslated region, and three putative promoters (P1, P2, and P3). The longest transcription unit is greater than 67 kilobase pairs, and the location of introns within the coding region of the mCTR gene (exons E3-E14) are identical to those of the porcine and human CTR genes. We have identified novel cDNA sequences that form three new exons as well as others that add 512 base pairs to the 5' side of the previously published cDNA, thereby extending exon E1 to 682 base pairs. Two of these novel exons are upstream of exon E2 and form a tripartite exon E2 (E2a, E2b, and E2c) in which E2a is utilized by promoter P2 with variable splicing of E2b. The third new exon (E3b') lies between E3a and E3b and is utilized by promoter P3. Analysis of mCTR mRNAs has revealed that the three alternative promoters give rise to at least seven mCTR isoforms in the 5' region of the gene and generate 5'-untranslated regions of very different lengths. Analysis by reverse transcription-polymerase chain reaction shows that promoters P1 and P2 are utilized in osteoclasts, brain, and kidney, whereas promoter P3 appears to be osteoclast-specific. Using transiently transfected reporter constructs, promoter P2 has activity in both a murine kidney cell line (MDCT209) and a chicken osteoclast-like cell line (HD-11EM), whereas promoter P3 is active only in the osteoclast-like cell line. These transfection data confirm the osteoclast specificity of promoter P3 and provide the first evidence that the CTR gene is regulated in a tissue-specific manner by alternative promoter utilization.

Organization and evolution of the human growth hormone receptor gene 5'-flanking region

Previous studies have identified eight variant human GH receptor (hGHR) messenger RNA (mRNAs; V1-V8), that differ in their 5'-untranslated regions (5'UTRs) but splice into the same site just upstream of the translation start site in exon 2; thus, they encode the same protein. Here we report a novel variant, V9, and describe the mapping of all nine 5'UTR sequences within 40 kb upstream of exon 2. A cluster of three sequences, V2-V9-V3 (termed module A), lies furthest 5', and approximately 16 kb downstream is a second cluster of four exons, V7-V1-V4-V8 (module B). V6 is midway between modules A and B. Module B is about 18 kb upstream of V5, which lies adjacent to exon 2. hGHR expression is under developmental- and tissue-specific regulation, and expression of the variant mRNAs is related to their position within the 5'-flanking region; whereas module A (V2,V9,V3) and V5 variants are widely expressed, module B (V7,V1,V4,V8) and V6 variant mRNAs are detectable only in postnatal liver. Transcriptional start sites for V1 and V9 (representing the two different modules) were identified, showing that postnatal liver-specific expression of V1 is driven from two TATA boxes, whereas the ubiquitous V9 transcript has a single start site and a TATA-less promoter. V9 promoter activity was shown by in vivo and in vitro transfection assays, and an NF-Y binding site was demonstrated by electromobility shift assay. Thus, the regulatory regions of the hGHR gene are complex, and the clustering of seven 5'UTR exons within two modules with distinctly different mRNA expression patterns is the most striking feature.

Developmental aspects of parathyroid hormone-related protein biology

Parathyroid hormone-related protein (PTHrP) has been discovered as a parathyroid hormone (PTH)-like factor responsible for the humoral hypercalcaemia of malignancies. Further studies revealed that PTHrP is ubiquitously expressed, in mature as well as in developing normal tissues from various species. Although not completely understood, the biological roles of PTHrP concern a variety of domains, including calcium phosphorus metabolism and bone mineralization, smooth muscle relaxation, cell growth and differentiation, and embryonic development. As a poly-hormone, PTHrP is now acknowledged to act via the paracrine, autocrine, and even the intracrine pathways. This review focuses on the main developmental features of the biology of PTHrP. During embryonic development, PTHrP is considered to be involved as a growth factor that promotes cell proliferation and delays cell terminal maturation. PTHrP has been shown to intervene in the development of various tissues and organs such as the skeleton, skin, hair follicles, tooth, pancreas, and the kidney. In addition, through its midregion sequence, which is able to promote an active transplacental calcium transport, PTHrP may intervene indirectly in the mineralization of the foetal skeleton. PTHrP has also been shown to be necessary for the normal development of the mammary gland, while huge amounts of PTHrP are found in the human milk. Finally, observations of physiologic, vasodilating effects of PTHrP in the kidney suggest its involvment in the control of renal hemodynamics, especially in the perinatal period.

Parathyroid hormone/parathyroid hormone-related peptide type 1 receptor in human bone

The parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor (denoted as PTH-1R) is a key signaling factor through which calcium-regulating hormones PTH and PTHrP exert their effects on bone. There are contradictory reports regarding the capability of osteoclasts to express PTH-1R. To address this issue in humans, bone biopsy specimen samples from 9 normal controls and 16 patients with moderate to severe secondary renal hyperparathyroid bone disease (2 degrees HPT) with elevated PTH levels were studied to determine whether osteoclasts in the bone microenvironment express PTH-1R messenger RNA (mRNA) and protein. We report that osteoclasts express the PTH-1R mRNA but the protein is detected only in patients with 2 degrees HPT. The PTH-1R mRNA and protein also were found in osteoblasts, osteocytes, and bone marrow cells. Receptor expression was higher in osteoclasts and osteoblasts of patients with 2 degrees HPT than normal controls (98.0 +/- 1.1% vs. 65.7 +/- 14.3% and 65.8 +/- 3.4% vs. 39.1 +/- 6.2%; p < 0.01, respectively). Approximately half of osteoclasts found in bone of patients with 2 degrees HPT have the PTH-1R protein. In patients with 2 degrees HPT, a positive relationship exists between erosion depth, a parameter of osteoclastic activity, and the percentage of osteoclasts with PTH-1R protein (r = 0.58; p < 0.05). In normal controls, an inverse relationship exists between the percentage of osteoblasts with receptor mRNA, mRNA signals/cell, and serum PTH levels (r = -0.82 and p < 0.05 and r = -0.78 and p < 0.01, respectively). The results provide the novel evidence of PTH-1R in human osteoclasts and suggest a functional role for the receptors in 2 degrees HPT.

Analysis of the P3 promoter of the human parathyroid hormone (PTH)/PTH-related peptide receptor gene in pseudohypoparathyroidism type 1b

Hypocalcemia and hyperphosphatemia caused by PTH resistance are the only discernible abnormalities in pseudohypoparathyroidism type 1b (PHP-1b). Because of the selective resistance toward PTH, inactivating mutations in its receptor, the PTH/PTH-related peptide receptor (PTHR1), were thought to be responsible for PHP-1b. However, gene abnormalities responsible for PHP-1b have not been identified in the coding region and well conserved promoters (P1 and P2) of the PTHR1 gene. The purpose of the present study was to analyze the structure of the P3 promoter, the main promoter of the human PTHR1 gene in kidney, in patients with PHP-1b. Southern analysis of genomic DNA from lymphoblastoid cell lines of eight nonfamilial patients with PHP-1b revealed neither gross rearrangements nor methylation abnormalities in the P3 promoter region of the PTHR1 gene. Sequencing revealed no abnormalities in the P3 promoter region, although one patient was homozygous for an (AAAG)n polymorphic variant. In conclusion, despite the selective resistance toward PTH in the kidney, which mainly uses the PTHR1 P3 promoter, PHP-1b in eight cases is not associated with structural abnormalities in this promoter. This study also indicates that inactivation of the P3 promoter is not achieved by methylation as tested in patients' genomic DNA from lymphoblastoid cell lines. The influence of alterations in the polymorphic A-rich repeat sequence on promoter activity warrants further study.

Dissection of differentially regulated (G+C)-rich promoters of the human parathyroid hormone (PTH)/PTH-related peptide receptor gene

The PTH/PTH-related peptide (PTHrP) receptor (PTHR) is required for normal skeletal development, and a wide array of physiological responses mediated by PTH and PTHrP. We have previously identified three promoters, P1-P3, which control human PTHR gene transcription. P2 and P3 are (G+C)-rich, function in a number of tissues, lie within the same CpG island, and display many hallmarks of housekeeping promoters. However, they are differentially regulated during development as P2, but not P3, functions in fetal tissues. Here, we have used both stably and transiently transfected human osteoblast-like cells to delineate regions of P2 and P3 required for promoter activity. Deletion analyses performed in stably transfected cells indicated that sequences extending from -91 to -12 relative to the transcription start site were required for function of the P2 promoter. No negative regulatory elements were detected in P2. In contrast, deletion of an A-rich region of P3 extending from -147 to -115 was required for optimal basal activity, suggesting that this sequence acts as a repressor of P3. Strikingly, however, whereas the A-rich region also functioned as a negative element when inserted upstream of the (G+C)-rich P2 promoter, it enhanced expression from the thymidine kinase promoter, suggesting that its function depends on other transcription factors bound to promoter sequences. Fine deletion of P3 sequences proximal to -115 implicated Spl motifs and downstream initiation sites in P3 function. These studies indicate that function of P2 and P3 is controlled by ubiquitously expressed transcription factors and raise the possibility that P3 activity is repressed during fetal development.

Methylation patterns of human parathyroid hormone (PTH)/PTH-related peptide receptor gene promoters are established several weeks prior to onset of their function

Expression of the human parathyroid hormone (PTH)/PTH-related peptide receptor (PTHR) gene is controlled by three promoters, P1-P3. P1 functions specifically in kidney, whereas P2 is ubiquitously active. P3 is also widely active, although more so in kidney than other tissues. However, only P2 functions at midgestation. We examined the role of methylation in controlling PTHR promoter activity. Function of all promoters was inhibited by CpG methylation in vitro. Significantly, P1 is selectively hypomethylated in adult kidney in vivo, strongly suggesting that demethylation is required for renal P1 function. Moreover, this pattern is established by 11. 75 weeks of fetal age, several weeks prior to the onset P1 activity. P3 is unmethylated at midgestation, although it is inactive at this stage of development, and thus exhibits characteristics of both tissue-specific and ubiquitously active promoters. These results show that adult methylation patterns of P1 and P3 are established several weeks prior to their induction, indicating that their function requires factors expressed late in development.

Zebrafish express the common parathyroid hormone/parathyroid hormone-related peptide receptor (PTH1R) and a novel receptor (PTH3R) that is preferentially activated by mammalian and fugufish parathyroid hormone-related peptide

To further explore the evolution of receptors for parathyroid hormone (PTH) and PTH-related peptide (PTHrP), we searched for zebrafish (z) homologs of the PTH/PTHrP receptor (PTH1R). In mammalian genes encoding this receptor, exons M6/7 and M7 are highly conserved and separated by 81-84 intronic nucleotides. Genomic polymerase chain reaction using degenerate primers based on these exons led to two distinct DNA fragments comprising portions of genes encoding the zPTH1R and the novel zPTH3R. Sequence comparison of both full-length teleost receptors revealed 69% similarity (61% identity), but less homology with zPTH2R. When compared with hPTH1R, zPTH1R showed 76% and zPTH3R 67% amino acid sequence similarity; similarity with hPTH2R was only 59% for both teleost receptors. When expressed in COS-7 cells, zPTH1R bound [Tyr(34)]hPTH-(1-34)-amide (hPTH), [Tyr(36)]hPTHrP-(1-36)-amide (hPTHrP), and [Ala(29),Glu(30), Ala(34),Glu(35), Tyr(36)]fugufish PTHrP-(1-36)-amide (fuguPTHrP) with a high apparent affinity (IC(50): 1.2-3.5 nM), and was efficiently activated by all three peptides (EC(50): 1.1-1.7 nM). In contrast, zPTH3R showed higher affinity for fuguPTHrP and hPTHrP (IC(50): 2.1-11.1 nM) than for hPTH (IC(50): 118.2-127.0 nM); cAMP accumulation was more efficiently stimulated by fugufish and human PTHrP (EC(50): 0.47 +/- 0.27 and 0.45 +/- 0.16, respectively) than by hPTH (EC(50): 9.95 +/- 1.5 nM). Agonist-stimulated total inositol phosphate accumulation was observed with zPTH1R, but not zPTH3R.

Receptors for PTH and PTHrP: their biological importance and functional properties

The type 1 receptor (PTH1R) for parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) is a G protein-coupled receptor that is highly expressed in bone and kidney and mediates in these tissues the PTH-dependent regulation of mineral ion homeostasis. The PTH1R also mediates the paracrine actions of PTHrP, which play a particularly vital role in the process of endochondral bone formation. These important functions, the likely involvement of the PTH1R in certain genetic diseases affecting skeletal development and calcium homeostasis, and the potential utility of PTH in treating osteoporosis have been the driving force behind intense investigations of both the receptor and its peptide ligands. Recent lines of work have led to the identification of constitutively active PTH1Rs in patients with Jansen's metaphyseal chondrodysplasia, the demonstration of inverse agonism by certain ligand analogs, and the discovery of the PTH-2 receptor subtype that responds to PTH but not PTHrP. As reviewed herein, a detailed exploration of the receptor-ligand interaction process is currently being pursued through the use of site-directed mutagenesis and photoaffinity cross-linking methods; ultimately, such work could enable the development of novel PTH receptor ligands that have therapeutic value in treating diseases such as osteoporosis and certain forms of hypercalcemia.

A G protein-coupled receptor from zebrafish is activated by human parathyroid hormone and not by human or teleost parathyroid hormone-related peptide. Implications for the evolutionary conservation of calcium-regulating peptide hormones

Genomic and cDNA clones encoding portions of a putative catfish parathyroid hormone (PTH) 2 receptor (PTH2R) led to the isolation of a cDNA encoding a full-length zebrafish PTH2R (zPTH2R). The zPTH2R shared 63 and 60% amino acid sequence identity with human and rat PTH2Rs, respectively, 47-52% identity with mammalian and frog PTH/PTHrP receptors (PTH1R), and less than 37% with other members of this family of G protein-coupled receptors. COS-7 cells expressing zPTH2R(43), a 5' splice variant that lacked 17 amino acids in the amino-terminal extracellular domain, showed cAMP accumulation when challenged with [Tyr(34)]hPTH(1-34)-amide (hPTH) (EC(50), 1.64 +/- 0. 95 nM) and [Ile(5),Trp(23),Tyr(36)]hPTHrP-(1-36)-amide ([Ile(5), Trp(23)]hPTHrP) (EC(50), 46.8 +/- 12.1 nM) but not when stimulated with [Tyr(36)]hPTHrP-(1-36)-amide (hPTHrP), [Trp(23), Tyr(36)]hPTHrP-(1-36)-amide ([Trp(23)]hPTHrP), or [Ala(29),Glu(30), Ala(34),Glu(35),Tyr(36)]fugufish PTHrP-(1-36)amide (fuguPTHrP). FuguPTHrP also failed to activate the human PTH2R but had similar efficiency and efficacy as hPTH and hPTHrP when tested with cells expressing the human PTH1R. Agonist-dependent activation of zPTH2R was less efficient than that of zPTH2R(43), and both receptor variants showed no cAMP accumulation when stimulated with either secretin, growth hormone-releasing hormone, or calcitonin. The zPTH2R thus has ligand specificity similar to that of the human homolog, which raises the possibility that a PTH-like molecule exists in zebrafish, species which lack parathyroid glands.