Cell cycle-dependent coupling of the calcitonin receptor to different G proteins

Cell active and functionally-relevant small-molecule agonists of calcitonin receptor

The natural calcitonin (CT) receptor and its peptide agonists are considered validated targets for drug discovery. A small molecule agonist, SUN-B8155, has previously been shown to efficiently activate cellular CTR. Herein, we report the synthesis of a series of compounds (S8155 1-9) derived from SUN-B8155, and investigate the structural-functional relationship, bias properties and their cellular activity profile. We discover that the N-hydroxyl group from the pyridone ring is required for G protein activity and its affinity to the CT receptor. Among the compounds studied, S8155-7 exhibits improved G protein activity while S8155-4 displays a significant β-arrestin-2 signaling bias. Finally, we show that both S8155-4 and S8155-7 inhibit tumour cell invasion through CTR activation. These two compounds are anticipated to find extensive applications in chemical biology research as well drug development efforts targeting CT receptor.

Characterization of signalling and regulation of common calcitonin receptor splice variants and polymorphisms

The calcitonin receptor (CTR) is a class B G protein-coupled receptor that is a therapeutic target for the treatment of hypercalcaemia of malignancy, Paget's disease and osteoporosis. In primates, the CTR is subject to alternative splicing, with a unique, primate-specific splice variant being preferentially expressed in reproductive organs, lung and kidney. In addition, humans possess a common non-synonymous single-nucleotide polymorphism (SNP) encoding a proline/leucine substitution in the C-terminal tail. In low power studies, the leucine polymorphism has been associated with increased risk of osteoporosis in East Asian populations and, independently, with increased risk of kidney stone disease in a central Asian population. The CTR is pleiotropically coupled, though the relative physiological importance of these pathways is poorly understood. Using both COS-7 and HEK293 cells recombinantly expressing human CTR, we have characterized both splice variant and polymorphism dependent response to CTs from several species in key signalling pathways and competition binding assays. These data indicate that the naturally occurring changes to the intracellular face of CTR alter ligand affinity and signalling, in a pathway and agonist dependent manner. These results further support the potential for these primate-specific CTR variants to engender different physiological responses. In addition, we report that the CTR exhibits constitutive internalization, independent of splice variant and polymorphism and this profile is unaltered by peptide binding.

Calcitonin receptor expression in medullary thyroid carcinoma

Background

Calcitonin expression is a well-established marker for medullary thyroid carcinoma (MTC); yet the role of calcitonin receptor (CTR), its seven-transmembrane G-protein coupled receptor, remains to be established in C-cells derived thyroid tumors. The aim of this work was to investigate CTR expression in MTC and to correlate such expression with clinicopathological features in order to evaluate its possible role as a prognostic indicator of disease aggressiveness and outcome.

Methods

Calcitonin receptor expression was analyzed in a series of 75 MTCs by immunohistochemistry, and by qPCR mRNA quantification in specimens from four patients. Statistical tests were used to evaluate the correlation between CTR expression and the clinicopathological and molecular characteristics of patients and tumors.

Results

Calcitonin receptor expression was detected in 62 out of 75 samples (82.7%), whereas 13 of the 75 samples (17.3%) were completely negative. CTR expression was significantly associated with expression of cytoplasmatic phosphatase and tensin homologue deleted on chromosome 10 and osteopontin, as well as with wild type RET/RAS genes and absence of tumor stroma, suggesting that CTR expression do not associate with clinicopathological signs of worse prognosis.

Discussion

Calcitonin receptor expression appears to be associated in MTC with more differentiated status of the neoplastic cells.

The experimental power of FR900359 to study Gq-regulated biological processes

Despite the discovery of heterotrimeric αβγ G proteins ∼25 years ago, their selective perturbation by cell-permeable inhibitors remains a fundamental challenge. Here we report that the plant-derived depsipeptide FR900359 (FR) is ideally suited to this task. Using a multifaceted approach we systematically characterize FR as a selective inhibitor of Gq/11/14 over all other mammalian Gα isoforms and elaborate its molecular mechanism of action. We also use FR to investigate whether inhibition of Gq proteins is an effective post-receptor strategy to target oncogenic signalling, using melanoma as a model system. FR suppresses many of the hallmark features that are central to the malignancy of melanoma cells, thereby providing new opportunities for therapeutic intervention. Just as pertussis toxin is used extensively to probe and inhibit the signalling of Gi/o proteins, we anticipate that FR will at least be its equivalent for investigating the biological relevance of Gq.

Calcitonin controls bone formation by inhibiting the release of sphingosine 1-phosphate from osteoclasts

The hormone calcitonin (CT) is primarily known for its pharmacologic action as an inhibitor of bone resorption, yet CT-deficient mice display increased bone formation. These findings raised the question about the underlying cellular and molecular mechanism of CT action. Here we show that either ubiquitous or osteoclast-specific inactivation of the murine CT receptor (CTR) causes increased bone formation. CT negatively regulates the osteoclast expression of Spns2 gene, which encodes a transporter for the signalling lipid sphingosine 1-phosphate (S1P). CTR-deficient mice show increased S1P levels, and their skeletal phenotype is normalized by deletion of the S1P receptor S1P₃. Finally, pharmacologic treatment with the nonselective S1P receptor agonist FTY720 causes increased bone formation in wild-type, but not in S1P₃-deficient mice. This study redefines the role of CT in skeletal biology, confirms that S1P acts as an osteoanabolic molecule in vivo and provides evidence for a pharmacologically exploitable crosstalk between osteoclasts and osteoblasts.

The regulatory role of calcitonin in bone homeostasis is well studied, yet its molecular activity is poorly understood. The authors show that calcitonin regulates bone cells function by inhibiting the osteoclast secretion of sphingosine 1-phosphate, a lipid mediator of osteoclast–osteoblast crosstalk.

Analyses on the mechanisms that underlie the chondroprotective properties of calcitonin

INTRODUCTION

Calcitonin (CT) has recently been shown to display chondroprotective effects. Here, we investigate the putative mechanisms by which CT delivers these actions.

METHODS

Immortalized C-28/I2 cells or primary adult human articular chondrocytes (AHAC) were cultured in high-density micromasses to investigate: (i) CT anabolic effects using qPCR and immuhistochemistry analysis; (ii) CT anti-apoptotic effects using quantitation of Bax/Bcl gene products ratio, TUNEL assay and caspase-3 expression; (iii) CT effects on CREB, COL2A1 and NFAT transcription factors.

RESULTS

CT (10(-10)-10(-8)nM) induced significant up-regulation of cartilage phenotypic markers (SOX9, COL2A1 and ACAN), with down-regulation of catabolic (MMP1 and MMP13 and ADAMTS5) gene products both in resting and inflammatory conditions. This was mirrored by an augmented production of type II collagen and accumulation of glycosaminoglycan- and proteoglycan-rich extracellular matrix in vitro. Mechanistic analyses revealed only partial involvement of cyclic AMP formation in these effects of CT. Congruently, using reporter assays for specific transcription factors, there was no indication for CREB activation, whereas the COL2A1 promoter was genuinely and directly activated by cell exposure to CT. Phenotypically, these mechanisms supported the ability of CT, whilst inactive on its own, to counteract the pro-apoptotic effects of IL-1β, demonstrated by TUNEL-positive staining of chondrocytes and ratio of BAX/BCL genes products.

CONCLUSION

These data may provide a novel lead for the development of CT-based chondroprotective strategies that rely on the engagement of mechanisms that lead to augmented chondrocyte anabolism and inhibited chondrocyte apoptosis.

Prolonged calcitonin receptor signaling by salmon, but not human calcitonin, reveals ligand bias

Salmon calcitonin (sCT) and human calcitonin (hCT) are pharmacologically distinct. However, the reason for the differences is unclear. Here we analyze the differences between sCT and hCT on the human calcitonin receptor (CT_(a)R) with respect to activation of cAMP signaling, β-arrestin recruitment, ligand binding kinetics and internalization. The study was conducted using mammalian cell lines heterologously expressing the human CT_(a) receptor. CT_(a)R downstream signaling was investigated with dose response profiles for cAMP production and β-arrestin recruitment for sCT and hCT during short term (<2 hours) and prolonged (up to 72 hours) stimulation. CT_(a)R kinetics and internalization was investigated with radio-labeled sCT and hCT ligands on cultured cells and isolated membrane preparations from the same cell line. We found that sCT and hCT are equipotent during short-term stimulations with differences manifesting themselves only during long-term stimulation with sCT inducing a prolonged activation up to 72 hours, while hCT loses activity markedly earlier. The prolonged sCT stimulation of both cAMP accumulation and β-arrestin recruitment was attenuated, but not abrogated by acid wash, suggesting a role for sCT activated internalized receptors. We have demonstrated a novel phenomenon, namely that two distinct CT_(a)R downstream signaling activation patterns are activated by two related ligands, thereby highlighting qualitatively different signaling responses in vitro that could have implications for sCT use in vivo.

G protein-coupled receptors: what a difference a 'partner' makes

G protein-coupled receptors (GPCRs) are important cell signaling mediators, involved in essential physiological processes. GPCRs respond to a wide variety of ligands from light to large macromolecules, including hormones and small peptides. Unfortunately, mutations and dysregulation of GPCRs that induce a loss of function or alter expression can lead to disorders that are sometimes lethal. Therefore, the expression, trafficking, signaling and desensitization of GPCRs must be tightly regulated by different cellular systems to prevent disease. Although there is substantial knowledge regarding the mechanisms that regulate the desensitization and down-regulation of GPCRs, less is known about the mechanisms that regulate the trafficking and cell-surface expression of newly synthesized GPCRs. More recently, there is accumulating evidence that suggests certain GPCRs are able to interact with specific proteins that can completely change their fate and function. These interactions add on another level of regulation and flexibility between different tissue/cell-types. Here, we review some of the main interacting proteins of GPCRs. A greater understanding of the mechanisms regulating their interactions may lead to the discovery of new drug targets for therapy.

Functional expression of the calcitonin receptor by human T and B cells

The calcitonin receptor (CTR) is a seven-transmembrane-domain G-protein-coupled receptor that regulates calcium metabolism and bone resorption by osteoclasts. Here we demonstrate that high levels are expressed by normal human T and B lymphocytes from tonsils and peripheral blood in relation to their activation status, as CTR(+) T cells are prone to produce IFN-gamma after TCR stimulation. The receptor is also highly expressed on B cells from chronic lymphocytic leukemia patients, thus suggesting a correlation between its expression, their proliferative extent as well as their memory, antigen-experienced phenotype. Moreover, we found that binding of the receptor with salmon calcitonin induces an increase of intracellular calcium(2+) in peripheral lymphocytes. This effect is involved in several lymphocyte immune functions, as cytosolic calcium(2+) levels regulate both cell proliferation and cytokine production. In our hands, the increase of calcium(2+) levels by CTR binding with sCT induced a dose-dependent cell proliferation. We therefore suppose that expression of this functional receptor may contribute to the modulation of cytoplasmic calcium(2+) levels needed to regulate T and B cell activation and perhaps other immune functions.

Calcitonin promotes outgrowth of trophoblast cells on endometrial epithelial cells: involvement of calcium mobilization and protein kinase C activation

Embryo implantation is a complex process that requires coordinated trophoblast-endometrial interactions. During implantation, trophoblast cells of the attached blastocyst penetrate the luminal epithelium of the endometrium before invasion into the endometrial stroma. Previous studies demonstrated that calcitonin was actively secreted by rat and human endometrial epithelial cells (EEC) during the implantation window and targeted disruption of endometrial calcitonin expression dramatically decreased embryo implantation rates; however, the role and signal transduction of calcitonin in trophoblast-endometrial interactions remained unclear and are therefore examined in this study. BeWo trophoblast and RL95-2 EEC lines were used because they preserve many properties of their respective normal tissues. We co-cultured BeWo trophoblast spheroids with RL95-2 EEC monolayers to mimic the blastocyst-endometrial interaction, and found that most spheroids quickly attached to EEC monolayers and then progressively expanded, with marked displacement of EEC adjacent to the outgrowing trophoblast cells. Interestingly, pretreatment of EEC monolayers with calcitonin before the addition of spheroids significantly enhanced trophoblast expansion on EEC monolayers. Cytosolic calcium (Ca(2+)) levels in EEC increased rapidly upon exposure to calcitonin, and blockade of Ca(2+) release by BAPTA-AM effectively prevented the promoting effect of calcitonin on trophoblast expansion on EEC. The Ca(2+)-dependent protein kinase C (PKC) was also activated in EEC after calcitonin treatment, and the PKC inhibitors staurosporine and calphostin C could completely abolish calcitonin-induced augmentation of trophoblast expansion on EEC. Our results suggest that calcitonin promotes trophoblastic displacement of EEC through calcium mobilization and PKC activation, thereby facilitating embryo implantation.

Fish calcitonin receptor has novel features

Calcitonin (CT), a 32-amino acid peptide, was initially isolated from fish. Fish CT has higher affinity to mammalian CT receptor (CTR), and has activity on calcium homeostasis. Therefore, fish CT has been used as a drug for the treatment of human bone diseases. However, the physiological roles of CT in fish as well as the characteristics of the fish CTR have not been clarified. Here, we cloned and characterized CTR from mefugu (Takifugu obscurus). Full-length cDNA sequencing revealed that mfCTR (mf, mefugu) consists of N-terminal four tandem putative hormone-binding domains (HBDs). Database mining showed that the multiple HBD-containing CTR is a common feature for some other fishes. Detailed pharmacological studies revealed that mfCTR generated cAMP in response to (1) fish CT, (2) calcitonin gene-related peptide (CGRP) in combinations with receptor activity-modifying proteins (mfRAMPs) 1 and 4, and (3) amylin in combinations with mfRAMPs 1-5. Unlike mammalian CTR, mfCTR showed dual affinity sites. Corresponding EC(50) values of those are in close proximity of the in vivo concentration of CT in fish. Analyses of the deletion mutants of mfCTR demonstrated that only the nearmost HBD to the first transmembrane region is functional to the ligands. Although, fish CT has higher affinity to the human CTR, human CT did not bind to the mfCTR. This is the first report that demonstrates the structure and property of fish receptor for CT, CGRP, and amylin. Fish CTR is the first example that has multiple HBD-like sequences.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Calpain is required for normal osteoclast function and is down-regulated by calcitonin

Osteoclast motility is thought to depend on rapid podosome assembly and disassembly. Both mu-calpain and m-calpain, which promote the formation and disassembly of focal adhesions, were observed in the podosome belt of osteoclasts. Calpain inhibitors disrupted the podosome belt, blocked the constitutive cleavage of the calpain substrates filamin A, talin, and Pyk2, which are enriched in the podosome belt, induced osteoclast retraction, and reduced osteoclast motility and bone resorption. The motility and resorbing activity of mu-calpain(-/-) osteoclast-like cells were also reduced, indicating that mu-calpain is required for normal osteoclast activity. Histomorphometric analysis of tibias from mu-calpain(-/-) mice revealed increased osteoclast numbers and decreased trabecular bone volume that was apparent at 10 weeks but not at 5 weeks of age. In vitro studies suggested that the increased osteoclast number in the mu-calpain(-/-) bones resulted from increased osteoclast survival, not increased osteoclast formation. Calcitonin disrupted the podosome ring, induced osteoclast retraction, and reduced osteoclast motility and bone resorption in a manner similar to the effects of calpain inhibitors and had no further effect on these parameters when added to osteoclasts pretreated with calpain inhibitors. Calcitonin inhibited the constitutive cleavage of a fluorogenic calpain substrate and transiently blocked the constitutive cleavage of filamin A, talin, and Pyk2 by a protein kinase C-dependent mechanism, demonstrating that calcitonin induces the inhibition of calpain in osteoclasts. These results indicate that calpain activity is required for normal osteoclast activity and suggest that calcitonin inhibits osteoclast bone resorbing activity in part by down-regulating calpain activity.

Hypercalcemia and status epilepticus relates to salmon calcitonin administration in breast cancer

Calcitonin is currently used to treat hypercalcemia of many clinical types. However, we encountered a woman who suffered severe hypercalcemia and status epilepticus, both of which developed 8 days after the administration of salmon calcitonin for the treatment of breast cancer. When the patient first presented her serum calcium level was 15.5mg/dl, intact parathyroid hormone level 118 pg/ml, calcitonin <2 pg/ml, magnesium 1.2mg/dl, and phosphate 1mg/dl. Her serum calcium level returned to the reference range within 48 h after correction. At follow-up no hypercalcemia had developed, although the patient had received no further treatment for her breast cancer and multiple metastases were subsequently detected. Her hypercalcemia is ascribed to exogenous calcitonin supplementation. These conflicting events may be due to functionally heterogeneous calcitonin receptors or to activation of 1 alpha-hydroxylase by exogenous calcitonin.

Tissue-specific activity of the proximal human calcitonin receptor promoter is mediated by Sp1 and an epigenetic phenomenon

To identify cis-acting sequences transcriptionally regulating the human calcitonin receptor (hCTR) gene, hCTR promoter/luciferase gene constructs were transiently or stably transfected into hCTR-positive and -negative cell lines. Luciferase assays demonstrated that the proximal hCTR promoter (hCTRP1) was transcriptionally active in all cell lines tested. High-level hCTRP1 activity depended on an 11 bp Sp1/Sp3 binding site. Electrophoretic mobility shift assay showed that this region bound the transcription factors Sp1 and Sp3. We further showed that hCTRP1 was strongly activated by the 11 bp Sp1/Sp3 binding site in hCTRP1/luciferase-, Sp1-transfected Drosophila S2 cells. Bisulphite-mediated sequencing of genomic DNA from hCTR-expressing and -non-expressing cell lines demonstrated that the endogenous hCTRP1 was hypomethylated in all cell lines tested. These results suggest that the hCTRP1 is activated by the tissue-ubiquitous transcription factor Sp1 and that an epigenetic process unrelated to CpG methylation represses its activity in hCTR-negative tissues.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

The alternatively spliced deltae13 transcript of the rabbit calcitonin receptor dimerizes with the C1a isoform and inhibits its surface expression

Numerous alternatively spliced transcripts are generated from the gene for the G protein-coupled calcitonin receptor, and some of the splice variants show differences in receptor-mediated signaling events. This study showed that the deltae13 splice variant of the rabbit calcitonin receptor is expressed together with the more common C1a in osteoclast-like cells. Since other G protein-coupled receptors form homo- or heterodimers, we examined whether heterodimerization of the calcitonin receptor splice variants occurs and, if so, whether it affects the function of the receptor. Homodimers of both isoforms and deltae13/C1a heterodimers were detected by co-immunoprecipitation and fluorescence resonance energy transfer analysis. In contrast to the C1a isoform, the deltae13 isoform was not efficiently transported to the cell surface. When co-expressed with the C1a splice variant, the deltae13 isoform colocalized with the C1a isoform within the cell but not at the cell surface. Furthermore, the overexpression of the deltae13 variant led to a significant reduction of the C1a surface expression and consequently a reduction of the cAMP response and Erk phosphorylation after ligand stimulation. We therefore suggest that the deltae13 variant of the rabbit calcitonin receptor acts to regulate the surface expression of the C1a isoform.

Predicting therapeutic value in the lead optimization phase of drug discovery

Recombinant and natural cellular assays for human G-protein-coupled receptors are used to optimize initial lead molecules obtained from screening. Although the activity of these molecules can be assessed on human genotype receptors, there is increasing evidence that cells impose a phenotypic selectivity to molecules in various cellular backgrounds. This opens the possibility of dissimulations between activity seen in lead optimization assays and the intended therapeutic value in humans. This review discusses the mechanisms by which cells can impose phenotypic selectivity on molecules and approaches to reduce this practical problem for drug discovery.

Osteoclastogenesis, bone resorption, and osteoclast-based therapeutics

Over the past decade, advances in molecular tools, stem cell differentiation, osteoclast and osteoblast signaling mechanisms, and genetically manipulated mice models have resulted in major breakthroughs in understanding osteoclast biology. This review focuses on key advances in our understanding of molecular mechanisms underlying the formation, function, and survival of osteoclasts. These include key signals mediating osteoclast differentiation, including PU.1, RANK, CSF-1/c-fms, and src, and key specializations of the osteoclast including HCl secretion driven by H+-ATPase and the secretion of collagenolytic enzymes including cathepsin K and matrix metalloproteinases (MMPs). These pathways and highly expressed proteins provide targets for specific therapies to modify bone degradation. The main outstanding issues, basic and translational, will be considered in relation to the osteoclast as a target for antiresorptive therapies.

Forty years of calcitonin--where are we now? A tribute to the work of Iain Macintyre, FRS

Calcitonin was discovered as a hypocalcemic principal that was initially thought to originate from the parathyroid gland. This view was corrected subsequently, and an origin from the thyroid C cells was documented. The purification and sequencing of various calcitonins soon followed. Calcitonin is a 32-amino-acid-long peptide with an N-terminal disulfide bridge and a C-terminal prolineamide residue. The peptide was shown to potently inhibit bone resorption; however, a direct osteoclastic action of the peptide was confirmed only in the early 1980s. Several osteoclast calcitonin receptors have subsequently been cloned and sequenced. Specific regions of the receptor necessary for ligand binding and intracellular signaling through cyclic AMP and calcium have been identified through systematic deletion mutagenesis and chimeric receptor studies. Calcitonin's potent antiresorptive effect has led to its use in treating Paget's disease of bone, osteoporosis, and hypercalcemia. This review retraces key aspects of the synthesis and structure of calcitonin, its cellular and molecular actions, and its therapeutic uses as they have emerged over the 40 years since its discovery. The review also examines the implications of these findings for future clinical applications as a tribute to early workers to whom credit must be given for creation of an important and expanding field. Notable are the new approaches currently being used to enhance calcitonin action, including novel allosteric activators of the calcitonin receptor, modulation of the release of endogenous calcitonin by calcimimetic agents, as well as the development of oral calcitonins.

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.

Neuropeptide FF receptors couple to a cholera toxin-sensitive G-protein in rat dorsal raphe neurones

In rat dorsal raphe neurones, nociceptin (300 nM) reduced the peak [Ca(2+)](i) transient, triggered by depolarization, by 36.7+/-1.8% (n=46). This effect of nociceptin decreased to 16.7+/-2.9% (n=18) after pre-treatment of the neurones with pertussis toxin (5 microg/ml, 2-6 h) but was unchanged (37.4+/-2.1%, n=44) after pre-incubation with cholera toxin (5 microg/ml, 2-6 h). This suggests that, in dorsal raphe neurones, the ORL1 receptor couples to inhibitory (G(i/o)) G-proteins. The neuropeptide FF analogue, [D-Tyr1, (N-Me)Phe(3)]neuropeptide FF (10, 100, 1000 nM), acted as an anti-opioid and reduced the effect of nociceptin (300 nM, 30 s) by 62.0+/-3.3% (n=28). Following pre-incubation with cholera toxin (5 microg/ml, 2-6 h) [D-Tyr1, (N-Me)Phe3] neuropeptide FF was unable, at the three concentrations tested, to block nociceptin activity. We conclude that, in rat dorsal raphe neurones, neuropeptide FF receptors couple to stimulatory G-proteins (Gs).

Cell cycle-dependent coupling of the vasopressin V1a receptor to different G proteins

Arginine vasopressin (AVP) regulates biological processes by binding to G protein-coupled receptors. In Swiss 3T3 fibroblasts, expressing the V(1a) subtype of vasopressin receptors, AVP mobilizes calcium from intracellular stores. In proliferating cells, the AVP-induced increase in intracellular calcium concentration ([Ca(2+)](i)) was mediated by G proteins of the G(q) family, which are insensitive to pertussis toxin (PTX) pretreatment of the cells. In quiescent cells, the AVP-induced increase in [Ca(2+)](i) was partially PTX-sensitive, suggesting an involvement of G(i) proteins. We confirmed this by photoaffinity labeling of G proteins in Swiss 3T3 cell membranes activated by AVP. In Swiss 3T3 cells arrested in the G(0)/G(1) phase of the cell cycle, the AVP-induced increase in [Ca(2+)](i) was also partially PTX-sensitive but was PTX-insensitive in cells arrested in other phases of the cell cycles. The blocking effect of PTX pretreatment in G(0)/G(1) cells was mimicked by microinjection of antisense oligonucleotides suppressing the expression of the Galpha(i3) subunits. These results were confirmed by microinjection of antibodies directed against the C terminus of G protein alpha-subunits. The data presented indicate that in Swiss 3T3 fibroblasts synchronized in the G(0)/G(1) phase of the cell cycle the V(1a) receptor couples to G(q/11) and G(i3) to activate the phospholipase C-beta, leading to release of intracellular calcium.

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

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

Cockroach diuretic hormones: characterization of a calcitonin-like peptide in insects

Insect diuretic hormones are crucial for control of water balance. We isolated from the cockroach Diploptera punctata two diuretic hormones (DH), Dippu-DH(31) and Dippu-DH(46), which increase cAMP production and fluid secretion in Malpighian tubules of several insect species. Dippu-DH(31) and -DH(46) contain 31 and 46 amino acids, respectively. Dippu-DH(46) belongs to the corticotropin-releasing factor (CRF)-like insect DH family, whereas Dippu-DH(31) has little sequence similarity to the CRF-like DH, but is similar to the calcitonin family. Dippu-DH(46) and -DH(31) have synergistic effects in D. punctata but have only additive effects in Locusta migratoria. Dippu-DH(31) represents a distinct type of insect DH with actions that differ from those of previously identified insect peptides with diuretic activity.

Calcitonin-suppressed expression of parathyroid hormone-related protein in breast cancer cells

Parathyroid hormone-related protein (PTHrP) is a key factor behind humoral hypercalcemia of malignancy (HHM). It is produced in most breast tumors and may be an important local mediator of skeletal metastases due to breast cancer. PTHrP may mediate local bone destruction in the absence of increased circulating PTHrP. Calcitonin (CT) is used for treatment of HHM, but there are data showing that CT can increase PTHrP expression and secretion in vitro. We have therefore studied the effect of CT on PTHrP gene expression and secretion in MCF-7 breast cancer cells. PTHrP mRNA decreased significantly after 4, 8, and 16 h incubation with 10 nM salmon calcitonin (sCT) when compared with the respective controls. PTHrP mRNA also decreased significantly and dose-dependently after incubation with sCT at 0.1 to 10 nM for 16 h. The PTHrP levels in the conditioned medium also decreased in a similar dose-dependent manner. The adenylate cyclase agonist forskolin lowered the PTHrP mRNA dose-dependently. In cells exposed to varying concentrations of sCT for 15 min, the cAMP levels increased dose-dependently. In conclusion, sCT can suppress PTHrP gene expression in MCF-7 breast cancer cells. The suppressive effect is probably exerted mainly via the cAMP-protein kinase A pathways.

Efficacy in drug receptor theory: outdated concept or under-valued tool?

In classical occupancy receptor theory, efficacy is a dimensionless proportionality constant denoting the power of agonists to produce a pharmacological response. In theoretical terms, it is difficult to separate affinity and efficacy estimates of agonists for receptors, hence questioning the value of clearly flawed estimates of efficacy by conventional methods. In this paper, the use of efficacy estimates, the limitations of the current methods to estimate efficacy, and the types of systems in which serious errors in efficacy estimation would be expected, is discussed. Specifically, in constitutively active receptor systems or in those where the receptor interacts with more than one G protein, there are theoretical objections to the use of relative maximal responses as indicators of intrinsic efficacy of agonists.

Protein kinase C antagonizes pertussis-toxin-sensitive coupling of the calcitonin receptor to adenylyl cyclase

The calcitonin receptor is known to couple to Gs and Gq, activating adenylyl cyclase and phospholipase C, respectively. The observation of pertussis-toxin-sensitive responses to calcitonin suggests that the receptor is capable of coupling to Gi/o as well. However, the calcitonin-dependent activation of adenylyl cyclase in HEK-293 cells that stably express the cloned rabbit calcitonin receptor, as in many other cells that express calcitonin receptors, shows little pertussis toxin sensitivity. Calcitonin treatment of these cells stimulates protein kinase C, which is reported to antagonize the receptor-dependent activation of Gi. The possibility that protein kinase C could be antagonizing Galphai-adenylyl cyclase coupling was tested by examining the effects of protein kinase C inhibitors (chelerythrine chloride and sphingosine) or of chronic treatment with phorbol ester to deplete protein kinase C. All three treatments led to a reduction of calcitonin-induced adenylyl cyclase activity that was reversed by pertussis toxin. Inhibiting or depleting protein kinase C had no effect on the activation of adenylyl cyclase by cholera toxin, indicating that Gs and adenylyl cyclase were not affected by these treatments. Calcitonin treatment of HEK-293 cells, that stably express a myc-tagged rabbit calcitonin receptor, induced the formation of complexes of the receptor and Galphai subunits, confirming that the calcitonin receptor interacts with Gi. Thus, the calcitonin receptor can couple to Gi, but the inhibition of adenylyl cyclase by Galphai is negatively regulated by protein kinase C.

Functional Expression of Receptors for Calcitonin Gene-Related Peptide, Calcitonin, and Vasoactive Intestinal Peptide in the Human Thymus and Thymomas from Myasthenia Gravis Patients

The molecular and functional expression of serpentine membrane receptors for vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP), and calcitonin (CT) were characterized in human thymus and thymomas from myasthenia gravis (MG) patients and thymic epithelial cells either in primary culture (PTEC) or transformed by the siman virus 40 large T (SV40LT) oncogene (LT-TEC). Using RT-PCR combined with Southern analysis, we identified the PCR products corresponding to the receptor (-R) transcripts for VIP, CGRP, and CT in thymus from control subjects and MG patients with either hyperplasia or thymoma. Similar expressions of the VIP- and CGRP-R transcripts were observed in PTEC, whereas the CT-R message was not detected. In LT-TEC, the signals for VIP-R, CGRP-R, and CT-R transcripts were seen with a lower intensity than those in control and MG thymus. In agreement with our molecular analysis, 1) VIP was the most potent peptide among VIP-related peptides (VIP &gt; PACAP &gt; PHM &gt; PHV) to stimulate cAMP production through specific type 1 VIP receptors in both PTEC and LT-TEC; 2) cAMP generation was induced by CGRP in PTEC and by CT in LT-TEC; 3) in frozen thymic sections and by flow cytometry, type 1 VIP-R, CGRP-R, and CT-R were localized in epithelial cells; and 4) in parallel, the transcription of the acetylcholine receptor α subunit (the main autoantigen in MG) was induced by CGRP and CT in PTEC and LT-TEC, respectively. Our data suggest that the neuroendocrine peptides VIP, CGRP, and CT may exert functional roles during MG and malignant transformation of the human thymus.

Calcitonin receptor binding properties in bone and kidney of the chicken during the oviposition cycle

The binding property of calcitonin (CT) in the membrane fraction of calvaria and kidney of egg-laying and nonlaying hens was analyzed using a [125I] CT binding assay system. Binding properties of CT receptors in both tissues satisfy the authentic criteria of a receptor-ligand interaction in terms of specificity, reversibility, and saturation. Scatchard plots revealed a single class of binding sites. Values of the equilibrium dissociation constant (Kd) and binding capacity (Bmax) in laying hens showed a decrease during the period between 3 h before and 2 h after oviposition. No change was observed in nonlaying hens. In vivo administration of 17beta-estradiol or progesterone caused the decrease in Kd and Bmax values. The results suggest that the binding affinity and capacity of the CT receptor in the calvaria and the kidney of the hen may be modulated by the ovarian steroid hormone.

Phospholipase D- and protein kinase C isoenzyme-dependent signal transduction pathways activated by the calcitonin receptor

The calcitonin receptor expressed by the porcine LLC-PK1 renal tubule cells is a seven-transmembrane domain, G protein-coupled receptor activating adenylyl-cyclase and phospholipase C. Salmon calcitonin stimulated dose- and time-dependent release of the phospholipase D-dependent phosphatidylcholine product [3H] choline with an EC50 = 2.5 +/-0.3 x 10(-8) M, similar to that determined for phosphoinositide metabolism (EC50 = 4.5 +/-1.0 x 10(-8)M). The hormone failed to induce release of [3H]phosphocholine and [3H]glycerophosphocholine, ruling out activation of phosphatydilcholine-specific phospholipase C and phospholipase A. Calcitonin stimulated phosphatidic acid, a product of phospholipase D-dependent phosphatydilcholine hydrolysis. Activation of phospholipase D was confirmed by release of [3H]phosphatydilethanol, a specific and stable product in the presence of a primary alcohol. Activation of calcitonin receptor induced diacylglycerol formation, with a rapid peak followed by a prolonged increase, due to activation of phospholipase C and of phospholipase D. Consequently, the protein kinase-C alpha, but not the delta isoenzyme, was cytosol-to-membrane translocated by approximately 50% after 20 min exposure to calcitonin, whereas protein kinase-C zeta, which was approximately 40% membrane-linked in unstimulated cells, translocated by approximately 19%. The human calcitonin receptor expressed by BIN-67 ovary tumor cells, although displaying higher affinity for calcitonin, failed to activate phospholipase D and protein kinase-C in response to the hormone. This receptor lacks the G protein binding consensus site due to the presence of a 48-bp cassette encoding for a 16-amino acid insert in the predicted first intracellular loop. This modification is likely to prevent the calcitonin receptor from associating to phospholipase-coupled signaling.

Breast cancer cell response to calcitonin: modulation by growth-regulating agents

Calcitonin may induce cyclic AMP production by breast cancer cells and inhibit their growth. The molecular complex leading to cyclic AMP production in response to calcitonin is made of the calcitonin receptor coupled to the adenylate cyclase by at least one guanine nucleotide-binding protein (G-protein, of the Gs type). Our aim was to determine whether and how the responses of cells to calcitonin were modulated by growth-regulating agents not directly acting through the cyclic AMP pathway. We found that the cyclic AMP response to calcitonin was reduced after preincubation of cells with the mitogens 17beta-estradiol and epidermal growth factor (EGF), while it was enhanced after preincubation with the growth inhibitors tamoxifen and 1,25(OH)2D3, as well as with an antisense oligonucleotide to the proto-oncogene c-myc. Scatchard-plots revealed no significant change in the calcitonin receptor number or affinity. On the other hand, the cyclic AMP production of cells in response to activators unrelated to calcitonin, such as forskolin, a direct adenylate cyclase effector, and isoproterenol, a beta-adrenergic receptor agonist, was modulated only weakly or not at all by the growth-regulating agents. This suggested that the effects observed were essentially calcitonin-specific and associated with events located between the calcitonin receptor and the adenylate cyclase. Since a Go- or Gi-protein has been previously implicated in the calcitonin signal transduction, we tested the action of pertussis toxin, a specific inhibitor of these G-proteins. Pertussis toxin produced a general increase in the cyclic AMP response of cells to calcitonin; moreover, the toxin almost abolished the effect of mitogens and antimitogens on that parameter. We conclude that in breast cancer cells, the calcitonin receptor and the adenylate cyclase are coupled by at least one Go/Gi-protein sensitive to growth-regulating agents; this results in a modulation of the cyclic AMP response to calcitonin by these agents. On the other hand, the growth-inhibitory effect of calcitonin on breast cancer cells was reduced by 17beta-estradiol and enhanced by tamoxifen. We suggest that this could be a consequence of changes in cyclic AMP levels and deserves further investigation.

Role of phosphodiesterase 4 isoenzyme in alkaline phosphatase activation by calcitonin in porcine kidney LLC-PK1 cells

To confirm the intracellular signal transduction in regulation of alkaline phosphatase (ALP) activity by calcitonin in kidney tubular cells, effects of several inhibitors of cyclic nucleotide phosphodiesterase (PDE) isoenzymes and cyclic AMP-dependent protein kinase (PKA) on the action of salmon calcitonin in porcine kidney tubular epithelial cells LLC-PK1 were examined. A confluent culture of LLC-PK1 cells was treated with calcitonin and inhibitors in Dulbecco's modified Eagle's medium supplemented with 0.1% bovine serum albumin, and intracellular cyclic AMP content and ALP activity were measured after incubation for 30 min and 48 hr, respectively. Calcitonin and PDE 4 inhibitors increased cyclic AMP level and ALP activity in the cells, and PDE 4 inhibitors synergistically potentiated the effects of calcitonin. Calcitonin induced ALP activation by treatment for the first 1 hr, as well as continuous treatment for 48 hr, while it never increased the enzyme activity just after 1-hr exposure. Rolipram, an inhibitor of PDE 4 isoenzyme, induced ALP activation by itself and in combination with calcitonin by only a long term treatment (48 hr). The activation of ALP by calcitonin and rolipram each alone and in combination was completely abolished by a PKA inhibitor, H-89. These results confirm that calcitonin induces ALP activation through the cyclic AMP-PKA pathway and that PDE 4 isoenzyme is closely associated with the calcitonin-receptor system and plays a major role in hydrolysis of cyclic AMP produced in the kidney tubular cells.

Function of the rat calcitonin receptors, C1a and C1b, expressed in Xenopus oocytes

The function of the cloned rat calcitonin receptors, C1a and C1b, was studied in Xenopus oocytes using the two-electrode voltage clamp method. In oocytes expressing the C1a receptors and the cystic fibrosis transmembrane conductance regulator (CFTR), C1a/ CFTR, application (30 sec) of either salmon calcitonin (sCT) or human calcitonin (hCT) activated currents through CFTR. In C1b/CFTR, sCT activated the currents, whereas hCT failed to elicit a response. The sCT induced currents in C1a/CFTR were similar in size to those in C1b/CFTR. Both the activation and the deactivation of sCT-induced currents were slower in C1a/ CFTR. In oocytes expressing C1a or C1b alone, application of relatively high concentrations of sCT induced small oscillatory inward currents. Application of hCT induced small inward currents in C1a alone, but failed to activate currents in C1b alone. These results demonstrate new insights into the signal transduction of calcitonin receptors.

Ca2+ transport by the luminal membrane of the distal nephron: action and interaction of protein kinases A and C

We previously reported that parathyroid hormone and calcitonin increase Ca2+ uptake by purified distal luminal membranes. This effect is mimicked by high concentrations of cAMP. However, both hormones stimulate adenylate cyclase and phospholipase C. The purpose of the present study was to investigate the role of the phospholipase C pathway in the hormone action, and the interrelationship between the two messengers. Distal tubules from rabbit kidneys were incubated with dibutyryl cAMP (dbcAMP) or PMA, or both, and Ca2+ uptake by purified luminal membranes was measured by the rapid filtration technique. Incubation of the distal tubules with 1 mM dbcAMP significantly increased Ca2+ transport by the luminal membranes. A dose-response curve showed a half-maximal stimulation with 0.82 mM dbcAMP. In contrast, treatment of the tubules with 10 nM, 100 nM or 1 microM PMA did not influence Ca2+ uptake by these membranes. However, the addition of 100 nM PMA to low concentrations of dbcAMP strongly increased this uptake. The presence of cAMP or protein kinase C inhibitors prevented the effects of either a high concentration of dbcAMP alone or a low concentration of dbcAMP combined with 100 nM PMA. Our laboratory has already reported that Ca2+ uptake by the distal luminal membranes displays two-component kinetics. dbcAMP increased the Vmax of the low-affinity component, whereas a combination of the two messengers stimulated the Vmax of both the low- and high-affinity components. From these results, we conclude that: (1) in the distal tubule cells, activation of both protein kinases A and C is necessary for the stimulation of Ca2+ transport by the luminal membrane; (2) the combined effect of protein kinases A and C involves both components of the Ca2+-transport kinetics.

Expression cloning and receptor pharmacology of human calcitonin receptors from MCF-7 cells and their relationship to amylin receptors

Human breast cell carcinoma MCF-7 cells were found to bind 125I-labeled rat amylin (rAmylin) and the peptide amylin antagonist radioligand 125I-AC512 with high affinity. This high affinity binding possessed characteristics unique to the already defined high affinity binding site for amylin in the rat nucleus accumbens [Mol. Pharmacol. 44:493-497 (1993); J. Pharmacol. Exp. Ther. 270:779-787 (1994); Eur. J. Pharmacol. 262:133-141 (1994)]. To further define this receptor, we report results of expression cloning studies from an MCF-7 cell library. We isolated two variants of a seven-transmembrane receptor that were identical to two previously described human calcitonin receptors (hCTR1 and hCTR2). These receptors were characterized by expression in different surrogate host cell systems. Transient expression of hCTR1 in COS cells yielded membranes that bound 125I-AC512 and 125I-salmon calcitonin with high affinity, but no high affinity binding was observed with 125I-human calcitonin (hCAL) or 125I-rAmylin. Stable expression of hCTR1 in HEK 293 cells produced similar data. In contrast, expression of hCTR2 in COS cells yielded membranes that bound 125I-AC512, 125I-hCAL, and 125I-rAmylin with high affinity. The agonists 125I-hCAL and 125I-rAmylin bound 65% and 1.5%, respectively, of the sites bound by the antagonist radioligand 125I-AC512 in this expression system. This pattern of binding was repeated in HEK 293 cells stably transfected with hCTR2 (125I-hCAL = 24.8% Bmax, 125I-rAmylin = 8% Bmax). In both expression systems, the agonists hCAL and rAmylin were much more potent in displacing their radioligand counterparts than was the antagonist radioligand 125I-AC512. For example, the pKi value for displacement of 125I-AC512 by rAmylin was 7.2 in HEK 293 cells but rose to 9.1 when displacing 125I-rAmylin. Finally, hCTR2 was expressed in baculovirus-infected Ti ni cells. In this system, only specific binding to the antagonist 125I-AC512 and agonist 125I-hCAL was observed; no binding to 125I-rAmylin could be detected. These data are discussed in terms of two working hypotheses. The first is that amylin is a weak agonist for hCTR2 and that this receptor is unrelated to the amylin receptor found in this cell line. The second is that hCTR2 couples to different G proteins for calcitonin and amylin function in different cells. At present, these data cannot be used to disprove conclusively either hypothesis.

Phosphorylation of the human calcitonin receptor by multiple kinases is localized to the C-terminus

The calcitonin receptor is a seven-transmembrane G-protein coupled receptor which is located on osteoclasts, in kidney, and in brain. The receptor signals through multiple pathways, including activation of adenylate cyclase, leading to inhibition of bone resorption. In the present study, we used antibodies raised against the C-terminus of the human calcitonin (CT) receptor to study receptor phosphorylation. In baby hamster kidney cells transfected with the human CT receptor, phosphorylation of the receptor increased approximately 2.5-fold after cells were treated with calcitonin, phorbol ester, forskolin, or calcitonin plus phorbol ester. Phosphorylation reached a maximum 20 minutes after treatment with sCT and half-maximal phosphorylation was observed at 0.1 nM sCT, a hormone concentration related to receptor occupancy. Digestion of the immunoprecipitated receptor with cyanogen bromide (CNBr) yielded a single 32P-labeled fragment which migrates at Mr 14 kD on gel electrophoresis. This corresponds to the predicted size of the CNBr fragment containing the C-terminal domain of the receptor. No 32P-labeled bands were observed for CNBr fragments predicted to contain the first, second, or third intracellular loops. An identical labeling pattern was seen with cells expressing an alternatively spliced isoform of the human receptor (insert-positive isoform). Phosphorylation of the receptor by phorbol ester and forskolin was further localized to a Mr 6 kD proteolytic fragment within the C-terminus. The protein kinase A and C inhibitors staurosporine, chelerythrine, and H-89 had little effect on CT-induced phosphorylation, suggesting that nonsecond messenger-activated kinases are involved in hormone-dependent CT receptor phosphorylation.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Effect of calcitonin on calcium transport by the luminal and basolateral membranes of the rabbit nephron

In the rabbit, calcitonin has been shown to enhance calcium (Ca2+) reabsorption in the early distal tubule. The aim of the present study was to investigate the mechanism of this action, using isolated luminal and basolateral membranes of distal tubules. The tubule suspensions were preincubated in the presence or absence of 10(-7) M calcitonin. The luminal or basolateral membranes were subsequently purified and 45Ca transport through the vesicles was measured using the rapid filtration technique. Results were compared with those obtained from proximal tubule membranes. In the proximal tubules, calcitonin had no effect on Ca2+ uptake by luminal membranes. In the distal tubules, the presence of Na+ in the incubation medium strongly decreased the uptake of Ca2+ by luminal membranes. Preincubation of distal tubules with calcitonin partially restored this uptake. We previously reported a dual kinetics of Ca2+ uptake by the distal luminal membranes. Calcitonin enhanced Ca2+ transport by the low affinity component, increasing the Vmax and leaving the K(m) unchanged. Renal calcitonin receptors usually couple to both adenylate cyclase and phospholipase C. To determine through which messenger(s) calcitonin enhances Ca2+ transport by the distal tubules, we first confirmed that the hormone stimulates cAMP and IP3 release. Incubation of the distal tubules with 10(-7) M calcitonin significantly increased both messengers. In contrast, calcitonin did not influence the IP3 nor the cAMP content of proximal tubules. Therefore, we studied the actions of cAMP and phorbol 12-myristate 13 acetate (PMA) on Ca2+ transport by the distal luminal membranes. Incubation of distal tubule suspensions with dibutyryl cAMP significantly increased Ca2+ uptake by the luminal membranes. However, incubation of these tubules with various concentrations of PMA (10 nM, 100 nM and 1 microM) had no effect on this uptake. Calcitonin also influenced Ca2+ transport by the distal basolateral membrane. Incubation of distal tubule suspensions with 10(-7) M calcitonin activated the Na+/Ca2+ exchanger activity, almost doubling the Na+ dependent Ca2+ uptake. Here again this action was mimicked by cAMP. We conclude that calcitonin increases Ca2+ transport by the distal tubule through two mechanisms: the opening of low affinity Ca2+ channels in the luminal membrane and the stimulation of the Na+/Ca2+ exchanger in the basolateral membrane, both actions depending on the activation of adenylate cyclase.

Structure/function relationships of calcitonin analogues as agonists, antagonists, or inverse agonists in a constitutively activated receptor cell system

The structure/function relationship of salmon calcitonin (sCT) analogues was investigated in heterologous calcitonin receptor (CTR) expression systems. sCT analogues with progressive amino-terminal truncations intermediate of sCT-(1-32) to sCT-(8-32) were examined for their ability to act as agonists, antagonists, or inverse agonists. Two CTR cell clones, B8-H10 and G12-E12, which express approximately 5 million and 25,000 C1b receptors/cell, respectively, were used for this study. The B8-H10 clone has an approximately 80-fold increase in basal levels of intracellular cAMP due to constitutive activation of the overexpressed receptor. In whole-cell competition binding studies, sCT-(1-32) was more potent than any of its amino-terminally truncated analogues in competition for 125I-sCT binding. In cAMP accumulation studies, sCT-(1-32) and modified analogues sCT-(2-32) and sCT-(3-32) had agonist activities. SDZ-216-710, with an amino-terminal truncation of four amino acids, behaved as a partial agonist/antagonist, whereas amino-terminal truncations of six or seven amino acid residues produced a 16-fold reduction in basal cAMP levels and attenuated the response to the agonist sCT-(1-32) in the constitutively active CTR system. This inverse agonist effect was insensitive to pertussis toxin inhibition. In contrast, the inverse agonist activity of these peptides was not observed in the nonconstitutively active CTR system, in which sCT analogues with amino-terminal truncations of four or more amino acids behaved as neutral competitive antagonists. These results suggest that the inverse agonist activity is mediated by stabilization of the inactive state of the receptor, which does not couple to G protein, and attenuates basal signaling initiated by ligand-independent activation of the effector adenylyl cyclase.

The deletion of 14 amino acids in the seventh transmembrane domain of a naturally occurring calcitonin receptor isoform alters ligand binding and selectively abolishes coupling to phospholipase C

The cDNA that encodes the rabbit calcitonin receptor was cloned by screening a rabbit osteoclast library. Reverse transcription-polymerase chain reaction amplification of calcitonin receptor sequences from rabbit osteoclast RNA yielded cDNAs that encode two isoforms of the calcitonin receptor. One isoform is homologous to the C1a isoform previously identified in multiple cell types and species, while the second, designated CTRDeltae13, is a previously unidentified isoform that is apparently generated by alternative splicing during mRNA processing that deletes exon 13, resulting in the absence of 14 amino acids in the predicted seventh transmembrane domain. Expression of mRNA transcripts encoding the two isoforms varies in a tissue-specific manner, with CTRDeltae13 accounting for less than 15% of the total calcitonin receptor mRNA in osteoclasts, kidney, and brain, but comprising at least 50% of the transcripts in skeletal muscle and lung. The two isoforms were expressed, and the ligand binding and signal transduction properties were characterized. Deletion of the residues in the seventh transmembrane domain in CTRDeltae13 reduced the binding affinity for salmon and human calcitonin by more than 10-fold and approximately 2-fold, respectively, resulting in a receptor that failed to discriminate between the two forms of calcitonin. Both isoforms activated adenylyl cyclase, with EC50 values consistent with the difference in ligand affinities. In contrast, only the C1a isoform, but not the CTRDeltae13 isoform, activated phospholipase C. Thus, while the CTRDeltae13 remains active despite the deletion of a significant portion of its seventh transmembrane domain, it has significantly altered ligand recognition and signal transduction properties.

The neuropeptide calcitonin gene-related peptide stimulates insulin-like growth factor I production by primary fetal rat osteoblasts

Calcitonin gene-related peptide (CGRP)-immunoreactive sensory nerve terminals infiltrate all tissues including bone, in which CGRP may play a local regulatory role. To initiate studies on the role of this neuropeptide in bone, osteoblasts were isolated from fetal rat calvariae, treated with CGRP, and analyzed for cAMP and insulin-like growth factor I (IGF-I) production. CGRP alpha and -beta induced a cAMP accumulation in osteoblastic cells, suggesting that they express functional receptors for CGRP. CGRP induced an increase in both IGF-I transcripts and immunoreactive polypeptide. In contrast to prostaglandin E2 (PGE2) treatment, this increase was not accompanied by an augmentation in IGF binding proteins. Although PGE2 induced a more significant increase in IGF-I transcripts than did CGRP, the concentration of IGF-I polypeptide produced by osteoblasts was similar in response to both treatments. It is concluded from this study that CGRP has potent anabolic effects on osteoblasts, an observation which opens possibilities to study the potential therapeutic role of CGRP in osteoporosis.