Intracellular calcium increases mediated by a recombinant human calcitonin receptor

Determination of the site of action of calcitonin gene-related peptide in the alteration of intracellular calcium levels in adult and neonatal rodent myocytes

The purpose of this study is to elucidate the mechanism of action and site of action of calcitonin gene-related peptide (CGRP) and its effects on calcium concentrations in two types of cardiomyocytes, neonatal and adult, by employing real-time fluorescence imaging. CGRP caused an increase in intramyocytic calcium with adult cells, but a decrease with neonates. Treatment of adult myocytes with ouabain and ryanodine yielded results suggesting that CGRP action is not at the ryanodine receptor (RyR) and does not involve Na+ +K+ ATPase. Furthermore, in neonatal cardiomyocytes CGRP caused a reduction in intramyocytic calcium levels, and challenges with ryanodine and ouabain gave results supporting the hypothesis that CGRP acts at the sarcolemmal L-type calcium channel. Employing real-time fluorescence measurements in cultured, dedifferentiated adult cardiomyocytes, which are known to express a fetal phenotype and exhibit neonatal-like calcium transients, our acquisitions demonstrated a major reduction in intracellular calcium levels. Finally, our collaborative studies in human myocardium using fluorescence deconvolution microscopy revealed that CGRP localization was found in a pattern similar to that of the sarcolemmal L-type calcium channel.

Calcitonin therapy in osteoporosis

Osteoporosis is the most prevalent metabolic bone disease and is characterized by diminished bone strength predisposing to an increased risk of fracture. Its incidence is particularly high in postmenopausal women but it can also affect other groups, such as men and patients receiving corticosteroid therapy. Calcitonin is a naturally occurring peptide which acts via specific receptors to strongly inhibit osteoclast function. It has been used in the treatment of osteoporosis for many years. Historically, calcitonin was administered as a parenteral injection, but the intranasal formulation is now the most widely used because of its improved tolerability. New approaches are currently being investigated to enhance the bioavailability and effects of calcitonin, including oral, pulmonary, and transdermal routes of administration, and novel allosteric activators of the calcitonin receptor. Several controlled trials have reported that calcitonin stabilizes and in some cases produces a short-term increase in bone density at the lumbar spine level. The most relevant clinical trial to evaluate the effect of calcitonin in the prevention of fractures was the Prevent Recurrence of Osteoporotic Fractures (PROOF) study, a 5-year double-blind, randomized, placebo-controlled trial showing that salmon calcitonin nasal spray at a dosage of 200 IU/day can reduce the risk of vertebral osteoporotic fractures by 33% (relative risk [RR] = 0.67; 95% CI 0.47, 0.97; p = 0.03). However, the 100 and 400 IU/day dosages did not significantly reduce vertebral fracture risk. Effects on nonvertebral fractures were not significant (RR = 0.80; 95% CI 0.59, 1.09; p = 0.16). There is mounting evidence to show that calcitonin diminishes bone pain in osteoporotic vertebral fractures, which may have clinical utility in vertebral crush fracture syndrome. A recent study suggests that nasal salmon calcitonin appears to be a promising therapeutic approach for the treatment of men with idiopathic osteoporosis, although long-term trials are necessary to confirm these results and evaluate fracture rate as an endpoint in men. The role of calcitonin in corticosteroid-induced osteoporosis remains controversial, hence it can only be considered a second-line agent for the treatment of patients with low bone mineral density who are receiving long-term corticosteroid therapy.

Calcitonin: the other thyroid hormone

Calcitonin was originally discovered as a hypocalcemic factor synthesized by thyroid parafollicular C cells. Early experiments demonstrated that calcitonin inhibited bone resorption and decreased calcium efflux from isolated cat tibiae and subsequent histologic and culture studies confirmed the osteoclast as its major site of action. Its potent antiresorptive effect and analgesic action have led to its clinical use in treatment of Paget's bone disease, osteoporosis, and hypercalcemia of malignancy. This review surveys the cellular and molecular basis of these physiologic and clinical actions.

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.

Calcitonin gene-related peptide receptor independently stimulates 3',5'-cyclic adenosine monophosphate and Ca2+ signaling pathways

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse biological properties including potent vasodilating activity. Recently, we reported the cloning of complementary DNAs (cDNAs) encoding the human and porcine CGRP receptors which share significant amino acid sequence homology with the human calcitonin receptor, a member of the recently described novel subfamily of G-protein-coupled 7TM receptors. Activation of this family of receptors has been shown to result in an increase in intracellular cAMP accumulation and calcium release. In this study, we demonstrate that HEK-293 cells expressing recombinant CGRP receptors (HEK-293HR or PR) respond to CGRP with increased intracellular calcium release (EC50 = 1.6 nM) in addition to the activation of adenylyl cyclase (EC50 = 1.4 nM). The effect of CGRP on adenylyl cyclase activation and calcium release was inhibited by CGRP (8-37), a CGRP receptor antagonist. Both effects were mediated by cholera toxin-sensitive G-proteins, but these two signal transduction pathways were independent of each other. While cholera toxin pretreatment of HEK-293PR cells resulted in permanent activation of adenylyl cyclase, the same pretreatment resulted in an inhibition of CGRP-mediated [Ca2+]i release. Pertussis toxin was without effect on CGRP-mediated responses. In addition, CGRP-mediated calcium release appears to be due to release from a thapsigargin-sensitive intracellular calcium pool. These results show that the recombinant human as well as porcine CGRP receptor can independently increase both cAMP production and intracellular calcium release when stably expressed in the HEK-293 cell line.

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.