Kinetics of calcitonin receptor internalization in lung cancer (BEN) and osteogenic sarcoma (UMR 106-06) cells

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-induced change in serum calcium levels and its relationship to osteoclast morphology and number of calcitonin receptors

It has been shown that, in live subjects, the ability of calcitonin (CT) to decrease serum calcium (Ca) levels can be lost in response to its continued or repeated administration. The present study investigated the relationship between such changes of in vivo serum Ca levels and the response of osteoclasts to CT administration, including the downregulation of their CT receptors (CTRs). Rats were either given a single injection of CT or repeated injections at either 6- or 24-h intervals, after which their serum Ca levels were evaluated. Their parietal bones were dissected, and the amount of 125I-labeled elcatonin (125I-eCT) binding to their osteoclasts measured using autoradiography. Ultrastructural changes in the osteoclasts were also examined. Twenty-four hours after a single CT administration, serum Ca levels had dropped, and there was an absence of ruffled borders on the osteoclasts. Less 125I-eCT binding to the osteoclast was found than in the control group. Forty-eight and 72 h after CT administration, serum Ca levels had almost returned to control levels, and the osteoclasts showed ruffled borders once again. The amount of 125I-eCT binding to the osteoclast also recovered to control levels. When these osteoclasts were then incubated in CT, their ruffled borders once again disappeared. In the 6-h interval multiple CT administration schedule subjects, upon inspection 72 h after their first administration (6 h following the final one), serum Ca levels were found to have almost returned to control levels with the presence of osteoclast ruffled borders. The amount of 125I-eCT binding to these osteoclasts was remarkably limited, and no disappearance of the ruffled borders occurred in response to additional CT incubation. In the 24-h interval multiple administration schedule subjects, upon inspection 72 h after their first CT administration (24 h following the final one), there was less 125I-eCT binding than in the single-dose subjects tested 24 h after their injection, and the ability of CT to lower their serum Ca levels was reduced. The ability of CT to lower serum Ca levels was therefore related to the response of osteoclasts to the CT (the disappearance of the ruffled borders), and this response was related to the amount of CTRs available for binding with CT on the osteoclast surface. Furthermore, the reduced effectiveness of CT in response to repeated CT administration was found to be related to the downregulation of the CTRs on the osteoclast surface.

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.

Expression of the CT/CGRP gene and its regulation by dibutyryl cyclic adenosine monophosphate in human osteoblastic cells

There is general agreement that calcitonin (CT) inhibits bone resorption by its effects on osteoclast function. CT was also found to have direct effects on osteoblast-like cells. In this study, we investigated the expression of CT and calcitonin gene-related peptide (CGRP), the two peptides encoded by the CT/CGRP gene, in human osteosarcoma cell lines and in normal human trabecular osteoblastic cells (HOB), and we studied the modulation of CT/CGRP gene expression by dibutyryl cyclic adenosine monophosphate ((Bu)2, cAMP), a cAMP analog. We first detected by Northern blot hybridization the presence of CT and CGRP mRNAs in different osteosarcoma cell lines (OHS-4, MG-63, Saos-2, HOS-TE85) and HOB cells. In the steady state, OHS-4 cells express slightly more CT and CGRP mRNAs than other cell lines or normal human osteoblasts, in parallel with messengers of differentiated osteoblasts, such as osteocalcin (OC) and alkaline phosphatase (ALP). OHS-4 cells also express CT and CGRP proteins, as demonstrated by immunocytochemistry. Stimulation of OHS-4 cells with 1 mM (Bu)2 cAMP induced a significant increase in mRNA levels for CT (x 2.5) and CGRP (x 3), as determined by a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) procedure. The involvement of a transcriptional mechanism in this effect was evidenced by nuclear run-off transcription assay. In addition, (Bu)2 cAMP increased OC (x 4) and ALP (x 3) mRNA levels in OHS-4 cells. These effects were observed at 24 h and were maximal at 48 h, indicating that (Bu)2, cAMP induced cell differentiation and increased the transcription of the CT/CGRP gene in OHS-4 osteoblast-like cells. The results indicate that human osteosarcoma cells and primary human osteoblastic cells express CT and CGRP mRNA and proteins, and that (Bu)2 cAMP, an activator of protein kinase A, induces up-regulation of osteoblastic phenotypic genes and enhances CT and CGRP gene transcription, indicating that induction of osteoblastic differentiation by (Bu)2 cAMP is associated with enhanced expression of CT and CGRP in human osteoblastic cells.

Stimulation of neonatal mouse calvarial bone resorption by the glucocorticoids hydrocortisone and dexamethasone

In vitro stimulation of bone resorption was observed with the glucocorticoids hydrocortisone and dexamethasone. Dosage-dependent release of 45Ca from neonatal mouse calvarial bones was found for both steroids, with half-maximal responses for hydrocortisone and dexamethasone of 0.3 and 0.08 microM, respectively. Significant release of stable calcium (Ca2+), inorganic phosphate (Pi), and the lysosomal enzyme beta-N-acetylglucosaminidase was noted following treatment of mouse calvariae with either 1 microM hydrocortisone or 1 microM dexamethasone. Additionally, both 1 microM hydrocortisone and 1 microM dexamethasone elicited release of 3H from calvarial bones prelabeled with [3H]proline. The stimulation of bone resorption by the glucocorticoids, as assessed by 45Ca release, was sustained over 120 h of culture. Inhibition of 45Ca release from calvariae treated with either 1 microM hydrocortisone or 0.1 microM dexamethasone was observed with 0.01-30 nM salmon calcitonin (sCT), 0.1 mM acetazolamide, and 0.1 mM of the bisphosphonate AHPrBP. Inhibition of glucocorticoid-induced bone resorption by sCT occurred without "escape from calcitonin-induced inhibition." The 45Ca release stimulated by 1 microM hydrocortisone and 0.1 microM dexamethasone was also inhibited by 10 microM progesterone in a competitive manner and by 1 microM of the antiglucocorticoid RU38486, both of which are modulators of glucocorticoid binding. Prostaglandin E2 (PGE2) formation by 10 nM parathyroid hormone (PTH) in neonatal mouse calvarial bones was inhibited by both 1 microM hydrocortisone and 1 microM dexamethasone, but neither compound altered basal PGE2 formation. Exposure of calvarial bones to the mitotic inhibitors hydroxyurea and mitomycin C inhibited 45Ca release stimulated by 1 microM hydrocortisone and 1 microM dexamethasone. In contrast, addition of 1 ng/ml of recombinant murine granulocyte macrophage colony stimulating factor (rmGM-CSF) had no effect on 45Ca release elicited by the glucocorticoids. These results suggest that hydrocortisone and dexamethasone stimulate osteoclastic resorption in neonatal mouse calvariae by a receptor-mediated mechanism that is dependent on cellular replication.

Short treatment of osteoclasts in bone marrow culture with calcitonin causes prolonged suppression of calcitonin receptor mRNA

Cells exhibiting osteoclast characteristics of calcitonin receptors (CTRs) and tartrate-resistant acid phosphatase (TRAP) histochemistry are formed in murine bone marrow cultures treated with 1 alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. We have previously demonstrated that CTR mRNA is highly expressed in these cultures. The aim of this study was to investigate homologous regulation of the CTR, and regulation of TRAP expression in osteoclast-like cells after brief treatment with salmon CT (sCT). Murine bone marrow cells were cultured in 9 cm dishes in the presence of 10 nmol/L 1,25-(OH)2D3. On day 6 of culture, when multinucleated cells were abundant, the cells were treated with 1 nmol/L sCT for 1 h. Both control and treated cells were then harvested at intervals up to 72 h posttreatment, and both CTR and TRAP mRNA levels assessed by reverse-transcription PCR (RT-PCR). In parallel cultures, cells with CTR expression detectable by autoradiography, and TRAP positivity by histochemistry, were counted. The effects of brief sCT treatment could be seen 6 h after treatment when the CTR RT-PCR product was markedly reduced. Total recovery of CTR mRNA levels had not occurred even after 72 h. Calcitonin treatment had little effect on TRAP mRNA levels. There was no difference in the numbers of multinucleated TRAP(+) osteoclast-like cells between treated and control cells. These results indicate that brief sCT treatment, while not influencing multinucleated osteoclast-like cell number, causes specific, acute reduction of CTR mRNA in bone marrow culture-derived osteoclasts. The prolonged decrease in CTR mRNA levels suggests that recovery may require new osteoclast formation, and indicates that regulation of the CTR in cells of the osteoclast lineage is different from that in nonosteoclastic cells and tissues.

Glucocorticoid regulation of calcitonin receptor in mouse osteoclast-like multinucleated cells

Abundant multinucleated cells (MNCs) are formed in cocultures of mouse osteoblastic cells and marrow cells in the presence of 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25(OH)2D3], and these cells have the properties of osteoclasts (OCs). In this study using the mammalian OCs, we tried to clarify the role of glucocorticoids (GCs) in calcitonin receptors (CTR) and CT-responsive cAMP production in OCs. Dexamethasone (DEX) dose and time dependently enhanced the specific binding of [125I]salmon calcitonin (sCT). When the MNCs were preincubated with DEX for 24 h, the effect was evident at 10(-9) M and the maximum effect was obtained at 10(-7) M. The effect developed over 12-48 h at doses of 10(-9) and 10(-6) M DEX. The numbers of CTR-positive mononuclear cells and MNCs were not altered by the DEX treatment. Prednisolone and triamcinolone reproduced the DEX effect, but 17 beta-estradiol, progesterone, testosterone, aldosterone, and 1 alpha, 25(OH)2D3 did not. RU486, a GC receptor antagonist, attenuated the effect of DEX to enhance the specific binding of [125I]sCT. From a Scatchard plot analysis, DEX enhanced CTR number (212 +/- 64%) with a minimal change in the affinity to sCT. Autoradiographic studies using [125I]sCT showed that DEX enhanced the density of the grains on the tartrate-resistant acid phosphatase (TRAP)-positive MNCs and mononuclear cells, but not on other types of cells. DEX preincubation also enhanced sCT-stimulated but not prostaglandin E2- or forskolin-stimulated cAMP production.(ABSTRACT TRUNCATED AT 250 WORDS)

Histochemical and autoradiographic studies on elcatonin internalization and intracellular movement in osteoclasts

The binding sites and chronologic localization of elcatonin (eCT) in osteoclasts were examined by autoradiography using [125I]elcatonin (125I-eCT). In addition to the structural changes induced by calcitonin (CT) reported so far, changes were also observed in the structure of Golgi apparatus. These changes continued until 48-72 h after incubation with eCT. Developed silver grains of 125I-eCT were localized into multinucleated osteoclasts and mononuclear cells that were ultrastructurally defined as "preosteoclasts." The silver grains located on plasma membranes of those cells and were then internalized; they accumulated, especially in the Golgi apparatus, and remained for 48-72 h. A few silver grains were also detected in lysosomes and small vesicles. The decrease in the number of silver grains in the Golgi apparatus accompanied the recovery of osteoclast structures--Golgi apparatus and then ruffled borders. These findings suggest that (1) CT especially inhibits the sorting function of Golgi apparatus in osteoclasts, resulting in prolonged retention of CT in this organelle. (2) The CT in Golgi apparatus may keep its activity and cause the prolonged effect of CT on osteoclast activity.

Two biochemical indices of mouse bone formation are increased, in vivo, in response to calcitonin

In a series of four studies, adult female Swiss-Webster mice were used to measure the effects of salmon calcitonin on two biochemical indices of local and systematic bone formation: (1) skeletal alkaline phosphatase activity--in serum and in extracts of calvaria and tibiae, and (2) calvarial collagenase-digestible protein synthesis--measured, acutely, in vitro. Subcutaneous calcitonin doses ranged from 50 to 400 mU/mouse/day (0.95-18.1 U/kg/day), and treatment schedules were continuous (daily) for 2-14 days, acute, or intermittent (2 days/week for 6 weeks). The effects of calcitonin on these bone formation indices (skeletal alkaline phosphatase and collagenase-digestible protein synthesis) were biphasic with respect to dose and treatment time, being increased in response to short-term, low-dose treatment, but not long-term, continuous treatment. The effects of long-term intermittent calcitonin treatment were dose-dependent increases in skeletal alkaline phosphatase in calvaria and serum (r = 0.948, P less than 0.02, and r = 0.960, P less than 0.01, respectively).

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitonin (but not calcitonin gene-related peptide) increases mouse bone cell proliferation in a dose-dependent manner, and increases mouse bone formation, alone and in combination with fluoride

Previous in vitro studies have shown that salmon calcitonin had direct effects to increase parameters associated with embryonic chicken bone formation and to increase mouse and chicken osteoblast-line cell proliferation. The current studies demonstrate increased cell proliferation (i.e., [3H]-thymidine incorporation into DNA and tetrazolium salt reduction/deposition) in the osteoblastic murine cell line MC-3T3-E1 in response to salmon calcitonin (P less than 0.005) and to human calcitonin (P less than 0.005), but not to human calcitonin gene-related peptide. The current studies also show that salmon calcitonin increased several indices of murine bone formation. We found that 72 hours of exposure to salmon calcitonin [at 5 mU/ml-about 0.37 nM; mU/ml = milliunits of calcitonin activity/ml incubation medium (at 4,000 U/mg protein)] increased net 45Ca deposition (121% of control, P less than 0.05), net [3H]-proline incorporation 149% of control, P less than 0.001), and alkaline phosphatase activity (146% of control, P less than 0.01), in neonatal mouse half-calvaria. The calcitonin-dependent increase in alkaline phosphatase activity was not affected by co-incubation with 1 nM parathyroid hormone. Co-incubation with fluoride (which also increased net [3H]-proline incorporation and alkaline phosphatase activity in neonatal mouse half-calvaria, P less than 0.05, for each) enhanced the osteogenic response to low-dose calcitonin, (i.e., co-incubation with fluoride shifted the biphasic calcitonin dose-response curve to a range of lower calcitonin concentrations). The calcitonin-fluoride combinations had proportional effects on net [3H]-proline incorporation and alkaline phosphatase in the treated mouse calvaria (r = 0.78, P less than 0.005).

Down-regulation of calcitonin receptors in T47D cells by internalization of calcitonin-receptor complexes

T47D cells possess specific calcitonin (CT) receptors and a CT-responsive adenylate cyclase. Internalization of part of their CT receptors has been suggested. At 37 degrees C, bound 125I-labelled salmon CT (sCT) becomes increasingly resistant to acid washing, which can remove surface-bound hormone, thus indicating internalization. Monensin and chloroquine, which raise the pH of the lysosomes and thereby inhibit cellular processing of endosomes, inhibit the decrease of total bound activity seen in the controls. Acid-resistant (internalized) activity increases to the levels of total binding. Preincubation with sCT leads to a loss of specific binding. Recovery of CT binding is prevented by monensin, which also inhibits transport of cellular proteins to the cell membrane. Recovery is not influenced by chloroquine. As chloroquine prevents recycling, we conclude that after binding of CT the receptors are internalized, transferred to a lysosomal compartment, and degraded intracellularly without recycling. All receptors seem to undergo internalization. Desensitization to CT in T47D cells is at least partly mediated by intracellular metabolism of CT receptors.

Down-regulation of rat kidney calcitonin receptors by salmon calcitonin infusion evidenced by autoradiography

In treating age-related osteoporosis and Paget disease of bone, it is of major importance to avoid an escape phenomenon that would reduce effectiveness of the treatment. The factors involved in the loss of therapeutic efficacy with administration of large pharmacological doses of the hormone require special consideration. Down-regulation of the hormone receptors could account for the escape phenomenon. Specific binding sites for salmon calcitonin (sCT) were characterized and localized by autoradiography on rat kidney sections incubated with 125I-labeled sCT. Autoradiograms demonstrated a heterogenous distribution of 125I-labeled sCT binding sites in the kidney, with high densities in both the superficial layer of the cortex and the outer medulla. Infusion of different doses of unlabeled sCT by means of Alzet minipumps for 7 days produced rapid changes in plasma calcium, phosphate, and magnesium levels, which were no longer observed after 2 or 6 days of treatment. Besides, infusion of high doses of sCT induced down-regulation of renal sCT binding sites located mainly in the medulla, where calcitonin (CT) has been shown to exert its physiological effects on water and ion reabsorption. These data suggest that the resistance to high doses of sCT often observed during long-term treatment of patients may be the consequence of not only bone-cell desensitization but also down-regulation of CT-sensitive kidney receptor sites.