Stimulate Calcitonin Secretion Research Articles

Asymptomatic parathyroid adenoma in mother as a cause of late-onset persistent hypocalcemia in a newborn

Pregnant women with primary hyperparathyroidism may be asymptomatic or have mild symptoms such as fatigue, thirst, constipation, or transient depression. Transfer of calcium from mother to fetus leads to increased fetal calcium concentrations, suppressing fetal parathyroid hormone synthesis, and stimulating calcitonin secretion leading to neonatal hypocalcemia. Here, we present a report of a newborn admitted on day 10 of life with recurrent tonic convulsions. On investigation, it was found that the baby had severe persistent hypocalcemia which when further evaluated was due to asymptomatic maternal hyperparathyroidism due to parathyroid adenoma.

Igf-I influences everolimus activity in medullary thyroid carcinoma.

Medullary thyroid carcinoma (MTC) is a rare tumor originating from thyroid parafollicular C cells. It has been previously demonstrated that insulin-like growth factor I (IGF-I) protects MTC from the effects of antiproliferative drugs. Everolimus, an mTOR inhibitor, has shown potent antiproliferative effects in a human MTC cell line, TT, and in two human MTC primary cultures. To verify whether IGF-I may influence the effects of everolimus in a group of human MTC primary cultures. We collected 18 MTCs that were dispersed in primary cultures, treated without or with 10 nM-1 μM everolimus and/or 50 nM IGF-I. Cell viability was evaluated after 48 h, and calcitonin (CT) secretion was assessed after a 6 h incubation. IGF-I receptor downstream signaling protein expression profile was also investigated. Everolimus significantly reduced cell viability in eight MTC [by ~20%; P < 0.01 vs. control; everolimus-responders (E-R) MTCs], while cell viability did not change in 10 MTCs [everolimus-non-responders (E-NR) MTCs]. In E-R MTCs, IGF-I blocked the antiproliferative effects of everolimus that did not affect CT secretion, but blocked the stimulatory effects of IGF-I on this parameter. IGF-I receptor downstream signaling proteins were expressed at higher levels in E-NR MTC as compared to E-R MTCs. IGF-I protects a subset of MTC primary cultures from the antiproliferative effects of everolimus and stimulates CT secretion by an mTOR mediated pathway that, in turn, may represent a therapeutic target in the treatment of aggressive MTCs.

Presentation of points of general discussion and voting among the speakers of the European Thyroid Association-Cancer Research Network (ETA-CRN) meeting in Lisbon, 2009, entitled ”European comments to ATA medullary thyroid cancer guidelines”

The main subjects of discussion, held online within the ETA-CRN board invited 16 expert-panelists are shown. The ad hoc emerged ETA-CRN panel of experts (EPE) first congratulated Professor Kloos and the ATA Taskforce for the extensive work on medullary thyroid cancer, and appreciated discussing the ATA guidelines during the ETA-CRN meeting. As it was not possible for all experts to visit the meeting, they enclosed their comments in the online ETA forum. The overall intention was to evaluate certain discrepancies between the ATA guidelines and were biased European clinical practice.All discussants were aware that the ATA guidelines had followed evidence based medicine rules; however, it was intended to reach an European consensus in this matter. The results of online voting among the EPE are shown.We received answers from nine experts. The particular ATA guidelines devoted to the management of MTC ranged in agreement in 0/9 to 4/9. This did not reflect the general, good assessment of the guidelines, as of votes a set of questions.The strongest discrepancies were found in assessment of the usefulness of pentagastrin (Peptavlon®) stimulated calcitonin secretion. The majority of the EPE (5/9) chose an option: “the increase of the basal Ct >100 ng/L means the substantial risk of MTC. However, there should also have been a recommendation for the grey zone 10-100 ng/L, where stimulation with pentagastrin is useful. The cut-off to perform stimulation test at ≤ 15-20 ng/L and values >100 ng/L means a significant suspicion of MTC”.Similarly, attention from the EPE was raised towards the surgical procedures in MTC, particularly the extent and indications for lymph node surgical intervention. Four questions were related to the indications to lymphadenectomy and extent of surgery. The equal number (4/8) of EPE agreed with the ATA R61 and half of the ETA-CRN panel of experts disagreed because the indications to lymphadenectomy (Lx) depended in their opinion on the tumors detected by the Ct screening, in which prophylactic Lx might not be necessary.“Notwithstanding the evidence based guidelines, their final acceptation requires unrestricted discussion and consideration of differences in clinical practice and experience between countries”.

Strontium is a biased agonist of the calcium-sensing receptor in rat medullary thyroid carcinoma 6-23 cells.

The calcium-sensing receptor (CaSR)-specific allosteric modulator cinacalcet has revolutionized the treatment of secondary hyperparathyroidism in patients with chronic kidney disease. However, its application is limited to patients with end-stage renal disease because of hypocalcemic side effects presumably caused by CaSR-mediated calcitonin secretion from thyroid parafollicular C-cells. These hypocalcemic side effects might be dampened by compounds that bias the signaling of CaSR, causing similar therapeutic effects as cinacalcet without stimulating calcitonin secretion. Because biased signaling of CaSR is poorly understood, the objective of the present study was to investigate biased signaling of CaSR by using rat medullary thyroid carcinoma 6-23 cells as a model of thyroid parafollicular C-cells. By doing concentration-response experiments we focused on the ability of two well known CaSR agonists, calcium and strontium, to activate six different signaling entities: G(q/11) signaling, G(i/o) signaling, G(s) signaling, extracellular signal-regulated kinases 1 and 2 (ERK1/2) signaling, intracellular calcium ([Ca(2+)](i)) mobilization, and calcitonin secretion. The experiments showed that strontium biases CaSR signaling toward ERK1/2 signaling and possibly another pathway independent of G(q/11) signaling and [Ca(2+)](i) mobilization. It is noteworthy that the potency of strontium-stimulated calcitonin secretion was elevated compared with calcium. Combining these results with experiments investigating signaling pathway components involved in calcitonin secretion, we found that the enhanced potency of strontium-mediated calcitonin secretion was caused by a different signaling pattern than that produced by calcium. Together, our results suggest that calcitonin secretion can be affected by CaSR-stimulated signaling bias, which may be used to develop novel drugs for the treatment of secondary hyperparathyroidism.

Dipeptidyl peptidase-4 inhibitors and prevention of bone fractures: Effects beyond glyemic control.

Dipeptidyl peptidase-4 (DPP-4) inhibitors improve glycemic control in patients with type 2 diabetes by preventing degradation of two incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). GLP-1 and GIP are secreted from the intestine on ingestion of various nutrients, and enhance insulin secretion from pancreatic β-cells glucose-dependently1. Both incretin hormones show various biological functions in addition to their glucose-dependent insulinotropic action. Thus, DPP-4 inhibitors are expected to exert extra effects on various tissues and cell types. Among them, their effects on bones are of particular interest. A recent meta-analysis of randomized clinical trials comparing DPP-4 inhibitors with placebo or active comparator drugs in patients with type 2 diabetes suggested that treatment with DPP-4 inhibitors could be associated with a reduced risk of bone fractures2. Type 2 diabetes is associated with higher bone mineral density and, paradoxically, with increased fracture risk, presumably because of impaired bone quality that causes fragility fractures even when bone mass remains normal3. Duration of more than 10 years, presence of diabetic nephropathy, presence of diabetic neuropathy and high serum levels of pentosidine are shown to be risk factors for bone fractures3. One plausible mechanism of increased risk of bone fractures in patients with type 2 diabetes relates to chronic hyperglycemia, raising concentrations of advanced glycation end-products, such as pentosidine, that increases non-enzymatic collagen cross-linking and impairs bone quality. Furthermore, accumulating evidence shows negative impacts of antidiabetic drugs, thiazolidinediones, on bone turnover and bone fractures in patients with type 2 diabetes. However, so far, no oral antidiabetic drugs have been associated clinically with a reduction of bone fractures. Thus, the current finding on DPP-4 inhibitors by Monami et al. is highly promising despite some limitations, including short duration of the trials included, bone fractures being not principal end-points, and no discrimination between sexes and between pre- and postmenopausal women; and the Monami study provides a premise to initiate randomized, prospective, long-term clinical trials evaluating the effects of DPP-4 inhibitors on bone metabolism and bone fractures in patients with type 2 diabetes. The effects of GIP and GLP-1 on bone metabolism have been well characterized mainly in rodents (Figure 1)1. Investigations on GIP receptor-deficient mice and GIP transgenic mice showed that GIP increases bone mass by acting on osteoblasts to promote bone formation after meal ingestion, and inhibiting parathyroid hormone-induced bone resorption. Furthermore, GIP administration has been shown to attenuate ovariectomy-induced bone loss in rats. In contrast, studies on GLP-1 receptor-deficient mice showed that GLP-1 controls bone resorption, likely through a calcitonin-dependent pathway. Administration of GLP-1 receptor agonist exenatide has been shown to promote bone formation in normal and streptozotocin-induced diabetic rats, suggesting its insulin-independent action. Although these lines of evidence suggest an association of GIP and GLP-1 with bone turnover, the effects of GIP and GLP-1 on human bone turnover are largely unknown. A recent study showed that 44-week exenatide treatment did not affect bone mineral density in patients with type 2 diabetes4. As aforementioned, GLP-1 action on the bone is presumably mediated through calcitonin. A series of clinical trials on liraglutide, another GLP-1 receptor agonist, showed few changes in serum calcitonin levels in patients with type 2 diabetes, suggesting that GLP-1 might not play a role in human bone metabolism. Regarding GIP, Henriksen et al.5 previously reported that postprandial reduction of bone resorption was not mediated by GIP, but GLP-2 – another intestinal hormone cosecreted with GLP-1. However, caution should be taken when interpreting their results, as they investigated the effects of subcutaneous single injections of native GIP that should be rapidly inactivated by DPP-4 before it reaches the bones. Therefore, further investigations are definitely required to understand GIP and GLP-1 actions on bone metabolism in humans. The effects of two incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like polypeptide-1 (GLP-1), on bone metabolism. GIP binds to GIP receptors expressed on osteoblasts, thereby activating new bone formation. GIP also acts on osteocrasts, presumably through osteoblasts, to suppress bone resorption. In contrast, GLP-1 stimulates calcitonin secretion from the thyroid gland, which then suppresses bone resorption by osteocrasts. Prevention of bone fractures could be the tip of the iceberg among potentially beneficial effects of DPP-4 inhibitors in patients with type 2 diabetes. It has been shown that DPP-4 inhibitors target not only two incretin hormones, GIP and GLP-1, but also other DPP-4 substrates, such as pituitary adenylate cyclase-activating peptide and stromal cell-derived factor-1α in patients with type 2 diabetes. Enhancement of these bioactive polypeptides could prevent progression of diabetic micro- and macrovascular complications independently of improvement in glycemic control. In the future, clinical trials with adequately powered, prospective, controlled relevant end-points will clarify the effects of DPP-4 inhibitors beyond glycemic control. The authors have no competing financial interests to disclose.

Involvement of the Cyclic-AMP-dependent Protein Kinase A Pathway in Thyroxine Effects on Calcitonin Secretion from TT Cells

Previous studies have demonstrated that plasma calcitonin is lower in hypothyroid patients and that thyroxine stimulates the human thyroid to release calcitonin. Therefore, thyroid hormones may regulate the secretion of calcitonin, but further work is needed to address this possibility in more detail. TT cells, a model of human thyroid C cells, were incubated in a medium containing vehicle, thyroxine, or thyroxine methyl-hemisuccinate-bovine serum albumin (BSA-L-T(4), thyroxine was immobilized and linked to BSA); then, the levels of secreted calcitonin (hCT), calcitonin mRNA, and cAMP were measured. To study links that connect the cAMP-dependent protein kinase A (PKA) pathway to the observed thyroxine effects, cells were treated with either vehicle or thyroxine plus SQ22536 [an adenylyl cyclase (AC) inhibitor], KT5720 (a PKA inhibitor), or 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor). The activity levels of AC and PKA, and secreted calcitonin were then measured. The results indicate that thyroxine increases calcitonin secretion, cellular cAMP accumulation, and the activities of AC and PKA, but does not increase hCT mRNA levels in TT cells. BSA-L-T(4) also increases calcitonin secretion. These effects are inhibited by SQ22536, and KT5720 and suggest that the nongenomic thyroxine effects that stimulate calcitonin secretion from TT cells involve the cAMP-dependent PKA pathway.

Norepinephrine induced calcitonin secretion in rat medullary thyroid carcinoma 6-23 cells: interaction between intracellular calcium and cAMP.

Catecholamines are known to stimulate calcitonin secretion in C-cells by a receptor mediated pathway, but details regarding the postreceptor events are unknown. Since norepinephrine (NE) influences intracellular calcium concentration [Ca2+]i and cAMP levels in C-cells, we used different adrenergic agonists and antagonists to investigate the effect of NE on [Ca2+]i and cAMP accumulation, and on calcitonin secretion in rat MTC-6-23 cells. NE stimulated intracellular cAMP accumulation and calcitonin secretion dose-dependent, with 10(-7) mol/l causing maximal stimulation. The NE induced increase in cAMP accumulation/calcitonin secretion could be decreased to baseline by equimolar amounts of the beta-adrenergic blocker propanolol. The alpha-blocker phentolamine did not significantly influence NE stimulated calcitonin secretion even at high concentrations. The beta-adrenergic agonist fenoterol proved to be as effective as NE in stimulating cAMP accumulation/calcitonin secretion. Activation of inhibitory G-proteins by the adenosine A1 receptor analogue N6-phenylisopropyladenosine at 10(-6) mol/l completely blocked NE stimulated calcitonin secretion. NE stimulated calcitonin secretion was also completely blocked by the cAMP antagonist RpcAMPs. The calcium channel blocker verapamil significantly inhibited NE stimulated calcitonin secretion, but interestingly increased NE stimulated cAMP accumulation. We conclude that NE induced calcitonin secretion is mediated through beta-receptors coupled to adenylate cyclase via G-proteins. cAMP and changes in [Ca2+]i are necessary for NE induced calcitonin secretion. There seems to be a complex interaction between the two pathways even regarding events occurring distal to cell membrane.

Medullary Thyroid Cancer Responsiveness to Pentagastrin Stimulation: An Early Surrogate Parameter of Tumor Dissemination?

Because of its outstanding sensitivity, stimulation of calcitonin secretion with iv injection of pentagastrin is widely used for biochemical diagnosis of medullary thyroid cancer. The objective of this study was to explore the relationship between the results of the pentagastrin stimulation test and extent of disease in patients with previously untreated medullary thyroid cancer. This was a retrospective study. The investigation took place at a tertiary referral center. Included were 89 patients with increased basal calcitonin levels who had a pentagastrin test at this institution before initial neck surgery for medullary thyroid cancer. Measurements included basal and stimulated calcitonin levels, carcinoembryonic antigen levels, primary tumor diameter, extrathyroidal extension, lymph node metastases, and distant metastases. There was a strong dose-dependent relationship between a less than 10-fold increase in preoperative calcitonin levels after iv stimulation with pentagastrin and both the frequency (41-54 vs. 4-27%; P = 0.001) and number (means of 3.0-10.8 vs. 0-1.1 positive nodes, P < 0.001) of lymph node metastases. Weaker associations were identified with the respective frequency of extrathyroidal extension (14-27 vs. 0-7%; P = 0.027), distant metastasis (9-23 vs. 0%; P = 0.017), and postoperative normalization of calcitonin (40-55 vs. 53-82%; P = 0.029). On multivariate analysis, only lymph node metastases were associated with a less than 10-fold increase in preoperative calcitonin levels. Based on these clinical data and preclinical literature, reduced responsiveness to stimulation with pentagastrin may reflect early dedifferentiation. Evidence of this condition may enable early risk stratification in patients with medullary thyroid cancer.

Nutrition-Related Peptides and Bone Homeostasis

THE SKELETON SERVES two main functions. First, it has a structural role, to support, protect, and facilitate the function of the soft tissues. Second, it is a mineral reservoir, which can be drawn on when dietary intake is inadequate to meet the body’s needs. Optimal function in each of these roles requires regulation of skeletal mass and function in relation to the demands being placed on it. From the structural perspective, the size and strength of the skeleton should be proportional to the mass of soft tissue that it supports, so that the skeleton is structurally adequate without being larger or heavier than necessary. To function as an effective mineral reservoir, there must be a mechanism by which mineral release from the skeleton can occur during times of privation and its replacement be affected when there is dietary abundance. Both the structural and reservoir roles of the skeleton suggest that skeletal homeostasis should be regulated by factors that reflect soft tissue mass on the one hand and nutritional status on the other. Because soft tissue mass and nutrition are themselves linked, there is the possibility of a complex interaction of regulatory factors that act across these two dimensions. Recent research indicates that this is the case, and this Perspective will review our current understanding of this relatively new area of bone science.

CCK receptors in human neuroendocrine tumors: Clinical implications

Receptors for gastrin and cholecystokinin have been shown to be expressed in several types of human cancers. CCK-B receptors for instance have been identified in several types of neuroendocrine tumors. One of the highest incidences of CCK-B receptors has been reported in medullary thyroid cancers at the protein and at the mRNA level using in vitro receptor autoradiography and RT-PCR. It is likely that these receptors mediate the stimulation of calcitonin secretion from neoplastic C-cells by pentagastrin, a well established clinical test to detect occult medullary thyroid cancers. In order to target these tumors in vivo in patients, several peptide radiopharmaceuticals such as DTPA-linked minigastrins or non-sulfated CCK-8 analogs radiolabeled with 111 Indium or 90 Yttrium have recently been developed. As a proof of concept, it could be demonstrated that a majority of medullary thyroid cancer primary tumors and metastases are visualized in vivo with CCK-B receptor scintigraphy using these radioligands. More recently, radiotherapy of CCK-B receptor expressing medullary thyroid cancers with radiolabeled minigastrin has been successfully reported in a small number of patients, giving support to the proposal that CCK-B receptors overexpressed in tumors represent a useful target for clinical application.

Stimulatory effect of insulin on calcitonin secretion.

Infusion of insulin directly into thyroid arterial blood perfusing the surgically isolated in situ pig thyroid gland produced an increase in the secretion rate of calcitonin (CT) measured by immunoassay in thyroid venous effluent blood. Insulin in concentrations ranging from approximately 1 to 400 ng/ml produced a maximal stimulation of 4-5 fold. The stimulatory effect of insulin on CT could not be duplicated by infusion of either IGF-I or amylin. Specific binding of radiolabeled insulin was demonstrated using isolated pig thyroid plasma membranes and both rat (6-23) and human (TT) medullary thyroid carcinoma C-cells. Increased CT release was observed from C-cells exposed to a high concentration of insulin. The administration of glucose iv to pigs in order to stimulate secretion of endogenous insulin produced an increase in circulating insulin, which was accompanied by an increase in the secretion of CT. The results show that insulin, delivered directly to the pig thyroid gland, can stimulate CT release. The in vitro binding and secretion studies indicate that C-cells can bind insulin and respond with an increase in CT secretion, and the iv glucose experiments suggest that endogenous insulin is capable of stimulating CT secretion. The findings imply that insulin is capable of acting as a CT secretagogue and suggest that changes in CT secretion may accompany altered states of insulin production such as diabetes or insulin-secreting tumors.

Effects of glucagon and glucagon-like peptide-1-(7-36) amide on C cells from rat thyroid and medullary thyroid carcinoma CA-77 cell line.

Glucagon is known to stimulate calcitonin secretion by thyroid C cells over a wide range of concentrations, raising the possibility of its interaction with several types of receptors. This study was designed to characterize receptors that mediate the effect of glucagon on a rat C cell line (CA-77). Binding studies, using radiolabeled [125I]glucagon and [125I]glucagon-like peptide-1-(7-36) amide ([125I]tGLP-1), to CA-77 plasma membranes demonstrated the presence of 1) a glucagon receptor with a dissociation constant (Kd) of 2.3 nM and relative potencies for structurally related peptides as follows: glucagon > oxyntomodulin > > tGLP-1; and 2) a tGLP-1 receptor with a Kd of 0.33 nM and relative potencies as follows: tGLP-1 > oxyntomodulin > glucagon. Glucagon stimulated calcitonin secretion from CA-77 cells in a dose-dependent manner over 4 orders of magnitude, with a maximal response of 312% over the basal value and an ED50 close to 50 nM. tGLP-1 induced a calcitonin release over 2 orders of magnitude, with a maximal response of 170% over the basal value and an ED50 close to 0.2 nM. Glucagon and tGLP-1 stimulated cAMP production in CA-77 cells to similar maximal levels over 4 and 2 orders of magnitude, respectively. The stimulation of cAMP production by glucagon at concentrations over 10 nM was suppressed by the tGLP-1 antagonist exendin-(9-39) amide, whereas the stimulation of calcitonin secretion was only partly abolished. Using a perifusion system of rat thyroid, glucagon and tGLP-1 stimulated calcitonin secretion in a calcium-dependent manner. It is concluded that glucagon and tGLP-1 receptors are expressed in the rat C cell line (CA-77) and in the normal rat thyroid. The effects of glucagon on calcitonin secretion observed at high concentrations are mediated in part through interaction with tGLP-1 receptors and via an additional non-cAMP-mediated mechanism.

Estrogen Receptors in the Stingray (Dasyatis akajei) Ultimobranchial Gland

On the basis of our previous observation that estrogen stimulates calcitonin secretion by the ultimobranchial gland in the stingray,Dasyatis akajei,experiments were conducted to examine the presence of estrogen receptor (ER) and its mRNA in the gland, employing exchange assay and Northern blot analysis, respectively. The optimal incubation conditions of the exchange assay for cytosolic ER were found to be 25° for 2 hr. Scatchard analysis of cytosolic ER revealed two components with dissociation constants (Kd) of 0.090 ± 0.015 and 17.780 ± 1.910 nMand with a maximum number of binding sites (Bmax) of 62.2 ± 7.9 and 2290 ± 180 fmol/mg protein, respectively. Furthermore, Sephadex G-100 filtration of the cytosol and immunoblot analysis using antiserum against a recombinant rat ER confirmed the presence of two specific binding components for estrogen (41 and 57 kDa). Total RNA extracted from the ultimobranchial gland was subjected to Northern blot analysis using a rat ER cDNA as a probe. Two positive signals were detected at 2.5 and 2.0 kb. Thus, the presence of ER was confirmed for the first time in nonmammalian calcitonin-secreting cells.

Adenosine A1-Receptors Inhibit cAMP and Ca2+Mediated Calcitonin Secretion in C-Cells

The effect of the adenosine A1 receptor activation on calcitonin secretion was studied in medullary thyroid carcinoma cells of the rat (rMTC 6-23). Calcitonin was determined by radioimmunoassay, intracellular cAMP by protein binding assay, intracellular calcium in fura-2 loaded single cells using microspectrofluorimetry, and calcium channel activity by patch clamp technique. The adenosine A1 receptor analogue N-6 phenylisopropyl-adenosine (PIA) (10(-10)-10(-6) M) inhibits dose-dependently glucagon (10(-7) M) and rGRH (10(-7) M) stimulated cAMP formation and calcitonin secretion. These effects were partly abolished by pretreatment with pertussis toxin (PT) (100 ng/ml). PIA (10(-10)-10(-6) M) also suppressed extracellular calcium-stimulated calcitonin secretion, rises in intracellular calcium, and calcium channel currents. PT (100 ng/ml) pretreatment again partly abolished this inhibitory effect. The addition to the medium of adenosine deaminase (0.4 U/ml) stimulated calcitonin secretion. Our results suggest that in calcitonin-secreting cells A1 receptors couple to adenylate cyclase and calcium channels via PT-sensitive G proteins and thus inhibit calcitonin secretion. Adenosine seems to act as an autocrine/paracrine factor in calcitonin-secreting cells.

Polyamines regulate human medullary thyroid carcinoma TT-cell proliferation and secretion of calcitonin and calcitonin gene-related peptide

The significance of polyamines for the neoplastic proliferation and secretion of calcitonin (CT) and calcitonin-gene-related peptide (CGRP) by the human medullary thyroid carcinoma TT cell line was investigated. TT cells were cultured in vitro for 6 days with or without additions of pathway inhibitors of polyamine biosynthetic enzymes. Treatment of the cells with 1 mM of the specific Lornithine decarboxylase (ODC) inhibitor DL-α-difluoromethylornithine (DFMO) resulted in a 97% decrease in ODC activity, lowered contents of putrescine (96%) and spermidine (85%) and cell proliferation rates (90%) along with a compensatory 15-fold increase in 5-adenosyl-L-methionine decarboxylase (SAMDC) activity. DFMO treatment also led to a decrease in cellular content of CT (33%) and CGRP (26%), while the drug enhanced secretion of CT (31%) but depressed that of CGRP (26%), and elevated the ratio of CT to CGRP secreted into the medium by 74%. Ethylglyoxal bis(guanylhydrazone) (EGBG), a SAMDC inhibitor, at 100 ,μM evoked a similar reduction of cell proliferation and lowered the content of spermine by 81%. Furthermore, EGBG treatment caused a 34-fold increase in ODC activity and a subsequent 35-fold build-up of putrescine, but also seemed to stabilize SAMDC as evidenced by a highly enhanced SAMDC activity (≈200-fold) during enzyme assays in the absence of the inhibitor. EGBG exposure resulted in an increase in cellular CT content (110%) and secretion of the hormone (82%), while not affecting CGRP content or release. EGBG also increased the ratio of cellular content of CT to CGRP (64%) and also that secreted into the medium (44%), which is an indicator of TT cell differentiation. The EGBG effects were partially counteracted by DFMO. It is concluded that putrescine, spermidine and spermine are required for the neoplastic proliferation of cultured human TT cells, and that putrescine and/or spermidine seem to act as permissive or stimulatory factors in the regulation of cellular contents of CT and CGRP while stimulating CT secretion and inhibiting CGRP release. Spermine appears to suppress the cellular content of CT and secretion while not influencing the content or release of CGRP. Hence, polyamine depletion induces differentiation of cultured human TT cells.

The Relationship Between Serum Calcitonin and Calcium in the Hemodialysis Patient

An increase in the serum calcium level is known to stimulate calcitonin secretion, but whether calcitonin contributes to calcium regulation in the dialysis patient is not known. Because we recently observed in the rat that (1) a sigmoidal calcitonin-calcium curve was present, and (2) calcitonin stimulation decreased the calcemic response to parathyroid hormone (PTH), the relationship between serum calcitonin and calcium was studied in six hemodialysis patients. To evaluate both the calcitonin-calcium and PTH-calcium relationships, hemodialysis with a low-calcium dialysate (2 mg/dL) was performed and followed 1 week later by hemodialysis with a high-calcium dialysate (8 mg/dL). The calcitonin-calcium relationship was best represented as a sigmoidal curve that was opposite in direction to the PTH-calcium curve. The range of the calcitonin-calcium curve was less than the PTH-calcium curve; as a percent of maximal, the minimal calcitonin was higher than the minimal PTH, 61% +/- 5% versus 26% +/- 4% (P < 0.001). As opposed to the PTH-calcium curve in which the basal PTH value was intermediate between the minimal and maximal PTH, the basal calcitonin value was close to the minimal calcitonin. Despite a wide range of ionized serum calcium concentrations (3.5 to 5.3 mg/dL), maximal stimulation was greater than maximal inhibition of calcitonin, 60% +/- 13% versus 3% +/- 2% (P = 0.01). Finally, a significant correlation was observed between basal and maximal calcitonin (r = 0.99, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in cytoplasmic Ca 2+ and stimulation of calcitonin secretion from human medullary thyroid carcinoma cells by the gastrin-releasing peptide

Examination was made of the effects of gastrin-releasing peptide (GRP) on human medullary thyroid carcinoma cells (TT cells). GRP stimulated calcitonin(CT) release in a concentration-dependent manner at 0.1–1000nmol/l. On adding forskolin along with GRP, CT release was greater than by GRP alone. The stimulatory effect of A23187 was not additive. Intracellular free calcium concentration ([Ca 2+]i) was measured for individual TT cells loaded with fura-2. The addition of GRP caused a rapid and transient rise in [Ca 2+]i in a concentration-dependent manner followed by a sustained increase in [Ca 2+]i. In the medium without Ca 2+, this sustained increase did not occur and the concentration of CT release from TT cells by GRP was reduced by approximately a half. GRP would thus appear to be importantly involved in the regulation of thyroid C cell function through modulation of [Ca 2+]i.

Inhibition of Ca 2+-induced calcitonin secretion by somatostatin: Roles of voltage dependent Ca 2+ channels and G-proteins

Somatostatin has recently been applied therapeutically for hypercalcitonemia in patients with calcitonin-producting tumours. Using calcitonin-secreting cells (C-cells) of the medullary thyroid carcinoma cell line rMTC 44-2, we investigated the inhibitory action of somatostatin on calcitonin release, cytosolic Ca 2+ and Ca 2+ channel currents. The Ca 2+-induced rises of the cytosolic Ca 2+ and calcitonin secretion were greatly inhibited by somatostatin or its stable analogue octreotide. The effects of somatostatin were pertussis toxin-sensitive. Under voltage clamp conditions, C-cells exhibited slowly inactivating Ca 2+ channel currents. Bath application of 100 nM somatostatin reversibly reduced the Ca 2+ channel current by about 30%. The Ca 2+ channel current and its inhibition by somatostatin were not affected by intracellularly applied cyclic AMP. Moreover, pretreating the cells with pertussis toxin had no effect on the control Ca 2+ channel currents but greatly abolished its inhibition by somatostatin. The data show that somatostatin suppresses the Ca 2+-stimulated calcitonin secretion by inhibiting voltage-dependent Ca 2+ channel currents and by lowering cytosolic Ca 2+. These actions of somatostatin involved pertussis toxin-sensitive G-proteins and occur independently of changes in the cyclic AMP concentration.

Regulation of calcitonin secretion and calcitonin gene expression.

Medullary thyroid carcinoma (MTC) is a tumor of the calcitonin (CT)-secreting cells (C-cells or parafollicular cells). During embryogenesis C-cells migrate from the neural crest to the last branchial pouch and ultimately into the thyroid. In contrast to the follicular cells of the thyroid gland that produce the thyroid hormones under the control of thyroid-stimulating hormone (TSH), the C-cells secrete CT and related peptides under different control systems (Deftos and Roos 1989). CT helps to maintain the calcium homeostasis and as a feedback, physiological changes in blood-calcium concentration directly regulate CT secretion. In addition, 1,25 (OH)2D3 is involved in CT secretion and synthesis. CT release is also closely tied to feeding and gastrointestinal activity; gastrointestinal hormonal factors including gastrin, cholecystokinin (CCK), and glucagon participate in promoting CT release. Moreover neuroendocrine factors may be involved in the control of CT secretion. β-Adrenergic agonists stimulate CT secretion, while α-adrenergic agonists and dopamine inhibit it, which suggests a participation of the autonomic nervous system in the regulation of CT secretion. Somatostatin appears to have a paracrine or autocrine regulatory function as it is secreted by C-cells and it is able to inhibit CT secretion. While these phenomena can be studied in vivo, emphasizing their physiological relevance, the cellular and molecular mechanisms underlying regulation of CT secretion, synthesis, and transcription can only be studied in in vitro systems. The available in vitro systems include rat thyroid explants (Cooper et al. 1977), primary cell cultures of MTC (Raue et al. 1989), and permanent C-cell carcinoma cell lines (human TT: Leong et al. 1981; rMTC 6–23: Gagel et al. 1980). Cellular signaling processes thought to be involved in initiation of C-cell neoplasia and subsequent cellular changes during tumor progression are activation of adenylate cyclase, protein kinase C, and oncogenes.KeywordsMedullary Thyroid CarcinomaCalcitonin SecretionHuman Medullary Thyroid CarcinomaMedullary Thyroid Carcinoma Cell LineHuman Medullary Thyroid Carcinoma CellThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

91174773 Prevention of bone loss by hormone replacement therapy is probably not due to stimulation of calcitonin secretion

91174773 Prevention of bone loss by hormone replacement therapy is probably not due to stimulation of calcitonin secretion