Expression of the calcium-sensing receptor in gastrinomas

A Case of Pancreatic Neuroendocrine Tumor with Liver Metastases Demonstrating the Possibility of Enhanced ACTH Production by the SACI Test

Objective

ACTH-producing pancreatic NETs have a propensity to metastasize, and in patients with metastases, there is no established method yet to precisely determine if the excess ACTH is produced by the primary or the metastatic tumors. Localizing the source of production of ACTH in such cases is important for devising suitable treatment strategies and evaluating the benefit of local therapies from the viewpoint of control of Cushing's syndrome.

Methods

We performed the selective arterial calcium injection (SACI) test combined with selective portal and hepatic venous sampling in a 32-year-old female patient with ectopic ACTH-producing pancreatic NET and liver metastases.

Results

The blood level of ACTH after Ca loading was significantly elevated only in the vessels thought to be directly feeding the pancreatic tumor, and Ca loading from any artery did not significantly increase ACTH concentrations in the hepatic veins compared to the main trunk of the portal vein.

Conclusions

The present case demonstrates that there might be an ACTH-producing p-NET that responds to Ca loading. Further in vitro studies are required to validate this possibility.

A Comprehensive Review on Neuroendocrine Neoplasms: Presentation, Pathophysiology and Management

Neuroendocrine neoplasms (NENs) are a group of heterogeneous tumors with neuroendocrine differentiation that can arise from any organ. They account for 2% of all malignancies in the United States. A significant proportion of NEN patients experience endocrine imbalances consequent to increased amine or peptide hormone secretion, impacting their quality of life and prognosis. Over the last decade, pathologic categorization, diagnostic techniques and therapeutic choices for NENs-both well-differentiated neuroendocrine tumors (NETs) and poorly differentiated neuroendocrine carcinomas (NECs)-have appreciably evolved. Diagnosis of NEN mostly follows a suspicion from clinical features or incidental imaging findings. Hormonal or non-hormonal biomarkers (like serum serotonin, urine 5-HIAA, gastrin and VIP) and histology of a suspected NEN is, therefore, critical for both confirmation of the diagnosis and classification as an NET or NEC. Therapy for NENs has progressed recently based on a better molecular understanding, including the involvement of mTOR, VEGF and peptide receptor radionuclide therapy (PRRT), which add to the growing evidence supporting the possibility of treatment beyond complete resection. As the incidence of NENs is on the rise in the United States and several other countries, physicians are more likely to see these cases, and their better understanding may support earlier diagnosis and tailoring treatment to the patient. We have compiled clinically significant evidence for NENs, including relevant changes to clinical practice that have greatly updated our diagnostic and therapeutic approach for NEN patients.

A Parathyroid-Gut Axis: Hypercalcemia and the Pathogenesis of Gastrinoma in Multiple Endocrine Neoplasia 1

Patients with multiple endocrine neoplasia 1 (MEN1) syndrome have a germline mutation in the MEN1 gene. Loss of the wild-type allele can initiate endocrine tumorigenesis. Microscopic and macroscopic pituitary, parathyroid, and pancreatic tumors (referred to as the 3 P's) show loss of the wild-type MEN1 allele up to 100%. In contrast, the duodenal gastrinoma pathogenesis in MEN1 syndrome follows a hyperplasia-to-neoplasia sequence. Gastrinomas have loss of heterozygosity of the MEN1 locus in <50%, and invariably coincide with linear, diffuse, or micronodular gastrin-cell hyperplasia. The factor initiating the gastrin-cell hyperplasia-to-neoplasia sequence is unknown. In this perspective, we argue that hypercalcemia may promote the gastrin-cell hyperplasia-to-neoplasia sequence through the calcium sensing receptor. Hypercalcemia is present in almost all patients with MEN1 syndrome due to parathyroid adenomas. We propose a parathyroid-gut axis, which could well explain why patients with MEN1 syndrome are regularly cured of duodenal gastrinoma after parathyroid surgery, and might cause MEN1 syndrome phenocopies in MEN1-mutation negative individuals with parathyroid adenomas. This perspective on the pathogenesis of the gastrin-cell hyperplasia and neoplasia sequence sheds new light on tumorigenic mechanisms in neuroendocrine tumors and might open up novel areas of gastrinoma research. It may also shift focus in the treatment of MEN1 syndrome-related gastrinoma to biochemical prevention.

Catching the Zebra: Clinical Pearls and Pitfalls for the Successful Diagnosis of Zollinger-Ellison Syndrome

Zollinger-Ellison syndrome (ZES) results from an ectopic gastrin-secreting tumor leading to peptic ulcer disease, reflux, and chronic diarrhea. While early recognition portends an excellent prognosis with >80% survival at 15 years, symptoms are often nonspecific making the diagnosis difficult to establish. Diagnosis involves a series of tests, including fasting gastrin, gastric pH, chromogranin A, and secretin stimulation. Performing these tests in the correct sequence and at the proper time is essential to avoid inaccurate results. Tumor localization is equally nuanced. Although providers have classically used ¹¹¹indium-radiolabeled octreotide with somatostatin receptor scintigraphy to evaluate tumor size and metastases, recent studies have shown superior results with newer imaging modalities. In particular, ⁶⁸gallium (⁶⁸Ga)-labeled somatostatin radiotracers (i.e., ⁶⁸Ga-DOTATOC, ⁶⁸Ga-DOTANOC and ⁶⁸Ga-DOTATATE) used with positron emission tomography/computed tomography can provide excellent results. Endoscopic ultrasound is another useful modality, particularly in patients with ZES in the setting of multiple endocrine neoplasia type 1. This review aims to provide clinicians with an overview of ZES with a focus on both clinical presentation and the proper utilization of the various biochemical and imaging tests available.

Molecular mechanisms of calcium-sensing receptor-mediated calcium signaling in the modulation of epithelial ion transport and bicarbonate secretion

Epithelial ion transport is mainly under the control of intracellular cAMP and Ca(2+) signaling. Although the molecular mechanisms of cAMP-induced epithelial ion secretion are well defined, those induced by Ca(2+) signaling remain poorly understood. Because calcium-sensing receptor (CaSR) activation results in an increase in cytosolic Ca(2+) ([Ca(2+)]cyt) but a decrease in cAMP levels, it is a suitable receptor for elucidating the mechanisms of [Ca(2+)]cyt-mediated epithelial ion transport and duodenal bicarbonate secretion (DBS). CaSR proteins have been detected in mouse duodenal mucosae and human intestinal epithelial cells. Spermine and Gd(3+), two CaSR activators, markedly stimulated DBS without altering duodenal short circuit currents in wild-type mice but did not affect DBS and duodenal short circuit currents in cystic fibrosis transmembrane conductance regulator (CFTR) knockout mice. Clotrimazole, a selective blocker of intermediate conductance Ca(2+)-activated K(+) channels but not chromanol 293B, a selective blocker of cAMP-activated K(+) channels (KCNQ1), significantly inhibited CaSR activator-induced DBS, which was similar in wild-type and KCNQ1 knockout mice. HCO3 (-) fluxes across epithelial cells were activated by a CFTR activator, but blocked by a CFTR inhibitor. CaSR activators induced HCO3 (-) fluxes, which were inhibited by a receptor-operated channel (ROC) blocker. Moreover, CaSR activators dose-dependently raised cellular [Ca(2+)]cyt, which was abolished in Ca(2+)-free solutions and inhibited markedly by selective CaSR antagonist calhex 231, and ROC blocker in both animal and human intestinal epithelial cells. Taken together, CaSR activation triggers Ca(2+)-dependent DBS, likely through the ROC, intermediate conductance Ca(2+)-activated K(+) channels, and CFTR channels. This study not only reveals that [Ca(2+)]cyt signaling is critical to modulate DBS but also provides novel insights into the molecular mechanisms of CaSR-mediated Ca(2+)-induced DBS.

The role of the calcium-sensing receptor in human disease

Following the discovery of the calcium-sensing receptor (CaSR) in 1993, its pivotal role in disorders of calcium homeostasis such as Familial Hypocalciuric Hypercalcemia (FHH) was quickly demonstrated. Since then, it has become clear that the CaSR has immense functional versatility largely through its ability to activate many different signaling pathways in a ligand- and tissue-specific manner. This allows the receptor to play diverse and crucial roles in human physiology and pathophysiology, both in calcium homeostasis and in tissues and biological processes unrelated to calcium balance. This review covers current knowledge of the role of the CaSR in disorders of calcium homeostasis (FHH, neonatal severe hyperparathyroidism, autosomal dominant hypocalcemia, primary and secondary hyperparathyroidism, hypercalcemia of malignancy) as well as unrelated diseases such as breast and colorectal cancer (where the receptor appears to play a tumor suppressor role), Alzheimer's disease, pancreatitis, diabetes mellitus, hypertension and bone and gastrointestinal disorders. In addition, it examines the use or potential use of CaSR agonists or antagonists (calcimimetics and calcilytics) and other drugs mediated through the CaSR, in the management of disorders as diverse as hyperparathyroidism, osteoporosis and gastrointestinal disease.

The extracellular calcium-sensing receptor is required for cholecystokinin secretion in response to L-phenylalanine in acutely isolated intestinal I cells

The extracellular calcium-sensing receptor (CaSR) has recently been recognized as an L-amino acid sensor and has been implicated in mediating cholecystokinin (CCK) secretion in response to aromatic amino acids. We investigated whether direct detection of L-phenylalanine (L-Phe) by CaSR results in CCK secretion in the native I cell. Fluorescence-activated cell sorting of duodenal I cells from CCK-enhanced green fluorescent protein (eGFP) transgenic mice demonstrated CaSR gene expression. Immunostaining of fixed and fresh duodenal tissue sections confirmed CaSR protein expression. Intracellular calcium fluxes were CaSR dependent, stereoselective for L-Phe over D-Phe, and responsive to type II calcimimetic cinacalcet in CCK-eGFP cells. Additionally, CCK secretion by an isolated I cell population was increased by 30 and 62% in response to L-Phe in the presence of physiological (1.26 mM) and superphysiological (2.5 mM) extracellular calcium concentrations, respectively. While the deletion of CaSR from CCK-eGFP cells did not affect basal CCK secretion, the effect of L-Phe or cinacalcet on intracellular calcium flux was lost. In fact, both secretagogues, as well as superphysiological Ca(2+), evoked an unexpected 20-30% decrease in CCK secretion compared with basal secretion in CaSR(-/-) CCK-eGFP cells. CCK secretion in response to KCl or tryptone was unaffected by the absence of CaSR. The present data suggest that CaSR is required for hormone secretion in the specific response to L-Phe by the native I cell, and that a receptor-mediated mechanism may inhibit hormone secretion in the absence of a fully functional CaSR.

Amino acids stimulate cholecystokinin release through the Ca2+-sensing receptor

Cholecystokinin (CCK) is produced by discrete endocrine cells in the proximal small intestine and is released following the ingestion of food. CCK is the primary hormone responsible for gallbladder contraction and has potent effects on pancreatic secretion, gastric emptying, and satiety. In addition to fats, digested proteins and aromatic amino acids are major stimulants of CCK release. However, the cellular mechanism by which amino acids affect CCK secretion is unknown. The Ca(2+)-sensing receptor (CaSR) that was originally identified on parathyroid cells is not only sensitive to extracellular Ca(2+) but is activated by extracellular aromatic amino acids. It has been postulated that this receptor may be involved in gastrointestinal hormone secretion. Using transgenic mice expressing a CCK promoter driven/enhanced green fluorescent protein (GFP) transgene, we have been able to identify and purify viable intestinal CCK cells. Intestinal mucosal CCK cells were enriched >200-fold by fluorescence-activated cell sorting. These cells were then used for real-time PCR identification of CaSR. Immunohistochemical staining with an antibody specific for CaSR confirmed colocalization of CaSR to CCK cells. In isolated CCK cells loaded with a Ca(2+)-sensitive dye, the amino acids phenylalanine and tryptophan, but not nonaromatic amino acids, caused an increase in intracellular Ca(2+) ([Ca(2+)](i)). The increase in [Ca(2+)](i) was blocked by the CaSR inhibitor Calhex 231. Phenylalanine and tryptophan stimulated CCK release from intestinal CCK cells, and this stimulation was also blocked by CaSR inhibition. Electrophysiological recordings from isolated CCK-GFP cells revealed these cells to possess a predominant outwardly rectifying potassium current. Administration of phenylalanine inhibited basal K(+) channel activity and caused CCK cell depolarization, consistent with changes necessary for hormone secretion. These findings indicate that amino acids have a direct effect on CCK cells to stimulate CCK release by activating CaSR and suggest that CaSR is the physiological mechanism through which amino acids regulate CCK secretion.

Calcium-sensing receptor is a physiologic multimodal chemosensor regulating gastric G-cell growth and gastrin secretion

The calcium-sensing receptor (CaR) is the major sensor and regulator of extracellular Ca(2+), whose activity is allosterically regulated by amino acids and pH. Recently, CaR has been identified in the stomach and intestinal tract, where it has been proposed to function in a non-Ca(2+) homeostatic capacity. Luminal nutrients, such as Ca(2+) and amino acids, have been recognized for decades as potent stimulants for gastrin and acid secretion, although the molecular basis for their recognition remains unknown. The expression of CaR on gastrin-secreting G cells in the stomach and their shared activation by Ca(2+), amino acids, and elevated pH suggest that CaR may function as the elusive physiologic sensor regulating gastrin and acid secretion. The genetic and pharmacologic studies presented here comparing CaR-null mice and wild-type littermates support this hypothesis. Gavage of Ca(2+), peptone, phenylalanine, Hepes buffer (pH 7.4), and CaR-specific calcimimetic, cinacalcet, stimulated gastrin and acid secretion, whereas the calcilytic, NPS 2143, inhibited secretion only in the wild-type mouse. Consistent with known growth and developmental functions of CaR, G-cell number was progressively reduced between 30 and 90 d of age by more than 65% in CaR-null mice. These studies of nutrient-regulated G-cell gastrin secretion and growth provide definitive evidence that CaR functions as a physiologically relevant multimodal sensor. Medicinals targeting diseases of Ca(2+) homeostasis should be reviewed for effects outside traditional Ca(2+)-regulating tissues in view of the broader distribution and function of CaR.

The role of the calcium-sensing receptor in the development and progression of cancer

The calcium-sensing receptor (CaR) is responsive to changes in the extracellular Ca(2+) (Ca(2+)(o)) concentration. It is a member of the largest family of cell surface receptors, the G protein-coupled receptors, and it has been shown to be involved in Ca(2+)(o) homeostasis. Apart from its primary role in Ca(2+)(o) homeostasis, the CaR may be involved in phenomena that allow for the development of many types of benign or malignant tumors, from parathyroid adenomas to breast, prostate, and colon cancers. For example, whereas the CaR is expressed in both normal and malignant breast tissue, increased CaR levels have been reported in highly metastatic primary breast cancer cells and breast cancer cell lines, possibly contributing to their malignancy and associated alterations in their biological properties. In these settings the CaR exhibits oncogenic properties. Enhanced CaR expression and altered proliferation of prostate cancer cells in response to increased Ca(2+)(o) have also been described. In contrast, colon and parathyroid cancers often present with reduced or absent CaR expression, and activation of this receptor decreases cell proliferation, suggesting a role for the CaR as a tumor suppressor gene. Thus, the CaR may play an important role in the development of many types of neoplasia. Herein, we review the role of the CaR in various benign and malignant tumors in further detail, describing its contribution to parathyroid tumors, breast, prostate, and colon cancers, and we evaluate how pharmacological manipulations of this receptor may be of interest for the treatment of certain cancers in the future.

THE CALCIUM-SENSING RECEPTOR IN NORMAL PHYSIOLOGY AND PATHOPHYSIOLOGY: A Review

The discovery of a G protein-coupled, calcium-sensing receptor (CaR) a decade ago and of diseases caused by CaR mutations provided unquestionable evidence of the CaR's critical role in the maintenance of systemic calcium homeostasis. On the cell membrane of the chief cells of the parathyroid glands, the CaR "senses" the extracellular calcium concentration and, subsequently, alters the release of parathyroid hormone (PTH). The CaR is likewise functionally expressed in bone, kidney, and gut--the three major calcium-translocating organs involved in calcium homeostasis. Intracellular signal pathways to which the CaR couples via its associated G proteins include phospholipase C (PLC), protein kinase B (AKT); and mitogen-activated protein kinases (MAPKs). The receptor is widely expressed in various tissues and regulates important cellular functions in addition to its role in maintaining systemic calcium homeostasis, i.e., protection against apoptosis, cellular proliferation, and membrane voltage. Functionally significant mutations in the receptor have been shown to induce diseases of calcium homeostasis owing to changes in the set point for calcium-regulated PTH release as well as alterations in the renal handling of calcium. Gain-of-function mutations cause hypocalcemia, whereas loss-of-function mutations produce hypercalcemia. Recent studies have shown that the latter clinical presentation can also be caused by inactivating autoantibodies directed against the CaR Newly discovered type II allosteric activators of the CaR have been found to be effective as a medical treatment for renal secondary hyperparathyroidism.

Ca2+mobilization through dorsal root ganglion Ca2+-sensing receptor stably expressed in HEK293 cells

The rat dorsal root ganglion (DRG) Ca2+-sensing receptor (CaR) was stably expressed in-frame as an enhanced green fluorescent protein (EGFP) fusion protein in human embryonic kidney (HEK)293 cells, and is functionally linked to changes in intracellular Ca2+concentration ([Ca2+]i). RT-PCR analysis indicated the presence of the message for the DRG CaR cDNA. Western blot analysis of membrane proteins showed a doublet of 168–175 and 185 kDa, consistent with immature and mature forms of the CaR.EGFP fusion protein, respectively. Increasing extracellular [Ca2+] ([Ca2+]e) from 0.5 to 1 mM resulted in increases in [Ca2+]ilevels, which were blocked by 30 μM 2-aminoethyldiphenyl borate. [Ca2+]e-response studies indicate a Ca2+sensitivity with an EC50of 1.75 ± 0.10 mM. NPS R-467 and Gd3+activated the CaR. When [Ca2+]ewas successively raised from 0.25 to 4 mM, peak [Ca2+]i, attained with 0.5 mM, was reduced by ∼50%. Similar reductions were observed with repeated applications of 10 mM Ca2+, 1 and 10 μM NPS R-467, or 50 and 100 μM Gd3+, indicating desensitization of the response. Furthermore, Ca2+mobilization increased phosphorylated protein kinase C (PKC)α levels in the cells. However, the PKC activator, phorbol myristate acetate did not inhibit CaR-mediated Ca2+signaling. Rather, a spectrum of PKC inhibitors partially reduced peak responses to Cae2+. Treatment of cells with 100 nM PMA for 24 h, to downregulate PKC, reduced [Ca2+]itransients by 49.9 ± 5.2% (at 1 mM Ca2+) and 40.5 ± 6.5% (at 2 mM Ca2+), compared with controls. The findings suggest involvement of PKC in the pathway for Ca2+mobilization following CaR activation.

Serum gastrin in Zollinger-Ellison syndrome: II. Prospective study of gastrin provocative testing in 293 patients from the National Institutes of Health and comparison with 537 cases from the literature. evaluation of diagnostic criteria, proposal of new criteria, and correlations with clinical and tumoral features

In two-thirds of patients with Zollinger-Ellison syndrome (ZES), fasting serum gastrin (FSG) levels overlap with values seen in other conditions. In these patients, gastrin provocative tests are needed to establish the diagnosis of ZES. Whereas numerous gastrin provocative tests have been proposed, only the secretin, calcium, and meal tests are widely used today. Many studies have analyzed gastrin provocative test results in ZES, but they are limited by small patient numbers and methodologic differences. To address this issue, we report the results of a prospective National Institutes of Health (NIH) study of gastrin provocative tests in 293 patients with ZES and compare these data with those from 537 ZES and 462 non-ZES patients from the literature. In 97%-99% of gastrinoma patients, an increase in serum gastrin post secretin (Delta secretin) or post calcium (Delta calcium) occurred. In NIH ZES patients with <10-fold increase in FSG, the sensitivity/specificity of the widely used criteria were as follows: Delta secretin > or =200 pg/mL (83%/100%), Delta secretin >50% (86%/93%), Delta calcium > or =395 pg/mL (54%/100%), and Delta calcium >50% (78%/83%). A systematic analysis of the sensitivity and specificity of other possible criteria for a positive secretin or calcium test allowed us to identify a new criterion for secretin testing (Delta > or =120 pg/mL) with the highest sensitivity/specificity (94%/100%) and to confirm the commonly used criterion for calcium tests (Delta > or =395 pg/mL) (62%/100%). This analysis further showed that the secretin test was more sensitive than the calcium test (94% vs. 62%). Our results suggest that secretin stimulation should be used as the first-line provocative test because of its greater sensitivity and simplicity and lack of side effects. In ZES patients with a negative secretin test, 38%-50% have a positive calcium test. Therefore the calcium test should be considered in patients with a strong clinical suspicion of ZES but a negative secretin test. Furthermore, we found that some clinical (diarrhea, duration of medical treatment), laboratory (basal acid output), and tumoral (size, extent) characteristics correlate with the serum gastrin increase post secretin and post calcium. However, using the proposed criteria, the result of these provocative tests (that is, positive or negative) is minimally influenced by these factors, so secretin and calcium provocative tests are reliable in patients with different clinical, laboratory, and tumor characteristics. A systematic analysis of meal testing showed that 54%-77% of ZES patients have a <50% postprandial serum gastrin increase. However, 9%-20% of ZES patients had a >100% increase post meal, causing significant overlap with antral syndromes. Furthermore, we could not confirm the usefulness of meal tests for localization of duodenal gastrinomas. We conclude that the secretin test is a crucial element in the diagnosis of most ZES patients, the calcium test may be useful in selected patients, but the meal test is not helpful in the management of ZES. For secretin testing, the criterion with the highest sensitivity and specificity is an increase of > or =120 pg/mL, which should replace other criteria commonly used today.

Role of calcium and other trace elements in the gastrointestinal physiology

Calcium is an essential ion in both marine and terrestrial organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the regulation of neuronal function. The Ca²⁺ balance is maintained by three organ systems, including the gastrointestinal tract, bone and kidney.

Since first being cloned in 1993 the Ca²⁺-sensing receptor has been expressed along the entire gastrointestinal tract, until now the exact function is only partly elucidated. As of this date it still remains to be determined if the Ca²⁺-sensing receptor is involved in calcium handling by the gastrointestinal tract. However, there are few studies showing physiological effects of the Ca²⁺-sensing receptor on gastric acid secretion and fluid transport in the colon. In addition, polyamines and amino acids have been shown to activate the Ca²⁺-sensing receptor and also act as allosteric modifiers to signal nutrient availability to intestinal epithelial cells. Activation of the colonic Ca²⁺-sensing receptor can abrogate cyclic nucleotide-mediated fluid secretion suggesting a role of the receptor in modifying secretory diarrheas like cholera. For many cell types changes in extracellular Ca²⁺ concentration can switch the cellular behavior from proliferation to terminal differentiation or quiescence. As cancer remains predominantly a disease of disordered balance between proliferation, termination and apoptosis, disruption in the function of the Ca²⁺-sensing receptor may contribute to the progression of neoplastic disease. Loss of the growth suppressing effects of elevated extracellular Ca²⁺ have been demonstrated in colon carcinoma, and have been correlated with changes in the level of CaSR expression.

Dissimilar PTH, gastrin, and calcitonin responses to oral calcium and peptones in hypocalciuric hypercalcemia, primary hyperparathyroidism, and normal subjects: a useful tool for differential diagnosis

We analyzed gastrin, PTH, and calcitonin responses to oral calcium and peptones in hypocalciuric hypercalcemia, mild primary hyperparathyroidism, and normal controls. We observed diverse hormonal responses that may help in the differential diagnosis of these conditions.

INTRODUCTION

Hypocalciuric hypercalcemia (HH) is consequent to calcium-sensing receptor (CaSR) genetic mutations or anti-CaSR antibodies. CaSR is expressed in parathyroid tissue, thyroid C cells, and gastrin-secreting cells, where it has been suggested that on calcium and/or amino acid allosteric activation, promotes gastrin secretion.

MATERIALS AND METHODS

We evaluated gastrin, PTH, and calcitonin responses to oral calcium (1 g) and peptones (10 g) in 10 patients with HH (mean age, 58.5 +/- 10.3 years; F/M = 9/1), 15 patients with primary hyperparathyroidism (PH; mean age, 60.4 +/- 8.3 years; F/M = 11/4), and 30 healthy controls (mean age, 60.3 +/- 8.1 years). Statistical analyses for differences during oral loading tests were calculated with ANOVA for repeated measurements and comparisons between two groups were performed with Student's t-test.

RESULTS

PTH response to peptones was markedly increased in patients with PH compared with flat responses in controls and HH patients (p < 0.05). Gastrin increase after oral calcium was absent in HH and PH subjects (p < 0.05 versus controls), and gastrin responses to peptones were blunted in HH and PH subjects compared with controls (p < 0.05). PTH drop and calcitonin increase after calcium load observed in controls were absent in HH and PH subjects (p < 0.05).

CONCLUSIONS

The marked difference in PTH response elicited by peptones observed in PH compared with subjects with HH may help in the differential diagnosis of these conditions without genetic studies. Peptones may stimulate CaSR-controlled hormones as an allosteric regulatory pathway. CaSR abnormalities may help to explain the different calcium- and peptones-induced hormonal responses observed in PH and HH compared with normal subjects.

Increased expression of insulin-like growth factor I and/or its receptor in gastrinomas is associated with low curability, increased growth, and development of metastases

PURPOSE

Growth factors, particularly insulin-like growth factor I (IGF-I) and IGF-I receptor (IGF-IR) in some nonendocrine and a few endocrine tumors, are thought important in recurrence, growth, and aggressiveness. Whether this is true of neuroendocrine tumors such as gastrinomas is unclear. The aim of this study was to address this question in gastrinomas.

EXPERIMENTAL DESIGN

IGF-I and IGF-IR expression in gastrinomas from 54 patients with Zollinger-Ellison syndrome were analyzed and correlated with clinical/tumor characteristics. IGF-I and IGF-IR mRNA levels were determined by competitive reverse transcription-PCR. IGF-IR expression, assessed by immunohistochemistry, was done on a subset.

RESULTS

IGF-IR mRNA was found in 100% and IGF-I in 89%. IGF-I mRNA expression varied by >254-fold, IGF-IR by 2,670-fold, and the levels correlated in a given tumor. The IGF-IR level was lower in gastrinomas of patients who were rendered disease free and increased levels correlated with tumor growth, aggressiveness, extent, and with liver metastases. Increased IGF-I levels correlated with increased growth, tumor extent, and aggressiveness. Neither IGF-IR nor IGF-I levels correlated with tumor location, size, or its clinical/functional features. The IGF-IR correlated with disease-free survival. IGF-IRbeta was found in 31 of 32 tumors (97%) by immunohistochemistry.

CONCLUSIONS

These results indicate that IGF-I and IGF-IR are expressed in almost all gastrinomas. Furthermore, assessment of IGF-I/IGF-IR expression in gastrinomas may be clinically useful in identifying those patients with more aggressive tumors who might benefit from more aggressive treatment.

Calcium sensing by endocrine cells

The elucidation of the structure and function of the Ca2+(o)-sensing receptor (CaR) has provided important insights into the normal control of Ca2+(o) homeostasis, particularly the key role of the receptor in kidney and parathyroid. Further studies are needed to define more clearly the homeostatic role of the CaR in additional tissues, both those that are involved and those that are uninvolved in systemic Ca2+(o) homeostasis. The availability of the cloned CaR has also permitted documentation of the molecular basis of inherited disorders of Ca2+(o) sensing, including those in which the receptor is less and or more sensitive than normal to Ca2+(o). Antibodies to the CaR that either activate it or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the receptor is abnormally low in 1 degree and 2 degrees hyperparathyroidism, which could contribute to the defective Ca2+(o) sensing in these conditions. The recent discovery of calcimimetics, which sensitize the CaR to Ca2+(o), has provided what will likely be an effective medical therapy for the secondary/tertiary hyperparathyroidism of end stage renal failure as well as for 1 degree hyperparathyroidism.

Molecular cloning and characterization of a rat sensory nerve Ca2+-sensing receptor

A full-length cDNA encoding a Ca2+-sensing receptor (CaSR) expressed in rat dorsal root ganglia (DRG) was identified using rapid amplification of 5'-cDNA ends and primer extension and then cloned into the plasmid vector pCR3.1. The DNA sequence of the DRG CaSR was 99.9% homologous with published rat kidney CaSR in the coding region and 247 bp upstream of the start site but showed little homology 5' to this site, which maps to exonic junction I/II, supporting the hypothesis that CaSR message arises as a splice variant and showing tissue-to-tissue heterogeneity. Western blot revealed a doublet of 140 and 160 kDa in a thyroparathyroid preparation and a single 140-kDa band in DRG. Deglycosylation using N-glycanase increased the mobility of CaSR protein from both DRG and thyroparathyroid, whereas endo-H was without effect, indicating that the DGR CaSR is a mature form of the receptor. A DRG CaSR-pEGFP fusion product was constructed, and when transfected into HEK-293 cells, it was distributed at the cell membrane and resulted in extracellular Ca2+ (0.5-3 mM)-evoked increases in intracellular Ca2+, which in some instances exhibited oscillatory behavior. We conclude that DRG CaSR cDNA arises from tissue-specific alternative splicing of a single gene, that the amino acid sequence of DRG CaSR is homologous to other known CaSRs, and that the DRG CaSR undergoes differential posttranslational processing relative to the thyroparathyroid CaSR and is functionally active when transfected into a human-derived cell line.

The extracellular calcium Ca2+o-sensing receptor is expressed in myeloma cells and modulates cell proliferation

The calcium-sensing receptor (CaR) is a G protein-coupled receptor that plays key roles in extracellular calcium ion (Ca(2+)(o)) homeostasis by enabling parathyroid, kidney, and other cells to directly "sense" changes in Ca(2+)(o). In multiple myeloma-associated bone disease, myeloma cells could raise the level of Ca(2+)(o) within their immediate vicinity in the bone marrow microenvironment, through their known capacity to cause bone destruction by stimulating osteoclastic bone resorption. Thus if myeloma cells expressed the CaR, they might sense these locally elevated levels of Ca(2+)(o), which could, in turn, potentially modify their function(s) in ways that could contribute to myeloma bone disease or other aspects of the pathophysiology of this disabling hematological malignancy. In this study, we examined the expression of the CaR in three myeloma cell lines, human U266, IM-9, and RPMI8226 cells. CaR protein was present in all three cell lines as assessed by immunocytochemistry and Western blot analysis using a monoclonal antibody specific for the CaR. Moreover, the use of reverse transcription-polymerase chain reaction (RT-PCR) with CaR-specific primers, followed by nucleotide sequencing of the amplified products, also identified CaR transcripts in the three cell lines. Exposure to known polycationic agonists of the CaR, including high Ca(2+)(o) (2.5mM), neomycin, and gadolinium (Gd(3+)) as well as a specific CaR activator, NPS R467, augmented cell proliferation in all three cell lines. RT-PCR revealed that U266 cells, but not IM-9 cells or RPMI8226 cells, expressed interleukin-6 (IL-6), the expression of which was not enhanced by treatments with CaR agonists. Therefore, taken together, our data first document the fact that the myeloma cell lines, U266, IM-9, and RPMI8226, all express CaR protein and mRNA. Moreover, the CaR expressed on myeloma cells could sense the locally high levels of Ca(2+)(o) in the vicinity of sites of osteoclastic bone resorption and stimulate their proliferation in an IL-6-independent manner. These processes may result in promoting further growth of the tumor and aggravating the associated bone disease.

A new cause of Zollinger-Ellison syndrome: non-small cell lung cancer

Numerous epidemiologic studies suggest a relationship between lung cancer and peptic ulcer disease. Furthermore, various lung cancers synthesize and release a number of peptides such as gastrin and gastrin-releasing peptide that could cause acid hypersecretion; however, Zollinger-Ellison syndrome (ZES), because of a lung tumor, has never been described. We report such a patient for the first time. A 60-year-old man with a non-small cell lung carcinoma (large cell type) presented with diarrhea, heartburn, abdominal pain, and duodenal ulcers. Evaluation showed ZES was present (fasting hypergastrinemia, hyperchlorhydria) and control of all symptoms by omeprazole. No abdominal or cardiac tumor, the other known locations of gastrinomas causing ZES, was found on detailed tumor imaging studies. Resection of the lung tumor resulted in a decrease in gastrin levels to normal values. Plasma radioimmunoassays showed elevated gastrin, chromogranin A and normal levels of gastrin-releasing peptide, and 9 other hormones. The tumor showed similar immunocytochemical results. The characteristics of this case are compared with 100 cases of sporadic abdominal gastrinomas, and the evidence reviewed suggests why ZES should be considered in patients with lung cancer with peptic symptoms.