Calcium-sensing receptor stimulates PTHrP release by pathways dependent on PKC, p38 MAPK, JNK, and ERK1/2 in H-500 cells

Perspectives on the Novel Multifunctional Nerve Guidance Conduits: From Specific Regenerative Procedures to Motor Function Rebuilding

Peripheral nerve injury potentially destroys the quality of life by inducing functional movement disorders and sensory capacity loss, which results in severe disability and substantial psychological, social, and financial burdens. Autologous nerve grafting has been commonly used as treatment in the clinic; however, its rare donor availability limits its application. A series of artificial nerve guidance conduits (NGCs) with advanced architectures are also proposed to promote injured peripheral nerve regeneration, which is a complicated process from axon sprouting to targeted muscle reinnervation. Therefore, exploring the interactions between sophisticated NGC complexes and versatile cells during each process including axon sprouting, Schwann cell dedifferentiation, nerve myelination, and muscle reinnervation is necessary. This review highlights the contribution of functional NGCs and the influence of microscale biomaterial architecture on biological processes of nerve repair. Progressive NGCs with chemical molecule induction, heterogenous topographical morphology, electroactive, anisotropic assembly microstructure, and self-powered electroactive and magnetic-sensitive NGCs are also collected, and they are expected to be pioneering features in future multifunctional and effective NGCs.

Comprehensive Review of γ-Glutamyl Peptides (γ-GPs) and Their Effect on Inflammation Concerning Cardiovascular Health

γ-Glutamyl peptides (γ-GPs) are a group of peptides naturally found in various food sources. The unique γ-bond potentially enables them to resist gastrointestinal digestion and offers high stability in vivo with a longer half-life. In recent years, these peptides have caught researchers' attention due to their ability to impart kokumi taste and elicit various physiological functions via the allosteric activation of the calcium-sensing receptor (CaSR). This review discusses the various food sources of γ-glutamyl peptides, different synthesis modes, allosteric activation of CaSR for taste perception, and associated multiple biological functions they can exhibit, with a special emphasis on their role in modulating chronic inflammation concerning cardiovascular health.

Gene expression signature of exercise and change of diet on non-alcoholic fatty liver disease in mice

Non-alcoholic fatty liver diseases (NAFLD) are particularly prevalent in the general Western adult population, with around one third of the population suffering from the disease. Evidence shows that NAFLD is associated with metabolic syndromes such as obesity, insulin resistance, and hypertension. Currently, the sole therapy for NAFLD involves exercise intervention. Studies showed that, with and without weight loss, exercise interventions produced a significant cutback in intrahepatic lipid content in humans, but better controlled studies that can investigate the cellular and molecular mechanisms are still lacking. In the current study we perform RNA sequencing analysis on liver samples from C57BL/6 mice submitted to aerobic exercise and diet interventions that are human-translatable and determine the genetic expression signature of exercise in the NAFLD onset. We show that aerobic exercise affects genes and pathways related to liver metabolism, muscle contraction and relaxation, immune response and inflammation, and development of liver cancer, counteracting non-alcoholic steatohepatitis and hepatocellular carcinoma development. While genes and pathways implicating immune response are activated by aerobic exercise in all interventions, the most effective intervention in terms of improvement of NASH is the combination of aerobic exercise with change of diet.

Phloretin Alleviates Arsenic Trioxide-Induced Apoptosis of H9c2 Cardiomyoblasts via Downregulation in Ca2+/Calcineurin/NFATc Pathway and Inflammatory Cytokine Release

Arsenic trioxide (ATO) is among the first-line chemotherapeutic drugs for treating acute promyelocytic leukemia patients, but its clinical use is hampered due to cardiotoxicity. The present investigation unveils the mechanism underlying ATO-induced oxidative stress that promotes calcineurin (a ubiquitous Ca²⁺/calmodulin-dependent serine/threonine phosphatase expressed only during sustained Ca²⁺ elevation) expression, inflammatory cytokine release and apoptosis in H9c2 cardiomyoblasts, and its possible modulation with phloretin (PHL, an antioxidant polyphenol present in apple peel). ATO caused Ca²⁺ overload resulting in elevated expression of calcineurin and its downstream transcriptional effector NFATc causing the release of cytokines such as IL-2, IL-6, MCP-1, IFN-γ, and TNF-α in H9c2 cardiomyoblast. There was a visible increase in the nuclear fraction of NF-κB and ROS-mediated apoptotic cell death. The expression levels of cardiac-specific genes (troponin, desmin, and caveolin-3) and genes of the apoptotic signaling pathway (BCL-2, BAX, IGF1, AKT, ERK1, -2, RAF1, and JNK) in response to ATO and PHL were studied. The putative binding mode and the potential ligand-target interactions of PHL with calcineurin using docking software (Autodock and iGEMDOCKv2) showed the high binding affinity of PHL to calcineurin. PHL co-treatment significantly reduced Ca²⁺ influx and normalized the expression of calcineurin, NFATc, NF-κB, and other cytokines. PHL co-treatment resulted in activation of BCL-2, IGF1, AKT, RAF1, ERK1, and ERK2 and inhibition of BAX and JNK. Overall, these results revealed that PHL has a protective effect against ATO-induced apoptosis and we propose calcineurin as a druggable target for the interaction of PHL in ATO cardiotoxicity in H9c2 cells.

Protective Effects of Polyphenols against Ischemia/Reperfusion Injury

Myocardial infarction (MI) is a leading cause of morbidity and mortality across the world. It manifests as an imbalance between blood demand and blood delivery in the myocardium, which leads to cardiac ischemia and myocardial necrosis. While it is not easy to identify the first pathogenic cause of MI, the consequences are characterized by ischemia, chronic inflammation, and tissue degeneration. A poor MI prognosis is associated with extensive cardiac remodeling. A loss of viable cardiomyocytes is replaced with fibrosis, which reduces heart contractility and heart function. Recent advances have given rise to the concept of natural polyphenols. These bioactive compounds have been studied for their pharmacological properties and have proven successful in the treatment of cardiovascular diseases. Studies have focused on their various bioactivities, such as their antioxidant and anti-inflammatory effects and free radical scavenging. In this review, we summarized the effects and benefits of polyphenols on the cardiovascular injury, particularly on the treatment of myocardial infarction in animal and human studies.

International Union of Basic and Clinical Pharmacology. CVIII. Calcium-Sensing Receptor Nomenclature, Pharmacology, and Function

The calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor that responds to multiple endogenous agonists and allosteric modulators, including divalent and trivalent cations, L-amino acids, γ-glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca2+ o) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca2+ o homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca2+ o homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation. Thus, although the CaSR is targeted clinically by the positive allosteric modulators (PAMs) cinacalcet, evocalcet, and etelcalcetide in hyperparathyroidism, it is also a putative therapeutic target in diabetes, asthma, cardiovascular disease, and cancer. The CaSR is somewhat unique in possessing multiple ligand binding sites, including at least five putative sites for the "orthosteric" agonist Ca2+ o, an allosteric site for endogenous L-amino acids, two further allosteric sites for small molecules and the peptide PAM, etelcalcetide, and additional sites for other cations and anions. The CaSR is promiscuous in its G protein-coupling preferences, and signals via Gq/11, Gi/o, potentially G12/13, and even Gs in some cell types. Not surprisingly, the CaSR is subject to biased agonism, in which distinct ligands preferentially stimulate a subset of the CaSR's possible signaling responses, to the exclusion of others. The CaSR thus serves as a model receptor to study natural bias and allostery. SIGNIFICANCE STATEMENT: The calcium-sensing receptor (CaSR) is a complex G protein-coupled receptor that possesses multiple orthosteric and allosteric binding sites, is subject to biased signaling via several different G proteins, and has numerous (patho)physiological roles. Understanding the complexities of CaSR structure, function, and biology will aid future drug discovery efforts seeking to target this receptor for a diversity of diseases. This review summarizes what is known to date regarding key structural, pharmacological, and physiological features of the CaSR.

The calcium-sensing receptor in physiology and in calcitropic and noncalcitropic diseases

The Ca2+-sensing receptor (CaSR) is a dimeric family C G protein-coupled receptor that is expressed in calcitropic tissues such as the parathyroid glands and the kidneys and signals via G proteins and β-arrestin. The CaSR has a pivotal role in bone and mineral metabolism, as it regulates parathyroid hormone secretion, urinary Ca2+ excretion, skeletal development and lactation. The importance of the CaSR for these calcitropic processes is highlighted by loss-of-function and gain-of-function CaSR mutations that cause familial hypocalciuric hypercalcaemia and autosomal dominant hypocalcaemia, respectively, and also by the fact that alterations in parathyroid CaSR expression contribute to the pathogenesis of primary and secondary hyperparathyroidism. Moreover, the CaSR is an established therapeutic target for hyperparathyroid disorders. The CaSR is also expressed in organs not involved in Ca2+ homeostasis: it has noncalcitropic roles in lung and neuronal development, vascular tone, gastrointestinal nutrient sensing, wound healing and secretion of insulin and enteroendocrine hormones. Furthermore, the abnormal expression or function of the CaSR is implicated in cardiovascular and neurological diseases, as well as in asthma, and the CaSR is reported to protect against colorectal cancer and neuroblastoma but increase the malignant potential of prostate and breast cancers.

Calcium-sensing receptor in nutrient sensing: an insight into the modulation of intestinal homoeostasis

The animal gut effectively prevents the entry of hazardous substances and microbes while permitting the transfer of nutrients, such as water, electrolytes, vitamins, proteins, lipids, carbohydrates, minerals and microbial metabolites, which are intimately associated with intestinal homoeostasis. The gut maintains biological functions through its nutrient-sensing receptors, including the Ca-sensing receptor (CaSR), which activates a variety of signalling pathways, depending on cellular context. CaSR coordinates food digestion and nutrient absorption, promotes cell proliferation and differentiation, regulates energy metabolism and immune response, stimulates hormone secretion, mitigates secretory diarrhoea and enhances intestinal barrier function. Thus, CaSR is crucial to the maintenance of gut homoeostasis and protection of intestinal health. In this review, we focused on the emerging roles of CaSR in the modulation of intestinal homoeostasis including related underlying mechanisms. By elucidating the relationship between CaSR and animal gut homoeostasis, effective and inexpensive methods for treating intestinal health imbalance through nutritional manipulation can be developed. This article is expected to provide experimental data of the effects of CaSR on animal or human health.

Lycium barbarum polysaccharide encapsulated Poly lactic-co-glycolic acid Nanofibers: cost effective herbal medicine for potential application in peripheral nerve tissue engineering

Nerve regeneration is a serious clinical challenge following peripheral nerve injury. Lycium barbarum polysaccharide (LBP) is the major component of wolfberry extract, which has been shown to be neuroprotective and promising in nerve recovery in many studies. Electrospun nanofibers, especially core-shell structured nanofibers being capable of serving as both drug delivery system and tissue engineering scaffolds, are well known to be suitable scaffolds for regeneration of peripheral nerve applications. In this study, LBP was incorporated into core-shell structured nanofibrous scaffolds via coaxial electrospinning. Alamar blue assays were performed to investigate the proliferation of both PC12 and Schwann cells cultured on the scaffolds. The neuronal differentiation of PC12 cells was evaluated by NF200 expression with immunostaining and morphology changes observed by SEM. The results indicated that the released LBP dramatically enhanced both proliferation and neuronal differentiation of PC12 cells induced by NGF. Additionally, the promotion of Schwann cells myelination and neurite outgrowth of DRG neurons were also observed on LBP loaded scaffolds by LSCM with immunostaining. In summary, LBP, as a drug with neuroprotection, encapsulated into electrospun nanofibers could be a potential candidate as tissue engineered scaffold for peripheral nerve regeneration.

Protective role and mechanism of snakegourd peel against myocardial infarction in rats

BACKGROUND

Injection of snakegourd peel (SP), an herb used in traditional Chinese medicine, is used to treat coronary artery disease and stable angina in China. However, its therapeutic role and mechanism of action for the treatment of myocardial infarction (MI) is not fully understood.

PURPOSE

The present study was designed to investigate the effect of SP on MI-induced cardiac injury and elucidate its underlying molecular mechanisms.

METHODS

To create an in vivo model of MI, we ligated the left coronary artery of Wistar rats. For our in vitro model of MI, we treated primary neonatal rat ventricular myocytes with hypoxia. Myocardial infarct size was measured by triphenyltetrazolium chloride (TTC) staining. Intracellular calcium concentration (Ca²⁺) was measured by confocal microscopy, and cardiomyocyte apoptosis was assessed by TUNEL assay. Western blot was applied to determine protein levels.

RESULTS

Three days post-MI, SP significantly improved MI-induced impairment of cardiac function, as indicated by increased left ventricular systolic pressure (LVSP), maximum rate of left ventricular pressure rise and fall (± dp/dt max), and decreased left ventricular end-diastolic pressure (LVEDP). In addition, SP treatment markedly reduced the infarct size and serum lactate dehydrogenase (LDH) activity; inhibited cardiomyocyte apoptosis and Caspase-3 activation both in vivo and in vitro; and decreased intracellular calcium overload, Cav1.2, phosphorylated JNK (p-JNK), and p38 MAPK (p-p38 MAPK) levels in ischemic myocardium.

CONCLUSION

SP alleviated cardiac ischemic injury and inhibited cardiomyocyte apoptosis by attenuating intracellular calcium overload, suppressing Caspase-3 activation, and downregulating protein expression of p-JNK and p-p38MAPK. These results suggest that SP may serve as a potential novel therapeutic drug for MI.

Identification and Functional Characterization of a Novel Mutation in the Human Calcium-Sensing Receptor That Co-Segregates With Autosomal-Dominant Hypocalcemia

The human calcium-sensing receptor (CASR) is the key controller of extracellular Ca_o²⁺ homeostasis, and different mutations in the CASR gene have been linked to different calcium diseases, such as familial hypocalciuric hypercalcemia, severe hyperparathyroidism, autosomal-dominant hypocalcemia (ADH), and Bartter's syndrome type V. In this study, two generations of a family with biochemically and clinically confirmed ADH who suffered severe muscle pain, arthralgia, tetany, abdominal pain, and fatigue were evaluated for mutations in the CASR gene. The study comprises genotyping of all family members, functional characterization of a potential mutant receptor by in vitro analysis related to the wild-type receptor to reveal an association between the genotype and phenotype in the affected family members. The in vitro analysis of functional characteristics includes measurements of inositol trisphosphate accumulation, Ca²⁺ mobilization in response to [Ca²⁺]_o-stimulation and receptor expression. The results reveal a significant leftward shift of inositol trisphosphate accumulation as a result of the "gain-of-function" mutant receptor and surprisingly a normalization of the response in (Ca²⁺)_i release in the downstream pathway and additionally the maximal response of (Ca²⁺)_i release was significantly decreased compared to the wild type. However, no gross differences were seen in D126V and the D126V/WT CASR dimeric >250 kDa band expression compared to the WT receptor, however, the D126V and D126V/WT CASR immature ~140 kDa species appear to have reduced expression compared to the WT receptor. In conclusion, in this study, a family with a clinical diagnosis of ADH in two generations was evaluated to identify a mutation in the CASR gene and reveal an association between genotype and phenotype in the affected family members. The clinical condition was caused by a novel, activating, missense mutation (D126V) in the CASR gene and the in vitro functional characteristics of the mutation co-segregated with their individual phenotype.

The Calcium-Sensing Receptor and the Parathyroid: Past, Present, Future

Parathyroid hormone (PTH) defends the extracellular fluid from hypocalcemia and has powerful and well-documented actions on the skeleton and renal tubular system. To achieve a satisfactory stable plasma calcium level, the secretion of PTH, and the resulting serum PTH level, is titrated carefully to the prevailing plasma ionized Ca²⁺ concentration via a Ca²⁺ sensing mechanism that mediates feedback inhibition of PTH secretion. Herein, I consider the properties of the parathyroid Ca²⁺ sensing mechanism, the identity of the Ca²⁺ sensor, the intracellular biochemical mechanisms that it controls, the manner of its integration with other components of the PTH secretion control mechanism, and its modulation by other nutrients. Together the well-established, recently elucidated, and yet-to-be discovered elements of the story constitute the past, present, and future of the parathyroid and its calcium-sensing receptor (CaSR).

Calcium-Sensing Receptor in Breast Physiology and Cancer

The calcium-sensing receptor (CaSR) is expressed in normal breast epithelial cells and in breast cancer cells. During lactation, activation of the CaSR in mammary epithelial cells increases calcium transport into milk and inhibits parathyroid hormone-related protein (PTHrP) secretion into milk and into the circulation. The ability to sense changes in extracellular calcium allows the lactating breast to actively participate in the regulation of systemic calcium and bone metabolism, and to coordinate calcium usage with calcium availability during milk production. Interestingly, as compared to normal breast cells, in breast cancer cells, the regulation of PTHrP secretion by the CaSR becomes rewired due to a switch in its G-protein usage such that activation of the CaSR increases instead of decreases PTHrP production. In normal cells the CaSR couples to Gα_i to inhibit cAMP and PTHrP production, whereas in breast cancer cells, it couples to Gα_s to stimulate cAMP and PTHrP production. Activation of the CaSR on breast cancer cells regulates breast cancer cell proliferation, death and migration, in part, by stimulating PTHrP production. In this article, we discuss the biology of the CaSR in the normal breast and in breast cancer, and review recent findings suggesting that the CaSR activates a nuclear pathway of PTHrP action that stimulates cellular proliferation and inhibits cell death, helping cancer cells adapt to elevated extracellular calcium levels. Understanding the diverse actions mediated by the CaSR may help us better understand lactation physiology, breast cancer progression and osteolytic bone metastases.

The Calcium-Sensing Receptor and the Reproductive System

Active placental transport of maternal serum calcium (Ca²⁺) to the offspring is pivotal for proper development of the fetal skeleton as well as various organ systems. Moreover, extracellular Ca²⁺ levels impact on distinct processes in mammalian reproduction. The calcium-sensing receptor (CaSR) translates changes in extracellular Ca²⁺-concentrations into cellular reactions. This review summarizes current knowledge on the expression of CaSR and its putative functions in reproductive organs. CaSR was detected in placental cells mediating materno-fetal Ca²⁺-transport such as the murine intraplacental yolk sac (IPYS) and the human syncytiotrophoblast. As shown in casr knock-out mice, ablation of CaSR downregulates transplacental Ca²⁺-transport. Receptor expression was reported in human and rat ovarian surface epithelial (ROSE) cells, where CaSR activation stimulates cell proliferation. In follicles of various species a role of CaSR activation in oocyte maturation was suggested. Based on studies in avian follicles, the activation of CaSR expressed in granulosa cells may support the survival of follicles after their selection. CaSR in rat and equine sperms was functionally linked to sperm motility and sperm capacitation. Implantation involves complex interactions between the blastocyst and the uterine epithelium. During early pregnancy, CaSR expression at the implantation site as well as in decidual cells indicates that CaSR is important for blastocyst implantation and decidualization in the rat uterus. Localization of CaSR in human extravillous cytotrophoblasts suggests a role of CaSR in placentation. Overall, evidence for functional involvement of CaSR in physiologic mammalian reproductive processes exists. Moreover, several studies reported altered expression of CaSR in cells of reproductive tissues under pathologic conditions. However, in many tissues we still lack knowledge on physiological ligands activating CaSR, CaSR-linked G-proteins, activated intracellular signaling pathway, and functional relevance of CaSR activation. Clearly, more work is required in the future to decode the complex physiologic and pathophysiologic relationship of CaSR and the mammalian reproductive system.

The role of extracellular calcium in bone metastasis

This review summarizes the role of extracellular calcium, as found present in the bone tissue, in the process of bone metastasis.

The calcium-sensing receptor and integrins modulate cerebellar granule cell precursor differentiation and migration

In the developing cerebellum granule cell precursors (GCPs) proliferate in the external granule cell layer before differentiating and migrating to the inner granule cell layer. Aberrant GCP proliferation leads to medulloblastoma, the most prevalent form of childhood brain cancer. Here, we demonstrate that the calcium-sensing receptor (CaSR), a homodimeric G-protein coupled receptor, functions in conjunction with cell adhesion proteins, the integrins, to enhance GCP migration and cell homing by promoting GCP differentiation. During the second postnatal week a robust peak in CaSR expression was observed in GCPs; reciprocal immunoprecipitation experiments conducted during this period established that the CaSR and β1 integrins are present together in a macromolecular protein complex. Analysis of cell-surface proteins demonstrated that activation of the CaSR by positive allosteric modulators promoted plasma membrane expression of β1 integrins via ERK2 and AKT phosphorylation and resulted in increased GCP migration toward an extracellular matrix protein. The results of in vivo experiments whereby CaSR modulators were injected i.c.v. revealed that CaSR activation promoted radial migration of GCPs by enhancing GCP differentiation, and conversely, a CaSR inhibitor disrupted GCP differentiation and promoted GCP proliferation. Our results demonstrate that an ion-sensing G-protein coupled receptor acts to promote neuronal differentiation and homing during cerebellar maturation. These findings together with those of others also suggest that CaSR/integrin complexes act to transduce extracellular calcium signals into cellular movement, and may function in this capacity as a universal cell migration/homing complex in the developing brain.

Novel activating mutation of human calcium-sensing receptor in a family with autosomal dominant hypocalcaemia

INTRODUCTION

Autosomal dominant hypocalcaemia (ADH) is caused by activating mutations in the calcium sensing receptor gene (CaR) and characterised by mostly asymptomatic mild to moderate hypocalcaemia with low, inappropriately serum concentration of PTH.

OBJECTIVE

The purpose of the present study was to biochemically and functionally characterise a novel mutation of CaR.

PATIENTS

A female proband presenting with hypocalcaemia was diagnosed to have "idiopathic hypoparathyroidism" at the age of 10 with a history of muscle pain and cramps. Further examinations demonstrated hypocalcaemia in nine additional family members, affecting three generations.

MAIN OUTCOME MEASURE

P136L CaR mutation was predicted to cause gain of function of CaR.

RESULTS

Affected family members showed relevant hypocalcaemia (mean ± SD; 1.9 ± 0.1 mmol/l). Patient history included mild seizures and recurrent nephrolithiasis. Genetic analysis confirmed that hypocalcaemia cosegregated with a heterozygous mutation at codon 136 (CCC → CTC/Pro → Leu) in exon 3 of CaR confirming the diagnosis of ADH. For in vitro studies P136L mutant CaR was generated by site-directed mutagenesis and examined in transiently transfected HEK293 cells. Extracellular calcium stimulation of transiently transfected HEK293 cells showed significantly increased intracellular Ca(2+) mobilisation and MAPK activity for mutant P136L CaR compared to wild type CaR.

CONCLUSIONS

The present study gives insight about a novel activating mutation of CaR and confirms that the novel P136L-CaR mutation is responsible for ADH in this family.

NKCC2 activity is inhibited by the Bartter's syndrome type 5 gain-of-function CaR-A843E mutant in renal cells

BACKGROUND INFORMATION

The gain-of-function A843E mutation of the calcium sensing receptor (CaR) causes Bartter syndrome type 5. Patients carrying this CaR variant show a remarkably reduced renal NaCl reabsorption in the thick ascending limb (TAL) of Henle's loop resulting in renal loss of NaCl in the absence of mutations in renal Na(+) and Cl(-) ion transporters. The molecular mechanisms underlying this clinical phenotype are incompletely understood. We investigated, in human embryonic kidney 293 (HEK 293) cells and porcine kidney epithelial (LLC-PK1) cells, the functional cross-talk of CaR-A843E with the Na(+):K(+):2Cl(-) co-transporter, NKCC2, which provides NaCl reabsorption in the TAL.

RESULTS

The expression of the CaR mutant did not alter the apical localisation of NKCC2 in LLC-PK1 cells. However, the steady-state NKCC2 phosphorylation and activity were decreased in cells transfected with CaR-A843E compared with the control wild-type CaR (CaR WT)-transfected cells. Of note, low-Cl(-)-dependent NKCC2 activation was also strongly inhibited upon the expression of CaR-A843E mutant. The use of either P450 ω-hydroxylase (CYP4)- or phospholipase A2 (PLA2)-blockers suggests that this effect is likely mediated by arachidonic acid (AA) metabolites.

CONCLUSIONS

The data suggested that the activated CaR affects intracellular pathways modulating NKCC2 activity rather than NKCC2 intracellular trafficking in renal cells, and throw further light on the pathological role played by active CaR mutants in Bartter syndrome type 5.

Effects of parathyroid hormone on calcium ions in rat bone marrow mesenchymal stem cells

The present study was conducted in order to explore the mechanisms whereby parathyroid hormone (PTH) maintains in vitro proliferation of bone marrow mesenchymal stem cells (BMSCs). Bone marrow was isolated from Sprague Dawley (SD) rat femurs, cultured in vitro, and passaged using a cell adherent culture method. The BMSC proliferation was evaluated by the methyl thiazolyl tetrazolium (MTT) assay and the fluorescence intensity of calcium ions in BMSCs was analyzed by laser scanning confocal microscopy (LSCM). Our results show that BMSC proliferation in the experimental group treated with PTH was more significant than controls. The calcium ion fluorescence intensity in BMSCs was significantly higher for the experimental group as compared to the control group. For each group, there was significant difference in the fluorescence intensity of calcium ions in BMSCs between 7 d and 14 d. In conclusion, parathyroid hormone increased the fluorescence intensity of calcium ions in BMSCs, which might represent a key mechanism whereby BMSC proliferation is maintained.

High calcium concentration in bones promotes bone metastasis in renal cell carcinomas expressing calcium-sensing receptor

Background

The prognosis for renal cell carcinoma (RCC) is related to a high rate of metastasis, including 30% of bone metastasis. Characteristic for bone tissue is a high concentration of calcium ions. In this study, we show a promoting effect of an enhanced extracellular calcium concentration on mechanisms of bone metastasis via the calcium-sensing receptor (CaSR) and its downstream signaling molecules.

Methods

Our analyses were performed using 33 (11/category) matched specimens of normal and tumor tissue and 9 (3/category) primary cells derived from RCC patients of the 3 categories: non-metastasized, metastasized into the lung and metastasized into bones during a five-year period after nephrectomy. Expression of CaSR was determined by RT-PCR, Western blot analyses and flow cytometry, respectively. Cells were treated by calcium and the CaSR inhibitor NPS 2143. Cell migration was measured in a Boyden chamber with calcium (10 μM) as chemotaxin and proliferation by BrdU incorporation. The activity of intracellular signaling mediators was quantified by a phospho-kinase array and Western blot.

Results

The expression of CaSR was highest in specimens and cells of patients with bone metastases. Calcium treatment induced an increased migration (19-fold) and proliferation (2.3-fold) exclusively in RCC cells from patients with bone metastases. The CaSR inhibitor NPS 2143 elucidated the role of CaSR on the calcium-dependent effects. After treatment with calcium, the activity of AKT, PLCγ-1, p38α and JNK was clearly enhanced and PTEN expression was almost completely abolished in bone metastasizing RCC cells.

Conclusions

Our results indicate a promoting effect of extracellular calcium on cell migration and proliferation of bone metastasizing RCC cells via highly expressed CaSR and its downstream signaling pathways. Consequently, CaSR may be regarded as a new prognostic marker predicting RCC bone metastasis.

Excessive signal transduction of gain-of-function variants of the calcium-sensing receptor (CaSR) are associated with increased ER to cytosol calcium gradient

In humans, gain-of-function mutations of the calcium-sensing receptor (CASR) gene are the cause of autosomal dominant hypocalcemia or type 5 Bartter syndrome characterized by an abnormality of calcium metabolism with low parathyroid hormone levels and excessive renal calcium excretion. Functional characterization of CaSR activating variants has been so far limited at demonstrating an increased sensitivity to external calcium leading to lower Ca-EC50. Here we combine high resolution fluorescence based techniques and provide evidence that for the efficiency of calcium signaling system, cells expressing gain-of-function variants of CaSR monitor cytosolic and ER calcium levels increasing the expression of the Sarco-Endoplasmic Reticulum Calcium-ATPase (SERCA) and reducing expression of Plasma Membrane Calcium-ATPase (PMCA). Wild-type CaSR (hCaSR-wt) and its gain-of-function (hCaSR-R990G; hCaSR-N124K) variants were transiently transfected in HEK-293 cells. Basal intracellular calcium concentration was significantly lower in cells expressing hCaSR-wt and its gain of function variants compared to mock. In line, FRET studies using the D1ER probe, which detects [Ca2+]ER directly, demonstrated significantly higher calcium accumulation in cells expressing the gain of function CaSR variants compared to hCaSR-wt. Consistently, cells expressing activating CaSR variants showed a significant increase in SERCA activity and expression and a reduced PMCA expression. This combined parallel regulation in protein expression increases the ER to cytosol calcium gradient explaining the higher sensitivity of CaSR gain-of-function variants to external calcium. This control principle provides a general explanation of how cells reliably connect (and exacerbate) receptor inputs to cell function.

The calcium-sensing receptor in the breast

Normal breast epithelial cells and breast cancer cells express the calcium-sensing receptor (CaSR), the master regulator of systemic calcium metabolism. During lactation, activation of the CaSR in mammary epithelial cells downregulates parathyroid hormone-related protein (PTHrP) levels in milk and in the circulation, and increases calcium transport into milk. In contrast, in breast cancer cells the CaSR upregulates PTHrP production. A switch in G-protein usage underlies the opposing effects of the CaSR on PTHrP expression in normal and malignant breast cells. During lactation, the CaSR in normal breast cells coordinates a feedback loop that matches the transport of calcium into milk and maternal calcium metabolism to the supply of calcium. A switch in CaSR G-protein usage during malignant transformation converts this feedback loop into a feed-forward cycle in breast cancer cells that may promote the growth of osteolytic skeletal metastases.

LPS induces cardiomyocyte injury through calcium-sensing receptor

Calcium-sensing receptor (CaSR) belongs to the family C of G-protein coupled receptors. We have previously demonstrated that CaSR could induce apoptosis of cultured neonatal rat ventricular cardiomyocytes in simulated ischemia/reperfusion. It remains unknown whether the CaSR has function in lipopolysaccharide (LPS)-induced myocardial injure. The aim of this study was to investigate whether the CaSR plays a role in LPS-induced myocardial injury. Cultured neonatal rat cardiomyocytes were treated with LPS, with or without pretreatment with the CaSR-specific agonist gadolinium chloride (GdCl3) or the CaSR-specific antagonist NPS2390. Release of TNF-α and IL-6 from cardiomyocytes was observed. Levels of malonaldehyde (MDA), lactate dehydrogenase (LDH), and activity of superoxide dismutase (SOD) were measured. In addition, apoptosis of the cardiomyocytes, [Ca(2+)]i and level of CaSR expression were determined. The results showed that LPS increased cardiomyocytes apoptosis, [Ca(2+)]i, MDA, LDH, TNF-α, IL-6 release, and CaSR protein expression. Compared with LPS treatment alone, pretreatment with GdCl3 further increased apoptosis of cardiomyocytes, MDA, LDH, TNF-α, IL-6 release, [Ca(2+)]i, and the expression of the CaSR protein. Conversely, pretreatment with NPS2390 decreased apoptosis of cardiomyocytes, MDA, LDH, TNF-α, IL-6 release, [Ca(2+)]i and the expression of the CaSR protein. These results demonstrate that LPS could induce cardiomyocyte injury. Moreover, LPS-induced cardiomyocyte injury was related to CaSR-mediated cardiomyocytes apoptosis, TNF-α, IL-6 release, and increase of intracellular calcium.

Activation of calcium-sensing receptors is associated with apoptosis in a model of simulated cardiomyocytes ischemia/reperfusion

Objective

Calcium-sensing receptors (CaSRs) are G-protein coupled receptors which maintain systemic calcium homeostasis and participate in hormone secretion, activation of ion channels, cell apoptosis, proliferation, and differentiation. Previous studies have shown that CaSRs induce apoptosis in isolated adult rat heart and in normal neonatal rat cardiomyocytes by G-protein-PLC-IP₃ signaling transduction. However, little knowledge is presently available concerning the role of CaSRs in the apoptosis induced by ischemia and reperfusion in neonatal cardiomyocytes.

Methods

Primary neonatal rat ventricular cardiomyocytes were incubated in ischemiamimetic solution for 2 h, and then re-incubated in normal culture medium for 24 h to establish a model of simulated ischemia/reperfusion (I/R). Cardiomyocyte apoptosis was detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). The expression of CaSRs mRNA was detected by real-time reverse transcription polymerase chain reaction (RT-PCR). In addition, the expressions of caspase-3 and Bcl-2 were analyzed by western blot.

Results

The simulated I/R enhanced the expression of CaSRs and cardiomyocyte apoptosis. GdCl₃, a specific activator of CaSRs, further increased the expression of CaSRs and cardiomyocyte apoptosis, along with up-regulation of caspase-3 and down-regulation of Bcl-2.

Conclusion

CaSRs are associated with I/R injury and apoptosis in neonatal rat ventricular cardiomyocytes via suppressing Bcl-2 and promoting caspase-3 expression.

Effect of the calcium sensing receptor on rat bone marrow-derived mesenchymal stem cell proliferation through the ERK1/2 pathway

Migration and proliferation of bone marrowderived mesenchymal stem cells (BMSCs) is critical to treatment of ischemic injury. The calcium sensing receptor (CaSR) has an important role in maintaining systemic calcium homeostasis, which is related to cell proliferation, apoptosis and paracrine signaling. We hypothesize that CaSR may enhance BMSC proliferation. Rat BMSCs were incubated with various calcium concentrations for 48 h in vitro to activate CaSR. To investigate potential mechanisms responsible for growth enhancement by calcium, the rat BMSC cell cycle progression was analyzed by fluorescence-activated cell sorting (FACS), and induction of apoptosis confirmed by cytofluorimetric analysis using propidium iodide and Annexin V-FITC double staining. Since the mitogen-activated protein kinase (MAPK) signaling pathway was one of the most significantly affected by CaSR, MAPK activation was measured by western blotting. Calcium exposure significantly enhanced rat BMSCs proliferation, as well as the proportion of the population in S phase, in a dose-dependent manner, effects which were abolished by NPS2390 (a CaSR antagonist) and U0126 (a MEK1/2 inhibitor). These results demonstrate that CaSR is involved in rat BMSC proliferation, as seen by an increased proliferation index, decreased apoptosis, and ERK1/2 activation, and provide important insight into the cellular and molecular mechanisms by which CaSR affects cell proliferation. A CaSR agonist may prove useful to enhance BMSC survival during transplantation.

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 calcium-sensing receptor and calcimimetics in blood pressure modulation

Calcium is a crucial second messenger in the cardiovascular system. However, calcium may also be an extracellular first messenger through a G-protein-coupled receptor that senses extracellular concentration (Ca(2+)(o)), the calcium-sensing receptor (CaR). The most prominent physiological function of the CaR is to maintain the extracellular Ca(2+) level in a very tight range by regulating the circulating levels of parathyroid hormone (PTH). This control over PTH and Ca(2+) levels is partially lost in patients suffering from primary and secondary hyperparathyroidism. Allosteric modulators of the CaR (calcimimetics) are the first drugs in their class to become available for clinical use and have been shown to successfully treat certain forms of primary and secondary hyperparathyroidism. In addition, several studies suggest beneficial effects of calcimimetics on cardiovascular risk factors associated with hyperparathyroidism. Although a plethora of studies demonstrated the CaR in heart and blood vessels, exact roles of the receptor in the cardiovascular system still remain to be elucidated. However, several studies point toward a possibility that the CaR might be involved in the regulation of vascular tone. This review will summarize the current knowledge on the possible functions of the CaR and calcimimetics on blood pressure regulation.

Regulation of stability and trafficking of calcium-sensing receptors by pharmacologic chaperones

Gain- or loss-of-function mutations and polymorphisms of the calcium-sensing receptor (CaSR) cause Ca(2+) handling diseases. Altered expression and/or signaling of wild-type CaSR can also contribute to pathology. Recent studies have demonstrated that a significant proportion of mutations cause altered targeting and/or trafficking of CaSR to the plasma membrane. Pharmacological approaches to rescue of CaSR function include treatment with allosteric modulators, which potentiate the effects of the orthosteric agonist Ca(2+). Dissection of the mechanism(s) contributing to allosteric agonist-mediated rescue of loss-of-function CaSR mutants has demonstrated pharmacologic chaperone actions coincident with CaSR biosynthesis. The distinctive responses to the allosteric agonist (NPS R-568), which promotes CaSR stability, and the allosteric antagonist (NPS 2143), which promotes CaSR degradation, have led to a model for a conformational checkpoint during CaSR biosynthesis. The conformational checkpoint would "tune" CaSR biosynthesis to cellular signaling state. Navigation of a distinct checkpoint for endoplasmic release can also be augmented by pharmacologic chaperones. The diverse, post-endoplasmic reticulum quality control site(s) for pharmacologic chaperone modulation of CaSR stability and trafficking redefines the role(s) of allosteric modulators in regulation of overall GPCR function.

Signaling through the extracellular calcium-sensing receptor (CaSR)

The extracellular calcium ([Formula: see text])-sensing receptor (CaSR) was the first GPCR identified whose principal physiological ligand is an ion, namely extracellular Ca(2+). It maintains the near constancy of [Formula: see text] that complex organisms require to ensure normal cellular function. A wealth of information has accumulated over the past two decades about the CaSR's structure and function, its role in diseases and CaSR-based therapeutics. This review briefly describes the CaSR and key features of its structure and function, then discusses the extracellular signals modulating its activity, provides an overview of the intracellular signaling pathways that it controls, and, finally, briefly describes CaSR signaling both in tissues participating in [Formula: see text] homeostasis as well as those that do not. Factors controlling CaSR signaling include various factors affecting the expression of the CaSR gene as well as modulation of its trafficking to and from the cell surface. The dimeric cell surface CaSR, in turn, links to various heterotrimeric and small molecular weight G proteins to regulate intracellular second messengers, lipid kinases, various protein kinases, and transcription factors that are part of the machinery enabling the receptor to modulate the functions of the wide variety of cells in which it is expressed. CaSR signaling is impacted by its interactions with several binding partners in addition to signaling elements per se (i.e., G proteins), including filamin-A and caveolin-1. These latter two proteins act as scaffolds that bind signaling components and other key cellular elements (e.g., the cytoskeleton). Thus CaSR signaling likely does not take place randomly throughout the cell, but is compartmentalized and organized so as to facilitate the interaction of the receptor with its various signaling pathways.

P38 mitogen-activated protein kinase inhibitor, FR167653, inhibits parathyroid hormone related protein-induced osteoclastogenesis and bone resorption

p38 mitogen-activated protein kinase (MAPK) acts downstream in the signaling pathway that includes receptor activator of NF-κB (RANK), a powerful inducer of osteoclast formation and activation. We investigated the role of p38 MAPK in parathyroid hormone related protein (PTHrP)-induced osteoclastogenesis in vitro and PTHrP-induced bone resorption in vivo. The ability of FR167653 to inhibit osteoclast formation was evaluated by counting the number of tartrate-resistant acid phosphatase positive multinucleated cells (TRAP-positive MNCs) in in vitro osteoclastgenesis assays. Its mechanisms were evaluated by detecting the expression level of c-Fos and nuclear factor of activated T cells c1 (NFATc1) in bone marrow macrophages (BMMs) stimulated with sRANKL and M-CSF, and by detecting the expression level of osteoprotegerin (OPG) and RANKL in bone marrow stromal cells stimulated with PTHrP in the presence of FR167653. The function of FR167653 on bone resorption was assessed by measuring the bone resorption area radiographically and by counting osteoclast number per unit bone tissue area in calvaria in a mouse model of bone resorption by injecting PTHrP subcutaneously onto calvaria. Whole blood ionized calcium levels were also recorded. FR167653 inhibited PTHrP-induced osteoclast formation and PTHrP-induced c-Fos and NFATc1 expression in bone marrow macrophages, but not the expression levels of RANKL and OPG in primary bone marrow stromal cells treated by PTHrP. Furthermore, bone resorption area and osteoclast number in vivo were significantly decreased by the treatment of FR167653. Systemic hypercalcemia was also partially inhibited. Inhibition of p38 MAPK by FR167653 blocks PTHrP-induced osteoclastogenesis in vitro and PTHrP-induced bone resorption in vivo, suggesting that the p38 MAPK signaling pathway plays a fundamental role in PTHrP-induced osteoclastic bone resorption.

The calcium-sensing receptor: a molecular perspective

Compelling evidence of a cell surface receptor sensitive to extracellular calcium was observed as early as the 1980s and was finally realized in 1993 when the calcium-sensing receptor (CaR) was cloned from bovine parathyroid tissue. Initial studies relating to the CaR focused on its key role in extracellular calcium homeostasis, but as the amount of information about the receptor grew it became evident that it was involved in many biological processes unrelated to calcium homeostasis. The CaR responds to a diverse array of stimuli extending well beyond that merely of calcium, and these stimuli can lead to the initiation of a wide variety of intracellular signaling pathways that in turn are able to regulate a diverse range of biological processes. It has been through the examination of the molecular characteristics of the CaR that we now have an understanding of how this single receptor is able to convert extracellular messages into specific cellular responses. Recent CaR-related reviews have focused on specific aspects of the receptor, generally in the context of the CaR's role in physiology and pathophysiology. This review will provide a comprehensive exploration of the different aspects of the receptor, including its structure, stimuli, signalling, interacting protein partners, and tissue expression patterns, and will relate their impact on the functionality of the CaR from a molecular perspective.

Inhibitory effect of hepatocyte growth factor on cardiomyocytes apoptosis is partly related to reduced calcium sensing receptor expression during a model of simulated ischemia/reperfusion

Calcium-sensing receptors (CaSR) are G-protein coupled receptors which maintain systemic calcium haemeostasis, participate in hormone secretion, activation of iron channel, cell apoptosis, proliferation and differentiation. Previous studies have show CaSR induce apoptosis in isolated rat adult heart and in normal rat neonatal cardiomyocytes by G-protein-PLC-IP3 signaling transinduction. A few of studies had demonstrated that CaSR induce apoptosis in cultured neonatal rat cardiomyocytes during ischemia/reperfusion. Hepatocyte growth factor (HGF), as a mesenchymally derived heterodimeric glycoprotein, play vital role in mitogenesis, angiogenesis, cellular motility and growth and anti-apoptosis after postinfarction heart failure via activation of transmembrane tyrosine kinase cell surface receptor c-Met. However, little knowledge exists about whether anti-apoptotic role of HGF in preventing cardiomyocytes injury induced by ischemia/reperfusion is associated with downregulation of CaSR expression. We incubated primary neonatal rat ventricular cardiomyocytes in ischemia-mimetic solution for 2 h, then reincubated them in normal culture medium for 24 h to establish a model of simulated ischemia/reperfusion (I/R). Cardiomyocyte apoptosis was detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. The expression of CaSR mRNA was detected by reverse transcriptase polymerase chain reaction (RT-PCR). In addition, we analyzed the expression of Caspase-3, Bcl-2 and Phosphoinositide 3-kinase (PI3K) by Western blotting. The simulated I/R enhances the expression of CaSR and cardiomyocyte apoptosis. GdCl3, a specific activator of CaSR, further increase the expression of CaSR and Cardiomyocyte apoptosis, along with upregulation of Caspase-3, downregulation of Bcl-2 and inhibiting PI3K phosphorylation. Combination of GdCl3 with LY294002 (a selective PI3K inhibitor) increased Cardiomyocytes apoptosis but did not increased CaSR expression. Treatment of HGF decreased I/R- and GdCl3-induced apoptosis by suppressing Caspase-3 and promoting Bcl-2 and PI3K phosphorylation expression in accordance with downregulation of CaSR expression. HGF exerts protective role in I/R-induced apoptosis at least in part by inhibiting CaSR expression along with promoting Bcl-2, suppressing Caspase-3 expression and stimulating PI3K phosphorylation signaling pathway.

Strontium ranelate decreases receptor activator of nuclear factor-ΚB ligand-induced osteoclastic differentiation in vitro: involvement of the calcium-sensing receptor

Strontium ranelate exerts both an anticatabolic and an anabolic effect on bone cells. To further investigate the mechanism by which strontium ranelate inhibits bone resorption, the effects of varying concentrations of Sr(o)(2+) on osteoclastic differentiation were studied using RAW 264.7 cells and peripheral blood monocytic cells (PBMCs). We report that increasing concentrations of Sr(o)(2+) down-regulate osteoclastic differentiation and tartrate-resistant acid phosphatase activity, leading to inhibition of bone resorption (-48% when PBMCs were cultured for 14 days in the presence of 2 mM Sr(o)(2+)). Using a dominant-negative form of the calcium-sensing receptor (CaR) and a small interfering RNA approach, we provide evidences that the inhibition of osteoclast differentiation by Sr(o)(2+) is mediated by stimulation of the CaR. Moreover, our results suggest that the effects of Sr(o)(2+) on osteoclasts are, at least in part, mediated by inhibition of the receptor activator of nuclear factor-κB ligand (RANKL)-induced nuclear translocation of nuclear factor-κB and activator protein-1 in the early stages of osteoclastic differentiation. In conclusion, our data indicate that Sr(2+) directly inhibits the formation of mature osteoclasts through down-regulation of RANKL-induced osteoclast differentiation and decreases osteoclast differentiation through the activation of the CaR.

Alterations in phosphorus, calcium and PTHrP contribute to defects in dental and dental alveolar bone formation in calcium-sensing receptor-deficient mice

To determine whether the calcium-sensing receptor (CaR) participates in tooth formation and dental alveolar bone development in mandibles in vivo, we examined these processes, as well as mineralization, in 2-week-old CaR-knockout (CaR(-/-)) mice. We also attempted to rescue the phenotype of CaR(-/-) mice by genetic means, in mice doubly homozygous for CaR and 25-hydroxyvitamin D 1alpha-hydroxylase [1alpha(OH)ase] or parathyroid hormone (Pth). In CaR(-/-) mice, which exhibited hypercalcemia, hypophosphatemia and increased serum PTH, the volumes of teeth and of dental alveolar bone were decreased dramatically, whereas the ratio of the area of predentin to total dentin and the number and surface of osteoblasts in dental alveolar bone were increased significantly, as compared with wild-type littermates. The normocalcemia present in CaR(-/-);1alpha(OH)ase(-/-) mice only slightly improved the defects in dental and alveolar bone formation observed in the hypercalcemic CaR(-/-) mice. However, these defects were completely rescued by the additional elimination of hypophosphatemia and by an increase in parathyroid hormone-related protein (PTHrP) expression in the apical pulp, Hertwig's epithelial root sheath and mandibular tissue in CaR(-/-); Pth(-/-) mice. Therefore, alterations in calcium, phosphorus and PTHrP contribute to defects in the formation of teeth and alveolar bone in CaR-deficient mice. This study indicates that CaR participates in the formation of teeth and in the development of dental alveolar bone in mandibles in vivo, although it appears to do so largely indirectly.

Calcium-sensing receptor in cancer: good cop or bad cop?

The extracellular calcium-sensing receptor (CaR) is a versatile 'sensor' for di- and polycationic molecules in the body. CaR plays a key role in the defense against hypercalcemia by "sensing" extracellular calcium levels in the parathyroid and kidney, the key organs maintaining systemic calcium homeostasis. Although mutation of CaR gene has so far not been associated with any malignancy, aberrant functions of CaR have implications in malignant progression. One situation is loss of CaR expression, resulting in loss of growth suppressing effects of elevated extracellular Ca(2+) by CaR, reported in parathyroid adenoma and in colon carcinoma. Another situation is activation of CaR, resulting in increased production of parathyroid hormone-related peptide (PTHrP), a primary causal factor in hypercalcemia of malignancy and a contributor to metastatic processes involving bone. CaR signaling and effects have been studied in several cancers including ovarian cancers, gastrinomas, and gliomas in addition to comparatively detailed studies in breast, prostate, and colon cancers. Studies on H-500 rat Leydig cells, a xenotransplantable model of humoral hypercalcemia of malignancy has shed much light on the mechanisms of CaR-induced cancer cell growth and survival. Pharmacological agonists and antagonists of CaR hold therapeutic promise depending on whether activation of CaR is required such as in case of colon cancer or inactivating the receptor is required as in the case of breast- and prostate tumors.

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.

Development and progression of secondary hyperparathyroidism in chronic kidney disease: lessons from molecular genetics

The identification of the calcium-sensing receptor (CaSR) and the clarification of its role as the major regulator of parathyroid gland function have important implications for understanding the pathogenesis and evolution of secondary hyperthyroidism in chronic kidney disease (CKD). Signaling through the CaSR has direct effects on three discrete components of parathyroid gland function, which include parathyroid hormone (PTH) secretion, PTH synthesis, and parathyroid gland hyperplasia. Disturbances in calcium and vitamin D metabolism that arise owing to CKD diminish the level of activation of the CaSR, leading to increases in PTH secretion, PTH synthesis, and parathyroid gland hyperplasia. Each represents a physiological adaptive response by the parathyroid glands to maintain plasma calcium homeostasis. Studies of genetically modified mice indicate that signal transduction via the CaSR is a key determinant of parathyroid cell proliferation and parathyroid gland hyperplasia. Because enlargement of the parathyroid glands has important implications for disease progression and disease severity, it is possible that clinical management strategies that maintain adequate calcium-dependent signaling through the CaSR will ultimately prove useful in diminishing parathyroid gland hyperplasia and in modifying disease progression.

Kaempferol and quercetin stimulate granulocyte-macrophage colony-stimulating factor secretion in human prostate cancer cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF) holds immunotherapeutic promise in prostate cancer as it activates the host immune system. Increased production of GM-CSF by cancer cells may facilitate host immunosurveillence by the dendritic cells (DC). Here, we studied the effects of kaempferol (K) and quercetin (Q) on the production of GM-CSF in PC-3 cells. Human cytokine antibody array revealed that treatment with K or Q increased GM-CSF release by PC-3 cells. We further observed by ELISA that K and Q in a concentration-dependent manner increased GM-CSF production without affecting its mRNA levels. Inhibitors of vesicular traffic through the endoplasmic reticulum and Golgi-blocked GM-CSF secretory stimulation. A microtubule-stabilizing agent stimulated GM-CSF release, whereas tubulin and actin depolymerizers suppressed K- or Q-stimulated secretion of GM-CSF. Depletion of extracellular or intracellular calcium ion inhibited the GM-CSF secretion upregulated by both K and Q. Furthermore, we showed that K- and Q-stimulated GM-CSF production involves PLC, PKC, and MEK1/2 activation. Treating human DC with the conditioned medium of K- or Q-incubated PC-3 cells increased chemotaxis of DC, which was significantly attenuated when the conditioned medium was incubated with the neutralizing antibody against GM-CSF. Taken together, our results demonstrate that K and Q activate an immune response in the prostate cancer cells by stimulating GM-CSF production, which in turn could result in the recruitment of DCs to the tumor site.

Intracellular signal transduction by the extracellular calcium-sensing receptor of Xenopus melanotrope cells

The extracellular calcium-sensing receptor (CaR) is expressed in various types of endocrine pituitary cell, but the intracellular mechanism this G protein-coupled receptor uses in these cells is not known. In the present study we investigated possible intracellular signal transduction pathway(s) utilized by the CaR of the endocrine melanotrope cells in the intermediate pituitary lobe of the South African-clawed toad Xenopus laevis. For this purpose, the effects of various pharmacological agents on CaR-evoked secretion of radiolabeled secretory peptides from cultured melanotrope cells were assessed. CaR-evoked secretion, induced by the potent CaR agonist L-phenylalanine (L-Phe), could not be inhibited by cholera toxin, nor by NPC-15437 and PMA, indicating that neither G(s)/PKA nor G(q)/PKC pathways are involved. However, pertussis toxin (G(i/o) protein inhibitor), genistein (inhibitor of PTKs), wortmannin/LY-294002 (PI3-K inhibitor) and U-0126 (inhibitor of extracellular signal-regulated kinase, ERK) all substantially inhibited CaR-evoked secretion, indicating that the Xenopus melanotrope cell possesses a PI3-K/MAPK system that plays some role in CaR-signaling. Since no direct effect of L-Phe on ERK phosphorylation could be shown it is concluded that CaR must act primarily through another, still unknown, signaling pathway in Xenopus melanotropes. Our results indicate that the PI3-K/MAPK system has a facilitating effect on CaR-induced secretion, possibly by sensitizing the CaR.

Extracellular calcium regulates parathyroid hormone-related peptide expression in osteoblasts and osteoblast progenitor cells

Parathyroid hormone-related peptide (PTHrP) has been shown to have anabolic effects on bone in women with postmenopausal osteoporosis. On the cellular level PTHrP promotes the recruitment of osteogenic cells and prevents apoptotic death of osteoblasts and osteocytes. The calcium concentration is considerably higher in the vicinity of resorbing osteoclasts than in the plasma. Therefore the osteoblasts are likely to be confronted by elevated extracellular calcium concentrations in the areas of resorptive activity. The present study was designed to assess the possibility that extracellular calcium could regulate PTHrP expression in osteoblastic cells. Adult human mesenchymal stem cells (hMSC) were cultured and differentiated by standard methods. The PTHrP release into the culture media was measured by an immunoradiometric assay and the expression of PTHrP, osteocalcin and Runx2 mRNA was assayed by real-time PCR. Increasing the extracellular calcium from 1 mM to 5 mM for 24 h resulted in a 4-6-fold increase in the PTHrP release. PTHrP mRNA was also increased by elevated calcium levels. The effect of calcium stimulation on PTHrP release could be seen within 60 min of treatment. The extracellular calcium sensing receptor (CaR) agonist neomycin mimicked the effects of calcium and the MEK/MAPK inhibitor PD98059 abolished the effect of calcium and neomycin. High extracellular calcium increased the mineralization of hMSC and the expression of osteocalcin, but this effect was not mimicked by neomycin. Our results show that in hMSC, elevated extracellular calcium levels increases both released PTHrP and PTHrP mRNA expression. The effect of calcium on PTHrP can be mimicked by activation of the CaR and can be diminished by inhibition of the MAPK signalling pathway.

Molecular biology of bone metastasis

Metastasis is a final stage of tumor progression. Breast and prostate cancer cells preferentially metastasize to bone, wherein they cause incurable osteolytic and osteoblastic lesions. The bone matrix is rich in factors, such as transforming growth factor-beta and insulin-like growth factors, which are released into the tumor microenvironment by osteolysis. These factors stimulate the growth of tumor cells and alter their phenotype, thus promoting a vicious cycle of metastasis and bone pathology. Physical factors within the bone microenvironment, including low oxygen levels, acidic pH, and high extracellular calcium concentrations, may also enhance tumor growth. These elements of the microenvironment are potential targets for chemotherapeutic intervention to halt tumor growth and suppress bone metastasis.

The extracellular calcium-sensing receptor (CaSR) on human esophagus and evidence of expression of the CaSR on the esophageal epithelial cell line (HET-1A)

Gastrointestinal reflux disease and eosinophilic esophagitis are characterized by basal cell hyperplasia. The extracellular calcium-sensing receptor (CaSR), a G protein-coupled receptor, which may be activated by divalent agonists, is expressed throughout the gastrointestinal system. The CaSR may regulate proliferation or differentiation, depending on cell type and tissue. The current experiments demonstrate the expression of the CaSR on a human esophageal epithelial cell line (HET-1A) and the location and expression of the CaSR in the human esophagus. CaSR immunoreactivity was seen in the basal layer of normal human esophagus. CaSR expression was confirmed in HET-1A cells by RT-PCR, immunocytochemistry, and Western blot analysis. CaSR stimulation by extracellular calcium or agonists, such as spermine or Mg(2+), caused ERK1 and 2 activation, intracellular calcium concentration ([Ca(2+)](i)) mobilization (as assessed by microspecfluorometry using Fluo-4), and secretion of the multifunctional cytokine IL-8 (CX-CL8). HET-1A cells transiently transfected with small interfering (si)RNA duplex against the CaSR manifested attenuated responses to Ca(2+) stimulation of phospho- (p)ERK1 and 2, [Ca(2+)](i) mobilization, and IL-8 secretion, whereas responses to acetylcholine (ACh) remained sustained. An inhibitor of phosphatidylinositol-specific phospholipase C (PI-PLC) (U73122) blocked CaSR-stimulated [Ca(2+)](i) release. We conclude that the CaSR is present on basal cells of the human esophagus and is present in a functional manner on the esophageal epithelial cell line, HET-1A.

Inhibition of epidermal growth factor receptor signalling reduces hypercalcaemia induced by human lung squamous-cell carcinoma in athymic mice

The purpose of this study was to evaluate the role of the epidermal growth factor receptor (EGFR) in parathyroid hormone-related protein (PTHrP) expression and humoral hypercalcaemia of malignancy (HHM), using two different human squamous-cell carcinoma (SCC) xenograft models. A randomised controlled study in which nude mice with RWGT2 and HARA xenografts received either placebo or gefitinib 200 mg kg⁻¹ for 3 days after developing HHM. Effectiveness of therapy was evaluated by measuring plasma calcium and PTHrP, urine cyclic AMP/creatinine ratios, and tumour volumes. The study end point was at 78 h. The lung SCC lines, RWGT2 and HARA, expressed high levels of PTHrP mRNA as well as abundant EGFR protein, but very little erbB2 or erbB3. Both lines expressed high transcript levels for the EGFR ligand, amphiregulin (AREG), as well as, substantially lower levels of transforming growth factor-α (TGF-α), and heparin binding-epidermal growth factor (HB-EGF) mRNA. Parathyroid hormone-related protein gene expression in both lines was reduced 40–80% after treatment with 1 μM of EGFR tyrosine kinase inhibitor PD153035 and precipitating antibodies to AREG. Gefitinib treatment of hypercalcaemic mice with RWGT2 and HARA xenografts resulted in a significant reduction of plasma total calcium concentrations by 78 h. Autocrine AREG stimulated the EGFR and increased PTHrP gene expression in the RWGT2 and HARA lung SCC lines. Inhibition of the EGFR pathway in two human SCC models of HHM by an anilinoquinazoline demonstrated that the EGFR tyrosine kinase is a potential target for antihypercalcaemic therapy.