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

The extracellular calcium-sensing receptor reciprocally regulates the secretion of BMP-2 and the BMP antagonist Noggin in colonic myofibroblasts

To understand whether postprandial extracellular Ca(2+) (Ca(o)(2+)) changes were related to intestinal epithelial homeostasis, we performed array analysis on extracellular calcium-sensing receptor (CaSR)-expressing colonic myofibroblasts (18Co cells) and observed increases in bone morphogenetic protein (BMP)-2 transcripts. The present experiments demonstrated that regulated secretion of BMP-2 occurs in response to CaSR activation of these cells and revealed a new property of BMP-2 on the intestinal barrier. Activation by Ca(o)(2+), spermine, GdCl(3), or neomycin sulfate of 18Co cells or primary isolates of myofibroblasts from the normal human colon stimulated both the synthesis (RT-PCR) and secretion (ELISA) of BMP-2. Transient transfection with short interfering RNA against CaSR completely inhibited BMP-2 secretion. Transient transfection with dominant negative CaSR (R185Q) increased the EC(50) of Ca(o)(2+) (5.7 vs. 2.3 mM). Upregulation of BMP-2 transcript and secretion occurring within 3 h of CaSR activation was prevented by actinomycin D. CaSR-mediated BMP-2 synthesis and secretion required phosphatidylinositol 3-kinase activation (as assessed by phospho-Akt generation). Exogenous BMP-2 and conditioned medium from CaSR-stimulated 18Co cells accelerated restitution in wounded postconfluent Caco-2 cells. Exogenous BMP-2 and conditioned medium from CaSR-stimulated 18Co cells increased the transepithelial resistance of low- and high-resistance T-84 epithelial monolayers. CaSR stimulation of T-84 epithelia and colonic myofibroblasts downregulated the BMP family antagonist Noggin, as assessed by RT-PCR and Western blot analysis. Together, our data suggest that the CaSR mediates the effective concentration of BMP-2 in the intestine, which leads to enhanced repair and barrier development.

Calcium receptor stimulates chemotaxis and secretion of MCP-1 in GnRH neurons in vitro: potential impact on reduced GnRH neuron population in CaR-null mice

The factors controlling the migration of mammalian gonadotropin-releasing hormone (GnRH) neurons from the nasal placode to the hypothalamus are not well understood. We studied whether the extracellular calcium-sensing receptor (CaR) promotes migration/chemotaxis of GnRH neurons. We demonstrated expression of CaR in GnRH neurons in the murine basal forebrain and in two GnRH neuronal cell lines: GT1-7 (hypothalamus derived) and GN11 (olfactory bulb derived). Elevated extracellular Ca(2+) concentrations promoted chemotaxis of both cell types, with a greater effect in GN11 cells. This effect was CaR mediated, as, in both cell types, overexpression of a dominant-negative CaR attenuated high Ca(2+)-stimulated chemotaxis. We also demonstrated expression of a beta-chemokine, monocyte chemoattractant protein-1 (MCP-1), and its receptor, CC motif receptor-2 (CCR2), in the hypothalamic GnRH neurons as well as in GT1-7 and GN11 cells. Exogenous MCP-1 stimulated chemotaxis of both cell lines in a dose-dependent fashion; the effect was greater in GN11 than in GT1-7 cells, consistent with the higher CCR2 mRNA levels in GN11 cells. Activating the CaR stimulated MCP-1 secretion in GT1-7 but not in GN11 cells. MCP-1 secreted in response to CaR stimulation is biologically active, as conditioned medium from GT1-7 cells treated with high Ca(2+) promoted chemotaxis of GN11 cells, and this effect was partially attenuated by a neutralizing antibody to MCP-1. Finally, in the preoptic area of anterior hypothalamus, the number of GnRH neurons was approximately 27% lower in CaR-null mice than in mice expressing the CaR gene. We conclude that the CaR may be a novel regulator of GnRH neuronal migration likely involving, in part, MCP-1.

Extracellular calcium as a candidate mediator of prostate cancer skeletal metastasis

Prostate cancer almost exclusively metastasizes to skeletal sites, indicating that the bone provides a favorable microenvironment for its localization and progression. A natural yet understudied factor in bone that could facilitate tumor localization is elevated extracellular calcium ([Ca2+]o). The present study found that elevated [Ca2+]o (2.5 mmol/L) enhanced proliferation of skeletal metastatic prostate cell lines (PC-3 and C4-2B), but not the nonskeletal metastatic, epithelial-derived prostate cell line LNCaP. The proliferative effect of elevated [Ca2+]o was associated with higher expression of the calcium-sensing receptor (CaSR), a heterotrimeric G-protein-coupled receptor that is the predominant cell-surface sensor for [Ca2+]o. Knockdown of the CaSR via RNA interference reduced cell proliferation in vitro and metastatic progression in vivo. CaSR signaling in PC-3 cells was evaluated by measuring the elevated [Ca2+]o-dependent inhibition of cyclic AMP accumulation, induced by either prostaglandin E2 or forskolin. Elevated [Ca2+]o stabilized expression of cyclin D1, a protein required for cell cycle transition. Furthermore, elevated [Ca2+]o triggered activation of the Akt signaling pathway and enhanced PC-3 cell attachment. Both pertussis toxin (a G-protein inhibitor) and LY294002 (an inhibitor of Akt signaling) reduced cell attachment. These data suggest that elevated [Ca2+]o following increased bone remodeling could facilitate metastatic localization of prostate cancer via the CaSR and the Akt signaling pathway. Taken together, [Ca2+]o is a candidate mediator of prostate cancer bone metastasis.

Calcium insensitivity of FA-6, a cell line derived from a pancreatic cancer associated with humoral hypercalcemia, is mediated by the significantly reduced expression of the Calcium Sensitive Receptor transduction component p38 MAPK

The Calcium-Sensing Receptor is a key component of Calcium/Parathyroid hormone homeostatic system that helps maintain appropriate plasma Ca²⁺ concentrations. It also has a number of non-homeostatic functions, including cell cycle regulation through the p38 MAPK pathway, and recent studies have indicated that it is required for Ca²⁺ mediated growth arrest in pancreatic carcinoma cells. Some pancreatic cancers produce pathogenic amounts of parathyroid like hormones, however, which significantly increase Ca²⁺ plasma concentrations and might be expected to block further cell growth. In this study we have investigated the expression and function of the p38 MAPK signaling pathway in Ca²⁺ sensitive (T3M-4) and insensitive (FA6) pancreatic cancer cell lines. FA-6 cells, which are derived from a pancreatic adenocarcinoma that secretes a parathyroid hormone related peptide, exhibit only very low levels of p38 MAPK expression, relative to T3M-4 cells. Transfecting FA-6 cells with a p38 MAPK expression construct greatly increases their sensitivity to Ca²⁺. Furthermore, the reduction of p38 MAPK in T3M-4 cells significantly reduces the extent to which high levels of Ca²⁺ inhibit proliferation. These results suggest that the low levels of p38 MAPK expression in FA-6 cells may serve to reduce their sensitivity to high concentrations of external Ca²⁺ that would otherwise block proliferation.

Extracellular calcium sensing in rat aortic vascular smooth muscle cells

Extracellular calcium (Ca(2+)(o)) can act as a first messenger in many cell types through a G protein-coupled receptor, calcium-sensing receptor (CaR). It is still debated whether the CaR is expressed in vascular smooth muscle cells (VSMCs). Here, we report the expression of CaR mRNA and protein in rat aortic VSMCs and show that Ca(2+)(o) stimulates proliferation of the cells. The effects of Ca(2+)(o) were attenuated by pre-treatment with MAPK kinase 1 (MEK1) inhibitor, as well as an allosteric modulator, NPS 2390. Furthermore, stimulation of the VSMCs with Ca(2+)(o)-induced phosphorylation of ERK1/2, but surprisingly did not cause inositol phosphate accumulation. We were not able to conclusively state that the CaR mediates Ca(2+)(o)-induced cell proliferation. Rather, an additional calcium-sensing mechanism may exist. Our findings may be of importance with regard to atherosclerosis, an inflammatory disease characterized by abnormal proliferation of VSMCs and high local levels of calcium.

Effects of calcium-sensing receptor on the secretion of parathyroid hormone-related peptide and its impact on humoral hypercalcemia of malignancy

The extracellular calcium-sensing receptor (CaR) plays a key role in the defense against hypercalcemia by "sensing" extracellular calcium (Ca2+(o)) levels in the parathyroid and kidney, the key organs maintaining systemic calcium homeostasis. However, CaR function can be aberrant in certain pathophysiological states, e.g., in some types of cancers known to produce humoral hypercalcemia of malignancy (HHM) in humans and animal models in which high Ca2+(o), via the CaR, produces a homeostatically inappropriate stimulation of parathyroid hormone-related peptide (PTHrP) secretion from these tumors. Increased levels of PTHrP set a cycle in motion whereby elevated systemic levels of Ca2+(o) resulting from its increased bone-resorptive and positive renal calcium-reabsorbing effects give rise to hypercalcemia, which in turn begets worsening hypercalcemia by stimulating further release of PTHrP by the cancer cells. I review the relationship between CaR activation and PTHrP release in normal and tumor cells giving rise to HHM and/or malignant osteolysis and the actions of the receptor on key cellular events such as proliferation, angiogenesis, and apoptosis of cancer cells that will favor tumor growth and osseous metastasis. I also illustrate diverse signaling mechanisms underlying CaR-stimulated PTHrP secretion and other cellular events in tumor cells. Finally, I raise several necessary questions to demonstrate the roles of the receptor in promoting tumors and metastases that will enable consideration of the CaR as a potential antagonizing/neutralizing target for the treatment of HHM.

Calcium receptor is functionally expressed in rat neonatal ventricular cardiomyocytes

Both intra- and extracellular calcium play multiple roles in the physiology and pathophysiology of cardiomyocytes, especially in stimulus-contraction coupling. The intracellular calcium level is closely controlled through the concerted actions of calcium channels, exchangers, and pumps; however, the expression and function(s) of the so-called calcium-sensing receptor (CaR) in the heart remain less well characterized. The CaR is a seven-transmembrane receptor, which, in response to noncovalent binding of extracellular calcium, activates intracellular effectors, including G proteins and extracellular signal-regulated kinases (ERK1/2). We have shown that cultured neonatal cardiomyocytes express the CaR messenger RNA and the CaR protein. Furthermore, increasing concentrations of extracellular calcium and a type II CaR activator “calcimimetic” caused inositol phosphate (IP) accumulation, downregulated tritiated thymidine incorporation, and supported ERK1/2 phosphorylation, suggesting that the CaR protein is functionally active. Interestingly, the calcimimetic induced a more rapid ERK1/2 phosphorylation than calcium and left-shifted the IP concentration-response curve for extracellular calcium, supporting the hypothesis that CaR is functionally expressed in cardiac myocytes. This notion was underscored by studies using a virus containing a dominant-negative CaR construct, because this protein blunted the calcium-induced IP response. In conclusion, we have shown that the CaR is functionally expressed in neonatal ventricular cardiomyocytes and that the receptor activates second messenger pathways, including IP and ERK, and decreases DNA synthesis. A specific calcium-sensing receptor on cardiac myocytes could play a role in regulating cardiac development, function, and homeostasis.

Calcium sensing receptors and calcium oscillations: calcium as a first messenger

Calcium sensing receptors (CaR) are unique among G-protein-coupled receptors (GPCRs) since both the first (extracellular) and second (intracellular) messengers are Ca(2+). CaR serves to translate small fluctuations in extracellular Ca(2+) into intracellular Ca(2+) oscillations. In many cells and tissues, CaR also acts as a coincidence detector, sensing both changes in extracellular Ca(2+) plus the presence of various allosteric activators including amino acids, polyamines, and/or peptides. CaR oscillations are uniquely shaped by the activating agonist, that is, Ca(2+) triggers sinusoidal oscillations while Ca(2+) plus phenylalanine trigger transient oscillations of lower frequency. The distinct oscillation patterns generated by Ca(2+)versus Ca(2+) plus phenylalanine are the results of activation of distinct signal transduction pathways. CaR is a member of Family C GPCRs, having a large extracellular agonist binding domain, and functioning as a disulfide-linked dimer. The CaR dimer likely can be driven to distinct active conformations by various Ca(2+) plus modulator combinations, which can drive preferential coupling to divergent signaling pathways. Such plasticity with respect to both agonist and signaling outcomes allows CaR to uniquely contribute to the physiology of organs and tissues where it is expressed. This chapter will examine the structural features of CaR, which contribute to its unique properties, the nature of CaR-induced intracellular Ca(2+) signals and the potential role(s) for CaR in development and differentiation.

High calcium activates the EGF receptor potentially through the calcium-sensing receptor in Leydig cancer cells

Epidermal growth factor (EGF) plays an important role in the physiology and pathophysiology of the Leydig cell. In H-500 rat Leydig cancer cells, a model for humoral hypercalcemia of malignancy (HHM), we previously showed that the calcium-sensing receptor (CaR) stimulates PTHrP release and proliferation, both involving multiple mitogen-activated protein kinases. An emerging concept of signaling by G-protein coupled receptors (GPCR) is that it occurs via transactivation of receptor tyrosine kinases. Therefore, we investigated whether stimulation with calcium activates the EGFR in H-500 Leydig cancer cells. We show that treatment of H-500 cells with Ca(2+) results in EGFR phosphorylation. The CaR-induced activation of ERK1/2, induction of PTHrP release and stimulation of cellular proliferation in H-500 cells are likewise mediated, in large part, through the EGFR. In conclusion, the calcium activates the EGFR, possibly through the CaR, to regulate downstream signaling events and important biological functions in a model of HHM.

Calcium-sensing receptor activation induces nitric oxide production in H-500 Leydig cancer cells

Nitric oxide (NO) is a versatile second messenger. NO is produced by Leydig cells, where NO is a negative regulator of steroidogenesis. In cancer cells, NO is thought to have mutagenic and proliferative effects. We have previously shown that the calcium-sensing receptor (CaR) has promalignant effects in rat H-500 Leydig cancer cells, a model for humoral hypercalcemia of malignancy. Calcium, the major physiological ligand of the CaR, is a recognized intracellular cofactor in the process of NO production by virtue of its positive modulation of neuronal and endothelial nitric oxide synthase (NOS), but importantly, not of inducible (i) NOS activity. iNOS activity is regulated by changes in its expression level. Therefore, we investigated whether CaR activation changes iNOS expression. We found that high extracellular calcium (Cao2+) upregulates the level of mRNA for iNOS, whereas no change was seen in neuronal or endothelial NOS, as assessed by microarray and real-time PCR, respectively. The high Cao2+-induced iNOS upregulation was also detected by Northern and Western blotting. By quantitative real-time PCR, we showed that calcium maximally upregulates iNOS at 18 h. The effect of calcium was abolished by overexpression of a dominant-negative CaR (R185Q), confirming that the effect of Cao2+ was mediated by the CaR. Cells treated with high calcium had higher NO production than those treated with low calcium, as detected with the NO-specific DAF2-AM dye. This was confirmed in single-cell fluorescence determinations using confocal microscopy. In conclusion, high calcium upregulates the levels of iNOS mRNA and protein as well as NO production in H-500 cells, and the effect of Cao2+ on iNOS expression is mediated by the CaR.

Calcium sensing by the mammary gland

Calcium is an important nutrient that is secreted into milk in quantities that put a considerable stress upon maternal calcium homeostasis. Here we summarize the evidence that two important entities, the extracellular calcium-sensing receptor (CaR) and parathyroid hormone-related protein (PTHrP) are involved in a feedback loop that regulates calcium fluxes to the mammary gland. The CaR may also play a role in regulating milk secretion, and may regulate the proliferation of normal and neoplastic mammary epithelial cells. Finally, the relationship between the CaR and PTHrP in breast cancer cells may promote the formation of osteolytic bone metastases.

The effects of salmon calcitonin-induced hypocalcemia on bone metabolism in ovariectomized rats

The ovariectomized rat has proved to be a most useful model for preclinical testing of potential therapies for osteoporosis. We describe the immediate effects of a single treatment with salmon calcitonin (sCT) on calcium homeostasis and bone turnover markers in 6-month-old sham and ovariectomized (ovx) rats at 15 days postovariectomy. Rats were fasted for 24 h prior to and following administration of 0.3 microg/kg body weight sCT. Blood specimens were collected at 0 (pretreatment), 2, 4, and 8 h. Urine samples were collected during the intervening periods. sCT treatment produced a decrease in blood ionized calcium at 2 h posttreatment in sham and ovx rats (P < 0.001), which was exaggerated in the ovx rats (P < 0.001). Increased parathyroid hormone (PTH) levels (P < 0.001) accompanied the hypocalcemia in ovx rats. Furthermore, PTH levels were significantly higher in ovx rats compared with sham rats for the same ionized calcium range of 1.275-1.300 mmol/l (P < 0.05). sCT treatment in sham rats increased urine hydroxyproline (UHyp) at 6 h posttreatment (P < 0.01). In conclusion, the calcitonin-induced hypocalcemia and secondary hyperparathyroidism was more pronounced in the ovariectomized rats, consistent with the actions of calcitonin in states of increased bone turnover induced by estrogen deficiency. This study highlights the importance of considering the actions of PTH and estrogen status when interpreting changes in calcium homeostasis and bone turnover following treatment with calcitonin in rodent models and provides further evidence for a potential role of estrogen in parathyroid function.

Evidence for a functional calcium-sensing receptor that modulates myogenic tone in rat subcutaneous small arteries

Myogenic tone of small arteries is dependent on the presence of extracellular calcium (Ca(o)(2+)), and, recently, a receptor that senses changes in Ca(2+), the calcium-sensing receptor (CaR), has been detected in vascular tissue. We investigated whether the CaR is involved in the regulation of myogenic tone in rat subcutaneous small arteries. Immunoblot analysis using a monoclonal antibody against the CaR demonstrated its presence in rat subcutaneous arteries. To determine whether the CaR was functionally active, segments of artery (< 250 microm internal diameter) mounted in a pressure myograph with an intraluminal pressure of 70 mmHg were studied after the development of myogenic tone. Increasing Ca(o)(2+) concentration ([Ca(2+)](o)) cumulatively from 0.5 to 10 mM induced an initial constriction (0.5-2 mM) followed by dilation (42 +/- 5% loss of tone). The dose-dependent dilation was mimicked by other known CaR agonists including magnesium (1-10 mM) and the aminoglycosides neomycin (0.003-10 mM) and kanamycin (0.003-3 mM). PKC activation with the phorbol ester phorbol-12,13-dibutyrate (20nM) inhibited the dilation induced by high [Ca(2+)](o) or neomycin, whereas inhibition of PKC with GF109203X (10 microM) increased the responses to Ca(o)(2+) or neomycin, consistent with the role of PKC as a negative regulator of the CaR. We conclude that rat subcutaneous arteries express a functionally active CaR that may be involved in the modulation of myogenic tone and hence the regulation of peripheral vascular resistance.

Activating calcium-sensing receptor mutation in the mouse is associated with cataracts and ectopic calcification

The extracellular calcium-sensing receptor (CaSR) plays a pivotal role in the regulation of extracellular calcium such that abnormalities, which result in a loss or gain of function, lead to hypercalcemia or hypocalcemia, respectively, in patients. Mice carrying CaSR knockout alleles develop hypercalcemia that mimics the disorders observed in humans. To date, there is no mouse model for an activating CaSR mutation. Here, we describe such a mouse model, named Nuf, originally identified for having opaque flecks in the nucleus of the lens in a screen for eye mutants. Nuf mice also display ectopic calcification, hypocalcemia, hyperphosphatemia, and inappropriately reduced levels of plasma parathyroid hormone. These features are similar to those observed in patients with autosomal dominant hypocalcemia. Inheritance studies of Nuf mice revealed that the trait was transmitted in an autosomal-dominant manner, and mapping studies located the locus to chromosome 16, in the vicinity of the CaSR gene (Mouse Genome Database symbol Gprc2a). DNA sequence analysis revealed the presence of a Gprc2a missense mutation, Leu723Gln. Transient expression of wild-type and mutant CaSRs in human embryonic kidney 293 cells demonstrated that the mutation resulted in a gain of function of the CaSR, which had a significantly lower EC(50). Thus, our results have identified a mouse model for an activating CaSR mutation, and the development of ectopic calcification and cataract formation, which tended to be milder in the heterozygote Nuf mice, indicates that an evaluation for such abnormalities in autosomal dominant hypocalcemia patients who have activating CaSR mutations is required.

Calcium-sensing receptor activation stimulates parathyroid hormone-related protein secretion in prostate cancer cells: role of epidermal growth factor receptor transactivation

We have previously reported that high extracellular Ca2+ stimulates parathyroid hormone-related protein (PTHrP) release from human prostate and breast cancer cell lines as well as from H-500 rat Leydig cancer cells, an action mediated by the calcium-sensing receptor (CaR). Activating the CaR leads to phosphorylation of mitogen-activated protein kinases (MAPKs) that participate in PTHrP synthesis and secretion. Because the CaR is a G protein-coupled receptor (GPCR), it is likely to transactivate the epidermal growth factor receptor (EGFR) or the platelet-derived growth factor receptor (PDGFR). In this study, we hypothesized that activation of the CaR transactivates the EGFR or PDGFR, and examined whether transactivation affects PTHrP secretion in PC-3 human prostate cancer cells. Using Western analysis, we observed that an increase in extracellular Ca2+ resulted in delayed activation of extracellular signal-regulated kinase (ERK) in PC-3 cells. Pre-incubation with AG1478 (an EGFR kinase inhibitor) or an EGFR neutralizing antibody inhibited the high Ca2+ -induced phosphorylation of ERK1/2. GM6001, a pan matrix metalloproteinase (MMP) inhibitor, also partially suppressed the ERK activation, but AG1296 (a PDGFR kinase inhibitor) did not. High extracellular Ca2+ stimulates PTHrP release during a 6-h incubation (1.5- to 2.5- and 3- to 4-fold increases in 3.0 and 7.5 mM Ca2+, respectively). When cells were preincubated with AG1478, GM6001, or an antihuman heparin-binding EGF (HB-EGF) antibody, PTHrP secretion was significantly inhibited under basal as well as high Ca2+ conditions, while AG1296 had no effect on PTHrP secretion. Taken together, these findings indicate that activation of the CaR transactivates the EGFR, but not the PDGFR, leading to phosphorylation of ERK1/2 and resultant PTHrP secretion, although CaR-EGFR-ERK might not be the only signaling pathway for PTHrP secretion. This transactivation is most likely mediated by activation of MMP and cleavage of proheparin-binding EGF (proHB-EGF) to HB-EGF.

Expression of pituitary tumor transforming gene (PTTG) and its binding protein in human astrocytes and astrocytoma cells: function and regulation of PTTG in U87 astrocytoma cells

Human securin, pituitary tumor transforming gene (PTTG), is a protooncogene. Here we report expressions of PTTG and its interacting protein, PTTG-binding factor in human astrocytic cells. PTTG expression was higher in malignant cells than in primary astrocytes, whereas PTTG-binding factor was not. Using a xenotransplantable, glioma cell line (U87), we observed that knocking down PTTG mRNA by RNA silencing inhibited serum-induced proliferation by approximately 50%. Furthermore, in U87 cells PTTG expression was up-regulated by promalignant ligands epithelial growth factor (EGF) and TGFalpha, both at the protein and mRNA levels. PTTG induction by EGF receptor (EGFR) ligands could be blocked by the specific EGFR inhibitor, AG1478. Hepatocyte growth factor (HGF) also induced PTTG but to a lesser extent than EGF. Although EGF stimulates HGF secretion in U87 cells, the effect of EGF on PTTG mRNA expression is independent of HGF as neutralizing antibody against HGF failed to abolish EGF-induced up-regulation of PTTG mRNA. PTTG mRNA was unchanged by incubating U87 cells with the promalignant growth factor TGFbeta, apoptosis inducing TNFalpha and ligands for nuclear receptors, such as retinoic acid and retinoid X receptors and peroxisome proliferator-activated receptor-gamma, known for their growth-inhibitory and apoptosis-inducing effects on gliomas. In addition, 17beta-estradiol and Ca2+, known to activate PTTG expression, did not change PTTG mRNA levels in U87 cells. In summary, we show higher PTTG expression in astrocytoma than normal astrocytes and secondly, PTTG is involved in glioma cell growth. Finally, regulation of its expression has glioma-specific features and is selectively regulated by promalignant cytokines including EGFR ligands and HGF.

Calcium receptor-mediated intracellular signalling

As a G protein-coupled receptor (GPCR), the extracellular calcium-sensing receptor (CaR) responds to changes in extracellular free calcium concentration by inducing intracellular signalling. These CaR-induced signals then specifically modulate cellular functions such as parathyroid hormone secretion from the parathyroid glands and calcium reabsorption in the kidney and thus to understand how the CaR functions one must understand how it signals. CaR-induced signalling involves intracellular Ca2+ mobilisation/oscillations as well as the activation of various phospholipases and protein kinases and the suppression of cAMP formation. This review will detail the intracellular pathways by which the CaR is believed to elicit its physiological functions and summarises the evidence for cell- and agonist-specific differential signalling.

Diseases associated with the extracellular calcium-sensing receptor

The human calcium-sensing receptor (CaSR) is a 1078 amino acid cell surface protein, which is predominantly expressed in the parathyroids and kidney, and is a member of the family of G protein-coupled receptors. The CaSR allows regulation of parathyroid hormone (PTH) secretion and renal tubular calcium reabsorption in response to alterations in extracellular calcium concentrations. The human CaSR gene is located on chromosome 3q21.1 and loss-of-function CaSR mutations have been reported in the hypercalcaemic disorders of familial benign (hypocalciuric) hypercalcaemia (FHH, FBH or FBHH) and neonatal severe primary hyperparathyroidism (NSHPT). However, some individuals with loss-of-function CaSR mutations remain normocalcaemic. In addition, there is genetic heterogeneity amongst the forms of FHH. Thus, the majority of FHH patients have loss-of-function CaSR mutations, and this is referred to as FHH type 1. However, in one family, the causative gene for FHH is located on 19p13, referred to as FHH type 2, and in another family it is located on 19q13, referred to as FHH type 3. Gain-of-function CaSR mutations have been shown to result in autosomal dominant hypocalcaemia with hypercalciuria (ADHH) and Bartter's syndrome type V. CaSR auto-antibodies have been found in FHH patients who did not have loss-of-function CaSR mutations, and in patients with an acquired form (i.e. autoimmune) of hypoparathyroidism. Thus, abnormalities of the CaSR are associated with three hypercalcaemic and three hypocalcaemic disorders.

Mitogenic action of calcium-sensing receptor on rat calvarial osteoblasts

The parathyroid calcium-sensing receptor (CaR) plays a nonredundant role in systemic calcium homeostasis. In bone, Ca(2+)(o), a major extracellular factor in the bone microenvironment during bone remodeling, could potentially serve as an extracellular first messenger, acting via the CaR, that stimulates the proliferation of preosteoblasts and their differentiation to osteoblasts (OBs). Primary digests of rat calvarial OBs express the CaR as assessed by RT-PCR, Northern, and Western blot analysis, and immunocolocalization of the CaR with the OB marker cbfa-1. Real-time PCR revealed a significant increase in CaR mRNA in 5- and 7-d cultures compared with 3-d cultures post harvesting. High Ca(2+)(o) did not affect the expression of CaR mRNA during this time but up-regulated cyclin D (D1, D2, and D3) genes, which are involved in transition from the G1 to the S phase of the cell cycle, as well as the early oncogenes, c-fos and early growth response-1; high Ca(2+)(o) did not, however, alter IGF-I expression, a mitogenic factor for OBs. The high Ca(2+)(o)-dependent increase in the proliferation of OBs was attenuated after transduction with a dominant-negative CaR (R185Q), confirming that the effect of high Ca(2+)(o) is CaR mediated. Stimulation of proliferation by the CaR involves the Jun-terminal kinase (JNK) pathway, as high Ca(2+)(o) stimulated the phosphorylation of JNK in a CaR-mediated manner, and the JNK inhibitor SP600125 abolished CaR-induced proliferation. Our data, therefore, show that the parathyroid/kidney CaR expressed in rat calvarial OBs exerts a mitogenic effect that involves activation of the JNK pathway and up-regulation of several mitogenic genes.

Regulation of a Ca2+-activated K+ channel by calcium-sensing receptor involves p38 MAP kinase

By using pharmacological and molecular approaches, we previously showed that the G-protein-coupled, extracellular calcium (Ca2+(o))-sensing receptor (CaR) regulates a large-conductance (approximately 140 pS), Ca(2+)-activated K+ channel [IK(Ca); CAKC] in U87 astrocytoma cells. Here we show that elevated Ca2+(o) stimulates extracellular-signal-regulated kinase (ERK1/2) and p38 MAP kinase (MAPK). The effect of high Ca2+(o) on p38 MAPK but not ERK1/2 is CaR mediated, insofar as transduction with a dominant-negative CaR (R185Q) using recombinant adeno-associated virus (rAAV) attenuated the activation of p38 MAPK but not of ERK1/2. p38 MAPK activation by the CaR is likely to be protein kinase C (PKC) independent, in that the pan-PKC inhibitor GF109203X failed to abolish the high-Ca2+(o)-induced phosphorylation of p38 MAPK. Consistently with our data on the activation of this kinase, we observed that inhibiting p38 MAPK blocked the activation of the CAKC induced by the specific pharmacological CaR activator NPS R-467. In contrast, inhibiting MEK1 only transiently inhibited the activation of this K+ channel by NPS R-467, despite the continued presence of the antagonist. Similarly to the lack of any effect of the PKC inhibitor on the activation of ERK1/2 and p38 MAPK, inhibiting PKC had no effect on NPS R-467-induced activation of this channel. Therefore, our data show that the CaR, acting via p38 MAPK, regulates a large-conductance CAKC in U87 cells, a process that is PKC independent. Large-conductance CAKCs play an important role in the regulation of cellular volume, so our results have important implications for glioma cell volume regulation.

Calcium-sensing receptor induces proliferation through p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase but not extracellularly regulated kinase in a model of humoral hypercalcemia of malignancy

Using H-500 rat Leydig cancer cells as a model of humoral hypercalcemia of malignancy (HHM), we previously showed that high Ca(2+) induces PTH-related peptide (PTHrP) secretion via the calcium-sensing receptor (CaR) and mitogen- and stress-activated kinases, e.g. MAPK kinase 1 (MEK1), p38 MAPK, and stress-activated protein kinase 1/c-Jun N-terminal kinase. Because cellular proliferation is a hallmark of malignancy, we studied the role of the CaR in regulating the proliferation of H-500 cells. Elevated Ca(2+) has a mitogenic effect on these cells that is mediated by the CaR, because the calcimimetic NPS R-467 also induced proliferation. Inhibition of phosphatidylinositol 3-kinase (PI3K) and p38 MAPK but not MEK1 abolished the mitogenic effect. Activation of PI3K by elevated Ca(2+) was documented by phosphorylation of its downstream kinase, protein kinase B. Because protein kinase B activation promotes cell survival, we speculated that elevated Ca(2+) might protect H-500 cells against apoptosis. Using terminal uridine deoxynucleotidyl nick end labeling staining, we demonstrated that high Ca(2+) (7.5 mM) and NPS R-467 indeed protect cells against apoptosis induced by serum withdrawal compared with low Ca(2+) (0.5 mM). Because the CaR induces PTHrP secretion, it is possible that the mitogenic and antiapoptotic effects of elevated Ca(2+) could be indirect and mediated via PTHrP. However, blocking the type 1 PTH receptor with PTH (7-34) peptide did not alter either high Ca(2+)-induced proliferation or protection against apoptosis. Taken together, our data show that activation of PI3K and p38 MAPK but not of MEK1/ERK by the CaR promotes proliferation of H-500 cells as well as affords protection against apoptosis. These effects are likely direct without the involvement of PTHrP in an autocrine mode.

Parathyroid hormone-related protein expression is correlated with clinical course in patients with glial tumors

BACKGROUND

Parathyroid hormone-related protein (PTHrP) expression modulates cell survival in a number of human solid tumors. Although PTHrP is expressed in normal developing and neoplastic central nervous system tissue, clinical data indicating the importance of this protein with respect to local control and/or survival in patients with glial tumors are scarce.

METHODS

Using a standard immunoperoxidase technique, the authors examined PTHrP expression in a population of 51 patients with Daumas-Duport Grade II-IV astrocytomas over a 15-year period. Both local control and survival were calculated from the date of definitive irradiation to the last time of known follow-up examination using the actuarial method. PTHrP expression was scored on examination under 40x magnification, with the incidence of cellular staining averaged over 10 high-power fields. The intensity and extent of staining were characterized semiquantitatively using the standard World Health Organization classification criteria. The median follow-up duration was approximately 5.5 years. Multivariate analyses were performed to ascertain the statistical significance of several standard clinicohistopatholgic factors (Karnofsky functional status, age, gender, extent of surgical resection, radiotherapy dose, grade, and PTHrP expression) with respect to local control and survival. P < 0.05 was considered indicative of statistical significance.

RESULTS

Patients with high levels of PTHrP expression had significantly lower glial tumor local control rates and corresponding decreases in progression-free and overall actuarial survival after definitive irradiation (P < 0.01). In a Cox 3-variable model, the PTHrP staining score was independent of tumor grade or Karnofsky functional status. It is notable that the strongest predictor of survival was tumor grade (P < 0.001).

CONCLUSIONS

PTHrP may be an important adjunct to standard immunopathologic criteria in the determination of glial tumor responses. A number of mechanisms were explored to derive a more mechanistic understanding of these translational results. Subsequent prospective studies involving larger patient populations will be necessary before findings can be translated to clinical practice.

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.

Calcium-sensing receptor induces messenger ribonucleic acid of human securin, pituitary tumor transforming gene, in rat testicular cancer

Pituitary tumor transforming gene (PTTG), the human ortholog of securin, is an oncogene. Few normal tissues express PTTG, although in the testis, it is more abundantly expressed. In cancer, however, its wide expression has been directly correlated with the proliferation and angiogenesis, although very little is known about the overall regulation of the PTTG gene. In this study, we investigate the role of the calcium-sensing receptor (CaR), a G protein-coupled receptor (GPCR), in regulating PTTG in a widely used model of humoral hypercalcemia of malignancy, the rat H-500 Leydig cell testicular cancer. We show that extracellular calcium (Ca2+o) up-regulates PTTG mRNA. This up-regulation has a rapid onset, starting at 0.5 h, and remains up-regulated until 40 h. The up-regulation was also Ca2+o concentration dependent, with increases (mean +/- se) of 4.22 +/- 1.61-fold, 5.11 +/- 1.11-fold, and 5.64 +/- 1.92-fold at 5, 7.5, and 10 mm calcium, respectively, compared with 0.5 mm Ca2+o. This effect was abolished by overexpression of a dominant-negative CaR (R185Q), thereby confirming that the effect of high Ca2+o is CaR mediated. Another GPCR agonist, ADP, had no effect on PTTG expression. Because PTTG has been reported to induce angiogenesis, we investigated the effect of elevated Ca2+o on vascular endothelial growth factor (VEGF) expression. Indeed high calcium up-regulated VEGF mRNA by 1.59 +/- 0.22-fold. In conclusion, we show for the first time that a GPCR, the CaR, stimulates the synthesis of PTTG mRNA in a nonmetastasizing model for humoral hypercalcemia of malignancy and, in the process, might induce angiogenesis via VEGF.