The hunting of the snark: the elusive calcium receptor(s)

Raisz, Lawrence G.

doi:10.1172/jci18235

Cited by 10 publications

(6 citation statements)

References 22 publications

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“…Some studies attribute the response to CASR, ( 23 ) whereas others indicate the presence of a novel cation‐sensing receptor mechanism distinct from CASR. ( 4,10,17,24,26 ) Our findings suggest that strontium can act on osteoblasts in vitro through the novel cation‐sensing mechanism that is distinct from CASR. In this regard, both MC3T3‐E1 cells, which do not express CASR transcripts but express genes that characterize them as osteoblasts (Fig.…”

Section: Discussionmentioning

confidence: 81%

“…The molecular targets in bone cells that mediate the response to strontium and other extracellular cations are uncertain. ( 17 ) Some authors have found evidence for expression of the classical calcium‐sensing receptor CASR in osteoblasts ( 18 ) and osteoclasts ( 19 ) as well as other cells in bone marrow. ( 20 ) CASR is a G‐protein‐coupled receptor (GPCR) that responds to changes in extracellular calcium concentrations and belongs to the class C metabotrophic glutamate/pheromone family that is predominately located in the parathyroid gland, kidney, and thyroidal C‐cells, where it regulates PTH secretion, renal calcium and water handling, and calcitonin secretion, respectively.…”

Section: Introductionmentioning

confidence: 99%

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A Novel Cation-Sensing Mechanism in Osteoblasts Is a Molecular Target for Strontium

Quarles

2004

Journal of Bone and Mineral Research

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Defining the molecular target for strontium in osteoblasts is important for understanding the anabolic effects of this cation on bone. The current studies demonstrate that a G-protein-mediated response to strontium persists in osteoblasts that lack CASR, suggesting a predominant role for a novel cation-sensing receptor in mediating the osseous response to strontium.Introduction: Strontium has anabolic effects on bone and is currently being developed for the treatment of osteoporosis. The molecular target for strontium in osteoblasts has not been determined, but the existence of CASR, a G-protein-coupled receptor calcium-sensing receptor, raises the possibility that strontium actions on bone are mediated through this or a related receptor. Materials and Methods:We used activation of a transfected serum response element (SRE)-luciferase reporter in HEK-293 cells to determine if CASR is activated by strontium. In addition, we examined strontium-mediated responses in MC3T3-E1 osteoblasts and osteoblasts derived from wild-type and CASR null mice to determine if other cation-sensing mechanisms are present in osteoblasts. Results and Conclusions:We found that strontium stimulated SRE-luc activity in HEK-293 cells transfected with full-length CASR but not in cells expressing the alternatively spliced CASR construct lacking exon 5. In contrast, we found that MC3T3-E1 osteoblasts that lack CASR as well as osteoblasts derived from CASR null mice respond to millimolar concentrations of strontium. The response to strontium in osteoblasts was nonadditive to a panel of extracellular cations, including aluminum, gadolinium, and calcium, suggesting a common mechanism of action. In contrast, neither the CASR agonist magnesium nor the calcimimetic NPS-R568 activated SRE activity in osteoblasts, but the response to these agonists was imparted by transfection of CASR into these osteoblasts, consistent with the presence of distinct cation-sensing mechanisms. Co-expression of the dominant negative G␣q(305-359) minigene also inhibited cation-stimulated SRE activity in osteoblasts lacking known CASR. These findings are consistent with strontium activation of a novel G␣q-coupled extracellular cation-sensing receptor in osteoblasts with distinct cation specificity.

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Section: Discussionmentioning

confidence: 81%

Section: Introductionmentioning

confidence: 99%

A Novel Cation-Sensing Mechanism in Osteoblasts Is a Molecular Target for Strontium

Quarles

2004

Journal of Bone and Mineral Research

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“…We found no effect of [Mg 2+ ] e or [Gd 3+ ] e on COX‐2 promoter activity in POBs but did find a response to [Sr 2+ ]. Moreover, the changes in ion concentration necessary to induce COX‐2 expression were much larger than the 1–2% changes in concentration that can alter PTH secretion (73) . Hence, the [Ca 2+ ] e induction of COX‐2 in POBs does not seem to occur through the classical CaR.…”

Section: Discussionmentioning

confidence: 96%

“…The role of the classical CaR in bone is unclear (73) . Studies on immortalized osteoblasts from CaR knockout mice failed to detect any effects of disruption of the CaR (12) .…”

Section: Discussionmentioning

confidence: 99%

Extracellular Calcium Is a Potent Inducer of Cyclo-oxygenase-2 in Murine Osteoblasts Through an ERK Signaling Pathway

Choudhary

Wadhwa

Raisz

et al. 2003

Journal of Bone and Mineral Research

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ABSTRACT] e on bone may be mediated by COX-2.Introduction: Local changes in extracellular calcium ([Ca 2ϩ ] e ) may play an important role in bone turnover. We examined the possibility that prostaglandins produced by cyclo-oxygenase-2 (COX-2) could mediate some of the effects of [Ca 2ϩ ] e on osteoblasts. Methods: We examined the [Ca 2ϩ ] e induction of COX-2 expression and prostaglandin E 2 (PGE 2 ) production in primary osteoblasts (POBs) obtained by sequential enzymatic digestion of mouse calvariae. We measured mRNA and protein levels by Northern and Western analyses and PGE 2 production in culture medium by radioimmunoassay (RIA). COX-2 promoter activity was measured as luciferase activity in calvarial osteoblasts derived from mice transgenic for 371 bp of the COX-2 promoter fused to a luciferase reporter gene. ] e (5 mM) induced COX-2 mRNA within 30 minutes; levels peaked at 6 -9 h and remained elevated at 24 h. Cumulative medium PGE 2 was increased at 3 h, with levels rising to 30 nM at 24 h. PGE 2 production in POBs from mice with only COX-1 gene expression was 1/40th of that in POBs from mice with both COX-1 and COX-2 gene expression. [Ca 2ϩ ] e increased alkaline phosphatase activity and osteocalcin mRNA, and this increase was blocked by inhibiting PGE 2 production. [Ca 2ϩ ] e stimulation of COX-2 promoter activity correlated with the induction of COX-2 mRNA expression. [Ca 2ϩ ] e induced rapid and transient phosphorylation of extracellular signal-regulated kinase (ERK) in POBs, which peaked at 5-10 minutes. Inhibition of ERK phosphorylation with the specific inhibitors, PD-98059 and U-0126, decreased the [Ca 2ϩ ] e induction of both COX-2 mRNA and luciferase activity by 70 -80%. Although less effective than [Ca 2ϩ ] e , strontium [Sr 2ϩ ] e also induced COX-2 mRNA and promoter activity in POBs through an ERK signaling pathway. We conclude that [Ca 2ϩ ] e is a potent transcriptional inducer of COX-2 expression and PGE 2 production in osteoblasts through an ERK signaling pathway.

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“…Recently two studies have shown that CaRdeficient mice exhibit an essentially normal skeletal phenotype when the hyperparathyroidism resulting from the lack of the parathyroid CaR is prevented (10,11). Thus, it remains unclear whether the osteoblast CaR is a true regulator of bone function or whether its expression is vestigial (12).…”

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confidence: 99%

Physiological changes in extracellular calcium concentration directly control osteoblast function in the absence of calciotropic hormones

Dvorák

Siddiqua

Ward

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

393

262

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We investigated the direct effects of changes in free ionized extracellular calcium concentrations ([Ca 2؉ ]o) on osteoblast function and the involvement of the calcium-sensing receptor (CaR) in mediating these responses. CaR mRNA and protein were detected in osteoblast models, freshly isolated fetal rat calvarial cells and murine clonal osteoblastic 2T3 cells, and in freshly frozen, undecalcified preparations of human mandible and rat femur. In fetal rat calvarial cells, elevating [Ca 2؉ ]o and treatment with gadolinium, a nonpermeant CaR agonist, resulted in phosphorylation of the extracellular signal-regulated kinases 1 and 2, Akt, and glycogensynthase kinase 3␤, consistent with signals of cell survival and proliferation. In agreement, cell number was increased under these conditions. Expression of the osteoblast differentiation markers core binding factor ␣1, osteocalcin, osteopontin, and collagen I mRNAs was increased by high [ (2, 3) and alter the levels of expression of some differentiation markers (4, 5). During mineralization, decreases in [Ca 2ϩ ] o are also likely to occur (6), but the effect of lowering [Ca 2ϩ ] o in bone cells has not been extensively addressed.The mechanism of [Ca 2ϩ ] o -sensing by osteoblasts is unclear. The parathyroid extracellular calcium-sensing receptor (CaR) is a key player in the maintenance of a constant systemic [Ca 2ϩ ] o , predominantly through regulation of parathyroid hormone (PTH) secretion and urinary calcium excretion (7,8). CaR is also present in osteoblasts (9 and references therein), where a functional role is currently debated. Recently two studies have shown that CaRdeficient mice exhibit an essentially normal skeletal phenotype when the hyperparathyroidism resulting from the lack of the parathyroid CaR is prevented (10, 11). Thus, it remains unclear whether the osteoblast CaR is a true regulator of bone function or whether its expression is vestigial (12).In this study, we investigated the effects of both decreasing and increasing [Ca 2ϩ ] o on osteoblast proliferation and intracellular signaling events, the expression of several osteoblast differentiation markers [core binding factor ␣1 (Cbfa1, also termed Runx2 and Osf2), osteocalcin (OC), osteopontin (OP), and type I collagen (collaI)], the activity of alkaline phosphatase (AlP), and mineralized nodule formation in the absence of systemic calciotropic factors, namely PTH and vitamin D. We further investigated the role played by the CaR in these events using an alternative, nonpermeant CaR agonist, gadolinium (Gd 3ϩ ) and a CaR inhibitor, NPS 89636 (a ''calcilytic''). We used well characterized osteoblast models, freshly isolated fetal rat calvarial cells (FRC) (13) and the clonal murine osteoblast cell line, 2T3 cells (14). The expression of CaR in freshly frozen sections of rat and human bone was also determined. Materials and MethodsAnimals. Sprague-Dawley rats (Charles River Breeding Laboratories) were killed by cervical dislocation and used in accordance to the U.K. Animals Scientific Procedures...

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The hunting of the snark: the elusive calcium receptor(s)

Cited by 10 publications

References 22 publications

A Novel Cation-Sensing Mechanism in Osteoblasts Is a Molecular Target for Strontium

A Novel Cation-Sensing Mechanism in Osteoblasts Is a Molecular Target for Strontium

Extracellular Calcium Is a Potent Inducer of Cyclo-oxygenase-2 in Murine Osteoblasts Through an ERK Signaling Pathway

Physiological changes in extracellular calcium concentration directly control osteoblast function in the absence of calciotropic hormones

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