Calcium-induced activation of a mutant G-protein-coupled receptor causes in vitro transformation of NIH/3T3 cells

Recombinant human parathyroid hormone therapy (1-34) in an adult patient with a gain-of-function mutation in the calcium-sensing receptor-a case report

OBJECTIVE

To describe a case of hypocalcemia in a patient with a gain-of-function mutation in the calcium-sensing receptor that was undetected until adulthood and successfully treated with recombinant parathyroid hormone.

METHODS

The clinical findings, laboratory data, and a review of the pertinent literature are presented.

RESULTS

A 55-year-old woman was hospitalized and seen by the endocrinology consult service for hypocalcemia that was refractory to repeated doses of intravenous calcium gluconate. She expressed concern about chronic leg muscle cramps and paresthesias of the lips and fingertips. In addition, she had no history of neck surgery, neck irradiation, or any autoimmune disease. She was a well-appearing female with no dysmorphic features or skin changes. Laboratory tests revealed hypocalcemia, hyperphosphatemia, hypomagnesemia, and hypovitaminosis D. Her parathyroid hormone concentration (PTH) was low at 14.2 pg/mL. Her PTH and calcium concentrations remained low despite repletion of magnesium and treatment with calcitriol and oral calcium replacement. A 24-hour collection for urinary calcium showed inappropriate hypercalciuria. Medical records showed her hypocalcemia to be chronic. Additionally, several family members had also complained of muscle cramps. A congenital cause of her hypoparathyroidism was considered, and genetic testing confirmed heterozygosity for a gain-of-function mutation in the calcium-sensing receptor gene associated with autosomal dominant familial isolated hypoparathyroidism (ADH). Treatment with subcutaneous recombinant human parathyroid hormone teriparatide (rhPTH [1-34]) 20 mcg twice daily for three days normalized her calcium and phosphorus concentrations.

CONCLUSION

rhPTH (1-34) is an effective treatment for patients with hypoparathyroidism due to gain-of-function mutations in the calcium-sensing receptor. ADH can be insidious in presentation and the diagnosis can be missed unless there is a high index of suspicion.

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 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.

Upregulation of parathyroid VDR expression by extracellular calcium is mediated by ERK1/2-MAPK signaling pathway

We have previously demonstrated that the activation of rat parathyroid calcium-sensing receptor (CaSR) upregulates VDR expression in vivo (Garfia B, Cañadillas S, Luque F, Siendones E, Quesada M, Almadén Y, Aguilera-Tejero E, Rodríguez M. J Am Soc Nephrol 13: 2945–2952, 2002; Rodriguez ME, Almaden Y, Cañadillas S, Canalejo A, Siendones E, Lopez I, Aguilera-Tejero E, Martin D, Rodriguez M. Am J Physiol Renal Physiol 292: F1390–F1395, 2007). The present study was designed to characterize the signaling system that mediates the stimulation of parathyroid VDR gene expression by extracellular calcium. Experiments were performed in vitro by the incubation of rat parathyroid glands and in vivo with normal and uremic (Nx) rats receiving injections of CaCl2or EDTA to obtain hypercalcemic or hypocalcemic clamps. A high calcium concentration increased VDR expression. The addition of arachidonic acid (AA) to the low-calcium medium produced an increase in VDR mRNA of the same magnitude as that observed with high calcium. The addition of ionophore to the low-calcium medium also increased VDR mRNA expression. High calcium or the addition of AA to the low-calcium medium induced the activation (phosphorylation) of ERK1/2-MAPK. The specific inhibition of the ERK1/2-MAPK activity prevented the stimulation of VDR expression by high calcium or AA. These results suggest that AA regulates parathyroid VDR gene expression through the activation of the ERK1/2-MAPK. CaSR activation induced the activation of transcription factor Sp1, but not of NF-κB p50 or p65 or activator protein-1. The addition of AA to the low-calcium medium increased specific DNA-binding activity of Sp1 to almost the same level as high calcium, which was prevented by the inhibition of ERK1/2. Furthermore, mithramycin A (a Sp1 inhibitor) prevented the upregulation of VDR mRNA by high calcium. Finally, both sham and Nx hypercalcemic rats showed similar increased levels of VDR mRNA compared with sham and Nx hypocalcemic rats. Our results demonstrate that extracellular calcium stimulates VDR expression in parathyroid glands through the elevation of the cytosolic calcium level and the stimulation of the PLA2-AA-dependent ERK1/2-pathway. Furthermore, the transcription factor Sp1 mediates this effect.

Cloning of novel tumor metastasis-related genes from the highly metastatic human lung adenocarcinoma cell line Anip973

A cDNA library was successfully constructed from Anip973, a human lung adenocarcinoma cell line with high metastatic potential. NIH3T3 cells were stably transfected using this cDNA library and screened for morphological changes in a soft agar assay. Genomic DNA was isolated from putative clones and the integrated sequence was retrieved by PCR and sequencing. Three known genes, ribosomal protein L23, hypothetical protein FLJ22104, and serine protease inhibitor, kazal type 6 and a number of 5'-terminally truncated sequences were identified. Furthermore, cells transfected with ribosomal protein L23 was highly invasive compared with the empty vector as control (P<0.02). These results indicate that the expression cloning of cDNA libraries in NIH3T3 cells and subsequent screening for loss of contact inhibition in soft agar is a viable tool for identifying tumor-related genes and ribosomal protein L23 gene plays a role in cell movement and metastasis.

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.

G-protein-coupled receptors and cancer

G-protein-coupled receptors (GPCRs), the largest family of cell-surface molecules involved in signal transmission, have recently emerged as crucial players in tumour growth and metastasis. Malignant cells often hijack the normal physiological functions of GPCRs to survive, proliferate autonomously, evade the immune system, increase their blood supply, invade their surrounding tissues and disseminate to other organs. This Review will address our current understanding of the many roles of GPCRs and their signalling circuitry in tumour progression and metastasis. We will also discuss how interfering with GPCRs might provide unique opportunities for cancer prevention and treatment.

Expression of transforming K-Ras oncogene affects mitochondrial function and morphology in mouse fibroblasts

K-ras transformed fibroblasts have been shown to have a stronger dependence from glycolysis, reduced oxidative phosphorylation ability and a fragility towards glucose depletion compared to their immortalized, normal counterparts. In this paper, using RNA profiling assays and metabolic perturbations, we report changes in expression of genes encoding mitochondrial proteins and alterations in mitochondrial morphology that correlate with mitochondrial functionality. In fact, unlike normal cells, transformed cells show reduced ATP content and inability to modify mitochondria morphology upon glucose depletion. Being reverted by GEF-DN expression, such morphological and functional changes are directly connected to Ras activation. Taken together with reported partial mitochondrial uncoupling and more sustained apoptosis of transformed cells, our results indicate that activation of the Ras pathway strikingly impacts on energy and signaling-related aspects of mitochondria functionality, that in turn may affect the terminal phenotype of transformed cells.

Calcium-sensing Receptor-mediated ERK1/2 Activation Requires Gαi2 Coupling and Dynamin-independent Receptor Internalization

The calcium-sensing receptor (CaR) recently has been shown to activate MAP kinase (ERK1/2) in various cell types as well as in heterologous expression systems. In this study we show that the CaR agonist NPS R-467 (1 microm), which does not activate the CaR by itself, robustly activates ERK1/2 in the presence of a low concentration of Ca(2+) (0.5 mm CaCl(2)) in human embryonic kidney (HEK) cells permanently expressing the human CaR (HEK-hCaR). Ca(2+) (4 mm) also activates ERK1/2 but with differing kinetics. CaR-dependent ERK1/2 activation begins to desensitize to 4 mm Ca(2+) after 10 min, whereas there is no desensitization to NPS R-467/CaCl(2) as late as 4 h. Moreover, recovery from desensitization occurs as rapidly as 30 min with 4 mm CaCl(2). Pretreatment of HEK-hCaR cells with concanavalin A (250 microg/ml) to block CaR internalization completely eliminated the NPS R-467/CaCl(2)-mediated ERK1/2 activation but did not block the 2-min time point of 4 mm Ca(2+)-mediated ERK1/2 activation. Neither dominant-negative dynamin (K44A) nor dominant-negative beta-arrestin inhibited ERK1/2 activation by either CaR agonist treatment, suggesting that CaR-elicited ERK1/2 signaling occurs via a dynamin-independent pathway. Pertussis toxin pretreatment partially attenuated the 4 mm Ca(2+)-ERK1/2 activation; this attenuated activity was completely restored by co-expression of the Galpha(i2) (C351I) but not Galpha(i1) (C351I) or Galpha(i3) (C351I) G proteins, PTX-insensitive G protein mutants. Taken together, these data suggest that both 4 mm Ca(2+) and NPS R-467/CaCl(2) activate ERK1/2 via distinguishable pathways in HEK-hCaR cells and may represent a nexus to differentially regulate differentiation versus proliferation via CaR activation.

G-protein-coupled receptors and signaling networks: emerging paradigms

G-protein-coupled receptors (GPCRs) constitute the largest family of cell-surface molecules involved in signal transmission. These receptors play key physiological roles and their dysfunction results in several diseases. Recently, it has been shown that many of the cellular responses mediated by GPCRs do not involve the sole stimulation of conventional second-messenger-generating systems, but instead result from the functional integration of an intricate network of intracellular signaling pathways. Effectors for GPCRs that are independent of G proteins have now also been identified, thus changing the conventional view of the GPCR-heterotrimeric-G-protein-associated effector. The emerging information is expected to help elucidate the most basic mechanism by which these receptors exert their numerous physiological roles, in addition to determining why the perturbation of their function results in many pathological conditions.