CaR-mediated COX-2 expression in primary cultured mTAL cells

The calcium-sensing receptor in inflammation: Recent updates

The Calcium-Sensing Receptor (CaSR) is a member of the class C of G-proteins coupled receptors (GPCRs), it plays a pivotal role in calcium homeostasis by directly controlling calcium excretion in the kidneys and indirectly by regulating parathyroid hormone (PTH) release from the parathyroid glands. The CaSR is found to be ubiquitously expressed in the body, playing a plethora of additional functions spanning from fluid secretion, insulin release, neuronal development, vessel tone to cell proliferation and apoptosis, to name but a few. The present review aims to elucidate and clarify the emerging regulatory effects that the CaSR plays in inflammation in several tissues, where it mostly promotes pro-inflammatory responses, with the exception of the large intestine, where contradictory roles have been recently reported. The CaSR has been found to be expressed even in immune cells, where it stimulates immune response and chemokinesis. On the other hand, CaSR expression seems to be boosted under inflammatory stimulus, in particular, by pro-inflammatory cytokines. Because of this, the CaSR has been addressed as a key factor responsible for hypocalcemia and low levels of PTH that are commonly found in critically ill patients under sepsis or after burn injury. Moreover, the CaSR has been found to be implicated in autoimmune-hypoparathyroidism, recently found also in patients treated with immune-checkpoint inhibitors. Given the tight bound between the CaSR, calcium and vitamin D metabolism, we also speculate about their roles in the pathogenesis of severe acute respiratory syndrome coronavirus-19 (SARS-COVID-19) infection and their impact on patients' prognosis. We will further explore the therapeutic potential of pharmacological targeting of the CaSR for the treatment and management of aberrant inflammatory responses.

Extracellular Calcium Receptor as a Target for Glutathione and Its Derivatives

Extracellular glutathione (GSH) and oxidized glutathione (GSSG) can modulate the function of the extracellular calcium sensing receptor (CaSR). The CaSR has a binding pocket in the extracellular domain of CaSR large enough to bind either GSH or GSSG, as well as the naturally occurring oxidized derivative L-cysteine glutathione disulfide (CySSG) and the compound cysteinyl glutathione (CysGSH). Modeling the binding energies (ΔG) of CySSG and CysGSH to CaSR reveals that both cysteine derivatives may have greater affinities for CaSR than either GSH or GSSG. GSH, CySSG, and GSSG are found in circulation in mammals and, among the three, CySSG is more affected by HIV/AIDs and aging than either GSH or GSSG. The beta-carbon linkage of cysteine in CysGSH may model a new class of calcimimetics, exemplified by etelcalcetide. Circulating glutathionergic compounds, particularly CySSG, may mediate calcium-regulatory responses via receptor-binding to CaSR in a variety of organs, including parathyroids, kidneys, and bones. Receptor-mediated actions of glutathionergics may thus complement their roles in redox regulation and detoxification. The glutathionergic binding site(s) on CaSR are suggested to be a target for development of drugs that can be used in treating kidney and other diseases whose mechanisms involve CaSR dysregulation.

Understanding the molecular association between hyperkalemia and lung squamous cell carcinomas

Background

Previous studies indicated a strong association between hyperkalemia and lung squamous cell carcinomas (LSCC). However, the underlying mechanism is not fully understood so far.

Methods

Literature-based data mining was conducted to identify genes, molecule, and cell processes linked to both hyperkalemia and LSCC. Pathway analysis was performed to explore the interactive network, common-target network, and common-regulator network for both disorders. Then, a mega-analysis using 11 independent LSCC RNA expression datasets (358 LSCCs and 278 healthy controls) was performed to test the hypothesis that genes influencing hyperkalemia may also play roles in LSCC.

Results

There was a significant overlap between the genes implicated with both diseases (20 genes, p-value = 4.98e-15), which counts for 16% of all hyperkalemia genes (125 genes). Network analysis identified 12 molecules as common targets for hyperkalemia and LSCC, and 19 molecules as common regulators. Moreover, 19 molecules were identified within an interactive network, through which hyperkalemia and LSCC could exert influence on each other. In addition, meta-analysis identified one hyperkalemia promoter, SPP1, as a novel contributor for LSCC (LFC = 2.64; p-value = 2.81e-6). MLR analysis suggests geographical region as an influential factor for the expression levels of SPP1 in LSCC patients (p value = 0.036, 0.054).

Conclusion

Our results showed that there was a common molecular basis for the pathology of both hyperkalemia and LSCC, and that genes promoting hyperkalemia might also play roles in the development of LSCC. However, this study did not suggest hypercalcemia as a casual factor for LSCC.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

PGE(2) EP(3) receptor downregulates COX-2 expression in the medullary thick ascending limb induced by hypertonic NaCl

We tested the hypothesis that inhibition of EP3 receptors enhances cyclooxygenase (COX)-2 expression in the thick ascending limb (TAL) induced by hypertonic stimuli. COX-2 protein expression in the outer medulla increased approximately twofold in mice given free access to 1% NaCl in the drinking water for 3 days. The increase was associated with an approximate threefold elevation in COX-2 mRNA accumulation and an increase in PGE2 production by isolated medullary (m)TAL tubules from 77.3 ± 8.4 to 165.7 ± 10.8 pg/mg protein. Moreover, administration of NS-398 abolished the increase in PGE2 production induced by 1% NaCl. EP3 receptor mRNA levels also increased approximately twofold in the outer medulla of mice that ingested 1% NaCl. The selective EP3 receptor antagonist L-798106 increased COX-2 mRNA by twofold in mTAL tubules, and the elevation in COX-2 protein induced by 1% NaCl increased an additional 50% in mice given L-798106. COX-2 mRNA in primary mTAL cells increased twofold in response to media made hypertonic by the addition of NaCl (400 mosmol/kg H2O). L-798106 increased COX-2 mRNA twofold in isotonic media and fourfold in cells exposed to 400 mosmol/kg H2O. PGE2 production by mTAL cells increased from 79.3 ± 4.6 to 286.7 ± 6.3 pg/mg protein after challenge with 400 mosmol/kg H2O and was inhibited in cells transiently transfected with a lentivirus short hairpin RNA construct targeting exon 5 of COX-2 to silence COX-2. Collectively, the data suggest that local hypertonicity in the mTAL is associated with an increase in COX-2 expression concomitant with elevated EP3 receptor expression, which limits COX-2 activity in this segment of the nephron.

Vitamin D increases plasma renin activity independently of plasma Ca2+ via hypovolemia and β-adrenergic activity

1, 25-Dihydroxycholechalciferol (calcitriol) and 19-nor-1, 25-dihydroxyvitamin D2 (paricalcitol) are vitamin D receptor (VDR) agonists. Previous data suggest VDR agonists may actually increase renin-angiotensin activity, and this has always been assumed to be mediated by hypercalcemia. We hypothesized that calcitriol and paricalcitol would increase plasma renin activity (PRA) independently of plasma Ca(2+) via hypercalciuria-mediated polyuria, hypovolemia, and subsequent increased β-adrenergic sympathetic activity. We found that both calcitriol and paricalcitol increased PRA threefold (P < 0.01). Calcitriol caused hypercalcemia, but paricalcitol did not. Both calcitriol and paricalcitol caused hypercalciuria (9- and 7-fold vs. control, P < 0.01) and polyuria (increasing 2.6- and 2.2-fold vs. control, P < 0.01). Paricalcitol increased renal calcium-sensing receptor (CaSR) expression, suggesting a potential cause of paricalcitol-mediated hypercalciuria and polyuria. Volume replacement completely normalized calcitriol-stimulated PRA and lowered plasma epinephrine by 43% (P < 0.05). β-Adrenergic blockade also normalized calcitriol-stimulated PRA. Cyclooxygenase-2 inhibition had no effect on calcitriol-stimulated PRA. Our data demonstrate that vitamin D increases PRA independently of plasma Ca(2+) via hypercalciuria, polyuria, hypovolemia, and increased β-adrenergic activity.

The association between osteoporosis and hypertension: the role of a low dairy intake

Hypertension and related cardiovascular diseases are reported to be associated with osteoporosis. A nutritional pathway related to dairy intake has been postulated for both diseases. The aim of this study was to assess calcium intake from dairy sources as a possible pathogenic link between osteoporosis and hypertension. This was a cross-sectional observational study performed on 3,301 postmenopausal women referred for a densitometry screening. Osteoporosis was diagnosed by lumbar dual-energy X-ray absorptiometry and hypertension was defined by blood pressure data and/or the use of antihypertensive medication. Dairy food consumption was evaluated using a weekly food-frequency questionnaire. The odds of being affected by osteoporosis, hypertension, or both diseases were calculated for quartiles of dairy intake by logistic regression analyses. Women with hypertension were affected more frequently by osteoporosis (33.2 vs. 23.3 %; p = 0.000), and there was a higher prevalence of hypertension among women with osteoporosis (32.2 vs. 22.5 %; p = 0.000). The proportion of women with hypertension, osteoporosis, and both diseases significantly increased across decreasing quartiles of dairy intake. A dairy intake in the lowest quartile was a significant predictor of osteoporosis [OR (95 % CI): 1.43 (1.12, 1.82)] and hypertension [OR (95 % CI): 1.46 (1.15, 1.85)] when compared to the highest quartile. Similarly, a low dairy intake was associated with increased odds to have both the diseases [OR (95 % CI): 1.60 (1.10, 2.34)]. From these results we conclude that osteoporosis and hypertension are associated in postmenopausal women, and a low dairy intake may increase the risk of both diseases, acting as a possible pathogenic link.

Tumor necrosis factor-α: regulation of renal function and blood pressure

Tumor necrosis factor-α (TNF-α) is a pleiotropic cytokine that becomes elevated in chronic inflammatory states such as hypertension and diabetes and has been found to mediate both increases and decreases in blood pressure. High levels of TNF-α decrease blood pressure, whereas moderate increases in TNF-α have been associated with increased NaCl retention and hypertension. The explanation for these disparate effects is not clear but could simply be due to different concentrations of TNF-α within the kidney, the physiological status of the subject, or the type of stimulus initiating the inflammatory response. TNF-α alters renal hemodynamics and nephron transport, affecting both activity and expression of transporters. It also mediates organ damage by stimulating immune cell infiltration and cell death. Here we will summarize the available findings and attempt to provide plausible explanations for such discrepancies.

Calcium nutrition and extracellular calcium sensing: relevance for the pathogenesis of osteoporosis, cancer and cardiovascular diseases

Through a systematic search in Pubmed for literature, on links between calcium malnutrition and risk of chronic diseases, we found the highest degree of evidence for osteoporosis, colorectal and breast cancer, as well as for hypertension, as the only major cardiovascular risk factor. Low calcium intake apparently has some impact also on cardiovascular events and disease outcome. Calcium malnutrition can causally be related to low activity of the extracellular calcium-sensing receptor (CaSR). This member of the family of 7-TM G-protein coupled receptors allows extracellular Ca2+ to function as a "first messenger" for various intracellular signaling cascades. Evidence demonstrates that Ca2+/CaSR signaling in functional linkage with vitamin D receptor (VDR)-activated pathways (i) promotes osteoblast differentiation and formation of mineralized bone; (ii) targets downstream effectors of the canonical and non-canonical Wnt pathway to inhibit proliferation and induce differentiation of colorectal cancer cells; (iii) evokes Ca2+ influx into breast cancer cells, thereby activating pro-apoptotic intracellular signaling. Furthermore, Ca2+/CaSR signaling opens Ca2+-sensitive K+ conductance channels in vascular endothelial cells, and also participates in IP(3)-dependent regulation of cytoplasmic Ca2+, the key intermediate of cardiomyocyte functions. Consequently, impairment of Ca2+/CaSR signaling may contribute to inadequate bone formation, tumor progression, hypertension, vascular calcification and, probably, cardiovascular disease.

Tumor necrosis factor-alpha induces renal cyclooxygenase-2 expression in response to hypercalcemia

The effect of tumor necrosis factor-alpha (TNF) on cyclooxygenase-2 (COX-2) expression in the renal outer medulla (OM) was determined in a model of dihydrotachysterol (DHT)-induced hypercalcemia. Increases in serum calcium and water intake were observed during ingestion of a DHT-containing diet in both wild type (WT) and TNF deficient mice (TNF(-/-)). Polyuria and a decrease in body weight were observed in response to DHT treatment in WT and TNF(-/-) mice. A transient elevation in urinary TNF was observed in WT mice treated with DHT. Moreover, increased urinary levels of prostaglandin E(2) (PGE(2)) and a corresponding increase in COX-2 expression in the OM were observed in WT mice fed DHT. Increased COX-2 expression was not observed in TNF(-/-) mice fed DHT, and the characteristics of PGE(2) synthesis were distinct from those in WT mice. This study demonstrates that COX-2 expression in the OM, secondary to hypercalemia, is TNF-dependent.

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.

Eicosanoids and tumor necrosis factor-alpha in the kidney

The thick ascending limb of Henle's loop (TAL) is capable of metabolizing arachidonic acid (AA) by cytochrome P450 (CYP450) and cyclooxygenase (COX) pathways and has been identified as a nephron segment that contributes to salt-sensitive hypertension. Previous studies demonstrated a prominent role for CYP450-dependent metabolism of AA to products that inhibited ion transport pathways in the TAL. However, COX-2 is constitutively expressed along all segments of the TAL and is increased in response to diverse stimuli. The ability of Tamm-Horsfall glycoprotein, a selective marker of cortical TAL (cTAL) and medullary (mTAL), to bind TNF and localize it to this nephron segment prompted studies to determine the capacity of mTAL cells to produce TNF and determine its effects on mTAL function. The colocalization of calcium-sensing receptor (CaR) and COX-2 in the TAL supports the notion that activation of CaR induces TNF-dependent COX-2 expression and PGE₂ synthesis in mTAL cells. Additional studies showed that TNF produced by mTAL cells inhibits ⁸⁶Rb uptake, an in vitro correlate of natriuresis, in an autocrine- and COX-2-dependent manner. The molecular mechanism for these effects likely includes inhibition of Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2) expression and trafficking.

Associations among 25-hydroxyvitamin D, diet quality, and metabolic disturbance differ by adiposity in adults in the United States

CONTEXT

Recent evidence indicates that a higher plasma level of 25-hydroxyvitamin D [25(OH)D] is associated with lower adiposity and a reduced number of metabolic disturbances (MetD).

OBJECTIVES

We examined associations among dietary quality, 25(OH)D, percent body fat (%BF), and MetD, and a pathway linking them, across central obesity.

DESIGN

This cross-sectional nationally representative study used extensive data from the National Health and Nutrition Examination Surveys of 2001-2004.

PARTICIPANTS

U.S. adults aged at least 20 yr were stratified by central obesity (CO) status. Sample sizes ranged from 1943 (all MetD combined) to 7796 (each component).

MAIN OUTCOME MEASURES

%BF was measured using dual-energy x-ray absorptiometry, and MetD was measured with individual continuous nonadiposity outcomes (e.g. fasting plasma glucose) and with a composite count index of binary MetD with prespecified cutoff points (Index I).

RESULTS

A higher 25(OH)D was associated with better dietary quality, lower %BF, and lower number of MetD. These inverse 25(OH)D-%BF and 25(OH)D-MetD associations (i.e. fasting blood glucose, homeostatic model assessment of insulin resistance, C-reactive protein, and Index I) were significantly stronger among the CO+ group. Finally, the pathway linking the dairy component of the Healthy Eating Index (HEIdairy) to Index I through 25(OH)D and %BF indicated complete mediation among the CO- group, but HEIdairy and 25(OH)D had direct inverse associations with Index I among the CO+ group.

CONCLUSIONS

Due to potential genetic differences between CO- and CO+ groups, empowering U.S. adults with central obesity to make related behavioral changes may be especially effective in improving their vitamin D status and metabolic profile.

Vitamin D and calcium insufficiency-related chronic diseases: molecular and cellular pathophysiology

A compromised vitamin D status, characterized by low 25-hydroxyvitamin D (25-(OH)D) serum levels, and a nutritional calcium deficit are widely encountered in European and North American countries, independent of age or gender. Both conditions are linked to the pathogenesis of many degenerative, malignant, inflammatory and metabolic diseases. Studies on tissue-specific expression and activity of vitamin D metabolizing enzymes, 25-(OH)D-1 alpha-hydroxylase and 25-(OH)D-24-hydroxylase, and of the extracellular calcium-sensing receptor (CaR) have led to the understanding of how, in non-renal tissues and cellular systems, locally produced 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) and extracellular Ca(2+) act jointly as key regulators of cellular proliferation, differentiation and function. Impairment of cooperative signalling from the 1,25-(OH)(2)D(3)-activated vitamin D receptor (VDR) and from the CaR in vitamin D and calcium insufficiency causes cellular dysfunction in many organs and biological systems, and, therefore, increases the risk of diseases, particularly of osteoporosis, colorectal and breast cancer, inflammatory bowel disease, insulin-dependent diabetes mellitus type I, metabolic syndrome, diabetes mellitus type II, hypertension and cardiovascular disease. Understanding the underlying molecular and cellular processes provides a rationale for advocating adequate intake of vitamin D and calcium in all populations, thereby preventing many chronic diseases worldwide.

Novel regulatory aspects of the extracellular Ca2+-sensing receptor, CaR

The capacity to sense and adapt to changes in environmental cues is of paramount importance for every living organism. From yeast to man, cells must be able to match cellular activities to growth environment and nutrient availability. Key to this process is the development of membrane-bound systems that can detect modifications in the extracellular environment and to translate these into biological responses. Evidence gathered over the last 15 years has demonstrated that many of these cell surface "sensors" belong to the G protein-coupled receptor superfamily. Crucial to our understanding of nutrient sensing in mammalian species has been the identification of the extracellular Ca(2+)/cation-sensing receptor, CaR. CaR was the first ion-sensing molecule identified in man and genetic studies in humans have revealed the importance of the CaR in mineral ion metabolism. Latter, it has become apparent that the CaR also plays an important role outside the Ca(2+) homeostatic system, as an integrator of multiple environmental signals for the regulation of many vital cellular processes, from cell-to-cell communication to secretion and cell survival/cell death. Recently, novel aspects of receptor function reveal an unexpected role for the CaR in the regulation of growth and development in utero.

Thick ascending limb: the Na(+):K (+):2Cl (-) co-transporter, NKCC2, and the calcium-sensing receptor, CaSR

The thick ascending limb of Henle's loop is a nephron segment that is vital to the formation of dilute and concentrated urine. This ability is accomplished by a consortium of functionally coupled proteins consisting of the apical Na(+):K(+):2Cl(-) co-transporter, the K(+) channel, and basolateral Cl(-) channel that mediate electroneutral salt absorption. In thick ascending limbs, salt absorption is importantly regulated by the calcium-sensing receptor. Genetic or pharmacological disruption impairing the function of any of these proteins results in Bartter syndrome. The thick ascending limb is also an important site of Ca(2+) and Mg(2+) absorption. Calcium-sensing receptor activation inhibits cellular Ca(2+) absorption induced by parathyroid hormone, as well as passive paracellular Ca(2+) transport. The present review discusses these functions and their genetic and molecular regulation.

Expression and function of NFAT5 in medullary thick ascending limb (mTAL) cells

The contribution of nuclear factor of activated T cells 5 (NFAT5) to the regulation of tumor necrosis factor-alpha (TNF) production in medullary thick ascending limb (mTAL) cells is unclear. RT-PCR analysis was performed on primary cultures of mouse mTAL cells and freshly isolated mTAL tubules to determine which NFAT isoforms are present in this nephron segment. Primer pairs were designed, based on published sequences for mouse NFAT1-5, to produce fragments of approximately 200 bp. Analysis of PCR products by gel electrophoresis and subsequent DNA sequencing indicated that cells and tubules contained mRNA for all five NFAT isoforms. The relative expression of NFAT isoforms was then determined using quantitative real-time RT-PCR. The data indicate that NFAT isoforms 5 >/= 1 are the predominant isoforms present in mTAL cells and tubules. Western blot analysis demonstrated constitutive expression of NFAT5 in nuclear extracts from mTAL tubules and primary culture cells; expression in mTAL cells also was detected by immunofluorescence. Expression of NFAT5 was increased in mTAL cells transiently transfected with an NFAT5 overexpression vector (pcDNA3.1-NFAT5), resulting in increased basal and calcium-sensing receptor (CaR)-mediated TNF production. Transient transfection of mTAL cells with a small hairpin RNA vector that targeted exon 8 of NFAT5 (U6-N5 ex8) significantly inhibited TNF promoter activity. Transient transfection with U6-N5 ex8 also reduced nuclear expression of NFAT5, TNF mRNA accumulation, and attenuated CaR-mediated activation of Cl(-) entry into polarized mTAL cells. Collectively, these data suggest that activation of NFAT5 is part of a TNF-dependent pathway that inhibits apical Cl(-) influx in the mTAL after activation of CaR.

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.

Calcium-sensing receptor signaling pathways in medullary thick ascending limb cells mediate COX-2-derived PGE2 production: functional significance

We determined the functional implications of calcium-sensing receptor (CaR)-dependent, Gq- and Gi-coupled signaling cascades, which work in a coordinated manner to regulate activity of nuclear factor of activated T cells and tumor necrosis factor (TNF)-alpha gene transcription that cause expression of cyclooxygenase (COX)-2-derived prostaglandin E2 (PGE2) synthesis by rat medullary thick ascending limb cells (mTAL). Interruption of Gq, Gi, protein kinase C (PKC), or calcineurin (CaN) activities abolished CaR-mediated COX-2 expression and PGE2 synthesis. We tested the hypothesis that these pathways contribute to the effects of CaR activation on ion transport in mTAL cells. Ouabain-sensitive O2 consumption, an in vitro correlate of ion transport in the mTAL, was inhibited by approximately 70% in cells treated for 6 h with extracellular Ca2+ (1.2 mM), an effect prevented in mTAL cells transiently transfected with a dominant negative CaR overexpression construct (R796W), indicating that the effect was initiated by stimulation of the CaR. Pretreatment with the COX-2-selective inhibitor, NS-398 (1 microM), reversed CaR-activated decreases in ouabain-sensitive O2 consumption by approximately 60%, but did not alter basal levels of ouabain-sensitive O2 consumption. Similarly, inhibition of either Gq, Gi, PKC, or CaN, which are components of the mechanism associated with CaR-stimulated COX-2-derived PGE2 synthesis, reversed the inhibitory effects of CaR on O2 consumption without affecting basal O2 consumption. Our findings identified signaling elements required for CaR-mediated TNF production that are integral components regulating mTAL function via a mechanism involving COX-2 expression and PGE2 production.

CaR activation increases TNF production by mTAL cells via a Gi-dependent mechanism

We evaluated the contribution of calcium-sensing receptor (CaR)-mediated G(i)-coupled signaling to TNF production in medullary thick ascending limb (mTAL) cells. A selective G(i) inhibitor, pertussis toxin (PTX), but not the inactive B-oligomer binding subunit, abolished CaR-mediated increases in TNF production. The inhibitory effect of PTX was partially reversed by using an adenylate cyclase inhibitor. CaR-mediated TNF production also was partially reversed by a cAMP analog, 8-Br-cAMP. IP(1) accumulation was CaR dependent and blocked by PI-PLC; partial inhibition also was observed with PTX. CaR increased calcineurin (CaN) activity by approximately threefold, and PTX prevented CaR-mediated increases in CaN activity, an nuclear factor of activated T cells (NFAT)-cis reporter construct, and a TNF promoter construct. The interaction between G(i) and PKC was determined, as we previously showed that CaR-mediated TNF production was CaN and NFAT- mediated and G(q) dependent. CaR activation increased PKC activity by twofold, an effect abolished by transient transfection with a dominant negative CaR construct, R796W, or pretreatment with PTX. Inhibition with the pan-specific PKC inhibitor GF 109203X (20 nM) abolished CaR-mediated increases in activity of CaN, an NFAT reporter, and a TNF promoter construct. Collectively, the data suggest that G(i)-coupled signaling contributes to NFAT-mediated TNF production in a CaN- and PKC-dependent manner and may be part of a CaR mechanism to regulate mTAL function. Moreover, concurrent G(q) and G(i) signaling is required for CaR-mediated TNF production in mTAL cells via a CaN/NFAT pathway that is PKC dependent. Understanding CaR-mediated signaling pathways that regulate TNF production in the mTAL is crucial to defining novel mechanisms that regulate extracellular fluid volume and salt balance.

Characterization of a long-term rat mTAL cell line

A medullary thick ascending limb (mTAL) cell line, termed raTAL, has been established from freshly isolated rat mTAL tubules and cultured continuously for up to 75 passages; it retains characteristics of mTAL cells even after retrieval from storage in liquid nitrogen for several months. The cells express Tamm-Horsfall glycoprotein (THP), a TAL-specific marker, grow to confluence, exhibit a polygonal morphology characteristic of epithelial cells, and form “domes.” Detection of THP, Na+-K+-2Cl−cotransporter (NKCC2), Na+-K+-ATPase, and renal outer medullary K+channel (ROMK) was achieved using indirect immunofluorescence and confocal microscopy. Western blot analysis of NKCC2 expression using two different antibodies revealed a band of ∼160 kDa, and RT-PCR analysis demonstrated the presence of NKCC2 isoforms A and F, which was confirmed by DNA sequencing; transport of Cl−into raTAL cells was inhibited by furosemide. Ouabain- and bumetanide-sensitive oxygen consumption, an index of ion transport activity in the mTAL, was observed in raTAL cells, and the number of domes present was reduced significantly when cells were incubated in the presence of ouabain or bumetanide. The specific activity of Na+-K+-ATPase activity was determined in raTAL cells (0.67 ± 0.18 nmol Pi·μg protein−1·min−1), primary cultures of mTAL cells (0.39 ± 0.08 nmol Pi·μg protein−1·min−1), and freshly isolated mTAL tubules (1.10 ± 0.29 nmol Pi·μg protein−1·min−1), and ∼30–50% of total cellular ATPase activity was inhibited by ouabain, in accord with other mTAL preparations. This cell line will be used in studies that address biochemical, molecular, and physiological mechanisms in the mTAL.

Novel role of AQP-1 in NO-dependent vasorelaxation

Nitric oxide (NO) produced by endothelial cells diffuses to vascular smooth muscle cells to cause dilatation of the renal vasculature and other vessels. Although it is generally assumed that NO moves from cell to cell by free diffusion, we recently showed that aquaporin-1 (AQP-1) transports NO across cell membranes. AQP-1 is expressed in endothelial and vascular smooth muscle cells. We hypothesized that diffusion of NO into vascular smooth muscle cells and out of endothelial cells is facilitated by AQP-1, and transport of NO by AQP-1 is involved in endothelium-dependent relaxation. In intact aortic rings from AQP-1 −/− mice, vasorelaxation induced by acetylcholine (which increases endogenous NO) was reduced ( P < 0.0001 vs. control). No differences were found in the relaxation caused by intracellular delivery of NO or intracellular cGMP between strains. In endothelium-denuded aortic rings from AQP-1 −/− mice, the vasorelaxant capability of NO released in the extracellular space was reduced ( P < 0.0001 vs. control). Influx of NO (5 μM) into vascular smooth muscle cells was 0.17 ± 0.02 f.u./s for control and 0.07 ± 0.01 f.u./s for AQP-1 −/− mice, 62% lower ( P < 0.002). NO released by endothelial cells in response to 1 μM acetylcholine was 96.2 ± 17.7 pmol NO/mg for control and 41.9 ± 13.4 pmol NO/mg for AQP-1 −/− mice, 56% reduction ( P < 0.04). NOS3 expression was 1.33 ± 0.29 O.D. units for control and 3.84 ± 0.76 O.D. units for AQP-1 −/− mice, 188% increase ( P < 0.01). We conclude that 1) AQP-1 facilitates NO influx into vascular smooth muscle cells, 2) AQP-1 facilitates NO diffusion out of endothelial cells, and 3) transport of NO by AQP-1 is required for full expression of endothelium-dependent relaxation.

Henle loop basement membrane as initial site for Randall plaque formation

Since the early description of Randall plaques in 1937, studies of the pathogenesis of stone formation mainly focused on the chemistry involving salt precipitation and crystallization, rather than tubular and interstitial medullar mechanisms of calcium concentration and supersaturation. In 2003, Bushinsky published a suggestive and inspiring sequence of events aimed to show that the basement membrane of the thin limb of the loop of Henle can be the first site of nucleation, as recently shown by the impressive work by Evan et al. The aim of this minireview is to verify the consistency of the Evan and Bushinsky theory with the current literature in the field.

Ca2+ stimulates COX-2 expression through calcium-sensing receptor in fibroblasts

Fibroblasts isolated from jaw cysts expressed calcium-sensing receptor (CasR). In the fibroblasts elevated extracellular Ca(2+) ([Ca(2+)](o)) increased fluo-3 fluorescence intensity, and the production of inositol(1,4,5)trisphosphate and active protein kinase C. Phospholipase C inhibitor U-73122 attenuated the Ca(2+)-induced increase in fluo-3 fluorescence intensity. Elevated [Ca(2+)](o) enhanced the expression of cyclooxygenase-2 (COX-2) mRNA and protein, and the secretion of prostaglandin E(2) in the fibroblasts. CasR activator neomycin also increased the expression of COX-2 mRNA, and U-73122 attenuated the Ca(2+)-induced expression of COX-2 mRNA. Elevated [Ca(2+)](o)-induced phosphorylation of extracellular signal-regulated protein kinase-1/2 (ERK1/2), p38 mitogen-activated protein kinase (MAPK), and c-Jun N-terminal kinase (JNK), and U-73122 inhibited the Ca(2+)-induced phosphorylation. The inhibitors for each kinase, PD98059, SB203580, and SP600125, attenuated the Ca(2+)-induced expression of COX-2 mRNA. These results suggest that in jaw cyst fibroblasts elevated extracellular Ca(2+) may enhance COX-2 expression via the activation of ERK1/2, p38 MAPK, and JNK through CasR.

Cyclooxygenase-2 induction by bradykinin in aortic vascular smooth muscle cells

Vascular smooth muscle cell proliferation and migration play an important role in the pathophysiology of several vascular diseases, including atherosclerosis. Prostaglandins that have been implicated in this process are synthesized by two isoforms of cyclooxygenase (COX), with the expression of the regulated COX-2 isoform increased in atherosclerotic plaques. Bradykinin (BK), a vasoactive peptide increased in inflammation, induces the formation of prostaglandins through specific receptor activation. We hypothesized that BK plays an important role in the regulation of COX-2, contributing to the increase in production of prostaglandins in vascular smooth muscle cells. Herein we examined the signaling pathways that participate in the BK regulation of COX-2 protein levels in primary cultured aortic vascular smooth muscle cells. We observed an increase in COX-2 protein levels induced by BK that was maximal at 24 h. This increase was blocked by a B2 kinin receptor antagonist but not a B1 receptor antagonist, suggesting that the B2 receptor is involved in this pathway. In addition, we conclude that the activation of mitogen-activated protein kinases p42/p44, protein kinase C, and nitric oxide synthase is necessary for the increase in COX-2 levels induced by BK because either of the specific inhibitors for these enzymes blocked the effect of BK. Using a similar approach, we further demonstrated that reactive oxygen species and cAMP were not mediators on this pathway. These results suggest that BK activates several intracellular pathways that act in combination to increase COX-2 protein levels. This study suggests a role for BK on the evolution of the atheromatous plaque by virtue of controlling the levels of COX-2.

NFAT regulates calcium-sensing receptor-mediated TNF production

Because nuclear factor of activated T cells (NFAT) has been implicated in TNF production as well as osmoregulation and salt and water homeostasis, we addressed whether calcium-sensing receptor (CaR)-mediated TNF production in medullary thick ascending limb (mTAL) cells was NFAT dependent. TNF production in response to addition of extracellular Ca(2+) (1.2 mM) was abolished in mTAL cells transiently transfected with a dominant-negative CaR construct (R796W) or pretreated with the phosphatidylinositol phospholipase C (PI-PLC) inhibitor U-73122. Cyclosporine A (CsA), an inhibitor of the serine/threonine phosphatase calcineurin, and a peptide ligand, VIVIT, that selectively inhibits calcineurin-NFAT signaling, also prevented CaR-mediated TNF production. Increases in calcineurin activity in cells challenged with Ca(2+) were inhibited after pretreatment with U-73122 and CsA, suggesting that CaR activation increases calcineurin activity in a PI-PLC-dependent manner. Moreover, U-73122, CsA, and VIVIT inhibited CaR-dependent activity of an NFAT construct that drives expression of firefly luciferase in transiently transfected mTAL cells. Collectively, these data verify the role of calcineurin and NFAT in CaR-mediated TNF production by mTAL cells. Activation of the CaR also increased the binding of NFAT to a consensus oligonucleotide, an effect that was blocked by U-73122 and CsA, suggesting that a calcineurin- and NFAT-dependent pathway increases TNF production in mTAL cells. This mechanism likely regulates TNF gene transcription as U-73122, CsA, and VIVIT blocked CaR-dependent activity of a TNF promoter construct. Elucidating CaR-mediated signaling pathways that regulate TNF production in the mTAL will be crucial to understanding mechanisms that regulate extracellular fluid volume and salt balance.

Vitamin D and calcium deficits predispose for multiple chronic diseases

There is evidence from both observational studies and clinical trials that calcium malnutrition and hypovitaminosis D are predisposing conditions for various common chronic diseases. In addition to skeletal disorders, calcium and vitamin D deficits increase the risk of malignancies, particularly of colon, breast and prostate gland, of chronic inflammatory and autoimmune diseases (e.g. insulin-dependent diabetes mellitus, inflammatory bowel disease, multiple sclerosis), as well as of metabolic disorders (metabolic syndrome, hypertension). The aim of the present review was to provide improved understanding of the molecular and cellular processes by which deficits in calcium and vitamin D cause specific changes in cell and organ functions and thereby increase the risk for chronic diseases of different aetiology. 1,25-Dihydroxyvitamin D(3) and extracellular Ca(++) are both key regulators of proliferation, differentiation and function at the cellular level. However, the efficiency of vitamin D receptor-mediated intracellular signalling is limited by the negative effects of hypovitaminosis D on extrarenal 25-hydroxyvitamin D-1alpha-hydroxylase activity and thus on the production of 1,25-dihydroxyvitamin D(3). Calcium malnutrition eventually causes a decrease in calcium concentration in extracellular fluid compartments, resulting in organ-specific modulation of calcium-sensing receptor activity. Hence, attenuation of signal transduction from the ligand-activated vitamin D receptor and calcium-sensing receptor seems to be the prime mechanism by which calcium and vitamin D insufficiencies cause perturbation of cellular functions in bone, kidney, intestine, mammary and prostate glands, endocrine pancreas, vascular endothelium, and, importantly, in the immune system. The wide range of diseases associated with deficits in calcium and vitamin D in combination with the high prevalence of these conditions represents a special challenge for preventive medicine.

Calcium and salinity sensing by the thick ascending limb: a journey from mammals to fish and back again

The roles of the CaSR in endocrine, epithelial, CNS, and other cells have been reviewed previously [17-19, 20, 27-30, 31-33]. This brief review focuses on the roles of the CaSR in the thick ascending limb of Henle (TAL), and is written in honor of my mentor and long-term friend and colleague, Thomas E. Andreoli, on the occasion of his retirement. My early studies of TAL function with Tom Andreoli were the inspiration for this work.

Extracellular calcium induces COX-2 in osteoblasts via a PKA pathway

We have shown that extracellular calcium [Ca(+2)](e) induces cyclooxygenase-2 (COX-2) expression and prostaglandin E(2) (PGE(2)) production via an ERK signaling pathway in osteoblasts. In this study, we examined the roles of protein kinase C (PKC) and A (PKA) signaling pathways in the [Ca(+2)](e) induction of COX-2 in primary calvarial osteoblasts from mice transgenic for -371 bp of the COX-2 promoter fused to a luciferase reporter. Neither PKC specific inhibitors nor downregulation of the PKC pathway by phorbol myristate acetate (PMA) affected the [Ca(+2)](e) stimulation of COX-2 mRNA or promoter activity. In contrast, PKA inhibitors, used at doses that inhibited forskolin-stimulated luciferase activity by 90%, reduced [Ca(+2)](e)-stimulated COX-2 mRNA expression and promoter activity by 80-90%. [Ca(+2)](e) also stimulated a 2- to 3-fold increase in cAMP production. Hence, the [Ca(+2)](e) induction of COX-2 mRNA expression and promoter activity was independent of the PKC pathway and dependent on the PKA signaling pathway.

Extracellular calcium is a potent inducer of cyclo-oxygenase-2 in murine osteoblasts through an ERK signaling pathway

[Ca2+]e may be important in bone turnover. We found [Ca2+]e induces COX-2 transcription and PGE2 production in primary calvarial osteoblasts through an ERK signaling pathway. Inhibition of PGE2 production inhibited the [Ca2+]e stimulation of osteoblastic differentiation but not the increase in cell number. Hence, some effects of [Ca2+]e on bone may be mediated by COX-2.

INTRODUCTION

Local changes in extracellular calcium ([Ca2+]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 [Ca2+]e on osteoblasts.

METHODS

We examined the [Ca2+]e induction of COX-2 expression and prostaglandin E2 (PGE2) 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 PGE2 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.

RESULTS AND CONCLUSIONS

COX-2 mRNA and protein expression were induced by 3-40 mM of [Ca2+]e. [Ca2+]e (5 mM) induced COX-2 mRNA within 30 minutes; levels peaked at 6-9 h and remained elevated at 24 h. Cumulative medium PGE2 was increased at 3 h, with levels rising to 30 nM at 24 h. PGE2 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. [Ca2+]e increased alkaline phosphatase activity and osteocalcin mRNA, and this increase was blocked by inhibiting PGE2 production. [Ca2+]e stimulation of COX-2 promoter activity correlated with the induction of COX-2 mRNA expression. [Ca2+]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 [Ca2+]e induction of both COX-2 mRNA and luciferase activity by 70-80%. Although less effective than [Ca2+]e, strontium [Sr2+]e also induced COX-2 mRNA and promoter activity in POBs through an ERK signaling pathway. We conclude that [Ca2+]e is a potent transcriptional inducer of COX-2 expression and PGE2 production in osteoblasts through an ERK signaling pathway.

Calcium-sensing receptor-mediated TNF production in medullary thick ascending limb cells

Medullary thick ascending limb (mTAL) cells in primary culture express the Ca(2+)-sensing receptor (CaR), a G protein-coupled receptor that senses changes in extracellular Ca(2+) (Ca(o)(2+)) concentration, resulting in increases of intracellular Ca(2+) concentration and PKC activity. Exposure of mTAL cells to either Ca(o)(2+) or the CaR-selective agonist poly-L-arginine increased TNF-alpha synthesis. Moreover, the response to Ca(o)(2+) was enhanced in mTAL cells transfected with a CaR overexpression vector. Transfection of mTAL cells with a TNF promoter construct revealed an increase in reporter gene activity after exposure of the cells to Ca(o)(2+), suggesting that intracellular signaling pathways initiated by means of activation of a CaR contribute to TNF synthesis by a mechanism that involves transcription of the TNF gene. Neutralization of TNF activity with an anti-TNF antibody attenuated Ca(2+)-mediated increases in cyclooxygenase-2 (COX-2) protein expression and PGE(2) synthesis, suggesting that TNF exerts an autocrine effect in the mTAL, which contributes to COX-2-mediated PGE(2) production. Preincubation with the PKC inhibitor bisindolylmaleimide I inhibited Ca(2+)-mediated TNF production. Significant inhibition of COX-2 protein expression and PGE(2) synthesis also was observed when cells were challenged with Ca(o)(2+) in the presence of bisindolylmaleimide I. The data suggest that increases in TNF production subsequent to activation of the CaR may be the basis of an important renal mechanism that regulates salt and water excretion.