Cellular regulation of sodium-calcium exchange

Rise of palmitoylation: A new trick to tune NCX1 activity

The cardiac Na+/Ca2+ Exchanger (NCX1) controls transmembrane calcium flux in numerous tissues. The only reversible post-translational modification established to regulate NCX1 is palmitoylation, which alters the ability of the exchanger to inactivate. Palmitoylation creates a binding site for the endogenous XIP domain, a region of the NCX1 intracellular loop established to inactivate NCX1. The binding site created by NCX1 palmitoylation sensitizes the transporter to XIP. Herein we summarize our recent knowledge on NCX1 palmitoylation and its association with cardiac pathologies, and discuss these findings in the light of the recent cryo-EM structures of human NCX1.

Fetal cardiomyocyte phenotype, ketone body metabolism, and mitochondrial dysfunction in the pathology of atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia diagnosed in clinical practice. Even though hypertension, congestive heart failure, pulmonary disease, and coronary artery disease are the potential risk factors for AF, the underlying molecular pathology is largely unknown. The reversion of the mature cardiomyocytes to fetal phenotype, impaired ketone body metabolism, mitochondrial dysfunction, and the cellular effect of reactive oxygen species (ROS) are the major underlying biochemical events associated with the molecular pathology of AF. On this background, the present manuscript sheds light into these biochemical events in regard to the metabolic derangements in cardiomyocyte leading to AF, especially with respect to structural, contractile, and electrophysiological properties. In addition, the article critically reviews the current understanding, potential demerits, and translational strategies in the management of AF.

O-GlcNAcylation of cardiac Nav1.5 contributes to the development of arrhythmias in diabetic hearts

BACKGROUND

Cardiovascular complications are major causes of mortality and morbidity in diabetic patients. The mechanisms underlying the progression of diabetic heart (DH) to ventricular arrhythmias are unclear. O-linked GlcNAcylation (O-GlcNAc) is a reversible post-translational modification for the regulation of diverse cellular processes. The purpose of this study was to assess whether the cardiac voltage-gated sodium channel (Nav1.5) is subjected to O-linked GlcNAcylation (O-GlcNAc), which plays an essential role in DH-induced arrhythmias.

METHODS AND RESULTS

In this study, Sprague-Dawley rats (male, 200-230 g) were treated with a single high-dose of streptozotocin (STZ, 80 mg/kg) to generate a rat model of diabetes. STZ-induced 3-month diabetic rats displayed increased susceptibility to ventricular arrhythmias. The elevated O-GlcNAc modification was correlated with decreases in both total and cytoplasmic Nav1.5 expression in vivo and in vitro. In addition, both co-immunoprecipitation and immunostaining assays demonstrated that hyperglycemia could increase the O-GlcNAc-modified Nav1.5 levels and decrease the interaction between Nav1.5 and Nav1.5-binding proteins Nedd4-2/SAP-97. Furthermore, patch-clamp measurements in HEK-293 T cells showed that Nav1.5 current densities decreased by 30% after high-glucose treatment, and the sodium currents increased via O-GlcNAc inhibition.

CONCLUSION

Our data suggested that hyperglycemia increased the O-GlcNAc modification of Nav1.5 expression and decreased the interaction between Nav1.5 and Nedd4-2/SAP-97, which led to the abnormal expression and distribution of Nav1.5, loss of function of the sodium channel, and prolongation of the PR/QT interval. Excessive O-GlcNAc modification of Nav1.5 is a novel signaling event, which may be an underlying contributing factor for the development of the arrhythmogenesis in DH.

Proteomic analysis of imatinib-resistant CML-T1 cells reveals calcium homeostasis as a potential therapeutic target

Chronic myeloid leukemia (CML) therapy has markedly improved patient prognosis after introduction of imatinib mesylate for clinical use. However, a subset of patients develops resistance to imatinib and other tyrosine kinase inhibitors (TKIs), mainly due to point mutations in the region encoding the kinase domain of the fused BCR-ABL oncogene. To identify potential therapeutic targets in imatinib-resistant CML cells, we derived imatinib-resistant CML-T1 human cell line clone (CML-T1/IR) by prolonged exposure to imatinib in growth media. Mutational analysis revealed that the Y235H mutation in BCR-ABL is probably the main cause of CML-T1/IR resistance to imatinib. To identify alternative therapeutic targets for selective elimination of imatinib-resistant cells, we compared the proteome profiles of CML-T1 and CML-T1/IR cells using 2-DE-MS. We identified eight differentially expressed proteins, with strongly upregulated Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) in the resistant cells, suggesting that this protein may influence cytosolic pH, Ca²⁺ concentration or signaling pathways such as Wnt in CML-T1/IR cells. We tested several compounds including drugs in clinical use that interfere with the aforementioned processes and tested their relative toxicity to CML-T1 and CML-T1/IR cells. Calcium channel blockers, calcium signaling antagonists and modulators of calcium homeostasis, namely thapsigargin, ionomycin, verapamil, carboxyamidotriazole and immunosuppressive drugs cyclosporine A and tacrolimus (FK-506) were selectively toxic to CML-T1/IR cells. The putative cellular targets of these compounds in CML-T1/IR cells are postulated in this study. We propose that Ca²⁺ homeostasis can be a potential therapeutic target in CML cells resistant to TKIs. We demonstrate that a proteomic approach may be used to characterize a TKI-resistant population of CML cells enabling future individualized treatment options for patients.

Cardiomyocyte Remodeling in Atrial Fibrillation and Hibernating Myocardium: Shared Pathophysiologic Traits Identify Novel Treatment Strategies?

Atrial fibrillation (AF) is the most common arrhythmia and is associated with a high risk of morbidity and mortality. However, there are limited treatment strategies for prevention of disease onset and progression. Development of novel therapies for primary and secondary prevention of AF is critical and requires improved understanding of the cellular and molecular mechanisms underlying the AF disease process. Translational and clinical studies conducted over the past twenty years have revealed that atrial remodeling in AF shares several important pathophysiologic traits with the remodeling processes exhibited by hibernating myocardium that develop in response to chronic ischemia. These shared features, which include an array of structural, metabolic, and electrophysiologic changes, appear to represent a conserved adaptive myocyte response to chronic stress that involves dedifferentiation towards a fetal phenotype to promote survival. In this review, we discuss the pathophysiology of AF, summarize studies supporting a common remodeling program in AF and hibernating myocardium, and propose future therapeutic implications of this emerging paradigm. Ultimately, better understanding of the molecular mechanisms of atrial myocyte remodeling during the onset of AF and the transition from paroxysmal to persistent stages of the disease may facilitate discovery of new therapeutic targets.

Diacylglycerol kinase γ regulates antigen-induced mast cell degranulation by mediating Ca(2+) influxes

Diacylglycerol (DAG) is an important lipid that acts as a signaling messenger during mast cell degranulation after allergen cross-linking of immunoglobulin (Ig) E-bound FcεRI receptors. In this study, we determined the role of diacylglycerol kinase (DGK), which negatively regulates DAG-dependent signaling by converting DAG to phosphatidic acid (PA), in the regulation of mast cell degranulation. Treating RBL (rat basophilic leukemia)-2H3 mast cells with a type I DGK inhibitor significantly reduced antigen-induced degranulation and PA production. Among type I DGK isoforms, we observed that DGKα and DGKγ mRNAs were expressed in RBL-2H3 mast cells using reverse transcription polymerase chain reaction. DGKγ knockdown, but not DGKα, by isoform-specific short hairpin RNAs reduced mast cell degranulation and Ca(2+) influxes from the extracellular environment. These results suggest that DGKγ regulates mast cell degranulation after FcεRI cross-linking through mobilization of intracellular Ca(2+) through Ca(2+) influxes.

Role of intracellular Ca2+ and Na+/Ca2+ exchanger in the pathogenesis of contrast-induced acute kidney injury

The precise mechanisms underlying contrast-induced acute kidney injury (CI-AKI) are not well understood. Intracellular Ca²⁺ overload is considered to be a key factor in CI-AKI. Voltage-dependent Ca²⁺ channel (VDC) and Na⁺/Ca²⁺ exchanger (NCX) system are the main pathways of intracellular Ca²⁺ overload in pathological conditions. Here, we review the potential underlying mechanisms involved in CI-AKI and discuss the role of NCX-mediated intracellular Ca²⁺ overload in the contrast media-induced renal tubular cell injury and renal hemodynamic disorder.

Functional comparison of the reverse mode of Na+/Ca2+ exchangers NCX1.1 and NCX1.5 expressed in CHO cells

AIM

To investigate the reverse mode function of Na(+)/Ca(2+) exchangers NCX1.1 and NCX1.5 expressed in CHO cells as well as their modulations by PKC and PKA.

METHODS

CHO-K1 cells were transfected with pcDNA3.1 (+) plasmid carrying cDNA of rat cardiac NCX1.1 and brain NCX1.5. The expression of NCX1.1 and NCX1.5 was examined using Western blot analysis. The intracellular Ca(2+) level ([Ca(2+)]i) was measured using Ca(2+) imaging. Whole-cell NCX currents were recorded using patch-clamp technique. Reverse mode NCX activity was elicited by perfusion with Na(+)-free medium. Ca(2+) paradox was induced by Ca(2+)-free EBSS medium, followed by Ca(2+)-containing solution (1.8 or 3.8 mmol/L CaCl2).

RESULTS

The protein levels of NCX1.1 and NCX1.5 expressed in CHO cells had no significant difference. The reverse modes of NCX1.1 and NCX1.5 in CHO cells exhibited a transient increase of [Ca(2+)]i, which was followed by a Ca(2+) level plateau at higher external Ca(2+) concentrations. In contrast, the wild type CHO cells showed a steady increase of [Ca(2+)]i at higher external Ca(2+) concentrations. The PKC activator PMA (0.3-10 μmol/L) and PKA activator 8-Br-cAMP (10-100 μmol/L) significantly enhanced the reverse mode activity of NCX1.1 and NCX1.5 in CHO cells. NCX1.1 was 2.4-fold more sensitive to PKC activation than NCX1.5, whereas the sensitivity of the two NCX isoforms to PKA activation had no difference. Both PKC- and PKA-enhanced NCX reverse mode activities in CHO cells were suppressed by NCX inhibitor KB-R7943 (30 μmol/L).

CONCLUSION

Both NCX1.1 and NCX1.5 are functional in regulating and maintaining stable [Ca(2+)]i in CHO cells and differentially regulated by PKA and PKC. The two NCX isoforms might be useful drug targets for heart and brain protection.

Selective inhibition of the reverse mode of Na(+)/Ca(2+) exchanger attenuates contrast-induced cell injury

BACKGROUND

The precise mechanisms underlying radiocontrast nephropathy (RCN) are not well understood. Intracellular Ca(2+) overload is considered to be a key factor in RCN. The Na(+)/Ca(2+) exchanger (NCX) system is one of the main pathways of intracellular Ca(2+) overload. We investigated whether intracellular Ca(2+) overload via the NCX system was involved in contrast-induced renal tubular cytotoxicity.

METHODS

NRK-52E cells were exposed to ioversol (100 mg iodine/ml) for 4 h. KB-R7943 (inhibitor of reverse mode of NCX, 4 × 10(-5), 4 × 10(-6)M) was added 1 h before incubation with ioversol. Cell viability and permeability were determined by 3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyl tetrazolium bromide and lactate dehydrogenase assay. Apoptosis was determined by flow cytometry. Intracellular Ca(2+) concentration ([Ca(2+)](i)] and reactive oxygen species (ROS) were detected by confocal microscopy. The expression of NCX1 mRNA and caspase-3 protein was evaluated by reverse transcription-polymerase chain reaction and Western blot, respectively.

RESULTS

Ioversol exposure induced significantly increased lactate dehydrogenase release and decreased 3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyl tetrazolium bromide conversion in NRK-52E cells. Significantly increased apoptosis and caspase-3 protein expression were observed in the NRK-52E cells exposed to ioversol for 4 h. Ioversol treatment induced a significant increase in [Ca(2+)](i) and intracellular ROS. KB-R7943 dose-dependently and significantly suppressed the increase in [Ca(2+)](i), intracellular ROS and caspase-3 overexpression induced by ioversol and attenuated the contrast-induced NRK-52E cell apoptosis. No significant changes in NCX1 mRNA expression were observed following contrast exposure.

CONCLUSION

Intracellular Ca(2+) overload via the reverse mode of NCX, followed by ROS overproduction and caspase-3 overexpression played an important role in the contrast-induced renal tubular cytotoxicity. The reverse mode of the NCX inhibitor KB-R7943 attenuated contrast-induced renal tubular cytotoxicity.

Effects of estradiol and IGF-1 on the sodium calcium exchanger in rat cultured cortical neurons

The Na(+)/Ca(2+) exchanger (NCX) is an important bidirectional transporter of calcium in neurons and has been shown to be involved in neuroprotection. Calcium can activate a number of cascades that can result in apoptosis and cell death, and NCX is a key factor in regulating the cytoplasmic concentration of this ion. 17-β-estradiol and insulin-like growth factor 1 (IGF-1) are known neuroprotective hormones with interacting mechanisms and effects on intracellular calcium; however, their relationship with the NCX has not been explored. In this article, the effects of these two hormones on neuronal NCX were tested using the whole-cell patch clamp technique on rat primary culture neurons. Both 17-β-estradiol and IGF-1 produced an increase in the NCX-mediated inward current and a decrease in the NCX-mediated outward current. However, the IGF-1 effect was lower than that of 17-β-estradiol, and the effect of both agents together was greater than the sum of each agent alone. Neither of the agents affected the pattern of regulation by extracellular or intrapipette calcium. Inhibitors of the IGF-1 and 17-β-estradiol receptors and inhibitors of the main signaling pathways failed to change the observed effects, indicating that these actions were not mediated by the classical receptors of these hormones. These effects on the NCX could be a mechanism explaining the neuroprotective actions of 17-β-estradiol and IGF-1, and these findings could help researchers to understand the role of the NCX in neuroprotection.

Sodium-calcium exchange mediated contraction in left anterior descending and left ventricular branch arteries

We tested the hypothesis that the de-endothelialized artery rings from the left anterior descending (LAD) coronary artery and its left ventricular branch (LVB) differ in their contractile responses to Na⁺–Ca²⁺-exchanger (NCX) mediated Ca²⁺-entry, muscarinic receptor activation with carbachol, and sarco/endoplasmic reticulum Ca²⁺ pump (SERCA) inhibition with thapsigargin. In LVB, the force of contraction (in N/g tissue) produced by the NCX mediated Ca²⁺-entry (17.5 ± 1.4) and carbachol (18 ± 1.5) was only slightly smaller than that due to membrane depolarization with KCl (24.0 ± 1.0). In contrast, in LAD the force of contraction produced with NCX (8.7 ± 0.7) and carbachol (6.1 ± 1.1) was much smaller than with KCl (15.7 ± 0.7). Thapsigargin also contracted LVB with greater force than LAD. When isolated microsomes were used, the binding to the muscarinic receptor antagonist quinuclidinyl benzilate was greater in LVB than in LAD. Microsomes were also used for Western blots. The intensities of signals for both SERCA and NCX were greater in LVB than in LAD. These biochemical observations were consistent with the contractile experiments. Thus, it appears that the differences between LAD and the resistance arteries may begin as early as LVB.

Defects in myelination, paranode organization and Purkinje cell innervation in the ether lipid-deficient mouse cerebellum

Ether lipids (ELs), particularly plasmalogens, are essential constituents of the mammalian central nervous system. The physiological role of ELs, in vivo, however is still enigmatic. In the present study, we characterized a mouse model carrying a targeted deletion of the peroxisomal dihydroxyacetonephosphate acyltransferase gene that results in the complete lack of ELs. Investigating the cerebellum of these mice, we observed: (i) defects in foliation patterning and delay in precursor granule cell migration, (ii) defects in myelination and concomitant reduction in the level of myelin basic protein, (iii) disturbances in paranode organization by extending the Caspr distribution and disrupting axo-glial septate-like junctions, (iv) impaired innervation of Purkinje cells by both parallel fibers and climbing fibers and (v) formation of axon swellings by the accumulation of inositol-tris-phosphate receptor 1 containing smooth ER-like tubuli. Functionally, conduction velocity of myelinated axons in the corpus callosum was significantly reduced. Most of these phenotypes were already apparent at P20 but still persisted in 1-year-old animals. In summary, these data show that EL deficiency results in severe developmental and lasting structural alterations at the cellular and network level of the cerebellum, and reveal an important role of ELs for proper brain function. Common molecular mechanisms that may underlie these phenotypes are discussed.

Involvement of protein kinase C in the regulation of Na+/Ca2+ exchanger in bovine adrenal chromaffin cells

1. The Na(+)/Ca(2+) exchanger (NCX) exchanges Na+ and Ca(2+) bidirectionally through the forward mode (Ca(2+) extrusion) or the reverse mode (Ca(2+) influx). The present study was undertaken to clarify the role of protein kinase C (PKC) in the regulation of NCX in bovine adrenal chromaffin cells. The Na(+)-loaded cells were prepared by treatment with 100 micromol/L ouabain and 50 micromol/L veratridine. Incubation of Na(+)-loaded cells with Na(+)-free solution in the presence of the Ca(2+) channel blockers nicardipine (3 micromol/L) and omega-conotoxin MVIIC (0.3 micromol/L) caused Ca(2+) uptake and catecholamine release. 2. The Na(+)-dependent Ca(2+) uptake and catecholamine release were inhibited by 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400; 1 micromol/L) and 2-[2-[4-(4-nitrobenzyloxy)phenyl]isothiourea (KB-R7943; 10 micromol/L), both NCX inhibitors. These results indicate that the Na(+)-dependent responses are mostly due to activation of the NCX working in the reverse mode. 3. In addition, we examined the effects of PKC inhibitors and an activator on the NCX-mediated Ca(2+) uptake and catecholamine release. Bisindolylmaleimide I (0.3-10 micromol/L) and chelerythrine (3-100 micromol/L), both PKC inhibitors, inhibited NCX-mediated responses. In contrast, phorbol 12,13-dibutyrate (0.1-10 micromol/L), a PKC activator, enhanced the responses. Bisindolylmaleimide I and chelerythrine, at effective concentrations for inhibition of Na(+)-dependent catecholamine release, had a little or no effect on high K(+)-induced catecholamine release in intact cells or on Ca(2+)-induced catecholamine release in beta-escin-permeabilized cells. 4. These results suggest that PKC is involved in the activation of NCX in bovine adrenal chromaffin cells.

Unraveling the complexities of sphingosine-1-phosphate function: the mast cell model

Sphingosine-1-phosphate (S1P) is a lipid mediator involved in diverse biological processes, from vascular and neural development to the regulation of lymphocyte trafficking. Many of its functions are regulated by five widely expressed S1P G-protein-coupled receptors (S1P(1-5)). S1P is produced mostly intracellularly, thus, much of its potential as an autocrine and paracrine mediator depends on how, when, and where it is generated or secreted out of the cells. However, S1P can also have intracellular activity independent of its receptors, adding to the complexity of S1P function. The mast cell, a major effector cell during an allergic response, has proven instrumental towards understanding the complex regulation and function of S1P. Antigen (Ag) engagement of the IgE receptor in mast cells stimulates sphingosine kinases, which generate S1P and are involved in the activation of calcium fluxes critical for mast cell responses. In addition, mast cells secrete considerable amounts of S1P upon activation, thus affecting the surrounding tissues and recruiting inflammatory cells. Export of S1P is also involved in the autocrine transactivation of S1P receptors present in mast cells. The in vivo response of mast cells, however, is not strictly dependent on their ability to generate S1P, but they are also affected by changes in S1P in the environment previous to Ag challenge. This review will discuss the recent advances towards understanding the intricacies of S1P generation, secretion and regulation in mast cells. In addition, how S1P receptors are activated and their involvement in mast cell functions will also be covered, including new insights on the role of S1P in the mast cell-mediated allergic response of systemic anaphylaxis.

The sphingosine kinase-sphingosine-1-phosphate axis is a determinant of mast cell function and anaphylaxis

Sphingosine-1-phosphate, a key mediator in immune cell trafficking, is elevated in the lungs of asthmatic patients and regulates pulmonary epithelium permeability. Stimulation of mast cells by allergens induces two mammalian sphingosine kinases (Sphk1 and Sphk2) to produce sphingosine-1-phosphate (S1P). Little is known about the individual role of these kinases in regulating immune cell function. Here we show that in mast cells, Sphk2 is required for production of S1P, for calcium influx, for activation of protein kinase C, and for cytokine production and degranulation. However, susceptibility to in vivo anaphylaxis is determined both by S1P within the mast cell compartment and by circulating S1P generated by Sphk1 predominantly from a non-mast cell source(s). Thus, sphingosine kinases are determinants of mast cell responsiveness, demonstrating a previously unrecognized relationship with anaphylaxis.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Inhibition by SEA0400, a Selective Inhibitor of Na+/Ca2+ Exchanger, of Na+-Dependent Ca2+ Uptake and Catecholamine Release in Bovine Adrenal Chromaffin Cells

The effects of SEA0400, a selective inhibitor of the Na(+)/Ca(2+) exchanger (NCX), on Na(+)-dependent Ca(2+) uptake and catecholamine (CA) release were examined in bovine adrenal chromaffin cells that were loaded with Na(+) by treatment with ouabain and veratridine. SEA0400 inhibited Na(+)-dependent (45)Ca(2+) uptake and CA release, with the IC(50) values of 40 and 100 nM, respectively. The IC(50) values of another NCX inhibitor KB-R7943 were 1.8 and 3.7 microM, respectively. These results indicate that SEA0400 is about 40 times more potent than KB-R7943 in inhibiting NCX working in the reverse mode. In intact cells, SEA0400 and KB-R7943 inhibited CA release induced by acetylcholine and DMPP. The IC(50) values of SEA0400 were 5.1 and 4.5 microM and the values of KB-R7943 were 2.6 and 2.1 microM against the release induced by acetylcholine and DMPP, respectively, indicating that the potency of SEA0400 is about a half of that of KB-R7943 in inhibiting the nicotinic receptor-mediated CA release. The binding of [(3)H]nicotine with nicotinic receptors was inhibited by SEA0400 (IC(50) = 90 microM) and KB-R7943 (IC(50) = 12 microM). From these results, it is concluded that unlike KB-R7943, SEA0400 has a potent and selective action on NCX in bovine adrenal chromaffin cells.

Na+/Ca2+ exchanger activity modulates connective tissue growth factor mRNA expression in transforming growth factor beta1- and Des-Arg10-kallidin-stimulated myofibroblasts

Transforming growth factor (TGF)-beta and des-Arg(10)-kallidin stimulate the expression of connective tissue growth factor (CTGF), a matrix signaling molecule that is frequently overexpressed in fibrotic disorders. Because the early signal transduction events regulating CTGF expression are unclear, we investigated the role of Ca(2+) homeostasis in CTGF mRNA expression in TGF-beta1- and des-Arg(10)-kallidin-stimulated human lung myofibroblasts. Activation of the kinin B1 receptor with des-Arg(10)-kallidin stimulated a rise in cytosolic Ca(2+) that was extracellular Na(+)-dependent and extracellular Ca(2+)-dependent. The des-Arg(10)-kallidin-stimulated increase of cytosolic Ca(2+) was blocked by KB-R7943, a specific inhibitor of Ca(2+) entry mode operation of the plasma membrane Na(+)/Ca(2+) exchanger. TGF-beta1 similarly stimulated a KB-R7943-sensitive increase of cytosolic Ca(2+) with kinetics distinct from the des-Arg(10)-kallidin-stimulated Ca(2+) response. We also found that KB-R7943 or 2',4'-dichlorobenzamil, an amiloride analog that inhibits the Na(+)/Ca(2+) exchanger activity, blocked the TGF-beta1- and des-Arg(10)-kallidin-stimulated increases of CTGF mRNA. Pretreatment with KB-R7943 also reduced the basal and TGF-beta1-stimulated levels of alpha1(I) collagen and alpha smooth muscle actin mRNAs. These data suggest that, in addition to regulating ion homeostasis, Na(+)/Ca(2+) exchanger acts as a signal transducer regulating CTGF, alpha1(I) collagen, and alpha smooth muscle actin expression. Consistent with a more widespread role for Na(+)/Ca(2+) exchanger in fibrogenesis, we also observed that KB-R7943 likewise blocked TGF-beta1-stimulated levels of CTGF mRNA in human microvascular endothelial and human osteoblast-like cells. We conclude that Ca(2+) entry mode operation of the Na(+)/Ca(2+) exchanger is required for des-Arg(10)-kallidin- and TGF-beta1-stimulated fibrogenesis and participates in the maintenance of the myofibroblast phenotype.

Lanthanum is transported by the sodium/calcium exchanger and regulates its activity

La3+ uptake was measured in fura 2-loaded Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchanger (NCX1.1). La3+ was taken up by the cells after an initial lag phase of 50-60 s and achieved a steady state within 5-6 min. Neonatal cardiac myocytes accumulated La3+ in a similar manner. La3+ uptake was due to the activity of the exchanger, because no uptake was seen in nontransfected cells or in transfected cells that had been treated with gramicidin to remove cytosolic Na+. The low rate of La3+ uptake during the lag period resulted from insufficient cytosolic Ca2+ to activate the exchanger at its regulatory sites, as shown by the following observations. La3+ uptake occurred without a lag period in cells expressing a mutant of NCX1.1 that does not exhibit regulatory activation by cytosolic Ca2+. The rate of La3+ uptake by wild-type cells was increased, and the lag phase was reduced or eliminated, when the cytosolic Ca2+ concentration was increased before initiating La3+ uptake. La3+ could substitute for Ca2+ at very low concentrations to activate exchange activity. Thus preloading cells expressing NCX1.1 with a small quantity of La3+ increased the rate of exchange-mediated Ca2+ influx by 20-fold; in contrast, cytosolic La3+ partially inhibited Ca2+ uptake by the regulation-deficient mutant. With an estimated KD of 30 pM for the binding of La3+ to fura 2, we conclude that cytosolic La3+ activates exchange activity at picomolar concentrations. We speculatively suggest that endogenous trace metals might activate exchange activity under physiological conditions.