Proton Pump Inhibitors and Serum Magnesium Levels in Patients With Torsades de Pointes.

Background: Torsades de pointes (TdP) is a life-threatening ventricular tachycardia occurring in long QT-syndrome patients. It usually develops when multiple QT-prolonging factors are concomitantly present, more frequently drugs and electrolyte imbalances. Since proton-pump inhibitors (PPIs)-associated hypomagnesemia is an increasingly recognized adverse event, PPIs were recently included in the list of drugs with conditional risk of TdP, despite only few cases of TdP in PPI users have been reported so far. Objectives: Aim of the present study is to evaluate whether PPI-induced hypomagnesemia actually has a significant clinical impact on the risk of TdP in the general population. Methods: Forty-eight unselected patients who experienced TdP were consecutively enrolled (2008-2017). Shortly after the first TdP episode, in those patients who did not receive magnesium sulfate and/or potassium or calcium replacement therapy, serum electrolytes were measured and their relationship with PPI usage analyzed. Results: Many patients (28/48, 58%) were under current PPI treatment when TdP occurred. Among TdP patients in whom serum electrolyte determinations were obtained before replacement therapy (27/48), those taking PPIs had significantly lower serum magnesium levels than those who did not. Hypomagnesemia occurred in ~40% of patients receiving PPIs (6/14), in all cases after an extended treatment (>2 weeks). In patients taking PPIs the mean QT-prolonging risk factor number was significantly higher than in those who did not, a difference which was mainly driven by lower magnesium levels. Conclusions: In unselected TdP patients, PPI-induced hypomagnesemia was common and significantly contributed to their cumulative arrhythmic risk. By providing clinical support to current recommendations, our data confirm that more awareness is needed when a PPI is prescribed, specifically as regards the risk of life-threatening arrhythmias.

Torsades de Pointes in Patients with Polymyalgia Rheumatica.

Polymyalgia rheumatica (PMR) represents the most common inflammatory rheumatic disease of the elderly. It is characterized by synovitis of proximal joints and extra-articular synovial structures, along with chronic high-grade systemic inflammation. PMR is closely related to giant cell arteritis (GCA), a large-vessel vasculitis that involves the major branches of the aorta, particularly the extracranial branches of carotid artery including temporal arteries. It is currently believed that PMR and GCA may represent different manifestations of the same disease process. Chronic systemic inflammation is presently recognized as one of the key pathogenic mechanisms underlying cardiovascular disease and associated complications, including cardiac arrhythmias and sudden death. In this regard, several studies demonstrated that besides promoting structural heart disease, inflammatory activation may also be per se arrhythmogenic, via cytokine-mediated effects on cardiac electrophysiology. In particular, increasing evidence points to inflammation as a novel risk factor for QTc prolongation and related life-threatening arrhythmias, specifically Torsade de Pointes (TdP). Starting from the report of two cases of TdP occurring in PMR patients with active disease and elevated circulating IL-6 levels, we here reviewed literature data regarding heart involvement and arrhythmic events in PMR/GCA, as well as TdP risk in inflammatory diseases. Potential underlying mechanisms were dissected, by focusing on the driving role of inflammatory activation.

Identification and characterization of three novel mutations in the CASQ1 gene in four patients with tubular aggregate myopathy.

Here, we report the identification of three novel missense mutations in the calsequestrin-1 (CASQ1) gene in four patients with tubular aggregate myopathy. These CASQ1 mutations affect conserved amino acids in position 44 (p.(Asp44Asn)), 103 (p.(Gly103Asp)), and 385 (p.(Ile385Thr)). Functional studies, based on turbidity and dynamic light scattering measurements at increasing Ca2+ concentrations, showed a reduced Ca2+ -dependent aggregation for the CASQ1 protein containing p.Asp44Asn and p.Gly103Asp mutations and a slight increase in Ca2+ -dependent aggregation for the p.Ile385Thr. Accordingly, limited trypsin proteolysis assay showed that p.Asp44Asn and p.Gly103Asp were more susceptible to trypsin cleavage in the presence of Ca2+ in comparison with WT and p.Ile385Thr. Analysis of single muscle fibers of a patient carrying the p.Gly103Asp mutation showed a significant reduction in response to caffeine stimulation, compared with normal control fibers. Expression of CASQ1 mutations in eukaryotic cells revealed a reduced ability of all these CASQ1 mutants to store Ca2+ and a reduced inhibitory effect of p.Ile385Thr and p.Asp44Asn on store operated Ca2+ entry. These results widen the spectrum of skeletal muscle diseases associated with CASQ1 and indicate that these mutations affect properties critical for correct Ca2+ handling in skeletal muscle fibers.

Searching Novel Therapeutic Targets for Scleroderma: P2X7-Receptor Is Up-regulated and Promotes a Fibrogenic Phenotype in Systemic Sclerosis Fibroblasts.

Objectives: Systemic sclerosis (SSc) is a connective tissue disorder presenting fibrosis of the skin and internal organs, for which no effective treatments are currently available. Increasing evidence indicates that the P2X7 receptor (P2X7R), a nucleotide-gated ionotropic channel primarily involved in the inflammatory response, may also have a key role in the development of tissue fibrosis in different body districts. This study was aimed at investigating P2X7R expression and function in promoting a fibrogenic phenotype in dermal fibroblasts from SSc patients, also analyzing putative underlying mechanistic pathways. Methods: Fibroblasts were isolated by skin biopsy from 9 SSc patients and 8 healthy controls. P2X7R expression, and function (cytosolic free Ca2+ fluxes, α-smooth muscle actin [α-SMA] expression, cell migration, and collagen release) were studied. Moreover, the role of cytokine (interleukin-1ß, interleukin-6) and connective tissue growth factor (CTGF) production, and extracellular signal-regulated kinases (ERK) activation in mediating P2X7R-dependent pro-fibrotic effects in SSc fibroblasts was evaluated. Results: P2X7R expression and Ca2+ permeability induced by the selective P2X7R agonist 2'-3'-O-(4-benzoylbenzoyl)ATP (BzATP) were markedly higher in SSc than control fibroblasts. Moreover, increased αSMA expression, cell migration, CTGF, and collagen release were observed in lipopolysaccharides-primed SSc fibroblasts after BzATP stimulation. While P2X7-induced cytokine changes did not affect collagen production, it was completely abrogated by inhibition of the ERK pathway. Conclusion: In SSc fibroblasts, P2X7R is overexpressed and its stimulation induces Ca2+-signaling activation and a fibrogenic phenotype characterized by increased migration and collagen production. These data point to the P2X7R as a potential, novel therapeutic target for controlling exaggerated collagen deposition and tissue fibrosis in patients with SSc.

GLUT10-Lacking in Arterial Tortuosity Syndrome-Is Localized to the Endoplasmic Reticulum of Human Fibroblasts.

GLUT10 belongs to a family of transporters that catalyze the uptake of sugars/polyols by facilitated diffusion. Loss-of-function mutations in the SLC2A10 gene encoding GLUT10 are responsible for arterial tortuosity syndrome (ATS). Since subcellular distribution of the transporter is dubious, we aimed to clarify the localization of GLUT10. In silico GLUT10 localization prediction suggested its presence in the endoplasmic reticulum (ER). Immunoblotting showed the presence of GLUT10 protein in the microsomal, but not in mitochondrial fractions of human fibroblasts and liver tissue. An even cytosolic distribution with an intense perinuclear decoration of GLUT10 was demonstrated by immunofluorescence in human fibroblasts, whilst mitochondrial markers revealed a fully different decoration pattern. GLUT10 decoration was fully absent in fibroblasts from three ATS patients. Expression of exogenous, tagged GLUT10 in fibroblasts from an ATS patient revealed a strict co-localization with the ER marker protein disulfide isomerase (PDI). The results demonstrate that GLUT10 is present in the ER.

Glucose transporter type 10-lacking in arterial tortuosity syndrome-facilitates dehydroascorbic acid transport.

Loss-of-function mutations in the gene encoding GLUT10 are responsible for arterial tortuosity syndrome (ATS), a rare connective tissue disorder. In this study GLUT10-mediated dehydroascorbic acid (DAA) transport was investigated, supposing its involvement in the pathomechanism. GLUT10 protein produced by in vitro translation and incorporated into liposomes efficiently transported DAA. Silencing of GLUT10 decreased DAA transport in immortalized human fibroblasts whose plasma membrane was selectively permeabilized. Similarly, the transport of DAA through endomembranes was markedly reduced in fibroblasts from ATS patients. Re-expression of GLUT10 in patients' fibroblasts restored DAA transport activity. The present results demonstrate that GLUT10 is a DAA transporter and DAA transport is diminished in the endomembranes of fibroblasts from ATS patients.

Subcellular compartmentation of ascorbate and its variation in disease states.

Beyond its general role as antioxidant, specific functions of ascorbate are compartmentalized within the eukaryotic cell. The list of organelle-specific functions of ascorbate has been recently expanded with the epigenetic role exerted as a cofactor for DNA and histone demethylases in the nucleus. Compartmentation necessitates the transport through intracellular membranes; members of the GLUT family and sodium-vitamin C cotransporters mediate the permeation of dehydroascorbic acid and ascorbate, respectively. Recent observations show that increased consumption and/or hindered entrance of ascorbate in/to a compartment results in pathological alterations partially resembling to scurvy, thus diseases of ascorbate compartmentation can exist. The review focuses on the reactions and transporters that can modulate ascorbate concentration and redox state in three compartments: endoplasmic reticulum, mitochondria and nucleus. By introducing the relevant experimental and clinical findings we make an attempt to coin the term of ascorbate compartmentation disease.

Multiple roles of glucose-6-phosphatases in pathophysiology: state of the art and future trends.

BACKGROUND: The endoplasmic reticulum enzyme glucose-6-phosphatase catalyzes the hydrolysis of glucose-6-phosphate to glucose and inorganic phosphate. The enzyme is a part of a multicomponent system that includes several integral membrane proteins; the catalytic subunit (G6PC) and transporters for glucose-6-phosphate, inorganic phosphate and glucose. The G6PC gene family presently includes three members, termed as G6PC, G6PC2, and G6PC3. Although the three isoforms show a moderate amino acid sequence homology, their membrane topology and catalytic site are very similar. The isoforms are expressed differently in various tissues. Mutations in all three genes have been reported to be associated with human diseases. SCOPE OF REVIEW: The present review outlines the biochemical features of the G6PC gene family products, the regulation of their expression, their role in the human pathology and the possibilities for pharmacological interventions. MAJOR CONCLUSIONS: G6PCs emerge as integrators of extra- and intracellular glucose homeostasis. Beside the well known key role in blood glucose homeostasis, the members of the G6PC family seem to play a role as sensors of intracellular glucose and of intraluminal glucose/glucose-6-phosphate in the endoplasmic reticulum. GENERAL SIGNIFICANCE: Since mutations in the three G6PC genes can be linked to human pathophysiological conditions, the better understanding of their functioning in connection with genetic alterations, altered expression and tissue distribution has an eminent importance.

The glucose-6-phosphate transport is not mediated by a glucose-6-phosphate/phosphate exchange in liver microsomes.

A phosphate-linked antiporter activity of the glucose-6-phosphate transporter (G6PT) has been recently described in liposomes including the reconstituded transporter protein. We directly investigated the mechanism of glucose-6-phosphate (G6P) transport in rat liver microsomal vesicles. Pre-loading with inorganic phosphate (Pi) did not stimulate G6P or Pi microsomal inward transport. Pi efflux from pre-loaded microsomes could not be enhanced by G6P or Pi addition. Rapid G6P or Pi influx was registered by light-scattering in microsomes not containing G6P or Pi. The G6PT inhibitor, S3483, blocked G6P transport irrespectively of experimental conditions. We conclude that hepatic G6PT functions as an uniporter.

Expression of hexose-6-phosphate dehydrogenase in rat tissues.

Hexose-6-phosphate dehydrogenase (H6PD) is the main NADPH generating enzyme in the lumen of the endoplasmic reticulum. H6PD is regarded as an ancillary enzyme in prereceptorial glucocorticoid activation and probably acts as a nutrient sensor and as a prosurvival factor. H6PD expression was determined in a variety of rat and human tissues by detecting mRNA and protein levels, and by measuring its dehydrogenase and lactonase activities. It was found that H6PD was present in all investigated tissues; both expression and activity remained within an order of magnitude. Correlation was found between the dehydrogenase activity and protein or mRNA levels. The results confirmed the supposed housekeeping feature of the enzyme.

Contribution of fructose-6-phosphate to glucocorticoid activation in the endoplasmic reticulum: possible implication in the metabolic syndrome.

Both fructose consumption and increased intracellular glucocorticoid activation have been implicated in the pathogenesis of the metabolic syndrome. Glucocorticoid activation by 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) depends on hexose-6-phosphate dehydrogenase (H6PD), which physically interacts with 11ß-HSD1 at the luminal surface of the endoplasmic reticulum (ER) membrane and generates reduced nicotinamide adenine dinucleotide phosphate for the reduction of glucocorticoids. The reducing equivalents for the reaction are provided by glucose-6-phosphate (G6P) that is transported by G6P translocase into the ER. Here, we show that fructose-6-phosphate (F6P) can substitute for G6P and is sufficient to maintain reductase activity of 11ß-HSD1 in isolated microsomes. Our findings indicate that the mechanisms of F6P and G6P transport across the ER membrane are distinct and provide evidence that F6P is converted to G6P in the ER lumen, thus yielding substrate for H6PD-dependent reduced nicotinamide adenine dinucleotide phosphate generation. Using the purified enzyme, we show that F6P cannot be directly dehydrogenated by H6PD, and we also excluded H6PD as a phosphohexose isomerase. Therefore, we postulate the existence of an ER luminal hexose-phosphate isomerase different from the cytosolic enzyme. The results suggest that cytosolic F6P promotes prereceptor glucocorticoid activation in white adipose tissue, which might have a role in the pathophysiology of the metabolic syndrome.

Hexose-6-phosphate dehydrogenase in the endoplasmic reticulum.

Hexose-6-phosphate dehydrogenase (H6PD) is a luminal enzyme of the endoplasmic reticulum that is distinguished from cytosolic glucose-6-phosphate dehydrogenase by several features. H6PD converts glucose-6-phosphate and NADP(+) to 6-phosphogluconate and NADPH, thereby catalyzing the first two reactions of the pentose-phosphate pathway. Because the endoplasmic reticulum has a separate pyridine nucleotide pool, H6PD provides NADPH for luminal reductases. One of these enzymes, 11beta-hydroxysteroid dehydrogenase type 1 responsible for prereceptorial activation of glucocorticoids, has been the focus of much attention as a probable factor in the pathomechanism of several human diseases including insulin resistance and the metabolic syndrome. This review summarizes recent advances related to the functions of H6PD.

Metyrapone prevents cortisone-induced preadipocyte differentiation by depleting luminal NADPH of the endoplasmic reticulum.

Preadipocyte differentiation is greatly affected by prereceptorial glucocorticoid activation catalyzed by 11beta-hydroxysteroid dehydrogenase type 1 in the lumen of the endoplasmic reticulum. The role of the local NADPH pool in this process was investigated using metyrapone as an NADPH-depleting agent. Metyrapone administered at low micromolar concentrations caused the prompt oxidation of the endogenous NADPH, inhibited the reduction of cortisone and enhanced the oxidation of cortisol in native rat liver microsomal vesicles. However, in permeabilized microsomes, it only slightly decreased both NADPH-dependent cortisone reduction and NADP(+)-dependent cortisol oxidation. Accordingly, metyrapone administration caused a switch in 11beta-hydroxysteroid dehydrogenase activity from reductase to dehydrogenase in both 3T3-L1-derived and human stem cell-derived differentiated adipocytes. Metyrapone greatly attenuated the induction of 11beta-hydroxysteroid dehydrogenase type 1 and the accumulation of lipid droplets during preadipocyte differentiation when 3T3-L1 cells were stimulated with cortisone, while it was much less effective in case of cortisol or dexamethasone. In conclusion, the positive feedback of glucocorticoid activation during preadipocyte differentiation is interrupted by metyrapone, which depletes NADPH in the endoplasmic reticulum. The results also indicate that the reduced state of luminal pyridine nucleotides in the endoplasmic reticulum is important in the process of adipogenesis.

Constant expression of hexose-6-phosphate dehydrogenase during differentiation of human adipose-derived mesenchymal stem cells.

The reductase activity of 11beta-hydroxysteroid dehydrogenase type 1 (HSD11B1) plays an important role in the growth and differentiation of adipose tissue via the prereceptorial activation of glucocorticoids. This enzyme colocalizes with hexose-6-phosphate dehydrogenase (H6PD) at the luminal surface of the endoplasmic reticulum membrane, and the latter enzyme provides NADPH to the former, which can thus act as an 11beta-reductase. It was suggested that, during adipogenesis, the increased expression of H6PD causes a dehydrogenase-to-reductase switch in the activity of HSD11B1. However, only the expression of the HSD11B1 has been extensively studied, and little is known about the expression of H6PD. Here, we investigated the expression and the activity of H6PD in the course of the differentiation of human adipose-derived mesenchymal stem cells (ADMSCs) and murine 3T3-L1 cells. It was found that H6PD is already present in adipose-derived stem cells and in 3T3-L1 fibroblasts even before the induction of adipogenesis. Moreover, mRNA and protein levels, as well as the microsomal H6PD activities remained unchanged during the differentiation. At the same time a great induction of HSD11B1 was observed in both cell types. The observed constant expression of H6PD suggests that HSD11B1 acts as a reductase throughout the adipogenesis process in human ADMSCs and murine 3T3-L1 cells.

Maintenance of luminal NADPH in the endoplasmic reticulum promotes the survival of human neutrophil granulocytes.

The present study demonstrates the expression of hexose-6-phosphate dehydrogenase and 11 beta-hydroxysteroid dehydrogenase type 1 in human neutrophils, and the presence and activity of these enzymes in the microsomal fraction of the cells. Their concerted action together with the previously described glucose-6-phosphate transporter is responsible for cortisone-cortisol interconversion detected in human neutrophils. Furthermore, the results suggest that luminal NADPH generation by the cortisol dehydrogenase activity of 11 beta-hydroxysteroid dehydrogenase type 1 prevents neutrophil apoptosis provoked by the inhibition of the glucose-6-phosphate transporter. In conclusion, the maintenance of the luminal NADPH pool is an important antiapoptotic factor in neutrophil granulocytes.

Both translocon and a cation channel are involved in the passive Ca2+ leak from the endoplasmic reticulum: a mechanistic study on rat liver microsomes.

Steady-state levels of calcium ions in endoplasmic reticulum reflect a balance between active inward transport, mediated by MgATP-dependent Ca(2+) pumps, and passive backflux of the ions, through putative "leak channels". We have investigated the efflux of Ca(2+) from rat liver microsomal vesicles, passively pre-equilibrated in the presence radiolabelled Ca(2+). Similarly, we have also evaluated the efflux of a low-Mwt uncharged compound, i.e., sucrose. The results show that two major passive Ca(2+) efflux pathways exist. One appeared to involve the translocon pore, since it was stimulated by the translocon opener puromycin, and also allowed the passage of sucrose. Putative channels likely mediated the other one, since it required counter ion influx and was inhibited by Gd(3+) and La(3+). The latter pathway did not appear to involve inactive Ca(2+) pumps, Bcl2 proteins, or known channels, such as the InsP3 and ryanodine receptors. While sucrose efflux was highly represented in a rough microsomal subfraction--enriched in the translocon component Sec61alpha--the efflux of Ca(2+) was represented both in smooth and in rough microsomes. We conclude that the passive efflux of Ca(2+) from the (liver) ER could be mediated by both the translocon pore and putative Ca(2+) leak channels. However, the relative role of these Ca(2+) efflux pathways in the intact cell as well as the molecular nature of the Ca(2+) leak channel(s) remain to be clarified.

Transport and transporters in the endoplasmic reticulum.

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

The glucose-6-phosphate transporter-hexose-6-phosphate dehydrogenase-11beta-hydroxysteroid dehydrogenase type 1 system of the adipose tissue.

11beta-hydroxysteroid dehydrogenase type 1, expressed mainly in the endoplasmic reticulum of adipocytes and hepatocytes, plays an important role in the prereceptorial activation of glucocorticoids. In liver endoplasmic reticulum-derived microsomal vesicles, nicotinamide adenine dinucleotide phosphate reduced supply to the enzyme is guaranteed by a tight functional connection with hexose-6-phosphate dehydrogenase and the glucose-6-phosphate transporter (G6PT). In adipose tissue, the proteins and their activities supporting the action of 11beta-hydroxysteroid dehydrogenase type 1 have not been explored yet. Here we report the occurrence of the hexose-6-phosphate dehydrogenase in rat epididymal fat, as detected at the level of mRNA, protein, and activity. In the isolated microsomes, the activity was evident only on the permeabilization of the membrane because of the poor permeability to the cofactor nicotinamide adenine dineucleotide phosphate (NADP(+)), which is consistent with the intralumenal compartmentation of both the enzyme and a pool of pyridine nucleotides. In fat cells, the access of the substrate, glucose-6-phosphate to the intralumenal hexose-6-phosphate dehydrogenase appeared to be mediated by the liver-type G6PT. In fact, the G6PT expression was revealed at the level of mRNA and protein. Accordingly, the transport of glucose-6-phosphate was demonstrated in microsomal vesicles, and it was inhibited by S3483, a prototypic inhibitor of G6PT. Furthermore, isolated adipocytes produced cortisol on addition of cortisone, and the production was markedly inhibited by S3483. The results show that adipocytes are equipped with a functional G6PT-hexose-6-phosphate dehydrogenase-11beta-hydroxysteroid dehydrogenase type 1 system and indicate that all three components are potential pharmacological targets for modulating local glucocorticoid activation.

Uncoupled redox systems in the lumen of the endoplasmic reticulum. Pyridine nucleotides stay reduced in an oxidative environment.

The redox state of the intraluminal pyridine nucleotide pool was investigated in rat liver microsomal vesicles. The vesicles showed cortisone reductase activity in the absence of added reductants, which was dependent on the integrity of the membrane. The intraluminal pyridine nucleotide pool could be oxidized by the addition of cortisone or metyrapone but not of glutathione. On the other hand, intraluminal pyridine nucleotides were slightly reduced by cortisol or glucose 6-phosphate, although glutathione was completely ineffective. Redox state of microsomal protein thiols/disulfides was not altered either by manipulations affecting the redox state of pyridine nucleotides or by the addition of NAD(P)+ or NAD(P)H. The uncoupling of the thiol/disulfide and NAD(P)+/NAD(P)H redox couples was not because of their subcompartmentation, because enzymes responsible for the intraluminal oxidoreduction of pyridine nucleotides were distributed equally in smooth and rough microsomal subfractions. Instead, the phenomenon can be explained by the negligible representation of glutathione reductase in the endoplasmic reticulum lumen. The results demonstrated the separate existence of two redox systems in the endoplasmic reticulum lumen, which explains the contemporary functioning of oxidative folding and of powerful reductive reactions.

Cooperativity between 11beta-hydroxysteroid dehydrogenase type 1 and hexose-6-phosphate dehydrogenase is based on a common pyridine nucleotide pool in the lumen of the endoplasmic reticulum.

11Beta-hydroxysteroid dehydrogenase type 1 (11betaHSD1) is a NADP(H)-dependent oxidoreductase of the ER lumen, which may have an important role in the pathogenesis of metabolic syndrome. Here, the functional coupling of 11beta-hydroxysteroid dehydrogenase type 1 and hexose-6-phosphate dehydrogenase (H6PDH) was investigated in rat liver microsomal vesicles. The results demonstrate the existence of a separate intraluminal pyridine nucleotide pool in the hepatic endoplasmic reticulum and a close cooperation between 11betaHSD1 and H6PDH based on their co-localization and the mutual generation of cofactors for each other.