Glucose Transport and Transporters in the Endomembranes.

Glucose is a basic nutrient in most of the creatures; its transport through biological membranes is an absolute requirement of life. This role is fulfilled by glucose transporters, mediating the transport of glucose by facilitated diffusion or by secondary active transport. GLUT (glucose transporter) or SLC2A (Solute carrier 2A) families represent the main glucose transporters in mammalian cells, originally described as plasma membrane transporters.

Species-Specific Glucose-6-Phosphatase Activity in the Small Intestine-Studies in Three Different Mammalian Models.

Besides the liver, which has always been considered the major source of endogenous glucose production in all post-absorptive situations, kidneys and intestines can also produce glucose in blood, particularly during fasting and under protein feeding. However, observations gained in different experimental animals have given ambiguous results concerning the presence of the glucose-6-phosphatase system in the small intestine. The aim of this study was to better define the species-related differences of this putative gluconeogenic organ in glucose homeostasis.

Early rehabilitation for severe acquired brain injury in intensive care unit: multicenter observational study.

Background: The increased survival after a severe acquired brain injury (sABI) raise the problem of making most effective the treatments in Intensive Care Unit (ICU)/Neurointensive Care Unit (NICU), also integrating rehabilitation care. Despite previous studies reported that early mobilization in ICU was effective in preventing complications and reducing hospital stay, few studies addressed the rehabilitative management of sABI patients in ICU/NICU.

Aim: To collect clinical and functional data about the early rehabilitative management of sABI patients during ICU/NICU stay.

Vitamin E Improves Clinical Outcome of Patients Affected by Glycogen Storage Disease Type Ib.

Background: It has been suggested, on a few GSD1b patients, that vitamin E improves neutrophil count and reduces frequency and severity of infections.The main objective of the present study was to investigate the efficacy of vitamin E on the neutropenia, neutrophil dysfunction and IBD in the entire Italian caseload of GSD1b patients.

Patients And Methods: Eighteen GSD1b patients, median age at the time of the study protocol 14.

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.

Transient knockdown of presenilin-1 provokes endoplasmic reticulum stress related formation of autophagosomes in HepG2 cells.

The involvement of presenilins in the endoplasmic reticulum (ER) related autophagy was investigated by their transient knockdown in HepG2 cells. The silencing of PSEN1 but not of PSEN2 led to cell growth impairment and decreased viability. PSEN1 silencing resulted in ER stress response as evidenced by the elevated levels of glucose regulated protein 78 (Grp78), protein disulfide isomerase (PDI), and CCAAT/enhancer-binding protein homologous protein (CHOP) and by the activation of activating transcription factor 6 (ATF6).

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.

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.

G6PT-H6PDH-11βHSD1 triad in the liver and its implication in the pathomechanism of the metabolic syndrome.

The metabolic syndrome, one of the most common clinical conditions in recent times, represents a combination of cardiometabolic risk determinants, including central obesity, glucose intolerance, insulin resistance, dyslipidemia, non-alcoholic fatty liver disease and hypertension. Prevalence of the metabolic syndrome is rapidly increasing worldwide as a consequence of common overnutrition and consequent obesity. Although a unifying picture of the pathomechanism is still missing, the key role of the pre-receptor glucocorticoid activation has emerged recently.

Production of H₂O₂ in the endoplasmic reticulum promotes in vivo disulfide bond formation.

Aims: Oxidative protein folding in the luminal compartment of endoplasmic reticulum (ER) is thought to be accompanied by the generation of H₂O₂, as side-product of disulfide bond formation. We aimed to examine the role of H₂O₂ produced in the lumen, which on one hand can lead to redox imbalance and hence can contribute to ER stress caused by overproduction of secretory proteins; on the other hand, as an excellent electron acceptor, H₂O₂ might serve as an additional pro-oxidant in physiological oxidative folding.

Results: Stimulation of H₂O₂ production in the hepatic ER resulted in a decrease in microsomal GSH and protein-thiol contents and in a redox shift of certain luminal oxidoreductases in mice.

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.

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 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) depends on hexose-6-phosphate dehydrogenase (H6PD), which physically interacts with 11β-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.

Altered redox state of luminal pyridine nucleotides facilitates the sensitivity towards oxidative injury and leads to endoplasmic reticulum stress dependent autophagy in HepG2 cells.

Maintenance of the reduced state of luminal pyridine nucleotides in the endoplasmic reticulum - an important pro-survival factor in the cell - is ensured by the concerted action of glucose-6-phosphate transporter and hexose-6-phosphate dehydrogenase. The mechanism by which the redox imbalance leads to cell death was investigated in HepG2 cells. The chemical inhibition of the glucose-6-phosphate transporter, the silencing of hexose-6-phosphate dehydrogenase and/or the glucose-6-phosphate transporter, or the oxidation of luminal NADPH by themselves did not cause a significant loss of cell viability.

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.

Hexose-6-phosphate dehydrogenase: linking endocrinology and metabolism in the endoplasmic reticulum.

Hexose-6-phosphate dehydrogenase (H6PD) got into the focus of interest due to its role in the prereceptorial activation of glucocorticoids, which has been implicated in the pathomechanism of metabolic syndrome. Genetic observations, results gained in H6PD knockout mice, and studies on differentiating adipocytes demonstrated the importance of the enzyme in metabolic regulation. A nutrient-sensing function can be postulated for the enzyme, which links metabolism to endocrinology in the endoplasmic reticulum.

Intraluminal hydrogen peroxide induces a permeability change of the endoplasmic reticulum membrane.

Gulonolactone treatment of mice resulted in the elevation of hepatic ascorbate and hydrogen peroxide levels accompanied by transient liver swelling and reversible dilatation of endoplasmic reticulum cisternae. Although a decrease in glutathione (reduced form)/total glutathione ratio was observed in microsomes, the redox state of luminal foldases remained unchanged and the signs of endoplasmic reticulum stress were absent. Increased permeability of the microsomal membrane to various compounds of low molecular weight was substantiated.

Effect of nifedipine on capacitive calcium entry in Jurkat T lymphocytes.

The effect of nifedipine-an antagonist of L-type calcium (Ca(2+)) channels-on capacitative Ca(2+) entry (CCE) was studied in Jurkat T lymphocytes. CCE was induced by a variety of treatments each of which depleted intracellular Ca(2+) stores. Cells were treated with thapsigargin, ionomycin, anti-CD3 antibodies, and phytohaemagglutinin, or pre-incubated in a Ca(2+)-free medium.

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.

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.

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.

The translocon and the non-specific transport of small molecules in the endoplasmic reticulum (Review).

Transmembrane fluxes are major determinants of several enzyme activities localized in the luminal compartment of the endoplasmic reticulum (ER). Although a large number of metabolites were shown to be transported across the ER membrane, only a few transporters have been identified so far. It can be assumed that the basal permeability of ER membrane vesicles (microsomes) to a variety of small molecules is due to the presence of a low-selectivity channel or pore rather than many strictly specific transporters.

Endoplasmic reticulum stress.

Stress is the imbalance of homeostasis, which can be sensed even at the subcellular level. The stress-sensing capability of various organelles including the endoplasmic reticulum (ER) has been described. It has become evident that acute or prolonged ER stress plays an important role in many human diseases; especially those involving organs/tissues specialized in protein secretion.

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.

Stress on redox.

Redox imbalance in the endoplasmic reticulum lumen is the most frequent cause of endoplasmic reticulum stress and consequent apoptosis. The mechanism involves the impairment of oxidative protein folding, the accumulation of unfolded/misfolded proteins in the lumen and the initiation of the unfolded protein response. The participation of several redox systems (glutathione, ascorbate, FAD, tocopherol, vitamin K) has been demonstrated in the process.

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.