GSH depletion, K-Cl cotransport, and regulatory volume decrease in high-K/high-GSH dog red blood cells

Sulfate influx on band 3 protein of equine erythrocyte membrane (Equus caballus) using different experimental temperatures and buffer solutions

The aim of this study was to assess the anion transport in equine erythrocytes through the measurement of the sulfate uptake operating from band 3 using different experimental temperatures and buffer solutions. Blood samples of six clinically healthy horses were collected via jugular vein puncture, and an emochrome-citometric examination was performed. The blood was divided into four aliquots and by centrifugation and aspiration the plasma and buffy coat were carefully discarded. The red blood cells were washed with an isosmotic medium and centrifuged. The obtained cell suspensions were incubated with two different experimental buffer solutions (buffer A: 115 mM Na2SO4, 10 mM NaCl, 20 mM ethylenediaminetetraacetic acid, 30 mM glucose; and buffer B: 115 mM Na2SO4, 10 mM NaCl, 20 mM ethylenediaminetetraacetic acid, 30 mM MgCl2) in a water bath for 1 h at 25 °C and 37 °C. Normal erythrocytes, suspended at 3% hematocrit, were used to measure the SO4= influx by absorption spectrophotometry at 425 nm wavelength. Unpaired Student's t-test showed a statistically significant decrease (P < 0.01) of rate constants in equine erythrocytes at 25 °C versus 37 °C using both experimental buffer solutions. Comparing the buffer A with buffer B unpaired Student's t-test showed statistically lower values (P < 0.0001) for A solution versus B solution both at 25 °C and at 37 °C. The greater inhibition of SO4 (=) influx measured in equine erythrocytes indicates the increased formation of the sulfydryl bonds in band 3 and the modulation of the sulfydryl groups, culminating in the conformational changes in band 3.

Hsp90 inhibitors and drug resistance in cancer: the potential benefits of combination therapies of Hsp90 inhibitors and other anti-cancer drugs

Hsp90 is a chaperone protein that interacts with client proteins that are known to be in the cell cycle, signaling and chromatin-remodeling pathways. Hsp90 inhibitors act additively or synergistically with many other drugs in the treatment of both solid tumors and leukemias in murine tumor models and humans. Hsp90 inhibitors potentiate the actions of anti-cancer drugs that target Hsp90 client proteins, including trastuzumab (Herceptin™) which targets Her2/Erb2B, as Hsp90 inhibition elicits the drug effects in cancer cell lines that are otherwise resistant to the drug. A phase II study of the Hsp90 inhibitor 17-AAG and trastuzumab showed that this combination therapy has anticancer activity in patients with HER2-positive metastatic breast cancer progressing on trastuzumab. In this review, we discuss the results of Hsp90 inhibitors in combination with trastuzumab and other cancer drugs. We also discuss recent results from yeast focused on the genetics of drug resistance when Hsp90 is inhibited and the implications that this might have in understanding the effects of genetic variation in treating cancer in humans.

Swelling-induced Ca²+ influx and K+ efflux in American alligator erythrocytes

The American alligator can hibernate during winter, which may lead to osmotic imbalance because of reduced kidney function and lack of food consumption during this period. Accordingly, we hypothesized that their red blood cells would have a well-developed regulatory volume decrease (RVD) to cope with the homeostatic challenges associated with torpor. Osmotic fragility was determined optically, mean cell volume was measured by electronic sizing, and changes in intracellular Ca²+ concentration were visualized using fluorescence microscopy and fluo-4-AM. Osmotic fragility increased and the ability to regulate volume was inhibited when extracellular Na+ was replaced with K+, or when cells were exposed to the K+ channel inhibitor quinine, indicating a requirement of K+ efflux for RVD. Addition of the ionophore gramicidin to the extracellular medium decreased osmotic fragility and also potentiated volume recovery, even in the presence of quinine. In addition, hypotonic shock (0.5 x Ringer) caused an increase in cytosolic Ca²+, which resulted from Ca²+ influx because it was not observed when extracellular Ca²+ was chelated with EGTA (ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid). Furthermore, cells loaded with BAPTA-AM (1,2-bis(2-aminophenoxymethyl)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester) or exposed to a low Ca²+-EGTA hypotonic Ringer had a greater osmotic fragility and also failed to recover from cell swelling, indicating that extracellular Ca²+ was needed for RVD. Gramicidin reversed the inhibitory effect of low extracellular Ca²+. Finally, and surprisingly, the Ca²+ ionophore A23187 increased osmotic fragility and inhibited volume recovery. Taken together, our results show that cell swelling activated a K+ permeable pathway via a Ca²+-dependent mechanism, and this process mediated K+ loss during RVD.

Aquaporin 1 expression in tissues of canines possessing inherited high K+ erythrocytes

We investigated the expression of aquaporin 1 (AQP1) in tissues from canines with an inherited anomaly that causes their erythrocytes to have high K⁺. Northern blot analysis revealed abundant AQP1 expression in lung and kidney, though little expression was found in spleen. Using anti-C-terminus for dog AQP1, abundant expression was shown in kidney, trachea, and eye, but little expression was shown in pancreas and cerebrum, indicating that AQP1 expression in canine tissues is similar to that noted in other mammals.

Antibiotic effects on SO4(2-) uptake in human erythrocytes

The erythrocyte is a cell highly exposed to oxygen pressure that, in turn, provokes oxidative stress involving loss of SH-groups, cell shrinkage by activation of K(+)-Cl(-) cotransport (KCC) and membrane destabilization which plays an important role in the premature haemolysis of red blood cells (RBCs). Oxidative stress provoked by chemicals frequently occurs in human erythrocytes. The aim of this study was to test whether the antibiotics alter the redox state and investigate their influences on band 3 protein that is involved in the facilitated electro neutral exchange of Cl(-) for HCO(3)(-) across the membrane of mammalian erythrocytes. Normal erythrocytes were treated with some antibiotics and thiol oxidizing agent N-ethylmaleimide (NEM) and tested for sulphate uptake, K(+) efflux and for glutathione (GSH) concentration as an index of oxidative stress. The rate constant of SO(4)(=) uptake measured in erythrocytes treated with antibiotics as well as NEM was decreased with respect to control cells as a result of band 3 SH-groups oxidation or the stress-induced K(+)-Cl(-) symport-mediated cell shrinkage. In fact, this hypothesis was verified by increased K(+) efflux and decreased GSH values measured in treated erythrocytes compared to controls.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

Redox imbalance, macrocytosis, and RBC homeostasis

The erythrocyte represents a major component of the antioxidant capacity of the blood through the enzymes contained in the cell, the glutathione system, and the low-molecular-weight antioxidants of the erythrocyte membrane. A further major red blood cell contribution is in regenerating consumed redox equivalents via the oxidative pentose phosphate pathway and glutathione reductase. Moreover, its extracellular antioxidant capacity, its mobility, and the existence of reducing equivalents far in excess of its normal requirements make erythrocytes function as an effective oxidative sink in the organism. That is why red blood cell metabolism and homeostasis strongly affect the antioxidant properties of the whole body. Conversely, the relation between macrocytosis and oxidative stress has not been fully delineated. Reviewing the mechanisms involved in red blood cell homeostasis in cases of redox imbalance is crucial in identification of factors that could potentially improve erythrocyte survival and defense against oxidant damage.

Regulation of K-Cl cotransport: from function to genes

This review intends to summarize the vast literature on K-Cl cotransport (COT) regulation from a functional and genetic viewpoint. Special attention has been given to the signaling pathways involved in the transporter's regulation found in several tissues and cell types, and more specifically, in vascular smooth muscle cells (VSMCs). The number of publications on K-Cl COT has been steadily increasing since its discovery at the beginning of the 1980s, with red blood cells (RBCs) from different species (human, sheep, dog, rabbit, guinea pig, turkey, duck, frog, rat, mouse, fish, and lamprey) being the most studied model. Other tissues/cell types under study are brain, kidney, epithelia, muscle/smooth muscle, tumor cells, heart, liver, insect cells, endothelial cells, bone, platelets, thymocytes and Leishmania donovani. One of the salient properties of K-Cl-COT is its activation by cell swelling and its participation in the recovery of cell volume, a process known as regulatory volume decrease (RVD). Activation by thiol modification with N-ethylmaleimide (NEM) has spawned investigations on the redox dependence of K-Cl COT, and is used as a positive control for the operation of the system in many tissues and cells. The most accepted model of K-Cl COT regulation proposes protein kinases and phosphatases linked in a chain of phosphorylation/dephosphorylation events. More recent studies include regulatory pathways involving the phosphatidyl inositol/protein kinase C (PKC)-mediated pathway for regulation by lithium (Li) in low-K sheep red blood cells (LK SRBCs), and the nitric oxide (NO)/cGMP/protein kinase G (PKG) pathway as well as the platelet-derived growth factor (PDGF)-mediated mechanism in VSMCs. Studies on VSM transfected cells containing the PKG catalytic domain demonstrated the participation of this enzyme in K-Cl COT regulation. Commonly used vasodilators activate K-Cl COT in a dose-dependent manner through the NO/cGMP/PKG pathway. Interaction between the cotransporter and the cytoskeleton appears to depend on the cellular origin and experimental conditions. Pathophysiologically, K-Cl COT is altered in sickle cell anemia and neuropathies, and it has also been proposed to play a role in blood pressure control. Four closely related human genes code for KCCs (KCC1-4). Although considerable information is accumulating on tissue distribution, function and pathologies associated with the different isoforms, little is known about the genetic regulation of the KCC genes in terms of transcriptional and post-transcriptional regulation. A few reports indicate that the NO/cGMP/PKG signaling pathway regulates KCC1 and KCC3 mRNA expression in VSMCs at the post-transcriptional level. However, the detailed mechanisms of post-transcriptional regulation of KCC genes and of regulation of KCC2 and KCC4 mRNA expression are unknown. The K-Cl COT field is expected to expand further over the next decades, as new isoforms and/or regulatory pathways are discovered and its implication in health and disease is revealed.

Chemical and pathological oxidative influences on band 3 protein anion-exchanger

BACKGROUND/AIMS

The erythrocyte is a cell exposed to a high level of oxygen pressure and to oxidative chemical agents. This stress involves SH-groups oxidation, cell shrinkage by activation of K-Cl co-transport (KCC) and elevation of the band 3 tyrosine phosphorylation level. The aim of our study was to test whether oxidative stress could influence band 3-mediated anion transport in human red blood cells.

METHODS

To evaluate this hypothesis, normal and pathological (glucose 6 phosphate dehydrogenase (G6PDH) defficient) erythrocytes were treated with known sulphydryl-blocking or thiol-oxidizing agents, such as N-ethylmaleimide (NEM), azodicarboxylic acid bis[dimethylamide] (diamide), orthovanadate, Mg2+ and tested for sulphate (SO4-) uptake, K+ efflux, G6PDH activity and glutathione (GSH) concentration.

RESULTS

In normal red blood cells, the rate constants of SO4- uptake decreased by about 28 % when cells were incubated with NEM, diamide and orthovanadate. In G6PDH-deficient red blood cells, in which oxidative stress occurs naturally, the rate constant of sulphate uptake was decreased by about 40% that of normal red cells. Addition of oxidizing and phosphatase inhibitor agents to pathological erythrocytes further decreased anion transport. In contrast, G6PDH activity was increased under oxidative stress in normal as well as in pathological cells and was lower in the presence of exogenous Mg2+ in parallel to a significant increase in sulphate transport. In both cells, the oxidizing agents increased K+ efflux with depletion of GSH.

CONCLUSION

The data are discussed in light of the possible opposite effects exerted by oxidative agents and Mg2+ on KCC and on the protein tyrosine kinase (PTK)-protein tyrosine phosphatase (PTP) equilibrium. The decreased sulphate uptake observed in the experimental and pathological conditions could be due to band 3 SH-groups oxidation or to oxidative stress-induced K-Cl symport-mediated cell shrinkage with concomitant band 3 tyrosine phosphorylation.

A Model for Fluid Secretion in Rhodnius Upper Malpighian Tubules (UMT)

We have measured fluid secretion rate in Rhodnius prolixus upper Malpighian tubules (UMT) stimulated to secrete with 5-OH-tryptamine. We used double perfusions in order to have access separately to the basolateral and to the apical cell membranes. Thirteen pharmacological agents were applied: ouabain, Bafilomycin A(1), furosemide, bumetanide, DIOA, Probenecid, SITS, acetazolamide, amiloride, DPC, BaCl(2), pCMBS and DTT. These agents are known to block different ion transport functions, namely ATPases, co- and/or counter-transporters and ion and water channels. The basic assumption is that water movement changes reflect changes in ion transport mechanisms, which we localize as follows: (i) At the basolateral cell membrane, fundamental are a Na(+)-K(+)-2Cl(-) cotransporter and a Cl(-)-HCO(3) (-) exchanger; of intermediate importance are the Na(+)-K(+)-ATPase, Cl(-) channels and Rp-MIP water channels; K(+) channels play a lesser role: (ii) At the apical cell membrane, most important are a K(+)-Cl(-) cotransport that is being located for the first time, a V-H(+)-ATPase; and a Na(+)-H(+) exchanger; a urate-anion exchanger and K(+) channels are less important, while Cl(-) channels are not important at all. A tentative model for the function of the UMT cell is presented.

KCl cotransport mediates abnormal sulfhydryl-dependent volume regulation in sickle reticulocytes

KCl cotransport (KCC) activation by cell swelling and pH was compared in sickle (SS) and normal (AA) red blood cells (RBCs). KCC fluxes had the same relationship to mean corpuscular hemoglobin concentration (MCHC) in SS and AA RBCs when normalized to the maximal volume-stimulated (VSmax) flux (MCHC &lt; 270 g/L [27 g/dL]). Acid-stimulated (pH 6.9) KCC flux in SS RBCs was 60% to 70% of VSmax KCC versus 20% in AA RBCs. Density gradients were used to track changes in reticulocyte MCHC during KCC-mediated regulatory volume decrease (RVD). Swelling to MCHC of 260 g/L (26 g/dL) produced Cl-dependent RVD that resulted in higher MCHC in SS than AA reticulocytes. In acid pH, RVD was also greater in SS than AA reticulocytes. Sulfhydryl reduction by dithiothreitol (DTT) lowered VSmax KCC flux in AA and SS RBCs by one third but did not alter swelling-induced RVD. DTT lowered acid-activated KCC in SS RBCs by 50% and diminished acid-induced RVD in SS reticulocytes. Thus, swelling activation of KCC is normal in SS RBCs but KCC-mediated RVD produces higher MCHC in SS than AA reticulocytes. Acid activation of KCC is exaggerated in SS RBCs and causes dehydration in SS reticulocytes. KCC response to acid stimulation was mitigated by DTT, suggesting that it arises from sulfhydryl oxidation.

Pivotal role of reduced glutathione in oxygen-induced regulation of the Na(+)/K(+) pump in mouse erythrocyte membranes

This study addresses the mechanisms of oxygen-induced regulation of ion transport pathways in mouse erythrocyte, specifically focusing on the role of cellular redox state and ATP levels. Mouse erythrocytes possess Na(+)/K(+) pump, K(+)-Cl(-) and Na(+)-K(+)-2Cl(-) cotransporters that have been shown to be potential targets of oxygen. The activity of neither cotransporter changed in response to hypoxia-reoxygenation. In contrast, the Na(+)/K(+) pump responded to hypoxic treatment with reversible inhibition. Hypoxia-induced inhibition was abolished in Na(+)-loaded cells, revealing no effect of O(2) on the maximal operation rate of the pump. Notably, the inhibitory effect of hypoxia was not followed by changes in cellular ATP levels. Hypoxic exposure did, however, lead to a rapid increase in cellular glutathione (GSH) levels. Decreasing GSH to normoxic levels under hypoxic conditions abolished hypoxia-induced inhibition of the pump. Furthermore, GSH added to the incubation medium was able to mimic hypoxia-induced inhibition. Taken together these data suggest a pivotal role of intracellular GSH in oxygen-induced modulation of the Na(+)/K(+) pump activity.

Extracellular ATP as a signaling molecule for epithelial cells

The charge of this invited review is to present a convincing case for the fact that cells release their ATP for physiological reasons. Many of our "purinergic" colleagues as well as ourselves have experienced resistance to this concept, because it is teleologically counter-intuitive. This review serves to integrate the three main tenets of extracellular ATP signaling: ATP release from cells, ATP receptors on cells, and ATP receptor-driven signaling within cells to affect cell or tissue physiology. First principles will be discussed in the Introduction concerning extracellular ATP signaling. All possible cellular mechanisms of ATP release will then be presented. Use of nucleotide and nucleoside scavengers as well as broad-specificity purinergic receptor antagonists will be presented as a method of detecting endogenous ATP release affecting a biological endpoint. Innovative methods of detecting released ATP by adapting luciferase detection reagents or by using "biosensors" will be presented. Because our laboratory has been primarily interested in epithelial cell physiology and pathophysiology for several years, the role of extracellular ATP in regulation of epithelial cell function will be the focus of this review. For ATP release to be physiologically relevant, receptors for ATP are required at the cell surface. The families of P2Y G protein-coupled receptors and ATP-gated P2X receptor channels will be introduced. Particular attention will be paid to P2X receptor channels that mediate the fast actions of extracellular ATP signaling, much like neurotransmitter-gated channels versus metabotropic heptahelical neurotransmitter receptors that couple to G proteins. Finally, fascinating biological paradigms in which extracellular ATP signaling has been implicated will be highlighted. It is the goal of this review to convert and attract new scientists into the exploding field of extracellular nucleotide signaling and to convince the reader that extracellular ATP is indeed a signaling molecule.