Evidence for a sodium-dependent calcium influx in isolated rat hepatocytes undergoing ATP depletion

Neuroprotective effect of heparin Trisulfated disaccharide on ischemic stroke

Cells undergoing hypoxia experience intense cytoplasmic calcium (Ca2+) overload. High concentrations of intracellular calcium ([Ca2+]i) can trigger cell death in the neural tissue, a hallmark of stroke. Neural Ca2+ homeostasis involves regulation by the Na+/Ca2+ exchanger (NCX). Previous data published by our group showed that a product of the enzymatic depolymerization of heparin by heparinase, the unsaturated trisulfated disaccharide (TD; ΔU, 2S-GlcNS, 6S), can accelerate Na+/Ca2+ exchange via NCX, in hepatocytes and aorta vascular smooth muscle cells. Thus, the objective of this work was to verify whether TD could act as a neuroprotective agent able to prevent neuronal cell death by reducing [Ca2+]i. Pretreatment of N2a cells with TD reduced [Ca2+]i rise induced by thapsigargin and increased cell viability under [Ca2+]I overload conditions and in hypoxia. Using a murine model of stroke, we observed that pretreatment with TD decreased cerebral infarct volume and cell death. However, when mice received KB-R7943, an NCX blocker, the neuroprotective effect of TD was abolished, strongly suggesting that this neuroprotection requires a functional NCX to happen. Thus, we propose TD-NCX as a new therapeutic axis for the prevention of neuronal death induced by [Ca2+]i overload.

Arecoline Increases Glycolysis and Modulates pH Regulator Expression in HA22T/VGH Hepatoma Cells, Leading to Increase of Intracellular Ca2+, Reactive Oxygen Species, and Anoikis

Background: Cancer cells proliferate rapidly and are resistant to cell death, relying on aggravated glycolysis to satisfy their increased demand for energy and biosynthetic precursors. However, this process may create unfavorable microenvironments, such as increased acidity, leading to cytotoxicity. Our previous study demonstrated that arecoline induces anoikis of HA22T/VGH hepatoma cells. The present study aimed to examine if arecoline induced anoikis is related to the glycolytic pathway and explore the underlying mechanisms.

Methods: HA22T/VGH cells were treated with arecoline and changes in the glycolytic end products lactate and ATP, glycolytic-related gene expression, intracellular and extracellular pH, pH-regulating gene expression, reactive oxygen species (ROS) levels, intracellular Ca²⁺ concentration ([Ca²⁺]i) and mitochondrial membrane potential were examined, relative to untreated cells. Cell viability and morphology were also assessed.

Results: Arecoline increased lactate and ATP production through induction of glycolytic genes, including glucose transporter 3 (Glut3), hexokinase 1 (HK1), hexokinase 2 (HK2), and pyruvate kinase (PK). The intracellular pH was not changed, despite increased lactate levels, implying that intracellular H⁺ was exported out of the cells. mRNA expression of pH regulators including monocarboxylate transporter 1 and 4 (MCT 1 and 4), sodium bicarbonate cotransporter 1 (NBC1), carbonic anhydrases (CA) IX and XII and vacuolar ATPase (V-ATPase) were down-regulated. Na⁺/H⁺ exchanger 1 (NHE1) mRNA levels remained unchanged while Na⁺/Ca²⁺ exchanger (NCX) was up-regulated and eventually [Ca²⁺]i was increased. ROS generation was increased and mitochondrial membrane potential was decreased followed by cell detachment and death. Addition of 2-deoxy-d-glucose, a glucose competitor that interferes with glycolysis, attenuated arecoline induction of lactate [Ca²⁺]i, ROS and cell detachment. Similarly, ROS scavengers could block the effects of arecoline.

Conclusions: This study demonstrated that arecoline induced glycolysis and modulated the mRNA expression of pH-regulator genes in HA22T/VGH cells. This phenomenon led to the elevation of [Ca²⁺]i, ROS generation, and subsequent cell detachment.

Reactive oxygen and nitrogen species in steatotic hepatocytes: a molecular perspective on the pathophysiology of ischemia-reperfusion injury in the fatty liver

SIGNIFICANCE

Hepatic ischemia-reperfusion (IR) injury results from the temporary deprivation of hepatic blood supply and is a common side effect of major liver surgery (i.e., transplantation or resection). IR injury, which in most severe cases culminates in acute liver failure, is particularly pronounced in livers that are affected by non-alcoholic fatty liver disease (NAFLD). In NAFLD, fat-laden hepatocytes are damaged by chronic oxidative/nitrosative stress (ONS), a state that is acutely exacerbated during IR, leading to extensive parenchymal damage.

RECENT ADVANCES

NAFLD triggers ONS via increased (extra)mitochondrial fatty acid oxidation and activation of the unfolded protein response. ONS is associated with widespread protein and lipid (per)oxidation, which reduces the hepatic antioxidative capacity and shifts the intracellular redox status toward an oxidized state. Moreover, activation of the transcription factor peroxisome proliferator-activated receptor α induces expression of mitochondrial uncoupling protein 2, resulting in depletion of cellular energy (ATP) reserves. The reduction in intracellular antioxidants and ATP in fatty livers consequently gives rise to severe ONS and necrotic cell death during IR.

CRITICAL ISSUES

Despite the fact that ONS mediates both NAFLD and IR injury, the interplay between the two conditions has never been described in detail. An integrative overview of the pathophysiology of NAFLD that renders steatotic hepatocytes more vulnerable to IR injury is therefore presented in the context of ONS.

FUTURE DIRECTIONS

Effective methods should be devised to alleviate ONS and the consequences thereof in NAFLD before surgery in order to improve resilience of fatty livers to IR injury.

Volume-sensitive tyrosine kinases regulate liver cell volume through effects on vesicular trafficking and membrane Na+ permeability

In liver cells, the influx of Na+ mediated by nonselective cation (NSC) channels in the plasma membrane contributes importantly to regulation of cell volume. Under basal conditions, channels are closed; but both physiologic (e.g. insulin) and pathologic (e.g. oxidative stress) stimuli that are known to stimulate tyrosine kinases are associated with large increases in membrane Na+ permeability to approximately 80 pA/pF or more. Consequently, the purpose of these studies was to evaluate whether volume-sensitive tyrosine kinases mediate cell volume increases through effects on the activity or distribution of NSC channel proteins. In HTC hepatoma cells, decreases in cell volume evoked by hypertonic exposure increased total cellular tyrosine kinase activity approximately 20-fold. Moreover, hypertonic exposure (320-400 mosM) was followed after a delay by NSC channel activation and partial recovery of cell volume toward basal values (regulatory volume increase (RVI)). The tyrosine kinase inhibitors genistein and erbstatin prevented both NSC channel activation and RVI. Similarly, hypertonic exposure resulted in an increase in p60(c-src) activity, and intracellular dialysis with recombinant p60(c-src) led to activation of NSC currents in the absence of an osmolar gradient. Utilizing FM1-43 fluorescence, exposure to hypertonic media caused a rapid increase in the rate of exocytosis of approximately 40% (p < 0.01), and genistein inhibited both exocytosis and channel activation. These findings indicate that volume-sensitive increases in p60(c-src) and/or related tyrosine kinases play a key role in the regulation of membrane Na+ permeability, suggesting that increases in the NSC conductance may be mediated in part through rapid recruitment of a distinct pool of channel-containing vesicles.

2,5-Anhydro-D-mannitol increases hepatocyte sodium: transduction of a hepatic hunger stimulus?

To test the hypothesis that decreased hepatocyte ATP is transduced into a hepatic neuronal signal via a change in sodium pump activity, we examined the effect of 2,5-anhydro-D-mannitol (2,5-AM), which stimulates feeding behavior in rats, on intracellular sodium levels using 23Na nuclear magnetic resonance (NMR) spectroscopy. Isolated hepatocytes suspended in agarose beads were superfused with either 2.5 mM 2,5-AM or fructose in the presence of the paramagnetic shift reagent, thulium(III)(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylenephosphonate)). Superfusion with 2,5-AM decreased hepatocyte ATP and increased intracellular sodium levels compared with superfusion with either fructose or shift reagent alone starting within 15 min of exposure, reaching a maximum level of 120% of baseline by 30 min and declining gradually thereafter over the next 90 min. Superfusion with fructose, which also decreased hepatocyte ATP but by less than half the amount seen with 2,5-AM, had no significant effect on cellular sodium levels. The results support the hypothesis that changes in sodium pump activity could participate in transducing a hunger stimulus associated with hepatocyte energy status into a signal for hunger.

Molecular mechanisms of ischemic neuronal injury

Brain ischemia triggers a complex cascade of molecular events that unfolds over hours to days. Identified mechanisms of postischemic neuronal injury include altered Ca(2+) homeostasis, free radical formation, mitochondrial dysfunction, protease activation, altered gene expression, and inflammation. Although many of these events are well characterized, our understanding of how they are integrated into the causal pathways of postischemic neuronal death remains incomplete. The primary goal of this review is to provide an overview of molecular injury mechanisms currently believed to be involved in postischemic neuronal death specifically highlighting their time course and potential interactions.

Alterations of Na(+) homeostasis in hepatocyte reoxygenation injury

Reperfusion injury represents an important cause of primary graft non-function during liver transplantation. However, the mechanism responsible for cellular damage during reoxygenation has not yet been completely understood. We have investigated whether changes in intracellular Na(+) distribution might contribute to cause hepatocyte damage during reoxygenation buffer after 24 h of cold storage. Hepatocyte reoxygenation resulted in a rapid increase in cellular Na(+) content that was associated with cytotoxicity. Na(+) accumulation and hepatocyte death were prevented by the omission of Na(+) from the incubation medium, but not by the addition of antioxidants. Blocking Na(+)/H(+) exchanger and Na(+)/HCO(3)(-) co-transporter by, respectively, 5-(N,N-dimethyl)-amiloride or omitting HCO(3)(-) from the reoxygenation medium significantly decreased Na(+) overload and cytotoxicity. Stimulation of ATP re-synthesis by the addition of fructose also lowered Na(+) accumulation and cell death during reoxygenation. A significant protection against Na(+)-mediated reoxygenation injury was evident in hepatocytes maintained in an acidic buffer (pH 6.5) or in the presence of glycine. The cytoprotective action of glycine or of the acidic buffer was reverted by promoting Na(+) influx with the Na(+)/H(+) ionophore monensin. Altogether, these results suggest that Na(+) accumulation during the early phases of reoxygenation might contribute to liver graft reperfusion injury.

Ischemic preconditioning reduces Na(+) accumulation and cell killing in isolated rat hepatocytes exposed to hypoxia

Short periods of ischemia followed up by reperfusion are known to protect the heart against injury caused by a subsequent sustained ischemia. This phenomenon, known as ischemic preconditioning, has also been recently shown to reduce ischemic liver damage, but the mechanisms involved are still unknown. By using isolated hepatocytes as an in vitro model of liver preconditioning, we have investigated the possible effect of preconditioning on intracellular pH and Na(+) homeostasis. Freshly isolated rat hepatocytes were preconditioned by 10 minutes of incubation under hypoxic conditions followed up by 10 minutes of reoxygenation and subsequently exposed to 90 minutes of hypoxia. Although preconditioning did not ameliorate adenosine triphosphate (ATP) depletion, preconditioned hepatocytes exhibited an increased resistance to cell killing during hypoxic incubation. Intracellular acidosis and Na(+) accumulation developing during hypoxia were appreciably reduced in preconditioned cells. The effects of preconditioning on intracellular pH, Na(+) homeostasis, and cytotoxicity were mimicked by stimulating protein kinase C (PKC) with 4beta-phorbol-12-myristate-13-acetate (PMA) or 1,2 dioctanoyl-glycerol (1,2 DOG). Conversely, inhibiting PKC with chelerythrine or blocking vacuolar proton ATPase (V-ATPase) with bafilomycin A(1) abolished the protection given by preconditioning or by PMA treatment on hypoxic acidosis, Na(+) overload, and hepatocyte killing. Similarly, the addition of Na(+) ionophore monensin also reverted the cytoprotection exerted by preconditioning. This indicated that ischemic preconditioning of isolated hepatocytes decreased cell killing during hypoxia by preventing intracellular Na(+) accumulation. We propose that, after preconditioning, the stimulation of PKC might activate proton extrusion through V-ATPase, thus, limiting intracellular acidosis and Na(+) overload promoted by Na(+)-dependent acid buffering systems.

Alterations of cell volume regulation in the development of hepatocyte necrosis

Intracellular Na+ accumulation has been shown to contribute to hepatocyte death caused by anoxia or oxidative stress. In this study we have investigated the mechanism by which Na+ overload can contribute to the development of cytotoxicity. ATP depletion in isolated hepatocytes exposed to menadione-induced oxidative stress or to KCN was followed by Na+ accumulation, loss of intracellular K+, and cell swelling. Hepatocyte swelling occurred in two phases: a small amplitude swelling (about 15% of the initial size) with preservation of plasma membrane integrity and a terminal large amplitude swelling associated with cell death. Inhibition of Na+ accumulation by the use of a Na+-free medium prevented K+ loss, cell swelling, and cytotoxicity. Conversely, blocking K+ efflux by the addition of BaCl2 did not influence Na+ increase and small amplitude swelling, but greatly stimulated large amplitude swelling and cytotoxicity. Menadione or KCN killing of hepatocytes was also enhanced by inducing cell swelling in an hypotonic medium. However, increasing the osmolarity of the incubation medium did not protect against large amplitude swelling and cytotoxicity, since stimulated Na+ accumulation and K+ efflux. Altogether these results indicate that the impairment of volume regulation in response to the osmotic load caused by Na+ accumulation is critical for the development of cell necrosis induced by mitochondrial inhibition or oxidative stress.

Pancreatitis-induced ascitic fluid and hepatocellular dysfunction in severe acute pancreatitis

BACKGROUND

Multiple organ failure (MOF) is the most serious complication in severe acute pancreatitis, contributing to its high mortality. It has been suggested that changes of high-energy phosphates, intracellular pH, and intracellular cation homeostasis are closely related to hepatocellular injury associated with MOF.

METHODS

Phosphorus metabolites, intracellular pH (pHi), and intracellular Na+ concentration ([Na+]i) were measured in rat livers in vivo using 31P and 23Na NMR spectroscopy after deoxycholic acid (DCA)-induced pancreatitis or intraperitoneal injection (ip) of pancreatitis-induced ascitic fluid (PAF).

RESULTS

Two hours after induction of DCA-pancreatitis, the liver experienced significant intracellular acidosis (pHi = 6.99 +/- 0.16) and sodium loading (75 +/- 9 mM) and a reduction in its energy state (beta-ATP/Pi = 0.2 +/- 0.03 and Pi = 164 +/- 12). Although ip injection of PAF into healthy rats did not induce systemic hypotension, the livers under these conditions also developed severe disturbances in hepatocellular ion homeostasis and depletion of its bioenergetics. The longer the abdomen was exposed to the PAF, the worse the changes were. At 3 h after ip injection of PAF, hepatic [Na+]i significantly increased (42 +/- 3 mM) along with a significant decrease in pHi (7.30 +/- 0. 03). At 6 h after ip injection of PAF, the hepatic beta-ATP/Pi ratio decreased to 0.34 +/- 0.05 and Pi increased to 97 +/- 27.

CONCLUSIONS

PAF induced severe hepatocellular acidosis, rapid accumulation of hepatic intracellular sodium, impaired hepatic cytosolic phosphorylation potential, and increased hepatic utilization of ATP. These effects may account for the eventual development of liver dysfunction associated with necrotizing pancreatitis.

Effect of sodium bicarbonate infusion on hepatocyte Ca2+ overload during resuscitation from hemorrhagic shock

Ischemia/reperfusion events alter the cellular ion homeostasis by intracellular acidosis and a subsequent rise of sodium and calcium concentrations. Since disturbance of intracellular Ca2+ signaling pathways impairs cellular function, we investigated the effect of sodium bicarbonate infusion on hepatocellular Ca2+ dysregulation induced by resuscitation from hemorrhagic shock. Anesthetized Sprague-Dawley rats were bled to a mean arterial blood pressure of 40 mmHg for 60 min. Rats were resuscitated by retransfusion of shed blood (60%) in 20 min and three-fold the bleed out volume as lactated Ringers' during 60 min and received either a bolus infusion of sodium bicarbonate (2 mval/kg body weight) or an equal volume of sodium chloride (0.9%). After hepatocyte isolation by portal collagenase perfusion, the rate of Ca2+ influx (Ca2+in) in the absence and presence of epinephrine (100 nM), cellular Ca2+ uptake (Ca2+up) and membrane Ca2+ flux (Ca2+flux) were determined using 45Ca2+ incubation techniques. Hemorrhage/resuscitation substantially increased hepatocyte Ca2+up (3.44 +/- 0.2 nmol/mg protein) and Ca2+flux (32.8 +/- 5 pmol/mg protein x min) compared to sham-operated controls (2.57 +/- 0.1 and 15.2 +/- 3.5; P < 0.05). Resuscitation with sodium bicarbonate significantly prevented altered hepatocyte Ca2+ regulation (2.31 +/- 0.1 and 14.4 +/- 4.6; P < 0.05). These findings suggested that postischemic hepatocyte Ca2+ overload could partly be due to enhanced membrane Ca2+ movements to correct for altered intracellular pH homeostasis.

Mitochondrial permeability transition in pH-dependent reperfusion injury to rat hepatocytes

To simulate ischemia and reperfusion, cultured rat hepatocytes were incubated in anoxic buffer at pH 6.2 for 4 h and reoxygenated at pH 7.4. During anoxia, intracellular pH (pHi) decreased to 6.3, mitochondria depolarized, and ATP decreased to < 1% of basal values, but the mitochondrial permeability transition (MPT) did not occur as assessed by confocal microscopy from the redistribution of cytosolic calcein into mitochondria. Moreover, cell viability remained > 90%. After reperfusion at pH 7.4, pHi returned to pH 7.2, the MPT occurred, and most hepatocytes lost viability. In contrast, after reperfusion at pH 6.2 or with Na(+)-free buffer at pH 7.4, pHi did not rise and cell viability remained > 80%. After acidotic reperfusion, the MPT did not occur. When hepatocytes were reperfused with cyclosporin A (0.5-1 microM) at pH 7.4, the MPT was prevented and cell viability remained > 80%, although pHi increased to 7.2. Reperfusion with glycine (5 mM) also prevented cell killing but did not block recovery of pHi or the MPT. Retention of cell viability was associated with recovery of 30-40% of ATP. In conclusion, preventing the rise of pHi after reperfusion blocked the MPT, improved ATP recovery, and prevented cell death. Cyclosporin A also prevented cell killing by blocking the MPT without blocking recovery of pHi. Glycine prevented cell killing but did not inhibit recovery of pHi or the MPT.

Role of Na+/Ca2+ exchanger in preventing Na+ overload and hepatocyte injury: opposite effects of extracellular and intracellular Ca2+ chelation

We have previously shown that an increase of intracellular Na+ occurs in isolated rat hepatocytes undergoing ATP depletion and that Na+ accumulation is associated with an uncontrolled influx of Ca2+ through the activation in reverse mode of the Na+/Ca2+ exchanger. In the present study we have investigated the relationship between alterations of Na+ and Ca2+ homeostasis and hepatocyte killing using treatments which differentially chelate extracellular or intracellular Ca2+. Chelation of extracellular Ca2+ by ethylene glycol bis-(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) potentiated Na+ overload and cell killing induced in isolated rat hepatocytes by hypoxia or menadione. Similar effects were also observed when Na+ accumulation was induced by the combined addition of Na+ ionophore monensin and the inhibition of plasma membrane Na+/K+ ATPase by ouabain. Conversely, the use of the intracellular Ca2+ chelator EGTA acetoxymethyl ester (EGTA/AM) reduced Na+ overload and hepatocyte death induced by hypoxia or cell treatment with menadione or monensin plus ouabain. The effects of EGTA/AM were reverted in the presence of bepridil, an inhibitor of Na+/Ca2+ exchanger. Altogether these results indicated that differential chelation of intracellular or extracellular Ca2+ influences in opposite ways hepatocyte killing due to ATP depletion by modulating intracellular Na+ levels through the reversed activity of the Na+/Ca2+ exchanger.

Mechanism of toxicity of precocene II in rat hepatocyte cultures

Precocene II was more toxic in 24 hour cultures than in 72 hour cultures of rat hepatocytes. In 24 hour cultures, there was no observable toxicity at 75 microM precocene II after exposure for 6 hours, but after 24 hours, 65% of the cells were dead. In contrast, although 794 microM killed 50% of the cells in the 72 hour cultures after a 24 hour exposure, 1 mM killed 96% of the cells within 6 hours. In both 24 and 72 hour cultures, cell death was preceded by a rapid, early loss of mitochondrial membrane potential, followed by decreases in glutathione, reduced pyridine nucleotide status, and plasma membrane Na+/K+-ATPase activity. There was also a rapid loss of ATP in the 72 hour cultures but not in the 24 hour cultures; therefore, onset of cell death may be closely linked to loss of ATP. Inhibition of cytochrome P-450 prevented the toxicity, and partially protected against the loss of membrane potential and glutathione, in 24 hour cultures but was ineffective in 72 hour cultures. Therefore, in addition to depletion of glutathione, precocene II appears to damage mitochondria and plasma membrane functions and can do so by more than one pathway.