Subcellular characteristics of phospholipase A2 activity in the rat kidney. Enhanced cytosolic, mitochondrial, and microsomal phospholipase A2 enzymatic activity after renal ischemia and reperfusion

Pathogenesis of Multiple Organ Failure: The Impact of Systemic Damage to Plasma Membranes

Multiple organ failure (MOF) is the major cause of morbidity and mortality in intensive care patients, but the mechanisms causing this severe syndrome are still poorly understood. Inflammatory response, tissue hypoxia, immune and cellular metabolic dysregulations, and endothelial and microvascular dysfunction are the main features of MOF, but the exact mechanisms leading to MOF are still unclear. Recent progress in the membrane research suggests that cellular plasma membranes play an important role in key functions of diverse organs. Exploration of mechanisms contributing to plasma membrane damage and repair suggest that these processes can be the missing link in the development of MOF. Elevated levels of extracellular phospholipases, reactive oxygen and nitrogen species, pore-forming proteins (PFPs), and dysregulation of osmotic homeostasis occurring upon systemic inflammatory response are the major extracellular inducers of plasma membrane damage, which may simultaneously operate in different organs causing their profound dysfunction. Hypoxia activates similar processes, but they predominantly occur within the cells targeting intracellular membrane compartments and ultimately causing cell death. To combat the plasma membrane damage cells have developed several repair mechanisms, such as exocytosis, shedding, and protein-driven membrane remodeling. Analysis of knowledge on these mechanisms reveals that systemic damage to plasma membranes may be associated with potentially reversible MOF, which can be quickly recovered, if pathological stimuli are eliminated. Alternatively, it can be transformed in a non-resolving phase, if repair mechanisms are not sufficient to deal with a large damage or if the damage is extended to intracellular compartments essential for vital cellular functions.

A synthetic epoxyeicosatrienoic acid analogue prevents the initiation of ischemic acute kidney injury

AIM

Imbalances in cytochrome P450 (CYP)-dependent eicosanoid formation may play a central role in ischemic acute kidney injury (AKI). We reported previously that inhibition of 20-hydroxyeicosatetraenoic acid (20-HETE) action ameliorated ischemia/reperfusion (I/R)-induced AKI in rats. Now we tested the hypothesis that enhancement of epoxyeicosatrienoic acid (EET) actions may counteract the detrimental effects of 20-HETE and prevent the initiation of AKI.

METHODS

Male Lewis rats underwent right nephrectomy and ischemia was induced by 45 min clamping of the left renal pedicle followed by up to 48 h of reperfusion. Circulating CYP-eicosanoid profiles were compared in patients who underwent cardiac surgery with (n = 21) and without (n = 38) developing postoperative AKI.

RESULTS

Ischemia induced an about eightfold increase of renal 20-HETE levels, whereas free EETs were not accumulated. To compensate for this imbalance, a synthetic 14,15-EET analogue was administered by intrarenal infusion before ischemia. The EET analogue improved renal reoxygenation as monitored by in vivo parametric MRI during the initial 2 h reperfusion phase. The EET analogue improved PI3K- as well as mTORC2-dependent rephosphorylation of Akt, induced inactivation of GSK-3β, reduced the development of tubular apoptosis and attenuated inflammatory cell infiltration. The EET analogue also significantly alleviated the I/R-induced drop in creatinine clearance. Patients developing postoperative AKI featured increased preoperative 20-HETE and 8,9-EET levels.

CONCLUSIONS

Pharmacological interventions targeting the CYP-eicosanoid pathway could offer promising new options for AKI prevention. Individual differences in CYP-eicosanoid formation may contribute to the risk of developing AKI in clinical settings.

Rapid Identification of Ischemic Injury in Renal Tissue by Mass-Spectrometry Imaging

The increasing analytical speed of mass-spectrometry imaging (MSI) has led to growing interest in the medical field. Acute kidney injury is a severe disease with high morbidity and mortality. No reliable cut-offs are known to estimate the severity of acute kidney injury. Thus, there is a need for new tools to rapidly and accurately assess acute ischemia, which is of clinical importance in intensive care and in kidney transplantation. We investigated the value of MSI to assess acute ischemic kidney tissue in a porcine model. A perfusion model was developed where paired kidneys received warm (severe) or cold (minor) ischemia ( n = 8 per group). First, ischemic tissue damage was systematically assessed by two blinded pathologists. Second, MALDI-MSI of kidney tissues was performed to study the spatial distributions and compositions of lipids in the tissues. Histopathological examination revealed no significant difference between kidneys, whereas MALDI-MSI was capable of a detailed discrimination of severe and mild ischemia by differential expression of characteristic lipid-degradation products throughout the tissue within 2 h. In particular, lysolipids, including lysocardiolipins, lysophosphatidylcholines, and lysophosphatidylinositol, were dramatically elevated after severe ischemia. This study demonstrates the significant potential of MSI to differentiate and identify molecular patterns of early ischemic injury in a clinically acceptable time frame. The observed changes highlight the underlying biochemical processes of acute ischemic kidney injury and provide a molecular classification tool that can be deployed in assessment of acute ischemic kidney injury.

What we need to know about lipid-associated injury in case of renal ischemia-reperfusion

Renal segmental metabolism is reflected by the complex distribution of the main energy pathways along the nephron, with fatty acid oxidation preferentially used in the cortex area. Ischemia/reperfusion injury (IRI) is due to the restriction of renal blood flow, rapidly leading to a metabolic switch toward anaerobic conditions. Subsequent unbalance between energy demand and oxygen/nutrient delivery compromises kidney cell functions, resulting in a complex inflammatory cascade including the production of reactive oxygen species (ROS). Renal IRI especially involves lipid accumulation. Lipid peroxidation is one of the major events of ROS-associated tissue injury. Here, we briefly review the current knowledge of renal cell lipid metabolism in normal and ischemic conditions. Next, we focus on renal lipid-associated injury, with emphasis on its mechanisms and consequences during the course of IRI. Finally, we discuss preclinical observations aiming at preventing and/or attenuating lipid-associated IRI.

Protective effect of propofol on ischemia-reperfusion injury detected by HPLC-MS/MS targeted metabolic profiling

Ischemia-reperfusion injury(IRI), described as tissue damage caused by reversible ischemic injury or hypoxia prior to the blood supply restoration, is a common pathological phenomenon. In recent study, a hypoxia-reoxygenation (H/R) in the presence or absence of propofol posthypoxia treatment (P-PostH) cell model was built to simulate the condition of IRI, and researchers found propofol may protect cells by suppressing autophagic cell death. To investigate the mechanism underling the protective effect of propofol. A metabolomic analysis was performed in this study using ultra performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC/Q-TOF- MS) to compare the metabolism during the process of H/R in the presence or absence of P-PostH. A total of 22 metabolites were detected varied after propofol posthypoxia treatment. Pathway analysis revealed these metabolites were mainly involved in the purine metabolic pathway, three carboxylic acid metabolic pathways, alanine, aspartate and glutamate metabolism pathway and lipid metabolism pathway. We measured the level of Hypoxanthine to verify the metabolomics work, for pathway analysis, we detect the level of reactive oxygen species with H/R and P-PostH treatment. Our study achieved a global comparison of metabolism profiling of H/R cell model with or without propofol posthypoxic treatment. The result indicated that propofol can attenuate endothelial injury caused by IRI by reducing oxidative damage.

Oxidized phosphatidylcholines are produced in renal ischemia reperfusion injury

BACKGROUND

The aim of this study was to determine the individual oxidized phosphatidylcholine (OxPC) molecules generated during renal ischemia/ reperfusion (I/R) injury.

METHODS

Kidney ischemia was induced in male Sprague-Dawley rats by clamping the left renal pedicle for 45 min followed by reperfusion for either 6h or 24h. Kidney tissue was subjected to lipid extraction. Phospholipids and OxPC species were identified and quantitated using liquid chromatography coupled to electrospray ionization tandem mass spectrometry using internal standards.

RESULT

We identified fifty-five distinct OxPC in rat kidney following I/R injury. These included a variety of fragmented (aldehyde and carboxylic acid containing species) and non-fragmented products. 1-stearoyl-2-linoleoyl-phosphatidylcholine (SLPC-OH), which is a non-fragmented OxPC and 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PAzPC), which is a fragmented OxPC, were the most abundant OxPC species after 6h and 24 h I/R respectively. Total fragmented aldehyde OxPC were significantly higher in 6h and 24h I/R groups compared to sham operated groups (P = 0.03, 0.001 respectively). Moreover, levels of aldehyde OxPC at 24h I/R were significantly greater than those in 6h I/R (P = 0.007). Fragmented carboxylic acid increased significantly in 24h I/R group compared with sham and 6h I/R groups (P = 0.001, 0.001). Moreover, levels of fragmented OxPC were significantly correlated with creatinine levels (r = 0.885, P = 0.001). Among non-fragmented OxPC, only isoprostanes were elevated significantly in 6h I/R group compared with sham group but not in 24h I/R group (P = 0.01). No significant changes were observed in other non-fragmented OxPC including long chain products and terminal furans.

CONCLUSION

We have shown for the first time that bioactive OxPC species are produced in renal I/R and their levels increase with increasing time of reperfusion in a kidney model of I/R and correlate with severity of I/R injury. Given the pathological activity of fragmented OxPCs, therapies focused on their reduction may be a mechanism to attenuate renal I/R injury.

Protective effect of quinacrine against glycerol-induced acute kidney injury in rats

BACKGROUND

Acute kidney injury (AKI) is a serious clinical problem with high rate of mortality and morbidity. Currently used prophylactic and therapeutic strategies to address AKI are limited and warrant further studies. In the present study an attempt was made to investigate the effect of quinacrine, a phospholipase A2 inhibitor against glycerol induced AKI in rats.

METHODS

Adult female Wistar rats were divided in to five groups. After 24 h of water deprivation rats in groups 3, 4 and 5 received an intraperitoneal injection of quinacrine (3 mg/kg, 10 mg/kg and 30 mg/kg of body weight respectively). Thirty minutes after the first injection of quinacrine animals in groups 3, 4 and 5 received an intramuscular injection of 25% glycerol (10 ml/kg of body weight). The animals in group 2 received 25% glycerol (10 ml/kg of body weight) only whereas rats in group 1 served as control . The quinacrine administration was continued once daily for three days, on the fourth day animals were sacrificed, blood and kidney were collected for various biochemical and histopathological studies.

RESULTS

Glycerol treatment produced significant renal structural abnormalities and functional impairment (increased urea and creatinine). Increase in myeloperoxidase (MPO) and malondialdehyde (MDA) clearly suggested the involvement of oxidative stress and neutrophilic activity following glycerol administration. Quinacrine dose dependently attenuated glycerol induced structural and functional changes in kidney.

CONCLUSION

The reversal of glycerol induced AKI by quinacrine points towards a role of phospholipase A2 (PLA2) in the pathogenesis of renal injury. The result of this study suggests that quinacrine may offer an alternative mode of treatment for AKI.

Distinct urinary lipid profile in children with focal segmental glomerulosclerosis

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) accounts for the majority of new-onset end-stage renal disease (ESRD) during adolescence. FSGS treatment is a great challenge for pediatric nephrologists due to intertwined molecular pathways underlining its complex pathophysiology. There is emerging evidence showing that perturbed lipid metabolism plays a role in the pathophysiology of FSGS.

METHODS

We postulate that the nephrotic milieu in FSGS differs from minimal change disease (MCD) and that urinary lipidomics can be used as a tool for early diagnosis of FSGS. We explored the urinary lipid profile of patients with FSGS and MCD using an unbiased metabolomics approach.

RESULTS

We discovered a unique lipid signature characterized by increased concentration of fatty acid (FA) and lysophosphatidylcholines (LPC) and a decrease in urinary concentration of phosphatidylcholine (PC) in patients with FSGS. These findings indicate increased metabolism of membrane phospholipid PC by phospholipase A2 (PLA2), resulting in higher urinary concentrations of LPC and FA.

CONCLUSIONS

We propose that increased PC by-products can be used as a biomarker to diagnose FSGS and shed light on the mechanism of tubular and podocyte damage. Validation of identified urinary lipids as a biomarker in predicting the diagnosis and progression of FSGS in a larger patient population is warranted.

Involvement of AQP 1 in the cardio-protective effect of remifentanil post-conditioning in ischemia/reperfusion rats

BACKGROUND

our research aim to study the role of AQP1 in the cardioprotective effect of remifentanil post-conditioning for myocardial ischemia/reperfusion injury.

METHODS

Ninety Sprague-Dawley (SD) rats were divided into 6 groups: sham operation group (Sham group), myocardial ischemia and reperfusion group (I/R group), postconditioning of remifentanil group (R-post), postconditioning of remifentanil plus AQP1 inhibitor acetazolamide group (R-post +Ace), postconditioning of remifentanil plus opioid-receptor antagonist compounds (R-post +AC), postconditioning of remifentanil plus AQP1 enhancer arginine vasopressin (R-post +AV). All groups except the sham operation group were given 30 min ischemia in left anterior descending (LAD) coronary arteries. All groups were then given 120 min reperfusion to the LAD. Before reperfusion, the R-post, R-post +Ace, R-post +AC, R-post +AV groups were given 10 min remifentanil post-conditioning. Hemodynamic data were measured every 30 min after initiation of ischemia. The rats' hearts were exercised for detecting infarct size and water content in the left ventricle, and AQP1 expression were also detected.

RESULTS

The R-post group showed a significant reduction of the infarct size compared to the I/R group. The effect of R-post for reducing infarct size was slightly enhanced by adding acetazolamide to R-post, so significant differences could still be found when compared R-post+Ace group to the I/R group. The effect of infarct size reduction brought by R-post was blocked by the opioid-receptor antagonist compounds. This effect was also blocked by the AQP1 enhancer. Similar outcomes were found considering the water content of the left ventricle and the AQP1 expression.

CONCLUSION

Cardioprotective effect of remifentanil post-conditioning may initiate through inhibiting the function of AQP1.

CYP-13A12 of the nematode Caenorhabditis elegans is a PUFA-epoxygenase involved in behavioural response to reoxygenation

CYP-13A12 of the nematode Caenorhabditis elegans was characterized after heterologous expression in insect cells as a PUFA epoxygenase producing eicosanoids. These metabolites function as signalling molecules in the regulation of the O2-ON response, a rapid increase of locomotion in response to anoxia/reoxygenation.

Polyunsaturated fatty acid metabolism signature in ischemia differs from reperfusion in mouse intestine

Polyunsaturated fatty acid (PUFA) metabolites are bioactive autoacoids that play an important role in the pathogenesis of a vast number of pathologies, including gut diseases. The induction and the resolution of inflammation depend on PUFA metabolic pathways that are favored. Therefore, understanding the profile of n-6 (eicosanoids)/n-3 (docosanoids) PUFA-derived metabolites appear to be as important as gene or protein array approaches, to uncover the molecules potentially implicated in inflammatory diseases. Using high sensitivity liquid chromatography tandem mass spectrometry, we characterized the tissue profile of PUFA metabolites in an experimental model of murine intestinal ischemia reperfusion. We identified temporal and quantitative differences in PUFA metabolite production, which correlated with inflammatory damage. Analysis revealed that early ischemia induces both pro-inflammatory and anti-inflammatory eicosanoid production. Primarily, LOX- (5/15/12/8-HETE, LTB4, LxA4) and CYP- (5, 6-EET) metabolites were produced upon ischemia, but also PGE3, and PDx. This suggests that different lipids simultaneously play a role in the induction and counterbalance of ischemic inflammatory response from its onset. COX-derived metabolites were more present from 2 to 5 hours after reperfusion, fitting with the concomitant inflammatory peaks. All metabolites were decreased 48 hours post-reperfusion except for to the pro-resolving RvE precursor 18-HEPE and the PPAR-γαμμα agonist, 15d-PGJ2. Data obtained through the pharmacological blockade of transient receptor potential vanilloid-4, which can be activated by 5, 6-EET, revealed that the endogenous activation of this receptor modulates post-ischemic intestinal inflammation. Altogether, these results demonstrate that different lipid pathways are involved in intestinal ischemia-reperfusion processes. Some metabolites, which expression is severely changed upon intestinal ischemia-reperfusion could provide novel targets and may facilitate the development of new pharmacological treatments.

The existence of phospholipase A(2) activity in plant mitochondria and its activation by hyperosmotic stress in durum wheat (Triticum durum Desf.)

The activity of mitochondrial phospholipase A(2) (PLA(2)) was shown for the first time in plants. It was observed in etiolated seedlings from durum wheat, barley, tomato, spelt and green seedlings of maize, but not in potato and topinambur tubers and lentil etiolated seedlings. This result was achieved by a novel spectrophotometric assay based on the coupled PLA(2)/lipoxygenase reactions using 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine as substrate; the mitochondrial localisation was assessed by checking recovery of marker enzymes. Durum wheat mitochondrial PLA(2) (DWM-PLA(2)) showed maximal activity at pH 9.0 and 1mM Ca(2+), hyperbolic kinetics (K(m)=90±6μM, V(max)=29±1nmolmin(-1)mg(-1) of protein) and inhibition by methyl arachidonyl fluorophosphonate, 5-(4-benzyloxyphenyl)-4S-(7-phenylheptanoylamino)pentanoic acid and palmityl trifluoromethyl ketone. Reactive oxygen species had no effect on DWM-PLA(2), that instead was activated by about 50% and 95%, respectively, under salt (0.21M NaCl) and osmotic (0.42M mannitol) stress imposed during germination. Contrarily, a secondary Ca(2+)-independent activity, having optimum at pH 7.0, was stress-insensitive. We propose that the activation of DWM-PLA(2) is responsible for the strong increase of free fatty acids recently measured in mitochondria under the same stress conditions [Laus, et al., J. Exp. Bot. 62 (2011) 141-154] that, in turn, activate potassium channel and uncoupling protein, able to counteract hyperosmotic stress.

Activation of the plant mitochondrial potassium channel by free fatty acids and acyl-CoA esters: a possible defence mechanism in the response to hyperosmotic stress

The effect of free fatty acids (FFAs) and acyl-CoA esters on K(+) uptake was studied in mitochondria isolated from durum wheat (Triticum durum Desf.), a species that has adapted well to the semi-arid Mediterranean area and possessing a highly active mitochondrial ATP-sensitive K(+) channel (PmitoK(ATP)), that may confer resistance to environmental stresses. This was made by swelling experiments in KCl solution under experimental conditions in which PmitoK(ATP) activity was monitored. Linoleate and other FFAs (laurate, palmitate, stearate, palmitoleate, oleate, arachidonate, and the non-physiological 1-undecanesulphonate and 5-phenylvalerate), used at a concentration (10 μM) unable to damage membranes of isolated mitochondria, stimulated K(+) uptake by about 2-4-fold. Acyl-CoAs also promoted K(+) transport to a much larger extent with respect to FFAs (about 5-12-fold). In a different experimental system based on safranin O fluorescence measurements, the dissipation of electrical membrane potential induced by K(+) uptake via PmitoK(ATP) was found to increase in the presence of 5-phenylvalerate and palmitoyl-CoA, both unable to elicit the activity of the Plant Uncoupling Protein. This result suggests a direct activation of PmitoK(ATP). Stimulation of K(+) transport by FFAs/acyl-CoAs resulted in a widespread phenomenon in plant mitochondria from different mono/dicotyledonous species (bread wheat, barley, triticale, maize, lentil, pea, and topinambur) and from different organs (root, tuber, leaf, and shoot). Finally, an increase in mitochondrial FFAs up to a content of 50 nmol mg(-1) protein, which was able to activate PmitoK(ATP) strongly, was observed under hyperosmotic stress conditions. Since PmitoK(ATP) may act against environmental/oxidative stress, its activation by FFAs/acyl-CoAs is proposed to represent a physiological defence mechanism.

Inhibition of 20-HETE synthesis and action protects the kidney from ischemia/reperfusion injury

20-Hydroxyeicosatetraenoic acid (20-HETE) production is increased in ischemic kidney tissue and may contribute to ischemia/reperfusion (I/R) injury by mediating vasoconstriction and inflammation. To test this hypothesis, uninephrectomized male Lewis rats were exposed to warm ischemia following pretreatment with either an inhibitor of 20-HETE synthesis (HET0016), an antagonist (20-hydroxyeicosa-6(Z),15(Z)-dienoic acid), an agonist (20-hydroxyeicosa-5(Z),14(Z)-dienoic acid), or vehicle via the renal artery and the kidneys were examined 2 days after reperfusion. Pretreatment with either the inhibitor or the antagonist attenuated I/R-induced renal dysfunction as shown by improved creatinine clearance and decreased plasma urea levels, compared to controls. The inhibitor and antagonist also markedly reduced tubular lesion scores, inflammatory cell infiltration, and tubular epithelial cell apoptosis. Administering the antagonist accelerated the recovery of medullary perfusion, as well as renal medullary and cortical re-oxygenation, during the early reperfusion phase. In contrast, the agonist did not improve renal injury and reversed the beneficial effect of the inhibitor. Thus, 20-HETE generation and its action mediated kidney injury due to I/R. Whether or not these effects are clinically important will need to be tested in appropriate human studies.

Role of cytochrome P450 enzymes in the bioactivation of polyunsaturated fatty acids

Cytochrome P450 (CYP)-dependent metabolites of arachidonic acid (AA), such as epoxyeicosatrienoic acids and 20-hydroxyeicosatetraenoic acid, serve as second messengers of various hormones and growth factors and play pivotal roles in the regulation of vascular, renal and cardiac function. As discussed in the present review, virtually all of the major AA metabolizing CYP isoforms accept a variety of other polyunsaturated fatty acids (PUFA), including linoleic, eicosapentaenoic (EPA) and docosahexaenoic acids (DHA), as efficient alternative substrates. The metabolites of these alternative PUFAs also elicit profound biological effects. The CYP enzymes respond to alterations in the chain-length and double bond structure of their substrates with remarkable changes in the regio- and stereoselectivity of product formation. The omega-3 double bond that distinguishes EPA and DHA from their omega-6 counterparts provides a preferred epoxidation site for CYP1A, CYP2C, CYP2J and CYP2E subfamily members. CYP4A enzymes that predominantly function as AA ω-hydroxylases show largely increased (ω-1)-hydroxylase activities towards EPA and DHA. Taken together, these findings indicate that CYP-dependent signaling pathways are highly susceptible to changes in the relative bioavailability of the different PUFAs and may provide novel insight into the complex mechanisms that link essential dietary fatty acids to the development of cardiovascular disease.

Reduced colonic apoptosis in mice overexpressing bovine growth hormone occurs through changes in several kinase pathways

OBJECTIVE

Growth hormone (GH) has antiapoptotic effects in several cell lines, including human colonic adenocarcinoma cells. In addition, it has been reported that patients with acromegaly have reduced apoptosis in colonic mucosa. The aim of the study was to investigate colonic apoptosis and underlying molecular mechanisms in transgenic mice overexpressing bovine GH (Acro) aged 3 months (young) or 9 months (elder).

DESIGN AND METHODS

Apoptosis in colonic epithelial cells was evaluated by TUNEL and Annexin V; expression of pro- and anti-apoptotic proteins was assessed by Western blot. GH action was blocked treating Acro with a selective GH receptor antagonist.

RESULTS

Young and elder Acro had lower colonic apoptosis [driven by GH through p38, p44/42 and PI3 kinase pathways], than littermate controls; changes were abolished by treating Acro with a selective GH receptor antagonist. The effects of GH were consistent with an anti-apoptotic phenotype (reduced cytosolic cytochrome-c, Bad and Bax and increased Bcl-2, and Bcl-XL level) leading to lower activation of caspase-9 and caspase-3. Changes in apoptotic proteins reversed after treatment with a GH receptor antagonist, suggesting a direct effect of GH. In addition, antiapoptotic phenotype of Acro had a protective role against doxorubicin-induced apoptosis.

CONCLUSIONS

Our results suggest that GH leads to increased and reduced levels of anti- and pro-apoptotic proteins, respectively, lowering apoptosis in either young or elder transgenic animals through activation of several kinase pathways.

Alterations in membrane transport function and cell viability induced by ATP depletion in primary cultured rabbit renal proximal tubular cells

This study was undertaken to elucidate the underlying mechanisms of ATP depletion-induced membrane transport dysfunction and cell death in renal proximal tubular cells. ATP depletion was induced by incubating cells with 2.5 mM potassium cyanide (KCN)/0.1 mM iodoacetic acid (IAA), and membrane transport function and cell viability were evaluated by measuring Na(+)-dependent phosphate uptake and trypan blue exclusion, respectively. ATP depletion resulted in a decrease in Na(+)-dependent phosphate uptake and cell viability in a time-dependent manner. ATP depletion inhibited Na(+)-dependent phosphate uptake in cells, when treated with 2 mM ouabain, a Na(+) pump-specific inhibitor, suggesting that ATP depletion impairs membrane transport functional integrity. Alterations in Na(+)-dependent phosphate uptake and cell viability induced by ATP depletion were prevented by the hydrogen peroxide scavenger such as catalase and the hydroxyl radical scavengers (dimethylthiourea and thiourea), and amino acids (glycine and alanine). ATP depletion caused arachidonic acid release and increased mRNA levels of cytosolic phospholipase A(2) (cPLA(2)). The ATP depletion-dependent arachidonic acid release was inhibited by cPLA(2) specific inhibitor AACOCF(3). ATP depletion-induced alterations in Na(+)-dependent phosphate uptake and cell viability were prevented by AACOCF(3). Inhibition of Na(+)-dependent phosphate uptake by ATP depletion was prevented by antipain and leupetin, serine/cysteine protease inhibitors, whereas ATP depletion-induced cell death was not altered by these agents. These results indicate that ATP depletion-induced alterations in membrane transport function and cell viability are due to reactive oxygen species generation and cPLA(2) activation in renal proximal tubular cells. In addition, the present data suggest that serine/cysteine proteases play an important role in membrane transport dysfunction, but not cell death, induced by ATP depletion.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

Early activation of bradykinin B2 receptor aggravates reactive oxygen species generation and renal damage in ischemia/reperfusion injury

The kallikrein/kinin system is beneficial in ischemia/reperfusion injury in heart, controversial in brain, but detrimental in lung, liver, and intestine. We examined the role of the kallikrein/kinin system in acute ischemia/reperfusion renal injury induced by 40 min occlusion of the renal artery followed by reperfusion. Rats were infused with tissue kallikrein protein 5 days before (pretreated group) or after (treated group) ischemia. Two days later, the pretreated group exhibited the worst renal dysfunction, followed by the treated group, then the control group. Kallikrein increased tubular necrosis and inflammatory cell infiltration with generation of more tumor necrosis factor-alpha and monocyte chemoattractant protein-1. Reactive oxygen species (ROS), malondialdehyde, and reduced/oxidized glutathione measurement revealed that the oxidative stress was augmented by kallikrein administration in both ischemic and reperfusion phases. The groups with more ROS generation also had more apoptotic renal cells. The deleterious effects of kallikrein on ischemia/reperfusion injury were reversed by cotreatment with bradykinin B2 receptor (B2R) antagonist, but not B1 receptor antagonist, and were not associated with hemodynamic changes. We conclude that early activation of B2R augmented ROS generation in ischemia/reperfusion renal injury, resulting in subsequent apoptosis, inflammation, and tissue damage. This finding suggests the potential application of B2R antagonists in acute ischemic renal disease associated with bradykinin activation.

Accumulation of nonesterified fatty acids causes the sustained energetic deficit in kidney proximal tubules after hypoxia-reoxygenation

Kidney proximal tubules exhibit decreased ATP and reduced, but not absent, mitochondrial membrane potential (Deltapsi(m)) during reoxygenation after severe hypoxia. This energetic deficit, which plays a pivotal role in overall cellular recovery, cannot be explained by loss of mitochondrial membrane integrity, decreased electron transport, or compromised F1F0-ATPase and adenine nucleotide translocase activities. Addition of oleate to permeabilized tubules produced concentration-dependent decreases of Deltapsi(m) measured by safranin O uptake (threshold for oleate = 0.25 microM, 1.6 nmol/mg protein; maximal effect = 4 microM, 26 nmol/mg) that were reversed by delipidated BSA (dBSA). Cell nonesterified fatty acid (NEFA) levels increased from <1 to 17.4 nmol/mg protein during 60- min hypoxia and remained elevated at 7.6 nmol/mg after 60 min reoxygenation, at which time ATP had recovered to only 10% of control values. Safranin O uptake in reoxygenated tubules, which was decreased 85% after 60-min hypoxia, was normalized by dBSA, which improved ATP synthesis as well. dBSA also almost completely normalized Deltapsi(m) when the duration of hypoxia was increased to 120 min. In intact tubules, the protective substrate combination of alpha-ketoglutarate + malate (alpha-KG/MAL) increased ATP three- to fourfold, limited NEFA accumulation during hypoxia by 50%, and lowered NEFA during reoxygenation. Notably, dBSA also improved ATP recovery when added to intact tubules during reoxygenation and was additive to the effect of alpha-KG/MAL. We conclude that NEFA overload is the primary cause of energetic failure of reoxygenated proximal tubules and lowering NEFA substantially contributes to the benefit from supplementation with alpha-KG/MAL.

Indomethacin induces free radical-mediated changes in renal brush border membranes

Nonsteroidal anti-inflammatory drugs (NSAIDs) are used extensively in clinical medicine. One disadvantage of their use, however, is the occurrence of adverse effects in the kidneys. The side effects produced in this organ have been classically attributed to the inhibitory effect of these drugs on the activity of cyclooxygenase, a key enzyme in prostaglandin synthesis. Our earlier work with indomethacin, a commonly used NSAID, has shown that oxidative stress and mitochondrial dysfunction occur in the kidney in response to the drug. In view of this, this study looked into the effect of indomethacin on brush border membranes (BBM) from the kidney, as these biomembranes are prime targets of oxygen free radicals. Rats, fasted overnight, were dosed with indomethacin (20 mg/kg) by gavage and sacrificed 24 h later. BBM were isolated from the kidneys by polyethylene glycol precipitation. It was found that there was an increase in levels of products of peroxidation and a fall in the level of alpha-tocopherol in the BBM from indomethacin-dosed rats. These BBM also exhibited impaired glucose transport. The lipid composition of the membranes was also found to be altered. Alterations in lipids were associated with up-regulation of phospholipase A2. Pretreatment with L-arginine, a nitric oxide donor, protected against these effects of indomethacin. Thus, this study suggests that indomethacin induces impairment in structure and function of BBM in the kidney, with these effects possibly mediated by free radicals and activation of phospholipases. We postulate that such alterations may be important in the pathogenesis of NSAID-induced nephropathy.

Studies on the endogenous phospholipids of mammalian kidney and theirin vitro hydrolysis by endogenous phospholipases: a thin layer chromatographic and densitometric study

The phosphoglycerides profile of six species of mammalian kidney (guinea pig, pig, cat, dog, mouse and rat) and their in vitro response to the endogenous phospholipases were determined by TLC technology in conjunction with densitometric measurements. Changes in their phospholipids profile subsequent to in vitro incubation of whole tissue homogenate of these kidneys for 60 min, at pH 7.4, 38 degrees C, and prior to phospholipids extraction have shown that the deacylation of the endogenous cardiolipin (CL) is the most prevalent lipolytic event of all mammalian kidneys studied. Concurrent with the deacylation of CL, there was also formation of monolysocardiolipin (MLCL) and a reduction in CL level. To a much lesser extent, lyso alkenyl phosphatidyl ethanolamine (LPE) was also produced concomitant with a decrease of the endogenous alkenyl phosphatidyl ethanolamine (PE) level. The deacylation of PE plasmalogen to its lyso form confirms the action of endogenous PLA(2) releasing sn-2 fatty acids.

Cyclooxygenase 1-dependent production of F2-isoprostane and changes in redox status during warm renal ischemia-reperfusion

The detrimental role of oxidative stress has been widely described in tissue damage caused by ischemia-reperfusion. A nonenzymatic, reactive oxygen species-related pathway has been suggested to produce 8-iso-prostaglandin F(2alpha) (8-iso-PGF(2alpha)), an epimer of prostaglandin F(2alpha) (PGF(2alpha)), which has been proposed as an indicator of oxidative stress. Using an in vivo ischemia-reperfusion model in rat kidneys, we investigated intrarenal accumulation of 8-iso-PGF(2alpha) and PGF(2alpha). Both prostanoids accumulated in the ischemic kidney and disappeared upon reperfusion. In addition, a nonselective (acetylsalicylic acid) or selective cyclooxygenase (COX) 1 inhibitor (SC-560) completely abrogated the 8-iso-PGF(2alpha) and PGF(2alpha) formation in kidneys subjected to ischemia. COX2 inhibition had no effect on the production of these prostanoids. Therefore the two metabolites of arachidonic acid seemed to be produced via an enzymatic COX1-dependent pathway. Neither COX overexpression nor COX activation was detected. We also investigated renal glutathione, which is considered to be the major thiol-disulfide redox buffer of the tissue. Total and oxidized glutathione was decreased during the ischemic period, whereas no further decrease was seen for up to 60 min of reperfusion. These data demonstrate that a dramatic decrease in antioxidant defense was initiated during warm renal ischemia, whereas the 8-iso-PGF(2alpha) was related only to arachidonate conversion by COX1.

Age-related changes of the endogenous cardiolipin and plasmalogens of guinea pig kidney and theirin vitro hydrolysis by endogenous phospholipases: a thin layer chromatographic analysis in conjunction with densitometric measurement

The phosphoglycerides profile of guinea pig kidney, fetal, young adult, and aged, and their in vitro response to the endogenous lipolytic enzymes, mainly in the phospholipase group were determined by TLC technology in conjunction with densitometric measurement. Changes in phosphoglycerides profile subsequent to in vitro incubation of these tissues at pH 7.4, and 38 degrees C for 45 min and prior to phospholipid extraction has provided evidence relating to their respective lipolytic enzymes capabilities and age. These changes are mainly related to endogenous cardiolipin (CL), alkenyl phospholipids (phosphatidyl ethanolamine and phosphatidyl choline) and their endogenous deacylation to their respective lyso derivatives monolysocardiolipin (MLCL), lyso alkenyl phosphatidyl ethanolamine (LPE), and lyso alkenyl phosphatidyl choline (LPC) by endogenous phospholipases. The hydrolysis of the plasmalogen confirms the action of endogenous PLA(2) on sn-2 fatty acids of these compounds.

Calpain released from dying hepatocytes mediates progression of acute liver injury induced by model hepatotoxicants

Liver injury is known to progress even after the hepatotoxicant is long gone and the mechanisms of progressive injury are not understood. We tested the hypothesis that hydrolytic enzymes such as calpain, released from dying hepatocytes, destroy the surrounding cells causing progression of injury. Calpain inhibitor, N-CBZ-VAL-PHE-methyl ester (CBZ), administered 1 h after a toxic but nonlethal dose of CCl(4) (2 ml/kg, ip) to male Sprague Dawley rats substantially mitigated the progression of liver injury (6 to 48 h) and also led to 75% protection against CCl(4)-induced lethality following a lethal dose (LD75) of CCl(4) (3 ml/kg). Calpain leakage in plasma and in the perinecrotic areas increased until 48 h and decreased from 72 h onward paralleling progression and regression of liver injury, respectively, after CCl(4) treatment. Mitigation of progressive injury was accompanied by substantially low calpain in perinecrotic areas and in plasma after CBZ treatment. Normal hepatocytes incubated with the plasma collected from CCl(4)-treated rats (collected at 12 h when most of the CCl(4) is eliminated) resulted in extensive cell death prevented by CBZ. Cell-impermeable calpain inhibitor E64 also protected against progression of CCl(4)-induced liver injury, thereby confirming the role of released calpain in progression of liver injury. Following CCl(4) treatment, calpain-specific breakdown of alpha-fodrin increased, while it was negligible in rats receiving CBZ after CCl(4). Hepatocyte cell death in incubations containing calpain was completely prevented by CBZ. Eighty percent of Swiss Webster mice receiving a lethal dose (LD80) of acetaminophen (600 mg/kg, ip) survived if CBZ was administered 1 h after acetaminophen, suggesting that calpain-mediated progression of liver injury is neither species nor chemical specific. These findings suggest the role of calpain in progression of liver injury.

Dedifferentiation and proliferation of surviving epithelial cells in acute renal failure

In contrast to the heart or brain, the kidney can completely recover from an ischemic or toxic insult that results in cell death. During recovery from ischemia/reperfusion injury, surviving tubular epithelial cells dedifferentiate and proliferate, eventually replacing the irreversibly injured tubular epithelial cells and restoring tubular integrity. Repair of the kidney parallels kidney organogenesis in the high rate of DNA synthesis and apoptosis and in patterns of gene expression. As has been shown by proliferating cell nuclear antigen and 5-bromo 2'-deoxyuridine labeling studies and, in unpublished studies, by counting mitotic spindles identified by labeling with antitubulin antibody, the proliferative response is rapid and extensive, involving many of the remaining cells of the proximal tubule. This extensive proliferative capacity is interpreted to reflect the intrinsic ability of the surviving epithelial cell to adapt to the loss of adjacent cells by dedifferentiating and proliferating. Adhesion molecules likely play important roles in the regulation of renal epithelial cell migration, proliferation, and differentiation, as do cytokines and chemokines. Better understanding of all of the characteristics resulting in dedifferentiation and proliferation of the proximal tubule epithelial cell and cell-cell and cell-matrix interactions important for this repair function will lead to novel approaches to therapies designed to facilitate the processes of recovery in humans.

Effect of betaxolol on aspartate aminotransferase activity in hypoxic rat retina in vitro

We investigated the effect of betaxolol on the decrease of mitochondrial aspartate aminotransferase (mAAT) activity in rat retinas induced by hypoxia in vitro. It is reported that mAAT decreases in ischemic or hypoxic retina and that the decrease is caused by Ca(2+)-dependent proteases such as calpain. Betaxolol is a compound that has beta(1)-adrenergic receptor blocking and voltage-dependent calcium channel blocking properties. The rat eye cups were maintained with Locke's solution saturated with 95% air - 5% CO(2). The eye cups were immersed in glucose-free Locke's solution saturated with 95% N(2) / 5% CO(2) (hypoxic solution). Ninety minutes of hypoxia caused a 20% decrease in mAAT activity. The eye cups incubated with the hypoxic solution containing 1 mM EGTA, 10 micro M MK-801 or 100 micro M betaxolol were protected from the decrease in mAAT activity, so that the residual mAAT activity was 50%, 45% or 40%, respectively, compared to the eye cups incubated in hypoxic solution alone, which had a 100% decrease in mAAT activity. In addition, co-incubation with EGTA and betaxolol had a greater protective effect against the mAAT decrease than a single application. This additive effect of betaxolol was dose-dependent. These results suggested that betaxolol had a protective effect against the decrease of mAAT caused by hypoxia and indicated that betaxolol might inhibit the Ca(2+) release from intracellular Ca(2+) stores.

Kupffer cells play an important role in the cytokine production and activation of nuclear factors of liver grafts from non-heart-beating donors

In the non-heart-beating donor (NHBD) deterioration of microcirculation of the liver graft is strongly associated with secretion of cytokines and eicosanoids. In this study we investigated the excretion of cytokines, eicosanoids, and DNA binding activity of transcription factors in the grafts from NHBD and evaluated the effects of the elimination of Kupffer cells on them. The purpose of this study was to clarify the impact of Kupffer cells on transcription factor activity and the that of cytokine and eicosanoid production on reperfusion injury of liver grafts from NHBD. Wistar rats were allocated to four groups: (a). control group: livers were extracted under heart-beating conditions and perfused without cold storage, (b). heart beating (HB) group: livers extracted under heart-beating conditions were perfused after 6 h of cold storage, (c). non-heart-beating (NHB) group: livers extracted after cardiac arrest were perfused after cold storage, (d). Kupffer cell eliminated (KE) group: liposome-encapsulated dichloromethylene diphosphonate was intravenously administered to eliminate Kupffer cells before extraction, and the liver was perfused after cold storage. Cytokines and eicosanoids in perfusate were measured. DNA binding activity of nuclear factor kappa B, activating protein 1, and nuclear factor-interleukin 6 of tissue were investigated. Concentrations of interleukin 1 beta and thromboxane B(2) in the perfusate were significantly higher in NHB group, but they were completely suppressed in the KE group. A rise in binding activity of nuclear factor kappa B and activating protein 1 was not observed during cold storage in any groups, but these activities did increase remarkably after reperfusion. Significant buildup of those activities were recognized in the NHB group, and this phenomenon was inhibited in the KE group. The histological structures of the sinusoid in the KE group were well maintained, as with those of the control group. These results indicate that cytokines, eicosanoids, and the DNA binding activity of the transcripton factor are strongly associated with reperfusion injury, and Kupffer cells play an important role in this mechanism in grafts from NHBDs.

Renal cytochrome p450 oxygenases and preglomerular vascular response to arachidonic acid and endothelin-1 following ischemia/reperfusion

This study tested the hypothesis that cytochrome P450 (P450) metabolites of arachidonic acid (AA) contribute to the vascular changes in ischemia/reperfusion (I/R) injury in the rat. In this study, P450-dependent omega-hydroxylase-mediated vascular reactivity of the rat renal interlobular and arcuate vessels [preglomerular vessels (PGMV)] was measured in left kidneys subjected to I/R. Clipping the left renal artery and vein for 30 min followed by reperfusion (I/R) for 3, 6, and 24 h markedly reduced renal microsomal omega-hydroxylase-mediated conversion of [(14)C]AA to 20-hydroxyeicosatetraenoic acid (HETE) that amounted to 34, 37, and 58% of the control enzyme activity, respectively. CYP4A protein expression was also reduced. There was no significant change in epoxygenase activity. Despite these changes, constriction of the rat PGMV by AA or endothelin-1 (ET-1) was not different in vessels from the clipped and nonclipped (contralateral) kidney. Clofibrate (250 mg/kg i.p.), an inducer of CYP4A protein and omega-hydroxylase enzymes, did not increase 20-HETE production but selectively enhanced the vasoconstriction produced by AA and ET-1 in the clipped but not the contralateral kidney without affecting the constriction produced by 9,11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F(2alpha). On the other hand, administration of 2% NaCl (w/v, orally for 7 days) to induce P450-dependent epoxygenase activity attenuated AA-induced vasoconstriction but enhanced ET-1-induced vasoconstriction only in the clipped kidney. These data indicate that the reduction in CYP4A protein expression and enzyme activity in I/R is an adaptive mechanism to preserve renal vasculature from excessive vasoconstriction. Moreover, the increase in epoxygenase activity following salt loading may account for the diminished vasoconstriction evoked by AA. However, the enhancing effect of salt on ET-1-induced vasoconstriction in I/R appears to result from an overwhelming effect of salt-induced sensitization of the renal vasculature to ET-1 over the enhanced production of dilator epoxygenase products.

Nuclear translocation of cytosolic phospholipase A2 is induced by ATP depletion

Phospholipase A(2) (PLA(2)) enzymes may play a role in cellular injury due to ATP depletion. Renal Madin-Darby canine kidney cells were subjected to ATP depletion to assess the effects of cellular energy metabolism on cytosolic PLA(2) (cPLA(2)) regulation. ATP depletion results in a decrease in soluble cPLA(2) activity and an increase in membrane-associated activity, which is reversed upon restoration of ATP levels by addition of dextrose. In ATP-depleted cells cPLA(2) mass shifts from cytosol to nuclear fractions. GFP-cPLA(2) is localized at the nuclear membrane of stably transfected ATP-depleted LLC-PK(1) cells under conditions where [Ca(2+)](i) is known to increase. cPLA(2) translocation does not occur if the increase in [Ca(2+)](i) increase is inhibited. If [Ca(2+)](i) is allowed to increase when ATP is depleted and the cells are then lysed, cPLA(2) remains associated with nuclear fractions even if the homogenate [Ca(2+)] is markedly reduced. In contrast, cPLA(2), which becomes associated with the nucleus when [Ca(2+)](i) is increased using ionophore, readily dissociates from the nuclear fractions of ATP-replete cells upon reduction of homogenate [Ca(2+)]. Okadaic acid inhibits the ATP depletion-induced association of cPLA(2) with nuclear fractions. Thus energy deprivation results in [Ca(2+)]-induced nuclear translocation, which is partially prevented by a phosphatase inhibitor.

Renal concentrating defect in mice lacking group IV cytosolic phospholipase A(2)

Eicosanoids regulate various cellular functions that are important in physiological and pathophysiological processes. Arachidonic acid is released from membranes by phospholipase A(2) (PLA(2)) activity. Activated macrophages derived from mice lacking the 85-kDa group IV cytosolic PLA(2) (cPLA(2)) have a markedly reduced release of prostaglandin E(2) and leukotrienes B(4) and C(4). Under basal conditions and after furosemide, urinary prostaglandin E(2) excretion is reduced in cPLA(2)-knockout (cPLA(2)(-/-)) mice. Serum creatinine, Na(+), K(+), and Ca(2+) concentrations, glomerular filtration rate, and fractional excretion of Na(+) and K(+) are not different in cPLA(2)(-/-) and cPLA(2)(+/+) mice. Maximal urinary concentration is lower in 48-h water-deprived cPLA(2)(-/-) mice compared with cPLA(2)(+/+) animals (1,934 +/- 324 vs. 3,541 +/- 251 mmol/kgH(2)O). Plasma osmolality is higher (337 +/- 5 vs. 319 +/- 3 mmol/kgH(2)O) in cPLA(2)(-/-) mice that lose a greater percentage of their body weight (20 +/- 2 vs. 13 +/- 1%) compared with cPLA(2)(+/+) mice after water deprivation. Vasopressin does not correct the concentrating defect. There is progressive reduction in urinary osmolality with age in cPLA(2)(-/-) mice. Membrane-associated aquaporin-1 (AQP1) expression, identified by immunocytochemical techniques, is reduced markedly in proximal tubules of older cPLA(2)(-/-) animals but is normal in thin descending limbs. However, Western blot analysis of kidney cortical samples revealed an equivalent AQP1 signal intensity in cPLA(2)(+/+) and cPLA(2)(-/-) animals. Young cPLA(2)(-/-) mice have normal proximal tubule AQP1 staining. Collecting duct AQP2, -3, and -4 were normally expressed in the cPLA(2)(-/-) mice. Thus mice lacking cPLA(2) develop an age-related defect in renal concentration that may be related to abnormal trafficking and/or folding of AQP1 in the proximal tubule, implicating cPLA(2) in these processes.

Redistribution of cPLA(2) in rat renal tubular cell cultures in response to PCBs

The influence of different polychlorinated biphenyls (PCBs) upon the cytosolic phospholipase A(2) (cPLA(2)) redistribution to the particulate fraction has been investigated in rat renal proximal tubule culture cells. Treatment with Aroclor 1248 increased PLA(2) activity in the particulate fraction in a concentration-dependent manner using two radioactive substrates. However, the activity of PLA(2) in the cytosolic fraction decreased. This work also shows that 2,2',4,4',5,5'-hexachlorobiphenyl (HCB) (a di-ortho-substituted nonplanar congener) can increase the activity of PLA(2) in the particulate fraction and decrease the enzyme activity in the cytosolic fraction. The exposure of cell cultures to 3,3',4,4'-tetrachlorobiphenyl (TCB) (a non-ortho-subtituted planar congener) does not alter PLA(2) activity. These results suggest that PCBs, depending on their planar or nonplanar structures, cause a translocation of the enzyme from the cytosol to membranes. To evaluate this possibility, the contents of immunoreactive cPLA(2) were examined by immunoblot analysis in the high-speed supernatant and the particulate fraction of treated cell cultures. The increases/decreases in the amounts of cPLA(2) protein agree with the increases/decreases of PLA(2) activity previously cited. These data demonstrate that the PCB-stimulated redistribution of cPLA(2) to membranes is associated, at least in part, with the changes detected in the activity of the enzyme.

Cytosolic phospholipase A(2) regulates golgi structure and modulates intracellular trafficking of membrane proteins

The Golgi complex and the trans-Golgi network are critical cellular organelles involved in the endocytic and biosynthetic pathways of protein trafficking. Lipids have been implicated in the regulation of membrane-protein trafficking, vesicular fusion, and targeting. We have explored the role of cytosolic group IV phospholipase A(2) (cPLA(2)) in membrane-protein trafficking in kidney epithelial cells. Adenoviral expression of cPLA(2) in LLC-PK(1) kidney epithelial cells prevents constitutive trafficking to the plasma membrane of an aquaporin 2-green fluorescent protein chimera, with retention of the protein in the rough endoplasmic reticulum. Plasma membrane Na(+)-K(+)-ATPase alpha-subunit localization is markedly reduced in cells expressing cPLA(2), whereas the trafficking of a Cl(-)/HCO(3)(-) anion exchanger to the plasma membrane is not altered in these cells. Expression of cPLA(2) results in dispersion of giantin and beta-COP from their normal, condensed Golgi localization, and in marked disruption of the Golgi cisternae. cPLA(2) is present in Golgi fractions from noninfected LLC-PK(1) cells and rat kidney cortex. The distribution of tubulin and actin was not altered by cPLA(2), indicating that the microtubule and actin cytoskeleton remain intact. Total cellular protein synthesis is unaffected by the increase in cPLA(2) activity. Thus cPLA(2) plays an important role in determining Golgi architecture and selective control of constitutive membrane-protein trafficking in renal epithelial cells.

Cell biology and molecular mechanisms of injury in ischemic acute renal failure

The pathogenesis of acute renal failure has been attributed to persistent vasoconstriction and leukocyte-endothelial interactions, resulting in inflammation and compromise of local blood flow to the outer medulla, the loss of tubular epithelial cell polarity with multiple functional sequelae, necrosis or apoptosis of epithelial cells, and the de-differentiation, migration and proliferation of surviving cells. In this paper, the authors present their views of pathophysiology of ischemic acute renal failure.

Complement-induced phospholipase A2 activation in experimental membranous nephropathy

BACKGROUND

In the passive Heymann nephritis (PHN) model of membranous nephropathy, C5b-9 induces glomerular epithelial cell (GEC) injury and proteinuria, which is partially mediated by eicosanoids. By analogy, in cultured rat GEC, sublytic C5b-9 injures plasma membranes and releases arachidonic acid (AA) and eicosanoids, due to activation of phospholipase A2 (PLA2). This study addresses the mechanisms of PLA2 activation.

METHODS

PLA2 expression was assessed with the polymerase chain reaction or immunoblotting, and activity was determined using an in vitro assay or by measurement of free AA.

RESULTS

Under basal conditions, GEC in culture expressed a relatively low level of cytosolic PLA2 (cPLA2) protein, while mRNAs of groups IB, IIA and V secretory PLA2s (sPLA2) were not detectable. Incubation of GEC with sublytic C5b-9 induced 1.5- to 2.0-fold increases in free [3H]AA at 40 minutes, and three and 24 hours. C5b-9 did not increase cPLA2 protein, and did not induce group IB, IIA or V sPLA2 mRNAs. Stable overexpression of cPLA2 in GEC amplified the C5b-9-induced increases in free [3H]AA, while analogous overexpression of group IIA sPLA2 had no effect. PLA2 activity was increased in glomeruli of rats with PHN, and this enhanced activity was characterized as cPLA2. There were no differences in cPLA2 protein expression between PHN and control glomeruli.

CONCLUSIONS

Release of AA by C5b-9 in GEC in culture and in vivo is mediated by cPLA2, and the mechanism is consistent with post-translational regulation of cPLA2 activity. C5b-9 does not induce expression or stimulate activity of sPLA2 isoforms in GEC.

Attenuation of intestinal ischemia/reperfusion injury with sodium nitroprusside: studies on mitochondrial function and lipid changes

Reactive oxygen species have been implicated in cellular injury during ischemia/reperfusion (I/R). Mitochondria are one of the main targets of oxygen free radicals and damage to this organelle leads to cell death. Reports suggest that nitric oxide (NO) may offer protection from damage during I/R. This study has looked at the functional changes and lipid alteration to mitochondria during intestinal I/R and the protection offered by NO. It was observed that I/R of the intestine is associated with functional alterations in the mitochondria as suggested by MTT reduction, respiratory control ratio and mitochondrial swelling. Mitochondrial lipid changes suggestive of activation of phospholipase A(2) and phospholipase D were also seen after (I/R) mediated injury. These changes were prevented by the simultaneous presence of a NO donor in the lumen of the intestine. These studies have suggested that structural and functional alterations of mitochondria are prominent features of I/R injury to the intestine which can be ameliorated by NO.

Role of phospholipase A2 in the cytotoxic effects of oxalate in cultured renal epithelial cells

BACKGROUND

Oxalate, a common constituent of kidney stones, is cytotoxic for renal epithelial cells. Although the exact mechanism of oxalate-induced cell death remains unclear, studies in various cell types, including renal epithelial cells, have implicated phospholipase A2 (PLA2) as a prominent mediator of cellular injury. Thus, these studies examined the role of PLA2 in the cytotoxic effects of oxalate.

METHODS

The release of [3H]-arachidonic acid (AA) or [3H]-oleic acid (OA) from prelabeled Madin-Darby canine kidney (MDCK) cells was measured as an index for PLA2 activity. The cell viability was assessed by the exclusion of ethidium homodimer-1.

RESULTS

Oxalate exposure (175 to 550 microM free) increased the release of [3H]-AA in MDCK cells but had no effect on the release of [3H]-OA. Oxalate-induced [3H]-AA release was abolished by arachidonyl trifluoromethyl ketone (AACOCF3), a selective inhibitor of cytosolic PLA2 (cPLA2), but was not affected by selective inhibitors of secretory PLA2 and calcium-independent PLA2. The [3H]-AA release could be demonstrated within 15 minutes after exposure to oxalate, which is considerably earlier than the observed changes in cell viability. Furthermore, AACOCF3 significantly reduced oxalate toxicity in MDCK cells.

CONCLUSIONS

Oxalate increases AA release from MDCK cells by a process involving cPLA2. In addition, based on the evidence obtained using a selective inhibitor of this isoform, it would appear that the activity of this enzyme is responsible, at least in part, for the cytotoxic effects of oxalate. The finding that oxalate can trigger a known lipid-signaling pathway may provide new insight into the initial events in the pathogenesis of nephrolithiasis.

Two uncompetitive, activated, and transport sites of the Na+/H+ exchanger for pH regulation in perfused rat kidney

The purpose of this study is to assess the effect of an apparent alteration in intracellular pH and the effect of amiloride on the activity of the Na+/H+ antiporter in perfused rat kidney. Rat kidney-Na+ retention was determined using tracer 22Na in perfusate composed of HCl-glycine buffer (pH 3.80 to pH 5.92) or NH4OH-glycine buffer (pH 6.22-7.95) containing Na+ to match physiologic concentrations. Plotting renal Na+ retention for 10 min versus pH in absence of amiloride showed two classical uncompetitive activator curves for H+, one curve from pH 4.19 to 5.10 and another from pH 6.22 to 7.95. H+ acts as an uncompetitive reversible binding substrate with the receptor triggering activation of the exchanger already sequestered with Na+, thus yielding two Ka values for the exchanger suggesting non-first order kinetics. Using an equation derived for uncompetitive-activation binding of Nao+ and Hi+, plotting [mM Na+ mg protein-1 10 min-1]-1 versus [H+], two linear plots are observed on Cartesian coordinates with abscissa intersecting at 47 +/- 1 microM, pKa = 4.32 +/- 0.02 (pH 4.19-5.10) and 4.21 +/- 0.02 microM, pKa = 5.38 +/- 0.01 (pH 6.22-7.95), respectively. Perfusing buffer containing 2 mM amiloride, completely inactivated the antiporter showing stronger inhibition between pH 3.80 and 5.92. Results suggest the presence of two uncompetitive binding sites for H+ with the Na+/H+ exchanger. One is a high affinity binding site at physiological intracellular apparent pH, and another is a low affinity binding site at ischaemic apparent pH, implying the existence of two titration sites for intracellular pH regulation.

Possible mechanism for the decrease of mitochondrial aspartate aminotransferase activity in ischemic and hypoxic rat retinas

Glutamate is believed to be an excitatory amino acid neurotransmitter in the retina. Enzymes for glutamate metabolism, such as glutamate dehydrogenase, ornithine aminotransferase, glutaminase, and aspartate aminotransferase (AAT), exist mainly in the mitochondria. The abnormal increase of intracellular calcium ions in ischemic retinal cells may cause an influx of calcium ions into the mitochondria, subsequently affecting various mitochondrial enzyme activities through the activity of mitochondrial calpain. As AAT has the highest level of activity among enzymes involved in glutamate metabolism, we investigated the change of AAT activity in ischemic and hypoxic rat retinas and the protection against such activity by calpain inhibitors. We used normal RCS (rdy+/rdy+) rats. For the in vivo studies, we clamped the optic nerve of anesthetized rats to induce ischemia. In the in vitro studies, the eye cups were incubated with Locke's solution saturated with 95% N2/5% CO2. The activity of cytosolic AAT (cAAT) was about 20% of total activity, whereas mitochondrial AAT (mAAT) was about 75% in rat retina. Ninety minutes of ischemia or hypoxia caused a 20% decrease in mAAT activity, whereas cAAT activity remained unchanged. To examine the contribution of intracellular calcium ions to the degradation of mAAT, we used Ca2+-free Locke's solution containing 1 mM EGTA, ryanodine (Ca2+ channel blocker), and thapsigargin (Ca2+-ATPase inhibitor). In the present study, thapsigargin in Ca2+-free Locke's solution, but not ryanodine in this solution, was found to prevent AAT degradation. AAT degradation was also prevented by calpain inhibitors (Ca2+-dependent protease inhibitor) such as calpeptin at 1 nM, 10 nM, 0.1 microM, 1 microM and 10 microM, and by calpain inhibitor peptide, but not by other protease inhibitors (10 microM leupeptin, pepstatin, chymostatin). Additionally, we determined the subcellular localization of calpain activity and examined the change of calpain activity in ischemic rat retinas. Our results suggest that decreased activity of mAAT in ischemic and hypoxic rat retinas might be evoked by the degradation by calpain-catalyzed proteolysis in mitochondria.

Plasma membrane phospholipid integrity and orientation during hypoxic and toxic proximal tubular attack

BACKGROUND

Acute cell injury can activate intracellular phospholipase A2 (PLA2) and can inhibit plasma membrane aminophospholipid translocase(s). The latter maintains inner/outer plasma membrane phospholipid (PL) asymmetry. The mechanistic importance of PLA2-mediated PL breakdown and possible PL redistribution ("flip flop") to lethal tubule injury has not been well defined. This study was performed to help clarify these issues.

METHODS

Proximal tubule segments (PTS) from normal CD-1 mice were subjected to either 30 minutes of hypoxia, Ca2+ ionophore (50 microM A23187), or oxidant attack (50 microM Fe). Lethal cell injury [the percentage of lactate dehydrogenase (LDH) release], plasma membrane PL expression [two-dimensional thin layer chromatography (TLC)], and free fatty acid (FFA) levels were then assessed. "Flip flop" was gauged by preferential decrements in phosphatidylserine (PS) versus phosphatidylcholine (PC; PS/PC ratios) in response to extracellular (Naja) PLA2 exposure.

RESULTS

Hypoxia induced approximately 60% LDH release, but no PL losses were observed. FFA increments suggested, at most 3% or less PL hydrolysis. Naja PLA2 reduced PLs in hypoxic tubules, but paradoxically, mild cytoprotection resulted. In contrast to hypoxia, Ca2+ ionophore and Fe each induced significant PL losses (6 to 15%) despite minimal FFA accumulation or cell death (26 to 27% LDH release). Arachidonic acid markedly inhibited PLA2 activity, potentially explaining an inverse correlation (r = -0.91) between tubule FFA accumulation and PL decrements. No evidence for plasma membrane "flip flop" was observed. In vivo ischemia reperfusion and oxidant injury (myohemoglobinuria) induced 0 and 24% cortical PL depletion, respectively, validating these in vitro data.

CONCLUSIONS

(a) Plasma membrane PLs are well preserved during acute hypoxic/ischemic injury, possibly because FFA accumulation (caused by mitochondrial inhibition) creates a negative feedback loop, inhibiting intracellular PLA2. (b) Exogenous PLA2 induces PL losses during hypoxia, but decreased cell injury can result. Together these findings suggest that PL loss may not be essential to hypoxic cell death. (c) Oxidant/Ca2+ overload injury induces early PL losses, perhaps facilitated by ongoing mitochondrial FFA metabolism, and (d) membrane "flip flop" does not appear to be an immediate mediator of acute necrotic tubular cell death.

Antibodies against type II phospholipase A2 prevent renal injury due to ischemia and reperfusion in rats

This study was performed to determine the involvement of type II phospholipase A2 (PLA2-II) in renal injury caused by ischemia and reperfusion. Ischemia and reperfusion significantly elevated levels of blood urea nitrogen and serum creatinine in rats. These increases were significantly reduced by i.v. administration of rabbit IgG F(ab')2 fragments against rat PLA2-II. Increased levels of acid-stable PLA2 activity in the kidney were caused by ischemia and reperfusion, and were suppressed by administration of anti-PLA2-II F(ab')2. Increased levels of myeloperoxidase activity, a marker of neutrophil infiltration, in the kidney were also reduced after anti-PLA2-II F(ab')2 treatment. These results suggest that PLA2-II plays a pivotal role in pathogenesis of ischemia and reperfusion injury through induction of neutrophil infiltration.

Effects of renal cytoprotective agents on erythrocyte membrane stability

To elucidate potential mechanisms of ischemic renal injury, investigators often use drugs that interfere with specific pathological pathways and study their protective efficacy in in vitro models of ischemia, such as isolated renal proximal tubules subjected to hypoxia. However, the protective effects of certain drugs may depend on non-specific membrane-stabilizing properties. We have studied the effects of several drugs on membrane integrity using osmotic lysis of erythrocytes as a model system. Freshly isolated rabbit erythrocytes were subjected to a hypotonic shock, and the protective effects of various calcium channel blockers, phospholipase inhibitors, free fatty acids, the NO-synthase inhibitor L-NAME, the amino acid glycine and its receptor-analogue strychnine, and two chloride channel blockers were examined. Most agents protected erythrocytes against hypotonic hemolysis when added to the medium in the same concentration range as used in suspensions of hypoxic proximal tubules. Only the protective agents that proposedly act via a blockade of chloride influx (glycine, strychnine and the chloride channel blockers), did not attenuate hypotonic hemolysis. The erythrocyte hemolysis assay may provide an easy and rapid method to screen for non-specific membrane-stabilizing effects of potentially cytoprotective agents.

Proteases in renal cell death: calpains mediate cell death produced by diverse toxicants

The role of proteases in renal cell death has received limited investigation. Calpains are non-lysosomal cysteine proteases that are Ca+2 activated. Calpain inhibitors that block the active site of calpains (calpain inhibitor 1 and 2) or the Ca+2 binding domain of calpains (PD150606) decreased calpain activity in rabbit renal proximal tubule (RPT) suspensions. The inhibition of calpain activity decreased cell death produced by the diverse toxicants antimycin A (mitochondrial inhibitor), tetrafluroethyl-L-cysteine (nephrotoxic halocarbon), bromohydroquinone (nephro-toxic quinone), t-butylhydroperoxide (model oxidant) and ionomycin (Ca+2 ionophore). In summary, calpains appear to play a common and critical role in cell injury produced by diverse toxicants with different mechanisms of action. The general cysteine protease inhibitor trans-epoxysuccinyl-L-leucylamido (4-guanidino)-butane (E-64) decreased antimycin A- and tetrafluoroethyl-L-cysteine-induced cell death but had no effect on bromohydroquinone- or t-butylhydroperoxide-induced cell death. Serine/cysteine protease inhibitors (antipain, leupeptin) were not cytoprotective to RPT exposed to any of the toxicants. The cytoprotection associated with E-64 correlated with inhibition of lysosomal cathepsins and E-64 was only cytoprotective after some cell death had occurred. Since some cell death occurred prior to the E-64 cytoprotective effect, lysosomal cathepsins may be released from dying cells and subsequently target the remaining viable cells.

Platelet-activating factor mediates intercellular adhesion molecule-1-dependent radical production in the nonhypoxic ischemia rat lung

It has been reported that reperfusion is the most important factor in ischemia-reperfusion (I/R) injury. However, causes of I/R injury in the lung are controversial, because oxygen is always supplied if ventilation continues. Therefore, we hypothesized that nonhypoxic ischemia without reperfusion is sufficient for lung injury. To test our hypothesis, we measured both hydrogen peroxide (H2O2) production in the pulmonary circulation, by digital imaging fluorescent dichlorofluorescein, and microvascular permeability (MVP), by the Evans blue extravasation technique in the nonhypoxic ischemia rat lung. We made a nonhypoxic ischemia rat lung by clamping the left pulmonary artery. Both H2O2 production and MVP increased in the nonhypoxic ischemia rat lung. We also determined the effect of oxygen removal by clamping the bronchus in advance of pulmonary artery occlusion, intercellular adhesion molecule-1 (ICAM-1) neutralization with monoclonal antibody 1A29, and platelet-activating factor (PAF) receptor antagonist CV6209 on H2O2 production and MVP. These treatments inhibited both H2O2 production and MVP increase. At high-power viewing of the fluorescent dichlorofluorescein image, H2O2 was detected in the leukocytes within pulmonary capillaries. These data indicate that the nonhypoxic ischemia without reperfusion alone causes radical production and increases MVP. Furthermore, PAF and ICAM-1 contribute to these reactions.

Role of phospholipase A2 isozymes in agonist-mediated signaling in proximal tubular epithelium

Angiotensin II in proximal tubule epithelium is known to stimulate the release of arachidonic acid after stimulation of phospholipase A2 (PLA2) independent of phospholipase C-mediated signaling. Furthermore, an angiotensin II type 2 receptor subtype has been linked to this signaling cascade. We investigated the regulation and differential stimulation of PLA2s by comparing the PLA2 activities associated with the membranes and cytosol of rabbit renal proximal tubular epithelial cells after stimulation with angiotensin II, epidermal growth factor, and bradykinin. Both fractions demonstrated PLA2 activity that was dithiothreitol insensitive, required micromolar concentrations of Ca2+ for optimal activity, and was inhibited in a dose-dependent manner by an antiserum to a cytosolic PLA2 with a molecular mass of 85 kD. However, membrane-associated PLA2 did not demonstrate significant substrate specificity, whereas 1-steroyl-2-[14C]arachidonylphosphatidyl choline was the preferred substrate for cPLA2. An antiserum generated against mastoparan, a known PLA2 activator, inhibited membrane- but not cytosol-associated PLA2 activity. Membrane fractions showed a broad pH range (7.5 to 8.5) for optimal PLA2 activity, whereas cytosol was maximum at pH 9.5. Angiotensin II stimulated membrane-associated PLA2 activity by 88%, whereas bradykinin and epidermal growth factor inhibited activity by 54% and 41%, respectively. The three agonists stimulated cPLA2. Moreover, angiotensin II-induced activation of membrane-associated PLA2 preceded the activation of cPLA2. These results demonstrate differential localization and regulation of proximal tubular epithelial PLA2 isozymes, which may determine the pattern of subsequent arachidonic acid metabolism by the cytochrome P450 system.

Effect of chronic ethanol exposure on mouse brain arachidonic acid specific phospholipase A2

The enzyme phospholipase A2 (PLA2), which catalyzes the hydrolysis of an ester bond at the sn-2 position of 1,2-sn-diacylglycerols, has been suggested to play an important role in regulating cellular functions. Although ethanol (EtOH)-induced activation of PLA2 activity was reported previously by us in mouse brain (Hungund et al., Neurochem Int 25: 321-325, 1994), its subcellular localization and biochemical properties have not been investigated. Therefore, in the present study, we examined the subcellular localization and characterization of EtOH-activated PLA2 activity in mouse brain. The results indicated that EtOH treatment decreased the specific activity of PLA2 for the first 48 hr, and then the activity increased and reached a peak level in both cytosol (1.6-fold) and membrane (1.7-fold) fractions at 96 hr of exposure. Specific activity was found to be higher in the membrane fraction than in the cytosol. Using differential density gradient centrifugation, subcellular localization of the membrane-associated PLA2 revealed that most of the EtOH-activated PLA2 specific activity was associated with the synaptic membrane (44%) followed by the nuclear membrane (13%). No significant increase in the PLA2 specific activity of mitochondrial and microsomal membranes was observed. No activity was detected in the myelin membrane. PLA2 specific activity of membranes from control and EtOH-exposed mouse brain exhibited preference for arachidonic acid over linoleic acid at the sn-2 position of glycero-3-phosphocholine (PC). No detectable PLA2 specific activity was found when PC containing oleic acid at the sn-2 position was used as a substrate. The present results also indicated that the PLA2 specific activity of membrane from control and EtOH-exposed mouse brain was insensitive to dithiothreitol, strongly stimulated by Ca2+, enhanced by glycerol, and inhibited by the cytosolic PLA2 (cPLA2) inhibitor methyl arachidonyl fluorophosphonate with an IC50 value of 3.33 microM. In summary, results suggest that the properties of EtOH-activated PLA2 activity found in mouse brain membrane fraction are similar to those of cPLA2 found in variety of cells, including mammalian brain.

Mechanisms by which thionin induces susceptibility of S49 cell membranes to extracellular phospholipase A2

Whereas cells normally resist attack by PLA2, they become susceptible under certain pathological conditions. To ascertain the regulatory mechanisms that induce cellular susceptibility to PLA2, the effect of thionin on S49 cells was examined in the presence of PLA2. Thionin alone was unable to evoke hydrolysis of the lipid bilayer. Likewise, the addition of PLA2 alone caused production of only a minimal amount of free fatty acid. However, thionin and PLA2 together resulted in significant hydrolysis of the cell membrane. Thionin caused perturbation of the bilayer structure as suggested by the changes in the emission spectra of laurdan and the permeability of the membrane to propidium iodide. These changes correlated quantitatively with the susceptibility of the lipid bilayer to PLA2. Furthermore, thionin induced a modest increase in intracellular Ca2+. The source of this Ca2+ was the extracellular fluid since EDTA in the extracellular medium inhibited the Ca2+ influx. Moreover, cobalt chloride, a universal Ca2+ channel blocker, prevented the rise in intracellular Ca2+, the uptake of propidium iodide, and the susceptibility to PLA2 induced by thionin. In contrast, the changes in the laurdan emission caused by the thionin were not affected by the cobalt. Furthermore, incubation of the cells with the calcium ionophore A23187 also caused the cells to become susceptible to PLA2. We hypothesize that thionin causes S49 cell membranes to become susceptible to PLA2 by a Ca2+-dependent perturbation of the bilayer structure.