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

Early loss of glutathione -s- transferase (GST) activity during diverse forms of acute renal tubular injury

Glutathione‐S‐transferases (GSTs) are a diverse group of phase II detoxification enzymes which primarily evoke tissue protection via glutathione conjugation to xenobiotics and reactive oxygen species. Given their cytoprotective properties, potential changes in GST expression during AKI has pathophysiologic relevance. Hence, we evaluated total GST activity, and the mRNA responses of nine cytosolic GST isotypes (GST alpha1, kappa1, mu1/5, omega1, pi1 sigma1, theta1, zeta1 mRNAs), in five diverse mouse models of AKI (glycerol, ischemia/reperfusion; maleate, cisplatin, endotoxemia). Excepting endotoxemia, each AKI model significantly reduced GST activity (~35%) during both the AKI “initiation” (0‐4 h) and “maintenance” phases (18 or 72 h). During the AKI maintenance phase, increases in multiple GST mRNAs were observed. However, no improvement in GST activity resulted. Increased urinary GST excretion followed AKI induction. However, this could not explain the reduced renal GST activity given that it also fell in response to ex vivo renal ischemia (i.e., absent urinary excretion). GST alpha, a dominant proximal tubule GST isotype, manifested 5–10‐fold protein increases following AKI, arguing against GST proteolysis as the reason for the GST activity declines. Free fatty acids (FFAs) and lysophospholipids, which markedly accumulate during AKI, are known to bind to, and suppress, GST activity. Supporting this concept, arachidonic acid addition to renal cortical protein extracts caused rapid GST activity reductions. Based on these results, we conclude that diverse forms of AKI significantly reduce GST activity. This occurs despite increased GST transcription/translation and independent of urinary GST excretion. Injury‐induced generation of endogenous GST inhibitors, such as FFAs, appears to be a dominant cause.

Insights into the structure and function of the human organic anion transporter 1 in lipid bilayer membranes

The human SLC22A6/OAT1 plays an important role in the elimination of a broad range of endogenous substances and xenobiotics thus attracting attention from the pharmacological community. Furthermore, OAT1 is also involved in key physiological events such as the remote inter-organ communication. Despite its significance, the knowledge about hOAT1 structure and the transport mechanism at the atomic level remains fragmented owing to the lack of resolved structures. By means of protein-threading modeling refined by μs-scaled Molecular Dynamics simulations, the present study provides the first robust model of hOAT1 in outward-facing conformation. Taking advantage of the AlphaFold 2 predicted structure of hOAT1 in inward-facing conformation, we here provide the essential structural and functional features comparing both states. The intracellular motifs conserved among Major Facilitator Superfamily members create a so-called “charge-relay system” that works as molecular switches modulating the conformation. The principal element of the event points at interactions of charged residues that appear crucial for the transporter dynamics and function. Moreover, hOAT1 model was embedded in different lipid bilayer membranes highlighting the crucial structural dependence on lipid-protein interactions. MD simulations supported the pivotal role of phosphatidylethanolamine components to the protein conformation stability. The present model is made available to decipher the impact of any observed polymorphism and mutation on drug transport as well as to understand substrate binding modes.

Voluntary running protects against neuromuscular dysfunction following hindlimb ischemia-reperfusion in mice

Ischemia-reperfusion (IR) due to temporary restriction of blood flow causes tissue/organ damages under various disease conditions, including stroke, myocardial infarction, trauma, and orthopedic surgery. In the limbs, IR injury to motor nerves and muscle fibers causes reduced mobility and quality of life. Endurance exercise training has been shown to increase tissue resistance to numerous pathological insults. To elucidate the impact of endurance exercise training on IR injury in skeletal muscle, sedentary and exercise-trained mice (5 wk of voluntary running) were subjected to ischemia by unilateral application of a rubber band tourniquet above the femur for 1 h, followed by reperfusion. IR caused significant muscle injury and denervation at neuromuscular junction (NMJ) as early as 3 h after tourniquet release as well as depressed muscle strength and neuromuscular transmission in sedentary mice. Despite similar degrees of muscle atrophy and oxidative stress, exercise-trained mice had significantly reduced muscle injury and denervation at NMJ with improved regeneration and functional recovery following IR. Together, these data suggest that endurance exercise training preserves motor nerve and myofiber structure and function from IR injury and promote functional regeneration. NEW & NOTEWORTHY This work provides the first evidence that preemptive voluntary wheel running reduces neuromuscular dysfunction following ischemia-reperfusion injury in skeletal muscle. These findings may alter clinical practices in which a tourniquet is used to modulate blood flow.

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.

Adenine-induced chronic kidney disease in rats

Many animal models have been developed to study the causes and treatments of chronic kidney disease (CKD) in humans, an insidious disease resulting from kidney injury and characterized by persistent functional decline for more than 3 months, with or without evidence of structural deficit. The eventual outcome of CKD may be end-stage kidney disease (ESKD), where patients need dialysis or transplantation to survive. Cardiovascular disease is accelerated in patients with CKD and contributes to increased mortality, with the relationship between CKD and cardiovascular disease being bi-directional. Most animal models do not mimic the complexity of the human disease as many do not develop CKD-associated cardiovascular disease. The adenine diet model of CKD in rodents is an exception. The original adenine diet model produced rapid-onset kidney disease with extensive tubulointerstitial fibrosis, tubular atrophy, crystal formation and marked vessel calcification. Since then, lower adenine intake in rats has been found to induce slowly progressive kidney damage and cardiovascular disease. These chronic adenine diet models allow the characterization of relatively stable kidney and cardiovascular disease, similar to CKD in humans. In addition, interventions for reversal can be tested. Here the key features of the adenine diet model of CKD are noted, along with some limitations of other available models. In summary, the data presented here support the use of chronic low-dose adenine diet in rats as an easy and effective model for understanding human CKD, especially the links with cardiovascular disease, and developing potential therapeutic interventions.

Lipid Deposition in Kidney Diseases: Interplay Among Redox, Lipid Mediators, and Renal Impairment

Significance: The relationship between lipid disturbances and renal diseases has been studied for several decades, and it is well recognized that when the balance of renal lipid uptake, synthesis, oxidation, and outflow is disrupted, lipids will undergo oxidation, be sequestrated as lipid droplets, generate toxic metabolites, and cause nephrotoxicity in diverse renal diseases. Recent Advances: During renal disorders, redox signaling is a pivotal event promoting or resulting from lipid disorders. Accordingly, a vicious cycle of lipid redox dysregulation could be developed, accelerating the renal damage. Critical Issues: The aim of this concise review is to introduce the connection among redox, lipid abnormalities and kidney damage in various conditions. And we summarized current understanding of the lipid redox loop implicated in acute kidney injury, chronic kidney disease, metabolic abnormalities, aging, and genetic pitfalls. Future Directions: Despite recent advances, further investigations are required to clarify the complicated molecular and regulatory mechanisms among redox, lipid mediators and renal disorders. Moreover, exploring an ideal target for potential therapies should be discussed and studied in future. Antioxid. Redox Signal. 28, 1027-1043.

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.

Mitochondrial dysfunction induced by pancreatic and crotalic (Crotalus durissus terrificus) phospholipases A2 on rabbit proximal tubules suspensions

In the present study we show that phospholipases A2 isolated from porcine pancreas (PP-PLA2) and Crotalus durissus terrificus snake venom (SV-PLA2) induced dose-dependent increases of LDH release from rabbit proximal tubules in suspension. Both porcine and crotalic PLA(2)s induced increases in non-esterified fatty acid (NEFA) levels (microg of NEFA/mg of tubule protein). It was observed that the NEFA levels in the pellets were higher than in the supernatant for both PLA2, and were dose-dependent for the crotalic PLA2 group. Furthermore, snake venom PLA2 induced a decrease in mitochondrial membrane potential (DeltaPsi(m)) assessed by both JC-1 uptake and safranin O uptake. Porcine PLA2 produced no effects on JC-1 uptake with the highest concentrations and an unexpected increase in the group treated with the lowest concentration. In contrast, the safranin O method revealed decreases of energization with both phospholipases, so it had higher sensitivity to the presence of the increased NEFA levels. Addition of delipidated bovine serum albumin (dBSA) completely reversed the effects induced by phospholipases on DeltaPsi(m) measured with safranin O. Incubation with pancreatic and crotalic phospholipases A2 produced no changes on cell ATP levels. We conclude that the treatment of proximal tubule suspensions with porcine or crotalic phospholipases disturbed membrane integrity as well as mitochondrial function. Specific early NEFA-mediated mitochondrial effects of the phospholipases used in the present study are indicated by the benefit provided by dBSA.

Hemoglobin induces cytotoxic damage of glycine-preserved renal tubules

In isolated tubular segments (ITS) of rat kidney cortex, we studied the effect of hemoglobin (Hb) on reoxygenation damage. All tubules were suspended in Ringer's solution containing 5-mm glycine and oxygenated for 30 min with 95% O(2):5% CO(2), followed by a 30-min period with 95% N(2):5% CO(2), and final reoxygenation for 60 min. Untreated tubules served as controls. Different concentrations of free Hb and equivalent amounts of intact erythrocytes were added to the incubation medium. Secondly, we added deferoxamine (DFO) to Hb and erythrocytes. Membrane leakage and lipid peroxidation were measured by lactate dehydrogenase and glutamate dehydrogenase and the development of thiobarbituric acid reactive substances. Cell function was quantified by gluconeogenesis and intracellular potassium accumulation. Hb exerted concentration-dependent cytotoxic effects indicated by significantly increased enzyme leakage rates, lipid peroxidation and a significantly decreased cell function (P < 0.05), in ITS during hypoxia, and subsequent reoxygenation. Moreover, we found that toxicity of both Fe(2+) and Fe(3+) ions increased with rising concentration. However, Fe(2+) showed a higher tissue toxicity than Fe(3+). DFO reduced significantly the reoxygenation damage of free Hb and iron ions. Our data clearly demonstrate a pronounced cytotoxic effect of free Hb in ITS, which critically depended on the reduction state of the iron ions.

Levosimendan protects against experimental endotoxemic acute renal failure

Endotoxemia induces a hemodynamic form of acute renal failure (ARF; renal vasoconstriction +/- reduced glomerular ultrafiltration coefficient, K(f); minimal/no histological damage). We tested whether levosimendan (LS), an ATP-sensitive K+ (K(ATP)) channel opener with cardiac ionotropic and possible anti-inflammatory properties, might have utility in combating this form of ARF. CD-1 mice were injected with LPS +/- LS. LS effects on LPS-induced systemic inflammation (plasma TNF-alpha/MCP-1; cardiorenal mRNAs), plasma NO levels, and azotemia were assessed. Because K(ATP) channel opening has been reported to mediate hypoxic tubular injury, possible adverse LS effects on ischemic ARF and ATP depletion injury were sought. Effects of diazoxide (another K(ATP) channel agonist) and glibenclamide (a channel antagonist) on hypoxic tubular injury also were assessed. Finally, the ability of LS to alter rat mesangial cell (MC) contraction in response to ANG II (elevated in sepsis) was tested. LS conferred almost complete protection against LPS-induced ARF, without any apparent reduction in the LPS-induced inflammatory response. Neither LS nor diazoxide altered ATP depletion-mediated tubule injury (in vivo or in vitro). Conversely, glibenclamide induced a marked and direct cytotoxic effect. LS completely blocked ANG II-induced MC contraction, an action likely to increase K(f). We concluded that 1) LS can confer marked protection against LPS-induced ARF; 2) this likely stems from vasoactive properties, rather than reductions in LPS-induced inflammation; and 3) K(ATP) channel agonists (but not antagonists) appear to be devoid of toxic proximal tubular cell effects. This suggests that LS, and other K(ATP) channel agonists, have a margin of safety if employed in situations (sepsis syndrome, heart failure) in which severe renal vasoconstriction might lead to ischemic ARF.

Bothrops venom induces direct renal tubular injury: role for lipid peroxidation and prevention by antivenom

Acute renal failure (ARF) is one of the most serious complications of Bothrops snakebites. Pathogenesis of ARF in snakebite envenomation may involve hemodynamic disturbances, immunologic reactions and direct nephrotoxicity. This study aimed at evaluating Bothrops jararaca venom direct toxicity on isolated rat renal proximal tubules (PT). PT was kept oxygenated and subjected to hypoxia (H, 15 min) and reoxygenation (R, 45 min). Bothropic antivenom effects, role of extracellular calcium and peroxide production were also evaluated. Cell injury was determined by LDH release (%) and peroxide production determined by xylenol-orange method. B. jararaca venom caused tubular injury (LDH 31.6 vs. 17.2%, P <0.05), which was prevented by simultaneous or delayed antivenom administration, but not with low extracellular calcium medium. Venom increased tubules peroxide production: 2.21 vs. 1.27 microM/mg protein (P <0.05) which was also prevented by antivenom administration. Venom toxic concentration did not enhance H/R injury. In contrast, non-toxic venom concentration afforded protection (LDH 41.3 vs. 51.5%, P <0.05). In conclusion, B. jararaca venom caused direct injury to normoxic renal tubules, but not to hypoxic/reoxygenated tubules. Tubular toxicity is independent of extracellular calcium and mediated in part by lipid peroxidation. Venom induced tubular injury was prevented by simultaneous or delayed antivenom administration.

Radiographic contrast media-induced tubular injury: evaluation of oxidant stress and plasma membrane integrity

BACKGROUND

Experimental and clinical investigations suggest that oxidant stress is a critical determinant of radiocontrast nephropathy (RCN), and that N acetyl cysteine (NAC) can prevent this damage. This study addresses these issues directly at the tubular cell level. Potential alternative mechanisms for RCN have also been sought.

METHODS

Isolated mouse proximal tubule segments (PTS), or cultured proximal tubule (HK-2) cells, were subjected to radiocontrast media (RCM) (Ioversol, Optiray 320) exposure, followed by assessments of cellular viability [% lactate dehydrogenase (LDH) release, tetrazolium dye (MTT), uptake] and lipid peroxidation. These experiments were conducted in the absence or presence of a variety of antioxidants [NAC, glutathione (GSH), superoxide dismutase, catalase] or pro-oxidant (GSH depletion, heme oxygenase inhibition) strategies. RCM effects on mitochondrial and plasma membrane integrity were also assessed.

RESULTS

RCM exposure did not induce PTS lipid peroxidation. Neither antioxidant nor pro-oxidant interventions mitigated or exacerbated RCM-induced tubular cell injury, respectively. RCM impaired mitochondrial integrity, as assessed by ouabain-resistant ATP reductions, and by cytochrome c release (before cell death). RCM also induced plasma membrane damage, as indicated by loss of key resident proteins (NaK-ATPase, caveolin) and by increased susceptibility to phospholipase A2 (PLA2) attack (increase of >/=2 times in free fatty acid and NaK-ATPase release). Hyperosmolality could not account for RCM's toxic effects.

CONCLUSION

RCM toxicity can be dissociated from tubular cell oxidant stress. Alternative mechanisms may include mitochondrial injury/cytochrome c release and plasma membrane damage. The latter results in critical protein loss, as well as a marked increase in plasma membrane susceptibility to exogenous/endogenous PLA2 attack.

Modulation of IFN-gamma-induced immunogenicity by phosphatidylethanolamine-linked hyaluronic acid

BACKGROUND

The present study was conducted to examine the possibility of modulating interferon (IFN-gamma)-induced immunogenicity by a novel compound that is composed of a PLA2 inhibitor linked to hyaluronic acid (HYPE).

METHODS

HYPE was tested for its effect on IFN-gamma-induced expression of MHC class I, class II, and intercellular adhesion molecule (ICAM-1) in cultured endothelial and renal proximal tubular cells by flow cytometric analysis (FACS) as well as its ability to influence T cell activation in mixed lymphocyte reaction (MLR) or after mitogen stimulation.

RESULTS

In FACS, a profound inhibition in MHC class I and ICAM-1 staining was observed in stimulated or unstimulated cells that were incubated with HYPE. This was not due to down-regulation of antigen expression and only occurred when monoclonal antibodies, but not when polyclonal antibodies, were used. HYPE inhibited the induction of MHC class II in both cell types after IFN-gamma stimulation in a dose-dependent manner. Moreover, the induction of class II transactivator (CIITA) was completely inhibited under these conditions, most likely because it blocked the binding of IFN-gamma to the cell membrane. Addition of HYPE to MLR inhibited the proliferation of T cells and the secretion of interleukin (IL)-2, IFN-gamma, and IL-10. This was not observed when HYPE was added together with anti-CD3 or phytohemagglutinin (PHA).

CONCLUSION

Our study provides experimental evidence that HYPE has immunosuppressive features. This makes the compound an interesting candidate as an immunosuppressive drug, not only in organ transplantation, but also in diseases where IFN-gamma is overexpressed.

Fatty-acid-binding proteins do not protect against induced cytotoxicity in a kidney cell model

Intracellular accumulation of fatty acids (FAs) is a well-described consequence of renal ischaemia and may lead to lethal cell injury. Fatty-acid-binding proteins (FABPs) are small cytosolic proteins with high affinity for FAs. They may protect vital cellular functions by binding to and promoting the metabolism of FAs, thereby reducing their intracellular concentration. In this study we investigated the putative cytoprotective role of FABPs in a Madin-Darby canine kidney (MDCK) cell model for renal damage. We studied the effects of transfection with cDNA encoding heart FABP, adipocyte FABP or liver FABP on cytotoxicity induced by chemical anoxia or FAs. Transfection of MDCK type II cells with these cDNA types caused a 5-20-fold increase in FABP content, but did not change the rate or extent of palmitate uptake. After 1 h of incubation with KCN, all cell types showed reduced viability and cellular ATP content and an intracellular accumulation of non-esterified FAs. High extracellular concentrations of oleate, but not palmitate, caused a markedly decreased cell viability and cellular ATP content. Oleate accumulated in non-esterified form in these cells. Simultaneous addition of glucose ameliorated the damaging effects of KCN or oleate, indicating that glycolytic ATP could substitute for uncoupled oxidative phosphorylation. No significant differences in the effects of chemical anoxia or oleate were observed between non-transfected, mock-transfected and FABP-cDNA-transfected cells. Non-esterified FA accumulation was not reduced in any of the FABP-cDNA-transfected cell lines. In conclusion, our data do not provide evidence for a cytoprotective role of FABP in this kidney cell model.

Linoleic acid prevents chloride influx and cellular lysis in rabbit renal proximal tubules exposed to mitochondrial toxicants

Despite many studies elucidating the mechanisms of necrotic cell death, the role of fatty acids released during necrosis remains to be determined. The goals of this study were to determine whether linoleic acid could protect rabbit renal proximal tubules (RPT) from necrotic cell death associated with mitochondrial dysfunction and oxidative injury and to determine the mechanisms involved. Exposure to antimycin A (10 microM) for 1 h or hypoxia (perfusion with 95% N(2)/5% CO(2)) for 1 or 2 h induced approximately 70% cellular lysis, as measured by lactate dehyrogenase release, versus 10% in controls. Preincubation with linoleic acid (100 microM) fully protected RPT from cellular lysis. RPT were also protected from lysis if linoleic acid was added 15 min after the addition of antimycin A. Measurements of free intracellular Ca(2+) concentrations showed that linoleic acid did not prevent the rise in intracellular Ca(2+) associated with a 30-min exposure to antimycin A. However, the influx of extracellular (36)Cl(-) following a 30-min exposure to antimycin A was ameliorated in the presence of linoleic acid. Linoleic acid did not prevent cellular lysis after exposure to hypoxia/reoxygenation (1 h/1 h) or t-butyl hydroperoxide (500 microM, 3 h). These data suggest that linoleic acid protects RPT during the late phase of cell death associated with inhibition of the electron transport chain but not oxidative injury. Several other fatty acids also protected RPT from lysis, and structure-activity relationship studies suggest that a free carboxyl terminus and at least one double bond are required for this action.

P glycoprotein-mediated cholesterol cycling determines proximal tubular cell viability

BACKGROUND

MDR P glycoproteins may help transport plasma membrane free cholesterol (FC) to the endoplasmic reticulum (ER), where it undergoes acylation, forming cholesterol esters (CE). This study assessed whether P glycoprotein inhibitors alter renal tubular FC/CE expression, thereby altering cell integrity.

METHODS

Mouse proximal tubule segments (PTS) were exposed to chemically dissimilar P glycoprotein inhibitors [progesterone (prog), trifluoperazine (TFP), or cyclosporine A (CsA)]. Their effects on FC/CE and adenosine 5'-triphosphate (ATP) levels, phospholipid expression, lipid peroxidation, and cell viability (lactate dehydrogenase release; LDH) were assessed. P glycoprotein inhibitor effects on cultured proximal tubular (HK-2) cell viability and susceptibility to Fe-induced oxidant stress were also addressed.

RESULTS

When applied to PTS, prog, TFP, and, to lesser extent, CsA induced dose-dependent ATP reductions (< or =90%), CE decrements (approximately 40%), and LDH release (< or =60%). No concomitant changes in lipid peroxidation or phospholipid profiles were observed. Ouabain did not preserve tubular ATP, suggesting that decreased ATP production, rather than increased consumption, was operative. Mechanisms leading to cell lysis were not identical, as glycine and arachidonic acid blocked prog- but not TFP-mediated cell death. When prog-driven CE reductions were attenuated in PTS with a procycling agent (cholesterol oxidase), decreased cell death resulted. P glycoprotein inhibitors also caused dose-dependent HK-2 cell death. Blocking Fe-mediated CE formation ( approximately x10) with sublethal CsA doses led to a marked increase in Fe-mediated cell death.

CONCLUSIONS

P glycoproteins may be critical to tubule cholesterol transport. If blocked with pharmacologic agents, decreased ATP production, overt cell lysis, and/or a marked propensity to superimposed tubular cell injury can result.

Renal cholesterol accumulation: a durable response after acute and subacute renal insults

Proximal tubular cholesterol levels rise within 18 hours of diverse forms of acute renal tubular injury (eg, myoglobinuria, ischemia/reperfusion, urinary tract obstruction). These increments serve to protect against further bouts of tubular attack (so-called "acquired cytoresistance"). Whether these cholesterol increments are merely transitory, or persist into the maintenance phase of acute renal failure (ARF), has not been previously defined. Furthermore, whether subacute/insidious tubular injury [eg, cyclosporine A (CSA), tacrolimus toxicity], nontubular injury (eg, acute glomerulonephritis), or physiological stress (eg, mild dehydration) impact renal cholesterol homeostasis have not been addressed. This study sought to resolve these issues. Male CD-1 mice were subjected to glycerol-induced ARF. Renal cortical-free cholesterol (FC) and cholesterol ester (CE) levels were determined 3, 5, 7, or 14 days later, and the values contrasted to prevailing blood-urea nitrogen concentrations. The impact of 40 minutes of unilateral renal ischemia plus reflow (3 to 6 days) on mouse cortical FC/CE content was also assessed. Additionally, FC/CE levels were measured in rat renal cortex either 10 days after CSA or tacrolimus therapy, or 48 hours after induction of nephrotoxic serum nephritis. Finally, the impact of overnight dehydration on mouse renal cortical/medullary FC/CE profiles was determined. Compared to sham-treated animals, glycerol, CSA, tacrolimus, ischemia-reperfusion, and nephrotoxic serum each induced dramatic CE +/- FC elevations, rising as much as 10x control values. In the glycerol model, striking correlations (r

CONCLUSIONS

FC/CE accumulation is a hallmark of the maintenance phase of ischemic and nephrotoxic ARF, and can reflect its severity. That cholesterol accumulation can result from glomerular injury and dehydration suggests that it is a generic renal stress response, with potential relevance extending beyond just the phenomenon of acquired cytoresistance.

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Key Words Collapse

MESH Headings

• Acute Kidney Injury/chemically induced
• Acute Kidney Injury/metabolism
• Animals
• Blood Urea Nitrogen
• Cholesterol/metabolism
• Cholesterol Esters/metabolism
• Cyclosporine/toxicity
• Dehydration/physiopathology
• Glomerulonephritis/metabolism
• Glycerol/toxicity
• Ischemia/metabolism
• Kidney/blood supply
• Kidney/metabolism
• Kidney/pathology
• Kidney Cortex/metabolism
• Kidney Medulla/metabolism
• Kidney Tubules/drug effects
• Kidney Tubules/metabolism
• Kidney Tubules/pathology
• Male
• Mice
• Nephritis/metabolism
• Nephritis/pathology
• Rats
• Rats, Sprague-Dawley
• Reperfusion
• Tacrolimus/toxicity

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Grants

• R01 DK038432 NIDDK NIH HHS
• R01 54200 PHS HHS
• R01 DK 37652 NIDDK NIH HHS
• DK38432 NIDDK NIH HHS
• R37 DK038432 NIDDK NIH HHS

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Affiliation(s)

R A Zager Department of Medicine, Fred Hutchinson Cancer Center, University of Washington, 1100 Fairview Ave. N, Rm. D2-190, Seattle, WA 98109, USA.
T Andoh
W M Bennett

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Cholesterol ester accumulation: an immediate consequence of acute in vivo ischemic renal injury

BACKGROUND

Cholesterol is a major constituent of plasma membranes, and recent evidence indicates that it is up-regulated during the maintenance phase of acute renal failure (ARF). However, cholesterol's fate and that of the cholesterol ester (CE) cycle [shuttling between free cholesterol (FC) and CEs] during the induction phase of ARF have not been well defined. The present studies sought to provide initial insights into these issues.

METHODS

FC and CE were measured in mouse renal cortex after in vivo ischemia (15 and 45 minutes)/reperfusion (0 to 120 minutes) and glycerol-induced myoglobinuria (1 to 2 hours). FC/CE were also measured in (1) cultured human proximal tubule (HK-2) cells three hours after ATP depletion and in (2) isolated mouse proximal tubule segments (PTSs) subjected to plasma membrane damage (with cholesterol oxidase, sphingomyelinase, phospholipase A2, or cytoskeletal disruption with cytochalasin B). The impact of cholesterol synthesis inhibition (with mevastatin) and FC traffic blockade (with progesterone) on injury-evoked FC/CE changes was also assessed.

RESULTS

In vivo ischemia caused approximately threefold to fourfold CE elevations, but not FC elevations, that persisted for at least two hours of reperfusion. Conversely, myoglobinuria had no effect. Isolated CE increments were observed in ATP-depleted HK-2 cells. Neither mevastatin nor progesterone blocked this CE accumulation. Plasma membrane injury induced with sphingomyelinase or cholesterol oxidase, but not with phospholipase A(2) or cytochalasin B, increased tubule CE content. High CE levels, induced with cholesterol oxidase, partially blocked hypoxic PTS attack.

CONCLUSIONS

In vivo ischemia/reperfusion acutely increases renal cortical CE, but not FC, content, indicating perturbed CE/FC cycling. The available data suggest that this could stem from specific types of plasma membrane damage, which then increase FC flux via aberrant pathways to the endoplasmic reticulum, where CE formation occurs. That CE levels are known to inversely correlate with both renal and nonrenal cell injury suggests the potential relevance of these observations to the induction phase of ischemic ARF.

Progressive disruption of the plasma membrane during renal proximal tubule cellular injury

The goal of this study was to examine the progression of plasma membrane disruption during cell injury using rabbit renal proximal tubules (RPT). The results demonstrated that the plasma membrane became permeable to larger and larger molecules as anoxia proceeded. At least three distinctive phases of membrane disruption were differentiated during anoxia. In phases 1, 2, and 3, plasma membranes became permeable to propidium iodide (PI, molecular weight = 668), 3 kDa dextrans, and 70 kDa dextrans or lactate dehydrogenase (LDH, molecular weight = 140 kDa), respectively. Phase 1 was reversible by reoxygenation but not prevented by the glycine. Phase 2 was inhibited by glycine. Phase 3 was inhibited by several membrane-permeable homobifunctional crosslinkers, dimethyl-pimelimidate (DMP), ethylene-glycolbis(succinimidylsuccinate), and dithiobis(succinimidylpropionate), but not by the membrane-impermeable crosslinker dithiobis(sulfosuccinimidylpropionate). In addition, DMP decreased RPT LDH release produced by mitochondrial inhibition (antimycin A), an oxidant (t-butylhydroperoxide) and a nephrotoxicant that is metabolized to an electrophile (tetrafluoroethyl-l-cysteine). These results identify (1) different phases of plasma membrane damage with increasing permeability during cell injury, (2) the reversibility of phase 1, (3) the relative site of action of the cytoprotectant glycine (prevents phase 2), and (4) the protective effects of chemical crosslinkers in RPT cell death produced by different toxicants.

Changes in free and esterified cholesterol: hallmarks of acute renal tubular injury and acquired cytoresistance

Acute tubular cell injury is accompanied by plasma membrane phospholipid breakdown. Although cholesterol is a dominant membrane lipid which interdigitates with, and impacts, phospholipid homeostasis, its fate during the induction and recovery phases of acute renal failure (ARF) has remained ill defined. The present study was performed to ascertain whether altered cholesterol expression is a hallmark of evolving tubular damage. Using gas chromatographic analysis, free cholesterol (FC) and esterified cholesterol (CE) were quantified in: 1) isolated mouse proximal tubule segments (PTS) after 30 minutes of hypoxic or oxidant (ferrous ammonium sulfate) injury; 2) cultured proximal tubule (HK-2) cells after 4 or 18 hours of either ATP depletion/Ca(2+) ionophore- or ferrous ammonium sulfate-mediated injury; and 3) in renal cortex 18 hours after induction of glycerol-induced myoglobinuric ARF, a time corresponding to the so-called "acquired cytoresistance" state (ie, resistance to further renal damage). Hypoxic and oxidant injury each induced approximately 33% decrements in CE (but not FC) levels in PTS, corresponding with lethal cell injury ( approximately 50 to 60% LDH release). When comparable CE declines were induced in normal PTS by exogenous cholesterol esterase treatment, proportionate lethal cell injury resulted. During models of slowly evolving HK-2 cell injury, progressive CE increments occurred: these were first noted at 4 hours, and reached approximately 600% by 18 hours. In vivo myoglobinuric ARF produced comparable renal cortical CE (and to a lesser extent FC) increments. Renal CE accumulation strikingly correlated with the severity of ARF (eg, blood urea nitrogen versus CE; r, 0.84). Mevastatin blocked cholesterol accumulation in injured HK-2 cells, indicating de novo synthesis was responsible. Acute tubule injury first lowers, then raises, tubule cholesterol content. Based on previous observations that cholesterol has cytoprotectant properties, the present findings have potential relevance for both the induction and maintenance phases of ARF.

Polyene antibiotics: relative degrees of in vitro cytotoxicity and potential effects on tubule phospholipid and ceramide content

Polyene antibiotic administration is limited by dose-dependent nephrotoxicity. The latter is believed to be mediated by polyene anchoring to plasma membrane cholesterol, resulting in pore formation, abnormal ion/solute flux, adenosine triphosphate (ATP) declines, and, ultimately, a loss of tubule viability. The relative nephrotoxicity of these agents and their liposomal preparations has remained poorly defined. Thus, freshly isolated mouse proximal tubules or cultured human proximal tubule (HK-2) cells were exposed to either nystatin, amphotericin B, or three different polyene liposomal preparations (Nyotran, AmBisome, or Abelcet; 4 to 64 microg/mL). The impact of these agents on (1) plasma membrane injury (sodium-driven ATP consumption, assessed by ATP-adenosine diphosphate [ADP] ratios); (2) cellular susceptibility to superimposed injury (chemical hypoxia or ferrous ammonium sulfate-mediated oxidative stress; assessed by lactate dehydrogenase release); and (3) membrane cholesterol, phospholipid, and ceramide expression was assessed. Amphotericin B was more cytotoxic than nystatin (approximately 25% to 50% greater ATP-ADP ratio declines). Most of this toxicity could be eliminated by polyene liposomal formulation. Nevertheless, the liposomal polyenes still fully sensitized tubule cells to superimposed chemical hypoxic (antimycin/deoxyglucose), but not oxidant, attack. Nystatin and amphotericin B caused acute increments in tubule sphingomyelin-phosphatidylcholine ratios and ceramide content (indicating an impact on the plasma membrane extending beyond the classic view of pore formation with ion flux). In conclusion, (1) nystatin is seemingly less cytotoxic than amphotericin B (in contrast to the prevailing clinical view); (2) liposomal formulation markedly decreases this cytotoxicity; (3) despite this reduced toxicity, liposomal polyenes are still able to render tubule cells more vulnerable to selected forms of superimposed injury; and (4) acute alterations in plasma membrane phospholipid and ceramide expression are previously unrecognized consequences and potential mediators of polyene-mediated tubular cell attack.

Plasma membrane cholesterol: a critical determinant of cellular energetics and tubular resistance to attack

BACKGROUND

Cholesterol is a major component of plasma membranes, forming membrane microdomains ("rafts" or "caveolae") via hydrophobic interactions with sphingolipids. We have recently demonstrated that tubule cholesterol levels rise by 18 hours following diverse forms of injury, and this change helps to protect kidneys from further damage (so-called acquired cytoresistance). The present study was undertaken to better define the effects of membrane cholesterol/microdomains on tubule homeostasis and cell susceptibility to superimposed attack.

METHODS

Plasma membrane cholesterol was perturbed in normal mouse proximal tubular segments with either cholesterol esterase (CE) or cholesterol oxidase (CO). Alternatively, cholesterol-sphingomyelin complexes were altered by sphingomyelinase (SMase) treatment. Changes in cell energetics (ATP/ADP ratios + ouabain), viability [lactate dehydrogenase (LDH) release], phospholipid profiles, and susceptibility to injury (Fe-induced oxidant stress, PLA2, Ca2+ ionophore) were determined. The impacts of selected cytoprotectants were also assessed.

RESULTS

Within 15 minutes, CE and CO each induced approximately 90% ATP/ADP ratio suppressions. These were seen prior to lethal cell injury (LDH release), and it was ouabain resistant (suggesting decreased ATP production, not increased consumption). SMase also depressed ATP without inducing cell death. After 45 minutes, CE and CO each caused marked cytotoxicity (up to 70% LDH release). However, different injury mechanisms were operative since (1) CE, but not CO, toxicity significantly altered cell phospholipid profiles, and (2) 2 mmol/L glycine completely blocked CE- but not CO-mediated cell death. Antioxidants also failed to attenuate CO cytotoxicity. Disturbing cholesterol/microdomains with a sublytic CE dose dramatically increased tubule susceptibility to Fe-mediated oxidative stress and Ca2+ overload, but not PLA2-mediated damage.

CONCLUSION

Intact plasma membrane cholesterol/microdomains are critical for maintaining cell viability both under basal conditions and during superimposed attack. When perturbed, complex injury pathways can be impacted, with potential implications for both the induction of acute tubular damage and the emergence of the postinjury cytoresistance state.

Sphingomyelinase and membrane sphingomyelin content: determinants ofProximal tubule cell susceptibility to injury

Ceramides acutely accumulate in proximal tubules during injury. Pathogenic relevance of this change is suggested by observations that adding ceramide to tubular cells alters superimposed hypoxic and toxic attack. Ceramide accumulation during cell injury is thought to arise from sphingomyelinase (SMase)-mediated sphingomyelin (SM) hydrolysis +/- decreased catabolism. Thus, ceramide addition to cells cannot precisely simulate pathophysiologic events. Therefore, this study assessed direct effects of SMase activity on tubular cell viability under basal conditions and during superimposed attack. Cultured human proximal tubule (HK-2) cells were exposed to differing SMase doses. Its effects on cell phospholipids, ceramides, proliferation rates, and susceptibility to injury (ATP depletion, Fe-mediated oxidant stress) were assessed. Because SMase reduces cell SM content, the effect of exogenous SM on membrane injury (intact cells, isolated vesicles) was also tested. Finally, because SM decreases membrane fluidity, the impact of a fluidizing agent (A(2)C) on membrane injury (phospholipase A(2), lipid peroxidation) was addressed. SMase reduced HK-2 SM content by approximately 33%, but only modest ceramide increments resulted (suggesting high endogenous ceramidase activity). SMase, by itself, caused no cell death (lactate dehydrogenase release). However, it was mildly antiproliferative, and it dramatically predisposed to both ATP depletion- and Fe-mediated attack. SMase also predisposed isolated vesicles to damage, suggesting that its impact on intact cells reflects a direct membrane effect. Adding SM to intact cells (or vesicles) mitigated ATP depletion and Fe- and phospholipase A(2)-induced damage. In contrast, A(2)C rendered membranes more vulnerable to attack. SMase predisposes tubular cells to superimposed ATP depletion and oxidant injury. This may be explained by SM losses, and not simply cytotoxic ceramide gains, given that SM can directly decrease cell/membrane damage. The ability of SM to decrease membrane fluidity may explain, at least in part, its cytoprotective effect.

Renal cortical ceramide patterns during ischemic and toxic injury: assessments by HPLC-mass spectrometry

Ceramides are a class of signaling molecules that can acutely accumulate in tissues as part of a "stress response." They are classically measured by the diacylglycerol kinase assay, which, in general, measures total ceramide rather than individual moieties within the diverse ceramide family. The present study was undertaken to 1) adapt current HPLC-mass spectrometry technology for measuring individual renal ceramides, and 2) use this technique to more fully characterize the nature of the renal ceramide "stress" reaction. Renal cortical tissues were obtained from CD-1 mice under control conditions and 2 or 18 h after renal injury (ischemia-reperfusion and glycerol-mediated myohemoglobinuria). C24, C22, and C16 ceramides were identified in normal renal cortex, constituting 70, 10, and 20% of the total ceramide pool, respectively. Within each of these families, heterogeneity was apparent because of differing degrees of unsaturation (0-3 double bonds) in the constituent fatty acid of ceramide. Renal injury dramatically changed ceramide profiles: 1) total ceramide increased by approximately 300%; 2) although all ceramides participated in this reaction, they did so to differing degrees; 3) this caused pronounced changes in ceramide distribution patterns; 4) injury induced a striking shift toward unsaturated (vs. saturated) fatty acids within the C22 and C24 (but not the C16) ceramide pools; and 5) the extent of these qualitative changes differed according to the etiology of the initiating renal damage. Thus we conclude that ceramide stress response involves major qualitative (and not simply quantitative) changes in ceramide expression that are partially disease dependent. These findings underscore the fact that simply measuring total renal ceramide content (e.g., by diacylglycerol kinase assay) substantially oversimplifies the nature and, hence, the potential implications of the ceramide stress reaction.

Increased proximal tubular cholesterol content: implications for cell injury and "acquired cytoresistance"

BACKGROUND

Acute renal failure (ARF) leads to secondary adaptive changes that serve to protect proximal tubules from subsequent ischemic or toxic damage [so-called "acquired cytoresistance" (CR)]. A characteristic of CR is increased plasma membrane resistance to attack. Therefore, this study sought to identify potential changes in plasma membrane lipid composition in CR tubules/renal cortex and, if present, to test whether they might mechanistically contribute to the CR state.

METHODS

Renal cortices/isolated tubules were obtained from CR mouse kidneys (18-hr postinduction of ischemia reperfusion, myoglobinuria, or ureteral obstruction). Their plasma membrane phospholipid/cholesterol profiles were compared with those observed in either control tissues or tissues obtained one to two hours post-renal damage (that is, prior to emergence of CR).

RESULTS

Either no changes or inconsistent changes in phospholipid profiles were observed in CR tissues. Conversely, CR (vs. control) tissues demonstrated a consistent 25 to 50% increase in membrane cholesterol content. To ascertain whether cholesterol impacts tubule susceptibility to injury, its levels were reduced in proximal tubule (HK-2) cells with either (a) mevastatin, (b) a cholesterol "stripping" agent, (c) cholesterol oxidase, or (d) cholesterol esterase. Then cell susceptibility to injury [adenosine 5'-triphosphate (ATP) depletion; Fe-mediated oxidant stress] was assessed. In each instance, cholesterol reductions dramatically sensitized to superimposed injury (for example, a 2 to 3 times increase in the % of lactate dehydrogenase release). When cholesterol levels were restored to normal in CR tubules (with a "stripping" agent), an increased tubule susceptibility to injury resulted. Because cholesterol decreases membrane fluidity, the impact of a membrane-fluidizing agent (A2C) on cell injury was assessed. A2C dramatically sensitized HK-2 cells to superimposed attack.

CONCLUSIONS

ARF leads to an up-regulation of proximal tubule cholesterol content. The latter may then contribute to acquired CR, possibly by stabilizing the plasma membrane via its antifluidizing effect.