Differential cytoprotection by glycine against oxidant damage to proximal tubule cells

Ferroptosis-based advanced therapies as treatment approaches for metabolic and cardiovascular diseases

Ferroptosis has attracted attention throughout the last decade because of its tremendous clinical importance. Here, we review the rapidly growing body of literature on how inhibition of ferroptosis may be harnessed for the treatment of common diseases, and we focus on metabolic and cardiovascular unmet medical needs. We introduce four classes of preclinically established ferroptosis inhibitors (ferrostatins) such as iron chelators, radical trapping agents that function in the cytoplasmic compartment, lipophilic radical trapping antioxidants and ninjurin-1 (NINJ1) specific monoclonal antibodies. In contrast to ferroptosis inducers that cause serious untoward effects such as acute kidney tubular necrosis, the side effect profile of ferrostatins appears to be limited. We also consider ferroptosis as a potential side effect itself when several advanced therapies harnessing small-interfering RNA (siRNA)-based treatment approaches are tested. Importantly, clinical trial design is impeded by the lack of an appropriate biomarker for ferroptosis detection in serum samples or tissue biopsies. However, we discuss favorable clinical scenarios suited for the design of anti-ferroptosis clinical trials to test such first-in-class compounds. We conclude that targeting ferroptosis exhibits outstanding treatment options for metabolic and cardiovascular diseases, but we have only begun to translate this knowledge into clinically relevant applications.

Mechanisms and Models of Kidney Tubular Necrosis and Nephron Loss

Understanding nephron loss is a primary strategy for preventing CKD progression. Death of renal tubular cells may occur by apoptosis during developmental and regenerative processes. However, during AKI, the transition of AKI to CKD, sepsis-associated AKI, and kidney transplantation ferroptosis and necroptosis, two pathways associated with the loss of plasma membrane integrity, kill renal cells. This necrotic type of cell death is associated with an inflammatory response, which is referred to as necroinflammation. Importantly, the necroinflammatory response to cells that die by necroptosis may be fundamentally different from the tissue response to ferroptosis. Although mechanisms of ferroptosis and necroptosis have recently been investigated in detail, the cell death propagation during tubular necrosis, although described morphologically, remains incompletely understood. Here, we argue that a molecular switch downstream of tubular necrosis determines nephron regeneration versus nephron loss. Unraveling the details of this "switch" must include the inflammatory response to tubular necrosis and regenerative signals potentially controlled by inflammatory cells, including the stimulation of myofibroblasts as the origin of fibrosis. Understanding in detail the molecular switch and the inflammatory responses to tubular necrosis can inform the discussion of therapeutic options.

Abnormal Iron and Lipid Metabolism Mediated Ferroptosis in Kidney Diseases and Its Therapeutic Potential

Ferroptosis is a newly identified form of regulated cell death driven by iron-dependent phospholipid peroxidation and oxidative stress. Ferroptosis has distinct biological and morphology characteristics, such as shrunken mitochondria when compared to other known regulated cell deaths. The regulation of ferroptosis includes different molecular mechanisms and multiple cellular metabolic pathways, including glutathione/glutathione peroxidase 4(GPX4) signaling pathways, which are involved in the amino acid metabolism and the activation of GPX4; iron metabolic signaling pathways, which are involved in the regulation of iron import/export and the storage/release of intracellular iron through iron-regulatory proteins (IRPs), and lipid metabolic signaling pathways, which are involved in the metabolism of unsaturated fatty acids in cell membranes. Ferroptosis plays an essential role in the pathology of various kidneys diseases, including acute kidney injury (AKI), chronic kidney disease (CKD), autosomal dominant polycystic kidney disease (ADPKD), and renal cell carcinoma (RCC). Targeting ferroptosis with its inducers/initiators and inhibitors can modulate the progression of kidney diseases in animal models. In this review, we discuss the characteristics of ferroptosis and the ferroptosis-based mechanisms, highlighting the potential role of the main ferroptosis-associated metabolic pathways in the treatment and prevention of various kidney diseases.

The pathological role of ferroptosis in ischemia/reperfusion-related injury

Ischemia/reperfusion (I/R) is a pathological process that occurs in numerous organs throughout the human body, and it is frequently associated with severe cellular damage and death. Recently it has emerged that ferroptosis, a new form of regulated cell death that is caused by iron-dependent lipid peroxidation, plays a significantly detrimental role in many I/R models. In this review, we aim to revise the pathological process of I/R and then explore the molecular pathogenesis of ferroptosis. Furthermore, we aim to evaluate the role that ferroptosis plays in I/R, providing evidence to support the targeting of ferroptosis in the I/R pathway may present as a therapeutic intervention to alleviate ischemia/reperfusion injury (IRI) associated cell damage and death.

Emerging Role of Ferroptosis in Acute Kidney Injury

Acute kidney injury (AKI) is a heterogeneous group of critical disease conditions with high incidence and mortality. Vasoconstriction, oxidative stress, apoptosis, and inflammation are generally thought to be the main pathogenic mechanisms of AKI. Ferroptosis is a type of iron-dependent nonapoptotic cell death characterized by membrane lipid peroxide accumulation and polyunsaturated fatty acid consumption, and it plays essential roles in many diseases, including cancers and neurologic diseases. Recent studies have revealed an emerging role of ferroptosis in the pathophysiological processes of AKI. Here, in the present review, we summarized the most recent discoveries on the role of ferroptosis in the pathogenesis of AKI as well as its therapeutic potential in AKI.

Immunological consequences of kidney cell death

Death of renal cells is central to the pathophysiology of acute tubular necrosis, autoimmunity, necrotizing glomerulonephritis, cystic kidney disease, urosepsis, delayed graft function and transplant rejection. By means of regulated necrosis, immunogenic damage-associated molecular patterns (DAMPs) and highly reactive organelles such as lysosomes, peroxisomes and mitochondria are released from the dying cells, thereby causing an overwhelming immunologic response. The rupture of the plasma membrane exhibits the "point of no return" for the immunogenicity of regulated cell death, explaining why apoptosis, a highly organized cell death subroutine with long-lasting plasma membrane integrity, elicits hardly any immune response. Ferroptosis, an iron-dependent necrotic type cell death, results in the release of DAMPs and large amounts of lipid peroxides. In contrast, anti-inflammatory cytokines are actively released from cells that die by necroptosis, limiting the DAMP-induced immune response to a surrounding microenvironment, whereas at the same time, inflammasome-associated caspases drive maturation of intracellularly expressed interleukin-1β (IL-1β). In a distinct setting, additionally interleukin-18 (IL-18) is expressed during pyroptosis, initiated by gasdermin-mediated plasma membrane rupture. As all of these pathways are druggable, we provide an overview of regulated necrosis in kidney diseases with a focus on immunogenicity and potential therapeutic interventions.

Comparative effect of selenium and glycine on hydrogen peroxide-induced cell death and activation of macrophage U937 cells

The effects of selenium and glycine (either separately or in combination) on hydrogen peroxide-induced cell death on U937 cells and activation of U937-derived macrophages were investigated. In the first instance, U937 cells were incubated with or without selenium (Se) or glycine (GLY) or both (Se + GLY) for 24 h before exposure to hydrogen peroxide. Control cells were not incubated with Se, GLY or exposed to hydrogen peroxide. Cell viability was later assessed via trypan blue and MTT assays. For the other experiment, U937 cells were transformed to the macrophage form using phorbol 12-myristate 13-acetate before incubating with or without Se, GLY, Se + GLY. Contents were subsequently exposed to hydrogen peroxide and 24 h later assessed for the production of TNF-α, IL-1, IL-6 and the expression of iNOS and NF-κB. The results revealed that hydrogen peroxide caused significant cell death which was ameliorated by both Se and GLY. Pre-incubation of the cells with both Se and GLY did not significantly enhance cell numbers compared to GLY (p > 0.05). On the other hand, Se and GLY reduced hydrogen peroxide-mediated production of TNF-α, IL-1, IL-6 and expression of iNOS and NF-κB. Incubating the U937-derived macrophages with Se + GLY significantly ameliorated hydrogen peroxide-mediated activation of macrophages when compared to pre-treatments with Se or GLY (p < 0.05). The findings demonstrate that both Se and GLY reduced hydrogen peroxide-induced alterations in U937 cells and U937-derived macrophages. Implications of the findings are discussed.

Nonapoptotic cell death in acute kidney injury and transplantation

Acute tubular necrosis causes a loss of renal function, which clinically presents as acute kidney failure (AKI). The biochemical signaling pathways that trigger necrosis have been investigated in detail over the past 5 years. It is now clear that necrosis (regulated necrosis, RN) represents a genetically driven process that contributes to the pathophysiology of AKI. RN pathways such as necroptosis, ferroptosis, parthanatos, and mitochondrial permeability transition-induced regulated necrosis (MPT-RN) may be mechanistically distinct, and the relative contributions to overall organ damage during AKI in living organisms largely remain elusive. In a synchronized manner, some necrotic programs induce the breakdown of tubular segments and multicellular functional units, whereas others are limited to killing single cells in the tubular compartment. Importantly, the means by which a renal cell dies may have implications for the subsequent inflammatory response. In this review, the recent advances in the field of renal cell death in AKI and key enzymes that might serve as novel therapeutic targets will be discussed. As a consequence of the interference with RN, the immunogenicity of dying cells in AKI in renal transplants will be diminished, rendering inhibitors of RN indirect immunosuppressive agents.

The role of glycine in regulated cell death

The cytoprotective effects of glycine against cell death have been recognized for over 28 years. They are expressed in multiple cell types and injury settings that lead to necrosis, but are still not widely appreciated or considered in the conceptualization of cell death pathways. In this paper, we review the available data on the expression of this phenomenon, its relationship to major pathophysiologic pathways that lead to cell death and immunomodulatory effects, the hypothesis that it involves suppression by glycine of the development of a hydrophilic death channel of molecular dimensions in the plasma membrane, and evidence for its impact on disease processes in vivo.

Renoprotective approaches and strategies in acute kidney injury

Acute kidney injury (AKI) is a major renal disease associated with high mortality rate and increasing prevalence. Decades of research have suggested numerous chemical and biological agents with beneficial effects in AKI. In addition, cell therapy and molecular targeting have been explored for reducing kidney tissue damage and promoting kidney repair or recovery from AKI. Mechanistically, these approaches may mitigate oxidative stress, inflammation, cell death, and mitochondrial and other organellar damage, or activate cytoprotective mechanisms such as autophagy and pro-survival factors. However, none of these findings has been successfully translated into clinical treatment of AKI. In this review, we analyze these findings and propose experimental strategies for the identification of renoprotective agents or methods with clinical potential. Moreover, we propose the consideration of combination therapy by targeting multiple targets in AKI.

Regulated cell death and inflammation: an auto-amplification loop causes organ failure

Regulated cell death (RCD) is either immunologically silent or immunogenic. RCD in parenchymal cells may lead to the release of damage- associated molecular patterns that drive both tissue inflammation and the activation of further pathways of RCD. Following an initial event of regulated necrosis, RCD and inflammation can induce each other and drive a local auto-amplification loop that leads to exaggerated cell death and inflammation. In this Opinion article, we propose that such crosstalk between pro-inflammatory and RCD pathways has pathophysiological relevance in solid organ failure, transplantation and cancer. In our opinion, clinicians should not only prescribe immunosuppressive treatments to disrupt this circuit, but also implement the neglected therapeutic option of adding compounds that interfere with RCD.

Substrate modulation of fatty acid effects on energization and respiration of kidney proximal tubules during hypoxia/reoxygenation

Kidney proximal tubules subjected to hypoxia/reoxygenation develop a nonesterified fatty acid-induced energetic deficit characterized by persistent partial mitochondrial deenergization that can be prevented and reversed by citric acid cycle substrates. To further assess the role of competition between fatty acids and substrates on inner membrane substrate carriers in the deenergization and the contribution to deenergization of fatty acid effects on respiratory function, digitonin-permeabilized rabbit and mouse tubules were studied using either addition of exogenous oleate after control normoxic incubation or increases of endogenous fatty acids produced by hypoxia/reoxygenation. The results demonstrated major effects of matrix oxaloacetate accumulation on succinate-supported energization and respiration and their modification by fatty acids. Improvements of energization in the presence of fatty acids by glutamate were shown to result predominantly from lowering matrix oxaloacetate rather than from amelioration of transmembrane cycling of fatty acids and uncoupling. Mouse tubules had 2.5 fold higher rates of succinate utilization, which resulted in stronger effects of oxaloacetate accumulation than rabbit tubules. Hypoxia/reoxygenation induced respiratory inhibition that was more severe for complex I-dependent substrates. Fatty acids themselves did not acutely contribute to this respiratory inhibition, but lowering them during 60 min. reoxygenation to allow recovery of ATP during that period alleviated it. These data clarify the basis for the nonesterified fatty acid-induced mitochondrial energetic deficit in kidney proximal tubules that impairs structural and functional recovery and provide insight into interactions that need to be considered in the design of substrate-based interventions to improve mitochondrial function.

Ferrostatins inhibit oxidative lipid damage and cell death in diverse disease models

Ferrostatin-1 (Fer-1) inhibits ferroptosis, a form of regulated, oxidative, nonapoptotic cell death. We found that Fer-1 inhibited cell death in cellular models of Huntington's disease (HD), periventricular leukomalacia (PVL), and kidney dysfunction; Fer-1 inhibited lipid peroxidation, but not mitochondrial reactive oxygen species formation or lysosomal membrane permeability. We developed a mechanistic model to explain the activity of Fer-1, which guided the development of ferrostatins with improved properties. These studies suggest numerous therapeutic uses for ferrostatins, and that lipid peroxidation mediates diverse disease phenotypes.

Two independent pathways of regulated necrosis mediate ischemia-reperfusion injury

Regulated necrosis (RN) may result from cyclophilin (Cyp)D-mediated mitochondrial permeability transition (MPT) and receptor-interacting protein kinase (RIPK)1-mediated necroptosis, but it is currently unclear whether there is one common pathway in which CypD and RIPK1 act in or whether separate RN pathways exist. Here, we demonstrate that necroptosis in ischemia-reperfusion injury (IRI) in mice occurs as primary organ damage, independent of the immune system, and that mice deficient for RIPK3, the essential downstream partner of RIPK1 in necroptosis, are protected from IRI. Protection of RIPK3-knockout mice was significantly stronger than of CypD-deficient mice. Mechanistically, in vivo analysis of cisplatin-induced acute kidney injury and hyperacute TNF-shock models in mice suggested the distinctness of CypD-mediated MPT from RIPK1/RIPK3-mediated necroptosis. We, therefore, generated CypD-RIPK3 double-deficient mice that are viable and fertile without an overt phenotype and that survived prolonged IRI, which was lethal to each single knockout. Combined application of the RIPK1 inhibitor necrostatin-1 and the MPT inhibitor sanglifehrin A confirmed the results with mutant mice. The data demonstrate the pathophysiological coexistence and corelevance of two separate pathways of RN in IRI and suggest that combination therapy targeting distinct RN pathways can be beneficial in the treatment of ischemic injury.

Glycine, a simple physiological compound protecting by yet puzzling mechanism(s) against ischaemia-reperfusion injury: current knowledge

Ischaemia is amongst the leading causes of death. Despite this importance, there are only a few therapeutic approaches to protect from ischaemia-reperfusion injury (IRI). In experimental studies, the amino acid glycine effectively protected from IRI. In the prevention of IRI by glycine in cells and isolated perfused or cold-stored organs (tissues), direct cytoprotection plays a crucial role, most likely by prevention of the formation of pathological plasma membrane pores. Under in vivo conditions, the mechanism of protection by glycine is less clear, partly due to the physiological presence of the amino acid. Here, inhibition of the inflammatory response in the injured tissue is considered to contribute decisively to the glycine-induced reduction of IRI. However, attenuation of IRI recently achieved in experimental animals by low-dose glycine treatment regimens suggests additional/other (unknown) protective mechanisms. Despite the convincing experimental evidence and the large therapeutic width of glycine, there are only a few clinical trials on the protection from IRI by glycine with ambivalent results. Thus, both the mechanism(s) behind the protection of glycine against IRI in vivo and its true clinical potential remain to be addressed in future experimental studies/clinical trials.

A TAT-frataxin fusion protein increases lifespan and cardiac function in a conditional Friedreich's ataxia mouse model

Friedreich's ataxia (FRDA) is the most common inherited human ataxia and results from a deficiency of the mitochondrial protein, frataxin (FXN), which is encoded in the nucleus. This deficiency is associated with an iron-sulfur (Fe-S) cluster enzyme deficit leading to progressive ataxia and a frequently fatal cardiomyopathy. There is no cure. To determine whether exogenous replacement of the missing FXN protein in mitochondria would repair the defect, we used the transactivator of transcription (TAT) protein transduction domain to deliver human FXN protein to mitochondria in both cultured patient cells and a severe mouse model of FRDA. A TAT-FXN fusion protein bound iron in vitro, transduced into mitochondria of FRDA deficient fibroblasts and reduced caspase-3 activation in response to an exogenous iron-oxidant stress. Injection of TAT-FXN protein into mice with a conditional loss of FXN increased their growth velocity and mean lifespan by 53% increased their mean heart rate and cardiac output, increased activity of aconitase and reversed abnormal mitochondrial proliferation and ultrastructure in heart. These results show that a cell-penetrant peptide is capable of delivering a functional mitochondrial protein in vivo to rescue a very severe disease phenotype, and present the possibility of TAT-FXN as a protein replacement therapy.

Glycine maintains mitochondrial activity and bile composition following warm liver ischemia-reperfusion injury

BACKGROUND AND AIM

Experimental studies have shown protective effect by the non-essential amino acid glycine to liver ischemia-reperfusion (I/R) injury but the mechanism of action is unknown.

METHODS

A rabbit model of hepatic lobar I/R was used. Three groups of animals (n=6) were studied: Sham group (laparotomy alone), ischemia reperfusion (I/R) group (1 h of liver lobar ischemia and 6 h of reperfusion), and a glycine I/R group (intravenous glycine 5 mg/kg prior to the I/R protocol). Systemic and hepatic hemodynamics, degree of liver injury (bile flow, transaminases), hepatic microcirculation, mitochondrial activity (redox state of cytochrome oxidase), bile composition and cytokines (tumor necrosis factor-α and interleukin-8) were measured during the experiment.

RESULTS

Glycine administration increased portal blood flow, bile production, hepatic microcirculation and maintained cytochrome oxidase activity as compared with the I/R group during reperfusion. Glycine also reduced bile lactate surge and stimulated acetoacetate release in bile during reperfusion versus the I/R group. Cytokine levels (tumor necrosis factor-α, interleukin-8) and hepatocellular injury (aspartate aminotransferase and alanine aminotransferase) were significantly reduced by glycine administration.

CONCLUSION

Intravenous glycine administration reduces liver warm I/R injury by reducing the systemic inflammatory response, and maintaining cellular energy production.

Glycine transporter GLYT1 is essential for glycine-mediated protection of human intestinal epithelial cells against oxidative damage

Glycine protects mammalian intestine against oxidative damage caused by ischaemia-reperfusion (IR) injury and prevents or reverses experimentally-induced colitis. However the mechanism of protection remains largely unknown. The objectives of the current study were to demonstrate directly glycine-mediated protection of human intestinal epithelial cells and to determine the requirement for glycine uptake by the specific transporter GLYT1. Exogenous glycine protected human intestinal Caco-2 and HCT-8 cells against the oxidative agent tert-butylhydroperoxide and reduced the intracellular concentration of reactive oxygen species, when applied prior to but not concomitant with the oxidative challenge. Glycine given prior to oxidative challenge preserved intracellular glutathione concentration but had no effect on the rate of glycine uptake. Protection was dependent on GLYT1 activity, being blocked by a specific GLYT1 inhibitor, supporting a requirement for intracellular glycine accumulation. Maintained intracellular glutathione content is indicated as a mechanism through which the protective effect may in part be mediated. However expression of the genes encoding GLYT1 and the glutathione synthesising enzymes glutamate-cysteine ligase, both catalytic and modifier subunits, and glutathione synthetase was not altered by glycine or tert-butylhydroperoxide, suggesting transcriptional regulation is not involved. This work has demonstrated a novel role of GLYT1 in intestine and shown that intestinal epithelial cells respond directly to oxidative challenge without reliance on extra-epithelial tissues or functions such as neurone, blood-flow or immune responses for antioxidant defence. The protective actions of glycine and maintenance of epithelial antioxidant defences suggest it may be beneficial in treatment of inflammatory bowel disease.

Uremia induces proximal tubular cytoresistance and heme oxygenase-1 expression in the absence of acute kidney injury

Acute kidney injury (AKI) induces adaptive responses within proximal tubular (PT) cells that serve to protect them from further ischemic or toxic damage. However, it is not known whether uremia, a potential consequence of AKI, independently alters susceptibility to tubular injury. To address this issue, we subjected CD-1 mice to bilateral ureteral transection (BUTx), which produces uremia (blood urea nitrogen approximately 150 mg/dl) in the absence of direct renal damage. PT segments were then isolated from BUTx and control mice and subjected to in vitro hypoxic injury. Additionally, "in vitro uremia" was modeled in isolated tubules or in cultured PT (HK-2) cells by addition of 1) peritoneal dialysate (obtained from mice with bilateral ureteral obstruction), 2) peritoneal fluid (from BUTx mice), or 3) normal human urine (pH 7.4, with and without boiling). Effects on injury severity (lactate dehydrogenase release) were assessed. Finally, because uremia is a prooxidant state, it was hypothesized that BUTx would increase renal lipid peroxidation (malondialdehyde) and induce heme oxygenase-1 (HO-1), a redox-sensitive cytoprotective protein. BUTx conferred striking protection against hypoxic damage. This could be partially modeled in tubules and HK-2 cells by induction of in vitro uremia. Urine's protective action was heat labile (largely destroyed by boiling). BUTx caused a tripling of renal malondialdehyde and HO-1 protein levels. Increased HO-1 transcription was likely involved, as indicated by a tripling of HO-1 mRNA and RNA polymerase II binding along the HO-1 gene (chromatin immunoprecipitation assay). "Gene-activating" histone modifications [H3K4 trimethylation (H3K4m3) and histone 2 variant (H2A.Z)] at HO-1 gene loci were also observed. Uremia, per se, can contribute to the AKI-induced cytoresistance. Low-molecular-weight, heat-labile, cytoprotective factor(s) and uremia-induced renal stress responses (e.g., HO-1 gene activation) are likely involved. Finally, renal HO-1 induction following AKI may reflect direct cell injury effects and adaptations to uremia.

Maleate nephrotoxicity: mechanisms of injury and correlates with ischemic/hypoxic tubular cell death

Maleate injection causes dose-dependent injury in proximal tubular cells. This study sought to better define underlying pathogenic mechanisms and to test whether maleate toxicity recapitulates critical components of the hypoxic/ischemic renal injury cascade. CD-1 mice were injected with maleate or used as a source for proximal tubule segments (PTS) for in vitro studies. Maleate induced dose-dependent PTS injury [lactate deydrogenase (LDH) release, ATP reductions, nonesterified fatty acid (NEFA) accumulation]. These changes were partially dependent on maleate metabolism (protection conferred by metabolic inhibitors: succinate, acetoacetate). Maleate toxicity reproduced critical characteristics of the hypoxia/ATP depletion-induced injury cascade: 1) glutathione (GSH) conferred protection, but due to its glycine, not cysteine (antioxidant), content; 2) ATP reductions reflected decreased production, not Na-K-ATPase-driven increased consumption; 3) cell death was completely blocked by extracellular acidosis (pH 6.6); 4) intracellular Ca(2+) chelation (BAPTA) mitigated cell death; 5) maleate and hypoxia each caused plasma membrane cholesterol shedding and in both instances, this was completely glycine suppressible; 6) maleate + hypoxia caused neither additive NEFA accumulation nor LDH release, implying shared pathogenic pathways; and 7) maleate, like ischemia, induced renal cortical cholesterol loading; increased HMG CoA reductase (HMGCR) activity (statin inhibitable), increased HMGCR mRNA levels, and increased RNA polymerase II recruitment to the HMGCR locus (chromatin immunoprecipitation, ChIP, assay) were involved. These results further define critical determinants of maleate nephrotoxicity and suggest that it can serve as a useful adjunct for studies of ischemia/ATP depletion-induced, proximal tubule-specific, cell death.

Toll-like receptor (TLR4) shedding and depletion: acute proximal tubular cell responses to hypoxic and toxic injury

Acute renal failure (ARF) induces tubular hyperresponsiveness to TLR4 ligands, culminating in exaggerated renal cytokine/chemokine production. However, the fate of TLR4 protein during acute tubular injury remains unknown. The study sought new insights into this issue. Male CD-1 mice were subjected to 1) unilateral ischemia-reperfusion (I/R), 2) cisplatin (CP) nephrotoxicity, or 3) glycerol-induced myohemoglobinuric ARF. Renal cortical TLR4 protein (Western blotting, immunohistochemistry) and TLR4 mRNA levels (RT-PCR) were determined thereafter (90 min-4 days). Urinary TLR4 excretion post-I/R or CP injection was also assessed. To gain proximal tubule-specific results, TLR4 protein and mRNA were quantified in posthypoxic or oxidant (Fe)-challenged isolated mouse tubules. Finally, TLR4 mRNA was determined in antimycin A-injured cultured proximal tubular (HK-2) cells. Acute in vivo renal injury reduced proximal tubule TLR4 content. These changes corresponded with the appearance of TLR4 fragment(s) in urine and a persistent increase in renal cortical TLR4 mRNA. Isolated proximal tubules responded to injury with rapid TLR4 reductions, dramatic extracellular TLR4 release, and increases in TLR4 mRNA. Glycine blocked these processes, implying membrane pore formation was involved. HK-2 cell injury increased TLR4 mRNA, but not protein levels, suggesting intact transcriptional, but not translational, pathways. Diverse forms of acute tubular injury rapidly reduce proximal tubular TLR4 content. Plasma membrane TLR4 release through glycine-suppressible pores, possibly coupled with a translation block, appears to be involved. Rapid postinjury urinary TLR4 excretion suggests its potential utility as a "biomarker" of impending ARF.

Glycine--an important neurotransmitter and cytoprotective agent

BACKGROUND

Glycine, the simplest of the amino acids, is an essential component of important biological molecules, a key substance in many metabolic reactions, the major inhibitory neurotransmitter in the spinal cord and brain stem, and an anti-inflammatory, cytoprotective, and immune modulating substance.

MATERIAL AND METHODS

Based on available literature, we discuss some of the important biological properties of glycine. In addition, we describe some clinical disorders where glycine plays a central role, either as an essential structural element, or through its metabolism or receptors.

RESULTS

The past few years have witnessed a broadening of glycine research. The traditional prime interest in aspects related to its role as an inhibitory neurotransmitter in the central nervous system has been expanded to equally emphasize other organs and tissues. With the demonstration of glycine-gated chloride channels on neurons in the central nervous system, on most leukocytes, and subsequently on other cells as well, a unifying mechanism of action accounting for many of the widespread effects of glycine has been found.

CONCLUSIONS

Glycine is a simple, easily available, and inexpensive substance with few and innocuous side-effects. The diversity of biological activities is well documented in the literature. Despite this, glycine has only gained a modest place in clinical medicine.

Renal tubular triglyercide accumulation following endotoxic, toxic, and ischemic injury

BACKGROUND

Cholesterol accumulates in renal cortical proximal tubules in response to diverse forms of injury or physiologic stress. However, the fate of triglycerides after acute renal insults is poorly defined. This study sought new insights into this issue.

METHODS

CD-1 mice were subjected to three diverse models of renal stress: (1) endotoxemia [Escherichia coli lipopolysaccharide (LPS), injection]; (2) ischemia/reperfusion (I/R); or (3) glycerol-induced rhabdomyolysis. Renal cortical, or isolated proximal tubule, triglyceride levels were measured approximately 18 hours later. To gain mechanistic insights, triglyceride levels were determined in (1) proximal tubules following exogenous phospholipase A(2) (PLA(2)) treatment; (2) cultured HK-2 cells after mitochondrial blockade (antimycin A) +/- serum; or (3) HK-2 cells following "septic" (post-LPS) serum, or exogenous fatty acid (oleate) addition.

RESULTS

Each form of in vivo injury evoked three-to fourfold triglyceride increases in renal cortex and/or proximal tubules. PLA(2) treatment of proximal tubules evoked acute, dose-dependent, triglyceride formation. HK-2 cell triglyceride levels rose with antimycin A. With serum present, antimycin A induced an exaggerated triglyceride loading state (vs. serum alone or antimycin A alone). "Septic" serum stimulated HK-2 triglyceride formation (compared to control serum). Oleate addition caused striking HK-2 cell triglyceride accumulation. Following oleate washout, HK-2 cells were sensitized to adenosine triphosphate (ATP) depletion or oxidant attack.

CONCLUSION

Diverse forms of renal injury induce dramatic triglyceride loading in proximal tubules/renal cortex, suggesting that this is a component of a cell stress response. PLA(2) activity, increased triglyceride/triglyceride substrate (e.g., fatty acid) uptake, and possible systemic cytokine (e.g., from LPS) stimulation, may each contribute to this result. Finally, in addition to being a marker of prior cell injury, accumulation of triglyceride (or of its constituent fatty acids) may predispose tubules to superimposed ATP depletion or oxidant attack.

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.

Heme: a novel inducer of MCP-1 through HO-dependent and HO-independent mechanisms

This study examined the effect of hemin on the expression of heme oxygenase-1 (HO-1) and monocyte chemoattractant protein-1 (MCP-1) in immortalized rat proximal tubular epithelial cells (IRPTCs). Hemin elicited a dose- and time-dependent induction of HO-1 and MCP-1 mRNA. HO activity contributed to MCP-1 mRNA expression at early time points (4-6 h) because inhibition of HO activity by zinc protoporphyrin (ZnPP) prevented hemin-induced expression of MCP-1 mRNA. Catalytically active intracellular iron was markedly increased in hemin-treated IRPTCs and contributed to the induction of HO-1 and MCP-1 mRNA because an iron chelator blocked hemin-induced upregulation of both genes, whereas a cell-permeant form of iron directly induced these genes. N-acetylcysteine completely blocked hemin-induced expression of HO-1 and MCP-1 mRNA, thereby providing added evidence for redox regulation of expression of these genes. The redox-sensitive transcription factor NF-kappaB was recruited in hemin-induced upregulation of MCP-1 because two different compounds that abrogate the activation of NF-kappaB (TPCK and BAY 11-7082) completely blocked hemin-induced upregulation of MCP-1 mRNA. In contrast to this HO-mediated induction of MCP-1 through redox-sensitive, iron-dependent, and NF-kappaB-involved pathways observed after 4-6 h, hemin also elicited a delayed induction of MCP-1 at 18 h through HO-independent pathways. We conclude that hemin is a potent inducer of MCP-1 in IRPTCs: HO-dependent, heme-degrading pathways lead to an early, robust, and self-remitting induction of MCP-1, whereas HO-independent mechanisms lead to a delayed expression of MCP-1.

Parenteral iron formulations: a comparative toxicologic analysis and mechanisms of cell injury

BACKGROUND

Multiple parenteral iron (Fe) formulations exist for administration to patients with end-stage renal disease. Although there are concerns regarding their potential toxicities, no direct in vitro comparisons of these agents exist. Thus, the present study contrasted pro-oxidant and cytotoxic potentials of four available Fe preparations: Fe dextran (Fe dext), Fe sucrose (Fe sucr), Fe gluconate (Fe gluc), and Fe oligosaccharide (Fe OS).

METHODS

Differing dosages (0.06 to 1 mg/mL) of each compound were added to either (1) isolated mouse proximal tubule segments, (2) renal cortical homogenates, or (3) cultured human proximal tubule (HK-2) cells (0.5- to 72-hour incubations). Oxidant injury (malondialdehyde generation) and lethal cell injury (percentage of lactate dehydrogenase release; tetrazolium dye uptake) were assessed. Effects of selected antioxidants (glutathione [GSH], catalase, dimethylthiourea (DMTU), and sodium benzoate also were assessed.

RESULTS

Each test agent induced massive and similar degrees of lipid peroxidation. Nevertheless, marked differences in cell death resulted (Fe sucr >> Fe gluc > Fe dext approximately Fe OS). This relative toxicity profile also was observed in cultured aortic endothelial cells. Catalase, DMTU, and sodium benzoate conferred no protection. However, GSH and its constituent amino acid glycine blocked Fe sucr-mediated cell death. The latter was mediated by mitochondrial blockade, causing free radical generation and a severe adenosine triphosphate depletion state.

CONCLUSIONS

(1) parenteral Fes are highly potent pro-oxidants and capable of inducing tubular and endothelial cell death, (2) markedly different toxicity profiles exist among these agents, and (3) GSH can exert protective effects. However, the latter stems from GSH's glycine content, rather than from a direct antioxidant effect.

Reactive oxygen species and acute renal failure

Acute renal failure is commonly due to acute tubular necrosis (ATN), the latter representing an acute, usually reversible loss of renal function incurred from ischemic or nephrotoxic insults occurring singly or in combination. Such insults instigate a number of processes-hemodynamic alterations, aberrant vascular responses, sublethal and lethal cell damage, inflammatory responses, and nephron obstruction-that initiate and maintain ATN. Eventually, reparative and regenerative processes facilitate the resolution of renal injury and the recovery of renal function. Focusing mainly on ischemic ATN, this article reviews evidence indicating that the inordinate or aberrant generation of reactive oxygen species (ROS) may contribute to the initiation and maintenance of ATN. This review also discusses the possibility that ROS may instigate adaptive as well as maladaptive responses in the kidney with ATN, and raises the possibility that ROS may participate in the recovery phase of ATN.

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.

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.

Lipid peroxidative stress and antioxidative enzymes in brains of milk-supplemented rats

Skim milk cultured with lactic acid bacteria has been previously reported to reduce lipid peroxidation in rat livers. In this study, the effects of skim milk and cultured milk supplementation on peroxidative stress in brains of weanling rats were investigated. We observed a reduction of brain thiobarbituric acid reacting substances (TBARS) concentration in milk-supplemented animals as compared with controls. In brains of control rats, the superoxide dismutase (SOD) enzyme levels were significantly higher than those from the milk-supplemented animals. In addition, SOD activity in control animal brains had a positive correlation with the TBARS concentration. There was no significant differences in the brain glutathione-S-transferase (GST) levels of all the three groups of animals. The results suggest that milk supplementation may be beneficial in reducing peroxidative stress in the developing rat brain.

Attenuation of cisplatin-induced acute renal failure is associated with less apoptotic cell death

To clarify the pathophysiologic role of apoptosis in acute renal failure (ARF), we examined whether the attenuation of cisplatin-induced ARF is associated with the change in the degree of apoptotic cell death. The administration of cisplatin (CDDP) (6 mg/kg body weight) in rats induced ARF at day 5, as manifested by a significant increase in serum creatinine (Scr) and tubular damage. CDDP-induced apoptotic cell death was confirmed by electron microscopic examination, agarose gel electrophoresis, and increased cells positive for TaT-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) in the outer medulla of the kidney. Treatment with dimethylthiourea (DMTU)--a scavenger of hydroxyl radicals--or glycine abrogated CDDP-induced increases in Scr, the tubular damage score, and the number of TUNEL-positive cells. Pretreatment with uranyl acetate (UA) induced a significant expression of Bcl-2 in the kidney and ameliorated CDDP-induced increases in Scr, the tubular damage score, and TUNEL-positive cells in the outer stripe of the outer medulla. Our findings indicate (1) that the attenuation of CDDP-induced ARF was associated with less apoptotic cell death and (2) that the induction of the anti-apoptotic protein Bcl-2 attenuated apoptosis and tubular damage. Our results suggest that apoptotic cell death may play an important role in the development of cisplatin-induced ARF.

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.

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.

Isoflurane alters proximal tubular cell susceptibility to toxic and hypoxic forms of attack

BACKGROUND

Fluorinated anesthetics can profoundly alter plasma membrane structure and function, potentially impacting cell injury responses. Because major surgery often precipitates acute renal failure, this study assessed whether the most commonly used fluorinated anesthetic, isoflurane, alters tubular cell responses to toxic and hypoxic attack.

METHODS

Mouse proximal tubule segments were incubated under control conditions or with a clinically relevant isoflurane dose. Cell viability (lactate dehydrogenase release), deacylation (fatty acid, such as C20:4 levels), and adenosine triphosphate (ATP) concentrations were assessed under one or more of the following conditions: (a) exogenous phospholipase A2 (PLA2) or C20:4 addition, (b) Ca2+ overload (A23187 ionophore), (c) increased metabolic work (Na ionophore), and (d) hypoxia- or antimycin A-induced attack. Isoflurane's effect on NBD phosphatidylserine uptake (an index of plasma membrane aminophospholipid translocase activity) was also assessed.

RESULTS

Isoflurane alone caused trivial deacylation and no lactate dehydrogenase release. However, it strikingly sensitized to both PLA2- and A23187-induced deacylation and cell death. Isoflurane also exacerbated C20:4's direct membrane lytic effect. Under conditions of mild ATP depletion (Na ionophore-induced increased ATP consumption; PLA2-induced mitochondrial suppression), isoflurane provoked moderate/severe ATP reductions and cell death. Conversely, under conditions of maximal ATP depletion (hypoxia, antimycin), isoflurane conferred a modest cytoprotective effect. Isoflurane blocked aminophospholipid translocase activity, which normally maintains plasma membrane lipid asymmetry (that is, preventing its "flip flop").

CONCLUSIONS

Isoflurane profoundly and differentially affects tubular cell responses to toxic and hypoxic attack. Direct drug-induced alterations in lipid trafficking/plasma membrane orientation and in cell energy production are likely involved. Although the in vivo relevance of these findings remains unknown, they have potential implications for intraoperative renal tubular cell structure/function and how cells may respond to superimposed attack.

Inhibition of oxidant-induced lipid peroxidation in cultured renal tubular epithelial cells (LLC-PK1) by quercetin

The protective effect of quercetin against oxidant-induced cell injury (hypoxanthine/xanthine oxidase system) was studied in the renal tubular epithelial cell line LLC-PK1. Pretreatment with quercetin provided protection from structural and functional cell damage in a concentration-dependent manner (10-100 microM). Comparison with structural variants revealed that the protective property of quercetin depends on the number of hydroxyl substituents in the B-ring, the presence of an extended C-ring chromophore, 3-D-planarity and lipophilicity, indicating that membrane affinity is essential for protection. The hypothesis that quercetin exerts its protective effects via inhibition of lipid peroxidation was further examined. Protection by quercetin was found when lipid peroxidation, assessed by the release of malondialdehyde, was initiated by H2O2 or by the combination of 1-chloro-2,4-dinitrobenzene and aminotriazole. In contrast, the bioflavonoid was not protective when oxidative cell damage was induced by menadione and occurred in the absence of lipid peroxidation. These data suggest that cytoprotective effects of quercetin are related to membrane affinity and may be explained by interruption of membrane lipid peroxidation rather than by intracellular scavenging of oxygen free radicals.

Epidermal growth factor accelerates recovery of LLC-PK1 cells following oxidant injury

In renal tubular epithelial cells, oxidant injury results in several metabolic alterations including ATP depletion, decreased Na+K+ATPase activity, and altered intracellular sodium and potassium content. To investigate the recovery of LLC-PK1 cells following oxidant injury and to determine if recovery can be accelerated, we induced oxidant stress in LLC-PK1 cells with 500 microM hydrogen peroxide for 60 min. Identical cohorts of oxidant-stressed cells were incubated in recovery medium without epidermal growth factor (EGF) or recovery medium containing 25 ng EGF per ml. ATP levels, Na+K+ATPase activity in whole cells, Na+K+ATPase activity in disrupted cells, and intracellular sodium and potassium ion content were determined at 0, 5, 24, 48, and 72 h following oxidant injury in each cohort of cells. In oxidant-stressed cells recovering in medium without EGF, ATP levels, Na+K+ATPase activity, and intracellular ion content improved but continued to remain substantially lower than control values at all time points following oxidant stress. In cells recovering in medium with EGF, ATP levels, Na+K+ATPase activity, and the intracellular potassium-to-sodium ratio were significantly higher at nearly all time points than values in cells recovering in medium alone. In cells recovering with added EGF, Na+K+ATPase activity had improved to control levels, whereas ATP levels and intracellular ion content approached control values by 72 h following oxidant stress. We conclude that oxidant-mediated ATP depletion, altered Na+K+ATPase activity, and intracellular ion content remain depressed for several d following oxidant stress and that EGF accelerated recovery of LLC-PK1 cells from oxidant injury.

Differential effects of glutathione and cysteine on Fe2+, Fe3+, H2O2 and myoglobin-induced proximal tubular cell attack

Glutathione (GSH) is widely advocated as a cytoprotectant for preventing oxidant forms of renal damage. However, in the case of myoglobinuric tubular injury, both beneficial and adverse effects have been noted. The purpose of this study was to help elucidate this seeming paradox by assessing the impact on thiol supplementation on normal tubules and on tubules subjected to individual components of heme protein-induced oxidant attack (Fe2+, Fe3+, and H2O2). Isolated mouse proximal tubular segments (PTS) were exposed to either GSH or cysteine under normal conditions or in the presence of exogenous Fe2+, Fe3+, or H2O2. Lethal cell injury (LDH release) and lipid peroxidation (malondialdehyde) were then assessed. GSH and cysteine exerted iron dependent, H2O2 independent, pro-oxidant effects on normal PTS. Both were also pro-oxidant in the presence of an exogenous Fe3+ challenge. In contrast, each attenuated Fe2+ cytotoxicity. The importance of iron's redox status on the expression of tubular injury was further underscored by the fact that Fe3+ partially blocked Fe2+'s cytotoxic effects. GSH mitigated H2O2 toxicity (consistent with a fueling of GSH peroxidase activity). Conversely, cysteine promoted H2O2's injurious effects. To assess the impact of thiol supplementation on a fully integrated model of heme protein toxicity, proximal tubular (HK-2) cells were cultured with myoglobin x 24 hours +/- test reactants. Exogenous GSH worsened, while GSH depletion (BSO) protected, against myoglobin toxicity (indicating a predominance of GSH's pro-oxidant effects). Conversely, cysteine (but not homocysteine) decreased myoglobin toxicity. These GSH/cysteine effects were confirmed in LLC-PK1 cells subjected to iron attack. We conclude that: (1) GSH and cysteine can exert pro- and anti-oxidant effects, depending on the nature of the oxidant challenge and iron's redox status; (2) Fe3+ can function as a cytoprotectant, partially offsetting Fe2+ toxicity; and (3) cysteine, although potentially pro-oxidant, can mitigate heme protein-induced injury. Since the kidney rapidly catabolizes GSH to cysteine, the latter may be at least partially responsible for GSH's reported cytoprotective actions against myoglobinuric acute renal failure.

Cold preservation of isolated rabbit proximal tubules induces radical-mediated cell injury

BACKGROUND

Reactive oxygen species (ROS) are involved in reperfusion injury after preservation. Recent studies in isolated endothelial cells and hepatocytes suggested the occurrence of ROS-mediated injury during the period of cold incubation. In the present study, formation of ROS and subsequent cell injury were studied in freshly isolated rabbit proximal tubules (PTs).

METHODS

PTs were incubated in University of Wisconsin (UW) solution, Euro-Collins solution, or a modified Krebs-Henseleit buffer under aerobic conditions for up to 94 hr at 4 degrees C. ROS formation and cell death were assessed as lipid peroxidation (formation of thiobarbituric acid-reactive substances [TBARS]) and release of lactate dehydrogenase, respectively. The involvement of ROS was further investigated in UW solution using compounds that might interfere with ROS formation. In addition, tubules were studied under anaerobic conditions (gassing with 95% N2/5% CO2).

RESULTS

Cold preservation of rabbit PTs in any of the solutions under aerobic conditions caused progressive lipid peroxidation and concomitant cell injury. Addition to UW solution of inhibitors of ROS formation, in particular 2,2'-dipyridyl, or removal of oxygen by gassing with 95% N2/5% CO2, prevented lipid peroxidation and protected rabbit PTs against cold injury. Both the nitric oxide (NO) synthase inhibitor L-NAME and dexamethasone, which blocks the inducible NO synthase, were ineffective. The cytoprotectant glycine affected neither TBARS formation nor lactate dehydrogenase release.

CONCLUSIONS

Cold preservation of renal PTs under aerobic conditions caused cell injury even in the specially designed preservation solution UW. Cell injury is caused by iron-dependent, NO synthase-independent ROS formation.

Glycine-protected, hypoxic, proximal tubules develop severely compromised energetic function

Glycine-treated, hypoxic, proximal tubules developed a progressive energetic defect that resulted in failure to restore ATP levels to greater than 10 to 20% of control values during reoxygenation after 60 minutes of hypoxia despite continued cytoprotection by glycine. The defect was not corrected by supplementation with exogenous purines and was not modified by lowering the pH during hypoxia or reoxygenation. In the continued presence of glycine, the failure to restore ATP was associated with impaired recovery of structural changes that developed during hypoxia and, if glycine was withdrawn, lethal membrane damage occurred. The lesion was significantly ameliorated by the presence during hypoxia of two agents known to suppress development of the mitochondrial permeability transition, cyclosporine A and butacaine, which were most effective when used in combination. The data suggest that development of the mitochondrial permeability transition in glycine-protected tubules during hypoxia contributes to continued metabolic and structural impairment and cell death that occur despite glycine replete conditions such as exist frequently during in vivo insults and may be a target for therapeutic maneuvers.

Functional alpha 1 protease inhibitor produced by a human hepatoma cell line

Alpha 1 protease inhibitor antigen was identified in the culture medium of the human ascites hepatoma cell line SK-HEP-1. Trypsin inhibitory activity and alpha 1 Pl antigen accumulated in serum-free medium concomitantly over a period of several days. Radioactive alpha 1 Pl antigen was detected in conditioned medium from cultures supplemented with 35S-L-methionine, indicating a synthesis and release of the protein. Alpha 1 Pl antigen in conditioned medium appeared to be antigenically identical to that in human plasma, and the newly synthesized (radiolabeled) antigen co-migrated with plasma, alpha 1 Pl after immunoelectrophoresis or SDS-polyacrylamide gel electrophoresis. Moreover, evidence is presented that the synthesized inhibitor exhibits functional activity, since the 35S-labeled alpha 1 Pl in conditioned medium complexes with trypsin. We conclude that SK-HEP-1 cells in culture produce functionally active alpha 1 Pl which may be identical to that in plasma.