Proximal tubule H-ferritin mediates iron trafficking in acute kidney injury

Complement C5 inhibition protects against hemolytic anemia and acute kidney injury in anthrax peptidoglycan-induced sepsis in baboons

Late-stage anthrax infections are characterized by dysregulated immune responses and hematogenous spread of Bacillus anthracis, leading to extreme bacteremia, sepsis, multiple organ failure, and, ultimately, death. Despite the bacterium being nonhemolytic, some fulminant anthrax patients develop a secondary atypical hemolytic uremic syndrome (aHUS) through unknown mechanisms. We recapitulated the pathology in baboons challenged with cell wall peptidoglycan (PGN), a polymeric, pathogen-associated molecular pattern responsible for the hemostatic dysregulation in anthrax sepsis. Similar to aHUS anthrax patients, PGN induces an initial hematocrit elevation followed by progressive hemolytic anemia and associated renal failure. Etiologically, PGN induces erythrolysis through direct excessive activation of all three complement pathways. Blunting terminal complement activation with a C5 neutralizing peptide prevented the progressive deposition of membrane attack complexes on red blood cells (RBC) and subsequent intravascular hemolysis, heme cytotoxicity, and acute kidney injury. Importantly, C5 neutralization did not prevent immune recognition of PGN and shifted the systemic inflammatory responses, consistent with improved survival in sepsis. Whereas PGN-induced hemostatic dysregulation was unchanged, C5 inhibition augmented fibrinolysis and improved the thromboischemic resolution. Overall, our study identifies PGN-driven complement activation as the pathologic mechanism underlying hemolytic anemia in anthrax and likely other gram-positive infections in which PGN is abundantly represented. Neutralization of terminal complement reactions reduces the hemolytic uremic pathology induced by PGN and could alleviate heme cytotoxicity and its associated kidney failure in gram-positive infections.

Iron deficiency exacerbates cisplatin- or rhabdomyolysis-induced acute kidney injury through promoting iron-catalyzed oxidative damage

Iron deficiency is the most common micronutrient deficiency worldwide. While iron deficiency is known to suppress embryonic organogenesis, its effect on the adult organ in the context of clinically relevant damage has not been considered. Here we report that iron deficiency is a risk factor for nephrotoxic intrinsic acute kidney injury of the nephron (iAKI). Iron deficiency exacerbated cisplatin-induced iAKI by markedly increasing non-heme catalytic iron and Nox4 protein which together catalyze production of hydroxyl radicals followed by protein and DNA oxidation, apoptosis and ferroptosis. Crosstalk between non-heme catalytic iron/Nox4 and downstream oxidative damage generated a mutual amplification cycle that facilitated rapid progression of cisplatin-induced iAKI. Iron deficiency also exacerbated a second model of iAKI, rhabdomyolysis, via increasing catalytic heme-iron. Heme-iron induced lipid peroxidation and DNA oxidation by interacting with Nox4-independent mechanisms, promoting p53/p21 activity and cellular senescence. Our data suggests that correcting iron deficiency and/or targeting specific catalytic iron species are strategies to mitigate iAKI in a wide range of patients with diverse forms of kidney injury.

Pumping iron in the kidney

Iron balance is tightly controlled to provide adequate amounts of this essential nutrient, but to limit the adverse effects of excess iron. Key mediators of systemic iron homeostasis are the iron regulatory hormone hepcidin and its receptor, the iron export protein ferroportin. A new study by Mohammad et al. demonstrates the functional role of the hepcidin-ferroportin axis in the kidney, and how this contributes to kidney iron levels and the systemic iron economy.

The Macrophage Iron Signature in Health and Disease

Throughout life, macrophages are located in every tissue of the body, where their main roles are to phagocytose cellular debris and recycle aging red blood cells. In the tissue niche, they promote homeostasis through trophic, regulatory, and repair functions by responding to internal and external stimuli. This in turn polarizes macrophages into a broad spectrum of functional activation states, also reflected in their iron-regulated gene profile. The fast adaptation to the environment in which they are located helps to maintain tissue homeostasis under physiological conditions.

Heme Burden and Ensuing Mechanisms That Protect the Kidney: Insights from Bench and Bedside

With iron at its core, the tetrapyrrole heme ring is a cardinal prosthetic group made up of many proteins that participate in a wide array of cellular functions and metabolism. Once released, due to its pro-oxidant properties, free heme in sufficient amounts can result in injurious effects to the kidney and other organs. Heme oxygenase-1 (HO-1) has evolved to promptly attend to such injurious potential by facilitating degradation of heme into equimolar amounts of carbon monoxide, iron, and biliverdin. HO-1 induction is a beneficial response to tissue injury in diverse animal models of diseases, including those that affect the kidney. These protective attributes are mainly due to: (i) prompt degradation of heme leading to restraining potential hazardous effects of free heme, and (ii) generation of byproducts that along with induction of ferritin have proven beneficial in a number of pathological conditions. This review will focus on describing clinical aspects of some of the conditions with the unifying end-result of increased heme burden and will discuss the molecular mechanisms that ensue to protect the kidneys.

Iron sucrose ('RBT-3') activates the hepatic and renal HAMP1 gene, evoking renal hepcidin loading and resistance to cisplatin nephrotoxicity

BACKGROUND

Iron sucrose (FeS) administration induces a state of renal preconditioning, protecting against selected forms of acute kidney injury (AKI). Recent evidence suggests that recombinant hepcidin also mitigates acute renal damage. Hence the goals of this study were to determine whether a new proprietary FeS formulation ('RBT-3') can acutely activate the hepcidin (HAMP1) gene in humans, raising plasma and renal hepcidin concentrations; assess whether the kidney participates in this posited RBT-3-hepcidin generation response; test whether RBT-3 can mitigate a clinically relevant AKI model (experimental cisplatin toxicity) and explore whether mechanisms in addition to hepcidin generation are operative in RBT-3's cytoprotective effects.

METHODS

Healthy human volunteers (n = 9) and subjects with Stages 3-4 CKD (n = 9) received 120, 240 or 360 mg of RBT-3 (intravenously over 2 h). Plasma and urine samples were collected and assayed for hepcidin levels (0-72 h post-RBT-3 injection). In complementary mouse experiments, RBT-3 effects on hepatic versus renal hepcidin (HAMP1) messenger RNA (mRNA) and protein levels were compared. RBT-3's impact on the mouse Nrf2 pathway and on experimental cisplatin nephrotoxicity was assessed. Direct effects of exogenous hepcidin on in vivo and in vitro (HK-2 cells) cisplatin toxicity were also tested.

RESULTS

RBT-3 induced rapid, dose-dependent and comparable plasma hepcidin increases in both healthy volunteers and chronic kidney disease subjects (∼15 times baseline within 24 h). Human kidney hepcidin exposure was confirmed by 4-fold urinary hepcidin increases. RBT-3 up-regulated mouse hepcidin mRNA, but much more so in kidney (>25 times) versus liver (∼2 times). RBT-3 also activated kidney Nrf2 [increased Nrf2 nuclear binding; increased Nrf2-responsive gene mRNAs: heme oxygenase-1, sulfiredoxin-1, glutamate-cysteine ligase catalytic subunit and NAD(P)H quinone dehydrogenase 1]. RBT-3 preconditioning (18 h time lapse) markedly attenuated experimental cisplatin nephrotoxicity (∼50% blood urea nitrogen/creatinine decrements), in part by reducing renal cisplatin uptake by 40%. Exogenous hepcidin (without RBT-3) treatment conferred protection against mild in vivo (but not in vitro) cisplatin toxicity.

CONCLUSIONS

RBT-3 acutely and dramatically up-regulates cytoprotective hepcidin production, increasing renal hepcidin levels. However, additional cytoprotective mechanisms are activated by RBT-3 (e.g. Nrf2 activation; reduced cisplatin uptake). Thus RBT-3-induced preconditioning likely confers renal resistance to cisplatin via an interplay of multiple cytoprotective activities.

The Cross-Link between Ferroptosis and Kidney Diseases

Acute and chronic kidney injuries result from structural dysfunction and metabolic disorders of the kidney in various etiologies, which significantly affect human survival and social wealth. Nephropathies are often accompanied by various forms of cell death and complex microenvironments. In recent decades, the study of kidney diseases and the traditional forms of cell death have improved. Nontraditional forms of cell death, represented by ferroptosis and necroptosis, have been discovered in the field of kidney diseases, which have reshuffled the role of traditional cell death in nephropathies. Although interactions between ferroptosis and acute kidney injury (AKI) have been continuously explored, studies on ferroptosis and chronic kidney disease (CKD) remain limited. Here, we have reviewed the therapeutic significance of ferroptosis in AKI and anticipated the curative potential of ferroptosis for CKD in the hope of providing insights into ferroptosis and CKD.

Iron Metabolism: An Under Investigated Driver of Renal Pathology in Lupus Nephritis

Nephritis is a common manifestation of systemic lupus erythematosus, a condition associated with inflammation and iron imbalance. Renal tubules are the work horse of the nephron. They contain a large number of mitochondria that require iron for oxidative phosphorylation, and a tight control of intracellular iron prevents excessive generation of reactive oxygen species. Iron supply to the kidney is dependent on systemic iron availability, which is regulated by the hepcidin-ferroportin axis. Most of the filtered plasma iron is reabsorbed in proximal tubules, a process that is controlled in part by iron regulatory proteins. This review summarizes tubulointerstitial injury in lupus nephritis and current understanding of how renal tubular cells regulate intracellular iron levels, highlighting the role of iron imbalance in the proximal tubules as a driver of tubulointerstitial injury in lupus nephritis. We propose a model based on the dynamic ability of iron to catalyze reactive oxygen species, which can lead to an accumulation of lipid hydroperoxides in proximal tubular epithelial cells. These iron-catalyzed oxidative species can also accentuate protein and autoantibody-induced inflammatory transcription factors leading to matrix, cytokine/chemokine production and immune cell infiltration. This could potentially explain the interplay between increased glomerular permeability and the ensuing tubular injury, tubulointerstitial inflammation and progression to renal failure in LN, and open new avenues of research to develop novel therapies targeting iron metabolism.

Regulated cell death in cisplatin-induced AKI: relevance of myo-inositol metabolism

Regulated cell death (RCD), distinct from accidental cell death, refers to a process of well-controlled programmed cell death with well-defined pathological mechanisms. In the past few decades, various terms for RCDs were coined, and some of them have been implicated in the pathogenesis of various types of acute kidney injury (AKI). Cisplatin is widely used as a chemotherapeutic drug for a broad spectrum of cancers, but its usage was hampered because of being highly nephrotoxic. Cisplatin-induced AKI is commonly seen clinically, and it also serves as a well-established prototypic model for laboratory investigations relevant to acute nephropathy affecting especially the tubular compartment. Literature reports over a period of three decades have indicated that there are multiple types of RCDs, including apoptosis, necroptosis, pyroptosis, ferroptosis, and mitochondrial permeability transition-mediated necrosis, and some of them are pertinent to the pathogenesis of cisplatin-induced AKI. Interestingly, myo-inositol metabolism, a vital biological process that is largely restricted to the kidney, seems to be relevant to the pathogenesis of certain forms of RCDs. A comprehensive understanding of RCDs in cisplatin-induced AKI and their relevance to myo-inositol homeostasis may yield novel therapeutic targets for the amelioration of cisplatin-related nephropathy.

Ciclopirox olamine induces ferritinophagy and reduces cyst burden in polycystic kidney disease

Despite the recent launch of tolvaptan, the search for safer polycystic kidney disease (PKD) drugs continues. Ciclopirox (CPX) or its olamine salt (CPX-O) is contained in a number of commercially available antifungal agents. CPX is also reported to possess anticancer activity. Several mechanisms of action have been proposed, including chelation of iron and inhibition of iron-dependent enzymes. Here, we show that CPX-O inhibited in vitro cystogenesis of primary human PKD cyst-lining epithelial cells cultured in a 3D collagen matrix. To assess the in vivo role of CPX-O, we treated PKD mice with CPX-O. CPX-O reduced the kidney-to-body weight ratios of PKD mice. The CPX-O treatment was also associated with decreased cell proliferation, decreased cystic area, and improved renal function. Ferritin levels were markedly elevated in cystic kidneys of PKD mice, and CPX-O treatment reduced renal ferritin levels. The reduction in ferritin was associated with increased ferritinophagy marker nuclear receptor coactivator 4, which reversed upon CPX-O treatment in PKD mice. Interestingly, these effects on ferritin appeared independent of iron. These data suggest that CPX-O can induce ferritin degradation via ferritinophagy, which is associated with decreased cyst growth progression in PKD mice. Most importantly these data indicate that CPX-O has the potential to treat autosomal dominant PKD.

UAB-UCSD O'Brien Center for Acute Kidney Injury Research

Acute kidney injury (AKI) remains a significant clinical problem through its diverse etiologies, the challenges of robust measurements of injury and recovery, and its progression to chronic kidney disease (CKD). Bridging the gap in our knowledge of this disorder requires bringing together not only the technical resources for research but also the investigators currently endeavoring to expand our knowledge and those who might bring novel ideas and expertise to this important challenge. The University of Alabama at Birmingham-University of California-San Diego O'Brien Center for Acute Kidney Injury Research brings together technical expertise and programmatic and educational efforts to advance our knowledge in these diverse issues and the required infrastructure to develop areas of novel exploration. Since its inception in 2008, this O'Brien Center has grown its impact by providing state-of-the-art resources in clinical and preclinical modeling of AKI, a bioanalytical core that facilitates measurement of critical biomarkers, including serum creatinine via LC-MS/MS among others, and a biostatistical resource that assists from design to analysis. Through these core resources and with additional educational efforts, our center has grown its investigator base to include >200 members from 51 institutions. Importantly, this center has translated its pilot and catalyst funding program with a $37 return per dollar invested. Over 500 publications have resulted from the support provided with a relative citation ratio of 2.18 ± 0.12 (iCite). Through its efforts, this disease-centric O'Brien Center is providing the infrastructure and focus to help the development of the next generation of researchers in the basic and clinical science of AKI. This center creates the promise of the application at the bedside of the advances in AKI made by current and future investigators.

Deletion of ferritin H in neurons counteracts the protective effect of melatonin against traumatic brain injury-induced ferroptosis

Accumulating evidence demonstrates that ferroptosis may be important in the pathophysiological process of traumatic brain injury (TBI). As a major hormone of the pineal gland, melatonin exerts many beneficial effects on TBI, but there is no information regarding the effects of melatonin on ferroptosis after TBI. As expected, TBI resulted in the time-course changes of ferroptosis-related molecules expression and iron accumulation in the ipsilateral cortex. Importantly, we found that treating with melatonin potently rescued TBI induced the changes mentioned above and improved functional deficits versus vehicle. Similar results were obtained with a ferroptosis inhibitor, liproxstatin-1. Moreover, the protective effect of melatonin is likely dependent on melatonin receptor 1B (MT2). Although ferritin plays a vital role in iron metabolism by storing excess cellular iron, its precise function in the brain, and whether it involves melatonin's neuroprotection remain unexplored. Considering ferritin H (Fth) is expressed predominantly in the neurons and global loss of Fth in mice induces early embryonic lethality, we then generated neuron-specific Fth conditional knockout (Fth-KO) mice, which are viable and fertile but have altered iron metabolism. In addition, Fth-KO mice were more susceptible to ferroptosis after TBI, and the neuroprotection by melatonin was largely abolished in Fth-KO mice. In vitro siFth experiments further confirmed the results mentioned above. Taken together, these data indicate that melatonin produces cerebroprotection, at least partly by inhibiting neuronal Fth-mediated ferroptosis following TBI, supporting the notion that melatonin is an excellent ferroptosis inhibitor and its anti-ferroptosis provides a potential therapeutic target for treating TBI.

Heme Oxygenase 1: A Defensive Mediator in Kidney Diseases

The incidence of kidney disease is rising, constituting a significant burden on the healthcare system and making identification of new therapeutic targets increasingly urgent. The heme oxygenase (HO) system performs an important function in the regulation of oxidative stress and inflammation and, via these mechanisms, is thought to play a role in the prevention of non-specific injuries following acute renal failure or resulting from chronic kidney disease. The expression of HO-1 is strongly inducible by a wide range of stimuli in the kidney, consequent to the kidney's filtration role which means HO-1 is exposed to a wide range of endogenous and exogenous molecules, and it has been shown to be protective in a variety of nephropathological animal models. Interestingly, the positive effect of HO-1 occurs in both hemolysis- and rhabdomyolysis-dominated diseases, where the kidney is extensively exposed to heme (a major HO-1 inducer), as well as in non-heme-dependent diseases such as hypertension, diabetic nephropathy or progression to end-stage renal disease. This highlights the complexity of HO-1's functions, which is also illustrated by the fact that, despite the abundance of preclinical data, no drug targeting HO-1 has so far been translated into clinical use. The objective of this review is to assess current knowledge relating HO-1's role in the kidney and its potential interest as a nephroprotection agent. The potential therapeutic openings will be presented, in particular through the identification of clinical trials targeting this enzyme or its products.

Ferroptosis and its emerging roles in cardiovascular diseases

Ferroptosis is a new form of regulated cell death (RCD) driven by iron-dependent lipid peroxidation, which is morphologically and mechanistically distinct from other forms of RCD including apoptosis, autophagic cell death, pyroptosis and necroptosis. Recently, ferroptosis has been found to participate in the development of various cardiovascular diseases (CVDs) including doxorubicin-induced cardiotoxicity, ischemia/reperfusion-induced cardiomyopathy, heart failure, aortic dissection and stroke. Cardiovascular homeostasis is indulged in delicate equilibrium of assorted cell types composing the heart or vessels, and how ferroptosis contributes to the pathophysiological responses in CVD progression is unclear. Herein, we reviewed recent discoveries on the basis of ferroptosis and its involvement in CVD pathogenesis, together with related therapeutic potentials, aiming to provide insights on fundamental mechanisms of ferroptosis and implications in CVDs and associated disorders.

New insights into the role of heme oxygenase-1 in acute kidney injury

Acute kidney injury (AKI) is attended by injury-related biomarkers appearing in the urine and serum, decreased urine output, and impaired glomerular filtration rate. AKI causes increased morbidity and mortality and can progress to chronic kidney disease and end-stage kidney failure. AKI is without specific therapies and is managed by supported care. Heme oxygenase-1 (HO-1) is a cytoprotective, inducible enzyme that degrades toxic free heme released from destabilized heme proteins and, during this process, releases beneficial by-products such as carbon monoxide and biliverdin/bilirubin and promotes ferritin synthesis. HO-1 induction protects against assorted renal insults as demonstrated by in vitro and preclinical models. This review summarizes the advances in understanding of the protection conferred by HO-1 in AKI, how HO-1 can be induced including via its transcription factor Nrf2, and HO-1 induction as a therapeutic strategy.

Red blood cell exosome hemoglobin content increases after cardiopulmonary bypass and mediates acute kidney injury in an animal model

OBJECTIVE

Hemolysis, characterized by formation of free hemoglobin (Hb), occurs in patients undergoing cardiopulmonary bypass (CPB). However, there is no study of the dynamic changes in red blood cell (RBC)-derived exosomes (Exos) released during CPB, nor whether these particles mediate acute kidney injury (AKI).

METHODS

This study is a comprehensive time-course analysis, at baseline, 30 minutes, to 24 hours post-crossclamp release (XCR) to determine (1) Exos Hb content; (2) free Hb/heme, haptoglobin, hemopexin; and (3) urinary markers of AKI over the same time period. In addition, we developed a model system in Sprague-Dawley rats to test for AKI after intravenous injection of Exos Hb released during CPB.

RESULTS

In 30 patients undergoing CPB, there is a significant increase in plasma Hb-positive Exos but not microvesicles 30 minutes post-XCR versus other time points, with a simultaneous decrease in the haptoglobin/Hb ratio. These changes presage a significant increase in urine neutrophil gelatinase-associated lipocalin and kidney injury molecule-1 at 24 hours. Intravenous injection of plasma Exos (10^9-10 particles obtained 30 minutes post-XCR) into rats causes AKI at 72 hours, manifested by multifocal degeneration of proximal tubular epithelium. At 21 days, there is persistent tubular injury and interstitial fibrosis. Intravenous injection of Exos from 35-day-old stored RBCs into rats results in glomerular-tubular injury, increased kidney ferritin and hemoxygenase-1 expression, and significant elevation of kidney injury molecule-1 and proteinuria at 72 hours.

CONCLUSIONS

These combined studies raise the potential for RBC-derived Exos, released during CPB, to target the kidney and mediate AKI.

Nrf-2 as a therapeutic target in acute kidney injury

Multifaceted cellular pathways exhibit a crucial role in the preservation of homeostasis at the molecular, cellular, and organism levels. One of the most important of these protective cascades is Nuclear factor E2-related factor (Nrf-2) that regulates the expression of several genes responsible for cellular detoxification, antioxidant function, anti-inflammation, drug/xenobiotic transportation, and stress-related factors. A growing body of evidence provides information regarding the protective role of Nrf-2 against a number of kidney diseases. Acute kidney injury (AKI) is a substantial clinical problem that causes a huge social burden. In the kidneys, Nrf-2 exerts a dynamic role in improving the injury triggered by inflammation and oxidative stress. Understanding of the exact molecular mechanisms underlying AKI is vital in order to determine the equilibrium between renal adaptation and malfunction and thus reduce disease progression. This review highlights the role of Nrf-2 targeting against AKI and provides evidence that targeting Nrf-2 to prevail oxidative damage and its consequences might exhibit protective effects in kidney diseases.

Metabolic profiling during malaria reveals the role of the aryl hydrocarbon receptor in regulating kidney injury

Systemic metabolic reprogramming induced by infection exerts profound, pathogen-specific effects on infection outcome. Here, we detail the host immune and metabolic response during sickness and recovery in a mouse model of malaria. We describe extensive alterations in metabolism during acute infection, and identify increases in host-derived metabolites that signal through the aryl hydrocarbon receptor (AHR), a transcription factor with immunomodulatory functions. We find that Ahr^-/- mice are more susceptible to malaria and develop high plasma heme and acute kidney injury. This phenotype is dependent on AHR in Tek-expressing radioresistant cells. Our findings identify a role for AHR in limiting tissue damage during malaria. Furthermore, this work demonstrates the critical role of host metabolism in surviving infection.

Iron, ferroptosis, and new insights for prevention in acute kidney injury

Acute kidney injury (AKI) is a very common condition with high morbidity and mortality, which can be seen in 5-7% of all hospitalized patients and in up to 57% of all intensive care unit admissions. Despite recent advances in clinical care, the prevalence of AKI has been shown to increase with virtually no change in mortality. AKI is a complex syndrome occurring in a variety of clinical settings. Early detection is crucial to prevent irreversible loss of renal function. The pathogenesis of AKI is highly multifactorial and complex, including vasoconstriction, reactive oxygen species formation, cell death, abnormal immune modulators and growth factors. Emerging evidence from both human and animal studies suggests that dysregulation of iron metabolism may play a potentially important role in AKI. Therefore, targeting the iron homeostasis may provide a new therapeutic intervention for AKI. New therapeutic strategies including iron chelation therapy, targeting iron metabolism related proteins and direct inhibitors of ferroptosis are imperative to improve the outcomes of patients. Taking into consideration the complexity of AKI, one intervention may not be enough for therapeutic success. Future preclinical studies in animal disease models followed by well-designed clinical trials should be conducted to extend findings from animal AKI models to humans.

Deletion of H-ferritin in macrophages alleviates obesity and diabetes induced by high-fat diet in mice

AIMS/HYPOTHESIS

Iron accumulation affects obesity and diabetes, both of which are ameliorated by iron reduction. Ferritin, an iron-storage protein, plays a crucial role in iron metabolism. H-ferritin exerts its cytoprotective action by reducing toxicity via its ferroxidase activity. We investigated the role of macrophage H-ferritin in obesity and diabetes.

METHODS

Conditional macrophage-specific H-ferritin (Fth, also known as Fth1) knockout (LysM-Cre Fth KO) mice were used and divided into four groups: wild-type (WT) and LysM-Cre Fth KO mice with normal diet (ND), and WT and LysM-Cre Fth KO mice with high-fat diet (HFD). These mice were analysed for characteristics of obesity and diabetes, tissue iron content, inflammation, oxidative stress, insulin sensitivity and metabolic measurements. RAW264.7 macrophage cells were used for in vitro experiments.

RESULTS

Iron concentration reduced, and mRNA expression of ferroportin increased, in macrophages from LysM-Cre Fth KO mice. HFD-induced obesity was lower in LysM-Cre Fth KO mice than in WT mice at 12 weeks (body weight: KO 34.6 ± 5.6 g vs WT 40.1 ± 5.2 g). mRNA expression of inflammatory cytokines and infiltrated macrophages and oxidative stress increased in the adipose tissue of HFD-fed WT mice, but was not elevated in HFD-fed LysM-Cre Fth KO mice. However, WT mice fed an HFD had elevated iron concentration in adipose tissue and spleen, which was not observed in LysM-Cre Fth KO mice fed an HFD (adipose tissue [μmol Fe/g protein]: KO 1496 ± 479 vs WT 2316 ± 866; spleen [μmol Fe/g protein]: KO 218 ± 54 vs WT 334 ± 83). Moreover, HFD administration impaired both glucose tolerance and insulin sensitivity in WT mice, which was ameliorated in LysM-Cre Fth KO mice. In addition, energy expenditure, mRNA expression of thermogenic genes, and body temperature were higher in KO mice with HFD than WT mice with HFD. In vitro experiments showed that iron content was reduced, and lipopolysaccharide-induced Tnf-α (also known as Tnf) mRNA upregulation was inhibited in a macrophage cell line transfected with Fth siRNA.

CONCLUSIONS/INTERPRETATION

Deletion of macrophage H-ferritin suppresses the inflammatory response by reducing intracellular iron levels, resulting in the prevention of HFD-induced obesity and diabetes. The findings from this study highlight macrophage iron levels as a potential therapeutic target for obesity and diabetes.

Hydrogen sulfide inhibits aortic valve calcification in heart via regulating RUNX2 by NF-κB, a link between inflammation and mineralization

Introduction

Hydrogen sulfide (H₂S) was revealed to inhibit aortic valve calcification and inflammation was implicated in the pathogenesis of calcific aortic valve disease (CAVD).

Objectives

We investigate whether H₂S inhibits mineralization via abolishing inflammation.

Methods and results

Expression of pro-inflammatory cytokines, interleukin-1β (IL-1β) and tumor necrosis factor α (TNF-α) were increased in patients with CAVD and in calcified aortic valve of ApoE-/- mice. Administration of H_{2 2}S releasing donor (4-methoxyphenyl piperidinylphosphinodithioc acid (AP72)) exhibited inhibition on both calcification and inflammation in aortic valve of apolipoprotein E knockout mice (ApoE-/-) mice is reflected by lowering IL-1β and TNF-α levels. Accordingly, AP72 prevented the accumulation of extracellular calcium deposition and decreased nuclear translocation of nuclear factor-κB (NF-κB) in human valvular interstitial cells (VIC). This was also accompanied by reduced cytokine response. Double-silencing of endogenous H₂S producing enzymes, Cystathionine gamma-lyase (CSE) and Cystathionine beta-synthase (CBS) in VIC exerted enhanced mineralization and higher levels of IL-1β and TNF-α. Importantly, silencing NF-κB gene or its pharmacological inhibition prevented nuclear translocation of runt-related transcription factor 2 (Runx2) and subsequently the calcification of human VIC. Increased levels of NF-κB and Runx2 and their nuclear accumulation occurred in ApoE-/- mice with a high-fat diet. Administration of AP72 decreased the expression of NF-κB and prevented its nuclear translocation in VIC of ApoE-/- mice on a high-fat diet, and that was accompanied by a lowered pro-inflammatory cytokine level. Similarly, activation of Runx2 did not occur in VIC of ApoE-/- mice treated with H₂S donor. Employing Stimulated Emission Depletion (STED) nanoscopy, a strong colocalization of NF-κB and Runx2 was detected during the progression of valvular calcification.

Conclusions

Hydrogen sulfide inhibits inflammation and calcification of aortic valve. Our study suggests that the regulation of Runx2 by hydrogen sulfide (CSE/CBS) occurs via NF-κB establishing a link between inflammation and mineralization in vascular calcification.

Myo-inositol oxygenase expression profile modulates pathogenic ferroptosis in the renal proximal tubule

Overexpression of myo-inositol oxygenase (MIOX), a proximal tubular enzyme, exacerbates cellular redox injury in acute kidney injury (AKI). Ferroptosis, a newly coined term associated with lipid hydroperoxidation, plays a critical role in the pathogenesis of AKI. Whether or not MIOX exacerbates tubular damage by accelerating ferroptosis in cisplatin-induced AKI remains elusive. Cisplatin-treated HK-2 cells exhibited notable cell death, which was reduced by ferroptosis inhibitors. Also, alterations in various ferroptosis metabolic sensors, including lipid hydroperoxidation, glutathione peroxidase 4 (GPX4) activity, NADPH and reduced glutathione (GSH) levels, and ferritinophagy, were observed. These perturbations were accentuated by MIOX overexpression, while ameliorated by MIOX knockdown. Likewise, cisplatin-treated CD1 mice exhibited tubular damage and derangement of renal physiological parameters, which were alleviated by ferrostatin-1, a ferroptosis inhibitor. To investigate the relevance of MIOX to ferroptosis, WT mice, MIOX-overexpressing transgenic (MIOX-Tg) mice, and MIOX-KO mice were subjected to cisplatin treatment. In comparison with cisplatin-treated WT mice, cisplatin-treated MIOX-Tg mice had more severe renal pathological changes and perturbations in ferroptosis metabolic sensors, which were minimal in cisplatin-treated MIOX-KO mice. In conclusion, these findings indicate that ferroptosis, an integral process in the pathogenesis of cisplatin-induced AKI, is modulated by the expression profile of MIOX.

Loss of Cardiac Ferritin H Facilitates Cardiomyopathy via Slc7a11-Mediated Ferroptosis

RATIONALE

Maintaining iron homeostasis is essential for proper cardiac function. Both iron deficiency and iron overload are associated with cardiomyopathy and heart failure via complex mechanisms. Although ferritin plays a central role in iron metabolism by storing excess cellular iron, the molecular function of ferritin in cardiomyocytes remains unknown.

OBJECTIVE

To characterize the functional role of Fth (ferritin H) in mediating cardiac iron homeostasis and heart disease.

METHODS AND RESULTS

Mice expressing a conditional Fth knockout allele were crossed with 2 distinct Cre recombinase-expressing mouse lines, resulting in offspring that lack Fth expression specifically in myocytes (MCK-Cre) or cardiomyocytes (Myh6-Cre). Mice lacking Fth in cardiomyocytes had decreased cardiac iron levels and increased oxidative stress, resulting in mild cardiac injury upon aging. However, feeding these mice a high-iron diet caused severe cardiac injury and hypertrophic cardiomyopathy, with molecular features typical of ferroptosis, including reduced glutathione (GSH) levels and increased lipid peroxidation. Ferrostatin-1, a specific inhibitor of ferroptosis, rescued this phenotype, supporting the notion that ferroptosis plays a pathophysiological role in the heart. Finally, we found that Fth-deficient cardiomyocytes have reduced expression of the ferroptosis regulator Slc7a11, and overexpressing Slc7a11 selectively in cardiomyocytes increased GSH levels and prevented cardiac ferroptosis.

CONCLUSIONS

Our findings provide compelling evidence that ferritin plays a major role in protecting against cardiac ferroptosis and subsequent heart failure, thereby providing a possible new therapeutic target for patients at risk of developing cardiomyopathy.

A Pharmacologic "Stress Test" for Assessing Select Antioxidant Defenses in Patients with CKD

BACKGROUND AND OBJECTIVES

Oxidative stress is a hallmark and mediator of CKD. Diminished antioxidant defenses are thought to be partly responsible. However, there is currently no way to prospectively assess antioxidant defenses in humans. Tin protoporphyrin (SnPP) induces mild, transient oxidant stress in mice, triggering increased expression of select antioxidant proteins (e.g., heme oxygenase 1 [HO-1], NAD[P]H dehydrogenase [quinone] 1 [NQO1], ferritin, p21). Hence, we tested the hypothesis that SnPP can also variably increase these proteins in humans and can thus serve as a pharmacologic "stress test" for gauging gene responsiveness and antioxidant reserves.

DESIGN

, setting, participants, & measurementsA total of 18 healthy volunteers and 24 participants with stage 3 CKD (n=12; eGFR 30-59 ml/min per 1.73 m²) or stage 4 CKD (n=12; eGFR 15-29 ml/min per 1.73 m²) were injected once with SnPP (9, 27, or 90 mg). Plasma and/or urinary antioxidant proteins were measured at baseline and for up to 4 days post-SnPP dosing. Kidney safety was gauged by serial measurements of BUN, creatinine, eGFR, albuminuria, and four urinary AKI biomarkers (kidney injury molecule 1, neutrophil gelatinase-associated lipocalin, cystatin C, and N-acetyl glucosaminidase).

RESULTS

Plasma HO-1, ferritin, p21, and NQO1 were all elevated at baseline in CKD participants. Plasma HO-1 and urine NQO1 levels each inversely correlated with eGFR (r=-0.85 to -0.95). All four proteins manifested statistically significant dose- and time-dependent elevations after SnPP injection. However, marked intersubject differences were observed. p21 responses to high-dose SnPP and HO-1 responses to low-dose SnPP were significantly suppressed in participants with CKD versus healthy volunteers. SnPP was well tolerated by all participants, and no evidence of nephrotoxicity was observed.

CONCLUSIONS

SnPP can be safely administered and, after its injection, the resulting changes in plasma HO-1, NQO1, ferritin, and p21 concentrations can provide information as to antioxidant gene responsiveness/reserves in subjects with and without kidney disease.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

A Study with RBT-1, in Healthy Volunteers and Subjects with Stage 3-4 Chronic Kidney Disease, NCT0363002 and NCT03893799.

Antitumour and Toxicity Evaluation of a Ru(II)-Cyclopentadienyl Complex in a Prostate Cancer Model by Imaging Tools

BACKGROUND

Ruthenium complexes have been extensively investigated for their prospective value as alternatives to cisplatin. Recently, we reported the in vitro anticancer properties of a family of organometallic ruthenium( II)-cyclopentadienyl complexes and have explored their mechanism of action.

OBJECTIVE

The purpose of this study was to evaluate the in vivo antitumour efficacy and toxicity of one of these Ru(II) compounds, [RuCp(mTPPMSNa)(2,2'-bipy)][CF3SO2] (TM85) which displayed an interesting spectrum of activity against several cancer cells.

METHODS

Studies to assess the antitumour activity and toxicity were performed in a metastatic prostate (PC3) mice model using ICP-MS, nuclear microscopy, elemental analysis and Transmission Electron Microscopy (TEM).

RESULTS

TM85 showed low systemic toxicity but no significant tumour reduction, when administered at tolerated dose (20mg/kg) over 10 days. Ru was mainly retained in the liver and less in kidneys, with low accumulation in tumour. Increased bilirubin levels, anomalous Ca and Fe concentrations in liver and mitochondria alterations were indicative of liver injury. The hepatotoxicity observed was less severe than that of cisplatin and no nephrotoxicity was found.

CONCLUSION

Under the experimental conditions of this study, TM85 is less toxic than cisplatin, induces similar tumour reduction and avoids the formation of metastatic foci. No renal toxicity was observed by the analysis of creatinine levels and the effective renal plasma flow by 99mTc-MAG3 clearance. Hence, it can be considered a valuable compound for further studies in the field of Ru-based anticancer drugs.

Ferritins in Kidney Disease

Ferritins are evolutionarily conserved proteins that regulate cellular iron metabolism. It is the only intracellular protein that is capable of storing large quantities of iron. Although the ratio of different subunits determines the iron content of each ferritin molecule, the exact mechanism that dictates organization of these subunits still is unclear. In this review, we address renal ferritin expression and its implication in kidney disease. Specifically, we address the role of ferritin subunits in preventing kidney injury and also promoting tolerance against infection-associated kidney injury. We describe functions for ferritin that are independent of its ability to ferroxidize and store iron. We further discuss the implications of ferritin in body fluids, including blood and urine, during inflammation and kidney disease. Although there are several in-depth review articles on ferritin in the context of iron metabolism, we chose to focus on the role of ferritin particularly in kidney health and disease and highlight unanswered questions in the field.

Modulation of iron homeostasis with hepcidin ameliorates spontaneous murine lupus nephritis

Lupus nephritis is the end organ manifestation of systemic lupus erythematosus. Iron metabolism and its master regulator, hepcidin, are known to regulate cell proliferation and inflammation, but their direct role in the pathophysiology of lupus nephritis remains under-investigated. Exogenous hepcidin reduced the severity of lupus nephritis in MRL/lpr mice, a preclinical model of spontaneous systemic lupus erythematosus without worsening anemia of inflammation. Hepcidin treatment reduced renal iron accumulation, systemic and intrarenal cytokines, and renal immune cell infiltration, independent of glomerular immune complex deposits and circulating autoantibodies. Hepcidin increased renal H-ferritin (a ferroxidase), reduced expression of free iron dependent DNA synthesis enzymes, Ribonucleotide Reductase 1 and 2, and intra-renal macrophage proliferation. These findings were recapitulated in vitro upon treatment of macrophages with hepcidin and murine colony stimulation factor-1. Furthermore, hepcidin-treated macrophages secreted less IL-1β and IL-6 upon stimulation with the TLR3 agonist polyinosine-polycytidylic acid. Of clinical relevance, hepcidin reduced progression and severity of nephritis in old mice with established systemic autoimmunity and overt proteinuria, highlighting its therapeutic potential. Thus, our findings provide a proof-of-concept that targeting cellular iron metabolism with hepcidin represents a promising therapeutic strategy in lupus nephritis.

The Acute Phase Response Is a Prominent Renal Proteome Change in Sepsis in Mice

(1) Background: Sepsis-induced acute kidney injury (AKI) is the most common form of acute kidney injury (AKI). We studied the temporal profile of the sepsis-induced renal proteome changes. (2) Methods: Male mice were injected intraperitoneally with bacterial lipopolysaccharide (LPS) or saline (control). Renal proteome was studied by LC-MS/MS (ProteomeXchange: PXD014664) at the early phase (EP, 1.5 and 6 h after 40 mg/kg LPS) and the late phase (LP, 24 and 48 h after 10 mg/kg LPS) of LPS-induced AKI. Renal mRNA expression of acute phase proteins (APP) was assessed by qPCR. (3) Results: Renal proteome change was milder in EP vs. LP. APPs dominated the proteome in LP (proteins upregulated at least 4-fold (APPs/all): EP, 1.5 h: 0/10, 6 h: 1/10; LP, 24 h: 22/47, 48 h: 17/44). Lipocalin-2, complement C3, fibrinogen, haptoglobin and hemopexin were the most upregulated APPs. Renal mRNA expression preceded the APP changes with peak effects at 24 h, and indicated renal production of the majority of APPs. (4) Conclusions: Gene expression analysis revealed local production of APPs that commenced a few hours post injection and peaked at 24 h. This is the first demonstration of a massive, complex and coordinated acute phase response of the kidney involving several proteins not identified previously.

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.

Mechanisms and consequences of oxidant-induced renal preconditioning: an Nrf2-dependent, P21-independent, anti-senescence pathway

Background

P21, a cyclin kinase inhibitor, is upregulated by renal 'ischemic preconditioning' (IPC), and induces a 'cytoresistant' state. However, P21-induced cell cycle inhibition can also contribute to cellular senescence, a potential adverse renal event. Hence, this study assessed whether: (i) IPC-induced P21 upregulation is associated with subsequent renal senescence; and (ii) preconditioning can be established 'independent' of P21 induction and avoid a post-ischemic senescent state?

Methods

CD-1 mice were subjected to either IPC (5-15 min) or to a recently proposed 'oxidant-induced preconditioning' (OIP) strategy (tin protoporphyrin-induced heme oxygenase inhibition +/- parental iron administration). P21 induction [messenger RNA (mRNA)/protein], cell proliferation (KI-67, phosphohistone H3 nuclear staining), kidney senescence (P16ink4a; P19Arf mRNAs; senescence-associated beta-galactosidase levels) and resistance to ischemic acute kidney injury were assessed.

Results

IPC induced dramatic (10-25×) and persistent P21 activation and 'downstream' tubular senescence. Conversely, OIP did not upregulate P21, it increased, rather than decreased, cell proliferation markers, and it avoided a senescence state. OIP markedly suppressed ischemia-induced P21 up-regulation, it inhibited the development of post-ischemic senescence and it conferred near-complete protection against ischemic acute renal failure (ARF). To assess OIP's impact on a non-P21-dependent cytoprotective pathway, its ability to activate Nrf2, the so-called 'master regulator' of endogenous cell defenses, was assessed. Within 4 h, OIP activated each of three canonical Nrf2-regulated genes (NQO1, SRXN1, GCLC; 3- to 5-fold mRNA increases). Conversely, this gene activation pathway was absent in Nrf2-/- mice, confirming Nrf2 specificity. Nrf2-/- mice also did not develop significant OIP-mediated protection against ischemic ARF.

Conclusions

OIP (i) activates the cytoprotective Nrf2, but not the P21, pathway; (ii) suppresses post-ischemic P21 induction and renal senescence; and (iii) confers marked protection against ischemic ARF. In sum, these findings suggest that OIP may be a clinically feasible approach for safely activating the Nrf2 pathway, and thereby confer protection against clinical renal injury.

Parenterial iron sucrose-induced renal preconditioning: differential ferritin heavy and light chain expression in plasma, urine, and internal organs

Experimental data suggest that iron sucrose (FeS) injection, used either alone or in combination with other prooxidants, can induce "renal preconditioning," in part by upregulating cytoprotective ferritin levels. However, the rapidity, degree, composition (heavy vs. light chain), and renal ferritin changes after FeS administration in humans remain to be defined. To address these issues, healthy human volunteers (n = 9) and patients with stage 3-4 chronic kidney disease(n = 9) were injected once with FeS (120, 240, or 360 mg). Plasma ferritin was measured from 0 to 8 days postinjection as an overall index of ferritin generation. Urinary ferritin served as a "biomarker" of renal ferritin production. FeS induced rapid (≤2 h), dose-dependent, plasma ferritin increases in all study participants, peaking at approximately three to five times baseline within 24-48 h. Significant urinary ferritin increases (~3 times), without dose-dependent increases in albuminuria, neutrophil gelatinase-associated lipocalin, or N-acetyl-β-d-glucosaminidase excretion, were observed. Western blot analysis with ferritin heavy chain (Fhc)- and light chain (Flc)-specific antibodies demonstrated that FeS raised plasma Flc but not Fhc levels. Conversely, FeS increased both Fhc and Flc in urine. To assess sites of FeS-induced ferritin generation, organs from FeS-treated mice were probed for Fhc, Flc, and their mRNAs. FeS predominantly raised hepatic Flc. Conversely, marked Fhc and Flc elevations developed in the kidney and spleen. No cardiopulmonary ferritin increases occurred. Ferritin mRNAs remained unchanged throughout, implying posttranscriptional ferritin production. We conclude that FeS induces rapid, dramatic, and differential Fhc and Flc upregulation in organs. Renal Fhc and Flc increases, in the absence of nephrotoxicity, suggest potential FeS utility as a clinical renal "preconditioning" agent.

Iron Chelation as a Potential Therapeutic Strategy for AKI Prevention

AKI remains a major public health concern. Despite years of investigation, no intervention has been demonstrated to reliably prevent AKI in humans. Thus, development of novel therapeutic targets is urgently needed. An important role of iron in the pathophysiology of AKI has been recognized for over three decades. When present in excess and in nonphysiologic labile forms, iron is toxic to the kidneys and multiple other organs, whereas iron chelation is protective across a broad spectrum of insults. In humans, small studies have investigated iron chelation as a novel therapeutic strategy for prevention of AKI and extrarenal acute organ injury, and have demonstrated encouraging initial results. In this review, we examine the existing data on iron chelation for AKI prevention in both animal models and human studies. We discuss practical considerations for future clinical trials of AKI prevention using iron chelators, including selection of the ideal clinical setting, patient population, iron chelating agent, and dosing regimen. Finally, we compare the key differences among the currently available iron chelators, including pharmacokinetics, routes of administration, and adverse effects.

The multifaceted role of iron in renal health and disease

Iron is an essential element that is indispensable for life. The delicate physiological body iron balance is maintained by both systemic and cellular regulatory mechanisms. The iron-regulatory hormone hepcidin assures maintenance of adequate systemic iron levels and is regulated by circulating and stored iron levels, inflammation and erythropoiesis. The kidney has an important role in preventing iron loss from the body by means of reabsorption. Cellular iron levels are dependent on iron import, storage, utilization and export, which are mainly regulated by the iron response element-iron regulatory protein (IRE-IRP) system. In the kidney, iron transport mechanisms independent of the IRE-IRP system have been identified, suggesting additional mechanisms for iron handling in this organ. Yet, knowledge gaps on renal iron handling remain in terms of redundancy in transport mechanisms, the roles of the different tubular segments and related regulatory processes. Disturbances in cellular and systemic iron balance are recognized as causes and consequences of kidney injury. Consequently, iron metabolism has become a focus for novel therapeutic interventions for acute kidney injury and chronic kidney disease, which has fuelled interest in the molecular mechanisms of renal iron handling and renal injury, as well as the complex dynamics between systemic and local cellular iron regulation.

Dynamic signature of lymphangiogenesis during acute kidney injury and chronic kidney disease

Acute kidney injury (AKI) and chronic kidney disease (CKD) are interconnected syndromes with significant attributable morbidity and mortality. The disturbing trend of increasing incidence and prevalence of these clinical disorders highlights the urgent need for better understanding of the underlying mechanisms that are involved in pathogenesis of these conditions. Lymphangiogenesis and its involvement in various inflammatory conditions is increasingly recognized while its role in AKI and CKD remains to be fully elucidated. Here, we studied lymphangiogenesis in three models of kidney injury. Our results demonstrate that the main ligands for lymphangiogenesis, VEGF-C and VEGF-D, are abundantly present in tubules at baseline conditions and the expression pattern of these ligands is significantly altered following injury. In addition, we show that both of these ligands increase in serum and urine post-injury and suggest that such increment may serve as novel urinary biomarkers of AKI as well as in progression of kidney disease. We also provide evidence that irrespective of the nature of initial insult, lymphangiogenic pathways are rapidly and robustly induced as evidenced by higher expression of lymphatic markers within the kidney.

Renal Inflammation and Fibrosis: A Double-edged Sword

Renal tissue injury initiates inflammatory and fibrotic processes that occur to promote regeneration and repair. After renal injury, damaged tissue releases cytokines and chemokines, which stimulate activation and infiltration of inflammatory cells to the kidney. Normal tissue repair processes occur simultaneously with activation of myofibroblasts, collagen deposition, and wound healing responses; however, prolonged activation of pro-inflammatory and pro-fibrotic cell types causes excess extracellular matrix deposition. This review focuses on the physiological and pathophysiological roles of specialized cell types, cytokines/chemokines, and growth factors, and their implications in recovery or exacerbation of acute kidney injury.

Mechanisms of haemolysis-induced kidney injury

Intravascular haemolysis is a fundamental feature of chronic hereditary and acquired haemolytic anaemias, including those associated with haemoglobinopathies, complement disorders and infectious diseases such as malaria. Destabilization of red blood cells (RBCs) within the vasculature results in systemic inflammation, vasomotor dysfunction, thrombophilia and proliferative vasculopathy. The haemoprotein scavengers haptoglobin and haemopexin act to limit circulating levels of free haemoglobin, haem and iron - potentially toxic species that are released from injured RBCs. However, these adaptive defence systems can fail owing to ongoing intravascular disintegration of RBCs. Induction of the haem-degrading enzyme haem oxygenase 1 (HO1) - and potentially HO2 - represents a response to, and endogenous defence against, large amounts of cellular haem; however, this system can also become saturated. A frequent adverse consequence of massive and/or chronic haemolysis is kidney injury, which contributes to the morbidity and mortality of chronic haemolytic diseases. Intravascular destruction of RBCs and the resulting accumulation of haemoproteins can induce kidney injury via a number of mechanisms, including oxidative stress and cytotoxicity pathways, through the formation of intratubular casts and through direct as well as indirect proinflammatory effects, the latter via the activation of neutrophils and monocytes. Understanding of the detailed pathophysiology of haemolysis-induced kidney injury offers opportunities for the design and implementation of new therapeutic strategies to counteract the unfavourable and potentially fatal effects of haemolysis on the kidney.

Role of the kidneys in the redistribution of heme-derived iron during neonatal hemolysis in mice

Moderate intravascular hemolysis is a common condition in newborns. It is followed by the accumulation of bilirubin, which is a secondary product of the activity of heme oxygenase-1, an enzyme that catalyzes the breakdown of heme released from disrupted erythrocytes and taken up by hepatic macrophages. Although these cells are a major site of enzymatic heme breakdown in adults, we show here that epithelial cells of proximal tubules in the kidneys perform the functions of both heme uptake and catabolism in mouse neonates. A time-course study examining mouse pups during the neonatal period showed a gradual recovery from hemolysis, and concomitant decreases in the expression of heme-related genes and non-heme iron transporters in the proximal tubules. By adjusting the expression of iron-handling proteins in response to the disappearance of hemolysis in mouse neonates, the kidneys may play a role in the detoxification of iron and contribute to its recirculation from the primary urine to the blood.

HO-1 and CD39: It Takes Two to Protect the Realm

Cellular protective mechanisms exist to ensure survival of the cells and are a fundamental feature of all cells that is necessary for adapting to changes in the environment. Indeed, evolution has ensured that each cell is equipped with multiple overlapping families of genes that safeguard against pathogens, injury, stress, and dysfunctional metabolic processes. Two of the better-known enzymatic systems, conserved through all species, include the heme oxygenases (HO-1/HO-2), and the ectonucleotidases (CD39/73). Each of these systems generates critical bioactive products that regulate the cellular response to a stressor. Absence of these molecules results in the cell being extremely predisposed to collapse and, in most cases, results in the death of the cell. Recent reports have begun to link these two metabolic pathways, and what were once exclusively stand-alone are now being found to be intimately interrelated and do so through their innate ability to generate bioactive products including adenosine, carbon monoxide, and bilirubin. These simple small molecules elicit profound cellular physiologic responses that impact a number of innate immune responses, and participate in the regulation of inflammation and tissue repair. Collectively these enzymes are linked not only because of the mitochondria being the source of their substrates, but perhaps more importantly, because of the impact of their products on specific cellular responses. This review will provide a synopsis of the current state of the field regarding how these systems are linked and how they are now being leveraged as therapeutic modalities in the clinic.

Nrf2 Plays a Protective Role Against Intravascular Hemolysis-Mediated Acute Kidney Injury

Massive intravascular hemolysis is associated with acute kidney injury (AKI). Nuclear factor erythroid-2-related factor 2 (Nrf2) plays a central role in the defense against oxidative stress by activating the expression of antioxidant proteins. We investigated the role of Nrf2 in intravascular hemolysis and whether Nrf2 activation protected against hemoglobin (Hb)/heme-mediated renal damage in vivo and in vitro. We observed renal Nrf2 activation in human hemolysis and in an experimental model of intravascular hemolysis promoted by phenylhydrazine intraperitoneal injection. In wild-type mice, Hb/heme released from intravascular hemolysis promoted AKI, resulting in decreased renal function, enhanced expression of tubular injury markers (KIM-1 and NGAL), oxidative and endoplasmic reticulum stress (ER), and cell death. These features were more severe in Nrf2-deficient mice, which showed decreased expression of Nrf2-related antioxidant enzymes, including heme oxygenase 1 (HO-1) and ferritin. Nrf2 activation with sulforaphane protected against Hb toxicity in mice and cultured tubular epithelial cells, ameliorating renal function and kidney injury and reducing cell stress and death. Nrf2 genotype or sulforaphane treatment did not influence the severity of hemolysis. In conclusion, our study identifies Nrf2 as a key molecule involved in protection against renal damage associated with hemolysis and opens novel therapeutic approaches to prevent renal damage in patients with severe hemolytic crisis. These findings provide new insights into novel aspects of Hb-mediated renal toxicity and may have important therapeutic implications for intravascular hemolysis-related diseases.

Iron Homeostasis in Healthy Kidney and its Role in Acute Kidney Injury

Iron is required for key aspects of cellular physiology including mitochondrial function and DNA synthesis and repair. However, free iron is an aberration because of its ability to donate electrons, reduce oxygen, and generate reactive oxygen species. Iron-mediated cell injury or ferroptosis is a central player in the pathogenesis of acute kidney injury. There are several homeostatic proteins and pathways that maintain critical balance in iron homeostasis to allow iron's biologic functions yet avoid ferroptosis. Hepcidin serves as the master regulator of iron homeostasis through its ability to regulate ferroportin-mediated iron export and intracellular H-ferritin levels. Hepcidin is a protective molecule in acute kidney injury. Drugs targeting hepcidin, H-ferritin, and ferroptosis pathways hold great promise to prevent or treat kidney injury. In this review we discuss iron homeostasis under physiological and pathologic conditions and highlight its importance in acute kidney injury.

Ferritin in Kidney and Vascular Related Diseases: Novel Roles for an Old Player

Iron is at the forefront of a number of pivotal biological processes due to its ability to readily accept and donate electrons. However, this property may also catalyze the generation of free radicals with ensuing cellular and tissue toxicity. Accordingly, throughout evolution numerous pathways and proteins have evolved to minimize the potential hazardous effects of iron cations and yet allow for readily available iron cations in a wide variety of fundamental metabolic processes. One of the extensively studied proteins in the context of systemic and cellular iron metabolisms is ferritin. While clinicians utilize serum ferritin to monitor body iron stores and inflammation, it is important to note that the vast majority of ferritin is located intracellularly. Intracellular ferritin is made of two different subunits (heavy and light chain) and plays an imperative role as a safe iron depot. In the past couple of decades our understanding of ferritin biology has remarkably improved. Additionally, a significant body of evidence has emerged describing the significance of the kidney in iron trafficking and homeostasis. Here, we briefly discuss some of the most important findings that relate to the role of iron and ferritin heavy chain in the context of kidney-related diseases and, in particular, vascular calcification, which is a frequent complication of chronic kidney disease.

Heme oxygenase-1 repeat polymorphism in septic acute kidney injury

Acute kidney injury (AKI) is a syndrome that frequently affects the critically ill. Recently, an increased number of dinucleotide repeats in the HMOX1 gene were reported to associate with development of AKI in cardiac surgery. We aimed to test the replicability of this finding in a Finnish cohort of critically ill septic patients. This multicenter study was part of the national FINNAKI study. We genotyped 300 patients with severe AKI (KDIGO 2 or 3) and 353 controls without AKI (KDIGO 0) for the guanine–thymine (GTn) repeat in the promoter region of the HMOX1 gene. The allele calling was based on the number of repeats, the cut off being 27 repeats in the S–L (short to long) classification, and 27 and 34 repeats for the S–M–L₂ (short to medium to very long) classification. The plasma concentrations of heme oxygenase-1 (HO-1) enzyme were measured on admission. The allele distribution in our patients was similar to that published previously, with peaks at 23 and 30 repeats. The S-allele increases AKI risk. An adjusted OR was 1.30 for each S-allele in an additive genetic model (95% CI 1.01–1.66; p = 0.041). Alleles with a repeat number greater than 34 were significantly associated with lower HO-1 concentration (p<0.001). In septic patients, we report an association between a short repeat in HMOX1 and AKI risk.

Hydrogen Gas Alleviates Chronic Intermittent Hypoxia-Induced Renal Injury through Reducing Iron Overload

Iron-induced oxidative stress has been found to be a central player in the pathogenesis of kidney injury. Recent studies have indicated H₂ can be used as a novel antioxidant to protect cells. The present study was designed to investigate the protective effects of H₂ against chronic intermittent hypoxia (CIH)-induced renal injury and its correlation mechanism involved in iron metabolism. We found that CIH-induced renal iron overloaded along with increased apoptosis and oxidative stress. Iron accumulates mainly occurred in the proximal tubule epithelial cells of rats as showed by Perl’s stain. Moreover, we found that CIH could promote renal transferrin receptor and divalent metal transporter-1 expression, inhibit ceruloplasmin expression. Renal injury, apoptosis and oxidative stress induced by CIH were strikingly attenuated in H₂ treated rats. In conclusion, hydrogen may attenuate CIH-induced renal injury at least partially via inhibiting renal iron overload.

Ferritin regulates organismal energy balance and thermogenesis

OBJECTIVE

The ferritin heavy/heart chain (FTH) gene encodes the ferroxidase component of the iron (Fe) sequestering ferritin complex, which plays a central role in the regulation of cellular Fe metabolism. Here we tested the hypothesis that ferritin regulates organismal Fe metabolism in a manner that impacts energy balance and thermal homeostasis.

METHODS

We developed a mouse strain, referred herein as Fth^R26 fl/fl, expressing a tamoxifen-inducible Cre recombinase under the control of the Rosa26 (R26) promoter and carrying two LoxP (fl) sites: one at the 5'end of the Fth promoter and another the 3' end of the first Fth exon. Tamoxifen administration induces global deletion of Fth in adult Fth^R26Δ/Δ mice, testing whether FTH is required for maintenance of organismal homeostasis.

RESULTS

Under standard nutritional Fe supply, Fth deletion in adult Fth^R26Δ/Δ mice led to a profound deregulation of organismal Fe metabolism, oxidative stress, inflammation, and multi-organ damage, culminating in death. Unexpectedly, Fth deletion was also associated with a profound atrophy of white and brown adipose tissue as well as with collapse of energy expenditure and thermogenesis. This was attributed mechanistically to mitochondrial dysfunction, as assessed in the liver and in adipose tissue.

CONCLUSION

The FTH component of ferritin acts as a master regulator of organismal Fe homeostasis, coupling nutritional Fe supply to organismal redox homeostasis, energy expenditure and thermoregulation.

Renal control of disease tolerance to malaria

Malaria, the disease caused by Plasmodium spp. infection, remains a major global cause of morbidity and mortality. Host protection from malaria relies on immune-driven resistance mechanisms that kill Plasmodium However, these mechanisms are not sufficient per se to avoid the development of severe forms of disease. This is accomplished instead via the establishment of disease tolerance to malaria, a defense strategy that does not target Plasmodium directly. Here we demonstrate that the establishment of disease tolerance to malaria relies on a tissue damage-control mechanism that operates specifically in renal proximal tubule epithelial cells (RPTEC). This protective response relies on the induction of heme oxygenase-1 (HMOX1; HO-1) and ferritin H chain (FTH) via a mechanism that involves the transcription-factor nuclear-factor E2-related factor-2 (NRF2). As it accumulates in plasma and urine during the blood stage of Plasmodium infection, labile heme is detoxified in RPTEC by HO-1 and FTH, preventing the development of acute kidney injury, a clinical hallmark of severe malaria.

Association of urinary ionomic profiles and acute kidney injury and mortality in patients after cardiac surgery

OBJECTIVE

The rarity of sensitive biomarkers for acute kidney injury (AKI) has impeded the timely therapy of AKI. Emerging evidence suggests that ion homeostasis may play pertinent roles in AKI. We aimed to screen out representative urinary ions and build a cardiac surgery-associated AKI indication model.

METHODS

We performed urinary ionomic analysis from patients undergoing cardiac surgeries in Zhongshan Hospital, Fudan University, Shanghai, China (N = 261). By bioinformatics analysis, we identified differentially changed elements and established the AKI indication model we named the urinary ion index (UII). Follow-ups were performed to evaluate 30-day survival.

RESULTS

The concentrations of most ions dynamically changed whether a patient developed AKI or not. A significant number of differentially changed elements between AKI and non-AKI groups were detected, especially at 2 hours after cardiac surgery, based on which we generated UII, with the area under the curve of 0.815 ± 0.006 and a cut-off value of 1.24. UII was associated with need for renal replacement therapy, with an area under the curve of 0.83 at a cutoff value of 1.62. Kaplan-Meier and log-rank methods, as well as Cox proportional hazards model, reflected that patients in the UII > 1.24 group had significantly higher risk of mortality within 30 days after surgery (hazard ratio, 5.15; P = .0097 and hazard ratio, 3.56; P = .033) than the UII ≤ 1.24 group.

CONCLUSIONS

Our data demonstrate that UII appears to be a novel and valid index of early cardiac surgery-associated AKI. UII > 1.24 at 2 hours after surgery indicates high risk of AKI and less 30-day survival.

Potential Role of H-Ferritin in Mitigating Valvular Mineralization

Objective- Calcific aortic valve disease is a prominent finding in elderly and in patients with chronic kidney disease. We investigated the potential role of iron metabolism in the pathogenesis of calcific aortic valve disease. Approach and Results- Cultured valvular interstitial cells of stenotic aortic valve with calcification from patients undergoing valve replacement exhibited significant susceptibility to mineralization/osteoblastic transdifferentiation in response to phosphate. This process was abrogated by iron via induction of H-ferritin as reflected by lowering ALP and osteocalcin secretion and preventing extracellular calcium deposition. Cellular phosphate uptake and accumulation of lysosomal phosphate were decreased. Accordingly, expression of phosphate transporters Pit1 and Pit2 were repressed. Translocation of ferritin into lysosomes occurred with high phosphate-binding capacity. Importantly, ferritin reduced nuclear accumulation of RUNX2 (Runt-related transcription factor 2), and as a reciprocal effect, it enhanced nuclear localization of transcription factor Sox9 (SRY [sex-determining region Y]-box 9). Pyrophosphate generation was also increased via upregulation of ENPP2 (ectonucleotide pyrophosphatase/phosphodiesterase-2). 3H-1, 2-dithiole-3-thione mimicked these beneficial effects in valvular interstitial cell via induction of H-ferritin. Ferroxidase activity of H-ferritin was essential for this function, as ceruloplasmin exhibited similar inhibitory functions. Histological analysis of stenotic aortic valve revealed high expression of H-ferritin without iron accumulation and its relative dominance over ALP in noncalcified regions. Increased expression of H-ferritin accompanied by elevation of TNF-α (tumor necrosis factor-α) and IL-1β (interleukin-1β) levels, inducers of H-ferritin, corroborates the essential role of ferritin/ferroxidase via attenuating inflammation in calcific aortic valve disease. Conclusions- Our results indicate that H-ferritin is a stratagem in mitigating valvular mineralization/osteoblastic differentiation. Utilization of 3H-1, 2-dithiole-3-thione to induce ferritin expression may prove a novel therapeutic potential in valvular mineralization.

Systemic and local hepcidin as emerging and important peptides in renal homeostasis and pathology

Recent data suggest that the importance of hepcidin goes beyond its classical role in controlling systemic iron metabolism. Local hepcidins are emerging as important peptides for organ homeostasis in the brain, heart, blood vessels, and in cancer as well. Similarly, accumulating data indicate that hepcidin does seem to be an important factor in renal homeostasis. This review encompasses present knowledge concerning the role of hepcidin in renoprotection and its use as a biomarker of kidney diseases. Understanding the role of hepcidin in kidneys is important due to its relevance for kidney physiology and its potential therapeutic application in kidney pathologies. © 2018 BioFactors, 45(2):118-134, 2019.

Iron uptake by ZIP8 and ZIP14 in human proximal tubular epithelial cells

In patients with iron overload disorders, increasing number of reports of renal dysfunction and renal iron deposition support an association between increased iron exposure and renal injury. In systemic iron overload, elevated circulating levels of transferrin-bound (TBI) and non-transferrin-bound iron (NTBI) are filtered to the renal proximal tubules, where they may cause injury. However, the mechanisms of tubular iron handling remain elusive. To unravel molecular renal proximal tubular NTBI and TBI handling, human conditionally immortalized proximal tubular epithelial cells (ciPTECs) were incubated with ⁵⁵Fe as NTBI and fluorescently labeled holo-transferrin as TBI. Ferrous iron importers ZIP8 and ZIP14 were localized in the ciPTEC plasma membrane. Whereas silencing of either ZIP8 or ZIP14 alone did not affect ⁵⁵Fe uptake, combined silencing significantly reduced ⁵⁵Fe uptake compared to control (p < 0.05). Furthermore, transferrin receptor 1 (TfR1) and ZIP14, but not ZIP8, colocalized with early endosome antigen 1 (EEA1). TfR1 and ZIP14 also colocalized with uptake of fluorescently labeled transferrin. Furthermore, ZIP14 silencing decreased ⁵⁵Fe uptake after ⁵⁵Fe-Transferrin exposure (p < 0.05), suggesting ZIP14 could be involved in early endosomal transport of TBI-derived iron into the cytosol. Our data suggest that human proximal tubular epithelial cells take up TBI and NTBI, where ZIP8 and ZIP14 are both involved in NTBI uptake, but ZIP14, not ZIP8, mediates TBI-derived iron uptake. This knowledge provides more insights in the mechanisms of renal iron handling and suggests that ZIP8 and ZIP14 could be potential targets for limiting renal iron reabsorption and enhancing urinary iron excretion in systemic iron overload disorders.