Novel cellular defenses against iron and oxidation: ferritin and autophagocytosis preserve lysosomal stability in airway epithelium

Underlying Mechanism of Lysosomal Membrane Permeabilization in CNS Injury: A Literature Review

Lysosomes play a crucial role in various intracellular pathways as their final destination. Various stressors, whether mild or severe, can induce lysosomal membrane permeabilization (LMP), resulting in the release of lysosomal enzymes into the cytoplasm. LMP not only plays a pivotal role in various cellular events but also significantly contributes to programmed cell death (PCD). Previous research has demonstrated the participation of LMP in central nervous system (CNS) injuries, including traumatic brain injury (TBI), spinal cord injury (SCI), subarachnoid hemorrhage (SAH), and hypoxic-ischemic encephalopathy (HIE). However, the mechanisms underlying LMP in CNS injuries are poorly understood. The occurrence of LMP leads to the activation of inflammatory pathways, increased levels of oxidative stress, and PCD. Herein, we present a comprehensive overview of the latest findings regarding LMP and highlight its functions in cellular events and PCDs (lysosome-dependent cell death, apoptosis, pyroptosis, ferroptosis, and autophagy). In addition, we consolidate the most recent insights into LMP in CNS injury by summarizing and exploring the latest advances. We also review potential therapeutic strategies that aim to preserve LMP or inhibit the release of enzymes from lysosomes to alleviate the consequences of LMP in CNS injury. A better understanding of the role that LMP plays in CNS injury may facilitate the development of strategic treatment options for CNS injury.

Serum ferritin as a significant biomarker for patients with idiopathic inflammatory myopathy-associated interstitial lung disease: A systematic review and meta-analysis

OBJECTIVE

The biomarkers for predicting the occurrence, progression, and death of idiopathic inflammatory myopathy-associated interstitial lung disease (IIM-ILD) remain unclear. Serum ferritin (SF) is a potential candidate and this systematic review and meta-analysis aimed to reveal the clinical significance of SF in IIM-ILD.

METHODS

Eligible English studies were selected from PubMed, Embase, Web of science and Scopus up to 9 June 2023. The SF levels in patients with IIM-ILD were extracted and pooled. Subgroup analysis was performed based on disease types, sensitivity analysis was conducted by excluding one class of literature at a time, and publication bias was assessed by funnel plot and Egger's test.

RESULTS

Pooled analysis of 1,933 patients with IIM from 19 studies showed that SF levels were significantly higher in IIM-ILD group (WMD=263.53ng/mL, 95% CI: 146.44-380.62, p<0.001) than IIM without ILD, subgroup analysis showed that SF levels in DM-ILD (WMD = 397.67ng/mL, 95% CI:142.84-652.50, p = 0.002) and PM/DM-ILD (WMD = 117.68 ng/mL, 95% CI: 86.32-149.04, p < 0.001) were significantly higher compared to those without ILD. SF levels were significantly higher in rapidly progressive interstitial lung disease group (RP-ILD)(WMD = 484.99 ng/mL, 95% CI: 211.12-758.87, p= 0.001) than chronic ILD(C-ILD) group, subgroup analysis showed that SF levels in DM-RP-ILD (WMD= 509.75 ng/mL, 95% CI: 215.34-804.16, p=0.001) were significantly higher than those in DM-C-ILD group. SF levels were significantly higher in death group (WMD= 722.16 ng/mL, 95% CI: 572.32-872.00, p< 0.001) compared to the survival group, subgroup analysis showed that death patients with DM-ILD(WMD= 735.62 ng/mL, 95% CI:574.92-896.32, p<0.001) and PM-ILD (WMD= 632.56 ng/mL, 95% CI:217.92-1047.19, p=0.003) had significantly higher SF levels than survival group respectively.

CONCLUSION

Increased SF levels can serve as a biomarker for predicting the occurrence, progression and death of patients with IIM-ILD, which can provide early warning sign for intervention and prognosis evaluation for IIM-ILD patients.

Recent Progress of Ferroptosis in Lung Diseases

Ferroptosis is a new form of programmed cell death due to iron-dependent excess accumulation of lipid peroxides and differs from other programmed cell deaths in morphological and biochemical characteristics. The process of ferroptosis is precisely regulated by iron metabolism, lipid metabolism, amino acid metabolism, and numerous signaling pathways, and plays a complex role in many pathophysiological processes. Recent studies have found that ferroptosis is closely associated with the development and progression of many lung diseases, including acute lung injury, pulmonary ischemia-reperfusion injury, lung cancer, chronic obstructive pulmonary disease, and pulmonary fibrosis. Here, we present a review of the main regulatory mechanisms of ferroptosis and its research progress in the pathogenesis and treatment of lung diseases, with the aim of providing new ideas for basic and clinical research of lung-related diseases.

Autophagy of metallothioneins prevents TNF-induced oxidative stress and toxicity in hepatoma cells

Lysosomal membrane permeabilization (LMP) induced by oxidative stress has recently emerged as a prominent mechanism behind TNF cytotoxicity. This pathway relies on diffusion of hydrogen peroxide into lysosomes containing redox-active iron, accumulated by breakdown of iron-containing proteins and subcellular organelles. Upon oxidative lysosomal damage, LMP allows relocation to the cytoplasm of low mass iron and acidic hydrolases that contribute to DNA and mitochondrial damage, resulting in death by apoptosis or necrosis. Here we investigate the role of lysosomes and free iron in death of HTC cells, a rat hepatoma line, exposed to TNF following metallothionein (MT) upregulation. Iron-binding MT does not normally occur in HTC cells in significant amounts. Intracellular iron chelation attenuates TNF and cycloheximide (CHX)-induced LMP and cell death, demonstrating the critical role of this transition metal in mediating cytokine lethality. MT upregulation, combined with starvation-activated MT autophagy almost completely suppresses TNF and CHX toxicity, while impairment of both autophagy and MT upregulation by silencing of Atg7, and Mt1a and/or Mt2a, respectively, abrogates protection. Interestingly, MT upregulation by itself has little effect, while stimulated autophagy alone depresses cytokine toxicity to some degree. These results provide evidence that intralysosomal iron-catalyzed redox reactions play a key role in TNF and CHX-induced LMP and toxicity. The finding that chelation of intralysosomal iron achieved by autophagic delivery of MT, and to some degree probably of other iron-binding proteins as well, into the lysosomal compartment is highly protective provides a putative mechanism to explain autophagy-related suppression of death by TNF and CHX.

Toxic Effects of the Major Components of Diesel Exhaust in Human Alveolar Basal Epithelial Cells (A549)

We investigated the toxicity of benzo[a]pyrene (B[a]P), 1-nitropyrene (1-NP) and 3-nitrobenzanthrone (3-NBA) in A549 cells. Cells were treated for 4 h and 24 h with: B[a]P (0.1 and 1 μM), 1-NP (1 and 10 μM) and 3-NBA (0.5 and 5 μM). Bulky DNA adducts, lipid peroxidation, DNA and protein oxidation and mRNA expression of CYP1A1, CYP1B1, NQO1, POR, AKR1C2 and COX2 were analyzed. Bulky DNA adducts were induced after both treatment periods; the effect of 1-NP was weak. 3-NBA induced high levels of bulky DNA adducts even after 4-h treatment, suggesting rapid metabolic activation. Oxidative DNA damage was not affected. 1-NP caused protein oxidation and weak induction of lipid peroxidation after 4-h incubation. 3-NBA induced lipid peroxidation after 24-h treatment. Unlike B[a]P, induction of the aryl hydrocarbon receptor, measured as mRNA expression levels of CYP1A1 and CYP1B1, was low after treatment with polycyclic aromatic hydrocarbon (PAH) nitro-derivatives. All test compounds induced mRNA expression of NQO1, POR, and AKR1C2 after 24-h treatment. AKR1C2 expression indicates involvement of processes associated with reactive oxygen species generation. This was supported further by COX2 expression induced by 24-h treatment with 1-NP. In summary, 3-NBA was the most potent genotoxicant, whereas 1-NP exhibited the strongest oxidative properties.

Vesicular disruption of lysosomal targeting organometallic polyarginine bioconjugates

Compounds which are able to destabilize the lysosomal membrane have been proposed as interesting candidates for targeted anticancer drugs due to the pronounced lysosomal changes in cancer cells. For this purpose, metallocene derivatives of a cell penetrating polyarginine peptide M–(Arg)9(Phe)2Lys–NH2 (where M = ferrocene carboxylate or ruthenocene carboxylate) were designed and their biological activities were investigated in detail. The ferrocenoyl- and ruthenocenoyl polyarginine bioconjugates were synthesized via Fmoc solid-phase peptide synthesis (SPPS) protocols on a microwave-assisted synthesizer. After HPLC purification >98% purity was observed for all conjugates. Their interaction with supported biomimetic membranes was investigated on a quartz crystal microbalance (QCM) and revealed a very strong binding of the metallocene peptides and their metal-free congeners to an artificial eukaryotic membrane model (DMPC–cholesterol). To demonstrate their antiproliferative utility as cytotoxic compounds for a targeted anticancer drug, cell viability (by the crystal violet assay), apoptosis (flow cytometry, Ann V/PI staining), induction of reactive oxygen species (ROS, by flow cytometry with dihydroethidium staining), and changes in cancer cell metabolism, e.g. respiration and glycolysis, were studied. Our results reveal only a weak toxicity for the metal-free polyarginine peptide, which could be significantly enhanced (to ca. 50 μM against HeLa cells in the best case) by coupling ferrocene or ruthenocene carboxylates to the N-terminus of the peptide. The investigation of the cellular uptake and intracellular localization by fluorescence microscopy revealed an enhanced vesicular disruption by the metallocene bioconjugate compared to the metal-free derivative which could be triggered by light and chemicals. Further studies of apoptosis, respiration, glycolysis and ROS formation reveal the superior characteristics of the metallocene compounds. While most cells remain viable even at 300 μM of the metal free bioconjugate 1, most cells are dead or in late stages of apoptosis at 200 μM of the ruthenocene derivative 3, and at 100 μM of the most active ferrocene derivative 2, however, all show very little sign of necrosis. Also, the metal free compound 1 does not induce ROS formation but both metallocene–polyarginine bioconjugates are clearly associated with enhanced intracellular ROS levels, with levels for the redox-active ferrocene derivative being two times higher than for the structurally very similar but redox-silent ruthenocene derivative. We propose that such metallocene–polyarginine peptides induce lysosomal membrane permeabilization and thereby could be developed towards targeted anticancer drugs.

Oxidative stress and the homeodynamics of iron metabolism

Iron and oxygen share a delicate partnership since both are indispensable for survival, but if the partnership becomes inadequate, this may rapidly terminate life. Virtually all cell components are directly or indirectly affected by cellular iron metabolism, which represents a complex, redox-based machinery that is controlled by, and essential to, metabolic requirements. Under conditions of increased oxidative stress—i.e., enhanced formation of reactive oxygen species (ROS)—however, this machinery may turn into a potential threat, the continued requirement for iron promoting adverse reactions such as the iron/H2O2-based formation of hydroxyl radicals, which exacerbate the initial pro-oxidant condition. This review will discuss the multifaceted homeodynamics of cellular iron management under normal conditions as well as in the context of oxidative stress.

Serum ferritin is an important inflammatory disease marker, as it is mainly a leakage product from damaged cells

"Serum ferritin" presents a paradox, as the iron storage protein ferritin is not synthesised in serum yet is to be found there. Serum ferritin is also a well known inflammatory marker, but it is unclear whether serum ferritin reflects or causes inflammation, or whether it is involved in an inflammatory cycle. We argue here that serum ferritin arises from damaged cells, and is thus a marker of cellular damage. The protein in serum ferritin is considered benign, but it has lost (i.e. dumped) most of its normal complement of iron which when unliganded is highly toxic. The facts that serum ferritin levels can correlate with both disease and with body iron stores are thus expected on simple chemical kinetic grounds. Serum ferritin levels also correlate with other phenotypic readouts such as erythrocyte morphology. Overall, this systems approach serves to explain a number of apparent paradoxes of serum ferritin, including (i) why it correlates with biomarkers of cell damage, (ii) why it correlates with biomarkers of hydroxyl radical formation (and oxidative stress) and (iii) therefore why it correlates with the presence and/or severity of numerous diseases. This leads to suggestions for how one might exploit the corollaries of the recognition that serum ferritin levels mainly represent a consequence of cell stress and damage.

Iron metabolism in neuromyelitis optica patients

OBJECTIVE

The aim of this study was to investigate whether there was any relationship between iron metabolism and clinical features of NMO patients.

METHODS

We measured serum iron, ferritin, and transferrin levels in 25 patients with NMO and 25 controls.

RESULTS

The patients with NMO had significantly decreased serum iron and increased serum ferritin (P=0.001 and 0.002, respectively), both of which were in normal ranges. The serum iron level of NMO patients was not correlated with disease duration, relapse number, or magnetic resonance imaging lesions. After multivariate linear regression analysis, the serum iron level was correlated with age (P=0.003) but not EDSS score.

CONCLUSIONS

The iron metabolism might reflect the chronic oxidative injury in NMO patients. It is still unclear whether iron metabolism is implicated in the pathogenesis of NMO.

Increased Lysosomal Membrane Permeabilization in Oxidant-exposed Macrophages of Human Fibrotic Lungs

A disrupted balance of reduced glutathione (GSH) and iron (Fe) and subsequent enhanced susceptibility of lysosomes of lung macrophages (LMs) to oxidants may play a role in lung fibrogenesis. We assessed cellular Fe/GSH, lysosomal membrane permeabilization (LMP), and cell death in cultures of oxidant exposed LMs. LMs from 7 lung fibrosis patients and healthy subjects were exposed to a physiologic concentration of H₂O₂ for 1 h. LMP was assessed with acridine orange green fluorescence, apoptosis/necrosis were estimated by apoptotic DNA and typical morphology, Fe was assessed with Prussian blue staining/atomic absorption spectrophotometry, and GSH was evaluated using a GSH assay kit. Oxidant-induced LMP and cell death were more pronounced in cultures of LMs from patients with lung fibrosis, and these cells contained less GSH and more cytochemically stained Fe. These observations indicate that LMP may be involved in fibrosis development, possibly through activation of the inflammasome complex. Further studies are warranted for a detailed understanding.

The complex interplay of iron metabolism, reactive oxygen species, and reactive nitrogen species: insights into the potential of various iron therapies to induce oxidative and nitrosative stress

Production of minute concentrations of superoxide (O2(*-)) and nitrogen monoxide (nitric oxide, NO*) plays important roles in several aspects of cellular signaling and metabolic regulation. However, in an inflammatory environment, the concentrations of these radicals can drastically increase and the antioxidant defenses may become overwhelmed. Thus, biological damage may occur owing to redox imbalance-a condition called oxidative and/or nitrosative stress. A complex interplay exists between iron metabolism, O2(*-), hydrogen peroxide (H2O2), and NO*. Iron is involved in both the formation and the scavenging of these species. Iron deficiency (anemia) (ID(A)) is associated with oxidative stress, but its role in the induction of nitrosative stress is largely unclear. Moreover, oral as well as intravenous (iv) iron preparations used for the treatment of ID(A) may also induce oxidative and/or nitrosative stress. Oral administration of ferrous salts may lead to high transferrin saturation levels and, thus, formation of non-transferrin-bound iron, a potentially toxic form of iron with a propensity to induce oxidative stress. One of the factors that determine the likelihood of oxidative and nitrosative stress induced upon administration of an iv iron complex is the amount of labile (or weakly-bound) iron present in the complex. Stable dextran-based iron complexes used for iv therapy, although they contain only negligible amounts of labile iron, can induce oxidative and/or nitrosative stress through so far unknown mechanisms. In this review, after summarizing the main features of iron metabolism and its complex interplay with O2(*-), H2O2, NO*, and other more reactive compounds derived from these species, the potential of various iron therapies to induce oxidative and nitrosative stress is discussed and possible underlying mechanisms are proposed. Understanding the mechanisms, by which various iron formulations may induce oxidative and nitrosative stress, will help us develop better tolerated and more efficient therapies for various dysfunctions of iron metabolism.

Mobilization of stored iron in mammals: a review

From the nutritional standpoint, several aspects of the biochemistry and physiology of iron are unique. In stark contrast to most other elements, most of the iron in mammals is in the blood attached to red blood cell hemoglobin and transporting oxygen to cells for oxidative phosphorylation and other purposes. Controlled and uncontrolled blood loss thus has a major impact on iron availability. Also, in contrast to most other nutrients, iron is poorly absorbed and poorly excreted. Moreover, amounts absorbed (~1 mg/day in adults) are much less than the total iron (~20 mg/day) cycling into and out of hemoglobin, involving bone marrow erythropoiesis and reticuloendothelial cell degradation of aged red cells. In the face of uncertainties in iron bioavailability, the mammalian organism has evolved a complex system to retain and store iron not immediately in use, and to make that iron available when and where it is needed. Iron is stored innocuously in the large hollow protein, ferritin, particularly in cells of the liver, spleen and bone marrow. Our current understanding of the molecular, cellular and physiological mechanisms by which this stored iron in ferritin is mobilized and distributed-within the cell or to other organs-is the subject of this review.

The radioprotective agent, amifostine, suppresses the reactivity of intralysosomal iron

Amifostine (2-[(3-aminopropyl)amino]ethane-thiol dihydrogen phosphate ester; WR-2721) is a radioprotective agent used clinically to minimize damage from radiation therapy to adjacent normal tissues. This inorganic thiophosphate requires dephosphorylation to produce the active, cell-permeant thiol metabolite, WR-1065. The activation step is presumably catalyzed by membrane-bound alkaline phosphatase, activity of which is substantially higher in the endothelium of normal tissues. This site-specific delivery may explain the preferential protection of normal versus neoplastic tissues. Although it was developed several decades ago, the mechanisms through which this agent exerts its protective effects remain unknown. Because WR-1065 is a weak base (pKa = 9.2), we hypothesized that the drug should preferentially accumulate (via proton trapping) within the acidic environment of intracellular lysosomes. These organelles contain abundant 'loose' iron and represent a likely initial target for oxidant- and radiation-mediated damage. We further hypothesized that, within the lysosomal compartment, the thiol groups of WR-1065 would interact with this iron, thereby minimizing iron-catalyzed lysosomal damage and ensuing cell death. A similar mechanism of protection via intralysosomal iron chelation has been invoked for the hexadentate iron chelator, desferrioxamine (DFO; although DFO enters the lysosomal compartment by endocytosis, not proton trapping). Using cultured J774 cells as a model system, we found substantial accumulation of WR-1065 within intracellular granules as revealed by reaction with the thiol-binding fluorochrome, BODIPY FL L-cystine. These granules are lysosomes as indicated by co-localization of BODIPY staining with LysoTracker Red. Compared to 1 mM DFO, cells pre-treated with 0.4 microM WR-1065 are protected from hydrogen peroxide-mediated lysosomal rupture and ensuing cell death. On a molar basis in this experimental system, WR-1065 is approximately 2500 times more effective than DFO in preventing oxidant-induced lysosomal rupture and cell death. This increased effectiveness is most likely due to the preferential concentration of this weak base within the acidic lysosomal apparatus. By electron spin resonance, we found that the generation of hydroxyl radical, which normally occurs following addition of hydrogen peroxide to J774 cells, is totally blocked by pretreatment with either WR-1065 or DFO. These findings suggest a single and plausible explanation for the radioprotective effects of amifostine and may provide a basis for the design of even more effective radio- and chemoprotective drugs.

Leaky lysosomes in lung transplant macrophages: azithromycin prevents oxidative damage

Background

Lung allografts contain large amounts of iron (Fe), which inside lung macrophages may promote oxidative lysosomal membrane permeabilization (LMP), cell death and inflammation. The macrolide antibiotic azithromycin (AZM) accumulates 1000-fold inside the acidic lysosomes and may interfere with the lysosomal pool of Fe.

Objective

Oxidative lysosomal leakage was assessed in lung macrophages from lung transplant recipients without or with AZM treatment and from healthy subjects. The efficiency of AZM to protect lysosomes and cells against oxidants was further assessed employing murine J774 macrophages.

Methods

Macrophages harvested from 8 transplant recipients (5 without and 3 with ongoing AZM treatment) and 7 healthy subjects, and J774 cells pre-treated with AZM, a high-molecular-weight derivative of the Fe chelator desferrioxamine or ammonium chloride were oxidatively stressed. LMP, cell death, Fe, reduced glutathione (GSH) and H-ferritin were assessed.

Results

Oxidant challenged macrophages from transplants recipients without AZM exhibited significantly more LMP and cell death than macrophages from healthy subjects. Those macrophages contained significantly more Fe, while GSH and H-ferritin did not differ significantly. Although macrophages from transplant recipients treated with AZM contained both significantly more Fe and less GSH, which would sensitize cells to oxidants, these macrophages resisted oxidant challenge well. The preventive effect of AZM on oxidative LMP and J774 cell death was 60 to 300 times greater than the other drugs tested.

Conclusions

AZM makes lung transplant macrophages and their lysososomes more resistant to oxidant challenge. Possibly, prevention of obliterative bronchiolitis in lung transplants by AZM is partly due to this action.

TNF-α-stimulated macrophages protect A549 lung cells against iron and oxidation

Previously, we have shown that TNF-α protects iron-exposed J774 macrophages against iron-catalyzed oxidative lysosomal disruption and cell death by increasing reduced glutathione and H-ferritin in cells. Because J774 cells are able to harbor large amounts of iron, which is potentially harmful in a redox-active state, we hypothesized that TNF-α-stimulated J774 macrophages will prevent iron-driven oxidative killing of alveolar epithelial A549 cells in co-culture. In the present study, iron trichloride (which is endocytosed by cells as hydrated iron-phosphate complexes) was mainly deposited inside the lysosomes of J774 macrophages, while A549 cells, equally iron exposed, accumulated much less iron. When challenged by oxidants, however, reactive lysosomal iron in A549 cells promoted lysosomal disruption and cell death, particularly in the presence of TNF-α. This effect resulted from an elevation in ROS generation by TNF-α, while a compensatory upregulation of protective molecules (H-ferritin and/or reduced glutathione) by TNF-α was absent. A549 cell death was particularly pronounced when iron and TNF-α were present in the conditioned medium during oxidant challenge; thus, iron-driven oxidative reactions in the culture medium were a much greater hazard to A549 cells than those taking place inside their lysosomes. Consequently, the iron chelator, deferoxamine, efficiently prevented A549 cell death when added to the culture medium during an oxidant challenge. In co-cultures of TNF-α-stimulated lung cells, J774 macrophages sequestered iron inside their lysosomes and protected A549 cells from oxidative reactions and cell death. Thus, the collective effect of TNF-α on co-cultured lung cells was mainly cytoprotective.

Lysosomal proteolysis is the primary degradation pathway for cytosolic ferritin and cytosolic ferritin degradation is necessary for iron exit

Cytosolic ferritins sequester and store iron, consequently protecting cells against iron-mediated free radical damage. However, the mechanisms of iron exit from the ferritin cage and reutilization are largely unknown. In a previous study, we found that mitochondrial ferritin (MtFt) expression led to a decrease in cytosolic ferritin. Here we showed that treatment with inhibitors of lysosomal proteases largely blocked cytosolic ferritin loss in both MtFt-expressing and wild-type cells. Moreover, cytosolic ferritin in cells treated with inhibitors of lysosomal proteases was found to store more iron than did cytosolic ferritins in untreated cells. The prevention of cytosolic ferritin degradation in MtFt-expressing cells significantly blocked iron mobilization from the protein cage induced by MtFt expression. These studies also showed that blockage of cytosolic ferritin loss by leupeptin resulted in decreased cytosolic ferritin synthesis and prolonged cytosolic ferritin stability, potentially resulting in diminished iron availability. Lastly, we found that proteasomes were responsible for cytosolic ferritin degradation in cells pretreated with ferric ammonium citrate. Thus, the current studies suggest that cytosolic ferritin degradation precedes the release of iron in MtFt-expressing cells; that MtFt-induced cytosolic ferritin decrease is partially preventable by lysosomal protease inhibitors; and that both lysosomal and proteasomal pathways may be involved in cytosolic ferritin degradation.

Regulation of apoptosis-associated lysosomal membrane permeabilization

Lysosomal membrane permeabilization (LMP) occurs in response to a large variety of cell death stimuli causing release of cathepsins from the lysosomal lumen into the cytosol where they participate in apoptosis signaling. In some settings, apoptosis induction is dependent on an early release of cathepsins, while under other circumstances LMP occurs late in the cell death process and contributes to amplification of the death signal. The mechanism underlying LMP is still incompletely understood; however, a growing body of evidence suggests that LMP may be governed by several distinct mechanisms that are likely engaged in a death stimulus- and cell-type-dependent fashion. In this review, factors contributing to permeabilization of the lysosomal membrane including reactive oxygen species, lysosomal membrane lipid composition, proteases, p53, and Bcl-2 family proteins, are described. Potential mechanisms to safeguard lysosomal integrity and confer resistance to lysosome-dependent cell death are also discussed.

TNF-alpha preserves lysosomal stability in macrophages: a potential defense against oxidative lung injury

Iron-catalyzed oxidative damage on the respiratory epithelium is prevented by alveolar macrophages depositing iron inside their lysosomes. Bound in an un-reactive state to various metalloproteins, e.g. ferritin, most lysosomal iron is kept separated from reactive oxygen species (ROS) by intracellular anti-oxidative enzyme systems. Some ROS may, however, escape this protective shield of antioxidants, react with small amounts of free redox-active iron within lysosomes, thereby causing peroxidative damage on lysosomes and possibly also ensuing cell death. Since macrophages, containing large amounts of lysosomal iron, are very resistant to TNF-alpha, we hypothesized that this cell type has developed specific defense mechanisms against TNF-alpha-induced ROS generation. Murine macrophages were exposed (or not) to non-toxic concentrations of TNF-alpha and/or iron and were then challenged with H(2)O(2). Iron-exposed oxidatively stressed cells exhibited extensive lysosomal disruption resulting in pronounced cell death. In contrast, TNF-alpha stabilized lysosomes and protected cells, particularly those iron-exposed, by reducing cellular iron and increasing H-ferritin. Intracellular generation of H(2)O(2) under oxidative stress was kept unchanged by TNF-alpha and/or iron. However, TNF-alpha increased basal levels of glutathione by up-regulating the synthesis of gamma-glutamylcystein synthetase, thereby strengthening the anti-oxidative capacity. TNF-alpha inhibitors would block this novel anti-oxidative defense system, possibly explaining their adverse effects on the lung.

Oxidative stress and cell death in cells expressing L-ferritin variants causing neuroferritinopathy

Neuroferritinopathies are dominantly inherited movement disorders associated with nucleotide insertions in the L-ferritin gene that modify the protein's C-terminus. The insertions alter physical and functional properties of the ferritins, causing an imbalance in brain iron homeostasis. We describe the effects produced by the over-expression in HeLa and neuroblastoma SH-SY5Y cells of two pathogenic L-ferritin variants, 460InsA and 498InsTC. Both peptides co-assembled with endogenous ferritins, producing molecules with reduced iron incorporation capacity, acting in a dominant negative manner. The cells showed an increase in cell death and a decrease in proteasomal activity. The formation of iron-ferritin aggregates became evident after 10 days of variant expression and was not associated with increased cell death. The addition of iron chelators or antioxidants restored proteasomal activity and reduced aggregate formation. The data indicate that cellular iron imbalance and oxidative damage are primary causes of cell death, while aggregate formation is a secondary effect.

Lysosomes in iron metabolism, ageing and apoptosis

The lysosomal compartment is essential for a variety of cellular functions, including the normal turnover of most long-lived proteins and all organelles. The compartment consists of numerous acidic vesicles (pH approximately 4 to 5) that constantly fuse and divide. It receives a large number of hydrolases ( approximately 50) from the trans-Golgi network, and substrates from both the cells' outside (heterophagy) and inside (autophagy). Many macromolecules contain iron that gives rise to an iron-rich environment in lysosomes that recently have degraded such macromolecules. Iron-rich lysosomes are sensitive to oxidative stress, while 'resting' lysosomes, which have not recently participated in autophagic events, are not. The magnitude of oxidative stress determines the degree of lysosomal destabilization and, consequently, whether arrested growth, reparative autophagy, apoptosis, or necrosis will follow. Heterophagy is the first step in the process by which immunocompetent cells modify antigens and produce antibodies, while exocytosis of lysosomal enzymes may promote tumor invasion, angiogenesis, and metastasis. Apart from being an essential turnover process, autophagy is also a mechanism by which cells will be able to sustain temporary starvation and rid themselves of intracellular organisms that have invaded, although some pathogens have evolved mechanisms to prevent their destruction. Mutated lysosomal enzymes are the underlying cause of a number of lysosomal storage diseases involving the accumulation of materials that would be the substrate for the corresponding hydrolases, were they not defective. The normal, low-level diffusion of hydrogen peroxide into iron-rich lysosomes causes the slow formation of lipofuscin in long-lived postmitotic cells, where it occupies a substantial part of the lysosomal compartment at the end of the life span. This seems to result in the diversion of newly produced lysosomal enzymes away from autophagosomes, leading to the accumulation of malfunctioning mitochondria and proteins with consequent cellular dysfunction. If autophagy were a perfect turnover process, postmitotic ageing and several age-related neurodegenerative diseases would, perhaps, not take place.

Lysosomes and oxidative stress in aging and apoptosis

The lysosomal compartment consists of numerous acidic vesicles (pH approximately 4-5) that constantly fuse and divide. It receives a large number of hydrolases from the trans-Golgi network, while their substrates arrive from both the cell's outside (heterophagy) and inside (autophagy). Many macromolecules under degradation inside lysosomes contain iron that, when released in labile form, makes lysosomes sensitive to oxidative stress. The magnitude of generated lysosomal destabilization determines if reparative autophagy, apoptosis, or necrosis will follow. Apart from being an essential turnover process, autophagy is also a mechanism for cells to repair inflicted damage, and to survive temporary starvation. The inevitable diffusion of hydrogen peroxide into iron-rich lysosomes causes the slow oxidative formation of lipofuscin in long-lived postmitotic cells, where it finally occupies a substantial part of the volume of the lysosomal compartment. This seems to result in a misdirection of lysosomal enzymes away from autophagosomes, resulting in depressed autophagy and the accumulation of malfunctioning mitochondria and proteins with consequent cellular dysfunction. This scenario might put aging into the category of autophagy disorders.

Intracellular free iron and acidic pathways mediate TNF-induced death of rat hepatoma cells

Rat hepatoma HTC cells are intrinsically resistant to various apoptosis-inducing agents. Strategies to induce death in hepatoma cells are needed and the present experimental study was aimed to investigate the sensitivity of HTC cells to TNF and to clarify the mechanisms of action of this cytokine. Cells were treated with TNF and death mechanisms characterized employing an integration of morphological and biochemical techniques. HTC cells, sensitized to TNF toxicity with cycloheximide, died in a caspase-independent apoptosis-like manner. Although we found no evidence for a direct involvement of lysosomal cathepsins, bafilomycin A1 and ammonium chloride significantly attenuated TNF toxicity. Also desferrioxamine mesylate, an iron chelator, partly protected the cells from TNF, while a complete protection was afforded by combining ammonium chloride and iron chelator. Moreover, HTC were protected from TNF also by lipophylic antioxidants and diphenylene iodonium chloride, a NADPH oxidase inhibitor. These data depict a novel mechanism of TNF-mediated cytotoxicity in HTC cells, in which the endo-lysosomal compartment, NADPH oxidase and an iron-mediated pro-oxidant status contribute in determining a caspase-independent, apoptosis-like cell death.

The environmental carcinogen 3-nitrobenzanthrone and its main metabolite 3-aminobenzanthrone enhance formation of reactive oxygen intermediates in human A549 lung epithelial cells

The environmental contaminant 3-nitrobenzanthrone (3-NBA) is highly mutagenic and a suspected human carcinogen. We aimed to evaluate whether 3-NBA is able to deregulate critical steps in cell cycle control and apoptosis in human lung epithelial A549 cells. Increased intracellular Ca(2+) and caspase activities were detected upon 3-NBA exposure. As shown by cell cycle analysis, an increased number of S-phase cells was observed after 24 h of treatment with 3-NBA. Furthermore, 3-NBA was shown to inhibit cell proliferation when added to subconfluent cell cultures. The main metabolite of 3-NBA, 3-ABA, induced statistically significant increases in tail moment as judged by alkaline comet assay. The potential of 3-NBA and 3-ABA to enhance the production of reactive oxygen species (ROS) was demonstrated by flow cytometry using 2',7'-dichlorofluorescein-diacetate (DCFH-DA). The enzyme inhibitors allopurinol, dicumarol, resveratrol and SKF525A were used to assess the impact of metabolic conversion on 3-NBA-mediated ROS production. Resveratrol decreased dichlorofluorescein (DCF) fluorescence by 50%, suggesting a role for CYP1A1 in 3-NBA-mediated ROS production. Mitochondrial ROS production was significantly attenuated (20% reduction) by addition of rotenone (complex I inhibition) and thenoyltrifluoroacetone (TTFA, complex II inhibition). Taken together, the results of the present study provide evidence for a genotoxic potential of 3-ABA in human epithelial lung cells. Moreover, both compounds lead to increased intracellular ROS and create an environment favorable to DNA damage and the promotion of cancer.

Involvement of heat shock protein-70 in the mechanism of hydrogen peroxide-induced DNA damage: the role of lysosomes and iron

Heat shock protein-70 (Hsp70) is the main heat-inducible member of the 70-kDa family of chaperones that assist cells in maintaining proteins functional under stressful conditions. In the present investigation, the role of Hsp70 in the molecular mechanism of hydrogen peroxide-induced DNA damage to HeLa cells in culture was examined. Stably transfected HeLa cell lines, overexpressing or lacking Hsp70, were created by utilizing constitutive expression of plasmids containing the functional hsp70 gene or hsp70-siRNA, respectively. Compared to control cells, the Hsp70-overexpressing ones were significantly resistant to hydrogen peroxide-induced DNA damage, while Hsp70-depleted cells showed an enhanced sensitivity. In addition, the "intracellular calcein-chelatable iron pool" was determined in the presence or absence of Hsp70 and found to be related to the sensitivity of nuclear DNA to H(2)O(2). It seems likely that the main action of Hsp70, at least in this system, is exerted at the lysosomal level, by protecting the membranes of these organelles against oxidative stress-induced destabilization. Apart from shedding additional light on the mechanistic details behind the action of Hsp70 during oxidative stress, our results indicate that modulation of cellular Hsp70 may represent a way to make cancer cells more sensitive to normal host defense mechanisms or chemotherapeutic drug treatment.

Ferritin and ferritin isoforms I: Structure-function relationships, synthesis, degradation and secretion

Ferritin is the intracellular protein responsible for the sequestration, storage and release of iron. Ferritin can accumulate up to 4500 iron atoms as a ferrihydrite mineral in a protein shell and releases these iron atoms when there is an increase in the cell's need for bioavailable iron. The ferritin protein shell consists of 24 protein subunits of two types, the H-subunit and the L-subunit. These ferritin subunits perform different functions in the mineralization process of iron. The ferritin protein shell can exist as various combinations of these two subunit types, giving rise to heteropolymers or isoferritins. Isoferritins are functionally distinct and characteristic populations of isoferritins are found depending on the type of cell, the proliferation status of the cell and the presence of disease. The synthesis of ferritin is regulated both transcriptionally and translationally. Translation of ferritin subunit mRNA is increased or decreased, depending on the labile iron pool and is controlled by an iron-responsive element present in the 5'-untranslated region of the ferritin subunit mRNA. The transcription of the genes for the ferritin subunits is controlled by hormones and cytokines, which can result in a change in the pool of translatable mRNA. The levels of intracellular ferritin are determined by the balance between synthesis and degradation. Degradation of ferritin in the cytosol results in complete release of iron, while degradation in secondary lysosomes results in the formation of haemosiderin and protection against iron toxicity. The majority of ferritin is found in the cytosol. However, ferritin with slightly different properties can also be found in organelles such as nuclei and mitochondria. Most of the ferritin produced intracellularly is harnessed for the regulation of iron bioavailability; however, some of the ferritin is secreted and internalized by other cells. In addition to the regulation of iron bioavailability ferritin may contribute to the control of myelopoiesis and immunological responses.

Therapeutic targets and potential of the novel brain- permeable multifunctional iron chelator?monoamine oxidase inhibitor drug, M-30, for the treatment of Alzheimer's disease

Novel therapeutic approaches for the treatment of neurodegenerative disorders comprise drug candidates designed specifically to act on multiple CNS targets. We have synthesized a multifunctional non-toxic, brain permeable iron chelator drug, M-30, possessing propargyl monoamine oxidase (MAO) inhibitory neuroprotective and iron-chelating moieties, from our prototype iron chelator VK-28. In the present study M-30 was shown to possess a wide range of pharmacological activities, including pro-survival neurorescue effects, induction of neuronal differentiation and regulation of amyloid precursor protein (APP) and beta-amyloid (Abeta) levels. M-30 was found to decrease apoptosis of SH-SY5Y neuroblastoma cells in a neurorescue, serum deprivation model, via reduction of the pro-apoptotic proteins Bad and Bax, and inhibition of the apoptosis-associated phosphorylated H2A.X protein (Ser 139) and caspase 3 activation. In addition, M-30 induced the outgrowth of neurites, triggered cell cycle arrest in G(0)/G(1) phase and enhanced the expression of growth associated protein-43. Furthermore, M-30 markedly reduced the levels of cellular APP and beta-C-terminal fragment (beta-CTF) and the levels of the amyloidogenic Abeta peptide in the medium of SH-SY5Y cells and Chinese hamster ovary cells stably transfected with the APP 'Swedish' mutation. Levels of the non-amyloidogenic soluble APPalpha and alpha-CTF in the medium and cell lysate respectively were coordinately increased. These properties, together with its brain selective MAO inhibitory and propargylamine- dependent neuroprotective effects, suggest that M-30 might serve as an ideal drug for neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases, in which oxidative stress and iron dysregulation have been implicated.

Radiation-induced lysosomal iron reactivity: implications for radioprotective therapy

A novel mechanism of radiosensitization involves radiation-enhanced autophagy of damaged mitochondria and various metalloproteins, by which iron accumulates within lysosomes. Hydrogen peroxide, formed by the radiolytic cleavage of water, generates in the presence of lysosomal redox-active iron extremely reactive hydroxyl radicals by Fenton-type chemistry. Subsequent peroxidative damage of lysosomal membranes initiates release of harmful content from ruptured lysosomes that triggers a cascade of events eventuating in DNA damage and apoptotic or necrotic cell death. This article reviews the role of lysosomal destabilization in radiation-induced cell damage and death. The potential effects of iron chelation therapy targeted to the lysosomes for protection of normal tissues against unwanted effects by radiation is also discussed.

Crucial role of lysosomal iron in the formation of dinitrosyl iron complexes in vivo

Dinitrosyl non-heme-iron complexes (DNIC) are found in many nitric oxide producing tissues. A prerequisite of DNIC formation is the presence of nitric oxide, iron and thiol/imidazole groups. The aim of this study was to investigate the role of the cellular labile iron pool in the formation of DNIC in erythroid K562 cells. The cells were treated with a nitric oxide donor in the presence of a permeable (salicylaldehyde isonicotinoyl hydrazone) or a nonpermeable (desferrioxamine mesylate) iron chelator and DNIC formation was recorded using electron paramagnetic resonance. Both chelators inhibited DNIC formation up to 50% after 6 h of treatment. To further investigate the role of lysosomal iron in DNIC formation, we prevented lysosomal proteolysis by pretreatment of whole cells with NH4Cl. Pretreatment with NH4Cl inhibited the formation of DNIC in a time-dependent manner that points to the importance of the degradation of iron metalloproteins in DNIC formation in vivo. Fractionation of the cell content after treatment with the nitric oxide donor revealed that DNIC is formed predominantly in the endosomal/lysosomal fraction. Taken together, these data indicate that lysosomal iron plays a crucial role in DNIC formation in vivo. Degradation of iron-containing metalloproteins seems to be important for this process.

Iron-binding drugs targeted to lysosomes: a potential strategy to treat inflammatory lung disorders

In many inflammatory lung disorders, an abnormal assimilation of redox-active iron will exacerbate oxidative tissue damage. It may be that the most important cellular pool of redox-active iron exists within lysosomes, making these organelles vulnerable to oxidative stress. In experiments employing respiratory epithelial cells and macrophages, the chelation of intra-lysosomal iron efficiently prevented lysosomal rupture and the ensuing cell death induced by hydrogen peroxide, ionising radiation or silica particles. Furthermore, cell-permeable iron-binding agents (weak bases) that accumulate within lysosomes due to proton trapping were much more efficient for cytoprotection than the chelator, desferrioxamine. On a molar basis, the weak base alpha-lipoic acid plus was 5000 times more effective than desferrioxamine at preventing lysosomal rupture and apoptotic cell death in cell cultures exposed to hydrogen peroxide. Thus, iron-chelating therapy that targets the lysosome might be a future treatment strategy for inflammatory pulmonary diseases.

Release of iron from ferritin requires lysosomal activity

How ferritin-Fe becomes available for cell functions is unknown. Our previous studies with rat hepatoma cells indicated ferritin had to be degraded to release its Fe. In these studies, we investigated whether this occurs in other cell types and whether lysosomes are required. Release of ferritin-Fe was induced with desferoxamine (DFO) in (59)Fe-preloaded hepatoma, Caco2, and erythroid K562 cells and measured by rocket immunoelectrophoresis and autoradiography. The half-lives for ferritin-(59)Fe and protein were parallel (23, 16, and 11 h for the hepatic, Caco2, and K562 cells, respectively). Co-treatment with 180 microM Fe, leupeptin, chymostatin, or chloroquine markedly decreased rates of ferritin-Fe release and ferritin degradation. Lactacystin had no effect except for a small one in erythroid cells. Fractionation of hepatoma cell lysates on iodixanol gradients showed rapid depletion of cytosolic ferritin by DFO treatment but no accumulation in lysosomes. We conclude that regardless of cell type, release of Fe from ferritin occurs mainly through lysosomal proteolysis.

Radiation-induced cell death: importance of lysosomal destabilization

The mechanisms involved in radiation-induced cellular injury and death remain incompletely understood. In addition to the direct formation of highly reactive hydroxyl radicals (HO*) by radiolysis of water, oxidative stress events in the cytoplasm due to formation of H2O2 may also be important. Since the major pool of low-mass redox-active intracellular iron seems to reside within lysosomes, arising from the continuous intralysosomal autophagocytotic degradation of ferruginous materials, formation of H2O2 inside and outside these organelles may cause lysosomal labilization with release to the cytosol of lytic enzymes and low-mass iron. If of limited magnitude, such release may induce 'reparative autophagocytosis', causing additional accumulation of redox-active iron within the lysosomal compartment. We have used radio-resistant histiocytic lymphoma (J774) cells to assess the importance of intralysosomal iron and lysosomal rupture in radiation-induced cellular injury. We found that a 40 Gy radiation dose increased the 'loose' iron content of the (still viable) cells approx. 5-fold when assayed 24 h later. Cytochemical staining revealed that most redox-active iron was within the lysosomes. The increase of intralysosomal iron was associated with 'reparative autophagocytosis', and sensitized cells to lysosomal rupture and consequent apoptotic/necrotic death following a second, much lower dose of radiation (20 Gy) 24 h after the first one. A high-molecular-mass derivative of desferrioxamine, which specifically localizes intralysosomally following endocytic uptake, added to the culture medium before either the first or the second dose of radiation, stabilized lysosomes and largely prevented cell death. These observations may provide a biological rationale for fractionated radiation.

Iron-dependent lysosomal destabilization initiates silica-induced apoptosis in murine macrophages

Alveolar macrophages play a critical role in silica-induced lung fibrosis, and apoptotic mechanisms have been implicated in silica-induced pathogenesis. Here, employing a model of murine macrophages (J774 cells), it is shown that serum-coated alpha-quartz silica particles cause lysosomal rupture and apoptosis following endocytotic uptake. The loss of lysosomal integrity involves intralysosomal iron-catalyzed peroxidative damage to lysosomal membranes. Thus, lysosomal damage is most pronounced in cells exposed to silica particles with high amounts of surface-bound iron, whereas silica particles previously treated with the iron chelator desferrioxamine only induce modest rupture. Furthermore, inhibition of intralysosomal Fenton type chemistry, either by pre-treatment with desferrioxamine complexed to starch--an iron chelator targeted to the lysosomal compartment--or by concomitant treatment with diphenylene iodonium--a potent inhibitor of NADPH oxidase --both prevent silica-induced lysosomal leakage and ensuing apoptotic cell death. This study also demonstrates that silica-induced lysosomal rupture is a very early apoptotic event, preceding activation of caspases, disruption of transmembrane mitochondrial potential and DNA fragmentation. Indeed, these later apoptotic events appear to be directly correlated to the magnitude of lysosomal leakage, and are not observed in cells treated with high molecular weight desferrioxamine or diphenylene iodonium.

Ketone bodies stimulate chaperone-mediated autophagy

Chaperone-mediated autophagy (CMA) is a selective lysosomal protein degradative process that is activated in higher organisms under conditions of prolonged starvation and in cell culture by the removal of serum. Ketone bodies are comprised of three compounds (beta-hydroxybutyrate, acetoacetate, and acetone) that circulate during starvation, especially during prolonged starvation. Here we have investigated the hypothesis that ketone bodies induce CMA. We found that physiological concentrations of beta-hydroxybutyrate (BOH) induced proteolysis in cells maintained in media with serum and without serum; however, acetoacetate only induced proteolysis in cells maintained in media with serum. Lysosomes isolated from BOH-treated cells displayed an increased ability to degrade both glyceraldehyde-3-phosphate dehydrogenase and ribonuclease A, substrates for CMA. Isolated lysosomes from cells maintained in media without serum also demonstrated an increased ability to degrade glyceraldehyde-3-phosphate dehydrogenase and ribonuclease A when the reaction was supplemented with BOH. Such treatment did not affect the levels of lysosome-associated membrane protein 2a or lysosomal heat shock cognate protein of 70 kDa, two rate-limiting proteins in CMA. However, pretreatment of glyceraldehyde-3-phosphate and ribonuclease A with BOH increased their rate of degradation by isolated lysosomes. Lysosomes pretreated with BOH showed no increase in proteolysis, suggesting that BOH acts on the substrates to increase their rates of proteolysis. Using OxyBlot analysis to detect carbonyl formation on proteins, one common marker of protein oxidation, we showed that treatment of substrates with BOH increased their oxidation. Neither glycerol, another compound that increases in circulation during prolonged starvation, nor butanol or butanone, compounds closely related to BOH, had an effect on CMA. The induction of CMA by ketone bodies may provide an important physiological mechanism for the activation of CMA during prolonged starvation.

Molecular pharmacology of the interaction of anthracyclines with iron

Although anthracyclines such as doxorubicin are widely used antitumor agents, a major limitation for their use is the development of cardiomyopathy at high cumulative doses. This severe adverse side effect may be due to interactions with cellular iron metabolism, because iron loading promotes anthracycline-induced cell damage. On the other hand, anthracycline-induced cardiotoxicity is significantly alleviated by iron chelators (e.g., desferrioxamine and dexrazoxane). The molecular mechanisms by which anthracyclines interfere with cellular iron trafficking are complex and still unclear. Doxorubicin can directly bind iron and can perturb iron metabolism by interacting with multiple molecular targets, including the iron regulatory proteins (IRP) 1 and 2. The RNA-binding activity of these molecules regulates synthesis of the transferrin receptor 1 and ferritin, which are crucial proteins involved in iron uptake and storage, respectively. At present, it is not clear whether doxorubicin affects IRP1-RNA-binding activity by intracellular formation of doxorubicinol and/or by generation of the doxorubicin-iron(III) complex. Furthermore, doxorubicin prevents the mobilization of iron from ferritin by a mechanism that may involve lysosomal degradation of this protein. Prevention of iron mobilization from ferritin would probably disturb vital cellular functions as a result of inhibition of essential iron-dependent proteins, such as ribonucleotide reductase. This review discusses the molecular interactions of anthracyclines with iron metabolism and the development of cardioprotective strategies such as iron chelators.

Diet restriction induced autophagy: a lysosomal protective system against oxidative- and pollutant-stress and cell injury

Nutrient deprivation or dietary restriction (DR) confers protection against ageing and stress in many animals and induced lysosomal autophagy is part of this mechanism. The effects of dietary restriction on the toxicity of copper and the polycyclic aromatic hydrocarbon phenanthrene have been investigated in the common marine mussel Mytilus edulis. The findings show that DR-induced autophagy facilitates the recovery of the digestive gland (i.e., molluscan liver analogue) from cell injury caused by both copper and phenanthrene. It is inferred that DR-induced autophagy and lysosomal proteolysis results in improved cellular "housekeeping" through the more efficient removal of oxidatively and pollutant damaged proteins (e.g., protein carbonyls, protein adducts, etc.) and that this contributes to stress resistance.

Relocalized redox-active lysosomal iron is an important mediator of oxidative-stress-induced DNA damage

Oxidative damage to nuclear DNA is known to involve site-specific Fenton-type chemistry catalysed by redox-active iron or copper in the immediate vicinity of DNA. However, the presence of transition metals in the nucleus has not been shown convincingly. Recently, it was proposed that a major part of the cellular pool of loose iron is confined within the acidic vacuolar compartment [Yu, Persson, Eaton and Brunk (2003) Free Radical Biol. Med. 34, 1243-1252; Persson, Yu, Tirosh, Eaton and Brunk (2003) Free Radical Biol. Med. 34, 1295-1305]. Consequently, rupture of secondary lysosomes, as well as subsequent relocation of labile iron to the nucleus, could be an important intermediary step in the generation of oxidative damage to DNA. To test this concept we employed the potent iron chelator DFO (desferrioxamine) conjugated with starch to form an HMM-DFO (high-molecular-mass DFO complex). The HMM-DFO complex will enter cells only via fluid-phase endocytosis and remain within the acidic vacuolar compartment, thereby chelating redox-active iron exclusively inside the endosomal/lysosomal compartment. Both free DFO and HMM-DFO equally protected lysosomal-membrane integrity against H2O2-induced oxidative disruption. More importantly, both forms of DFO prevented H2O2-induced strand breaks in nuclear DNA, including telomeres. To exclude the possibility that lysosomal hydrolases, rather than iron, caused the observed DNA damage, limited lysosomal rupture was induced using the lysosomotropic detergent O-methyl-serine dodecylamine hydrochloride; subsequently, hardly any DNA damage was found. These observations suggest that rapid oxidative damage to cellular DNA is minimal in the absence of redox-active iron and that oxidant-mediated DNA damage, observed in normal cells, is mainly derived from intralysosomal iron translocated to the nucleus after lysosomal rupture.

Examination of the mechanism(s) involved in doxorubicin-mediated iron accumulation in ferritin: studies using metabolic inhibitors, protein synthesis inhibitors, and lysosomotropic agents

Anthracyclines are potent anticancer agents, but their use is limited by cardiotoxicity at high cumulative doses. The mechanisms involved in anthracycline-mediated cardiotoxicity are still poorly understood, but numerous investigations have indicated a role for iron in this process. Our previous studies using neoplastic and myocardial cells showed that anthracyclines inhibit iron mobilization from the iron storage protein, ferritin, resulting in marked accumulation of ferritin-iron. Although the process of ferritin-iron mobilization is little understood, catabolism of ferritin by lysosomes may be a likely mechanism. Because anthracyclines have been shown to accumulate in lysosomes, this latter organelle may be a potential target for these drugs. The present study demonstrated, using native polyacrylamide gel electrophoresis-59Fe autoradiography, that ferritin-59Fe mobilization is an energy-dependent process that also requires protein synthesis. Depression of lysosomal activity via the enzyme inhibitors E64d [(2S,3S)-trans-epoxysuccinyl-l-leucylamido-2-methylbutane ethyl ester] and leupeptin or the lysosomotropic agents ammonium chloride, chloroquine, and methylamine resulted in a 3- to 5-fold increase in 59Feferritin accumulation compared with control cells. In addition, the proteasome inhibitors N-benzoyloxycarbonyl (Z)-Leu-Leuleucinal (MG132) and lactacystin also significantly increased 59Fe-ferritin levels compared with control cells. These effects of lysosomotropic agents or inhibitors of lysosomal activity were comparable with that observed with the anthracycline doxorubicin. Collectively, our study indicates a role for lysosomes and proteasomes in ferritin-iron mobilization, and this pathway is dependent on metabolic energy and protein synthesis. Furthermore, the lysosome/proteasome pathway may be a novel anthracycline target, inhibiting iron mobilization from ferritin that is essential for vital iron-requiring processes such as DNA synthesis.

Intralysosomal iron: a major determinant of oxidant-induced cell death

As a result of continuous digestion of iron-containing metalloproteins, the lysosomes within normal cells contain a pool of labile, redox-active, low-molecular-weight iron, which may make these organelles particularly susceptible to oxidative damage. Oxidant-mediated destabilization of lysosomal membranes with release of hydrolytic enzymes into the cell cytoplasm can lead to a cascade of events eventuating in cell death (either apoptotic or necrotic depending on the magnitude of the insult). To assess the importance of the intralysosomal pool of redox-active iron, we have temporarily blocked lysosomal digestion by exposing cells to the lysosomotropic alkalinizing agent, ammonium chloride (NH(4)Cl). The consequent increase in lysosomal pH (from ca. 4.5 to > 6) inhibits intralysosomal proteolysis and, hence, the continuous flow of reactive iron into this pool. Preincubation of J774 cells with 10 mM NH(4)Cl for 4 h dramatically decreased apoptotic death caused by subsequent exposure to H(2)O(2), and the protection was as great as that afforded by the powerful iron chelator, desferrioxamine (which probably localizes predominantly in the lysosomal compartment). Sulfide-silver cytochemical detection of iron revealed a pronounced decrease in lysosomal content of redox-active iron after NH(4)Cl exposure, probably due to diminished intralysosomal digestion of iron-containing material coupled with continuing iron export from this organelle. Electron paramagnetic resonance experiments revealed that hydroxyl radical formation, readily detectable in control cells following H(2)O(2) addition, was absent in cells preexposed to 10 mM NH(4)Cl. Thus, the major pool of redox-active, low-molecular-weight iron may be located within the lysosomes. In a number of clinical situations, pharmacologic strategies that minimize the amount or reactivity of intralysosomal iron should be effective in preventing oxidant-induced cell death.

Prevention of oxidant-induced cell death by lysosomotropic iron chelators

Intralysosomal iron powerfully synergizes oxidant-induced cellular damage. The iron chelator, desferrioxamine (DFO), protects cultured cells against oxidant challenge but pharmacologically effective concentrations of this drug cannot readily be achieved in vivo. DFO localizes almost exclusively within the lysosomes following endocytic uptake, suggesting that truly lysosomotropic chelators might be even more effective. We hypothesized that an amine derivative of alpha-lipoamide (LM), 5-[1,2] dithiolan-3-yl-pentanoic acid (2-dimethylamino-ethyl)-amide (alpha-lipoic acid-plus [LAP]; pKa = 8.0), would concentrate via proton trapping within lysosomes, and that the vicinal thiols of the reduced form of this agent would interact with intralysosomal iron, preventing oxidant-mediated cell damage. Using a thiol-reactive fluorochrome, we find that reduced LAP does accumulate within the lysosomes of cultured J774 cells. Furthermore, LAP is approximately 1000 and 5000 times more effective than LM and DFO, respectively, in protecting lysosomes against oxidant-induced rupture and in preventing ensuing apoptotic cell death. Suppression of lysosomal accumulation of LAP (by ammonium-mediated lysosomal alkalinization) blocks these protective effects. Electron paramagnetic resonance reveals that the intracellular generation of hydroxyl radical following addition of hydrogen peroxide to J774 cells is totally eliminated by pretreatment with either DFO (1 mM) or LAP (0.2 microM) whereas LM (200 microM) is much less effective.

Alpha-lipoic acid and alpha-lipoamide prevent oxidant-induced lysosomal rupture and apoptosis

Alpha-lipoic acid (LA) and its corresponding derivative, alpha-lipoamide (LM), have been described as antioxidants, but the mechanisms of their putative antioxidant effects remain largely uncharacterised. The vicinal thiols present in the reduced forms of these compounds suggest that they might possess metal chelating properties. We have shown previously that cell death caused by oxidants may be initiated by lysosomal rupture and that this latter event may involve intralysosomal iron which catalyzes Fenton-type chemistry and resultant peroxidative damage to lysosomal membranes. Here, using cultured J774 cells as a model, we show that both LA and LM stabilize lysosomes against oxidative stress, probably by chelating intralysosomal iron and, consequently, preventing intralysosomal Fenton reactions. In preventing oxidant-mediated apoptosis, LM is significantly more effective than LA, as would be expected from their differing capacities to enter cells and concentrate within the acidic lysosomal compartment. As previously reported, the powerful iron-chelator, desferrioxamine (Des) (which also locates within the lysosomal compartment), also provides protection against oxidant-mediated cell death. Interestingly, although Des enhances the partial protection afforded by LA, it confers no additional protection when added with LM. Therefore, the antioxidant actions of LA and LM may arise from intralysosomal iron chelation, with LM being more effective in this regard.