Metallothionein/disulfide interactions, oxidative stress, and the mobilization of cellular zinc

An increase of lysosomes through EGF-triggered endocytosis attenuated zinc-mediated lysosomal membrane permeabilization and neuronal cell death

In the context of acute brain injuries, where zinc neurotoxicity and oxidative stress are acknowledged contributors to neuronal damage, we investigated the pivotal role of lysosomes as a potential protective mechanism. Our research commenced with an exploration of epidermal growth factor (EGF) and its impact on lysosomal dynamics, particularly its neuroprotective potential against zinc-induced cytotoxicity. Using primary mouse cerebrocortical cultures, we observed the rapid induction of EGFR endocytosis triggered by EGF, resulting in a transient increase in lysosomal vesicles. Furthermore, EGF stimulated lysosomal biogenesis, evident through elevated expression of lysosomal-associated membrane protein 1 (LAMP-1) and the induction and activation of prominent lysosomal proteases, particularly cathepsin B (CTSB). This process of EGFR endocytosis was found to promote lysosomal augmentation, thus conferring protection against zinc-induced lysosomal membrane permeabilization (LMP) and subsequent neuronal death. Notably, the neuroprotective effects and lysosomal enhancement induced by EGF were almost completely reversed by the inhibition of clathrin-mediated and caveolin-mediated endocytosis pathways, along with the disruption of retrograde trafficking. Furthermore, tyrosine kinase inhibition of EGFR nullified EGFR endocytosis, resulting in the abrogation of EGF-induced lysosomal upregulation and neuroprotection. An intriguing aspect of our study is the successful replication of EGF's neuroprotective effects through the overexpression of LAMP-1, which significantly reduced zinc-induced LMP and cell death, demonstrated in both primary mouse cerebrocortical neuronal cultures and human embryonic kidney (HEK) cells. Our research extended beyond zinc-induced neurotoxicity, as we observed EGF's protective effects against other oxidative stressors linked to intracellular zinc release, including hydrogen peroxide (H2O2) and 1-methyl-4-phenylpyridinium ion (MPP+). Collectively, our findings unveil the intricate interplay between EGF-triggered EGFR endocytosis, lysosomal upregulation, an increase in the regulatory capacity for zinc homeostasis, and the subsequent alleviation of zinc-induced neurotoxicity. These results present promising avenues for therapeutic interventions to enhance neuroprotection by targeting lysosomal augmentation.

The Role of Zinc on Liver Fibrosis by Modulating ZIP14 Expression Throughout Epigenetic Regulatory Mechanisms

Zinc plays a pivotal role in tissue regeneration and maintenance being as a central cofactor in a plethora of enzymatic activities. Hypozincemia is commonly seen with chronic liver disease and is associated with an increased risk of liver fibrosis development and hepatocellular carcinoma. Previously favorable effects of zinc supplementation on liver fibrosis have been shown. However, the underlying mechanism of this effect is not elucidated. Liver fibrosis was induced in mice by using CCl4 injection, followed by treatment with zinc chloride (ZnCl2) both at fibrotic and sham groups, and their hepatocytes were isolated. Our results showed that the administration of ZnCl2 restored the depleted cytosolic zinc levels in the hepatocytes isolated from the fibrotic group. Also, alpha-smooth muscle actin (αSMA) expression in hepatocytes was decreased, indicating a reversal of the fibrotic process. Notably, ZIP14 expression significantly increased in the fibrotic group following ZnCl2 treatment, whereas in the sham group ZIP14 expression decreased. Chromatin immunoprecipitation (ChIP) experiments revealed an increased binding percentage of Metal-regulatory transcription factor 1 (MTF1) on ZIP14 promoter in the hepatocytes isolated from fibrotic mice compared to the sham group after ZnCl2 administration. In the same group, the binding percentage of the histone deacetylase HDAC4 on ZIP14 promoter decreased. Our results suggest that the ZnCl2 treatment ameliorates liver fibrosis by elevating intracellular zinc levels through MTF1-mediated regulation of ZIP14 expression and the reduction of ZIP14 deacetylation via HDAC4. The restoration of intracellular zinc concentrations and the modulation of ZIP14 expression by zinc orchestrated through MTF1 and HDAC4, appear to be essential determinants of the therapeutic response in hepatic fibrosis. These findings pave the way for potential novel interventions targeting zinc-related pathways for the treatment of liver fibrosis and associated conditions.

Metal profiling in coronary ischemia-reperfusion injury: Implications for KEAP1/NRF2 regulated redox signaling

Coronary ischemia-reperfusion (IR) injury results from a blockage of blood supply to the heart followed by restoration of perfusion, leading to oxidative stress induced pathological processes. Nuclear factor erythroid 2-related factor 2 (NRF2), a master antioxidant transcription factor, plays a key role in regulating redox signaling. Over the past decades, the field of metallomics has provided novel insights into the mechanism of pro-oxidant and antioxidant pathological processes. Both redox-active (e.g. Fe and Cu) and redox-inert (e.g. Zn and Mg) metals play unique roles in establishing redox balance under IR injury. Notably, Zn protects against oxidative stress in coronary IR injury by serving as a cofactor of antioxidant enzymes such as superoxide dismutase [Cu-Zn] (SOD1) and proteins such as metallothionein (MT) and KEAP1/NRF2 mediated antioxidant defenses. An increase in labile Zn2+ inhibits proteasomal degradation and ubiquitination of NRF2 by modifying KEAP1 and glycogen synthase kinase 3β (GSK3β) conformations. Fe and Cu catalyse the formation of reactive oxygen species via the Fenton reaction and also serve as cofactors of antioxidant enzymes and can activate NRF2 antioxidant signaling. We review the evidence that Zn and redox-active metals Fe and Cu affect redox signaling in coronary cells during IR and the mechanisms by which oxidative stress influences cellular metal content. In view of the unique double-edged characteristics of metals, we aim to bridge the role of metals and NRF2 regulated redox signaling to antioxidant defenses in IR injury, with a long-term aim of informing the design and application of novel therapeutics.

Arsenic binding to human metallothionein-3

Arsenic poisoning is of great concern with respect to its neurological toxicity, which is especially significant for young children. Human exposure to arsenic occurs worldwide from contaminated drinking water. In human physiology, one response to toxic metals is through coordination with the metallochaperone metallothionein (MT). Central nervous system expression of MT isoform 3 (MT3) is thought to be neuroprotective. We report for the first time on the metalation pathways of As3+ binding to apo-MT3 under physiological conditions, yielding the absolute binding constants (log Kn, n = 1-6) for each sequential As3+ binding event: 10.20, 10.02, 9.79, 9.48, 9.06, and 8.31 M-1. We report on the rate of the reaction of As3+ with apo-MT3 at pH 3.5 with rate constants (kn, n = 1-6) determined for each sequential As3+ binding event: 116.9, 101.2, 85.6, 64.0, 43.9, and 21.0 M-1 s-1. We further characterize the As3+ binding pathway to fully metalated Zn7MT3 and partially metalated Zn-MT3. As3+ binds rapidly with high binding constants under physiological conditions in a noncooperative manner, but is unable to replace the Zn2+ in fully-metalated Zn-MT3. As3+ binding to partially metalated Zn-MT3 takes place with a rearrangement of the Zn-binding profile. Our work shows that As 3+ rapidly and efficiently binds to both apo-MT3 and partially metalated Zn-MT3 at physiological pH.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Non-Cell-Autonomous Regulation of Optic Nerve Regeneration by Amacrine Cells

Visual information is conveyed from the eye to the brain through the axons of retinal ganglion cells (RGCs) that course through the optic nerve and synapse onto neurons in multiple subcortical visual relay areas. RGCs cannot regenerate their axons once they are damaged, similar to most mature neurons in the central nervous system (CNS), and soon undergo cell death. These phenomena of neurodegeneration and regenerative failure are widely viewed as being determined by cell-intrinsic mechanisms within RGCs or to be influenced by the extracellular environment, including glial or inflammatory cells. However, a new concept is emerging that the death or survival of RGCs and their ability to regenerate axons are also influenced by the complex circuitry of the retina and that the activation of a multicellular signaling cascade involving changes in inhibitory interneurons - the amacrine cells (AC) - contributes to the fate of RGCs. Here, we review our current understanding of the role that interneurons play in cell survival and axon regeneration after optic nerve injury.

The Function and Regulation of Zinc in the Brain

Nearly sixty years ago Fredrich Timm developed a histochemical technique that revealed a rich reserve of free zinc in distinct regions of the brain. Subsequent electron microscopy studies in Timm- stained brain tissue found that this "labile" pool of cellular zinc was highly concentrated at synaptic boutons, hinting a possible role for the metal in synaptic transmission. Although evidence for activity-dependent synaptic release of zinc would not be reported for another twenty years, these initial findings spurred decades of research into zinc's role in neuronal function and revealed a diverse array of signaling cascades triggered or regulated by the metal. Here, we delve into our current understanding of the many roles zinc plays in the brain, from influencing neurotransmission and sensory processing, to activating both pro-survival and pro-death neuronal signaling pathways. Moreover, we detail the many mechanisms that tightly regulate cellular zinc levels, including metal binding proteins and a large array of zinc transporters.

Antioxidant Defenses in the Human Eye: A Focus on Metallothioneins

The human eye, the highly specialized organ of vision, is greatly influenced by oxidants of endogenous and exogenous origin. Oxidative stress affects all structures of the human eye with special emphasis on the ocular surface, the lens, the retina and its retinal pigment epithelium, which are considered natural barriers of antioxidant protection, contributing to the onset and/or progression of eye diseases. These ocular structures contain a complex antioxidant defense system slightly different along the eye depending on cell tissue. In addition to widely studied enzymatic antioxidants, including superoxide dismutase, glutathione peroxidase, catalase, peroxiredoxins and selenoproteins, inter alia, metallothioneins (MTs) are considered antioxidant proteins of growing interest with further cell-mediated functions. This family of cysteine rich and low molecular mass proteins captures and neutralizes free radicals in a redox-dependent mechanism involving zinc binding and release. The state of the art of MTs, including the isoforms classification, the main functions described to date, the Zn-MT redox cycle as antioxidant defense system, and the antioxidant activity of Zn-MTs in the ocular surface, lens, retina and its retinal pigment epithelium, dependent on the number of occupied zinc-binding sites, will be comprehensively reviewed.

Nutrition Can Help DNA Repair in the Case of Aging

Micronutrients such as vitamins and trace elements are crucial for maintaining the health of all organisms. Micronutrients are involved in every cellular/biochemical process. They play roles in proper heart and brain functioning, influence immunological responses, and antioxidant defense systems. Therefore, prolonged deficiency in one or more micronutrients leads to cardiovascular or neurodegenerative disorders. Keeping micronutrients at adequate levels is especially important for seniors. They are prone to deficiencies due to age-associated functional decline and often to a diet poor in nutrients. Moreover, lack of micronutrients has an indirect impact on the genome. Their low levels reduce the activity of antioxidant enzymes, and therefore inhibit the efficiency of defense against free radicals which can lead to the formation of DNA lesions. The more DNA damage in the genetic material, the faster aging at the cellular level and a higher risk of pathological processes (e.g., carcinogenesis). Supplementation of crucial antioxidative micronutrients such as selenium, zinc, vitamin C, and vitamin E seems to have the potential to positively influence the condition of an aging organism, including minimizing inflammation, enhancing antioxidative defense, and limiting the formation of DNA lesions. In consequence, it may lead to lowering the risk and incidence of age-related diseases such as cardiovascular diseases, neurodegenerative diseases, and malnutrition. In this article, we attempt to present the synergistic action of selected antioxidant micronutrients (vitamin C, vitamin E, selenium, and zinc) for inhibiting oxidative stress and DNA damage, which may impede the process of healthy aging.

Identification of novel activators of the metal responsive transcription factor (MTF-1) using a gene expression biomarker in a microarray compendium

Environmental exposure to metals is known to cause a number of human toxicities including cancer. Metal-responsive transcription factor 1 (MTF-1) is an important component of metal regulation systems in mammalian cells. Here, we describe a novel method to identify chemicals that activate MTF-1 based on microarray profiling data. MTF-1 biomarker genes were identified that exhibited consistent, robust expression across 10 microarray comparisons examining the effects of metals (zinc, nickel, lead, arsenic, mercury, and silver) on gene expression in human cells. A subset of the resulting 81 biomarker genes was shown to be altered by knockdown of the MTF1 gene including metallothionein family members and a zinc transporter. The ability to correctly identify treatment conditions that activate MTF-1 was determined by comparing the biomarker to microarray comparisons from cells exposed to reference metal activators of MTF-1 using the rank-based Running Fisher algorithm. The balanced accuracy for prediction was 93%. The biomarker was then used to identify organic chemicals that activate MTF-1 from a compendium of 11 725 human gene expression comparisons representing 2582 chemicals. There were 700 chemicals identified that included those known to interact with cellular metals, such as clioquinol and disulfiram, as well as a set of novel chemicals. All nine of the novel chemicals selected for validation were confirmed to activate MTF-1 biomarker genes in MCF-7 cells and to lesser extents in MTF1-null cells by qPCR and targeted RNA-Seq. Overall, our work demonstrates that the biomarker for MTF-1 coupled with the Running Fisher test is a reliable strategy to identify novel chemical modulators of metal homeostasis using gene expression profiling.

Rapid Intramitochondrial Zn2+ Accumulation in CA1 Hippocampal Pyramidal Neurons After Transient Global Ischemia: A Possible Contributor to Mitochondrial Disruption and Cell Death

Mitochondrial Zn2+ accumulation, particularly in CA1 neurons, occurs after ischemia and likely contributes to mitochondrial dysfunction and subsequent neurodegeneration. However, the relationship between mitochondrial Zn2+ accumulation and their disruption has not been examined at the ultrastructural level in vivo. We employed a cardiac arrest model of transient global ischemia (TGI), combined with Timm's sulfide silver labeling, which inserts electron dense metallic silver granules at sites of labile Zn2+ accumulation, and used transmission electron microscopy (TEM) to examine subcellular loci of the Zn2+ accumulation. In line with prior studies, TGI-induced damage to CA1 was far greater than to CA3 pyramidal neurons, and was substantially progressive in the hours after reperfusion (being significantly greater after 4- than 1-hour recovery). Intriguingly, TEM examination of Timm's-stained sections revealed substantial Zn2+ accumulation in many postischemic CA1 mitochondria, which was strongly correlated with their swelling and disruption. Furthermore, paralleling the evolution of neuronal injury, both the number of mitochondria containing Zn2+ and the degree of their disruption were far greater at 4- than 1-hour recovery. These data provide the first direct characterization of Zn2+ accumulation in CA1 mitochondria after in vivo TGI, and support the idea that targeting these events could yield therapeutic benefits.

Zn2+ entry through the mitochondrial calcium uniporter is a critical contributor to mitochondrial dysfunction and neurodegeneration

Excitotoxic Ca²⁺ accumulation contributes to ischemic neurodegeneration, and Ca²⁺ can enter the mitochondria through the mitochondrial calcium uniporter (MCU) to promote mitochondrial dysfunction. Yet, Ca²⁺-targeted therapies have met limited success. A growing body of evidence has highlighted the underappreciated importance of Zn²⁺, which also accumulates in neurons after ischemia and can induce mitochondrial dysfunction and cell death. While studies have indicated that Zn²⁺ can also enter the mitochondria through the MCU, the specificity of the pore's role in Zn²⁺-triggered injury is still debated. Present studies use recently available MCU knockout mice to examine how the deletion of this channel impacts deleterious effects of cytosolic Zn²⁺ loading. In cultured cortical neurons from MCU knockout mice, we find significantly reduced mitochondrial Zn²⁺ accumulation. Correspondingly, these neurons were protected from both acute and delayed Zn²⁺-triggered mitochondrial dysfunction, including mitochondrial reactive oxygen species generation, depolarization, swelling and inhibition of respiration. Furthermore, when toxic extramitochondrial effects of Ca²⁺ entry were moderated, both cultured neurons (exposed to Zn²⁺) and CA1 neurons of hippocampal slices (subjected to prolonged oxygen glucose deprivation to model ischemia) from MCU knockout mice displayed decreased neurodegeneration. Finally, to examine the therapeutic applicability of these findings, we added an MCU blocker after toxic Zn²⁺ exposure in wildtype neurons (to induce post-insult MCU blockade). This significantly attenuated the delayed evolution of both mitochondrial dysfunction and neurotoxicity. These data-combining both genetic and pharmacologic tools-support the hypothesis that Zn²⁺ entry through the MCU is a critical contributor to ischemic neurodegeneration that could be targeted for neuroprotection.

The redox biology of redox-inert zinc ions

Zinc(II) ions are redox-inert in biology. Yet, their interaction with sulfur of cysteine in cellular proteins can confer ligand-centered redox activity on zinc coordination sites, control protein functions, and generate signalling zinc ions as potent effectors of many cellular processes. The specificity and relative high affinity of binding sites for zinc allow regulation in redox biology, free radical biology, and the biology of reactive species. Understanding the role of zinc in these areas of biology requires an understanding of how cellular Zn²⁺ is homeostatically controlled and can serve as a regulatory ion in addition to Ca²⁺, albeit at much lower concentrations. A rather complex system of dozens of transporters and metallothioneins buffer the relatively high (hundreds of micromolar) total cellular zinc concentrations in such a way that the available zinc ion concentrations are only picomolar but can fluctuate in signalling. The proteins targeted by Zn²⁺ transients include enzymes controlling phosphorylation and redox signalling pathways. Networks of regulatory functions of zinc integrate gene expression and metabolic and signalling pathways at several hierarchical levels. They affect enzymatic catalysis, protein structure and protein-protein/biomolecular interactions and add to the already impressive number of catalytic and structural functions of zinc in an estimated three thousand human zinc proteins. The effects of zinc on redox biology have adduced evidence that zinc is an antioxidant. Without further qualifications, this notion is misleading and prevents a true understanding of the roles of zinc in biology. Its antioxidant-like effects are indirect and expressed only in certain conditions because a lack of zinc and too much zinc have pro-oxidant effects. Teasing apart these functions based on quantitative considerations of homeostatic control of cellular zinc is critical because opposite consequences are observed depending on the concentrations of zinc: pro- or anti-apoptotic, pro- or anti-inflammatory and cytoprotective or cytotoxic. The article provides a biochemical basis for the links between redox and zinc biology and discusses why zinc has pleiotropic functions. Perturbation of zinc metabolism is a consequence of conditions of redox stress. Zinc deficiency, either nutritional or conditioned, and cellular zinc overload cause oxidative stress. Thus, there is causation in the relationship between zinc metabolism and the many diseases associated with oxidative stress.

Agonist-Evoked Increases in Intra-Platelet Zinc Couple to Functional Responses

Background  Zinc (Zn ²⁺ ) is an essential trace element that regulates intracellular processes in multiple cell types. While the role of Zn ²⁺ as a platelet agonist is known, its secondary messenger activity in platelets has not been demonstrated.

Objectives  This article determines whether cytosolic Zn ²⁺ concentrations ([Zn ²⁺ ] _i ) change in platelets in response to agonist stimulation, in a manner consistent with a secondary messenger, and correlates the effects of [Zn ²⁺ ] _i changes on activation markers.

Methods  Changes in [Zn ²⁺ ] _i were quantified in Fluozin-3 (Fz-3)-loaded washed, human platelets using fluorometry. Increases in [Zn ²⁺ ] _i were modelled using Zn ²⁺ -specific chelators and ionophores. The influence of [Zn ²⁺ ] _i on platelet function was assessed using platelet aggregometry, flow cytometry and Western blotting.

Results  Increases of intra-platelet Fluozin-3 (Fz-3) fluorescence occurred in response to stimulation by cross-linked collagen-related peptide (CRP-XL) or U46619, consistent with a rise of [Zn ²⁺ ] _i . Fluoresence increases were blocked by Zn ²⁺ chelators and modulators of the platelet redox state, and were distinct from agonist-evoked [Ca ²⁺ ] _i signals. Stimulation of platelets with the Zn ²⁺ ionophores clioquinol (Cq) or pyrithione (Py) caused sustained increases of [Zn ²⁺ ] _i , resulting in myosin light chain phosphorylation, and cytoskeletal re-arrangements which were sensitive to cytochalasin-D treatment. Cq stimulation resulted in integrin α _IIb β ₃ activation and release of dense, but not α, granules. Furthermore, Zn ²⁺ -ionophores induced externalization of phosphatidylserine.

Conclusion  These data suggest that agonist-evoked fluctuations in intra-platelet Zn ²⁺ couple to functional responses, in a manner that is consistent with a role as a secondary messenger. Increased intra-platelet Zn ²⁺ regulates signalling processes, including shape change, α _IIb β ₃ up-regulation and dense granule release, in a redox-sensitive manner.

Mitochondrial Zn2+ Accumulation: A Potential Trigger of Hippocampal Ischemic Injury

Ischemic stroke is a major cause of death and disabilities worldwide, and it has been long hoped that improved understanding of relevant injury mechanisms would yield targeted neuroprotective therapies. While Ca²⁺ overload during ischemia-induced glutamate excitotoxicity has been identified as a major contributor, failures of glutamate targeted therapies to achieve desired clinical efficacy have dampened early hopes for the development of new treatments. However, additional studies examining possible contributions of Zn²⁺, a highly prevalent cation in the brain, have provided new insights that may help to rekindle the enthusiasm. In this review, we discuss both old and new findings yielding clues as to sources of the Zn²⁺ that accumulates in many forebrain neurons after ischemia, and mechanisms through which it mediates injury. Specifically, we highlight the growing evidence of important Zn²⁺ effects on mitochondria in promoting neuronal injury. A key focus has been to examine Zn²⁺ contributions to the degeneration of highly susceptible hippocampal pyramidal neurons. Recent studies provide evidence of differences in sources of Zn²⁺ and its interactions with mitochondria in CA1 versus CA3 neurons that may pertain to their differential vulnerabilities in disease. We propose that Zn²⁺-induced mitochondrial dysfunction is a critical and potentially targetable early event in the ischemic neuronal injury cascade, providing opportunities for the development of novel neuroprotective strategies to be delivered after transient ischemia.

Critical Role of Zinc as Either an Antioxidant or a Prooxidant in Cellular Systems

Zinc is recognized as an essential trace metal required for human health; its deficiency is strongly associated with neuronal and immune system defects. Although zinc is a redox-inert metal, it functions as an antioxidant through the catalytic action of copper/zinc-superoxide dismutase, stabilization of membrane structure, protection of the protein sulfhydryl groups, and upregulation of the expression of metallothionein, which possesses a metal-binding capacity and also exhibits antioxidant functions. In addition, zinc suppresses anti-inflammatory responses that would otherwise augment oxidative stress. The actions of zinc are not straightforward owing to its numerous roles in biological systems. It has been shown that zinc deficiency and zinc excess cause cellular oxidative stress. To gain insights into the dual action of zinc, as either an antioxidant or a prooxidant, and the conditions under which each role is performed, the oxidative stresses that occur in zinc deficiency and zinc overload in conjunction with the intracellular regulation of free zinc are summarized. Additionally, the regulatory role of zinc in mitochondrial homeostasis and its impact on oxidative stress are briefly addressed.

Mechanistic insights into the protective impact of zinc on sepsis

Sepsis, a systemic inflammation as a response to a bacterial infection, is a huge unmet medical need. Data accumulated over the last decade suggest that the nutritional status of patients as well as composition of their gut microbiome, are strongly linked with the risk to develop sepsis, the severity of the disease and prognosis. In particular, the essential micronutrient zinc is essential in the resistance against sepsis and has shown to be protective in animal models as well as in human patients. The potential mechanisms by which zinc protects in sepsis are discussed in this review paper: we will focus on the inflammatory response, chemotaxis, phagocytosis, immune response, oxidative stress and modulation of the microbiome. A full understanding of the mechanism of action of zinc may open new preventive and therapeutic interventions in sepsis.

Thermodynamics of Pb(ii) and Zn(ii) binding to MT-3, a neurologically important metallothionein

Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of Pb(2+) and Zn(2+) binding to metallothionein-3 (MT-3). Pb(2+) binds to zinc-replete Zn7MT-3 displacing each zinc ion with a similar change in free energy (ΔG) and enthalpy (ΔH). EDTA chelation measurements of Zn7MT-3 and Pb7MT-3 reveal that both metal ions are extracted in a tri-phasic process, indicating that they bind to the protein in three populations with different binding thermodynamics. Metal binding is entropically favoured, with an enthalpic penalty that reflects the enthalpic cost of cysteine deprotonation accompanying thiolate ligation of the metal ions. These data indicate that Pb(2+) binding to both apo MT-3 and Zn7MT-3 is thermodynamically favourable, and implicate MT-3 in neuronal lead biochemistry.

Tetrabromobisphenol A disturbs zinc homeostasis in cultured cerebellar granule cells: A dual role in neurotoxicity

The brominated flame retardant tetrabromobisphenol A (TBBPA) has recognized neurotoxic properties mediated by intracellular Ca²⁺ imbalance and oxidative stress. Although these factors are known to trigger the release of Zn²⁺ from intracellular stores, the effects of TBBPA on Zn²⁺ homeostasis in neurons and the role of Zn²⁺in TBBPA neurotoxicity have not yet been studied. Therefore, we investigated zinc transients in primary cultures of rat cerebellar granule cells and assessed their involvement in TBBPA neurotoxicity. The results demonstrate that TBBPA releases Zn²⁺ from the intracellular stores and increases its intracellular concentration, followed by Zn²⁺ displacement from the cells. TBBPA-evoked Zn²⁺ transients are partially mediated by Ca²⁺ and ROS. Application of TPEN, Zn²⁺ chelator, potentiates TBBPA- and glutamate-induced ⁴⁵Ca uptake, enhances TBBPA-induced ROS production and potentiates decreases in the ΔΨm in cells treated with 25 μM TBBPA, revealing the potential neuroprotective capacity of endogenous Zn²⁺. However, the administration of TPEN does not aggravate TBBPA neurotoxicity, and even slightly decreases neuronal death induced by 25 μM TBBPA. In summary, it was shown for the first time that TBBPA interferes with the cellular Zn²⁺ homeostasis in neuronal cultures, and we revealed complex roles for endogenous Zn²⁺ in cytoprotection and TBBPA toxicity in cultured neurons.

Dysregulated Zn2+ homeostasis impairs cardiac type-2 ryanodine receptor and mitsugumin 23 functions, leading to sarcoplasmic reticulum Ca2+ leakage

Aberrant Zn²⁺ homeostasis is associated with dysregulated intracellular Ca²⁺ release, resulting in chronic heart failure. In the failing heart a small population of cardiac ryanodine receptors (RyR2) displays sub-conductance-state gating leading to Ca²⁺ leakage from sarcoplasmic reticulum (SR) stores, which impairs cardiac contractility. Previous evidence suggests contribution of RyR2-independent Ca²⁺ leakage through an uncharacterized mechanism. We sought to examine the role of Zn²⁺ in shaping intracellular Ca²⁺ release in cardiac muscle. Cardiac SR vesicles prepared from sheep or mouse ventricular tissue were incorporated into phospholipid bilayers under voltage-clamp conditions, and the direct action of Zn²⁺ on RyR2 channel function was examined. Under diastolic conditions, the addition of pathophysiological concentrations of Zn²⁺ (≥2 nm) caused dysregulated RyR2-channel openings. Our data also revealed that RyR2 channels are not the only SR Ca²⁺-permeable channels regulated by Zn²⁺. Elevating the cytosolic Zn²⁺ concentration to 1 nm increased the activity of the transmembrane protein mitsugumin 23 (MG23). The current amplitude of the MG23 full-open state was consistent with that previously reported for RyR2 sub-conductance gating, suggesting that in heart failure in which Zn²⁺ levels are elevated, RyR2 channels do not gate in a sub-conductance state, but rather MG23-gating becomes more apparent. We also show that in H9C2 cells exposed to ischemic conditions, intracellular Zn²⁺ levels are elevated, coinciding with increased MG23 expression. In conclusion, these data suggest that dysregulated Zn²⁺ homeostasis alters the function of both RyR2 and MG23 and that both ion channels play a key role in diastolic SR Ca²⁺ leakage.

Antioxidant and anti-inflammatory effects of zinc. Zinc-dependent NF-κB signaling

Zinc is a nutritionally fundamental trace element, essential to the structure and function of numerous macromolecules, including enzymes regulating cellular processes and cellular signaling pathways. The mineral modulates immune response and exhibits antioxidant and anti-inflammatory activity. Zinc retards oxidative processes on a long-term basis by inducing the expression of metallothioneins. These metal-binding cysteine-rich proteins are responsible for maintaining zinc-related cell homeostasis and act as potent electrophilic scavengers and cytoprotective agents. Furthermore, zinc increases the activation of antioxidant proteins and enzymes, such as glutathione and catalase. On the other hand, zinc exerts its antioxidant effect via two acute mechanisms, one of which is the stabilization of protein sulfhydryls against oxidation. The second mechanism consists in antagonizing transition metal-catalyzed reactions. Zinc can exchange redox active metals, such as copper and iron, in certain binding sites and attenuate cellular site-specific oxidative injury. Studies have demonstrated that physiological reconstitution of zinc restrains immune activation, whereas zinc deficiency, in the setting of severe infection, provokes a systemic increase in NF-κB activation. In vitro studies have shown that zinc decreases NF-κB activation and its target genes, such as TNF-α and IL-1β, and increases the gene expression of A20 and PPAR-α, the two zinc finger proteins with anti-inflammatory properties. Alternative NF-κB inhibitory mechanism is initiated by the inhibition of cyclic nucleotide phosphodiesterase, whereas another presumed mechanism consists in inhibition of IκB kinase in response to infection by zinc ions that have been imported into cells by ZIP8.

Zinc and Excitotoxic Brain Injury: A New Model

It has been nearly 15 years since the suggestion that synaptically released Zn2+ might contribute to excitotoxic brain injury after seizures, stroke, and brain trauma. In the original “zinc-translocation” model, it was proposed that synaptically released Zn2+ ions penetrated postsynaptic neurons, causing injury. According to the model, chelating zinc in the cleft was predicted to be neuroprotective. This proved to be true: zinc chelators have proved to be remarkably potent at reducing excitotoxic neuronal injury in many paradigms. Promising new zinc-based therapies for stroke, head trauma, and epileptic brain injury are under development. However, new evidence suggests that the original translocation model was incomplete. As many as three sources of toxic zinc ions may contribute to excitotoxicity: presynaptic vesicles, postsynaptic zincsequestering proteins, and (more speculatively) mitochondrial pools. The authors present a new model of zinc currents and zinc toxicity that offers expanded opportunities for zinc-selective therapeutic chelation interventions.

Intracellular Zinc Modulates Cardiac Ryanodine Receptor-mediated Calcium Release

Aberrant Zn²⁺ homeostasis is a hallmark of certain cardiomyopathies associated with altered contractile force. In this study, we addressed whether Zn²⁺ modulates cardiac ryanodine receptor gating and Ca²⁺ dynamics in isolated cardiomyocytes. We reveal that Zn²⁺ is a high affinity regulator of RyR2 displaying three modes of operation. Picomolar free Zn²⁺ concentrations potentiate RyR2 responses, but channel activation is still dependent on the presence of cytosolic Ca²⁺. At concentrations of free Zn²⁺ >1 nm, Zn²⁺ is the main activating ligand, and the dependence on Ca²⁺ is removed. Zn²⁺ is therefore a higher affinity activator of RyR2 than Ca²⁺. Millimolar levels of free Zn²⁺ were found to inhibit channel openings. In cardiomyocytes, consistent with our single channel results, we show that Zn²⁺ modulates both the frequency and amplitude of Ca²⁺ waves in a concentration-dependent manner and that physiological levels of Zn²⁺ elicit Ca²⁺ release in the absence of activating levels of cytosolic Ca²⁺. This highlights a new role for intracellular Zn²⁺ in shaping Ca²⁺ dynamics in cardiomyocytes through modulation of RyR2 gating.

The influence of zinc(ii) on thioredoxin/glutathione disulfide exchange: QM/MM studies to explore how zinc(ii) accelerates exchange in higher dielectric environments

QM/MM calculations on thiolate disulfide exchange reveal that a polar, 4-centered, cyclic transition state is formed when Zn(ii) is present, helping to explain faster exchange rates in higher dielectric solvents for metal-assisted exchange.

Age-dependent astroglial vulnerability to hypoxia and glutamate: the role for erythropoietin

Extracellular accumulation of toxic concentrations of glutamate (Glu) is a hallmark of many neurodegenerative diseases, often accompanied by hypoxia and impaired metabolism of this neuromediator. To address the question whether the multifunctional neuroprotective action of erythropoietin (EPO) extends to the regulation of extracellular Glu-level and is age-related, young and culture-aged rat astroglial primary cells (APC) were simultaneously treated with 1mM Glu and/or human recombinant EPO under normoxic and hypoxic conditions (NC and HC). EPO increased the Glu uptake by astrocytes under both NC and especially upon HC in culture-aged APC (by 60%). Moreover, treatment with EPO up-regulated the activity of glutamine synthetase (GS), the expression of glutamate-aspartate transporter (GLAST) and the level of EPO mRNA. EPO alleviated the Glu- and hypoxia-induced LDH release from astrocytes. These protective EPO effects were concentration-dependent and they were strongly intensified with age in culture. More than a 4-fold increase in apoptosis and a 2-fold decrease in GS enzyme activity was observed in APC transfected with EPO receptor (EPOR)-siRNA. Our in vivo data show decreased expression of EPO and a strong increase of EPOR in brain homogenates of APP/PS1 mice and their wild type controls during aging. Comparison of APP/PS1 and age-matched WT control mice revealed a stronger expression of EPOR but a weaker one of EPO in the Alzheimer's disease (AD) model mice. Here we show for the first time the direct correlation between the extent of differentiation (age) of astrocytes and the efficacy of EPO in balancing extracellular glutamate clearance and metabolism in an in-vitro model of hypoxia and Glu-induced astroglial injury. The clinical relevance of EPO and EPOR as markers of brain cells vulnerability during aging and neurodegeneration is evidenced by remarkable changes in their expression levels in a transgenic model of AD and their WT controls.

The role of metallothionein in oxidative stress

Free radicals are chemical particles containing one or more unpaired electrons, which may be part of the molecule. They cause the molecule to become highly reactive. The free radicals are also known to play a dual role in biological systems, as they can be either beneficial or harmful for living systems. It is clear that there are numerous mechanisms participating on the protection of a cell against free radicals. In this review, our attention is paid to metallothioneins (MTs) as small, cysteine-rich and heavy metal-binding proteins, which participate in an array of protective stress responses. The mechanism of the reaction of metallothioneins with oxidants and electrophilic compounds is discussed. Numerous reports indicate that MT protects cells from exposure to oxidants and electrophiles, which react readily with sulfhydryl groups. Moreover, MT plays a key role in regulation of zinc levels and distribution in the intracellular space. The connections between zinc, MT and cancer are highlighted.

Effects of a copper-deficient diet on the biochemistry, neural morphology and behavior of aged mice

Copper dyshomeostasis has been suggested as an aetiological risk factor for some neurodegenerative diseases, such as Alzheimer’s disease. However, the precise mechanism at the base of this involvement is still obscure. In this work, we show the effects of a copper-deficient diet in aged CD1 mice and the influence of such a diet on: a) the concentration of various metal ions (aluminium, copper, iron, calcium, zinc) in the main organs and in different brain areas; b) the alteration of metallothioneins I-II and tyrosine hydroxylase immunopositivity in the brain; c) behavioural tests (open field, pole, predatory aggression, and habituation/dishabituation smell tests). Our data suggested that the copper-deficiency was able to produce a sort of “domino effect” which altered the concentration of the other tested metal ions in the main organs as well as in the brain, without, however, significantly affecting the animal behaviour.

Redox regulation of intracellular zinc: molecular signaling in the life and death of neurons

Zn(2+) has emerged as a major regulator of neuronal physiology, as well as an important signaling agent in neural injury. The intracellular concentration of this metal is tightly regulated through the actions of Zn(2+) transporters and the thiol-rich metal binding protein metallothionein, closely linking the redox status of the cell to cellular availability of Zn(2+). Accordingly, oxidative and nitrosative stress during ischemic injury leads to an accumulation of neuronal free Zn(2+) and the activation of several downstream cell death processes. While this Zn(2+) rise is an established signaling event in neuronal cell death, recent evidence suggests that a transient, sublethal accumulation of free Zn(2+) can also play a critical role in neuroprotective pathways activated during ischemic preconditioning. Thus, redox-sensitive proteins, like metallothioneins, may play a critical role in determining neuronal cell fate by regulating the localization and concentration of intracellular free Zn(2+).

The toxicity redox mechanisms of cadmium alone or together with copper and zinc homeostasis alteration: its redox biomarkers

Cadmium (Cd) is a toxic metal and can induce and/or promote diseases in humans (cancer, aging diseases, kidney and bone diseases, etc.). Its toxicity involves many mechanisms including the alteration of copper (Cu) and zinc (Zn) homeostasis leading to reactive oxygen species (ROS) production, either directly or through the inhibition of antioxidant activities. Importantly, ROS can induce oxidative damages in cells. Cadmium, Cu and Zn are also able to induce glutathione (GSH) and metallothioneins (MT) synthesis in a cell-type-dependent manner. As a consequence, the effects induced by these three metals result simultaneously from the inhibition of antioxidant activities and the induction of other factors such as GSH and MT synthesis. MT levels are regulated not only by the p53 protein in a cell-type-dependent manner, or by transcription factors such as metal-responsive transcription factor 1 (MTF-1) and cellular Zn levels but also by cellular GSH level. As described in the literature, DNA damage, GSH and MT levels are sensitive biomarkers used to identify Cd-induced toxicity alone or together with Cu and Zn homeostasis alteration.

Redox biochemistry of mammalian metallothioneins

Metallothionein (MT) is a generic name for certain families of structurally rather variable metal-binding proteins. While purely chemical or biological approaches failed to establish a single physiologic function for MTs in any species, a combination of chemical and biological approaches and recent progress in defining the low but significant concentrations of cytosolic free zinc(II) ions have demonstrated that mammalian MTs function in cellular zinc metabolism in specific ways that differ from conventional knowledge about any other metalloprotein. Their thiolate coordination environments make MTs redox-active zinc proteins that exist in different molecular states depending on the availability of cellular zinc and the redox poise. The zinc affinities of MTs cover a range of physiologic zinc(II) ion concentrations and are modulated. Oxidative conditions make more zinc available, while reductive conditions make less zinc available. MTs move from the cytosol to cellular compartments, are secreted from cells, and are taken up by cells. They provide cellular zinc ions in a chemically available form and participate in cellular metal muffling: the combination of physiologic buffering in the steady state and the cellular redistribution and compartmentalization of transiently elevated zinc(II) ion concentrations in the pre-steady state. Cumulative evidence indicates that MTs primarily have a redox-dependent function in zinc metabolism, rather than a zinc-dependent function in redox metabolism.

Measuring steady-state and dynamic endoplasmic reticulum and Golgi Zn2+ with genetically encoded sensors

Zn(2+) plays essential roles in biology, and cells have adopted exquisite mechanisms for regulating steady-state Zn(2+) levels. Although much is known about total Zn(2+) in cells, very little is known about its subcellular distribution. Yet defining the location of Zn(2+) and how it changes with signaling events is essential for elucidating how cells regulate this essential ion. Here we create fluorescent sensors genetically targeted to the endoplasmic reticulum (ER) and Golgi to monitor steady-state Zn(2+) levels as well as flux of Zn(2+) into and out of these organelles. These studies reveal that ER and Golgi contain a concentration of free Zn(2+) that is 100 times lower than the cytosol. Both organelles take up Zn(2+) when cytosolic levels are elevated, suggesting that the ER and Golgi can sequester elevated cytosolic Zn(2+) and thus have the potential to play a role in influencing Zn(2+) toxicity. ER Zn(2+) homeostasis is perturbed by small molecule antagonists of Ca(2+) homeostasis and ER Zn(2+) is released upon elevation of cytosolic Ca(2+) pointing to potential exchange of these two ions across the ER. This study provides direct evidence that Ca(2+) signaling can influence Zn(2+) homeostasis and vice versa, that Zn(2+) dynamics may modulate Ca(2+) signaling.

Mobilizing Cu(I) for carbon-carbon bond forming catalysis in the presence of thiolate. Chemical mimicking of metallothioneins

Copper(I) is rendered catalytically viable in the presence of thiolate by the design of a small molecule chemical analogue of the metallothionein system in which an N-O reactant serves the same conceptual purpose of the S-S reactant of the biological system.

Zinc oxide nanoparticles in modern sunscreens: an analysis of potential exposure and hazard

Sunscreens containing metal oxide nanoparticles appear transparent on the skin and provide excellent protection against sunburn caused by UV radiation. While it is likely that nanoparticles remain on the surface of the skin of healthy adult humans, and thus are considered safe for use in sunscreens, there has been no comprehensive assessment of the impact on human health from exposure to the metal oxide nanoparticles destined for use in sunscreens, either in the workplace during the manufacturing process, in long-term use across a range of skin conditions, or upon release into the broader environment, either accidentally or consequent of normal sunscreen use. In this review, we focus on zinc oxide nanoparticles destined for use in modern sunscreens, and discuss the potential for human exposure and the health hazard at each stage of their manufacture and use. We highlight where there is a need for further research.

Cardioprotective effect of zinc requires ErbB2 and Akt during hypoxia/reoxygenation

Recent literature suggests that exogenous zinc can prevent ischemia reperfusion injury by activating phosphoinositide-3 kinase (PI3K)/Akt and by targeting the mitochondrial permeability transition pore (mPTP). It is known that ErbB2 expression promotes association and activation of PI3-kinase/Akt, resulting in growth and survival of cardiac myocytes. In this study, we found that zinc-induced ErbB2 protein expression and Akt activation are required for preventing reperfusion injury. Neonatal rat cardiac myocytes subjected to 8 h of hypoxia, followed by 16 h of reoxygenation induced cardiomyocyte apoptosis, as assessed by increased caspase-3 activity, annexin V staining and lowered MTT reduction and ATP levels. However, addition of zinc-pyrithione (ZPT) before onset of reoxygenation effectively lowered the apoptotic indices and restored the ATP levels. ZPT induced a significant increase in ErbB2 protein expression and Akt activation. Pretreatment with Hsp 90 inhibitor, geldanamycin or PI3-kinase inhibitor, wortmannin prevented the increase in ATP levels and abrogated the protective effect of zinc-pyrithione. Taken together, these data suggest that zinc prevents reperfusion injury by modulating the ErbB2 protein expression and Akt activation.

The relationship between transient zinc ion fluctuations and redox signaling in the pathways of secondary cellular injury: relevance to traumatic brain injury

A major obstacle that hampers the design of drug therapy for traumatic brain injury is the incomplete understanding of the biochemical pathways that lead to secondary cellular injury and contribute to cell death. One such pathway involves reactive species that generate potentially cytotoxic zinc ion fluctuations as a major executor of neuronal, and possibly glial, cell death. Whether zinc ions released during traumatic brain injury are toxic or protective is controversial but can be approached by investigating the exact concentrations of free zinc ions, the thresholds of compromised zinc buffering capacity, and the mechanism of cellular homeostatic control of zinc. Rapidly stretch-injured rat pheochromocytoma (PC12) cells express cellular zinc ion fluctuations that depend on the production of nitric oxide. Chelation of cellular zinc ions after rapid stretch injury, however, increases cellular reactive oxygen species. In a rat model of traumatic brain injury, parasagittal fluid percussion, analysis of the metal load of metallothionein was used as an indicator of changes in cellular zinc ion concentrations. The combined results from the cellular and in vivo investigations caution against interpreting zinc ion fluctuations in the early phase (24h) after injury as a primarily cytotoxic event.

Metallothionein deficiency leads to soleus muscle contractile dysfunction following acute spinal cord injury in mice

Metallothionein (MT) is a small molecular weight protein possessing metal binding and free radical scavenging properties. We hypothesized that MT-1/MT-2 null (MT(-/-)) mice would display exacerbated soleus muscle atrophy, oxidative injury, and contractile dysfunction compared with the response of wild-type (WT) mice following acute spinal cord transection (SCT). Four groups of mice were studied: WT laminectomy, WT transection, MT(-/-) laminectomy (MT(-/-) lami), and MT(-/-) transection (MT(-/-) trans). Laminectomy animals served as surgical controls. Mice in SCT groups experienced similar percent body mass (BM) losses at 7 days postinjury. Soleus muscle mass (MM) and MM-to-BM ratio were lower at 7 days postinjury in SCT vs. laminectomy mice, with no differences observed between strains. However, soleus muscles from MT(-/-) trans mice showed reduced maximal specific tension compared with MT(-/-) lami animals. Mean cross-sectional area (microm(2)) of type I and type IIa fibers decreased similarly in SCT groups compared with laminectomy controls, and no difference in fiber distribution was observed. Lipid peroxidation (4-hydroxynoneal) was greater in MT(-/-) trans vs. MT(-/-) lami mice, but protein oxidation (protein carbonyls) was not altered by MT deficiency or SCT. Expression of key antioxidant proteins (catalase, manganese, and copper-zinc superoxide dismutase) was similar between the groups. In summary, MT deficiency did not impact soleus MM loss, but resulted in contractile dysfunction and increased lipid peroxidation following acute SCT. These findings suggest a role of MT in mediating protective adaptations in skeletal muscle following disuse mediated by spinal cord injury.

Zinc: the brain's dark horse

Zinc is a life-sustaining trace element, serving structural, catalytic, and regulatory roles in cellular biology. It is required for normal mammalian brain development and physiology, such that deficiency or excess of zinc has been shown to contribute to alterations in behavior, abnormal central nervous system development, and neurological disease. In this light, it is not surprising that zinc ions have now been shown to play a role in the neuromodulation of synaptic transmission as well as in cortical plasticity. Zinc is stored in specific synaptic vesicles by a class of glutamatergic or "gluzinergic" neurons and is released in an activity-dependent manner. Because gluzinergic neurons are found almost exclusively in the cerebral cortex and limbic structures, zinc may be critical for normal cognitive and emotional functioning. Conversely, direct evidence shows that zinc might be a relatively potent neurotoxin. Neuronal injury secondary to in vivo zinc mobilization and release occurs in several neurological disorders such as Alzheimer's disease and amyotrophic lateral sclerosis, in addition to epilepsy and ischemia. Thus, zinc homeostasis is integral to normal central nervous system functioning, and in fact its role may be underappreciated. This article provides an overview of zinc neurobiology and reviews the experimental evidence that implicates zinc signals in the pathophysiology of neuropsychiatric diseases. A greater understanding of zinc's role in the central nervous system may therefore allow for the development of therapeutic approaches where aberrant metal homeostasis is implicated in disease pathogenesis.

Comparison of uptake and neuroprotective potential of seven zinc-salts

Zinc plays an important role as an antioxidant in different cells treated with various kinds of oxidative stressors. Although intracellular Zn(2+) is important in many cellular events, little is known about the cellular uptake of this trace metal and the intracellular status that is required for its optimal function. Since previous reports usually employed only one type of zinc-salt, in this work was compared cellular uptake and antioxidative potential of seven zinc-salts in order to discriminate whether different counterions and ligands may influence its function. Oxidative stress was induced by peroxide or iron in neuronal PC12 cells. We compared uptake of zinc-salts into the labile Zn(2+) pool of PC12 cells as well as their effects on the prevention of cell death, glutathione depletion, lipid peroxidation and ROS production. Zinc-salts provided better protection against oxidative stress-induced in PC12 cultures by peroxide than by iron. Preincubations with zinc-salts displayed better neuroprotection in all cases than coincubations. Zinc-histidine complex was shown to be the most potent compound. Our results indicated that protective effect of zinc is not related to its uptake into PC12 cells, what is indicated by the rather low salt concentrations required for the cell protection and by the observation that despite a superior antioxidant effect of zinc-histidine, the uptake of this salt by PC12 cells was remarkably lower in comparison with other zinc-salts. Although zinc-sulfate exerted weak neuroprotective potential, accumulation of Zn(2+) from this salt within cells was significantly higher compared to other salts. The differences in accumulation of zinc-salts were not specific and unique to PC12 cells, since similar results were obtained in rat primary hepatocytes and endothelial HUVEC cells.