Depletion of intracellular zinc induces macromolecule synthesis- and caspase-dependent apoptosis of cultured retinal cells

Effect of Zn2+ on emodin molecules studied by time-resolved fluorescence spectroscopy and quantum chemical calculations

Emodin is a natural drug for treating neurodegenerative diseases and plays a vital role in the mitigation of nerve damage. Metal ions can modify the drug properties of emodin, where Zn²⁺ can synergize with the emodin molecule and enhance the drug effect of emodin. Besides, complex changes can be observed in the fluorescence intensity and fluorescence lifetime of the emodin molecule as the concentration of Zn²⁺ increases. Herein, the synergistic effects of ligand structural in Zn(II)-Emodin complexes and the electronic effects of metal elements on the antioxidant properties of the complexes are discussed in detail based on UV-vis absorption spectroscopy, fluorescence spectroscopy, time-correlated single photon counting (TCSPC) technique and quantum chemical calculations at the B3LYP/6-31G(d) level. The experimental results confirm that Zn²⁺ can coordinate with the hydroxyl groups on the emodin to make the molecule structure more rigid, thus inhibiting the non-radiative processes such as high-frequency vibrations of the emodin molecule in solution. The suppression of non-radiative processes leads to an increase in the average fluorescence lifetime of the emodin molecule, and finally results in the enhanced fluorescence intensity. The chemical softness of Zn(II)-Emodin is then confirmed to be higher than that of emodin by Gaussian calculations, indicating its higher chemical reactivity and lower stability. The stronger electron donating ability of Zn(II)-Emodin compared to emodin may explain the higher antioxidant activity of Zn(II)-Emodin, which gives it a stronger pharmacological activity. The results of this study show that emodin can well complex with Zn²⁺ to remove excess Zn²⁺ in human body and the resulting complex has better antioxidant properties, which helps to understand the role of Zn²⁺ in drug-metal coordination and provides guidance for the design of new drugs.

Effects of zinc deficiency on impaired spermatogenesis and male infertility: the role of oxidative stress, inflammation and apoptosis

Zinc (Zn) is necessary for the normal function of the male reproductive system and spermatozoa. Although influences of zinc deficiency on impaired spermatogenesis and male infertility have been widely considered, the molecular and cellular mechanisms of these abnormalities are not well understood. General abnormalities, including hypogonadism, Leydig cells damage, deficiency of sex hormone production and impaired spermatogenesis, as well as inflammation, antioxidant depletion, sperm death and male infertility can be observed during zinc deficiency. However, it is not obvious which pathways are relevant to the pathogenesis of zinc deficiency. Oxidative stress (OS) induced by reactive oxygen species is likely as the main mechanism of zinc deficiency which is associated with sperm DNA fragmentation, decrease in sperm membrane integrity, apoptosis, depletion of antioxidants, and consequently poor sperm quality and male infertility. Therefore, identification of these pathways will give valuable information regarding the mechanisms of zinc deficiency on the male reproductive system and the potential way for developing a better clinical approach. In this review, we aim to discuss the proposed cellular and molecular mechanisms of zinc deficiency on the male reproductive system, the importance of OS and mechanisms by which zinc deficiency induces OS and depletion of other antioxidants.

The mucolipin-1 (TRPML1) ion channel, transmembrane-163 (TMEM163) protein, and lysosomal zinc handling

Lysosomes are emerging as important players in cellular zinc ion (Zn²⁺) homeostasis. The series of work on Zn2+ accumulation in the neuronal lysosomes and the mounting evidence on the role of lysosomal Zn²⁺ in cell death during mammary gland involution set a biological precedent for the central role of the lysosomes in cellular Zn²⁺ handling. Such a role appears to involve cytoprotection on the one hand, and cell death on the other. The recent series of work began to identify the molecular determinants of the lysosomal Zn²⁺ handling. In addition to zinc transporters (ZnT) of the solute-carrier family type 30A (SLC30A), the lysosomal ion channel TRPML1 and the poorly understood novel transporter TMEM163 have been shown to play a role in the Zn²⁺ uptake by the lysosomes. In this review, we summarize the current knowledge on molecular determinants of the lysosomal Zn²⁺ handling, uptake, and release pathways, as well as discuss their possible roles in health and disease.

Low zinc environment induces stress signaling, senescence and mixed cell death modalities in colon cancer cells

Currently it is not clear what type of the final cellular response (i.e. cell death modality or senescence) is induced upon chronic intracellular zinc depletion in colon cancer cells. To address this question, isogenic colon cancer lines SW480 and SW620 exposed to low zinc environment were studied over the period of 6 weeks. Low zinc environment reduced total as well as free intracellular zinc content in both cell lines. Decreased intracellular zinc content resulted in changes in cellular proliferation, cell cycle distribution and activation of stress signaling. In addition, colonocytes with low zinc content displayed increased levels of oxidative stress, changes in mitochondrial activity but in the absence of significant DNA damage. Towards the end of treatment (4th-6th week), exposed cells started to change morphologically, and typical markers of senescence as well as cell death appeared. Of two examined colon cancer cell lines, SW480 cells proved to activate predominantly senescent phenotype, with frequent form of demise being necrosis and mixed cell death modality but not apoptosis. Conversely, SW620 cells activated mostly cell death, with relatively equal distribution of apoptosis and mixed types, while senescent phenotypes and necrosis were present only in a small fraction of cell populations. Addition of zinc at the beginning of 4th week of treatment significantly suppressed cell death phenotypes in both cell lines but had no significant effect on senescence. In conclusion, presented results demonstrate variability of responses to chronic zinc depletion in colon cancer as modeled in vitro.

Zinc deficiency leads to lipofuscin accumulation in the retinal pigment epithelium of pigmented rats

Background

Age-related macular degeneration (AMD) is associated with lipofuscin accumulation whereas the content of melanosomes decreases. Melanosomes are the main storage of zinc in the pigmented tissues. Since the elderly population, as the most affected group for AMD, is prone to zinc deficit, we investigated the chemical and ultrastructural effects of zinc deficiency in pigmented rat eyes after a six-month zinc penury diet.

Methodology/Principal Findings

Adult Long Evans (LE) rats were investigated. The control animals were fed with a normal alimentation whereas the zinc-deficiency rats (ZD-LE) were fed with a zinc deficient diet for six months. Quantitative Energy Dispersive X-ray (EDX) microanalysis yielded the zinc mole fractions of melanosomes in the retinal pigment epithelium (RPE). The lateral resolution of the analysis was 100 nm. The zinc mole fractions of melanosomes were significantly smaller in the RPE of ZD-LE rats as compared to the LE control rats. Light, fluorescence and electron microscopy, as well as immunohistochemistry were performed. The numbers of lipofuscin granules in the RPE and of infiltrated cells (Ø>3 µm) found in the choroid were quantified. The number of lipofuscin granules significantly increased in ZD-LE as compared to control rats. Infiltrated cells bigger than 3 µm were only detected in the choroid of ZD-LE animals. Moreover, the thickness of the Bruch's membrane of ZD-LE rats varied between 0.4–3 µm and thin, rangy ED1 positive macrophages were found attached at these sites of Bruch's membrane or even inside it.

Conclusions/Significance

In pigmented rats, zinc deficiency yielded an accumulation of lipofuscin in the RPE and of large pigmented macrophages in the choroids as well as the appearance of thin, rangy macrophages at Bruch's membrane. Moreover, we showed that a zinc diet reduced the zinc mole fraction of melanosomes in the RPE and modulated the thickness of the Bruch's membrane.

Zinc-desferrioxamine attenuates retinal degeneration in the rd10 mouse model of retinitis pigmentosa

Iron-associated oxidative injury plays a role in retinal degeneration such as age-related macular degeneration and retinitis pigmentosa. The metallo-complex zinc-desferrioxamine (Zn/DFO) may ameliorate such injury by chelation of labile iron in combination with release of zinc. We explored whether Zn/DFO can affect the course of retinal degeneration in the rd10 mouse model of retinitis pigmentosa. Zn/DFO-treated animals showed significantly higher electroretinographic responses at 3 and 4.5 weeks of age compared with saline-injected controls. Corresponding retinal (photoreceptor) structural rescue was observed by quantitative histological and immunohistochemical techniques. When administered alone, the components of the complex, Zn and DFO, showed a lesser, partial effect. TBARS, a marker of lipid peroxidation, and levels of oxidative DNA damage as quantified by 8-OHdG immunostaining were significantly lower in Zn/DFO-treated retinas compared with saline-injected controls. Reduced levels of retinal ferritin as well as reduced iron content within ferritin molecules were measured in Zn/DFO-treated retinas. The data, taken together, suggest that the protective effects of the Zn/DFO complex are mediated through modulation of iron bioavailability, leading to attenuation of oxidative injury. Reducing iron-associated oxidative stress using complexes such as Zn/DFO may serve as a "common pathway" therapeutic approach to attenuate injury in retinal degeneration.

Catalytic properties of the retinal rod outer segment disk ADP-ribosyl cyclase

Cyclic ADP-ribose (cADPR) is a second messenger modulating intracellular calcium levels. We have previously described a cADPR-dependent calcium signaling pathway in bovine rod outer segments (ROS), where calcium ions play a pivotal role. ROS ADP-ribosyl cyclase (ADPR-cyclase) was localized in the membrane fraction. In the present work, we examined the properties of the disk ADPR-cyclase through the production of cyclic GDP-ribose from the NAD(+) analogue NGD(+). The enzyme displayed an estimated K(m) for NGD(+) of 12.5 ± 0.3 μM, a V(max) of 26.50 ± 0.70 pmol cyclic GDP-ribose synthesized/min/mg, and optimal pH of 6.5. The effect of divalent cations (Zn(2+), Cu(2+), and Ca(2+)) was also tested. Micromolar Zn(2+) and Cu(2+) inhibited the disk ADPR-cyclase activity (half maximal inhibitory concentration, IC50=1.1 and 3.6 μM, respectively). By contrast, Ca(2+) ions had no effect. Interestingly, the properties of the intracellular membrane-associated ROS disk ADPR-cyclase are more similar to those of the ADPR-cyclase found in CD38-deficient mouse brain, than to those of CD38 or CD157. The novel intracellular mammalian ADPR-cyclase would elicit Ca(2+) release from the disks at various rates in response to change in free Ca(2+) concentrations, caused by light versus dark adaptation, in fact there was no difference in disk ADPR-cyclase activity in light or dark conditions. Data suggest that disk ADPR-cyclase may be a potential target of retinal toxicity of Zn(2+) and may shed light to the role of Cu(2+) and Zn(2+) deficiency in retina.

Endogenous zinc mediates apoptotic programmed cell death in the developing brain

Endogenous zinc can mediate the apoptotic programmed cell death (PCD) in the developing brain. Intensive accumulation of labile zinc occurs in almost all neurons undergoing PCD in the developing rat brain. Based on the greater frequency of neurons with intensive zinc accumulation compared to apoptotic neurons, it is inferred that cytosolic zinc accumulation precedes apoptotic PCD. To determine the role of intracellular labile zinc in developmental apoptosis, we subcutaneously injected the membrane-permeant zinc chelator, N,N,N',N-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN) into postnatal rats for 7 days after birth. TPEN chelated intraneuronal zinc without modulating the expression of the zinc-regulating proteins, ZnT-1, ZnT-3, and synaptophysin. The frequency of apoptotic neurons significantly decreased in TPEN-treated rat brains compared with that in normal postnatal rats. Activating cleavages of caspase-9 and -3, and mitochondrial pro-apoptotic Bax expression were reduced, whereas expression of anti-apoptotic Bcl-2 was increased. Thus, intracerebral zinc chelation may arrest PCD in the developing brain by interfering with the caspase-dependent apoptotic pathway. The present study demonstrates that intracellular zinc acts as a key mediator of developmental apoptosis and therefore provides the first in vivo evidence that endogenous labile zinc causes neuronal apoptosis.

A Novel Zinc Compound (Zinc Monocysteine) Enhances the Antioxidant Capacity of Human Retinal Pigment Epithelial Cells

PURPOSE

The aim of this study was to document the effect of a novel zinc amino acid combination on the concentrations of important antioxidants in cultured human retinal pigment epithelial (hRPE) cells.

METHODS

Primary confluent hRPE cells were treated with 30 microM of zinc acetate, zinc chloride, zinc cysteine, and zinc sulfate. The antioxidants catalase, glutathione, glutathione peroxidase, and metallothionein were measured. MTT assays were performed to determine the relative protection of the zinc compounds from the cytotoxic effects of H202 and t-butyl hydroperoxide.

RESULTS

Catalase and glutathione peroxidase activities were increased by the zinc formulations compared with the untreated control. Glutathione and metallothionein content were also increased. The greatest increases occurred with zinc conjugated to the amino acid cysteine. The MTT assays showed that zinc monocysteine protected cultured RPE cells from the toxicity of H2O2 and t-butyl hydroperoxide.

CONCLUSIONS

Our results demonstrate that zinc treatment of RPE cells increases antioxidants and protects cultured RPE cells from the cytotoxic effects of H2O2 and t-butyl hydroperoxide. The results show that zinc conjugated to cysteine offers greater benefits than either zinc salts or cysteine alone.

PAC-1 activates procaspase-3 in vitro through relief of zinc-mediated inhibition

The direct induction of apoptosis has emerged as a powerful anticancer strategy, and small molecules that either inhibit or activate certain proteins in the apoptotic pathway have great potential as novel chemotherapeutic agents. Central to apoptosis is the activation of the zymogen procaspase-3 to caspase-3. Caspase-3 is the key "executioner" caspase, catalyzing the hydrolysis of a multitude of protein substrates within the cell. Interestingly, procaspase-3 levels are often elevated in cancer cells, suggesting a compound that directly stimulates the activation of procaspase-3 to caspase-3 could selectively induce apoptosis in cancer cells. We recently reported the discovery of a compound, PAC-1, which enhances procaspase-3 activity in vitro and induces apoptotic death in cancer cells in culture and in mouse xenograft models. Described herein is the mechanism by which PAC-1 activates procaspase-3 in vitro. We show that zinc inhibits the enzymatic activity of procaspase-3 and that PAC-1 strongly activates procaspase-3 in buffers that contain zinc. PAC-1 and zinc form a tight complex with one another, with a dissociation constant of approximately 42 nM. The combined data indicate that PAC-1 activates procaspase-3 in vitro by sequestering inhibitory zinc ions, thus allowing procaspase-3 to autoactivate itself to caspase-3. The small-molecule-mediated activation of procaspases has great therapeutic potential and thus this discovery of the in vitro mechanism of action of PAC-1 is critical to the development and optimization of other procaspase-activating compounds.

Regulation and function of Zip4, the acrodermatitis enteropathica gene

The SLC39A (solute carrier 39A) [ZIP (Zrt-Irt-like protein)] family consists of 14 members which are thought to control zinc uptake into the cytoplasm. Among these, ZIP4 is known to be particularly important for zinc homoeostasis. Mutations in this gene cause acrodermatitis enteropathica, a rare recessive-lethal human genetic disorder. In the present paper, our studies of the regulation and function of the mouse Zip4 gene are briefly reviewed. Mouse Zip4 is expressed at highest levels in tissues involved in absorption of dietary or maternal zinc, and the gene and protein are dynamically regulated by multiple post-transcriptional mechanisms in response to zinc availability. ZIP4 accumulates at the apical surface of enterocytes and endoderm cells when zinc is deficient, because of increased stability of the mRNA and stabilization of the protein. In contrast, when zinc is replenished, the mRNA is destabilized and the protein is internalized and degraded rapidly. The critical importance of ZIP4 in zinc homoeostasis is revealed in mice with targeted deletions of this gene. Homozygous Zip4-knockout embryos die during early morphogenesis and heterozygous offspring are significantly underrepresented and display an array of developmental defects, including exencephalia, anophthalmia and severe growth retardation. Mice heterozygous for Zip4-knockout are hypersensitive to zinc deficiency, which suggests that humans heterozygous for this gene may also be very sensitive to zinc deficiency.

Reduced zinc and copper in the retinal pigment epithelium and choroid in age-related macular degeneration

PURPOSE

To measure zinc and copper levels in the retinal pigment epithelium (RPE) and choroid complex and in the neural retina in subjects with and without age-related macular degeneration (AMD).

DESIGN

Laboratory investigation.

METHODS

Eighty-eight donor eyes (44 subjects) were analyzed. After retinal dissection, the RPE and choroid complex was photographed. Using the Minnesota Grading System (MGS), the RPE and choroid complex was classified into 1 of 4 stages as defined by the Age-Related Eye Disease Study. Subjects without AMD were defined as both eyes having MGS stage 1; subjects with AMD were defined as both eyes having MGS stages 2 through 4. Zinc and copper levels were determined by using an inductively coupled plasma-mass spectrometer. Metal levels from two eyes of the same subject were averaged and treated as one observation. Differences in metal levels were examined by using Wilcoxon rank-sum tests.

RESULTS

The mean RPE and choroid complex zinc level in subjects with AMD (+/- standard deviation, 223.7 +/- 94.0 microg/g; n = 15) was reduced 24% when compared with that of subjects without AMD (292.1 +/- 98.5 microg/g; n = 29; P = .01). The mean RPE and choroid complex copper level in subjects with AMD (5.1 +/- 1.1 microg/g) was reduced 23% when compared with that of subjects without AMD (6.6 +/- 1.4 microg/g; P = .002). No difference was detected in retinal zinc and copper levels in subjects with and without AMD (P > .09).

CONCLUSIONS

Reduced RPE and choroid complex zinc and copper levels in AMD eyes combined with previous information that oral supplementation of zinc plus copper reduces the risk of progression of AMD suggests that metal homeostasis plays a role in AMD and in retinal health.

Ethambutol-induced toxicity is mediated by zinc and lysosomal membrane permeabilization in cultured retinal cells

Ethambutol, an efficacious antituberculosis agent, can cause irreversible visual loss in a small but significant fraction of patients. However, the mechanism of ocular toxicity remains to be established. We previously reported that ethambutol caused severe vacuole formation in cultured retinal cells, and that the addition of zinc along with ethambutol aggravated vacuole formation whereas addition of the cell-permeable zinc chelator, N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN), reduced vacuole formation. To investigate the origin of vacuoles and to obtain an understanding of drug toxicity, we used cultured primary retinal cells from newborn Sprague-Dawley rats and imaged ethambutol-treated cells stained with FluoZin-3, zinc-specific fluorescent dye, under a confocal microscope. Almost all ethambutol-induced vacuoles contained high levels of labile zinc. Double staining with LysoTracker or MitoTracker revealed that almost all zinc-containing vacuoles were lysosomes and not mitochondria. Intracellular zinc chelation with TPEN markedly blocked both vacuole formation and zinc accumulation in the vacuole. Immunocytochemistry with antibodies to lysosomal-associated membrane protein-2 (LAMP-2) and cathepsin D, an acid lysosomal hydrolase, disclosed lysosomal activation after exposure to ethambutol. Immunoblotting after 12 h exposure to ethambutol showed that cathepsin D was released into the cytosol. In addition, cathepsin inhibitors attenuated retinal cell toxicity induced by ethambutol. This is consistent with characteristics of lysosomal membrane permeabilization (LMP). TPEN also inhibited both lysosomal activation and LMP. Thus, accumulation of zinc in lysosomes, and eventual LMP, may be a key mechanism of ethambutol-induced retinal cell death.

The involvement of caspase-11 in TPEN-induced apoptosis

The depletion of intracellular zinc with N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) induces protein synthesis-dependent apoptosis. Here we examined the involvement of caspase induction in apoptosis. Among the examined caspases, only caspase-11 was increased by TPEN. Caspase-11 activity also increased, which resulted in caspase-3 activation. Cycloheximide or actinomycin D blocked caspase-11 induction, reduced caspase-11 and -3 activation, and attenuated TPEN-induced neuronal apoptosis. Blockade of caspase-11 by a chemical inhibitor or genetic deletion attenuated TPEN-induced apoptosis, indicating a critical role of caspase-11 in TPEN-induced apoptosis. Although mitochondria-mediated caspase-9/-3 activation also contributed to TPEN-induced apoptosis, caspase-11 is likely a key inducible apoptosis-inducing protein.

The mouse acrodermatitis enteropathica gene Slc39a4 ( Zip4 ) is essential for early development and heterozygosity causes hypersensitivity to zinc deficiency

The human Zip4 gene (Slc39a4) is mutated in the rare recessive genetic disorder of zinc metabolism acrodermatitis enteropathica, but the physiological functions of Zip4 are not well understood. Herein we demonstrate that homozygous Zip4-knockout mouse embryos die during early morphogenesis and heterozygous offspring are significantly underrepresented. At mid-gestation, an array of developmental defects including exencephalia, anophthalmia and severe growth retardation were noted in heterozygous embryos, and at weaning, many (63/280) heterozygous offspring were hydrocephalic, growth retarded and missing one or both eyes. Maternal dietary zinc deficiency during pregnancy exacerbated these effects, whereas zinc excess ameliorated these effects and protected embryonic development of heterozygotes but failed to rescue homozygous embryos. Heterozygous Zip4 embryos were not underrepresented in litters from wild-type mothers, but were approximately 10 times more likely to develop abnormally than were their wild-type littermates during zinc deficiency. Thus, both embryonic and maternal Zip4 gene expressions are critical for proper zinc homeostasis. These studies suggest that heterozygous mutations in the acrodermatitis gene Zip4 may be associated with a wider range of developmental defects than was previously appreciated, particularly when dietary zinc is limiting.

Cytotoxic effects of intra and extracellular zinc chelation on human breast cancer cells

Zinc is an essential trace element with cofactor functions in a large number of proteins of intermediary metabolism, hormone secretion pathways, immune defence mechanisms, and as a cofactor of transcription factors it is also involved in the control of gene expression. Our study demonstrates that the modulation of intra and extracellular zinc alone is sufficient to induce metabolic changes or even apoptosis in two model human breast cancer cell lines MCF-7 and MDA-MB468. Treatment of breast cancer cells with different concentrations of a cell membrane permeable zinc chelator, N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) and the membrane impermeable zinc chelator, diethylenetriaminepentacetic acid, (DTPA) resulted in a significant increase of cell death. Features of apoptosis, such as chromatin condensation and nuclear fragmentation accompanied the DTPA and TPEN-induced cell death. A significant increase in the activity of caspase-9 was observed in both cell lines; whereas, caspase-3 activity was only increased in MDA-MB468 cells since caspase-3 is not expressed in MCF-7 cells. Caspase-8 activation was negligible in both cell lines. Addition of Zn(2+) or Cu(2+) prevented DTPA and TPEN-induced cytotoxicity, indicating that both bivalent cations can be replaced functionally to a certain extent in our experimental system. Interestingly, addition of Ca(2+), or Mg(2+) had no effect. The antioxidant N-Acetyl-L-Cysteine inhibited the cytotoxic effect of DTPA and TPEN, indicating that oxidative stress is the likely mediator of Zn-deficiency-related cell death.

Proinsulin/insulin is synthesized locally and prevents caspase- and cathepsin-mediated cell death in the embryonic mouse retina

Programmed cell death is an essential, highly regulated process in neural development. Although the role of insulin-like growth factor I in supporting the survival of neural cells has been well characterized, studies on proinsulin/insulin are scarce. Here, we characterize proinsulin/insulin effects on cell death in embryonic day 15.5 mouse retina. Both proinsulin mRNA and proinsulin/insulin immunoreactivity were found in the developing retina. Organotypic embryonic day 15.5 retinas cultured under growth factor deprivation showed an increase in cell death that was reversed by proinsulin, insulin and insulin-like growth factor I, with similar median effective concentration values via phosphatidylinositol-3-kinase activation. Although insulin and insulin-like growth factor I provoked a sustained Akt phosphorylation, proinsulin-induced phosphorylation of Akt was not found. Analysis of the growth factor deprivation-induced cell death mechanisms, using caspase and cathepsin inhibitors, demonstrated that both protease families were required for the effective execution of cell death. The insulin survival effect, which decreased the extent and distribution of cell death to levels similar to those found in vivo, was not enhanced by simultaneous treatment with caspase and cathepsin inhibitors, suggesting that insulin interferes with these protease pathways in the embryonic mouse retina. The mechanisms characterized in this study provide new details on early neural cell death and its genuine regulation by insulin/proinsulin.

Antiproliferative and apoptotic effects of zinc-citrate compound (CIZAR(R)) on human epithelial ovarian cancer cell line, OVCAR-3

OBJECTIVE

Zinc inhibits the growth of several carcinoma cells through induction of cell cycle arrest and apoptosis. The intracellular concentration of zinc and its dynamic changes are critically important in cell biology. We investigated the effects of zinc-citrate compound (CIZAR) on normal human ovarian epithelial cells (NOSE) and human epithelial ovarian cancer cell line, OVCAR-3.

METHODS

To investigate the potential effect of CIZAR on cell growth and survival, cells were treated with different doses and exposed to different times. Intracellular concentration of zinc was measured by colorimetric assay. Mitochondrial aconitase activity was determined in cell extracts using aconitase assay. The flow cytometric assay, DNA laddering, and morphological analysis were done to investigate cytotoxic effects of CIZAR. Molecular mechanism of cell death was investigated by p53, Bcl-xL, Bcl-2, Bax protein, activity of caspase-3 and -12, and activity of telomerase.

RESULTS

CIZAR-induced zinc accumulation in OVCAR-3 cells was higher than that in NOSE cells. CIZAR(R) treatment resulted in a time- and dose-dependent decrease in cell number in OVCAR-3 cells in comparison with NOSE cells. M-aconitase activity was significantly decreased in OVCAR-3 cells within 4 h exposure to CIZAR but relatively constant in NOSE cells. The flow cytometric assay, DNA laddering, and morphological analysis indicated apoptosis in OVCAR-3 cells but not in NOSE cells. CIZAR increased the expression of p21(waf1) which is a part of p53-independent pathway and induced reduction of telomerase activity. CIZAR reduced expression of Bcl-2 and Bcl-xL proteins but induced expression of Bax protein. CIZAR induced apoptosis of OVCAR-3 cells by activation of caspase-12 and caspase-3 pathway.

CONCLUSIONS

Exposure to CIZAR induces apoptosis in OVCAR-3 cells which accumulate high intracellular levels of zinc, but not in NOSE cells, which do not accumulate high levels of zinc. CIZAR(R) prevents the proliferation of OVCAR-3 cells by inactivation of m-aconitase activity and induces apoptosis by induction of proapoptotic gene (Bax), repression of antiapoptotic genes (Bcl-2, Bcl-xL), and consequently activation of caspase-3. CIZAR also induced activation of caspase-12. The CIZAR will offer new window in prevention and treatment of epithelial ovarian cancer.

Neuritic outgrowth from goldfish retinal explants, interaction of taurine and zinc

Various studies provide evidence for an interaction between taurine and zinc during development, affecting the morphology and function of the retina. The objectives of the present work were to determine taurine and zinc levels in the retina of goldfish during regeneration and to investigate the effect of the intracellular zinc chelator N,N,N,N-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) on the trophic role of taurine on outgrowth from post-crush goldfish retinal explants. Taurine was determined by HPLC (nmol/mg protein) and zinc by spectrophotometry ICP (microg/mg protein) at various days post-crushing the optic nerve. The levels of taurine were significantly increased at 72 h and the zinc levels at 24 h. Explants from retinas, 10 days post-crush, were cultured for 5 days in the presence of various concentrations and combinations of TPEN and taurine. TPEN, 1 nM, decreased the outgrowth but simultaneously with taurine (1-8 mM) there was an increase. These results demonstrate that zinc was necessary for normal outgrowth of retinal fibers and that taurine counteracted the chelator effect.

Taurine and Zinc Modulate Outgrowth from Goldfish Retinal Explants

Taurine and zinc, highly concentrated in the retina, possess similar properties in this structure, such as neuro-protection, membrane stabilization, influencing regeneration, and modulating development, maybe by acting in parallel or as interacting agents. We previously demonstrated that there are some correlations between taurine and zinc levels in hippocampus, dentate gyrus and retina of the developing rat. In the present study we evaluate the possible effects of taurine and zinc on outgrowth from goldfish retinal explants. The optic nerve was crushed 10 days before plating and culturing retinal explants in Leibovitz medium with 10% fetal calf serum and gentamicin. Neurites were measured with SigmaScanPro after 5 days in culture. Taurine (HPLC) and zinc (ICP) concentrations were determined in the retina between 1 and 180 days after crushing the optic nerve. Zinc sulfate (0.01-100 microM), N,N, N',N'-tetrakis (pyridylmethyl) ethylenediamine (TPEN, 0.1-5 nM) and diethylenetriamine penta-acetic acid (DTPA, 10-300 microM), intracellular and extracellular zinc chelators, respectively, were added to the medium. TPEN was also injected intraocular (0.1 nM). Combinations of them were added with taurine (1-16 mM). Taurine concentrations were elevated in the retina 72 h after the crush, but were normalized by 180 days, those of zinc increased at 24 h, preceding the increase of taurine. The axonal transport of [3H]taurine from the optic tectum to the retina was not affected in fish with or without crush of the optic nerve at early periods after the injection, indicating an increase of it post-lesion. Zinc sulfate produced a bell-shaped concentration dependency on in vitro outgrowth, with stimulation at 0.05 microM, and inhibition at higher levels, also increased the effect of 4 mM taurine at 0.02 microM, but diminished it at higher concentrations in the medium. TPEN decreased outgrowth at 1 nM, but not at 0.5 nM, although the simultaneous presence of 4 mM taurine and 0.5 nM TPEN decreased outgrowth respecting the stimulation by taurine alone. The intraocular administration of TPEN decreased outgrowth in vitro, an effect counteracted by the addition of 4 mM taurine to the culture medium. DTPA decreased outgrowth from 10 microM in the medium. The present results indicate that an optimal zinc concentration is necessary for outgrowth of goldfish retinal explants and that, in zinc deficient retina, taurine could stimulate outgrowth. In addition, the observations of variations in tissue concentrations and of the effects of intraocular administration of TPEN indicate that these effects could occur in vivo.

Mechanism of apoptosis induced by S100A8/A9 in colon cancer cell lines: the role of ROS and the effect of metal ions

The protein complex S100A8/A9, abundant in the cytosol of neutrophils, is secreted from the cells upon cellular activation and induces apoptosis in tumor cell lines and normal fibroblasts in a zinc-reversible manner. In the present study, we present evidence that the S100A8/A9 also exerts its apoptotic effect by a zinc-independent mechanism. Treatment of the colon carcinoma cells with different concentrations of human S100A8/A9 or the metal ion chelator diethylenetriaminepentacetic acid (DTPA) resulted in a significant increase of cell death. Annexin V/phosphatidylinositol and Hoechst 33258 staining revealed that cell death was mainly of the apoptotic type. A significant increase in the activity of caspase-3 and -9 was observed in both cell lines after treatment. Caspase-8 activation was negligible in both cell lines. The cytotoxicity/apoptotic effect of human S100A8/A9 and DTPA was inhibited significantly (P<0.05) by Zn(+2) and Cu(+2), more effectively than by Ca(2+) and Mg(2+). The antioxidant N-acetyl-L-cysteine inhibited the cytotoxicity/apoptotic effect of S100A8/A9 and DTPA. However, as a result of the different time-courses of both agents and that the S100A8/A9-induced apoptosis was not completely reversed, we conclude that S100A8/A9 exerts its apoptotic effect on two colon carcinoma cell lines through a dual mechanism: one via zinc exclusion from the target cells and the other through a yet-undefined mechanism, probably relaying on the cell-surface receptor(s).

Zinc inhibits Bax and Bak activation and cytochrome c release induced by chemical inducers of apoptosis but not by death-receptor-initiated pathways

Zinc has been known for many years to inhibit apoptosis but the mechanism remains unclear. Originally thought to inhibit an apoptotic endonuclease, zinc has subsequently been shown to inhibit steps earlier in the pathway. Since many additional steps in apoptosis have now been defined, we have re-evaluated the steps inhibited by zinc. In response to activation of the chemical-mediated death pathway by anisomycin, 0.3 mM zinc inhibited Bax and Bak activation, cytochrome c release, and all of the subsequent steps in apoptosis. In the receptor-mediated death pathway initiated by Fas or tumor necrosis factor, 3 mM zinc was required to inhibit apoptosis as judged by inhibition of caspase 3 activity and DNA digestion, but it failed to inhibit cytochrome c release, activation of Bax and Bak, or upstream signaling events in this pathway. These results are consistent with zinc selectively inhibiting activation of BH3-only proteins required in the chemical pathway but inhibiting downstream caspase activation in the death-receptor pathway.

Neurotrophic and neurotoxic effects of zinc on neonatal cortical neurons

Although zinc exerts direct neurotoxic action, this metal is also essential for the activity of numerous biological systems and zinc deficiency has been associated with various pathologies. We investigated the cellular responses and neuronal viability following exposure to different concentrations of zinc in primary cultures of neonatal rat cortical neurons. Higher concentrations of zinc (0.15 and 0.2 mM) triggered excessive zinc influx, glutathione depletion and ATP loss leading to necrotic neuronal death. In contrast, lower concentrations of zinc (0.05 and 0.1 mM) attenuated serum-deprivation induced apoptotic neuronal death. The antiapoptotic action of low amounts of zinc was found both in mixed cultures and neuron-enriched cultures indicating the independence of glial mediator. Neurotrophic action was not accompanied by significant alteration in those cellular responses but required chelatable zinc. The N-methyl-D-aspartate (NMDA) antagonist, MK-801, mimicked the beneficial effect of zinc in protecting neuronal death. Moreover, both MK-801 and zinc eliminated NMDA-induced neuronal injury. The results suggest that zinc is an intrinsic factor for neuron survival and exogenous zinc, in low amounts, is an active neuroprotectant against serum deprivation in part through the antagonism of NMDA receptor activation.

Implication of extracellular zinc exclusion by recombinant human calprotectin (MRP8 and MRP14) from target cells in its apoptosis-inducing activity

BACKGROUND

Calprotectin is a calcium-binding and zinc-binding protein complex that is abundant in the cytosol of neutrophils. This factor is composed of 8 and 14 kDa subunits, which have also been termed migration inhibitory factor-related proteins MRP8 and MRP14. We previously reported that rat calprotectin purified from inflammatory neutrophils induces apoptosis of various tumor cells or normal fibroblasts in a zinc-reversible manner.

AIM

The present study was undertaken to elucidate which subunit is responsible for the apoptosis-inducing activity, and to explore the mechanism of zinc-reversible apoptosis induction.

METHODS

The apoptosis-inducing activity of recombinant human MRP8 (rhMRP8) and recombinant human MRP14 (rhMRP14) was examined against EL-4 lymphoma cells in vitro. To determine whether zinc deprivation by calprotectin was essential for the cytotoxicity, the activity of calprotectin was tested under conditions where physical contact between the factor and the cells was precluded by a low molecular weight cut-off dialysis membrane.

RESULTS

The cytotoxicity of rhMRP14 against EL-4 cells was first detected at 10 microM in a standard medium, whereas rhMRP8 caused only marginal cytotoxicity at 40 microM. A mixture of both proteins showed higher specific activity (onset of cytotoxicity at 5 microM). When the cells were cultured in divalent cation-depleted medium, each dose-response curve was shifted to about a four-fold lower concentration range. Calprotectin was found to induce cell death even when the complex and the target cells were separated by dialysis membrane. A membrane-impermeable zinc chelator, diethylenetriamine pentaacetic acid (DTPA), also induced target cell apoptosis in a similar time-course as calprotectin. Moreover, the activities of calprotectin and DTPA were completely inhibited by the presence of zinc ions.

CONCLUSION

These data indicate that calprotectin has higher specific activity to induce apoptosis than the Individual subunits, and that the mechanism is exclusion of zinc from target cells.

LEDGF, a survival factor, activates stress-related genes

LEDGF is a survival factor and it enhances survival of various cell types against stress. LEDGF is also a transcriptional activator and it binds to promoter elements of heat shock and stress-related genes to activate expression of these genes. The elevated levels of the stress-related family of proteins, such as heat shock proteins, antioxidant proteins, and detoxication enzymes might suppress apoptosis induced by stress. The protective mechanisms against stress in mammalian cells and in yeast are surprisingly similar.

[Role of intracellular zinc in programmed cell death]

Apoptosis is a type of cell death involved in several biological events during tissue development, remodelling or involution. It could be induced by several extracellular or intracellular stimuli with an important role for metals like zinc or calcium. Cellular zinc is described as an inhibitor of apoptosis, while its depletion induces death in many cell lines. Using different chemical tools like specific zinc-chelators or ionophores, it is possible to study and understand the mechanisms of programmed cell death induction. The decrease in intracellular zinc concentration induces a characteristic apoptosis with apoptotic bodies formation and nuclear DNA condensation and fragmentation. This zinc depletion activates the caspases-3, -8 and -9, responsible for the proteolysis of several target proteins like poly(ADP-ribose) polymerase or transcription factors. Zinc addition in cell culture medium prevents the apparition of morphological and biochemical signs induced by intracellular zinc chelation, but also by other apoptosis inducers like etoposide or tumour necrosis factor alpha (TNF alpha). However, excess of zinc can also be cytotoxic. The balance between life and cell death is maintained by several zinc channels, controlling the intracellular zinc movements and the free amount of the metal.

BAPTA/AM, an intracellular calcium chelator, induces delayed necrosis by lipoxygenase-mediated free radicals in mouse cortical cultures

1. Disruption of calcium homeostasis during neurodegenerative diseases is known to trigger apoptotic or necrotic death in neuronal cells. Recently, the authors reported that intracellular calcium restriction by NMDA receptor antagonists induces apoptosis in cortical cultures. To evaluate whether further restriction of intracellular free calcium can induce apoptosis or necrosis, we examined the neurotoxic characterization of BAPTA/AM, a permeable free calcium chelator, in mouse cortical cultures. 2. Exposure of mixed (glia and neuron) cortical cultures (DIV 13-16) to 3-10 microM BAPTA/AM (non-toxic concentration for glial cells) for 24-48 hr resulted in delayed and necrotic neuronal death. The necrotic findings included swelling and loss of mitochondria and endoplasmic reticulum (ER) with neuronal membrane rupture 24 hr after treatment with BAPTA/AM. Simultaneously, we observed a few TUNEL-positive cells in the neuronal subpopulation of the same cultures. 3. The neurotoxicity evoked by BAPTA/AM (10 microM) was significantly attenuated by the addition of 0.5 microM cycloheximide (a protein synthesis inhibitor), 10 microM actinomycin D (an RNA transcription inhibitor), a high extracellular potassium concentration (total 15 mM KCl), 100 microM t-ACPD (a metabotrophic agonist), 100 microM alpha-tocopherol (a free radical scavenger), 100 microM deferoxamine (a ferric ion chelator), 100 microM L-NAME (a nitric oxide synthase (NOS) inhibitor), 50 microM DNQX (a non-NMDA receptor blocker), and 3-30 microM esculetin (a lipoxygenase inhibitor). However, 0.3-3 mM ASA (a cyclooxygenase inhibitor), 100 ng/ml nerve growth factor (NGF), 10 microM MK-801 (a NMDA receptor antagonist), 20 microM zVAD-fmk (caspase inhibitor) and 50 U/ml catalase failed to inhibit the injury. 4. However, NGF and catalase blocked the neurotoxicity induced by BAPTA/AM in young neuronal cells (DIV 6). BAPTA/AM (10 microM) did not alter the expression of inducible nitric oxide synthase (iNOS) on glial cells. 5. These results suggest that the feature of neuronal death induced by BAPTA/AM exhibits predominantly delayed necrosis mediated by lipoxygenase-dependent free radicals.

Retinol binding protein expression is induced in HepG2 cells by zinc deficiency

Zinc (Zn) deficiency is often associated with low plasma vitamin A (retinol) concentrations. It has been suggested that the reduction in plasma retinol is secondary to reduced liver retinol binding protein (RBP) synthesis. In the present study, RBP expression was determined in HepG2 cells cultured in either Zn adequate media or chelated media containing varying concentrations of Zn. Levels of RBP mRNA increased in a time- and Zn concentration-dependent manner such that 0.5 microM Zn-treated cells exhibited a >7.5-fold increase while cells treated with 15 microM Zn were increased 2.9-fold at 72 h compared to controls. RBP protein also progressively increased by 72 h to levels >8-fold and 3-fold higher than controls, in 0.5 microM and 15 microM Zn-treated cells, respectively. The increase in RBP occurred without any change in DNA concentration between groups through 72 h. The Zn deficiency-induced elevations in RBP transcript levels could be reversed within 24-48 h of repletion in Zn adequate media. Thus, the reductions in plasma retinol observed in Zn deficiency are in part a direct consequence of the deficiency.