The important role of the apoptotic effects of zinc in the development of cancers

Zinc ion enhances GABA tea-mediated oxidative DNA damage

GABA tea is a tea product that contains a high level of γ-aminobutyric acid (GABA). Previous study has demonstrated a synergistic effect of GABA tea and copper ions on DNA breakage. This study further explored whether zinc (Zn), a nonredox metal, modulated DNA cleavage induced by GABA tea extract. In a cell-free system, Zn(2+) significantly enhanced GABA tea extract and (-)-epigallocatechin-3-gallate (EGCG)- or H(2)O(2)-induced DNA damage at 24 h of incubation. Additionally, low dosages of GABA tea extract (1-10 μg/mL) possessed pro-oxidant activity to increase H(2)O(2)/Zn(2+)-induced DNA cleavage in a dose-dependent profile. By use of various reactive oxygen scavengers, it was observed that glutathione, catalase, and potassium iodide effectively inhibited DNA degradation caused by the GABA tea extract/H(2)O(2)/Zn(2+) system. Moreover, the data showed that the GABA tea extract itself (0.5-5 mg/mL) could induce DNA cleavage in a long-term exposure (48 h). EGCG, but not the GABA tea extract, enhanced H(2)O(2)-induced DNA cleavage. In contrast, GABA decreased H(2)O(2)- and EGCG-induced DNA cleavage, suggesting that GABA might contribute the major effect on the antioxidant activity of GABA tea extract. Furthermore, a comet assay revealed that GABA tea extract (0.25 mg/mL) and GABA had antioxidant activity on H(2)O(2)-induced DNA breakage in human peripheral lymphocytes. Taken together, these findings indicate that GABA tea has the potential of both pro-oxidant and antioxidant. It is proposed that a balance between EGCG-induced pro-oxidation and GABA-mediated antioxidation may occur in a complex mixture of GABA tea extract.

New applications of old metal-binding drugs in the treatment of human cancer

Significant advances in the use of metal complexes, precipitated by platinum, have fostered a renewed interest in harnessing their rich potential in the treatment of cancer. In addition to platinum-based complexes, the anticancer properties of other metals such as ruthenium have been realized, and ruthenium-based compounds are currently being investigated in clinical trials. Since the process of drug development can be expensive and cumbersome, finding new applications of existing drugs may provide effective means to expedite the regulatory process in bringing new drugs to the clinical setting. Encouraging findings from laboratory studies reveal significant anticancer activity from different classes of metal-chelating compounds, such as disulfiram, clioquinol, and dithiocarbamate derivatives that are currently approved for the treatment of various pathological disorders. Their use as coordination complexes with metals such as copper, zinc, and gold that target the ubiquitin-proteasome pathway have shown significant promise as potential anticancer agents. This review discusses the unique role of several selected metals in relation to their anti-cancer properties as well as the new therapeutic potential of several previously approved metal-chelating drugs. In vitro and in vivo experimental evidence along with mechanisms of action (e.g., via targeting the tumor proteasome) will also be discussed with anticipation of strengthening this exciting new concept.

Illuminating mobile zinc with fluorescence from cuvettes to live cells and tissues

With the aid of chemoselective sensors, fluorescence microscopy has emerged as an indispensable tool to visualize the distribution and dynamics of various biologically important molecules in live specimens. Motivated by our interest in understanding the chemistry and biology of mobile zinc underlying its physiological and pathological roles, over the past decade, our laboratory has developed an extensive library of zinc fluorescence probes. In this chapter, we provide essential information about our sensor toolbox in order to assist investigators interested to apply our constructs to study various aspects of mobile zinc biology. We illustrate their use with several examples of imaging both exogenous and endogenous mobile zinc in live cells and tissues using various versions of fluorescence microscopy, including confocal and two-photon microscopy.

Human prostate cancer ZIP1/zinc/citrate genetic/metabolic relationship in the TRAMP prostate cancer animal model

Prostate cancer is the second leading cause of cancer deaths among men. The availability of animal models that represent the events and factors that exist in the natural history and biology of human prostate cancer is essential in dealing with prostate cancer. In recent decades and presently, emphasis has been directed at the development and employment of prostate cancer induced in transgenic mice. However, the important consistent hallmark characteristic and event of decrease in zinc and citrate and downregulation of ZIP1 zinc transporter in prostate malignancy has not been studied or identified in any animal model. We investigated the status of these parameters in TRAMP tumors as compared with human prostate cancer. The results show that citrate levels are markedly decreased in the developing and advancing stages of malignancy in TRAMP. Zinc levels are also decreased and ZIP1 transporter is lost in TRAMP tumors. In vitro studies show that zinc treatment of TRAMP C2 cells exhibits cytotoxic effects. Collectively, these results mimic the ZIP1, zinc, and citrate status and relationship that exist in human prostate cancer. This is the first report that establishes the existence of the human prostate zinc/citrate hallmark characteristic and relationship in an animal model. It now appears that the TRAMP model will be suitable for studies relating to the implications and role of zinc- and citrate-related metabolism in the development and progression of human prostate cancer.

A histidine-rich motif mediates mitochondrial localization of ZnT2 to modulate mitochondrial function

Female reproductive tissues such as mammary glands, ovaries, uterus, and placenta are phenotypically dynamic, requiring tight integration of bioenergetic and apoptotic mechanisms. Mitochondrial zinc (Zn) pools have emerged as a central player in regulating bioenergetics and apoptosis. Zn must first be imported into mitochondria to modulate mitochondrion-specific functions; however, mitochondrial Zn import mechanisms have not been identified. Here we documented that the Zn transporter ZnT2 is associated with the inner mitochondrial membrane and acts as an auxiliary Zn importer into mitochondria in mammary cells. We found that attenuation of ZnT2 expression significantly reduced mitochondrial Zn uptake and total mitochondrial Zn pools. Moreover, expression of a ZnT2-hemagglutinin (HA) fusion protein was localized to mitochondria and significantly increased Zn uptake and mitochondrial Zn pools, directly implicating ZnT2 in Zn import into mitochondria. Confocal microscopy of truncated and point mutants of ZnT2-green fluorescent protein (GFP) fusion proteins revealed a histidine-rich motif ((51)HHXH(54)) in the NH(2) terminus that is important for mitochondrial targeting of ZnT2. More importantly, the expansion of mitochondrial Zn pools by ZnT2 overexpression significantly reduced ATP biogenesis and mitochondrial oxidation concurrent with increased apoptosis, suggesting a functional role for ZnT2-mediated Zn import into mitochondria. These results identify the first Zn transporter directly associated with mitochondria and suggest that unique secretory tissues such as the mammary gland require novel mechanisms to modulate mitochondrion-specific functions.

Novel metals and metal complexes as platforms for cancer therapy

Metals are essential cellular components selected by nature to function in several indispensable biochemical processes for living organisms. Metals are endowed with unique characteristics that include redox activity, variable coordination modes, and reactivity towards organic substrates. Due to their reactivity, metals are tightly regulated under normal conditions and aberrant metal ion concentrations are associated with various pathological disorders, including cancer. For these reasons, coordination complexes, either as drugs or prodrugs, become very attractive probes as potential anticancer agents. The use of metals and their salts for medicinal purposes, from iatrochemistry to modern day, has been present throughout human history. The discovery of cisplatin, cis-[Pt(II) (NH(3))(2)Cl(2)], was a defining moment which triggered the interest in platinum(II)- and other metal-containing complexes as potential novel anticancer drugs. Other interests in this field address concerns for uptake, toxicity, and resistance to metallodrugs. This review article highlights selected metals that have gained considerable interest in both the development and the treatment of cancer. For example, copper is enriched in various human cancer tissues and is a co-factor essential for tumor angiogenesis processes. However the use of copper-binding ligands to target tumor copper could provide a novel strategy for cancer selective treatment. The use of nonessential metals as probes to target molecular pathways as anticancer agents is also emphasized. Finally, based on the interface between molecular biology and bioinorganic chemistry the design of coordination complexes for cancer treatment is reviewed and design strategies and mechanisms of action are discussed.

Modulation of glioma risk and progression by dietary nutrients and antiinflammatory agents

Gliomas are tumors of glial origin formed in the central nervous system and exhibit profound morphological and genetic heterogeneity. The etiology of this heterogeneity involves an interaction between genetic alterations and environmental risk factors. Scientific evidence suggests that certain natural dietary components, such as phytoestrogens, flavonoids, polyunsaturated fatty acids, and vitamins, may exert a protective effect against gliomas by changing the nature of the interaction between genetics and environment. Similarly, certain antiinflammatory drugs and dietary modifications, such as methionine restriction and the adoption of low-calorie or ketogenic diets, may take advantage of glioma and normal glial cells' differential requirements for glucose, methionine, and ketone bodies and may, therefore, be effective as part of preventive or treatment strategies for gliomas. Treatment trials of glioma patients and chemoprevention trials of individuals with a known genetic predisposition to glioma using the most promising of these agents, such as the antiinflammatory drugs curcumin and gamma-linolenic acid, are needed to validate or refute these agents' putative role in gliomas.

Zinc in innate and adaptive tumor immunity

Zinc is important. It is the second most abundant trace metal with 2-4 grams in humans. It is an essential trace element, critical for cell growth, development and differentiation, DNA synthesis, RNA transcription, cell division, and cell activation. Zinc deficiency has adverse consequences during embryogenesis and early childhood development, particularly on immune functioning. It is essential in members of all enzyme classes, including over 300 signaling molecules and transcription factors. Free zinc in immune and tumor cells is regulated by 14 distinct zinc importers (ZIP) and transporters (ZNT1-8). Zinc depletion induces cell death via apoptosis (or necrosis if apoptotic pathways are blocked) while sufficient zinc levels allows maintenance of autophagy. Cancer cells have upregulated zinc importers, and frequently increased zinc levels, which allow them to survive. Based on this novel synthesis, approaches which locally regulate zinc levels to promote survival of immune cells and/or induce tumor apoptosis are in order.

Zip4 (Slc39a4) expression is activated in hepatocellular carcinomas and functions to repress apoptosis, enhance cell cycle and increase migration

Background

The zinc transporter ZIP4 (Slc39a4) is important for proper mammalian development and is an essential gene in mice. Recent studies suggest that this gene may also play a role in pancreatic cancer.

Methods/Principal Findings

Herein, we present evidence that this essential zinc transporter is expressed in hepatocellular carcinomas. Zip4 mRNA and protein were dramatically elevated in hepatocytes in the majority of human hepatocellular carcinomas relative to noncancerous surrounding tissues, as well as in hepatocytes in hepatocellular carcinomas occurring in farnesoid X receptor-knockout mice. Interestingly, meta-analysis of microarray data in the Geo and Oncomine databases suggests that Zip4 mRNA may also be elevated in many types of cancer. Potential mechanisms of action of ZIP4 were examined in cultured cell lines. RNAi knockdown of Zip4 in mouse Hepa cells significantly increased apoptosis and modestly slowed progression from G₀/G₁ to S phase when cells were released from hydroxyurea block into zinc-deficient medium. Cell migration assays revealed that RNAi knockdown of Zip4 in Hepa cells depressed in vitro migration whereas forced over-expression in Hepa cells and MCF-7 cells enhanced in vitro migration.

Conclusions

ZIP4 may play a role in the acquisition of zinc by hepatocellular carcinomas, and potentially many different cancerous cell-types, leading to repressed apoptosis, enhanced growth rate and enhanced invasive behavior.

The role of survivin and Bcl-2 in zinc-induced apoptosis in prostate cancer cells

OBJECTIVES

To study the effects of zinc treatment on the gene expression levels of survivin and Bcl-2 in prostate cancer cells.

MATERIALS AND METHODS

The effects of zinc exposure on apoptosis were assessed using two human prostate cancer cell lines, LNCaP and PC-3. Zinc-induced apoptosis was measured by Annexin V staining. The direct effect of zinc on the expression levels of zinc transporters (ZnT-1 and ZnT-4) and apoptosis-related genes (Bax, Bcl-2, and survivin) was determined by RT-PCR analysis.

RESULTS

When LNCaP and PC-3 cells were exposed to various concentrations of zinc sulfate for 48 hors, their growth was inhibited in a dose-dependent manner. The levels of zinc in both cell lines treated with zinc sulfate for 24 hours were higher than in untreated cells. Exposure to zinc induced apoptosis and necrosis in LNCaP and PC-3 cells. Apoptosis became more extensive as the treatment time with zinc increased. There was a significant increase in the gene expression levels of ZnT-1 and ZnT-4 in both cell lines treated with zinc sulfate compared with untreated cells. The expression of Bax mRNA was up-regulated, while the expression of Bcl-2 and survivin were decreased in both cell lines following zinc treatment.

CONCLUSIONS

Exposure to zinc sulfate in human prostate cancer cells increased intracellular levels of zinc, which resulted in increased apoptosis. The apoptogenic effect of elevated concentration of zinc could be due either to increased expression of zinc transporters and increased levels of Bax or decreased Bcl-2 and survivin expression.

An integrated imaging approach to the study of oxidative stress generation by mitochondrial dysfunction in living cells

BACKGROUND

The mechanisms of action of many environmental agents commonly involve oxidative stress resulting from mitochondrial dysfunction. Zinc is a common environmental metallic contaminant that has been implicated in a variety of oxidant-dependent toxicological responses. Unlike ions of other transition metals such as iron, copper, and vanadium, Zn(2+) does not generate reactive oxygen species (ROS) through redox cycling.

OBJECTIVE

To characterize the role of oxidative stress in zinc-induced toxicity.

METHODS

We used an integrated imaging approach that employs the hydrogen peroxide (H2O2)-specific fluorophore Peroxy Green 1 (PG1), the mitochondrial potential sensor 5,5 ,6,6 -tetrachloro-1,1 ,3,3 -tetraethylbenzimidazolylcarbocyanine iodide (JC-1), and the mitochondria-targeted form of the redox-sensitive genetically encoded fluorophore MTroGFP1 in living cells.

RESULTS

Zinc treatment in the presence of the Zn(2+) ionophore pyrithione of A431 skin carcinoma cells preloaded with the H(2)O(2)-specific indicator PG1 resulted in a significant increase in H(2)O(2) production that could be significantly inhibited with the mitochondrial inhibitor carbonyl cyanide 3-chlorophenylhydrazone. Mitochondria were further implicated as the source of zinc-induced H(2)O(2) formation by the observation that exposure to zinc caused a loss of mitochondrial membrane potential. Using MTroGFP1, we showed that zinc exposure of A431 cells induces a rapid loss of reducing redox potential in mitochondria. We also demonstrated that zinc exposure results in rapid swelling of mitochondria isolated from mouse hearts.

CONCLUSION

Taken together, these findings show a disruption of mitochondrial integrity, H(2)O(2) formation, and a shift toward positive redox potential in cells exposed to zinc. These data demonstrate the utility of real-time, live-cell imaging to study the role of oxidative stress in toxicological responses.

Imaging mobile zinc in biology

Trafficking and regulation of mobile zinc pools influence cellular functions and pathological conditions in multiple organs, including brain, pancreas, and prostate. The quest for a dynamic description of zinc distribution and mobilization in live cells fuels the development of increasingly sophisticated probes. Detection systems that respond to zinc binding with changes of their fluorescence emission properties have provided sensitive tools for mobile zinc imaging, and fluorescence microscopy experiments have afforded depictions of zinc distribution within live cells and tissues. Both small-molecule and protein-based fluorescent probes can address complex imaging challenges, such as analyte quantification, site-specific sensor localization, and real-time detection.