Transient peaks in zinc and metallothionein levels during differentiation of 3T3L1 cells

Zinc Favors Triple-Negative Breast Cancer's Microenvironment Modulation and Cell Plasticity

Triple-negative breast cancer (TNBC) tends to metastasize to the brain, a step that worsens the patient’s prognosis. The specific hallmarks that determine successful metastasis are motility and invasion, microenvironment modulation, plasticity, and colonization. Zinc, an essential trace element, has been shown to be involved in all of these processes. In this work, we focus our attention on the potential role of zinc during TNBC metastasis. We used MDA-MB-BrM2 (BrM2) cells, a brain metastasis model derived from the parental TNBC cell line MDA-MB-231. Our studies show that BrM2 cells had double the zinc content of MDA-MB-231 cells. Moreover, exploring different metastatic hallmarks, we found that the zinc concentration is especially important in the microenvironment modulation of brain metastatic cells, enhancing the expression of SerpinB2. Furthermore, we show that zinc promotes the tumorigenic capacity of breast cancer stem cells. In addition, by causing a disturbance in MDA-MB-231 zinc homeostasis by overexpressing the Zip4 transporter, we were able to increase tumorigenicity. Nevertheless, this strategy did not completely recapitulate the BrM2 metastatic phenotype. Altogether, our work suggests that zinc plays an important role in the transformative steps that tumoral cells take to acquire tumorigenic potential and niche specificity.

Role of Zinc Homeostasis in the Pathogenesis of Diabetes and Obesity

Zinc deficiency is a risk factor for obesity and diabetes. However, until recently, the underlying molecular mechanisms remained unclear. The breakthrough discovery that the common polymorphism in zinc transporter SLC30A8/ZnT8 may increase susceptibility to type 2 diabetes provided novel insights into the role of zinc in diabetes. Our group and others showed that altered ZnT8 function may be involved in the pathogenesis of type 2 diabetes, indicating that the precise control of zinc homeostasis is crucial for maintaining health and preventing various diseases, including lifestyle-associated diseases. Recently, the role of the zinc transporter ZIP13 in the regulation of beige adipocyte biogenesis was clarified, which indicated zinc homeostasis regulation as a possible therapeutic target for obesity and metabolic syndrome. Here we review advances in the role of zinc homeostasis in the pathophysiology of diabetes, and propose that inadequate zinc distribution may affect the onset of diabetes and metabolic diseases by regulating various critical biological events.

Metallothioneins regulate the adipogenic differentiation of 3T3-L1 cells via the insulin signaling pathway

Knockout of metallothionein (MT) genes contributes to a heavier body weight in early life and the potential to become obese through the intake of a high fat diet (HFD) in mice. It has thus been suggested that MT genes regulate the formation of adipose tissue, which would become the base for later HFD-induced obesity. We evaluated the fat pads of mice during the lactation stage. The fat mass and adipocyte size of MT1 and MT2 knockout mice were greater than those of wild type mice. Next, we assayed the ability of small interfering RNA (siRNA) to silence MT genes in the 3T3-L1 cell line. The expressions of MT1 and MT2 genes were transiently upregulated during adipocyte differentiation, and the siRNA pretreatment led to the suppression of the expression of both MT mRNAs and proteins. The MT siRNA promoted lipid accumulation in adipocytes and caused proliferation of post-confluent preadipocytes; these effects were suppressed by an inhibitor of phosphatidylinositol 3-kinase (LY294002). In addition, MT siRNA promoted insulin-stimulated phosphorylation of Akt, a downstream kinase of the insulin signaling pathway. Enhanced lipid accumulation in 3T3-L1 cells resulting from MT-gene silencing was inhibited by pretreatment with an antioxidant, N-acetylcysteine, used as a substitute for antioxidant protein MTs. These results suggest that interference in MT expression enhanced the activation of the insulin signaling pathway, resulting in higher lipid accumulation in 3T3-L1 adipocytes.

Metallothionein 1G promotes the differentiation of HT-29 human colorectal cancer cells

Metallothioneins (MTs) are a family of low- molecular-weight, cysteine-rich proteins involved in zinc and redox metabolism, that are epigenetically downregulated during colorectal cancer (CRC) progression, but may be re-induced with a variety of agents. Since loss of MT expression is associated with a worse prognosis, in the present study we investigated the effects of overexpression of the most significantly downregulated isoform in CRC, namely MT1G, on the HT-29 cell line. Overexpression of MT1G resulted in xenograft tumors with an aberrant morphology, characterized by an evident increase in mucin-containing cells that were identified as goblet cells under electron microscopy. Immunohistochemical detection of CDX2 and cytokeratin 20 was also increased, as were goblet-cell and enterocyte-specific genes by qRT-PCR. Microarray analysis of gene expression confirmed the alteration of several differentiation signaling pathways, including the Notch pathway. Using sodium butyrate and post-confluent growth as inducers of differentiation, we demonstrated that MT1G does indeed play a functional role in promoting goblet over enterocyte differentiation in vitro. Labile zinc is also induced upon differentiation of CRC cells, functionally contributing to enterocyte over goblet differentiation, as revealed using zinc-specific chelating agents. Overall, our results uncover a new tumor-suppressor activity of MT1G in promoting the differentiation of at least some CRC tumors, and implicate MTs and zinc signaling as new players in colorectal differentiation. This further contributes to the hypothesis that re-induction of MTs may have therapeutic value by diminishing the aggressiveness of CRC tumors.

The Physiological Roles of Extracellular Metallothionein

Metallothionein (MT) is a low-molecular-weight protein with a number of roles to play in cellular homeostasis. MT is synthesized as a consequence of a variety of cellular stressors, and has been found in both intracellular compartments and in extracellular spaces. The intracellular pool of this cysteine-rich protein can act as a reservoir of essential heavy metals, as a scavenger of reactive oxygen and nitrogen species, as an antagonist of toxic metals and organic molecules, and as a regulator of transcription factor activity. The presence of MT outside of cells due to the Influence of stressors suggests that this protein may make important contributions as a “danger signal” that influences the management of responses to cellular damage. While conventional wisdom has held that extracellular MT is the result of cell death or leakage from stressed cells, there are numerous examples of selective release of proteins by nontraditional mechanisms, including stress response proteins. This suggests that MT may similarly be selectively released, and that the pool of extracellular MT represents an important regulator of various cellular functions. For example, extracellular MT has effects both on the severity of autoimmune disease, and on the development of adaptive immune functions. Extracellular MT may operate as a chemotactic factor that governs the trafficking of inflammatory cells that move to resolve damaged tissues, as a counter to extracellular oxidant-mediated damage, and as a signal that influences the functional behavior of wounded cells. A thorough understanding of the mechanisms of MT release from cells, the conditions under which MT is released to the extracellular environment, and the ways in which MT Interacts with sensitive cells may both illuminate our understanding of an important control mechanism that operates in stressful conditions, and should indicate new opportunities for therapeutic management via the manipulation of this pool of extracellular MT.

Zinc transporter 7 deficiency affects lipid synthesis in adipocytes by inhibiting insulin-dependent Akt activation and glucose uptake

Mice deficient for zinc transporter 7 protein (ZnT7) are mildly zinc deficient with low body weight gain and body fat accumulation. To investigate the underlying mechanism of ZnT7 deficiency in body adiposity, we examined fatty acid composition and insulin sensitivity in visceral (epididymal) and subcutaneous fat pads from Znt7 knockout and control mice. We showed that ZnT7 deficiency had adverse effects on fatty acid metabolism and insulin action in subcutaneous fat but not in epididymal fat in mice, consistent with the ZnT7 protein expression pattern in adipose tissues. Importantly, we found that the expression of ZnT7 protein was induced by lipogenic differentiation and reached a peak when the adipocyte was fully differentiated in mouse 3T3-L1 adipocytes. We demonstrated, using Znt7 knockdown (Znt7KD) 3T3-L1 adipocytes, that reduction in Znt7 expression blunted activations of the signal transduction pathways that regulated both basal and insulin-stimulated glucose uptake in adipocytes, resulting in low glucose uptake and lipid accumulation. The expression of the signaling mediators critical for the initiation of pre-adipocyte differentiation, including Pparγ and C/Ebpα, appeared not to be affected by Znt7KD in 3T3-L1 adipocytes. These findings strongly suggest a role for ZnT7 in adipocyte lipogenesis.

Redox modulation of adipocyte differentiation: hypothesis of "Redox Chain" and novel insights into intervention of adipogenesis and obesity

In view of the global prevalence of obesity and obesity-associated disorders, it is important to clearly understand how adipose tissue forms. Accumulating data from various laboratories implicate that redox status is closely associated with energy metabolism. Thus, biochemical regulation of the redox system may be an attractive alternative for the treatment of obesity-related disorders. In this work, we will review the current data detailing the role of the redox system in adipocyte differentiation, as well as identifying areas for further research. The redox system affects adipogenic differentiation in an extensive way. We propose that there is a complex and interactive "redox chain," consisting of a "ROS-generating enzyme chain," "combined antioxidant chain," and "transcription factor chain," which contributes to fine-tune the regulation of ROS level and subsequent biological consequences. The roles of the redox system in adipocyte differentiation are paradoxical. The redox system exerts a "tridimensional" mechanism in the regulation of adipocyte differentiation, including transcriptional, epigenetic, and posttranslational modulations. We suggest that redoxomic techniques should be extensively applied to understand the biological effects of redox alterations in a more integrated way. A stable and standardized "redox index" is urgently needed for the evaluation of the general redox status. Therefore, more effort should be made to establish and maintain a general redox balance rather than to conduct simple prooxidant or antioxidant interventions, which have comprehensive implications.

Gene expression of the zinc transporter ZIP14 (SLC39a14) is affected by weight loss and metabolic status and associates with PPARγ in human adipose tissue and 3T3-L1 pre-adipocytes

Background

The expansion and function of adipose tissue are important during the development of insulin resistance and inflammation in obesity. Zinc dyshomeostasis is common in obese individuals. In the liver, zinc influx transporter ZIP14, affects proliferation and glucose metabolism but the role of ZIP14 in adipose tissue is still unknown. This study investigates ZIP14 gene expression in human adipose tissue before and after weight loss as well as the regulation of ZIP14 during early adipogenesis.

Methods

Fourteen obese individuals were investigated before and after a 10 week weight loss intervention and compared to 14 non-obese controls. Gene expressions of ZIP14 and peroxisome proliferator-activated receptor γ (PPARγ) were measured in subcutaneous adipose tissue and correlated with metabolic and inflammatory markers. Further, we investigated gene expression of ZIP14 and PPARγ during early adipogenesis of 3T3-L1 pre-adipocytes, together with an in silico analysis of PPARγ binding motifs in the promoter sequence of ZIP14.

Results

ZIP14 was down-regulated in obese individuals compared to non-obese controls (p = 0.0007) and was up-regulated after weight loss (p = 0.0005). Several metabolic markers of clinical importance, including body mass index, triglyceride, and insulin resistance, were inversely correlated with ZIP14. During early adipogensis an up-regulation of ZIP14 gene expression was found. PPARγ gene expression was positively correlated with the ZIP14 gene expression in both adipose tissue and during adipogenesis. However, in silico analysis revealed that the ZIP14 promoter does not contain PPARγ-binding motifs.

Conclusions

We hypothesize that ZIP14-mediated zinc influx might directly influence PPARγ activity and that ZIP14 may regulate expansion and function of adipose tissue and serve as a potential biomarker for metabolic stress.

Electronic supplementary material

The online version of this article (doi:10.1186/s40608-015-0076-y) contains supplementary material, which is available to authorized users.

Zinc and its role in immunity and inflammation

Zinc (Zn) nutritional importance has been known for a long time, but in the last decades its importance in immune modulation has arisen. This review aims at describing the mechanisms involved in the regulation of Zn homeostasis and their effects on the immune response focusing on those which are implicated in the physiopathology of rheumatoid arthritis. Zn functions as a modulator of the immune response through its availability, which is tightly regulated by several transporters and regulators. When this mechanism is disturbed, Zn availability is reduced, altering survival, proliferation and differentiation of the cells of different organs and systems and, in particular, cells of the immune system. Zn deficiency affects cells involved in both innate and adaptive immunity at the survival, proliferation and maturation levels. These cells include monocytes, polymorphonuclear-, natural killer-, T-, and B-cells. T cell functions and the balance between the different T helper cell subsets are particularly susceptible to changes in Zn status. While acute Zn deficiency causes a decrease in innate and adaptive immunity, chronic deficiency increases inflammation. During chronic deficiency, the production of pro-inflammatory cytokines increases, influencing the outcome of a large number of inflammatory diseases, including rheumatoid arthritis.

Loss of pluripotency in human embryonic stem cells directly correlates with an increase in nuclear zinc

The pluripotency of human embryonic stem cells (hESCs) is important to investigations of early development and to cell replacement therapy, but the mechanism behind pluripotency is incompletely understood. Zinc has been shown to play a key role in differentiation of non-pluripotent cell types, but here its role in hESCs is directly examined. By mapping the distribution of metals in hESCs at high resolution by x-ray fluorescence microprobe (XFM) and by analyzing subcellular metal content, we have found evidence that loss of pluripotency is directly correlated with an increase in nuclear zinc. Zinc elevation not only redefines our understanding of the mechanisms that support pluripotency, but also may act as a biomarker and an intervention point for stem cell differentiation.

Cellular zinc homeostasis is a regulator in monocyte differentiation of HL-60 cells by 1 alpha,25-dihydroxyvitamin D3

It was reported previously that zinc-deficient mice show impaired lymphopoiesis. At the same time, monocyte numbers in these animals are increased, indicating a negative impact of zinc on monocyte development. Here, we investigate the role of zinc homeostasis in the differentiation of myeloid precursors into monocytes. Reduced gene expression of several zinc transporters, predominantly from the Zip family, was observed during 1 alpha, 25-dihydroxyvitamin D(3) (1,25D(3))-induced differentiation of HL-60 cells. This was accompanied by a reduction of intracellular-free zinc, measured by FluoZin-3. Amplifying this reduction with the zinc chelator TPEN or zinc-depleted cell-culture medium enhanced 1,25D(3)-induced expression of monocytic surface markers CD11b and CD14 on HL-60, THP-1, and NB4 cells. In contrast, differentiation of NB4 cells to granulocytes was not zinc-sensitive, pointing toward a specific effect of zinc on monocyte differentiation. Further, monocyte functions, such as TNF-alpha secretion, phagocytosis, and oxidative burst, were also augmented by differentiation in the presence of TPEN. The second messenger cAMP promotes monocyte differentiation. We could show that zinc inhibits the cAMP-synthesizing enzyme adenylate cyclase, and chelation of zinc by TPEN increases cAMP generation after stimulation with the adenylate cyclase activator forskolin. Based on our in vitro results and the in vivo observations from the literature, we suggest a model in which the intracellular-free zinc concentration limits AC activity, and the decrease of zinc after 1,25D(3) treatment promotes differentiation by relieving AC inhibition. Thus, cellular zinc homeostasis acts as an endogenous modulator of monocyte differentiation.

Evidence for a potential role of metallothioneins in inflammatory bowel diseases

Inflammatory bowel diseases (IBDs) are a group of chronic, relapsing, immune-mediated disorders of the intestine, including Crohn's disease and ulcerative colitis. Recent studies underscore the importance of the damaged epithelial barrier and the dysregulated innate immune system in their pathogenesis. Metallothioneins (MTs) are a family of small proteins with a high and conserved cysteine content that are rapidly upregulated in response to an inflammatory stimulus. Herein, we review the current knowledge regarding the expression and potential role of MTs in IBD. MTs exert a central position in zinc homeostasis, modulate the activation of the transcription factor nuclear factor (NF)-kappaB, and serve as antioxidants. In addition, MTs could be involved in IBD through their antiapoptotic effects or through specific immunomodulating extracellular effects. Reports on MT expression in IBD are contradictory but clearly demonstrate a deviant MT expression supporting the idea that these aberrations in IBD require further clarification.

In vitro assessment of the effects of cadmium and zinc on mammalian nerve fibres

Zinc and cadmium are environmental contaminants that have a wide range of effects on the nervous system, but zinc is also considered to be an important metal in the human body. In this study the effect of CdCl(2) and ZnCl(2), at concentrations of 50,150, 250 and 500 microM, on the nerve fibres of the sciatic nerve of the rat isolated in a three-chamber recording bath were studied. At the same concentrations, CdCl(2) and ZnCl(2) were found to have almost the same inhibitory effect on the compound action potential (CAP) of the nerve fibres. Their concentration-effect curves almost overlap and there was no significant difference in their EC(50) which for CdCl(2) is 250.1+/-18 microM (n=5) and for ZnCl(2) is 282.2+/-25 microM (n=5) correspondingly (P>0.05). The no-observed-effect concentration (NOEC) was estimated to be 50-100 microM for both metals. The identical inhibitory effect of both metals on the sciatic nerve fibres indicates a common mode of action which is related to their potential to generate reactive oxygen species (ROS).

Clusterin/Apolipoprotein J up-regulation after zinc exposure, replicative senescence or differentiation of human haematopoietic cells

Clusterin/Apolipoprotein J (CLU) is a cellular senescence biomarker implicated in several physiological processes. In this work we have investigated CLU expression and function in human haematopoietic cells. We found that early passage human T cell clones (TCC) express minimal endogenous amounts of CLU, which are significantly elevated in late passage cells. Moreover, exposure of TCC to increased levels of the essential micronutrient zinc in culture resulted in intense induction of CLU. Because haematopoietic cells cease proliferation following induction of terminal differentiation, we also studied the expression profile of CLU in the leukemic progenitor cell lines K562 and HL-60. We found that, like TCC, both cell lines express minimal endogenous levels of CLU in their actively proliferating state. However, when induced to differentiate into their distinct cell types, CLU was found to be up-regulated specifically in those cells expressing the main differentiation markers. Enforced stable over-expression of CLU in K562 cells inhibited the expression of the CD14 differentiation marker and blocked differentiation to either monocytes/megacaryoblasts or to erythrocytes. Overall, our results suggest that CLU is actively involved in both replicative senescence and terminal differentiation in different types of human haematopoietic cells.

Transcriptome analysis and promoter sequence studies on early adipogenesis in 3T3-L1 cells

To identify regulatory molecules which play key roles in the development of obesity, we investigated the transcriptional profiles in 3T3-L1 cells at early stage of differentiation and analyzed the promoter sequences of differentially regulated genes. One hundred and sixty-one (161) genes were found to have significant changes in expression at the 2nd day following treatment with differentiation cocktail. Among them, 86 transcripts were up-regulated and 75 transcripts were down-regulated. The 161 transcripts were classified into 10 categories according to their functional roles; cytoskeleton, cell adhesion, immune, defense response, metabolism, protein modification, protein metabolism, regulation of transcription, signal transduction and transporter. To identify transcription factors likely involved in regulating these differentially expressed genes, we analyzed the promoter sequences of up- or -down regulated genes for the presence of transcription factor binding sites (TFBSs). Based on coincidence of regulatory sites, we have identified candidate transcription factors (TFs), which include those previously known to be involved in adipogenesis (CREB, OCT-1 and c-Myc). Among them, c-Myc was also identified by our microarray data. Our approach to take advantage of the resource of the human genome sequences and the results from our microarray experiments should be validated by further studies of promoter occupancy and TF perturbation.

Zinc coordination environments in proteins as redox sensors and signal transducers

Zinc/cysteine coordination environments in proteins are redox-active. Oxidation of the sulfur ligands mobilizes zinc, while reduction of the oxidized ligands enhances zinc binding, providing redox control over the availability of zinc ions. Some zinc proteins are redox sensors, in which zinc release is coupled to conformational changes that control varied functions such as enzymatic activity, binding interactions, and molecular chaperone activity. Whereas the released zinc ion in redox sensors has no known function, the redox signal is transduced to specific and sensitive zinc signals in redox transducers. Released zinc can bind to sites on other proteins and modulate signal transduction, generation of metabolic energy, mitochondrial function, and gene expression. The paradigm of such redox transducers is the zinc protein metallothionein, which, together with its apoprotein, thionein, functions at a central node in cellular signaling by redistributing cellular zinc, presiding over the availability of zinc, and interconverting redox and zinc signals. In this regard, the transduction of nitric oxide (NO) signals into zinc signals by metallothionein has received particular attention. It appears that redox-inert zinc has been chosen to control some aspects of cellular thiol/disulfide redox metabolism. Tight control of zinc is essential for redox homeostasis because both increases and decreases of cellular zinc elicit oxidative stress. Depending on its availability, zinc can be cytoprotective as a pro-antioxidant or cytotoxic as a pro-oxidant. Any condition with acute or chronic oxidative stress is expected to perturb zinc homeostasis.

Lipid raft-dependent endocytosis of metallothionein in HepG2 cells

Human hepatocellular carcinoma (HepG2) cells take up metallothionein (MT) by endocytosis. MT co-localizes with albumin but not with transferrin, indicating uptake via a non-classical pathway rather than via clathrin-mediated endocytosis. A lipid raft-dependent uptake is indicated by pravastatin inhibition of cholesterol synthesis and methyl-beta-cyclodextrin inhibition of cholesterol translocation to the plasma membrane, reducing MT uptake by 29% and 69%, respectively. Subcellular fractionation after MT uptake reveals significant amounts of MT in vesicular fractions including lysosomes but virtually no MT in the cytosol. Metals bound to MT are released into the cytosol, however. The findings define a pathway for cellular metal acquisition. Together with results from other studies demonstrating secretion of MT from different cells and the presence of MT in extracellular fluids, the results suggest a function of MT in intercellular communication.

SLC39A14, a LZT protein, is induced in adipogenesis and transports zinc

During adipocyte differentiation, there is an underlying complex series of gene expressions. We have previously isolated many genes whose expression levels are quickly elevated by the addition of inducers to mouse 3T3-L1 preadipocyte cells. Here we report the isolation and characterization of SLC39A14, a member of the LZT proteins, one of the subfamilies of ZIP transporters. The expression of the SLC39A14 gene was strongly and rapidly induced at the early stages of differentiation. Moreover, it was highly restricted to the potential differentiation state of 3T3-L1 cells and the expression level was quite low in the nonadipogenic NIH-3T3 cells, indicating a dominant expression in adipocyte differentiation. The zinc uptake assay revealed that SLC39A14 functions as a zinc transporter. Taken together, these results suggest that SLC39A14 plays a role as a zinc transporter during the early stages of adipogenesis.

Loss of PNA staining in mouse aortico-pulmonary septum is associated with mesenchymal cell apoptosis

Massive apoptosis of mesenchymal cells in the septum of the aortico-pulmonary trunk was found in mouse fetuses at stage 14.5dpc. It was associated with the appearance of cavities in the mesenchymal tissue, presumably due to cell loss, a strong reduction in the extent of lectin PNA staining, and the induction of metallothioneins in specialized mesenchymal cells. Cell loss was spatially restricted to an inner area of the septum and was due to a distinct apoptotic pattern of cells, different from that in the heart wall. These events led to a rapid reduction of the aortico-pulmonary septum as occurs during the late stages of heart morphogenesis. It coincided with the migration of other cell types that invaded the cell-depleted septum, and contributed to the histiogenesis of the mature heart.

Gene expression profiling of cathepsin D, metallothioneins-1 and -2, osteopontin, and tenascin-C in a mouse spinal cord injury model by cDNA microarray analysis

The purpose of this study was to use a cDNA microarray to identify new genes involved in healing of spinal cord injury. C57BL/6 mice (7-8 weeks, male) were subjected to spinal cord compression injury (SCI) at the T7/8 level (20 g, 5 min; SCI group). For the control group, mice underwent only laminectomy. Mice were killed at 1, 3 and 7 days. cDNA transcribed from mRNA was hybridized to NIA mice 15K microarrays at each time point. We found 84 genes showing significant expressional changes, including higher and lower expression levels in the SCI groups than in the control [more than 1.0 or less than -1.0 using log ratio (base 2)]. Five genes were selected for further quantitative gene expression analysis by real-time reverse transcription (RT)-PCR. For histological examination, we applied in situ hybridization and fluorescence immunohistochemistry. Cathepsin D, metallothionein-1 (MT-1), metallothionein-2 (MT-2), osteopontin (OPN), and tenascin-C were selected for quantitative and histological analysis. Microarray analysis revealed that SCI led to the up-regulation of OPN and cathepsin D expression at 7 days and also of MT-1, MT-2, and tenascin-C expression at 1 day. Tenascin-C was re-up-regulated at 7 days. These values agreed with those of real-time RT-PCR analysis. By double labeling with in situ hybridization and fluorescence immunohistochemistry, MT-1, MT-2 and tenascin-C expression was observed in neurons and glial cells at 1 day, whereas at 7 days the main MT-2 and tenascin-C expression was found in fibronectin-positive fibroblasts. The main cathepsin D and OPN expression was observed in activated macrophages/microglia at 3 and 7 days. The five genes picked up by microarray gene expression profiling were shown to exhibit temporal and spatial changes of expression after SCI. This system is potentially useful for identifying genes that are involved in the response to SCI.

INVOLVEMENT OF ACTIVATION OF NADPH OXIDASE AND EXTRACELLULAR SIGNAL-REGULATED KINASE (ERK) IN RENAL CELL INJURY INDUCED BY ZINC

Zinc is employed as a supplement; however, zinc-related nephropathy is not generally known. In this study, we investigated zinc-induced renal cell injury using a pig kidney-derived cultured renal epithelial cell line, LLC-PK(1), with proximal kidney tubule-like features, and examined the involvement of free radicals and extracellular signal-regulated kinase (ERK) in the cell injury. The LLC-PK(1) cells showed early uptake of zinc (30 microM), and the release of lactate dehydrogenase (LDH), an index of cell injury, was observed 24 hr after uptake. Three hours after zinc exposure, generation of reactive oxygen species (ROS) was increased. An antioxidant, N, N'-diphenyl-p-phenylenediamine (DPPD), inhibited a zinc-related increase in ROS generation and zinc-induced renal cell injury. An NADPH oxidase inhibitor, diphenyleneiodonium (DPI), inhibited a zinc-related increase in ROS generation and cell injury. We investigated translocation from the cytosol fraction of the p67(phox) subunit, which is involved in the activation of NADPH oxidase, to the membrane fraction, and translocation was induced 3 hr after zinc exposure. We examined the involvement of ERK1/2 in the deterioration of zinc-induced renal cell injury, and the association between ERK1/2 and an increase in ROS generation. Six hours after zinc exposure, the activation (phosphorylation) of ERK1/2 was observed. An antioxidant, DPPD, inhibited the zinc-related activation of ERK1/2. An MAPK/ERK kinase (MEK1/2) inhibitor, U0126, almost completely inhibited zinc-related cell injury (the release of LDH), but did not influence ROS generation. These results suggest that early intracellular uptake of zinc by LLC-PK(1) cells causes the activation of NADPH oxidase, and that ROS generation by the activation of the enzyme leads to the deterioration of renal cell injury via the activation of ERK1/2.

S-Nitroso compounds interfere with zinc probing by Zinquin

The intracellular homeostasis of zinc is postulated to be controlled by signaling through nitric oxide (NO). Administration of the NO donor S-nitrosocysteine (SNOC) caused a rapid drop in the fluorescence of the zinc-specific fluorescence of the zinc probe zinquin in C6 glioma cells. Tentatively, a strong effect of NO on the level of mobile intracellular zinc ions was concluded. However, zinc analysis with atomic absorption spectrometry demonstrated that the total cellular zinc level was not changed under these conditions. Sodium nitrite or an NO donor devoid of sulfhydryl groups (diethylamine NONOate) exerted no degrading effect on the Zn/zinquin fluorescence, but cysteine alone evoked a similar decline as SNOC. Hence, the sulfhydryl groups of cysteine seem to compete for zinc from the Zn/zinquin complex. Analysis of the reaction products by mass spectrometry demonstrated that cysteine caused a depletion of zinc from the Zn/zinquin complex, whereas an NO donor without sulfhydryl groups (diethylamine NONOate) did not. It is concluded that great caution should be employed when using S-nitroso compounds together with zinquin in investigations of intracellular zinc homeostasis.

Alterations in the Levels of Metallothionein and Metals in the Liver, and Unique Serum Liver Enzyme Response in Metallothionein Knock-Out Mice after Burn Injury

OBJECTIVES

Metallothionein (MT) is a small cysteine-rich protein that sequesters and distributes metal ions. Its overexpression stimulates cell proliferation and inhibits apoptosis. We investigated the effects of burn injury on MT expression and metal localization. We also sought to determine roles of MT in the pathophysiologic alterations in the liver after injury.

METHODS

Mice (C57BLKS/J, MT-I/II knock-out, KO, and wild-type control mice) were subjected to an 18% burn. Liver tissues harvested after injury were analyzed for the MT expression and the levels of zinc, copper, manganese, and iron. Levels of alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase were measured in serum samples from MT-I/MT-II KO mice and controls after injury.

RESULTS

Transient induction of MT-I and MT-II mRNAs was observed 3-6 h after injury, while MT-I/MT-II protein peaked on day 1. The induction was localized to hepatocytic nuclei. The intrahepatic levels of zinc, copper, and iron were transiently elevated on day 1, when a downregulation of manganese was evident. Interestingly, only the serum levels of aspartate aminotransferase were significantly augmented in MT-I/MT-II KO mice compared to controls after injury.

CONCLUSIONS

These data suggest that MT and metals may participate in the pathogenesis of the liver after burn injury.

Zinc-metallothionein genoprotective effect is independent of the glutathione depletion in HaCaT keratinocytes after solar light irradiation

UV radiations are the major environmental factors that induce DNA damage of skin cells either by direct absorption (UVB), or after inducing an oxidative stress (UVA and UVB). Cells maintain a reducing intracellular environment to avoid genomic damage. MTs have been expected not only to control metal homeostasis but also counteract the glutathione (GSH) depletion induced by oxidative stress because of their high thiol content. Induction and redistribution of MTs in cultured human keratinocytes (HaCaT) in response to SSL, is an important cellular defense mechanism against DNA damage. Reduced glutathione (GSH) is another way of cellular protection against UV-induced oxidative stress. This study which extend our previous finding focused on the relation between intracellular GSH and Zn genoprotective effects after solar irradiation. HaCaT cells, depleted or not in GSH by a chemical treatment were used to compare MTs induction by Northern blot, expression by Western blot and localization using immunocytochemistry. Zn genoprotection experiments after SSL irradiation was carried out by the comet assay. We demonstrated that in absence of GSH, Zn-MTs could protect DNA after SSL irradiation and that GSH depletion has no effect on MTs induction and localization. Nuclear Zn-MTs could be responsible for this observed genoprotection in GSH depleted cells. So the GSH/Zn and the MT/Zn systems could be two independent but interacting mechanisms of cellular protection against SSL injury.

Intracellular trafficking of micronutrients: from gene regulation to nutrient requirements

The intracellular distribution of micronutrients, as well as their uptake, is important for cell function. In some cases the distribution of micronutrients or their related proteins is determined by gene expression mechanisms. The 3' untranslated region (3'UTR) of metallothionein-1 mRNA determines localisation of the mRNA, and in turn intracellular trafficking of the protein product. Using transfected cells we have evidence for the trafficking of metallothionein-1 into the nucleus and for its involvement in protection from oxidative stress and DNA damage. When nutritional supply of Se is limited, selenoprotein expression is altered, but not all selenoproteins are affected equally; the available Se is prioritised for synthesis of particular selenoproteins. The prioritisation involves differences in mRNA translation and stability due to 3'UTR sequences. Potentially, genetic variation in these regulatory mechanisms may affect nutrient requirements. Genetic polymorphisms in the 3'UTR from two selenoprotein genes have been observed; one polymorphism affects selenoprotein synthesis. These examples illustrate how molecular approaches can contribute at several levels to an increased understanding of nutrient metabolism and requirements. First, they provide the tools to investigate regulatory features in genes and their products. Second, understanding these processes can provide model systems to investigate nutrient metabolism at the cellular level. Third, once key features have been identified, the availability of human genome sequence information and single nucleotide polymorphism databases present possibilities to define the extent of genetic variation in genes of nutritional relevance. Ultimately, the functionality of any variations can be defined and subgroups of the population with subtly different nutrient requirements identified.

Effects of cadmium and zinc on solar-simulated light-irradiated cells: potential role of zinc-metallothionein in zinc-induced genoprotection

Zinc is an essential oligoelement for cell growth and cell survival and has been demonstrated to protect cells from oxidative stress induced by UVA or from genotoxic stress due to UVB. In a recent work we demonstrated that the antioxidant role of zinc could be related to its ability to induce metallothioneins (MTs). In this study we identified the mechanism of zinc protection against solar-simulated light (SSL) injury. Cultured human keratinocytes (HaCaT) were used to examine MTs expression and localization in response to solar-simulated radiation. We found translocation to the nucleus, with overexpression of MTs in irradiated cells, a novel observation. The genoprotective effect of zinc was dependent on time and protein synthesis. DNA damage was significantly decreased after 48 h of ZnCl(2) (100 microM) treatment and is inhibited by actinomycin D. ZnCl(2) treatment (100 microM) led to an intense induction, redistribution, and accumulation of MT in the nucleus of irradiated cells. MT expression correlated with the time period of ZnCl(2) treatment. CdCl(2), a potent MT inducer, did not show any genoprotection, although the MTs were expressed in the nucleus. Overall our findings demonstrate that MTs could be a good candidate for explaining the genoprotection mediated by zinc on irradiated cells.

DNA damage-induced [Zn(2+)](i) transients: correlation with cell cycle arrest and apoptosis in lymphoma cells

Reactive changes in free intracellular zinc cation concentration ([Zn(2+)](i)) were monitored, using the fluorescent probe Zinquin, in human lymphoma cells exposed to the DNA-damaging agent VP-16. Two-photon excitation microscopy showed that Zinquin-Zn(2+) forms complexes in cytoplasmic vesicles. [Zn(2+)](i) increased in both p53(wt) (wild type) and p53(mut) (mutant) cells after exposure to low drug doses. In p53(mut) cells noncompetent for DNA damage-induced apoptosis, elevated [Zn(2+)](i) was maintained at higher drug doses, unlike competent p53(wt) cells that showed a collapse of the transient before apoptosis. In p53(wt) cells, the [Zn(2+)](i) rise paralleled an increase in p53 and bax-to-bcl-2 ratio but preceded an increase in p21(WAF1), active cell cycle arrest in G(2), or nuclear fragmentation. Reducing [Zn(2+)](i), using N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine, caused rapid apoptosis in both p53(wt) and p53(mut) cells, although cotreatment with VP-16 exacerbated apoptosis only in p53(wt) cells. This may reflect changed thresholds for proapoptotic caspase-3 activation in competent cells. We conclude that the DNA damage-induced transient is p53-independent up to a damage threshold, beyond which competent cells reduce [Zn(2+)](i) before apoptosis. Early stress responses in p53(wt) cells take place in an environment of enhanced Zn(2+) availability.

[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.

Role of a copper-specific metallothionein of the blue crab, Callinectes sapidus, in copper metabolism associated with degradation and synthesis of hemocyanin

We have identified three MT encoding genes in the blue crab: MT-I, inducible by cadmium, zinc and copper; MT-II, inducible by cadmium and zinc; and MT-III, inducible by copper only [Syring et al., Comp. Biochem. Physiol. C, 125 (2000) 325-332]. To examine the role of the CuMT-I and CuMT-III isoforms in copper metabolism associated with the synthesis and degradation of the oxygen-binding copper protein, hemocyanin, we (1) cloned and sequenced hemocyanin cDNA, (2) examined interaction of the CuMTs with endoplasmic reticulum (ER) vesicles and (3) measured changes in levels of hemocyanin, MT-I, MT-III protein and mRNA that occur in crabs during different stages of the molt cycle. The cDNA-derived hemocyanin amino-acid sequence revealed the presence of a leader peptide indicating that hemocyanin is a secretory protein that is synthesized on the ER. Copper uptake studies show that ER vesicles take up both Cu1+ and Cu2+ in an ATP-independent process. The copper transporter has a Km of 10.8+/-2.4 microM copper and a Vmax of 6.1+/-0.5 nmol Cu/mg protein/10 min. ER vesicles contain hemocyanin, and bind CuMT-I and, preferentially, CuMT-III. However, binding does not result in copper transfer to the ER. There are statistically significant changes in hepatopancreas MT-III and hemocyanin mRNA, and in hemolymph hemocyanin concentrations during the molt cycle. MT-I mRNA remains constant. Changes in MT-III mRNA are positively correlated with changes in hemocyanin mRNA and hemocyanin protein, which points to coordinate control of MT-III and hemocyanin transcription. No CuMT-III protein is observed in hepatopancreas of intermolt crabs when levels of both MT-III and hemocyanin mRNA are high, suggesting rapid utilization of copper bound to MT-III when cells are actively synthesizing hemocyanin. CuMT-III is present in premolt and softshell crabs, and its emergence appears to coincide with a decrease in hemocyanin synthesis and increase in hemocyanin degradation. These results support the hypothesis that the copper-specific metallothionein is intricately involved in copper homeostasis associated with both the synthesis and degradation of hemocyanin.

Zinc induces apoptosis that can be suppressed by lanthanum in C6 rat glioma cells

Zinc ions have both essential and toxic effects on mammalian cells. Here we report the ability of zinc to act as an inducer of apoptosis in C6 rat glioma cells. Incubation with 150 to 300 microM ZnCl2 caused cell death that was characterized as apoptotic by internucleosomal DNA fragmentation, formation of apoptotic bodies, nuclear fragmentation and breakdown of the mitochondrial membrane potential. On the other hand, zinc deprivation by the membrane permeable chelator TPEN [N,N,N',N',-tetrakis (2-pyridyl-methyl)-ethylenediamine] also induced programmed death in this cell line, indicating the existence of intracellular zinc levels below and above which apoptosis is induced. Zinc-induced apoptosis in C6 cells was independent of major signaling pathways (protein kinase C, mitogen activated protein kinase and guanylate cyclase) and protein synthesis, but was increased by facilitating zinc uptake with the ionophore pyrithione. Lanthanum(III)chloride was also able to increase the net zinc uptake, but nevertheless apoptotic features and zinc toxicity were reduced. Remarkably, lanthanum suppressed the zinc-induced breakdown of the mitochondrial membrane potential. We conclude that in C6 cells lanthanum acts in two different ways, as a promoter of net zinc uptake and as a suppressor of zinc-induced apoptosis.

Roles of the metallothionein family of proteins in the central nervous system

Metallothioneins (MTs) constitute a family of proteins characterized by a high heavy metal [Zn(II), Cu(I)] content and also by an unusual cysteine abundance. Mammalian MTs are comprised of four major isoforms designated MT-1 trough MT-4. MT-1 and MT-2 are expressed in most tissues including the brain, whereas MT-3 (also called growth inhibitory factor) and MT-4 are expressed predominantly in the central nervous system and in keratinizing epithelia, respectively. All MT isoforms have been implicated in disparate physiological functions, such as zinc and copper metabolism, protection against reactive oxygen species, or adaptation to stress. In the case of MT-3, an additional involvement of this isoform in neuromodulatory events and in the pathogenesis of Alzheimer's disease has also been suggested. It is essential to gain insight into how MTs are regulated in the brain in order to characterize MT functions, both in normal brain physiology, as well as in pathophysiological states. The focus of this review concerns the biology of the MT family in the context of their expression and functional roles in the central nervous system.

Metallothionein isogene transcription in red blood cell precursors from human cord blood

The in vitro transcription patterns for 10 functional metallothionein (MT) isogenes have been investigated in red blood cell (RBC) precursors from human cord blood. Active transcription status of the isogenes, MT-0, MT-1A, MT-1B, MT-1E, MT-1G, MT-1X, and MT-2A, was detected in both ex vivo expanded RBC precursors (burst-forming unit-erythroid) and glycophorin A(+) and CD71(+) cells separated by magnetic cell sorting. Transcription patterns of these isogenes were analyzed at different times of incubation with the addition of Zn supplement. In neither the ex vivo expanded precursors nor glycophorin A(+) and CD71(+) cells could MT-1F and MT-3 be detected. Transcripts of MT-4 were detected in glycophorin A(+) and CD71(+) cells. Erythropoietin-responsive constitutive transcription of MT-1X and possible interleukin-3-responsive downregulation of MT-2A in ex vivo expanded precursors reveal their effect on MT biosynthesis. Biosynthesis and induction of MT at the protein level in the RBC precursors was also demonstrated by immunoblotting.

Zn2+ and Cd2+ increase the cyclic GMP level in PC12 cells by inhibition of the cyclic nucleotide phosphodiesterase

In the present study, the influence of the heavy metal ions Cd2+ and Zn2+ on cGMP metabolism in the neurosecretory rat pheochromocytoma (PC12) cell line has been investigated. Cadmium and zinc ions showed a concentration-dependent increase of intracellular cGMP levels as determined by radioimmunoassay: a 20-fold increase in cGMP concentration was found after 15 min of incubation with 20 microM Cd2+, and a 7-fold increase in cGMP was found after incubation with 50 microM Zn2+ (control: 6.05+/-2.1 pmol cGMP/mg protein). To obtain further mechanistic informations, the effects of Cd2+ and Zn2+ on intracellular 3',5'-cyclic nucleotide phosphodiesterase have been studied by a high performance liquid chromatography-based phosphodiesterase-assay. The cellular cGMP hydrolysis was found to be inhibited by these ions with an IC(50) value of 6+/-0.7 microM for Cd2+ and 13+/-2.5microM for Zn2+ . Hence, dose-dependent increase in cellular cGMP content is due to an inhibition of cGMP hydrolysis and not due to an increase in cGMP synthesis. Cd2+ and Zn2+ were taken up by PC12 cells as determined by atomic absorption spectroscopy, all measurements were performed in a subtoxic concentration range. Our data illustrate that zinc and cadmium ions are efficient inhibitors of the cGMP-stimulated cyclic nucleotide PDEII in PC12 cells resulting in elevated cellular cGMP concentrations. Therefore, subtoxic doses of these metals may disturb intracellular cGMP/cAMP-signalling pathways leading to an impaired or altered gene expression.

The transport mechanism of metallothionein is different from that of classical NLS-bearing protein

A nuclear localization signal (NLS) has been detected in several nuclear proteins. Classical NLS-mediated nuclear pore targeting is performed by using the cytosolic factors, importin alpha and importin beta, whereas nuclear translocation requires the small GTPase, Ran. In the present study, we demonstrated that nuclear localization of metallothionein (MT) differs from that of classical NLS-mediated substrates. In digitonin-permeabilized BALB/c3T3 cells, biotinylated MT was localized in the nucleus in the presence of ATP and erythrocyte cytosol in the same manner as for SV40 large T NLS-conjugated allophycocyanin (APC-NLS). Under ATP-free conditions, nuclear rim-binding was observed in both transport substrates. Rim-binding of labeled MT was competitively inhibited by the addition of an excess amount of unlabeled MT. Different elution profiles were observed for the localization-promoting activities of MT in the cytosol compared to those of APC-NLS. Furthermore, nuclear localization of MT was determined to be a wheat germ agglutinin-insensitive, GTPgammaS-sensitive, and anti-Ran antibody-sensitive process. Green fluorescent protein-metallothionein (GFP-MT) fusion protein was also localized in the nucleus in the stable transformant of CHL-IU cells. These results strongly suggest that the targeting by MT of the nuclear pore is mediated by cytosolic factor(s) other than importins and that MT requires Ran for its nuclear localization.

Metallothionein gene expression and secretion in white adipose tissue

White adipose tissue (WAT) has been examined to determine whether the gene encoding metallothionein (MT), a low-molecular-weight stress response protein, is expressed in the tissue and whether MT may be a secretory product of adipocytes. The MT-1 gene was expressed in epididymal WAT, with MT-1 mRNA levels being similar in lean and obese (ob/ob) mice. MT-1 mRNA was found in each of the main adipose tissue sites (epididymal, perirenal, omental, subcutaneous), and there was no major difference between depots. Separation of adipocytes from the stromal-vascular fraction of WAT indicated that the MT gene (MT-1 and MT-2) was expressed in adipocytes themselves. Treatment of mice with zinc had no effect on MT-1 mRNA levels in WAT, despite strong induction of MT-1 expression in the liver. MT-1 gene expression in WAT was also unaltered by fasting or norepinephrine. However, administration of a beta(3)-adrenoceptor agonist, BRL-35153A, led to a significant increase in MT-1 mRNA. On differentiation of fibroblastic preadipocytes to adipocytes in primary culture, MT was detected in the medium, suggesting that the protein may be secreted from WAT. It is concluded that WAT may be a significant site of MT production; within adipocytes, MT could play an antioxidant role in protecting fatty acids from damage.

Metalloregulation of the tumor suppressor protein p53: zinc mediates the renaturation of p53 after exposure to metal chelators in vitro and in intact cells

The tumor suppressor p53 is a transcription factor which binds DNA through a structurally complex domain stabilized by a zinc atom. Zinc chelation disrupts the architecture of this domain, inducing the protein to adopt an immunological phenotype identical to that of many mutant forms of p53. In this report, we used 65Zn to show that incorporation of zinc within the protein was required for folding in the 'wild-type' conformation capable of specific DNA-binding. Using a cellular assay, we show that addition of extracellular zinc at concentrations within the physiological range (5 microM) was required for renaturation and reactivation of wild-type p53. Among other divalent metals tested (Cd2+, Cu2+, Co2+, Fe2+ and Ni2+), only Co2+ at 125 microM had a similar effect. Recombinant metallothionein (MT), a metal chelator protein, was found to modulate p53 conformation in vitro. In cultured cells, overexpression of MT by transfection could modulate p53 transcriptional activity. Taken together, these results suggest that zinc binding plays a regulatory role in the control of p53 folding and DNA-binding activity.

Factors that determine the plasma-membrane potential in bloodstream forms of Trypanosoma brucei

The plasma-membrane potential (Delta(psi)p) in bloodstream forms of Trypanosoma brucei was studied using several different radiolabelled probes: 86Rb+ and [14C]SCN- were used to report Delta(psi)p directly because they distribute in easily measured quantities across the plasma membrane only, and [3H]methyltriphenylphosphonium (MePh3P+) was used to report Delta(psi)p only when Delta(psi)m had been abolished with FCCP because it reports the algebraic sum of the two potentials when used alone. The unperturbed Delta(psi)p had a value of -82 mV and was found to be essentially identical with, and determined almost completely by, the potassium diffusion potential, as evidenced by: (a) the lack of effect of valinomycin on the value obtained under appropriate conditions when any of these probes were used; (b) the close agreement of this measured value with that predicted from the measured distribution of K+ across the plasma membrane (-76 mV); (c) the large effect of changes in the extracellular K+ concentration by substitution with Na+ on Delta(psi)p together with the complete lack of effect of substitution of extracellular Na+ by the choline cation or substitution of extracellular Cl- by the gluconate anion on Delta(psi)p. The contribution to Delta(psi)p by electrogenic pumping of Na+/K+-ATPase was found to be small (of the order of 6 mV). H+ was not found to be pumped across the plasma membrane or to contribute to Delta(psi)p.

Delay of M-phase onset by aphidicolin can retain the nuclear localization of zinc and metallothionein in 3T3-L1 fibroblasts

The transient nuclear localization of metallothionein during cell growth and differentiation may be related to the increased requirement of zinc for DNA synthesis, activation of metalloenzymes, and transcription factors. Treatment of 3T3-L1 fibroblasts with aphidicolin, an inhibitor of nuclear DNA synthesis, caused a cell-cycle block at G1/S phase and a delay in the onset of M phase. This also resulted in the accumulation of both zinc and metallothionein in the nucleus. After removal of aphidicolin, the cells rapidly reentered S phase, and during the G2/M phase of cell cycle both zinc and metallothionein began to relocate to the cytoplasm. Delaying the onset of M phase in 3T3-L1 cells could prevent the cytoplasmic relocation of metallothionein. The nuclear translocation of both zinc and metallothionein during the cell cycle can be considered as a normal process and this may be a general mechanism in response to mitogenic signals.

Modulation of telomerase activity by zinc in human prostatic and renal cancer cells

Because the up-regulation of telomerase in most cancer tissues is considered to be responsible for the unlimited proliferation of cancer cells, suppression of telomerase activity is an attractive potential target for cancer therapy. The mechanism for the activation of telomerase in cancer cells, however, is still unclear. In the present study, we demonstrated that Zn induces an enhancement of telomerase activity in the human renal cell carcinoma (NRC-12) and prostatic cancer (DU145) cell lines. The maximum elevation of the activity was observed 6 hr after treatment with 100 microM Zn; it was diminished by the addition of either metal chelator or cycloheximide. Other metals such as Cd and Cu also enhanced telomerase activity but to a lesser extent, and no correlation between the activation of telomerase and the induction of metallothionein was observed. Our findings provide the first evidence that metals, especially Zn, can modulate telomerase activity in cancer cells.