Reactive oxygen species as essential mediators of cell adhesion: the oxidative inhibition of a FAK tyrosine phosphatase is required for cell adhesion

Signal transduction by reactive oxygen species (ROS; "redox signaling") has recently come into focus in cellular biology studies. The signaling properties of ROS are largely due to the reversible oxidation of redox-sensitive target proteins, and especially of protein tyrosine phosphatases, whose activity is dependent on the redox state of a low pKa active site cysteine. A variety of mitogenic signals, including those released by receptor tyrosine kinase (RTKs) ligands and oncogenic H-Ras, involve as a critical downstream event the intracellular generation of ROS. Signaling by integrins is also essential for the growth of most cell types and is constantly integrated with growth factor signaling. We provide here evidence that intracellular ROS are generated after integrin engagement and that these oxidant intermediates are necessary for integrin signaling during fibroblast adhesion and spreading. Moreover, we propose a synergistic action of integrins and RTKs for redox signaling. Integrin-induced ROS are required to oxidize/inhibit the low molecular weight phosphotyrosine phosphatase, thereby preventing the enzyme from dephosphorylating and inactivating FAK. Accordingly, FAK phosphorylation and other downstream events, including MAPK phosphorylation, Src phosphorylation, focal adhesion formation, and cell spreading, are all significantly attenuated by inhibition of redox signaling. Hence, we have outlined a redox circuitry whereby, upon cell adhesion, oxidative inhibition of a protein tyrosine phosphatase promotes the phosphorylation/activation and the downstream signaling of FAK and, as a final event, cell adhesion and spreading onto fibronectin.

Redox regulation of cAMP-responsive element-binding protein and induction of manganous superoxide dismutase in nerve growth factor-dependent cell survival

Reactive oxygen species (ROS) act as both signaling molecules and mediators of cell damage in the nervous system and are implicated in the pathogenesis of neurodegenerative diseases. Neurotrophic factors such as the nerve-derived growth factor (NGF) support neuronal survival during development and promote regeneration after neuronal injury through the activation of intracellular signals whose molecular effectors and downstream targets are still largely unknown. Here we present evidence that early oxidative signals initiated by NGF in PC12 cells, an NGF-responsive cell line, play a critical role in preventing apoptosis induced by serum deprivation. This redox-signaling cascade involves phosphatidylinositol 3-kinase, the small GTPase Rac-1, and the transcription factor cAMP-responsive element-binding protein (CREB), a molecule essential to promote NGF-dependent survival. We found that ROS are necessary for NGF-dependent phosphorylation of CREB, an event directly correlated with CREB activity, whereas hydrogen peroxide induces a robust CREB phosphorylation. Cells exposed to NGF show a late decrease in the intracellular content of ROS when compared with untreated cells and increased expression of the mitochondrial antioxidant enzyme manganese superoxide dismutase, a general inhibitor of cell death. Accordingly, serum deprivation-induced apoptosis was selectively inhibited by low concentrations of the mitochondrially targeted antioxidant Mito Q (mitoquinol/mitoquinone). Taken together, these data demonstrate that the oxidant-dependent activation of CREB is a component of NGF survival signaling in PC12 cells and outline an intriguing circuitry by which a cytosolic redox cascade promotes cell survival at least in part by increasing mitochondrial resistance to oxidative stress.

Reactive oxygen species as downstream mediators of angiogenic signaling by vascular endothelial growth factor receptor-2/KDR

Recent evidence shows the involvement of reactive oxygen species (ROS) in the mitogenic cascade initiated by the tyrosine kinase receptors of several growth factor peptides. We have asked whether also the vascular endothelial growth factor (VEGF) utilizes ROS as messenger intermediates downstream of the VEGF receptor-2 (VEGFR-2)/KDR receptor given that the proliferation of endothelial cells during neoangiogenesis is physiologically regulated by oxygen and likely by its derivative species. In porcine aortic endothelial cells stably expressing human KDR, receptor activation by VEGF is followed by a rapid increase in the intracellular generation of hydrogen peroxide as revealed by the peroxide-sensitive probe dichlorofluorescein diacetate. Genetic and pharmacological studies suggest that such oxidant burst requires as upstream events the activation of phosphatidylinositol 3-kinase and the small GTPase Rac-1 and is likely initiated by lipoxygenases. Interestingly, ROS generation in response to VEGF is not blocked but rather potentiated by endothelial nitric-oxide synthase inhibitors diphenyleneiodonium and N(G)methyl-l-arginine, ruling out the possibility of nitric oxide being the oxidant species here detected in VEGF-stimulated cells. Inhibition of KDR-dependent generation of ROS attenuates early signaling events including receptor autophosphorylation and binding to a phospholipase C-gamma-glutathione S-transferase fusion protein. Moreover, catalase, the lipoxygenase inhibitor nordihydroguaiaretic acid, the synthetic ROS scavenger EUK-134, and phosphatidylinositol 3-kinase inhibitor wortmannin all reduce ERK phosphorylation in response to VEGF, and antioxidants prevent VEGF-dependent mitogenesis. Finally, cell culture and stimulation in a nearly anoxic environment mimic the effect of ROS scavenger on receptor and ERK phosphorylation, reinforcing the idea that ROS are necessary components of the mitogenic signaling cascade initiated by KDR. These data identify ROS as a new class of intracellular angiogenic mediators and may represent a potential premise for new antioxidant-based antiangiogenic therapies.

Cell compartmentalization in redox signaling

From a growing body of evidence on the role of Reactive Oxygen Species as intracellular signaling molecules, the concept starts to emerge that cell responses to redox changes are function of the intracellular site where oxidants are produced and/or meet their molecular targets. In particular, a major distinction between oxidative events in the cytosolic versus the mitochondrial compartment appears to exist in terms of physiological stimuli, signaling mechanisms and functional consequences. Experimental data supporting this view are reviewed here, and the potential implications of this new perspective in redox signaling are discussed.

The level of manganese superoxide dismutase content is an independent prognostic factor for glioblastoma. Biological mechanisms and clinical implications

We address the issue of the role of manganese superoxide dismutase in tumorigenesis by studying a relatively homogeneous group of tumours for the correlation between amount of this anti-oxidant enzyme and prognosis. The clinical outcome of 30 patients affected by glioblastomas whose manganese superoxide dismutase content had been established at the time of first diagnosis is compared. When the survival of patients is stratified according to manganese superoxide dismutase level in the tumour, a link of these levels and prognosis can be observed. Patients with high levels of manganese superoxide dismutase show a median survival time of 6.11 months, while patients whose tumours display a low amount of MnSOD have a median survival time of 12.17 months. To assess the upstream mechanisms that sustain the increase in manganese superoxide dismutase content in brain neuroepithelial tumours, we also studied the expression of p53 in a series of 17 astrocytomas of various grading. In all tested astrocytomas, high manganese superoxide dismutase content is associated with cytoplasmic accumulation of p53. Thus glioblastomas can be divided into two distinct groups on the basis of their content of manganese superoxide dismutase, having 'better' or 'worse' prognosis, respectively. The use of this protein as a marker may help to define therapeutic strategies in the clinical management of glioblastoma.

A redox signaling mechanism for density-dependent inhibition of cell growth

Reactive oxygen species (ROS) have recently drawn significant attention as putative mitogenic mediators downstream of activated growth factor receptors and oncogenic Ras; however, the possibility that a redox-related mechanism also operates in the negative control of cell proliferation by inhibitory signals has not been investigated thus far. Here we show that the arrest of growth induced by cell confluence ("contact inhibition") is due, at least in part, to a decrease in the steady-state levels of intracellular ROS and the consequent impairment of mitogenic redox signaling. In confluent fibroblast cultures, the decrease in the concentration of oxygen species was associated with diminished activity of the small GTPase Rac-1, a signal transducer directly involved in the ligand-dependent generation of oxygen-derived molecules, and was effectively mimicked by exposure of sparse cultures to dithiothreitol (DTT) and inhibitors of enzymes (phospholipase A2 and lipoxygenase) acting in the arachidonic acid cascade downstream of growth factor receptors and Rac-1. Sparse fibroblasts treated with nontoxic amounts of DTT underwent growth arrest, whereas a low concentration of hydrogen peroxide significantly increased thymidine incorporation in confluent cultures, demonstrating a causal link between redox changes and growth control by cell density. Removal of oxygen species from sparse cultures was accompanied by a drastic decrease of protein tyrosine phosphorylation after epidermal growth factor stimulation, which, at a biochemical level, reproduced the signaling hallmarks of contact inhibition. Moreover, the cytosolic tyrosine phosphatase SHP-2 was identified as a putative target for redox signaling by cell density because the enzyme itself and the associated substrates appear markedly dephosphorylated in both confluent and reductant-treated cells after exposure to epidermal growth factor, and SHP-2 enzymatic activity is strongly activated by DTT in vitro. Taken together, these data support a model in which impaired generation of ROS and increased protein tyrosine phosphatase activity impede mitogenic signaling in contact-inhibited cells.

Deregulated manganese superoxide dismutase expression and resistance to oxidative injury in p53-deficient cells

Loss of function of the tumor suppressor protein p53 represents a very frequent event in human carcinogenesis, but the molecular mechanisms linking impaired p53 activity to increased cell malignancy are still incompletely understood. p53 is normally involved in both cell cycle control and the induction of cell death and is involved in the latter mainly through the transcriptional regulation of pro- and antiapoptotic proteins. Reactive oxygen species are known to be powerful inducers of p53 activity; moreover, they play a role in the execution of p53-dependent apoptosis. Here we show that transformed mouse fibroblasts lacking p53 are significantly more resistant than wild-type (wt) controls to the cytotoxic effect of a number of pro-oxidant treatments. Interestingly, these cells also exhibit deregulated expression of the antioxidant enzyme manganese superoxide dismutase (MnSOD), a protein known to protect cancer cells from the oxidative injury inflicted by antitumoral cytokines and anticancer drugs. MnSOD activity was also increased in liver tissue from p53-deficient mice in comparison with wt tissue. Transient transfection of wt p53 in HeLa cells led to a significant reduction in steady-state MnSOD mRNA levels and enzymatic activity, confirming that the expression of this antioxidant enzyme is negatively regulated by p53. Forced expression of MnSOD rendered HeLa cells resistant to p53-dependent cytotoxic treatments and, in cotransfection experiments, counteracted the growth-inhibitory effect of p53. Taken together, these data identify MnSOD as a potential target for tumor suppressor protein p53 and underscore the relevance of MnSOD modulation in the context of normal p53 functions because it is consistent with many reports of abnormally increased MnSOD expression in human cancers.

Redox regulation of lymphocyte signaling

Compelling evidence exists that reactive oxygen species can deliver intracellular signals in mammalian cells, and elicit a broad array of physiological responses according to the cell type, the oxidative burden and the cellular compartment where radicals are generated. When applied to immune cells, these concepts gain a particular relevance, in relation to the plasticity of immune functions and the biological complexity of lymphocyte response to antigens. Here we review some recent and somehow conflicting observations on the involvement of oxygen radicals and redox balance in lymphocyte activation, and propose models for how radical species could contribute to normal and pathological immunity.

Endogenous oxygen radicals modulate protein tyrosine phosphorylation and JNK-1 activation in lectin-stimulated thymocytes

Molecular events mediating the T-lymphocyte response to lectins are still incompletely understood, although much evidence suggests that both the mitogenic and the death-promoting effects of these agents involve the biochemical cascade initiated by the CD3/T-cell antigen receptor (TCR) complex. Reactive oxygen species (ROS) and in particular H(2)O(2) have been shown to have a role in cell response to cytokines and growth factors. Here we report that the proliferation of mouse thymocytes in response to the mitogenic lectin concanavalin A (ConA) is strongly and selectively inhibited by the intracellular ROS scavenger N-acetylcysteine (NAC) and by diphenyleneiodonium (DPI), a potent inhibitor of NADPH-dependent membrane oxidases activated by surface receptors. A rapid 'burst' of intracellular oxygen radicals was observed in mouse thymocytes stimulated by ConA, with kinetics that paralleled the appearance of tyrosine-phosphorylated proteins. This burst was abrogated by the pretreatment of cells with NAC or DPI. Only a modest increase in intracellular oxygen species was found in thymocytes stimulated by strong cross-linking of TCR together with CD4 or CD28. Pharmacological interference with ROS production in ConA-stimulated thymocytes resulted in a decreased tyrosine phosphorylation of multiple protein species, including a 38 kDa band able to recruit the adapter protein Grb2 and corresponding to the recently identified transducer LAT (linker for activation of T-cells), a molecule involved in linking activated TCR to the production of interleukin 2 and the proliferation of T-cells. Furthermore, ROS inhibition markedly attenuated the activation of stress-activated protein kinase/JNK-1 (c-Jun N-terminal kinase 1) in response to lectins. Taken together, these results identify ROS as important modulators of the signalling cascade initiated by mitogenic lectins in thymocytes and, by extension, as a novel class of mediators downstream of antigen receptors.

Regulation of manganese superoxide dismutase (MnSOD) in chronic experimental alcoholism: effects of vitamin E-supplemented and -deficient diets

In order to investigate the pathogenic mechanism responsible for liver injury associated with chronic alcoholism, we studied the effects of different dietary vitamin E levels in chronically ethanol (EtOH)-fed rats on the activity and mRNA regulation of the manganese superoxide dismutase (MnSOD) enzyme. Evidence is accumulating that intermediates of oxygen reduction may in fact be associated with the development of alcoholic liver disease. Since low vitamin E liver content seems to potentiate EtOH-linked oxidative stress, we studied the effect of EtOH treatment in livers from rats fed a diet deficient or supplemented with vitamin E. Chronic EtOH feeding enhanced hepatic consumption of vitamin E in both groups of EtOH-treated animals, irrespectively of the vitamin E level of the basal diet and the effect was observed in both the microsomal and mitochondrial fractions. Both EtOH-fed groups exhibited increased MnSOD gene expression, while the enzyme activity was enhanced only in the vitamin E-deprived group of EtOH-treated animals. The significant increase in manganese liver content found only in this last group could explain the rise of enzyme activity. In fact, in the absence of a parallel increase of the prosthetic ion manganese, MnSOD mRNA induction was not accompanied by a higher enzymatic activity. These findings support the role of oxidative alteration in the EtOH-induced chronic hepatotoxicity in which MnSOD response might represent a primary defence mechanism against the damaging effect of oxygen radical species.

Increased growth capacity of cervical-carcinoma cells over-expressing manganous superoxide dismutase

Increases in the expression of manganese-dependent superoxide dismutase (MnSOD) have been detected in several classes of human and experimental tumors and appear to correlate with poorer prognosis in human neuro-epithelial, ovarian and cervical tumors. To delineate the relevance of MnSOD expression to tumor-cell growth and survival, a human MnSOD cDNA was over-expressed in the HeLa cervical-carcinoma cell line. MnSOD over-expression had marginal effects on the growth of HeLa cells in standard medium, but markedly protected the cells from growth suppression and cell death in conditions of serum deprivation. Serum starvation did not affect expression of endogenous MnSOD in wild-type HeLa cells, but was associated with increases in cell death and in the generation of intracellular oxygen radicals. By contrast, in HT29 colon-carcinoma cells, which are relatively resistant to growth-factor withdrawal, serum deprivation was associated with increases in MnSOD expression and activity. Together these observations suggest that MnSOD provides a mechanism for counteracting the intracellular oxidative processes that impair cell growth and viability in the context of growth-factor withdrawal and, in this context, may promote tumor-cell survival in vivo in conditions normally unfavorable to cell growth.

Oxidative stress and overexpression of manganese superoxide dismutase in patients with Alzheimer's disease

Substantial evidence supports the hypothesis that oxygen free radicals are involved in various neurodegenerative disorders. To assess the presence of oxidative stress in Alzheimer's disease (AD) we examined the activity of the enzyme copper-zinc superoxide dismutase (CuZnSOD) in red blood cells, the levels of the mitochondrial inducible enzyme manganese superoxide dismutase (MnSOD) mRNA in lymphocytes, and the total radical-trapping antioxidant capacity (TRAP) in plasma of AD patients and in a group of age-matched non-demented controls. We found that CuZnSOD activity (P < 0.01 vs. controls) was significantly increased as well as the MnSOD mRNA levels while the total antioxidant status (P < 0.001 vs. controls) was decreased in AD patients. These findings support the role of oxidative alterations in the pathogenetic mechanism underlying AD neurodegeneration.

Iron modulation of LPS-induced manganese superoxide dismutase gene expression in rat tissues

Mitochondrial superoxide dismutase (MnSOD) is usually diminished in cancer cells. We observed that in vivo treatment with LPS produces a strong increase of MnSOD mRNA levels and a weak induction of an inactive protein in rat hepatocarcinomas. In normal liver iron deficiency, obtained with desferrioxamine administration, produces a decrease in the MnSOD induction by LPS, indicating that such induction could depend on tissue iron content. However, no change in MnSOD mRNA has been observed in iron-overloaded tumor tissue. Thus, iron is possibly involved in the transcriptional regulation of the protein, in combination with some other unknown factor that appears to be deficient in tumor cells.

The level of MnSOD is directly correlated with grade of brain tumours of neuroepithelial origin

The oxy-radical scavenger enzyme manganese superoxide dismutase (MnSOD) may act in the capacity of a tumour-suppressor gene. To address the issue of its role in tumour transformation and progression in vivo, we evaluated the content of this enzyme in 33 brain tumours of neuroepithelial origin with different degrees of differentiation (WHO grade II-IV) by means of Western blot and immunohistology. Our results show that immunoreactive MnSOD increases in a direct relationship with tumour grade and is therefore inversely correlated with differentiation. The increase in induced at a pretranscriptional level and is apparently specific to brain tumours of neuroepithelial origin. Approximately 30% of grade IV tumours display low levels of MnSOD content, and preoperative radiotherapy and brachytherapy result in low amounts of enzyme. Based upon these observations, we suggest that MnSOD cannot be considered a classical tumour-suppressor gene.

Diethyldithiocarbamate treatment up regulates manganese superoxide dismutase gene expression in rat liver

In vivo experiments demonstrate that rat liver manganese-containing superoxide dismutase (MnSOD) is up-regulated at the transcriptional level following the inactivation of copper-zinc superoxide dismutase (CuZnSOD). CuZnSOD activity was inhibited by the administration of the copper chelating agent diethyldithiocarbamate (DDC). This CuZnSOD inactivation is likely associated with an intracellular oxidative stress. Indeed the antioxidant N-acetyl-cysteine (NAC) completely prevents the MnSOD mRNA up-regulation observed after DDC administration. Evidence is also provided that an approximately 50% diminution of the total iron content in the tissue, which follows the in vivo administration of the iron chelator desferrioxamine (DESF), reduces the amount of MnSOD induction achieved by DDC treatment. Both NAC and DESF significantly down-regulate MnSOD gene expression also in normal untreated rat liver. While the observed inhibitory effect of NAC in MnSOD mRNA up-regulation can be ascribed mainly to its antioxidant property, iron chelation could act with an antioxidant effect and/or affecting some iron-dependent factor(s) possibly involved in MnSOD gene regulation. It is proposed that this metal could have a role among factors that sense and/or trigger transcription of the MnSOD gene.

Manganese content and high-affinity transport in liver and hepatoma

Morris hepatomas 3924A and 9618A have much lower endogenous contents of Mn than normal rat liver. This work studied the uptake of Mn by slices of these three tissues over a range of concentrations from 0.05 to 100 microM. The influx was assessed with 54Mn while atomic absorption measurements determined the total content. At medium Mn from 0.05 to 5 microM, entry of 54Mn in 2 min was taken as the initial rate and within this period the apparent concentration of Mn in the cell water exceeded that in the medium. Liver showed three apparently saturable uptake systems, the medium concentrations of Mn for half-maximal uptake rate being 0.075, approximately 2, and 100 microM. Hepatoma 3924A appeared to have only two systems, the half-maximal concentration for the higher affinity mechanism being, at 0.34 microM, substantially greater than that for liver. At no concentration was the uptake rate of Mn by hepatoma 3924A less than that of liver although there was some indication that Mn uptake by 9618A was somewhat less than that by the other two tissues. It is concluded that liver and hepatoma 3924A have systems for Mn uptake with affinities that enable them to be active at the plasma concentration (approximately 0.1 microM) as well as uptake systems of less affinity. However, differences in these systems between liver and hepatomas do not account for the differences in endogenous Mn content.

The role of delta-6- and delta-9-desaturase in the fatty acid metabolism of hepatomas with different growth rate

The fatty acid composition of microsomal membranes from Morris hepatomas 9618A, slow growing, and 3924A, fast growing, confirm the higher content in oleic acid and the loss of PUFAs of the tumours with respect to controls. The specific activities of delta-9-desaturase indicate alternative metabolic pathways for the increased production of oleic acid in the two hepatomas. The delta-6-desaturase activity is much lower in tumours than in controls. However the loss of PUFAs found in tumours seems to be mostly due to a low content in linoleic acid.

Ethanol treatment up-regulates the expression of mitochondrial manganese superoxide dismutase in rat liver

On the basis of the well-known effect of ethanol poisoning on the prooxidant/antioxidant balance of human and rodent liver we tested the response of the mitochondrial manganese-containing superoxide dismutase (MnSOD) in the liver of rats following an acute ethanol load or chronically intoxicated with an alcohol-supplemented solid diet for three weeks. In both conditions the enzyme activity and messenger RNA were monitored. In the acutely treated animals MnSOD was induced (post-)translationally already at 3 hours after ethanol injection, reached the maximum level (about 50% increment) at 9 hours and decreased thereafter. Chronic ethanol feeding caused an up-regulation of the enzyme at the mRNA level, with a good correlation between the transcript and the enzyme activity during the first two weeks of treatment. After 20 days the mRNA level dropped to normal, whereas the activity still remained high. Chronic alcohol intake also led to a significant decrease in the content of vitamin E in the liver mitochondrial and microsomal fractions, suggesting the occurrence of an enhanced lipid peroxidation, consequent to the ethanol-induced oxidative stress. The response of MnSOD appears to be a protective mechanism that the genetic machinery builds up to partially overcome such a condition.

Monoclonal antibody 35.8 recognizes human, mouse and rat MnSODs in western blot and immunostaining

MnSOD is an antioxidant enzyme whose decrease in activity appears involved in tumorigenesis. We had previously reported the production of a monoclonal antibody, named 35.8, against rat MnSOD. In the present paper we show that it recognizes human and mouse MnSODs, although with different detection limits. We also use the antibody for immunofluorescence studies and observed that the antibody yields a positive staining of a non-nuclear protein, in rat and human organs where high concentration of MnSOD activity have been reported, and a lack of staining in rat kidney where MnSOD activity is decreased. Two tumors, an experimental rat hepatocarcinoma and a human liver metastasis from a gastrointestinal adenocarcinoma, are found negative for immunostaining.

Vinblastine-dependent down-modulation of TNF receptors in human osteosarcoma cells is mediated by protein kinase C activity

The binding of tumor necrosis factor (TNF) to a human osteogenic sarcoma cell line (Saos-2) was investigated. These cells express two types of receptors as determined by specific monoclonal antibodies. Vinblastine induced a down-modulation of these receptors weaker than the one produced by phorbol esters or okadaic acid treatment. On exposure of cells to 10 microM vinblastine for two hours an approximately 55-65% diminution of TNF binding was observed, but only 20% reduction occurred under long-term vinblastine treatment. TNF receptor down-modulation induced by vinblastine was partially prevented by protein kinase C inhibitors or protein kinase C depletion. It is suggested that the regulation of TNF binding to each one of its receptors in Saos-2 cells always occurs in a phosphorylation-dependent manner.

Preparation of a monoclonal antibody against rat MnSOD, using a COOH-terminal peptide

In the present paper we report the production of a monoclonal antibody against rat MnSOD, a supposed tumor-suppressor protein, using a purified synthetic peptide encompassing amino acids 184-198 to immunize mice, without conjugation to a carrier. The resulting antibody is able to recognize the native form of the protein, since it can immunoprecipitate the MnSOD activity in rat liver homogenate. In Western blot studies, the antibody recognizes a protein of 24 KD M(r), whose concentration varies according to the MnSOD activity and it apparently recognizes also human and mouse MnSODs. The protocol of immunization gives high yield of secreting lines. This monoclonal antibody will allow the detection of structural and functional alterations of MnSOD.

Defective gene expression of MnSOD in cancer cells

Reactive oxygen species (ROS) have been frequently implicated in the initiation and promotion phases of carcinogenesis. Antioxidant enzymes, which can antagonize this process, are lowered in a number of malignancies. The enzyme most commonly decreased is the mitochondrial Mn-containing superoxide dismutase (MnSOD) encoded by a nuclear gene mapped on the band 6q21, a region frequently deleted in several human tumours. The close association of del(6q) with diminution of MnSOD has led to suggest that MnSOD might be a new type of tumour-suppressor gene. This hypothesis is also sustained by the finding that transfection of MnSOD cDNA into human melanoma cell lines suppress the malignant phenotype. There are, however, conflicting observations that tend to ascribe the deficiency of the MnSOD activity more to a defect in the expression of the gene than to its deletion. In many transformed cell lines, including some with marked del(6q), there is no change in the dosage of the MnSOD gene and the enzyme is highly inducible by various pro-oxidant agents. Transition metals (Mn, Fe) have been found to be highly deficient in human and rodent tumours. Owing to the second messenger function of ROS in activating transcription factors (NF-kB, AP-1) and to the ability of Mn to facilitate the dismutation of O2- to H2O2 and of Fe to participate in the Fenton reaction, we propose that in the early stage of carcinogenesis an impairment of the signal transduction machinery, related to the metal deficiency, might limit the binding to DNA of transcription factors and cause the defect in the MnSOD gene expression.