Cell-cell contacts prevent t-BuOOH-triggered ferroptosis and cellular damage in vitro by regulation of intracellular calcium.

Tert-butyl hydroperoxide (t-BuOOH) is an organic hydroperoxide widely used as a model compound to induce oxidative stress. It leads to a plethora of cellular damage, including lipid peroxidation, DNA double-strand breaks (DNA DSBs), and breakdown of the mitochondrial membrane potential (MMP). We could show in several cell lines that t-BuOOH induces ferroptosis, triggered by iron-dependent lipid peroxidation. We have further revealed that not only t-BuOOH-mediated ferroptosis, but also DNA DSBs and loss of MMP are prevented by cell-cell contacts. The underlying mechanisms are not known. Here, we show in murine fibroblasts and a human colon carcinoma cell line that t-BuOOH (50 or 100 µM, resp.) causes an increase in intracellular Ca2+, and that this increase is key to lipid peroxidation and ferroptosis, DNA DSB formation and dissipation of the MMP. We further demonstrate that cell-cell contacts prevent t-BuOOH-mediated raise in intracellular Ca2+. Hence, we provide novel insights into the mechanism of t-BuOOH-triggered cellular damage including ferroptosis and propose a model in which cell-cell contacts control intracellular Ca2+ levels to prevent lipid peroxidation, DNA DSB-formation and loss of MMP. Since Ca2+ is a central player of toxicity in response to oxidative stress and is involved in various cell death pathways, our observations suggest a broad protective function of cell-cell contacts against a variety of exogenous toxicants.

Cell-cell contacts protect against t-BuOOH-induced cellular damage and ferroptosis in vitro.

Ferroptosis is a recently discovered pathway of regulated necrosis dependent on iron and lipid peroxidation. It has gained broad attention since it is a promising approach to overcome resistance to apoptosis in cancer chemotherapy. We have recently identified tertiary-butyl hydroperoxide (t-BuOOH) as a novel inducer of ferroptosis. t-BuOOH is a widely used compound to induce oxidative stress in vitro. t-BuOOH induces lipid peroxidation and consequently ferroptosis in murine and human cell lines. t-BuOOH additionally results in a loss of mitochondrial membrane potential, formation of DNA double-strand breaks, and replication block. Here, we specifically address the question whether cell-cell contacts regulate t-BuOOH-induced ferroptosis and cellular damage. To this end, murine NIH3T3 or human HaCaT cells were seeded to confluence, but below their saturation density to allow the establishment of cell-cell contacts without inducing quiescence. Cells were then treated with t-BuOOH (50 or 200 µM, respectively). We revealed that cell-cell contacts reduce basal and t-BuOOH-triggered lipid peroxidation and consequently block ferroptosis. Similar results were obtained with the specific ferroptosis inducer erastin. Cell-cell contacts further protect against t-BuOOH-induced loss of mitochondrial membrane potential, and formation of DNA double-strand breaks. Interestingly, cell-cell contacts failed to prevent t-BuOOH-mediated replication block or formation of the oxidative base lesion 8-oxo-dG. Since evidence of protection against cell death was both (i) observed after treatment with hydrogen peroxide, methyl methanesulfonate or UV-C, and (ii) seen in several cell lines, we conclude that protection by cell-cell contacts is a widespread phenomenon. The impact of cell-cell contacts on toxicity might have important implications in cancer chemotherapy.

t-BuOOH induces ferroptosis in human and murine cell lines.

Reactive oxygen species (ROS)-induced apoptosis has been extensively studied. Increasing evidence suggests that ROS, for instance, induced by hydrogen peroxide (H2O2), might also trigger regulated necrotic cell death pathways. Almost nothing is known about the cell death pathways triggered by tertiary-butyl hydroperoxide (t-BuOOH), a widely used inducer of oxidative stress. The lipid peroxidation products induced by t-BuOOH are involved in the pathophysiology of many diseases, such as cancer, cardiovascular diseases, or diabetes. In this study, we exposed murine fibroblasts (NIH3T3) or human keratinocytes (HaCaT) to t-BuOOH (50 or 200 µM, respectively) which induced a rapid necrotic cell death. Well-established regulators of cell death, i.e., p53, poly(ADP)ribose polymerase-1 (PARP-1), the stress kinases p38 and c-Jun N-terminal-kinases 1/2 (JNK1/2), or receptor-interacting serine/threonine protein kinase 1 (RIPK1) and 3 (RIPK3), were not required for t-BuOOH-mediated cell death. Using the selective inhibitors ferrostatin-1 (1 µM) and liproxstatin-1 (1 µM), we identified ferroptosis, a recently discovered cell death mechanism dependent on iron and lipid peroxidation, as the main cell death pathway. Accordingly, t-BuOOH exposure resulted in a ferrostatin-1- and liproxstatin-1-sensitive increase in lipid peroxidation and cytosolic ROS. Ferroptosis was executed independently from other t-BuOOH-mediated cellular damages, i.e., loss of mitochondrial membrane potential, DNA double-strand breaks, or replication block. H2O2 did not cause ferroptosis at equitoxic concentrations (300 µM) and induced a (1) lower and (2) ferrostatin-1- or liproxstatin-1-insensitive increase in lipid peroxidation. We identify that t-BuOOH and H2O2 produce a different pattern of lipid peroxidation, thereby leading to different cell death pathways and present t-BuOOH as a novel inducer of ferroptosis.

The Brassica-derived phytochemical indolo[3,2-b]carbazole protects against oxidative DNA damage by aryl hydrocarbon receptor activation.

Epidemiological studies suggest that a high intake of Brassica vegetables protects against colon carcinogenesis. Brassica vegetables are rich in glucosinolates which are hydrolysed during digestion to various products including indole-3-carbinol. In animal studies, a protective effect of indole-3-carbinol has been demonstrated in colon carcinogenesis. Indole-3-carbinol is highly unstable and, therefore, the observed protection likely results from condensation products of indole-3-carbinol, e.g. diindolylmethane or indolo[3,2-b]carbazole (ICZ). Interestingly, ICZ is a potent activator of the aryl hydrocarbon receptor (AhR), a transcription factor known to mediate toxic effects of environmental pollutants, such as dioxin and polycyclic aromatic hydrocarbons. Here, we show that ICZ protects against oxidative DNA damage in various cell lines including the colon carcinoma cell line Caco-2. When preincubated for 24 h, ICZ decreases DNA single-strand break (SSB) and 8-oxo-dG formation induced by tertiary-butylhydroperoxide (t-BOOH), hydrogen peroxide or benzo[a]pyrene. Simultaneous addition of ICZ does not protect against t-BOOH-induced SSB formation, which disproves a direct radical scavenging effect. The repair of SSBs was not enhanced, but the data indicate that ICZ attenuates the ROS level following t-BOOH. The antioxidant response factor Nrf2 was not activated following ICZ. Functional inhibition of the AhR and AhR-/ARNT-defective cell lines demonstrate that the AhR/ARNT pathway is mandatory for the observed ROS defence caused by ICZ, supporting the hypothesis that AhR-mediated regulation of defence genes is involved. The data point to a hitherto unknown protective function of ICZ and a novel role of the AhR in the defence against oxidative DNA damage.

AhR-mediated changes in global gene expression in rat liver progenitor cells.

Although the tumor-promoting effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), coplanar polychlorinated biphenyls (PCBs), and related compounds in liver tissue are primarily attributed to the activation of the aryl hydrocarbon receptor (AhR), the underlying molecular mechanisms are still unclear. Liver progenitor (oval) cells have been suggested to constitute a potential target for hepatocarcinogenic chemicals. To better understand AhR-driven pathways, we analyzed the transcriptional program in response to coplanar PCB 126 in contact-inhibited rat liver progenitor WB-F344 cells using high-density microarrays. After 6-h treatment, we identified 145 significantly deregulated genes considered to be direct AhR-dependent target genes. The number of differentially regulated genes increased to 658 and 968 genes after 24 and 72 h, respectively. Gene ontology analysis revealed that these genes were primarily involved in drug and lipid metabolism, cell cycle and growth control, cancer developmental processes, cell-cell communication, and adhesion. Interestingly, the Wnt and TGF-ß signaling pathways, both being involved in developmental and tumorigenic processes, belonged to the most affected pathways. AhR- and ARNT-dependent regulation of selected target genes of interest was then confirmed using TCDD as a model AhR agonist, together with pharmacological inhibition of the AhR and by RNA-interference techniques. We demonstrated AhR-dependent regulation of emerging and novel AhR target genes, such as Fst, Areg, Hbegf, Ctgf, Btg2, and Foxq1. Among them, the transcription factor Foxq1, recently suggested to contribute to tumor promotion and/or progression, was found to be regulated at both mRNA and protein levels by AhR/ARNT activation.

Involvement of the transcription factor FoxM1 in contact inhibition.

Contact inhibition is a crucial mechanism regulating proliferation in vitro and in vivo. Although it is generally accepted that contact inhibition plays a pivotal role in maintaining tissue homeostasis, the molecular mechanisms of contact inhibition are still not fully understood. FoxM1 is known as a proliferation-associated transcription factor and is upregulated in many cancer types. Vice versa, anti-proliferative signals, such as TGF-ß and differentiation signals decrease FoxM1 expression. Here we investigated the role of FoxM1 in contact inhibition in fibroblasts. We show that protein expression of FoxM1 is severely and rapidly downregulated upon contact inhibition, probably by inhibition of ERK activity, which then leads to decreased expression of cyclin A and polo-like kinase 1. Vice versa, ectopic expression of FoxM1 prevents the decrease in cyclin A and polo-like kinase 1 and causes a two-fold increase in saturation density indicating loss of contact inhibition. Hence, we show that downregulation of FoxM1 is required for contact inhibition by regulating expression of cyclin A and polo-like kinase 1.

Differential p38-dependent signalling in response to cellular stress and mitogenic stimulation in fibroblasts.

p38 MAP kinase is known to be activated by cellular stress finally leading to cell cycle arrest or apoptosis. Furthermore, a tumour suppressor role of p38 MAPK has been proposed. In contrast, a requirement of p38 for proliferation has also been described. To clarify this paradox, we investigated stress- and mitogen-induced p38 signalling in the same cell type using fibroblasts. We demonstrate that - in the same cell line - p38 is activated by mitogens or cellular stress, but p38-dependent signalling is different. Exposure to cellular stress, such as anisomycin, leads to a strong and persistent p38 activation independent of GTPases. As a result, MK2 and downstream the transcription factor CREB are phosphorylated. In contrast, mitogenic stimulation results in a weaker and transient p38 activation, which upstream involves small GTPases and is required for cyclin D1 induction. Consequently, the retinoblastoma protein is phosphorylated and allows G1/S transition. Our data suggest a dual role of p38 and indicate that the level and/or duration of p38 activation determines the cellular response, i.e either proliferation or cell cycle arrest.

Regulation of ERK1/2 activity upon contact inhibition in fibroblasts.

Contact inhibition is a crucial mechanism regulating proliferation in vitro and in vivo. Despite its generally accepted importance for maintaining tissue homeostasis knowledge about the underlying molecular mechanisms of contact inhibition is still scarce. Since the MAPK ERK1/2 plays a pivotal role in the control of proliferation, we investigated regulation of ERK1/2 phosphorylation which is downregulated in confluent NIH3T3 cultures. We found a decrease in upstream signaling including phosphorylation of the growth factor receptor adaptor protein ShcA and the MAPK kinase MEK1/2 in confluent compared to exponentially growing cultures whereas involvement of ERK1/2 phosphatases in ERK1/2 inactivation is unlikely. Treatment of confluent, serum-deprived cultures with PDGF-B resulted in similar phosphorylation of ERK1/2 and induction of DNA-synthesis as detected in sparse, serum-deprived cultures. In contrast, ERK1/2 phosphorylation and DNA-synthesis could not be stimulated in confluent, serum-deprived cultures exposed to EGF. Our data indicate that PDGFR- and EGFR signaling are differentially inhibited in confluent cultures of NIH3T3 cells.

The transcriptional programme of contact-inhibition.

Proliferation of non-transformed cells is regulated by cell-cell contacts, which are referred to as contact-inhibition. Vice versa, transformed cells are characterised by a loss of contact-inhibition. Despite its generally accepted importance for cell-cycle control, little is known about the intracellular signalling pathways involved in contact-inhibition. Unravelling the molecular mechanisms of contact-inhibition and its loss during tumourigenesis will be an important step towards the identification of novel target genes in tumour diagnosis and treatment. To better understand the underlying molecular mechanisms we identified the transcriptional programme of contact-inhibition in NIH3T3 fibroblast using high-density microarrays. Setting the cut off: >or=1.5-fold, P or=2-fold, P

The aryl hydrocarbon receptor-dependent deregulation of cell cycle control induced by polycyclic aromatic hydrocarbons in rat liver epithelial cells.

Disruption of cell proliferation control by polycyclic aromatic hydrocarbons (PAHs) may contribute to their carcinogenicity. We investigated role of the aryl hydrocarbon receptor (AhR) in disruption of contact inhibition in rat liver epithelial WB-F344 'stem-like' cells, induced by the weakly mutagenic benz[a]anthracene (BaA), benzo[b]fluoranthene (BbF) and by the strongly mutagenic benzo[a]pyrene (BaP). There were significant differences between the effects of BaA and BbF, and those of the strongly genotoxic BaP. Both BaA and BbF increased percentage of cells entering S-phase and cell numbers, associated with an increased expression of Cyclin A and Cyclin A/cdk2 complex activity. Their effects were significantly reduced in cells expressing a dominant-negative AhR mutant (dnAhR). Roscovitine, a chemical inhibitor of cdk2, abolished the induction of cell proliferation by BbF. However, neither BaA nor BbF modulated expression of the principal cdk inhibitor involved in maintenance of contact inhibition, p27(Kip1), or pRb phosphorylation. The strongly mutagenic BaP induced apoptosis, a decrease in total cell numbers and significantly higher percentage of cells entering S-phase than either BaA or BbF. Given that BaP induced high levels of Cyclin A/cdk2 activity, downregulation of p27(Kip1) and hyperphosphorylation of pRb, the accumulation of cells in S-phase was probably due to cell proliferation, although S-phase arrest due to blocked replication forks can not be excluded. Both types of effects of BaP were significantly attenuated in dnAhR cells. Transfection of WB-F344 cells with siRNA targeted against AhR decreased induction of Cyclin A induced by BbF or BaP, further supporting the role of AhR in proliferative effects of PAHs. This suggest that activation of AhR plays a significant role both in disruption of contact inhibition by weakly mutagenic PAHs and in genotoxic effects of BaP possibly leading to enhanced cell proliferation. Thus, PAHs may increase proliferative rate and the likelihood of fixation of mutations.

p38alpha MAPK is required for contact inhibition.

Proliferation of nontransformed cells is regulated by cell-cell contacts, which are referred to as contact-inhibition. Despite its generally accepted importance for cell cycle control, knowledge about the intracellular signalling pathways involved in contact inhibition is scarce. In the present work we show that p38alpha mitogen-activated protein kinase (MAPK) is involved in the growth-inhibitory signalling cascade of contact inhibition in fibroblasts. p38alpha activity is increased in confluent cultures of human fibroblasts compared to proliferating cultures. Time course studies show a sustained activation of p38alpha in response to cell-cell contacts in contrast to a transient activation after serum stimulation. The induction of contact inhibition by addition of glutaraldehyde-fixed cells is impaired by pharmacological inhibition of p38 as well as in p38alpha-/- fibroblasts. Further evidence for a central role of p38alpha in contact inhibition comes from the observation that p38alpha-/- fibroblasts show a higher saturation density compared to wild-type (wt) fibroblasts, which is reversed by reconstituted expression of p38alpha. In agreement with a defect in contact inhibition, p27(Kip1) accumulation is impaired in p38alpha-/- fibroblasts compared to wt fibroblasts. Hence, our work shows a new role for p38alpha in contact inhibition and provides a mechanistic basis for the recently proposed tumour suppressive function of this MAPK pathway.

TCDD induces c-jun expression via a novel Ah (dioxin) receptor-mediated p38-MAPK-dependent pathway.

The aryl hydrocarbon receptor (AhR) has a fundamental role during postnatal liver development and is essential for mediating dioxin toxicity. However, the genetic programs mediating, both, the toxic and physiological effects downstream of the transcription factor AhR are in major parts unknown. We have identified the proto-oncogene c-jun as a novel target gene of AhR. Induction of c-jun depends on activation of p38-mitogen-activated protein kinase (MAPK) by an AhR-dependent mechanism. None of the kinases that are known to phosphorylate p38-MAPK is activated by AhR. Neither the dephosphorylation rate of p38-MAPK is reduced. Furthermore, increased p38-MAPK phosphorylation in response to dioxins does not require ongoing transcription. These findings establish activating 'cross-talk' with MAPK signaling as a novel principle of AhR action, which is apparently independent of the AhR's function as a DNA-binding transcriptional activator.

Aryl hydrocarbon receptor-dependent cell cycle arrest in isolated mouse oval cells.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor, which mediates toxic responses to environmental pollutants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Besides its well known role in induction of xenobiotic metabolizing enzymes, for instance CYP1A1, the AhR is also involved in tumor promotion in rodents although the underlying mechanisms are still poorly understood. Additionally, the AhR is known to regulate cellular proliferation, which might result in either inhibition or stimulation of proliferation depending on the cell-type studied. Potential targets in hepatocarcinogenesis are liver oval (stem/progenitor) cells. In the present work we analyzed the effect of TCDD on proliferation in oval cells derived from mouse liver. We show that TCDD inhibits proliferation in these cells. In line, the amount of G0/G1 cells increases in response to TCDD. We further show that the expression of cyclin D1 and cyclin A is decreased, while p27 is increased. As a result, the retinoblastoma protein is not phosphorylated thereby inducing G0/G1 arrest. Pharmacological inhibition of the AhR and knock-down of AhR expression by RNA interference decreased the inhibitory effect on cell cycle and protein expression, indicating that the AhR at least partially mediates cell cycle arrest.

Evaluation of the role of c-Src and ERK in TCDD-dependent release from contact-inhibition in WB-F344 cells.

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) is the most potent tumor promoter ever tested in rodents. Although it is known that most of the effects of TCDD are mediated by binding to the aryl hydrocarbon receptor (AhR), the mechanisms leading to tumor promotion remain to be elucidated. Loss of contact-inhibition is one characteristic hallmark in tumorigenesis. In WB-F344 cells, TCDD induces a release from contact-inhibition which is manifested by a twofold increase in DNA-synthesis and cell number when TCDD (1 nmol L-1) is given to confluent cells. Because TCDD leads to phosphorylation of the epidermal growth factor receptor and an increase in c-Src-activation in WB-F344 cells, we investigated the functional relevance of this observation. Pharmacological inhibition of c-Src using PP1 (10 micromol L-1) or genistein (10 micromol L-1) did not prevent TCDD-dependent release from contact-inhibition. In accordance, elevation of cyclin A-a previously identified target of TCDD and marker of S-phase entry-was not reduced in the presence of PP1 or genistein. Western blot analysis revealed that phosphorylation of the EGF-receptor downstream target ERK was not induced in response to TCDD. Furthermore, TCDD-dependent increase in DNA-synthesis was not inhibited by the MEK1/2 inhibitor U0126 (10 micromol L-1). Our data show that neither c-Src-activation, nor ERK-activation are required for TCDD-dependent release from contact-inhibition arguing against a functional role of EGF-receptor activation in response to TCDD in WB-F344 cells.

Transforming growth factor-beta1 is not involved in TCDD-dependent release from contact inhibition in WB-F344 cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most potent tumor promoter ever tested in rodents. Although it is known that most of the effects of TCDD are mediated by binding to the aryl hydrocarbon receptor (AhR), the mechanisms leading to tumor promotion still remain to be elucidated. Loss of contact inhibition is one characteristic hallmark in tumorigenesis. In WB-F344 cells TCDD induces a release from contact inhibition which is manifested by a two- to threefold increase in DNA synthesis when TCDD (1 nM) is applied to confluent cells. Since proliferation of epithelial cells is known to be inhibited by TGF-beta, we investigated whether decreased TGF-beta1 mediates TCDD-dependent release from contact inhibition in WB-F344 cells. Expression of TGF-beta (type II) receptor in WB-F344 cells was analyzed by Western blot analysis. Exposure of 0.1 ng/ml TGF-beta1 to exponentially growing WB-F344 cells resulted in a 40% decrease in DNA synthesis, which was blocked by preincubation with a neutralizing anti-TGF-beta1 antibody, indicating that the TGF-beta receptor in WB-F344 cells is functionally active. Preincubation of confluent, G1-arrested cultures with the neutralizing anti-TGF-beta1-antibody did not lead to an increase in DNA synthesis, ruling out an involvement of TGF-beta1 in mediating contact inhibition in WB-F344 cells. In accord with this, Western blot analysis revealed that protein expression of TGF-beta1 is neither upregulated in confluent cultures nor decreased after TCDD treatment. We conclude that TGF-beta1 is not involved in contact inhibition or in TCDD-dependent release from contact inhibition in WB-F344 cells.

Subcellular localization of beta-catenin is regulated by cell density.

It is generally accepted that subcellular distribution of beta-catenin regulates its function. Membrane-bound beta-catenin mediates cell-cell adhesion, whereas elevation of the cytoplasmic and nuclear pool of the protein is associated with an oncogenic function. Although the role of beta-catenin in transformed cells is relatively well characterized, little is known about its importance in proliferation and cell-cycle control of nontransformed epithelial cells. Using different approaches we show that in human keratinocytes (HaCaT) beta-catenin is distributed throughout the cells in subconfluent, proliferating cultures. In contrast, beta-catenin is nearly exclusively located at the plasma membrane in confluent, contact-inhibited cells. Hence, we demonstrate for the first time that beta-catenin is translocated from the cytoplasm to the plasma membrane in response to high cell density. We conclude that beta-catenin plays an important role in proliferation and mediating contact-inhibition by changing intracellular localization.

Transforming growth factor beta1 is not involved in 2,3,7,8-tetrachlorodibenzo- p-dioxin-dependent release from contact-inhibition in WB-F344 cells.

TCDD (2,3,7,8-tetrachlorodibenzo- p-dioxin) is the most potent tumor promoter ever tested in rodents. Although it is known that most of the effects of TCDD are mediated by binding to the aryl hydrocarbon receptor (AhR), the mechanisms leading to tumor promotion still remain to be elucidated. Loss of contact-inhibition is one characteristic hallmark in tumorigenesis. In WB-F344 cells, TCDD induces a release from contact-inhibition, which is manifested by a two- to three-fold increase in DNA-synthesis when TCDD (1 nM) is given to confluent cells. Since proliferation of epithelial cells is known to be inhibited by transforming growth factor beta (TGF-beta) we investigated whether decreased TGF-beta expression mediates TCDD-dependent release from contact-inhibition in WB-F344 cells. Expression of TGF-beta (type II) receptor in WB-F344 cells was shown by Western blot analysis. Exposure of exponentially growing WB-F344 cells to 0.1 ng/ml TGF-beta1 resulted in a 40% decrease in DNA synthesis, which could be blocked by pre-incubation with a neutralizing anti-TGF-beta1 antibody indicating that the TGF-beta receptor in WB-F344 cells is functionally active. Pre-incubation of confluent, G1-arrested cultures with the neutralizing anti-TGF-beta1 antibody did not lead to an increase in DNA synthesis, ruling out an involvement of TGF-beta1 in mediating contact-inhibition in WB-F344 cells. In accordance with this, Western blot analysis revealed that protein expression of TGF-beta1 was neither upregulated in confluent cultures nor decreased after TCDD treatment. We therefore conclude that TGF-beta1 is not involved in contact-inhibition nor in TCDD-dependent release from contact-inhibition in WB-F344 cells.

TCDD-dependent downregulation of gamma-catenin in rat liver epithelial cells (WB-F344).

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) is the most potent tumor promoter ever tested in rodents. Although it is known that most of the effects of TCDD are mediated by binding to the aryl hydrocarbon receptor (AHR), the mechanisms leading to tumor promotion still remain to be elucidated. Loss of contact-inhibition is a characteristic hallmark in tumorigenesis. In WB-F344 cells, TCDD induces a release from contact-inhibition manifested by a 2- to 3-fold increase in DNA-synthesis and the emergence of foci when TCDD (1 nM) is given to confluent cells. We focussed our interest on potential cell membrane proteins mediating contact-inhibition in WB-F344 cells, namely E-cadherin, alpha,- beta,- and gamma-catenin (plakoglobin). Using indirect immunofluorescence, E-cadherin, alpha-, beta- and gamma-catenin were detected at cell adhesion sites in untreated, confluent cells. After TCDD-exposure, gamma-catenin was exclusively localized in the cytoplasm whereas localization of E-cadherin, alpha- and beta-catenin remained unaffected. Cytoplasmic gamma-catenin could be extracted by Triton X-100 treatment, demonstrating that gamma-catenin was no longer bound to the actin cytoskeleton. Western blot analysis showed downregulation of gamma-catenin protein levels. This effect was not blocked by pre-incubation with the selective proteasome inhibitor MG-132, indicating that proteolytical degradation of gamma-catenin by the proteasome system was not increased by TCDD. Because mRNA-levels of gamma-catenin were markedly diminished after TCDD-exposure, we conclude that transcriptional downregulation or destabilization of the mRNA contributes to the decrease in gamma-catenin protein levels in response to TCDD. Because gamma-catenin is considered to be a tumor suppressor, our findings might give more insight into the tumor promoting actions of TCDD.

Occupational exposure to heavy metals: DNA damage induction and DNA repair inhibition prove co-exposures to cadmium, cobalt and lead as more dangerous than hitherto expected.

Co-exposure to cadmium, cobalt, lead and other heavy metals occurs in many occupational settings, such as pigment and batteries production, galvanization and recycling of electric tools. However, little is known about interactions between several heavy metals. In the present study we determined DNA single strand break (DNA-SSB) induction and repair capacity for 8-oxoguanine in mononuclear blood cells of 78 individuals co-exposed to cadmium (range of concentrations in air: 0.05-138.00 micro g/m(3)), cobalt (range: 0-10 micro g/m(3)) and lead (range: 0-125 micro g/m(3)). Exposure to heavy metals was determined in air, blood and urine. Non-parametric correlation analysis showed a correlation between cadmium concentrations in air with DNA-SSB (P = 0.001, R = 0.371). Surprisingly, cobalt air concentrations correlated even better (P < 0.001, R = 0.401), whereas lead did not correlate with DNA-SSB. Logistic regression analysis including 11 possible parameters of influence resulted in a model showing that cobalt in air, cadmium in air, cadmium in blood and lead in blood influence the level of DNA-SSB. The positive result with cobalt was surprising, since exposure levels were much lower compared with the TRK-value of 100 micro g/m(3). To examine, whether the positive result with cobalt is stable, we applied several logistic regression models with two blocks, where all factors except cobalt were considered preferentially. All strategies resulted in the model described above. Logistic regression analysis considering also all possible interactions between the relevant parameters of influence finally resulted in the following model: Odds ratio = 1.286(Co in air) x 1.040(Cd in air) x 3.111(Cd in blood) x 0.861(Pb in air) x 1.023(Co in air x Pb in air). This model correctly predicts an increased level of DNA-SSB in 91% of the subjects in our study. One conclusion from this model is the existence of more than multiplicative effects for co-exposures of cadmium, cobalt and lead. For instance increasing lead air concentrations from 1.6 to 50 micro g/m(3) in the presence of constant exposures to cobalt and cadmium (8 micro g/m(3) and 3.8 micro g/m(3)) leads to an almost 5-fold increase in the odds ratio, although lead alone does not increase DNA-SSB. The mechanism behind these interactions might be repair inhibition of oxidative DNA damage, since a decrease in repair capacity will increase susceptibility to reactive oxygen species generated by cadmium or cobalt. Indeed, repair of 8-oxoguanine decreased with increasing exposures and inversely correlated with the level of DNA-SSB (P = 0.001, R = -0.427). Protein expression patterns of individuals exposed to cobalt concentrations of approximately 10 micro g/m(3) were compared with those of unexposed individuals using two-dimensional gel electrophoresis. Qualitative and apparent quantitative alterations in protein expression were selective and certainly occurred in <0.1% of all proteins. In conclusion, the hazard due to cobalt exposure - that has been classified only as IIB by the IARC - seems to be underestimated, especially when individuals are co-exposed to cadmium or lead. Co-exposure may cause genotoxic effects, even if the concentrations of individual heavy metals do not exceed TRK-values.