Cadmium decreases gap junctional intercellular communication in mouse liver

TFAM-mediated intercellular lipid droplet transfer promotes cadmium-induced mice nonalcoholic fatty liver disease

Cadmium (Cd) is an important environmental pollutant. Herein, we discovered a new way of lipid accumulation, where lipid droplets can be transferred across cells. In this study, mice and AML12 cells were used to establish models of Cd poisoning. After Cd treatment, the level of TFAM was reduced, thereby regulating the reconstitution of the cytosolic actin filament network. MYH9 is a myosin involved in cell polarization, migration, and movement of helper organelles. Rab18 is a member of the Rab GTPase family, which localizes to lipid droplets and regulates lipid drop dynamics. In this study, we found that Cd increases the interaction between MYH9 and Rab18. However, TFAM overexpression alleviated the increase in Cd-induced interaction between MYH9 and Rab18, thereby reducing the transfer of intercellular lipid droplets and the accumulation of intracellular lipids. Through a co-culture system, we found that the transferred lipid droplets can act as a signal to form an inflammatory storm-like effect, and ACSL4 can act as an effector to transfer lipid droplets and promote lipid accumulation in surrounding cells. These results suggest that TFAM can be used as a new therapeutic target for Cd-induced lipid accumulation in the liver.

Gap junction intercellular communication mediates cadmium-induced apoptosis in hepatocytes via the Fas/FasL pathway

As a common environmental pollutant, cadmium (Cd) causes damage to many organs of the body. Gap junction intercellular communication (GJIC) represents one of the most important routes of rapid signaling between cells. However, the mechanisms underlying GJIC's role in hepatotoxicity induced by Cd remain unknown. We established a Cd poisoning model in vitro by co-culturing Cd-exposed and unexposed hepatocytes and found that 18β-glycyrrhetinic acid (GA), a GJIC inhibitor, can effectively reduce the apoptosis rate of healthy cells co-cultured with apoptotic cells treated with Cd. We also found that anti-FasL antibody had the same effect. However, in mono-cultured cells, GA treatment in combination with Cd was found to aggravate the damage induced by Cd exposure, increase the level of oxidative stress and protein expression of HO-1, decrease the mitochondrial membrane potential, incur more serious morphological damage to mitochondria than Cd treatment alone. Moreover, compared with Cd-only exposure, GA and Cd co-treatment further increased the expression levels of the apoptosis-related proteins Fas, FasL, FADD and the ratio of Bax/Bcl-2, inhibited the protein expression of ASK1 and Daxx. We also found that the protein expression of Daxx in siFADD + Cd hepatocytes was significantly higher than in Cd-treated cells. Thus, our study suggests that gap junction inhibition may play a dual role in Cd-induced cell damage by inhibiting the transmission of death signals from damaged cells to healthy cells but also aggravating the transmission of death signals between damaged cells, and that the Fas/FasL-mediated death receptor pathway may play an important role in this process.

Connexin-Based Channels in the Liver

Connexin proteins oligomerize in hexameric structures called connexin hemichannels, which then dock to form gap junctions. Gap junctions direct cell-cell communication by allowing the exchange of small molecules and ions between neighboring cells. In this way, hepatic gap junctions support liver homeostasis. Besides serving as building blocks for gap junctions, connexin hemichannels provide a pathway between the intracellular and the extracellular environment. The activation of connexin hemichannels is associated with acute and chronic liver pathologies. This article discusses the role of gap junctions and connexin hemichannels in the liver. © 2022 American Physiological Society. Compr Physiol 12:1-17, 2022.

Cadmium-mediated pancreatic islet transcriptome changes in mice and cultured mouse islets

Type II diabetes mellitus (T2DM) is a multifactorial disease process that is characterized by insulin resistance and impairment of insulin-producing pancreatic islets. There is evidence that environmental exposure to cadmium contributes to the development of T2DM. The presence of cadmium in human islets from the general population and the uptake of cadmium in β-cells have been reported. To identify cadmium-mediated changes in gene expression and molecular regulatory networks in pancreatic islets, we performed next-generation RNA-Sequencing (RNA-Seq) in islets following either in vivo (1 mM CdCl2 in drinking water) or ex-vivo (0.5 μM CdCl2) exposure. Both exposure regiments resulted in islet cadmium concentrations that are comparable to those found in human islets from the general population. 6-week in vivo cadmium exposure upregulates the expression of five genes: Synj2, Gjb1, Rbpjl, Try5 and 5430419D17Rik. Rbpjl is a known regulator of ctrb, a gene associated with diabetes susceptibility. With 18-week in vivo cadmium exposure, we found more comprehensive changes in gene expression profile. Pathway enrichment analysis showed that these secondary changes were clustered to molecular mechanisms related to intracellular protein trafficking to the plasma membrane. In islet culture, cadmium ex vivo significantly induces the expression of Mt1, Sphk1, Nrcam, L3mbtl2, Rnf216 and Itpr1. Mt1 and Itpr1 are known to be involved in glucose homeostasis. Collectively, findings reported here revealed a complex cadmium-mediated effect on pancreatic islet gene expression at environmentally relevant cadmium exposure conditions, providing the basis for further studies into the pathophysiological processes arising from cadmium accumulation in pancreatic islets.

Cadmium exposure modulates the gut-liver axis in an Alzheimer's disease mouse model

The human Apolipoprotein E4 (ApoE4) variant is the strongest known genetic risk factor for Alzheimer's disease (AD). Cadmium (Cd) has been shown to impair learning and memory at a greater extent in humanized ApoE4 knock-in (ApoE4-KI) mice as compared to ApoE3 (common allele)-KI mice. Here, we determined how cadmium interacts with ApoE4 gene variants to modify the gut-liver axis. Large intestinal content bacterial 16S rDNA sequencing, serum lipid metabolomics, and hepatic transcriptomics were analyzed in ApoE3- and ApoE4-KI mice orally exposed to vehicle, a low dose, or a high dose of Cd in drinking water. ApoE4-KI males had the most prominent changes in their gut microbiota, as well as a predicted down-regulation of many essential microbial pathways involved in nutrient and energy homeostasis. In the host liver, cadmium-exposed ApoE4-KI males had the most differentially regulated pathways; specifically, there was enrichment in several pathways involved in platelet activation and drug metabolism. In conclusion, Cd exposure profoundly modified the gut-liver axis in the most susceptible mouse strain to neurological damage namely the ApoE4-KI males, evidenced by an increase in microbial AD biomarkers, reduction in energy supply-related pathways in gut and blood, and an increase in hepatic pathways involved in inflammation and xenobiotic biotransformation.

Gap Junction Intercellular Communication Negatively Regulates Cadmium-Induced Autophagy and Inhibition of Autophagic Flux in Buffalo Rat Liver 3A Cells

Cadmium is an important environmental pollutant that poses a serious threat to the health of humans and animals. A large number of studies have shown that the liver is one of the important target organs of cadmium. Stimulation of cells can lead to rapid changes in gap junction intercellular communication (GJIC) and autophagy. Previous studies have shown that cadmium can inhibit GJIC and induce autophagy. In order to understand the dynamic changes of GJIC and autophagy in the process of cadmium-induced hepatotoxic injury and the effects of GJIC on autophagy, a time-gradient model of cadmium cytotoxicity was established. The results showed that within 24 h of cadmium exposure, 5 μmol/L cadmium inhibited GJIC by down regulating the expression levels of connexin 43 (Cx43) and disturbing the localization of Cx43 in Buffalo rat liver 3A (BRL 3A) cells. In addition, cadmium induced autophagy and then inhibited autophagic flux in the later stage. During this process, inhibiting of GJIC could exacerbate the cytotoxic damage of cadmium and induce autophagy, but further blocked autophagic flux, promoting GJIC in order to obtain the opposite results.

Protective Effect of Combined Long Time Administration of Selenium and Vitamin C on Liver and Kidney Toxicity of Cadmium in Rats

Background & Objective:

Increased industrial activities leads to prolonged human exposure to industrial pollutant such as cadmium (Cd). ‍Chronic exposure to Cd in Mammals and also human being, can cause damages to various organs and particularly kidneys and liver. The goal of this study was to investigate the prophylactic effects of combined selenium (Se) and ascorbic acid supplement in rat cadmium toxicity.

Methods:

Sixty adult male Wistar rats were divided to 10 groups: one control, one sham and two clusters of 4 intervention groups which were fed with 1 or 5 mg Cd /kg water, for 28 days. Ascorbic acid supplement was added to drinking water of four groups (10 mg/L). Four groups received intraperitoneal Se (1 mg/kg) at day 1, 5, 10, 15, 20 and 25. Finally, Cd concentration was measured by atomic absorption spectrophotometry in liver and kidney sections. Furthermore, pathological changes were investigated in these sections.

Results:

The results showed weight gain in Cd groups which received ascorbic acid and Se, in contrast to weight loss in parallel groups without vitamin C and Se. The stronger necrosis and inflammation have been observed in group received 5 mg/kg Cd compared to group with 1 mg/kg Cd (P<0.05). In addition, cadmium level was higher in untreated groups without any supplements, significantly (P<0.05).

Conclusion:

Drinking water with ascorbic acid may have prophylactic effects across cadmium, and combination of Se and ascorbic acid are associated with higher prophylactic effects in both kidney and liver in rats to decrease the Cd toxicity.

Targeting gap junctional intercellular communication by hepatocarcinogenic compounds

Gap junctions in liver, as in other organs, play a critical role in tissue homeostasis. Inherently, these cellular constituents are major targets for systemic toxicity and diseases, including cancer. This review provides an overview of chemicals that compromise liver gap junctions, in particular biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. The focus in this review is placed upon the mechanistic scenarios that underlie these adverse effects. Further, the potential use of gap junctional activity as an in vitro biomarker to identify non-genotoxic hepatocarcinogenic chemicals is discussed.

Association of liver and kidney functions with Klotho gene methylation in a population environment exposed to cadmium in China

Exposure to the heavy metal cadmium has adverse effects on human health, including DNA methylation. This study aimed to investigate the effects of cadmium on liver and kidney functions and Klotho gene methylation and to explore the relationship of methylation level with indicators of liver and kidney functions. Graphite furnace atomic absorption spectrometry was conducted to determine urinary cadmium, and an automatic biochemical analyzer was used to detect indices of liver and kidney functions. PCR pyrosequencing was performed to detect the methylation rate of Klotho. One-way ANOVA was adopted to compare the differences between groups, and the linear correlation to variables was analyzed. Cadmium exposure was negatively correlated with albumin level (r=-0.143, p=0.021) and positively correlated with urinary β₂-microglobulin level (r=0.229, p<0.001). However, the methylation levels of Klotho gene was decreased and increased by low and high doses of cadmium exposure, respectively. And Klothomethylation levels were negatively correlated with albumin levels and positively correlated with β₂-microglobulin levels.In this study, cadmium exposure affects liver and kidney functions as well as Klotho methylation levels, but the effect on Klotho methylation levels is not linear. Klotho methylation levels also influence liver and kidney functions.

Pigmented macrophages and related aggregates in the spleen of european sea bass dosed with heavy metals: Ultrastructure and explorative morphometric analysis

The ultrastructure and morphometrics of pigmented macrophages (PMs) were assessed in the spleen of European sea bass experimentally dosed with Cd and Hg. PMs occurred either as solitary cells or as variably structured aggregations, defined as macrophage aggregates (MAs). Light microscopy revealed a high degree of morphological heterogeneity amongst MAs of all experimental groups. At the ultrastructural level, MAs showed a heterogeneous pigment content that was not influenced by the treatment. Cytoplasm rarefaction/vacuolation and euchromatic nuclei, were observed in PMs of dosed fish. Undosed and Cd-dosed samples differ significantly with regard to the following morphometric features: the Minor axis of the best fitting ellipse, Aspect Ratio, and Roundness. In Cd-dosed fish, MAs showed reduced size and complexity. Lacunarity showed significant differences between undosed and both Cd and Hg-dosed samples. These results suggest that heavy metals, and especially Cd, may influence the dynamics of PM aggregation/disaggregation. Variability in splenic MAs was observed both by light and electron microscopy. However, only the morphometric techniques adequately and objectively described the phenomenon, allowing a quantitative/statistical comparison of morphology among experimental groups. These morphometric analyses could be usefully applied in toxicological and ecotoxicological, as well as morpho-functional studies.

Connexin-based signaling and drug-induced hepatotoxicity

Being critical mediators of liver homeostasis, connexins and their channels are frequently involved in liver toxicity. In the current paper, specific attention is paid to actions of hepatotoxic drugs on these communicative structures. In a first part, an overview is provided on the structural, regulatory and functional properties of connexin-based channels in the liver. In the second part, documented effects of acetaminophen, hypolipidemic drugs, phenobarbital and methapyriline on connexin signaling are discussed. Furthermore, the relevance of this subject for the fields of clinical and in vitro toxicology is demonstrated.

Relevance for patients: The role of connexin signaling in drug-induced hepatotoxicity may be of high clinical relevance, as it offers perspectives for the therapeutic treatment of such insults by interfering with connexin channel opening.

Prediction of Nephrotoxicant Action and Identification of Candidate Toxicity-Related Biomarkers

A vast majority of pharmacological compounds and their metabolites are excreted via the urine, and within the complex structure of the kidney, the proximal tubules are a main target site of nephrotoxic compounds. We used the model nephrotoxicants mercuric chloride, 2-bromoethylamine hydrobromide, hexachlorobutadiene, mitomycin, amphotericin, and puromycin to elucidate time- and dose-dependent global gene expression changes associated with proximal tubular toxicity. Male Sprague–Dawley rats were dosed via intraperitoneal injection once daily for mercuric chloride and amphotericin (up to 7 doses), while a single dose was given for all other compounds. Animals were exposed to 2 different doses of these compounds and kidney tissues were collected on day 1, 3, and 7 postdosing. Gene expression profiles were generated from kidney RNA using 17K rat cDNA dual dye microarray and analyzed in conjunction with histopathology. Analysis of gene expression profiles showed that the profiles clustered based on similarities in the severity and type of pathology of individual animals. Further, the expression changes were indicative of tubular toxicity showing hallmarks of tubular degeneration/regeneration and necrosis. Use of gene expression data in predicting the type of nephrotoxicity was then tested with a support vector machine (SVM)-based approach. A SVM prediction module was trained using 120 profiles of total profiles divided into four classes based on the severity of pathology and clustering. Although mitomycin C and amphotericin B treatments did not cause toxicity, their expression profiles were included in the SVM prediction module to increase the sample size. Using this classifier, the SVM predicted the type of pathology of 28 test profiles with 100% selectivity and 82% sensitivity. These data indicate that valid predictions could be made based on gene expression changes from a small set of expression profiles. A set of potential biomarkers showing a time- and dose-response with respect to the progression of proximal tubular toxicity were identified. These include several transporters ( Slc21a2, Slc15, Slc34a2), Kim 1, IGFbp-1, osteopontin, α -fibrinogen, and Gstα.

Gap junction blockage promotes cadmium-induced apoptosis in BRL 3A derived from Buffalo rat liver cells

Gap junctions mediate direct communication between cells; however, toxicological cascade triggered by nonessential metals can abrogate cellular signaling mediated by gap junctions. Although cadmium (Cd) is known to induce apoptosis in organs and tissues, the mechanisms that underlie gap junction activity in Cd-induced apoptosis in BRL 3A rat liver cells has yet to be established. In this study, we showed that Cd treatment decreased the cell index (a measure of cellular electrical impedance) in BRL 3A cells. Mechanistically, we found that Cd exposure decreased expression of connexin 43 (Cx43), increased expression of p-Cx43 and elevated intracellular free Ca²⁺ concentration, corresponding to a decrease in gap junctional intercellular communication. Gap junction blockage pretreatment with 18β-glycyrrhizic acid (GA) promoted Cd-induced apoptosis, involving changes in expression of Bax, Bcl-2, caspase-3 and the mitochondrial transmembrane electrical potential (Δψm). Additionally, GA was found to enhance ERK and p38 activation during Cd-induced activation of mitogen-activated protein kinases, but had no significant effect on JNK activation. Our results indicated the apoptosis-related proteins and the ERK and p38 signaling pathways may participate in gap junction blockage promoting Cd-induced apoptosis in BRL 3A cells.

Mechanisms of environmental chemicals that enable the cancer hallmark of evasion of growth suppression

As part of the Halifax Project, this review brings attention to the potential effects of environmental chemicals on important molecular and cellular regulators of the cancer hallmark of evading growth suppression. Specifically, we review the mechanisms by which cancer cells escape the growth-inhibitory signals of p53, retinoblastoma protein, transforming growth factor-beta, gap junctions and contact inhibition. We discuss the effects of selected environmental chemicals on these mechanisms of growth inhibition and cross-reference the effects of these chemicals in other classical cancer hallmarks.

Salidroside Protects against Cadmium-Induced Hepatotoxicity in Rats via GJIC and MAPK Pathways

It is known that cadmium (Cd) induces cytotoxicity in hepatocytes; however, the underlying mechanism is unclear. Here, we studied the molecular mechanisms of Cd-induced hepatotoxicity in rat liver cells (BRL 3A) and in vivo. We observed that Cd treatment was associated with a time- and concentration-dependent decrease in the cell index (CI) of BRL 3A cells and cellular organelle ultrastructure injury in the rat liver. Meanwhile, Cd treatment resulted in the inhibition of gap junction intercellular communication (GJIC) and activation of mitogen-activated protein kinase (MAPK) pathways. Gap junction blocker 18-β-glycyrrhetinic acid (GA), administered in combination with Cd, exacerbated cytotoxic injury in BRL 3A cells; however, GA had a protective effect on healthy cells co-cultured with Cd-exposed cells in a co-culture system. Cd-induced cytotoxic injury could be attenuated by co-treatment with an extracellular signal-regulated kinase (ERK) inhibitor (U0126) and a p38 inhibitor (SB202190) but was not affected by co-treatment with a c-Jun N-terminal kinase (JNK) inhibitor (SP600125). These results indicate that ERK and p38 play critical roles in Cd-induced hepatotoxicity and mediate the function of gap junctions. Moreover, MAPKs induce changes in GJIC by controlling connexin gene expression, while GJIC has little effect on the Cd-induced activation of MAPK pathways. Collectively, our study has identified a possible mechanistic pathway of Cd-induced hepatotoxicity in vitro and in vivo, and identified the participation of GJIC and MAPK-mediated pathways in Cd-induced hepatotoxicity. Furthermore, we have shown that salidroside may be a functional chemopreventative agent that ameliorates the negative effects of Cd via GJIC and MAPK pathways.

Autophagy and gap junctional intercellular communication inhibition are involved in cadmium-induced apoptosis in rat liver cells

Cadmium (Cd) is known to induce hepatotoxicity, yet the underlying mechanism of how this occurs is not fully understood. In this study, Cd-induced apoptosis was demonstrated in rat liver cells (BRL 3A) with apoptotic nuclear morphological changes and a decrease in cell index (CI) in a time- and concentration-dependent manner. The role of gap junctional intercellular communication (GJIC) and autophagy in Cd-induced apoptosis was investigated. Cd significantly induced GJIC inhibition as well as downregulation of connexin 43 (Cx43). The prototypical gap junction blocker carbenoxolone disodium (CBX) exacerbated the Cd-induced decrease in CI. Cd treatment was also found to cause autophagy, with an increase in mRNA expression of autophagy-related genes Atg-5, Atg-7, Beclin-1, and microtubule-associated protein light chain 3 (LC3) conversion from cytosolic LC3-I to membrane-bound LC3-II. The autophagic inducer rapamycin (RAP) prevented the Cd-induced CI decrease, while the autophagic inhibitor chloroquine (CQ) caused a further reduction in CI. In addition, CBX promoted Cd-induced autophagy, as well as changes in expression of Atg-5, Atg-7, Beclin-1 and LC3. CQ was found to block the Cd-induced decrease in Cx43 and GJIC inhibition, whereas RAP had opposite effect. These results demonstrate that autophagy plays a protective role during Cd-induced apoptosis in BRL 3A cells during 6 h of experiment, while autophagy exacerbates Cd-induced GJIC inhibition which has a negative effect on cellular fate.

Salidroside ameliorates Cd-induced calcium overload and gap junction dysfunction in BRL 3A rat liver cells

It is known that cadmium (Cd) induces cytotoxicity via Ca(2+) signaling, although the underlying mechanism is unclear. Here, we studied the molecular mechanisms of Cd-induced cytotoxicity in BRL 3A cells, a rat liver cell line. We observed that Cd treatment was associated with a time-dependent decrease in cell index (CI) in BRL 3A cells. Mechanistically, we observed that Cd exposure was associated with decreased expression of Cx43, P-Cx43, and Cx32. Specifically, Cx43 was decreased at the site of cell-cell junctions at the cell membrane, corresponding to a decrease in gap junctional intercellular connections (GJICs). We also found that Cd triggered a rise in the intracellular free Ca(2+) concentration ([Ca(2+)]i), and the intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis, acetoxymethyl ester (BAPTA-AM), prevented the Cd-induced decrease in CI. On the other hand, the gap junction blocker 18-β-glycyrrhetinic acid (GA) and the endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin exacerbated cytotoxic injury induced by Cd via further elevating [Ca(2+)]i, The extracellular calcium chelator ethylene glycol tetraacetic acid could partly attenuate Cd-induced calcium elevation but had little effect on GA combined Cd. Furthermore, salidroside as a protective agent prevented Cd-induced GJIC inhibition and calcium overload. Our findings suggest that Cd triggers elevation of [Ca(2+)]i via mainly stimulating Ca(2+) release from intracellular Ca(2+) storage organelles and inhibiting GJIC, causing cytotoxic injury, and salidroside could be used to prevent Cd-induced cytotoxicity.

Inorganic arsenic trioxide induces gap junction loss in association with the downregulation of connexin43 and E-cadherin in rat hepatic "stem-like" cells

Chronic exposure to inorganic arsenic trioxide causes tumors of the skin, urinary bladder, lung, and liver. Several cancer initiators and promoters have been shown to alter cell-cell signaling by interference with gap junction intercellular communication (GJIC) and/or modulation of cell adhesion molecules, such as connexin43 (Cx43), E-cadherin, and β-catenin. The aim of this study was to determine whether the disruption of cell-cell interactions occurs in liver epithelial cells after exposure to arsenic trioxide. WB-F344 cells were treated with arsenic trioxide (6.25-50 μM) for up to 8 hours, and gap junction function was analyzed using the scrape-load/dye transfer assay. In addition, the changes in mRNA and protein levels of Cx43, E-cadherin, and β-catenin were determined. A significant dose- and time-dependent decrease in GJIC was observed when WB-F344 cells were exposed to arsenic trioxide (p < 0.05). Consistent with the inhibition of GJIC, cells' exposure to arsenic trioxide resulted in dose- and time-dependent decreases in Cx43 and E-cadherin mRNA expression and protein levels. However, arsenic trioxide did not alter the mRNA or protein levels of β-catenin. In an immunofluorescence study, nuclei were heavily stained with anti-β-catenin antibody, indicating significant nuclear translocation. In this study, we also demonstrated that arsenic trioxide-induced GJIC loss was a reversible process. Taken together, these data support the hypothesis that disruption of cell-cell communication may contribute to the tumor-promoting effect of inorganic arsenic trioxide.

17-β-Estradiol counteracts the effects of high frequency electromagnetic fields on trophoblastic connexins and integrins

We investigated the effect of high-frequency electromagnetic fields (HF-EMFs) and 17-β-estradiol on connexins (Cxs), integrins (Ints), and estrogen receptor (ER) expression, as well as on ultrastructure of trophoblast-derived HTR-8/SVneo cells. HF-EMF, 17-β-estradiol, and their combination induced an increase of Cx40 and Cx43 mRNA expression. HF-EMF decreased Int alpha1 and β1 mRNA levels but enhanced Int alpha5 mRNA expression. All the Ints mRNA expressions were increased by 17-β-estradiol and exposure to both stimuli. ER-β mRNA was reduced by HF-EMF but augmented by 17-β-estradiol alone or with HF-EMF. ER-β immunofluorescence showed a cytoplasmic localization in sham and HF-EMF exposed cells which became nuclear after treatment with hormone or both stimuli. Electron microscopy evidenced a loss of cellular contact in exposed cells which appeared counteracted by 17-β-estradiol. We demonstrate that 17-β-estradiol modulates Cxs and Ints as well as ER-β expression induced by HF-EMF, suggesting an influence of both stimuli on trophoblast differentiation and migration.

The unfolded protein response triggered by environmental factors

Endoplasmic reticulum (ER) stress and consequent unfolded protein response (UPR) are involved in a diverse range of pathologies including ischemic diseases, neurodegenerative disorders, and metabolic diseases, such as diabetes mellitus. The UPR is also triggered by various environmental factors; e.g., pollutants, infectious pathogens, therapeutic drugs, alcohol, physical stress, and malnutrition. This review summarizes current knowledge on environmental factors that induce ER stress and describes how the UPR is linked to particular pathological states after exposure to environmental triggers.

Nitric oxide protects endothelium from cadmium mediated leakiness

Cadmium targets the vascular endothelium causing endothelial dysfunction and leakiness of endothelial barrier. Nitric oxide plays a major role in mediating endothelial functions including angiogenesis, migration and permeability. The present study investigates the nitric oxide effects on cadmium induced endothelial leakiness. Results of ex vivo and in vitro permeability assays showed that even a sub-lethal dose of cadmium chloride (1 µM) was sufficient to induce leakiness of endothelial cells. Cadmium drastically altered the actin polymerisation pattern and membrane tension of these cells compared to controls. Addition of nitric oxide donor Spermine NONOate (SP) significantly blunted cadmium-mediated effects and recover endothelial cells integrity. Cadmium-induced cytoskeletal rearrangements and membrane leakiness are associated with the low nitric oxide availability and high reactive oxygen species generation. In brief, we show the protective role of nitric oxide against cadmium-mediated endothelial leakiness.

Tumor promoting effects of cyanobacterial extracts are potentiated by anthropogenic contaminants--evidence from in vitro study

Inhibition of gap junctional intercellular communication (GJIC) is affiliated with tumor promotion process and it has been employed as an in vitro biomarker for evaluation of tumor promoting effects of chemicals. In the present study we investigated combined effects of anthropogenic environmental contaminants 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) and fluoranthene, cyanotoxins microcystin-LR and cylindrospermopsin, and extracts of laboratory cultures of cyanobacteria Aphanizomenon gracile and Cylindrospermopsis raciborskii, on GJIC in the rat liver epithelial cell line WB-F344. Binary mixtures of PCB 153 with fluoranthene and the mixtures of the two cyanobacterial strains elicited simple additive effects on GJIC after 30 min exposure, whereas microcystin-LR and cylindrospermopsin neither inhibited GJIC nor altered effects of PCB 153 or fluoranthene. However, synergistic effects were observed in the cells exposed to binary mixtures of anthropogenic contaminants (PCB 153 or fluoranthene) and cyanobacterial extracts. The synergistic effects were especially pronounced after prolonged (6-24h) co-exposure to fluoranthene and A. gracile extract, when mixture caused nearly complete GJIC inhibition, while none of the individual components caused any downregulation of GJIC at the same concentration and exposure time. The effects of cyanobacterial extracts were independent of microcystin-LR or cylindrospermopsin, which were not detected in cyanobacterial biomass. It provides further evidence on the presence of unknown tumor promoting metabolites in cyanobacteria. Clear potentiation of the GJIC inhibition observed in the mixtures of two anthropogenic contaminants and cyanobacteria highlight the importance of combined toxic effects of chemicals in complex environmental mixtures.

Connexin43 hemichannels contribute to cadmium-induced oxidative stress and cell injury

We investigated the potential involvement of connexin hemichannels in cadmium ions (Cd(2+))-elicited cell injury. Transfection of LLC-PK1 cells with a wild-type connexin43 (Cx43) sensitized them to Cd(2+)-elicited cell injury. The cell susceptibility to Cd(2+) was increased by depletion of glutathione (GSH) with DL-buthionine-[S,R]-sulfoximine, and decreased by N-acetyl-cysteine or glutathione reduced ethyl ester. Fibroblasts derived from Cx43 wild-type (Cx43+/+) and knockout (Cx43-/-) fetal littermates displayed different susceptibility to Cd(2+). Cd(2+) induced a higher concentration of reactive oxygen species, a stronger activation c-Jun N-terminal kinase, and significantly more severe cell injury in Cx43+/+ fibroblasts, as compared with Cx43-/- fibroblasts. Cd(2+) caused a reduction in intracellular GSH, whereas it elevated extracellular GSH. This effect of Cd(2+) was more dramatic in Cx43+/+ than Cx43-/- fibroblasts. Treatment of Cx43+/+ fibroblasts with Cd(2+) caused a Cx43 hemichannel-dependent influx of Lucifer Yellow and efflux of ATP. Collectively, our study demonstrates that Cx43 sensitizes cells to Cd(2+)-initiated cytotoxicity, possibly through hemichannel-mediated effects on intracellular oxidative status.

Advanced glycation end products down-regulate gap junctions in human hepatoma SKHep 1 cells via the activation of Src-dependent ERK1/2 and JNK/SAPK/AP1 signaling pathways

Hyperglycemia and advanced glycation end products (AGEs) are associated with an elevated risk of developing several cancers in diabetic patients. However, the detailed mechanisms remain to be elucidated. The mechanism of AGE-bovine serum albumin (BSA) on gap junction intercellular communication in human hepatoma cell line, SKHep 1, was investigated. Both Cx32 and Cx43 are major gap junction forming proteins in the liver, the loss of which has been shown to facilitate tumorigenesis. Although the MTT assay results showed that AGE-BSA significantly increased cell growth by 31%, AGE-BSA down-regulated Cx32 and Cx43 expression in a dose- and time-dependent manner. The present study also demonstrated that ERK1/2 and JNK/SAPK were significantly activated by AGE-BSA and that Src, ERK1/2, and JNK/SAPK inhibitors significantly reversed the reduction of Cx32 and Cx43 proteins by AGE-BSA. Taken together, these results strongly support the hypothesis that Src-dependent ERK1/2 and JNK/SAPK/AP1 signaling pathways play a key role in AGE-BSA-mediated down-regulation of Cx32 and Cx43 protein expression in SKHep 1 cells.

Role of metallothionein in cadmium traffic and toxicity in kidneys and other mammalian organs

Metallothioneins are cysteine-rich, small metal-binding proteins present in various mammalian tissues. Of the four common metallothioneins, MT-1 and MT-2 (MTs) are expressed in most tissues, MT-3 is predominantly present in brain, whereas MT-4 is restricted to the squamous epithelia. The expression of MT-1 and MT-2 in some organs exhibits sex, age, and strain differences, and inducibility with a variety of stimuli. In adult mammals, MTs have been localized largely in the cell cytoplasm, but also in lysosomes, mitochondria and nuclei. The major physiological functions of MTs include homeostasis of essential metals Zn and Cu, protection against cytotoxicity of Cd and other toxic metals, and scavenging free radicals generated in oxidative stress. The role of MTs in Cd-induced acute and chronic toxicity, particularly in liver and kidneys, is reviewed in more details. In acute toxicity, liver is the primary target, whereas in chronic toxicity, kidneys are major targets of Cd. The intracellular MTs bind Cd ions and form CdMT. In chronic intoxication, Cd stimulates de novo synthesis of MTs; it is assumed that toxicity in the cells starts when loading with Cd ions exceeds the buffering capacity of intracellular MTs. CdMT, released from the Cd-injured organs, or when applied parenterally for experimental purposes, reaches the kidneys via circulation, where it is filtered, endocytosed in the proximal tubule cells, and degraded in lysosomes. Liberated Cd can immediately affect the cell structures and functions. The resulting proteinuria and CdMT in the urine can be used as biomarkers of tubular injury.

MAPK activation is involved in cadmium-induced Hsp70 expression in HepG2 cells

Cadmium is one of the most toxic elements to which man can be exposed at work or in the environment. By far, the most salient toxicological property of Cd is its exceptionally long half-life in the human body. Once absorbed, Cd accumulates in the human body, particularly in the liver and other vital organs. The cellular actions of Cd are extensively documented, but the molecular mechanisms underlying these actions are still not resolved. It is known that Cd activates the activator protein-1 (AP-1), but no data about the pathway involved are reported for liver. The objective was to provide a greater insight into the effect of cadmium on mitogen-activated protein kinases (MAPK's) involved in signal transduction, its relationship with AP-1 activation, and heat shock protein (Hsp) 70 expression, in HepG2 cells. AP-1 activation as a result of 5 microM CdCl(2) exposure was increased 24.5-fold over control cells after 4 h treatment. To investigate the role of the extracellular signal-regulated protein kinases (ERK's), c-Jun N-terminal kinases (JNK's) and p38 kinases in cadmium-induced AP-1 activation, specific MAPKs inhibitors were used. AP-1 activation decreased by 74% with ERK inhibition, by 83% with p38 inhibition, while inhibition of JNK decreased by 70%. Only ERK and JNK participated in Hsp70 production, conferring cell protection against cadmium damage.

Gap junctional intercellular communication as a target for liver toxicity and carcinogenicity

Direct communication between hepatocytes, mediated by gap junctions, constitutes a major regulatory platform in the control of liver homeostasis, ranging from hepatocellular proliferation to hepatocyte cell death. Inherent to this pivotal task, gap junction functionality is frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity and carcinogenicity. In the present paper, the deleterious effects of a number of chemical and biological toxic compounds on hepatic gap junctions are discussed, including environmental pollutants, biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. Particular attention is paid to the molecular mechanisms that underlie the abrogation of gap junction functionality. Since hepatic gap junctions are specifically targeted by tumor promoters and epigenetic carcinogens, both in vivo and in vitro, inhibition of gap junction functionality is considered as a suitable indicator for the detection of nongenotoxic hepatocarcinogenicity.

Structure-activity-dependent regulation of cell communication by perfluorinated fatty acids using in vivo and in vitro model systems

BACKGROUND

Perfluoroalkanoates, [e.g., perfluorooctanoate (PFOA)], are known peroxisome proliferators that induce hepatomegaly and hepatocarcinogenesis in rodents, and are classic non-genotoxic carcinogens that inhibit in vitro gap-junctional intercellular communication (GJIC). This inhibition of GJIC is known to be a function of perfluorinated carbon lengths ranging from 7 to 10.

OBJECTIVES

The aim of this study was to determine if the inhibition of GJIC by PFOA but not perfluoropentanoate (PFPeA) observed in F344 rat liver cells in vitro also occurs in F344 rats in vivo and to determine mechanisms of PFOA dysregulation of GJIC using in vitro assay systems.

METHODS

We used an incision load/dye transfer technique to assess GJIC in livers of rats exposed to PFOA and PFPeA. We used in vitro assays with inhibitors of cell signaling enzymes and antioxidants known to regulate GJIC to identify which enzymes regulated PFOA-induced inhibition of GJIC.

RESULTS

PFOA inhibited GJIC and induced hepatomegaly in rat livers, whereas PFPeA had no effect on either end point. Serum biochemistry of liver enzymes indicated no cytotoxic response to these compounds. In vitro analysis of mitogen-activated protein kinase (MAPK) indicated that PFOA, but not PFPeA, can activate the extracellular receptor kinase (ERK). Inhibition of GJIC, in vitro, by PFOA depended on the activation of both ERK and phosphatidylcholine-specific phospholipase C (PC-PLC) in the dysregulation of GJIC in an oxidative-dependent mechanism.

CONCLUSIONS

The in vitro analysis of GJIC, an epigenetic marker of tumor promoters, can also predict the in vivo activity of PFOA, which dysregulated GJIC via ERK and PC-PLC.

Endoplasmic reticulum stress and unfolded protein response in renal pathophysiology: Janus faces

A number of pathophysiological insults lead to accumulation of unfolded proteins in the endoplasmic reticulum (ER) and cause ER stress. In response to accumulation of unfolded/misfolded proteins, cells adapt themselves to the stress condition via the unfolded protein response (UPR). For the cells, UPR is a double-edged sword. It triggers both prosurvival and proapoptotic signals. ER stress and UPR may, therefore, be involved in a diverse range of pathological situations. However, currently, information is limited regarding roles of ER stress and UPR in the renal pathophysiology. This review describes current knowledge on the relationship between ER stress and diseases and summarizes evidence for the link between ER stress/UPR and renal diseases.

Endoplasmic reticulum stress in the kidney

Endoplasmic reticulum (ER) stress is involved in a wide range of pathological circumstances including neurodegenerative disorders, diabetes mellitus, ischemic injury, cancers, atherosclerosis, inflammation, infection, toxicity of chemicals and metals, and psychotic diseases. ER stress is also involved in some physiological events including development of particular cell types. A number of pathophysiological triggers cause accumulation of unfolded proteins in the ER, i.e., ER stress. In response to accumulation of unfolded/misfolded proteins, cells adapt themselves to the stress conditions via a coordinated adaptive program, the unfolded protein response (UPR). UPR is a double-edged sword. It induces both prosurvival and proapoptotic signaling. It also triggers both proinflammatory and anti-inflammatory signals. In this review, I summarize current knowledge on putative, pathophysiological roles of ER stress in the kidney.

The vascular system as a target of metal toxicity

Vascular system function involves complex interactions among the vascular endothelium, smooth muscle, the immune system, and the nervous system. The toxic metals cadmium (Cd), arsenic (As), and lead (Pb) can target the vascular system in a variety of ways, ranging from hemorrhagic injury to subtle pathogenic remodeling and metabolic changes. Acute Cd exposure results in hemorrhagic injury to the testis, although some strains of animals are resistant to this effect. A comparison of Cd-sensitive with Cd-resistant mouse strains showed that expression of the Slc39a8 gene, encoding the ZIP8 transporter, in the testis vasculature endothelium is responsible for this difference. Endogenously, ZIP8 is a Mn(2+)/HCO(3)(-)symporter that may also contribute to Cd damage in the kidney. Chronic Cd exposure is associated with various cardiovascular disorders such as hypertension and cardiomyopathy and it is reported to have both carcinogenic and anticarcinogenic activities. At noncytotoxic concentrations of 10-100nM, Cd can inhibit chemotaxis and tube formation of vascular endothelial cells. These angiostatic effects may be mediated through disruption of vascular endothelial cadherin, a Ca(2+)-dependent cell adhesion molecule. With regard to As, ingestion of water containing disease-promoting concentrations of As promotes capillarization of the liver sinusoidal endothelium. Because capillarization is a hallmark precursor for liver fibrosis and contributes to an imbalance of lipid metabolism, this As effect on hepatic endothelial cells may be a pathogenic mechanism underlying As-related vascular diseases. With regard to Pb, perinatal exposure may cause sustained elevations in adult blood pressure, and genetically susceptible animals may show enhanced sensitivity to this effect. Taken together, these data indicate that the vascular system is a critical target of metal toxicity and that actions of metals on the vascular system may play important roles in mediating the pathophysiologic effects of metals in specific target organs.

Adherens junction breakdown in the periderm following cadmium administration in the chick embryo: distribution of cadherins and associated molecules

BACKGROUND

The teratogenic metal cadmium (Cd) has been found to cause ventral body wall defects similar to human omphalocele when administered to post-gastrulation chick embryos prior to body wall folding. From 4h after Cd, affected embryos demonstrate varying degrees of cell junction breakdown and desquamation in the periderm. We examined the effect of Cd on tissue and cell distribution of cadherins and their intracellular associates.

METHODS

Chicks were explanted and given 50microl of 50microM Cd solution at 60h incubation (Hamburger-Hamilton stage 16-17). To examine peridermal junctions, embryos were processed into resin and ultra-thin sections examined by transmission electron microscopy (TEM). Tissue was processed into paraffin and 6microm sections treated according to standard protocols for immunohistochemical detection of L-CAM, pan-cadherin, beta-catenin, alpha-1 and alpha-2 catenin. To examine actin distribution, frozen sections were cut at 10-20microm, stained with oragon green phalloidin and nuclei counter-stained with propidium iodide.

RESULTS

The overall tissue distribution of L-CAM, pan-cadherin and the alpha-catenins did not appear to be altered following Cd. However, beta-catenin changed from its normal sub-membranous location to a more general cytoplasmic distribution, with translocation to the nucleus in both peridermal and ectodermal cells. Similarly, actin distribution in the periderm in embryos demonstrating cell junction breakdown was markedly altered, with clumping and disorganization after 4h.

CONCLUSIONS

Although L-CAM is distributed normally after Cd, post-translational modification may occur causing breakdown of its normal association with the catenins and actin, and allowing beta-catenin to translocate to the nucleus in peri-ectodermal tissue, mimicking the canonical Wnt pathway.

The protective effects of caffeic acid phenethyl ester (CAPE) against liver damage induced by cigarette smoke inhalation in rats

The aim of this study was to investigate the histological and biochemical changes in liver of rats exposed to cigarette smoke and effects of caffeic acid phenetyl ester (CAPE) on these changes. For this purpose, 21 male Wistar rats were divided into three groups. Animals in Group I were used as control. Rats in Group II were exposed to cigarette smoke and rats in Group III were exposed to cigarette smoke and injected daily with CAPE. At the end of the 60-days experimental period, all rats were killed by decapitation and blood samples were obtained. Serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin levels and hepatic superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px ), malondialdehyde (MDA) contents were determined. Following routine histological procedures, liver tissue specimens were examined under a light microscope. The levels of ALT, AST, total bilirubin, SOD, GSH-Px and MDA were significantly increased in rats exposed to cigarette smoke compared with those of the controls. Light microscopic examination of liver specimens from rats exposed to cigarette smoke revealed mononuclear cell infiltration and that some of the hepatocytes had a hyperchromatic nucleus and enlarged sinusoids. The rats which were treated with CAPE along with cigarettes had partially attenuated histological changes associated with cigarette exposure. In conclusion, the damage inflicted by cigarette in the rat liver can be partially prevented by CAPE administration.

Effects of gap junction intercellular communication in rat liver on the proliferation of hepatic oval cells in vivo

AIM: To investigate the effect of gap junction intercellular communication (GJIC) in rat liver on the proliferation of hepatic oval cells (HOC) in vivo.

METHODS: Male Wistar rats were randomized into control group (n = 6), model group and phenobarbital (PB) group. HOC proliferation was induced in the rats of model group: 9 days of treatment with 2-AAF, 20 mg/kg per day by gavage, interrupted on day 5 to perform a 70% hepatectomy (2-AAF/PH). The rats in PB group were administered with PB (0.8 g/L, till the end of experiment) in drinking water, and on the 8^th day they received the same treatment as model group. The rats in model and PB group were sacrificed and necropsied at the 4^th hour, on the 4^th, 8^th, 12^th and 16^th day (6 rats at each time point) followed hepatectomy. The morphological changes of liver tissues were observed by pathological examination and the proliferation of HOC was counted using immunohistochemistry and morphological recognition. GJIC was confirmed by incision loading/dye transfer (IL/DT), and the levels of CX32 protein and mRNA were detected by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR), respectively. The expression of CX43 protein and mRNA were determined by immunohistochemistry, Western blot and RT-PCR, respectively.

RESULTS: No HOC proliferation was seen in the rat liver of control, 4-hour model and PB group. HOC appeared at portal area in model group on day 4, increased to the peak on day 8, intensely proliferated from the portal spaces and invaded the liver parenchyma on day 12, and decreased on day 16 as compared with day 12. HOC proliferation had a significant increase in PB group (from day 4 to 16) as compared with that in model group. The distance of dye transfer in model group (4 h, 4, 8, 12, 16 d) was significantly reduced in comparison with that in control group, and moreover, it was further decreased in PB group. The signal number of CX32 in the rat liver of model and PB groups were reduced as compared with that in control group (P < 0.05), and there was also significant differences between model and PB group (P < 0.05 or P < 0.01). The expression of CX32 mRNA in model group at the 4^th hour, on the 4^th, 8^th, 12^th and 16^th day was 0.82 ± 0.13, 0.33 ± 0.11, 0.51 ± 0.13, 0.68 ± 0.14 and 1.12 ± 0.18 folds of that in control group, respectively. As compared with that in model group, the level of CX32 mRNA expression in PB group had no statistical difference at the 4^th hour (P > 0.05), but had a significant increase on day 4 to 16 (P < 0.05). The expression of CX43 protein in the liver of model group at the 4^th hour, on the 4^th, 8^th, 12^th and 16^th day was 1.14 ± 0.17, 3.87 ± 0.35, 5.28 ± 0.48, 2.96 ± 0.33 and 2.12 ± 0.19 folds of that in control group, respectively. As compared with that in model group, the quantity of CX43 protein in PB group had no statistical difference at the 4th hour (P > 0.05), but had a significant decrease on day 4 to 16 (P < 0.05). The level of CX43 mRNA expression in model group at the 4^th hour, on the 4^th, 8^th, 12^th and 16^th day was 1.09 ± 0.16, 2.82 ± 0.23, 5.46 ± 0.58, 3.34 ± 0.64 and 0.91 ± 0.11 folds of that in control group, respectively. As compared with that in model group, the level of CX43 mRNA in PB group was increased (P < 0.05).

CONCLUSION: The GJIC of hepatocyte and HOC can be decreased by altering the spatial and temporal expression patterns of CX in rat liver after 2-AAF/PH, which leads to the acceleration of HOC proliferation.

Function and significance of gap junction intercellular communication in rat liver for hepatic oval cell proliferation in vivo

AIM: To study the expression of connexin 32 (CX32) and connexin 43 (CX43) and function of gap junction intercellular communication (GJIC) in rat liver during 2-acetylaminofluorene/partial hepatectomy (2AAF/PH) for hepatic oval cell (HOC) proliferation, and explore the potential mechanism of HOC proliferation in vivo.

METHODS: Male Wistar rats were randomized into normal control group (n = 6) and model group. Rats in model group were used to induce HOC proliferation: 9 days of treatment with 2-AAF, 20 mg/kg per day by gavage, interrupted on day 5 to perform a 70% hepatectomy (2-AAF/PH). At the 4^th hour, 4^th, 8^th, 12^th and 16^th day, 6 rats of model group were sacrificed respectively. The morphological changes of liver tissues were observed by pathological examination and the proliferation of HOC was counted using immunohistochemistry and morphological recognition. GJIC was confirmed by incision loading/dye transfer (IL/DT), and the levels of CX32 protein and mRNA were detected by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR), respectively. The expression of CX43 protein and mRNA were determined by immunohistochemistry, Western blot and RT-PCR, respectively.

RESULTS: No HOC proliferation was seen in the rat liver of control and 4-hour model group. Pathologic examination revealed that HOC appeared at portal area in model group on day 4, increased to the peak on day 8, intensely proliferated from the portal spaces and invaded the liver parenchyma on day 12, and decreased on day 16 as compared with day 12. In comparison with that in control group, the distance of dye transfer in model groups (4 h, 4, 8, 12, 16 d) was significantly reduced (84.5 ± 3.4, 60.6 ± 3.3, 108.6 ± 4.2, 150.6 ± 2.6, 199.6 ± 3.7 μm vs 250.0 ± 5.0 μm, P < 0.01). The signal number of CX32 in the rat liver of model groups began to decrease at the 4th hour, reached to the minimum (2.85 ± 0.39) on day 4, and recovered starting from day 8, and it was markedly reduced as compared with that in control group (P < 0.05). CX32 mRNA in model groups was decreased at the 4^th hour, reached the lowest level (0.33 ± 0.11) on day 4 and started to recover on day 8. On day 16, CX32 mRNA expression was also higher than that in control group, but the difference was not significant (P > 0.05). Western blot analysis showed an increased CX43 protein expression at the 4^th hour (P > 0.05), on day 4, 8, 12 and 16 (P < 0.01). In comparison with that in control group, the level of CX43 mRNA in model group had a slight increase at the 4^th hour (P > 0.05), an obvious increase on day 4, reached the peak on day 12 (5.46 ± 0.58), and started to decrease on day 16 (P > 0.05).

CONCLUSION: Satisfactory rat model of HOC proliferation is successfully obtained using AAF/PH, and this method is convenient, stable and repeatable. Inhibition of GJIC function, which may activate the proliferation of HOC, is regulated by CX expression patterns.

Atypical, bidirectional regulation of cadmium-induced apoptosis via distinct signaling of unfolded protein response

Cadmium is a widely distributed nephrotoxic metal that causes renal tubular injury. In this report, we investigated involvement of endoplasmic reticulum (ER) stress and individual unfolded protein responses in cadmium-initiated apoptosis of tubular epithelial cells. Cadmium chloride (CdCl(2)) induced expression of endogenous ER stress markers, GRP78, GRP94 and CHOP in vitro and in vivo, and subsequently caused cytological changes typical of apoptosis. Attenuation of ER stress by transfection with ER chaperone GRP78 or ORP150 suppressed CdCl(2)-triggered apoptosis. In response to CdCl(2), phosphorylation of RNA-dependent protein kinase-like ER kinase (PERK) and eukaryotic translation initiation factor 2alpha (eIF2alpha) was observed. Enhanced phosphorylation of eIF2alpha attenuated, whereas inhibition of eIF2alpha exacerbated CdCl(2)-induced apoptosis. Activating transcription factor 6 (ATF6) was also activated by CdCl(2) and blockade of this process suppressed induction of CHOP and thereby improved cell survival. CdCl(2) also triggered activation of the inositol-requiring ER-to-nucleus signal kinase 1 (IRE1)-X-box-binding protein 1 (XBP1) pathway and inhibition of XBP1 attenuated apoptosis independent of GRP78 and CHOP. c-Jun N-terminal kinase (JNK), another molecule downstream of IRE1, was also phosphorylated by CdCl(2) and its inhibition attenuated apoptosis. These results evidenced bidirectional regulation of apoptosis in cadmium-exposed cells. The ATF6 and IRE1 pathways cooperatively caused apoptosis via induction of CHOP, activation of XBP1 and phosphorylation of JNK, and the PERK-eIF2alpha pathway counteracted the proapoptotic processes.

The vascular endothelium as a target of cadmium toxicity

Cadmium (Cd) is an important industrial and environmental pollutant that can produce a wide variety of adverse effects in humans and animals. A growing volume of evidence indicates that the vascular endothelium may be one of the primary targets of Cd toxicity in vivo. Studies over the past 20 years have shown that Cd, at relatively low, sublethal concentrations, can target vascular endothelial cells at a variety of molecular levels, including cell adhesion molecules, metal ion transporters and protein kinase signaling pathways. The purpose of this review is to summarize the results of these recent studies and to discuss the implications of these findings with regard to the mechanisms of Cd toxicity in specific organs including the lung, liver, kidney, testis and heart. In addition the possible roles of the vascular endothelium in mediating the tumor promoting and anticarcinogenic effects of Cd are discussed.

Tissues and hair residues and histopathology in wild rats (Rattus rattus L.) and Algerian mice (Mus spretus Lataste) from an abandoned mine area (Southeast Portugal)

Data gathered in this study suggested the exposure of rats and Algerian mice, living in an abandoned mining area, to a mixture of heavy metals. Although similar histopathological features were recorded in the liver and spleen of both species, the Algerian mouse has proved to be the strongest bioaccumulator species. Hair was considered to be a good biological material to monitor environmental contamination of Cr in rats. Significant positive associations were found between the levels of this element in hair/kidney (r=0.826, n=9, p<0.01) and hair/liver (r=0.697, n=9, p=0.037). Although no association was found between the levels of As recorded in the hair and in the organs, the levels of this element recorded in the hair, of both species, were significantly higher in animals captured in the mining area, which met the data from the organs analysed. Nevertheless, more studies will be needed to reduce uncertainty about cause-effect relationships.

Kinetics of the early subcellular distribution of cadmium in rat hepatocytes

The kinetics of the early subcellular distribution of cadmium (Cd) was characterized in primary cultures of rat hepatocytes exposed to 10, 50 and 100 microM Cd in a serum-free WME medium for 10, 30 or 60 min. Our results demonstrate a time- and concentration-dependent increase in Cd content with the highest metal concentration measured in the cytosol, whereas the lowest was observed in the mitochondria. With the exception of early localization in the plasma membrane, Cd concentrations in fractions were characterized by the following decreasing order of magnitude: cytosol > low density molecules approximately nuclei > lysosomes approximately mitochondria. We also found evidence for: (i) a two-step process for Cd distribution in the nuclei and mitochondria; and (ii) a time-dependent 'slow' process of transfer from the plasma membrane to the cytosol. Saturation in Cd uptake was observed at 50 microM in most cell fractions at 10 and 30 min, except for the plasma membrane. The lack of apparent saturation for Cd accumulation at 60 min was not related to an increase in metallothionein synthesis. Altogether, our data provide insights into the dynamics of transfer between intracellular compartments, and allow a better identification of the organelles that are the most subjected to Cd toxicity for early exposure conditions.

Nonylphenol alters connexin 43 levels and connexin 43 phosphorylation via an inhibition of the p38-mitogen-activated protein kinase pathway

Endocrine-disrupting chemicals are exogenous compounds that mimic or inhibit the action of estrogens or other hormones. Nonylphenol, an environmental contaminant distributed along the St. Lawrence River, has been reported to act as a weak estrogen. Previous studies from our laboratory have shown that rats that were fed fish taken from nonylphenol contaminated sites have altered spermatogenesis and decreased sperm count. The mechanism responsible for this effect is unknown. Gap junctional intercellular communication (GJIC) in the testis is critical for coordinating spermatogenesis. The objectives of the study were to determine the effects of nonylphenol on GJIC and connexin 43 (Cx43) in a murine Sertoli cell line, TM4. Cells were exposed for 24 h to different concentrations (1 to 50 microM) of either nonylphenol or 17beta-estradiol. GJIC was determined using a microinjection approach in which Lucifer yellow was injected directly into a single cell, and GJIC was assessed 3 min postinjection. Nonylphenol exposure decreased GJIC between adjacent cells by almost 80% relative to controls. A significant concentration-dependent reduction in GJIC was observed at nonylphenol concentrations between 1 and 50 microM. Cx43 immunofluorescent staining was reduced at both 10 and 50 microM doses of nonylphenol. Cx43 phosphorylation, as determined by Western blot analysis, was reduced at both 10 and 50 microM concentrations, which may explain, at least in part, the inhibition of GJIC. In contrast, no effect on GJIC or Cx43 protein was observed in cells exposed to 17beta-estradiol at these concentrations. Cx43 has been reported to be phosphorylated via the p38-mitogen-activated protein kinase (MAPK) pathway. P38-MAPK activity was assessed in both control and nonylphenol-exposed cells. A dose-dependent decrease in p38-MAPK activity was observed in nonylphenol-exposed Sertoli cells. Protein kinase C activity was also measured and was not influenced by nonylphenol. These results suggest that nonylphenol inhibits GJIC between Sertoli cells and that this is modulated via nonestrogenic pathways.

Influence of cadmium exposure on in vitro ovine gamete dysfunction

This study was conducted to determine the in vitro effects of three different cadmium concentrations (0, 2, and 20 microM CdCl(2)) on oocyte maturation, fertilisation, and acrosome integrity and sperm viability in sheep. Cumulus-oocyte complexes were recovered from ovaries of slaughtered sheep and sperm were collected by artificial vagina from adult rams. The oocyte maturation rate was significantly affected (P < 0.001) by Cd at both concentrations, with a metaphase II (MII) rate of 96.8, 63.8, and 32.0% for 0, 2, and 20 microM cadmium, respectively. In the second experiment, the presence of Cd significantly decreased (P < 0.01) the rate of oocytes resting in MII after 24-h postmaturation culture, compared with the control group (93.8 versus 29.0 and 19.8%, respectively, for 0, 2, and 20 microM Cd). Oocytes cultured with Cd 2 microM showed a higher activation rate (59.5%, P < 0.001) with one or two pronucleus than with 0 and 20 microM Cd (6.2 and 22.9%, respectively). During fertilisation the presence of fertilised oocytes was decreased in both culture systems with Cd compared with the control (76.1, 25.9, and 4.7% for 0, 2, and 20 microM Cd, respectively; P < 0.001) while polyspermy was increased in the 2 microM Cd group (23.5 for 2 microM versus 6.7 and 0%, respectively, for 0 and 20 microM groups). In both experiments Cd significantly increased (P < 0.001) the rates of oocyte degeneration. In the third experiment, Cd 20 microM significantly decreased (P < 0.01) the viability rate (35.6%) of spermatozoa compared with 2 microM (57.6%) and 0 microM (54.4%) while Cd 2 microM increased (P < 0.01) acrosome-reacted spermatozoa (45.2%) compared with 20 microM (32.5%) and control (31.9%). The results suggest that in vitro cadmium at the lowest dose tested affects the physiological function of both ovine gametes but at higher dose tested can compromise cell viability.

Gap junctions, homeostasis, and injury

Gap junctions (Gj) play an important role in the communication between cells of many tissues. They are composed of channels that permit the passage of ions and low molecular weight metabolites between adjacent cells, without exposure to the extracellular environment. These pathways are formed by the interaction between two hemichannels on the surface of opposing cells. These hemichannels are formed by the association of six identical subunits, named connexins (Cx), which are integral membrane proteins. Cell coupling via Gj is dependent on the specific pattern of Cx gene expression. This pattern of gene expression is altered during several pathological conditions resulting in changes of cell coupling. The regulation of Cx gene expression is affected at different levels from transcription to post translational processes during injury. In addition, Gj cellular communication is regulated by gating mechanisms. The alteration of Gj communication during injury could be rationalized by two opposite theories. One hypothesis proposes that the alteration of Gj communication attenuates the spread of toxic metabolites from the injured area to healthy organ regions. The alternative proposition is that a reduction of cellular communication reduces the loss of important cellular metabolisms, such as ATP and glucose.

Cellular and molecular mechanisms mediating the effects of polychlorinated biphenyls on oocyte developmental competence in cattle

Polychlorinated biphenyls (PCBs) can interfere with normal reproductive functions acting as endocrine disruptors. Aroclor-1254 (A-1254), is a pool of more than 60 congeners used for in vitro studies because its composition is representative of PCBs environmental pollution. We previously demonstrated that the exposure of bovine oocytes to A-1254 during in vitro maturation (IVM) was detrimental not only to the maturation process but also induced a significant increase of polyspermy and a reduction of developmental competence. Therefore, we investigated whether A-1254 acts on two processes that occur during IVM and may be related with its negative effects: maternal mRNA polyadenylation and cortical granules (CGs) migration and exocytosis. Bovine cumulus-oocyte complexes (COCs) were exposed to 0.1 microg/ml of A-1254 during IVM, a level of exposure known to affect oocyte maturation, fertilization, and developmental competence. Oocyte exposure to A-1254 altered the poly(A) tail length of 5 out of 10 genes examined. PCBs effect on mRNA polyadenylation was different depending on the gene considered and resulted either in a shorter or in a longer poly(A) tail. At the end of maturation, Aroclor treated oocytes presented clustered CG in a significantly higher percentage than the control group. In addition, CG exocytosis after 8 hr of fertilization occurred at significantly lower extent in zygotes derived from the exposed group compared to control. Our results indicated that the lower developmental competence of oocytes exposed to PCBs during IVM can be related to the interaction of these contaminants with mechanisms regulating maternal mRNA storage in the ooplasm and normal CGs function.

Effects of cadmium on gap junctional intercellular communication in WB-F344 rat liver epithelial cells

Cadmium has been associated with a number of tumors but its role in tumor promotion has not been elucidated clearly or the results obtained from various studies have been conflicting. This study was designed to investigate the effects of cadmium on the gap junctional intercellular communication (GJIC), number of gap junctions per cell, and cell proliferation in WB-F344 rat liver epithelial cells from the viewpoint of tumor promotion. GJIC was monitored by counting the cells stained with Lucifer yellow CH dye, using the scrape-loading and dye-transfer method. The numbers of gap junctions per cell were visually quantitated after an indirect immunostaining for gap junction protein using an antibody to connexin 43. Cell proliferation was assayed by direct counting of the living cells using the trypan blue dye exclusion method. In the time course study, cells treated with 200 microM CdCl2 showed rapid and nearly complete inhibition of GJIC (approximately 14% of the control) and a decrease in the number of gap junctions per cell (approximately 21% of the control) at 30 min, and the decrease continued up to 4 h without any changes in the cell viability. Treatment with CdCl2 (7.4-200 microM) for 4 h resulted in the decrease of GJIC and gap junction numbers per cell in a dose-response pattern without changes in the cell viability. In the long-term (14 days) exposure studies at doses of 0.01-7.4 microM CdCl2, an increase in cell proliferation was observed at low doses of 0.03-2.5 microM CdCl2, with GJIC also decreasing. These data demonstrate that cadmium inhibits GJIC, reduces the number of gap junctions per cell, and induces cell proliferation while decreasing the function of the gap junction.

Cadmium-induced alterations of connexin expression in the promotion stage of in vitro two-stage transformation

During the multistage carcinogenesis, functions of several key genes involved in the cell cycle control and cell-cell communication can be damaged. Gap junction intercellular communication (GJIC) is known to transfer small, water-soluble molecules through intercellular channels composed of proteins called connexins (Cxs). Therefore, aberrant expression of Cx may be one of the critical factors for the clonal expansion of initiated cells during the two-stage transformation. We already improved the classical in vitro two-stage transformation method using N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) as an initiator and cadmium as a promoter on Balb/3T3 A31 cells, and reconfirmed the promotional effect of cadmium with this method (Fang, M.Z., Cho, M.H., Lee, H.W., 2001. Improvement of in vitro two-stage transformation assay and detection of the promotional effect of cadmium, Toxicol. In Vitro (in press). In this study, precise roles of Cd on Cx expression in normal Balb/3T3 A31 and during the promotion stage of the in vitro two-stage transformation were elucidated. For this purpose, the Cx43, Cx32 and Cx26 protein levels, Cx43 and Cx26 mRNA levels and the cellular distribution location of Cx43 protein were determined. Normal Balb/3T3 cells expressed Cx43 and Cx32, but not Cx26. After a short-term treatment of cadmium on normal cells, phosphorylation of Cx43 protein increased and Cx32 protein level decreased. However, during the promotion stage of the in vitro two-stage transformation, transformed cells treated with cadmium for long periods expressed Cx43 and Cx32 highly, similar to the level of normal Balb/3T3 cells, compared to the nontransformed cells. Moreover, Cx43 of the transformed cells was distributed mostly in the perinuclear region rather than the intercellular membrane. These data suggest that cadmium may inhibit the GJIC by increasing the phosphorylation of Cx43 and decreasing the expression of Cx32 in the normal Balb/3T3 A31 cells. Our results also suggest that these changes are not associated with the cell transformation; transformed cells may reexpress Cx43 and Cx32 similar to the normal cells, though Cx43 protein is distributed aberrantly during the transformation process. Further studies are needed to clarify the relationship between transformation and posttranslational modification of the Cx proteins.