Increased DNA-repair capacity and the modulation of 2 proteins in and metallothionein overexpressing Chinese hamster cell line

Changes in Metallothionein Levels in Freshwater Mussels Exposed to Urban Wastewaters: Effects from Exposure to Heavy Metals?

Municipal effluents are complex mixtures of compounds such as heavy metals, aromatic and aliphatic hydrocarbons, and micro-organisms and are released in aquatic ecosystems. The purpose of this study was to verify whether changes in metallothioneins (MT) were associated with the accumulation of labile metals in tissue of freshwater mussels exposed to the dispersion plume of a major municipal effluent. Mussels were placed in experimental cages deployed at sites 1.5 km upstream, 8 km downstream and 12 km downstream of the outfall of a major, primary-treated municipal effluent in the St. Lawrence River (Québec, Canada). Mussels were analysed for MT and labile zinc levels in their gonads, gills and digestive glands. Lipogenic enzyme (isocitrate and glucose-6-phosphate dehydrogenase) and arachidonic acid cyclooxygenase (COX) activities were also measured in gonad and gill tissues. Although MT was induced in all the tissues examined, the results showed that labile zinc levels were significantly reduced in gill and gonad tissues, with an increase observed only at the 12 km downstream site in the digestive gland. COX activity was readily induced in gills and gonads. Glucose-6-phosphate dehydrogenase activity was reduced at both downstream sites, but isocitrate dehydrogenase activity was significantly induced at the farthest (12 km) site. Analysis of covariance revealed that MT levels in gills were more influenced by COX activity than with distance in the dispersion plume and was negatively correlated with labile zinc levels. In conclusion, MT induction was inversely related to the levels of labile zinc but positively so with the inflammation biomarker COX. Hence, the induction of MT in mussels exposed to the municipal effluent of a large city appears to be associated with either inflammatory processes or as compensation for the loss of labile essential metals. We propose that the simple and complimentary parameters of labile zinc and COX evaluations be used to link MT induction with divalent heavy metal exposure in environmental studies dealing with various type of contaminants in such complex contaminant mixture effluents.

Metallothionein induction reduces caspase-3 activity and TNFalpha levels with preservation of cognitive function and intact hippocampal neurons in carmustine-treated rats

Hippocampal integrity is essential for cognitive functions. On the other hand, induction of metallothionein (MT) by ZnSO(4) and its role in neuroprotection has been documented. The present study aimed to explore the effect of MT induction on carmustine (BCNU)-induced hippocampal cognitive dysfunction in rats. A total of 60 male Wistar albino rats were randomly divided into four groups (15/group): The control group injected with single doses of normal saline (i.c.v) followed 24 h later by BCNU solvent (i.v). The second group administered ZnSO(4) (0.1 micromol/10 microl normal saline, i.c.v, once) then BCNU solvent (i.v) after 24 h. Third group received BCNU (20 mg/kg, i.v, once) 24 h after injection with normal saline (i.c.v). Fourth group received a single dose of ZnSO(4) (0.1 micromol/10 microl normal saline, i.c.v) then BCNU (20 mg/kg, i.v, once) after 24 h. The obtained data revealed that BCNU administration resulted in deterioration of learning and short-term memory (STM), as measured by using radial arm water maze, accompanied with decreased hippocampal glutathione reductase (GR) activity and reduced glutathione (GSH) content. Also, BCNU administration increased serum tumor necrosis factor-alpha (TNFalpha), hippocampal MT and malondialdehyde (MDA) contents as well as caspase-3 activity in addition to histological alterations. ZnSO(4) pretreatment counteracted BCNU-induced inhibition of GR and depletion of GSH and resulted in significant reduction in the levels of MDA and TNFalpha as well as the activity of caspase-3. The histological features were improved in hippocampus of rats treated with ZnSO(4) + BCNU compared to only BCNU-treated animals. In conclusion, MT induction halts BCNU-induced hippocampal toxicity as it prevented GR inhibition and GSH depletion and counteracted the increased levels of TNFalpha, MDA and caspase-3 activity with subsequent preservation of cognition.

Kinetics of gamma-H2AX focus formation upon treatment of cells with UV light and alkylating agents

Histone H2AX is rapidly phosphorylated in response to DNA double-strand breaks (DSBs) induced by ionizing radiation (IR). Here we show that DNA damage induced by alkylating agents [methyl methanesulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)] and ultraviolet light (UV-C) leads to a dose and time dependent accumulation of phosphorylated H2AX (gamma-H2AX). Time course experiments revealed that the number of gamma-H2AX foci reached peak levels 8 hr after MMS or MNNG treatment and declined to almost control values within 24 hr after exposure. Upon UV-C treatment, a biphasic response was observed with a maximum 12 hr after treatment. In 43-3B cells deficient in nucleotide excision repair (NER) the number of gamma-H2AX foci increased steadily. gamma-H2AX foci were preferentially formed in BrdU labeled cells. In proliferation compromised cells, the gamma-H2AX level was significantly reduced, indicating that most of the gamma-H2AX foci induced by UV-C and alkylating agent treatments were replication dependent. The data are in line with the view that DNA damage induced by UV-C light and simple alkylating agents, leads to the formation of DSBs during DNA replication giving rise to H2AX phosphorylation. In replicating NER defective cells, DSBs accumulate due to nonrepaired primary DNA lesions that produce a high level of DSBs during replication. The data support that gamma-H2AX foci are a useful marker of DSBs that are induced by S-phase dependent genotoxins during replication.

Chinese hamster cells expressing antisense to metallothionein become spontaneous mutators

The functions of metallothioneins (MTs) have been debated for at least a decade. Because it seems unlikely that they evolved only to protect cells against exogenous heavy metals, it has been suggested that MTs have roles in scavenging reactive intermediates, controlling zinc and copper homeostasis, and controlling transfer of zinc to transcription factors and other proteins. Previously, we demonstrated that Chinese hamster G12 cells which overexpress MT have greatly reduced spontaneous mutation rates, suggesting that MT evolved to prevent spontaneous mutagenesis induced by free nuclear zinc ions. We have now isolated G12 transfectants which express antisense RNA to MT. Immunofluorescent staining reveals MT protein in both the nucleus and the cytoplasm in parental cells. A clone expressing high levels of antisense RNA (AMT30) shows reduced basal and induced levels of MT protein. AMT30 cells are hypersensitive to cadmium, zinc, copper and mercury chlorides as well as to menadione. Glutathione levels in AMT30 and G12 cells do not differ. AMT30 cells are spontaneous mutators, showing a spontaneous mutation rate 5-10 times that of G12 cells or G12 cells transfected with vector alone. Only transfectants which show a high level of MT antisense expression (i.e., AMT30) had greatly elevated spontaneous mutation rates. These results support our hypothesis that a major role of MT is to act as an endogenous antimutagen probably via scavenging of reactive intermediates in the nucleus. AMT30 cells should be useful in delineating the sources of spontaneous mutagenesis.

Hepatocytes from metallothionein-I and II knock-out mice are sensitive to cadmium- and tert-butylhydroperoxide-induced cytotoxicity

Metallothionein (MT) has been proposed to play an important role in heavy metal detoxication and in the scavenging of free radicals. Effects of MT on the cytotoxicity of cadmium (Cd), tert-butylhydroperoxide (t-BHP) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were examined using primary hepatocyte cultures from control (C57BL/6J) and MT-I and II knock-out (MT-null) mice. Compared to control hepatocytes, MT-null hepatocytes had minimal Cd-binding proteins (MT equivalents), but cellular glutathione concentration was similar to the control hepatocytes. MT-null hepatocytes were more sensitive than controls to the cytotoxic effects of Cd (50-300 microM) and t-BHP (125-500 microM), as indicated by the levels of lactate dehydrogenase released into the medium. Cd and t-BHP also produced more lipid peroxidation in MT-null hepatocytes than in control cells, as demonstrated by the abundance of thiobarbituric acid-reactive substances. However, MT-null hepatocytes were equally sensitive as controls to the cytotoxicity of MNNG (0.5-2.0 mM), suggesting that MT does not protect against MNNG-induced cytotoxicity. These results support the hypothesis that constitutive MT levels affect the sensitivity of mammalian cells to Cd and oxidative stress.