Ecotoxicology of hexavalent chromium in freshwater fish: a critical review

Chromium (Cr) is a naturally occurring element found in rocks, animals, plants, and soil, predominantly in its insoluble trivalent form [Cr(III)]. Intense industrialization and other anthropogenic activities have led to the global occurrence of soluble hexavalent chromium Cr(VI), which is readily leached from soil to groundwater or surface water, in concentrations above permissible levels. The ecotoxicology of Cr(VI) is linked to its environmental persistence and the ability to induce a variety of adverse effects in biologic systems, including fish. In aquatic ecosystems, Cr(VI) exposure poses a significant threat to aquatic life. This paper reviews the fate and transport of Cr(VI) in the environment and its acute and chronic effects on fish. We also discuss Cr(VI) toxicity at the cellular, biochemical, and genetic levels. An attempt is made in this review to comprehend the staggered data on the toxic effects of Cr(VI) to various species of fish. Such data are extremely useful to the scientific community and public officials involved in health risk assessment and management of environmental contaminants as a guide to the best course of action to restore ecosystems and, in turn, to preserve human health.

Chromium-induced biochemical, genotoxic and histopathologic effects in liver and kidney of goldfish, carassius auratus

Fish constitute an excellent model to understand the mechanistic aspects of metal toxicity vis-à-vis oxidative stress in aquatic ecosystems. Hexavalent chromium (Cr (VI)), due to its redox potential can induce oxidative stress (OS) in fish and impair their health. In the present investigation, we hypothesize that OS plays a key role in chromium induced toxicity in goldfish; leading to the production of reactive oxygen species (ROS) such as O(.)(2), H(2)O(2), OH(.), and subsequent modulation of the activities of antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD), metallothioneins (MT), glutathione proxidase (GPx), genotoxicity and histopathology. To test this hypothesis, antioxidant enzymes, DNA damage and histopathology assays were performed in liver and kidney tissues of goldfish exposed to different concentrations of Cr (VI) (LC(12.5), LC(25) and LC(50)) following 96h static renewal bioassay. The results of this study clearly show that the fish experienced OS as characterized by significant modulation of enzyme activities, induction of DNA damage and microscopic morphological changes in the liver and kidney. In both tissues, CAT activity was decreased whereas SOD activity and hydroperoxide levels were increased. In addition, GPx activity also increased significantly in higher test concentrations, especially in the kidney. MT induction and DNA damage were observed in both tissues in a concentration dependent manner. Microscopic examination of organ morphology indicated degeneration of liver tissue and necrosis of central vein. Necrosis of kidney tubular epithelial cells and tubules was observed at higher Cr (VI) concentrations. Taking together the findings of this study are helpful in organ-specific risk assessment of Cr (VI)-induced oxidative stress, genotoxicity and histopathology in fish.

Low Doses of Cisplatin Induce Gene Alterations, Cell Cycle Arrest, and Apoptosis in Human Promyelocytic Leukemia Cells

Cisplatin is a known antitumor drug, but its mechanisms of action are not fully elucidated. In this research, we studied the anticancer potential of cisplatin at doses of 1, 2, or 3 µM using HL-60 cells as a test model. We investigated cisplatin effects at the molecular level using RNA sequencing, cell cycle analysis, and apoptotic assay after 24, 48, 72, and 96 hours of treatment. The results show that many genes responsible for molecular and cellular functions were significantly altered. Cisplatin treatment also caused the cells to be arrested at the DNA synthesis phase, and as the time increases, the cells gradually accumulated at the sub-G1 phase. Also, as the dose increases, a significant number of cells entered into the apoptotic and necrotic stages. Altogether, the data show that low doses of cisplatin significantly impact the viability of HL-60 cells, through modulation of gene expression, cell cycle, and apoptosis.

Coilin participates in the suppression of RNA polymerase I in response to cisplatin-induced DNA damage

This study's findings identify a novel and unexpected function for coilin, independentof its role in snRNP biogenesis, establishing a new link between the DNA damage response and the inhibition of rRNA synthesis.

Coilin is a nuclear phosphoprotein that concentrates within Cajal bodies (CBs) and impacts small nuclear ribonucleoprotein (snRNP) biogenesis. Cisplatin and γ-irradiation, which cause distinct types of DNA damage, both trigger the nucleolar accumulation of coilin, and this temporally coincides with the repression of RNA polymerase I (Pol I) activity. Knockdown of endogenous coilin partially overrides the Pol I transcriptional arrest caused by cisplatin, while both ectopically expressed and exogenous coilin accumulate in the nucleolus and suppress rRNA synthesis. In support of this mechanism, we demonstrate that both cisplatin and γ-irradiation induce the colocalization of coilin with RPA-194 (the largest subunit of Pol I), and we further show that coilin can specifically interact with RPA-194 and the key regulator of Pol I activity, upstream binding factor (UBF). Using chromatin immunoprecipitation analysis, we provide evidence that coilin modulates the association of Pol I with ribosomal DNA. Collectively, our data suggest that coilin acts to repress Pol I activity in response to cisplatin-induced DNA damage. Our findings identify a novel and unexpected function for coilin, independent of its role in snRNP biogenesis, establishing a new link between the DNA damage response and the inhibition of rRNA synthesis.

Coilin association with Box C/D scaRNA suggests a direct role for the Cajal body marker protein in scaRNP biogenesis

Spliceosomal small nuclear ribonucleoproteins (snRNPs) are enriched in the Cajal body (CB). Guide RNAs, known as small Cajal body-specific RNAs (scaRNAs), direct modification of the small nuclear RNA (snRNA) component of the snRNP. The protein WRAP53 binds a sequence motif (the CAB box) found in many scaRNAs and the RNA component of telomerase (hTR) and targets these RNAs to the CB. We have previously reported that coilin, the CB marker protein, associates with certain non-coding RNAs. For a more comprehensive examination of the RNAs associated with coilin, we have sequenced the RNA isolated from coilin immunocomplexes. A striking preferential association of coilin with the box C/D scaRNAs 2 and 9, which lack a CAB box, was observed. This association varied by treatment condition and WRAP53 knockdown. In contrast, reduction of WRAP53 did not alter the level of coilin association with hTR. Additional studies showed that coilin degrades/processes scaRNA 2 and 9, associates with active telomerase and can influence telomerase activity. These findings suggest that coilin plays a novel role in the biogenesis of box C/D scaRNPs and telomerase.

Oxidative Stress and DNA Damage Induced by Chromium in Liver and Kidney of Goldfish, Carassius auratus

Chromium (Cr) is an abundant element in the Earth’s crust. It exhibits various oxidation states, from divalent to hexavalent forms. Cr has diverse applications in various industrial processes and inadequate treatment of the industrial effluents leads to the contamination of the surrounding water resources. Hexavalent chromium (Cr (VI)) is the most toxic form, and its toxicity has been associated with oxidative stress. The present study was designed to investigate the toxic potential of Cr (VI) in fish. In this research, we investigated the role of oxidative stress in chromium-induced genotoxicity in the liver and kidney cells of goldfish, Carassius auratus. Goldfish were acclimatized to the laboratory conditions and exposed them to 5% and 10% of 96 hr-LC₅₀ (85.7 mg/L) of aqueous Cr (VI) in a continuous flow through system. Fish were sampled every 7 days for a period of 28 days to analyze the lipid hydroperoxides (LHP) levels and genotoxic potentials in the liver and kidney. LHP levels were analyzed by spectrophotometry while genotoxicity was assessed by single cell gel electrophoresis (comet) assay. LHP levels in the liver increased significantly at week 1, followed by a decrease. LHP levels in the kidney increased significantly at weeks 1, 2, and 3, and decreased at week 4 compared to the control. The percentage of DNA damage increased in both liver and kidney at both test concentrations. The results clearly indicate that Cr (VI) induces significant levels of DNA damage in liver and kidney cells of goldfish. The induced LHP levels in both organs were concentration-dependent and were directly correlated with the levels of DNA damage. The two tested Cr (VI) concentrations induced significant levels of oxidative stress in both organs, however the kidney appears to be more vulnerable and sensitive to Cr-induced toxicity than the liver.

Coilin interacts with Ku proteins and inhibits in vitro non-homologous DNA end joining

Coilin is a nuclear protein that plays a role in Cajal body formation. The function of nucleoplasmic coilin is unknown. Here we report that coilin interacts with Ku70 and Ku80, which are major players in the DNA repair process. Ku proteins compete with SMN and SmB' proteins for coilin interaction sites. The binding domain on coilin for Ku proteins cannot be localized to one discrete region, and only full-length coilin is capable of inhibiting in vitro non-homologous DNA end joining (NHEJ). Since Ku proteins do not accumulate in CBs, these findings suggest that nucleoplasmic coilin participates in the regulation of DNA repair.

Coilin phosphomutants disrupt Cajal body formation, reduce cell proliferation and produce a distinct coilin degradation product

Coilin is a nuclear phosphoprotein that accumulates in Cajal bodies (CBs). CBs participate in ribonucleoprotein and telomerase biogenesis, and are often found in cells with high transcriptional demands such as neuronal and cancer cells, but can also be observed less frequently in other cell types such as fibroblasts. Many proteins enriched within the CB are phosphorylated, but it is not clear what role this modification has on the activity of these proteins in the CB. Coilin is considered to be the CB marker protein and is essential for proper CB formation and composition in mammalian cells. In order to characterize the role of coilin phosphorylation on CB formation, we evaluated various coilin phosphomutants using transient expression. Additionally, we generated inducible coilin phosphomutant cell lines that, when used in combination with endogenous coilin knockdown, allow for the expression of the phosphomutants at physiological levels. Transient expression of all coilin phosphomutants except the phosphonull mutant (OFF) significantly reduces proliferation. Interestingly, a stable cell line induced to express the coilin S489D phosphomutant displays nucleolar accumulation of the mutant and generates a N-terminal degradation product; neither of which is observed upon transient expression. A N-terminal degradation product and nucleolar localization are also observed in a stable cell line induced to express a coilin phosphonull mutant (OFF). The nucleolar localization of the S489D and OFF coilin mutants observed in the stable cell lines is decreased when endogenous coilin is reduced. Furthermore, all the phosphomutant cells lines show a significant reduction in CB formation when compared to wild-type after endogenous coilin knockdown. Cell proliferation studies on these lines reveal that only wild-type coilin and the OFF mutant are sufficient to rescue the reduction in proliferation associated with endogenous coilin depletion. These results emphasize the role of coilin phosphorylation in the formation and activity of CBs.

Hexavalent chromium-induced multiple biomarker responses in liver and kidney of goldfish, Carassius auratus

Hexavalent chromium [Cr (VI)] is a constituent of chromite ore. Although it is known to have several industrial and technological applications, its release into the aquatic environment as a result of chemical spill or inadequate waste discharge may hamper the health of aquatic organisms. In this study, we have investigated the effects of Cr (VI) on multiple biomarkers responses in goldfish under subchronic exposure conditions. Laboratory-acclimatized fish were exposed to 4.25 ppm and 8.57 ppm Cr (VI) for four weeks using a continuous flow-through system. During exposure, fish samples were collected on a weekly basis and analyzed for multiple biomarkers including catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), metallothionein (MT), and total protein in liver and kidney. Study results indicated that the CAT activity and total protein levels in Cr (VI) - treated goldfish did not significantly differ (P > 0.05) from their respective controls during experimentation. However, highly significant up-regulations (P < 0.05) of SOD, GPx, and MT expression in Cr (VI) - treated goldfish were recorded at different exposure times depending on Cr (VI) concentration, test organ, and/or biomarker of interest. For example, significantly higher liver GPx levels were found at weeks 2 and 3 in the 4.25 ppm concentration, and at weeks 3 and 4 in the 8.57 ppm, while kidney GPx levels were significantly higher at weeks 1, 2 and 3 in the 4.25 ppm concentration, and at weeks 2, 3 and 4 in the 8.57 ppm concentration. In summary, Cr (VI)-induced oxidative stress was characterized by statistically significant increases in SOD, GPx, and MT expression in goldfish tissues; with the kidney showing a relatively higher sensitivity to Cr (VI) toxicity compared with the liver.

Regulated specific proteolysis of the Cajal body marker protein coilin

Cajal bodies (CB) are subnuclear domains that contain various proteins with diverse functions including the CB marker protein coilin. In this study, we investigate the proteolytic activity of calpain on coilin. Here, we report a 28-kDa cleaved coilin fragment detected by two coilin antibodies that is cell cycle regulated, with levels that are consistently reduced during mitosis. We further show that an in vitro calpain assay with full-length or C-terminal coilin recombinant protein releases the same size cleaved fragment. Furthermore, addition of exogenous RNA to purified coilin induces proteolysis by calpain. We also report that the relative levels of this cleaved coilin fragment are susceptible to changes induced by various cell stressors, and that coilin localization is affected by inhibition or knockdown of calpain both under normal and stressed conditions. Collectively, our data suggest that coilin is subjected to regulated specific proteolysis by calpain, and this processing may play a role in the regulation of coilin activity and CB formation.

Coilin levels modulate cell cycle progression and γH2AX levels in etoposide treated U2OS cells

Coilin is considered the Cajal body (CB) marker protein. In this report, we investigated the role of coilin in the DNA damage response and found that coilin reduction correlated with significantly increased levels of soluble γH2AX in etoposide treated U2OS cells. Additionally, coilin levels influenced the proliferation rate and cell cycle distribution of cells exposed to etoposide. Moreover, coilin overexpression inhibited nucleolar localization of endogenous coilin in etoposide treated U2OS cells. Collectively, these data provide additional evidence for coilin and CBs in the DNA damage response.

Preclinical Assessment of Low Doses of Cisplatin in the Management of Acute Promyelocytic Leukemia

Cis-diamminedichloroplatinum (II) (cisplatin) is the most widely used chemotherapeutic drug for various cancers, but its effectiveness is limited by tumor cell resistance and the severe side effects it causes. Since high level of cisplatin is cytotoxic to both cancer and normal cells, the goal of the present study was to explore the effectiveness of prolonged low doses of cisplatin in the management of leukemia. To achieve our goal, human leukemia (HL-60) cells were treated with different doses (1, 2, or 3 µM) of cisplatin for 24, 48, 72 and 96 hours. Cell viability was assessed by MTS assay. Both oxidative stress damage and genotoxicity were estimated by antioxidants, lipid peroxidation, and comet assays, respectively. Data obtained from the MTS assay demonstrated that cisplatin treatment decreased the number of viable tumor cells by direct cell killing or by simply decreasing the rate of cellular proliferation in a dose- and time-dependent fashion. The results of the lipid peroxidation showed a significant increase (p<0.05) of malondialdehyde levels with increasing cisplatin doses. Results obtained from super oxide dismutase and catalase assays showed a gradual increase in antioxidant enzyme activity in cisplatin-treated cells compared to control cells. Data generated from the Comet assay demonstrated a significant dose-dependent increase in genotoxicity with respect to DNA damage as a result of cisplatin treatment. Taken together, our research demonstrated that cisplatin-induced cytotoxicity in HL-60 cells is mediated at least in part via induction of oxidative stress and oxidative damage.