Critical pathways in heart function: bis(2-chloroethoxy)methane-induced heart gene transcript change in F344 rats

Thallium exposure interfered with heart development in embryonic zebrafish (Danio rerio): From phenotype to genotype

Thallium (Tl) is a rare trace metal element but increasingly detected in wastewater produced by coal-burning, smelting, and more recently, high-tech manufacturing industries. However, the adverse effects of Tl, especially cardiotoxicity, on aquatic biota remain unclear. In this study, zebrafish model was used to elucidate the effects and mechanisms of Tl(I) cardiotoxicity in developing embryos. Exposure of embryonic zebrafish to low-dose Tl(I) (25-100 μg/L) decreased heart rate and blood flow activity, and subsequently impaired swim bladder inflation and locomotive behavior of larvae. Following high-level Tl(I) administration (200-800 μg/L), embryonic zebrafish exhibited pericardial edema, incorrect heart looping, and thinner myocardial layer. Based on RNA-sequencing, Tl(I) altered pathways responsible for protein folding and transmembrane transport, as well as negative regulation of heart rate and cardiac jelly development. The gene expression of nppa, nppb, ucp1, and ucp3, biomarkers of cardiac damage, were significantly upregulated by Tl(I). Our findings demonstrate that Tl(I) at environmentally relevant concentrations interfered with cardiac development with respect to anatomy, function, and transcriptomic alterations. The cardiotoxic mechanisms of Tl(I) provide valuable information in the assessment of Tl-related ecological risk in freshwater environment.

Occult Cardiotoxicity: Subtoxic Dosage of Bis(2-chloroethoxy)methane Impairs Cardiac Response to Simulated Ischemic Injury

The effect of Bis(2-chloroethoxy)methane (CEM) on myocardial response to ischemia was tested in rats. CEM was dermally applied for 3 days to F344/N male rats, at 0, 100, 400, or 600 mg/kg/d. Subsequently, left ventricular sections were prepared from each rat heart. Part of the sections from each heart were exposed to 90 minutes of simulated ischemia, followed by 90 minutes of reoxygenation. The rest of the sections were continuously oxygenated. Mitochondrial activity was assessed in the sections by the MTT colorimetric assay, reflecting dehydrogenases redox activity. Myocardial toxicity occurred in response to 400 and 600 mg/kg, characterized by myofiber vacuoles, necrosis, and mononuclear infiltrates. The latter dose was lethal. In sections from rats treated with 400 mg/kg CEM, redox activity was decreased by 21% ( p < 0.01) in oxygenated conditions and by 45% ( p < 0.01) in ischemia-reoxygenation, compared to controls. Hearts of rats treated with 100 mg/kg/d CEM showed normal histology. Their mitochondrial activity did not differ from that of untreated rat hearts in oxygenated conditions. However, in ischemia-reoxygenation, their redox activity was significantly lower (by 46%, p < 0.01) than that of untreated rat hearts. These results demonstrate that subtoxic dosage of a cardiotoxic agent may cause occult cardiotoxicity, reflected by impaired response to ischemia.

Identifying ion channel genes related to cardiomyopathy using a novel decision forest strategy

Ion channels play many crucial functions in life. Their dysfunction may lead to a number of diseases, such as arrhythmia and beta cell dysfunction. In this study, we firstly selected the ion channel gene expression profiles using a dimensionality reduction method. After that, we applied a novel decision forest strategy to mine cardiomyopathy related ion channel genes. The novel proposed Zi integrated the information of the decision trees' height and the frequency at which a gene was located in the tree. It achieved a much higher ability of feature selection. In the result, 26 cardiomyopathy related ion channel genes were identified. Their Zi were higher than the threshold Z*. Furthermore, most of these genes had been reported to have relationships with cardiomyopathies. In conclusion, our proposed decision forest strategy had a better classification performance. Our result can provide a theoretical basis for cardiovascular researchers.

Meta-Analysis of Large-Scale Toxicogenomic Data Finds Neuronal Regeneration Related Protein and Cathepsin D to Be Novel Biomarkers of Drug-Induced Toxicity

Undesirable toxicity is one of the main reasons for withdrawing drugs from the market or eliminating them as candidates in clinical trials. Although numerous studies have attempted to identify biomarkers capable of predicting pharmacotoxicity, few have attempted to discover robust biomarkers that are coherent across various species and experimental settings. To identify such biomarkers, we conducted meta-analyses of massive gene expression profiles for 6,567 in vivo rat samples and 453 compounds. After applying rigorous feature reduction procedures, our analyses identified 18 genes to be related with toxicity upon comparisons of untreated versus treated and innocuous versus toxic specimens of kidney, liver and heart tissue. We then independently validated these genes in human cell lines. In doing so, we found several of these genes to be coherently regulated in both in vivo rat specimens and in human cell lines. Specifically, mRNA expression of neuronal regeneration-related protein was robustly down-regulated in both liver and kidney cells, while mRNA expression of cathepsin D was commonly up-regulated in liver cells after exposure to toxic concentrations of chemical compounds. Use of these novel toxicity biomarkers may enhance the efficiency of screening for safe lead compounds in early-phase drug development prior to animal testing.

Toxicokinetics of bis(2-chloroethoxy)methane following intravenous administration and dermal application in male and female F344/N rats and B6C3F1 mice

In the National Toxicology Program's toxicity studies, rats were more sensitive than mice to Bis(2-chloroethoxy)methane (CEM) - induced cardiac toxicity following dermal application to male and female F344/N rats and B6C3F1 mice. Thiodiglycolic acid (TDGA) is a major metabolite of CEM in rats. It has been implicated that chemicals metabolized to TDGA cause cardiac toxicity in humans. Therefore, the toxicokinetics of CEM and TDGA were investigated in male and female F344/N rats and B6C3F1 mice following a single intravenous administration or dermal application of CEM to aid in the interpretation of the toxicity data. Absorption of CEM following dermal application was rapid in both species and genders. Bioavailability following dermal application was low but was higher in rats than in mice with females of both species showing higher bioavailability than males. CEM was rapidly distributed to the heart, thymus, and liver following both routes of administration. Plasma CEM C(max) and AUC(∞) increased proportionally with dose, although at the dermal dose of 400mg/kg in rats and 600mg/kg in mice non-linear kinetics were apparent. Following dermal application, dose-normalized plasma CEM C(max) and AUC(∞) was significantly higher in rats than in mice (p-value<0.0001 for all comparisons except for C(max) in the highest dose groups where p-value=0.053). In rats, dose-normalized plasma CEM C(max) and AUC(∞) was higher in females than in males: however, the difference was significant only at the lowest dose (p-value=0.009 for C(max) and 0.056 for AUC(∞)). Similar to rats, female mice also showed higher C(max) and AUC(∞) in females than in male: the difference was significant only for C(max) at the lowest dose (p-value=0.002). Dose-normalized heart CEM C(max) was higher in rats than in mice and in females than their male counterparts. The liver CEM C(max) was lower compared to that of heart and thymus in both rats and mice following intravenous administration and in rats following dermal application. This is likely due to the rapid metabolism of CEM in the liver as evidenced by the high concentration of TDGA measured in the liver. Dose-normalized plasma and heart TDGA C(max) values were higher in rats compared to mice. In rats, females had higher plasma and heart TDGA C(max) than males; however, there was no gender difference in plasma or heart TDGA C(max) in mice. These findings support the increased sensitivity of rats compared to mice to CEM-induced cardiac toxicity. Data also suggest that, either CEM C(max) or AUC can be used to predict the CEM-induced cardiac toxicity. Although, both plasma and heart TDGA C(max) was consistent with the observed species difference and the gender difference in rats, the gender difference in mice to cardiac toxicity could not be explained based on the TDGA data. This animal study suggests that toxicologically significant concentrations of CEM and TDGA could possibly be achieved in the systemic circulation and/or target tissues in humans as a result of dermal exposure to CEM.

Comparative phenotypic assessment of cardiac pathology, physiology, and gene expression in C3H/HeJ, C57BL/6J, and B6C3F1/J mice

Human cardiomyopathies often lead to heart failure, a major cause of morbidity and mortality in industrialized nations. Described here is a phenotypic characterization of cardiac function and genome-wide expression from C3H/HeJ, C57BL/6J, and B6C3F1/J male mice. Histopathologic analysis identified a low-grade background cardiomyopathy (murine progressive cardiomyopathy) in eight of nine male C3H/HeJ mice (age nine to ten weeks), but not in male C57BL/6J and in only of ten male B6C3F1/J mice. The C3H/HeJ mouse had an increased heart rate and a shorter RR interval compared to the B6C3F1/J and C57BL/6J mice. Cardiac genomic studies indicated the B6C3F1/J mice exhibited an intermediate gene expression phenotype relative to the 2 parental strains. Disease-centric enrichment analysis indicated a number of cardiomyopathy-associated genes were induced in B6C3F1/J and C3H/HeJ mice, including Myh7, My14, and Lmna and also indicated differential expression of genes associated with metabolic (e.g., Pdk2) and hypoxic stress (e.g. Hif1a). A novel coexpression and integrated pathway network analysis indicated Prkaa2, Pdk2, Rhoj, and Sgcb are likely to play a central role in the pathophysiology of murine progressive cardiomyopathy in C3H/HeJ mice. Our studies indicate that genetically determined baseline differences in cardiac phenotype have the potential to influence the results of cardiotoxicity studies.

The pivotal role of electron microscopic evaluation in investigation of the cardiotoxicity of bis(2-chloroethoxy)methane in rats and mice

Electron microscopy and light microscopy have been used to evaluate the cardiotoxicity of bis(2-chloroethoxy)methane (CEM) in F344/N rats and B6C3F1 mice. Rats received vehicle control or CEM at 50 mg/kg/day, and mice, vehicle control or CEM at doses up to 100 mg/kg/day, by oral gavage for up to sixteen days. Cardiotoxicity in rats at 50 mg/kg consisted of myocardial degeneration, including myocardial inflammation, myofiber vacuolation, and/or myofiber necrosis. There was no light microscopic evidence for cardiotoxicity in mice even at doses twice that of rats, but cardiotoxic damage was seen after electron microscopic evaluations including mitochondrial disintegration and vacuolation. Mice with mitochondrial damage may be more susceptible to subsequent cardiotoxic events and have a reduced capacity to respond when energy demands increase. Oral treatment of rats with CEM caused cardiotoxic lesions similar to those reported after dermal administration (Dunnick, Johnson, et al. 2004). The F344/N rat is more sensitive than the B6C3F1 mouse to the cardiotoxic effects of CEM.

Occult Cardiotoxicity—Toxic Effects on Cardiac Ischemic Tolerance

The outcome of cardiac ischemic events depends not only on the extent and duration of the ischemic stimulus but also on the myocardial intrinsic tolerance to ischemic injury. Cardiac ischemic tolerance reflects myocardial functional reserves that are not always used when the tissue is appropriately oxygenated. Ischemic tolerance is modulated by ubiquitous signal transduction pathways, transcription factors and cellular enzymes, converging on the mitochondria as the main end effector. Therefore, drugs and toxins affecting these pathways may impair cardiac ischemic tolerance without affecting myocardial integrity or function in oxygenated conditions. Such effect would not be detected by current toxicological studies but would considerably influence the outcome of ischemic events. The authors refer to such effect as “occult cardiotoxicity.” In this review, the authors summarize current knowledge about main mechanisms that determine cardiac ischemic tolerance, methods to assess it, and the effects of drugs and toxins on it. The authors offer a view that low cardiac ischemic tolerance is a premorbid status and, therefore, that occult cardiotoxicity is a significant potential source of cardiac morbidity. The authors propose that toxicologic assessment of compounds would include the assessment of their effect on cardiac ischemic tolerance.

Microarray analysis of differentially expressed genes in the kidneys and testes of mice after long-term irradiation with low-dose-rate gamma-rays

Measuring global gene expression using cDNA or oligonucleotide microarrays is an effective approach to understanding the complex mechanisms of the effects of radiation. However, few studies have been carried out that investigate gene expression in vivo after prolonged exposure to low-dose-rate radiation. In this study, C57BL/6J mice were continuously irradiated with gamma-rays for 485 days at dose-rates of 0.032-13 microGy/min. Gene expression profiles in the kidney and testis from irradiated and unirradiated mice were analyzed, and differentially expressed genes were identified. A combination of pathway analysis and hierarchical clustering of differentially expressed genes revealed that expression of genes involved in mitochondrial oxidative phosphorylation was elevated in the kidney after irradiation at the dose-rates of 0.65 microGy/min and 13 microGy/min. Expression of cell cycle-associated genes was not profoundly modulated in the kidney, in contrast to the response to acute irradiation, suggesting a threshold in the dose-rate for modulation of the expression of cell cycle-related genes in vivo following exposure to radiation. We demonstrated that changes to the gene expression profile in the testis were largely different from those in the kidney. The Gene Ontology categories "DNA metabolism", "response to DNA damage" and "DNA replication" overlapped significantly with the clusters of genes whose expression decreased with an increase in the dose-rate to the testis. These observations provide a fundamental insight into the organ-specific responses to low-dose-rate radiation.

Toxicogenomics: transcription profiling for toxicology assessment

Toxicogenomics, the application of transcription profiling to toxicology, has been widely used for elucidating the molecular and cellular actions of chemicals and other environmental stressors on biological systems, predicting toxicity before any functional damages, and classification of known or new toxicants based on signatures of gene expression. The success of a toxicogenomics study depends upon close collaboration among experts in different fields, including a toxicologist or biologist, a bioinformatician, statistician, physician and, sometimes, mathematician. This review is focused on toxicogenomics studies, including transcription profiling technology, experimental design, significant gene extraction, toxicological results interpretation, potential pathway identification, database input and the applications of toxicogenomics in various fields of toxicological study.

Temporal Gene Expression Profiling Indicates Early Up-regulation of Interleukin-6 in Isoproterenol-induced Myocardial Necrosis in Rat

Gene expression was evaluated in the myocardium of male Wistar rats after a single subcutaneous administration of 0.5 mg of isoproterenol, a β-adrenergic agonist that causes acute tachycardia with subsequent myocardial necrosis. Histology of the heart, clinical chemistry, and hematology were evaluated at 9 time points (0.5 hours to 14 days postinjection). Myocardial gene expression was evaluated at 4 time points (1 hour to 3 days). Contraction bands and loss of cross-striation were identified on phosphotungstic acid-hematoxylin-stained sections 0.5 hours postdosing. Plasma troponin I elevation was detected at 0.5 hours, peaked at 3 hours, and returned to baseline values at 3 days postdosing. Interleukin 6 (Il6) expression spiked at 1 to 3 hours and was followed by a short-lived, time-dependent dysregulation of its downstream targets. Concurrently and consistent with the kinetics of the histologic findings, many pathways indicative of necrosis/apoptosis (p38 mitogen-activated protein kinase [MAPK] signaling, NF-κB signaling) and adaptation to hypertension (PPAR signaling) were overrepresented at 3 hours. The 1-day and 3-day time points indicated an adaptive response, with down-regulation of the fatty acid metabolism pathway, up-regulation of the fetal gene program, and superimposed inflammation and repair at 3 days. These results suggest early involvement of Il6 in isoproterenol-induced myocardial necrosis and emphasize the value of early time points in transcriptomic studies.