Cytochrome P450 and GlutathioneS-Transferase mRNA Expression in Human Fetal Liver Hematopoietic Stem Cells

Developmental estrogen exposure in mice disrupts uterine epithelial cell differentiation and causes adenocarcinoma via Wnt/β-catenin and PI3K/AKT signaling

Tissue development entails genetically programmed differentiation of immature cell types to mature, fully differentiated cells. Exposure during development to non-mutagenic environmental factors can contribute to cancer risk, but the underlying mechanisms are not understood. We used a mouse model of endometrial adenocarcinoma that results from brief developmental exposure to an estrogenic chemical, diethylstilbestrol (DES), to determine causative factors. Single-cell RNA sequencing (scRNAseq) and spatial transcriptomics of adult control uteri revealed novel markers of uterine epithelial stem cells (EpSCs), identified distinct luminal and glandular progenitor cell (PC) populations, and defined glandular and luminal epithelium (LE) cell differentiation trajectories. Neonatal DES exposure disrupted uterine epithelial cell differentiation, resulting in a failure to generate an EpSC population or distinguishable glandular and luminal progenitors or mature cells. Instead, the DES-exposed epithelial cells were characterized by a single proliferating PC population and widespread activation of Wnt/β-catenin signaling. The underlying endometrial stromal cells had dramatic increases in inflammatory signaling pathways and oxidative stress. Together, these changes activated phosphoinositide 3-kinase/AKT serine-threonine kinase signaling and malignant transformation of cells that were marked by phospho-AKT and the cancer-associated protein olfactomedin 4. Here, we defined a mechanistic pathway from developmental exposure to an endocrine disrupting chemical to the development of adult-onset cancer. These findings provide an explanation for how human cancers, which are often associated with abnormal activation of PI3K/AKT signaling, could result from exposure to environmental insults during development.

The multifaceted role of glutathione S-transferases in cancer

Glutathione S-transferases (GSTs) are phase II detoxifying enzymes involved in the maintenance of cell integrity, oxidative stress and protection against DNA damage by catalyzing the conjugation of glutathione to a wide variety of electrophilic substrates. Though enzymes of the glutathione synthesis and salvage pathways have been well characterized in the past, there is still a lack of comprehensive understanding of their independent and coordinate regulatory mechanisms in carcinogenesis. The present review discusses implication of GST in cancer development and progression, gene polymorphism, drug resistance, signaling and epigenetic regulation involving their role in cancer. It is anticipated that GST especially the GSTP1 class can be developed as a biomarker either used alone or in combination with other biomarkers for early cancer detection and/or diagnosis as well as for future targeted preventive and therapeutic interventions with dietary agents.

Role of Chromatin Structural Changes in Regulating Human CYP3A Ontogeny

Variability in drug-metabolizing enzyme developmental trajectories contributes to interindividual differences in susceptibility to chemical toxicity and adverse drug reactions, particularly in the first years of life. Factors linked to these interindividual differences are largely unknown, but molecular mechanisms regulating ontogeny are likely involved. To evaluate chromatin structure dynamics as a likely contributing mechanism, age-dependent changes in modified and variant histone occupancy were evaluated within known CYP3A4 and 3A7 regulatory domains. Chromatin immunoprecipitation using fetal or postnatal human hepatocyte chromatin pools followed by quantitative polymerase chain reaction DNA amplification was used to determine relative chromatin occupancy by modified and variant histones. Chromatin structure representing a poised transcriptional state (bivalent chromatin), indicated by the occupancy by modified histones associated with both active and repressed transcription, was observed for CYP3A4 and most 3A7 regulatory regions in both postnatal and fetal livers. However, the CYP3A4 regulatory regions had significantly greater occupancy by modified histones associated with repressed transcription in the fetal liver. Conversely, some modified histones associated with active transcription exhibited greater occupancy in the postnatal liver. CYP3A7 regulatory regions also had significantly greater occupancy by modified histones associated with repressed transcription in the fetus. The observed occupancy by modified histones is consistent with chromatin structural dynamics contributing to CYP3A4 ontogeny, although the data are less conclusive regarding CYP3A7. Interpretation of the latter data may be confounded by cell-type heterogeneity in the fetal liver.

Dynamics of Cytosine Methylation in the Proximal Promoters of CYP3A4 and CYP3A7 in Pediatric and Prenatal Livers

Members of the human CYP3A family of metabolizing enzymes exhibit developmental changes in expression whereby CYP3A7 is expressed in fetal tissues, followed by a transition to expression of CYP3A4 in the first months of life. Despite knowledge about the general pattern of CYP3A activity in human development, the mechanisms that regulate developmental expression remain poorly understood. Epigenetic changes, including cytosine methylation, have been suggested to play a role in the regulation of CYP3A expression. The objective of this study was to investigate changes in cytosine methylation of the CYP3A4 and CYP3A7 genes in human pediatric and prenatal livers. The methylation status of cytosine-phospho-guanine dinucleotides was determined in 16 pediatric liver samples using methyl-seq and confirmed by bisulfite sequencing of 48 pediatric and 34 prenatal liver samples. Samples were separated by age into five groups (prenatal, < 1 year of age, 1.8-6 years, 7-11 years, and 12-17 years). Methyl-seq anaylsis revealed that cytosines in the proximal promoter of CYP3A7 are hypomethylated in neonates compared with adolescents (P < 0.001). In contrast, a cytosine 383 base pair upstream of CYP3A4 is hypermethylated in liver samples from neonates compared with adolescents (P = 0.00001). Developmental changes in methylation of cytosines in the proximal promoters of CYP3A4 and CYP3A7 in pediatric livers were confirmed by bisulfite sequencing. In addition, the methylation status of cytosine in the CYP3A4 and CYP3A7 proximal promoters correlated with changes in developmental expression of mRNA for the two enzymes.

Chlorpyrifos Induces MLL Translocations Through Caspase 3-Dependent Genomic Instability and Topoisomerase II Inhibition in Human Fetal Liver Hematopoietic Stem Cells

Household pesticide exposure during pregnancy has been associated with a more than 2-fold increased risk in infant leukemia, and chlorpyrifos (CPF) is among the most frequently applied insecticides. During early fetal development, liver is a hematopoietic organ with majority of cells being CD34(+) hematopoietic stem cells (CD34(+)HSC). The in utero injury to CD34(+)HSC has been known to underlie the pathogenesis of several blood disorders, often involving rearrangements of the mixed-lineage leukemia (MLL) gene on 11q23. In this study, we evaluated the leukemogenic potential of CPF in human fetal liver-derived CD34(+)HSC. Specifically, exposure to 10 μM CPF led to decrease in viability, inhibition in proliferation and induction of DNA double-strand breaks (DSBs) and occurrence of MLL(+) rearrangements. In particular, we observed CPF-mediated cell cycle disturbance as shown by G0/G1 arrest, in contrast to etoposide (VP-16), an anticancer drug used as a positive control and known to induce G2/M arrest. Further study on mechanisms underlying DNA DSBs and MLL(+) rearrangements revealed that CPF might act as topoisomerase II poison, a mechanism of action similar to VP-16. On the other hand, CPF was also shown to induce early apoptosis through active caspase-3 activation, a pathway known to underlie DNA DSBs and MLL(+) translocations. Our data indicate that in utero injury of CD34(+)HSC by CPF may contribute to the increased risk of infant leukemia. Future work will elucidate the mechanism and the type of CPF-induced MLL(+) translocations in HSC.

Monocrotophos induces the expression and activity of xenobiotic metabolizing enzymes in pre-sensitized cultured human brain cells

The expression and metabolic profile of cytochrome P450s (CYPs) is largely missing in human brain due to non-availability of brain tissue. We attempted to address the issue by using human brain neuronal (SH-SY5Y) and glial (U373-MG) cells. The expression and activity of CYP1A1, 2B6 and 2E1 were carried out in the cells exposed to CYP inducers viz., 3-methylcholanthrene (3-MC), cyclophosphamide (CPA), ethanol and known neurotoxicant- monocrotophos (MCP), a widely used organophosphorous pesticide. Both the cells show significant induction in the expression and CYP-specific activity against classical inducers and MCP. The induction level of CYPs was comparatively lower in MCP exposed cells than cells exposed to classical inducers. Pre-exposure (12 h) of cells to classical inducers significantly added the MCP induced CYPs expression and activity. The findings were concurrent with protein ligand docking studies, which show a significant modulatory capacity of MCP by strong interaction with CYP regulators-CAR, PXR and AHR. Similarly, the known CYP inducers- 3-MC, CPA and ethanol have also shown significantly high docking scores with all the three studied CYP regulators. The expression of CYPs in neuronal and glial cells has suggested their possible association with the endogenous physiology of the brain. The findings also suggest the xenobiotic metabolizing capabilities of these cells against MCP, if received a pre-sensitization to trigger the xenobiotic metabolizing machinery. MCP induced CYP-specific activity in neuronal cells could help in explaining its effect on neurotransmission, as these CYPs are known to involve in the synthesis/transport of the neurotransmitters. The induction of CYPs in glial cells is also of significance as these cells are thought to be involved in protecting the neurons from environmental insults and safeguard them from toxicity. The data provide better understanding of the metabolizing capability of the human brain cells against xenobiotics.

Hexabromocyclododecane concentrations in Canadian human fetal liver and placental tissues

Detectable concentrations of the flame retardant hexabromocyclododecane (HBCD) have been reported in human tissues worldwide, but investigations to determine fetal exposure to this brominated flame retardant are lacking. This study was undertaken to determine the concentrations of α-, β- and γ-HBCD in human tissues (fetal liver and placenta) from Canada. Tissue samples were collected over a thirteen year period following elective pregnancy terminations in Montreal, Quebec, Canada. Samples were extracted using homogenisation with solvent, cleaned up using adsorption chromatography and analysis was performed with liquid chromatography-tandem mass spectrometry. Total HBCD concentrations ranged from below the limit of detection (<LOD; <1 ng g(-1)) to 4500 ng g(-1) lipid in fetal livers and <LOD (<1 ng g(-1)) to 5600 ng g(-1) lipid in placental tissue. No clear temporal trend was established in liver samples, nor was a significant relationship observed between fetal age and ΣHBCD concentrations. Temporal differences, however, were noted in placental tissues before and after 2005. HBCD concentrations in liver:placental paired tissue samples did not show a correlation. HBCD was found in placental tissues from Canadian women and results indicate that HBCD was present, at measurable concentrations, in developing fetuses from as early as 6.5 weeks.

DNA adducts in aldehyde dehydrogenase-positive lung stem cells of A/J mice treated with the tobacco specific lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

Lung cancer is the leading cause of cancer death in the world. Evidence suggests that lung cancer could originate from mutations accumulating in a subpopulation of self-renewing cells, lung stem cells. Aldehyde dehydrogenase (ALDH) is a marker of stem cells. To investigate the presence of DNA modifications in these cells, we isolated ALDH-positive lung cells from A/J mice exposed to the lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. Using LC-NSI-HRMS/MS-PRM, O(6)-methyl-G, 7-POB-G, and O(2)-POB-dT were positively identified in ALDH-positive cell DNA. This is the first example of detection of carcinogen-DNA adducts in lung stem cells, supporting the hypothesis of their role in lung carcinogenesis.

Expression and inducibility of cytochrome P450s (CYP1A1, 2B6, 2E1, 3A4) in human cord blood CD34(+) stem cell-derived differentiating neuronal cells

The status of xenobiotic metabolism in developing human brain cells is not known. The reason is nonavailability of developing human fetal brain. We investigate the applicability of the plasticity potential of human umbilical cord blood stem cells for the purpose. Characterized hematopoietic stem cells are converted into neuronal subtypes in eight days. The expression and substrate-specific catalytic activity of the cytochrome P450s (CYPs) CYP1A1 and 3A4 increased gradually till day 8 of differentiation, whereas CYP2B6 and CYP2E1 showed highest expression and activity at day 4. There was no significant increase in the expression of CYP regulators, namely, aryl hydrocarbon receptor (AHR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), and glutathione-S-transferase (GSTP1-1) during differentiation. Differentiating cells showed significant induction in the expression of CYP1A1, 2B6, 2E1, 3A4, AHR, CAR, PXR, and GSTP1-1 when exposed to rifampin, a known universal inducer of CYPs. The xenobiotic-metabolizing capabilities of these differentiating cells were confirmed by exposing them to the organophosphate pesticide monocrotophos (MCP), a known developmental neurotoxicant, in the presence and absence of a universal inhibitor of CYPs-cimetidine. Early-differentiating cells (day 2) were found to be more vulnerable to xenobiotics than mature well-differentiated cells. For the first time, we report significant expression and catalytic activity of selected CYPs in human cord blood hematopoietic stem cell-derived neuronal cells at various stages of maturity. We also confirm significant induction in the expression and catalytic activity of selected CYPs in human cord blood stem cell-derived differentiating neuronal cells exposed to known CYP inducers and MCP.

Early lifestage exposure and potential developmental susceptibility to tetrachloroethylene

BACKGROUND

Tetrachloroethylene, also known as perchloroethylene or "perc", is a highly volatile and lipophilic solvent widely used in dry cleaning, textile processing, and metal-cleaning operations. The limited epidemiological and toxicological data available for exposure to perc during developmental lifestages, as well as the evidence for critical windows of exposure, highlight early life as a period of potential susceptibility.

METHODS

A literature search was performed to identify all peer-reviewed epidemiological and toxicologial studies examining outcomes from early lifestage exposure to perc, and reviewed by developmental stage for both exposure and outcome.

RESULTS

Exposure scenarios to perc unique to early lifestages include transplacental and breast milk intake, along with inhalation, ingestion, or dermal exposure. Toxicokinetics factors that may influence early lifestage susceptibility to perc, along with existing physiologically based pharmacokinetic (PBPK) models, are described. Adverse outcomes examined include: reproductive outcomes examined prior to conception including reduced fertility, adverse effects on sperm, or altered reproductive hormones; prenatal outcomes examined after exposure prior to conception or prenatally including fetal death, birth defects, and decreased birth weight; postnatal outcomes examined after exposure prior to conception, prenatally, or during childhood including neurotoxicity, immunotoxicity, cancer, hepatotoxicity, congential anomalies and mortality; and adult schizophrenia examined after exposure prior to conception.

CONCLUSIONS

The limited evidence on early lifestage exposure to perc does not provide sufficient evidence of this sensitive period as being more or less important than exposure at a later lifestage, such as during adulthood. However, there are a number of adverse health effects observed uniquely in early lifestages, and increased sensitivity to visual system deficits is suggested in children. Other outcomes observed in adults may not have been adequately assessed in children to directly compare sensitivity.

In vitro sensitivity of granulo-monocytic progenitors as a new toxicological cell system and endpoint in the ACuteTox Project

The ACuteTox Project (part of the EU 6th Framework Programme) was started up in January 2005. The aim of this project is to develop a simple and robust in vitro strategy for prediction of human acute systemic toxicity, which could replace animal tests used for regulatory purposes. Our group is responsible for the characterization of the effect of the reference chemicals on the hematopoietic tissue. CFU-GM assay based on the culture of human mononuclear cord blood cells has been used to characterize the effects of the selected compounds on the myeloid progenitors. Previous results have shown the relevance of the CFU-GM assay for the prediction of human acute neutropenia after treatment of antitumoral compounds, and this assay has been recently approved by the ECVAM's Scientific Advisory Committee. Among the compounds included in the study there were pharmaceuticals, environmental pollutants and industrial chemicals. Eleven out of 55 chemicals did not show any cytotoxic effect at the maximum concentration tested. The correlation coefficients of CFU-GM IC50, IC70 and IC90 values with human LC50 values (50% lethal concentration calculated from time-related sublethal and lethal human blood concentrations) were 0.4965, 0.5106 and 0.5142 respectively. Although this correlation is not improve respect to classical in vitro basal cytotoxicity tests such as 3T3 Neutral Red Uptake, chemicals which deviate substantially in the correlation with these assays (colchicine, digoxin, 5-Fluorouracil and thallium sulfate) fitted very well in the linear regression analysis of the CFU-GM progenitors. The results shown in the present study indicate that the sensitivity of CFU-GM progenitors correlates better than the sensitivity of HL-60 cells with human LC50 values and could help to refine the predictability for human acute systemic toxicity when a given chemical may affect to the hematopoietic myeloid system.

Expression quantitative trait loci mapping identifies new genetic models of glutathione S-transferase variation

Expression quantitative trait loci (eQTL) mapping can be used to identify the genetic variations that underlie inherited differences in gene transcription. We performed eQTL mapping by combining whole genome transcriptional data from the hypothalami of 33 strains of inbred mice with a detailed haplotype map of those same strains, revealing 10,655 trans associations and 31 cis eQTLs. One of the cis associations was found to be driven by strain-specific variation in the expression of Glutathione S-transferase, mu 5 (Gstm5). Gstm5 is one of seven members of the glutathione S-transferase, Mu family of genes. The glutathione S-transferases are phase II metabolic enzymes and are key regulators of drug and toxin clearance. In mouse, all seven family members are tightly clustered on mouse chromosome 3. Investigation of the Gstm5 cis association in multiple tissues types revealed that an 84-kilobase region on MMU3 acts as a haplotype-specific locus control region for the glutathione S-transferase, Mu cluster. In the strains that share the minor haplotype, drastic reductions in mRNA levels in multiple members of the Gst Mu family were observed. The strain-specific differences in Gst Mu transcription characterized here accurately model the human population, in which extreme variations in expression of GST Mu family members have been observed. Furthermore, the reduction in Gst Mu levels has important relevance for pharmacology and toxicology studies conducted in these strains. For instance, the reduced levels of Gst Mu in general and Gstm5 in particular have implications in models of dopamine metabolism, Parkinson's disease, and chemical neurotoxicity.

Differential expression of glutathione S-transferases P1-1 and A1-1 at protein and mRNA levels in hepatocytes derived from human bone marrow mesenchymal stem cells

The aim of this study was to find out the profile of cellular glutathione (GSH) and GSH S-transferase (GST) in hepatocytes differentiated from adult mesenchymal stem cells (MSC). For this purpose, we have derived functionally active hepatocyte-like cells from normal human multipotent adult MSC. Then the differentiated cells were characterized by specific hepatic markers. The cellular GSH and GST catalytic activity toward 1-chloro-2,4-dinitrobenzene (CDNB) were determined in hepatocyte-like cells differentiated from MSC compared with undifferentiated MSC. Reverse transcription polymerase chain reaction (RT-PCR) and immunoblotting techniques were used to study GST-P1-1 and GST-A1-1 expression in differentiated and undifferentiated cells. The results showed that there is more than threefold increase in GST catalytic activity in hepatocytes recovered by day 14 of differentiation. GST-P1-1 mRNA expression was detected in both differentiated hepatocyte-like cells and their undifferentiated progenitors. Under similar conditions, only differentiated hepatocyte-like cells expressed GST-A1-1 mRNA. These results were further confirmed by showing that the undifferentiated cells expressed both GST-A and GST-P proteins. Unlike GST, the level of cellular GSH was declined (approximately 20%) in hepatocytes derived from MSC as compared to that of undifferentiated cells. These data may suggest that hepatogenic differentiation of human bone marrow MSC is accompanied with the regulation of factors participating in GSH conjugation pathway.

The ontogeny of drug metabolism enzymes and implications for adverse drug events

Profound changes in drug metabolizing enzyme (DME) expression occurs during development that impacts the risk of adverse drug events in the fetus and child. A review of our current knowledge suggests individual hepatic DME ontogeny can be categorized into one of three groups. Some enzymes, e.g., CYP3A7, are expressed at their highest level during the first trimester and either remain at high concentrations or decrease during gestation, but are silenced or expressed at low levels within one to two years after birth. SULT1A1 is an example of the second group of DME. These enzymes are expressed at relatively constant levels throughout gestation and minimal changes are observed postnatally. ADH1C is typical of the third DME group that are not expressed or are expressed at low levels in the fetus, usually during the second or third trimester. Substantial increases in enzyme levels are observed within the first one to two years after birth. Combined with our knowledge of other physiological factors during early life stages, knowledge regarding DME ontogeny has permitted the development of robust physiological based pharmacokinetic models and an improved capability to predict drug disposition in pediatric patients. This review will provide an overview of DME developmental expression patterns and discuss some implications of the data with regards to drug therapy. Common themes emerging from our current knowledge also will be discussed. Finally, the review will highlight gaps in knowledge that will be important to advance this field.

The role of mitochondrial and oxidative injury in BDE 47 toxicity to human fetal liver hematopoietic stem cells

The polybrominated diphenyl ethers (PBDEs) are a group of flame retardants whose residues have markedly increased in the environment and in human tissues during the last decade. Of the various congeners, BDE 47 (2,2',4,4'-tetrabromodiphenyl ether) is typically the predominant congener observed in fish and wildlife samples, as well as in human tissues. Several studies indicate in utero transfer of PBDEs during pregnancy with residues accumulating in fetal tissues, and thus the potential for BDE 47-mediated injury in utero is of concern. In this study, we examined the mechanisms of BDE 47-mediated injury to primary human fetal liver hematopoietic stem cells (HSCs), which comprise a large proportion of fetal hepatic cells and play a key role in hematopoiesis during fetal development. Incubation of fetal liver HSCs with BDE 47 led to a loss of mitochondrial membrane potential and the onset of apoptosis. These effects were observed in the low micromolar range of BDE 47 exposures. At higher concentrations, BDE 47 elicited a loss of viability, which was accompanied by the generation of reactive oxygen species and peroxidation of HSC lipids. Preincubation of fetal liver HSCs with N-acetylcysteine, a glutathione (GSH) precursor, caused an increase in cellular GSH concentrations, restored mitochondrial redox status, and ameliorated the toxicity of BDE 47. BDE 47-mediated cytotoxicity or oxidative injury was not evident at the lower concentrations (< 1microM). Collectively, these data support a role for oxidative stress in the cytotoxicity of BDE 47 and indicate that oxidative stress-associated biomarkers may be useful in assessing the sublethal effects of BDE 47 toxicity in other models. However, the fact that BDE 47 undergoes a concentration-dependent accumulation in other primary cells in media that can underestimate cellular concentrations (W. R. Mundy et al., 2004, Toxicol. Sci. 82, 164-169) suggests that the HSC cell injury observed in our study may be of less relevance to human in utero PBDE exposures.