Kim Boekelheide

TOXICITY TESTING IN THE 21ST CENTURY: A VISION AND A STRATEGY

With the release of the landmark report Toxicity Testing in the 21st Century: A Vision and a Strategy, the U.S. National Academy of Sciences, in 2007, precipitated a major change in the way toxicity testing is conducted. It envisions increased efficiency in toxicity testing and decreased animal usage by transitioning from current expensive and lengthy in vivo testing with qualitative endpoints to in vitro toxicity pathway assays on human cells or cell lines using robotic high-throughput screening with mechanistic quantitative parameters. Risk assessment in the exposed human population would focus on avoiding significant perturbations in these toxicity pathways. Computational systems biology models would be implemented to determine the dose-response models of perturbations of pathway function. Extrapolation of in vitro results to in vivo human blood and tissue concentrations would be based on pharmacokinetic models for the given exposure condition. This practice would enhance human relevance of test results, and would cover several test agents, compared to traditional toxicological testing strategies. As all the tools that are necessary to implement the vision are currently available or in an advanced stage of development, the key prerequisites to achieving this paradigm shift are a commitment to change in the scientific community, which could be facilitated by a broad discussion of the vision, and obtaining necessary resources to enhance current knowledge of pathway perturbations and pathway assays in humans and to implement computational systems biology models. Implementation of these strategies would result in a new toxicity testing paradigm firmly based on human biology.

The Mammalian SIR2α Protein Has a Role in Embryogenesis and Gametogenesis

ABSTRACT The yeast Sir2p protein has an essential role in maintaining telomeric and mating type genes in their transcriptionally inactive state. Mammalian cells have a very large proportion of their genome inactive and also contain seven genes that have regions of homology with the yeast sir2 gene. One of these mammalian genes, sir2α, is the presumptive mammalian homologue of the yeast sir2 gene. We set out to determine if sir2α plays a role in mammalian gene silencing by creating a strain of mice carrying a null allele of sir2α. Animals carrying two null alleles of sir2α were smaller than normal at birth, and most died during the early postnatal period. In an outbred background, the sir2α null animals often survived to adulthood, but both sexes were sterile. We found no evidence for failure of gene silencing in sir2α null animals, suggesting that either SIR2α has a different role in mammals than it does in Saccharomyces cerevisiae or that its role in gene silencing in confined to a small subset of mammalian genes. The phenotype of the sir2α null animals suggests that the SIR2α protein is essential for normal embryogenesis and for normal reproduction in both sexes.

NTP-CERHR expert panel report on the reproductive and developmental toxicity of bisphenol A.

Robert E. Chapin, Jane Adams, Kim Boekelheide, L. Earl Gray Jr, Simon W. Hayward, Peter S.J. Lees, Barry S. McIntyre, Kenneth M. Portier, Teresa M. Schnorr, Sherry G. Selevan, John G. Vandenbergh, and Susan R. Woskie Pfizer, Inc., Groton, CT University of Massachusetts, Boston, MA Brown University, Providence, RI U.S. Environmental Protection Agency, Research Triangle Park, NC Vanderbilt University Medical Center, Nashville, TN Johns Hopkins University, Baltimore, MD Schering Plough Research Institute, Summit, NJ American Cancer Society, Atlanta, GA National Institute for Occupational Safety and Health, Cincinnati, OH U.S. Public Health Service (Ret), Silver Spring, MD North Carolina State University, Raleigh, NC University of Massachusetts, Lowell, MA

Sensitivity of Testicular Germ Cells to Toxicant-Induced Apoptosis in gld Mice That Express a Nonfunctional Form of Fas Ligand

Germ cell apoptosis in testis is essential for functional spermatogenesis. Recent evidence suggests that the Fas signaling system is critical for the regulation of testicular germ cell apoptosis. To further evaluate the Fas signaling system in testis, we examined the incidence of germ cell apoptosis in gld mice that lack a functional Fas-signaling pathway. gld mice have a small, but significant, increase in testis weight and numbers of spermatid heads per testis compared with wild-type mice. In addition, gld mice have a small increase in the spontaneous incidence of germ cell apoptosis, as indicated by characteristic DNA fragmentation via the terminal deoxxynucleotidyltransferase-mediated deoxy-UTP nick end labeling assay. To test the role of the Fas system in toxicant-induced germ cell apoptosis, mice were exposed to either a Sertoli cell- or germ cell-specific toxicant[ mono-(2-ethylhexyl)phthalate (MEHP; 1 g/kg) or 5 Gy radiation, respectively]. These two exposure paradigms induced extensive increases ...

Studies of the molecular pathogenesis of hexane neuropathy: II. Evidence that pyrrole derivatization of lysyl residues leads to protein crosslinking☆☆☆

Abstract In the reaction between ethanolamine and 2,5-hexanedione, 1-(2-hydroxyethyl)-2,5-dimethylpyrrole was formed, and the pyrrole was found to autoxidize to form an orange chromophore. Similar orange chromophores were observed in the reaction of 2,5-hexanedione, 2,5-heptanedione, and 3,6-octanedione with a variety of primary amines and with proteins. The development of the orange chromophore in the reaction of 2,5-hexanedione with proteins was attended by a proportional derivatization of lysyl residues and by extensive intramolecular and intermolecular crosslinking. These observations suggest that the sequence of events in the crosslinking of neurofilaments during chronic n -hexane intoxication may be metabolism to 2,5-hexanedione, formation of an imine with lysyl residues, cyclization to form a pyrrole, autoxidation of the pyrrole, and finally covalent crosslinking involving pyrrole rings.

Toxicity Testing in the 21st Century: Defining New Risk Assessment Approaches Based on Perturbation of Intracellular Toxicity Pathways

The approaches to quantitatively assessing the health risks of chemical exposure have not changed appreciably in the past 50 to 80 years, the focus remaining on high-dose studies that measure adverse outcomes in homogeneous animal populations. This expensive, low-throughput approach relies on conservative extrapolations to relate animal studies to much lower-dose human exposures and is of questionable relevance to predicting risks to humans at their typical low exposures. It makes little use of a mechanistic understanding of the mode of action by which chemicals perturb biological processes in human cells and tissues. An alternative vision, proposed by the U.S. National Research Council (NRC) report Toxicity Testing in the 21st Century: A Vision and a Strategy, called for moving away from traditional high-dose animal studies to an approach based on perturbation of cellular responses using well-designed in vitro assays. Central to this vision are (a) “toxicity pathways” (the innate cellular pathways that may be perturbed by chemicals) and (b) the determination of chemical concentration ranges where those perturbations are likely to be excessive, thereby leading to adverse health effects if present for a prolonged duration in an intact organism. In this paper we briefly review the original NRC report and responses to that report over the past 3 years, and discuss how the change in testing might be achieved in the U.S. and in the European Union (EU). EU initiatives in developing alternatives to animal testing of cosmetic ingredients have run very much in parallel with the NRC report. Moving from current practice to the NRC vision would require using prototype toxicity pathways to develop case studies showing the new vision in action. In this vein, we also discuss how the proposed strategy for toxicity testing might be applied to the toxicity pathways associated with DNA damage and repair.

Exposure In Utero to Di(n-Butyl) Phthalate Alters the Vimentin Cytoskeleton of Fetal Rat Sertoli Cells and Disrupts Sertoli Cell-Gonocyte Contact

Abstract Di(n-butyl) phthalate (DBP) is commonly used in personal care products and as a plasticizer to soften consumer plastic products. Male rats exposed to DBP in utero have malformations of the male reproductive tract and testicular atrophy characterized by degeneration of seminiferous epithelium and decreased sperm production. In the fetal testis, in utero exposure to DBP reportedly resulted in reduced testosterone levels, Leydig cell aggregates, and multinucleated gonocytes (MNG). We investigated whether exposure in utero to DBP affects rat fetal Sertoli cells and compromises interactions between Sertoli and germ cells in the developing testis. Histological examination showed that MNG occurred at low frequency in the normal fetal rat testis. Exposure in utero at the dose level of DBP above estimated environmental or occupational human exposure levels significantly increased the number of these abnormal germ cells. Postnatally, MNG exhibited aberrant mitoses and were detected at the basal lamina. MNG were not apoptotic in the fetal and postnatal rat testes, as indicated by TUNEL. Sertoli cells in DBP-exposed fetal testis had retracted apical processes, altered organization of the vimentin cytoskeleton, and abnormal cell-cell contacts with gonocytes. The effect of DBP on Sertoli cell morphology at the level of light microscopy was reversed after birth and cessation of exposure. Our data indicate that fetal Sertoli cells are targeted by exposure in utero to DBP and suggest that abnormal interactions between Sertoli and germ cells during fetal life play a role in the development of MNG.

Predicting later-life outcomes of early-life exposures.

Background: In utero exposure of the fetus to a stressor can lead to disease in later life. Epigenetic mechanisms are likely mediators of later-life expression of early-life events. Objectives: We examined the current state of understanding of later-life diseases resulting from early-life exposures in order to identify in utero and postnatal indicators of later-life diseases, develop an agenda for future research, and consider the risk assessment implications of this emerging knowledge. Methods: This review was developed based on our participation in a National Research Council workshop titled “Use of in Utero and Postnatal Indicators to Predict Health Outcomes Later in Life: State of the Science and Research Recommendations.” We used a case study approach to highlight the later-life consequences of early-life malnutrition and arsenic exposure. Discussion: The environmental sensitivity of the epigenome is viewed as an adaptive mechanism by which the developing organism adjusts its metabolic and homeostatic systems to suit the anticipated extrauterine environment. Inappropriate adaptation may produce a mismatch resulting in subsequent increased susceptibility to disease. A nutritional mismatch between the prenatal and postnatal environments, or early-life obesogen exposure, may explain at least some of the recent rapid increases in the rates of obesity, type 2 diabetes, and cardiovascular diseases. Early-life arsenic exposure is also associated with later-life diseases, including cardiovascular disease and cancer. Conclusions: With mounting evidence connecting early-life exposures and later-life disease, new strategies are needed to incorporate this emerging knowledge into health protective practices.

Sertoli Cell Toxicants

This chapter discusses toxicants that specifically target the Sertoli cell. The term toxicant encompasses any toxic agent of natural, biological, or synthetic origin. The accurate designation of the Sertoli cell as the target of a testicular toxicant depends on the quantity and quality of available information. Theoretically, any testicular toxicant may produce subtle alterations in germ cell function that trigger secondary manifestations of Sertoli cell dysfunction as the earliest observed sign of testicular injury. Such an unrecognized germ cell toxicant is considered a Sertoli cell toxicant until the knowledge base is improved by further experimentation. Alternatively, any testicular toxicant producing subtle, undetected alterations in Sertoli cell function followed by germ cell abnormalities as the earliest sign of testicular injury is incorrectly classified as a germ cell toxicant. The frequency of incorrect assignment of toxicant to target cell is unknown; however, given the central role of the Sertoli cell in germ cell maintenance, one would predict that cryptic Sertoli cell toxicants are more often misidentified as germ cell toxicants than vice versa.

Fas Is Involved in the p53-Dependent Apoptotic Response to Ionizing Radiation in Mouse Testis

Abstract Apoptosis induced in male germ cells following ionizing radiation is dependent on functional p53 (Trp53) being present. We sought to determine whether Fas (Tnfrsf6/CD95/APO-1), an apoptotic factor, is involved in this p53-dependent germ cell death. In p53 knock-out mice exposed to 5 Gy of x-radiation, germ cells were protected from cell death, as assessed by counting apoptotic seminiferous tubules 12 h following radiation. Similarly, spermatid head counts in p53 knock-out mice remained near normal 29 days after exposure to 0.5 Gy of radiation, whereas wild-type animals had a more than twofold reduction in spermatid head counts. Fas mRNA expression remained at pretreatment levels in p53 knock-out mice; however, Fas increased in a time-dependent manner in wild-type mice following exposure to 5 Gy of radiation, indicating that radiation-induced Fas expression is p53-dependent. The functional significance of Fas involvement was demonstrated when lprcg mice, having a nonfunctional Fas receptor, were exposed to 5 Gy of radiation; the number of apoptotic seminiferous tubules 12 h following radiation was significantly reduced compared to that of wild-type mice. Additionally, lprcg mice exposed to 0.5 Gy of radiation had increased spermatid head counts 29 days following radiation compared to wild-type mice. Interestingly, gld mice with a non-functional Fas ligand (Tnfsf6/FasL/CD95L) were as sensitive to radiation as wild-type animals, and levels of FasL mRNA were not affected by radiation treatment. These results indicate that apoptosis and up-regulation of Fas following radiation are both p53-dependent events. Although Fas is necessary, in part, for radiation-induced p53-dependent apoptosis, FasL is not.