Assessment of the genotoxicity of tert-butyl alcohol in an in vivo thyroid comet assay.

Chronic exposure to high (20,000 ppm) concentrations of tert-butyl alcohol (TBA) in drinking water, equivalent to ~2100 mg/kg bodyweight per day, is associated with slight increases in the incidence of thyroid follicular cell adenomas and carcinomas in mice, with no other indications of carcinogenicity. In a recent toxicological review of TBA, the U.S. EPA determined that the genotoxic potential of TBA was inconclusive, largely based on non-standard studies such as in vitro comet assays. As such, the potential role of genotoxicity in the mode of action of thyroid tumors and therefore human relevance was considered uncertain. To address the potential role of genotoxicity in TBA-associated thyroid tumor formation, CD-1 mice were exposed up to a maximum tolerated dose of 1500 mg/kg-day via oral gavage for two consecutive days and DNA damage was assessed with the comet assay in the thyroid. Blood TBA levels were analyzed by headspace GC-MS to confirm systemic tissue exposure. At study termination, no significant increases (DNA breakage) or decreases (DNA crosslinks) in %DNA tail were observed in TBA exposed mice. In contrast, oral gavage of the positive control ethyl methanesulfonate significantly increased %DNA tail in the thyroid. These findings are consistent with most genotoxicity studies on TBA and provide mechanistic support for non-linear, threshold toxicity criteria for TBA. While the mode of action for the thyroid tumors remains unclear, linear low dose extrapolation methods for TBA appear more a matter of policy than science.

Comparison of transcriptomic profiles between HFPO-DA and prototypical PPARα, PPARγ, and cytotoxic agents in mouse, rat, and pooled human hepatocytes.

Like many per- or polyfluorinated alkyl substances (PFAS), toxicity studies with HFPO-DA (ammonium, 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)-propanoate), a short-chain PFAS used in the manufacture of some types of fluorinated polymers, indicate that the liver is the primary target of toxicity in rodents following oral exposure. Although the current weight of evidence supports the PPARα mode of action (MOA) for liver effects in HFPO-DA-exposed mice, alternate MOAs have also been hypothesized including PPARγ or cytotoxicity. To further evaluate the MOA for HFPO-DA in rodent liver, transcriptomic analyses were conducted on samples from primary mouse, rat and pooled human hepatocytes treated for 12, 24 or 72 hours with various concentrations of HFPO-DA, or agonists of PPARα (GW7647), PPARγ (rosiglitazone), or cytotoxic agents (ie, acetaminophen or d-galactosamine). Concordance analyses of enriched pathways across chemicals within each species demonstrated greatest concordance between HFPO-DA and PPARα agonist GW7647-treated hepatocytes compared to the other chemicals evaluated. These findings were supported by benchmark concentration modeling and predicted upstream regulator results. In addition, transcriptomic analyses across species demonstrated a greater transcriptomic response in rodent hepatocytes treated with HFPO-DA or agonists of PPARα or PPARγ, indicating rodent hepatocytes are more sensitive to HFPO-DA or PPARα/γ agonist treatment. These results are consistent with previously published transcriptomic analyses and further support that liver effects in HFPO-DA-exposed rodents are mediated through rodent-specific PPARα signaling mechanisms as part of the MOA for PPARα activator-induced rodent hepatocarcinogenesis. Thus, effects observed in mouse liver are not appropriate endpoints for toxicity value development for HFPO-DA in human health risk assessment.

Comparison of transcriptomic profiles between HFPO-DA and prototypical PPARα, PPARγ, and cytotoxic agents in wild-type and PPARα knockout mouse hepatocytes.

Recent in vitro transcriptomic analyses for the short-chain polyfluoroalkyl substance (PFAS), HFPO-DA (ammonium, 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)-propanoate), support conclusions from in vivo data that HFPO-DA-mediated liver effects in mice are part of the early key events of the peroxisome proliferator-activated receptor alpha (PPARα) activator-induced rodent hepatocarcinogenesis mode of action (MOA). Transcriptomic responses in HFPO-DA-treated rodent hepatocytes have high concordance with those treated with a PPARα agonist and lack concordance with those treated with PPARγ agonists or cytotoxic agents. To elucidate whether HFPO-DA-mediated transcriptomic responses in mouse liver are PPARα-dependent, additional transcriptomic analyses were conducted on samples from primary PPARα knockout (KO) and wild-type (WT) mouse hepatocytes exposed for 12, 24 or 72 hours with various concentrations of HFPO-DA, or well-established agonists of PPARα (GW7647) and PPARγ (rosiglitazone), or cytotoxic agents (acetaminophen or d-galactosamine). Pathway and predicted upstream regulator-level responses were highly concordant between HFPO-DA and GW7647 in WT hepatocytes. A similar pattern was observed in PPARα KO hepatocytes, albeit with a distinct temporal and concentration-dependent delay potentially mediated by compensatory responses. This delay was not observed in PPARα KO hepatocytes exposed to rosiglitazone, acetaminophen, d-galactosamine. The similarity in transcriptomic signaling between HFPO-DA and GW7647 in both the presence and absence of PPARα in vitro indicates these compounds share a common MOA.

Assessment of the in vivo genotoxic potential of three smoke flavoring primary product mixtures.

Smoke flavorings are mixtures generated from wood pyrolysis that are filtered to remove tar and are often considered healthier alternatives to conventional smoking processes. While the latter is mostly unregulated, smoke-flavoring primary products (SFPPs) are undergoing the 10-year required re-evaluation in the European Union (EU). To comply with recent smoke flavor guidance, in vivo micronucleus studies in rats and transgenic rodent (TGR) mutation assays in Muta™Mice were conducted on three SFPPs. For most studies, typical limit doses were exceeded to comply with regulatory requests. Exposure to SFPPs by oral gavage did not result in significant increases in bone marrow micronucleus formation. Except for one group, exposure to SFPPs via feed for 28 days did not result in significant increases in mutant frequency (MF) in the glandular stomach or liver. One group exposed to a maximal feasible dietary dose of 50,000 ppm (>10,000 mg/kg bodyweight per day) exhibited a statistically significant increase in liver MF; however, the MF in all mice in this group were within the historical vehicle control 95% quantile confidence intervals and therefore not considered biologically relevant. Based on estimates of human dietary exposure to each SFPP, the margin of exposure (MOE) values in the TGR assays exceed 10,000. The MOE for one unintentionally present constituent, 2,5(H)-furanone, also exceeds 10,000. Collectively, these data indicate that these SFPPs pose no genotoxic risk and are safe alternatives to conventional smoking.

Derivation of oral cancer slope factors for hexavalent chromium informed by pharmacokinetic models and in vivo genotoxicity data.

Hexavalent chromium [Cr(VI)] is present in drinking water from natural and anthropogenic sources at approximately 1 ppb. Several regulatory bodies have recently developed threshold-based safety criteria for Cr(VI) of 30-100 ppb based on evidence that small intestine tumors in mice following exposure to ≥20,000 ppb are the result of a non-mutagenic mode of action (MOA). In contrast, U.S. EPA has recently concluded that Cr(VI) acts through a mutagenic MOA based, in part, on scoring numerous in vivo genotoxicity studies as having low confidence; and therefore derived a cancer slope factor (CSF) of 0.5 (mg/kg-day)-1, equivalent to ∼0.07 ppb. Herein, we demonstrate how physiologically based pharmacokinetic (PBPK) models and intestinal segment-specific tumor incidence data can form a robust dataset supporting derivation of alternative CSF values that equate to Cr(VI) concentrations ranging from below background to concentrations similar to those derived using threshold approaches-depending on benchmark response level and risk tolerance. Additionally, we highlight weaknesses in the rationale EPA used to discount critical in vivo genotoxicity studies. While the data support a non-genotoxic MOA, these alternative toxicity criteria require only PBPK models, robust tumor data, and fair interpretation of published in vivo genotoxicity data for Cr(VI).

A putative adverse outcome network for neonatal mortality and lower birth weight in rodents: Applicability to per- and polyfluoroalkyl substances and relevance to human health.

BACKGROUND: Some per- and poly-fluoroalkyl substances (PFAS) cause neonatal mortality and lower birth weight in rodents. We constructed an Adverse Outcome Pathway (AOP) network for neonatal mortality and lower birth weight in rodents, comprising three putative AOPs. We then assessed strengths of the evidence for the AOPs and applicability to PFAS. Finally, we considered the relevance of this AOP network to human health. METHODS: Literature searches targeted PFAS, peroxisome proliferator-activated receptor (PPAR) agonists, other nuclear receptors, relevant tissues, and developmental targets. We used reviews of established biology and described results of studies with prenatal PFAS exposure that assessed birth weight and neonatal survival. Molecular initiating events (MIEs) and key events (KEs) were proposed and strengths of KE relationships (KERs), applicability to PFAS, and human relevance were assessed. RESULTS: Neonatal mortality has been observed in rodents following gestational exposure to most longer chain PFAS studied, often coincident with lower birth weight. In AOP 1, PPARα activation and PPARγ activation or downregulation are MIEs; placental insufficiency, fetal nutrient restriction, neonatal hepatic glycogen deficit, and hypoglycemia are KEs leading to neonatal mortality and lower birth weight. In AOP 2, constitutive androstane receptor (CAR) and pregnane X receptor (PXR) activation upregulates Phase II metabolism, lowering maternal circulating thyroid hormones. In AOP 3, disrupted pulmonary surfactant function and PPARγ downregulation cause neonatal airway collapse and mortality from respiratory failure. CONCLUSIONS: It is likely that different components of this AOP network will apply to different PFAS, largely determined by which nuclear receptors they activate. The MIEs and KEs in this AOP network can occur in humans, but differences in PPAR structure and function, and the timeline of liver and lung development, suggest that humans may be less susceptible to this AOP network. This putative AOP network elucidates knowledge gaps and research needed to better understand the developmental toxicity of PFAS.

Assessment of Mouse Liver Histopathology Following Exposure to HFPO-DA With Emphasis on Understanding Mechanisms of Hepatocellular Death.

Ammonium 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)-propanoate (HFPO-DA) is a short chain member of per- and polyfluoroalkyl substances (PFAS). To better understand the relevance of histopathological effects seen in livers of mice exposed to HFPO-DA for human health risk assessment, histopathological effects were summarized from hematoxylin and eosin (H&E)-stained sections in several repeat-dose toxicity studies in mice. Findings across studies revealed histopathological changes consistent with peroxisomal proliferation, whereas two reports of steatosis could not be confirmed in the published figures. In addition, mechanisms of hepatocellular death were assessed in H&E sections as well as with the apoptotic marker cleaved caspase-3 (CCasp3) in newly cut sections from archived liver blocks from select studies. A comparison of serially CCasp3 immunolabeled and H&E-stained sections revealed that mechanisms of hepatocellular death cannot be clearly discerned in H&E-stained liver sections alone as several examples of putatively necrotic cells were positive for CCasp3. Published whole genome transcriptomic data were also reevaluated for enrichment of various forms of hepatocellular death in response to HFPO-DA, which revealed enrichment of apoptosis and autophagy, but not ferroptosis, pyroptosis, or necroptosis. These morphological and molecular findings are consistent with transcriptomic evidence for peroxisome proliferator-activated receptor alpha (PPARα) signaling in HFPO-DA exposed mice.

Assessment of the mode of action underlying development of liver lesions in mice following oral exposure to HFPO-DA and relevance to humans.

HFPO-DA (ammonium, 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)propanoate) is a short-chain polyfluorinated alkyl substance (PFAS) used in the manufacture of some types of fluorinated polymers. Like many PFAS, toxicity studies with HFPO-DA indicate the liver is the primary target of toxicity in rodents following oral exposure. Due to the structural diversity of PFAS, the mode of action (MOA) can differ between PFAS for the same target tissue. There is significant evidence for involvement of peroxisome proliferator-activated receptor alpha (PPARα) activation based on molecular and histopathological responses in the liver following HFPO-DA exposure, but other MOAs have also been hypothesized based on limited evidence. The MOA underlying the liver effects in mice exposed to HFPO-DA was assessed in the context of the Key Events (KEs) outlined in the MOA framework for PPARα activator-induced rodent hepatocarcinogenesis. The first 3 KEs (ie, PPARα activation, alteration of cell growth pathways, and perturbation of cell growth/survival) are supported by several lines of evidence from both in vitro and in vivo data available for HFPO-DA. In contrast, alternate MOAs, including cytotoxicity, PPARγ and mitochondrial dysfunction are generally not supported by the scientific literature. HFPO-DA-mediated liver effects in mice are not expected in humans as only KE 1, PPARα activation, is shared across species. PPARα-mediated gene expression in humans produces only a subset (ie, lipid modulating effects) of the responses observed in rodents. As such, the adverse effects observed in rodent livers should not be used as the basis of toxicity values for HFPO-DA for purposes of human health risk assessment.

Evaluation of Transcriptomic Responses in Livers of Mice Exposed to the Short-Chain PFAS Compound HFPO-DA.

HFPO-DA (ammonium 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)-propanoate; CASRN 62037-80-3) is a component of the GenX technology platform used as a polymerization aid in the manufacture of some types of fluoropolymers. The liver is the primary target of toxicity for HFPO-DA in rodents and previous examination of hepatic transcriptomic responses in mice following oral exposure to HFPO-DA for 90 days showed induction of peroxisome proliferator-activated receptor signaling pathways, predominantly by PPARα, as well as increased gene expression of both peroxisomal and mitochondrial fatty acid metabolism. To further investigate the mechanism of liver toxicity, transcriptomic analysis was conducted on liver tissue from mice orally exposed to 0, 0.1, 0.5 or 5 mg/kg-bw/day HFPO-DA in a reproduction/developmental toxicity study. Hepatic gene expression changes demonstrated activation of the PPARα signaling pathway. Peroxisomal and mitochondrial fatty acid ß-oxidation gene sets were enriched at lower HFPO-DA concentrations, and complement cascade, cell cycle and apoptosis related gene sets were enriched at higher HFPO-DA concentrations. These results support the reported histopathological findings in livers of mice from this study and indicate that the effects of HFPO-DA are mediated through rodent-specific PPARα signaling mechanisms regardless of reproductive status in mice.

Crypt and Villus Transcriptomic Responses in Mouse Small Intestine Following Oral Exposure to Hexavalent Chromium.

Oral exposure to hexavalent chromium (Cr(VI)) induces tumors in the mouse duodenum. Previous microarray-based transcriptomic analyses of homogenized mouse duodenal tissue have demonstrated Cr(VI)-induced alterations in various cellular pathways and processes. However, X-ray fluorescence microscopy indicates that chromium localizes primarily to the duodenal villi following exposure to Cr(VI), suggesting that previous transcriptomic analyses of homogenized tissue provide an incomplete picture of transcriptomic responses in the duodenum. Herein, transcriptomic analyses were conducted separately on crypt and villus tissue from formalin-fixed paraffin-embedded transverse duodenal sections from the same study in which microarray-based analyses were previously conducted. A total of 28 groups (7 doses × 2 timepoints × 2 tissue compartments) were analyzed for differential gene expression, dose-response, and gene set enrichment. Tissue compartment isolation was confirmed by differences in expression of typical markers of crypt and villus compartments. Fewer than 21 genes were altered in the crypt compartment of mice exposed to 0.1-5 ppm Cr(VI) for 7 or 90 days, which increased to hundreds or thousands of genes at ≥20 ppm Cr(VI). Consistent with histological evidence for crypt proliferation, a significant, dose-dependent increase in genes that regulate mitotic cell cycle was prominent in the crypt, while subtle in the villus, when compared with samples from time-matched controls. Minimal transcriptomic evidence of DNA damage response in either the crypts or the villi is consistent with published in vivo genotoxicity data. These results are also discussed in the context of modes of action that have been proposed for Cr(VI)-induced small intestine tumors in mice.

Development of updated RfD and RfC values for medium carbon range aromatic and aliphatic total petroleum hydrocarbon fractions.

Using total petroleum hydrocarbon (TPH) measurements as a tool for assessing potential human health risks associated with exposures to petroleum products in the environment poses unique challenges, as TPH represents highly variable and complex mixtures containing hundreds of individual chemicals with wide-ranging chemical and physical properties. Current risk assessment practice generally involves analysis of environmental samples for various TPH fractions and summation of risk across those fractions. The United States Environmental Protection Agency (USEPA) derived provisional toxicity criteria for low, medium, and high carbon range aromatic and aliphatic hydrocarbon fractions over a decade ago. These criteria have been used, in whole or in part, to derive risk-based cleanup levels for TPH contamination in soil and groundwater. Herein, we evaluate and update oral and inhalation toxicity criteria for two of these fractions - medium carbon range aromatics and aliphatics - using, where applicable, newer data, updated modeling techniques, and new/alternative analyses of certain endpoints, human relevance, and uncertainty. The results of the analyses support an ~10-fold increase in the USEPA provisional reference concentration (p-RfC) values from 0.1 mg/m3 to 1 mg/m3 for both medium carbon range aromatics (different uncertainty factor) and aliphatics (new study and different judgment of toxicity data from existing study). Compared to the USEPA provisional oral reference dose (p-RfD) values for the medium carbon range aromatics and aliphatics of 0.03 mg/kg-day and 0.01 mg/kg-day, respectively, the present analyses suggest the RfD for medium carbon range aromatics could be increased >6.6-fold to 0.2 mg/kg-day (updated modeling and different uncertainty factors), and the RfD for medium carbon range aliphatics could be increased ~20-fold to 0.2 mg/kg-day (new study). These updated toxicity criteria could be used by regulatory agencies to reevaluate risk-based screening levels or by risk managers to support cleanup levels for medium carbon range aromatics and aliphatics, while still ensuring adequate health protection.Implications: Petroleum products represent complex mixtures of hydrocarbons broadly comprised of aliphatic compounds (straight-chain, branched-chain, and cyclic alkanes and alkenes) and aromatic compounds such as benzene, alkylbenzenes, and polycyclic aromatic hydrocarbons. The complex nature of petroleum products presents challenges for assessing potential health risks associated with exposure to petroleum hydrocarbon contamination in the environment. It has been over ten years since the U.S. Environmental Protection Agency derived provisional toxicity criteria for low, medium, and high carbon range aromatic and aliphatic hydrocarbon fractions. In that time, risk assessment guidance and tools have evolved, and new studies have been published. Our analyses indicate that current provisional toxicity criteria for medium carbon range aromatics and aliphatics fractions are overly conservative by approximately an order of magnitude.

Inhalation cancer risk assessment for environmental exposure to hexavalent chromium: Comparison of margin-of-exposure and linear extrapolation approaches.

Hexavalent chromium [Cr(VI)] exists in the ambient air at low concentrations (average upperbound ~0.1 ng/m3) yet airborne concentrations typically exceed EPA's Regional Screening Level for residential exposure (0.012 ng/m3) and other similar benchmarks, which assume a mutagenic mode of action (MOA) and use low-dose linear risk assessment models. We reviewed Cr(VI) inhalation unit risk estimates developed by researchers and regulatory agencies for environmental and occupational exposures and the underlying epidemiologic data, updated a previously published MOA analysis, and conducted dose-response modeling of rodent carcinogenicity data to evaluate the need for alternative exposure-response data and risk assessment approaches. Current research supports the role of non-mutagenic key events in the MOA, with growing evidence for epigenetic modifiers. Animal data show a weak carcinogenic response, even at cytotoxic exposures, and highlight the uncertainties associated with the current epidemiological data used in risk assessment. Points of departure from occupational and animal studies were used to determine margins of exposure (MOEs). MOEs range from 1.5 E+3 to 3.3 E+6 with a median of 5 E+5, indicating that current environmental exposures to Cr(VI) in ambient air should be considered of low concern. In this comprehensive review, the divergent results from default linear and MOE assessments support the need for more relevant and robust epidemiologic data, additional mechanistic studies, and refined risk assessment strategies.

Assessment of Mechanistic Data for Hexavalent Chromium-Induced Rodent Intestinal Cancer Using the Key Characteristics of Carcinogens.

Oral exposure to hexavalent chromium (Cr[VI]) induces intestinal tumors in mice. Mutagenic and nonmutagenic modes of action (MOAs) have been accepted by different regulatory bodies globally, the latter involving cytotoxicity-induced regenerative cell proliferation. However, concerns persist that all possible MOAs have not been fully considered. To address the potential for alternative MOAs, mechanistic data not represented in the existing two MOAs were evaluated. Relevant data were identified and organized by key characteristics of carcinogens (KCCs); literature related to epigenetics, immunosuppression, receptor-mediated effects, and immortalization were reviewed to identify potential key events associated with an alternative MOA. Over 200 references were screened for these four KCCs and further prioritized based on relevance to the research objective (ie, in vivo, oral exposure, gastrointestinal tissue). Minimal data were available specific to the intestine for these KCCs, and there was no evidence of any underlying mechanisms or key events that are not already represented in the two proposed MOAs. For example, while epigenetic dysregulation of DNA repair genes has been demonstrated, epigenetic effects were not measured in intestinal tissue, and it has been shown that Cr(VI) does not cause DNA damage in intestinal tissue. High-throughput screening data related to the KCCs were also evaluated, with activity generally limited to the two recognized MOAs. Collectively, no plausible alternative MOAs (or key events) were identified in addition to those previously proposed for Cr(VI) small intestine tumors.

A review of mammalian in vivo genotoxicity of hexavalent chromium: implications for oral carcinogenicity risk assessment.

Assessment of genotoxicity is a critical component of mode of action (MOA) analysis and carcinogen risk assessment due to its influence on quantitative risk extrapolation approaches. To date, clear guidance and expert consensus on the determination of a mutagenic MOA remains elusive, resulting in different estimates of carcinogenic risk for the same chemical among different stakeholders. Oral toxicity criteria for hexavalent chromium [Cr(VI)], for example, differ by orders of magnitude due largely to the interpretation of in vivo genotoxicity data. Herein, we review in vivo genotoxicity studies for Cr(VI) to inform the MOA for Cr(VI)-induced tumors observed in a two-year cancer bioassay in mice and rats exposed via drinking water. Overall, genotoxicity results in carcinogenic target tissues (viz., oral cavity and duodenum) are negative. Results in the intestine are consistent with imaging data indicating little to no chromium present in the crypt compartment following oral exposure. Positive genotoxicity results in nontarget tissues have been reported at high doses mostly following nonphysiological routes of exposure. Given the negative genotoxicity results in carcinogenic target organs from oral exposure to Cr(VI), there is scientific justification to support the use of nonlinear low-dose extrapolation methods in the derivation of oral toxicity criteria for Cr(VI). These results highlight important differences between genotoxicity testing for hazard identification purposes and quantitative risk assessment.

An adverse outcome pathway for small intestinal tumors in mice involving chronic cytotoxicity and regenerative hyperplasia: a case study with hexavalent chromium, captan, and folpet.

Small intestinal (SI) tumors are relatively uncommon outcomes in rodent cancer bioassays, and limited information regarding chemical-induced SI tumorigenesis has been reported in the published literature. Herein, we propose a cytotoxicity-mediated adverse outcome pathway (AOP) for SI tumors by leveraging extensive target species- and site-specific molecular, cellular, and histological mode of action (MOA) research for three reference chemicals, the fungicides captan and folpet and the transition metal hexavalent chromium (Cr(VI)). The gut barrier functions through highly efficient homeostatic regulation of SI epithelial cell sloughing, regenerative proliferation, and repair, which involves the replacement of up to 1011 cells per day. This dynamic turnover in the SI provides a unique local environment for a cytotoxicity mediated AOP/MOA. Upon entering the duodenum, cytotoxicity to the villous epithelium is the molecular initiating event, as indicated by crypt elongation, villous atrophy/blunting, and other morphologic changes. Over time, the regenerative capacity of the gut epithelium to compensate declines as epithelial loss accelerates, especially at higher exposures. The first key event (KE), sustained regenerative crypt proliferation/hyperplasia, requires sufficient durations, likely exceeding 6 or 12 months, due to extensive repair capacity, to create more opportunities for the second KE, spontaneous mutation/transformation, ultimately leading to proximal SI tumors. Per OECD guidance, biological plausibility, essentiality, and empirical support were assessed using modified Bradford Hill considerations. The weight-of-evidence also included a lack of induced mutations in the duodenum after up to 90 days of Cr(VI) or captan exposure. The extensive evidence for this AOP, along with the knowledge that human exposures are orders of magnitude below those associated with KEs in this AOP, supports its use for regulatory applications, including hazard identification and risk assessment.

Exposure to environmentally-relevant concentrations of hexavalent chromium does not induce ovarian toxicity in mice.

Exposure to high concentrations of hexavalent chromium [Cr(VI)] in drinking water (≥250 ppm) is reported to decrease ovarian follicle counts and increase follicular atresia in mice. To assess effects at lower concentrations, herein we exposed B6C3F1 mice to 0.1-150 ppm Cr(VI) in drinking water for 90 days in a GLP-compliant study. Ovarian follicular counts, differentiation, and degeneration were assessed from every 10th serial section (up to 14 sections per ovary). Ovarian follicular counts, differentiation, and rate of atresia were not altered in any exposure group. Gross and microscopic changes were not apparent in any of the evaluated reproductive or glandular organs. The no observable adverse effect level (NOAEL) for follicular effects was 150 ppm. In addition to these findings, published Cr(VI) studies examining follicles were scored using two methods for assessing study quality for use in risk assessment-including the Toxic Substance Control Act (TSCA) scoring method. Both methods revealed that studies reporting adverse effects on follicles generally received low scores. Overall, the current study indicates no/low potential for Cr(VI) to induce follicular toxicity in mice below 150 ppm Cr(VI) in drinking water (17.7 mg/kg bodyweight).

Comparison of threshold of toxicological concern (TTC) values to oral reference dose (RfD) values.

Thousands of chemicals have limited, or no hazard data readily available to characterize human risk. The threshold of toxicological concern (TTC) constitutes a science-based tool for screening level risk-based prioritization of chemicals with low exposure. Herein we compare TTC values to more rigorously derived reference dose (RfD) values for 288 chemicals in the U.S. Environmental Protection Agency's (US EPA) Integrated Risk Information System (IRIS) database. Using the Cramer decision tree and the Kroes tiered decision tree approaches to determine TTC values, the TCC for the majority of these chemicals were determined to be lower than their corresponding RfD values. The ratio of log10(RfD/TCC) was used to measure the differences between these values and the mean ratio for the substances evaluated was ~0.74 and ~0.79 for the Cramer and Kroes approach, respectively, when considering the Cramer Classes only. These data indicate that the RfD values for Cramer Class III compounds were, on average, ~6-fold higher than their TTC value. These analyses indicate that provisional oral toxicity values might be estimated from TTCs in data-poor or emergency situations; moreover, RfD values that are well below TTC values (e.g., 2 standard deviations below the log10(Ratio)) might be overly conservative and targets for re-evaluation.

Assessment of the Mode of Action Underlying the Effects of GenX in Mouse Liver and Implications for Assessing Human Health Risks.

GenX is an alternative to environmentally persistent long-chain perfluoroalkyl and polyfluoroalkyl substances. Mice exposed to GenX exhibit liver hypertrophy, elevated peroxisomal enzyme activity, and other apical endpoints consistent with peroxisome proliferators. To investigate the potential role of peroxisome proliferator-activated receptor alpha (PPARα) activation in mice, and other molecular signals potentially related to observed liver changes, RNA sequencing was conducted on paraffin-embedded liver sections from a 90-day subchronic toxicity study of GenX conducted in mice. Differentially expressed genes were identified for each treatment group, and gene set enrichment analysis was conducted using gene sets that represent biological processes and known canonical pathways. Peroxisome signaling and fatty acid metabolism were among the most significantly enriched gene sets in both sexes at 0.5 and 5 mg/kg GenX; no pathways were enriched at 0.1 mg/kg. Gene sets specific to the PPARα subtype were significantly enriched. These findings were phenotypically anchored to histopathological changes in the same tissue blocks: hypertrophy, mitoses, and apoptosis. In vitro PPARα transactivation assays indicated that GenX activates mouse PPARα. These results indicate that the liver changes observed in GenX-treated mice occur via a mode of action (MOA) involving PPARα, an important finding for human health risk assessment as this MOA has limited relevance to humans.