ToxSci Advance Access originally published online on November 13, 2007
Toxicological Sciences 2008 102(1):3-14; doi:10.1093/toxsci/kfm270
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Perfluoroalkyl Acids and Related Chemistries—Toxicokinetics and Modes of Action
,1
* The Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina 27709
Medical Department, 3M Company, St. Paul, Minnesota 55144
IneosChlor, Manchester, UK
DuPont Haskell Laboratories, DuPont, Newark, Delaware 19714
¶ Reproduction Toxicology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711
|| Risk Assessment Division, Office of Pollution Prevention and Toxics, U.S. Environmental Protection Agency, Washington, DC 20460
||| Department of Biochemistry and Molecular Biology, School of Medicine, University of Minnesota, Duluth, Minnesota 55812
1 To whom correspondence should be addressed at Medical Department, 3M Company, 3M Center 220-06-W-08, St Paul, MN 55144. Fax: (651) 733-1773. E-mail: jlbutenhoff{at}mmm.com.
Received August 16, 2007; accepted October 28, 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONPHARMACOKINETICS AND TISSUE...MODES OF ACTION: ROLE...CONCLUSIONREFERENCES |
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The perfluoroalkyl acid salts (both carboxylates and sulfonates, hereafter designated as PFAAs) and their derivatives are important chemicals that have numerous consumer and industrial applications. However, recent discoveries that some of these compounds have global distribution, environmental persistence, presence in humans and wildlife, as well as toxicity in laboratory animal models, have generated considerable scientific, regulatory, and public interest on an international scale. The Society of Toxicology Contemporary Concepts in Toxicology Symposium, entitled "Perfluoroalkyl Acids and Related Chemistries: Toxicokinetics and Modes-of-Action Workshop" was held February 14–16, 2007 at the Westin Arlington Gateway, Arlington, VA. In addition to the Society of Toxicology, this symposium was sponsored by 3M Company, DuPont, Plastics Europe, and the U.S. Environmental Protection Agency. The objectives of this 3-day meeting were to (1) provide an overview of PFAA toxicity and description of recent findings with the sulfonates, carboxylates, and telomer alcohols; (2) address the toxicokinetic profiles of various PFAAs among animal models and humans, and the biological processes that are responsible for these observations; (3) examine the possible modes of action that determine the PFAA toxicities observed in animal models, and their relevance to human health risks; and (4) identify the critical research needs and strategies to fill the existing informational gaps that hamper risk assessment of these chemicals. This report summarizes the discourse that occurred during the symposium.
Key Words: perfluoroalkyl acids; toxicokinetics; modes of action.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONPHARMACOKINETICS AND TISSUE...MODES OF ACTION: ROLE...CONCLUSIONREFERENCES |
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The observation of organically bound fluorine in human blood was first reported by Taves (1968)
in the journal, Nature. Nonvolatile, fluorinated organic compounds have been used for a variety of industrial and commercial applications; therefore, the potential contribution of these industrial and commercial chemicals to the organically bound fluorine in blood was further investigated. Using advanced liquid chromatography tandem mass spectrometry methodology, 3M Company scientists and their coworkers confirmed the widespread presence of perfluorooctane sulfonate (PFOS, Fig. 1), perfluorohexane sulfonate (PFHxS), and perfluorooctanoate (PFOA, Fig. 1) at low ng/ml (ppb) concentrations in sera from nonoccupationally exposed individuals and wildlife (Giesy and Kannan, 2001; Hansen et al., 2001; Olsen et al., 2003a). Following this discovery, 3M Company announced a voluntary phase-out of these compounds on May 16, 2000 (http://www.epa.gov/newsroom/newsreleases.htm) that was completed by the end of 2002. Going forward, a number of human population biomonitoring studies have continued to identify the presence of these and other perfluoroalkyl carboxylates and sulfonates (PFAA) in samples derived from human blood and other matrices (liver, urine, milk) around the world. In this report, we will refer to the appropriate blood matrix as reported in the original work. From a practical perspective, plasma and serum concentration have been reported to be essentially identical, and very little organofluorine is found associated with the cellular components of blood (Ehresman et al., 2007). At present, the Center for Disease Control and Prevention (CDC) has included biomonitoring for a number of the PFAAs in their National Health and Nutrition Examination Survey (NHANES). Geometric mean serum concentrations for PFOS, PFHxS, and PFOA from samples collected between 1999 and 2000 were 0.0304, 0.0021, and 0.0052 µg/ml serum, respectively, and these PFAAs were found in all samples (Calafat et al., 2007a). The central 95th percentile estimates for these three PFAAs were 0.0756, 0.0087, and 0.0119 µg/ml serum, respectively. These findings were remarkably consistent with those of Olsen et al. (2003a). The most recent data from NHANES suggest that concentrations of PFOS, PFHxS, and PFOA in the United States general population are in a decline since the years 1999–2000 which is consistent with the reported elimination half-lives of these compounds in humans (Calafat et al., 2007b; Olsen et al., 2007a); on the other hand, those of perfluorononanoic acid (PFNA), though relatively low, appear to have increased. The observation of a general decline of most PFAAs in the general population since 1999–2000 is further supported by data published by Olsen et al. (2007b).
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In contrast to general population serum concentrations, occupational exposure has produced mean serum concentrations of PFOA and PFOS that are two to three orders of magnitude higher than those reported for the general population (Olsen and Zobel, 2007
; Olsen et al., 2003b). These serum concentrations tend to be job and location specific. The sources and exposure routes of these PFAAs that lead to their presence in human general population sera are not completely understood; however, these likely include some combinations of manufacturing and product releases, as well as environmental and metabolic degradation of precursor compounds. Potential precursor compounds derive from two major technologies, electrochemical fluorination and telomerization. Both technologies produce materials used in a variety of specialized surface protection, sealant, and surfactant uses.
Due to the strength of the carbon–fluorine bond, PFAAs are exceptionally stable to metabolic or environmental degradation. The widespread presence of certain PFAAs in humans and wildlife may be in part related to this chemical stability, and in part to poor elimination once absorbed. In fact, PFOS, PFHxS, and PFOA have quite long residence times in humans, with elimination half-lives of several years (Andersen et al., 2006; Olsen et al., 2007a). For PFOA, considerable variability in elimination rates has been noted across species (Hundley et al., 2006; Kudo and Kawashima, 2003). The greater retention of PFOA by some species, or sexes within species, may be, in part, related to differences in the expression of organic anion transporters and postulated renal tubular resorption (Andersen et al., 2006; Katakura et al., 2007; Kudo et al., 2002). The carbon-chain length of PFAAs also appears to influence their pharmacokinetic properties, with smaller molecules having a tendency to be eliminated more efficiently (Ohmori et al., 2003). Thus, a better understanding of these species differences in pharmacokinetics will be critical in evaluating the hazard and risk profile of the various PFAAs.
The widespread distribution and extended residence times in the body of some PFAAs have led to increased focus in the regulatory and scientific community on the potential health risk that continued exposure to these compounds may pose. Toward that end, there has been a growing body of toxicological data on these substances from studies with laboratory animals (Kennedy et al., 2004; Lau et al., 2004, 2007; Organisation for Economic Co-operation and Development, 2002). As with other compounds found in the environment, plasma concentrations in humans are generally much lower than those associated with toxicity in the animal studies. The human health database currently includes three decades of occupational mortality and morbidity studies as well as medical surveillance and monitoring of chemical production workers whose mean serum concentrations of PFAAs are significantly higher than those found in nonoccupationally exposed groups. New studies of occupational and nonoccupational populations are becoming available. To date, no consistent associations with adverse human health outcomes have been established based on occupational-health studies (Alexander et al., 2003; Grice et al., 2007; Leonard et al., 2007; Olsen and Zobel, 2007; Sakr et al., 2007a, b; C. J.) and one community-health study (Emmett et al., 2006a,b).
Despite the increasing number of studies that have been made available, major informational gaps still exist that are important to the risk assessment efforts by the regulatory agencies. First, although the persistence of some PFAAs (with long half-life estimates) in humans and animals is well known, their toxicokinetic profiles and the underlying mechanisms for such chemical persistence are not completely understood. Second, even though some of the PFAAs have been shown to be agonists for the peroxisome proliferator activated receptor-alpha (PPAR
) signaling pathway, and some may alter mitochondrial function and intercellular communication, little else is known about their modes of action that can account for the observed outcomes in toxicological studies. Because the PFAA family is composed of over 20 individual chemicals (known to exist in the environment) with different carbon-chain lengths, functional substitutes, and derivatives, an understanding of their kinetic properties as well as common modes of action will facilitate cross-species extrapolation and ultimately provide a reliable basis for their risk assessment for human health. As being alluded to previously, PFAAs are not reactive and are resistant to degradation in biological systems; therefore, their modes of action and pharmacokinetic features are determined largely by their physical and chemical properties. They structurally resemble fatty acids, and appear to have certain behaviors that are similar to fatty acids.
In order to address the information gaps in toxicokinetics and modes of action for PFAAs, a Society of Toxicology Contemporary Concepts in Toxicology PFAA workshop was held on February 14–16, 2007 in Arlington, VA. Several workshops had been held previously to address specifically the trace analysis of PFAAs in various media, as well as fate and transport of these chemicals in the environment. These previous workshops, sponsored by SETAC (2003), U.S. EPA/3M (2004), and the University of Toronto (FLUOROS 2005) lacked a specific focus on the toxicology of these compounds as influenced by potential modes of action and pharmacokinetic properties. Therefore, the objectives of the Society of Toxicology Contemporary Concepts in Toxicology PFAA workshop were to provide (1) an overview of PFAA toxicity and description of recent findings with the sulfonates, carboxylates and telomer alcohols; (2) a summary on the toxicokinetic profiles of various PFAAs among animal models and humans, and the biological processes that are responsible for these toxicokinetic observations; (3) an examination of the possible modes of action that determine the toxicities observed in animal models and their relevance to human health risks; and (4) an identification of the critical research needs and strategies to fill the existing informational gaps that hamper risk assessment of these chemicals. With respect to toxicokinetics, leading topics were addressed, and these included (1) the determinants of the long half-life of some PFAAs; (2) the influence of carbon-chain length on rates of elimination; (3) the basis of species and gender differences in elimination profiles; and (4) potential means of extrapolating between species, including humans. Key areas explored in this regard included the role of organic anion transport systems and the potential influence of renal tubular resorption. The mode-of-action discussion focused on PPAR agonism, in particular, PPAR, and its roles in the hepatic, developmental, and immunological activities observed with some of the PFAAs (notably PFOA). In addition, the potential influence of non-PPAR mediated pathways on observed toxicities was addressed, and the use of genomic and molecular approaches to explore the biochemical pathways mediating PFAA actions was elaborated. Workshop sessions were led by expert investigators in these areas, and discussions were initiated by several invited speakers who lent insights to these subject matters. A substantial portion of this meeting was devoted to an open forum discussion among all participants, as well as individual exchanges at the poster sessions.
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PHARMACOKINETICS AND TISSUE DOSIMETRY |
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TOPABSTRACTINTRODUCTIONPHARMACOKINETICS AND TISSUE...MODES OF ACTION: ROLE...CONCLUSIONREFERENCES |
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Risk assessments based on mode of action require a clear statement of the form of the chemical responsible for those tissue interactions that lead to adverse responses. For extrapolations across species, it is also necessary to understand whether the relationship between tissue dose and adverse responses are independent of species. PFAAs are stable to biotransformation, so the relative toxicity of parent compound versus metabolites is not an issue. Thus, tissue dosimeters of these chemicals related to toxicity are likely to be (1) free concentrations of PFAAs in target tissues; (2) the extent of binding of PFAAs to specific receptors (e.g., PPAR
or target proteins); or (3) some time-averaged value of free concentration or proportion of receptors bound. The relative importance of particular biological targets may well vary for PFAAs that have different chain lengths and for PFAAs with carboxylate versus sulfonate functional groups.
Tissue concentrations of a parent compound achieved following single or multiple doses of PFAAs will depend on bioavailability after dosing, the overall distribution to various tissues, and the rates of elimination by different routes. The importance of these factors can be established by combinations: (1) experiments to establish time course of PFAAs at various concentrations; (2) pharmacokinetic modeling, with either empirical or physiologically based compartments, to describe these time courses quantitatively; and (3) directed in vivo or in vitro experiments to assess factors that contribute to PFAA dosimetry in intact animals. Other important considerations with PFAAs, as with other compounds, such as dioxin (Andersen et al., 1997), that also induce transcriptional upregulation of various proteins, is the extent to which the upregulated proteins may alter dosimetry by changing concentrations of transporter or binding proteins. Dose-dependent changes in disposition complicate modeling and can impede discovery of the biological factors involved in PFAA dosimetry.
Pharmacokinetic studies therefore will help to interpret the relationship of dose and toxicity in animal studies and to place tissue concentrations observed in toxicity studies in context compared with the lower concentrations of PFAAs in the human population. These studies can also provide data to understand key issues for PFAA dosimetry, including (1) comparison among various PFAAs based on chain lengths and functional groups; (2) comparison of pharmacokinetics across dose levels and treatment paradigms (repeated vs. single exposure), and age groups (young, adult, or elderly) for individual compounds; (3) the roles of organic anion transporters for both tissue influx, tissue efflux, and excretion from the body; and (4) cross-species extrapolation.
The marked gender difference in serum elimination half-life of PFAA in rats is highly dependent on chain length (Katakura et al., 2007; Kudo and Kawashima, 2003; Kudo et al., 2000, 2001, 2006; Ohmori et al., 2003). These differences are quite large for PFOA (5.6 vs. 0.08 days for males and females, respectively) and PFNA (30 vs. 2.5 days). Elimination rates of PFAAs with shorter carbon-chain appear to be faster. The half-life of perfluorohexanoate (PFHxA) has been reported to be between 1 and 2 h for both males and females; whereas unpublished data presented in a poster at this symposium (Chang et al., 3M) suggested half-lives of 2 and 8 h for perfluorobutyrate in males and females, respectively. In contrast, both male and female rats show much longer half-life (46–60 days) and slower urinary clearance of perfluorodecanoic acid (PFDA). These observations and the dependence of the slow excretion phenotype on testosterone indicate the induction of key proteins in the male rat that regulate PFAA excretion. However, these gender differences in elimination in the rats do not seem to be quite as large in other experimental animals and may even be reversed in some species, such as the hamster and the cynomolgus monkey, in which males appear to have more rapid elimination of PFOA than females (Butenhoff et al., 2004a; Hundley et al., 2006).
The relationship of induced peroxisomal β-oxidation with achieved liver PFAA concentration (on a molar basis) in rats was constant across PFAAs for chain lengths C8–C10 and was equivalent for males and females (Kudo et al., 2006). Expression of various rat transporter proteins in frog (Xenopus laevis) oocytes provided support for roles played by at least two rat kidney proteins in control of PFAA disposition-oatp1 (in resorption) and OAT3 (in excretion) (Katakura et al., 2007). Another important characteristic that was recently discovered was a dose-dependent uptake of PFOA in the liver and a dose-dependent association of PFOA with various liver subcellular fractions (Kudo et al., 2007). Intravenous doses over the range of 0.041–16.56 mg/kg produced variable hepatic accumulation. The lowest dose (and likely most relevant for human exposures) produced the highest proportion of PFOA in liver (about 45%) and the hepatic PFOA was predominantly found in the 8000 x g precipitate (membrane fraction). At the highest dose, only 20% of the PFOA was found in the liver, predominantly in the 15,000 x g supernatant (the cytoplasm). In addition, biliary clearance was very small at the low liver concentrations and bile/plasma concentration ratios only increased at higher doses. These observations indicate exposure ranges over which there are dose dependencies, either from saturation of binding sites or from induction of tissue binding components, and, as such, they represent good candidate studies for developing quantitative models of these dose-dependent processes. Studies in PPAR–null mice could also provide information on whether these dose dependencies are due to induction of new proteins versus saturation of existing binding sites.
Despite indication of dose dependencies, some insights about PFAA disposition can be gleaned from straight-forward kinetic analyses. Serum elimination half-lives of some of these PFAAs differ widely across species (Butenhoff et al., 2004a; Hundley et al., 2006; Kudo and Kawashima, 2003; Olsen et al., 2007a). For PFOA, half-lives have been estimated to be 0.08 days in female rats and 1273 (geometric mean) days in humans. Excretion occurs both in bile and in urine; the latter route of excretion predominates in the rat, whereas fecal excretion predominates in monkeys and humans. In monkeys, the time course behavior in plasma following a single iv dose is consistent with a two-compartment pharmacokinetic model with a volume of distribution between 0.1 and 0.2 l/kg. Complexities in the shapes of the elimination curves indicate time- and dose-dependent alterations in factors that control tissue dosimetry of these PFAAs. In multiday oral dosing, the volume of distribution at steady-state is larger than that estimated after the iv dose. Another unusual characteristic was the observation of biphasic urinary excretion curves after the cessation of dosing. The profiles of plasma concentration and fecal elimination did not recapitulate the biphasic nature of urinary elimination. A somewhat more physiological pharmacokinetic model was applied to describe dose-dependent urinary resorption (Andersen et al., 2006). This physiologically based filtrate reuptake model is the first to focus on resorption. Previous pharmacokinetic studies with some anionic herbicides have described dose-dependent renal excretion that saturated at higher plasma concentrations. In this saturable resorption model, the filtration flow has a sensitivity coefficient of – 2, due to a model structure where glomerular filtration first moves compound into the filtrate, and bulk flow of the filtrate (at close to the initial filtration rate) moves filtrate out of the region from which resorption can take place. The high sensitivity of the modeled filtration flow may account for some of the large species differences in serum elimination half-lives (e.g., between rats, monkeys and humans), and offer another viable explanation besides the differences in transporter expression. Be that as it may, challenges remain in understanding the biological processes that are time and dose dependent, and in accounting for any interactions in kinetics between the PFAAs and endogenous fatty acids that may compete for binding proteins or transporters.
Although pharmacokinetic models and studies in which transporters have been cloned into frog oocytes infer a role of transporters in tissue uptake and excretion, more detailed studies are necessary to show which transporters are involved and how they function to move PFAAs of different chain lengths into tissues and out of the body. In recent years, as with many protein families, there has been full characterization of the organic anion transporter proteins from various species, especially from rat, human, and mouse. Strong chain-length–dependent inhibition of 17-β-estradiol by various PFAAs was noted for the liver transporter, OATP1B1, and the inhibition was somewhat consistent with whole body half-lives for carboxylate PFAAs between PFHxA and PFDA, with inhibition following an order of PFDA > PFNA > PFOA > PFHxA. Shorter and longer chain length PFAAs did not inhibit transport. Ongoing studies of PFAA specificity in inhibiting transporter-mediated uptake, and specificity in interacting with fatty acid binding proteins as well as with PPAR proteins, should add considerably to the understanding of the intersection of dosimetry and biological responses. A more biologically complete kinetic model should support risk assessments and aid in interpreting human biomonitoring study results that are now becoming available for selected populations around sites that manufactured PFAAs (Emmett et al., 2006a,b) and for the general U.S. population through the NHANES program at CDC (Calafat et al., 2007a,b).
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MODES OF ACTION: ROLE OF NUCLEAR RECEPTOR ACTIVATION IN OBSERVED TOXICITY |
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TOPABSTRACTINTRODUCTIONPHARMACOKINETICS AND TISSUE...MODES OF ACTION: ROLE...CONCLUSIONREFERENCES |
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Activation of PPARs is a transcriptional signature for both PFOA and PFOS in rats and mice, as well as common carp and zebrafish. The PPAR
response is reflected in the increased transcription of mitochondrial and peroxisomal lipid metabolism, sterol, and bile acid biosynthesis, and retinol metabolism genes. However, the effects of PFAAs on nuclear receptors are not limited to PPAR. The nonselective pan-activation of numerous nuclear receptors is apparent not only by the transcriptional activation of many genes in PPAR–null mice, but also by the scope of metabolic and regenerative pathways elicited by PFOA and PFOS exposure, including those stimulated in human liver cell cultures.
A nuclear receptor ligand-binding domain/GAL4 DNA-binding domain chimeric reporter system was used to study the activation of human, mouse and rat PPAR, PPARβ, PPAR
, liver X receptor (LXRβ), and retinoid X receptor (RXR) by PFOA and PFOS (Vanden Heuvel et al., 2006). In addition, activation of these constructs by PFOA and PFOS was compared with several structural classes of natural fatty acids and appropriate positive control ligands. The data show that of the nuclear receptors studied, PPAR is the most likely target of PFOA and PFOS, although PPAR is also activated to some extent. Compared with naturally occurring long-chain fatty acids, for example, linoleic, and -linolenic acids, PFOA and PFOS were more selective and less potent in their activation of nuclear receptors.
PFOA exerts marked morphological and biochemical effects upon several tissues. These effects include a variety of perturbations of intermediary metabolism, particularly alterations in the transport and metabolism of lipids and changes in the regulation of the cell cycle. The profile of biological responses to PFAA exposure differs amongst animal species and between individual tissues, depending on the balance and affinities of the various nuclear receptors, of which there are nearly 50 subtypes. For example, PPAR is the primary PPAR nuclear receptor subtype only in liver, kidney, and heart. Regardless, manifestation of nuclear receptor-mediated activities occurs at doses of PFOA and PFOS well below those associated with discernable histomorphological alterations of the liver or changes in serum enzymes reflecting hepatic function, indicating that the nuclear receptor response is a sensitive indicator of the biological effect of PFAA exposures.
The PPAR responses to PFOA, being pronounced in rodents, are characterized by weight loss resulting from increased peroxisomal and mitochondrial oxidation of fat as well as lower plasma triglycerides and cholesterol, and increased serum leptin. All of these effects are absent in PPAR–null mice. Hepatomegaly observed in PPAR–null mice is most likely a manifestation of expansion of the smooth endoplasmic reticulum by PFOA-mediated activation of the nuclear receptor, constitutive androstane receptor (CAR). The scope of the PPAR response is reflected in the 975 genes exclusively expressed in wild-type, but not PPAR-null mice exposed to PFOA, the extent of which relies mostly on the gender and nutritional state of the animal. The greater PPAR response in male rats compared with females is under the regulation of testosterone, which causes a diminution of the urinary clearance of PFAAs. Castration increases urinary clearance, whereas treating with testosterone increases PFOA accumulation.
Liver Effects
PFAAs, such as PFOA and PFOS, are weak, nonmetabolizable ligands with structural components that resemble fatty acids. The characteristics that seem to be shared by most of the PFAAs are (1) a lack of direct genotoxicity; and (2) an ability to cause hepatomegaly and, in some cases, hepatotoxicity, in rodents and primates, including an increased incidence of hepatocellular adenoma in rats (PFOA and PFOS). PFOA was also shown to increase the incidence of pancreatic acinar cell adenomas and testicular Leydig cell adenomas in rats. Many PPAR agonists, like PFOA, induce low incidences of tumors of the rat liver, pancreas, and testis. These compounds are nongenotoxic and produce these tumors by a non-DNA reactive mode-of-action involving binding to and stimulation of PPAR, leading to increased cell proliferation and, ultimately, low incidences of tumors. The administration of PFOA to rats leads to hepatomegaly and can result in reduced weight gain or actual weight loss, presumably via the increased oxidation of fat. Treatment with PFOA resulted in marked biochemical changes associated with activation of the PPAR pathways, as well as pathways mediated by other nuclear receptors such as the constitutive androstane receptor (CAR) and pregnenolone X receptor (PXR). Other PFAAs studied seem to share the ability to activate the PPAR pathway, for which endogenous fatty acids are the natural ligands. Therefore, it is reasonable to place a primary emphasis on the role of PPAR in understanding the mode(s) of action responsible for the observed PFAA toxicities.
The use of gene array technology, coupled with advances in bioinformatics, has shown that, in the liver, many of the transcriptionally induced genes (such as those involved in fatty acid transport and oxidation) were found to be regulated by the transcription factor PPAR. Toward that end, transcript profiling can be used to dissect the molecular basis of events that occur after PPAR activation in the liver. A compendium approach to the biological interpretation of approximately 400 transcript profiles from the livers of mice, rats, monkeys, and humans exposed to eight peroxisome proliferating compounds (PPC), including PFOA is being developed using Affymetrix chip data. This data set is being analyzed using a number of bioinformatic approaches. As part of this project, the gene expression profiles in mouse liver after exposure to a subset of PPCs that included WY-14,643 (WY) and PFOA were compared with determine the roles of PPAR in mediating the transcriptional changes induced by these chemicals. In the wild-type mice, common functional categories of genes altered by WY and PFOA included those involved in fatty acid oxidation and transport, peroxisomal biogenesis, proteome maintenance, coagulation, complement cascade, and oxidative stress. Almost all of these common genes were dependent on PPAR for changes in transcript levels as the changes were not observed in PPAR-null mice after exposure to WY or PFOA. In both wild-type and PPAR-null mice, PFOA but not WY exposure led to changes in the expression of a set of genes, mainly enriched in those involved in xenobiotic metabolism and included phase I and phase II genes. Many of these PFOA-regulated genes are known targets of other nuclear receptors including CAR and PXR. A comparison of the PPAR-independent genes altered by PFOA with genes altered by CAR activators indicated that the PFOA-regulated genes overlap with those regulated by CAR. This leads to the hypothesis that CAR, and possibly other transcription factors, may be responsible for the activation of the xenobiotic metabolism enzymes. This preliminary toxicogenomic analysis presented in this workshop supports the notion that a majority of the effects of PFOA are mediated by PPAR. However, the PPAR-independent pathways altered by PFOA should not be discounted until their functional significance to liver effects, including tumorigenesis, is understood.
Pancreas
Gene expression changes observed in the pancreas are quite different from those in the liver, and suggest possible effects on gluconeogenesis and glutamine metabolism. In the pancreas, a marked upregulation of phosphoenolpyruvate carboxykinase, a PPAR-regulated gene, has been observed. The overall profile of regulated genes associated with pancreatic tumorigenesis is indicative of metabolic acidosis, oxidative stress, and endogenous processes of DNA damage against a background of increased mitogenesis.
With regard to pancreatic tumors, acinar cell tumors are common in the rat, but exceedingly rare in humans (< 0.1% of pancreatic tumors). In fact, ductal carcinomas are the most frequently noted tumors of the exocrine pancreas in humans. The key events responsible for the induction of pancreatic tumors in rats by PPAR agonists are not as clearly understood as they are for liver tumors. There is evidence that the mode-of-action involves stimulation of PPAR leading to bile stasis and/or bile acid compositional changes, ultimately resulting in an increased cholecystokinin (CCK), which stimulates pancreatic acinar cell proliferation and eventual production of tumors. In rats, a variety of stimuli can lead to increased CCK, such as administration of high lipid diets, or corn oil by gavage, will lead to an increase in pancreatic acinar cell proliferation and tumors. However, this appears to be a rat-specific phenomenon that is dependent on the unusual levels of CCK receptors on the rat pancreatic acinar cells. Such a sequence of events does not occur in humans in response to increases in CCK; e.g., Nevertheless, additional research is needed to firmly establish whether this mode-of-action is indeed operative in the rats following exposure to PFOA. There remains also the question of the relevance of rat pancreatic acinar cell tumors to humans, in general.
Testicular Leydig Cells
The testicular tumors that occur in response to PFOA and other PPAR agonists, the third tumor of the triad, are located in the Leydig cells. These tumors are quite common in rats, even occurring spontaneously in certain strains, such as the F344, at very high incidences. Two possible modes of action have been suggested for PPAR-mediated induction of Leydig cell tumors, and both involve alterations in steroidogenesis. One pathway involves an induction of CYP19A1 (aromatase) in the liver, leading to increased circulating estradiol levels, and an attendant increase in TGF, a growth factor for Leydig cell proliferation. The second pathway involves inhibition of peripheral benzodiazepine receptor and/or C-17,20 lyase, leading to decreased levels of testosterone with consequent increases in circulating LH and stimulation of Leydig cell proliferation. However, both modalities are presently at an hypothetical stage, and neither has yet been definitively shown to be operative in rats following exposure to PFOA. Again, the hormonal effects on rat Leydig cells appear to be species specific. For example, inherited disorders involving high levels of circulating LH are not associated with an increased incidence of Leydig cell tumors in humans. Furthermore, Leydig cell tumors are uncommon in humans, in contrast to their high frequency in rats and other rodents.
Human Relevance of Tumors Observed with PFOA in Rats
The potential human relevance of the tumor triad (rat and mouse hepatocellular adenoma, rat pancreatic acinar cell adenoma, and rat testicular Leydig cell adenoma) that is often observed from treatment of rats with PPAR agonists (such as PFOA and PFOS) has been the subject of a recent review (Klaunig et al., 2003). Although epidemiological studies indicate a lack of carcinogenic activity by PFOA, PFOS, or other PPAR agonists in humans, cancer risk assessment is dependent largely on the results of chronic studies in laboratory animals, and a clear understanding of the mode-of-action for the potential human relevance is warranted.
PFOA was used as a case study for the development of a human relevance framework by the International Life Sciences Institutes/Risk Sciences Institute sponsored by the U.S. EPA and Health Canada (Cohen et al., 2003; Klaunig et al., 2003). In the case of liver tumors observed in rats treated with PFOA, the weight of evidence suggests that the mode-of-action involves interaction of PFOA with PPAR leading to activation of several genes, ultimately producing an increase in hepatocyte proliferation, which leads to the development of a low, but significant incidence of hepatocellular tumors. There is strong evidence that this mode-of-action is not relevant to humans, partly because of the much lower level of expression of these receptors in humans and major differences in downstream response elements when compared with rodents (Cheung et al., 2004; Morimura et al., 2006; Shah et al., 2007).
Developmental Toxicity
Interests in the developmental effects of PFAAs originate from a two-generation reproductive toxicity study of PFOS in rats (Luebker et al., 2005a,b). In this study, all first generation offspring (F1 pups) at the highest dose (3.2 mg·kg–1·day–1) died within a day after birth, whereas 34% of the F1 pups in the 1.6 mg·kg–1·day–1 dose group died within 4 days after birth. The findings in this study led to a great deal of research to understand the species specificity and mode-of-action of this effect, and to determine whether other PFAAs may also elicit the same response (Butenhoff et al., 2004b; Grasty et al., 2003, 2005; Lau et al., 2003, 2006; Luebker et al., 2005b; Thibodeaux et al., 2003).
Teratological studies in the rat, rabbit, and mouse with PFOS are generally unremarkable (Case et al., 2001; Thibodeaux et al., 2003). Observed developmental effects include reduction in fetal weight, cleft palate, anasarca (edema), delayed ossification, and cardiac abnormalities; the structural abnormalities have been noted in the highest PFOS doses tested; however, these effects, with the exception of fetal weight reduction, have not been noted with PFOA. When rodent dams (rat or mice) are exposed to PFOS throughout pregnancy and allowed to give birth, there is a time- and dose-dependent increase in neonatal mortality (Grasty et al., 2003; Lau et al., 2003). Although the treatment paradigm in these studies was different than that in the two-generation reproductive toxicity study, both study designs resulted in similar body burdens of PFOS as judged by serum levels; thus, body burden can be used to compare results across species and study designs (Lau et al., 2007). Cross-fostering studies demonstrated that the reduced pup survival is mainly a result of in utero exposure to PFOS (Lau et al., 2003; Luebker et al., 2005a). In contrast, exposure during lactation alone, through milk from exposed dams, does not appear to have any adverse effect on pup viability. Critical period studies in rats indicated that late gestation was the developmental period most sensitive to PFOS, because offspring of dams dosed on gestation days 17–20 had the highest incidence of mortality (Grasty et al., 2003). Subsequent mode-of-action studies have focused on the lung maturation (Grasty et al., 2005). Lungs removed from PFOS-treated neonates were poorly inflated and showed histological evidence of immaturity and focal atelectasis. However, biochemical analyses of these lungs showed normal levels of pulmonary surfactant, indicating that the lungs were not immature.
Recently, research has been extended to examine the potential developmental effects of PFOA. A two-generation reproductive toxicity study in rats showed a reduction in weight gain of the F1 pups during lactation and an increase in mortality during the first week following weaning at 30 mg·kg–1·day–1 (Butenhoff et al., 2004b). The increase in neonatal mortality that has been observed with PFOS was not seen in this study. However, this is probably due to the pronounced gender difference in elimination of PFOA that is observed in rats. To test this possibility, studies were extended to the mouse, which does not exhibit the gender difference in elimination (Lau et al., 2006). When pregnant mice were exposed to PFOA throughout pregnancy and allowed to give birth, there was a pattern of neonatal morality (time and dose dependent) that was similar to that observed with PFOS. A cross-fostering study demonstrated that the combination of gestational and lactational exposure decreased postnatal survival (Wolf et al., 2007). Some of the surviving neonates were examined after birth with no additional PFOA treatment. Preliminary results presented at the workshop indicated that development of the mammary gland in female mice was compromised by in utero exposure to PFOA. In addition, the mice grew poorly during weaning, but in adulthood, these offspring became obese, accumulating large depots of abdominal fat. Microarray expression analysis of fetal lung and liver from litters of PFOA-treated mice suggested that genes related to fatty acid catabolism were altered in their expression (Rosen et al., 2007). Results from these studies thus implicated PPAR signaling as a potential mode-of-action.
Hence, in addition to the hepatic effects, activation of PPAR may play a seminal role in the PFAA-induced developmental toxicity, at least for PFOA. Indeed, subsequent studies with PPAR-null mice demonstrated that the PFOA-induced neonatal mortality is dependent upon the expression of PPAR (Abbott et al., 2007). The only non-PPAR effect in these PPAR-null mice was full-liter resorption. These studies have also shown that the reductions in weight gain that are observed during lactation are also influenced by, and possibly dependent on PPAR expression. Additional investigations using genetically modified (such as humanized PPAR) mice may provide further insight into the potential human relevance of the developmental effects of PFAAs.
Human Experience
Apelberg et al. (2007a,b) conducted a cross-sectional study at Johns Hopkins University Hospital that involved the analysis of cord blood PFOS and PFOA concentrations from 293 singleton births between November 2004 and March 2005. Geometric mean serum concentrations were 4.9 and 1.6 ng/ml, respectively. Cord serum concentrations of both PFOS and PFOA were highly correlated (r = 0.64). After adjusting for potential confounders, statistically significant negative associations were observed with each compound for birth weight, ponderal index, and head circumference. No associations were observed between PFOS or PFOA concentrations and newborn length or gestational age. An increase in PFOS concentrations from the 25th to 75th percentile (i.e., the interquartile range), equivalent from 3.4 to 7.9 ng/ml, was associated with a 58 g decrease in birth weight. Likewise, an increase from the 25th to 75th percentile in cord blood PFOA concentrations (from 1.2 to 2.1 ng/ml) was associated with a 58 g decrease in birth weight. For head circumference, the PFOS and PFOA respective changes by interquartile range was 0.27 and 0.23 cm, respectively. Apelberg et al. (2007a) emphasized that their data should be cautiously interpreted, because the infants born in their study were healthy and the variations observed were within normal range.
In a study of self-reported medical condition among 3M Company employees who had been exposed occupationally to PFOS and PFOA, former and current female employees were asked to complete a brief pregnancy history that included recalled birth weight of children (Grice et al., 2007). Overall, there was a 75% response rate from the 353 eligible female employees. In this study, no current biomonitoring data were used. Rather, Grice et al. (2007) relied on historical biomonitoring data at this 3M manufacturing site to construct an exposure categorization matrix. This matrix was used to categorize female workers up to the year of the self-reported birth. Three exposure categories, "high," "low," and "nonexposed" were constructed. The high-potential exposure category included those jobs that averaged serum PFOS concentrations of approximately 1300–1970 ng/ml. The low-potential workplace exposure category had jobs with serum PFOS concentrations in the 390–890 ng/ml range. The no-direct-workplace exposure category had jobs with serum PFOS concentrations in the 110–290 ng/ml range. There was also a strong correlation at this manufacturing site between PFOS and PFOA concentrations with similar magnitudes of concentrations (Olsen et al., 2003b). There were 421 singleton births with 32 born to female employees who worked in the high exposure categorization for at least 1 year. The median birth weight recalled for these 32 babies was 3.50 kg compared with 3.35 kg of the 312 babies born to female employees categorized as having no direct exposure. The median self-recalled birth weight for babies born to female employees who ever had low exposure was 3.40 kg. Regression analyses that adjusted for potential confounders reported no dose–response. Grice et al. (2007) concluded there was no association between working in a PFOS-exposed job and birth weight.
In a report that appeared after the workshop, Fei et al. (2007) randomly selected 1400 women from the Danish National Birth Cohort who gave birth to a singleton baby between 1996 and 2002. Stored maternal plasma samples that were collected in the first trimester of pregnancy were analyzed for PFOS and PFOA. The median serum PFOS and PFOA concentrations were 35.3 and 5.6 ng/ml, respectively. Using a subset of 200 maternal samples that were subsequently collected during the second trimester, as well as 50 umbilical cord blood samples, Fei et al., observed declining maternal PFOS and PFOA concentrations through gestation and the lowest concentrations found in the umbilical cord blood samples of 11.0 ng/ml PFOS and 3.7 ng/ml PFOA, respectively. After adjusting for several potential confounders, Fei et al. did not observe an association between PFOS and birth weight. A 1.0 ng/ml increase in PFOS resulted in only a 0.46 g decline in birth weight. The authors did observe an association between PFOA and birth weight with a 1.0 ng/ml increase in PFOA resulted in a 10.63 g decline in birth weight. However, a categorical analysis of the data indicated this association may only be in the lowest quartile for PFOA (< 3.9 ng/ml). Other data have indicated that PFOA may more readily cross the placenta than PFOS (Midasch et al., 2007).
Collectively, the study results provided by Apelberg et al., Grice et al., and Fei et al. are noteworthy for their inconsistencies. This may be due, in part, to different study designs and exposure assessments. Whereas Apelberg et al. (2007a) reported small negative associations between birth weight and PFOA and PFOS umbilical cord serum concentrations, Fei et al. (2007) reported no associations for PFOS and birth weight and a much smaller negative association between PFOA and birth weight when using maternal serum collected during the first trimester. In their general population studies, both Apelberg et al. (2007b) and Fei et al. measured serum PFOS and PFOA concentrations at very low ng/ml levels. On the other hand, Grice et al. (2007) reported no associations with self-recalled child's birth weight from female employees whose jobs may have resulted in serum PFOS and PFOA concentrations that were two to three orders of magnitude higher than those measured in the general population by Apelberg et al. (2007b) and Fei et al. The nine covariates used in the Fei et al. study and the 11 used by Apelberg et al. in their regression analyses demonstrate the many influences on birth weight and other indicators of fetal growth, including head circumference and ponderal index. One of these parameters is maternal weight gain, but only Apelberg et al. included this as a covariate. Maternal weight gain is positively associated with plasma volume expansion, greater perfusion of placental tissue, and increased birth weight. It is therefore biologically plausible that higher birth weights observed will be associated with lower concentrations of a compound measured in cord blood and vice versa. Because so many covariates need to be adjusted for in the analyses of fetal growth indicators, it becomes unlikely that all confounding factors have been adequately controlled because of either residual (measured) or unmeasured error (Fewell et al., 2007).
Subsequent research needs to carefully consider pregnancy-related physiological events, including how expanding plasma volume might affect the distribution of protein-bound organic anions, such as PFOS and PFOA, that are found predominantly in extracellular space. There could there be noncausal, physiologically related associations with the measurement of these compounds with highly correlated birth parameters including birth weight, head circumference, abdominal circumference, birth length, and ponderal index.
Immunotoxicity
The immunotoxic potential of PFOA was first reported in 2000 (Yang et al., 2000, 2002a), where decreases in thymus and spleen weights were noted in the male mice after exposure to PFOA in the diet. Decreased body weight due to fat loss was also observed. However, the relatively high doses used in these studies and the reduced food intake by the animals complicated interpretation. The time course of thymic and splenic atrophy (likely related to an inhibition of cell proliferation) was similar to that of PFOA-induced hepatomegaly and peroxisome proliferation. In addition, PFOA was shown to be immunosuppressive: the primary humoral response to horse red blood cell immunization was prevented by PFOA pretreatment, whereas ex vivo spleen cell proliferation in response to both T- and B-cell activation was attenuated by the fluorochemical. Investigation with transgenic mice demonstrated that these effects were attenuated in the PPAR–null model (Yang et al., 2002b), thus reiterating the prominent involvement of PPAR signals as a mode-of-action for a host of PFAA toxicity.
The variety of effects and mechanisms investigated in the posters presented at the workshop underscored the interest that has been generated in the potential immunological effects of PFAAs. As this work becomes published, it will form a foundation for future research into the modes of action and potential human relevance of PFAA-induced modulation of the immune system. It should also be noted that for all the endpoints examined in rodents, some indication ought to be provided regarding the tissue concentrations at which the effects occur, the relationship of these tissue levels to those levels causing weight loss and other toxicities, including lethality, and the relationship of these levels to concentrations observed in representative human populations.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONPHARMACOKINETICS AND TISSUE...MODES OF ACTION: ROLE...CONCLUSIONREFERENCES |
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The workshop was the first of its kind to bring together a group of scientists with expertise in a number of areas such as toxicokinetics, developmental biology, and molecular biochemistry in order to address toxicological research concerning PFAAs. The workshop had more than 150 attendees that included many international participants.
Several areas were identified for future research that could improve on the current understanding of pharmacokinetics and modes of action. Pursuit of these areas of research will undoubtedly aid risk assessment activities that require extrapolation of knowledge across molecular structures and across species. Several "take-home" messages from the workshop are summarized as follows.
Toxicokinetics
- Challenges remain in understanding the biological processes that are time and dose dependent, and in accounting for any interactions in kinetics between the PFAAs and endogenous fatty acids that may compete for binding proteins or transporters.
- Nonlinear pharmacokinetic behavior has been observed for the various PFAAs. These observations result either from saturation of binding sites or from induction of tissue binding components, and, as such, they represent areas for further study that would allow for developing quantitative models of these dose-dependent processes.
- Studies in PPAR-null mice can provide information on whether these dose dependencies are due to induction of new proteins versus saturation of existing binding sites.
- Although pharmacokinetic models and studies in which transporters have been cloned into frog oocytes infer a role of transporters in tissue uptake and excretion, more detailed studies are necessary to identify which transporters are involved and how they function to move PFAAs of different chain lengths into tissues and out of the body. Inducibility of these transport processes by the PFAAs should also be investigated.
- Continued studies of PFAA specificity in inhibiting transporter-mediated uptake, and specificity in interacting with fatty acid binding proteins and with PPAR proteins, should add considerably to the understanding of the intersection of dosimetry and biological responses.
- A more biologically complete kinetic model will support risk assessments and aid in interpreting human biomonitoring study results that are now becoming available for selected populations around sites that manufactured PFAAs (Emmett et al., 2006a,b) and for the general U.S. population through the NHANES program at CDC (Calafat et al., 2007a,b).
- Especially in light of the large species differences in half-lives, toxicity studies should report both daily dosages and plasma and tissue concentrations at which responses are observed. These concentrations should be reported in molar units. Similarly, the effective doses that cause specific responses across gender and species should be compared on the basis of achieved concentration in target tissues, and on the basis of how they compare with human tissue concentrations. In this way, differences in pharmacokinetics can be resolved more easily between species and genders, which may account for differences in pharmacodynamic responses to PFAAs.
Modes of Action
- Most of the toxicological work has been conducted at doses that result in serum or plasma concentrations that are high relative to those observed in the general population, from populations exposed occupationally or to identifiable environmental sources. It is unlikely that effects observed under these experimental conditions are meaningful at the lower levels of exposure encountered by humans in the environment. Evaluating the dose–response relationship over wide ranges of tissue concentrations, including those that are more representative of human exposure will be of great value, especially in light of the nonlinear dose–response characteristics observed at the higher doses in rodents.
- When the common mode(s) of action for PFAAs are identified, the issue of exposures to a mixture of these chemicals (a realistic scenario) should be addressed to support human health risk assessment.
- Additional studies using genetically modified mice may provide further insight into the potential human relevance of effects.
- The continued evaluation of exposed human populations, including occupationally and nonoccupationally exposed groups, is to be encouraged.
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NOTES |
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The information in this document has been subjected to review by the Office of Pollution Prevention and Toxics and the Office of Research and Development of U.S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents reflect the views of the Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.
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ACKNOWLEDGMENTS |
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The authors wish to thank Dr Linda Birnbaum, Dr John Heinz, Dr Chester Rodriguez, and Dr Yu-Mei (Cecilia) Tan for their detailed notes from the symposium, which were of great value in preparing this report.
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REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONPHARMACOKINETICS AND TISSUE...MODES OF ACTION: ROLE...CONCLUSIONREFERENCES |
|---|
Abbott BD, Wolf CJ, Schmid JE, Das KP, Zehr RD, Helfant L, Nakayama S, Lindstrom AB, Strynar MJ, Lau CS. Perfluorooctanoic acid (PFOA)-induced developmental toxicity in the mouse is dependent on expression of peroxisome proliferator activated receptor-alpha (PPAR{alpha}). Toxicol. Sci. (2007) 98:571–581.
Alexander BH, Olsen GW, Burris JM, Mandel JH, Mandel JS. Mortality of employees of a perfluorooctanesulphonyl fluoride manufacturing facility. Occup. Environ. Med. (2003) 60:722–729.
Andersen ME, Birnbaum LS, Barton HA, Eklund CR. Regional hepatic CYP1A1 and CYP1A2 induction with 2,3,7,8-tetrachlorodibenzo-p-dioxin evaluated with a multicompartment geometric model of hepatic zonation. Toxicol. Appl. Pharmacol. (1997) 144:145–155.[CrossRef][Web of Science][Medline]
Andersen ME, Clewell HJ 3rd, Tan YM, Butenhoff JL, Olsen GW. Pharmacokinetic modeling of saturable, renal resorption of perfluoroalkylacids in monkeys—Probing the determinants of long plasma half-lives. Toxicology (2006) 227:156–164.[CrossRef][Web of Science][Medline]
Apelberg BJ, Goldman LR, Calafat AM, Herbstman JB, Kuklenyik Z, Heidler J, Needham LL, Halden RU, Witter FR. Determinants of fetal exposure to polyfluoroalkyl compounds in Baltimore, Maryland. Environ. Sci. Technol. (2007b) 41:3891–3897.[Medline]
Apelberg BJ, Witter FR, Herbstman JB, Calafat AM, Halden RU, Needham LL, Goldman LR. Cord serum concentrations of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) in relation to weight and size at birth. Environ. Health Perspect. (2007a) doi:10.1289/ehp.10334. Available at: http://dx.doi.org/. Online 31 July 2007.
Butenhoff JL, Kennedy GL Jr, Hinderliter PM, Lieder PH, Jung R, Hansen KJ, Gorman GS, Noker PE, Thomford PJ. Pharmacokinetics of perfluorooctanoate in cynomolgus monkeys. Toxicol. Sci. (2004a) 82:394–406.
Butenhoff JL, Kennedy GL Jr, Frame SR, O'Connor JC, York RG. The reproductive toxicology of ammonium perfluorooctanoate (APFO) in the rat. Toxicology (2004b) 196:95–116.[CrossRef][Web of Science][Medline]
Calafat AM, Kuklenyik Z, Reidy JA, Caudill SP, Tully JS, Needham LL. Serum concentrations of 11 polyfluoroalkyl compounds in the u.s. population: Data from the National Health and Nutrition Examination Survey (NHANES) 1999-2000. Environ. Sci. Technol. (2007a) 41:2237–2242.[Medline]
Calafat A, Wong LY, Kuklenylk Z, Reidy JA, Needham LL. Polyfluoroalkyl chemicals in the U.S. population: Data from the National Health and Nutrition Examination Survey (NHANES) 2003-2004 and comparisons to NHANES 1999-2000. Environ. Health Perspect. (2007b) doi:10.1289/ehp.10598 [epub ahead of print].
Case MT, York RG, Christian MS. Rat and rabbit oral developmental toxicology studies with two perfluorinated compounds. Int. J. Toxicol. (2001) 20:101–109.[CrossRef][Web of Science][Medline]
Cheung C, Akiyama TE, Ward JM, Nicol CJ, Feigenbaum L, Vinson C, Gonzalez FJ. Diminished hepatocellular proliferation in mice humanized for the nuclear receptor peroxisome proliferator-activated receptor alpha. Cancer Res. (2004) 64:3849–3854.
Cohen SM, Meek ME, Klaunig JE, Patton DE, Fenner-Crisp PA. The human relevance of information on carcinogenic modes of action: Overview. Crit. Rev. Toxicol. (2003) 33:581–589.[Web of Science][Medline]
Ehresman DJ, Froelich JW, Olsen GW, Chang S-C, Butenhoff JL. Comparison of whole blood, plasma, and serum matrices for the determination of perfluorooctanesulfonate (PFOS), perfluorooctanoate (PFOA), and other fluorochemicals. Environ. Res. (2007) 103:176–184.[Medline]
Emmett EA, Shofer FS, Zhang H, Freeman D, Desai C, Shaw LM. Community exposure to perfluorooctanoate: Relationships between serum concentrations and exposure sources. J. Occup. Environ. Med. (2006b) 48:759–770.[CrossRef][Web of Science][Medline]
Emmett EA, Zhang H, Shofer FS, Freeman D, Rodway NV, Desai C, Shaw LM. Community exposure to perfluorooctanoate: Relationships between serum levels and certain health parameters. J. Occup. Environ. Med. (2006a) 48:771–779.[CrossRef][Web of Science][Medline]
Fei C, McLaughlin JK, Tarone RE, Olsen J. Perfluorinated chemicals and fetal growth: A study within the Danish National Birth Cohort. Environ. Health Perspect. (2007) doi:10.1289/ehp.10506.
Fewell Z, Smith GD, Sterne JAC. The impact of residual and unmeasured confounding in episdemiologic studies: A simulation study. Am. J. Epidemiol. (2007) 166:646–655.
Giesy JP, Kannan K. Global distribution of perfluorooctane sulfonate in wildlife. Environ. Sci. Technol. (2001) 35:1339–1342.[Medline]
Grasty RC, Bjork JA, Wallace KB, Wolf DC, Lau CS, Rogers JM. Effects of prenatal perfluorooctane sulfonate (PFOS) exposure on lung maturation in the perinatal rat. Birth Defects Res. B Dev. Reprod. Toxicol. (2005) 74:405–416.[CrossRef][Web of Science][Medline]
Grasty RC, Wolf DC, Grey BE, Lau CS, Rogers JM. Prenatal window of susceptibility to perfluorooctane sulfonate-induced neonatal mortality in the Sprague-Dawley rat. Birth Defects Res. B Dev. Reprod. Toxicol. (2003) 68:465–471.[CrossRef][Web of Science][Medline]
Grice MM, Alexander BH, Hoffbeck R, Kampa DM. Self-reported medical conditions in perfluorooctanesulfonyl fluoride manufacturing workers. J. Occup. Environ. Med. (2007) 49:722–729.[CrossRef][Web of Science][Medline]
Hansen KJ, Clemen LA, Ellefson ME, Johnson HO. Compound-specific, quantitative characterization of organic fluorochemicals in biological matrices. Environ. Sci. Technol. (2001) 35:766–770.[Medline]
Hundley SG, Sarrif AM, Kennedy GL. Absorption, distribution, and excretion of ammonium perfluorooctanoate (APFO) after oral administration to various species. Drug Chem. Toxicol. (2006) 29:137–145.[CrossRef][Web of Science][Medline]
Katakura M, Kudo N, Tsuda T, Hibino Y, Mitsumoto A, Kawashima Y. Rat organic anion transporter 3 and organic anion transporting polypeptide 1 mediate perfluorooctanoic acid transport. J. Health Sci. (2007) 53:77–83.[CrossRef]
Kennedy GL Jr., Butenhoff JL, Olsen GW, O'Connor JC, Seacat AM, Perkins RG, Biegel LB, Murphy SR, Farrar DG. The toxicology of perfluorooctanoate. Crit. Rev. Toxicol. (2004) 34:351–384.[CrossRef][Web of Science][Medline]
Klaunig JE, Babich MA, Baetcke KP, Cook JC, Corton JC, David RM, DeLuca JG, Lai DY, McKee RH, Peters JM, et al. PPARalpha agonist-induced rodent tumors: Modes of action and human relevance. Crit. Rev. Toxicol. (2003) 33:655–680.[Web of Science][Medline]
Kudo N, Bandai N, Suzuki E, Katakura M, Kawashima Y. Induction by perfluorinated fatty acids with different carbon chain length of peroxisomal beta-oxidation in the liver of rats. Chem. Biol. Interact. (2000) 124:119–132.[CrossRef][Web of Science][Medline]
Kudo N, Katakura M, Sato Y, Kawashima Y. Sex hormone-regulated renal transport of perfluorooctanoic acid. Chem. Biol. Interact. (2002) 139:301–316.[CrossRef][Web of Science][Medline]
Kudo N, Kawashima Y. Toxicity and toxicokinetics of perfluorooctanoic acid in humans and animals. J. Toxicol. Sci. (2003) 28:49–57.[CrossRef][Medline]
Kudo N, Sakai A, Mitsumoto A, Hibino Y, Tsuda T, Kawashima Y. Tissue distribution and hepatic subcellular distribution of perfluorooctanoic Acid at low dose are different from those at high dose in rats. Biol. Pharm. Bull. (2007) 30:1535–1540. JST.JSTAGE/bpb/30.1535 [pii].[CrossRef][Web of Science][Medline]
Kudo N, Suzuki E, Katakura M, Ohmori K, Noshiro R, Kawashima Y. Comparison of the elimination between perfluorinated fatty acids with different carbon chain length in rats. Chem. Biol. Interact. (2001) 134:203–216.[CrossRef][Web of Science][Medline]
Kudo N, Suzuki-Nakajima E, Mitsumoto A, Kawashima Y. Responses of the liver to perfluorinated fatty acids with different carbon chain length in male and female mice: In relation to induction of hepatomegaly, peroxisomal beta-oxidation and microsomal 1-acylglycerophosphocholine acyltransferase. Biol. Pharm. Bull. (2006) 29:1952–1957.[CrossRef][Web of Science][Medline]
Lau C, Anitole K, Hodes C, Lai D, Pfahles-Hutchens A, Seed J. Perfluoroalkyl acids: A review of monitoring and toxicological findings. Toxicol. Sci. (2007) 99:366–394. doi:10.1093/toxsci/kfm128.
Lau C, Butenhoff JL, Rogers JM. The developmental toxicity of perfluoroalkyl acids and their derivatives. Toxicol. Appl. Pharmacol. (2004) 198:231–241.[CrossRef][Web of Science][Medline]
Lau C, Thibodeaux JR, Hanson RG, Narotsky MG, Rogers JM, Lindstrom AB, Strynar MJ. Effects of perfluorooctanoic acid exposure during pregnancy in the mouse. Toxicol. Sci. (2006) 90:510–518.
Lau C, Thibodeaux JR, Hanson RG, Rogers JM, Grey BE, Stanton ME, Butenhoff JL, Stevenson LA. Exposure to perfluorooctane sulfonate during pregnancy in rat and mouse. II: Postnatal evaluation. Toxicol. Sci. (2003) 74:382–392.
Leonard RC, Kreckmann KH, Sakr CJ, Symons JM. Retrospective Cohort Mortality Study of Workers in a Polymer Production Plant Including a Reference Population of Regional Workers. Ann Epidemiol. (2007) [Epub ahead of print].
Luebker DJ, Case MT, York RG, Moore JA, Hansen KJ, Butenhoff JL. Two-generation reproduction and cross-foster studies of perfluorooctanesulfonate (PFOS) in rats. Toxicology (2005a) 215:126–148.[CrossRef][Web of Science][Medline]
Luebker DJ, York RG, Hansen KJ, Moore JA, Butenhoff JL. Neonatal mortality from in utero exposure to perfluorooctanesulfonate (PFOS) in Sprague-Dawley rats: Dose-response, and biochemical and pharmacokinetic parameters. Toxicology (2005b) 215:149–169.[CrossRef][Web of Science][Medline]
Midasch O, Drexler H, Hart N, Beckmann MW, Angerer J. Transplacental exposure of neonates to perfluorooctanesulfonate and perfluorooctanoate: A pilot study. Int. Arch. Occup. Environ. Health (2007) 80:643–648.[CrossRef][Web of Science][Medline]
Morimura K, Cheung C, Ward JM, Reddy JK, Gonzalez FJ. Differential susceptibility of mice humanized for peroxisome proliferator-activated receptor alpha to Wy-14,643-induced liver tumorigenesis. Carcinogenesis (2006) 27:1074–1080.
Ohmori K, Kudo N, Katayama K, Kawashima Y. Comparison of the toxicokinetics between perfluorocarboxylic acids with different carbon chain length. Toxicology (2003) 184:135–140.[CrossRef][Web of Science][Medline]
Olsen GW, Burris JM, Ehresman DJ, Froehlich JW, Seacat AM, Butenhoff JL, Zobel LR. Half-life of serum elimination of perfluorooctanesulfonate, perfluorohexanesulfonate, and perfluorooctanoate in retired fluorochemical production workers. Environ. Health Perspect. (2007a) 115:1298–1305.[Web of Science][Medline]
Olsen GW, Church TR, Miller JP, Burris JM, Hansen KJ, Lundberg JK, Armitage JB, Herron RM, Medhdizadehkashi Z, Nobiletti JB, et al. Perfluorooctanesulfonate and other fluorochemicals in the serum of American Red Cross adult blood donors. Environ. Health Perspect. (2003a) 111:1892–1901.[Web of Science][Medline]
Olsen GW, Logan PW, Hansen KJ, Simpson CA, Burris JM, Burlew MM, Vorarath PP, Venkateswarlu P, Schumpert JC, Mandel JH. An occupational exposure assessment of a perfluorooctanesulfonyl fluoride production site: Biomonitoring. Am. Ind. Hyg. Assoc. J. (2003b) 64:651–659.
Olsen GW, Mair DC, Reagen WK, Ellefson ME, Ehresman DJ, Butenhoff JL, Zobel LR. Preliminary evidence of a decline in perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA) concentrations in American Red Cross blood donors. Chemosphere (2007b) 68:105–111.[Medline]
Olsen GW, Zobel LR. Assessment of lipid, hepatic, and thyroid parameters with serum perfluorooctanoate (PFOA) concentrations in fluorochemical production workers. Int. Arch. Occup. Environ. Health. (2007) 81:231–246.[CrossRef][Web of Science][Medline]
Organisation for Economic Co-operation and Development. (2002) ENV/JM/RD(2002)17/FINAL: Hazard assessment of perfluorooctane sulfonate (PFOS) and its salts. Report of the Environment Directorate, Joint Meeting of the Chemicals Committee and the Working Party on Chemicals, Pesticides and Biotechnology, Co-operation on Existing Chemicals. Paris, November 21, 2002.
Rosen MB, Thibodeaux JR, Wood CR, Zehr RD, Schmid JE, Lau C. Gene expression profiling in the lung and liver of PFOA-exposed mouse fetuses. Toxicology (2007) 239:15–33.[CrossRef][Web of Science][Medline]
Sakr CJ, Leonard RC, Kreckmann KH, Slade MD, Cullen MR. Longitudinal study of serum lipids and liver enzymes in workers with occupational exposure to ammonium perfluorooctanoate. J. Occup. Environ. Med. (2007a) 49:872–879.[CrossRef][Web of Science][Medline]
Sakr CJ, Kreckmann KH, Green JW, Gillies PJ, Reynolds JL, Leonard RC. Cross-Sectional Study of Lipids and Liver Enzymes Related to a Serum Biomarker of Exposure (ammonium perfluorooctanoate or APFO) as Part of a General Health Survey in a Cohort of Occupationally Exposed Workers. J Occup Environ Med. (2007b) 49:1086–1096.[Web of Science][Medline]
Shah YM, Morimura K, Yang Q, Tanabe T, Takagi M, Gonzalez FJ. Peroxisome proliferator-activated receptor alpha regulates a microRNA-mediated signaling cascade responsible for hepatocellular proliferation. Mol. Cell. Biol. (2007) 27:4238–4247.
Taves DR. Evidence that there are two forms of fluoride in human serum. Nature (1968) 217:1050–1051.[Medline]
Thibodeaux JR, Hanson RG, Rogers JM, Grey BE, Barbee BD, Richards JH, Butenhoff JL, Stevenson LA, Lau C. Exposure to perfluorooctane sulfonate during pregnancy in rat and mouse. I: Maternal and prenatal evaluations. Toxicol. Sci. (2003) 74:369–381. [Erratum to Toxicol. Sci. 2003; 74; 369-381: Toxicol. Sci. 82, 359.].
Vanden Heuvel JP, Thompson JT, Frame SR, Gillies PJ. Differential activation of nuclear receptors by perfluorinated fatty acid analogs and natural fatty acids: A comparison of human, mouse, and rat peroxisome proliferator-activated receptor-alpha, -beta, and -gamma, liver X receptor-beta, and retinoid X receptor-alpha. Toxicol. Sci. (2006) 92:476–489.
Wolf CJ, Fenton SE, Schmid JE, Calafat AM, Kuklenyik Z, Bryant XA, Thibodeaux J, Das KP, White SS, Lau CS, et al. Developmental toxicity of perfluorooctanoic acid in the CD-1 mouse after cross-foster and restricted gestational exposures. Toxicol. Sci. (2007) 95:462–473.
Yang Q, Xie Y, Depierre JW. Effects of peroxisome proliferators on the thymus and spleen of mice. Clin. Exp. Immunol. (2000) 122:219–226.[CrossRef][Web of Science][Medline]
Yang Q, Abedi-Valugerdi M, Xie Y, Zhao XY, Moller G, Nelson BD, DePierre JW. Potent suppression of the adaptive immune response in mice upon dietary exposure to the potent peroxisome proliferator, perfluorooctanoic acid. Int. Immunopharmacol. (2002a) 2:389–397.[CrossRef][Web of Science][Medline]
Yang Q, Xie Y, Alexson SE, Nelson BD, DePierre JW. Involvement of the peroxisome proliferator-activated receptor alpha in the immunomodulation caused by peroxisome proliferators in mice. Biochem. Pharmacol. (2002b) 63:1893–1900.[CrossRef][Web of Science][Medline]
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