ToxSci Advance Access originally published online on June 26, 2009
Toxicological Sciences 2009 111(1):1-3; doi:10.1093/toxsci/kfp142
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Published by Oxford University Press 2009.
Does Exposure to Perfluoroalkyl Acids Present a Risk to Human Health?
* Integrated Systems Toxicology Division
Toxicity Assessment Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina
1 To whom correspondence should be addressed at U.S. Environmental Protection Agency, MD 72, Research Triangle Park, NC 27711. Fax: (919) 541-4017. E-mail: rosen.mitch{at}epa.gov.
Received June 16, 2009; accepted June 18, 2009
ABSTRACT
The information in this document has been funded by the U.S. Environmental Protection Agency. It has been subjected to review by the National Health and Environmental Effects Research Laboratory and approved for publication. Approval does not signify that the contents reflect the views of the Agency or does mention of trade names or commercial products constitute endorsement or recommendation for use.
Key Words: PPAR
; tumor; rodent; PFOA; PFOS; peroxisome proliferator; liver; risk assessment.
In the current issue of Toxicological Sciences, Bjork and Wallace (2009)
provide data suggesting that traditional rodent models overestimate the hepatocarcinogenic risk of perfluoroalkyl acids (PFAAs) in humans. Considerable attention has recently been paid to PFAAs as a class of persistent organic pollutants. Reports concerning the environmental exposure and toxicology of these chemicals emerged a decade ago, and these investigations have intensified in the past 5 years. Specific PFAAs are ubiquitous in the environment, resistant to biological and environmental degradation, and found in the blood of both wildlife and humans (Lau et al., 2007). Two 8-carbon PFAAs, perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), are most commonly reported in monitoring studies, although lower levels of a number of perfluorinated sulfonic and carboxylic acids of varying chain length are found well. Because of their surfactant properties, these compounds have a role in the manufacture of a wide array of industrial and household products. This includes the use of PFOA in the manufacture of polytetrafluoroethylene (Teflon) a fluoropolymer perhaps best known for its use in nonstick cookware. So great is the concern among the general public that "PFOA-free cookware" is now readily available in the marketplace.
Blood levels of PFOS and PFOA have gradually declined in recent years, in part because the primary U.S. manufacturer of PFOS began phasing out production of the compound in 2002 and because reduced industrial emissions of PFOA and related chemicals have begun to be realized (Calafat et al., 2007). Furthermore, under the U.S. Environmental Protection Agency 2010/2015 PFOA Stewardship Program, eight principle manufacturers of PFAAs have committed to a 95% reduction in the emission and product content of PFOA and/or related chemicals by 2010, with a further goal of 100% reduction by 2015. Nevertheless, the persistence of certain PFAAs in the environment along with their long biological half-lives in humans suggests that attention will be paid to the potential health effects of these compounds for some time to come.
PFAAs are capable of activating the nuclear receptor peroxisome proliferator–activated receptor alpha (PPAR). Once activated, PPAR is responsible for regulating the transcription of genes involved in a number of biological processes including lipid metabolism, inflammation, and cell growth. A group of structurally diverse compounds known as peroxisome proliferators also function, at least in part, by activating PPAR. These include fibrate pharmaceuticals, phthalate ester plasticizers, solvents such as trichloroethylene and perchloroethylene, and certain herbicides. In the liver of rats and mice, exposure to PFAAs and other peroxisome proliferators is related to increased fatty acid catabolism, hepatocyte hypertrophy/proliferation, and tumor induction. The mode of action leading to liver tumor formation is not fully understood for these compounds, although it is thought to be PPAR dependent since none of the short-term effects associated with tumor induction are observed in PPAR-null mice exposed to fibrates nor is tumor induction itself found in these animals (Hays et al., 2005; Peters et al., 1997). Compared to other test species, rats and mice are particularly robust in their response to peroxisome proliferators leading to the hypothesis that laboratory studies overestimate the risk to human health. Humans, e.g., have been exposed to peroxisome proliferators in the form of fibrate drugs for decades without apparent adverse effects.
In this issue, Bjork and Wallace (2009) examined human responsiveness to PFAAs using primary human hepatocytes and human hepatoma HepG2/C3a cells. PPAR activity was estimated by measuring changes in the expression of specific PPAR marker genes and comparing these effects to those in primary rat hepatocytes. To avoid potential confounding effects due to cell injury, Bjork and Wallace were also careful to evaluate the expression of DNA damage–inducible transcript 3 (Ddit3), a transcription factor upregulated as part of the stress-activated unfolded protein response. Their investigation builds upon existing data indicating that human hepatocytes do not respond to peroxisome proliferators in the same way that rodent cells do. In their study, increased expression of acyl-CoA oxidase 1 (ACOX1 or ACOX), acyl-CoA thioesterase 1 (ACOT1 or CTE1), and CYP4A1 were observed in primary rat hepatocytes following exposure to varying concentrations of either PFOA or perfluorobutane sulfonate. In contrast, with the exception of modest increases in the expression of CYP4A11, the human ortholog of CYP4A1, similar changes were not found in primary human hepatocytes. Moreover, no changes were observed in HepG2 cells. All three genes are involved in fatty acid metabolism and are known to be regulated by PPAR in rats. In a second experiment, the authors conducted a structure-activity analysis in which rat and human cells were monitored for responsiveness after exposure to perfluorinated carboxylic and sulfonic acids of varying chain length. They found a tendency in primary rat hepatocytes for longer chain perfluorinated carboxylic acids to be more potent activators of PPAR compared to shorter chain compounds or perfluorinated sulfonates, a result in agreement with recent transient transfection data (Wolf et al., 2008). As in the first experiment, however, the predicted PPAR-related response was not observed in human cells. They concluded that rodent data may not be a relevant indicator of PFAA risk in humans.
The observation that the rat or mouse liver responds differently to peroxisome proliferators than that of the human, dog, guinea pig, Syrian hamster, or nonhuman primate is hardly new and has been used to conclude that the mode of action associated with PPAR-dependent liver tumor induction in rodents is not relevant to humans (Klaunig et al., 2003). In contrast to rats and mice, PPAR agonists are at best weak inducers of ACOX1 in human liver cells. Nor is peroxisome proliferation, a hallmark of PPAR activation in rats and mice, readily observed in patients treated with fibrate drugs. The potential explanation for such differences has been well described and includes reduced levels of liver PPAR messenger RNA, the presence of a truncated variant of the human PPAR protein that functions as a dominant-negative form of the receptor and species-related differences in the structure of the peroxisome proliferator response element in certain genes. In one school of thought, even if human PPAR levels could approach those in rodents, there is no evidence that the receptor is "wired" the same way in the human liver. Evidence for this comes from studies using the PPAR-humanized mouse in which PPAR-null mice have been engineered to express human PPAR at levels comparable to those in rodents. Two different models have been studied including a liver-specific humanized mouse model and a mouse carrying human PPAR under the control of its own promoter resulting in a tissue expression pattern similar to that found in wild-type mice. While fibrates similarly alter the expression of genes associated with lipid metabolism in both wild-type and PPAR-humanized mice, these compounds do not induce hepatocyte proliferation in the PPAR-humanized mouse. In addition, as has been shown so far for the liver-specific mouse model, liver tumor formation is not observed in PPAR-humanized mice, thus reinforcing the concept that the effects of peroxisome proliferators are species specific (reviewed by Gonzalez and Shah, 2008).
Given such evidence, we might conclude that the PPAR-dependent mode of action proposed for hepatic tumor induction in rodents is not relevant to humans. Available data regarding peroxisome proliferators, however, are primarily based on exposure to fibric acid derivatives and not environmental contaminants. Recent arguments have also attempted to bring this mode of action into question, in part due to the observation that liver tumors can be observed in PPAR-null mice following a chronic exposure to the plasticizer and peroxisome proliferator di(2-ethylhexyl)phthalate (DEHP) (Guyton et al., 2009). However, this argument might be likened to throwing the baby out with the bathwater. Recent studies indicate that DEHP, along with several PFAAs, activate the constitutive androstane receptor (CAR), the nuclear receptor required for phenobarbital-induced liver cancer (Cheng and Klaassen, 2008; Eveillard et al., 2009; Ren et al., 2009). The picture emerging from these studies is that in wild-type mice, PPAR mediates the vast majority of transcriptional and phenotypic effects of peroxisome proliferators, while CAR plays a minor role. In contrast, CAR appears to be a more dominate player in PPAR-null mice and is the likely culprit in the induction of the PPAR-independent liver tumors induced by DEHP. Overall, the current weight of evidence from humanized and nullizygous mouse studies, as well as studies in human primary hepatocytes such as those by Bjork and Wallace, supports the point of view that the rodent PPAR mode of action associated with tumor induction is not operational in human hepatocytes.
Future PFAA research should also focus on those toxicities not related to liver tumor formation. In utero exposure to PFOA in mice alters mammary gland development (White et al., 2007) and has effects at low dose on body weight and serum parameters such as circulating levels of insulin and leptin when measured in animals during mid-life (Hines et al., 2009). PFOA and PFOS are also considered immunotoxicants in wild-type mice as well as in PPAR-null mice treated with PFOA (DeWitt et al., 2009). Moreover, compounds such as PFOA and PFOS cause deficits in neonatal growth and viability in both rats and mice (Lau et al., 2007). Interestingly, the mode of action associated with PFOS-induced neonatal mortality has been shown in mice to be independent of PPAR (Abbott et al., 2008). Along those lines, recent epidemiological data suggest a negative association between estimates of maternal exposure to PFAAs and fetal growth or fertility in humans. However, a number of concerns have been raised about these data including the possibility that they may not be the result of a true causal relationship but might, in the case of an outcome such as birth weight, reflect confounding with covariables such as maternal plasma volume expansion (Olsen et al., 2009).
So are we ready to conclude that the results of rodent studies overestimate the risk of PFAAs in the human population? That may very well be the case in terms of hepatocarcinogenic risk, although questions may still remain. More work is needed to further understand the modes of action related to other toxicities observed in laboratory animals and to establish relative human risk. Like most toxicants, PFAAs are likely to have multiple modes of action. That appears to be the case with PFOS in the developing mouse. Future studies should also take the combinatorial effects of various PFAAs into consideration because biomonitoring studies have revealed a mixture of PFAAs in humans and wildlife. The study of Bjork and Wallace is in close agreement with existing data for other peroxisome proliferators, which indicate that in human hepatocytes the PPAR response is qualitatively and quantitatively different than that of rats or mice. Such a conclusion is a significant contribution to risk assessment since it suggests that certain PFAAs need not be considered outside the discussion already underway for peroxisome proliferators and PPAR-dependent liver toxicity.
FUNDING
The information in this document has been funded by the U.S. Environmental Protection Agency. It has been subjected to review by the National Health and Environmental Effects Research Laboratory 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.
REFERENCES
Abbott BD, Wolf CJ, Das KP, Zehr RD, Schmid JE, Lindstrom AB, Strynar MJ, Lau C. Developmental toxicity of perfluorooctane sulfonate (PFOS) is not dependent on expression of peroxisome proliferator activated receptor-alpha (PPARalpha) in the mouse. Reprod. Toxicol. (2008) 27:258–265.[CrossRef][Web of Science][Medline]
Bjork J, Wallace K. Structure-activity relationships and human relevance for perfluoroalkyl acid-induced transcriptional activation of peroxisome proliferation in liver cell cultures. Toxicol. Sci. (2009) doi: 10.1093/toxsci/kfp093.
Calafat AM, Wong LY, Kuklenyik 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 with NHANES 1999-2000. Environ. Health Perspect. (2007) 115:1596–1602.[Web of Science][Medline]
Cheng X, Klaassen CD. Perfluorocarboxylic acids induce cytochrome P450 enzymes in mouse liver through activation of PPAR-alpha and CAR transcription factors. Toxicol. Sci. (2008) 106:29–36.
DeWitt JC, Shnyra A, Badr MZ, Loveless SE, Hoban D, Frame SR, Cunard R, Anderson SE, Meade BJ, Peden-Adams MM, et al. Immunotoxicity of perfluorooctanoic acid and perfluorooctane sulfonate and the role of peroxisome proliferator-activated receptor alpha. Crit. Rev. Toxicol. (2009) 39:76–94.[CrossRef][Web of Science][Medline]
Eveillard A, Mselli-Lakhal L, Mogha A, Lasserre F, Polizzi A, Pascussi JM, Guillou H, Martin PG, Pineau T. Di-(2-ethylhexyl)-phthalate (DEHP) activates the constitutive androstane receptor (CAR): A novel signalling pathway sensitive to phthalates. Biochem. Pharmacol. (2009) 77:1735–1746.[CrossRef][Web of Science][Medline]
Gonzalez FJ, Shah YM. PPARalpha: Mechanism of species differences and hepatocarcinogenesis of peroxisome proliferators. Toxicology (2008) 246:2–8.[Web of Science][Medline]
Guyton KZ, Chiu WA, Bateson TF, Jinot J, Scott CS, Brown RC, Caldwell JC. A reexamination of the PPAR-alpha activation mode of action as a basis for assessing human cancer risks of environmental contaminants. Environ. Health Perspect. (2009) 10.1289/ehp 0900758. (Online 15 May 2009).
Hays T, Rusyn I, Burns AM, Kennett MJ, Ward JM, Gonzalez FJ, Peters JM. Role of peroxisome proliferator-activated receptor-alpha (PPARalpha) in bezafibrate-induced hepatocarcinogenesis and cholestasis. Carcinogenesis (2005) 26:219–227.
Hines EP, White SS, Stanko JP, Gibbs-Flournoy EA, Lau C, Fenton SE. Phenotypic dichotomy following developmental exposure to perfluorooctanoic acid (PFOA) in female CD-1 mice: Low doses induce elevated serum leptin and insulin, and overweight in mid-life. Mol. Cell Endocrinol. (2009) 304:97–105.[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) 33:655–780.[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.
Olsen GW, Chang SC, Noker PE, Gorman GS, Ehresman DJ, Lieder PH, Butenhoff JL. A comparison of the pharmacokinetics of perfluorobutanesulfonate (PFBS) in rats, monkeys, and humans. Toxicology (2009) 256:65–74.[CrossRef][Web of Science][Medline]
Peters JM, Cattley RC, Gonzalez FJ. Role of PPAR alpha in the mechanism of action of the nongenotoxic carcinogen and peroxisome proliferator Wy-14,643. Carcinogenesis (1997) 18:2029–2033.
Ren H, Vallanat B, Nelson DM, Yeung LW, Guruge KS, Lam PK, Lehman-McKeeman LD, Corton JC. Evidence for the involvement of xenobiotic-responsive nuclear receptors in transcriptional effects upon perfluoroalkyl acid exposure in diverse species. Reprod. Toxicol. (2009) 27:266–277.[CrossRef][Web of Science][Medline]
White SS, Calafat AM, Kuklenyik Z, Villanueva L, Zehr RD, Helfant L, Strynar MJ, Lindstrom AB, Thibodeaux JR, et al. Gestational PFOA exposure of mice is associated with altered mammary gland development in dams and female offspring. Toxicol. Sci. (2007) 96:133–144.
Wolf CJ, Takacs ML, Schmid JE, Lau C, Abbott BD. Activation of mouse and human peroxisome proliferator-activated receptor alpha by perfluoroalkyl acids of different functional groups and chain lengths. Toxicol. Sci. (2008) 106:162–171.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||