Subacute exposure to N-ethyl perfluorooctanesulfonamidoethanol results in the formation of perfluorooctanesulfonate and alters superoxide dismutase activity in female rats

Abstract

Perfluorooctanesulfonamides, such as N-ethyl perfluorooctanesulfonamidoethanol (N-EtFOSE), are large scale industrial chemicals but their disposition and toxicity are poorly understood despite significant human exposure. The hypothesis that subacute exposure to N-EtFOSE, a weak peroxisome proliferator, causes a redox imbalance in vivo was tested using the known peroxisome proliferator, ciprofibrate, as a positive control. Female Sprague-Dawley rats were treated orally with N-EtFOSE, ciprofibrate or corn oil (vehicle) for 21 days, and levels of N-EtFOSE and its metabolites as well as markers of peroxisome proliferation and oxidative stress were assessed in serum, liver and/or uterus. The N-EtFOSE metabolite profile in liver and serum was in good agreement with reported in vitro biotransformation pathways in rats and the metabolite levels decreasing in the order perfluorooctanesulfonate >> perfluorooctanesulfonamide ~ N-ethyl perfluorooctanesulfonamidoacetate >> perfluorooctanesulfonamidoethanol approximately N-EtFOSE. Although N-EtFOSE treatment significantly decreased the growth rate, increased relative liver weight and activity of superoxide dismutases (SOD) in liver and uterus (total SOD, CuZnSOD and MnSOD), a metabolic study revealed no differences in the metabolome in serum from N-EtFOSE-treated and control animals. Ciprofibrate treatment increased liver weight and peroxisomal acyl Co-A oxidase activity in the liver and altered antioxidant enzyme activities in the uterus and liver. According to NMR metabolomic studies, ciprofibrate treated animals had altered serum lipid profiles compared to N-EtFOSE-treated and control animals, whereas putative markers of peroxisome proliferation in serum were not affected. Overall, this study demonstrates the biotransformation of N-EtFOSE to PFOS in rats that is accompanied by N-EtFOSE-induced alterations in antioxidant enzyme activity.

Figure 1

Simplified biotransformation pathways of N-EtFOSE: PFOS is the major metabolite of N-EtFOSE detected in the liver and serum of female rats. Several other metabolites, including FOSA and N-EtFOSAA, were also detected in agreement with the N-EtFOSE biotransformation pathway proposed by Xu and co-workers (Xu et al., 2004). Major metabolites are shown in bold. Metabolites shown in parentheses were either not detected (N-EtFOSA) or not analyzed (FOSAA). O- and N-glucuronide metabolites are omitted for clarity reasons. N-EtFOSE was administered orally over a three week period, animals were euthanized on day 21 and liver and serum samples were analyzed for selected metabolites as described under Materials and Methods. The chemical names of the metabolites are listed under Methods and Materials.

Figure 2

Projections of Principal Component 1 versus Principal Component 2 from the Principal Component Analysis (PCA) of the data from the metabolomic study reveal two groupings: One grouping with ciprofibrate-treated animals and a second grouping of N-EtFOSE-treated and control animal. Serum was mixed in a 1:1 ratio (v/v) with phosphate buffer (pH = 7.4) containing TSP as internal standard and CPMG spectra were recorded on a 600-NMR instrument as described under Materials and Methods. The spectra were integrated manually and adjusted for TSP prior to PCA. Each word-label in the Figure denotes one animal.

Figure 3

AOX activity in liver of female Sprague-Dawley treated with N-EtFOSE was not increased, whereas a significant increase was observed in ciprofibrate-treated animals. Data are presented as mean ± S.E.M.. *** p < 0.001.

Figure 4

N-EtFOSE increased the activity of total SOD, MnSOD and CuZnSOD in (A) the uterus and (B) the liver. The only exception was MnSOD activity in the uterus. In contrast, ciprofibrate significantly decreased MnSOD activity and increased CuZnSOD activity in the uterus. Otherwise, ciprofibrate did not significantly alter SOD activities. Data are presented as mean ± S.E.M.; * p < 0.05; ** p < 0.01; *** p < 0.001.

Figure 5

N-EtFOSE significantly increased the activity of catalase in the uterus but had no significant effect on hepatic catalase activity. Ciprofibrate did not alter catalase activity. Data are presented as mean ± S.E.M.. *** p < 0.001.

Figure 6

N-EtFOSE significantly decreased the activity of total GPx in the uterus (A), but not the liver (B), and had no effect on GPx-1 activity. Ciprofibrate significantly decreased total GPx activity in the liver and uterus and GPx-1 activity in the liver. Data are presented as mean ± S.E.M.. * p < 0.05; ** p < 0.01; *** p < 0.001.