Although many of the hepatic responses observed on the
Although many of the hepatic responses observed on the first day of recovery attenuated over the course of the recovery period, minimal-to-mild centrilobular hepatocellular hypertrophy tended to persist. Mean absolute and relative liver weights had returned to control levels after 28 days of recovery. The apparent increase based on statistical significance in relative liver weight on recovery Day 84 likely was reflective of a heavier control body weight (558g in control versus 524g and 510g in the 20 and 100ppm dietary dose groups, respectively). This interpretation is supported by the lack of an increased relative liver weight on recovery Days 28 and 56. In addition, the relative liver weights for K+PFOS-treated rats on recovery Day 84 were similar to those from controls and treated groups on recovery Day 56. Mean liver DNA concentration remained low in the 100ppm group relative to controls through Day 56 of recovery. Mean liver P450 concentrations remained elevated relative to controls throughout the recovery period in both K+PFOS-treated groups; although, they appeared to attenuate over time. Mean liver total DNA, similar to controls on recovery Day 1, was decreased at both treatment levels relative to controls on Days 28 and 56 of recovery, returning to control levels on recovery Day 84. The incidence and severity of centrilobular hepatocellular hypertrophy remained elevated and the incidence and severity glycogen-induced vacuolation remained decreased relative to controls in a dose-dependent manner through Day 84 of recovery. The enzyme activities responsive to PPARα and CAR/PXR induction gradually attenuated during recovery. At the end of recovery Day 84, the activity of testosterone 6 β-hydroxylase remained elevated relative to controls, while activities of acyl CoA oxidase, lauric new relationships 12-hydroxylase, and pentoxyresorufin-O-depentylase approximated control levels of activity or were slightly lower than control levels. The persistent increase in P450 concentrations at the end of recovery corresponded to hepatocellular hypertrophy and may be the result of poor clearance of PFOS. The observed dose-dependent increase in liver proliferative index on recovery Day 1 was no longer present at other recovery time points. In contrast, the dose-dependent decreased liver apoptotic index observed on recovery Day 1 persisted through the 84-day recovery period. Both of these effects are key events in the mode of action for rodent liver tumor formation for agents that activate PPARα and CAR/PXR (Holsapple et al., 2006, Klaunig et al., 2003, Lake, 2009). The persistence of decreased hepatic apoptosis through 84 days of recovery in the present study suggests that damaged hepatocytes are not being removed and replaced and may increase the potential opportunity for clonal expansion of initiated/transformed hepatocytes on proliferative stimulus (Schattenberg et al., 2011). There were no significant elevations in liver enzymes (ALT or AST) during this study, suggesting absence of overt hepatic toxicity. A reduction of serum total cholesterol was evident at the beginning of the recovery period in K+PFOS-treated rats of both dietary dose groups. This effect appeared to be stronger on Day 28 of recovery than on Day 1 and to return to levels similar to the controls on Days 56 and 84 of recovery; although, serum total cholesterol was reduced with significance in the 100ppm group on recovery Day 84. The latter observation may represent a lingering depression of serum total cholesterol; however, it could also have been a chance observation, as a similar finding was not present on Day 56. Cholesterol lowering is consistent with previously reported observations (Bijland et al., 2011, Curran et al., 2008, Seacat et al., 2002, Seacat et al., 2003) and likely is modulated, in part, by enhanced PPAR-mediated fatty acid oxidation. The attenuation of effects in recovery followed diminishing concentrations of PFOS in serum, liver, and liver cytosol. Similar to the previously reported serum elimination half-lives for PFOS (Benskin et al., 2009, Chang et al., in press), the serum PFOS elimination half-lives during the 84-day recovery phase in the study reported herein were of the order of one month. Liver PFOS elimination half-lives were on the order of two months.