Thyroid Hormone Administration Induces Rat Liver Nrf2 Activation: Suppression by N-Acetylcysteine Pretreatment

Background: Oxidative stress associated with 3,3',5-triiodo-L-thyronine (T-3)-induced calorigenesis upregulates the hepatic expression of mediators of cytoprotective mechanisms. The aim of this study was to evaluate the hypothesis that in vivo T-3 administration triggers a redox-mediated translocation of the cytoprotective nuclear transcription factor erythroid 2-related factor 2 (Nrf2) from the cytosol to the nucleus in rat liver. Such translocation of transcription factors is considered to be an activating step. Materials and Methods: The effect of T-3 administration in the presence and absence of N-acetylcysteine (NAC) on cytosol-to-nuclear translocation of Nrf2 was evaluated, with inhibition of this process by NAC taken as evidence that the process was redox mediated. Male Sprague-Dawley rats weighing 180-200 g were given a single intraperitoneal dose of 0.1 mg T-3/kg. Another group of rats were given the same dose of T-3 and were also pretreated with NAC (0.5 g/kg) at 0.5 hour before T-3 administration. Two other groups of rats received vehicle treatment and NAC, respectively. Following these treatments, rectal temperature of the animals, liver O-2 consumption, serum and hepatic levels of 8-isoprostanes, and liver protein levels of Nrf2, Akt, p38, and thioredoxin (Western blot) were determined at different times up to 48 hours. Results: T-3 administration induced a significant increase in the hepatic nuclear levels of Nrf2 at 1 and 2 hours after treatment and a concomitant decrease in cytosolic Nrf2. It also increased hepatic thioredoxin, a protein whose gene transcription is induced by nuclear Nrf2. Levels of nuclear Nrf2 were at a plateau from 4 to 6 hours after T-3. Rectal temperature of the animals rose from 36.6 degrees C to 37.5 degrees C as did liver O-2 consumption. Serum and liver 8-isoprostanes levels increased (p < 0.05) from 38.4 +/- 4.0 pg/mL (n - 4) to 69.2 +/- 2.0 pg/mL (n - 3) and from 0.75 +/- 0.09 ng/g liver (n - 3) to 1.53 +/- 0.10 ng/g liver (n - 5), respectively. In the group of rats pretreated with NAC, the increase in cytosol-to-nuclear translocation of Nrf2 was only 28% that induced by T-3. In addition, T-3 induced liver Akt and p38 activation during the period of 1-4 hours after T-3 administration. p38 activation at 2 hours after T-3 administration was abolished in NAC-pretreated animals. Conclusions: In vivo T-3 administration leads to a rapid and transient cytosol-to-nuclear translocation of liver Nrf2. This appears to be promoted by a redox-dependent mechanism as it is blocked by NAC. It may also be contributed by concomitant p38 activation, which in turn promoted Nrf2 phosphorylation. Nrf2 cytosol-to-nuclear translocation may represent a novel cytoprotective mechanism of T-3 to limit free radical or electrophile toxicity, as this would likely entail promoting thioredoxin production.