Liver glutathione S-transferase α expression is decreased by 3,5,3′-triiodothyronine in hypothyroid but not in euthyroid mice

Abstract

As previously reported, the activity of liver glutathione S-transferases, an important family of enzymes for detoxification processes, is regulated by thyroid hormone levels. Here, we specifically studied glutathione S-transferase α (Gsta) gene expression in livers of mice. First, in wild-type (WT) mice, hypothyroidism was induced by 5 weeks of a diet containing 5-propyl-2-thiouracil plus water containing metimazole, whereas hyperthyroidism was induced by daily injections of 50 μg (100 g body weight)−1 of 3,3 , 5-triiodo-L-thyronine (L-T 3 ) for 15 days. Importantly, hypothyroidism induced liver Gsta mRNA (>500%) and protein levels (70%; P < 0.01), indicating an important role of baseline thyroid hormone levels to repress this gene; however, surprisingly, no differences were seen in hyperthyroid mice. To further investigate Gsta repression by T 3 , we used animals expressing a naturally occurring mutation of the gene for thyroid hormone receptor (TR)-β ( 337T), which prevents T 3 binding and causes a general resistance to thyroid hormone. At baseline, homozygous animals showed increased Gsta levels (mRNA 3.5 times, protein 1.3 times) similar to those found in hypothyroid animals. After a T 3 suppression test, we found a blunted response of liver Gsta after the lower doses of T 3 in homozygous animals, as expected. However, after the highest dose of T 3 , we observed a decrease in Gsta expression (80%), similar to normal animals, explained by a higher expression of TR-α1 (60%; P < 0.01) and a lower expression of Src1 (steroid coactivator receptor) in the mutant animals (50% decrease). In summary, a decrease in Gsta expression caused by T 3 was observed only in the hypothyroid state. In addition, an essential role of TR-β1 is to mediate Gsta suppression in response to T 3 and, in the absence of a functional TR-β, there is a compensatory action of TR-α1 that depends on low levels of Src1.

Thyroid hormone and estradiol have overlapping effects on kidney glutathioneS-transferase-α gene expression

“…The total serum T 3 levels of male and female TR␤ ⌬337T/⌬337T mice were similar and five to seven times higher compared with wild-type animals, as reported before (20,50). The TR␤ ⌬337T/⌬337T mouse is a model in which T 3 , even when present in high levels, is unable to bind to TR␤.…”

“…Recent data from our group have shown that liver GST␣ is inhibited by T 3 in a hypothyroid state but not when mice are euthyroid (EU). This finding was dependent on the cooperative actions of the two different TH receptor isoforms, TR␣1 and TR␤1 (20).…”

“…The kidneys were excised and homogenized in an Ultra-Turrax T25 basic (IKA Werke, Staufen, Germany) using lysis buffer (50 mM Tris·HCl, 0.25 mM sucrose, 10 mM KCl, and 5 mM MgCl2, pH 7.5) with the Complete protease inhibitor cocktail (Roche, Indianapolis, IN). Samples containing 20 g of total protein were resolved by SDS-PAGE on a 15.0% gel, as described previously for the liver (20). The antibodies and dilutions were as follows: goat anti-Gsta (1:3,000 dilution for male samples and 1:30,000 for female samples; Abcam), rabbit anti-cyclophilin B (1:20,000 dilution; Affinity BioReagents, Golden, CO), rabbit anti-ER␣ (1:200; Santa Cruz Biotechnology), rabbit anti-TR␤ (1:500; Affinity BioReagents), peroxidase-labeled anti-rabbit IgG (1:10,000 dilution; Amersham Biosciences, Piscataway, NJ), and peroxidase-labeled anti-goat IgG (1:15,000 dilution; Abcam).…”

“…3C) were induced to become hyperthyroid (HYPER) by daily sc injections of T3 (50 g/100 g body wt) for 15 days. Hypothyroidism (HYPO) was induced by feeding OVX animals a diet containing 0.15% 5-propyl-2-thiouracil (PTU; Sigma) for 4 wk, as described previously (20). After the induction of hypo-and hyperthyroidism, the OVX mice were injected sc with 100 g/kg E2 benzoate (Sigma) every 4 days for an additional 15 days throughout T 3 and PTU treatments to ensure that, at the last time point, the OVX animals were hypo-or hyperthyroid plus E2 benzoate supplemented (Fig.…”

“…It is agreed that most of the TH effects are mediated by nuclear action via a superfamily of ligand-modulated transcription factors that modify target gene transcription (36). Regarding GST␣ regulation by thyroid hormones, TH suppress both the activity and gene expression of GST␣ in the liver (1,11,20,43,48,57). Recent data from our group have shown that liver GST␣ is inhibited by T 3 in a hypothyroid state but not when mice are euthyroid (EU).…”