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.
