Biphasic modulation of insulin receptor substrate-1 during goitrogenesis.

Insulin receptor substrate-1 (IRS-1) is the main intracellular substrate for both insulin and insulin-like growth factor I (IGF-I) receptors and is critical for cell mitogenesis. Thyrotropin is able to induce thyroid cell proliferation through the cyclic AMP intracellular cascade; however, the presence of either insulin or IGF-I is required for the mitogenic effect of thyroid-stimulating hormone (TSH) to occur. The aim of the present study was to determine whether thyroid IRS-1 content is modulated by TSH in vivo. Strikingly, hypothyroid goitrous rats, which have chronically high serum TSH levels (control, C = 2.31 +/- 0.28; methimazole (MMI) 21d = 51.02 +/- 6.02 ng/mL, N = 12 rats), when treated with 0.03% MMI in drinking water for 21 days, showed significantly reduced thyroid IRS-1 mRNA content. Since goiter was already established in these animals by MMI for 21 days, we also evaluated IRS-1 expression during goitrogenesis. Animals treated with MMI for different periods of time showed a progressive increase in thyroid weight (C = 22.18 +/- 1.21; MMI 5d = 32.83 +/- 1.48; MMI 7d = 31.1 +/- 3.25; MMI 10d = 33.8 +/- 1.25; MMI 14d = 45.5 +/- 2.56; MMI 18d = 53.0 +/- 3.01; MMI 21d = 61.9 +/- 3.92 mg, N = 9-15 animals per group) and serum TSH levels (C = 1.57 +/- 0.2; MMI 5d = 9.95 +/- 0.74; MMI 7d = 10.38 +/- 0.84; MMI 10d = 17.72 +/- 1.47; MMI 14d = 25.65 +/- 1.23; MMI 18d = 35.38 +/- 3.69; MMI 21d = 31.3 +/- 2.7 ng/mL, N = 9-15 animals per group). Thyroid IRS-1 mRNA expression increased progressively during goitrogenesis, being significantly higher by the 14th day of MMI treatment, and then started to decline, reaching the lowest values by the 21st day, when a significant reduction was detected. In the liver of these animals, however, a significant decrease of IRS-1 mRNA was detected after 14 days of MMI treatment, a mechanism probably involved in the insulin resistance that occurs in hypothyroidism. The increase in IRS-1 expression during goitrogenesis may represent an important event associated with the increased rate of cell mitosis promoted by TSH and indicates that insulin and IGF-I are important co-mitogenic factors in vivo, possibly acting through the activation of IRS-1.

Thyroid hormone modulates ClC-2 chloride channel gene expression in rat renal proximal tubules.

Thyroid hormones has its main role in controlling metabolism, but it can also modulate extracellular fluid Volume (ECFV) through its action on the expression and activity of Na(+) transporters. Otherwise, chloride is the main anion in the ECFV and the influence of thyroid hormones in the regulation of chloride transporters is not yet understood. In this work, we studied the effect of thyroid hormones in the expression of ClC-2, a cell Volume-, pH- and voltage-sensitive Cl(-) channel, in rat kidney. To analyze the modulation of ClC-2 gene expression by thyroid hormones, we used hypothyroid (Hypo) rats with or without thyroxine (T(4)) replacement and hyperthyroid (Hyper) rats as our experimental models. Total RNA was isolated and the expression of ClC-2 mRNA was evaluated by a ribonuclease protection assay, and/or semi-quantitative RT-PCR. Renal ClC-2 expression decreased in Hypo rats and increased in Hyper rats. In addition, semi-quantitative RT-PCR of different nephron segments showed that these changes were due exclusively to the modulation of ClC-2 mRNA expression by thyroid hormone in convoluted and straight proximal tubules. To investigate whether thyroid hormones action was direct or indirect, renal proximal tubule primary culture cells were prepared and subjected to different T(4) concentrations. ClC-2 mRNA expression was increased by T(4) in a dose-dependent fashion, as analyzed by RT-PCR. Western blotting demonstrated that ClC-2 protein expression followed the same profile of mRNA expression.

Biphasic modulation of insulin receptor substrate-1 during goitrogenesis

Insulin receptor substrate-1 (IRS-1) is the main intracellular substrate for both insulin and insulin-like growth factor I (IGF-I) receptors and is critical for cell mitogenesis. Thyrotropin is able to induce thyroid cell proliferation through the cyclic AMP intracellular cascade; however, the presence of either insulin or IGF-I is required for the mitogenic effect of thyroid-stimulating hormone (TSH) to occur. The aim of the present study was to determine whether thyroid IRS-1 content is modulated by TSH in vivo. Strikingly, hypothyroid goitrous rats, which have chronically high serum TSH levels (control, C = 2.31 ± 0.28; methimazole (MMI) 21d = 51.02 ± 6.02 ng/mL, N = 12 rats), when treated with 0.03 percent MMI in drinking water for 21 days, showed significantly reduced thyroid IRS-1 mRNA content. Since goiter was already established in these animals by MMI for 21 days, we also evaluated IRS-1 expression during goitrogenesis. Animals treated with MMI for different periods of time showed a progressive increase in thyroid weight (C = 22.18 ± 1.21; MMI 5d = 32.83 ± 1.48; MMI 7d = 31.1 ± 3.25; MMI 10d = 33.8 ± 1.25; MMI 14d = 45.5 ± 2.56; MMI 18d = 53.0 ± 3.01; MMI 21d = 61.9 ± 3.92 mg, N = 9-15 animals per group) and serum TSH levels (C = 1.57 ± 0.2; MMI 5d = 9.95 ± 0.74; MMI 7d = 10.38 ± 0.84; MMI 10d = 17.72 ± 1.47; MMI 14d = 25.65 ± 1.23; MMI 18d = 35.38 ± 3.69; MMI 21d = 31.3 ± 2.7 ng/mL, N = 9-15 animals per group). Thyroid IRS-1 mRNA expression increased progressively during goitrogenesis, being significantly higher by the 14th day of MMI treatment, and then started to decline, reaching the lowest values by the 21st day, when a significant reduction was detected. In the liver of these animals, however, a significant decrease of IRS-1 mRNA was detected after 14 days of MMI treatment, a mechanism probably involved in the insulin resistance that occurs in hypothyroidism. The increase in IRS-1 expression during goitrogenesis may represent...