Biphasic modulation of insulin receptor substrate-1 during goitrogenesis

Abstract

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.

Regulation of thyroid sodium‐iodide symporter in different stages of goiter: Possible involvement of reactive oxygen species

“…In both periods of MMI treatment, thyroid weight was significantly increased and the magnitude of this increase was greater after 21 days of MMI, as previously described . In both periods of treatment, 1 day of MMI withdrawal did not normalize thyroid weight (Table ).…”

“…In both periods of MMI treatment, thyroid weight was significantly increased and the magnitude of this increase was greater after 21 days of MMI, as previously described. 9 In both periods of treatment, 1 day of MMI withdrawal did not normalize thyroid weight (Table 2). MMI treatment for both 5 and 21 days significantly decreased colloid size (Table 2), as expected, reflecting the stimulation of the endocytosis in response to elevated TSH levels.…”

“…The animals were distributed into the following groups: control (water available ad libitum), MMI 5d or 21d (0.03% MMI in drinking water for 5 or 21 days) and R1d (0.03% MMI in the drinking water for 5 or 21 days followed by 1 day without MMI). Five and 21 days of MMI were chosen because they lead to goiters in the early and advanced stages of development, respectively and 1 day of MMI removal was chosen because thyroid iodide uptake was rapidly down‐regulated after 1 day of MMI removal in rats treated with MMI for 21 days. Animals were killed by decapitation; thyroids were excised, immediately frozen in liquid nitrogen, and stored at −80 ° C for mRNA extraction, H 2 O 2 generation or TPO activity measurement or fixed for histomorphometric analysis.…”

“…Experimentally, goiter can be induced by treating animals with methimazole (MMI), which inhibits thyroperoxidase (TPO) activity, the key enzyme for thyroid hormone synthesis . TPO inhibition by MMI reduces serum thyroid hormones, thus increasing serum TSH and, consequently, leading to the enlargement of the gland …”