Effects Of Different Iodine Intake On Rat Thyroid Function, Morphology, Apoptosis And Proliferation

Suitable level of iodine intake is essential for normal functions of thyroid. Iodine deficiency or excess will change the structure and functions of thyroid, resulting in iodine deficiency disorder or iodine excess disorder. The changes above are usually related with the imbalance between apoptosis and proliferation. Apoptosis is a process controlled by many genes in which Fas/FasL and Bcl-2 family are the most important signal transferring systems. ObjectivesAfter successfully establishing animal models of iodine deficiency and iodine excess in Wistar rats and cultured FRTL cells, we explored the effects of iodine on structure and functions of thyroid, apoptosis and proliferation of thyrocytes in vivo and in vitro. MethodsWistar rats, weaning one month and weighing 120-140g, were randomly divided into six groups according to their sex and body weight, i.e, (1)low iodine(LI);(2) normal iodine (NI);(3)five fold high iodine(5HI);(4)ten fold high iodine;(5)fifty-fold high iodine(50HI);(6)one hundred fold high iodine(100HI). By controlling foodstuff and drinking water(adding different amounts of potassium iodate), all groups got prospective iodine intake respectively, that is, 0.6μg/d, 6.15μg/d, 30.75μg/d, 61.5μg/d, 307.5μg/d and 615μg/d. After 7 days, 14 days, 28 days, 3 months, 6 months and 12 months, rats were sacrificed. The following parameters were measured, i.e, thyroid absolute and relative weight, iodine content in urine andthyroid tissue, thyroid hormones in serum and thyroid tissue, and thyroid morphology. We also tested thyroid proliferation, apoptosis, and apoptosis related genes expression by TUNEL, immunohistochemistry and RT-PCR. At the same time, FRTL cells were passaged and cultured. Incubations were performed for 48 hours with potassium iodate (lO^-lO^mol/L). Using Flow Cytometry(FCM), RT-PCR and MTT, we observed the effects of iodine on apoptosis, apoptosis related genes expression and proliferation of FRTL cells. Results1. After long-term administration^, 6 and 12 months), in LI group iodine concentrations in urine and thyroid tissue, and thyroid hormones contents in serum and thyroid tissue were significantly lower than those in NI group;rats in LI group demonstrated a typical goiter of iodine deficiency;the follicles were small and the number increased obviously. In HI group iodine concentrations in urine and thyroid tissue were higher than those in NI group, but thyroid hormones content in serum and thyroid tissue showed a decreasing tendency;The follicles in HI group showed polymorphic changes, and colloid in follicles increased;but the thyroid absolute and relative weight did not change significantly.No variation of bcl-2 mRNA expression was found in either long-term iodine deficiency group or iodine excess groups. Both long-term iodine deficiency and excess enhanced bax and fas mRNA expression at 6 months, but not at 3 and 12 months. fasL mRNA expression augmented only in LI group at 6 months and in HI groups at 3 and 6 months. Effects of long-term iodine deficiency and excess on Bcl-2, Bax, Fas and FasL proteins expression were consistent. Expressions of proteins were negative or weak positive in LI group;but they were positive in HI groups, and theirstaining density rised with the increase of iodine intake. Apoptosis was not found in all groups. At 3 and 6 months, expression of PCNA was higher in LI group and lower in severe iodine excess groups;and at 12 months expression of PCNA was only higher in LI group.2. After short-term administration(7, 14 and 28 days), in LI group T4 concentration in serum was obviously lower than that in NI group, but T3 not;goiter of iodine deficiency gradually appeared with the extension of iodine administration;but there were no differences in serum thyroid hormones contents between HI groups and NI group, and colloid deposit in follicles increased. Both long-term iodine deficiency and excess did not change the mRNA expression of bcl-2, bax, and fas genes;and mRNA expression of fasL went up in severe iodine excess groups. With short-term administration Bcl-2, Bax, Fas and FasL proteins expressions were consistent in LI and NI groups, and all of them were negative or weak positive;in HI groups their staining density rised with the increase of iodine intake and length of administration period. Apoptosis was not found in all groups. Expression of PCNA was enhanced by short-term iodine deficiency, but not by short-term iodine excess.3. Iodine excess inhibited bcl-2 and enhanced fasL mRNA expressions of FRTL cells in vitro;but no changes in bax and fas mRNA expressions occured with iodine excess. Cultured in medium with 10"2mol/L potassium iodate for 48 hrs, apoptosis of FRTL cells increased and proliferation was inhibited.Conclusions1. Iodine metabolism and thyroid functions change with iodine intake and are regulated compensatively. Both iodine deficiency and iodine excess lead to autoregulation of thyroid gland at first. When the normal level of thyroid hormones inserum can not be maintained, activity of hypothalamus-pituitary-thyroid axis start up. But this axis function started much later in iodine excess than that in iodine deficiency.2. Both iodine deficiency and excess can result in hypothyroidism, but in different ways. Iodine deficiency cut down the synthesis of thyroid hormones, whereas iodine excess inhibite the function of thyroid. Wistar rats demonstrate strong tolerance to long-term iodine excess, and the severity of hypothyroidism caused by iodine excess is less than the iodine deficiency.3. Both iodine deficiency and excess can affect apoptosis-related genes in mRNA expression and/or protein expression, but no apoptosis occur in vivo. Iodine excess promote apoptosis of FRTL cells. Apoptosis may be induced by many ways and blocked by TSH or other factors.4. Proliferation can be induced by short-term iodine deficiency. But proliferation is restrained by long-term iodine excess, and the suppressive action can be antagonized by TSH.5. Compared the results in vivo with those in vitro, we can conclude that Wistar rats demonstrate stronger tolerance to iodine excess than that to iodine deficiency, and scathe can be alleviated obviously through the adaptation mechanism in vivo which does not exist in vitro.