Effects Of Oxidative Stress Induced By High-fat Diet On Thyroid Hormone Homeostasis And Modulation By Quercetin

Thyroid hormone(TH) is important to energy metabolism, but the role of it in the occurrence and development of obesity is unknown. Oxidative stress affects TH metabolism, and might be the mechanism by which TH affects obesity. The thesis studied the role of thyroid hormone metabolism disturbance induced by oxidative stress in the different propensities to obesity. The effects of quercetin pretreatment on thyroid hormone homeostasis in mice fed with high-fat diets(HFD) for different periods and the hepatic TH-dependent energy metablism were also explored. Furthermore, the role of quercetin in reversing hepatic steatosis in HFD-induced obesity mice was also studied. Finally, the effects of quercetin pretreatment in attenuating HFD-induced cognitive deficits in the perspective of TH homeostasis were also determined.HFD induced obesity-prone(OP) and obesity-resistant(OR) phetotypes. We evaluated the differences of thyroid hormone metabolism after short-term(7 weeks) and long-term(27 weeks) HFD consumption, as well as the effects of these discrepancies on hepatic energy metabolism. The results showed that when mice were fed with their respective diets for 7 weeks, compared to control group, OR mice significantly elevated hepatic D1 activity to increase serum T3 levels through enhancing Gpx, SOD activity and decreasing MDA levels(P<0.05); OP mice primarily strengthened thyroid function(illustrated by higher expression of Nis, Tpo and Duox2) by activating HPT axis(demonstrated by up-regualted Trh expression in hypothalamus and Tsh expression in pituitary, accompanied by elevated serum TSH levels), and ultimately increasing circulating T4 levels. When the mice were fed for a longer time, both OP and OR mice had lower hepatic D1 activity, OR mice activated HPT axis to up-regulate the expression of Nis, Tpo and Tgb to improve thyroid function and thus elevated circulating TH levels. Furthermore, the expressions of TH-dependent genes that involved in energy metabolism were normalized. Because of the destroyed HPT axis and oxidative stress in thyroid(evidenced by significant down-regulation of Gpx3, Nrf2 and upregulation of Duox2), OP mice had impaired thyroid function because of significant downregulation of Tpo, Nis and Tgb, thus, failed to produce enough TH, which interfere THdependent energy metabolism process, and led to body weight gain.Mice were fed HFD with or without quercetin for 3, 7, 17, and 27 weeks to study the effects of quercetin on thyroid hormone homeostasis. The doses of quercetin added to HFD were 0.025%, 0.05% and 0.1%(w/w). The resutls were as follows: In the 3rd week, TH levels were not different from control mice. As the feeding time developed, TSH levels in HFD mice were always higher than control group. Serum T4 levels were significantly higher in the 7th and 17 th week, but the significance disappeared in the 27 th week. Serum T3 levels in HFD mice were only remarkably elevated in the 7th week, and showed no difference in other feeding cycles. Simultaneously, oxidative stress occurred in liver, kidney and brain in the 7th week, but the D1 activity was significantly decreased in the 17 th week, which might result in local T3 deficiency and be responsible for reduced energy expenditure. Hepatic lipid infiltration was observed in the 7th week and gradually developed in the later experiments, accompanied by significant alteration of T3-dependent target genes, including genes involved in lipogenesis(Fasn, Srebf1 and Acc1), fatty acid oxidation(Cpt1α and Pparα), energy metabolism(Cox7c and Atp5c1). Quercetin administration could increase energy expenditure, improve the redox status in liver, kidney and brain, elevate local D1 and D2 activity, and regulate expression of T3 responsive genes, including Fasn, Srebf1, Acc1, Cpt1α, Pparα, Cox7 c, Atp5c1 in the all feeding cycles excluding the 27 th week, resulting in increased fatty acid oxidation and energy metabolism, reduced hepatic lipid production, and reduced body weight to a certain degree. Among the three doses used in the study, 0.05% quercetin supplementation exerted the most beneficial effects. In the 27 th week, the ability of quercetin in regulating TH homeostasis was impaired, although serum T4 was higher than HFD group, decreased D1 activity led to similar T3 level, and ultimately induced T3 responsive genes alteration, hepatic lipid accumulation and body weight elevation.When obese model was successfully induced by HFD for 10 weeks, they were treated with standard diet and quercetin–rich high-fat diet, respectively. The results showed that when the intervention was conducted for 7 weeks, standard diet could fully reverse all the changes influenced by HFD, including body weight, respiratory exchange ratio, energy expenditure, and serum total T4, T3, TSH and cytokines(IL-6 and TNF-α), as well as normalizing hepatic redox status, D1 activity and expressions of T3 target genes involved in lipid metabolism. Compared to HFD group, although quercetin failed to affect energy expenditure, it succeeded in increasing serum T4 levels and hepatic D1 activity, which might led to increased T3 levels in liver. The genes(Fasn and Srebf1) that played roles in lipogenesis were down-regulated, but the expressions of Cpt1α and Pparα were unchanged, leading to significant reduced body weight, which was still higher than that of control mice.Local thyroid hormones in specific brain district affect cognition. When C57BL/6J mice were fed HFD for 7 weeks, hippocampus-dependent learning and memory ability was impaired. In Morris water maze, HFD fed mice had significantly longer escape latency than control mice. In the probe trial, the time that each mouse spent in the target quadrant was shorter than the control. Meanwhile, hippocampal BDNF and its m RNA levles were significantly reduced. HFD induced many significantly altered biological pathways in hippocampi, including long-term potentiation, thyroid hormone synthesis, peroxisome and et al. HFD mice had significantly higher serum TSH levels than control mice, but T4 and T3 levels were not significantly different. HFD induced severe oxidative stress in hippocampus as illustrated by decreased Gpx and SOD activity, as well as increased MDA levels, with reduced D2 activity, which might lead to T3 deficiency and subsequent declined expression of Glut4, impaired glucose transport in hippocampus and cognitive deficits. Quercetin accumulated in plasma and brain after taken orally, and efficiently attenuated oxidative stress(illustrated by increased Gpx and SOD activity, decreased MDA levels and normalization of expressions of antioxidant-related genes), and normalized D2 activity. Futhermore, quercetin significantly altered some biological pathways when compared to HFD, such as MAPK signaling pathway, thyroid hormone synthesis and et al. All these changes might contribute to improved learning and memory.In conclusion, thyroid hormone homeostasis disturbance might induce different propensities to obesity, quercetin intervention could prevent and reverse thyroid hormone metabolism imbalance in a certain period of time to attenuate obesity and cognition through improving energy expenditure.