| The prevalence of diabetes is reaching epidemic expansion worldwide. Type2diabetes is the most common form of diabetes. In type2diabetes, either the body does not produce enough insulin or the cells ignore the insulin. Obesity is commonly associated with type2diabetes. The prevalence of obesity has increased dramatically in recent years. Insulin resistance is a key feature of obesity and is a main mechanism of type2diabetes. Insulin resistance is defined as a state that requires more insulin to obtain the biological effects achieved by a lower amount of insulin in the normal state.Increasing evidence from human studies and animal researches has established correlative as well as causative links between inflammation and insulin resistance. The proinflammatory cytokine tumor necrosisi factor (TNF)-a has been demonstrated to mediate insulin resistance as a result of obesity in many rodent obesity models. The concept of fat as a site for the production of cytokines and other bioactive substances quickly extended beyond TNF-a to include IL (interleukin)-6, leptin, serum amyloid A (SAA), resistin, monocyte chemoattractant protein-1(MCP-1), plasminogen activator inhibitor-1(PAI-1), angiotensinogen, visfatin, retinol-binding protein-4, and others. The investigations that focused on intracellular pathways activated by inflammation, instead of individual cytokines, have helped to restructure the framework for thinking about insulin resistance. It has been demonstrated that c-JUN N-terminal kinase (JNK) and nuclear factor-KB (NF-κB) pathway play the crucial roles in the insulin resistance.Besides inflammation, reactive oxygen species (ROS) production is one of many factors that have been suggested to play a role in the development of insulin resistance, based on the following evidence:(1) high doses of hydrogen peroxide and reagents that accumulate ROS can induce insulin resistance in3T3-L1adipocytes, and (2) increased markers of oxidative stress were observed in obese humans and rodents. It has been also demonstrated that the up-regulation of genes responsible for ROS production occurs in both the liver and adipose tissue before the onset of insulin resistance and obesity in mice fed with high-fat diet (HFD). Otherwise, several clinical trials have demonstrated improvement of insulin sensitivity in insulin-resistant and diabetic patients treated with antioxidants.Apocynin (4-hydroxy-3-methoxy-acetophenone) is a constituent of the Himalayan herb Picrorhiza kurrooa Royle (Scrophulariaceae) which is regarded as an inhibitor of nicotinamide adenine dinucleotide phosphate-oxidase (NADPH oxidase) and was widely used as an antioxidant in research. In addition, apocynin also exhibits anti-inflammatory effects in many previous studies. Apocynin has been demonstrated to suppress NF-κB activation in spontaneously hypertensive rats, down-regulate TNF-a protein expression in coronary arteries, decrease vascular cell adhesion molecule-1(VCAM-1) induction by TNF-a in vein endothelial cells, reduce polymorphonuclear granulocyte chemotaxis, inhibited peroxynitrite in airway lumen in mice, attenuated inflammation-mediated cartilage destructionm activity in human articular cartilage, and reduced cyclooxygenase (COX)-2expression in human monocytes. The therapeutic effect of apocynin on insulin resistance has not been explored. Hence, one main goal of this study is to determine that if apocynin could ameliate insulin resistance in HFD mice through its effects on the oxidative stress and inflammation.Erythropoietin (EPO), a kind of hematopoietic growth factor from kidney, was first used for clinic treatment in last century. In the latter utility and research, EPO was found to have many other effects like inhibiting inflammation and oxidative stress, anti-apoptosis. Interestingly, it was found that EPO could attenuate insulin resistance in diabetic or non-diabetic end-stage renal disease (ESRD) patients who were taking haemodialysis therapy, while the underlying mechanisms are still unclear. The third part of this thesis is aimed to explore the effects of EPO treatment on insulin resistance in mice. Part Ⅰ Anti-oxidative effect of apocynin on insulin resistance in HFD miceThis part examined the effects of apocynin on oxidative stress and antioxidant enzymes in HFD induced obese mice. After12weeks on HFD, the C57BL/6J mice that clearly exhibited insulin resistance received apocynin (2.4g/L) in their drinking water for5weeks. The results show that apocynin treatment significantly ameliorated hyperglycemia, hyperinsulemia and dyslipidemia in HFD mice. Also, the intraperitoneal glucose tolerance test (IPGTT) and homeostasis model assessment of insulin resistance (HOMA-IR) indicate significant improvement of insulin sensitivity in HFD fed mice after apocynin treatment. Compared to the HFD control mice, serum malondialdehyde (MDA) was significantly lower and serum superoxide dismutase (SOD) was significantly higher in apocynin treated HFD mice, which revealed apocynin suppressed systemic oxidative stress. In the liver, apocynin was found to significantly reduce the level of MDA. Accordingly, apocynin treatment strengthened the antioxidative defense system with an increased activity of SOD, glutathione-peroxidase (GSHpx) and content of reduced glutathione (GSH). We also found that hepatic catalase (CAT).activity significantly decreased after apocynin treatment which may reflect apocynin significantly reduced hydrogen peroxide and oxidative stress in the liver. These results suggest that apocynin may ameliorate insulin resistance through reducing systemic and hepatic oxidative stress in HFD fed mice.Part Ⅱ Apocynin improves insulin resistance through suppressing inflammation in HFD induced obese miceIn this part, we investigated the effects of apocynin on inflammation in HFD induced insulin resistance mice model. After12weeks of HFD fed, the C57BL/6J mice that exhibited insulin resistance then received5weeks of apocynin (2.4g/L, in water). Following apocynin treatment, fasting glucose, insulin and glucose tolerance test showed a significant improvement in insulin sensitivity in HFD fed mice. We demonstrated that serum levels of TNF-α, IL-6and leptin were remarkably reduced with apocynin treatment. We also found that mRNA expression of TNF-α, IL-6, MCP-1in the liver and mRNA expression of TNF-a, IL-6, MCP-1, leptin in adipose tissue were suppressed by apocynin. Furthermore, the activity of transcription factor NF-κB in the liver was significantly suppressed with apocynin treatment. These results suggest that apocynin may reduce inflammatory factors in the blood, liver and adipose tissue, resulting in amelioration of insulin resistance in HFD fed mice.Part III EPO attenuates insulin resistance through multiple protective functions in HFD fed miceIncreasing data indicated that erythropoietin (EPO) has multiple biological functions besides erythropoiesis. A few studies recently showed that EPO could ameliorate insulin resistance, but the related mechanisms are still obscure which were explored in the present study. The high-fat diet (HFD)-induced insulin resistance mice were divided into groups with different EPO dosages and treatment durations. The results showed that EPO exhibited its protective effects on insulin resistance in2-,5-and7-week EPO treatment groups. To demonstrating the undergoing mechanisms, the status of insulin signaling, lipid metabolism, oxidative stress and inflammation in the HFD mice with or without EPO treatment were assessed. Our data showed that:1) EPO could increase activity of AKT in the liver which is critical in insulin signaling;2) In the liver, EPO reduced the expression of sterol regulatory element binding protein isoform (SREBP)-1c and fatty acid synthase (FAS), which both related to the lipid metabolism. At also, EPO decreased SREBP-1c and increased lipoprotein lipase (LPL) expression levels in the adipose tissue;3) with regard to the effects of EPO on oxidative stress, increased superoxide dismutase (SOD), Cu-Zn SOD, glutathione (GSH), glutathione peroxidase(GPx), and decreased malondialdehyde (MDA) were detected in the liver;4) EPO decreased serum tumor necrosis factor(TNF)-a, interleukin(IL)-6and leptin. The gene expressions of inflammatory factors in the liver and adipose tissue were also reduced after EPO treatment. Moreover, EPO suppressed the activations of hepatic JNK1/2and nuclear factor-KB (NF-κB). In addition, our results demonstrated that EPO treatment with low-dose and long-term did not cause the side-effect of high hematocrit. In conclusion, our results found that EPO could ameliorate insulin resistance through its protective functions on effecting insulin signaling, lipid metabolism, oxidative stress and inflammation. Thus, EPO could be a promising treatment for insulin resistance, and even for type2diabetes. |
PDF Full Text Request