Inflammatory Response And High Fat Diet Induced Obesity And Insulin Resistance

The prevalence of obesity is growing rapidly and has become a major public health problem worldwide. In addition to the fat accumulation, obesity also plays a crucial role in the development of metabolic syndrome (MS).Dietary fat has been recognized as a major risk factor for the development of obesity. However, both human beings and rodents appear to be different in developing obesity when they exposed to high fat diet, described as dietary induced obesity (DIO) and dietary induced obesity resistant (DIO-R). The potential mechanism related to the different susceptibility to obesity induced by high fat diet has not been clearly elucidated. Recently, many studies revealed that increased inflammatory response in hypothalamus produces insulin and leptin resistance and contributes to the energy imbalance. TLRs are pattern recognition receptors providing the first line of host defense. Emerging study proposed that TLR4 act as a predominant molecular target for saturated fatty acids in the hypothalamus trigger the inflammatory response and ultimately results to leptin resistance and weight gain. Moreover, pharmacological inhibition designed to disrupt TLR4 signaling could reverse leptin resistance and protect against high fat diet induced food intake. NF-κB was a master switch of innate inmmunity and also associated with diminished hypothalamic insulin and leptin signal transduction. Further, experimental and genetic interventions that block the hypothalamic NF-κB signaling reversed hypothalamic insulin and leptin resistance and was associated with reduced food intake and weight loss. These data collectively implicated that the activation of hypothalamic inflammation is necessary and sufficient for the control of energy intake and likely involved in the mechanism underlying the pathogenesis of obesity during high fat diet feeding. We wonder wether the hypothalamic inflammatory response was involved in the different obesity susceptibility induced by high fat diet.Additionally, high fat diet induced obesity usually was associated with peripheral insulin resistance. Many studies have demonstrated that saturated fat diet and polyunsaturated fat diet had different effect on insulin sensitivity and the related mechanism remain unclear. Recently, obesity is considered to be a low grade chronic inflammatory state and associated with insulin resistance. Adipose tissue is major endocrine tissue promoting the interaction between metabolism and inflammation. Further research showed that the macrophages in adipose tissue are a major source of inflammatory cytokine. Among all the factors associated with migration and aggregation of macrophages, OPN (osteopontin) received more attention for its strong chemotactic ability. Further research found that OPN was also related with insulin resistance. OPN-/- mice could prevent the insulin resistance induced by high fat diet compared with the OPN+/+mice which hadthe same body weight. Moreover, the antibody of OPN could improve the insulin resistance in obese mice. These findings collectively demonstrated that OPN play an important role in the development of insulin resistance. However, it remains unclear wether OPN is involved in the different insulin sesentivity induced by saturated fat diet and polyunsaturated fat diet.Therefore, we established different animal model for investigate the underlying mechanism of different obesity susceptibility and insulin sensitivity induced by high fat diet and observe the influence of low fat diet on inflammation in hypothalamus and adipose tissue. Part IHypothalamic inflammatory response and different obesity susceptibility induced by high fat dietObjective: To investigate the hypothalamic inflammation in dietary induced obesity (DIO) and dietary induced obesity resistant (DIO-R) rats for investigating the potential mechanism related to different susceptibility to obesity and their responses to low fat diet intervention.Methods:Fifty-five six-week-old outbred male SD rats (purchased from Shanghai Sippr-BK lab animal Co. Ltd.) weighing 150-160g were housed individually with regulated temperature (22±5℃) and humidity (50±10%) on a daily cycle of 12 h light and darkness (08:00-20:00 h). All rats were allowed ad libitum access to water and food throughout the experimental period. After a week acclimation, tail blood was collected and serum was stored under-80℃for further assay. Then the rats were randomly divided into two groups:the HF group (n=45) were placed on a high-fat diet containing 4.62kcal/g (49.85% fat,20.00% protein, and 30.15% carbohydrate) and the CF group (n=10) were remained on normal laboratory chow food (purchased from Tongji Medical college lab animal center, Wuhan) containing 3.29kcal/g (13.68% fat,21.88% protein, and 64.44% carbohydrate). Dietary intake was recorded daily and body weight was measured weekly in the morning throughout the study. After 10 weeks of free access to their corresponding diet, rats in HF group with body weights more than x+1.96s of CF group were designated as DIO and those with body weight less than x+1.0s of CF group designated as DIO-R rats. Then one half of the DIO and DIO-R rats were switched to chow diet and the other half of the DIO rats and DIO-R rats were kept on HF diets for the following 8 weeks. All rats were provided water and food ad libitum. Terminally, all animals were killed after 12h fasting. Trunk blood was collected and centrifuged and serum was stored for further use. Perirenal and epididymal white adipose tissue was dissected and weighed. Hypothalamus was located and isolated according to brain coronal plane iconography of rat and related articles Samples of hypothalamic tissues were snap-frozen in liquid nitrogen immediately and stored at-80℃for RNA extraction.Results:1. Rats fed high fat diet had a wide distribution in body weight and weight gain.18 of the 45 were designated as DIO rats and 12 of which were DIO-R rats. The DIO rats had higher energy intake and energy efficiency compared with DIO-R and CF rats. TC,TG,LDL-C,FBG,FINS和HOMA-IR in DIO rats was increased compared with DIO-R and CF rats, TC,LDL-C and FINS in DIO-R rats was higher than that in CF rats.2. At the end of experiment, Hypothalamic TLR4, NF-κB, IL-1βand IL-6 mRNA expression in DIO/HF rats was significantly increased compared to DIO-R/HF and CF rats (P<0.05).3. Switching to chow food from high fat diet reduced the body weight and fat mass in DIO-R rats. The TC,FINS and HOMA-IR in DIO/CF rats was decreased compared with their counterpart on high fat diet. Low fat diet intervention failed to affect hypothalamic TLR4, NF-κB, IL-1βand IL-6 mRNA expression in DIO rats.Conclusion:SD rat fed a high fat diet showed different susceptibility to obesity. Upregulated hypothalamic inflammation may contribute to the overeating and development of obesity susceptibility induced by high fat diet and obesity resistant rats could main the appropriate energy intake and body weight by inhibiting the increased hypothalamic inflammatory response. While reduced body weight in DIO rats by chow food was not through correcting hypothalamic inflammation during the intervention period. PartⅡInflammatory response in adipose tissuce and different insulin sensitivity induced by saturated fat diet and polyunsaturated fat dietObjective:To explore the expression of OPN and related inflammatory factor in adipose tissue for investigating the underlying mechanism of the different insulin sensitivity induced by saturated and polyunsaturated fatty acid and their response to low fat diet.Methods:100 male SD rats weighing 150-160 g were housed individually in cages under controlled conditions. All rats had ad libitum access to their respective food and water throughout the study. After a week acclimation, tail blood was collected and serum was stored under-80℃for further assay. Then the rats were randomly divided into three groups, the SF group (n=45) were placed on a saturated fat diet containing 4.62kcal/g (49.85% fat,20.00% protein, and 30.15% carbohydrate), PF group (n=45) were placed on a polyunsaturated fat diet containing 4.62kcal/g (49.85% fat,20.00% protein, and 30.15% carbohydrate)and the CF group (n=10) remained on normal laboratory chow food (purchased from Tongji Medical college lab animal center, Wuhan) containing 3.29kcal/g (13.68% fat,21.88% protein, and 64.44% carbohydrate).At the end of 10th week, rats that fed high-fat diet with body weights more than x+1.96s of CF group were classified as saturated fat diet induced obesity (SF-DIO) and polyunsaturated fat diet induced obesity (PF-DIO). The SF-DIO and PF-DIO rats were then randomly subdivided into two groups respectively:one subgroup of each was shifted to chow food (SF-DIO/CF and PF-DIO/CF) and the other maintained on their respective high-fat diet (SF-DIO/SF and PF-DIO/PF). After another 8 weeks of feeding, all rats were sacrificed after 12 h fasting and trunk blood was collected and serum samples were stored at-80℃. The epididymal adipose tissue were rapidly separated, immediately frozen in liquid nitrogen, and then stored at-80℃until analysis. OPN, NF-κB, IL-6 and IL-10 mRNA expression in adipose tissue were determined by RT-PCR. Western blotting was used to determine OPN protein expression in adipose tissue and ELISA for serum OPN.Results:1. At the end of the 10th week,19 of SF group (42.2%) and 22 of PF group (48.9%) rats were designated as SF-DIO and PF-DIO respectively according to the body weight as compared with that of CF group. Serum FBG, FINS and HOMA-IR in SF-DIO group were significantly higher than PF-DIO and CF group, while the FINS and HOMA-IR in PF-DIO group were significantly higher than CF group.2. SF-DIO/SF rats had higher OPN, NF-κB and IL-6 expression compared with PF-DIO/PF and CF group, and PF-DIO/PF also had higher OPN, NF-κB and IL-6 expression compared with CF group, IL-10 expression in SF-DIO and PF-DIO rats was significantly reduced compared with CF control.3. Switching to chow food from high fat diet reduced the body weight and fat mass both in SF-DIO and PF-DIO rats without alteration of energy. FINS and HOMA-IR in SF-DIO/CF rats was reduced compared with SF-DIO/SF rats, and HOMA-IR in PF-DIO/CF rats was lower than that in PF-DIO/PF rats. The expression of OPN, NF-κB and IL-6 in SF-DIO/CF and PF-DIO/CF rat was decreased compared with their counterpart on high fat diet, SF-DIO/CF rats remained higher OPN and NF-κB expression than PF-DIO/CF rats. No difference in serum OPN was found among the group.Conclusion:Polyunsaturated fat diet showed improved insulin sensitivity compared with the saturated fat diet although they had the same effect on the development of obesity. The increased expression of OPN and related inflammatory factor in SF-DIO rats might be responsible for the development of insulin resistance, while inhibited expression of OPN and related inflammatory factors in PF-DIO rats compared with SF-DIO rats might be contributed to improved insulin sensitivity. And the improved insulin sensitivity by chow food both in SF-DIO and PF-DIO rats may be through the reduced the inflammatory response in adipose tissue during the intervention period.