The Relationships Among Osteopontin, Obesity And Insulin Resistance

With the development of social economy, more and more people have become obesity. Obesity has become a major threat to public health. Obesity developing to a certain degree is called metabolic disease. Researches have found that obesity plays a crucial role in the occurrence and development of metabolic syndrome(MS). Metabolic syndrome, also called insulin resistance syndrome, is a syndrome consists of hypertension, hyperglycemia and dyslipidosis and there is no unified definition for diagnosis. Insulin resistance is the core of metabolic syndrome. A series of metabolic related diseases such as obesity, hypertension, type 2 diabetes and cardioascular disease are all closely related to insulin resistance.Lots of researches have showed that diet induced obesity (DIO) is usually accompanied by the increasing of blood glucose, insulin, insulin resistance and even type 2 diabetes. Evidence suggests that DIO is associated with a low-grade inflammation characterized by pro-inflammatory cytokines production, such as TNF-α, IL-1βand IL-6. Pro-inflammatory cytokines mediate the occurrence of insulin resistance through the insulin signaling pathway. Animal experiments have found that not all DIO rats develop insulin resistance. DIO rats induced by high fat diet rich in unsaturated fatty acid had much better insulin sensitivities than those fed on high fat diet rich in saturated fatty acid. The mechanism may related to their low inflammation level.Adipose tissue plays an important role in endocrine system, which promotes the interaction between metabolic and inflammation. Research showed that inflammatory response was triggered predominantly in adipose tissue, especially macrophages in adipose tissue. Understanding the main factors mediating peripheral macrophages moving towards adipose tissue will be very useful for the better control of insulin resistance and metabolic syndrome. Among the factors, OPN causes the attention of people for its strong adhesion chemotactic ability and the role in mediating cell migration.There were already some researches about the relationship between OPN and macrophages infiltration into adipose tissue. It's generally regarded that OPN has a close relationship with adipose tissue macrophages infiltration, pro-inflammatory factors expression and insulin resistance. However, there were few reports about the serum OPN level of obese individuals and the results were inconsistent. To date, there are no report comparing OPN expression in obesity rats induced by high-fat diet with different types of fat.The aims of this study were as follows:①to observe the effects of different fat types on body adiposity by rats fed with high-fat diet rich in either lard and soybean oil;②to compare adipose tissue OPN expression and peripheral insulin levels in DIO rats induced by high-fat diets with different types of fat;③to assess the effects of switching to low-fat diet on adipose tissue OPN expression and peripheral insulin levels change of DIO rats fed with high-fat diet.Part 1 Development of obesity in SD rats induced with high-fat diets rich in lard or soybean oilObjective: To observe the effects of dietary fatty acid profile on body adiposity and peripheral insulin levels.Methods: 100 male Sprague-Dawley rats weighing 120-130 g were housed individually in cages under controlled conditions (a 12-h light/dark cycle; 17-27°C; 40-60% relative humidity). All rats had ad libitum access to their respective food and water throughout the study. After 10 days of standard laboratory chow feeding to allow them to adapt to the new environment, the rats were weight-matched and divided into 3 groups and fed with either low-fat standard chow (LF, n=10) or high-fat diets that included 49.85% of calories from lard oil (rich in saturated and monounsaturated fatty acids, HL, n=45) and soybean oil (rich in polyunsaturated fatty acids, HS, n=45). The standard chow food containing 13.77 kJ/g (13.68% fat, 21.88% protein and 64.44% carbohydrate) and the high fat diet containing 19.33 kJ/g (49.85% fat, 20.00% protein and 30.15% carbohydrate). Except fat, the other compositions of HL and HS are all the same. At the end of 10th weeks, rats that fed high-fat diet with body weights more than x +1.96s of LF group were classified as diet induced obesity (DIO) and selected for further study. Tail blood was collected at 0 week and the end of the 10th week, and serum samples were stored at ?80°C. Serum concentrations of glucose and insulin were measured by their corresponding kits.Results:①There were no differences between HL(42.2%) and HS(48.9%) in inducing obesity of SD rats.②Serum insulin level and HOMA-IR of HS(50.80±12.00 uIU/ml and 15.22±4.64) were Significantly lower than HL(87.68±21.60 uIU/ml and 26.94±9.43).Conclusions:①When the three macronutrients were given with the same proportions, high-fat diet rich in lard oil(the main fatty acid compositions were saturated and monounsaturated fatty acids) and high-fat diet rich in soybean oil(the main fatty acid compositions were polyunsaturated fatty acids) had the same effects in inducing obesity in SD rats.②Polyunsaturated fatty acids could improve the insulin sensitivity of obese SD rats, and to a certain degree prevent insulin resistance.Part 2 OPN expression in adipose tissue and insulin sensitivity between DIO rats fed with different high-fat dietsObjective: To explore the role of OPN in the development of inflammatory response and insulin resistance associated with obesity by comparing OPN expression in adipose tissue and insulin sensitivity of the two types of DIO rats induced by high-fat diets with different fat types.Methods: The obesity model is established as in part 1. At the end of 10th weeks, six DIO rats from HL group were selected to be fed with their original high-fat diet(HL-DIO-HL,n=6). Six DIO rats from HS group also continued to be fed with their original high-fat diet(HS-DIO-HS, n=6). The LF rats still continued to be fed with normal chow diet(LF, n=10). After another 8 weeks of feeding, all rats were sacrificed after 12h fasting and trunk blood was collected and serum samples were stored at ?80°C . Perirenal adipose tissue and epididymal adipose tissues were rapidly separated, weighed, immediately frozen in liquid nitrogen, and then stored at -80°C until analysis. Serum concentrations of glucose, insulin and OPN were measured by their corresponding kits. OPN, NF-κB, IL-6 and IL-10 mRNA expression of adipose tissue were determined using real-time polymerase chain reaction. Western blotting was used to determine OPN protein expression.Results:①A lthough rats from HL-DIO-HL and HS-DIO-HS groups developed the same extent of obesity, the serum insulin level and HOMA-IR were significantly higher in HL-DIO-HL group.②Compared with HS-DIO-HS group, adipose OPN and IL-6 expressions in HL-DIO-HL rats were significantly higher, while IL-10 expression was lower.③NF-κB mRNA expression in adipose tissue of HL-DIO-HL group was much higher than that in HS-DIO-HS group.Conclusions: Polyunsaturated fatty acids could significantly improve insulin sensitivity of obesity SD rats, and this effect may related to the lower inflammatory factors expressions in adipose tissue. Upregulated OPN may contribute to increased inflammatory response through NF-κB pathway.Part 3 The effect of dietary intervention on adipose tissue OPN expression and insulin sensitivity in obese SD ratsObjective: To explore the role of OPN by observing the effects of changes in diet on adipose tissue pro-inflammatory cytokines expressions and peripheral insulin levels in DIO rats.Methods: The obesity model is established in part 1. At the end of 10th weeks, the DIO rats from HL group was then randomly subdivided into two groups: one subgroup of which was shifted to chow food (HL-DIO-LF, n=6) and the other continued to be fed with their original high-fat diet(HL-DIO-HL, n=6). The LF rats still continued to be fed with normal chow diet(LF, n=10). 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°C. Perirenal adipose tissue and epididymal adipose tissues were rapidly separated, weighed, immediately frozen in liquid nitrogen, and then stored at -80°C until analysis. Serum concentrations of glucose, insulin and OPN were measured by their corresponding kits. OPN, NF-κB, IL-6 and IL-10 mRNA expression of adipose tissue were determined using real-time polymerase chain reaction. Western blotting was used to determine OPN protein expression.Results:①S hifting to normal low fat diet resulted in reduction in fat content and total energy intake, effectively descreased body weight and increased Insulin sensitivity.②Compared with HL-DIO-HL group, HL-DIO-LF rats showed significantly lower adipose OPN and IL-6 expressions and higher IL-10 expression.③NF-κB mRNA expression in adipose tissue of HL-DIO-HL group was much higher than that in HL-DIO-LF group.Conclusions: Reduced dietary fat content could effectively decrese adipose tissue inflammatory factors expressions and improve insulin sensitivity of obesity SD rats. It may due to downregulation of OPN in adipose tissue and reduced macrophages infiltrating into adipose tissue.