Study On The Relationship Between The Levels Of Serum STNFR2, Leptin And Insulin Resistance In Obese Rats

IntroductionThe incidence of obesity has obviously increased with the development of e-conomy and the level of human life. Recently, many studies have shown that o-besity is highly correlated with insulin resistance (IR). The mechanism of obesity leading to IR is not well known. Adipose tissue plays an important role in the mechanism of IR. Some adipocytokines such as leptin, TNF - α, FFA and resistin all related to IR. TNF - a and its receptor are hotspot in these years. sT-NFR2 may participate in IR related to obesity. TNF - a can stimulate the secretion of leptin. TNF - α maybe accompany with leptin in IR. The purpose of this study was to investigate the levels of serum sTNFR2, TNF - a and leptin in diet - induced obese rats, the relationship of these cytokines and the association with IR, and to provide theoretical basis for the study of IR and the prevention and cure of obesity.Objective1. To investigate the relationship between serum sTNFR2 and IR in diet -induced obese rats.2. To investigate the relationship between the levels of serum TNF - a, leptin, FFA and IR in diet - induced obese rats.3. To study the interacted role of TNF - α,leptin and FFA in the mechanism of IR related to obesity.Materials and MethodsAnimals and Diets. 52 male Wistar rats weighting 180 ±20 g were obtained from the Animal Department of China Medical University. They were housed in cages in environmentally controlled condition (temperature,22 ± 3℃ ; moisture, 45% -55% ) and allowed ad libitum access to food and water throughout the study. 39 rats were given a high - fat diet and 13 rats were given a standard diet as controls.Experimental protocol. 52 animals were initially fed a standard diet for one week, then 39 rats were given a high - fat diet to induce obesity, the other 13 rats were given a standard diet as controls. At each weekend in this period, body weights were measured. 24 rats(6 control, 18 experiment )were killed in 3 w, while others were killed in 12 w after sampled blood. Abdominal fat pads ( peri-renal , mesenteric, epididymidis) were weighed. All blood samples were centri-fuged and the serum was stored at — 70℃.Over the 3 w and 12 w period, rats fed the high - fat diet diverged into two groups based on body weight gain. 1/3 rats with the greatest weight gain were referred to as diet - induced obesity, while 1/3 rats with the lowest weight gain were referred to as diet - induced obesity resistant. IR was destifined by insulin sensitivity index (ISI).Assays. The levels of serum leptin, TNF - a and insulin were measured by radioimmunoassay. The level of serum sTNFR2 was determined by ELISA.Statistical method. All values were described as means ± SE. Datas among multiple groups were analyzed by one -way ANOVA,adopting LSD test . Correlation was tested by Pearson method. All analyses were carried out through SPSS statistical software.Results1. Body weights of DIO, DIO - R and control ratsInitial body weights of DIO rats did not differ from those of DIO - R andcontrol rats,but by the end of 3 w,differences in body weights became apparently. At that point, body weights of DIO rats were significantly greater than those of DIO - R and control group(P <0.01). From then on, body weights continuously divergenced. There was no significant difference in body weights between DIO - R and control group ( P > 0.05 ).2. Abdominal fat pad weights of DIO, DIO - R and control ratsIn 3 w , the weights of the mesenteric, perirenal, epididymal, total fat pad, total fat pad/body weight of DIO group were heavier than those of DIO - R and control rats. The weights of perirenal and total fat pad/body weight of DIO -R rats were higher than those of control group(P <0.05). In 12 w , the levels of all fat pads in DIO rats were significantly higher than those of DIO - R and control rats(P <0. 01). There was no significant difference in body fat between DIO -R and control groups(P >0.05) .3. Serum FINS, ISI and FFA in DIO,DIO -R and control rats.In 3,12 w ,the levels of serum FINS and FFA were higher in DIO rats than those of DIO - R and control rats, ISI was significantly lower than the latter groups (P <0. 01) ; There was no significant difference between DIO - R and control rats(P>0. 05).4. Serum sTNFR2, TNF - a and leptin in DIO, DIO - R and control rats In 3 w , There were no significant differences of the levels of serum sTNFR2and TNF - a in the three groups (P > 0. 05 ). The level of serum leptin of DIO rats was higher than that of the control rats ( P < 0. 05 ) , but had no difference compared to DIO - R rats (P > 0. 05 ). There was no significant difference between DIO - R and control rats( P > 0. 05 ). In 12 w , concentrations of serum sTNFR2,TNF - a and leptin were significantly higher in DIO rats than those in DIO - R and control rats ( P < 0. 01). There was no significant difference between DIO - R and control rats( P > 0. 05 ).5. The relationship of the factors at the end of 12 w in DIO ratsTNF - a was in positive correlation with the levels of serum sTNFR2, leptin and FFA (P <0.05) , respectively. ISI was in negative correlation with the levels of serum sTNFR2 ,TNF - a, leptin and FFA ( P <0.05 ) , respectively.DiscussionIn this study, part of the rats fed with the high - fat diet were obese and part rats not. It showed that the establishment of DIO and DIO - R animal models was successful.1. leptin and IRThere was an axis of fat - insulin in normal person, leptin elevated following the increased fat weight. Leptin could inhibit the secretion of insulin to reduce synthesis of fat;insulin could stimulate the secretion of leptin. At the end of our study, the levels of leptin and insulin in DIO rats were significantly higher than those in DIO - R and control rats. DIO rats exhibited hyperinsulinemia and hyperleptinemia. ISI decreased and was in negative correlation with leptin. All these suggested that leptin may relate to IR. The high level of leptin could not lessen body weight of DIO rats, there maybe exist leptin resistance in DIO rats and the ability of leptin inhibiting the secretion of insulin decreased, so the axis of fat - insulin was destroyed. IR emerged in DIO rats.2. sTNFR2, TNF-aandIRThe concentration of sTNFR2 was more stable than TNF - a and detected easily. It was reported that the level of serum sTNFR2 in obese women was higher than that in normal weight controls. sTNFR2 might serve as a diagnostic marker for obese individuals in whom there was TNF - a related to IR. There were no significant differences of the levels of serum sTNFR2 and TNF - a in the three groups in 3 w of our study. In 12 w, concentrations of serum sTNFR2 and TNF - a were significantly higher in DIO rats than those in DIO - R and control rats, negatively correlated with ISI. sTNFR2 may participate in the pathogenerosis of IR related to obesity. The possible mechanism of the high level of sTNFR2 induced IR was that the elevated level of TNF - a selectively upregulated TN-FR2. After TNF - a combinating to TNFR2, TNF - a inhibited tyrosine phos-phorylation of insulin receptor and IRS - 1 and the expression of GLUT4, leading to IR. Furthermore, sTNFR2 may prolong the bioactivity of TNF - a. It could suppose that sTNFR2 may play an important role in pathogenerosis of IR related to obesity.