Study On OB-Rb, SOCS3 MRNA Expression In Liver Of Obese SD Rats Fed With High-fat Diet

Background: Obesity means an over-deposition of fat results from disequilibrium of energy metabolism and too much intake of calorie. As the improvement of standard of living and change of meal structure, obesity in children has shown an upward trend year by year. Obesity not only influences the health of children, but also as a risk factor for hypertension, diabetes, coronary disease, cholelithiasis, gout and diseases such as sudden death of adult. The discovery of leptin provided a new visual field for the understanding of fat mechanism. Leptin has the broad biological effects: inhibiting food intake, promoting energy expenditure, regulating metabolism and affecting hormone secretion, reproduction, immunity and vascular proliferation etc. Leptin acts through it's receptor OB-R. Animal experiments showed that gene mutation lead to the deficiency of leptin in blood circulation, which was one of the reasons of obesity. On the contrary, serum leptin levels were high in the acquired obese rodent and majority of obese patients. The phenomenon of low or no reaction to leptin is called leptin resistance, revealing a closer relationship between obesity and leptin resistance.Janus kinase-signal transducer and activator of transcription protein (JAK-STAT) pathway is the principal approach for leptin signal transduction. Suppressors-of-cytokine-signaling3 (SOCS3) is a leptin- inducible inhibitor of leptin signaling and a potential mediator of leptin resistance in obesity. In virto, SOCS3 can be induced by leptin and act in a negative feedback loop to inhibit the receptor. Overexpression of SOCS3 inhibits leptin-mediated tyrosine phosphorylation. Compared with wild–type mice, SOCS3+/- mice show both enhanced weight loss and increased hypothalamic leptin receptor signaling in response to exogenous leptin administration. Furthermore, SOCS3+/- are significantly protected against the development of diet-induced obesity and associated metabolic compli- cations. The level of SOCS3 expression is thus a critical determinant of leptin sensitivity and obesity susceptibility and this molecule is a potential target for therapeutic intervention.Objectives: To imitate the feature of children obesity, 4-week-old (equal to juvenile stage) SD rats were selected as rodent in the experiment .One of them were fed with high–fat diet to set up the animal model of dietary obesity. We study the changes of leptin as well as expression of long receptor isoforms(OB-Rb) mRNA and SOCS3 mRNA in peripheral tissue (liver). From the aspect of expression in OB-Rb mRNA and post-receptor signal transduction pathway, we try to probe into the possible pathogenesis of children leptin resistance and to study the relationship between OB-Rb mRNA or SOCS3 mRNA expression and serum leptin concentrations respectively.Methods: Thirty two four -week-old healthy male SD rats were divided into two groups randomly. One (20) acting as experimental group fed with high-fat diet to set up the animal model of dietary obesity; the other (12) serving as control group fed with ordinary diet .Body weights were monitored weekly. At the end of 8 weeks, obese group was obtained according to the standard that average body weight go beyond 20% compared with the control. For both obese and control rats, serum leptin concentrations were measured with radiation immunoassay ; expression of OB-Rb, SOCS3 mRNA in the liver was detected with semi-quantitative RT-PCR ( using ?–actin as an internal standard).Results:1. Rats fed with high-fat diet were observed significant increases in body weight from 4 weeks of high-fat diet administration compared with control group. At the end of 8 weeks, according to the standard above, 15 rats (75%) were diet-induced-obesity rats and treated as obese group. The body weight of obese group was (416.33±23.01)g, increased significantly compared with control group (321.92±19.65)g , and there were significantdifferences in body weights between obese and control groups (P<0.01). There were higher levels of glucose , triglyceride and insulin in obese group.2. The concentrations of serum leptin,insulin and C petide in rats of obese group were elevated obviously compared with control group ( (P<0.01). There were significant positive relationship between insulin,C petide and serum leptin concentrations in obese group (r=0.608,P<0.05; r=0.686,P<0.01)3. The expression (relative gray value) of OB-RbmRNA in obese group were 0.79±0.29, reduced markedly compared with the control group (1.63±0.62, P<0.01) and there were significant negative relationship between OB-Rb mRNA expression in liver and serum leptin concentrations in obese group (r2 =0.523, P<0.01).4. SOCS3 existed both in liver of two groups .The expression (relative gray value) of SOCS3mRNA in obese group were 1.42±0.65, increased markedly compared with the control group (0.54±0.12, P<0.01) and there were significant positive relationship between SOCS3 mRNA expression in liver and serum leptin concentrations in obese group (r2 =0.624, P<0.01). 5. There were no relationship between OB-Rb mRNA and SOCS3 mRNA expression in obese group (r2 =0.154, P>0.05).Conclusion:1. Serum leptin levels were increased in obese rat fed with high-fat diet. There were significant positive relationship between insulin,C petide and serum leptin concentrations, suggesting that high-fat diet may lead to leptin resistance. High leptin levels maybe act as one of the factors of insulin resistance in obese rats.2. The expression of OB-Rb mRNA in obese group was reduced remarkably as compared with the control group, and there was significant negative relationship between OB-Rb mRNA expression in liver and serum leptin concentrations in obese group, suggesting the down-regulation of OB-R in peripheral site act as one of the factors that result in leptin resistance in obese rats.3. The expression of SOCS3 mRNA in obese group was increased remarkably as compared with the control group, and there was significant positive relationship between SOCS3 mRNA expression in liver and serumleptin concentrations in obese group. The up-regulation of SOCS3's expression may act as the factors that result in leptin resistance in obese rats.