| Obesity is a public health problem.It increase the risk of life threatening diseases such as hypertension,type 2 diabetes and atherosclerosis.Increased fat intake induces overweight and obesity.Switching to low-fat or fat-free diets may not be ideal because of vitamin loss and increased risk for heart disease.One of the alternative strategies is to replace part of the conventional dietary long-chain triglyceride(LCT) with medium-chain TG(MCT) oil.Despite controversy,many studies have documented that MCTs cause significant reduction in body weight or fat pad size in animals and humans, as recently reviewed by St-Onge et al.MCTs also facilitate vitamin delivery and inhibit the production of inflammatory cytokines.MCT have several unique physiological and biological characteristics,as reported elsewhere over the past half-century.MCT,composed exclusively of MCFA(medium-chain fatty acid),are metabolized differently from LCT,composed exclusively of LCFA(long chain fatty acid).MCFA are absorbed via the portal system and are transported to the liver directly,whereas LCT are absorbed via the intestinal lymphatic ducts and transported as chylomicrons through the thoracic duct to reach systemic circulation.MCFA and hence MCT are easily oxidized because their intramitochondrial transport does not require carnitine palmitoyltransferase(CPT),a rate-limiting enzyme of mitochondrial-oxidation.MCT are absorbed and metabolized as rapidly as glucose.Several studies showed that MCT have more energy expenditure. They are utilized to prevent obesity and several lifestyle-related diseases.Previous studies have concentrated on the pheripheral mechanism,such as adiposity,liver, gastrointestinal hormones.However,the effect of MCT on central neuropeptide,such as neuropeptide Y(NPY),leptin receptor(Ob-R) are still unknow.NPY,a potent stimulator of feed intake,is mainly synthesized in the arcuate nucleus and injected into the paraventricular nucleus induced hyperphagia and reduced energy expenditure.Leptin is a signal for the regulation of food intake through hypothalamic leptin receptor(Ob-R) to regulate body fat mass.Kalra et al indicated that the over expression of Ob-Rb in the hypothalamus may contribute to the inhibitory effects of leptin on food intake and body weight.It has been well established that leptin receptor (Ob-R) regulating food intake and energy expenditure with NPY.It has been concluded that the type and amount of dietary fat can modulate the NPY mRNA expression.It is possible that the inhibitory effect of MCT on body weight gain is caused by altered downstream of the actions of Ob-Rb.We hypothesized that dietary MCT might alter leptin and leptin signaling to the brain(leptin-related hypothalamic neuropetide mRNA expression) resulting in less body weight gain.Insulin resistance is characterized by a reduced sensitivity of insulin to target tissues. Although obesity is a common cause of insulin resistance,the molecular link between increased adiposity and insulin sensitivity remain unclear.Animal and human studies have provided evidence that MCT impact not only lipid metabolism but also glucose metablism.Han et al found improved glucose tolerance and insulin sensitivity in MCT-fed rats,compared with LCT.Eckel et al reported that an MCT diet,compared with an LCT diet,increased insulin sensitivity in both diabetic patients and nondiabetic subjects.Adipose tissue synthesizes and secretes biologically active molecular that might be called "adipocytokines".Dysregulation of adipocytokines in obesity might play a role in the development of insulin resistance.More recently,resistin,which is a novel adipose-specific cysteine-rich protein,has been found to impair insulin sensitivity and glucose tolerance.Adiponectin is a product of the apM1 gene,which is specifically and highly expressed in human adipose tissue.Plasma adiponectin concentration and mRNA expression were decreased in obese humans,with increased adiposity,and in patients with type 2 diabetes with insulin resistance.The peroxisome proliferator-activated receptor(PPAR)-γis the master regulator of adipocyte differentiation and controls many adipocytes genes.Its synthetic ligands,thiazolidinediones(TZDs),are a new class of antidiabetic drugs that improve insulin action.Many studies demonstrated TZDs can markedly enhance the expression and secretion of adiponectin in vitro and in vivo through the activation of its promoter.Furthermore,retinoid X receptor(RXR),liver receptor homolog-1(LRH-1),tumor necrosis factor a(TNF-a) are all involved in transactivation machinery of adiponectin gene.Here,we tried to determine whether dietary MCT woule affect leptin and leptin's signaling pathway.Meanwhile,we also investigated the effect of MCT diet on adipose adiponectin,resistin,PPARγ,TNF-a mRNA PART 1 METABOLIC EFFECTS OF A MEDIUM-CHAIM TRIGLYCERIDES DIETObject:It is well know that high fat-diet induce obesity,but the result will be adiverse when there is too much fat in diet,because studies on establishment of obese models have revealed that too much fat down-regulatees the appetite of subjects.Thus,we induced obesity by feeding rats a high fat diet containing about 15%fat in order to provide a solid foundation for the next work.The aim of the present part was to investigated the effects of MCT diet on some physiological factors such as body weight, food intake,adipose depots,size of fat cells,serum lipid profiles,serum glucose,and insulin levels.Methods:Thirty male wistar rats weighting 85～115g were fed standard laboratory chow for the 1st wk to allow them to adjust to the new environment.Mice were then randomly assigned to a control group(5%of weight as fat),MCT group(14%of weight as MCT) and LCT group(15%of weight as LCT).All rats were maintained in cages under appropriate temperatue,humidity,and light-dark cycle conditions.Weekly body weight and daily food intake were measured for 8 weeks.At the end of feeding period,the rats were killed by anesthetizing with sodium pentobarbital(7ml/100g of body weight i.p.) after 12h of food deprivation.Blood(inferior vena cava) was collected to obtain serum, which was stored at -80℃for analysis.One aliquot of blood was collected directly in a tube containing heparin(10μl/ml of total blood) and aprotinin(500U/ml of total blood) placed on ice and centrifuged;and the obtained blood plasma was stored at -20℃for leptin and NPY determination.The abdominal adipose tissues(perirenal,epididymal) were carefully removed using scissors and weighed.At the same time,the whole brains of five rats in each group was obtained and stored in liquid nitrogen.They were putted on ice plate the next day;the hypothalamuses were sampled and weighed,then putted in glass homogenizer.It was added with 1ml 1NHCL and homogenized,then transferred to plastic tube and stored 100min under room temperature to make the bioactive peptides dissolved completely in acid solution.After adding 1ml 1N NAOH,it was centrifuged for 10min by 4000×g under 4℃.The supernatant was stored at -20℃for determination of NPY and leptin contents.The other hypothalami was also sampled and frozen for extraction of RNA and quantitative real-time RT-PCR experiments.Results:Significantly decrease of body weight,the weight of adipose pads,serum total cholesterol(TC),low density lipoprotein-cholesterol(LDL) and high density lipoprotein-cholesterol(HDL-C) were noted in the group fed MCT as compared with those fed LCT.In contrast,serum total triaglyceride(TG) were significantly increased in MCT group compared to those in LCT group.There were no significant differences in serum glucose,food intake,energy intake between them.The insulin resistance in MCT group was ameliorated compared with those in LCT group.MCT diet reduced adipocyte diameter enlarged by LCT diet.conclusions:The results suggested dietary MCT can reduce body weigh gain,the weight of adipose pads,diameter of adipocyte,serum total cholesterol and LDL after a eight-week feeding period,and can reverse insulin resistance.PART 2 High Expression of Leptin Receptor in Rats Fed with Medium-chain TAG Compared with Long-chain TAGObject:Previous studies demonstrated that,compared with long-chain TAG(LCT), dietary medium-chain TAG(MCT) could reduce body weight gain.It has been well established that leptin acts as a signal for body fat storage through the hypothalamic leptin receptor(OB-R) regulating food intake and energy expenditure with neuropeptide Y.Methods:Male Wistar rats were fed with diets containing 30%MCT or LCT for 8 wk. The effects of dietary MCT on serum leptin concentration,leptin receptor and NPY mRNA levels in hypothalamus were studied in rats.NPY and leptin was extracted and measured with a specific radioimmunoassay and ELISA.Results:Rats fed with the MCT diets had less body weight gain and body fat accumulation in the MCT diet group than those fed with the LCT diets(P＜0.01). Hypothalamus and serum NPY concentrations in rats fed with the MCT diets were lower than those in LCT group.MCT rats had significantly higher ratio of hypothalamus/serum leptin compared with LCT rats.The Ob-Rb mRNA level in the hypothalamus was significantly higher in the rats fed with the MCT diets than in those fed with the LCT diets.There was no significant difference in NPY expression between the LCT and MCT groups.Conculsions:These findings suggest that dietary MCT,compared with LCT,results in higher Ob-Rb mRNA level in rats.The effects of MCT on other actions of leptin such as its effects on the SCOS-3,melanocortin or endocannabinoid system should be investigated.PART 3 High Expression of Adiponectin and PPARγin Rats Fed with Medium-chain TAG Compared with Long-chain TAGObjective:Previous studies demonstrated that,compared with long-chain TAG (LCT),dietary medium-chain TAG(MCT) could improve glucose tolerance in rats and humans.It has been well established that adiponetin acts to increase insulin sensitivity. The peroxisome proliferator-activated receptorγ(PPARγ) is the master regulator of adipocyte differentiation and controls many adipocytes genes,including adiponectin and TNF-a.Furthermore,TNF-a are all involved in transactivation machinery of adiponectin gene.The effects of dietary MCT on adiponection,PPARγ,TNF-a levels in serum and adipose tissue were studied in rats.Methods:Serum adiponectin and TNF-a were assayed by enzyme linked immunosorbant assay(ELISA).Total RNA was isolated from the retroperitoneal fat by the method of GIT,and real-time reverse transcription polymerase chain reaction was performed to quantify mRNA of PPARγ,adiponectin.Western-blot was perfomed to quantify protein of PPARγ,adiponectin in white adipose tissue.Results:Rats fed the MCT diet had smaller cell diameter of the perirenal adipose tissue (P＜0.01).The serum adiponectin concentration was high(P＜0.01) in the MCT diet group than in the LCT diets group.Adipose adiponectin,PPARγmRNA expression was much higher in the MCT diet group than in the LCT diet group.In contrast,no differences in serum TNF-a levels expressions were found between the two groups.Conclusions:Dietary MCT result in a high serum adiponectin level with transcriptional activation of the adiponectin gene in rats compared with LCT diet.We speculate that improved glucose tolerance in rats fed an MCT diet may be ascribe to the high serum adiponetin level.Our results also indicate that PPARγplay significant roles in the transcriptional activation of adiponectin gene. |
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