| Summary: | 碩士 === 中國醫藥大學 === 營養研究所 === 93 === Abstract
Peroxisome proliferator-activated receptor (PPAR) is a member of steroid hormone receptor superfamily of ligand-activated transcription factors. To date, three isoforms have been identified, PPARa, d and g, encoded by three separate genes. PPARa is expressed predominantly in lipid metabolizing tissue, such as liver where it plays a role in lipid catabolism. PPARg, on the other hand, is mainly expressed in adipose tissue and has a critical role in adipocyte differentiation and glucose metabolism. We had previously proved that dietary fish oil and oxidized frying oil (OFO) could activate PPARa and increase fatty acid oxidation in liver. Thus, lowering the liver (serum) lipids in rats. The anti-adiposity effect of OFO was also observed as well as fish oil. However, the effect of OFO on adipocyte differentiation, and glucose metabolism had never been explored. The aim of this study was to investigate the morphological change and differentiation status of adipocytes in rats fed with OFO or fish oil. In addition, the insulin sensitivity was also evaluated in rats fed with OFO. 48 SD male rats (about 100 g) were divided into four groups, receiving diet contain 5% (g/g) fresh soybean oil (LF), 20% fresh soybean oil (HF), OFO (HO) or fish oil (HFO), respectively. The OFO was prepared by frying wheat dough sheets in soybean oil at 205 ± 5 ℃ for 24 h. After 12 weeks feeding, rats in HO and HFO groups showed a significantly increased in liver and kidney weight (p<0.0001), and a significantly decreased in body weight gain (p<0.0001), feeding efficiency (p<0.0001), abdominal fat weight including epididymal (EP) and retroperitoneal (RE) (p<0.0001), serum and liver lipids (p<0.0001). The TG (triglyceride) content of adipocytes isolated from EP and RE was also significantly lower than those of rats fed with fresh soybean oil (p<0.0001 for RE). The volume of adipocytes isolated from EP and RE was also significantly lower in OFO than those of rats fed with fresh soybean oil (p=0.05 for EP and RE).
Though the activities of enzymes participated in lipogenesis of adipose tissue including glycerol-3-phosphate dehydrogenase (p<0.01 for RE) and lipoprotein lipase (p<0.05 for EP and p=0.05 for RE) were significantly increased in rats fed with OFO, the basal lipolysis rate (p<0.05 for EP and RE) in adipose tissue of HO and HFO group was significantly higher than HF group. The enhanced lipolysis rate in adipose tissue of rats fed with fish oil and OFO partly explained the anti-adiposity effect observed in those rats.
The gene expression of adipocyte differentiation markers and adipocytokines was analyzed by Northern blot. Compared with HF group, PPARg (p<0.05 for RE), LPL (p=0.05 for RE), leptin (p<0.005 for EP and p<0.0001 for RE) and adiponectin (p<0.0005 for RE) mRNA in adipose tissue was significantly reduced in HFO group. On the other hand, PPARg mRNA was significantly increased (p=0.05 for EP) but leptin mRNA was significantly decreased (p<0.0001 for RE) in adipose of HO group. Thus, the adipocyte differentiation seems to be slightly inhibited by fish oil, but not by OFO, though the adiposity was significantly reduced in OFO-fed rats.
The fasting blood glucose and serum insulin were followed for 9 wk in LF, HF and HO groups of rats. There was no significant difference among three groups, but the insulin and blood glucose tended to be higher in HO group of rats at the 9th wk. Rats fed with OFO also showed a significantly higher area under curve (AUC) in oral glucose tolerance test (OGTT) performed at the 3th and 9th wk (p<0.001 and p<0.01 for the 3th and 9th wk respectively).
In conclusion, these results showed OFO inhibit the hypertrophy of adipocytes induced by high-fat diet, but didn’t restrain the differentiation of adipocytes. The anti-adiposity effect in rats fed with fish oil and OFO-rich diet could be attributed to PPARa activation in liver and increased lipolysis rate in adipose tissue. Compared with fresh soybean oil, OFO may result in insulin resistance.
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