| Objective: With the rapid development of our country’s economy,the dietary pattern and lifestyle have changed dramatically,obesity and associated metabolic diseases are increasing year by year.Prevalence of childhood and adolescent obesity has increased greatly over the past 30 years.An increasing occurrence in children of obesity associated disorders such as diabetes has attracted great attention.However,relationships of age,dietary habits,obesity,and glucose/lipid metabolism are still unknown.The risks of developing metabolic disorders and diabetes induced by high fat diet in individuals at different ages remain unclear.The objective of this study is to examine the effects of high fat diet on insulin sensitivity and pancreatic β cell function,better understand the role of the age in response to metabolic stress,and search for early markers of β cell dysfunction induced by metabolic stress at different ages.Method : Three different age groups of C57BL/6J male mice,21-day-old,3-month-old,and 6-month-old,were used in this study.The mice in each group were randomly divided into two sub-groups and fed with either normal chow or high-fat chow for 12 weeks.Body weight and fasting blood glucose of the mice were monitored during the couse of the study.Plasma insulin and oral glucose tolerance tests(OGTT)were performed at the 12 th week of the experiment.At the end of the study,the mice were sarcrified,the islets were isolated and the ratio of proinsulin to insulin was examined.The pancrease,liver,and abdominal fat were also collected to examine the histological changes by immunohistochemistry.Results: Body weights of the mice with high fat diet(HFD)began to increase significantly at 1-2 weeks after the start of feeding compared with those of control mice in each group.At the end of 12 week of feeding(21DHF46.18±3.34 g vs.21DCD28.51±1.66g;3MHF51.51±2.2g vs.3MCD32.99±1.26g;6MHF46.06±8.07 g vs.6MCD36.87±4.67 g,P<0.01),21 DHF group had a highest increase in body weight from baseline 21DHF33.47±2.29 g vs.21DCD15.89±1.26g;3MHF22.71±1.63 g vs.3MCD4.40±1.44g;6MHF12.67±6.08 g vs.6MCD3.95±3.73 g,P<0.01),3MHF group had a highest increase in body weight induced by the high fat diet.The extra weight gains induced by high fat diet was 4 folds,2 folds and 1 fold respectively in 3MHF,,6MHF and 21 DHF group,suggesting that 3-month-old mice appear to be more susceptible to HFD and gain more weight.There was significant increase of 6-hour fasting blood glucose in HFD groups compared with that in the control groups(21DHF9.68±0.82 vs.21DCD6.60±0.86mmol/L;3MHF12.86±1.34 vs.3MCD6.30±1.42 mmol/L;6MHF9.68±1.32 vs.6MCD5.40±0.97mmol/L,P<0.05).Fasting blood glucose of 3MHF group began to increase significantly earliest,and was higher(P < 0.05)than the other two HFD groups.OGTT showed that the mice with HFD were glucose intolerance compared with the mice in control groups.The AUC during the OGTT was significantly higher(P<0.05)in the HFD groups than that in the control groups.The mice in the 3-month-old group appeared to be more susceptible to have hyperglycemia and glucose intolerance than that of the mice in the 21-day-old and 6-month-old.The fasting serum insulin level was significantly increaed(P<0.05)in HFD groups.The HOMA-IR was significantly higher(P<0.05)in HFD groups.The ISI was significantly lower(P<0.05)in HFD groups.3-month-old mice appear to be more susceptible to gain insulin resistance under HFD feeding.The percentage of proinsulin(PI)in total insulin(PI+I)in isolated islets was higher in HFD mice than the control mice of the same age(21DHF51.53±4.18% vs.21DCD28.20 ± 6.20%,P<0.01;3MHF69.84 ± 1.55% vs.3MCD30.76 ± 0.18%,P<0.01;6MHF 46.65±2.49% vs.6MCD 39.64±1.28%,P<0.01).Agin,the percentage of proinsulin was significantly increased in the islets isolated from 3MHF than that from other two HFD groups.The mean islet areas were higher in HFD than that in control mice of the same age(21DHF 14040±22274um2 vs.21 DCD 11061±7004um2,P=0.62;3MHF 22759±36270um2 vs.3MCD 11191±12509um2,P=0.02;6MHF 22072±30223um2 vs.6MCD 11424±17425um2,P=0.02.The increase in islet size is due to increased area of the β cells(ratio of β cell to islet area: 21DHF91.40±6.41% vs.21DCD85.41±9.47%,P<0.001;3MHF 94.68±9.06% vs.3MCD 85.83±9.55% P<0.001;6MHF 88.87±7.72% vs.6MCD 83.08±14.99% P=0.04),the ratio of α cell to β cell numbers was significantly lower in HFD than that in control mice of the same age((21DHF0.21±0.16 vs.21 DCD 0.30±0.16,P<0.05;3MHF 0.18±0.07 vs.3MCD 0.28±0.16 P<0.05;6MHF 0.22±0.09 vs.6MCD 0.27±0.13 P=0.09),but the mean β cell size was significantly higher only in 3MHF than in 3MCD mice.These data suggest that HFD induced increase in islet size is due to increased number of β cells and altered β cell volume in the 3MHF and 6MHF mice,but only due to increased number of β cells in the 21 DHF mice.Increased lipid accumulation was observed in the liver of all HFD groups.The severity of hepatic lipid accumulation caused by HFD followed this order: 3MHF>6MHF>21DHF.Increased accumulation of inflammatory cells was observed in the abdominal fat of all HFD mice.Conclusion:(1)C57BL/6J male mice fed with a 60% HFD for 12 weeks can develop obesity,insulin resistance and abnormal glucose metabolism.(2)3-month-old mice appear to be more susceptible to developing obesity,hyperglycemia,glucose intolerance,and insulin resistance than that in 21-day-old and 6-month-old mice.(3)Although there are compensatoy increases of islet size and β cell mass in HFD mice,a disproportional increased proinsulin,impaired glucose tolerance,and elevated fasting blood glucose suggest that β cell function appeared to be impaired in the mice fed with HFD.Altogether,this study reveals that age is a confounding factor playing a role in adaptation to the metabolic stress.Uncovering molecular mechanisms of age differences in compensatory caparcity of β cells may shed light on better understanding pathgenesis of type 2 diabetes. |
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