DNA methylation of peroxisome proliferator-activated receptor γ promoter region and down-regulation of its mRNA expression in liver decrease insulin sensitivity of fetal growth restricted rats

Objective To explore the effect of methylation of peroxisome proliferator-activated receptor γ(PPARy) gene promoter in liver and its mRNA expression changes on decreasing of insulin sensitivity in fetal growth restriction (FGR) rats.Methods Twenty pregnant rats were randomly divided into two groups on their first day of pregnancy:normal-protein group (NP) and low-protein group (LP),ten in each.During pregnancy the LP group rats were fed with low-protein diet (8.00％ protein),while the NP group rats were fed with normal-protein diet (20.00％ protein).The offspring rats were fed with standard feed after 21 days of birth.Male offsprings in NP group were as control offsprings,and male FGR offsprings in LP group ware as FGR offsprings.At day 3,7,14,30,60 and 90,fasting blood of offsprings was collected to measure fasting plasma glucose (FPG) and fasting insulin(FINS).Then insulin resistance index of homeostasis model assessment (HOMA-IR) and insulin sensitivity index (ISI) were calculated to evaluate insulin sensitivity.At day 7 and 90,liver tissue of male offsprings was collected to extract DNA and total RNA.The methylation level of PPARγ gene promoter and its mRNA expression were detected by methylation specific-polymerase chain reaction (MS-PCR) and reverse transcription-RCR,respectively.The relationships between methylation of PPARγ gene promoter and mRNA expression and insulin sensitivity were analyzed by Pearson correlation and nonparametric test method.Results (1) The mean offspring birth-weight of LP group was (4.92±0.36) g,which was lower than that [(6.43±0.59) g] of control group (t=14.73,P＜0.05).In LP group,the incidence of FGR offspring was 88.2％ (97/110) and the FGR incidence of male ones was 94.1％ (48/51).(2) At day 90,compared with control offsprings,FPG [(8.95±1.83) mmol/L vs (6.21±1.14) mmol/L,t=-3.291,P＜0.05],FINS [(59.57±9.89) mU/Lvs (36.10±7.32) mU/L,t=-4.916,P＜0.05] and HOMA-IR (0.967±0.297 vs 0.410±0.135,t=-4.472,P＜0.05) of FGR offsprings were significantly higher; while ISI of FGR offspring was lower than that of control offsprings (-3.043±0.294 vs -2.172±0.354,t=4.774,P＜0.05).(3) There was no significant difference in methylation degree of PPARγ gene promoter in liver between FGR and control offsprings at day 7 (0/8 vs 2/8,Fisher exact test,P＞0.05).The methylation degree of PPARγ gene promoter in liver in FGR offsprings was significantly higher than that of control offsprings at day 90 (8/8 vs 2/8,Fisher exact test,P＜0.05).Compared with control offsprings,PPARγ gene mRNA expression level of FGR offsprings decreased significantly at day 90 (4.3.07±7.51 vs 146.72± 40.66,t=7.09,P＜0.05).mRNA expression of PPARγ gene was the lowest in exhaustive methylation offsprings (27.2± 1.6),and then in partial methylation ones (47.3±33.0),the highest in no methylation ones (144.6 ± 21.2) (P＜0.05).(4) The correlation analysis showed that PPARγ mRNA expression level negatively correlated to the level of FPG (r=-0.819),FINS (r=-0.906) and HOMA-IR (r=-0.860),P＜0.05 respectively; but positively correlated to ISI level (r=0.947,P＜0.05).Conclusions Hypermethylation in promoter region of PPARγ gene might inhibit gene transcription,and be involved in the occurrence of insulin resistance in FGR rats. Key words: Fetal growth retadation; Insulin resistance; PPAR gamma; DNA methylation