Animals handling
The animal experiments were approved by the Ethical Committee of the Department of Scientific Management of the Institute. After adaptive feeding with AIN-93M formula [37] for 5 days, 24 Wistar rats were divided randomly into control or 0.5% quercetin groups according to body weight and maintained on an AIN-93 diet (control) or 0.5% quercetin-supplemented (Sigma-Aldrich, St. Louis, MO, SA) AIN-93 diet (0.5%Q) for 6 weeks. Each group consisted of 6 female and 6 male rats, whose initial weights ranged from 180 to 200 g. Dietary intake was recorded daily and body weight weekly. The animal source, handling methods, and environmental conditions matched a previously reported protocol [20]. At the end of the experiment, all rats were fasted overnight. The animals were sacrificed by cervical dislocation. Liver tissues were sampled immediately, washed in ice-cold saline, and frozen in liquid nitrogen until use.
DNA preparation and methylated DNA immunoprecipitation-ChIP analysis
Liver samples from 2 female and 2 male rats in each group were randomly selected for genome-wide methylation analysis. Genomic DNA (gDNA) extraction, purification, quantification, and quality assessments were conducted according to the procedure of Aksomics, Ltd. (Shanghai, China). Sonicated gDNA was used for immunoprecipitation with a mouse monoclonal anti-5-methylcytosine antibody (Diagenode, Liège, Belgium). Methylated DNA immunoprecipitation DNA (MeDIP DNA) was purified and quantified using Qiagen MinElute columns (Qiagen, Hilden, Germany) and a nanodrop ND-1000 spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). A NimbleGen Dual-Color DNA Labeling Kit was used for DNA labeling according to the NimbleGen MeDIP-chip protocol (Nimblegen Systems, Inc., Madison, WI, USA). The microarrays were then hybridized with Cy3/5- labelled DNA and washed using the Nimblegen Wash Buffer Kit (Nimblegen Systems, Inc).
The Arraystar Rat 4 × 180K RN4 RefSeq Promoter Array (Aksomics, Shanghai, China) is designed to investigate DNA methylation and transcription factor binding sites within RefSeq Gene promoter regions and includes 15,987 gene promoter regions (from about − 1300 bp to + 500 bp of the transcript start sequences) covered by approximately 180,000 probes with approximately 158 bp spacing.
Data Normalization and Analysis
The MeDIP-chip data were analyzed by sliding-window (1500 bp) peak-finding algorithm provided by NimbleScan v2.5 (Roche-NimbleGen Inc.) from the normalized log2 ratio data. NimbleScan detects peaks by searching for at least two probes above a p-value minimum cutoff (-log10) of 2 and maximum spacing of 500 bp between nearby probes within the peaks. To compare differentially enriched regions between the 0.5%Q group and the control group, the log2 ratios were averaged and then used to calculate M0 for each probe: M0 = Average (log2MeDIP(1%Met)/Input(1%Met))-Average(log2MeDIP(control)/Input(control)). The NimbleScan sliding-window peak-finding algorithm was run on these data to find the differential enrichment peaks (DEPs). The DEPs, identified by the NimbleScan algorithm, were filtered according to the following criteria: (1) At least one of the two groups has a median log2 MeDIP/Input ≥ 0.3 and a median M0 > 0. (2) At least half of the probes in a peak may have a CV ≤ 0.8 in both groups. To separate strong CpG islands from weak CpG islands, promoters were categorized into three levels: high CpG promoters/regions (HCP, high CpG density promoter), intermediate CpG promoters/regions (ICP, intermediate CpG density promoter), and low CpG promoters/regions (LCP, low CpG density promoter).
Microarray data processing and Gene Ontology (GO) and Pathway analysis
Raw microarray data were normalized by the Bioconductor packages Ringo, limma, and MEDME. Normalized MeDIP-chip data were analyzed by NimbleScan v2.5 (NimbleGen). DAVID software was used to perform GO and Pathway analysis of regulatory networks. The GO project provides a controlled vocabulary to describe gene and gene product attributes in any organism and covers three domains: Biological Process, Cellular Component and Molecular Function. Pathway analysis is functional analysis that maps genes to KEGG pathways. The p-value (EASE-score, Fisher-Pvalue or Hypergeometric-Pvalue) denotes the significance of the Pathway correlated to the conditions. Lower the p-value, more significant is the Pathway (The recommend p-value cut-off is 0.05), using an unbiased, automated survey of published scientific literature (Global Literature Analysis). This analysis identifies functional relations among genes, such as direct binding, up-regulation or down-regulation and also builds subnetworks of genes and cellular processes based on their interconnections.
ChIP-quantitative PCR assay
A MeDIP assay combined with real-time quantitative PCR (qPCR) was used to evaluate the methylation status of candidate genes in the rat liver. MeDIP was performed as described above. Quantitative PCR was used to analyze the expression of purified DNA with an Applied Biosystems 7900 Real-Time PCR System (Applied Biosystems, Foster City, CA, USA). Expression levels of methylated Flot1, Kcnj11, Gys1, Erp29, and GAPDH were evaluated. The primers used for ChIP validation are shown in Table 1.
Table 1
Sequences of the primers used in qChIP analyses
Genes
|
Primer sequence
|
Product length (bp)
|
Flot1
|
F:5’ GGCGACTCGACCTCTTGCT 3’
R:5’ CGTGCGTCCGAACACTTCT 3’
|
104
|
Kcnj11
|
F:5’ TGTGCCCCGGATTTCCCAT 3’
R:5’ GGTTGCAGGGTGACTCGAAGG 3’
|
121
|
Gys1
|
F:5’ TCCCGGTCATCCAGTCATCT 3’
R:5’ CACCCGCTTTCCAATTTAGC 3’
|
106
|
Erp29
|
F:5’ GCTATGATCCCTGTGTCTTCTCCAG 3’
R:5’ GTGATAAAGGCTCGAAGGAATGAA 3’
|
56
|
GAPDH
|
F:5’GGACCCTGTGGTGCTTCATCT3’
R:5’GGGCAGTAAGTGCTCCTAATCG3’
|
181
|
Quantitative PCR analysis
Hepatic mRNA expressions of genes were determined by quantitative PCR (qPCR) [38]. Total hepatic RNA was isolated using the TRIzol reagent. The first cDNA strand was synthesized using a cDNA synthesis kit. Quantitative PCR was performed using a FastStart Universal SYBR Green Master Mix kit. TRIzol reagent, cDNA synthesis kit, and FastStart Universal SYBR Green Master Mix kit were purchased from Roche, Ltd. (Basel, Switzerland). Finally, melting curve analysis was performed by slowly cooling the PCR mixture with simultaneous measurement of the SYBR Green I signal intensity using an ABI Real-time PCR System (Applied Biosystems). The Δ threshold cycle (Ct) method was used to evaluate relative quantification, and GAPDH was used as a reference. The primers used for qPCR validation are shown in Table 2.
Table 2
Sequences of the primers used in qPCR analyses
Genes
|
Primers sequence
|
Product length (bp)
|
Erp29
|
F:5' GACAAGAAGTGGGCCAGTCA 3'
R :5’ GAAGGCGGTGAGGATGTTGA 3’
|
168
|
Kcnj11
|
F: 5' GTCAGGGGCTCAGTAAGCAA 3'
R :5’ CTTGCACCAACCTCTGGACT 3’
|
105
|
Gys1
|
F:5' GAGGGCAGAATGTCGGTCAA 3'
R :5’ GTACACGTGGGGCTTCAAGA 3’
|
172
|
GAPDH
|
F: 5’CCCCCAATGTATCCGTTGTG 3’
R :5’TAGCCCAGGATGCCCTTTAGT 3’
|
192
|
Statistical analysis
The one-sided Kolmogorov-Smirnov test was applied to analyze the microarray data. Fisher’s exact test was used to perform GO and Pathway analysis. ChIP-quantitative PCR and qPCR data are presented as means ± standard deviation. Statistical analysis was performed using the SPSS 10.01 software (SPSS Inc., Chicago, IL, USA). Student’s t-test was used to compare differences between two groups. Differences between two groups were considered statistically significant at p < 0.05.