Glucocorticoid receptor is involved in the breed-dependent transcriptional regulation of mtDNA- and nuclear-encoded mitochondria genes in the liver of newborn piglets

Abstract

Background: Mitochondria, which are essential for the functionality of eukaryotic cells, are particularly important in metabolically active tissues such as liver. Different breeds of pigs demonstrate distinct metabolic profiles in the liver, yet little is known whether the expression and transcriptional regulation of mitochondrial genes differ between breeds.

Results: Here we used male newborn Large White (LW) and Erhualian (EHL) piglets to delineate the difference in hepatic mitochondrial gene regulation between breeds. The hepatic content of ATP was significantly higher (p < 0.01) in EHL piglets, which was associated with lower mtDNA copy number (p < 0.05). Most of the mtDNA-encoded genes (10 of 13), however, were more abundantly expressed in EHL compared to LW piglets. We also detected 3 differentially expressed nuclear-encoded mitochondrial genes, among which isocitrate dehydrogenase 2 (IDH2) and ATP synthase, H+ transporting, mitochondrial Fo complex, subunit d (ATP5H) were expressed significantly lower, while adenylate kinase 1 (AK1) was significantly over expressed in EHL piglets. Compared to LW, the over expression of mtDNA-encoded genes in EHL was associated with significantly higher (p < 0.01) glucocorticoid receptor (GR) binding to the control region of mtDNA with no alterations in the methylation status. For nuclear-encoded genes, however, a negative correlation was observed between GR binding and mRNA expression of AK1 and ATP5H. Moreover, higher expression of AK1 in EHL piglets was also associated with lower cytosine methylation (p < 0.05) and hydroxymethylation (p < 0.05). In the promoter region.

Conclusions: These results indicate a role of the GR in the breed-dependent regulation of mitochondrial genes in the liver of newborn piglets.

Figure 1

Hepatic ATP content (A) and the relative mtDNA copy number (B) in newborn EHL and LW piglets. Values are mean ± SEM, n = 6/breed. * means p < 0.05, ** means p < 0.01.

Figure 2

Hepatic expression of mtDNA- (A) and nuclear-encoded (B) mitochondrial genes in EHL and LW pigs. Values are mean ± SEM, n = 6/breed. * means p < 0.05, p values between 0.05 and 0.1 are marked above the bar.

Figure 3

Schematic structure of pig mtDNA (A), GR binding (B), and DNA (hydroxyl)methylation status of CpG island on the control region of mtDNA (C). Values are mean ± SEM, n = 6/breed. ** means p < 0.01 between breeds; GREs, glucocorticoid response elements; HSP, heavy strand promoter; LSP, light strand promoter.

Figure 4

GR binding and (hydroxyl)methylation status of the promoter of nuclear-encoded mitochondrial genes. Values are mean ± SEM, n = 6/breed. * means p < 0.05 between breeds, ** means p < 0.01. GREs, glucocorticoid response elements. (A) Schematic structure of IDH2 promoter. (B) GR binding to IDH2 promoter. (C) DNA methylation and hydroxymethylation status of the CpG island on IDH2 promoter. (D) Schematic structure of AK1 promoter. (E) GR binding to AK1 promoter. (F) DNA methylation and hydroxymethylation status of the CpG island on AK1 promoter. (G) Schematic structure of ATP5H promoter. (H) GR binding to ATP5H promoter. (I) DNA methylation and hydroxymethylation status of ATP5H promoter.

Figure 5

Hierarchical cluster analysis of the characteristics in liver from Large White (LW) and Erhualian (EHL) piglets. The figure was drawn by MeV software (version 4.2.6). Correlation (centred) similarity matrix and average linkage algorithms were used in the cluster analysis. Each row represents an individual characteristic, and each column represents a sample. The dendrogram at the left side and the top displays similarity of expression among characteristics and samples individually. The color legend represents the characteristic level, with red indicating high expression levels and green indicating low expression levels, while the gray blocks in the figure indicate missing values. The codes on the legend are log2-transformed values.