Transcriptomic and Metabolomic Analyses Reveal Inhibition of Hepatic Adipogenesis and Fat Catabolism in Yak for Adaptation to Forage Shortage During Cold Season

Abstract

Animals have adapted behavioral and physiological strategies to conserve energy during periods of adverse conditions. Hepatic glucose is one such adaptation used by grazing animals. While large vertebrates have been shown to have feed utilization and deposition of nutrients-fluctuations in metabolic rate-little is known about the regulating mechanism that controls hepatic metabolism in yaks under grazing conditions in the cold season. Hence, the objective of this research was to integrate transcriptomic and metabolomic data to better understand how the hepatic responds to chronic nutrient stress. Our analyses indicated that the blood parameters related to energy metabolism (glucose, total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, lipoprotein lipase, insulin, and insulin-like growth factor 1) were significantly (p < 0.05) lower in the cold season. The RNA-Seq results showed that malnutrition inhibited lipid synthesis (particularly fatty acid, cholesterol, and steroid synthesis), fatty acid oxidation, and lipid catabolism and promoted gluconeogenesis by inhibiting the peroxisome proliferator-activated receptor (PPAR) and PI3K-Akt signaling pathways. For metabolite profiles, 359 metabolites were significantly altered in two groups. Interestingly, the cold season group remarkably decreased glutathione and phosphatidylcholine (18:2 (2E, 4E)/0:0). Moreover, integrative analysis of the transcriptome and metabolome demonstrated that glycolysis or gluconeogenesis, PPAR signaling pathway, fatty acid biosynthesis, steroid biosynthesis, and glutathione metabolism play an important role in the potential relationship between differential expression genes and metabolites. The reduced lipid synthesis, fatty acid oxidation, and fat catabolism facilitated gluconeogenesis by inhibiting the PPAR and PI3K-Akt signaling pathways to maintain the energy homeostasis of the whole body in the yak, thereby coping with the shortage of forages and adapting to the extreme environment of the Qinghai-Tibetan Plateau (QTP).

Keywords: energy metabolism; forage shortage; liver; metabolomics; transcriptome; yak.

FIGURE 1

Hepatic metabolic profiles in YW/YC group. (A) All samples' principal component analysis (PCA) (at the metabolites level). (B) Orthogonal partial least-squares discriminant analysis (OPLS-DA). (C) Heat map of differential metabolites in YW/YC group. The abscissa indicates the sample name, and the ordinate indicates the differential metabolite. The color from blue to red indicates the abundance of expression of metabolites from low to high; that is, the redder indicates the higher expression abundance of differential metabolites. (D) Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway terms enriched by metabolites of liver between YW and YC yaks. X axis means rich factor (rich factor = DEGs enriched in the pathway ÷ background genes in the pathway). Y axis represents the KEGG pathway terms. The color of roundness represents p value. The area of roundness represents the number of DEGs enriched in this pathway.

FIGURE 2

Transcriptomic comparisons of liver between YW and YC yaks. (A) All samples principal component analysis (PCA) (at the genes level). (B) Histogram of DEGs in liver of YW and YC yaks. (C) The volcanic map of DEGs in liver of YW and YC groups. Gray was the nonsignificantly different gene; red and green were the significantly different genes. The X axis represents log₂ fold change, and the Y axis represents −log₁₀ p value. (D) Heat map for hierarchical cluster analysis of DEGs between samples. Red: up-regulated genes; blue: down-regulated genes. (E) Gene Ontology (GO) analysis of the DEGs in liver of yaks. The top 30 GO terms with the lowest FDR in molecular function and biological process are shown, respectively. Y axis represents GO terms, and X axis represents the −log₁₀ (p value).

FIGURE 3

Heat map and KEGG pathways enrichment for DEGs. (A) Heat map of DEG-related hepatic energy metabolism. Red: up-regulated genes; blue: down-regulated genes. (B) KEGG pathways enrichment for DEGs. YC group versus YW group.

FIGURE 4

KEGG pathways enrichment and correlation analysis for DEGs and differential metabolite–related energy metabolism. (A) KEGG pathways enrichment for DEGs and differential metabolite–related energy metabolism. YC group versus YW group. (B) Correlation analysis of the differential metabolites and the DEG-related energy metabolism. Y axis represents different genes. X axis represents the different metabolites. Color intensity indicates the following: red: positively correlated; blue: negatively correlated.

FIGURE 5

The regulation of the PPAR signaling pathway and PI3K-Akt signaling pathway in the liver of the yak under long-term nutritional stress. ↑: up-regulation of genes expression or enhanced the pathways; ↓: down-regulation of genes expression or diminished the pathways. →: promote or result in. ⊥: the genes expression or metabolic pathways were inhibited.