Distinctive Lipogenic Gene Expression Patterns in the Mammary Glands of Dairy Cows Are Associated with the Unique Fatty Acid Composition of Bovine Milk Fat

Abstract

Fat composition is largely responsible for the technological and rheological properties of cow milk and dairy products. Bovine milk fat is unique in terms of its fatty acid composition and positional distribution, with about 25% of its fatty acids being short- and medium-chain, which are synthesized de novo in the mammary gland and are not present in extra-mammary tissues. With the aim to identify potential genetic factors responsible for the unique composition of bovine milk fat, we extracted genes with GO annotations related to lipid metabolism and performed a gene expression mega-analysis. Overall, different lipogenic tissues (i.e., mammary, liver, and adipose) displayed discerned expression patterns. In a PCA, the liver was significantly separated from adipose and mammary tissues. In a correlation analysis with the fatty acid synthetase (FASN) gene, notable differences among the tissues were found. In the mammary gland, the majority of genes (~70%) were negatively correlated with FASN expression, whereas only 18% were negatively correlated in adipose. Only a few genes were positively correlated with FASN exclusively in the mammary gland, including AGPAT1 and AGPAT6, which also had the highest expression in the mammary gland compared with adipose. Looking at the expression levels in tissues (TPM) revealed significant differences in the expressions of genes responsible for the activation of fatty acids by ligation to CoA, according to their carbon chain length. Notably, the ACSS1 gene, which converts acetate to acetyl-CoA, had the highest expression in the mammary gland, whereas genes responsible for the activation of long-chain fatty acids had lower expressions. The findings of the present study suggest that the unique properties of dairy fat are the results of the distinct expression patterns of genes involved in de novo synthesis of fatty acids and their downstream utilization.

Keywords: AGPATs; Acyl-CoA synthase; adipose; dairy; de novo synthesis; lipids; triglycerides.

Figure 1

PCA analysis of expression data. Principal component analysis (PCA) plot, based on covariances, demonstrating differences in lipid metabolism gene expression across lipogenic tissues (adipose, mammary, liver, and milk cells). The PCA analysis revealed a distinct separation of the tissues in the reduced dimensional space defined by the principal components. Each data point represents a tissue sample, with different colors indicating different tissues.

Figure 2

Venn diagram based on correlation with FASN gene expression. Gene expressions of 734 genes with GO annotations related to lipid metabolism were correlated with FASN gene expression in adipose and mammary tissues. A total of 622 genes that were significantly correlated with FASN in adipose and/or mammary are visualized in Venn diagrams. (A) Venn diagram of both positively and negatively correlated genes in adipose and mammary (illustrated using Venny2.0.1 [19]). (B,C) Venn diagrams of negatively (B) and positively (C) correlated genes in adipose and mammary.

Figure 3

Clustering of candidate genes based on correlation with FASN gene expression. Hierarchal clustering analysis of 115 analyzed genes from six functional categories [availability of building blocks for the de novo synthesis of fatty acids (pink), acyl-CoA thioesterases (blue), elongases and desaturases (red), acyl-CoA synthases (green), phospholipid and sphingolipid biosynthesis (grey), and TAG biosynthesis (purple)] based on their expression level correlations coefficients with FASN gene expression level in adipose and mammary tissues. Genes are presented with their name, or if absent, with their stable IDs. Text color refers to the category according to the legend.

Figure 4

Expressions of thioesterase genes. Expressions [Log(TPM+1)] of acyl-CoA thioesterase genes in adipose and mammary tissues and milk cells. Means and error bars of all samples are shown by gene and tissue. Statistical analysis of the differences among the means of different tissues for each gene are provided in Supplementary File S3.

Figure 5

Expressions of acyl-CoA synthase genes. Expressions [Log(TPM+1)] of acyl-CoA synthetase genes in adipose and mammary tissues and milk cells. Means and error bars of all samples are shown by gene and tissue. Statistical analysis of the differences among the means of different tissues for each gene are provided in Supplementary File S3.

Figure 6

Expressions of TAG synthesis genes. Expressions [Log(TPM+1)] of triglyceride biosynthesis genes in adipose and mammary tissues and milk cells. Means and error bars of all samples are shown by gene and tissue. Statistical analysis of the differences among the means of different tissues for each gene are provided in Supplementary File S3.