Uncovering the mechanism whereby dietary nicotinic acid increases the intramuscular fat content in finishing steers by RNA sequencing analysis

In our previous study, we found that a higher dosage of nicotinic acid (NA) in the diet dramatically increases intramuscular fat (IMF) content and improves meat quality in finishing steers. We hypothesised that increased IMF results from the regulation of genes associated with adipogenesis. To address this hypothesis, RNA-seq was used to investigate gene-expression profiles of longissimus muscles from the same 16 cattle that were also used in our previous study and treated with or without dietary NA. Four cDNA libraries were constructed and sequenced. The repeatability and reproducibility of RNA-seq data were confirmed by quantitative reverse-transcription polymerase-chain reaction. In total, 123 differentially expressed genes (DEGs) were identified between longissimus muscles treated and those not treated with dietary NA. Of the 123 DEGs, 117 genes were upregulated by the NA treatment. These DEGs were enriched in 21 pathways, including the extracellular matrix (ECM) –receptor interaction, PPAR signalling pathway, adipocytokine signalling pathway and transforming growth factor-β signalling pathway, all of which are associated with lipid metabolism. Furthermore, candidate genes related to adipocyte differentiation and adipogenesis (PLIN1, PLIN2, ADPN, LEP, LCN2 and SOCS3), lipid metabolism (FABP4, RBP4, GAL, ANXA1, ANXA2 and PTX3) and fatty acid synthesis and esterification (ELOVL6, ACSM1, SOT1 and PTGIS) were upregulated in the NA group. Three genes involved in glucose metabolism (PGAM1, UGDH and GLUT3) were also transcriptionally upregulated. However, MYH4 that encodes glycolytic Type IIb muscle fibres was downregulated by dietary NA. These gene expression results indicated a confirmation of our hypothesis that dietary NA increases the IMF content of longissimus muscle through upregulating the expression of the genes related to adipocyte differentiation, adipogenesis and lipid and glucose metabolism.