Investigating the potential role of propionylcarnitine in milk pentadecanoic acid synthesis in Chinese holstein dairy cows using multi-omics analysis

Pentadecanoic acid (C15:0) is an odd-chain fatty acid (OCFA) with significant health benefits, its content in milk is regulated by both ruminal microbial metabolism and mammary gland synthesis. To investigate the regulatory mechanisms of C15:0 synthesis, we conducted 16S rRNA sequencing and ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) of ruminal fluid of Chinese Holstein dairy cows with low (1.03 ± 0.01%, Low group) and high (1.39 ± 0.06%, High group) milk C15:0 content. Additionally, an in vitro study was performed using key ruminal metabolite to culture bovine mammary epithelial cells (BMECs). Ruminal propionate level increased in the High group. 16S rRNA sequencing identified 3 bacterial species with differential abundance, including higher levels of Prevotella_bryantii and Lachnospiraceae_bacterium_DJF_B223 in the High group, while lower level of Fibrobacter_sp_UWH4. Metabolomic analysis revealed 7 differential metabolites (mean decrease accuracy > 0 and p < 0.05), primarily lipid metabolites, amino acid derivatives, and compounds linked to energy metabolism and signaling. These metabolites were enriched in carbohydrate and amino acid metabolic pathways. Correlation analysis indicated positive associations among milk C15:0 content, the increased bacterial species, and the differential metabolite propionylcarnitine (PLC). Consequently, PLC was selected for in vitro study which demonstrated that 100 µM PLC for 24 h treatment enhanced BMECs proliferation, increased C15:0 content, and decreased C17:0 content compared with BMECs treated without PLC. The RNA-seq identified 536 differentially expressed genes (DEGs; p < 0.05 and fold change > 1.2) between the two BMECs groups. PLC upregulated the expression of DEGs related to lipid metabolism, including hydroxysteroid 17-beta dehydrogenase 4 (HSD17B4), glycerol-3-phosphate acyltransferase, mitochondrial (GPAM), and CD36 molecule (CD36), while western blotting demonstrated that the protein expression of HSD17B4 was also upregulated. KEGG enrichment analysis showed 536 DEGs were enriched in peroxisome proliferator-activated receptors signaling pathway, biosynthesis of unsaturated fatty acids, lipid and atherosclerosis. The Higher abundance of Prevotella_bryantii and Lachnospiraceae_bacterium_DJF_B223 in the rumen of dairy cows related with the higher level of PLC, and more PLC was transported to the mammary gland. In BMECs, PLC enhanced C15:0 synthesis via upregulating the peroxisomal β‑oxidation mediated by HSD17B4.