Differences of PPARD Expression in the Liver of Cattle with Different Marbling Grades

Fat deposition determines beef marbling grade and meat quality, but the underlying molecular mechanisms remain unclear. This study aimed to investigate the role of bovine PPARD in lipid deposition, especially its effect on the expression of fatty acid transport genes (CD36, FATP1, and FABP1) and the lipid droplet-associated gene (PLIN2) in liver tissues from cattle with different marbling grades. The mRNA abundance and protein levels in liver tissues from thirty-one Wagyu × Angus crossbred beef cattle (25–26 months old) with different marbling grades (according to GB/T 29392-2022, based on the marbling richness of the longissimus dorsi muscle at the 12th–13th rib interface) were analyzed by RT-qPCR and Western blot, respectively. Additionally, PPARD was knocked down and overexpressed in bovine mammary epithelial cells to validate its effects on lipid-metabolism-related genes. The results showed that the mRNA levels of CD36, FATP1, PPARD, RXRA, RXRB, and PLIN2 were significantly higher (p < 0.01) in livers tissues from the A3 and A4 groups (high marbling) than in those from the A1 and A2 groups (low to moderate marbling). Western blot analysis revealed significantly higher PPARD protein expression in the A3 and A4 groups (high marbling) than that in the A1 and A2 groups (low to moderate marbling) (p < 0.05). It should be noted that the sample size of Group A4 is only 2, and the results of this group should be considered as a preliminary trend that needs to be validated with larger sample sizes. Cellular experiments confirmed that PPARD knockdown significantly decreased mRNA expressions of FABP1, CD36, and PLIN2 (p < 0.01), while PPARD overexpression significantly increased their mRNA levels (p < 0.05). These results indicate a positive correlation between PPARD expression and the transcriptional levels of genes involved in fatty acid transport and lipid droplet storage, suggesting that PPARD may be associated with hepatic lipid metabolism and potentially contribute to marbling development. These findings suggest that the PPARD signaling pathway contributes to hepatic lipid deposition and may play a role in marbling formation in beef cattle.