Vitamin A-activated PPARγ signaling enhances intramuscular fat accumulation by overriding AMPK-mediated inhibition in late-fattening beef cattle.

BACKGROUND

Intramuscular fat (IMF) deposition determines beef marbling quality, with current industry practices relying on vitamin A (VA) restriction throughout fattening to enhance marbling development. This study challenges the conventional approach by investigating late-fattening vitamin A supplementation effects on marbling formation in Woking black cattle.

RESULTS

Initial in vitro experiments using bovine skeletal muscle cells (BSMCs) demonstrated that all-trans-retinoic acid (ATRA) treatment during late differentiation (0.1-1 μmol/L) enhanced lipid accumulation with upregulated PPARγ and FABP4 expression. In vivo trials with late-fattening VA supplementation (3,000 IU/kg DM) significantly improved marbling grades, achieving 75% high-grade marbling (A3 or above) with enhanced nutritionally beneficial fatty acids including EPA and DHA levels. Large-scale analysis using 336 genetically homogeneous cattle revealed that superior marbling development correlated with serum VA depletion after VA supplementation, indicating active utilization rather than restriction. A4-grade cattle showed significantly lower serum VA levels than A1-grade cattle, with coordinated upregulation of lipogenic proteins (FASN, SCD, ACACA, PPARγ, FABP4). Transcriptomic analysis unexpectedly revealed significant AMPK pathway activation alongside enhanced marbling development, contradicting conventional understanding of AMPK as an adipogenesis inhibitor. Functional validation using AMPK modulators in BSMCs confirmed that while AMPK inhibition (Compound C) dramatically enhanced VA-induced adipogenesis, AMPK activation (AICAR) suppressed lipogenesis, demonstrating AMPK functions as a negative feedback regulator during VA-mediated adipogenesis rather than preventing intramuscular fat accumulation.

CONCLUSIONS

Strategic late-fattening VA supplementation enhances marbling development through PPARγ-mediated transcriptional networks, with AMPK serving as a metabolic sensor and negative feedback regulator rather than an absolute inhibitor. This stage-specific intervention achieved superior marbling quality and improved fatty acid composition in Woking black cattle, suggesting potential for optimization of premium beef production. Validation across diverse genetic backgrounds and production systems will be essential for broader industry implementation.