25-Hydroxycholecalciferol Enhances Male Broiler Breast Meat Yield through the mTOR Pathway.

BACKGROUND: In recent years, there has been a growing body of evidence indicating that replacing cholecalciferol (vitamin D₃) with 25-hydroxycholecalciferol [25(OH)D₃] through dietary supplementation enhances breast meat yield in broiler chickens. However, the underlying molecular mechanisms are still unknown.
OBJECTIVE: We investigated the effect of 25(OH)D₃ on male broiler growth performance (body weight, feed intake, feed conversion ratio, and breast meat yield), muscle protein synthesis, and the potential underlying molecular mechanisms.
METHODS: Male Cobb 500 broiler chickens were divided into 4 body weight-matched groups and received a control diet with normal cholecalciferol (2760 IU/kg feed) for 42 d, a diet with high concentrations of cholecalciferol (5520 IU/kg feed) for 42 d, or a diet with 25(OH)D₃ (5520 IU/kg feed) for 42 d (HyD-42). A fourth group consumed the HyD-42 for 21 d and then control feed for 21 d (HyD-21) (n = 360 birds, 12 replicates/treatment). Food and clean water were available for ad libitum consumption. At the end of the 42-d experiment, protein turnover was measured by phenylalanine flooding dose. Breast muscle tissues were collected and protein synthesis-related gene and protein expression were measured by real time polymerase chain reaction and Western blot, respectively. Functional studies were performed in vitro with the use of a quail myoblast (QM7) cell line. QM7 cells were treated with 2 doses (1 nM and 10 nM) of cholecalciferol or 25(OH)D₃ alone or in combination with 100 nM rapamycin, and cell proliferation was determined by cell proliferation assay. Protein synthesis-related gene and protein expression were also determined.
RESULTS: The HyD-42 increased 25(OH)D₃ circulating concentrations by 126% (P < 0.05), enhanced breast meat yield (P < 0.05), and increased the fractional rate of protein synthesis by 3-fold (P < 0.05) compared with the control diet. Molecular analyses revealed that breast muscle from chickens consuming the HyD-42 expressed significantly higher concentrations of vitamin D receptor (VDR), phospho mechanistic target of rapamycin(Ser2481), phospho ribosomal P70 S6 kinase (RPS6K)(Thr421/Ser424), and antigen Ki-67 (Ki67) compared with the other groups. In line with the in vivo data, in vitro functional studies showed that cells treated with 25(OH)D₃ for 24 h had increased VDR expression, and activated the mechanistic target of rapamycin (mTOR)/S6 kinase (S6K) pathway, enhanced Ki67 protein concentrations, and induced QM7 cell proliferation compared with untreated or cholecalciferol-treated cells. Blocking the mTOR pathway with rapamycin reversed these effects.
CONCLUSION: Taken together, our findings provide evidence that the effects of 25(OH)D₃ on male broiler breast muscle are likely mediated through the mTOR-S6K pathway.