Effects of methionyl-methionine on milk protein synthesis in bovine mammary gland

Abstract Text: This study was conducted to evaluate effect of methionyl-methionine (Met-Met) on milk protein synthesis in cultured lactating bovine mammary tissues. Mammary tissues were obtained from three lactating Holstein dairy cows and incubated in DMEM/F12 for 72 h. Triplicate isolated cultured mammary tissues were treated with conditional medium containing certain essential amino acid (AA) for 3 h before treatment exposure. The concentration of Met-Met was adjusted through replacing the medium Met with Met-Met at ratios of 0, 5, 10, 15, 20, and 25%, respectively. After incubation with experimental medium, tissues were collected for determination of α_s1 casein expression by RT-qPCR and western blot. The AA absorption was determined by the change of free AA concentration in the medium. The mRNA abundance of AA transporters, mTOR and JAK2-STAT5 signaling pathway were measured for the control and optimal ratio of Met-Met treatment. Replacement of Met-Met for single Met significantly increased α_s1 casein expression, with the highest expression at 15% of Met-Met substitution. Compared with the control, mRNA abundance of AA transporters including ATB (0, +) and CAT1 was increased, and AA absorption were significantly improved in 15% Met-Met group (P<0.05), in which the mRNA abundance of mTOR, S6K1, 4E-BP1, JAK2 and STAT5 (P<0.05) was also increased. From these results, it is indicated that Met-Met promoted milk protein synthesis and AA absorption in cultured bovine mammary tissues, and mTOR and JAK2-STAT5 signaling pathways may be involved in the Met-Met-stimulated milk protein synthesis.

Keywords: αs1 casein, bovine mammary gland, methionyl-methionine