Abundance and phosphorylation state of translation initiation factors in mammary glands of lactating and nonlactating dairy cows.

To test if control of mRNA translation is involved in the increase in protein synthesis by mammary glands during lactation, cellular contents and phosphorylation states of translation factors and their upstream regulators were measured in mammary parenchyma from 12 nonpregnant dairy cows. For a 42-d period, 6 cows in late lactation continued to be milked (L) and 6 at the same stage of lactation were dried off (NL). All cows were then slaughtered and mammary glands and tissue samples obtained. Alveoli and lobules tended to be larger in L cows. Mammary parenchymal mass, cell number, cell size, and RNA, DNA, and protein contents were greater in L cows. Increases (3.1- and 1.8-fold) in the abundance of active, phosphorylated ribosomal protein S6 and its kinase, S6K1, respectively, in L vs. NL parenchyma indicated an ability to sustain greater rates of synthesis of translational machinery, which was also evident in the 102% increase in parenchymal RNA:DNA between the 2 groups. Cellular abundances of the main eukaryotic translation initiation factors (eIF), eIF2 and eIF4E, were 2.6- and 3-fold greater, respectively, in L cows. That these differences were greater than the 102% greater RNA:DNA in L mammary parenchyma suggests an elevated translational efficiency in L glands. Abundance of phosphorylated rpS6 was not different between mammary parenchyma and liver, whereas eIF2alpha was 50% greater in mammary tissue. In semimembranosus muscle, abundances of phosphorylated rpS6 and eIF2alpha were 3 to 4 times lower than in mammary parenchyma. In both L and NL mammary glands, 11% of eIF2alpha was in the inhibitory, phosphorylated form and 48 to 60% of eIF4E was complexed with its binding protein, 4EBP1. It is concluded that up-regulation of initiation of mRNA translation occurs in the fully differentiated milk secretory cell and that, where crucial initiation factors are not present in a maximally active form, the initiation rate might be flexible in response to external stimuli.