Expression of translation initiation factors in the bovine mammary gland

This thesis represents the first investigation into the regulation of translation initiation in the mammary glands of dairy cows, with emphasis on the roles of physiological stage and nutrients. After 42 d of involution in the mammary glands of late-lactation cows, levels of global RNA (RNA:DNA) are maintained at 49% of those of lactating cows by sustained cell signaling to ribosomal protein S6 (rpS6) through its kinase p70S6K. In contrast, translational competency is greatly reduced through large decreases in eukaryotic translation initiation factors (eIF) 2 and 4E. The similar decrease in expression of eIF2 and eIF4E due to milk stasis seems to indicate that these initiation factors most likely share responsibility for the increased translational efficiency in lactation. As indicated by the presence of biologically inactive eIF2alpha and unavailable eIF4E, there appears to be an excess capacity for the up-regulation of translation in both late-lactation and dry cows.; In the mammary gland of the high-producing dairy cow, glucose and histidine infusion generated responses in yields of milk components for which they are not direct substrates. In a fasting-infusion model, results implicate glucose and histidine as powerful activators of the protein and lactose synthetic processes. Glucose or histidine infusion alone also generated responses in yields of milk protein. Up-regulation of milk protein synthesis by a signaling mechanism is thus implicated. Components of the translational machinery that are phosphorylated in other tissues in response to glucose and amino acid supply are also phosphorylated in the bovine mammary gland in vivo. Glucose is a powerful activator of milk protein synthesis but not ribosome biogenesis and a decreased phosphorylation of eIF2alpha may mediate the effect. The essential amino acids (EAA) plus glucose infusion increased milk protein yield and p70S6K- and rpS6 phosphorylation, suggesting that EAAs play a far greater role than does glucose in the regulation of these two proteins. The role of histidine in stimulating milk protein mRNA translation also appeared to be through eIF2alpha phosphorylation. Leucine by itself did not stimulate milk protein yield but components of the signaling cascade to 5'-terminal oligopyrimidine-containing mRNA translation were activated and translational capacity was increased.