Regulation of protein synthesis in the mammary gland : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand

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Massey University
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This thesis examines the signaling pathways involved in the regulation of milk protein synthesis in the lactating mammary gland and their control. The protein synthetic machinery can be regulated during the transcription, translation and degradation stages of mRNA processing. Translation control in eukaryotes involves changes in the activity or other functional properties of the translation factors. These include proteins involved in initiation, peptide-chain elongation and termination of mRNA processing. Changes in the nutritional, physiological and hormonal status of the body are sensed by receptors that signal to a central protein, known as mammalian target of rapamycin (mTOR). The mTOR signaling pathway then activates or inhibits the activity of translation factors and kinases involved in the initiation and elongation stage of translation. A major objective of this thesis was to elucidate which genes and pathways are involved in the regulation of milk protein synthesis in the mammary gland and the mechanism(s) that regulate their action. The results presented here show that changes in milk protein production occurring during lactation in response to external stimuli are potentially regulated at the level of translation or subsequent processing rather than by transcriptional regulation (mRNA abundance). The results also show that in response to growth hormone (GH) treatment, which increased the yield of milk protein, the phosphorylation status of the ribosomal protein S6 (S6) is increased as well as the protein abundance of eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 4E (eIF4E). These results suggest an important relationship between milk protein yield and changes in the initiation and elongation stages of translation. Another major finding was the elucidation that mTOR is involved in the signaling pathways activated by GH and that this effect involves signaling through the PI-3 kinase pathway. In these experiments, increased protein synthesis was potentially achieved with the use of GH. Thus, this study suggests the mTOR signaling pathway is a key mediator of the GH effects in protein synthesis stimulation. Finally, the requirement for a functional mTOR signaling (TOS) motif in the eukaryotic initiation factor 4E binding protein (4E-BP1) was identified. This finding could help the identification of other proteins that may be controlled by mTOR and consequently are regulators of mRNA translation. In summary, this thesis unveils key signaling pathways involved in the regulation of milk protein synthesis and provides further insight into the control of the mTOR signaling pathway. These findings open new frontiers for the manipulation of milk composition.
Regulation of protein synthesis, Milk proteins, Gene expression