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
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