281. The role of insulin in milk protein synthesis
K. K. Menzies A , K. L. Macmillan B , K. R. Nicholas A , C. Lefevre A C and C. Ormandy DA Department of Zoology, CRC for Innovative Dairy Products, University of Melbourne, VIC, Australia
B University of Melbourne, School of Veterinary Science, Werribee, VIC, Australia
C Monash University, Victorian Bioinformatics Consortium, Clayton, VIC, Australia
D Garvan Institute of Medical Research, Darlinghurst, NSW, Australia
Reproduction, Fertility and Development 17(9) 117-117 https://doi.org/10.1071/SRB05Abs281
Submitted: 26 July 2005 Accepted: 26 July 2005 Published: 5 September 2005
Abstract
The mammary explant culture model has been frequently used to mimic lactation and to examine the endocrine control of milk protein gene expression. Studies in the mouse show the expression of the milk protein genes in explants requires insulin in the presence of prolactin and cortisol. The role of insulin in milk protein synthesis in the dairy cow is not as clear. The bovine mammary explant culture model has been utilised to show that insulin is essential for alpha-s1-casein gene expression and the synthesis of the casein proteins. In addition, mouse culture experiments were undertaken to provide an insight into the underlying molecular mechanisms of insulin action in hte mammary gland. A global analysis of the genes induced in the cultured explants was done using Affymetrix microarray and showed 132 genes, including the major milk protein genes, required the complement of insulin, cortisol and prolactin for maximal expression. Twenty-seven genes showed a 3-fold change in gene expression in response to insulin. The function of these genes can be largely categorised into maintenance of cell integrity, signal transduction, transport mechanisms, cellular metabolism and a direct effect on gene transcription in the nucleus. The requirement for insulin in milk protein synthesis is highlighted by its role in inducing the STAT5 gene, known to be a key transcription factor for the milk protein genes. Interestingly, dairy cows of high genetic merit have unusually low serum concentrations of insulin. This has occured in association with a high selection pressure for milk volume that has altered the regulation of blood glucose homeostasis. Our study indicates that this intensity of selection for high milk volume could be compromising the dairy cow’s potential for milk protein production: Has selecting for milk volume in many populations of dairy cows been achieved by lowering circulating insulin levels with consequent effects on the efficiency for milk protein yield as well as compromised reproductive performance.
