Mammary transcriptome analysis of lactating dairy cows

Growth hormone (GH) has a well established galactopoietic effect on ruminant lactation, however the molecular mechanisms that mediate these effects are not fully understood. This study aimed to define the effects of GH on the production of milk protein by the bovine mammary gland and to establish the potential molecular mechanisms mediating these effects. Elevated levels of GH increased milk yield with corresponding increases in protein, fat and lactose yield, while milk composition remained unaffected by treatment. Increased yield (but not concentration) of all the individual milk proteins was not accompanied by changes in the levels of the milk protein transcripts, with the exception of S2-casein, suggesting post-transcriptional regulation of the milk protein genes is a main factor in the GH-mediated effects on bovine lactation. There were significant transcriptional changes in a wide range of pathways including cell signaling, cell death, cell growth and proliferation, cell cycle regulation and metabolism, consistent with the broad effects of GH. Differential expression of genes involved in the PI3K, ERK/MAPK, insulin receptor, JAK/STAT and IGF-1 signaling pathways were also evident in response to GH treatment supporting potential cross-talk between the GH and insulin signaling pathways in the bovine mammary gland. Transcript levels of the SHC1 gene were elevated in GH-treated cows. SHC1 activates many pathways including those associated with translational regulation and gene transcription and has been implicated in mediating cross-talk between the insulin and GH signaling pathways, providing some insights into the potential pathways mediating the effects of GH on milk protein synthesis in the bovine mammary gland. Eight non-pregnant second lactation spring-calved Jersey cows between 178 to 200 days postpartum, were sourced from a commercial herd. Cows were weighed twice weekly for one month prior to treatment, and randomly assigned to two groups of four. On day 0, a slow-release formula of commercially available GH (Lactatropin®, Elanco Animal Health, Bryanston, South Africa) was administered via a single subcutaneous injection to 4 lactating Jersey cows. A further 4 Jersey cows were given a single subcutaneous injection of saline as controls. 18 slides were used in a loop design which requires three aliquots of each test sample labeled alternately with the Cy5 and Cy3 dyes and co-hybridized on arrays with two other test samples.