Transcriptional regulations of milk protein and lactose synthesis by the diet in dairy cows

Diet is one of the main factors affecting milk yield and lactose and protein synthesis in dairy cows. It is well known that the synthesis of milk lactose and protein results from the differentiation of mammary epithelial cells (MEC) which are able to express genes coding for alpha-lactalbumin (LALBA), several caseins and specific glucose transporters. While the effects of mammary development and galactopoietic hormones on the expression of these genes are known, the impact of nutrition has been less investigated. We and other carried out studies to investigate the expression of genes involved in lactose and protein synthesis in response to feed restriction (4 trials) or increasing metabolizable protein supply in dairy cows (2 trials). The gene expression was evaluated by qPCR in mammary tissue samples or MEC purified from milk. Feed restriction induced reductions in milk lactose and protein synthesis in all trials but its effects on gene expression differs between studies. In two studies, moderate feed restriction (70% of allowance) induced a decrease in the transcript levels of SLC2A1, one of the main transmembrane transporters of glucose in MEC. Moderate feed restriction increased milk casein and LALBA transcripts in early lactation (in 1 trial) but not on a later stage (in 4 trials). In contrast, a severe feed restriction (- 50%) in early lactation inducing a 38% milk yield loss was associated with a decreases in LALBA and K casein gene expression without modifying alphaS1 casein gene expression. Decreasing protein supply in 2 trials (from 125 to 70% or from 120 to 100% of allowance) decreased milk and lactose yields and surprisingly increased the expression of LALBA transcript. Thus, the effects of feed restriction or dietary protein levels on mammary transcripts involved in milk lactose and protein synthesis likely depends on the severity of the restriction and the stage of lactation. In some cases, manipulating the diet induced a variations in transcript expression opposite to that observed for milk lactose and protein yields suggesting that other mechanisms than modification of gene expression may modulate protein and lactose synthesis.