The objectives of this thesis research were to determine the molecular mechanisms by which dietary fatty acids (FA) regulate lipogenic gene expression and milk fat synthesis. Principal component and multivariate analyses were conducted to establish the relationship between milk fat and FA concentrations in lactating cows fed milk fat depressing (MFD) diets. This analysis showed that in addition to the established inhibitory effect of t10c12 CLA, t7-18:1 and t7c9-CLA isomers might be involved in MFD. Lactating mice were used to test the effects of several individual trans-18:1 isomers and t10c12-CLA on milk fat synthesis, lipogenic genes in liver and mammary tissues. Both MFD and extensive conversion of t7-18:1 to t7c9-CLA in mammary and liver tissues were shown in mice fed the t7-18:1. As expected, t10c12-CLA feeding caused MFD and reduced the expression of lipogenic transcription factor (TF) SREBP-1C. Potential roles of the TF ChREBP, PPARG, and INSIG1 were also established.

A subsequent study aimed to establish whether these mechanisms operated in lactating dairy cows. Compared with uninfused controls and a t10c12 CLA negative control, post-ruminal infusion of butterfat which contains all fatty acids in the same proportion to those found in milk fat to a mixture of fats containing only the long chain fatty acids were examined. Milk fat content, milk yield and mammary lipogenic gene expression were increased by butterfat but not by the long chain fatty acid mixture. This suggested that rates of short and medium chain fatty acid synthesis might be limiting for milk fat production. Unlike the results from the mouse study, SREPB-1C expression was not altered by FA treatments including t10c12-CLA.

The effects of individual FA effects and a PPARγ-specific agonist (Rosiglitazone) on mRNA expression via qPCR of 19 genes with roles in de novo synthesis, FA uptake and transport, desaturation, triacylglycerol synthesis, transcriptional regulation, and nuclear receptor signaling in a MACT cell culture system were examined. The FA regulated mammary lipogenic gene expression to different extents. PPAR-γ activation of de novo lipogenesis coupled with exogenous FA availability might play a role in regulating milk fat synthesis. These experiments demonstrate the role of FA in regulating mammary lipogenic pathways, highlighting the complexity and multiple transcriptional factor involvement in milk fat synthesis.