mRNA abundance of genes involved in mammary lipogenesis during fish oil- or trans-10,cis-12 CLA-induced milk fat depression in dairy ewes.

Milk fat depression (MFD) caused by trans-10,cis-12 18:2 is known to be mediated in cows and ewes by downregulation of mammary lipogenic genes. However, transcriptional mechanisms underlying marine lipid-induced MFD have not been well defined yet and the few available studies in ovine are not consistent. This trial was conducted to directly compare changes in animal performance, milk fatty acid composition, and particularly mammary mRNA abundance of candidate lipogenic genes and transcription factors in response to the inclusion of fish oil or trans-10,cis-12 18:2 in the dairy sheep diet. To meet this objective, 12 lactating Assaf ewes (on average, 64 days in milk, producing 1.72 kg of milk/d with 5.17% of fat) were divided into 3 groups and offered a total mixed ration without supplementation (control) or supplemented with 2.4% dry matter of fish oil (FO treatment) or 1% dry matter of a commercial product rich in trans-10,cis-12 18:2 (CLA treatment) for 39 d. Measurements and samplings were conducted before starting the treatments and at the end of the trial. Milk samples were used for RNA extraction from somatic cells. Feed intake was not affected by lipid supplements, and as designed, reductions in milk fat concentration (-31%) were similar in the 2 treatments, although the unpredicted increase in milk production with FO counteracted the anticipated reduction in milk fat yield. Nevertheless, this did not preclude the detection of FO-induced decreases in the mRNA abundance of candidate lipogenic genes [e.g., acyl-CoA synthetase short-chain family member 2 (ACSS2), fatty acid synthase (FASN), and lipin 1 (LPIN1)], thus supporting the hypothesis that transcriptional regulation would be a relevant component of this type of MFD in sheep. Expected CLA-induced downregulation of some genes, such as FASN or sterol regulatory element binding transcription factor 1 (SREBF1), could not be detected in our samples, which might be related, at least in part, to high inter-individual variation and relatively advanced lactation stage (on average 102-103 d in milk on d 38 and 39). Overall, direct comparison of the effects of dietary FO and CLA on transcript abundance of candidate lipogenic genes and transcription factors suggest that there might be relevant differences in the transcriptional control mechanisms underlying the MFD induced by each kind of supplement (i.e., fish oil or CLA).