Fish oil-induced milk fat depression and associated downregulation of mammary lipogenic genes in dairy ewes

Several studies in dairy cows have shown a relationship between milk fat depression (MFD) and alterations caused in lipogenic gene expression by dietary nutrients. However, information on small ruminants is not only scarce but also inconsistent. Therefore, this experiment was conducted in dairy ewes to study the effect of a diet known to induce MFD on milk fatty acid (FA) composition and mRNA abundance of key candidate genes involved in mammary lipogenesis. Twelve lactating Assaf ewes (on average 63 d in milk) were randomly assigned to 2 treatments consisting of a total mixed ration based on alfalfa hay and concentrates (50:50), supplemented with 0 (control) or 17 g of fish oil/kg of diet dry matter (FO). Profiles of milk FA and mRNA abundance of candidate genes in biopsied mammary tissue were examined before starting the treatments and after 1 and 4.5 wk on the diets. As expected, FO induced MFD and modified milk FA composition. Compared with the control, reductions in milk fat concentration and yield were not detected on d 7, but reached up to 25 and 22%, respectively, on d 30. However, increases in confirmed or putative antilipogenic FA (trans-10,cis-12 and trans-9,cis-11 18:2, cis-9 16:1, cis-11 18:1, and oxo-FA) were already established on the early stage of the treatment and lasted until the end of the feeding period. These changes were accompanied by decreases in the mRNA abundance of genes encoding lipogenic enzymes. The coordinated nature of the downregulation, which tended to affect most studied metabolic pathways, including FA activation (ACSS1), de novo synthesis (ACACA and FASN), uptake and transport (LPL and FABP3), desaturation (SCD1), and esterification (AGAPT6), supports the involvement of a central regulator of milk fat synthesis. In this regard, without ruling out the potential contribution of PPARG, our results suggest that SREBF1 would have a relevant role in the MFD syndrome in sheep fed FO. Among the other studied transcription factors, the tendency to a downregulation of INSIG1 was associated with that of SREBF1, whereas no variation was detected for SCAP or THRSP. Fish oil had no significant effects on the transcript abundance of CD36, GPAM, DGAT1, LPIN1, and XDH. Overall, changes in potential antilipogenic FA and mRNA abundance of candidate lipogenic genes support a relationship between them and suggest that FO-induced MFD in dairy ewes would be mediated by transcriptional mechanisms.