Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/7/2006
Publication Date: 9/16/2006
Citation: Murrieta, C.M., Hess, B.W., Scholljegerdes, E.J., Engle, T.E., Hossner, K.L., Moss, G.E., Rule, D.C. 2006. Evaluation of milk somatic cells as a source of mRNA for study of lipogenesis in the mammary gland of lactating beef cows supplemented with dietary high-linoleate safflower seeds. Jr. Anim. Sci. 84:2399-2405. Interpretive Summary: Postpartum linoleate supplementation in lactating beef cows resulted in changes in milk fatty acid composition that would reflect a decrease in de novo fatty acid synthesis by dietary lipids. However, of the mRNA types evaluated, only lipoprotein lipase mRNA tended to be affected by diet. The correlation between mRNA types in milk somatic cells and mammary tissue indicated that milk somatic cells are a valuable tool for evaluating the transcriptional regulation of lipogenic enzymes in the mammary gland of lactating beef cows. An advantage of using milk somatic cells for RNA extraction is that repeated sampling in large-scale studies would be possible and more feasible than if mammary tissue biopsies were used. This approach would be less stressful for the cow, thereby reducing the potential to influence the outcome of the analysis.
Technical Abstract: Our objectives were 2-fold: to determine the effect of dietary linoleate on milk fat composition and on transcript abundance of acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), lipoprotein lipase (LPL), and stearoyl-CoA desaturase (SCD) mRNA in mammary tissue, and to evaluate milk somatic cell mRNA as a source of mammary tissue mRNA for these enzymes. Eighteen primiparous, crossbred beef cows (BW = 411 ± 24 kg; BCS = 5.25) were offered Foxtail millet hay at 1.68% of BW daily and either a low-fat control (n = 9) or a high-linoleate (79% 18:2n-6), cracked safflower seed supplement (n = 9). Diets were isonitrogenous and isocaloric, and the linoleate diet contained 5.4% of DMI as fat. At slaughter (37 ± 3 d postpartum), mammary tissue was sampled and immediately frozen in liquid N2 before being stored at -80°C. Milk samples were obtained from the same mammary glands and immediately centrifuged at 1,200 × g to pellet somatic cells. A ribonuclease protection assay was used to quantify the mRNA in the mammary gland and milk somatic ells. Effects of diet, tissue, or their interaction were not observed for ACC (P = 0.28, 0.89, and 0.35, respectively), FAS (P = 0.38, 0.66, and 0.20, respectively), LPL (P = 0.09, 0.15, and 0.43, respectively), or SCD (P = 0.45, 0.19, and 0.29, respectively). Dietary effects on fatty acid profile of the milk fat suggested that linoleate supplementation might have decreased de novo lipogenesis while increasing uptake of dietary fatty acids; this effect was consistent with a trend toward greater LPL mRNA for linoleate-fed cows (P = 0.09). Correlations (r values) between mammary tissue and milk somatic cell data for each mRNA for the low-fat control diet were: ACC, 0.76 (P = 0.02); FAS, 0.69 (P = 0.04); LPL, 0.68 (P = 0.04); and SCD, 0.73 (P = 0.05), and for the linoleate diet were: ACC, 0.85 (P = 0.003); FAS, 0.75 (P = 0.02); LPL, 0.90 (P = 0.001); and SCD, 0.73 (P = 0.03). We conclude that milk somatic cells obtained from lactating beef cows can be used as a source of RNA to study nutritional regulation of mammary gland lipogenesis in cows fed dietary fat supplements.