Nutritional regulation of stearoyl-CoA desaturase in the bovine mammary gland

Increasing the proportion of unsaturated fatty acids (UFA) in milk is believed to be beneficial in terms of human health, thereby increasing the nutritional quality of milk. The proportion of UFA in milk is mainly dependent on the proportion of UFA in the diet, the degree of biohydrogenation of UFA in the rumen, and on activity of the stearoyl-CoA desaturase (SCD) enzyme in the mammary gland. This thesis focuses on SCD in the mammary gland of dairy cows, and how SCD can be influenced by nutrition. In the first study it was shown that supplementing the diet of dairy cows with soybean oil significantly decreases mammary SCD1 expression compared with rapeseed oil or linseed oil and this was partly reflected by the lower desaturase indices in milk. In contrast, mammary SCD5 expression was much lower (<103) than that of SCD1 and was not affected by dietary plant oil supplementation. To study the changes in genome-wide expression of genes in response to dietary UFA supplementation, mammary tissue samples of the same experiment were used for micro-array analysis. It was found that 972 genes were significantly affected through UFA supplementation, indicating that large transcriptional adaptations occurred in the mammary gland when diets of dairy cows were supplemented with unprotected dietary UFA. Since biopsy of the mammary gland is an invasive and costly method which presents a risk of udder infection, the use of milk somatic cells as a non-invasive, alternative source of mRNA was investigated in the second experiment. Since there was a significant relationship between SCD1 expression in milk somatic cells and mammary tissue, it can be concluded that milk somatic cells can be used as a source of mRNA to study SCD1 expression in dairy cows. To study the effects of acetate (Ac) and β-hydroxybutyrate (BHBA) as well as various long-chain fatty acids (LCFA) on mammary SCD expression, a bovine mammary epithelial cell line (MAC-T) was used in the third experiment. This study showed that Ac up-regulates expression of SCD1 and acetyl-CoA carboxylase in MAC-T cells, which indicates that Ac may increase desaturation and de novo synthesis of fatty acids in the bovine mammary gland. In addition it was shown that expression of sterol regulatory binding protein 1 (SREBP-1) and insulin-induced gene 1 protein (INSIG-1) was related to the expression of several lipogenic genes, thereby strengthening the support for the role of SREBP-1 and INSIG-1 as central regulators of lipogenesis in the bovine mammary gland. Overall, it can be concluded that saturated LCFA have little or no effect on SCD1 expression in the bovine mammary gland, whereas unsaturated LCFA inhibit mammary SCD1 expression. The regulation of SCD1 in the bovine mammary gland by LCFA appears to be, at least partly, regulated by the transcription factors SREBP-1 and INSIG-1. Based on the in vitro research it appears that short-chain fatty acids, in particular Ac, upregulate mammary SCD1 expression, although further research is needed to verify if short-chain fatty acids induce SCD1 expression in the bovine mammary gland. The recently discovered isoform SCD5 is expressed in bovine mammary tissue, although contribution to ∆9-desaturation of fatty acids appears to be quite low.