AMP-activated protein kinase controls lipid and lactose synthesis in bovine mammary epithelial cells.

The synthesis of milk components in bovine mammary epithelial cells (BMEC) requires an adequate supply of energy. The AMP-activated protein kinase (AMPK) is a cellular energy gauge that controls anabolic and catabolic processes to maintain a balance between energy supply and demand. The objectives of this study were to assess the role of AMPK on de novo lipid and lactose synthesis, as well as its regulation by glucose and acetate availability in BMEC. We isolated primary BMEC from the mammary tissue of 3 lactating Holstein cows and differentiated them with lactogenic hormones for 4 d. We measured protein abundance, site-specific phosphorylation, and proteolytic processing by immunoblotting. We quantified the expression of genes involved in lipid and lactose synthesis using real-time quantitative PCR. We measured de novo lipid and lactose synthesis by incorporation of radioactive substrates. We analyzed data by ANOVA using a randomized complete block design with PROC MIXED in SAS. To assess the effect of AMPK activation on milk component synthesis, we treated BMEC with 100 µM A-769662 (A76; an allosteric activator of AMPK) or vehicle control for 16 h. Consistent with activation of AMPK, A76 increased phosphorylation of its downstream targets ACC Ser79 and TSC2 Ser1387 by 144% and 26%, respectively. Activation of AMPK decreased lipid synthesis by 19%. This effect was accompanied by increased expression of FABP3. Activation of AMPK reduced the proportion of mature SREBP-1c. In addition, AMPK activation reduced lactose synthesis by 24% and lowered the expression of SLC2A1, the gene encoding GLUT1. To assess the regulation of AMPK by energy substrate availability, we incubated BMEC in a control medium containing 4 mM D-glucose and 1 mM sodium acetate, or medium lacking glucose or acetate, for 4 h. Compared with the control medium, deprivation of glucose or acetate promoted AMPKα phosphorylation at Thr172 by 84% or 58%, respectively. Activation of AMPK was significantly increased in BMEC only when the medium was devoid of glucose for at least 4 h. We concluded that activation of AMPK inhibits de novo lipid and lactose synthesis in BMEC. Further studies are needed to assess the physiological relevance of AMPK activation for milk composition in vivo and to identify the mechanisms mediating its effects on milk component synthesis.

Effect of feeding system and grain source on lactation characteristics and milk components in dairy cattle.

The objective of this study was to examine the effect of feeding systems [component and total mixed rations (TMR)] and dietary grain sources (barley, commercial concentrate, corn grain, and high-moisture corn) on lactation characteristics and milk composition. A total of 852,242 test-day records, information on animal characteristics, feed composition, and feeding systems from 104,129 Holstein cows in 4,319 herds covering a period of 5 yr were obtained from Quebec's Dairy Herd Improvement Association (Valacta). We performed descriptive statistics and graphical representations of the data for each type of feeding system and grain source by parity (1 to 3). The milk records were binned in 15-d in milk blocks. Mixed models using a combination of forward and backward stepwise selections were developed to predict milk and milk component yields. The TMR-fed cows had greater yield of milk, fat, protein, and lactose and lower milk urea N (MUN) concentration than component-fed cows at all parities. Cows fed a TMR had higher peak milk yields and greater persistency after peak lactation compared with component-fed cows. In addition, greater yields of milk fat and protein from peak to mid-lactation were found in TMR- versus component-fed cows. In general, greater milk fat and protein yields as well as lower MUN concentration were observed in cows fed corn grain or high-moisture corn compared with barley or commercial concentrate, but parity influenced these relationships. The feeding system by day in milk blocks interaction was significant in models of milk and components yields for all parities, but only for second-lactation cows for MUN concentration. This means that effect of TMR and component feeding differs with stage of lactation. In conclusion, feeding TMR and corn-based diets are associated with greater yield of milk and milk components under commercial conditions.