Role of insulin in the regulation of mammary synthesis of fat and protein

Feed restriction of lactating cows triggers acute downregulation of mammary mammalian target of rapamycin signaling and chronic reduction of mammary epithelial mass

While there is generally no consensus about how nutrients determine milk synthesis in the mammary gland, it is likely that the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) plays a role as a key integrator of nutritional and mitogenic signals that can influence a multitude of catabolic and anabolic pathways. The objectives of this study were to evaluate acute changes (<24 h) in translational signaling, in addition to chronic changes (14 d) in mammary gland structure and composition, in response to a severe feed restriction. Fourteen lactating Holstein dairy cows were assigned to either ad libitum feeding (n = 7) or a restricted feeding program (n = 7). Feed-restricted cows had feed removed after the evening milking on d 0. Mammary biopsies and blood samples were collected 16 h after feed removal, after which cows in the restricted group were fed 60% of their previously observed ad libitum intake for the remainder of the study. On d 14, animals were slaughtered and their mammary glands dissected. In response to feed removal, an acute increase in plasma nonesterified fatty acid concentration was observed, concurrent to a decrease in milk yield. In mammary tissue, we observed downregulation of the mTORC1-S6K1 signaling cascade, in addition to reductions in mRNA expression of markers of protein synthesis, endoplasmic reticulum biogenesis, and cell turnover (i.e., transcripts associated with apoptosis or cell proliferation). During the 14 d of restricted feeding, animals underwent homeorhetic adaptation to 40% lower nutrient intake, achieving a new setpoint of 14% reduced milk yield with 18% and 29% smaller mammary secretory tissue DM and CP masses, respectively. On d 14, no treatment differences were observed in markers of protein synthesis or mammary cell turnover evaluated using gene transcripts and immunohistochemical staining. These findings implicate mTORC1-S6K1 in the early phase of the adaptation of the mammary gland's capacity for milk synthesis in response to changes in nutrient supply. Additionally, changes in rates of mammary cell turnover may be transient in nature, returning to basal levels following brief alterations that have sustained effects.

Animal board invited review: The contribution of adipose stores to milk fat: implications on optimal nutritional strategies to increase milk fat synthesis in dairy cows

A wide range of nutritional and non-nutritional factors influence milk fat synthesis and explain the large variation observed in dairy herds. The capacity of the animal to synthesize milk fat will largely depend on the availability of substrates for lipid synthesis, some of which originate directly from the diet, ruminal fermentation or from adipose tissue stores. The mobilization of non-esterified fatty acids from adipose tissues is important to support the energy demands of milk synthesis and will therefore have an impact on the composition of milk lipids, especially during the early lactation period. Such mobilization is tightly controlled by insulin and catecholamines, and in turn, can be affected indirectly by factors that influence these signals, namely diet composition, lactation stage, genetics, endotoxemia, and inflammation. Environmental factors, such as heat stress, also impact adipose tissue mobilization and milk fat synthesis, mainly through endotoxemia and an immune response-related increase in concentrations of plasma insulin. Indeed, as proposed in the present review, the central role of insulin in the control of lipolysis is key to improving our understanding of how nutritional and non-nutritional factors impact milk fat synthesis. This is particularly the case during early lactation, as well as in situations where mammary lipid synthesis is more dependent on adipose-derived fatty acids.

Inclusion of Sunflower Oil, Organic Selenium, and Vitamin E on Milk Production and Composition, and Blood Parameters of Lactating Cows

Simple Summary

Feeding sunflower oil, selenium, and vitamin E to lactating dairy cows has improved the nutritional profile of milk for human consumption and positively impacted animal performance. This may be attributed to the increased healthier fat components, i.e., “good fats”, and antioxidant substances in milk. This study evaluated the effects of supplementing sunflower oil, selenium, and vitamin E on milk production and composition, and the blood parameters of lactating dairy cows. Supplementing sunflower oil to lactating dairy cows provided beneficial effects on milk fatty acid profiles, increasing healthier fatty acids concentrations, which have been reported as important anticarcinogenic, antiatherogenic, and antidiabetic nutrients in human diet. However, this strategy reduced the milk fat content. Selenium and vitamin E supplementation improved milk production and provided higher selenium and vitamin E content in blood and milk. These compounds are important antioxidants and nutrients for animal and human health.

Abstract

Aiming to improve milk quality and animal health, the effects of the inclusion of sunflower oil with added organic selenium (Se) and vitamin E in the diets of lactating cows were evaluated. Twenty-four multiparous lactating Jersey cows were randomly enrolled into four treatments: CON (control); SEL [2.5 mg organic Se kg⁻¹ dry matter (DM) + 1000 IU vitamin E daily]; SUN (sunflower oil 3% DM); and SEL + SUN (sunflower oil 3% DM + 2.5 mg organic Se kg⁻¹ DM + 1000 IU vitamin E daily). The experimental period was 12 weeks with 14 days for acclimation. Cows were milked twice a day. Dry matter intake, milk production, and composition were measured daily and analyzed in a pooled 4-week sample. On day 84, white blood cell counts, as well as serum and milk Se and vitamin E levels, were assessed. Supplementation with selenium and vitamin E alone or combined with sunflower oil increased milk production, and increased the serum and milk concentrations of those nutrients. The inclusion of sunflower oil reduced fat content and DM intake but also altered the milk fatty acid profile, mainly increasing levels of trans 11 C18:1 (vaccenic) and cis 9 trans 11 conjugated linoleic acid (CLA). Our results indicate that supplementation with sunflower oil, Se and vitamin E provides beneficial effects on animal performance and milk composition, which could be an important source of CLA and antioxidants (Se and vitamin E) for human consumption.

The Processes of Nutrition and Metabolism Affecting the Biosynthesis of Milk Components and Vitality of Cows with High- and Low-Fat Milk

In order to clarify the mechanism of the depression of milk fat formation and preserve the health of animals, the aim of the research was to study the characteristics of rumen digestion, energy metabolism, and milk composition in high-producing dairy cows with high and low levels of milk fat that are fed the same diet. Two groups of cows with normal milk fat content (3.94 ± 0.12; n = 10) and low milk fat content (2.95 ± 0.14, n = 10) contained in the same diet were identified. Gas exchange (O2 uptake and CO2 output) was studied in cows and blood samples, rumen contents (pH, NH3-N), and VFA and milk (fat, protein, and fatty acid composition) were collected and analyzed. It was determined that cows with low fat milk are more efficient at using the metabolized energy of their diets due to the tendency to have a decrease in the proportion of heat production (by 6.2 MJ; p = 0.055) and an earlier start of a positive energy balance. At the same time, the fat content in milk did not depend on the level of hormones in the blood or on the formation of acetate in the rumen. An analysis of the duration of the productive use of cows on this farm (n = 650) showed that the number of lactations was inversely correlated with the level of fat in milk (r = −0.68; p < 0.05, n = 1300). These results indicate the advantages of cows that can reduce the fat content of their milk in the first months of lactation.

A Comparative Review of the Extrinsic and Intrinsic Factors Regulating Lactose Synthesis

Milk is critical for the survival of all mammalian offspring, where its production by a mammary gland is also positively associated with its lactose concentration. A clearer understanding of the factors that regulate lactose synthesis stands to direct strategies for improving neonatal health while also highlighting opportunities to manipulate and improve milk production and composition. In this review we draw a cross-species comparison of the extra- and intramammary factors that regulate lactose synthesis, with a special focus on humans, dairy animals, and rodents. We outline the various factors known to influence lactose synthesis including diet, hormones, and substrate supply, as well as the intracellular molecular and genetic mechanisms. We also discuss the strengths and limitations of various in vivo and in vitro systems for the study of lactose synthesis, which remains an important research gap.

Effects of walnut shell and chicken manure biochar on in vitro fermentation and in vivo nutrient digestibility and performance of dairy ewes

Two experiments were conducted to investigate the effects of addition of walnut shell biochar (WSB) and chicken manure biochar (CMB) to dairy ewes' diet. In in vitro experiment, the effects of different levels of WSB and CMB (0.5, 1, and 1.5% diet dry matter (DM)) on rumen fermentation characteristics were assessed in a completely randomized design with seven treatments and three replicates. Treatments were as follows: basal diet without biochar (control), basal diet with 0.5, 1, and 1.5% WSB, and basal diet with 0.5, 1, and 1.5% CMB. Addition of 1% WSB and 1.5% CMB to the diet linearly decreased methane production and ammonia-N concentrations and increased pH compared to control (P < 0.001). Inclusion of WSB and CMB to the diet did not change volume of gas production and total volatile fatty acids (VFA) and proportion of acetate, propionate, and butyrate. In the second experiment, six milking Kermanian ewes were used in a replicated Latin square design with three treatments and three 21-day periods to evaluate the effects of 1% WSB and 1.5% CMB (based on results obtained from in vitro trial) on intake, digestibility, and milk yield and composition. Dietary inclusion of 1% WSB and 1.5% CMB resulted in more milk yield (P < 0.01), milk protein (P < 0.05), and solids not fat (SNF) (P < 0.001). Blood glucose and total protein increased (P < 0.01) in ewes fed 1% WSB and 1.5% CMB in comparison to ewes fed control diet. Apparent digestibility coefficients of DM (P < 0.01) and OM (P < 0.10) were increased with inclusion of 1% WSB and 1.5% CMB in diet. Neutral detergent fiber (NDF) digestibility was also increased in WSB-fed ewes (P < 0.01). The lack of negative effects of 1% WSB and 1.5% CMB coupled with the observed reduction in methane emission and ammonia concentration and also improvement in milk production suggested that biochars can be beneficially incorporated in dairy ewes' ration as a low-cost feed additive.

Phosphorylation of AKT serine/threonine kinase and abundance of milk protein synthesis gene networks in mammary tissue in response to supply of methionine in periparturient Holstein cows

The main objective was to evaluate the effect of increasing the supply of Met around parturition on abundance and phosphorylation of insulin- and mechanistic target of rapamycin complex 1 (mTORC1)-related signaling proteins along with mRNA abundance of milk protein and fat synthesis-related genes in postpartal mammary tissue. A basal control diet (control) or the basal diet plus ethyl-cellulose rumen-protected Met (0.9 g/kg of dry matter intake; Mepron, Evonik Nutrition & Care GmbH, Hanau-Wolfgang, Germany) were fed (n = 30 cows/diet) from d -28 to 60 relative to parturition. Mammary tissue and blood plasma were harvested from the same cows (n = 5/diet) in the control and Met groups at d 21 postpartum for mRNA, protein, and AA analysis. Increasing the supply of Met led to greater milk protein percentage and milk yield along with greater ratio of phosphorylated (p-)AKT to total AKT. The ratio of p-mTORC1 to total mTORC1 did not differ, but ratio of p-RPS6 to total ribosomal protein S6 (RPS6) was lower in response to Met supply. These responses were associated with greater mRNA abundance of the signaling proteins Janus kinase 2 (JAK2) and insulin receptor substrate 1 (IRS1). Greater Met supply also upregulated mRNA abundance of high-affinity cationic (SLC7A1) and sodium-coupled AA transporters (SLC38A1, SLC38A2); leucyl-tRNA (LARS), valyl-tRNA (VARS), and isoleucyl-tRNA synthetases (IARS); glucose transport solute carrier family 2 member 3 (SLC2A1); glucose transport solute carrier family 2 member 3 (SLC2A3); and casein α-s1 (CSN1S1). The mRNA abundance of components of the unfolded protein response, such as x-box binding protein 1 (XBP1) and activating transcription factor 6 (ATF6), were upregulated, and protein phosphatase 1, regulatory subunit 15A (PPP1R15A) was downregulated in response to greater Met supply. Overall, the data suggest that increased dry matter intake, greater phosphorylation status of AKT, upregulation of glucose and AA transporters, and transcripts of tRNases in response to enhanced Met supply might have compensated for a reduction in ribosome biogenesis due to a lower ratio of p-RPS6 to total RPS6. Together, these cellular responses constitute a mechanism whereby Met supply can regulate milk protein synthesis in early lactation.

Amino Acid Metabolism in Dairy Cows and their Regulation in Milk Synthesis

Background:

Reducing dietary Crude Protein (CP) and supplementing with certain Amino Acids (AAs) has been known as a potential solution to improve Nitrogen (N) efficiency in dairy production. Thus understanding how AAs are utilized in various sites along the gut is critical.

Objective:

AA flow from the intestine to Portal-drained Viscera (PDV) and liver then to the mammary gland was elaborated in this article. Recoveries in individual AA in PDV and liver seem to share similar AA pattern with input: output ratio in mammary gland, which subdivides essential AA (EAA) into two groups, Lysine (Lys) and Branchedchain AA (BCAA) in group 1, input: output ratio > 1; Methionine (Met), Histidine (His), Phenylalanine (Phe) etc. in group 2, input: output ratio close to 1. AAs in the mammary gland are either utilized for milk protein synthesis or retained as body tissue, or catabolized. The fractional removal of AAs and the number and activity of AA transporters together contribute to the ability of AAs going through mammary cells. Mammalian Target of Rapamycin (mTOR) pathway is closely related to milk protein synthesis and provides alternatives for AA regulation of milk protein synthesis, which connects AA with lactose synthesis via α-lactalbumin (gene: LALBA) and links with milk fat synthesis via Sterol Regulatory Element-binding Transcription Protein 1 (SREBP1) and Peroxisome Proliferatoractivated Receptor (PPAR).

Conclusion:

Overall, AA flow across various tissues reveals AA metabolism and utilization in dairy cows on one hand. While the function of AA in the biosynthesis of milk protein, fat and lactose at both transcriptional and posttranscriptional level from another angle provides the possibility for us to regulate them for higher efficiency.

Production of dairy cows fed distillers dried grains with solubles in low- and high-forage diets

The objective of the study was to investigate the effects of dietary forage and distillers dried grains with solubles (DDGS) concentration on the performance of lactating dairy cows. Twelve Holstein cows were blocked by parity and milk production and assigned to replicated 4 × 4 Latin squares with a 2 × 2 factorial arrangement of treatments. Diets were formulated to contain low forage [LF; 17% forage neutral detergent fiber (NDF)] or high forage (HF; 24.5% forage NDF) and DDGS at 0 or 18% of diet dry matter. The forage portion of the diet consisted of 80% corn silage and 20% alfalfa hay (dry matter basis). A portion of the ground corn and all of the expeller soybean meal and extruded soybeans from 0% DDGS diets were replaced with DDGS to formulate 18% DDGS diets. Overall, we found no interactions of forage × DDGS concentrations for any of the production measures. We observed no effect of diet on dry matter intake. Milk yield was greater when cows were fed LF diets compared with HF diets (43.3 vs. 41.5 kg/d). Milk fat concentration (3.03 vs. 3.38%) was lower for cows fed LF diets compared with HF diets, whereas protein concentration (3.11 vs. 2.98%) and yield (1.34 vs. 1.24 kg/d) were greater for cows fed LF diets compared with HF diets. Yields of fat, total solids, energy-corrected milk, and feed efficiency were not affected by diets. Cows partitioned equally for milk, maintenance, and body reserves. Replacing starch from ground corn and protein from soybean feeds with DDGS at either 17 or 24.5% of forage NDF concentration in the diet was cost-effective and did not affect the production performance of lactating dairy cows.

Optimising Milk Composition

During recent decades, the UK dairy industry has had to adjust to the introduction of milk quotas in 1984, the deregulation of milk markets in 1994, and accommodate changes in the demand for dairy products. The combination of these factors, in addition to Bovine Spongiform Encephalopathy and Foot and Mouth disease, and a fall in milk price has inevitably resulted in a restructuring of the industry, but also reinforced the need for all sectors of the industry to respond to the prevailing economic climate and changes in consumer preferences.

Food intake regulation in late pregnancy and early lactation

The dip in food intake, which starts in late pregnancy and continues into early lactation, has traditionally been interpreted as a depression in intake due to physical constraints. However, the rôle of physical constraints on intake has been overemphasized, particularly in early lactation. There is mounting evidence that the presence and mobilization of body reserves in early lactation play an important rôle in regulating intake at this time.

Conceptually, the dip in intake in early lactation observed when cows have access to non-limiting foods can be accounted for by assuming that the cow has a desired level of body reserves. When the cow is not compromised, the changes with time in body reserves and the dip in intake represent the normal case and provide the basis against which to assess true depressions in intake which may occur when the cow is compromised by limiting nutrition or environment.

The regulation of body reserves and intake in the periparturient cow is orchestrated through nervous and hormonal signals. Likely factors that are involved in intake regulation are reproductive hormones, neuropeptides, adrenergic signals, insulin and insulin resistance and leptin. Furthermore, oxidation of NEFA in the liver may result in feedback signals that reduce intake. The relative importance of these is discussed. A better understanding of the physiological signals involved in intake regulation and their interrelations with body weight regulation may provide important indicators of the degree of compromise that periparturient cows may experience.

Effects of supplemental calcium salts of palm oil and chromium-propionate on insulin sensitivity and productive and reproductive traits of mid- to late-lactating Holstein × Gir dairy cows consuming excessive energy

This experiment compared insulin sensitivity, milk production, and reproductive outcomes in dairy cows consuming excessive energy during mid to late lactation and receiving in a 2 × 2 factorial design (1) concentrate based on ground corn (CRN; n = 20) or including 8% (DM basis) of Ca salts of palm oil (CSPO; n = 20), and (2) supplemented (n = 20) or not (n = 20) with 2.5 g/d of Cr-propionate. During the experiment (d 0-203), 40 multiparous, nonpregnant, lactating 3/4 Holstein × 1/4 Gir cows (initial days in milk = 81 ± 2; mean ± SE) were offered corn silage for ad libitum consumption, and individually received concentrate formulated to allow diets to provide 160% of their daily net energy for lactation requirements. From d -15 to 203, milk production was recorded daily, blood samples collected weekly, and cow body weight (BW) and body condition score (BCS) recorded on d 0 and 203. For dry matter intake evaluation, cows from both treatments were randomly divided in 5 groups of 8 cows each, and allocated to 8 individual feeding stations for 3 d. Intake was evaluated 6 times/group. Glucose tolerance tests (GTT; 0.5 g of glucose/kg of BW) were performed on d -3, 100, and 200. Follicle aspiration for in vitro embryo production was performed via transvaginal ovum pick-up on d -1, 98, and 198. Mean DMI, net energy for lactation intake, as well as BW and BCS change were similar across treatments. On average, cows gained 40 kg of BW and 0.49 BCS during the experiment. Within weekly blood samples, CRN cows had lower serum concentrations of glucose, insulin, fatty acids, and insulin-to-glucose ratio compared with CSPO cows, suggesting increased insulin sensitivity in CRN cows. During the GTT, insulin-sensitivity traits were also greater in CRN versus CSPO cows. Supplemental Cr-propionate resulted in lower serum insulin concentrations and insulin-to-glucose ratio within CRN cows only, indicating that Cr-propionate improved basal insulin sensitivity in CRN but not in CSPO cows. During the GTT, however, Cr-propionate supplementation reduced hyperinsulinemia and insulin-to-glucose ratio across CSPO and CRN cows. Milk production, as well as number of viable oocytes collected and embryos produced within each aspiration, were not affected by treatments. Hence, replacing corn by Ca salts of palm oil in the concentrate did not improve insulin sensitivity in Holstein × Gir dairy cows consuming excessive energy during mid to late lactation, whereas Cr-supplementation was effective in improving basal insulin sensitivity in cows not receiving Ca salts of palm oil.

Field Trial on Glucose-Induced Insulin Response in High-Yielding Dairy Cows under Different Environmental Temperatures

This study aimed to evaluate glucose-induced insulin response in cows exposed to different temperature-humidity index. Twenty early lactating Holstein-Friesian cows were divided into 2 equal groups based on season, as summer (SU) and spring (SP). SP cows were not exposed to heat stress, while SU cows were exposed to moderate or severe heat stress. Milk production was recorded daily. Starting from day 30 of lactation, intravenous glucose tolerance test (IVGTT) was carried out three times at 30-day intervals. Blood samples were taken before (basal) and after glucose infusion, and glucose and insulin were measured at each sample point. The homeostatic model assessment (HOMA) index was calculated. Milk yield from days 30 to 40 and 64 to 90 of lactation were higher in SP cows than in SU cows. Basal glucose did not differ on days 30 and 60 of lactation, while basal insulin and HOMA were lower in SU compared to SP cows. On day 90 of lactation, SU cows had higher basal glucose, whereas basal insulin and HOMA did not differ. IVGTT results revealed that glucose tolerance was affected by heat stress such that SU cows had higher glucose clearance. Insulin responses to IVGTT did not differ on days 30 and 60 of lactation. Heat stress had a marked effect on insulin secretion on day 90 of lactation, illustrated by higher increments, peak concentrations and area under the curve for insulin in SU cows. Overall, season differences in glucose tolerance depend not only on heat stress and milk production but also on the stage of lactation.

Effect of abomasally infused casein on post-ruminal digestibility of total non-structural carbohydrates and milk yield and composition in dairy cows

A study was conducted to evaluate the effect of abomasal infusion of casein on post-ruminal digestibility of starch and on milk yield and composition. Six multiparous Israeli Holstein cows in mid lactation, fitted with ruminal and abomasal cannulas, were used in a 3 ✕ 3 Latin-square experiment. Each cow received 1600 g maize starch infused into the abomasum. Treatments were: abomasal infusion of sodium caseinate at 0, 350, or 700 g casein daily. Chromium mordant neutral-detergent fibre (NDF) was used as a digesta marker. Casein infusion was associated with increases in post-ruminal and total-tract digestibility of non-structural carbohydrates and protein. Concentrations of rumen ammonia and of plasma insulin, glucose and urea were higher in casein-infused cows. Concentrations of milk protein and lactose and milk protein yield increased with casein infusion. Results indicate that increased protein flow to the abomasum can improve the yield of milk constituents in dairy cows. Digestibility and yield responses to infusion of 350 g casein per day were similar to those at 700 g/day. There was a tendency toward reduced milk protein efficiency in casein-infused cows. It is suggested that part of the production response can be related to a direct effect of protein supply and the rest can be explained by the indirect effect of improved carbohydrate availability in the small intestine.

Transcriptome adaptation of the bovine mammary gland to diets rich in unsaturated fatty acids shows greater impact of linseed oil over safflower oil on gene expression and metabolic pathways

BACKGROUND

Nutritional strategies can decrease saturated fatty acids (SFAs) and increase health beneficial fatty acids (FAs) in bovine milk. The pathways/genes involved in these processes are not properly defined. Next-generation RNA-sequencing was used to investigate the bovine mammary gland transcriptome following supplemental feeding with 5% linseed oil (LSO) or 5% safflower oil (SFO). Holstein cows in mid-lactation were fed a control diet for 28 days (control period) followed by supplementation with 5% LSO (12 cows) or 5% SFO (12 cows) for 28 days (treatment period). Milk and mammary gland biopsies were sampled on days-14 (control period), +7 and +28 (treatment period). Milk was used to measure fat(FP)/protein(PP) percentages and individual FAs while RNA was subjected to sequencing.

RESULTS

Milk FP was decreased by 30.38% (LSO) or 32.42% (SFO) while PP was unaffected (LSO) or increased (SFO). Several beneficial FAs were increased by LSO (C18:1n11t, CLA:10t12c, CLA:9c11t, C20:3n3, C20:5n3, C22:5n3) and SFO (C18:1n11t, CLA:10t12c, C20:1c11, C20:2, C20:3n3) while several SFAs (C4:0, C6:0, C8:0, C14:0, C16:0, C17:0, C24:0) were decreased by both treatments (P < 0.05). 1006 (460 up- and 546 down-regulated) and 199 (127 up- and 72 down-regulated) genes were significantly differentially regulated (DE) by LSO and SFO, respectively. Top regulated genes (≥ 2 fold change) by both treatments (FBP2, UCP2, TIEG2, ANGPTL4, ALDH1L2) are potential candidate genes for milk fat traits. Involvement of SCP2, PDK4, NQO1, F2RL1, DBI, CPT1A, CNTFR, CALB1, ACADVL, SPTLC3, PIK3CG, PIGZ, ADORA2B, TRIB3, HPGD, IGFBP2 and TXN in FA/lipid metabolism in dairy cows is being reported for the first time. Functional analysis indicated similar and different top enriched functions for DE genes. DE genes were predicted to significantly decrease synthesis of FA/lipid by both treatments and FA metabolism by LSO. Top canonical pathways associated with DE genes of both treatments might be involved in lipid/cholesterol metabolism.

CONCLUSION

This study shows that rich α-linolenic acid LSO has a greater impact on mammary gland transcriptome by affecting more genes, pathways and processes as compared to SFO, rich in linoleic acid. Our study suggest that decrease in milk SFAs was due to down-regulation of genes in the FA/lipid synthesis and lipid metabolism pathways while increase in PUFAs was due to increased availability of ruminal biohydrogenation metabolites that were up taken and incorporated into milk or used as substrate for the synthesis of PUFAs.

The insulin receptor plays an important role in secretory differentiation in the mammary gland

Insulin is known to be an important regulator of milk secretion in the lactating mammary gland. Here we examine the role of insulin signaling in mammary development in pregnancy using a mouse with a floxed insulin receptor (IR) crossed with a mouse expressing Cre specifically in the mammary gland. In the mammary glands of these IR(fl/fl) Cre(+) mice, expression of IR is significantly diminished throughout development. Glands from these mice had 50% fewer alveoli at midpregnancy; casein and lipid droplets were diminished by 60 and 75%, respectively, indicating a role for IR both in alveolar development and differentiation. In an acinar preparation from mammary epithelial cells (MEC) isolated from pregnant mice, insulin stimulated lumen formation, mammary cell size, acinar size, acinar casein content, and the formation of lipid droplets with a Km of ∼1.7 nM. IGF-I and IGF-II had no effect at concentrations below 50 nM, and a function blocking antibody to the IGF type 1 receptor did not alter the response to insulin. We conclude that insulin interacting with IR is essential for mammary differentiation during murine pregnancy. Using array analysis, we then examined the expression of genes up- or downregulated >1.5-fold in the IR(fl/fl) Cre(+) MECs, finding significant downregulation of differentiation specific genes and upregulation of cell cycle and extracellular matrix genes. We conclude that insulin fosters differentiation and may inhibit cell proliferation in the mammary gland of the midpregnant mouse.

Insulin regulation of amino-acid metabolism in the mammary gland of sheep in early lactation and fed fresh forage

Insulin plays an important role in regulating the partitioning of nutrients to the mammary gland, particularly in lactating ruminants fed concentrate-based diets. There is evidence that the nutritional status of the animals might also affect their response to insulin. This is largely untested in early lactating ruminants fed fresh forage. To investigate nutritional effects on insulin response, 12 lactating sheep, housed indoors, were allocated to one of two treatment groups (hyperinsulinaemic euglycaemic clamp (HEC) or control) in a randomised block design and fed perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) pasture. Mammary amino acid (AA) net uptake from plasma and utilisation for milk protein synthesis was measured during the 4th day of the HEC using arterio-venous concentration differences, and 1-13C-leucine was used to estimate whole body and mammary gland leucine kinetics. There was no change in feed intake, milk protein output and mammary blood flow during the HEC (P > 0.1). The HEC decreased (P < 0.1) the arterial concentrations of all essential AA (EAA) except histidine. The mammary net uptake of some EAA (isoleucine, leucine, methionine and phenylalanine) was reduced by the HEC (P < 0.1). Leucine oxidation in the mammary gland was not altered during the HEC (P > 0.1) but mammary protein synthesis was reduced by the HEC (P < 0.05). These results show that sheep mammary gland can adapt to changing AA precursor supply to maintain milk protein production during early lactation, when fed fresh forage. How this occurs remains unclear, and this area deserves further study.

Selected imprinting of INS in the marsupial

BACKGROUND

In marsupials, growth and development of the young occur postnatally, regulated by milk that changes in composition throughout the long lactation. To initiate lactation in mammals, there is an absolute requirement for insulin (INS), a gene known to be imprinted in the placenta. We therefore examined whether INS is imprinted in the mammary gland of the marsupial tammar wallaby (Macropus eugenii) and compared its expression with that of insulin-like growth factor 2 (IGF2).

RESULTS

INS was expressed in the mammary gland and significantly increased, while IGF2 decreased, during established milk production. Insulin and IGF2 were both detected in the mammary gland macrophage cells during early lactation and in the alveolar cells later in lactation. Surprisingly, INS, which was thought only to be imprinted in the therian yolk sac, was imprinted and paternally expressed in the liver of the developing young, monoallelically expressed in the tammar mammary gland and biallelic in the stomach and intestine. The INS transcription start site used in the liver and mammary gland was differentially methylated.

CONCLUSIONS

This is the first study to identify tissue-specific INS imprinting outside the yolk sac. These data suggest that there may be an advantage of selective monoallelic expression in the mammary gland and that this may influence the growth of the postnatal young. These results are not consistent with the parental conflict hypothesis, but instead provide support for the maternal-infant co-adaptation hypothesis. Thus, imprinting in the mammary gland maybe as critical for postnatal growth and development in mammals as genomic imprinting in the placenta is prenatally.

Effects of jugular-infused lysine, methionine, and branched-chain amino acids on milk protein synthesis in high-producing dairy cows

In addition to lysine and methionine, current ration-balancing programs suggest that branched-chain amino acid (BCAA) supply may also be limiting in dairy cows. The objective of this study was to investigate whether BCAA, leucine, isoleucine, and valine become limiting for milk protein synthesis when methionine and lysine supply were not limiting. Nine multiparous Holstein cows with an average milk production of 53.5±7.1 kg/d were randomly assigned to 7-d continuous jugular infusions of saline (CTL), methionine and lysine (ML; 12 g and 21 g/d, respectively), or ML plus leucine, isoleucine, and valine (ML+BCAA; 35 g, 15 g, and 15 g/d, respectively) in a 3×3 Latin square design with 3 infusion periods separated by 7-d noninfusion periods. The basal diet consisted of 40% corn silage, 14% alfalfa hay, and a concentrate mix, and respectively supplied lysine, methionine, isoleucine, leucine, and valine as 6.1, 1.8, 4.7, 8.9, and 5.3% of metabolizable protein. Dry matter intake (23.9 kg/d), milk yield (52.8 kg/d), fat content (2.55%), fat yield (1.33 kg/d), lactose content (4.77%), lactose yield (2.51 kg/d), and milk protein efficiency (0.38) were similar across treatments. Protein yield and protein content were not significantly different between ML (1.52 kg/d and 2.88%, respectively) and ML+BCAA (1.51 kg/d and 2.83%, respectively), but they were significantly greater than that of CTL (1.39 kg/d and 2.71%). Cows that received ML+BCAA had less milk urea nitrogen content (10.9 mg/dL) compared with milk of CTL cows (12.4 mg/dL) and ML cows (11.8 mg/dL). Whereas high-producing cows responded positively to methionine and lysine supplementation, no apparent benefits of BCAA supplementation in milk protein synthesis were found. Infusion of BCAA may have stimulated synthesis of other body proteins, probably muscle proteins, as evidenced by decreased milk urea nitrogen.

Regulation of protein synthesis in mammary glands of lactating dairy cows by starch and amino acids

The objective of this study was to evaluate local molecular adaptations proposed to regulate protein synthesis in the mammary glands. It was hypothesized that AA and energy-yielding substrates independently regulate AA metabolism and protein synthesis in mammary glands by a combination of systemic and local mechanisms. Six primiparous mid-lactation Holstein cows with ruminal cannulas were randomly assigned to 4 treatment sequences in a replicated incomplete 4 x 4 Latin square design experiment. Treatments were abomasal infusions of casein and starch in a 2 x 2 factorial arrangement. All animals received the same basal diet (17.6% crude protein and 6.61 MJ of net energy for lactation/kg of DM) throughout the study. Cows were restricted to 70% of ad libitum intake and abomasally infused for 36 h with water, casein (0.86 kg/d), starch (2 kg/d), or a combination (2 kg/d starch+0.86 kg/d casein) using peristaltic pumps. Milk yields and composition were assessed throughout the study. Arterial and venous plasma samples were collected every 20 min during the last 8h of infusion to assess mammary uptake. Mammary biopsy samples were collected at the end of each infusion and assessed for the phosphorylation state of selected intracellular signaling molecules that regulate protein synthesis. Animals infused with casein had increased arterial concentrations of AA, increased mammary extraction of AA from plasma, either no change or a trend for reduced mammary AA clearance rates, and no change in milk protein yield. Animals infused with starch had increased milk and milk protein yields, increased mammary plasma flow, reduced arterial concentrations of AA, and increased mammary clearance rates and net uptake of some AA. Infusions of starch increased plasma concentrations of glucose, insulin, and insulin-like growth factor-I. Starch infusions increased phosphorylation of ribosomal protein S6 and endothelial nitric oxide synthase, consistent with changes in milk protein yields and plasma flow, respectively. Phosphorylation of the mammalian target of rapamycin was increased in response to starch only when casein was also infused. Thus, cell signaling molecules involved in the regulation of protein synthesis differentially responded to these nutritional stimuli. The hypothesized independent effects of casein and starch on animal metabolism and cell signaling were not observed, presumably because of the lack of a milk protein response to infused casein.

Nutritional stimulation of milk protein yield of cows is associated with changes in phosphorylation of mammary eukaryotic initiation factor 2 and ribosomal s6 kinase 1

Production of protein by the lactating mammary gland is stimulated by intake of dietary energy and protein. Mass-action effects of essential amino acids (EAA) cannot explain all of the nutritional response. Protein synthesis in tissues of growing animals is regulated by nutrients through the mammalian target of rapamycin (mTOR) and integrated stress response (ISR) networks. To explore if nutrients signal through the mTOR and ISR networks in the mammary gland in vivo, lactating cows were feed-deprived for 22 h and then infused i.v. for 9 h with EAA+ glucose (Glc), Glc only, l-Met+l-Lys, l-His, or l-Leu. Milk protein yield was increased 33 and 27% by EAA+Glc and Glc infusions, respectively. Infusions of Met+Lys and His generated 35 and 41%, respectively, of the EAA+Glc response. Infusion of EAA+Glc reduced phosphorylation of the ISR target, eukaryotic initiation factor(eIF) 2, in mammary tissue and increased phosphorylation of the mTOR targets, ribosomal S6 kinase 1 (S6K1) and S6. Both responses are stimulatory to protein synthesis. Glucose did not significantly increase mammary S6K1 phosphorylation but reduced eIF2 phosphorylation by 62%, which implicates the ISR network in the stimulation of milk protein yield. In contrast, the EAA infusions increased (P < 0.05) or tended to increase (P < 0.1) mammary mTOR activity and only His, like Glc, decreased eIF2 phosphorylation by 62%. Despite activation of these protein synthesis signals to between 83 and 127% of the EAA+Glc response, EAA infusions produced less than one-half of the milk protein yield response generated by EAA+Glc, indicating that ISR and mTOR networks exert only a portion of the control over protein yield.

Insulin regulates milk protein synthesis at multiple levels in the bovine mammary gland

The role of insulin in milk protein synthesis is unresolved in the bovine mammary gland. This study examined the potential role of insulin in the presence of two lactogenic hormones, hydrocortisone and prolactin, in milk protein synthesis. Insulin was shown to stimulate milk protein gene expression, casein synthesis and (14)C-lysine uptake in mammary explants from late pregnant cows. A global assessment of changes in gene expression in mammary explants in response to insulin was undertaken using Affymetrix microarray. The resulting data provided insight into the molecular mechanisms stimulated by insulin and showed that the hormone stimulated the expression of 28 genes directly involved in protein synthesis. These genes included the milk protein transcription factor, ELF5, translation factors, the folate metabolism genes, FOLR1 and MTHFR, as well as several genes encoding enzymes involved in catabolism of essential amino acids and biosynthesis of non-essential amino acids. These data show that insulin is not only essential for milk protein gene expression, but stimulates milk protein synthesis at multiple levels within bovine mammary epithelial cells.

Effect of peripartum dietary energy supplementation of dairy cows on metabolites, liver function and reproductive variables

Multiparous Holstein cows (n=58) were used to study the effects of peripartum dietary supplementation on metabolic status, liver function and reproduction variables. Diets for cows were as follows: (a) no supplementation (CTL), (b) prilled fatty acids as 1.9% of DM (PrFA), (c) calcium salts of long chain n-6 fatty acids as 2.24% of DM (CaLFA) or (d) daily topdressing with 769 g of 65% propylene glycol (PGLY). Supplements were fed during the last 21 days before expected calving except for PGLY that continued until 21 days after parturition. Ovarian activity was monitored by transrectal ultrasonography and days to first ovulation were recorded. Liver biopsies were obtained on day 8 and 21 postpartum and analyzed for triglyceride content and mRNA expression of pyruvate carboxylase, cytosolic phosphoenolpyruvate carboxykinase, carnitine palmytoyltransferase 1A, and peroxisome proliferator-activated receptor-alpha. At 71 days following parturition, stage of ovarian cycles was synchronized and at day15 of the cycle oxytocin was injected i.v., blood samples were obtained at frequent intervals, and analyzed for 13,14 dihydro, 15-keto PGF(2alpha) (PGFM). Milk production and milk components were not different among treatment groups. Cows in PGLY gained body condition score (BCS) prepartum and net energy balance prepartum tended to be greater, but was not different postpartum from other groups. PGLY supplementation increased plasma insulin concentration prepartum, but not during the postpartum period. No significant differences were observed in plasma concentrations of glucose, NEFA, and insulin-like growth factor or hepatic triglyceride content, but all supplements tended to decrease beta hydroxybutyrate postpartum compared to CTL cows. Abundance of mRNA of gluconeogenic and lipid oxidation genes was not different among treatment groups. Days to first ovulation and uterine PGF(2alpha) production in response to an oxytocin treatment were not significantly different among treatment groups. Peripartum supplementation did not result in the substantial improvement of metabolic profile in early lactation nor significantly affect days to first ovulation and PGFM response to an oxytocin treatment.

Introduction: secretory activation: from the past to the future

This issue of the Journal of Mammary Gland Biology brings a synthesis of the historical data leading to our understanding of the physiology of lactation up to about 1980 with the new technologies and understanding resulting from the molecular revolution in the late 20th century. We focus specifically on the activation of secretion at parturition, and show that the field is ripe for new investigation.

A Redefinition of the Representation of Mammary Cells and Enzyme Activities in a Lactating Dairy Cow Model

The Molly model predicts various aspects of digestion and metabolism in the cow, including nutrient partitioning between milk and body stores. It has been observed previously that the model underpredicts milk component yield responses to nutrition and consequently overpredicts body energy store responses. In Molly, mammary enzyme activity is represented as an aggregate of mammary cell numbers and activity per cell with minimal endocrine regulation. Work by others suggests that mammary cells can cycle between active and quiescent states in response to various stimuli. Simple models of milk production have demonstrated the utility of this representation when using the model to simulate variable milking and nutrient restriction. It was hypothesized that replacing the current representation of mammary cells and enzyme activity in Molly with a representation of active and quiescent cells and improving the representation of endocrine control of cell activity would improve predictions of milk component yield. The static representation of cell numbers was replaced with a representation of cell growth during gestation and early lactation periods and first-order cell death. Enzyme capacity for fat and protein synthesis was assumed to be proportional to cell numbers. Enzyme capacity for lactose synthesis was represented with the same equation form as for cell numbers. Data used for parameter estimation were collected as part of an extended lactation trial. Cows with North American or New Zealand genotypes were fed 0, 3, or 6 kg of concentrate dry matter daily during a 600-d lactation. The original model had root mean square prediction errors of 17.7, 22.3, and 19.8% for lactose, protein, and fat yield, respectively, as compared with values of 8.3, 9.4, and 11.7% for the revised model, respectively. The original model predicted body weight with an error of 19.7% vs. 5.7% for the revised model. Based on these observations, it was concluded that representing mammary synthetic capacity as a function of active cell numbers and revisions to endocrine control of cell activity was meritorious.

Effects of Dietary Protein and Starch on Intake, Milk Production, and Milk Fatty Acid Profiles of Dairy Cows Fed Corn Silage-Based Diets

Feed intake, milk production, and milk fatty acid profiles of dairy cows fed corn silage-based diets with different protein and starch concentrations were measured in a 3-period experiment in a changeover design using 12 Holstein cows. Each experimental period lasted for 3 wk. The diet fed as a total mixed ration consisted of 45% corn silage, 5% coarsely chopped wheat straw, and 50% concentrate, on a dry matter (DM) basis. The 4 treatments, formulated to be isoenergetic and to differ in concentrations of dietary crude protein (CP) and starch (DM basis), were as follows: low CP and low starch (LPLS; 14% CP and 15% starch), low CP and high starch (LPHS; 14% CP and 25% starch), high CP and low starch (HPLS; 16% CP and 15% starch), and high CP and high starch (HPHS; 16% CP and 25% starch). The LPLS treatment led to lower DM intake, milk yield, milk protein concentration, and milk lactose yield, probably due to a shortage of both rumen-degradable protein supply to rumen microbes and glucogenic nutrients to the animal. There were no differences between protein-rich diets and LPHS, suggesting that this diet satisfied the rumen-degradable protein requirements of rumen microbes and did not limit feed intake, and the increased supply of glucogenic nutrients spared AA so that the nutrient requirements of mid lactation dairy cows were met. Further increases in CP concentration increased plasma urea concentration and resulted in decreased efficiency of conversion of dietary N into milk N. Milk fatty acid profiles were affected by starch and protein supply, with starch having the largest effect. Additionally, increasing dietary starch concentration decreased the apparent transfer of dietary polyunsaturated fatty acids to milk, suggesting an increased channeling of fatty acids to adipose tissue. The results further suggest that C(15:0) and C(17:0) are synthesized de novo in animal tissues.

Response to Conjugated Linoleic Acid in Dairy Cows Differing in Energy and Protein Status

The trans-10, cis-12 conjugated linoleic acid (CLA) isomer inhibits milk fat synthesis, whereas milk yield and synthesis of other milk components generally remain unchanged in established lactation. However, in some CLA studies increases in milk yield, milk protein yield, or both have been observed in cows limited in energy, either in early lactation or when grazing pasture. Our objective was to evaluate the performance and monitor peripheral tissue responses to homeostatic signals regulating lipolysis and glucose uptake with CLA supplementation when cows were limited in metabolizable energy in combination with moderate or excess metabolizable protein supply. Holstein cows (n = 48; 112 +/- 5 d in milk; mean +/- SE) were provided ad libitum access to a diet that met energy and protein requirements for a 16-d standardization interval. Based on performance during this interval, the Cornell Net Carbohydrate and Protein System was used to design energy-limiting rations that provided 80% of metabolizable energy requirements, and these were fed throughout the treatment periods. Cows were randomly allocated to 4 treatments, in a 2-period crossover design. Treatments were 1) moderate metabolizable protein (MP) supply, 2) moderate MP supply + CLA, 3) excess MP supply, and 4) excess MP supply + CLA. Moderate and excess MP supply were at 88 and 117%, respectively, of the MP requirement established during the standardization period, as estimated by the Cornell Net Carbohydrate and Protein System. Each experimental period comprised 16 d, with crossover of CLA within each protein level. The lipid-encapsulated CLA supplement provided 12 g/d of trans-10, cis-12 CLA. Conjugated linoleic acid treatment reduced milk fat yield by 21% but increased milk yield and milk protein yield by 2.6 and 2.8%, respectively. Milk yield and content and yield of both milk protein and fat were unaltered by either protein treatment alone or in combination with CLA. Basal concentrations of glucose, insulin, and nonesterified fatty acids were unaffected by CLA supplementation. The fractional rate of glucose clearance in response to an insulin challenge and the nonesterified fatty acid response to an epinephrine challenge were also not altered by either CLA treatment or MP supply. Overall, the results demonstrate that CLA supplementation when cows are energy-limited allows for repartitioning of nutrients, resulting in increased yields of milk and milk protein, and this can occur without changes in whole-body glucose homeostasis and adipose tissue response to lipolytic stimuli.