The endocrine regulation of milk lipid synthesis and secretion in tammar wallaby (Macropus eugenii)

Lipids in tammar milk are predominantly triacylglycerols, and the fatty acid composition varies during the lactation cycle. Little is known about the regulation of their synthesis. This study investigates the endocrine regulation of lipid synthesis in mammary explants from pregnant tammars. Treatment of mammary explants with insulin resulted in a high level of lipid synthesis, but the lipids accumulated in the cytosol. Culture with prolactin resulted in a small increase in lipid synthesis, but electron microscopy showed lipid globules were synthesized in the mammary epithelial cells and secreted into the lumen. Culture with both insulin and prolactin demonstrated elevated levels of synthesis and secretion of lipid. Analysis of the type of fatty acids synthesized in these mammary explants showed that the initiation of synthesis of C(16:0), which also occurs in the first week of lactation, could be reproduced in the pregnant explants cultured with prolactin alone. However, treatment of mammary explants with hydrocortisone did not show a significant effect on lipid synthesis, secretion or the fatty acid synthesized. These results provide new information identifying the role of insulin and prolactin in regulating milk lipid synthesis and secretion in the tammar.     

Interactions of insulin, corticosterone and prolactin in promoting milk-fat synthesis by mammary explants from pregnant rabbits

1. The rate of fatty acid synthesis by mammary explants from rabbits pregnant for 16 days or from rabbits pseudopregnant for 11 days was stimulated up to 15-fold by culturing for 2-4 days with prolactin. This treatment initiated the predominant synthesis of C(8:0) and C(10:0) fatty acids, which are characteristic of rabbit milk. 2. Inclusion of insulin in the culture medium increased the rate of synthesis of these medium-chain fatty acids. By contrast the inclusion of corticosterone led to the predominant synthesis of long-chain fatty acids. When explants were cultured for 2-4 days with insulin, corticosterone and prolactin, the rate of fatty acid synthesis increased up to 42-fold, but both medium- and long-chain fatty acids were synthesized. 3. These results show that the stimulus to mammary-gland lipogenesis and the initiation of synthesis of medium-chain fatty acids observed between days 16 and 23 of pregnancy in the rabbit can be simulated in vitro by prolactin alone. 4. When mammary explants from rabbits pregnant for 23 days were cultured for 2 days with insulin, corticosterone and prolactin, the rate of fatty acid synthesis increased fivefold, but there was a preferential synthesis of long-chain fatty acids. Culture with prolactin alone had little effect on the rate or pattern of fatty acids synthesized. 5. The results are compared with findings in vivo on the control of lipogenesis in the rabbit mammary gland, and are contrasted with the known effects of hormones in vitro on the mammary gland of the mid-pregnant mouse.     

Comparative analysis of caveolins in mouse and tammar wallaby: Role in regulating mammary gland function

Recent studies using the mouse showed an inverse correlation between the Caveolin 1 gene expression and lactation, and this was regulated by prolactin. However, current study using mammary explants from pregnant mice showed that while insulin (I), cortisol (F) and prolactin (P) resulted in maximum induction of the β-casein gene, FP and IFP resulted in the downregulation of Caveolin 1. Additionally, IF, FP and IFP resulted in the downregulation of Caveolin 2. Immunohistochemistry confirmed localisation of Caveolin 1 specific to myoepithelial cells and adipocytes. Comparative studies with the tammar wallaby showed Caveolin 1 and 2 had 70–80% homology with the mouse proteins. However, in contrast to the mouse, Caveolin 1 and 2 genes showed a significantly increased level of expression in the mammary gland during lactation. The regulation of tammar Caveolin 1 and 2 gene expression was examined in mammary explants from pregnant tammars, and no significant difference was observed either in the absence or in the presence of IFP.     

The expression of beta-1,3 galactosyltransferase and beta-1,4 galactosyltransferase enzymatic activities in the mammary gland of the tammar wallaby (Macropus eugenii) during early lactation

The regulation of beta-1,3 galactosyltransferase (3betaGalT) and beta-1,4 galactosyltransferase enzymatic (4betaGalT) activities in the mammary gland of the tammar wallaby (Macropus eugenii) have been characterised. These two beta-galactosyltransferases are active at different times during the lactation cycle and play a central role in regulating the carbohydrate composition in tammar milk, which changes progressively throughout lactation to assist the physiological development of the altrical young. The 4betaGalT activity was present at parturition and increased 3-fold by day 10 of lactation (d10L), whereas 3betaGalT activity was barely detectable at day d5L and then increased 6-fold by d10L. This increase in activity of both enzymes was sucking dependent. While 3betaGalT activity was not observed in the mammary gland prior to d7L, this activity was found in mammary explants from late pregnant tammar cultured with insulin, hydrocortisone and prolactin (IFP) and was further stimulated by the addition of tri-iodothyronine (T) and 17beta-oestradiol (E). The activity of 4betaGalT in these explants was stimulated maximally with IFP. These data suggest the temporal activity of both 3betaGalT and 4betaGalT is most likely regulated by both endocrine stimuli and factors intrinsic to the mammary gland.     

The effects of hormones on milk-fat synthesis in mammary explants from pseudopregnant rabbits

1. When freshly prepared explants from pseudopregnant-rabbit mammary gland were incubated with sodium [1-¹⁴C]acetate plus glucose, they synthesized triglyceride and phospholipid containing long-chain fatty acids. Explants cultured with insulin and corticosterone also synthesized these products. The addition of prolactin to this culture medium increased the rate of fatty acid synthesis up to 40-fold and the explants synthesized predominantly triglyceride enriched with C_8:0 and C_10:0 fatty acids characteristic of rabbit milk. 2. The maximum rates of fatty acid synthesis obtained by explants from pseudopregnant-rabbit mammary gland after culture with insulin, corticosterone and prolactin were similar to those observed with freshly prepared explants from lactating-rabbit mammary gland. The time in culture required to attain these maximum rates varied between animals, and did not appear to be connected with the time required (6–7 days) to synthesize the maximum proportions of C_8:0 and C_10:0 acids. 3. As the pattern of short- and medium-chain milk fatty acids is characteristic for many species, the techniques described to determine the time-course for the development of this pattern can be used to investigate hormonal response.     

The expression of β-1,3 galactosyltransferase and β-1,4 galactosyltransferase enzymatic activities in the mammary gland of the tammar wallaby (Macropus eugenii) during early lactation

The regulation of β-1,3 galactosyltransferase (3βGalT) and β-1,4 galactosyltransferase enzymatic (4βGalT) activities in the mammary gland of the tammar wallaby (Macropus eugenii) have been characterised. These two β-galactosyltransferases are active at different times during the lactation cycle and play a central role in regulating the carbohydrate composition in tammar milk, which changes progressively throughout lactation to assist the physiological development of the altrical young. The 4βGalT activity was present at parturition and increased 3-fold by day 10 of lactation (d10L), whereas 3βGalT activity was barely detectable at day d5L and then increased 6-fold by d10L. This increase in activity of both enzymes was sucking dependent. While 3βGalT activity was not observed in the mammary gland prior to d7L, this activity was found in mammary explants from late pregnant tammar cultured with insulin, hydrocortisone and prolactin (IFP) and was further stimulated by the addition of tri-iodothyronine (T) and 17β-oestradiol (E). The activity of 4βGalT in these explants was stimulated maximally with IFP. These data suggest the temporal activity of both 3βGalT and 4βGalT is most likely regulated by both endocrine stimuli and factors intrinsic to the mammary gland.     

INHIBITION OF LACTOGENIC ACTIVITIES OF BOVINE MAMMARY GLAND EXPLANTS BY THE WHEY FRACTION OF BOVINE MILK

The regulation of milk constituents, synthesis and secretion in tissue cultures of the bovine mammary gland was altered by a whey fraction of bovine milk. α-Casein gene expression, casein secretion and fatty acid synthesis were inhibited by the whey fraction in a dose-dependent manner. The whey fraction inhibited the enhancement activity of prolactin on α-casein gene expression and fatty acid synthesis, and also inhibited casein secretion to the medium, in explants cultured in a medium with or without prolactin. No effect on the expression of the β-lactoglobulin gene was found.     

The effect of progesterone on prolactin stimulation of fatty acid synthesis, glycerolipid synthesis and lipogenic-enzyme activities in mammary glands of pseudopregnant rabbits, after explant culture or intraductal injection.

The activities of lipogenic enzymes, such as acetyl-CoA carboxylase, fatty acid synthetase and glucose-6-phosphate dehydrogenase, and glycerolipid synthesis increased significantly in mammary explants of 11-day-pseudopregnant rabbits in response to prolactin, in the presence of near-physiological concentrations of insulin and corticosterone in culture. Increasing the concentration of progesterone in culture resulted in suppression of glycerolipid synthesis and activities of acetyl-CoA carboxylase and fatty acid synthetase, but not the pentose phosphate dehydrogenases. However, at near-physiological concentration of progesterone, only acetyl-CoA carboxylase activity was decreased. Injection of prolactin intraductally into 11-day-pseudopregnant rabbits stimulated glycerolipid synthesis, fatty acid synthesis and enzymes involved in fatty acid synthesis, after 3 days. Intraductal injection of progesterone separately or together with prolactin had no significant effect on basal or stimulated lipogenesis in mammary glands. Intramuscular injection of progesterone at 10 mg/day did not suppress fatty acid synthesis stimulated when prolactin was injected intraductally, but a significant inhibition was observed at a higher dose (80 mg/day).     

Lactation alters the relationship between liver lipid synthesis and hepatic fat stores in the postpartum period

In mothers who are nursing their infants, increased clearance of plasma metabolites into the mammary gland may reduce ectopic lipid in the liver. No study to date has investigated the role of lactation on liver lipid synthesis in humans, and we hypothesized that lactation would modify fatty acid and glucose handling to support liver metabolism in a manner synchronized with the demands of milk production. Lactating (n = 18) and formula-feeding women (n = 10) underwent metabolic testing at 6-week postpartum to determine whether lactation modified intrahepatic triacylglycerols (IHTGs), measured by proton magnetic resonance spectroscopy. Subjects ingested oral deuterated water to measure fractional de novo lipogenesis (DNL) in VLDL-TG during fasting and during an isotope-labeled clamp at an insulin infusion rate of 10 mU/m²/min. Compared with formula-feeding women, we found that lactating women exhibited lower plasma VLDL-TG concentrations, similar IHTG content and similar contribution of DNL to total VLDL-TG production. These findings suggest that lactation lowers plasma VLDL-TG concentrations for reasons that are unrelated to IHTG and DNL. Surprisingly, we determined that the rate of appearance of nonesterified fatty acids was not related to IHTG in either group, and the expected positive association between DNL and IHTG was only significant in formula-feeding women. Further, in lactating women only, the higher the prolactin concentration, the lower the IHTG, while greater DNL strongly associated with elevations in VLDL-TG. In conclusion, we suggest that future studies should investigate the role of lactation and prolactin in liver lipid secretion and metabolism.     

The differential actions of cortisol on the synthesis and turnover of alpha-lactalbumin and casein and on accumulation of their mRNA in mouse mammary gland in organ culture.

Cortisol was previously shown to exert different, concentration-dependent, effects on the accumulation of casein and alpha-lactalbumin in mammary glands from mid-pregnant mice cultured in the presence of insulin and prolactin [Ono & Oka (1980) Cell 19, 473-480]. The present study demonstrated that the addition of 30nM-cortisol to the medium containing insulin and prolactin resulted in a marked enhancement of the rate of synthesis of both alpha-lactalbumin and casein in cultured tissue. The addition of 3 microM-cortisol in combination with insulin and prolactin caused a marked decrease in the rate of alpha-lactalbumin synthesis, but increased casein synthesis substantially. Similar changes were also observed in the amount of translatable mRNA for alpha-lactalbumin and casein in mammary explants cultured with insulin, prolactin and the two concentrations of cortisol. The study of the turnover of the milk proteins in cultured explants showed that virtually all of the casein synthesized remained intact in tissue explants cultured with 3 microM cortisol, whereas about 45% of casein disappeared in 40h from explants cultured with 30nM-cortisol. In contrast, the two concentrations of cortisol did not differentially affect the disappearance of alpha-lactalbumin, which was about 55% in 40h. These results indicate that the concentration-dependent differential actions of cortisol on the accumulation of alpha-lactalbumin and casein are exerted through its effects on the rate of synthesis and turnover of the two proteins as well as on the accumulation of their mRNA species.     

The regulation of Na(+)-dependent anionic amino acid transport by the rat mammary gland.

The regulation of anionic amino acid transport, using radiolabelled D-aspartate as a tracer, by rat mammary tissue explants has been examined. Na(+)-dependent D-aspartate uptake by mammary tissue increased between late pregnancy and early lactation and again at peak lactation but thereafter declined during late lactation. In contrast, the Na(+)-independent component of D-aspartate uptake by mammary explants did not change significantly with the physiological state of the donor animals. Premature weaning of rats during peak lactation markedly decreased Na(+)-dependent D-aspartate uptake by mammary tissue. In addition, premature weaning also reduced the effect of reversing the trans-membrane Na(+)-gradient on the fractional loss of D-aspartate from mammary tissue explants. Unilateral weaning of rats during peak lactation revealed that milk accumulation per se reduced the Na(+)-dependent moiety of D-aspartate uptake by mammary tissue suggesting that the transport of anionic amino acids is regulated to match supply with demand. Treating lactating rats with bromocryptine reduced D-aspartate uptake by mammary tissue explants suggesting that the transport of anionic amino acids by the rat mammary gland is regulated by prolactin.     

Regulation of milk protein synthesis by progesterone in cultured mouse mammary gland

The effect of progesterone on the synthesis of milk proteins, casein and alpha-lactalbumin was investigated by culturing mammary explants from mid-pregnant mice in serum-free medium. The addition of progesterone at concentrations above 10 ng/ml inhibited both the casein and alpha-lactalbumin accumulation that were induced by the synergistic actions of insulin, prolactin and cortisol. The maximal inhibition was attained at a progesterone concentration of 100 ng/ml. The maximal level of inhibition of the alpha-lactalbumin accumulation was about 90% in the presence of insulin and prolactin or insulin, prolactin and 0.01 microgram/ml of cortisol. The inhibition of the casein accumulation by progesterone was about 80% in the presence of insulin and prolactin, and about 40% in the presence of insulin, prolactin and 1 microgram/ml of cortisol, indicating that cortisol partially antagonized the action of progesterone on the casein synthesis. When the inhibitory effect of progesterone on the accumulation of both alpha-lactalbumin and casein was examined in cultured mammary tissues from virgin, early pregnant, mid-pregnant and late pregnant mice, the degree of inhibition was markedly reduced in tissue from late pregnant mice. This indicates that the susceptibility of mammary gland to the inhibitory action of progesterone varies with the developmental stage of the tissue.     

Changes in milk protein composition during acute involution at different phases of tammar wallaby (Macropus eugenii) lactation

This study exploited the unusual lactation cycle of the tammar wallaby (Macropus eugenii) to characterise milk composition during acute involution, a time when the mammary gland is subjected to increased risk of infection. In early-lactation, tammar milk contains elevated levels of complex oligosaccharides and low protein and lipid content. Later in lactation, protein and lipid concentrations increase significantly, whereas carbohydrate content is reduced dramatically and changes to monosaccharides. Following initiation of involution at early-lactation, the carbohydrate concentration greatly decreased, while lipid and protein concentrations were elevated, suggesting that complex oligosaccharides are the major osmole in milk at this time. In contrast, involution at late lactation, when carbohydrate concentration was very low, led to an increase in the lipid concentration, but the concentration of protein was not significantly altered. This indicates that protein synthesis during acute involution at late lactation in the tammar may be down-regulated much more rapidly than during early-lactation. Analysis of milk at day 3 after the onset of involution at early-lactation identified a number of potential antimicrobials secreted at high concentrations, including lysozyme, dermcidin, polymeric immunoglobulin receptor and fragments of beta-lactoglobulin. These proteins may protect the mammary gland by minimising the risk of potential infection during involution.     

SYNTHESIS OF LACTOFERRIN AND CASEIN BY EXPLANTS OF BOVINE MAMMARY TISSUE

Synthesis of lactoferrin and casein by the bovine mammary gland was determined in an experimental model where lactation was maintained in one mammary half, while involution was induced in the contralateral half. Culture of explants with prolactin had no consistent effect on synthesis of casein or lactoferrin in tissue from either mammary half. Endotoxin and tumor necrosis factor-α generally decreased synthesis of casein and lactoferrin, suggesting that these inflammatory mediators are not directly responsible for increasing lactoferrin synthesis during mammary inflammation or involution. Synthesis of lactoferrin was increased and casein decreased in the involuting mammary half vs. the lactating half. These results suggest that local factors in the mammary gland play a role in the regulation of lactoferrin synthesis during involution.     

The effect of lactogenic hormones on protein synthesis and amino acid uptake in rat mammary acinar cell culture at various physiological stages

• 1.1. The activities of protein synthesis and amino acid uptake at various physiological stages were determined by the incorporation of radioactive materials ([³H]-lysine, [¹⁴C]-cycloleucine) in rat mammary epithelial cell cultures. The activity of protein synthesis and amino acid uptake was higher in early lactation than in virgin, pregnant and late lactation stages.
• 2.2. Lactogenic hormones (prolactin, hydrocortisone and insulin) treatment related with mammary growth and differentiation increased the activities of protein synthesis and amino acid uptake. But increase of these activities was different at each physiological stage.
• 3.3. The effect of prolactin and hydrocortisone on the activities were greater in virgin, pregnant and late lactation than in early lactation. And effect of insulin was greater in pregnant and early lactation than in virgin and weanling.