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

Parental factors that impact the ecology of human mammary development, milk secretion, and milk composition-a report from "Breastmilk Ecology: Genesis of Infant Nutrition (BEGIN)" Working Group 1

The goal of Working Group 1 in the Breastmilk Ecology: Genesis of Infant Nutrition (BEGIN) Project was to outline factors influencing biological processes governing human milk secretion and to evaluate our current knowledge of these processes. Many factors regulate mammary gland development in utero, during puberty, in pregnancy, through secretory activation, and at weaning. These factors include breast anatomy, breast vasculature, diet, and the lactating parent's hormonal milieu including estrogen, progesterone, placental lactogen, cortisol, prolactin, and growth hormone. We examine the effects of time of day and postpartum interval on milk secretion, along with the role and mechanisms of lactating parent-infant interactions on milk secretion and bonding, with particular attention to the actions of oxytocin on the mammary gland and the pleasure systems in the brain. We then consider the potential effects of clinical conditions including infection, pre-eclampsia, preterm birth, cardiovascular health, inflammatory states, mastitis, and particularly, gestational diabetes and obesity. Although we know a great deal about the transporter systems by which zinc and calcium pass from the blood stream into milk, the interactions and cellular localization of transporters that carry substrates such as glucose, amino acids, copper, and the many other trace metals present in human milk across plasma and intracellular membranes require more research. We pose the question of how cultured mammary alveolar cells and animal models can help answer lingering questions about the mechanisms and regulation of human milk secretion. We raise questions about the role of the lactating parent and the infant microbiome and the immune system during breast development, secretion of immune molecules into milk, and protection of the breast from pathogens. Finally, we consider the effect of medications, recreational and illicit drugs, pesticides, and endocrine-disrupting chemicals on milk secretion and composition, emphasizing that this area needs much more research attention.

Transcriptomic changes underlying glucocorticoid-induced suppression of milk production by dairy cows

Milk production by dairy cows is sensitive to increased levels of stress hormones such as glucocorticoids (GC) that also regulate the transcription of several genes required for milk synthesis. Whereas previous studies identified that an exogenous GC such as dexamethasone (DEX) transiently suppresses milk yield in several species without any pronounced effect on milk protein or fat percentage, the mechanism underlying this effect has not been established. In this study we sought to establish changes within the mammary glands of non-pregnant dairy cows in their second lactation (n = 3-4; 648-838 kg) following a single dose of exogenous DEX. Changes in the udder were monitored by serial biopsy of alternating quarters, concurrent with quarter-level monitoring of milk yield and composition. Dexamethasone increased serum glucose levels from 12-36 h (p <0 .05), reduced milk yield from 12-48 h (p <0 .05), increased % milk protein content at 24 h post-DEX, and transiently decreased both milk lactose and α-lactalbumin content, while not altering the level of milk fat. After 72 h, all aspects of milk production had returned to pre-treatment levels. Transcriptomic changes in the mammary glands in response to DEX were identified by RNA sequencing followed by differential gene expression analysis. Coincident with the milk yield and composition changes was the differential expression of 519 and 320 genes at 12 and 24 h after DEX (adjusted p <0 .05), respectively, with the return of all gene expression to baseline levels by 72 h. Among the transcriptomic changes in response to DEX, there was notable downregulation of elements in the lactose synthesis pathway, specifically AQP3, GALE and LALBA (α-lactalbumin) at 12 h, and sustained downregulation of LALBA at 24 h. One gene in the pathway, UGP2, was upregulated at 12-24 h post-DEX. This work supports the hypothesis that there is a direct relationship between the response to DEX and the concurrent suppression of milk yield due to the reduced synthesis of α-lactalbumin and lactose by the mammary epithelium. The ability of glucocorticoids to modulate the homeorrhetic requirements for glucose during stressful states concurrent with immune activation bears significance for dairy animals as well as a broad range of lactating mammals.

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.

Mitogen- and stress-activated protein kinase 1 MSK1 regulates glucocorticoid response element promoter activity in a glucocorticoid concentration-dependent manner

The glucocorticoid receptor is a nuclear receptor, and can be activated by glucocorticoid ligands. Mitogen- and stress-activated protein kinase (MSK1), when activated by p38 and ERK mitogen-activated protein kinases (MAPKs), plays a major role in chromatin relaxation via phosphorylation of histone H3 S10. The glucocorticoid receptor can target MSK1 as part of its anti-inflammatory mechanism. Here, we studied the converse mechanism, i.e. the impact of MSK1 on glucocorticoid receptor-mediated transactivation. Upstream MSK1-activating kinases concentration-dependently enhanced glucocorticoid response element (GRE)-regulated promoter activity. Correspondingly, MSK1 inhibition, via H89, or combined p38 and ERK MAPK inhibition, via SB203580 and U0126, diminished maximally stimulated GRE-regulated promoter activity using high concentrations of glucocorticoids. Concomitantly, the combination of these agents does not seem to alter site-specific phosphorylations of murine glucocorticoid receptor S212 or S220. Paradoxically, we reveal that a sub-maximally activated GRE-mediated promoter activity, by using lower concentrations of glucocorticoids, is consistently enhanced by H89 or a combination of SB203580 and U0126, irrespective of the GRE promoter context. Furthermore, we show that the glucocorticoid-induced nucleocytoplasmic translocation of MSK1 occurs in a glucocorticoid concentration-dependent manner. The observed glucocorticoid concentration-dependent effect of MSK1 or MAPK inhibition on glucocorticoid receptor transactivation warrants further research into the applicability of combined glucocorticoid and kinase inhibitor strategies for anti-inflammatory purposes.

The extracellular matrix locally regulates asynchronous concurrent lactation in tammar wallaby (Macropus eugenii)

Asynchronous concurrent lactation (ACL) is an extreme lactation strategy in macropod marsupials including the tammar wallaby, that may hold the key to understanding local control of mammary epithelial cell function. Marsupials have a short gestation and a long lactation consisting of three phases; P2A, P2B and P3, representing early, mid and late lactation respectively and characterised by profound changes in milk composition. A lactating tammar is able to concurrently produce phase 2A and 3 milk from adjacent glands in order to feed a young newborn and an older sibling at heel. Physiological effectors of ACL remain unknown and in this study the extracellular matrix (ECM) is investigated for its role in switching mammary phenotypes between phases of tammar wallaby lactation. Using the level of expression of the genes for the phase specific markers tELP, tWAP, and tLLP-B representing phases 2A, 2B and 3 respectively we show for the first time that tammar wallaby mammary epithelial cells (WallMECs) extracted from P2B acquire P3 phenotype when cultured on P3 ECM. Similarly P2A cells acquire P2B phenotype when cultured on P2B ECM. We further demonstrate that changes in phase phenotype correlate with phase-specific changes in ECM composition. This study shows that progressive changes in ECM composition in individual mammary glands provide a local regulatory mechanism for milk protein gene expression thereby enabling the mammary glands to lactate independently.

Uteroplacental insufficiency alters the mammary gland response to lactogenic hormones in vitro

Adequate mammary development and coordinated actions of lactogenic hormones are essential for the initiation of lactation. Pregnancies compromised by uteroplacental insufficiency impair mammary development and lactation, further slowing postnatal growth. It is not known whether the initiation of lactation or galactopoesis is compromised. Uteroplacental insufficiency induced in rats by bilateral uterine vessel ligation (Restricted) or sham surgery (Control) on Day 18 of gestation preceded collection of mammary tissue on Day 20 of pregnancy. Mammary explants were cultured with combinations of insulin, cortisol and prolactin and analysed for α-lactalbumin and β-casein gene expression. Mammary tissue from late pregnant Restricted rats had elevated α-lactalbumin, but not β-casein, mRNA, which is consistent with premature lactogenesis resulting from an early decline in peripheral maternal progesterone. Explants from Restricted rats were more responsive to hormone stimulation after 3 days in culture, indicating that compromised galactopoesis, not lactogenesis, most likely leads to the reduced growth of suckled pups.

The role of glucocorticoids in secretory activation and milk secretion, a historical perspective

In this review we present our current understanding of the role of glucocorticoids in secretory activation and milk secretion by looking at the literature from a historical perspective. We begin with the early endocrine ablation experiments and continue from there to show that glucocorticoids are not just necessary for secretory activation and milk secretion--but mandatory. Specifically, we discuss the importance of glucocorticoids to: (1) induce the formation of ultrastructural components necessary to support milk synthesis and secretion, including rough endoplasmic reticulum and tight junction sealing; (2) regulate milk protein gene expression; and (3) prevent the second phase of involution, possibly by preventing the breakdown of the extracellular matrix.

Lactogenic hormones regulate xanthine oxidoreductase and beta-casein levels in mammary epithelial cells by distinct mechanisms

Xanthine oxidoreductase (XOR) is a prominent component of the milk lipid globule, whose concentration is selectively increased in mammary epithelial cells during the transition from pregnancy to lactation. To understand how XOR expression is controlled in the mammary gland, we investigated its properties and regulation by lactogenic hormones in cultured HC11 mammary epithelial cells. XOR was purified as the NAD(+)-dependent dehydrogenase by benzamidine-Sepharose chromatography and was shown to be intact and to have biochemical properties similar to those of enzyme from other sources. Treating confluent HC11 cells with prolactin and cortisol produced a progressive, four- to fivefold, increase in XOR activity, while XOR activity in control cells remained constant. Elevated cellular XOR activity was correlated with increased XOR protein and was due to both increased synthesis and decreased degradation of XOR. Prolactin and cortisol increased XOR protein and mRNA in the presence of epidermal growth factor, which blocked the stimulation of beta-casein synthesis by these hormones. Further, hormonal stimulation of XOR was inhibited by genistein (a protein tyrosine kinase inhibitor) and by PD 98059 (a specific inhibitor of the MAP kinase cascade). These findings indicate that lactogenic hormones stimulate XOR and beta-casein expression via distinct pathways and suggest that a MAP kinase pathway mediates their effects on XOR. Our results provide evidence that lactogenic hormones regulate milk protein synthesis by multiple signaling pathways.

Growth control and differentiation in mammary epithelial cells

Growth and differentiation of the mammary gland are controlled by various hormones and other environmental factors. The role of hormones and growth factors in mammary development is discussed with regard to animal species, physiological stages, and the various experimental systems in vitro. In the female embryo, mammary morphogenesis is induced by the mesenchyme and is hormone independent, whereas androgens cause the partial necrosis of mammary epithelium in the male. Ductal growth during adolescence requires estrogen and prolactin or growth hormone. During pregnancy, progesterone participates in the development of the lobuloalveolar structure of the gland. After parturition, changes in the hormonal environment lead to production and secretion of milk. Proliferation and differentiation of mammary epithelium can be induced in culture systems. Insulin and epidermal growth factor (EGF) stimulate mammary cell proliferation in vitro. EGF is required for the optimal growth of the mammary gland during pregnancy. EGF also appears to play an important role in mammary tumorigenesis in certain mouse strains. Production of milk proteins can be induced in vitro by the synergistic interactions of prolactin, insulin, and glucocorticoids and is inhibited by EGF and progesterone. Complete or partial sequencing of several milk protein genes and comparative analysis have led to identification of a sequence of high homology and conservation in the 5' flanking region that is likely to be involved in the regulation of milk protein gene expression.

Steroid receptors, and the generation of closely coupled/biphasic dose-response curves

In most steroid-modulated systems, responses are closely coupled to receptor occupancy, with half-maximal responses at Kd, and near-maximal at greater than or equal to 10 Kd. Though glucocorticoids have been reported to increase, and progesterone to lower, levels of various milk proteins, clear differences in glucocorticoid dose-response profiles have been previously described. In mouse mammary gland explants in vitro, Ono and Oka showed that casein synthesis was stimulated by cortisol to plateau levels with progressively higher doses; in contrast, the dose-response curve for alpha-lactalbumin (alpha-LA) was biphasic, with stimulation at low doses and a return to baseline at higher cortisol levels. Since cortisol is both non-physiologic in the mouse, and has higher affinity for Type I (mineralocorticoid-like) than Type II (glucocorticoid) receptors, this might reflect an induction of alpha-LA synthesis via Type I receptor occupancy, and a suppression via Type II receptor occupancy. To examine this possibility we have used highly selective Type II receptor ligands, and in addition have defined the role of progestins as candidate inhibitors of alpha-LA synthesis. We have thus incubated mid-pregnant rat mammary gland explants in medium including insulin and prolactin, with increasing concentrations of highly specific Type II ligand (RU26988 or RU28362) and/or the selective progestin ORG 2058, and determined tissue and medium content of alpha-LA 48 h later. RU26988/RU28362 increased alpha-LA to a maximum of 3-6-fold basal at approximately 3 nM, well below the Kd (37 degrees C) for Type II receptors (8-28 nM); levels fell progressively with increasing concentration of ligand, returning to baseline by 30-300 nM. ORG 2058 lowered both basal and RU26988-stimulated levels, with the half-maximally effective dose being approximately 10(-10) M, again well below Kd for receptor binding. To accommodate the observed biphasic dose-response curve for highly specific glucocorticoids, and the shift-to-the-left for both classes of effector, we have proposed a model of a single "turn-on" nuclear acceptor site for glucocorticoid receptors (glucocorticoid regulatory element: GRE) and multiple, pre-emptive "turn-off" GREs, occupancy of any one of which will abrogate transcription; for progestins, we propose that all the GRE are alternate "turn-off" sites. Consistent with such a hypothesis, Qasba and Safaya have shown that the alpha-LA genome contains six core consensus sequences (TGT TCCT) which are putative GRE, and von der Ahe et al. that both progesterone receptors and glucocorticoid receptors may bind to GRE sites.

Binding and action of glucocorticoids and mineralocorticoids in rabbit mammary gland. Exclusive participation of glucocorticoid type II receptors for stimulation of casein synthesis

In order to ascertain whether the effect of corticoids upon casein synthesis in pregnant rabbit mammary gland culture is due to interactions with classical glucocorticoid or type I (mineralocorticoid) receptors we have demonstrated the existence of both types of receptors in the tissue and have studied the effects of aldosterone and the specific glucocorticoid agonist RU 28362 upon casein synthesis in tissue culture. Both compounds significantly stimulated prolactin-induced casein synthesis. On dose-response studies RU 28362 proved to be as active as dexamethasone, cortisol was active at intermediate concentrations and aldosterone was the least active. The three glucocorticoids were able to stimulate DNA synthesis in the tissue, but aldosterone had no effect. Finally, RU 486, a potent glucocorticoid antagonist, blocked the action of aldosterone and the other corticoids upon casein synthesis, whereas spironolactone, a mineralocorticoid antagonist, was unable to do so. These results demonstrate that the stimulatory effect of corticoids upon casein synthesis in pregnant rabbit mammary tissue culture is mediated through classical (type II) glucocorticoid receptors. Transferrin accumulation in the tissue was not modified by any treatment, indicating that the action of the steroids was specific for casein, and not a general stimulation of protein synthesis.

Identification of a major N-glycosylated protein of rabbit mammary gland and its appearance during development in vivo

A major N-glycosylated protein was purified from hormonally stimulated pregnant and lactating rabbit mammary tissue. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate, immunodiffusion and amino acid analysis showed the protein to be transferrin. The rate of synthesis of the protein during tissue development was studied and found to parallel the whey proteins casein and alpha-lactalbumin. The function of the protein during lactation is discussed.

The study of differentiative potential of the lactating mouse mammary gland in organ culture

The organ culture of the mammary gland of lactating mice was used to examine the response of the differentiated gland to lactogenic stimuli, insulin, cortisol, and prolactin. Time course studies showed that casein synthesis in cultured tissue decreased rapidly during the first 2 d despite the presence of the three hormones, but on the 3rd d tissue cultured with either insulin and prolactin or all three hormones regained the ability to synthesize milk proteins, casein, and alpha-lactalbumin: a greater increase occurred in the three hormone system. The delayed addition of prolactin on Day 2 to the culture system containing insulin and cortisol also stimulated casein synthesis. The addition of cytarabine, which inhibited insulin-dependent cell proliferation in cultured explants, did not block the rebound of milk protein synthesis. These results indicate that in the presence of insulin, cortisol, and prolactin mammary epithelial cells in culture first lose and then regain the ability of synthesizing milk protein without requiring the formation of new daughter cells.