Loss of protein kinase C delta alters mammary gland development and apoptosis

As apoptotic pathways are commonly deregulated in breast cancer, exploring how mammary gland cell death is regulated is critical for understanding human disease. We show that primary mammary epithelial cells from protein kinase C delta (PKCδ) −/− mice have a suppressed response to apoptotic agents in vitro. In the mammary gland in vivo, apoptosis is critical for ductal morphogenesis during puberty and involution following lactation. We have explored mammary gland development in the PKCδ −/− mouse during these two critical windows. Branching morphogenesis was altered in 4- to 6-week-old PKCδ −/− mice as indicated by reduced ductal branching; however, apoptosis and proliferation in the terminal end buds was unaltered. Conversely, activation of caspase-3 during involution was delayed in PKCδ −/− mice, but involution proceeded normally. The thymus also undergoes apoptosis in response to physiological signals. A dramatic suppression of caspase-3 activation was observed in the thymus of PKCδ −/− mice treated with irradiation, but not mice treated with dexamethasone, suggesting that there are both target- and tissue-dependent differences in the execution of apoptotic pathways in vivo. These findings highlight a role for PKCδ in both apoptotic and nonapoptotic processes in the mammary gland and underscore the redundancy of apoptotic pathways in vivo.

Effect of antibiotic treatment of subclinical mastitis on human immunodeficiency virus type 1 RNA in human milk

Although subclinical mastitis is associated with increased HIV load in breast milk, it is not known whether empirical treatment with antibiotics will reduce breast milk HIV load. We examined the effect of antibiotic treatment for subclinical mastitis on HIV load in breast milk. Seventy-five HIV-infected post-partum women in Malawi with subclinical mastitis were treated with oral amoxicillin/clavulanic acid and were followed between 1 and 24 weeks later. Breastmilk HIV-1 load and sodium concentration were measured and microbiological studies were performed at presentation. At 1 week (n = 34), the proportion of women with elevated breast milk leukocyte counts decreased significantly to 41.2% (p < 0.0001) and there was a nonsignificant increase in breast milk HIV-1 RNA load (p = 0.9264) and sodium concentration (p = 0.08) in the affected breast. At 4 to 12 weeks (n = 63), breast milk HIV-1 RNA load and sodium concentration decreased significantly (p < 0.05) and 17.5% had elevated breast milk leukocyte counts. Treatment with amoxicillin/clavulanic acid was associated with a significant decrease in inflammation of the breast, but breast milk HIV load remained elevated despite a significant decrease from baseline. These findings have important implications regarding how mothers should be counselled on safety of resuming breastfeeding after resolution of subclinical mastitis.

Epithelial cells as phagocytes: apoptotic epithelial cells are engulfed by mammary alveolar epithelial cells and repress inflammatory mediator release

Clearance of apoptotic cells is critical to tissue homeostasis and resolution of inflammatory lesions. Macrophages are known to remove dying cells and release anti-inflammatory mediators in response; however, many cells traditionally thought of as poor phagocytes can mediate this function as well. In the lactating mammary gland following weaning, alveolar epithelial cell death is massive, yet the gland involutes rapidly, attaining its prepregnancy state in a matter of days. We found histologic evidence of apoptotic cell phagocytosis by viable mammary epithelial cells (MEC) in the involuting mouse mammary gland. Cultured MEC were able to engulf apoptotic cells in vitro, utilizing many of the same receptors used by macrophages, including the phosphatidylserine receptor (PSR), CD36, the vitronectin receptor alpha(v)beta3, and CD91. In addition, MEC, like macrophages, produced TGFbeta in response to stimulation of the PSR by apoptotic cells or the anti-PSR ab 217G8E9, and downregulated endotoxin-stimulated proinflammatory cytokine production. These data support the hypothesis that amateur phagocytes play a significant role in apoptotic cell clearance and its regulation of inflammation.

Epidemiology and microbiology of subclinical mastitis among HIV-infected women in Malawi

The epidemiology and microbiology of subclinical mastitis, a risk factor for perinatal HIV transmission, have not been well characterized. In all, 250 HIV-infected women were followed from two weeks to 12 months postpartum in Blantyre, Malawi, and subclinical mastitis was assessed by breast milk leukocyte counts. The point prevalence of subclinical mastitis at 2, 4, 6, 10, and 14 weeks, and 6, 9, and 12 months was 12.2%, 7.8%, 6.8%, 3.7%, 10.6%, 5.1%, 4.9%, and 1.9%, respectively (P = 0.002), and 27.2% of women had at least one episode of subclinical mastitis. There was no significant relationship between maternal plasma HIV load or parity and subclinical mastitis. Staphylococcus aureus was isolated in 30% of women with subclinical mastitis, and the proportion of women with positive cultures decreased during follow-up (P = 0.02). Subclinical mastitis is prevalent among breastfeeding mothers and further studies are needed to characterize the differences between infectious and non-infectious subclinical mastitis.

Trajectory clustering: a non-parametric method for grouping gene expression time courses, with applications to mammary development

Trajectory clustering is a novel and statistically well-founded method for clustering time series data from gene expression arrays. Trajectory clustering uses non-parametric statistics and is hence not sensitive to the particular distributions underlying gene expression data. Each cluster is clearly defined in terms of direction of change of expression for successive time points (its 'trajectory'), and therefore has easily appreciated biological meaning. Applying the method to a dataset from mouse mammary gland development, we demonstrate that it produces different clusters than Hierarchical, K-means, and Jackknife clustering methods, even when those methods are applied to differences between successive time points. Compared to all of the other methods, trajectory clustering was better able to match a manual clustering by a domain expert, and was better able to cluster groups of genes with known related functions.

Functional regulation of xanthine oxidoreductase expression and localization in the mouse mammary gland: evidence of a role in lipid secretion

Xanthine oxidoreductase (XOR), a key enzyme of purine metabolism, has been implicated in the secretion of the milk fat droplet in lactating mammary epithelial cells, possibly through structural interactions with other milk fat globule proteins including butyrophilin (Btn) and adipophilin (ADPH). To help determine the mechanism by which XOR is regulated, we examined the expression and localization of XOR in the non-secretory states of late pregnancy and induced involution compared with the state of active secretion. XOR mRNA levels started to increase at mid-pregnancy, turned sharply upwards at the onset of lactation and decreased rapidly with forced involution, indicating transcriptional control of the enzyme level by differentiation and secretory function. During pregnancy and involution the enzyme was diffusely distributed in the cytoplasm, but moved rapidly to the apical membrane of the cells when secretion was activated, where it colocalized with both Btn and ADPH, similar to the situation in the milk fat globule itself. Size-exclusion chromatography of solubilized milk fat globule membrane proteins showed that XOR formed a sulphydryl-bond-dependent complex with Btn and ADPH in the milk fat globule membrane. XOR returned to a diffuse cytoplasmic localization shortly after induced involution, while Btn remained localized to the apical membrane, suggesting that localization of XOR is not dependent on the presence of Btn in the apical membrane. Our findings indicate that the expression and membrane association of XOR in the mammary gland are tightly regulated by secretory activity, and suggest that the apical membrane association of XOR regulates the coupling of lipid droplets to the apical plasma membrane during milk lipid secretion.

Does human lactoferrin in the milk of transgenic mice deliver iron to suckling neonates?

Lactoferrin is an iron-binding glycoprotein abundantly present in human milk, and has been postulated both to increase and to decrease intestinal iron absorption. To examine this problem, the interaction of milk iron with pup hemoglobin was studied in controls and in transgenic mice overexpressing human lactoferrin in their milk (2 lines expressing 12 mg/mL and 4 mg/mL, respectively). At day 14 of gestation, pregnant mice were switched from a diet of commercial chow containing iron at 300 mg/kg to diets containing 5, 15, or 50mg iron/kg; controls continued on chow. Nontransgenic pups were cross-fostered to transgenic dams to ensure that any results found in the pups were the effect of milk components. The hemoglobin level in the blood of 10-day-old suckling neonates was measured and calculated as total hemoglobin per pup. The total hemoglobin levels were lower in the pups receiving milk high in human lactoferrin, but the difference reached significance (P < 0.02) only at the highest level of dietary iron. Our findings do not support the hypothesis that lactoferrin functions as an intestinal iron scavenger, at least at high doses.

Vesicular transport of soluble substances into mouse milk

Utilizing a novel protocol to study transport of substances into mouse milk in situ, we have shown that many "fluid-phase" markers are taken up by mammary epithelial cells and deposited in milk. Since the tight junctions are closed and impermeable even to small molecules, extra-alveolar substances (those not synthesized by the alveolar cells) must be transported into the milk by the epithelial cells themselves. The markers we have used include dextran, lucifer yellow dye, horseradish peroxidase, and albumin. Using these markers and immunostaining for endogenous proteins, we have visualized transcytotic vesicles involved in transporting these markers to milk.

Human lactoferrin in the milk of transgenic mice increases intestinal growth in ten-day-old suckling neonates

Regulatory roles and a signaling receptor have been proposed for the milk protein lactoferrin (Lf), but none has been definitively characterized. Nichols and colleagues (1987) observed that human lactoferrin (hLf) stimulated thymidine incorporation into the DNA of rat intestinal crypt cells. We tested the hypothesis that chronic Lf administration stimulates intestinal growth by studying neonatal mice suckling transgenic dams secreting about 12 mg/mL hLf in their milk. Specifically, nontransgenic litters were adjusted to eight pups each and cross-fostered to transgenic dams. Controls were pups suckling nontransgenic dams of the same strain. On day 10 postpartum pups were weighed, sacrificed, and the small intestines were weighed, measured, and stored for later determination of enzyme activities. The results indicate that intestinal growth was increased in neonates suckling transgenic dams. The weight of the small intestine was increased about 27% when the pups received milk containing hLf. Intestinal length only increased about 6.5% suggesting that Lf in milk enhanced mucosal growth. The ratio of maltase to lactase in the duodenal segment of the small intestine, an indicator of maturation, was also significantly increased in the pups suckling transgenic milks. Our results imply that chronic oral consumption of human Lf promotes the growth and maturation of the intestinal mucosa, and suggest a possible therapeutic role for the agent in premature infants as well as in patients with bowel damage.

Physiology and endocrine changes underlying human lactogenesis II

Lactogenesis stage II, the onset of copious milk secretion, takes place during the first 4 d postpartum in women and involves a carefully programmed set of changes in milk composition and volume. The evidence is summarized that progesterone withdrawal at parturition provides the trigger for lactogenesis in the presence of high plasma concentrations of prolactin and adequate plasma concentrations of cortisol. Although the process is generally robust, delayed lactogenesis does occur with stressful deliveries and in poorly controlled diabetes. Failure of early removal of colostrum from the breast is associated with high milk sodium and poor prognosis for successful lactation in many women. We speculate that this problem may result from accumulation of a substance in the mammary alveolus that inhibits lactogenesis, even in the face of appropriate hormonal changes after parturition.

Hormonal regulation of tight junction closure in the mouse mammary epithelium during the transition from pregnancy to lactation

Closure of the tight junctions of the mammary epithelium has been shown to accompany the onset of copious milk secretion or lactogenesis, stage 2, in both goats and humans. Here we use injection of [(14)C]sucrose and FITC-albumin (fluorescein isothiocyanate-albumin) into the mammary duct to follow the course of tight junction closure during lactogenesis in mice. To examine the hormonal changes responsible, we ovariectomized day 16 or 17 pregnant mice and found that closure followed ovariectomy with a mean delay of 13.6+/-1.5 (s.e.m. ) h. That progesterone withdrawal is the trigger for closure was shown by the finding that injection of progesterone within 4 h of ovariectomy delayed closure and that closure occurred after injection of the progesterone antagonist RU 486 in intact late pregnant mice. Endocrine ablation studies showed that low to moderate concentrations of corticosterone and either placental lactogen or prolactin are necessary for tight junction closure triggered by progesterone withdrawal. Thus the hormonal requirements for tight junction closure are similar to those shown by other investigators to promote lactogenesis, stage 2. Further, the tight temporal control of tight junction permeability suggests that ovariectomy of the late pregnant mouse may be a good model for molecular studies of the lactogenic switch.

A lipoprotein-containing particle is transferred from the serum across the mammary epithelium into the milk of lactating mice

To investigate the role of low-density lipoprotein (LDL) in the delivery of cholesterol to the mammary gland during pregnancy and lactation, we examined the distribution of radioactivity from (125)I-tyramine cellobiose-LDL injected into the tail vein of female mice at various stages of the reproductive cycle. Changes in the proportion of isotope taken up by the mammary gland largely reflected the increased weight of the gland in pregnancy and lactation. In addition, during lactation, radioactivity was found in the milk and was associated with a protein of the molecular weight of apoB-100. Quantitatively similar results were obtained with mice homozygous for disruption of the LDL receptor gene (LDLR null). Analysis of endogenous lipoproteins showed that the milk lipoprotein particles were denser than the corresponding serum lipoproteins and largely depleted of triglyceride and cholesterol. Using fluorescence microscopy we visualize the sorting of apoB protein from the LDL lipid phase at the basal surface of the mammary epithelial cell of both wild-type and LDLR-null mice. Our findings provide evidence that the mammary epithelium of the lactating mouse is able to take up LDL from the plasma by a non-LDLR-mediated process. An apoB-containing particle from which the cholesterol has been removed is transferred into milk.

Lactogenesis. The transition from pregnancy to lactation

The most important factors in initiation of the cascade of changes in the mammary epithelium that constitute lactogenesis stage II seem to be a prepared mammary epithelium, progesterone withdrawal, maintained plasma prolactin (in most species), and removal of milk from the breast within an undefined interval after birth. Although the molecular mechanisms by which prolactin regulates milk protein synthesis are the subject of intense and productive studies, the specific mechanisms by which progesterone and milk removal interact with the mammary epithelial cell at parturition have not been studied, perhaps because no in vitro model system exists that mimics lactogenesis stage II, or because of the complexity of the changes that must be coordinated during this process, or because of a lack of general understanding of the complex progression of changes in the function of the breast as it goes from the quiescent state of pregnancy to the active secretory state of lactation. With new technologies designed to investigate the biology of complex systems arising from the growing knowledge of the genome of human and animal species and the growing availability of animal and tissue culture models for these processes, physicians can expect a rapid increase in the molecular understanding of lactogenesis in the near future. These fundamental studies must be coupled with good prospective clinical studies if physicians are to obtain a useful, comprehensive understanding of lactogenesis in women.

Anatomy and physiology of lactation

Milk secretion is a robust process that proceeds normally in at least 85% of women postpartum. Anecdotal evidence suggests that, with assistance in the techniques of breastfeeding, at least 97% of women can successfully breastfeed their infants. The causes of lack of success in breastfeeding are not well understood because, at least in Western societies, when infants fail to thrive on the breast, formula substitution is easy. Although this article is not the place to discuss possible pathologic mechanisms, breastfeeding failure usually occurs at approximately the first week postpartum, and a much better understanding of the mechanisms by which milk secretion is initiated during this period may help researchers to understand why some women have severe problems with lactation. The general understanding of the mechanisms of milk secretion is fairly good, but the regulatory mechanisms at the cellular and molecular levels have not been given adequate attention and are ripe for future investigation. Other areas that require attention are the behavior correlates of breastfeeding and the transfer of drugs and toxins into milk. The latter may have a long-term impact on infant health and should receive increased attention.

Proteomic analysis of two functional states of the Golgi complex in mammary epithelial cells

Organellar compartments involved in secretion are expanded during the transition from late pregnancy (basal secretory state) to lactation (maximal secretory state) to accommodate for the increased secretory function required for copious milk production in mammary epithelial cells. The Golgi complex is a major organelle of the secretory pathway and functions to sort, package, distribute, and post-translationally modify newly synthesized proteins and membrane lipids. These complex functions of the Golgi are reflected in the protein complement of the organelle. Therefore, using proteomics, the protein complements of Golgi fractions isolated at two functional states (basal and maximal) were compared to identify some of the molecular changes that occur during this transition. This global analysis has revealed that only a subset of the total proteins is upregulated from steady state during the transition. Identification of these proteins by tandem mass spectrometry has revealed several classes of proteins involved in the regulation of membrane fusion and secretion. This first installment of the functional proteomic analysis of the Golgi complex begins to define the molecular basis for the transition from basal to maximal secretion.

Proteomics reveal a link between the endoplasmic reticulum and lipid secretory mechanisms in mammary epithelial cells

The synthesis and secretion of lipids by mammary epithelial cells is a highly ordered process that involves several distinct steps. Triacylglycerols are synthesized in the endoplasmic reticulum and incorporated into microlipid droplets which coalesce into cytoplasmic lipid droplets. These are vectorially transported to the apical plasma membrane where they are secreted into the milk surrounded by a membrane bilayer. The origin of this membrane as well as the mechanism by which cytoplasmic lipid droplets form and become surrounded by membrane is poorly understood. Proteomic analysis of the protein composition of milk fat globules and cytoplasmic lipid droplet has revealed that the endoplasmic reticulum is not only involved in the synthesis of the lipid but also potentially contributes to the membrane component of milk fat globules. The proteins identified suggest possible mechanisms of multiple steps during this process. Completion of the proteome of milk fat globule membranes and cytoplasmic lipid droplets will provide the necessary reporter molecules to follow and dissect the mechanisms of the sorting and ultimate secretion of cytoplasmic lipid droplets.

The effect of serum iron concentration on iron secretion into mouse milk

1. The concentration of iron in mouse milk is approximately 3 times that of the serum. Although there is clear evidence for the presence of the transferrin receptor in the rodent mammary gland, the precise mechanisms of iron transfer into milk are not known. 2. Milk iron was linearly related to the serum iron:transferrin ratio in lactating mice whose serum iron ranged from 8 to 66 microM. 3. Increasing the iron binding capacity of the milk by 340 microM by targeting the lactoferrin transgene to the mammary gland did not alter the relation between milk iron and the serum iron:transferrin ratio. 4. The steady-state distribution ratio of 125I-transferrin between plasma and milk was about 0.2, indicating that transcytosed transferrin contributed a maximum of 6% of the milk iron. 5. Fluorescently labelled transferrin incubated with the in situ gland localized mainly near the basal surface of the mammary alveolar cells. 6. These experiments provide evidence that the initial and rate-limiting step in the transfer of iron into milk is binding to a basal transferrin receptor. 7. A theoretical model of the relation between milk and serum iron suggests that the affinity of apotransferrin for the basal recycling system may be higher than observed in many other cell types.

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.

Mouse mammary gland xanthine oxidoreductase: purification, characterization, and regulation

Xanthine oxidoreductase (XOR) has been purified from lactating mouse mammary tissue and its properties and developmental expression have been characterized. XOR was purified 80-fold in two steps using benzamidine-Sepharose affinity chromatography. The purified enzyme had a specific activity of 5.7 U/mg and an activity to flavin ratio of 192. SDS-polyacrylamide gel electrophoresis showed that it was composed of a single (150 kDa) band and N-terminal sequence analysis verified that it was intact mouse XOR. Isoelectric focusing showed that purified XOR was composed of three catalytically active, electrophoretic variants with pI values of 7.55, 7.65, and 7.70. The majority of the XOR activity in both pregnant and lactating mammary glands was shown to exist as NAD+-dependent dehydrogenase (XD form), while the enzyme in freshly obtained mouse milk exits as O2-dependent oxidase (XO form). The activity and protein levels of XOR selectively increased in mammary tissue during pregnancy and lactation. The time course of these increases was biphasic and correlated with the functional maturation of the mammary gland. These results indicate that XOR may have novel, mammary gland-specific functions, in addition to its role in purine metabolism.

Physiology of lactation

The physiology of human lactation is described with secretions on mammary gland anatomy and development, the mechanisms of milk secretion and ejection, and the temporal sequence of events during the transition from pregnancy to lactation (lactogenesis). Finally, interactions between lactation and maternal metabolism are briefly described and the interaction of lactation with fertility discussed.

Breast-feeding, mastitis, and HIV transmission: nutritional implications

In many developing countries, transmission of the human immunodeficiency virus (HIV) from mother to infant occurs through breast-feeding. Mastitis, an inflammatory process in the breast, may be common in lactating women in Africa and is associated with both higher HIV load in breast milk and mother-to-child transmission of HIV. Antioxidant micronutrient deficiencies may increase the risk of mastitis. Whether prevention, early diagnosis, and prompt treatment of mastitis will help reduce mother-to-child transmission of HIV in breast-feeding women needs further study.

Adaptation of maternal lipid flux to pregnancy: research needs

Adaptations of maternal lipid metabolism during pregnancy are directed toward both the needs of the fetus for lipid substrates and maternal requirements for lipid stores serving as energy reserves for lactation. The mechanisms are poorly understood, but must be elucidated before new dietary recommendations can be made about dietary supplementation with long-chain polyunsaturated fatty acids (LC-PUFA). Problems of high priority for research are: (1) Fetal requirements for specific fatty acids, including essential fatty acids (EFA) and long-chain polyunsaturated fatty acids (LC-PUFA); (2) The mechanisms for transfer of fatty acids across the placenta; (3) The role of very low density lipoproteins (VLDL) in transfer of EFA and LC-PUFA from the maternal liver to the placenta; (4) Adjustments of fatty acid metabolism in the maternal liver during pregnancy; and (5) The effect of dietary LC-PUFA on maternal fatty acid metabolism during pregnancy.

Report of the IDECG Working Group on the modulating effect of stressors on the upper and lower limits of lipid and carbohydrate intake. International Dietary Energy Consultative Group

The recommended caloric intake should be increased by 2870kJ (670Kcal)/d in the exclusively breast-feeding woman. Of this increase, 10% should be ingested as protein, up to 50% as lipid and no less than 40% as carbohydrate. If weight loss is desired lipid intake can be reduced but carbohydrate intake should be maintained. Of the total energy increment 3% should be supplied by essential fatty acids (linoleic and alpha-linolenic acids) and 0.5% by n-3 fatty acids. The issue of dietary supplementation with long-chain polyunsaturated fatty acids remains for future research.

Tight junction regulation in the mammary gland

Tight junctions form a narrow, continuous seal that surrounds each endothelial and epithelial cell at the apical border, and act to regulate the movement of material through the paracellular pathway. In the mammary gland, the tight junctions of the alveolar epithelial cells are impermeable during lactation, and thus allow milk to be stored between nursing periods without leakage of milk components from the lumen. Nonetheless mammary epithelial tight junctions are dynamic and can be regulated by a number of stimuli. Tight junctions of the mammary gland from the pregnant animal are leaky, undergoing closure around parturition to become the impermeable tight junctions of the lactating animal. Milk stasis, high doses of oxytocin, and mastitis have been shown to increase tight junction permeability. In general changes in tight junction permeability in the mammary gland appear to be the results of a state change and not assembly and disassembly of tight junctions. Both local factors, such as intramammary pressure and TGF-beta, and systemic factors, such as prolactin, progesterone, and glucocorticoids, appear to play a role in the regulation of mammary tight junctions. Finally, the tight junction state appears to be closely linked to milk secretion. An increase in tight junction permeability is accompanied by decrease in the milk secretion rate, and conversely, a decrease in tight junction permeability is accompanied by an increase in the milk secretion rate.

The mammary fat pad

The mammary fat pad is essential for development of the mammary epithelium, providing signals that mediate ductal morphogenesis and, probably, alveolar differentiation. The "cleared" fat pad is often used as a transplantation site. Considering the crucial role of the fat pad, its properties have received relatively little attention from researchers in the field. Some of the questions whose investigation is pertinent to understanding both normal mammary development and carcinogenesis are outlined in this commentary in the spirit of stimulating enquiry into this important subject. It is clear from a brief perusal of the available literature that until studies are specifically designed to clearly differentiate between functional effects of the fibrous and the adipose stroma, more substantive information about their differential effects on mammary development and tumorigenesis will not be forthcoming.

Active transport of nitrofurantoin across a mouse mammary epithelial monolayer

The antibiotic nitrofurantoin is transported against an electrochemical gradient into milk. A monolayer of CIT3 cells, a subline of the Comma 1D normal mouse mammary epithelial cell line, transports [14C]-nitrofurantoin against a concentration gradient from the basal to the apical solution when grown on membrane filters. In a side-by-side diffusion chamber with well-stirred solutions on both sides, the transfer rate is 50% higher in the basal-to-apical than in the apical-to-basal direction. Nonlabeled nitrofurantoin (500 microM) in the basal chamber equalized the transport in both directions, suggesting that a specific transporter is responsible for the basal-to-apical increment in flux. From inhibition studies, the apparent affinity of this transporter for nitrofurantoin is 50 microM. Changes in pH between 6.4 and 7.8 had no effect on the active transport component of the flux but did affect the passive flux component. Passive flux of the nonionized molecule was 2.6 times faster than that of the ionized molecule, but the ionized molecule did appear to cross the membrane passively. Our findings show that nitrofurantoin is actively transported across a mammary epithelial cell monolayer by a transporter whose affinity for nitrofurantoin does not depend on the anionic charge on nitrofurantoin. The pH dependence of a parallel passive pathway suggests that both nonionized and ionized forms of nitrofurantoin cross the membranes of the mammary epithelial cell by passive diffusion.

Active transport of nitrofurantoin across the mammary epithelium in vivo

Nitrofurantoin is a commonly used urinary tract antibiotic that has been found at high concentrations in human milk. In vivo studies in rats were carried out to determine the mechanism by which this drug crosses the mammary epithelium. Lactating rats were gavage-fed with nitrofurantoin, and their milk and plasma levels of the antibiotic were measured at intervals up to 8 hr. The average milk-to-plasma (M/P) ratio, calculated from the areas under the milk and plasma curves, respectively, was 23 compared with a ratio predicted to be about 0.3 on the basis of lipid partitioning and protein binding determinations. M/P ratios for two nitrofurantoin congeners were also calculated. The neutral compound furazolidone had a M/P ratio of about 1, as predicted, whereas the basic compound furaltadone had a M/P ratio of 3.49 compared with a predicted ratio of 1.4. These data suggest that nitrofurantoin and, to a lesser extent, furaltadone are actively transported across the mammary epithelium into milk.

Regulation of milk lipid secretion and composition

Triacylglycerols make up 98% of the lipid content of milk, ranging in different species from 0 to 50% of the total milk volume. The fatty aid composition of the triacylglycerols depends on the species, the dietary fatty acid composition, and the carbohydrate-to-lipid ratio of the diet. The rate of lipid synthesis in the lactating mammary gland depends on the stage of mammary development and is decreased by fasting and starvation in ruminants and rodents but not in species that fast during lactation, such as seals and hibernating bears. Regulatory agents include insulin, prolactin, and non-esterified fatty acids. Dietary trans fatty acids may depress milk lipid synthesis under certain conditions. Evidence is presented that fatty acids may play a major regulatory role in acute changes in de novo mammary fatty acid synthesis, acting primarily on the activity of acetyl coenzyme A carboxylase.

Effects of xenobiotics on milk secretion and composition

Xenobiotics have the potential of disrupting the differentiated function of the mammary gland, that is, milk secretion, by inhibiting mammary development, by directly interfering with the secretory function of the mammary alveolar cell, by altering the hormonal milieu that supports lactation, or by impeding nutrient transport to the mammary cell. Although solid information is lacking in most of these areas, available research suggests that antiestrogens may interfere with mammary development, that oxidants and phorbol esters may inhibit milk secretion, that ergot alkaloids and alcohol may alter the hormonal milieu, and that dioxins may interfere with lipid transport to the mammary gland. Models for the investigation of these and other environmental toxins and drugs are suggested.

Calcium partitioning in human and bovine milk

The partitioning of calcium in human milk was studied experimentally and compared with the distribution of calcium among the compartments of bovine milk. Care was taken to handle the milk samples to produce minimal disturbance of the milk fat globule membranes and the CO2 content. About 15% of the milk calcium was associated with casein in human milk; the remainder was in the aqueous phase. The equilibrium between ionized calcium and calcium complexes with smaller anions was examined. Ionized calcium varied between 2.3 and 4.0 mM across individuals (mean 3.0 +/- .1 mM) at 90 d of lactation. The remainder of the calcium in the aqueous phase was associated with citrate and phosphate. Changes in total calcium in human milk during lactogenesis and late lactation reflected changes in citrate and casein rather than alterations in ionized calcium.

Regulation of lipoprotein lipase activity and mRNA in the mammary gland of the lactating mouse

We examined the effects of reproductive stage and fasting on lipoprotein lipase (LPL) activity and mRNA in the mouse mammary gland. Heparin-releasable and cell-associated LPL activity rose immediately after birth, followed 1-2 days later by an increase in LPL mRNA. Fasting decreased LPL activity in the mammary gland at all reproductive stages. During lactation, both milk and heparin-releasable LPL were substantially decreased by an overnight fast, whereas cell-associated LPL was less affected and LPL mRNA did not change. These studies indicate that the extracellular, heparin-releasable, fraction of mammary LPL activity responds most rapidly to alterations in physiological state, usually accompanied by smaller changes in cellular enzyme activity. Changes in the level of LPL mRNA were seen only during the transition from pregnancy to lactation, and these tended to follow, rather than precede, changes in enzyme activity. We conclude that in the mammary gland as in adipose tissue, LPL is regulated primarily at the translational and post-translational level.

Effects of fasting, elevated plasma glucose and plasma insulin concentrations on milk secretion in women

In order to determine whether short term variations in plasma glucose and/or insulin influence milk lactose secretion in women, the effects of fasting and increased blood insulin and glucose on milk volume and composition were studied with glucose clamp methodology in exclusively and partially breast-feeding women. Twenty hours of fasting had no discernable effect on the output of milk or its macronutrient composition. Four hours of increased plasma insulin, studied under conditions where plasma glucose was maintained at the fasting level, had no effect on milk volume, milk glucose concentration, total fat content or lactose secretion rate. Increased plasma glucose, maintained at twice fasting levels for 4 to 6 h, produced a threefold increase in milk glucose concentration but had no significant effect on the rate of lactose synthesis. In partially breast-feeding women producing no more than 200 ml milk per day, a similar degree of hyperglycaemia increased milk glucose more than fourfold but did not significantly increase the milk secretion rate. It is concluded that human milk production is isolated from the homeostatic mechanisms that regulate glucose metabolism in the rest of the body, in part because the lactose synthetase system has a Km for glucose lower than the concentration available in the Golgi compartment.

Studies in human lactation: milk composition and daily secretion rates of macronutrients in the first year of lactation

Time-dependent changes in milk composition and secretion from pregnancy through greater than or equal to 6 mo of exclusive breast-feeding were studied in 13 multiparous women. Concentrations and secretion rates of lipid, lactose, protein, sodium, chloride, potassium, total calcium, ionized calcium, magnesium, glucose, citrate, inorganic phosphate, creatinine, and urea and pH were analyzed longitudinally from day 6 until weaning commenced. The composition of the antepartum secretion was related to the permeability of the junctional complexes between mammary cells. Significant increases in lactose, glucose, pH, and ionized calcium and significant decreases in protein, sodium, potassium, chloride, and calcium concentrations were observed between 1 and 6 mo. Significant differences among individuals, which persisted through lactation, were observed for the concentrations of sodium, potassium, chloride, and inorganic phosphate. The amount of lactose, ionized calcium, and magnesium transferred to the infant was also characteristic of each mother-infant pair. Our data imply that lactation performance is determined in the first month postpartum.

Studies in human lactation: milk volume and nutrient composition during weaning and lactogenesis

Concentrations and secretion rates of macronutrients and major ions in human milk during lactogenesis (birth to 8 d) and late lactation (greater than 6 mo postpartum) are reported. Postpartum changes in lactose, sodium, and chloride concentrations signalled closure of the paracellular pathway during days 1-2. From days 2 to 4 postpartum, initiation of copious milk secretion was accompanied by significant increases in citrate, free phosphate, glucose, and calcium concentrations and a decrease in pH. During weaning, significant changes in milk protein, lactose, chloride, and sodium concentrations were observed only when milk volume fell below 400 mL/d; more than one feed per day was necessary to maintain milk secretion. Temporal changes in the concentration of other milk components, except glucose and magnesium, were not different in weaning and non-weaning women. Differences between the relation of milk volume and composition during lactogenesis and weaning suggest that volume is differently regulated in the two periods.

Distribution and source of lipoprotein lipase in mouse mammary gland

During lactation lipoprotein lipase (LPL) is elevated in mammary tissue and depressed in adipose tissue to redirect lipids for incorporation into milk fat. The cellular origin of lipoprotein lipase in mammary tissue is thought to be the mammary epithelial cell which is the predominant cell type noticeable in the lactating gland; however, mammary adipocytes are also present. If lipoprotein lipase is produced by adipocytes in other sites of the body, then the question remains as to why mammary adipocytes have not been shown to produce lipoprotein lipase. In this study we present several lines of evidence that indicate that the mammary adipocyte is a source of LPL in the lactating mammary gland of mice. This evidence includes the absence of extracellular and intracellular lipoprotein lipase activity in two types of primary mammary epithelial cell cultures and a similarity in the changes of lipoprotein lipase activity in genital adipose tissue from nonpregnant mice and lactating mammary tissue to the nutritional state of the animal. Other evidence presented here includes strong localization of lipoprotein lipase protein and messenger RNA by fluorescence immunohistochemistry and in situ hybridization, respectively, to interstitial cells located between epithelial structures. We postulate that these interstitial cells are regressed, lipid-deleted mammary adipocytes.

Lipoprotein lipase in human milk: compartmentalization and effect of fasting, insulin, and glucose

The object of this study was to investigate the effect of maternal metabolic state on the activity of lipoprotein lipase (LPL) in human milk. Although the total LPL activity in milk was not significantly affected by up to three cycles of freezing and thawing, the amount of LPL associated with the cream fraction of the milk increased from an average of less than 10% to about 70% after this treatment. The enzyme was relatively stable when the milk was stored on ice, losing activity at a rate of about 1% per hour. At 37 degrees C degradation was more rapid, about 7% per hour. When LPL activity was measured in samples taken at hourly intervals by breast pump, using oxytocin to achieve a complete letdown at each pumping, activity was found to double from the first to the third pumping. Thereafter the activity was stable under fasting conditions. Hyperglycemic and euglycemic, hyperinsulinemic glucose clamp protocols were used to evaluate the effects of glucose and insulin. Both high plasma glucose and high plasma insulin in the presence of normal glucose significantly increased LPL activity within 4 hours. We conclude that, like adipose, tissue LPL, mammary LPL is regulated by plasma insulin.

Divalent cation activation of galactosyltransferase in native mammary Golgi vesicles

Millimolar concentrations of manganese are required for maximal activation of purified galactosyltransferase (lactose synthase, EC 2.4.1.22), the enzyme that catalyzes addition of galactosyl groups to proteins and, in lactose synthesis, to glucose. To examine manganese activation of this enzyme under in vivo conditions, we studied intact, partially purified Golgi membranes from mouse mammary glands. In intact vesicles treated with the divalent cation ionophore, A23187, activation followed Michaelis-Menton kinetics with a Km of 3 microM; maximal activation was achieved below 10 microM manganese. In both detergent-solubilized and leaky vesicles the kinetics of manganese activation were consistent with the presence of two manganese-binding sites with dissociation constants about 40 microM and 20 mM. The difference is consistent with the presence in intact vesicles of an endogenous activator too large to traverse the membrane via A23187; this activator could bind to the low affinity manganese site allowing manganese or another divalent cation such as zinc to activate the enzyme at micromolar concentrations. The Km for UDP-galactose was found to be similar in the vesicular and solubilized preparations at micromolar and millimolar manganese concentrations, respectively, providing additional evidence for this hypothesis.

Milk composition and volume during the onset of lactation in a diabetic mother

Milk volume and composition were examined in a diabetic mother on days 3-7 postpartum. By day 5 milk volume produced and concentrations of sodium, potassium, chloride, lactose, protein, calcium, magnesium, and citrate were within limits of a reference population. Fat content of the milk was slightly lower. Free fatty acids were 2% of total lipid on day 3 but increased to 23% on days 4-7, suggesting impaired esterification in the mammary gland. Total milk lipoprotein lipase increased approximately fourfold during days 4-5. Other changes were 1) low cholesterol content, only one-fifth of normal milk; 2) decreased medium-chain fatty acids, suggesting impairment of fatty acid synthesis in the mammary gland; 3) increased oleic acid; and 4) high concentrations of polyunsaturated fatty acids, suggesting increased chain elongation. These results suggest that diabetes produces changes in lipid metabolism in the mammary gland that alter the composition of milk produced by the diabetic mother.

Studies in human lactation: secretion of zinc, copper, and manganese in human milk

Zinc, copper, and manganese were measured in milk samples collected longitudinally from 13 highly selected, healthy lactating women. Samples were obtained at least weekly up to 1 mo and then monthly up to 18 mo postpartum or to weaning. Zn concentrations in milk declined throughout lactation from 71.9 +/- 18.3 mumol/L (means +/- SD) at 7 d to 44.3 +/- 10.7 mumol/L at 1 mo and 7.64 +/- 4.59 mumol/L at 12 mo. Cu levels also declined from 9.76 +/- 2.05 mumol/L at 7 d to 3.46 +/- 0.79 mumol/L at 5 mo with little change thereafter. Zn and Cu were not affected by weaning. Mn concentrations declined from 67.4 +/- 23.7 nmol/L at 1 mo to approximately 36 nmol/L by 3 mo and then remained constant to 7 mo when mean levels started to rise. Gradual weaning was associated with large and irregular changes in Mn levels. Total daily outputs in milk averaged 26.0 mumol Zn, 3.9 mumol Cu, and 35 nmol Mn over the first 3 mo postpartum and 13.8 mumol Zn, 2.7 mumol Cu, and 29 nmol Mn over the second 3 mo.

Studies in human lactation: milk volumes in lactating women during the onset of lactation and full lactation

After validation of test-weighing procedures milk volumes produced by 13 multiparous Caucasian women were followed longitudinally through the first year of lactation. All practiced exclusive breast-feeding for at least 5 mo. Milk transfer to the infant was low on days 1 and 2 and increased rapidly to 498 +/- 129 g/d (means +/- SD) on day 5 and then more slowly to 753 +/- 89 g/d during months 3-5. There was a characteristic milk volume for each mother-infant pair that was significantly related neither to milk yield on days 4-6 nor to birth weight. It was, however, strongly related to infant weight at 1 mo, suggesting that infant and/or maternal factors coming into play during the first month of life are strong determinants of subsequent milk transfer to the infant.

Mammary Cultures on Floating Gels: A Model System for Mammary Function

Cultures of mammary epithelium on collagen gels can be induced to secrete the normal complement of milk proteins if the gels are released from the culture dish, allowing them to contract. During the contraction the epithelial cells undergo a shape change from flattened to columnar that initiates secretion of milk proteins. These floating cultures are dependent on hormones and offer a valuable model system for the study of milk secretion and its regulation.