Prolactin signalling to milk protein secretion but not to gene expression depends on the integrity of the Golgi region

Physiology of milk secretion

Milk is a unique and complete nutritive source for the mammal neonate, also providing immune protection and developmental signals. Lactation is a complex process, proper to the mother and child dyad, and including numerous variables ranging from psychological aspects to the secretory functioning of the mammary epithelial cells, all contributing to a successful breastfeeding. This review gives an integrated overview of the physiology of lactation with a particular focus on cellular and molecular mechanisms involved in milk product secretion and their regulations.

Prolactin (PRL)-stimulated ubiquitination of ZnT2 mediates a transient increase in zinc secretion followed by ZnT2 degradation in mammary epithelial cells

The zinc transporter ZnT2 imports zinc into secretory vesicles and regulates zinc export from the mammary epithelial cell. Mutations in ZnT2 substantially impair zinc secretion into milk. The lactogenic hormone prolactin (PRL) transcriptionally increases ZnT2 expression through the Jak2/STAT5 signaling pathway, increasing zinc accumulation in secretory vesicles and zinc secretion. Herein, we report that PRL post-translationally stimulated ZnT2 ubiquitination, which altered ZnT2 trafficking and augmented vesicular zinc accumulation and secretion from mammary epithelial cells in a transient manner. Ubiquitination then down-regulated zinc secretion by stimulating degradation of ZnT2. Mutagenesis of two N-terminal lysine residues (K4R and K6R) inhibited ZnT2 ubiquitination, vesicular zinc accumulation and secretion, and protein degradation. These findings establish that PRL post-translationally regulates ZnT2-mediated zinc secretion in a multifactorial manner, first by enhancing zinc accumulation in vesicles to transiently enhance zinc secretion and then by activating ubiquitin-dependent ZnT2 degradation. This provides insight into novel mechanisms through which ZnT2 and zinc transport is tightly regulated in mammary epithelial cells.

Effects of dietary fish oil and corn oil on rat mammary tissue

The effects, on the maternal mammary gland, of diets containing similar lipid percentages but differing in composition of polyunsaturated fatty acids (PUFA) have been assessed in rats during pregnancy and lactation. For this purpose, tuna fish oil (an n-3-PUFA-enriched oil) and corn oil (an n-6-PUFA-enriched oil) were included in diets at ratios such that the caloric inputs were the same as that of the control diet. As expected, the maternal diet affected the tissue composition of dams. Unexpectedly, only the tuna fish oil diet had an effect on pup growth, being associated with the pups being underweight between the ages of 11 and 21 days. The maternal mammary gland of rats fed the tuna fish oil diet displayed two main modifications: the size of cytoplasmic lipid droplets was increased when compared with those in control rats and the mammary epithelium showed an unusual formation of multilayers of cells. These results show that the tuna fish oil diet, during pregnancy and lactation, exerts specific effects on mammary cells and on the formation of lipid droplets. They suggest that this maternal diet affects the functioning of the mammary tissue.

Mammary gland secretion: hormonal coordination of endocytosis and exocytosis

The mammary epithelium coordinates the uptake of milk precursors and the transport of milk components in order to produce milk of relatively constant composition at a particular stage of lactation, as long as the mammary gland is healthy. The mammary epithelial cell controls the uptake of blood-borne molecules at its basal side and the release of products into milk at its apical side, through mechanisms of internalization (endocytosis) and mechanisms of release (exocytosis). These events are strictly dependent on the physiological stage of the mammary gland. This review addresses the mechanisms responsible for these processes and points out new questions that remain to be answered concerning possible interconnections between them, for an optimal milk secretion.

Changes of endoplasmic reticulum and mitochondria in mammary epithelial cells during mammogenesis in Chinese Holstein dairy cows

The objective of the research was to study the changes of the major organelles, endoplasmic reticulum (ER) and mitochondria, in mammary epithelial cells of the Chinese Holstein dairy cow during mammogenesis. For this purpose, a mammary epithelial cell bank was established from 9 selected Chinese Holstein dairy cows using collagenase I digestion and attachment culture biotechniques. This cell bank included 9 samples at stages of pregnancy, lactation and involution. The changes of ER and mitochondria in the mammary cells were observed at the subcellular level using living cell fluorescent labeling and laser confocal microscopy. Subsequently, the area of integrated optical density of each sample was calculated to determine changes of ER and mitochondria in the mammary epithelial cells. The results showed clear differences in the epithelial major organelles during the various mammary gland development stages. The ER and mitochondria, as an indicator of lactogenic activity of alveolar secretory cells, increased in number from pregnancy to lactation by an average 37.32% and 18.44%, respectively, which was followed by a reduction at involution by an average 38.04% and 22.91% compared to lactation. Our study shows that the stages of mammogenesis are accompanied by changes in activity of the major organelles of the mammary epithelial cells.

Mammary gland zinc metabolism: regulation and dysregulation

Zinc (Zn) is required for numerous metabolic processes serving both a structural and catalytic role. The mammary gland has a unique Zn requirement resulting from the need to also transfer an extraordinary amount of Zn into milk (~0.5-1 mg Zn/day) during lactation. Impairments in this process can result in severe Zn deficiency in the nursing offspring which has adverse consequences with respect to growth and development. Moreover, dysregulated mammary gland Zn metabolism has recently been implicated in breast cancer transition, progression and metastasis, thus there is a critical need to understand the molecular mechanisms which underlie these observations. Tight regulation of Zn transporting mechanisms is critical to providing an extraordinary amount of Zn for secretion into milk as well as maintaining optimal cellular function. Expression of numerous Zn transporters has been detected in mammary gland or cultured breast cells; however, understanding the molecular mechanisms which regulate mammary Zn metabolism as well as the etiology and downstream consequences resulting from their dysregulation is largely not understood. In this review, we will summarize the current understanding of the regulation of mammary gland Zn metabolism and its regulation by reproductive hormones, with a discussion of the dysregulation of this process in breast cancer.

Gene expression in the human mammary epithelium during lactation: the milk fat globule transcriptome

The molecular physiology underlying human milk production is largely unknown because of limitations in obtaining tissue samples. Determining gene expression in normal lactating women would be a potential step toward understanding why some women struggle with or fail at breastfeeding their infants. Recently, we demonstrated the utility of RNA obtained from breast milk fat globule (MFG) to detect mammary epithelial cell (MEC)-specific gene expression. We used MFG RNA to determine the gene expression profile of human MEC during lactation. Microarray studies were performed using Human Ref-8 BeadChip arrays (Illumina). MFG RNA was collected every 3 h for 24 h from five healthy, exclusively breastfeeding women. We determined that 14,070 transcripts were expressed and represented the MFG transcriptome. According to GeneSpring GX 9, 156 ontology terms were enriched (corrected P < 0.05), which include cellular (n = 3,379 genes) and metabolic (n = 2,656) processes as the most significantly enriched biological process terms. The top networks and pathways were associated primarily with cellular activities most likely involved with milk synthesis. Multiple sampling over 24 h enabled us to demonstrate core circadian clock gene expression and the periodicity of 1,029 genes (7%) enriched for molecular functions involved in cell development, growth, proliferation, and cell morphology. In addition, we found that the MFG transcriptome was comparable to the metabolic gene expression profile described for the lactating mouse mammary gland. This paper is the first to describe the MFG transcriptome in sequential human samples over a 24 h period, providing valuable insights into gene expression in the human MEC.

ANXA7 expression represents hormone-relevant tumor suppression in different cancers

Tumor suppressor function of ubiquitously expressed Annexin-A7, ANXA7 (10q21) that is involved in exocytosis and membrane fusion was based on cancer prone phenotype in Anxa7(+/-) mice as well as ANXA7 role in human prostate and breast cancers. To clarify ANXA7 biomarker and tumor suppressor function, we analyzed its expression pattern in comparison to the prostate-specific biomarker NKX3.1. Immunohistochemistry-based ANXA7 and NKX3.1 protein expression was analyzed on human tissue microarrays of 4,061 specimens from a wide spectrum of the histopathologically well-characterized tumors in different stages compared to corresponding normal tissues. Decreased ANXA7 expression was mostly associated with high invasive potential in multiple tumors. Although some metastases retained relatively high ANXA7 rates compared to primary cancer tissues, the lymph node metastases from different sites (including prostate and breast) had decreased ANXA7 expression in comparison to the intact lymphatic tissues. Major ANXA7 downregulation pattern was deviated in tumors of glandular (especially neuroendocrine) origin. ANXA7 and NKX3.1 proteins were synexpressed in the male urogenital system and adrenal gland. Gene expression profiling in prostate and breast cancers (SMD) revealed distinct hormone-related profiles for NKX3.1 and ANXA7, where ANXA7 expression correlated with steroid sulfatase which has a pivotal role in steroidogenesis. Abundant protein presence in adrenal gland and its loss in hormone-refractory prostate cancer indicated that ANXA7 can be relevant to steroidogenesis and androgen sensitivity in particular. With tumor suppressor pattern validated in different tumors, ANXA7 can be an attractive diagnostic and therapeutic target associated with the hormone and/or neurotransmitter-mediated modulation of tumorigenesis.

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.

Oxytocin stimulates secretory processes in lactating rabbit mammary epithelial cells

Oxytocin plays a major role in lactation mainly by its action on milk ejection via the contraction of myoepithelial cells. The effect of oxytocin on milk production and the presence of oxytocin receptors on different epithelial cells suggest that this hormone may play a role in mammary epithelial cells. To determine precisely the various roles of oxytocin, we studied localization of oxytocin receptors in lactating rabbit and rat mammary tissue and the influence of oxytocin on secretory processes in lactating rabbit mammary epithelial cells. Immunolocalization of oxytocin receptors on mammary epithelial cells by immunofluorescence and in mammary tissue by immunogold in addition to in situ hybridization showed that lactating rat and rabbit mammary epithelial cells expressed oxytocin receptors. Moreover, oxytocin bound specifically to epithelial cells. To determine whether oxytocin had an effect on lactating rabbit mammary epithelial cells, isolated mammary fragments were incubated in the presence or absence of 10(-6) i.u. ml(-1) of oxytocin. After 1 min of incubation with oxytocin, the morphology of epithelial cells and the localization of caseins and proteins associated with the secretory traffic suggested a striking acceleration of the transport leading to exocytosis, whereas the contraction of myoepithelial cells was only detectable after 7 min. Addition of 10(-8) g ml(-1) of atosiban before the addition of oxytocin prevented the oxytocin effect on secretory processes and on myoepithelial cell contraction. Addition of 10(-6) i.u. ml(-1) of vasopressin to the incubation medium did not mimic the stimulating effect of oxytocin on secretory traffic. These results show that lactating rabbit and rat mammary epithelial cells express oxytocin receptors and that oxytocin binds to these receptors. They strongly suggest that oxytocin has a dual effect on lactating mammary tissue: an acceleration of the intracellular transfer of caseins in mammary epithelial cells followed by the contraction of myoepithelial cells.

Cathepsin D released by lactating rat mammary epithelial cells is involved in prolactin cleavage under physiological conditions

The 16 kDa prolactin fragment arises from partial proteolysis of the native 23 kDa prolactin pituitary hormone. The mammary gland has been involved in this processing, although it has not been clarified whether it occurs in stroma or epithelial cells or extracellularly. Also, the processing enzyme has not been defined yet. Here we show that the incubation medium of stroma-deprived mammary acini from lactating rat contains an enzymatic activity able to cleave, in a temperature- and time-dependent fashion, the 23 kDa prolactin to generate a 16 kDa prolactin detectable under reducing conditions. This cleavage was not impaired in the presence of hirudin, a thrombin inhibitor, but strongly weakened in the presence of pepstatin A, a cathepsin D inhibitor. Cathepsin D immuno-depletion abolished the capability of acini-conditioned medium to cleave the 23 kDa prolactin. Brefeldin A treatment of acini, a condition that largely abolished the apical secretion of milk proteins, did not impair the secretion of the enzymatically active single chain of cathepsin D. These results show that mature cathepsin D from endosomes or lysosomes is released, likely at the baso-lateral site of mammary epithelial cells, and that a cathepsin D-dependent activity is required to effect, under physiological conditions, the cleavage of 23 kDa prolactin in the extracellular medium. This is the first report demonstrating that cathepsin D can perform a limited proteolysis of a substrate at physiological pH outside the cell.

Serotonin Regulates Mammary Gland Development via an Autocrine-Paracrine Loop

Mammary gland development is controlled by a dynamic interplay between endocrine hormones and locally produced factors. Biogenic monoamines (serotonin, dopamine, norepinephrine, and others) are an important class of bioregulatory molecules that have not been shown to participate in mammary development. Here we show that mammary glands stimulated by prolactin (PRL) express genes essential for serotonin biosynthesis (tryptophan hydroxylase [TPH] and aromatic amine decarboxylase). TPH mRNA was elevated during pregnancy and lactation, and serotonin was detected in the mammary epithelium and in milk. TPH was induced by PRL in mammosphere cultures and by milk stasis in nursing dams, suggesting that the gene is controlled by milk filling in the alveoli. Serotonin suppressed beta-casein gene expression and caused shrinkage of mammary alveoli. Conversely, TPH1 gene disruption or antiserotonergic drugs resulted in enhanced secretory features and alveolar dilation. Thus, autocrine-paracrine serotonin signaling is an important regulator of mammary homeostasis and early involution.

Mammary physiology and milk secretion

The presence of drugs or other potentially toxic materials in milk is an obvious public health risk, especially to infants and neonates. There is also increasing concern that human breast cancer is principally epigenetic in origin and results from environmentally produced lesions. Little is known about the mechanisms by which toxic substances enter milk or mammary tissue but knowledge of these processes is important to toxicologists and researchers involved in drug design and metabolism. Five general pathways have been described for transport of proteins, lipids, ions, nutrients and water into milk. Four of these pathways are transcellular, involving transport across at least two membrane barriers; the fifth is paracellular and allows direct exchange of interstitial and milk components. Solute transport by these pathways is mediated by a diverse, and complex array of transport and secretory processes that are regulated by hormonal, developmental, and physiological factors. Current research is beginning to define the mechanisms underlying some of these processes, however the regulation and coordination of solute transport mechanisms remains poorly understood. In this article we review our current understanding of the normal solute transport and secretory processes involved in milk production, and discuss potential regulatory mechanisms.