The biology of zinc transport in mammary epithelial cells: implications for mammary gland development, lactation, and involution

Zinc Supplementation Improves Texture, Oxidative Stability of Caciotta Cheese and Reduces Biogenic Amines Production

Zinc is essential for animals, playing a vital role in enzyme systems and various biochemical reactions. It is crucial to ensure a sufficient intake of zinc through the diet to maintain efficient homeostasis. Only few studies on zinc effect in cow lactating diet evaluated the effects on milk and cheese quality, with conflicting findings. 24 cows of the Friesian breed were divided into two groups (CTR: control and TRT: treated group). Cows were selected for age, body weight, parity and phase of lactations (mid lactation, 140-160 days). CTR diet contained 38 mg/kg of Zn and TRT diet was supplied with 120 mg/kg of complete feed for 60 days. The objective of current investigation was to evaluate the impact of a dietary Zinc Oxide (ZnO) integration of lactating Friesian cows on chemical composition, zinc content, fatty acid and proteic profile, ammine content, pH, aw, texture, and sensory profile of cheese and to improve the chemical-nutritional quality of milk and cheese. The results showed that ZnO supplementation reduced mesophilic aerobic bacteria and Presumptive Pseudomonas spp. growth, proteolysis, biogenic amines content, lipid oxidation, odour intensity and sour and increased hardness, gumminess, chewiness, elasticity of cheese. Biogenic amines are considered an important aspect of food safety. ZnO integration in cow diet could represent a promising strategy for improving the quality, the safety and shelf-life of caciotta cheese.

Cross-talk between zinc and calcium regulates ion transport: A role for the zinc receptor, ZnR/GPR39

Zinc is essential for many physiological functions, with a major role in digestive system, skin health, and learning and memory. On the cellular level, zinc is involved in cell proliferation and cell death. A selective zinc sensing receptor, ZnR/GPR39 is a Gq-coupled receptor that acts via the inositol trisphosphate pathway to release intracellular Ca2+. The ZnR/GPR39 serves as a mediator between extracellular changes in Zn2+ concentration and cellular Ca2+ signalling. This signalling pathway regulates ion transporters activity and thereby controls the formation of transepithelial gradients or neuronal membrane potential, which play a fundamental role in the physiological function of these tissues. This review focuses on the role of Ca2+ signalling, and specifically ZnR/GPR39, with respect to the regulation of the Na+/H+ exchanger, NHE1, and of the K+/Cl- cotransporters, KCC1-3, and also describes the physiological implications of this regulation.

Exploring zinc deficiency using serum Zn levels: consequences and potential solutions in suckling pigs

Three trials were undertaken to provide an answer to different questions: 1) Are suckling pigs able to maintain physiological serum Zn levels throughout lactation and do these levels vary between high and low body weight (BW) pigs?, 2) Are serum Zn levels in pigs soon after weaning a predisposing factor for diarrhea?, and 3) Is it possible to increase serum Zn levels at weaning by supplementing Zn during lactation. In trial 1, blood samples were taken from pigs during lactation. Eight pigs (one piglet per litter) had blood drawn on days 0 (farrowing), 7, 14, 21, and 28 (weaning), and 60 pigs (selected from the whole farrowing batch with 35 sows), categorized as either heavy (8.63 kg) or light (5.50 kg) had blood drawn on day 28. Serum Zn levels at birth were 1.2 mg/L and decreased (P < 0.01) to 0.67 mg/L on day 28. Heavier pigs showed greater (P < 0.01) serum Zn levels (0.98 mg/L) than light BW pigs (0.79 mg/L). In trial 2, blood samples were obtained from 240 pigs at weaning (26.2 ± 2.5 d) with an average initial BW of 6.94 ± 1.87 kg and were distributed into 24 pens (10 pigs/pen) by BW. Diarrhea incidence was recorded daily from days 0 to 35 post-weaning. From the 240 pigs, a group of 110 pigs with uniform BW (6.5 ± 1.9 kg) was selected and separated into two groups based on serum Zn levels at weaning with 55 pigs with low serum Zn (LZn: <0.71 mg/L) and 55 pigs with high serum Zn (HZn: >0.9 mg/L). Pigs with LZn were 2.49 times as likely to have diarrhea as pigs with HZn (P < 0.02). In trial 3, a total of 96 suckling pigs were allotted four treatments that consisted of the daily administration of 0, 6, 18, or 30 mg of Zn as Zn citrate in capsule form during the last 7 d of lactation. Pigs were individually weighed, and blood samples were obtained on days 14, 21 (weaning), and 7 after weaning. Serum Zn levels linearly increased by day as Zn citrate supplementation increased (interaction, P < 0.001). However, only light pigs supplemented with 18 and 30 mg/L of Zn experienced an increase in serum Zn levels during lactation. In conclusion, a decrease in serum Zn levels occurs during lactation and is more severe in low BW pigs. Low Zn status (< 0.7 mg/L) at weaning may be a predisposing factor for diarrhea. However, Zn supplementation during lactation can mitigate this decrease in light pigs.

Causes of Low Milk Supply: The Roles of Estrogens, Progesterone, and Related External Factors

Low milk supply (LMS) poses a significant challenge to exclusive and continued breastfeeding, affecting ∼10% to 15% of mothers. Milk production is intricately regulated by both endocrine and autocrine control mechanisms, with estrogens and progesterone playing pivotal roles in this process. In addition to endogenously produced hormones, external substances capable of interfering with normal hormonal actions, including phytoestrogens, mycoestrogens, synthetic estrogens, and hormonal contraceptives, can influence milk production. The effects of these extrinsic hormones on milk production may vary based on maternal body mass index. This comprehensive review examines the multifaceted causes of LMS, focusing on the involvement of estrogens, progesterone, and related external factors in milk production. Furthermore, it investigates the interplay between hormonal factors and obesity, aiming to elucidate the endocrine mechanisms underlying obesity-associated LMS. Insights from this review provide valuable perspectives for developing interventions to improve milk production and address the challenges associated with LMS.

The ins and outs of mammary gland calcium and zinc transport: A brief review

Milk is an excellent source of all macrominerals and trace elements, which are essential for proper function of a wide variety of vital processes. The concentrations of minerals in milk are influenced by numerous factors, including stage of lactation, time of day, nutritional and health status of the mother, as well as maternal genotype and environmental exposures. Additionally, tight regulation of mineral transport within the secretory mammary epithelial cell itself is critical for the production and secretion of milk. In this brief review, we focus on the current understanding of how the essential divalent cations calcium (Ca) and zinc (Zn) are transported in the mammary gland (MG) with a focus on molecular regulation and the consequence of genotype. A deeper grasp of mechanisms and factors affecting Ca and Zn transport in the MG is important to understanding milk production, mineral output, and MG health to inform intervention design and novel diagnostic and therapeutic strategies in production animals and humans.

Effects of sustained hyperprolactinemia in late gestation on the mammary parenchymal tissue transcriptome of gilts

BACKGROUND

Gilts experiencing sustained hyperprolactinemia from d 90 to 109 of gestation showed an early onset of lactogenesis coupled with premature mammary involution. To better understand the molecular mechanisms underlying the premature mammary involution observed in these gilts, a transcriptomic analysis was undertaken. Therefore, this study aimed to explore the effect of hyperprolactinemia on the global transcriptome in the mammary tissue of late gestating gilts and identify the molecular pathways involved in triggering premature mammary involution.

METHODS

On d 90 of gestation, gilts received daily injections of (1) canola oil until d 109 ± 1 of gestation (CTL, n = 18); (2) domperidone (to induce hyperprolactinemia) until d 96 ± 1 of gestation (T7, n = 17) or; (3) domperidone (until d 109 ± 1 of gestation (T20, n = 17). Mammary tissue was collected on d 110 of gestation and total RNA was isolated from six CTL and six T20 gilts for microarray analysis. The GeneChip® Porcine Gene 1.0 ST Array was used for hybridization. Functional enrichment analyses were performed to explore the biological significance of differentially expressed genes, using the DAVID bioinformatics resource.

RESULTS

The expression of 335 genes was up-regulated and that of 505 genes down-regulated in the mammary tissue of T20 vs CTL gilts. Biological process GO terms and KEGG pathways enriched in T20 vs CTL gilts reflected the concurrent premature lactogenesis and mammary involution. When looking at individual genes, it appears that mammary cells from T20 gilts can simultaneously upregulate the transcription of milk proteins such as WAP, CSN1S2 and LALBA, and genes triggering mammary involution such as STAT3, OSMR and IL6R. The down-regulation of PRLR expression and up-regulation of genes known to inactivate the JAK-STAT5 pathway (CISH, PTPN6) suggest the presence of a negative feedback loop trying to counteract the effects of hyperprolactinemia.

CONCLUSIONS

Genes and pathways identified in this study suggest that sustained hyperprolactinemia during late-pregnancy, in the absence of suckling piglets, sends conflicting pro-survival and cell death signals to mammary epithelial cells. Reception of these signals results in a mammary gland that can simultaneously synthesize milk proteins and initiate mammary involution.

Non-coding transcriptomic profiles in the sheep mammary gland during different lactation periods

Sheep milk production is a dynamic and multifactorial trait regulated by diverse biological mechanisms. To improve the quality and production of sheep milk, it is necessary to understand the underlying non-coding transcriptomic mechanisms. In this study, ribonucleic acid-sequencing (RNA-seq) was used to profile the expression of microRNAs (miRNAs) and circular RNAs (circRNAs) in the sheep mammary gland at three key lactation time points (perinatal period, PP; early lactation, EL; and peak lactation, PL). A total of 2,369 novel circRNAs and 272 miRNAs were profiled, of which 348, 373, and 36 differentially expressed (DE) circRNAs and 30, 34, and 7 DE miRNAs were detected in the comparison of EL vs. PP, PL vs. PP, and PL vs. EL, respectively. A series of bioinformatics analyses including functional enrichment, machine learning prediction, and competing endogenous RNA (ceRNA) network analyses were conducted to identify subsets of the potential candidate miRNAs (e.g., oar_miR_148a, oar_miR_362, and oar_miR_432) and circRNAs (e.g., novel_circ_0011066, novel_circ_0010460, and novel_circ_0006589) involved in sheep mammary gland development. Taken together, this study offers a window into the dynamics of non-coding transcriptomes that occur during sheep lactation and may provide further insights into miRNA and circRNA that influence sheep mammary gland development.

Expression profiles of the genes associated with zinc homeostasis in normal and cancerous breast and prostate cells

Zn2+ dyshomeostasis is an intriguing phenomenon in breast and prostate cancers, with breast cancer cells exhibiting higher intracellular Zn2+ level compared to their corresponding normal epithelial cells, in contrast to the low Zn2+ level in prostate cancer cells. In order to gain molecular insights into the zinc homeostasis of breast and prostate cancer cells, this study profiled the expression of 28 genes, including 14 zinc importer genes (SLC39A1-14) which encode ZIP1-14 to transport Zn2+ into the cytoplasm, 10 zinc exporter genes (SLC30A1-10) which encode ZnT1-10 to transport Zn2+ out of the cytoplasm and 4 metallothionein genes (MT1B, MT1F, MT1X, MT2A) in breast (MCF10A, MCF-7, MDA-MB-231) and prostate (RWPE-1, PC3, DU145) cell lines in response to extracellular zinc exposures at a mild cytotoxic dosage and a benign dosage. The RNA samples were prepared at 0 min (T0), 30 min (T30) and 120 min (T120) in a time course with or without zinc exposure, which were used for profiling the baseline and dynamic gene expression. The up-regulation of MT genes was observed across the breast and prostate cancer cell lines. The expression landscape of SLC39A and SLC30A was revealed by the qRT-PCR data of this study, which sheds light on the divergence of intracellular Zn2+ levels for breast and prostate cancer cells. Taken together, the findings are valuable in unravelling the molecular intricacy of zinc homeostasis in breast and prostate cancer cells.

The expression and clinical significance of GPR39 in colon cancer

BACKGROUND

Colorectal cancer is the third most common cancer and requires more prognostic biomarkers for precise treatment. GPR39 is a GPCR which can interact with Zn and modulate the colonocytes' survival. The clinical significance of GPR39 in colon cancer has never been reported.

MATERIALS

In our study, we compared GPR39 expression between colon cancers and tumor-adjacent tissues by retrieving TCGA data and detected the expression of GPR39 in colon cancers with qPCR and immunohistochemistry. The clinical significance of GPR39 was evaluated by analyzing the correlations with clinicopathological factors with the chi-square test. The prognostic significance of GPR39 was estimated with univariate and multivariate analyses. The expression of several other biomarkers including PPARG, EPCAM, and PD-L1 was investigated by re-analyzing TCGA data, qPCR, and IHC. The prognostic value of PPARG, EPCAM, and PD-L1 was also estimated with univariate analysis.

RESULTS

In both TCGA database and our 15 colon cancer pairs, GPR39 expression was significantly upregulated in colon cancer tissues. GPR39 was an independent prognostic biomarker in colon cancer for poor prognosis. With TCGA data re-analysis, qPCR, and IHC, we showed that GPR39 expression was significantly correlated with the expression of EPCAM and PD-L1, but not PPARG. EPCAM and PD-L1 were also unfavorable prognostic biomarkers of colon cancer.

CONCLUSIONS

GPR39 was upregulated in colon cancer tissues compared with tumor-adjacent tissues. GPR39 was an independent prognostic biomarker in colon cancer for poor prognosis. EPCAM and PD-L1 were substantially associated with GPR39 expression, and they were also identified as prognostic biomarkers in colon cancers.

Evolutionary rate covariation identifies SLC30A9 (ZnT9) as a mitochondrial zinc transporter

Recent advances in genome sequencing have led to the identification of new ion and metabolite transporters, many of which have not been characterized. Due to the variety of subcellular localizations, cargo and transport mechanisms, such characterization is a daunting task, and predictive approaches focused on the functional context of transporters are very much needed. Here we present a case for identifying a transporter localization using evolutionary rate covariation (ERC), a computational approach based on pairwise correlations of amino acid sequence evolutionary rates across the mammalian phylogeny. As a case study, we find that poorly characterized transporter SLC30A9 (ZnT9) coevolves with several components of the mitochondrial oxidative phosphorylation chain, suggesting mitochondrial localization. We confirmed this computational finding experimentally using recombinant human SLC30A9. SLC30A9 loss caused zinc mishandling in the mitochondria, suggesting that under normal conditions it acts as a zinc exporter. We therefore propose that ERC can be used to predict the functional context of novel transporters and other poorly characterized proteins.

Zinc Signaling in the Mammary Gland: For Better and for Worse

Zinc (Zn²⁺) plays an essential role in epithelial physiology. Among its many effects, most prominent is its action to accelerate cell proliferation, thereby modulating wound healing. It also mediates affects in the gastrointestinal system, in the testes, and in secretory organs, including the pancreas, salivary, and prostate glands. On the cellular level, Zn²⁺ is involved in protein folding, DNA, and RNA synthesis, and in the function of numerous enzymes. In the mammary gland, Zn²⁺ accumulation in maternal milk is essential for supporting infant growth during the neonatal period. Importantly, Zn²⁺ signaling also has direct roles in controlling mammary gland development or, alternatively, involution. During breast cancer progression, accumulation or redistribution of Zn²⁺ occurs in the mammary gland, with aberrant Zn²⁺ signaling observed in the malignant cells. Here, we review the current understanding of the role of in Zn²⁺ the mammary gland, and the proteins controlling cellular Zn²⁺ homeostasis and signaling, including Zn²⁺ transporters and the Gq-coupled Zn²⁺ sensing receptor, ZnR/GPR39. Significant advances in our understanding of Zn²⁺ signaling in the normal mammary gland as well as in the context of breast cancer provides new avenues for identification of specific targets for breast cancer therapy.

β-Lactoglobulin and Glycodelin: Two Sides of the Same Coin?

The two lipocalins, β-lactoglobulin (βLg) and glycodelin (Gd), are possibly the most closely related members of the large and widely distributed lipocalin family, yet their functions appear to be substantially different. Indeed, the function of β-lactoglobulin, a major component of ruminant milk, is still unclear although neonatal nutrition is clearly important. On the other hand, glycodelin has several specific functions in reproduction conferred through distinct, tissue specific glycosylation of the polypeptide backbone. It is also associated with some cancer outcomes. The glycodelin gene, PAEP, reflecting one of its names, progestagen-associated endometrial protein, is expressed in many though not all primates, but the name has now also been adopted for the β-lactoglobulin gene (HGNC, www.genenames.org). After a general overview of the two proteins in the context of the lipocalin family, this review considers the properties of each in the light of their physiological functional significance, supplementing earlier reviews to include studies from the past decade. While the biological function of glycodelin is reasonably well defined, that of β-lactoglobulin remains elusive.

Aging-Associated Alterations in Mammary Epithelia and Stroma Revealed by Single-Cell RNA Sequencing

Aging is closely associated with increased susceptibility to breast cancer, yet there have been limited systematic studies of aging-induced alterations in the mammary gland. Here, we leverage high-throughput single-cell RNA sequencing to generate a detailed transcriptomic atlas of young and aged murine mammary tissues. By analyzing epithelial, stromal, and immune cells, we identify age-dependent alterations in cell proportions and gene expression, providing evidence that suggests alveolar maturation and physiological decline. The analysis also uncovers potential pro-tumorigenic mechanisms coupled to the age-associated loss of tumor suppressor function and change in microenvironment. In addition, we identify a rare, age-dependent luminal population co-expressing hormone-sensing and secretory-alveolar lineage markers, as well as two macrophage populations expressing distinct gene signatures, underscoring the complex heterogeneity of the mammary epithelia and stroma. Collectively, this rich single-cell atlas reveals the effects of aging on mammary physiology and can serve as a useful resource for understanding aging-associated cancer risk.

Using single-cell RNA-sequencing, Li et al. compare mammary epithelia and stroma in young and aged mice. Age-dependent changes at cell and gene levels provide evidence suggesting alveolar maturation, functional deterioration, and potential pro-tumorigenic and inflammatory alterations. Additionally, identification of heterogeneous luminal and macrophage subpopulations underscores the complexity of mammary lineages.

Quantitative Imaging of Labile Zn2+ in the Golgi Apparatus Using a Localizable Small-Molecule Fluorescent Probe

Fluorescent Zn²⁺ probes used for the quantitative analysis of labile Zn²⁺ concentration ([Zn²⁺]) in target organelles are crucial for understanding the role of Zn²⁺ in biological processes. Although several fluorescent Zn²⁺ probes have been developed to date, there is still a lack of consensus concerning the [Zn²⁺] in intracellular organelles. In this study, we describe the development of ZnDA-1H, a small-molecule fluorescent probe for Zn²⁺, which exhibits less pH sensitivity, high Zn²⁺ selectivity, and large fluorescence enhancement upon binding to Zn²⁺. Through protein labeling technology, ZnDA-1H was precisely targeted in various intracellular organelles, such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus. ZnDA-1H exhibited a reversible fluorescence response toward labile Zn²⁺ in these organelles in live cells. Using this probe, the [Zn²⁺] in the Golgi apparatus was estimated to be 25 ± 1 nM, suggesting that labile Zn²⁺ plays a physiological role in the secretory pathway.

Reduced hepatic metallothionein expression in first trimester fetuses in response to intrauterine smoking exposure: a consequence of low maternal zinc levels?

STUDY QUESTION

Does maternal smoking in early pregnancy affect metallothionein 1 and 2 (MT1 and MT2) mRNA and protein expression in first trimester placenta or embryonic/fetal liver?

SUMMARY ANSWER

In the first trimester, MT protein expression is seen only in liver, where smoking is associated with a significantly reduced expression.

WHAT IS KNOWN ALREADY

Zinc homeostasis is altered by smoking. Smoking induces MT in the blood of smokers properly as a result of the cadmium binding capacities of MT. In term placenta MT is present and smoking induces gene and protein expression (MT2 in particular), but the MT presence and response to smoking have never been examined in first trimester placenta or embryonic/fetal tissues.

STUDY DESIGN, SIZE, DURATION

Cross sectional study where the presence of MT mRNA and protein was examined at the time of the abortion. The material was collected with informed consent after surgical intervention and frozen immediately. For protein expression analysis, liver tissue originating from smoking exposed n = 10 and unexposed n = 12 pregnancies was used. For mRNA expression analyses, placental tissue originating from smokers n = 19 and non-smokers n = 23 and fetal liver tissue from smoking exposed n = 16 and smoking unexposed pregnancies n = 13, respectively, were used.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Tissues were obtained from women who voluntarily and legally chose to terminate their pregnancy between gestational week 6 and 12. Western blot was used to determine the protein expression of MT, and real-time PCR was used to quantify the mRNA expression of MT2A and eight MT1 genes alongside the expression of key placental zinc transporters: zinc transporter protein-1 (ZNT1), Zrt-, Irt-related protein-8 and -14 (ZIP8 and ZIP14).

MAIN RESULTS AND THE ROLE OF CHANCE

A significant reduction in the protein expression of MT1/2 in liver tissue (P = 0.023) was found by western blot using antibodies detecting both MT forms. Overall, a similar tendency was observed on the mRNA level although not statistically significant. Protein expression was not present in placenta, but the mRNA regulation suggested a down regulation of MT as well. A suggested mechanism based on the known role of MT in zinc homeostasis could be that the findings reflect reduced levels of easily accessible zinc in the blood of pregnant smokers and hence a reduced MT response in smoking exposed fetal/embryonic tissues.

LIMITATIONS AND REASONS FOR CAUTION

Smoking was based on self-reports; however, our previous studies have shown high consistency regarding cotinine residues and smoking status. Passive smoking could interfere but was found mainly among smokers. The number of fetuses was limited, and other factors such as medication and alcohol might affect the findings. Information on alcohol was not consistently obtained, and we cannot exclude that it was more readily obtained from non-users. In the study, alcohol consumption was reported by a limited number (less than 1 out of 5) of women but with more smokers consuming alcohol. However, the alcohol consumption reported was typically limited to one or few times low doses. The interaction between alcohol and smoking is discussed in the paper. Notably we would have liked to measure zinc status to test our hypothesis, but maternal blood samples were not available.

WIDER IMPLICATIONS OF THE FINDINGS

Zinc deficiency-in particular severe zinc deficiency-can affect pregnancy outcome and growth. Our findings indicate that zinc homeostasis is also affected in early pregnancy of smokers, and we know from pilot studies that even among women who want to keep their babies, the zinc status is low. Our findings support that zinc supplements should be considered in particular to women who smoke.

STUDY FUNDING/COMPETING INTEREST(S)

We thank the Department of Biomedicine for providing laboratory facilities and laboratory technicians and the Lundbeck Foundation and Læge Sofus Carl Emil Friis og Hustru Olga Doris Friis Legat for financial support. The authors have no competing interests to declare.

TRIAL REGISTRATION NUMBER

N/A.

Effect of Zinc Nanoparticles on Milk Yield, Milk Composition and Somatic Cell Count in Early-Lactating Barbari Does

Nano minerals have better bioavailability, efficient utilization, smaller dose rate, and stable interactions within the biological system. Efficacy of dietary supplementation of nano zinc particles on milk yield, milk composition, and somatic cell count was studied in periparturient Barbari does. Twenty-four Barbari goats (mean body weight 35 ± 1.5 kg) of about 4 months of pregnancy were divided into four groups (Gr I, Gr II, Gr III, and Gr IV) of six animals each. All the does were fed a common basal diet (containing 38.96 mg Zn/kg DM) of concentrate pellet and Bengal gram (Cicer arietinum) straw. No zinc was supplemented in does of Gr I (control group), while Gr II, Gr III, and Gr IV does were supplemented with 20 ppm inorganic zinc oxide, 10 ppm of nano zinc, and 20 ppm of nano zinc respectively. The experimental feeding was done for 75 days (30 days before kidding to 45 days after kidding). Results showed that yield and composition of milk in different groups of lactating does were not significantly (P < 0.05) affected by zinc supplementation. The percentages of solid-nonfat, fat, protein, lactose, and total solid were comparable (P > 0.05) among different groups of lactating does. The milk mineral levels of Zn, Cu, Na, Ca, and K were statistically similar (P > 0.05) in all the groups of goats. There was significantly (P < 0.05) lower somatic cell count (SCC) in zinc-supplemented groups (Gr II, Gr III, and Gr IV) as compared with the control (Gr I) group of does. SCC was lowest in Gr IV followed by Gr III, Gr II, and Gr I. The present study concluded that supplementation of inorganic zinc @ 20 ppm and nano zinc @ 10 and 20 ppm had no significant effect on milk yield/composition but reduced milk somatic cell count. The efficacy of nano zinc was found to be better as compared with normal inorganic form.

A common genetic variant in zinc transporter ZnT2 (Thr288Ser) is present in women with low milk volume and alters lysosome function and cell energetics

Suboptimal lactation is a common, yet underappreciated cause for early cessation of breastfeeding. Molecular regulation of mammary gland function is critical to the process lactation; however, physiological factors underlying insufficient milk production are poorly understood. The zinc (Zn) transporter ZnT2 is critical for regulation of mammary gland development and maturation during puberty, lactation, and postlactation gland remodeling. Numerous genetic variants in the gene encoding ZnT2 (SLC30A2) are associated with low milk Zn concentration and result in severe Zn deficiency in exclusively breastfed infants. However, the functional impacts of genetic variation in ZnT2 on key mammary epithelial cell functions have not yet been systematically explored at the cellular level. Here we determined a common mutation in SLC30A2/ZnT2 substituting serine for threonine at amino acid 288 (Thr288Ser) was found in 20% of women producing low milk volume (n = 2/10) but was not identified in women producing normal volume. Exploration of cellular consequences in vitro using phosphomimetics showed the serine substitution promoted preferential phosphorylation of ZnT2, driving localization to the lysosome and increasing lysosome biogenesis and acidification. While the substitution did not initiate lysosome-mediated cell death, cellular ATP levels were significantly reduced. Our findings demonstrate the Thr288Ser mutation in SLC30A2/ZnT2 impairs critical functions of mammary epithelial cells and suggest a role for genetic variation in the regulation of milk production and lactation performance.

Dietary zinc deficiency or supplementation during gestation increases breast cancer susceptibility in adult female mice offspring following a J-shaped pattern and through distinct mechanisms

Zinc is required for fetal development and is involved in key processes associated with breast carcinogenesis. We evaluated whether maternal zinc deficiency or supplementation during gestation influences female offspring susceptibility to breast cancer in adulthood. C57BL/6 mice consumed during gestation control (30 p.p.m. zinc), zinc-deficient (8 p.p.m) or zinc-supplemented (45 p.p.m.) diets. Maternal zinc supplementation increased in female mice offspring the incidence of chemically-induced mammary adenocarcinomas that were heavier, compared to control group. This was accompanied by a decreased number of terminal end buds, increased cell proliferation and apoptosis, and increased tumor suppressors p21, p53 and Rassf1, Zfp382 and Stat3 expression in mammary glands, as well as increased zinc status. Although maternal zinc deficiency did not alter the incidence of these lesions, it also induced heavier mammary adenocarcinomas, compared to control group. These effects were accompanied by a decreased number of terminal end buds, increased proto-oncogenes c-Myc and Lmo4 expression and H3K9Me3 and H4K20Me3 epigenetic marks in mammary glands of offspring, and decreased zinc status and increased levels of oxidative marker malondialdehyde. The data suggest that both maternal zinc deficiency and supplementation during gestation programmed increased breast cancer susceptibility in adult mice offspring following a J-shaped pattern through distinct mechanisms.

Mast cells play role in wound healing through the ZnT2/GPR39/IL-6 axis

Zinc (Zn) is an essential nutrient and its deficiency causes immunodeficiency and skin disorders. Various cells including mast cells release Zn-containing granules when activated; however, the biological role of the released Zn is currently unclear. Here we report our findings that Zn transporter ZnT2 is required for the release of Zn from mast cells. In addition, we found that Zn and mast cells induce IL-6 production from inflammatory cells such as skin fibroblasts and promote wound healing, a process that involves inflammation. Zn induces the production of a variety of pro-inflammatory cytokines including IL-6 through signaling pathways mediated by the Zn receptor GPR39. Consistent with these findings, wound healing was impaired in mice lacking IL-6 or GPR39. Thus, our results show that Zn and mast cells play a critical role in wound healing through activation of the GPR39/IL-6 signaling axis.

ZnR/GPR39 upregulation of K+/Cl--cotransporter 3 in tamoxifen resistant breast cancer cells

Expression of the zinc receptor, ZnR/GPR39, is increased in higher grade breast cancer tumors and cells. Zinc, its ligand, is accumulated at larger concentrations in the tumor tissue and can therefore activate ZnR/GPR39-dependent Ca²⁺ signaling leading to tumor progression. The K⁺/Cl^- co-transporters (KCC), activated by intracellular signaling, enhance breast cancer cell migration and invasion. We asked if ZnR/GPR39 enhances breast cancer cell malignancy by activating KCC. Activation of ZnR/GPR39 by Zn²⁺ upregulated K⁺/Cl^- co-transport activity, measured using NH₄⁺ as a surrogate to K⁺ while monitoring intracellular pH. Upregulation of NH₄⁺ transport was monitored in tamoxifen resistant cells with functional ZnR/GPR39-dependent Ca²⁺ signaling but not in MCF-7 cells lacking this response. The NH₄⁺ transport was Na⁺-independent, and we therefore focused on KCC family members. Silencing of KCC3, but not KCC4, expression abolished Zn²⁺-dependent K⁺/Cl^- co-transport, suggesting that KCC3 is mediating upregulated NH₄⁺ transport. The ZnR/GPR39-dependent KCC3 activation accelerated scratch closure rate, which was abolished by inhibiting KCC transport with [(DihydroIndenyl) Oxy] Alkanoic acid (DIOA). Importantly, silencing of either ZnR/GPR39 or KCC3 attenuated Zn²⁺-dependent scratch closure. Thus, a novel link between KCC3 and Zn²⁺, via ZnR/GPR39, promotes breast cancer cell migration and proliferation.

The effects of a lipid-based nutrient supplement and antiretroviral therapy in a randomized controlled trial on iron, copper, and zinc in milk from HIV-infected Malawian mothers and associations with maternal and infant biomarkers

We evaluated effects of antiretroviral (ARV) therapy and lipid‐based nutrient supplements (LNSs) on iron, copper, and zinc in milk of exclusively breastfeeding HIV‐infected Malawian mothers and their correlations with maternal and infant biomarkers. Human milk and blood at 2, 6, and 24 weeks post‐partum and blood during pregnancy (≤30 weeks gestation) were collected from 535 mothers/infant‐pairs in the Breastfeeding, Antiretrovirals, and Nutrition study. The participants received ARV, LNS, ARV and LNS, or no intervention from 0 to 28 weeks post‐partum. ARVs negatively affected copper and zinc milk concentrations, but only at 2 weeks, whereas LNS had no effect. Among all treatment groups, approximately 80–90% of copper and zinc and <50% of iron concentrations met the current adequate intake for infants at 2 weeks and only 1–19% at 24 weeks. Pregnancy haemoglobin was negatively correlated with milk iron at 2 and 6 weeks (r = −.18, p < .02 for both). The associations of the milk minerals with each other were the strongest correlations observed (r = .11–.47, p < .05 for all); none were found with infant biomarkers. At 2 weeks, moderately anaemic women produced milk higher in iron when ferritin was higher or TfR lower. At 6 weeks, higher maternal α‐1‐acid glycoprotein and C‐reactive protein were associated with higher milk minerals in mildly anaemic women. Infant TfR was lower when milk mineral concentrations were higher at 6 weeks and when mothers were moderately anaemic during pregnancy. ARV affects copper and zinc milk concentrations in early lactation, and maternal haemoglobin during pregnancy and lactation could influence the association between milk minerals and maternal and infant iron status and biomarkers of inflammation.

The emerging role of zinc transporters in cellular homeostasis and cancer

Zinc is an essential micronutrient that plays a role in the structural or enzymatic functions of many cellular proteins. Cellular zinc homeostasis involves the opposing action of two families of metal transporters: the ZnT (SLC30) family that functions to reduce cytoplasmic zinc concentrations and the ZIP (SLC39) family that functions to increase cytoplasmic zinc concentrations. Fluctuations in intracellular zinc levels mediated by these transporter families affect signaling pathways involved in normal cell development, growth, differentiation and death. Consequently, changes in zinc transporter localization and function resulting in zinc dyshomeostasis have pathophysiological effects. Zinc dyshomeostasis has been implicated in the progression of cancer. Here we review recent progress toward understanding the structural basis for zinc transport by ZnT and ZIP family proteins, as well as highlight the roles of zinc as a signaling molecule in physiological conditions and in various cancers. As zinc is emerging as an important signaling molecule in the development and progression of cancer, the ZnT and ZIP transporters that regulate cellular zinc homeostasis are promising candidates for targeted cancer therapy.

The mucolipin-1 (TRPML1) ion channel, transmembrane-163 (TMEM163) protein, and lysosomal zinc handling

Lysosomes are emerging as important players in cellular zinc ion (Zn²⁺) homeostasis. The series of work on Zn2+ accumulation in the neuronal lysosomes and the mounting evidence on the role of lysosomal Zn²⁺ in cell death during mammary gland involution set a biological precedent for the central role of the lysosomes in cellular Zn²⁺ handling. Such a role appears to involve cytoprotection on the one hand, and cell death on the other. The recent series of work began to identify the molecular determinants of the lysosomal Zn²⁺ handling. In addition to zinc transporters (ZnT) of the solute-carrier family type 30A (SLC30A), the lysosomal ion channel TRPML1 and the poorly understood novel transporter TMEM163 have been shown to play a role in the Zn²⁺ uptake by the lysosomes. In this review, we summarize the current knowledge on molecular determinants of the lysosomal Zn²⁺ handling, uptake, and release pathways, as well as discuss their possible roles in health and disease.

Biological underpinnings of breastfeeding challenges: the role of genetics, diet, and environment on lactation physiology

Lactation is a dynamic process that has evolved to produce a complex biological fluid that provides nutritive and nonnutritive factors to the nursing offspring. It has long been assumed that once lactation is successfully initiated, the primary factor regulating milk production is infant demand. Thus, most interventions have focused on improving breastfeeding education and early lactation support. However, in addition to infant demand, increasing evidence from studies conducted in experimental animal models, production animals, and breastfeeding women suggests that a diverse array of maternal factors may also affect milk production and composition. In this review, we provide an overview of our current understanding of the role of maternal genetics and modifiable factors, such as diet and environmental exposures, on reproductive endocrinology, lactation physiology, and the ability to successfully produce milk. To identify factors that may affect lactation in women, we highlight some information gleaned from studies in experimental animal models and production animals. Finally, we highlight the gaps in current knowledge and provide commentary on future research opportunities aimed at improving lactation outcomes in breastfeeding women to improve the health of mothers and their infants.

Alpha-ketoglutarate enhances milk protein synthesis by porcine mammary epithelial cells

Alpha-ketoglutarate (AKG), a key intermediate in the Krebs cycle, has been reported to promote protein synthesis through activating mechanistic targeting of rapamycin (mTOR) in enterocytes. The study tested the hypothesis that AKG may enhance growth and milk protein synthesis in porcine mammary epithelial cells (PMECs). PMECs were cultured for 96 h in Dulbecco's modified Eagle's-F12 Ham medium (DMEM-F12) containing prolactin (2 µg/ml) and AKG (0 or 1.5 mM). At the end of 96-h culture, the abundance of apoptosis-related proteins (caspase-3, caspase-9), milk-specific proteins (α-lactalbumin and β-casein), mTOR signaling proteins (mTOR, p-mTOR, PERK, p-PERK, eIF2a, P70S6K and p-P70S6K), and endoplasmic reticulum stress (ERS)-associated proteins (BiP and CHOP) in PMEC were determined. Addition of AKG dose-dependently enhanced cell viability in the absence or presence of prolactin, with optimal concentrations of AKG being at 1.0 and 1.5 mM, respectively. In the presence of prolactin, addition of 1.5 mM AKG: (1) decreased (P < 0.05) the abundance of caspase-3 and caspase-9 by 21 and 39 %; (2) enhanced (P < 0.05) the phosphorylation of p-mTOR and p-P70S6K by 39 and 89 %, respectively; (3) increased (P < 0.05) the production of β-casein and α-lactalbumin by 16 and 20 %, respectively; (4) attenuated (P < 0.05) the expression of CHOP by 34 % but promoted (P < 0.05) the expression of BiP by 46 %; (5) increased (P < 0.05) the secretion of lactose by 15 %, when compared to the 0 mM AKG group. Rapamycin (50 nM; an inhibitor of mTOR) attenuated (P < 0.05) the stimulatory effect of AKG on mTOR signaling and syntheses of milk protein and lactose, while relieving (P < 0.05) an inhibitory effect of AKG on expression of proteins related to ERS. Collectively, our results indicate that AKG enhances milk protein production by modulating mTOR and ERS signaling pathways in PMECs.

Molecular Basis of Transient Neonatal Zinc Deficiency: NOVEL ZnT2 MUTATIONS DISRUPTING ZINC BINDING AND PERMEATION

A gradually increasing number of transient neonatal zinc deficiency (TNZD) cases was recently reported, all of which were associated with inactivating ZnT2 mutations. Here we characterized the impact of three novel heterozygous ZnT2 mutations G280R, T312M, and E355Q, which cause TNZD in exclusively breastfed infants of Japanese mothers. We used the bimolecular fluorescence complementation (BiFC) assay to provide direct visual evidence for the in situ dimerization of these ZnT2 mutants, and to explore their subcellular localization. Moreover, using three complementary functional assays, zinc accumulation using BiFC-Zinquin and Zinpyr-1 fluorescence as well as zinc toxicity assay, we determined the impact of these ZnT2 mutations on vesicular zinc accumulation. Although all three mutants formed homodimers with the wild type (WT) ZnT2 and retained substantial vesicular localization, as well as vesicular zinc accumulation, they had no dominant-negative effect over the WT ZnT2. Furthermore, using advanced bioinformatics, structural modeling, and site-directed mutagenesis we found that these mutations localized at key residues, which play an important physiological role in zinc coordination (G280R and E355Q) and zinc permeation (T312M). Collectively, our findings establish that some heterozygous loss of function ZnT2 mutations disrupt zinc binding and zinc permeation, thereby suggesting a haploinsufficiency state for the unaffected WT ZnT2 allele in TNZD pathogenesis. These results highlight the burning need for the development of a suitable genetic screen for the early diagnosis of TNZD to prevent morbidity.

ROS and intracellular ion channels

Oxidative stress is a well-known driver of numerous pathological processes involving protein and lipid peroxidation and DNA damage. The resulting increase of pro-apoptotic pressure drives tissue damage in a host of conditions, including ischemic stroke and reperfusion injury, diabetes, death in acute pancreatitis and neurodegenerative diseases. Somewhat less frequently discussed, but arguably as important, is the signaling function of oxidative stress stemming from the ability of oxidative stress to modulate ion channel activity. The evidence for the modulation of the intracellular ion channels and transporters by oxidative stress is constantly emerging and such evidence suggests new regulatory and pathological circuits that can be explored towards new treatments for diseases in which oxidative stress is an issue. In this review we summarize the current knowledge on the effects of oxidative stress on the intracellular ion channels and transporters and their role in cell function.

Marginal zinc intake reduces the protective effect of lactation on mammary gland carcinogenesis in a DMBA-induced tumor model in mice

Breastfeeding can reduce breast cancer risk; however, unknown factors modify this protective effect. Zinc (Zn) modulates an array of cellular functions including oxidative stress, cell proliferation, motility and apoptosis. Marginal Zn intake is common in women and is associated with breast cancer. We reported that marginal Zn intake in mice leads to mammary gland hypoplasia and hallmarks of pre-neoplastic lesions. In the present study, we tested the hypothesis that marginal Zn intake confounds the protective effect of lactation on breast cancer. Nulliparous mice fed control (ZA, 30 mg Zn/kg) or a marginal Zn diet (ZD, 15 mg Zn/kg), were bred and offspring were weaned naturally. Post-involution, mice were gavaged with corn oil or 7,12-dimethylbenz(a)anthracene (DMBA, 1 mg/wk for 4 weeks) and tumor development was monitored. A ZD diet led to insufficient involution, increased fibrosis and oxidative stress. Following DMBA treatment, mice fed ZD had higher oxidative stress in mammary tissue that correlated with reduced levels of peroxiredoxin-1 and p53 and tended to have shorter tumor latency and greater incidence of non-palpable tumors. In summary, marginal Zn intake creates a toxic mammary gland microenvironment and abrogates the protective effect of lactation on carcinogenesis.

Brief exposure to copper activates lysosomal exocytosis

Copper (Cu) is essential mineral, but its toxicity necessitates existence of powerful machinery responsible for the extraction of excess Cu from the cell. Cu exposure was recently shown to induce the translocation of Cu pump ATP7B to the lysosomes followed by lysosomal exocytosis. Here we sought to investigate the mechanisms underlying the effect of Cu on lysosomal exocytosis. We found that brief exposure to Cu activates lysosomal exocytosis, which was measured as a release of the lysosomal digestive enzyme β-hexosaminidase (β-hex) into the extracellular medium and by the presence lysosomal protein LAMP1 at the plasma membrane. Such release depends on calcium (Ca) and on the lysosomal SNARE VAMP7. ATP7B knockdown using RNAi suppressed the basal lysosomal exocytosis, but did not affect the ability of Cu to activate it. ATP7B knockdown was associated with sustained oxidative stress. The removal of Ca from the extracellular medium suppressed the Cu-dependent component of the lysosomal exocytosis. We propose that Cu promotes lysosomal exocytosis by facilitating a Ca-dependent step of the lysosomal exocytosis.

Genetic causes and gene–nutrient interactions in mammalian zinc deficiencies: acrodermatitis enteropathica and transient neonatal zinc deficiency as examples

Discovering genetic causes of zinc deficiency has been a remarkable scientific journey. It started with the description of a rare skin disease, its treatment with various agents, the successful therapy with zinc, and the identification of mutations in a zinc transporter causing the disease. The journey continues with defining the molecular and cellular pathways that lead to the symptoms caused by zinc deficiency. Remarkably, at least two zinc transporters from separate protein families are now known to be involved in the genetics of zinc deficiency. One is ZIP4, which is involved in intestinal zinc uptake. Its mutations can cause acrodermatitis enteropathica (AE) with autosomal recessive inheritance. The other one is ZnT2, the transporter responsible for supplying human milk with zinc. Mutations in this transporter cause transient neonatal zinc deficiency (TNZD) with symptoms similar to AE but with autosomal dominant inheritance. The two diseases can be distinguished in affected infants. AE is fatal if zinc is not supplied to the infant after weaning, whereas TNZD is a genetic defect of the mother limiting the supply of zinc in the milk, and therefore the infant usually will obtain enough zinc once weaned. Although these diseases are relatively rare, the full functional consequences of the numerous mutations in ZIP4 and ZnT2 and their interactions with dietary zinc are not known. In particular, it remains unexplored whether some mutations cause milder disease phenotypes or increase the risk for other diseases if dietary zinc requirements are not met or exceeded. Thus, it is not known whether widespread zinc deficiency in human populations is based primarily on a nutritional deficiency or determined by genetic factors as well. This consideration becomes even more significant with regard to mutations in the other 22 human zinc transporters, where associations with a range of diseases, including diabetes, heart disease, and mental illnesses have been observed. Therefore, clinical tests for genetic disorders of zinc metabolism need to be developed.

Paradoxical zinc toxicity and oxidative stress in the mammary gland during marginal dietary zinc deficiency

Zinc (Zn) regulates numerous cellular functions. Zn deficiency is common in females; ∼80% of women and 40% of adolescent girls consume inadequate Zn. Zn deficiency enhances oxidative stress, inflammation and DNA damage. Oxidative stress and inflammation is associated with breast disease. We hypothesized that Zn deficiency increases oxidative stress in the mammary gland, altering the microenvironment and architecture. Zn accumulated in the mammary glands of Zn deficient mice and this was associated with macrophage infiltration, enhanced oxidative stress and over-expression of estrogen receptor α. Ductal and stromal hypercellularity was associated with aberrant collagen deposition and disorganized e-cadherin. Importantly, these microenvironmental alterations were associated with substantial impairments in ductal expansion and mammary gland development. This is the first study to show that marginal Zn deficiency creates a toxic microenvironment in the mammary gland impairing breast development. These changes are consistent with hallmarks of potential increased risk for breast disease and cancer.