XBP1 Regulates the Biosynthetic Capacity of the Mammary Gland During Lactation by Controlling Epithelial Expansion and Endoplasmic Reticulum Formation

Loss of STING impairs lactogenic differentiation

Heightened energetic and nutrient demand during lactogenic differentiation of the mammary gland elicits upregulation of various stress responses to support cellular homeostasis. Here, we identify the stimulator of interferon genes (STING) as an immune supporter of the functional development of mouse mammary epithelial cells (MECs). An in vitro model of MEC differentiation revealed that STING is activated in a cGAS-independent manner to produce both type I interferons and proinflammatory cytokines in response to the accumulation of mitochondrial reactive oxygen species. Induction of STING activity was found to be dependent on the breast tumor suppressor gene single-minded 2 (SIM2). Using mouse models of lactation, we discovered that loss of STING activity results in early involution of #3 mammary glands, severely impairing lactational performance. Our data suggest that STING is required for successful functional differentiation of the mammary gland and bestows a differential lactogenic phenotype between #3 mammary glands and the traditionally explored inguinal 4|9 pair. These findings affirm unique development of mammary gland pairs that is essential to consider in future investigations into normal development and breast cancer initiation.

Nuclear Factor Erythroid 2-Related Factor 1 Regulates the Expression of Proteasomal Genes in Ketotic Cows and Protects Mammary Cells Against Free Fatty Acid-Induced Endoplasmic Reticulum Stress

Ketosis is a common metabolic disorder in high-yielding cows and is characterized by high concentrations of BHB and free fatty acids (FFA). High concentrations of FFA induce endoplasmic reticulum (ER) stress in multiple organs including mammary tissue, and result in reduced milk production and lower milk quality. In non-ruminants, loss of nuclear factor erythroid 2-related factor 1 (NFE2L1) results in ER stress. The physiological functions and molecular mechanisms controlled by NFE2L1 in bovine mammary tissue are poorly understood. Thus, the present study aimed to elucidate the role of the NFE2L1 on proteasomal homeostasis and ER stress in mammary tissue from early-lactation (DIM 6 to 14) healthy cows (CON, blood concentration of BHB <1.2 mM, n = 10) and cows with clinical ketosis (CK blood concentration of BHB >3 mM, n = 10). Compared with CON, serum concentration of glucose was lower due to CK, while serum concentrations of BHB and FFA were greater. Protein and mRNA abundance of NFE2L1 along with abundance of proteasomal subunits (PSMD1, PSMD14, PSMA1, PSMB1, and PSMB5 genes and PSMB4 and PSMB6 proteins) were lower in cows with CK, indicating that expression of NFE2L1 and proteasomal homeostasis was impaired by ketosis. In vitro, primary bovine mammary epithelial cells were exposed to various concentrations of FFA (0, 0.3, 0.6, or 1.2 mM). Compared with the 0 mM FFA, the ratio of phosphorylated (p)-protein kinase R-like ER kinase (PERK)/PERK along with the expression of inositol-requiring enzyme 1 (IRE1) α, activating transcription factor 6 (ATF6), glucose regulated protein 78 (GRP78), and C/EBP homologous protein (CHOP) was higher with 1.2 mM FFA. A similar response was observed for ER stress-associated genes (CHOP, GRP78, and XBP1) indicating that high concentrations of FFA induced ER stress. In line with in vivo results, 1.2 mM FFA downregulated the protein and mRNA abundance of NFE2L1, the abundance of PSMB6 protein, and PSM genes (PSMC1, PSMC3, and PSMD1), and increased the accumulation of ubiquitin. This suggested a marked negative effect of high FFA on NFE2L1 and proteasomal homeostasis. Silencing of NFE2L1 triggered upregulation of ER stress-associated genes as well as protein abundance of GRP78 and CHOP. Further, compared with CON-siRNA, the abundance of PSM genes was downregulated in the NFE2L1-siRNA group. In contrast, abundance of markers of ER stress and PSM genes and proteins indicated that overexpression of NFE2L1 relieved the FFA-induced ER stress and improved 26S proteasome homeostasis. Our data suggested that the mammary gland experiences ER stress during ketosis partly due to disruption of proteasomal homeostasis from the excess FFA. As such, NFE2L1 could represent a target for potential therapeutic applications in the field to alleviate the accumulation of malformed proteins that may impair the long-term lactogenic capacity of the udder.

Immune Cell Contribution to Mammary Gland Development

Postpartum breast cancer (PPBC) is a unique subset of breast cancer, accounting for nearly half of the women diagnosed during their postpartum years. Mammary gland involution is widely regarded as being a key orchestrator in the initiation and progression of PPBC due to its unique wound-healing inflammatory signature. Here, we provide dialogue suggestive that lactation may also facilitate neoplastic development as a result of sterile inflammation. Immune cells are involved in all stages of postnatal mammary development. It has been proposed that the functions of these immune cells are partially directed by mammary epithelial cells (MECs) and the cytokines they produce. This suggests that a more niche area of exploration aimed at assessing activation of innate immune pathways within MECs could provide insight into immune cell contributions to the developing mammary gland. Immune cell contribution to pubertal development and mammary gland involution has been extensively studied; however, investigations into pregnancy and lactation remain limited. During pregnancy, the mammary gland undergoes dramatic expansion to prepare for lactation. As a result, MECs are susceptible to replicative stress. During lactation, mitochondria are pushed to capacity to fulfill the high energetic demands of producing milk. This replicative and metabolic stress, if unresolved, can elicit activation of innate immune pathways within differentiating MECs. In this review, we broadly discuss postnatal mammary development and current knowledge of immune cell contribution to each developmental stage, while also emphasizing a more unique area of study that will be beneficial in the discovery of novel therapeutic biomarkers of PPBC.

Milk fat globule size: Unraveling the intricate relationship between metabolism, homeostasis, and stress signaling

Fat is an important component of milk which delivers energy, nutrients, and bioactive molecules from the lactating mother to the suckling neonate. Milk fat consists of a complex mixture of different types of lipids; hundreds of fatty acids, triglycerides, phospholipids, sphingolipids, cholesterol and cholesteryl ester, and glycoconjugates, secreted by the mammary gland epithelial cells (MEC) in the form of a lipid-protein assembly termed the milk fat globule (MFG). The mammary gland in general, and specifically that of modern dairy cows, faces metabolic stress once lactation commences, which changes the lipogenic capacity of MECs directly by reducing available energy and reducing factors required for both lipid synthesis and secretion or indirectly by activating a proinflammatory response. Both processes have the capacity to change the morphometric features (e.g., number and size) of the secreted MFG and its precursor-the intracellular lipid droplet (LD). The MFG size is tightly associated with its lipidome and proteome and also affects the bioavailability of milk fat and protein. Thus, MFG size has the potential to regulate the bioactivity of milk and dairy products. MFG size also plays a central role in the functional properties of milk and dairy products such as texture and stability. To understand how stress affects the structure-function of the MFG, we cover: (i) The mechanism of production and secretion of the MFG and the implications of MFG size, (ii) How the response mechanisms to stress can change the morphometric features of MFGs, and (iii) The possible consequences of such modifications.

SIM2s directed Parkin-mediated mitophagy promotes mammary epithelial cell differentiation

The functionally differentiated mammary gland adapts to extreme levels of stress from increased demand for energy by activating specific protective mechanisms to support neonatal health. Here, we identify the breast tumor suppressor gene, single-minded 2 s (SIM2s) as a novel regulator of mitophagy, a key component of this stress response. Using tissue-specific mouse models, we found that loss of Sim2 reduced lactation performance, whereas gain (overexpression) of Sim2s enhanced and extended lactation performance and survival of mammary epithelial cells (MECs). Using an in vitro model of MEC differentiation, we observed SIM2s is required for Parkin-mediated mitophagy, which we have previously shown as necessary for functional differentiation. Mechanistically, SIM2s localizes to mitochondria to directly mediate Parkin mitochondrial loading. Together, our data suggest that SIM2s regulates the rapid recycling of mitochondria via mitophagy, enhancing the function and survival of differentiated MECs.

Neuronal-epithelial cross-talk drives acinar specification via NRG1-ERBB3-mTORC2 signaling

Acinar cells are the principal secretory units of multiple exocrine organs. A single-cell, layered, lumenized acinus forms from a large cohort of epithelial progenitors that must initiate and coordinate three cellular programs of acinar specification, namely, lineage progression, secretion, and polarization. Despite this well-known outcome, the mechanism(s) that regulate these complex programs are unknown. Here, we demonstrate that neuronal-epithelial cross-talk drives acinar specification through neuregulin (NRG1)-ERBB3-mTORC2 signaling. Using single-cell and global RNA sequencing of developing murine salivary glands, we identified NRG1-ERBB3 to precisely overlap with acinar specification during gland development. Genetic deletion of Erbb3 prevented cell lineage progression and the establishment of lumenized, secretory acini. Conversely, NRG1 treatment of isolated epithelia was sufficient to recapitulate the development of secretory acini. Mechanistically, we found that NRG1-ERBB3 regulates each developmental program through an mTORC2 signaling pathway. Thus, we reveal that a neuronal-epithelial (NRG1/ERBB3/mTORC2) mechanism orchestrates the creation of functional acini.

Organelles coordinate milk production and secretion during lactation: Insights into mammary pathologies

The mammary gland undergoes a spectacular series of changes during its development and maintains a remarkable capacity to remodel and regenerate during progression through the lactation cycle. This flexibility of the mammary gland requires coordination of multiple processes including cell proliferation, differentiation, regeneration, stress response, immune activity, and metabolic changes under the control of diverse cellular and hormonal signaling pathways. The lactating mammary epithelium orchestrates synthesis and apical secretion of macromolecules including milk lipids, milk proteins, and lactose as well as other minor nutrients that constitute milk. Knowledge about the subcellular compartmentalization of these metabolic and signaling events, as they relate to milk production and secretion during lactation, is expanding. Here we review how major organelles (endoplasmic reticulum, Golgi apparatus, mitochondrion, lysosome, and exosome) within mammary epithelial cells collaborate to initiate, mediate, and maintain lactation, and how study of these organelles provides insight into options to maintain mammary/breast health.

TRB3 Deletion Has a Limited Effect on Milk Fat Synthesis and Milk Fat Depression in C57BL/6N Mice

BACKGROUND

Regulation of the endoplasmic reticulum (ER) stress pathway is critical to mammary epithelial cell function throughout pregnancy, lactation, and involution. Treatment with trans-10, cis-12 conjugated linoleic acid (t10c12CLA) suppresses mammary lipogenesis and stimulates the ER stress pathway. The ER stress pathway includes tribbles pseudokinase 3 (TRB3), a protein that regulates cellular energy and insulin signaling.

OBJECTIVES

Our objective was to describe the effect of TRB3 deficiency on milk fat synthesis and determine if TRB3 deficiency protects against suppression of mammary lipogenesis.

METHODS

First, mammary Trb3 expression was observed throughout pregnancy and lactation using ancillary microarray data (n = 4/time point). Second, intake, litter growth, and milk clot fatty acid (FA) profile of Trb3 knockout (KO) C57BL/6N mice were compared with wild-type (WT) and heterozygous (HET) mice throughout first (n ≥ 8/group) and second (n ≥ 6/group) lactation. Lastly, the interaction between Trb3 genotype and 2 treatments that suppress mammary lipogenesis, t10c12CLA and high safflower oil (HO) diet, was investigated in a 2 × 2 factorial design (n ≥ 6/group).

RESULTS

Trb3 expression was higher during late pregnancy and lactation. Trb3 KO and HET mice had lower feed intake, dam weight, and litter growth throughout first, but not second, lactation than WT mice. Treatment with t10c12CLA decreased litter growth (28%; P < 0.0001) and feed intake (8%; P < 0.0001) regardless of Trb3 genotype. When fed the HO diet, Trb3 KO mice had 17% higher mammary de novo synthesized FAs (<16 carbons; P int = 0.002) than WT mice. Mammary ER stress and lipogenic genes were mostly unaltered by Trb3 deficiency.

CONCLUSIONS

Overall, TRB3 plays a minor role in regulating mammary lipogenesis, because Trb3 deficiency had only a limited protective effect against diet-induced suppression of lipogenesis.

Immune and metabolic effects of rumen-protected methionine during a heat stress challenge in lactating Holstein cows

Multiparous, lactating Holstein cows (n = 32) were randomly assigned to one of two dietary treatments [TMR with rumen-protected Met (RPM) or TMR without RPM (CON)], and within each dietary treatment group cows were randomly assigned to one of two environmental treatment groups in a split-plot crossover design. In phase 1 (9 d), all cows were fed ad libitum and in thermoneutral conditions (TN). In phase 2 (9 d), group 1 (n = 16) was exposed to a heat stress (HS) challenge (HSC). Group 2 cows (n = 16) were pair-fed (PFTN) to HSC counterparts and remained in TN. After a 21-d washout period, the study was repeated (period 2) and the environmental treatments were inverted relative to treatments from phase 2 of period 1, while dietary treatments remained the same for each cow. During phase 1, cows in RPM had greater plasma Met concentration compared with cows in CON (59 and 30 µM, respectively; P < 0.001). Cows in PFTN had a greater decrease (P < 0.05) in plasma insulin than cows in HSC at 4 h (-2.7 µIU/mL vs. -0.7 µIU/mL) and 8 h (-7.7 µIU/mL vs. -0.4 µIU/mL) during phase 2. Compared with cows in PFTN, cows in HSC had an increase (P < 0.05) in plasma serum amyloid A (-59 µg/mL vs. +58 µg/mL), serum haptoglobin (-3 µg/mL vs. +33 µg/mL), plasma lipopolysaccharide binding protein (-0.27 and +0.11 µg/mL), and plasma interleukin-1β (-1.9 and +3.9 pg/mL) during phase 2. In conclusion, HSC elicited immunometabolic alterations; however, there were limited effects of RPM on cows in HSC.

IGF-1 induces IRE1-XBP1-dependent endoplasmic reticulum biogenesis in bovine mammary epithelial cells

Insulin-like growth factor-1 (IGF-1) plays a key role in proliferation and galactopoiesis in mammary epithelial cells (MEC), but its definitive functions on endoplasmic reticulum (ER) during protein synthesis remain unknown. The present study aimed to elucidate the effects of IGF-1 on ER biogenesis in MEC in vitro and examined the expression of ER biogenesis-associated genes in the mammary gland during early lactation. We treated mammary alveolar cells-large T antigen cells (immortalized bovine MEC line established via stable transfection with simian virus-40 large T-antigen) with IGF-1 and examined ER biogenesis using the fluorescence intensity of an ER tracker and quantitative real-time PCR. We found IGF-1 significantly increased ER tracker staining and upregulated mRNA levels of ER biogenesis-related genes, such as CHKA (choline kinase α), PCYT1A (choline-phosphate cytidylyltransferase A), and SURF4 (surfeit locus protein 4). We focused on unfolded protein response to explore molecular mechanisms by which IGF-1 induces ER biogenesis. We found IGF-1 significantly increased mRNA levels of the XBP1 splicing form (XBP1s). Based on western blot analysis, IGF-1 induced the expression of (inositol-requiring kinase 1 α) protein, upstream of XBP1s, and phosphorylated-IRE1α. The inhibition of IRE1 endoribonuclease activity with 4-methylumbelliferone 8-carbaldehyde (4μ8C) significantly suppressed the increase in ER tracker fluorescence and ER biogenesis-related gene expression induced by IGF-1. Also, IGF-1-induced XBP1s and ER biogenesis-associated gene expression was inhibited by rapamycin, an inhibitor of mTORC1 (mammalian target of rapamycin complex 1), indicating that IRE1-XBP1 activation by IGF-1 is mediated by mTORC1. Moreover, to clarify the expression of XBP1s and ER biogenesis-associated genes expression under normal physiological conditions, mammary gland tissue from biopsies of dairy cows during late gestation and lactation were analyzed. In vivo data highlighted the significant increases in the mRNA levels of XBP1s and ER biogenesis-related genes in mammary gland tissue immediately after calving through 6 wk of lactation. The mRNA levels of IGF1R (IGF-1 receptor) in mammary glands increased during 6 wk of lactation. Therefore, the present study indicated for the first time that IGF-1 induces ER biogenesis by activating the IRE1-XBP1 axis under the regulation of mTORC1 in bovine MEC line.

Impact of post-ruminally infused macronutrients on bovine mammary gland expression of genes involved in fatty acid synthesis, energy metabolism, and protein synthesis measured in RNA isolated from milk fat

Background

Characterising the regulation of milk component synthesis in response to macronutrient supply is critical for understanding the implications of nutritional interventions on milk production. Gene expression in mammary gland secretory cells was measured using RNA isolated from milk fat globules from 6 Holstein-Friesian cows receiving 5-d abomasal infusions of saline, essential amino acids (AA), or glucose (GG) or palm olein (LG) without (LAA) or with (HAA) essential AA, according to a 6 × 6 Latin square design. RNA was isolated from milk fat samples collected on d 5 of infusion and subjected to real-time quantitative PCR. We hypothesised that mRNA expression of genes involved in de novo milk fatty acid (FA) synthesis would be differently affected by GG and LG, and that expression of genes regulating transfer of tricarboxylic acid cycle intermediates would increase at the HAA level. We also hypothesised that the HAA level would affect genes regulating endoplasmic reticulum (ER) homeostasis but would not affect genes related to the mechanistic target of rapamycin complex 1 (mTORC1) or the integrated stress response (ISR) network.

Results

Infusion of GG did not affect de novo milk FA yield but decreased expression of FA synthase (FASN). Infusion of LG decreased de novo FA yield and tended to decrease expression of acetyl-CoA carboxylase 1 (ACC1). The HAA level increased both de novo FA yield and expression of ACC1, and tended to decrease expression of mitochondrial phosphoenolpyruvate carboxykinase (PCK2). mRNA expression of mTORC1 signaling participants was not affected by GG, LG, or AA level. Expression of the ε subunit of the ISR constituent eukaryotic translation initiation factor 2B (EIF2B5) tended to increase at the HAA level, but only in the presence of LG. X-box binding protein 1 (XBP1) mRNA was activated in response to LG and the HAA level.

Conclusions

Results show that expression of genes involved in de novo FA synthesis responded to glucogenic, lipogenic, and aminogenic substrates, whereas genes regulating intermediate flux through the tricarboxylic acid cycle were not majorly affected. Results also suggest that after 5 d of AA supplementation, milk protein synthesis is supported by enhanced ER biogenesis instead of signaling through the mTORC1 or ISR networks.

Expression of genes related to energy metabolism and the unfolded protein response in dairy cow mammary cells is affected differently during dietary supplementation with energy from protein and fat

Secretory capacity of bovine mammary glands is enabled by a high number of secretory cells and their ability to use a range of metabolites to produce milk components. We isolated RNA from milk fat to measure expression of genes involved in energy-yielding pathways and the unfolded protein response in mammary glands of lactating cows given supplemental energy from protein (PT) and fat (FT) tested in a 2 × 2 factorial arrangement. We hypothesized that PT and FT would affect expression of genes in the branched-chain AA catabolic pathway and tricarboxylic acid (TCA) cycle based on the different energy types (aminogenic versus lipogenic) used to synthesize milk components. We also hypothesized that the response of genes related to endoplasmic reticulum (ER) homeostasis via the unfolded protein response would reflect the increase in milk production stimulated by PT and FT. Fifty-six multiparous Holstein-Friesian dairy cows were fed a basal total mixed ration (34% grass silage, 33% corn silage, 5% grass hay, and 28% concentrate on a dry matter basis) for a 28-d control period. Experimental rations were then fed for 28 d, consisting of (1) low protein, low fat (LP/LF); (2) high protein, low fat (HP/LF); (3) low protein, high fat (LP/HF); or (4) high protein and high fat (HP/HF). To obtain the high-protein (HP) and high-fat (HF) diets, intake of the basal ration was restricted and supplemented isoenergetically (net energy basis) with 2.0 kg/d rumen-protected protein (soybean + rapeseed, 50:50 mixture on dry matter basis) and 0.68 kg/d hydrogenated palm fatty acids on a dry matter basis. RNA from milk fat samples collected on d 27 of each period underwent real-time quantitative PCR. Energy from protein increased expression of BCAT1 (branched-chain amino acid transferase 1) mRNA, but only at the LF level, and tended to decrease expression of mRNA encoding the main subunit of the branched-chain keto-acid dehydrogenase complex. mRNA expression of malic enzyme, a proposed channeling route for AA though the TCA cycle, was decreased by PT, but only at the LF level. Expression of genes associated with de novo fatty acid synthesis was not affected by PT or FT. Energy from fat had no independent effect on genes related to ER homeostasis. At the LF level, PT activated XBP1 (X-box binding protein 1) mRNA. At the HF level, PT increased mRNA expression of the gene encoding GADD34 (growth arrest and DNA damage-inducible 34). These findings support our hypothesis that mammary cells use aminogenic and lipogenic precursors differently for milk component production when dietary intervention alters AA and fatty acid supply. They also suggest that mammary cells respond to increased AA supply through mechanisms of ER homeostasis, dependent on the presence of FT.

Comprehensive profiling of transcriptional networks specific for lactogenic differentiation of HC11 mammary epithelial stem-like cells

The development of mammary gland as a lactogenic tissue is a highly coordinated multistep process. The epithelial cells of lactiferous tubules undergo profound changes during the developmental window of puberty, pregnancy, and lactation. Several hormones including estrogen, progesterone, glucocorticoids and prolactin act in concert, and orchestrate the development of mammary gland. Understanding the gene regulatory networks that coordinate proliferation and differentiation of HC11 Mammary Epithelial stem-like Cells (MEC) under the influence of lactogenic hormones is critical for elucidating the mechanism of lactogenesis in detail. In this study, we analyzed transcriptome profiles of undifferentiated MEC (normal) and compared them with Murine Embryonic Stem Cells (ESC) using next-generation mRNA sequencing. Further, we analyzed the transcriptome output during lactogenic differentiation of MEC following treatment with glucocorticoids (primed state) and both glucocorticoids and prolactin together (prolactin state). We established stage-specific gene regulatory networks in ESC and MEC (normal, priming and prolactin states). We validated the top up-and downregulated genes in each stage of differentiation of MEC by RT-PCR and found that they are comparable with that of RNA-seq data. HC11 MEC display decreased expression of Pou5f1 and Sox2, which is crucial for the differentiation of MEC, which otherwise ensure pluripotency to ESC. Cited4 is induced during priming and is involved in milk secretion. MEC upon exposure to both glucocorticoids and prolactin undergo terminal differentiation, which is associated with the expression of several genes, including Xbp1 and Cbp that are required for cell growth and differentiation. Our study also identified differential expression of transcription factors and epigenetic regulators in each stage of lactogenic differentiation. We also analyzed the transcriptome data for the pathways that are selectively activated during lactogenic differentiation. Further, we found that selective expression of chromatin modulators (Dnmt3l, Chd9) in response to glucocorticoids suggests a highly coordinated stage-specific lactogenic differentiation of MEC.

Do inositol supplements enhance phosphatidylinositol supply and thus support endoplasmic reticulum function?

This review attempts to explain why consuming extra myoinositol (Ins), an essential component of membrane phospholipids, is often beneficial for patients with conditions characterised by insulin resistance, non-alcoholic fatty liver disease and endoplasmic reticulum (ER) stress. For decades we assumed that most human diets provide an adequate Ins supply, but newer evidence suggests that increasing Ins intake ameliorates several disorders, including polycystic ovary syndrome, gestational diabetes, metabolic syndrome, poor sperm development and retinopathy of prematurity. Proposed explanations often suggest functional enhancement of minor facets of Ins Biology such as insulin signalling through putative inositol-containing 'mediators', but offer no explanation for this selectivity. It is more likely that eating extra Ins corrects a deficiency of an abundant Ins-containing cell constituent, probably phosphatidylinositol (PtdIns). Much of a cell's PtdIns is in ER membranes, and an increase in ER membrane synthesis, enhancing the ER's functional capacity, is often an important part of cell responses to ER stress. This review: (a) reinterprets historical information on Ins deficiency as describing a set of events involving a failure of cells adequately to adapt to ER stress; (b) proposes that in the conditions that respond to dietary Ins there is an overstretching of Ins reserves that limits the stressed ER's ability to make the 'extra' PtdIns needed for ER membrane expansion; and (c) suggests that eating Ins supplements increases the Ins supply to Ins-deficient and ER-stressed cells, allowing them to make more PtdIns and to expand the ER membrane system and sustain ER functions.

Transcriptional profiling of swine mammary gland during the transition from colostrogenesis to lactogenesis using RNA sequencing

Background

Colostrum and milk are essential sources of antibodies and nutrients for the neonate, playing a key role in their survival and growth. Slight abnormalities in the timing of colostrogenesis/lactogenesis potentially threaten piglet survival. To further delineate the genes and transcription regulators implicated in the control of the transition from colostrogenesis to lactogenesis, we applied RNA-seq analysis of swine mammary gland tissue from late-gestation to farrowing. Three 2nd parity sows were used for mammary tissue biopsies on days 14, 10, 6 and 2 before (−) parturition and on day 1 after (+) parturition. A total of 15 mRNA libraries were sequenced on a HiSeq2500 (Illumina Inc.). The Dynamic Impact Approach and the Ingenuity Pathway Analysis were used for pathway analysis and gene network analysis, respectively.

Results

A large number of differentially expressed genes were detected very close to parturition (−2d) and at farrowing (+ 1d). The results reflect the extraordinary metabolic changes in the swine mammary gland once it enters into the crucial phases of lactogenesis and underscore a strong transcriptional component in the control of colostrogenesis. There was marked upregulation of genes involved in synthesis of colostrum and main milk components (i.e. proteins, fat, lactose and antimicrobial factors) with a pivotal role of CSN1S2, LALBA, WAP, SAA2, and BTN1A1. The sustained activation of transcription regulators such as SREBP1 and XBP1 suggested they help coordinate these adaptations.

Conclusions

Overall, the precise timing for the transition from colostrogenesis to lactogenesis in swine mammary gland remains uncharacterized. However, our transcriptomic data support the hypothesis that the transition occurs before parturition. This is likely attributable to upregulation of a wide array of genes including those involved in ‘Protein and Carbohydrate Metabolism’, ‘Immune System’, ‘Lipid Metabolism’, ‘PPAR signaling pathway’ and ‘Prolactin signaling pathway’ along with the activation of transcription regulators controlling lipid synthesis and endoplasmic reticulum biogenesis and stress response.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4719-5) contains supplementary material, which is available to authorized users.

The unfolded protein response is involved in both differentiation and apoptosis of bovine mammary epithelial cells

The unfolded protein response (UPR) describes a process involved in the homeostasis of endoplasmic reticulum (ER) and the differentiation of secretory cells. At present, the roles of UPR in the mammary gland tissue of dairy cattle are unknown. In the current study, we investigated the expression of UPR-related genes in Holstein cows during the developmental and lactating stages of the mammary gland tissue. To investigate the roles of UPR during the differentiation of mammary epithelial cells (MEC), we used MAC-T cells, a line of MEC. We collected samples of mammary gland tissue in dairy cows by biopsy during the late gestation and lactation periods and examined the expression of UPR-related genes by quantitative real-time PCR. Expression levels of the spliced X-box binding protein 1 (XBP1) and activating transcription factor 4 (ATF4) were found to be significantly higher in the mammary gland tissue 10 d before delivery compared with 40 d before delivery. An investigation before and after differentiation in MAC-T cells showed that the expression of ATF4 increased after differentiation of MEC, whereas that of the spliced XBP1 did not significantly change. Western blot analysis revealed that the differentiation-inducing stimulus induced phosphorylation of eukaryotic initiation factor 2α (eIF2α) but reduced that of protein kinase RNA-like endoplasmic reticulum kinase (PERK). Additionally, in ATF4-knockdown bovine MEC, differentiation was significantly suppressed; ATF4 knockdown also significantly suppressed the expression of glucocorticoid and insulin receptors. These results revealed that ER stress-independent ATF4 is involved in the cell differentiation mechanism, either directly or indirectly, via the control of the expression of lactogenic hormone receptors in bovine MEC. Immediately after parturition, gene expression levels of the spliced XBP1, ATF4, and C/EBP homologous protein (CHOP) markedly increased in mammary gland tissue, with a strong negative correlation between expression of CHOP and initial milk yield; CHOP is an apoptosis-related protein induced by ER stress. The above findings indicate that UPR is intrinsically associated with apoptosis of MEC, thus affecting the differentiation of these cells, as well as milk yield.