Short communication: Endoplasmic reticulum stress gene network expression in bovine mammary tissue during the lactation cycle

The Experimental and In Silico-Based Evaluation of NRF2 Modulators, Sulforaphane and Brusatol, on the Transcriptome of Immortalized Bovine Mammary Alveolar Cells

Changes during the production cycle of dairy cattle can leave these animals susceptible to oxidative stress and reduced antioxidant health. In particular, the periparturient period, when dairy cows must rapidly adapt to the sudden metabolic demands of lactation, is a period when the production of damaging free radicals can overwhelm the natural antioxidant systems, potentially leading to tissue damage and reduced milk production. Central to the protection against free radical damage and antioxidant defense is the transcription factor NRF2, which activates an array of genes associated with antioxidant functions and cell survival. The objective of this study was to evaluate the effect that two natural NRF2 modulators, the NRF2 agonist sulforaphane (SFN) and the antagonist brusatol (BRU), have on the transcriptome of immortalized bovine mammary alveolar cells (MACT) using both the RT-qPCR of putative NRF2 target genes, as well as RNA sequencing approaches. The treatment of cells with SFN resulted in the activation of many putative NRF2 target genes and the upregulation of genes associated with pathways involved in cell survival, metabolism, and antioxidant function while suppressing the expression of genes related to cellular senescence and DNA repair. In contrast, the treatment of cells with BRU resulted in the upregulation of genes associated with inflammation, cellular stress, and apoptosis while suppressing the transcription of genes involved in various metabolic processes. The analysis also revealed several novel putative NRF2 target genes in bovine. In conclusion, these data indicate that the treatment of cells with SFN and BRU may be effective at modulating the NRF2 transcriptional network, but additional effects associated with cellular stress and metabolism may complicate the effectiveness of these compounds to improve antioxidant health in dairy cattle via nutrigenomic approaches.

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.

KRAS Affects the Lipid Composition by Regulating Mitochondrial Functions and MAPK Activation in Bovine Mammary Epithelial Cells

Kirsten rat sarcoma viral oncogene homolog (KRAS), or guanosine triphosphatase KRAS, is a proto-oncogene that encodes the small guanosine triphosphatase transductor protein. Previous studies have found that KRAS can promote cytokine secretion, cell chemotaxis, and survival. However, its effects on milk fat synthesis in bovine mammary epithelial cells are unclear. In this study, the effects of KRAS inhibition on cell metabolism, autophagy, oxidative stress, endoplasmic reticulum stress, mitochondrial function, and lipid composition as well as the potential mechanisms were detected in an immortalized dairy cow mammary epithelial cell line (MAC-T). The results showed that inhibition of KRAS changed the lipid composition (especially the triglyceride level), mitochondrial functions, autophagy, and endoplasmic reticulum stress in cells. Moreover, KRAS inhibition regulated the levels of the mammalian target of rapamycin and mitogen-activated protein kinase (extracellular regulated protein kinases, c-Jun N-terminal kinases, p38) activation. These results indicated that regulation of KRAS would affect the synthesis and composition of milk fat. These results are also helpful for exploring the synthesis and secretion of milk fat at the molecular level and provide a theoretical basis for improving the percentage of fat in milk and the yield of milk from cows.

Non-Esterified Fatty Acid Induces ER Stress-Mediated Apoptosis via ROS/MAPK Signaling Pathway in Bovine Mammary Epithelial Cells

Elevated concentrations of non-esterified fatty acid (NEFA) induced by negative energy balance (NEB) during the transition period of dairy cows is known to be toxic for multiple bovine cell types. However, the effect of NEFA in bovine mammary epithelial cells (BMECs) remains unclear. The present study aimed to explore the role and molecular mechanism of NEFA in endoplasmic reticulum (ER) stress and the subsequent apoptosis in BMECs. The results showed that NEFA increased ER stress and activated the three unfolded protein response (UPR) signaling sub-pathways by upregulating the expression of GRP78, HSP70, XBP1, ATF6, phosphor-PERK, and phosphor-IRE1α. We also found that NEFA dose-dependently induced apoptosis in BMECs, as indicated by flow cytometry analysis and increased apoptotic gene expression. RNA-seq analysis revealed that NEFA induced apoptosis in BMECs, probably via the ATF4-CHOP axis. Mechanistically, our data showed that NEFA increased reactive oxygen species (ROS) levels, resulting in the activation of the MAPK signaling pathway. Moreover, quercetin, a well-known antioxidant, was found to alleviate ER stress-mediated apoptosis in NEFA-treated BMECs. Collectively, our results suggest that NEFA induces ER stress-mediated apoptosis, probably via the ROS/MAPK signaling pathway, as quercetin has been shown to alleviate ER stress-mediated apoptosis in NEFA-treated BMECs.

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.

Reducing hepatic endoplasmic reticulum stress ameliorates the impairment in insulin signaling induced by high levels of β-hydroxybutyrate in bovine hepatocytes

Ketotic dairy cows exhibit a state of negative energy balance (NEB) characterized by elevated circulating levels of β-hydroxybutyrate (BHB) and fatty acids. Impaired hepatic insulin signaling in dairy cows occurs frequently during the transition into lactation, but its role on liver function during this period is not well known. In nonruminants, endoplasmic reticulum (ER) stress is a causal factor contributing to impaired insulin signaling in the liver. Thus, the aim of this study was to investigate the status of hepatic insulin and ER stress signaling and whether ER stress contributes to impaired insulin signaling in dairy cows with ketosis. Healthy (control cows, n = 10, BHB ≤0.6 mM) and ketotic (ketotic cows, n = 10, BHB ≥1.2 mM) cows at 3 to 10 d in milk were selected for liver biopsy and blood sampling before feeding. In vitro experiments were conducted with isolated hepatocytes from 5 healthy calves (1 d old, fasted female, 30-40 kg of body weight). Treatments included BHB (0, 0.9, 1.8, 3.6 mM), tauroursodeoxycholic acid (TUDCA, a canonical inhibitor of ER stress), and different incubation times (0.5, 1, 2, 3, 5, 7, 9, or 12 h). Ketotic cows had lower daily milk yield (median: 29.50 vs. 23.00 kg), higher plasma nonesterified fatty acid (NEFA) (median: 0.33 vs. 1.17 mM), BHB (median: 0.43 vs. 3.22 mM), aspartate aminotransferase (median: 70.58 vs. 155.70 U/L), alanine aminotransferase (median: 18.31 vs. 37.90 U/L), lower plasma glucose (median: 4.32 vs. 2.37 mg/dL), and revised quantitative insulin sensitivity check index (median: 0.39 vs. 0.37) compared with healthy cows. Increased abundance of phosphorylated insulin receptor substrate-1 (IRS1) and decreased abundance of phosphorylated protein kinase B (AKT) and glycogen synthase kinase-3β (GSK3β) in ketotic cows indicated a state of insulin resistance. In addition, abundance of phosphorylated protein kinase RNA-like ER kinase (PERK) and inositol requiring protein-1α (IRE1α), and cleavage of activating transcription factor-6 (ATF6) were greater in the liver of ketotic cows. In vitro, at the early stages of incubation, treatment with BHB upregulated abundance of phosphorylated of IRE1α, PERK, and the cleavage of ATF6, as well as several unfolded protein response genes [X-box-binding protein-1 (XBP1), 78 kDa glucose-regulated protein (GRP78), and C/EBP homologous protein (CHOP)]. Furthermore, in response to increasing doses of BHB, the phosphorylation level of PERK, IRE1α, and the cleavage of ATF6, and the abundance of XBP1, GRP78, and CHOP increased. In addition, BHB treatment increased phosphorylation of IRS1 and decreased phosphorylation of AKT and GSK3β, and upregulated abundance of gluconeogenic genes (phosphoenolpyruvate carboxykinase and glucose-6-phosphatase). Importantly, these changes were reversed by inhibiting ER stress with TUDCA treatment. Overall, the present study indicated that reversing ER stress during ketosis might help alleviate hepatic insulin resistance. Targeting ER stress may represent a potential therapeutic target for controlling the negative aspects of ketosis on liver function.

Remodeling of Zn2+ homeostasis upon differentiation of mammary epithelial cells

Zinc is the second most abundant transition metal in humans and an essential nutrient required for growth and development of newborns. During lactation, mammary epithelial cells differentiate into a secretory phenotype, uptake zinc from blood circulation, and export it into mother's milk. At the cellular level, many zinc-dependent cellular processes, such as transcription, metabolism of nutrients, and proliferation are involved in the differentiation of mammary epithelial cells. Using mouse mammary epithelial cells as a model system, we investigated the remodeling of zinc homeostasis during differentiation induced by treatment with the lactogenic hormones cortisol and prolactin. RNA-Seq at different stages of differentiation revealed changes in global gene expression, including genes encoding zinc-dependent proteins and regulators of zinc homeostasis. Increases in mRNA levels of three zinc homeostasis genes, Slc39a14 (ZIP14) and metallothioneins (MTs) I and II were induced by cortisol but not by prolactin. The cortisol-induced increase was partially mediated by the nuclear glucocorticoid receptor signaling pathway. An increase in the cytosolic labile Zn2+ pool was also detected in lactating mammary cells, consistent with upregulation of MTs. We found that the zinc transporter ZIP14 was important for the expression of a major milk protein, whey acid protein (WAP), as knockdown of ZIP14 dramatically decreased WAP mRNA levels. In summary, our study demonstrated remodeling of zinc homeostasis upon differentiation of mammary epithelial cells resulting in changes in cytosolic Zn2+ and differential expression of zinc homeostasis genes, and these changes are important for establishing the lactation phenotype.

Transcriptome Analysis Reveals the Potential Role of Long Non-coding RNAs in Mammary Gland of Yak During Lactation and Dry Period

The mammary gland is a remarkably dynamic organ of milk synthesis and secretion, and it experiences drastic structural and metabolic changes during the transition from dry periods to lactation, which involves the expression and regulation of numerous genes and regulatory factors. Long non-coding RNA (lncRNA) has considered as a novel type of regulatory factors involved in a variety of biological processes. However, their role in the lactation cycle of yak is still poorly understood. To reveal the involved mechanism, Ribo-zero RNA sequencing was employed to profile the lncRNA transcriptome in mammary tissue samples from yak at two physiological stages, namely lactation (LP) and dry period (DP). Notably, 1,599 lncRNA transcripts were identified through four rigorous steps and filtered through protein-coding ability. A total of 59 lncRNAs showed significantly different expression between two stages. Accordingly, the results of qRT-PCR were consistent with that of the transcriptome data. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses indicated that target genes of differentially expressed lncRNAs (DELs) were involved in pathways related to lactation, such as ECM-receptor interaction, PI3K-Akt signaling pathway, biosynthesis of amino acids and focal adhesion etc. Finally, we constructed a lncRNA-gene regulatory network containing some well known candidate genes for milk yield and quality traits. This is the first study to demonstrate a global profile of lncRNA expression in the mammary gland of yak. These results contribute to a valuable resource for future genetic and molecular studies on improving milk yield and quality, and help us to gain a better understanding of the molecular mechanisms underlying lactogenesis and mammary gland development of yak.

Evaluating endoplasmic reticulum stress and unfolded protein response through the lens of ecology and evolution

Considerable progress has been made in understanding the physiological basis for variation in the life-history patterns of animals, particularly with regard to the roles of oxidative stress and hormonal regulation. However, an underappreciated and understudied area that could play a role in mediating inter- and intraspecific variation of life history is endoplasmic reticulum (ER) stress, and the resulting unfolded protein response (UPR^ER ). ER stress response and the UPR^ER maintain proteostasis in cells by reducing the intracellular load of secretory proteins and enhancing protein folding capacity or initiating apoptosis in cells that cannot recover. Proper modulation of the ER stress response and execution of the UPR^ER allow animals to respond to intracellular and extracellular stressors and adapt to constantly changing environments. ER stress responses are heritable and there is considerable individual variation in UPR^ER phenotype in animals, suggesting that ER stress and UPR^ER phenotype can be subjected to natural selection. The variation in UPR^ER phenotype presumably reflects the way animals respond to ER stress and environmental challenges. Most of what we know about ER stress and the UPR^ER in animals has either come from biomedical studies using cell culture or from experiments involving conventional laboratory or agriculturally important models that exhibit limited genetic diversity. Furthermore, these studies involve the assessment of experimentally induced qualitative changes in gene expression as opposed to the quantitative variations that occur in naturally existing populations. Almost all of these studies were conducted in controlled settings that are often quite different from the conditions animals experience in nature. Herein, we review studies that investigated ER stress and the UPR^ER in relation to key life-history traits including growth and development, reproduction, bioenergetics and physical performance, and ageing and senescence. We then ask if these studies can inform us about the role of ER stress and the UPR^ER in mediating the aforementioned life-history traits in free-living animals. We propose that there is a need to conduct experiments pertaining to ER stress and the UPR^ER in ecologically relevant settings, to characterize variation in ER stress and the UPR^ER in free-living animals, and to relate the observed variation to key life-history traits. We urge others to integrate multiple physiological systems and investigate how interactions between ER stress and oxidative stress shape life-history trade-offs in free-living animals.

Effects of fatty acids on inducing endoplasmic reticulum stress in bovine mammary epithelial cells

Fatty acids play important roles in the regulation of endoplasmic reticulum (ER) stress-induced apoptosis in different cells. Currently, the effects of fatty acids on bovine mammary epithelial cells (MEC) remain unknown. Our study examined bovine MEC viability and measured unfolded protein response (UPR)-related gene and protein expressions following fatty acid treatments. To evaluate the role of fatty acids, we treated MAC-T cells (a line of MEC) with 100 to 400 μM of saturated (palmitic and stearic acid) and unsaturated (palmitoleic, oleic, linoleic, and linolenic acid) fatty acids and 1 to 5 mM of short- and medium-chain fatty acids (acetic, propionic, butyric, and octanoic acid). Thereafter, we determined UPR-related gene expression using quantitative real-time PCR. Palmitic acid stimulated expression of XBP1s, ATF4, ATF6A, and C/EBP homologous protein (CHOP). Stearic acid increased expression of XBP1s and CHOP and decreased expression of ATF4 and ATF6A. Results of Western blot analysis and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay revealed that palmitic and stearic acid reduced MAC-T cell viability and induced extreme ER stress by increasing the protein expression of ER stress markers, such as phospho-PKR-like endoplasmic reticulum kinase, phospho-eIF2α, cleaved CASP-3, and CHOP. Among unsaturated long-chain fatty acids, palmitoleic acid increased expression of ATF4 and ATF6A. Oleic acid increased expression of XBP1s, ATF4, and ATF6A. Linoleic and linolenic acids increased expression of XBP1s, ATF4, and ATF6A but decreased expression of XBP1s and ATF6A at the highest dose. Although palmitoleic, oleic, and linoleic acid decreased CHOP expression, only palmitoleic acid increased MAC-T cell viability. Therefore, unsaturated long-chain fatty acids did not induce severe ER stress. Acetic, propionic, and butyric acids decreased expression of ATF4, ATF6A, and CHOP and increased XBP1s expression. Although only octanoic acid increased ATF4 and ATF6A expressions, it lowered expression of XBP1s and CHOP. Although fatty acid treatment did not increase the levels of ER stress proteins, butyric and octanoic acids reduced cell viability, possibly because of early differentiation. These results suggest that saturated fatty acids play important roles in MEC viability by inducing severe ER stress compared with unsaturated fatty acids. In addition, acetic and propionic acids (short- and medium-chain fatty acids) reduced ER stress. Therefore, the present study reflects the new insight that serum fatty acid concentration plays an important role in maintaining the lactation physiology of dairy cows.

Orai calcium release-activated calcium modulator 1 (ORAI1) plays a role in endoplasmic reticulum stress in bovine mammary epithelial cells challenged with physiological levels of ketone bodies

The Orai calcium release-activated calcium modulator 1 (ORAI1) is a key component of the store-operated Ca²⁺ entry mechanism regulating cellular Ca²⁺ balance in nonruminants. Alterations in ORAI1 abundance have been associated with endoplasmic reticulum (ER) stress and changes in lipid metabolism in hepatocytes, an important lipogenic organ in nonruminants. Objectives were to (1) determine abundance of ORAI1 and components of the ER stress response in mammary tissue of ketotic cows, and (2) the potential role of ORAI1 on mammary cell responses to high levels of β-hydroxybutyrate (BHB). Healthy (n = 6, plasma BHB < 0.60 mmol/L) and clinically ketotic (n = 6, plasma BHB > 2.0 mmol/L) Holstein cows (days in milk = 10.13 ± 1.90) were used for mammary gland tissue and blood sample collection. Although milk production (22.5 ± 1.26, 33 ± 1.59, kg of milk/cow per day) and dry matter intake (19.5 ± 1.05, 21.9 ± 0.95, kg/d) were lower in ketotic cows, abundance of ORAI1 protein was greater and was associated with greater mRNA abundance of ER stress proteins (PERK, IRE1, ATF6, and GRP78) and lipogenic genes (FASN, SREBP1, and ACACA). Cellular mechanisms to establish links between BHB and mammary cell responses were evaluated using the immortalized cell line bovine mammary epithelial cells (MAC-T). First, a dose response study was performed with 0, 0.6, 1.2, 1.8, 2.4, or 4.8 mM BHB for 24 h. The mRNA abundance of FASN, SREBP1, and ACACA and lipid droplet formation peaked at 1.2 mM BHB. A subsequent study involved transfecting MAC-T with small interfering Orai 1 (siORAI1) or the ORAI1 inhibitor BTP₂ for 24 h followed by a challenge with 1.2 mM BHB for 24 h. Transcription and protein abundance of FASN, SREBP1, ACACA, and ER stress proteins returned to basal levels when ORAI1 was silenced or inhibited. Furthermore, the Ca²⁺ ionophore ionomycin (raises the intracellular level of Ca²⁺) also increased abundance of ORAI1, FASN, SREBP1, ACACA, and ER stress proteins. Data suggest that the mammary gland experiences ER stress during ketosis, partly due to the greater supply of BHB originating from ketogenesis in the liver. Intracellular Ca²⁺ signaling and ORAI1 seem to mediate in part the BHB-induced ER stress in mammary cells.

PRL/microRNA-183/IRS1 Pathway Regulates Milk Fat Metabolism in Cow Mammary Epithelial Cells

The aim of the study was to understand the internal relationship between milk quality and lipid metabolism in cow mammary glands. A serial of studies was conducted to assess the molecular mechanism of PRL/microRNA-183/IRS1 (Insulin receptor substrate) pathway, which regulates milk fat metabolism in dairy cows. microRNA-183 (miR-183) was overexpressed and inhibited in cow mammary epithelial cells (CMECs), and its function was detected. The function of miR-183 in inhibiting milk fat metabolism was clarified by triglycerides (TAG), cholesterol and marker genes. There is a CpG island in the 5'-flanking promoter area of miR-183, which may inhibit the expression of miR-183 after methylation. Our results showed that prolactin (PRL) inhibited the expression of miR-183 by methylating the 5' terminal CpG island of miR-183. The upstream regulation of PRL on miR-183 was demonstrated, and construction of the lipid metabolism regulation network of microRNA-183 and target gene IRS1 was performed. These results reveal the molecular mechanism of PRL/miR-183/IRS1 pathway regulating milk fat metabolism in dairy cows, thus providing an experimental basis for the improvement of milk quality.

Occludin protects secretory cells from ER stress by facilitating SNARE-dependent apical protein exocytosis

Tight junctions (TJs) are fundamental features of both epithelium and endothelium and are indispensable for vertebrate organ formation and homeostasis. However, mice lacking Occludin (Ocln) develop relatively normally to term. Here we show that Ocln is essential for mammary gland physiology, as mutant mice fail to produce milk. Surprisingly, Ocln null mammary glands showed intact TJ function and normal epithelial morphogenesis, cell differentiation, and tissue polarity, suggesting that Ocln is not required for these processes. Using single-cell transcriptomics, we identified milk-producing cells (MPCs) and found they were progressively more prone to endoplasmic reticulum (ER) stress as protein production increased exponentially during late pregnancy and lactation. Importantly, Ocln loss in MPCs resulted in greatly heightened ER stress; this in turn led to increased apoptosis and acute shutdown of protein expression, ultimately leading to lactation failure in the mutant mice. We show that the increased ER stress was caused by a secretory failure of milk proteins in Ocln null cells. Consistent with an essential role in protein secretion, Occludin was seen to reside on secretory vesicles and to be bound to SNARE proteins. Taken together, our results demonstrate that Ocln protects MPCs from ER stress by facilitating SNARE-dependent protein secretion and raise the possibility that other TJ components may participate in functions similar to Ocln.

1,25-hydroxyvitamin D3 decreases endoplasmic reticulum stress-induced inflammatory response in mammary epithelial cells

Recent studies indicated that intramammary administration of active vitamin D₃ hormone (1,25D₃) inhibits the inflammatory process associated with mastitis. We hypothesized that attenuation of endoplasmic reticulum (ER) stress by 1,25D₃ in mammary epithelial cells (MECs) is an important cellular mechanism contributing to this beneficial effect of intramammary treatment with 1,25D₃. To test this hypothesis, the effect of 1,25D₃ was studied on induction of ER stress in a transformed human MEC line, MCF-7 cells. Treatment with two different ER stress inducers, thapsigargin (TG) and tunicamycin (TM), caused a dose-dependent induction of ER stress as evident from up-regulation of protein kinase RNA-like ER kinase (PERK), heat shock protein family A (Hsp70) member 5 (HSPA5), activating transcription factor (ATF4), ATF6, DNA damage inducible transcript 3 (DDIT3) and spliced X-box binding protein 1 (sXBP1) and impaired cell viability and decreased expression of vitamin D receptor (VDR) in MCF-7 cells (P < 0.05). Treatment with 1,25D₃ (100 nM) inhibited TG (10 nM)- and TM (1 μg/mL)-induced mRNA and/or protein levels of ATF4, ATF6, DDIT3 and HSPA5 in MCF-7 cells (P < 0.05). In addition, 1,25D₃ (100 nM) antagonized the effect of TG (10 nM) and TM (1 μg/mL) on mRNA and protein levels of VDR and mRNA levels of genes involved in production and degradation of 1,25D₃ in MCF-7 cells (P < 0.05). Moreover, 1,25D₃ (100 nM) inhibited nuclear factor-κB (NF-κB) activation in response to TM (10 nM) and TG (1 μg/mL) in MCF-7 cells. In conclusion, the present findings show that 1,25D₃ is effective in attenuating ER stress and the NF-κB-driven inflammatory response in MCF-7 cells. This indicates that attenuation of ER stress by 1,25D₃ in MECs may contribute to the recently observed inhibitory effect of intramammary treatment of dairy cows with 1,25D₃ on the inflammatory process associated with mastitis.

Seipin deficiency leads to increased endoplasmic reticulum stress and apoptosis in mammary gland alveolar epithelial cells during lactation

Seipin is an integral endoplasmic reticulum (ER) membrane protein encoded by Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2/Bscl2) gene. Most litters (59%) from Bscl2-/- dams mated with wild type (WT) (Bscl2+/+) males did not survive postnatal day 5 (PND5) and pups (Bscl2+/-) lacked milk in their stomachs. The survived litters had reduced pup survival rate at PND21. It was hypothesized that seipin was critical for lactation. Bscl2 was upregulated and highly detected in the lactation day 1 (LD1) WT mammary gland alveolar epithelial cells. LD1 Bscl2-/- mammary glands lacked adipocytes and alveolar clusters and had varied alveolar morphology: from interconnected mammary gland alveoli with dilated lumen and sloughed epithelial cells to undifferentiated mammary gland alveoli with unexpanded lumen. Comparable levels of whey acidic protein (WAP, a major component in rodent milk) staining and Nile Red lipid droplet staining between WT and Bscl2-/- LD1 alveolar epithelial cells indicated normal milk protein synthesis and lipid syntheses in LD1 Bscl2-/- mammary glands. Significantly reduced percentage of larger lipid droplets was detected in LD1 Bscl2-/- alveoli with unexpanded lumen. There was no obviously impaired proliferation detected by PCNA staining but increased apoptosis detected by cleaved caspase-3 staining in LD1 Bscl2-/- alveolar epithelial cells. Increased expression of protein disulfide isomerase and binding immunoglobulin protein in the LD1 Bscl2-/- mammary gland alveolar epithelial cells indicated increased ER stress. This study demonstrates increased ER stress and apoptosis in LD1 Bscl2-/- mammary gland alveolar epithelial cells and reveals a novel in vivo function of seipin in lactation.

Phosphorylation of AKT serine/threonine kinase and abundance of milk protein synthesis gene networks in mammary tissue in response to supply of methionine in periparturient Holstein cows

The main objective was to evaluate the effect of increasing the supply of Met around parturition on abundance and phosphorylation of insulin- and mechanistic target of rapamycin complex 1 (mTORC1)-related signaling proteins along with mRNA abundance of milk protein and fat synthesis-related genes in postpartal mammary tissue. A basal control diet (control) or the basal diet plus ethyl-cellulose rumen-protected Met (0.9 g/kg of dry matter intake; Mepron, Evonik Nutrition & Care GmbH, Hanau-Wolfgang, Germany) were fed (n = 30 cows/diet) from d -28 to 60 relative to parturition. Mammary tissue and blood plasma were harvested from the same cows (n = 5/diet) in the control and Met groups at d 21 postpartum for mRNA, protein, and AA analysis. Increasing the supply of Met led to greater milk protein percentage and milk yield along with greater ratio of phosphorylated (p-)AKT to total AKT. The ratio of p-mTORC1 to total mTORC1 did not differ, but ratio of p-RPS6 to total ribosomal protein S6 (RPS6) was lower in response to Met supply. These responses were associated with greater mRNA abundance of the signaling proteins Janus kinase 2 (JAK2) and insulin receptor substrate 1 (IRS1). Greater Met supply also upregulated mRNA abundance of high-affinity cationic (SLC7A1) and sodium-coupled AA transporters (SLC38A1, SLC38A2); leucyl-tRNA (LARS), valyl-tRNA (VARS), and isoleucyl-tRNA synthetases (IARS); glucose transport solute carrier family 2 member 3 (SLC2A1); glucose transport solute carrier family 2 member 3 (SLC2A3); and casein α-s1 (CSN1S1). The mRNA abundance of components of the unfolded protein response, such as x-box binding protein 1 (XBP1) and activating transcription factor 6 (ATF6), were upregulated, and protein phosphatase 1, regulatory subunit 15A (PPP1R15A) was downregulated in response to greater Met supply. Overall, the data suggest that increased dry matter intake, greater phosphorylation status of AKT, upregulation of glucose and AA transporters, and transcripts of tRNases in response to enhanced Met supply might have compensated for a reduction in ribosome biogenesis due to a lower ratio of p-RPS6 to total RPS6. Together, these cellular responses constitute a mechanism whereby Met supply can regulate milk protein synthesis in early lactation.

Adiponectin: A prosurvival and proproliferation signal that increases bovine mammary epithelial cell numbers and protects them from endoplasmic reticulum stress responses

Cell-cell interactions between epithelial and stromal cells are predominant in the mammary gland, and various stromal cell-derived factors can elicit mitogenic responses in adjacent epithelial cells. Adiponectin is a hormone secreted mainly by adipocytes that mediates stromal-epithelial interactions in a number of tissues. Adiponectin receptors are expressed by bovine mammary epithelial cells, but the regulatory effects of adiponectin on the development and function of the mammary gland remain unclear. We therefore sought to investigate the effects of adiponectin on bovine mammary epithelial (MAC-T) cells and the regulatory mechanisms that underlie these adiponectin-induced actions. Our results revealed an increase in MAC-T cell proliferation and cell cycle progression in response to adiponectin. The expression of nuclear proliferating cell nuclear antigen (PCNA) and cyclin D1 was induced in MAC-T cells, and intracellular signaling molecules such as serine/threonine protein kinase (AKT), 70 kDa ribosomal S6 kinase (P70S6K), ribosomal protein S6 (S6), extracellular signal-regulated kinases 1 and 2 (ERK1/2), 90 kDa ribosomal S6 kinase (P90S6K), and cyclin D1 were activated in a dose-dependent manner. The abundance of adiponectin-induced signaling proteins was suppressed following inhibition of AKT or ERK1/2 mitogen-activated protein kinase (MAPK) signaling. In addition, inhibition of AKT or ERK1/2 signaling significantly reduced adiponectin-stimulated MAC-T cell proliferation. Furthermore, adiponectin reduced tunicamycin-induced expression and activation of endoplasmic reticulum stress-related proteins in MAC-T cells and attenuated the repressive effect of tunicamycin on proliferation of MAC-T cells. Collectively, these results suggest that adiponectin-mediated signaling may affect the development and function of the mammary gland in dairy cows by increasing mammary epithelial cell numbers. These findings may result in important implications for improving our fundamental understanding of lactation physiology in livestock species.

The functional effects and mechanisms by which fibroblast growth factor 2 (FGF2) controls bovine mammary epithelial cells: Implications for the development and functionality of the bovine mammary gland

Fibroblast growth factor (FGF) signaling plays essential roles in tissue development and homeostasis. Accumulating evidence reveals that fibroblast growth factor 2 (FGF2) regulates ductal elongation, which requires cell proliferation and epithelial expansion in the mammary gland. However, the function and mechanisms by which FGF2 controls functionality of epithelial cells is less well defined. Here, we demonstrate the functional effects of FGF2 on bovine mammary epithelial (MAC-T) cells and the intracellular signaling mechanisms for these FGF2-induced actions. The current results show that treatment of MAC-T cells with a recombinant FGF2 induced cell proliferation and cell-cycle progression with increased expression of proliferating cell nuclear antigen and cyclin D1. Moreover, FGF2 increased phosphorylation of serine/threonine protein kinase (protein kinase B [AKT]), extracellular signal-regulated kinases 1 and 2 (ERK1/2), Jun N-terminal kinase (JNK), 70 kDa ribosomal S6 kinase (P70S6K), 90 kDa ribosomal S6 kinase (P90S6K), ribosomal protein S6 (S6), and cyclin D1 proteins. These FGF2-induced activations of signaling pathway proteins were inhibited by blocking AKT, ERK1/2, or JNK phosphorylation. The effect of FGF2 to stimulate MAC-T cell proliferation was mediated by activation of FGF receptors (FGFR) and AKT, ERK1/2, and JNK mitogen-activated protein kinase pathways in response to FGF2 stimulation. Furthermore, expression and activation of endoplasmic reticulum (ER) stress-related factors and ER stress-induced MAC-T cell death was reduced by FGF2. Together, these results suggest that the FGF2-FGFR-intracellular signaling cascades may contribute to maintaining and/or increasing numbers of mammary epithelial cells by inducing proliferation of mammary epithelial cells and by protecting cells from ER stress responses. Therefore, this study provides evidence that FGF2 signaling is a positive factor for mammary gland remodeling and for increasing persistency of milk production.

A genetic variant in SLC30A2 causes breast dysfunction during lactation by inducing ER stress, oxidative stress and epithelial barrier defects

SLC30A2 encodes a zinc (Zn) transporter (ZnT2) that imports Zn into vesicles in highly-specialized secretory cells. Numerous mutations and non-synonymous variants in ZnT2 have been reported in humans and in breastfeeding women; ZnT2 variants are associated with abnormally low milk Zn levels and can lead to severe infantile Zn deficiency. However, ZnT2-null mice have profound defects in mammary epithelial cell (MEC) polarity and vesicle secretion, indicating that normal ZnT2 function is critical for MEC function. Here we report that women who harbor a common ZnT2 variant (T²⁸⁸S) present with elevated levels of several oxidative and endoplasmic reticulum (ER) stress markers in their breast milk. Functional studies in vitro suggest that substitution of threonine for serine at amino acid 288 leads to hyperphosphorylation retaining ZnT2 in the ER and lysosomes, increasing ER and lysosomal Zn accumulation, ER stress, the generation of reactive oxygen species, and STAT3 activation. These changes were associated with decreased abundance of zona occludens-1 and increased tight junction permeability. This study confirms that ZnT2 is important for normal breast function in women during lactation, and suggests that women who harbor defective variants in ZnT2 may be at-risk for poor lactation performance.

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.

Transcriptomic profiles of the bovine mammary gland during lactation and the dry period

The initiation and maintenance of lactation are complex phenomena governed by biochemical and endocrine processes in the mammary gland (MG). Although DNA-based approaches have been used to study the onset of lactation, more comprehensive RNA-based techniques may be critical in furthering our understanding of gene alterations that occur to support lactation in the bovine MG. To further determine how gene profiles vary during lactation compared with the dry period, RNA-seq transcriptomic analysis was used to identify differentially expressed genes (DEG) in bovine MG tissues from animals that were lactating and not lactating. A total of 881 DEG (605 upregulated and 276 downregulated) were identified in MG of 3 lactating Chinese Holstein dairy cows versus the 3 dry cows. The subcellular analysis showed that the upregulated genes were most abundantly located in "integral to membrane" and "mitochondrion," and the top number of downregulated genes existed in "nucleus" and "cytoplasm." The functional analysis indicated that the DEG were primarily associated with the support of lactation processes. The genes in higher abundance were most related to "metabolic process," "oxidation-reduction process," "transport" and "signal transduction," protein synthesis-related processes (transcription, translation, protein modifications), and some MG growth-associated processes (cell proliferation/cycle/apoptosis). The downregulated genes were mainly involved in immune-related processes (inflammatory/immune/defense responses). The KEGG analysis suggested that protein synthesis-related pathways (such as protein digestion and absorption; protein processing in endoplasmic reticulum; and glycine, serine, and threonine metabolism) were highly and significantly enriched in the bovine MG of lactating cows compared to dry cows. The results suggested that the dry cows had decreased capacity for protein synthesis, energy generation, and cell growth but enhanced immune response. Collectively, this reduced capacity in dry cows supports the physiological demands of the next lactation and the coordinated metabolic changes that occur to support these demands. A total of 51 identified DEG were validated by RT-PCR, and consistent results were found between RT-PCR and the transcriptomic analysis. This work provides a profile of gene-associated changes that occur during lactation and can be used to facilitate further investigation of the mechanisms underlying lactation in dairy cows.

Bifunctional role of ephrin A1-Eph system in stimulating cell proliferation and protecting cells from cell death through the attenuation of ER stress and inflammatory responses in bovine mammary epithelial cells

Structural and functional development of the mammary gland is constant in the mammary gland life cycle. Eph receptors and their ligands, ephrins, control events through cell-to-cell interactions during embryonic development, and adult tissue homeostasis; however, little information on participation of ephrin A1, a representative ligand of the Eph receptor, in the development and function of normal mammary glands is known. In this study, we demonstrated functional effects of the ephrin A1-Eph system and mechanisms of its action on bovine mammary epithelial (MAC-T) cells. The in vitro cultured MAC-T cells expressed the ephrin A1 ligand and EphA1, A2, A4, A7, and A8 among the eight members of the Eph A family. Our results revealed that ephrin A1 induced MAC-T cell cycle progression and stimulated cell proliferation with abundant expression of nucleic PCNA and cyclin D1 proteins. Additionally, ephrin A1 induced activation of intracellular signaling molecules involved in PI3 K/AKT and MAPK signaling, and the proliferation-stimulating effect of ephrin A1 was mediated by activation of these pathways. Furthermore, ephrin A1 influenced expression and activation of various ER stress-related proteins and protected MAC-T cells from stress-induced cell death. Finally, ephrin A1 alleviated LPS-induced cell death through down-regulation of inflammatory cytokines. In conclusion, the results of this study suggest that the Eph A-ephrin A1 system is a positive factor in the increase and maintenance of epithelial cells in mammary glands of cows; the signaling system contributes to development, remodeling, and functionality of normal mammary glands and could overcome mastitis in cows and other mammals.

Effect of endoplasmic reticulum stress on metabolic and stress signaling and kidney-specific functions in Madin-Darby bovine kidney cells

Recent studies demonstrated induction of endoplasmic reticulum (ER) stress in tissues of cows after parturition, but knowledge about the effect of ER stress on important cellular processes, such as critical signaling and metabolic pathways, in cattle is scarce. Thus, the present study aimed to investigate the effect of ER stress induction on nuclear factor-κB (NF-κB), nuclear factor E2-related factor 2 (Nrf2), and sterol regulatory element-binding protein (SREBF1) pathway in Madin-Darby bovine kidney (MDBK) cells, a widely used in vitro model in ruminant research. To consider the kidney origin of MDBK cells, the effect on renal distal tubular cell-specific functions, such as transport processes and regulation of 1,25(OH)₂D₃ levels, was also studied. Treatment of MDBK cells with 2 different ER stress inducers, thapsigargin (TG) and tunicamycin (TM), strongly induced ER stress as evident from induction of ER stress target genes, increased phosphorylation of PKR-like ER kinase, and enhanced splicing of X-box binding protein 1. The TM decreased the protein concentration of NF-κB p50 and the mRNA levels of the NF-κB target genes. Likewise, TG decreased the mRNA concentration of tumor necrosis factor and tended to decrease NF-κB p50 protein and mRNA levels of NF-κB target genes. The mRNA levels of most of the Nrf2 target genes investigated were reduced by TG and TM in MDBK cells. Both ER stress inducers reduced the mRNA levels of SREBF1 and its target genes in MDBK cells. Interestingly, TG decreased, but TM increased the mRNA level of the Ca²⁺ binding protein calbindin 1, whereas the mRNA level of the plasma membrane Ca²⁺-transporting ATPase 1 remained unchained. The mRNA level of the cytochrome P450 component 24A1 involved in 1α-hydroxylation of 25(OH)D₃ was strongly elevated, whereas the mRNA level of the cytochrome P450 component 27A1 catalyzing the breakdown of 1,25(OH)₂D₃ was markedly reduced by both ER stress inducers. The concentration of 1,25(OH)₂D₃ in the supernatant of MDBK cells was increased by approximately 15% by both TG and TM. The present study indicates that under conditions of ER stress, critical signaling pathways, such as NF-κB, Nrf2, and SREBF1, are inhibited, whereas the formation of 1,25(OH)₂D₃ is stimulated in bovine MDBK cells. Future studies are necessary to clarify the physiological relevance of these findings.

Determination of genetic effects of ATF3 and CDKN1A genes on milk yield and compositions in Chinese Holstein population

Background

Our previous RNA-sequencing study revealed that the ATF3 and CDKN1A genes were remarkably differentially expressed between the mammary glands of lactating Holstein cows with extremely high and low milk protein and fat percentage so that both of them were considered as candidates for milk composition. Herein, we further verified whether these genes have genetic effects on milk production traits in a Chinese Holstein cow population.

Results

By re-sequencing the entire coding and regulatory regions, we identified four SNPs in 5’promoter region, two in exons, seven in 3′ un-translated region (UTR), and six in 3’flanking region of ATF3 gene, and one SNP in exon 5, two in 3’UTR, and two in 3’flanking region of CDKN1A gene. Of these, only the SNP, c.271C > T (rs442346530), in exon 5 of CDKN1A gene was predicted to result in an amino acid replacement from arginine to tryptophan. Subsequent genotype-phenotype association analysis revealed that 19 SNPs in ATF3 and 5 SNPs in CDKN1A were evidently associated with 305-days milk yield, fat yield, protein yield, or protein percentage (P = < 0.0001 ~ 0.0494). Whilst, no significant SNPs in ATF3 gene were associated with fat percentage in both first and second lactations (P > 0.05), and only two SNPs of CDKN1A gene, c.271C > T (P = 0.0377) and c.*654C > T (P = 0.0144), were markedly associated with fat percentage in the first lactation. Further, linkage disequilibrium (LD) analyses were conducted among the identified SNPs in ATF3 and/or CDKN1A genes to further confirm the association results. We also observed that the four SNPs, g.72834301C > A, g.72834229C > A, g.72833969A > G, and g.72833562G > T altered the specific transcription factor (TF) binding sites in ATF3 promoter, and one SNP, c.271C > T, changed the CDKN1A protein secondary structure, suggesting they might be the promising potential functional mutations.

Conclusion

Our findings first profiled the genetic effects of ATF3 and CDKN1A genes for milk production traits in dairy cattle and will be available for marker-assisted breeding in dairy cattle.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-017-0516-4) contains supplementary material, which is available to authorized users.

Compression loading-induced stress responses in intervertebral disc cells encapsulated in 3D collagen constructs

Cells protect themselves from stresses through a cellular stress response. In the interverebral disc, such response was also demonstrated to be induced by various environmental stresses. However, whether compression loading will cause cellular stress response in the nucleus pulposus cells (NPCs) is not well studied. By using an in vitro collagen microencapsulation model, we investigated the effect of compression loading on the stress response of NPCs. Cell viability tests, and gene and protein expression experiments were conducted, with primers for the heat shock response (HSR: HSP70, HSF1, HSP27 and HSP90), and unfolded protein response (UPR: GRP78, GRP94, ATF4 and CHOP) genes and an antibody to HSP72. Different gene expression patterns occurred due to loading type throughout experiments. Increasing the loading strain for a short duration did not increase the stress response genes significantly, but over longer durations, HSP70 and HSP27 were upregulated. Longer loading durations also resulted in a continuous upregulation of HSR genes and downregulation of UPR genes, even after load removal. The rate of apoptosis did not increase significantly after loading, suggesting that stress response genes might play a role in cell survival following mechanical stress. These results demonstrate how mechanical stress might induce and control the expression of HSR and UPR genes in NPCs.

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.

Characterization and Comparative Analysis of the Milk Transcriptome in Two Dairy Sheep Breeds using RNA Sequencing

This study presents a dynamic characterization of the sheep milk transcriptome aiming at achieving a better understanding of the sheep lactating mammary gland. Transcriptome sequencing (RNA-seq) was performed on total RNA extracted from milk somatic cells from ewes on days 10, 50, 120 and 150 after lambing. The experiment was performed in Spanish Churra and Assaf breeds, which differ in their milk production traits. Nearly 67% of the annotated genes in the reference genome (Oar_v3.1) were expressed in ovine milk somatic cells. For the two breeds and across the four lactation stages studied, the most highly expressed genes encoded caseins and whey proteins. We detected 573 differentially expressed genes (DEGs) across lactation points, with the largest differences being found, between day 10 and day 150. Upregulated GO terms at late lactation stages were linked mainly to developmental processes linked to extracellular matrix remodeling. A total of 256 annotated DEGs were detected in the Assaf and Churra comparison. Some genes selectively upregulated in the Churra breed grouped under the endopeptidase and channel activity GO terms. These genes could be related to the higher cheese yield of this breed. Overall, this study provides the first integrated overview on sheep milk gene expression.

The pro-inflammatory cytokine tumor necrosis factor α stimulates expression of the carnitine transporter OCTN2 (novel organic cation transporter 2) and carnitine uptake via nuclear factor-κB in Madin-Darby bovine kidney cells

Carnitine uptake into tissues is mediated mainly by the novel organic cation transporter 2 (OCTN2), whose expression is upregulated in the liver of early-lactating dairy cows. It has been shown recently that pro-inflammatory cytokines, including tumor necrosis factor α (TNFα), stimulate OCTN2 expression and carnitine uptake in intestinal cells and inflamed intestinal mucosa. Given that many early-lactating dairy cows show typical signs of hepatic and systemic inflammation, such as elevated concentrations of circulating TNFα and activation of the key regulator of inflammation, nuclear factor κB (NF-κB), in tissues, it is possible that upregulation of OCTN2 and increase of carnitine uptake by TNFα is mediated by NF-κB, a mechanism that might contribute to the upregulation of OCNT2 in the liver of early-lactating dairy cows. Thus, in the present study, we tested the hypothesis that TNFα stimulates OCTN2 gene expression and carnitine uptake via NF-κB in the bovine Madin-Darby bovine kidney (MDBK) cell line. Treatment with TNFα caused activation of NF-κB, increased the mRNA and protein concentration of OCTN2, and stimulated the uptake of carnitine in MDBK cells. In contrast, combined treatment of MDBK cells with TNFα and the NF-κB inhibitor BAY 11-7085 completely blocked the effect of TNFα on OCTN2 mRNA and protein concentration and uptake of carnitine. These findings suggest that the bovine OCTN2 gene and carnitine uptake are regulated by NF-κB. Future studies are required to show the in vivo relevance of this regulatory mechanism in cattle.

Unmasking Upstream Gene Expression Regulators with miRNA-corrected mRNA Data

Expressed micro-RNA (miRNA) affects messenger RNA (mRNA) abundance, hindering the accuracy of upstream regulator analysis. Our objective was to provide an algorithm to correct such bias. Large mRNA and miRNA analyses were performed on RNA extracted from bovine liver and mammary tissue. Using four levels of target scores from TargetScan (all miRNA:mRNA target gene pairs or only the top 25%, 50%, or 75%). Using four levels of target scores from TargetScan (all miRNA:mRNA target gene pairs or only the top 25%, 50%, or 75%) and four levels of the magnitude of miRNA effect (ME) on mRNA expression (30%, 50%, 75%, and 83% mRNA reduction), we generated 17 different datasets (including the original dataset). For each dataset, we performed upstream regulator analysis using two bioinformatics tools. We detected an increased effect on the upstream regulator analysis with larger miRNA:mRNA pair bins and higher ME. The miRNA correction allowed identification of several upstream regulators not present in the analysis of the original dataset. Thus, the proposed algorithm improved the prediction of upstream regulators.

IRE1 phosphatase PP2Ce regulates adaptive ER stress response in the postpartum mammary gland

We recently reported that the PPM1l gene encodes an endoplasmic reticulum (ER) membrane targeted protein phosphatase (named PP2Ce) with highly specific activity towards Inositol-requiring protein-1 (IRE1) and regulates the functional outcome of ER stress. In the present report, we found that the PP2Ce protein is highly expressed in lactating epithelium of the mammary gland. Loss of PP2Ce in vivo impairs physiological unfolded protein response (UPR) and induces stress kinase activation, resulting in loss of milk production and induction of epithelial apoptosis in the lactating mammary gland. This study provides the first in vivo evidence that PP2Ce is an essential regulator of normal lactation, possibly involving IRE1 signaling and ER stress regulation in mammary epithelium.

Up-regulation of endoplasmic reticulum stress induced genes of the unfolded protein response in the liver of periparturient dairy cows

BACKGROUND

In dairy cows, the periparturient phase is a stressful period, which is commonly associated with strong metabolic adaptations and the development of pathophysiologic conditions and disorders. Some of the symptoms occurring in the liver, such as the development of fatty liver, are similar to those observed under the condition of endoplasmic reticulum (ER) stress. Therefore, we hypothesized, that in the liver of dairy cows ER stress is induced during the periparturient phase, which in turn leads to an induction of the unfolded protein response (UPR). In order to investigate this hypothesis, we determined relative mRNA concentrations of 14 genes of the ER stress-induced UPR in liver biopsy samples of 13 dairy cows at 3 wk antepartum and 1, 5 and 14 wk postpartum.

RESULTS

We found, that the mRNA concentrations of 13 out of the 14 genes involved in the UPR in the liver were significantly increased (1.9 to 4.0 fold) at 1 wk postpartum compared to 3 wk antepartum. From 1 wk postpartum to later lactation, mRNA concentrations of all the genes considered were declining. Moreover, at 1 wk postpartum, mRNA concentration of the spliced variant of XBP1 was increased in comparison to 3 wk antepartum, indicating that splicing of XBP1 - a hallmark of ER stress - was induced following the onset of lactation.

CONCLUSION

The present study reveals, that ER stress might be induced during the periparturient phase in the liver of dairy cows. We assume that the ER stress-induced UPR might contribute to the pathophysiologic conditions commonly observed in the liver of periparturient cows, such as the development of fatty liver, ketosis or inflammation.