Chi-miR-3031 regulates beta-casein via the PI3K/AKT-mTOR signaling pathway in goat mammary epithelial cells (GMECs)

Hepatic-derived extracellular vesicles in late pregnancy promote mammary gland development by stimulating prolactin receptor-mediated JAK2/STAT5/mTOR signalling

The mammary glands develop rapidly in late pregnancy to prepare adequately for lactation. At this stage the liver is crucial for mammary gland development, and it can achieve distal mammary gland regulation through hepatic factors and hormones. Recently, an increasing number of studies have found that hepatic-derived extracellular vesicles play an essential role in organ-to-organ communication, however, its effect on mammary gland development remains unclear. In this study, we extracted hepatic-derived extracellular vesicles from pregnant (P-hEVs) and non-pregnant mice (NP-hEVs), respectively, and explored their regulatory role on mammary gland development. The results revealed that P-hEVs was able to promote the proliferation and differentiation of HC11 cells. In addition, intraperitoneal injection of P-hEVs into pubertal female mice increased mammary gland weight and promoted mammary gland development. Mechanistically, P-hEVs activated the PI3K/AKT signalling pathway to enhance the proliferation of mammary epithelial cells, and also activated prolactin receptor-mediated JAK2/STAT5/mTOR signalling to promote mammary epithelial cell lactation and the synthesis of milk proteins and milk lipids. Overall, mouse liver during pregnancy can transmit signals to the mammary gland in the form of extracellular vesicles to promote its development and provide for subsequent lactation.

Identifying differentially expressed genes in goat mammary epithelial cells induced by overexpression of SOCS3 gene using RNA sequencing

The suppressor of cytokine signaling 3 (SOCS3) is a key signaling molecule that regulates milk synthesis in dairy livestock. However, the molecular mechanism by which SOCS3 regulates lipid synthesis in goat milk remains unclear. This study aimed to screen for key downstream genes associated with lipid synthesis regulated by SOCS3 in goat mammary epithelial cells (GMECs) using RNA sequencing (RNA-seq). Goat SOCS3 overexpression vector (PC-SOCS3) and negative control (PCDNA3.1) were transfected into GMECs. Total RNA from cells after SOCS3 overexpression was used for RNA-seq, followed by differentially expressed gene (DEG) analysis, functional enrichment analysis, and network prediction. SOCS3 overexpression significantly inhibited the synthesis of triacylglycerol, total cholesterol, non-esterified fatty acids, and accumulated lipid droplets. In total, 430 DEGs were identified, including 226 downregulated and 204 upregulated genes, following SOCS3 overexpression. Functional annotation revealed that the DEGs were mainly associated with lipid metabolism, cell proliferation, and apoptosis. We found that the lipid synthesis-related genes, STAT2 and FOXO6, were downregulated. In addition, the proliferation-related genes BCL2, MMP11, and MMP13 were upregulated, and the apoptosis-related gene CD40 was downregulated. In conclusion, six DEGs were identified as key regulators of milk lipid synthesis following SOCS3 overexpression in GMECs. Our results provide new candidate genes and insights into the molecular mechanisms involved in milk lipid synthesis regulated by SOCS3 in goats.

Alfalfa xenomiR-162 targets G protein subunit gamma 11 to regulate milk protein synthesis in bovine mammary epithelial cells

OBJECTIVE

It was shown that microRNAs (miRNAs) play an important role in milk protein synthesis. However, the post-transcriptional regulation of casein expression by exogenous miRNA (xeno-miRNAs) in ruminants remains unclear. This study explores the regulatory roles of alfalfa xeno-miR162 on casein synthesis in bovine mammary epithelial cells (bMECs).

METHODS

The effects of alfalfa xenomiR-162 and G protein subunit gamma 11 (GNG11) on proliferation and milk protein metabolism of bMECs were detected by 5-Ethynyl-2'-Deoxyuridine (EdU) staining, flow cytometry, cell counting kit-8 (CCK-8), enzyme-linked immunosorbent assay, quantitative real-time polymerase chain reaction (qRT-PCR), and Western blot. Dual-luciferase reporter assay was used to verify the targeting relationship between GNG11 and xenomiR-162.

RESULTS

Results showed that over-expression of xenomiR-162 inhibited cell proliferation but promoted apoptosis, which also up-regulated the expression of several casein coding genes, including CSN1S1, CSN1S2, and CSN3, while decreasing the expression of CSN2. Furthermore, the targeting relationship between GNG11 and xenomiR-162 was determined, and it was confirmed that GNG11 silencing also inhibited cell proliferation but promoted apoptosis and reduced the expression of casein coding genes and genes related to the mammalian target of rapamycin (mTOR) pathway.

CONCLUSION

Alfalfa xenomiR-162 appears to regulate bMECs proliferation and milk protein synthesis via GNG11 in the mTOR pathway, suggesting that this xeno-miRNA could be harnessed to modulate CSN3 expression in dairy cows, and increase κ-casein contents in milk.

Molecular complexity of mammary glands development: a review of lactogenic differentiation in epithelial cells

The mammary gland is a dynamic organ with various physiological processes like cellular proliferation, differentiation, and apoptosis during the pregnancy-lactation-involution cycle. It is essential to understand the molecular changes during the lactogenic differentiation of mammary epithelial cells (MECs, the milk-synthesizing cells). The MECs are organized as luminal milk-secreting cells and basal myoepithelial cells (responsible for milk ejection by contraction) that form the alveoli. The branching morphogenesis and lactogenic differentiation of the MECs prepare the gland for lactation. This process is governed by many molecular mediators including hormones, growth factors, cytokines, miRNAs, regulatory proteins, etc. Interestingly, various signalling pathways guide lactation and understanding these molecular transitions from pregnancy to lactation will help researchers design further research. Manipulation of genes responsible for milk synthesis and secretion will promote augmentation of milk yield in dairy animals. Identifying protein signatures of lactation will help develop strategies for persistent lactation and shortening the dry period in farm animals. The present review article discusses in details the physiological and molecular changes occurring during lactogenic differentiation of MECs and the associated hormones, regulatory proteins, miRNAs, and signalling pathways. An in-depth knowledge of the molecular events will aid in developing engineered cellular models for studies related to mammary gland diseases of humans and animals.

Progress in epigenetic regulation of milk synthesis, with particular emphasis on mRNA regulation and DNA methylation

Inadequate milk secretion and a lack of nutrients in humans and mammals are serious problems. It is of great significance to clarify the mechanisms of milk synthesis and treatment methods. Epigenetic modification, represented by RNA methylation, is an important way of gene expression regulation that profoundly affects human gene expression and participates in various physiological and pathological mechanisms. Epigenetic disorders also have an important impact on the production and secretion of milk. This review systematically summarized the research results of epigenetics in the process of lactation in PubMed, Web of Science, NSTL, and other databases and reviewed the effects of epigenetics on human and mammalian lactation, including miRNAs, circRNAs, lncRNAs, DNA methylations, and RNA methylations. The abnormal expression of miRNAs was closely related to the synthesis and secretion of milk fat, milk protein, and other nutrients in the milk of cattle, sheep, and other mammals. MiRNAs are also involved in the synthesis of human milk and the secretion of nutrients. CircRNAs and lncRNAs mainly target miRNAs and regulate the synthesis of nutrients in milk by ceRNA mechanisms. The abnormal expression of DNA and RNA methylation also has an important impact on milk synthesis. Epigenetic modification has the potential to regulate the milk synthesis of breast epithelial cells. Analyzing the mechanisms of human and mammalian milk secretion deficiency and nutrient deficiency from the perspective of epigenetics will provide a new perspective for the treatment of postpartum milk deficiency in pregnant women and mammalian milk secretion deficiency.

Toxic effects of zinc oxide nanoparticles as a food additive in goat mammary epithelial cells

Zinc oxide nanoparticles (ZnO NPs) have recently been used as food preservatives and additives because of their good antibacterial and nutritional functions. This study performed RNA-seq analyses to evaluate the potential toxicity of ZnO NPs on goat mammary epithelial cells (GMECs) in vitro. Our results suggested that the ZnO NP treatment significantly reduced GMEC viability in a time- and dose-dependent manner. Transcriptomic analysis showed that ZnO NP exposure changed the expression levels of more than 500 genes in GMECs, including various biological pathways. We observed that decreased mitochondrial membrane potential caused mitochondrial dysfunction. Further study indicated that the treatment of cells with ZnO NPs resulted in the accumulation of reactive oxygen species (ROS), which led to oxidative stress. Meanwhile, the expression of genes (TNFα, TNFR1, FADD, Caspase 8 and Caspase 6) associated with the death receptor pathway was upregulated, which indicated the death receptor-mediated extrinsic apoptosis pathway was activated. Moreover, the expression levels of Bax, Cytc, Caspase 3 and Caspase 9 were upregulated, while the expression levels of Bcl2 were downregulated, which indicated mitochondria-mediated intrinsic apoptosis pathway was activated. More notably, ZnO NP exposure increased the expression levels of ER stress-related genes (PERK, ATF4, eIF2α and CHOP) and proteins (p-PERK, p-eIF2α, PERK and CHOP). Furthermore, gene ontology (GO) terms and genes related to autophagy were altered, suggesting that exposure to ZnO NPs might activate autophagy in GMECs. In summary, our findings showed that ZnO NPs could exert significant toxic effects on GMECs through multiple mechanisms. These pathways are related to each other and influence each other to participate in ZnO NPs-induced the damage of GMECs. Thus, their safe use in the feed and food industry should be considered. Meanwhile, RNA-seq might represent a new method of assessing the toxicity mechanisms of nanomaterials.

Exosome-Encapsulated microRNA-140-5p Alleviates Neuronal Injury Following Subarachnoid Hemorrhage by Regulating IGFBP5-Mediated PI3K/AKT Signaling Pathway

Recent literature has highlighted the therapeutic implication of exosomes (Exos) released by adipose tissue-originated stromal cells (ADSCs) in regenerative medicine. Herein, the current study sought to examine the potential protective effects of ADSC-Exos on neuronal injury following subarachnoid hemorrhage (SAH) by delivering miR-140-5p. Firstly, isolated primary neurons were co-cultured together with well-identified ADSC-Exos. TDP-43-treated neurons were subsequently treated with PKH67-ADSC-Exos and Cy3-miR-140-5p to assess whether ADSC-Exos could transmit miR-140-5p to the recipient neurons to affect their behaviors. Moreover, a luciferase assay was carried out to identify the presumable binding of miR-140-5p to IGFBP5. IGFBP5 rescue experimentation was also performed to testify whether IGFBP5 conferred the impact of miR-140-5p on neuronal damage. The role of PI3K/AKT signaling pathway was further analyzed with the application of its inhibitor miltefosine. Lastly, SAH rat models were developed for in vivo validation. It was found that ADSC-Exos conferred protection against TDP-43-caused neuronal injury by augmenting viability and suppressing cell apoptosis. In addition, miR-140-5p was transmitted from ADSC-Exos to neurons and post-transcriptionally downregulated the expression of IGFBP5. As a result, by means of suppressing IGFBP5 and activating the PI3K/AKT signaling pathway, miR-140-5p from ADSC-Exos induced a neuroprotective effect. Furthermore, in vivo findings substantiated the aforementioned protective role of ADSC-Exos-miR-140-5p, contributing to protection against SAH-caused neurological dysfunction. Collectively, our findings indicated that ADSC-Exos-miR-140-5p could inhibit TDP-43-induced neuronal injury and attenuate neurological dysfunction of SAH rats by inhibiting IGFBP5 and activating the PI3K/Akt signaling pathway.

Progress on the Regulation of Ruminant Milk Fat by Noncoding RNAs and ceRNAs

Milk fat is not only a key factor affecting the quality of fresh milk but also a major target trait forbreeding. The regulation of milk fat involves multiple genes, network regulation and signal transduction. To explore recent discoveries of pathway regulation, we reviewed the published literature with a focus on functional noncoding RNAs and epigenetic regulation in ruminants. Results indicate that miRNAs play key roles in the regulation of milk fat synthesis and catabolism in ruminants. Although few data are available, merging evidence indicates that lncRNAs and circRNAs act on milk fat related genes through indirect action with microRNAs or RNAs in the ceRNA network to elicit positive effects on transcription. Although precise regulatory mechanisms remain unclear, most studies have focused on the regulation of the function of target genes through functional noncoding RNAs. Data to help identify factors that can regulate their own expression and function or to determine whether self-regulation involves positive and/or negative feedback are needed. Despite the growing body of research on the role of functional noncoding RNA in the control of ruminant milk fat, most data are still not translatable for field applications. Overall, the understanding of mechanisms whereby miRNA, lncRNA, circRNA, and ceRNA regulate ruminant milk fat remains an exciting area of research.

Exploration of the lactation function of protein phosphorylation sites in goat mammary tissues by phosphoproteome analysis

BACKGROUND

Protein phosphorylation plays an important role in lactation. Differentially modified phosphorylation sites and phosphorylated proteins between peak lactation (PL, 90 days postpartum) and late lactation (LL, 280 days postpartum) were investigated using an integrated approach, namely, liquid chromatography with tandem mass spectrometry (LC-MS/MS) and tandem mass tag (TMT) labeling, to determine the molecular changes in the mammary tissues during the different stages of goat lactation.

RESULTS

A total of 1,938 (1,111 upregulated, 827 downregulated) differentially modified phosphorylation sites of 1,172 proteins were identified (P values < 0.05 and fold change of phosphorylation ratios > 1.5). Multiple phosphorylation sites of FASN, ACACA, mTOR, PRKAA, IRS1, RPS6KB, EIF4EBP1, JUN, and TSC2 were different in PL compared with LL. In addition, the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that the calcium signaling pathway, oxytocin signaling pathway and MAPK signaling pathway were enriched. The western blot results showed that the phosphorylation levels of ACACA (Ser80), EIF4EBP1 (Thr46) and IRS1 (Ser312) increased and JUN (Ser63) decreased in PL compared with LL. These results were consistent with the phosphoproteome results.

CONCLUSIONS

In this study, we identified for the first time the differentially modified phosphorylation sites in goat mammary tissues between PL and LL. These results indicate that the multiple differentially modified phosphorylation sites of FASN, ACACA, mTOR, PRKAA, IRS1, RPS6KB, EIF4EBP1, TSC2, and JUN and proteins involved in the calcium signaling pathway, oxytocin signaling pathway, and MAPK signaling pathway are worthy of further exploration.

Integrated Analysis of mRNA- and miRNA-Seq in the Ovary of Rare Minnow Gobiocypris rarus in Response to 17α-Methyltestosterone

17α-Methyltestosterone (MT) is a synthetic androgen. The objective of this study was to explore the effects of exogenous MT on the growth and gonadal development of female rare minnow Gobiocypris rarus. Female G. rarus groups were exposed to 25–100 ng/L of MT for 7 days. After exposure for 7 days, the total weight and body length were significantly decreased in the 50-ng/L MT groups. The major oocytes in the ovaries of the control group were vitellogenic oocytes (Voc) and cortical alveolus stage oocytes (Coc). In the MT exposure groups, some fish had mature ovaries with a relatively lower proportion of mature oocytes, and the diameter of the perinucleolar oocytes (Poc) was decreased compared with those of the control group. Ovarian VTG, FSH, LH, 11-KT, E2, and T were significantly increased after exposure to 50 ng/L of MT for 7 days. Unigenes (73,449), 24 known mature microRNAs (miRNAs), and 897 novel miRNAs in the gonads of G. rarus were found using high-throughput sequencing. Six mature miRNAs (miR-19, miR-183, miR-203, miR-204, miR-205, and miR-96) as well as six differentially expressed genes (fabp3, mfap4, abca1, foxo3, tgfb1, and zfp36l1) that may be associated with ovarian development and innate immune response were assayed using qPCR. Furthermore, the miR-183 cluster and miR-203 were differentially expressed in MT-exposed ovaries of the different G. rarus groups. This study provides some information about the role of miRNA–mRNA pairs in the regulation of ovarian development and innate immune system, which will facilitate future studies of the miRNA–RNA-associated regulation of teleost reproduction.

miR-204-5p and miR-211 Synergistically Downregulate the αS1-Casein Content and Contribute to the Lower Allergy of Goat Milk

α_S1-Casein (encoded by the CSN1S1 gene) is associated with higher rates of allergy than other milk protein components for humans. microRNAs (miRNAs) as small noncoding RNA molecules regulate gene expression and influence diverse biological processes. However, little is known about the regulation of milk protein synthesis by miRNAs in ruminants. In this study, we aim to investigate the regulatory roles of miR-204 family members (miR-204-5p and miR-211) on α_S1-casein in goat mammary epithelial cells (GMEC). Here, we observed that the CSN1S1 mRNA level is upregulated, while miR-204-5p and miR-211 (miR-204-5p/-211) abundance is downregulated during peak lactation compared with middle lactation of dairy goats. We found that miR-204-5p/-211 synergistically inhibit α_S1-casein expression via directly binding to the 3'-untranslated region (3'UTR) of CSN1S1 in GMEC. miR-204-5p/-211 increase β-casein mRNA (CSN2) and protein abundance, as well as the signal transducer and activator of transcription 5a (STAT5a) activity. Further, miR-204-5p/-211 enhance β-casein expression via the CSN1S1-STAT5a signaling axis and promote β-casein transcription by activating the STAT5 response element located in the CSN2 promoter. In conclusion, miR-204-5p/-211 regulate α_S1-casein and β-casein synthesis via targeting CSN1S1 in GMEC, which provide the strategy for manipulating miR-204 family members to reduce milk allergy potential and improve ruminant milk quality for human consumption.

CircAgtpbp1 Acts as a Molecular Sponge of miR-543-5p to Regulate the Secretion of GH in Rat Pituitary Cells

CircRNAs have been identified to be expressed differently and stably in numerous species and tissues, but their functions in growth hormone (GH) secretion are still largely unknown. In summary, we have revealed a circRNA-miRNA-mRNA network that may play a biological role in the rat pituitary gland. First, we verified the chromosome location information of circAgtpbp1 according to sequencing analysis. The circAgtpbp1 characteristics were authenticated through PCR, qRT-PCR, treating with RNase and fluorescent in situ hybridization (FISH). Second, we detected the expression pattern of circAgtpbp1 in the rat anterior pituitary by qRT-PCR. We also designed circAgtpbp1 siRNA and constructed overexpression plasmid to evaluate the effect of circAgtpbp1 function on GH secretion by qRT-PCR, ELISA and Western blot. CircAgtpbp1 is a stable, truly circular molecule. We found that circAgtpbp1 interacted with miR-543-5p and can regulate GH secretion in pituitary cells through a circAgtpbp1-miR-543-5p-GH axis. Overall, the evidence generated by our study suggests that circAgtpbp1 can act as a sponge of miR-543-5p to reduce the inhibitory effect of miR-543-5p on Gh1 and further promote GH secretion. These findings expand our existing knowledge on the mechanisms of hormone regulation in the pituitary gland.

Epigenetics: New Insights into Mammary Gland Biology

The mammary gland undergoes important anatomical and physiological changes from embryogenesis through puberty, pregnancy, lactation and involution. These steps are under the control of a complex network of molecular factors, in which epigenetic mechanisms play a role that is increasingly well described. Recently, studies investigating epigenetic modifications and their impacts on gene expression in the mammary gland have been performed at different physiological stages and in different mammary cell types. This has led to the establishment of a role for epigenetic marks in milk component biosynthesis. This review aims to summarize the available knowledge regarding the involvement of the four main molecular mechanisms in epigenetics: DNA methylation, histone modifications, polycomb protein activity and non-coding RNA functions.

CircRNA8220 Sponges MiR-8516 to Regulate Cell Viability and Milk Synthesis via Ras/MEK/ERK and PI3K/AKT/mTOR Pathways in Goat Mammary Epithelial Cells

Simple Summary

Yield and quality of goat milk are important indexes for screening dairy goat breeds. Therefore, it is necessary for us to improve the yield and quality of goat milk. In this study, we demonstrated that circRNA8220/miR-8516/STC2 could promote the synthesis of β-casein and triglyceride through PI3K/AKT/mTOR pathway. In addition, we found that circRNA8220/miR-8516/STC2 also promote proliferation via Ras/MEK/ERK pathway in goat mammary epithelial cells (GMECs). These findings contribute to a better understanding of circRNA-controlled breast development and lactation mechanisms and provide new potential insights into the regulation of breast development and milk composition in dairy goats.

Abstract

Circular RNAs (circRNAs), which are considered a large class of endogenous noncoding RNAs, function as regulators in various biological procedures. In this study, the function and molecular mechanisms of circRNA8220 in goat mammary epithelial cells (GMECs) were explored. CircRNA8220 could spong miR-8516 and block the function of miR-8516 by binding to the target site of miR-8516 a negative feedback relationship existed between circRNA8220 and miR-8516. Stanniocalcin 2 (STC2) was a target gene of miR-8516. circRNA8220 could up-regulate the expression of STC2 by sponging miR-8516 in GMECs. circRNA8220/miR-8516/STC2 could promote proliferation and enhance the synthesis of β-casein and triglycerides (TG) via Ras/MEK/ERK and PI3K/AKT/mTOR signaling pathways, respectively.

OGR1 negatively regulates β-casein and triglyceride synthesis and cell proliferation via the PI3K/AKT/mTOR signaling pathway in goat mammary epithelial cells

Goat milk in some cases is less allergenic than cow milk, therefore, more people drink goat milk in the world, so it is necessary for us to improve the yield and quality of goat milk. Previous studies have shown that some genes are closely related to lactation. Ovarian cancer G protein-coupled 1 (OGR1) is a G protein-coupled receptor discovered recently. OGR1 is widely found in various tissues of organisms and is involved in cell skeleton reorganization, carcinogenesis, cell proliferation, and apoptosis by regulating multiple signaling pathways in cells. However, the modulating effect of OGR1 in lactation is still unknown. Therefore, the objective of this study is to investigate the function of OGR1 in goat mammary epithelial cells (GMECs). Flow cytometry, CCK8, EDU, enzyme-linked immunosorbent assay, and triglyceride test kit assays were performed and we found that OGR1 regulated Bcl-2/Bax ratio, Fas protein expression as well as the phosphorylation of AKT and mammalian target of rapamycin (mTOR). si-OGR1 could enhance the proliferation of GMECs by promoting G1/S phase progression and the synthesis of β-casein and triglyceride. By contrast, OGR1 repressed GMECs proliferation and down-regulated the synthesis of β-casein and triglyceride by blocking the PI3K/AKT/mTOR signaling pathway in GMECs.

chi-miR-143-3p Promotes Apoptosis of Mammary Gland Epithelial Cells from Dairy Goats by Targeting Ndfip1

The mammary gland is an important organ for lactation in dairy goats. Mammary gland development and lactation functions are primarily regulated by natural hormones and certain crucial regulatory factors. Nedd4 family-interacting protein 1 (Ndfip1) can specifically bind to neural precursor cell-expressed, developmentally downregulated protein 4 (Nedd4) family members to participate in ubiquitination, which in turn regulates a range of biological processes in the body. However, the effects of Ndfip1 expression regulation at the post-transcriptional level on the development of mammary gland cells have not been previously reported. To study the regulation of Ndfip1 at post-transcriptional level, the overexpression and interference vectors of Ndfip1 were constructed, and co-transfected into the primary mammary gland epithelial cells cultured in vitro with miR-143 mimics and inhibitor. Dual luciferase reporter gene system, real-time quantitative polymerase chain reaction, western blotting, cholecystokinin octapeptide assays, and flow cytometry were used to identify their regulation and function. As a result, Ndfip1 was targeted and regulated by miR-143, which influences the development of mammary gland epithelial cells in dairy goats cultured in vitro. This study will lay an experimental foundation for further understanding the functions of Ndfip1 and miR-143.

miR-142-3p Regulates Milk Synthesis and Structure of Murine Mammary Glands via PRLR-Mediated Multiple Signaling Pathways

Murine mammary gland is an ideal model for studying the development and milk synthesis in dairy animals. MicroRNAs play an important role in milk synthesis and mammary gland development; however, the molecular mechanism of miR-142-3p continues to be poorly understood. Here, we knocked down miR-142-3p expression in vitro and vivo, increased the prolactin receptor expression and activated many downstream cellular proteins, such as mammalian target of rapamycin, sterol regulatory element-binding transcription factor 1, cyclin D1, and signal transducer and activator of transcription 5. Additionally, miR-142-3p knockdown in mouse mammary gland epithelial cells increased proliferation but not viability, induced cell cycle progression, decreased apoptosis, and increased the expression of triglycerides and β-casein. Moreover, miR-142-3p knockdown in murine mammary gland tissue in vivo affected the structure and function of the mammary gland, which showed an increased number of lobules and ducts and was more capable of producing milk. However, overexpression of miR-142-3p had the opposite effects. In summary, these data reveal that miR-142-3p regulates milk synthesis and the structure of murine mammary glands via PRLR-mediated multiple signaling pathways.