Transcription factor activities and gene expression during mouse mammary gland involution

Lactose on the basolateral side of mammary epithelial cells inhibits milk production concomitantly with signal transducer and activator of transcription 5 inactivation

Mammary epithelial cells (MECs) are the only cells capable of synthesizing lactose. During lactation, alveolar MECs secrete lactose through the apical membrane into the alveolar lumen, whereas alveolar tight junctions (TJs) block the leakage of lactose into the basolateral sides of the MECs. However, lactose leaks from the alveolar lumen into the blood plasma in the mastitis and after weaning. This exposes the basolateral membrane of MECs to lactose. The relationship between lactose in blood plasma and milk production has been suggested. The present study determined whether lactose exposure on the basolateral membrane of mouse MECs adversely affects milk production in vitro. Restricted exposure to lactose on the basolateral side of the MECs was performed using a culture model, in which MECs on the cell culture insert exhibit milk production and less-permeable TJs. The results indicated that lactose exposure on the basolateral side inhibited casein and lipid production in the MECs. Interestingly, lactose exposure on the apical side did not show detectable effects on milk production in the MECs. Basolateral lactose exposure also caused the inactivation of STAT5, a primary transcriptional factor for milk production. Furthermore, p38 and JNK were activated by basolateral lactose exposure. The activation of p38 and JNK following anisomycin treatment reduced phosphorylated STAT5, and inhibitors of p38 blocked the reduction of phosphorylated STAT5 by basolateral lactose exposure. These findings suggest that lactose functions as a partial inhibitor for milk production but only when it directly makes contact with the basolateral membrane of MECs.

Short Time-Series Expression Transcriptome Data Reveal the Gene Expression Patterns of Dairy Cow Mammary Gland as Milk Yield Decreased Process

The existing research on dairy cow mammary gland genes is extensive, but there have been few reports about dynamic changes in dairy cow mammary gland genes as milk yield decrease. For the first time, transcriptome analysis based on short time-series expression miner (STEM) and histological observations were performed using the Holstein dairy cow mammary gland to explore gene expression patterns in this process of decrease (at peak, mid-, and late lactation). Histological observations suggested that the number of mammary acinous cells at peak/mid-lactation was significantly higher than that at mid-/late lactation, and the lipid droplets area secreted by dairy cows was almost unaltered across the three stages of lactation (p > 0.05). Totals of 882 and 1439 genes were differentially expressed at mid- and late lactation, respectively, compared to peak lactation. Function analysis showed that differentially expressed genes (DEGs) were mainly related to apoptosis and energy metabolism (fold change ≥ 2 or fold change ≤ 0.5, p-value ≤ 0.05). Transcriptome analysis based on STEM identified 16 profiles of differential gene expression patterns, including 5 significant profiles (false discovery rate, FDR ≤ 0.05). Function analysis revealed DEGs involved in milk fat synthesis were downregulated in Profile 0 and DEGs in Profile 12 associated with protein synthesis. These findings provide a foundation for future studies on the molecular mechanisms underlying mammary gland development in dairy cows.

Lactation Associated Genes Revealed in Holstein Dairy Cows by Weighted Gene Co-Expression Network Analysis (WGCNA)

Simple Summary

Weighted gene coexpression network analysis (WGCNA) is a novel approach that can quickly analyze the relationships between genes and traits. In the past few years, studies on the gene expression changes of dairy cow mammary glands were only based on transcriptome comparisons between two lactation stages. Few studies focused on the relationships between gene expression of the dairy mammary gland and lactation stage or milk composition in a lactation cycle. In this study, we detected milk yield and composition in a lactation cycle. For the first time, we constructed a gene coexpression network using WGCNA on the basis of 18 gene expression profiles during six stages of a lactation cycle by transcriptome sequencing, generating 10 specific modules. Genes in each module were performed with gene ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. Module–trait relationship analysis showed a series of potential candidates related to milk yield and composition. The current study provides an important theoretical basis for the further molecular breeding of dairy cows.

Abstract

Weighted gene coexpression network analysis (WGCNA) is a novel approach that can quickly analyze the relationships between genes and traits. In this study, the milk yield, lactose, fat, and protein of Holstein dairy cows were detected in a lactation cycle. Meanwhile, a total of 18 gene expression profiles were detected using mammary glands from six lactation stages (day 7 to calving, −7 d; day 30 post-calving, 30 d; day 90 post-calving, 90 d; day 180 post-calving, 180 d; day 270 post-calving, 270 d; day 315 post-calving, 315 d). On the basis of the 18 profiles, WGCNA identified for the first time 10 significant modules that may be related to lactation stage, milk yield, and the main milk composition content. Genes in the 10 significant modules were examined with gene ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. The results revealed that the galactose metabolism pathway was a potential candidate for milk yield and milk lactose synthesis. In −7 d, ion transportation was more frequent and cell proliferation related terms became active. In late lactation, the suppressor of cytokine signaling 3 (SOCS3) might play a role in apoptosis. The sphingolipid signaling pathway was a potential candidate for milk fat synthesis. Dairy cows at 315 d were in a period of cell proliferation. Another notable phenomenon was that nonlactating dairy cows had a more regular circadian rhythm after a cycle of lactation. The results provide an important theoretical basis for the further molecular breeding of dairy cows.

Analyzing the genomic and transcriptomic architecture of milk traits in Murciano-Granadina goats

Background

In this study, we aimed to investigate the molecular basis of lactation as well as to identify the genetic factors that influence milk yield and composition in goats. To achieve these two goals, we have analyzed how the mRNA profile of the mammary gland changes in seven Murciano-Granadina goats at each of three different time points, i.e. 78 d (T1, early lactation), 216 d (T2, late lactation) and 285 d (T3, dry period) after parturition. Moreover, we have performed a genome-wide association study (GWAS) for seven dairy traits recorded in the 1st lactation of 822 Murciano-Granadina goats.

Results

The expression profiles of the mammary gland in the early (T1) and late (T2) lactation were quite similar (42 differentially expressed genes), while strong transcriptomic differences (more than one thousand differentially expressed genes) were observed between the lactating (T1/T2) and non-lactating (T3) mammary glands. A large number of differentially expressed genes were involved in pathways related with the biosynthesis of amino acids, cholesterol, triglycerides and steroids as well as with glycerophospholipid metabolism, adipocytokine signaling, lipid binding, regulation of ion transmembrane transport, calcium ion binding, metalloendopeptidase activity and complement and coagulation cascades. With regard to the second goal of the study, the performance of the GWAS allowed us to detect 24 quantitative trait loci (QTLs), including three genome-wide significant associations: QTL1 (chromosome 2, 130.72-131.01 Mb) for lactose percentage, QTL6 (chromosome 6, 78.90-93.48 Mb) for protein percentage and QTL17 (chromosome 17, 11.20 Mb) for both protein and dry matter percentages. Interestingly, QTL6 shows positional coincidence with the casein genes, which encode 80% of milk proteins.

Conclusions

The abrogation of lactation involves dramatic changes in the expression of genes participating in a broad array of physiological processes such as protein, lipid and carbohydrate metabolism, calcium homeostasis, cell death and tissue remodeling, as well as immunity. We also conclude that genetic variation at the casein genes has a major impact on the milk protein content of Murciano-Granadina goats.

Moderate High Temperature Condition Induces the Lactation Capacity of Mammary Epithelial Cells Through Control of STAT3 and STAT5 Signaling

In lactating mammary glands, alveolar mammary epithelial cells (MECs) synthesize and secrete milk components. MECs also form less permeable tight junctions (TJs) to prevent the leakage of milk components. During lactation, MECs are exposed to temperature changes by metabolic heat production and air ambient temperature. In this study, we investigated whether temperature changes influence milk production ability and TJ barriers in MECs by using two lactating culture models. The results showed that 39 °C treatment activated milk production and enhanced the formation of less-permeable TJs. In contrast, 41 °C treatment caused adverse effects on the TJ barrier and cell viability, although the milk production ability of MECs was temporarily up-regulated. MECs cultured at 37 °C showed relatively low milk production ability and high proliferation activity. Furthermore, we investigated three kinds of transcription factors relating to lactogenesis, signal transducer and activator of transcription 5 (STAT5), STAT3 and glucocorticoid receptor (GR). STAT5 signaling was activated at 39 and 41 °C by an increase in total STAT5. However, long-term treatment led to a decrease in total STAT5. STAT3 signaling was inactivated by high temperature treatment through a decrease in total STAT3 and inhibited phosphorylation of STAT3. GR signaling was continuously activated regardless of temperature. These results indicate that a moderate high temperature condition at 39 °C induces a high lactation capacity of MECs through control of STAT5 and STAT3 signaling. In contrast, long-term exposure at 41 °C leads to a decline in milk production capacity by inactivation of STAT5 and a decrease in the total number of MECs.

The cJUN NH2-terminal kinase (JNK) pathway contributes to mouse mammary gland remodeling during involution

Involution returns the lactating mammary gland to a quiescent state after weaning. The mechanism of involution involves collapse of the mammary epithelial cell compartment. To test whether the cJUN NH₂-terminal kinase (JNK) signal transduction pathway contributes to involution, we established mice with JNK deficiency in the mammary epithelium. We found that JNK is required for efficient involution. JNK deficiency did not alter the STAT3/5 or SMAD2/3 signaling pathways that have been previously implicated in this process. Nevertheless, JNK promotes the expression of genes that drive involution, including matrix metalloproteases, cathepsins, and BH3-only proteins. These data demonstrate that JNK has a key role in mammary gland involution post lactation.

Overview of Mammary Gland Development: A Comparison of Mouse and Human

The mouse mammary gland is widely used as a model for human breast cancer and has greatly added to our understanding of the molecular mechanisms involved in breast cancer development and progression. To fully appreciate the validity and limitations of the mouse model, it is essential to be aware of the similarities and also the differences that exist between the mouse mammary gland and the human breast. This introduction therefore describes the parallels and contrasts in mouse mammary gland and human breast morphogenesis from an early embryonic phase through to puberty, adulthood, pregnancy, parturition, and lactation, and finally the regressive stage of involution.

RNA-Sequencing for profiling goat milk transcriptome in colostrum and mature milk

BACKGROUND

In this work we aimed at sequencing and assembling the goat milk transcriptome corresponding at colostrum and 120 days of lactation. To reconstruct transcripts we used both the genome as reference, and a de novo assembly approach. Additionally, we aimed at identifying the differentially expressed genes (DEGs) between the two lactation stages and at analyzing the expression of genes involved in oligosaccharides metabolism.

RESULTS

A total of 44,635 different transcripts, organized in 33,757 tentative genes, were obtained using the goat genome as reference. A significant sequence similarity match was found for 40,353 transcripts (90%) against the NCBI NT and for 35,701 (80%) against the NR databases. 68% and 69% of the de novo assembled transcripts, in colostrum and 120 days of lactation samples respectively, have a significant match with the merged transcriptome obtained using Cufflinks/Cuffmerge. CSN2, PAEP, CSN1S2, CSN3, LALBA, TPT1, FTH1, M-SAA3, SPP1, GLYCAM1, EEF1A1, CTSD, FASN, RPS29, CSN1S1, KRT19 and CHEK1 were found between the top fifteen highly expressed genes. 418 loci were differentially expressed between lactation stages, among which 207 and 122 were significantly up- and down-regulated in colostrum, respectively. Functional annotation and pathway enrichment analysis showed that in goat colostrum somatic cells predominate biological processes involved in glycolysis, carbohydrate metabolism, defense response, cytokine activity, regulation of cell proliferation and cell death, vasculature development, while in mature milk, biological process associated with positive regulation of lymphocyte activation and anatomical structure morphogenesis are enriched. The analysis of 144 different oligosaccharide metabolism-related genes showed that most of these (64%) were more expressed in colostrum than in mature milk, with eight expressed at very high levels (SLCA3, GMSD, NME2, SLC2A1, B4GALT1, B3GNT2, NANS, HEXB).

CONCLUSIONS

To our knowledge, this is the first study comparing goat transcriptome of two lactation stages: colostrum and 120 days. Our findings suggest putative differences of expression between stages and can be envisioned as a base for further research in the topic. Moreover because a higher expression of genes involved in immune defense response, carbohydrate metabolism and related to oligosaccharide metabolism was identified in colostrum we here corroborate the potential of goat milk as a natural source of lactose-derived oligosaccharides and for the development of functional foods.

Regulatory roles of Oct proteins in the mammary gland

The expression of Oct-1 and -2 and their binding to the octamer motif in the mammary gland are developmentally and hormonally regulated, consistent with the expression of milk proteins. Both of these transcription factors constitutively bind to the proximal promoter of the milk protein gene β-casein and might be involved in the inhibition or activation of promoter activity via interactions with other transcription factors or cofactors at different developmental stages. In particular, the lactogenic hormone prolactin and glucocorticoids induce Oct-1 and Oct-2 binding and interaction with both the signal transducer and activator of transcription 5 (STAT5) and the glucocorticoid receptor on the β-casein promoter to activate β-casein expression. In addition, increasing evidence has shown the involvement of another Oct factor, Oct-3/4, in mammary tumorigenesis, making Oct-3/4 an emerging prognostic marker of breast cancer and a molecular target for the gene-directed therapeutic intervention, prevention and treatment of breast cancer. This article is part of a Special Issue entitled: The Oct Transcription Factor Family, edited by Dr. Dean Tantin.

Decrease in an Inwardly Rectifying Potassium Conductance in Mouse Mammary Secretory Cells after Forced Weaning

Mammary glands are physiologically active in female mammals only during nursing. Immediately after weaning, most lactation-related genes are downregulated and milk production ceases. In our previous study, we have detected an inwardly rectifying potassium channel (Kir) 2.1-like current in mammary secretory (MS) cells freshly isolated from lactating mice. This current is highly sensitive to external Ba2+. The potassium permeability of the Kir channels may contribute to the secretion and/or preservation of ions in milk. We hypothesized that the functions of the Kir channels in MS cells are regulated after weaning. To test this hypothesis, we examined the effect of forced weaning on the Ba2+-sensitive Kir current and Kir2.1 expression in the mouse mammary glands. Twenty-four hours after weaning, the lumina of mammary acini were histologically enlarged by milk accumulation. The whole-cell patch-clamp analyses showed that the Ba2+-sensitive Kir current in the post-weaning MS cells was smaller than in the lactating MS cells. The inward conductances of the current in the lactating and post-weaning cells were 4.25 ± 0.77 and 0.93 ± 0.34 nS, respectively. Furthermore, real-time PCR and Western blot analyses showed that Kir2.1 mRNA and protein expression decreased in the post-weaning mammary gland (mRNA, 90% reduction; protein, 47% reduction). Moreover, the local milk accumulation caused by teat sealing decreased Kir conductance in MS cells (2.74 ± 0.45 and 0.36 ± 0.27 nS for control and sealed mammary glands, respectively). This was concomitant with the reduction in the Kir2.1 mRNA expression. Our results suggest that milk stasis after weaning immediately decreases the Kir conductance in MS cells. This decrease in the Kir conductance may be partly caused by the reduction in the Kir2.1 mRNA and protein expression. These alterations during the post-weaning period may be involved in the cessation of ion secretion and/or preservation in the milk.

DIGE based proteome analysis of mammary gland tissue in water buffalo (Bubalus bubalis): lactating vis-a-vis heifer

Mammary gland is an exocrine and sebaceous gland made up of branching network of ducts that end in alveoli. Milk is synthesized in the alveoli and secreted into alveolar lumen. Mammary gland represents an ideal system for the study of organogenesis that undergoes successive cycles of pregnancy, lactation and involution. To gain insights on the molecular events that take place in pubertal and lactating mammary gland, we have identified 43 differentially expressed proteins in mammary tissue of heifer (non-lactating representing a virgin mammary gland), and lactating buffaloes (Bubalus bubalis) by 2D-difference gel electrophoresis (2D-DIGE) and mass spectrometry. Twenty one proteins were upregulated during lactation whereas 8 proteins were upregulated in heifer mammary gland significantly (p<0.05). Bioinformatics analyses of the identified proteins showed that a majority of the proteins are involved in metabolic processes. The differentially expressed proteins were validated by real-time PCR and Western blotting. We observed differential expressions of certain new proteins including EEF1D, HSPA5, HSPD1 and PRDX6 during lactation which have not been reported before. The differentially expressed proteins were mapped to available biological pathways and networks involved in lactation. This study signifies the importance of some proteins which are preferentially expressed during lactation and in heifer mammary gland.

BIOLOGICAL SIGNIFICANCE

This work is important because we have generated information in water buffalo (B. bubalis) for the first time which is the major milk producing animal in Indian Subcontinent. Out of a present production of 133milliontons of milk produced in India, contribution of buffalo milk is around 54%. Its physiology is somewhat different from the lactating cows. Buffalo milk composition varies from cow milk in terms of higher fat and total solid content, which confers an advantage in preparation of specialized cheese, curd and other dairy products. Being a major milk producing animal in India it is highly essential to understand the lactation associated proteins in the mammary gland of buffalo. In the present investigation our attempt has been to identify new protein evidences which are expressed in lactating buffalo mammary gland and have not been reported before. The findings reported in the present study will help in understanding the lactation biology of buffalo mammary gland in particular and the mammary gland biology in general.

Cleavage of Histone 3 by Cathepsin D in the involuting mammary gland

The post-lactational regression of mammary gland is a complex multi-step process designed to conserve the biological function of the gland for next pregnancy. This developmental stage is a biological intrigue with great relevance to breast cancer research, and thus has been the subject of intensive scrutiny. Multipronged studies (microarray, proteomics profiling, animal knock-out models) have provided a repertoire of genes critical to involution. However, the caveat of these approaches remains in their failure to reveal post-translational modification(s), an emerging and critical aspect of gene regulation in developmental processes and mammary gland remodeling. The massive surge in the lysosomal enzymes concurrent with the onset of involution has been known for decades, and considered essential for “clearance” purposes. However, functional significance of these enzymes in diverse biological processes distinct from their proteolytic activity is just emerging. Studies from our laboratory had indicated specific post-translational modifications of the aspartyl endopeptidase Cathepsin D (CatD) at distinct stages mammary gland development. This study addresses the biological significance of these modifications in the involution process, and reveals that post-translational modifications drive CatD into the nucleus to cleave Histone 3. The cleavage of Histone 3 has been associated with cellular differentiation and could be critical instigator of involution process. From functional perspective, deregulated expression and increased secretion of CatD are associated with aggressive and metastatic phenotype of breast cancer. Thus unraveling CatD’s physiological functions in mammary gland development will bridge the present gap in understanding its pro-tumorigenic/metastatic functions, and assist in the generation of tailored therapeutic approaches.

Transcriptomic analysis reveals key regulators of mammogenesis and the pregnancy-lactation cycle

An organ unique to mammals, the mammary gland develops 90% of its mass after birth and experiences the pregnancylactation-involution cycle (PL cycle) during reproduction. To understand mammogenesis at the transcriptomic level and using a ribo-minus RNA-seq protocol, we acquired greater than 50 million reads each for the mouse mammary gland during pregnancy (day 12 of pregnancy), lactation (day 14 of lactation), and involution (day 7 of involution). The pregnancy-, lactation- and involution-related sequencing reads were assembled into 17344, 10160, and 13739 protein-coding transcripts and 1803, 828, and 1288 non-coding RNAs (ncRNAs), respectively. Differentially expressed genes (DEGs) were defined in the three samples, which comprised 4843 DEGs (749 up-regulated and 4094 down-regulated) from pregnancy to lactation and 4926 DEGs (4706 up-regulated and 220 down-regulated) from lactation to involution. Besides the obvious and substantive up- and down-regulation of the DEGs, we observe that lysosomal enzymes were highly expressed and that their expression coincided with milk secretion. Further analysis of transcription factors such as Trps1, Gtf2i, Tcf7l2, Nupr1, Vdr, Rb1, and Aebp1, and ncRNAs such as mir-125b, Let7, mir-146a, and mir-15 has enabled us to identify key regulators in mammary gland development and the PL cycle.

FMRFamide-related peptides: anti-opiate transmitters acting in apoptosis

Members of the FMRFamide-related peptide (FaRP) family are neurotransmitters, hormone-like substances and tumor suppressor peptides. In mammals, FaRPs are considered as anti-opiate peptides due to their ability to inhibit opioid signaling. Some FaRPs are asserted to attenuate opiate tolerance. A recently developed chimeric FaRP (Met-enkephalin-FMRFa) mimics the analgesic effects of opiates without the development of opiate-dependence, displaying a future therapeutical potential in pain reduction. In this review we support the notion, that opiates and representative members of the FaRP family show overlapping effects on apoptosis. Binding of FaRPs to opioid receptors or to their own receptors (G-protein linked membrane receptors and acid-sensing ion channels) evokes or suppresses cell death, in a cell- and receptor-type manner. With the dramatically increasing incidence of opiate abuse and addiction, understanding of opioid-induced cell death, and in this context FaRPs will deserve growing attention.

Acute involution in the tammar wallaby: identification of genes and putative novel milk proteins implicated in mammary gland function

Marsupials provide a suitable alternative model to studying mammary gland involution. They have evolved a different reproductive strategy from eutherians, giving birth to an altricial young and secreting milk that changes in composition during lactation. In this study, we used a marsupial-specific EST microarray to identify 47 up-regulated genes during mammary gland involution in the tammar wallaby (Macropus eugenii). These include the pro-apoptotic tumour necrosis factor receptor superfamily 21 (TNFRSF21) gene, whose expression in the mammary gland has not previously been reported. Genes encoding putative novel milk proteins which may protect the mammary gland from infection were also found to be up-regulated, such as amiloride binding protein 1 (ABP1), complement component 1QB (C1QB), complement component 4A (C4A) and colony stimulating factor 2 receptor β (CSF2Rβ). Our results show that the marsupial reproductive strategy was successfully exploited to identify genes and putative novel milk proteins implicated in mammary gland involution.

Singleminded-2s (Sim2s) promotes delayed involution of the mouse mammary gland through suppression of Stat3 and NFκB

Postlactational involution of the mammary gland provides a unique model to study breast cancer susceptibility and metastasis. We have shown that the short isoform of Singleminded-2s (Sim2s), a basic helix loop helix/PAS transcription factor, plays a role in promoting lactogenic differentiation, as well as maintaining mammary epithelial differentiation and malignancy. Sim2s is dynamically expressed during mammary gland development, with expression peaking during lactation, and decreasing in early involution. To determine the role of SIM2S in involution, we used transgenic mice expressing SIM2S under the mouse mammary tumor virus-Sim2s promoter. Overexpression of Sim2s in the mouse mammary gland resulted in delayed involution, indicated by a lower proportion of cleaved caspase-3-positive cells and slower reestablishment of the mammary fat pad. Immunohistochemical and quantitative RNA analysis showed a decrease in apoptotic markers and inflammatory response genes, and an increase in antiapoptotic genes, which were accompanied by inhibition of signal transducer and activator of transcription 3 activity. Microarray analysis confirmed that genes in the signal transducer and activator of transcription 3 signaling pathway were repressed by SIM2S expression, along with nuclear factor-κB and other key pathways involved in mammary gland development. Multiparous mouse mammary tumor virus-Sim2s females displayed a more differentiated phenotype compared with wild-type controls, characterized by enhanced β-casein expression and alveolar structures. Together, these results suggest a role for SIM2S in the normal involuting gland and identify potential downstream pathways regulated by SIM2S.

Mechanical strain induces involution-associated events in mammary epithelial cells

Background

Shortly after weaning, a complex multi-step process that leads to massive epithelial apoptosis is triggered by tissue local factors in the mouse mammary gland. Several reports have demonstrated the relevance of mechanical stress to induce adaptive responses in different cell types. Interestingly, these signaling pathways also participate in mammary gland involution. Then, it has been suggested that cell stretching caused by milk accumulation after weaning might be the first stimulus that initiates the complete remodeling of the mammary gland. However, no previous report has demonstrated the impact of mechanical stress on mammary cell physiology. To address this issue, we have designed a new practical device that allowed us to evaluate the effects of radial stretching on mammary epithelial cells in culture.

Results

We have designed and built a new device to analyze the biological consequences of applying mechanical stress to cells cultured on flexible silicone membranes. Subsequently, a geometrical model that predicted the percentage of radial strain applied to the elastic substrate was developed. By microscopic image analysis, the adjustment of these calculations to the actual strain exerted on the attached cells was verified. The studies described herein were all performed in the HC11 non-tumorigenic mammary epithelial cell line, which was originated from a pregnant BALB/c mouse. In these cells, as previously observed in other tissue types, mechanical stress induced ERK1/2 phosphorylation and c-Fos mRNA and protein expression. In addition, we found that mammary cell stretching triggered involution associated cellular events as Leukemia Inhibitory Factor (LIF) expression induction, STAT3 activation and AKT phosphorylation inhibition.

Conclusion

Here, we show for the first time, that mechanical strain is able to induce weaning-associated events in cultured mammary epithelial cells. These results were obtained using a new practical and affordable device specifically designed for such a purpose. We believe that our results indicate the relevance of mechanical stress among the early post-lactation events that lead to mammary gland involution.

Inhibition of basal p38 or JNK activity enhances epithelial barrier function through differential modulation of claudin expression

Tight junctions (TJs) form a barrier to the paracellular diffusion of ions and solutes across epithelia. Although transmembrane proteins of the claudin family have emerged as critical determinants of TJ permeability, little is known about the signaling pathways that control their expression. The aim of this study was to assess the role of three mitogen-activated protein kinases (MAPKs), i.e., extracellular signal-regulated kinase-1/2 (ERK1/2), c-Jun NH(2)-terminal kinases (JNKs), and p38 kinases, in the regulation of epithelial barrier function and claudin expression in mammary epithelial cells. Addition of either PD169316 (a p38 inhibitor) or SP600125 (a JNK inhibitor) induced formation of domes (a phenomenon dependent on TJ barrier function) and enhanced transepithelial electrical resistance, whereas U0126 (an inhibitor of the ERK1/2 activators MEK1/MEK2) had no significant effect. Similar results were obtained using mechanistically unrelated p38 or JNK inhibitors. PD169316 increased the expression of claudin-4 and -8, whereas SP600125 increased claudin-4 and -9 and downregulated claudin-8. Silencing of p38alpha by isoform-specific small interfering RNAs increased claudin-4 and -8 mRNAs, whereas silencing of p38beta only increased claudin-4 mRNA. Silencing of either JNK1 or JNK2 increased claudin-9 mRNA expression while decreasing claudin-8 mRNA. Moreover, selective silencing of JNK2 increased claudin-4 and -7 mRNAs. Finally, both PD169316 and SP600125 inhibited the paracellular diffusion of Na(+) and Cl(-) across epithelial monolayers. Collectively, these results provide evidence that inhibition of either p38 or JNK enhances epithelial barrier function by selectively modulating claudin expression, implying that the basal activity of these MAPKs exerts a tonic effect on TJ ionic permeability.

PD98059 enhanced insulin, cytokine, and growth factor activation of xanthine oxidoreductase in epithelial cells involves STAT3 and the glucocorticoid receptor

PD98059 and U0126 are organic compound inhibitors frequently used to block the activity of the MEK-1/2 protein kinase. In the present work, promoter activation analyses of xanthine oxidoreductase (XOR) in epithelial cells uncovered the unexpected opposite effect of these inhibitors on activation of XOR. Activation of an XOR-luciferase fusion gene was studied in stably transfected epithelial cells. The XOR reporter gene was activated by the epidermal growth factors (EGF), prolactin, and dexamethasone and by the acute phase cytokines (APC) IL-1, IL-6, and TNFalpha as previously reported for its native gene, and insulin further stimulated activation induced with acute phase cytokines or growth factors. Activation of the proximal promoter was blocked by inhibitors of the glucocorticoid receptor (GR), p38 MAP kinase, and U0126. Unexpectedly, PD98059 activated the promoter and significantly enhanced expression induced by insulin, APC, or growth factors. Analysis of the XOR upstream DNA and proximal promoter revealed primary roles for the GR and STAT3 in mediating the effects of PD98059 on XOR activation and protein complex formation with the promoter. STAT3 phosphotyrosine-705 was rapidly induced by PD98059, dexamethasone, and insulin. XOR activation by PD98059, dexamethasone, or insulin was superinduced by a constitutively active derivative of STAT3, while a dominant negative derivative of STAT3 blocked the enhancing effect of PD98059 on XOR activation. These data demonstrate a previously unrecognized effect of PD98059 on STAT3 and the GR that could have unanticipated consequences when used to infer the involvement of the MEK-1/2 protein kinase.

Induction of serum amyloid A genes is associated with growth and apoptosis of HC11 mammary epithelial cells

In this study, we examined the expression and functions of serum amyloid A (SAA) isoforms during apoptosis of HC11 mammary gland epithelial cells. Expression of SAA mRNAs and apoptosis were increased in HC11 cells by serum withdrawal and gradually decreased upon the addition of serum, or epidermal growth factor (EGF). TNFalpha treatment of HC11 cells also induced expression of SAA genes, and the effect on SAA1 and SAA2 expression was suppressed by treatment with MG132, and in cells transfected with a dominant negative mutant form of IkappaBalpha. Similar results were observed in response to interleukin-1 (IL-1), IL-6 and interferon gamma (IFNgamma). Furthermore, overexpression of the SAA1 and SAA2 isoforms suppressed growth and accelerated apoptosis of HC11 cells by increasing caspase 3/7 and caspase 8 activities, but the apoptotic effect of tumor necrosis factor alpha (TNFalpha) on HC11 cells was not enhanced. We found that expression of SAA1 and SAA2, but not SAA3, was regulated by an NFkappaB-dependent pathway, and that overexpression of SAA isoforms accelerated the apoptosis of HC11 cells.

Activin and glucocorticoids synergistically activate follicle-stimulating hormone beta-subunit gene expression in the immortalized LbetaT2 gonadotrope cell line

FSH is produced by the pituitary gonadotrope to regulate gametogenesis. Production of the beta-subunit of FSH is the rate-limiting step in FSH synthesis, and a number of peptide and steroid hormones within the reproductive axis have been found to regulate transcription of the FSH beta-subunit gene. Although both activin and glucocorticoids are notable regulators of FSHbeta by themselves, we find that cotreatment results in a synergistic interaction on the mouse FSHbeta promoter at the level of the gonadotrope using transient transfection of a reporter gene into the LbetaT2 immortalized gonadotrope-derived cell line. This synergistic interaction is specific to FSHbeta, because only additive effects of these two hormones are observed on LH beta-subunit, GnRH receptor, and mouse mammary tumor virus gene expression. Components of both activin and glucocorticoid signaling are found to be necessary for synergy, and there are specific cis elements on the mouse FSHbeta promoter that contribute to the synergistic response as well. We also identify novel activin-responsive regions in the mouse FSHbeta promoter and find that the -120 site can bind Smad2/3 in vitro. In addition, the glucocorticoid receptor and Smad3 are sufficient to confer a striking synergy with glucocorticoids on the mouse FSHbeta promoter. Our studies provide the first evidence of a synergistic interaction between activin and glucocorticoids within the gonadotrope cell and demonstrate that this synergy can occur directly at the level of the mouse FSHbeta promoter.

Role of amino acids in translational mechanisms governing milk protein synthesis in murine and ruminant mammary epithelial cells

The role of amino acids (AA) on translational regulation in mammary epithelial cells cultured under lactogenic conditions was studied. The rates of total protein synthesis and beta-lactoglobulin (BLG) synthesis in mouse CID-9 cells were 2.1- or 3.1-fold higher, respectively, than in their bovine L-1 counterparts. Total AA deprivation or selective deprivation of Leu had a negative protein-specific effect on BLG synthesis that was more pronounced in bovine cells than in murine cells. Dephosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and S6 kinase (S6K1) on Thr(389) but not on Ser(411) was also more prominent in bovine cells. Noteably, deprivation of Leu had a less marked effect on BLG synthesis and 4E-BP1 or S6K1 phosphorylation than deprivation of all AA. In AA-deprived CID-9 cells, Leu specifically restored BLG synthesis from pre-existing mRNA whereas AA also restored total protein synthesis. This restoration was associated with a more pronounced effect on 4E-BP1 and S6K1 phosphorylation in bovine versus murine cells. Rapamycin specifically reduced Leu- and AA-stimulated BLG translation initiation in a dose-dependent manner. A further reduction was observed for Leu-treated cells in the presence of LY294002, a PI3K (phosphatidylinositol 3-kinase) inhibitor, which also reduced total protein synthesis. These findings suggest that direct signaling from AA to the translational machinery is involved in determining the rates of milk protein synthesis in mammary epithelial cells.

The Genes Induced by Signal Transducer and Activators of Transcription (STAT)3 and STAT5 in Mammary Epithelial Cells Define the Roles of these STATs in Mammary Development

Prolactin and leukemia inhibitory factor (LIF) have different roles in the adult mammary gland, which are mediated in part by the signal transducers and activators of transcription (STAT)5 and STAT3. In vivo studies have shown that STAT5 contributes to prolactin-dependent lobuloalveolar development and lactation whereas STAT3 mediates LIF-dependent epithelial apoptosis during postlactational involution. To understand the molecular basis of these STAT-dependent pathways, we demonstrate the ligand-independent effects of STAT5 and STAT3 in mammary epithelial cells in vitro and also identify the genes regulated by these related transcription factors. Thus, using conditionally active STAT3- or STAT5a-GyraseB fusion proteins, we observed that enforced and specific dimerization of STAT3 induced apoptosis whereas STAT5 induced differentiation of mammary epithelial cells. Furthermore, STAT5 attenuated apoptosis mediated by LIF, the physiological inducer of STAT3. Microarray analysis of STAT3- and STAT5-induced genes using this system demonstrated a marked specificity, which reflected their different physiological effects in vitro and in vivo. STAT5-specific gene targets included the milk protein genes alpha-casein and kallikrein-8 and the survival factors prosaposin and Grb10. STAT3-specific genes included the apoptosis regulators CCAAT enhancer binding protein-delta, phosphatidylinositol 3-kinase-regulatory subunits, purine nucleoside phosphorylase, and c-fos. These data illustrate that specific activation of STAT3 and STAT5 alone is sufficient to induce and suppress apoptosis, respectively, and that these transcription factors elicit their actions by inducing distinct subsets of target genes in mammary epithelial cells.

MUC1 overexpression results in mammary gland tumorigenesis and prolonged alveolar differentiation

MUC1 is a transmembrane mucin that was initially cloned from malignant mammary epithelial cells as a tumor antigen. More than 90% of human breast carcinomas overexpress MUC1. Numerous studies have demonstrated an interaction between MUC1 and other oncogenic proteins such as beta-catenin, erbB receptors and c-Src, but a functional role for MUC1 in transformation has not been identified. We previously reported the development of transgenic mice that overexpress human MUC1 in the mouse mammary gland (MMTV-MUC1). Analysis of these transgenic mice at an early age demonstrated the ability of MUC1 to potentiate EGF-dependent activation of MAP kinase signaling pathways in the lactating mammary gland. We now report that multiparous MMTV-MUC1 transgenic mice stochastically develop unifocal mammary gland carcinomas late in life. Molecular analysis of these tumors shows a tumor-specific coimmunoprecipitation between MUC1 and beta-catenin. Examination of the contralateral glands in MMTV-MUC1 transgenics demonstrates that the development of frank carcinomas is accompanied by a failure of multiparous glands to undergo postlactational involution. Furthermore, uniparous MMTV-MUC1 transgenic mice display decreased postlactational apoptosis, elevated whey acidic protein expression and aberrant pErk2 activation. These findings are the first to determine that MUC1 overexpression promotes in vivo transformation of the mammary gland.

Sex steroids and growth factors in the regulation of mammary gland proliferation, differentiation, and involution

The mammary gland is subjected to major morphological and biochemical changes during the lactation cycle. It is therefore not surprising that this dynamic process is strictly controlled. The importance of the sex steroid hormones 17beta-estradiol and progesterone for normal development of the mammary gland was recognized several decades ago and has been unequivocally confirmed since. Furthermore, it is now also established that the influence of sex steroids is not restricted to mammogenesis, but that these hormones also control involution. Another important regulatory role is played by growth factors that have been shown to modulate survival (epidermal growth factor, amphiregulin, transforming growth factor alpha, insulin like growth factor, and tumor necrosis factor alpha) or apoptosis (tumor necrosis factor alpha, transforming growth factor beta) of mammary cells. However, the molecular mechanism underlying the influence of sex steroid hormones and/or growth factors on the development and function of the mammary gland remains largely unknown to date. Also scarce is information on the interaction between both groups of modulators. Nevertheless, based on the current indications compiled in this review, an important functional role for sex steroid hormones in the lactation cycle in co-operation with growth factors can be suggested.

Decreased expression of ?1-integrin and focal adhesion kinase in epithelial cells may initiate involution of mammary glands

The mechanisms regulating involution of mammary glands after weaning are not clear, but engorgement with milk is a key trigger. Many cell types require to be anchored to an extracellular matrix (ECM) as a prerequisite for survival and this is achieved via intregrins binding to specific motifs and signalling their attachment, intracellularly, via focal adhesion kinase (FAK). We sought to determine firstly, if expression of beta1-integrin and FAK is reduced during the first stage of involution. Expression of beta1-integrin and FAK was significantly reduced at 6 h after sealing teats and this was accompanied with a decreased abundance of cytochrome C in mitochondria. Secondly, we sought to determine if expression of beta1-integrin and FAK was restored during the first, partially reversible stage of involution (at 24 h), but not during the second irreversible stage, which occurs after 72 h. Re-suckling restored full expression of the 80 kDa fragment of FAK, but not of the 125 kDa protein or beta1-integrin at 24 h after weaning. Re-suckling did not restore expression of either peptide after 72 h. Changes in expression of cytochrome C and pro-caspase-3 (apoptotic markers) were similar to that of the 80 kDa fragment of FAK. These data suggest that epithelial cells can restore partial contact with their basement membrane during the first, reversible stage, but not during the second irreversible stage of involution. We speculate that decreased contact between epithelial cells and their basement membrane initiates apoptosis in mammary glands at weaning. This process begins within 6 h of pup withdrawal.

Involution of the mouse mammary gland is associated with an immune cascade and an acute-phase response, involving LBP, CD14 and STAT3

Introduction

Involution of the mammary gland is a complex process of controlled apoptosis and tissue remodelling. The aim of the project was to identify genes that are specifically involved in this process.

Methods

We used Affymetrix oligonucleotide microarrays to perform a detailed transcript analysis on the mechanism of controlled involution after withdrawal of the pups at day seven of lactation. Some of the results were confirmed by semi-quantitative reverse transcriptase polymerase chain reaction, Western blotting or immunohistochemistry.

Results

We identified 145 genes that were specifically upregulated during the first 4 days of involution; of these, 49 encoded immunoglobulin genes. A further 12 genes, including those encoding the signal transducer and activator of transcription 3 (STAT3), the lipopolysaccharide receptor (CD14) and lipopolysaccharide-binding protein (LBP), were involved in the acute-phase response, demonstrating that the expression of acute-phase response genes can occur in the mammary gland itself and not only in the liver. Expression of LBP and CD14 was upregulated, at both the RNA and protein level, immediately after pup withdrawal; CD14 was strongly expressed in the luminal epithelial cells. Other genes identified suggested neutrophil activation early in involution, followed by macrophage activation late in the process. Immunohistochemistry and histological staining confirmed the infiltration of the involuting mammary tissue with neutrophils, plasma cells, macrophages and eosinophils.

Conclusion

Oligonucleotide microarrays are a useful tool for identifying genes that are involved in the complex developmental process of mammary gland involution. The genes identified are consistent with an immune cascade, with an early acute-phase response that occurs in the mammary gland itself and resembles a wound healing process.

Inhibition of liver trans-sulphuration pathway by propargylglycine mimics gene expression changes found in the mammary gland of weaned lactating rats: role of glutathione

In the lactating mammary gland, weaning produces mitochondrial cytochrome c release and nuclear DNA fragmentation, as determined by gel electrophoresis. This is followed by a significant decrease in lactation. Weaning for 2 h produces an early induction of the tumour suppressor/transcription factor p53, whereas the oncoprotein c-Jun and c-Jun N-terminal kinase are elevated after 24 h of weaning when compared with controls. The expression of p21(cip1) and p27(kip1), cyclin-dependent kinase inhibitors, was significantly higher in weaned rats when compared with control lactating rats. All the changes mentioned above also happen in the lactating mammary gland when propargylglycine, an inhibitor of the liver trans-sulphuration pathway, is administered. This effect is partially reversed by N -acetylcysteine administration. The administration of buthionine sulphoximine, an irreversible inhibitor of gamma-glutamylcysteine synthetase, to lactating rats produces a decrease in GSH levels and changes in protein concentrations and gene transcripts similar to those in rats with impaired trans-sulphuration pathway. These data suggest that the inter-tissue flux of GSH is an important mechanism of L-cysteine delivery to the lactating mammary gland and emphasize the importance of this physiological event in maintaining the gene expression required to sustain lactation.

Microarray analysis of the involution switch

Mammary epithelial cells (MEC) undergo a series of developmental decisions during a pregnancy cycle. The switches from proliferation to differentiation to secretion and then to cell death are precisely controlled. In order to identify critical changes associated with the transition from a secretory phenotype during lactation to dedifferentiation and cell death, we have undertaken a microarray analysis of mouse mammary gland development. We have focused on the involution switch and on the transcription profiles of genes that are targets of transcription factors known to influence involution and apoptosis. Our results show that both Stat3 and NF-kB target genes are induced by the involution switch while Stat5 target genes are distinct from Stat3 induced genes. Furthermore, a substantial number of genes that were specifically upregulated at the start of involution are regulators of inflammation and the acute phase response. These results provide a novel insight into the involution process and demonstrate the value of microarray analysis in defining molecular events associated with critical developmental transitions in mammary gland.

Involvement of Oct-1 in transcriptional regulation of beta-casein gene expression in mouse mammary gland

Mouse beta-casein gene promoter contains a region termed block C which is crucial for its gene transcription induced by lactogenic hormones. Nuclear extracts from mouse mammary glands contain at least two binding complexes (DS1 and DS2) which specifically bind to double-stranded block C region DNA. The binding sequence of these complexes was identified to be 5'-AAATTAGCATGT-3' which contains a sequence element related to the consensus octamer motif's complement ATTTGCAT. In the present study, we demonstrate that this sequence element indeed is the binding site for octamer-binding transcription factors (Octs) and Octs represent the double-stranded DNA binding proteins specifically binding to the block C region. Formation of the specific double-stranded binding complexes can be completely blocked by Oct binding motif oligonucleotides and anti-rOct-1 antiserum. We also show that Oct-1B represents at least partial, if not all, double-stranded binding protein, DS1, in mammary nuclear extract. Oct-1B may function as a transcriptional activator on casein gene promoter. The Oct binding activity to beta-casein gene promoter in the mammary gland is affected under influence of hormones both in vitro and in vivo. The DS1 binding activity can be induced by the combination of lactogenic hormones insulin, hydrocortisone and prolactin in organ culture of virgin mouse mammary gland. The binding activity in vivo can be induced by injection of progesterone or its combination with estradiol in virgin mice.

Transcription factor NFIC undergoes N-glycosylation during early mammary gland involution

Expression of a 74-kDa nuclear factor I (NFI) protein is triggered in early involution in the mouse mammary gland, and its expression correlates with enhanced occupation of a twin (NFI) binding element in the clusterin promoter, a gene whose transcription is induced at this time (Furlong, E. E., Keon, N. K., Thornton, F. D., Rein, T., and Martin, F. (1996) J. Biol. Chem. 271, 29688-29697). We now identify this 74-kDa NFI as an NFIC isoform based on its interaction in Western analysis with two NFIC-specific antibodies. A transition from the expression of a 49-kDa NFIC in lactation to the expression of the 74-kDa NFIC in early involution is demonstrated. We show that the 74-kDa NFIC binds specifically to concanavalin A (ConA) and that this binding can be reversed by the specific ConA ligands, methyl alpha-D-mannopyranoside and methyl alpha-D-glucopyranoside. In addition, its apparent molecular size was reduced to approximately 63 kDa by treatment with the peptide N-glycosidase. The 49-kDa lactation-associated NFIC did not bind ConA nor was it affected by peptide N-glycosidase. Tunicamycin, a specific inhibitor of N-glycosylation, blocked formation of the 74-kDa NFI in involuting mouse mammary gland in vivo when delivered from implanted Elvax depot pellets. Finally, the production of the ConA binding activity could be reiterated in "mammospheres" formed from primary mouse mammary epithelial cells associated with a laminin-rich extracellular matrix. Synthesis of the 74-kDa NFIC was also inhibited in this setting by tunicamycin. Thus, involution triggers the production of an NFIC isoform that is post-translationally modified by N-glycosylation. We further show, by using quantitative competitive reverse transcriptase-PCR, that there is increased expression of the major mouse mammary NFIC mRNA transcript, mNFIC2, in early involution, suggesting that the involution-associated change in NFIC expression also has a transcriptional contribution.

Mouse mammary gland involution is associated with cytochrome c release and caspase activation

At weaning, milk producing mammary epithelial cells undergo apoptosis and are removed by phagocytosis. Here, we show that mouse mammary gland involution is associated with mitochondrial cytochrome c release and processing of numerous caspases, including caspase-1, -3, -7, -8 and -9. Induction of caspase-3-like activity paralleled cleavage of poly-(ADP--ribose) polymerase. Dexamethasone inhibited processing of caspase-3, -7 and -8 and apoptosis, but had no effect on caspase-1 accumulation and cytochrome c release. In Bcl-2 transgenic animals, cytochrome c release, caspase activation and apoptosis were impaired. Thus, the pro-apoptotic signaling pathway in mammary epithelial cells during involution involves the release of cytochrome c and activation of caspases. It is inhibited by Bcl-2 at the mitochondrial level and by dexamethasone at a post-mitochondrial level.

Dysregulation of mammary Stats 1,3 and 5 and PRL receptors by overexpression of TGFalpha

Mammary TGFalpha overexpression results in delayed involution and eventually mammary cancer in transgenic mice. We hypothesized that STATs and PRL receptors (PRLR), critical regulators of mammary function, are altered in these animals and may contribute to this phenotype. We examined these factors late in the first pregnancy (d.18) and during normal involution (d.4 post-lactation) in WAP-TGFalpha transgenic mice and non-transgenic controls. Long form PRLR mRNA in WAP-TGFalpha glands at both pregnant d.18 and d.4 post-lactation was significantly reduced compared to controls, and PRLR-S3 failed to rise during involution. Total and pTyr STAT 1,3,5a and 5b also were altered. STAT 3 was higher at both times in WAP-TGFalpha glands. STAT 5a and 5b were lower at late pregnancy, but higher post-lactation; however, pTyr(694) STAT 5 was abnormally low at both times. Thus overexpression of TGFalpha has direct or indirect effects on both STATs and PRL responsiveness in vivo, which may reflect mechanisms of TGFalpha-induced mammary epithelial abnormalities.

Inability of overexpressed des(1-3)human insulin-like growth factor I (IGF-I) to inhibit forced mammary gland involution is associated with decreased expression of IGF signaling molecules

Overexpression of des(1-3) human insulin-like growth factor I (IGF-I) in the mammary glands of transgenic mice (WAP-DES) inhibits apoptosis during natural, but not forced, mammary involution. We hypothesized that this differential response would correlate with the expression of IGF signal transducers. Forced and natural involution were analyzed in nontransgenic and WAP-DES mice beginning on day 16 postpartum. During natural involution, mammary gland wet weight was higher and apoptosis was lower in WAP-DES than in nontransgenic mice. The WAP-DES transgene had no effect on these parameters during forced involution. Mammary tissue concentrations of the transgene protein were 2- to 10-fold higher than those of endogenous IGF-I. Western blot analysis of pooled mammary tissue extracts demonstrated only slightly higher phosphorylation of the IGF signal transducers insulin receptor substrate-1 (IRS-1) and Akt in the WAP-DES than in nontransgenic mice. Dramatic early reductions in phospho-IRS-1, phospho-Akt, IRS-1, IRS-2, and Akt proteins occurred during forced, but not natural, involution. The abundance of the IGF-I receptor and the messenger RNAs for the IGF-I receptors, IRS-1 and -2, were not affected by either genotype or involution. These findings support the conclusions that mammary cells lose their responsiveness to insulin-like signals during forced involution, and that posttranscriptional or posttranslational regulation of IRS-1 and IRS-2 may play a role in this loss.

The tenascin family of ECM glycoproteins: structure, function, and regulation during embryonic development and tissue remodeling

The determination of animal form depends on the coordination of events that lead to the morphological patterning of cells. This epigenetic view of development suggests that embryonic structures arise as a consequence of environmental influences acting on the properties of cells, rather than an unfolding of a completely genetically specified and preexisting invisible pattern. Specialized cells of developing multicellular organisms are surrounded by a complex extracellular matrix (ECM), comprised largely of different collagens, proteoglycans, and glycoproteins. This ECM is a substrate for tissue morphogenesis, lends support and flexibility to mature tissues, and acts as an epigenetic informational entity in the sense that it transduces and integrates intracellular signals via distinct cell surface receptors. Consequently, ECM-receptor interactions have a profound influence on major cellular programs including growth, differentiation, migration, and survival. In contrast to many other ECM proteins, the tenascin (TN) family of glycoproteins (TN-C, TN-R, TN-W, TN-X, and TN-Y) display highly restricted and dynamic patterns of expression in the embryo, particularly during neural development, skeletogenesis, and vasculogenesis. These molecules are reexpressed in the adult during normal processes such as wound healing, nerve regeneration, and tissue involution, and in pathological states including vascular disease, tumorigenesis, and metastasis. In concert with a multitude of associated ECM proteins and cell surface receptors that include members of the integrin family, TN proteins impart contrary cellular functions, depending on their mode of presentation (i.e., soluble or substrate-bound) and the cell types and differentiation states of the target tissues. Expression of tenascins is regulated by a variety of growth factors, cytokines, vasoactive peptides, ECM proteins, and biomechanical factors. The signals generated by these factors converge on particular combinations of cis-regulatory elements within the recently identified TN gene promoters via specific transcriptional activators or repressors. Additional complexity in regulating TN gene expression is achieved through alternative splicing, resulting in variants of TN polypeptides that exhibit different combinations of functional protein domains. In this review, we discuss some of the recent advances in TN biology that provide insights into the complex way in which the ECM is regulated and how it functions to regulate tissue morphogenesis and gene expression.

An atlas of mouse mammary gland development

The mouse mammary gland is a complex tissue, which is continually undergoing changes in structure and function. Embryonically, the gland begins with invasion of the underlying fat pad by a rudimentary ductal structure. Postnatal growth occurs in two phases: ductal growth and early alveolar development during estrous cycles, and cycles of proliferation, differentiation, and death that occur with each pregnancy, lactation, and involution. The variety of epithelial structures and stromal changes throughout the life of a mammary gland makes it a challenge to study. The purpose of this histological review is to give a brief representation of the morphological changes that occur throughout the cycle of mouse mammary gland development so that developmental changes observed in mouse models of mammary development can be appreciated.