Local IGF-I axis in peripubertal ruminant mammary development

14-3-3β is essential for milk composition stimulated by Leu/IGF-1 via IGF1R signaling pathway in BMECs

The cell proliferation of bovine mammary epithelial cells (BMECs) and consequent milk synthesis are regulated by multiple factors. The purpose of this study was to examine the effect of 14-3-3β on cellular proliferation and milk fat/β-casein synthesis in BMECs and reveal its underlying mechanisms. In this study, we employed gene function analysis to explore the regulatory effect and molecular mechanisms of 14-3-3β on milk synthesis and proliferation in BMECs. We found that leucine and IGF-1 enhance cell proliferation and milk synthesis in a 14-3-3β-dependent manner and only exhibiting such effect in the presence of 14-3-3β. We further determined that 14-3-3β interacts with the IGF1R self-phosphorylation site and it additionally mediated leucine and IGF-1 to stimulate the synthesis of milk through the IGF1R-AKT-mTORC1 signaling pathway. In summary, our data indicated that 14-3-3β mediates the expression of milk fat and protein stimulated by leucine and IGF-1, leading to lactogenesis through IGF1R signaling pathway in BMECs.

Late gestation hyperthermia: epigenetic programming of daughter's mammary development and function

Exposure to stressors during early developmental windows, such as prenatally (i.e., in utero), can have life-long implications for an animal's health and productivity. The mammary gland starts developing in utero and, like other developing tissues and organs, may undergo fetal programming. Previous research has implicated factors, such as prenatal exposure to endocrine disruptors or alterations in maternal diet (e.g., maternal over or undernutrition), that can influence the developmental trajectory of the offspring mammary gland in postnatal life. However, the direct links between prenatal insults and future productive outcomes are less documented in livestock species. Research on in utero hyperthermia effects on early-life mammary development is scarce. This review will provide an overview of key developmental milestones taking place in the bovine mammary gland during the pre- and postnatal stages. We will showcase how intrauterine hyperthermia, experienced by the developing fetus during the last trimester of gestation, derails postnatal mammary gland development and impairs its synthetic capacity later in life. We will provide insights into the underlying histological, cellular, and molecular mechanisms taking place at key postnatal developmental life stages, including birth, weaning and the first lactation, that might explain permanent detriments in productivity long after the initial exposure to hyperthermia. Collectively, our studies indicate that prenatal hyperthermia jeopardizes the normal developmental trajectory of the mammary gland from fetal development to lactation. Further, in utero hyperthermia epigenetically programs the udder, and possibly other organs critical to lactation, yielding a less resilient and less productive cow for multiple lactations.

Adrenergic Tone as an Intermediary in the Temperament Syndrome Associated With Flight Speed in Beef Cattle

The temperament of farm animals can influence their resilience to everyday variations within the managed production environment and has been under strong direct and indirect selection during the course of domestication. A prominent objective measure used for assessing temperament in beef cattle is the behavioral flight response to release from confinement in a crush or chute. This behavioral measure, termed flight speed (also known as escape velocity) is associated with physiological processes including body temperature, feeding behavior, growth rate, carcass composition, immune function, and health outcomes. This review examines the functional links between this suite of traits and adrenergic activity of the sympathetic nervous system and the adrenomedullary hormonal system. It is suggested that flight speed is the behavioral aspect of an underlying “flightiness” temperament syndrome, and that elevated adrenergic tone in animals with a high level of flightiness (i.e., flighty animals) tunes physiological activities toward a sustained “fight or flight” defense profile that reduces productivity and the capacity to flourish within the production environment. Nonetheless, despite a common influence of adrenergic tone on this suite of traits, variation in each trait is also influenced by other regulatory pathways and by the capacity of tissues to respond to a range of modulators in addition to adrenergic stimuli. It is suggested that tuning by adrenergic tone is an example of homeorhetic regulation that can help account for the persistent expression of behavioral and somatic traits associated with the flight speed temperament syndrome across the life of the animal. At a population level, temperament may modulate ecological fit within and across generations in the face of environmental variability and change. Associations of flight speed with the psychological affective state of the animal, and implications for welfare are also considered. The review will help advance understanding of the developmental biology and physiological regulation of temperament syndromes.

Mulberry Leaf Flavonoids Improve Milk Production, Antioxidant, and Metabolic Status of Water Buffaloes

This study was aimed to evaluate the effect of mulberry leaf flavonoids (MLF) on oxidative stress, metabolic hormones, and milk production in Murrah buffaloes. Forty multiparous Murrah buffaloes (4 ± 1 lactations) with similar body weight (average 600 ± 50 Kg) and stage of lactation (90 ± 20 d) were randomly selected for this trial. Four treatment groups (10 buffaloes per group) with different doses of MLF included; control (0 g/d), MLF15 (15 g/d), MLF30 (30 g/d), and MLF45 (45 g/d). Buffaloes were fed with total mix ration consisting of grass (Pennisetum purpureum schum), brewery's grain and concentrate mixture for 5 weeks. Meteorological data including ambient temperature and relative humidity were recorded using the online dust monitoring system to calculate temperature-humidity index (THI). After 1 week of the adaptation, milk yield was recorded daily while physiological parameters (respiratory rate, rectal, and body surface temperature), and milk composition were measured weekly. At the end of the trial, blood samples were collected to analyze serum metabolic hormones including estradiol (E2), growth hormone (GH), prolactin (PRL), Tri-iodothyronine (T3), and Thyroxine (T4). Moreover, serum heat shock proteins (HSP), antioxidants enzymes including malondialdehyde (MDA), total antioxidant capacity (T-AOC), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) and blood biochemical indices were also analyzed. Results revealed a decrease (P = 0.012) in serum MDA level while increasing (P < 0.01) the HSP and serum GHS-Px contents in supplemented buffaloes. Treatment showed a linear and quadratic decrease (p = 0.001) in the serum T-AOC while reducing CAT contents linearly (p = 0.012) as compared to the control. However, no effect of treatment on serum SOD content was observed. Treatment resulted a linear increase (p = 0.001) in serum GH and PRL hormones while increasing serum E2 levels linearly (P < 0.001) and quadratically (P = 0.025). Treatment increased (p = 0.038) the daily milk yield as compared to the control. However, increase (P < 0.05) in serum T3 and T4 contents, fat corrected milk (4%) and milk protein (%) was observed only in MLF45. Moreover, we observed no change in serum biochemical indices except insulin which linearly increased (p = 0.002) in MLF45. Our findings indicated that MLF at 45 g per day is an appropriate level to enhance milk performance and alleviate heat stress in buffaloes.

In Utero Heat Stress Programs Reduced Performance and Health in Calves

Heat stress during late gestation adversely impacts the developing calf. Calves that experience heat stress are born at a lower bodyweight and those deficits persist at least until puberty. In utero heat stress reduces passive transfer and calf survival. Late gestation heat stress programs a phenotype with lower milk yield, relative to herd mates born to cooled dams, in the first lactation and subsequent lactations.

Mammary Epithelial Cell Lineage Changes During Cow's Life

Milk production is highly dependent on the optimal development of the mammary epithelium. It is therefore essential to better understand mammary epithelial cell growth and maintenance from the related epithelial lineage during the animal life. Here, we characterized the epithelial lineage at puberty, lactation and dry-off in bovine using the cell surface markers CD49_f, CD24, and CD10. The pubertal period was characterized by a high proportion of CD49_f^pos cells corresponding to various epithelial subpopulations, notably the CD24^pos subpopulations. The proportion of CD49_f^pos cells was weaker during lactation and dry-off, and CD24^pos cells were relatively few. Of note, the (sub)population profile at dry-off appeared close to that during lactation. Using a targeted gene approach, we associated specific genes with epithelial subpopulations, their expression level varying, or not, according to physiological stages. Caseins were only expressed in the CD49_f^medCD24^neg subpopulation. Basal marker genes (keratin(KRT)5, KRT14 and αSMA) were found in the CD49_f^highCD24^neg subpopulations. Luminal gene markers (KRT7, KRT8 and KRT19, CDH1 and the PRLR) were expressed in the CD49_f^lowCD24^neg subpopulation. The CD49_f^lowCD24^pos subpopulation, only abundant at puberty, expressed luminal gene markers and KI67 at high level. In contrast to others, the CD49_f^highCD24^pos cells accounted for a small proportion of total cells, decreasing from puberty to dry-off. They were characterized by expression of luminal and basal gene markers and low KI67 level. Interestingly, this subpopulation showed a remarkable stability of gene expression profile throughout physiological stages and bear the hallmark of quiescence that designate them as the potential bovine mammary stem cells.

Nutritional impact on mammary development in pigs: a review

Milk yield is a crucial component of a sow operation because it is a limiting factor for piglet growth rate. Stimulating mammary development is one avenue that could be used to improve sow milk production. A number of studies have shown that nutrition of gilts or sows during the periods of rapid mammary accretion occurring during prepuberty, gestation, and lactation can affect mammary development. The present review provides an overview of all the information currently published on the subject. Various nutritional treatments can bring about increases in mammary tissue weight ranging from 27% to 52%. It was clearly established that feed restriction from 90 d of age (but not before 90 d) until puberty has detrimental effects on mammary development in pigs. Ad libitum feeding during that period increased mammary parenchymal weight by 36% to 52%. Body condition is also important because gilts that were obese (36-mm backfat) or too lean (12- to 15-mm backfat) in late gestation had less developed mammary tissue. Furthermore, overfeeding energy in late gestation seems to be detrimental. On the other hand, increasing energy and protein intakes of sows during lactation was beneficial for development of mammary tissue. Feeding certain plant extracts with estrogenic or hyperprolactinemic properties may also prove beneficial in stimulating mammary development at specific physiological periods. For example, feeding genistein to prepubertal gilts increased parenchymal DNA by 44%. Even though research was carried out on the nutritional control of mammogenesis in pigs, it is evident that much remains to be learned before the best nutritional strategy to enhance mammary development can be developed.

TRIENNIAL LACTATION SYMPOSIUM/BOLFA: Plasticity of mammary development in the prepubertal bovine mammary gland

Although peripubertal mammary development represents only a small fraction of the total mass of mammary parenchyma present in the udder at the end of gestation and into lactation, there is increasing evidence that the tissue foundations created in early life can affect future mammary development and function. Studies on expression of estrogen and progesterone receptors seem to confirm the relevance of these steroids in prepubertal mammary development, but connections with other growth factors, hormones, and local tissue factors remain elusive. Enhanced preweaning feeding in the bovine appears to enhance the capacity of mammary tissue to response to mammogenic stimulation. This suggests the possibility that improved early nutrition might allow for creation of stem or progenitor cell populations to better support the massive ductal growth and lobulo-alveolar development during gestation. Increasing evidence that immune cells are involved in mammary development suggests there are unexpected and poorly understood connections between the immune system and mammary development. This is nearly unexplored in ruminants. Development of new tools to identify, isolate, and characterize cell populations within the developing bovine mammary gland offer the possibility of identifying and perhaps altering populations of mammary stem cells or selected progenitor cells to modulate mammary development and, possibly, mammary function.

The effects of milking frequency on insulin-like growth factor I signaling within the mammary gland of dairy cows

In dairy cows, short-term changes in milking frequency (MF) in early lactation have been shown to produce both an immediate and a long-term effect on milk yield. The effect of MF on milk yield is controlled locally within mammary glands and could be a function of changes in either number or activity of secretory mammary epithelial cells (MEC). Insulin-like growth factor I (IGF-I) signaling is one candidate factor that could mediate these effects, as it can be controlled locally within mammary glands. Both MEC number and activity can be affected by IGF-I signaling by activating the phosphoinositide 3-kinase (PI3K)/Akt and extracellular-signal-regulated kinase (ERK)1/2 pathways. To investigate the relationship between MF and IGF-I signaling, udder halves of 17 dairy cows were milked either 4 times a day (4×) or once a day (1×) for 14 d in early lactation. On d 14, between 3 and 5 h following milking, mammary biopsies were obtained from 10 cows from both udder halves, and changes in the expression of genes associated with IGF-I signaling and the activation of the PI3K/Akt and ERK1/2 pathways were measured. The mRNA abundance of IGF type I receptor, IGF binding protein (IGFBP)-3, and IGFBP-5 were lower following 4× milking relative to 1× milking. However, the mRNA abundance of IGF-I was not affected by MF. Both IGFBP3 and IGFBP5 are thought to inhibit IGF-I; therefore, decreases in their mRNA abundance may serve to stimulate the IGF-I signal in the 4×-milked mammary gland. The activation of PI3K/Akt pathway was lower in response to 4× milking relative to 1×, and the activation of the ERK1/2 was unaffected by MF, suggesting that they do not mediate the effects of MF.

Up-regulation of integrin α6β4 expression by mitogens involved in dairy cow mammary development

In dairy cows, the extracellular microenvironment varies significantly from the virgin state to lactation. The function of integrin α6β4 is dependent on cell type and extracellular microenvironment, and the precise expression profile of α6β4 and its effects on mammary development remain to be determined. In the present study, real-time PCR and immunohistochemistry were used to analyze the expression and localization of integrin α6β4 in Holstein dairy cow mammary glands. The effects of integrin α6β4 on the proliferation induced by mammogenic mitogens were identified by blocking integrin function in purified dairy cow mammary epithelial cells (DCMECs). The results showed that the localization of β4 subunit and its exclusive partner the α6 subunit were not consistent but were co-localized in basal luminal cells and myoepithelial cells, appearing to prefer the basal surface of the plasma membrane. Moreover, α6 and β4 subunit messenger RNA (mRNA) levels changed throughout the stages of dairy cow mammary development, reflected well by protein levels, and remained higher in the virgin and pregnancy states, with duct/alveolus morphogenesis and active cell proliferation, than during lactation, when growth arrest is essential for mammary epithelial cell differentiation. Finally, the upregulation of integrin expression by both mammogenic growth hormone and insulin-like growth factor-1 and the inhibited growth of DCMECs by function-blocking integrin antibodies confirmed that integrin α6β4 was indeed involved in dairy cow mammary development.

Cellular proliferation rate and insulin-like growth factor binding protein (IGFBP)-2 and IGFBP-3 and estradiol receptor alpha expression in the mammary gland of dairy heifers naturally infected with gastrointestinal nematodes during development

Mammary ductal morphogenesis during prepuberty occurs mainly in response to insulin-like growth factor-1 (IGF-1) and estradiol stimulation. Dairy heifers infected with gastrointestinal nematodes have reduced IGF-1 levels, accompanied by reduced growth rate, delayed puberty onset, and lower parenchyma-stroma relationship in their mammary glands. Immunohistochemical studies were undertaken to determine variations in cell division rate, IGF-1 system components, and estradiol receptors (ESR) during peripubertal development in the mammary glands of antiparasitic-treated and untreated Holstein heifers naturally infected with gastrointestinal nematodes. Mammary biopsies were taken at 20, 30, 40, and 70 wk of age. Proliferating cell nuclear antigen immunolabeling, evident in nuclei, tended to be higher in the parenchyma of the glands from treated heifers than in those from untreated. Insulin-like growth factor binding proteins (IGFBP) type 2 and type 3 immunolabeling was cytoplasmic and was evident in stroma and parenchyma. The IGFBP2-labeled area was lower in treated than in untreated heifers. In the treated group, a maximal expression of this protein was seen at 40 wk of age, whereas in the untreated group the labeling remained constant. No differences were observed for IGFBP3 between treatment groups or during development. Immunolabeling for α ESR (ESR1) was evident in parenchymal nuclei and was higher in treated than in untreated heifers. In the treated group, ESR1 peaked at 30 wk of age and then decreased. These results demonstrate that the parasite burden in young heifers negatively influence mammary gland development, affecting cell division rate and parameters related to estradiol and IGF-1 signaling in the gland.

Short communication: Expression of T-box 2 and 3 in the bovine mammary gland

To increase our understanding of the mechanisms by which growth hormone (GH) and insulin-like growth factor (IGF)-I influence bovine mammary gland development, the potential roles of T-box2 (TBX2) and T-box3 (TBX3) were investigated. Although no information regarding expression of either transcription factor in the bovine mammary gland exists, it is known that TBX3 and its closely related family member, TBX2, are required for mammary gland development in humans and mice. Additionally, TBX3 mutations in humans and mice lead to ulnar mammary syndrome. Evidence is present in bone that TBX3 is required for proliferation and its expression is regulated by GH, an important regulator of mammary gland development and milk production. We hypothesized that TBX2 and TBX3 are expressed in the bovine mammary gland and that GH, IGF-I, or both increase TBX2 and TBX3 expression in bovine mammary epithelial cells (MEC). Bovine mammary gland tissue, MAC-T cells, primary MEC, and fibroblasts were obtained and TBX2 and TBX3 expression was determined by real-time reverse transcription PCR. In addition, TBX2 and TBX3 expression was examined in cells treated with 100 or 500 ng/mL of GH or 100 or 200 ng/mL of IGF-I for 24 or 48 h. Both TBX2 and TBX3 were expressed in bovine mammary tissue. Surprisingly, expression of TBX2 was only detected in mammary fibroblast cells, whereas TBX3 was expressed in all 3 cell types. Growth hormone did not alter TBX3 expression in MAC-T cells or MEC. However, IGF-I increased TBX3 expression in MAC-T, but not in primary MEC. We did not observe a change in TBX2 or TBX3 expression in fibroblasts treated with GH and IGF. Therefore, we concluded that (1) TBX2 and TBX3 are expressed in bovine mammary gland, (2) their expression is cell-type specific, and (3) IGF-I stimulates TBX3 expression in MAC-T cells.

Bovine mammary stem cells: cell biology meets production agriculture

Mammary stem cells (MaSC) provide for net growth, renewal and turnover of mammary epithelial cells, and are therefore potential targets for strategies to increase production efficiency. Appropriate regulation of MaSC can potentially benefit milk yield, persistency, dry period management and tissue repair. Accordingly, we and others have attempted to characterize and alter the function of bovine MaSC. In this review, we provide an overview of current knowledge of MaSC gained from studies using mouse and human model systems and present research on bovine MaSC within that context. Recent data indicate that MaSC retain labeled DNA for extended periods because of their selective segregation of template DNA strands during mitosis. Relying on this long-term retention of bromodeoxyuridine-labeled DNA, we identified putative bovine MaSC. These label-retaining epithelial cells (LREC) are in low abundance within mammary epithelium (<1%). They are predominantly estrogen receptor (ER)-negative and localized in a basal or suprabasal layer of the epithelium throughout the gland. Thus, the response of MaSC to estrogen, the major mitogen in mammary gland, is likely mediated by paracrine factors released by cells that are ER-positive. This is consistent with considerable evidence for cross-talk within and between epithelial cells and surrounding stromal cells. Excision of classes of cells by laser microdissection and subsequent microarray analysis will hopefully provide markers for MaSC and insights into their regulation. Preliminary analyses of gene expression in laser-microdissected LREC and non-LREC are consistent with the concept that LREC represent populations of stem cells and progenitor cells that differ with regard to their properties and location within the epithelial layer. We have attempted to modulate the MaSC number by infusing a solution of xanthosine through the teat canal and into the ductal network of the mammary glands of prepubertal heifers. This treatment increased the number of putative stem cells, as evidenced by an increase in the percentage of LREC and increased telomerase activity within the tissue. The exciting possibility that stem cell expansion can influence milk production is currently under investigation.

Effect of exogenous somatotropin in Holstein calves on mammary gland composition and proliferation

Pubertal mammary gland growth and development are hormonally regulated, but the details are poorly understood in calves. Our purpose was to evaluate the effects of exogenous growth hormone (GH) on the biochemical composition of the prepubertal mammary gland, mRNA expression of selected genes, and histological characteristics of the developing parenchyma (PAR). In this experiment, 19 calves (7 ± 4 d of age) were randomly assigned to 1 of 2 treatments: bovine somatotropin (bST, 500 mg; n = 10) or placebo (Sal; 0.9% saline; n = 9). Animals were treated every 3 wk beginning on d 23. Calves were assigned to an early (65 d; tissue harvested after 2 treatment injections) or late collection time (107 d; tissue harvested after 4 treatment injections). Calves were fed milk replacer and calf starter for 8 wk and starter and hay thereafter. Parenchyma and mammary fat pad (MFP) from one udder half were harvested for analysis of protein, lipid, and DNA. Additional tissues were preserved for histological analysis or snap-frozen for quantitative real-time PCR. Somatotropin treatment did not significantly alter the mass of PAR or MFP or the general pattern of development of epithelial structures. Significant increases were observed in protein/100 kg of body weight (BW), total protein, DNA concentration, DNA/100 kg of BW, and total DNA in 107-d calves, and a significant treatment by day interaction was observed for DNA and lipid concentrations in PAR. In MFP, a significant decrease was observed in protein/100 kg of BW in bST-treated calves and in total MFP protein in 65-d calves. A treatment by day interaction was found for total protein, DNA, and protein/100 kg of BW. In PAR, relative expression of ATPase-binding cassette 3 and growth hormone receptor were reduced by bST and both were lower in 107-d-harvest calves. Epithelial cell retention of bromodeoxyuridine (BrdU; possible indicator of stem-like cells) was greatest in 65-d bST-treated calves, and a significant time of sampling response and treatment × time interaction were observed. Expression of the proliferation marker protein Ki67 was numerically higher in bST-treated calves but the difference was nonsignificant. Retention of the BrdU label was reduced in 107-d calves. Exogenous growth hormone given to calves may affect mammary tissue composition and epithelial cell gene expression in subtle ways but exogenous supplementation with bST alone is not likely to alter overall development patterns or affect the mass of mammary parenchymal tissue. Whether such subtle changes have an effect on subsequent development or function is unknown.

Transcriptome analysis of epithelial and stromal contributions to mammogenesis in three week prepartum cows

Transcriptome analysis of bovine mammary development has provided insight into regulation of mammogenesis. However, previous studies primarily examined expression of epithelial and stromal tissues combined, and consequently did not account for tissue specific contribution to mammary development. Our objective was to identify differences in gene expression in epithelial and intralobular stromal compartments. Tissue was biopsied from non-lactating dairy cows 3 weeks prepartum, cut into explants and incubated for 2 hr with insulin and hydrocortisone. Epithelial and intralobular stromal tissues were isolated with laser capture microdissection. Global gene expression was measured with Bovine Affymetrix GeneChips, and data were preprocessed using RMA method. Moderated t-tests from gene-specific linear model analysis with cell type as a fixed effect showed more than 3,000 genes were differentially expressed between tissues (P<0.05; FDR<0.17). Analysis of epithelial and stromal transcriptomes using Database for Annotation, Visualization and Integrated Discovery (DAVID) and Ingenuity Pathways Analysis (IPA) showed that epithelial and stromal cells contributed distinct molecular signatures. Epithelial signatures were enriched with gene sets for protein synthesis, metabolism and secretion. Stromal signatures were enriched with genes that encoded molecules important to signaling, extracellular matrix composition and remodeling. Transcriptome differences also showed evidence for paracrine interactions between tissues in stimulation of IGF1 signaling pathway, stromal reaction, angiogenesis, neurogenesis, and immune response. Molecular signatures point to the dynamic role the stroma plays in prepartum mammogenesis and highlight the importance of examining the roles of cell types within the mammary gland when targeting therapies and studying mechanisms that affect milk production.

Ultrasound imaging of mammary glands in dairy heifers at different stages of growth

To obtain B-mode ultrasound images of mammary glands in dairy heifers at different stages of growth, 25 clinically normal Holstein heifers were used. The heifers were divided into 5 groups (n=5/group) by stage of their growth: 2-month-old (group 1), 5-month-old (group 2), postpuberty (group 3), mid (group 4), and late (group 5) pregnancy. Furthermore, the sections of mammary glands were observed grossly at postmortem examination in one heifer in each group. Ultrasound images varied with the development of mammary glands. In group 1, the mammary glands had distinctive ultrasonographic findings: an oval to fusiform homogeneous hypoechoic structure. In all groups except group 1, mammary tissue consists of two major areas: a homogeneous, medium echogenic area and a poorly-defined, heterogeneous, hypoechoic area mostly in the superficial part. The superficial hypoechoic area spread more extensively and more irregularly with the development of mammary glands. Most pregnant heifers had irregular and extremely hypoechoic or anechoic areas like lactiferous sinus in the glands. The gross findings of mammary glands suggested that the hypoechoic areas of various shapes represented the lactiferous sinus and ducts. Thus, these results indicate that B-mode ultrasound imaging can visualize the internal structures of udders and could be a useful tool for evaluation of mammary glands in heifers.

Functional and gene network analyses of transcriptional signatures characterizing pre-weaned bovine mammary parenchyma or fat pad uncovered novel inter-tissue signaling networks during development

BACKGROUND

The neonatal bovine mammary fat pad (MFP) surrounding the mammary parenchyma (PAR) is thought to exert proliferative effects on the PAR through secretion of local modulators of growth induced by systemic hormones. We used bioinformatics to characterize transcriptomics differences between PAR and MFP from approximately 65 d old Holstein heifers. Data were mined to uncover potential crosstalk through the analyses of signaling molecules preferentially expressed in one tissue relative to the other.

RESULTS

Over 9,000 differentially expressed genes (DEG; False discovery rate <or= 0.05) were found of which 1,478 had a >or=1.5-fold difference between PAR and MFP. Within the DEG highly-expressed in PAR vs. MFP (n = 736) we noted significant enrichment of functions related to cell cycle, structural organization, signaling, and DNA/RNA metabolism. Only actin cytoskeletal signaling was significant among canonical pathways. DEG more highly-expressed in MFP vs. PAR (n = 742) belong to lipid metabolism, signaling, cell movement, and immune-related functions. Canonical pathways associated with metabolism and signaling, particularly immune- and metabolism-related were significantly-enriched. Network analysis uncovered a central role of MYC, TP53, and CTNNB1 in controlling expression of DEG highly-expressed in PAR vs. MFP. Similar analysis suggested a central role for PPARG, KLF2, EGR2, and EPAS1 in regulating expression of more highly-expressed DEG in MFP vs. PAR. Gene network analyses revealed putative inter-tissue crosstalk between cytokines and growth factors preferentially expressed in one tissue (e.g., ANGPTL1, SPP1, IL1B in PAR vs. MFP; ADIPOQ, IL13, FGF2, LEP in MFP vs. PAR) with DEG preferentially expressed in the other tissue, particularly transcription factors or pathways (e.g., MYC, TP53, and actin cytoskeletal signaling in PAR vs. MFP; PPARG and LXR/RXR Signaling in MFP vs. PAR).

CONCLUSIONS

Functional analyses underscored a reciprocal influence in determining the biological features of MFP and PAR during neonatal development. This was exemplified by the potential effect that the signaling molecules (cytokines, growth factors) released preferentially (i.e., more highly-expressed) by PAR or MFP could have on molecular functions or signaling pathways enriched in the MFP or PAR. These bidirectional interactions might be required to coordinate mammary tissue development under normal circumstances or in response to nutrition.

In vivo expansion of the mammary stem/ progenitor cell population by xanthosine infusion

Mammary stem cells provide for growth and maintenance of the mammary gland and are therefore of considerable interest as determinants of productivity and efficiency of dairy animals and as targets of carcinogenesis in humans. Xanthosine treatment was previously shown to promote expansion of hepatic stem cells in vitro. The objective of this study was to determine if in vivo treatment with xanthosine can increase the mammary stem cell population. Xanthosine was infused into the right mammary glands of four female Holstein calves for 5 consecutive days. Immediately after each xanthosine treatment, calves were injected intravenously with 5-bromo-2-deoxyuridine (BrdU). Forty days after the final treatment, calves were euthanized and mammary tissue harvested. BrdU-label retaining epithelial cells (LREC) were detected immunohistochemically and quantified. Retention of BrdU was used as a marker for putative bovine mammary stem cells. Infusion of xanthosine into the bovine mammary gland significantly increased the number of LREC in treated glands compared to contralateral control glands (P < 0.05). LREC averaged 0.4% of epithelial cells in control glands and 0.8% in xanthosine-treated glands. The increase in LREC in xanthosine-treated glands was supported by a concomitant increase in telomerase activity (P < 0.01) and a correlation between LREC and telomerase (P < 0.05; r (2) = 0.7). Data indicate that in vivo treatment with xanthosine can be used to increase the number of mammary stem cells. This is the first demonstration of an in vivo treatment to increase the endogenous population of mammary stem cells, with utility for biomedical research and dairy management.

Intramammary infusion of leptin decreases proliferation of mammary epithelial cells in prepubertal heifers

High energy intake and excessive body fatness impair mammogenesis in prepubertal ruminants. High energy intake and excessive fatness also increase serum leptin. Our objective was to determine if an infusion of leptin decreases proliferation of mammary epithelial cells of prepubertal heifers in vivo. Ovine leptin at 100 microg/ quarter per d with or without 10 microg of insulin-like growth factor (IGF)-I was infused via the teat canal into mammary glands of prepubertal dairy heifers; contralateral quarters were used as controls. After 7 d of treatment, bromodeoxyuridine was infused intravenously and heifers were slaughtered approximately 2 h later. Tissue from 3 regions of the mammary parenchyma was collected and immunostained for bromodeoxyuridine (BrdU), proliferating cell nuclear antigen (Ki-67), and caspase-3. Leptin decreased the number of mammary epithelial cells in the S-phase of the cell cycle by 48% in IGF-I-treated quarters and by 19% in saline-treated quarters. Leptin did not alter the number of mammary epithelial cells within the cell cycle, as indicated by Ki-67 labeling. Caspase-3 immunostaining within the mammary parenchyma was very low in these heifers, but leptin significantly increased labeling in saline-treated quarters. Leptin enhanced SOCS-3 expression in IGF-I-treated quarters but did not alter SOCS-1 or SOCS-5 expression. We conclude that a high concentration of leptin in the bovine mammary gland reduces proliferation of mammary epithelial cells. The reduced proliferation is accompanied by an increase in SOCS-3 expression, suggesting a possible mechanism for leptin inhibition of IGF-I action. Whether leptin might be a physiological regulator of mammogenesis remains to be determined.

Identification of putative bovine mammary epithelial stem cells by their retention of labeled DNA strands

Stem cells appear to retain labeled DNA for extended periods because of their selective segregation of template DNA strands during mitosis. In this study, proliferating cells in the prepubertal bovine mammary gland were labeled using five daily injections of 5-bromo-2-deoxyuridine (BrdU). Five weeks later, BrdU-labeled mammary epithelial cells were still evident. The percentage of BrdU-labeled epithelial cells was greatest in the lower region of the mammary gland, near the gland cistern, and was decreased toward the periphery of the parenchymal region, where the ducts were invading the mammary fat pad. Increased numbers of BrdU-labeled epithelial cells in basal regions of the gland are likely a consequence of decreased proliferation rates and increased cell cycle arrest in this area. In peripheral regions of mammary parenchyma, the percentage of heavily labeled epithelial cells averaged 0.24%, a number that is consistent with estimates of the frequency of stem cells in the mouse mammary gland. Epithelial label-retaining cells seemingly represent a slowly proliferating population of cells, as 5.4% of heavily labeled cells were positive for the nuclear proliferation antigen Ki67. Because epithelial label-retaining cells contain estrogen receptor (ER)-negative and ER-positive cells, they apparently comprise a mixed population, which I suggest is composed of ER-negative stem cells and ER-positive progenitors. Continuing studies will address the usefulness of this technique to identify bovine mammary stem cells and to facilitate studies of stem cell biology.

Identification of estrogen-responsive genes in the parenchyma and fat pad of the bovine mammary gland by microarray analysis

Identification of estrogen-responsive genes is an essential step toward understanding mechanisms of estrogen action during mammary gland development. To identify these genes, 16 prepubertal heifers were used in a 2 × 2 factorial experiment, with ovarian status (intact or ovariectomized) as the first factor and estrogen treatment as the second (control or estradiol). Heifers were ovariectomized at ∼4.5 mo of age, and estrogen treatments were initiated 1 mo later. After 3 days of treatment, gene expression was analyzed in the parenchyma and fat pad of the bovine mammary gland using a high-density oligonucleotide microarray. Oligonucelotide probes represented 40,808 tentative consensus sequences from TIGR Bos taurus Gene Index and 4,575 singleton expressed sequence tags derived from libraries of pooled mammary gland and gut tissues. Microarray data were analyzed by use of the SAS mixed procedure, with an experiment-wide permutation-based significance level of P < 0.1. Considerable differences in basal gene expression were noted between mammary parenchyma and fat pad. A total of 124 estrogen-responsive genes were identified, with most responding only in the parenchyma or the fat pad. The majority of genes identified were not previously reported to be estrogen responsive. These undoubtedly include genes that are regulated indirectly but also include known estrogen-targeted genes and novel genes with potential estrogen-responsive elements in their promoter regions. The distinctive expression patterns regulated by estrogen in parenchyma and fat pad shed light on the need for both tissues to obtain normal mammary development.

Major advances associated with hormone and growth factor regulation of mammary growth and lactation in dairy cows

In recent years, the number of researchers interested in mammary development and mammary function in dairy animals has declined. More importantly this cadre of workers has come to rely more than ever on scientists focused on and funded by breast cancer interests to provide fundamental mechanistic and basic cellular insights. Philosophically and practically this is a risky path to better understand, manipulate, and control a national resource as important as the dairy cow. The efficiency, resourcefulness, and dedication of dairy scientists have mirrored the actions of many dairy producers but there are limits. Many of the applications of research, use of bovine somatotropin, management of transition cows, estrus synchronization techniques, and so on, are based on decades-old scientific principles. Specific to dairy, do rodents or breast cancer cell lines adequately represent the dairy cow? Will these results inspire the next series of lactation-related dairy improvements? These are key unanswered questions. Study of the classic mammogenic and lactogenic hormones has served dairy scientists well. But there is an exciting, and bewildering universe of growth factors, transcription factors, receptors, intracellular signaling intermediates, and extracellular molecules that must ultimately interact to determine the size of the mature udder and the functional capacity of mammary gland in the lactating cow. We can only hope that enough scientific, fiscal, and resource scraps fall from the biomedical research banquet table to allow dairy-focused mammary gland research to continue.

Leptin Does Not Act Directly on Mammary Epithelial Cells in Prepubertal Dairy Heifers

The mammary gland of prepubertal dairy heifers consists of parenchyma expanding into the stroma, a matrix of connective and adipose tissue. High planes of nutrition increase stromal mass, but inhibit growth of parenchyma. The parenchyma consists of epithelial cells proliferating in response to growth factors such as insulin like growth factor-I (IGF-I). These observations have led to the hypothesis that elevated planes of nutrition increase leptin production, which in turn inhibits IGF-I-mediated epithelial cell proliferation. To assess this possibility, heifers were offered planes of nutrition sustaining average daily gains of 715 g/d (normal; NP) or 1,202 g/d (high; HP) from 42 d of age until slaughter at 240 kg. At slaughter, HP heifers had 2-fold greater plasma leptin concentration and 3-fold greater leptin mRNA abundance in mammary stroma and parenchyma. To assess the causal nature between leptin and parenchymal development, the induction of signaling events and functional responses in the MAC-T cell line and in primary mammary epithelial cells by leptin was examined. Leptin did not induce phosphorylation of signal transducers and activators of transcription (STAT)3, STAT5, extracellular signal-regulated kinase (ERK1/2), or AKT/Protein kinase B. Consistent with its inability to signal, leptin did not alter basal- or IGF-I-stimulated thymidine incorporation or increase suppressors of cytokine signaling 3 (SOCS3) expression in these cells. Transcripts corresponding to the short leptin receptor form were present in mammary tissue, but those corresponding to the long signaling form were not detected in either mammary tissue or cells. In conclusion, elevated planes of nutrition increase leptin synthesis in mammary stroma, but leptin does not act directly on bovine mammary epithelial cells.

Mammary epithelial proliferation and estrogen receptor alpha expression in prepubertal heifers: effects of ovariectomy and growth hormone

The objectives of this study were to determine the effects of ovariectomy and growth hormone on mammary epithelial cell proliferation and estrogen receptor alpha (ER alpha) expression within the bovine mammary gland. Two experiments were performed. In the first experiment, eight Holstein heifer calves aged between 1 and 3 mo were ovariectomized, while six calves served as controls. At 6 mo of age, calves were treated with bromodeoxyuridine (BrdU) to label proliferating cells and sacrificed 2 h later. Coinciding with reduced mammary mass (304 +/- 25 vs. 130 +/- 21 g), proliferation of mammary epithelial cells was significantly lower in ovariectomized heifers compared to control heifers (2.24 vs. 0.25%). ER alpha expression was restricted to mammary epithelial cells and was not observed within intra-lobular stroma of parenchymal tissue. The proportion of ER alpha positive cells was significantly higher in ovariectomized heifers than in controls (36.1% +/- 2.2 vs. 46.7% +/- 2.4). In the second experiment, mammary biopsies were taken from five 6-mo-old heifers, immediately preceding and 7 d following a single injection of bovine growth hormone. Mammary epithelial cell proliferation (assessed by incorporation of 3H-thymidine) was increased by growth hormone. The proportion of ER alpha positive mammary epithelial cells was not increased by growth hormone. In conclusion, reduced mammary epithelial cell proliferation following ovariectomy was associated with an increase in ER alpha expression, whereas increased proliferation caused by bovine growth hormone was not associated with changes in the proportion of ER alpha positive cells.

Extending the postpartum anovulatory period in dairy cattle with estradiol cypionate

Estradiol cypionate (ECP), a long-acting estrogen, has been used therapeutically in early postpartum (PP) dairy cows. In experiment 1, effects of ECP on circulating reproductive hormones, cyclicity, and ovarian function in early PP dairy cows were investigated. Lactating Holsteins received 10 mg of ECP (ECP; n = 17) or placebo (CON; n = 16) on d 7 PP. Serum and ultrasound data were acquired from 5 to 90 d of lactation. Compared to CON, ECP cows had greater serum estradiol for 10 d and lower serum FSH for 15 d posttreatment. After ECP, the appearance of follicles > or = 10 mm and time to first ovulation were delayed. Nevertheless, by 90 d PP, normal estrous cycles were found in only 50% of CON versus 88% of ECP cows. Primiparous, but not multiparous, cows receiving ECP had higher milk yields. Experiment 2 investigated effects of ECP on reproduction and milk production on a commercial dairy. Cows were blocked by parity and randomized to three treatments: 0 (n = 85), 4 (n = 85), or 10 (n = 86) mg of ECP on d 5 to 8 PP. Data included cycling status (two ultrasound examinations at 30 to 33 d PP and 7 d later), individual reproduction records, and daily milk yields from 10 to 90 d PP. In primiparous cows, ECP had no significant effects on ovulatory status or milk yields. By 40 d PP, a greater percentage of multiparous cows receiving 10 mg of ECP remained anovulatory compared with those receiving 0 or 4 mg. Milk yields were highest for multiparous cows receiving 4 mg of ECP, intermediate for the 10-mg dose, and lowest for controls. Lower conception was observed in multiparous cows receiving 4 mg of ECP. In summary, ECP delayed time to first ovulation particularly in multiparous cows, an effect associated with observed inhibition of circulating FSH. Milk yield responses to ECP were inconsistent within parity groups across the two experiments.

Genetic manipulation of the IGF-I axis to regulate mammary gland development and function

Insulin-like growth factor I (IGF-I) is known to regulate mammary gland development. This regulation occurs through effects on both cell cycle progression and apoptosis. Our laboratory has studied the IGF-I-dependent regulation of these processes by using transgenic and knockout mouse models that exhibit alterations in the IGF-I axis. Our studies of transgenic mice that overexpress IGF-I during pregnancy and lactation have demonstrated that this growth factor slows the apoptotic loss of mammary epithelial cells during the declining phase of lactation but has minimal effects during early lactation on milk composition or lactational capacity. In contrast, our analysis of early developmental processes in mammary tissue from mice carrying a targeted mutation in the IGF-I receptor gene suggests that IGF-dependent stimulation of cell cycle progression is more important to early mammary gland development than potential anti-apoptotic effects. With both models, the effects of perturbing the IGF-I axis are dependent on the physiological state of the animal. The diminished ductal development that occurs in response to loss of the IGF-I receptor is dramatically restored during pregnancy, whereas the ability of overexpressed IGF-I to protect mammary cells from apoptosis does not occur if the mammary gland is induced to undergo forced involution. Data from our laboratory on the expression of IGF-signaling molecules in the mammary gland suggest that this effect of physiological context may be related to the expression of members of the insulin receptor substrate family.

Establishing a framework for the functional mammary gland: from endocrinology to morphology

From its embryonic origins, the mammary gland in females undergoes a course of ductal development that supports the establishment of alveolar structures during pregnancy prior to the onset of lactogenesis. This development includes multiple stages of proliferation and morphogenesis that are largely directed by concurrent alterations in key hormones and growth factors across various reproductive states. Ductal elongation is directed by estrogen, growth hormone, insulin-like growth factor-I, and epidermal growth factor, whereas ductal branching and alveolar budding is influenced by additional factors such as progesterone, prolactin, and thyroid hormone. The response by the ductal epithelium to various hormones and growth factors is influenced by epithelial-stromal interactions that differ between species, possibly directing species-specific morphogenesis. Evolving technologies continue to provide the opportunity to further delineate the regulation of ductal development. Defining the hormonal control of ductal development should facilitate a better understanding of the mechanisms underlying mammary gland tumorigenesis.

A local increase in the mammary IGF-1: IGFBP-3 ratio mediates the mammogenic effects of estrogen and growth hormone

A single epithelium-free mammary fat pad was surgically prepared in each of twenty-five one-month-old, Friesian heifers. At 18 mo of age, heifers were randomly assigned to one of four treatment groups. Treatments were: control (C), growth hormone (GH), estrogen (E) or growth hormone + estrogen (GE). Hormones were administered for 40 hr before the animals were sacrificed to provide mammary samples of parenchyma (PAR), intact fat pad (MFP), and epithelium-free or "cleared" fat pad (CFP). IGF-1 and IGF binding protein-3 (IGFBP-3) mRNA was highest in CFP and MFP whereas the protein products were highest in PAR. IGFBP-2, a 28-kDa IGFBP and a 24-kDa IGFBP were more abundant in CFP and MFP. E and GH increased incorporation of [(3)H]thymidine into DNA of PAR. Incorporation of [(3)H]thymidine into the DNA of MFP or CFP was minimal. Coincident with the changes observed in mammary epithelial proliferation, E increased IGF-1 protein in MFP and PAR, and to a lesser extent in CFP. E tended to increase IGF-1 mRNA levels in MFP, but not CFP implying that the regulation of IGF-1 expression is modulated by adjacent epithelium. GH and E reduced IGFBP-3 protein in PAR and increased the 24-kDa IGFBP in CFP and MFP. Increased proliferation of mammary parenchymal cells was associated with increased IGF-1 and reduced IGFBP-3 protein in mammary tissue. An increase in the ratio of mammary IGF-1: IGFBP-3 likely increases the proportion of the mammary IGF-1 available to stimulate proliferation. These data also indicate that stromal: epithelial interactions regulate the IGF-1 axis in mammary tissue.

IGF-I stimulation of extracellular acidification is not linked to cell proliferation for autocrine cells

Insulin-like growth factor-I (IGF-I) increases extracellular acidification rate (ECAR), a measure correlated with proliferation for nonautocrine cells. To evaluate the effect of autocrine IGF-I secretion on cell responsiveness, a cell line that secretes IGF-I was tested. SV40-lGF-I cells also registered concentration-dependent increases in ECAR; however, unlike the parental cell line, signal attenuation upon repeat challenges was not evident. Furthermore, SV40-IGF-I cells did not proliferate in response to IGF-I. We investigated if lack of proliferation was due to differences in the protocols of the assays ([3H]thymidine incorporation and microphysiometry). We identified three key differences in the protocols: surface substrate, cell density, and fluid residence time. We found no increase in [3H]thymidine incorporation for cells on either tissue-culture plastic or polycarbonate transwells. Control levels of [3H]thymidine incorporation were cell-density-dependent, but IGF-I did not increase proliferation at any density studied. Finally, we investigated IGF-I stimulation for cells under microphysiometer flow conditions and found no proliferative response to IGF-I. We found that the cells do respond to IGF-I with increased amino acid uptake. These data suggest that IGF-I signaling is operational in the SV40-IGF-I cells, but the transduction pathway for IGF-I-induced proliferation is compromised, despite the fact that these cells respond to fetal bovine serum with increased growth. Ongoing studies are focused on identifying which elements in the signaling cascade are altered by autocrine secretion of IGF-I.

Selection for milk production from a lactation biology viewpoint

The success of selection for increased milk production in dairy cows is apparent. Certainly, many herds now have average production levels that would have only been associated with the best producers in the herd 30 yr ago. There are, of course, many reasons for this success. Among these are improvements in genetic selection methods and associated use of artificial insemination, better fulfillment of nutritional needs and diet formulation, and careful attention to mastitis control and milking management. Development of new management tools (i.e., bovine somatotropin, improved crops, estrus detection devices, estrus synchronization, monitoring of individual animal performance, and disease prevention) should not be forgotten. Although many aspects of a dairy operation determine overall performance and profitability, the focus of this paper is the udder. Information indicates that both the structure and function of the bovine mammary gland have been directly impacted by long-term selection for increased milk production but improved functionality may have been more important. This review also considered studies that attempt to develop techniques and measurements for possible selection of genetically superior animals including measurement of circulating hormones and direct assay of mammary tissue function.