The declining phase of lactation: peripheral or central, programmed or pathological?

Milk production and anatomical udder capacity changes of udder halves subjected to increased milking frequency at two stages of lactation

This study aimed to characterize the effects of increased milking frequency (IMF) at early and mid-lactation on milk yield and its association with changes in cistern and alveolar capacity. Fourteen multiparous Holstein cows were subjected to IMF using the unilateral frequent milking method from 3 to 24 d in milk (DIM). At mid-lactation, cows were randomly assigned to 1 of 2 treatments: control or repeated. From 150 to 170 DIM, IMF treatment was reimposed in the repeated group. During IMF, left udder halves were milked 2× and right udder halves were milked 4× daily. To separate individual milk yields of udder halves, separate buckets were used to collect samples from each udder half. Milk samples and milk yield from right and left udder halves were collected on d 150, 170, 200, 230, 260, and 290 of lactation. Alveolar and cistern capacity were measured 26 h after the last milking at 140 and 172 DIM using an oxytocin inhibitor. Cistern and alveolar capacity were measured by evaluating the milk harvested after oxytocin inhibitor and oxytocin administration, respectively. Udder half difference yields were calculated by subtracting left half yield from right half yield. At 170 DIM, the udder half difference in repeated was 2.27 kg greater than the udder half difference in control. Udder halves milked 4× produced more milk and protein than 2× udder halves in the repeated group at 170, 200, 230, and 260 DIM. Cumulative (150 to 290 DIM) and carry over (200 to 290 DIM) udder half differences in milk yield were similar between the control and repeated treatments. Alveolar volume was similar between udder halves milked 2× or 4× at 140 DIM, while cistern volume was larger for udder halves milked 4× than 2× in early lactation. There was no difference between alveolar or cistern volume proportion in udder halves milked 2× or 4× before mid-lactation IMF. After 20 d IMF for the repeated group, alveolar volume was similar between control and repeated independent of udder half milking frequency. However, repeated held 4.9 kg more cistern milk than control. Control treatment udder halves had a greater alveolar proportion than repeated treatment udder halves. As expected, the cistern proportion was smaller in control and larger in repeated after mid-lactation IMF. IMF at early and mid-lactation enhances milk and protein yield largely during differential milking frequency regimens. The lack of enhancement in milk yield after IMF might be associated with a different response to IMF in the mammary gland at early versus mid-lactation. Based on our results, we conclude that udder halves subjected to early and mid-lactation IMF had increased cistern volume capacity.

Transcription factor EB (TFEB)-mediated autophagy protects bovine mammary epithelial cells against H2O2-induced oxidative damage in vitro

Background

Bovine mammary epithelial cells after calving undergo serious metabolic challenges and oxidative stress both of which could compromise autophagy. Transcription factor EB (TFEB)-mediated autophagy is an important cytoprotective mechanism against oxidative stress. However, effects of TFEB-mediated autophagy on the oxidative stress of bovine mammary epithelial cells remain unknown. Therefore, the main aim of the study was to investigate the role of TFEB-mediated autophagy in bovine mammary epithelial cells experiencing oxidative stress.

Results

H₂O₂ challenge of the bovine mammary epithelial cell MAC-T increased protein abundance of LC3-II, increased number of autophagosomes and autolysosomes while decreased protein abundance of p62. Inhibition of autophagy via bafilomycin A1 aggravated H₂O₂-induced reactive oxygen species (ROS) accumulation and apoptosis in MAC-T cells. Furthermore, H₂O₂ treatment triggered the translocation of TFEB into the nucleus. Knockdown of TFEB by siRNA reversed the effect of H₂O₂ on protein abundance of LC3-II and p62 as well as the number of autophagosomes and autolysosomes. Overexpression of TFEB activated autophagy and attenuated H₂O₂-induced ROS accumulation. Furthermore, TFEB overexpression attenuated H₂O₂-induced apoptosis by downregulating the caspase apoptotic pathway.

Conclusions

Our results indicate that activation of TFEB mediated autophagy alleviates H₂O₂-induced oxidative damage by reducing ROS accumulation and inhibiting caspase-dependent apoptosis.

Oxidative stress, NF-κB signaling, NLRP3 inflammasome, and caspase apoptotic pathways are activated in mammary gland of ketotic Holstein cows

Ketosis is a serious metabolic disorder characterized by systemic and hepatic oxidative stress, inflammation, and apoptosis, as well as reduced milk yield. Because of the paucity of data on mammary responses during ketosis, the aim of this study was to evaluate alterations in oxidative stress, NF-κB signaling, NLRP3 inflammasome, and caspase apoptotic pathways in mammary gland of dairy cows with ketosis. Blood, mammary gland tissue, and milk samples were collected from healthy cows [Control, blood concentration of β-hydroxybutyrate (BHB) <0.6 mM, n = 10] and cows with subclinical ketosis (SCK, blood concentration of BHB >1.2 mM and <3 mM, n = 10) or clinical ketosis (CK, blood concentration of BHB >3 mM, n = 10) at median 8 d in milk (range = 6-12). Compared with Control, serum concentration of glucose was lower (3.91 vs. 2.86 or 2.12 mM) in cows with SCK or CK, whereas concentrations of fatty acids (0.25 vs. 0.57 or 1.09 mM) and BHB (0.42 vs. 1.81 or 3.85 mM) were greater. Compared with Control, the percentage of milk fat was greater in cows with SCK or CK. In contrast, the percentage of milk protein was lower in cows with SCK or CK. We detected no differences in milk lactose content across groups. Compared with Control, activities of glutathione peroxidase, superoxide dismutase, and catalase were lower in mammary gland tissue of cows with SCK or CK. In contrast, concentrations of hydrogen peroxide and malondialdehyde were greater in cows with SCK or CK. Compared with Control, mRNA abundances of TNFA, IL6, and IL1B were greater in mammary tissues of cows with SCK or CK. In addition, activity of IKKβ and the ratio of phosphorylated inhibitor of κBα to IκBα, and of phosphorylated NF-κB p65 to NF-κB p65, were also greater in mammary tissues of cows with SCK or CK. Subclinical or clinical ketosis also led to greater activity of caspase 1 and protein abundance of caspase 1, NLRP3, Bax, caspase 3, and caspase 9. In contrast, abundance of the antiapoptotic protein was lower in SCK or CK cows. The data indicate that the mammary gland of SKC or CK cows undergoes severe oxidative stress, inflammation, and cell death.

NMR-based metabolomics to evaluate the milk composition from Friesian and autochthonous cows of Northern Italy at different lactation times

It is well established that different factors affect milk composition in cows and that milk composition, in turn, affect both technological and nutritional qualities. In this respect the comprehension of the metabolic variability of milk composition in relation to the lactation time as well as to the genetic background may be of paramount importance for the agri-food industries. In the present study we investigated the variations of the metabolic profiles during lactation in milks obtained from Friesian and autochthonous races from Northern Italy by ¹H NMR metabolomics. Furthermore, the external factors influencing the milk composition were minimized: the cows were breeded in the same farm, were fed with the same diet and were paired for the lactation interval and lactation stage. Our results showed a difference in milk composition between races and in relation to late lactation. The PLS-DA analysis permitted to distinguish the Friesian and autochthonous cow milks at the investigated different lactation times. Interestingly, the metabolites significantly involved into the discrimination between races appeared to be also technological property parameters, highlighting the importance of maintaining the biodiversity of cow breeds. Therefore, NMR-based metabolomics of milk could represent an informative tool to identify metabolites involved in milk quality both from a nutritional and industrial perspective.

Prototheca zopfii isolated from bovine mastitis induced oxidative stress and apoptosis in bovine mammary epithelial cells

Bovine protothecal mastitis results in considerable economic losses worldwide. However, Prototheca zopfii induced morphological alterations and oxidative stress in bovine mammary epithelial cells (bMECs) is not comprehensively studied yet. Therefore, the aim of this current study was to investigate the P. zopfii induced pathomorphological changes, oxidative stress and apoptosis in bMECs. Oxidative stress was assessed by evaluating catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), malondialdehyde (MDA) contents and lactate dehydrogenase (LDH) activity, while ROS generation and apoptosis was measured by confocal laser scanning microscopy. The results revealed that infection of P. zopfii genotype II (GTII) significantly changed bMECs morphology, increased apoptotic rate and MDA contents at 12 h (p < 0.05) and 24 h (p < 0.01) in comparison with control group, in time-dependent manner. LDH activity and ROS generation was also increased (p < 0.01) at 12 h and 24 h. However, SOD and CAT contents in bMECs infected with GTII were decreased (p < 0.05) at 12 h, while GPx (p < 0.01), SOD (p < 0.05) and CAT (p < 0.01) levels were reduced at 24 h. In case of GTI, only CAT and GPx activities were significantly decreased when the duration prolonged to 24 h but lesser than GTII. This suggested that GTII has more devastating pathogenic effects in bMECs, and the findings of this study concluded that GTII induced apoptosis and oxidative stress in bMECs via the imbalance of oxidant and antioxidant defenses as well as the production of intracellular ROS.

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

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

Silymarin and lycopene administration in periparturient dairy cows: effects on milk production and oxidative status

AIMS

To establish during late gestation and early lactation the effects of supplementing silymarin, a natural hepatoprotective substance, combined with a powerful antioxidant, lycopene, on milk production and on serum biomarkers of oxidative status.

METHODS

Italian Friesian dairy cows were given individually a supplement containing a mixture of silymarin (7.77 g/day/cow) and lycopene (1.27 g/day/cow) (n=10) or no supplement (control; n=10). Treatment was administered from 7 days before the expected calving date to the first 14 days in milk. At 7 days before the expected calving date, at calving, and 7 days postpartum, body condition score (BCS) and concentrations in serum of reactive oxygen metabolites (ROM), total antioxidant capacity (OXY) and thiobarbituric acid reactive substances (TBARS) were evaluated. Bodyweight, milk production, and somatic cell count (SCC) were determined at 7, 14 and 21 days postpartum.

RESULTS

The mean time of treatment prior to calving was 6.3 (min 4, max 11) days. Compared with control cows, treatment increased energy corrected milk yield (33.3 vs. 37.8 (SEM 1.10) kg/day; p=0.006) and milk fat yield (1.14 vs. 1.32 (SEM 0.06) kg/day; p=0.05) over the first 21 days of lactation. Treated cows had lower mean log10 SCC compared to control cows (4.9 vs. 5.24 (SEM 0.11) log10 cells/mL) and had lower overall concentration of TBARS (1.47 vs. 1.59 (SEM 0.016) nM/mL; p<0.001), but similar concentrations of ROM and OXY in serum. However there was a treatment by time interaction (p=0.09) for OXY, and at calving mean OXY was higher in treated cows compared with pre-calving values (p<0.001).

CONCLUSIONS

Despite the acknowledged limitation of the study, i.e. the small number of animals, our results suggest that silymarin and lycopene, as health-beneficial feed supplements, may help dairy cows in metabolic adaptation during the first stages of lactation.

CLINICAL RELEVANCE

During the peripartum period dairy cows suffer moderate-to-severe fatty liver and from an overproduction of free radicals. The supplementation of a mixture of silymarin and lycopene during these stressful days may be useful to mitigate these metabolic disorders with beneficial effects on the subsequent lactation.

Comparative 2D-DIGE proteomic analysis of bovine mammary epithelial cells during lactation reveals protein signatures for lactation persistency and milk yield

Mammary gland is made up of a branching network of ducts that end with alveoli which surrounds the lumen. These alveolar mammary epithelial cells (MEC) reflect the milk producing ability of farm animals. In this study, we have used 2D-DIGE and mass spectrometry to identify the protein changes in MEC during immediate early, peak and late stages of lactation and also compared differentially expressed proteins in MEC isolated from milk of high and low milk producing cows. We have identified 41 differentially expressed proteins during lactation stages and 22 proteins in high and low milk yielding cows. Bioinformatics analysis showed that a majority of the differentially expressed proteins are associated in metabolic process, catalytic and binding activity. The differentially expressed proteins were mapped to the available biological pathways and networks involved in lactation. The proteins up-regulated during late stage of lactation are associated with NF-κB stress induced signaling pathways and whereas Akt, PI3K and p38/MAPK signaling pathways are associated with high milk production mediated through insulin hormone signaling.

Natural and abrupt involution of the mammary gland affects differently the metabolic and health consequences of weaning

In most mammals under natural conditions weaning is gradual. Weaning occurs after the mammary gland naturally produces much less milk than it did at peak and established lactation. Involution occurs following the cessation of milk evacuation from the mammary glands. The abrupt termination of the evacuation of milk from the mammary gland at peak and established lactation induces abrupt involution. Evidence on mice has shown that during abrupt involution, mammary gland utilizes some of the same tissue remodeling programs that are activated during wound healing. These results led to the proposition of the "involution hypothesis". According to the involution hypothesis, involution is associated with increased risk for developing breast cancer. However, the involution hypothesis is challenged by the metabolic and immunological events that characterize the involution process that follows gradual weaning. It has been shown that gradual weaning is associated with pre-adaption to the forthcoming break between dam and offspring and is followed by an orderly reprogramming of the mammary gland tissue. As discussed herein, such response may actually protect the mammary glands against the development of breast cancer and thus, may explain the protective effect of extended breastfeeding. On the other hand, the termination of breastfeeding during the first 6 months of lactation is likely associated with an abrupt involution and thus with an increased risk for developing breast cancer. Review of the literature on the epidemiology of breast cancer principally supports those conclusions.

Cellular calcium dynamics in lactation and breast cancer: from physiology to pathology

Breast cancer is the second leading cause of cancer mortality in women, estimated at nearly 40,000 deaths and more than 230,000 new cases diagnosed in the U.S. this year alone. One of the defining characteristics of breast cancer is the radiographic presence of microcalcifications. These palpable mineral precipitates are commonly found in the breast after formation of a tumor. Since free Ca(2+) plays a crucial role as a second messenger inside cells, we hypothesize that these chelated precipitates may be a result of dysregulated Ca(2+) secretion associated with tumorigenesis. Transient and sustained elevations of intracellular Ca(2+) regulate cell proliferation, apoptosis and cell migration, and offer numerous therapeutic possibilities in controlling tumor growth and metastasis. During lactation, a developmentally determined program of gene expression controls the massive transcellular mobilization of Ca(2+) from the blood into milk by the coordinated action of calcium transporters, including pumps, channels, sensors and buffers, in a functional module that we term CALTRANS. Here we assess the evidence implicating genes that regulate free and buffered Ca(2+) in normal breast epithelium and cancer cells and discuss mechanisms that are likely to contribute to the pathological characteristics of breast cancer.

In silico QTL mapping of maternal nurturing ability with the mouse diversity panel

Significant variation exists for maternal nurturing ability in inbred mice. Although classical mapping approaches have identified quantitative trait loci (QTL) that may account for this variation, the underlying genes are unknown. In this study, lactation performance data among the mouse diversity panel were used to map genomic regions associated with this variation. Females from each of 32 inbred strains (n = 8-19 dams/strain) were studied during the first 8 days of lactation by allowing them to raise weight- and size-normalized cross-foster litters (10 pups/litter). Average daily weight gain (ADG) of litters served as the primary indicator of milk production. The number of pups successfully reared to 8 days (PNUM8) also served as a related indicator of maternal performance. Initial haplotype association analysis using a Bonferroni-corrected, genome-wide threshold revealed 10 and 15 associations encompassing 11 and 13 genes for ADG and PNUM8, respectively. The most significant of these associated haplotype blocks were found on MMU 8, 11, and 19 and contained the genes Nr3c2, Egfr, Sec61g, and Gnaq. Lastly, two haplotype blocks on MMU9 were detected in association with PNUM8. These overlapped with the previously described maternal performance QTL, Neogq1. These results suggest that the application of in silico QTL mapping is a useful tool in discovering the presence of novel candidate genes involved in determining lactation capacity in mice.

Triennial Lactation Symposium: A local affair: How the mammary gland adapts to changes in milking frequency

Regular removal of milk from the mammary gland is critical to maintaining milk secretion. Early studies in rodents demonstrated that changes in milking frequency influenced mammary blood flow, as well as mammary cell number and activity. Later studies in ruminants confirmed those observations and that the response was regulated locally within the mammary gland. In addition, it was discovered that increased milking frequency (IMF) during early lactation stimulated an increase in milk production that partially persisted through late lactation, indicating long-term effects on mammary function. The local mechanisms regulating the mammary response to IMF are poorly understood, although several have been proposed. To gain insight into the mechanisms underlying the mammary response to IMF, and to identify genes associated with the response, we used a functional genomics approach and conducted experiments on dairy cows exposed to unilateral frequent milking [UFM; twice daily milking (2X) of the left udder half and 4-times daily milking (4X) of the right udder half]. Across multiple experiments, we were unable to detect an effect of UFM on mammary cell proliferation or apoptosis. We have, however, identified distinct transcriptional signatures associated with the mammary response to milk removal and to UFM during early lactation. Sequential sampling of mammary tissue revealed that when UFM was imposed during early lactation, at least 2 sets of genes were coordinately regulated with changes in differential milk production of 4X vs. 2X udder halves. Moreover, some genes were persistently differentially expressed in 4X vs. 2X udder halves after UFM and were associated with the persistent increase in milk yield. We conclude that a coordinated transcriptional response is associated with the increase in milk yield elicited by IMF during early lactation and that the 2 sets of differentially expressed genes may be a marker for the autocrine up-regulation of milk production. Moreover, we propose that we have identified a novel form of imprinting associated with persistent alteration of mammary function, which we term "lactational imprinting."

Selenomethionine increases proliferation and reduces apoptosis in bovine mammary epithelial cells under oxidative stress

The decline in mammary epithelial cell number as lactation progresses may be due, in part, to oxidative stress. Selenium is an integral component of several antioxidant enzymes. The present study was conducted to examine the effect of oxidative stress and selenomethionine (SeMet) on morphology, viability, apoptosis, and proliferation of bovine mammary epithelial cells (BMEC) in primary culture. Cells were isolated from mammary glands of lactating dairy cows and grown for 3 d in a low-serum gel system containing lactogenic hormones and 0 or 100 μM H2O2 with 0, 10, 20, or 50 nM SeMet. Hydrogen peroxide stress increased intracellular H2O2 to 3 times control concentrations and induced a loss of cuboidal morphology, cell-cell contact, and viability of BMEC by 25%. Apoptotic cell number more than doubled during oxidative stress, but proliferating cell number was not affected. Supplementation with SeMet increased glutathione peroxidase activity 2-fold and restored intracellular H2O2 to control levels with a concomitant return of morphology and viability to normal. Apoptotic BMEC number was decreased 76% below control levels by SeMet and proliferating cell number was increased 4.2-fold. These findings suggest that SeMet modulated apoptosis and proliferation independently of a selenoprotein-mediated reduction of H2O2. In conclusion, SeMet supplementation protects BMEC from H2O2-induced apoptosis and increased proliferation and cell viability under conditions of oxidative stress.

Developmental regulation of mitochondrial biogenesis and function in the mouse mammary gland during a prolonged lactation cycle

The regulation of mitochondrial biogenesis and function in the lactating mammary cell is poorly understood. The goal of this study was to use proteomics to relate temporal changes in mammary cell mitochondrial function during lactation to changes in the proteins that make up this organelle. The hypothesis tested was that changes in mammary cell mitochondrial biogenesis and function during lactation would be accounted for by coordinated changes in the proteins of the electron transport chain and that some of these proteins might be linked by their expression patterns to PPARGC1α and AMP kinase. The mitochondrial proteome was studied along with markers of mitochondrial biogenesis and function in mammary tissue collected from mice over the course of a single prolonged lactation cycle. Mammary tissue concentrations of AMP and ADP were increased (P < 0.05) during early lactation and then declined with prolonged lactation. Similar changes were also observed for mitochondrial ATP synthesis activity, mitochondrial mass and DNA copy number. Analysis of the mammary cell mitochondrial proteome identified 244 unique proteins. Of these, only two proteins of the electron transport chain were found to increase during early lactation. In contrast, coordinated changes in numerous electron transport chain proteins were observed both during mid- and late lactation. There were six proteins that could be directly linked to PPARGC1α through network analysis. Abundance of PPARGC-1α and phosphorylation of AMP kinase was highest on day 2 postpartum. The results suggest that the increases in mammary mitochondria ATP synthesis activity during early lactation results from changes in only a limited number proteins. In addition, decreases in a handful of proteins linked to lipid oxidation could be temporally linked to decreases in PPARGC1α and phospho-AMP kinase suggesting potential roles for these proteins in coordinating mammary gland metabolism during early lactation.

The bovine mammary gland expresses multiple functional isoforms of serotonin receptors

Recent studies in dairy cows have demonstrated that serotonergic ligands affect milk yield and composition. Correspondingly, serotonin (5-HT) has been demonstrated to be an important local regulator of lactational homeostasis and involution in mouse and human mammary cells. We determined the mRNA expression of bovine 5-HT receptor (HTR) subtypes in bovine mammary tissue (BMT) and used pharmacological agents to evaluate functional activities of 5-HT receptors. The mRNAs for five receptor isoforms (HTR1B, 2A, 2B, 4, and 7) were identified by conventional real-time (RT)-PCR, RT quantitative PCR, and in situ hybridization in BMT. In addition to luminal mammary epithelial cell expression, HTR4 was expressed in myoepithelium, and HTR1B, 2A, and 2B were expressed in small mammary blood vessels. Serotonin suppressed milk protein mRNA expression (alpha-lactalbumin and beta-casein mRNA) in lactogen-treated primary bovine mammary epithelial cell (BMEC) cultures. To probe the functional activities of individual receptors, caspase-3 activity and expression of alpha-lactalbumin and beta-casein were measured. Both SB22489 (1B antagonist) and ritanserin (2A antagonist) increased caspase-3 activity. Expression of alpha-lactalbumin and beta-casein mRNA levels in BMEC were stimulated by low concentrations of SB224289, ritanserin, or pimozide. These results demonstrate that there are multiple 5-HT receptor isoforms in the bovine mammary gland, and point to profound differences between serotonergic systems of the bovine mammary gland and the human and mouse mammary glands. Whereas human and mouse mammary epithelial cells express predominately the protein for the 5-HT(7) receptor, cow mammary epithelium expresses multiple receptors that have overlapping, but not identical, functional activities.

Mammary gland involution is associated with rapid down regulation of major mammary Ca2+-ATPases

Sixty percent of calcium in milk is transported across the mammary cells apical membrane by the plasma membrane Ca(2+)-ATPase 2 (PMCA2). The effect of abrupt cessation of milk production on the Ca(2+)-ATPases and mammary calcium transport is unknown. We found that 24 h after stopping milk production, PMCA2 and secretory pathway Ca(2+)-ATPases 1 and 2 (SPCA1 and 2) expression decreased 80-95%. PMCA4 and Sarco/Endoplasmic Reticulum Ca(2+)-ATPase 2 (SERCA2) expression increased with the loss of PMCA2, SPCA1, and SPCA2 but did not increase until 72-96 h of involution. The rapid loss of these Ca(2+)-ATPases occurs at a time of high mammary tissue calcium. These results suggest that the abrupt loss of Ca(2+)-ATPases, required by the mammary gland to regulate the large amount of calcium associated with milk production, could lead to accumulation of cell calcium, mitochondria Ca(2+) overload, calcium mediated cell death and thus play a part in early signaling of mammary involution.

Gene regulatory networks in lactation: identification of global principles using bioinformatics

Background

The molecular events underlying mammary development during pregnancy, lactation, and involution are incompletely understood.

Results

Mammary gland microarray data, cellular localization data, protein-protein interactions, and literature-mined genes were integrated and analyzed using statistics, principal component analysis, gene ontology analysis, pathway analysis, and network analysis to identify global biological principles that govern molecular events during pregnancy, lactation, and involution.

Conclusion

Several key principles were derived: (1) nearly a third of the transcriptome fluctuates to build, run, and disassemble the lactation apparatus; (2) genes encoding the secretory machinery are transcribed prior to lactation; (3) the diversity of the endogenous portion of the milk proteome is derived from fewer than 100 transcripts; (4) while some genes are differentially transcribed near the onset of lactation, the lactation switch is primarily post-transcriptionally mediated; (5) the secretion of materials during lactation occurs not by up-regulation of novel genomic functions, but by widespread transcriptional suppression of functions such as protein degradation and cell-environment communication; (6) the involution switch is primarily transcriptionally mediated; and (7) during early involution, the transcriptional state is partially reverted to the pre-lactation state. A new hypothesis for secretory diminution is suggested – milk production gradually declines because the secretory machinery is not transcriptionally replenished. A comprehensive network of protein interactions during lactation is assembled and new regulatory gene targets are identified. Less than one fifth of the transcriptionally regulated nodes in this lactation network have been previously explored in the context of lactation. Implications for future research in mammary and cancer biology are discussed.