Exosome signaling in mammary gland development and cancer

Exosomes are 40-100 nm intraluminal vesicles that are released by cells upon fusion of multivesicular endosomes (MVEs) with the plasma membrane. The Rab family of small GTPases, including Rab27A and Rab27B, control different steps of exosome release, including transport of MVEs and docking at the plasma membrane. Exosomes are long range message particles that mediate communication between cells in physiological conditions such as mammary gland development and lactation, but also in pathology such as breast cancer. Metastasis is the culmination of cancer progression and involves a complex interaction with the local and distant environment. Exosome messaging contributes to tumor environment interactions such as immune escape, thrombosis and myofibroblast differentiation, thereby modulating metastatic niche preparation.     

Shp2 signaling suppresses senescence in PyMT-induced mammary gland cancer in mice

In this study, we have used techniques from cell biology, biochemistry, and genetics to investigate the role of the tyrosine phosphatase Shp2 in tumor cells of MMTV-PyMT mouse mammary glands. Genetic ablation or pharmacological inhibition of Shp2 induces senescence, as determined by the activation of senescence-associated β-gal (SA-β-gal), cyclin-dependent kinase inhibitor 1B (p27), p53, and histone 3 trimethylated lysine 9 (H3K9me3). Senescence induction leads to the inhibition of self-renewal of tumor cells and blockage of tumor formation and growth. A signaling cascade was identified that acts downstream of Shp2 to counter senescence: Src, focal adhesion kinase, and Map kinase inhibit senescence by activating the expression of S-phase kinase-associated protein 2 (Skp2), Aurora kinase A (Aurka), and the Notch ligand Delta-like 1 (Dll1), which block p27 and p53. Remarkably, the expression of Shp2 and of selected target genes predicts human breast cancer outcome. We conclude that therapies, which rely on senescence induction by inhibiting Shp2 or controlling its target gene products, may be useful in blocking breast cancer.     

Endogenous MMTV in the normal mammary gland and in dimethylanthracene-induced mammary cancer in BALB/c mice

Immunization of DMBA-treated mice by the glycoprotein fraction from mammary glands of mice BALB/c did not decrease the frequencies of induction of mammary tumors. This is in contrast to the results obtained during immunization by the formaldehyde treated preparation of Mouse Mammary Tumor Virus (MMTV). EcoRI and HindIII cleaved DNA from the DMBA-induced mammary tumors did not contain the additional virus specific fragments. In mammary tumors the expression of p27 MMTV was registered in contrast to normal mammary glands and mammary epithelium cultures in which the proteins of MMTV are not expressed even after induction.     

Influence of milking frequency on mammary gland dynamics

The effects of milking frequency on milk production is a key question for the dairy industry. Milk production is related to the number of active alveoli in the mammary gland and movement between active and quiescent alveolar pools is influenced by the milking frequency. In this paper, we analyse a mechanistic model based on known biological inputs that describes the effect of milking frequency on the alveolar composition of the mammary gland. It is shown that the model can qualitatively reproduce the correct alveolar dynamics. We also investigate the model robustness and parameter sensitivity. Additionally, by making the plausible assumption that the senescence rate of alveoli is proportional to the number of quiescent alveoli present, we obtain an analytical solution requiring periodic resetting.     

Effects of serum on mammary epithelial proliferation in vitro during mammary gland development

The proliferative response of mammary gland epithelium from nonpregnant, pregnant, and lactating mice to mammary serum factor and insulin was studied in vitro. Mammary gland epiithelium from nonpregnant and lactating animals has a delayed proliferative response to mammary serum factor and insulin when compared to the response of epithelium from pregnant animals. The results show that as the animals go through pregnancy into lactation the mammary gland epithelium becomes less responsive to mammary serum factor while it retains its responsiveness to insulin. The concentration of mammary serum factor in sera from animals at various physiological stages is constant. Sera from hypophysectomized rats, on the other hand, show a 50% drop in mammary serum factor activity. This loss of activity cannot be reversed by injecting prolactin, 17-beta-estradiol, or growth hormone into the hypophysectomized animals. A hypothesis that the mammary gland is composed of two proliferative epithelial populations is developed, and the possible role of prolactin in stimulating DNA synthesis is discussed.     

Signaling pathways in mammary gland development.

Unlike most other organs, development of the mammary gland occurs predominantly after birth, under the control of steroid and peptide hormones. Once the gland is established, cycles of proliferation, functional differentiation, and death of alveolar epithelium occur repeatedly with each pregnancy. Although it is unique in this respect, the signaling pathways utilized by the gland are shared with other cell types, and have been tailored to meet the needs of this secretory tissue. Here we discuss the signaling pathways that have been adopted by the mammary gland for its own purposes, and the functions they perform.     

Diet-induced obesity impairs mammary development and lactogenesis in murine mammary gland

We have developed a mouse model of diet-induced obesity that shows numerous abnormalities relating to mammary gland function. Animals ate approximately 40% more calories when offered a high-fat diet and gained weight at three times the rate of controls. They exhibited reduced conception rates, increased peripartum pup mortality, and impaired lactogenesis. The impairment of lactogenesis involved lipid accumulation in the secretory epithelial cells indicative of an absence of copius milk secretion. Expression of mRNAs for beta-casein, whey acid protein, and alpha-lactalbumin were all decreased immediately postpartum but recovered as lactation was established over 2-3 days. Expression of acetyl-CoA carboxylase (ACC)-alpha mRNA was also decreased at parturition as was the total enzyme activity, although there was a compensatory increase in the proportion in the active state. By day 10 of lactation, the proportion of ACC in the active state was also decreased in obese animals, indicative of suppression of de novo fatty acid synthesis resulting from the supply of preformed fatty acids in the diet. Although obese animals consumed more calories in the nonpregnant and early pregnant states, they showed a marked depression in fat intake around day 9 of pregnancy before food intake recovered in later pregnancy. Food intake increased dramatically in both lean and obese animals during lactation although total calories consumed were identical in both groups. Thus, despite access to high-energy diets, the obese animals mobilized even more adipose tissue during lactation than their lean counterparts. Obese animals also exhibited marked abnormalities in alveolar development of the mammary gland, which may partially explain the delay in differentiation evident during lactogenesis.