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Autophagy is a process responsible for the degradation and recycling of cytoplasmic content by lysosomes. It is thought to facilitate cell survival during periods of nutrient starvation, but it can also be involved in other physiological processes including regulation of protein homeostasis, degrading intracellular bacteria, tumor suppression and regulation of programmed cell death. Our group was the first to report autophagy in bovine mammary epithelial cells, both in vitro, on BME-UV1 mammary epithelial cell line, and in vivo. The highest intensity of autophagy in bovine mammary glands is noted during dry periods, when the gland undergoes intensive involution and the deprivation of bioactive compounds (hormones, growth factors, cytokines) and nutrients occur. Our studies on the regulation of autophagy show that the conditions observed during bovine mammary gland involution, such as: 1) decrease in the release of lactogenic hormones and growth factors (prolactin, GH and IGF-I); 2) increase of apoptogenic factors (i.e. TGF-β₁) and their receptors, 3) increased synthesis of sex steroids (17-β estradiol, progesterone); and 4) the enhanced competition of intensively developing fetus and mother organism for nutritional and bioactive compounds, may create a state of temporary malnutrition of mammary epithelial cells that forces the cells to the induction of autophagy as a mechanism stabilizing intracellular supplies of energy and aminoacids, especially during the enhanced activity of apoptogenic factors.
Progress in studies concerning the process of mammogenesis have been stimulated by the development of the three-dimensional (3D) culture systems, which enable mammary epithelial cells to form structures mimicking the alveoli of mammary gland in vivo. Mammary epithelial cells (MECs) supported on a laminin-rich extracellular matrix (ECM) form 3D acinar structures - mammospheres - which mature to form polarized and functional monolayers surrounding a lumen and have the ability to produce milk proteins. These structures develop an axis of apico-basal polarity, subsequently become unresponsive to proliferative signals, and finally a bona fide lumen is formed by cavitation, involving the removal of centrally localized cells via multiple cell death processes. Lumen formation is associated with the selective apoptosis of centrally located cells. Autophagy, which is a process responsible for maintaining cell homeostasis, also seems to be crucial in mammary gland development and remodeling. This review describes the role of autophagy in the formation of acinar structures by mammary epithelial cells. Studies on MECs from different species (human, mouse, cow) cultured on Matrigel™ have shown the protective role of autophagy in centrally located cells of differentiating mammospheres. Autophagy seems to be the cells’ first response to the lack of contact with ECM, which in consequence leads to apoptotic cell death, anoikis, and lumen formation in developing alveoli.
Autophagy is an important cellular process responsible for the maintenance of homeostasis in the mammary gland during its development and remodeling. The main function of autophagy is to degrade long-lived proteins and damaged organelles in double-membrane autophagic vacuoles containing hydrolytic enzymes. This process is also involved in the regulation of cell development and death. Three-dimensional (3D) cell cultures made it possible to recreate in vitro the process of alveoli formation by mammary epithelial cells (MECs). When cultured on extracellular matrix (ECM) components, MECs form 3D acini structures called mammospheres, composed of a single layer of polarized cells and a hollow lumen in the center of the acini. It has been shown that during the process of mammosphere formation, autophagy is induced in the centrally located cells in response to the stress related to their loss of contact with the ECM. Studies have shown that the induction of autophagy is augmented in the presence of sex steroids, which regulate cell survival during starvation conditions. Additionally, these hormones control the process of lumen formation, regulating the rate of apoptotic death in mammospheres. TGF-â1 also induces autophagy in 3D cultures, but the presence of this cytokine inhibits the development of acinar structures. On the other hand, IGF-I stimulates the growth of mammospheres, inducing autophagy in the numerous cells located in the centre of acinar structures, where the availability of nutrition is insufficient. The present review article describes some latest studies that point to the role of the close regulation of autophagy by endocrine and intramammary signals during mammogenesis.
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