Mammary gland adipocytes in lactation cycle, obesity and breast cancer

Abstract

The mammary gland (MG) is an exocrine gland present in female mammals responsible for the production and secretion of milk during the process of lactation. It is mainly composed by epithelial cells and adipocytes. Among the features that make the MG unique there are 1) its highly plastic properties displayed during pregnancy, lactation and involution (all steps belonging to the lactation cycle) and 2) its requirement to grow in close association with adipocytes which are absolutely necessary to ensure MG's proper development at puberty and remodeling during the lactation cycle. Although MG adipocytes play such a critical role for the gland development, most of the studies have focused on its epithelial component only, leaving the role of the neighboring adipocytes largely unexplored. In this review we aim to describe evidences regarding MG's adipocytes role and properties in physiologic conditions (gland development and lactation cycle), obesity and breast cancer, emphasizing the existing gaps in the literature which deserve further investigation.

Keywords: Adipocytes; Breast cancer; Breastfeeding; Lactation; Mammary gland; Obesity.

Fig. 1

Resting Mammary Gland (light microscopy, hematoxylin and eosin staining). a: anterior mammary gland of a virgin C57BL/6 mouse containing white and brown adipocytes and ductal structures. b: magnification of squared area in A, 1: epithelial ducts; 2: brown adipocytes; 3: white adipocytes. c: Human mammary gland (resting): ductal structures and mammary stroma composed by white adipocytes and fibroblasts. d: magnification of squared area in C; 1: resting ductal structures; 2: white adipocytes; 3: fibrous stroma with fibroblasts; 4: blood vessel. Scale bars: 20μm

Fig. 2

Mammary gland during the lactation cycle. Cartoon representing the mammary gland accompanied by hematoxylin and eosin staining images (mouse). Virgin: ductal structures infiltrating the mammary stroma (mainly composed by white adipocytes) converging to the nipple. Pregnancy (day 17–18): formation of alveolar structures and progressive disappearance of white adipocytes. Lactation (day 14–15): prevalence of ductal-alveolar structures and enlarged milk-producing alveoli; adipocytes are not visible. Involution: return to the pre-pregnancy mammary phenotype. Scale bars: 50μm

Fig. 3

Mammary gland (mouse, 4^th) during the lactation cycle. Light microscopy, hematoxylin and eosin staining. a: virgin: ductal structures infiltrating the mammary stroma mainly composed by white adipocytes. b: pregnancy (day 10): initial formation of alveolar structures arising from ductal stem cells. c: enlargement of squared area in B: 1: ductal cuboidal epithelial cells; 2: newly formed alveoli arising from ductal stem cells. 3: ductal lumen. d: Pregnancy (day 18): increased presence of alveolar structures containing large lipid vacuoles (arrow), ducts (arrowheads), accompanied by a progressive disappearance of white adipocytes. e: Lactation (day 14–15): prevalence of ductal-alveolar structures and enlarged milk-producing alveoli; adipocytes less visible. f: Cartoon representing a ductal-alveolar structure. Scale bars: 50μm

Fig. 4

Mammary gland at day 17–18 of pregnancy. a: light microscopy image of a resin embedded and toluidine stained tissue: 1: compartimentalized adipocytes; 2: lipid rich alveoli; 3: alveolar lumen; 4: blood vessel. b: Transmission electron microscopy of one representative early alveolus (before lumen formation). L: lipid droplet; N: nucleus of epithelial cells; cap: capillary; arrows: myoepithelial cells. c: compartimentalized adipocyte by TEM; arrows indicate an unusually enlarged cytoplasm containing numerous organelles. Scale bars: 5μm

Fig. 5

Schematic illustration of a lineage tracing experiment; WAP^CreR26^LacZ mice. In this model, most of the milk-producing cells expressing the Whey Aciding Protein, WAP, undergoes recombination (mediated by the Cre recombinase) and start constitutively expressing the reporter gene (LacZ) which allow to effectively visualize such cells by β-Gal staining reactions. Virgin mice do not display any positively stained cell as compared to pregnant mice

Fig. 6

Scheme of mammary gland in condition of regular diet (ND) and high fat diet (HFD). The mammary gland in condition of HFD display a reduced number of ductal side branches, enlarged adipocytes and altered mammary stroma due to inflammation and fibrosis, as indicated by the presence of macrophages surrounding hypertrophic adipocytes and collagen fibers. These alterations compromise breastfeeding, alter milk composition and predispose to an increased risk for breast cancer development

Fig. 7

Scheme of mammary gland adipocytes in obesity and breast cancer. Obesity induces mammary gland adipocytes hypertrophy, thus fatty acid release, inflammation, fibrosis and hypoxia, accompanied by a state of hyperleptinemia and hyperestrogenemia, all features representing risk factors for breast cancer development. Cancer Associated Adipocyte (CAA) undergo delipidation and dedifferentiation upon the crosstalk with breast cancer cells. CAA release inflammatory cytokines, proteases, adipokines, estrogens and metabolites (Met) able to promote epithelio-mesenchymal transition (EMT), proliferation and migration of breast cancer cells