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The role of adipose tissue in breast development. (A) Mature breast adipose tissue secretes leptin which is essential for ductal epithelial development. High levels of adiponectin inhibit this process. White adipose tissue (WAT) is essential for the formation of terminal end buds (TEBs) during prepuberty and puberty stages. After menarche, the breast starts to mature, and the duct starts its side branching, which requires WAT. (B) During pregnancy, WAT trans differentiates into pink adipose tissue (PAT). PAT has milk secretory potential. The process of trans differentiation from WAT to PAT is carried out by the transcription factor SPP1. Moreover, WAT is also essential for alveolar development during the lactation phase and also for the lactation process. During the phase of pregnancy and lactation, brown adipose tissue (BAT) trans differentiates to a basal myoepithelial phenotype helping in the alveolar development. Both trans differentiated cells revert to their original state after lactation with the aid of the transcription factor PPARγ. (C) There are two stages of breast involution (i) post-lactation and (ii) age-related. SFRP1 secreted by breast adipose tissue helps in these involution processes by the apoptosis of luminal epithelial cells. Furthermore, the epithelial cells are replaced by adipose tissue during the involution process.
The adipogenesis repressors also display mixed roles in BC. GATA-binding factor (GATA) 2 promotes BC by inhibiting PTEN activity , while GATA3 acts as a tumor suppressor and is required for the normal development of the mammary gland, specifically luminal epithelial cells . Further, Forkhead Box (Fox) A2 suppresses BC , whereas FoxC2 promotes BC . CHOP (C/EBPζ) and Wnt signaling suppress adipogenesis by promoting the differentiation of mesenchymal stem cells into myocytes and osteocytes but blocking the commitment to the adipocyte lineage [69,70]. CHOP correlates with the invasiveness of human colorectal cancer , but not much information is reported in BC. Wnt signaling (dependent or independent of CTNNB1) is required for the development of the mammary gland, its branching and functions [72,73,74,75,76,77]. It has been reported that high levels of Catenin Beta 1 (CTNNB1) lead to high tumor grade and poor prognosis in BC patients [78,79,80]. Moreover, Preadipocyte factor 1 (PREF-1), also known as DLK-1, is highly expressed in mesenchymal adipocyte precursors, which are important for the development of embryonic WAT and the expansion of adult adipose tissue [81,82]. During breast development, platelet-derived growth factor (PDGF) receptor α+ (PDGFRα+) and PREF-1+ mesenchymal stem cells, located near the parenchymal epithelium, can differentiate into adipocytes or epithelial cells depending on the stimuli from steroid hormones . PREF-1 has been shown to exert its effect in a dose-dependent manner in BC, where high levels of PREF-1 result in a decrease in cell proliferation and invasion, whereas a low-level expression is necessary for these processes . The role of Sirtuin 1 (SIRT-1) in BC is controversial. Latifkar et a.l, 2019 showed that a knockdown of SIRT-1 changes the secretome of BC cells, leading to increased invasiveness and survival . On the other hand, Jin et al., 2018, showed that SIRT-1 expression leads to tumor promotion by modulating the expression of AKT . Tafazzin (TAZ) is highly expressed in most aggressive BCs and has a role in BC migration, invasion and tumorigenesis .
SFRP1 is an adipokine mainly expressed in mature adipocytes with a role in adipogenesis . SFRP1 mediates its effect on adipogenesis in a paracrine manner by inhibiting the Wnt/β-catenin pathway, thereby determining the fate of ADSC to become a