Mammary Epithelial Cells Are Not Able to Undergo Pregnancy-Dependent Differentiation in the Absence of the Helix-Loop-Helix Inhibitor Id2

Abstract

Mammary alveolar development during pregnancy is triggered by hormone signals. The prolactin receptor/Jak2/signal transducer and activator of transcription (Stat) 5 signal transduction pathway is the principal mediator of these cues and alveolar development is abrogated in its absence. The loss of the basic helix-loop-helix protein inhibitor of differentiation (Id)2 results in a similar defect. To investigate the role of Id2 in mammary epithelium, we performed structural and molecular analyses. Id2-null mammary epithelial cells were unable to form alveoli; the epithelial architecture was disorganized and dissimilar from early stages of alveologenesis in wild-type glands. The epithelial cells retained the ductal marker Na-K-Cl cotransporter (NKCC)1. Nuclear localization of Stat5a and down-regulation of NKCC1 was observed in some areas, indicating a limited response to pregnancy signals. The differentiation status of Id2-null tissue at term was further characterized with cDNA microarrays enriched in mammary specific sequences (mammochip). Some of the early differentiation markers for mammary epithelium were expressed in the Id2-null tissue, whereas genes that are expressed at later stages of pregnancy were not induced. From these results, we conclude that, in the absence of Id2, mammary epithelial development is arrested at an early stage of pregnancy.

High ID2 protein expression correlates with a favourable prognosis in patients with primary breast cancer and reduces cellular invasiveness of breast cancer cells

“…3,24 Further, overexpression of ID2 accelerates differentiation in mouse mammary epithelial cells, and during pregnancy ID2 levels continuously rise until the end of pregnancy. 2 Thus, during normal breast development, the main function of ID2 might be to promote differentiation, which contrasts with the normal function of ID proteins, i.e., to inhibit differentiation and to promote proliferation.…”

Regulation of cyclin expression and cell cycle progression in breast epithelial cells by the helix–loop–helix protein Id1

“…However, since cyclin E is essential for cell cycle progression (Ohtsubo et al, 1995), the longer-term decrease in cyclin E expression may contribute to growth inhibition after treatment with Id1 siRNA. Id2 is essential for normal mammary development and differentiation (Mori et al, 2000;Miyoshi et al, 2002). In murine mammary glands, Id1 expression is high in early pregnancy, and low in mid-late pregnancy, when Id2 expression is high (Mori et al, 2000;Parrinello et al, 2001), suggesting complementary roles for Id1 and Id2 in mammary gland proliferation and differentiation, and challenging the view of Id proteins as functioning redundantly to inhibit differentiation.…”

Mad Upregulation and Id2 Repression Accompany Transforming Growth Factor (TGF)-β-mediated Epithelial Cell Growth Suppression

“…Id2 function has also been shown to be important for lobulo-alveolar development during pregnancy (31). Recently, studies have argued that Id2 protein is induced late during pregnancy (32) and functions as a differentiation factor in the mammary gland (33). Interestingly, enforced TGF-␤ signaling during late pregnancy in transgenic mice induces apoptosis that occurs at the same time that mammary gland apoptosis is observed in Id2-null mice (31,50).…”

“…Through interactions with Rb, Id proteins can also actively promote cell proliferation (29,30) and recently, a role for Id2 as a survival and differentiation factor in the mammary gland during pregnancy has been demonstrated (31)(32)(33). TGF-␤ inhibits Id1, Id2, and Id3 expression in several cell types (34,35).…”