Mammalian Target of Rapamycin Regulates the Growth of Mammary Epithelial Cells through the Inhibitor of Deoxyribonucleic Acid Binding Id1 and Their Functional Differentiation through Id2

Abstract

Organ development requires the integration of multiple extracellular signals to assure a proper balance between proliferation and differentiation and to achieve and maintain specialized functions. Considerable progress has been made in the study of hormones and growth factors and in the understanding of the regulated intracellular pathways and transcriptional events that contribute to mammogenesis. Cell culture experiments have pointed out crucial pathways and components, which were subsequently validated in vivo experiments. We found that the mammalian target of rapamycin (mTOR) pathway is essential for both growth and differentiation of mammary epithelial cells and that the action of mTOR is mediated through the induction of the helix-loop-helix transcriptional regulators Id1 and Id2. Pharmacological inhibition of mTOR activity in HC11 mammary epithelial cells reduced cellular proliferation and prevented the lactogenic hormone-induced expression of milk proteins. Treatment of female mice with rapamycin impaired mammary gland differentiation and milk protein synthesis. The effects of mTOR on proliferation and differentiation require the functions of the helix-loop-helix proteins Id1 and Id2. Rapamycin treatment of HC11 cells resulted in a suppression of Id1 expression and an inhibition of proliferation. This effect of rapamycin was reversed by the forced expression of Id1. Rapamycin also prevented the induction of Id2 by lactogenic hormones and milk protein gene expression. Expression of a Id2 transgene bypassed the requirement of mTOR activity for beta-casein induction. These data suggest that mTOR activity has distinguishable functions in the proliferative and the differentiated state of mammary epithelial cells: it is a prerequisite for proliferation through the induction of Id1 and for differentiation-specific gene expression through the induction of Id2. The relative strengths of these proliferation and differentiation signals reflected by the expression levels of the individual Id proteins are crucial to the functional life cycle of mammary epithelial cells and might be disturbed in tumorigenesis.

Mammalian target of rapamycin activity is required for expansion of CD34+ hematopoietic progenitor cells

“…For example, proliferation of human endothelial progenitors, epithelial cells, osteoblasts and myoblasts is inhibited by rapamycin. [19][20][21][22] Our data demonstrate that mTOR activity is also required for the proliferation of hematopoietic progenitors during myelopoiesis. Progenitor expansion in the presence of rapamycin was significantly reduced, as measured by 3 Hthymidine incorporation experiments ( Figure 1B) and single cell proliferation assays (Figure 4).…”

“…16 Rapamycin has also been demonstrated to regulate the differentiation and proliferation of various adult cell types, including human endothelial progenitors, epithelial cells, chondrocytes, osteoblasts and myoblasts. [17][18][19][20][21][22][23][24] Although these data demonstrate the importance of mTOR signaling in the regulation of multiple cellular processes, a role for mTOR in the regulation of myelopoiesis remains to be investigated. In this study, we investigated the role of the mTOR signal transduction pathway in the regulation of myelopoiesis utilizing a human ex vivo granulocyte differentiation system.…”

Rapamycin inhibits multiple stages of c-Neu/ErbB2–induced tumor progression in a transgenic mouse model of HER2-positive breast cancer

“…In support of this concept, rapamycin prevented primary tumor formation in MMTV-NeuYD mice (16) as well as secondary tumor growth in MMTV-c-Neu mice (15). The presence of phosphorylated S6 levels in normal epithelial cells was unexpected because rapamycin does not negatively affect mammary gland development in early pubertal mice (36). These observations suggest that mTOR signaling may play a more significant role in malignant c-Neu/ErbB2 -driven epithelial growth than in the normal mammary gland.…”

Toxicogenomics and cancer pathogenesis

“…Rapamycin treatment of HC11 cells resulted in a suppression of Id1 expression and an inhibition of proliferation. Rapamycin also prevented the induction of Id2 by lactogenic hormones and milk protein gene expression [103]. Basic helix-loop-helix E proteins are transcription factors that play crucial roles in T cell development by controlling thymocyte proliferation, differentiation and survival.…”