Id-1, ITF-2, and Id-2 comprise a network of helix-loop-helix proteins that regulate mammary epithelial cell proliferation, differentiation, and apoptosis.
Mammary epithelial cells proliferate, invade the stroma, differentiate, and die in adult mammals by mechanisms that are poorly understood. We found that Id-1, an inhibitor of basic helix-loop-helix transcription factors, regulates mammary epithelial cell growth, differentiation, and invasion in culture. Here, we show that Id-1 is expressed highly during mammary development in virgin mice and during early pregnancy, when proliferation and invasion are high. During mid-pregnancy, Id-1 expression declined to undetectable levels as the epithelium differentiated fully. Surprisingly, Id-1 increased during involution, when the epithelium undergoes extensive apoptosis. To determine whether Id-1 regulates both proliferation and apoptosis, we constitutively expressed Id-1 in mammary epithelial cell cultures. Id-1 stimulated proliferation in sparse cultures but induced apoptosis in dense cultures, which reflect epithelial cell density during early pregnancy and involution, respectively. To understand how Id-1 acts, we screened a yeast two-hybrid library from differentiating mammary epithelial cells and identified ITF-2, a basic helix-loop-helix transcription factor, as an Id-1-interacting protein. Overexpression of ITF-2 significantly reduced Id-1-stimulated proliferation and apoptosis. We show further that, in contrast to Id-1, Id-2 was expressed highly in differentiated mammary epithelial cells in vivo and in culture. In culture, Id-2 antisense transcripts blocked differentiation. Our results suggest that Id-1, ITF-2, and Id-2 comprise a network of interacting molecular switches that govern mammary epithelial cell phenotypes.
Parrinello
S
S
Geraldine Brush Cancer Research Institute, California Pacific Medical Center, San Francisco, California 94115, USA.
Lin
C Q
CQ
Murata
K
K
Itahana
Y
Y
Singh
J
J
Krtolica
A
A
Campisi
J
J
Desprez
P Y
PY
D000818:Animals; D017209:Apoptosis; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D015152:Blotting, Northern; D015139:Blotting, Southern; D015153:Blotting, Western; D001940:Breast / Q000166:cytology; D002454:Cell Differentiation; D002455:Cell Division; D002478:Cells, Cultured; D053938:DNA Fragmentation; D018076:DNA, Complementary / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000502:physiology*; D015536:Down-Regulation; D004847:Epithelial Cells; D005260:Female; D018507:Gene Expression Regulation, Developmental; D018257:Helix-Loop-Helix Motifs; D007150:Immunohistochemistry; D051798:Inhibitor of Differentiation Protein 1; D051797:Inhibitor of Differentiation Protein 2; D051379:Mice; D008807:Mice, Inbred BALB C; D009419:Nerve Tissue Proteins; D016376:Oligonucleotides, Antisense / Q000378:metabolism; D010641:Phenotype; D011485:Protein Binding; D012333:RNA, Messenger / Q000378:metabolism; D012097:Repressor Proteins; D012190:Retroviridae / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D051820:TCF Transcription Factors; D013997:Time Factors; D015534:Trans-Activators / Q000737:chemistry / Q000502:physiology*; D000073940:Transcription Factor 4; D014157:Transcription Factors / Q000737:chemistry / Q000502:physiology*; D020798:Two-Hybrid System Techniques; D015854:Up-Regulation
D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D018076:DNA, Complementary; D004268:DNA-Binding Proteins; C497455:Idb1 protein, mouse; C497452:Idb2 protein, mouse; D051798:Inhibitor of Differentiation Protein 1; D051797:Inhibitor of Differentiation Protein 2; D009419:Nerve Tissue Proteins; D016376:Oligonucleotides, Antisense; D012333:RNA, Messenger; D012097:Repressor Proteins; D051820:TCF Transcription Factors; C090743:Tcf4 protein, mouse; D015534:Trans-Activators; D000073940:Transcription Factor 4; D014157:Transcription Factors