Id-1, ITF-2, and Id-2 comprise a network of helix-loop-helix proteins thatregulate 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 th at Id-1, an inhibitor of basic helix-loop-helix transcription factors, regu lates mammary epithelial cell growth, differentiation, and invasion in cult ure. 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 l evels as the epithelium differentiated fully. Surprisingly, Id-1 increased during involution, when the epithelium undergoes extensive apoptosis. To de termine whether Id-1 regulates both proliferation and apoptosis, we constit utively 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-l-interacting pro tein. Overexpression of ITF-2 significantly reduced Id-1-stimulated prolife ration and apoptosis. We show further that, in contrast to Id-1, Id-2 was e xpressed highly in differentiated mammary epithelial cells in vivo and in c ulture. In culture, Id-2 antisense transcripts blocked differentiation. Our results suggest that Id-1, ITF-2, and Id-2 comprise a network of interacti ng molecular switches that govern mammary epithelial cell phenotypes.