Regulation of mammary epithelial cell phenotypes by the helix-loop-helix protein, Id-1

Mammary epithelial cells undergo cycles of proliferation, invasion, differe ntiation and apoptotic cell-death throughout adult life. The molecular mech anisms that regulate these complex and co-ordinated developmental programs are poorly understood. We have identified Id-1 protein, a negative regulato r of basic helix-loop-helix transcription factors, as a critical regulator of these normal mammary epithelial cell phenotypes. We also found that Id-1 is an important regulator of the aggressive and invasive phenotype, as wel l as mediator of the effects of sex steroid hormones, in human breast cance r cells.