NOVEL PATHWAY FOR MAMMARY EPITHELIAL-CELL INVASION INDUCED BY THE HELIX-LOOP-HELIX PROTEIN ID-1

Mammary epithelial cells undergo changes in growth, invasion, and diff erentiation throughout much of adulthood, and most strikingly during p regnancy, lactation, and involution. Although the pathways of milk pro tein expression are being elucidated, little is known, at a molecular level, about control of mammary epithelial cell phenotypes during norm al tissue morphogenesis and evolution of aggressive breast cancer. We developed a murine mammary epithelial cell line, SCp2, that arrests gr owth and functionally differentiates in response to a basement membran e and lactogenic hormones. In these cells, expression of Id-1, an inhi bitor of basic helix-loop-helix transcription factors, declines prior to differentiation, and constitutive Id-1 expression blocks differenti ation. Here, we show that SCp2 cells that constitutively express Id-1 slowly invade the basement membrane but remain anchorage dependent for growth and do not form tumors in nude mice. Cells expressing Id-1 sec reted a similar to 120-kDa gelatinase, From inhibitor studies, this ge latinase appeared to be a metalloproteinase, and it was the only metal loproteinase detectable in conditioned medium from these cells. A nont oxic inhibitor diminished the activity of this metalloproteinase in vi tro and repressed the invasive phenotype of Id-1-expressing cells in c ulture. The implications of these findings for normal mammary-gland de velopment and human breast cancer were investigated. A gelatinase of s imilar to 120 kDa was expressed by the mammary gland during involution , a time when Id-1 expression is high and there is extensive tissue re modeling. Moreover, high levels of Id-1 expression and the activity of a similar to 120-kDa gelatinase correlated with a less-differentiated and more-aggressive phenotype in human breast cancer cells. We sugges t that Id-1 controls invasion by normal and neoplastic mammary epithel ial cells, primarily through induction of a similar to 120-kDa gelatin ase, This Id-1-regulated invasive phenotype could contribute to involu tion of the mammary gland and possibly to the development of invasive breast cancer.