Role of Id-2 in the Maintenance of a Differentiated and Noninvasive Phenotype in Breast Cancer Cells

Id proteins are inhibitors of basic helix-loop-helix transcription factors and generally stimulate cell proliferation and inhibit differentiation. We have shown that ectopic expression of Id-1 in murine mammary epithelial cells resulted in loss of differentiation and gain of invasive and proliferative abilities. Moreover, Id-1 was highly expressed in aggressive breast cancer cells in culture and in biopsies from infiltrating carcinomas. In contrast to Id-1, we found that, in vitro and in vivo, Id-2 mRNA and protein were up-regulated as mammary epithelial cells lost proliferative capacity and initiated differentiation. We further determined that this up-regulation of Id-2 was a necessary step toward a fully differentiated phenotype in breast cells. Here we show that one of the components of the extracellular matrix network, laminin, is responsible for the increase in Id-2 expression during differentiation. We also show that Id-2 expression is inversely correlated with the rate of proliferation in murine mammary epithelial cells and that Id-2 is expressed at a higher level in differentiated human breast cancer cells in comparison with very aggressive and metastatic cells. When reintroduced in aggressive breast cancer cells, Id-2 is able to reduce their proliferative and invasive phenotypes and decrease their level of matrix metalloproteinase 9 secretion as well as increase syndecan-1 expression. Moreover, little Id-2 protein expression is detectable in human biopsies from aggressive and invasive carcinomas in comparison with in situ carcinomas. In conclusion, Id-2 expression not only follows a pattern opposite to that of Id-1 during mammary gland development and breast cancer progression but also appears to act as an important protein for the maintenance of a differentiated and noninvasive phenotype in normal and transformed breast cells.