WHY ARE ESTROGEN-RECEPTOR-NEGATIVE BREAST CANCERS MORE AGGRESSIVE THAN THE ESTROGEN-RECEPTOR-POSITIVE BREAST CANCERS

It is well established that some of the hormone-resistant breast cance rs lack functional estrogen receptor (ER) and exhibit a more aggressiv e metastatic phenotype. The estrogen activation of ER in ER-positive b reast cancers enhances their aggressiveness, while the activation of e xogenously introduced ERs into aggressive ER-negative cells diminishes their aggressiveness. The basis of the protective effect of the ER ap pears complex and could be due to the altered expression of genes invo lved in cell proliferation, differentiation and invasiveness. In this review, we propose different mechanisms for the higher aggressiveness of most of the ER-negative breast cancers.