In malignant
breast cancer,
estrogen metabolism is altered, favoring the accumulation of hydroxyestradiols, which can generate
free radicals. These reactive species can activate
matrix metalloproteinases (
MMPs), which in turn can hydrolyze the
proteins of the extracellular matrix (ECM) that act as a barrier to
tumor cell passage. The aim of this study was to determine whether
reactive oxygen species generated by
4-hydroxyestradiol (4-OHE(2)) can activate MMP-2 and then enhance the invasiveness of
breast cancer cells MDA-MB-231 in vitro. Enzymatic assay and gel zymography demonstrated that 4-OHE(2) at a concentration as low as 10(-8) M led to the conversion of
proMMP-2 to active MMP-2. Activation of
proMMP-2 by 4-OHE(2) was inhibited by the Cu,Zn-SOD supporting the involvement of the
free radical superoxide anion (O(2)(*-)). Using invasion chambers coated with
matrigel (artificial ECM), 4-OHE(2) (10(-8) M) enhanced the invasiveness of MDA-MB-231
breast cancer cells by 3-fold. The addition of Cu,Zn-SOD reduced the invasiveness of MDA-MB-231 cells by more than 2-fold, supporting the involvement of O(2)(*-) generated by 4-OHE(2). Addition of an MMP-2 inhibitor completely inhibited the enhancement of invasiveness induced by 4-OHE(2), which demonstrates the importance of activating MMP-2 by 4-OHE(2). On the other hand,
estradiol, which does not have a
catechol structure, did not generate
free radicals, and it could not activate
proMMP-2 or enhance the invasiveness of beast
cancer cells. Although these data need to be confirmed in an animal model, this study suggests that the accumulation of 4-OHE(2) in
breast tumors could enhance the invasiveness of
breast cancer cells.