Inhibition of cellular enzymes by equine catechol estrogens in human breast cancer cells: Specificity for GST-Pi,

초록

Glutathione S-transferases (GSTs) are a family of detoxification isozymes that protect cells by conjugating GSH to a variety of toxic compounds, and they may also play a role in the regulation of both cellular proliferation and apoptosis. We have previously shown that human GST P1-1, which is the most widely distributed extrahepatic isozyme, could be inactivated by the catechol estrogen metabolite 4-hydroxyequilenin (4-OHEN) in vitro [Chang, M., Shin, Y. G., van Breemen, R. B., Blond, S. Y., and Bolton, J. L. (2001) Biochemistry 40, 4811−4820]. In the present study, we found that 4-OHEN and another catechol estrogen, 4,17β-hydroxyequilenin (4,17β-OHEN), significantly decreased GSH levels and the activity of GST within minutes in both estrogen receptor (ER) negative (MDA-MB-231) and ER positive (S30) human breast cancer cells. In addition, 4-OHEN caused significant decreases in GST activity in nontransformed human breast epithelial cells (MCF-10A) but not in the human hepatoma HepG2 cells, which lack GST P1-1. We also showed that GSH partially protected the inactivation of GST P1-1 by 4-OHEN in vitro, and depletion of cellular GSH enhanced the 4-OHEN-induced inhibition of GST activity. In addition, 4-OHEN GSH conjugates contributed about 27% of the inactivation of GST P1-1 by 4-OEHN in vitro. Our in vitro kinetic inhibition experiments with 4-OHEN showed that GST P1-1 had a lower Ki value (20.8 μM) compared to glyceraldehyde-3-phosphate dehydrogenase (GAPDH, 52.4 μM), P450 reductase (PR, 77.4 μM), pyruvate kinase (PK, 159 μM), glutathione reductase (GR, 230 μM), superoxide dismutase (SOD, 448 μM), catalase (562 μM), GST M1-1 (620 μM), thioredoxin reductase (TR, 694 μM), and glutathione peroxidase (GPX, 1410 μM). In contrast to the significant inhibition of total GST activity in these human breast cancer cells, 4-OHEN only slightly inhibited the cellular GAPDH activity, and other cellular enzymes including PR, PK, GR, SOD, catalase, TR, and GPX were resistant to 4-OHEN-induced inhibition. These data suggest that GST P1-1 may be a preferred protein target for equine catechol estrogens in vivo.

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