Increased expression of Nrf2/ARE-dependent anti-oxidant proteins in tamoxifen-resistant breast cancer cells

Acquired resistance to tamoxifen (TAM) is a serious therapeutic problem in breast cancer patients. In this study, we found that the expressions of anti-oxidant proteins (gamma-glutamylcysteine ligase heavy chain (gamma-GCL(h)), heme oxygenase-1, thioredoxin and peroxiredoxin1) in TAM-resistant MCF-7 (TAMR-MCF-7) cells were higher than control MCF-7 cells. Molecular analyses using antioxidant response element (ARE)-containing reporters and gel-shift supported the Critical role of NF-E2-related factor2 (Nrf2)/ARE in the overexpression of antioxidant proteins in TAMR-MCF-7 cells. Intracellular peroxide production was significantly decreased in TAMR-MCF-7 cells and TAM resistance was partially reversed by Nrf2 siRNA. The basal phosphorylation of extracellular signal-regulated kinase (ERK) and p38 kinase were increased in the TAMR-MCF-7 cells and the inhibition of ERK significantly decreased the activity of minimal ARE reporter and gamma-GCL(h) protein expression in TAMR-MCF-7 cells. However, exposure of TAMR-MCF-7 cells to 17-beta-estradiol or ICI-182,780 did not significantly change gamma-GCL(h) expression. These results suggest that the persistent activation of Nrf2/ARE is critical for the enhanced expression of anti-oxidant proteins in TAM-resistant breast cancer cells and the pathway of ERK, but not of estrogen receptor signaling are involved in the up-regulation of Nrf2/ARE. (c) 2008 Elsevier Inc. All rights reserved.