Changes in gene expression induced by thioredoxin-1 in MCF-7 human breast cancer cells

Thioredoxin-1 (Trx-1) is a small redox protein that is overexpressed in a number of human cancers. Elevated levels of Trx-1 in tumors is associated with increased cell proliferation, decreased apoptosis, and decreased patient survival. However, the mechanism(s) for the growth stimulating and anti-apoptosis effects of Trx-1 are unknown. We used DNA microarray technology to identify genes whose expression was altered in MCF-7 breast cancer cells stably transfected with wild-type Trx-1 (MCF-7/Trx 9) or a redox inactive mutant Trx-1 (MCF-7/SerB 4) compared to empty-vector transfected cells (MCF-7/neo). The expression of cytochrome P450 1B1 (CYP1B1) mRNA and protein is increased by Trx-1 transfection of MCF-7 human breast cancer cells and decreased by a redox inactive mutant Trx-1. CYP1B1 is a tumor specific CYP which converts 17β-estradiol (E₂) to the carcinogenic 4-hydroxyestradiol (4-OHE₂). The expression of peroxiredoxin 1 (PRDX1) mRNA is increased as a result of Trx-1 overexpression in MCF-7 cells. The peroxiredoxins belong to a conserved family of antioxidant proteins that use thiol groups as reducing equivalents to scavenge oxidants. Transfection of mouse WEHI7.2 thymoma cells with human PRDX1 protects cells from apoptosis induced by H₂O₂. Spermine/spermidine N'-acetyltransferase (SSAT) mRNA expression and enzyme activity is decreased by Trx-1 transfection of MCF-7 human breast cancer cells. SSAT is an important enzyme in the polyamine catabolic pathway. The inhibition of SSAT enzyme activity is associated with decreased putrescine levels in the Trx-1 transfected cells. Therefore, it appears as if the modification of cellular redox signaling brought about by the overexpression of Trx-1 in breast cancer cells induces changes in gene expression that contribute to the transformed phenotype. Trx-1 redirects estrogen metabolism in a more toxic pathway due to the induction of CYP1B1, provides resistance to apoptosis induced by reactive oxygen species via the upregulation of PRDX1, and alters polyamine metabolism by inhibiting the expression of SSAT