Received for publication January 10, 2006.
Revised May 8, 2006.
Accepted for publication May 9, 2006.

Endoxifen, a secondary metabolite of tamoxifen, and 4-OH-tamoxifen induce similar changes in global gene expression patterns in MCF-7 breast cancer cells

Abstract

We recently demonstrated that endoxifen (4-hydroxy-N-desmethyl-tamoxifen), a pharmacogenetically regulated metabolite of tamoxifen, is equipotent to 4-hydroxy-tamoxifen (4-OH-Tam) with respect to estrogen receptor binding and inhibition of 17-estradiol (E2)-induced cell proliferation. Endoxifen was also found to be more abundant in human plasma than 4-OH-Tam and its formation has been shown to be primarily catalyzed by cytochrome P450 2D6 (CYP2D6). Here, we report studies evaluating the effects of endoxifen, 4-OH-Tam, and E2 on gene expression in MCF-7 cells, using Affymetrix U133A Genechip® Arrays. We detected 4,062 genes that were E2-regulated (1,924 induced, 2,138 suppressed) and the ratio of E2-induced versus E2-suppressed genes was consistent regardless of the cutoff value. In the presence of E2, 2,444 and 2,390 genes were affected by 4-OH-Tam and endoxifen, respectively, when no minimum fold-change cutoff was implemented. The majority of genes regulated by the tamoxifen metabolites were also E2-responsive (74.4%, 73.3%, respectively). Endoxifen and 4-OH-Tam had overlapping effects on 1,365 E2-sensitive genes, whose fold-effects between these metabolites were highly correlated (R²=0.99). A significant correlation was also found between the fold-effects of 249 E2-insensitive genes co-regulated by both metabolites (R²=0.99). Hierarchical clustering analysis demonstrated similar gene regulation patterns between these metabolites, which were distinct from E2 or vehicle treatment patterns. Using RT-PCR, we validated the gene expression patterns of 5 genes that were differentially regulated by endoxifen and 4-OH-Tam. We conclude that endoxifen and 4-OH-Tam have similar effects on global gene expression patterns in MCF-7 cells and that the majority of the affected genes are estrogen-regulated genes.

Key words: Breast Cancer, Endoxifen, Microarray, antiestrogen, estradiol, tamoxifen

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