Abstract 5511: Endoxifen inhibits polyamine biosynthetic enzyme activity and up-regulates metabolizing enzymes, spermine oxidase (SMO) and acetyl spermine oxidase (APAO)in breast cancer
Authors: Thomas T. J. , Hsu Hui-Chen , Hyvonen Mervi , Keinanen Tuomo 
Published: August 2015

Tamoxifen is the most widely used selective estrogen receptor modulator for anti-estrogen therapy for women with estrogen receptor alpha (ERalpha)-positive breast cancer. Clinical trials recognized that tamoxifen was a pro-drug and got converted to active metabolites in the human body. Earlier studies recognized 4-hydroxytamoxifen as the active metabolite of tamoxifen, although recent studies showed that the concentration of another metabolite, endoxifen was up to 10-fold higher than that of 4-hydorxytamoxifen in patients treated with tamoxifen. These metabolites are formed by the action of cytochrome P450 2D6 (CYP2D6). To elucidate the mechanism of action of endoxifen, we determined its effects on cell growth, activity of polyamine biosynthetic and metabolizing enzymes, and oncogene expression in ERalpha-positive MCF-7 breast cancer cells. Polyamines (putrescine, spermidine and spermine) are ubiquitous cellular components and play important roles in cell proliferation. Estradiol (4 nM) increased the proliferation of MCF-7 cells by 2- to 3-fold by 3 days of treatment. Endoxifen significantly suppressed the effects of estradiol at 100 nM and higher concentrations (P &lt; 0.01; n = 3). Estradiol increased the activity of the polyamine biosynthetic enzymes, ornithine decarboxlase (ODC) and S-adenosylmethionine decarboxlase (AdoMetDC), whereas endoxifen suppressed ODC and AdoMetDC activities in a concentration- and time-dependent manner. The change in biosynthetic enzyme activities was reflected in polyamine pools, as estradiol increased and endoxifen suppressed the levels of putrescine and spermidine in MCF-7 cells. There was no significant change in spermine level. Endoxifen had a significant effect on increasing the activities of two polyamine metabolizing enzymes, SMO and APAO. Estradiol suppressed the activities of SMO and APAO by 40% compared to control group at 24 hours of treatment. Endoxifen increased SMO and APAO activities by 2- to 4-fold compared to the activities of these enzymes in estradiol treated group. The expression of c-myc, c-fos and Tiff1 genes was significantly increased by 4 nM estradiol, while the addition of endoxifen suppressed estradiol-mediated increase in gene expression, as measured by quantitative real-time PCR analysis. These results indicate that the polyamine pathway plays an important role in the growth inhibitory and anti-estrogenic effects of endoxifen in breast cancer cells. Upregulation of SMO and APAO might be useful biomarkers for the action of endoxifen.

Citation Format: T. J. Thomas, Hui-Chen Hsu, Mervi Hyvonen, Tuomo Keinanen. Endoxifen inhibits polyamine biosynthetic enzyme activity and up-regulates metabolizing enzymes, spermine oxidase (SMO) and acetyl spermine oxidase (APAO)in breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5511. doi:10.1158/1538-7445.AM2015-5511