Tamoxifen transcriptional regulation in normal human mammary epithelial cells (NHMEC)

Tamoxifen (TAM), used for adjuvant therapy of breast cancer, also increases the risk of endometrial cancer. To compare TAM-induced transcriptional changes we examined human and monkey uterus, as well as cultured normal human mammary epithelial cells (NHMECs) and human endometrial stromal cells (HESCs). Uterine DNA from TAM-exposed women (n=15) and monkeys (Erythrocebus patas n=5, and Macaca fascicularis n=12) showed no difference in 5-methyl-cytosine (5-meC) levels compared to unexposed controls. Studies comparing NHMECs and HESCs exposed to 10 M TAM for 48 hr showed cell-specific differences by microarray, with confirmation by RT-PCR. In TAM-exposed NHMECs there was significant up-regulation of interferon signaling and immune response pathways, while the TAM-exposed HESCs showed significant up-regulation of steroid and fatty acid biosynthesis pathways. Promoter region CpG islands of several genes highly up-regulated by TAM in each cell type (NHMECs: MX1 and STAT1; HESCs: PPARG, SREBF2, HMCGS and Prune2), were examined for 5-meC, but the levels ( 7%), were too low to measure accurately. We also examined several histone H3 lysine dimethylases by Western blot, and showed a significant depletion of total H3 histone, H3K4, H3K27 and H3K36 in TAM-exposed HESCs, with similar but non-significant changes in TAM-exposed NHMECs. Whereas TAM exposure had no discernible effect on 5-meC levels in primate uterus, TAM exposure induced up-regulation of different transcriptional pathways in NHMECs and HESCs, and concomitantly depleted H3 histone lysine dimethylase levels. Therefore, transcriptional dysregulation by TAM, including reduction of histone H3 dimethylase levels, may be related to TAM-induced endometrial carcinogenesis.

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