Effects of Estrogen Receptor Modulators on Uterine Pathology and Gene Expression.

Tamoxifen, a selective estrogen receptor modulator (SERM), is an effective treatment for breast cancers. In the CD-1 mouse model, neonatal oral dosing with tamoxifen leads to the development of adenomyosis. Subcutaneous dosing with tamoxifen or 4-hydroxyestradiol leads to adenocarcinomas after 12 to 18 months. Both 4-hydroxyestradiol and tamoxifen can form DNA-reactive metabolites and may be involved in carcinogenesis of the uterus. We compared the uterotrophic response of these compounds and their effects on uterine pathology and gene expression. 14-day-old CD-1 female mice were orally dosed daily for 3 days with estradiol benzoate (1-100μg/kg), tamoxifen (250-5000μg/kg), or 4-hydroxyestradiol (76.3-1920μg/kg). After 4 days uterine weights were recorded, then sections processed for PCNA staining. Maximal uterotrophic doses for weight increase over controls (fold change ± s.d.) were: estradiol (100g/kg) 2.6 ± 0.003, 4-hydroxyestradiol (385g/kg) 6.7 ± 0.03 and tamoxifen (250g/kg) 2.6 ± 0.005. PCNA staining showed maximal cell proliferative effects at this time were not related to uterine weight increases. Estradiol inhibited the development of glandular epithelium, whereas tamoxifen was able significantly to increase the number of glands. Maximal uterotrophic dose of estradiol (100g/kg) was given orally to newborn CD1 mice on days 1 – 5 after birth and gene and pathological changes examined in the uterus at 3 months after dosing. No adenomyosis was seen. Gene expression changes showed up-regulation of typical estrogenic response genes including actb, sp1 while other genes such as ngf and spp1 were down-regulated. This study shows that neonatal estradiol treatment brings about long-term changes in gene expression in the absence of apparent pathological changes.