Local estrogen production plays a key role in proliferative endometrial disorders, such as endometrial hyperplasia and cancer. Hydroxysteroid (17␤) dehydrogenase 1 (HSD17B1) is an enzyme that catalyzes with high efficiency the conversion of weakly active estrone into highly potent estradiol. Here we report that female transgenic mice expressing human HSD17B1 invariably develop endometrial hyperplasia in adulthood. These mice also fail to ovulate and have enhanced peripheral conversion of estrone into estradiol in a variety of target tissues, including the uterus. As in humans, endometrial hyperplasia in HSD17B1 transgenic female mice was reversible on ovulation induction , which triggers a rise in circulating progesterone levels , and in response to exogenous progestins. Strikingly , a treatment with an HSD17B1 inhibitor failed to restore ovulation yet completely reversed the hyperplastic morphology of epithelial cells in the glandular compartment , although less so in the luminal epithelium. The data indicate that human HSD17B1 expression enhances endometrial estrogen production, and consequently, estrogen-dependent proliferation. Therefore, HSD17B1 is a promising new therapeutic target in the management of estrogen-dependent endometrial diseases.

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Hydroxysteroid (17-beta) dehydrogenase type 1 (HSD17B1) converts low-active estrogen estrone to highly active estradiol. Estradiol is necessary for normal postpubertal mammary gland development; however, elevated estradiol levels increase mammary tumorigenesis. To investigate the significance of the human HSD17B1 enzyme in the mammary gland, transgenic mice universally overexpressing human HSD17B1 were used (HSD17B1TG mice). Mammary glands obtained from HSD17B1TG females at different ages were investigated for morphology and histology, and HSD17B1 activity and estrogen receptor activation in mammary gland tissue were assessed. To study the significance of HSD17B1 enzyme expression locally in mammary gland tissue, HSD17B1-expressing mammary epithelium was transplanted into cleared mammary fat pads of wild-type females, and the effects on mammary gland estradiol production, epithelial cells and the myoepithelium were investigated. HSD17B1TG females showed increased estrone to estradiol conversion and estrogen-response element-driven estrogen receptor signaling in mammary gland tissue, and they showed extensive lobuloalveolar development that was further enhanced by age along with an increase in serum prolactin concentrations. At old age, HSD17B1TG females developed mammary cancers. Mammary-restricted HSD17B1 expression induced lesions at the sites of ducts and alveoli, accompanied by peri- and intraductal inflammation and disruption of the myoepithelial cell layer. The lesions were shown to be estrogen dependent, as treatment with an antiestrogen, ICI 182,780, starting when lesions were already established reversed the phenotype. These data elucidate the ability of human HSD17B1 to enhance estrogen action in the mammary gland and indicate that HSD17B1 is a factor inducing phenotypic alterations associated with mammary tumorigenesis.

We have recently generated transgenic (TG) mice overexpressing human hydroxysteroid (17beta) dehydrogenase 2 enzyme (HSD17B2TG mice) under the ubiquitous chicken beta-actin promoter. As shown in the present study, the HSD17B2TG female mice presented with slower gain of body weight as compared with the wild-type (WT) littermates and suffered from ovarian dysfunction and mammary gland hyperplasia associated with increased expression of multiple pregnancy-associated genes. The macroscopic phenotype observed in the mammary gland was likely to be dependent on the increased progesterone and prolactin secretion, and a normal histological appearance was observed in HSD17B2TG mammary gland transplanted into a WT host. However, a significant suppression of several known estrogen target genes in the HSD17B2TG mammary transplants in WT females was observed, suggesting that HSD17B2 modulates estrogen action in vivo. Interestingly, the growth retardation of HSD17B2TG females was not efficiently rescued in the bi-TG mice expressing both HSD17B2 and HSD17B1 enzymes, and the bi-TG mice presented with certain masculinized phenotypes, including lack of nipples and closed vagina, recently reported for HSD17B1TG females. The present data suggest that HSD17B2 expression affects both sex steroid-independent and steroid-dependent pathways.

Hydroxysteroid (17beta) dehydrogenase type 1 (HSD17B1) is an enzyme that converts estrone to estradiol, while adenomyosis is an estrogen-dependent disease with poorly understood pathophysiology. In the present study, we show that mice universally over-expressing human estrogen biosynthetic enzyme HSD17B1 (HSD17B1TG mice) present with adenomyosis phenotype, characterized by histological and molecular evaluation. The first adenomyotic changes with endometrial glands partially or fully infiltrated into the myometrium appeared at the age of 5.5 months in HSD17B1TG females and became more prominent with increasing age. Preceding the phenotype, increased myometrial smooth muscle actin positivity and increased amount of glandular myofibroblast cells were observed in HSD17B1TG uteri. This was accompanied by transcriptomic upregulation of inflammatory and estrogen signaling pathways. Further, the genes upregulated in the HSD17B1TG uterus were enriched with genes previously observed to be induced in the human adenomyotic uterus, including several genes of the NFKB pathway. A 6-week-long HSD17B1 inhibitor treatment reduced the occurrence of the adenomyotic changes by 5-fold, whereas no effect was observed in the vehicle-treated HSD17B1TG mice, suggesting that estrogen is the main upstream regulator of adenomyosis-induced uterine signaling pathways. HSD17B1 is considered as a promising drug target to inhibit estrogen-dependent growth of endometrial disorders. The present data indicate that HSD17B1 over-expression in TG mice results in adenomyotic changes reversed by HSD17B1 inhibitor treatment and HSD17B1 is, thus, a potential novel drug target for adenomyosis.

Aromatase enzyme coded by the CYP19A1 gene catalyzes the final step of estrogen biosynthesis and is of major significance in endocrine regulation of breast cancer (BCa). Human CYP19A1 gene expression is regulated through the utilization of various non-translated tissue selective first exons, each controlled by unique sets of e.g. hormones and cytokines. Extragonadal expression pattern and regulation of human CYP19A1 gene is significantly different from that of rodents, which has been a major obstacle in studies focusing on regulation of aromatase gene expression in vivo. To study the tissue-selective regulation of human CYP19A1 gene in vivo, we have generated a transgenic reporter mouse model, hARO-Luc mouse, with a construct composing of a 100-kbp-long 5’-regulatory region of the human CYP19A1 gene attached to a luciferase (Luc) reporter gene. The aim of this study was to investigate human CYP19A1 gene expression in human BCa xenografts, to determine how the BCa cells regulate CYP19A1 gene expression in tumor associated stroma in vivo. In vitro, BCa cells have been shown to induce CYP19A1 gene expression in human breast derived fibroblasts. Furthermore, in order to study the regulation of the human CYP19A1 gene expression in hARO-Luc derived undifferentiated fibroblasts, mesenchymal stromal cells were isolated from bone marrow of female reporter mice. In these cells, human CYP19A1 gene expression was induced by dexamethasone ± TNFα, and PGE2 that are among factors known to drive aromatase gene expression in human mammary fibroblasts through promoters I.4 and I.3/PII. In order to investigate human CYP19A1 expression in BCa xenografts and tumor surrounding mammary gland, the hARO-Luc reporter mice were cross-bred with athymic nude mice. Estrogen-responsive MCF-7 or T47D cells or non-estrogen-responsive MDA-MB-231 cells were injected into inguino-abdominal mammary fat pads of athymic female hARO-Luc mice, and mice that received MCF-7 or T47D cells were also implanted s.c. with estradiol releasing pellet to induce the estrogen-dependent tumor growth. The reporter gene activity was measured from tissue homogenates. MCF-7 and T47D tumors had 5-10 -fold higher Luc activity as compared with the intact mammary gland, indicating a strong up-regulation of the reporter gene in host derived tumor stroma. In contrast, in MDA-MB-231 tumor bearing mice the reporter activity in tumors was only slightly higher as compared with the intact mammary gland. Furthermore, reporter gene activity in mammary gland surrounding MDA-MB-231 tumors was 2-fold higher than in the tumor itself. In conclusion, the data indicate that human BCa cells up-regulate expression of human aromatase gene in the tumor associated stroma in vivo. Citation Format: Päivi Järvensivu, Leena Strauss, Sari Mäkelä, Matti Poutanen, Niina Saarinen. A reporter mouse model reveals that human CYP19A1 (aromatase) gene expression is induced in breast cancer xenograft stroma and surrounding mammary gland by the cancer cells in vivo. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1402. doi:10.1158/1538-7445.AM2013-1402