Aromatase expression and regulation in breast and endometrial cancer.

Aromatase is the rate-limiting enzyme in estrogen biosynthesis ( Simpson & Santen 2015 ). The biologically active estrogen, 17β-estradiol (E2), exerts its actions by binding to its receptors, estrogen receptor-α (ERα) ( Lubahn et al . 1993 , Couse et al . 1999 , Dupont et al . 2000 ) and estrogen receptor-β ( Krege et al . 1998 ). Beyond its essential role in reproductive function ( Bulun et al . 2005 ), estrogen is also involved in vascular biology ( O’Lone et al . 2007 ), lipid and carbohydrate metabolism ( Jones et al . 2001 ), bone mineralization ( Nakamura et al . 2007 ), and cognitive and other brain-related functions ( Arevalo et al . 2015 ). Estrogen also plays an important role in initializing development and further growth of a number of benign and malignant hormone-dependent disorders, including breast and endometrial cancers ( Bulun et al . 2005 , Cavalieri et al . 2006 , Kim et al . 2013 , Santen et al . 2015 ). Breast cancer is the most common cancer among women in the United States (other than skin cancer). It can be divided into four main molecular subtypes based on the presence or absence of routinely evaluated biological markers: hormone (estrogen or progesterone) receptors (HR+/HR−) and excess levels of human epidermal growth factor receptor 2 (HER2+/HER2−), a protein promoting breast epithelial cell growth ( Perou et al . 2000 , Cancer Genome Atlas 2012 ). These four subtypes are luminal A (HR+/HER2−, 74%); luminal B (HR+/HER2+, 10%); HER2-enriched (HR−/HER2+, 4%); and triple negative (HR−/HER2−, 12%) ( Anderson et al . 2014 , Kohler et al . 2015 ). The majority of breast cancers (84%) express estrogen and progesterone receptors, indicating the essential role of estrogen in breast cancer development. Endometrial cancer is the sixth most common cancer in women worldwide and is divided into two types. Type 1 endometrial cancer is thought to be caused by excess estrogen ( Jarzabek et al . 2013 ). Risk factors that affect a woman’s estrogen balance play a critical role in this type of endometrial cancer. Type 2 endometrial cancer is not related to estrogen ( Morice et al . 2016 ). This review describes the molecular basis of tissue-specific estrogen production and its role in breast and endometrial cancers. Understanding the mechanisms that control the levels of estrogen in specific tissues may lead to the development of tissue-targeted therapies for estrogen-dependent diseases, such as breast and type 1 endometrial cancer.

Endometrial cancer is the most common gynecological malignancy in US women ( Morice et al . 2016 ). Type 1 endometrial cancer is the most common type, thought to be caused by excess estrogen, usually not very aggressive, and slow to spread to other tissues; however, type 2 endometrial cancer is not related to estrogen stimulation and usually presents as a higher-grade cancer with a poorer prognosis ( Bokhman 1983 , Rizner 2013 ). Type 1 endometrial adenocarcinoma occurs in the context of chronic exposure to excess estrogen (endogenous and exogenous) with insufficient opposing progesterone. High levels of endogenous estrogen arise as a consequence of various disease states, including anovulation, polycystic ovarian syndrome and obesity ( Creasman 1997 , Morice et al . 2016 ). Exogenous estrogen-only hormone replacement therapy (HRT) can promote the development of endometrial cancer compared with an estrogen plus progestin regimen ( Beresford et al . 1997 , Rossouw et al . 2002 ). With this regimen, progestin serves as an estrogen antagonist in the endometrium. Progestins have been used clinically to treat endometrial neoplasias. Tamoxifen exerts anti-estrogen effects in breast but acts as an E2 agonist in the uterus. Thus, treatment of breast cancer with tamoxifen increases the risk of developing endometrial cancer ( Swerdlow et al . 2005 , Chen et al . 2014 ). Although the pro-proliferative ( Siiteri 1978 , Key & Pike 1988 ) and DNA damaging ( Roy & Liehr 1999 , Shibutani et al . 2000 ) effects of estrogen and its metabolites lead to the hyperproliferation and transformation of cells, the underlying mechanisms involved in endometrial carcinogenesis from chronic estrogen exposure are unclear. Three major sites in the body produce estrogen in women with endometrial cancers ( Bulun et al . 2005 , Bulun 2009 ). First, E2 secreted by the ovary reaches endometrial cancer tissue through the systemic circulation. Secondly, aromatase in adipose tissue and skin catalyzes the conversion of circulating androstenedione to E1 that is subsequently converted to E2, and both E1 and E2 can enter the circulation and reach sites of endometrial cancer. Finally, the third source is estrogen that is synthesized locally in endometrial cancer tissues. As indicated above, a cascade of steroidogenic genes is needed to synthesize estrogen from cholesterol, including aromatase – the key enzyme for the last step of estrogen formation ( Bulun et al . 2005 ). In premenopausal women, ovarian estrogen is the main source of estrogen for endometrial tissue or endometrial cancer, as disease-free endometrium lacks aromatase and thus does not produce estrogen locally ( Bulun 2009 ). Although aromatase mRNA is absent in tissue homogenates of both proliferative-phase endometrium and secretory-phase endometrium, it is present in first-trimester decidua ( Gibson et al . 2013 ). During the luteal phase, the progesterone-dependent 17β-HSD 2 enzyme catalyzes the conversion of the biologically active E2 to the less estrogenic E1, which partially inhibits systemic estrogen action. Quantitative PCR with high cycle numbers amplifies aromatase mRNA in endometrial biopsies, but we cannot detect aromatase enzyme activity in cultured endometrial stromal cells from disease-free women ( Noble et al . 1996 ). Aromatase activity or mRNA cannot be induced by PGE2 or cAMP analogs in stromal cells from disease-free women, which is the main molecular mechanism responsible for increased expression of steroidogenic genes in stromal cells from diseased tissue such as endometriosis ( Noble et al . 1996 , Bulun et al . 2001 , Sebastian & Bulun 2001 , Sebastian et al . 2002 ). In postmenopausal women, extraovarian adipose tissues and skin may be the main sites of estrogen formation, which exerts biological effects on the endometrium or endometrial cancer tissues in an endocrine manner. The local concentration of E2 in endometrial cancer tissues is also reported to be higher than in blood or in the endometrium of cancer-free women ( Nagasako et al . 1988 , Potischman et al . 1996 , Sherman et al . 1997 , Berstein et al . 2002 ). It is, therefore, conceivable that endometrial cancer synthesizes E2 in situ , which then contributes to cancer development. One study demonstrated significant conversion of androstenedione to E1 in endometrial cancer tissue ( Yamamoto et al . 1993 ). Aromatase protein and mRNA are detected in endometrial cancer using immunohistochemistry and RT-PCR, whereas aromatase expression is low or undetectable in endometrial hyperplasia (a precursor lesion of endometrial cancer) ( Bulun et al . 1994 , Watanabe et al . 1995 ). Immunoreactive aromatase is also found in malignant epithelial, endometrial stromal and myometrial cells. These observations suggest that intratumoral aromatase may play a role in the pathology of endometrial cancer. Taken together, as supported by aromatase availability, tissues from ovaries, skin, adipose and endometrial cancer are the major sources of estrogen for premenopausal endometrial cancers; extraovarian skin and adipose tissue and endometrial cancer tissue are predominant sites of estrogen formation in postmenopausal endometrial cancers.

Estrogen plays a critical role in tumor formation and growth, and provides a therapeutic target for the prevention and treatment of breast and endometrial cancers. Aromatase is the rate-limiting enzyme in estrogen biosynthesis, with expression driven by multiple tissue-specific, untranslated and alternative promoters. Aromatase inhibitors have been used successfully to treat breast cancer in postmenopausal women but they reduce aromatase activity indiscriminately throughout the body, resulting in severe estrogen deprivation and major side effects such as bone loss and abnormal lipid metabolism. The shift of aromatase promoter usage in malignant tissues is a major mechanism driving abnormal aromatase overexpression and estrogen excess. Targeting these partially tissue-selective promoters in breast and endometrial cancers provides a potential opportunity to inhibit aromatase activity specifically in breast and endometrial tissue without the common side effects of aromatase inhibitors in other organs, such as the bone. Future basic research will focus on the mechanism of aromatase gene regulation in breast and endometrial cancer to identify ideal drug targets for developing selective aromatase modulators or inhibitors and for blocking estrogen production in targeted tissues.