Aromatase, induction

Biological state

Aromatase (synonyms: Cytochrome P450 aromatase, CYP19) plays a central role in steroidogenesis by converting androgens to estrogens through a three-step reaction that allows the aromatization of the A-ring of the steroid molecule (Mendelson et al., 1985; Thompson and Siiteri, 1974; Simpson and Santen, 2015) (see Fig. 1, Caciolla et al.2020). It is considered the rate-limiting enzyme in estrogen biosynthesis (Simpson and Santen, 2015; Zhao et al., 2016) since it catalyzes the final and key step, i.e., the conversion of C19 steroids to estrogen (Bulun et al., 2005). More specifically, aromatase converts through aromatization androstenedione and testosterone to estrone (E1) and estradiol (E2), respectively. Aromatase has been proposed as an important molecular target for many environmental endocrine disruptors chemicals (Laville et al., 2006).

Figure 1. Estrogen biosynthesis. In particular, the role of human aromatase is highlighted (Caciolla et al., 2020)

Biological compartment

The enzyme is present in all vertebrates as the product of expression of a single gene, with some exceptions represented by pigs and teleosts, where duplication events have produced three and two isoforms, respectively. The protein is expressed in different tissues in vertebrates, where it plays an essential role in reproductive biology as estrogens are responsible for ovarian differentiation, development of the reproductive system, sex differentiation, and reproduction. Moreover, a critical role of estrogens has also been demonstrated in brain, bone, skin, fat, and cardiovascular tissues (Di Nardo et al., 2021). In the normal endometrium aromatase is not expressed (Bulun 2009; Zhao et al., 2016).

General role in biology

Estrogen is synthesized by aromatase in the gonads and in several extragonadal organs, such as skin, adipose tissues, liver, heart and brain (Bulun et al., 2005; Simpson 2003, Bulun et al., 2009). Extragonadal estrogen is biologically active in a paracrine or intracrine fashion, although it may escape the local metabolism and enter the circulation (Simpson and Davis, 2001; Simpson, 2003).

In premenopausal women, ovary is the primary source of estrogens (primarily in the granulosa cells and corpus luteum), and the cyclic expression of estrogen by the ovaries drives endometrial proliferation (Mihm et al., 2011). Disease-free endometrium lacks aromatase and thus does not produce estrogen locally (Bulun 2009; Zhao et al., 2016).

In postmenopausal women, peripheral tissues, especially adipose tissue, become the main site of estrogen synthesis (Davis et al., 2015). The estrogen precursor androstenedione is primarily secreted by the adrenal glands (Zhao et al., 2016), and estrogen (E1, E2) is produced in many extragonadal organs (skin, adipose tissues, liver, heart and brain) (Bulun et al., 2005). After menopause, adipocytes, preadipocytes, and mesenchymal stem cells within fat tissue are the predominant source of aromatase, the enzyme responsible for the conversion of androgens to estrogen E1 and, to a lesser extent E2 (Zhao et al., 2016).

Expression:

Aromatase mRNA expression can be measured by RT-PCR [in JEG-3 choriocarcinoma cells cultures (Laville et al., 2006); in H295R Human Adrenocortical Carcinoma Cells (Sanderson et al., 2002); in KGN Human Ovarian Granulose-Like Tumor Cell Line (Morinaga et al., 2004)] and in situ hybridization [endometrial carcinoma tissue samples (Segawa et al., 2005)]

Aromatase protein levels can be measured by Western blot from cell cultures and liver microsomal samples (You et al., 2001) and by immunohistochemistry on tissue sections (You et al., 2001, Segawa et al., 2005)

Activity:

The catalytic aromatase activity was determined by using the tritium release assay (measurement of tritium released after the conversion of tritium-labelled androstenedione into estrone by cells or hepatic microsomes as described previously (Drenth et al., 1998; Lephart and Simpson, 1991) with minor modifications [in JEG-3 choriocarcinoma cells cultures (Laville, 2006); in H295R Human Adrenocortical Carcinoma Cells (Sanderson et al., 2002); in hepatic microsomal samples (You et al., 2001); in KGN Human Ovarian Granulose-Like Tumor Cell Line (Morinaga et al., 2004)].

Indirect methods:

Levels of estrogens in serum can offer an indirect measurement of aromatase induction.

Aromatase levels and activity increase as a function of age and adiposity (Simpson et al., 1987; Bulun and Simpson, 1994) and, therefore, contribute to estrogen-induced endometrial proliferation in the postmenopausal woman (Blakemore and Naftolin, 2016; Zhao et al., 2016). Consistent with the role of adipose tissue in estrogen synthesis, obesity is more strongly associated with the development of endometrial cancer than any other cancer type in women (Reeves et al., 2007).

In the mouse aromatase is expressed in fewer tissues (gonads, brain) than human aromatase; thus, mouse models do not mirror estrogen production in humans. Aromatase is only present in gonads, brain and male gonadal fat in mice (Zhao et al., 2016). Generation of humanized aromatase (Aromhum) mouse model that contains the full human aromatase gene, allowed to mimic human aromatase expression pattern in the mouse model (Zhao et al., 2012). In the rat, in addition to expression in gonads and brain, aromatase was induced after ovariectomy in liver and subcutaneous adipose tissue (Zhao et al., 2005)

Biological domains of applicability

1. Taxonomic applicability: Vertebrates. Invertebrates of the genus Branchiostoma (Di Nardo et al., 2021).
2. Life stage applicability: All life stages, mainly adulthood (UniProt)
3. Sex applicability: Males, females.

Evidence for the biological domain of applicability

Aromatase converts, through aromatization, androstenedione and testosterone to estrone (E1) and estradiol (E2), respectively. Aromatase induction could increase circulating estrogens (E1, E2) available to estrogenic activation pathways in estrogenic sensitive tissues (uterus).  

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