The Journal of Steroid Biochemistry and Molecular Biology
Inactivation by UDP-glucuronosyltransferase enzymes: The end of androgen signaling☆
Introduction
The major serum androgen produced by the testis is testosterone which is converted in androgen target tissues to 5α-reduced-dihydrotestosterone (DHT), the endogenous steroid that possess the highest affinity for the androgen receptor [1]. Testosterone is not the only steroid to be served as precursors of DHT since several tissues are also capable of converting adrenal dehydroepiandrosterone (DHEA) and its sulfate (DHEAS) to androgen [2]. Thus, after adrenarche and puberty, the adrenal and testis provide DHT precursors into the circulation but the local concentrations of DHT are modulated by a process which implicates various steroidogenic enzymes such as, steroid sulfatase, type 1 3β-hydroxysteroid dehydrogenase (3β-HSD), type 5 17β-HSD and, finally, 5α-reductase that ensures maximum formation of DHT [2]. However, the fine regulation of local tissue DHT concentrations is not only dependent of synthesizing, but also of inactivating enzymes. Phase I DHT catabolites might include androsterone (ADT) and androstane-3α,17β-diol (3α-DIOL) formed by the action of a series of 3α/β-hydroxysteroid dehydrogenase (HSD) and 17β-HSD isoforms [3]. Most if not all of the androgen-target tissues express HSD isoforms that are capable of back converting the Phase I metabolites into DHT. In the liver, several enzymes, including 5α-reductase, produces DHT and its metabolites. Exceptionally, this tissue is also the site of 5β-reduction of testosterone to 5β-reduced-DHT, an inactive isomer of DHT [4]. This pathway implies the production of a series of inactive metabolites with etiocholanolone as the major metabolic representative. It is considered that the pathway of 5β-reduction is an important mean to inactivate circulating testosterone by the liver (Fig. 1).
Despite the large production of testosterone, DHEA and is sulfate in the body, it was however surprising to observe relatively low amount of their non-conjugated metabolites in blood. Indeed, it is generally recognized that serum levels of 3α-DIOL, ADT and etiocholanolone are below 1 ng/mL in serum from adult men [5], [6]. Several observations published over the last 20 years indicated that a large portion of the androgen metabolites found in urine and serum was under the form of glucuronide derivative (Table 1). Glucuronide derivative of three 5α-reduced metabolites of DHT, namely androstane-3α,17β-diol-3glucuronide (3α-DIOL3G), androstane-3α,17β-diol-17glucuronide (3α-DIOL17G) and androsterone glucuronide (ADTG) are found in larger concentrations in blood than their corresponding non-conjugated form and were suggested to be the major metabolites of DHT in the body [3]. The presence of etiocholanolone glucuronide in circulation (Table 1), the major liver 5β-reduced metabolite of DHT, also indicates that the liver is also capable to release androgen glucuronide in blood stream.
Approximately 50–70% of androgen glucuronides in the circulation originates from testosterone secreted by the testis whereas the remaining proportion was from the adrenal androgen precursors DHEA and DHEAS [7]. This was supported by the lower serum ADTG, 3α-DIOL3G and 3α-DIOL17G concentrations in adult women and castrated men compared to adult men [7], [8]. Any increases in endogenous and exogenous precursors of DHT, particularly DHEA, DHEAS and testosterone, have also been shown to have a significant impact on glucuronide androgens [9]. Thus, several authors have reported that in women with acne or hirsutism where adrenal or ovarian androgens are elevated, there was also a significant increase in circulating androgen glucuronide levels [8], [10], [11], [12], [13], [14]. In the presence of normal serum concentrations of adrenal steroid precursors in hirsute women or with acne androgen glucuronide may also be increased likely due to an elevated steroidogenic enzymatic activity in skin that enhances local androgen production [8], [15].
This observation prompted several groups to investigate the sources of circulating androgen glucuronide formation. Evidence suggests that several tissues, including the liver, may contribute to the presence of androgen glucuronide in circulation [16], [17], [18] but it was difficult to clearly determine the relative role of each tissue. In the process of demonstrating the contribution of extrasplanchnic tissues in the production of androgen glucuronide, the presence of ADTG and 3α-DIOLG in tissues such as prostate, ovary and mammary gland was investigated by our group. We observed significant concentrations of these glucuronide conjugates in all three tissues, which suggested a local production and release to the circulation of these polar metabolites [19], [20], [21]. In order to further demonstrate the presence of steroid-conjugating enzymes in androgen target tissues, we then analyzed and localized within androgenic tissues the enzymes responsible for the glucuronidation of androgen in human. We revealed the existence of specific androgen-conjugating enzymes and we demonstrated that they are expressed in a series of androgen target tissues, where their synthesis is influenced by several endogenous factors, including steroid themselves.
Section snippets
Specificity of UDP-glucuronosyltransferase enzymes towards steroids
Conjugation of compounds by glucuronidation is a pathway that has been found in all vertebrates studied to date [22]. It is an important mechanism to protect the body from accumulating compounds, namely estrogen and androgen, which may have adverse effects at large concentrations [3], [23]. This reaction, which corresponds to the transfer of the group from UDP-glucuronic acid to small hydrophobic molecules, is catalyzed by enzymes of the UDP-glucuronosyltransferase (UGT) enzymes superfamily.
Serum levels of androgen glucuronides
Taking into account the high serum levels of the two 17-ketosteroids DHEA and DHEAS, it is not surprising to also detect elevated concentrations of the corresponding 17-ketoandrogen glucuronide ADTG. It is probable that the liver and other tissues convert in large proportion DHEA and DHEAS by a pathway that involves only 17-ketosteroids (DHEA to androstenedione to androstanedione to androsterone to ADTG). As a reflect of this metabolism, the concentrations of circulating ADTG and ETIOG are
Tissue and cellular expression of UGT2B7, B15 and B17 enzymes
Transcripts of all three UGT2B7, B15 and B17 have been detected in the liver and skin but, in other tissues examined, there is a specific pattern of expression for each conjugating enzyme. UGT2B7 transcripts and/or protein are detected in the mammary glands, uterus and ovary whereas it is absent in the adipose tissue and prostate [38]. Interestingly, UGT2B15 is the only UGT2B expressed in the adipose tissue but both UGT2B15 and B17 are observed in the prostate [38]. This tissue has attracted
Regulation in UGT2Bs expression and activity on steroids
The role of UGT enzymes in the liver has been recognized as a highly efficient and constitutive mechanism of detoxification of endogenous and exogenous compounds, particularly for steroid hormones [3], [48]. It was believed that the wide specificity of the conjugating enzymes should facilitate inactivation and elimination of steroids by multi-isoforms of UGTs with almost no limitation in term of activity. With the observation that a small number of UGT2Bs are specific for androgen and their
Polymorphism in UGT2B15 gene and deletion of UGT2B17 gene
The localization of UGT2B15 in luminal epithelial cells of the prostate suggests that this enzyme may conjugate DHT and its metabolite 3α-DIOL and that slightly changes in conjugating activity could affect the intracellular DHT concentration and its action. In addition to the first UGT2B15(D85) isolated by Chen et al., Lévesque et al. discovered a second form of UGT2B15, UGT2B15(Y85) and demonstrated that Vmax of the Y allele is approximately twofold greater than the enzyme resulting from the D
In vivo model to study the role of UGT2B enzymes
Contrary to human and monkey, the serum levels of ADTG, 3α-DIOL3G and 3α-DIOL17G are under the limit of detection in all species examined such as rodents, dog and bovine, thus suggesting that, in these species, the production of androgen glucuronide might be low in the body [67]. Indeed, subcutaneous administration of testosterone or DHT to rats does not cause increases of androgen glucuronide in serum and in the prostate whereas, in bile, significant increases of androgen glucuronide
Conclusions
The presence of androgen-conjugating UGT2B enzymes in several tissues of the human body is in agreement with the hypothesis that androgens can be inactivated not only in the liver but also locally in androgen target tissues [38]. The particular substrate specificity as well as the tissue and cellular distribution of UGT2B7, 2B15 and 2B17 support the concept that cells from an androgen target tissue may express all steroidogenic enzymes responsible for steroid production and elimination of
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2019, Pharmacology and TherapeuticsCitation Excerpt :Because steroid glucuronides do not bind to nuclear receptors and glucuronidation is largely irreversible in these tissues, it is considered an important mechanism to terminate steroid (mainly androgen) signalling. Consistent with this function, several steroid conjugating UGTs have been linked to breast and/or prostate cancer risk and progression (Chouinard et al., 2008; Gauthier-Landry, Belanger, & Barbier, 2015; Hu, Mackenzie, McKinnon, & Meech, 2016; Sun, Liu, McCloskey, & Lazarus, 2011; Yao et al., 2010; Hu, Selth et al., 2016; Zhu et al., 2018). The conjugation of exogenous chemicals by UGTs is also linked to cancer risk; many UGTs are expressed at interfaces with the environment such as airways and gut, where they detoxify carcinogenic xenobiotics.
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Presented at the ‘12th International Congress on Hormonal Steroids and Hormones & Cancer’ (Athens, Greece, 13–16 September 2006).