Inactivation by UDP-glucuronosyltransferase enzymes: The end of androgen signaling

Abstract

Conjugation by UDP-Glucuronosyltransferase (UGT) is the major pathway of androgen metabolism and elimination in the human. High concentrations of glucuronide conjugates of androsterone (ADT) and androstane-3α,17β-diol (3α-diol) are present in circulation and several studies over the last 30 years have concluded that the serum levels of these metabolites might reflect the androgen metabolism in several tissues, including the liver and androgen target tissues. Three UGT2B enzymes are responsible for the conjugation of DHT and its metabolites ADT and 3α-diol: UGT2B7, B15 and B17. UGT2B7 is expressed in the liver and skin whereas UGT2B15 and B17 were found in the liver, prostate and skin. Very specific antibodies against each UGT2B enzyme have been obtained and used for immunohistochemical studies in the human prostate. It was shown that UGT2B17 is expressed in basal cells whereas UGT2B15 is only localized in luminal cells, where it inactivates DHT. By using LNCaP cells, we have also demonstrated that the expression and activity of UGT2B15 and B17 are modulated by several endogenous prostate factors including androgen. Finally, to study the physiological role of UGT2B enzymes, transgenic mice bearing the human UGT2B15 gene were recently obtained. A decrease in reproductive tissue weight from transgenic animals compared to those from control animals was observed. In conclusion, the conjugation by UGT2B7, B15 and B17, which represents a non-reversible step in androgen metabolism, is an important means by which androgens are regulated locally. It is also postulated that UGT enzymes protect the tissue from deleteriously high concentrations of active androgen.

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.