Inactivation of androgens by UDP-glucuronosyltransferases in the human prostate

In the human prostate, dihydrotestosterone (DHT) – the natural androgen having the highest affinity for the androgen receptor – is not released directly into the systemic circulation from peripheral target tissues but it is rather converted in situ into two metabolites which have a low affinity for the androgen receptor: androsterone (ADT) and androstane-3α,17β-diol (3α-DIOL). Several clinical observations indicate that these two androgen metabolites are further inactivated in the prostate by glucuronidation. In the human, the family of UDP-glucuronosyltransferase (UGT) enzymes comprises 18 members in three subfamilies: UGT1A, UGT2A and UGT2B. Identification of the substrates for each member has revealed that three UGT2B enzymes are mainly responsible for DHT, ADT and 3α-DIOL glucuronidation: UGT2B7, UGT2B15 and UGT2B17. Tissue distribution and cellular localization of UGT2B transcripts and proteins clearly indicate that only UGT2B15 and UGT2B17 are expressed in the prostate. Using the human prostate carcinoma LNCaP cell line, it was shown that UGT2B expression and activity are negatively regulated by several factors, including androgens. On the other hand, inhibition of UGT2B115/17 expression by small interfering RNA (siRNA) resulted in an induced response to DHT of androgen-receptor target genes such as PSA, KLK4, NKX3.1, TMPRSS2, SLC16A6 and VEGF. It is suggested that the conjugating activity of UGT enzymes in androgen target tissues is a mechanism for modulating the action of steroids and/or protecting the tissues from deleterious high concentrations of androgens.