CHARACTERIZATION AND REGULATION OF UDP-GLUCURONOSYLTRANSFERASES IN STEROID TARGET TISSUES

Conjugation of compounds by glucuronidation is a pathway found in all vertebrates studied to date. Although, it is widely recognized that th e liver is a major site of glucuronidation, it is now clear that extra hepatic tissues are also involved in the conjugation of compounds to w hich these tissues are exposed. High levels of androsterone glucuronid e and androstane-3 alpha,17 beta-diol glucuronide found in the human p rostate, breast cyst fluid and ovary follicular fluid suggest that glu curonidation of 5 alpha-reduced C-19 steroids occurs in these tissues. Recently, we have reported the tissue distribution of UGT2B15, which can conjugate steroids in several human extrahepatic steroid target ti ssues including the skin, breast and prostate. We have also isolated a new UGT2B cDNA encoding UGT2B17, that conjugates ADT which is the maj or 5 alpha-reduced C-19 steroid glucuronide in the circulation of huma ns. UGT2B17 is also widely distributed in several human steroid target tissues. This gene was mapped to human chromosome 4q13 and has an exo n/intron structure similar to that of rat UGT2B1 and UGT2B2. Both UGT2 B15 and UGT2B17, which are able to catalyze the glucuronidation of DHT , are expressed in LNCaP cells. Interestingly, glucuronidation of ster oids is markedly regulated by several factors including androgens and growth factors. Treatment of LNCaP cells with dihydrotestosterone (DHT ) and epidermal growth factor (EGF) caused a decrease of DHT glucuroni dation and UGT2B mRNA levels. RNase protection assays showed a specifi c decrease of UGT2B17 transcript in LNCaP cells treated with DHT and E GF however, the level of UGT2B15 mRNA was not affected. As well, Weste rn blot analysis demonstrated a diminution of UGT2B17 protein level in response to DHT and EGF. These results demonstrate a differential reg ulation of different isoforms of steroid conjugating UGTs present in h uman prostate LNCaP cells. In addition, UGT2B17 was shown to be more l abile than UGT2B15 indicating that regulation of UGT2B17 expression wo uld lead to a more rapid change in the level of glucuronidated steroid s. Expression of exogenous UGT2B17 in LNCaP cells by gene transfer led to a significant decrease in the androgen response. This result indic ates the ability of UGT enzymes to regulate the androgen response by c onjugating androgens which abolishes their interaction with their rece ptor and facilitates their clearance from the cell. The glucuronidatio n of steroids by UGT enzymes is an important mechanism by which the le vels of steroids is regulated in steroid target tissues. (C) 1998 Else vier Science Ltd. All rights reserved.