Received for publication July 2, 2004.
Revised September 11, 2004.
Accepted for publication September 16, 2004.

Tibolone is metabolized by the 3/3-hydroxysteroid dehydrogenase (HSD) activities of the four human isozymes of the aldo-keto reductase (AKR)1C subfamily: inversion of stereo-specificity with a ⁵⁽¹⁰⁾-3-ketosteroid

Abstract

Tibolone is used to treat climacteric complaints and prevent osteoporosis. These beneficial effects are exerted via its 3- and a 3-hydroxymetabolites. Undesirable stimulation of the breast and endometrium is not apparent. Endometrial stimulation is prevented by the progestagenic activity of its 4-ene metabolite. The enzymes responsible for the formation of these active metabolites are unknown. Recently, human aldo-keto reductases (AKR)1C isoforms were shown to act as 3/3-HSDs on 5-dihydrotestosterone (5-DHT). We show that AKR1C's also efficiently catalyze the reduction of the ⁵⁽¹⁰⁾-3-ketosteroid tibolone to yield 3- and 3-hydroxytibolone. Homogeneous recombinant AKR1C1, AKR1C3 and AKR1C4 gave similar catalytic profiles to those observed with 5-DHT. AKR1C1 catalyzed exclusively the formation of 3-hydroxytibolone, AKR1C3 showed weak 3/3-HSD activity, and AKR1C4 acted predominantly as a 3-HSD. Whereas AKR1C2 acted as a 3-HSD towards 5-DHT, it functioned exclusively as a 3-HSD on tibolone. Furthermore, strong substrate inhibition was observed for the AKR1C2 catalyzed reduction of tibolone. Using NAD+, the 3-hydroxymetabolites were efficiently oxidized by homogeneous recombinant AKR1C2 and AKR1C4. However, due to potent inhibition of this activity by NADPH, AKR1Cs will likely act only as 3-ketosteroid reductases in vivo. Molecular docking simulations using crystal structures of AKR1C1 and AKR1C2 explained why AKR1C2 inverted its stereo-specificity from a 3-HSD with 5-DHT to a 3-HSD with tibolone. The preference for AKR1C1 and AKR1C2 to form 3-hydroxytibolone, and the preference of the liver specific AKR1C4 to form 3-hydroxytibolone, may explain why 3-hydroxytibolone is the major metabolite in human target tissues and why 3-hydroxytibolone is the major circulating metabolite.

Key words: Sex hormones, Structure-activity relationships and modeling, Enzymology, Structure/function/mechanism, Endocrine cells