Tibolone is metabolized by the 3alpha/3beta-hydroxysteroid dehydrogenase activities of the four human isozymes of the aldo-keto reductase 1C subfamily: inversion of stereospecificity with a delta5(10)-3-ketosteroid.
Tibolone is used to treat climacteric complaints and prevent osteoporosis. These beneficial effects are exerted via its 3alpha-and 3beta-hydroxymetabolites. Undesirable stimulation of the breast and endometrium is not apparent. Endometrial stimulation is prevented by the progestogenic activity of its Delta4-ene metabolite. The enzymes responsible for the formation of these active metabolites are unknown. Human aldo-keto reductase (AKR)1C isoforms have been shown to act as 3alpha/3beta-hydroxysteroid dehydrogenases (HSDs) on 5alpha-dihydrotestosterone (5alpha-DHT). We show that AKR1Cs also efficiently catalyze the reduction of the Delta(5(10))-3-ketosteroid tibolone to yield 3alpha- and 3beta-hydroxytibolone. Homogeneous recombinant AKR1C1, AKR1C3, and AKR1C4 gave similar catalytic profiles to those observed with 5alpha-DHT. AKR1C1 catalyzed exclusively the formation of 3beta-hydroxytibolone, AKR1C3 showed weak 3beta/3alpha-HSD activity, and AKR1C4 acted predominantly as a 3alpha-HSD. Whereas AKR1C2 acted as a 3alpha-HSD toward 5alpha-DHT, it functioned exclusively as a 3beta-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, because of potent inhibition of this activity by NADPH, AKR1Cs will probably act only as 3-ketosteroid reductases in vivo. Molecular docking simulations using crystal structures of AKR1C1 and AKR1C2 explained why AKR1C2 inverted its stereospecificity from a 3alpha-HSD with 5alpha-DHT to a 3beta-HSD with tibolone. The preference for AKR1C1 and AKR1C2 to form 3beta-hydroxytibolone, and the preference of the liver-specific AKR1C4 to form 3alpha-hydroxytibolone, may explain why 3beta-hydroxytibolone is the major metabolite in human target tissues and why 3alpha-hydroxytibolone is the major circulating metabolite.
Steckelbroeck
Stephan
S
Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6084, USA.
Jin
Yi
Y
Oyesanmi
Busola
B
Kloosterboer
Helenius J
HJ
Penning
Trevor M
TM
D015089:20-Hydroxysteroid Dehydrogenases / Q000378:metabolism*; D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D000449:Aldehyde Reductase; D000074408:Aldo-Keto Reductases; D020847:Estrogen Receptor Modulators / Q000494:pharmacology; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D007664:Ketosteroids / Q000493:pharmacokinetics; D007700:Kinetics; D009652:Norpregnenes / Q000493:pharmacokinetics*; D010024:Osteoporosis / Q000517:prevention & control; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity
D020847:Estrogen Receptor Modulators; D007527:Isoenzymes; D007664:Ketosteroids; D009652:Norpregnenes; D011994:Recombinant Proteins; D000429:Alcohol Oxidoreductases; D015089:20-Hydroxysteroid Dehydrogenases; C115140:3 alpha-beta, 20 beta-hydroxysteroid dehydrogenase; D000074408:Aldo-Keto Reductases; D000449:Aldehyde Reductase; C027385:tibolone