Tibolone Is Metabolized by the 3α/3β-Hydroxysteroid Dehydrogenase Activities of the Four Human Isozymes of the Aldo-Keto Reductase 1C Subfamily: Inversion of Stereospecificity with a Δ5(10)-3-Ketosteroid

Abstract

Tibolone is used to treat climacteric complaints and prevent osteoporosis. These beneficial effects are exerted via its 3␣-and 3␤-hydroxymetabolites. Undesirable stimulation of the breast and endometrium is not apparent. Endometrial stimulation is prevented by the progestogenic activity of its ⌬ 4 -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 3␣/3␤-hydroxysteroid dehydrogenases (HSDs) on 5␣-dihydrotestosterone (5␣-DHT). We show that AKR1Cs 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 toward 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 ϩ , 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 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.

Effects of Tibolone Metabolites on Human Endometrial Cell Lines in Co-culture

“…Recently it has been shown that the ketosteroid dehydrogenase enzymes of the AKR1C family are capable of metabolizing the 3hydroxy-tibolone metabolites 19 . Although AKR1C1, AKR1C3, and AKR1C4 act as reductases, the AKR1C2 enzyme is capable of oxidizing the 3β-OH-tibolone metabolite back to tibolone.…”

“…We show that the stromal and epithelial cells express the ketosteroid dehydrogenase AKR1C2, which may allow for the conversion of 3βOH-tibolone back to tibolone 19 . From our observations, using the co-culture system, we conclude that tibolone counteracts E 2 -driven proliferation in the uterus in a manner similar to progestins, and that the balance of metabolism of tibolone in the endometrium favors the production of the progestagenic metabolites.…”

The hormone replacement therapy drug tibolone acts very similar to medroxyprogesterone acetate in an estrogen-and progesterone-responsive endometrial cancer cell line

“…It is possible that over time 3b-hydroxy tibolone is converted back into D 4 -tibolone (Schatz et al 2005). However, our array data show that, like MPA, they are tibolone's own progestagenic properties which are responsible for an increased expression of the aldo-keto reductase family 1 member C1 (AKR1C1, an enzyme that catalyzes the formation of 3b-hydroxy tibolone exclusively) (Steckelbroeck et al 2004). …”

The other side of progestins: effects in the brain

“…Conversion of TIB into 3α-or 3β-hydroxytibolone depends on the tissue where its metabolism occurs. In peripheral tissues, AKR1C1 and AKR1C2 form 3β-hydroxytibolone which exhibits estrogenic activity, whereas in liver, TIB is reduced to 3α-hydroxytibolone by AKR1C4 which functions predominantly as 3α-hydroxysteroid dehydrogenase (Steckelbroeck et al 2004). This explains why 3α-hydroxytibolone is the major circulating metabolite, whereas 3β-hydroxytibolone is the major metabolite in target tissues (Penning et al 2014).…”

“…Interestingly, AKR1C2 exhibits different stereochemical preference depending on the ketosteroid substrate. Indeed, it produces 3α-products with the potent androgen 5α-dihydrotestosterone (5α-DHT) behaving as an efficient 3α-hydroxysteroid dehydrogenase, whereas it inverts its stereospecificity with TIB acting as a 3β-hydroxysteroid dehydrogenase (Steckelbroeck et al 2004, Rižner & Penning 2014. Finally, TIB can also be metabolized, in less amount, to the potent estrogen 7α-methylethinylestradiol (Wiegratz et al 2002).…”