Progestins as inhibitors of the human 20-ketosteroid reductases, AKR1C1 and AKR1C3

The human aldo-keto reductases 1C1 and 1C3 (AKR1C1 and AKR1C3) are important 20-ketosteroid reductases in pre-receptor regulation of progesterone action. Both AKR1C1 and AKR1C3 convert progesterone to the less potent metabolite 20α-hydroxyprogesterone, although AKR1C1 has a higher catalytic efficiency than AKR1C3. Recently, we reported significant up-regulation of AKR1C1 and AKR1C3 in ovarian endometriosis, a complex estrogen-dependent disease. The typical characteristics of endometriosis are increased formation of estradiol, which stimulates proliferation of endometriotic tissue, and disturbed action of the protective progesterone. Although progestins have been used for treatment of endometriosis since the 1960s, their detailed mechanisms of action are still not completely understood. In the present study, we evaluated the potential inhibitory effects of progestins on the pre-receptor regulatory enzymes AKR1C1 and AKR1C3. We examined the following progestins as inhibitors of progesterone reduction catalyzed by recombinant AKR1C1 and AKR1C3: progesterone derivatives (dydrogesterone, its metabolite, 20α-hydroxydydrogesterone; and medroxyprogesterone acetate), 19-nortestosterone derivatives (desogestrel, norethinodrone and levonorgestrel), and the androgen danazol. Dydrogesterone, medroxyprogesterone acetate, 20α-hydroxydydrogesterone and norethinodrone inhibited AKR1C1 and AKR1C3 with K(i) values of 1.9 μM, 7.9 μM, 20.8 μM and 48.0 μM, and of 0.5 μM, 1.4 μM, 18.2 μM and 6.6 μM, respectively. Levonorgestrel and desogestrel preferentially inhibited AKR1C3 with K(i) values of 5.6μM and 39.1μM, respectively. Our data thus show that dydrogesterone, medroxyprogesterone acetate, 20α-hydroxydydrogesterone and norethinodrone inhibit AKR1C1 and AKR1C3 in vitro, although their physiological inhibitory effects still need to be evaluated further. Additionally, we investigated whether progestin dydrogesterone can be metabolized to its active 20α-hydroxymetabolite by AKR1C1 and AKR1C3. AKR1C1 converted dydrogesterone with a high catalytic efficiency while AKR1C3 was less active, which suggests that in vivo dydrogesterone is metabolized mainly by AKR1C1. Docking simulations of dydrogesterone into AKR1C1 and AKR1C3 also support these experimental data.