Canine aldo-keto reductase 1C3 (AKR1C3/PGFS) exhibits 17β/20α-hydroxysteroid dehydrogenase activity and is inhibited...

Most members of the aldo-keto reductase (AKR) 1 C subfamily are hydroxysteroid dehydrogenases (HSDs), and their numbers are more than four in many individual mammals. In contrast, there is only one gene for the AKR1C protein (AKR1C3) in dogs, which have been used as a preclinical model for human biomedical research. Here, we report that dog AKR1C3 (known as prostaglandin-F synthase) catalyzes the conversion of the 17-keto group of estrone, 4- and 5-androstenes, and 5α-androstanes to their 17β-hydroxy-metabolites with NADPH as a coenzyme. Dog AKR1C3 also exhibited 20α-HSD activity toward 20-keto-C₂₁-steroids (deoxycorticosterone, its 5α-dihydro- and 5α-tetrahydro-derivatives, and progesterone), but, notably, did not display 3-ketosteroid reductase activity. Additionally, dog AKR1C3 reduced various nonsteroidal carbonyl compounds including endogenous 4-oxo-2-nonenal, all-trans-retinal, and isatin, of which isatin was the most excellent substrate. In the reverse reaction, the enzyme weakly oxidized 17β- and 20α-hydroxysteroids and some alicyclic alcohols. Further site-directed mutagenesis study revealed that residue M55 is responsible for the lack of 3-ketosteroid reductase activity. The enzyme was inhibited by flavonoids, nonsteroidal anti-inflammatory drugs, bile acids, benzbromarone, abiraterone, and trilostane, of which trilostane inhibited most potently (IC₅₀ 0.30 µM), and its inhibition was uncompetitive and competitive with respect to the substrates in the forward and reverse reactions, respectively. Thus, dog AKR1C3 may play a role in the metabolism of steroid hormones (as a reductive 17β/20α-HSD) and nonsteroidal carbonyl compounds, and is a novel target of trilostane that is used to treat Cushing's disease in dogs.