Long-chain fatty acids inhibit human members of the aldo-keto reductase 1C subfamily

Four human hydroxysteroid dehydrogenases in the aldoketo reductase (AKR) superfamily, AKR1C1-AKR1C4, are involved in the metabolism of steroids and other carbonyl compounds including drugs, and altered expression of AKRs (1C1, 1C2 and/or 1C3) is related to the pathogenesis of several extrahepatic cancers. Here, we report that unsaturated fatty acids (FAs) are potent competitive inhibitors of the AKR enzymes. The sensitivities to the FAs were different among the enzymes, especially between AKR1C1 and AKR1C2. The most potent inhibitors for AKR1C1, AKR1C2 and AKR1C4 were docosahexaenoic acid (K_i 0.77 μM), palmitoleic acid (K_i 0.41 μM) and linoleic acid (K_i 0.33 μM), respectively. AKR1C3 was the most sensitive to FA inhibition, showing low K_i values (0.23-0.29 μM) for oleic, linoleic, eicosapentaenoic and docosahexaenoic acids. Linoleic and oleic acids also inhibited AKR1C3-mediated metabolism of 9,10-phenanthrenequinone in colon DLD1 cells. Molecular docking and site-directed mutagenesis studies suggested upon FA binding to AKR1C1 and AKR1C3: (i) the carboxyl group of the FA binds to the oxyanion-binding site in the active site; (ii) the difference in FA sensitivity between AKR1C1 and AKR1C2 is due to their residue difference at position 54; (iii) Ser118, Phe306 and Phe311 of AKR1C3 are important for determining the inhibitory potency of FAs.