Selective Inhibition of Human AKR1B10 by n-Humulone, Adhumulone and Cohumulone Isolated from Humulus lupulus Extract

Abstract

Hop-derived compounds have been subjected to numerous biomedical studies investigating their impact on a wide range of pathologies. Isomerised bitter acids (isoadhumulone, isocohumulone and isohumulone) from hops, used in the brewing process of beer, are known to inhibit members of the aldo-keto-reductase superfamily. Aldo-keto-reductase 1B10 (AKR1B10) is upregulated in various types of cancer and has been reported to promote carcinogenesis. Inhibition of AKR1B10 appears to be an attractive means to specifically treat RAS-dependent malignancies. However, the closely related reductases AKR1A1 and AKR1B1, which fulfil important roles in the detoxification of endogenous and xenobiotic carbonyl compounds oftentimes crossreact with inhibitors designed to target AKR1B10. Accordingly, there is an ongoing search for selective AKR1B10 inhibitors that do not interact with endogeneous AKR1A1 and AKR1B1-driven detoxification systems. In this study, unisomerised α-acids (adhumulone, cohumulone and n-humulone) were separated and tested for their inhibitory potential on AKR1A1, AKR1B1 and AKR1B10. Also AKR1B10-mediated farnesal reduction was effectively inhibited by α-acid congeners with K_i-values ranging from 16.79 ± 1.33 µM (adhumulone) to 3.94 ± 0.33 µM (n-humulone). Overall, α-acids showed a strong inhibition with selectivity (115⁻137 fold) for AKR1B10. The results presented herein characterise hop-derived α-acids as a promising basis for the development of novel and selective AKR1B10-inhibitors.

Keywords: aldo-keto reductases; alpha-acids; cancer; farnesal reduction; hops; humulone; humulus lupulus; selective inhibition; tight-binding inhibition.

Figure 1

Structures of α-acids (n-humulone = compound 3, cohumulone = compound 2 and adhumulone = compound 1) and iso-α-acids (isohumulone, isocohumulone, and isoadhumulone) after thermal isomerisation through wort boiling.

Figure 2

Chromatogram of α-acid separation. Compound 1 = adhumulone; compound 2 = cohumulone; compound 3 = n-humulone. See text for further details.

Figure 3

IC₅₀-values of the isolated hop compounds for AKR1B10-catalysed farnesal reduction as a function of substrate concentration [compound 1 (adhumulone) (circles), compound 2 (cohumulone) (squares) and compound 3 (n-humulone) (triangles)].

Figure 4

Determination of inhibition mode. To determine the mode of inhibition the normalized velocity is plotted as a function of inhibitor concentration as exemplified in the upper diagram (farnesal = 1.25 µM). The data were fitted to v_i/v₀ = 1/(1 + ([I]/IC₅₀)) and lines are drawn from [I] = 0 to the intersection of v_i/v₀ = ½ and so on. The intersection of the dotted lines with the x-axis define the constant K as indicated [34,35]. This was done for all used farnesal and inhibitor concentrations and K was plotted as a function of substrate concentration (lower diagram). This secondary plot shows an increase of K with substrate concentration, which is characteristic for a competitive inhibitor (adhumulone). In the case of a non-competitive inhibitor K would be independent of substrate concentration (humulone and cohumulone).