Determination of the oral administration dose in CDDO-Me treatment
Cytoplasmic Nrf2 binds to a Keap1 homodimer that interacts with Cullin 3, constituting a ubiquitin E3 ligase complex, the formation of which results in Nrf2 degradation through the ubiquitin-proteasome system. CDDO-Me, an activator of Nrf2, covalently binds to Cys-151 within the Keap1 sequence, induces Nrf2 translocation to the nucleus, and increases its transcriptional activity (Figure 1a) (Cleasby et al., 2014; Suzuki & Yamamoto, 2017). To determine the optimal concentration of CDDO-Me required to activate Nrf2, the Nqo1 mRNA expression level was examined in Hepa1c1c7 cells in the presence of CDDO-Me. CDDO-Me upregulated Nqo1 gene expression in a dose-dependent manner to the extent of 0.1–3 nM, and the increased expression in 3 nM was maintained until 10 nM (Figure 1b). Consistent with mRNA expression, NQO1 enzyme activity in Hepa1c1c7 cells was increased by CDDO-Me, with an EC150 value of 0.62 nM (Figure 1c). Single oral administration of 3 or 10 mg・kg-1 of CDDO-Me to mice increased Nqo1 expression in the liver to approximately 3- or 6-fold higher than that in the control mice at 24 h, respectively (Figure 1d). The maximum plasma concentrations of CDDO-Me attained in mice after administering 3 and 10 mg・kg-1 were 10.2 ± 1.6 and 32.9 ± 0.6 nM, respectively (Figure 1e), and CDDO-Me disappeared from the plasma after 24 h of administration (Figure 1e). Based on these observations, doses of 3 and 10 mg・kg-1 were selected to investigate the hepatoprotective effects of CDDO-Me.