Augmentation of 3‐methylcholanthrene‐induced bioactivation in the human hepatoma cell line HepG2 by the calcium channel blocker nicardipine

(Cancer Sci 2010; 101: 652–657) The abilities of the dihydropyridine calcium channel blocker nicardipine (Nic) to induce cytochrome P450 1 family enzymes (CYP1s) and to enhance the 3‐methylcholanthrene (MC)‐mediated induction of CYP1s and formation of MC‐DNA adduct were examined in the human hepatoma cell line HepG2. The results from real time RT‐PCR analysis demonstrated that Nic could induce CYP1 mRNAs and enhance the MC‐mediated induction of the CYP1 mRNAs. The luciferase‐reporter gene assay using the HepG2‐A10 cell line, which has been previously established for the screening of aryl hydrocarbon receptor (AhR) activators, also indicated the augmentation of MC‐mediated activation of AhR (induction of luciferase) by Nic, although Nic showed limited capacity for the activation of AhR. Furthermore, the results from the Western blot analysis of CYP1s, the enzyme activity assay, and the assay for MC‐DNA adduct formation indicated that Nic could enhance the MC‐mediated induction of CYP1s, especially CYP1A1. Furthermore, the intracellular accumulation level of [³H]MC after treatment of HepG2 cells with [³H]MC significantly increased in the presence of Nic. The present findings demonstrate that Nic can enhance the MC‐mediated induction of CYP1s, especially CYP1A1, and the formation of MC‐DNA adduct in HepG2 cells. Furthermore, the augmentation of the MC‐mediated bioactivation by Nic is demonstrated to occur mainly through an increase in intracellular accumulation of MC.