Modulation of cytochrome P4501A1 activity by ascorbigen in murine hepatoma cells

Modulation of cytochrome P4501A1 (CYP1A1) activity is a mechanism whereby indoles present in cruciferous vegetables could affect the metabolism of xenobiotics. Ascorbigen (ASG) is the predominant indole formed during the degradation of glucobrassicin, although the mechanism by which ASG modulates CYP1A1 activity is not known. The major focus of this study was to examine the mechanism of CYP induction by ASG using a murine hepatoma-derived cell line (Hepa 1c1c7). ASG was shown to induce the activity of 7-ethoxyresorufin O-deethylase, a marker for CYP1A1, in a concentration-responsive manner with a maximum induction at 700 microM. Maximum ASG induction after 24-hr treatment was 7% of maximal CYP1Al activity induced by the well-known potent CYP1A1 inducer, indolo[3,2-b]carbazole (ICZ) (1 microM), and the EC50 values differed by 2-fold. The CYP1A1 activity increased continuously up to 72 hr, where ASG showed an induction efficiency in the same range as for the positive control (1 microM ICZ) after 24 hr, whereas the CYP1A1 protein level, measured by Western blot analysis, was maximally induced after 24 hr. ASG significantly inhibited CYP1A1 activity in whole cells at concentrations above 1 microM. ASG increased the chloramphenicol acetyl transferase (CAT) activity via a CAT reporter construct containing a dioxin-responsive element in Hepa 1c1c7 cells, indicating involvement of the aryl hydrocarbon receptor. ASG was shown to be transformed into ICZ, or a compound with the same chromatographic mobility as ICZ, in the medium. Taken together, the results indicate that ASG inhibits CYP1A1 activity at low concentrations, but induces the same activity at higher concentrations.