| Aryl hydrocarbon receptor (AhR) ligands and heavy metals are environmental co-contaminants and their molecular interaction may disrupt the coordinated regulation of AhR-dependent phase I and II drug metabolizing enzymes. As such, the effect of the heavy metal, As(III), on the AhR-regulated genes cytochrome P450 1A1 (CYP1A1), CYP1A2, CYP1B1, NAD(P)H: quinone oxidoreductase (NQO1), and glutathione S-transferase a1 subunit (GSTA1) was investigated. In HepG2 cells, As(III) significantly inhibited CYP1A1 at the mRNA, protein, and catalytic activity levels in a concentration- and time-dependent manner. As(III) significantly increased heme oxygenase-1 (HO-1), which coincided with a decrease in the CYP1A1 catalytic activity levels. The use of an HO-1 inhibitor, tin mesoporphyrin, a heme precursor, hemin, or transfecting the HepG2 cells with siRNA against HO-1 partially reversed the As(III)-mediated inhibition of the TCDD-mediated induction of CYP1A1 catalytic activity. Similarly, in rat primary hepatocytes, As(III) decreased CYP1A1, CYP1A2, CYP3A23, and CYP3A2 expression, and inhibiting As(III)-mediated induction of HO-1 partially restored the enzymatic activity of these P450s. In C57Bl/6 mice, As(III) modulated the constitutive and TCDD-induced AhR-regulated genes in a time-, tissue-, and AhR-regulated enzyme-specific manner. Additionally, As(III) increased the serum hemoglobin (Hb) levels in animals treated for 24h, and upon treatment of mouse isolated hepatocytes with Hb alone, there was an increase in the nuclear accumulation of AhR and AhR-dependent luciferase activity. Furthermore, Hb potentiated the TCDD-induced AhR-dependent luciferase activity, implicating Hb as an in vivo-specific modulator. Investigating the possible role of As(III) metabolites as an alternative modulator to CYP1A1, we found that methylated pentavalent arsenic metabolites are bifunctional inducers as they increase CYP1A1 through activating the AhR/XRE signaling pathway and they increase NQO1 through activating the Nrf2/ARE signaling pathway in addition to the AhR/XRE pathway. Future studies are required to determine the exact role of AhR-and Nrf-2-regulated genes in the initiation and progression of malignancies. |
