| Polycyclic Aromatic Hydrocarbons (PAHs) and heavy metals are frequent environmental co-contaminants. PAHs are carcinogens; a consequence of their bioactivation to reactive intermediates by cytochrome P4501A (CYP1A) enzymes, which are also induced by PAHs. Heavy metals may affect CYP1A1 enzyme induction, thereby altering the potential carcinogenicity of PAHs. Studies on the effects of the four most hazardous heavy metals, arsenic, cadmium, lead or mercury, on the PAH-mediated induction of CYP1A1 mRNA expression and protein activity in human HepG2 cell lines revealed the greatest effect with arsenic. Co-exposure of arsenic and the PAH, benzo[k]fluoranthene (BKF), resulted in the most signifcant decreases in CYP1A1 mRNA, protein and activity compared to exposure with cadmium, lead or mercury and BKF vs. exposure to BKF alone. Therefore, arsenic was the heavy metal used in the mechanistic studies.; Co-treatment of HepG2 cells with BKF and arsenic markedly decreased BKF-mediated induction of CYP1A1 mRNA, but to a lesser extent than the corresponding decrease in enzyme activity. To identify transcriptional effects, plasmids containing 1450 bp of the 5' upstream promoter region of the human CYP1A1 gene, truncated versions thereof or plasmids containing the xenobiotic response element (XRE) consensus sequence were constructed. It was concluded that transcriptional effects of arsenic on CYP1A1 expression are mediated via factors acting at a characterized oxidative response region upstream of the promoter, independently of the Ah receptor. Arsenic did not enhance mRNA degradation, eliminating mRNA destabilization as a factor in its down-regulation of CYP1A1 induction by BKF; Heme Oxygenase-1 (HO-1), which catalyzes heme degradation is induced by arsenic, providing a mechanism for post-translational CYP regulation. HepG2 cells were stably transfected with siRNA targeted to knock down HO-1 expression. In cells treated with BKF and increasing amounts of arsenite, decreases in CYP1A1 expression occurred in control and to a significantly lesser extent in siRNA transfected cells. The failure of siRNA to completely reverse the down regulation by arsenite is probably a consequence of the contribution of transcriptional regulation or residual HO-1 activity.; In summary, arsenite diminishes levels of CYP1A1 and therefore will probably decrease the bioactivation of the PAH and its subsequent carcinogenicity. |
