TCDD receptor ligands present in extracts of urban air particulate matter induce aryl hydrocarbon hydroxylase activity and cytochrome P-450c gene expression in rat hepatoma cells.

In previous studies we have shown that substances associated with particulates collected from urban air and automobile exhaust bind with high affinity to the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) receptor present in rat liver cytosol. In this study we used a rat hepatoma cell line, H4IIE, to investigate the effect of such substances on an enzyme system, aryl hydrocarbon hydroxylase (AHH), which is regulated via the TCDD receptor. The results demonstrate that AHH activity in the H4IIE cell line can be induced by extracts of particulates collected from urban air and automobile exhausts in a dose-dependent manner, and that the AHH activity is inducible by five polycyclic aromatic hydrocarbons (PAHs) including 1-/3-nitrobenzo[a]pyrene and 6-chlorochrysene, all present in extracts of particulates from urban air and automobile exhausts. The induction of AHH activity is correlated to apparent TCDD receptor affinity for investigated PAHs (r = 0.85) and particulate extracts. Biochemically, treatment of the cells with 5,6-benzoflavone significantly increased the level of cytochrome P-450c but not P-450d as shown by immunoblotting and analysis of mRNA levels. The data indicate that substances present in extracts of urban air particulates can interact with the TCDD receptor in intact cells and cause an accumulation of cytochrome P-450c mRNA leading to an increased synthesis of the gene product and thus an increase in enzyme activity.