Superfund Research Program

Activation of PCBs to Genotoxins in vivo

Project Leader: Larry W. Robertson (University of Iowa)
Grant Number: P42ES007380
Funding Period: 1997 - 2005

Project-Specific Links

Final Progress Reports

Year:   2004  1999 

Researchers in this project are investigating the concept that the lower halogenated biphenyls (especially mono- and di-chlorobiphenyls) may be activated by hepatic enzymes, metabolized to oxygenated species that are electrophilic and bind to proteins--including nuclear proteins and DNA. There are two electrophiles of particular interest: one is an arene oxide and the other is a quinone, which is an oxidation product of dihydroxylated biphenyls. This project therefore addresses the fundamental question of the mechanisms of toxicity--specifically, metabolic activation, protein and DNA-binding and ultimately the genotoxicity of individual PCBs.

In an effort to learn more about the mechanisms of PCB carcinogenesis, the following insights have been attained: 1) a PCB, namely the 4-chlorobiphenyl, can act as an initiator in the Solt-Farber rat liver carcinogenesis model. 2) tested PCBs that are fairly inactive themselves, bind covalently to nuclear protein and DNA in the livers of mice. This means that they were metabolically activated to electrophiles. The strong binding to protein suggests that quinones may have been formed, since these metabolites have a high reactivity with cellular sulfhydryl groups. Preliminary data also shows that PCBs are metabolized to compounds that bind to sulfydryl groups in hemoglobin, further strengthening the hypothesis that PCB-quinones play an important role in vivo. 3) the first possible mechanism of cancer initiation was DNA adduct formation by PCB metabolites. Researchers showed that PCB-quinones reacted in vitro with DNA to form adducts. The migration pattern of these adducts of synthesized quinones with DNA can now be used as standards. However, 4-chlorophenyl-hydroquinone was about 4-8 times more active in DNA adduct formation then the 4-chlorophenyl-quinone. This could mean that a semiquinone or other hydroquinone derivative is the major adduct forming species. 4) ROS formation may be a result of redox-reactions by dihydroxy-PCBs/quinones. It has been shown that dihydroxy-PCBs can be oxidized in vitro by peroxidases, including lactoperoxidase, to quinones, which redox-cycle and bind to GSH, all resulting in the production of ROS. Project investigators also found that in the presence of myeloperoxidase containing HL-60 cells, all PCB hydroquinone and quinone metabolites tested resulted in a significant increase in fluorescence above buffer-controls, indicating intracellular peroxide formation. 5) Oxidative stress, however, could also result from changes in pro-oxidant/anti-oxidant enzymes and cofactors. PCB quinones, and to a lesser degree PCB hydroquinones, strongly react with intracellular sulfhydryl containing proteins like Topo II and peptides like GSH. GSH depletion would make the cells more vulnerable to damage by ROS. Treatment with PCB 77 and PCB 158 increased cytochromes P-450 (pro-oxidant), glutathione transferase (GST) and GSSG reductase activity and decreased catalase activity in livers of female rats.

These results confirm the hypothesis that PCBs can be metabolized to compounds that directly (DNA adduction) or indirectly (ROS production and/or changing of the anti-oxidant status of the cell/tissue) contribute to carcinogenic action. Experiments are under way to further elucidate the mechanisms of PCB carcinogenesis. The findings of ROS generation have led us to broaden research efforts to include other indirect actions of PCBs including the formation of the polar endogenous adducts in our future investigations.