Superfund Research Program

Molecular Signals of Epigenetic Toxicity of Superfund Chemicals

Project Leader: Burra V. Madhukar
Grant Number: P42ES004911
Funding Period: 1995 - 2000

Project-Specific Links

Final Progress Reports

Year:   1999 

Project researchers have continued to investigate the biochemical and molecular mechanisms of action of polychlorinated biphenyls (PCBs) and other halogenated aromatic hydrocarbons in two cell culture models--namely, the WB rat liver and MCF-7 human breast cancer cells. The working hypothesis was that PCB congeners that do not bind with the Ah-receptor (dioxin receptor) exert epigenetic toxicity through modulation of cell signaling pathways that regulate cell growth and proliferation. This work clearly demonstrated that non-coplanar PCB congeners activated the extracellular signal-regulated kinases (ERKs), a family of the mitogen activated protein kinase (MAPK) superfamily of kinases. Limited structure-activity studies showed that ERKs activation by PCBs requires ortho- and para-chlorine substitution with 2,2'4,4'-tetrachlorobiphenyl as the most active congener. On the other hand, chlorine substitution, at meta and para positions, that increases their Ah-receptor affinity decreases their potency to activate ERKs.

Studies of ERK activation by PCBs have been extended using the MCF-7 human breast cancer cells. These cells are estrogen responsive, require estrogen stimulus for proliferation and thus represent the early stages of breast cancer cells in vivo whose growth is stimulated by endogenous estrogen. The estrogen requirement of MCF-7 cells may be circumvented by mitogenic growth factors such as EGF whose receptors activate the ERK pathway. In studies with ERK activation by PCBs in MCF-7 cells, the researchers observed that non-coplanar PCBs such as the 2,2',4,4'-TCB activate the ERK MAPKs, suggesting that they may influence growth of breast cancer cells independent of estrogen. In this respect, their actions are similar to those of mitogenic growth factors.

While growth factor induced MAP kinase activation is modulated through the tyrosine kinase activity of the GF receptors (that activate the downstream kinases MAPKK kinase and MAPK kinase (MEKs)), the mechanism of activation of MAP kinases by PCBs does not seem to involve activation of growth factor receptors because PCBs do not bind to the growth factor receptors. Project investigators observed that 1. PCB activation of ERKs was, in part, dependent on extracellular calcium, 2. activation was abrogated by PD-98059 a specific inhibitor of MAP kinase kinase the upstream regulator of ERKs, and genistein an inhibitor of tyrosine kinases but not by a PKC inhibitor bisindolylmalemide Thus, these results suggest that PCB activation of ERKs may involve more than one mechanism.