Superfund Research Program

Membrane efflux pumps and hormonal activity of organochlorine contaminants in New York Harbor sediments

Project Leader: Avrom Caplan
Grant Number: P42ES007384
Funding Period: 1995 - 2006

Project-Specific Links

Final Progress Reports

Year:   2005  1999 

In the past year Dr. Caplan’s work focused on the role played by polybrominated diphenyl ethers in activation of genes involved in the xenobiotic stress response.  Initial studies focused on the commercial mixture called DE-71, which has four major components: BDE-47 (24-38%), BDE-99 (50-60%), BDE-153 and BDE-154. The tetra (BDE47) and penta (BDE99) represent the major components accounting for approximately 90% of the commercial mixture. Previous studies have demonstrated that DE-71 induces CYP1A1 in liver, presumably via the aryl hydrocarbon receptor.

 

Dr. Caplan sought to establish whether or not components of the DE-71 mixture could signal via the xenobiotic sensing nuclear receptors PXR and CAR. Accordingly, his team assayed for expression of PXR and CAR responsive genes in cultured human colon and liver cells exposed to different doses of DE-71. Our results with the colon carcinoma cell line (see Figure 1) show that DE-71 induces CYP2B6, CYP3A4 and MDR1, which are genes that are induced by PXR and CAR. In particular, CYP2B6 is induced by CAR, while CYP3A4 and MDR1 are more associated with PXR function, although there is considerable crossover between these receptors.  In all cases, we see significant increases in gene expression at levels of DE-71 that approximate to the low µM range of the commercial mixture.

 

The results shown in Figure1 were similar those obtained using the liver carcinoma cell line HepG2, although the levels of induction of CYP3A4 and MDR1 were considerably lower (not shown). However, these results contrast with our finding that the PXR nuclear receptor is approximately 500-fold more abundant in HepG2 cells compared with LS174T cells. Dr. Caplan’s team therefore attempted to determine whether PXR overexpression would stimulate CYP3A4 and MDR1 expression in LS174T and HepG2 cells. They transfected the cDNA for PXR into these cells, although they observed no change in CYP3A4 or MDR1 expression in the presence of DE-71.  These combined data indicate that some other factor besides the PXR and CAR receptors is limiting in HepG2 and LS174T cells.  Dr.Caplan speculates these factors to include co-activators known to interact with PXR and CAR, and current studies are aimed toward identifying these factors.