Superfund Research Program
TCDD Impedes the Minimal Activation Threshold Required for Initiation of B Cell Differentiation: An Integrated Experimental and Computational Modeling Approach
Project Leader: Norbert E. Kaminski
Grant Number: P42ES004911
Funding Period: 2000-2021
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There is a growing body of evidence suggesting that halogenated aromatic hydrocarbons can modify biological responses through aryl hydrocarbon (AhR)-dependent as well as AhR–independent mechanisms. The objective of this project has been to further characterize the AhR-dependent and independent mechanisms of immune modulation by employing both AhR agonist including 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3,3’,4,4’-tetrachlorobiphenyl as well as weak agonists for the AhR such as the diortho-substituted noncoplanar PCBs 2,2’,4,4’-tetrachlorobiphenyl.
The researchers’ studies demonstrated that a diortho-substituted PCB increased expression of cyclooxygenase-2 (COX-2) in neutrophils. Efforts were continued to address regulation of critical genes during exposure of neutrophils to PCBs. In HL-60 cells 2,2’,4,4’-tetrachlorobiphenyl, but not 3,3’,4,4’-tetrachlorobiphenyl, increased the expression of the immediate-early gene, COX-2, in a time- and concentration-dependent manner. Exposure of HL-60 cells to 2,2’,4,4’-tetrachlorobiphenyl was also associated with a significant increase in release of arachidonic acid and production of reactive oxygen species. The possibility that release of arachidonic acid and/or production of reactive oxygen species contribute to upregulation of COX-2 was evaluated. Neither of these events was required for the PCB-induced increase in COX-2 mRNA. On the other hand, inhibition of the MAP kinase, p38, significantly reduced COX-2 expression, suggesting a role for this pathway in regulation of this gene during exposure to PCBs.
In addition, the team has demonstrated that 2,2’,4,4’-tetrachlorobiphenyl activated ERK sub-family of MAPKs in rat liver epithelial cells. Studies with pharmacological inhibitors of upstream regulatory pathways of MAPK signaling have identified that 2,2’,4,4’-tetrachlorobiphenyl activation of ERKs was mediated through EGF receptor and c-src tyrosine kinases. The researchers also observed that non-coplanar PCBs induce the activation of the transcription factor AP-1 that regulates the expression of genes involved in cell proliferation as well as apoptosis. Furthermore, they established that ERK-MAPK was directly involved in the activation of AP-1. These studies have identified biological effects induced by non-coplanar PCBs, which are induced independently of AhR activation.
In B lymphocytes, microarray analysis was performed on the murine B cell line CH12.LX. treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The study revealed the induction of several genes, including the suppressor of cytokine signaling-2 (SOCS-2), which the research team believes may play a crucial role in disrupting B cell differentiation into plasma cells. Specifically. SOCS-2 blocks signaling that is induced by interferon gamma binding to the interferon gamma receptors. SOCS-2 mRNA levels were elevated in B cells by TCDD or coplanar PCB126 treatment with similar kinetics as CYP1A1, a hallmark of Ah receptor activation. TCDD-mediated SOCS-2 induction was cell type specific as it was only induced in B cells (primary and CH12.LX) but not in hepa1c1c7 or MCF-7 cells. Promoter analysis revealed the presence of four dioxin responsive elements within 1 kb of the transcriptional start site. A reporter gene regulated by the SOCS-2 promoter was inducible by TCDD and was dependent on a functional Ah receptor signaling pathway. These results suggest that disruption of the B cell differentiation program by Ah receptor ligands may be mediated in part through the induction of SOCS-2. Collectively, the aforementioned findings suggest that halogenated aromatic hydrocarbons influence leukocyte signaling through AhR-dependent and independent mechanisms.