Grantee Research Project Results
Mechanisms of Polycyclic Aromatic Hydrocarbon-Induced ImmunosuppressionEPA Grant Number: U915003
Title: Mechanisms of Polycyclic Aromatic Hydrocarbon-Induced Immunosuppression
Investigators: Mann, Koren K.
Institution: Boston University
EPA Project Officer: Packard, Benjamin H
Project Period: January 1, 1996 through February 3, 1999
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1996) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Fellowship - Toxicology , Human Health
The objective of this research project is to dissect the pathways involved in the immunotoxic effects of polycyclic aromatic hydrocarbons (PAHs), focusing on the developing B lymphocyte compartment, both in vivo and in vitro.
Initially, in vivo (inside the living organism) experiments were conducted to determine the effects of prototypic PAH—7,12-dimethylbenzanthracene (DMBA)—on the bone marrow in mice. At 18 hours after injection, DMBA caused an increase in cells undergoing programmed cell death (apoptosis) when compared to the control. A distinct lack of marrow cells was evident at 48 hours after treatment with DMBA, leading to the conclusion that DMBA affects the site of B cell development by depleting the bone marrow via the induction of apoptosis.
To mimic B cell development in vitro (outside the living organism), a culture system was developed by coculturing a cloned preB cell line and a cloned stromal cell line. When treated with DMBA, the preB cell lines underwent apoptosis in a dose- and time-dependent manner that was dependent on contact with the stromal cell layer, as both preB cells grown in suspension and preB cells separated from stromal cells by transwell membranes are resistant to DMBA-induced apoptosis. Thus, not only are the stromal cells the mediators of this DMBA-induced death signal, but contact or close proximity to the stroma is required.
Once PAHs bind to the aromatic hydrocarbon receptor (AhR), it acts as a transcription factor to induce transcription of genes, including oncogenes (cancer-inducing genes) such as c-myc and ras as well as P4501A enzymes. -Napthoflavone, an antagonist of the AhR and P4501A enzymes, blocks DMBA-induced apoptosis, suggesting a role for the AhR or P4501A enzymes.
Cytochrome P450 enzymes are phase I enzymes involved in metabolizing PAHs, converting lipophilic (fat-soluble) compounds to hydrophilic (water-soluble) compounds, which can be excreted from the body. These P450 enzymes are upregulated by the very compounds they metabolize, and create reactive intermediate compounds that are genotoxic and may cause oxidative stress within the cell. A general P450 inhibitor, 1-aminobenzotriazole, does not block DMBA-induced apoptosis, suggesting no role for these enzymes in PAH-induced apoptosis, but rather, the AhR as a mediator of the death signal.