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In vitro effects of chlorpyrifos oxon on peroxisome proliferator-activated signaling in MCF-7 cells

Herriage, Stacey
The U.S. population is chronically exposed to a wide variety of environmental chemicals, including organophosphorus pesticides (OPs) such as chlorpyrifos. Chlorpyrifos oxon (CPO), the active metabolite of chlorpyrifos, inhibits acetylcholinesterase (AChE) to elicit toxicity, but is also a potent inhibitor of fatty acid amide hydrolase (FAAH). FAAH plays an important role in the degradation of fatty acid signaling lipids, and is the key enzyme in terminating endocannabinoid (eCB) signaling by hydrolyzing N-archidonylethanolamine (anandamide, AEA). AEA and other lipid intermediates, such as the eCB-like metabolites oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), are potent agonists at peroxisome proliferator-activated receptors (PPAR). PPARs regulate genes involved in numerous physiological processes associated with lipid metabolism and energy homeostasis. We hypothesized that inhibition of FAAH by CPO would disrupt degradation of intracellular AEA and OEA, leading to increased PPAR transcriptional activity, and altered expression of PPAR target genes. To evaluate the effects of CPO on PPAR-mediated gene expression, we exposed MCF-7 cells to a range of CPO concentrations in culture. In vitro inhibition assays were first conducted to evaluate concentration-dependent inhibition of FAAH and, for comparison, AChE. Cell cultures were then exposed to one of three selected CPO concentrations either alone, or along with AEA or OEA. Samples were collected at 8 hours, 1 day, and 3 days after dosing for gene expression analysis. Changes in expression of four selected target genes coupled to PPAR activation were measured using real-time quantitative polymerase chain reaction, and the comparative Ct method to quantify fold change in expression. Our results demonstrated inhibition of FAAH and AChE, as well as upregulation of all four target genes in a concentration and time-dependent manner. Inclusion of either AEA or OEA with CPO generally decreased the extent of gene upregulation noted with CPO alone. These are the first data to suggest that exposure to chlorpyrifos could influence lipid metabolism through PPAR activation, potentially contributing to metabolic disorders including obesity.