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Toxicological Sciences 66, 320-326 (2002)
Copyright © 2002 by the Society of Toxicology

RISK ASSESSMENT

Vinyl Acetate-Induced Intracellular Acidification: Implications for Risk Assessment

Matthew S. Bogdanffy,1

DuPont Haskell Laboratory, E. I. du Pont de Nemours and Co., Inc., P.O. Box 50, Newark, Delaware 19714

ABSTRACT

Cancerbioassays have demonstrated the carcinogenic activity of vinyl acetate in rodents. Tumors appear only at the site of contact and mechanistic data suggest that the carcinogenic mechanism involves carboxylesterase-mediated metabolism of vinyl acetate to acetic acid. It has been hypothesized that intracellular formation of acetate causes a reduction of intracellular pH (pHi) at noncytotoxic levels, but that prolonged exposure to reduced pHi is cytotoxic and/or mitogenic and drives proliferative responses. Coupled with exposure to metabolically formed acetaldehyde at high administered concentrations, nonlinear dose-response curves for epithelial tumors are produced. Freshly isolated rat hepatocytes were used as a model system to test the concept that exposure of cells to vinyl acetate causes a reduction in pHi. Quantitative fluorescence imaging ratio microscopy showed that exposure of hepatocytes to vinyl acetate concentrations ranging from 10 to 1000 µM caused rapid and sustained reductions of approximately 0.03 to 0.65 pH units. Cellular acidification was rapidly reversed to control pHi upon removal of vinyl acetate. There was minimal accumulation of protons during the exposure period, as suggested by minor differences in pHi of cells with or without prior exposure to vinyl acetate. The effect of vinyl acetate on pHi was attenuated by prior exposure to the carboxylesterase inhibitor bis(p-nitrophenyl)phosphate. These results support the concept that intracellular acidification is a sentinel pharmacodynamic response of cells to vinyl acetate exposure and that pHi is an appropriate metric dose for use in quantitative risk assessments of cancer and noncancer human health risk assessment.

Key Words: vinyl acetate; intracellular acidification; mode of action; cytotoxicity; mitogenicity.


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