Dose-Response Assessment of Naphthalene-Induced Genotoxicity and Glutathione Detoxication in Human TK6 Lymphoblasts

The dose-response relationship for the induction of micronuclei (MN) and the impact of glutathione (GSH) detoxication on naphthalene-induced cytotoxicity and genotoxicity were investigated in human TK6 cells. TK6 cells were exposed to 10 concentrations ranging from 0.0625 to 30 mu M naphthalene in the presence of beta-naphthoflavone- and phenobarbital (beta NP/PB)-induced rat liver S9 with a nicotinamide adenine dinucleotide phosphate-generating system. Three approaches were used to identify a no-observed-effect level (NOEL) for naphthalene-induced genotoxicity: (1) laboratory criteria of >= twofold increase over the concurrent solvent controls (NOEL = 10 mu M), (2) ANOVA with Bonferroni correction (NOEL = 2.5 mu M), and (3) the benchmark dose approach (BMCL10 = 3.35 mu M). The NOEL and point of departure micronucleus frequency for naphthalene-induced MN are between the tested naphthalene concentrations of 2.5-10.0 mu M in this experimental system. Supplementation of the exposure system with physiological relevant concentrations of 5mM GSH eliminated naphthalene-induced cytotoxicity and genotoxicity; no increased cytotoxicity or genotoxicity was observed at concentrations of up to 500 mu M naphthalene in the presence of GSH compared with 2.5-10.0 mu M in the absence of GSH. Naphthalene bioactivation by beta NP/PB-induced rat liver S9 exhibits a nonlinear dose-response for the induction of MN in TK6 cells with a NOEL of 2.5-10 mu M that in the presence of GSH is shifted upward greater than 50- to 200-fold. These data demonstrate a nonlinear dose-response for naphthalene-induced genotoxicity that is eliminated by GSH, and both observations should be considered when assessing human risk from naphthalene exposures.