Induction of glutathione S-transferase activity and protein expression in brown bullhead (Ameiurus nebulosus) liver by ethoxyquin

The inducibility of hepatic cytosolic glutathione S-transferases (GSTs) was examined in brown bullheads, a freshwater fish that is highly susceptible to hepatic neoplasia following exposure to carcinogen-contaminated sediment s. Juvenile bullheads were fed a semi-purified antioxidant-free diet supple mented with ethoxyquin (0.5% w/w dissolved in 3% corn oil), a prototypical rodent GST-inducing agent, twice daily for 14 days. Control bullheads recei ved the antioxidant-free diet supplemented with corn oil (3% w/w). A signif icant increase (1.6-fold, p less than or equal to 0.01) in hepatic cytosoli c GST activity toward 1-chloro-2,4-dinitrobenzene (CDNB) was observed in th e ethoxyquin-treated bullheads relative to control fish. A trend toward inc reased GST-NBC activity was observed in the ethoxyquin-treated fish (1.2-fo ld, p = 0.06), whereas no treatment-related effects were observed on GST ac tivities toward ethacrynic acid (ECA). In contrast, GST activity toward (+/ -)-anti-benzo[a]pyrene-Crans-7,8-dihydrodiol-9,10-epoxide (BPDE) was repres sed in affinity-purified cytosolic fractions prepared from ethoxyquin-treat ed bullheads relative to control bullheads. Silver staining and densitometr ic analysis of isoelectric-focused, affinity-purified GST proteins revealed increased expression of two basic GST-like isoforms in ethoxyquin-treated fish. In summary, exposure to ethoxyquin increases brown bullhead GST-CDNB catalytic activity and hepatic cationic GST protein expression. However, th e increase in overall GST-CDNB activity by ethoxyquin is associated with re pression of GST-BPDE activity, suggesting differential effects on hepatic b ullhead GST isoforms by ethoxyquin. The potential repression of bullhead GS T isoforms that conjugate the carcinogenic metabolites of PAH metabolism un der conditions of environmental chemical exposure could be a contributing f actor in the sensitivity of bullheads to pollutant-associated neoplasia.