Dose-responses in rat hepatic protein modification and expression following exposure to the rat hepatocarcinogen methapyrilene.

Dose-related effects of methapyrilene (MP) on protein modification and expression were examined using two-dimensional gel electrophoresis (2-D PAGE) coupled with computer analysis. Methapyrilene was administered ad libitum at doses of 0, 62.5, 125, 250 and 1000 p.p.m. to male F-344 rats for 12 weeks beginning at 8 weeks of age. Following treatment, livers were removed and frozen for 2-D PAGE analysis. Separation of hepatic proteins was conducted using ISO-DALT technology. Changes in abundance and modification of hepatic proteins were determined using the Kepler software package. Covalent modifications of three specific mitochondrial proteins were quantified using a charge modification index. Dose-response relationships were analyzed using Tukey's trend test. Results demonstrated that covalent modification of the three mitochondrial proteins was linearly related to dose and that a dose effect could be found at all dose levels in 2 out of 3 proteins. Two forms of change in protein expression were observed: (i) a dose-dependent change with effects at all doses and (ii) a change only at the toxic dose of 1000 p.p.m. MP. These results demonstrate a molecular effect of MP at doses that do not produce cellular responses including toxicity or increases in cell replication suggesting that these specific mitochondrial modifications are molecular dosimeters but are probably not direct factors and/or sufficient factors in carcinogenesis. This study also demonstrates the potential use of 2-D PAGE electrophoresis to delineate the effect of dose on expression of specific proteins.