Since histone acetylation has been implicated in the facilitation of specific gene transcription, we investigated the effect of increasing histone acetylation through inhibition of histone deacetylase on 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induction of P4501A activity in cultured rat hepatocytes. Inhibition of histone deacetylation was accomplished with addition of trichostatin A (TSA) to the incubation medium, and P4501A activity was measured spectrofluorometrically by determination of the rate of resorufin formation by ethoxyresorufin-O-deethylase (EROD). While TSA alone (5-200 ng/mL) had no effect on EROD activity, TSA potentiated the effect of various concentrations (10(-12) to 10(-10) M) of TCDD. Addition of 200 ng TSA/mL with TCDD resulted in an increased EROD activity of approximately 200% compared with TCDD alone. When TSA was removed from the cells after various incubation times (2, 6, 24 hr) by successive washings with TSA-free medium, it was determined that TSA was required for 24 hr in order to potentiate the effects of a 48-hr incubation with TCDD. In addition to measurement of EROD activity, P4501A1 and 1A2 microsomal protein were determined by western immunoblotting analysis. While neither P4501A1 nor 1A2 was detectable in the presence of TSA alone, P4501A1 was present after incubation of cells with TCDD in the presence or absence of TSA. TCDD plus TSA also resulted in the formation of P4501A2. The results of this study suggest an important role for histone acetylation in the action of TCDD on induction of P4501A enzymes.