Cytokine-mediated suppression of cytochrome P450 1A1 in Hepa-1c1c7 cells by pokeweed mitogen.

This study investigated the effects of pokeweed mitogen (PWM) on the regulation of cytochrome P450 (P450) 1A1 expression in an in vitro model, using murine hepatoma cell line Hepa-1c1c7 and murine macrophage cell line RAW 264.7 cell cultures. PWM added directly to Hepa-1c1c7 cells had no effect on 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced P450 1A1-specific 7-ethoxyresorufin O-deethylase (EROD) activity. However, TCDD-induced EROD activity and P450 1A1 mRNA levels were markedly suppressed when Hepa-1c1c7 cells were cultured with PWM-treated conditioned media from RAW 264.7 in a dose-dependent manner. Concomitant treatment with PWM and pentoxifylline, a TNFalpha synthesis inhibitor, to RAW 264.7 cells decreased the suppressive effects of PWM on TCDD-induced EROD activity. In PWM-exposed RAW 264.7 cell cultures, TNFalpha and IL-6 levels increased in a dose-dependent fashion. When antibodies to TNFalpha or/and IL-6 were added to PWM-treated conditioned media from RAW 264.7, the suppression of EROD activity was inhibited. These results suggested the suppression of P450 1A1 by PWM was mediated exclusively by TNFalpha and IL-6, released from macrophages.