CYTOCHROME-P450 1A1 INDUCTION IN RAT LYMPHOID-TISSUES FOLLOWING IN-VIVO AND IN-VITRO EXPOSURE TO 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN REQUIRES PROTEIN-KINASE C-1,C-2

The induction of cytochrome P450 1A1 (CYP1A1) is one of the most sensi tive responses associated with exposure to 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) and related compounds. The mechanisms that underlie th is response are not completely understood, particularly in lymphoid ti ssues that may be used in biomarker studies in humans. CYP1A1 mRNA exp ression and enzyme activity (ethoxyresorufin-o-deethylase, EROD) were investigated in rat thymus and spleen and isolated thymocytes and sple nocytes in culture. Thymus-or spleen-derived microsomes from rats trea ted in vivo with TCDD showed induced EROD activity after as little as 12 h following a single exposure to TCDD (5 mu g/kg body weight). Rest ing rat thymocytes in culture had detectable levels of EROD activity a nd CYP1A1 mRNA expression which increased following in vitro exposure to greater than or equal to 0.1 nM TCDD for 24 or 48 h. Interestingly, concomitant in vitro exposure of rat thymocytes to TCDD and the mitog en concanavalin A (Con A) inhibited the induction of EROD activity, wh ich is in contrast to the response of cultured human peripheral blood lymphocytes (Landi et al., 1994; Pharmacogenetics 4, 242-246). Resting rat splenocytes in culture had no detectable EROD activity and CYP1A1 ctivity could not be induced by in vitro TCDD exposure, in the presen ce or absence of Con A. These results suggest that the relative matura tion state of the cells is important in regulating the expression of C YPlAl, since splenocytes represent a more mature population of B and T lymphocytes. TCDD-induced CYP1A1 pression in cultured rat thymocytes was inhibited by the addition of calphostin C, a specific protein kina se C (PKC) inhibitor, suggesting a role for PKC as a second messenger in the CYP1A1 induction pathway. In vivo co-exposure with phorbol-myri state-acetate (PMA) and TCDD also inhibited CYP1A1 induction. Again, s uggesting a role for PKC in CYP1A1 induction. Together, these results indicate that relative lymphocyte maturation state and the PKC pathway are important factors in regulating the expression of CYP1A1. (C) 199 7 Elsevier Science Ireland Ltd.