Dose-dependent localization of TCDD in isolated centrilobular and periportal hepatocytes.

Dose-response relationships for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suggest a differential sensitivity of liver cell types to the induction of cytochrome P450 gene expression, and that the induction of hepatic protein CYP1A2 causes sequestration of TCDD. In addition, immunolocalization of hepatic CYP1A1/1B1/1A2 proteins is not uniform after exposure to TCDD. The mechanism for the regio-specific induction of hepatic P450s by TCDD is unknown, but may involve the differential distribution of participants in the AhR-mediated pathway and/or regional P450 isozymes, as well as, non-uniform distribution/sequestration of TCDD. Therefore, this study examined the effects of TCDD in unfractionated, centrilobular and periportal hepatocytes isolated from female Sprague-Dawley rats acutely exposed (3 days) to a single oral dose of 0.01-10.0 microg [3H]TCDD/kg. A dose-dependent increase in concentration of TCDD was accompanied by a dose-dependent increase in CYP1A1, CYP1A2, and CYP1B1 mRNA expression and associated enzymes in all liver-cell populations. Centrilobular hepatocytes showed a 2.7- to 4.5-fold higher concentration of TCDD as compared to the periportal hepatocytes at doses up to 0.3 microg TCDD/kg. Centrilobular hepatocytes also exhibited an elevated MROD activity as compared to the periportal hepatocytes at doses up to 0.3 microg TCDD/kg. Furthermore, centrilobular hepatocytes showed an elevated concentration of induced CYP1A2 and CYP1B1 mRNA as compared to periportal hepatocytes within the 0.01- and 0.3-microg TCDD/kg-treatment groups. This is the first study to demonstrate that a dose-dependent difference in distribution of TCDD exists between centrilobular and periportal cells that might be related to regional differences in P450 induction.