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 ofcytochrome P450 gene expression, and that the induction of hepatic proteinCYP1A2 causes sequestration of TCDD. In addition, immunolocalization ofhepatic CYP1A1/1B1/1A2 proteins is not uniform after exposure to TCDD. Themechanism for the regio-specific induction of hepatic P450s by TCDD isunknown, but may involve the differential distribution of participants inthe AhR-mediated pathway and/or regional P450 isozymes, as well as,non-uniform distribution/sequestration of TCDD. Therefore, this studyexamined the effects of TCDD in unfractionated, centrilobular andperiportal hepatocytes isolated from female Sprague-Dawley rats acutelyexposed (3 days) to a single oral dose of 0.01-10.0 microg [3H]TCDD/kg. Adose- dependent increase in concentration of TCDD was accompanied by adose- dependent increase in CYP1A1, CYP1A2, and CYP1B1 mRNA expression andassociated enzymes in all liver-cell populations. Centrilobular hepatocytesshowed a 2.7- to 4.5-fold higher concentration of TCDD as compared to theperiportal hepatocytes at doses up to 0.3 microg TCDD/kg. Centrilobularhepatocytes also exhibited an elevated MROD activity as compared to theperiportal hepatocytes at doses up to 0.3 microg TCDD/kg. Furthermore,centrilobular hepatocytes showed an elevated concentration of inducedCYP1A2 and CYP1B1 mRNA as compared to periportal hepatocytes within the0.01- and 0.3-microg TCDD/kg- treatment groups. This is the first study todemonstrate that a dose- dependent difference in distribution of TCDDexists between centrilobular and periportal cells that might be related toregional differences in P450 induction.