Induction of P-450IIB genes within the rat liver acinus is not dependent on the chemical inducer or on the acinar organization

Abstract

P-450IIB genes (P-450b and P-450e in rat) are induced following treatment with phenobarbital predominantly in hepatocytes located in zones 2 and 3 of the liver acinus. The previous finding of phenobarbital-mediated induction of P-450IIB mRNAs and apoproteins in the same zonal hepatocytes suggests that this differential induction is most likely due to zonal differences in the activation of gene transcription. To determine whether differential P-450IIB gene transcription was dependent on the type of inducer (i.e., inducer specificity) or on the capacity of hepatocytes located in different acinar zones to respond to inducers (i.e. zonal specificity), the pattern of acinar induction was evaluated following treatment with inducers that have diverse physiochemical properties and inductive capacities. The zonal distribution of P-450b and P-450e mRNAs in liver was determined by in situ hybridization after the administration of either phenobarbital, polychlorinated biphenyls, chlordane, or chlorpromazine. Liver sections were hybridized with 3H-labeled RNA transcripts of a P-450e cDNA that recognizes sequences of both P-450b and P-450e mRNAs and the pattern of zonal mRNA induction was measured by quantitative image analysis. Each inducer increased P-450b,e mRNA levels predominantly in hepatocytes of zones 2 and 3 of the hepatic acinus. The P-450b and P-450e apoproteins were induced in the same zonal hepatocytes as the P-450b,e mRNAs as shown by immunofluorescence studies using monoclonal antibodies. Therefore, differential transcriptional induction of the P-450IIB genes in the liver acinus does not seem to be dependent on a specific chemical inducer, but rather is a characteristic capacity of hepatocytes located in different acinar zones. To determine whether the induction of P-450b and P-450e was dependent on the liver tissue organization, hepatocytes were isolated by collagenase perfusion of the liver and transplanted into the spleens of syngeneic rats. Induction of P-450b and P-450e mRNAs and apoproteins, assessed 1 month after transplantation, was evident with each of the chemical inducers. These data suggest that, once hepatocytes have attained the capacity to respond to inducers, the organization of hepatocytes into acini and the sinusoidal microenvironment of the liver are not required for hepatocytes to maintain the ability to respond to inducers with an increase in transcription of P-450IIB genes. Moreover, the splenic hepatocytes demonstrated a heterogeneous pattern of P-450b,e apoprotein induction; this raises the possibility that the acinar organization is also not required for the heterogenous expression of P-450IIB genes.