Related Experiment Videos
Phenobarbital-inducible gene expression in developing rat liver: relationship to hepatocyte function
1INSERM U 75, CHU Necker, Paris, France.
Insights
The number of liver cells, not biochemical changes, controls how phenobarbital-inducible genes are expressed during development. This study shows phenobarbital-inducible cytochrome P-450 and UDP-glucuronosyltransferase RNAs are mainly regulated by hepatocyte proliferation.
Area of Science:
- Biochemistry
- Developmental Biology
- Pharmacology
Background:
- Cytochromes P-450 and UDP-glucuronosyltransferases are crucial for drug metabolism.
- Their developmental expression and induction patterns in rat liver are not fully understood.
- Understanding these processes is key to predicting drug efficacy and toxicity during development.
Purpose of the Study:
- To investigate the developmental expression of specific drug-metabolizing enzyme RNAs in rat liver.
- To determine the influence of different chemical inducers on these RNA levels during ontogenesis.
- To elucidate the primary regulatory mechanism controlling phenobarbital-inducible gene expression in developing liver.
Main Methods:
- Quantitative analysis of RNA expression for cytochrome P-450 and UDP-glucuronosyltransferase.
- Administration of various inducers (phenobarbital, pregnenolone 16 alpha-carbonitrile, methylcholanthrene, clofibrate) to rats at different developmental stages.
- Correlation of RNA expression levels with hepatocyte cell number and other developmental parameters.
Main Results:
- RNAs for inducible enzymes were present in fetal liver and stable in non-induced animals.
- Inducers showed differential effects: methylcholanthrene was active in fetal liver, clofibrate had no effect.
- Phenobarbital induction increased with ontogenesis, paralleling hepatocyte proliferation.
- Phenobarbital's induction of P-450 and UDPGT RNAs is restricted to hepatocytes and constant throughout development.
- Phenobarbital potentiated methylcholanthrene's induction capacity exclusively in hepatocytes.
- Pregnenolone 16 alpha-carbonitrile was effective in fetal/neonatal rats, with peak effect at 5 days.
Conclusions:
- Hepatocyte number, not biochemical maturation, controls phenobarbital-inducible gene expression during rat liver development.
- This study provides direct evidence for proliferation-dependent regulation of drug-metabolizing enzymes.
- Pregnenolone 16 alpha-carbonitrile exhibits a distinct temporal regulation compared to phenobarbital.
Abstract:
The expression of phenobarbital-, pregnenolone 16 alpha-carbonitrile- and polycyclic aromatic hydrocarbon-inducible cytochromes P-450 and of phenobarbital-inducible UDP-glucuronosyltransferase was examined in developing rat liver. RNAs coding for these proteins were present in fetal rat liver and their respective concentrations remained quite stable in non-induced animals. Inducers differently affected the concentration of RNAs: clofibrate had no action, whereas methylcholanthrene was highly active in fetal liver. Induction by phenobarbital gradually increased during ontogenesis, in parallel with the augmentation of the number of hepatocyte cells in the liver. Our contribution definitively demonstrates that the ability of phenobarbital to enhance P-450 and UDPGT RNAs is strictly restricted to hepatocytes and remains roughly unchanged throughout ontogenesis. In addition, phenobarbital was also able to potentiate the inducing capacity of methylcholanthrene (i.e., raising the TCDD-binding protein) exclusively in hepatocytes. This is the first direct evidence that the number of hepatocytes in the liver, rather than a biochemical maturation, controls the expression of phenobarbital-inducible genes. Pregnenolone 16 alpha-carbonitrile was also effective as inducer in fetal and neonatal rats and its maximal effect was observed in 5-d-old neonates, suggesting a regulation mechanism temporally different from that of phenobarbital.