A comparative study of tbe regulation of cytochrome P-450 and glutathione transferase gene expression in rat liver

Abstract

A cDNA clone for the Ya subunit of glutathione transferase from rat liver was constructed in E. coli. The clone hybridized to Ya and Yc subunit messenger RNAs. On the basis of experiments involving cell-free translation and hybridization to the cloned probe, it was shown that prototype inducers of cytochrome P-450 such as phenobarbitone and 3-methylcholanthrene as well as inhibitors such as CoCl2 and 3-amino-1,2,4-triazole enhanced the glutathione transferase (Ya+Yc) messenger RNA contents in rat liver. A comparative study with the induction of cytochrome P-450 (b+e) by phenobarbitone revealed that the drug manifested a striking increase in the nuclear pre-messenger RNAs for the cytochrome at 12 hr, but did not significantly affect the same in the case of glutathione transferase (Ya+Yc). 3-Amino-1,2,4-triazole and CoCl2 blocked the phenobarbitone mediated increase in cytochrome P-450 (b+e) nuclear pre-messenger RNAs. These compounds did not significantly affect the glutathione transferase (Ya+Yc) nuclear pre-messenger RNA levels. The polysomal, poly (A)- containing messenger RNAs for cytochrome P-450 (b+e) increased by 12-15 fold after phenobarbitone administration, reached a maximum around 16 hr and then decreased sharply. In comparison, the increase in the case a glutathione transferase (Ya+Yc) messenger RNAs was sluggish and steady and a value of 3-4 fold was reached around 24 hr. Run-off transcription rates for cytochrome P-450 (b+e) increased by nearly 15 fold in 4 hr after phenobarbitone administration, whereas the increase for glutathione transferase (Ya+Yc) was only 2.0 fold. At 12 hr after the drug administration, the glutathione transferase (Ya+Yc) transcription rates were near normal. Administration of 3-amino-1,2,4-triazole and CoCl2 blocked the phenobarbitone-mediated increase in the transcription of cytochrome P-450 (b+e) messenger RNAs. These compounds at best had only marginal effects on the transcription of glutathione transferase (Ya+Yc) messenger RNAs. The half-life of cytochrome P-450 (b+e) messenger RNA was estimated to be 3-4 hr, whereas that for glutathione transferase (Ya+Yc) was found to be 8-9 hr. Administration of phenobarbitone enhanced the half-life of glutathione transferase (Ya+Yc) messenger RNA by nearly two fold. It is suggested that while transcription activation may play a primary role in the induction of cytochrome P-450 (b+e), the induction of glutathione transferase (Ya+Yc) may essentially involve stabilization of the messenger RNAs.

Modifications of hepatic alpha‐1‐acid glycoprotein and albumin gene expression in rats treated with phenobarbital

“…A direct effect of phenobarbital in increasing the rate of gene transcription has previously been reported for cytochrome P450IIB [49 -521 while a post-transcriptional effect involving mRNA stabilization has been described for glutathione transferase mRNA [53]. In our study, the 11-fold increase in cytochrome P450 mRNA 12 h after the first injection of phenobarbital is in favour of a direct effect.…”

Phenobarbital induction of α1-acid glycoprotein in primary rat hepatocyte cultures

“…However, whereas transcriptional activation of the cytochrome P-450 IIB gene was detected only a few hours after a single administration of PB, induction of GST required several days of regular phenobarbital treatment. These kinetic differences could be explained as follows: in the case of cytochrome P-450 IIB, phenobarbital seems to act essentially at the transcriptional level (34)(35)(36)(37), whereas in GST induction posttranscriptional mechanisms predominate (38). With regard to AGP modulation, direct transcriptional activation by PB seems to predominate (19), and the lag time before increased gene expression could be the consequence of a more complex pathway involving several intermediate steps.…”

Special article the glutathione S-transferases: An update

“…However, an increase in transcriptional activity may not be the only explanation for the increase in mRNA levels. The effect of phenobarbital on transcription rates and stability of mRNA for cytochrome P-450 and GST Ya/Yc was assessed in rats (80). The amount of mRNA for P-450 increased rapidly (by 4 hr) following phenobarbital administration and peaked at 16 hr (a 15-fold increase), followed by a steep decline.…”