Nucleic Acids Research

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 CoCl? and 3-amino-l,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-l, 2,4-tnazole and CoClblocked 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 16hr and then decreased sharply. In comparison, the increase in the case of glutathione transferase (Ya+Yc) mesenger 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-l,2,4-triazole and C0CI2 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. INTRODUCTION Cytochrome P-450 and glutathione transferase constitute two major protein constituents of the drug metabolizing system (1,2). The induction of specific species of cytochrome P-450 in response to drug administration is a well known phenomenon. For example, phenobarbitone induces predominantly cytochrome P-450 b and e © IRL Prew Limited, Oxford, England. 2497 Nucleic Acids Research species in rat liver, whereas 3-methylcholanthrene induces cytochrorne P-450 c and d species (3-5). It has also been shown that these drugs act at the transcription level in this regard (6-9). Glutathione transferase represents a family of proteins consisting of two polypeptides recruited from Ya, Yb and Yc subunits in rat liver. These protein species can be homo or hetero-dimers and the possible existence of variants of the Ya, Yb and Yc subunits as well as the presence of additional new subunits have been recognized (10-13). Prototype inducers such as phenobarbitone and 3-methylcholanthrene have been reported to induce Ya and Yc subunits, with Ya being the major subunit induced. In addition, it is known that Ya and Yc have extensive homologies, but are derived from independent messenger RNAs (14-16). In terms of the mechanism of action of foreign chemicals in inducing the cytochrome P-

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