Cloning of a cDNA encoding the rat placental glutathione S-transferase: overproduction of the mRNA in hepatocellular carcinomas.

Rat hepatocellular carcinomas and normal liver have been screened for mRNA sequences which are more abundant in the carcinomas than in livers. The screen was done by differential colony hybridization of a cDNA library constructed from the mRNA of a primary tumor induced by diethylnitrosamine (DEN). One cDNA was isolated which hybridizes to a 900 nucleotide mRNA present in abundance in six chemically induced rat hepatomas (both primary and transplantable), but is not detectable in normal adult or fetal (17 day) liver. The nucleotide sequence of this cDNA is identical to that reported by Suguoka et al. for the placental isoenzyme of rat glutathione S-transferase (GST-P) (Nucleic Acids Res. 13:6049). A comparison of the GST-P amino acid sequence with those of two liver GST isoenzymes (Yal and Yc) shows only 26% overall homology; however, this homology is concentrated in three regions of 50%, 68.2% and 34.5%. Genomic blotting experiments show that the overproduction of GST-P mRNA in three Morris hepatomas is not due to amplification of genomic DNA sequences hybridizing with the GST-P cDNA. However, hybridizing sequences are contained on at least 72 kb of genomic DNA, suggesting great complexity of the rat GST-P locus.