Specific cDNA probes were used to determine steadystate mRNA levels for the multiple glutathione S-transferases in primary hepatocyte cultures. In the first 24 hr of culture, gene transcripts for the Y;, family decreased sharply, Yb3 disappeared completely, but changes in levels of mRNA for Y,,, and Y h p were smaller. These results suggest that the isoenzymes are regulated independently. Y, mRNA, which is present at greatly elevated levels in hyperplastic nodules and hepatocellular carcinomas but not in normal adult livers, was hardly detectable in freshly isolated hepatocytes, but Y, transcripts rapidly accumulated in the first 24 hr in culture and continued to increase for 72 hr. Decreased levels in Y, and Y,. and increases in Y, were detected by immunoblotting methods, indicating that translation products changed together with mRNA levels in the cultured cells. The appearance of Y, transcripts in hepatocytes was effectively blocked by addition of dexamethasone to the culture medium. Elevations of Y, levels are characteristic of the cell culture system and factors regulating Y, transcription in nodules and carcinomas may also be operative in cultured hepatocytes.
Glutathione S-transferases (GSTsj are a family of intracellular binding and transport proteins that catalyze conjugation reactions between glutathione and a variety of electrophils [for reviews, see Refs. (1-5jl. Multiple forms of rat GSTs originate from homo-and heterohmeric combinations of subunits of at least three different classes; the Y,,Y,, Yh and Y , , forms may be distinguished from each other on the basis of primary structure, catalytic, binding, immunologic and other properties (4-7 ). Expression of individual isoenzymes of this supergene family (8, 9) varies in a tissue-specific manner (10-12).
The predominant subunits of adult rat liver are Y,, Y,, Y,,,, YhZ with lesser amounts of Y , , , (4, [13][14][15]. Y,, is present at very low levels in normal livers but is greatly elevated in hyperplastic nodules and hepatomas (16)(17)(18)(19). This study shows that the differential regulation of individual GSTs is marked by the appearance of Y , gene transcripts in cultured hepatocytes and that dexamet h-