The same γ‐glutamyl transpeptidase RNA species is expressed in fetal liver, hepatic carcinomas, and rasT24‐transformed rat liver epithelial cells

In rats, γ-glutamyl transpeptidase (γGT) exists as a single-copy gene, and three distinct species of RNA (types I, II, and III) that differ in their 5' untranslated regions have been identified. To compare steady-state levels of these γGT RNAs in rat tissues, hepatic carcinomas, and cultured cells, we used RNA dot-blot hybridization and a reverse transcriptase-polymerase chain reaction (RT-PCR) technique with oligonucleotides specifically designed for each type of RNA. Fetal live, hepatic carcinomas, rasT24-transformed rat liver epithelial (RLE) cells and pancreas make only type III RNA. Liver and untransformed RLE cells do not make detectable levels of γGT RNA. We found that both fetal and adult kidneys synthesize all three types of RNA, indicating that increases in γGT RNA known to occur after birth do not result from recruitment of additional RNA species. When we increased the sensitivity of the assay approximately 1000 fold by sequencing the RT-PCR product directly after an additional round of amplification, we found that very low levels of types I and II RNA were present in fetal liver, ras T24-transformed RLE cells, and pancreas, and that adult liver and untransformed RLE cells synthesized very low levels of all three RNA species. Rat-1 fibroblasts did not make levels of γGT RNA detectable by this method. These results demonstrate that different γGT RNA species are regulated differently during development and neoplastic transformation and that there is a commitment in some cell types to very-low-level expression of γGT RNAs.