Expression of gamma-glutamyl transpeptidase in midgestation mouse yolk sac and mouse visceral yolk sac carcinoma cells.

gamma-Glutamyl transpeptidase (gamma GT) is a crucial enzyme for the metabolism of xenobiotics and endogenous mediators of biological functions (leukotrienes, prostaglandins, and hepoxillins). Yet little is known about its potential role during development. It is a single copy gene expressed from at least seven promoters. Using histochemistry and immunohistochemistry we demonstrate that gamma GT first appears in the midgestational yolk sacs of mouse embryos. Established cell lines with phenotypic features of yolk sac endoderm (JC-44) or embryonic stem cells were also assayed for the expression of gamma GT. Significant levels were detected in JC-44 cells and higher levels were found in JC-44-derived embryoid bodies. Because this cell line appears to be a good in vitro counterpart of yolk sac differentiation, we characterized the gamma GT mRNA types expressed in JC-44 cells. By ribonuclease protection analysis, gamma GT RNA types IV and VI represent about 80% of the total gamma GT RNA in JC-44 embryoid bodies. Reverse transcription-mediated polymerase chain reaction detected low amounts of gamma GT RNA types I, III, and V. Expression of gamma GT in yolk sac follows a pattern seen in many tissues in which one or two gamma GT RNA types dominate the expression profile; however, the reason for this tissue specificity is unknown.