Cytoplasmic prostaglandin E2 synthase is dominantly expressed in cultured KAT-50 thyrocytes, cells that express constitutive prostaglandin-endoperoxide H synthase-2. Basis for low protaglandin E2 production.

The recent identification and cloning of two glutathione-dependent prostaglandin E(2) synthase (PGES) genes has yielded important insights into the terminal step of PGE(2) synthesis. These enzymes form efficient functional pairs with specific members of the prostaglandin-endoperoxide H synthase (PGHS) family. Microsomal PGES (mPGES) is inducible and works more efficiently with PGHS-2, the inflammatory cyclooxygenase, while the cytoplasmic isoform (cPGES) pairs functionally with PGHS-1, the cyclooxygenase that ordinarily exhibits constitutive expression. KAT-50, a well differentiated thyroid epithelial cell line, expresses high levels of PGHS-2 but surprisingly low levels of PGE(2) when compared with human orbital fibroblasts. Moreover, PGHS-1 protein cannot be detected in KAT-50. We report here that KAT-50 cells express high basal levels of cPGES but mPGES mRNA and protein are undetectable. Thus, KAT-50 cells express the inefficient PGHS-2/cPGES pair, and this results in modest PGE(2) production. The high levels of cPGES and the absence of mPGES expression result from dramatic differences in the activities of their respective gene promoters. When mPGES is expressed in KAT-50 by transiently transfecting the cells, PGE(2) production is up-regulated substantially. These observations indicate that naturally occurring cells can express a suboptimal profile of PGHS and PGES isoforms, resulting in diminished levels of PGE(2) generation.