A Model for the Transcriptional Regulation of a Mouse PGH Synthase Gene

Abstract

Factors such as serum (1), EGF (2, 3), PGDF (4, 5), hormones (6, 7), IL-1 (8, 9), and phorbol esters (8, 10) promote prostaglandin synthesis by two mechanisms. Within minutes these compounds activate specific phospholipases leading to the release of arachidonate and production of prostaglandin endoperoxide H2 (PGH2). This first committed step in the prostaglandin biosynthetic pathway is catalyzed by the enzyme prostaglandin endoperoxide H (PGH) synthase. Within a few hours, these same factors also increase prostaglandin synthesis by stimulating de novo synthesis of PGH synthase. New enzyme production increases the capacity of a cell to produce prostaglandins both by increasing the absolute PGH synthase levels, and by replacing inactive enzyme. PGH synthase is catalytically inactivated during production of PGH2 (11–13); therefore new enzymeis required for prolonged synthesis, and sustained production of prostaglandins is believed to be necessary for the elaboration of many biological events such as inflammation, ovulation and luteolysis, and for maintaining vascular homeostasis (reviewed in 14). The exact mechanisms controlling PGH synthase synthesis are not known, but many of the factors that increase de novo synthesis of the enzyme also increase PGH synthase mRNA levels (1, 4, 5, 15, 16). Thus, one likely mechanism for regulating PGH synthase enzyme expression may be by regulating transcription of the PGH synthase gene. We and others who have examined regulation of PGH synthase gene expression in cultured mouse 3T3 fibroblasts, have shown that mRNA levels of one PGH synthase gene can be elevated in response to serum (1), PDGF and phorbol esters (5). To begin to investigate how PGH synthase mRNA levels are regulated our laboratory has isolated genomic clones and determined the structure of one mouse PGH synthase gene. We have also determined the transcription start site and more than 2 kb of sequence from the 5’-flanking region of this gene. We have identified several enhancer sequences that may regulate PGH synthase transcription and here propose a model for the transcriptional regulation of PGH synthase gene expression.