Regulation of prostaglandin H synthase-1 gene expression.

Abstract

Prostaglandin H synthase (PGHS, also called cyclooxygenase, COX; EC 1.14.99.1) is a bifunctional enzyme which catalyzes the conversion of arachidonic acid (AA) to prostaglandin G2 (PGG2) and then PGH2 (1). PGH2 is the precursor of biologically active and physiologically important prostanoids, i.e. thromboxane A2 (TXA2), prostacyclin (PGI2), PGE2, PGF2α and PGD2. PGHS, hence, occupies a pivotal position in prostanoid biosynthesis. Two isoforms of PGHS have been identified. The original enzyme, designated PGHS-1 was purified to homogeneity from bovine and ovine seminal vesicles (2–3). The purified enzyme is a glycoprotein and exists as a dimer in detergent with a subunit molecular mass of approximately 70 kDa. Histochemical studies show that it is distributed widely in mammalian cells and tissues. PGHS-1 cDNA obtained from sheep, mouse and human tissues has a high degree (>90%) of homology (4–8). It encodes a 600 amino acid protein including a signal peptide containing ~24 amino acids. There are 3 potential N-glycosylation sites. PGHS-2 cDNA codes for a 602 amino acid protein with a molecular weight indistinguishable from that of PGHS-I on Western blots. Although several laboratories including ours suspected that there might be a second type of PGHS (9–11), cDNA for PGHS-2, however was cloned accidentally. Xie et al. reported the cloning from chicken embryonic fibroblasts a src-inducible cDNA encoding a 70 kDa protein with 59% identity with ovine, murine and human PGHS-1 (12). Kujubu et al. independently reported the characterization of a 4.5 kb phorbol ester-inducible immediate early transcript from murine fibroblasts which codes for a polypeptide with 65% sequence homology to PGHS-1 (13). This murine transcript was more closely related (82% identity) to the chicken PGHS-2 than to PGHS-1. PGHS-2 cDNA cloned from cultured human endothelial cells is also more closely related to murine (88% identity), and chicken (81% identity) PGHS-2 cDNA than human endothelial cell PGHS-1 (61% identity) (14,15). PGHS-2 cDNA codes for a 602 amino acid protein with a molecular weight indistinguishable from that of PGHS-1 on Western blots. Comparison of predicted amino acid sequence of PGHS-2 with that of PGHS-1 revealed a major difference in the N-terminal where PGHS-2 has a shorter signal peptide than PGHS-1 and in the C-terminal where PGHS-2 has a 18-amino acid insert but amino acid residues critical for the catalytic activity of PGHS are aligned in both isozymes (16,17).