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, PGF_2α 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).