Nitric Oxide: A Prostaglandin H Synthase 1 and 2 Reducing Cosubstrate That Does Not Stimulate Cyclooxygenase Activity or Prostaglandin H Synthase Expression in Murine Macrophages

Several recent reports have investigated the possibility that nitric oxide (NO) can regulate prostaglandin H synthase (PHS) activity. The potential significance of NO regulation of PHS is considerable, when one considers the numerous important biological processes that are influenced by PHS. In this study, we used microsomal and purified PHS to investigate the direct effect of NO and NO-generating compounds on PHS catalytic activity. We found that NO neither significantly inhibited nor enhanced prostaglandin (PG) formation, despite the fact that NO stimulated PHS peroxidase activity. We also investigated the effect of NO and NO generators on PHS product, protein, and mRNA levels in the RAW264.7 murine macrophage cell line. We found that NO or NO generators had little or no effect on PG formation, PHS expression, or PHS mRNA expression in unstimulated RAW264.7 cells. The same results were obtained with macrophages that were stimulated by 18 h pretreatment with lipopolysaccharide, a known inducer of PHS-2 in macrophages. These data clearly indicate that NO acts as a cosubstrate for PHS peroxidase. However, NO does not enhance or inhibit either cycloxygenase activity or expression of PHS in the model systems used in this study.