Timing of prostaglandin exposure is critical for the inhibition of LPS‐ or IFN‐γ‐induced macrophage NO synthesis by PGE 2

Macrophage nitric oxide (NO) synthesis is an integral component of the host defense system. We have previously found that NO and prostaglandins interact in a variety of ways. NO modulates Kupffer cell prostaglandin E 2 (PGE 2 ) production and we have recently described the inhibitory effects of PGE 2 on NO synthesis in both Kupffer cells and hepatocytes. Activated macrophages produce a number of prostaglandins but studies regarding the capacity of prostaglandins to regulate macrophage NO synthesis have yielded conflicting results. We found that exogenous PGE 2 decreased lipopolysaccharide (LPS)‐induced NO synthesis in murine resident peritoneal macrophages and in the RAW 264.7 murine macrophage cell line. PGE 2 also suppressed NO synthesis in response to interferon‐γ (IFN‐γ) alone and a combination of LPS + IFN‐γ. Inhibition of endogenous PGE 2 synthesis with indomethacin or ibuprofen had no effect on NO synthesis. PGE 2 added with the activating stimulus was most effective. PGE 2 lost the capacity to block NO synthesis if added more than 180 min after LPS. PGE 2 decreased inducible NO synthase (iNOS) mRNA and immunoreactive iNOS protein, consistent with the hypothesis that exogenous PGE 2 inhibits macrophage iNOS expression but that the inhibition depends on the time and concentration of prostaglandin exposure. J. Leukoc. Biol . 61: 712–720; 1997.