Timing of prostaglandin exposure is critical for the inhibition of LPS- or IFN-γ-induced macrophage NO synthesis by PGE₂

Macrophage nitric oxide (NO) synthesis is an integral component of the hose defense system. We have previously found that NO and prostaglandins interact in a variety of ways. NO modulates Kupffer cell prostaglandin E₂ (PGE₂) production and we have recently described the inhibitory effects of PGE₂ 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 PGE₂ decreased lipopolysaccharide (LPS)-induced NO synthesis in murine resident peritoneal macrophages and in the RAW 264.7 murine macrophage cell line. PGE₂ also suppressed NO synthesis in response to interferon-γ (IFN-γ) alone and a combination of LPS + IFN-γ. Inhibition of endogenous PGE₂ synthesis with indomethacin or ibuprofen had no effect on NO synthesis. PGE₂ added with the activating stimulus was most effective. PGE₂ lost the capacity to block NO synthesis if added more than 180 min after LPS. PGE₂ decreased inducible NO synthase (iNOS) mRNA and immunoreactive iNOS protein, consistent with the hypothesis that exogenons PGE₂ inhibits macrophage iNOS expression but that the inhibition depends on the time and concentration of prostaglandin exposure.