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

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 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-gamma (IFN-gamma) alone and a combination of LPS + IFN-gamma. 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 synthesis (iNOS) mRNA and immunoreactive iNOS protein, consistent with the hypothesis that exogenous PGE2 inhibits macrophage iNOS expression but that the inhibition depends on the time and concentration of prostaglandin exposure.