Modulation of nitric oxide synthase activity in macrophages

Induction of nitric oxide synthesis in murine macrophages by Helicobacter pylori

Intact Helicobacter pylori cells, as well as cellular components, stimulated nitric oxide (NO) synthesis in an in vitro murine macrophage system by the L-arginine-nitric oxide pathway. Macrophage-mediated NO formation was dependent on the presence of H. pylori and exhibited a dose-dependent increase at H. pylori concentrations between 10(6) and 5 and 10(7) cells/ml. H. pylori mediated NO synthesis also required L-arginine and was inhibited by NG-monomethyl-L-arginine (NMMA), a selective inhibitor of nitric oxide synthase. NO synthesis was induced by whole H. pylori cells. H. pylori media filtrate, extracted membrane proteins, and H. pylori lipopolysaccharide (LPS). Maximal NO synthesis was induced by viable H. pylori cells with media filtrate and membrane protein extracts inducing significant NO responses. NO stimulation by media filtrate and membrane protein extracts support secreted H. pylori products as potential activators of inflammatory cell NO synthesis in vivo. NO synthesis in response to H. pylori suggests that chronic H. pylori infection may increase endogenous formation of NO. Elevated NO exposure may represent an etiologic factor explaining the epidemiologic association between long-term H. pylori infection and gastric cancer.

Dual effects of oxidized low-density lipoprotein on immune-stimulated nitric oxide and prostaglandin release in macrophages

Oxidized low-density lipoprotein (LDL) is currently regarded as a tentative key player in atherosclerosis by virtue of its ability to induce intracellular lipid accumulation and to modulate cell functions in the vessel wall. We previously demonstrated that inducible nitric oxide (NO) synthase activity is attenuated in lipid-laden J774 macrophages obtained by incubation with oxidized LDL 200 micrograms ml-1 for 24 h. In the present study we investigated the effect of oxidized LDL in a lower concentration (20 micrograms ml-1) or for a shorter time (6 h) and the possible mediator role of prostaglandin E2 and prostacyclin. Prostaglandins and the NO synthase metabolites citrulline and nitrite were elevated in the 24 h supernatant after immune stimulation with interferon-gamma 100 U ml-1 with or without lipopolysaccharide 10 micrograms ml-1. Pretreatment with oxidized LDL 20 micrograms ml-1 for 18 h decreased nitrite release by 31 +/- 2%, whereas prostaglandin production was not affected. A 6 h pre-exposure to 200 micrograms ml-1 had an opposite effect: it significantly potentiated interferon-gamma-stimulated prostaglandin E2 (10-fold), prostacyclin (7-fold), nitrite (1.5-fold), and citrulline (2.4-fold) release. Indomethacin 10 microM abolished the prostaglandin production and largely prevented the oxidized LDL-dependent increase in NO synthase activity. Acetylated LDL was without effect. The data show that the immune-induced release of NO is potentiated or suppressed, depending on the conditions of exposure to oxidized LDL. The potentiation due to short, high-dose exposure is partly mediated by prostaglandins since indomethacin inhibited both processes.

Human and rat macrophages mediate fungistatic activity against Rhizopus species differently: in vitro and ex vivo studies

Both rat alveolar macrophages and a human macrophages cell line with characteristics of human tissue (e.g., alveolar) macrophages (THP-1) were found to inhibit the germination of Rhizopus spores. However, the conditions under which fungistatic activity occurs are different for these two cell types. The inhibition of Rhizopus spore germination by rat alveolar macrophages requires the activation of macrophages and the presence of serum and L-arginine. During rat alveolar macrophage-mediated fungistatic activity, L-arginine is oxidized to nitric. Human macrophage-mediated fungistatic activity is similar to that mediated by rat macrophages in terms of the serum requirement, but it does not require L-arginine. Human macrophages did not produce any nitrite detectable by the colorimetric assay. Their ability to inhibit germination was enhanced by the combination of endotoxin and gamma interferon. The inhibition of Rhizopus spore germination by rat alveolar macrophages is thus mediated by the generation of nitric oxide, whereas the mechanism of similar inhibition by human macrophages remains poorly understood. Serum samples from diabetic rats as well as from patients with diabetes or uremia decreased the inhibitory effect of macrophages on spore germination. Dialysis of the serum samples against a buffered salt solution antagonized this phenomenon, indicating that a low-molecular-weight factor in the sera of patients with diabetes or uremia may modulate local antifungal defense mechanisms. The absence of L-arginine-dependent nitrogen oxidation in human macrophages, compared with its presence in rat alveolar macrophages, under conditions during which fungistatic activity occurs suggests that this phenomenon is species specific.