EXPRESSION OF NITRIC-OXIDE SYNTHASE IN HUMAN PERIPHERAL-BLOOD MONONUCLEAR-CELLS AND NEUTROPHILS

It has been clearly demonstrated in rodents that nitric oxide (NO) pla ys an important role in host defense and immunity. However, evidence t hat human leukocytes express inducible nitric oxide synthase (iNOS) or its products has been inconclusive and a source of controversy. We re port that iNOS could not be detected in human monocytes, HL-60 cells, neutrophils, and T cells by Western blotting ana lysis (less than or e qual to 10 pg) or by radiolabeled L-arginine-to-citrulline conversion (less than or equal to 20 pmol L-citrulline) under conditions sufficie nt to induce iNOS in the rodent system and in human hepatocytes, which include activation with cytokines, endotoxins, and/or chemoattractant s. However, sensitive methods such as RT-PCR and Northern blot analysi s show ''constitutively, expressed'' iNOS mRNA from human monocytes, n eutrophils, Jurkat cells, and HL-60 cells. This iNOS mRNA is 4.4 kb an d is similar to that seen in human hepatocytes and rodent macrophages. In spire of the constitutive expression of mRNA in neutrophils and th e lack of detectable NOS activity (based on Western blotting and L-arg inine-to-L-citrulline conversion assay), stimulation of human neutroph ils with FMLP in vitro induced the ADP-ribosylation of an intracellula r NO target glyceraldehyde-3-PO4 dehydrogenase (GAPDH), in a NO-depend ent manner. These studies indicate that low levels of NOS protein are expressed in neutrophils (and perhaps T cells and monocytes) and produ ce NO following stimulation. The data indicate that, in addition to it s phagocytic and tumoricidal activity, NO may also function as an auta coid signaling molecule within the cells. (C) 1997 Wiley-Liss, Inc.