Polyamine regulation of nitric oxide production in LPS-activated macrophages

Abstract

Polyamines are physiological cellular constituents essential for cell growth and differentiation, and regulate a multitude of cellular functions (1-4). Nitric oxide (NO) is an effector molecule in both the cardiovascular and nervous systems (5,6). Intracellularly, NO and polyamines are derived from arginine, the latter via the rate-limiting enzyme ornithine decarboxylase (ODC; 7). This enzyme, like the inducible nitric oxide synthase (iNOS), is induced by proinflammatory cytokines and bacterial lipopolysaccharide (LPS), resulting in enhanced enzyme activity and increased polyamine biosynthesis (8,9). While the increase in polyamine synthesis would have important implications for cell growth and proliferation, it is not clear how this might affect iNOS pathway. Inhibition of polyamine biosynthesis impairs the phagocytic capacity of macrophages (10) and can block macrophage activation by tumour necrosis factor (11). Recently, exogenous polyamines have been shown to inhibit NO production in LPS-activated J774 cells (12) and by isolated neuronal NO synthase (13). However, these effects required relatively high concentrations of polyamines compared to those found in plasma and in intact cells (14), and appear to be due to aldehyde metabolites resulting from polyamine oxidation by the amine oxidase present in calf serum (15-17). In this study we have explored the effects of both endogenous and exogenous polyamines on the inducible L-arginine-NO pathway by examining whether inhibition of ornithine decarboxylase (ODC) and thus of polyamine biosynthesis (7), regulates NO production in lipopolysaccharide-activated J774 cells, a murine macrophage cell line