Nitric oxide and mechanisms of redox signalling: matrix and matrix-metabolizing enzymes as prime nitric oxide targets

Abstract

One of the greatest biomedical breakthroughs of the twentieth century was the discovery of endothelium-derived relaxing factor and its identification as nitric oxide (NO). NO has received special attention ever since: besides its potent vasodilatory and vasoprotective effects, NO was identified as a key player in innate immunity and was found to act as an unconventional type of neurotransmitter. This article focuses on mechanisms of NO signalling that form the basis of functional cell responses to accommodate changes in the cellular microenvironment. Redox-based regulation of signal transduction and, on a more long-term scale, changes in gene expression will be exemplified by NO-modulation of matrix components and matrix-metabolizing enzymes. It seems to be a safe bet that ongoing analyses of NO signalling and gene expression will provide a wealth of promising therapeutic targets in human diseases.

Introduction

Nitric oxide (NO) is a diffusible and short-lived free radical gas that can be generated in mammalian cells by a family of NO synthases (NOS). This family comprises the constitutively expressed neuronal NOS (nNOS) and endothelial NOS (eNOS), and the cytokine-inducible NOS (iNOS). iNOS requires a delay of several hours before the onset of NO production but, once induced, this enzyme is active for hours to days and produces NO in 1000-fold larger quantities than the constitutive enzymes eNOS and nNOS Moncada et al., 1991, Beck et al., 1999. Under physiological conditions cells produce only minute amounts of NO by the constitutive enzymes and only trace amounts of reactive oxygen species are available to scavenge NO, thus indicating that direct NO chemistry will dominate functional cell responses (Grisham et al., 1999). The physiologically most relevant action of NO is the activation of the soluble guanylate cyclase by binding to the enzyme's haem moiety. The subsequent increase in cyclic GMP concentrations alters the activity of several target proteins (see below). In a similar way the formation of nitrosyl complexes affects other metalloproteins such as catalase, cytochrome oxidase, cytochrome P450 and NOS itself. Moreover, NO can scavenge superoxide and other free radicals and also inhibit superoxide-driven Fenton chemistry and lipid peroxidation and thus, may depict quite remarkable antioxidant features. In contrast, large amounts of NO as produced by iNOS in an inflammatory setting, often accompanied by a co-generation of reactive oxygen species will shift NO chemistry towards indirect effects such as nitrosation, nitration and oxidation (Grisham et al., 1999). The interaction of NO with molecular oxygen or superoxide will cause the generation of the potent nitrosating agent N₂O₃ and peroxynitrite, respectively. Furthermore, S-nitrosothiol adducts are formed by the interaction between N₂O₃ and certain protein thiol groups and trigger signalling through alterations of protein kinases and phosphatases, G-proteins and ion channels (see below). Finally, these signalling devices directly or indirectly modulate the activity of transcription factors and thus alter gene expression which constitute a fundamental cause of disease-associated pathophysiologies. A detailed discussion of regulation of gene expression by NO is provided elsewhere Marshall et al., 2000, Bogdan, 2001, Pfeilschifter et al., 2001.