Current Molecular Medicine - Volume 4, Issue 7, 2004

Nitric oxide has emerged as one of the most important and diverse players in physiology. This small diatomic radical stunned researchers because of its existence and unique biological properties in human physiology. Over the last two decades it was found that NO often has fickle behavior in pathophysiological mechanisms. Where benefiting the host in one case yet inducing and augmenting injury in another. This has lead to confusion in is NO good or bad? Much of the answers to this dichotomy lies in the chemistry of NO and its related nitrogen oxide species. To help understand the complex chemistry with perspective to biology, a discussion on the chemical biology of NO is useful. The chemical biology defines the relevant chemical reaction of NO and nitrogen monoxide in the context of the biological conditions. We discuss in this article the chemistry of nitrogen oxide with different types of biological motifs. Reaction of NO with metal complexes and radicals require low concentration, where formation of reactive nitrogen oxide species require considerably higher amounts and generally are isolated to specific microenvironments in vivo. Though many reactive nitrogen oxide species are formed from chemical reactions with NO, there are several which appear to not require NO to be present, HNO and NO2. These two species have unique physiological effects and represent additional complexity to this biological picture. From this discussion, a picture can be formed concerning the possible chemical dynamics, which can be plausible in different biological mechanisms.

During the past years nitric oxide (NO) signaling became an integral component in understanding physiological and pathophysiological processes of cell proliferation, death or cellular adaptation. Among other activities NO affects multiple targets that allow regulation of gene expression. Although there is no evidence for direct NO-responsive DNA elements within promotor regions of eukaryotic genes numerous indirect signaling pathways exist to explain NO-regulated gene expression. A characteristic feature of some transcription factors such as hypoxia inducible factor-1α (HIF-1α) or p53 (tumor suppressor p53) is their low protein abundance in unstressed cells due to efficient 26S proteasomal degradation of the protein. Characteristically, the protein amount of HIF-1α or p53 is increased steeply upon hypoxic stress or mechanisms that require activation of “guardian of the genome”, i.e. p53. Current available data illustrate that NO is endowed with the ability to mimic a hypoxic response by stabilizing HIF-1α and / or to accumulate p53 and thus to affect viability decisions. Here we review recent advances in understanding molecular mechanisms how NO affects stability regulation of HIF-1α and p53. Moreover, we summarize existing concepts how HIF-1α and p53 interact to direct proliferation, death or adaptation. Considering HIF-1α and p53 as targets of reactive nitrogen intermediates (RNI) may provide insights into basic chemical reactions, biochemical signal transduction pathways with broad implications for medicine.

Nitric oxide (NO.) and its reaction products are key players in the physiology and pathophysiology of inflammatory settings such as sepsis and shock. The consequences of the expression of inducible NO. synthase (iNOS, NOS-2) can be either protective or damaging to the liver. We have delineated two distinct hepatoprotective actions of NO.: the stimulation of cyclic guanosine monophosphate and the inhibition of caspases by S-nitrosation. In contrast, iNOS / NO. promotes hepatocyte death under conditions of severe redox stress, such as hemorrhagic shock or ischemia / reperfusion. Redox stress activates an unknown molecular switch that transforms NO., which is hepatoprotective under resting conditions, into an agent that induces hepatocyte death. We hypothesize that the magnitude of the redox stress is a major determinant for the effects of NO. on cell survival by controlling the chemical fate of NO.. To address this hypothesis, we have carried out studies in relevant in vivo and in vitro settings. Moreover, we have constructed an initial mathematical model of caspase activation and coupled it to a model describing some of the reactions of NO• in hepatocytes. Our studies suggest that modulation of iron, oxygen, and superoxide may dictate whether NO. is hepatoprotective or hepatotoxic.

The expression of the inducible nitric oxide synthase (iNOS) is one of the direct consequences of an inflammatory process. Early studies have focused on the potential toxicity of the ensuing high-output NO-synthesis serving as a means to eliminate pathogens or tumor cells but also contributing to local tissue destruction during chronic inflammation. More recently, however, data are accumulating on a protective effect of high-output NO synthesis and - equally important - on a generegulatory function that helps to mount a protective stress response and simultaneously aids in downregulating the proinflammatory response. These findings appear to contrast to the often observed sustained iNOS-expression during chronic inflammatory diseases, as for instance in Psoriasis vulgaris and other conditions with a chronic Th1-like reactivity. We here pose the question as to whether the iNOS is really active in these diseases. We review the data accumulated on iNOS expression in chronic diseases. We also report on the various factors that potentially interfere with proper NO formation by the expressed enzyme. We also highlight the recent findings of how, why and where evidences emerge that impeded NO formation contributes to chronic disease processes and finally present details on our current understanding of such abnormally low NO synthesis and its contribution to the pathophysiological processes of the human proinflammatory skin disease Psoriasis vulgaris.

Atherosclerosis is an inflammatory disorder, and the inflammation associated with systemic lupus erythematosus (SLE) accelerates the development of atherosclerosis. Nitric oxide (NO) is an important mediator of inflammation including the inflammation associated with atherosclerosis and SLE. Endothelial nitric oxide synthase (NOS3)-mediated constitutive expression of NO promotes endothelial integrity and normal vascular function. In contrast, inducible nitric oxide synthase- (NOS2) mediated expression of NO promotes endothelial dysfunction and atherogenesis. Statins appear to have antiinflammatory properties and reverse many of the deleterious effects associated with NO metabolism in atherosclerosis. Statins augment NOS3 expression and inhibit the induction of NOS2. Therefore, the balance between normal vascular function and atherogenesis may be mediated by differences in the quantity, location, and timing of NO production within vessel walls.

Malaria has re-emerged as a global health problem, leading to an increased focus on the cellular and molecular biology of the mosquito Anopheles and the parasite Plasmodium with the goal of identifying novel points of intervention in the parasite life cycle. Anti-parasite defenses mounted by both mammalian hosts and Anopheles can suppress the growth of Plasmodium. Nonetheless, the parasite is able to escape complete elimination in vivo, perhaps by thwarting or co-opting these mechanisms for its own survival, as do numerous other pathogens. Among the defense systems used by the mammalian host against Plasmodium is the synthesis of nitric oxide (NO), catalyzed by an inducible NO synthase (iNOS). Nitric oxide produced by the action of an inducible Anopheles stephensi NO synthase (AsNOS) may be central to the anti-parasitic arsenal of this mosquito. In mammals, iNOS can be modulated by members of the transforming growth factor-β (TGF-β) cytokine superfamily. Transforming growth factor-β is produced as an inactive precursor that is activated following dissociation of certain inhibitory proteins, a process that can be promoted by reaction products of NO as well as by hemin. Ingestion by Anopheles of blood containing Plasmodium initiates parasite development, blood digestion which results in the accumulation of hematin (hemin) in the insect midgut, and induction of both AsNOS and TGF-β-like (As60A) gene expression in the midgut epithelium. Active mammalian TGF-β1 can be detected in the A. stephensi midgut up to 48h postingestion and latent TGF-β1 can be activated by midgut components in vitro, a process that is potentiated by NO and that may involve hematin. Further, mammalian TGF-β1 is perceived as a cytokine by A. stephensi cells in vitro and can alter Plasmodium development in vivo. Bloodfeeding by Anopheles, therefore, results in a juxtaposition of evolutionarily conserved mosquito and mammalian TGF-β superfamily homologs that may influence transmission dynamics of Plasmodium in endemic regions.