Regulation of endothelial nitric-oxide synthase during hypoxia

Biosynthesis and action of nitric oxide in mammalian cells

Nitric oxide (NO) can act as a vasorelaxant, a modulator of neurotransmission and a defence against pathogens. However, under certain conditions, NO can also have damaging effects to cells. Whether NO is useful or harmful depends on its chemical fate, and on the rate and location of its production. Here, we discuss progress in NO chemistry and the enzymology of NO synthases, and we will also attempt to explain its actions in the cardiovascular, nervous and immune systems.

Expression of endothelial constitutive nitric oxide synthase mRNA in gastrointestinal mucosa and its downregulation by endotoxin

Endothelial nitric oxide exerts local vasodilatory actions in the gastrointestinal (GI) microvasculature and is proposed to play a role in enteric vasomotor regulation. The aims of this study were to characterize the tissue distribution of mRNA for endothelial nitric oxide synthase (NOS-III) and to examine its response to endotoxin challenge in vivo. We demonstrate the expression of NOS-III mRNA and protein in mucosa throughout the gastrointestinal tract and show for the first time that NOS-III mRNA expression in the GI mucosa was down-regulated in the rats treated with endotoxin. The ubiquitous expression of NOS-III mRNA in digestive tissues is consistent with the proposed role of NOS-III in the physiology of the gastrointestinal tract. The decreased NOS-III mRNA, in parallel to induction of inducible NOS (NOS-II) mRNA, may contribute to the impaired endothelium-dependent relaxation and damaged mucosal integrity during sepsis.

Hypoxia stimulates ecNOS mRNA expression by differentiated human trophoblasts

Cytotrophoblasts isolated from normal human placenta cultured under normoxic conditions (20% O2, pO2 = 130 mmHg) for 48-72 h differentiate to a form which expresses high levels of hCG and which morphologically resembles syncytiotrophoblast. We had previously shown that hypoxia (0-1% O2, pO2 = 12-14 mmHg) blocks this differentiation process, although trophoblasts exposed to hypoxia for up to 96 h were completely viable. In this article we showed that trophoblast responds to hypoxia by expressing the hypoxia-sensitive DNA binding protein HIF-1. We also showed that in trophoblast cultured under normoxic conditions, expression of endothelial cell nitric oxide synthase (ecNOS) mRNA increases with time, reaching a maximum in 48-72 h. However, in trophoblast maintained under hypoxic conditions for 48 h (after an initial 24 h in normoxia), expression of ecNOS mRNA is greatly reduced. These observations are consistent with the expression of ecNOS by syncytiotrophoblast but not by cytotrophoblast. In contrast, exposure of differentiated trophoblasts to hypoxia for 24 h (after 48-72 h in normoxia) significantly stimulates expression of ecNoS mRNA over that of cells maintained continuously in normoxia. These results suggest that in differentiated trophoblast hypoxia can stimulate ecNOS expression.

Nitric oxide in renal health and disease

Nitric oxide (NO) is a labile radical gas that is widely acclaimed as one of the most important molecules in biology. Through covalent modifications of target proteins and redox reactions with oxygen and superoxide radical and transition metal prosthetic groups, NO plays a critical role in many vital biological processes, including the control of vascular tone, neurotransmission, ventilation, hormone secretion, inflammation, and immunity. Moreover, NO has been shown to influence a host of fundamental cellular functions, such as RNA synthesis, mitochondrial respiration, glycolysis, and iron metabolism. NO is formed from L-arginine by NO synthases (NOSs), a family of related enzymes encoded by separate unlinked genes. The different NOS isozymes exhibit disparate tissue and intrarenal distributions and are governed by unique regulatory mechanisms. In the kidney, NO participates in several vital processes, including the regulation of glomerular and medullary hemodynamics, the tubuloglomerular feedback response, renin release, and the extracellular fluid volume. While NO serves beneficial roles as a messenger and host defense molecule, excessive NO production can be cytotoxic, the result of NO's reaction with reactive oxygen and nitrogen species, leading to peroxynitrite anion formation, protein tyrosine nitration, and hydroxyl radical production. Indeed, NO may contribute to the evolution of several commonly encountered renal diseases, including immune-mediated glomerulonephritis, postischemic renal failure, radiocontrast nephropathy, obstructive nephropathy, and acute and chronic renal allograft rejection. Moreover, impaired NO production has been implicated in the pathogenesis of volume-dependent hypertension. This duality of NO's beneficial and detrimental effects has created extraordinary interest in this molecule and the need for a detailed understanding of NO biosynthesis.

Interrelation between nitric oxide synthase and heme oxygenase in rat endothelial cells

The gene expression and interrelation of the constitutive type nitric oxide (NO) synthase-III as a NO-forming enzyme and heme oxygenase-2 as a carbon monoxide-forming enzyme were studied in cultured rat aortic endothelial cells. Both NO synthase-III and heme oxygenase-2 mRNAs were demonstrated in the endothelial cells by RNAase protection analysis. NO synthase-III mRNA was upregulated in the presence of the heme oxygenase inhibitor, zinc protoporphyrin IX, but not in the presence of the NO synthase inhibitor, N(G)-nitro-L-arginine. Although heme oxygenase-2 mRNA was significantly upregulated in the presence of both NO synthase inhibitor and heme oxygenase inhibitor, the increase was greater with the NO synthase inhibitor. These results provide the first evidence for the concomitant gene expression of NO synthase-III and heme oxygenase-2, and their compensatory interrelation in endothelial cells.

Vascular dysfunction induced by elevated glucose levels in rats is mediated by vascular endothelial growth factor

The purpose of these experiments was to investigate a potential role for vascular endothelial growth factor (VEGF) in mediating vascular dysfunction induced by increased glucose flux via the sorbitol pathway. Skin chambers were mounted on the backs of Sprague-Dawley rats and 1 wk later, granulation tissue in the chamber was exposed twice daily for 7 d to 5 mM glucose, 30 mM glucose, or 1 mM sorbitol in the presence and absence of neutralizing VEGF antibodies. Albumin permeation and blood flow were increased two- to three-fold by 30 mM glucose and 1 mM sorbitol; these increases were prevented by coadministration of neutralizing VEGF antibodies. Blood flow and albumin permeation were increased approximately 2.5-fold 1 h after topical application of recombinant human VEGF and these effects were prevented by nitric oxide synthase (NOS) inhibitors (aminoguanidine and N(G)-monomethyl L-arginine). Topical application of a superoxide generating system increased albumin permeation and blood flow and these changes were markedly attenuated by VEGF antibody and NOS inhibitors. Application of sodium nitroprusside for 7 d or the single application of a calcium ionophore, A23187, mimicked effects of glucose, sorbitol, and VEGF on vascular dysfunction and the ionophore effect was prevented by coadministration of aminoguanidine. These observations suggest a potentially important role for VEGF in mediating vascular dysfunction induced by "hypoxia-like" cytosolic metabolic imbalances (reductive stress, increased superoxide, and nitric oxide production) linked to increased flux of glucose via the sorbitol pathway.

Progress in transcriptionally targeted and regulatable vectors for genetic therapy

Safety is an important consideration in the development of genetic therapy protocols; for example, proteins that are therapeutic in the context of one tissue may be harmful in another. This is particularly relevant to suicide gene strategies for cancer, which require in vivo delivery of DNA and which, in general, demand that the therapeutic product be limited as far as possible to malignant cells. This has led to a requirement for "transcriptionally targeted" vectors that can restrict the expression of the therapeutic sequence to appropriate cells. Furthermore, there may be a therapeutic window for certain proteins such that levels of expression below and above certain thresholds may be ineffective or toxic, respectively. Therefore, it would also be desirable to create vectors that allow exogenous control of expression, so that levels of the therapeutic protein can be raised or lowered according to therapeutic need. In the context of transcriptional targeting, one may sometimes use cis-acting sequences to limit transgene expression to the target cell type. In genetic therapy for cancer, for example, it may be possible to identify and use transcriptional control elements that drive expression of proteins unique to, or over-expressed in, malignant cells. These controls would greatly reduce collateral expression of the transgene, and hence reduce toxicity to healthy cells. With regard to exogenous control of expression subsequent to transduction, several synthetic gene regulation systems have now been produced. In these systems, an inducer or repressor acts on a synthetic transcription factor that recognizes motifs unique to the promoter of the transgene; this allows regulated expression of the therapeutic protein without nonspecific effects on cellular promoters. It is likely that a vector will soon be produced in which tissue-restricted expression of the synthetic transcription factor is combined with regulatable transgene expression thereby allowing precise control of therapeutic protein production in specific tissues via administration of an inducing or repressing agent.

The nature of renal cell injury

The main functional change in patients with acute renal failure (ARF) is a decrease in glomerular filtration rate (GFR). The virtual complete recovery of renal function in those patients who survive ARF, as well as the minimal renal histological abnormalities during ARF when the GFR is less than 10 ml/min, suggest that a major component of the renal tubular cell injury is sublethal or reversible. Experimental models of acute tubular necrosis frequently have placed the emphasis on irreversible proximal tubular cell death. The nature of the renal tubular cell injury in ischemic acute renal failure, however, includes not only cell death (necrosis or apoptosis) but also sublethal injury causing cell dysfunction. The role of intracellular calcium, the calcium-dependent enzymes calpain, phospholipase A2 and nitric oxide synthase (NOS), in the pathophophysiology of this renal tubular cell injury during hypoxia/ischemia is described. The effects of calpain and nitric oxide (NO) on the cytoskeleton and cell adhesion are discussed. Potential mechanisms whereby tubular injury leads to a profound fall in GFR, including increased tubuloglomerular feedback and increased distal tubular obstruction, in ischemic acute renal failure are proposed.