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Brief Reviews |
From the Department of Anesthesiology and Molecular Pharmacology and Experimental Therapeutics (Z.S.K.); Department of Medicine, Division of Cardiovascular Diseases (N.M.C.); Division of Nephrology (K.A.N.); and Molecular Medicine Program (Z.S.K., K.A.N.), Mayo Clinic, Rochester, Minn.
Correspondence to Zvonimir S. Katusic, MD, PhD, Department of Anesthesiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905. E-mail katusic.zvonimir{at}mayo.edu
Series Editor: Frank M. Faraci
ATVB In Focus
Endothelium: Signaling, Oxidative Stress, and Gene Expression
Previous Brief Reviews in this Series:
Wolfrum S. Jensen KS, Liao JK. Endothelium-dependent effects of statins. 2003;23:729736.
Frank PG, Woodman SE, Park DS, Lisanti MP. Caveolin, caveolae, and endothelial cell function. 2003;23:11611168.
| Abstract |
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Key Words: adenovirus endothelium superoxide anion protein kinase Akt vascular endothelial growth factor
| Introduction |
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| Definitions |
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Indirect gene transfer (natural gene transfer) is defined as any method that transfers genes into recipient genomes of target organisms by using biological vectors (eg, viruses).2 Transduction is defined as the abortive (nonreplicative or dead-end) viral infection that introduces functional genetic information expressed from the recombinant vectors into the target cell.3 In this review, we will focus mostly on in vivo adenovirus-mediated gene transfer to vascular endothelium. Although numerous studies have been performed on cultured endothelial cells using direct gene transfer methods, this approach has not been very successful in achieving high levels of recombinant protein expression under in vivo conditions. In contrast, virus-mediated gene delivery into endothelial cells has been extensively used to study vascular biology of endothelium and to test the potential for therapeutic application of expressed recombinant proteins.
| In Vivo Gene Delivery to Vascular Endothelium |
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Validation of this concept opened up exciting new prospects for studying expression and function of novel proteins in vascular endothelium. Coupled with the acceleration of the Human Genome Project in the mid 1990s, it became clear that gene transfer technology provided a feasible approach in elucidating a key issue in genomics and proteomics: what is the function of a given gene or protein? The advent of adenovirus-based vectors provided additional stimulus for the application of gene transfer technology in addressing basic questions of vascular biology. Consistent high-efficiency transduction of nonreplicating cells in vivo by adenovirus as well as ability to obtain high-titer adenoviral preparations enabled numerous investigators to routinely use this technique to study pathogenesis of vascular disease. A recent report by Falkenberg et al11 nicely illustrates this point; whereas in vivo overexpression of urokinase plasminogen activator (uPA) in rabbit carotid artery endothelial cells decreased intravascular thrombosis, expression of uPA is increased in atherosclerotic human arteries. The latter issue raised the question of the pathogenetic significance of uPA in atherosclerotic vessels. To address this question regarding the functional significance of uPA in atherogenesis, recombinant uPA was expressed in carotid arteries of cholesterol-fed rabbits. The results demonstrated that, indeed, the high expression of uPA in endothelial cells contributes to intimal growth and constrictive remodeling, resulting in lumen loss. Thus, although the overexpression of uPA may exert the seemingly beneficial effect of mitigating intravascular thrombosis, overexpression of uPA, for as yet unclear reasons, may accelerate atherosclerosis. Obviously, genetic modification of endothelial function in vivo offers a powerful tool that will continue to have a major impact on experimental vascular biology.
| Gene Transfer of Recombinant Nitric Oxide Synthase |
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| eNOS Gene Transfer |
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Adenovirus-mediated gene transfer proved to be a valuable tool in characterization of signal transduction pathways involved in activation of eNOS. Studies of cultured endothelial cells demonstrated that vascular endothelial growth factor (VEGF) and physiological fluid shear stress stimulate production of nitric oxide via the phosphorylation of eNOS by serine/threonine protein kinase Akt (protein kinase B).24,25 To test the importance of this concept for control of blood flow in vivo, Luo et al26 made adenoviral constructs encoding constitutively active Akt and dominant-negative mutant Akt that cannot be activated by phosphorylation. In vivo expression of constitutively active Akt in endothelial cells of rabbit femoral artery increased baseline lumen diameter, elevated cyclic GMP in the vascular wall, and increased resting blood flow. In contrast, expression of dominant-negative Akt impaired endothelium-dependent relaxations mediated by nitric oxide and inhibited acetylcholine-induced increase in blood flow. These studies established an important role for Akt in control of nitric oxide production and local blood flow in vivo.
Studies by Fulton et al24 and Dimmeler et al25 demonstrated that eNOS serine 1179 (bovine) or 1177 (human) is a target for phosphorylation by Akt. Mutation of this residue to aspartate constitutively increased eNOS enzymatic activity. Adenoviral vectors encoding phosphomimetic eNOS or nonphosphorylatable eNOS obtained by mutating serine 1179 to an alanine residue have been successfully used in studies designed to characterize the role of eNOS phosphorylation in basal as well as stimulated production of nitric oxide in vivo.27 By overexpressing different eNOS mutants in the carotid artery of eNOS knockout animals, these investigators demonstrated that eNOS phosphorylation at serine residue 1179 is an important mechanism regulating basal eNOS activity and activation of eNOS by VEGF and acetylcholine. Adenovirus-mediated overexpression of nonphosphorylatable eNOS served as a negative control and provided important additional evidence for the key role of serine 1179 phosphorylation in control of nitric oxide production. This resulted in elevation of basal cyclic GMP and inhibition of vasoconstrictor effect of endothelin-1. Impaired function of protein kinase B (Akt) has been observed in endothelial cells exposed to hypercholesterolemia,28,29 aging,30 and high glucose.31 Indeed, overexpression of a recombinant phosphomimetic eNOS inhibits apoptosis in aged endothelial cells30 and inhibits vasoconstrictor effect of endothelin-1 in cerebral arteries.32 These observations suggest that gene therapy strategies using the constitutively active phosphomimetic eNOS may have additional value in the prevention and treatment of vascular diseases caused by inactivation of the protein kinase Akt.
| iNOS Gene Transfer |
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10 fold; Figure 2), most likely reflecting high production of nitric oxide.32 As a consequence, vascular reactivity to vasoconstrictor agonists, phenylephrine, U46619, and uridine 5'-triphosphate is significantly reduced.35,36 Endothelium-dependent relaxations are also impaired in arteries expressing iNOS.35,36 The exact mechanism of this impairment is not completely understood but may be attributable to reduced reactivity of smooth muscle cells to nitric oxide or increased formation of superoxide anions.36 The source of superoxide anions in arteries transduced with iNOS is unclear. It is likely that a component of higher superoxide anion production can be attributable to uncoupling of iNOS. Suboptimal concentrations of NOS substrate, L-arginine, or cofactor tetrahydrobiopterin favor uncoupling of NADPH consumption from nitric oxide synthesis.37 This uncoupling may lead to iNOS-mediated reduction of oxygen and formation of superoxide anions (Figure 3). Other reasons for increased superoxide anion production including reduction of antioxidants or increased activity of NAD(P)H or xanthine oxidase cannot be ruled out and remain to be determined. Regardless of underlying mechanisms, simultaneous production of nitric oxide and superoxide anions creates favorable conditions for formation of a potent oxidant, peroxynitrite. Thus, although during inflammation initial upregulation of iNOS could be an adaptive response designed to preserve production of nitric oxide, sustained high activity of iNOS may have a detrimental effect on vascular function by imposing oxidative stress. Indeed, consistent with this hypothesis is the observation that inactivation of iNOS gene in apolipoprotein Edeficient mice retards progression and severity of atherosclerosis.38,39
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It is important to keep in mind that delivering iNOS gene into normal blood vessels causes "out of context" expression of this NOS isoform.40 Although such "out of context" expression of iNOS is clearly a useful strategy to interrogate physiological and pathophysiological questions, there are caveats that need to be considered. For example, under pathological conditions, proinflammatory stimuli cannot only upregulate expression of the iNOS gene but also many proteins that are important for iNOS enzymatic activity or vascular effects of nitric oxide, including the L-arginine transporter or GTP-cyclohydrolase I, the rate-limiting enzyme in synthesis of tetrahydrobiopterin.41 Expression of these genes may not increase in normal arteries transduced with recombinant iNOS, creating favorable conditions for uncoupling of iNOS. Thus, adenovirus-mediated gene transfer of a specific protein may help to unmask effects arising from the protein of interest, which are hidden when protein is expressed in the milieu of vascular disease. However, overexpressing an inducible protein in normal arteries does not recapitulate the true context surrounding the upregulation of protein in the diseased vasculature. These caveats need to be kept in mind when assessing the pathophysiologic significance of "out of context" expression of specific protein.
| nNOS Gene Transfer |
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| Limitations of Adenovirus-Mediated Gene Transfer |
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Another important limitation of the adenovirus is its ability to induce expression of a variety of endothelial genes. Ramalingam et al53 used a cDNA subtraction library to isolate cDNAs differentially expressed in primary HUVECs exposed to adenovirus and demonstrated that adenovirus upregulates expression of the proteins that participate in intracellular signaling, growth regulation, and organization of cytoskeleton. Thus, it is likely that adenoviral vectors induce expression of target cell genes, some of which may modify endothelial function and interfere with interpretation of the transgene specific results. Performing proper control experiments including comparison between effects of null adenoviral vectors with adenovirus encoding the gene of interest is therefore essential. However, development of improved vectors in the future will certainly improve our ability to perform gene transfer experiments with a minimum of unwanted effects.
| Summary |
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| Acknowledgments |
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This work was supported by the National Institutes of Health (HL-53524, HL-58080, HL-55552, HL-066958, and DK-47050), the American Heart Association Bugher Award for Investigation of Stroke, and the Mayo Foundation. The authors would like to thank Dr Sean Hynes for helpful discussions, Leslie Smith for preparation of figures, and Janet Beckman for editorial assistance.
Received February 28, 2003; accepted May 12, 2003.
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