Targeting of nitric oxide synthase to endothelial cell caveolae via palmitoylation: implications for nitric oxide signaling.

The membrane association of endothelial nitric oxide synthase (eNOS) plays an important role in the biosynthesis of nitric oxide (NO) in vascular endothelium. Previously, we have shown that in cultured endothelial cells and in intact blood vessels, eNOS is found primarily in the perinuclear region of the cells and in discrete regions of the plasma membrane, suggesting trafficking of the protein from the Golgi to specialized plasma membrane structures. Here, we show that eNOS is found in Triton X-100-insoluble membranes prepared from cultured bovine aortic endothelial cells and colocalizes with caveolin, a coat protein of caveolae, in cultured bovine lung microvascular endothelial cells as determined by confocal microscopy. To examine if eNOS is indeed in caveolae, we purified luminal endothelial cell plasma membranes and their caveolae directly from intact, perfused rat lungs. eNOS is found in the luminal plasma membranes and is markedly enriched in the purified caveolae. Because palmitoylation of eNOS does not significantly influence its membrane association, we next examined whether this modification can affect eNOS targeting to caveolae. Wild-type eNOS, but not the palmitoylation mutant form of the enzyme, colocalizes with caveolin on the cell surface in transfected NIH 3T3 cells, demonstrating that palmitoylation of eNOS is necessary for its targeting into caveolae. These data suggest that the subcellular targeting of eNOS to caveolae can restrict NO signaling to specific targets within a limited microenvironment at the cell surface and may influence signal transduction through caveolae.     

Novel mechanism of endothelial nitric oxide synthase activation mediated by caveolae internalization in endothelial cells

Caveolin-1, the caveolae scaffolding protein, binds to and negatively regulates eNOS activity. As caveolin-1 also regulates caveolae-mediated endocytosis after activation of the 60-kDa albumin-binding glycoprotein gp60 in endothelial cells, we addressed the possibility that endothelial NO synthase (eNOS)-dependent NO production was functionally coupled to caveolae internalization. We observed that gp60-induced activation of endocytosis increased NO production within 2 minutes and up to 20 minutes. NOS inhibitor N(G)-nitro-L-arginine (L-NNA) prevented the NO production. To determine the role of caveolae internalization in the mechanism of NO production, we expressed dominant-negative dynamin-2 mutant (K44A) or treated cells with methyl-beta-cyclodextrin. Both interventions inhibited caveolae-mediated endocytosis and NO generation induced by gp60. We determined the role of signaling via Src kinase in the observed coupling of endocytosis to eNOS activation. Src activation induced the phosphorylation of caveolin-1, Akt and eNOS, and promoted dissociation of eNOS from caveolin-1. Inhibitors of Src kinase and Akt also prevented NO production. In isolated perfused mouse lungs, gp60 activation induced NO-dependent vasodilation, whereas the response was attenuated in eNOS(-/-) or caveolin-1(-/-) lungs. Together, these results demonstrate a critical role of caveolae-mediated endocytosis in regulating eNOS activation in endothelial cells and thereby the NO-dependent vasomotor tone.     

Red blood cells express a functional endothelial nitric oxide synthase

The synthesis of nitric oxide (NO) in the circulation has been attributed exclusively to the vascular endothelium. Red blood cells (RBCs) have been demonstrated to carry a nonfunctional NO synthase (NOS) and, due to their huge hemoglobin content, have been assumed to metabolize large quantities of NO. More recently, however, RBCs have been identified to reversibly bind, transport, and release NO within the cardiovascular system. We now provide evidence that RBCs from humans express an active and functional endothelial-type NOS (eNOS), which is localized in the plasma membrane and the cytoplasm of RBCs. This NOS is regulated by its substrate L-arginine, by calcium, and by phosphorylation via PI3 kinase. RBC-NOS activity regulates deformability of RBC membrane and inhibits activation of platelets. The NOS-dependent conversion of L-arginine in RBCs is comparable to that of cultured human endothelial cells. RBCs in eNOS-/- mice in contrast to wild-type mice lack NOS protein and activity, strengthening the evidence of an eNOS in RBCs. These data show an eNOS-like protein and activity in RBCs serving regulatory functions in RBCs and platelets, which may stimulate new approaches in the treatment of NO deficiency states inherent to several vascular and hematologic diseases.     

Estrogen modulates endothelial and neuronal nitric oxide synthase expression via an estrogen receptor beta-dependent mechanism in hypothalamic slice cultures

Although it is evident that estrogen has important physiological effects in the brain, the signaling mechanisms mediating these effects remain unclear. We recently showed that estrogen mediates attenuated blood pressure responses to psychological stress in ovariectomized female rats through brain nitric oxide (NO). An area likely to mediate these effects is the hypothalamic paraventricular nucleus (PVN), because here NO exerts inhibitory effects on autonomic output to the periphery. Because little is known about how estrogen acts on the NO system in the PVN, our aim was to study the effects of estrogen on the NO system in the PVN of hypothalamic slices cultures. We show that 17beta-estradiol (E2; 1 nm) increases endothelial NO synthase (eNOS) protein expression and decreases the numbers of neuronal NOS (nNOS)-positive neurons in the PVN after 8 and 24 h, respectively. Using the nonselective estrogen receptor (ER) antagonist, ICI 182,780 (10 nm), we determined that E2-induced changes in NOS expression in the PVN are ER dependent. Using the ERbeta agonist, genistein (0.1 microm), we determined that activation of ERbeta induces increased eNOS expression and a decreased number of nNOS-positive neurons. We used the selective ERalpha agonist, propyl-pyrazole-triol (10 nm), and antagonist, methyl-piperidino-pyrazole (1 microm), to exclude the possibility that ERalpha is involved in the E2-induced increase in eNOS and nNOS in the PVN. These results demonstrate that E2 induces changes in NOS expression in the PVN and that these effects are ERbeta dependent.     

Endothelial nitric oxide synthase and endothelial dysfunction

Nitric oxide (NO) regulates vascular tone and local blood flow, platelet aggregation and adhesion, and leukocyte-endothelial cell interactions. Abnormalities in NO production by the vascular endothelium result in endothelial dysfunction, which occurs in hypertension, diabetes, aging, and as a prelude to atherosclerosis. The common feature of endothelial dysfunction is a decrease in the amount of bioavailable NO. In this article, the physiologic roles of NO and the mechanisms of endothelial dysfunction are reviewed. Regulation of endothelial NO synthase (eNOS) activity by fatty acid modifications, intracellular localization, interactions with heat shock protein 90 (hsp90) and caveolin, substrate and cofactor dependence, and phosphorylation might all affect the level of bioavailable NO. A hypothesis is proposed that the final common pathway of diverse causes of endothelial dysfunction involves abnormalities in eNOS phosphorylation at Ser 1179 and other key phosphorylation sites     

Fas signaling induces Akt activation and upregulation of endothelial nitric oxide synthase expression

A growing body of evidence has shown that Fas, a death receptor, mediates apoptosis-unrelated biological effects. Here, we report that Fas engagement with Fas ligand induced activation of Akt and upregulation of endothelial nitric oxide synthase expression without induction of apoptosis. In the presence of the phosphatidylinositol 3-kinase inhibitor wortmannin, Fas ligand, however, induced apoptosis instead of upregulation of endothelial nitric oxide synthase expression. In vivo, systolic blood pressure was slightly higher in mutant mice with decreased cell surface Fas expression (lpr mice) compared with wild-type mice. In addition, chronic inhibition of nitric oxide synthesis by N(G)-nitro-l-arginine induced a progressive increase in the levels of blood pressure in wild-type mice, whereas no further increase in the levels of blood pressure was observed in lpr mice. Furthermore, acetylcholine caused a lesser endothelium-dependent relaxation of the strips from lpr mice compared with wild-type mice, although the vasoconstrictor potency of phenylephrine was not different between the two groups. These findings indicate that Fas signaling may have a role in the regulation of endothelial function and blood pressure through modulating endothelial nitric oxide synthase expression in the Akt signal-dependent manner.     

Endothelial nitric oxide synthase and caveolin-1 are co-localized in sinusoidal endothelial fenestrae.

BACKGROUND/AIMS: Nitric oxide is synthesized in diverse mammalian tissues by a family of calmodulin-dependent nitric oxide synthases (NOS). Caveolin, the principal structural protein in caveolae, interacts with endothelial NOS leading to enzyme inhibition in a reversible process modulated by Ca++-calmodulin. The aim of the present study was to clarify the ultrastructural localization of eNOS and caveolin-1 in hepatic sinusoidal endothelium by an electron immunogold method. METHODS: Male Wistar rats were used. Liver tissues and hepatic sinusoidal endothelial cells isolated from rat livers by collagenase infusion were studied. For immunohistochemistry, liver specimens were reacted with anti-eNOS or anti-caveolin-1 antibody. The ultrastructural localization of eNOS or caveolin-1 was identified by electron microscopy using an immunogold post-embedding method. RESULTS: Immunohistochemical studies using liver tissues localized endothelial NOS in hepatic sinusoidal lining cells, portal veins and hepatic arteries; and caveolin-1 in sinusoidal lining cells, bile canaliculi, portal vein and hepatic arteries. Immunogold particles indicating the presence of eNOS and caveolin-1 were demonstrated on the plasma membrane of sinusoidal endothelial fenestrae in liver tissue and also in isolated sinusoidal endothelial cells. CONCLUSION: Endothelial NOS and caveolin are co-localized on sinusoidal endothelial fenestrae, suggesting that interaction of the two may modulate cellular regulation of NO synthesis.     

The central hypotensive effect induced by alpha 2-adrenergic receptor stimulation is dependent on endothelial nitric oxide synthase

OBJECTIVE: The aim of the present study was to determine whether the central antihypertensive effect of drugs that act via central alpha 2-adrenergic receptors is mediated by the nitric oxide-ergic system. METHODS: The hypotensive effects of dexmedetomidine, a 'pure' alpha2-adrenergic agonist, were compared in endothelial nitric oxide synthase knockout and wild-type control mice. RESULTS: When injected intravenously (5 mug/kg) in wild-type mice, dexmedetomidine elicited a depressor response (60 +/- 4 to 34 +/- 1 mmHg, P < 0.05), but had no hypotensive effect in endothelial nitric oxide synthase (eNOS) knockout mice (84 +/- 7 to 84 +/- 7 mmHg, P > 0.05). In the presence of N-omega-nitro-L-arginine, a nonselective nitric oxide synthase (NOS) blocker that does not cross the blood-brain barrier, the hypotensive effect of dexmedetomidine was not abolished (Delta MAP = 21 +/- 2 mmHg vs. Delta MAP = 26 +/- 3 mmHg, P > 0.05). CONCLUSIONS: It is concluded that the central cardiovascular effects of alpha 2-adrenergic agonists, such as dexmedetomidine, require an intact expression of eNOS within the brain. This study raises the interesting question of whether central eNOS itself might be considered as a target for new cardiovascular drugs regardless of any activation of alpha 2-adrenergic receptors.     

Expression of endothelial nitric oxide synthase and inducible nitric oxide synthase in synovium of rheumatoid arthritis]

OBJECTIVES: To examine the localization and distribution of endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS), which participate in nitric oxide (NO) production, in synovium of rheumatoid arthritis (RA). MATERIALS AND METHODS: Immunohistochemical analysis for eNOS and iNOS in synovial tissues obtained from 10 patients with RA who were underwent total knee replacement. Synovial tissues of osteoarthritis (OA) were used as control. The percentage of cells that were positive for eNOS and iNOS was estimated in five hundred endothelial cells, synovial lining cells and interstitial cells, respectively. And mRNA expression of NOS was confirmed by in situ hybridization. In addition, to test NO production, nitration of tyrosines was assessed by immunohistochemistry. RESULTS: Not only endothelial cells but also synovial lining cells and interstitial cells exhibited immune-reactive both eNOS and iNOS. Cells which were seemed immune-reactive eNOS and iNOS expressed nitrotyrosin. By in situ hybridization, we detected mRNA expression for eNOS and iNOS. CONCLUSIONS: Endothelial cells, synovial lining cells and interstitial cells expressed both eNOS and iNOS with high frequency in RA synovium compared with OA synovium. It seemed to correlate with NO production. These results suggest that expression of iNOS may be involved in the induction of arthritis and eNOS may be participated in augmentation of inflammation in RA.     

Expression and regulation of endothelial nitric oxide synthase

Endothelium-derived nitric oxide (NO) is a key determinant of blood pressure homeostasis and platelet aggregation and is synthesized by the endothelial isoform of nitric oxide synthase (eNOS). In the vascular wall, eNOS is activated by diverse cell-surface receptors and by increases in blood flow, and the consequent generation of NO leads to vascular smooth-muscle relaxation. Endothelium-dependent vasorelaxation is deranged in a variety of disease states, including hypertension, diabetes, and atherosclerosis, but the roles of eNOS in endothelial dysfunction remain to be clearly defined. The past several years have witnessed important advances in understanding the molecular and cellular biology of eNOS regulation. In endothelial cells, eNOS undergoes a complex series of covalent modifications, including myristoylation, palmitoylation, and phosphorylation. Palmitoylation of eNOS dynamically targets the enzyme to distinct domains of the endothelial plasma membrane termed caveolae; caveolae may serve as sites for the sequestration of signal-transducing proteins and are themselves subject to dynamic regulation by ligands and lipids. Originally thought to be expressed only in endothelial cells, eNOS is now known to be expressed in a variety of tissues, including blood platelets, cardiac myocytes, and brain hippocampus. Paradigms established in endothelial cells for the molecular regulation and subcellular targeting of eNOS are being extended to the investigation of eNOS expressed in nonendothelial tissues. This review summarizes recent advances in understanding the molecular regulation of eNOS and the other NOS isoforms and identifies important parallels between eNOS and other cell-signaling molecules. ? 1997, Elsevier Science Inc. (Trends Cardiovasc Med 1997;7:28-37).