Leukocyte integrin Mac-1 recruits toll/interleukin-1 receptor superfamily signaling intermediates to modulate NF-kappaB activity

The leukocyte integrin Mac-1 (alphaMbeta2, CD11b/CD18) regulates important cell functions in inflammation, including adhesion, phagocytosis, and oxidative burst. Deficiency of Mac-1 reduces vessel wall inflammation and neointimal thickening after murine carotid artery injury. Although Mac-1 has been implicated in modulating AP-1 and NF-kappaB activity, the signal transduction pathways involved are undefined. cDNA array analysis of Mac-1-clustered compared with -nonclustered monocytic THP-1 cells showed increased expression of the signal transducer TRAF6 (TNF receptor-associated factor 6), leading us to consider the possibility that Mac-1 used a Toll/IL-1 receptor family-like signaling pathway. Mac-1-dependent activation of NF-kappaB was potentiated by wild-type, and attenuated by dominant negative, TRAF6- and TGF-beta-activated kinase (TAK1) constructs. IRAK1 (IL-1 receptor associated kinase), a kinase immediately upstream of TRAF6, coimmunoprecipitated with Mac-1. Taken together, these observations indicate that Mac-1 recruits a Toll/IL-1 receptor family-like cascade to modulate NF-kappaB activity. This represents a new pathway for integrin-dependent modulation of gene expression.

Simplified risk score models accurately predict the risk of major in-hospital complications following percutaneous coronary intervention

The objectives of this analysis were to develop and validate simplified risk score models for predicting the risk of major in-hospital complications after percutaneous coronary intervention (PCI) in the era of widespread stenting and use of glycoprotein IIb/IIIa antagonists. We then sought to compare the performance of these simplified models with those of full logistic regression and neural network models. From January 1, 1997 to December 31, 1999, data were collected on 4,264 consecutive interventional procedures at a single center. Risk score models were derived from multiple logistic regression models using the first 2,804 cases and then validated on the final 1,460 cases. The area under the receiver operating characteristic (ROC) curve for the risk score model that predicted death was 0.86 compared with 0.85 for the multiple logistic model and 0.83 for the neural network model (validation set). For the combined end points of death, myocardial infarction, or bypass surgery, the corresponding areas under the ROC curves were 0.74, 0.78, and 0.81, respectively. Previously identified risk factors were confirmed in this analysis. The use of stents was associated with a decreased risk of in-hospital complications. Thus, risk score models can accurately predict the risk of major in-hospital complications after PCI. Their discriminatory power is comparable to those of logistic models and neural network models. Accurate bedside risk stratification may be achieved with these simple models.

Photodynamic Therapy With Motexafin Lutetium Induces Redox-Sensitive Apoptosis of Vascular Cells

Abstract —Motexafin lutetium is a photosensitizer that accumulates in atherosclerotic plaque and, after activation by far-red light, produces cytotoxic singlet oxygen. The combination of photosensitizer and illumination, known as photodynamic therapy (PDT), has been shown to reduce atheroma formation in animal models and is under clinical investigation. However, the effects of PDT with motexafin lutetium on isolated vascular cells are unknown. This study was designed to characterize the effects of PDT on vascular cell viability and to define the cell-death pathway for this agent. Fluorescence microscopy of RAW macrophages and human vascular smooth muscle cells revealed time-dependent uptake of motexafin lutetium. Illumination of motexafin lutetium–loaded cells with 732-nm light (2 J/cm 2 ) impaired cellular viability and growth (IC 50 5 to 20 μmol/L). Depletion of intracellular glutathione potentiated ( P =0.035) and the addition of antioxidant N -acetylcysteine attenuated ( P =0.002) cell death, suggesting that the intracellular redox state influences motexafin lutetium action. PDT was associated with the loss of mitochondrial membrane potential, mitochondrial release of cytochrome c , and caspase activation. PDT promoted phosphatidylserine externalization and induced apoptotic DNA fragmentation, with the number of apoptotic cells increasing from 7±2% to 34±3% of total cells. Reducing plaque cellularity by the induction of apoptosis may be one mechanism by which PDT reduces plaque burden, possibly modulates plaque vulnerability, and inhibits restenosis in vivo.

A comparative study of light transmission aggregometry and automated bedside platelet function assays in patients undergoing percutaneous coronary intervention and receiving abciximab, eptifibatide, or tirofiban

Platelet inhibition is central to the efficacy of glycoprotein (GP) IIb-IIIa antagonist therapy, but is not routinely measured during percutaneous coronary intervention (PCI). Data directly comparing the antiplatelet effects of these agents are also limited. Therefore, we compared ex vivo platelet function by standard light transmission aggregometry (LTA) and two automated bedside platelet function assays in 36 patients undergoing PCI with GP IIb-IIIa inhibitors. At baseline and 10 min following clinically recommended bolus and infusion of abciximab (0.25 mg/kg, 0.125 microg/kg/min), eptifibatide (180 microg/kg, 2 microg/kg/min), or tirofiban (10 microg/kg, 0.1 microg/kg/min), we measured 20 microM ADP- and 1.9 mg/mL collagen-induced platelet aggregation using LTA. Platelet function was also assessed using the bedside Accumetrics Ultegra-Rapid Platelet Function Assay (RPFA) and the Xylum Clot Signature Analyzer (CSA). The degree of platelet inhibition, as assessed by LTA, varied significantly between the clinically recommended doses of these GP IIb-IIIa antagonists. RPFA measurements agreed closely with LTA for abciximab, but tended to overestimate the degree of platelet inhibition for small molecules. CSA demonstrated profoundly inhibited shear-induced platelet function, but lacked sensitivity to discriminate between agents. These findings may have implications for the results of trials comparing the efficacy of these agents in patients undergoing PCI.

Neutrophil, not macrophage, infiltration precedes neointimal thickening in balloon-injured arteries

Macrophages are abundant after stent-induced arterial injury. Inhibition of macrophage recruitment blocks neointimal growth in this model. In contrast, after superficial injury from balloon endothelial denudation, macrophages are sparse. However, many anti-inflammatory therapies remain effective against neointimal growth after balloon injury. To investigate further the role of leukocytes after injury, 41 New Zealand White rabbits underwent iliac artery balloon denudation. In 18, subcutaneous pumps were placed to deliver intravenous heparin (0.3 mg/kg per hour). Arteries were harvested at 6 hours and at 3, 7, and 14 days. In 8 animals, either M1/70 (a monoclonal antibody [mAb] against adhesion molecule Mac-1) or a nonspecific IgG was given (5 mg/kg IV bolus and then 1 mg/kg SC QOD), and arteries were harvested at 6 hours and 3 days. Computer-aided morphometry was performed as was immunohistochemistry to assess smooth muscle cell (SMC) proliferation (bromodeoxyuridine-positive cells), neutrophil content (RPN357, mAb against rabbit neutrophil/thymocyte), and macrophage content (RAM-11, mAb against rabbit macrophage). Heparin inhibited neointimal growth at 7 and 14 days (64% and 32.5% reduction, respectively; P:<0.05). Neutrophils were observed in the media early after balloon injury, and heparin and M1/70 inhibited this infiltration (82% and 83% reduction, respectively; P:<0.05 each) with a coincident inhibition of medial SMC proliferation at 3 days (49% and 84% reduction, respectively; P:<0.05 each). Macrophages were absent at all time points. Neutrophil, but not macrophage, infiltration occurs early after endothelial denudation. Inhibition of this process is associated with a reduction in medial SMC proliferation. These data suggest a central role for neutrophils in restenosis and help to explain prior reports of an inhibitory effect of anti-inflammatory therapies on neointimal growth after balloon injury.

Plasminogen is a critical determinant of vascular remodeling in mice

Extracellular proteolysis is likely to be a feature of vascular remodeling associated with atherosclerotic and restenotic arteries. To investigate the role of plasminogen-mediated proteolysis in remodeling, polyethylene cuffs were placed around the femoral arteries of mice with single and combined deficiencies in plasminogen and fibrinogen. Neointimal development occurred in all mice and was unaffected by genotype. Significant compensatory medial remodeling occurred in the cuffed arteries of control mice but not in plasminogen-deficient mice. Furthermore, focal areas of medial atrophy were frequently observed in plasminogen-deficient mice but not in control animals. A simultaneous deficit of fibrinogen restored the potential of the arteries of plasminogen-deficient mice to enlarge in association with neointimal development but did not eliminate the focal medial atrophy. An intense inflammatory infiltrate occurred in the adventitia of cuffed arteries, which was associated with enhanced matrix deposition. Adventitial collagen deposition was apparent after 28 days in control and fibrinogen-deficient arteries but not in plasminogen-deficient arteries, which contained persistent fibrin. These studies demonstrate that plasmin(ogen) contributes to favorable arterial remodeling and adventitial collagen deposition via a mechanism that is related to fibrinogen, presumably fibrinolysis. In addition, these studies reveal a fibrin-independent role of plasminogen in preventing medial atrophy in challenged vessels.

Platelet glycoprotein ibalpha is a counterreceptor for the leukocyte integrin Mac-1 (CD11b/CD18)

The firm adhesion and transplatelet migration of leukocytes on vascular thrombus are both dependent on the interaction of the leukocyte integrin, Mac-1, and a heretofore unknown platelet counterreceptor. Here, we identify the platelet counterreceptor as glycoprotein (GP) Ibalpha, a component of the GP Ib-IX-V complex, the platelet von Willebrand factor (vWf) receptor. THP-1 monocytic cells and transfected cells that express Mac-1 adhered to GP Ibalpha-coated wells. Inhibition studies with monoclonal antibodies or receptor ligands showed that the interaction involves the Mac-1 I domain (homologous to the vWf A1 domain), and the GP Ibalpha leucine-rich repeat and COOH-terminal flanking regions. The specificity of the interaction was confirmed by the finding that neutrophils from wild-type mice, but not from Mac-1-deficient mice, bound to purified GP Ibalpha and to adherent platelets, the latter adhesion being inhibited by pretreatment of the platelets with mocarhagin, a protease that specifically cleaves GP Ibalpha. Finally, immobilized GP Ibalpha supported the rolling and firm adhesion of THP-1 cells under conditions of flow. These observations provide a molecular target for disrupting leukocyte-platelet complexes that promote vascular inflammation in thrombosis, atherosclerosis, and angioplasty-related restenosis.

Identification of a urokinase receptor-integrin interaction site. Promiscuous regulator of integrin function

Adhesion and signaling by integrins require their dynamic association with nonintegrin membrane proteins. One such protein, the glycolipid-anchored urokinase receptor (uPAR), associates with and modifies the function of the beta(2)-integrin Mac-1 (CD11b/CD18). In this study, a critical non-I-domain binding site for uPAR on CD11b (M25; residues 424-440) is identified by homology with a phage display peptide known to bind uPAR. Recombinant soluble uPAR and cells expressing uPAR bound to immobilized M25, binding being promoted by urokinase and blocked by soluble M25, but not a scrambled control or homologous peptides from other beta(2)-associated alpha-chains. Mac-1, but not a mutated Mac-1 in which M25 was replaced with the homologous sequence of CD11c, co-precipitated with uPAR. In the beta-propeller model of alpha-chain folding, M25 spans an exposed loop on the ligand-binding, upper surface of alphaM, identifying uPAR as an atypical alphaM ligand. Although not blocking ligand binding to Mac-1, M25 (25-100 microM) inhibited leukocyte adhesion to fibrinogen, vitronectin, and cytokine-stimulated endothelial cells. M25 also blocked the association of uPAR with beta(1)-integrins and impaired beta(1)-integrin-dependent spreading and migration of human vascular smooth muscle cells on fibronectin and collagen. These observations indicate that uPAR associates with integrins directly and that disruption of this association broadly impairs integrin function, suggesting a novel strategy for regulation of integrins in the settings of inflammation and tumor progression.

Decreased neointimal formation in Mac-1(-/-) mice reveals a role for inflammation in vascular repair after angioplasty

Inflammation plays an essential role in the initiation and progression of atherosclerosis, but its role in vascular repair after mechanical arterial injury (i.e., percutaneous transluminal coronary angioplasty, PTCA) is unknown. In animal models of vascular injury, leukocytes are recruited as a precursor to intimal thickening. Furthermore, markers of leukocyte activation - in particular, increased expression of the beta2-integrin Mac-1 (alphaMbeta2, or CD11b/CD18), which is responsible for firm leukocyte adhesion to platelets and fibrinogen on denuded vessels - predict restenosis after PTCA. To determine whether Mac-1-mediated leukocyte recruitment is causally related to neointimal formation, we subjected mice lacking Mac-1 to a novel form of mechanical carotid artery dilation and complete endothelial denudation. We now report that the selective absence of Mac-1 impairs transplatelet leukocyte migration into the vessel wall, reducing leukocyte accumulation over time. Diminished medial leukocyte accumulation was accompanied by markedly reduced neointimal thickening after vascular injury. These data establish a role for inflammation in neointimal thickening and suggest that leukocyte recruitment to mechanically injured arteries may prevent restenosis.

Role of urokinase receptor and caveolin in regulation of integrin signaling

Emerging evidence indicates a prominent role for non-integrin membrane adaptors in the dynamic regulation of integrin signaling. Two such integrin-associated proteins are the glycosylphosphatidyl-inositol (GPI)-linked urokinase receptor (u-PAR) and the cholesterol-binding protein, caveolin-1. Recent studies indicate that caveolin is required for the association of Src-family kinases with beta 1 integrins. Loss of caveolin/beta 1 integrin association results in loss of ligand-induced focal adhesion kinase (FAK) phosphorylation and impaired development of focal adhesion sites. Similarly, fibronectin-dependent fyn signaling through alpha 5/beta 1 leading to mitogen-activated protein (MAP) kinase activation requires the presence of caveolin-1. Caveolin binds Src-family kinases and such binding maintains these kinases in an inactive state. Current evidence favors a model in which ligand-induced integrin clustering, a central event in integrin activation, promotes caveolin oligomerization leading to release and/or activation of Src-family kinases and initiation of integrin signaling. The presence of u-PAR promotes these events because the extracellular domain(s) of u-PAR binds to beta 1 and beta 2 integrins and the GPI anchor of u-PAR, like that of other GPI-anchored proteins, interacts with cholesterol-rich membrane domains enriched in caveolin and tyrosine kinases. Integrins, caveolin, and u-PAR form interdependent functional complexes, promoting the association of integrins with caveolin-rich signaling domains. During states of accelerated cellular migration, such as during inflammation and tumorigenesis, expression of u-PAR may be a key facilitator of integrin signaling. Interruption of u-PAR/integrin interactions may be a strategy to regulate cellular migration in these settings.

Vitamin E and vascular homeostasis: implications for atherosclerosis

Considerable epidemiologic data suggest that dietary consumption of vitamin E reduces the incidence of cardiovascular disease. The precise mechanisms are not clear, but emerging data indicate that vitamin E has numerous activities that may, in part, explain its effect on vascular disease. In particular, vitamin E enhances the bioactivity of nitric oxide, inhibits smooth muscle proliferation, and limits platelet aggregation. One common mechanism to account for these effects of vitamin E is the inhibition of protein kinase C stimulation. In the setting of atherosclerosis, inhibition of protein kinase C by vitamin E would be expected to maintain normal vascular homeostasis and thus reduce the clinical incidence of cardiovascular disease.

Ring-toric lens for focus-error sensing in optical data storage

We discuss the design and performance of diffractive ring-toric lenses for focus-error sensing in optical data storage. A ring-toric lens images a point source of light to a ring-shaped image. Focus-error sensing is accomplished by means of monitoring the change in ring radius: The ring expands in response to a diverging wave front, and the ring contracts in response to a converging wave front. We describe the use of a segmented phi detector to generate a focus-error signal (FES). We found that the FES slope, a measure of sensitivity to disk defocus, is higher for the ring-toric lenses described in this paper than for other techniques such as the astigmatic and the obscuration methods. We measured an FES slope of 0.7 per micrometer of disk defocus (microm(-1)). The corresponding theoretical FES slope is 0.96 microm(-1).

Expression of the elastolytic cathepsins S and K in human atheroma and regulation of their production in smooth muscle cells

Formation of the atherosclerotic intima must involve altered metabolism of the elastin-rich arterial extracellular matrix. Proteases potentially involved in these processes remain unclear. This study examined the expression of the potent elastases cathepsins S and K in human atheroma. Normal arteries contained little or no cathepsin K or S. In contrast, macrophages in atheroma contained abundant immunoreactive cathepsins K and S. Intimal smooth muscle cells (SMC), especially cells appearing to traverse the internal elastic laminae, also contained these enzymes. Extracts of atheromatous tissues had approximately twofold greater elastase-specific activity than extracts of uninvolved arteries, mostly due to cysteine proteases. Cultured human SMC displayed no immunoreactive cathepsins K and S and exhibited little or no elastolytic activity when incubated with insoluble elastin. SMC stimulated with the atheroma-associated cytokines IL-1beta or IFN-gamma secreted active cathepsin S and degraded substantial insoluble elastin (15-20 microg/10(6) cells/24 h). A selective inhibitor of cathepsin S blocked > 80% of this elastolytic activity. The presence of cathepsins K and S at sites of vascular matrix remodeling and the ability of SMC and macrophages to use these enzymes to degrade elastin supports a role for elastolytic cathepsins in vessel wall remodeling and identifies novel therapeutic targets in regulating plaque stability.

A mAb to the beta2-leukocyte integrin Mac-1 (CD11b/CD18) reduces intimal thickening after angioplasty or stent implantation in rabbits

Leukocytes are recruited early and abundantly to experimentally injured vessels, in direct proportion to cell proliferation and intimal growth. Activated circulating leukocytes and Mac-1 (CD11b/CD18, alphaMbeta2) expression are markers of restenosis risk in patients undergoing angioplasty. As angioplastied vessels lack endothelium but have extensive fibrin(ogen) and platelet deposition, we hypothesized that Mac-1-dependent adhesion to fibrin(ogen) is an important determinant of leukocyte recruitment and function, which may in turn promote intimal growth. To study this hypothesis we administered M1/70, an anti-CD11b blocking mAb, to rabbits (1 mg/kg i.v.) immediately before, and every 48 hr for 3, 6, or 14 days after, iliac artery balloon denudation or deeper stent-induced injury. M1/70, which bound to isolated rabbit monocytes and dose-dependently inhibited Mac-1-mediated fibrinogen binding in vitro, reduced leukocyte recruitment more than 2-fold 3, 6, and 14 days after injury. Neointimal growth 14 days after injury was markedly attenuated by treatment with M1/70 (intimal area after balloon injury, 0.12 +/- 0.09 mm2, compared with 0.32 +/- 0.08 mm2 in vehicle-treated controls, P < 0.01, and 0.38 +/- 0.08 mm2 in IgG-treated controls, P < 0.005; intimal area after stent injury, 0. 56 +/- 0.16 mm2, compared with 0.84 +/- 0.13 mm2 in vehicle-treated controls, P < 0.05, and 0.90 +/- 0.15 mm2 in IgG-treated controls, P < 0.02). Mac-1 blockade reduces experimental neointimal thickening, suggesting that leukocyte recruitment to and infiltration of injured arteries may be a valid target for preventing intimal hyperplasia.

Expression of the elastolytic cathepsins S and K in human atheroma and regulation of their production in smooth muscle cells

Formation of the atherosclerotic intima must involve altered metabolism of the elastin-rich arterial extracellular matrix. Proteases potentially involved in these processes remain unclear. This study examined the expression of the potent elastases cathepsins S and K in human atheroma. Normal arteries contained little or no cathepsin K or S. In contrast, macrophages in atheroma contained abundant immunoreactive cathepsins K and S. Intimal smooth muscle cells (SMC), especially cells appearing to traverse the internal elastic laminae, also contained these enzymes. Extracts of atheromatous tissues had approximately twofold greater elastase-specific activity than extracts of uninvolved arteries, mostly due to cysteine proteases. Cultured human SMC displayed no immunoreactive cathepsins K and S and exhibited little or no elastolytic activity when incubated with insoluble elastin. SMC stimulated with the atheroma-associated cytokines IL-1beta or IFN-gamma secreted active cathepsin S and degraded substantial insoluble elastin (15-20 microg/10(6) cells/24 h). A selective inhibitor of cathepsin S blocked > 80% of this elastolytic activity. The presence of cathepsins K and S at sites of vascular matrix remodeling and the ability of SMC and macrophages to use these enzymes to degrade elastin supports a role for elastolytic cathepsins in vessel wall remodeling and identifies novel therapeutic targets in regulating plaque stability.

Microtags with 150-nm line gratings fabricated by use of extreme-ultraviolet lithography

The microtag concept is an anticounterfeiting and security measure. Microtags are computer-generated holograms (CGH's) consisting of 150-nm lines arranged to form 300-nm-period gratings. The microtags that we describe were designed for readout at 442nm . The smallest microtag measures 56micromx80 microm when viewed at normal incidence. The CGH design process uses a modified iterative Fourier-transform algorithm to create either phase-only or phase-and-amplitude microtags. We also report on a simple and compact readout system for recording the diffraction pattern formed by a microtag. The measured diffraction patterns agree very well with predictions.

Hemostatic/fibrinolytic predictors of allograft coronary artery disease after cardiac transplantation

Allograft coronary artery disease (CAD) remains the leading cause of morbidity and mortality affecting the long-term survival of patients after cardiac transplantation. Because there is increasing evidence that imbalances in hemostatic and fibrinolytic pathways are associated with graft failure, we hypothesized that atherothrombotic risk factors may contribute to allograft CAD. This study sought to determine if plasma hemostatic and fibrinolytic parameters are associated with the severity of allograft CAD. The extent of allograft CAD was investigated by angiography and intravascular ultrasound (IVUS) in 16 cardiac transplant recipients. Intimal thickening was quantified using IVUS by measuring the intimal index (li = intimal area/[intimal area + luminal area]) in two to five segments of the left anterior descending (LAD) coronary artery. The maximal li per patient was calculated and index to the time post-transplant (Mxli/Yr). Plasma fibrinogen (FGN), tissue-type plasminogen activator (t-PA), plasminogen activator inhibitor-1 (PAI-1), lipoprotein(a) (Lp(a)), and net fibrinolytic activity of plasma were assayed 6-24 months after transplant as indicators of the fibrinolytic system and then correlated with the IVUS measurements. The FGN level correlated with the severity of intimal thickening, Mxli/Yr (r2 = 0.41, p = 0.008), and was inversely correlated with angiographic tertiary vessel filling (r2 = 0.25, p = 0.051). In patients with lower plasma fibrinolytic activity (lytic zone less than 100 mm2), Mxli/Yr was increased eightfold (0.218 +/- 0.137 versus 0.025 +/- 0.021, p = 0.001). t-PA (r2 = 0.0004, p = 0.94), PAI-1 (r2 = 0.008, p = 0.75) and Lp(a) levels (r2 = 0.11, p = 0.21) did not predict Mxli/Yr. Thus, we demonstrate that plasma FGN and net fibrinolytic activity correlate with the degree of intimal thickening measured by IVUS after cardiac transplantation. These data suggest that fibrin deposition may play a role in allograft CAD after cardiac transplantation.

7E3 monoclonal antibody directed against the platelet glycoprotein IIb/IIIa cross-reacts with the leukocyte integrin Mac-1 and blocks adhesion to fibrinogen and ICAM-1

Recent clinical trials suggest that blockade of integrins is a promising strategy for the treatment of acute coronary syndromes. Administration of 7E3 monoclonal antibody (mAb) Fab fragment (c7E3 Fab) directed against platelet integrin IIb/IIIa (alpha IIb beta 3, CD41/CD61) reduces acute ischemic complications of coronary angioplasty and clinical restenosis at 6 months. However, 7E3 mAb is not selective for platelet IIb/IIIa but also cross-reacts with the leukocyte integrin Mac-1 (alpha M beta 2, CD11b/CD18) and the vitronectin receptor (alpha v beta 3, CD51/CD61). Information regarding how this mAb may affect other cells important in vascular repair is scant. Potential interactions of c7E3 Fab with inflammatory (i.e., monocytes and neutrophils), vascular smooth muscle, and endothelial cells may contribute to the in vivo actions of c7E3 Fab. In this study we explored the binding of 7E3 to monocytic cells and the functional effect of 7E3 and c7E3 Fab on Mac-1-mediated adhesion to fibrinogen (FGN) and intercellular adhesion molecule-1 (ICAM-1), ligands abundant in the injured vessel wall. Flow cytometry demonstrated that 7E3 bound to THP-1 monocytic cells and identified a subpopulation (approximately 10%) of Mac-1 that was qualitatively similar to that recognized by CBRM1/5, a mAb directed to an activation-specific neoepitope present on a subset of Mac-1 molecules. mAb 7E3 bound to K562 cells transfected with just the alpha subunit (CD11b) of Mac-1 but not to nontransfected cells, confirming a direct interaction between 7E3 and Mac-1. mAb 7E3 and c7E3 Fab blocked the adhesion of Mac-1-bearing cells to FGN (80 +/- 11% and 78 +/- 9% inhibition, respectively) and ICAM-1 (62 +/- 14% and 62 +/- 17%). Both 7E3 and c7E3 Fab significantly inhibited (70 +/- 6% and 62 +/- 26%) soluble FGN binding to human peripheral blood monocytes. Thus, c7E3 Fab cross-reacts with the CD11b subunit of Mac-1 and interrupts cell-extracellular matrix and cell-cell adhesive interactions and may thereby influence the recruitment of circulating monocytes to sites of vessel injury. Given the recent evidence that adherent and infiltrating monocyte number directly correlates with the extent of neointimal hyperplasia, inhibition of Mac-1-dependent adhesion and IIb/IIIa-dependent function by c7E3 Fab may jointly contribute to the regulation of vascular repair and to the sustained clinical benefits observed with c7E3 Fab after angioplasty.

Mac-1 (CD11b/CD18) and the urokinase receptor (CD87) form a functional unit on monocytic cells

The leukocyte integrin Mac-1 (CD11b/CD18) and the urokinase receptor (uPAR, CD87) mediate complementary functions in myelomonocytic cells. Both receptors promote degradation of fibrin(ogen) and also confer adhesive properties on cells because Mac-1 and uPAR bind fibrin and vitronectin, respectively. Staining of lung biopsy specimens from patients with acute lung injury indicated that fibrin and vitronectin colocalize at exudative sites in which macrophages bearing these receptors accumulate. Because of the parallel roles and physical proximity of Mac-1 and uPAR, the capacity of these receptors to functionally interact was explored. Induction of Mac-1 and uPAR expression on monocytic cell lines by transforming growth factor- beta 1 and 1.25-(OH)2 vitamin D3 conferred urokinase and uPAR-dependent adhesion to vitronectin, which was further promoted by engagement of Mac-1. Vitronectin attachment promoted subsequent Mac-1-mediated fibrinogen degradation threefold to fourfold. In contrast, enhancement of uPAR occupancy by exogenous urokinase or receptor binding fragments thereof inhibited Mac-1 function. Addition of urokinase progressively inhibited Mac-1-mediated fibrinogen binding and degradation (maximal inhibition, 91% +/- 14% and 72% +/- 15%, respectively). Saturation of uPAR with urokinase also inhibited binding of the procoagulant Mac-1 ligand, Factor X. These inhibitory effects of uPAR were reproduced in fresh monocytes, cultured monocytic cells, and in Chinese hamster ovary (CHO) cells transfected with both human Mac-1 and human uPAR. These data show that the procoagulant and fibrinolytic potential of monocytic cells is co-ordinately regulated by ligand binding to both Mac-1 and uPAR and identify uPAR as a regulator of integrin function. Vitronectin-enhanced fibrin(ogen) turnover by Mac-1 may operate as a salvage pathway in the setting of urokinase and plasmin inhibitors to promote clearance of the provisional matrix and subsequent healing.

Regulation of integrin function by the urokinase receptor

Integrin function is central to inflammation, immunity, and tumor progression. The urokinase-type plasminogen activator receptor (uPAR) and integrins formed stable complexes that both inhibited native integrin adhesive function and promoted adhesion to vitronectin via a ligand binding site on uPAR. Interaction of soluble uPAR with the active conformer of integrins mimicked the inhibitory effects of membrane uPAR. Both uPAR-mediated adhesion and altered integrin function were blocked by a peptide that bound to uPAR and disrupted complexes. These data provide a paradigm for regulation of integrins in which a nonintegrin membrane receptor interacts with and modifies the function of activated integrins.

Polynitrosylated proteins: characterization, bioactivity, and functional consequences

Chemical modification of proteins is a common theme in their regulation. Nitrosylation of protein sulfhydryl groups has been shown to confer nitric oxide (NO)-like biological activities and to regulate protein functions. Several other nucleophilic side chains -- including those with hydroxyls, amines, and aromatic carbons -- are also potentially susceptible to nitrosative attack. Therefore, we examined the reactivity and functional consequences of nitros(yl)ation at a variety of nucleophilic centers in biological molecules. Chemical analysis and spectroscopic studies show that nitrosation reactions are sustained at sulfur, oxygen, nitrogen, and aromatic carbon centers, with thiols being the most reactive functionality. The exemplary protein, BSA, in the presence of a 1-, 20-, 100-, or 200-fold excess of nitrosating equivalents removes 0.6 +/- 0.2, 3.2 +/- 0.4, 18 +/- 4, and 38 +/- 10, respectively, moles of NO equivalents per mole of BSA from the reaction medium; spectroscopic evidence shows the proportionate formation of a polynitrosylated protein. Analogous reaction of tissue-type plasminogen activator yields comparable NO protein stoichiometries. Disruption of protein tertiary structure by reduction results in the preferential nitrosylation of up to 20 thus-exposed thiol groups. The polynitrosylated proteins exhibit antiplatelet and vasodilator activity that increases with the degree of nitrosation, but S-nitroso derivatives show the greatest NO-related bioactivity. Studies on enzymatic activity of tissue-type plasminogen activator show that polynitrosylation may lead to attenuated function. Moreover, the reactivity of tyrosine residues in proteins raises the possibility that NO could disrupt processes regulated by phosphorylation. Polynitrosylated proteins were found in reaction mixtures containing interferon-gamma/lipopolysaccharide-stimulated macrophages and in tracheal secretions of subjects treated with NO gas, thus suggesting their physiological relevance. In conclusion, multiple sites on proteins are susceptible to attack by nitrogen oxides. Thiol groups are preferentially modified, supporting the notion that S-nitrosylation can serve to regulate protein function. Nitrosation reactions sustained at additional nucleophilic centers may have (patho)physiological significance and suggest a facile route by which abundant NO bioactivity can be delivered to a biological system, with specificity dictated by protein substrate.

Mechanisms of pertussis toxin-induced myelomonocytic cell adhesion: role of Mac-1(CD11b/CD18) and urokinase receptor (CD87)

Stimulation of monoblastic U937 cells with transforming growth factor beta 1 and 1,25-(OH)2 vitamin D3 (TGF-beta 1/D3) upregulates urokinase receptor (uPAR) and confers urokinase-dependent adhesiveness to the cells for serum- or vitronectin-coated surfaces. Recent studies show that uPAR itself is a high-affinity adhesion receptor for vitronectin and that urokinase (uPA) is an activator of this adhesive function. In the course of exploring possible G-protein involvement in this adhesion it was observed that TGF-beta 1/D3-primed U937 cells became adhesive to vitronectin in an uPAR-dependent manner when exposed to pertussis toxin (PTX). The adherent response is concentration- and time-dependent, and was not due to the ADP-ribosyltransferase activity of the toxin because the purified B-subunit of PTX was equally effective. Although promoting adhesion to serum- or vitronectin-coated surfaces, PTX blocked spontaneous cell adhesion to fibrinogen, an endogenous ligand for the Mac-1 receptor (CD11b/CD18). Flow cytometry study showed that expression of the alpha-subunit of Mac-1 (CD11b) on primed cells was increased by nearly threefold. Monoclonal antibody to CD11b abolished the PTX-induced cell adhesion and the binding of the primed cells to PTX-coated plates. Activation of Mac-1 receptor by its endogenous ligand fibrinogen induced cell adherent response similar to PTX. PTX, but not uPA, triggered a rapid rise in [Ca2+]i in primed U937 cells, and PTX-induced adhesion was significantly attenuated by 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/acetoxy-methyl ester (BAPTA/AM), a selective membrane-permeant [Ca2+]i chelator. PTX-induced cell adhesion was also prevented by antibodies to uPAR and by conditioned medium containing soluble uPAR. Together these data indicate that PTX B-subunit may bind to Mac-1 integrin, which leads to a rapid rise in [Ca2+]i and subsequent activation of uPAR for adherence to vitronectin, suggesting a functional link between Mac-1 and activation of uPAR important to cellular trafficking and host defence in response to Bordetella pertussis infection.

Cathepsin D-like aspartyl protease activity mediates the degradation of tissue-type plasminogen activator/plasminogen activator inhibitor-1 complexes in human monocytes

Plasminogen activator inhibitor-1 (PAI-1) is the most important inhibitor of tissue-type plasminogen activator (t-PA) in plasma and plays a major role in the regulation of fibrinolysis. Plasma t-PA/PAI-1 complexes are cleared via a receptor-dependent mechanism in hepatocytes, while the fate of complexes formed in the extracellular matrix and in thrombi is less well understood. In this study, the degradation of t-PA/PAI-1 complexes by monocytes was examined. THP-1 monocytoid cells and freshly isolated human monocytes internalize and degrade [125I]t-PA/PAI-1 complexes at rates of 11.4 +/- 5.9 (mean +/- S.D.) and 44.6 +/- 6.3 ng/10(6) cells/h, respectively. Degradation is blocked by receptor-associated protein (RAP), indicating a member of the low density lipoprotein (LDL) receptor family is involved in the uptake/degradation of t-PA/PAI-1 complexes by monocytes. Degradation of t-PA/PAI-1 complexes is also inhibited by chloroquine and by pepstatin A, suggesting that a lysosomal aspartyl protease is likely involved. SDS-PAGE and Western blotting demonstrated that the purified lysosomal aspartyl protease, cathepsin D, is capable of digesting t-PA (t1/2 15 min), active PAI-1 (t1/2 2 h), and t-PA/PAI-1 complex (t1/2 30 min). Cathepsin D sequentially cleaves PAI-1 after hydrophobic amino acids, yielding lower molecular weight fragments. PAI-1 conformation influences the degradative efficiency of cathepsin D, with vitronectin-bound PAI-1 and latent PAI-1 exhibiting resistance to proteolysis and > 10-fold prolongation in t1/2. These data provide evidence that t-PA/PAI-1 complexes are internalized by human monocytes via a member of the low density lipoprotein (LDL) receptor family, and identifies cathepsin D-like aspartyl protease activity as largely responsible for the degradation of these complexes. Furthermore, vitronectin-bound PAI-1 and latent PAI-1 are relatively resistant to degradation by cathepsin D, which may be of importance in complex physiological environments.

Resolution of spontaneous bleeding events but failure of pregnancy in fibrinogen-deficient mice

To explore the role of the key coagulation factor, fibrinogen, in development, hemostasis, wound repair, and disease pathogenesis, we disrupted the fibrinogen A alpha chain gene in mice. Homozygous, A alpha chain-deficient (A alpha-/-) mice are born normal in appearance, and there is no evidence of fetal loss of these animals based on the Mendelian pattern of transmission of the mutant A alpha chain allele. All of the component chains of fibrinogen (A alpha, B beta, and gamma) are immunologically undetectable in the circulation of both neonatal and adult A alpha-/- mice, and blood samples fail to either clot or support platelet aggregation in vitro. Overt bleeding events develop shortly after birth in approximately 30% of A alpha-/- mice, most frequently in the peritoneal cavity, skin, and soft tissues around joints. Remarkably, most newborns displaying signs of bleeding ultimately control the loss of blood, clear the affected tissues, and survive the neonatal period. Juveniles and young adult A alpha-/- mice are predisposed to spontaneous fatal abdominal hemorrhage, but long-term survival is variable and highly dependent on genetic background. The periodic rupture of ovarian follicles in breeding-age A alpha-/- females does not appear to significantly diminish life expectancy relative to males; however, pregnancy uniformly results in fatal uterine bleeding around the tenth day of gestation. Microscopic analysis of spontaneous lesions found in A alpha-/- mice suggests that fibrin(ogen) plays a fundamental role in the organization of cells at sites of injury.

Effect of nitric oxide synthase inhibition on bleeding time in humans

Endothelium-derived nitric oxide (NO) is a potent in vitro inhibitor of platelet adhesion and aggregation, but its role in regulating platelet reactivity in vivo and in humans in particular is undefined. Our primary objective was to determine whether the in vivo inhibition of NO production shortens template bleeding time (BT). The hemodynamic and platelet effects of NG-mono-methyl-L-arginine (L-NMMA), an NO synthase inhibitor, were studied in 12 normal volunteers. Forearm template BT was determined before and 15 min after the intravenous (i.v.) infusion of 4.3 mg/kg L-NMMA. L-NMMA infusion increased mean arterial pressure from 88 +/- 4 to 99 +/- 3 mm Hg (p = 0.0001). Plasma NO levels, determined by chemiluminescence, decreased 65 +/- 10% from basal values (p < 0.05), confirming inhibition of endogenous NO production. Intravenous L-NMMA shortened BT from 5.5 +/- 0.9 to 4.0 +/- 0.6 min (p = 0.026), or by 24 +/- 8% (p = 0.008). L-NMMA infusion did not significantly change ex vivo platelet aggregation. To determine whether vasoconstriction affected BT, we investigated forearm blood flow (FBF; determined by venous occlusion plethymography), and template BT in 3 subjects after the local infusion of L-NMMA (2-4 mg/min intraarterially, i.a.). Intraarterial administration of L-NMMA caused a 39 +/- 3% (p = 0.006) reduction in FBF in the infused arm but did not change BT. These data show that systemic inhibition of NO production shortens BT in humans. However, the precise mechanism by which L-NMMA shortens BT is not completely defined.

The fibrin(ogen)olytic properties of cathepsin D

Fibrin(ogen) is important for hemostasis and is cleared from sites of vascular injury primarily by the plasminogen activator system. However, there is emerging evidence in plasminogen activator-deficient transgenic mice that non-plasmin pathways may also be important for endogenous fibrinolysis. We have recently described an alternative, plasmin-independent fibrinolytic pathway in activated human monocytes that utilizes the integrin Mac-1 (CD11b/CD18), which directly binds and internalizes fibrin, resulting in its lysosomal degradation. The identity of the lysosomal fibrinolytic enzyme(s) responsible for monocyte/macrophage-mediated fibrinolytic is unknown. Protease inhibitor studies now suggest that an aspartyl protease is responsible for this fibrinolytic activity. We, therefore, examined the fibrinolytic properties of cathepsin D, a lysosomal aspartyl protease, and report that cathepsin D possesses both fibrinogenolytic and fibrinolytic activity. Cathepsin D cleavage of fibrinogen follows Michaelis-Menten kinetics with a Michaelis constant, Km, of 1.5 microM; catalytic rate constant, kcat, of 1.4 x 10(-3) s-1; and catalytic efficiency, kcat/Km, of 9.3 x 10(-4) microM-1 s-1. A pH-activity profile of fibrinogen digestion by cathepsin D demonstrates a pH optimum of 3.5 with 50% residual activity at pH 5.0. Fibrinolysis was assessed by fibrin plate and fibrin clot lysis assays. Cathepsin D possesses significant fibrinolytic activity over a dose range of 100 nM to 10 microM and is able to lyse fibrin, as well as albumin-enriched and albumin/red cell-enriched fibrin clots. Cathepsin D cleaves the alpha-, beta-, and gamma-chains of FGN, generating multiple low-molecular-weight fragments.(ABSTRACT TRUNCATED AT 250 WORDS)

Fibrin(ogen) is internalized and degraded by activated human monocytoid cells via Mac-1 (CD11b/CD18): a nonplasmin fibrinolytic pathway

Fibrin(ogen) (FGN) is important for hemostasis and wound healing and is cleared from sites of injury primarily by the plasminogen activator system. However, there is emerging evidence in plasminogen activator-deficient transgenic mice that nonplasmin pathways may be important in fibrin(ogen)olysis, as well. Given the proximity of FGN and monocytes within the occlusive thrombus at sites of vascular injury, we considered the possibility that monocytes may play an ancillary role in the degradation and clearance of fibrin. We found that monocytes possess an alternative fibrinolytic pathway that uses the integrin Mac-1, which directly binds and internalizes FGN, resulting in its lysosomal degradation. At 4 degrees C, FGN binds to U937 monocytoid cells in a specific and saturable manner with a kd of 1.8 mumol/L. Binding requires adenosine diphosphate stimulation and is calcium-dependent. At 37 degrees C, FGN and fibrin monomer (FM) are internalized and degraded at rates of 0.37 +/- 0.13 and 0.55 +/- 0.03 microgram/10(6) cells/h by U937 cells, 1.38 +/- 0.02 and 1.20 +/- 0.30 microgram/10(6) cells/h by THP-1 cells, and 2.10 +/- 0.20 and 2.52 +/- 0.18 micrograms/10(6) cells/h by human peripheral blood mononuclear cells, respectively. The serine protease inhibitors, PPACK and aprotinin, and the specific elastase inhibitor, AAPVCK, do not significantly inhibit degradation. However, degradation is inhibited by chloroquine, suggesting that a lysosomal pathway is involved. Factor X, a competitive ligand with FGN for the Mac-1 receptor, also blocks degradation, as does a monoclonal antibody to the alpha-subunit of Mac-1. Autoradiography of radioiodinated, internalized FGN shows that FGN proteolysis by the pathway produces a unique degradation pattern distinct from that observed with plasmin. In a fibrin clot lysis assay, Mac-1-mediated fibrinolysis contributed significantly to total fibrinolysis. In summary, FGN is internalized and degraded by activated human monocytoid cells via Mac-1 in the absence of plasmin, thereby providing an alternative fibrinolytic pathway. Thus, in addition to the function of cell adhesion, integrins may also act as receptors that mediate the internalization and degradation of bound ligands.

Antiplatelet properties of protein S-nitrosothiols derived from nitric oxide and endothelium-derived relaxing factor

S-nitrosothiols may serve as carriers in the mechanism of action of endothelium-derived relaxing factor (EDRF) by stabilizing the labile nitric oxide (NO) radical from inactivation by reactive species in the physiological milieu and by delivering NO to the heme activator site of guanylyl cyclase. Low-molecular-weight thiols, such as cysteine and glutathione, form S-nitrosothiol adducts with vasodilatory and antiplatelet properties, and protein thiols can interact in the presence of NO and/or EDRF to form uniquely stable S-nitroso-proteins. We now show that the S-nitroso-proteins, S-nitroso-albumin, S-nitroso-tissue type plasminogen activator, and S-nitroso-cathepsin B, have potent antiplatelet effects with an IC50 of approximately 1.5 microM. In the dog, S-nitroso-albumin inhibits ex vivo platelet aggregation and significantly prolongs the template bleeding time from 2.15 +/- 0.13 (mean +/- SEM) to 9.70 +/- 1.24 minutes. The antiplatelet action of S-nitroso-proteins is associated with the stimulation of guanylyl cyclase and a significant decrease in fibrinogen binding to platelets. S-Nitroso-proteins undergo thiol-nitrosothiol exchange with low-molecular-weight thiols to form low-molecular-weight S-nitroso-thiols, and they also interact directly with the platelet surface, both of which processes facilitate generation of NO. These data suggest that S-nitroso-proteins are potent antiplatelet agents and may be intermediates in the antiplatelet mechanism of EDRF action.

Lipoprotein(a) binds to human platelets and attenuates plasminogen binding and activation

Lipoprotein(a) [Lp(a)] is a unique lipoprotein consisting of a low-density lipoprotein moiety (LDL) covalently linked to apoprotein(a). Previous work has demonstrated that Lp(a) can compete with plasminogen (PGN) for binding to endothelial and mononuclear cells and can inhibit PGN activation in cell-free systems. We have assessed the binding of Lp(a) to platelets and the influence of binding on the activation of PGN by tissue-type plasminogen activator (t-PA) in this system. In direct binding experiments, Lp(a) bound specifically, saturably, and reversibly to platelets with an estimated apparent Kd of 0.20 microM. Scatchard analysis revealed a single class of binding sites with 81,000 +/- 22,000 particles of Lp(a) bound at saturation. Interestingly, Lp(a) bound to a similar extent to thromboasthenic platelets. Activation of platelets with ADP or thrombin reduced Lp(a) binding capacity by approximately 50% without changing affinity. Lp(a) also inhibited the binding of PGN to platelets with an IC50 of approximately 0.23 microM. Over a similar concentration range, LDL did not inhibit PGN binding to platelets. In addition, Lp(a) inhibited PGN binding to plasmin-treated platelets with an IC50 of approximately 0.2 microM. Kinetic experiments demonstrated that Lp(a) acted as a competitive inhibitor of PGN activation by t-PA on the platelet surface, with an estimated Ki of 0.49 microM. In the presence of platelets, Lp(a) decreased the kcat/Km for t-PA by 3-fold, owing primarily to an increase in the Km of t-PA for PGN. In contrast, LDL did not alter the kinetics of PGN activation by t-PA on the platelet surface.(ABSTRACT TRUNCATED AT 250 WORDS)

NO forms an adduct with serum albumin that has endothelium-derived relaxing factor-like properties

Recent evidence suggests that sulfhydryl species can react with oxides of nitrogen under physiologic conditions and thereby stabilize endothelium-derived relaxing factor (EDRF) activity, but the presence of a specific in vivo thiol carrier for nitric oxide (NO) remains controversial. The single free sulfhydryl of serum albumin is the most abundant thiol species in plasma (approximately 0.5 mM) and is particularly reactive towards NO. To examine the potential role of serum albumin in endogenous nitric oxide metabolism, we synthesized S-nitroso-BSA (S-NO-BSA), a model S-nitroso-protein, and examined its effects on platelet function and coronary and systemic vascular tone in 16 mongrel dogs. Intravenous bolus S-NO-BSA markedly reduced mean arterial pressure in a dose-dependent manner and proved seven and a half-fold less potent than intravenous nitroglycerin and 10-fold less potent than intravenous S-nitroso-cysteine (half-maximal response of 75 nmol/kg compared to 10 and 7.5 nmol/kg, respectively; P < 0.05); when given by intravenous infusion (half-maximal response = 10 nmol/kg per min), however, S-NO-BSA and nitroglycerin were equipotent. Intravenous bolus S-NO-BSA had a greater duration of action than either nitroglycerin or S-nitroso-cysteine and produced marked prolongation of the template bleeding time associated with dose-dependent inhibition of ex vivo platelet aggregation (half-maximal response approximately 70 nmol/kg). Intracoronary S-NO-BSA increased coronary blood flow (mean +/- SEM) less effectively than nitroprusside, acetylcholine, or S-nitroso-cysteine (165% +/- 24% vs. 315% +/- 82%, 483% +/- 55%, or 475% +/- 66%, respectively; P < 0.05) although with much longer duration of action. On a molar basis, S-nitroso-cysteine proved more effective than S-nitroso-BSA, nitroprusside, or acetylcholine as an epicardial coronary vasodilator. Thus, serum albumin reacts with oxides of nitrogen to form a stable S-nitroso-thiol with properties reminiscent of authentic EDRF supporting the view that protein associated thiol may participate in the action and metabolism of EDRF.

S-nitrosylation of tissue-type plasminogen activator confers vasodilatory and antiplatelet properties on the enzyme

Tissue-type plasminogen activator (t-PA) reacts upon exposure to endothelium-derived relaxing factor (EDRF) by way of the enzyme's single free sulfhydryl (Cys-83) to form a stable S-nitrosothiol protein adduct. S-nitrosylation endows t-PA with potent vasodilatory and antiplatelet properties that are accompanied by elevations in intracellular cyclic GMP analogous to those induced by low molecular weight (e.g., S-nitroso amino acid) S-nitrosothiols. Moreover, this chemical modification does not adversely affect the catalytic efficiency of t-PA, the fibrin stimulation of this activity, the binding of t-PA to fibrinogen, or the interaction of the enzyme with its physiologic serine protease inhibitor, plasminogen-activator inhibitor type I. The coupling of vasodilatory, antiplatelet, and fibrinolytic properties in one molecule makes the S-nitrosylated t-PA a unique molecular species and may provide insight into the mechanisms by which the endothelium maintains vessel patency. These data also suggest a pharmacologic approach to treatment of thromboocclusive disorders.

Nitric oxide circulates in mammalian plasma primarily as an S-nitroso adduct of serum albumin

We have recently shown that nitric oxide or authentic endothelium-derived relaxing factor generated in a biologic system reacts in the presence of specific protein thiols to form S-nitrosoprotein derivatives that have endothelium-derived relaxing factor-like properties. The single free cysteine of serum albumin, Cys-34, is particularly reactive toward nitrogen oxides (most likely nitrosonium ion) under physiologic conditions, primarily because of its anomalously low pK; given its abundance in plasma, where it accounts for approximately 0.5 mM thiol, we hypothesized that this plasma protein serves as a reservoir for nitric oxide produced by the endothelial cell. To test this hypothesis, we developed a methodology, which involves UV photolytic cleavage of the S--NO bond before reaction with ozone for chemiluminescence detection, with which to measure free nitric oxide, S-nitrosothiols, and S-nitrosoproteins in biologic systems. We found that human plasma contains approximately 7 microM S-nitrosothiols, of which 96% are S-nitrosoproteins, 82% of which is accounted for by S-nitroso-serum albumin. By contrast, plasma levels of free nitric oxide are only in the 3-nM range. In rabbits, plasma S-nitrosothiols are present at approximately 1 microM; 60 min after administration of NG-monomethyl-L-arginine at 50 mg/ml, a selective and potent inhibitor of nitric oxide synthetases, S-nitrosothiols decreased by approximately 40% (greater than 95% of which were accounted for by S-nitrosoproteins, and approximately 80% of which was S-nitroso-serum albumin); this decrease was accompanied by a concomitant increase in mean arterial blood pressure of 22%. These data suggest that naturally produced nitric oxide circulates in plasma primarily complexed in S-nitrosothiol species, principal among which is S-nitroso-serum albumin. This abundant, relatively long-lived adduct likely serves as a reservoir with which plasma levels of highly reactive, short-lived free nitric oxide can be regulated for the maintenance of vascular tone.

S-nitrosylation of proteins with nitric oxide: synthesis and characterization of biologically active compounds

Endothelium-derived relaxing factor (EDRF) activity has been attributed to the highly labile nitric oxide radical (NO). In view of the fact that the plasma and cellular milieux contain reactive species that can rapidly inactivate NO, it has been postulated that NO is stabilized by a carrier molecule that preserves its biological activity. Reduced thiol species are candidates for this role, reacting readily in the presence of NO to yield biologically active S-nitrosothiols that are more stable than NO itself. Because sulfhydryl groups in proteins represent an abundant source of reduced thiol in biologic systems, we examined the reaction of several sulfhydryl-containing proteins of diverse nature and function upon exposure to authentic NO and EDRF. We demonstrate that S-nitroso proteins form readily under physiologic conditions and possess EDRF-like effects of vasodilation and platelet inhibition. These observations suggest that S-nitrosothiol groups in proteins may serve as intermediates in the cellular metabolism of NO and raise the possibility of an additional type of cellular regulatory mechanism.

Tissue-type plasminogen activator binds to and is inhibited by surface-bound lipoprotein(a) and low-density lipoprotein

Elevated levels of lipoprotein(a) [Lp(a)] are associated with an increased risk of atherothrombotic disease, but the mechanism(s) by which Lp(a) potentiates atherogenesis is unknown. The extensive homology of apolipoprotein(a) [apo(a)] to plasminogen has led us and others to postulate that Lp(a) may impair fibrinolysis. We have previously shown that Lp(a) inhibits fibrin stimulation of plasminogen activation by tissue-type plasminogen activator (t-PA); however, we and other investigators have been unable to demonstrate direct inhibition of t-PA by Lp(a) in solution. We now report that t-PA binds reversibly and saturably to surface-bound Lp(a) and to low-density lipoprotein (LDL) and that as a result of this binding activation of plasminogen by t-PA is inhibited. The catalytic efficiency (kcat/Km) of t-PA when bound to polystyrene surface-bound fibrinogen increased 2.9-fold compared to t-PA bound to control wells. When bound to surface-bound Lp(a), however, the catalytic efficiency of t-PA was reduced 9.5-fold compared to t-PA bound to control wells; likewise, by binding to surface-bound LDL, the catalytic efficiency of t-PA was reduced 16-fold compared to the control. Studies with defined monoclonal antibodies suggest that major determinants of t-PA binding are its active site, the LDL receptor binding domain of apolipoprotein B-100 (apoB-100), and apo(a). These data suggest a unique mechanism by which Lp(a) and LDL incorporated in an atheroma can inhibit endogenous fibrinolysis and thereby contribute to the genesis of atherothrombotic disease.

Sulfadiazine crystalluria revisited. The treatment of Toxoplasma encephalitis in patients with acquired immunodeficiency syndrome

Toxoplasma gondii encephalitis is an important opportunistic infection in the acquired immunodeficiency syndrome, estimated to occur in 20,000 to 40,000 patients with acquired immunodeficiency syndrome in the United States by 1991. The combination of sulfadiazine and pyrimethamine is regarded as the treatment of choice. Acute renal failure due to crystal deposition in the urinary tract was well described 30 to 40 years ago and is likely to resurface as a clinical entity if appropriate prophylactic measures are not taken. We describe two cases of sulfadiazine-induced crystalluria and renal failure in patients with acquired immunodeficiency syndrome, review the pertinent literature, and discuss the pathogenesis. Recommendations are made for the prophylaxis and treatment of sulfadiazine-related renal toxic reaction. Physicians using this "new" drug must be aware of the potential danger of sulfonamide-induced injury to the urinary tract.