Metabolism of bradykinin by peptidases in the lung

SARS-CoV-2 Spike Protein Destabilizes Microvascular Homeostasis

SARS-CoV-2 infection can cause compromised respiratory function and thrombotic events. SARS-CoV-2 binds to and mediates downregulation of angiotensin converting enzyme 2 (ACE2) on cells that it infects. Theoretically, diminished enzymatic activity of ACE2 may result in increased concentrations of pro-inflammatory molecules, angiotensin II, and Bradykinin, contributing to SARS-CoV-2 pathology. Using immunofluorescence microscopy of lung tissues from uninfected, and SARS-CoV-2 infected individuals, we find evidence that ACE2 is highly expressed in human pulmonary alveolar epithelial cells and significantly reduced along the alveolar lining of SARS-CoV-2 infected lungs. Ex vivo analyses of primary human cells, indicated that ACE2 is readily detected in pulmonary alveolar epithelial and aortic endothelial cells. Exposure of these cells to spike protein of SARS-CoV-2 was sufficient to reduce ACE2 expression. Moreover, exposure of endothelial cells to spike protein-induced dysfunction, caspase activation, and apoptosis. Exposure of endothelial cells to bradykinin caused calcium signaling and endothelial dysfunction (increased expression of von Willibrand Factor and decreased expression of Krüppel-like Factor 2) but did not adversely affect viability in primary human aortic endothelial cells. Computer-assisted analyses of molecules with potential to bind bradykinin receptor B2 (BKRB2), suggested a potential role for aspirin as a BK antagonist. When tested in our in vitro model, we found evidence that aspirin can blunt cell signaling and endothelial dysfunction caused by bradykinin in these cells. Interference with interactions of spike protein or bradykinin with endothelial cells may serve as an important strategy to stabilize microvascular homeostasis in COVID-19 disease.

IMPORTANCE SARS-CoV-2 causes complex effects on microvascular homeostasis that potentially contribute to organ dysfunction and coagulopathies. SARS-CoV-2 binds to, and causes downregulation of angiotensin converting enzyme 2 (ACE2) on cells that it infects. It is thought that reduced ACE2 enzymatic activity can contribute to inflammation and pathology in the lung. Our studies add to this understanding by providing evidence that spike protein alone can mediate adverse effects on vascular cells. Understanding these mechanisms of pathogenesis may provide rationale for interventions that could limit microvascular events associated with SARS-CoV-2 infection.

An exploration of bioactive peptides: My collaboration with Ervin G. Erdös

This paper provides a brief historical sketch of the science of biologically active peptides. It also offers the story of how Ervin G. Erdös, a pioneer in the study of metabolism of various peptides, influenced me through collaborations that span many years. I worked in Dr. Erdös's research laboratories in Oklahoma City, Dallas, and Chicago, and we shared research interests through visits across the Atlantic between the former Yugoslavia and the United States. Among other findings, we discovered angiotensin-converting enzyme in the retina, which opened up a new research direction for many scientists interested in serious ocular diseases. This tribute to my mentor paints a portrait of a man who, in addition to his dedication to science and his seminal discoveries about the metabolism of peptides, took the time to invest in training many young scientists. His fine personal qualities explain why all of those who worked with him hold him in such high regard.

Genetically increased angiotensin I-converting enzyme alters peripheral and renal vascular reactivity to angiotensin II and bradykinin in mice

Angiotensin I-converting enzyme (ACE) levels in humans are under strong genetic influence. Genetic variation in ACE has been linked to risk for and progression of cardiovascular and renal diseases. Causality has been documented in genetically modified mice, but the mechanisms underlying causality are not completely elucidated. To further document the vascular and renal consequences of a moderate genetic increase in ACE synthesis, we studied genetically modified mice carrying three copies of the ACE gene (three-copy mice) and littermate wild-type animals (two-copy mice). We investigated peripheral and renal vascular reactivity to angiotensin II and bradykinin in vivo by measuring blood pressure and renal blood flow after intravenous administration and also reactivity of isolated glomerular arterioles by following intracellular Ca2+ mobilization. Carrying three copies of the ACE gene potentiated the systemic and renal vascular responses to angiotensin II over the whole range of peptide concentration tested. Consistently, the response of isolated glomerular afferent arterioles to angiotensin II was enhanced in three-copy mice. In these mice, signaling pathways triggered by endothelial activation by bradykinin or carbachol in glomerular arterioles were also altered. Although the nitric oxide (NO) synthase (NOS)/NO pathway was not functional in arterioles of two-copy mice, in muscular efferent arterioles of three-copy mice NOS3 gene expression was induced and NO mediated the effect of bradykinin or carbachol. These data document new and unexpected vascular consequences of a genetic increase in ACE synthesis. Enhanced vasoconstrictor effect of angiotensin II may contribute to the risk for cardiovascular and renal diseases linked to genetically high ACE levels.

NEW & NOTEWORTHY A moderate genetic increase in angiotensin I-converting enzyme (ACE) in mice similar to the effect of the ACE gene D allele in humans unexpectedly potentiates the systemic and renal vasoconstrictor responses to angiotensin II. It also alters the endothelial signaling pathways triggered by bradykinin or carbachol in glomerular efferent arterioles.

Angiotensin-converting enzyme inhibitors reduce mortality compared to angiotensin receptor blockers: Systematic review and meta-analysis

Background There are few reviews comparing the long-term outcomes of the use of angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers in a hypertensive population because both are effective in reducing blood pressure. None of them compared angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers with a placebo group in patients with essential hypertension, because few studies exist with this design. Methods A systematic search of PUBMED, LILACS, SCIELO, ICTRP, Cochrane, EMBASE and ClinicalTrials.gov from 1 January 2000 until 31 December 2015 selected prospective studies that reported an association between the use of angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers in the following cardiovascular outcomes: heart failure/hospitalisation, stroke, acute myocardial infarction, total cardiovascular deaths, total deaths and total outcomes. Summary odds ratios (ORs) and 95% confidence intervals (CIs) were combined by using a fixed-effects model. Results Seventeen studies ( n = 73,761) were included of which 12 studies were randomly assigned to angiotensin II receptor blocker therapy ( n = 24,697) and five to angiotensin-converting enzyme inhibitors ( n = 12,170). Angiotensin-converting enzyme inhibitors proved to be significant in reducing total deaths (OR 0.85, 95% CI 0.78-0.93) and cardiovascular deaths (OR 0.77, 95% CI 0.69-0.87). Angiotensin II receptor blocker therapy did not show a reduction in total deaths (OR 1.02, 95% CI 0.96-1.09) or cardiovascular deaths (OR 0.95, 95% CI 0.86-1.06). For acute myocardial infarction, stroke and heart failure/hospitalisation, the reductions were significant for both classes. Conclusion Angiotensin-converting enzyme inhibitor or angiotensin II receptor blocker use is similar in preventing major cardiovascular outcomes regarding acute myocardial infarction, stroke and heart failure/hospitalisation. However, the use of angiotensin-converting enzyme inhibitors is more effective in reducing total deaths and cardiovascular deaths than angiotensin II receptor blockers.

Characterization of thoracic spinal neurons with noxious convergent inputs from heart and lower airways in rats

Respiratory symptoms experienced in some patients with cardiac diseases may be due to convergence of noxious cardiac and pulmonary inputs onto neurons of the central nervous system. For example, convergence of cardiac and respiratory inputs onto single solitary tract neurons may be in part responsible for integration of regulatory and defensive reflex control. However, it is unknown whether inputs from the lungs and heart converge onto single neurons of the spinal cord. The present aim was to characterize upper thoracic spinal neurons responding to both noxious stimuli of the heart and lungs in rats. Extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated male rats. A catheter was placed in the pericardial sac to administer bradykinin (BK, 10 microg/ml, 0.2 ml, 1 min) as a noxious cardiac stimulus. The lung irritant, ammonia, obtained as vapor over a 30% solution of NH(4)OH was injected into the inspiratory line of the ventilator (0.5-1.0 ml over 20 s). Intrapericardial bradykinin (IB) altered activity of 58/65 (89%) spinal neurons that responded to inhaled ammonia (IA). Among those cardiopulmonary convergent neurons, 81% (47/58) were excited by both IA and IB, and the remainder had complex response patterns. Bilateral cervical vagotomy revealed that vagal afferents modulated but did not eliminate responses of individual spinal neurons to IB and IA. The convergence of pulmonary and cardiac nociceptive signaling in the spinal cord may be relevant to situations where a disease process in one organ influences the behavior of the other.

Parasite cysteine proteinase interactions with α2-macroglobulin or kininogens: differential pathways modulating inflammation and innate immunity in infection by pathogenic trypanosomatids

Plasma extravasation is a common endothelium response to tissue injury provoked by pathogens. Herein I will review studies showing that host proteinase inhibitors (e.g., alpha2-macroglobulin (alpha2M) or kininogens) interact with protozoan cysteine proteinases (CPs) in extravascular infection sites, linking inflammation to innate immunity by different mechanisms. Using human monocytes as antigen presenting cells, we first demonstrated that alpha2M entrapment of cruzipain, a Trypanosoma cruzi CP, reduced the activation threshold of cruzipain-specific CD4 T cells due to facilitated uptake of alpha2M-cruzipain complexes by the multiscavenger receptor (CD91). More recently, studies of the mechanisms underlying inflammation elicited by T. cruzi revealed that kininogens, once bound to glycosaminoglycans, are not able to efficiently inactivate cruzipain via their inhibitory cystatin-like domains. Instead, we found that cruzipain readily processes surface-bound kininogens, liberating bioactive kinins. Acting as paracrine hormones, kinins vigorously activate host cells through bradykinin (BK) receptors, thus stimulating endocytic uptake of the pathogen. Rather than unilaterally enhancing parasite infectivity, the liberated kinins activate innate immunity by potently stimulating dendritic cell maturation via the BK B2 receptor. The discovery of chagasin, a novel family of endogenous inhibitors expressed by trypanosomatids, is likely another regulatory player involved in the dynamics of the inflammatory response.

Association of vitamin D receptor gene polymorphisms with childhood and adult asthma

Vitamin D receptor (VDR) polymorphisms have been associated with several immune-related diseases, and VDR and vitamin D itself modulate T cell differentiation. VDR maps to chromosome 12q, near a region commonly linked to asthma. We evaluated VDR as part of a 12q positional candidate survey, and in response to observations of VDR polymorphism associations with asthma and atopy in a founder population of Quebec. Twenty-eight loci in 7 positional candidates (7 in VDR) were genotyped in 582 families. Whereas other candidates demonstrated no association, the VDR ApaI polymorphism demonstrated significant transmission distortion, with undertransmission of the C allele in a ratio of 4:5 (p = 0.01). This association was most prominent in girls, in whom distortion was more marked (p = 0.009). Sex-specific associations between multiple VDR polymorphisms and immunoglobulin E levels were also observed (p = 0.006-0.01). Asthma associations were replicated in a second cohort (517 females with asthma and 519 matched control subjects): 4 of 6 VDR variants demonstrated significant association (p = 0.02-0.04). The direction of association in this second cohort was opposite to the effects seen in the trios, but similar to findings in the Quebec study. These results suggest that VDR influences asthma and allergy susceptibility in a complex manner.

New aspects on angiotensin-converting enzyme: from gene to disease

Angiotensin-converting enzyme (ACE) is a well known zinc-metallopeptidase that converts angiotensin I to the potent vasoconstrictor angiotensin II and that degrades bradykinin, a powerful vasodilator, both for regulation of vascular tone and cardiac functions. Other natural substrates of ACE were identified broadening the functions of this enzyme within different physiological contexts such as neuronal metabolism, hematopoiesis, digestion and reproduction. Synthetic substrates were developed for the determination of ACE activity in various biological fluids, mostly human plasma, for the diagnosis of sarcoidosis and other granulomatous diseases. After the successful use of captopril, the first ACE inhibitor in the treatment of hypertension, a number of molecules were synthesized and used in the treatment of congestive heart failure and for preventing cardiac impairment after myocardial infarction. This class of antihypertensive drugs benefited from structural data on carboxypeptidases active site, as ACE molecule has not yet been crystallized. In the last two decades ACE gene has been cloned that allowed the identification (i) of two isoenzymes, one called somatic ACE resulting from gene duplication and primarily expressed in endothelial cells, and the other, called germinative or testicular ACE, resulting from the transcription in the male reproductive system of a more simple gene, (ii) of an hydrophobic C-terminal peptide for membrane-anchoring and specifically cleaved by a metalloprotease to release soluble forms of both isoenzymes, and (iii) of several allelic polymorphisms, one of them consisting of an insertion/deletion (I/D) polymorphism in a short intronic Alu sequence that could account for half the variance in plasma ACE level and resulting in a large inter-individual variability; moreover this I/D polymorphism was proposed as a genetic marker for identifying individuals at high risk of ischemic heart disease and of anticipating in one individual the efficacy of the antihypertensive therapy, although conflicting data arose from the past decade literature. Moreover, ACE gene cloning has confirmed the expression of the enzyme in endothelial cell, in particular as an ecto-enzyme facing the vascular lumen, but not to the same extent with regard to the vascular origin of the cells. Plasma ACE in healthy subjects arises essentially from the endothelium. On the other hand, in granulomatous diseases where a local stimulation of macrophages leads to an abnormal ACE secretion, it can also be found in other biological fluids such as cerebrospinal and broncho-alveolar fluids. Low plasma ACE levels result from endothelium impairment such as in deep vein thrombosis or in endothelio-toxic anticancer therapies. Another cause of low, sometimes undetectable, plasma ACE levels is the use of an ACE inhibitor, but this is without any significance with regard to its clinical benefits. Albeit molecular cloning has provided a number of new details on ACE structure and function, many questions still remain, in particular about its tertiary structure including glycosylations, about its tissue-specific expression and regulation, and also about the exact significance of the I/D polymorphism in cardiovascular pathology including the pharmacogenomic field.

The kallikrein-kininogen-kinin system: lessons from the quantification of endogenous kinins

The purpose of the present review is to describe the place of endogenous kinins, mainly bradykinin (BK) and des-Arg(9)-BK in the kallikrein-kininogen-kinin system, to review and compare the different analytical methods reported for the assessment of endogenous kinins, to explain the difficulties and the pitfalls for their quantifications in biologic samples and finally to see how the results obtained by these methods could complement and extend the pharmacological evidence of their pathophysiological role.

Degradation of bradykinin by peritoneal and alveolar macrophages of the guinea pig

Peptidase inhibitors and identification of the peptide fragments were used for the characterization of the bradykinin metabolism by alveolar and peritoneal macrophages. Both cell types show differences in the rate of inactivation and in the quantity of the metabolites generated. BK(1-5), BK(1-8), and BK(1-7) are the predominant direct metabolites. Metalloendopeptidase 24.15, carboxypeptidase M, and an unidentified peptidase are responsible for their formation. Angiotensin-converting enzyme and neutral endopeptidase 24.11 do not play a crucial role in the degradation of bradykinin by macrophages. In the bronchoalveolar space, other cells than the macrophages are more important to the breakdown of this peptide.

Peptidase activities in serum and bronchoalveolar lavage fluid from allergic asthmatics--comparison with healthy non-smokers and smokers and effects of inhaled glucocorticoids

BACKGROUND

Neuropeptides may be involved in the pathogenesis of asthma by evoking neurogenic inflammation. Since the effects of neuropeptides are limited by peptidases, reduced activity of peptidases may contribute to the inflammatory process.

OBJECTIVE

We hypothesized that soluble peptidase activities are decreased in asthmatics and that inhaled glucocorticoids exert part of their anti-inflammatory action by increasing soluble peptidase activities.

METHODS

Serum and bronchoalveolar lavage (BAL) fluid was obtained from non-smoking and smoking volunteers and from allergic asthmatics both before and after treatment for 12 weeks with placebo or inhaled fluticasone propionate. Activities of neutral endopeptidase (NEP), aminopeptidase N (APN) and dipeptidyl peptidase IV (DPP IV) were determined using colourometric assays.

RESULTS

Reduced DPP IV activity in serum and reduced NEP activity in BAL fluid were found in healthy smokers compared with non-smokers. In contrast, no differences in peptidase activities in serum or BAL fluid were observed between allergic asthmatics and healthy non-smokers. Fluticasone propionate treatment did not affect peptidase activities in the asthmatic patients.

CONCLUSIONS

We conclude that reduced peptidase activities in serum or BAL fluid can be found in healthy smokers, but not in allergic asthmatics, and that inhaled glucocorticoids do not affect peptidase activities in BAL fluid or serum of asthmatics. Our results do not support the hypothesized dysfunction of peptidases in the asthmatic airways.

Effects of aminopeptidase P inhibition on kinin-mediated vasodepressor responses

We studied in anesthetized rats whether aminopeptidase P (AMP) may be involved in bradykinin (BK) metabolism and responses. For this we inhibited AMP with the specific inhibitor apstatin (Aps). Studies were done with Aps alone or together with the angiotensin-converting enzyme inhibitor lisinopril (Lis). Aps increased the vasodepressor response to an intravenous bolus of BK (400 ng/kg): vehicle, -3.0 +/- 0.7 mmHg; Aps, -7.8 +/- 0.7 mmHg (P < 0.01 vs. vehicle); Lis, -23.8 +/- 1.8 mmHg; Aps + Lis, -37.5 +/- 1.9 mmHg (P < 0.01 vs. Lis). Aps did not affect the vasodepressor response to BK given into the descending aorta. Plasma BK increased only in Aps + Lis-treated rats (in pg/ml): control, 48.0 +/- 1.4; Lis, 57.5 +/- 7.6; Aps + Lis, 121. 8 +/- 30.6 (P < 0.05 vs. control or Lis), whereas in rats infused with BK (400 ng. kg-1. min-1 for 5 min), Aps increased plasma BK (in pg/ml): control, 51.9 +/- 2.5; Aps, 83.5 +/- 20.5; Lis, 725 +/- 225; Aps + Lis, 1,668 +/- 318 (P < 0.05, Aps vs. control and Lis vs. Aps + Lis). In rats with aortic coarctation hypertension, the acute antihypertensive effects of Aps plus Lis were greater than Lis alone (P < 0.01). Hoe-140, a BK B2-receptor antagonist, abolished the difference. We concluded that in the rat AMP contributes to regulation of BK metabolism and responses.

Kallidin- and bradykinin-degrading pathways in human heart: degradation of kallidin by aminopeptidase M-like activity and bradykinin by neutral endopeptidase

BACKGROUND

Since kinins kallidin (KD) and bradykinin (BK) appear to have cardioprotective effects ranging from improved hemodynamics to antiproliferative effects, inhibition of kinin-degrading enzymes should potentiate such effects. Indeed, it is believed that this mechanism is partly responsible for the beneficial effects of angiotensin-converting enzyme (ACE) inhibitors. In the heart, enzymes other than ACE may contribute to local degradation of kinins. The purpose of this study was to investigate which enzymes are responsible for the degradation of KD and BK in human heart tissue.

METHODS AND RESULTS

Cardiac membranes were prepared from the left ventricles of normal (n=5) and failing (n=10) hearts. The patients had end-stage congestive heart failure as the result of coronary heart disease or idiopathic dilated cardiomyopathy. Heart tissue was incubated with KD or BK in the presence or absence of enzyme inhibitors. We found no difference in the enzymes responsible for kinin metabolism or their activities between normal and failing hearts. Thus KD was mostly converted into BK by the aminopeptidase M-like activity. When BK was used as substrate, it was converted into an inactive metabolite BK-(1-7) mostly (80% to 90%) by the neutral endopeptidase (NEP) activity, with ACE unexpectedly playing only a minor role. The low enzymatic activity of ACE in the cardiac membranes, compared with that of NEP, was not due to chronic ACE inhibitor therapy, because the cardiac ACE activities of patients, whether receiving ACE inhibitors or not, and of normal subjects were all equal.

CONCLUSIONS

The present in vitro study shows that in human cardiac membranes, the most critical step in kinin metabolism, that is, inactivation of BK, appears to be mediated mostly by NEP. This observation suggests a role for NEP in the local control of BK concentration in heart tissue. Thus inhibition of cardiac NEP activity could be cardioprotective by elevating the local concentration of BK in the heart.

An inhibitor of angiotensin converting enzyme (enalapril) augments endotoxin-induced hypotension in the pig

Septic shock causes an extensive inflammatory reaction including increased capillary leakage and a decrease in systemic blood pressure. Human septic shock can be replicated in the endotoxaemic pig. Angiotensin converting enzyme (ACE) is involved in the degradation of bradykinin, an inflammatory mediator, and in the regulation of blood pressure. Inhibition of ACE is a common approach to reduce hypertension as well as left ventricular insufficiency. Fifteen anaesthetised pigs received a continuous 3 h endotoxin infusion. The animals were randomly given an inhibitor of ACE (enalpril) [at a dose (0.5 mg x kg-1) that did not per se reduce mean arterial blood pressure (MAP); (n = 7)], or the corresponding volume of saline (n = 8). Another seven pigs were randomised for treatment with enalapril (0.5 mg x kg-1) + saline (n = 3). Four pigs were randomised to serve as untreated controls (saline + saline). Basic physiologic variables were registered. Endotoxaemia progressively reduced MAP. This decrease was significantly augmented by enalapril. Hypovolemia caused by increased permeability or salt/water excretion did not seem to explain this effect as neither blood haemoglobin nor plasma sodium differed between the two groups of endotoxaemic pigs. Inhibitors of ACE are known to potentiate the cardio-depressant effect of bradykinin. This may explain the reduction in MAP by enalapril during porcine endotoxaemia.

B1 kinin receptor activity in pigs is associated with pre-existing infection

Bradykinin (BK) and related kinins are potent inflammatory mediators produced during acute and chronic inflammation. The effects of these kinins are mediated via the stimulation of either a B2 or a B1 receptor. The B1 receptor is not normally present but its expression can be induced within 4 h by a variety of noxious stimuli, specifically, gram-negative bacteria or bacterial lipopolysaccharide (LPS) given to healthy animals. This study compared the cardiovascular response of healthy pigs and pigs diagnosed with a pre-existing spontaneously acquired infection to BK, a B2 receptor agonist, and des-Arg9-BK, a B1 receptor agonist. Eighty-eight percent of the animals diagnosed with an established infection based on a standardized clinical evaluation demonstrated increased sensitivity and responsiveness to des-Arg9-BK but normal responsiveness to BK and acetylcholine. In contrast, only 15% of healthy animals showed elevated responses to des-Arg9-BK. The response to des-Arg9-BK and BK in each group was characterised as B1 and B2, respectively, using the selective B1 and B2 antagonists Lys0-Leu8-des-Arg9-BK and Hoe 140, respectively. This study demonstrates the existence and function of the B1 receptor in animals with a previously acquired infection. These observations lend validity to animal experiments with LPS infusion in order to model bacterial inflammation.

Metabolism of bradykinin by the rat coronary vascular bed

OBJECTIVE

To study the metabolism of bradykinin (BK) after a single passage through the coronary bed in isolated Langendorff rat hearts.

METHODS

BK was infused into the aortic flow line to obtain a final concentration of 10 nM, and the coronary, effluent was collected to quantify BK and des-Arg9-BK by competitive enzyme immunoassay. The nature of immunoreactive material was confirmed by immunograms after HPLC separation. The experiments were performed with hearts perfused at either one of the following coronary flow rates: 1, 5 or 10 ml/min.

RESULTS

BK recovery without inhibitors was 86.3 +/- 2.9, 60.8 +/- 6.3, and 29.6 +/- 6.8% at 10, 5, and 1 ml/min, respectively. The Vmax/Km ratios at these coronary flow rates were 2.19 +/- 0.72, 4.81 +/- 0.64, and 2.59 +/- 0.33 min-1 g-1), respectively. The angiotensin-converting enzyme (ACE) inhibitor, enalaprilat (130 nM), reduced BK degradation at all flow rates. Inhibition of neutral endopeptidase with retrothiorphan (25 nM) had no effect on BK degradation. However, the combined treatment with enalapril and retrothiorphan reduced BK degradation to lower values than enalaprilat alone. The effect of enzyme inhibitors on BK recovery was inversely related to coronary flow: inhibiting BK degradation markedly increased BK recovery at 1 ml/min, but had no effect at 10 ml/min. The kininase I metabolite of BK, des-Arg9-BK, could not be detected under these experimental conditions.

CONCLUSIONS

ACE is the major enzyme responsible for BK degradation during a single passage through the coronary bed. Neutral endopeptidase contributes to BK degradation only when ACE activity is impaired. The effect of enzyme inhibitors on the coronary concentration of BK is highly dependent on coronary flow rate.

Peptidases in human bronchoalveolar lining fluid, macrophages, and epithelial cells: dipeptidyl (amino)peptidase IV, aminopeptidase N, and dipeptidyl (carboxy)peptidase (angiotensin-converting enzyme)

The modulation of proteolytic activity is an important factor in regulating the metabolism and function of peptide hormones. In this study, the activities of dipeptidyl (carboxy)peptidase (angiotensin-converting enzyme [ACE]), aminopeptidase N (APN), and dipeptidyl (amino)peptidase IV (DPP IV) were measured in the blood, the human bronchial epithelial and alveolar cells, bronchoalveolar macrophages, and the soluble phase of bronchoalveolar lavage (BAL) samples obtained from normal human volunteers and patients with pulmonary pathologic conditions. BAL fluid expressed ACE activity and very low levels of APN and DPP IV activities in the volunteer population, but higher levels could be measured in samples from patients. In patients, increased APN corresponded to a high granulocyte count, while DPP IV and ACE were associated with a high percentage of lymphocytes. Neither AIDS nor smoking induced an increased level of these enzymes. Immunohistochemical staining of bronchoalveolar smears with anti-human ACE monoclonal antibody showed that only macrophages expressed this enzyme. Enzyme histochemistry for DPP IV and APN showed that all leukocytes expressed these activities. APN, DPP IV, and ACE activities were also found in cell extracts of bronchoalveolar macrophages. In extracts of bronchial epithelial and alveolar cells, only APN and DPP IV activities were detected. Kinetic properties of the soluble enzymes in lavage supernatants were comparable to those of serum enzymes. These results demonstrate that soluble forms of cellular enzymes found in BAL fluid are regulated independently of blood and that different cell types may release these enzymes.

Contribution of angiotensin-converting enzyme to the cardiac metabolism of bradykinin: an interspecies study

The role of angiotensin-converting enzyme (ACE) in the metabolism of bradykinin (BK) has been studied in several tissues. However, and contrary to angiotensin I, the metabolism of BK at the cardiac level has not been investigated. In this study, we define the participation of ACE in the carboxy-terminal degradation of BK in heart membranes of the dog, human, rabbit, and rat. The calculation of the kinetic parameters characterizing the metabolism of BK and the generated des-Arg9-BK can be summarized as follows: the half-life (t1/2) of BK [dog (218 +/- 32 s) > human (143 +/- 9 s) = rat (150 +/- 4 s) > rabbit (22 +/- 2 s)] and of des-Arg9-BK [dog (1,042 +/- 40 s) > human (891 +/- 87 s) > rat (621 +/- 65 s) > rabbit (89 +/- 8 s)] both showed significant differences according to species. Enalaprilat, an ACE inhibitor, significantly prevented the rapid degradation of BK and des-Arg9-BK in all species studied, whereas retrothiorphan, a neutral endopeptidase inhibitor, and losartan, an angiotensin II type I receptor antagonist, did not affect this metabolism. The relative importance of ACE in the cardiac metabolism of BK was species related: dog (68.4 +/- 3.2%) = human (72.2 +/- 2.0%) > rabbit (47.7 +/- 5.0%) = rat (45.3 +/- 3.9%). ACE participation in the metabolism of des-Arg9-BK was as follows: rabbit (57.0 +/- 4.0%) > dog (39.9 +/- 8.8%) = human (25.4 +/- 5.5%) = rat (36.0 +/- 7.0%). The participation of cardiac kininase I (carboxypeptidase M) in the transformation of BK into des-Arg9-BK was minor: human (2.6 +/- 0.1%) > dog (0.9 +/- 0.1%) = rabbit (1.0 +/- 0.1%) = rat (1.0 +/- 0.1%). These results demonstrate that ACE is the major BK-degrading enzyme in cardiac membranes. However, the metabolism of exogenous BK by heart membranes is species dependent. Our observations could explain some discrepancies regarding the contribution of kinins in the cardioprotective effects of ACE inhibitors.

Pulmonary C-fiber activation before and after peptidase inhibition in rats

Inhibition of peptidases within the lungs not only potentiates the effects of neuropeptides released from C-fibers but also the effects of bradykinin and capsaicin both of which stimulate C-fibers. To determine if peptidase inhibition potentiates C-fiber activation, we challenged pulmonary C-fibers in rats with capsaicin or bradykinin before and after inhibition of neutral endopeptidase (NEP) or angiotensin converting enzyme (ACE). Inhibition of NEP by phosphoramidon (10 mg/kg, i.v.) potentiated the effect of capsaicin (0.5-1 micrograms, i.v.) on C-fiber activity but did not change the response to bradykinin (1-2 micrograms, i.v.). Inhibition of ACE by captopril (5 mg/kg, i.v.) potentiated C-fiber activation by either bradykinin or capsaicin. Aerosol administration of either phosphoramidon (1 x 10(-5) M, 2 min) or captopril (4.6 x 10(-3) M, 2 min) potentiated C-fiber activation by capsaicin aerosol (1.6 x 10(-4) M, 1 min) but not by bradykinin aerosol (9.4 x 10(-5) M, 1 min). Therefore, inhibition of NEP or ACE may potentiate airway obstructive mechanisms initiated by C-fiber stimulation.

Metabolism of Bradykinin by Peptidases in Health and Disease

This chapter provides an overview of the metabolism of bradykinin (BK) by peptidases in health and disease. The enzymatic breakdown of kinins affects the duration of their biological actions as the plasma half-life of intravenously injected BK is in the range of seconds. Kinins are cleaved in vitro and in vivo by enzymes that belong to families, such as zinc-metallopeptidases, astacin-like metallopeptidases, and catheptic enzymes. Vane noted the importance of the pulmonary circulation in the metabolism of vasoactive substances, such as BK as well as angiotensin 1 and 5- hydroxytryptamine. It is clear after decades of research that angiotensin 1-converting enzyme (ACE) on the vascular endothelial cell surface is the most important inactivator of blood-borne BK. BK may act primarily in an autocrine and paracrine fashion, establishing the importance of local regulation of its activity by enzymes on cell surfaces. Thus, the assortment of other enzymes that can inactivate BK is important in a variety of physiological and pathological situations. Most physiological systems have redundant pathways of metabolism so that the abolishment of one pathway is compensated for by the presence of others. This is demonstrated by the pharmacological inhibition of ACE in hypertension.

Effects of captopril on responses to bradykinin in the hindquarters vascular bed of the rat

The effects of captopril, and angiotensin converting enzyme (ACE) inhibitor, on responses to bradykinin (BK), to angiotensin (Ang) I and II, and to other agonists were investigated in the hindquarters vascular bed of the rat. Under conditions of controlled flow, intra-arterial (i.a.) injections of BK in doses of 0.1-1.0 microgram, produced dose related decreases in hindquarters perfusion pressure and evoked decreases in systemic arterial pressure. Intra-arterial injections of Ang I and II produced dose-related increases in hindquarters perfusion pressure. Following administration of captopril in a dose of 2 mg/kg i.v., vasodilator responses to i.a. injections of BK were only slightly enhanced in the hindquarters vascular bed, whereas the evoked systemic vasodepressor responses to i.a. injections of BK were markedly enhanced by the ACE inhibitor. Captopril significantly reduced vasoconstrictor responses to i.a. injections of Ang I, whereas vasoconstrictor responses to i.a. injections of Ang II were significantly enhanced. The ACE inhibitor did not significantly alter vasodilator responses to i.a. injections of acetylcholine, nitroglycerin, nitric oxide, albuterol, or pinacidil. The present data show that BK has potent vasodilator activity in the hindquarters vascular bed of the rat and suggest that the site of action is most likely upstream from the site of inactivation, whereas the site of action of Ang I is at or near the site of conversion to Ang II in the hindquarters vascular bed. The observation that the evoked systemic vasodepressor responses to i.a. injections of BK were greatly enhanced, suggest that the lung is the major site of inactivation of BK.

Influence of anti-inflammatory and antioxidant agents on endothelial permeability alterations induced by bradykinin

Increased vascular permeability to plasma proteins and altered hemodynamics at the site of inflammation are characteristics of inflammation. In the present study, alterations in endothelial barrier permeability were evaluated in different organs/tissues 6 h after a systemic inflammatory response induced by intravenous injection of bradykinin (BK; 1.7 mg/kg). The effect of intravenous pretreatment with indomethacin or ibuprofen (cyclooxygenase inhibitors), N-acetyl-L-cysteine (NAC, an oxygen free radical scavenger), and allopurinol (a xanthine oxidase inhibitor) was determined. Endothelial permeability was evaluated by determining tissue water content (TWC), 125I-labeled human serum albumin (HSA) flux, and albumin leakage index (ALI) in various organs/tissues. The vasodilation in the local tissues was reflected by tissue blood content (TBC), measured by 51Cr-labeled red blood cells. The results indicate that albumin flux significantly increased in the peritoneum, pancreas, stomach, PSI, DSI, colon, kidneys, liver, lungs, and brain, TBC significantly increased in the kidneys, liver, lungs, and heart, as well as in the intestine, and an increased ALI, assaying endothelial permeability considering local hemodynamic alterations was noted in the pancreas, kidneys, liver, lungs, PSI, and DSI in the group with BK alone. These changes were to varying degrees reversed by pretreatment with indomethacin, ibuprofen, N-acetyl-L-cysteine, or allopurinol, where the protective effect tended to be organ-dependent.

The release of kininase from rat isolated hearts during myocardial ischaemia

BK destroying activity was observed in rat isolated heart perfusates. BK was optimally degraded at pH 8.4 in rat heart. The results indicated that myocardial kinin degradation was due to ACE and a serine protease. These results suggest that bradykinin may have some cardioprotective role during myocardial ischaemia at acidic pH.

Cyclophilin-facilitated bradykinin inactivation in the perfused rat lung

Cis and trans isomers of X-proline (X-Pro) bonds can influence some aspects of the kinetics of peptide metabolism. We previously used the peptidyl-prolyl cis-trans isomerase, cyclophilin, to show that angiotensin converting enzyme (ACE) preferentially hydrolyzes the trans isomer of a synthetic tripeptide that contains a C-terminal proline (Dawson et al., Am J Physiol 257:H853-H865, 1989; Merker et al., J Appl Physiol 75: 1519-1524, 1993). Bradykinin (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) exists as both cis and trans isomers at all three X-Pro bonds, and although its inactivation in the lung by pulmonary endothelial peptidases is extensive, commonly a small fraction of the peptide survives passage through the lung. To determine whether the presence of cis X-Pro bonds might limit the extent of bradykinin metabolism in the lung, we studied inactivation of bradykinin by the isolated perfused rat lung using the rabbit jugular vein superfused with the pulmonary venous effluent as a bioassay for bradykinin. A large fraction (> 90%) of the bradykinin in a bolus injection was inactivated in a single transit through the pulmonary circulation, but a detectable fraction emerged in the venous effluent. The addition of cyclophilin to the bradykinin in the bolus reduced the bradykinin emerging from the lungs to virtually undetectable levels. When the isomerase inhibitor cyclosporin A was included with bradykinin and cyclophilin in the injectate, this effect of cyclophilin was reversed. These observations suggest that the fraction of bradykinin that normally survives passage through the lungs contains isomers that have at least one X-Pro bond that is refractory to enzymatic inactivation and whose isomerization time constant is significantly longer than the pulmonary capillary transit time.

Carboxypeptidase M is identical to the MAX.1 antigen and its expression is associated with monocyte to macrophage differentiation

The two monoclonal antibodies MAX.1 and MAX.11 recognize cell surface antigens that are almost undetectable on monocytes but highly expressed on differentiated macrophages. Biochemical characterization revealed that both antibodies detect the same 58-64-kDa glycoprotein anchored to the plasma membrane by glycosyl-phosphatidylinositol linkage. We purified the MAX.1/11 antigen by immunoaffinity chromatography using monoclonal antibody MAX.11. The NH2-terminal amino acid sequence was determined and turned out to be identical to the NH2-terminal sequence of the membrane-bound carboxypeptidase M. By precipitation with antibodies MAX.1 and MAX.11, membrane preparations of macrophages and placental microvilli were almost completely depleted of enzyme activity, indicating that the two antibodies indeed recognize carboxypeptidase M. Immunoreactivity of both antibodies correlates with the reported tissue distribution of enzyme activity. Expression of carboxypeptidase M on mRNA level and enzymatic activity markedly increase during in vitro differentiation of monocytes, according to the described increase in MAX.1 and MAX.11 antigen expression. Moreover, in vitro differentiated macrophages show the highest specific activity yet described in any tissue. In addition, carboxypeptidase M expression could be detected in HL-60, U937, and THP-1 myeloid cell lines. Vitamin D3-induced monocytic differentiation resulted in an increased carboxypeptidase M expression in all three cell lines. Further studies are needed to elucidate the functional role of carboxypeptidase M during monocytic differentiation and activation.

des-Arg9-bradykinin produces tone-dependent kinin B1 receptor-mediated responses in the pulmonary vascular bed

Responses to des-Arg9-bradykinin, a selective kinin B1 receptor agonist, were characterized in the pulmonary vascular bed of the intact-chest cat. Injections of des-Arg9-bradykinin into the perfused lobar artery under low-resting tone conditions caused dose-related increases in lobar arterial pressure; whereas in the same experiment under elevated tone conditions, injections of the B1 agonist caused dose-related decreases in lobar arterial pressure. Vasoconstrictor responses to des-Arg9-bradykinin under low-tone conditions and vasodilator responses under elevated-tone conditions were antagonized by des-Arg9,[Leu8]-bradykinin, a kinin B1 receptor antagonist, whereas responses under low- and high-tone conditions were not altered by Hoe 140, a kinin B2 receptor antagonist. Vasoconstrictor responses to des-Arg9-bradykinin under low-tone conditions were attenuated by phentolamine, prazosin, and reserpine but not by sodium meclofenamate, suggesting that release of catecholamines and activation of alpha-adrenergic receptors are involved. Pulmonary vasodilator responses under elevated-tone conditions were inhibited by N omega-nitro-L-arginine methyl ester, suggesting that des-Arg9-bradykinin stimulates the release of nitric oxide, whereas meclofenamate and U-37883A, a nonsulfonylurea ATP-sensitive K+ channel antagonist, did not alter vasodilator responses to the B1 receptor agonist. These results suggest that vasoconstrictor responses to des-Arg9-bradykinin under low-tone conditions are mediated by the activation of kinin B1 receptors, the release of catecholamines within the lung, and the activation of alpha-adrenergic receptors, whereas vasodilator responses under elevated tone conditions are mediated by activation of B1 receptors and the release of nitric oxide from the endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of either angiotensin-converting enzyme or neutral endopeptidase induces both enzymes

Synthesis of angiotensin-converting enzyme is induced during its chronic inhibition. Like angiotensin-converting enzyme, neutral endopeptidase (EC 3.4.24.11) is a plasma membrane peptidase. We studied changes of the two enzymes in lung, kidney and serum in a coronary ligation model of experimental congestive heart failure, and during chronic inhibition of the enzymes. Coronary-ligated rats (n = 19) and sham-operated controls (n = 18) were given SCH 34826 [(S)-N-[N-[1-[[(2,2-dimethyl-1,3-dioxolan-4-yl) methoxy]carbonyl]-2-phenylethyl]-L-phenylalanine]-beta-alanine], a specific neutral endopeptidase inhibitor (n = 13), captopril (n = 12), or vehicle (n = 12) for 4 days, and exsanguinated. Pulmonary angiotensin-converting enzyme was induced both by captopril (52% compared to vehicle) and by SCH 34826 (21%). Serum angiotensin-converting enzyme was induced by captopril (44%). Neutral endopeptidase was induced in lung by captopril (73%), and in kidney by SCH 38426 (32%). Compared to controls, the relative heart weight of rats with heart failure was increased by 29%, and the plasma level of atrial natriuretic peptide elevated by 74%, but enzyme activities were not different. We conclude that, in the rat, separate inhibition of either angiotensin-converting enzyme or neutral endopeptidase induces both enzymes, and that the induction varies in different tissues. Alterations in the substrates of the two enzymes, e.g. in bradykinin, might cause these changes.

High concentration of carboxypeptidase M in lungs: presence of the enzyme in alveolar type I cells

The presence of high concentrations of membrane-bound carboxypeptidase M in human, baboon, dog, and rat lung was established by employing a variety of techniques. The activity of the enzyme in the membrane-enriched fractions of human, baboon, dog, and rat lung, measured with fluorescent dansyl substrate (DNS-Ala-Arg), was 198, 261, 484, and 153 nmol/h/mg protein, respectively. This activity in the lung was much higher than that found in the heart, liver, or kidney. The enzyme, optimally active around neutral pH, was completely inhibited by 10 microM 2-mercaptomethyl-3-guanidinoethylthiopropanoic acid and was activated by 1 mM CoCl2 to 170%. Antibody to human carboxypeptidase M immunoprecipitated the solubilized carboxypeptidase from human (98%), baboon (81%), and dog (88%) lung membrane fractions. Carboxypeptidase M is attached to lung membranes by a phosphatidylinositol glycan anchor; thus, it is released with bacterial phospholipase C. Membrane fractions from cultured human pulmonary arterial endothelial cells also contained high carboxypeptidase M activity (254 nmol/h/mg protein). A Northern blot of poly(A)+ RNA from various human tissues showed the presence of a high level of carboxypeptidase M mRNA in human lung and placenta. Finally, immunohistochemistry, employing purified antibody to the enzyme, revealed in fluorescent light microscopy that carboxypeptidase M is present in alveolar type I pneumocytes and in macrophages in apparently lower concentration. In contrast, type II alveolar epithelial cells gave negative results. Because carboxypeptidase M cleaves a variety of active peptides (e.g., bradykinin, anaphylatoxins), it may protect the alveolar surface from the effects of these peptides. In addition, carboxypeptidase M could be a marker enzyme for type I cells.