Angiotensin and bradykinin metabolism by peptidases identified in cultured human skeletal muscle myocytes and fibroblasts

Angiotensin-converting enzyme 2 (ACE2): Two decades of revelations and re-evaluation

Angiotensin-converting enzyme-2, or ACE2, is primarily a zinc-dependent peptidase and ectoenzyme expressed in numerous cell types and functioning as a counterbalance to ACE in the renin-angiotensin system. It was discovered 21 years ago more than 40 years after the discovery of ACE itself. Its primary physiological activity is believed to be in the conversion of angiotensin II to the vasodilatory angiotensin-(1-7) acting through the Mas receptor. As such it has been implicated in numerous pathological conditions, largely in a protective mode which has led to the search for ACE2 activatory mechanisms. ACE2 has a diverse substrate specificity allowing its participation in multiple peptide pathways. It also regulates aspects of amino acid transport through its homology with a membrane protein, collectrin. It also serves as a viral receptor for the SARS virus, and subsequently SARS-CoV2, driving the current COVID-19 pandemic. ACE2 therefore provides a therapeutic target for the treatment of COVID and understanding the biological events following viral binding can provide insight into the multiple pathologies caused by the virus, particularly inflammatory and vascular. In part this may relate to the ability of ACE2, like ACE, to be shed from the cell membrane. The shed form of ACE2 (sACE2) may be a factor in determining susceptibility to certain COVID pathologies. Hence, for just over 20 years, ACE2 has provided numerous surprises in the field of vasoactive peptides with, no doubt, more to come but it is its central role in COVID pathology that is producing the current intense interest in its biology.

Angiotensin-II Drives Human Satellite Cells Toward Hypertrophy and Myofibroblast Trans-Differentiation by Two Independent Pathways

Skeletal muscle regeneration is ensured by satellite cells (SC), which upon activation undergo self-renewal and myogenesis. The correct sequence of healing events may be offset by inflammatory and/or fibrotic factors able to promote fibrosis and consequent muscle wasting. Angiotensin-II (Ang) is an effector peptide of the renin angiotensin system (RAS), of which the direct role in human SCs (hSCs) is still controversial. Based on the hypertrophic and fibrogenic effects of Ang via transient receptor potential canonical (TRPC) channels in cardiac and renal tissues, we hypothesized a similar axis in hSCs. Toward this aim, we demonstrated that hSCs respond to acute Ang stimulation, dose-dependently enhancing p-mTOR, p-AKT, p-ERK1/2 and p-P38. Additionally, sub-acute Ang conditioning increased cell size and promoted trans-differentiation into myofibroblasts. To provide a mechanistic hypothesis on TRPC channel involvement in the processes, we proved that TRPC channels mediate a basal calcium entry into hSCs that is stimulated by acute Ang and strongly amplified by sub-chronic Ang conditioning. Altogether, these findings demonstrate that Ang induces a fate shift of hSCs into myofibroblasts and provide a basis to support a benefit of RAS and TRPC channel blockade to oppose muscle fibrosis.

Neuropeptides: metabolism to bioactive fragments and the pharmacology of their receptors

The proteolytic processing of neuropeptides has an important regulatory function and the peptide fragments resulting from the enzymatic degradation often exert essential physiological roles. The proteolytic processing generates, not only biologically inactive fragments, but also bioactive fragments that modulate or even counteract the response of their parent peptides. Frequently, these peptide fragments interact with receptors that are not recognized by the parent peptides. This review discusses tachykinins, opioid peptides, angiotensins, bradykinins, and neuropeptide Y that are present in the central nervous system and their processing to bioactive degradation products. These well-known neuropeptide systems have been selected since they provide illustrative examples that proteolytic degradation of parent peptides can lead to bioactive metabolites with different biological activities as compared to their parent peptides. For example, substance P, dynorphin A, angiotensin I and II, bradykinin, and neuropeptide Y are all degraded to bioactive fragments with pharmacological profiles that differ considerably from those of the parent peptides. The review discusses a selection of the large number of drug-like molecules that act as agonists or antagonists at receptors of neuropeptides. It focuses in particular on the efforts to identify selective drug-like agonists and antagonists mimicking the effects of the endogenous peptide fragments formed. As exemplified in this review, many common neuropeptides are degraded to a variety of smaller fragments but many of the fragments generated have not yet been examined in detail with regard to their potential biological activities. Since these bioactive fragments contain a small number of amino acid residues, they provide an ideal starting point for the development of drug-like substances with ability to mimic the effects of the degradation products. Thus, these substances could provide a rich source of new pharmaceuticals. However, as discussed herein relatively few examples have so far been disclosed of successful attempts to create bioavailable, drug-like agonists or antagonists, starting from the structure of endogenous peptide fragments and applying procedures relying on stepwise manipulations and simplifications of the peptide structures.

Renin-angiotensin system: an old player with novel functions in skeletal muscle

Skeletal muscle is a tissue that shows the most plasticity in the body; it can change in response to physiological and pathological stimuli. Among the diseases that affect skeletal muscle are myopathy-associated fibrosis, insulin resistance, and muscle atrophy. A common factor in these pathologies is the participation of the renin-angiotensin system (RAS). This system can be functionally separated into the classical and nonclassical RAS axis. The main components of the classical RAS pathway are angiotensin-converting enzyme (ACE), angiotensin II (Ang-II), and Ang-II receptors (AT receptors), whereas the nonclassical axis is composed of ACE2, angiotensin 1-7 [Ang (1-7)], and the Mas receptor. Hyperactivity of the classical axis in skeletal muscle has been associated with insulin resistance, atrophy, and fibrosis. In contrast, current evidence supports the action of the nonclassical RAS as a counter-regulator axis of the classical RAS pathway in skeletal muscle. In this review, we describe the mechanisms involved in the pathological effects of the classical RAS, advances in the use of pharmacological molecules to inhibit this axis, and the beneficial effects of stimulation of the nonclassical RAS pathway on insulin resistance, atrophy, and fibrosis in skeletal muscle.

Different cross-talk sites between the renin-angiotensin and the kallikrein-kinin systems

Targeting the renin-angiotensin system (RAS) constitutes a major advance in the treatment of cardiovascular diseases. Evidence indicates that angiotensin-converting enzyme inhibitors and angiotensin AT1 receptor blockers act on both the RAS and the kallikrein-kinin system (KKS). In addition to the interaction between the RAS and KKS at the level of angiotensin-converting enzyme catalyzing both angiotensin II generation and bradykinin degradation, the RAS and KKS also interact at other levels: 1) prolylcarboxypeptidase, an angiotensin II inactivating enzyme and a prekallikrein activator; 2) kallikrein, a kinin-generating and prorenin-activating enzyme; 3) angiotensin-(1-7) exerts kininlike effects and potentiates the effects of bradykinin; and 4) the angiotensin AT1 receptor forms heterodimers with the bradykinin B2 receptor. Moreover, angiotensin II enhances B1 and B2 receptor expression via transcriptional mechanisms. These cross-talks explain why both the RAS and KKS are up-regulated in some circumstances, whereas in other circumstances both systems change in the opposite manner, expressed as an activated RAS and a depressed KKS. As the cross-talks between the RAS and the KKS play an important role in response to different stimuli, taking these cross-talks between the two systems into account may help in the development of drugs targeting the two systems.

Intramuscular renin-angiotensin system is activated in human muscular dystrophy

To investigate the role of the muscular renin-angiotensin system (RAS) in human muscular dystrophy, we used immunohistochemistry and Western blotting to examine the cellular localization of angiotensin-converting enzyme (ACE), the angiotensin II type 1 receptor (AT1) and the angiotensin II type 2 receptor (AT2) in muscle biopsies from patients with Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), and congenital muscular dystrophy (CMD). In normal muscle, ACE was expressed in vascular endothelial cells and neuromuscular junctions (NMJs), whereas AT1 was immunolocalized to the smooth muscle cells of blood vessels and intramuscular nerve twigs. AT2 was immunolocalized in the smooth muscle cells of blood vessels. These findings suggest that the RAS has a functional role in peripheral nerves and NMJs. ACE and AT1, but AT2 immunoreactivity were increased markedly in dystrophic muscle as compared to controls. ACE and the AT1 were strongly expressed in the cytoplasm and nuclei of regenerating muscle fibers, fibroblasts, and in macrophages infiltrating necrotic fibers. Double immunolabeling revealed that activated fibroblasts in the endomysium and perimysium of DMD and CMD muscle were positive for ACE and AT1. Triple immunolabeling demonstrated that transforming growth factor-beta1 (TGF-beta1) and ACE were colocalized on the cytoplasm of activated fibroblasts in dystrophic muscle. Furthermore, Western blotting showed increases in the expression of AT1 and TGF-beta1 protein in dystrophic muscle, which coincided with our immunohistochemical results. The overexpression of ACE and AT1 in dystrophic muscle would likely result in the increased production of Ang II, which may act on these cells in an autocrine manner via AT1. The activation of AT1 may induce fibrous tissue formation through overexpression of TGF-beta1, which potently activates fibrogenesis and suppresses regeneration. In conclusion, our results imply that the intramuscular RAS-TGF-beta1 pathway is activated in human muscular dystrophy and plays a role at least partly in the pathophysiology of this disease.

Neprilysin participates in skeletal muscle regeneration and is accumulated in abnormal muscle fibres of inclusion body myositis

Neprilysin (NEP, EP24.11), a metallopeptidase originally shown to modulate signalling events by degrading small regulatory peptides, is also an amyloid-beta- (Abeta) degrading enzyme. We investigated a possible role of NEP in inclusion body myositis (IBM) and other acquired and hereditary muscle disorders and found that in all myopathies NEP expression was directly associated with the degree of muscle fibre regeneration. In IBM muscle, NEP protein was also strongly accumulated in Abeta-bearing abnormal fibres. In vitro, during the experimental differentiation of myoblasts, NEP protein expression was regulated at the post-transcriptional level with a rapid increase in the early stage of myoblast differentiation followed by a gradual reduction thereafter, coincident with the progression of the myogenic programme. Treatment of differentiating muscle cells with the NEP inhibitor dl-3-mercapto-2-benzylpropanoylglycine resulted in impaired differentiation that was mainly associated with an abnormal regulation of Akt activation. Therefore, NEP may play an important role during muscle cell differentiation, possibly through the regulation, either directly or indirectly, of the insulin-like growth factor I-driven myogenic programme. In IBM muscle increased NEP may be instrumental in (i) reducing the Abeta accumulation in vulnerable fibres and (ii) promoting a repair/regenerative attempt of muscle fibres possibly through the modulation of insulin-like growth factor I-dependent pathways.

Angiotensin I-converting enzyme-dependent and neutral endopeptidase-dependent generation and degradation of angiotensin II contrarily modulate noradrenaline release: implications for vasopeptidase-inhibitor therapy?

OBJECTIVES

Vasopeptidase inhibitors inhibit neutral endopeptidase (NEP) and angiotensin I-converting enzyme (ACE). Since angiotensin (ANG) II availability is decreased by ACE inhibition but is increased by NEP inhibition, we evaluated the influence of the vasopeptidase inhibitor omapatrilat on ANG II-dependent noradrenaline (NA) release.

DESIGN

The functional relevance of ACE-dependent and NEP-dependent generation and degradation of ANG II on NA overflow was determined in pithed rats by applications of ANG I (0.1-100 microg/kg) or ANG II (0.01-10 microg/kg) after single injections of ramipril (1 mg/kg), the NEP inhibitor candoxatril (100 mg/kg), or the vasopeptidase inhibitor omapatrilat (30 mg/kg).

RESULTS

Blood pressure was equipotently decreased by ramipril and omapatrilat, but not by candoxatril. NA overflow was increased after ANG I infusions in controls (EC50 = 9.0 microg/kgANG I, Emax = 5680 pg/ml), but almost completely suppressed by ramipril and omapatrilat. Candoxatril decreased EC50 (4.1 microg/kg) and increased Emax (7259 pg/ml). NA overflow after ANG II infusions was enhanced by candoxatril or omapatrilat. Ex vivo ACE activity was extensively inhibited by ramipril or omapatrilat, whereas ex vivo NEP activity was reduced by omapatrilat and candoxatril only. In vitro, omapatrilat inhibited NEP and ACE with similar potencies (IC50 NEP/IC50 ACE = 0.4).

CONCLUSIONS

Vasopeptidase inhibitors influence ANG II-related NA release depending on their ability to modulate the availability of ANG II via ACE or NEP. After acute application, the vasopeptidase inhibitor suppresses NA release in response to ANG I due to a predominant reduction of ANG II formation. These results indicate that the ratio of ACE-inhibitory and NEP-inhibitory potencies of vasopeptidase inhibitors may be relevant for sympathetic activation in chronic therapy.

Production of Antihypertensive Angiotensin I-Converting Enzyme Inhibitor from Malassezia pachydermatis G-14

To produce a novel antihypertensive angiotensin I-converting enzyme (ACE) inhibitor from yeast, a yeast isolate, designated G-14 showing the highest ACE inhibitory activity was obtained and identified as Malassezia pachydermatis based on morphological, biochemical and cultural characteristics. The maximal extracellular ACE inhibitor production was obtained from M. pachydermatis G-14 when the strain was cultured in YEPD medium containing 0.5% yeast extract, 3.0% peptone and 2.0% glucose at 30℃ for 24 h and the final ACE inhibitory activity was 48.9% under the above condition.

Effect of bestatin on angiotensin I-, II- and III-induced collagen gel contraction in cardiac fibroblasts

OBJECTIVE

The purpose of this investigation was to determine whether the aminopeptidase inhibitor with broad specificity, bestatin, affects angiotensin I (Ang I)-, angiotensin II (Ang II)- or angiotensin III (Ang III)-stimulated collagen gel contraction in cardiac fibroblasts.

DESIGN AND METHODS

Cardiac fibroblasts (from normal male adult rats) were cultured to confluency in Dulbeccos modified Eagles medium (DMEM) with 10% foetal bovine serum (FBS). These fibroblasts (100,000 cells) were then further incubated in a floating collagen gel lattice with the test products Ang I (1 micromol/L), Ang II (100 nmol/L), Ang III (100 nmol/L) and bestatin (100 micromol/L) for three days in DMEM without FBS. The area of the collagen gels embedded with cardiac fibroblasts was determined by a densitometric analysis. Aminopeptidase activity was estimated by spectrophotometric determination of the liberation of p-nitroaniline from alanine- or arginine-p-nitroanilide.

RESULTS

Ang I, II and III stimulated (p<0.05) collagen gel contraction by 30.4+/-4.8 (SEM)%, 27.1+/-3.1% and 15.4+/-3.6% respectively. Ang I- and II-induced stimulation of collagen gel contraction was of the same order but more pronounced (p<0.05) than Ang III- stimulated collagen gel contraction. The Ang I-, II- and III-stimulated collagen contraction was reduced by bestatin. Bestatin, however, did not affect basal collagen gel contraction in cardiac fibroblasts. Bestatin dose-dependently inhibited the hydrolysis of arginine- and alanine-p-nitroanilide in cardiac fibroblasts. When a neutralising antibody to transforming growth factor TGF-b1 was added to the collagen gel simultaneously with the angiotensins, the stimulated collagen contraction was not affected. Beta-aminoproprionitrile, an inhibitor of lysyl oxidase, completely abolished basal as well as Ang I-, II- and III-stimulated collagen contraction in cardiac fibroblasts.

RESULTS

Our data suggest that aminopeptidases are involved in the Ang I-, II- and III-induced stimulation of collagen contraction in cardiac fibroblasts.

Kinin peptides in human trapezius muscle during sustained isometric contraction and their relation to pain

To determine the muscular concentration of bradykinin and kallidin during static contraction, microdialysis probes were implanted bilaterally in the trapezius muscles of healthy women. Three hours after probe implantation, 200 μM of the angiotensin-converting enzyme (ACE) inhibitor enalaprilat were added to the perfusion solution in one of the sides for 30 min. Thirty minutes later, the subjects performed a sustained bilateral shoulder abduction at 10% of the maximal voluntary contraction until exhaustion. This protocol was repeated twice, with an interval of at least 17 days. High intersession repeatability was observed in the concentration of bradykinin but not of kallidin. Enalaprilat induced a significant increase in bradykinin levels in the dialysate, without affecting kallidin levels. The sustained contraction induced a significant increase in dialysate levels of both kinin peptides. The contraction also induced a significant increase in pain ratings, as measured by a visual analog scale. During contraction, positive correlations were found between pain ratings and levels of kinin peptides in dialysate, predominantly in the side previously perfused with enalaprilat. Subjects with the higher pain ratings also showed larger increases in kinin peptides in the side previously perfused with enalaprilat. The present results show that both plasma and tissue kinin-kallikrein are activated during muscle contraction, but that their metabolic pathways are differently regulated during rest and contraction, because they showed a different response to ACE inhibition. They also indicate that intramuscular kinin peptides levels, and ACE activity, may contribute to muscle pain.

Angiotensin Converting Enzyme Genotype and Strength in Chronic Obstructive Pulmonary Disease

Quadriceps muscle weakness is an important contributor to exercise limitation in patients with chronic obstructive pulmonary disease. The deletion allele of the angiotensin converting enzyme gene polymorphism has previously been associated with a greater response to strength training in healthy subjects and might, therefore, protect against detraining in these patients. In 103 stable outpatients (mean [SD] FEV(1) 34.4 [16.5] % predicted), the angiotensin deletion allele was associated with greater isometric quadriceps strength; mean (SD) 31.4 (10.8) kg for insertion homozygotes, 34.1 (13.0) kg for heterozygotes, and 38.3 (11.6) kg for deletion homozygotes (p = 0.04 linear trend). Adjusted for fat-free mass, the relationship was stronger (linear trend p = 0.007). There was no correlation between strength and genotype in a group of 101 age-matched healthy control subjects. Twitch quadriceps force in response to magnetic femoral nerve stimulation, measured in 39 patients, was also genotype dependent; 8.3 (2.6) kg for insertion homozygotes, 10.1 (3.6) kg for heterozygotes, and 12.4 (3.5) kg for deletion homozygotes (p = 0.002 linear trend). Body mass index and fat-free mass did not differ significantly between genotypes in either group. There was no association in either patients or control subjects between genotype and inspiratory muscle strength. In chronic obstructive pulmonary disease the deletion allele is associated with greater quadriceps strength independent of confounding factors.

Isolation and characterization of a novel angiotensin I-converting enzyme inhibitory peptide derived from the edible mushroom Tricholoma giganteum

The fruiting body of Tricholoma giganteum has many pharmaceutical uses and has long been utilized as a home remedy in Asia. This study describes the extraction and characterization of the first angiotensin I-converting enzyme (ACE) inhibitory peptide from T. giganteum. The maximum ACE inhibitory activity (IC50: 0.31 mg) was obtained when the fruiting body of T. giganteum was extracted with distilled water at 30 degrees C for 3 h. After the purification of ACE inhibitory peptides with ultrafiltration, Sephadex G-25 column chromatography, and reverse-phase HPLC, an active fraction with an IC50 of 0.04 mg and a yield of 0.3% was obtained. The ACE inhibitory peptide was a novel tripeptide, showing very low similarity to other ACE inhibitory peptide sequences, and was sequenced as Gly-Glu-Pro. The purified ACE inhibitor from T. giganteum competitively inhibited ACE, and it maintained inhibitory activity even after incubation with proteases. ACE inhibitor from T. giganteum showed a clear antihypertensive effect in spontaneously hypertensive rats (SHR), at a dosage of 1 mg/kg.

Skeletal muscle RAS and exercise performance

A local renin-angiotensin system (RAS) may be suggested by evidence of gene expression of RAS components within the tissue as well as physiological responsiveness of this gene expression. This review will focus on the evidence supporting the existence of the constituent elements of a physiologically functional paracrine muscle RAS. The effect of local skeletal muscle RAS on human exercise performance will be explored via its relation with pharmacological intervention and genetic studies. The most likely configuration of the muscle RAS is a combination of in situ synthesis and uptake from the circulation of RAS components. A reduction in angiotensin-converting enzyme (ACE) activity reverses the decline in physical performance due to peripheral muscle factors in those with congestive heart failure and may halt or slow decline in muscle strength in elderly women. Genetic studies suggest that increased ACE and angiotensin II (Ang II) mediate greater strength gains perhaps via muscle hypertrophy whereas lower ACE levels and reduced bradykinin (BK) degradation mediate enhanced endurance performance perhaps via changes in substrate availability, muscle fibre type and efficiency.

Contraction-related factors affect the concentration of a kallidin-like peptide in rat muscle tissue

In order to study the effects of the manipulation of various factors related to muscular activity on the concentration of kinins in muscular tissue, a microdialysis probe was implanted in the adductor muscle of the hindlimb in anaesthetized rats. After collection of baseline samples, the perfusion fluid was changed to a Ringer solution containing sodium lactate (10 or 20 mM), adenosine (50 or 100 microM) or a lower pH (7.0 or 6.6). Whereas perfusion with lactate did not have any significant effect on the concentration of kinins in the dialysate, the perfusion with a lower pH or with adenosine dose-dependently increased the kinin content in the samples. In a second microdialysis experiment, by using specific radioimmunoassays (RIA) for bradykinin and kallidin, we observed that about 70 % of the total kinins dialysed from rat muscle are a kallidin-like peptide. Also, the simultaneous perfusion with 100 microM caffeine totally abolished the increase in kinin levels induced by the perfusion at pH 6.6. In a third experiment, soleus muscles from rat were stimulated in vitro during 30 min in the presence or absence of 77 microM caffeine. Electrically stimulated contraction, but not the addition of 10 mU ml(-1) insulin, induced an increase in the concentration of the kallidin-like peptide in the buffer. This effect was totally prevented by the addition of the adenosine antagonist caffeine. These results show that a kallidin-like peptide is released from rat muscle, and that its production is enhanced by muscle activity. Furthermore, the increase in kinin peptides during muscle contraction may be mediated by an increase in adenosine levels.

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.

Transforming growth factor-beta(1) induces angiotensin-converting enzyme synthesis in rat cardiac fibroblasts during their differentiation to myofibroblasts

OBJECTIVES

Appearance of angiotensin-converting enzyme (ACE) in fibrotic tissue can be the result of the action either of one particular growth factor or of cross-talk between multiple factors. Transforming growth factor-beta1 (TGF-beta(1)) is an effective inducor of the differentiation of cultured fibroblasts to myofibroblasts, which are heterogeneous cells with different phenotypes. The present study investigated whether TGF-beta(1) is able to induce, in vitro, the differentiation of cultured fibroblasts to myofibroblasts with a phenotype containing ACE.

DESIGN

Adult rat cardiac ventricular fibroblasts were obtained from hearts perfused with collagenase. Cells from second passage were always used. Rat cardiac ventricular fibroblasts were incubated with TGF-beta(1) (10 ng/ml) for seven days. Cell characterisation was performed using light microscopy and indirect immunostaining. Presence of vimentin, desmin, factor VIII, alpha-smooth muscle actin, and ACE was checked with both immunostaining and Western blotting. ACE activity was measured fluorometrically with hippuryl-histidyl-leucine as substrate. Synthesis of DNA was measured as (3)H-thymidine incorporation.

RESULTS

Fibroblasts contained two types of activity of hip-his-leu degradation, namely a lisinopril-dependent activity (ACE activity) and a lisinopril-independent activity ('ACE-like' activity) which is performed by peptidase(s) other than ACE. The ACE activity constituted approximately 30% of the total activity. TGF-beta(1) induced an increase in both ACE activity and ACE protein (detected by immunoblotting) by 4.5 +/- 0.9- and 6.9 +/- 2.0-fold, respectively (p<0.05). This induction of ACE was accompanied by a profound modification of the fibroblasts phenotype, which consisted of a change in cell morphology, an enlargement of cell volume and an increase in cell protein content. TGF-beta(1) profoundly inhibited (3)H-thymidine incorporation and the number of cells in growing cultures. The induction of alpha-smooth muscle actin by TGF-beta(1) (6.8 +/- 2.8-fold increase, p<0.05) simultaneously with these modifications in cell morphology and proliferation indicates the appearance of myofibroblasts. These myofibroblasts did not contain desmin.

CONCLUSION

TGF-beta(1) is able to induce the appearance of ACE in cultures of adult rat cardiac ventricular fibroblasts. The appearance of the enzyme is accompanied by the differentiation of fibroblasts to myofibroblasts.

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.

Possible involvement of proteases in the regulation of spermatogenesis

Proteolytic enzymes, which are synthesized and secreted by cells of the seminiferous tubule of the testis, have important functions in spermatogenesis. We performed metabolic studies using small peptide hormones as a substrate to investigate the activity of proteases in cultured Sertoli cells of the rat. High-performance liquid chromatographic analysis of the cell culture supernatants showed cleavage of met- and leu-enkephalin, substance P, and bradykinin. No peptidolysis was observed for the cyclic peptide oxytocin. The hormone cleavage pattern and the use of specific protease inhibitors in peptide degradation experiments demonstrated activities of several proteases in Sertoli cells. These are mainly metalloproteinases including neutral metalloendopeptidases, angiotensin-converting enzyme and aminopeptidases. In addition, activities of serine and aspartic proteases were detected. Only marginal proteolytic activities were observed in Sertoli cell conditioned supernatants, indicating that the investigated proteases are mainly located on Sertoli cell membranes. The peptide hormones used in this study have been found to play a potential role in the endocrine, paracrine or autocrine regulation of testicular cells. The membrane-associated proteases reported here may therefore be involved in the metabolism and inactivation of these peptides.

Neutral endopeptidase-24.11 (NEP) activity in human fibroblasts during development and ageing

Neutral endopeptidase-24.11 (NEP, EC 3.4.24.11) is a cell surface Zn metallopeptidase that hydrolyzes bioactive regulatory peptides. Using a spectrofluorimetric procedure, we assessed NEP activity in plasma membranes of normal human skin and lung fibroblasts. We found a considerable increase in NEP activity during fetal-to-adult transition. Adult skin fibroblasts from an old donor exhibited significantly higher levels of NEP activity than cells from young donors. Interestingly, however, the NEP activity of fibroblasts from a centenarian donor was similar to that of cells from young donors. Increased levels of NEP activity were also found in in vitro aged lung fibroblasts. Finally, adrenocorticotropin hormone (ACTH (1-24)), a regulatory peptide that can be cleaved by NEP, provoked an increase in enzymic activity in fetal and young adult donor fibroblasts and a decrease in this activity in fibroblasts from adult and old donors. This finding suggests that ageing may affect NEP activity.

Localization of a constitutively active, phagocyte-like NADPH oxidase in rabbit aortic adventitia: enhancement by angiotensin II

Superoxide anion (O2-) plays a key role in the endogenous suppression of endothelium-derived nitric oxide (NO) bioactivity and has been implicated in the development of hypertension. In previous studies, we found that O2- is produced predominantly in the adventitia of isolated rabbit aorta and acts as a barrier to NO. In the present studies, we characterize the enzyme responsible for O2- production in the adventitia and show that this enzyme is a constitutively active NADPH oxidase with similar composition as the phagocyte NADPH oxidase. Constitutive O2--generating activity was localized to aortic adventitial fibroblasts and was enhanced by the potent vasoconstrictor angiotensin II. Immunohistochemistry of aortic sections demonstrated the presence of p22(phox), gp91(phox), p47(phox), and p67(phox) localized exclusively in rabbit aortic adventitia, coincident with the site of staining for O2- production. Furthermore, immunodepletion of p67(phox) from adventitial fibroblast particulates resulted in the loss of NADPH oxidase activity, which could be restored by the addition of recombinant p67(phox). Further study into the regulation of this adventitial source of O2- is important in elucidating the mechanisms regulating the bioactivity of NO and may contribute to our understanding of the pathogenesis of hypertension.

Substance P and neurokinin A metabolism by cultured human skeletal muscle myocytes and fibroblasts

A recent study determined that cultured human skeletal muscle adult myoblasts, myotubes, and fibroblasts degraded angiotensins and kinins via neutral endopeptidase-24.11 (NEP-24.11: EC 3.4.24.11) and aminopeptidase N (APN: EC 3.4.11.2). Due to the possible importance of other peptides to skeletal muscle blood flow and function, the present study looked specifically at the metabolism of the neurokinins substance P (SP) and neurokinin A (NKA) by skeletal muscle peptidases. The results show that SP is degraded not only by NEP-24.11, but also sequentially by dipeptidyl(amino)peptidase IV (DAP IV: EC 3.4.14.5)/APN. NKA is unaffected by DAP IV but is metabolized by NEP-24.11 and APN. NEP-24.11 was inhibited by phosphoramidon (IC50 = 80 nM), thiorphan and ZINCOV, DAP IV by diprotin A (IC50 = 8 microM), and APN by amastatin (IC50 = 50 nM) and bestatin (IC50 = 100 microM). Skeletal muscle myocyte and fibroblast metabolism of SP and NKA may regulate local skeletal muscle vascular and extravascular functions including SP- and NKA-mediated nerve-induced vasodilation. Inhibition of both NEP-24.11 and DAP IV/APN may increase skeletal muscle blood flow and decrease peripheral vascular resistance via potentiation of local neurokinin levels.