Cardioprotective effects of the aminopeptidase P inhibitor apstatin: studies on ischemia/reperfusion injury in the isolated rat heart

A Potent Inhibitor of Aminopeptidase P2 Reduces Reperfusion Injury in Models of Myocardial Infarction and Stroke

During a myocardial infarction or ischemic stroke, blood flow to the heart or brain is partially blocked. This results in reduced delivery of oxygen and nutrients and, ultimately, tissue damage. Initial treatment involves removing the clot and restoring blood flow (reperfusion). However, this treatment is not as effective as one would hope because the reperfusion process itself can cause a different type of damage (reperfusion injury) that contributes up to 50% of the total damage. Bradykinin is an autocoid that is released from blood vessel endothelial cells during ischemia and reperfusion and has the potential to prevent reperfusion injury. However, bradykinin is rapidly inactivated by enzymes on endothelial cells, limiting its beneficial effects. One of these enzymes is aminopeptidase P2. We designed a potent and specific inhibitor of aminopeptidase P2 called ST-115, [(S)-2-mercapto-4-methylpentanoyl]-4(S)-fluoro-Pro-Pro-3(R)-beta-Pro. When ST-115 is administered intravenously at the start of reperfusion, it reduces bradykinin degradation. This increases bradykinin's concentration in the capillaries and enhances its protective effects. We tested ST-115 in a mouse model of myocardial infarction and found that the damaged area of the heart was reduced by 58% compared with mice given saline. In a rat model of ischemic stroke, ST-115 reduced functional deficits in a skilled walking test by 60% and reduced brain edema by 51%. It reduced brain infarct size by 48% in a major subset of rats with small strokes. The results indicate that ST-115 can ameliorate reperfusion injury and can ultimately serve as a therapeutic for acute myocardial infarction and ischemic stroke. SIGNIFICANCE STATEMENT: We have shown that our aminopeptidase P2 inhibitor, ST-115, can reduce tissue injury caused by episodes of ischemia followed by reperfusion. It was successful in rodent models of myocardial infarction and stroke. The clinical use would involve the intravenous administration of ST-115 at the induction of reperfusion. In the case of stroke, the successful technique of thrombectomy could be combined with ST-115 administration to simultaneously reduce both ischemic and reperfusion injury.

Investigation of the proton relay system operative in human cystosolic aminopeptidase P

Aminopeptidase P, a metalloprotease, targets Xaa-Proline peptides for cleavage [1-4]. There are two forms of human AMPP, a membrane-bound form (hmAMPP) and a soluble cytosolic form (hcAMPP)[5]. Similar to the angiotensin-I-converting enzyme, AMPP plays an important role in the catabolism of inflammatory and vasoactive peptides, known as kinins. The plasma kinin, bradykinin, was used as the substrate to conduct enzymatic activity analyses and to determine the Michaelis constant (Km) of 174 μM and the catalytic rate constant (kcat) of 10.8 s-1 for hcAMPP. Significant differences were observed in the activities of Y527F and R535A hcAMPP mutants, which displayed a 6-fold and 13.5-fold for decrease in turnover rate, respectively. Guanidine hydrochloride restored the activity of R535A hcAMPP, increasing the kcat/Km 20-fold, yet it had no impact on the activities of the wild-type or Y527F mutant hcAMPPs. Activity restoration by guanidine derivatives followed the order guanidine hydrochloride >> methyl-guanidine > amino-guanidine > N-ethyl-guanidine. Overall, the results indicate the participation of R535 in the hydrogen bond network that forms a proton relay system. The quaternary structure of hcAMPP was determined by using analytical ultracentrifugation (AUC). The results show that alanine replacement of Arg535 destabilizes the hcAMPP dimer and that guanidine hydrochloride restores the native monomer-dimer equilibrium. It is proposed that Arg535 plays an important role in hcAMMP catalysis and in stabilization of the catalytically active dimeric state.

Structure and substrate fingerprint of aminopeptidase P from Plasmodium falciparum

Malaria is one of the world's most prevalent parasitic diseases, with over 200 million cases annually. Alarmingly, the spread of drug-resistant parasites threatens the effectiveness of current antimalarials and has made the development of novel therapeutic strategies a global health priority. Malaria parasites have a complicated lifecycle, involving an asymptomatic ‘liver stage’ and a symptomatic ‘blood stage’. During the blood stage, the parasites utilise a proteolytic cascade to digest host hemoglobin, which produces free amino acids absolutely necessary for parasite growth and reproduction. The enzymes required for hemoglobin digestion are therefore attractive therapeutic targets. The final step of the cascade is catalyzed by several metalloaminopeptidases, including aminopeptidase P (APP). We developed a novel platform to examine the substrate fingerprint of APP from Plasmodium falciparum (PfAPP) and to show that it can catalyze the removal of any residue immediately prior to a proline. Further, we have determined the crystal structure of PfAPP and present the first examination of the 3D structure of this essential malarial enzyme. Together, these analyses provide insights into potential mechanisms of inhibition that could be used to develop novel antimalarial therapeutics.

Extracellular and intracellular proteases in cardiac dysfunction due to ischemia-reperfusion injury

Various procedures such as angioplasty, thrombolytic therapy, coronary bypass surgery, and cardiac transplantation are invariably associated with ischemia-reperfusion (I/R) injury. Impaired recovery of cardiac function due to I/R injury is considered to be a consequence of the occurrence of both oxidative stress and intracellular Ca(2+)-overload in the myocardium. These changes in the ischemic myocardium appear to activate both extracellular and intracellular proteases which are responsible for the cleavage of extracellular matrix and subcellular structures involved in the maintenance of cardiac function. It is thus intended to discuss the actions of I/R injury on several proteases, with a focus on calpain, matrix metalloproteinases, and cathepsins as well as their role in inducing alterations both inside and outside the cardiomyocytes. In addition, modifications of subcellular organelles such as myofibrils, sarcoplasmic reticulum and sarcolemma as well as extracellular matrix, and the potential regulatory effects of endogenous inhibitors on protease activities are identified. Both extracellular and intracellular proteolytic activities appear to be imperative in determining the true extent of I/R injury and their inhibition seems to be of critical importance for improving the recovery of cardiac function. Thus, both extracellular and intracellular proteases may serve as potential targets for the development of cardioprotective interventions for reducing damage to the heart and retarding the development of contractile dysfunction caused by I/R injury.

Evidence for catalytic roles for Plasmodium falciparum aminopeptidase P in the food vacuole and cytosol

The metalloenzyme aminopeptidase P catalyzes the hydrolysis of amino acids from the amino termini of peptides with a prolyl residue in the second position. The human malaria parasite Plasmodium falciparum expresses a homolog of aminopeptidase P during its asexual intraerythrocytic cycle. P. falciparum aminopeptidase P (PfAPP) shares with mammalian cytosolic aminopeptidase P a three-domain, homodimeric organization and is most active with Mn(II) as the cofactor. A distinguishing feature of PfAPP is a 120-amino acid amino-terminal extension that appears to be removed from the mature protein. PfAPP is present in the food vacuole and cytosol of the parasite, a distribution that suggests roles in vacuolar hemoglobin catabolism and cytosolic peptide turnover. To evaluate the plausibility of these putative functions, the stability and kinetic properties of recombinant PfAPP were evaluated at the acidic pH of the food vacuole and at the near-neutral pH of the cytosol. PfAPP exhibited high stability at 37 degrees C in the pH range 5.0-7.5. In contrast, recombinant human cytosolic APP1 was unstable and formed a high molecular weight aggregate at acidic pH. At both acidic and slightly basic pH values, PfAPP efficiently hydrolyzed the amino-terminal X-Pro bond of the nonapeptide bradykinin and of two globin pentapeptides that are potential in vivo substrates. These results provide support for roles for PfAPP in peptide catabolism in both the food vacuole and the cytosol and suggest that PfAPP has evolved a dual distribution in response to the metabolic needs of the intraerythrocytic parasite.

Kinetic and crystallographic analysis of mutant Escherichia coli aminopeptidase P: insights into substrate recognition and the mechanism of catalysis

Aminopeptidase P (APPro) is a manganese-dependent enzyme that cleaves the N-terminal amino acid from polypeptides where the second residue is proline. APPro shares a similar fold, substrate specificity, and catalytic mechanism with methionine aminopeptidase and prolidase. To investigate the roles of conserved residues at the active site, seven mutant forms of APPro were characterized kinetically and structurally. Mutation of individual metal ligands selectively abolished binding of either or both Mn(II) atoms at the active site, and none of these metal-ligand mutants had detectable catalytic activity. Mutation of the conserved active site residues His243 and His361 revealed that both are required for catalysis. We propose that His243 stabilizes substrate binding through an interaction with the carbonyl oxygen of the requisite proline residue of a substrate and that His361 stabilizes substrate binding and the gem-diol catalytic intermediate. Sequence, structural, and kinetic analyses reveal that His350, conserved in APPro and prolidase but not in methionine aminopeptidase, forms part of a hydrophobic binding pocket that gives APPro its proline specificity. Further, peptides in which the required proline residue is replaced by N-methylalanine or alanine are cleaved by APPro, but they are extremely poor substrates due to a loss of interactions between the prolidyl ring of the substrate and the hydrophobic proline-binding pocket.

Aminopeptidase P isozyme expression in human tissues and peripheral blood mononuclear cell fractions

Aminopeptidase P (APP) isoforms specifically remove the N-terminal amino acid from peptides that have a proline residue in the second position. The mRNA levels of three different isoforms, each coded by a different gene, were determined in 16 human tissues and in peripheral blood mononuclear cell (PBMC) fractions by RT-PCR. The cytosolic isoform, APP1, and the cell surface membrane-bound isoform, APP2, are expressed in all of the human tissues and PBMC fractions examined. The very high expression of APP2 mRNA in kidney compared to other tissues was confirmed by enzyme activity measurements. Among the PBMC fractions, APP2 expression is highest in resting CD8(+) T cells, but decreases in these cells following their activation with phytohemagglutinin; in contrast, expression of APP2 increases in CD4(+) T cells upon activation. The third isoform, APP3, is a hypothetical protein identified by nucleotide sequencing. A detailed analysis of its amino acid sequence confirmed that the protein is an aminopeptidase P-like enzyme with greater similarity to Escherichia coli APP than to either APP1 or APP2. Two splice variants of APP3 exist, one of which is predicted to have a mitochondrial localization (APP3m) while the other is cytosolic (APP3c). Both forms are variably expressed in all of the human tissues and PBMC fractions examined.

Rat and mouse membrane aminopeptidase P: structure analysis and tissue distribution

Membrane-bound aminopeptidase P (mAPP) is a highly specific exopeptidase that removes the N-terminal amino acid only from a peptide (three amino acids or longer) that has a prolyl residue in the second position. mAPP can inactivate bradykinin, a potent vasodilating and cardioprotective peptide hormone, by hydrolyzing the Arg(1)-Pro(2) bond. Studies on the rat have shown that the metabolism of bradykinin is an important physiological role of this enzyme. We report here the complete coding sequences for rat and mouse mAPP determined from mRNA isolated from lung tissue. Key structural features that determine post-translational processing and substrate recognition and catalysis were identified based on sequence homologies and the crystal structure of Escherichia coli aminopeptidase P complexed with Pro-Leu. The tissue-specific expression of mAPP was studied using the polymerase chain reaction. The mAPP gene is widely, but variably, expressed in adult tissues of the rat and mouse and in mouse embryos.

Apstatin, a selective inhibitor of aminopeptidase P, reduces myocardial infarct size by a kinin-dependent pathway

1. Inhibitors of the angiotensin converting enzyme (ACE) have been shown to exert their cardioprotective actions through a kinin-dependent mechanism. ACE is not the only kinin degrading enzyme in the rat heart. 2. Since aminopeptidase P (APP) has been shown to participate in myocardial kinin metabolism to the same extent as ACE, the aims of the present study were to investigate whether (a) inhibition of APP leads to a reduction of myocardial infarct size in a rat model of acute ischaemia and reperfusion, (b) reduction of infarct size is mediated by bradykinin, and (c) a combination of APP and ACE inhibition leads to a more pronounced effect than APP inhibition alone. 3. Pentobarbital-anaesthetized rats were subjected to 30 min left coronary artery occlusion followed by 3 h reperfusion. The APP inhibitor apstatin, the ACE-inhibitor ramiprilat, or their combination were administered 5 min before ischaemia. Rats receiving HOE140, a specific B(2) receptor antagonist, were pretreated 5 min prior to enzyme inhibitors. Myocardial infarct size (IS) was determined by tetrazolium staining and expressed as percentage of the area at risk (AAR). 4. IS/AAR% was significantly reduced in rats that received apstatin (18+/-2%), ramiprilat (18+/-3%), or apstatin plus ramiprilat (20+/-4%) as compared with those receiving saline (40+/-2%), HOE (43+/-3%) or apstatin plus HOE140 (49+/-4%). 5. Apstatin reduces IS in an in vivo model of acute myocardial ischaemia and reperfusion to the same extent than ramiprilat. Cardioprotection achieved by this selective inhibitor of APP is mediated by bradykinin. Combined inhibition of APP and ACE did not result in a more pronounced reduction of IS than APP-inhibition alone.

Bradykinin metabolism in the isolated perfused rabbit heart

BACKGROUND

Bradykinin is a potent cardioprotective hormone, the beneficial role of which in vivo appears to be limited by its rapid metabolism. Inhibitors of peptidases that degrade endogenously formed bradykinin are themselves cardioprotective, presumably by increasing local bradykinin concentrations. As bradykinin-degrading peptidases are potential therapeutic targets, it is important to identify these enzymes in different animal models of cardiac function.

OBJECTIVE

To determine the mechanism of bradykinin degradation in the coronary circulation of the rabbit, using an isolated perfused heart preparation.

DESIGN AND METHODS

[3H]Bradykinin (16 nmol/l) was perfused as a bolus through the isolated rabbit heart in the presence and absence of specific peptidase inhibitors. The effluent was collected and the radiolabeled metabolites of [3H]bradykinin were separated by high performance liquid chromatography, identified, and quantified.

RESULTS

[3H]Bradykinin was metabolized to the extent of 62 +/- 3% in a single passage through the rabbit coronary circulation at a physiological flow rate. The metabolites were identified as [3H]bradykinin(1-5) and [3H]bradykinin(1-7),accounting for 50 +/- 4 and 12 +/- 2% of the radioactivity, respectively. Co-perfusion with the angiotensin converting enzyme inhibitor, ramiprilat, completely blocked formation of these metabolites.

CONCLUSIONS

Angiotensin-converting enzyme fully accounts for the metabolism of [3H]bradykinin in the rabbit coronary circulation. This result contrasts with data obtained using rat heart, which demonstrated a prominent role for aminopeptidase P in bradykinin metabolism in this species.

Effects of the aminopeptidase P inhibitor apstatin on bradykinin-induced inositol 1,4,5-triphosphate in neonatal rat cardiomyocytes

We investigated the effect of apstatin (an aminopeptidase P inhibitor) on bradykinin-induced inositol 1,4,5-triphosphate (IP3) formation and glucose uptake in isolated neonatal rat cardiomyocytes. Apstatin enhanced bradykinin-induced IP3 formation in a dose-dependent manner. We found that 1 microM Hoe 140 (a bradykinin B2-receptor antagonist) significantly decreased the potentiation of bradykinin-induced IP3 production by 5 microM apstatin from 781.8+/-67.2 to 127.4+/-33.0 pmol/mg protein; 5 microM apstatin increased bradykinin-induced glucose uptake from 197.0+/-25.5 to 297.3+/-64.0 pmol/h per milligram of protein. The stimulation of glucose uptake with apstatin was blocked to 132.5+/-26.2 pmol/h per milligram of protein by 1 microM Hoe 140. We conclude that apstatin stimulates bradykinin-induced IP3 formation and glucose uptake by preventing the degradation of bradykinin.