Specific cellular proteins associate with angiotensin-converting enzyme and regulate its intracellular transport and cleavage-secretion

ACE Phenotyping in Human Blood and Tissues: Revelation of ACE Outliers and Sex Differences in ACE Sialylation

Angiotensin-converting enzyme (ACE) metabolizes a number of important peptides participating in blood pressure regulation and vascular remodeling. Elevated ACE expression in tissues (which is generally reflected by blood ACE levels) is associated with an increased risk of cardiovascular diseases. Elevated blood ACE is also a marker for granulomatous diseases. Decreased blood ACE activity is becoming a new risk factor for Alzheimer's disease. We applied our novel approach-ACE phenotyping-to characterize pairs of tissues (lung, heart, lymph nodes) and serum ACE in 50 patients. ACE phenotyping includes (1) measurement of ACE activity with two substrates (ZPHL and HHL); (2) calculation of the ratio of hydrolysis of these substrates (ZPHL/HHL ratio); (3) determination of ACE immunoreactive protein levels using mAbs to ACE; and (4) ACE conformation with a set of mAbs to ACE. The ACE phenotyping approach in screening format with special attention to outliers, combined with analysis of sequencing data, allowed us to identify patient with a unique ACE phenotype related to decreased ability of inhibition of ACE activity by albumin, likely due to competition with high CCL18 in this patient for binding to ACE. We also confirmed recently discovered gender differences in sialylation of some glycosylation sites of ACE. ACE phenotyping is a promising new approach for the identification of ACE phenotype outliers with potential clinical significance, making it useful for screening in a personalized medicine approach.

Urinary ACE Phenotyping as a Research and Diagnostic Tool: Identification of Sex-Dependent ACE Immunoreactivity

BACKGROUND

Angiotensin-converting enzyme (ACE) is highly expressed in renal proximal tubules, but ACE activity/levels in the urine are at least 100-fold lower than in the blood. Decreased proximal tubular ACE has been associated with renal tubular damage in both animal models and clinical studies. Because ACE is shed into urine primarily from proximal tubule epithelial cells, its urinary ACE measurement may be useful as an index of tubular damage.

OBJECTIVE AND METHODOLOGY

We applied our novel approach-ACE phenotyping-to characterize urinary ACE in volunteer subjects. ACE phenotyping includes (1) determination of ACE activity using two substrates (ZPHL and HHL); (2) calculation of the ratio of hydrolysis of the two substrates (ZPHL/HHL ratio); (3) quantification of ACE immunoreactive protein levels; and (4) fine mapping of local ACE conformation with mAbs to ACE.

PRINCIPAL FINDINGS

In normal volunteers, urinary ACE activity was 140-fold less than in corresponding plasma/serum samples and did not differ between males and females. However, urinary ACE immunoreactivity (normalized binding of 25 mAbs to different epitopes) was strongly sex-dependent for the several mAbs tested, an observation likely explained by differences in tissue ACE glycosylation/sialylation between males and females. Urinary ACE phenotyping also allowed the identification of ACE outliers. In addition, daily variability of urinary ACE has potential utility as a feedback marker for dieting individuals pursuing weight loss.

CONCLUSIONS/SIGNIFICANCE

Urinary ACE phenotyping is a promising new approach with potential clinical significance to advance precision medicine screening techniques.

ACE2 Nascence, trafficking, and SARS-CoV-2 pathogenesis: the saga continues

With the emergence of the novel coronavirus SARS-CoV-2 since December 2019, more than 65 million cases have been reported worldwide. This virus has shown high infectivity and severe symptoms in some cases, leading to over 1.5 million deaths globally. Despite the collaborative and concerted research efforts that have been made, no effective medication for COVID-19 (coronavirus disease-2019) is currently available. SARS-CoV-2 uses the angiotensin-converting enzyme 2 (ACE2) as an initial mediator for viral attachment and host cell invasion. ACE2 is widely distributed in the human tissues including the cell surface of lung cells which represent the primary site of the infection. Inhibiting or reducing cell surface availability of ACE2 represents a promising therapy for tackling COVID-19. In this context, most ACE2-based therapeutic strategies have aimed to tackle the virus through the use of angiotensin-converting enzyme (ACE) inhibitors or neutralizing the virus by exogenous administration of ACE2, which does not directly aim to reduce its membrane availability. However, through this review, we present a different perspective focusing on the subcellular localization and trafficking of ACE2. Membrane targeting of ACE2, and shedding and cellular trafficking pathways including the internalization are not well elucidated in literature. Therefore, we hereby present an overview of the fate of newly synthesized ACE2, its post translational modifications, and what is known of its trafficking pathways. In addition, we highlight the possibility that some of the identified ACE2 missense variants might affect its trafficking efficiency and localization and hence may explain some of the observed variable severity of SARS-CoV-2 infections. Moreover, an extensive understanding of these processes is necessarily required to evaluate the potential use of ACE2 as a credible therapeutic target.

Lysozyme and bilirubin bind to ACE and regulate its conformation and shedding

Angiotensin I-converting enzyme (ACE) hydrolyzes numerous peptides and is a critical participant in blood pressure regulation and vascular remodeling. Elevated tissue ACE levels are associated with increased risk for cardiovascular and respiratory disorders. Blood ACE concentrations are determined by proteolytic cleavage of ACE from the endothelial cell surface, a process that remains incompletely understood. In this study, we identified a novel ACE gene mutation (Arg532Trp substitution in the N domain of somatic ACE) that increases blood ACE activity 7-fold and interrogated the mechanism by which this mutation significantly increases blood ACE levels. We hypothesized that this ACE mutation disrupts the binding site for blood components which may stabilize ACE conformation and diminish ACE shedding. We identified the ACE-binding protein in the blood as lysozyme and also a Low Molecular Weight (LMW) ACE effector, bilirubin, which act in concert to regulate ACE conformation and thereby influence ACE shedding. These results provide mechanistic insight into the elevated blood level of ACE observed in patients on ACE inhibitor therapy and elevated blood lysozyme and ACE levels in sarcoidosis patients.

Tissue-specific expression of transgenic secreted ACE in vasculature can restore normal kidney functions, but not blood pressure, of Ace-/- mice

Angiotensin-converting enzyme (ACE) regulates normal blood pressure and fluid homeostasis through its action in the renin-angiotensin-system (RAS). Ace-/- mice are smaller in size, have low blood pressure and defective kidney structure and functions. All of these defects are cured by transgenic expression of somatic ACE (sACE) in vascular endothelial cells of Ace-/- mice. sACE is expressed on the surface of vascular endothelial cells and undergoes a natural cleavage secretion process to generate a soluble form in the body fluids. Both the tissue-bound and the soluble forms of ACE are enzymatically active, and generate the vasoactive octapeptide Angiotensin II (Ang II) with equal efficiency. To assess the relative physiological roles of the secreted and the cell-bound forms of ACE, we expressed, in the vascular endothelial cells of Ace-/- mice, the ectodomain of sACE, which corresponded to only the secreted form of ACE. Our results demonstrated that the secreted form of ACE could normalize kidney functions and RAS integrity, growth and development of Ace-/- mice, but not their blood pressure. This study clearly demonstrates that the secreted form of ACE cannot replace the tissue-bound ACE for maintaining normal blood pressure; a suitable balance between the tissue-bound and the soluble forms of ACE is essential for maintaining all physiological functions of ACE.

Mesencephalic astrocyte-derived neurotrophic factor protects the heart from ischemic damage and is selectively secreted upon sarco/endoplasmic reticulum calcium depletion

The endoplasmic reticulum (ER) stress protein mesencephalic astrocyte-derived neurotrophic factor (MANF) has been reported to protect cells from stress-induced cell death before and after its secretion; however, the conditions under which it is secreted are not known. Accordingly, we examined the mechanism of MANF release from cultured ventricular myocytes and HeLa cells, both of which secrete proteins via the constitutive pathway. Although the secretion of proteins via the constitutive pathway is not known to increase upon changes in intracellular calcium, MANF secretion was increased within 30 min of treating cells with compounds that deplete sarcoplasmic reticulum (SR)/ER calcium. In contrast, secretion of atrial natriuretic factor from ventricular myocytes was not increased by SR/ER calcium depletion, suggesting that not all secreted proteins exhibit the same characteristics as MANF. We postulated that SR/ER calcium depletion triggered MANF secretion by decreasing its retention. Consistent with this were co-immunoprecipitation and live cell, zero distance, photo affinity cross-linking, demonstrating that, in part, MANF was retained in the SR/ER via its calcium-dependent interaction with the SR/ER-resident protein, GRP78 (glucose-regulated protein 78 kDa). This unusual mechanism of regulating secretion from the constitutive secretory pathway provides a potentially missing link in the mechanism by which extracellular MANF protects cells from stresses that deplete SR/ER calcium. Consistent with this was our finding that administration of recombinant MANF to mice decreased tissue damage in an in vivo model of myocardial infarction, a condition during which ER calcium is known to be dysregulated, and MANF expression is induced.

Molecular and cellular mechanisms of ectodomain shedding

The extracellular domain of several membrane-anchored proteins is released from the cell surface as soluble proteins through a regulated proteolytic mechanism called ectodomain shedding. Cells use ectodomain shedding to actively regulate the expression and function of surface molecules, and modulate a wide variety of cellular and physiological processes. Ectodomain shedding rapidly converts membrane-associated proteins into soluble effectors and, at the same time, rapidly reduces the level of cell surface expression. For some proteins, ectodomain shedding is also a prerequisite for intramembrane proteolysis, which liberates the cytoplasmic domain of the affected molecule and associated signaling factors to regulate transcription. Ectodomain shedding is a process that is highly regulated by specific agonists, antagonists, and intracellular signaling pathways. Moreover, only about 2% of cell surface proteins are released from the surface by ectodomain shedding, indicating that cells selectively shed their protein ectodomains. This review will describe the molecular and cellular mechanisms of ectodomain shedding, and discuss its major functions in lung development and disease.

mu-Opioid receptor cell surface expression is regulated by its direct interaction with Ribophorin I

The trafficking of the mu-opioid receptor (MOR), a member of the rhodopsin G protein-coupled receptor (GPCR) family, can be regulated by interaction with multiple cellular proteins. To determine the proteins involved in receptor trafficking, using the targeted proteomic approach and mass spectrometry analysis, we have identified that Ribophorin I (RPNI), a component of the oligosaccharide transferase complex, could directly interact with MOR. RPNI can be shown to participate in MOR export by the intracellular retention of the receptor after small interfering RNA knockdown of endogenous RPNI. Overexpression of RPNI rescued the surface expression of the MOR 344KFCTR348 deletion mutant independent of calnexin. Furthermore, RPNI regulation of MOR trafficking is dependent on the glycosylation state of the receptor, as reflected by the inability of overexpression of RPNI to affect the trafficking of the N-glycosylation-deficient mutants, or GPCRs that have minimal glycosylation sites. Hence, this novel RPNI chaperone activity is a consequence of N-glycosylation-dependent direct interaction with MOR.

A small region in the angiotensin-converting enzyme distal ectodomain is required for cleavage-secretion of the protein at the plasma membrane

Both germinal and somatic isoforms of ACE are type I ectoproteins expressed on the cell surface from where the enzymatically active ectodomains are released to circulation by a regulated cleavage-secretion process. Our previous studies have shown that ACE-secretase activity is regulated by the ACE distal ectodomain and not by sequences at or around the cleavage site. In the current study we have identified that the ACE residues encompassing 343 to 655 of the germinal form are needed for its cleavage-secretion. To narrow down this region further, we have examined the cleavage-secretion of ACE-CD4 chimeric proteins in mammalian cells and Pichia pastoris. These experiments identified five residues (HGEKL) in the ACE region of the chimeric proteins that were essential for their cleavage-secretion. When the corresponding residues were substituted by alanine in native germinal and somatic ACE, the mutant proteins were not cleaved, although they were displayed on the cell surface and enzymatically active. These results demonstrated that a small region in the ectodomain of ACE is required for its cleavage at the juxtamembrane domain. This conclusion was further supported by our observation that secreted ACE inhibited cell-bound ACE cleavage-secretion, although the secreted form did not contain the cleavage site.

Ribophorin I acts as a substrate-specific facilitator of N-glycosylation

The mammalian oligosaccharyltransferase (OST) complex is composed of about eight subunits and mediates the N-glycosylation of nascent polypeptide chains entering the endoplasmic reticulum (ER). The conserved STT3 subunit of eukaryotic OST complexes has been identified as its catalytic centre, yet although many other subunits are equally well conserved their functions are unknown. We used RNA interference to investigate the function of ribophorin I, an ER-translocon-associated subunit of the OST complex previously shown to associate with newly synthesised membrane proteins. We show that ribophorin I dramatically enhances the N-glycosylation of selected membrane proteins and provide evidence that it is not essential for N-glycosylation per se. Parallel studies confirm that STT3 is essential for transferase activity of the complex, but reveal that the two mammalian isoforms are not functionally equivalent when modifying bona fide polypeptide substrates. We propose a new model for OST function where ribophorin I acts as a chaperone or escort to promote the N-glycosylation of selected substrates by the catalytic STT3 subunits.

Signaling by the angiotensin-converting enzyme

Inhibition of the angiotensin-converting enzyme (ACE) protects against the progression of several cardiovascular diseases. Because of its dual role in regulating angiotensin II and bradykinin levels, the positive clinical effects of ACE inhibitors were thought to be the consequence of concomitant reductions in the production of angiotensin II and the degradation of bradykinin. Recent evidence suggests that some of the beneficial effects of ACE inhibitors on cardiovascular function and homeostasis can be attributed to novel mechanisms. These include the accumulation of the ACE substrate N-acetyl-seryl-aspartyl-lysyl-proline, which blocks collagen deposition in the injured heart, as well as the activation of an ACE signaling cascade that involves the activation of the kinase CK2 and the c-Jun N-terminal kinase in endothelial cells and leads to changes in gene expression. Moreover, at least one other ACE homologue (ACE2) is proposed to counteract the detrimental effects associated with the activation of the classical renin-angiotensin system. These data reveal hitherto unexpected levels of internal regulation of the renin-angiotensin system.

Signaling via the angiotensin-converting enzyme results in the phosphorylation of the nonmuscle myosin heavy chain IIA

The phosphorylation of the short C-terminal cytoplasmic domain of the somatic angiotensin-converting enzyme (ACE) is involved in the regulation of enzyme shedding. We determined whether the phosphorylation of the cytoplasmic domain of ACE (ACEct) on Ser1270 regulates the cleavage/secretion of the enzyme by affecting its association with other proteins. ACE was associated with beta-actin and the nonmuscle myosin heavy chain IIA (MYH9) in endothelial cells, as determined by coimmunoprecipitation experiments as well as an ACEct affinity column. The ACE-associated MYH9 immunoprecipitated from (32)P-labeled endothelial cells was basally phosphorylated and cell stimulation with ACE inhibitors, or with bradykinin, increased the phosphorylation of MYH9. Casein kinase 2 (CK2) but not protein kinase C phosphorylated MYH9 in vitro, CK2 coprecipitated with MYH9 from endothelial cells and the phosphorylation of MYH9 in intact cells paralleled the phosphorylation of ACE on Ser1270 by CK2. The CK2 inhibitor 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole attenuated the phosphorylation of ACE and MYH9, disrupted their association, and enhanced the cleavage/secretion of ACE from the plasma membrane. Cytochalasin D decreased the interaction between ACE and MYH9 and stimulated ACE shedding. Although MYH9 was still able to associate with residual amounts of a nonphosphorylatable S1270A ACE mutant, no ACE inhibitor-induced increase in MYH9 phosphorylation could be detected in S1270A-expressing cells. These data indicate that the interaction of ACE with MYH9 determines ACE shedding and is modulated by phosphorylation processes. Furthermore, because ACE inhibitors affect the phosphorylation of MYH9, the phosphorylation of this class II myosin might contribute to the phenomenon of ACE signaling in endothelial cells.

Calmodulin binds to the cytoplasmic domain of angiotensin-converting enzyme and regulates its phosphorylation and cleavage secretion

The rate of cleavage secretion of the enzymatically active ectodomain of angiotensin-converting enzyme (ACE) is regulated by tyrosine phosphorylation of the protein and by the phorbol ester, phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C. Here, we report that both calmodulin inhibitor (CaMI) and calmodulin kinase inhibitor could also enhance cleavage secretion of ACE. This effect was accompanied by the dissociation of calmodulin from a specific region within the cytoplasmic domain of ACE to which it had been bound. The same domain of ACE was phosphorylated, and both CaMI and PMA caused dephosphorylation of ACE as well. Mass spectrometric and mutational analyses identified Ser730 as the only phosphorylated residue in the cytoplasmic domain of ACE. The Ser730 --> Ala mutant of ACE was not phosphorylated, but it still bound calmodulin, and its cleavage secretion was enhanced by both CaMI and PMA. Similarly, when Ser730 was replaced by the phosphoserine mimetic, Asp, cleavage secretion of the resultant mutant remained susceptible to the enhancing effect of CaMI and PMA. These results demonstrate that, although CaMI and PMA can enhance both cleavage secretion of ACE and its dephosphorylation, the two effects are not mutually interdependent.

Ribophorin I associates with a subset of membrane proteins after their integration at the sec61 translocon

The biosynthesis of membrane proteins at the endoplasmic reticulum (ER) involves the integration of the polypeptide at the Sec61 translocon together with a number of maturation events, such as N-glycosylation and signal sequence cleavage, that can occur both during and after synthesis. To better understand the events occurring after the release of the nascent chain from the ER translocon, we investigated the ER components adjacent to the transmembrane-spanning domain of a well characterized fragment of the amyloid precursor protein. Using individual cysteine residues as site-specific cross-linking targets, we found that several ER components can be cross-linked to the fully integrated polypeptide. We identified strong adducts with both the ribophorin I subunit of the oligosaccharyltransferase complex and the 25-kDa subunit of the signal peptidase complex. Focusing on the association with ribophorin I, we found that adduct formation occurred exclusively after the exit of the nascent chain from the Sec61 translocon and was unaffected by the N-glycosylation status of the associated precursor. Only a subset of newly made membrane proteins associated with ribophorin I in vitro, and we could recapitulate a specific association between the amyloid precursor protein fragment and ribophorin I in vivo. Taken together, our data suggest a model where ribophorin I may function to retain potential substrates in close proximity to the catalytic subunit of the oligosaccharyltransferase and thereby stochastically improve the efficiency of the N-glycosylation reaction in vivo. Alternatively ribophorin I may be multifunctional and facilitate additional processes, for example, ER quality control.

Role of Tyrosine Phosphorylation in the Regulation of Cleavage Secretion of Angiotensin-converting Enzyme

Both germinal (gACE) and somatic (sACE) isozymes of angiotensin-converting enzyme (ACE) are type I ectoproteins whose enzymatically active ectodomains are cleaved and shed by a membrane-bound protease. Here, we report a role of protein tyrosine phosphorylation in regulating this process. Strong enhancements of ACE cleavage secretion was observed upon enhancing protein Tyr phosphorylation by treating gACE- or sACE-expressing cells with pervanadate, an inhibitor of protein Tyr phosphatases. Secreted gACE, cell-bound mature gACE and its precursors were all Tyr-phosphorylated, as was the endoplasmic reticulum protein, immunoglobulin heavy chain-binding protein, that co-immunoprecipitated with ACE. The enhancement of cleavage secretion by pervanadate did not require the presence of the cytoplasmic domain of ACE, and it was not accomplished by enhancing the rate of intracellular processing of the protein. The observed enhancement of cleavage secretion of ACE in pervanadate-treated cells was specifically blocked by an inhibitor of the p38 mitogen-activated protein (MAP) kinase but not by inhibitors of many other Ser/Thr and Tyr protein kinases, including a specific inhibitor of protein kinase C that, however, could block the enhancement of cleavage secretion elicited by phorbol ester. These results indicate that ACE Tyr phosphorylation, probably in the endoplasmic reticulum, enhances the rate of its cleavage secretion at the plasma membrane using a regulatory pathway that may involve p38 MAP kinase.

Deletion of the cytoplasmic domain increases basal shedding of angiotensin-converting enzyme

Ectodomain shedding generates soluble isoforms of cell-surface proteins, including angiotensin-converting enzyme (ACE). Increasing evidence suggests that the juxtamembrane stalk of ACE, where proteolytic cleavage-release occurs, is not the major site of sheddase recognition. The role of the cytoplasmic domain has not been completely defined. We deleted the cytoplasmic domain of human testis ACE and found that this truncation mutant (ACE-DeltaCYT) was shed constitutively from the surface of transfected CHO-K1 cells. Phorbol ester treatment produced only a slight increase in shedding of ACE-DeltaCYT, unlike the marked stimulation seen with wild-type ACE. However, for both wild-type ACE and ACE-DeltaCYT, shedding was inhibited by the peptide hydroxamate TAPI and the major cleavage site was identical, indicating the involvement of similar or identical sheddases. Cytochalasin D markedly increased the basal shedding of wild-type ACE but had little effect on the shedding of ACE-DeltaCYT. These data suggest that the cytoplasmic domain of ACE interacts with the actin cytoskeleton and that this interaction is a negative regulator of ectodomain shedding.

The serum angiotensin-converting enzyme and angiotensin II response to altered posture and acute exercise, and the influence of ACE genotype

The deletion (D) rather than insertion (I) allele of the angiotensin-converting enzyme (ACE) gene is associated with greater ACE activity. We examined: (1) the influence of posture change (recumbent to seated) and acute exercise on serum ACE and angiotensin II (Ang II) activity; (2) the relationship between ACE and Ang II levels; and (3) the influence of ACE genotype on changes in ACE and Ang II levels with posture and exercise. Recreationally active young male Caucasians (10 each of II, ID and DD genotypes) rested for 35 min supine then 15 min upright, took 20 min bicycle ergometric exercise at 70% maximum oxygen uptake, then rested for 40 min. Samples were taken throughout for ACE activity and Ang II levels. Supine ACE levels were dependent upon ACE genotype [24.8 (5.7), 26.9 (4.5), 45.5 (6.4) nmol His-Leu ml(-1) min(-1); II, ID, DD, respectively; P<0.00005] and thereafter. ACE activity rose with assumption of a seated posture [from 32.4 (10.9) nmol His-Leu ml(-1) min(-1) to 35.0 (11.5) nmol His-Leu ml(-1) min(-1), P<0.00001], the absolute rise being independent of genotype [3.22 (1.92), 1.6 (1.6), 2.4 (2.3) nmol His-Leu ml(-1) min(-1); II, ID, DD; P=0.22], unlike percentage change [12.8 (6.8), 5.6 (5.5), 5.3 (5.0)%; II, ID, DD; P<0.01, and P=0.004 for II vs presence of the D allele]. A further genotype-independent rise occurred with exercise [+2.9 (3.7) units, P<0.0003]. An associated rise in Ang II levels [30.3 (15.9), or 2587.9 (489.76)%, P<0.00001] was independent of ACE genotype or activity. Upright posture increases ACE activity, and this may be influenced by ACE genotype. ACE activity and Ang II levels rise independently with exercise in a non-genotype-dependent fashion.

Mechanisms of myocardial remodeling: ramiprilat blocks the expressional upregulation of protein kinase C-epsilon in the surviving myocardium early after infarction

Inhibition of angiotensin-converting enzyme (ACEI) after myocardial infarction reduces remodeling of the surviving myocardium. The cellular signaling mechanisms contributing to remodeling are not fully elucidated. Goal of the current study was to test whether protein kinase C (PKC) is regulated in the surviving myocardium shortly after infarction and whether this regulation is influenced by ACEI. Rats were subjected to anterior wall myocardial infarction in vivo or sham operation. After 15-45 min, mRNA levels and protein expression of the major cardiac PKC isoforms were measured in the ischemic and the remote myocardium. The influence of ACEI on PKC was tested by pretreating the rats with ramiprilat. In the ischemic region of the myocardium, a significant increase of the mRNA for PKC-delta and PKC-epsilon was observed in close correlation with increased isoform protein expression. In the surviving, remote myocardium, however, only PKC-epsilon expression was significantly augmented both at the mRNA level (158%) and at the protein level (149%). PKC-delta and PKC-alpha were unchanged. Treatment with ramiprilat could abolish this isoform-specific PKC regulation in both areas. These data characterize for the first time an isoform-specific transcriptional regulation process of PKC in the surviving myocardium after infarction. This induction of PKC-epsilon can be prevented by ACEI. It is speculated that PKC-epsilon plays a role in the signal transduction of early remodeling after infarction.

CK2 phosphorylates the angiotensin-converting enzyme and regulates its retention in the endothelial cell plasma membrane

Soluble angiotensin-converting enzyme (ACE) is derived from the membrane-bound form by proteolytic cleavage of its C-terminal domain. Because intracellular events might be involved in the regulation of the cleavage process, we determined whether the cytoplasmic tail of ACE is phosphorylated and whether this process regulates secretion. Immunoprecipitation of ACE (180 kDa) from (32)P-labeled endothelial cells revealed that ACE is phosphorylated. Phosphorylation was not observed in endothelial cells overexpressing a mutant form of ACE (ACEDeltaS, all five cytoplasmic serine residues replaced by alanine). CK2 coprecipitated with ACE from endothelial cells, and CK2 phosphorylated both ACE and a peptide corresponding to the cytoplasmic tail. Mutation of serine(1270) within the CK2 consensus sequence almost abolished ACE phosphorylation. In ACE-overexpressing endothelial cells, ACE was mostly localized to the plasma membrane. However, no ACE was detected in the plasma membrane of ACEDeltaS-overexpressing cells, although a precursor ACE (170 kDa) was prominent in the endoplasmic reticulum and the cell supernatant contained substantial amounts of the soluble protein (175 kDa). A correlation between ACE-phosphorylation and secretion was confirmed in endothelial cells treated with the CK2-inhibitor, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, which time-dependently decreased the phosphorylation of ACE and increased its shedding. These results indicate that the CK2-mediated phosphorylation of ACE regulates its retention in the plasma membrane and may determine plasma ACE levels.

Long-term programming of blood pressure by maternal dietary iron restriction in the rat

We have reported that blood pressure was elevated in 3-month-old rats whose mothers were Fe-restricted during pregnancy. These animals also had improved glucose tolerance and decreased serum triacylglycerol. The aim of the present study was to determine whether these effects of maternal nutritional restriction, present in these animals at 3 months of age, can be observed in the same animals in later life. Pulmonary and serum angiotensin converting enzyme (ACE) concentrations were also measured to investigate whether the renin-angiotensin system was involved in the elevation of blood pressure observed in the offspring of Fe-restricted dams. Systolic blood pressure was higher in the offspring of Fe-restricted dams at 16 months of age. Heart and kidney weight were increased as a proportion of body weight in the offspring of Fe-restricted dams. The pulmonary ACE concentration was not significantly different between the groups. The serum ACE concentration was significantly elevated in the offspring of Fe-restricted dams at 3 but not 14 months of age. There was a strong correlation between serum ACE levels at 3 and 14 months of age. Glucose tolerance and serum insulin were not different between the maternal diet groups. Serum triacylglycerol tended to be lower in the offspring of Fe-restricted dams. There were no differences in serum non-esterified fatty acids or serum cholesterol between the maternal diet groups. This study provides further evidence that maternal nutrition has effects on the offspring that persist throughout life. At 16 months of age, the elevation of blood pressure in Fe-restricted offspring does not appear to be mediated via changes in ACE levels. Both cardiac hypertrophy and decreased serum triacylglycerol have also been observed in Fe-restricted fetuses, suggesting that these changes may be initiated in utero.

Epitope-specific antibody-induced cleavage of angiotensin-converting enzyme from the cell surface

Angiotensin I-converting enzyme (ACE; CD143, EC 3.4.15.1) is a type-1 integral membrane protein that can also be released into extracellular fluids (such as plasma, and seminal and cerebrospinal fluids) as a soluble enzyme following cleavage mediated by an unidentified protease(s), referred to as ACE secretase, in a process known as "shedding". The effects of monoclonal antibodies (mAbs) to eight different epitopes on the N-terminal domain of ACE on shedding was investigated using Chinese hamster ovary cells (CHO cells) expressing an ACE transgene and using human umbilical vein endothelial cells. Antibody-induced shedding of ACE was strongly epitope-specific: most of the antibodies increased the shedding by 20-40%, mAbs 9B9 and 3A5 increased the shedding by 270 and 410% respectively, whereas binding of mAb 3G8 decreased ACE shedding by 36%. The ACE released following mAb treatment lacked a hydrophobic transmembrane domain anchor. The antibody-induced shedding was completely inhibited at 4 degrees C and by zinc chelation using 1,10-phenanthroline, suggesting involvement of a metalloprotease in this process. A hydroxamate-based metalloprotease inhibitor (batimastat, BB-94) was 15 times more efficacious in inhibiting mAb-induced ACE shedding than basal (constitutive) ACE release. Treatment of CHO-ACE cells with BB-94 more effectively prevented elevation in antibody-dependent (but not basal) ACE release induced by 3,4-dichloroisocoumarin and iodoacetamide. These data suggest that different secretases might be responsible for ACE release under basal compared with antibody-induced shedding. Further experiments with more than 40 protease inhibitors suggest that calpains, furin and the proteasome may participate in this process.

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.

Altered expression of ADAMs (A Disintegrin And Metalloproteinase) in fibrillating human atria

BACKGROUND

ADAMs (A Disintegrin And Metalloproteinase) are ectoproteases that have recently been reported to be expressed in cardiac tissue. Although they are known to regulate cell-cell and cell-matrix interactions, their pathophysiological role in various cardiac diseases is unclear. The purpose of the present study was to determine whether structural remodeling of the atria during atrial fibrillation (AF) is associated with altered ADAM expression.

METHODS AND RESULTS

Atrial tissue samples of 30 patients undergoing open-heart surgery were examined. Fifteen patients had persistent AF (> or =6 months), and the remaining 15 patients had no history of AF. ADAM9, ADAM10, and ADAM15 expression was analyzed quantitatively at the mRNA and protein levels. ADAM expression was localized by immunohistochemistry. ADAM expression was correlated with amounts of integrins beta1 and beta3. The amount of ADAM10 protein more than doubled during AF (82+/-15 versus 36+/-8 U; P<0.01). Amounts of ADAM15 protein (102+/-12 versus 40+/-6 U; P<0.01) and mRNA (24.0+/-5.6 versus 10.5+/-2.5 U; P<0.05) increased significantly during AF compared with sinus rhythm. ADAM9 protein was not detected in any sample. ADAM/integrin ratios showed an increase of 4- to 6-fold (P<0.05) in patients with AF who had significantly dilated atria (4.94+/-0.6 versus 4.3+/-0.7 cm; P<0.05). ADAM/integrin ratios correlated with atrial diameter.

CONCLUSIONS

AF is associated with an increase in the expression of ADAM10 and ADAM15. Enhanced ADAM-dependent disintegrin and metalloproteinase activity may be a molecular mechanism that contributes to the dilation of fibrillating human atria.

The germinal isozyme of angiotensin-converting enzyme can substitute for the somatic isozyme in maintaining normal renal structure and functions

The angiotensin-converting enzyme (ACE) gene encodes two structurally related isozymes, somatic ACE and germinal ACE, that are uniquely expressed in discrete locations in the body. The importance of ACE in these cell types was revealed by generating Ace -/- mice, which exhibit multiple abnormalities including renal structural defects and functions, hypotension, and male sterility. To test the hypothesis that specific physiological functions of ACE are mediated by isozyme-specific and tissue-specific expression patterns, we have used a transgenic approach to develop mouse strains that express just one ACE isoform in the target tissue of Ace -/- mice. The mice described in this report produce germinal ACE in sperm and serum. These mice were as healthy as wild type mice, and the males were fertile. Interestingly, they had normal kidney structure, fluid homeostasis, and partially restored urine concentration despite having low blood pressure. This result demonstrated that circulating germinal ACE is sufficient for maintaining normal kidney structure and fluid homeostasis but insufficient for restoring blood pressure to normal levels.

Colocalization and interaction of cyclooxygenase-2 with caveolin-1 in human fibroblasts

Results from our previous study suggest that cyclooxygenase-2 (COX-2) induced by phorbol 12-myristate 13-acetate (PMA) may be localized to caveolae-like structures (Liou, J.-Y., Shyue, S.-K., Tsai, M.-J., Chung, C.-L., Chu, K.-Y., and Wu, K. K. (2000) J. Biol. Chem. 275, 15314-15320). In this study, we determined subcellular localization of COX-2 and caveolin-1 by confocal microscopy. COX-2 in human foreskin fibroblasts stimulated by PMA (100 nm) or interleukin-1beta (1 ng/ml) for 6 h was localized to plasma membrane in addition to endoplasmic reticulum and nuclear envelope. Caveolin-1 was localized to plasma membrane, and image overlay showed colocalization of COX-2 with caveolin-1. This was confirmed by the presence of COX-2 and caveolin-1 in the detergent-insoluble membrane fraction of cells stimulated by PMA. Immunoprecipitation showed complex formation of COX-2 with caveolin-1 in a time-dependent manner. A larger quantity of COX-2 was complexed with caveolin-1 in PMA-treated than in interleukin-1beta-treated cells. Purified COX-2 complexed with glutathione S-transferase-fused caveolin-1, which was not inhibited by the scaffolding domain peptide. Caveolin-1-bound COX-2 was catalytically active, and its activity was not inhibited by the scaffolding domain peptide. These results suggest that COX-2 induced by PMA and interleukin-1beta is colocalized with caveolin-1 in the segregated caveolae compartment. Because caveolae are rich in signaling molecules, this COX-2 compartment may play an important role in diverse pathophysiological processes.

Point mutation in the stalk of angiotensin-converting enzyme causes a dramatic increase in serum angiotensin-converting enzyme but no cardiovascular disease

BACKGROUND

Angiotensin-converting enzyme (ACE) metabolizes many small peptides and plays a key role in blood pressure regulation. Elevated serum ACE is claimed to be associated with an increased risk for cardiovascular disease. Previously, two families with dramatically increased serum ACE were described, but no systematic survey of affected individuals was performed, and the molecular background of this trait is unknown.

METHODS AND RESULTS

Eight families were identified with autosomal dominant inheritance of a dramatic (5-fold) increase of serum ACE activity. Strikingly, no clinical abnormalities were apparent in the affected subjects. Isolated blood cells were used for genetic and biochemical analysis. The level of ACE expression on the blood leukocytes and dendritic cells and total cell-associated ACE of the affected individuals was similar to that in nonaffected relatives; however membrane-bound mutant ACE was much more efficiently clipped from the cell surface compared with its wild-type counterpart. A point mutation causing Pro1199Leu in the stalk region of the ACE molecule cosegregates with the increase in serum ACE (LOD score, 6.63).

CONCLUSIONS

A point mutation in the stalk region of the ACE protein causes increased shedding, leading to increased serum ACE, whereas cell-bound ACE is unaltered, and affected individuals exhibit no clinical abnormalities. These findings qualify the importance of serum ACE and establish a new determinant of ACE solubilization.

Increased shedding of angiotensin-converting enzyme by a mutation identified in the stalk region

Angiotensin-converting enzyme (ACE), an enzyme that plays a major role in vasoactive peptide metabolism, is a type 1 ectoprotein, which is released from the plasma membrane by a proteolytic cleavage occurring in the stalk sequence adjacent to the membrane anchor. In this study, we have discovered the molecular mechanism underlying the marked increase of plasma ACE levels observed in three unrelated individuals. We have identified a Pro(1199) --> Leu mutation in the juxtamembrane stalk region. In vitro analysis revealed that the shedding of [Leu(1199)]ACE was enhanced compared with wild-type ACE. The solubilization process of [Leu(1199)]ACE was stimulated by phorbol esters and inhibited by compound 3, an inhibitor of ACE-secretase. The results of Western blot analysis were consistent with a cleavage at the major described site (Arg(1203)/Ser(1204)). Two-dimensional structural analysis of ACE showed that the mutated residue was critical for the positioning of a specific loop containing the cleavage site. We therefore propose that a local conformational modification caused by the Pro(1199) --> Leu mutation leads to more accessibility at the stalk region for ACE secretase and is responsible for the enhancement of the cleavage-secretion process. Our results show that different molecular mechanisms are responsible for the common genetic variation of plasma ACE and for its more rare familial elevation.