Organ-specific distribution of ACE2 mRNA and correlating peptidase activity in rodents

New mass spectrometric assay for angiotensin-converting enzyme 2 activity

A novel assay was developed for evaluation of mouse angiotensin-converting enzyme (ACE) 2 and recombinant human ACE2 (rACE2) activity. Using surface-enhanced laser desorption/ionization time of flight mass spectrometry (MS) with ProteinChip Array technology, ACE1 and ACE2 activity could be measured using natural peptide substrates. Plasma from C57BL/6 mice, kidney from wild-type and ACE2 knockout mice, and rACE2 were used for assay validation. Plasma or tissue extracts were incubated with angiotensin I (Ang I; 1296 m/z) or angiotensin II (Ang II; 1045 m/z). Reaction mixtures were spotted onto the ProteinChips WCX2 and peptides detected using surface-enhanced laser desorption/ionization time of flight MS. MS peaks for the substrates, Ang I and Ang II, and the generated peptides, Ang (1-7) and Ang (1-9), were monitored. The ACE2 inhibitor MLN 4760 (0.01 to 100 micromol/L) significantly inhibited rACE2 activity (IC50=3 nmol/L). Ang II was preferably cleaved by rACE2 (km=5 mumol/L), whereas Ang I was not a good substrate for rACE2. There was no detectable ACE2 activity in plasma. Assay specificity was validated in a model of ACE2 gene deletion. In kidney extract from ACE2-deficient mice, there was no generation of Ang (1-7) from Ang II. However, Ang (1-7) was produced when Ang I was used as a substrate. In conclusion, we developed a specific and sensitive assay for ACE2 activity, which used the natural endogenous peptide substrate Ang II. This approach allows for the rapid screening for ACE2, which has applications in drug testing, high-throughput enzymatic assays, and identification of novel substrates/inhibitors of the renin-angiotensin system.

Deletion of angiotensin-converting enzyme 2 accelerates pressure overload-induced cardiac dysfunction by increasing local angiotensin II

Angiotensin-converting enzyme 2 (ACE2) is a carboxypeptidase that cleaves angiotensin II to angiotensin 1-7. Recently, it was reported that mice lacking ACE2 (ACE2(-/y) mice) exhibited reduced cardiac contractility. Because mechanical pressure overload activates the cardiac renin-angiotensin system, we used ACE2(-/y) mice to analyze the role of ACE2 in the response to pressure overload. Twelve-week-old ACE2(-/y) mice and wild-type (WT) mice received transverse aortic constriction (TAC) or sham operation. Sham-operated ACE2(-/y) mice exhibited normal cardiac function and had morphologically normal hearts. In response to TAC, ACE2(-/y) mice developed cardiac hypertrophy and dilatation. Furthermore, their hearts displayed decreased cardiac contractility and increased fetal cardiac gene induction, compared with WT mice. In response to chronic pressure overload, ACE2(-/y) mice developed pulmonary congestion and increased incidence of cardiac death compared with WT mice. On a biochemical level, cardiac angiotensin II concentration and activity of mitogen-activated protein (MAP) kinases were markedly increased in ACE2(-/y) mice in response to TAC. Administration of candesartan, an AT1 subtype angiotensin receptor blocker, attenuated the hypertrophic response and suppressed the activation of MAP kinases in ACE2(-/y) mice. Activation of MAP kinases in response to angiotensin II was greater in cardiomyocytes isolated from ACE2(-/y) mice than in those isolated from WT mice. ACE2 plays an important role in dampening the hypertrophic response to pressure overload mediated by angiotensin II. Disruption of this regulatory function may accelerate cardiac hypertrophy and shorten the transition period from compensated hypertrophy to cardiac failure.

Effects of renin-angiotensin system blockade on renal angiotensin-(1-7) forming enzymes and receptors

BACKGROUND

Angiotensin-converting enzyme (ACE)2, a homologue of ACE, which is insensitive to ACE inhibitors and forms angiotensin-(1-7) [Ang-(1-7)] from angiotensin II (Ang II) with high efficiency was investigated in response to chronic blockade with lisinopril, losartan, and both drugs combined.

METHODS

Thirty-six adult Lewis rats were assigned to receive these medications in their drinking water for 2 weeks while their arterial pressure, water intake, and urine volume were recorded throughout the study. Measures of renal excretory variables included assessing excretion rates of angiotensin I (Ang I), Ang II and Ang-(1-7) while blood collected at the completion of the study was used for measures of plasma angiotensin concentrations. Samples from renal cortex were assayed for renin, angiotensinogen (Aogen), neprilysin, angiotensin types 1 and 2 (AT(1) and AT(2)) and mas receptor mRNAs by semiquantitative reverse transcriptase (RT) real-time polymerase chain reaction (PCR). ACE2 activity was determined as the rate of Ang II conversion into Ang-(1-7).

RESULTS

Comparable blood pressure reductions were obtained in rats medicated with either lisinopril or losartan, whereas both drugs produced a greater decrease in arterial pressure. Polyuria was recorded in all three forms of treatment associated with reduced osmolality but no changes in creatinine excretion. Lisinopril augmented plasma levels and urinary excretion rates of Ang I and Ang-(1-7), while plasma Ang II was reduced with no effect on urinary Ang II. Losartan produced similar changes in plasma and urinary Ang-(1-7) but increased plasma Ang II without changing urinary Ang II excretion. Combination therapy mimicked the effects obtained with lisinopril on plasma and urinary Ang I and Ang-(1-7) levels. Renal cortex Aogen mRNA increased in rats medicated with either lisinopril or the combination, whereas all three treatments produced a robust increase in renal renin mRNA. In contrast, ACE, ACE2, neprilysin, AT(1), and mas receptor mRNAs remained unchanged with all three treatments. Renal cortex ACE2 activity was significantly augmented in rats medicated with lisinopril or losartan but not changed in those given the combination.

CONCLUSION

Our data revealed a role for ACE2 in Ang-(1-7) formation from Ang II in the kidney of normotensive rats as primarily reflected by the increased ACE2 activity measured in renal membranes from the kidney of rats given either lisinopril or losartan. The data further indicate that increased levels of Ang-(1-7) in the urine of animals after ACE inhibition or AT(1) receptor blockade reflect an intrarenal formation of the heptapeptide.

Distinct roles for ANG II and ANG-(1-7) in the regulation of angiotensin-converting enzyme 2 in rat astrocytes

Angiotensin-converting enzyme 2 (ACE2) is a homolog of ACE that preferentially forms angiotensin-(1-7) [ANG-(1-7)] from angiotensin II (ANG II). Incubation of neonatal rat cerebellar or medullary astrocytes with ANG II reduced ACE2 mRNA by approximately 60%, suggesting transcriptional regulation of the enzyme. In contrast, ANG II had no effect on ACE mRNA in astrocytes isolated from either brain region, demonstrating a differential regulation of the two enzymes by ANG II. The ANG II-mediated reduction in ACE2 mRNA was blocked by the angiotensin type 1 (AT(1)) receptor antagonists losartan or valsartan; the angiotensin type 2 (AT(2)) antagonist PD123319 was ineffective. The reduction in ACE2 mRNA by ANG II also was associated with a 50% decrease in cerebellar and medullary ACE2 protein, which was blocked by losartan. Treatment of medullary astrocytes with ANG-(1-7), the product of ACE2 hydrolysis of ANG II, did not affect ACE2 mRNA; however, ANG-(1-7) prevented the ANG II-mediated reduction in ACE2 mRNA. The addition of [d-Ala(7)]-ANG-(1-7), a selective AT((1-7)) receptor antagonist, blocked the inhibitory actions of ANG-(1-7). These data are the first to demonstrate transcriptional regulation of ACE2 by ANG II and ANG-(1-7). Because ACE2 preferentially converts ANG II to ANG-(1-7), downregulation of the enzyme by ANG II constitutes a novel positive feed-forward system within the brain that may favor ANG II-mediated neural responses. Furthermore, the modulatory role of ANG-(1-7) in the transcriptional regulation of ACE2 by ANG II suggests a complex interplay between these peptides that is mediated by distinct receptor systems.

Angiotensin-converting enzyme 2 (ACE2), but not ACE, is preferentially localized to the apical surface of polarized kidney cells

Angiotensin-converting enzyme-2 (ACE2) is a homologue of angiotensin-I converting enzyme (ACE), the central enzyme of the renin-angiotensin system (RAS). ACE2 is abundant in human kidney and heart and has been implicated in renal and cardiac function through its ability to hydrolyze Angiotensin II. Although ACE2 and ACE are both type I integral membrane proteins and share 61% protein sequence similarity, they display distinct modes of enzyme action and tissue distribution. This study characterized ACE2 at the plasma membrane of non-polarized Chinese hamster ovary (CHO) cells and polarized Madin-Darby canine kidney (MDCKII) epithelial cells and compared its cellular localization to its related enzyme, ACE, using indirect immunofluorescence, cell-surface biotinylation, Western analysis, and enzyme activity assays. This study shows ACE2 and ACE are both cell-surface proteins distributed evenly to detergent-soluble regions of the plasma membrane in CHO cells. However, in polarized MDCKII cells under steady-state conditions the two enzymes are differentially expressed. ACE2 is localized predominantly to the apical surface ( approximately 92%) where it is proteolytically cleaved within its ectodomain to release a soluble form. Comparatively, ACE is present on both the apical ( approximately 55%) and basolateral membranes ( approximately 45%) where it is also secreted but differentially; the ectodomain cleavage of ACE is 2.5-fold greater from the apical surface than the basolateral surface. These studies suggest that both ACE2 and ACE are ectoenzymes that have distinct localization and secretion patterns that determine their role on the cell surface in kidney epithelium and in urine.