The angiotensin converting enzyme (ACE) is a 180-kD expopeptidase with two homologous
domains that catalyzes the conversion of angiotensin I (Ang I) to angiotensin II (Ang
II), a potent vasoconstrictor, in a substrate concentration-dependent manner. ACE
also degrades bradykinin, an important vasodilator, as well as other vasoactive peptides.
Angiotensin I converting enzyme 2 (ACE-2) is an exopeptidase that catalyzes the conversion
of angiotensin I to the nonapeptide angiotensin (1-9) or the conversion of angiotensin
II to angiotensin (1-7). Ang II interacts with two receptors; the AT1 receptor performs
the well-appreciated vasoconstrictor, aldosterone-releasing, and sodium-reabsorptive
activities. The AT2 receptor seems to perform ameliorative or other unknown functions.
Ang (1-7) has also been implicated in cardioprotection and interacts with the Mas
receptor, encoded by a proto-oncogene of the same name (Fig. 1). Subsequent signaling
is also imperfectly defined.
Fig. 1
The aspartyl protease renin cleaves angiotensinogen to the ten amino acid peptide,
Ang I. The angiotensin converting enzyme (ACE) in turn cleaves Ang I to the eight
amino acid peptide, Ang II. Ang II can interact with two receptors (AT1 and AT2) that
appear to have divergent functions. Ang I and Ang II can be cleaved to smaller products
by a second angiotensin converting enzyme (ACE-2). One of these small products (Ang
1-7) can interact with its own receptor termed Mas
We know quite a bit about ACE. The Bernstein laboratory performed a series of novel
strategic promoter-swapping studies indicating that ACE plays a role in many other
physiologic processes beyond simple blood pressure control [1]. ACE-2, an ACE homolog
with about a 40 % homology and only one active catalytic domain, was cloned in 2000.
ACE-2 has direct effects on cardiac function and is expressed predominantly in vascular
endothelial cells, the heart, the kidneys, and testis [2]. Then, there is the more
ethereal ACE-3 that is an interacting protein in sperm with the IZUMO1 protein. The
interrelationship apparently facilitates sperm–egg interaction. In man, ACE-3 apparently
only exists as a pseudo-gene with no functional relevance [3, 4]. ACE and ACE-2 are
absent in the Drosophila. However, this invertebrate has homologous sequences probably
belonging to a different family, an angiotensin-related converting enzyme (ACER).
The ACER proteins are active endopeptidases important during fly development but seem
to lack a clear function in adulthood [5]. Bacteria also contain an ACE-like protein
that cleaves Ang I to Ang II, as shown for Xanthomonas axonopodi [6]. The findings
suggest that the ACEs have been around far longer than the angiotensins. Furthermore,
ACE-2 seems to be senior to ACE.
ACE-2 won our hearts over with the finding that in hypertensive rat strains, ACE-2
messenger RNA and protein expression were markedly reduced. Targeted disruption of
ACE-2 in mice resulted in a severe cardiac contractility defect, increased Ang II
levels, and upregulation of hypoxia-induced genes in the heart. Genetic ablation of
ACE on an ACE-2-mutant background rescued the cardiac phenotype. Interestingly, disruption
of ACER, the Drosophila ACE-2 homologue, resulted in fruit fly heartbreak [7]. These
genetic data for ACE-2 showed that it is an essential regulator of cardiac function
in vivo. A second independent ACE-2 gene deletion story gave substantially less dramatic
results, but nevertheless “hope springs eternal” [8].
Then, there is Hartnup disease. When I was a medical student (50 years ago), I encountered
a family with this “pellagra-like” dermatosis. We knew at the time that the problem
lay in the failed absorption of tryptophan and other non-polar amino acids. Tryptophan
is necessary to make serotonin, melatonin, and niacin. The Hartnup amino acid transporter
(SLC6A19) is the major luminal sodium-dependent neutral amino acid transporter of
small intestine and renal proximal tubule. SLC6A19 renal expression depends on its
association with collectrin (Tmem27), a protein homologous to the membrane-anchoring
domain of ACE-2. And so, Camargo et al. [9] showed that ACE-2 is necessary for the
expression of the Hartnup transporter in intestine and suggested that the differential
functional association of mutant SLC6A19 transporters with ACE-2 and collectrin in
intestine and kidney participate in the phenotypic heterogeneity of Hartnup disease.
Several years ago, a coronavirus (CoV) that can unleash a lethal severe adult respiratory
distress syndrome (SARS) caused worldwide panic. Subsequently, Kuba et al. [10] provided
genetic evidence that ACE-2 serves as a SARS receptor. ACE-2 gene-deleted mice were
resistant to virus infection. SARS-CoV infections and the Spike protein of the SARS-CoV
also reduced the ACE-2 expression. Notably, injection of SARS-CoV Spike into mice
worsened acute lung injury (ALI) in vivo, which was attenuated by blocking the renin–angiotensin
system pathway. These results provided a molecular explanation why SARS-CoV infections
caused severe and oftentimes lethal ALI. Downregulation of ACE-2 by SARS-CoV could
accelerate the SARS pathogenesis. Furthermore, the results suggested a possible therapy
for SARS and other respiratory disease viruses that elicit ALI. All these findings
are fascinating but confusing. First, we are dealing with a matrix metalloproteinase
that has been around a very long time from bacteria, to invertebrates, and to vertebrates.
The findings imply a role in cardiovascular regulation from invertebrates to vertebrates.
Furthermore, a simple conversion of Ang I to Ang II or degradation of bradykinin cannot
be the end of the story. We are left with “rather messy” renin–angiotensin–aldosterone
system remains that include Ang (1-9), Ang (1-7), the Mas proto-oncogene, and other
unfathomable variables. Then, we skirt by Hartnup disease and collectrin to learn
that ACE-2 embraces fundamental processes in gastrointestinal and renal tubular reabsorption.
Are these functions of ACE-2 in any way related to Ang (1-7)? Could there be other
substrates, and if so, what are the substrates and what are the end products? Or perhaps
the molecule has still another function? All that information should be reason for
a major migraine, but there is more to come.
In this issue, Rey-Parra et al. hypothesized that ACE-2 prevents bleomycin-induced
lung injury. Bleomycin belongs to a family of chemotherapeutic agents that induce
DNA strand breaks. When I was a junior physician, we included bleomycin in our regimen
for patients with Hodgkin's lymphoma. Pulmonary toxicity was common—a model of drug-induced
pulmonary fibrosis in man. Rey-Parra et al. [11] studied mice and found that ACE-2
gene deletion worsens bleomycin-induced lung injury and more so in males than females
(so much for the myth of male privilege). Pulmonary fibrosis, TGF-β1, collagen, and
hydroxyproline expression were all more severe in ACE-2 gene-deleted mice, compared
to controls. Furthermore, exercise tolerance and dynamic compliance were markedly
reduced in male ACE-2 gene-deleted mice given bleomycin. In female ACE-2 gene-deleted
mice, these effects were less pronounced. To apply ACE-2 therapeutically, the authors
expressed the amino acid residues 1-740 (rhACE-2) in Chinese hamster ovary cells.
The rhACE-2 was administered intraperitoneally for 221 days at 2 mg/kg. The ACE-2
treatment protected against bleomycin-induced fibrosis; in female mice, this treatment
caused a complete reduction in fibrosis. However, these experiments were performed
in wild-type mice so that a rescue experiment in ACE-2 gene-deleted mice was evidently
not performed. The authors next included some data on AT1 and AT2 receptor expression.
These experiments were done in the female mice without gene deletion. The AT2 receptor
expression appears to have been enhanced, particularly in female mice given bleomycin.
The pathogenesis of bleomycin-induced lung fibrosis is not well understood. Histological
studies show that bleomycin treatment results in damage to endothelial cells in lung
vasculature, followed by the accumulation of inflammatory cells, including macrophages
and lymphocytes, in the interstitium. However, the mechanisms are still unclear. After
bleomycin, lung fibroblast cells are activated by cytokines such as tumor necrosis
factor and interleukins, and collagen is deposited in the alveolar space and capillaries.
This chain of events results in reduced oxygen exchange and impaired lung function.
Corticosteroids are sometimes prescribed to suppress the lung toxicity. The authors
imply that the renin–angiotensin system could participate in the bleomycin toxicity
process and that ACE-2 mediates protection. The Penninger laboratory reported earlier
that ACE-2 and the AT2 receptor protected mice from severe ALI induced by acid aspiration
or sepsis [12, 13]. They showed that mice deficient for ACE show markedly improved
disease, and also that recombinant ACE-2 (as demonstrated in the current report) protected
mice from severe ALI (Fig. 2).
Fig. 2
ACE-2 has protective functions in lung that are complex. Some protection may be afforded
by generating Ang (1-7) that signals via the Mas receptor. ACE-2 is also a receptor
for the CoV responsible for SARS (ARDS) an ALI. CoV after cell entry downregulates
ACE-2 by unclear mechanisms. The Absence of ACE-2 increases susceptibility to ALI
as induced by bleomycin in the work by Rey-Parra et al. [11]. Finally, ACE-2 is a
homolog of an amino acid transporter. Much needs to be learned about this complex
signaling process (adapted from [13])
Rey-Parra et al. [11] suggest that recombinant ACE-2 could have therapeutic potential
to attenuate respiratory morbidity in ALI and resultant acute respiratory distress
syndrome (ARDS) from various causes. Can one matrix metalloproteinase with only one
domain (as opposed to the two domains in ACE) do so many different things? First,
what is the most important function of ACE-2 when ACE came later and ACE-3 exists
in humans solely as a pseudo-gene? ACE-2 could counterbalance the effects of ACE (party-line
thinking here) and possibly that state-of-affairs could justify why ACE-2 is around
today. Second, it appears that ACE-2 plays a protective role in the risk of SARS-CoV,
although ACE-2 is apparently necessary for CoV infection in the first place. The appearance
of ACE-2 as a partner in transport systems is most fascinating. This finding is of
major clinical relevance because aside from dramatic genetic diseases (Hartnup disease),
these transporters have accrued relatively little attention. ALI and ARDS are exceptionally
important. The idea of a recombinant ACE-2 therapy has appeal, although I could not
quite follow how rhACE-2 moved so readily from the peritoneum into the lung. There
is much work to be done in this field.
Respectfully,
Friedrich C. Luft