Download The malonyl CoA axis as a potential target for treating ischaemic

Review
Cardiovascular Research (2008) 79, 259–268
doi:10.1093/cvr/cvn130
The malonyl CoA axis as a potential target for treating
ischaemic heart disease
John R. Ussher and Gary D. Lopaschuk*
Cardiovascular Research Group, Department of Pediatrics, University of Alberta, Edmonton, Canada
Received 23 January 2008; revised 11 May 2008; accepted 16 May 2008; online publish-ahead-of-print 22 May 2008
Time for primary review: 20 days
KEYWORDS
Ischaemic heart disease;
Malonyl CoA;
Malonyl CoA decarboxylase;
Acetyl CoA carboxylase;
AMP-activated protein kinase;
Fatty acid oxidation
Cardiovascular disease is the leading cause of death and disability for people living in western societies,
with ischaemic heart disease accounting for the majority of this health burden. The primary treatment
for ischaemic heart disease consists of either improving blood and oxygen supply to the heart or reducing the heart’s oxygen demand. Unfortunately, despite recent advances with these approaches, ischaemic heart disease still remains a major health problem. Therefore, the development of new treatment
strategies is still required. One exciting new approach is to optimize cardiac energy metabolism, particularly by decreasing the use of fatty acids as a fuel and by increasing the use of glucose as a fuel.
This approach is beneficial in the setting of ischaemic heart disease, as it allows the heart to
produce energy more efficiently and it reduces the degree of acidosis associated with ischaemia/reperfusion. Malonyl CoA is a potent endogenous inhibitor of cardiac fatty acid oxidation, secondary to inhibiting carnitine palmitoyl transferase-I, the rate-limiting enzyme in the mitochondrial uptake of fatty
acids. Malonyl CoA is synthesized in the heart by acetyl CoA carboxylase, which in turn is phosphorylated
and inhibited by 50 AMP-activated protein kinase. The degradation of myocardial malonyl CoA occurs via
malonyl CoA decarboxylase (MCD). Previous studies have shown that inhibiting MCD will significantly
increase cardiac malonyl CoA levels. This is associated with an increase in glucose oxidation, a decrease
in acidosis, and an improvement in cardiac function and efficiency during and following ischaemia.
Hence, the malonyl CoA axis represents an exciting new target for the treatment of ischaemic heart
disease.
1. Introduction
Cardiovascular disease (CVD) is a major health problem
worldwide, and it is predicted to be the number one killer
by 2010.1 The underlying cause for the majority living with
CVD is a diminished oxygen supply to the cardiac muscle,
termed ‘ischaemic heart disease’.
Fortunately, epidemiological studies and randomized
clinical trials have provided compelling evidence that
ischaemic heart disease is largely manageable.2 Current
treatment regimens, which consist of either percutaneous
or surgical techniques to improve myocardial blood supply,
or pharmacotherapy to limit myocardial oxygen demand,
have greatly aided in the overall prognosis of ischaemic
heart disease patients. Yet, there are still patients who
prove to be ineligible or refractory to conventional treatment, and percutaneous or surgical revascularization is
associated with a distinct set of risks. Therefore, new
approaches to treat such patients are necessary. One such
* Corresponding author: 423 Heritage Medical Research Center, University
of Alberta, Edmonton, Canada T6G 2S2. Tel: þ1 780 492 2170; fax: þ1 780
492 9753.
E-mail address: [email protected]
exciting new therapy is the optimization of cardiac energy
metabolism.
In the setting of ischaemic heart disease, the general
premise for the optimization of cardiac energy metabolism
is to either stimulate the oxidation of glucose or inhibit
the oxidation of fatty acids for energy production.3 As the
oxidation of one molecule of glucose consumes less oxygen
than that of a fatty acid, this allows the heart to produce
energy more efficiently. Furthermore, stimulating glucose
oxidation either directly, or secondarily due to an inhibition
of fatty acid oxidation, results in improved coupling
between glycolysis and glucose oxidation, which decreases
proton production and alleviates myocardial acidosis,
improving cardiac efficiency.4,5
There are numerous ways to inhibit cardiac fatty acid oxidation, some of which include2 the inhibition of fatty acid
transport into the cardiac myocyte,3 the inhibition of fatty
acid uptake into the mitochondria,4 and the inhibition of
the enzymatic machinery of the b-oxidative pathway
itself. Although there are existing agents that target all
three of these approaches, this review will focus on the
inhibition of mitochondrial fatty acid uptake approach,3
and in particular, the use of agents that increase levels of
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2008.
For permissions please email: [email protected].
260
malonyl CoA, a potent endogenous inhibitor of carnitine
palmitoyltransferase I (CPT-I), the rate-limiting enzyme in
the mitochondrial uptake of fatty acids.6 We will review
the literature on the regulation of malonyl CoA via both
its synthesis and degradation, how the malonyl CoA axis
has been manipulated in animal models of ischaemia/
reperfusion (focusing on the most recent studies involving
transgenic mice to manipulate malonyl CoA), and the
potential for this axis to be manipulated in humans.
2. Cardiac energy metabolism
In the normal healthy heart, almost all (.95%) ATP generated in the heart comes from mitochondrial oxidative phosphorylation, with the remainder derived from glycolysis.7
Despite producing more ATP than carbohydrates, fatty
acids are not as oxygen-efficient, requiring 10% more
oxygen to produce an equivalent amount of ATP.8 This is of
particular importance when oxygen becomes a limiting
factor for oxidative metabolism, as seen with ischaemic
heart disease. In addition, fatty acids directly inhibit the
oxidation of carbohydrates through a phenomenon termed
the ‘Randle cycle’.9 This uncouples glycolysis from glucose
oxidation, which results in an increased proton production
that reduces cardiac efficiency during reperfusion following
ischaemia10 (see 11,12 for review).
As will be discussed, an emerging approach to optimizing
cardiac energy metabolism is to keep levels of malonyl CoA
high in the heart. This inhibits the mitochondrial uptake of
fatty acids, leading to a subsequent inhibition of fatty acid
b-oxidation and secondary increase in glucose oxidation,
thereby making oxygen utilization and cardiac energy production more efficient, while preventing the production of
protons and development of acidosis.10,11
3. Cardiac carbohydrate metabolism
The metabolism of glucose can be separated into two major
components, glycolysis and glucose oxidation (see 3,8 for
review). Glycolysis results in the production of pyruvate
and accounts for ,10% of the total ATP produced by the
aerobic heart.13 If glycolysis is coupled to glucose oxidation,
the pyruvate generated from glycolysis is converted to
acetyl-CoA (which is subsequently oxidized in the TCA
cycle) by the enzymatic action of the multi-enzyme
complex, pyruvate dehydrogenase (PDH).
The PDH complex is under tight regulation by an upstream
kinase, PDH kinase, which phosphorylates and inhibits its
activity.9 This PDH kinase is positively regulated by acetyl
CoA and NADH. As mitochondrial acetyl CoA/CoA and
NADH/NAD ratios are increased by elevated rates of fatty
acid oxidation, it leads to a potent inhibition of PDH and
glucose oxidation. This phenomenon was first described
by Randle et al.9 in the 1960s and has been termed the
‘Randle cycle’.
4. Cardiac fatty acid metabolism
Fatty acids enter the cardiac myocyte by either passive diffusion or protein-mediated transport across the sarcolemma14 (see 3,8 for review). Once transported across the
sarcolemma, fatty acids are subsequently activated by
esterification to fatty acyl CoA by fatty acyl CoA synthetase.
J.R. Ussher and G.D. Lopaschuk
This acyl CoA can either be esterified to intracellular lipids
or converted to long-chain fatty acyl carnitine by CPT-I.7
Fatty acid b-oxidation occurs predominantly in the mitochondria and to a smaller extent in the peroxisomes.15 For
mitochondrial fatty acid b-oxidation to begin, the cytoplasmic long-chain fatty acyl CoA must first be transported
into the mitochondrial matrix through a carnitinedependent transport system.8,16 This carnitine transport
system involves three enzymes: CPT-I, carnitine acyltranslocase, and CPT-II. Of these three enzymes, CPT-I is rate limiting in regard to mitochondrial fatty acid uptake and is
subject to potent inhibition via malonyl CoA, the compound
whose regulation will be the major focus of this review.6
Once in the mitochondria, b-oxidation repeatedly cleaves
off two carbon acetyl CoA units from fatty acyl CoA, generating NADH and reduced flavine adenine dinucleotide in the
process. The b-oxidation process involves four enzymatically
catalysed reactions, with the last step regenerating acyl CoA
for another round of b-oxidation and releasing acetyl CoA
for the citric acid cycle. As mentioned earlier, oxidation of
fatty acids increases the acetyl CoA/CoA ratio, which inhibits PDH and glucose oxidation.
5. Regulation of malonyl CoA
As mentioned previously, malonyl CoA is a potent endogenous inhibitor of CPT-I, the rate-limiting enzyme in the mitochondrial uptake of fatty acids. Thus, malonyl CoA
decreases the uptake of fatty acids into the mitochondria,
thereby reducing mitochondrial fatty acid b-oxidation.
Because the turnover of malonyl CoA is quite rapid, with
a half life of 1.25 min,17 both the production and the
degradation of malonyl CoA control its levels, and therefore
of fatty acid oxidation rates.
The production of malonyl CoA is primarily attributed to
the enzymatic activity of acetyl CoA carboxylase (ACC),
which catalyses the carboxylation of acetyl CoA to malonyl
CoA (Figure 1).18,19 There are two isoforms of ACC in the
heart, a and b, with a predominance of ACCb.19 This leads
to the suggestion that the malonyl CoA produced by this
isoform is more involved in the regulation of fatty acid oxidation, as opposed to the high abundance of ACCa in the
liver, where the malonyl CoA produced by this isoform is
more involved in the regulation of fatty acid synthesis.
Studies from our laboratory have confirmed the key role of
ACCb in regulating cardiac fatty acid oxidation.20 The regulation of ACC is under phosphorylation/dephosphorylation
control, with 50 AMP-activated protein kinase (AMPK) having
a major role in its regulation in the heart (Figure 1).21,22
As will be discussed in the following section, this AMPK regulation of ACC becomes very important during times of energy
starvation in the heart, as seen in ischaemia/reperfusion.
Until recently, it was much less clear as to what enzymes
might be responsible for the degradation of malonyl CoA.
One enzyme that has emerged as being important in controlling cardiac malonyl CoA degradation is malonyl CoA decarboxylase (MCD), whose catalytic activity is responsible for
the decarboxylation of malonyl CoA back into acetyl CoA
(Figure 1).23 Studies in both rat and mouse have demonstrated that MCD is indeed involved in regulating cardiac
malonyl CoA levels, and that inhibition of MCD can limit
rates of fatty acid oxidation, leading to a secondary increase
in glucose oxidation (Figure 2). This decrease in fatty acid
Malonyl CoA and ischaemic heart disease
261
Figure 1 The regulation of malonyl CoA in the heart and its alterations during ischaemia/reperfusion. Malonyl CoA is synthesized via acetyl CoA carboxylase,
which carboxylates acetyl CoA into malonyl CoA. Malonyl CoA is degraded via malonyl CoA decarboxylase, which decarboxylates malonyl CoA back into acetyl
CoA. In addition, acetyl CoA carboxylase is negatively regulated by phosphorylation via 50 AMP-activated protein kinase. Increased production of malonyl CoA
inhibits mitochondrial uptake of fatty acids through carnitine palmitoyltransferase I, thereby reducing rates of fatty acid b-oxidation. During ischaemia,
decreased ATP production and a subsequent increase in AMP lead to a rapid activation of 50 AMP-activated protein kinase, which phosphorylates and inhibits
acetyl CoA carboxylase, resulting in a dramatic drop in malonyl CoA levels. Following aerobic reperfusion of the ischaemic heart, 50 AMP-activated protein
kinase activity is sustained, while malonyl CoA decarboxylase activity is maintained. This keeps malonyl CoA levels low, allowing fatty acids to dominate as
the main source of oxidative ATP production, at the expense of glucose oxidation, which increases the production of lactate and protons observed during
reperfusion.
oxidation and increase in glucose oxidation are also associated with an improvement in the functional recovery of
the heart during ischaemia/reperfusion injury.12,24–26 In
addition, peroxisome proliferators-activated receptor
alpha (PPARa), which is a major transcription factor involved
in the regulation of fatty acid oxidation, has been shown to
regulate the expression of MCD.27,28
Previous work in our laboratory has demonstrated that the
high rates of fatty acid oxidation observed during reperfusion are attributed to a dramatic reduction in the levels of
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J.R. Ussher and G.D. Lopaschuk
Figure 2 Targeting of the malonyl CoA axis to improve ischaemia/reperfusion injury. Novel malonyl CoA decarboxylase inhibitors prevent the degradation of
malonyl CoA, keeping malonyl CoA levels high during reperfusion, thereby preventing the uptake and subsequent oxidation of fatty acids in the mitochondria.
This reduces mitochondrial acetyl CoA/CoA and NADH/NAD ratios, thus alleviating the fatty acid-induced inhibition of pyruvate dehydrogenase via the ‘Randle
cycle’. Increased pyruvate dehydrogenase activity increases the oxidation of pyruvate, improving the coupling between glycolysis and glucose oxidation, reducing lactate and proton production. This decrease in proton production alleviates the acidosis seen during reperfusion, allowing ATP in the heart to be used for
contractile purposes as opposed to restoring ionic homeostasis, which improves cardiac functional recovery and overall efficiency.
malonyl CoA, as opposed to direct alterations in the characteristics of CPT-I.21 In addition, we have shown that this
reduction in malonyl CoA levels is associated with a rapid
activation of AMPK during ischaemia, which persists into
reperfusion, causing the phosphorylation-induced inactivation of ACC (Figure 1).21 It has also been suggested that
MCD is a direct target of AMPK, but our laboratory has
been unable to reproduce that data.29 In summary,
decreased ACC activity via AMPK phosphorylation, coupled
with a maintained MCD activity, is a key factor responsible
for the rapid decrease in cardiac malonyl CoA levels
observed during ischaemia/reperfusion.
263
Malonyl CoA and ischaemic heart disease
Cardiac malonyl CoA levels can also increase secondary to
an increase in glucose oxidation. Previous work demonstrated that the acetyl CoA produced by PDH was more
accessible to carnitine acetyltransferase than that produced
from fatty acid b-oxidation.30 Thus, we hypothesized and
showed that increased glucose oxidation could increase
cytosolic acetyl CoA supply for ACCb, increasing malonyl
CoA levels that then inhibit fatty acid b-oxidation.20
It is of interest to note that although malonyl CoA is a
potent inhibitor of CPT-I, the majority of malonyl CoA in
the heart is inaccessible to CPT-I. The cytosolic concentration of malonyl CoA in the heart is 5 mM, whereas the
IC50 of CPT-I inhibition via malonyl CoA is between 50 and
100 nM.20,31 Therefore, if all the malonyl CoA in the heart
were accessible to CPT-I, fatty acid oxidation would always
be completely inhibited, which of course is not the case.
The reason why the majority of malonyl CoA is not accessible to CPT-I is not known, but may have to do with compartmentation of malonyl CoA. Recent evidence supporting this
may come from the observation that a large proportion of
MCD exists in the peroxisomes.32 In addition, it has been
suggested that the majority of malonyl CoA produced in
the heart originates from the peroxisomes.33
6. Cardiac energy metabolism during
ischaemia/reperfusion
Cardiac energy metabolism is drastically altered during
ischaemia, starting with an acceleration of glycolysis, in
an attempt to provide an anaerobic source of ATP to make
up for the reduction in oxidative ATP production. Also of
importance is that fatty acids dominate as the substrate
for what residual oxidative metabolism remains in the
oxygen-deprived ischaemic heart,34 at the expense of
glucose oxidation. This results in an uncoupling between glycolysis and glucose oxidation, contributing to the acidosis
observed in the ischaemic heart, which reduces cardiac
efficiency.4,10
During reperfusion of the ischaemic heart, glycolytic rates
remain elevated, while fatty acids dominate as a source for
oxidative energy production.10 These high rates of fatty acid
metabolism can account for .90% of energy production in
the reperfused heart, and this results in a drastic inhibition
of glucose oxidation. Thus, similar to ischaemia, reperfusion
of the ischaemic heart is accompanied by an increased production of protons that lowers cardiac efficiency. A major
focus of this review has been the emphasis that high rates
of fatty acid oxidation can cause myocardial acidosis by
uncoupling glucose oxidation from glycolysis. Although
other studies have suggested that acidosis may protect the
heart during reperfusion by reducing contractile force35–38
to preserve the viability of hibernating myocardium, or
keeping the mitochondrial permeability transition pore in
a closed state,39,40 such matters are beyond the scope of
this review.
The reason for the excessive use of fatty acids as a source
of fuel during and following ischaemia is primarily the result
of two factors: (1) plasma levels of fatty acids increase dramatically during and following ischaemia,41–44 and (2) subcellular changes occur in the heart itself, resulting in a
decreased control over fatty acid oxidation.21,45 In particular, the levels of malonyl CoA decrease in the heart due to
the rapid activation of AMPK during ischaemia, which persists into reperfusion, resulting in the phosphorylation
induced inhibition of ACC.11,21,23,45 This leads to the accelerated mitochondrial uptake of fatty acids and subsequent
oxidation.
7. Targeting the malonyl CoA axis to treat
cardiac ischaemia
There are a number of ways to manipulate malonyl CoA in
the heart, which include manipulation of the enzymes
involved in regulating its production, i.e. AMPK, ACC, and
MCD. The following sections will examine each of the
latter three enzymes individually as potential treatments
for ischaemia/reperfusion in more detail.
7.1 Targeting 50 AMP-activated protein kinase
to treat ischaemia/reperfusion
Since its initial identification in 1988 by Sim and Hardie,46
AMPK has become a protein, with wide interest among
many investigators, due to its ability to increase energy
metabolism in times of stress. In regard to the stress of
ischaemia/reperfusion injury, many groups have postulated
that AMPK activation would be beneficial in this scenario
via increasing glucose uptake to provide an anaerobic
source of ATP for the energy-starved heart.47–49 However,
our group previously showed that AMPK activity can
decrease cardiac malonyl CoA and increase fatty acid oxidation in the ischaemic heart.21 We therefore hypothesized
that inhibition of AMPK would be beneficial for ischaemia/
reperfusion injury by increasing malonyl CoA levels and
reducing fatty acid oxidation rates, thereby alleviating
myocardial acidosis. Supporting the former proposal,
isolated working heart studies in a transgenic mouse model
of a dominant negative (DN) AMPKa2 with nearly a complete
loss of myocardial AMPK activity have been shown to recover
worse during reperfusion following a low-flow ischaemia.50
Hearts from the transgenic DN-AMPKa2 mice were unable
to increase GLUT4 translocation and glucose uptake
during the low-flow ischaemia or reperfusion periods, and
they had accelerated rates of apoptosis as indicated by
increased caspase 3 cleavage and TUNEL staining.
However, DN-AMPKa2 transgenic mice had significant
contractile dysfunction compared with wild-type mice in
the normal setting, which may explain why they did not
recover as well during ischaemia/reperfusion. Furthermore,
there were no differences in myocardial metabolism
between the control and DN-AMPKa2 transgenic mice,
which suggests that there may have been no differences in
malonyl CoA levels from the hearts of these animals.
Another study investigating the beneficial effects of
adiponectin during ischaemia/reperfusion injury noted a
reduced phosphorylation of AMPK at its threonine 172
residue (which is necessary for AMPK activation) in an
adiponectin-deficient mouse model 48 h after a 30 min
occlusion of the left anterior descending (LAD) coronary
artery.51 The authors demonstrated that inhibition of AMPK
prevented the anti-apoptotic effects of adiponectin on
cardiac myocytes and fibroblasts subjected to hypoxia/reoxygenation. Nonetheless, the anti-apoptotic effects of AMPK
were performed only in culture, and the beneficial effects
of adiponectin during reperfusion following LAD occlusion
264
could also be explained via cyclooxygenase II-dependent
effects.
More recently, a study in an AMPKa2 subunit-deficient
mouse model reported that although AMPKa2 deficiency
accelerated the appearance of contracture during ischaemia, there was no effect on reperfusion recovery of these
hearts, suggesting that inhibition of AMPK is not detrimental
in the heart as an earlier study had reported.52 Moreover,
the perfusions in this study were glucose-only perfusions,
and thus high rates of fatty acid oxidation would not be a
problem in ischaemia/reperfusion. It is possible that the
beneficial effect of reducing the extremely high rates of
fatty acid oxidation during reperfusion may have been
masked because of this.
The regulation of AMPK by its upstream kinases, the AMPK
kinases (AMPKKs), likely plays an important role in activating
AMPK during ischaemia/reperfusion. Previous work in our
laboratory has shown that although AMPK is activated in
the rat heart during reperfusion following ischaemia, myocardial levels of AMP quickly return to normal. Thus,
alternative mechanisms of AMPK activation likely exist
outside of changes in AMP levels. One possibility is AMPKK,
which phosphorylates and activates AMPK. Identified
AMPKKs include the tumour-suppressor LKB1, calmodulindependent protein kinase kinase b (CamKKb), and more
recently,
transforming
growth
factor-activated
kinase-1.22,53,54 Our group55 and others47 have shown that
AMPKK is rapidly activated in the ischaemic heart. To date,
the only AMPKK that has been extensively studied in the
heart is LKB1. We have shown that AMPKK activation
during ischemia is not due to LKB1 activation.55 In contrast,
a previous study investigating the role of LKB1 on AMPK activation in the heart during ischaemia and anoxia reported
that LKB1 was responsible for phosphorylating and activating
the AMPKa2 subunit at its threonine 172 residue, but not the
AMPKa1 subunit.56 Although hearts from these mice had
enlarged atria and reduced weights, echocardiographic
assessment revealed no cardiac dysfunction. However,
these studies did not examine functional differences
during reperfusion, and perfusions were again performed
with glucose as the sole substrate. Further studies are therefore required to determine what role LKB1 and the other
AMPKKs have on AMPK during ischaemia/reperfusion,
malonyl CoA levels, and functional recovery.
Another way to inhibit AMPK is via insulin administration.
A number of studies have examined the role of glucose–
insulin–potassium (GIK) for the treatment of acute myocardial infarction.57–59 We initially hypothesized that insulin
would benefit the aerobically reperfused ischaemic heart.
This would arise from the inhibition of myocardial AMPK
during ischaemia and subsequent reperfusion, thereby reducing rates of fatty acid oxidation and increasing glucose oxidation rates, alleviating myocardial acidosis and improving
functional recovery. Unfortunately, the majority of studies
examining the beneficial effects of insulin on functional
recovery of the heart during reperfusion have performed
their perfusions with glucose as the sole substrate. A
recent study in our laboratory has demonstrated that fatty
acids in the perfusate interfere with insulin’s ability to
inhibit AMPK, and although insulin is still able to reduce
fatty acid oxidation, a greater stimulation of glycolysis
than glucose oxidation actually increases proton production
and worsens functional recovery during reperfusion.60
J.R. Ussher and G.D. Lopaschuk
Therefore, it is unlikely that any beneficial effects of
insulin during reperfusion involve an inhibition of AMPK,
and it is possible that our previous results can explain the
lack of mortality benefit with GIK for patients undergoing
an acute myocardial infarction during the recent CREATEECLA trial.61 In fact, a recent study suggests that GIK may
actually increase mortality in the early post-AMI period.62
Owing to the limitations of the first two studies, and the
results of the aforementioned third study, we believe that
there is insufficient evidence to state that AMPK activation
is beneficial during ischaemia/reperfusion injury. What is
needed to reconcile these discrepancies is more studies
done using in vivo ischaemia/reperfusion models investigating the effect of AMPK on myocardial function, as well
as actual measurements of myocardial malonyl CoA in
these systems.
7.2 Targeting acetyl CoA carboxylase to treat
ischaemia/reperfusion
In regard to ACC, there is very little literature investigating
its effects on ischaemia/reperfusion, as the majority of literature on ACC modulation has focused on inhibiting ACC
in the liver and adipose tissue. It has been postulated that
inhibiting ACC in these tissues to decrease malonyl CoA
would decrease fatty acid biosynthesis.63–65 Unfortunately,
no studies have investigated the effects of ACC inhibition
on cardiac function. There are no selective pharmacological
inhibitors of ACC available, and they would have to be
specific to the b-isoform, which predominates in the heart
and is more tightly linked to the regulation of fatty acid oxidation.20 Nonetheless, mice deficient for ACCb are available
and have been characterized and show significant increases
in fatty acid oxidation.66 It would be of interest to conduct
ischaemia/reperfusion studies in these mice, as it is possible
that hearts from these mice would have a depressed recovery of cardiac function during ischaemia/reperfusion due to
high rates of fatty acid oxidation and subsequent acidosis.
7.3 Targeting malonyl CoA decarboxylase
to treat ischaemia/reperfusion
Recent studies in our laboratory have shown that MCD is a
major regulator of fatty acid oxidation rates in the heart,
and that inhibition of this enzyme is a viable target for the
treatment of ischaemia/reperfusion injury.25,26 First, using
novel inhibitors of MCD, we have shown that inhibition of
MCD in an isolated working rat heart perfusion system significantly increases malonyl CoA levels.26 This was associated
with a significant decrease in fatty acid oxidation rates
and a subsequent increase in glucose oxidation rates.
These metabolic effects induced via inhibition of MCD
caused a significant reduction in proton production during
both the aerobic setting and during low-flow ischaemia.
Furthermore, inhibition of MCD resulted in a significant
improvement in cardiac functional recovery of aerobically
reperfused ischaemic rat hearts. Another study from our
laboratory and Stanley’s demonstrated in an in vivo pig
model of demand-induced ischaemia that MCD inhibition
once again significantly increased malonyl CoA levels
and glucose oxidation rates.24,26 Moreover, this was
accompanied by a significant reduction in myocardial
lactate production and a complete restoration of left
ventricular work. Last, a third study from our laboratory
265
Malonyl CoA and ischaemic heart disease
investigated the effects of chronic MCD inhibition in a
whole-body MCD-deficient mouse model.25 Although hearts
from these animals had a significant increase in malonyl
CoA levels, isolated aerobic working heart perfusions
demonstrated no differences in glucose and fatty acid
metabolism compared with control hearts. This may have
been attributed to the significant upregulation in the
mRNA of a number of different PPARa target transcripts
such as CD36 and CPT-I. Nonetheless, when hearts from
the MCD-deficient mice were subjected to the stress of
ischaemia/reperfusion injury, a significant improvement in
the recovery of cardiac power and function was observed.
This enhanced recovery was associated with a significant
increase in glucose oxidation rates, whereby glucose
oxidation became the major source of ATP production in
the heart. Thus, hearts from the MCD-deficient mice
had improved coupling between glycolysis and glucose
oxidation, reduced proton production, and less myocardial
acidosis.
As mentioned earlier, although the inhibition of fatty acid
oxidation in the heart improves function in the setting of
ischaemic heart disease, others have postulated that the
inhibition of fatty acid oxidation in peripheral tissues, such
as the muscle and liver, will exacerbate insulin resistance
and type 2 diabetes.67 This is an important point to consider,
a significant number of patients with ischaemic heart
disease are also obese/type 2 diabetic. From a clinical
standpoint, oral delivery of MCD inhibitors for the prophylactic treatment of ischaemic heart disease would be practical. However, this would affect the peripheral tissues such
as the muscle, and could cause insulin resistance or
worsen the diabetic state. A recent collaboration by our laboratory and that of Muoio and colleagues68 has shown that
this is not the case. Obesity and insulin resistance induced
by high-fat diet are actually accompanied by increased
rates of incomplete fatty acid oxidation as opposed to
impaired fatty acid oxidation.68 In fact, we demonstrated
that the MCD-deficient mouse was protected from
obesity-induced insulin resistance, which was associated
with decreased rates of incomplete fatty acid oxidation.
Furthermore, potential toxic intramuscular fatty acid
metabolites that have been postulated to cause insulin
resistance due to impaired fatty acid oxidation, such as
long-chain acyl CoAs, actually trended to increase in the
MCD KO mice, suggesting that they are not direct mediators
of insulin resistance.
As mentioned earlier, MCD expression is transcriptionally
regulated by PPARa.27,28 Thus, manipulating PPARa offers
another approach to regulating malonyl CoA levels and
rates of fatty acid oxidation. Indeed, previous work in our
laboratory has shown that hearts from mice deficient for
PPARa have reduced expression of MCD and increased
levels of malonyl CoA.69 These animals subsequently have
lower rates of fatty acid oxidation and increased rates of
glucose oxidation. Moreover, PPARa-deficient mice have an
improved recovery of cardiac power during reperfusion following a global no-flow ischaemia, whereas mice overexpressing PPARa have a worse recovery of cardiac power
under identical conditions.70 These results are contrary to
reports utilizing acute systemic activation of PPARa with
ligands that caused beneficial effects against ischaemia/
reperfusion injury.71,72 The benefit seen with PPARa inhibition could be due to the optimization of cardiac energy
metabolism during reperfusion, whereas the benefit seen
with PPARa activation could be the result of its antiinflammatory properties.73–75 Because PPARa has antiinflammatory properties and regulates the expression of
many genes involved in fatty acid uptake and oxidation
besides MCD,73,76,77 inhibiting MCD to decrease fatty acid
oxidation in the heart may be a more plausible approach
than inhibiting PPARa.
8. The potential for malonyl CoA axis
manipulation in humans
Currently, there are a number of agents used clinically that
optimize cardiac energy metabolism. This includes, trimetazidine, a 3-ketoacyl CoA thiolase inhibitor. The Cochrane
collaboration meta-analysis recently showed trimetazidine
to be an effective therapy for stable angina compared with
placebo (40% reduction in the mean number of angina
attacks per week), alone or combined with conventional antianginal agents.78 In addition, perhexiline, a CPT-1 inhibitor,
has been shown to have clinical utility in refractory angina
pectoris,79 aortic stenosis,80 and chronic heart failure,81
where it improves symptomatic status, left ventricular ejection fraction, and quality of life. Direct stimulation of glucose
oxidation with dichloroacetate, a PDH kinase inhibitor,
improves left ventricular stroke volume and cardiac efficiency
in the setting of heart failure.82
To date, there are no agents clinically available for the
treatment of ischaemia/reperfusion injury that target the
malonyl CoA axis to optimize cardiac energy metabolism.
On the basis of the positive human trials with trimetazidine,
a direct fatty acid b-oxidation inhibitor, and the positive
results seen with both acute and chronic MCD inhibition in
animals, manipulating the malonyl CoA axis in humans
appears plausible and could yield positive effects for the
treatment of ischaemia/reperfusion injury. In regard to
MCD inhibition, two potential setbacks are that CPT-1 inhibition has been shown in past studies to cause hypertrophy,
and that MCD deficiency in humans is associated with cardiomyopathy.83 A number of studies in the early 1980s
reported that chronic treatment in dogs and rats with the
CPT-1 inhibitor, oxfenicine, results in the development of
cardiac hypertrophy.84,85 However, more recent studies
have actually shown that the CPT-1 inhibitors, oxfenicine
and etomoxir, can actually delay adverse left ventricular
modelling in a dog model of pacing-induced heart failure86
and rat model of aortic constriction.87 Some of the reported
discrepancy may lie in the fact that the earlier studies did
not address whether physiological or pathological hypertrophy was occurring. Furthermore, these inhibitors of CPT-1
were irreversible, whereas manipulating malonyl CoA via
MCD would be an indirect inhibition of CPT-1. In addition,
MCD-deficient mice have normal cardiac function and show
no signs of cardiac hypertrophy.26 To address the cardiomyopathy issue, only 18 patients with MCD deficiency have been
reported in the literature,88 and of these 18, only five developed a cardiomyopathy.83,88–92 Of further interest, in those
patients who completely lack MCD, no cardiomyopathies
were observed, and those patients deficient in MCD that
developed cardiomyopathies often possessed mutations in
the gene that result in subcellular mistargeting and not
the complete absence of MCD protein.92 Therefore,
266
inhibition of MCD in the heart appears to be safe, and future
studies are warranted to explore the role of MCD inhibition
as a therapy for ischaemic heart disease. As of yet, there
are no studies looking at regulating the malonyl CoA axis
via ACC in the heart to warrant manipulation of this
enzyme in human studies of ischaemia/reperfusion. Moreover, animal studies of AMPK manipulation have yet to investigate the effects of AMPK on malonyl CoA levels in the
heart, and results of AMPK’s effects on cardiac function
are mixed. Thus, further work in animal models is necessary
before AMPK manipulation is used as a therapeutic target in
humans for ischaemia/reperfusion injury.
9. Summary
The optimization of cardiac energy metabolism represents
an exciting new therapeutic approach for the treatment of
ischaemia/reperfusion injury. One approach to optimize
cardiac energy metabolism is to regulate the levels of
malonyl CoA, a potent endogenous inhibitor of cardiac
fatty acid oxidation, secondary to its inhibition of CPT-1,
the rate-limiting enzyme in the mitochondrial uptake of
fatty acids. A number of studies have recently been published showing that the inhibition of MCD increases cardiac
malonyl CoA levels. These studies reported improved functional recovery of the heart during ischaemia/reperfusion
injury, which was attributed to increased glucose oxidation
and decreased proton production. Thus, targeting the
malonyl CoA axis in the heart represents a potential exciting
new therapy for the treatment of ischaemic heart disease.
Conflict of interest: J.R.U. is a trainee of the Alberta Heritage
Foundation for Medical Research and Tomorrow’s Research Cardiovascular Health Professionals (TORCH). G.D.L. is a medical scientist
of the Alberta Heritage Foundation for Medical Research. J.R.U. has
no conflicts to declare. G.D.L. is the President and CEO of Metabolic
Modulators Research Ltd, a company which has interests in developing metabolic drugs to treat heart disease. This includes agents that
modify the malonyl CoA axis.
Funding
Supported by a grant from the Canadian Institutes for Health
Research Grant to G.D.L.
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