Download Fatty acid oxidation inhibition with PPARa activation (FOXIB/PPARa

Cardiovascular Research 66 (2005) 423 – 426
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Editorial
Fatty acid oxidation inhibition with PPARa activation (FOXIB/PPARa)
for normalizing gene expression in heart failure?
Heinz Rupp*, Thomas P. Rupp, Bernhard Maisch
Molecular Cardiology Laboratory, Department of Internal Medicine and Cardiology, Philipps University of Marburg, Marburg, Germany
Received 23 March 2005; accepted 31 March 2005
Available online 19 April 2005
See also article by Lionetti et al. [7] (pages 454 –461) in
this issue.
1. Gene expression of overloaded cardiomyocytes
In the majority of patients with heart failure, the left
ventricle is overloaded and cardiac hypertrophy occurs,
which is associated with a dysregulated gene expression. A
hallmark of hypertrophied animal hearts is the fetal
phenotype, which has been characterized on the basis of a
reduced or inadequate expression of a-myosin heavy chain
(a-MHC) and the Ca2+ pump (SERCA2) of the sarcoplasmic
reticulum (SR) that appears to be a marker of a great number
of dysregulated genes [1] also involving Na+ –Ca2+ exchange
[2]. During progression of heart failure involving neuroendocrine activation, reprogramming of an even larger group
of genes ensues. Microarray data has revealed at least 251
genes that are up- or downregulated upon heart failure [3].
The finding that many of the differentially expressed genes
code for enzymes involved in energy metabolism might not
be unexpected, since they are also reduced in the fetal
period. Repression of genes that are responsible for the
oxidation of fatty acids was particularly pronounced [1,4],
which is indicative of reduced fatty acid utilization. Glucose
oxidation is increased, which might nevertheless be inadequate when insulin resistance occurs [5]. Since PPARa was
reduced as a consequence of pressure overload, the switch in
* Corresponding author. Molecular Cardiology Laboratory, Department
of Internal Medicine and Cardiology, Karl-von-Frisch-Strasse 1, 35033
Marburg, Germany. Tel.: +49 6421 286 5032; fax: +49 6421 286 8964.
E-mail address: [email protected] (H. Rupp).
fuel metabolism has been attributed to a reduced influence of
PPARa [6]. Which of these many alterations represent initial
events and are causative for the progression of heart failure?
Which represent a compensatory reprogramming of gene
expression?
Until recently, it was still a matter of dispute whether
the overloaded cardiomyocyte contributes to heart failure,
and it was thought that deterioration of pump function
arises primarily from an adverse remodeling of the
extracellular matrix. This controversy can be settled by
examining drugs that interfere with the altered gene
expression of cardiomyocytes and by assessing consequences for the progression of heart failure. In these
experiments, the overload of the heart should not be
reduced by the treatment. As an example, antihypertensive ACE inhibitor treatment of infarcted overloaded
heart reduces the overload and defects in gene expression
of cardiomyocytes, therefore, cannot be traced. Ideally, a
given compound should not have acute inotropic or
vasodilatory actions. As reported in this issue of
Cardiovascular Research, Lionetti et al. [7] studied such
a compound in a large animal model of severe heart
failure. They addressed the question whether partial
inhibition of fatty acid oxidation of the heart in a rapidly
progressing model of heart failure due to rapid pacing
accelerates or slows heart failure. Keeping in mind that
fatty acids are the main fuel for the heart and that the
failing heart might already be energy starved [5], the
outcome is not straightforward. By blocking mitochondrial carnitine palmitoyltransferase I (CPT I) activity, the
already reduced fatty acid utilization is impaired further
and an even more prominent shift to glucose utilization is
expected.
0008-6363/$ - see front matter D 2005 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.cardiores.2005.03.023
424
H. Rupp et al. / Cardiovascular Research 66 (2005) 423 – 426
2. Oxfenicine treatment in a dog model of ischemic heart
failure
Lionetti et al. [7] provide clear evidence that the CPT I
inhibitor oxfenicine slowed progression of heart failure and
preserved pump function. The treatment delayed the onset
of failure by approximately 1 week, reduced left ventricular remodeling and prevented various changes in
protein phenotype. In particular, activation of matrix
metalloproteinase (MMP)-2 and -9, which are known to
be involved in cardiac dilatation, was prevented and enddiastolic diameter was reduced. Since the hearts still failed,
it has to be concluded that activation of MMP-2 and -9
only partly contributes to cardiac dilatation, but certainly
does not represent an epiphenomenon. The beneficial
effects can most probably not be attributed to putatively
antioxidative properties of oxfenicine. Another CPT I
inhibitor, etomoxir, also retarded dilatation of the pressure-overloaded heart [8], and this compound is not
expected to exhibit antioxidative effects. Since the
reduction in the mRNA levels of the genes PPARa,
RXRa, GAPDH, citrate synthase, m-CPT I, PDK-4 and
UCP3 of untreated dogs with heart failure were prevented
by oxfenicine, the question emerges whether these genes
belong to the group of causative genes for heart failure
progression or represent a compensatory reprogramming
due to the impaired heart function. At first glance, the
findings on oxfenicine might be puzzling. Mechanisms
underlying the improvement in heart function become
apparent when comparing oxfenicine with etomoxir.
Insulin receptor
Mitochondrion
Etomoxir has been developed as a hypoglycemic and
hypolipidemic compound. In contrast to the physiological
inhibitor malonyl-CoA, etomoxir inhibits irreversibly a
certain proportion of CPT I molecules and thereby reduces
the mitochondrial uptake and oxidation of fatty acids (Fig.
1). CPT I inhibition alone, however, would reduce an
already diminished fatty acid oxidation of pressure-overloaded hearts. As in the case of statins, the compound
appears to have a pleiotropic action. As a consequence of
CPT I inhibition, cytoplasmic triacylglycerols and fatty
acids are increased and thereby provide additional endogenous PPARa ligands, leading to activation of the important
transcription factor PPARa. Because of its fatty acid
residue, etomoxir is also expected to be a direct ligand for
PPARa. In contrast to established PPARa agonists such as
the hypolipidemic fibrates, which increase fatty acid
oxidation, etomoxir leads to PPARa activation without
increasing fatty acid oxidation; i.e., it belongs to the drug
class of ‘‘fatty acid oxidation inhibitors with PPARa
activation (FOXIB/PPARa)’’ exhibiting a dual mechanism
Glucose
Nucleus
Sugar
intermediates
Acetyl-CoA
PPARα
betaThiolases
NADH
Acyl-CoA dehydrogenases oxidation
Ligand
CPT I
Malonyl-CoA
Oxidation of:
inner
CPT II
Glucose
Fatty acids
outer membrane
CPT I inhibitor
Acylcarnitine Carnitine+Acyl-CoA
PPARα promoter act.
CPT I inhibition
+PPARα activation
Carnitine + Fatty acid
Carnitine
Pressure overload
PPARα deactivation
PPARα promoter act.
Carnitine Acylcarnitine Carnitine + Acyl-CoA
CACT
Gene promoters
RXR
PDK
Citrate
Fatty acid
oxidation
3. Etomoxir counteracts a dysregulated gene expression
of overloaded cardiomyocytes
Glucose transporter Glucose
PFK
Pyruvate
Glycolysis
Glucose
oxidation
Although oxfenicine differs in the chemical structure (lhydroxyphenylglycine) from etomoxir (2-[6-(4-chlorophenoxy)hexyl]oxirane-2-carboxylate), both have been developed as CPT I inhibitors and appear to have a similar
action in overloaded and failing heart.
Fatty acid
Endothelial cell
Fatty acid
Fibrate
Fig. 1. Possible protein or metabolite targets (italics) for drugs (underlined) interfering with cardiac energy metabolism. Best characterized is CPT I inhibition,
which has a dual mechanism of action by reducing fatty acid oxidation and activating PPARa. Malonyl-CoA is the physiological CPT I inhibitor and can be
increased by inhibiting malonyl-CoA decarboxylase. Etomoxir also inhibits acetyl-CoA carboxylase, which controls fatty acid synthesis thereby limiting
triacylglycerol accumulation. Lipid-lowering interventions reduce fatty acid supply to the heart and fatty acid oxidation and enhance glucose utilization (Randle
cycle) if glucose uptake is not limited due to insulin resistance. Gene promoter mechanisms possibly involving sugar intermediates, PPARa and other metabolic
signals are summarized in Refs. [11,16]. Abbreviations: PDK, pyruvate dehydrogenase kinase; CACT, carnitine-acylcarnitine translocase; CPT, muscle-type
carnitine palmitoyltransferase; PFK, 6-phosphofructo-1-kinase; PPAR, peroxisome proliferator-activated receptor; RXR, retinoid X receptor; PPARa promoter
act., activity of gene promoters with response elements for PPARa.
H. Rupp et al. / Cardiovascular Research 66 (2005) 423 – 426
of action. In this respect, an intriguing observation of
Lionetti et al. [7] was the marked upregulation of pyruvate
dehydrogenase kinase-4 (PDK-4) mRNA, which is a typical
response to PPARa activation. However, PPARa activation
on its own with an established PPARa agonist had
detrimental effects in the pressure-overloaded heart [9].
The dose used was high enough to counteract the reduction
of fatty acid oxidation of the pressure-overloaded heart and
actually appeared to enhance fatty acid oxidation beyond
control values [9].
Chronic etomoxir treatment rescued the down-regulated
SERCA2 of hypertrophied, pressure-overloaded hearts back
to near normal levels. The SR Ca2+ ATPase activity, Ca2+
uptake rate, number of active Ca2+ pumps [E¨P] and
SERCA2 protein and mRNA abundance were increased
(see Refs. [10,11] and references therein). Since a reduction
in SERCA2 expression results in an impaired Ca2+
handling by the cardiomyocyte, it appears to be a causative
defect leading to depression of pump function. Etomoxir
also increased the proportion of a-MHC or myosin V1 (2
a-MHC), demonstrating a coordinated expression of genes
required for fast relaxation and contraction of the heart.
Based on putative regulatory sequences on the SERCA2
promoter, the transcriptional effects of etomoxir have been
attributed to a shift in energy metabolism with increased
glucose utilization and/or PPARa activation [11]. The
etomoxir-induced increase in gene expression of SERCA2
and a-MHC was associated with an improved pump
function of the pressure-overloaded heart [8]. Since the
high afterload arising from constriction of the ascending
aorta cannot be reduced by a drug, direct actions on the
cardiomyocyte have to be inferred. At low dosage,
etomoxir had a selective influence on the pressure-overloaded left ventricle. Both myocardial working capacity and
rates of contraction and relaxation of isovolumically beating
hearts were increased. Etomoxir also influenced the
transition from apparently compensated to decompensated
cardiac hypertrophy [12]. After severe constriction of the
ascending aorta, the pronounced left ventricular hypertrophy was associated with pulmonary congestion indicating heart failure. The SR Ca2+ uptake rate per gram wet
weight was reduced, which was independent of phospholamban phosphorylation and the inhibition of the SR Ca2+
release mechanism. The SERCA2 protein amount was
likewise reduced. The SR Ca2+ uptake rate was inversely
correlated with left ventricular weight, but was not
influenced by the occurrence of pulmonary edema. Since
the calculated SR Ca2+ uptake rate of the whole ventricle
was not reduced, a hypertrophy proportional dilution of SR
Ca2+ pumps that precedes the occurrence of pulmonary
edema appears likely. Etomoxir did not affect left ventricular weight, but reduced right ventricular hypertrophy,
which is a consequence of pulmonary edema. In parallel,
the SR Ca2+ uptake rate and the proportion of myosin V1
increased. It was concluded that etomoxir represents a
candidate drug for the prevention of heart failure pro-
425
gression by increasing the SR Ca2+ uptake rate. In
accordance, selective restoration of SERCA2 by adenoviral
gene transfer resulted in functional improvement in pressure
overload-induced heart failure and normalization of 51 gene
transcripts [3], demonstrating the occurrence of compensatory reprogramming of gene expression as a result of
impaired function.
4. CPT I inhibition in patients with heart failure
Heart failure is a disease with many causes and one
cannot simply extrapolate from animal experiments to heart
failure in general. Nonetheless, the study by Lionetti et al.
[7] is unique because it demonstrates its effects in a largeanimal model of severe heart failure, which follows a
predictable time course of transition to decompensation.
Thus, compounds referred to as CPT I inhibitors represent
not only an effective therapy for heart failure in rat models.
Based on the animal experiments, clinical trials in heart
failure appear to be a rational consequence.
In a pilot study on etomoxir in 10 patients with impaired
heart performance but without diabetes mellitus type II, an
improved cardiac function was observed after a 3-month
treatment [13]. Based on echocardiographic data, it was
concluded that the etomoxir treatment had no influence on
left ventricular muscle mass. Also, no significant side
effects were observed. In acute studies, etomoxir showed
neither a positive inotropic effect nor vasodilatory properties
[13]. A randomized, placebo-controlled trial including heart
failure patients of NYHA classes II – III with no metabolic
or other severe diseases was terminated prematurely,
however, because side effects had occurred in a small
number of patients.
Should this drug approach be pursued further? The
observed adverse events do not argue against the validity of
basic mechanisms underlying the beneficial effects observed
in the dog and rat. One should, however, take into account
that irreversible CPT I inhibition has its risks if the dosage
cannot be maintained due to an impaired drug elimination.
Obviously, excessive CPT I inhibition has adverse consequences. Thus, etomoxir can cause lipid accumulation in
the rat liver [14]: 125 mg/kg/day of active enantiomeric (+)etomoxir was administered, which is much higher than the
dose [15 mg/kg/day racemic (T)-etomoxir corresponding to
7.5 mg/kg/day of active (+)-form] with selective effects in
overloaded rat hearts and which only moderately increased
lipid droplets in the heart [10]. Lipid accumulation due to
CPT I inhibition is expected to be counteracted by the
hypolipidemic action of etomoxir [15] arising from a
reduced de novo fatty acid synthesis. In future trials,
therefore, blood levels of CPT I inhibitors should be
determined to detect accumulation of the compound due
to possible drug interactions.
In summary, the study by Lionetti et al. [7] represents
another important step in the search for drugs that not only
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H. Rupp et al. / Cardiovascular Research 66 (2005) 423 – 426
interfere with neuroendocrine activation but target defects
within the overloaded or failing cardiomyocyte. Such
studies also help in dealing with the misconception that
heart failure is just a symptom and not a disease on its own
with causative changes within the cardiomyocytes. Efforts
should be strengthened to assess signals associated with
cardiac energy metabolism for their potential of correcting a
dysregulated phenotype of overloaded and failing cardiomyocytes. In particular, studies on promoter interactions of
SERCA2 and related genes involving Ca2+ in addition to
metabolic signals [11,16] are required for further drug
screening. Restoring the function of hypertrophied cardiomyocytes during early asymptomatic progression of heart
failure would be of particular relevance in elderly patients
with inadequately treated hypertension.
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