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Heart Failure : A PKGarious Balancing Act
David A. Kass
Circulation. 2012;126:797-799; originally published online July 17, 2012;
doi: 10.1161/CIRCULATIONAHA.112.124909
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Editorial
Heart Failure
A PKGarious Balancing Act
David A. Kass, MD
T
he pathologically hypertrophied and failing heart is a
battlefield in a war that would make even George Lucas
proud. On the one side, you have hemodynamic, neurohormonal, morphological, and cellular/molecular dark forces
urging the ventricle toward decompensation and ultimate
demise. On the other, Jedi signaling cascades try valiantly to
stave off the impending disaster. Alas, unlike the movies, the
dark side often wins, and we need better-equipped counterforces to change this. Current heart failure therapies are fairly
defensive— blocking neurohumoral stimuli and hemodynamic overload. However, adaptive/offensive strategies are advancing, including those aimed at enhancing metabolism,
vascular supply, and cell regeneration, and those activating
molecular signaling to counter maladaptation.
Article see p 830
An example of disease-countering (Jedi) signaling is that
related to cyclic GMP-dependent protein kinase (PKG).
cGMP and its cousin cAMP are second-messenger molecules
involved in a broad array of cell signaling. In the heart, the
role of cAMP is firmly established, with localized signaling
targeting protein kinase A to modulate excitation– contraction
coupling and metabolism, and exchange proteins activated by
cAMP (Epacs) to mediate chronic stress. Although sustained
cAMP activation contributes to maladaptive stress, cGMP
activation of PKG blunts these responses. cGMP is synthesized in heart muscle by 2 pathways—nitric oxide-stimulated
soluble guanylate cyclase (sGC) and a natriuretic peptide
receptor-coupled cyclase. Once generated, cGMP binds to
regulatory domains in PKG to activate the kinase and to
affect signaling. There is no known Epac equivalent for
cGMP. cGMP is catabolized by members of the phosphodiesterase (PDE) superfamily, and by its binding to regulatory
(PDE2 and PDE5) or catalytic (PDE3) domains in several of
these proteins; cGMP also regulates its own hydrolysis and
that of cAMP.1
Unlike in the vasculature, where resting cGMP and corresponding PKG activity contribute to vascular tone and endothelial function, basal activity of this signaling cascade in
cardiac myocytes is very low. PKG knock-down or loss-ofThe opinions expressed in this article are not necessarily those of the
editors or of the American Heart Association.
From The Johns Hopkins University School of Medicine, Baltimore,
MD.
Correspondence to David A. Kass, MD, Division of Cardiology, Johns
Hopkins Medical Institutions, Ross Building 858, 720 Rutland Avenue,
Baltimore, MD 21205. E-mail [email protected]
(Circulation. 2012;126:797-799.)
© 2012 American Heart Association, Inc.
Circulation is available at http://circ.ahajournals.org
DOI: 10.1161/CIRCULATIONAHA.112.124909
function studies have found minimal impact on either cardiac
morphology or function under resting conditions,2,3 and some
even questioned its role in the heart.4 However, similar to an
automotive brake, the influence of cGMP and PKG in hearts
under stress is greater. During the past decade, studies have
shown that cGMP/PKG activation counters a broad array of
acute and chronic cardiac stress responses, including those
from ␤-adrenergic stimulation,5 ischemic injury,6 pressure
and volume overload,7,8 and doxorubicin cardiotoxicity.9
Methods to activate cGMP/PKG signaling have already been
developed as clinical therapies, including those that enhance
cGMP synthesis, such as organic nitrates (eg, isosorbide dinitrate), activate guanylate cyclase (eg, cinaciguat or natriuretic
peptides), and curtail cGMP hydrolysis (eg, sildenafil, tadalafil).
All are either used or being studied in patients with heart failure.
PDE5 inhibitors were first thought to have negligible effects on
the heart; however, research during the past decade has revealed
substantial benefits in a variety of experimental and human
cardiac diseases. Results of single-center trials testing sildenafil
in patients with dilated heart failure indicated improvements in
symptoms and exercise capacity, microvascular function, pulmonary hypertension, and cardiac morphology and function.10,11
The PhosphodiesteRasE-5 Inhibition to Improve Quality of Life
And EXercise Capacity in Diastolic Heart Failure (RELAX) trial
of 225 patients, a National Institutes of Health–sponsored
multicenter study testing the utility of sildenafil in patients with
heart failure and a preserved ejection fraction (HFpEF), completed enrollment in early 2012, and results are anticipated later
this year. Another multicenter National Institutes of Health trial
of 2000 patients, PDE5 Inhibition with Tadalafil Changes
Outcomes in Heart Failure (PITCH-HF), will start shortly and
will test the efficacy of tadalafil in patients with heart failure and
a reduced ejection fraction (HFrEF) who also have resting or
exercise-induced pulmonary hypertension. On the stimulation
side, the combination of isosorbide dinitrate and hydralazine is
approved for treating HFrEF in blacks, direct sGC activators are
currently being tested in HFrEF, and B-type and more recently,
CD-type natriuretic peptides are being examined as chronic
therapies.
The efficacy of treatments engaging the cGMP/PKG pathway depends on specifics of the signaling, and this itself can
be modified by heart disease. For example, a nitric oxide
(NO) mimetic leverages sGC to generate cGMP. However, in
vascular and cardiac disease, sGC is often subject to oxidative
stress that reduces its responsiveness to NO.12,13 Direct sGC
agonists that are heme (oxidative state) independent may
circumvent this limitation.13 Generating more cGMP becomes less effective if cGMP-PDEs are upregulated, and in
this setting, inhibiting the appropriate PDEs becomes very
useful. Neither elevated cGMP synthesis nor hydrolysis is a
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798
Circulation
August 14, 2012
feature of the normal heart, but it is in human HFrEF14,15 and
experimental pressure overload, supporting current efforts
with PDE5 inhibition. Last, PKG itself needs to have something useful to do; that is, a targetable pathway must exist in
which modification by the kinase can offset cardiac maladaptation. Some diseases activate these pathways more than
others. To date, PKG modulation of the calcineurin/nuclear
factor of activated T-cells signaling cascade, transient receptor potential channel 6, mitochondrial ATP-sensitive potassium channel, Ras homolog gene family, mamber A, regulator of G-protein signaling 2 and 4, and other factors have all
been identified as key contributors to its amelioration of
cardiac disease.16 Thus, whether intrinsic cGMP/PKG signaling works as a competent Jedi knight that can be enhanced by
therapeutic interventions depends on a balancing act among
factors controlling the signaling cascade.
In the current issue of Circulation, van Heerebeek et al17
present provocative new data regarding this balance, and in
particular show how things go awry in HFpEF. The data set
is unique—left ventricular endocardial biopsies from nearly
150 patients with nonischemic HFrEF, HFpEF, or aortic
stenosis (the latter obtained at surgery from the left ventricular outflow tract). The measurements include passive tension
in isolated cardiac myocytes and assays to assess various
levels, activity, and posttranslational modifications of proteins involved with the NO/natriuretic peptide– cGMP/PKG
cascade. As these investigators reported previously,18 maximal passive myocyte stiffness is higher in HFpEF than
HFrEF (in the new study, they find it higher than in aortic
stenosis [AS] patients as well). Their earlier study showed
this disparity could be eliminated by incubation with PKA,
and in the current research they find a similar result using
PKG, which suggests a deficit of PKG activity in HFpEF that
is supported more directly by several kinase assays. With regard
to why PKG activity might be less in HFpEF, the investigators
find cGMP is also much reduced. This correlates with lower
pro-brain natriuretic peptide expression; however, levels are
similarly depressed in AS patients whose PKG activity is much
higher. PDE5 protein expression appears similar among groups.
Rather, van Heerebeek et al18 highlight greater oxidative/nitrosative stress in HFpEF, suggesting reduced NO-stimulated
cGMP synthesis as the culprit.
The authors also perform subgroup analysis, examining
myocyte passive stiffness, PKG activity, and cGMP levels in
each group with or without diabetes mellitus (DM). Patients
with HFpEF have the highest stiffness regardless of DM
status. However, unlike DM– patients, both HFpEF and AS
with DM⫹ similarly reduce PKG activity, raising questions
about its role. Furthermore, despite low PKG activity in
patients with AS and DM, these subjects have greater cGMP
levels unlike patients with HFpEF and DM. These investigators have shown previously that DM exacerbates myocyte
passive stiffness in patients with AS,19 and this seems true in
the current analysis, although it is not mentioned. PKG
activity also appears to be about half in DM-AS versus AS
alone, although significance is not noted.
The current study by van Heerebeek et al17 poses an
intriguing new explanation for HFpEF pathophysiology. Reduced PKG activity in stressed myocardium would be antic-
ipated to exacerbate the pathophysiology of HFpEF, not only
through effects on passive properties but also on remodeling
and on systole. In mice harboring myocyte-targeted and
controllable PDE5 expression, upregulation (lowered PKG
activity) worsened hypertrophy, function, and fibrosis in a
pressure overload model; reducing PDE5 expression (increasing PKG activity) did the opposite.2 Although a prior genetic
loss-of-function model concluded PKG was unimportant to
chronic cardiac stress remodeling,4 more recent studies,2
including one from some of the same investigators3 using a
more direct genetic approach to suppress PKG in myocytes,
support an important protective role.
The finding of similar improvement in myocyte stiffness in
HFpEF (and HFrEF) to PKA (prior data18) and now to PKG
suggests the residue targets may be similar. Titin, for example, serves as a molecular spring, and both PKG and PKA
modify the same residues in the variable N2b region that
reduce its stiffness constant.20 The previous work18 raised
questions about the use of ␤-blockade in HFpEF, given its
suppression of PKA. Rather than stimulate PKA further,
however, the current study suggests a similar benefit can be
obtained by cGMP/PKG stimulation. van Heerebeek et al17
speculate that oxidation of titin may also play a role in
HFpEF, although the rescue of stiffening by PKG alone in
vitro argues for phosphorylation as a dominant mechanism.
The data also raise intriguing questions regarding how one
might increase PKG activity to treat HFpEF. Although PDE5
immunohistochemical staining is similar among the groups in
the current study, others have reported substantial upregulation of PDE5 expression in HFrEF compared with low levels
in normal control subjects.14,15 Thus, the current data could be
consistent with upregulation in all groups. PDE5 activity was
not assessed, and expression does not always reflect enzyme
activity as a result of posttranslational changes and alterations
in protein localization and function (eg, hydrolysis of cGMP
generated by sGC versus natriuretic peptide receptor-coupled
cyclase).21 Last, the left ventricular biopsy analysis reflects
limited sampling, whereas prior studies supporting PDE5
upregulation have examined tissue from explanted hearts.14,15
Relatively low pro-brain natriuretic peptide is observed in
HFpEF myocardium, and although this may or may not have
contributed to the decline in cGMP, natriuretic peptide
stimulation still seems a reasonable therapeutic option. The
mechanism favored by van Heerebeek et al17 is that NOreactive oxygen species (ROS) interactions rose in HFpEF, as
reflected by enhanced tissue nitrotyrosine, presumably reflecting diminished nitric oxide synthase-NO-cGMP synthesis. This is interesting, although still speculative. Specifically,
confirmation that nitric oxide synthase-derived NO and
cGMP were indeed suppressed more prominently (and ROS
enhanced reciprocally) in HFpEF was not demonstrated. The
nitrotyrosine assay has limitations, one being a lack of
specificity for peroxinitrite formation. Although the authors
suggest that a greater prevalence of DM may have contributed to redox imbalance in HFpEF, their subset analysis
found a similar depression of cGMP and PKG activity in
HFpEF with or without DM. It would have been interesting to
see nitrotyrosine data for these 2 subgroups. Other studies
have found ROS activation and ROS/NO imbalance in
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Kass
HFrEF models; indeed, this has been suggested as pertinent to
patients who respond to isosorbide dinitrate and hydralazine.22 If HFpEF reflects greater NO-ROS imbalance, then
perhaps direct heme-independent sGC activators such as
cinaciguat would be a better choice, given likely redox
changes in sGC that blunt its NO response.
As more and more therapeutic avenues become available to
modulate cGMP/PKG signaling, interest in this pathway and
ways to leverage it for treating heart disease will continue to
increase. The recent promising results from PDE5 inhibitor
trials in HFrEF, and evidence of potency in HFpEF as well,
are moving this field forward, with pivotal clinical trials now
underway. To date, natriuretic peptide therapy has been used
subacutely, given the need for intravenous administration.
Whether brain natriuretic peptide or newer, more stable and
potent natriuretic peptides are beneficial when used chronically remains to be tested. Last, novel methods to improve
nitric oxide synthase activity or sGC generation of cGMP are
moving forward. Understanding how to use these approaches
best requires appreciation of the balance between cGMP
synthesis, hydrolysis, and PKG targeting. The Paulus Laboratory and colleagues are to be congratulated for providing us
with valuable new insights in this regard.
Acknowledgments
Dr Kass thanks Eiki Takimoto for his helpful review.
This work was supported by National Institutes of Health grants
HL077180, HL-089297, and HL-093432; Muscular Dystrophy Association Award 186454; and Fondation Leducq.
Disclosures
None.
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KEY WORDS: Editorials 䡲 cyclic nucleotides 䡲 heart failure 䡲 left
ventricular diastolic dysfunction 䡲 oxidative stress 䡲 protein kinase G
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