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Cardiovascular Research 2004 63(3):450-457; doi:10.1016/j.cardiores.2004.04.002
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Copyright © 2004, European Society of Cardiology

Interference of antihypertrophic molecules and signaling pathways with the Ca2+–calcineurin–NFAT cascade in cardiac myocytes

Beate Fiedler and Kai C Wollert*

Department of Cardiology and Angiology, Hannover Medical School, Carl-Neuberg Str. 1, 30625, Hannover, Germany

* Corresponding author. Tel.: +49-511-532-4055; fax: +49-511-532-5412. Email address: wollert.kai{at}mh-hannover.de

Received 5 January 2004; revised 12 March 2004; accepted 3 April 2004


   Abstract
Top
 Abstract
1. Introduction
 2. Inhibition upstream from...
 3.2. Cabin-1/Cain, AKAP79, CHP,...
 References
 
Cardiac hypertrophy occurs in a number of disease states associated with chronic increases in cardiac work load. Although cardiac hypertrophy may initially represent an adaptive response of the myocardium, ultimately, it often progresses to ventricular dilatation and heart failure. Much investigation has focused on the signaling pathways controlling cardiac hypertrophy at the level of the single cardiac myocyte. One prohypertrophic pathway that has received much attention involves the ubiquitously expressed Ca2+/calmodulin-activated phosphatase calcineurin. Upon activation by Ca2+, calcineurin dephosphorylates nuclear factor of activated T cell (NFAT) transcription factors, leading to their nuclear translocation. As common in complex biological systems, cardiac hypertrophy is controlled simultaneously by stimulatory (prohypertrophic) and counter-regulatory (antihypertrophic) pathways. Given the potent prohypertrophic effects of the Ca2+–calcineurin–NFAT pathway in cardiac myocytes, it is not surprising that the activity of this pathway is tightly controlled at multiple levels. Inhibitory mechanisms upstream (nitric oxide (NO), cGMP, cGMP-dependent protein kinase type I (PKG I), heme oxygenase-1 (HO-1), biliverdin, carbon monoxide (CO)) and downstream from calcineurin (glycogen synthase kinase-3 (GSK3), c-Jun N-terminal kinases (JNKs), p38 mitogen-activated protein kinase (MAPKs)) have been described. Moreover, several inhibitors directly target calcineurin enzymatic activity (cyclosporine A (CsA), tacrolimus (FK506), calcineurin-binding protein-1 (Cabin-1)/calcineurin-inhibitory protein (Cain), A-kinase-anchoring protein-79 (AKAP79), calcineurin B homology protein (CHP), MCIPs, VIVIT). Considering the dominant role of the calcineurin pathway in cardiac hypertrophy and failure, calcineurin-inhibitory strategies may lead to the identification of novel therapeutic approaches for patients with cardiac disease.

KEYWORDS Hypertrophy; Calcium (cellular); Protein phosphatases; Signal transduction


   1. Introduction
Top
 Abstract
 1. Introduction
2. Inhibition upstream from...
 3.2. Cabin-1/Cain, AKAP79, CHP,...
 References
 
Cardiac hypertrophy is a generic response of the myocardium that occurs in diverse cardiovascular disease states such as hypertension, myocardial infarction, valvular heart disease, and various cardiomyopathies of genetic, viral, or metabolic origin. All these conditions impose chronic increases of external or internal work load to the myocardium. From a teleological standpoint, cardiac hypertrophy has been regarded as an adaptive response to maintain cardiac output and tissue perfusion under these circumstances. The adaptive nature of cardiac hypertrophy has been challenged, however. First of all, it has been recognized that hypertrophy can ultimately progress to ventricular dilatation, contractile dysfunction, and heart failure [1,2]. Secondly, some studies have indicated that hypertrophy may not even be required for a successful adaptation to increased work load [3,4].

The sine qua non of cardiac hypertrophy is cardiac myocyte hypertrophy, defined simply by an increase in cardiac myocyte size [5]. Accordingly, much investigation has focused on the molecular mechanisms of cardiac myocyte hypertrophy. It has emerged that cardiac myocyte hypertrophy is controlled by growth factor receptors and mechanical stress sensors, collectively leading to the activation of an intracellular network of protein kinases, phospholipid kinases, and protein phosphatases which transmit the hypertrophic signal to the cell nucleus [6,7]. Beyond increases in cell size and sarcomere assembly, activation of these signaling pathways induces a multitude of qualitative and quantitative changes in the expression levels of contractile proteins, ion pumps and channels, secreted proteins, signaling proteins, enzymes involved in cardiac energetics, components of the extracellular matrix, and regulators of cell survival.

One prohypertrophic signaling pathway that has received much attention involves the ubiquitously expressed Ca2+/calmodulin-activated serine/threonine-phosphatase calcineurin (PP2B). Calcineurin becomes activated by sustained elevations of intracellular Ca2+ concentration [8]. Once activated, calcineurin dephosphorylates its primary downstream effectors, cytoplasmic latent transcription factors belonging to the nuclear factor of activated T cells (NFAT) family, leading to their nuclear translocation. In addition to NFAT, calcineurin regulates the activity of other downstream targets, including the transcription factors MEF2 and NF-{kappa}B, and the proapoptotic regulator bad [9–13]. Importantly, the Ca2+–calcineurin–NFAT pathway interacts with additional pathways, e.g. protein kinase C and mitogen-activated protein kinases (MAPK), to coordinate the hypertrophic response. The role of calcineurin and its interacting partners in cardiac hypertrophy has been the topic of several excellent reviews [7,14], and will not be further discussed here.

As common in complex biological systems, cardiac hypertrophy is controlled simultaneously by stimulatory (prohypertrophic) and counter-regulatory (antihypertrophic) pathways. In theory, antihypertrophic effects could be mediated via autonomous inhibitory pathways (e.g. through activation of transcriptional repressors) and/or via suppression of prohypertrophic pathways. For the majority of antihypertrophic pathways, the latter appears to be the case (e.g. peroxisome proliferator-activated receptor-{gamma} (PPAR{gamma}) inhibits activation of the prohypertrophic AP1, STAT3, and NF-{kappa}B transcription factors and MAPK phosphatase terminates MAPK activation) [15–17]. Considering the potent prohypertrophic effects of the Ca2+–calcineurin–NFAT signaling pathway in cardiac myocytes, it is not surprising that the activity also of this pathway is tightly controlled at multiple levels (Fig. 1). This review provides a comprehensive overview of calcineurin-inhibitory molecules and signaling pathways, some expressed endogenously in cardiac myocytes, some employed as pharmacological and/or research tools.


Figure 1
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  		Fig. 1 Inhibitory control of the prohypertrophic Ca2+–calcineurin–NFAT signaling pathway at multiple levels. Ca2+ influx through the L-type Ca2+ channel (LTCC) triggers Ca2+ release from the sarcoplasmic reticulum via the ryanodine receptor (RyR). The actual source of Ca2+ promoting calcineurin activation in cardiac myocytes is not clear; the LTCC appears to be critically involved, however. Once activated, calcineurin dephosphorylates NFAT, leading to its translocation to the nucleus. In addition, calcineurin activates MEF2 and other, as yet poorly defined, downstream targets to promote cardiac myocyte hypertrophy. The Ca2+–calcineurin–NFAT cascade is negatively controlled at multiple levels. Inhibitory mechanisms upstream and downstream from calcineurin have been described. Moreover, several inhibitors directly target calcineurin enzymatic activity. NO denotes nitric oxide; PKG I, cGMP-dependent protein kinase type I; HO-1, heme oxygenase-1; CO, carbon monoxide; CsA, cyclosporine A; Cabin-1, calcineurin-binding protein-1 (also known as calcineurin-inhibitory protein, Cain), AKAP79, A-kinase-anchoring protein-79; CHP, calcineurin B homology protein; MCIP, modulatory calcineurin-interacting proteins; GSK3, glycogen synthase kinase-3; JNK, c-Jun N-terminal kinases; p38, p38 mitogen-activated protein kinases. See text for details.

 

   2. Inhibition upstream from calcineurin
Top
 Abstract
 1. Introduction
 2. Inhibition upstream from...
3.2. Cabin-1/Cain, AKAP79, CHP,...
 References
 
2.1. Nitric oxide, cGMP, cGMP-dependent protein kinase
In cell culture models, the free radical gas nitric oxide (NO) has been shown to promote antihypertrophic effects via activation of soluble guanylyl cyclase and cGMP formation [18,19]. Cyclic GMP-dependent protein kinase type I (PKG I) has been identified as the prime downstream target mediating the antihypertrophic effects of NO and cGMP [19,20]. Studies in gene-targeted mice lacking the neuronal and/or the endothelial isoforms of NO synthase have established that NO acts as an inhibitor of cardiac (myocyte) hypertrophy in the in vivo situation as well (reviewed in Ref. [21]). The role of cGMP as an antihypertrophic second messenger is supported by studies in gene-targeted mice lacking the natriuretic peptide receptor guanylyl cyclase-A (GC-A) specifically in cardiac myocytes [22]. Similar to NO, which stimulates cGMP formation in cardiac myocytes via soluble guanylyl cyclase, natriuretic peptides promote cGMP formation by binding to and activating GC-A [23]. Guanylyl cyclase-A-deficient mice spontaneously develop cardiac hypertrophy that is further enhanced in response to pressure overload [22]. Recent studies have explored the downstream mechanisms, whereby NO and cGMP promote antihypertrophic effects in cardiac myocytes. Interference with a muscle LIM protein-dependent prohypertrophic pathway constitutes one important mechanism [24]. Moreover, inhibition of the Ca2+–calcineurin–NFAT pathway at multiple levels appears to be of critical importance.

Nitric oxide and cGMP (acting via PKG I) attenuate NFAT nuclear translocation and transcriptional activity in response to growth factor (phenylephrine) stimulation in cardiac myocytes. The site of inhibition of the phenylephrine-Ca2+–calcineurin–NFAT signaling pathway by NO and cGMP appears to be upstream from calcineurin, since NFAT activation induced by a Ca2+-independent, constitutively-active calcineurin mutant is not affected by NO and cGMP [20]. The source of intracellular Ca2+ for stimulating calcineurin is somewhat cell-type specific [25]. Although the actual source of Ca2+ that activates calcineurin in cardiac myocytes is incompletely understood [14], it has been proposed that calcineurin activation in cardiac myocytes depends on Ca2+ entry via the L-type Ca2+ channel [26,27]. Along this line, we have recently shown that the growth-inhibitory effects of the NO–cGMP–PKG I pathway upstream from calcineurin are mediated by inhibition of the L-type Ca2+ channel current [20]. Notably, cGMP/PKG I-dependent inhibition of the L-type Ca2+ channel current has been implicated also in the negative inotropic effects of NO in cardiac myocytes [28,29]. It should be noted that the role of the L-type Ca2+ channel in controlling cardiac (myocyte) hypertrophy is still a point of debate in the literature. Some studies suggest a prohypertrophic role of the L-type Ca2+ channel. For example, cardiomyocyte-selective overexpression of the L-type Ca2+ channel has been shown to promote cardiac hypertrophy and cardiomyopathy in transgenic mice [30], and the L-type Ca2+ channel inhibitor diltiazem has been reported to blunt cardiac hypertrophy in a mouse model of familial hypertrophic cardiomyopathy [31]. On the other hand, L-type Ca2+ channel gating properties are unchanged in cardiac myocytes isolated from pressure-overloaded hypertrophied rat hearts which may argue against an involvement of the L-type Ca2+ channel in the hypertrophic response [32].

The antihypertrophic effects of NO and cGMP in cultured cardiac myocytes can be augmented by adenoviral overexpression of PKG I [19]. Such overexpression of PKG I enhances the inhibitory effects of NO/cGMP upstream from calcineurin and exposes an additional site of inhibition downstream from calcineurin which is independent from NFAT and may involve inhibition of MEF2 activation by calcineurin [20]. The physiological relevance of this mode of inhibition may be limited, however, as it is observed only in PKG I-overexpressing cardiac myocytes.

Growth factor (phenylephrine)-induced cardiac myocyte hypertrophy is associated with increased expression levels of the catalytic A subunit of the calcineurin heterodimer [27]. Increased calcineurin A subunit expression levels may contribute to the increase in calcineurin phosphatase activity in the hypertrophied and failing heart in vivo [33,34]. It is worth mentioning, in this regard, that NO and cGMP inhibit phenylephrine-induced calcineurin A subunit expression in cultured cardiac myocytes, which may constitute another mechanism whereby NO/cGMP interfere with the Ca2+–calcineurin–NFAT signaling pathway (Fiedler B and Wollert KC, unpublished observations).

2.2. Heme oxygenase-1, biliverdin, carbon monoxide
Heme oxygenase-1 (HO-1) is the rate-limiting enzyme catalyzing the degradation of heme, releasing biliverdin, carbon monoxide (CO), and free ferrous iron [35]. Heme oxygenase-1, acting via its reaction products biliverdin and CO, has recently been shown to suppress growth factor (endothelin 1)-induced cardiac myocyte hypertrophy [36]. The growth-inhibitory effects of biliverdin and CO are mediated, in part, via MAPK inhibition. In addition, biliverdin and CO inhibit the endothelin 1-Ca2+–calcineurin–NFAT signaling pathway upstream from calcineurin. Interestingly, this inhibition does not involve PKG I activation (in contrast to upstream inhibition mediated by NO and cGMP), although the precise mechanism(s) remain to elucidated [36].

3. Direct inhibition of calcineurin
3.1. Cyclosporine A and tacrolimus
Cyclosporine A (CsA) and tacrolimus (FK506) are important immunosuppressive agents that are used widely in organ transplantation and various immune disorders. CsA is a 11 amino acid cyclic polypeptide extracted from Trichoderma polysporum. FK506 is a macrolide antibiotic obtained from Streptomyces tsukubaensis. Cyclosporine A and FK506 form high-affinity complexes with their ubiquitous cytosolic binding partners (immunophilins), cyclophilin and FK506-binding protein-12 (FKBP12), respectively [8]. The CsA-cyclophilin and FK506–FKBP12 complexes associate with the catalytic subunit of calcineurin to inhibit its phosphatase activity and interaction with various substrates, including NFAT transcription factors. In that way, CsA and FK506 block the dephosphorylation and nuclear translocation of NFATs (which prevents transcription of the immune-modulatory cytokine, interleukin-2, in T cells) [37–39]. Cyclosporine A and FK506 have been used extensively to explore the role of Ca2+–calcineurin–NFAT signaling in various cell culture and animal models of cardiac hypertrophy (reviewed in Ref. [14]). Most of these studies have provided evidence in support of a causal role for calcineurin in the development of cardiac hypertrophy, whereas some reports found no correlation between calcineurin and the hypertrophic response. Differences in hypertrophy models and surgical procedures, genetic background, drug dosage and side effects, timing, route, and duration of drug administration, and inhibition by CyA and FK506 of additional targets likely to be involved in cardiac hypertrophy (e.g. TGFβ, MAPKs, ryanodine receptor, L-type Ca2+ channel) have been invoked to reconcile the disparate results provided by some of these reports [14]. Overall, CsA and FK506 are poor research tools in the field of hypertrophy signaling, and genetic approaches, some of which are discussed below, are more suitable strategies to decipher the role of the Ca2+–calcineurin–NFAT signaling pathway in cardiac hypertrophy.


   3.2. Cabin-1/Cain, AKAP79, CHP, and MCIP family
Top
 Abstract
 1. Introduction
 2. Inhibition upstream from...
 3.2. Cabin-1/Cain, AKAP79, CHP,...
References
 
In recent years, several endogenous calcineurin inhibitors have been identified. Calcineurin-binding protein-1 (Cabin-1) [40], also known as calcineurin-inhibitory protein (Cain) [41], is a large scaffolding protein and non-competitive inhibitor of calcineurin phosphatase activity. Likewise, A-kinase-anchoring protein-79 (AKAP79) is a scaffolding protein which binds to and inhibits calcineurin enzymatic activity; AKAP79 also interacts with cAMP-dependent protein kinase (A-kinase) and protein kinase C [42]. Finally, calcineurin B homology protein (CHP), which shares a high degree of homology to calmodulin and the Ca2+-binding regulatory B subunit of calcineurin, has been identified as a potent inhibitor of calcineurin enzymatic activity [43]. Most likely, Cabin-1/Cain and AKAP79 are not involved in the regulation of calcineurin activity in cardiac myocytes, since these proteins are not expressed at significant levels in the heart [41,44]. However, the calcineurin-inhibitory properties of Cabin-1/Cain and AKAP79 have been exploited to assess the involvement of calcineurin in cardiac hypertrophy, thus avoiding the limitations of CsA and FK506. Forced expression of the calcineurin-binding domains of Cabin-1/Cain or AKAP79 in cardiac myocytes has been shown to prevent growth factor-induced cardiac myocyte hypertrophy in vitro and pressure-overload hypertrophy in vivo, providing strong evidence that calcineurin–NFAT activation is indeed required for cardiac (myocyte) hypertrophy [27,45].

Modulatory calcineurin-interacting proteins have recently been described as a family of proteins that inhibit calcineurin through a direct physical interaction [46]. MCIP1 (which is encoded by the Down's syndrome critical region-1 gene locus, DSCR1) and MCIP2 (encoded by ZAKI-4/DSCR1L1) are preferentially expressed in striated muscles and brain, whereas MCIP3 (encoded by DSCR1L2) is expressed more broadly in a number of tissues [46–48]. Members of this family are capable of binding to and inhibiting the catalytic A subunit of calcineurin. MCIP1 and MCIP2 expression levels in cardiac myocytes are controlled by distinct signaling pathways. An intragenic segment located between exons 3 and 4 of the human MCIP1 gene functions as an alternative promoter that responds to calcineurin activation. This region includes a cluster of 15 NFAT binding sites. Because MCIP1 inhibits calcineurin activity, these results suggest that MCIP1 may participate in a negative feedback circuit to diminish potentially deleterious effects of unrestrained calcineurin activity in cardiac myocytes [49]. Expression of MCIP2, by contrast, is not altered by activated calcineurin but responds to thyroid hormone, which has no effect on MCIP1 [49].

Transgenic mice with cardiac-selective overexpression of the calcineurin-inhibitory domain of MCIP1 display a mild reduction in heart size as compared to wild-type mice, implicating calcineurin in developmental cardiac growth. MCIP1 overexpression also reduces the hypertrophic response and progression to heart failure that is observed in calcineurin-overexpressing mice [50,51] or after myocardial infarction [52]. Finally, the MCIP1-overexpressing mouse model suggests an involvement of calcineurin in exercise-induced cardiac hypertrophy [50] (although another study has indicated that the calcineurin–NFAT pathway may not be a primary mediator of physiological hypertrophy [53]). The recent generation of MCIP1-deficient mice has provided further insight into the role of the calcineurin-interacting protein MCIP1 [54]. In the absence of stress, MCIP1-deficient animals exhibit no overt phenotype. However, lack of MCIP1 exacerbates the hypertrophic response observed in calcineurin-overexpressing mice, consistent with a role of MCIP1 as a negative regulator of calcineurin signaling. Paradoxically, cardiac hypertrophy in response to pressure overload or chronic adrenergic stimulation is blunted in MCIP1-deficient mice, suggesting that MCIP1 can suppress or facilitate cardiac calcineurin signaling depending on the nature of the hypertrophic stimulus [54].

3.3. VIVIT
The synthetic peptide, MAGPHPVIVITGPHEE (VIVIT) was selected from a combinatorial peptide library based on the calcineurin docking motif of NFAT [55]. VIVIT binds to calcineurin with high-affinity and potently inhibits NFAT activation and NFAT-dependent gene expression in T cells. Remarkably, expression of genes that are regulated by calcineurin independent from NFAT is not affected, indicating that VIVIT (in contrast to CsA and FK506) selectively targets NFAT activation by calcineurin. Overexpression of VIVIT in cardiac myocytes blunts the hypertrophic response to growth factor (phenylephrine) stimulation and results in increased cardiac myocyte apoptosis [56]. By contrast, non-selective inhibition of calcineurin by CsA inhibits hypertrophy, but does not promote cardiac myocyte apoptosis after phenylephrine stimulation, demonstrating that calcineurin activates both pro- and antiapoptotic pathways in cardiac myocytes during phenylephrine stimulation, and that NFAT is a critical component of the antiapoptotic pathway that regulates whether the outcome of calcineurin activation is cardiac myocyte apoptosis or survival [56].

4. Inhibition downstream from calcineurin
4.1. Glycogen synthase kinase-3
Glycogen synthase kinase-3 (GSK3) is a ubiquitously expressed serine/threonine protein kinase. Although initially described as an enzyme involved in glycogen metabolism, GSK3 is now known to regulate a diverse array of cell functions [57]. Unlike most protein kinases, GSK3 is active in unstimulated cells and becomes inactivated in response to a variety of stimuli. Akt (also known as protein kinase B), phosphoinositide 3-kinase-dependent protein kinase, and other kinases phosphorylate GSK3 leading to its inactivation [58–60]. NFAT transcription factors have been identified as prime targets for GSK3 kinase activity. In the nucleus, GSK3 phosphorylates conserved serines in the N-terminal regulatory region of NFAT proteins, thereby promoting their nuclear export [61]. The ability of GSK3 to oppose calcineurin signaling by reversing nuclear accumulation of NFATs suggests that GSK3 may act as a negative regulator of cardiac myocyte hypertrophy [62]. Indeed, overexpression of a constitutively active form of GSK3β reduces nuclear accumulation of NFAT transcription factors in cardiac myocytes [63]. Similarly, transgenic overexpression of constitutively active GSK3β in the heart inhibits hypertrophy induced by calcineurin overexpression, chronic β-adrenergic stimulation, and pressure overload in vivo [64]. Interestingly, overexpression of constitutively active GSK3β stimulates the expression levels of the natriuretic peptides, ANP and BNP, which have been reported to exert antihypertrophic effects (see above), and may therefore contribute to the antihypertrophic effects of GSK3β [64]. In agreement with antihypertrophic properties of GSK3, growth factor (endothelin 1) stimulation of cardiac myocytes results in a suppression of GSK3 activity which, in concert with calcineurin activation, contributes to enhanced nuclear localization of NFAT transcription factors; similarly, cardiac GSK3 activity is suppressed in chronic pressure overload hypertrophy [63]. A recent study in yeast has added another level of complexity to the regulation of calcineurin activity by GSK3 [65]. In this study, protein kinases from the GSK3 family have been shown to phosphorylate the yeast homologe of MCIP1 and to convert it from an inhibitor to a stimulator of calcineurin activity; calcineurin, in turn, dephosphorylates the GSK3 consensus site of the yeast homologe of MCIP1 [65]. Interestingly, phosphorylation by GSK3 and dephosphorylation by calcineurin has also been demonstrated for human MCIP1 [66]. Although stimulation of calcineurin signaling by GSK3 has not yet been demonstrated in mammalian cells, these data raise the intriguing possibility that cycles of MCIP1 phosphorylation (by GSK3) and dephosphorylation (by calcineurin) may modulate calcineurin signaling [65].

In addition to GSK3, several other protein kinases (e.g. cAMP-dependent protein kinase) have been implicated in NFAT phosphorylation, although the relevance of these observations for cardiac hypertrophy signaling remains unresolved [61]. Similarly, casein kinase-1 and MEKK1 inhibit calcineurin–NFAT signaling by reducing nuclear import of NFAT [67]. Casein kinase-1 directly binds to and phosphorylates NFAT, resulting in the inhibition of its nuclear translocation, whereas MEKK1 indirectly suppresses NFAT nuclear import by stabilizing the interaction between NFAT and casein kinase-1 [67].

4.2. JNKs and p38 mitogen-activated protein kinases
Recent studies have identified an intimate cross-talk of the Ca2+–calcineurin–NFAT signaling pathway and all three branches of the MAP kinase signaling pathway: c-Jun N-terminal kinases (JNKs) and p38 MAPKs inhibit calcineurin–NFAT signaling, whereas Erk MAPKs potentiate calcineurin-NFAT activation via as yet unknown mechanisms (reviewed in this issue of Cardiovascular Research, Ref. [68]). Transgenic animal studies indicate that JNKs and p38 MAPKs serve as negative regulators of the hypertrophic response to pressure overload in vivo and suggest that these antihypertrophic effects are mediated, at least in part, by an inhibition of calcineurin–NFAT signaling [69,70]. In this regard, JNKs and p38 MAPKs have been shown to directly phosphorylate NFAT transcription factors, thereby promoting their nuclear export, a paradigm that is also operating in cardiac myocytes [69–74].

5. Conclusions and future perspectives
Numerous studies have established that the Ca2+–calcineurin–NFAT signaling pathway plays a critical role in the development of cardiac hypertrophy. Remarkably little is known regarding the role of this pathway during the transition from hypertrophy to failure. Recent studies have shown that not cardiac hypertrophy per se, but activation of certain signaling pathways is detrimental in pathological conditions of chronic overload (reviewed in Ref. [75]). Certain hypertrophy signaling pathways (e.g. melusin-GSK3) may in fact be required for a successful adaptation to overload without progression to failure [76]. Other hypertrophy signaling pathways (e.g. G{alpha}q) are clearly maladaptive, as they accelerate functional decline and transition to heart failure [4]. A very recent study has shown that the calcineurin–NFAT pathway is activated in the pressure-overloaded, hypertrophied heart, i.e. in "pathological" hypertrophy, but not in "physiological" cardiac hypertrophy after exercise training [77]. Interestingly, exercise-induced cardiac hypertrophy appears to be regulated by a distinct pathway, involving phosphoinositide 3-kinase and Akt [78]. Overexpression of calcineurin in transgenic mice promotes profound cardiac hypertrophy that rapidly progresses to heart failure, indicating that excessive activation of the calcineurin–NFAT pathway is detrimental [51]. Whether activation of the endogenous calcineurin–NFAT pathway plays an adaptive or maladaptive role in the pressure overloaded heart is less well established, however. In other words, it is not clear whether interference with the calcineurin pathway is a desirable treatment strategy for patients with "pathological" cardiac hypertrophy or established heart failure (especially since antiapoptotic effects of the calcineurin–NFAT pathway have been described [56,79]). Investigation of these important issues requires hemodynamic characterization and long-term follow-up of animal models with overload hypertrophy following specific (genetic) inhibition of the calcineurin pathway as discussed in this review. Finally, the role of other calcineurin downstream targets besides NFAT needs to be better defined in the context of cardiac hypertrophy and failure in order to delineate future therapeutic strategies (non-selective inhibition/activation of calcineurin vs. selective inhibition/activation of specific downstream targets). Considering the dominant role of the calcineurin–NFAT pathway in cardiac hypertrophy and (possibly) cardiac failure, these studies may lead to the identification and clinical development of novel therapeutic approaches for patients with cardiac disease.


   Acknowledgements
 
This work was supported by the Deutsche Forschungsgemeinschaft (Wo 552/2-2).


   Notes
 
Time for primary review 17 days


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 3.2. Cabin-1/Cain, AKAP79, CHP,...
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