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Insulin signalling in the heart

Luc Bertrand, Sandrine Horman, Christophe Beauloye, Jean-Louis Vanoverschelde
DOI: http://dx.doi.org/10.1093/cvr/cvn093 238-248 First published online: 5 April 2008

Abstract

The main role of insulin in the heart under physiological conditions is obviously the regulation of substrate utilization. Indeed, insulin promotes glucose uptake and its utilization via glycolysis. In addition, insulin participates in the regulation of long-chain fatty acid uptake, protein synthesis, and vascular tonicity. Significant advancements have been made over the last 20 years in the understanding of the signal transduction elements involved in these insulin effects. Among these molecular mechanisms, the phosphatidylinositol 3-kinase/protein kinase B (Akt) pathway is thought to play a crucial role. Under pathological conditions, such as type-2 diabetes, myocardial ischaemia, and cardiac hypertrophy, insulin signal transduction pathways and action are clearly modified. These molecular signalling alterations are often linked to atypical crosstalks with other signal transduction pathways. On the other hand, pharmacological modifications of parallel and interdependent signalling components, such as the AMP-activated protein kinase pathway, are now considered to be a good therapeutic approach to treat insulin-signalling defects such as insulin resistance and type-2 diabetes. In this review, we will focus on the description of the molecular signalling elements involved in insulin action in the heart and vasculature under these different physiological, pathological, and therapeutical conditions.

Keywords
  • Energy metabolism
  • PKB/Akt
  • Ischaemia
  • Protein synthesis
  • Insulin resistance
  • AMPK

1. Introduction

Insulin plays a key role in the regulation of various aspects of cardiovascular metabolism and function including glucose and long-chain fatty acid (LCFA) metabolism, protein translation, and vascular tone. Since its discovery in 1921, the effects of insulin have been extensively studied.1 However, the full identification of the molecular signalling events involved in these insulin actions is still in progress nowadays. For example, it had been demonstrated in 1949 for the first time that insulin induced glucose uptake,2 whereas the insulin-sensitive glucose transporter Glut4 was discovered only in the 1980s; in 2003, the tandem formed by two other proteins, namely the protein kinase B (PKB)/Akt and the Rab GTPase-activating protein (GAP) Akt substrate 160 (AS160), was only suspected to be part of the missing link of this insulin-induced glucose uptake.3 Even if certain insulin-dependent molecular steps still need to be identified, the recent understanding of the signal transduction mechanisms involved in insulin action informs us of the possibility to exploit this information to develop molecular therapeutic approaches to treat cardiac diseases such as insulin resistance, cardiac hypertrophy, or myocardial ischaemia.

2. Heart metabolism under physiological conditions

The heart is an energy-consuming organ that requires a constant supply of fuel and oxygen in order to maintain its intracellular ATP level which is essential for the uninterrupted myocardial contraction/relaxation cycle. The human heart produces and consumes between 3.5 and 5 kg of ATP every day to sustain pumping.4 The way to generate this energy depends on the cardiac environment including coronary flow, blood substrate supply, hormones, and nutritional status.58 Under physiological conditions, this ATP production comes from the mitochondrial oxidation of different substrates, LCFAs (60–70%) being predominant over glucose (20%) and lactate (10%) (Figure 1). This substrate preference is due to the fact that LCFA oxidation inhibits glucose uptake and catabolism via the Randle cycle.9 Ketone body circulating levels, which are low under control conditions, rise upon starvation, in the presence of high blood concentrations of LCFAs, in chronic heart failure or in poorly controlled diabetes.6,7 In these conditions, ketone bodies become a major substrate for the heart by inhibiting the oxidation of other substrates. Ketone bodies inhibit glucose oxidation at the pyruvate dehydrogenase level via acetyl-CoA production, whereas the inhibition of LCFA oxidation is still poorly understood.6,7 On the other hand, when glucose and insulin concentration rises, glucose becomes the favoured oxidized substrate of the heart (Figure 1). The signal transduction mechanism involved in this insulin-induced adaptation in substrate utilization is complex and acts on several cellular targets, including the glucose transporter Glut4, the LCFA transporter 88 kDa FA translocase (FAT)/CD36, and the glycolytic enzyme 6-phosphofructo-1-kinase (PFK-1) (Figures 1 and 2). The proximal part of the insulin signal transduction pathway is common to all these metabolic effects.

Figure 1

Cardiac metabolism under control (A) and insulin (B) conditions. Under control conditions, ATP production comes from fatty acids and glucose oxidation. Fatty acid is the privileged substrate used by the heart, its β-oxidation inhibiting glucose oxidation via the Randle cycle (dashed lines). When glucose and insulin plasma levels increase, glucose becomes the main energy-providing substrate. Indeed, insulin induces Glut4 translocation and 6-phosphofructo-2-kinase (PFK-2) activation, leading to the concomitant stimulation of glucose uptake and glycolysis (see the text for further details). FAT/CD36, LCFA transporter 88 kDa FA translocase/CD36; Fru-1,6-P2, fructose 1,6-bisphosphate; Fru-2,6-P2, fructose 2,6-bisphosphate; Glu-6-P, glucose 6-phosphate; Glut4, glucose transporter 4; LCFAs, long-chain fatty acids; PDH, pyruvate dehydrogenase; PFK-1, 6-phosphofructo-1-kinase; TCA cycle, tricarboxylic acid cycle. This figure is derived from Foretz et al.159

Figure 2

Insulin-signalling pathways regulating cardiovascular metabolism. The different insulin-signalling elements presented here are described in detail in the text. (A) The effects of insulin on substrate metabolism and protein translation in the cardiomyocyte. (B) Insulin actions on vasodilatation, which occur in the endothelial/vascular smooth muscle cell system, on apoptosis and cell growth. Dotted lines symbolize translocations and solid lines illustrate direct interactions, whereas dashed lines represent indirect regulation. The insulin-induced negative feedback loop is presented in red. Interactions between insulin and AMP-activated protein kinase signalling are also shown, red lines indicating inhibiting effects of one pathway to the other, green lines pointing out AMP-activated protein kinase effects analogous to that of insulin. Question marks represent signalling events that have not yet been established in the heart tissue.

3. The proximal insulin signalling

The insulin receptor (IR) is a tetrameric enzyme comprising two extracellular α-subunits and two transmembrane β-subunits.10 The binding of insulin to the extracellular part of IR induces the activation of the intrinsic tyrosine kinase activity of the β-subunits of the receptor (Figure 2). This leads to an autotransphosphorylation of the receptor where one β-subunit phosphorylates the other on several tyrosine residues. IR shares a similar structure with the insulin-like growth factor (IGF-1) receptor, explaining cross-reaction and partially overlapping functions between the two receptors and their ligands.11,12 Once activated and phosphorylated, IR binds via its phosphotyrosine residues, and phosphorylates a series of downstream elements, including the IR substrate (IRS) family and Shc.1316 This recruitment and activation lead to the activation of two main pathways, the phosphatidylinositol 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK) pathway, respectively. PI3K (especially class Ia) is considered to be the main player of the metabolic action of insulin, whereas the MAPK pathway is principally involved in cell growth and differentiation. The MAPK pathway has already been well reviewed.1720 So, we primarily focus the present review on the PI3K pathway. PI3K is a heterodimeric protein composed of a p110 catalytic and a p85 regulatory subunit. This regulatory subunit contains SH2 domains that bind to phosphorylated tyrosine residues of IR or IRS adaptor proteins.21 This insulin-induced PI3K recruitment at the plasma membrane leads to its activation. PI3K is a lipid kinase that phosphorylates PI phosphates at the 3 position.21 The main product of PI3K is the PI3,4,5P3 produced from the PI4,5P2. The increase in PI3,4,5P3 at the plasma membrane induced the recruitment and co-localization of the phosphoinositide-dependent kinase 1 (PDK1) and PKB/Akt. Once recruited, PDK1 phosphorylates and activates PKB/Akt. Insulin is a very potent PKB/Akt activator in the heart.22 Moreover, it has been shown in PDK1 muscle-specific knockout (KO) mice that PDK1 is required for the insulin-induced activation of cardiac PKB/Akt.23 The tandem composed by PDK1 and PKB/Akt is an important player of the metabolic effect of insulin. For example, PDK1 and the PKBβ/Akt2 isoform seem to be essential for the regulation of glucose uptake in the heart.2426 In parallel to PKB/Akt, PDK1 is also able to activate the atypical protein kinase Cs (PKCs) λ and ζ even if this insulin-induced activation has not yet been actually demonstrated in cardiac tissue.27

4. Regulation of glucose and long-chain fatty acid metabolism

We will now illustrate the downstream mechanism implicated in the insulin-induced regulation of glucose and LCFA entry and metabolism (Figure 2). Glucose has to enter into the cardiomyocyte to be oxidized. The uptake of glucose depends on the presence of glucose transporters at the plasma membrane. Among Glut1 and Glut4, the two glucose transporters expressed in the heart, Glut4 is considered to be the main contributor for the regulation of glucose uptake by insulin.28,29 Gluts are present into the cardiomyocyte in two different populations, one located in intracellular compartments, and the other being at the plasma membrane. Insulin induces the translocation of Glut4 from the intracellular storage to the sarcolemmal membrane and so favours glucose entry.28,30 The role of the PI3K/PKB/Akt-signalling cascade in the insulin-induced Glut4 translocation has been well established for many years.31 Conversely, the distal elements involved in this recruitment have been only recently partially determined.3134 Indeed, It has been shown that AS160, the GAP of the small G proteins Rabs required for membrane trafficking, is a substrate of PKB/Akt. This insulin-induced phosphorylation inactivates AS160, prevents Rab GAP function, and so favours Glut4 translocation in adipocytes3 and muscle.35 Phosphorylation of AS160 is also enhanced by insulin in cardiomyocytes (L. Bertrand, personal communication). However, more recent studies, principally made in adipocytes and myotubes, show that the PKB/AS160 axis is probably not the sole mechanism regulating the insulin-induced Glut4 translocation. Indeed, another Rab GAP, TBC1D1, an additional PKB/Akt substrate, could play a role parallel to AS160.36,37 Two other PKB/Akt substrates, the phosphoinositide 5-kinase PIKfive38 and the SNARE-associated protein synip,39 have also been proposed to participate in Glut4 translocation. PIKfive may regulate Glut4 vesicles trafficking by acting on F-actin dynamics. Synip seems to control the fusion of Glut4 vesicles with the plasma membrane. In addition, Bai et al.40 showed that the preparation of the Glut4 storage compartments for fusion with the plasma membrane is another key step regulated by insulin independently of the PI3K/PKB/Akt axis. In parallel to PKB/Akt, another PDK1 substrate, the atypical PKC λ/ζ family, has been implicated in Glut4 transfer to the plasma membrane as well.41 Besides the PI3K/PKB/Akt, there is another pathway, located in specific lipid raft microdomains, that involves the G protein TC10, coupled to IR and the IRS Cbl. This pathway has been proposed to play a role in the translocation of Glut4 in adipocytes.31,32,34,42,43 In cardiac muscle, insulin has been shown to increase the phosphorylation of both Cbl and TC10.44 All these parameters regulated by insulin seem to share the control of Glut4 translocation. However, the roles of TBC1D1, PIKfive, synip, PKC λ/ζ, or Cbl/TC10 in the insulin-induced stimulation of glucose uptake have not yet been studied in the heart.

Once entered into cardiomyocytes, glucose is metabolized to participate in glycogen synthesis and glycolysis in an insulin-dependent manner also (Figures 1 and 2). Insulin is able to stimulate glycogen synthase (GS) in muscle cells via its dephosphorylation, which is dependent of the PKB-induced inactivation of the GS kinase-3 (GSK-3).26,4547 However, this insulin-induced and PKB/Akt-dependent GS stimulation does not seem to be implicated in the regulation of the glycogen content in the heart.26 A mechanism involving allosteric activation of GS by glucose-6-phosphate could be the main factor regulating cardiac glycogen level.26 More importantly, insulin favours the use of glucose by stimulating cardiac glycolysis.4850 Indeed, insulin activates the cardiac 6-phosphofructo-2-kinase (PFK-2) isoform, which is the enzyme that synthesizes fructose 2,6-bisphosphate.51 Fructose 2,6-bisphosphate is a potent stimulator of PFK-1, the main enzyme-regulating glycolytic flux.50 PFK-2 activation results from its phosphorylation, which is undoubtedly under the control of a PI3K/PDK1-dependent pathway.23,48,5254 However, the identity of the downstream protein kinase responsible for the direct PFK-2 phosphorylation is still controversial, with one study showing a putative role of PKB,53 whereas others implicate another PDK1 dependent kinase.22,27,52 The role of PFK2 in the regulation of cardiac function seems to be important because the cardio-specific expression of a kinase-dead PFK2 decreases glycolytic flux, induces hypertrophy and fibrosis, and reduces cardiomyocyte function.55

In parallel to the stimulation of glucose uptake, insulin also induces LCFA uptake in cardiomyocytes.56,57 This stimulation comes from the insulin-dependent translocation to the plasma membrane of the LCFA transporter FAT/CD36 (Figure 2). This phenomenon requires the insulin-induced PI3K activation58 but needs additional studies to be further characterized. In contrary to glucose, the resulting increase in intracellular LCFA concentration does not result in the increase in LCFA oxidation but in the storage of this excess into the intracellular pool of lipids.59

5. Regulation of protein synthesis

Insulin can be considered as an anabolic hormone promoting protein synthesis and cell growth. The control of protein synthesis by insulin involves phosphorylation/dephosphorylation of several translation factors and ribosomal proteins.60,61 PKB/Akt is a pivotal element of this complex regulation (Figure 2). PKB/Akt can phosphorylate and inactivate the tuberous sclerosis factor 2 (TSC2), the GAP of the G protein Rheb.62,63 Active TSC2 promotes the formation of the GDP-bound inactive state of Rheb. By inactivating TSC2, PKB/Akt induces Rheb activation, which, in turn, induces the phosphorylation and activation of the mammalian target of rapamycin (mTOR) by a mechanism that is still not well defined.64 Insulin has been clearly shown to regulate the PKB/Akt/TSC2/mTOR pathway in cardiomyocytes.65 Once activated, mTOR mainly regulates two targets involved in the regulation of protein translation, the 4E-binding protein-1 (4E-BP1) and the p70 ribosomal S6 protein kinase (p70S6K).61 The mTOR-mediated phosphorylation of 4E-BP1 prevents its inhibitory action on the eukaryotic initiation factor 4E (eIF-4E), allowing this factor to bind the mRNA cap and stimulate the initiation step of protein synthesis. Activated p70S6K phosphorylates the S6 ribosomal protein involved in the regulation of the translation of the 5′TOP mRNAs that encode several translation factors and ribosomal proteins. Therefore, S6 phosphorylation increases both translation factor content and translational capacity by increasing ribosomal biogenesis. The eukaryotic elongation factor-2 (eEF2) kinase (eEF2K) is another p70S6K substrate. eEF2K is a dedicated calcium- and calmodulin-dependent kinase that controls the phosphorylation and inactivation of eEF2. The insulin-dependent eEF2K inactivation leads to the stimulation of protein elongation. Numerous studies already demonstrated the insulin-mediated regulation of 4E-BP1,46,66,67 p70S6K/S6,46,47,6770 and eEF2K/eEF246 in the heart or cardiomyocytes.

PKB/Akt controls two other substrates, GSK-3 and the forkhead transcription factor FOXO family, regulating protein translation and atrophy, respectively. In parallel to the modulation of GS activity, GSK-3 is also implicated in the regulation of protein translation. GSK-3 phosphorylates and inactivates the initiation factor eIF2B.61 By inhibiting GSK-3, insulin activates eIF2B and, so, stimulates the initiation of protein synthesis. Even if the relationship between insulin, PKB, GSK-3, and eIF2B has been demonstrated in various tissues including skeletal muscle, a similar study still needs to be extensively performed in cardiac tissue.46 To be complete, GSK-3 also participates in the negative regulation of cardiac hypertrophy by phosphorylating and inactivating the nuclear factor of activated T (NFAT) cells responsible for the pro-hypertrophic gene expression.17

Atrophy, and its associated proteosomal destruction of proteins, can be considered to be the opposite of protein synthesis. FOXO family members, comprising FOXO1, FOXO3a, and FOXO4, were initially known to promote the atrogene transcriptional program in skeletal muscle.71 Moreover, it was established that the phosphorylation of FOXOs by PKB/Akt promotes their nuclear exclusion and prevents atrophy.72 Similar to what happens in skeletal muscle, Skurk et al.73 revealed that insulin prevents cardiac muscle atrophy by inhibiting FOXO3a through a PKB/Akt-dependent pathway. More recently, cardiac FOXO1 has been shown to be regulated in the same way.74

6. Insulin and vasculature

Insulin has important vascular actions that, for most of them, lead to vasodilatation, increased blood flow, and subsequent augmentation of glucose disposal in classical insulin-target tissues. One of its essential roles is the stimulation of increased production of the potent vasodilator nitric oxide (NO) from vascular endothelium.75 In endothelial cells, endothelial NO synthase (eNOS) catalyses the conversion of the substrate l-arginine to the products NO and l-citrulline.76 The insulin-signalling pathway regulates the activation of eNOS by a phosphorylation-dependent mechanism. It requires activation of the PI3K/PKB/Akt pathway (Figure 2). PKB/Akt directly phosphorylates eNOS, resulting in enhanced eNOS activity77 and increased production of NO.75,78,79 This pathway is completely distinct and independent from classical calcium-dependent mechanisms used by G protein-coupled receptors such as acetylcholine receptor.80,81 PKBα/Akt-1 being the major isoform in the vasculature and endothelial cells, it is not surprising that PKBα/Akt-1 KO mice have significantly low levels of active eNOS.82 It is therefore likely that PKBα/Akt-1 mediates insulin-induced activation of eNOS in these cells. However, activation of PKB/Akt, if necessary, is not sufficient to activate eNOS. Indeed, although insulin-induced eNOS activation is calcium-independent, insulin stimulates calmodulin binding to eNOS.83 This requires the prior binding of HSP90 onto the enzyme. The formation of a ternary eNOS/HSP90/PKB/Akt complex facilitates eNOS phosphorylation by PKB/Akt.80

Rodent models of insulin resistance provide important insights into cardiovascular actions of insulin. IRS-1 (−/−)- and IRS-2 (−/−)-deficient mice not only exhibit resistance to the metabolic actions of insulin, but also demonstrate decreased eNOS activity and elevated blood pressure.84 In addition, in obese Zucker rat (carrying a recessive mutation in the gene for leptin) commonly used as a model of insulin resistance, the insulin-mediated attenuation of vascular contractility85 and increases in limb blood flow86 are substantially reduced. In contrast, insulin can also have effects that oppose vasodilator actions of NO, such as the stimulation of secretion of the vasoconstrictor endothelin-1 (ET-1) from vascular endothelium. These effects have been shown to depend exclusively of the MAPK pathway.87,88 Under insulin resistance, PI3K-dependent pathways are impaired, whereas MAPK-dependent pathways are intact. Therefore, a shift in the balance between vasoconstrictor and vasodilator actions of insulin could be an important factor in the vascular pathophysiology of insulin resistance and endothelial dysfunction. Human studies in overweight,89 obese,90 hypertensive,91 and diabetic subjects90 support this notion.

In the vasculature, NO originates mostly from the endothelium and diffuses into vascular smooth muscle cells (VSMCs) where it activates guanylate cyclase to increase cGMP levels that evoke relaxation. But NO production may also act in an autocrine fashion to regulate vasodilator functions in VSMCs. Indeed, expression of eNOS and inducible NOS (iNOS) has been detected in these cells, and insulin increases their activity and the resulting NO-dependent cGMP production9295 via a PI3K-dependent mechanism.92,94 Moreover, the NO/cGMP-dependent kinase (PKG) pathway attenuates contractility by regulating the calcium-dependent RhoA/Rho kinase (ROK) pathway stimulated in response to contractile agonists (Figure 2).96 Activation of RhoA-dependent pathways is involved in excessive contraction and thereby increases blood pressure. All Rho proteins are prenylated at their C-terminus and this prenylation is involved in the translocation of the active GTP-bound form of the Rho protein to the cell membrane.96 Active RhoA recruits and stimulates ROK-α, which then acts, in part, by inactivating myosin phosphatase by phosphorylation and also directly phosphorylating myosin light chains to enhance contraction.97,98 In VSMCs, phosphorylation of RhoA by PKG relocalizes the Rho protein to the cytosol and also inhibits its affinity for its downstream effector ROK-α.99 These effects are abrogated by wortmannin, a PI3K inhibitor, and small interfering RNA against PKB/Akt, indicating that, in VSMCs, the PI3K/PKB/Akt insulin-signalling pathway is likely to mediate the inhibition of ROK-mediated RhoA-dependent contraction.100,101

7. Insulin action under ischaemia: crosstalk between insulin and AMP-activated protein kinase signalling

Insulin signalling and action are greatly altered under myocardial ischaemia.49,102106 Insulin signalling can be modified by myocardial ischaemia via mainly two pathways, one dependent of the AMP-activated protein kinase (AMPK), the other being unrelated to this protein kinase. First, it has been shown that the acidosis produced by myocardial ischaemia provokes the inhibition of the tyrosine kinase activity of IR (Figure 2).47 This inhibition is directly correlated to the decrease in phosphorylation of different downstream elements including PKB/Akt, p70S6K, and GSK-3. Secondly, ischaemia, by affecting oxidative ATP production, and so, increasing AMP concentration, induces AMPK activation.107 Once activated, AMPK is considered to regulate different components of the insulin-signalling pathway that control glucose metabolism and protein synthesis (Figure 2).49,102105,107109 Similar to insulin, AMPK stimulates cardiac glucose uptake and glycolysis by regulating the same targets, namely Glut-4 translocation (see later in the section dedicated to insulin resistance) and PFK-2 activation.110,111 AMPK also stimulates LCFA uptake.112 In contrast, AMPK is known to antagonize the stimulating effect of insulin on protein synthesis by inhibiting the TSC2/mTOR/p70S6K113116 and eEF2 pathways.68,117 However, the exact role of AMPK on the regulation of protein synthesis in the ischaemic heart still needs to be clearly defined. Indeed, the pharmacological118 or genetic119 activation of AMPK clearly counteracts the PKB/Akt-mediated activation of p70S6K, phosphorylation of eEF2, and stimulation of protein synthesis in cardiomyocytes. However, recent data in our laboratory showed that the ischaemia-induced phosphorylation of eEF2 is clearly altered by the absence of the AMPKα2 isoform (the main catalytic isoform in the heart), whereas the inhibition of the mTOR/p70S6K is not modified (L. Bertrand, personal communication). This last result confirms the role of AMPK in the regulation of protein synthesis elongation but questions the implication of this protein kinase on the inhibition of the (pre)initiation step of protein translation by ischaemia.

In parallel to the ischaemia-dependent insulin regulation, insulin is known to antagonize AMPK signalling as well.120,121 Indeed, activated PKB/Akt can phosphorylate AMPK, this phosphorylation reducing the ischaemia-induced AMPK activation (Figure 2).45,122,123

8. Is insulin protective during ischaemia/reperfusion?

Even if the insulin-signalling pathway is blunted during ischaemia, insulin is still able to act during reperfusion. During several decades, the metabolic cocktail glucose/insulin/potassium (GIK) was considered to be a good approach to reduce infarct size after myocardial infarcts.124,125 Numerous studies involved the insulin-mediated activation of PKB/Akt to explain the protective effect of GIK.103,125127 A first metabolic reason is the shift of the myocardial metabolism from LCFAs to glucose oxidation during reperfusion (discussed earlier), which is more oxygen efficient and prevents the production of toxic LCFA intermediates. Secondly, activated PKB/Akt leads to several protective mechanisms. PKB/Akt phosphorylates, sequesters, and/or inactivates several pro-apoptotic proteins including BAD, BAX, and caspase-9 (Figure 2).128,129 As explained earlier, PKB/Akt also phosphorylates and activates eNOS, inducing protection most likely via the NO-dependent PKG activation and the inhibition of the mitochondrial transition pore.129 Finally, the PKB/Akt/mTOR/p70S6K is also supposed to be protective by promoting, among others, the post-ischaemic synthesis of contractile proteins. It has to be mentioned that, similar to insulin, IGF-1 exerts a protective action during ischaemia/reperfusion, by acting on the same targets, as for instance the PKB/Akt pathway.130,131

Despite the existence of all these arguments demonstrating a protective effect of insulin, a more recent randomized controlled trial, the CREATE-ECLA study,132 showed a neutral effect of GIK on mortality in patients with acute myocardial infarction. Several hypotheses can be presented to understand this apparent divergence. First of all, the time of administration of GIK could directly interfere with the efficacy of the treatment. The best cardiac recovery should be obtained if GIK perfusion occurs rapidly, before coronary intervention being optimal.133 Moreover, a recent study demonstrated that LCFAs attenuate insulin cardioprotection.134 Even if insulin is known to lower LCFA in the plasma, this new issue has to be taken into account in the near future. Finally, hyperglycaemia, characteristic to GIK infusion and known to provoke glucotoxicity,135 could counteract the beneficial effects of insulin. In conclusion, a full metabolic monitoring needs to be performed to definitively prove the beneficial effect of GIK infusion.103

9. Insulin resistance in type 2 diabetes

Obesity as well as myocardial insulin resistance and metabolic alteration are features of type 2 diabetes and are established risk factors for the development of cardiovascular diseases.5,56 The diabetic heart is characterized by a decrease in glucose uptake and oxidation. In contrast, LCFA uptake and oxidation are clearly enhanced. Nevertheless, the increase in LCFA oxidation is not sufficient to prevent lipid accumulation in the heart. It is generally accepted that this metabolic disorder plays an essential role in the establishment of myocardial insulin resistance despite the fact that the molecular processes leading to this resistance are not yet fully understood. Insulin resistance is characterized by an alteration of the insulin-induced activation of the PI3K/PKB/Akt-signalling pathway.136138 Mainly studied in skeletal muscle and adipose tissue, the decrease in insulin signalling is explained to come from at least three molecular events. First, LCFA-mediated ceramide accumulation seems to induce atypical PKC ζ-dependent inhibition of PKB/Akt.139 Secondly, LCFA-dependent diacylglycerol accumulation induces conventional and novel PKCs activation that, in turn, could provoke the serine phosphorylation of IRS1, thereby inhibiting the PIK3/PKB/Akt pathway.140,141 Thirdly, insulin signalling by itself could produce a negative feedback loop in which the insulin-dependent activation of mTOR/p70S6K induces serine phosphorylation and inhibition of IRS1 (Figure 2).142 Whatever the mechanism, the insulin-signalling alteration explains the impairment of the insulin-induced Glut4 translocation and glucose uptake stimulation that are found in diabetic hearts. Ex vivo experimental studies have yielded conflicting results on the susceptibility of diabetic hearts to ischaemia/reperfusion injury.143,144 However, in vivo animal and human studies have clearly shown that diabetes and insulin resistance aggravate myocardial ischaemic injury143,144 and reduce the beneficial effects of ischaemic pre-conditioning (IPC). Indeed, IPC is known to protect the heart against ischaemic injury via activation of the PI3K/PKB/Akt pathway.128 In diabetic hearts, stronger IPC stimuli are required to activate the PKB/Akt pathway and induce cardioproctection.145

Two types of therapeutic approaches could be performed to re-establish insulin sensitivity: by blocking the upstream source of insulin resistance or by directly modulating insulin signalling. Indeed, the first solution is to modulate glucose and/or LCFA metabolism to find a new equilibrium favouring glucose uptake and oxidation in opposition to LCFA oxidation.56 On the other hand, adjustment of insulin signalling can be done via the utilization of the anti-diabetic drug metformin. It has been shown that metformin induces AMPK activation, which could explain, at least in part, its insulin sensitizing action.146,147 Metformin-induced AMPK activation could have a double impact on myocardial metabolism (Figure 2). AMPK can directly stimulate glucose uptake by phosphorylating and inactivating AS160, similar to PKB/Akt.148 AS160 is therefore another converging point between insulin and AMPK-signalling pathways (Figure 2) that makes clear the synergistic effect of insulin and metformin on glucose uptake in cardiomyocytes.149 More importantly, AMPK, by inhibiting the mTOR/p70S6K axis (discussed earlier), could decrease the p70S6K-mediated phosphorylation and inhibition of IRS1 (Figure 2). The inhibition of this negative feedback loop should enhance the insulin-dependent PI3K/PKB/Akt activation and re-establish normal insulin signalling in insulin-resistant cells. Recently, we demonstrated that AMPK activation by metformin greatly enhances the insulin-induced PKB/Akt activation in insulin-resistant cardiomyocytes.149 This PKB/Akt overactivation is correlated to the decrease in serine phosphorylation of IRS1 (L. Bertrand, personal communication).

10. Insulin signalling and cardiac hypertrophy

Insulin shares signalling pathways with different growth hormones including IGF-1 and neurohormonal hypertrophic agonists such as angiotensin II.17,19 All these hormones can activate the MAPK and/or the PI3K/PKB/Akt signalling involved in the stimulation of both cell growth and protein synthesis and in the inhibition of protein breakdown (discussed earlier). Under normal conditions, the stimulation of the PI3K/PKB/Akt pathway by insulin and IGF-1 participates in both embryonic and postnatal cardiac growth.18,150 For example, cardiomyocyte-restricted IR KO151 and PDK1 KO23 are characterized by a smaller heart and smaller cardiomyocytes, respectively. It could be considered that IGF-1 acts under the hypothalamic axis, whereas insulin action reflects nutritional status. On the other hand, in the adult stage, some physiological (exercise) and pathological (hypertension, valvular dysfunction) conditions are linked to the chronic stimulation of PI3K/PKB/Akt and/or MAPK pathways, which undeniably participates in the establishment of myocardial hypertrophy.1719 The signalling pathways involved in the physiological and pathological hypertrophy are not similar even if they share some signalling elements. The PI3K/PKB/Akt axis seems more linked to the physiological hypertrophy, whereas MAPK signalling, in collaboration with the PKC and calcineurin/NFAT pathways, participates in the development of the pathological hypertrophy typically induced by angiotensin II.18

Using overexpressing dominant-negative PI3K152 or PKBα/Akt1 KO153 mice, it has been shown that these kinases are required for physiological but not for pathological hypertrophy. PKB/Akt action requires the mTOR/P70S6K pathway because the use of rapamycin, the mTOR inhibitor, is sufficient to block the myocardial hypertrophy induced by the cardiac overexpression of PKB/Akt.154 The important role played by mTOR in the development of cardiac hypertrophy offers a putative role to AMPK as a potential strategy to prevent hypertrophic development.155

In parallel to their direct implication in physiological hypertrophy, insulin and insulin signalling can be indirectly connected to pathological hypertrophy. Indeed, it has been recently shown that chronic hyperinsulinaemia stimulates the angiotensin II signalling that is involved in pathological hypertrophy.156 Moreover, hypertension-induced hypertrophy is often found in humans simultaneously with insulin resistance. This insulin resistance appears first in skeletal muscle and adipocytes inducing elevated plasma insulin levels. This hyperinsulinaemia probably facilitates hypertrophy by overactivating cardiac insulin signalling.157 On the other hand, pathological hypertrophy is characterized by a remodelling of substrate utilization with a shift to carbohydrate usage mimicking the insulin-stimulated phenotype.157,158 It seems more and more evident that this metabolic shift participates in the establishment of hypertrophy via the production of reactive oxygen species.158

11. Summary

A large part of the insulin-signalling components involved in the regulation of processes such as myocardial glucose and LCFA utilization, protein translation, and vascular activity is now identified. This knowledge allows us to better understand the connection between insulin and other signalling pathways as well as under physiological than pathological conditions such as ischaemia, diabetes, hypertension, and hypertrophy. The partial elucidation of these signal transduction pathways is now followed by the development of therapeutical strategies based on the modulation of certain of these signalling elements. The continuation of the elucidation of all these multifaceted signalling pathways in the near future should bring new information that will be used to treat myocardial pathologies with even more efficiency.

Conflict of interest: none declared.

Funding

The authors were supported by the Fonds National de la Recherche Scientifique et Médicale (Belgium) and the Actions de Recherche Concertées (Belgium). L.B. and S.H. are research associates of the Fonds National de la Recherche Scientifique, Belgium.

References

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