FoxO1-Dio2 signaling axis governs cardiomyocyte thyroid hormone metabolism and hypertrophic growth.

Forkhead box O (FoxO) proteins and thyroid hormone (TH) have well established roles in cardiovascular morphogenesis and remodeling. However, specific role(s) of individual FoxO family members in stress-induced growth and remodeling of cardiomyocytes remains unknown. Here, we report that FoxO1, but not FoxO3, activity is essential for reciprocal regulation of types II and III iodothyronine deiodinases (Dio2 and Dio3, respectively), key enzymes involved in intracellular TH metabolism. We further show that Dio2 is a direct transcriptional target of FoxO1, and the FoxO1-Dio2 axis governs TH-induced hypertrophic growth of neonatal cardiomyocytes in vitro and in vivo. Utilizing transverse aortic constriction as a model of hemodynamic stress in wild-type and cardiomyocyte-restricted FoxO1 knockout mice, we unveil an essential role for the FoxO1-Dio2 axis in afterload-induced pathological cardiac remodeling and activation of TRα1. These findings demonstrate a previously unrecognized FoxO1-Dio2 signaling axis in stress-induced cardiomyocyte growth and remodeling and intracellular TH homeostasis.

Malonyl CoA Decarboxylase Inhibition Improves Cardiac Function Post-Myocardial Infarction.

Alterations in cardiac energy metabolism after a myocardial infarction contribute to the severity of heart failure (HF). Although fatty acid oxidation can be impaired in HF, it is unclear if stimulating fatty acid oxidation is a desirable approach to treat HF. Both immediate and chronic malonyl coenzyme A decarboxylase inhibition, which decreases fatty acid oxidation, improved cardiac function through enhancing cardiac efficiency in a post-myocardial infarction rat that underwent permanent left anterior descending coronary artery ligation. The beneficial effects of MCD inhibition were attributed to a decrease in proton production due to an improved coupling between glycolysis and glucose oxidation.

Isx9 Regulates Calbindin D28K Expression in Pancreatic ß Cells and Promotes ß Cell Survival and Function.

Pancreatic ß-cell dysfunction and death contribute to the onset of diabetes, and novel strategies of ß-cell function and survival under diabetogenic conditions need to be explored. We previously demonstrated that Isx9, a small molecule based on the isoxazole scaffold, drives neuroendocrine phenotypes by increasing the expression of genes required for ß-cell function and improves glycemia in a model of ß cell regeneration. We further investigated the role of Isx9 in ß-cell survival. We find that Isx9 drives the expression of Calbindin-D28K (D28K), a key regulator of calcium homeostasis, and plays a cytoprotective role through its calcium buffering capacity in ß cells. Isx9 increased the activity of the calcineurin (CN)/cytoplasmic nuclear factor of the activated T-cells (NFAT) transcription factor, a key regulator of D28K, and improved the recruitment of NFATc1, cAMP response element-binding protein (CREB), and p300 to the D28K promoter. We found that nutrient stimulation increased D28K plasma membrane enrichment and modulated calcium channel activity in order to regulate glucose-induced insulin secretion. Isx9-mediated expression of D28K protected ß cells against chronic stress induced by serum withdrawal or chronic inflammation by reducing caspase 3 activity. Consequently, Isx9 improved human islet function after transplantation in NOD-SCID mice in a streptozotocin-induced diabetes model. In summary, Isx9 significantly regulates expression of genes relevant to ß cell survival and function, and may be an attractive therapy to treat diabetes and improve islet function post-transplantation.

Polycystin-2-dependent control of cardiomyocyte autophagy.

AIMS: Considerable evidence points to critical roles of intracellular Ca2+ homeostasis in the modulation and control of autophagic activity. Yet, underlying molecular mechanisms remain unknown. Mutations in the gene (pkd2) encoding polycystin-2 (PC2) are associated with autosomal dominant polycystic kidney disease (ADPKD), the most common inherited nephropathy. PC2 has been associated with impaired Ca2+ handling in cardiomyocytes and indirect evidence suggests that this protein may be involved in autophagic control. Here, we investigated the role for PC2 as an essential regulator of Ca2+ homeostasis and autophagy. METHODS AND RESULTS: Activation of autophagic flux triggered by mTOR inhibition either pharmacologically (rapamycin) or by means of nutrient depletion was suppressed in cells depleted of PC2. Moreover, cardiomyocyte-specific PC2 knockout mice (αMhc-cre;Pkd2F/F mice) manifested impaired autophagic flux in the setting of nutrient deprivation. Stress-induced autophagy was blunted by intracellular Ca2+ chelation using BAPTA-AM, whereas removal of extracellular Ca2+ had no effect, pointing to a role of intracellular Ca2+ homeostasis in stress-induced cardiomyocyte autophagy. To determine the link between stress-induced autophagy and PC2-induced Ca2+ mobilization, we over-expressed either wild-type PC2 (WT) or a Ca2+-channel deficient PC2 mutant (PC2-D509V). PC2 over-expression increased autophagic flux, whereas PC2-D509V expression did not. Importantly, autophagy induction triggered by PC2 over-expression was attenuated by BAPTA-AM, supporting a model of PC2-dependent control of autophagy through intracellular Ca2+. Furthermore, PC2 ablation was associated with impaired Ca2+ handling in cardiomyocytes marked by partial depletion of sarcoplasmic reticulum Ca2+ stores. Finally, we provide evidence that Ca2+-mediated autophagy elicited by PC2 is a mechanism conserved across multiple cell types. CONCLUSION: Together, this study unveils PC2 as a novel regulator of autophagy acting through control of intracellular Ca2+ homeostasis.

SF-1 expression in the hypothalamus is required for beneficial metabolic effects of exercise.

Exercise has numerous beneficial metabolic effects. The central nervous system (CNS) is critical for regulating energy balance and coordinating whole body metabolism. However, a role for the CNS in the regulation of metabolism in the context of the exercise remains less clear. Here, using genetically engineered mice we assessed the requirement of steroidogenic factor-1 (SF-1) expression in neurons of the ventromedial hypothalamic nucleus (VMH) in mediating the beneficial effects of exercise on metabolism. We found that VMH-specific deletion of SF-1 blunts (a) the reductions in fat mass, (b) improvements in glycemia, and (c) increases in energy expenditure that are associated with exercise training. Unexpectedly, we found that SF-1 deletion in the VMH attenuates metabolic responses of skeletal muscle to exercise, including induction of PGC-1α expression. Collectively, this evidence suggests that SF-1 expression in VMH neurons is required for the beneficial effects of exercise on metabolism.

Polycystin-1 Is a Cardiomyocyte Mechanosensor That Governs L-Type Ca2+ Channel Protein Stability.

BACKGROUND: L-type calcium channel activity is critical to afterload-induced hypertrophic growth of the heart. However, the mechanisms governing mechanical stress-induced activation of L-type calcium channel activity are obscure. Polycystin-1 (PC-1) is a G protein-coupled receptor-like protein that functions as a mechanosensor in a variety of cell types and is present in cardiomyocytes. METHODS AND RESULTS: We subjected neonatal rat ventricular myocytes to mechanical stretch by exposing them to hypo-osmotic medium or cyclic mechanical stretch, triggering cell growth in a manner dependent on L-type calcium channel activity. RNAi-dependent knockdown of PC-1 blocked this hypertrophy. Overexpression of a C-terminal fragment of PC-1 was sufficient to trigger neonatal rat ventricular myocyte hypertrophy. Exposing neonatal rat ventricular myocytes to hypo-osmotic medium resulted in an increase in α1C protein levels, a response that was prevented by PC-1 knockdown. MG132, a proteasomal inhibitor, rescued PC-1 knockdown-dependent declines in α1C protein. To test this in vivo, we engineered mice harboring conditional silencing of PC-1 selectively in cardiomyocytes (PC-1 knockout) and subjected them to mechanical stress in vivo (transverse aortic constriction). At baseline, PC-1 knockout mice manifested decreased cardiac function relative to littermate controls, and α1C L-type calcium channel protein levels were significantly lower in PC-1 knockout hearts. Whereas control mice manifested robust transverse aortic constriction-induced increases in cardiac mass, PC-1 knockout mice showed no significant growth. Likewise, transverse aortic constriction-elicited increases in hypertrophic markers and interstitial fibrosis were blunted in the knockout animals CONCLUSION: PC-1 is a cardiomyocyte mechanosensor that is required for cardiac hypertrophy through a mechanism that involves stabilization of α1C protein.

Constitutive phosphorylation of cardiac myosin regulatory light chain in vivo.

In beating hearts, phosphorylation of myosin regulatory light chain (RLC) at a single site to 0.45 mol of phosphate/mol by cardiac myosin light chain kinase (cMLCK) increases Ca(2+) sensitivity of myofilament contraction necessary for normal cardiac performance. Reduction of RLC phosphorylation in conditional cMLCK knock-out mice caused cardiac dilation and loss of cardiac performance by 1 week, as shown by increased left ventricular internal diameter at end-diastole and decreased fractional shortening. Decreased RLC phosphorylation by conventional or conditional cMLCK gene ablation did not affect troponin-I or myosin-binding protein-C phosphorylation in vivo. The extent of RLC phosphorylation was not changed by prolonged infusion of dobutamine or treatment with a ß-adrenergic antagonist, suggesting that RLC is constitutively phosphorylated to maintain cardiac performance. Biochemical studies with myofilaments showed that RLC phosphorylation up to 90% was a random process. RLC is slowly dephosphorylated in both noncontracting hearts and isolated cardiac myocytes from adult mice. Electrically paced ventricular trabeculae restored RLC phosphorylation, which was increased to 0.91 mol of phosphate/mol of RLC with inhibition of myosin light chain phosphatase (MLCP). The two RLCs in each myosin appear to be readily available for phosphorylation by a soluble cMLCK, but MLCP activity limits the amount of constitutive RLC phosphorylation. MLCP with its regulatory subunit MYPT2 bound tightly to myofilaments was constitutively phosphorylated in beating hearts at a site that inhibits MLCP activity. Thus, the constitutive RLC phosphorylation is limited physiologically by low cMLCK activity in balance with low MLCP activity.

Dichloroacetate selectively improves cardiac function and metabolism in female and male rainbow trout.

Cardiac tissue from female rainbow trout demonstrates a sex-specific preference for exogenous glucose and glycolysis, impaired Ca(2+) handling, and a greater tolerance for hypoxia and reoxygenation than cardiac tissue from male rainbow trout. We tested the hypothesis that dichloroacetate (DCA), an activator of pyruvate dehydrogenase, enhances cardiac energy metabolism and Ca(2+) handling in female preparations and provide cardioprotection for hypoxic male tissue. Ventricle strips from sexually immature fish with very low (male) and nondetectable (female) plasma sex steroids were electrically paced in oxygenated or hypoxic Ringer solution with or without 1 mM DCA. In the presence of 5 mM glucose, aerobic tissue from male trout could be paced at a higher frequency (1.79 vs. 1.36 Hz) with lower resting tension and less contractile dysfunction than female tissue. At 0.5 Hz, DCA selectively reduced resting tension below baseline values and lactate efflux by 75% in aerobic female ventricle strips. DCA improved the functional recovery of developed twitch force, reduced lactate efflux by 50%, and doubled citrate in male preparations after hypoxia-reoxygenation. Independent of female sex steroids, reduced myocardial pyruvate dehydrogenase activity and impaired carbohydrate oxidation might explain the higher lactate efflux, compromised function of the sarcoplasmic reticulum, and reduced mechanical performance of aerobic female tissue. Elevated oxidative metabolism and reduced glycolysis might also underlie the beneficial effects of DCA on the mechanical recovery of male cardiac tissue after hypoxia-reoxygenation. These results support the use of rainbow trout as an experimental model of sex differences of cardiovascular energetics and function, with the potential for modifying metabolic phenotypes and cardioprotection independent of sex steroids.

Histone deacetylase inhibition blunts ischemia/reperfusion injury by inducing cardiomyocyte autophagy.

BACKGROUND: Reperfusion accounts for a substantial fraction of the myocardial injury occurring with ischemic heart disease. Yet, no standard therapies are available targeting reperfusion injury. Here, we tested the hypothesis that suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor approved for cancer treatment by the US Food and Drug Administration, will blunt reperfusion injury. METHODS AND RESULTS: Twenty-one rabbits were randomly assigned to 3 groups: (1) vehicle control, (2) SAHA pretreatment (1 day before and at surgery), and (3) SAHA treatment at the time of reperfusion only. Each arm was subjected to ischemia/reperfusion surgery (30 minutes coronary ligation, 24 hours reperfusion). In addition, cultured neonatal and adult rat ventricular cardiomyocytes were subjected to simulated ischemia/reperfusion to probe mechanism. SAHA reduced infarct size and partially rescued systolic function when administered either before surgery (pretreatment) or solely at the time of reperfusion. SAHA plasma concentrations were similar to those achieved in patients with cancer. In the infarct border zone, SAHA increased autophagic flux, assayed in both rabbit myocardium and in mice harboring an RFP-GFP-LC3 transgene. In cultured myocytes subjected to simulated ischemia/reperfusion, SAHA pretreatment reduced cell death by 40%. This reduction in cell death correlated with increased autophagic activity in SAHA-treated cells. RNAi-mediated knockdown of ATG7 and ATG5, essential autophagy proteins, abolished SAHA's cardioprotective effects. CONCLUSIONS: The US Food and Drug Administration-approved anticancer histone deacetylase inhibitor, SAHA, reduces myocardial infarct size in a large animal model, even when delivered in the clinically relevant context of reperfusion. The cardioprotective effects of SAHA during ischemia/reperfusion occur, at least in part, through the induction of autophagic flux.

The effects of neuregulin on cardiac Myosin light chain kinase gene-ablated hearts.

BACKGROUND: Activation of ErbB2/4 receptor tyrosine kinases in cardiomyocytes by neuregulin treatment is associated with improvement in cardiac function, supporting its use in human patients with heart failure despite the lack of a specific mechanism. Neuregulin infusion in rodents increases cardiac myosin light chain kinase (cMLCK) expression and cardiac myosin regulatory light chain (RLC) phosphorylation which may improve actin-myosin interactions for contraction. We generated a cMLCK knockout mouse to test the hypothesis that cMLCK is necessary for neuregulin-induced improvement in cardiac function by increasing RLC phosphorylation. PRINCIPAL FINDINGS: The cMLCK knockout mice have attenuated RLC phosphorylation and decreased cardiac performance measured as fractional shortening. Neuregulin infusion for seven days in wildtype mice increased cardiac cMLCK protein expression and RLC phosphorylation while increasing Akt phosphorylation and decreasing phospholamban phosphorylation. There was no change in fractional shortening. In contrast, neuregulin infusion in cMLCK knockout animals increased cardiac performance in the absence of cMLCK without increasing RLC phosphorylation. In addition, CaMKII signaling appeared to be enhanced in neuregulin-treated knockout mice. CONCLUSIONS: Thus, Neuregulin may improve cardiac performance in the failing heart without increasing cMLCK and RLC phosphorylation by activating other signaling pathways.

RalGDS-dependent cardiomyocyte autophagy is required for load-induced ventricular hypertrophy.

Recent work has demonstrated that autophagy, a phylogenetically conserved, lysosome-mediated pathway of protein degradation, is a key participant in pathological cardiac remodeling. One common feature of cell growth and autophagy is membrane biogenesis and processing. The exocyst, an octomeric protein complex involved in vesicle trafficking, is implicated in numerous cellular processes, yet its role in cardiomyocyte plasticity is unknown. Here, we set out to explore the role of small G protein-dependent control of exocyst function and membrane trafficking in stress-induced cardiomyocyte remodeling and autophagy. First, we tested in cultured neonatal rat cardiomyocytes (NRCMs) two isoforms of Ral (RalA, RalB) whose actions are mediated by the exocyst. In these experiments, mTOR inhibition in response to starvation or Torin1 was preserved despite RalA or RalB knockdown; however, activation of autophagy was suppressed only in NRCMs depleted of RalB, implicating RalB as being required for mTOR-dependent cardiomyocyte autophagy. To define further the role of RalB in cardiomyocyte autophagy, we analyzed hearts from mice lacking RalGDS (Ralgds(-/-)), a guanine exchange factor (GEF) for the Ral family of small GTPases. RalGDS-null hearts were similar to wild-type (WT) littermates in terms of ventricular structure, contractile performance, and gene expression. However, Ralgds(-/-) hearts manifested a blunted growth response (p<0.05) to TAC-mediated pressure-overload stress. Ventricular chamber size and contractile performance were preserved in response to TAC in Ralgds(-/-) mice, and load-induced cardiomyocyte autophagy was suppressed. Interestingly, TAC-induced activation of the fetal gene program was similar in both genotypes despite the relative lack of hypertrophic growth in mutant hearts. Together, these data implicate RalGDS-mediated induction of autophagy and exocyst function as a critical feature of load-induced cardiac hypertrophy.

Diabetic cardiomyopathy and metabolic remodeling of the heart.

The incidence and prevalence of diabetes mellitus are both increasing rapidly in societies around the globe. The majority of patients with diabetes succumb ultimately to heart disease, much of which stems from atherosclerotic disease and hypertension. However, the diabetic milieu is itself intrinsically noxious to the heart, and cardiomyopathy can develop independent of elevated blood pressure or coronary artery disease. This process, termed diabetic cardiomyopathy, is characterized by significant changes in the physiology, structure, and mechanical function of the heart. Presently, therapy for patients with diabetes focuses largely on glucose control, and attention to the heart commences with the onset of symptoms. When the latter develops, standard therapy for heart failure is applied. However, recent studies highlight that specific elements of the pathogenesis of diabetic heart disease are unique, raising the prospect of diabetes-specific therapeutic intervention. Here, we review recently unveiled insights into the pathogenesis of diabetic cardiomyopathy and associated metabolic remodeling with an eye toward identifying novel targets with therapeutic potential.

Metabolic stress-induced activation of FoxO1 triggers diabetic cardiomyopathy in mice.

The leading cause of death in diabetic patients is cardiovascular disease; diabetic cardiomyopathy is typified by alterations in cardiac morphology and function, independent of hypertension or coronary disease. However, the molecular mechanism that links diabetes to cardiomyopathy is incompletely understood. Insulin resistance is a hallmark feature of diabetes, and the FoxO family of transcription factors, which regulate cell size, viability, and metabolism, are established targets of insulin and growth factor signaling. Here, we set out to evaluate a possible role of FoxO proteins in diabetic cardiomyopathy. We found that FoxO proteins were persistently activated in cardiac tissue in mice with diabetes induced either genetically or by high-fat diet (HFD). FoxO activity was critically linked with development of cardiomyopathy: cardiomyocyte-specific deletion of FoxO1 rescued HFD-induced declines in cardiac function and preserved cardiomyocyte insulin responsiveness. FoxO1-depleted cells displayed a shift in their metabolic substrate usage, from free fatty acids to glucose, associated with decreased accumulation of lipids in the heart. Furthermore, we found that FoxO1-dependent downregulation of IRS1 resulted in blunted Akt signaling and insulin resistance. Together, these data suggest that activation of FoxO1 is an important mediator of diabetic cardiomyopathy and is a promising therapeutic target for the disease.

Energy-preserving effects of IGF-1 antagonize starvation-induced cardiac autophagy.

AIMS: Insulin-like growth factor 1 (IGF-1) is known to exert cardioprotective actions. However, it remains unknown if autophagy, a major adaptive response to nutritional stress, contributes to IGF-1-mediated cardioprotection. METHODS AND RESULTS: We subjected cultured neonatal rat cardiomyocytes, as well as live mice, to nutritional stress and assessed cell death and autophagic rates. Nutritional stress induced by serum/glucose deprivation strongly induced autophagy and cell death, and both responses were inhibited by IGF-1. The Akt/mammalian target of rapamycin (mTOR) pathway mediated the effects of IGF-1 upon autophagy. Importantly, starvation also decreased intracellular ATP levels and oxygen consumption leading to AMP-activated protein kinase (AMPK) activation; IGF-1 increased mitochondrial Ca(2+) uptake and mitochondrial respiration in nutrient-starved cells. IGF-1 also rescued ATP levels, reduced AMPK phosphorylation and increased p70(S6K) phosphorylation, which indicates that in addition to Akt/mTOR, IGF-1 inhibits autophagy by the AMPK/mTOR axis. In mice harbouring a liver-specific igf1 deletion, which dramatically reduces IGF-1 plasma levels, AMPK activity and autophagy were increased, and significant heart weight loss was observed in comparison with wild-type starved animals, revealing the importance of IGF-1 in maintaining cardiac adaptability to nutritional insults in vivo. CONCLUSION: Our data support the cardioprotective actions of IGF-1, which, by rescuing the mitochondrial metabolism and the energetic state of cells, reduces cell death and controls the potentially harmful autophagic response to nutritional challenges. IGF-1, therefore, may prove beneficial to mitigate damage induced by excessive nutrient-related stress, including ischaemic disease in multiple tissues.

Reversibility of adverse, calcineurin-dependent cardiac remodeling.

RATIONALE: Studies to dissect the role of calcineurin in pathological cardiac remodeling have relied heavily on murine models, in which genetic gain- and loss-of-function manipulations are initiated at or before birth. However, the great majority of clinical cardiac pathology occurs in adults. Yet nothing is known about the effects of calcineurin when its activation commences in adulthood. Furthermore, despite the fact that ventricular hypertrophy is a well-established risk factor for heart failure, the relative pace and progression of these 2 major phenotypic features of heart disease are unknown. Finally, even though therapeutic interventions in adults are designed to slow, arrest, or reverse disease pathogenesis, little is known about the capacity for spontaneous reversibility of calcineurin-dependent pathological remodeling. OBJECTIVE: We set out to address these 3 questions by studying mice engineered to harbor in cardiomyocytes a constitutively active calcineurin transgene driven by a tetracycline-responsive promoter element. METHODS AND RESULTS: Expression of the mutant calcineurin transgene was initiated for variable lengths of time to determine the natural history of disease pathogenesis, and to determine when, if ever, these events are reversible. Activation of the calcineurin transgene in adult mice triggered rapid and robust cardiac growth with features characteristic of pathological hypertrophy. Concentric hypertrophy preceded the development of systolic dysfunction, fetal gene activation, fibrosis, and clinical heart failure. Furthermore, cardiac hypertrophy reversed spontaneously when calcineurin activity was turned off, and expression of fetal genes reverted to baseline. Fibrosis, a prominent feature of pathological cardiac remodeling, manifested partial reversibility. CONCLUSIONS: Together, these data establish and define the deleterious effects of calcineurin signaling in the adult heart and reveal that calcineurin-dependent hypertrophy with concentric geometry precedes systolic dysfunction and heart failure. Furthermore, these findings demonstrate that during much of the disease process, calcineurin-dependent remodeling remains reversible.

Histone deacetylase (HDAC) inhibitors attenuate cardiac hypertrophy by suppressing autophagy.

Histone deacetylases (HDACs) regulate cardiac plasticity; however, their molecular targets are unknown. As autophagy contributes to pathological cardiac remodeling, we hypothesized that HDAC inhibitors target autophagy. The prototypical HDAC inhibitor (HDACi), trichostatin A (TSA), attenuated both load- and agonist-induced hypertrophic growth and abolished the associated activation of autophagy. Phenylephrine (PE)-triggered hypertrophy and autophagy in cultured cardiomyocytes were each blocked by a panel of structurally distinct HDAC inhibitors. RNAi-mediated knockdown of either Atg5 or Beclin 1, two essential autophagy effectors, was similarly capable of suppressing ligand-induced autophagy and myocyte growth. RNAi experiments uncovered the class I isoforms HDAC1 and HDAC2 as required for the autophagic response. To test the functional requirement of autophagic activation, we studied mice that overexpress Beclin 1 in cardiomyocytes. In these animals with a fourfold amplified autophagic response to TAC, TSA abolished TAC-induced increases in autophagy and blunted load-induced hypertrophy. Finally, we subjected animals with preexisting hypertrophy to HDACi, finding that ventricular mass reverted to near-normal levels and ventricular function normalized completely. Together, these data implicate autophagy as an obligatory element in pathological cardiac remodeling and point to HDAC1/2 as required effectors. Also, these data reveal autophagy as a previously unknown target of HDAC inhibitor therapy.

Sustained hemodynamic stress disrupts normal circadian rhythms in calcineurin-dependent signaling and protein phosphorylation in the heart.

RATIONALE: Despite overwhelming evidence of the importance of circadian rhythms in cardiovascular health and disease, little is known regarding the circadian regulation of intracellular signaling pathways controlling cardiac function and remodeling. OBJECTIVE: To assess circadian changes in processes dependent on the protein phosphatase calcineurin, relative to changes in phosphorylation of cardiac proteins, in normal, hypertrophic, and failing hearts. METHODS AND RESULTS: We found evidence of large circadian oscillations in calcineurin-dependent activities in the left ventricle of healthy C57BL/6 mice. Calcineurin-dependent transcript levels and nuclear occupancy of the NFAT (nuclear factor of activated T cells) regularly fluctuated as much as 20-fold over the course of a day, peaking in the morning when mice enter a period of rest. Phosphorylation of the protein phosphatase 1 inhibitor 1 (I-1), a direct calcineurin substrate, and phospholamban, an indirect target, oscillated directly out of phase with calcineurin-dependent signaling. Using a surgical model of cardiac pressure overload, we found that although calcineurin-dependent activities were markedly elevated, the circadian pattern of activation was maintained, whereas, oscillations in phospholamban and I-1 phosphorylation were lost. Changes in the expression of fetal gene markers of heart failure did not mirror the rhythm in calcineurin/NFAT activation, suggesting that these may not be direct transcriptional target genes. Cardiac function in mice subjected to pressure overload was significantly lower in the morning than in the evening when assessed by echocardiography. CONCLUSIONS: Normal, opposing circadian oscillations in calcineurin-dependent activities and phosphorylation of proteins that regulate contractility are disrupted in heart failure.

Cardiac myosin light chain kinase is necessary for myosin regulatory light chain phosphorylation and cardiac performance in vivo.

In contrast to studies on skeletal and smooth muscles, the identity of kinases in the heart that are important physiologically for direct phosphorylation of myosin regulatory light chain (RLC) is not known. A Ca(2+)/calmodulin-activated myosin light chain kinase is expressed only in cardiac muscle (cMLCK), similar to the tissue-specific expression of skeletal muscle MLCK and in contrast to the ubiquitous expression of smooth muscle MLCK. We have ablated cMLCK expression in male mice to provide insights into its role in RLC phosphorylation in normally contracting myocardium. The extent of RLC phosphorylation was dependent on the extent of cMLCK expression in both ventricular and atrial muscles. Attenuation of RLC phosphorylation led to ventricular myocyte hypertrophy with histological evidence of necrosis and fibrosis. Echocardiography showed increases in left ventricular mass as well as end-diastolic and end-systolic dimensions. Cardiac performance measured as fractional shortening decreased proportionally with decreased cMLCK expression culminating in heart failure in the setting of no RLC phosphorylation. Hearts from female mice showed similar responses with loss of cMLCK associated with diminished RLC phosphorylation and cardiac hypertrophy. Isoproterenol infusion elicited hypertrophic cardiac responses in wild type mice. In mice lacking cMLCK, the hypertrophic hearts showed no additional increases in size with the isoproterenol treatment, suggesting a lack of RLC phosphorylation blunted the stress response. Thus, cMLCK appears to be the predominant protein kinase that maintains basal RLC phosphorylation that is required for normal physiological cardiac performance in vivo.

FoxO, autophagy, and cardiac remodeling.

In response to changes in workload, the heart grows or shrinks. Indeed, the myocardium is capable of robust and rapid structural remodeling. In the setting of normal, physiological demand, the heart responds with hypertrophic growth of individual cardiac myocytes, a process that serves to maintain cardiac output and minimize wall stress. However, disease-related stresses, such as hypertension or myocardial infarction, provoke a series of changes that culminate in heart failure and/or sudden death. At the other end of the spectrum, cardiac unloading, such as occurs with prolonged bed rest or weightlessness, causes the heart to shrink. In recent years, considerable strides have been made in deciphering the molecular and cellular events governing pro- and anti-growth events in the heart. Prominent among these mechanisms are those mediated by FoxO (Forkhead box-containing protein, O subfamily) transcription factors. In many cell types, these proteins are critical regulators of cell size, viability, and metabolism, and their importance in the heart is just emerging. Also in recent years, evidence has emerged for a pivotal role for autophagy, an evolutionarily conserved pathway of lysosomal degradation of damaged proteins and organelles, in cardiac growth and remodeling. Indeed, evidence for activated autophagy has been detected in virtually every form of myocardial disease. Now, it is clear that FoxO is an upstream regulator of both autophagy and the ubiquitin-proteasome system. Here, we discuss recent advances in our understanding of cardiomyocyte autophagy, its governance by FoxO, and the roles each of these plays in cardiac remodeling.

Diabetic Cardiomyopathy: Mechanisms and Therapeutic Targets.

The incidence and prevalence of diabetes mellitus are each increasing rapidly in our society. The majority of patients with diabetes succumb ultimately to heart disease, much of which stems from atherosclerotic disease and hypertension. However, cardiomyopathy can develop independent of elevated blood pressure or coronary artery disease, a process termed diabetic cardiomyopathy. This disorder is a complex diabetes-associated process characterized by significant changes in the physiology, structure, and mechanical function of the heart. Here, we review recently derived insights into mechanisms and molecular events involved in the pathogenesis of diabetic cardiomyopathy.