Julie R. McMullen

Akt/Protein Kinase B Promotes Organ Growth in Transgenic Mice

ABSTRACT One of the least-understood areas in biology is the determination of the size of animals and their organs. In Drosophila, components of the insulin receptor phosphoinositide 3-kinase (PI3K) pathway determine body, organ, and cell size. Several biochemical studies have suggested that Akt/protein kinase B is one of the important downstream targets of PI3K. To examine the role of Akt in the regulation of organ size in mammals, we have generated and characterized transgenic mice expressing constitutively active Akt (caAkt) or kinase-deficient Akt (kdAkt) specifically in the heart. The heart weight of caAkt transgenic mice was increased 2.0-fold compared with that of nontransgenic mice. The increase in heart size was associated with a comparable increase in myocyte cell size in caAkt mice. The kdAkt mutant protein attenuated the constitutively active PI3K-induced overgrowth of the heart, and the caAkt mutant protein circumvented cardiac growth retardation induced by a kinase-deficient PI3K mutant protein. Rapamycin attenuated caAkt-induced overgrowth of the heart, suggesting that the mammalian target of rapamycin (mTOR) or effectors of mTOR mediated caAkt-induced heart growth. In conclusion, Akt is sufficient to induce a marked increase in heart size and is likely to be one of the effectors of the PI3K pathway in mediating heart growth.

Phosphoinositide 3-kinase(p110α) plays a critical role for the induction of physiological, but not pathological, cardiac hypertrophy

An unresolved question in cardiac biology is whether distinct signaling pathways are responsible for the development of pathological and physiological cardiac hypertrophy in the adult. Physiological hypertrophy is characterized by a normal organization of cardiac structure and normal or enhanced cardiac function, whereas pathological hypertrophy is associated with an altered pattern of cardiac gene expression, fibrosis, cardiac dysfunction, and increased morbidity and mortality. The elucidation of signaling cascades that play distinct roles in these two forms of hypertrophy will be critical for the development of more effective strategies to treat heart failure. We examined the role of the p110α isoform of phosphoinositide 3-kinase (PI3K) for the induction of pathological hypertrophy (pressure overload-induced) and physiological hypertrophy (exercise-induced) by using transgenic mice expressing a dominant negative (dn) PI3K(p110α) mutant specifically in the heart. dnPI3K transgenic mice displayed significant hypertrophy in response to pressure overload but not exercise training. dnPI3K transgenic mice also showed significant dilation and cardiac dysfunction in response to pressure overload. Thus, PI3K(p110α) appears to play a critical role for the induction of physiological cardiac growth but not pathological growth. PI3K(p110α) also appears essential for maintaining contractile function in response to pathological stimuli.

Differences between pathological and physiological cardiac hypertrophy: novel therapeutic strategies to treat heart failure.

1 In general, cardiac hypertrophy (an increase in heart mass) is a poor prognostic sign. Cardiac enlargement is a characteristic of most forms of heart failure. Cardiac hypertrophy that occurs in athletes (physiological hypertrophy) is a notable exception. 2 Physiological cardiac hypertrophy in response to exercise training differs in its structural and molecular profile to pathological hypertrophy associated with pressure or volume overload in disease. Physiological hypertrophy is characterized by normal organization of cardiac structure and normal or enhanced cardiac function, whereas pathological hypertrophy is commonly associated with upregulation of fetal genes, fibrosis, cardiac dysfunction and increased mortality. 3 It is now clear that several signalling molecules play unique roles in the regulation of pathological and physiological cardiac hypertrophy. 4 The present review discusses the possibility of targeting cardioprotective signalling pathways and genes activated in the athletes heart to treat or prevent heart failure.

Therapeutic inhibition of the miR-34 family attenuates pathological cardiac remodeling and improves heart function

MicroRNAs are dysregulated in a setting of heart disease and have emerged as promising therapeutic targets. MicroRNA-34 family members (miR-34a, -34b, and -34c) are up-regulated in the heart in response to stress. In this study, we assessed whether inhibition of the miR-34 family using an s.c.-delivered seed-targeting 8-mer locked nucleic acid (LNA)-modified antimiR (LNA-antimiR-34) can provide therapeutic benefit in mice with preexisting pathological cardiac remodeling and dysfunction due to myocardial infarction (MI) or pressure overload via transverse aortic constriction (TAC). An additional cohort of mice subjected to MI was given LNA-antimiR-34a (15-mer) to inhibit miR-34a alone as a comparison for LNA-antimiR-34. LNA-antimiR-34 (8-mer) efficiently silenced all three miR-34 family members in both cardiac stress models and attenuated cardiac remodeling and atrial enlargement. In contrast, inhibition of miR-34a alone with LNA-antimiR-34a (15-mer) provided no benefit in the MI model. In mice subjected to pressure overload, LNA-antimiR-34 improved systolic function and attenuated lung congestion, associated with reduced cardiac fibrosis, increased angiogenesis, increased Akt activity, decreased atrial natriuretic peptide gene expression, and maintenance of sarcoplasmic reticulum Ca2+ ATPase gene expression. Improved outcome in LNA-antimiR-34–treated MI and TAC mice was accompanied by up-regulation of several direct miR-34 targets, including vascular endothelial growth factors, vinculin, protein O-fucosyltranferase 1, Notch1, and semaphorin 4B. Our results provide evidence that silencing of the entire miR-34 family can protect the heart against pathological cardiac remodeling and improve function. Furthermore, these data underscore the utility of seed-targeting 8-mer LNA-antimiRs in the development of new therapeutic approaches for pharmacologic inhibition of disease-implicated miRNA seed families.

Protective effects of exercise and phosphoinositide 3-kinase(p110α) signaling in dilated and hypertrophic cardiomyopathy

Physical activity protects against cardiovascular disease, and physiological cardiac hypertrophy associated with regular exercise is usually beneficial, in marked contrast to pathological hypertrophy associated with disease. The p110α isoform of phosphoinositide 3-kinase (PI3K) plays a critical role in the induction of exercise-induced hypertrophy. Whether it or other genes activated in the athletes heart might have an impact on cardiac function and survival in a setting of heart failure is unknown. To examine whether progressive exercise training and PI3K(p110α) activity affect survival and/or cardiac function in two models of heart disease, we subjected a transgenic mouse model of dilated cardiomyopathy (DCM) to swim training, genetically crossed cardiac-specific transgenic mice with increased or decreased PI3K(p110α) activity to the DCM model, and subjected PI3K(p110α) transgenics to acute pressure overload (ascending aortic constriction). Life-span, cardiac function, and molecular markers of pathological hypertrophy were examined. Exercise training and increased cardiac PI3K(p110α) activity prolonged survival in the DCM model by 15–20%. In contrast, reduced PI3K(p110α) activity drastically shortened lifespan by ≈50%. Increased PI3K(p110α) activity had a favorable effect on cardiac function and fibrosis in the pressure-overload model and attenuated pathological growth. PI3K(p110α) signaling negatively regulated G protein-coupled receptor stimulated extracellular responsive kinase and Akt (via PI3K, p110γ) activation in isolated cardiomyocytes. These findings suggest that exercise and enhanced PI3K(p110α) activity delay or prevent progression of heart disease, and that supraphysiologic activity can be beneficial. Identification of genes important for hypertrophy in the athletes heart could offer new strategies for treating heart failure.

Gata4 is required for maintenance of postnatal cardiac function and protection from pressure overload-induced heart failure

An important event in the pathogenesis of heart failure is the development of pathological cardiac hypertrophy. In cultured cardiomyocytes, the transcription factor Gata4 is required for agonist-induced hypertrophy. We hypothesized that, in the intact organism, Gata4 is an important regulator of postnatal heart function and of the hypertrophic response of the heart to pathological stress. To test this hypothesis, we studied mice heterozygous for deletion of the second exon of Gata4 (G4D). At baseline, G4D mice had mild systolic and diastolic dysfunction associated with reduced heart weight and decreased cardiomyocyte number. After transverse aortic constriction (TAC), G4D mice developed overt heart failure and eccentric cardiac hypertrophy, associated with significantly increased fibrosis and cardiomyocyte apoptosis. Inhibition of apoptosis by overexpression of the insulin-like growth factor 1 receptor prevented TAC-induced heart failure in G4D mice. Unlike WT-TAC controls, G4D-TAC cardiomyocytes hypertrophied by increasing in length more than width. Gene expression profiling revealed up-regulation of genes associated with apoptosis and fibrosis, including members of the TGF-β pathway. Our data demonstrate that Gata4 is essential for cardiac function in the postnatal heart. After pressure overload, Gata4 regulates the pattern of cardiomyocyte hypertrophy and protects the heart from load-induced failure.

PI3K(p110 alpha) Protects Against Myocardial Infarction-Induced Heart Failure Identification of PI3K-Regulated miRNA and mRNA

Objective—Myocardial infarction (MI) is a serious complication of atherosclerosis associated with increasing mortality attributable to heart failure. Activation of phosphoinositide 3-kinase [PI3K(p110&agr;)] is considered a new strategy for the treatment of heart failure. However, whether PI3K(p110&agr;) provides protection in a setting of MI is unknown, and PI3K(p110&agr;) is difficult to target because it has multiple actions in numerous cell types. The goal of this study was to assess whether PI3K(p110&agr;) is beneficial in a setting of MI and, if so, to identify cardiac-selective microRNA and mRNA that mediate the protective properties of PI3K(p110&agr;). Methods and Results—Cardiomyocyte-specific transgenic mice with increased or decreased PI3K(p110&agr;) activity (caPI3K-Tg and dnPI3K-Tg, respectively) were subjected to MI for 8 weeks. The caPI3K-Tg subjected to MI had better cardiac function than nontransgenic mice, whereas dnPI3K-Tg had worse function. Using microarray analysis, we identified PI3K-regulated miRNA and mRNA that were correlated with cardiac function, including growth factor receptor-bound 14. Growth factor receptor-bound 14 is highly expressed in the heart and positively correlated with PI3K(p110&agr;) activity and cardiac function. Mice deficient in growth factor receptor-bound 14 have cardiac dysfunction. Conclusion—Activation of PI3K(p110&agr;) protects the heart against MI-induced heart failure. Cardiac-selective targets that mediate the protective effects of PI3K(p110&agr;) represent new drug targets for heart failure.

Ibrutinib increases the risk of atrial fibrillation, potentially through inhibition of cardiac PI3K-Akt signaling

To the editor: The Bruton’s tyrosine kinase (BTK) inhibitor ibrutinib sets a new standard of care for the treatment of patients with relapsed or refractory chronic lymphocytic leukemia (CLL)[1][1],[2][2] and mantle cell lymphoma (MCL).[3][3] Ibrutinib is generally well tolerated and largely

Pathophysiology of cardiac hypertrophy and heart failure: signaling pathways and novel therapeutic targets

Abstract The onset of heart failure is typically preceded by cardiac hypertrophy, a response of the heart to increased workload, a cardiac insult such as a heart attack or genetic mutation. Cardiac hypertrophy is usually characterized by an increase in cardiomyocyte size and thickening of ventricular walls. Initially, such growth is an adaptive response to maintain cardiac function; however, in settings of sustained stress and as time progresses, these changes become maladaptive and the heart ultimately fails. In this review, we discuss the key features of pathological cardiac hypertrophy and the numerous mediators that have been found to be involved in the pathogenesis of cardiac hypertrophy affecting gene transcription, calcium handling, protein synthesis, metabolism, autophagy, oxidative stress and inflammation. We also discuss new mediators including signaling proteins, microRNAs, long noncoding RNAs and new findings related to the role of calcineurin and calcium-/calmodulin-dependent protein kinases. We also highlight mediators and processes which contribute to the transition from adaptive cardiac remodeling to maladaptive remodeling and heart failure. Treatment strategies for heart failure commonly include diuretics, angiotensin converting enzyme inhibitors, angiotensin II receptor blockers and β-blockers; however, mortality rates remain high. Here, we discuss new therapeutic approaches (e.g., RNA-based therapies, dietary supplementation, small molecules) either entering clinical trials or in preclinical development. Finally, we address the challenges that remain in translating these discoveries to new and approved therapies for heart failure.

Follistatin-mediated skeletal muscle hypertrophy is regulated by Smad3 and mTOR independently of myostatin

Smad3/Akt/mTOR/S6K/S6RP signaling plays a critical role in follistatin-mediated muscle growth that operates independently of myostatin-driven mechanisms.