Targets, trafficking, and timing of cardiac autophagy

Circadian regulation of cardiac muscle function and protein degradation

The circadian clock plays a fundamental role in physiology. In particular, the heart is a target organ where the clock orchestrates various aspects of cardiac function. At the molecular level, the clock machinery governs daily rhythms of gene expression. Such circadian regulation is in tune with the dynamic nature of heart structure and function, and provides the foundation for chronotherapeutic applications in cardiovascular diseases. In comparison, a regulatory role of the clock in cardiac protein degradation is poorly documented. Sarcomere is the structural and functional unit responsible for cardiac muscle contraction, and sarcomere components are closely regulated by protein folding and proteolysis. Emerging evidence supports a role of the circadian clock in governing sarcomere integrity and function. Particularly, recent studies uncovered a circadian regulation of a core sarcomere component TCAP. It is possible that circadian regulation of the cardiac muscle protein turnover is a key regulatory mechanism underlying cardiac remodeling in response to physiological and environmental stimuli. While the detailed regulatory mechanisms and the molecular links to cardiac (patho)physiology remain to be further studied, therapeutic strategies targeting circadian control in the heart may markedly enhance intervention outcomes against cardiovascular disease.

Ang II Promotes Cardiac Autophagy and Hypertrophy via Orai1/STIM1

The pathophysiology of cardiac hypertrophy is complex and multifactorial. Both the store-operated Ca²⁺ entry (SOCE) and excessive autophagy are the major causative factors for pathological cardiac hypertrophy. However, it is unclear whether these two causative factors are interdependent. In this study, we examined the functional role of SOCE and Orai1 in angiotensin II (Ang II)-induced autophagy and hypertrophy using in vitro neonatal rat cardiomyocytes (NRCMs) and in vivo mouse model, respectively. We show that YM-58483 or SKF-96365 mediated pharmacological inhibition of SOCE, or silencing of Orai1 with Orail-siRNA inhibited Ang II-induced cardiomyocyte autophagy both in vitro and in vivo. Also, the knockdown of Orai1 attenuated Ang II-induced pathological cardiac hypertrophy. Together, these data suggest that Ang II promotes excessive cardiomyocyte autophagy through SOCE/Orai1 which can be the prime contributing factors in cardiac hypertrophy.

TRPM2 promotes autophagic degradation in vascular smooth muscle cells

Transient receptor potential channel M2 (TRPM2) is a Ca²⁺-permeable channel that is activated by reactive oxygen species (ROS). In many cell types, ROS activate TRPM2 to induce excessive Ca²⁺ influx, resulting in Ca²⁺ overload and consequent cell death. Recent studies suggest that TRPM2 may also regulate autophagy in pericytes and cancer cells by acting on the early step of autophagy, i.e. autophagic induction. However, there is no report on the role of TRPM2 in autophagic degradation, which is the late stage of autophagy. In the present study, we found abundant TRPM2 expression in lysosomes/autolysosomes in the primary cultured mouse aortic smooth muscle cells (mASMCs). Nutrient starvation stimulated autophagic flux in mASMCs mainly by promoting autophagic degradation. This starvation-induced autophagic degradation was reduced by TRPM2 knockout. Importantly, starvation-induced lysosomal/autolysosomal acidification and cell death were also substantially reduced by TRPM2 knockout. Taken together, the present study uncovered a novel mechanism that lysosomal TRPM2 facilitates lysosomal acidification to stimulate excessive autolysosome degradation and consequent cell death.

Rheostatic Balance of Circadian Rhythm and Autophagy in Metabolism and Disease

Circadian rhythms are physical, behavioral and environmental cycles that respond primarily to light and dark, with a period of time of approximately 24 h. The most essential physiological functions of mammals are manifested in circadian rhythm patterns, including the sleep-wake cycle and nutrient and energy metabolism. Autophagy is a conserved biological process contributing to nutrient and cellular homeostasis. The factors affecting autophagy are numerous, such as diet, drugs, and aging. Recent studies have indicated that autophagy is activated rhythmically in a clock-dependent manner whether the organism is healthy or has certain diseases. In addition, autophagy can affect circadian rhythm by degrading circadian proteins. This review discusses the interaction and mechanisms between autophagy and circadian rhythm. Moreover, we introduce the molecules influencing both autophagy and circadian rhythm. We then discuss the drugs affecting the circadian rhythm of autophagy. Finally, we present the role of rhythmic autophagy in nutrient and energy metabolism and its significance in physiology and metabolic disease.

PKD deletion promotes autophagy and inhibits hypertrophy in cardiomyocyte

Protein kinase D (PKD) plays an important role in the development of cardiac hypertrophy induced by pressure overload. However, the mechanism involved is unclear. This study, using primary cardiomyocyte culture, PKD knockdown and overexpression, and other molecular techniques, tested our hypothesis that PKD pathway mediates cardiac hypertrophy by negatively regulating autophagy in cardiomyocyte. Neonatal cardiomyocytes were isolated from Wistar rats and cell hypertrophy was induced by norepinephrine treatment (PE, 10^-4 mol/L), and divided into the following groups: (1) Vehicle; (2) PE; (3) PE + control siRNA; (4) PE + Rapamycin (100 nM); (5) PE + PKD-siRNA (2 × 10⁸ U/0.1 ml); (6) PE + PKD siRNA + 3 MA (10 mM). The results showed that PE treatment induced cardiomyocyte hypertrophy, which were confirmed by cell size and biomarkers of cardiomyocyte hypertrophy including increased ANP and BNP mRNA. PKD knockdown or Rapamycin significantly inhibited PE-induced cardiomyocyte hypertrophy. In addition, PKD siRNA increased autophagy activity determined by electron microscopy, increased biomarkers of autophagy by Western blot, accompanied by down-regulated AKT/mTOR/S6K pathway. All the effects of PKD knockout were inhibited by co-treatment with 3-MA, an autophagy inhibitor. Oppositely, the autophagy in cardiomyocytes was inhibited by PKD overexpression. These results suggest that PKD participates in the development of cardiac hypertrophy by regulating autophagy via AKT/mTOR/S6K pathway.

MicroRNAs in Cardiac Autophagy: Small Molecules and Big Role

Autophagy, which is an evolutionarily conserved process according to the lysosomal degradation of cellular components, plays a critical role in maintaining cell homeostasis. Autophagy and mitochondria autophagy (mitophagy) contribute to the preservation of cardiac homeostasis in physiological settings. However, impaired or excessive autophagy is related to a variety of diseases. Recently, a close link between autophagy and cardiac disorders, including myocardial infarction, cardiac hypertrophy, cardiomyopathy, cardiac fibrosis, and heart failure, has been demonstrated. MicroRNAs (miRNAs) are a class of small non-coding RNAs with a length of approximately 21–22 nucleotides (nt), which are distributed widely in viruses, plants, protists, and animals. They function in mediating the post-transcriptional gene silencing. A growing number of studies have demonstrated that miRNAs regulate cardiac autophagy by suppressing the expression of autophagy-related genes in a targeted manner, which are involved in the pathogenesis of heart diseases. This review summarizes the role of microRNAs in cardiac autophagy and related cardiac disorders. Furthermore, we mainly focused on the autophagy regulation pathways, which consisted of miRNAs and their targeted genes.

Polycystin-2 Plays an Essential Role in Glucose Starvation-Induced Autophagy in Human Embryonic Stem Cell-Derived Cardiomyocytes

Autophagy is a process essential for cell survival under stress condition. The patients with autosomal dominant polycystic kidney disease, which is caused by polycystin-1 or polycystin-2 (PKD2) mutation, display cardiovascular abnormalities and dysregulation in autophagy. However, it is unclear whether PKD2 plays a role in autophagy. In the present study, we explored the functional role of PKD2 in autophagy and apoptosis in human embryonic stem cell-derived cardiomyocytes. HES2 hESC line-derived cardiomyocytes (HES2-CMs) were transduced with adenoviral-based PKD2-shRNAs (Ad-PKD2-shRNAs), and then cultured with normal or glucose-free medium for 3 hours. Autophagy was upregulated in HES2-CMs under glucose starvation, as indicated by increased microtubule-associated protein 1 light chain 3-II level in immunoblots and increased autophagosome and autolysosome formation. Knockdown of PKD2 reduced the autophagic flux and increased apoptosis under glucose starvation. In Ca²⁺ measurement, Ad-PKD2-shRNAs reduced caffeine-induced cytosolic Ca²⁺ rise. Co-immunoprecipitation and in situ proximity ligation assay demonstrated an increased physical interaction of PKD2 with ryanodine receptor 2 (RyR2) under glucose starvation condition. Furthermore, Ad-PKD2-shRNAs substantially attenuated the starvation-induced activation of AMP-activated protein kinase (AMPK) and inactivation of mammalian target of rapamycin (mTOR). The present study for the first time demonstrates that PKD2 functions to promote autophagy under glucose starvation, thereby protects cardiomyocytes from apoptotic cell death. The mechanism may involve PKD2 interaction with RyR2 to alter Ca²⁺ release from sarcoplasmic reticulum, consequently modulating the activity of AMPK and mTOR, resulting in alteration of autophagy and apoptosis. Stem Cells 2018;36:501-513.

TM9SF4 is a novel factor promoting autophagic flux under amino acid starvation

Autophagy is a highly complicated process with participation of large numbers of autophagy-related proteins. Under nutrient starvation, autophagy promotes cell survival by breaking down nonessential cellular components for recycling use. However, due to its high complexity, molecular mechanism of autophagy is still not fully understood. In the present study, we report a novel autophagy-related protein TM9SF4, which plays a functional role in the induction phase of autophagic process. TM9SF4 proteins were abundantly expressed in the kidney, especially in renal proximal tubular epithelial cells. At subcellular cells, TM9SF4 proteins were mostly localized in lysosome, Golgi, late endosome and autophagosome. Knockdown of TM9SF4 with TM9SF4-shRNAs markedly reduced the starvation-induced autophagy in HEK293 cells, the effect of which persisted in the presence of bafilomycin A1. TM9SF4-shRNAs also substantially attenuated the starvation-induced mTOR inactivation. In animal model, starvation was able to induce LC3-II accumulation and cause mTOR inactivation in renal cortical tissue in wild-type mice, the effect of which was minimal/absent in TM9SF4 knockout (TM9SF4^-/-) mice. Co-immunoprecipitation and proximity ligation assay demonstrated physical interaction of TM9SF4 proteins with mTOR. In addition, knockdown or knockout of TM9SF4 reduced the starvation-induced cell death in HEK293 cells and animal model. Taken together, the present study identifies TM9SF4 as a novel autophagy-related protein. Under nutrient starvation, TM9SF4 functions to facilitate mTOR inactivation, resulting in an enhanced autophagic flux, which serves to protect cells from apoptotic cell death.

Calcineurin signaling in the heart: The importance of time and place

The calcium-activated protein phosphatase, calcineurin, lies at the intersection of protein phosphorylation and calcium signaling cascades, where it provides an essential nodal point for coordination between these two fundamental modes of intracellular communication. In excitatory cells, such as neurons and cardiomyocytes, that experience rapid and frequent changes in cytoplasmic calcium, calcineurin protein levels are exceptionally high, suggesting that these cells require high levels of calcineurin activity. Yet, it is widely recognized that excessive activation of calcineurin in the heart contributes to pathological hypertrophic remodeling and the progression to failure. How does a calcium activated enzyme function in the calcium-rich environment of the continuously contracting heart without pathological consequences? This review will discuss the wide range of calcineurin substrates relevant to cardiovascular health and the mechanisms calcineurin uses to find and act on appropriate substrates in the appropriate location while potentially avoiding others. Fundamental differences in calcineurin signaling in neonatal verses adult cardiomyocytes will be addressed as well as the importance of maintaining heterogeneity in calcineurin activity across the myocardium. Finally, we will discuss how circadian oscillations in calcineurin activity may facilitate integration with other essential but conflicting processes, allowing a healthy heart to reap the benefits of calcineurin signaling while avoiding the detrimental consequences of sustained calcineurin activity that can culminate in heart failure.

The role of autophagy in angiotensin II-induced pathological cardiac hypertrophy

Pathological cardiac hypertrophy is associated with nearly all forms of heart failure. It develops in response to disorders such as coronary artery disease, hypertension and myocardial infarction. Angiotensin II (Ang II) has direct effects on the myocardium and promotes hypertension. Chronic elevation of Ang II can lead to pathological cardiac hypertrophy and cardiac failure. Autophagy is an important process in the pathogenesis of cardiovascular diseases. Under physiological conditions, autophagy is an essential homeostatic mechanism to maintain the global cardiac structure function by ridding damaged cells or unwanted macromolecules and organelles. Dysregulation of autophagy may play an important role in Ang II-induced cardiac hypertrophy although conflicting reports on the effects of Ang II on autophagy and cardiac hypertrophy exist. Some studies showed that autophagy activation attenuated Ang II-induced cardiac dysfunction. Others suggested that inhibition of the Ang II induced autophagy should be protective. The discrepancies may be due to different model systems and different signaling pathway involved. Ang II-induced cardiac hypertrophy may be alleviated through regulation of autophagy. This review focuses on Ang II to highlight the molecular targets and pathways identified in the prevention and treatment of Ang II-induced pathological cardiac hypertrophy by regulating autophagy.

Genetic Variants on Chromosome 1p13.3 Are Associated with Non-ST Elevation Myocardial Infarction and the Expression of DRAM2 in the Finnish Population

Myocardial infarction (MI) is divided into either ST elevation MI (STEMI) or non-ST elevation MI (NSTEMI), differing in a number of clinical characteristics. We sought to identify genetic variants conferring risk to NSTEMI or STEMI by conducting a genome-wide association study (GWAS) of MI stratified into NSTEMI and STEMI in a consecutive sample of 1,579 acute MI cases with 1,576 controls. Subsequently, we followed the results in an independent population-based sample of 562 cases and 566 controls, a partially independent prospective cohort (N = 16,627 with 163 incident NSTEMI cases), and examined the effect of disease-associated variants on gene expression in 513 healthy participants. Genetic variants on chromosome 1p13.3 near the damage-regulated autophagy modulator 2 gene DRAM2 associated with NSTEMI (rs656843; odds ratio 1.57, P = 3.11 × 10⁻¹⁰) in the case-control analysis with a consistent but not statistically significant effect in the prospective cohort (rs656843; hazard ratio 1.13, P = 0.43). These variants were not associated with STEMI (rs656843; odds ratio, 1.11, P = 0.20; hazard ratio 0.97, P = 0.87), appearing to have a pronounced effect on NSTEMI risk. A majority of the variants at 1p13.3 associated with NSTEMI were also associated with the expression level of DRAM2 in blood leukocytes of healthy controls (top-ranked variant rs325927, P = 1.50 × 10⁻¹²). The results suggest that genetic factors may in part influence whether coronary artery disease results in NSTEMI rather than STEMI.

Measuring cardiac autophagic flux in vitro and in vivo

Autophagy is a lysosomal-dependent catabolic pathway that recycles various cytoplasmic-borne components, such as organelles and proteins, through the lysosomes. This process creates energy and biomolecules that are used to maintain homeostasis and to serve as an energy source under conditions of acute stress. Autophagic flux is a measure of efficiency or throughput of the pathway. Here, we describe a method for determining autophagic flux in vitro and in vivo using the autophagosomal/lysosomal fusion inhibitors chloroquine or bafilomycin A1 and then probing for the autophagosomal marker LC3-II via Western Blot.

Mitochondrial maintenance failure in aging and role of sexual dimorphism

Gene expression changes during aging are partly conserved across species, and suggest that oxidative stress, inflammation and proteotoxicity result from mitochondrial malfunction and abnormal mitochondrial-nuclear signaling. Mitochondrial maintenance failure may result from trade-offs between mitochondrial turnover versus growth and reproduction, sexual antagonistic pleiotropy and genetic conflicts resulting from uni-parental mitochondrial transmission, as well as mitochondrial and nuclear mutations and loss of epigenetic regulation. Aging phenotypes and interventions are often sex-specific, indicating that both male and female sexual differentiation promote mitochondrial failure and aging. Studies in mammals and invertebrates implicate autophagy, apoptosis, AKT, PARP, p53 and FOXO in mediating sex-specific differences in stress resistance and aging. The data support a model where the genes Sxl in Drosophila, sdc-2 in Caenorhabditis elegans, and Xist in mammals regulate mitochondrial maintenance across generations and in aging. Several interventions that increase life span cause a mitochondrial unfolded protein response (UPRmt), and UPRmt is also observed during normal aging, indicating hormesis. The UPRmt may increase life span by stimulating mitochondrial turnover through autophagy, and/or by inhibiting the production of hormones and toxic metabolites. The data suggest that metazoan life span interventions may act through a common hormesis mechanism involving liver UPRmt, mitochondrial maintenance and sexual differentiation.

The need to recognize the pulmonary circulation and the right ventricle as an integrated functional unit: facts and hypotheses (2013 Grover Conference series)

For many patients with severe pulmonary arterial hypertension, heart failure-and, in particular, right heart failure-is the final chapter of their chronic illness. Targeted therapy for pulmonary hypertension is effective only if the right ventricular ejection fraction is maintained or improved. Because improvement of right heart function and reversal of right heart failure are treatment goals, it is important to investigate the cellular and molecular mechanisms that cause right heart failure. Here, we propose that right ventricular capillary rarefaction is an important hallmark of right heart failure and consider that the "sick lung circulation" and the pressure-overloaded right ventricle constitute a functional unit.

Inhibition of autophagy promoted sphingosylphosphorylcholine induced cell death in non-small cell lung cancer cells

Sphingosylphosphorylcholine (SPC) is a bioactive lipid mediated popular cell apoptosis in cancer cells. As a cell-specific sphingolipid, its function in lung cancer cells is unknown. Here we showed that SPC treatment triggered necrosis and autophagy but inhibited apoptosis in two non-small cell lung cancer cell lines: A549 cell line and H157 cell line. Then 3-methyladenine (3-MA), an autophagy inhibitor, was introduced to clarify the relationships between autophagy and necrosis or apoptosis. 3MA suppressed the survival furtherly by promoting apoptosis while had no influence on necrosis. Subsequent studies revealed that activity of AKT and mammalian target of rapamycin (mTOR) complex 1 (mTORC1) were downregulated during autophagy. Furthermore, SPC failed to promote autophagy in p53 deleted cells. Thus SPC induced autophagy in non-small cell lung cancer cells was through AKT/mTORC1 and P53 signal pathway. Besides, SPC reduced both the mitochondria membrane potential and ROS level in A549 cells. These findings provided a molecular basis of SPC-stimulated A549 cell death and support the notion that inhibition of autophagy is likely a novel anticancer mechanism.

Proteotoxicity: an underappreciated pathology in cardiac disease

In general, in most organ systems, intracellular protein homeostasis is the sum of many factors, including chromosomal state, protein synthesis, post-translational processing and transport, folding, assembly and disassembly into macromolecular complexes, protein stability and clearance. In the heart, there has been a focus on the gene programs that are activated during pathogenic processes, but the removal of damaged proteins and organelles has been underappreciated as playing an important role in the pathogenesis of heart disease. Proteotoxicity refers to the adverse effects of damaged or misfolded proteins and even organelles on the cell. At the cellular level, the ultimate outcome of uncontrolled or severe proteotoxicity is cell death; hence, the pathogenic impact of proteotoxicity is maximally manifested in organs with no or very poor regenerative capability such as the brain and the heart. Evidence for increased cardiac proteotoxicity is rapidly mounting for a large subset of congenital and acquired human heart disease. Studies carried out in animal models and in cell culture have begun to establish both sufficiency and, in some cases, the necessity of proteotoxicity as a major pathogenic factor in the heart. This dictates rigorous testing for the efficacy of proteotoxic attenuation as a new strategy to treat heart disease. This review article highlights some recent advances in our understanding of how misfolded proteins can injure and are handled in the cell, examining the emerging evidence for targeting proteotoxicity as a new therapeutic strategy for heart disease. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy."

Reprogramming and carcinogenesis--parallels and distinctions

Rapid progress made in various areas of regenerative medicine in recent years occurred both at the cellular level, with the Nobel prize-winning discovery of reprogramming (generation of induced pluripotent stem (iPS) cells) and also at the biomaterial level. The use of four transcription factors, Oct3/4, Sox2, c-Myc, and Klf4 (called commonly "Yamanaka factors") for the conversion of differentiated cells, back to the pluripotent/embryonic stage, has opened virtually endless and ethically acceptable source of stem cells for medical use. Various types of stem cells are becoming increasingly popular as starting components for the development of replacement tissues, or artificial organs. Interestingly, many of the transcription factors, key to the maintenance of stemness phenotype in various cells, are also overexpressed in cancer (stem) cells, and some of them may find the use as prognostic factors. In this review, we describe various methods of iPS creation, followed by overview of factors known to interfere with the efficiency of reprogramming. Next, we discuss similarities between cancer stem cells and various stem cell types. Final paragraphs are dedicated to interaction of biomaterials with tissues, various adverse reactions generated as a result of such interactions, and measures available, that allow for mitigation of such negative effects.

Cardiac metabolism and its interactions with contraction, growth, and survival of cardiomyocytes

The network for cardiac fuel metabolism contains intricate sets of interacting pathways that result in both ATP-producing and non-ATP-producing end points for each class of energy substrates. The most salient feature of the network is the metabolic flexibility demonstrated in response to various stimuli, including developmental changes and nutritional status. The heart is also capable of remodeling the metabolic pathways in chronic pathophysiological conditions, which results in modulations of myocardial energetics and contractile function. In a quest to understand the complexity of the cardiac metabolic network, pharmacological and genetic tools have been engaged to manipulate cardiac metabolism in a variety of research models. In concert, a host of therapeutic interventions have been tested clinically to target substrate preference, insulin sensitivity, and mitochondrial function. In addition, the contribution of cellular metabolism to growth, survival, and other signaling pathways through the production of metabolic intermediates has been increasingly noted. In this review, we provide an overview of the cardiac metabolic network and highlight alterations observed in cardiac pathologies as well as strategies used as metabolic therapies in heart failure. Lastly, the ability of metabolic derivatives to intersect growth and survival are also discussed.