Autophagy in Health and Disease

Roberta A. Gottlieb, Roberta A. Gottlieb

• Publisher: Academic Press
• Year: 2012
• ISBN:
  9780123851024
• Topic: Medicina
• Subtopic: Sanità, amministrazione e assistenza pubblica

Autophagy in Health and Disease offers an overview of the latest research in autophagy with a translational emphasis. This publication takes scientific research in autophagy a step further and offers integrated content with advancements in autophagy from cell biology and biochemical research to clinical treatments. A necessary reference for the bookshelf of medical and scientific researchers and students, Autophagy in Health and Disease presents high quality, reputable information on autophagy, allowing the reader quick access to the most applicable information.

• Discusses current understanding of the roles of autophagy in health and disease
• Covers the background of autophagy, the development of tools and therapeutics to measure and modulate autophagy, and autophagy in tissues and disease processes

SECTION II

Organ Systems

Chapter 3. Autophagy in the Cardiovascular System

Chapter 4. Autophagy in the Immune System

Chapter 5. Autophagy in the Gastrointestinal Tract

Chapter 6. Autophagy in the Homeostasis of Pancreatic β-Cells

Chapter 3

Autophagy in the Cardiovascular System

Kazuhiko Nishida, Ph.D.¹, Osamu Yamaguchi, M.D. Ph.D.¹ and Kinya Otsu, Ph.D.^1,2

¹Department of Cardiovascular Medicine, Osaka University Graduate School of Medicine, Suita, Osaka, Japan

²Correspondence to Kinya Otsu MD. PhD., Department of Cardiovascular Medicine, Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita 565-0871, Osaka, Japan

Introduction

Heart disease is the leading cause of death in the industrialized world¹. Heart disease of diverse sorts culminates in heart failure, a syndrome wherein the heart is unable to meet the metabolic demands of the body². Five million Americans currently suffer from chronic heart failure with a mortality of approximately 50% at five years. The heart is an organ of remarkable plasticity, and adult cardiomyocytes are terminally differentiated and replication-deficient². In response to pathological stress from neurohumoral activation, hypertension or other myocardial injury, the heart initially compensates with an adaptive hypertrophic increase in cardiac mass in order to normalize ventricular wall stress. However, cardiac hypertrophy is an independent risk factor for cardiovascular mortality³ and is a major factor for heart failure^{4, 5}. Under prolonged stress, the heart undergoes apparently irreversible changes culminating in heart failure. Although several theories exist regarding the mechanism governing the transition from hypertrophy to heart failure⁶, the precise mechanism remains elusive.

Autophagy has evolved as a conserving process for bulk degradation and recycling of cytoplasmic components, such as long-lived proteins and organelles^7-9. Autophagy is controlled by autophagy-related genes (Atgs), many of which, including Atg5, Atg7 and Beclin 1, are involved in autophagosome formation. Recent studies have demonstrated a variety of physiological and pathophysiological roles in autophagy such as adaptation to nutrient deprivation, intracellular clearance of proteins and organelles, elimination of micro-organelles including mitochondria, and maintenance of endoplasmic reticulum (ER). Paradoxically, autophagy also appears to modulate cell death through excessive self-digestion and degradation of essential cellular constituents^{10, 11}. In the heart, the level of autophagy is altered in response to stresses triggered by cardiovascular diseases such as cardiac hypertrophy¹² and heart failure^{13, 14}. The multiple features of apoptosis, necrosis and autophagy have been simultaneously observed in failing human hearts^{15, 16}. Since it is unclear whether autophagy is protective or detrimental, it is imperative to have a full understanding of the cell death mechanism regarding not only apoptosis and necrosis but also autophagy in cardiomyocytes.

Autophagy in the Heart at a Low Basal Level Under Normal Conditions

Cardiomyocytes are mitochondria-rich, and damaged mitochondria release proapoptotic factors such as cytochrome c¹⁷. Autophagy can remove the damaged mitochondria and prevent activation of apoptosis^{18, 19}.

In previous reports regarding autophagy in the heart’s basal state, LAMP2-deficient mice showed excessive accumulation of autophagic vacuoles and impaired autophagic degradation of long-lived proteins, resulting in cardiomyopathy^{14, 20}. In our examination, autophagy also appears to play a protective role under normal or mildly stressed conditions, in the heart’s so-called basal state. Temporally controlled and cardiac-specific Atg5-deficiency in tamoxifen-treated Atg5^flox/flox;MerCreMer⁺ mice²¹ leads to left ventricular dilatation and contractile dysfunction (Figure 3.1a)²². Ultra-structural analyses of Atg5-deficient hearts reveal a disorganized sarcomere structure, misalignment and aggregation of mitochondria, and aberrant concentric membranous structures (Figure 3.1b). Inactivation of Atg5 causes the accumulation of abnormal proteins and organelles and promotes ER stress and apoptosis²².

FIGURE 3.1 Cardiac dysfunction in tamoxifen-treated Atg5^flox/flox;MerCreMer⁺ mice.⁽²²⁾ a) Echocardiographic analysis. b) Electron micrographs of tamoxifen-treated Atg5^flox/flox;MerCreMer⁺ mouse hearts. Scale cars: 5μm in left, 1μm in right.

Autophagy also plays a role in cardiac aging. Abnormal proteins and damaged mitochondria accumulate during aging^{23, 24}. In cardiomyocytes, the production of reactive oxygen species from mitochondria increases with age and leads to more mitochondrial damage²⁵. In our examination, the cardiac-specific Atg5-deficient mice died earlier than the wild-type mice due to cardiac dysfunction with a disorganized sarcomere structure and collapsed mitochondria²⁶. These results indicate that constitutive cardiomyocyte autophagy is required for protein quality control and normal cellular structure and function under a basal state. Accumulation of abnormal proteins and organelles, especially mitochondria, may directly cause cardiac dysfunction²⁷.

Autophagy in Cardiac Hypertrophy

Cardiac hypertrophy is the consequence of an increase in cardiomyocyte size. Since cardiomyocytes have little or no capacity for cellular proliferation, their only means of growth is by hypertrophy. In response to stress from neurohumoral activation, hypertension or other myocardial injury, however, the heart initially compensates with an adaptive hypertrophic increase in cardiac mass in order to normalize ventricular wall stress. This increase is characterized by enhanced protein synthesis and an increase in the size and organization of cardiomyocyte sarcomeres^{2, 28}. The compensated hypertrophic response is beneficial. However, prolonged stress leads to irreversible decompensation, culminating in dilatation and contractile dysfunction. This is accompanied by thinning of the ventricular walls through a combination of proteolysis and/or cardiomyocyte cell death²⁹.

Autophagy has been observed in hypertrophied myocardium³⁰. Protein turnover is increased during hypertrophy, although in previous reports, autophagy was diminished in response to aortic stenosis¹² and isoproterenol infusion³¹. Pressure overload is accompanied by an elevated rate of myocardial protein synthesis and cardiac hypertrophy³². In our experiments, pressure overload due to transverse aortic constriction (TAC) induced hypertrophy without cardiac dysfunction 1 week after TAC in wild-type mice³³. Autophagic activity was suppressed in TAC-induced hypertrophied hearts at the 1-week time point compared with that in sham-operated hearts²². This suggests that autophagic activity is reduced during compensative hypertrophic response²⁷.

We also have reported that temporally controlled and cardiac-specific Atg5-deficiency in tamoxifen-treated Atg5^flox/flox;MerCreMer⁺ mice leads to cardiac hypertrophy²². Knockdown of Atg7 by adenovirus expressing short hairpin RNA targeted to Atg7 inhibits autophagy in rat neonatal cardiomyocytes and then induces cardiomyocyte hypertrophy with typical characteristics²². Rapamycin, a potent activator of autophagy, prevents cardiac hypertrophy induced by thyroid hormone treatment³⁴, or aortic banding³⁵, and regresses existing cardiac hypertrophy³⁶. This suggests that autophagy can also decrease cardiac mass and antagonize cardiac hypertrophy by increasing protein degradation. However, cardiac hypertrophic response is similar between cardiac-specific Atg5-deficient mice and control mice after TAC²². Zhu H et al reported that autophagic activity increases rapidly after severe TAC and is maintained at elevated levels for at least three weeks³⁷, whereas we reported that autophagic activity is suppressed in TAC-induced hypertrophied hearts as mentioned above²². Although mice that are haploinsufficient for Beclin 1 (Beclin 1^+/-) have partial suppression of autophagic activity, cardiac hypertrophic response is similar between Beclin 1^+/- mice and control mice three weeks after TAC³⁷.

These results suggest that autophagy does not play an important role in regulating cardiomyocyte hypertrophy induced by hemodynamic stress or that its function in the hypertrophic process is compensated by the action of other hypertrophic signaling mechanisms. The role of autophagy in cardiac hypertrophy remains to be elucidated.

Autophagy in Heart Failure

Autophagy has been observed in failing myocardium caused by dilated cardiomyopathy^{13, 15, 38}, by valvular disease³⁹, and by ischemic heart disease^40-42. In human failing hearts with idiopathic dilated cardiomyopathy, the prevalence of autophagic, apoptotic and necrotic cells also has been observed¹⁵. In animal models, dead and dying cardiomyocytes showing characteristics of autophagy also have been observed. In our experiments, pressure overload due to TAC induced heart failure four weeks after TAC in wild-type mice³³. Autophagy was up-regulated in failing wild-type hearts at the four-week time point^{22, 27}. However, the question remains as to whether autophagy is a sign of failed cardiomyocyte repair or is a suicide pathway for the failing cardiomyocytes.

We examined the role of autophagy in cardiac remodeling during sustained pressure overload. Cardiac-specific Atg5 deficiency was associated with cardiac dysfunction and left ventricular dilatation one week after TAC²². Polyubiquitinated proteins accumulated, ER stress was increased and apoptosis was promoted in Atg5-deficient hearts. We also examined the role of autophagy against β-adrenergic stimulation in the heart. Infusion of isoproterenol for seven days in cardiac-specific Atg5 deficient mice led to greater left ventricular dilatation and contractile dysfunction compared to wild-type mice. Furthermore, adult cardiomyocytes isolated from cardiac-specific Atg5 deficient mouse hearts were more susceptible to isoproterenol than were those from control hearts. These results indicate that autophagy in failing hearts is an adaptive response to protect cells either from pressure overload or from isoproterenol stimulation²².

Conversely, the pathological remodeling by severe pressure overload is moderately diminished in Beclin 1^+/- mice³⁷. In the mice engineered for forced over-expression of Beclin 1 in cardiomyocytes, pressure overload can trigger an amplified autophagic response and pathological remodeling is more severe. This suggests that autophagy can be a maladaptive response.

These experimental o...