Marina Bayeva

Cardiotoxicity of doxorubicin is mediated through mitochondrial iron accumulation

Doxorubicin is an effective anticancer drug with known cardiotoxic side effects. It has been hypothesized that doxorubicin-dependent cardiotoxicity occurs through ROS production and possibly cellular iron accumulation. Here, we found that cardiotoxicity develops through the preferential accumulation of iron inside the mitochondria following doxorubicin treatment. In isolated cardiomyocytes, doxorubicin became concentrated in the mitochondria and increased both mitochondrial iron and cellular ROS levels. Overexpression of ABCB8, a mitochondrial protein that facilitates iron export, in vitro and in the hearts of transgenic mice decreased mitochondrial iron and cellular ROS and protected against doxorubicin-induced cardiomyopathy. Dexrazoxane, a drug that attenuates doxorubicin-induced cardiotoxicity, decreased mitochondrial iron levels and reversed doxorubicin-induced cardiac damage. Finally, hearts from patients with doxorubicin-induced cardiomyopathy had markedly higher mitochondrial iron levels than hearts from patients with other types of cardiomyopathies or normal cardiac function. These results suggest that the cardiotoxic effects of doxorubicin develop from mitochondrial iron accumulation and that reducing mitochondrial iron levels protects against doxorubicin-induced cardiomyopathy.

Mitochondria as a Therapeutic Target in Heart Failure

Heart failure is a pressing public health problem with no curative treatment currently available. The existing therapies provide symptomatic relief, but are unable to reverse molecular changes that occur in cardiomyocytes. The mechanisms of heart failure are complex and multiple, but mitochondrial dysfunction appears to be a critical factor in the development of this disease. Thus, it is important to focus research efforts on targeting mitochondrial dysfunction in the failing heart to revive the myocardium and its contractile function. This review highlights the 3 promising areas for the development of heart failure therapies, including mitochondrial biogenesis, mitochondrial oxidative stress, and mitochondrial iron handling. Moreover, the translational potential of compounds targeting these pathways is discussed.

Taking Diabetes to Heart—Deregulation of Myocardial Lipid Metabolism in Diabetic Cardiomyopathy

Heart disease is the leading cause of death in patients with diabetes.[1][1] Although advances in medical management and lifestyle interventions have reduced cardiovascular mortality in diabetic patients by as much as 40% over the last decade, the actual number of deaths is predicted to rise as a

Disruption of ATP-binding cassette B8 in mice leads to cardiomyopathy through a decrease in mitochondrial iron export

Mitochondrial iron levels are tightly regulated, as iron is essential for the synthesis of Fe/S clusters and heme in the mitochondria, but high levels can cause oxidative stress. The ATP-binding cassette (ABC) transporter ABCB8 is a mitochondrial inner membrane protein with an unknown function. Here, we show that ABCB8 is involved in mitochondrial iron export and is essential for baseline cardiac function. Induced genetic deletion of ABCB8 in mouse hearts resulted in mitochondrial iron accumulation and cardiomyopathy, as assessed by echocardiography and invasive hemodynamics. Mice with ABCB8 deletion in the heart also displayed mitochondrial damage, and higher levels of reactive oxygen species and cell death. Down-regulation of ABCB8 in vitro resulted in decreased iron export from isolated mitochondria, whereas its overexpression had the opposite effect. Furthermore, ABCB8 is needed for the maturation of the cytosolic Fe/S proteins, as its deletion in vitro and in vivo led to decreased activity of cytosolic, but not mitochondrial, iron–sulfur-containing enzymes. These results indicate that ABCB8 is essential for normal cardiac function, maintenance of mitochondrial iron homeostasis and maturation of cytosolic Fe/S proteins. In summary, this report provides characterization of a protein involved in mitochondrial iron export.

mTOR regulates cellular iron homeostasis through tristetraprolin.

Iron is an essential cofactor with unique redox properties. Iron-regulatory proteins 1 and 2 (IRP1/2) have been established as important regulators of cellular iron homeostasis, but little is known about the role of other pathways in this process. Here we report that the mammalian target of rapamycin (mTOR) regulates iron homeostasis by modulating transferrin receptor 1 (TfR1) stability and altering cellular iron flux. Mechanistic studies identify tristetraprolin (TTP), a protein involved in anti-inflammatory response, as the downstream target of mTOR that binds to and enhances degradation of TfR1 mRNA. We also show that TTP is strongly induced by iron chelation, promotes downregulation of iron-requiring genes in both mammalian and yeast cells, and modulates survival in low-iron states. Taken together, our data uncover a link between metabolic, inflammatory, and iron-regulatory pathways, and point toward the existence of a yeast-like TTP-mediated iron conservation program in mammals.

Mitochondrial Dysfunction and Oxidative Damage to Sarcomeric Proteins

Hypertension is an important risk factor for the development of heart failure. Increased production of reactive oxygen species (ROS) contributes to cardiac dysfunction by activating numerous pro-hypertrophic signaling cascades and damaging the mitochondria, thus setting off a vicious cycle of ROS generation. The way in which oxidative stress leads to exacerbation of systolic and diastolic dysfunction is still unclear, however. In skeletal muscle and ischemic myocardium, increased ROS production causes preferential oxidation of myofibrillar proteins and provides a mechanistic link between oxidative damage and impaired contractility through disruption of actin-myosin interactions, enzymatic functions, calcium sensitivity, and efficiency of cross-bridge cycling. In this review, we summarize recent findings in the fields of heart failure and sarcomere biology and speculate that oxidative damage to myofibrils may contribute to the development of heart failure.

Molecular and cellular basis of viable dysfunctional myocardium

Heart failure is a leading cause of healthcare expenditures, hospitalization, and mortality in developed countries, and its burden is growing globally.1 With the aging of the population and increasing prevalence of chronic diseases, including hypertension, diabetes mellitus, and obesity, the current heart failure epidemic is guaranteed to significantly worsen in the near future. Thus, new disease-modifying treatments for heart failure are needed urgently and represent an area of intense investigation.2 Multiple studies in humans and animals have shown that the functionality of myocardial tissue of a failing heart can be restored. First, in ischemic heart failure because of severe coronary artery stenosis, revascularization therapy is known to improve heart function in a proportion of patients.3,4 Second, the mechanical unloading of the heart by left ventricular assist device (LVAD) is associated with improvements in cardiac function. In a recent report of 80 patients with heart failure who underwent implantation of a continuous-flow LVAD, the ejection fraction increased by >50% in about one third of the patients, with corresponding improvements in LV end-systolic and end-diastolic volumes and decreases in LV mass at 6 months after LVAD unloading.5 In addition, normalization of echocardiographic parameters has obviated completely the need for continuing LVAD support or cardiac transplantation in several patients.6,7 Importantly, the positive effects of mechanical unloading were noted in patients with both ischemic and nonischemic heart failure,5 suggesting that dysfunctional but potentially salvageable segments of myocardium exist in the failing heart regardless of pathogenesis. Third, in patients with broken heart syndrome (also known as Takotsubo cardiomyopathy), characterized by a rapid and severe loss of cardiac contractility secondary to emotional stress, myocardial function normalized spontaneously, again arguing for the reversibility of heart failure.8 In summary, although maladaptive changes observed in failing hearts were …

ATP-binding cassette B10 regulates early steps of heme synthesis

Rationale: Heme plays a critical role in gas exchange, mitochondrial energy production, and antioxidant defense in cardiovascular system. The mitochondrial transporter ATP-binding cassette (ABC) B10 has been suggested to export heme out of the mitochondria and is required for normal hemoglobinization of erythropoietic cells and protection against ischemia–reperfusion injury in the heart; however, its primary function has not been established. Objective: The aim of this study was to identify the function of ABCB10 in heme synthesis in cardiac cells. Methods and Results: Knockdown of ABCB10 in cardiac myoblasts significantly reduced heme levels and the activities of heme-containing proteins, whereas supplementation with &dgr;-aminolevulinic acid reversed these defects. Overexpression of mitochondrial &dgr;-aminolevulinic acid synthase 2, the rate-limiting enzyme upstream of &dgr;-aminolevulinic acid export, failed to restore heme levels in cells with ABCB10 downregulation. ABCB10 and heme levels were increased by hypoxia, and reversal of ABCB10 upregulation caused oxidative stress and cell death. Furthermore, ABCB10 knockdown in neonatal rat cardiomyocytes resulted in a significant delay of calcium removal from the cytoplasm, suggesting a relaxation defect. Finally, ABCB10 expression and heme levels were altered in failing human hearts and mice with ischemic cardiomyopathy. Conclusions: ABCB10 plays a critical role in heme synthesis pathway by facilitating &dgr;-aminolevulinic acid production or export from the mitochondria. In contrast to previous reports, we show that ABCB10 is not a heme exporter and instead is required for the early mitochondrial steps of heme biosynthesis.

Heme Levels are Increased in Human Failing Hearts

OBJECTIVES The goal of this study was to characterize the regulation of heme and non-heme iron in human failing hearts. BACKGROUND Iron is an essential molecule for cellular physiology, but in excess it facilitates oxidative stress. Mitochondria are the key regulators of iron homeostasis through heme and iron-sulfur cluster synthesis. Because mitochondrial function is depressed in failing hearts and iron accumulation can lead to oxidative stress, we hypothesized that iron regulation may also be impaired in heart failure (HF). METHODS We measured mitochondrial and cytosolic heme and non-heme iron levels in failing human hearts retrieved during cardiac transplantation surgery. In addition, we examined the expression of genes regulating cellular iron homeostasis, the heme biosynthetic pathway, and micro-RNAs that may potentially target iron regulatory networks. RESULTS Although cytosolic non-heme iron levels were reduced in HF, mitochondrial iron content was maintained. Moreover, we observed a significant increase in heme levels in failing hearts, with corresponding feedback inhibition of the heme synthetic enzymes and no change in heme degradation. The rate-limiting enzyme in heme synthesis, delta-aminolevulinic acid synthase 2 (ALAS2), was significantly upregulated in HF. Overexpression of ALAS2 in H9c2 cardiac myoblasts resulted in increased heme levels, and hypoxia and erythropoietin treatment increased heme production through upregulation of ALAS2. Finally, increased heme levels in cardiac myoblasts were associated with excess production of reactive oxygen species and cell death, suggesting a maladaptive role for increased heme in HF. CONCLUSIONS Despite global mitochondrial dysfunction, heme levels are maintained above baseline in human failing hearts.

ABCB10 Regulates Early Steps of Heme Synthesis

Rationale: Heme plays a critical role in gas exchange, mitochondrial energy production, and antioxidant defense in cardiovascular system. The mitochondrial transporter ATP-binding cassette (ABC) B10 has been suggested to export heme out of the mitochondria and is required for normal hemoglobinization of erythropoietic cells and protection against ischemia–reperfusion injury in the heart; however, its primary function has not been established. Objective: The aim of this study was to identify the function of ABCB10 in heme synthesis in cardiac cells. Methods and Results: Knockdown of ABCB10 in cardiac myoblasts significantly reduced heme levels and the activities of heme-containing proteins, whereas supplementation with &dgr;-aminolevulinic acid reversed these defects. Overexpression of mitochondrial &dgr;-aminolevulinic acid synthase 2, the rate-limiting enzyme upstream of &dgr;-aminolevulinic acid export, failed to restore heme levels in cells with ABCB10 downregulation. ABCB10 and heme levels were increased by hypoxia, and reversal of ABCB10 upregulation caused oxidative stress and cell death. Furthermore, ABCB10 knockdown in neonatal rat cardiomyocytes resulted in a significant delay of calcium removal from the cytoplasm, suggesting a relaxation defect. Finally, ABCB10 expression and heme levels were altered in failing human hearts and mice with ischemic cardiomyopathy. Conclusions: ABCB10 plays a critical role in heme synthesis pathway by facilitating &dgr;-aminolevulinic acid production or export from the mitochondria. In contrast to previous reports, we show that ABCB10 is not a heme exporter and instead is required for the early mitochondrial steps of heme biosynthesis.