From Contractile Enhancement to Pathological Hypertrophy: Angiotensin II-Induced Nox2-Mediated Reactive Oxygen Species

Role of Vasoactive Hormone-Induced Signal Transduction in Cardiac Hypertrophy and Heart Failure

Heart failure is the common concluding pathway for a majority of cardiovascular diseases and is associated with cardiac dysfunction. Since heart failure is invariably preceded by adaptive or maladaptive cardiac hypertrophy, several biochemical mechanisms have been proposed to explain the development of cardiac hypertrophy and progression to heart failure. One of these includes the activation of different neuroendocrine systems for elevating the circulating levels of different vasoactive hormones such as catecholamines, angiotensin II, vasopressin, serotonin and endothelins. All these hormones are released in the circulation and stimulate different signal transduction systems by acting on their respective receptors on the cell membrane to promote protein synthesis in cardiomyocytes and induce cardiac hypertrophy. The elevated levels of these vasoactive hormones induce hemodynamic overload, increase ventricular wall tension, increase protein synthesis and the occurrence of cardiac remodeling. In addition, there occurs an increase in proinflammatory cytokines and collagen synthesis for the induction of myocardial fibrosis and the transition of adaptive to maladaptive hypertrophy. The prolonged exposure of the hypertrophied heart to these vasoactive hormones has been reported to result in the oxidation of catecholamines and serotonin via monoamine oxidase as well as the activation of NADPH oxidase via angiotensin II and endothelins to promote oxidative stress. The development of oxidative stress produces subcellular defects, Ca2+-handling abnormalities, mitochondrial Ca2+-overload and cardiac dysfunction by activating different proteases and depressing cardiac gene expression, in addition to destabilizing the extracellular matrix upon activating some metalloproteinases. These observations support the view that elevated levels of various vasoactive hormones, by producing hemodynamic overload and activating their respective receptor-mediated signal transduction mechanisms, induce cardiac hypertrophy. Furthermore, the occurrence of oxidative stress due to the prolonged exposure of the hypertrophied heart to these hormones plays a critical role in the progression of heart failure.

Adaptive and maladaptive roles of different angiotensin receptors in the development of cardiac hypertrophy and heart failure

Angiotensin II (Ang II) is formed by the action of angiotensin-converting enzyme (ACE) in the renin-angiotensin system. This hormone is known to induce cardiac hypertrophy and heart failure and its actions are mediated by the interaction of both pro- and antihypertrophic Ang II receptors (AT1R and AT2R). Ang II is also metabolized by ACE 2 to Ang-(1-7), which elicits the activation of Mas receptors (MasR) for inducing antihypertrophic actions. Since heart failure under different pathophysiological situations is preceded by adaptive and maladaptive cardiac hypertrophy, we have reviewed the existing literature to gain some information regarding the roles of AT1R, AT2R, and MasR in both acute and chronic conditions of cardiac hypertrophy. It appears that the activation of AT1R may be involved in the development of adaptive and maladaptive cardiac hypertrophy as well as subsequent heart failure because both ACE inhibitors and AT1R antagonists exert beneficial effects. On the other hand, the activation of both AT2R and MasR may prevent the occurrence of maladaptive cardiac hypertrophy and delay the progression of heart failure, and thus therapy with different activators of these antihypertrophic receptors under chronic pathological stages may prove beneficial. Accordingly, it is suggested that a great deal of effort should be made to develop appropriate activators of both AT2R and MasR for the treatment of heart failure subjects.

Angiotensin II-Induced Signal Transduction Mechanisms for Cardiac Hypertrophy

Although acute exposure of the heart to angiotensin (Ang II) produces physiological cardiac hypertrophy and chronic exposure results in pathological hypertrophy, the signal transduction mechanisms for these effects are of complex nature. It is now evident that the hypertrophic response is mediated by the activation of Ang type 1 receptors (AT₁R), whereas the activation of Ang type 2 receptors (AT₂R) by Ang II and Mas receptors by Ang-(1-7) exerts antihypertrophic effects. Furthermore, AT₁R-induced activation of phospholipase C for stimulating protein kinase C, influx of Ca²⁺ through sarcolemmal Ca²⁺- channels, release of Ca²⁺ from the sarcoplasmic reticulum, and activation of sarcolemmal NADPH oxidase 2 for altering cardiomyocytes redox status may be involved in physiological hypertrophy. On the other hand, reduction in the expression of AT₂R and Mas receptors, the release of growth factors from fibroblasts for the occurrence of fibrosis, and the development of oxidative stress due to activation of mitochondria NADPH oxidase 4 as well as the depression of nuclear factor erythroid-2 activity for the occurrence of Ca²⁺-overload and activation of calcineurin may be involved in inducing pathological cardiac hypertrophy. These observations support the view that inhibition of AT₁R or activation of AT₂R and Mas receptors as well as depression of oxidative stress may prevent or reverse the Ang II-induced cardiac hypertrophy.

Hydrogen Attenuates Thyroid Hormone-Induced Cardiac Hypertrophy in Rats by regulating angiotensin II type 1 receptor and NADPH oxidase 2 mediated oxidative stress

Cardiac hypertrophy occurs as a result of high levels of thyroid hormone, which may contribute to heart failure and is closely related to oxidative stress. Hydrogen is a good antioxidant. In this study, we found that intragastric levothyroxine administration for two weeks caused obvious cardiac hypertrophy without reduced systolic function. Additionally, hydrogen inhalation ameliorated the levothyroxine-induced metabolic increase and cardiac hypertrophy in rats. Serum brain natriuretic peptide expression was also attenuated by hydrogen treatment. However, hydrogen had no significant effect on levothyroxine -induced serum troponin I and serum thyroid hormone changes. Hydrogen treatment also reduced the levothyroxine-induced increase in cardiac malondialdehyde, 8-hydroxy-2-deoxyguanosine and serum hydrogen peroxide levels and upregulated superoxide dismutase and glutathione peroxidase activity. Additionally, western blotting results showed that hydrogen inhalation inhibited the expression of cardiac nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2), angiotensin II type 1 receptor, sarcoplasmic reticulum Ca²⁺-ATPase (SERCA2), phospho-phospholamban and α-myosin heavy chain proteins. In conclusion, the present study revealed a protective effect of hydrogen on levothyroxine -induced cardiac hypertrophy by regulating angiotensin II type 1 receptors and NOX2-mediated oxidative stress in rats.

Catalase Mediates the Inhibitory Actions of PPARδ against Angiotensin II-Triggered Hypertrophy in H9c2 Cardiomyocytes

Hypertrophy of myocytes has been implicated in cardiac dysfunctions affecting wall stress and patterns of gene expression. However, molecular targets potentially preventing cardiac hypertrophy have not been fully elucidated. In the present study, we demonstrate that upregulation of catalase by peroxisome proliferator-activated receptor δ (PPARδ) is involved in the anti-hypertrophic activity of PPARδ in angiotensin II (Ang II)-treated H9c2 cardiomyocytes. Activation of PPARδ by a specific ligand GW501516 significantly inhibited Ang II-induced hypertrophy and the generation of reactive oxygen species (ROS) in H9c2 cardiomyocytes. These effects of GW501516 were almost completely abolished in cells stably expressing small hairpin (sh)RNA targeting PPARδ, indicating that PPARδ mediates these effects. Significant concentration and time-dependent increases in catalase at both mRNA and protein levels were observed in GW501516-treated H9c2 cardiomyocytes. In addition, GW501516-activated PPARδ significantly enhanced catalase promoter activity and protein expression, even in the presence of Ang II. GW501516-activated PPARδ also inhibited the expression of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), which are both marker proteins for hypertrophy. The effects of GW501516 on the expression of ANP and BNP were reversed by 3-amino-1,2,4-triazole (3-AT), a catalase inhibitor. Inhibition or downregulation of catalase by 3-AT or small interfering (si)RNA, respectively, abrogated the effects of PPARδ on Ang II-induced hypertrophy and ROS generation, indicating that these effects of PPARδ are mediated through catalase induction. Furthermore, GW501516-activated PPARδ exerted catalase-dependent inhibitory effects on Ang II-induced hypertrophy by blocking p38 mitogen-activated protein kinase. Taken together, these results indicate that the anti-hypertrophic activity of PPARδ may be achieved, at least in part, by sequestering ROS through fine-tuning the expression of catalase in cardiomyocytes.

Scoparone alleviates Ang II-induced pathological myocardial hypertrophy in mice by inhibiting oxidative stress

Long‐term poorly controlled myocardial hypertrophy often leads to heart failure and sudden death. Activation of ras‐related C3 botulinum toxin substrate 1 (RAC1) by angiotensin II (Ang II) plays a pivotal role in myocardial hypertrophy. Previous studies have demonstrated that scoparone (SCO) has beneficial effects on hypertension and extracellular matrix remodelling. However, the function of SCO on Ang II‐mediated myocardial hypertrophy remains unknown. In our study, a mouse model of myocardial hypertrophy was established by Ang II infusion (2 mg/kg/day) for 4 weeks, and SCO (60 mg/kg bodyweight) was administered by gavage daily. In vitro experiments were also performed. Our results showed that SCO could alleviate Ang II infusion‐induced cardiac hypertrophy and fibrosis in mice. In vitro, SCO treatment blocks Ang II‐induced cardiomyocyte hypertrophy, cardiac fibroblast collagen synthesis and differentiation to myofibroblasts. Meanwhile, we found that SCO treatment blocked Ang II‐induced oxidative stress in cardiomyocytes and cardiac fibroblasts by inhibiting RAC1‐GTP and total RAC1 in vivo and in vitro. Furthermore, reactive oxygen species (ROS) burst by overexpression of RAC1 completely abolished SCO‐mediated protection in cardiomyocytes and cardiac fibroblasts in vitro. In conclusion, SCO, an antioxidant, may attenuate Ang II‐induced myocardial hypertrophy by suppressing of RAC1 mediated oxidative stress.

Puerarin attenuates angiotensin II-induced cardiac fibroblast proliferation via the promotion of catalase activity and the inhibition of hydrogen peroxide-dependent Rac-1 activation

The aims of the present study were to evaluate the effects of puerarin on angiotensin II-induced cardiac fibroblast proliferation and to explore the molecular mechanisms of action. Considering the role of H₂O₂ in nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation, we hypothesized that modulating catalase activity would be a potential target in regulating the redox-sensitive pathways. Our results showed that the activation of Rac1 was dependent on the levels of intracellular H₂O₂. Puerarin blocked the phosphorylation of extracellular regulated protein kinases (ERK)1/2, abolished activator protein (AP)-1 binding activity, and eventually attenuated cardiac fibroblast proliferation through the inhibition of H₂O₂-dependent Rac1 activation. Further studies revealed that angiotensin II treatment resulted in decreased catalase protein expression and enzyme activity, which was disrupted by puerarin via the upregulation of catalase protein expression at the transcriptional level and the prolonged protein degradation. These findings indicated that the anti-proliferation mechanism of puerarin was mainly through blocking angiontensin II-triggered downregulation of catalase expression and H₂O₂-dependent Rac1 activation.

Beta3 adrenergic receptor stimulation in human macrophages inhibits NADPHoxidase activity and induces catalase expression via PPARγ activation

The beta3 adrenergic receptor (β3-AR) stimulation plays a protective role against preterm labor by blocking myometrial contraction, cytokine production, remodeling and apoptosis. We previously demonstrated that macrophage-induced ROS production in the myometrium was a key element leading to the induction of all these labor-associated features. We thus aimed to investigate if the β3-AR could be expressed in human macrophages and could trigger its protective role in the myometrium by directly inhibiting ROS production. Using lipopolysaccharide (LPS)-stimulated myometrial samples and cell co-culture experiments, we demonstrated that β3-AR stimulation inhibits the activation of the NADPH oxidase, leading to the subsequent inhibition of ROS production by macrophages. This antioxidant effect was associated with a potent anti-inflammatory response in macrophages. Furthermore, we observed that β3-AR leads to the expression of catalase not only in macrophages but also in myometrial cells, thereby preventing the transactivation of myometrial cells by hydrogen peroxide. Pharmacological experiments allowed us to demonstrate that these effects were driven by an Erk1/2-mediated activation of the antioxidant transcription factor PPARγ. These results suggest that β3-AR protective effects in the myometrium could be due to its dual antioxidant properties. Further, the effects observed in a macrophage could highlight new applications in chronic inflammatory diseases.