NADPH oxidases in heart failure: poachers or gamekeepers?

Abstract

Significance: Oxidative stress is involved in the pathogenesis of heart failure but clinical antioxidant trials have been unsuccessful. This may be because effects of reactive oxygen species (ROS) depend upon their source, location, and concentration. Nicotinamide adenine dinucleotide phosphate oxidase (Nox) proteins generate ROS in a highly regulated fashion and modulate several components of the heart failure phenotype.

Recent advances: Two Nox isoforms, Nox2 and Nox4, are expressed in the heart. Studies using gene-modified mice deficient in Nox2 activity indicate that Nox2 activation contributes to angiotensin II-induced cardiomyocyte hypertrophy, atrial fibrillation, and the development of interstitial fibrosis but may also positively modulate physiological excitation-contraction coupling. Nox2 contributes to myocyte death under stress situations and plays important roles in postmyocardial infarction remodeling, in part by modulating matrix metalloprotease activity. In contrast to Nox2, Nox4 is constitutively active at a low level and induces protective effects in the heart under chronic stress, for example, by maintaining myocardial capillary density. However, high levels of Nox4 could have detrimental effects.

Critical issues: The effects of Nox proteins during the development of heart failure likely depend upon the isoform, activation level, and cellular distribution, and may include beneficial as well as detrimental effects. More needs to be learnt about the precise regulation of abundance and biochemical activity of these proteins in the heart as well as the downstream signaling pathways that they regulate.

Future directions: The development of specific approaches to target individual Nox isoforms and/or specific cell types may be important for the achievement of therapeutic efficacy in heart failure.

FIG. 1.

Schematic representation of NADPH oxidase 1 (Nox1), Nox2, Nox4, and Nox5 and their accessory subunits. Nox5 is absent in rodents and does not require p22^phox, unlike the other isoforms. The cellular distribution of each isoform and the reactive oxygen species (ROS) that are generated are also shown.

FIG. 2.

Mechanism of Nox2 activation. Several signaling pathways, such as protein kinase C (PKC), protein kinase D (PKD), and phosphoinositide-3-kinase (PI3K), are activated downstream of G-protein coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs) and can each lead to post-translational modification of cystosolic regulatory subunits, in particular, p47^phox. At the same time, Rac is activated downstream of PI3K. The active Nox2 complex comprises the Nox2-p22^phox heterodimer together with p47^phox, p67^phox, p40^phox, and Rac. (To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars).

FIG. 3.

Propensity for Nox4 to generate hydrogen peroxide (H₂O₂) rather than superoxide. (A) Schematic illustration of the Nox4 protein and different loops between transmembrane domains. (B) Nox4 deletion constructs of the E-loop between transmembrane domains 5 and 6 that were studied by Takac et al. (190). FAD binding site (FAD), NADPH binding site (NADPH). (C) Determination of superoxide production by L-012 luminescence in transiently transfected HEK293 cells. (D) Determination of hydrogen peroxide production by luminol+HRP chemiluminescence. Deletion of parts of the E-loop converts Nox4 from a hydrogen peroxide to a superoxide generator. Figure adapted from Ref. (190) with permission. HRP, horseradish peroxidase.

FIG. 4.

Reaction of superoxide versus hydrogen peroxide with nitric oxide (NO). Superoxide inactivates NO to generate peroxynitrite that may have its own effects on signaling. When superoxide and NO are present in similar concentrations, this reaction has faster kinetics than those for superoxide dismutation to hydrogen peroxide. Hydrogen peroxide does not undergo a similar reaction with NO and may enhance nitric oxide synthase (NOS) activity and/or expression levels.

FIG. 5.

Schematic showing effects of Nox2 on different components of the failing heart phenotype. Signaling pathways modulated by Nox2 are shown.

FIG. 6.

Effects of Nox4 on angiogenesis. Nox4 levels rise in response to pressure overload and hypoxia and trigger an increase in hypoxia-inducible factor-1α (HIF1α) levels. This may be mediated through an increase in eNOS levels and NO production or could involve direct effects of Nox4-derived ROS on prolyl hydroxylases (PHD) enzymes. An increase in HIF1α leads to increased production and release of angiogenic factors, such as vascular endothelial growth factor, from myocytes or other cell types.

FIG. 7.

Schematic showing potential effects of Nox2-derived ROS on excitation-contraction coupling. Asterisks denote ROS. Other abbreviations as in the text. (To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars).

FIG. 8.

Aldosterone-stimulated Nox-dependent oxidation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) contributes to cardiac rupture following acute myocardial infarction (MI). CaMKII induces the production of matrix metalloproteinase 9 (MMP9) that causes cardiac rupture. Reproduced from Ref. (84) with permission. (To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars).