Novel Regulator for NADPH Oxidase and Angiotensin II-Induced Hypertension
The source of reactive oxygen species (ROS) produced in cardiovascular systems includes NADPH oxidase, xanthine oxidase, and uncoupling of endothelial NO synthase (eNOS) as well as mitochondria. In particular, NADPH oxidase has been considered a predominant source of ROS in the pathogenesis of hypertension, atherosclerosis, cardiac hypertrophy, and heart failure. Recent data suggest that angiotensin (Ang) II, a potent hypertensive hormone which is known to activate NADPH oxidase, induces mitochondrial dysfunction, which, in turn, promotes excess amounts of ROS, eg, superoxide (O2·−), hydrogen peroxide (H2O2), and peroxynitrite from mitochondria.1 This contributes to endothelial dysfunction by reducing NO bioavailability and activating apoptotic signaling, thereby progressing cardiovascular disease, neurodegenerative disease, and aging. The role of mitochondrial ROS is demonstrated by previous reports that transgenic mice overexpressing catalase targeted to the mitochondria exhibit an extended life span.2 Mice overexpressing peroxiredoxin 3, the mitochondria-specific peroxidase linked to thioredoxin 2 (Trx2), show improved survival after myocardial infarction.3 Furthermore, Ang II-converting enzyme inhibitors and Ang II type I receptor blockers prevent age-related mitochondrial dysfunction, hypertension-induced renal mitochondrial dysfunction, and cardiac mitochondrial dysfunction in the setting of acute ischemia.1 However, the role of mitochondria-derived ROS and its relationship with NADPH oxidase-derived ROS in Ang II-induced hypertension remain unclear.
One of the major antioxidant defense systems against mitochondrial ROS (in particular, H2O2) is thiol-reducing systems, including thioredoxin (Trx), glutaredoxin, and the glutathione system. The Trx system (Trx, Trx reductase, and NADPH) reduces oxidized cysteine groups on protein through an interaction with the redox-active center of Trx (Cys-Gly-Pro-Cys) to form a disulfide bond, which, in turn, can be reduced by Trx reductase and NADPH. In mammals there are ≥3 different thioredoxins: (1) Trx1 is present in the cytosol but can also translocate to the nucleus; (2) Trx2 has a consensus signal for translocation to the mitochondria; and (3) SP-Trx is found in spermatozoa. Mitochondrial Trx systems (Trx2, TrxR2, and Prx3) are critical in protecting cells from mitochondria-dependent ROS and apoptosis.4 Little is known about the functional roles of Trx2 in hypertension.
In this issue of Hypertension, using transgenic mice overexpressing Trx2 (hTrx2-Tg), Widder et al5 provide the novel evidence that mitochondrial antioxidant Trx2 plays a critical role in regulating endothelial function and systolic blood pressure in Ang II-induced hypertension. Overexpression of Trx2 decreases “total” as well as “mitochondrial” ROS in aortas from mice infused with Ang II, suggesting that mitochondrial ROS play a critical role in regulating Ang II-induced hypertension. A cross-talk between NADPH oxidase- and mitochondria-derived ROS appears to exist in Ang II-induced mitochondrial dysfunction, ROS production, and preconditioning and nitroglycerin-triggered vascular dysfunction.6–8 Ang II activates NADPH oxidase, thereby elevating cytosolic ROS (in particular, O2·−), which triggers mitochondrial ROS elevation. This mechanism seems to be mediated through either activation of the mitochondrial ATP-sensitive potassium channel or mitochondrial dysfunction induced by peroxynitrite produced by the reaction of O2·− with NO. This mitochondrial ROS further increases ROS by inducing the mitochondrial permeability transition (ROS-triggered ROS formation).9 In this study, uncoupling eNOS may not be a source for Ang II-induced O2·− production, because aortic tetrahydrobiopterin levels and the eNOS dimer:monomer ratio are not changed after chronic Ang II infusion. Widder et al5 have found that chronic Ang II infusion increases expression of the NADPH oxidase subunits Nox2, p22phox, p47phox, and Rac-1 in wild-type mice, which is attenuated in mice overexpressing Trx2.5 These results suggest that mitochondrial ROS increase expression of NADPH oxidase components. Given that NADPH oxidase can be stimulated by H2O2 and lipid peroxides,10 the mitochondrial ROS, including H2O2, might stimulate NADPH oxidase activity and expression in a feed-forward fashion. Thus, the decrease in NADPH oxidase expression and total ROS production in Ang II-infused hTrx2-Tg mice might be caused by inhibition of cross-talk between mitochondrial- and NADPH oxidase-derived ROS (Figure).
The hTrx2-Tg mice improve endothelial dysfunction induced by Ang II infusion,5 suggesting that mitochondrial ROS inhibit endothelial cell function, as reported previously.11 This protective effect of Trx2 is not attributable to an increase in vascular eNOS levels, because chronic Ang II infusion did not alter vascular eNOS levels in both wild-type and hTrx2-Tg mice. Because mitochondrial ROS may stimulate NADPH oxidase, overexpression of Trx2 may block ROS production derived from both mitochondria and NADPH oxidase, thereby efficiently preserving NO bioavailability. The hTrx2-Tg mice also prevent vasoconstriction induced by chronic Ang II infusion, indicating that mitochondrial ROS contribute to Ang II-mediated vasoconstriction. Zhang et al11 reported that transgenic mice overexpressing “endothelial-specific” Trx2 (EC-hTrx2-Tg) without Ang II infusion promote endothelium-dependent vasorelaxation and reduce vasoconstriction, superoxide production, and systolic blood pressure. However, hTrx2-Tg mice have no effect on these basal responses. This discrepancy may be attributed to the possibility that the Trx2 expression level in endothelial cells is much higher in EC-hTrx2-Tg mice than in hTrx2-Tg mice.
Widder et al5 also show that overexpression of Trx2 inhibits cardiac hypertrophy and cardiac superoxide levels caused by chronic Ang II infusion. Mitochondria dysfunction seems to contribute to cardiac hypertrophy in heart failure, as well as ischemia/reperfusion injury. Independent of its antioxidant properties, Trx2 has antiapoptoic activity through inhibition of apoptosis signal-regulated kinase 1.12 Given that apoptosis signal-regulated kinase 1 is involved in Ang II-induced cardiac hypertrophy and remodeling, it is tempting to speculate that the antihypertrophic effect of overexpression of Trx2 may be mediated through apoptosis signal-regulated kinase 1.
The data presented by Widder et al5 strongly support a critical role for Trx2, as a regulator of mitochondrial ROS, in Ang II-induced hypertension and cardiac hypertrophy (Figure). Moreover, their finding underscores the importance of targeting antioxidants to mitochondria as a new therapeutic strategy to restore vascular function and reduce the pathophysiology of hypertension. This may explain the failure of antioxidants as therapeutic agents in a series of clinical trials and emphasizes the relevance of the manipulation of ROS at the subcellular level.
There are many unanswered questions. What is the role of endogenous Trx2 in vascular function and hypertension? Because Trx2−/− mice are embryonically lethal, study using Trx2+/− mice will provide new information regarding the functional significance of Trx2 in Ang II-induced hypertension and other cardiovascular diseases. Can overexpression of other mitochondrial thiol-reducing systems, eg, glutathione peroxidase, mimic the effect of Trx2? Can overexpression of Trx2 affect other models of hypertension, eg, deoxycorticosterone acetate salt hypertension? Addressing these questions will be essential to understand the mechanism of oxidative stress-dependent cardiovascular diseases and aging in which mitochondrial ROS play an essential role.
I thank Dr Masuko Ushio-Fukai for critical review of the article and helpful comments.
Source of Funding
This work was supported by National Institutes of Health grant 5R01HL070187-08.
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
de Cavanagh EM, Ferder M, Inserra F, Ferder L. Angiotensin II, mitochondria, cytoskeletal, and extracellular matrix connections: an integrating viewpoint. Am J Physiol Heart Circ Physiol. 2009; 296: H550–H558.
Schriner SE, Linford NJ, Martin GM, Treuting P, Ogburn CE, Emond M, Coskun PE, Ladiges W, Wolf N, Van Remmen H, Wallace DC, Rabinovitch PS. Extension of murine life span by overexpression of catalase targeted to mitochondria. Science. 2005; 308: 1909–1911.
Matsushima S, Ide T, Yamato M, Matsusaka H, Hattori F, Ikeuchi M, Kubota T, Sunagawa K, Hasegawa Y, Kurihara T, Oikawa S, Kinugawa S, Tsutsui H. Overexpression of mitochondrial peroxiredoxin-3 prevents left ventricular remodeling and failure after myocardial infarction in mice. Circulation. 2006; 113: 1779–1786.
Widder JD, Fraccarollo D, Galuppo P, Hansen JM, Jones DP, Ertl G, Bauersachs J. Attenuation of angiotensin II-induced vascular dysfunction and hypertension by overexpression of thioredoxin 2. Hypertension. 2009; 54: 338–344.
Doughan AK, Harrison DG, Dikalov SI. Molecular mechanisms of angiotensin II-mediated mitochondrial dysfunction: linking mitochondrial oxidative damage and vascular endothelial dysfunction. Circ Res. 2008; 102: 488–496.
Kimura S, Zhang GX, Nishiyama A, Shokoji T, Yao L, Fan YY, Rahman M, Suzuki T, Maeta H, Abe Y. Role of NAD(P)H oxidase- and mitochondria-derived reactive oxygen species in cardioprotection of ischemic reperfusion injury by angiotensin II. Hypertension. 2005; 45: 860–866.
Wenzel P, Mollnau H, Oelze M, Schulz E, Wickramanayake JM, Muller J, Schuhmacher S, Hortmann M, Baldus S, Gori T, Brandes RP, Munzel T, Daiber A. First evidence for a crosstalk between mitochondrial and NADPH oxidase-derived reactive oxygen species in nitroglycerin-triggered vascular dysfunction. Antioxid Redox Signal. 2008; 10: 1435–1447.
Li WG, Miller FJ Jr, Zhang HJ, Spitz DR, Oberley LW, Weintraub NL. H(2)O(2)-induced O(2) production by a non-phagocytic NAD(P)H oxidase causes oxidant injury. J Biol Chem. 2001; 276: 29251–29256.
Zhang R, Al-Lamki R, Bai L, Streb JW, Miano JM, Bradley J, Min W. Thioredoxin-2 inhibits mitochondria-located ASK1-mediated apoptosis in a JNK-independent manner. Circ Res. 2004; 94: 1483–1491.