Response by Dikalova and Dikalov to Letter Regarding Article, “Mitochondrial Deacetylase Sirt3 Reduces Vascular Dysfunction and Hypertension While Sirt3 Depletion in Essential Hypertension Is Linked to Vascular Inflammation and Oxidative Stress”

In Response:

We appreciate the comments by Zhou et al on our recent publication.¹ They noted that acetylation-deacetylation represents only one of many post-translational regulations, and other modifications including phosphorylation, ubiquitination, oxidation, nitration and S-glutathionylation also contribute to the regulation of mitochondrial superoxide dismutase (SOD2) activity.² Indeed, multiple post-translational modifications of SOD2 can alter the efficiency of superoxide dismutation, however, the abundance of these modifications varies by many fold and SOD2 acetylation is the predominant and most abundant post-translational modification of mitochondrial SOD2.³ Acetylation of lysine residues K68 at the SOD2 catalytical center had the greatest inhibitory effect on the SOD2 superoxide dismutation activity ⁴ and supplementation of recombinant deacetylase Sirt3 to SOD2 isolated from hypertensive mice completely rescued SOD2 activity.⁵ The specific post-translational modifications of SOD2 will likely vary in different pathological conditions i.e. inflammation-induced nitrosative stress may increase contribution of SOD2 nitration while hyperoxia can promote SOD2 S-glutathionylation. We agree that additional aspects should be taken in consideration, meanwhile, it is clear that reduced Sirtuin 3 deacetylase activity leads to SOD2 hyperacetylation and this contributes to hypertension and vascular dysfunction.^{1, 5}

One of the novel findings of our study was that Sirtuin 3 impairment in essential hypertension is due to both Sirtuin 3 depletion and Sirtuin 3 inactivation. We observed a 40% decrease in vascular Sirtuin 3 level and 300% increase in SOD2 acetylation in hypertensive subjects while SOD2 levels were not affected.¹ We have shown previously that angiotensin II and inflammatory cytokines reduces Sirtuin 3 expression by 30%.⁵ Meanwhile, the precise molecular mechanism of Sirtuin 3 inactivation remains elusive.

The failure of common antioxidants such as vitamin E and vitamin C in clinical trials of cardiovascular events was discouraging and urge a reassessment of the role of oxidative stress in cardiovascular disease.⁶ We suggest to rescue the Sirtuin 3 activity to reduce the SOD2 acetylation and activate the key intrinsic antioxidant SOD2 which is thousand times more effective in scavenging superoxide radicals than any low molecular weight antioxidant. Therefore, Sirtuin 3, can be “the improved antioxidant”.⁶

Our data demonstrate that SOD2 lysine residue K68 is hyperacetylated in animal models and human hypertension.^{1, 5} Recent studies show that expression of acetylation mimetic mutant SOD2-K68Q or SOD2-K68-Acetyl is accompanied with a change of SOD2 stoichiometry from homotetramer complex to a monomeric form. Biochemical experiments suggest that these monomers function as a peroxidase (using pyrogallol as the substrate) rather than superoxide dismutase.⁷ These data suggest a provocative idea that SOD2-K68 acetylation in hypertension is not just a “loss-of-function” modification but may represent a switch of “antioxidant SOD2” to “prooxidant peroxidase SOD2”. Further studies are needed to define the potential role of peroxidase activity of SOD2-K68-Acetyl in human pathological conditions.

It is important to note that Sirtuin 3 is a key node in regulation of both metabolic and antioxidant pathways.⁸ It activates mitochondrial metabolism by deacetylation of Krebs cycle,⁹ complex I ¹⁰ and fatty acid β-oxidation enzymes,¹¹ maintains mitochondrial redox status by deacetylation of isocitrate dehydrogenase 2 (IDH2),¹² and the effect of Sirtuin 3 inactivation is not limited by SOD2 hyperacetylation. Furthermore, Sirtuin 3 activity can be inhibited in response to metabolic or oxidative stress.^{5, 13} We appreciate the concerns raised by Zhou et al and encourage other groups to consider other post-translational modifications of Sirt3 and SOD2. Discussing divergent ideas with curiosity and openness are in the best interests of science.