SOD2 Acetylation and Deacetylation: Another Tale of Jekyll and Hyde in Cancer

COMMENTARY COMMENTARY SOD2 acetylation and deacetylation: Another tale of Jekyll and Hyde in cancer Anita B. Hjelmelanda,b and Rakesh P. Patela,c,1 Subsets of highly invasive, therapy-resistant tumor tumor-suppressive and -promoting functions have been cells contribute to the development of metastasis and described (13). At physiologic levels, SOD2 is antitu- treatment failures. Recent evidence suggests that these morigenic; lower dismutase activity leads to stabiliza- tumor cell subsets are enriched for cancer stem cells tion of HIF1α and underlies cancer cell adaptation to (CSCs) (1–3). Similar to nonneoplastic stem cells, CSCs hypoxia (14). However, SOD2 expression is elevated express specific markers and transcription factors and in many cancers, and sites of metastasis have higher can self-renew or differentiate. For example, breast SOD2 levels compared to primary tumors (15–18). If CSCs are identified as positive for CD44, ALDH1 activ- SOD2 is an antioxidant, why does its overexpression ity, and/or expressing SOX2, OCT4, or Nanog (4). Com- not confer greater protection, but instead result in a pared to bulk tumor cells, CSCs are often more resistant flipping of its function to a procancer role? Insights to cell death, including that induced by chemo- or ra- into this conundrum are provided by He et al. (10). diotherapy. Furthermore, CSCs are metabolically plastic They show that CSC gene expression signatures were with different redox states associated with epithelial- or greater in SOD2-overexpressing MCF7 cells (breast mesenchymal-like breast CSCs (5). Hypoxia is an impor- cancer-derived epithelial cells) compared to paren- tant inducer of CSC phenotypes, and hypoxia-inducible tal controls. SOD2-overexpressing cells were more factor (HIF) 2α is known to mediate OCT4 up-regulation mesenchymal-like and displayed increased growth (6–9). Importantly, CSCs are enriched for the ability to and invasiveness in vitro, key functional end points propagate tumors in immunocompromised mice, ac- supporting prometastatic and tumorigenic potential. counting for their alternative designation as tumor- To determine how SOD2 overexpression could initiating cells. However, this name should not imply promote reprogramming toward a CSC-like state, He that the CSC/tumor-initiating cell is the cell of origin et al. (10) focus on HIF2α as a critical downstream for the cancer. While it is true that stem cell acquisition of mediator. HIF2α, but not HIF1α, protein was increased mutations can lead to tumorigenesis, the CSC hypoth- in SOD2-overexpressing MCF-7 cells. EPAS1 (HIF2α) esis indicates the importance of targeting the existing messenger RNA (mRNA) was also increased, indicat- population of cancer cells with stem cell-like character- ing that changes in EPAS1 transcription contribute to istics in order to prevent disease recurrence. Thus, and/or are a product of SOD2-mediated CSC pheno- understanding how CSC function and survival are types. Targeting EPAS1 but not HIF1A in cells with controlled is important. In PNAS, He et al. (10) extend SOD2 overexpression decreased expression of POU5F1 previous findings (11, 12) and further establish a causal (Oct4) and Nanog transcripts, demonstrating the im- relationship between manganese superoxide dismu- portance for HIF2α in breast CSC maintenance. As tase (SOD2) overexpression and CSC formation, and HIF2α is suggested to be stabilized at higher oxygen provide a mechanism that explainsthisassociation tensions than HIF1α (8, 19) and the majority of He involving acetylated SOD2, increased mitochondrial et al.’s (10) experiments were performed in normoxia, H2O2,andHIF2α expression. the results suggest that a SOD2/HIF2α axis could in- Changes in metabolism, bioenergetics, and redox crease stem cell/hypoxia signals even when oxygen signaling are established hallmarks of cancer. Within tensions are high, as in a perivascular niche. It will be this framework, the role of SOD2, which works in interesting to determine whether there are synergistic the mitochondria to catalyze the oxidation and reduc- effects under hypoxia, particularly for well-established tion (dismutase activity) of 2 superoxide anion mole- hypoxia-induced phenotypes such as invasion and stem cules to O2 and H2O2, respectively, is complex: Both cell maintenance. aCenter for Free Radical Biology, University of Alabama at Birmingham, Birmingham, AL 35294; bDepartment of Cell, Developmental and Integrative Biology, University of Alabama at Birmingham, Birmingham, AL 35294; and cDepartment of Pathology, University of Alabama at Birmingham, Birmingham, AL 35294 Author contributions: A.B.H. and R.P.P. wrote the paper. The authors declare no competing interest. Published under the PNAS license. See companion article on page 23534. 1To whom correspondence may be addressed. Email: [email protected]. First published November 6, 2019. 23376–23378 | PNAS | November 19, 2019 | vol. 116 | no. 47 www.pnas.org/cgi/doi/10.1073/pnas.1916214116 Downloaded by guest on October 2, 2021 K68Ac K68Ac SOD2 SOD2 - O2 H2O2 SOD2 Overexpression Catalase H2O2 2H+ Acetylation SOD2 K68Ac Deacetylation H2O+O2 - SOD2 O2 Peroxidase O2 Superoxide Dismutase Oxidative Catalase H2O2 stress H2O+O2 HIF2a CSC Fig. 1. Acetylated (Jekyll) and deacetylated (Hyde) SOD2 and cancer stem cell formation. Acetylated SOD2 loses dismutase activity and acquires peroxidase activity that promotes an H2O2-dependent feed-forward mechanism for generated further H2O2, activating HIF2α and cancer stem cell formation. Next, He et al. (10) address how SOD2 overexpression results different pools of SOD2, nonacetylated vs. acetylated, with the first in elevation of HIF2α in relation to SOD2 catalytic activity. A com- providing substrate for the latter? The model proposed by He et al. pelling set of data indicate that SOD2-dependent induction of the (10) could involve H2O2-induced H2O2 formation, a feed-forward CSC-like phenotype is not related to superoxide dismutation per pathway for which there is precedent. For example, endothelial se but a peroxidase activity that is associated with SOD2 acetyla- NOX4-derived H2O2 promotes subsequent NOX2-derived H2O2 tion. The authors conclude that it is not the fact that SOD2 ex- in the mitochondria to regulate angiogenesis (26). Such data are pression is higher in cancer that is important, but that acetylation leading to a deeper appreciation that mitochondria are hubs that of SOD2 is also elevated coincidentally with higher protein ex- integrate redox-signaling networks, via retrograde mechanisms, pression. Previous studies, by this group and others, demonstrate across the cell. Identification of SOD2K68Ac and its role in pro- that SOD acetylation on lysine 68 (SOD2K68) results in a loss of foundly altering cell phenotype adds to a growing list of examples. dismutase activity and a gain of peroxidase activity (20–22). In The conclusion that SOD2K68 links variations in SOD2 activity enzymes, whose primary function is as a peroxidase, substrate to SOD2 expression to HIF2α and a CSC phenotype was derived H2O2 is reduced to water and coupled to the oxidation of a spe- from 3 key complementary experiments. First, down-regulation of cific substrate. With SOD2, like other pseudoperoxidases, oxi- the mitochondrial deacetylase, Sirt3, increased SOD2 acetylation dation of nonspecific substrates may occur, and the result is and the molecular signatures indicative of more cancer stemness. oxidative damage. Indeed, the peroxidase activity of SOD2 has Critically, these responses were attenuated with concomitant been shown to increase oxidative damage to mitochondria and SOD2 knockdown, strongly supporting SOD2 acetylation as the sensitize cells to peroxide stress (23, 24). He et al. (10) demon- crucial step. Second, silencing the mitochondrial acetyl transferase, strate that scavenging of H2O2 prevented the increase in OCT4 GCN5L1, to decrease acetylation, led to lower CSC markers. Finally, and Nanog mRNA, as well as HIF2α and cancer stemness in expression of an acetylation mimic or resistant mutant, SOD2K68Q SOD2-overexpressing cells. This sets up an interesting proposi- and SOD2K68R, respectively, altered the CSC phenotype in a man- K68 tion whereby SOD2 uses H2O2 as a substrate for the peroxi- ner consistent with SOD2 acetylation leading to HIF2α expression. dase reaction, and somehow this leads to further H2O2 generation Underscoring the translational relevance of the proposed mecha- that selects for surviving CSC-like cells and/or is key for repro- nism, key insights from in vitro cell function and phenotyping ex- gramming. This model raises a number of questions and warrants periments were verified in mouse models and human tissue. Further consideration of redox signaling and oxidative stress paradigms. studies evaluating how the balance between acetyltransferase Is formation of H2O2 via SOD2-peroxidase activity a direct effect and deacetylase activity is regulated by or in coordination with of peroxidase activity, or indirect? Presumably it is the latter and SOD2 expression are needed. Interestingly, Sirt3 silencing, while involves damage to endogenous mitochondrial components that increasing acetylation, also changed SOD2 expression, suggesting K68Ac then results in increased H2O2. How SOD2 -derived H2O2 a coordinated mechanism. Acetylation will also be modulated by activates HIF2α is unclear; is this selective for SOD2K68Ac-derived metabolic status and supply of substrate acetyl CoA. Whether dif- H2O2, or can other sources of H2O2 also mediate this response? ferential

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