Cellular death may occur through a number of mechanisms and varied pathways. Apoptosis requires energy resulting in cellular shrinkage, nuclear fragmentation, chromatin condensation, chromosomal DNA disruption, and lastly cellular fragmentation into apoptotic bodies that may be engulfed by phagocytic cells. In contrast, necrosis is not energy dependent and results in loss of cellular membrane integrity, swelling, and eventual lysis. Although previously thought to be a passive process, accumulating evidence shows that necrosis may also be programmed.1-3 This regulated form of necrosis, necroptosis, may have important implications for a number of processes including myocardial infarction, stroke, neurodegenerative disorders, and infections.1-3
A number of ligands including TNF-α, FASL and TRAIL can promote both apoptosis and necrosis. Specifically, TNF-α can promote apoptosis through upregulation of the serine/threonine kinase RIP1 or necrosis through complex formation with the kinase RIP1 and RIP3.1-6 Although both RIP1 and RIP3 appear to be required for necrosis, the mechanisms that control these processes are unclear.
Sirtuin proteins possess either histone deacetylase or monoribosyltransferase activity and SIRT2 has been found to be important in a number of cellular processes including mylination,7 tubulin deacetylase,8 regulation of cell cycle progression,9 and tumor suppression.10 Most recently, Narayan et al found that SIRT2 co-precipitates with RIP3 when cells were programmed to undergo TNF-α induced necroptosis (current reference).
To further assess the interactions between SIRT2 and RIP, mouse embryonic fibroblasts were obtained from SIRT2 (−/−) or wild-type mice. In wild type mice, TNF-α induced RIP1-RIP3 complex formation which did not form in cells obtained from SIRT (−/−) mice. Use of a short hairpin RNA to block SIRT2 also inhibited necrosis. Treatment with the specific SIRT2 deacetylase inhibitor AGK23,11 inhibited the formation of the RIP1–RIP3 complex and programmed necrosis. Above results demonstrate the necessity of SIRT2 in the formation of RIP1-RIP3 complex and programmed necrosis following activation with TNF-α.
Using a variety of inhibitors, the authors also found that TNF-α brings RIP1 and the RIP3–SIRT2 complex into close proximity resulting in SIRT2-dependent deacetylation of RIP1. Lysine 530 was identified as being the target of SIRT2 and following deacetylation, RIP1 lysine residues can then be deacetylated by SIRT2.
Using a RIP1-deficient cell line, the authors found that necrosis was greatly diminished in lysine 530 RIP1 mutants vs wild-type RIP1 cells. This provides further support that SIRT2-dependent deacetylation of lysine 530 of RIP1 is necessary for stable RIP1-RIP3 complex formation and ligand-dependent programmed necrosis.
Previous studies have found that inhibition of necrosis can reduce ischemia-reperfusion injury in the brain and heart.12,13 When wild-type or SIRT2 (−/−) mice were subjected to myocardial ischemia-reperfusion model, the hearts of SIRT2 (−/−) mice had better performance and approximately a 50% reduction in myocardial infarction. As found in cellular studies following treatment with TNF-α, ischemia-reperfusion also lead to in vivo deacetylation of RIP1 which reducted in SIRT2 (−/−) mice or wild-type mice treated with the SIRT2 inhibitor AGK2. Ischemia-reperfusion also resulted in increased RIP1-RIP3 interaction which did not occur in SIRT2 (−/−) mice or wild-type mice treated with AGK2. AGK2 treatment also resulted in functional recovery following ischemia-reperfusion and small infarcts.
These results provide a role for SIRT2 in regulating necroptosis through binding to RIP3 and deacetylation of RIP1. As sirtuins are postulated to be regulated by NAD levels, the balance between apoptosis and necrosis maybe regulated by cellular energetic and tissue NAD levels.14 Similar models of necrosis have been noted in both ischemia-reperfusion following stroke neurodegenerative disorders, and infection.1-3 A number of other factors are likely involved and further studies will be necessary to develop a broader view of the regulation behind necroptosis. Despite these areas of uncertainty, inhibition of SIRT2 or RIP1-RIP3 may provide an interesting and possibly more comprehensive avenue to provide neuroprotection following a number of cerebral insults.
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