Sirtuins are involved in longevity assurance in organisms as evolutionarily diverse as yeast, worms, and mice. All members of the family have homology to histone deacetylases (HDACs), but each protein has unique characteristics as well. Individual family members have distinct tissue expression profiles, subcellular localization, and substrate specificity. Over the past few years, we’ve begun to learn a great deal about the specific targets and interactions of each sirtuin, and how these interaction contribute to their functions in prolonging lifespan.
The SIRT6 protein, one of seven sirtuins encoded by mammalian genomes, came onto biogerontologists’ radar with a report from Katrin Chua‘s group that its histone H3K9 deacetylase activity is required to maintain telomeric chromatin in a healthy state. Furthermore, SIRT6 is required for the proper localization of the Werner’s syndrome protein, WRN, to telomeres: in the absence of SIRT6, the WRN-telomere association becomes unstable, recapitulating several of the cellular phenotypes of Werner’s progeria. (SIRT6 isn’t the only sirtuin involved in WRN biology: SIRT1, the most well-studied member of the family, appears to directly deacetylate WRN).
A second association between SIRT6 and aging has been revealed by a new study from the Chua lab: SIRT6 associates with the transcription factor NF-κB and deacetylates histones at NF-κB-bound promoters, causing them to become less active. Genetic suppression studies suggest that SIRT6′s influence on lifespan might be primarily mediated by NF-κB. Kawahara et al.:
SIRT6 Links Histone H3 Lysine 9 Deacetylation to NF-κB-Dependent Gene Expression and Organismal Life Span
Members of the sirtuin (SIRT) family of NAD-dependent deacetylases promote longevity in multiple organisms. Deficiency of mammalian SIRT6 leads to shortened life span and an aging-like phenotype in mice, but the underlying molecular mechanisms are unclear. Here we show that SIRT6 functions at chromatin to attenuate NF-κB signaling. SIRT6 interacts with the NF-κB RELA subunit and deacetylates histone H3 lysine 9 (H3K9) at NF-κB target gene promoters. In SIRT6-deficient cells, hyperacetylation of H3K9 at these target promoters is associated with increased RELA promoter occupancy and enhanced NF-κB-dependent modulation of gene expression, apoptosis, and cellular senescence. Computational genomics analyses revealed increased activity of NF-κB-driven gene expression programs in multiple Sirt6-deficient tissues in vivo. Moreover, haploinsufficiency of RelA rescues the early lethality and degenerative syndrome of Sirt6-deficient mice. We propose that SIRT6 attenuates NF-κB signaling via H3K9 deacetylation at chromatin, and hyperactive NF-κB signaling may contribute to premature and normal aging.
NF-κB has been widely implicated in the aging process, especially in the context of inflammatory transcription resulting in “inflammaging.” Indeed, a very recent study has suggested that knocking down NF-κB activity is sufficient to reverse the effects of chronological aging in the skin, at least at the level of gene expression, possibly by blocking inflammatory transcription and allowing the tissue’s natural regenerative capacity to proceed without obstacle.
As with the WRN story, this isn’t the first time a sirtuin has been implicated in regulating the activity of NF-κB — but also as with WRN, the mechanisms of sirtuin action are distinct. Studies of chronic obstructive pulmonary disease have revealed that SIRT1 directly deacetylates NF-κB, reducing its activity. In contrast, SIRT6 appears to associated with NF-κB but then exploit this interaction to “follow” the transcription factor to promoters, where it deacetylates histone H3K9 and facilitates formation of a closed or inactive chromatin state. Kind of a neat team: SIRT1 directly deacetylates proteins of interest, while SIRT6 acts in the same location but operates on chromatin. Working together, the proteins may well have greater than additive impact.
Thus, there is partial redundancy of ultimate function, even though the proteins operate via different mechanisms. This might actually make it easier to intervene favorably in the affected processes, if separate agonists of SIRT1 and SIRT6 end up having a synergistic effect at target promoters (and telomeres).
(There’s also a nice preview/summary piece in the same issue of Cell, by Gioacchino Natoli.)