Sirtuins :Role in Mitochondrial Metabolism and Aging

The sirtuins are a family of proteins that act predominantly as nicotinamide adenine dinucleotide (NAD)-dependent deacetylases. In mammals seven sirtuin family members exist, including three members, Sirt3, Sirt4, and Sirt5, that localize exclusively within the mitochondria. Although originally linked to life-span regulation in simple organisms, this family of proteins appears to have various and diverse functions in higher organisms.

THE SIRTUIN FAMILY OF PROTEINS

Originally characterized in yeast as regulators of life span the sirtuins are an evolutionarily conserved family of proteins that appear to exert a wide range of biological function. In mammals seven sirtuin family members exist. Sirt1 appears to be the closest mammalian homolog to yeast Sir2, the first member of the sirtuins linked to aging. Because of this homology, initial studies of mammalian sirtuins focused predominantly on the biology of Sirt1. Insights from these studies have implicated Sirt1 in the regulation of a variety of metabolic phenotypes including insulin secretion, lipid mobilization from adipocytes, and regulation of glucose tolerance. With that said, the role of sirtuins in mammalian aging remains an open question, and even some of the earlier work in lower organisms involving Sir2 and life span has recently been questioned .

Although there has been considerable attention directed toward Sirt1 biology, there is also a growing interest in understanding the function of the related family members. It is clear that each of the mammalian sirtuins has a distinct subcellular localization. Sirt1, Sirt6, and Sirt7 are nuclear proteins, although a fraction of Sirt1 can be found in the cytosol. Sirt2, on the other hand, is predominantly cytosolic, although again it can be found in the nucleus in certain situations . Finally, three sirtuins—Sirt3, Sirt4, and Sirt5—appear to be found exclusively in the mitochondria. From a human genetics point of view, the strongest association between aging and sirtuins is the association between polymorphisms in the mitochondrial Sirt3 and longevity.

The ability of sirtuins to influence metabolism and potentially life span is believed to revolve around the ability of sirtuin family members to function as protein deacetylases. In addition to this enzymatic function, Sirt4 can further act to ADP ribosylate target proteins. Unlike other protein deacetylases, sirtuins require nicotinamide adenine dinucleotide (NAD) as a cofactor in the deacetylation reaction . The link among NAD, NADH, and sirtuin activity has led many to believe that this family of proteins acts in some fashion as a sensor of energetic status. This may particularly be true in the mitochondria, where levels of NAD and NADH are high and where a disproportionate fraction of proteins appear to be acetylated .

Sirt1 AND MITOCHONDRIAL FUNCTION

Evidence suggests that mitochondrial biogenesis is regulated at least in part by proliferator-activated receptor coactivator-1α (PGC-1α), a transcriptional coactivator of peroxisome proliferator-activated receptor-γ (PPARγ) as well as other transcription factors . It was therefore of considerable interest when it was shown that PGC-1α was in fact a deacetylation target of Sirt1 and that acetylation regulated PGC-1α activity . There are at least 13 lysine residues on PGC-1α that appear to be reversibly acetylated . Site-directed mutants that lack all 13 of these sites alter the ability of PGC-1α to regulate gene expression, although it remains unclear if all, or only a subset, of PGC-1α’s acetylation sites are truly regulatory in nature. Sirt1 appears to be the predominant in vitro and in vivo regulator of PGC-1α deacetylation. For instance, in vitro knockdown of Sirt1 in hepatic cells leads to increased PGC-1α acetylation with a corresponding reduction in a set of genes that are the rate-limiting enzymes responsible for hepatic gluconeogenesis . Similarly, both overexpression and knockdown studies support a role for Sirt1 in regulating PGC-1α activity through reversible deacetylation, which in turn has dramatic effects on in vivo hepatic glucose and lipid metabolism. A similar relationship appears to exist in skeletal muscle. In particular, in skeletal muscle, fasting was shown to lead to a Sirt1-dependent deacetylation of PGC-1α, and this deacetylation appeared to be required for PGC-1α-dependent gene expression, including gene products required for effective mitochondrial biogenesis . Together these studies link Sirt1 and PGC-1α activities in metabolically active tissues such as the liver and skeletal muscle .

Sirt3 AND MITOCHONDRIAL FUNCTION

The studies we have reviewed indicating that the predominantly nuclear Sirt1 was an important regulator of mitochondrial function spurred interest in the biology of those sirtuin family members that directly localize to this organelle. Comparative analysis of total liver mitochondrial protein acetylation following distinct genetic knockout of each of the three mitochondrial-enriched sirtuins—Sirt3, Sirt4, and Sirt5—showed that Sirt3 is the major mitochondrial deacetylase . At the same time, the dynamic flux in mitochondrial protein acetylation in response to changes in caloric load as illustrated by feeding and fasting , caloric restriction, and caloric excess suggest that, as is the case with Sirt1, Sirt3 may possess a nutrient-sensing regulatory role governing mitochondrial protein function.

Sirt3 is also emerging as a regulatory protein in the modulation of additional mitochondrial programs, including the deacetylation and inhibition of the mitochondrial ribosomal protein L10 (MRPL10) . This results in an NAD-dependent inhibition of mitochondrial protein synthesis, which might function as an energy-sparing response under nutrient-restricted conditions.

The mitochondrial sirtuins also appear to play an important role in the control of reactive oxygen species. This regulatory role may be particularly relevant to modulating the development of age-associated degenerative conditions.

Sirt5 AND MITOCHONDRIAL FUNCTION

The identification of targets of Sirt5 in the mitochondria has been more limited, although the urea cycle enzyme carbamoyl phosphate synthetase 1 (CPS-1) has been identified as a Sirt5 deacetylation target. The activation of CPS-1 catalyzes ammonia to urea and would be expected to have ameliorative effects via the elimination of oxidative stress-promoting ammonium.

CONCLUSIONS

Although originally described in yeast, the mammalian sirtuins represent an intriguing family of proteins that appear to function as sensors and regulators of metabolic status. Included among this family’s diverse function is the coordinated control of mitochondrial activity. This regulation includes the creation and targeted destruction of mitochondria by Sirt1, as well as the regulation of substrate utilization and oxidative phosphorylation by Sirt3.

In addition to these biochemical properties, there are tantalizing clues that sirtuins may play an important physiological role in overall metabolic homeostasis and perhaps in modulating age-related metabolic pathologies. Finally, although the predominant function of the sirtuin family revolves around NAD-dependent lysine deacetylation, other less-characterized enzymatic activities including ADP ribosylation, as well as other lysine modifications (e.g., demalonylation and desuccinylation), are just beginning to be explored. Although considerable gaps exist in our understanding, further dissection of sirtuin biology promises to provide important insight into how metabolic supply is coupled to mitochondrial activity.