Working with non-human primates, scientists have discovered that the protein SIRT2, a member of the sirtuin family, might play an important role in slowing cardiac aging .
The heart of the matter
The heart is arguably the hardest worker among the organs, constantly pumping enormous amounts of blood without ever skipping a beat (well, almost). This marvel of evolution works for decades before it begins to show its age. Heart aging happens due to all the usual culprits, including chronic inflammation, mitochondrial dysfunction, oxidative stress, and telomere damage .
In this study published in Nature Aging, the researchers used long-tailed macaques to elucidate the molecular aspects of cardiac aging using multi-omics analysis. Unlike short-lived mice and rats, non-human primates like these have hearts that closely resemble those of humans and, due to their relatively long lifespan, suffer from spontaneous heart conditions as well.
Identifying SIRT2 as a potential player
The researchers compared the hearts of eight young (4-6 years) and eight aged (18-21 years) monkeys, which roughly translates to 16 and 65 human years. In aged monkeys, hearts exhibited all the familiar signs of aging: they contained more senescent cells and more fibrotic areas, and their heart muscle cells were significantly enlarged with structural abnormalities. The levels of several inflammatory factors were elevated as well.
Using proteomic analysis, the researchers identified 126 upregulated and 43 downregulated aging-associated differentially expressed proteins (DEPs). Further analysis showed that the upregulated DEPs were mainly related to inflammation, blood clotting, and fibrosis, while protein synthesis, mitochondrial function, and lipid metabolism DEPs were downregulated.
The researchers then compared those DEPs to genes known to be involved in age-related cardiovascular diseases. SIRT2, a SIRT family protein that often pops up in studies of aging , was the only protein that was downregulated in aged monkey hearts and was also linked to all four types of cardiovascular diseases. It was also the only downregulated DEP that overlapped with the aging-related genes from the Aging Atlas database.
The SIRT2 – STAT3 – CDKN2B axis
The scientists then generated human SIRT2-deficient cardiomyocytes from embryonic stem cells. The resulting cells resembled old cardiomyocytes, including hypertrophy and an increased percentage of senescent cells. They also showed signs of mitochondrial dysfunction.
Transcriptomic analysis showed that many genes were either upregulated or downregulated in SIRT2-deficient cardiomyocytes compared to young healthy cells. The researchers were able to identify the transcription factor (a gene that regulates expression of other genes) STAT3 as a major driver of those changes. STAT3 is also a well-known mediator of inflammation.
Notably, STAT3 was the only transcription factor that controlled changes in gene expression in both SIRT2-deficient human cardiomyocytes and in aged hearts of monkeys of both sexes. The researchers then confirmed via a technique called co-immunoprecipitation that STAT3 was one of the few transcription factors to interact with SIRT2.
SIRT2 acts by deacetylating proteins (removing an acetyl group from lysine residues, which alters the protein’s function). Overexpression of SIRT2 led to decreased levels of acetylated STAT3, suggesting that SIRT2 recognizes STAT3 as a substrate. STAT3 levels did not react to overexpression of a SIRT2 mutant that lacked deacetylation ability. Aged monkey hearts had more acetylated STAT3 than young ones, showing impaired deacetylation.
SIRT2 rescues cardiac aging in mice
Since transcription factors act by changing the expression of other genes, the researchers searched for downstream targets that would be relevant to the cardiac aging phenotype. They identified the gene CDKN2B, which encodes the senescence-related protein p15, as an important target. Apparently, acetylated STAT3 induces the transcription of CDKN2B, which, in turn, induces cellular senescence in cardiomyocytes. By deacetylating STAT3, SIRT2 intervenes in this process and ameliorates cardiac aging.
The researchers tested this hypothesis by injecting the hearts of aged mice with viral vectors containing SIRT2. Decreased ejection fraction and fractional shortening, two major markers of cardiac aging observed in old mice, were partially reversed by the treatment, as was the age-related enlargement of cardiomyocytes, indicating a possible cardioprotective role for SIRT2.
In this study, we systemically surveyed the multi-dimensional profiles of the NHP heart and unveiled a panel of critical biological pathways that shifted during primate heart aging. We identified SIRT2 as a key mediator of geroprotection in primate heart aging and showed that SIRT2-deficient human cardiomyocytes recaptured key senescence features of aged primate hearts. We also found that SIRT2 formed complexes with STAT3 and deacetylated it on Lys685, which, in turn, transcriptionally inactivated the senescence inducer CDKN2B. Thus, our results suggest a SIRT2–STAT3–CDKN2B axis, regardless of sex, in the regulation of primate cardiomyocyte senescence.
 Ye, Y., Yang, K., Liu, H., Yu, Y., Song, M., Huang, D., … & Liu, G. H. (2023). SIRT2 counteracts primate cardiac aging via deacetylation of STAT3 that silences CDKN2B. Nature Aging, 1-19.
 Li, H., Hastings, M. H., Rhee, J., Trager, L. E., Roh, J. D., & Rosenzweig, A. (2020). Targeting age-related pathways in heart failure. Circulation research, 126(4), 533-551.
 de Oliveira, R. M., Sarkander, J., Kazantsev, A. G., & Outeiro, T. F. (2012). SIRT2 as a therapeutic target for age-related disorders. Frontiers in pharmacology, 3, 82.