Mammalian sirtuins are NAD+-dependent deacetylases/deacylases that have varied enzymatic activities. They function as cellular stress sensors to modify histones and other proteins to modulate diverse cellular processes. Epigenetic alterations are a conserved feature of biological aging in diverse organisms, and have been designated as a “hallmark of aging”. Chromatin organization – in particular, diminished heterochromatinization of repetitive regions – is progressively lost during cellular and organismal aging. The first example of age-associated heterochromatin perturbation in mammals was identified in a seminal study 25 years ago. This work focused on the Major Satellite Repeats (MSRs), pericentromeric repeats in the mouse that ensure proper chromosomal segregation and maintenance of euploidy. This study showed that MSR repression is lost during aging specifically in mouse myocardium. The mechanistic basis for this effect has never been elucidated.
We have found that MSR derepression is not associated with decreases in levels of the canonical repressive marks – DNA methylation, H3K9me3, or H3K56me3 – at the MSRs. Instead, levels of the activating marks, acH3K9 and acH4K16, targets of SIRT1, increase during aging in the heart. These findings have led to the hypothesis that loss of SIRT1 activity contributes to derepression of MSR loci in aged myocardium. The work presented in this thesis suggests the hypothesis that SIRT1 regulates genome stability in an age-dependent manner.
This thesis also focuses on the mitochondrial sirtuin, SIRT5, and its role in promoting melanoma cell survival. SIRT5 removes succinyl, malonyl, and glutaryl modifications from lysines on diverse protein targets, primarily in the mitochondrial matrix, thereby regulating multiple metabolic pathways. SIRT5 regulates various aspects of mitochondrial metabolism, and represents an attractive target for metabolic-directed therapies. In 10/10 human melanoma cell lines tested, SIRT5 knockdown resulted in rapid loss of proliferative potential and cell death. Likewise, SIRT5 loss impeded melanoma xenograft formation in mice. SIRT5 knockdown results in increased apoptotic cell death, which can be partially rescued by overexpressing anti-apoptotic BCL2, suggesting a novel role for SIRT5 in allowing cancer cells to evade apoptosis. Lastly, via metabolomics, we have found that SIRT5 regulates glucose and glutamine metabolism in melanoma. These data highlight the importance of a previously underappreciated sirtuin in modulating human healthspan.