Sirtuins participate in sensing nutrient availability and directing metabolic activity to match energy needs with energy production and consumption. However, the pivotal targets for sirtuins in cancer are mainly unknown. In this study, we identify the M2 isoform of pyruvate kinase (PKM2) as a critical target of the sirtuin SIRT2 implicated in cancer. PKM2 directs the synthesis of pyruvate and acetyl-CoA, the latter of which is transported to mitochondria for use in the Krebs cycle to generate ATP. Enabled by a shotgun mass spectrometry analysis founded on tissue culture models, we identified a candidate SIRT2 deacetylation target at PKM2 lysine 305 (K305). Biochemical experiments including site-directed mutants that mimicked constitutive acetylation suggested that acetylation reduced PKM2 activity by preventing tetramerization to the active enzymatic form. Notably, ectopic overexpression of a deacetylated PKM2 mutant in Sirt2-deficient mammary tumor cells altered glucose metabolism and inhibited malignant growth. Taken together, our results argued that loss of SIRT2 function in cancer cells reprograms their glycolytic metabolism via PKM2 regulation, partially explaining the tumor-permissive phenotype of mice lacking Sirt2. Cancer Res; 76(13); 3802–12. ©2016 AACR.

A fundamental observation in biology is that mitochondrial function, as measured by increased reactive oxygen species (ROS), changes significantly with age, suggesting a potential mechanistic link between the cellular processes governing longevity and mitochondrial metabolism homeostasis. In addition, it is well established that altered ROS levels are observed in multiple age-related illnesses including carcinogenesis, neurodegenerative, fatty liver, insulin resistance, and cardiac disease, to name just a few. Manganese superoxide dismutase (MnSOD) is the primary mitochondrial ROS scavenging enzyme that converts superoxide to hydrogen peroxide, which is subsequently converted to water by catalase and other peroxidases. It has recently been shown that MnSOD enzymatic activity is regulated by the reversible acetylation of specific, evolutionarily conserved lysine(s) in the protein. These results, suggest for the first time, that the mitochondria contain bidirectional post-translational signaling networks, similar to that observed in the cytoplasm and nucleus, and that changes in lysine acetylation alter MnSOD enzymatic activity. In addition, these new results demonstrate that the mitochondrial anti-aging or fidelity / sensing protein, SIRT3, responds to changes in mitochondrial nutrient and/or redox status to alter the enzymatic activity of specific downstream targets, including MnSOD that adjusts and/or maintains ROS levels as well as metabolic homeostatic poise.

One fundamental observation in cancer etiology is that the rate of malignancies in any mammalian population increases exponentially as a function of age, suggesting a mechanistic link between the cellular processes governing longevity and carcinogenesis. In addition, it is well established that aberrations in mitochondrial metabolism, as measured by increased reactive oxygen species (ROS), are observed in both aging and cancer. In this regard, genes that impact upon longevity have recently been characterized in S. cerevisiae and C. elegans, and the human homologs include the Sirtuin family of protein deacetylases. Interestingly, three of the seven sirtuin proteins are localized into the mitochondria suggesting a connection between the mitochondrial sirtuins, the free radical theory of aging, and carcinogenesis. Based on these results it has been hypothesized that Sirt3 functions as a mitochondrial fidelity protein whose function governs both aging and carcinogenesis by modulating ROS metabolism. Sirt3 has also now been identified as a genomically expressed, mitochondrial localized tumor suppressor and this review will outline potential relationships between mitochondrial ROS/superoxide levels, aging, and cell phenotypes permissive for estrogen and progesterone receptor positive breast carcinogenesis.