THE MITOCHONDRIAL SIRTUIN SIRT5 ENABLES METABOLIC REPROGRAMMING AND METASTASIS IN BREAST CANCER
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Sirtuins are NAD+-dependent enzymes involved in processes such as metabolism, aging, and cancer. Sirtuin 5 (SIRT5), localized mainly in mitochondria, removes negatively charged acyl groups like malonyl and succinyl from lysine residues of protein substrates. Upregulated in various cancers, including breast cancer, SIRT5 may have tumor-promoting functions, making it a potential therapeutic target. Prior studies showed that SIRT5 disruption in triple-negative breast cancer cells (MDA-MB-231) impaired their proliferation, anchorage-independent growth, and invasiveness. In the MMTV-PyMT mouse mammary tumor model, Sirt5 loss reduced primary tumor growth and lung metastasis. To investigate the role of SIRT5 in metastasis, we used allograft mouse models to bypass primary tumor effects and focus on metastasis. Intracardiac and tail vein injections of Sirt5-deficient AT-3 cells into syngeneic mice revealed that SIRT5 promotes liver and lung metastasis. Together, these findings suggest that SIRT5 is crucial in both primary tumor growth and subsequent metastatic steps. Additionally, SIRT5 loss in AT-3 cells caused significant cytoskeletal changes, with increased stress fibers and filopodia formation, indicating actin disruption. Western blotting revealed reduced E-cadherin and elevated N-cadherin and Vimentin levels, markers of the epithelial-mesenchymal transition (EMT). RNA sequencing showed that genes associated with motility and known for inducing EMT were significantly upregulated in Sirt5-deficient cells. In human breast tumors, cell clusters with low SIRT5 mRNA expression upregulate genes associated with EMT and motility, mirroring transcriptional changes observed in our Sirt5 KO AT3 cells. Similarly, epithelial-like mouse mammary tumor cells exhibit higher SIRT5 mRNA and protein expression than quasi-mesenchymal-like mouse mammary tumor cells. Finally, we explored the impact of SIRT5 in mitochondrial metabolism. RNA sequencing revealed downregulation of genes associated with mitochondrial-related processes including cellular respiration, in Sirt5-deficient cells. Sirt5 loss in AT-3 cells also led to impaired mitochondrial metabolism and glycolysis, highlighting the role of SIRT5 in energy production. Overall, SIRT5 is critical for regulating cancer cell metabolism, with major impacts on cytoskeletal organization, invasiveness, and metastasis. Targeting SIRT5 with inhibitors could offer a promising therapeutic approach for breast cancer.