Cancer cells undergo metabolic reprogramming to support increased energy demands due to rapid proliferation. However, recent findings have revealed that this dynamic metabolic modulation occurs not only in cancer cells but in several other cell types as well. The mitochondrial sirtuin (SIRT5) regulates protein post-translational modifications on metabolic enzymes by catalyzing the removal of succinyl, malonyl, and glutaryl moieties. While Sirt5 KO mice are physiologically normal, with only mild phenotypes observed, recent studies have shown that SIRT5 functions as a tumor promoter and that the SIRT5 gene is amplified in human breast cancers and across other malignancies. We recently reported that genetic and pharmacological perturbation of SIRT5 impairs tumorigenesis and metastasis in both human breast cancer cells and mouse models. Our published findings established SIRT5 as a promising therapeutic target for selectively targeting aberrant cancer cell metabolism. However, many other cell types beyond cancer are highly susceptible to metabolic reprogramming. We, therefore, hypothesized that SIRT5 also functions as a master metabolic regulator in non-transformed cells in the tumor microenvironment (TME). To distinguish between tumor-intrinsic versus tumor-extrinsic effects functions for SIRT5, we transplanted SIRT5-proficient mouse mammary tumor cells (AT-3) into immunocompetent Sirt5 WT or KO host mice and examined tumor growth and metastasis. SIRT5 loss in the TME resulted in a striking and significant decrease in tumor weight and lung metastases in Sirt5 KO compared to Sirt5 WT hosts, highlighting an important role for SIRT5 in establishing a tumor-promoting microenvironment. Prior literature in the field has linked SIRT5 to the regulation of immune cells, in many cases macrophages, and our bulk RNA sequencing revealed that immune and metabolic transcripts were differentially expressed in Sirt5 KO vs WT MMTV-PyMT mammary tumors. Consequently, this led me to generate my own independent hypothesis which is that SIRT5 regulates macrophages in the TME. To examine this possibility, I performed flow cytometry analysis on primary mammary tumors from our MMTV-PyMT transgenic mice and discovered a prominent increase in pan-macrophage numbers in SIRT5 KO tumors compared to SIRT5 WT controls. These results prompted me to pursue in vitro culture of primary bone-marrow-derived-macrophages (BMDM) from SIRT5 WT and KO mice to perform additional experiments. Preliminary qPCR experiments suggested that Sirt5 KO BMDM express more pro-inflammatory genes when polarized to the classical “M1” state compared to Sirt5 WT BMDM. In parallel, we are also currently directly testing whether SIRT5 promotes a permissive, pro-tumorigenic TME via the regulation of immune cells by conducting bone-marrow transplants from Sirt5 WT and KO donors to Sirt5 WT recipient mice followed by orthotopic mouse mammary cancer cell transplantations and analysis as above. Future experiments will investigate the molecular mechanism behind SIRT5-mediated macrophage regulation in the TME via immunogenomics and metabolomics to identify the cellular composition, associated gene expression, and metabolite profiles of primary mammary tumors that differ in SIRT5 status. Taken together these results will be the first to show that SIRT5 has critical synergistic functions in both cancer cells and the surrounding TME to promote optimal breast cancer progression, expanding SIRT5’s utility as a novel drug target to cancer immunotherapy and potentially even other inflammatory and autoimmune diseases.