SIRT5’s GOT1 up on PDAC

Pancreas cancer represents the seventh leading cause of cancer-related mortality worldwide, and the third most common cause of cancer-related death in the US¹. In 2021, 48,220 individuals will die of pancreas cancer in this country². The most common pancreatic neoplasm is ductal adenocarcinoma (PDAC), accounting for >90% of pancreatic malignancies³. PDAC is highly aggressive, with a 5-year survival rate of ~10%^{1, 4}. Surgery, radiation, and chemotherapy are the only treatment modalities generally available for PDAC, and there are no approved targeted or immune-based therapies. Unfortunately, only 10–20% of PDAC patients present with surgically resectable disease⁵. These dire statistics highlight the urgent unmet need to develop new therapeutic approaches for PDAC.

Famously, the PDAC microenvironment is characterized by striking desmoplasia, a dense fibrous extracellular matrix resulting in high intra-tumoral pressure⁶. This promotes vascular collapse, inducing hypoxia and nutrient deprivation. To cope with their hostile neighborhood, PDAC cells rely on non-canonical metabolic pathways for growth and survival. Understanding these metabolic adaptations may allow development of novel effective therapeutic interventions for this devastating disease.

In this issue of Gastroenterology, Hu et al⁷ demonstrate that the SIRT5 protein suppresses PDAC progression by hindering non-canonical glutamine metabolism, via deacetylation and inactivation of cytosolic aspartate aminotransferase (GOT1). SIRT5 is a member of sirtuin family of NAD⁺-dependent protein deacylases⁸. The seven mammalian sirtuins, SIRT1–7, exhibit diverse sub-cellular localization patterns, enzymatic activities, and substrate specificities. SIRT5 primarily localizes to the mitochondrial matrix; however, a fraction of active extra-mitochondrial SIRT5 also exists in cells⁸. SIRT5 exhibits only very weak deacetylase activity, but instead preferentially removes negatively charged lysine modifications: succinyl, malonyl, and glutaryl groups. Metabolic enzymes represent a major class of SIRT5 targets. Despite SIRT5’s unique catalytic activity profile, no major phenotypes or marked metabolic abnormalities are observed in SIRT5-deficient mice under basal conditions⁸. However, like other sirtuins, SIRT5 has been implicated in neoplasia, as both an oncogene and a tumor suppressor, in a context-specific manner⁸. As an oncogene, SIRT5 promotes folate metabolism via activation of mitochondrial serine hydroxymethyltransferase (SHMT2), facilitating tumor cell growth in vitro and in vivo⁹. Folate metabolism is a target of several approved chemotherapy drugs. Likewise, SIRT5 inhibits pyruvate kinase muscle isozyme 2 (PKM2), resulting in accumulation of glycolytic intermediates, driving tumor growth¹⁰. In colorectal cancer, SIRT5 promotes entry of glutamine into the TCA cycle by activating glutamate dehydrogenase 1 (GLUD1)¹¹. Additionally, SIRT5 desuccinylates citrate synthase (CS), the rate-limiting enzyme in the TCA cycle, promoting its activity¹². CS hypersuccinylation inhibits its function and suppresses colorectal cancer cell proliferation and migration¹². In breast cancer, SIRT5 regulates glutamine metabolism by desuccinylating glutaminase (GLS), protecting it from ubiquitin-mediated degradation¹³. In melanoma, SIRT5 is required to maintain histone acetylation and methylation to promote expression of key genes, including MITF, a lineage-specific oncogene, and c-MYC¹⁴. Likewise, recent studies have documented oncogenic roles for SIRT5 in breast cancer and in AML (Bob, Mike cites).

Conversely, as a tumor suppressor, SIRT5 maintains fatty acid oxidation and redox homeostasis by inhibiting dimerization of acyl-CoA oxidase 1 (ACOX1), attenuating its function¹⁵. Consistently, in hepatocellular carcinoma, low SIRT5 expression is associated with increased ACOX1 succinylation and activity¹⁵. In AML, GBM, and certain other cancer types, isocitrate dehydrogenase (IDH) gain-of-function mutants convert α-ketoglutarate (α-KG) into the oncometabolite R-2-hydroxyglutarate (R-2HG), which in turn inhibits α-KG-dependent enzymes, including DNA and histone demethylases, thereby inducing epigenetic dysregulation⁸. Ectopic expression of SIRT5 reverses R-2HG-induced resistance to apoptosis in IDH1 mutant glioma cells, impairing their growth¹⁶.

In an elegant new study published in this issue of Gastroenterology, Hu and colleagues characterize a novel tumor suppressor function of SIRT5 in PDAC⁷. They show that SIRT5 levels are reduced in human PDAC, as well as in pancreatic tumors from an autochthonous mouse model. Low SIRT5 levels are associated with worsened mortality in PDAC patients. SIRT5 depletion exacerbates PDAC cell growth and tumorigenesis by patient-derived organoids, whereas overexpression of active SIRT5, but not an inactive mutant, impairs PDAC cells growth, showing that SIRT5’s catalytic activity is required for its tumor suppressor function in PDAC⁷. The authors further demonstrate that Sirt5 deletion accelerates pancreas cancer progression in multiple genetically engineered mouse PDAC models⁷.

As noted, SIRT5 plays roles in metabolic reprograming across diverse cancers^{9–13, 15, 16}. Many cancer types rely on glutamine-dependent anaplerosis to feed the TCA cycle. In contrast, PDAC cells utilize glutamine to maintain proper redox balance via a non-canonical mechanism, in which GOT1 converts glutamine-derived aspartate into oxaloacetic acid, which is subsequently converted into pyruvate and NADPH⁶. Hu et al show that SIRT5 deacetylates GOT1 at lysine 369, inhibiting its activity and hence suppressing this pathway⁷. SIRT5 knockdown (KD) increases glutamine uptake, and SIRT5 KD PDAC cells exhibit increased levels of glutamine-derived metabolic intermediates⁷. The authors also describe a novel small-molecule SIRT5 activator, MC3138. Treatment of PDAC cells with MC3138 phenocopies the effects of SIRT5 overexpression, reducing lysine acetylation on GOT1 and inhibiting its enzymatic activity⁷. Excitingly, MC3138 impairs viability of PDAC cell lines, and synergizes with gemcitabine treatment of PDAC cells and PDAC organoids, and in autochthonous PDAC models.

In summary, though SIRT5 exerts seemingly modest effects in normal cell types, it has now emerged as a major player across many malignancies, a growing list of cancers that now includes PDAC⁸. The findings of Hu et al., together with prior results in the sirtuin field, suggest that small molecule-mediated SIRT5 inhibition and activation might each represent effective and well-tolerated cancer treatments, in a tumor type-specific fashion. How SIRT5 plays such disparate functions, in a manner seemingly idiosyncratic to distinct cancers, remains an important open question for future work. Likewise, most studies have shown that SIRT5 possesses minimal deacetylase activity¹⁷, although a few investigators have documented SIRT5-mediated deacetylation of specific targets¹⁸. Whether SIRT5 may interact with distinct binding partners, and/or be subject to regulatory post-translational modifications in a cell type-specific manner, directing its activity to distinct modifications and/or protein targets, is also an important topic that needs to be addressed. Nevertheless, this new work hints that optimized SIRT5 activators, in combination with other therapies, might represent a needed new addition to the armamentarium against PDAC.

Figure 1. Major roles for SIRT5 in cancer cell metabolism.

Via deacylation (blue), SIRT5 regulates the activities of various metabolic enzymes (yellow) and functions as either an oncogene (red) or a tumor suppressor (green), in a context-specific manner. SHMT2, mitochondrial serine hydroxymethyltransferase⁹; PKM2, pyruvate kinase muscle isozyme 2¹⁰; GLS, glutaminase¹³; GLUD1, glutamate dehydrogenase 1¹¹; CS, citrate synthase¹²; SOD1, Cu/Zn superoxide dismutase¹⁹; ACOX1, acyl-CoA oxidase 1¹⁵; GOT1, cytosolic aspartate aminotransferase⁷. Some graphics in this figure were obtained and modified from Servier Medical Art from Servier (http://www.servier.com/Powerpoint-image-bank).