Exploring cancer metabolism using stable isotope-resolved metabolomics (SIRM)

Abstract

Metabolic reprogramming is a hallmark of cancer. The changes in metabolism are adaptive to permit proliferation, survival, and eventually metastasis in a harsh environment. Stable isotope-resolved metabolomics (SIRM) is an approach that uses advanced approaches of NMR and mass spectrometry to analyze the fate of individual atoms from stable isotope-enriched precursors to products to deduce metabolic pathways and networks. The approach can be applied to a wide range of biological systems, including human subjects. This review focuses on the applications of SIRM to cancer metabolism and its use in understanding drug actions.Metabolism is the collection of predominantly enzyme-catalyzed biochemical transformations that meet the growth and survival demands of an organism. For nearly a century, scientists have documented profound metabolic changes that occur in tumors (1, 2). One of the earliest insights into cancer metabolism came when Otto Warburg noted increased uptake and fermentation of glucose (Glc) 3 to lactate in tumors relative to surrounding tissue, even in the presence of ample oxygen (2). Clinical cancer diagnosis exploits this "Warburg effect" by measuring uptake of the Glc analogue 18 F-deoxyglucose using positron emission tomography (PET), as the metabolic demands of differentiated, non-proliferating tissues generally require far less Glc than tumors (3).Oncoproteins and tumor suppressors are well-established regulators of metabolism, and mutations or dysregulated expression can lead to the altered metabolic phenotypes observed in many cancers (4, 5). Inactivating mutations in enzymes such as fumarate hydratase (FH) or succinate dehydrogenase (SDH) induce pathophysiological accumulation of substrates that inhibit other critical enzyme functions, whereas less common gain-of-function mutations such as in isocitrate dehydrogenases (IDH) produce metabolites that directly deregulate cellular processes (6). Additionally, cancer cells frequently express fetal isoforms of metabolic enzymes that are subject to different regulatory mechanisms to provide growth advantages (7). Gaining a systematic understanding of the metabolic changes that occur during carcinogenesis can thus be exploited for therapeutic and diagnostic purposes.Stable isotope-resolved metabolomics (SIRM) is a powerful approach developed by others and by us where isotopically enriched precursors such as [ 13 C 6 ]Glc are administered to a biological system, and the ensuing metabolic transformations are determined using appropriate analytic techniques to enable robust reconstruction of metabolic pathways (8 -11). Stable isotopes are non-radioactive and in SIRM practice are identical chemically and functionally to the most abundant isotope of that element. Incorporating stable isotopes such as 2 H, 13 C, or 15 N into biological precursors has long been used to trace their metabolism in living systems (12). SIRM is thus distinct from non-tracer-based metabolomics profiling for statistical models. Here, we review SIRM applications and demonstrate h...

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