Cancer cell metabolism: the essential role of the nonessential amino acid, glutamine

Abstract

Biochemistry textbooks and cell culture experiments seem to be telling us two different things about the significance of external glutamine supply for mammalian cell growth and proliferation. Despite the fact that glutamine is a nonessential amino acid that can be synthesized by cells from glucose-derived carbons and amino acid-derived ammonia, most mammalian cells in tissue culture cannot proliferate or even survive in an environment that does not contain millimolar levels of glutamine. Not only are the levels of glutamine in standard tissue culture media at least ten-fold higher than other amino acids, but glutamine is also the most abundant amino acid in the human bloodstream, where it is assiduously maintained at approximately 0.5 mM through a combination of dietary uptake, de novo synthesis, and muscle protein catabolism. The complex metabolic logic of the proliferating cancer cells' appetite for glutamine-which goes far beyond satisfying their protein synthesis requirements-has only recently come into focus. In this review, we examine the diversity of biosynthetic and regulatory uses of glutamine and their role in proliferation, stress resistance, and cellular identity, as well as discuss the mechanisms that cells utilize in order to adapt to glutamine limitation.

Keywords: cancer; glutamine metabolism; proliferation; stress response.

Figure 1. Glutamine supplies nitrogen and carbon for biosynthetic reactions

(A) Chemical structure of glutamine. (B) Usage of γ‐ and α‐nitrogen of glutamine in mammalian cells. GLS: glutaminase; GDH: glutamate dehydrogenase; ATs: aminotransferases. (C) Glutamine‐derived carbon enters the TCA cycle through α‐KG to supply anaplerotic substrates. Glucose‐derived pyruvate can enter the TCA cycle through OAA. This reaction is mediated by PC, which is suppressed when glutamine‐derived carbon enters the TCA cycle. Gln: glutamine; α‐KG: α‐ketoglutarate; Suc: succinate; Fum: fumarate; Mal: malate; OAA: oxaloacetate; Cit: citrate; Glu: glutamate; Asp: aspartate; Asn: asparagine; Glc: glucose; Pyr: pyruvate; Ac‐CoA: acetyl‐CoA; SDH: succinate dehydrogenase; FH: fumarase; MDH: malate dehydrogenase; GOT: aspartate aminotransferase; ASNS: asparagine synthetase; PC: pyruvate carboxylase; IDH1/2: isocitrate dehydrogenase 1/2. IDH1 is localized in cytosol.

Figure 2. The key role of glutamine‐derived glutamate in glutathione biosynthesis

Glutamine‐derived glutamate is a necessary substrate to synthesize glutathione. In addition, glutamate functions as an exchanging counter ion to import extracellular cystine through the xCT transporter. In the cell, cystine is converted to cysteine that is used as a second substrate for glutathione biosynthesis. Gln: glutamine; Glu: glutamate; Gly: glycine; GSH: glutathione; GSSG: glutathione disulfide; α‐KG: α‐ketoglutarate: Ser: serine; Met: methionine; HomoCys: homocysteine; GCL: glutamate–cysteine ligase; GS: glutathione synthetase; CBS: cystathionine beta‐synthase; PSAT1: phosphoserine aminotransferase 1.

Figure 3. Glutamine‐derived α‐KG is a substrate for α‐KG‐dependent dioxygenases

α‐KG is the substrate of Jumonji C histone demethylases (JHDM) and TET DNA demethylases and therefore mediates histone and DNA demethylation. In addition, α‐KG is the substrate of prolyl hydroxylase (PHD) that mediates HIF1α ubiquitination and degradation. These α‐KG‐dependent dioxygenases convert α‐KG to succinate that can feedback inhibit their dioxygenase activity. Either cancer‐associated IDH1/2 mutations or oxygen limitation can cause accumulation of 2‐HG, which can competitively inhibit α‐KG‐dependent dioxygenases.

Figure 4. Glutamine uptake and de novo biosynthesis

(A) Glutamine uptake can be mediated by ASCT2 and SNAT1/2 transporters, allowing the application of ¹⁸F‐FGln‐based PET imaging as a diagnostic tool for brain tumors. Furthermore, glucose‐derived carbon can be used as precursors to synthesize glutamine de novo. (B) A potential mechanism for asparagine as the nitrogen source for glutamine biosynthesis. However, this activity has only been reliably observed in unicellular organisms thus far. GLUL: glutamate‐ammonia ligase (glutamine synthetase); MSO: methionine sulfoximine.

Figure 5. Glutamine acquisition through proteolytic scavenging

Extracellular proteins (macropinocytosis) and live/dead cells (entosis and phagocytosis) can be engulfed and digested in lysosomes to release free amino acids, including glutamine. In addition, intracellular proteins and organelles can also be digested in lysosomes to release free amino acids via macroautophagy.