Glucose is an essential nutrient for all cells, as it is not only metabolized through glycolysis and the tricarboxylic acid (TCA) cycle to produce energy but is also converted into various biological substances (Fig. 1). For glucose to be utilized by cells, it must first be taken up across the cell membrane by membrane proteins called sugar transporters. These sugar transporters belong to two families: facilitative glucose transporter (GLUT encoded by SLC2A), which takes up glucose based on the difference in intracellular and extracellular glucose concentration, and the sodium-driven glucose symporter (SGLT encoded by SLC5A), which takes up glucose against glucose concentration using the Na+ gradient. Recently, the SWEET (Sugars Will Eventually be Exported Transporter; encoded by SLC50A1) family has been newly identified as a unique family of glucose transporters (Chen et al., 2015, Feng and Frommer, 2015). Unlike GLUTs and SGLTs, SWEETs are not classified as the major facilitator superfamily (MFS). GLUTs are expressed in most cells and are involved in glucose transport and homeostasis, whereas SGLTs are mainly expressed in the brush border membranes of the small intestine and kidney to absorb or reabsorb glucose into the body. SWEETs are involved in intracellular trafficking and cellular efflux of sugars in plants; however, they remain to be characterized in mammals. As GLUT proteins are intricately involved in tumor development and progression, several GLUT inhibitors have been discovered, mainly in the fields of chemical biology and anti-cancer drug discovery and development. In this chapter, we focus on the small-molecule inhibitors of GLUTs.

GLUT proteins, encoded by the SLC2A genes, are members of the MFS transporters and function as uniporters responsible for the facilitated diffusion of glucose across cell membranes. Since GLUT1 was cloned in 1985, GLUT proteins have been clubbed into a gene family consisting of 14 isoforms classified into three subclasses based on the characteristics of the gene structure: Class I (GLUTs 1, 2, 3, 4, and 14), Class II (GLUTs 5, 7, 9, and 11), and Class III (GLUTs 6, 8, 10, and 12, and HMIT/13) (Table 1) (Holman, 2020, Zhao and Keating, 2007). They have 12 transmembrane domains with intracellular N- and C-termini, and each has different affinities for glucose and other sugars, such as galactose and fructose (Table 1). Of the 14 members, class I GLUTs1–4 and GLUT5 have been extensively studied for the biological characterization of transporters.

GLUT1 is expressed in many cell types and exhibits a high affinity for glucose, with a Km value of approximately 2 mM, which is significantly lower than the blood glucose level (5 mM after fasting in humans), indicating that GLUT1 is responsible for basal glucose uptake (Mueckler & Thorens, 2013). Recent studies have shown that the activity and plasma membrane localization of GLUT1 is regulated by not only its transcriptional expression but also its phosphorylation by protein kinase C (PKC) (Lee et al., 2015) and S-palmitoylation by DHHC9 (Zhang et al., 2021). Compared to other GLUTs, GLUT2 has a low affinity for glucose with a Km value of approximately 17 mM (Uldry, Ibberson, Hosokawa, & Thorens, 2002). GLUT2 is involved in glucose uptake and release in hepatocytes. GLUT2 is present in the basolateral membrane of cells of the intestine and participates in the release of glucose. GLUT3 is expressed primarily in the brain. Similar to GLUT1, GLUT3 has a high affinity for glucose, suggesting that it is involved in basal glucose uptake. GLUT4 is an insulin-responsive glucose transporter that is translocated from the intracellular reservoir compartment to the plasma membrane (Huang & Czech, 2007). GLUT4 is present mainly in insulin-responsive tissues such as adipose and skeletal muscles. Of class II transporters (GLUTs 5, 7, 9, and 11), GLUT5 transports fructose into cells in the small intestine (Burant, Takeda, Brot-Laroche, Bell, & Davidson, 1992), and GLUT9 uses urate as a substrate (Anzai et al., 2008). Class III members (GLUTs 6, 8, 10, and 12, and HMIT/13) possess dileucine motifs at the N-terminus, which is a signal sequence, for the intracellular translocation of membrane proteins (Holman, 2020). This suggests that they function not only on the cell membrane but also in intracellular organelles such as vesicles and secretory granules.

Upregulation of GLUT proteins has been reported in numerous tumor types. GLUT1 and GLUT3 are overexpressed in a wide variety of cancers, such as lung, ovary, colon, breast, and brain cancers, and high expression of GLUT1, GLUT3, or both is often correlated with poor clinical outcomes in patients (Barron, Bilan, Tsakiridis, & Tsiani, 2016). Other GLUT isoforms, including GLUT2 (Godoy et al., 2006, Kim et al., 2017), GLUT5 (Godoy et al., 2006), GLUT6 (Caruana & Byrne, 2020), and GLUT12 (Rogers, Docherty, Slavin, Henderson, & Best, 2003), have also been linked to tumor development and progression.

Otto Warburg discovered aerobic glycolysis (the Warburg effect) in cancer cells over 100 years ago, and altered cellular metabolism is now a recognized hallmark of cancer (Hanahan, 2022). Advances in genome and omics research have revealed that the essence of tumor metabolism is the metabolic reprogramming orchestrated by oncogenes and tumor suppressor genes. For example, the K-ras and c-Myc oncoproteins upregulate GLUT1 to stimulate glycolysis. In contrast, the tumor suppressor p53 negatively regulates GLUT1 and GLUT3 transcription (Ancey, Contat, & Meylan, 2018). Advances in research have evidenced that tumor metabolism is not necessarily dependent on the Warburg effect but rather on metabolic diversity, plasticity, adaptability, and heterogeneity (DeBerardinis and Chandel, 2016, Martinez-Outschoorn et al., 2017). Some cancer cells use glutamine as their main energy source and switch and adapt their metabolic mechanisms depending on their environment. Furthermore, cancer cells adopt different metabolic mechanisms inside and outside of a single solid tumor. Therefore, it is important to consider the tumor type for the development of metabolic inhibitors, including GLUT inhibitors, as anti-cancer drugs.