L-Glucose: Another Path to Cancer Cells

Abstract

Cancerous tumors comprise cells showing metabolic heterogeneity. Among numerous efforts to understand this property, little attention has been paid to the possibility that cancer cells take up and utilize otherwise unusable substrates as fuel. Here we discuss this issue by focusing on l-glucose, the mirror image isomer of naturally occurring d-glucose; l-glucose is an unmetabolizable sugar except in some bacteria. By combining relatively small fluorophores with l-glucose, we generated fluorescence-emitting l-glucose tracers (fLGs). To our surprise, 2-NBDLG, one of these fLGs, which we thought to be merely a control substrate for the fluorescent d-glucose tracer 2-NBDG, was specifically taken up into tumor cell aggregates (spheroids) that exhibited nuclear heterogeneity, a major cytological feature of malignancy in cancer diagnosis. Changes in mitochondrial activity were also associated with the spheroids taking up fLG. To better understand these phenomena, we review here the Warburg effect as well as key studies regarding glucose uptake. We also discuss tumor heterogeneity involving aberrant uptake of glucose and mitochondrial changes based on the data obtained by fLG. We then consider the use of fLGs as novel markers for visualization and characterization of malignant tumor cells.

Uptake of fluorescent d- and l-glucose analogues, 2-NBDG and 2-NBDLG, into human osteosarcoma U2OS cells in a phloretin-inhibitable manner

“…Pharmacological experiments further revealed that both of the inhibitors against GLUTs and against SGLTs failed to suppress the 2-NBDLG uptake into MIN6 cells [18]. Eventually, we found that the uptake of 2-NBDLG was abolished by phloretin [18], an aglycone of natural polyphenol phlorizin and a broadspectrum inhibitor against membrane transport including GLUTs/water channels [19,20], implying that 2-NBDLG enters the tumor cells through non-GLUT/non-SGLT pathways [4]. Since then, we had been unable to find out other tumor cell lines showing such specific uptake of 2-NBDLG.…”

“…Persistence of the 2-NBDLG uptake in the presence of both cytochalasin B and phlorizin suggested an involvement of a non-GLUT, non-SGLT-mediated mechanism in the uptake of 2-NBDLG in U2OS cells. It is widely accepted that glucose transporters, at least those known to date, all operate by binding of d-glucose to their glucose binding site [4]. Alternatively saying, the uptake of fluorescently labeled glucose analogues could well be competitively inhibited by d-glucose, if it is mediated by glucose transporters.…”

“…On the contrary, l-glucose could not be taken in nor metabolized by cells except some soil bacteria [3]. As such, l-glucose has been used only as a negative control substrate for d-glucose [4].…”

“…2-NBDG has been proposed to visualize tumor cells as well [16]. However, there may be a number of ways that cells use to take in glucose [4]. Thus, to more precisely understand this issue, we developed a fluorescently labeled l-glucose analogue 2-[N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-2-deoxy-L-glucose (2-NBDLG, Fig.…”

“…In addition, no imaging modality has been available for reliably discriminating malignant bone/soft tissue tumors from benign ones at the single cell study [26]. 2-NBDLG is one of the candidate tracers for visualizing single living tumor cells expressing malignant phenotypes [4]. To our best knowledge, this is the first study to characterize a human tumor cell line that is capable of specifically taking up the fluorescent analogue of l-glucose into cells in a pharmacologically defined manner.…”