The primary requirements of an ideal bioprobe for glucose

The primary requirements of an ideal bioprobe for glucose transporter-mediated bioimaging include high specificity, sensitivity as well as low cytotoxicity [18]. According to these concepts, we recently reported the simple amide linked glycoconjugates which containing cyanine fluorophores (Cy3 and Cy5 fluorescent dyes) attached to the 1N-position of 1-amino-1-deoxyglucose [19]. Our study showed that the 1N-Gly-Cy3/5 dyes are GLUT-specific and can be potentially used for real-time monitoring and microplate-based fluorometric studies in GLUT-overexpress living cell systems. By adopting the design concept implicated from the first generation glucose transport imaging dye 6-NBDG (Fig. 1, the 6N-glycoconjugated fluorescent dye), we subsequently synthesized the similar bioprobes by coupling the cyanine fluorophores with 6-amino-6-deoxy-d-galactose (Fig. 1). Our previous studies have demonstrated that galactose as GLUT natural substrate is a suitable glycoconjugation motif for platinum(II)-based drug delivery via GLUT mediated cell uptake and Warburg effect related tumor targeting [20], [21]. The whole rationales for the current study include: 1) evaluate if galactose conjugated cyanine fluorophores are useful for GLUT mediated cell imaging, 2) leave C-1 hydroxyl group (1-OH) of galactose unsubstituted may potentially make the probe to be substrate for galactokinase (GALK), therefore amenable for galactose metabolic research in galactosemia, 3) cyanine coupling at the C-6 position may help the sugar-conjugate to be well recognized by hexose transporter due to the hydrophobic region of the transporter is in close proximity to this part [9], [22], 4) the amide KU55933 australia enables the linker not susceptible to enzymatic destruction in biological environments, thereby ensuring a stable labeling reagent [15], [23], 5) cyanine dyes were chosen as fluorophore because of their high compatibility with biological systems and excellent tolerance of intense light sources as compared with the fluorescent group of 7-nitrobenz-2-oxa-l,3-diazol-4-yl (NBD) [24], [25].
Materials and methods
Results and discussion
Conclusion Investigation results showed that both 6N-Gly-Cy3 and 6N-Gly-Cy5 as 6-amino-6-deoxy fluorescent glycoconjugates are promising glucose transport bioimaging probes with very intense fluorescence, high GLUT-specificity, as well as high sensitivity for cellular uptake and cenn-based fluorometric assay. Through comparison study, we confirmed for the first time that 6-NBDG as the first generation of glucose transport fluorescent probe is not applicable for cell-based uptake study under physiological conditions due to lack of compatibility of the molecule with the culture medium. The long-lasting quenching property of 6N-Gly-Cy5 suggested the potential applcability of the probe for cell labeling in xenograft transplantation as well as in vivo animal imaging studies. Differing from the 1N-derived bioprobes, the galactose conjugated 6N-Gly-Cy3 and 6N-Gly-Cy5 also offering potential for probing the galactokinase mediated glycolysis and galactose metabolism. Compared with the traditional glucose uptake assay via radioisotope-labeled probes, 6N-Gly-Cy3 and 6N-Gly-Cy5 allow the safe and simple real-time monitoring of glucose transport in live GLUT-overexpressing cells, thereby providing an important tool for chemical biology and biomedical science. Furthermore, a combination of 6N-Gly-Cy3 and 6N-Gly-Cy5 can be potentially used in single-molecule FRET-mediated spatial resolution studies in biological research. More applications and evaluation studies are in progress to demonstrate the robustness and significance of these bioprobes in biomedical, diagnostic and Warburg effect targeted drug screening studies.
Acknowledgements This research was supported by Grants from the Tianjin Municipal Applied Basic and Key Research Scheme of China (11JCYBJC14400, 12ZCDZSY11500, 13JCZD27500), and by the Project of National Basic Research (973) Program of China (2015CB856500).