Deoxy Sugars as Inhibitors of Biosynthesis

The Glc and Man analog 2-deoxy-d-glucose has been shown to be a general inhibitor of cellular metabolism. Early work by Woodward et al. showed that this deoxy sugar is a strong inhibitor of both the growth of yeast cells and their fermentation of Glc

[77]. It blocks glycoconjugate biosynthesis and has antiviral activity [39,78-83]. Since 2-deoxy-Glc uses the same transport system as Glc, it might simply block Glc uptake, depriving cells of a key metabolic substrate [84-87]. This has been directly demonstrated at high concentrations in yeast, chick fibroblasts, and Ehrlich ascites carcinoma cells [88,89]. However, at lower concentrations, 2-deoxy-Glc is taken up and converted to its UDP and GDP analogs. Incorporation into cell wall glucans in yeast and glycoconjugates in mammalian cells has been demonstrated [81,90-94].

Within glycoconjugates, 2-deoxy-Glc might serve as a chain terminator for further elongation. Studies of altered cell surface architecture induced by treatment of 2-deoxy-Glc have been performed [90]. Lectin binding studies suggest that 2-deoxy-Glc actually replaces Man, not Glc. Agglutination of cells by the lectins Ri-cinus communis (RCA) and Phaseolus vulgaris (PHA) [78], both of which bind to Gal and GalNAc residues, is significantly decreased during incubation with 2-deoxy-Glc. This result indicates that the underlying mannosyl residues are unable to support further chain elongation to include elaboration with Gal or GalNAc, residues necessary for binding to these lectins. Another possibility is that the underlying Man residues are simply missing. However, binding of concanavalin A (ConA) [78,95], a Man-specific lectin, increases on cells incubated with 2-deoxy-Glc, suggesting that Man residues are present, perhaps as terminal residues if the 2-deoxy modification prevents further elongation. Other studies provide more direct evidence that 2-deoxy-Glc functions as a Man substitute rather than a Glc substitute in cells. The addition of low doses of Man, but not Glc, to cells grown with 2-deoxy-Glc can reverse the effects of this anti-metabolite [96,97]. Addition of Man to cells grown with 2-deoxy-Glc restores natural lectin agglutinating properties, further confirming that the compound functions as a Man mimic.

Similar to 2-deoxy-Glc, it has been established that 2-deoxy-Gal is incorporated into glycoproteins in both rat and human cells [98-100]. When rats to which 2-deoxy-Gal had been administered were injected with [3H]Fuc, a significant decrease in Fuc incorporation was observed. This result suggested that the missing 2-OH group on Gal prevented 2-fucosylation, and this was confirmed by treatment with an a1,2-fucosidase. Interestingly, analysis with a1,3- and a1,4-fucosidases revealed a corresponding increase in a1,3/a1,4-fucosylation. Incubation of AS-30D rat ascites hepatoma cells with 2-deoxy-[1-14C]Gal [101] or the administration of this compound to rats [102,103] resulted in a large accumulation of 2-deoxy-Gal-1-phosphate. This metabolite comprised 78% of the total radioactivity detected in the liver, with the remaining metabolites being 20% UDP-2-deoxy-Gal and 0.7% UDP-2-deoxy-Glc [104]. The formation of the uridylate intermediates occurred only after high levels of 2-deoxy-Gal-1-phosphate had been achieved. The accumulation of UDP-2-deoxy-Gal-1-phosphate was attributed to the limited epimerization of UDP-2-deoxy-Gal-1-phosphate to UDP-2-deoxy-Glc-1-phosphate. This phenotype is characteristic of uri-dylyltransferase-deficient cells and tissues induced by Gal and may serve as an excellent model for studying the cellular injury in galactosemia.

2-Deoxy-Fuc, a residue found to be a constituent of several natural products, was tested for inhibition and cytotoxicity properties in L1210 leukemia, mouse mammary adenocarcinoma (TA3), and P288 leukemia cells (Table 7) [105]. This compound had no significant effect on the growth of any of the cell lines tested. However, the fully acetylated version of this substrate was active as a growth inhibitor, and it exhibited significant influence on glycoprotein biosynthesis in cultured P288 leukemia cells. The methyl glycoside of the fully acetylated 2-deoxy-Fuc had an even greater effect on cell viability, with an IC50 of 500 ^M in L1210 leukemia cells; cell growth was inhibited to 43% of control cells. Furthermore, this compound decreased cellular incorporation of [3H]GlcN and [3H]leucine by 20 and 9%, respectively.

In summary, much early work with subtly modified monosaccharide substrates suggested the capacity to alter cell surface glycosylation through metabolic processes.

Table 7 Growth Inhibitory Activity of 2-Deoxy Fuc Analogs Against L1210 Leukemia Cells

Table 7 Growth Inhibitory Activity of 2-Deoxy Fuc Analogs Against L1210 Leukemia Cells

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