Editors' ChoiceMetabolism

GlcNAc in Place of Glucose

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Sci. Signal.  21 Dec 2010:
Vol. 3, Issue 153, pp. ec391
DOI: 10.1126/scisignal.3153ec391

Because glucose and glutamine can serve as carbon sources for metabolic processes including mitochondrial ATP production, one might expect that cells deprived of glucose would increase their uptake and utilization of glutamine to compensate. However, Wellen et al. show that various primary and immortalized hematopoietic cells exhibit decreased glutamine uptake in response to glucose deprivation and that the cells become smaller. This was true even in the presence of growth factors and was explored in detail for cells dependent on interleukin 3 (IL-3). Mass spectrometry analysis of metabolites associated with different metabolic processes showed that the hexose-phosphate pathway and ribose-phosphate pathways (required for nucleotide and NADPH biosynthesis) were particularly dependent on glucose, whereas there was only partial labeling with radioactive glucose added after a deprivation period of a TCA cycle intermediate (citrate) and an intermediate in the hexosamine pathway (UDP-GlcNAc). Glucose deprivation caused a reduction in the abundance of the IL-3 receptor α subunit (IL-3Rα), which is a cell surface glycoprotein, and signaling downstream of the receptor even in the presence of IL-3 and the reduction in receptor abundance and signaling was rescued by the addition of N-acetylglucosamine (GlcNAc), which is required for protein glycosylation. Indeed, if protein glycosylation was pharmacologically inhibited, GlcNAc failed to restore receptor abundance or signaling. Metabolite analysis showed that GlcNAc was not substituting for glucose in glycolytic pathways, the pentose phosphate pathway, or the TCA cycle, but that it specifically acted in the hexosamine pathway. Cell size was not reduced in cells deprived of glucose if GlcNAc was substituted, and the addition of GlcNAc could restore growth to cells that had atrophied in response to glucose deprivation. However, GlcNAc could not substitute for glucose in promoting cell proliferation, which is consistent with the requirement for glucose for nucleotide biosynthesis. GlcNAc did not serve as a fuel source; instead, substitution of glucose for GlcNAc stimulated glutamine uptake, and this was blocked by inhibition of IL-3 receptor signaling and appeared to involve stimulation of the transcription of two amino acid transporters. The growth-promoting effects of GlcNAc were also blocked by the glutamine antagonist and analog 6-diazo-5-oxo-L-norleucine (DON). DON also blocked glucose- or GlcNAc-stimulated leucine uptake, which suggests that induction of glutamine uptake is required for leucine uptake, and blocked IL-3–mediated activation of downstream signaling. This inhibition of IL-3 signaling was likely due to the use of glutamine in mitochondrial respiration and the production of reactive oxygen species, because oxygen consumption increased in response to GlcNAc substitution and antioxidant treatment blocked the GlcNAc-mediated stimulation of IL-3 signaling. These results show that the availability of glucose regulates IL-3 receptor signaling through the hexosamine pathway and that in the absence of adequate flux through this pathway, uptake of glutamine as an alternate carbon source is prevented. It will be interesting to determine whether other growth factor receptors, many of which are glycosylated, are regulated by flux through the hexosamine pathway.

K. E. Wellen, C. Lu, A. Mancuso, J. M. S. Lemons, M. Ryczko, J. W. Dennis, J. D. Rabinowitz, H. A. Coller, C. B. Thompson, The hexosamine biosynthetic pathway couples growth factor-induced glutamine uptake to glucose metabolism. Genes Dev. 24, 2784–2799 (2010). [Abstract] [Full Text]

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