O-GlcNAc Protein Modification in Cancer Cells Increases in Response to Glucose Deprivation through Glycogen Degradation

Abstract

Although the level of O-GlcNAc transferase was unchanged, the enzyme contained less O-GlcNAc, and its activity was increased. Moreover, O-GlcNAcase activity was reduced. The studied cells contain glycogen, and we show that its degradation in response to glucose deprivation provides a source for UDP-GlcNAc required for increased O-GlcNAcylation under this condition. This required active glycogen phosphorylase and resulted in increased glutamine:fructose-6-phosphate amidotransferase, the first and rate-limiting enzyme in the hexosamine biosynthetic pathway. Interestingly, glucose deprivation reduced the amount of phosphofructokinase 1, a regulatory glycolytic enzyme, and blocked ATP synthesis. These findings suggest that glycogen is the source for increased O-GlcNAcylation but not for generating ATP in response to glucose deprivation and that this may be useful for cancer cells to survive.

Glucose Deprivation-induced Increase in Protein O-GlcNAcylation in Cardiomyocytes Is Calcium-dependent

“…Similarly, Kang et al found a significant decrease in OGA activity following glucose deprivation, but surprisingly they found no changes in OGA protein levels (24). Conversely, Cheung et al reported no decrease in OGA activity in response to glucose deprivation (26); however, similar to our findings, Taylor et al reported a decrease in OGA protein (25).…”

“…One limitation of our study is that we did not measure OGT activity; however, contrary to other reports (25-27) we also did not observe any change in OGT in response to glucose deprivation. It should be noted, however, that whereas Kang et al found no change in OGT protein levels following glucose deprivation, they did observe an increase in OGT activity (24). On the other hand, Cheung et al reported no change in OGT specific activity (26) in response to glucose deprivation.…”

“…On the other hand, Cheung and Hart reported that in Neuro-2a neuroblastoma cells, the effects of glucose deprivation on O-GlcNAcylation were mediated in part by AMPK as well as by p38 MAPK (26); however, in A549 lung cancer cells Kang et al, reported that the glucose deprivation-induced increase in O-GlcNAc was not APMK-dependent, but rather appeared to be due to increased glycogen degradation (24).…”

“…5B). Cheung and Hart showed that 20 M Compound C attenuated the O-GlcNAc response to glucose deprivation (26), whereas Kang et al found that it had no effect (24). Compound C is widely used to inhibit AMPK in cell culture; however, it has been reported that 40 M is required to block APMK activity completely (53).…”

“…In Neuro-2a cells it was shown that glucose deprivation increased O-GlcNAc levels by up-regulation of OGT expression in an AMP-activated protein kinase (AMPK)-dependent manner and also that substrate targeting of OGT was regulated by p38 MAPK (26). On the other hand Kang et al (24) reported that glycogen was a primary substrate for glucose deprivation-induced increase in O-GlcNAc in various cancer cell lines and that this was accompanied by increased OGT activity and decreased O-GlcNAcase activity, which catalyzes the removal of O-GlcNAc, in the absence of changes in the levels of either protein.…”