Regulation of ACSS2 by O-GlcNAc transferase in glioblastoma cells

O-GlcNAcylation plays an important role in the regulation of various signaling pathways and diseases such as cancer. Many cancers contain elevated O-GlcNAc levels, owed to an increase in O-GlcNAc Transferase (OGT), the enzyme that adds this sugar moiety to proteins. OGT regulates the activity of many pathways, such as those that generate lipid metabolites, which are significantly reduced upon knockdown of OGT. Given OGT's apparent role in lipid metabolism, we hypothesized that highly lipid-dependent cancers, such as glioblastoma, would be reliant on OGT's activity. Additionally, glioblastoma has a very poor survival rate, and has been void of significant treatment advances in the last decade, thus research into this disease is desperately needed. Here, we show that O-GlcNAcylation is required for glioblastoma growth, and regulates protein levels of Acetyl-CoA Synthetase 2 (ACSS2), the enzyme that converts acetate to acetyl-CoA, a key precursor of lipid metabolism. Increased O-GlcNAcylation leads to elevated ACSS2 levels and increased phosphorylation on serine 267, a putative cyclin-dependent kinase 5 (CDK5) site. Indeed, we show that CDK5 can phosphorylate ACSS2 in vitro and these two proteins interact both in vitro and in glioblastoma cells in an O-GlcNAc-dependent manner. This phosphorylation event appears to be important in the regulation of ACSS2, as inhibition of CDK5 activity negates OGT's influence on ACSS2 protein levels. Thus, elevated O-GlcNAcylation increases acetate metabolism through the CDK5-dependent regulation of ACSS2 in brain cancer. This work identifies both OGT and CDK5 as novel therapeutic targets for treating glioblastomas.