OGT/CDK5/ACSS2 Axis Regulates Metabolic Adaptation of Tumors in the Brain
Authors:   , Ciraku Lorela , Reginato Mauricio J 
Published: August 2022

Brain borne tumors such as Glioblastoma multiforme (GBM) or tumors that metastasize to the brain are often incurable conditions, with no effective drug treatments with a median survival after diagnosis measured in months. Thus, there is an urgent need to develop novel treatment strategies for tumors in the brain. Cancer cells employ altered metabolic pathways in order to support the increased demand for macromolecular biosynthesis. The brain has a unique ability to rewire its metabolism in response to changing metabolite availability. Similarly, tumors in the brain must also adapt to utilize non-glucose sources to generate energy. Both brain tumors and tumors that metastasize to the brain are unique in their ability to efficiently oxidize acetate as an energy source preferentially over glucose. The metabolic enzyme acetyl-CoA synthetase short chain family member 2 (ACSS2), which catalyzes the conversion of acetate and coenzyme A (CoA) into acetyl-CoA, is critical for utilizing acetate as an energy source. In this study, we show that the nutrient sensing enzyme O-GlcNAc transferase (OGT), which uses UDP-GlcNAc to modify cytoplasmic and nuclear proteins with O-linked sugar moieties, lies upstream of ACSS2 and regulates GBM and breast cancer brain metastases (BCBM) growth. Specifically, we show that OGT and O-GlcNAcylation are elevated in GBM and BCBM cancer cells and patient samples. Reducing OGT expression in GBM and BCBM cells transplanted in an orthotopic intracranial mouse model reduced tumor growth and extended survival. Mechanistically, we show that in GBM cells, OGT overexpression increases carbon-flux of acetate into acetyl-CoA and lipids. OGT regulates acetate metabolism via ACSS2 phosphorylation on Ser-267 in a cyclin dependent kinase 5 (CDK5)-dependent manner, which regulates ACSS2 stability by reducing polyubiquitination and degradation. ACSS2 Ser-267 is critical for OGT-mediated GBM growth as overexpression of ACSS2 Ser-267 phospho-mimetic rescues tumor growth in vivo. We show that human breast cancer cells selected to metastasize to the brain contain increased levels of O-GlcNAc, OGT and ACSS2-Ser267 phosphorylation compared to parental cells and human brain metastatic patient samples contain elevated ACSS2-Ser-267 levels. Moreover, overexpression of ACSS2-S267D phospho-mimetic mutant confers a growth advantage on brain metastatic breast cancer cells in the brain but not in the mammary fat pad suggesting ACSS2 is critical for metabolic adaptation in the brain microenvironment. ACSS2 converts acetate into acetyl-CoA to be used by fatty acid synthase (FASN) to generate lipids. We show that CDK5-ACSS2 axis also regulates lipid storage and FASN protein levels in BCBM cells potentially by regulating acetylation and stability of FASN. Using a novel ex vivo GBM and BCBM brain slice model we show that treatment of GBM and BCBM-transplanted slices with OGT, CDK5 and ACSS2 inhibitors shrinks preformed tumors in cultured brain slices. Additionally, we identified novel ACSS2 inhibitors with an improved CNS drug-like profile from a pharmacophore-based shape screen that effectively reduced lipid storage and tumor growth in cultured brain slices. These data suggest that nutrient sensor O-GlcNAc via CDK5-mediated phosphorylation of ACSS2 may regulate cancer cells metabolic adaptability to the brain microenvironment and identify OGT/CDK5/ACSS2 as novel therapeutic targets for treatment of brain tumors and brain metastases.