Fig. 4: Impact of the circadian clock on metabolic reprogramming in GBM.

The circadian clock impacts major metabolic pathways, which are important for GBM progression such as glycolysis and lipid metabolism, as well as early autophagy. Glycolysis is a pathway which converts glucose to pyruvate. It produces ATP and NADH for energy and biomolecule production. HK2 is a key enzyme of glycolysis. It traps glucose in the cell in the form of glucose-6-phosphate (Glc-6-P). Pyruvate can be converted to Acetyl-CoA (Acetyl-Co-enzyme A) and utilized in the TCA-cycle or converted to lactate. The conversion to lactate is achieved by LDHA. LDHA is one of the key enzymes of aerobic glycolysis (Warburg phenotype). This pathway is often preferred by cancer cells over oxidative phosphorylation even in the presence of oxygen to provide intermediates for amino acid and nucleotide production. The TCA-cycle consists of several oxidative decarboxylation steps to produce redox equivalents and ATP for later oxidative phosphorylation. It also serves as a hub for the production of various biomolecules. CLOCK/BMAL1 positively influence several glycolysis enzymes (PGM1, HK2) and the TCA cycle. They also act as positive transcriptional regulators of LDHA to promote aerobic glycolysis. CRY and REV-ERB show a negative influence on both glycolysis (HK2), LDHA and the TCA cycle. REV-ERBα inhibits the transcription of FABP7 and RORα activates it. FABP7 is implicated in FA (Fatty Acid) uptake and -transport and FA-loaded FABP7 activates the transcription of PPARγ (Peroxisome Proliferator Activated Receptor Gamma) to promote migration, invasion and proliferation. CLOCK/BMAL1 are important positive regulators of pathways, which are crucial for tumour cells, but less for healthy cells (autophagy and de-novo FA synthesis). Specifically, CLOCK/BMAL1 positively influence FAS (Fatty Acid Synthetase), SCD (Stearoyl-CoA Desaturase) and ACACA, whereas REV-ERB negatively influences SCD and FAS. FA synthesis originates from Acetyl-CoA, which is converted to Malonyl-CoA by ACACA. FAS then merges Acetyl-CoA with Malonyl-CoA to form palmitate, which is transported into the smooth endoplasmic reticulum (sER). Palmitate is processed to FAs in a multistep process involving SCD. They can be further converted to TAGs (Triacylglycerol) by addition of glycerol. Autophagy is a process in which the cell removes dysfunctional or no-longer necessary components. It consists of early autophagy in which the autophagosome is formed from the rER and late autophagy in which the autophagosome is fused with the lysosome to form the autolysosome. CLOCK/BMAL1 positively regulates the early autophagy proteins ULK1 (Unc-51 (Uncoordinated-51) Like Autophagy Activating Kinase 1), Beclin1, ATG7 (Autophagy Related 7) and ULK3 (Unc-51 Like Autophagy Activating Kinase 3), whereas REV-ERB negatively influences them. The clock network as a whole regulates several metabolic pathways i.e., neurosteroid synthesis from cholesterol, mitochondrial fusion and fission, mitochondrial transport (transduceosome), GPL synthesis and the redox state of the cell. Neurosteroids are formed from cholesterol. Cholesterol is produced from AcetylCoA in the sER. It is converted to pregnenolone and further to neurosteroids in the mitochondria. GPLs are produced from the glycolysis intermediate glyceraldehyde-3-phosphate (G3P), which is converted into phosphatidic acid (PA) in the sER. PA is converted to diacyl-glycerol (DAG) by PAP (Prostatic Acid Phosphatase). DAG serves as an intermediate to form either Lyso-PE (Phosphatidylethanolamine) or Lyso-PC (Phosphatidylcholine). PCYT-2 (Phosphate Cytidylyltransferase 2) converts DAG to PE. PE is then converted to Lyso-PE by LPEAT (Lyso-Phosphatidylethanolamine Acyltransferase). On the other hand, DAG can be converted to PC by CHOKα (Choline Kinase Alpha) and then to Lyso-PC by LPCAT (Lyso-Phosphatidylcholine Acyltransferase). GPLs regulate the fluidity of the plasma membrane and their synthesis is regulated by the redox state of the cell. The figure was created with BioRender.com.