Fulfilling the metabolic requirements for cell proliferation.

The activity of key metabolic enzymes is regulated by the ubiquitin ligases that control the function of the cyclins; therefore the activity of these ubiquitin ligases explains the coordination of cell-cycle progression with the supply of substrates necessary for cell duplication. APC/C (anaphase-promoting complex/cyclosome)-Cdh1, the ubiquitin ligase that controls G(1)- to S-phase transition by targeting specific degradation motifs in cell-cycle proteins, also regulates the glycolysis-promoting enzyme PFKFB3 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3) and GLS1 (glutaminase 1), a critical enzyme in glutaminolysis. A decrease in the activity of APC/C-Cdh1 in mid-to-late G(1) releases both proteins, thus explaining the simultaneous increase in the utilization of glucose and glutamine during cell proliferation. This occurs at a time consistent with the point in G(1) that has been described as the nutrient-sensitive restriction point and is responsible for the transition from G(1) to S. PFKFB3 is also a substrate at the onset of S-phase for the ubiquitin ligase SCF (Skp1/cullin/F-box)-β-TrCP (β-transducin repeat-containing protein), so that the activity of PFKFB3 is short-lasting, coinciding with a peak in glycolysis in mid-to-late G(1), whereas the activity of GLS1 remains high throughout S-phase. The differential regulation of the activity of these proteins indicates that a finely-tuned set of mechanisms is activated to fulfil specific metabolic demands at different stages of the cell cycle. These findings have implications for the understanding of cell proliferation in general and, in particular, of cancer, its prevention and treatment.