Aerobic glycolysis is carried out at an inordinately high rate in cancer cells, a phenomenon known as the Warburg effect. While the relevance of this effect is unclear, it is partly driven by upregulation of PFKFB3 (6-Phospho-2-Fructo Kinase/Fructose-2,6-Bisphosphatase). Intriguingly, the expression of PFKFB3 in adipocytes is extremely high compared to most other tissues. We observed an insulin-dependent induction of aerobic glycolysis in adipocytes through lactate production. We hypothesised that the insulin-dependent upregulation of glycolysis in adipocytes is critical for adipocytes to enter the anabolic state. We therefore examined the effect of perturbing PFKFB3 activity on glucose metabolism and insulin signalling.

The PFKFB3 inhibitor 3-(3-pyridinyl)-1-(4-pyridinyl)-2-propen-1-one (3PO) decreased insulin-stimulated glucose uptake, GLUT4 translocation and lactate production in adipocytes. These data suggest that glycolysis controls insulin-stimulated glucose entry into adipocytes. Because this process relies on the activation of Akt we examined the effects of 3PO on Akt signalling. 3PO inhibited insulin-stimulated Akt activation by 90% in 3T3-L1 adipocytes whilst insulin receptor and IRS phosphorylation were unaffected. To investigate the relationship between PFKFB3 activity and Akt signalling we examined the effect of over-expression or siRNA knock down of PFKFB3 in HEK-293 cells. Knock down of PFKFB3 decreased insulin-stimulated Akt phosphorylation at Ser473 while PFKFB3 overexpression increased Akt phosphorylation by ~5 fold. A strong positive correlation between lactate released and insulin stimulated Akt phosphorylation was observed. The effects on Akt are not due to an autocrine effect of lactate itself since addition of lactate to culture medium did not potentiate Akt activation.

The ‘feedforward’ mechanism by which insulin stimulation increases glycolysis is well described. Our study reveals a ‘feedback’ mechanism by which a defect in glycolysis decreases growth factor signalling through Akt. This has potential implications for both cancer, where Akt activity may be driven by over stimulation of glycolysis, and diabetes where impaired glucose uptake may compromise Akt activity.