Roles for fructose-2,6-bisphosphate in the control of fuel metabolism: beyond its allosteric effects on glycolytic and gluconeogenic enzymes.

Fructose-2,6-bisphosphate (F26P2) was identified as a regulator of glucose metabolism over 25 years ago. A truly bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PFK2/FBP2), with two active sites synthesizes F26P2 from fructose-6-phosphate (F6P) and ATP or degrades F26P2 to F6P and Pi. In the classic view, F26P2 regulates glucose metabolism by allosteric effects on 6-phosphofructo-1-kinase (6PFK1, activation) and fructose-1,6-bisphosphatase (FBPase, inhibition). When levels of F26P2 are high, glycolysis is enhanced and gluconeogenesis is inhibited. In this regard, altering levels of F26P2 via 6PFK2/FBP2 overexpression has been used for metabolic modulation, and has been shown capable of restoring euglycemia in rodent models of diabetes. Recently, a number of novel observations have suggested that F26P2 has much broader effects on the enzymes of glucose metabolism. This is evidenced by the effects of F26P2 on the gene expression of two key glucose metabolic enzymes, glucokinase (GK) and glucose-6-phosphatase (G6Pase). When levels of F26P2 are elevated in the liver, the gene expression and protein amount of GK is increased whereas G6Pase is decreased. These coordinated changes in GK and G6Pase protein illustrate how F26P2 regulates glucose metabolism. F26P2 also affects the gene expression of enzymes related to lipid metabolism. When F26P2 levels are elevated in liver, the expression of two key lipogenic enzymes, acetyl-CoA carboxylase 1 (ACC1) and fatty acid synthase (FAS) is reduced, contributing to a unique coordinated decrease in lipogenesis. When combined, F26P2 effects on glucose and lipid metabolism provide cooperative regulation of fuel metabolism. The regulatory roles for F26P2 have also expanded to transcription factors, as well as certain key proteins (enzymes) of signaling and/or energy sensoring. Although some effects may be secondary to changes in metabolite levels, high levels of F26P2 have been shown to regulate protein amount and/or phosphorylation state of hepatic nuclear factor 1-alpha (HNF1alpha), carbohydrate response element binding protein (ChREBP), peroxisome proliferators-activated receptor alpha (PPARalpha), and peroxisome proliferators-activated receptor gamma co-activator 1beta (PGC1beta), as well as Akt and AMP-activated protein kinase (AMPK). Importantly, changes in these transcription factors, signaling proteins, and sensor proteins are produced in a way that appropriately coordinates whole body fuel metabolism.