Stimulation of glucose transport in response to activation of distinct AMPK signaling pathways

Abstract

AMP-activated protein kinase (AMPK) plays a critical role in the stimulation of glucose transport in response to hypoxia and inhibition of oxidative phosphorylation. In the present study, we examined the signaling pathway(s) mediating the glucose transport response following activation of AMPK. Using mouse fibroblasts of AMPK wild type and AMPK knockout, we documented that the expression of AMPK is essential for the glucose transport response to both azide and 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR). In Clone 9 cells, the stimulation of glucose transport by a combination of azide and AICAR was not additive, whereas there was an additive increase in the abundance of phosphorylated AMPK (p-AMPK). In Clone 9 cells, AMPK wild-type fibroblasts, and H9c2 heart cells, azide or hypoxia selectively increased p-ERK1/2, whereas, in contrast, AICAR selectively stimulated p-p38; phosphorylation of JNK was unaffected. Azide's effect on p-ERK1/2 abundance and glucose transport in Clone 9 cells was partially abolished by the MEK1/2 inhibitor U0126. SB 203580, an inhibitor of p38, prevented the phosphorylation of p38 and the glucose transport response to AICAR and, unexpectedly, to azide. Hypoxia, azide, and AICAR all led to increased phosphorylation of Akt substrate of 160 kDa (AS160) in Clone 9 cells. Employing small interference RNA directed against AS160 did not inhibit the glucose transport response to azide or AICAR, whereas the content of P-AS160 was reduced by ∼80%. Finally, we found no evidence for coimmunoprecipitation of Glut1 and p-AS160. We conclude that although azide, hypoxia, and AICAR all activate AMPK, the downstream signaling pathways are distinct, with azide and hypoxia stimulating ERK1/2 and AICAR stimulating the p38 pathway.