AMP-activated protein kinase (AMPK) is activated by reduced cellular energy charge and mimics the action of insulin in muscle by stimulating increased trafficking of GLUT4 to the plasma membrane. In contrast, we have previously reported that short-term activation of AMPK in adipocytes has no effect on glucose uptake. Whether prolonged AMPK activation influences adipocyte glucose uptake remains poorly characterised. To investigate the effect of sustained AMPK activation on glucose uptake in adipocytes, glucose uptake and insulin signalling were assessed in 3T3-L1 adipocytes stimulated with AICAR and 991, which activate AMPK by different mechanisms, for 24 h. Furthermore, glucose uptake and GLUT4 levels were assessed in adipocytes or adipose tissue from mice lacking AMPKα1 as a model of prolonged AMPK downregulation. AICAR, but not 991, markedly inhibited insulin-stimulated glucose uptake in 3T3-L1 adipocytes. This effect of AICAR was associated with impaired trafficking of GLUT4 to the plasma membrane but did not alter cellular GLUT4 levels or insulin signalling via AKT. The effect of AICAR did, however, require phosphorylation to the nucleotide ZMP and was associated with altered insulin-stimulated MEK1/2-ERK1/2 phosphorylation. Sustained AMPK downregulation had no effect on adipocyte glucose uptake or GLUT4 levels. Taken together, these data demonstrate that sustained changes in AMPK activity do not alter adipocyte glucose uptake. Furthermore, AICAR reduces insulin-stimulated GLUT4 translocation and glucose uptake in adipocytes by a mechanism that is independent of AMPK but requires phosphorylation of AICAR to ZMP.
Keywords: AMPK; GLUT4; adipocyte; glucose uptake.