GLP-1 receptor signalling promotes β-cell glucose metabolism via mTOR-dependent HIF-1α activation

Glucagon-like peptide-1 (GLP-1) promotes insulin secretion from pancreatic ß-cells in a glucose dependent manner. Several pathways mediate this action by rapid, kinase phosphorylation-dependent, but gene expression-independent mechanisms. Since GLP-1-induced insulin secretion requires glucose metabolism, we aimed to address the hypothesis that GLP-1 receptor (GLP-1R) signalling can modulate glucose uptake and utilization in ß-cells. We have assessed various metabolic parameters after short and long exposure of clonal BRIN-BD11 ß-cells and rodent islets to the GLP-1R agonist Exendin-4 (50 nM). Here we report for the first time that prolonged stimulation of the GLP-1R for 18 hours promotes metabolic reprogramming of ß-cells. This is evidenced by up-regulation of glycolytic enzyme expression, increased rates of glucose uptake and consumption, as well as augmented ATP content, insulin secretion and glycolytic flux after removal of Exendin-4. In our model, depletion of Hypoxia-Inducible Factor 1 alpha (HIF-1a) impaired the effects of Exendin-4 on glucose metabolism, while pharmacological inhibition of Phosphoinositide 3-kinase (PI3K) or mTOR completely abolished such effects. Considering the central role of glucose catabolism for stimulus-secretion coupling in ß-cells, our findings suggest that chronic GLP-1 actions on insulin secretion include elevated ß-cell glucose metabolism. Moreover, our data reveal novel aspects of GLP-1 stimulated insulin secretion involving de novo gene expression

Differential effects of AMPK agonists on cell growth and metabolism

As a sensor of cellular energy status, the AMP-activated protein kinase (AMPK) is believed to act in opposition to the metabolic phenotypes favored by proliferating tumor cells. Consequently, compounds known to activate AMPK have been proposed as cancer therapeutics. However, the extent to which the anti-neoplastic properties of these agonists are mediated by AMPK is unclear. Here we examined the AMPK-dependence of six commonly used AMPK agonists (metformin, phenformin, AICAR, 2DG, salicylate and A-769662) and their influence on cellular processes often deregulated in tumor cells. We demonstrate that the majority of these agonists display AMPK-independent effects on cell proliferation and metabolism with only the synthetic activator, A-769662, exerting AMPK-dependent effects on these processes. We find that A-769662 promotes an AMPK-dependent increase in mitochondrial spare respiratory capacity (SRC). Finally, contrary to the view of AMPK activity being tumor suppressive, we find A-769662 confers a selective proliferative advantage to tumor cells growing under nutrient deprivation. Our results indicate that many of the anti-growth properties of these agonists cannot be attributed to AMPK activity in cells, and thus any observed effects using these agonists should be confirmed using AMPK-deficient cells. Ultimately, our data urge caution, not only regarding the type of AMPK agonist proposed for cancer treatment, but also the context in which they are used