Download files
Abstract
Type 2 diabetes (T2D) is a chronic metabolic disease that has become a major global health problem. It is well known that high blood glucose, insulin and glucagon levels are the main hallmarks of T2D. Most research has focused on glucose or insulin signalling pathways that might be defective in T2D, but the role of glucagon signalling in T2D progression/aetiology is largely unknown. The transcription factor 7-like 2 (TCF7L2) is strongly associated with T2D. Interestingly, β-catenin forms a complex with TCF7L2 to regulate gene transcription. Glucose has been found to regulate β-catenin/TCF7L2 in a variety of cell lines. In the liver, TCF7L2 overexpression leads to increased hepatic glucose production (HGP), whereas knockout studies showed reduced HGP. It remains unknown what is regulating β-catenin/TCF7L2 signalling in the liver to induce changes in HGP. Thus, we aimed to see if glucose (along with insulin or glucagon) could regulate hepatic β-catenin/TCF7L2 signalling. In isolated rat liver, we found glucagon (no effect of glucose or insulin) rapidly increased β-catenin/TCF7L2 signalling.
Next we used primary rat hepatocytes to investigate pathways regulating β-catenin/TCF7L2 signalling following glucagon exposure. It is well established that protein kinase A, protein kinase B and the Wnt pathway can regulate the phosphorylation of β-catenin. Thus we used a range of small molecule inhibitors to dissect how glucagon regulates β-catenin. We found glucagon phosphorylation of serine 552 on β-catenin is not via either protein kinase B or the Wnt pathway, but directly through protein kinase A. To examine this mechanism in vivo we used an obese mouse model and our data demonstrated glucagon also phosphorylates serine 552 on β-catenin in vivo and niclosamide (the FDA approved antihelminthic drug) blocked glucagon signalling and reduced HGP thus improving glucose metabolism in obese mice. As niclosamide is a mitochondrial uncoupler we hypothesise that this reduces ATP synthesis resulting in inhibition of adenylate cyclase activity and cAMP production thus inhibiting PKA activation upon stimulation of the glucagon receptor in the liver. Our experimental data is highly reproducible, from primary hepatocytes, isolated liver to whole animals. Thus, we have identified an important novel link between glucagon and β-catenin/TCF7L2 signalling that could provide a new drug target for treating T2D in people with obesity and defects in glucagon signalling.