Novel Antidiabetic Mechanism that Prevents Hypoglycemia
Hyperglycemia and type II diabetes meillitus are serious medical conditions that can be due to the disruption of glucose regulatory mechanisms, such as glycolysis and gluconeogenesis. A particular enzymatic site for regulation of glucose homeostasis mechanisms is glucokinase (GK), which converts glucose to glucose-6-phosphate in glycolysis. The endogenous inhibitor GK regulatory protein (GKRP) complexes with GK and deactivates GK’s ability to breakdown glucose, in times of low blood glucose levels. Prior research suggests that the GK:GKRP sequesters in the nucleus in order to regulate glucose levels back to normal via the absence of GK’s function in the cytosol during gluconeogensis. Since high blood glucose levels are a major symptom of type II diabetes meillitus, GK activators are a new therapeutic strategy to lower blood glucose levels, but many clinical accounts demonstrate that such therapeutic agents increase the dangerous risk of hypoglycemia. However, a new study by Lloyd et al. (Nature. 2013, published online November 13, DOI: 10.1038/nature12724) utilizes a novel therapeutic strategy that can lower the blood glucose levels by increasing glucose activity via GKRP inhibition and potentially avoiding the symptom of hypoglycemia. A characterized crystal structure of GKRP has not yet been identified and no small molecule inhibitors exist to date that can directly inhibitor for GKRP.
By screening the Amgen chemical library, the authors were able to identify AMG-1694, which with increasing concentrations mediates the dissociation of the GK-GKRP complex and indirectly increases the GK activity as demonstrated by a glucose-phosphorylation assay and a direct binding assay. The authors next concern is AMG-1694’s competition with a sorbitol-6-phosphate GK-GKRP stabilizer, but AMG-1694’s destabilizing effect overrides S6P’s stabilizing abilities, which the authors demonstrate by surface plasmon resonance. To have a better understanding of S6P and AMG-1694, the authors crystallize GKRP, discovering that there exists a pocket accessible to small molecules exclusively which S6P does not occupy. Then, in rat primary hepatocytes, the authors observe glucose uptake in the cell and GK cytoplasmic accumulation as a result of GK:GKRP dissociation and translocation from the nucleus. By observing GK translocation in vivo in diabetic fatty rats and normoglycemic rats by immunohistochemistry, the authors demonstrate that AMG-1694 effectively lowers blood glucose levels and increases GK translocation to the cytoplasm. Also, AMG-1694 is only effective in hyperglycemic animals based on the same data set. In three diabetes mice models, the authors demonstrate that a similar inhibitor AMG-3969 effectively lowers blood glucose levels and increases GK translocation to the cytoplasm. Lastly, using indirect calorimetry, the authors compare GKA and AMG-3969 efficacy in diabetic mice and discover that AMG-3696 was highly effective at promoting carbohydrate substrate use while GKA was not. In this study, the authors clearly demonstrate that AMG-1694 and AMG-3969 produce antidiabetic effects exclusively in rodent diabetic models by increasing GK activity via inhibition of GKRP, while avoiding hypoglycemic effects observed by GK activators. Although clear antidiabetic therapeutic potential exists, the authors demonstrate that further investigation must be conducted in order to understand the unknown inhibition mechanism of GK:GKRP dissociation by AMG-1694 and AMG-3969.