Kinetic modelling of β-cell metabolism reveals control points in the insulin-regulating pyruvate cycling pathways
Insulin, a key hormone in the regulation of glucose homeostasis, is secreted by pancreatic {beta}-cells in response to elevated glucose levels. Insulin is released in a biphasic manner in response to glucose metabolism in {beta}-cells. The first phase of insulin secretion is triggered by an increase in the ATP:ADP ratio; the second phase occurs in response to both a rise in ATP:ADP as well as other key metabolic signals, including a rise in the NADPH:NADP+ ratio. Experimental evidence indicates that pyruvate-cycling pathways play an important role in the elevation of the NADPH:NADP+ ratio in response to glucose. In this work we developed a kinetic model for the tricarboxylic acid cycle and pyruvate cycling pathways. We successfully validated our model against recent experimental observations and performed local and global sensitivity analysis to identify key regulatory interactions in the system. The model predicts that the dicarboxylate carrier (DIC) and pyruvate transporter (PYC) are the most important regulators of pyruvate cycling and NADPH production. In contrast, our analysis showed that variation in the pyruvate carboxylase (PC) flux was compensated by a response in the activity of mitochondrial isocitrate dehydrogenase (ICDm) resulting in minimal effect on overall pyruvate cycling flux. The model predictions suggest starting points for further experimental investigation, as well as potential drug targets for treatment of type 2 diabetes.