Elusive Proximal Signals of β-Cells for Insulin Secretion

The β-cell is unique because its major agonists, i.e., insulin secretagogues, undergo metabolism instead of interacting with a receptor. This perspectives presents the hypothesis that the first part of a metabolic signal of a secretagogue is specific to the secretagogue and the β-cell and can be envisioned as proximal. The second part, which occurs after transduction to more universal signaling mechanisms, is viewed as distal. Distal signaling and exocytosis in the β-cell operate the same as in other cells. Aerobic glycolysis is required for glucose-induced insulin release. Because glyceraldehyde, which enters metabolism at the triose phosphates in the glycolytic pathway, is a potent insulin secretagogue but pyruvate, which is metabolized in the mitochondrion, is not an insulin secretagogue, the proximal signal for glucose-induced insulin release originates with an interaction between the central part of the glycolytic pathway and mitochondrial metabolism. The proximal message in leucine-induced insulin release originates with leucine allosterically activating glutamate dehydrogenase, which activates endogenous glutamate metabolism, and by the metabolism of leucine itself. The methyl ester of succinate is a potent experimental insulin secretogogue. It is puzzling why the glucose signal requires the interplay of glycolysis and mitochondrial metabolism, whereas the signals from leucine and succinate originate entirely from within the mitochondrion. Leucine-induced insulin release is suppressed and glucose-induced insulin release is activated in islets cultured at a high concentration of glucose. Conversely, leucine-induced insulin release is activated and glucose-induced insulin release is suppressed in islets cultured at low glucose. We have correlated suppression of the insulinotropism of leucine and glucose with decreased expression of the genes that encode the catalytic subunit of the first component of the branched-chain ketoacid dehydrogenase complex and the pyruvate dehydrogenase complex, respectively. This indicates that the proximal signal is specific to the secretagogue, whereas distal signals are more universal and are shared by many secretagogues. The proximal signaling mechanisms have yet to be elucidated, but many distal mechanisms are known.