In pancreatic β cells, glucose metabolism is essential for the regulation of insulin secretion. Glucose is taken up by glucose transporters and metabolized to generate adenosine triphosphate (ATP), which is the main driver of glucose-induced insulin secretion (GIIS). Increased cytosolic ATP causes closure of ATP-sensitive K+ (KATP) channels, depolarizing the plasma membrane, leading to the opening of voltage-dependent Ca2+ channels (VDCCs), which allows Ca2+ influx. The resultant rise in intracellular Ca2+ concentration ([Ca2+]i) induces exocytosis of insulin granules in the triggering pathway of insulin secretion (Figure 7.1). In addition, other signals generated by glucose amplify insulin secretion. Lipid metabolism is also involved in GIIS by interacting with glucose metabolism.
Glucose induces insulin secretion in a biphasic manner: an initial component (1st phase) develops rapidly but lasts only a few minutes, and is followed by a sustained component (2ndphase). Pancreatic β cells contain at least two pools of insulin secretory granules that differ in release competence: a reserve pool (RP) that accounts for the vast majority of granules, and a readily releasable pool (RRP) that accounts for the remaining <5%. Although the prevailing hypothesis is that release of predocked granules accounts for the 1st phase and a subsequent supply of new granules mobilized for release accounts for the 2nd phase of GIIS, recent studies show that both phases involve granules that are located some distance from plasma membrane. Hormonal and neural inputs to the β cells are also important for modulating GIIS.
Glucose sensing glycolysis
The most prominent feature of pancreatic β cells is secretion of insulin in response to changes in the physiologic concentration of extracellular (blood) glucose, the cells possessing the capacity to sense circulating glucose levels. Glucose is transported into the β cells through facilitated glucose transporters and then is promptly phosphorylated by glucokinase in the glycolytic pathway. In rodents, the major glucose transporter is GLUT2, which is a high-capacity, low-affinity glucose transporter isoform. Although GLUT2 is the major glucose transporter in pancreatic β cells in rodents, GLUT1 is predominantly expressed in human β cells . On the other hand, the glucose-phosphorylating enzyme glucokinase (hexokinase IV: a high Km isoform of hexokinase) catalyzes the formation of glucose-6-phosphate from glucose without allosteric inhibition of the product. As glucokinase determines the rate of glycolysis, it is considered to be the molecular glucose sensor for insulin secretion in pancreatic β cells . Indeed, overexpression of hexokinase shifts glucose sensitivity in a mouse pancreatic β-cell line  and mutations in the glucokinase gene can cause diabetes . Phosphorylated glucose is then metabolized to produce pyruvate, the end product of glycolysis. As the expression of lactate dehydrogenase (LDH) is very low in pancreatic β cells , pyruvate readily enters the mitochondrion for subsequent oxidation.
In the mitochondrion, pyruvate is converted to acetyl-CoA by pyruvate dehydrogenase (PDH) and reacts with oxaloacetate to form citrate, an intermediate metabolite in the tricarboxylic acid (TCA) cycle. In addition, an anaplerotic pathway provides oxaloacetate for the TCA cycle directly from pyruvate by pyruvate carboxylase (PC). This reaction is also involved in the pyruvate/malate shuttle. The TCA cycle is an important metabolic circuit in terms of production of reducing equivalents in the form of NADH and FADH2 for generation of ATP in the electron transport chain. Citrate is oxidized and decarboxylated to form α-ketoglutarate, which undergoes further oxidative decarboxylation to succinyl-CoA or generates glutamate by glutamate dehydrogenase (GDH). NADH is formed in these processes. Succinyl-CoA is then metabolized to succinate and converted subsequently to fumarate by succinate dehydrogenase, by which FADH2 is generated. At the end of the cycle, oxaloacetate is regenerated via malate. The TCA cycle is an important aerobic pathway for the final step of the oxidation of fatty acids and certain amino acids as well as carbohydrates.
ATP generation in the respiratory chain
Activation of the TCA cycle stimulates the electron transport chain to pump H+ ions out of the mitochondrial matrix, which hyperpolarizes the inner mitochondrial membrane.The respiratory chain comprises complex I (NADH-ubiquinone reductase), II (succinate dehydrogenase), III (ubiquinol-cytochrome c reductase), and IV (cytochrome c oxidase). Complex I and II accept electrons from NADH and FADH2, respectively, and transport them to ubiquinone (coenzyme Q). Ubiquinone then transfers electrons to complex III, which is a multisubunit transmembrane protein encoded by both the mitochondrial and the nuclear genomes. Complex III transports electrons to cytochrome c, and then to complex IV, in which these electrons are transferred to oxygen (O2), producing H2O. At the same time, protons are translocated across the membrane, contributing to the proton gradient. This gradient is used by the FOF1 ATP synthase complex (sometimes called complex V) to make ATP via oxidative phosphorylation. Thus, in the respiratory chain, electrons move from an electron donor (NADH and FADH2) to a terminal electron acceptor (O2) via a series of redox reactions, which are coupled to the creation of a proton gradient across the mitochondrial inner membrane. The resulting transmembrane proton gradient is used in making ATP. Synthesized ATP is translocated to cytosol by the adenine nucleotide translocator (ANT). Since ATP production is a critical signal in the triggering pathway of GIIS from pancreatic β cells (Figure 7.1), disruption of mitochondrial function causes loss of GIIS [6,7].
NADH shuttles are linked to glycolysis to generate NAD+ in supplying electrons for the respiratory chain in the mitochondria (Figure 7.2). Pancreatic β cells cannot generate NAD+ via lactate formation because of extremely low LDH activity . Instead, β cells possess high activity of two NADH shuttles, the malate-aspartate (MA) shuttle and the glycerol-phosphate (GP) shuttle, both of which generate NAD+ via mitochondrial oxidation. In the MA shuttle, cytosolic oxaloacetate is reduced to malate by cytosolic malate dehydrogenase (MDH1) using NADH+H+ to generate NAD+. Malate is transported into the mitochondrial matrix in exchange for α-ketoglutarate, and then oxidized by mitochondrial malate dehydrogenase (MDH2) back to oxaloacetate, a process in which NADH is supplied for oxidative phosphorylation by the respiratory chain. Oxaloacetate is transformed to aspartate catalyzed by mitochondrial aspartate aminotransferase (AST2) and moves out to the cytosol via glutamate/aspartate carrier (Aralar1) in exchange for glutamate. In the cytosol, aspartate is transaminated by cytosolic aspartate aminotransferase (AST1) to restore oxaloacetate. In the GP shuttle, dihydroxyacetone phosphate is reduced to form glycerol 3-phosphate by cytosolic glycerol 3-phosphate dehydrogenase (GPD1) using NADH+H+ to generate NAD+. Glycerol 3-phosphate diffuses into the intermembrane space of the mitochondrion and is oxidized by mitochondrial glycerol 3-phosphate dehydrogenase (GPD2) to generate dihydroxyacetone phosphate, which diffuses back to the cytosol, and FADH2, which can be oxidized by the respiratory chain. The activity of GPD2 is extremely high in pancreatic islets [5,8], and decreased activity of GPD2 or the GP shuttle may be associated with type 2 diabetes . However, GPD2 deficient mice do not have impaired GIIS [10,11], indicating that the GP shuttle is not essential for GIIS in pancreatic β cells. Inhibition of the MA shuttle by aminooxyacetate in GPD2 deficient mice abolishes GIIS . Thus, the two NADH shuttles complementarily operate in pancreatic β cells in regulating GIIS.
In pancreatic β cells, glucose metabolism interacts with lipid metabolism, such that GIIS is associated with inhibition of fatty acid oxidation and increased lipid synthesis. The activated form of fatty acids is long-chain acyl-CoA (LC-CoA), which is generated by acyl-CoA synthetase (ACS). The fate of fatty acids is determined by malonyl-CoA, which blocks mitochondrial oxidation of fatty acids so that LC-CoA levels are increased. A high concentration of glucose activates the TCA cycle and increases anaplerotic input of OAA, which in turn elevates export of citrate from mitochondria to cytosol via the citrate-isocitrate carrier (CIC). Citrate is cleaved by ATP-citrate lyase (ACL) to OAA and acetyl-CoA, the acetyl-CoA then being carboxylated by acetyl-CoA carboxylase-1 (ACC1) to form malonyl-CoA. Thus, glucose stimulation of β cells elevates malonyl-CoA levels . Addition of LC-CoA to permeabilized β cells stimulates insulin granule exocytosis , suggesting a role for LC-CoA in GIIS. Inhibition of the tricarboxylate transporter in the mitochondrial outer membrane decreases GIIS but not potassium-induced insulin secretion in rat clonal β cells , suggesting that export of citrate from mitochondria to cytosol is involved in metabolic signaling in the regulation of GIIS.