Introduction
In pancreatic β cells, glucose metabolism is necessary for the regulation of insulin secretion. Glucose is taken up by glucose transporters and metabolized to yield adenosine triphosphate (ATP), which is the main driver of glucose-induced insulin secretion (GIIS). Increased cytosolic ATP triggers 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) sets off exocytosis of insulin granules in the triggering pathway of insulin secretion (Figure 7.1). In addition, others signals generated by glucose amplify insulin secretion. Lipid metabolism is likewise involved in GIIS by interacting along with glucose metabolism.
Glucose sets off insulin secretion in a biphasic manner: an first component (1st phase) develops quickly however lasts just a couple of minutes, and is followed by a sustained component (2ndphase). Pancreatic β cells contain a minimum of 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%. Despite the fact that the prevailing hypothesis is that release of predocked granules accounts for the 1st phase and a subsequent supply of brand-new granules mobilized for release accounts for the 2nd phase of GIIS, recent studies reveal that the two phases involve granules that are located some distance from plasma membrane. Hormonal and neural inputs to the β cells are likewise essential for modulating GIIS.
Beta-Cell metabolism
Glucose sensing glycolysis
The a lot of prominent feature of pancreatic β cells is secretion of insulin in response to adjustments in the physiologic concentration of extracellular (blood) glucose, the cells possessing the capacity to sense circulating glucose levels. Glucose is transported in to the β cells through facilitated glucose transporters then is promptly phosphorylated by glucokinase in the glycolytic pathway. In rodents, the significant glucose transporter is GLUT2, which is a high-capacity, low-affinity glucose transporter isoform. Despite the fact that GLUT2 is the significant glucose transporter in pancreatic β cells in rodents, GLUT1 is predominantly expressed in human β cells [1]. On the others hand, the glucose-phosphorylating enzyme glucokinase (hexokinase IV: a higher Km isoform of hexokinase) catalyzes the formation of glucose-6-phosphate from glucose devoid of allosteric inhibition of the product. As glucokinase determines the price of glycolysis, it is considered to be the molecular glucose sensor for insulin secretion in pancreatic β cells [2]. Indeed, overexpression of hexokinase shifts glucose sensitivity in a mouse pancreatic β-cell line [3] and mutations in the glucokinase gene can easily trigger diabetes [4]. Phosphorylated glucose is after that metabolized to create pyruvate, the end product of glycolysis. As the expression of lactate dehydrogenase (LDH) is rather reasonable in pancreatic β cells [5], pyruvate readily goes into the mitochondrion for subsequent oxidation.
Mitochondrial metabolism
In the mitochondrion, pyruvate is converted to acetyl-CoA by pyruvate dehydrogenase (PDH) and reacts along with oxaloacetate to form citrate, an intermediate metabolite in the tricarboxylic acid (TCA) cycle. In addition, an anaplerotic pathway supplies oxaloacetate for the TCA cycle straight from pyruvate by pyruvate carboxylase (PC). This reaction is likewise involved in the pyruvate/malate shuttle. The TCA cycle is an essential metabolic circuit in terms of production of lowering 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 after that 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 essential cardiovascular pathway for the last step of the oxidation of fatty acids and certain amino acids too as carbohydrates.
ATP generation in the respiratory chain
Activation of the TCA cycle stimulates the electron transport chain to pump H+ ions from 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 after that transfers electrons to complex III, which is a multisubunit transmembrane healthy protein encoded by the two the mitochondrial and the nuclear genomes. Complex III transports electrons to cytochrome c, then to complex IV, in which these electrons are transferred to oxygen (O2), making H2O. At the very 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 relocate from an electron donor (NADH and FADH2) to a terminal electron acceptor (O2) via a collection 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). Due to the fact that ATP production is a important signal in the triggering pathway of GIIS from pancreatic β cells (Figure 7.1), disruption of mitochondrial function triggers loss of GIIS [6,7].
NADH shuttles
NADH shuttles are linked to glycolysis to yield NAD+ in supplying electrons for the respiratory chain in the mitochondria (Figure 7.2). Pancreatic β cells cannot yield NAD+ via lactate formation due to very reasonable LDH activity [5]. Instead, β cells possess higher activity of two NADH shuttles, the malate-aspartate (MA) shuttle and the glycerol-phosphate (GP) shuttle, the two of which yield NAD+ via mitochondrial oxidation. In the MA shuttle, cytosolic oxaloacetate is reasonable to malate by cytosolic malate dehydrogenase (MDH1) utilizing NADH+H+ to yield NAD+. Malate is transported in to the mitochondrial matrix in exchange for α-ketoglutarate, then oxidized by mitochondrial malate dehydrogenase (MDH2) spine to oxaloacetate, a procedure 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 reasonable to form glycerol 3-phosphate by cytosolic glycerol 3-phosphate dehydrogenase (GPD1) utilizing NADH+H+ to yield NAD+. Glycerol 3-phosphate diffuses in to the intermembrane space of the mitochondrion and is oxidized by mitochondrial glycerol 3-phosphate dehydrogenase (GPD2) to yield dihydroxyacetone phosphate, which diffuses spine to the cytosol, and FADH2, which can easily be oxidized by the respiratory chain. The activity of GPD2 is very higher in pancreatic islets [5,8], and decreased activity of GPD2 or the GP shuttle could be associated along with type 2 diabetes [9]. However, GPD2 deficient mice do not have actually impaired GIIS [10,11], indicating that the GP shuttle is not necessary for GIIS in pancreatic β cells. Inhibition of the MA shuttle by aminooxyacetate in GPD2 deficient mice abolishes GIIS [10]. Thus, both NADH shuttles complementarily operate in pancreatic β cells in regulating GIIS.
Lipid metabolism
In pancreatic β cells, glucose metabolism interacts along with lipid metabolism, such that GIIS is associated along 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 to make sure that LC-CoA levels are increased. a higher concentration of glucose activates the TCA cycle and enhances anaplerotic input of OAA, which in transform 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 after that being carboxylated by acetyl-CoA carboxylase-1 (ACC1) to form malonyl-CoA. Thus, glucose stimulation of β cells elevates malonyl-CoA levels [12]. Addition of LC-CoA to permeabilized β cells stimulates insulin granule exocytosis [13], suggesting a role for LC-CoA in GIIS. Inhibition of the tricarboxylate transporter in the mitochondrial outer membrane decreases GIIS however not potassium-induced insulin secretion in rat clonal β cells [14], suggesting that export of citrate from mitochondria to cytosol is involved in metabolic signaling in the regulation of GIIS.
